Impact of the Ku complex on HIV-1 expression and latency

Association of Polymorphisms in NHEJ Pathway Genes with HIV-1 Infection and AIDS Progression in a Northern Chinese MSM Population

Background and Aims

Men who have sex with men (MSM) are at high risk of HIV infection. The nonhomologous end joining (NHEJ) pathway is the main way of double-stranded DNA break (DSB) repair in the higher eukaryotes and can repair the DSB timely at any time in cell cycle. It is also indicated that the NHEJ pathway is associated with HIV-1 infection since the DSB in host genome DNA occurs in the process of HIV-1 integration. The aim of the present investigation was to evaluate associations of single-nucleotide polymorphisms (SNPs) in NHEJ pathway genes with susceptibility to HIV-1 infection and AIDS progression among MSM residing in northern China.

Methods

A total of 481 HIV-1 seropositive men and 493 HIV-1 seronegative men were included in this case-control study. Genotyping of 22 SNPs in NHEJ pathway genes was performed using the SNPscan™ Kit.

Results

Positive associations were observed between XRCC6 rs132770 and XRCC4 rs1056503 genotypes and the susceptibility to HIV-1 infection. In gene-gene interaction analysis, significant SNP-SNP interactions of XRCC6 and XRCC4 genetic variations were found to play a potential role in the risk of HIV-1 infection. In stratified analysis, XRCC5 rs16855458 was significantly associated with CD4+ T cell counts in AIDS patients, whereas LIG4 rs1805388 was linked to the clinical phases of AIDS patients.

Conclusions

NHEJ gene polymorphisms can be considered to be risk factors of HIV-1 infection and AIDS progression in the northern Chinese MSM population.

Complex Relationships between HIV-1 Integrase and Its Cellular Partners

RNA viruses, in pursuit of genome miniaturization, tend to employ cellular proteins to facilitate their replication. HIV-1, one of the most well-studied retroviruses, is not an exception. There is numerous evidence that the exploitation of cellular machinery relies on nucleic acid-protein and protein-protein interactions. Apart from Vpr, Vif, and Nef proteins that are known to regulate cellular functioning via interaction with cell components, another viral protein, integrase, appears to be crucial for proper virus-cell dialog at different stages of the viral life cycle. The goal of this review is to summarize and systematize existing data on known cellular partners of HIV-1 integrase and their role in the HIV-1 life cycle.

The HCV Envelope Glycoprotein Down-Modulates NF-κB Signalling and Associates With Stimulation of the Host Endoplasmic Reticulum Stress Pathway

Following acute HCV infection, the virus establishes a chronic disease in the majority of patients whilst few individuals clear the infection spontaneously. The precise mechanisms that determine chronic HCV infection or spontaneous clearance are not completely understood but are proposed to be driven by host and viral genetic factors as well as HCV encoded immunomodulatory proteins. Using the HIV-1 LTR as a tool to measure NF-κB activity, we identified that the HCV E1E2 glycoproteins and more so the E2 protein down-modulates HIV-1 LTR activation in 293T, TZM-bl and the more physiologically relevant Huh7 liver derived cell line. We demonstrate this effect is specifically mediated through inhibiting NF-κB binding to the LTR and show that this effect was conserved for all HCV genotypes tested. Transcriptomic analysis of 293T cells expressing the HCV glycoproteins identified E1E2 mediated stimulation of the endoplasmic reticulum (ER) stress response pathway and upregulation of stress response genes such as ATF3. Through shRNA mediated inhibition of ATF3, one of the components, we observed that E1E2 mediated inhibitory effects on HIV-1 LTR activity was alleviated. Our in vitro studies demonstrate that HCV Env glycoprotein activates host ER Stress Pathways known to inhibit NF-κB activity. This has potential implications for understanding HCV induced immune activation as well as oncogenesis.

The Role of Ku70 as a Cytosolic DNA Sensor in Innate Immunity and Beyond

Human Ku70 is a well-known endogenous nuclear protein involved in the non-homologous end joining pathway to repair double-stranded breaks in DNA. However, Ku70 has been studied in multiple contexts and grown into a multifunctional protein. In addition to the extensive functional study of Ku70 in DNA repair process, many studies have emphasized the role of Ku70 in various other cellular processes, including apoptosis, aging, and HIV replication. In this review, we focus on discussing the role of Ku70 in inducing interferons and proinflammatory cytokines as a cytosolic DNA sensor. We explored the unique structure of Ku70 binding with DNA; illustrated, with evidence, how Ku70, as a nuclear protein, responds to extracellular DNA stimulation; and summarized the mechanisms of the Ku70-involved innate immune response pathway. Finally, we discussed several new strategies to modulate Ku70-mediated innate immune response and highlighted some potential physiological insights based on the role of Ku70 in innate immunity.

The Ku complex: recent advances and emerging roles outside of non-homologous end-joining

Since its discovery in 1981, the Ku complex has been extensively studied under multiple cellular contexts, with most work focusing on Ku in terms of its essential role in non-homologous end-joining (NHEJ). In this process, Ku is well-known as the DNA-binding subunit for DNA-PK, which is central to the NHEJ repair process. However, in addition to the extensive study of Ku's role in DNA repair, Ku has also been implicated in various other cellular processes including transcription, the DNA damage response, DNA replication, telomere maintenance, and has since been studied in multiple contexts, growing into a multidisciplinary point of research across various fields. Some advances have been driven by clarification of Ku's structure, including the original Ku crystal structure and the more recent Ku-DNA-PKcs crystallography, cryogenic electron microscopy (cryoEM) studies, and the identification of various post-translational modifications. Here, we focus on the advances made in understanding the Ku heterodimer outside of non-homologous end-joining, and across a variety of model organisms. We explore unique structural and functional aspects, detail Ku expression, conservation, and essentiality in different species, discuss the evidence for its involvement in a diverse range of cellular functions, highlight Ku protein interactions and recent work concerning Ku-binding motifs, and finally, we summarize the clinical Ku-related research to date.

Phosphorylation Targets of DNA-PK and Their Role in HIV-1 Replication

The DNA dependent protein kinase (DNA-PK) is a trimeric nuclear complex consisting of a large protein kinase and the Ku heterodimer. The kinase activity of DNA-PK is required for efficient repair of DNA double-strand breaks (DSB) by non-homologous end joining (NHEJ). We also showed that the kinase activity of DNA-PK is essential for post-integrational DNA repair in the case of HIV-1 infection. Besides, DNA-PK is known to participate in such cellular processes as protection of mammalian telomeres, transcription, and some others where the need for its phosphorylating activity is not clearly elucidated. We carried out a systematic search and analysis of DNA-PK targets described in the literature and identified 67 unique DNA-PK targets phosphorylated in response to various in vitro and/or in vivo stimuli. A functional enrichment analysis of DNA-PK targets and determination of protein-protein associations among them were performed. For 27 proteins from these 67 DNA-PK targets, their participation in the HIV-1 life cycle was demonstrated. This information may be useful for studying the functioning of DNA-PK in various cellular processes, as well as in various stages of HIV-1 replication.

Analysis of RNA binding properties of human Ku protein reveals its interactions with 7SK snRNA and protein components of 7SK snRNP complex

Human Ku heterodimeric protein composed of Ku70 and Ku80 subunits plays an important role in the non-homologous end-joining DNA repair pathway as a sensor of double strand DNA breaks. Ku is also involved in numerous cellular processes, and in some of them it acts in an RNA-dependent manner. However, RNA binding properties of the human Ku have not been well studied. Here we have analyzed interactions of a recombinant Ku heterodimer with a set of RNAs of various structure as well as eCLIP (enhanced crosslinking and immunoprecipitation) data for human Ku70. As a result, we have proposed a consensus RNA structure preferable for the Ku binding that is a hairpin possessing a bulge just near GpG sequence-containing terminal loop. 7SK snRNA is a scaffold for a ribonucleoprotein complex (7SK snRNP), which is known to participate in transcription regulation. We have shown that the recombinant Ku specifically binds a G-rich loop of hairpin 1 within 7SK snRNA. Moreover, Ku protein has been co-precipitated from HEK 293T cells with endogenous 7SK snRNA and such proteins included in 7SK snRNP as HEXIM1, Cdk9 and CTIP2. Ku and Cdk9 binding is found to be RNA-independent, meanwhile HEXIM1 and Ku co-precipitation depended on the presence of intact 7SK snRNA. The latter result has been confirmed using recombinant HEXIM1 and Ku proteins. Colocalization of Ku and CTIP2 was additionally confirmed by confocal microscopy. These results allow us to propose human Ku as a new component of the 7SK snRNP complex.

NHEJ pathway is involved in post-integrational DNA repair due to Ku70 binding to HIV-1 integrase

Background

HIV-1 integration results in genomic DNA gaps that are repaired by cellular DNA repair pathways. This step of the lentiviral life cycle remains poorly understood despite its crucial importance for successful replication. We and others reported that Ku70 protein of the non-homologous end joining pathway (NHEJ) directly binds HIV-1 integrase (IN). Here, we studied the importance of this interaction for post-integrational gap repair and the recruitment of NHEJ factors in this process.

Results

We engineered HIV-based pseudovirus with mutant IN defective in Ku70 binding and generated heterozygous Ku70, Ku80 and DNA-PKcs human knockout (KO) cells using CRISPR/Cas9. KO of either of these proteins or inhibition of DNA-PKcs catalytic activity substantially decreased the infectivity of HIV-1 with native IN but not with the mutant one. We used a recently developed qPCR assay for the measurement of gap repair efficiency to show that HIV-1 with mutant IN was defective in DNA post-integrational repair, whereas the wild type virus displayed such a defect only when NHEJ system was disrupted in any way. This effect was present in CRISPR/Cas9 modified 293T cells, in Jurkat and CEM lymphoid lines and in primary human PBMCs.

Conclusions

Our data provide evidence that IN recruits DNA-PK to the site of HIV-1 post-integrational repair due to Ku70 binding—a novel finding that explains the involvement of DNA-PK despite the absence of free double stranded DNA breaks. In addition, our data clearly indicate the importance of interactions between HIV-1 IN and Ku70 in HIV-1 replication at the post-integrational repair step.

Targeting the DNA-PK complex: Its rationale use in cancer and HIV-1 infection

The DNA-PK complex is the major component of the predominant mechanism of DSB repair in humans. In addition, this complex is involved in many other processes such as DNA recombination, genome maintenance, apoptosis and transcription regulation. Several studies have linked the decrease of the DNA-PK activity with cancer initiation, due to defects in the repair. On another hand, higher DNA-PK expression and activity have been observed in various other tumor cells and have been linked with a decrease of the efficiency of anti-tumor drugs. It has also been shown that DNA-PK is critical for the integration of the HIV-1 DNA into the cell host genome and promotes replication and transcription of the virus. Targeting this complex makes therefore sense to treat these two pathologies. However, according to the status of HIV-1 replication (active versus latent replication) or to the tumor grade cells (initiation versus metastasis), the way to target this DNA-PK complex might be rather different. In this review, we discuss the importance of DNA-PK complex in two major pathologies i.e. HIV-1 infection and cancer, and the rationale use of therapies aiming to target this complex.

Multiple Inhibitory Factors Act in the Late Phase of HIV-1 Replication: a Systematic Review of the Literature

The use of lentiviral vectors for therapeutic purposes has shown promising results in clinical trials. The ability to produce a clinical-grade vector at high yields remains a critical issue. One possible obstacle could be cellular factors known to inhibit human immunodeficiency virus (HIV). To date, five HIV restriction factors have been identified, although it is likely that more factors are involved in the complex HIV-cell interaction. Inhibitory factors that have an adverse effect but do not abolish virus production are much less well described. Therefore, a gap exists in the knowledge of inhibitory factors acting late in the HIV life cycle (from transcription to infection of a new cell), which are relevant to the lentiviral vector production process. The objective was to review the HIV literature to identify cellular factors previously implicated as inhibitors of the late stages of lentivirus production. A search for publications was conducted on MEDLINE via the PubMed interface, using the keyword sequence "HIV restriction factor" or "HIV restriction" or "inhibit HIV" or "repress HIV" or "restrict HIV" or "suppress HIV" or "block HIV," with a publication date up to 31 December 2016. Cited papers from the identified records were investigated, and additional database searches were performed. A total of 260 candidate inhibitory factors were identified. These factors have been identified in the literature as having a negative impact on HIV replication. This study identified hundreds of candidate inhibitory factors for which the impact of modulating their expression in lentiviral vector production could be beneficial.

A Cas9 Ribonucleoprotein Platform for Functional Genetic Studies of HIV-Host Interactions in Primary Human T Cells

New genetic tools are needed to understand the functional interactions between HIV and human host factors in primary cells. We recently developed a method to edit the genome of primary CD4⁺ T cells by electroporation of CRISPR/Cas9 ribonucleoproteins (RNPs). Here, we adapted this methodology to a high-throughput platform for the efficient, arrayed editing of candidate host factors. CXCR4 or CCR5 knockout cells generated with this method are resistant to HIV infection in a tropism-dependent manner, whereas knockout of LEDGF or TNPO3 results in a tropism-independent reduction in infection. CRISPR/Cas9 RNPs can furthermore edit multiple genes simultaneously, enabling studies of interactions among multiple host and viral factors. Finally, in an arrayed screen of 45 genes associated with HIV integrase, we identified several candidate dependency/restriction factors, demonstrating the power of this approach as a discovery platform. This technology should accelerate target validation for pharmaceutical and cell-based therapies to cure HIV infection.

Characterization of HIV-1 integrase interaction with human Ku70 protein and initial implications for drug targeting

Human Ku70/Ku80 protein is known to influence HIV-1 replication. One of the possible reasons may be the protection of integrase from proteasomal degradation by Ku70 subunit. We demonstrated that recombinant HIV-1 integrase and Ku70 form a stable complex, while no interaction of Ku70 with integrase from prototype foamy virus was observed. By analyzing protein subdomains we determined two binding sites in the structure of both Ku70 and integrase: the 51–160 a.a. region of integrase interacts with residues 251–438 of Ku70, whereas Ku70 N-terminal domain (1–250 a.a.) contacts an α6-helix in the 200–220 a.a. integrase region. Single substitutions within integrase (E212A or L213A) block the interaction with Ku70 thus indicating that the binding site formed by the 200–220 a.a. integrase region is crucial for complex formation. E212A/L213A substitutions decreased the integrase capacity to bind Ku70 in HEK293T cells. A conjugate of 2′-ОMe-GGUUUUUGUGU oligonucleotide with eosin is shown by molecular modeling to shield integrase residues E212/L213 and is effective in blocking complex formation of Ku70 with integrase what makes the complex between α6-helix and Ku70(1–250) a possible target for drug development.

Opposite transcriptional regulation of integrated vs unintegrated HIV genomes by the NF-κB pathway

Integration of HIV-1 linear DNA into host chromatin is required for high levels of viral expression, and constitutes a key therapeutic target. Unintegrated viral DNA (uDNA) can support only limited transcription but may contribute to viral propagation, persistence and/or treatment escape under specific situations. The molecular mechanisms involved in the differential expression of HIV uDNA vs integrated genome (iDNA) remain to be elucidated. Here, we demonstrate, for the first time, that the expression of HIV uDNA is mainly supported by 1-LTR circles, and regulated in the opposite way, relatively to iDNA, following NF-κB pathway modulation. Upon treatment activating the NF-κB pathway, NF-κB p65 and AP-1 (cFos/cJun) binding to HIV LTR iDNA correlates with increased iDNA expression, while uDNA expression decreases. On the contrary, inhibition of the NF-κB pathway promotes the expression of circular uDNA, and correlates with Bcl-3 and AP-1 binding to its LTR region. Finally, this study identifies NF-κB subunits and Bcl-3 as transcription factors binding the HIV promoter differently depending on viral genome topology, and opens new insights on the potential roles of episomal genomes during the HIV-1 latency and persistence.

Host Proteins Ku and HMGA1 As Participants of HIV-1 Transcription

Human immunodeficiency virus type 1 is known to use the transcriptional machinery of the host cell for viral gene transcription, and the only viral protein that partakes in this process is Tat, the viral trans-activator of transcription. During acute infection, the binding of Tat to the hairpin at the beginning of the transcribed viral RNA recruits the PTEFb complex, which in turn hyperphosphorylates RNA-polymerase II and stimulates transcription elongation. Along with acute infection, HIV-1 can also lead to latent infection that is characterized by a low level of viral transcription. During the maintenance and reversal of latency, there are no detectable amounts of Tat protein in the cell and the mechanism of transcription activation in the absence of Tat protein remains unclear. The latency maintenance is also a problematic question. It seems evident that cellular proteins with a yet unknown nature or role regulate both transcriptional repression in the latent phase and its activation during transition into the lytic phase. The present review discusses the role of cellular proteins Ku and HMGA1 in the initiation of transcription elongation of the HIV-1 provirus. The review presents data regarding Ku-mediated HIV-1 transcription and its dependence on the promoter structure and the shape of viral DNA. We also describe the differential influence of the HMGA1 protein on the induced and basal transcription of HIV-1. Finally, we offer possible mechanisms for Ku and HMGA1 proteins in the proviral transcription regulation.

HIV-1 and two avian retroviral 5' untranslated regions bind orthologous human and chicken RNA binding proteins

Essential host cofactors in retrovirus replication bind cis-acting sequences in the 5'untranslated region (UTR). Although host RBPs are crucial to all aspects of virus biology, elucidating their roles in replication remains a challenge to the field. Here RNA affinity-coupled-proteomics generated a comprehensive, unbiased inventory of human and avian RNA binding proteins (RBPs) co-isolating with 5'UTRs of HIV-1, spleen necrosis virus and Rous sarcoma virus. Applying stringent biochemical and statistical criteria, we identified 185 RBP; 122 were previously implicated in retrovirus biology and 63 are new to the 5'UTR proteome. RNA electrophoretic mobility assays investigated paralogs present in the common ancestor of vertebrates and one hnRNP was identified as a central node to the biological process-anchored networks of HIV-1, SNV, and RSV 5' UTR-proteomes. This comprehensive view of the host constituents of retroviral RNPs is broadly applicable to investigation of viral replication and antiviral response in both human and avian cell lineages.

Dual and Opposite Effects of hRAD51 Chemical Modulation on HIV-1 Integration

The cellular DNA repair hRAD51 protein has been shown to restrict HIV-1 integration both in vitro and in vivo. To investigate its regulatory functions, we performed a pharmacological analysis of the retroviral integration modulation by hRAD51. We found that, in vitro, chemical activation of hRAD51 stimulates its integration inhibitory properties, whereas inhibition of hRAD51 decreases the integration restriction, indicating that the modulation of HIV-1 integration depends on the hRAD51 recombinase activity. Cellular analyses demonstrated that cells exhibiting high hRAD51 levels prior to de novo infection are more resistant to integration. On the other hand, when hRAD51 was activated during integration, cells were more permissive. Altogether, these data establish the functional link between hRAD51 activity and HIV-1 integration. Our results highlight the multiple and opposite effects of the recombinase during integration and provide new insights into the cellular regulation of HIV-1 replication.

XMRV low level of expression in human cells delays superinfection interference and allows proviral copies to accumulate

Xenotropic Murine leukemia virus-Related Virus (XMRV) directly arose from genetic recombinations between two endogenous murine retroviruses that occurred during human xenografts in laboratory mice. Studies on XMRV could thus bring clues on how a new retrovirus could circumvent barrier species. We observed that XMRV exhibits a weak promoter activity in human cells, similar to the transcription level of a Tat-defective HIV-1. Despite this low fitness, XMRV can efficiently propagate through the huge accumulation of viral copies (≈40 copies per cell) that compensates for the low expression level of individual proviruses. We further demonstrate that there is an inverse relationship between the maximum number of viral copies per infected cell and the level of viral expression, which is explained by viral envelope interference mechanisms. Low viral expression compensation by viral copy accumulation through delayed interference could a priori contribute to the propagation of others viruses following species jumps.