Podcasts about cdk1

This week, please join author Vasan Ramachandran and Associate Editor Mercedes Carnethon as they discuss the article "Temporal Trends in the Remaining Lifetime Risk of Cardiovascular Disease Among Middle-Aged Adults Across 6 Decades: The Framingham Study." Dr. Carolyn Lam: Welcome to Circulation on the Run, your weekly podcast, summary and backstage pass to the journal and its editors. We're your co-hosts. I'm Dr. Carolyn Lam, Associate Editor from the National Heart Center in Duke National University of Singapore. Dr. Greg Hundley: And I'm Dr. Greg Hundley, Associate Editor, Director of the Pauley Heart Center at VCU Health in Richmond, Virginia. Dr. Carolyn Lam: Greg, I'm so excited about today's feature paper. You see, I trained at the Framingham Heart Study and today's feature paper talks about the temporal trends in the remaining lifetime risk of cardiovascular disease among middle aged adults across six decades in the Framingham Heart Study. Truly a landmark study and a discussion nobody wants to miss. But first, let's talk about the other papers in today's issue, and I understand that you've got one ready. Dr. Greg Hundley: You bet Carolyn. I'll get started first. Thank you. So my first paper comes from Dr. Daniel Mark from Duke University and it refers to the ISCHEMIA trial. Dr. Carolyn Lam: Ooh, could you please first remind us what is the ISCHEMIA trial and are you presenting a substudy, is that correct? Dr. Greg Hundley: Right, Carolyn. So the International Study of Comparative Health Effectiveness with Medical and Invasive Approaches, or ISCHEMIA, compared an initial invasive strategy with an initial conservative strategy in 5,179 participants with chronic coronary disease and moderate or severe ischemia. And this sub study of the ischemia research program included a comprehensive quality of life analysis. Dr. Carolyn Lam: So very interesting. What did they find Greg? Dr. Greg Hundley: Right, Carolyn. So this study included 1,819 participants. 907 in the invasive, 912 in the conservative. And collected a battery of disease specific and generic quality of life instruments by structured interviews at baseline. And then at three, 12, 24 and 36 months post randomization, and then finally at study closeout. Now Carolyn, these assessments included an angina related quality of life assessment from the 19 item Seattle Angina Questionnaire, a generic health status assessment, an assessment of depressive symptoms, and for North American patients, cardiac functional status from the Duke Activity Status Index, or DASI. In this study, Carolyn, in terms of results, the median age was 67 years and about 20% were women and about 16% were nonwhite. So Carolyn, getting to the results. The estimated mean difference for the SAQ 19 summary score favored invasive therapy. And remember the SAQ 19 was the Seattle Angina Questionnaire. Dr. Greg Hundley: Next, no differences were observed in patients with rare or absent baseline angina. Next, among patients with more frequent angina baseline, those randomized to invasive had a mean point higher score on the SAQ 19 summary score than the conservative approach, with consistent effects across all of the SAQ subscales including physical limitations, angina frequency and quality of life health perceptions. For the DASI, and remember DASI refers to the Duke Activity Status Index, no difference was estimated overall by treatment. But in patients with baseline marked angina, DASI scores were higher for the interventional arm. Whereas patients with rare or absent baseline angina showed really no treatment related differences. Dr. Carolyn Lam: Oh, okay. So a lot of results. What's the take-home message, Greg? Dr. Greg Hundley: Right, Carolyn. Glad you asked. So in the ISCHEMIA comprehensive quality of life substudy, patients with more frequent baseline angina reported greater improvements in the symptom physical functioning and psychological wellbeing dimensions of quality of life when treated with an invasive strategy. Whereas patients who had rare or absent angina baseline reported no consistent treatment related quality of life differences. Dr. Carolyn Lam: Wow. Thank you, Greg. Very interesting indeed. Now from angina to now cholesterol. Now, cholesterol guidelines typically prioritize primary prevention statin therapy based on 10 year risk of cardiovascular disease. Now the advent of generic pricing may in fact justify expansion of statin eligibility. Moreover, 10 year risk may not be the optimal approach for statin prioritization. So these issues were looked at in this next paper by authors led by Dr. Kohli Lynch from Northwestern University and colleagues who estimated the cost effectiveness of expanding preventive statin eligibility, and evaluated novel approaches to prioritization from a Scottish health sector perspective. A computer simulation model predicted long term health and cost outcomes in Scottish adults, age 40 years or more. Dr. Greg Hundley: So Carolyn, what did they find? Dr. Carolyn Lam: The advent of generic pricing has rendered preventive statin therapy cost effective for many adults. Absolute risk reduction guided statin therapy, which is based on 10 year cardiovascular disease risk and non HDL cholesterol levels, is cost effective and would improve population health. Whereas age stratified risk thresholds were more expensive and less effective than alternative approaches to statin prioritization. So guidelines committees may need to expand statin eligibility and consider new ways to allocate statins based on absolute risk reduction rather than 10 year risk thresholds. Dr. Greg Hundley: Very nice Carolyn. Always important, new information regarding statin therapy. Well Carolyn, my next paper comes to us from the world of preclinical science. And Carolyn, as you know, the regenerative capacity of the heart after myocardial infarction is limited. And these authors led by Dr. Tamer Mohamed from University of Louisville previously showed that ectopic introduction of Cdk1, CyclinB1 and Cdk4, CyclinD1 complexes and we'll refer to those now as 4F, promoted cardiomyocyte proliferation in 15 to 20% of infected cardiomyocytes in vitro and in vivo and improved cardiac function after MI in mice. So Carolyn, in this study using temporal single cell RNA sequencing, the investigative team aimed to identify the necessary reprogramming stages during the forced cardiomyocyte proliferation with 4F on a single cell basis. And also using rat and pig models of ischemic heart failure, they aim to start the first preclinical testing to introduce 4F gene therapy as a candidate for the treatment of ischemia induced heart failure. Dr. Carolyn Lam: Oh, wow Greg. So what did they find? Dr. Greg Hundley: Several things, Carolyn. First, temporal bulk and single cell RNA sequencing and further biochemical validations of mature HIPS cardiomyocytes treated with either LAcZ or 4F adenoviruses revealed full cell cycle reprogramming in 15% of the cardiomyocyte population at 48 hours post-infection with 4F. Which was mainly associated with sarcomere disassembly and metabolic reprogramming. Second Carolyn, transient overexpression of 4F specifically in cardiomyocytes was achieved using a polycistronic non-integrating lentivirus encoding the 4F with each driven by a TNNT2 promoter entitled TNNT2-4F polycistronic-NIL. Now this TNNT2-4F polycistronic-NIL or control virus was injected intra myocardial one week after MI in rats, so 10 per group, and pigs, six to seven per group. Dr. Greg Hundley: And four weeks post-injection the TNNT2-4F polycistronic-NIL treated animals showed significant improvement in left ventricular injection fraction and scar size compared with the control virus treated animals. And four months after treatment, the rats that received TNNT2-4F polycistronic-NIL still showed a sustained improvement in cardiac function without obvious development of cardiac arrhythmias or systemic tumorigenesis. And so Carolyn this study advances concepts related to myocellular regeneration by providing mechanistic insights into the process of forced cardiomyocyte proliferation and advances the clinical feasibility of this approach by minimizing the oncogenic potential of the cell factors, thanks to the use of a novel transient and cardiomyocyte specific viral construct. Dr. Carolyn Lam: Wow. What a rich study. Thanks so much, Greg. Dr. Greg Hundley: Well, Carolyn, how about if we see what else and what other articles are in this issue. And maybe I'll go first. So there's a research letter from Dr. Wu entitled Modeling Effects of Immunosuppressive Drugs on Human Hearts Using IPSC Derived Cardiac Organoids and Single Cell RNA Sequencing. Carolyn, there's an EKG challenge from Dr. Yarmohammadi, entitled “Fast and Furious, A Case of Group Beating in Cardiomyopathy.” And then finally from Dr. Tulloch, a really nice Perspective entitled “The Social Robots are Coming, Preparing For a New Wave of Virtual Care in Cardiovascular Medicine. Dr. Carolyn Lam: Oh, how interesting. Well, also in the mail back is an exchange of letters of among Drs. Lakkireddy, Dhruva, Natale, and Price regarding Amplatzer Amulet Left Atrial Appendage Occluder versus Watchman Device for stroke prophylaxis, a randomized control trial. All right. Thank you so much, Greg. Shall we go on to our feature discussion now? Dr. Greg Hundley: You bey. Welcome listeners to our feature discussion today. And we're so fortunate we have with us today, Dr. Vasan Ramachandran from Boston University and our own Associate Editor, Dr. Mercedes Carnethon from Northwestern University in Chicago. Welcome to you both. And Vasan, let's start with you. Could you describe for us some of the background information pertaining to your study and what was the hypothesis that you wanted to address? Dr. Vasan Ramachandran: Thank you, Greg, first of all for having me. So we know two facts. One is that heart disease and stroke disease death rates and incidents are declining over the last six decades in the United States. Juxtapose against that is also the observation that there is rising incidence of obesity and overweight, and also a rising burden of diabetes. There are a lot of advances in our ability to treat high blood pressure, high cholesterol, as well as high blood sugar. So we wanted to ask the question, given the historic trends in control awareness of risk factors and their control, interrupted by this escalating burden of obesity, overweight, and diabetes, what is the lived experiences of people over time in terms of the risk of developing heart disease or stroke using a metric we call as the remaining lifetime risk of developing heart disease or stroke. Dr. Greg Hundley: The hypothesis you wanted to address? Dr. Vasan Ramachandran: The hypothesis we wanted to address was that perhaps the decline in the incidence of heart disease and stroke may have decreased over time given the escalating burden of overweight, obesity and diabetes. Dr. Greg Hundley: Very nice. And can you describe for us your study population and your study design? Dr. Vasan Ramachandran: Thank you, Greg. So the Framingham Heart Study is one of the oldest running epidemiological studies in the world. We have multiple cohorts. The study began in 1948 with the original cohort, the offspring cohort enrolled in 1971, third generation cohort in 2002, and two minoritized cohorts in the 1990s and 2002. So we have an observation period of different cohorts over a six decade period. So we asked the question, if you were a participant in the Framingham study between 1960 and 1979 and then 1980 to 1999, and then 2000 to 2018, what was your lifetime risk of experiencing a heart disease or stroke in the three different time periods? Is it going down, is it steady or is it going up? Dr. Greg Hundley: Very nice. And so, Vasan, describe your study results. Dr. Vasan Ramachandran: Look, what we found was if you look at the first, the 20 year period from 1960 to 1979, and compare that with the latest, which is 2000 to 2018, in the initial time period, the lifetime risk of developing heart disease or stroke in a man was pretty high. It was about one in two. And that for a woman was about one in three. So when you come to the latest epoch, what we find that the risk of one in two men had dropped to about one in three men in the latest decade. For women, the risk declined from what was one in three in the earlier epoch to one in four. So approximately there was about a 36% reduction in the lifetime probability of developing heart disease or stroke across the six decade period of observation. Dr. Greg Hundley: Very nice. And so help us a little bit, put the context of your results into what that might mean for us today as we are managing patients with atherosclerotic disease. Dr. Vasan Ramachandran: Yes, Greg. What it means is that the permeation of the advances in science in terms of the screening of risk factors, awareness of risk factors, medications to lower these risk factors effectively, the clinical trials that have given us these new medications, they may have translated into a reduction in risk over time. That the lived experience of people in the later decades is better in terms of having a lower risk of heart disease or stroke as the consequence of multiple advances that have happened in heart disease and stroke. Dr. Greg Hundley: Well, thank you so much Vasan. Well listeners, now we're going to turn to our Associate Editor, Mercy Carnethon. And Mercy, you have many papers come across your desk. What attracted you to this particular paper and how do you put these results really in the context of other science pertaining to risk associated with populations that may have atherosclerotic cardiovascular disease? Dr. Mercedes Carnethon: Thanks so much for that question, Greg. And again, Vasan, I really thank you and your team for bringing forth such outstanding research. You know, as cardiovascular disease epidemiologists, we were all raised and taught that what we know about risk factors for cardiovascular disease are based on the Framingham cohort. And so I was really excited to see this very comprehensive piece of work that characterized what the Framingham study has identified and also leverages the unique characteristics of a study that started in 1948. Dr. Mercedes Carnethon: So, you know, we're almost 75 years in and actually has the ability that cross sectional studies don't have to look over longer periods of time at risk. And you know, when we think about papers that excite us, that we really want to feature in circulation, they are papers that teach us something new. And I will say there were aspects of this work that confirmed what I had heard but had not seen using empirical data. Namely that the remaining lifetime risks for developing cardiovascular disease were going down over time, and they were going down secondary to better management and recognition of the risk factors that the Framingham cohort study had really been instrumental in identifying in the first place. Dr. Mercedes Carnethon: There were surprising elements of the paper. The surprising elements being that I think as you brought up earlier, we were concerned that risk factors that were on the rise, such as obesity, were threatening these increases in life expectancy. And it was really nice to see that the findings held, even in the face of rising risk factors. And just to summarize, what I really like about this piece when we situate it within circulation, where we are addressing clinical treatment factors, where we're also featuring clinical trials and even other epidemiologic studies, is that your work identifies for us the overall context in which the clinicians who read the journal are thinking about managing patients and where we're going. It highlights our successes, but it also really brings up what we need to do next. And I look forward to hearing from you about where you think this may be headed. Dr. Greg Hundley: Well, Mercy, you're teeing us up for that next question. Vasan, what do you think is the next study or studies that need to be performed in this space? Dr. Vasan Ramachandran: Thank you, Greg and Mercy, for your kind comments. Like I shared, this is a success story for a predominantly white population in the Northeast. We are very much aware about the heterogeneity and the geographic variation in heart disease burden in our country. So one of the success stories interpretation might be this represents the upper bound. What can happen to a population that is compliant with screening of risk factors, awareness of risk factors, treatment and healthcare access. I think the next set of studies should broaden the study population to bring in additional populations that are more diverse, that are also followed up over a period of time to assess and put the current observations in the appropriate context. Do we see similar findings longitudinally in other cohorts with non-white participants? Is it different, is their lived experience different? If so, why? And that could inform us how we can reach the success story and replicate it across the entirety of our country. Dr. Greg Hundley: And Mercy, do you have anything to add? Dr. Mercedes Carnethon: I do. You know, I really like that focus on broadening to whom these results are applicable. We've undergone a lot of shifts within our country and also around the world. You know, circulation, we have a worldwide readership. I would love to see this sort of work replicated across different countries to the extent that we have the data to do so, recognizing that limitation. But I'd love to see work focus on comparing how these things change in low income countries, middle income and high income countries, so that we can really think about resource allocation and find strategies to try to replicate the successes that we are seeing based on the data from the Framingham heart and offspring studies. Dr. Greg Hundley: Excellent. Well listeners, we really appreciate the opportunity to get together today with Dr. Vasan Ramachandran from Boston University and our own Associate Editor, Dr. Mercedes Carnethon from Northwestern University in Chicago. And really appreciate them for bringing us these epidemiologic data from the Framingham cohort, indicating that over the past decades, mean life expectancy increased and the remaining lifetime risk of atherosclerotic cardiovascular disease decreased across individuals in the cohort, even after accounting for increasing incidences of other cardiovascular risk factors like obesity and smoking. Well on behalf of Carolyn and myself, we want to wish you a great week and we will catch you next week on the run. Dr. Greg Hundley This program is copyright of the American Heart Association, 2022. The opinions expressed by speakers in this podcast are their own and not necessarily those of the editors or of the American Heart Association. For more, please visit ahajournals.org.

A small regulatory protein can be found not only in all humans, animals, plants and fungi, but in each and every eukaryotic cell. Following the discovery of these ubiquitous proteins in 1974, researchers aptly named them ubiquitin. Enzymes and/or ligases (enzymes that catalyze the formation of chemical bonds), allow ubiquitins to bind to substrate proteins. This process, known as ubiquitination, can initiate or prevent protein interactions, denote them for degradation and alter their activity and cellular location. Aberrant ubiquitination may play an important role in tumorigenesis. At the core of solid tumors, hypoxia-inducible factor 1α (HIF-1α) is induced (by hypoxia) and allows cancer cells to adapt to the lack of oxygen. The accumulation of HIF-1α can also transcribe a number of genes well-known to be involved in cancer. Therefore, HIF-1α is an attractive, albeit difficult-to-pinpoint, therapeutic target in cancer. Researchers—from Brown University and ​​Lifespan Cancer Institute—previously observed the stabilization of HIF-1α by the protein coding gene cyclin-dependent kinases CDK1. They further proposed that CDK4 may also be an HIF-1α stabilizer. However, the mechanism of HIF-1α's regulation by CDK4 has yet to be fully elucidated. “Till now, development of therapies targeting HIF-1α remains hindered. Therefore, it is imperative to explore the mechanism of HIF-1α regulation in cancer cells and investigate new possibilities to therapeutically target HIF-1 signaling.” In 2021, the same team conducted a new study further investigating the molecular mechanisms of HIF-1α destabilization by CDK1 or CDK4/6 inhibitors in colorectal cancer. Their priority research paper was published as the cover of Oncotarget's Volume 12, Issue 20, and entitled, “Identification of Smurf2 as a HIF-1α degrading E3 ubiquitin ligase.” Full blog - https://www.oncotarget.org/2021/09/30/new-study-protein-analysis-reveals-novel-role-of-enzyme-in-cancer/ Sign up for free Altmetric alerts about this article - https://oncotarget.altmetric.com/details/email_updates?id=10.18632%2Foncotarget.28081 DOI - https://doi.org/10.18632/oncotarget.28081 Full text - https://www.oncotarget.com/article/28081/text/ Correspondence to - Wafik S. El-Deiry - wafik@brown.edu Keywords - Smurf2, CDK4/6 inhibition, HIF1alpha, hypoxia, cancer therapy About Oncotarget Oncotarget is a bi-weekly, peer-reviewed, open access biomedical journal covering research on all aspects of oncology. To learn more about Oncotarget, please visit https://www.oncotarget.com or connect with: SoundCloud - https://soundcloud.com/oncotarget Facebook - https://www.facebook.com/Oncotarget/ Twitter - https://twitter.com/oncotarget YouTube - https://www.youtube.com/c/OncotargetYouTube/ LinkedIn - https://www.linkedin.com/company/oncotarget Pinterest - https://www.pinterest.com/oncotarget/ Reddit - https://www.reddit.com/user/Oncotarget/ Oncotarget is published by Impact Journals, LLC please visit https://www.ImpactJournals.com or connect with @ImpactJrnls Media Contact MEDIA@IMPACTJOURNALS.COM 18009220957

Bulunduğumuz haftanın bilim dünyasından önemli gelişmeleri sizler için derledik. Blue Origin safely launches four commercial astronauts to space and back. https://www.blueorigin.com/news-archive/first-human-flight-updates Per and polyfluoroalkyl substances (PFAS) in breast milk, concerning trends for current use PFAS. https://doi.org/10.1021/acs.est.0c06978 Agency for Toxic Substances and Disease Registry. https://www.atsdr.cdc.gov Structural basis of human separase regulation by securin and CDK1 - cyclin B1. https://doi.org/10.1038/s41586-021-03764-0 Effectiveness of COVID-19 vaccines against the B.1.617.2 (Delta) variant. https://doi.org/10.1056/nejmoa2108891 Bize 101.podcast.info@gmail.com adresinden ulaşabilirsiniz.

This week's cover paper of Oncotarget (Volume 12, Issue 14) is entitled, "Esomeprazole enhances the effect of ionizing radiation to improve tumor control," by researchers from Baylor College of Medicine, University of Texas MD Anderson Cancer Center, and Texas Children's Hospital. Abstract: The resistance of cancer cells to radiation-based treatment is a major clinical challenge confounding standard of care in cancer. This problem is particularly notable in many solid tumors where cancer cells are only partially responsive to radiation therapy. Combination of radiation with radiosensitizers is able to enhance tumor cell killing. However, currently available radiosensitizers are associated with significant normal tissue toxicity. Accordingly, there is an unmet need to develop safer and more effective radiosensitizers to improve tumor control. Here, we evaluated the radiosensitizing effect of the FDA-approved drug esomeprazole in normal and radioresistant human head and neck squamous cell carcinoma (HNSCC) cells in vitro, and in a mouse model of HNSCC. For the in vitro studies, we used cancer cell colony formation (clonogenicity) assay to compare cancer cell growth in the absence or presence of esomeprazole. To determine mechanism(s) of action, we assessed cell proliferation and profiled cell cycle regulatory proteins. In addition, we performed reverse phase protein array (RPPA) study to understand the global effect of esomeprazole on over 200 cancer-related proteins. For the in vivo study, we engrafted HNSCC in a mouse model and compared tumor growth in animals treated with radiation, esomeprazole, and combination of radiation with esomeprazole. We found that esomeprazole inhibits tumor growth and dose-dependently enhances the cell killing effect of ionizing radiation in wildtype and p53-mutant radioresistant cancer cells. Mechanistic studies demonstrate that esomeprazole arrests cancer cells in the G1 phase of the cell cycle through upregulation of p21 protein and inhibition of cyclin-dependent kinases (Cdks) type 1 (Cdk1) and type 2 (Cdk2). In vivo data showed greater tumor control in animals treated with combination of radiation and esomeprazole compared to either treatment alone, and that this was associated with inhibition of cell proliferation in vivo. In addition, combination of esomeprazole with radiation significantly impaired repair following radiation-induced DNA damage. Our studies indicate that esomeprazole sensitizes cancer cells to ionizing radiation, and is associated with upregulation of p21 to arrest cells in the G1 phase of the cell cycle. Our findings have significant therapeutic implications for the repurposing of esomeprazole as a radiosensitizer in HNSCC and other solid tumors. Sign up for free Altmetric alerts about this article - https://oncotarget.altmetric.com/details/email_updates?id=10.18632%2Foncotarget.28008 DOI - https://doi.org/10.18632/oncotarget.28008 Full text - https://www.oncotarget.com/article/28008/text/ Correspondence to - Yohannes T. Ghebre - yohannes.ghebre@bcm.edu Keywords - esomeprazole, proton pump inhibitors, ionizing radiation, radiosensitization, tumor control About Oncotarget Oncotarget is a bi-weekly, peer-reviewed, open access biomedical journal covering research on all aspects of oncology. To learn more about Oncotarget, please visit https://www.oncotarget.com or connect with: SoundCloud - https://soundcloud.com/oncotarget Facebook - https://www.facebook.com/Oncotarget/ Twitter - https://twitter.com/oncotarget YouTube - https://www.youtube.com/c/OncotargetYouTube/ LinkedIn - https://www.linkedin.com/company/oncotarget Pinterest - https://www.pinterest.com/oncotarget/ Reddit - https://www.reddit.com/user/Oncotarget/ Oncotarget is published by Impact Journals, LLC please visit https://www.ImpactJournals.com or connect with @ImpactJrnls Media Contact MEDIA@IMPACTJOURNALS.COM 18009220957

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.10.08.329599v1?rss=1 Authors: Salamina, M., Montefiore, B. C., Liu, M., Wood, D. J., Heath, R., Ault, J. R., Wang, L.-Z., Korolchuk, S., Basle, A., Pastok, M. W., Reeks, J., Tatum, N. J., Sobott, F., Arold, S. T., Pagano, M., Noble, M. E. M., Endicott, J. A. Abstract: The SCFSKP2 ubiquitin ligase relieves G1 checkpoint control of CDK-cyclin complexes by promoting p27KIP1 degradation. We describe reconstitution of stable complexes containing SKP1-SKP2 and CDK1-cyclin B or CDK2-cyclin A/E, mediated by the CDK regulatory subunit CKS1. We further show that a direct interaction between a SKP2 N-terminal motif and cyclin A can stabilize SKP1-SKP2-CDK2-cyclin A complexes in the absence of CKS1. We identify the SKP2 binding site on cyclin A and demonstrate the site is not present in cyclin B or cyclin E. This site is distinct from but overlapping with features that mediate binding of p27KIP1 and other G1 cyclin regulators to cyclin A. We propose that the capacity of SKP2 to engage with CDK2-cyclin A by more than one structural mechanism provides a way to fine tune the degradation of p27KIP1 and distinguishes cyclin A from other G1 cyclins to ensure orderly cell cycle progression. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.11.246504v1?rss=1 Authors: Campbell, A., Ferraz Franco, C., Su, L.-I., Perkins, S., Jones, A. R., Brownridge, P. J., Perkins, N. D., Eyers, C. E. Abstract: Different types of DNA damage can initiate phosphorylation-mediated signalling cascades that result in stimulus specific pro- or anti-apoptotic cellular responses. Amongst its many roles, the NF-{kappa}B transcription factor RelA is central to these DNA damage response pathways. However, we still lack understanding of the co-ordinated signalling mechanisms that permit different DNA damaging agents to induce distinct cellular outcomes through RelA. Here, we use label-free quantitative phosphoproteomics to examine the temporal effects of exposure of U2OS cells to either etoposide (ETO) or hydroxyurea (HU) by monitoring the phosphorylation status of RelA and its protein binding partners. Although few stimulus-specific differences were identified in the constituents of phosphorylated RelA interactome after exposure to these DNA damaging agents, we observed subtle, but significant, changes in their phosphorylation states, as a function of both type and duration of treatment. The DNA double strand break (DSB)-inducing ETO invoked more rapid, sustained responses than HU, with regulated targets primarily involved in transcription, cell division and canonical DSB repair. Kinase substrate prediction of ETO-regulated phosphosites suggest abrogation of CDK1 and ERK1 signalling, in addition to the known induction of ATM/ATR. In contrast, HU-induced replicative stress mediated temporally dynamic regulation, with phosphorylated RelA binding partners having roles in rRNA/mRNA processing and translational initiation, many of which contained a 14-3-3{varepsilon} binding motif, and were putative substrates of the dual specificity kinase CLK1. Our data thus point to differential regulation of key cellular processes and the involvement of distinct signalling pathways in modulating DNA damage-specific functions of RelA. Copy rights belong to original authors. Visit the link for more info

Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.07.24.220186v1?rss=1 Authors: Dengler, L., Örd, M., Schwab, L. M., Loog, M., Ewald, J. C. Abstract: Protein phosphorylation enables a rapid adjustment of cellular activities to diverse intracellular and environmental stimuli. Many phosphoproteins are targeted on more than one site, which allows the integration of multiple signals and the implementation of complex responses. However, the hierarchy and interplay between multiple phospho-sites are often unknown. Here, we study multi-site phosphorylation using the yeast trehalase Nth1 and its activator, the 14-3-3 protein Bmh1, as a model. Nth1 is known to be phosphorylated by the metabolic kinase PKA on four serine residues and by the cell cycle kinase CDK on one residue. However, how these five phospho-sites adjust Nth1 activity remains unclear. Using a novel reporter construct, we investigated the contribution of the individual sites for the regulation of the trehalase and its 14-3-3 interactor. In contrast to the constitutively phosphorylated S20 and S83, the weaker sites S21 and S60 are only phosphorylated by increased PKA activity. For binding Bmh1, S83 functions as the high-affinity "gatekeeper" site, but successful binding of the Bmh1 dimer and thus Nth1 activation requires S60 as a secondary site. Under nutrient-poor conditions with low PKA activity, S60 is not efficiently phosphorylated and the cell cycle dependent phosphorylation of S66 by Cdk1 contributes to Nth1 activity, likely by providing an alternative Bmh1 binding site. Additionally, the PKA sites S20 and S21 modulate the dephosphorylation of Nth1 on downstream Bmh1 sites. In summary, our results expand our molecular understanding of Nth1 regulation and provide a new aspect of the interaction of 14-3-3 proteins with their targets. Copy rights belong to original authors. Visit the link for more info

Full text - http://bit.ly/2mlYU6k "ERβ inhibits cyclin dependent kinases 1 and 7 in triple negative breast cancer" Abstract - Triple negative breast cancer (TNBC), which comprises approximately 15% of all primary breast cancer diagnoses, lacks estrogen receptor alpha, progesterone receptor and human epidermal growth factor receptor 2 expression. However, we, and others, have demonstrated that approximately 30% of TNBCs express estrogen receptor beta (ERβ), a nuclear hormone receptor and potential drug target. Treatment of ERβ expressing MDA-MB-231 cells with estrogen or the ERβ selective agonist, LY500307, was shown to result in suppression of cell proliferation. This inhibitory effect was due to blockade of cell cycle progression. In vivo, estrogen treatment significantly repressed the growth of ERβ expressing MDA-MB-231 cell line xenografts. Gene expression studies and ingenuity pathway analysis identified a network of ERβ down-regulated genes involved in cell cycle progression including CDK1, cyclin B and cyclin H. siRNA mediated knockdown or drug inhibition of CDK1 and CDK7 in TNBC cells resulted in substantial decreases in proliferation regardless of ERβ expression. These data suggest that the tumor suppressive effects of ERβ in TNBC result from inhibition of cell cycle progression, effects that are in part mediated by suppression of CDK1/7. Furthermore, these data indicate that blockade of CDK1/7 activity in TNBC may be of therapeutic benefit, an area of study that has yet to be explored. Facebook - http://bit.ly/2xznxjV Twitter - http://bit.ly/2xzWvsu LinkedIn - http://bit.ly/2xzJ6kc Pintrest - http://bit.ly/2xzX8SS Reddit - http://bit.ly/2hoxI0N www.Oncotarget.com

The process of mitosis involves a comprehensive reorganization of the cell: chromosomes condense, the nuclear envelope breaks down, the mitotic spindle is assembled, cells round up and release their ties to the substrate and so on and so forth. This reorganization is triggered by the activation of a protein kinase called Cyclin-Dependent Kinase 1 (CDK1). The end of mitosis is marked by the proteolysis of the cyclin subunit of CDK1, which terminates kinase activity. At this point, the phosphate moieties that altered the properties of hundreds of proteins to bring about the cellular reorganization are removed by protein phosphatases. We recently began to pay attention to the control of these protein phosphatases, conscious that it was likely that they were shut off as cells enter mitosis, and reactivated when mitosis is complete, allowing return to interphase. It is difficult to see how proteins could be fully phosphorylated if both kinases and phosphatases were simultaneously active (much as filling a wash basin requires not only turning on the water taps, but also putting in the plug). It emerged that at least one protein phosphatase, PP2A-B55, was shut off in mitosis. Depletion of this particular form of PP2A accelerated entry into mitosis, and blocked exit from mitosis. We have discovered how this phosphatase is regulated. It entails binding a small inhibitor protein (endosulfine or ARPP-19) that is phosphorylated by a protein kinase called Greatwall that is itself a substrate of CDK1. Failure to inhibit PP2A-B55 causes arrest of the cell cycle in G2 phase. I will explain how we found this out, and discuss the role of this particular control mechanism in the control of mitosis. The “switches and latches” of my title refers to our still rather poor understanding of exactly how the timing of entry into mitosis is controlled, together with the realization that the Greatwall- Endosulfine circuit is not only required for entering mitosis, but also for staying there. Recent evidence from budding yeast suggests that the same control module is involved in controlling the switch into quiescence when the yeast are starving, and not in the normal control of cell division.

Cells drastically alter their structure at the onset of mitosis. Gavet and Pines reveal that the mitotic kinase CyclinB1-Cdk1 triggers its own nuclear import in prophase to ensure reorganization of the nucleus and cytoplasm is synchronized. This biosights episode presents the paper by Gavet and Pines from the April 19, 2010 issue of The Journal of Cell Biology, and includes an interview with senior author Jonathon Pines. Produced by Justin Paul and Ben Short.   Subscribe to biosights via iTunes or RSS View biosights archive The Rockefeller University Press biosights@rockefeller.edu

During mitotic spindle assembly, the sister chromatids have to be captured by kinetochore (K) -fibers (bundles of kinetochore microtubules; KMTs) to ensure stable attachment of the chromosomes. This is a prerequisite for chromosome congression at the metaphase plate, and the subsequent segregation of separated sister chromatids to the spindle poles. Although, this is a critical step during mitosis, the identity and regulation of the proteins that mediate the formation and stabilization of K-fibers are still largely unknown. This thesis describes a functional characterization of HURP (hepatoma upregulated protein) and CHICA (C20Orf129), two proteins recently identified in a proteomic survey of the human spindle apparatus (Sauer et al., 2005). The spindle association of both proteins was analyzed with polyclonal antibodies, showing that that of HURP and CHICA were mutually exclusive. While HURP decorated the KMT plus ends, CHICA localized to the spindle pole caps and had no major influence on K-fiber stability. The description of the CHICA project will be brief, as this work was primarily continued by Dr. Anna Santamaria. Her investigations demonstrated that CHICA is important for the spindle recruitment of the chromokinesin Kid, which is required for polar ejection forces. Our studies showed that HURP binds to, and bundles microtubules (MTs) in vitro. In vivo, HURP localizes predominantly to K-fibers in the vicinity of chromosomes and is required for K-fiber stabilization. Moreover, we revealed that importin β binds to the N-terminus of HURP and demonstrated that the nucleotide state of the small GTPase Ran controls HURP localization and function. We conclude that the spindle assembly pathway centered on RanGTP contributes to K-fiber stabilization and that HURP is a critical target of this pathway. To better understand the mechanism of HURP recruitment to the K-fibers we subsequently carried out a structure-function analysis of the different HURP domains. This study revealed that the N-terminus, which contains two coiled coil domains binds to, and bundles MTs and is essential for the initial loading of HURP onto the spindle, whereas the C-terminus (including a Guanylate kinase-associated protein domain; GKAP) is involved in the specific KMT plus end targeting. Furthermore, we identified a conserved mitosis-specific Cdk1 phosphorylation site in the GKAP domain of HURP, indicating that in addition to the RanGTP gradient, Cdk1 phosphorylation may also play a role in HURP recruitment to the K-fibers.

Late mitotic events are chiefly controlled by proteolysis of key regulatory proteins via the ubiquitin-proteasome pathway. In this pathway ubiquitin ligases modify substrates by attachment of ubiquitin (“ubiquitylation”), which usually results in their subsequent degradation by the 26S proteasome. The crucial ubiquitin ligase involved in late mitosis is the anaphase-promoting complex or cyclosome (APC/C). Among the many substrates of the APC/C is the anaphase inhibitor securin, whose destruction leads to activation of separase, which in turn triggers sister chromatid separation by proteolytic cleavage of cohesin. The APC/C also targets cyclin B1, an activating subunit of Cdk1 kinase, whose inactivation is a prerequisite for mitotic exit. The unstable APC/C substrates are often found in association with stable partner proteins. How single subunits of multi-protein complexes are selectively extracted and eventually degraded is largely unknown, but there is increasing evidence that additional factors assist to extract ubiquitin-carrying subunits from stable binding partners. One such factor is vertebrate p97 (Cdc48 in yeast), an abundant and highly conserved member of the AAA-ATPase family. It is involved in such diverse processes as transcriptional regulation, membrane fusion, and ER-associated protein degradation (ERAD). The unifying scheme in these seemingly unrelated functions is that p97 is able to “extract” preferentially ubiquitylated proteins from their environment. Roles of p97 in mitosis have recently emerged: p97 was reported to be required for spindle disassembly and for nuclear envelope reformation during mitotic exit in Xenopus. Furthermore, a genetic interaction between p97, separase and securin, as well as a requirement of p97 for separase stability, were discovered in fission yeast. Given these hints and the importance of ubiquitylation in both mitosis and p97 pathways, this study intended to elucidate additional mitotic roles of p97 in vertebrates. Towards this end, tools to interfere with p97 function in Xenopus egg extracts were developed. These included immunodepletion of the p97 adaptors Npl4, Ufd1 and p47 and addition of recombinant dominant-negative p97-mutants. ERAD, which could be established here for the first time in Xenopus egg extracts, was greatly impaired in the absence of p97 function. However, many aspects of mitosis were found to be unaffected. Importantly, p97’s proposed role in spindle disassembly was clearly falsified within this thesis. Furthermore, p97 was shown to be dispensable for activity and stability of vertebrate separase. Disassembly of the mitotic checkpoint complex, which prevents premature APC/C activation by sequestering its activator Cdc20, did also not require functional p97 despite its dependence on ubiquitylation of Cdc20. However, a novel function of p97 at fertilization was discovered. p97 was found to interact with nucleoplasmin, a histone-binding chaperone that catalyzes the exchange of sperm-specific basic proteins (SBPs) to histones. Indeed, interference with p97 function delayed sperm decondensation in Xenopus egg extracts, thereby confirming a novel role of this AAA-ATPase in sperm chromatin remodelling. In another project the role of securin in human cells was investigated. Human cells lacking securin had been reported to suffer from massive chromosome missegregation, which was in sharp contrast to the mild phenotype of securin knockout mice. In collaboration with the group of M. Speicher it could be demonstrated that chromosome losses in securin-/- cells are transient and give way to a stable segregation pattern after just a few passages. This was despite persisting biochemical defects such as reduced level and activity of separase. These data demonstrate that securin is dispensable for chromosomal stability in human cells.

Mitosis is the process by which sister chromatids are equally segregated into two daughter cells. Tight control in various events during mitotic progression is essential for maintaining chromosome stability. Mitotic kinases including Cyclin dependent kinase 1 (Cdk1) and Aurora family are required for regulating proper mitotic progression by phosphorylating mitotic substrates thereby, controlling their activities, localization or abundance. On the other hand, these mitotic kinases are modulated by de-novo synthesis, activators, phosphorylation and ubiquitin-dependent proteolysis. A thorough understanding of the function and regulation of mitotic kinases could further our knowledge on mitotic progression. In the first part of the thesis, we investigated the expression, localization and regulation of human Lats1 kinase, which is a close homologue of the yeast Dbf2 kinase family involved in the mitotic exit network (MEN). Despite the fact that Lats1 has been suggested to be a spindle protein that binds and inactivates Cdk1, we found that Lats1 is mainly cytoplasmic throughout the cell cycle by immunofluorescence microscopy. Both yeast two-hybrid and coimmunoprecipitation showed no significant interaction between Lats1 and Cdk1. Although Lats1 was highly phosphorylated during mitosis, no detectable kinase activity was observed. However, we identified Ste20 like kinase MST2 as the upstream regulator of human Lats1. Phosphorylation of Lats1 by Mst2 resulted in the activation of Lats1 kinase activity both in vivo and in vitro. This kinase-substrate relation was proven to be specific, as another distant Mst2 homolog, Mst4, did not possess this ability. Subsequent mass-spectrometry-based phosphosites analysis revealed that Mst2 phosphorylates Lats1 on more than five residues. Alanine mutations on Lats1T1079 and S909 impaired Lats1 kinase activity. Thus, we could not confirm the suggested role of Lat1 in mitosis. Instead, we show that similar to its Drosophila ortholog, Lats1 is involved in the Mst2 signaling pathway and might control developmentally regulated cell proliferation and apoptosis in mammals. In the second part of this thesis, we characterized hBora, a novel Aurora A interactor originally found in Drosophila. We show that hBora is upregulated and phosphorylated during mitosis. siRNA-mediated knockdown of hBora led to spindle formation defects and aneuploidy. hBora overexpression caused monoastral spindle formation and mislocalization not only of Aurora A but also Plk1. Further investigations showed that Cdk1 phosphorylation on hBoraSer252 leads to Plk1 binding and this may promote the SCF-mediated proteolysis of hBora. Indeed, Plk1 depletion led to an increase in hBora levels. Interestingly, the co-depletion of both hBora and Plk1 (to lower hBora levels in Plk1 depleted cells) rescued the localization of Aurora A to the centrosomes and bipolar spindle formation. Thus, we propose that hBora is a functional link between Plk1 and Aurora A and that by modulating the proteolysis of hBora, Plk1 could regulate Aurora A localization and activity. At the end, we also investigated the function of Aurora A and could show that Aurora A is required for centriole cohesion and centrosome separation.

Zusammenfassung Die Protease Separase trägt zur Regulation mitotischer und meiotischer Vorgänge entscheidend bei. Ihre klassische Funktion ist die Induktion der Schwesterchromosomen-trennung durch Spaltung des Cohesin-Proteinkomplexes, der die Schwesterchromatiden von der S-Phase bis zur Mitose gepaart hält. Separase wird am Ende der Metaphase durch Ubiquitin-abhängigen Abbau ihres Inhibitors Securin aktiviert. Ein zweiter Separase-Inhibitionsmechanismus ist die Hemmung durch Cyclin B1/Cdk1 („Cyclin Dependent Kinase 1“). Dafür ist Separase-Phosphorylierung durch Cdk1 notwendig (Stemmann et al., 2001). In vielen Modellorganismen hat Separase Funktionen, die über die Anaphase-Induktion hinausgehen. So trägt sie in S. cerevisiae beispielsweise zur Cdk1-Inaktivierung beim Meiose I-Meiose II-Übergang bei. Diese Separase-Funktion benötigt die proteolytische Separase-Aktivität nicht, ist jedoch abhängig vom Securin-Abbau. Für andere Funktionen der Separase hingegen könnte die Separase-abhängige Spaltung noch nicht identifizierter Substrate notwendig sein. In der vorliegenden Arbeit wird deshalb die Etablierung der IVEC-Methode („In Vitro Expression Cloning“) zur Identifizierung neuer Separase-Substrate vorgestellt. Mittels IVEC wurde - basierend auf der proteolytischen Separase-Aktivität - aus einer menschlichen cDNA-Bibliothek das In-vitro-Separase-Substrat GASP isoliert. Des Weiteren wurde die Separase-Hemmung durch Cyclin B1/Cdk1 näher untersucht. In der vorliegenden Arbeit konnte gezeigt werden, dass die Phosphorylierung von Separase durch Cyclin B1/Cdk1 für ihre Inhibition zwar notwendig, aber nicht hinreichend ist. Nach Phosphorylierung der Separase assoziiert die Kinase stabil mit der Protease, und erst diese Komplexbildung führt letztendlich zur Inhibition der proteolytischen Separase-Aktivität. Cyclin B1/Cdk1 ist also ein nicht-katalytisch wirkender Separase-Inhibitor. Die zeitlich korrekte Separase-Aktivierung ist für die fehlerlose Chromosomentrennung essentiell. Da Zellen ohne Securin ihre Chromosomen jedoch akkurat und zum richtigen Zeitpunkt trennen, muss es alternative Separase-Inhibitionsmechanismen geben. Die Separase-Hemmung durch Cyclin B1/Cdk1-Bindung könnte dieser gesuchte Securin-unabhängige Mechanismus sein, da der Separase-Cyclin B1/Cdk1-Komplex in Zellen bereits vor der Anaphase nachgewiesen werden kann und Cyclin B1 - wie Securin - am Ende der Metaphase Ubiquitin-vermittelt abgebaut wird. Securin und Cyclin B1/Cdk1 können nicht gleichzeitig an Separase binden. Die beiden Inhibitoren sind also Komponenten parallel und nicht konvergent wirkender Regulationsmechanismen. Die Phosphorylierung von Separase an Serin 1126 ist für ihre Cyclin B1/Cdk1-abhängige Inhibition essentiell (Stemmann et al., 2001). Daneben konnte in der hier vorgestellten Arbeit eine zweite Domäne in Separase identifiziert werden, die ebenfalls sowohl für die Inhibition der proteolytischen Separase-Aktivität als auch für die Komplexbildung mit Cyclin B1/Cdk1 nötig ist. Da diese zweite Cyclin B1/Cdk1-Bindungsdeterminante Sequenzhomologie zu dem Cdc6-Protein aufweist, wurde sie CLD („Cdc6 Like Domain“) genannt. Cdc6 ist ein konserviertes Protein, das in S. cerevisiae Cdk1-Inhibitionsaktivität besitzt. Dazu bindet es abhängig von der Phosphorylierung seines Aminoterminus direkt an B-Typ-Cycline, die sich im Komplex mit ihren Cdks befinden (Mimura et al., 2004). Durch Phosphatase-behandlung und Mutationsanalyse konnte bewiesen werden, dass die Interaktion zwischen Separase und Cyclin B1/Cdk1 auch von Phosphorylierung der Protease innerhalb ihrer CLD abhängt. Dies legt nahe, dass die Separase-CLD wie der Cdc6-Aminoterminus direkte Kontakte mit der Cyclin-Untereinheit der Kinase ausbildet. Serin 1126-Phosphorylierung ist dagegen indirekt an der Kinase-Bindung beteiligt. Denn erstens wird sie nach der Etablierung des Komplexes für seinen Erhalt nicht mehr benötigt (Holland et al., 2006), und zweitens ist sie für die Wechselwirkung zwischen CLD-enthaltenden Separasefragmenten und der Kinase abkömmlich. Ein zunächst favorisiertes Bindungsmodell, bei dem die Polo-Kinase an phosphoryliertes Serin 1126 bindet, um danach die Bindung von Cyclin B1 durch Phosphorylierung der CLD zu vermitteln, konnte ausgeschlossen werden. Stattdessen bewirkt die Phosphorylierung von Serin 1126 wohl eine Konformationänderung der CLD, die dadurch in die Lage versetzt wird, starke Wechselwirkungen mit der Cyclin B1-Untereinheit der Kinase einzugehen. Überraschenderweise ist im Separase-Cyclin B1/Cdk1-Komplex auch die Kinase inaktiv. Diese unerwartete Separase-Funktion als Cdk1-Inhibitor ist in Oozyten der Maus für den Übergang von der Meiose I in die Meiose II von entscheidender Bedeutung. Denn die Inhibition der Separase-Cyclin B1/Cdk1-Komplexbildung durch Mikroinjektion entsprechender Antikörper in Maus-Oozyten verhindert den Ausstoß des ersten Polkörpers, d.h., die Eizellen können den Meiose I-Meiose II-Übergang nicht vollziehen. In diesen Oozyten sinkt die Cdk1-Aktivität am Ende der Meiose I nicht wie bei Kontroll-Oozyten ab. Diese persistente Cdk1-Aktivität ist der Grund für den verhinderten Übergang von Meiose I nach -II, da künstliche Cdk1-Inhibition in Anwesenheit des inhibitorischen Antikörpers den Polkörperausstoß wiederherstellt. In mitotischen Zellen steigt der unter endogenen Bedingungen mit Separase assoziierte Anteil von Cyclin B1/Cdk1 in der Anaphase - d.h. nach dem Abbau seines Bindungskompetitors Securin - an. Übertragen auf die Meiose bedeutet das, dass Securin-Abbau die Induktion der Anaphase mit der Separase-abhängigen Cdk1-Inaktivierung koppelt.

Um Aneuploidie zu verhindern, muss die Trennung der Chromosomen in der Anaphase mit hoher Genauigkeit und daher streng reguliert ablaufen. Bisher galt folgendes Modell der eukaryontischen Schwesterchromatidentrennung: Der „anaphase promoting complex/cyclosome” (APC/C) wird erst aktiviert, wenn alle Chromosomen ordnungsgemäß bipolar an die Mikrotubuli des Spindelapparates angeheftet sind. In seiner Eigenschaft als Ubiquitinligase katalysiert der APC/C dann den proteasomalen Abbau des Anaphaseinhibitors Securin aus dem Komplex mit Separase. Die auf diese Weise als Protease aktivierte Separase löst daraufhin die Anaphase aus, indem sie den Proteinkomplex Kohäsin, welcher die Schwesterchromatiden zusammenhält, spaltet. Das Ausbleiben eines Phänotyps beim Verlust von Securin deutet jedoch auf die Existenz weiterer Regulationsmechanismen der Anaphase hin. Der APC/C sorgt gleichermaßen für den Abbau von Cyclin B1. Die damit verbundene Inaktivierung der Cyclin-abhängigen Kinase 1 (Cdk1) führt zum Austritt aus der Mitose. Im Gegensatz zur Bäckerhefe, in der die Cdc14-Phosphatase ebenfalls als essentieller Gegenspieler von Cdk1 fungiert, repräsentierte in höheren Eukaryonten der APC/C-abhängige Abbau von Cyclin B1 den einzig bekannten Mechanismus zur Cdk1-Inaktivierung. Bisher glaubte man, dass nach dem APC/C die zur Anaphase und zum Mitoseaustritt führenden Signalwege strikt getrennt voneinander verlaufen. Daher war die kürzlich gemachte Beobachtung unerwartet, wonach die durch nicht abbaubares Cyclin B1 konstitutiv aktivierte Cdk1-Kinase die Schwesterchromatidentrennung in Xenopus Eiextrakten blockiert und zwar durch eine Securin-unabhängige Inhibition von Separase. Obwohl die Mutation von Separase an Cdk1-Phosphorylierungsstellen die Kohäsinspaltung in Gegenwart von aktiver Cdk1 wiederherstellte, blieben die molekularen Details der Cdk1-abhängigen Separaseinhibition unklar. In der vorliegenden Arbeit wird gezeigt, dass die Phosphorylierung zwar notwendig aber nicht hinreichend ist, um Separase zu inaktivieren. Zur Inhibition kommt es erst, wenn in einem zweiten Schritt der Cdk1-Komplex stabil und unabhängig von seiner Kinaseaktivität an zuvor phosphorylierte Separase bindet. Es wurde eine Region in Separase identifiziert, die wahrscheinlich in Abhängigkeit von ihrer Phosphorylierung durch die regulatorische Cyclin B1-Untereinheit von Cdk1 erkannt wird. Da sich Securin- und Cdk1-Bindung an Separase gegenseitig ausschließen, stellen sie, anders als ursprünglich angenommen, nicht konvergente sondern parallele Inhibitions-mechanismen dar. Bei der Rekonstitution des Separase-Cdk1 Komplexes wurde eine neue Funktion von Vertebraten-Separase als ein direkter, stöchiometrischer Cdk1-Inhibitor entdeckt, welche unabhängig von der proteolytischen Aktivität ist. Eine durch Mutantenanalyse verifizierte Sequenzhomologie im Cyclin B-bindenden Bereich zwischen Separase und dem Cdk1-Inhibitor Cdc6 aus S. cerevisiae bestätigt dieses Ergebnis. Mikroinjektionsexperimente an Oozyten zeigen, dass die Separase-vermittelte Inhibition von Cdk1 eine essentielle Rolle während der Meiose I spielt. Separase ist also nicht nur ein universeller Auslöser der eukaryontischen Anaphase, sondern sie wirkt auch, trotz unterschiedlicher Mechanismen in Hefe und Vertebraten, als konservierter Cdk1-Antagonist und koppelt damit die Anaphase mit dem Austritt aus der Meiose I.

Cytokinesis is the process that divides the cytoplasm of a parent cell into two. In animal cells, cytokinesis requires the formation of the central spindle and the contractile ring structures. The onset of cytokinesis is marked during anaphase with the specification of the division site, followed by cleavage furrow formation and ingression, midbody formation and abscission. The astral microtubules that originate from the centrosomes and the anti-parallel microtubules of the central spindle are proposed to determine the site of cleavage furrow formation (Bringmann and Hyman, 2005). The acto-myosin based contractile ring assembles at the division site and constricts the cytoplasm which is supported by the fusion of membrane vesicles to the ingressing plasma membrane. All these processes together result in the formation of two daughter cells. The small GTPase RhoA is one of the most upstream regulators of contractile ring assembly at the cortex. Rho proteins are activated by GEF’s (guanine nucleotide exchange factors) and one GEF that is required for cytokinesis is Ect2 (epithelial cell transforming protein2) (Tatsumoto et al., 1999). The Drosophila pebble (pbl) gene product is the founding member of the Ect2 protein family and has been shown to be required for cytokinesis (Lehner, 1992). In mammals, Ect2 was originally identified as a transforming protein in an expression cloning assay (Miki et al., 1993) and subsequently shown to be essential for cytokinesis. In this study, we have explored the temporal and spatial mechanisms that regulate Ect2 function. In agreement with previous studies, we show that Ect2 is a cell cycle regulated protein and is phosphorylated during mitosis. We identify a number of potentially interesting endogenous phosphorylation sites in Ect2, including potential Plk1 and Cdk1 sites. Although we have not been able to determine the function of these phosphorylation sites, their strong conservation among different species implies that they accomplish evolutionarily conserved roles.The identification of these phosphorylation sites sets the stage for future functional analyses. In complementary studies, we have shown that the central spindle and cell cortex localizations of Ect2 are facilitated by the BRCT and PH domains, respectively. The targeting of Ect2 to the central spindle is mediated by the MKlp1/MgcRacGAP and MKlp2/Aurora-B complexes. Of the two complexes, we show that Ect2 interacts and colocalizes only with the MKlp1/MgcRacGAP complex in telophase and propose that this interaction is mediated by a phosphorylation dependent docking mechanism that targets Ect2 to the central spindle. Interestingly, the displacement of Ect2 from the central spindle did not prevent cytokinesis, suggesting that localized GEF activity is not absolutely essential for cleavage furrow ingression and cytokinesis. In the second part of this thesis, we have explored the role of Ect2 during cytokinesis and show that, in Ect2 depleted cells, levels of RhoA and Citron kinase are diminished at the cleavage site, concomitant with the impairment of cleavage furrow formation and ingression. Additionally, overexpression of appropriate amino-terminal Ect2 fragments in cells also hinders cytokinesis. In these cells, RhoA and Citron kinase localize to the cortex and cleavage furrow ingression occurs, but, the subsequent abscission fails. Taken together, these results suggest that proper function of Ect2 is not only important for cleavage furrow ingression, but also for cell abscission. Finally, we investigate the overexpression phenotypes of different Ect2 truncation mutants. We show that abscission failure correlates with the persistence of amino-terminal Ect2 fragments at striking ring-like structures surrounding the midbody, indicating that completion of cell division requires the displacement of Ect2 from the contractile ring and its re-import into the reforming cell nucleus. Collectively, our data indicate that multiple mechanisms cooperate to regulate Ect2 in a spatial-temporal manner.