Spt5 cooperates with human immunodeficiency virus type 1 Tat by preventing premature RNA release at terminator sequences

The HIV-1 Transcriptional Program: From Initiation to Elongation Control

A large body of work in the last four decades has revealed the key pillars of HIV-1 transcription control at the initiation and elongation steps. Here, I provide a recount of this collective knowledge starting with the genomic elements (DNA and nascent TAR RNA stem-loop) and transcription factors (cellular and the viral transactivator Tat), and later transitioning to the assembly and regulation of transcription initiation and elongation complexes, and the role of chromatin structure. Compelling evidence support a core HIV-1 transcriptional program regulated by the sequential and concerted action of cellular transcription factors and Tat to promote initiation and sustain elongation, highlighting the efficiency of a small virus to take over its host to produce the high levels of transcription required for viral replication. I summarize new advances including the use of CRISPR-Cas9, genetic tools for acute factor depletion, and imaging to study transcriptional dynamics, bursting and the progression through the multiple phases of the transcriptional cycle. Finally, I describe current challenges to future major advances and discuss areas that deserve more attention to both bolster our basic knowledge of the core HIV-1 transcriptional program and open up new therapeutic opportunities.

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.

Promoter-proximal regulation of gene transcription: Key factors involved and emerging role of general transcription factors in assisting productive elongation

The pausing of RNA polymerase II (Pol II) at the promoter-proximal sites is a key rate-limiting step in gene expression. Cells have dedicated a specific set of proteins that sequentially establish pause and then release the Pol II from promoter-proximal sites. A well-controlled pausing and subsequent release of Pol II is crucial for thefine tuning of expression of genes including signal-responsive and developmentally-regulated ones. The release of paused Pol II broadly involves its transition from initiation to elongation. In this review article, we will discuss the phenomenon of Pol II pausing, the underlying mechanism, and also the role of different known factors, with an emphasis on general transcription factors, involved in this overall regulation. We will further discuss some recent findings suggesting a possible role (underexplored) of initiation factors in assisting the transition of transcriptionally-engaged paused Pol II into productive elongation.

The PNUTS-PP1 complex acts as an intrinsic barrier to herpesvirus KSHV gene expression and replication

Control of RNA Polymerase II (pol II) elongation is a critical component of gene expression in mammalian cells. The PNUTS-PP1 complex controls elongation rates, slowing pol II after polyadenylation sites to promote termination. The Kaposi's sarcoma-associated herpesvirus (KSHV) co-opts pol II to express its genes, but little is known about its regulation of pol II elongation. We identified PNUTS as a suppressor of a KSHV reporter gene in a genome-wide CRISPR screen. PNUTS depletion enhances global KSHV gene expression and overall viral replication. Mechanistically, PNUTS requires PP1 interaction, binds viral RNAs downstream of polyadenylation sites, and restricts transcription readthrough of viral genes. Surprisingly, PNUTS also represses productive elongation at the 5´ ends of the KSHV reporter and the KSHV T1.4 RNA. From these data, we conclude that PNUTS' activity constitutes an intrinsic barrier to KSHV replication likely by suppressing pol II elongation at promoter-proximal regions.

The pleiotropic roles of SPT5 in transcription

Initially discovered by genetic screens in budding yeast, SPT5 and its partner SPT4 form a stable complex known as DSIF in metazoa, which plays pleiotropic roles in multiple steps of transcription. SPT5 is the most conserved transcription elongation factor, being found in all three domains of life; however, its structure has evolved to include new domains and associated posttranslational modifications. These gained features have expanded transcriptional functions of SPT5, likely to meet the demand for increasingly complex regulation of transcription in higher organisms. This review discusses the pleiotropic roles of SPT5 in transcription, including RNA polymerase II (Pol II) stabilization, enhancer activation, Pol II pausing and its release, elongation, and termination, with a focus on the most recent progress of SPT5 functions in regulating metazoan transcription.

So Pathogenic or So What?-A Brief Overview of SIV Pathogenesis with an Emphasis on Cure Research

HIV infection requires lifelong antiretroviral therapy (ART) to control disease progression. Although ART has greatly extended the life expectancy of persons living with HIV (PWH), PWH nonetheless suffer from an increase in AIDS-related and non-AIDS related comorbidities resulting from HIV pathogenesis. Thus, an HIV cure is imperative to improve the quality of life of PWH. In this review, we discuss the origins of various SIV strains utilized in cure and comorbidity research as well as their respective animal species used. We briefly detail the life cycle of HIV and describe the pathogenesis of HIV/SIV and the integral role of chronic immune activation and inflammation on disease progression and comorbidities, with comparisons between pathogenic infections and nonpathogenic infections that occur in natural hosts of SIVs. We further discuss the various HIV cure strategies being explored with an emphasis on immunological therapies and "shock and kill".

Induction of Autophagy to Achieve a Human Immunodeficiency Virus Type 1 Cure

Effective antiretroviral therapy has led to significant human immunodeficiency virus type 1 (HIV-1) suppression and improvement in immune function. However, the persistence of integrated proviral DNA in latently infected reservoir cells, which drive viral rebound post-interruption of antiretroviral therapy, remains the major roadblock to a cure. Therefore, the targeted elimination or permanent silencing of this latently infected reservoir is a major focus of HIV-1 research. The most studied approach in the development of a cure is the activation of HIV-1 expression to expose latently infected cells for immune clearance while inducing HIV-1 cytotoxicity—the “kick and kill” approach. However, the complex and highly heterogeneous nature of the latent reservoir, combined with the failure of clinical trials to reduce the reservoir size casts doubt on the feasibility of this approach. This concern that total elimination of HIV-1 from the body may not be possible has led to increased emphasis on a “functional cure” where the virus remains but is unable to reactivate which presents the challenge of permanently silencing transcription of HIV-1 for prolonged drug-free remission—a “block and lock” approach. In this review, we discuss the interaction of HIV-1 and autophagy, and the exploitation of autophagy to kill selectively HIV-1 latently infected cells as part of a cure strategy. The cure strategy proposed has the advantage of significantly decreasing the size of the HIV-1 reservoir that can contribute to a functional cure and when optimised has the potential to eradicate completely HIV-1.

AP-1 and NF-κB synergize to transcriptionally activate latent HIV upon T-cell receptor activation

Latent HIV-1 proviruses are capable of reactivating productive lytic infection, but the precise molecular mechanisms underlying emergence from latency are poorly understood. In this study, we determined the contribution of the transcription factors NF-κB, NFAT, and AP-1 in the reactivation of latent HIV following T-cell receptor (TCR) activation using Jurkat T-cell clones harboring single latent HIV proviruses. Our findings demonstrate that during reactivation from latency, NF-κB enhances HIV transcription while NFAT inhibits it by competing with NF-κB for overlapping binding sites on the HIV long terminal repeat (LTR). We have also demonstrated for the first time the molecular contribution of AP-1 in the reactivation of HIV from latency, whereby AP-1 synergizes with NF-κB to regulate HIV transcriptional elongation following TCR activation.

CRISPRi-mediated depletion of Spt4 and Spt5 reveals a role for DSIF in the control of HIV latency

Pivotal studies on the control of HIV transcription has laid the foundations for our understanding of how metazoan transcription is executed, and what are the factors that control this step. Part of this work established a role for DRB Sensitivity Inducing Factor (DSIF), consisting of Spt4 and Spt5, in promoting pause-release of RNA Polymerase II (Pol II) for optimal elongation. However, while there has been substantial progress in understanding the role of DSIF in mediating HIV gene transcription, its involvement in establishing viral latency has not been explored. Moreover, the effects of depleting Spt4 or Spt5, or simultaneously knocking down both subunits of DSIF have not been examined. In this study, we employed CRISPR interference (CRIPSRi) to knockdown the expression of Spt4, Spt5 or the entire DSIF complex, and monitored effects on HIV transcription and viral latency. Knocking down DSIF, or each of its subunits, inhibited HIV transcription, primarily at the step of Tat transactivation. This was accompanied by a decrease in promoter occupancy of Pol II and Cdk9, and to a lesser extent, AFF4. Interestingly, targeting the expression of one subunit of DSIF, reduced the protein stability of its counterpart partner. Moreover, depletion of Spt4, Spt5 or DSIF complex impaired cell growth, but did not cause cell death. Finally, knockdown of Spt4, Spt5 or DSIF, facilitated entry of HIV into latency. We conclude that each DSIF subunit plays a role in maintaining the stability of its other partner, achieving optimal function of the DSIF to enhance viral gene transcription.

Peptidyl inhibition of Spt4-Spt5: Protein-protein inhibitors for targeting the transcriptional pathway related to C9orf72 expansion repeats

Spt4/Spt5 is an useful target as it is likely a transcription factor that has implications for long non-coding RNA repeats related to frontotemporal dementia (FTD) found in the C9orf72 disease pathology. Inhibitors for Spt4/Spt5 using peptides as a starting point for assays as a means for developing small molecules, which could likely lead to therapeutic development for inhibition for Spt4/Spt5 with CNS characteristics. To elucidate the specific steps of identification and modification of key interacting residues from Spt4/Spt5 complex with further effect prediction, a set of different computational methods was applied. Newly characterized, theoretically derived peptides docked on Spt4/Spt5 models, based on X-ray crystallography sources, allowed us to complete molecular dynamics simulations and docking studies for peptide libraries that give us high confident set of peptides for use to screen for Spt4/Spt5 inhibition. Several peptides with increased specificity to the Spt4/Spt5 interface were found and can be screened in cell-based assays and enzymatic assays for peptide screens that lead to small molecule campaigns. Spt4/Spt5 comprises an attractive target for neurological diseases, and applying these peptides into a screening campaign will promote the goal of therapeutic searches for FTD drug discovery.

HIV-1 Proviral Transcription and Latency in the New Era

Three decades of extensive work in the HIV field have revealed key viral and host cell factors controlling proviral transcription. Various models of transcriptional regulation have emerged based on the collective information from in vitro assays and work in both immortalized and primary cell-based models. Here, we provide a recount of the past and current literature, highlight key regulatory aspects, and further describe potential limitations of previous studies. We particularly delve into critical steps of HIV gene expression including the role of the integration site, nucleosome positioning and epigenomics, and the transition from initiation to pausing and pause release. We also discuss open questions in the field concerning the generality of previous regulatory models to the control of HIV transcription in patients under suppressive therapy, including the role of the heterogeneous integration landscape, clonal expansion, and bottlenecks to eradicate viral persistence. Finally, we propose that building upon previous discoveries and improved or yet-to-be discovered technologies will unravel molecular mechanisms of latency establishment and reactivation in a “new era”.

SUPT4H1 Depletion Leads to a Global Reduction in RNA

SUPT4H1 is a transcription elongation factor that makes up part of the RNA polymerase II complex. Recent studies propose a selective role for SUPT4H1 in the transcription of repeat-containing DNA, the translated products of which contribute to neurodegenerative disorders such as C9orf72-amyotrophic lateral sclerosis. To investigate the potential of SUPT4H1 as a therapeutic target in repeat-associated neurodegeneration, we depleted SUPT4H1 by RNA interference to inhibit the function of the SUPT4H1/SUPT5H transcription elongation complex. Depletion of SUPT4H1 leads to a global reduction in all cellular RNA, highlighting the significant challenges that are associated with targeting this molecule for the treatment of human disease. Any requirement of SUPT4H1 for transcription of specific transcripts should be interpreted in the context of global modulatory effects on the transcriptome.

DNA dependent protein kinase (DNA-PK) enhances HIV transcription by promoting RNA polymerase II activity and recruitment of transcription machinery at HIV LTR

Despite reductions in mortality from the use of highly active antiretroviral therapy (HAART), the presence of latent or transcriptionally silent proviruses prevents HIV cure/eradication. We have previously reported that DNA-dependent protein kinase (DNA-PK) facilitates HIV transcription by interacting with the RNA polymerase II (RNAP II) complex recruited at HIV LTR. In this study, using different cell lines and peripheral blood mononuclear cells (PBMCs) of HIV-infected patients, we found that DNA-PK stimulates HIV transcription at several stages, including initiation, pause-release and elongation. We are reporting for the first time that DNA-PK increases phosphorylation of RNAP II C-terminal domain (CTD) at serine 5 (Ser5) and serine 2 (Ser2) by directly catalyzing phosphorylation and by augmenting the recruitment of the positive transcription elongation factor (P-TEFb) at HIV LTR. Our findings suggest that DNA-PK expedites the establishment of euchromatin structure at HIV LTR. DNA-PK inhibition/knockdown leads to the severe impairment of HIV replication and reactivation of latent HIV provirus. DNA-PK promotes the recruitment of Tripartite motif-containing 28 (TRIM28) at LTR and assists the release of paused RNAP II through TRIM28 phosphorylation. These results provide the mechanisms through which DNA-PK controls the HIV gene expression and, likely, can be extended to cellular gene expression, including during cell malignancy, where the role of DNA-PK has been well-established.

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.

Control of RNA Pol II Speed by PNUTS-PP1 and Spt5 Dephosphorylation Facilitates Termination by a "Sitting Duck Torpedo" Mechanism

Control of transcription speed, which influences many co-transcriptional processes, is poorly understood. We report that PNUTS-PP1 phosphatase is a negative regulator of RNA polymerase II (Pol II) elongation rate. The PNUTS W401A mutation, which disrupts PP1 binding, causes genome-wide acceleration of transcription associated with hyper-phosphorylation of the Spt5 elongation factor. Immediately downstream of poly(A) sites, Pol II decelerates from >2 kb/min to <1 kb/min, which correlates with Spt5 dephosphorylation. Pol II deceleration and Spt5 dephosphorylation require poly(A) site recognition and the PNUTS-PP1 complex, which is in turn necessary for transcription termination. These results lead to a model for termination, the "sitting duck torpedo" mechanism, where poly(A) site-dependent deceleration caused by PNUTS-PP1 and Spt5 dephosphorylation is required to convert Pol II into a viable target for the Xrn2 terminator exonuclease. Spt5 and its bacterial homolog NusG therefore have related functions controlling kinetic competition between RNA polymerases and the termination factors that pursue them.

Widespread Backtracking by RNA Pol II Is a Major Effector of Gene Activation, 5' Pause Release, Termination, and Transcription Elongation Rate

In addition to phosphodiester bond formation, RNA polymerase II has an RNA endonuclease activity, stimulated by TFIIS, which rescues complexes that have arrested and backtracked. How TFIIS affects transcription under normal conditions is poorly understood. We identified backtracking sites in human cells using a dominant-negative TFIIS (TFIIS_DN) that inhibits RNA cleavage and stabilizes backtracked complexes. Backtracking is most frequent within 2 kb of start sites, consistent with slow elongation early in transcription, and in 3' flanking regions where termination is enhanced by TFIIS_DN, suggesting that backtracked pol II is a favorable substrate for termination. Rescue from backtracking by RNA cleavage also promotes escape from 5' pause sites, prevents premature termination of long transcripts, and enhances activation of stress-inducible genes. TFIIS_DN slowed elongation rates genome-wide by half, suggesting that rescue of backtracked pol II by TFIIS is a major stimulus of elongation under normal conditions.

Roles of CDKs in RNA polymerase II transcription of the HIV-1 genome

Studies of RNA Polymerase II (Pol II) transcription of the HIV-1 genome are of clinical interest, as the insight gained may lead to strategies to selectively reactivate latent viruses in patients in whom viral replication is suppressed by antiviral drugs. Such a targeted reactivation may contribute to a functional cure of infection. This review discusses five Cyclin-dependent kinases - CDK7, CDK9, CDK11, CDK2, and CDK8 - involved in transcription and processing of HIV-1 RNA. CDK7 is required for Pol II promoter clearance of reactivated viruses; CDK7 also functions as an activating kinase for CDK9 when resting CD4⁺ T cells harboring latent HIV-1 are activated. CDK9 is targeted by the viral Tat protein and is essential for productive Pol II elongation of the HIV-1 genome. CDK11 is associated with the TREX/THOC complex and it functions in the 3' end processing and polyadenylation of HIV-1 transcripts. CDK2 phosphorylates Tat and CDK9 and this stimulates Tat activation of Pol II transcription. CDK8 may stimulate Pol II transcription of the HIV-1 genome through co-recruitment with NF-κB to the viral promoter. Some notable open questions are discussed concerning the roles of these CDKs in HIV-1 replication and viral latency.

Targeting the Latent Reservoir for HIV-1

Antiretroviral therapy can effectively block HIV-1 replication and prevent or reverse immunodeficiency in HIV-1-infected individuals. However, viral replication resumes within weeks of treatment interruption. The major barrier to a cure is a small pool of resting memory CD4+ T cells that harbor latent HIV-1 proviruses. This latent reservoir is now the focus of an intense international research effort. We describe how the reservoir is established, challenges involved in eliminating it, and pharmacologic and immunologic strategies for targeting this reservoir. The development of a successful cure strategy will most likely require understanding the mechanisms that maintain HIV-1 proviruses in a latent state and pathways that drive the proliferation of infected cells, which slows reservoir decay. In addition, a cure will require the development of effective immunologic approaches to eliminating infected cells. There is renewed optimism about the prospect of a cure, and the interventions discussed here could pave the way.

Cyclin-dependent kinase 7 (CDK7)-mediated phosphorylation of the CDK9 activation loop promotes P-TEFb assembly with Tat and proviral HIV reactivation

The HIV trans-activator Tat recruits the host transcription elongation factor P-TEFb to stimulate proviral transcription. Phosphorylation of Thr-186 on the activation loop (T-loop) of cyclin-dependent kinase 9 (CDK9) is essential for its kinase activity and assembly of CDK9 and cyclin T1 (CycT1) to form functional P-TEFb. Phosphorylation of a second highly conserved T-loop site, Ser-175, alters the competitive binding of Tat and the host recruitment factor bromodomain containing 4 (BRD4) to P-TEFb. Here, we investigated the intracellular mechanisms that regulate these key phosphorylation events required for HIV transcription. Molecular dynamics simulations revealed that the CDK9/CycT1 interface is stabilized by intramolecular hydrogen bonding of pThr-186 by an arginine triad and Glu-96 of CycT1. Arginine triad substitutions that disrupted CDK9/CycT1 assembly accumulated Thr-186-dephosphorylated CDK9 associated with the cytoplasmic Hsp90/Cdc37 chaperone. The Hsp90/Cdc37/CDK9 complex was also present in resting T cells, which lack CycT1. Hsp90 inhibition in primary T cells blocked P-TEFb assembly, disrupted Thr-186 phosphorylation, and suppressed proviral reactivation. The selective CDK7 inhibitor THZ1 blocked CDK9 phosphorylation at Ser-175, and in vitro kinase assays confirmed that CDK7 activity is principally responsible for Ser-175 phosphorylation. Mutation of Ser-175 to Lys had no effect on CDK9 kinase activity or P-TEFb assembly but strongly suppressed both HIV expression and BRD4 binding. We conclude that the transfer of CDK9 from the Hsp90/Cdc37 complex induced by Thr-186 phosphorylation is a key step in P-TEFb biogenesis. Furthermore, we demonstrate that CDK7-mediated Ser-175 phosphorylation is a downstream nuclear event essential for facilitating CDK9 T-loop interactions with Tat.

Relationships Between RNA Polymerase II Activity and Spt Elongation Factors to Spt- Phenotype and Growth in Saccharomyces cerevisiae

The interplay between adjacent transcription units can result in transcription-dependent alterations in chromatin structure or recruitment of factors that determine transcription outcomes, including the generation of intragenic or other cryptic transcripts derived from cryptic promoters. Mutations in a number of genes in Saccharomyces cerevisiae confer both cryptic intragenic transcription and the Suppressor of Ty (Spt^-) phenotype for the lys2-128∂ allele of the LYS2 gene. Mutants that suppress lys2-128∂ allow transcription from a normally inactive Ty1 ∂ promoter, conferring a LYS⁺ phenotype. The arrangement of transcription units at lys2-128∂ is reminiscent of genes containing cryptic promoters within their open reading frames. We set out to examine the relationship between RNA Polymerase II (Pol II) activity, functions of Spt elongation factors, and cryptic transcription because of the previous observation that increased-activity Pol II alleles confer an Spt^- phenotype. We identify both cooperating and antagonistic genetic interactions between Pol II alleles and alleles of elongation factors SPT4, SPT5, and SPT6. We find that cryptic transcription at FLO8 and STE11 is distinct from that at lys2-128∂, though all show sensitivity to reduction in Pol II activity, especially the expression of lys2-128∂ found in Spt^- mutants. We determine that the lys2-128∂ Spt^- phenotypes for spt6-1004 and increased activity rpo21/rpb1 alleles each require transcription from the LYS2 promoter. Furthermore, we identify the Ty1 transcription start site (TSS) within the ∂ element as the position of Spt^- transcription in tested Spt^- mutants.

Biochemical Analysis of Yeast Suppressor of Ty 4/5 (Spt4/5) Reveals the Importance of Nucleic Acid Interactions in the Prevention of RNA Polymerase II Arrest

RNA polymerase II (RNAPII) undergoes structural changes during the transitions from initiation, elongation, and termination, which are aided by a collection of proteins called elongation factors. NusG/Spt5 is the only elongation factor conserved in all domains of life. Although much information exists about the interactions between NusG/Spt5 and RNA polymerase in prokaryotes, little is known about how the binding of eukaryotic Spt4/5 affects the biochemical activities of RNAPII. We characterized the activities of Spt4/5 and interrogated the structural features of Spt5 required for it to interact with elongation complexes, bind nucleic acids, and promote transcription elongation. The eukaryotic specific regions of Spt5 containing the Kyrpides, Ouzounis, Woese domains are involved in stabilizing the association with the RNAPII elongation complex, which also requires the presence of the nascent transcript. Interestingly, we identify a region within the conserved NusG N-terminal (NGN) domain of Spt5 that contacts the non-template strand of DNA both upstream of RNAPII and in the transcription bubble. Mutating charged residues in this region of Spt5 did not prevent Spt4/5 binding to elongation complexes, but abrogated the cross-linking of Spt5 to DNA and the anti-arrest properties of Spt4/5, thus suggesting that contact between Spt5 (NGN) and DNA is required for Spt4/5 to promote elongation. We propose that the mechanism of how Spt5/NGN promotes elongation is fundamentally conserved; however, the eukaryotic specific regions of the protein evolved so that it can serve as a platform for other elongation factors and maintain its association with RNAPII as it navigates genomes packaged into chromatin.

The effects of cocaine on HIV transcription

Illicit drug users are a high-risk population for infection with the human immunodeficiency virus (HIV). A strong correlation exists between prohibited drug use and an increased rate of HIV transmission. Cocaine stands out as one of the most frequently abused illicit drugs, and its use is correlated with HIV infection and disease progression. The central nervous system (CNS) is a common target for both drugs of abuse and HIV, and cocaine intake further accelerates neuronal injury in HIV patients. Although the high incidence of HIV infection in illicit drug abusers is primarily due to high-risk activities such as needle sharing and unprotected sex, several studies have demonstrated that cocaine enhances the rate of HIV gene expression and replication by activating various signal transduction pathways and downstream transcription factors. In order to generate mature HIV genomic transcript, HIV gene expression has to pass through both the initiation and elongation phases of transcription, which requires discrete transcription factors. In this review, we will provide a detailed analysis of the molecular mechanisms that regulate HIV transcription and discuss how cocaine modulates those mechanisms to upregulate HIV transcription and eventually HIV replication.

Structures and Functions of the Multiple KOW Domains of Transcription Elongation Factor Spt5

The eukaryotic Spt4-Spt5 heterodimer forms a higher-order complex with RNA polymerase II (and I) to regulate transcription elongation. Extensive genetic and functional data have revealed diverse roles of Spt4-Spt5 in coupling elongation with chromatin modification and RNA-processing pathways. A mechanistic understanding of the diverse functions of Spt4-Spt5 is hampered by challenges in resolving the distribution of functions among its structural domains, including the five KOW domains in Spt5, and a lack of their high-resolution structures. We present high-resolution crystallographic results demonstrating that distinct structures are formed by the first through third KOW domains (KOW1-Linker1 [K1L1] and KOW2-KOW3) of Saccharomyces cerevisiae Spt5. The structure reveals that K1L1 displays a positively charged patch (PCP) on its surface, which binds nucleic acids in vitro, as shown in biochemical assays, and is important for in vivo function, as shown in growth assays. Furthermore, assays in yeast have shown that the PCP has a function that partially overlaps that of Spt4. Synthesis of our results with previous evidence suggests a model in which Spt4 and the K1L1 domain of Spt5 form functionally overlapping interactions with nucleic acids upstream of the transcription bubble, and this mechanism may confer robustness on processes associated with transcription elongation.

Cocaine promotes both initiation and elongation phase of HIV-1 transcription by activating NF-κB and MSK1 and inducing selective epigenetic modifications at HIV-1 LTR

Cocaine accelerates human immunodeficiency virus (HIV-1) replication by altering specific cell-signaling and epigenetic pathways. We have elucidated the underlying molecular mechanisms through which cocaine exerts its effect in myeloid cells, a major target of HIV-1 in central nervous system (CNS). We demonstrate that cocaine treatment promotes HIV-1 gene expression by activating both nuclear factor-kappa B (NF-ĸB) and mitogen- and stress-activated kinase 1 (MSK1). MSK1 subsequently catalyzes the phosphorylation of histone H3 at serine 10, and p65 subunit of NF-ĸB at 276th serine residue. These modifications enhance the interaction of NF-ĸB with P300 and promote the recruitment of the positive transcription elongation factor b (P-TEFb) to the HIV-1 LTR, supporting the development of an open/relaxed chromatin configuration, and facilitating the initiation and elongation phases of HIV-1 transcription. Results are also confirmed in primary monocyte derived macrophages (MDM). Overall, our study provides detailed insights into cocaine-driven HIV-1 transcription and replication.

Short chain fatty acids potently induce latent HIV-1 in T-cells by activating P-TEFb and multiple histone modifications

HIV patients with severe periodontitis have high levels of residual virus in their saliva and plasma despite effective therapy (HAART). Multiple short chain fatty acids (SCFAs) from periodontal pathogens reactivate HIV-1 in both Jurkat and primary T-cell models of latency. SCFAs not only activate positive transcription elongation factor b (P-TEFb), which is an essential cellular cofactor for Tat, but can also reverse chromatin blocks by inducing histone modifications. SCFAs simultaneously increase histone acetylation by inhibiting class-1/2 histone deacetylases (HDACs) and decrease repressive histone tri-methylation at the proviral LTR by downregulating expression of the class-3 HDAC sirtuin-1 (SIRT1), and the histone methyltransferases enhancer of Zeste homolog 2 (EZH2) and suppressor of variegation 3-9 homolog 1 (SUV39H1). Our findings provide a mechanistic link between periodontal disease and enhanced HIV-1 replication, and suggest that treatment of periodontal disease, or blocking the activities of SCFAs, will have a therapeutic benefit for HIV patients.

Development of a sensitive amplified luminescent proximity homogeneous assay to monitor the interactions between pTEFb and Tat

The viral transactivator protein (Tat) plays an essential role in the replication of human immunodeficiency type 1 virus (HIV-1) by recruiting the host positive transcription elongation factor (pTEFb) to the RNA polymerase II transcription machinery to enable an efficient HIV-1 RNA elongation process. Blockade of the interaction between Tat and pTEFb represents a novel strategy for developing a new class of antiviral agents. In this study, we developed a homogeneous assay in AlphaLISA (amplified luminescent proximity homogeneous assay) format using His-tagged pTEFb and biotinylated Tat to monitor the interaction between Tat and pTEFb. On optimizing the assay conditions, the signal-to-background ratio was found to be greater than 10-fold. The assay was validated with untagged Tat and peptides known to compete with Tat for pTEFb binding. The Z' of the assay is greater than 0.5, indicating that the assay is robust and can be easily adapted to a high-throughput screening format. Furthermore, the affinity between Tat and pTEFb was determined to be approximately 20 pM, and only 7% of purified Tat was found to be active in forming tertiary complex with pTEFb. Development of this assay should facilitate the discovery of a new class of antiviral agents providing HIV-1 patients with broader treatment choices.

A source of the single-stranded DNA substrate for activation-induced deaminase during somatic hypermutation

During somatic hypermutation (SHM), activation-induced deaminase (AID) mutates deoxycytidine on single-stranded DNA (ssDNA) generated by the transcription machinery, but the detailed mechanism remains unclear. Here we report a higher abundance of RNA polymerase II (Pol II) at the immunoglobulin heavy-chain variable (Igh-V) region compared with the constant region and partially transcribed Igh RNAs, suggesting a slower Pol II progression at Igh-V that could result in some early/premature transcription termination after prolonged pausing/stalling of Pol II. Knocking down RNA-exosome complexes, which could decrease premature transcription termination, leads to decreased SHM. Knocking down Spt5, which can augment premature transcription termination, leads to increase in both, SHM and the abundance of ssDNA substrates. Collectively, our data support the model that, following the reduction of Pol II progression (pausing or stalling) at the Igh-V, additional steps such as premature transcription termination are involved in providing ssDNA substrates for AID during SHM.

Negative elongation factor is required for the maintenance of proviral latency but does not induce promoter-proximal pausing of RNA polymerase II on the HIV long terminal repeat

The role of the negative elongation factor (NELF) in maintaining HIV latency was investigated following small hairpin RNA (shRNA) knockdown of the NELF-E subunit, a condition that induced high levels of proviral transcription in latently infected Jurkat T cells. Chromatin immunoprecipitation (ChIP) assays showed that latent proviruses accumulate RNA polymerase II (RNAP II) on the 5' long terminal repeat (LTR) but not on the 3' LTR. NELF colocalizes with RNAP II, and its level increases following proviral induction. RNAP II pause sites on the HIV provirus were mapped to high resolution by ChIP with high-throughput sequencing (ChIP-Seq). Like cellular promoters, RNAP II accumulates at around position +30, but HIV also shows additional pausing at +90, which is immediately downstream of a transactivation response (TAR) element and other distal sites on the HIV LTR. Following NELF-E knockdown or tumor necrosis factor alpha (TNF-α) stimulation, promoter-proximal RNAP II levels increase up to 3-fold, and there is a dramatic increase in RNAP II levels within the HIV genome. These data support a kinetic model for proviral transcription based on continuous replacement of paused RNAP II during both latency and productive transcription. In contrast to most cellular genes, HIV is highly activated by the combined effects of NELF-E depletion and activation of initiation by TNF-α, suggesting that opportunities exist to selectively activate latent HIV proviruses.

Transcriptional control of HIV latency: cellular signaling pathways, epigenetics, happenstance and the hope for a cure

Replication-competent latent HIV-1 proviruses that persist in the genomes of a very small subset of resting memory T cells in infected individuals under life-long antiretroviral therapy present a major barrier towards viral eradication. Multiple molecular mechanisms are required to repress the viral trans-activating factor Tat and disrupt the regulatory Tat feedback circuit leading to the establishment of the latent viral reservoir. In particular, latency is due to a combination of transcriptional silencing of proviruses via host epigenetic mechanisms and restrictions on the expression of P-TEFb, an essential co-factor for Tat. Induction of latent proviruses in the presence of antiretroviral therapy is expected to enable clearance of latently infected cells by viral cytopathic effects and host antiviral immune responses. An in-depth comprehensive understanding of the molecular control of HIV-1 transcription should inform the development of optimal combinatorial reactivation strategies that are intended to purge the latent viral reservoir.

Reactivation of latent HIV-1 in central memory CD4⁺ T cells through TLR-1/2 stimulation

BACKGROUND

Toll-like receptors (TLRs) are crucial for recognition of pathogen-associated molecular patterns by cells of the innate immune system. TLRs are present and functional in CD4⁺ T cells. Memory CD4⁺ T cells, predominantly central memory cells (TCM), constitute the main reservoir of latent HIV-1. However, how TLR ligands affect the quiescence of latent HIV within central memory CD4⁺ T cells has not been studied.

RESULTS

We evaluated the ability of a broad panel of TLR agonists to reactivate latent HIV-1. The TLR-1/2 agonist Pam3CSK4 leads to viral reactivation of quiescent HIV in a model of latency based on cultured TCM and in resting CD4⁺ T cells isolated from aviremic patients. In addition, we investigated the signaling pathway associated with Pam3CSK4 involved in HIV-1 reactivation. We show that the transcription factors NFκB, NFAT and AP-1 cooperate to induce viral reactivation downstream of TLR-1/2 stimulation. Furthermore, increasing levels of cyclin T1 is not required for TLR-mediated viral reactivation, but induction of viral expression requires activated pTEFb. Finally, Pam3CSK4 reactivates latent HIV-1 in the absence of T cell activation or proliferation, in contrast to antigen stimulation.

CONCLUSIONS

Our findings suggest that the signaling through TLR-1/2 pathway via Pam3CSK4 or other reagents should be explored as an anti-latency strategy either alone or in combination with other anti-latency drugs.

Transcriptional and posttranscriptional regulation of HIV-1 gene expression

Control of HIV-1 gene expression depends on two viral regulatory proteins, Tat and Rev. Tat stimulates transcription elongation by directing the cellular transcriptional elongation factor P-TEFb to nascent RNA polymerases. Rev is required for the transport from the nucleus to the cytoplasm of the unspliced and incompletely spliced mRNAs that encode the structural proteins of the virus. Molecular studies of both proteins have revealed how they interact with the cellular machinery to control transcription from the viral LTR and regulate the levels of spliced and unspliced mRNAs. The regulatory feedback mechanisms driven by HIV-1 Tat and Rev ensure that HIV-1 transcription proceeds through distinct phases. In cells that are not fully activated, limiting levels of Tat and Rev act as potent blocks to premature virus production.

Phosphorylation of CDK9 at Ser175 enhances HIV transcription and is a marker of activated P-TEFb in CD4(+) T lymphocytes

The HIV transactivator protein, Tat, enhances HIV transcription by recruiting P-TEFb from the inactive 7SK snRNP complex and directing it to proviral elongation complexes. To test the hypothesis that T-cell receptor (TCR) signaling induces critical post-translational modifications leading to enhanced interactions between P-TEFb and Tat, we employed affinity purification–tandem mass spectrometry to analyze P-TEFb. TCR or phorbal ester (PMA) signaling strongly induced phosphorylation of the CDK9 kinase at Ser175. Molecular modeling studies based on the Tat/P-TEFb X-ray structure suggested that pSer175 strengthens the intermolecular interactions between CDK9 and Tat. Mutations in Ser175 confirm that this residue could mediate critical interactions with Tat and with the bromodomain protein BRD4. The S175A mutation reduced CDK9 interactions with Tat by an average of 1.7-fold, but also completely blocked CDK9 association with BRD4. The phosphomimetic S175D mutation modestly enhanced Tat association with CDK9 while causing a 2-fold disruption in BRD4 association with CDK9. Since BRD4 is unable to compete for binding to CDK9 carrying S175A, expression of CDK9 carrying the S175A mutation in latently infected cells resulted in a robust Tat-dependent reactivation of the provirus. Similarly, the stable knockdown of BRD4 led to a strong enhancement of proviral expression. Immunoprecipitation experiments show that CDK9 phosphorylated at Ser175 is excluded from the 7SK RNP complex. Immunofluorescence and flow cytometry studies carried out using a phospho-Ser175-specific antibody demonstrated that Ser175 phosphorylation occurs during TCR activation of primary resting memory CD4+ T cells together with upregulation of the Cyclin T1 regulatory subunit of P-TEFb, and Thr186 phosphorylation of CDK9. We conclude that the phosphorylation of CDK9 at Ser175 plays a critical role in altering the competitive binding of Tat and BRD4 to P-TEFb and provides an informative molecular marker for the identification of the transcriptionally active form of P-TEFb.

The release of the transcription elongation factor P-TEFb from the 7SK RNP complex and its binding to the HIV Tat transactivator protein enables the efficient transcription of HIV proviruses. In resting memory T-cells, which carry the bulk of the latent HIV viral pool, limiting the cellular levels of P-TEFb ensures that the provirus remains silenced unless the host cell is activated. Here we demonstrate that T-cell receptor (TCR) activation induces phosphorylation of Ser175, a residue which is located at the interface between CycT1, CDK9 and Tat. Phosphorylation of Ser175 occurs on free or 7SK-dissociated P-TEFb and genetic experiments indicate that this modification enhances P-TEFb interaction with Tat resulting in Tat-dependent reactivation of HIV proviral transcription. Modification of Ser175 appears critical for controlling the competitive binding of Tat and the bromodomain protein BRD4 to P-TEFb. Activation of P-TEFb in resting T-cells thus involves both the initial assembly of the 7SK snRNP complex and the subsequent mobilization of P-TEFb by cellular signaling and Tat. Therefore, pSer175 provides an informative molecular marker for the identification of the transcriptionally active form of P-TEFb that can be used to monitor the extent of T-cell activation during therapeutic interventions aimed at virus eradication.

HIV latency

HIV-1 can establish a state of latent infection at the level of individual T cells. Latently infected cells are rare in vivo and appear to arise when activated CD4(+) T cells, the major targets cells for HIV-1, become infected and survive long enough to revert back to a resting memory state, which is nonpermissive for viral gene expression. Because latent virus resides in memory T cells, it persists indefinitely even in patients on potent antiretroviral therapy. This latent reservoir is recognized as a major barrier to curing HIV-1 infection. The molecular mechanisms of latency are complex and include the absence in resting CD4(+) T cells of nuclear forms of key host transcription factors (e.g., NFκB and NFAT), the absence of Tat and associated host factors that promote efficient transcriptional elongation, epigenetic changes inhibiting HIV-1 gene expression, and transcriptional interference. The presence of a latent reservoir for HIV-1 helps explain the presence of very low levels of viremia in patients on antiretroviral therapy. These viruses are released from latently infected cells that have become activated and perhaps from other stable reservoirs but are blocked from additional rounds of replication by the drugs. Several approaches are under exploration for reactivating latent virus with the hope that this will allow elimination of the latent reservoir.

Human immunodeficiency virus (HIV) latency: the major hurdle in HIV eradication

Failure of highly active antiretroviral therapy to eradicate the human immunodeficiency virus (HIV), even in patients who suppress the virus to undetectable levels for many years, underscores the problems associated with fighting this infection. The existence of persistent infection in certain cellular and anatomical reservoirs appears to be the major hurdle in HIV eradication. The development of therapeutic interventions that eliminate or limit the latent viral pools or prevent the reemergence of the viruses from producing cells will therefore be required to enhance the effectiveness of current antiretroviral strategies. To achieve this goal, there is a pressing need to understand HIV latency at the molecular level to design novel and improved therapies to either eradicate HIV or find a functional cure in which patients could maintain a manageable viral pool without AIDS in the absence of antiretroviral therapy. The integrated proviral genome remains transcriptionally silent for a long period in certain subsets of T cells. This ability to infect cells latently helps HIV to establish a persistent infection despite strong humoral and cellular immune responses against the viral proteins. The main purpose of this report is to provide a general overview of the HIV latency. We will describe the hurdles being faced in eradicating latent HIV proviruses. We will also briefly discuss the ongoing strategies aimed toward curing HIV infection.

The Spt4-Spt5 complex: a multi-faceted regulator of transcription elongation

In all domains of life, elongating RNA polymerases require the assistance of accessory factors to maintain their processivity and regulate their rate. Among these elongation factors, the Spt5/NusG factors stand out. Members of this protein family appear to be the only transcription accessory proteins that are universally conserved across all domains of life. In archaea and eukaryotes, Spt5 associates with a second protein, Spt4. In addition to regulating elongation, the eukaryotic Spt4-Spt5 complex appears to couple chromatin modification states and RNA processing to transcription elongation. This review discusses the experimental bases for our current understanding of Spt4-Spt5 function and recent studies that are beginning to elucidate the structure of Spt4-Spt5/RNA polymerase complexes and mechanism of Spt4-Spt5 action. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

Control of HIV latency by epigenetic and non-epigenetic mechanisms

Intensive antiretroviral therapy successfully suppresses viral replication but is unable to eradicate the virus. HIV persists in a small number of resting memory T cells where HIV has been transcriptionally silenced. This review will focus on recent insights into the HIV transcriptional control mechanisms that provide the biochemical basis for understanding latency. There are no specific repressors of HIV transcription encoded by the virus, instead latency arises when the regulatory feedback mechanism driven by HIV Tat expression is disrupted. Small changes in transcriptional initiation, induced by epigenetic silencing, lead to profound restrictions in Tat levels and force the entry of proviruses into latency. In resting memory T cells, which carry the bulk of the latent viral pool, additional restrictions, especially the limiting cellular levels of the essential Tat cofactor P-TEFb and the transcription initiation factors NF-κB and NFAT ensure that the provirus remains silenced unless the host cell is activated. The detailed understanding of HIV transcription is providing a framework for devising new therapeutic strategies designed to purge the latent viral pool. Importantly, the recognition that there are multiple restrictions imposed on latent proviruses suggest that proviral reactivation will not be achieved when only a single reactivation step is targeted and that any optimal activation strategy will require both removal of epigenetic blocks and the activation of P-TEFb.

The Roles of the Paf1 Complex and Associated Histone Modifications in Regulating Gene Expression

The conserved Paf1 complex (Paf1C) carries out multiple functions during transcription by RNA polymerase (pol) II, and these functions are required for the proper expression of numerous genes in yeast and metazoans. In the elongation stage of the transcription cycle, the Paf1C associates with RNA pol II, interacts with other transcription elongation factors, and facilitates modifications to the chromatin template. At the end of elongation, the Paf1C plays an important role in the termination of RNA pol II transcripts and the recruitment of proteins required for proper RNA 3′ end formation. Significantly, defects in the Paf1C are associated with several human diseases. In this paper, we summarize current knowledge on the roles of the Paf1C in RNA pol II transcription.

RNA polymerase II stalling promotes nucleosome occlusion and pTEFb recruitment to drive immortalization by Epstein-Barr virus

Epstein-Barr virus (EBV) immortalizes resting B-cells and is a key etiologic agent in the development of numerous cancers. The essential EBV-encoded protein EBNA 2 activates the viral C promoter (Cp) producing a message of ~120 kb that is differentially spliced to encode all EBNAs required for immortalization. We have previously shown that EBNA 2-activated transcription is dependent on the activity of the RNA polymerase II (pol II) C-terminal domain (CTD) kinase pTEFb (CDK9/cyclin T1). We now demonstrate that Cp, in contrast to two shorter EBNA 2-activated viral genes (LMP 1 and 2A), displays high levels of promoter-proximally stalled pol II despite being constitutively active. Consistent with pol II stalling, we detect considerable pausing complex (NELF/DSIF) association with Cp. Significantly, we observe substantial Cp-specific pTEFb recruitment that stimulates high-level pol II CTD serine 2 phosphorylation at distal regions (up to +75 kb), promoting elongation. We reveal that Cp-specific pol II accumulation is directed by DNA sequences unfavourable for nucleosome assembly that increase TBP access and pol II recruitment. Stalled pol II then maintains Cp nucleosome depletion. Our data indicate that pTEFb is recruited to Cp by the bromodomain protein Brd4, with polymerase stalling facilitating stable association of pTEFb. The Brd4 inhibitor JQ1 and the pTEFb inhibitors DRB and Flavopiridol significantly reduce Cp, but not LMP1 transcript production indicating that Brd4 and pTEFb are required for Cp transcription. Taken together our data indicate that pol II stalling at Cp promotes transcription of essential immortalizing genes during EBV infection by (i) preventing promoter-proximal nucleosome assembly and ii) necessitating the recruitment of pTEFb thereby maintaining serine 2 CTD phosphorylation at distal regions.

T-cell receptor signaling enhances transcriptional elongation from latent HIV proviruses by activating P-TEFb through an ERK-dependent pathway

Latent human immunodeficiency virus (HIV) proviruses are thought to be primarily reactivated in vivo through stimulation of the T-cell receptor (TCR). Activation of the TCR induces multiple signal transduction pathways, leading to the ordered nuclear migration of the HIV transcription initiation factors NF-κB (nuclear factor κB) and NFAT (nuclear factor of activated T-cells), as well as potential effects on HIV transcriptional elongation. We have monitored the kinetics of proviral reactivation using chromatin immunoprecipitation assays to measure changes in the distribution of RNA polymerase II in the HIV provirus. Surprisingly, in contrast to TNF-α (tumor necrosis factor α) activation, where early transcription elongation is highly restricted due to rate-limiting concentrations of Tat, efficient and sustained HIV elongation and positive transcription elongation factor b (P-TEFb) recruitment are detected immediately after the activation of latent proviruses through the TCR. Inhibition of NFAT activation by cyclosporine had no effect on either HIV transcription initiation or elongation. However, examination of P-TEFb complexes by gel-filtration chromatography showed that TCR signaling led to the rapid dissociation of the large inactive P-TEFb:7SK RNP (small nuclear RNA 7SK ribonucleoprotein) complex and the release of active low-molecular-weight P-TEFb complexes. Both P-TEFb recruitment to the HIV long terminal repeat and enhanced HIV processivity were blocked by the ERK (extracellular-signal-regulated kinase) inhibitor U0126, but not by AKT (serine/threonine protein kinase Akt) and PI3K (phosphatidylinositol 3-kinase) inhibitors. In contrast to treatment with HMBA (hexamethylene bisacetamide) and DRB (5,6-dichlorobenzimidazole 1-β-ribofuranoside), which disrupt the large 7SK RNP complex but do not stimulate early HIV elongation, TCR signaling provides the first example of a physiological pathway that can shift the balance between the inactive P-TEFb pool and the active P-TEFb pool and thereby stimulate proviral reactivation.

Celastrol inhibits Tat-mediated human immunodeficiency virus (HIV) transcription and replication

Current drugs used for antiretroviral therapy against human immunodeficiency virus (HIV) have a narrow spectrum of activity and, more often, have associated toxicities and severe side effects in addition to developing resistance. Thus, there is a need to develop new therapeutic strategies against HIV/AIDS to complement the already existing ones. Surprisingly, transactivator of transcription (Tat), an early virus-encoded protein required for the efficient transcription of the HIV genome, has not been developed as a target for small molecular therapeutics. We have previously described the ability of an endogenous Michael acceptor electrophile (MAE), 15-deoxy-Δ(12,14)-prostaglandin J(2) (15d-PGJ(2)), to inhibit Tat-dependent transcription by targeting its cysteine (Cys)-rich domain. In an effort to identify other MAEs possessing inhibitory activity against HIV-1 Tat, we tested a collection of plant-derived compounds with electrophilic properties, including curcumin, rosmarinic acid, and gambogic acid, for their ability to inhibit Tat. Celastrol (Cel), a triterpenoid MAE isolated from Tripterygium wilfordii, exhibited the highest inhibitory activity against Tat. Using biochemical techniques, we demonstrate that Cel, by covalently modifying the cysteine thiols, inhibits Tat transactivation function. Using circular dichroism spectroscopy, we show that alkylation of Tat brought about a change in the secondary structure of Tat, which inhibited the transcription elongation of the HIV proviral genome by effecting mechanisms other than Tat-TAR (transactivation-responsive region) interaction. Our results demonstrate the underlying mechanism of antiretroviral activity of the plant-derived MAEs and suggest that Cel could serve as a lead compound to develop novel antiviral therapeutics.

Inhibition of both HIV-1 reverse transcription and gene expression by a cyclic peptide that binds the Tat-transactivating response element (TAR) RNA

The RNA response element TAR plays a critical role in HIV replication by providing a binding site for the recruitment of the viral transactivator protein Tat. Using a structure-guided approach, we have developed a series of conformationally-constrained cyclic peptides that act as structural mimics of the Tat RNA binding region and block Tat-TAR interactions at nanomolar concentrations in vitro. Here we show that these compounds block Tat-dependent transcription in cell-free systems and in cell-based reporter assays. The compounds are also cell permeable, have low toxicity, and inhibit replication of diverse HIV-1 strains, including both CXCR4-tropic and CCR5-tropic primary HIV-1 isolates of the divergent subtypes A, B, C, D and CRF01_AE. In human peripheral blood mononuclear cells, the cyclic peptidomimetic L50 exhibited an IC₅₀ ∼250 nM. Surprisingly, inhibition of LTR-driven HIV-1 transcription could not account for the full antiviral activity. Timed drug-addition experiments revealed that L-50 has a bi-phasic inhibition curve with the first phase occurring after HIV-1 entry into the host cell and during the initiation of HIV-1 reverse transcription. The second phase coincides with inhibition of HIV-1 transcription. Reconstituted reverse transcription assays confirm that HIV-1 (−) strand strong stop DNA synthesis is blocked by L50-TAR RNA interactions in-vitro. These findings are consistent with genetic evidence that TAR plays critical roles both during reverse transcription and during HIV gene expression. Our results suggest that antiviral drugs targeting TAR RNA might be highly effective due to a dual inhibitory mechanism.

The HIV-1 transactivator protein (Tat), together with the elongation factor P-TEFb binds to an HIV-1 RNA secondary structure in the 5′-UTRs of nascent viral mRNAs (TAR) and promotes transcription elongation. This process has been an attractive target for drug development but previous inhibitors that bind either Tat or TAR have been plagued by poor inhibition of virus replication, limited cell penetration, and off-target effects. In this article, we describe a series of rationally designed cyclic peptides that block Tat-TAR interactions. L50, the most potent of these compounds, inhibits a wide range of HIV-1 strains from around the world. Remarkably, L50 inhibits two distinct steps in the HIV-1 lifecycle. As expected, L50 inhibits Tat-dependent HIV-1 transcription, but the majority of its anti-HIV activity is due to a block in reverse transcription, i.e. synthesis of the proviral DNA from the RNA genome. L50 inhibition of reverse transcription reveals an important role for TAR RNA during reverse transcription as well as providing one of first examples of a drug with a dual mechanism of action.

The molecular biology of HIV latency: breaking and restoring the Tat-dependent transcriptional circuit

PURPOSE OF REVIEW

Despite the remarkable success of intensive antiretroviral drug therapy in blocking the HIV replication, the virus persists in a small number of cells in which HIV has been transcriptionally silenced. This review will focus on recent insights into the HIV transcriptional control mechanisms that provide the biochemical basis for understanding latency.

RECENT FINDINGS

Latency arises when the regulatory feedback mechanism driven by HIV Tat expression is disrupted. Small changes in transcriptional initiation, induced by epigenetic silencing, can lead to restrictions in Tat levels and entry of proviruses into latency. In resting memory T-cells, which carry the bulk of the latent viral pool, additional restrictions limiting cellular levels of the essential Tat cofactor P-TEFb and the transcription initiation factors nuclear factor kappa B and nuclear factor of activated T cells ensure that the provirus remains silenced unless the host cell is activated.

SUMMARY

Strategies to purge the latent proviral pool require nontoxic activator molecules. The multiple restrictions imposed on latent proviruses that need to be overcome suggest that proviral reactivation will not be achieved when only a single reactivation step is targeted but will require both removal of epigenetic blocks and the activation of P-TEFb. Alternatively, new inhibitors that block proviral reactivation could be developed.

Curing HIV: Pharmacologic approaches to target HIV-1 latency

HIV-1 infection persists even after years of antiretroviral therapy (ART). Although ART can halt viral replication and thereby reduce viremia to clinically undetectable levels, proviral latency established within the host genome remains largely unaffected by ART and can replenish systemic infection following interruption of therapy. Pharmacologic strategies, which not only target viral replication but also deplete proviral infection, are required for successful clearance of HIV-1 infection. This review highlights the current understanding of molecular mechanisms that establish and maintain HIV-1 latency in its major reservoir, the resting memory CD4(+) T cell. We also identify the molecular targets that might be exploited to induce HIV-1 expression, remove epigenetic restrictions, or enhance effective transcription. Finally, we discuss the potential pharmacologic approaches toward targeting viral persistence in different cellular and anatomical reservoirs to achieve a cure of HIV-1 infection.

Promoter-proximal pausing of RNA polymerase II: an opportunity to regulate gene transcription

Transcription of eukaryotic genes by RNA polymerase II (pol II) is a complex, highly regulated multiphasic process. Pol II pauses in the proximity of the promoter on a large fraction of transcribed genes. Transcription initiation and elongation of transcripts are under distinct control. Induced gene expression can thus be due to enhanced initiation and/or stimulated elongation. Pausing and resumption of the elongation of transcripts is under the control of transcription elongation factors. Three of them, P-TEFb, DSIF, and NELF have been well characterized as protein complexes with multiple general but also gene specific functions. Elongation factors execute checkpoint functions but serve also as targets for signaling processes which regulate gene expression. Due to the general importance of transcription elongation factors, it is difficult to delineate the mechanisms by which elongation of specific genes is regulated by specific intracellular signals. However, it is clear that the controlled pausing of pol II provides an opportunity to finely control timing and quantity of transcriptional output.

CDK8 is a positive regulator of transcriptional elongation within the serum response network

The Mediator complex allows communication between transcription factors and RNA polymerase II (RNAPII). CDK8, the kinase found in some variants of Mediator, has been characterized mostly as a transcriptional repressor. Recently, CDK8 was demonstrated to be a potent oncoprotein. Here we show that CDK8 is a positive regulator of genes within the serum response network, including several members of the AP-1 and EGR family of oncogenic transcription factors. Mechanistic studies demonstrate that CDK8 is not required for RNAPII recruitment or promoter escape. Instead, CDK8 depletion leads to the appearance of slower elongation complexes carrying hypophosphorylated RNAPII. CDK8-Mediator regulates precise steps in the assembly of the RNAPII elongation complex, including the recruitment of P-TEFb and BRD4. Furthermore, CDK8-Mediator specifically interacts with P-TEFb. Thus, we uncovered a novel role for CDK8 in transcriptional regulation that may contribute to its oncogenic effects.

DSIF, the Paf1 complex, and Tat-SF1 have nonredundant, cooperative roles in RNA polymerase II elongation

Transcription elongation factor DSIF/Spt4-Spt5 is capable of promoting and inhibiting RNA polymerase II elongation and is involved in the expression of various genes. While it has been known for many years that DSIF inhibits elongation in collaboration with the negative elongation factor NELF, how DSIF promotes elongation is largely unknown. Here, an activity-based biochemical approach was taken to understand the mechanism of elongation activation by DSIF. We show that the Paf1 complex (Paf1C) and Tat-SF1, two factors implicated previously in elongation control, collaborate with DSIF to facilitate efficient elongation. In human cells, these factors are recruited to the FOS gene in a temporally coordinated manner and contribute to its high-level expression. We also show that elongation activation by these factors depends on P-TEFb-mediated phosphorylation of the Spt5 C-terminal region. A clear conclusion emerging from this study is that a set of elongation factors plays nonredundant, cooperative roles in elongation. This study also shows unambiguously that Paf1C, which is generally thought to have chromatin-related functions, is involve directlyd in elongation control.

Histone H3K4 and K36 methylation, Chd1 and Rpd3S oppose the functions of Saccharomyces cerevisiae Spt4-Spt5 in transcription

Spt4-Spt5, a general transcription elongation factor for RNA polymerase II, also has roles in chromatin regulation. However, the relationships between these functions are not clear. Previously, we isolated suppressors of a Saccharomyces cerevisiae spt5 mutation in genes encoding members of the Paf1 complex, which regulates several cotranscriptional histone modifications, and Chd1, a chromatin remodeling enzyme. Here, we show that this suppression of spt5 can result from loss of histone H3 lysines 4 or 36 methylation, or reduced recruitment of Chd1 or the Rpd3S complex. These spt5 suppressors also rescue the synthetic growth defects observed in spt5 mutants that also lack elongation factor TFIIS. Using a FLO8 reporter gene, we found that a chd1 mutation caused cryptic initiation of transcription. We further observed enhancement of cryptic initiation in chd1 isw1 mutants and increased histone acetylation in a chd1 mutant. We suggest that, as previously proposed for H3 lysine 36 methylation and the Rpd3S complex, H3 lysine 4 methylation and Chd1 function to maintain normal chromatin structures over transcribed genes, and that one function of Spt4-Spt5 is to help RNA polymerase II overcome the repressive effects of these histone modifications and chromatin regulators on transcription.

Phosphorylation of the transcription elongation factor Spt5 by yeast Bur1 kinase stimulates recruitment of the PAF complex

The Saccharomyces cerevisiae kinase Bur1 is involved in coupling transcription elongation to chromatin modification, but not all important Bur1 targets in the elongation complex are known. Using a chemical genetics strategy wherein Bur1 kinase was engineered to be regulated by a specific inhibitor, we found that Bur1 phosphorylates the Spt5 C-terminal repeat domain (CTD) both in vivo and in isolated elongation complexes in vitro. Deletion of the Spt5 CTD or mutation of the Spt5 serines targeted by Bur1 reduces recruitment of the PAF complex, which functions to recruit factors involved in chromatin modification and mRNA maturation to elongating polymerase II (Pol II). Deletion of the Spt5 CTD showed the same defect in PAF recruitment as rapid inhibition of Bur1 kinase activity, and this Spt5 mutation led to a decrease in histone H3K4 trimethylation. Brief inhibition of Bur1 kinase activity in vivo also led to a significant decrease in phosphorylation of the Pol II CTD at Ser-2, showing that Bur1 also contributes to Pol II Ser-2 phosphorylation. Genetic results suggest that Bur1 is essential for growth because it targets multiple factors that play distinct roles in transcription.