Cdk7 (Cyclin-dependent kinase 7) is conserved from yeast to human and involved in multiple functions. Cdk7 acts as a CAK (Cdk activating kinase) in a trimeric complex with Cyclin H and Mat1. The CAK activity is required for the full activation of the Cdks that directly regulate the cell cycle transitions. In addition, Cdk7 is the kinase subunit of TFIIH, the general transcription/DNA repair factor IIH. TFIIH is required for the general transcription of messenger RNAs by RNA polymerase II and for the transcription-coupled nucleotide excision repair functions. As in other systems, Drosophila Cdk7 has multiple functions. In order to understand how different functions of Cdk7 are regulated, I performed genetic screens to identify the regulators or downstream factors of multiple functions of Cdk7. Several candidate dominant suppressors and enhancers were identified in these screens. One strong suppressor of cdk7, xpd, encodes another subunit of TFIIH. The genetic suppression by xpd attracted me to further characterize the biological significance of this interaction. I showed that Xpd does have a novel function in regulating CAK activity of cdk7 , it down-regulates mitotic CAK activity. Furthermore, I found that Xpd protein levels are cell cycle dependent, being down-regulated at the beginning of the mitosis. Based on these data, I propose a model that mitotic down-regulation of Xpd results in increased CAK activity, positively regulating mitotic progression. Simultaneously, this down-regulation can be expected to contribute to the mechanisms of mitotic silencing of basal transcription.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 140-172).

Diss. (sammanfattning) Stockholm : Karol. inst., 2005. / Härtill 4 uppsatser.

Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (leaves 132-146).

Thesis (Ph. D.)--Cornell University, May, 2009. / Vita. Includes bibliographical references (leaves 152-178).

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 128-153).

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 179-239). Also available in electronic version.

Until now, the role of cyclin dependent kinase 5 (CDK5) in cardiac pathophysiology has not been explored. While CDK5 has been well studied in the neuroscience/Alzheimer’s field as a cyclin-independent kinase, there is currently no investigation into the cardiac-specific role of CDK5. Recently, it was established that inhibition of CDK5 in stem cell derived cardiomyocytes from individuals with Timothy Syndrome (TS) rescued the delayed inactivation phenotype; TS is a fatal genetic long QT syndrome (LQTS) caused by delayed inactivation of the L-type voltage gated Ca2+channel CaV1.2. While it is evident that CDK5 plays an important role in regulating CaV1.2 function, its role in cardiac tissue remains to be elucidated. To determine whether CDK5 is essential for cardiac function, two separate mouse models were established—a cardiac-deficient Cdk5 mouse model (Cdk5 flox x αMHC-MerCreMer+) and a Cdk5 activation mouse model via overexpression of Cdk5’s known activator, p35 (Cdk5r1/p35 OE x αMHC-MerCreMer+). Immediately after spatiotemporal induction of deficiency/activation of Cdk5 in adult mice, echocardiography, histology and proteomic analysis were performed to examine effects on cardiac structure and function. Analysis of cardiac function and morphology in Cdk5 deficient mice revealed severe systolic dysfunction and a dilated cardiomyopathy-like phenotype. These results were further validated by a pathway analysis of quantified global proteome changes. Conversely, mice with an activation of Cdk5 displayed only minor changes in cardiac function with a modest reduction in fractional shortening and ejection fraction. Notably, these mice did not have any significant changes in cardiac chamber morphology, nor any significant changes to their global proteome. Interestingly, however, phosphoproteomic analysis revealed over 3,000 differentially phosphorylated proteins. Pathway and gene ontology analysis of proteome changes revealed significant hits related to cell adhesion. Evidence for the extensively studied role of CDK5 in the brain has demonstrated a critical role for CDK5 kinase activity in the regulation of cell adhesion. Alterations in cell adhesion are observed in a number of cardiac pathologies including heart failure and dilated cardiomyopathy; it is therefore plausible that CDK5 potentially regulates cardiac function via cell adhesion mechanisms. A comparison of the phospho-proteome acutely after Cdk5 depletion vs the phospho-proteome acutely after Cdk5 activation, allowed for the identification of a novel cardiac-specific Cdk5 substrate, beta taxilin (Txlnb). Validation of this potential phospho-substrate with an in situ proximity ligation assay demonstrated the co-localization of Cdk5-Txlnb in wildtype mouse cardiac tissue sections. When looking at co-localization in Cdk5 deficient tissue sections, no signals were observed. Lastly, our lab obtained donor cardiac tissue samples from individuals who passed away due to either heart failure or non-cardiac causes (serving as control cardiac tissue). Analysis of cardiac tissue samples revealed a significant increase in both CDK5 and p35 expression in heart failure samples. Dysregulation of phosphorylation has been implicated in cardiac dysfunction, with known contribution to contractile failure and a number of cardiac pathologies including cardiomyopathies. These findings further support a role for CDK5 in cardiac function. In conclusion, it appears that CDK5 is imperative for the maintenance of healthy cardiac function. Cardiac-specific homozygous and heterozygous Cdk5 deficiency revealed severe systolic dysfunction along with a dilated cardiomyopathy-like phenotype. While the effects of Cdk5 activation in the heart need to be further investigated, initial findings report significant downstream effects on the phosphorylation of a number of proteins, including Txlnb. Moreover, Txlnb was identified as a potential novel cardiac-specific substrate of Cdk5. The importance of identifying a role for CDK5 in the heart extends beyond this study. CDK inhibitors have been at the forefront of drug development for cancer therapeutics and immunotherapy. While modulation of CDK5 activity may be beneficial in one physiological system, it may prove deleterious in another. It is therefore imperative that the full range of molecular and physiological roles of each CDK be fully elucidated prior to therapeutic application. Furthermore, outcomes from this study have the potential to be translational for drug discovery and the development of new therapeutic avenues for heart disease.

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