Cdk7: open questions beyond the prevailing model

CDK7 and miR-210 Co-regulate Cell-Cycle Progression of Neural Progenitors in the Developing Neocortex

The molecular mechanisms regulating neural progenitor (NP) proliferation are fundamental in establishing the cytoarchitecture of the mammalian neocortex. The rate of cell-cycle progression and a fine-tuned balance between cell-cycle re-entry and exit determine the numbers of both NPs and neurons as well as postmitotic neuronal laminar distribution in the cortical wall. Here, we demonstrate that the microRNA (miRNA) miR-210 is required for normal mouse NP cell-cycle progression. Overexpression of miR-210 promotes premature cell-cycle exit and terminal differentiation in NPs, resulting in an increase in early-born postmitotic neurons. Conversely, miR-210 knockdown promotes an increase in the radial glial cell population and delayed differentiation, resulting in an increase in late-born postmitotic neurons. Moreover, the cyclin-dependent kinase CDK7 is regulated by miR-210 and is necessary for normal NP cell-cycle progression. Our findings demonstrate that miRNAs are essential for normal NP proliferation and cell-cycle progress during neocortical development.

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miR-210 level is essential for cell-cycle progression in cortical neural progenitors

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Cdk7 and miR-210 control neural progenitor proliferation

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miR-210 promotes premature cell-cycle exit and differentiation in neural progenitors

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miR-210 expression induces a deep-layer neuronal fate in the neocortex

Upregulation of CDK7 in gastric cancer cell promotes tumor cell proliferation and predicts poor prognosis

CDK7 has been known as a component of CDK activating kinase (CAK) complex, the complex was composed of CDK7, Cyclin H and RING finger protein Mat1 that contribute to cell cycle progression by phosphorylating other CDKs. In addition, the complex is also an essential component of general transcription factor TFIIH which controls transcription via activating RNA polymerase II by serines 5 and 7 phosphorylation of the carboxyl-terminal domain (CTD) of its largest subunit. However, the role of CDK7 in the pathogenesis of gastric cancer has not been identified. Our study showed that CDK7 was significantly upregulated and positively correlated with tumor grade, infiltration depth, lymph node, Ki-67, and predicted poor prognosis in 173 gastric cancer specimens by immunohistochemistrical analyses. Furthermore, in vitro results indicated that CDK7 promoted proliferation of gastric cancer cells by CCK8, clone formation analyses and flow cytometric analyses, while CDK7 knockdown led to decreased cell proliferation. Our study will provide a theoretical basis for the study of CDK7 in gastric cancer.

Cross-talk of phosphorylation and prolyl isomerization of the C-terminal domain of RNA Polymerase II

Post-translational modifications of the heptad repeat sequences in the C-terminal domain (CTD) of RNA polymerase II (Pol II) are well recognized for their roles in coordinating transcription with other nuclear processes that impinge upon transcription by the Pol II machinery; and this is primarily achieved through CTD interactions with the various nuclear factors. The identification of novel modifications on new regulatory sites of the CTD suggests that, instead of an independent action for all modifications on CTD, a combinatorial effect is in operation. In this review we focus on two well-characterized modifications of the CTD, namely serine phosphorylation and prolyl isomerization, and discuss the complex interplay between the enzymes modifying their respective regulatory sites. We summarize the current understanding of how the prolyl isomerization state of the CTD dictates the specificity of writers (CTD kinases), erasers (CTD phosphatases) and readers (CTD binding proteins) and how that correlates to transcription status. Subtle changes in prolyl isomerization states cannot be detected at the primary sequence level, we describe the methods that have been utilized to investigate this mode of regulation. Finally, a general model of how prolyl isomerization regulates the phosphorylation state of CTD, and therefore transcription-coupled processes, is proposed.

Cyclin-dependent kinases, control of cell cycle and hepatic fibrosis

Multiple etiologies of liver disease lead to liver fibrosis by driving the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Liver fibrosis is associated with the proliferation of HSCs, and the cell cycle of activated HSCs is abnormal. Cyclin-dependent kinases (CDKs) play essential roles in cell proliferation. However, the molecular mechanisms responsible for the abnormal proliferation of activated HSCs during hepatic fibrogenesis remain to be defined. Here we will review recent progress in understanding the associations among CDKs, the control of cell cycle and hepatic fibrosis, with an aim to reveal the potential mechanisms of hepatic fibrosis.

The CDK Network: Linking Cycles of Cell Division and Gene Expression

Cyclin-dependent kinases (CDKs) play essential roles in cell proliferation and gene expression. Although distinct sets of CDKs work in cell division and transcription by RNA polymerase II (Pol II), they share a CDK-activating kinase (CAK), which is itself a CDK-Cdk7-in metazoans. Thus a unitary CDK network controls and may coordinate cycles of cell division and gene expression. Recent work reveals decisive roles for Cdk7 in both pathways. The CAK function of Cdk7 helps determine timing of activation and cyclin-binding preferences of different CDKs during the cell cycle. In the transcription cycle, Cdk7 is both an effector kinase, which phosphorylates Pol II and other proteins and helps establish promoter-proximal pausing; and a CAK for Cdk9 (P-TEFb), which releases Pol II from the pause. By governing the transition from initiation to elongation, Cdk7, Cdk9 and their substrates influence expression of genes important for developmental and cell-cycle decisions, and ensure co-transcriptional maturation of Pol II transcripts. Cdk7 engaged in transcription also appears to be regulated by phosphorylation within its own activation (T) loop. Here I review recent studies of CDK regulation in cell division and gene expression, and propose a model whereby mitogenic signals trigger a cascade of CDK T-loop phosphorylation that drives cells past the restriction (R) point, when continued cell-cycle progression becomes growth factor-independent. Because R-point control is frequently deregulated in cancer, the CAK-CDK pathway is an attractive target for chemical inhibition aimed at impeding the inappropriate commitment to cell division.