The multi-tasking P-TEFb complex

MOF-mediated acetylation of CDK9 promotes global transcription by modulating P-TEFb complex formation

Cyclin-dependent kinase 9 (CDK9), a catalytic subunit of the positive transcription elongation factor b (P-TEFb) complex, is a global transcriptional elongation factor associated with cell proliferation. CDK9 activity is regulated by certain histone acetyltransferases, such as p300, GCN5 and P/CAF. However, the impact of males absent on the first (MOF) (also known as KAT8 or MYST1) on CDK9 activity has not been reported. Therefore, the present study aimed to elucidate the regulatory role of MOF on CDK9. We present evidence from systematic biochemical assays and molecular biology approaches arguing that MOF interacts with and acetylates CDK9 at the lysine 35 (i.e. K35) site, and that this acetyl-group can be removed by histone deacetylase HDAC1. Notably, MOF-mediated acetylation of CDK9 at K35 promotes the formation of the P-TEFb complex through stabilizing CDK9 protein and enhancing its association with cyclin T1, which further increases RNA polymerase II serine 2 residues levels and global transcription. Our study reveals for the first time that MOF promotes global transcription by acetylating CDK9, providing a new strategy for exploring the comprehensive mechanism of the MOF-CDK9 axis in cellular processes.

Evolutionary analysis reveals the role of a non-catalytic domain of peptidyl arginine deiminase 2 in transcriptional regulation

Peptidyl arginine deiminases (PADIs) catalyze protein citrullination, a post-translational conversion of arginine to citrulline. The most widely expressed member of this family, PADI2, regulates cellular processes that impact several diseases. We hypothesized that we could gain new insights into PADI2 function through a systematic evolutionary and structural analysis. Here, we identify 20 positively selected PADI2 residues, 16 of which are structurally exposed and maintain PADI2 interactions with cognate proteins. Many of these selected residues reside in non-catalytic regions of PADI2. We validate the importance of a prominent loop in the middle domain that encompasses PADI2 L162, a residue under positive selection. This site is essential for interaction with the transcription elongation factor (P-TEFb) and mediates the active transcription of the oncogenes c-MYC, and CCNB1, as well as impacting cellular proliferation. These insights could be key to understanding and addressing the role of the PADI2 c-MYC axis in cancer progression.

The cell biology of HIV-1 latency and rebound

Transcriptionally latent forms of replication-competent proviruses, present primarily in a small subset of memory CD4+ T cells, pose the primary barrier to a cure for HIV-1 infection because they are the source of the viral rebound that almost inevitably follows the interruption of antiretroviral therapy. Over the last 30 years, many of the factors essential for initiating HIV-1 transcription have been identified in studies performed using transformed cell lines, such as the Jurkat T-cell model. However, as highlighted in this review, several poorly understood mechanisms still need to be elucidated, including the molecular basis for promoter-proximal pausing of the transcribing complex and the detailed mechanism of the delivery of P-TEFb from 7SK snRNP. Furthermore, the central paradox of HIV-1 transcription remains unsolved: how are the initial rounds of transcription achieved in the absence of Tat? A critical limitation of the transformed cell models is that they do not recapitulate the transitions between active effector cells and quiescent memory T cells. Therefore, investigation of the molecular mechanisms of HIV-1 latency reversal and LRA efficacy in a proper physiological context requires the utilization of primary cell models. Recent mechanistic studies of HIV-1 transcription using latently infected cells recovered from donors and ex vivo cellular models of viral latency have demonstrated that the primary blocks to HIV-1 transcription in memory CD4+ T cells are restrictive epigenetic features at the proviral promoter, the cytoplasmic sequestration of key transcription initiation factors such as NFAT and NF-κB, and the vanishingly low expression of the cellular transcription elongation factor P-TEFb. One of the foremost schemes to eliminate the residual reservoir is to deliberately reactivate latent HIV-1 proviruses to enable clearance of persisting latently infected cells-the "Shock and Kill" strategy. For "Shock and Kill" to become efficient, effective, non-toxic latency-reversing agents (LRAs) must be discovered. Since multiple restrictions limit viral reactivation in primary cells, understanding the T-cell signaling mechanisms that are essential for stimulating P-TEFb biogenesis, initiation factor activation, and reversing the proviral epigenetic restrictions have become a prerequisite for the development of more effective LRAs.

Histone acetyltransferase HAF2 associates with PDC to control H3K14ac and H3K23ac in ethylene response

Ethylene plays its essential roles in plant development, growth, and defense responses by controlling the transcriptional reprogramming, in which EIN2-C-directed regulation of histone acetylation is the first key-step for chromatin to perceive ethylene signaling. However, the histone acetyltransferase in this process remains unknown. Here, we identified histone acetyltransferase HAF2, and mutations in HAF2 confer plants with ethylene insensitivity. Furthermore, we found that HAF2 interacts with EIN2-C in response to ethylene. Biochemical assays demonstrated that the bromodomain of HAF2 binds to H3K14ac and H3K23ac peptides with a distinct affinity for H3K14ac; the HAT domain possesses acetyltransferase catalytic activity for H3K14 and H3K23 acetylation, with a preference for H3K14. ChIP-seq results provide additional evidence supporting the role of HAF2 in regulating H3K14ac and H3K23ac levels in response to ethylene. Finally, our findings revealed that HAF2 co-functions with pyruvate dehydrogenase complex (PDC) to regulate H3K14ac and H3K23ac in response to ethylene in an EIN2 dependent manner. Overall, this research reveals that HAF2 as a histone acetyltransferase that forms a complex with EIN2-C and PDC, collectively governing histone acetylation of H3H14ac and H3K23ac, preferentially for H3K14 in response to ethylene.

Distinct roles of two SEC scaffold proteins, AFF1 and AFF4, in regulating RNA polymerase II transcription elongation

The super elongation complex (SEC) containing positive transcription elongation factor b plays a critical role in regulating transcription elongation. AFF1 and AFF4, two members of the AF4/FMR2 family, act as central scaffold proteins of SEC and are associated with various human diseases. However, their precise roles in transcriptional control remain unclear. Here, we investigate differences in the genomic distribution patterns of AFF1 and AFF4 around transcription start sites (TSSs). AFF1 mainly binds upstream of the TSS, while AFF4 is enriched downstream of the TSS. Notably, disruption of AFF4 results in slow elongation and early termination in a subset of AFF4-bound active genes, whereas AFF1 deletion leads to fast elongation and transcriptional readthrough in the same subset of genes. Additionally, AFF1 knockdown increases AFF4 levels at chromatin, and vice versa. In summary, these findings demonstrate that AFF1 and AFF4 function antagonistically to regulate RNA polymerase II transcription.

Truncating the spliceosomal 'rope protein' Prp45 results in Htz1 dependent phenotypes

Spliceosome assembly contributes an important but incompletely understood aspect of splicing regulation. Prp45 is a yeast splicing factor which runs as an extended fold through the spliceosome, and which may be important for bringing its components together. We performed a whole genome analysis of the genetic interaction network of the truncated allele of PRP45 (prp45(1-169)) using synthetic genetic array technology and found chromatin remodellers and modifiers as an enriched category. In agreement with related studies, H2A.Z-encoding HTZ1, and the components of SWR1, INO80, and SAGA complexes represented prominent interactors, with htz1 conferring the strongest growth defect. Because the truncation of Prp45 disproportionately affected low copy number transcripts of intron-containing genes, we prepared strains carrying intronless versions of SRB2, VPS75, or HRB1, the most affected cases with transcription-related function. Intron removal from SRB2, but not from the other genes, partly repaired some but not all the growth phenotypes identified in the genetic screen. The interaction of prp45(1-169) and htz1Δ was detectable even in cells with SRB2 intron deleted (srb2Δi). The less truncated variant, prp45(1-330), had a synthetic growth defect with htz1Δ at 16°C, which also persisted in the srb2Δi background. Moreover, htz1Δ enhanced prp45(1-330) dependent pre-mRNA hyper-accumulation of both high and low efficiency splicers, genes ECM33 and COF1, respectively. We conclude that while the expression defects of low expression intron-containing genes contribute to the genetic interactome of prp45(1-169), the genetic interactions between prp45 and htz1 alleles demonstrate the sensitivity of spliceosome assembly, delayed in prp45(1-169), to the chromatin environment.

Review: Non-canonical role of Drosha ribonuclease III

The typical function of Drosha is participating in cleaving pri-miRNA, the initial step of miRNA biogenesis, in the nucleus. Since Drosha has a double-stranded RNA-binding domain and two RNase III domains, when it binds and/or cleaves other RNA species other than pri-miRNA, Drosha is able to induce a variety of novel biological effects. Moreover, by interacting with other protein, Drosha is able to modify the function of other protein complexes. Recently, diverse non-classical functions of Drosha have been demonstrated, such as promoting DNA damage repair, transcriptional activation and inhibition, pre-mRNA splicing regulation, mRNA destabilization, and virus-host interaction. In this review, we describe these newly discovered functions of Drosha in order to present a panoramic picture of the novel biological processes that Drosha is involved in.

Nucleophosmin 1 cooperates with BRD4 to facilitate c-Myc transcription to promote prostate cancer progression

Nucleophosmin 1 (NPM1) is a multifunctional protein that promotes tumor progression in various cancers and is associated with a poor prognosis of prostate cancer (PCa). However, the mechanism by which NPM1 exerts its malignant potential in PCa remains elusive. Here, we showed that NPM1 is overexpressed in PCa cell lines and tissues and that the dysregulation of NPM1 promotes PCa proliferation. We also demonstrated that NPM1 transcriptionally upregulates c-Myc expression in PCa cells that is diminished by blockade of bromodomain-containing protein 4 (BRD4). Furthermore, we detected a correlation between NPM1 and c-Myc in patient PCa specimens. Mechanistically, NPM1 influences and cooperates with BRD4 to facilitate c-Myc transcription to promote PCa progression. In addition, JQ1, a bromodomain and extra-terminal domain (BET) inhibitor, in combination with NPM1 inhibition suppresses PCa progression in vitro and in vivo. These results indicate that NPM1 promotes PCa progression through a c-Myc -mediated pathway via BRD4, and blockade of the NPM1-c-Myc oncogenic pathway may be a therapeutic strategy for PCa.

Degradation of CDK9 by Ubiquitin E3 Ligase STUB1 Regulates P-TEFb Level and Its Functions for Global Target Gene Expression within Mammalian Cells

Positive transcription elongation factor b (P-TEFb) regulates expression of diverse sets of genes within mammalian cells that have implications in several human disease pathogeneses. However, mechanisms of functional regulation of P-TEFb complex through regulation of its stability are poorly known. In this study, we show an important role of C-terminus of Hsc70-interacting protein (CHIP aka STUB1) in regulation of overall level of CDK9 and thus P-TEFb complex within mammalian cells. STUB1 acts as a ubiquitin E3 ligase for proteasomal degradation of CDK9 involving N-terminal lysine 3 (K3) residue. Whereas, overexpression of STUB1 enhances, its knockdown reduces overall CDK9 degradation kinetics within mammalian cells. Interestingly, owing to the same region of binding within CDK9, CyclinT1 protects CDK9 from STUB1-mediated degradation. Factors that cooperatively bind with CyclinT1 to form functional complex also protects CDK9 from degradation by STUB1. Knockdown of STUB1 enhances CDK9 expression and thus P-TEFb complex formation that leads to global increase in RNA polymerase II CTD phosphorylation and transcriptional activation of diverse P-TEFb target genes. Thus, we describe an important functional role of STUB1 in regulation of transcription through modulation of overall level of P-TEFb complex formation within mammalian cells.

ERK2 stimulates MYC transcription by anchoring CDK9 to the MYC promoter in a kinase activity-independent manner

The transcription factor MYC regulates cell proliferation, transformation, and survival in response to growth factor signaling that is mediated in part by the kinase activity of ERK2. Because ERK2 can also bind to DNA to modify gene expression, we investigated whether it more directly regulates MYC transcription. We identified ERK2 binding sites in the MYC promoter and detected ERK2 at the promoter in various serum-stimulated cell types. Expression of nuclear-localized ERK2 constructs in serum-starved cells revealed that ERK2 in the nucleus-regardless of its kinase activity-increased MYC mRNA expression and MYC protein abundance. ERK2 bound to the promoter through its amino-terminal insert domain and to the cyclin-dependent kinase CDK9 (which activates RNA polymerase II) through its carboxyl-terminal conserved docking domain. Both interactions were essential for ERK2-induced MYC expression, and depleting ERK impaired CDK9 occupancy and RNA polymerase II progression at the MYC promoter. Artificially tethering CDK9 to the MYC promoter by fusing it to the ERK2 insert domain was sufficient to stimulate MYC expression in serum-starved cells. Our findings demonstrate a role for ERK2 at the MYC promoter acting as a kinase-independent anchor for the recruitment of CDK9 to promote MYC expression.

STAT3 Inhibition Attenuates MYC Expression by Modulating Co-Activator Recruitment and Suppresses Medulloblastoma Tumor Growth by Augmenting Cisplatin Efficacy In Vivo

MB is a common childhood malignancy of the central nervous system, with significant morbidity and mortality. Among the four molecular subgroups, MYC-amplified Group 3 MB is the most aggressive type and has the worst prognosis due to therapy resistance. The present study aimed to investigate the role of activated STAT3 in promoting MB pathogenesis and chemoresistance via inducing the cancer hallmark MYC oncogene. Targeting STAT3 function either by inducible genetic knockdown (KD) or with a clinically relevant small molecule inhibitor reduced tumorigenic attributes in MB cells, including survival, proliferation, anti-apoptosis, migration, stemness and expression of MYC and its targets. STAT3 inhibition attenuates MYC expression by affecting recruitment of histone acetyltransferase p300, thereby reducing enrichment of H3K27 acetylation in the MYC promoter. Concomitantly, it also decreases the occupancy of the bromodomain containing protein-4 (BRD4) and phosphoSer2-RNA Pol II (pSer2-RNAPol II) on MYC, resulting in reduced transcription. Importantly, inhibition of STAT3 signaling significantly attenuated MB tumor growth in subcutaneous and intracranial orthotopic xenografts, increased the sensitivity of MB tumors to cisplatin, and improved the survival of mice bearing high-risk MYC-amplified tumors. Together, the results of our study demonstrate that targeting STAT3 may be a promising adjuvant therapy and chemo-sensitizer to augment treatment efficacy, reduce therapy-related toxicity and improve quality of life in high-risk pediatric patients.

Progress in 7SK ribonucleoprotein structural biology

The 7SK ribonucleoprotein (RNP) is a dynamic and multifunctional regulator of RNA Polymerase II (RNAPII) transcription in metazoa. Comprised of the non-coding 7SK RNA, core proteins, and numerous accessory proteins, the most well-known 7SK RNP function is the sequestration and inactivation of the positive transcription elongation factor b (P-TEFb). More recently, 7SK RNP has been shown to regulate RNAPII transcription through P-TEFb-independent pathways. Due to its fundamental role in cellular function, dysregulation has been linked with human diseases including cancers, heart disease, developmental disorders, and viral infection. Significant advances in 7SK RNP structural biology have improved our understanding of 7SK RNP assembly and function. Here, we review progress in understanding the structural basis of 7SK RNA folding, biogenesis, and RNP assembly.

Predictive model of transcriptional elongation control identifies trans regulatory factors from chromatin signatures

Promoter-proximal Polymerase II (Pol II) pausing is a key rate-limiting step for gene expression. DNA and RNA-binding trans-acting factors regulating the extent of pausing have been identified. However, we lack a quantitative model of how interactions of these factors determine pausing, therefore the relative importance of implicated factors is unknown. Moreover, previously unknown regulators might exist. Here we address this gap with a machine learning model that accurately predicts the extent of promoter-proximal Pol II pausing from large-scale genome and transcriptome binding maps and gene annotation and sequence composition features. We demonstrate high accuracy and generalizability of the model by validation on an independent cell line which reveals the model's cell line agnostic character. Model interpretation in light of prior knowledge about molecular functions of regulatory factors confirms the interconnection of pausing with other RNA processing steps. Harnessing underlying feature contributions, we assess the relative importance of each factor, quantify their predictive effects and systematically identify previously unknown regulators of pausing. We additionally identify 16 previously unknown 7SK ncRNA interacting RNA-binding proteins predictive of pausing. Our work provides a framework to further our understanding of the regulation of the critical early steps in transcriptional elongation.

Arabidopsis cyclin-dependent kinase C2 interacts with HDA15 and is involved in far-red light-mediated hypocotyl cell elongation

Histone deacetylases (HDAs) regulate many aspects of plant development and responses to environmental changes. Previous studies have demonstrated that the Arabidopsis histone deacetylase HDA15 is a positive regulator in far-red (FR) light-mediated inhibition of hypocotyl elongation. Furthermore, HDA15 can be phosphorylated and its enzymatic activity is negatively regulated by phosphorylation. However, the kinases that can phosphorylate HDA15 are still unknown. Cyclin-dependent kinases (CDKs) are a large family of serine/threonine protein kinases and have been identified as major regulators of the cell cycle and transcription. In this study, we show that the cyclin-dependent kinase CDKC2 interacts with HDA15 both in vitro and in vivo. In vitro kinase assays show that CDKC2 phosphorylates HDA15. Genetic evidence suggests that HDA15 acts downstream of CDKC2 in hypocotyl elongation under FR light. Furthermore, HDA15 and CDKC2 function synergistically in the regulation of FR-mediated cell elongation. The expression of cell wall organization- and auxin signaling-related genes under FR light is increased in hda15 and cdkc2/hda15 mutants. Taken together, our study indicates that CDKC2 can phosphorylate HDA15 and plays an important role in FR light-regulated hypocotyl elongation.

When cyclin-dependent kinases meet viral infections, including SARS-CoV-2

Cyclin‐dependent kinases (CDKs) are protein kinases that play a key role in cell division and transcriptional regulation. Recent studies have demonstrated the critical roles of CDKs in various viral infections. However, the molecular processes underpinning CDKs' roles in viral infection and host antiviral defense are unknown. This minireview briefly overviews CDKs' functions and highlights the most recent discoveries of CDKs' emerging roles during viral infections, thereby providing a scientific and theoretical foundation for antiviral regulation and shedding light on developing novel drug targets and therapeutic strategies against viral infection.

Aurora A and AKT Kinase Signaling Associated with Primary Cilia

Dysregulation of kinase signaling is associated with various pathological conditions, including cancer, inflammation, and autoimmunity; consequently, the kinases involved have become major therapeutic targets. While kinase signaling pathways play crucial roles in multiple cellular processes, the precise manner in which their dysregulation contributes to disease is dependent on the context; for example, the cell/tissue type or subcellular localization of the kinase or substrate. Thus, context-selective targeting of dysregulated kinases may serve to increase the therapeutic specificity while reducing off-target adverse effects. Primary cilia are antenna-like structures that extend from the plasma membrane and function by detecting extracellular cues and transducing signals into the cell. Cilia formation and signaling are dynamically regulated through context-dependent mechanisms; as such, dysregulation of primary cilia contributes to disease in a variety of ways. Here, we review the involvement of primary cilia-associated signaling through aurora A and AKT kinases with respect to cancer, obesity, and other ciliopathies.

Human FKBP5 negatively regulates transcription through inhibition of P-TEFb complex formation

Although large number of recent studies indicate strong association of FKBP5 (aka FKBP51) functions with various stress-related psychiatric disorder, the overall mechanisms are poorly understood. Beyond a few studies indicating its functions in regulating glucocorticoid receptor-, and AKT-signalling pathways, other functional roles (if any) are unclear. In this study, we report an anti-proliferative role of human FKBP5 through negative regulation of expression of proliferation-related genes. Mechanistically, we show that, owing to same region of interaction on CDK9, human FKBP5 directly competes with CyclinT1 for functional P-TEFb complex formation. In vitro biochemical coupled with cell-based assays, showed strong negative effect of FKBP5 on P-TEFb-mediated phosphorylation of diverse substrates. Consistently, FKBP5 knockdown showed enhanced P-TEFb complex formation leading to increased global RNA polymerase II CTD phosphorylation and expression of proliferation-related genes and subsequent proliferation. Thus, our results show an important role of FKBP5 in negative regulation of P-TEFb functions within mammalian cells.

Platelet transcriptome identifies progressive markers and potential therapeutic targets in chronic myeloproliferative neoplasms

Predicting disease progression remains a particularly challenging endeavor in chronic degenerative disorders and cancer, thus limiting early detection, risk stratification, and preventive interventions. Here, profiling the three chronic subtypes of myeloproliferative neoplasms (MPNs), we identify the blood platelet transcriptome as a proxy strategy for highly sensitive progression biomarkers that also enables prediction of advanced disease via machine-learning algorithms. The MPN platelet transcriptome reveals an incremental molecular reprogramming that is independent of patient driver mutation status or therapy. Subtype-specific markers offer mechanistic and therapeutic insights, and highlight impaired proteostasis and a persistent integrated stress response. Using a LASSO model with validation in two independent cohorts, we identify the advanced subtype MF at high accuracy and offer a robust progression signature toward clinical translation. Our platelet transcriptome snapshot of chronic MPNs demonstrates a proof-of-principle for disease risk stratification and progression beyond genetic data alone, with potential utility in other progressive disorders.

Latency Reversing Agents: Kick and Kill of HTLV-1?

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia/lymphoma (ATLL), is a retrovirus, which integrates into the host genome and persistently infects CD4⁺ T-cells. Virus propagation is stimulated by (1) clonal expansion of infected cells and (2) de novo infection. Viral gene expression is induced by the transactivator protein Tax, which recruits host factors like positive transcription elongation factor b (P-TEFb) to the viral promoter. Since HTLV-1 gene expression is repressed in vivo by viral, cellular, and epigenetic mechanisms in late phases of infection, HTLV-1 avoids an efficient CD8⁺ cytotoxic T-cell (CTL) response directed against the immunodominant viral Tax antigen. Hence, therapeutic strategies using latency reversing agents (LRAs) sought to transiently activate viral gene expression and antigen presentation of Tax to enhance CTL responses towards HTLV-1, and thus, to expose the latent HTLV-1 reservoir to immune destruction. Here, we review strategies that aimed at enhancing Tax expression and Tax-specific CTL responses to interfere with HTLV-1 latency. Further, we provide an overview of LRAs including (1) histone deacetylase inhibitors (HDACi) and (2) activators of P-TEFb, that have mainly been studied in context of human immunodeficiency virus (HIV), but which may also be powerful in the context of HTLV-1.

CSN6 promotes melanoma proliferation and metastasis by controlling the UBR5-mediated ubiquitination and degradation of CDK9

As a critical subunit of the constitutive photomorphogenesis 9 (COP9) signalosome (CSN), CSN6 is upregulated in some human cancers and plays critical roles in tumorigenesis and progression, but its biological functions and molecular mechanisms in melanoma remain unknown. Our study showed that CSN6 expression was upregulated in melanoma patients and cells, and correlated with poor survival in melanoma patients. In melanoma cells, CSN6 knockdown remarkably inhibited cell proliferation, tumorigenicity, migration, and invasion, whereas CSN6 recovery rescued the proliferative and metastatic abilities. Notably, we identified that CSN6 stabilized CDK9 expression by reducing CDK9 ubiquitination levels, thereby activating CDK9-mediated signaling pathways. In addition, our study described a novel CSN6-interacting E3 ligase UBR5, which was negatively regulated by CSN6 and could regulate the ubiquitination and degradation of CDK9 in melanoma cells. Furthermore, in CSN6-knockdown melanoma cells, UBR5 knockdown abrogated the effects caused by CSN6 silencing, suggesting that CSN6 activates the UBR5/CDK9 pathway to promote melanoma cell proliferation and metastasis. Thus, this study illustrates the mechanism by which the CSN6-UBR5-CDK9 axis promotes melanoma development, and demonstrate that CSN6 may be a potential biomarker and anticancer target in melanoma.

Bromodomain and BET family proteins as epigenetic targets in cancer therapy: their degradation, present drugs, and possible PROTACs

Alteration in the pattern of epigenetic marking leads to cancer, neurological disorders, inflammatory problems etc. These changes are due to aberration in histone modification enzymes that function as readers, writers and erasers. Bromodomains (BDs) and BET proteins that recognize acetylation of chromatin regulate gene expression. To block the function of any of these BrDs and/or BET protein can be a controlling agent in disorders such as cancer. BrDs and BET proteins are now emerging as targets for new therapeutic development. Traditional drugs like enzyme inhibitors and protein–protein inhibitors have many limitations. Recently Proteolysis-Targeting Chimeras (PROTACs) have become an advanced tool in therapeutic intervention as they remove disease causing proteins. This review provides an overview of the development and mechanisms of PROTACs for BRD and BET protein regulation in cancer and advanced possibilities of genetic technologies in therapeutics.

Alteration in the pattern of epigenetic marking leads to cancer, neurological disorders, inflammatory problems etc.

Function and regulation of the Spt-Ada-Gcn5-Acetyltransferase (SAGA) deubiquitinase module

The Spt-Ada-Gcn5 Acetyltransferase (SAGA) chromatin modifying complex is a critical regulator of gene expression and is highly conserved across species. Subunits of SAGA arrange into discrete modules with lysine aceyltransferase and deubiquitinase activities housed separately. Mutation of the SAGA deubiquitinase module can lead to substantial biological misfunction and diseases such as cancer, neurodegeneration, and blindness. Here, we review the structure and functions of the SAGA deubiquitinase module and regulatory mechanisms acting to control these.

ENL initiates multivalent phase separation of the super elongation complex (SEC) in controlling rapid transcriptional activation

Release of paused RNA polymerase II (Pol II) requires incorporation of the positive transcription elongation factor b (P-TEFb) into the super elongation complex (SEC), thus resulting in rapid yet synchronous transcriptional activation. However, the mechanism underlying dynamic transition of P-TEFb from inactive to active state remains unclear. Here, we found that the SEC components are able to compartmentalize and concentrate P-TEFb via liquid-liquid phase separation from the soluble inactive HEXIM1 containing the P-TEFb complex. Specifically, ENL or its intrinsically disordered region is sufficient to initiate the liquid droplet formation of SEC. AFF4 functions together with ENL in fluidizing SEC droplets. SEC droplets are fast and dynamically formed upon serum exposure and required for rapid transcriptional induction. We also found that the fusion of ENL with MLL can boost SEC phase separation. In summary, our results suggest a critical role of multivalent phase separation of SEC in controlling transcriptional pause release.

P-TEFb as A Promising Therapeutic Target

The positive transcription elongation factor b (P-TEFb) was first identified as a general factor that stimulates transcription elongation by RNA polymerase II (RNAPII), but soon afterwards it turned out to be an essential cellular co-factor of human immunodeficiency virus (HIV) transcription mediated by viral Tat proteins. Studies on the mechanisms of Tat-dependent HIV transcription have led to radical advances in our knowledge regarding the mechanism of eukaryotic transcription, including the discoveries that P-TEFb-mediated elongation control of cellular transcription is a main regulatory step of gene expression in eukaryotes, and deregulation of P-TEFb activity plays critical roles in many human diseases and conditions in addition to HIV/AIDS. P-TEFb is now recognized as an attractive and promising therapeutic target for inflammation/autoimmune diseases, cardiac hypertrophy, cancer, infectious diseases, etc. In this review article, I will summarize our knowledge about basic P-TEFb functions, the regulatory mechanism of P-TEFb-dependent transcription, P-TEFb’s involvement in biological processes and diseases, and current approaches to manipulating P-TEFb functions for the treatment of these diseases.

The Role of Macrophages in HIV-1 Persistence and Pathogenesis

Current antiretroviral therapy (ART) effectively suppresses Human Immunodeficiency Virus type 1 (HIV-1) in infected individuals. However, even long term ART does not eradicate HIV-1 infected cells and the virus persists in cellular reservoirs. Beside memory CD4⁺ T cells, cells of the myeloid lineage, especially macrophages, are believed to be an important sanctuary for HIV-1. Monocytes and macrophages are key players in the innate immune response to pathogens and are recruited to sites of infection and inflammation. Due to their long life span and ability to reside in virtually every tissue, macrophages have been proposed to play a critical role in the establishment and persistence of the HIV-1 reservoir. Current HIV-1 cure strategies mainly focus on the concept of “shock and kill” to purge the viral reservoir. This approach aims to reactivate viral protein production in latently infected cells, which subsequently are eliminated as a consequence of viral replication, or recognized and killed by the immune system. Macrophage susceptibility to HIV-1 infection is dependent on the local microenvironment, suggesting that molecular pathways directing differentiation and polarization are involved. Current latency reversing agents (LRA) are mainly designed to reactivate the HIV-1 provirus in CD4+ T cells, while their ability to abolish viral latency in macrophages is largely unknown. Moreover, the resistance of macrophages to HIV-1 mediated kill and the presence of infected macrophages in immune privileged regions including the central nervous system (CNS), may pose a barrier to elimination of infected cells by current “shock and kill” strategies. This review focusses on the role of monocytes/macrophages in HIV-1 persistence. We will discuss mechanisms of viral latency and persistence in monocytes/macrophages. Furthermore, the role of these cells in HIV-1 tissue distribution and pathogenesis will be discussed.

Targeting cyclin-dependent kinases for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating disease with poor prognosis and limited therapeutic options. We screened for pathways that may be responsible for the abnormal phenotype of pulmonary arterial smooth muscle cells (PASMCs), a major contributor of PAH pathobiology, and identified cyclin-dependent kinases (CDKs) as overactivated kinases in specimens derived from patients with idiopathic PAH. This increased CDK activity is confirmed at the level of mRNA and protein expression in human and experimental PAH, respectively. Specific CDK inhibition by dinaciclib and palbociclib decreases PASMC proliferation via cell cycle arrest and interference with the downstream CDK-Rb (retinoblastoma protein)-E2F signaling pathway. In two experimental models of PAH (i.e., monocrotaline and Su5416/hypoxia treated rats) palbociclib reverses the elevated right ventricular systolic pressure, reduces right heart hypertrophy, restores the cardiac index, and reduces pulmonary vascular remodeling. These results demonstrate that inhibition of CDKs by palbociclib may be a therapeutic strategy in PAH.

Cells of the pulmonary vasculature show a hyperproliferative phenotype in pulmonary arterial hypertension (PAH), thus contributing to the disease pathogenesis. Here the authors show that cyclin-dependent kinases are overactivated in PAH, and that their pharmacological inhibition attenuates the disease in two independent rodent models

The Role of Macrophages in HIV-1 Persistence and Pathogenesis

Current antiretroviral therapy (ART) effectively suppresses Human Immunodeficiency Virus type 1 (HIV-1) in infected individuals. However, even long term ART does not eradicate HIV-1 infected cells and the virus persists in cellular reservoirs. Beside memory CD4⁺ T cells, cells of the myeloid lineage, especially macrophages, are believed to be an important sanctuary for HIV-1. Monocytes and macrophages are key players in the innate immune response to pathogens and are recruited to sites of infection and inflammation. Due to their long life span and ability to reside in virtually every tissue, macrophages have been proposed to play a critical role in the establishment and persistence of the HIV-1 reservoir. Current HIV-1 cure strategies mainly focus on the concept of "shock and kill" to purge the viral reservoir. This approach aims to reactivate viral protein production in latently infected cells, which subsequently are eliminated as a consequence of viral replication, or recognized and killed by the immune system. Macrophage susceptibility to HIV-1 infection is dependent on the local microenvironment, suggesting that molecular pathways directing differentiation and polarization are involved. Current latency reversing agents (LRA) are mainly designed to reactivate the HIV-1 provirus in CD4+ T cells, while their ability to abolish viral latency in macrophages is largely unknown. Moreover, the resistance of macrophages to HIV-1 mediated kill and the presence of infected macrophages in immune privileged regions including the central nervous system (CNS), may pose a barrier to elimination of infected cells by current "shock and kill" strategies. This review focusses on the role of monocytes/macrophages in HIV-1 persistence. We will discuss mechanisms of viral latency and persistence in monocytes/macrophages. Furthermore, the role of these cells in HIV-1 tissue distribution and pathogenesis will be discussed.

HDAC stimulates gene expression through BRD4 availability in response to IFN and in interferonopathies

In contrast to the common role of histone deacetylases (HDACs) for gene repression, HDAC activity provides a required positive function for IFN-stimulated gene (ISG) expression. Here, we show that HDAC1/2 as components of the Sin3A complex are required for ISG transcriptional elongation but not for recruitment of RNA polymerase or transcriptional initiation. Transcriptional arrest by HDAC inhibition coincides with failure to recruit the epigenetic reader Brd4 and elongation factor P-TEFb due to sequestration of Brd4 on hyperacetylated chromatin. Brd4 availability is regulated by an equilibrium cycle between opposed acetyltransferase and deacetylase activities that maintains a steady-state pool of free Brd4 available for recruitment to inducible promoters. An ISG expression signature is a hallmark of interferonopathies and other autoimmune diseases. Combined inhibition of HDAC1/2 and Brd4 resolved the aberrant ISG expression detected in cells derived from patients with two inherited interferonopathies, ISG15 and USP18 deficiencies, defining a novel therapeutic approach to ISG-associated autoimmune diseases.

Bromodomain protein inhibition: a novel therapeutic strategy in rheumatic diseases

The reading of acetylation marks on histones by bromodomain (BRD) proteins is a key event in transcriptional activation. Small molecule inhibitors targeting bromodomain and extra-terminal (BET) proteins compete for binding to acetylated histones. They have strong anti-inflammatory properties and exhibit encouraging effects in different cell types in vitro and in animal models resembling rheumatic diseases in vivo. Furthermore, recent studies that focus on BRD proteins beyond BET family members are discussed.

P-TEFb et Brd4

La physiologie d’une cellule est dictée par l’intégration des signaux qu’elle reçoit et la mise en place de réponses adaptées par le biais, entre autres, de programmes transcriptionnels adéquats. Pour assurer un contrôle optimal de ces réponses, des mécanismes de régulation ont été sélectionnés, dont un processus de pause transcriptionnelle et de levée de cette pause par P-TEFb (positive transcription elongation factor) et Brd4 (bromodomain-containing protein 4). Le dérèglement de ce processus peut conduire à l’apparition de pathologies. P-TEFb et Brd4 ont ainsi émergé au cours des dernières années comme des cibles thérapeutiques potentielles dans le cadre des cancers et du syndrome d‘immunodéficience acquise (sida) notamment.

HIV Tat/P-TEFb Interaction: A Potential Target for Novel Anti-HIV Therapies

Transcription is a crucial step in the life cycle of the human immunodeficiency virus type 1 (HIV 1) and is primarily involved in the maintenance of viral latency. Both viral and cellular transcription factors, including transcriptional activators, suppressor proteins and epigenetic factors, are involved in HIV transcription from the proviral DNA integrated within the host cell genome. Among them, the virus-encoded transcriptional activator Tat is the master regulator of HIV transcription. Interestingly, unlike other known transcriptional activators, Tat primarily activates transcriptional elongation and initiation by interacting with the cellular positive transcriptional elongation factor b (P-TEFb). In this review, we describe the molecular mechanism underlying how Tat activates viral transcription through interaction with P-TEFb. We propose a novel therapeutic strategy against HIV replication through blocking Tat action.

LARP7 in papillary thyroid carcinoma induces NIS expression through suppression of the SHH signaling pathway

The incidence of thyroid cancer has increased the past few decades, the most frequent type has been identified to be the papillary thyroid carcinoma (PTC). Following thyroidectomy, radioiodine ablation treatment on PTC is routinely performed. However, many patients do not benefit from radioiodine therapy. Therefore, novel targeted therapies to suppress tumor growth and improve radioiodine uptake are required. La ribonucleoprotein domain family member (LARP)7 is a member of the LARP family and functions as a potential suppressor of the progression of carcinoma. In the present study, the expression status of LARP7 in PTC tissues and cell lines was investigated, and the cell viability, proliferation and apoptotic rate, radioiodine uptake ability of PTC cells with overexpression of LARP7 in vitro was determined. Expression levels of LARP7 were significantly downregulated in PTC tissues and cell lines. Overexpression of LARP7 inhibited the proliferation and increased the radioiodine uptake ability of PTC cells in vitro and inhibited the tumor growth in vivo. Furthermore, LARP7 overexpression inhibited the sonic hedgehog (SHH) signaling pathway and increased sodium/iodide symporter (NIS) expression. However, treatment with recombinant human SHH partially reduced radioiodine uptake ability and NIS expression induced by LARP7. In conclusion, LARP7 may act as a tumor suppressor in PTC by inhibiting the SHH signaling pathway and may be a promising therapeutic target in patients with PTC.

SUMO suppresses and MYC amplifies transcription globally by regulating CDK9 sumoylation

Regulation of transcription is fundamental to the control of cellular gene expression and function. Although recent studies have revealed a role for the oncoprotein MYC in amplifying global transcription, little is known as to how the global transcription is suppressed. Here we report that SUMO and MYC mediate opposite effects upon global transcription by controlling the level of CDK9 sumoylation. On one hand, SUMO suppresses global transcription via sumoylation of CDK9, the catalytic subunit of P-TEFb kinase essential for productive transcriptional elongation. On the other hand, MYC amplifies global transcription by antagonizing CDK9 sumoylation. Sumoylation of CDK9 blocks its interaction with Cyclin T1 and thus the formation of active P-TEFb complex. Transcription profiling analyses reveal that SUMO represses global transcription, particularly of moderately to highly expressed genes and by generating a sumoylation-resistant CDK9 mutant, we confirm that sumoylation of CDK9 inhibits global transcription. Together, our data reveal that SUMO and MYC oppositely control global gene expression by regulating the dynamic sumoylation and desumoylation of CDK9.

The novel BET bromodomain inhibitor BI 894999 represses super-enhancer-associated transcription and synergizes with CDK9 inhibition in AML

Bromodomain and extra-terminal (BET) protein inhibitors have been reported as treatment options for acute myeloid leukemia (AML) in preclinical models and are currently being evaluated in clinical trials. This work presents a novel potent and selective BET inhibitor (BI 894999), which has recently entered clinical trials (NCT02516553). In preclinical studies, this compound is highly active in AML cell lines, primary patient samples, and xenografts. HEXIM1 is described as an excellent pharmacodynamic biomarker for target engagement in tumors as well as in blood. Mechanistic studies show that BI 894999 targets super-enhancer-regulated oncogenes and other lineage-specific factors, which are involved in the maintenance of the disease state. BI 894999 is active as monotherapy in AML xenografts, and in addition leads to strongly enhanced antitumor effects in combination with CDK9 inhibitors. This treatment combination results in a marked decrease of global p-Ser2 RNA polymerase II levels and leads to rapid induction of apoptosis in vitro and in vivo. Together, these data provide a strong rationale for the clinical evaluation of BI 894999 in AML.

CDK9 inhibitors in acute myeloid leukemia

Current treatment for acute myeloid leukemia (AML) is less than optimal, but increased understanding of disease pathobiology and genomics has led to clinical investigation of novel targeted therapies and rational combinations. Targeting the cyclin-dependent kinase 9 (CDK9) pathway, which is dysregulated in AML, is an attractive approach. Inhibition of CDK9 leads to downregulation of cell survival genes regulated by super enhancers such as MCL-1, MYC, and cyclin D1. As CDK9 inhibitors are nonselective, predictive biomarkers that may help identify patients most likely to respond to CDK9 inhibitors are now being utilized, with the goal of improving efficacy and safety.

Ensemble-based modeling and rigidity decomposition of allosteric interaction networks and communication pathways in cyclin-dependent kinases: Differentiating kinase clients of the Hsp90-Cdc37 chaperone

The overarching goal of delineating molecular principles underlying differentiation of protein kinase clients and chaperone-based modulation of kinase activity is fundamental to understanding activity of many oncogenic kinases that require chaperoning of Hsp70 and Hsp90 systems to attain a functionally competent active form. Despite structural similarities and common activation mechanisms shared by cyclin-dependent kinase (CDK) proteins, members of this family can exhibit vastly different chaperone preferences. The molecular determinants underlying chaperone dependencies of protein kinases are not fully understood as structurally similar kinases may often elicit distinct regulatory responses to the chaperone. The regulatory divergences observed for members of CDK family are of particular interest as functional diversification among these kinases may be related to variations in chaperone dependencies and can be exploited in drug discovery of personalized therapeutic agents. In this work, we report the results of a computational investigation of several members of CDK family (CDK5, CDK6, CDK9) that represented a broad repertoire of chaperone dependencies—from nonclient CDK5, to weak client CDK6, and strong client CDK9. By using molecular simulations of multiple crystal structures we characterized conformational ensembles and collective dynamics of CDK proteins. We found that the elevated dynamics of CDK9 can trigger imbalances in cooperative collective motions and reduce stability of the active fold, thus creating a cascade of favorable conditions for chaperone intervention. The ensemble-based modeling of residue interaction networks and community analysis determined how differences in modularity of allosteric networks and topography of communication pathways can be linked with the client status of CDK proteins. This analysis unveiled depleted modularity of the allosteric network in CDK9 that alters distribution of communication pathways and leads to impaired signaling in the client kinase. According to our results, these network features may uniquely define chaperone dependencies of CDK clients. The perturbation response scanning and rigidity decomposition approaches identified regulatory hotspots that mediate differences in stability and cooperativity of allosteric interaction networks in the CDK structures. By combining these synergistic approaches, our study revealed dynamic and network signatures that can differentiate kinase clients and rationalize subtle divergences in the activation mechanisms of CDK family members. The therapeutic implications of these results are illustrated by identifying structural hotspots of pathogenic mutations that preferentially target regions of the increased flexibility to enable modulation of activation changes. Our study offers a network-based perspective on dynamic kinase mechanisms and drug design by unravelling relationships between protein kinase dynamics, allosteric communications and chaperone dependencies.

SIRT7-dependent deacetylation of CDK9 activates RNA polymerase II transcription

SIRT7 is an NAD⁺-dependent protein deacetylase that regulates cell growth and proliferation. Previous studies have shown that SIRT7 is required for RNA polymerase I (Pol I) transcription and pre-rRNA processing. Here, we took a proteomic approach to identify novel molecular targets and characterize the role of SIRT7 in non-nucleolar processes. We show that SIRT7 interacts with numerous proteins involved in transcriptional regulation and RNA metabolism, the majority of interactions requiring ongoing transcription. In addition to its role in Pol I transcription, we found that SIRT7 also regulates transcription of snoRNAs and mRNAs. Mechanistically, SIRT7 promotes the release of P-TEFb from the inactive 7SK snRNP complex and deacetylates CDK9, a subunit of the elongation factor P-TEFb, which activates transcription by phosphorylating serine 2 within the C-terminal domain (CTD) of Pol II. SIRT7 counteracts GCN5-directed acetylation of lysine 48 within the catalytic domain of CDK9, deacetylation promoting CTD phosphorylation and transcription elongation.

Bromodomain factors of BET family are new essential actors of pericentric heterochromatin transcriptional activation in response to heat shock

The heat shock response is characterized by the transcriptional activation of both hsp genes and noncoding and repeated satellite III DNA sequences located at pericentric heterochromatin. Both events are under the control of Heat Shock Factor I (HSF1). Here we show that under heat shock, HSF1 recruits major cellular acetyltransferases, GCN5, TIP60 and p300 to pericentric heterochromatin leading to a targeted hyperacetylation of pericentric chromatin. Redistribution of histone acetylation toward pericentric region in turn directs the recruitment of Bromodomain and Extra-Terminal (BET) proteins BRD2, BRD3, BRD4, which are required for satellite III transcription by RNAP II. Altogether we uncover here a critical role for HSF1 in stressed cells relying on the restricted use of histone acetylation signaling over pericentric heterochromatin (HC).

Metabolic and Epigenetic Coordination of T Cell and Macrophage Immunity

Recognition of pathogens by innate and adaptive immune cells instructs rapid alterations of cellular processes to promote effective resolution of infection. To accommodate increased bioenergetic and biosynthetic demands, metabolic pathways are harnessed to maximize proliferation and effector molecule production. In parallel, activation initiates context-specific gene-expression programs that drive effector functions and cell fates that correlate with changes in epigenetic landscapes. Many chromatin- and DNA-modifying enzymes make use of substrates and cofactors that are intermediates of metabolic pathways, providing potential cross talk between metabolism and epigenetic regulation of gene expression. In this review, we discuss recent studies of T cells and macrophages supporting a role for metabolic activity in integrating environmental signals with activation-induced gene-expression programs through modulation of the epigenome and speculate as to how this may influence context-specific macrophage and T cell responses to infection.

Chromatin remodeller Fun30Fft3 induces nucleosome disassembly to facilitate RNA polymerase II elongation

Previous studies have revealed that nucleosomes impede elongation of RNA polymerase II (RNAPII). Recent observations suggest a role for ATP-dependent chromatin remodellers in modulating this process, but direct in vivo evidence for this is unknown. Here using fission yeast, we identify Fun30^Fft3 as a chromatin remodeller, which localizes at transcribing regions to promote RNAPII transcription. Fun30^Fft3 associates with RNAPII and collaborates with the histone chaperone, FACT, which facilitates RNAPII elongation through chromatin, to induce nucleosome disassembly at transcribing regions during RNAPII transcription. Mutants, resulting in reduced nucleosome-barrier, such as deletion mutants of histones H3/H4 themselves and the genes encoding components of histone deacetylase Clr6 complex II suppress the defects in growth and RNAPII occupancy of cells lacking Fun30^Fft3. These data suggest that RNAPII utilizes the chromatin remodeller, Fun30^Fft3, to overcome the nucleosome barrier to transcription elongation.

Nucleosomes have been shown to impede the elongation of RNA polymerase II during transcription. Here, the authors provide evidence that in fission yeast chromatin remodeller Fun30^Fft3 localizes to transcribing regions to promote transcription by nucleosome disassembly in vivo.

Targeting CDK9: a promising therapeutic opportunity in prostate cancer

Cyclin-dependent kinase 9 (CDK9) is a key transcriptional regulator and a lucrative target for cancer treatment. Targeting CDK9 can effectively confine the hyperactivity of androgen receptor and the constitutive expression of anti-apoptotic proteins; both being main causes of prostate cancer (PCa) development and progression. In castrate-resistant PCa, traditional therapies that only target androgen receptor (AR) have become obsolete due to reprograming in AR activity to make the cells independent of androgen. CDK9 inhibitors may provide a new and better therapeutic opportunity over traditional treatment options by targeting both androgen receptor activity and anti-apoptotic proteins, improving the chances of positive outcomes, especially in patients with the advanced disease. This review focuses on biological functions of CDK9, its involvement with AR and the potential for therapeutic opportunities in PCa treatment.

Targeting the Brain Reservoirs: Toward an HIV Cure

One of the top research priorities of the international AIDS society by the action “Towards an HIV Cure” is the purge or the decrease of the pool of all latently infected cells. This strategy is based on reactivation of latently reservoirs (the shock) followed by an intensifying combination antiretroviral therapy (cART) to kill them (the kill). The central nervous system (CNS) has potential latently infected cells, i.e., perivascular macrophages, microglial cells, and astrocytes that will need to be eliminated. However, the CNS has several characteristics that may preclude the achievement of a cure. In this review, we discuss several limitations to the eradication of brain reservoirs and how we could circumvent these limitations by making it efforts in four directions: (i) designing efficient latency-reversal agents for CNS-cell types, (ii) improving cART by targeting HIV transcription, (iii) improving delivery of HIV drugs in the CNS and in the CNS-cell types, and (iv) developing therapeutic immunization. As a prerequisite to these efforts, we also believe that a better comprehension of molecular mechanisms involved in establishment and persistence of HIV latency in brain reservoirs are essential to design new molecules for strategies aiming to achieve a cure for instance the “shock and kill” strategy.

Covalent targeting of remote cysteine residues to develop CDK12 and CDK13 inhibitors

Cyclin-dependent kinases 12 and 13 (CDK12 and CDK13) play critical roles in the regulation of gene transcription. However, the absence of CDK12 and CDK13 inhibitors has hindered the ability to investigate the consequences of their inhibition in healthy cells and cancer cells. Here we describe the rational design of a first-in-class CDK12 and CDK13 covalent inhibitor, THZ531. Co-crystallization of THZ531 with CDK12-cyclin K indicates that THZ531 irreversibly targets a cysteine located outside the kinase domain. THZ531 causes a loss of gene expression with concurrent loss of elongating and hyperphosphorylated RNA polymerase II. In particular, THZ531 substantially decreases the expression of DNA damage response genes and key super-enhancer-associated transcription factor genes. Coincident with transcriptional perturbation, THZ531 dramatically induced apoptotic cell death. Small molecules capable of specifically targeting CDK12 and CDK13 may thus help identify cancer subtypes that are particularly dependent on their kinase activities.

Inhibition of P-TEFb disrupts global transcription, oocyte maturation, and embryo development in the mouse

Positive transcription elongation factor b (P-TEFb) is an RNA polymerase II kinase that phosphorylates Ser2 of the carboxyl-terminal domain and promotes the elongation phase of transcription. Despite the fact that P-TEFb has role in many cellular processes, the role of this kinase complex remains to be understood in early developmental events. In this study, using immunocytochemical analyses, we find that the P-TEFb components, Cyclin T1, CDK9, and its T-loop phosphorylated form, are localized to nuclear speckles, as well as in nucleoli in mouse germinal vesicle oocytes. Moreover, using fluorescence in situ hybridization, we show that in absence of CDK9 activity, nucleolar integration, as well as production of 28S rRNA is impaired in oocytes and embryos. We also present evidence indicating that P-TEFb kinase activity is essential for completion of mouse oocyte maturation and embryo development. Treatment with CDK9 inhibitor, flavopiridol resulted in metaphase I arrest in maturing oocytes. Inhibition of CDK9 kinase activity did not interfere with in vitro fertilization and pronuclear formation. However, when zygotes or 2-cell embryos were treated with flavopiridol only in their G2 phase of the cell cycle, development to the blastocyst stage was impaired. Inhibition of the CDK9 activity after embryonic genome activation resulted in failure to form normal blastocysts and aberrant phosphorylation of RNA polymerase II CTD. In all stages analyzed, treatment with flavopiridol abrogated global transcriptional activity. Collectively, our data suggest that P-TEFb kinase activity is crucial for oocyte maturation, embryo development, and regulation of global RNA transcription in mouse early development.