G2/M cell cycle arrest correlates with primate lentiviral Vpr interaction with the SLX4 complex

HIV-1 Vpr Functions in Primary CD4+ T Cells

HIV-1 encodes four accesory proteins in addition to its structural and regulatory genes. Uniquely amongst them, Vpr is abundantly present within virions, meaning it is poised to exert various biological effects on the host cell upon delivery. In this way, Vpr contributes towards the establishment of a successful infection, as evidenced by the extent to which HIV-1 depends on this factor to achieve full pathogenicity in vivo. Although HIV infects various cell types in the host organism, CD4+ T cells are preferentially targeted since they are highly permissive towards productive infection, concomitantly bringing about the hallmark immune dysfunction that accompanies HIV-1 spread. The last several decades have seen unprecedented progress in unraveling the activities Vpr possesses in the host cell at the molecular scale, increasingly underscoring the importance of this viral component. Nevertheless, it remains controversial whether some of these advances bear in vivo relevance, since commonly employed cellular models significantly differ from primary T lymphocytes. One prominent example is the "established" ability of Vpr to induce G2 cell cycle arrest, with enigmatic physiological relevance in infected primary T lymphocytes. The objective of this review is to present these discoveries in their biological context to illustrate the mechanisms whereby Vpr supports HIV-1 infection in CD4+ T cells, whilst identifying findings that require validation in physiologically relevant models.

A CRISPR-Cas Cure for HIV/AIDS

Human immunodeficiency virus (HIV) infections and HIV-induced acquired immunodeficiency syndrome (AIDS) continue to represent a global health burden. There is currently no effective vaccine, nor any cure, for HIV infections; existing antiretroviral therapy can suppress viral replication, but only as long as antiviral drugs are taken. HIV infects cells of the host immune system, and it can establish a long-lived viral reservoir, which can be targeted and edited through gene therapy. Gene editing platforms based on the clustered regularly interspaced palindromic repeat-Cas system (CRISPR-Cas) have been recognized as promising tools in the development of gene therapies for HIV infections. In this review, we evaluate the current landscape of CRISPR-Cas-based therapies against HIV, with an emphasis on the infection biology of the virus as well as the activity of host restriction factors. We discuss the potential of a combined CRISPR-Cas approach that targets host and viral genes to activate antiviral host factors and inhibit viral replication simultaneously. Lastly, we focus on the challenges and potential solutions of CRISPR-Cas gene editing approaches in achieving an HIV cure.

Blood transcriptome analysis revealed the crosstalk between COVID-19 and HIV

BACKGROUND

The severe coronavirus disease 2019 (COVID-19) is an infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has resulted in the most devastating pandemic in modern history. Human immunodeficiency virus (HIV) destroys immune system cells and weakens the body's ability to resist daily infections and diseases. Furthermore, HIV-infected individuals had double COVID-19 mortality risk and experienced worse COVID-related outcomes. However, the existing research still lacks the understanding of the molecular mechanism underlying crosstalk between COVID-19 and HIV. The aim of our work was to illustrate blood transcriptome crosstalk between COVID-19 and HIV and to provide potential drugs that might be useful for the treatment of HIV-infected COVID-19 patients.

METHODS

COVID-19 datasets (GSE171110 and GSE152418) were downloaded from Gene Expression Omnibus (GEO) database, including 54 whole-blood samples and 33 peripheral blood mononuclear cells samples, respectively. HIV dataset (GSE37250) was also obtained from GEO database, containing 537 whole-blood samples. Next, the "Deseq2" package was used to identify differentially expressed genes (DEGs) between COVID-19 datasets (GSE171110 and GSE152418) and the "limma" package was utilized to identify DEGs between HIV dataset (GSE37250). By intersecting these two DEG sets, we generated common DEGs for further analysis, containing Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Ontology (GO) functional enrichment analysis, protein-protein interaction (PPI) analysis, transcription factor (TF) candidate identification, microRNAs (miRNAs) candidate identification and drug candidate identification.

RESULTS

In this study, a total of 3213 DEGs were identified from the merged COVID-19 dataset (GSE171110 and GSE152418), and 1718 DEGs were obtained from GSE37250 dataset. Then, we identified 394 common DEGs from the intersection of the DEGs in COVID-19 and HIV datasets. GO and KEGG enrichment analysis indicated that common DEGs were mainly gathered in chromosome-related and cell cycle-related signal pathways. Top ten hub genes (CCNA2, CCNB1, CDC20, TOP2A, AURKB, PLK1, BUB1B, KIF11, DLGAP5, RRM2) were ranked according to their scores, which were screened out using degree algorithm on the basis of common DEGs. Moreover, top ten drug candidates (LUCANTHONE, Dasatinib, etoposide, Enterolactone, troglitazone, testosterone, estradiol, calcitriol, resveratrol, tetradioxin) ranked by their P values were screened out, which maybe be beneficial for the treatment of HIV-infected COVID-19 patients.

CONCLUSION

In this study, we provide potential molecular targets, signaling pathways, small molecular compounds, and promising biomarkers that contribute to worse COVID-19 prognosis in patients with HIV, which might contribute to precise diagnosis and treatment for HIV-infected COVID-19 patients.

Identification of critical genes and molecular pathways in COVID-19 myocarditis and constructing gene regulatory networks by bioinformatic analysis

BACKGROUND

There is growing evidence of a strong relationship between COVID-19 and myocarditis. However, there are few bioinformatics-based analyses of critical genes and the mechanisms related to COVID-19 Myocarditis. This study aimed to identify critical genes related to COVID-19 Myocarditis by bioinformatic methods, explore the biological mechanisms and gene regulatory networks, and probe related drugs.

METHODS

The gene expression data of GSE150392 and GSE167028 were obtained from the Gene Expression Omnibus (GEO), including cardiomyocytes derived from human induced pluripotent stem cells infected with SARS-CoV-2 in vitro and GSE150392 from patients with myocarditis infected with SARS-CoV-2 and the GSE167028 gene expression dataset. Differentially expressed genes (DEGs) (adjusted P-Value <0.01 and |Log2 Fold Change| ≥2) in GSE150392 were assessed by NetworkAnalyst 3.0. Meanwhile, significant modular genes in GSE167028 were identified by weighted gene correlation network analysis (WGCNA) and overlapped with DEGs to obtain common genes. Functional enrichment analyses were performed by using the "clusterProfiler" package in the R software, and protein-protein interaction (PPI) networks were constructed on the STRING website (https://cn.string-db.org/). Critical genes were identified by the CytoHubba plugin of Cytoscape by 5 algorithms. Transcription factor-gene (TF-gene) and Transcription factor-microRibonucleic acid (TF-miRNA) coregulatory networks construction were performed by NetworkAnalyst 3.0 and displayed in Cytoscape. Finally, Drug Signatures Database (DSigDB) was used to probe drugs associated with COVID-19 Myocarditis.

RESULTS

Totally 850 DEGs (including 449 up-regulated and 401 down-regulated genes) and 159 significant genes in turquoise modules were identified from GSE150392 and GSE167028, respectively. Functional enrichment analysis indicated that common genes were mainly enriched in biological processes such as cell cycle and ubiquitin-protein hydrolysis. 6 genes (CDK1, KIF20A, PBK, KIF2C, CDC20, UBE2C) were identified as critical genes. TF-gene interactions and TF-miRNA coregulatory network were constructed successfully. A total of 10 drugs, (such as Etoposide, Methotrexate, Troglitazone, etc) were considered as target drugs for COVID-19 Myocarditis.

CONCLUSIONS

Through bioinformatics method analysis, this study provides a new perspective to explore the pathogenesis, gene regulatory networks and provide drug compounds as a reference for COVID-19 Myocarditis. It is worth highlighting that critical genes (CDK1, KIF20A, PBK, KIF2C, CDC20, UBE2C) may be potential biomarkers and treatment targets of COVID-19 Myocarditis for future study.

Huntingtin-Interacting Protein 1 Promotes Vpr-Induced G2 Arrest and HIV-1 Infection in Macrophages

Human immunodeficiency virus type 1 (HIV-1) modulates the host cell cycle. The HIV-1 accessory protein Vpr arrests the cell cycle at the G2 phase in dividing cells, and the ability of Vpr to induce G2 arrest is well conserved among primate lentiviruses. Additionally, Vpr-mediated G2 arrest likely correlates with enhanced HIV-1 infection in monocyte-derived macrophages. Here, we screened small-interfering RNA to reveal candidates that suppress Vpr-induced G2 arrest and identified Huntingtin-interacting protein 1 (HIP1) required for efficient G2 arrest. Interestingly, HIP1 was not essential for Vpr-induced DNA double-strand breaks, which are required for activation of the DNA-damage checkpoint and G2 arrest. Furthermore, HIP1 knockdown suppressed HIV-1 infection in monocyte-derived macrophages. This study identifies HIP1 as a factor promoting Vpr-induced G2 arrest and HIV-1 infection in macrophages.

Binding to DCAF1 distinguishes TASOR and SAMHD1 degradation by HIV-2 Vpx

Human Immunodeficiency viruses type 1 and 2 (HIV-1 and HIV-2) succeed to evade host immune defenses by using their viral auxiliary proteins to antagonize host restriction factors. HIV-2/SIVsmm Vpx is known for degrading SAMHD1, a factor impeding the reverse transcription. More recently, Vpx was also shown to counteract HUSH, a complex constituted of TASOR, MPP8 and periphilin, which blocks viral expression from the integrated viral DNA. In a classical ubiquitin ligase hijacking model, Vpx bridges the DCAF1 ubiquitin ligase substrate adaptor to SAMHD1, for subsequent ubiquitination and degradation. Here, we investigated whether the same mechanism is at stake for Vpx-mediated HUSH degradation. While we confirm that Vpx bridges SAMHD1 to DCAF1, we show that TASOR can interact with DCAF1 in the absence of Vpx. Nonetheless, this association was stabilized in the presence of Vpx, suggesting the existence of a ternary complex. The N-terminal PARP-like domain of TASOR is involved in DCAF1 binding, but not in Vpx binding. We also characterized a series of HIV-2 Vpx point mutants impaired in TASOR degradation, while still degrading SAMHD1. Vpx mutants ability to degrade TASOR correlated with their capacity to enhance HIV-1 minigenome expression as expected. Strikingly, several Vpx mutants impaired for TASOR degradation, but not for SAMHD1 degradation, had a reduced binding affinity for DCAF1, but not for TASOR. In macrophages, Vpx R34A-R42A and Vpx R42A-Q47A-V48A, strongly impaired in DCAF1, but not in TASOR binding, could not degrade TASOR, while being efficient in degrading SAMHD1. Altogether, our results highlight the central role of a robust Vpx-DCAF1 association to trigger TASOR degradation. We then propose a model in which Vpx interacts with both TASOR and DCAF1 to stabilize a TASOR-DCAF1 complex. Furthermore, our work identifies Vpx mutants enabling the study of HUSH restriction independently from SAMHD1 restriction in primary myeloid cells.

Human Immunodeficiency Virus (HIV) is still a major public health issue. The understanding of the molecular battle occurring during viral infection, between HIV components and cellular antiviral factors, the so-called restriction factors, is a key determinant for new treatment development. Namely, HIV auxiliary proteins are powerful to induce the downregulation of cellular restriction factors by hijacking the Ubiquitin/proteasome pathway, in order to facilitate the completion of a well-processed HIV replication cycle. For instance, HIV-2 Vpx eases reverse transcription in myeloid cells by counteracting the SAMDH1 restriction factor. More recently, we discovered the ability of Vpx to induce the degradation of the HUSH epigenetic repressor complex to favor in turn, the expression of the provirus. In this study, we uncovered the mechanisms by which Vpx antagonizes TASOR, the core subunit of the HUSH complex. We highlighted key differences between Vpx-induced TASOR and SAMHD1 degradation. These findings will help to propose strategies to study or to target either HUSH or SAMHD1, especially in myeloid cells where SAMHD1 restriction operates.

Structural insights into Cullin4-RING ubiquitin ligase remodelling by Vpr from simian immunodeficiency viruses

Viruses have evolved means to manipulate the host's ubiquitin-proteasome system, in order to down-regulate antiviral host factors. The Vpx/Vpr family of lentiviral accessory proteins usurp the substrate receptor DCAF1 of host Cullin4-RING ligases (CRL4), a family of modular ubiquitin ligases involved in DNA replication, DNA repair and cell cycle regulation. CRL4DCAF1 specificity modulation by Vpx and Vpr from certain simian immunodeficiency viruses (SIV) leads to recruitment, poly-ubiquitylation and subsequent proteasomal degradation of the host restriction factor SAMHD1, resulting in enhanced virus replication in differentiated cells. To unravel the mechanism of SIV Vpr-induced SAMHD1 ubiquitylation, we conducted integrative biochemical and structural analyses of the Vpr protein from SIVs infecting Cercopithecus cephus (SIVmus). X-ray crystallography reveals commonalities between SIVmus Vpr and other members of the Vpx/Vpr family with regard to DCAF1 interaction, while cryo-electron microscopy and cross-linking mass spectrometry highlight a divergent molecular mechanism of SAMHD1 recruitment. In addition, these studies demonstrate how SIVmus Vpr exploits the dynamic architecture of the multi-subunit CRL4DCAF1 assembly to optimise SAMHD1 ubiquitylation. Together, the present work provides detailed molecular insight into variability and species-specificity of the evolutionary arms race between host SAMHD1 restriction and lentiviral counteraction through Vpx/Vpr proteins.

HIV-1 Vpr antagonizes innate immune activation by targeting karyopherin-mediated NF-κB/IRF3 nuclear transport

HIV-1 must replicate in cells that are equipped to defend themselves from infection through intracellular innate immune systems. HIV-1 evades innate immune sensing through encapsidated DNA synthesis and encodes accessory genes that antagonize specific antiviral effectors. Here, we show that both particle associated, and expressed HIV-1 Vpr, antagonize the stimulatory effect of a variety of pathogen associated molecular patterns by inhibiting IRF3 and NF-κB nuclear transport. Phosphorylation of IRF3 at S396, but not S386, was also inhibited. We propose that, rather than promoting HIV-1 nuclear import, Vpr interacts with karyopherins to disturb their import of IRF3 and NF-κB to promote replication in macrophages. Concordantly, we demonstrate Vpr-dependent rescue of HIV-1 replication in human macrophages from inhibition by cGAMP, the product of activated cGAS. We propose a model that unifies Vpr manipulation of nuclear import and inhibition of innate immune activation to promote HIV-1 replication and transmission.

Promiscuous Targeting of Cellular Proteins by Vpr Drives Systems-Level Proteomic Remodeling in HIV-1 Infection

HIV-1 encodes four “accessory proteins” (Vif, Vpr, Vpu, and Nef), dispensable for viral replication in vitro but essential for viral pathogenesis in vivo. Well characterized cellular targets have been associated with Vif, Vpu, and Nef, which counteract host restriction and promote viral replication. Conversely, although several substrates of Vpr have been described, their biological significance remains unclear. Here, we use complementary unbiased mass spectrometry-based approaches to demonstrate that Vpr is both necessary and sufficient for the DCAF1/DDB1/CUL4 E3 ubiquitin ligase-mediated degradation of at least 38 cellular proteins, causing systems-level changes to the cellular proteome. We therefore propose that promiscuous targeting of multiple host factors underpins complex Vpr-dependent cellular phenotypes and validate this in the case of G2/M cell cycle arrest. Our model explains how Vpr modulates so many cell biological processes and why the functional consequences of previously described Vpr targets, identified and studied in isolation, have proved elusive.

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HIV-1 Vpr is responsible for almost all proteomic changes in HIV-1-infected cells

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Vpr directly targets multiple nuclear proteins for degradation

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Vpr cellular phenotypes (e.g., cell cycle arrest) stem from broad substrate targeting

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Targeting of a few proteins is conserved across diverse primate lentiviral species

Concise review on optimized methods in production and transduction of lentiviral vectors in order to facilitate immunotherapy and gene therapy

Lentiviral vectors (LVs) have provided an efficient way to integrate our gene of interest into eukaryote cells. Human immunodeficiency virus (HIV)-derived LVs have been vastly studied to become an invaluable asset in gene delivery. This abled LVs to be used in both research laboratories and gene therapy. Pseudotyping HIV-1 based LVs, abled it to transduce different types of cells, especially hematopoietic stem cells. A wide range of tropism, plus to the ability to integrate genes into target cells, made LVs an armamentarium in gene therapy. The third and fourth generations of self-inactivating LVs are being used to achieve safe gene therapy. Not only advanced methods enabled the clinical-grade LV production on a large scale, but also considerably heightened transduction efficiency. One of which is microfluidic systems that revolutionized gene delivery approaches. Since gene therapy using LVs attracted lots of attention to itself, we provided a brief review of LV structure and life-cycle along with methods for improving both LV production and transduction. Also, we mentioned some of their utilization in immunotherapy and gene therapy.

The Severe Fever with Thrombocytopenia Syndrome Virus NSs Protein Interacts with CDK1 To Induce G2 Cell Cycle Arrest and Positively Regulate Viral Replication

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly identified phlebovirus associated with severe hemorrhagic fever in humans. While many viruses subvert the host cell cycle to promote viral growth, it is unknown whether this is a strategy employed by SFTSV. In this study, we investigated how SFTSV manipulates the cell cycle and the effect of the host cell cycle on SFTSV replication. Our results suggest that cells arrest at the G₂/M transition following infection with SFTSV. The accumulation of cells at the G₂/M transition did not affect virus adsorption and entry but did facilitate viral replication. In addition, we found that SFTSV NSs, a nonstructural protein that forms viroplasm-like structures in the cytoplasm of infected cells and promotes virulence by modulating the interferon response, induces a large number of cells to arrest at the G₂/M transition by interacting with CDK1. The interaction between NSs and CDK1, which is inclusion body dependent, inhibits formation and nuclear import of the cyclin B1-CDK1 complex, thereby leading to cell cycle arrest. Expression of a CDK1 loss-of-function mutant reversed the inhibitive effect of NSs on the cell cycle, suggesting that this protein is a potential antiviral target. Our study provides new insight into the role of a specific viral protein in SFTSV replication, indicating that NSs induces G₂/M arrest of SFTSV-infected cells, which promotes viral replication.IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne pathogen that causes severe hemorrhagic fever. Although SFTSV poses a serious threat to public health and was recently isolated, its pathogenesis remains unclear. In particular, the relationship between SFTSV infection and the host cell cycle has not been described. Here, we show for the first time that both asynchronized and synchronized SFTSV-susceptible cells arrest at the G₂/M checkpoint following SFTSV infection and that the accumulation of cells at this checkpoint facilitates viral replication. We also identify a key mechanism underlying SFTSV-induced G₂/M arrest, in which SFTSV NSs interacts with CDK1 to inhibit formation and nuclear import of the cyclin B1-CDK1 complex, thus preventing it from regulating cell cycle progression. Our study highlights the key role that NSs plays in SFTSV-induced G₂/M arrest.

CRL4 Ubiquitin Pathway and DNA Damage Response

DNA damage occurs in a human cell at an average frequency of 10,000 incidences per day by means of external and internal culprits, damage that triggers sequential cellular responses and stalls the cell cycle while activating specific DNA repair pathways. Failure to remove DNA lesions would compromise genomic integrity, leading to human diseases such as cancer and premature aging. If DNA damage is extensive and cannot be repaired, cells undergo apoptosis. DNA damage response (DDR) often entails posttranslational modifications of key DNA repair and DNA damage checkpoint proteins, including phosphorylation and ubiquitination. Cullin-RING ligase 4 (CRL4) enzyme has been found to target multiple DDR proteins for ubiquitination. In this chapter, we will discuss key repair and checkpoint proteins that are subject to ubiquitin-dependent regulation by members of the CRL4 family during ultraviolet light (UV)-induced DNA damage.

Vpr and Its Cellular Interaction Partners: R We There Yet?

Vpr is a lentiviral accessory protein that is expressed late during the infection cycle and is packaged in significant quantities into virus particles through a specific interaction with the P6 domain of the viral Gag precursor. Characterization of the physiologically relevant function(s) of Vpr has been hampered by the fact that in many cell lines, deletion of Vpr does not significantly affect viral fitness. However, Vpr is critical for virus replication in primary macrophages and for viral pathogenesis in vivo. It is generally accepted that Vpr does not have a specific enzymatic activity but functions as a molecular adapter to modulate viral or cellular processes for the benefit of the virus. Indeed, many Vpr interacting factors have been described by now, and the goal of this review is to summarize our current knowledge of cellular proteins targeted by Vpr.

Illuminating the Role of Vpr in HIV Infection of Myeloid Cells

Vpr is a 14 kDa accessory protein conserved amongst extant primate lentiviruses that is required for virus replication in vivo. Although many functions have been attributed to Vpr, its primary role, and the function under selective pressure in vivo, remains elusive. The minimal importance of Vpr in infection of activated CD4+ T cells in vitro suggests that its major importance lies in overcoming restriction to virus replication in non-cycling myeloid cell populations, such as macrophages and dendritic cells. HIV-1 replication is attenuated in the absence of Vpr in myeloid cells such as monocyte-derived dendritic cells (MDDCs) and macrophages, and is correlated with the ability of Vpr to overcome a post-integration transcriptional defect in these cells. Intriguingly, recent identification of the human hub silencing (HUSH) complex as a target for DCAF^CRL4-mediated degradation by numerous ancestral SIV Vpr alleles, and the Vpr paralog Vpx, signifies the potential function of HIV-1 Vpr to alter yet-to-be identified chromatin remodeling complexes and prevent host-mediated transcriptional repression of both invading viral genomes and pro-inflammatory responses. Myeloid cells constitute an important bridge between innate and adaptive immune responses to invading pathogens. Here, we seek to illustrate the numerous means by which Vpr manipulates the myeloid cellular environment and facilitates virus replication, myeloid cell-dependent HIV transmission, and systemic virus dissemination.

Intertwined: SAMHD1 cellular functions, restriction, and viral evasion strategies

SAMHD1 was initially described for its ability to efficiently restrict HIV-1 replication in myeloid cells and resting CD4⁺ T cells. However, a growing body of evidence suggests that SAMHD1-mediated restriction is by far not limited to lentiviruses, but seems to be a general concept that applies to most retroviruses and at least a number of DNA viruses. SAMHD1 anti-viral activity was long believed to be solely due to its ability to deplete cellular dNTPs by enzymatic degradation. However, since its discovery, several new functions have been attributed to SAMHD1. It has been demonstrated to bind nucleic acids, to modulate innate immunity, as well as to participate in the DNA damage response and resolution of stalled replication forks. Consequently, it is likely that SAMHD1-mediated anti-viral activity is not or not exclusively mediated through its dNTPase activity. Therefore, in this review, we summarize current knowledge on SAMHD1 cellular functions and systematically discuss how these functions could contribute to the restriction of a broad range of viruses besides retroviruses: herpesviruses, poxviruses and hepatitis B virus. Furthermore, we aim to highlight different ways how viruses counteract SAMHD1-mediated restriction to bypass the SAMHD1-mediated block to viral infection.

Proteasomal Degradation Machinery: Favorite Target of HIV-1 Proteins

Proteasomal degradation pathways play a central role in regulating a variety of protein functions by controlling not only their turnover but also the physiological behavior of the cell. This makes it an attractive target for the pathogens, especially viruses which rely on the host cellular machinery for their propagation and pathogenesis. Viruses have evolutionarily developed various strategies to manipulate the host proteasomal machinery thereby creating a cellular environment favorable for their own survival and replication. Human immunodeficiency virus-1 (HIV-1) is one of the most dreadful viruses which has rapidly spread throughout the world and caused high mortality due to its high evolution rate. Here, we review the various mechanisms adopted by HIV-1 to exploit the cellular proteasomal machinery in order to escape the host restriction factors and components of host immune system for supporting its own multiplication, and successfully created an infection.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

SLX4: multitasking to maintain genome stability

The SLX4/FANCP tumor suppressor has emerged as a key player in the maintenance of genome stability, making pivotal contributions to the repair of interstrand cross-links, homologous recombination, and in response to replication stress genome-wide as well as at specific loci such as common fragile sites and telomeres. SLX4 does so in part by acting as a scaffold that controls and coordinates the XPF-ERCC1, MUS81-EME1, and SLX1 structure-specific endonucleases in different DNA repair and recombination mechanisms. It also interacts with other important DNA repair and cell cycle control factors including MSH2, PLK1, TRF2, and TOPBP1 as well as with ubiquitin and SUMO. This review aims at providing an up-to-date and comprehensive view on the key functions that SLX4 fulfills to maintain genome stability as well as to highlight and discuss areas of uncertainty and emerging concepts.

Dual role of the chromatin-binding factor PHF13 in the pre- and post-integration phases of HIV-1 replication

Viruses interact with multiple host cell factors. Some of these are required to promote viral propagation, others have roles in inhibiting infection. Here, we delineate the function of the cellular factor PHF13 (or SPOC1), a putative HIV-1 restriction factor. Early in the HIV-1 replication cycle PHF13 increased the number of integrated proviral copies and the number of infected cells. However, after HIV-1 integration, high levels of PHF13 suppressed viral gene expression. The antiviral activity of PHF13 is counteracted by the viral accessory protein Vpr, which mediates PHF13 degradation. Altogether, the transcriptional master regulator and chromatin binding protein PHF13 does not have purely repressive effects on HIV-1 replication, but also promotes viral integration. By the functional characterization of the dual role of PHF13 during the HIV-1 replication cycle, we reveal a surprising and intricate mechanism through which HIV-1 might regulate the switch from integration to viral gene expression. Furthermore, we identify PHF13 as a cellular target specifically degraded by HIV-1 Vpr.

The interaction between mitochondria and oncoviruses

Mitochondria play important roles in multiple aspects of viral tumorigenesis. Mitochondrial genomes contribute to the host's genetic background. After viruses enter the cell, they modulate mitochondrial function and thus alter bioenergetics and retrograde signaling pathways. At the same time, mitochondria also regulate and mediate viral oncogenesis. In this context, oncogenesis by oncoviruses like Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human papilloma virus (HPV), Human Immunodeficiency virus (HIV) and Epstein-Barr virus (EBV) will be discussed.

Virion encapsidated HIV-1 Vpr induces NFAT to prime non-activated T cells for productive infection

The majority of T cells encountered by HIV-1 are non-activated and do not readily allow productive infection. HIV-1 Vpr is highly abundant in progeny virions, and induces signalling and HIV-1 LTR transcription. We hence hypothesized that Vpr might be a determinant of non-activated T-cell infection. Virion-delivered Vpr activated nuclear factor of activated T cells (NFAT) through Ca²⁺ influx and interference with the NFAT export kinase GSK3β. This leads to NFAT translocation and accumulation within the nucleus and was required for productive infection of unstimulated primary CD4⁺ T cells. A mutagenesis approach revealed correlation of Vpr-mediated NFAT activation with its ability to enhance LTR transcription and mediate cell cycle arrest. Upon NFAT inhibition, Vpr did not augment resting T-cell infection, and showed reduced G2/M arrest and LTR transactivation. Altogether, Vpr renders unstimulated T cells more permissive for productive HIV-1 infection and stimulates activation of productively infected as well as virus-exposed T cells. Therefore, it could be involved in the establishment and reactivation of HIV-1 from viral reservoirs and might have an impact on the levels of immune activation, which are determinants of HIV-1 pathogenesis.

Are Evolution and the Intracellular Innate Immune System Key Determinants in HIV Transmission?

HIV-1 is the single most important sexually transmitted disease in humans from a global health perspective. Among human lentiviruses, HIV-1 M group has uniquely achieved pandemic levels of human-to-human transmission. The requirement to transmit between hosts likely provides the strongest selective forces on a virus, as without transmission, there can be no new infections within a host population. Our perspective is that evolution of all of the virus-host interactions, which are inherited and perpetuated from host-to-host, must be consistent with transmission. For example, CXCR4 use, which often evolves late in infection, does not favor transmission and is therefore lost when a virus transmits to a new host. Thus, transmission inevitably influences all aspects of virus biology, including interactions with the innate immune system, and dictates the biological niche in which the virus exists in the host. A viable viral niche typically does not select features that disfavor transmission. The innate immune response represents a significant selective pressure during the transmission process. In fact, all viruses must antagonize and/or evade the mechanisms of the host innate and adaptive immune systems that they encounter. We believe that viewing host-virus interactions from a transmission perspective helps us understand the mechanistic details of antiviral immunity and viral escape. This is particularly true for the innate immune system, which typically acts from the very earliest stages of the host-virus interaction, and must be bypassed to achieve successful infection. With this in mind, here we review the innate sensing of HIV, the consequent downstream signaling cascades and the viral restriction that results. The centrality of these mechanisms to host defense is illustrated by the array of countermeasures that HIV deploys to escape them, despite the coding constraint of a 10 kb genome. We consider evasion strategies in detail, in particular the role of the HIV capsid and the viral accessory proteins highlighting important unanswered questions and discussing future perspectives.

Primate lentiviruses use at least three alternative strategies to suppress NF-κB-mediated immune activation

Primate lentiviruses have evolved sophisticated strategies to suppress the immune response of their host species. For example, HIV-2 and most simian immunodeficiency viruses (SIVs) use their accessory protein Nef to prevent T cell activation and antiviral gene expression by downmodulating the T cell receptor CD3. This Nef function was lost in HIV-1 and other vpu-encoding viruses suggesting that the acquisition of Vpu-mediated NF-κB inhibition reduced the selection pressure for inhibition of T cell activation by Nef. To obtain further insights into the modulation of NF-κB activity by primate lentiviral accessory factors, we analyzed 32 Vpr proteins from a large panel of divergent primate lentiviruses. We found that those of SIVcol and SIVolc infecting Colobinae monkeys showed the highest efficacy in suppressing NF-κB activation. Vpr-mediated inhibition of NF-κB resulted in decreased IFNβ promoter activity and suppressed type I IFN induction in virally infected primary cells. Interestingly, SIVcol and SIVolc differ from all other primate lentiviruses investigated by the lack of both, a vpu gene and efficient Nef-mediated downmodulation of CD3. Thus, primate lentiviruses have evolved at least three alternative strategies to inhibit NF-κB-dependent immune activation. Functional analyses showed that the inhibitory activity of SIVolc and SIVcol Vprs is independent of DCAF1 and the induction of cell cycle arrest. While both Vprs target the IKK complex or a factor further downstream in the NF-κB signaling cascade, only SIVolc Vpr stabilizes IκBα and inhibits p65 phosphorylation. Notably, only de-novo synthesized but not virion-associated Vpr suppressed the activation of NF-κB, thus enabling NF-κB-dependent initiation of viral gene transcription during early stages of the replication cycle, while minimizing antiviral gene expression at later stages. Our findings highlight the key role of NF-κB in antiviral immunity and demonstrate that primate lentiviruses follow distinct evolutionary paths to modulate NF-κB-dependent expression of viral and antiviral genes.

The cellular transcription factor NF-κB plays a complex role in the lentiviral replication cycle. On the one hand, activation of NF-κB is required for efficient transcription of viral genes and reactivation of latent proviruses. On the other hand, NF-κB is also a key driver of antiviral gene expression, immune activation and progression to AIDS. As a result, primate lentiviruses tightly regulate the activation of NF-κB throughout their replication cycle to enable transcription of viral genes while minimizing antiviral gene expression. Here, we show that human and simian immunodeficiency viruses have evolved at least three alternative strategies to suppress NF-κB-dependent immune activation: HIV-2 and most SIVs prevent T cell activation via Nef-mediated downmodulation of CD3. In comparison, HIV-1 and its vpu-containing SIV precursors inhibit NF-κB activation via their accessory protein Vpu and lost the CD3 downmodulation function of Nef. Finally, SIVcol and SIVolc, infecting mantled guerezas and olive colobus monkeys, respectively, utilize Vpr. Our findings emphasize the key role of NF-κB as inducer of antiretroviral immune responses and add to the accumulating evidence that lentiviral accessory proteins target innate signaling cascades by sophisticated mechanisms to evade restriction.

Virion-Associated Vpr Alleviates a Postintegration Block to HIV-1 Infection of Dendritic Cells

Viral protein R (Vpr) is an HIV-1 accessory protein whose function remains poorly understood. In this report, we sought to determine the requirement of Vpr for facilitating HIV-1 infection of monocyte-derived dendritic cells (MDDCs), one of the first cell types to encounter virus in the peripheral mucosal tissues. In this report, we characterize a significant restriction of Vpr-deficient virus replication and spread in MDDCs alone and in cell-to-cell spread in MDDC-CD4⁺ T cell cocultures. This restriction of HIV-1 replication in MDDCs was observed in a single round of virus replication and was rescued by the expression of Vpr in trans in the incoming virion. Interestingly, infections of MDDCs with viruses that encode Vpr mutants unable to interact with either the DCAF1/DDB1 E3 ubiquitin ligase complex or a host factor hypothesized to be targeted for degradation by Vpr also displayed a significant replication defect. While the extent of proviral integration in HIV-1-infected MDDCs was unaffected by the absence of Vpr, the transcriptional activity of the viral long terminal repeat (LTR) from Vpr-deficient proviruses was significantly reduced. Together, these results characterize a novel postintegration restriction of HIV-1 replication in MDDCs and show that the interaction of Vpr with the DCAF1/DDB1 E3 ubiquitin ligase complex and the yet-to-be-identified host factor might alleviate this restriction by inducing transcription from the viral LTR. Taken together, these findings identify a robust in vitro cell culture system that is amenable to addressing mechanisms underlying Vpr-mediated enhancement of HIV-1 replication.IMPORTANCE Despite decades of work, the function of the HIV-1 protein Vpr remains poorly understood, primarily due to the lack of an in vitro cell culture system that demonstrates a deficit in replication upon infection with viruses in the absence of Vpr. In this report, we describe a novel cell infection system that utilizes primary human dendritic cells, which display a robust decrease in viral replication upon infection with Vpr-deficient HIV-1. We show that this replication difference occurs in a single round of infection and is due to decreased transcriptional output from the integrated viral genome. Viral transcription could be rescued by virion-associated Vpr. Using mutational analysis, we show that domains of Vpr involved in binding to the DCAF1/DDB1/E3 ubiquitin ligase complex and prevention of cell cycle progression into mitosis are required for LTR-mediated viral expression, suggesting that the evolutionarily conserved G₂ cell cycle arrest function of Vpr is essential for HIV-1 replication.

Activation of the DNA Damage Response Is a Conserved Function of HIV-1 and HIV-2 Vpr That Is Independent of SLX4 Recruitment

There has been extraordinary progress in understanding the roles of lentiviral accessory proteins in antagonizing host antiviral defense proteins. However, the precise primary function of the accessory gene Vpr remains elusive. Here we suggest that engagement with the DNA damage response is an important function of primate lentiviral Vpr proteins because of its conserved function among diverse lentiviral lineages. In contrast, we show that, for HIV-1, HIV-2, and related Vpr isolates and orthologs, there is a lack of correlation between DNA damage response activation and interaction with the host SLX4 protein complex of structure specific endonucleases; some Vpr proteins are able to interact with SLX4, but the majority are not. Using the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 method to knock out SLX4, we formally showed that HIV-1 and HIV-2 Vpr orthologs can still activate the DNA damage response and cell cycle arrest in the absence of SLX4. Together, our data suggest that activation of the DNA damage response, but not SLX4 interaction, is conserved and therefore indicative of an important function of Vpr. Our data also indicate that Vpr activates the DNA damage response through an SLX4-independent mechanism that remains uncharacterized.

HIV-1 and HIV-2 belong to a family of viruses called lentiviruses that infect at least 40 primate species, including humans. Lentiviruses have been circulating in primates for at least 5 million years. In order to better fight HIV, we must understand the viral and host factors necessary for infection, adaptation, and transmission of these viruses. Using the natural variation of HIV-1, HIV-2, and related lentiviruses, we have investigated the role of the DNA damage response in the viral life cycle. We have found that the ability of lentiviruses to activate the DNA damage response is largely conserved. However, we also found that the SLX4 host factor is not required for this activation, as was previously proposed. This indicates that the DNA damage response is an important player in the viral life cycle, and yet the mechanism(s) by which HIV-1, HIV-2, and other primate lentiviruses engage the DNA damage response is still unknown.

SLX4-SLX1 Protein-independent Down-regulation of MUS81-EME1 Protein by HIV-1 Viral Protein R (Vpr)

Evolutionarily conserved structure-selective endonuclease MUS81 forms a complex with EME1 and further associates with another endonuclease SLX4-SLX1 to form a four-subunit complex of MUS81-EME1-SLX4-SLX1, coordinating distinctive biochemical activities of both endonucleases in DNA repair. Viral protein R (Vpr), a highly conserved accessory protein in primate lentiviruses, was previously reported to bind SLX4 to mediate down-regulation of MUS81. However, the detailed mechanism underlying MUS81 down-regulation is unclear. Here, we report that HIV-1 Vpr down-regulates both MUS81 and its cofactor EME1 by hijacking the host CRL4-DCAF1 E3 ubiquitin ligase. Multiple Vpr variants, from HIV-1 and SIV, down-regulate both MUS81 and EME1. Furthermore, a C-terminally truncated Vpr mutant and point mutants R80A and Q65R, all of which lack G2 arrest activity, are able to down-regulate MUS81-EME1, suggesting that Vpr-induced G2 arrest is not correlated with MUS81-EME1 down-regulation. We also show that neither the interaction of MUS81-EME1 with Vpr nor their down-regulation is dependent on SLX4-SLX1. Together, these data provide new insight on a conserved function of Vpr in a host endonuclease down-regulation.

Subversion of Cell Cycle Regulatory Mechanisms by HIV

To establish a productive infection, HIV-1 must counteract cellular innate immune mechanisms and redirect cellular processes toward viral replication. Recent studies have discovered that HIV-1 and other primate immunodeficiency viruses subvert cell cycle regulatory mechanisms to achieve these ends. The viral Vpr and Vpx proteins target cell cycle controls to counter innate immunity. The cell-cycle-related protein Cyclin L2 is also utilized to counter innate immunity. The viral Tat protein utilizes Cyclin T1 to activate proviral transcription, and regulation of Cyclin T1 levels in CD4(+) T cells has important consequences for viral replication and latency. This review will summarize this emerging evidence that primate immunodeficiency viruses subvert cell cycle regulatory mechanisms to enhance replication.

Sensing of HIV-1 Infection in Tzm-bl Cells with Reconstituted Expression of STING

Production of proinflammatory cytokines indicative of potent recognition by the host innate immune system has long been recognized as a hallmark of the acute phase of HIV-1 infection. The first components of the machinery by which primary HIV target cells sense infection have recently been described; however, the mechanistic dissection of innate immune recognition and viral evasion would be facilitated by an easily accessible cell line model. Here we describe that reconstituted expression of the innate signaling adaptor STING enhanced the ability of the well-established HIV reporter cell line Tzm-bl to sense HIV infection and to convert this information into nuclear translocation of IRF3 as well as expression of cytokine mRNA. STING-dependent immune sensing of HIV-1 required virus entry and reverse transcription but not genome integration. Particularly efficient recognition was observed for an HIV-1 variant lacking expression of the accessory protein Vpr, suggesting a role of the viral protein in circumventing STING-mediated immune signaling. Vpr as well as STING significantly impacted the magnitude and breadth of the cytokine mRNA expression profile induced upon HIV-1 infection. However, cytoplasmic DNA sensing did not result in detectable cytokine secretion in this cell system, and innate immune recognition did not affect infection rates. Despite these deficits in eliciting antiviral effector functions, these results establish Tzm-bl STING and Tzm-bl STING IRF3.GFP cells as useful tools for studies aimed at dissecting mechanisms and regulation of early innate immune recognition of HIV infection.

IMPORTANCE

Cell-autonomous immune recognition of HIV infection was recently established as an important aspect by which the host immune system attempts to fend off HIV-1 infection. Mechanistic studies on host cell recognition and viral evasion are hampered by the resistance of many primary HIV target cells to detailed experimental manipulation. We describe here that expression of the signaling adaptor STING renders the well-established HIV reporter cell line Tzm-bl competent for innate recognition of HIV infection. Key characteristics reflected in this cell model include nuclear translocation of IRF3, expression of a broad range of cytokine mRNAs, and an antagonistic activity of the HIV-1 protein Vpr. These results establish Tzm-bl STING and Tzm-bl STING IRF3.GFP cells as a useful tool for studies of innate recognition of HIV infection.

Vpr Enhances Tumor Necrosis Factor Production by HIV-1-Infected T Cells

The HIV-1 accessory protein Vpr displays different activities potentially impacting viral replication, including the arrest of the cell cycle in the G2 phase and the stimulation of apoptosis and DNA damage response pathways. Vpr also modulates cytokine production by infected cells, but this property remains partly characterized. Here, we investigated the effect of Vpr on the production of the proinflammatory cytokine tumor necrosis factor (TNF). We report that Vpr significantly increases TNF secretion by infected lymphocytes. De novo production of Vpr is required for this effect. Vpr mutants known to be defective for G2 cell cycle arrest induce lower levels of TNF secretion, suggesting a link between these two functions. Silencing experiments and the use of chemical inhibitors further implicated the cellular proteins DDB1 and TAK1 in this activity of Vpr. TNF secreted by HIV-1-infected cells triggers NF-κB activity in bystander cells and allows viral reactivation in a model of latently infected cells. Thus, the stimulation of the proinflammatory pathway by Vpr may impact HIV-1 replication in vivo.

IMPORTANCE

The role of the HIV-1 accessory protein Vpr remains only partially characterized. This protein is important for viral pathogenesis in infected individuals but is dispensable for viral replication in most cell culture systems. Some of the functions described for Vpr remain controversial. In particular, it remains unclear whether Vpr promotes or instead prevents proinflammatory and antiviral immune responses. In this report, we show that Vpr promotes the release of TNF, a proinflammatory cytokine associated with rapid disease progression. Using Vpr mutants or inhibiting selected cellular genes, we show that the cellular proteins DDB1 and TAK1 are involved in the release of TNF by HIV-infected cells. This report provides novel insights into how Vpr manipulates TNF production and helps clarify the role of Vpr in innate immune responses and inflammation.

Molecular determinants for recognition of divergent SAMHD1 proteins by the lentiviral accessory protein Vpx

The SAMHD1 triphosphohydrolase inhibits HIV-1 infection of myeloid and resting T cells by depleting dNTPs. To overcome SAMHD1, HIV-2 and some SIVs encode either of two lineages of the accessory protein Vpx that bind the SAMHD1 N or C terminus and redirect the host cullin-4 ubiquitin ligase to target SAMHD1 for proteasomal degradation. We present the ternary complex of Vpx from SIV that infects mandrills (SIVmnd-2) with the cullin-4 substrate receptor, DCAF1, and N-terminal and SAM domains from mandrill SAMHD1. The structure reveals details of Vpx lineage-specific targeting of SAMHD1 N-terminal "degron" sequences. Comparison with Vpx from SIV that infects sooty mangabeys (SIVsmm) complexed with SAMHD1-DCAF1 identifies molecular determinants directing Vpx lineages to N- or C-terminal SAMHD1 sequences. Inspection of the Vpx-DCAF1 interface also reveals conservation of Vpx with the evolutionally related HIV-1/SIV accessory protein Vpr. These data suggest a unified model for how Vpx and Vpr exploit DCAF1 to promote viral replication.

HIV-1 Vpr suppresses the cytomegalovirus promoter in a CRL4(DCAF1) E3 ligase independent manner

Although the Vpr protein of human immunodeficiency virus type 1 (HIV-1) has been shown to act as a transcriptional activator of the HIV-1 LTR and certain host genes, the current study demonstrates that it can also function as a potent inhibitor of the cytomegalovirus (CMV) promoter. Previous studies have shown that the cell cycle arrest and apoptotic functions of Vpr required recruitment of the CRL4(DCAF1) E3 ligase, but this complex is shown not to be required for inhibition of the CMV promoter. We identified conserved sites (A30/V31) from diverse Vpr from HIV/SIV that were critical for blocking the CMV promoter activity. Interestingly, the Vpr mutant A30S/V31S protein also impaired the ability of Vpr to down-regulate transcription of the host UNG2 gene. Our findings shed light on the dual functions of Vpr on the transcription of HIV-1, other viruses and host genes which may contribute to viral replication and disease progression in vivo.

Determinants for degradation of SAMHD1, Mus81 and induction of G2 arrest in HIV-1 Vpr and SIVagm Vpr

Vpr and Vpx are a group of highly related accessory proteins from primate lentiviruses. Despite the high degree of amino acid homology within this group, these proteins can be highly divergent in their functions. In this work, we constructed chimeric and mutant proteins between HIV-1 and SIVagm Vpr in order to better understand the structure-function relationships. We tested these constructs for their abilities to induce G2 arrest in human cells and to degrade agmSAMHD1 and Mus81. We found that the C-terminus of HIV-1 Vpr, when transferred onto SIVagm Vpr, provides the latter with the de novo ability to induce G2 arrest in human cells. We confirmed that HIV-1 Vpr induces degradation of Mus81 although, surprisingly, degradation is independent and genetically separable from Vpr׳s ability to induce G2 arrest.

How SLX4 cuts through the mystery of HIV-1 Vpr-mediated cell cycle arrest

Vpr is one of the most enigmatic viral auxiliary proteins of HIV. During the past twenty years, several activities have been ascribed to this viral protein, but one, its ability to mediate cell cycle arrest at the G2 to M transition has been the most extensively studied. Nonetheless, the genuine role of Vpr and its pathophysiological relevance in the viral life cycle have remained mysterious. Recent work by Laguette et al. (Cell 156:134-145, 2014) provides important insight into the molecular mechanism of Vpr-mediated G2 arrest. This study highlights for the first time how Vpr recruits the SLX4 endonuclease complex and how Vpr-induced inappropriate activation of this complex leads to G2 arrest. Here, we will discuss these findings in the light of previous work to show how they change the view of Vpr’s mechanism of action. We will also discuss how these findings open new questions towards the understanding of the biological function of Vpr regarding innate immune sensing.