Podcasts about GTPase

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.07.21.550042v1?rss=1 Authors: Rutkowski, D. M., Vincenzetti, V., Vavylonis, D., Martin, S. G. Abstract: Local Cdc42 GTPase activation promotes polarized exocytosis, resulting in membrane flows that deplete low-mobility membrane-associated proteins from the growth region. To investigate the self-organizing properties of the Cdc42 secretion-polarization system under membrane flow, we developed a reaction-diffusion particle model. The model includes positive feedback activation of Cdc42, hydrolysis by GTPase-activating proteins (GAPs), and flow-induced displacement by exo/endocytosis. Simulations show how polarization relies on flow-induced depletion of low mobility GAPs. To probe the role of Cdc42 mobility in the fission yeast Schizosaccharomyces pombe, we changed its membrane binding properties by replacing its prenylation site with 1, 2 or 3 repeats of the Rit1 C terminal membrane binding domain (ritC), yielding alleles with progressively lower unbinding and diffusion rates. Concordant modelling predictions and experimental observations show that lower Cdc42 mobility results in lower Cdc42 activation level and wider patches. Indeed, while Cdc42-1ritC cells are viable and polarized, Cdc42-2ritC polarize poorly and Cdc42-3ritC is inviable. The model further predicts that GAP depletion increases Cdc42 activity at the expense of loss of polarization. Experiments confirm this prediction, as deletion of Cdc42 GAPs restores viability to Cdc42-3ritC cells. Our combined experimental and modelling studies demonstrate how membrane flows are an integral part of Cdc42-driven pattern formation. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.07.03.547282v1?rss=1 Authors: Bjornson, K. J., Vanderplow, A. M., Yang, Y., Kermath, B. A., Cahill, M. E. E. Abstract: The effects of repeated stress on cognitive impairment are thought to be mediated, at least in part, by reductions in the stability of dendritic spines in brain regions critical for proper learning and memory, including the hippocampus. Small GTPases are particularly potent regulators of dendritic spine formation, stability, and morphology in hippocampal neurons. Through the use of small GTPase protein profiling in mice, we identify increased levels of synaptic Rap1 in the hippocampal CA3 region in response to escalating, intermittent stress. We then demonstrate that increased Rap1 in the CA3 is sufficient in and of itself to produce stress-relevant dendritic spine and cognitive phenotypes. Further, using super-resolution imaging, we investigate how the pattern of Rap1 trafficking to synapses likely underlies its effects on the stability of select dendritic spine subtypes. These findings illuminate the involvement of aberrant Rap1 regulation in the hippocampus in contributing to the psychobiological effects of stress. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.06.22.546081v1?rss=1 Authors: Rochon, K., Bauer, B. L., Roethler, N. A., Buckley, Y., Su, C.-C., Huang, W., Ramachandran, R., Stoll, M. S. K., Yu, E. W., Taylor, D. J., Mears, J. A. Abstract: Mitochondrial fission is crucial for distributing cellular energy throughout cells and for isolating damaged regions of the organelle that are targeted for degradation. This multistep process is initiated by the enhanced recruitment and oligomerization of dynamin-related protein 1 (Drp1) at the surface of mitochondria. As such, Drp1 is essential for inducing mitochondrial division in mammalian cells, and homologous proteins are found in all eukaryotes. De novo missense mutations in the Drp1 gene, DNM1L, are associated with severe neurodevelopmental diseases in patients, and no effective treatments are available. As a member of the dynamin superfamily of proteins (DSPs), controlled Drp1 self-assembly into large helical polymers stimulates its GTPase activity to promote membrane constriction. Still, little is known about the regulatory mechanisms that determine when and where Drp1 self-assembles, and proper mitochondrial dynamics requires correct spatial and temporal assembly of the fission machinery. Here we present a cryo-EM structure of a full-length, native Drp1 dimer in an auto-inhibited state. This dimer reveals two key conformational rearrangements that must be unlocked through intermolecular interactions to achieve the assembly competent state previously observed in crystal and filament structures. Specifically, the G domain is closed against the stalk domain and occludes intermolecular interactions necessary for self-assembly beyond a dimer. Similarly, adjacent stalks in the dimer form a more continuous interface that further occludes conserved intermolecular contact sites. This structural insight provides a novel mechanism for regulated self-assembly of the mitochondrial fission machinery. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.06.19.545304v1?rss=1 Authors: Heydasch, M., Hinderling, L., van Unen, J., Dobrzynski, M., Pertz, O. Abstract: Tightly regulated spatio-temporal Rho GTPase activity patterns regulate morphogenetic processes such as cell migration. Emerging evidence suggests that binding of Guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) to the cytoskeleton or adhesions mediate feedback regulation to spatio-temporal Rho GTPase activation. To explore such feedback regulation, we study the Rho specific GAP Deleted in Liver Cancer 1 (DLC1) which binds to focal adhesions (FAs) through mechanosensitive interactions. Using a FRET biosensor, we show that DLC1 loss of function leads to global increase in Rho activity and contractility throughout the cell without affecting a striking lamellar RhoA activity band in fibroblasts. To interrogate the Rho GTPase signaling flux, we build a genetic circuit consisting of an optogenetic actuator to control Rho activity, and a Rho activity biosensor. In spreading cells at steady state, optogenetic manipulation of Rho activity reveals that DLC1 controls the rate of Rho activation rather than duration, both at FAs and at the plasma membrane (PM). Local and reversible optogenetic control of contractility shows that DLC1 associates/dissociates with FAs during their reinforcement/relaxation. This might provide positive feedback that locally increases the rate of Rho activation at FAs that experience local tension to facilitate FA disassembly. Our results indicate that DLC1 operates both at the PM and at FAs to regulate global Rho activity levels at steady state, or to amplify local Rho activity at FAs experiencing a strong mechanical input, presumably to induce robust FA disassembly. This provides new insights in the complexity of spatio-temporal Rho GTPase signaling. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.05.05.539603v1?rss=1 Authors: Seager, R., Shree Ramesh, N., Cross, S., Guo, C., Wilkinson, K., Henley, J. Abstract: Mitochondrial fission regulates mitochondrial morphology, function, mitophagy and apoptosis. Fission is mediated by the GTPase dynamin related protein-1 (DRP1) and its recruitment to the outer mitochondrial membrane by DRP1 receptors. Mitochondrial fission factor (MFF) is considered the major pro-fission receptor, whereas the mitochondrial dynamics proteins (MiD49/51) sequester inactive DRP1 and facilitate the MFF-DRP1 interaction by forming a trimeric DRP1-MiD-MFF complex. Here, we identify MFF as a target of poly-SUMOylation at a single residue (Lys151). Following bioenergetic stress, AMPK phosphorylates MFF to promote its SUMOylation, a critical step in stress-induced fragmentation. MFF SUMOylation is not required for DRP1 recruitment from the cytosol but causes a rearrangement of the trimeric fission complex to displace MiD proteins. This alleviates MiD inhibition of DRP1 to facilitate formation of a fission-competent complex. Thus, our data demonstrate that MFF SUMOylation fine-tunes the ratio of MiD to DRP1 for the dynamic control of stress-induced mitochondrial fragmentation. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.04.14.536870v1?rss=1 Authors: Bonnycastle, K., Dobson, K. L., Blumrich, E.-M., Gajbhiye, A., Davenport, E. C., Pronot, M., Steinruecke, M., Trost, M., Gonzalez-Sulser, A., Cousin, M. A. Abstract: Pathogenic heterozygous missense mutations in the DNM1 gene result in a novel form of epileptic encephalopathy. DNM1 encodes for the large GTPase dynamin-1, an enzyme with an obligatory role in the endocytosis of synaptic vesicles (SVs) at mammalian nerve terminals. Pathogenic DNM1 mutations cluster within regions required for its essential GTPase activity, implicating disruption of this enzyme activity as being central to epileptic encephalopathy. We reveal that the most prevalent pathogenic mutation of DNM1, R237W, disrupts dynamin-1 enzyme activity and SV endocytosis when overexpressed in central neurons. To determine how this dominant-negative heterozygous mutant impacted cell, circuit and behaviour when expressed from its endogenous locus, we generated a mouse carrying the R237W mutation. Neurons isolated from heterozygous mice displayed dysfunctional SV endocytosis, which translated into altered excitatory neurotransmission and seizure-like phenotypes. Importantly, these phenotypes were corrected at the cell, circuit and in vivo level by the drug, BMS-204352, which accelerates SV endocytosis. This study therefore provides the first direct link between dysfunctional SV endocytosis and epilepsy, and importantly reveals that SV endocytosis is a viable therapeutic route for monogenic intractable epilepsies. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.03.31.535139v1?rss=1 Authors: Connor, O. M., Matta, S. K., Friedman, J. R. Abstract: Mitochondria are highly dynamic double membrane-bound organelles that maintain their shape in part through fission and fusion. Mitochondrial fission is performed by the dynamin-related protein Dnm1 (Drp1 in humans), a large GTPase that constricts and divides the mitochondria in a GTP hydrolysis-dependent manner. However, it is unclear whether factors inside mitochondria help coordinate the process and if Dnm1/Drp1 activity alone is sufficient to complete fission of both mitochondrial membranes. Here, we identify an intermembrane space protein required for mitochondrial fission in yeast, which we propose to name Mdi1. Loss of Mdi1 leads to hyper-fused mitochondria networks due to defects in mitochondrial fission, but not lack of Dnm1 recruitment to mitochondria. Mdi1 plays a conserved role in fungal species and its homologs contain a putative amphipathic alpha-helix, mutations in which disrupt mitochondrial morphology. One model to explain these findings is that Mdi1 associates with and distorts the mitochondrial inner membrane to enable Dnm1 to robustly complete fission. Our work reveals that Dnm1 cannot efficiently divide mitochondria without the coordinated function of a protein that resides inside mitochondria. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.03.31.535147v1?rss=1 Authors: Hladyshau, S., Stoop, J. P., Kamada, K., Nie, S., Tsygankov, D. V. Abstract: Rho-GTPases are central regulators within a complex signaling network that controls the cytoskeletal organization and cell movement. This network includes multiple GTPases, such as the most studied Rac1, Cdc42, and RhoA, and their numerous effectors that provide mutual regulation and feedback loops. Here we investigate the temporal and spatial relationship between Rac1 and Cdc42 during membrane ruffling using a simulation model which couples GTPase signaling with cell morphodynamics to capture the GTPase behavior observed with FRET-based biosensors. We show that membrane velocity is regulated by the kinetic rate of GTPase activation rather than the concentration of active GTPase. Our model captures both uniform and polarized ruffling. We also show that cell-type specific time delays between Rac1 and Cdc42 activation can be reproduced with a single signaling motif, in which the delay is controlled by feedback from Cdc42 to Rac1. The resolution of our simulation output matches those of the time-lapsed recordings of cell dynamics and GTPase activity. This approach allows us to validate simulation results with quantitative precision using the same pipeline for the analysis of simulated and experimental data. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.03.15.532838v1?rss=1 Authors: Kumar, A., Larrea, D., Pero, M. E., Infante, P., Conenna, M., Shin, G. J.-e., Grueber, W. B., Di Marcotullio, L., Area-Gomez, E., Bartolini, F. Abstract: Acetylated microtubules play key roles in the regulation of mitochondria dynamics. It has however remained unknown if the machinery controlling mitochondria dynamics functionally interacts with the alpha-tubulin acetylation cycle. Mitofusin-2 (MFN2), a large GTPase residing in the mitochondrial outer membrane and mutated in Charcot-Marie-Tooth type 2 disease (CMT2A), is a regulator of mitochondrial fusion, transport and tethering with the endoplasmic reticulum. The role of MFN2 in regulating mitochondrial transport has however remained elusive. Here we show that mitochondrial contacts with microtubules are sites of alpha-tubulin acetylation, which occurs through the MFN2-mediated recruitment of alpha-tubulin acetyltransferase 1 (ATAT1). We discover that this activity is critical for MFN2-dependent regulation of mitochondria transport, and that axonal degeneration caused by CMT2A MFN2 associated mutations, R94W and T105M, may depend on the inability to release ATAT1 at sites of mitochondrial contacts with microtubules. Our findings reveal a function for mitochondria in regulating acetylated alpha-tubulin and suggest that disruption of the tubulin acetylation cycle play a pathogenic role in the onset of MFN2-dependent CMT2A. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.02.27.530328v1?rss=1 Authors: Rodriguez-Polanco, W. R., Norris, A., Velasco, A. B., Gleason, A., Grant, B. Abstract: After endocytosis, many plasma membrane components are recycled via narrow-diameter membrane tubules that emerge from early endosomes to form recycling endosomes, eventually leading to their return to the plasma membrane. We previously showed that the F-BAR and SH3 domain Syndapin/PACSIN-family protein SDPN-1 is required in vivo for basolateral endocytic recycling in the C. elegans intestine. Here we sought to determine the significance of a predicted interaction between the SDPN-1 SH3 domain and a target sequence in PXF-1/PDZ-GEF1/RAPGEF2, a known exchange factor for Rap-GTPases. We found that endogenous mutations we engineered into the SDPN-1 SH3 domain, or its binding site in the PXF-1 protein, interfere with recycling in vivo, as does loss of the PXF-1 target RAP-1. Rap-GTPases have been shown in several contexts to negatively regulate RhoA activity. Our results show that RHO-1/RhoA is enriched on SDPN-1 and RAP-1 positive endosomes in the C. elegans intestine, and loss of SDPN-1 or RAP-1 elevates RHO-1(GTP) levels on intestinal endosomes. Furthermore, we found that depletion of RHO-1 suppressed sdpn-1 mutant recycling defects, indicating that control of RHO-1 activity is a key mechanism by which SDPN-1 acts to promote endocytic recycling. RHO-1/RhoA is well-known for controlling actomyosin contraction cycles, although little is known of non-muscle myosin II on endosomes. Our analysis found that non-muscle myosin II is enriched on SDPN-1 positive endosomes, with two non-muscle myosin II heavy chain isoforms acting in apparent opposition. Depletion of nmy-2 inhibited recycling like sdpn-1 mutants, while depletion of nmy-1 suppressed sdpn-1 mutant recycling defects, indicating actomyosin contractility in controlling recycling endosome function. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.02.20.529203v1?rss=1 Authors: Nanda, S., Calderon, A., Duong, T.-T., Koch, J., Sachan, A., Xin, X., Solouk, D., Wu, Y.-W., Nalbant, P., Dehmelt, L. Abstract: Rho GTPase crosstalk is thought to play a key role in the spatio-temporal coordination of cytoskeletal dynamics during cell migration. Here, we directly investigated crosstalk between the major Rho GTPases Rho, Rac and Cdc42 by combining acute activity perturbation with activity measurements in individual, mammalian cells. As expected for their proposed mutual inhibition, we confirmed that Rho inhibits Rac activity. However, surprisingly, we found that Rac strongly stimulates Rho activity. We hypothesized that this crosstalk might play a role in mediating the tight spatio-temporal coupling between cell protrusions and retractions that are typically observed during mesenchymal cell migration. Using new, improved activity sensors for endogenous Rho GTPases, we find that Rac activation is tightly and precisely coupled to local cell protrusions, followed by Rho activation during retraction. In a screen for potential crosstalk mediators, we find that a subset of the Rho activating Lbc-type GEFs, in particular Arhgef11 and Arhgef12, are enriched at transient cell protrusions and retractions. Furthermore, via an optogenetic approach, we show that these Lbc GEFs are recruited to the plasma membrane by active Rac, suggesting that they might link cell protrusion and retraction by mediating Rac/Rho activity crosstalk. Indeed, depletion of these GEFs impaired cell protrusion-retraction dynamics, which was accompanied by an increase in migration directionality and reduced migration velocity. Thus, our study shows that Arhgef11 and Arhgef12 facilitate effective exploratory cell migration by coordinating the central cell morphogenic processes of cell protrusion and retraction by coupling the activity of the associated small GTPases Rac and Rho. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.02.15.528749v1?rss=1 Authors: Maxson, M. E., Huynh, K., Grinstein, S. Abstract: While it has been known for decades that luminal acidification is required for normal traffic along the endocytic pathway, the precise underlying mechanism(s) remain unknown. We found that dissipation of the endomembrane pH gradient resulted in acute formation of large Rab5- or Rab7-positive vacuoles. Vacuole formation was associated with and required hyperactivation of the Rabs, which was attributable to impaired GTPase activity, despite normal recruitment of cognate GAPs. Surprisingly, LRRK2 -a kinase linked to Parkinsons disease-was recruited to endomembranes and markedly activated upon dissipation of luminal acidification. LRRK2 phosphorylated Rab GTPases, rendering them insensitive to deactivation. Importantly, genetic deletion of LRRK2 prevented the {Delta}pH-induced vacuolation, implying that the kinase is required to modulate vesicular traffic. We propose that by dictating the state of activation of LRRK2 and in turn that of Rab GTPases, the development of a progressive luminal acidification serves as a timing device to control endocytic maturation. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.02.06.527405v1?rss=1 Authors: Rangaraj, N., Vaibhava, V., Sudhakar, C., Moharir, S. C., Swarup, G. Abstract: Transferrin and its receptor play an important role in iron homeostasis. Binding of transferrin to its receptor (TFRC, transferrin receptor protein 1) initiates endocytic trafficking and subsequent recycling of TFRC to the plasma membrane. RAB8-positive tubules emanating from the endocytic recycling compartment play an important role in receptor recycling. However, the signaling pathways or mechanisms that mediate formation of RAB8-positive tubules are not clear. Here, we have investigated the role of transferrin-induced signaling in the regulation of RAB8- and MICAL-L1-positive tubules. Addition of transferrin to the serum starved HeLa cells resulted in enhanced recruitment of RAB8 as well as MICAL-L1 to the tubules, which was mediated by TFRC. Dynasore, an inhibitor of dynamin and endocytosis, completely blocked transferrin-induced formation of RAB8/MICAL-L1-positive tubules. RAB8 showed strong colocalization with MICAL-L1 on the tubules. Blocking of SRC or AKT kinase activity by specific inhibitors abolished transferrin-induced recruitment of RAB8 and MICAL-L1 to the tubules. Recycling of transferrin receptor was inhibited by blocking of AKT activity. TBC1D17, a GTPase activating protein for RAB8, inhibited RAB8/MICAL-L1-positive tubule formation. A phospho-mimicking mutant S366D of TBC1D17 did not inhibit formation of RAB8-positive tubules. Overall, these results show that transferrin induces TFRC mediated signaling dependent on endocytosis that is essential for the formation of RAB8- and MICAL-L1-positive tubules involved in recycling of transferrin receptor. Our results also show that AKT regulates transferrin-induced formation of RAB8- and MICAL-L1-positive tubules, which might be mediated by phosphorylation of TBC1D17. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.01.30.525978v1?rss=1 Authors: Reeb, T., Rhea, L., Adelizzi, E., Garnica, B., Dunnwald, E., Dunnwald, M. Abstract: BACKGROUND: RhoA GTPase plays critical roles in actin cytoskeletal remodeling required for controlling a diverse range of cellular functions including cell proliferation, cell adhesions, migration and changes in cell shape. RhoA cycles between an active GTP-bound and an inactive GDP-bound form, a process that is regulated by guanine nucleotide exchange factors (GEFs), and GTPase-activating proteins (GAPs). ARHGAP29 is a GAP expressed in keratinocytes of the skin and is decreased in the absence of Interferon Regulator Factor 6, a critical regulator of cell proliferation and migration. However, the role for ARHGAP29 in keratinocyte biology is unknown. RESULTS: Novel ARHGAP29 knockdown keratinocyte cell lines were generated using both CRISPR/Cas9 and shRNA technologies. Knockdown cells exhibited significant reduction of ARHGAP29 protein (50-80%) and displayed increased filamentous actin (stress fibers), phospho-myosin light chain (contractility), cell area and population doubling time. Furthermore, we found that ARHGAP29 knockdown keratinocytes displayed significant delays in scratch wound closure in both single cell and collective cell migration conditions. Particularly, our results show a reduction in path lengths, speed, directionality and persistence in keratinocytes with reduced ARHGAP29. The delay in scratch closure was rescued by both adding back ARHGAP29 or adding a ROCK inhibitor to ARHGAP29 knockdown cells. CONCLUSIONS: These data demonstrate that ARHGAP29 is required for keratinocyte morphology, proliferation and migration mediated through the RhoA pathway. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2023.01.12.523789v1?rss=1 Authors: Kumar, R., Francis, V., Ioannou, M. S., Aguila, A., Banks, E., Kulasekaran, G., Khan, M., McPherson, P. S. Abstract: Cytokinesis is the final stage of cell division. Successful cytokinesis requires membrane trafficking pathways regulated by Rabs, molecular switches activated by guanine nucleotide exchange factors (GEFs). Late in cytokinesis, an intercellular cytokinetic bridge (ICB) connecting the two daughter cells undergoes abscission, which requires depolymerization of actin. Rab35 recruits MICAL1 to oxidate and depolymerize actin filaments. We report that DENND2B, a protein previously implicated in cancer, mental retardation and multiple congenital disorders functions as a GEF for Rab35 and recruits and activates the GTPase at the ICB. Unexpectedly, the N terminal region of DENND2B interacts with an active mutant of Rab35, suggesting that DENND2B is both a Rab35 GEF and effector. Knockdown of DENND2B delays abscission resulting in increased multinucleated cells and over-accumulation of F-actin at the ICB. F-actin accumulation leads to formation of a chromatin bridge, a process known to activate the NoCut/abscission checkpoint, and DENND2B knockdown actives Aurora B kinase, a hallmark of checkpoint activation. This study identifies DENND2B as a crucial player in cytokinetic abscission and provides insight into the multisystem disorder associated with DENND2B mutation. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.11.26.517555v1?rss=1 Authors: Corteggio, A., Lo Monte, M., Schembri, L., Dathan, N., Di Paola, S., Grimaldi, G., Corda, D. Abstract: The GTPase Rab14 is localized at the trans-Golgi network and at the intermediate compartment associated to sorting/recycling endosomes-like structures of the transferrin- recycling pathway: as other Rab family members, it is involved in the regulation of intracellular vesicle trafficking, though its role and functional relationship with effector/endosomal proteins is still incomplete. We have analysed whether post-translational modifications could affect Rab14 activity: the results obtained define mono-ADP-ribosylation (MARylation) as the yet-unknown Rab14 modification, catalysed by the ADP-ribosyltransferase PARP12, which specifically modifies glutamic acid residues in position 159/162. This modification is essential for the Rab14- dependent endosome progression. Accordingly, recycling of the transferrin receptor is inhibited when MARylation of Rab14 is prevented by PARP12 knocking-down or inhibition, or by overexpression of Rab14 ADP-ribosylation-defective mutant. Under these conditions, Rab14 and transferrin receptors are withheld at the cell periphery at the level of the Rab4- RUFY1-positive sorting endosomes, indicating that the interaction of Rab14 with the dual effectors RUFY and then FIP1c (which specifically binds both Rab11 and Rab14) determines the progression between the Rab4-RUFY- and Rab11-FIP1c-specific vesicles. Therefore Rab14-MARylation determines the sequential binding of this GTPase to RUFY and FIP1c, thus controlling endosome progression (i.e., transferrin receptors recycling) through the Rab4- , Rab14- and Rab11-specific vesicles. This identifies a Rab14-specific compartment of the recycling pathway and a crucial enzymatic reaction amenable to pharmacological control. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.11.25.517942v1?rss=1 Authors: Wu, D., Dai, Y., Gao, N. Abstract: Bacterial HflX is a conserved ribosome-binding GTPase involved in splitting ribosomal complexes accumulated under stress condition. However, the atomic details of its ribosomal interaction remain to be elucidated. In this work, we present a high-resolution structure of the E. coli 50S subunit bound with HflX. The structure reveals highly specific contacts between HflX and the ribosomal RNA, and in particular, an insertion loop of the N-terminal domain of HflX is situated in the peptidyl transferase center (PTC) and makes direct interactions with PTC residues. Interestingly, this loop displays steric clash with a few PTC-targeting antibiotics on the 50S subunit, such as chloramphenicol. Deletion of hflX results in hypersensitivity to chloramphenicol treatment, and a loop residue G154 of HflX is important for the observed chloramphenicol resistance. Overall, our results suggest that HflX could be a general stress response factor that functions in both stalled ribosome splitting and PTC antibiotic displacing. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.11.14.516471v1?rss=1 Authors: Dou, D., Smith, E. M., Evans, C. S., Boecker, C. A., Holzbaur, E. L. F. Abstract: Gain-of-function mutations in the LRRK2 gene cause Parkinson's disease (PD), increasing phosphorylation of RAB GTPases through hyperactive kinase activity. We found that LRRK2-hyperphosphorylated RABs disrupt the axonal transport of autophagosomes by perturbing the coordinated regulation of cytoplasmic dynein and kinesin motors. In iPSC-derived human neurons, knock-in of the strongly-hyperactive LRRK2-p.R1441H mutation caused striking impairments in autophagosome transport, inducing frequent directional reversals and pauses. Knock-out of the opposing Protein Phosphatase 1H (PPM1H) phenocopied the effect of hyperactive LRRK2. Overexpression of ADP-ribosylation factor 6 (ARF6), a GTPase that acts as a switch for selective activation of dynein or kinesin, attenuated transport defects in both p.R1441H knock-in and PPM1H knock-out neurons. Together, these findings support a model where a regulatory imbalance between LRRK2-hyperphosphorylated RABs and ARF6 induces an unproductive "tug-of-war" between dynein and kinesin, disrupting processive autophagosome transport. This disruption may contribute to PD pathogenesis by impairing the essential homeostatic functions of axonal autophagy. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.11.02.514950v1?rss=1 Authors: Liu, Y.-X., Li, W.-J., Zhang, R.-K., Sun, S.-N., Fan, Z.-C. Abstract: Certain ciliary transmembrane and membrane-tethered signaling proteins migrate from the ciliary tip to base via retrograde intraflagellar transport (IFT), essential for maintaining their ciliary dynamics to enable cells to sense and transduce extracellular stimuli inside the cell. During this process, the BBSome functions as an adaptor between retrograde IFT trains and these signaling protein cargoes. The Arf-like 13 (ARL13) small GTPase resembles ARL6/BBS3 in facilitating these signaling cargoes to couple with the BBSome at the ciliary tip prior to loading onto retrograde IFT trains for transporting towards the ciliary base, while the molecular basis for how this intricate coupling event happens remains elusive. Here, we report that Chlamydomonas ARL13 only in a GTP-bound form (ARL13GTP) anchors to the membrane for diffusing into cilia. Upon entering cilia, ARL13 undergoes GTPase cycle for shuttling between the ciliary membrane (ARL13GTP) and matrix (ARL13GDP). To achieve this goal, the ciliary membrane-anchored BBS3GTP binds and activates the ciliary matrix-residing ARL13GDP as an ARL13 guanine nucleotide exchange factor. At the ciliary tip, ARL13GTP binds and recruits the ciliary matrix-residing and post-remodeled BBSome as an ARL13 effector to anchor to the ciliary membrane. This makes the BBSome spatiotemporally become available for the ciliary membrane-tethered phospholipase D (PLD) to couple with. Afterward, ARL13GTP hydrolyzes GTP for releasing the PLD-laden BBSome to load onto retrograde IFT trains. According to this model, hedgehog signaling defects associated with ARL13b and BBS3 mutations in humans could be satisfactorily explained, providing us a mechanistic understanding behind BBSome-cargo coupling required for proper ciliary signaling. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.10.17.512253v1?rss=1 Authors: Mahlandt, E. K., Palacios Martinez, S., Arts, J. J. G., Tol, S., van Buul, J. D., Goedhart, J. Abstract: The inner layer of blood vessels consists of endothelial cells, which form the physical barrier between blood and tissue. This vascular barrier is tightly regulated to allow the passage of essential molecules like oxygen, carbon-dioxide, water, ions, and nutrients. The vascular endothelial barrier is defined by cell-cell contacts through adherens and tight junctions. To further investigate the signaling in the endothelium that regulates vascular barrier strength, we focused on Rho GTPases, regulators of the actin cytoskeleton and known to control junction integrity. Rho GTPase signaling is confined in space and time. To manipulate the signaling in a temporal and spatial manner we applied optogenetics. Guanine exchange factor (GEF) domains from ITSN1, TIAM1 and p63RhoGEF, activating Cdc42, Rac and Rho respectively, were integrated into the optogenetic recruitment tool iLID. This tool allows for activation at the subcellular level in a reversible and non-invasive manner and thereby to recruit a GEF to local areas at the plasma membrane, enabling the local activation of specific Rho GTPases. The membrane tag of iLID was optimized and a HaloTag was applied to gain more flexibility for multiplex imaging. The resulting Opto-RhoGEFs were tested in an endothelial cell monolayer and demonstrated precise temporal control of vascular barrier strength by a cell-cell overlap-dependent, VE-cadherin-independent, mechanism. Furthermore, Opto-RhoGEFs enabled precise optogenetic control in endothelial cells over morphological features such as cell-size, -roundness, local extension, and cell contraction. In conclusion, we have optimized and applied the optogenetic iLID GEF recruitment tool i.e. Opto-RhoGEFs, to study the role of Rho GTPases in the vascular barrier of the endothelium and found that membrane protrusions at the junction region can rapidly increase barrier integrity independent of VE-cadherin. Copy rights belong to original authors. Visit the link for more info Podcast created by Paper Player, LLC

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2022.09.19.508514v1?rss=1 Authors: Kamiyama, D., Nishida, Y., Kamiyama, R., Fitch, M. A., Chihara, T. Abstract: The formation of primary dendrites (dendritogenesis) significantly affects the overall orientation and coverage of dendritic arborization, limiting the number and types of inputs a neuron can receive. Previously we reported how a Drosophila motoneuron spatially controls the positioning of dendritogenesis through the Dscam1/Dock/Pak1 pathway; however, how the neuron defines the timing of this process remains elusive. Here we show that the Eph receptor tyrosine kinase provides a temporal cue. We find that, at the onset of dendritogenesis, the Eph receptor recruits the Rho Family GEF Vav to the intracellular domain of Eph, which transiently activates the Cdc42 family of small GTPase. We also show that vap33 (vesicle-associated membrane protein-associated protein) mutants exhibit defects in Cdc42 activation and dendritic outgrowth, indicating Vap33 may play an upstream role in Eph signaling. Together, our result and previous studies argue that the formation of primary dendrites requires the proximity of active Cdc42 and membrane-anchored Pak1 driven by collaborative action between two distinct signaling complexes, Vap33/Eph/Vav and Dscam1/Dock. Signal integration from multiple input pathways would represent a general mechanism for the spatiotemporal precision of dendrite branch formation. Copy rights belong to original authors. Visit the link for more info Podcast created by PaperPlayer

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.11.17.386177v1?rss=1 Authors: Cuevas-Navarro, A., Van, R., Cheng, A., Urisman, A., Castel, P., McCormick, F. Abstract: The spindle assembly checkpoint (SAC) is an evolutionarily conserved safety mechanism that maintains genomic stability. However, despite the understanding of the fundamental mechanisms that control the SAC, it remains unknown how signaling pathways directly interact with and regulate the mitotic checkpoint activity. In response to extracellular stimuli, a diverse network of signaling pathways involved in cell growth, survival, and differentiation are activated and this process is prominently regulated by the Ras family of GTPases. Here we show that RIT1, a Ras-related GTPase, is essential for timely progression through mitosis and proper chromosome segregation. Furthermore, pathogenic levels of RIT1 silence the SAC, accelerate transit through mitosis, and promote chromosome segregation errors through direct association with SAC proteins MAD2 and p31comet. Our results highlight a unique function of RIT1 compared to other Ras GTPases and elucidate a direct link between a signaling pathway and the SAC through a novel regulatory mechanism. Copy rights belong to original authors. Visit the link for more info

This episode builds on content covered in episode 3, so be sure to check it out if you haven't listened to it already. This week, we explore an example of the GTPase content we discussed last time. Specifically, we look at the diploid mating of baker's yeast in stressful environments. Source for this episode: 1) Alberts, Johnson, Lewis Raff, Roberts and Walter (2008), Molecular Biology of the Cell, Fifth Edition. Abingdon: Garland Science, Taylor and Francis Group LLC.

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.10.14.339192v1?rss=1 Authors: Katsanevaki, D., Till, S. M., Buller-Peralta, I., Watson, T. C., Nawaz, M. S., Arkell, D., Tiwari, S., Kapgal, V., Biswal, S., Smith, J. A., Anstey, N. J., Mizen, L., Perentos, N., Jones, M. W., Cousin, M. A., Chattarji, S., Gonzalez-Sulser, A., Hardt, O., Wood, E. R., Kind, P. C. Abstract: Pathogenic variants in SYNGAP1 are one of the most common genetic causes of nonsyndromic intellectual disability (ID) and are considered a risk for autism spectrum disorder (ASD). SYNGAP1 encodes a synaptic GTPase activating protein that modulates the intrinsic GTPase activity of several small G-proteins and is implicated in regulating the composition of the postsynaptic density. By targeting the deletion of exons encoding the calcium/lipid binding (C2) and GTPase activating protein (GAP) domains, we generated a novel rat model to study SYNGAP related pathophysiology. We find that rats heterozygous for the C2/GAP domain deletion (Syngap+/{Delta}-GAP) exhibit reduced exploration and fear extinction, altered social behaviour, and spontaneous seizures, while homozygous mutants die within days after birth. This new rat model reveals that the enzymatic domains of SYNGAP are essential for normal brain function and provide an important new model system in the study of both ID/ASD and epilepsy. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.10.07.330126v1?rss=1 Authors: Dube, S., Racz, B., Brown, W. E., Gao, Y., Soderblom, E. J., Yasuda, R., Soderling, S. H. Abstract: In contrast to their postsynaptic counterparts, the contributions of activity-dependent cytoskeletal signaling to presynaptic plasticity remain controversial and poorly understood. To identify and evaluate these signaling pathways, we conducted a proteomic analysis of the presynaptic cytomatrix using in vivo biotin identification (iBioID). The resultant proteome was heavily enriched for actin cytoskeleton regulators, including Rac1, a Rho GTPase that activates the Arp2/3 complex to nucleate branched actin filaments. Strikingly, we find Rac1 and Arp2/3 are closely associated with presynaptic vesicle membranes and negatively regulate synaptic vesicle replenishment at both excitatory and inhibitory synapses. Using optogenetics and fluorescence lifetime imaging, we show this pathway bidirectionally sculpts short-term synaptic depression and that its presynaptic activation is coupled to action potentials by voltage-gated calcium influx. Thus, this study provides a new proteomic framework for understanding presynaptic physiology and uncovers a previously unrecognized mechanism of actin-regulated short-term presynaptic plasticity that is conserved across cell types. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.10.289546v1?rss=1 Authors: Ganichkin, O., Vancraenenbroeck, R., Rosenblum, G., Hofmann, H., Mikhailov, A. S., Daumke, O., Noel, J. K. Abstract: The mechano-chemical GTPase dynamin assembles on membrane necks of clathrin-coated vesicles into helical oligomers that constrict and eventually cleave the necks in a GTP-dependent way. It remains not clear whether dynamin achieves this via molecular motor activity and, if so, by what mechanism. Here, we used ensemble kinetics, single-molecule FRET and molecular dynamics simulations to characterize dynamin's GTPase cycle and determine the powerstroke strength. The results were incorporated into a coarse-grained structural model of dynamin filaments on realistic membrane templates. Working asynchronously, dynamin's motor modules were found to collectively constrict a membrane tube. Force is generated by motor dimers linking adjacent helical turns and constriction is accelerated by their strain-dependent dissociation. Consistent with experiments, less than a second is needed to constrict a membrane tube to the hemi-fission radius. Thus, a membrane remodeling mechanism relying on cooperation of molecular ratchet motors driven by GTP hydrolysis has been revealed. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.10.290866v1?rss=1 Authors: Kerloch, T., Farrugia, F., Maitre, M., Terral, G., Koehl, M., Heng, J. I.-T., Blanchard, M., Doat, H., Leste-Lasserre, T., Goron, A., Gonzales, D., Guillemot, F., Abrous, N., Pacary, E. Abstract: Despite the central role of Rho GTPases in neuronal development, their functions in adult hippocampal neurogenesis remain poorly explored. Here, by using a retrovirus-based loss-of-function approach in vivo, we show that the atypical Rho GTPase Rnd2 is crucial for the survival, positioning, somatodendritic morphogenesis and functional maturation of adult-born dentate granule neurons. Interestingly, most of these functions are specific to granule neurons generated during adulthood since the deletion of Rnd2 in neonatally-born granule neurons only affects dendritogenesis. In addition, suppression of Rnd2 in adult-born dentate granule neurons increases anxiety-like behaviour whereas its deletion in pups has no such effect, a finding supporting the adult neurogenesis hypothesis of anxiety disorders. Thus, our results provide mechanistic insight into the differential regulation of hippocampal neurogenesis during development and adulthood, and establishes a causal relationship between Rnd2 expression and anxiety. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.09.289835v1?rss=1 Authors: Zolotarov, Y., Ma, C., Gonzalez-Recio, I., Hardy, S., Franken, G., Uetani, N., Latta, F., Kostantin, E., Boulais, J., Thibault, M.-P., Cote, J.-F., Diaz Moreno, I., Diaz Quintana, A., Hoenderop, J. G. J., Martinez-Cruz, L. A., Tremblay, M. L., de Baaij, J. H. F. Abstract: Cyclin M (CNNM1-4) proteins maintain cellular and body magnesium (Mg2+) homeostasis. Using various biochemical approaches, we have identified members of the CNNM family as direct interacting partners of ADP-ribosylation factor-like protein 15 (ARL15), a small GTP-binding protein. ARL15 interacts with CNNMs at their carboxyl-terminal conserved cystathionine-{beta}- synthase (CBS) domains. In silico modeling of the interaction using the reported structures of both CNNM2 and ARL15 supports that the small GTPase specifically binds the CBS1 domain. Immunocytochemical experiments demonstrate that CNNM2 and ARL15 co-localize in the kidney, with both proteins showing subcellular localization in the Golgi-apparatus. Most importantly, we found that ARL15 is required for forming complex N-glycosylation of CNNMs. Overexpression of ARL15 promotes complex N-glycosylation of CNNM3. Mg2+ uptake experiments with a stable isotope demonstrate that there is a significant increase of 25Mg2+ uptake upon knockdown of ARL15 in multiple kidney cancer cell lines. Altogether, our results establish ARL15 as a novel negative regulator of Mg2+ transport by promoting the complex N-glycosylation of CNNMs. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.15.252247v1?rss=1 Authors: Motiwala, Z., Sandholu, A. S., Sengupta, D., Kulkarni, K. Abstract: Ras superfamily GTPases are molecular switches which regulate critical cellular processes. Extensive structural and computational studies on this G protein family have tried to establish a general framework for their switching mechanism. The current understanding of the mechanism is that two loops, Switch I and Switch II, undergo conformational changes upon GTP binding and hydrolysis, which results in alteration of their functional state. However, because of variation in the extent of conformational changes seen across the members of the Ras superfamily, there is no generic modus operandi defining their switching mechanism, in terms of loop conformation. Here, we have developed a novel method employing wavelet transformation to dissect the structures of these molecular switches to explore indices that defines the unified principle of working. Our analysis shows that the structural coupling between the Switch I and Switch II regions is manifested in terms of wavelet coherence phases. The resultant phase pertaining to these regions serve as a functional identity of the GTPases. The coupling defined in terms of wavelet coherence phases is conserved across the Ras superfamily. In oncogenic mutants of the GTPases the phase coupling gets disentangled, this perhaps provides an alternative explanation for their aberrant function. Although similar observations were made using MD simulations, there was no structural parameter to define the coupling, as delineated here. Furthermore, the technique reported here is computationally inexpensive and can provide significant functional insights on the GTPases by analyzing as few as two structures. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.04.235994v1?rss=1 Authors: Strausz, S., Ruotsalainen, S. E., Ollila, H. M., Karjalainen, J., Reeve, M., Kurki, M., Mars, N., Havulinna, A. S., Kiiskinen, T., Mansour Aly, D., Ahlqvist, E., Teder-Laving, M., Palta, P., Groop, L., Magi, R., Makitie, A., Salomaa, V., Bachour, A., Tuomi, T., Palotie, A., Palotie, T., Ripatti, S. Abstract: There is currently only limited understanding of the genetic aetiology of obstructive sleep apnoea (OSA). The aim of our study is to identify genetic loci associated with OSA risk and to test if OSA and its comorbidities share a common genetic background. We conducted the first large-scale genome-wide association study of OSA using FinnGen Study (217,955 individuals) with 16,761 OSA patients identified using nationwide health registries. We estimated 8.3% [0.06-0.11] heritability and identified five loci associated with OSA (P < 5.0x10^-8): rs4837016 near GTPase activating protein and VPS9 domains 1 (GAPVD1), rs10928560 near C-X-C motif chemokine receptor 4 (CXCR4), rs185932673 near Calcium/calmodulin-dependent protein kinase ID (CAMK1D) and rs9937053 near Fat mass and obesity-associated protein (FTO) - a variant previously associated with body mass index (BMI). In a BMI-adjusted analysis, an association was observed for rs10507084 near Rhabdomyosarcoma 2 associated transcript (RMST)/NEDD1 gamma-tubulin ring complex targeting factor (NEDD1). We found genetic correlations between OSA and BMI (rg=0.72 [0.62-0.83]) and with comorbidities including hypertension, type 2 diabetes (T2D), coronary heart disease (CHD), stroke, depression, hypothyroidism, asthma and inflammatory rheumatic diseases (IRD) (rg > 0.30). Polygenic risk score (PRS) for BMI showed 1.98-fold increased OSA risk between the highest and the lowest quintile and Mendelian randomization supported a causal relationship between BMI and OSA. Our findings support the causal link between obesity and OSA and joint genetic basis between OSA and comorbidities. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.07.28.225524v1?rss=1 Authors: Fromm, S., Lawrence, R. E., Hurley, J. H. Abstract: The mechanistic target of rapamycin complex 1 (mTORC1) couples cell growth to nutrient, energy and growth factor availability1-3. mTORC1 is activated at the lysosomal membrane when amino acids are replete via the Rag guanosine triphosphatases (GTPases)4-6. Rags exist in two stable states, an inactive (RagA/BGDP:RagC/DGTP) and active (RagA/BGTP:RagC/DGDP) state, during low and high cellular amino acid levels4,5. The lysosomal folliculin (FLCN) complex (LFC) consists of the inactive Rag dimer, the pentameric scaffold Ragulator7,8, and the FLCN:FNIP (FLCN-interacting protein) GTPase activating protein (GAP) complex9, and prevents activation of the Rag dimer during amino acid starvation10,11. How the LFC is released upon amino acid refeeding is a major outstanding question in amino-acid dependent Rag activation. Here we show that the cytoplasmic tail of the lysosomal solute carrier family 38 member 9 (SLC38A9), a known Rag activator12-14, destabilizes the LFC. By breaking up the LFC, SLC38A9 triggers the GAP activity of FLCN:FNIP toward RagC. We present the cryo electron microscopy (cryo-EM) structures of Rags in complex with their lysosomal anchor complex Ragulator and the cytoplasmic tail of SLC38A9 in the pre and post GTP hydrolysis state of RagC, which explain how SLC38A9 destabilizes the LFC and so promotes Rag dimer activation. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.06.02.128991v1?rss=1 Authors: Goetz, T. W. B., Puchkov, D., Luetzkendorf, J., Nikonenko, A. G., Quentin, C., Lehmann, M., Sigrist, S. J., Petzoldt, A. G. Abstract: Reliable delivery of presynaptic material, including active zone and synaptic vesicle proteins from neuronal somata to synaptic terminals is prerequisite for faithful synaptogenesis and neurotransmission. However, molecular mechanisms controlling the somatic assembly of presynaptic precursors remain insufficiently understood. Here we show that in mutants of the small GTPase RAB2 active zone and synaptic vesicle proteins accumulated in the neuronal somata at the trans-Golgi network and were consequently depleted at synaptic terminals, provoking neurotransmission deficits. The ectopic presynaptic material accumulations consisted of heterogeneous vesicles and short tubules of 40x60 nm and segregated in subfractions either positive for active zone proteins or co-positive for synaptic vesicle proteins and LAMP1, a lysosomal membrane protein. Genetically, rab2 behaved epistatic over arl8, a lysosomal adaptor controlling axonal export of precursors. Collectively, we here identified a Golgi-associated assembly sequence in presynaptic precursor vesicle biogenesis controlled by RAB2 dependent membrane remodelling and protein sorting at the trans-Golgi. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.05.26.117713v1?rss=1 Authors: Nakazawa, S., Yoshimura, Y., Takagi, M., Mizuno, H., Iwasato, T. Abstract: Spatially-organized spontaneous activity is a characteristic feature of developing mammalian sensory systems. However, the transitions of spontaneous-activity spatial organization during development and related mechanisms remain largely unknown. We reported previously that layer 4 (L4) glutamatergic neurons in the barrel cortex exhibit spontaneous activity with a patchwork-type pattern at postnatal day 5 (P5), which is during barrel formation. In the current work, we revealed that spontaneous activity in barrel-cortex L4 glutamatergic neurons exhibits at least three phases during the first two weeks of postnatal development. Phase I activity has a patchwork-type pattern and is observed not only at P5, but also P1, prior to barrel formation. Phase II is found at P9, by which time barrel formation is completed, and exhibits broadly synchronized activity across barrel borders. Phase III emerges around P11 when L4-neuron activity is desynchronized. The Phase I activity, but not Phase II or III activity, is blocked by thalamic inhibition, demonstrating that the Phase I to II transition is associated with loss of thalamic dependency. Dominant-negative Rac1 expression in L4 neurons hampers the Phase II to III transition. It also suppresses developmental increases in spine density and excitatory synapses of L4 neurons in the second postnatal week, suggesting that Rac1-mediated synapse maturation could underlie the Phase II to III transition. Our findings revealed the presence of distinct mechanisms for Phase I to II and Phase II to III transition. They also highlighted the role of a small GTPase in the developmental desynchronization of cortical spontaneous activity. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.05.14.095117v1?rss=1 Authors: Andrews, M. G., Subramanian, L., Kriegstein, A. R. Abstract: Outer radial glial (oRG) cells are a population of neural stem cells prevalent in the developing human cortex that contribute to its cellular diversity and evolutionary expansion. The mammalian Target of Rapamycin (mTOR) signaling pathway is active in human oRG cells. Mutations in mTOR pathway genes are linked to a variety of neurodevelopmental disorders and malformations of cortical development. We find that dysregulation of mTOR signaling specifically affects oRG cells, but not other progenitor types, by changing the actin cytoskeleton through the activity of the GTPase, CDC42. These effects change oRG cellular morphology, migration, and mitotic behavior. Thus, mTOR signaling can regulate the architecture of the developing human cortex by maintaining the cytoskeletal organization of oRG cells and the radial glia scaffold. Our study provides insight into how mTOR dysregulation may contribute to neurodevelopmental disease. Copy rights belong to original authors. Visit the link for more info

Dr Carolyn Lam:                Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. I'm Dr Carolyn Lam, associate editor from the National Heart Center and Duke National University of Singapore. Dr Greg Hundley:             And I'm Greg Hundley, associate editor and director of the Pauley Heart Center from VCU Health in Richmond, Virginia. Dr Carolyn Lam:                So, Greg, are we any closer to the holy grail of safe ED discharge for acute heart failure based on a risk score? Well, we're going to be discussing that coming right up after Greg and I share about the papers that we'd like to discuss today. Lovely issue, isn't it? Dr Greg Hundley:             Yup, and time to get your coffee and bring it up. My first paper, Carolyn, is from Michael Chu from London Health Sciences Center, and is really investigating the surgical management of thoracic aortic disease, and looking at the impact of gender or sex related differences. Sex related differences have not been thoroughly studied. This group looked at a total of 1653 patients, 30% were women, who underwent thoracic aortic surgery with hypothermic circulatory arrest between the years of 2002 and 2017 across Canada in 10 institutions.                                                 Well, women underwent less aortic root reconstruction, including aortic root replacement, Ross procedures, or valve sparing root operations. But, even with less invasive, the women experienced higher rates of mortality, 11% versus 7%, stroke, and that composite of the thoracic surgeons' adverse events. On multi variable analysis, female sex or women was an independent predictor of overall mortality, stroke, and those comorbidities. Dr Carolyn Lam:                Greg, you know how much I love these papers, so I'm going to repeat that. You're saying the women received less ominous procedures and yet had worse outcomes, and this was independent of the clinical covariances, right? Dr Greg Hundley:             Absolutely. Putting all this together, women underwent thoracic aortic surgery a little bit older, and with larger index aortic aneurysm size than men. Intraoperatively, women undergo fewer concomitant procedures, such as the aortic root repairs, and things that you just mentioned. But nevertheless, women experience significantly worse outcomes identified as an independent predictor of mortality, stroke, and that composite endpoint for mortality, morbidity, after multi variable analysis.                                                 What should we think about this? Well, sex specific considerations are important when considering thoracic aortic surgery and future research should focus on the development of a personalized approach to thoracic aortic surgery with respect to gender. For example, utilization of maybe lower size thresholds for women for aortic aneurysms should be considered, and for earlier interventions, and improved outcomes.                                                 Carolyn, tell me about one of your papers. Dr Carolyn Lam:                All right, so I chose a paper looking at the stroke outcomes in the COMPASS trial. Now, let's remind everybody that the COMPASS trial, where patients with stable coronary artery disease or peripheral artery disease, and randomly assigned to receive aspirin 100 milligrams daily, rivaroxaban five milligrams twice daily, or the combination of rivaroxaban 2.5 milligrams twice daily plus aspirin. Patients requiring anticoagulation with a stroke within a month had a previous lacunar stroke or intracerebral hemorrhage were excluded.                                                 Now, in the current paper, and this is from Dr Sharma from Population Health Research Institute, and their colleagues, basically they looked at a detailed analysis of the stroke by type, predictors, and anti-thrombotic effects in the key subgroups. They found that the combination of low dose rivaroxaban and aspirin prevented stroke and disabling stroke better than aspirin in patients without atrial fibrillation and with stable vascular disease, and without an increasing risk of hemorrhagic stroke; which is really important. This effect was consistent across subgroups of baseline risk, and particularly marked in those with a history of previous stroke. Dr Greg Hundley:             Carolyn, what about that rivaroxaban five milligrams twice daily alone? Dr Carolyn Lam:                There was no significant difference in the occurrence of stroke in the rivaroxaban alone group compared with aspirin. But all of this simply says perhaps low dose rivaroxaban and aspirin may be a really important new anti-thrombotic option for primary and secondary stroke prevention in patients with clinical stable atherosclerosis. Dr Greg Hundley:             Very interesting. I'm going to follow your lead and go into another sort of anticoagulant-related topic on iliofemoral deep vein thrombosis. This paper is by Suresh Vedantham from the Washington University of St. Louis.                                                 Let's talk about just what is the definition? This is a DVT that involves the iliac and/or the common femoral vein with or without involvement of additional veins. It basically obstructs the outflow of the veins. These patients are phenotypically distinct from patients with cath or femoral popliteal DVT because that totally obstructs flow, and they have more frequent recurrence of venous thromboembolic events, and more frequent post-thrombotic syndrome. Well, that's a horrible condition because of that obstruction, it leads to calf muscle dysfunction, edema, subcutaneous fibrosis, tissue hypoxia, and ulceration. Dr Carolyn Lam:                Great background. What did this study show? Dr Greg Hundley:             This is a sub-study of the ATTRACT trial. The ATTRACT trial basically is looking at anticoagulation plus perhaps mechanical intervention, or direct catheter directed thrombolysis therapy versus just anticoagulation alone. This sub-study is 391 patients with acute DVT involving just the iliac or the common femoral veins, and following these individuals for 24 months to compare short and long-term outcomes.                                                 What did the study show? Well, this interventional group did have a reduction in leg pain and swelling, and improvement in quality of life related to that lower extremity. But, no overall difference in overall quality of life, and very importantly, no difference in the occurrence of this post thrombotic syndrome. Dr Carolyn Lam:                That's kind of disappointing. I understand that the ATTRACT study is not the first to look at this, though. That was in an editorial discussing this. Could you tell us about that? Dr Greg Hundley:             Yeah, Carolyn. Jay Giri from University of Pennsylvania just had an incredible editorial. I think if you have an opportunity, listeners, to take a look at that, I highly recommend it. He reminded us of the CaVenT trial, which is basically performed as an open label randomized control trial of 209 patients across 20 hospitals in Norway.                                                 What was different in the CaVenT trial is that at 24 months of follow up, the intervention with thrombolysis and systemic anticoagulation improved iliofemoral patency. It reduced the incidence of this post thrombotic syndrome. In ATTRACT, in this sub-study, it was intravenous thrombolysis, systemic anticoagulation, and mechanical intervention on the vein versus in the other study from Norway, CaVenT, just the inter vein thrombolysis and the systemic anticoagulation.                                                 What Dr Giri points out is that maybe something related to intervention in that vein when you're stripping out thrombus, et cetera, are we damaging the veins in the vessel that prevents reflux, et cetera?                                                 I think really moving forward, you're going to have to personalize this decision in individual patients until we have more data on this subject. Dr Carolyn Lam:                Great learning. I learned a lot from this next paper, too, because I actually chose a basic science paper. This is a paper that uncovers a new fine tuning factor that modulates myocardial infarction induced inflammation. That is a small GTPase called RhoE.                                                 In this study, Drs Chang from Texas A&M University College of Medicine, and Song from Fuwai Hospital in Beijing used three genetic mouse model lines. Those are the global knockout, the cardiomyocyte specific RhoE heterozygous mouse, and the cardiomyocyte specific RhoE over expression mouse. With this combination, they showed that RhoE deficiency causes excessive inflammatory response in infarct animal heart, resulting in enlarged heart, decreased contractility, and increased mortality. The mechanism is that RhoE binds to P65 and P50, which impedes their dimerization and blocks these two proteins from nuclear translocation. Now, over expression of cardiac RhoE inhibits NF-κB, restrains post MI inflammation, and improves cardiac function and survival.                                                 Importantly as you always say, Greg, there is human data. They found that the expression of RhoE was elevated in the infarct patient heart and that patients with a higher expression of RhoE exhibited a better prognosis and better cardiac function recovery. Dr Greg Hundley:             Carolyn, tell me a little bit about the clinical significance of this. Dr Carolyn Lam:                You just wanted to ask me a tough question. I can see it on your face. Basically, I think this is really exciting because RhoE may serve as a new potential biomarker for the assessment of myocardial infarction in patients, and manipulation of RhoE could be a potential therapeutic approach for MI. There. Dr Greg Hundley:             Very good. Dr Carolyn Lam:                That's all the time we have for our little discussion here. Now, let's go onto the feature paper. ...                                                 Over 80% of emergency department patients with acute heart failure are admitted to the hospital. Now, contrast this with the fact that over 80% of all emergency department visits result in discharge. So, why is that many other emergency department based cardiovascular disease processes like for acute coronary syndrome have evolved from high rates of admission to timely and safe discharge whereas decision making in acute heart failure has not experienced a similar evolution. Do we need perhaps a better acute heart failure prognostic score that's validated?                                                 Well, guess what? We're going to talk about this right now in our feature discussion, and a beautiful feature paper that we're so proud to have the corresponding author, Dr Douglas Lee from University of Toronto right here to discuss; along with the managing editor, Dr Justin Ezekowitz, who's associate editor from University of Alberta, and the editorialist, Dr Sean Collins from Vanderbilt University Medical Center. Welcome everyone, and Doug, please, could you just start by telling us about this great paper? Dr Douglas Lee:                 We validated, and it's a tool, decision making tool, for acute heart failure patients in the emergency department. We, in this study, wanted to prospectively validate a decision making prognostic tool called the Emergency Heart Failure Mortality Risk Grade, or EHFMRG for short, to see how well it performed in the real world busy emergency department hospital setting.                                                 We studied just under 2,000 patients who came to emergency departments at multiple centers, and asked physicians to rate their prognostic estimation of what's going to happen to that patient in the next seven days. We compared that with the EHFMRG model, which predicts outcomes of seven days and 30 days. We were very careful to ask physicians to provide their prognostic estimates. This is their intuitive guesstimation of the risk of the patient before calculating the score because we didn't want the physicians to be influenced by the score.                                                 What we found was that when we looked at how well physicians' estimates performed, they actually performed quite well. The c-statistic for physician estimated risk was around .7, which is a reasonable discrimination. However, the physicians' estimates were not as good as the EHFMRG risk score, which had a C greater than .8. The mathematical model seemed to do better in terms of predicting what's going to happen to the patient than physicians' estimates.                                                 Interestingly, when we combined the physicians' estimates with the EHFMRG risk score, the c-statistic improved by another 1%, so there's some additive value of having both factors combined.                                                 The other interesting finding was that patients in the lowest risk groups had 0% mortality at seven days, and 0% mortality at 30 days. We may be able to identify, using the score, patients who have a very low risk of events in that seven to 30 day period after emergency department presentation. Dr Carolyn Lam:                Thanks so much, Doug. I have to tell you, I am a fan of the EHFMRG score. In fact, we're trying to study how well it performs in our local situation even here in Singapore.                                                 Justin, you've been thinking a lot about this. I would love for you to share the reactions that we got when we discussed this among the editors. Dr Justin Ezekowitz:        We had a lot of good discussion about this from a number of different aspects. First, it's an in-practice assessment, a physician-based risk assessment, as we survey hundreds of physicians in the ER, which is a busy environment, and get these types of information. That's a very unique piece of this study where, in addition to the just under 2,000 patients and collecting the other data in a robust way, this really does have a potential to contribute to the literature.                                                 A lot of the discussion was about how data rich this is, and that this is an area where unlike acute cardiovascular disease where there are good risk assessment tools and other therapies, it's a really need of a scoring system that was well validated, can be replicated, and both in clinical practice as well as in selective cohorts. Doug, my congrats to your and the other parts of the team that's helped put this together.                                                 One of the questions that came up when we were discussing it was the risk textiles and buckets were very important for people to think about the very low risk, as you mentioned, 0% all the way up through much higher percents for seven day mortality, but how discrepant the risk was of the physicians versus the mathematical model; and a very good reminder of the inaccuracy of sometimes our assessments of risk in practice, especially in acute care.                                                 I wonder if you could comment on what your fence was from the physicians who participated in the study, and then the data of those, the most striking findings of that piece about where physicians make judgements on risk in for that seven-day mortality. Just any comments you may have? Dr Douglas Lee:                 We didn't know what to expect because there haven’t been many studies of this type before. What we found in our study was that physicians tended to overestimate the risk of lower risk patients. They thought bad things would happen to healthier patients, just to put it very simply. Physicians also underestimated the risk of the highest risk patients. They thought that the highest risk patients would do well.                                                 We were surprised about that finding, but also, we were not surprised in the fact that it seems to explain some of our earlier findings that in our earlier work, we found that low risk patients are hospitalized, and we think it's probably that physicians are admitting those patients because they want to ensure that they're making a safe decision; and no harm will fall in the patient. Maybe physicians are erring on the side of admitting those patients, even though they know they're a little bit low risk.                                                 At the other extreme, physicians underestimated risk in the highest risk patients. We think it might explain the observation that we made previously that sometimes high risk patients are discharged home, and they die at home after discharge. That may be because patients who look well to physicians, I think there's great value in the clinical experience of a seasoned physician looking at a patient and knowing that, that patient is sick or not sick. But in certain cases, patients may look relatively well, but their numbers would indicate that they're actually higher risk. I think it's that group where we found they're higher risk, but physicians thought that they were healthier than they were. It seems physicians' estimations really have great value, but it seems that they can be improved. Dr Carolyn Lam:                Sean, you discussed this beautifully in your editorial. Share with us your thoughts, and especially thoughts on the question you posed: are we any closer to the holy grail of safe emergency department discharge based on acute heart failure risk rules? Dr Sean Collins:                 Doug, kudos to you. Nearly 2,000 patients, nine different hospitals, prospective data collection, as Justin said. I don't think this can be overstated. From a data cleaning perspective, this is truly a labor of love, and to get this done, congratulations to you and your team.                                                 I think the most interesting part of this is this exact disconnect of patients look well who are high risk, and patients may look a little bit unwell who may be low risk, ironically. That's where a risk tool is much needed, as Carolyn said in her introduction to sort of change the dynamic of 80 to 90% of patients are admitted to the hospital. If we even chipped away at 10 to 15% to able to be discharged, it would be a huge win for partly for management for an emergency department perspective.                                                 I think that the importantly, the next steps will be now looking at implementing this in some sort of a randomized manner, somewhat like what you did with asking physicians gestalt about what their level of risk is, but really finding out how does a physician gestalt when it comes to nuance and heart failure. A relative amount of congestion, even when the tool says the patient may be low risk, can they go home? I think that will be the crucial next step to find out how much does this augment and/or detract from physician decision making? We have a long way to go, as Carolyn said. It's just the complete opposite at almost every other disease process, including chest pain, from a discharge perspective. Even a little bit improvement would be great, so I'm looking forward to seeing the next steps, and I'm wondering what your thoughts are about the next steps, Doug. Dr Douglas Lee:                 There's actually great value in physicians' clinical judgment. It's been, I think relatively understudied. I'm hopeful that future studies where decision tools or prognostic tools are validated, we can see more potentially, more comparisons with clinicians because we don't have a real great understanding, I think, of how doctors think, especially in an acute setting. More research in this area, I think would be really helpful, especially as we ... As more and more clinical decision tools being published, it would be great to see how well they hold up against good clinician judgment.                                                 In terms of next steps and implementation, when we talk to our emergency colleagues, they have brought up an issue about it's great that patients are low risk, and that we could potentially discharge them from hospital; but where is the receptor to take that patient and to care for that patient once they've left the hospital? Are they going to get good care once they leave the hospital? Are there structures in place?                                                 We're now embarking on testing this in the clinical trial where we will be comparing two strategies. The first strategy will be using the risk score at a hospital-wide level, and then discharging home patients who are in the lower risk categories, and having them follow up, and receive their care in a rapid ambulatory follow up clinic within two to three days after discharge from the emergency. This will be compared to the control, which is not using the risk score, and having usual follow up care. This trial is called the Comparison of Outcomes and Access to Heart Failure Trial, or the COAHFT trial. It is currently ongoing. Dr Sean Collins:                 Great point, Doug. As Carolyn suggested with chest pain and heart failure as the interesting dichotomy is that unlike chest pain, when we safely rule somebody out and send them home, we're sort of done with that acute episode. Heart failure, it doesn't end. We've found that they're safe enough to go home, but now they need great collaboration and outpatient support with their heart failure provider, which may be as equally heavy lift as externally validating the EHFMRG score. You bring up a great point, which is we need to have outpatient follow up and collaboration for this to be successful. Thanks. Dr Carolyn Lam:                Awesome comments, guys. Could I switch tracks a bit and maybe just ask Justin to round up by sharing? Circulation, we get a lot of papers about risk scores and so on. There's a bit of fatigue, I think, about scores in all kinds of things. Now, could you maybe tell us, Justin, what makes us look at a paper twice, and in fact, feature this one with a good editorial? I mean it's clearly very clinically applicable. Could you share some thoughts there? Dr Justin Ezekowitz:        Yes, that's a great point. The things that make a risk score like this kind of elevated into kind of a circulation level of manuscript is A) the data quality has to be excellent. There has to be lots of completeness of data, but also capture of elements that we think are quite important. Two, the data science about how it's analyzed and put together, and interpreted, it has to be to the bar that we feel would be robust, and be able ... if somebody could repeat it and replicate it without an obvious challenge to the quality.                                                 The third, I think is the clinical applicability. It's okay to write a data model and come up with all these great risk scores, but if they haven't been thought through about how either a patient will be seeing this, or clinicians behave, or the environment that it has to be deployed in that, that isn't necessarily going to be something that is going to be implemented. Then, the question is: why would somebody do the study in the first place?                                                 Now, it's okay if somebody's forward thinking and saying, 'Look, EMRs are coming, or other EHRs around, so this could be implemented if there was enough impetuous and it's a good enough quality.' That's actually okay, but in the reverse where if you try to implement a model that is too complex, and it's in a hand-off to the environment, it just won't work. We just want to make sure people have thought that next knowledge translation and dissemination approach through.                                                 The final part is things that have a very local impact are, that are very unique to the environment they're in, such as it only would work in your hometown or your own country because of some environment, that's okay. But under that, the much more global focus that, that is, it could be picked up and trans located to any major city, providence, state, or country, because vis vises are global. Those things have a much greater impact because the circulation leadership is global. The patients are global. The clinicians who care for them are also global. People are all looking for very similar situations and can adapt to their own environments. Dr Carolyn Lam:                Awesome, Justin. I don't think any of us could have said it better. Those are the reasons that we're so grateful that you publish with us, Doug. Thank you so much, Sean, for your excellent editorial, too.                                                 Thank you, listeners, for joining us today. You've been listening to Circulation on the Run. Don't forget to tune in again next week.                                                 This program is copyright American Heart Association 2019.  

Description : Beaucoup connaissent cette mélodie, écrite au 16ème siècle par Canon Pachalbel. Il a été découvert scientifiquement ceci : Cette musique relaxante à la particularité d'avoir huit notes correspondant exactement aux fréquences d'activité des huit acides aminés primordiaux chez l'homme. On sait aujourd'hui que chaque acide aminé possède sa propre fréquence qui se fixe en oscillation dans la cellule. Ces huit notes essentielles font partie de notre chaîne polypeptidique (chaîne de 10 à 100 acides aminés reliés) Son écoute pénètre notre corps et va stimuler dans la protéine d'activation de la GTPase. Cette activation permet aux circuits enzymatiques de réguler le métabolisme de la biosynthèse, toutes les réactions chimique de notre corps, gestion du chaud et du froid, digestion, rythme cardiaque, etc. Elle est extraordinaire dans sa composition ou Canon Pachalbel a voulu s'assurer qu'elle fasse du bien quelque soit le moment où on l'écoute, en lui attribuant de nombreux couplets sur des rythmes différents. Les basses ayant une fréquence de résonance choisie par Pachelbel , elles viennent réconforter le thème initial au niveau du rapport entre les fréquences. La protéine nourrie par cette musique produit un puissant anti stress pour l'homme. Un seul conseil... écoutez- le le plus souvent et vous verrez... A contrario une recherche démontre également, une musique très populaire construit sur les mêmes bases que le Canon de Pachelbel c'est à dire, huit notes et couplets. Mais avec un effet top dans le négatif. Explication sur mon site, http://terrevivante.e-monsite.com/pages/page.html http://terrevivante.e-monsite.com/pages/livre.html http://terrevivante.jimdo.com/ https://youtu.be/sZASDH6tbrI

Lymphoid and non-lymphoid tissues accommodate defined numbers of dendritic cells (DCs). There, DC-life span is influenced by various components such as proliferation and cell division triggered by cytokines, maturation processes in response to extracellular inflammatory and microbial substances, as well as induction of migration and apoptosis. Previous work has demonstrated the importance of specific numbers of DCs in tissues, as changes of DC numbers or DC life span could alter immunity, tolerance or inflammation resulting in various immune diseases. However, currently it is still unknown how DC life span and homeostasis is regulated in vivo. RhoA is a member of Rho GTPase-family, which plays important roles in regulating cytoskeleton organization, proliferation, migration and survival. However, our current knowledge about RhoA-functions is based mostly on studies using dominant negative and constitutively active RhoA-mutants, which have possible unspecific effects on other members of the Rho GTPase family. Therefore, we used a LoxP/Cre recombinase approach to knock out RhoA selectively in DCs. Here we found that GTPase RhoA controlled the homeostasis of mature DCs, and deletion of RhoA caused significantly reduced numbers of CD8+CD11b- and CD11b+Esamhi DC subsets, while CD11b+Esamlo DCs remained largely unaltered. Loss of RhoA interfered with homeostatic proliferation, cytokinesis, survival and turnover of cDCs. By performing proteomic analysis, we found that a pro-survival PI3Kγ/Akt/BAD pathway was deregulated in RhoA-ko DCs. Taken together, our findings indicate that RhoA plays a critical role in regulating DC-homeostasis, deletion of which decreases DC numbers resulting in impaired immune protection.

Wed, 30 Sep 2015 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/19068/ https://edoc.ub.uni-muenchen.de/19068/1/Niesser_Juergen_G.pdf Niesser, Jürgen Gerd ddc:540, ddc:500, Fakult

Membrane remodeling is a dynamic process that occurs in bacterial cells to facilitate substrate transport and to provide protection to bacteria during environmental stress. In eukaryotic cells, membrane remodeling is carried out by dynamin-like proteins (DLPs). These proteins are involved in diverse membrane-associated functions such as cargo transport via vesicles, cytokinesis, division of cell organelles and resistance to pathogens. DLPs are also conserved in bacterial species; however, their function is still not clearly understood. The genome of B. subtilis contains a gene dynA (ypbR), which encodes a large DLP (136 KDa),DynA, that can tether membranes and induce membrane fusion in vitro. Deletion of dynA in B. subtilis strain 168 fails to produce any observable growth phenotype under standard laboratory conditions. B. subtilis is a soil bacterium and prey to several environmental stress factors to which laboratory strains are normally not exposed. Hence, it was conceivable that DynA might be required when bacteria are exposed to stress. To address this hypothesis, the behavior of DynA was examined under conditions causing membrane-stress, such as exposure to antibiotics and phage infection. A strain lacking dynA showed impaired growth in the presence of sublethal amounts of antibiotics that target the cell membrane and was more sensitive to phage infection compared to wild-type strains. Time-lapse microscopy and fluorescence loss in photobleaching (FLIP) experiments showed that ΔdynA cells have compromised membrane remodeling compared to wild-type strain. In conclusion, all results propose DynA to play a role in protecting the cell membrane under stress conditions. Also, for the first time, it is shown that a bacterial DLP contributes to innate immunity of bacteria. DynA not only has a unique membrane protection function but also distinctive structural features. A single DynA polypeptide contains two dynamin-like subunits, each consisting of a GTPase domain and a dynamin-like stalk region. Both subunits, D1 and D2, share strong intra-molecular cooperativity to facilitate GTPase activity. Here, a combination of mutational analysis and subsequent in vivo and in vitro investigation was applied to further characterise structural assembly and biochemical properties of DynA. Size-exclusion chromatography elucidated that DynA dimerisation requires C-terminal amino acids 591-620. In addition, in vivo localisation, in vitro lipid-binding and GTPase analysis revealed arginine at position 512 of DynA to be a key regulator of GTP hydrolysis as well as lipid-binding. Furthermore, in vivo localisation and bacterial two-hybrid experiments were employed to confirm interaction of DynA with putative interaction partners (YneK, YwpG and YmdA). YneK was found to interact with D1 and YwpG with D1 and D2 individually, whereas YmdA required a full-length DynA (D1+D2) for interaction. Taken together, the results presented here greatly expand on current knowledge regarding functional, biochemical and structural properties of a bacterial dynamin-like protein (BDLP). This thesis not only demonstrates the preserved membrane remodeling function of DLPs in bacteria but also explain their conservation from bacteria to higher-organisms.

Aspergillus fumigatus is a major opportunistic, filamentous fungal pathogen causing invasive aspergillosis (IA), a fatal systemic infection in immunocompromised patients with significant mortality rate. The fungal cell is protected by a rigid but highly dynamic cellular structure, the cell wall that forms the first level of defence against environmental stress. The cell wall being an essential and unique structure of the fungus has always been an ideal drug target. The major antifungal drugs used currently either target the fungal cell membrane or cell wall. However, due to the poor efficacy of current antifungal therapy, the CWI (cell wall integrity) pathway has emerged as the focus of research in recent years to discover potential molecular drug targets for designing antifungal therapy with novel mode of action. This signaling cascade is dedicated to monitoring and maintaining functional integrity of the cell wall, remodelling its structure in response to cell surface stress. This MAPK (mitogen activated protein kinase) cascade is highly coordinated to transduce the stress signals to the nucleus and consequently trigger necessary gene expression to counteract the stress. In this study, we explored the pivotal role of guanine nucleotide exchange factor (GEF), Rom2 in cell wall stress response and antifungal drug susceptibility. The findings of this work reveal that the Rom2 GEF is essential for viability of the pathogen. Additionally, characterization of a conditional rom2 mutant functionally links it to the previously identified CWI sensors, namely, Wsc1, Wsc3 and MidA in A. fumigatus. The conditional mutant shows severe growth defects under repressive conditions such as hyper-susceptibility to heat, Calcofluor white and Congo red, similar to the ∆midA mutant. Additionally, similar to the ∆wsc1, the rom2 mutant cultured under repressive conditions is increasingly susceptible to the actively used antifungal and inhibitor of cell wall β-1,3-glucan synthesis, echinocandin such as caspofungin. Furthermore, the Rom2 shows a sub-cellular localization similar to the Rho1 GTPase to hyphal tips and also physically interacts with the GTPase. Thus, these relevant findings establish the integral role of Rom2 as an intermediate relay molecule acting between the cell surface sensors and Rho1 GTPase as well as the downstream MAPK module. This study also reports a novel mechanism imparting echinocandin tolerance to the pathogen. This work explores two possibilities that may explain the fungistatic nature of echinocandins against Aspergillus: one either owing to incomplete inhibiton of β-1,3-glucan synthesis or that the cell wall β-1,3-glucan is not essential for A. fumigatus viability. In order to evaluate the role of the β-1,3-glucan synthase subunit, Fks1 in viability, growth and antifungal response of the mold, a conditional fks1 mutant was generated. Downregulation of fks1 expression results in characteristic growth behaviour which phenocopies the effect of wild type treated with echinocandins. The mutant cultured under repressive growth conditions also displays significant decrease in cell surface β-1,3-glucan and enhanced galactomannan shedding, marked with a compensatory increase in chitin content. Importantly, the growth of the conditional fks1 mutant is not completely abolished in presence of echinocandin and an fks1 deletion mutant is surprisingly viable. These results strongly reflect that β-1,3-glucan is not essential in A. fumigatus, and thereby justifies the limited activity of β-1,3-glucan synthesis inhibitor echinocandin on the mold. The novel findings of the work also suggest that presence of septa is an essential means of survival for A. fumigatus upon echinocandin treatment. Compounds inhibiting septum formation exhibit significant synergism with the echinocandin caspofungin. Thus, the present study identifies and proposes that septum inhibition is a promising strategy for enhancing echinocandin fungicidal potency and improving existing antifungal therapy.

William Pryor and Srinivasa Subramaniam report that a mutant form of huntingtin that is associated with early-onset Huntington's disease promotes anabolic signaling that contributes to disease symptoms in mice.

Wed, 6 Nov 2013 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/18855/ https://edoc.ub.uni-muenchen.de/18855/1/Schulz_Anna.pdf Schulz, Anna ddc:610, ddc:600,

Die von intrinsichen renalen Zellen und infiltrierenden Leukozyten exprimierten Zytokine sind zentrale Vermittler entzündlicher Nierenerkrankungen. Tumor Nekrose Faktor-α (TNF) ist ein solches proinflamatorisches Zytokin, das in der glomerulären Entzündungsreaktion involviert ist. Die funktionelle Rolle von TNF wurde in Tiermodellen der Glomerulonephritis belegt. Die biologischen Effekte von TNF werden durch die beiden funktionell eigenständigen TNF-Rezeptoren TNFR1 (CD120a) und TNFR2 (CD120b) vermittelt. Neuere Daten zeigen, dass in Modellen einer Immunkomplex-Glomerulonephritis wie der nephrotoxische Serumnephritis die beiden TNF-Rezeptoren in vivo unterschiedliche Funktionen bei der glomerulären Entzündung vermitteln können. Der vorliegenden Arbeit liegt die Hypothese zugrunde, dass Tnfr1 und Tnfr2 unterschiedliche inflammatorische TNF-Effekte in Glomeruli vermitteln. Daher war das Ziel dieser Arbeit, Expression und Funktion der beiden TNF-Rezeptoren in Maus-Glomeruli zu charakterisieren und die Tnfr-abhängig exprimierten Entzündungsmediatoren in Maus-Glomeruli zu identifizieren. Aufbauend auf den Ergebnissen dieser Arbeit könnten selektive, Tnfr-spezifische Therapien zur Hemmung der glomerulären Entzündungsreaktion entwickelt werden. Zudem wurde in dieser Arbeit die funktionelle Rolle der beiden TNF-Rezeptoren im MRL/lpr-Mausmodell der Lupusnephritis untersucht, um eine selektive Tnfr-Blockade als mögliche Therapiestrategie zu charakterisieren. Hierfür war eine Rückkreuzung von Tnfr1- und Tnfr2-defizienten C57BL/6J-Mäusen in den MRL/lpr-Hintergrund erforderlich. Um TNF-Rezeptor-1- und 2-vermittelte inflammatorische Signalwege in Glomeruli zu identifizieren wurde die Expression und die Funktion der beiden TNF-Rezeptoren in Mausnieren, in isolierten Glomeruli ex vivo und murinen glomerulären Endothel- und Mesangialzellen in vitro untersucht. In normaler Mausniere konnte eine Tnfr1- und Tnfr2-mRNA- und Protein-Expressionen präferentiell in Glomeruli im Vergleich zum Tubulointerstitium nachgewiesen werden. Die Expression von beiden TNF-Rezeptoren und die TNF-induzierte Induktion von Tnfr2-mRNA-Expression wurde auch in vitro sowohl in murinen glomerulären Endothel- als auch Mesangialzelllinien bestätigt. Die prominente glomeruläre TNF-Rezeptor-Expression korrelierte mit einer konstitutiven glomerulären mRNA-Expression von Adhäsionsmolekülen wie Icam-1, Vcam-1, E- und P-Selektin und Chemokinen wie Ccl2, Ccl3 und Ccl5. Eine intraperitoneale TNF-Injektion induzierte die Expression dieser Mediatoren präferentiell in Glomeruli. Diese in vivo TNF-Exposition führte zu einer raschen glomerulären Akkumulation von Leukozyten einschließlich Neutrophilen und mononukleären Phagozyten, die mittels einer kompartimentspezifischer Durchflußzytometrie analysiert wurden. Um Tnfr-abhängige inflammatorische Effekte in intrinsischen glomerulären Zellen unabhängig von infiltrierenden Leukozyten zu untersuchen, wurde eine Microarray-Gene-Expressionsanalyse an intakten Glomeruli durchgeführt, die aus Wildtyp und Tnfr-defizienten Mäusen isoliert und anschließend mit TNF ex vivo stimuliert wurden. Die meisten TNF-Effekte wurden ausschließlich durch Tnfr1 vermittelt, unter anderem die induzierte mRNA-Expression von Adhäsionsmolekülen, proinflammatorischen Chemokinen, Komplement-Faktoren und proapoptotischen Molekülen. Im Gegensatz dazu fanden wir nur vier Tnfr2-abhängig exprimierte Gene, einschließlich einer kleinen GTPase der Rab-Familie (Rab6b). Diese Ergebnisse wurden durch quantitative RT-PCR-Analysen von TNF-stimulierten Glomeruli und primären Mesangialzellen bestätigt. Weitere Untersuchungen zeigten allerdings auch einen Beitrag von Tnfr2 bei der gesteigerten glomerulären Expression von Adhäsionsmolekülen und Chemokine nach Stimulation mit niedrigen TNF-Konzentrationen auf. Im Gegensatz zur Wildtyp-Kontrolle fehlte in TNF-stimulierten Tnfr1-defizienten Glomeruli die Sekretion verschiedener proinflammatorischer Chemokine beinahe vollständig. Interessanterweise war die Proteinexpression auch in Tnfr2-defizienten Glomeruli signifikant herunterreguliert. Folglich sind die meisten inflammatorischen TNF-Effekte in Glomeruli via Tnfr1 durch die induzierte Expression von proinfammatorischen Mediatoren wie Adhäsionsmolekülen und Chemokinen vermittelt. Darüber hinaus dürfte Tnfr2 zu dieser inflammatorischen Antwort beitragen, wenn Glomeruli niedrigen TNF-Konzentrationen ausgesetzt sind. Ferner scheint Tnfr2 posttranskriptionell die Chemokinsekretion in Glomeruli nach einer TNF-Exposition zu beeinflussen, möglicherweise durch die Tnfr2-abhängig exprimierte Rab GTPase Rab6b, die am intrazellulären Transport und der Sekretion von inflammatorischen Molekulen beteiligt sein könnte. In Bezug auf Tnfr-spezifische, anti-inflammatorische Therapien weisen die hier präsentierten Ergebnisse somit darauf hin, dass eine selektive Tnfr1-Blockade eine glomeruläre, insbesondere durch Granulozyten und Makrophagen vermittelte Entzündung verbessern könnte, möglicherweise bei geringer Hemmung immunregulatorischer und antimikrobieller Funktionen von TNF, die redundant durch Tnfr2 vermittelt werden könnten. Dagegen erscheint aufgrund der erhobenen Daten im MRL/lpr-Mausmodell eine Blockade von TNF oder beider Rezeptoren bei der Lupusnephritis, in der glomeruläre Neutrophileninfiltrate keine entzündliche Rolle spielen, weniger erfolgversprechend. Gleichzeitig weisen die vorliegenden Ergebnisse auf eine immunsuppressive, die systemische Immunreaktivität beim SLE begrenzende Funktion von Tnfr2 hin.

The vacuolar ATPase is required for amino acid-mediated activation of mTORC1.

Influenza A virus infections are a major threat to the world poultry population. In the 19th century fatal influenza A virus infections have been described under the name “fowl plague” in chickens. The chicken is a natural host for this viral infection. The myxovirus resistance (Mx) gene which belongs to the group of interferon stimulated genes (ISG) was first described in mice. This protein confers protection against highly pathogenic influenza A viruses in mice. Mx proteins have been characterized in many species including mammals, poultry and fish. They belong to the family of large GTPase proteins exhibiting three highly conserved GTP-binding motifs at the amino-terminus and a leucin zipper at the carboxy-terminus. The GTPase activity was been shown to be essential for the antiviral activity of the Mx protein. Mx was also identified in the chicken, where it shows a significant degree of polymorphism. A polymorphism at aminoacid position 631 (serin versus asparagin) is thought to confer antiviral activity towards influenza A viruses. With the help of the RCAS retroviral vector system these two different chMx isoforms were examined in vitro and in vivo for their antiviral activity towards various pathogenic influenza A viruses. Neither in vitro nor in vivo antiviral activity of the chMx isoforms was detectable. In contrast overexpression of murine Mx1 and human MxA in the same form led to protection of the chicken embryo fibroblast cultures against influenza A viruses. Stimulation of chicken embryo fibroblasts with type I interferon induced chMx mRNA and protein expression as well as an antiviral state of the cells. However a chMx knock down mediated by siRNA did not lead to the loss of the antiviral state mediated by type I interferon. In summary, the in vitro studies did not provide evidence for a role of chMx in the antiviral state induced by type I interferon.

Fri, 20 Mar 2009 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/10322/ https://edoc.ub.uni-muenchen.de/10322/1/Schaefer_Anja.pdf Schäfer, Anja ddc:570, ddc:500, Fak

Vesicle traffic in eukaryotic cells is a tightly organized process involving a multitude of regulatory proteins. Key regulators of this traffic are small GTPases called Rabs. With about 60 members in the human genome, they constitute the largest subgroup in the superfamily of Ras like monomeric GTPases. They recruit effector proteins to specific membranes and thus define the identity of organelles. Rabs switch between an active, GTP bound state and an inactive GDP bound state. Key regulators of this conversion are RabGAPs, which accelerate the hydrolysis of bound GTP. All RabGAPs are characterized by the presence of a TBC domain. In the human genome 40 RabGAPs were identified, most of which had not been studied so far. To assign them to their specific Rab proteins, a novel reverse yeast two-hybrid screening method was developed. This identified a GAP for Rab5 termed RabGAP-5. RabGAP-5 stimulated the GTPase activity of Rab5. Its expression inactivated Rab5 and redistributed the Rab5 effector EEA1 from early endosomes to the cytoplasm. RabGAP-5 also blocked the Rab5 dependent uptake of EGF and transferrin from the plasma membrane. When RabGAP-5 was depleted, the size of endosomes was increased, indicating elevated Rab5-GTP levels. Endocytosed EGF was unable to exit the endosome, indicating that trafficking through endosomes was also blocked. To identify GAPs and Rabs implicated in the regulation of early secretory events simultaneously, a second novel screening method was established. It involved the analysis of phenotypes caused by the inactivation of endogenous target Rabs via the overexpression of RabGAPs. Changes in Golgi morphology, ERGIC organisation and the proceeding of secretion were only observed with one candidate RabGAP, the highly conserved protein TBC1D20. TBC1D20 showed activity towards Rab1 and Rab2 in vitro, and acted primarily on Rab1 in vivo. In contrast to all other RabGAPs it has a transmembrane domain, which localises it to the ER. TBC1D20 interacts with RTN-1 on ER membranes. This interaction modulates the activity of TBC1D20. These data indicate a novel function for Rab1 in regulating ER exit, and thus extend the classical view of RabGAPs as regulators of active Rab lifetime.

Thu, 24 Apr 2008 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/9971/ https://edoc.ub.uni-muenchen.de/9971/1/Bauer_Martina.pdf Bauer, Martina ddc:540, ddc:5

Die autosomal dominante Optikusatrophie ist mit Mutationen in dem Gen OPA1 assoziiert. OPA1 kodiert eine konservierte mitochondriale Dynamin-ähnliche GTPase. Das Ortholog von OPA1 in S. cerevisiae ist Mgm1. Mgm1 liegt im Intermembranraum der Mitochondrien assoziiert mit der Innenmembran in zwei Proteinisoformen vor: der langen (l-Mgm1) und der kurzen Isoform (s-Mgm1). Beide Isoformen sind für den Erhalt der mitochondrialen Morphologie und der mitochondrialen DNA erforderlich. l-Mgm1 wird von der mitochondrialen Rhomboidprotease Pcp1 durch limitierte N-terminale Proteolyse in s-Mgm1 umgesetzt. OPA1 ist ebenfalls für den Erhalt normaler mitochondrialer Morphologie in Säugetierzellen erforderlich. Zusätzlich reguliert es die Freisetzung von Cytochrom c während der Apoptose. Insgesamt acht Transkriptionsvarianten von OPA1 sind bekannt, die durch alternatives Spleißen der N-terminal gelegenen Exons 4, 4b und 5b entstehen. Auf Proteinebene ließen sich bis zu fünf OPA1-Proteinisoformen unterschiedlicher Größe voneinander abgrenzen. Die Proteinisoformen liegen zum einen Teil membranverankert in der Innenmembran und zum anderen Teil peripher mit der Innenmembran assoziiert im Intermembranraum der Mitochondrien vor. Die vorliegende Dissertation beschäftigt sich mit der Biogenese der verschiedenen OPA1-Proteinisoformen. Hierzu wurden OPA1-Transkriptionsvarianten in Hefe heterolog exprimiert. OPA1 wird in Hefe ähnlich wie in Säugetierzellen prozessiert. Die Prozessierung erfolgt N-terminal, an mehreren Stellen und schrittweise. Die menschliche mitochondriale Rhomboidprotease PARL kann Pcp1 in der Hefe voll komplementieren, aber weder Pcp1 noch PARL prozessieren OPA1. In PARL-/--Mauszellen wird OPA1 normal prozessiert. In der Hefe ist die Prozessierung von OPA1 von den Untereinheiten Yta10 und Yta12 der mitochondrialen AAA-Protease der Matrix (m-AAA-Protease) abhängig. Durch Expression der Untereinheiten der menschlichen m-AAA-Protease, Paraplegin und AFG3L2, lässt sich die Prozessierung von OPA1 in yta10yta12 rekonstituieren. Die Ergebnisse deuten darauf hin, dass die Biogenese von Mgm1/OPA1 nicht vollständig von der Hefe bis zu Säugetieren konserviert ist. Der Austausch der prozessierenden Protease könnte in Verbindung mit einem Mechanismus zur Qualitätssicherung der Mitochondrien in Metazoa stehen.

The first step of preprotein translocation across the membranes of chloroplasts is facilitated by the Toc translocon. Aim of this work was to elucidate the dynamics and the mechanism of action of this molecular machine. The central, stably associated part of the Toc translocon, the Toc core complex, consists of the pore forming Toc75 and two receptors with GTPase activity, Toc34 and Toc159. The question of Toc159 localization was addressed since controversal results on this topic were reported. In this study, membrane localization of Toc159 was confirmed, which has further implications on the mode of its action. To understand the necessity of multiple isoforms of Toc components as found in Arabidopsis thaliana, expression analysis and tissue-specific localization were conducted. Gathered data suggested the existence of several types of the complex, assembled from different types of subunits. These complexes have different preprotein specificities. Expression analysis provided further arguments for dynamic association of the intermembrane space complex with the Toc core complex. Comparison of gene expression and protein presence of translocon subunits contradicts the function of Tic20 as a general pore for stromal targeted proteins, but not as a protein conducting channel per se. For further analysis of the Toc translocon structure and function, its purification and reconstitution into proteoliposomes was reinvestigated. To this end, a technique for liposome size determination in a single spectrophotometric measurement was developed.

Makrophagen spielen innerhalb des zellulären unspezifischen Abwehrsystems eine wesentliche Rolle. Für die Ausübung ihrer Funktion sind dynamische Änderungen des Zytoskeletts sowie Aufnahmeprozesse wie Phago- und Pinozytose von entscheidender Bedeutung. Diese Prozesse werden u. a. von Rho-GTPasen und ihren Effektorproteinen reguliert. Zu diesen Effektorproteinen gehören die Proteine der WASp-Familie, die aus WASp, N-WASP und den drei WAVE-Isoformen besteht. In unserer Arbeitsgruppe konnten mittels eines pan-WAVE-Antikörpers Akkumulationen von WAVE an vesikulären Strukturen gezeigt werden (Dissertation B. Schell, 2003). Über eine Beteiligung von WAVE an der Regulation von Vesikeln ist jedoch bisher nichts bekannt. Deshalb beschäftigt sich diese Arbeit mit der Rolle und Funktion von WAVE im Rahmen der Vesikelbildung in Makrophagen. Mittels Färbungen gegen die verschiedenen WAVE-Isoformen konnte erstmals in J774- und primären Makrophagen gezeigt werden, dass WAVE1 an vesikulären Strukturen lokalisiert. Überexpressionen von WAVE1- und WAVE2-GFP bestätigten dieses Ergebnis. Darüber hinaus war es möglich, WAVE1 nach Stimulierung der Makrophagen durch chemoattraktive Stoffe wie fMLP und LPS an Vesikeln zu lokalisieren. Im Rahmen ihrer Rolle als Fresszellen sind Makrophagen insbesondere zu Phagozytose und Pinozytose befähigt. Da Vesikel gerade bei derartigen Prozessen auftreten, wurde untersucht, ob im Rahmen endozytotischer Vorgänge auch WAVE1-Vesikel vorkommen. Da es sich bei der Phagozytose um die Aktin-abhängige Internalisierung von Partikeln > 0,5 µm handelt, wurde ein Phagozytose-Assay mit latex-beads gewählt. Dabei werden von der Zelle Aktin-reiche Strukturen, sog. phagocytic cups, um den aufzunehmenden Partikel erzeugt. In den durchgeführten Experimenten wurde jedoch nur eine geringgradig gesteigerte Bildung von WAVE1-Vesikeln beobachtet. Eine Assoziation zwischen WAVE1 und den entstandenen phagocytic cups wurde dabei nicht festgestellt. Da die phagocytic cups auch nicht den gesuchten vesikulären Strukturen entsprachen, standen Phagozytose und phagocytic cups nicht im Fokus der weiteren Arbeit. Zur Stimulation der Pinozytose wurden sog. fluid phase marker wie z. B. Dextrane und Lysotracker verwendet. Damit konnte gezeigt werden, dass WAVE1-haltige Vesikel mit fluoreszenzmarkierten Dextranen in pinozytotischen Vesikeln kolokalisieren. Durch Verwendung von Lysotracker konnten die kolokalisierenden Vesikel sauren Kompartimenten im endosomallysosomalen Pathway, am ehesten Lysosomen entsprechend, zugeordnet werden. Endozytotische Vorgänge sind hochregulierte Prozesse. Da sich Makropinozytose sowie der anschließende Vesikeltransport entlang von Filamenten u. a. durch Manipulationen des Aktinund Mikrotubuli-Zytoskeletts inhibieren lässt, wurde der Einfluss des Aktin- bzw. Mikrotubuli- Zytoskeletts auf die WAVE1-Vesikel Bildung durch die Verwendung von Cytochalasin D und Nocodazol untersucht. Die Bildung von WAVE1-Vesikeln zeigte sich dabei unabhängig von der Manipulation sowohl des Aktin-Zytoskeletts als auch des Mikrotubuli-Netzwerkes. Im Gegensatz dazu steht die Bildung von Dextran-Vesikeln: diese konnte durch Zerstörung des Aktin- Zytoskeletts mittels Cytochalasin D reduziert werden. Damit konnte die in der Literatur beschriebene Aktin-Abhängigkeit von Dextran-Vesikeln bestätigt werden. Desweiteren scheint, wie erwartet, durch Zerstörung des Mikrotubuli-Netzwerkes mittels Nocodazol nicht die Aufnahme, sondern der intrazelluläre Transport der Dextran-Vesikel entlang von Filamenten inhibiert zu werden. WAVE1 stellt ein Multidomänenprotein dar. Um die Rolle der einzelnen Domänen von WAVE1 in Bezug auf die Bildung von WAVE1- und Dextran-Vesikel zu analysieren, wurden verschiedene Mutanten von WAVE1 als GST-Fusionsproteine in Makrophagen mikroinjiziert. Einen Effekt bezüglich der Bildung von Dextran-Vesikeln konnte mit der WA-Domäne von WAVE1 gezeigt werden. Dieses Resultat stimmt mit der zuvor beschriebenen Aktin- Abhängigkeit der Dextran-Vesikel überein. Die Konstrukte WAVE1-P ebenso wie WAVE1- PWA führten zu einer signifikanten Reduktion der Bildung von Dextran-Vesikeln. Dies lässt den Schluss zu, dass die Prolin-reiche Region eine essentielle Rolle in der Regulation sowohl von WAVE1- als auch Dextran-Vesikeln spielt. Zur Beschreibung eines möglichen Signalweges, der WAVE1- und Dextran-Vesikel beeinflusst, wurde nach Interaktionspartnern von WAVE1 gesucht. Mit NCK-1 und PAK-1 konnten in der Immunfluoreszenz zwei mit WAVE1 kolokalisierende Proteine gefunden werden. Transfektionsversuche lassen den Schluss zu, dass PAK1 die Bildung von WAVE1-Vesikeln beeinflusst. Weitere Experimente mit verschiedenen Mutanten von NCK-1 geben Hinweise auf einen Zusammenhang zwischen NCK-1 und WAVE1. Dabei scheinen vor allem die drei SH3- Domänen von NCK-1 einen Einfluss auf die Bildung der Dextran-Vesikel zu besitzen. WAVE1 wird durch die sog. mitogen activated protein kinase (MAPK) beeinflusst (Miki et al., 1999). Eine Phosphorylierung von WAVE1 durch die MAPK konnte in der vorliegenden Arbeit nicht nachgewiesen werden. Jedoch konnte durch Verwendung eines Inhibitors der MAPK ein deutlicher Einfluss sowohl auf die Bildung der WAVE1-Vesikel als auch auf die Bildung der Dextran-Vesikel gezeigt werden. Dies lässt den Schluss zu, dass die MAPK, ob direkt oder indirekt, eine wichtige Rolle im Rahmen der Bildung von WAVE1- und Dextran-Vesikeln spielt. Es konnte ein hypothetisches Modell eines Signalweges von WAVE1 erstellt werden: Phagozytotische Stimuli wie Dextrane aktivieren die GTPase Rac. Dies führt zur Rekrutierung und Aktivierung von Effektorproteinen wie PAK1 und NCK-1. Aktiviertes NCK-1 bindet WAVE1 und kann dieses seinerseits an die Plasmamembran rekrutieren. Dort könnten bspw. an der Zellfront WAVE1-abhängig membrane ruffles entstehen. Durch einen möglichen positiven feedback loop wird die Aufnahme von Dextran erleichtert. Aktiviertes PAK1 aktiviert die MAPK und beeinflusst WAVE1. Durch die Aktivierung von WAVE1, NCK-1 und PAK1 erfolgt die Bildung von WAVE1-Vesikeln. Diese WAVE1-Vesikel kolokalisieren im Laufe des endolysosomalen Pathway mit den internalisierten Dextran-Vesikeln und werden wahrscheinlich Lysosomen zur Degradierung zugeführt.

Rap1 is a ubiquitous Ras-like guanine-nucleotide-binding protein that is involved in a variety of signal-transduction processes especially during cytoskeletal rearrangements. Rap1 is regulated by guanine-nucleotide- exchange factors (GEFs) and GTPase-activating proteins (GAPs) which increase the slow intrinsic GTPase activity by many orders of magnitude and allow tight regulation of signaling. In this study a new Dictyostelium RapGAP1 gene was cloned and characterized. RapGAP1 was discovered by screening the sequences of the D. discoideum database. The Dictyostelium RapGAP1 gene encodes a protein with 1212 amino acids protein which shows at the C-terminal region 53% sequence similarity to human RapGAP. RapGAP1 mRNA was present during all stages of D. discoideum development with a strong upregulation at 9 hours of development. Furthermore, to investigate the role of RapGAP1 in cellular processes RapGAP1 null cells where generated by inserting a gene replacement construct into the endogenous gene. RapGAP1 minus mutants did not show any significant phenotypic abnormalities except that there was a slight delay in development. This delay by about three hours was confirmed by testing the expression of developmentally regulated genes like csA, a cell adhesion protein, and MUD1, a prespore-specific cell surface antigen. However, the mutant was able to complete normal developmental and to form fruiting bodies containing mature spores. Studies on cell motility showed that RapGAP1 null cells moved faster than AX2 wild type cells. This finding suggests that RapGAP1 belongs to a signal transduction chain which ultimately leads to changes in cytoskeletal dynamics.

Fri, 28 Oct 2005 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/7758/ https://edoc.ub.uni-muenchen.de/7758/1/Nopora_Adam.pdf Nopora, Adam ddc:500, ddc:570, Fa

The Drosophila gene minibrain codes for a protein kinase. Mutants with a reduced expression of minibrain are characterised by a strongly reduced brain size and learning and memory deficits. Dyrk1A is the mammalian homologue of minibrain; the encoded protein is the founding member of the family of Dyrk protein kinases. The human DYRK1A gene is implicated in the emergence of the cognitive deficits in Down syndrome, due to its location on chromosome 21 in the so-called Down syndrome critical region (DSCR), its overexpression in Down syndrome individuals and also because Dyrk1A-overexpressing mice show learning and memory deficits. Taken together, these data point to a gene dosage dependent function of Dyrk1A/minibrain in processes that are associated with cognitive functions such as learning and memory. The aim of this study was to elucidate the function of Dyrk1A in the adult mammalian brain. Three aproaches were chosen: First, an analysis of the expression of Dyrk1A and of the intracellular localisation of the respective protein in the adult mouse brain was performed. Dyrk1A is expressed in several brain areas, e.g. in the neocortex, in the hippocampal formation and in the cerebellum. In neurons, Dyrk1A is located either mostly nuclear, or predominantly or even exclusively in the cytoplasm, depending on the region and on the type of neuron. Second, to characterise the gene dosis dependent function of Dyrk1A, it was intended to generate transgenic mice that express a third copy of Dyrk1A, as it is the case in human trisomy 21 (Down syndrome). These transgenic mice should express the third copy of Dyrk1A in a region- and time-specific manner, e.g. in principal neurons in the adult forebrain. This should avoid developmental defects and disturbances of, e.g., the motor system and should allow for the specific analysis of the role of Dyrk1A in learning and memory and in brain regions involved in cognitive processes. A BAC (bacterial artificial chromosome) clone containing the whole Dyrk1A locus was isolated. This BAC was modified by homologous recombination in E. coli (GET recombination) to render it suitable for the generation of transgenic mice. Furthermore, a recombination cassette for a second modification step was constructed to allow for the region- and time-specific expression of the Dyrk1A gene on the BAC. Third, a yeast two-hybrid screen to identify proteins interacting with Dyrk1A was performed. Two of six clones obtained were shown to code for proteins interacting specifically with Dyrk1A: Sept4 (Pnutl2/H5/CDCrel-2), a GTPase of the Septin family, and Arip4, a steroid hormone receptor cofactor with ATP-dependent chromatin-remodelling activity. These interactions were confirmed in mammalian cells by coimmunoprecipitation. By in situ-hybridisation, the coexpression of Dyrk1A with the genes of both interactors was shown in various brain regions. In addition, Dyrk1A and Arip4 were shown to be colocalised in a speckle-like nuclear subcompartment in primary rat hippocampal neurons and in mammalian cell lines. These results point to an interaction of Dyrk1A with Arip4 also in vivo and implicate Dyrk1A in steroid hormone-mediated signalling. Furthermore, an interaction between Dyrk1A and Sept5/CDCrel-1, a close relative of Sept4, was shown. Sept5 is thought to play an inhibitory role in the fusion of synaptic vesicles with the presynaptic membrane and transmitter release. The results of this study contribute to the understanding of the role of Dyrk1A in physiological and pathophysiological contexts. Eventually, this should help (i) to elucidate the mechanisms that lead to mental retardation in Down syndrome and (ii) to develop tools to interfere with the function of Dyrk1A and to establish novel strategies for a therapy to alleviate mental retardation in Down syndrome.

The endothelium is among the largest organs in the body. Stimuli originating from the blood or from neighbouring cells, like inflammatory cytokines (IC), lead to structural and functional alterations of vascular endothelial cells (EC). These alterations are often referred to as “EC activation”. Activated EC play a key role in different physiological processes like during immune response, in menstruation and in pathological processes like inflammation, allergy, viral infections, atherosclerosis and tumour angiogenesis. The human guanylate binding protein-1 (GBP-1) is a protein of the family of large GTPases. GBP-1 is characterized by a high turnover GTPase activity. Previous work showed that GBP-1 mRNA expression is induced by IC in EC and that GBP-1 is the specific mediator of the anti-proliferative effect of IC on EC in vitro. The main goals of this work were first, to investigate whether GBP-1 may be a molecular marker of IC-activated EC at the protein level in vitro. Second, to investigate GBP-1 expression in human healthy and/or disease tissues and to determine whether GBP-1 may be a molecular marker of IC-activated EC in vivo. To this goal mono- and poly-clonal antibodies against GBP-1 were generated. In vitro studies showed that GBP-1 expression in EC is induced by IFN-, IFN-, IL-1, IL-1 or TNF- but not by other cytokines, chemokines or growth factors. Moreover, simultaneous addition of bFGF and VEGF and IC reduced the IC-induced GBP-1 expression. This indicated that GBP-1 characterizes cells that are preferentially exposed to IC. In vivo studies using immunohistochemistry and immunofluorescence showed that GBP-1 expression is highly associated with vascular EC in a broad range of human tissues. This was confirmed by the simultaneous detection of GBP-1 and the EC-associated marker CD31. Notably, GBP-1 expression was undetectable in healthy skin. In contrast, GBP-1 was highly expressed in vessels of skin diseases with a high inflammatory component including psoriasis, adverse drug reactions and Kaposi’s sarcoma. This indicated that GBP-1 characterizes IC-activated EC in vivo. Further immunohistochemical studies on Kaposi’s sarcoma demonstrated that GBP-1 expression and EC cell proliferation are inversely related. This indicated that GBP-1 may also mediate the anti-proliferative effect of IC on EC in vivo. Finally, GBP-1 was found to be secreted by EC stimulated with IFN- and IFN- in vitro. This finding was confirmed by immunoprecipitation of GBP-1 from cell culture supernatants and by a novel ELISA developed for the detection of GBP-1 in solution. Further characterization of the mechanism of secretion demonstrated that GBP-1 release is due to an 3 Summary energy-dependent mechanism and is not due to cell death. Most importantly, circulating GBP-1 could be detected in increased concentrations in the blood of patients that were subjected to IFN–-therapy or in patients with inflammatory diseases. These findings indicated that GBP-1 is a novel marker of inflammatory vessel activation. Specifically, the serological detection of GBP-1 may open new perspectives for the early detection of inflammatory activation of EC in patients with inflammatory diseases.

Reaktive – Sauerstoff - Spezies (ROS) spielen in der Physiologie und Pathophysiologie des vaskulären Systems eine wichtige Rolle. So kommt es z.B. bei Hypertonie, Atherosklerose, Ischämie / Reperfusion und weiteren Krankheiten und Stoffwechselstörungen, wie z.B. Hypercholesterinämie und Diabetes mellitus zu einem Ungleichgewicht zwischen Sauerstoffradikalbildung und anti - oxidativen Mechanismen. Superoxidanionen (O2 -) spielen insofern eine besondere Rolle, als sie durch direkte Interaktion endotheliales NO inaktivieren, so daß es seine vasodilatatorische, anti – proliferative und plättchenaggregationshemmende Funktion nicht mehr voll erfüllen kann. Damit ist O2 - maßgeblich an der Induktion der Endotheldysfunktion beteiligt. Bei Beginn dieser Arbeit gab es erste Hinweise, daß eine der leukozytären NAD(P)H - Oxidase ähnlichen Oxidase auch im Endothel existiert und wesentlich zur endothelialen O2 - - Bildung beiträgt. Wenig erforscht waren jedoch die Regulationsmechanismen dieser Oxidase. Ein bisher noch nicht bekannter Stimulus zur Steigerung der endothelialen O2 - - Bildung wurde 1996 beschrieben. In Endothelzellen aus bovinen Pulmonararterien führte eine Depolarisation zu einer gesteigerten O2 - - Bildung. Dies kann insofern von Bedeutung sein, als es sowohl unter physiologischen, als auch pathophysiologischen Bedingungen zu akuten oder chronischen Veränderungen des endothelialen Membranpotentials kommt. In dieser Arbeit wurde nun untersucht, ob eine NAD(P)H – Oxidase in der Tat auch in humanen Endothelzellen vorhanden ist, ob sie im Gegensatz zur leukozytären Form konstitutiv aktiv ist, und welchen Beitrag sie zur basalen endothelialen O2 - - Bildung leistet. Weitere Untersuchungen in HUVEC sollten zeigen, ob und wie sich sowohl De – als auch Hyperpolarisation der Zellmembran auf die O2 - - Bildung auswirken, welches Enzym hierbei eine Rolle spielt und welche Signaltransduktionsmechanismen beteiligt sind. Zur O2 - - Messung an vaskulären Zellen war die Verwendung der Lucigenin – Chemilumineszenz – Methode etabliert, so daß auch hier anfänglich mit dieser Methode gearbeitet wurde. Da jedoch dann Befunde veröffentlicht wurden, die zeigten, daß Lucigenin in Enzymsyste-men, die sonst kein oder nur wenig O2 - produzieren, zu einer erheblichen O2 - - Bildung führte, mußte mit weiteren Methoden der O2 - - Messung überprüft werden, ob diese Nachteile auch unter unseren Versuchsbedingungen auftraten. Verwendet wurden hierzu die MCLA – verstärkte Chemilumineszenz, die NBT – und Cytochrom C – Methode. Mit diesen verschiedenen, voneinander unabhängigen Methoden zeigte sich, daß in Anwesenheit von NADH Lucigenin selbst zu einer wesentlich gesteigerten O2 - - Bildung in Lysaten von humanen Umbilikalvenenendothelzellen (HUVEC) führt. Daher wurde zur Untersuchung der endothelialen O2 - - Bildung in dieser Arbeit schließlich nur die Cytochrom C Methode verwendet. Zur Überprüfung der Auswirkungen der verwendeten Substanzen auf das Membranpotential wurde die Membranpotentialmeßmethode mittels dem Potential – sensitiven Fluoreszenzfarbstoff Bis - oxonol aufgebaut und verwendet. Intakte HUVEC zeigten eine basale O2 - - Produktion, die durch bekannte Inhibitoren der leukozytären NAD(P)H – Oxidase, mit unterschiedlichen Wirkmechanismen signifikant gehemmt wurde (Diphenyleniodonium ca. 48%, Phenylarsenoxid ca. 34% ). Ebenso resultierte die Inaktivierung des GTP - bindenden - Proteins rac mit Clostridium difficile Toxin B in einer signifikanten Reduktion der basalen endothelialen O2 - - Produktion um ca. 30%. Weiterhin konnte gezeigt werden, daß nach Aufhebung der zellulären Integrität durch das Lysieren der HUVEC die Gabe von NADH eine um ca. 2.7 fach erhöhte O2 - - Produktion im Vergleich zu NADPH bewirkte. Mit Hilfe der Immunfluoreszenz bzw. rtPCR konnten außerdem zumindest ein Teil der leukozytären NAD(P)H – Oxidase Untereinheiten, p67phox und gp91phox auch in HUVEC nachgewiesen werden. Zur gezielten Depolarisation des Membranpotentials wurden ein Puffer mit erhöhter Kaliumkonzentration (90 mM), der nicht selektive Kalium – Kanal - Blocker Tetrabutylammonium Chlorid (1 mM) und das Kation – Ionophor Gramicidin (1 µM) verwendet. Die basale endotheliale O2 - - Produktion wurde durch diese Substanzen in ähnlichem Ausmaß (~ 60% ) signifikant gesteigert (n=23, p

Die extrazelluläre Matrix umgibt Zellen, stabilisiert Gewebe und reguliert Zellfunktionen. Bestandteile der ECM transduzieren Signale durch Zellmembranrezeptoren, sog. Integrine. Integrine vermitteln Änderungen der Zellmorphologie, Proliferation, Differenzierung und Apoptose in Zellen. Die Frage ob die ECM eine wichtige Rolle in der Physiologie und Tumorgenese der Hypophyse spielt, ist immer noch offen. In der vorliegenden Arbeit wurden zum ersten Mal die regulatorischen Möglichkeiten der ECM in der Hypophyse dargestellt. Es wurde der Einfluß der extrazellulären Matrix auf das Wachstum und die Zytokinsekretion von follikulostellaren Zellen, sowie Hormonsekretion, Proliferation und Signaltransduktion in kortikotropen Zellen untersucht. Desweiteren werden in dieser Arbeit Änderungen der Expression von Laminin, als auch deren mögliche funktionale Konsequenzen innerhalb der Prolaktinomgenese demonstriert. In der follikulostellaren Zellinie TtT-GF konnte gezeigt werden, daß Fibronektin und Kollagen I die Zellproliferation stimulieren. Simultan führte nur Kollagen I zu einer Erhöhung der Interleukin-6 Sekretion, welches ein bekannter Wachstumsfaktor für follikulostellare Zellen ist. Die signifikante Hemmung der Proliferation von FS-Zellen bei Kombination von Kollagen I mit einem anti-IL6-Antikörper läßt darauf schließen, daß Kollagen I die Proliferation und Zytokinsekretion von follikulostellaren Zellen reguliert. Die Fibronektin vermittelte Proliferationsteigerung scheint dagegen einem anderen Mechanismus zugrunde zu liegen. In der kortikotropen Tumorzellinie AtT-20 konnte nachgewiesen werden, daß die ACTH Sekretion durch Fibronektin, Laminin und Kollagen I inhibiert wird. Ein Reporterkonstrukt bestehend aus dem POMC Promoter und Luciferasegen zeigte ähnliche Ergebnisse, was auf eine Hemmung der ACTH Sekretion bereits auf Ebene der POMC-Transkription schließen läßt. Im Gegensatz dazu konnte keine signifikante Veränderung der ACTH Sekretion in normalen Hypophysenzellen festgestellt werden. AtT-20 Zellproliferation wurde durch Kollagen IV und Fibronektin stimuliert, wogegen Kollagen I und Laminin zu einer Inhibition führten. Parallel dazu fand eine Veränderung der Zellform statt. Ein möglicher integrinvermittelter Signalweg umfasst die Aktivierung von Rac, einer kleinen GTPase, mit der konsekutiven Produktion von reaktiven Sauerstoffradikalen (ROS) und einer runden Zellform. Es konnte gezeigt werden, daß eine Inhibition der AtT-20 Proliferation durch Laminin mit einer signifikanten Erhöhung der reaktiven Sauerstoffradikalen und einer runden Zellform einhergeht. Dieser Effekt war mit NAcetylcystein (NAC), einem ROS-Antagonisten, umkehrbar und am ehesten Rac vermittelt. Unter Kollagen IV fand ebenfalls eine Inhibition des Zellwachstums statt. AtT-20 Zellen nahmen auch hier eine runde Zellform an und produzierten, wenn auch weniger stark als Laminin und Kollagen I, ROS. Dieser Effekt war jedoch nicht durch NAC umkehrbar. Kollagen I führte dagegen zu einer Steigerung der Proliferation und ROS-Produktion, sowie zu ausgebreiteten als auch runden Zellen. Diese z.T. gegensätzlichen durch Kollagen I+IV vermittelten Effekte könnten durch simultane Aktivierung alternativer Mechanismen, wie z.B. eine integrinbedingte Aktivierung als auch Hemmung von Rezeptoren für Wachstumsfaktoren, verursacht sein. Ein weiterer, oft mit Fibronektin assoziierter Signalweg, beinhaltet die integrinvermittelte Aktivierung von Rho, einer weiteren kleinen GTPase. Die fehlende ROS Erhöhung, der Einsatz eines β1-integrin stimulierenden Antikörpers, sowie die integrinunabhängige Stimulation von Rho durch Lysophosphatidatsäure läßt auf eine Rho assoziierte Proliferationserhöhung in AtT-20 Zellen durch Fibronektin schließen. In GH3 Zellen führte Laminin zu einer Abnahme der Prolaktinsekretion und zur Inhibition der Proliferation. Im Gegensatz dazu konnten keine Veränderungen der Prolaktinsekretion in normalen Rattenhypophysenzellen beobachtet werden. Übereinstimmend zeigte sich im Dopamin2 Rezeptor defizienten Mausprolaktinom, einem Knock-Out in vivo Modell für spontane Prolaktinomentwicklung, und humanen Prolaktinom eine bereits sehr frühe Abnahme der Lamininexpression. Diese Hemmung der Lamininexpression während der Prolaktinomgenese könnte einen weiteren Faktor für erhöhte Hormonproduktion und Zellproliferation in Prolaktinomen darstellen. Die hier erstmalig beschriebenen Auswirkungen der extrazellulären Matrix auf Hypophysenzellproliferation und -hormonsekretion verdeutlichen die wichtige, aber wenig erforschte Rolle der ECM in der Hypophyse. Diese Resultate sind nicht nur neue Ansatzpunkte der Hypophysenphysiologie und -pathophysiologie, sondern lassen auch die Weitläufigkeit der unterschiedlichen regulativen Systeme innerhalb der Hypophyse erkennen.

Das Leukocyten-spezifische Integrin LFA-1 spielt eine wichtige Rolle bei der Immunantwort, durch die Vermittlung dynamischer Zell-Zell- bzw. Zell-Matrix-Interaktionen. Die kontrollierte Adhäsion bzw. Deadhäsion von Leukocyten bedarf einer spezifischen Regulation des LFA-1-Integrins und die Aufklärung der molekularen Grundlagen dieser Vorgänge ist von großem Interesse. Cytohesin-1 war unmittelbar vor Beginn dieser Arbeit als cytoplasmatischer Regulationsfaktor der durch LFA-1 vermittelten Zelladhäsion identifiziert worden und seine spezifische Interaktion mit der cytoplasmatischen Domäne von CD18 konnte in vitro dokumentiert werden. Im Rahmen dieser Arbeit gelang es zunächst, die Assoziation von Cytohesin-1 und LFA-1 auch endogen, im intakten Zellverband, mittels Kolokalisationsstudien in der lymphoblastoiden B-Zellinie LCL-721, zu demonstrieren. Ferner konnte mit Hilfe von Mutationsanalysen die, für die Interaktion kritische Region in der cytoplasmatischen Domäne von CD18 lokalisiert werden. Sie befindet sich im aminoterminalen Bereich und umfaßt die Aminosäuren WKA(723 - 725). Die Mutation dieser Aminosäurereste nach TRG resultierte in einem vollständigen Interaktionsverlust mit Cytohesin-1. Die Inhibition der Cytohesin-1/CD18-Bindung konnte dabei sowohl durch Protein-Protein-Interaktionsanalysen in Hefe als auch durch biochemische Bindungsstudien in vitro dokumentiert werden, wobei jeweils Fusionsproteine der cytoplasmatischen Domäne von CD18 charakterisiert wurden. Funktionale Analysen der WKA(723-725)-Region von CD18 ergaben, daß die Mutation von WKA(723-725) nach TRG im intakten LFA-1-Molekül eine signifikante Reduktion der Integrin- Aktivität zur Folge hatte. Sowohl T-Zellklone als auch nicht hämatopoetische Zellen, wie HeLa, wiesen nach Expression von LFA-1(TRG), mit Hilfe rekombinanter Vaccinia- Viren, eine stark reduzierte Adhäsionsfähigkeit an immobilisiertes ICAM-1 auf. Ferner ergaben funktionale Studien mit HeLa-Zellen, die LFA-1 stabil exprimierten, daß Cytohesin-1 nur dann eine gesteigerte Adhäsion dieser Zellen an ICAM-1 induzierte, wenn sie Wildtyp-LFA-1 exprimierten. HeLa-Zellen, die LFA-1(TRG) exprimierten, ließen sich durch Cytohesin-1 zu keiner verstärkten Adhäsion aktivieren. Diese Ergebnisse demonstrierten die Bedeutsamkeit der Cytohesin-1/CD18-Interaktion für eine effiziente, durch LFA-1 vermittelte Zelladhäsion. Unklar war jedoch der Mechanismus, durch den Cytohesin-1 die Integrin/Liganden-Bindung regulierte. Studien mit dem Reporterantikörper 24 ließen darauf schließen, daß Cytohesin-1 durch die Bindung an CD18 eine Konformationsänderung in der extrazellulären Domäne des LFA-1-Integrins induzieren konnte, die möglicherweise die Affinität des Rezeptors modulierte. Diese Modulation der LFA-1-Konformation schien jedoch nicht hinreichend für eine stabile Bindung an ICAM-1 zu sein, wie eingehendere Analysen von Dr. W. Kolanus zeigten. Vielmehr erforderte eine effiziente Zelladhäsion zusätzlich die Guaninnukleotid-Austauschfunktion (GEF-Funktion) von Cytohesin-1, da die GEF-defekte Punktmutante, Cytohesin-1(E157K), nicht mehr in der Lage war, die Adhäsion von Jurkat E6-Zellen an ICAM-1 stabil zu induzieren. Biochemische Interaktionsstudien konnten dabei zeigen, daß die Mutante weiterhin fähig war, die cytoplasmatische Domäne von CD18 zu binden. Diese und weitere Ergebnisse von Dr. W. Nagel, die einen Zusammenhang zwischen der GEF-Funktion von Cytohesin-1 und dem „Spreading“ von adhärenten Jurkat E6-Zellen aufzeigten, legen die Vermutung nahe, daß Cytohesin-1 durch einen dualen Mechanismus in die LFA-1-Regulation involviert ist. Sowohl die direkte Interaktion von Cytohesin-1 und dem Integrin als auch seine GEF-Funktion stellen essentielle Faktoren für eine stabile Zelladhäsion, die durch LFA-1 vermittelt wird, dar. Welche funktionalen Mechanismen dabei durch den Guaninnukleotid-Austausch und der damit verbundenen Aktivierung einer GTPase induziert werden, ist noch unklar. Primär wäre eine Modulation des Aktin-Cytoskelettes und eine damit verbundene erhöhte laterale Mobilität der Integrine denkbar, die eine verstärkte Rezeptormultimerisierung und dadurch eine Aviditätsänderung des Integrins ermöglicht. Weitere Studien dieser Arbeit analysierten die Regulation von Cytohesin-1 selbst. Es konnte gezeigt werden, daß PI3-Kinase in die Kontrolle der Cytohesin-1-Funktion involviert war. Die Überexpression einer konstitutiv aktiven Form dieser Kinase (P110*) führte zu einer gesteigerten Adhäsion von Jurkat E6-Zellen an ICAM-1. Eine Inkubation dieser Zellen mit dem PI3-Kinase-spezifischen Inhibitor Wortmannin resultierte dagegen in einer signifikanten Reduktion der Zelladhäsion. Weitere funktionale Analysen, die die Zelladhäsion von Jurkat E6-Zellen nach Koexpression von P110* und der PH-Domäne von Cytohesin-1 untersuchten, sowie eingehendere Studien von Dr. W. Nagel, ermöglichten die Entwicklung eines Modells zur Regulation von Cytohesin- 1. Demzufolge führt die Aktivierung der PI3-Kinase zu einer verstärkten Rekrutierung von Cytohesin-1 an die Plasmamembran. Als Rekrutierungsmodul fungiert dabei die PHDomäne, die durch Bindung von PtdIns(3,4,5)P3, einem Produkt der PI3-Kinase, die Assoziation mit der Membran gewährleistet. Die Rekrutierung von Cytohesin-1 an die Plasmamembran führt zur Aktivierung von LFA-1 und der damit verbundenen stabilen Zelladhäsion an ICAM-1.