Evidence for a cytopathogenicity determinant in HIV-1 Vpr

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.

In silico investigation of binding propensity of hematoxylin derivative and damnacanthal for their potential inhibitory effect on HIV-1 Vpr from different subtypes

HIV-1, the causative agent of AIDS leads to many deaths worldwide though few options are available as therapeutics. To deal with the continuous mutation in the virus genome, requirement of new drugs is always there. Subtype variation plays a crucial role in case of HIV-1 therapeutics development. In this study, we want to investigate some pre examined molecules that can be effective for HIV-1 VPR. Inhibition of several protein-protein interactions with the small molecules will lead to identify some molecules as therapeutics other than the conventional drugs. We retrieved the sequences of different subtypes from the database and representative sequences were identified. Representative structures were modelled and validated using MD simulations. Forty molecules, showing anti Vpr activity in vitro were identified from literature survey and those were docked with each subtype representative structures. Two molecules a stable Hematoxylin Derivative (SHD) and Damnacanthal (D3), these were shown to be bind more effectively for all the subtypes. The stability of the protein and those two small molecule complexes were identified again with MD simulation followed by the binding energy calculation. Thus, these molecules can be thought as any option other than the conventional drug targeting HIV-1 Vpr.Communicated by Ramaswamy H. Sarma.

Important role of microglia in HIV-1 associated neurocognitive disorders and the molecular pathways implicated in its pathogenesis

The development of effective combined anti-retroviral therapy (cART) led to a significant reduction in the death rate associated with human immunodeficiency virus type 1 (HIV-1) infection. However, recent studies indicate that considerably more than 50% of all HIV-1 infected patients develop HIV-1-associated neurocognitive disorder (HAND). Microglia are the foremost cells infected by HIV-1 in the central nervous system (CNS), and so, are also likely to contribute to the neurotoxicity observed in HAND. The activation of microglia induces the release of pro-inflammatory markers and altered secretion of cytokines, chemokines, secondary messengers, and reactive oxygen species (ROS) which activate signalling pathways that initiate neuroinflammation. In turn, ROS and inflammation also play critical roles in HAND. However, more efforts are required to understand the physiology of microglia and the processes involved in their activation in order to better understand the how HIV-1-infected microglia are involved in the development of HAND. In this review, we summarize the current state of knowledge about the involvement of oxidative stress mechanisms and role of HIV-induced ROS in the development of HAND. We also examine the academic literature regarding crucial HIV-1 pathogenicity factors implicated in neurotoxicity and inflammation in order to identify molecular pathways that could serve as potential therapeutic targets for treatment of this disease. KEY MESSAGES Neuroinflammation and excitotoxicity mechanisms are crucial in the pathogenesis of HAND. CNS infiltration by HIV-1 and immune cells through the blood brain barrier is a key process involved in the pathogenicity of HAND. Factors including calcium dysregulation and autophagy are the main challenges involved in HAND.

D. de Azevedo S, Delatorre E, Bello G. Few amino acid signatures distinguish HIV-1 subtype B pandemic and non-pandemic strains

The Human Immunodeficiency Virus Type I (HIV-1) subtype B comprises approximately 10% of all HIV infections in the world. The HIV-1 subtype B epidemic comprehends a pandemic variant (named B_PANDEMIC) disseminated worldwide and non-pandemic variants (named B_CAR) that are mostly restricted to the Caribbean. The goal of this work was the identification of amino acid signatures (AAs) characteristic to the B_CAR and B_PANDEMIC variants. To this end, we analyzed HIV-1 subtype B full-length (n = 486) and partial (n = 814) genomic sequences from the Americas classified within the B_CAR and B_PANDEMIC clades and reconstructed the sequences of their most recent common ancestors (MRCA). Analysis of contemporary HIV-1 sequences revealed 13 AAs between B_CAR and B_PANDEMIC variants (four on Gag, three on Pol, three on Rev, and one in Vif, Vpu, and Tat) of which only two (one on Gag and one on Pol) were traced to the MRCA. All AAs correspond to polymorphic sites located outside essential functional proteins domains, except the AAs in Tat. The absence of stringent AAs inherited from their ancestors between modern B_CAR and B_PANDEMIC variants support that ecological factors, rather than viral determinants, were the main driving force behind the successful spread of the B_PANDEMIC strain.

HIV Vpr Modulates the Host DNA Damage Response at Two Independent Steps to Damage DNA and Repress Double-Strand DNA Break Repair

The DNA damage response (DDR) is a signaling cascade that is vital to ensuring the fidelity of the host genome in the presence of genotoxic stress. Growing evidence has emphasized the importance of both activation and repression of the host DDR by diverse DNA and RNA viruses. Previous work has shown that HIV-1 is also capable of engaging the host DDR, primarily through the conserved accessory protein Vpr. However, the extent of this engagement has remained unclear. Here, we show that HIV-1 and HIV-2 Vpr directly induce DNA damage and stall DNA replication, leading to the activation of several markers of double- and single-strand DNA breaks. Despite causing damage and activating the DDR, we found that Vpr represses the repair of double-strand breaks (DSB) by inhibiting homologous recombination (HR) and nonhomologous end joining (NHEJ). Mutational analyses of Vpr revealed that DNA damage and DDR activation are independent from repression of HR and Vpr-mediated cell cycle arrest. Moreover, we show that repression of HR does not require cell cycle arrest but instead may precede this long-standing enigmatic Vpr phenotype. Together, our data uncover that Vpr globally modulates the host DDR at at least two independent steps, offering novel insight into the primary functions of lentiviral Vpr and the roles of the DNA damage response in lentiviral replication.IMPORTANCE The DNA damage response (DDR) is a signaling cascade that safeguards the genome from genotoxic agents, including human pathogens. However, the DDR has also been utilized by many pathogens, such as human immunodeficiency virus (HIV), to enhance infection. To properly treat HIV-positive individuals, we must understand how the virus usurps our own cellular processes. Here, we have found that an important yet poorly understood gene in HIV, Vpr, targets the DDR at two unique steps: it causes damage and activates DDR signaling, and it represses the ability of cells to repair this damage, which we hypothesize is central to the primary function of Vpr. In clarifying these important functions of Vpr, our work highlights the multiple ways human pathogens engage the DDR and further suggests that modulation of the DDR is a novel way to help in the fight against HIV.

Evolution of a concept: From accessory protein to key virulence factor, the case of HIV-1 Vpr

Back in 1989 some studies have shown that the viral protein Vpr was dispensable for HIV-1 replication in vitro. From then the concept of accessory or auxiliary protein for Vpr has emerged and it is still used to date. However, Vpr soon appeared to be very important for in vivo virus spread and pathogenesis. Vpr has been involved in many biological functions including regulation of reverse transcriptase activity, the nuclear import of the pre-integration complex (PIC), HIV-1 transcription, gene splicing, apoptosis and in cell cycle arrest. Thus, we might rather consider Vpr as a true virulence factor instead of just an accessory factor. At present, Vpr can be regarded as a potential and promising target in different strategies aiming to fight infected cells including latently infected cells.

Delineation of Homeostatic Immune Signatures Defining Viremic Non-progression in HIV-1 Infection

Viremic non-progressors (VNPs), a distinct group of HIV-1-infected individuals, exhibit no signs of disease progression and maintain persistently elevated CD4+ T cell counts for several years despite high viral replication. Comprehensive characterization of homeostatic cellular immune signatures in VNPs can provide unique insights into mechanisms responsible for coping with viral pathogenesis as well as identifying strategies for immune restoration under clinically relevant settings such as antiretroviral therapy (ART) failure. We report a novel homeostatic signature in VNPs, the preservation of the central memory CD4+ T cell (CD4+ T_CM) compartment. In addition, CD4+ T_CM preservation was supported by ongoing interleukin-7 (IL-7)-mediated thymic repopulation of naive CD4+ T cells leading to intact CD4+ T cell homeostasis in VNPs. Regulatory T cell (Treg) expansion was found to be a function of preserved CD4+ T cell count and CD4+ T cell activation independent of disease status. However, in light of continual depletion of CD4+ T cell count in progressors but not in VNPs, Tregs appear to be involved in lack of disease progression despite high viremia. In addition to these homeostatic mechanisms resisting CD4+ T cell depletion in VNPs, a relative diminution of terminally differentiated effector subset was observed exclusively in these individuals that might ameliorate consequences of high viral replication. VNPs also shared signatures of impaired CD8+ T cell cytotoxic function with progressors evidenced by increased exhaustion (PD-1 upregulation) and CD127 (IL-7Rα) downregulation contributing to persistent viremia. Thus, the homeostatic immune signatures reported in our study suggest a complex multifactorial mechanism accounting for non-progression in VNPs.

Retroviral restriction: nature's own solution

PURPOSE OF REVIEW

The present review will discuss recent advances in the development of anti-HIV therapies inspired by studies of the mechanisms of host restriction factor-mediated resistance to HIV infection.

RECENT FINDINGS

Manipulating the interplay between host cell restriction factors and viral accessory factors that overcome them can potentially be therapeutically useful. Preliminarily successful therapies - some of which are entering clinical trials - either inhibit the ability of virus to evade restriction factor-mediated immunity, or promote intracellular levels of restriction factors. These aims are achieved by multiple means, which are discussed.

SUMMARY

Many restriction factors appear to provide potentially useful targets for anti-HIV therapies, so time and interest should be invested in investigating ways to successfully therapeutically manipulate restriction factor-mediated immunity.

Yeast for virus research

Budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are two popular model organisms for virus research. They are natural hosts for viruses as they carry their own indigenous viruses. Both yeasts have been used for studies of plant, animal and human viruses. Many positive sense (+) RNA viruses and some DNA viruses replicate with various levels in yeasts, thus allowing study of those viral activities during viral life cycle. Yeasts are single cell eukaryotic organisms. Hence, many of the fundamental cellular functions such as cell cycle regulation or programed cell death are highly conserved from yeasts to higher eukaryotes. Therefore, they are particularly suited to study the impact of those viral activities on related cellular activities during virus-host interactions. Yeasts present many unique advantages in virus research over high eukaryotes. Yeast cells are easy to maintain in the laboratory with relative short doubling time. They are non-biohazardous, genetically amendable with small genomes that permit genome-wide analysis of virologic and cellular functions. In this review, similarities and differences of these two yeasts are described. Studies of virologic activities such as viral translation, viral replication and genome-wide study of virus-cell interactions in yeasts are highlighted. Impacts of viral proteins on basic cellular functions such as cell cycle regulation and programed cell death are discussed. Potential applications of using yeasts as hosts to carry out functional analysis of small viral genome and to develop high throughput drug screening platform for the discovery of antiviral drugs are presented.

The HIV-1 Vpr Protein: A Multifaceted Target for Therapeutic Intervention

The human immunodeficiency virus type 1 (HIV-1) Vpr protein is an attractive target for antiretroviral drug development. The conservation both of the structure along virus evolution and the amino acid sequence in viral isolates from patients underlines the importance of Vpr for the establishment and progression of HIV-1 disease. While its contribution to virus replication in dividing and non-dividing cells and to the pathogenesis of HIV-1 in many different cell types, both extracellular and intracellular forms, have been extensively studied, its precise mechanism of action nevertheless remains enigmatic. The present review discusses how the apparently multifaceted interplay between Vpr and host cells may be due to the impairment of basic metabolic pathways. Vpr protein modifies host cell energy metabolism, oxidative status, and proteasome function, all of which are likely conditioned by the concentration and multimerization of the protein. The characterization of Vpr domains along with new laboratory tools for the assessment of their function has become increasingly relevant in recent years. With these advances, it is conceivable that drug discovery efforts involving Vpr-targeted antiretrovirals will experience substantial growth in the coming years.

HIV-1 Vpr N-terminal tagging affects alternative splicing of the viral genome

To facilitate studies on Vpr function in replicating HIV-1, we aimed to tag the protein in an infectious virus. First we showed that N-, but not C-terminal HA/FLAG tagging of Vpr protein preserves Vpr cytopathicity. Cloning the tags into proviral DNA however ablated viral production and replication. By construction of additional viral variants we could show this defect was not protein- but RNA-dependent and sequence specific, and characterized by oversplicing of the genomic RNA. Simulation of genomic RNA folding suggested that introduction of the tag sequence induced an alternative folding structure in a region enriched in splice sites and splicing regulatory sequences. In silico predictions identified the HA/His₆-Vpr tagging in HIV-1 to affect mRNA folding less than HA/FLAG-Vpr tagging. In vitro infectivity and mRNA splice pattern improved but did not reach wild-type values. Thus, sequence-specific insertions may interfere with mRNA splicing, possibly due to altered RNA folding. Our results point to the complexity of viral RNA genome sequence interactions. This should be taken into consideration when designing viral manipulation strategies, for both research as for biological interventions.

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.

Identification of two unique naturally occurring Vpr sequence polymorphisms associated with clinical parameters in HIV-1 chronic infection

HIV-1 viral protein R (Vpr) plays important roles in HIV-1 replication. Despite the identification of a number of HLA class I-associated immune escape mutations; it is yet known whether immune-driven Vpr polymorphisms are associated with disease outcome. Hereby, we comprehensively analyzed Vpr sequence polymorphisms and their association with disease outcome and host HLA genotypes, by using plasma viral RNA isolated from 444 HLA-typed, treatment-naïve, chronically HIV-1 infected individuals. Vpr amino acid residues at positions 13, 37, 45, 55, 63, 77, 84, 85, 86, and 93 were significantly associated with patients' plasma viral load and/or CD4 count. Further analysis revealed Ala at position 55 was significantly associated with lower plasma viral load; and Thr at position 63 was significantly associated with lower plasma viral load and higher CD4 count. Also, the number of amino acid residues at the two positions, located in a functionally important α-helical domain, correlated inversely with plasma viral load and positively with CD4 count. Moreover, a phylogenetically corrected method revealed residues at positions 55 and 63 are associated with patients' HLA genotypes. Taken together, our results suggest that Vpr polymorphisms at functionally important and immune-reactive sites may contribute, at least in part, to viral replication and disease outcome in vivo. J. Med. Virol. 89:123-129, 2017. © 2016 Wiley Periodicals, Inc.

Retroviral DNA Integration

The integration of a DNA copy of the viral RNA genome into host chromatin is the defining step of retroviral replication. This enzymatic process is catalyzed by the virus-encoded integrase protein, which is conserved among retroviruses and LTR-retrotransposons. Retroviral integration proceeds via two integrase activities: 3'-processing of the viral DNA ends, followed by the strand transfer of the processed ends into host cell chromosomal DNA. Herein we review the molecular mechanism of retroviral DNA integration, with an emphasis on reaction chemistries and architectures of the nucleoprotein complexes involved. We additionally discuss the latest advances on anti-integrase drug development for the treatment of AIDS and the utility of integrating retroviral vectors in gene therapy applications.

Defining the roles for Vpr in HIV-1-associated neuropathogenesis

It is increasingly evident that the human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) has a unique role in neuropathogenesis. Its ability to induce G2/M arrest coupled with its capacity to increase viral gene transcription gives it a unique role in sustaining viral replication and aiding in the establishment and maintenance of a systemic infection. The requirement of Vpr for HIV-1 infection and replication in cells of monocytic origin (a key lineage of cells involved in HIV-1 neuroinvasion) suggests an important role in establishing and sustaining infection in the central nervous system (CNS). Contributions of Vpr to neuropathogenesis can be expanded further through (i) naturally occurring HIV-1 sequence variation that results in functionally divergent Vpr variants; (ii) the dual activities of Vpr as a intracellular protein delivered and expressed during HIV-1 infection and as an extracellular protein that can act on neighboring, uninfected cells; (iii) cell type-dependent consequences of Vpr expression and exposure, including cell cycle arrest, metabolic dysregulation, and cytotoxicity; and (iv) the effects of Vpr on exosome-based intercellular communication in the CNS. Revealing that the effects of this pleiotropic viral protein is an essential part of a greater understanding of HIV-1-associated pathogenesis and potential approaches to treating and preventing disease caused by HIV-1 infection.

Vpr overcomes macrophage-specific restriction of HIV-1 Env expression and virion production

The HIV-1 accessory protein Vpr enhances infection of primary macrophages through unknown mechanisms. Recent studies demonstrated that Vpr interactions with the cellular DCAF1-DDB1-CUL4 E3 ubiquitin ligase complex limit activation of innate immunity and interferon (IFN) induction. We describe a restriction mechanism that targets the HIV-1 envelope protein Env, but is overcome by Vpr and its interaction with DCAF1. This restriction is active in the absence of Vpr in HIV-1-infected primary macrophages and macrophage-epithelial cell heterokaryons, but not epithelial cell lines. HIV-1-infected macrophages lacking Vpr express more IFN following infection, target Env for lysosomal degradation, and produce fewer Env-containing virions. Conversely, Vpr expression reduces IFN induction, rescues Env expression, and enhances virion release. Addition of IFN or silencing DCAF1 reduces the amount of cell-associated Env and virion production in wild-type HIV-1-infected primary macrophages. These findings provide insight into an IFN-stimulated macrophage-specific restriction pathway targeting HIV-1 Env that is counteracted by Vpr.

R77Q and Q3R HIV1-VPR mutations in an otherwise asymptomatic 5-year-old child with repeated ear infections

INTRODUCTION

Viral protein R (Vpr) of human immunodeficiency virus type 1 (HIV-1) has been described as being involved in the progression of AIDS, and specific mutations are associated with long-term non-progressor patients.

CASE PRESENTATION

We describe the case of a child with repeated ear infections who was otherwise healthy. The patient, a 5-year-old boy, was HIV-1 positive and the viral load at admission was 1 073 899 RNA copies ml^-1 and 0 % CD4⁺ lymphocytes. A detailed study of the vpr gene sequence of the child revealed mutations leading to amino acid substitutions at positions 3 and 77.

CONCLUSION

The case reported provides clinical support of previous findings that show that the R77Q and Q3R HIV-1 Vpr variants are associated with patients with delayed disease progression.

Visualizing Vpr-induced G2 arrest and apoptosis

Vpr is an accessory protein of human immunodeficiency virus type 1 (HIV-1) with multiple functions. The induction of G2 arrest by Vpr plays a particularly important role in efficient viral replication because the transcriptional activity of the HIV-1 long terminal repeat is most active in G2 phase. The regulation of apoptosis by Vpr is also important for immune suppression and pathogenesis during HIV infection. However, it is not known whether Vpr-induced apoptosis depends on the ability of Vpr to induce G2 arrest, and the dynamics of Vpr-induced G2 arrest and apoptosis have not been visualized. We performed time-lapse imaging to examine the temporal relationship between Vpr-induced G2 arrest and apoptosis using HeLa cells containing the fluorescent ubiquitination-based cell cycle indicator2 (Fucci2). The dynamics of G2 arrest and subsequent long-term mitotic cell rounding in cells transfected with the Vpr-expression vector were visualized. These cells underwent nuclear mis-segregation after prolonged mitotic processes and then entered G1 phase. Some cells subsequently displayed evidence of apoptosis after prolonged mitotic processes and nuclear mis-segregation. Interestingly, Vpr-induced apoptosis was seldom observed in S or G2 phase. Likewise, visualization of synchronized HeLa/Fucci2 cells infected with an adenoviral vector expressing Vpr clearly showed that Vpr arrests the cell cycle at G2 phase, but does not induce apoptosis at S or G2 phase. Furthermore, time-lapse imaging of HeLa/Fucci2 cells expressing SCAT3.1, a caspase-3-sensitive fusion protein, clearly demonstrated that Vpr induces caspase-3-dependent apoptosis. Finally, to examine whether the effects of Vpr on G2 arrest and apoptosis were reversible, we performed live-cell imaging of a destabilizing domain fusion Vpr, which enabled rapid stabilization and destabilization by Shield1. The effects of Vpr on G2 arrest and subsequent apoptosis were reversible. This study is the first to characterize the dynamics of the morphological changes that occur during Vpr-induced G2 arrest and apoptosis.

Human immunodeficiency virus type 1 Vpr polymorphisms associated with progressor and nonprogressor individuals alter Vpr-associated functions

Following infection with Human immunodeficiency virus 1 (HIV-1) there is a remarkable variation in virus replication and disease progression. Both host and viral factors have been implicated in the observed differences in disease status. Here, we focus on understanding the contribution of HIV-1 viral protein R (Vpr) by evaluating the disease-associated Vpr polymorphism and its biological functions from HIV-1 positive rapid progressor (RP) and long-term nonprogressor (LTNP) subjects. Results presented here show distinct variation in phenotypes of Vpr alleles from LTNP and RP subjects. Most notably, the polymorphism of Vpr at R36W and L68M associated with RP shows higher levels of oligomerization, and increased virus replication, whereas R77Q exhibits poor replication kinetics. Interestingly, we did not observe correlation with cell cycle arrest function. Together these results indicate that polymorphisms in Vpr in part may contribute to altered virus replication kinetics leading to the observed differences in disease progression in LTNP and RP groups.

The progestin-only contraceptive medroxyprogesterone acetate, but not norethisterone acetate, enhances HIV-1 Vpr-mediated apoptosis in human CD4+ T cells through the glucocorticoid receptor

The glucocorticoid receptor (GR) regulates several physiological functions, including immune function and apoptosis. The HIV-1 virus accessory protein, viral protein R (Vpr), can modulate the transcriptional response of the GR. Glucocorticoids (GCs) and Vpr have been reported to induce apoptosis in various cells, including T-cells. We have previously shown that the injectable contraceptive, medroxyprogesterone acetate (MPA) is a partial to full agonist for the GR, unlike norethisterone acetate (NET-A). We investigated the functional cross talk between the GR and Vpr in inducing apoptosis in CD4(+) T-cells, in the absence and presence of GCs and these progestins, as well as progesterone. By using flow cytometry, we show that, in contrast to NET-A and progesterone, the synthetic GR ligand dexamethasone (Dex), cortisol and MPA induce apoptosis in primary CD4(+) T-cells. Furthermore, the C-terminal part of the Vpr peptide, or HIV-1 pseudovirus, together with Dex or MPA further increased the apoptotic phenotype, unlike NET-A and progesterone. By a combination of Western blotting, PCR and the use of receptor- selective agonists, we provide evidence that the GR and the estrogen receptor are the only steroid receptors expressed in peripheral blood mononuclear cells. These results, together with the findings that RU486, a GR antagonist, prevents Dex-, MPA- and Vpr-mediated apoptosis, provide evidence for the first time that GR agonists or partial agonists increase apoptosis in primary CD4(+) T-cells via the GR. We show that apoptotic induction involves differential expression of key apoptotic genes by both Vpr and GCs/MPA. This work suggests that contraceptive doses of MPA but not NET-A or physiological doses of progesterone could potentially accelerate depletion of CD4(+) T-cells in a GR-dependent fashion in HIV-1 positive women, thereby contributing to immunodeficiency. The results imply that choice of progestin used in contraception may be critical to susceptibility and progression of diseases such as HIV-1.

NOT THAT RIGID MIDGETS AND NOT SO FLEXIBLE GIANTS: ON THE ABUNDANCE AND ROLES OF INTRINSIC DISORDER IN SHORT AND LONG PROTEINS

Intrinsically disordered proteins or proteins with disordered regions are very common in nature. These proteins have numerous biological functions which are complementary to the biological activities of traditional ordered proteins. A noticeable difference in the amino acid sequences encoding long and short disordered regions was found and this difference was used in the development of length-dependent predictors of intrinsic disorder. In this study, we analyze the scaling of intrinsic disorder in eukaryotic proteins and investigate the presence of length-dependent functions attributed to proteins containing long disordered regions.

Molecular mechanisms of HIV immune evasion of the innate immune response in myeloid cells

The expression of intrinsic antiviral factors by myeloid cells is a recently recognized mechanism of restricting lentiviral replication. Viruses that enter these cells must develop strategies to evade cellular antiviral factors to establish a productive infection. By studying the cellular targets of virally encoded proteins that are necessary to infect myeloid cells, a better understanding of cellular intrinsic antiviral strategies has now been achieved. Recent findings have provided insight into how the lentiviral accessory proteins, Vpx, Vpr and Vif counteract antiviral factors found in myeloid cells including SAMHD1, APOBEC3G, APOBEC3A, UNG2 and uracil. Here we review our current understanding of the molecular basis of how cellular antiviral factors function and the viral countermeasures that antagonize them to promote viral transmission and spread.

Evidence for Vpr-dependent HIV-1 replication in human CD4+ CEM.NKR T-cells

BACKGROUND

Vpr is exclusively expressed in primate lentiviruses and contributes to viral replication and disease progression in vivo. HIV-1 Vpr has two major activities in vitro: arrest of cell cycle in the G2 phase (G2 arrest), and enhancement of viral replication in macrophages. Previously, we reported a potent HIV-1 restriction in the human CD4+ CEM.NKR (NKR) T cells, where wild-type (WT) HIV-1 replication was inhibited by almost 1,000-fold. From the parental NKR cells, we isolated eight clones by limiting dilution. These clones showed three levels of resistance to the WT HIV-1 infection: non-permissive (NP), semi-permissive (SP), and permissive (P). Here, we compared the replication of WT, Vif-defective, Vpr-defective, and Vpu-defective viruses in these cells.

RESULTS

Although both WT and Vpu-defective viruses could replicate in the permissive and semi-permissive clones, the replication of Vif-defective and Vpr-defective viruses was completely restricted. The expression of APOBEC3G (A3G) cytidine deaminase in NKR cells explains why Vif, but not Vpr, was required for HIV-1 replication. When the Vpr-defective virus life cycle was compared with the WT virus life cycle in the semi-permissive cells, it was found that the Vpr-defective virus could enter the cell and produce virions containing properly processed Gag and Env proteins, but these virions showed much less efficiency for reverse transcription during the next-round of infection. In addition, although viral replication was restricted in the non-permissive cells, treatment with arsenic trioxide (As2O3) could completely restore WT, but not Vpr-defective virus replication. Moreover, disruption of Vpr binding to its cofactor DCAF1 and/or induction of G2 arrest activity did not disrupt the Vpr activity in enhancing HIV-1 replication in NKR cells.

CONCLUSIONS

These results demonstrate that HIV-1 replication in NKR cells is Vpr-dependent. Vpr promotes HIV-1 replication from the 2nd cycle likely by overcoming a block at early stage of viral replication; and this activity does not require DCAF1 and G2 arrest. Further studies of this mechanism should provide new understanding of Vpr function in the HIV-1 life cycle.

Protein intrinsic disorder as a flexible armor and a weapon of HIV-1

Many proteins and protein regions are disordered in their native, biologically active states. These proteins/regions are abundant in different organisms and carry out important biological functions that complement the functional repertoire of ordered proteins. Viruses, with their highly compact genomes, small proteomes, and high adaptability for fast change in their biological and physical environment utilize many of the advantages of intrinsic disorder. In fact, viral proteins are generally rich in intrinsic disorder, and intrinsically disordered regions are commonly used by viruses to invade the host organisms, to hijack various host systems, and to help viruses in accommodation to their hostile habitats and to manage their economic usage of genetic material. In this review, we focus on the structural peculiarities of HIV-1 proteins, on the abundance of intrinsic disorder in viral proteins, and on the role of intrinsic disorder in their functions.

The host-pathogen interaction of human cyclophilin A and HIV-1 Vpr requires specific N-terminal and novel C-terminal domains

BACKGROUND

Cyclophilin A (CypA) represents a potential key molecule in future antiretroviral therapy since inhibition of CypA suppresses human immunodeficiency virus type 1 (HIV-1) replication. CypA interacts with the virus proteins Capsid (CA) and Vpr, however, the mechanism through which CypA influences HIV-1 infectivity still remains unclear.

RESULTS

Here the interaction of full-length HIV-1 Vpr with the host cellular factor CypA has been characterized and quantified by surface plasmon resonance spectroscopy. A C-terminal region of Vpr, comprising the 16 residues 75GCRHSRIGVTRQRRAR90, with high binding affinity for CypA has been identified. This region of Vpr does not contain any proline residues but binds much more strongly to CypA than the previously characterized N-terminal binding domain of Vpr, and is thus the first protein binding domain to CypA described involving no proline residues. The fact that the mutant peptide Vpr75-90 R80A binds more weakly to CypA than the wild-type peptide confirms that Arg-80 is a key residue in the C-terminal binding domain. The N- and C-terminal binding regions of full-length Vpr bind cooperatively to CypA and have allowed a model of the complex to be created. The dissociation constant of full-length Vpr to CypA was determined to be approximately 320 nM, indicating that the binding may be stronger than that of the well characterized interaction of HIV-1 CA with CypA.

CONCLUSIONS

For the first time the interaction of full-length Vpr and CypA has been characterized and quantified. A non-proline-containing 16-residue region of C-terminal Vpr which binds specifically to CypA with similar high affinity as full-length Vpr has been identified. The fact that this is the first non-proline containing binding motif of any protein found to bind to CypA, changes the view on how CypA is able to interact with other proteins. It is interesting to note that several previously reported key functions of HIV-1 Vpr are associated with the identified N- and C-terminal binding domains of the protein to CypA.

Vpr-host interactions during HIV-1 viral life cycle

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) is a multifunctional viral protein that plays important role at multiple stages of the HIV-1 viral life cycle. Although the molecular mechanisms underlying these activities are subject of ongoing investigations, overall, these activities have been linked to promotion of viral replication and impairment of anti-HIV immunity. Importantly, functional defects of Vpr have been correlated with slow disease progression of HIV-infected patients. Vpr is required for efficient viral replication in non-dividing cells such as macrophages, and it promotes, to some extent, viral replication in proliferating CD4+ T cells. The specific activities of Vpr include modulation of fidelity of viral reverse transcription, nuclear import of the HIV-1 pre-integration complex, transactivation of the HIV-1 LTR promoter, induction of cell cycle G2 arrest and cell death via apoptosis. In this review, we focus on description of the cellular proteins that specifically interact with Vpr and discuss their significance with regard to the known Vpr activities at each step of the viral life cycle in proliferating and non-proliferating cells.

Functional integrity of naturally occurring mutants of HIV-1 subtype C Vpr

HIV-1 Vpr, an accessory protein with multiple functions, is involved in the induction of apoptosis, cell cycle G2 arrest, and modulation of gene expression. Many functions of this protein have been documented for the wild-type subtype B Vpr, however the functionality of other subtypes has not sufficiently been addressed. In this study, the functionality of Subtype B Vpr, 6 subtype C mutant Vpr proteins and the consensus sequence of subtype C Vpr were compared with each other. All the subtype B and C Vpr proteins localized to the nucleus of human 293T cells. Subtype C Vpr proteins induced cell cycle G2 arrest in a lower proportion of human 293T cells compared to subtype B Vpr. Subtype B and the naturally mutant Vpr proteins induced apoptosis in a similar manner, ranging from 95.33% to 98.64%. However, an artificially designed Vpr protein containing the consensus sequences of subtype C Vpr indicated a reduced ability in induction of apoptosis. The study of mRNA profile of the transfected cells indicated that all Vpr proteins modulated the apoptotic genes triggering the intrinsic pathway of apoptosis. Our results indicate that subtype C Vpr is able to exert the same functions previously reported for subtype B Vpr. Most natural mutations in Vpr not only do not disturb the functions of the protein but also potentiate the protein for an increased functionality. The natural mutations of Vpr may thus not always be regarded as defective mutations. The study suggests the adaptive role of the natural mutations commonly found in subtype C Vpr.

Cell cycle G2/M arrest through an S phase-dependent mechanism by HIV-1 viral protein R

Background

Cell cycle G2 arrest induced by HIV-1 Vpr is thought to benefit viral proliferation by providing an optimized cellular environment for viral replication and by skipping host immune responses. Even though Vpr-induced G2 arrest has been studied extensively, how Vpr triggers G2 arrest remains elusive.

Results

To examine this initiation event, we measured the Vpr effect over a single cell cycle. We found that even though Vpr stops the cell cycle at the G2/M phase, but the initiation event actually occurs in the S phase of the cell cycle. Specifically, Vpr triggers activation of Chk1 through Ser³⁴⁵ phosphorylation in an S phase-dependent manner. The S phase-dependent requirement of Chk1-Ser³⁴⁵ phosphorylation by Vpr was confirmed by siRNA gene silencing and site-directed mutagenesis. Moreover, downregulation of DNA replication licensing factors Cdt1 by siRNA significantly reduced Vpr-induced Chk1-Ser³⁴⁵ phosphorylation and G2 arrest. Even though hydroxyurea (HU) and ultraviolet light (UV) also induce Chk1-Ser³⁴⁵ phosphorylation in S phase under the same conditions, neither HU nor UV-treated cells were able to pass through S phase, whereas vpr-expressing cells completed S phase and stopped at the G2/M boundary. Furthermore, unlike HU/UV, Vpr promotes Chk1- and proteasome-mediated protein degradations of Cdc25B/C for G2 induction; in contrast, Vpr had little or no effect on Cdc25A protein degradation normally mediated by HU/UV.

Conclusions

These data suggest that Vpr induces cell cycle G2 arrest through a unique molecular mechanism that regulates host cell cycle regulation in an S-phase dependent fashion.

Molecular mechanisms of HIV-1 persistence in the monocyte-macrophage lineage

The introduction of the highly active antiretroviral therapy (HAART) has greatly improved survival. However, these treatments fail to definitively cure the patients and unveil the presence of quiescent HIV-1 reservoirs like cells from monocyte-macrophage lineage. A purge, or at least a significant reduction of these long lived HIV-1 reservoirs will be needed to raise the hope of the viral eradication. This review focuses on the molecular mechanisms responsible for viral persistence in cells of the monocyte-macrophage lineage. Controversy on latency and/or cryptic chronic replication will be specifically evoked. In addition, since HIV-1 infected monocyte-macrophage cells appear to be more resistant to apoptosis, this obstacle to the viral eradication will be discussed. Understanding the intimate mechanisms of HIV-1 persistence is a prerequisite to devise new and original therapies aiming to achieve viral eradication.

Human immunodeficiency virus type 1 Vpr: functions and molecular interactions

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) is an accessory protein that interacts with a number of cellular and viral proteins. The functions of many of these interactions in the pathogenesis of HIV-1 have been identified. Deletion of the vpr gene reduces the virulence of HIV-1 dramatically, indicating the importance of this protein for the virus. This review describes the current findings on several established functions of HIV-1 Vpr and some possible roles proposed for this protein. Because Vpr exploits cellular proteins and pathways to influence the biology of HIV-1, understanding the functions of Vpr usually involves the study of cellular pathways. Several functions of Vpr are attributed to the virion-incorporated protein, but some of them are attributed to the expression of Vpr in HIV-1-infected cells. The structure of Vpr may be key to understanding the variety of its interactions. Due to the critical role of Vpr in HIV-1 pathogenicity, study of the interactions between Vpr and cellular proteins may help us to understand the mechanism(s) of HIV-1 pathogenicity.

Molecular and phylogenetic analysis of HIV type 1 vpr sequences of South African strains

HIV-1 subtype C is the prevalent subtype in South Africa, with non-C subtypes being sporadically detected. The accessory genes of subtype C have not been well studied in South Africa. In this study the HIV-1 vpr region of 58 strains was amplified, sequenced, and subtyped. Phylogenetic analysis characterized 54 strains as HIV-1 subtype C and 4 strains as HIV-1 subtype B. The amino acid sequence of the protein was also investigated. The overall amino acid sequence of Vpr was conserved as well as the motifs FPRPWL (34-39: IH1) and TYGDTW (49-54: IH2). The C-terminal was more variable. The importance of these motifs and variability needs to be addressed.

Tat-induced FOXO3a is a key mediator of apoptosis in HIV-1-infected human CD4+ T lymphocytes

The high mutation rate of HIV is linked to the generation of viruses expressing proteins with altered function whose impact on disease progression is unknown. We investigated how HIV-1 viruses lacking Env, Vpr, and Nef affect CD4(+) T cell survival. We found that in the absence of these proteins, HIV-1-infected CD4(+) primary T cells progress to the G(0) phase of the cell cycle and to cell death, indicating that viruses expressing inactive forms of these proteins can contribute to the CD4(+) T cell decline as the wild-type virus, suggesting that other HIV proteins are responsible for inducing apoptosis. Apoptosis in these cells is triggered by the alteration of the Egr1-PTEN-Akt (early growth response-1/phosphate and tensin homolog deleted on chromosome 10/Akt) and p53 pathways, which converge on the FOXO3a (Forkhead box transcription factor O class 3a) transcriptional activator. The FOXO3a target genes Fas ligand and TRAIL, involved in the extrinsic apoptotic pathway, and PUMA, Noxa, and Bim, which are part of the intrinsic apoptotic pathway, were also up-regulated, indicating that HIV infection leads to apoptosis by the engagement of multiple apoptotic pathways. RNAi-mediated knockdown of Egr1 and FOXO3a resulted in reduced apoptosis in HIV-infected HeLa and CD4(+) T cells, providing further evidence for their critical role in HIV-induced apoptosis and G(0) arrest. We tested the possibility that Tat is responsible for the T cell apoptosis observed with these mutant viruses. The induction of Egr1 and FOXO3a and its target genes was observed in Jurkat cells transduced by Tat alone. Tat-dependent activation of the Egr1-PTEN-FOXO3a pathway provides a mechanism for HIV-1-associated CD4(+) T cell death.

Direct Vpr-Vpr interaction in cells monitored by two photon fluorescence correlation spectroscopy and fluorescence lifetime imaging

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) encodes several regulatory proteins, notably Vpr which influences the survival of the infected cells by causing a G2/M arrest and apoptosis. Such an important role of Vpr in HIV-1 disease progression has fuelled a large number of studies, from its 3D structure to the characterization of specific cellular partners. However, no direct imaging and quantification of Vpr-Vpr interaction in living cells has yet been reported. To address this issue, eGFP- and mCherry proteins were tagged by Vpr, expressed in HeLa cells and their interaction was studied by two photon fluorescence lifetime imaging microscopy and fluorescence correlation spectroscopy.

RESULTS

Results show that Vpr forms homo-oligomers at or close to the nuclear envelope. Moreover, Vpr dimers and trimers were found in the cytoplasm and in the nucleus. Point mutations in the three alpha helices of Vpr drastically impaired Vpr oligomerization and localization at the nuclear envelope while point mutations outside the helical regions had no effect. Theoretical structures of Vpr mutants reveal that mutations within the alpha-helices could perturb the leucine zipper like motifs. The DeltaQ44 mutation has the most drastic effect since it likely disrupts the second helix. Finally, all Vpr point mutants caused cell apoptosis suggesting that Vpr-mediated apoptosis functions independently from Vpr oligomerization.

CONCLUSION

We report that Vpr oligomerization in HeLa cells relies on the hydrophobic core formed by the three alpha helices. This oligomerization is required for Vpr localization at the nuclear envelope but not for Vpr-mediated apoptosis.

A comprehensive analysis of the naturally occurring polymorphisms in HIV-1 Vpr: potential impact on CTL epitopes

The enormous genetic variability reported in HIV-1 has posed problems in the treatment of infected individuals. This is evident in the form of HIV-1 resistant to antiviral agents, neutralizing antibodies and cytotoxic T lymphocytes (CTLs) involving multiple viral gene products. Based on this, it has been suggested that a comprehensive analysis of the polymorphisms in HIV proteins is of value for understanding the virus transmission and pathogenesis as well as for the efforts towards developing anti-viral therapeutics and vaccines. This study, for the first time, describes an in-depth analysis of genetic variation in Vpr using information from global HIV-1 isolates involving a total of 976 Vpr sequences. The polymorphisms at the individual amino acid level were analyzed. The residues 9, 33, 39, and 47 showed a single variant amino acid compared to other residues. There are several amino acids which are highly polymorphic. The residues that show ten or more variant amino acids are 15, 16, 28, 36, 37, 48, 55, 58, 59, 77, 84, 86, 89, and 93. Further, the variant amino acids noted at residues 60, 61, 34, 71 and 72 are identical. Interestingly, the frequency of the variant amino acids was found to be low for most residues. Vpr is known to contain multiple CTL epitopes like protease, reverse transcriptase, Env, and Gag proteins of HIV-1. Based on this, we have also extended our analysis of the amino acid polymorphisms to the experimentally defined and predicted CTL epitopes. The results suggest that amino acid polymorphisms may contribute to the immune escape of the virus. The available data on naturally occurring polymorphisms will be useful to assess their potential effect on the structural and functional constraints of Vpr and also on the fitness of HIV-1 for replication.

Impaired nuclear import and viral incorporation of Vpr derived from a HIV long-term non-progressor

We previously reported an epidemiologically linked HIV-1 infected patient cohort in which a long-term non-progressor (LTNP) infected two recipients who then exhibited normal disease progression. Expression of patient-derived vpr sequences from each of the three cohort members in mammalian cells tagged with GFP revealed a significant reduction in Vpr nuclear import and virion incorporation uniquely from the LTNP, whereas Vpr from the two progressing recipients displayed normal localisation and virion incorporation, implying a link between efficient Vpr nuclear import and HIV disease progression. Importantly, an F72L point mutation in the LTNP was identified for the first time as being uniquely responsible for decreased Vpr nuclear import.

Mitochondrial dysfunction in lyssavirus-induced apoptosis

Lyssaviruses are highly neurotropic viruses associated with neuronal apoptosis. Previous observations have indicated that the matrix proteins (M) of some lyssaviruses induce strong neuronal apoptosis. However, the molecular mechanism(s) involved in this phenomenon is still unknown. We show that for Mokola virus (MOK), a lyssavirus of low pathogenicity, the M (M-MOK) targets mitochondria, disrupts the mitochondrial morphology, and induces apoptosis. Our analysis of truncated M-MOK mutants suggests that the information required for efficient mitochondrial targeting and dysfunction, as well as caspase-9 activation and apoptosis, is held between residues 46 and 110 of M-MOK. We used a yeast two-hybrid approach, a coimmunoprecipitation assay, and confocal microscopy to demonstrate that M-MOK physically associates with the subunit I of the cytochrome c (cyt-c) oxidase (CcO) of the mitochondrial respiratory chain; this is in contrast to the M of the highly pathogenic Thailand lyssavirus (M-THA). M-MOK expression induces a significant decrease in CcO activity, which is not the case with M-THA. M-MOK mutations (K77R and N81E) resulting in a similar sequence to M-THA at positions 77 and 81 annul cyt-c release and apoptosis and restore CcO activity. As expected, the reverse mutations, R77K and E81N, introduced in M-THA induce a phenotype similar to that due to M-MOK. These features indicate a novel mechanism for energy depletion during lyssavirus-induced apoptosis.

HIV-1 Vpr: mechanisms of G2 arrest and apoptosis

Since the first isolation of HIV-1 from a patient with generalized lymphadenopathy in 1983, great progress has been made in understanding the viral life cycle and the functional nuances of each of the nine genes encoded by HIV-1. Considerable attention has been paid to four small HIV-1 open reading frames, vif, vpr, vpu and nef. These genes were originally termed "accessory" because their deletion failed to completely disable viral replication in vitro. More than twenty years after the cloning and sequencing of HIV-1, a great deal of information is available regarding the multiple functions of the accessory proteins and it is well accepted that, collectively, these gene products modulate the host cell biology to favor viral replication, and that they are largely responsible for the pathogenesis of HIV-1. Expression of Vpr, in particular, leads to cell cycle arrest in G(2), followed by apoptosis. Here we summarize our current understanding of Vpr biology with a focus on Vpr-induced G(2) arrest and apoptosis.

Negative regulation of apoptosis in yeast

In recent years, yeast has been proven to be a useful model organism for studying programmed cell death. It not only exhibits characteristic markers of apoptotic cell death when heterologous inducers of apoptosis are expressed or when treated with apoptosis inducing drugs such as hydrogen peroxide (H(2)O(2)) or acetic acid, but contains homologues of several components of the apoptotic machinery identified in mammals, flies and nematodes, such as caspases, apoptosis inducing factor (AIF), Omi/HtrA2 and inhibitor-of-apoptosis proteins (IAPs). In this review, we focus on the role of negative regulators of apoptosis in yeasts. Bir1p is the only IAP protein in Saccharomyces cerevisiae and has long been known to play a role in cell cycle progression by acting as kinetochore and chromosomal passenger protein. Recent data established Bir1p's protective function against programmed cell death induced by H(2)O(2) treatment and in chronological ageing. Other factors that have a direct or indirect influence on intracellular levels of reactive oxygen species (ROS) and thus lead to apoptosis if they are misregulated or non-functional will be discussed.

Human immunodeficiency virus type 1 Vpr induces cell cycle G2 arrest through Srk1/MK2-mediated phosphorylation of Cdc25

Human immunodeficiency virus type 1 (HIV-1) Vpr induces cell cycle G(2) arrest in fission yeast (Schizosaccharomyces pombe) and mammalian cells, suggesting the cellular pathway(s) targeted by Vpr is conserved among eukaryotes. Our previous studies in fission yeast demonstrated that Vpr induces G(2) arrest in part through inhibition of Cdc25, a Cdc2-specific phosphatase that promotes G(2)/M transition. The goal of this study was to further elucidate molecular mechanism underlying the inhibitory effect of Vpr on Cdc25. We show here that, similar to the DNA checkpoint controls, expression of vpr promotes subcellular relocalization of Cdc25 from nuclear to cytoplasm and thereby prevents activation of Cdc2 by Cdc25. Vpr-induced nuclear exclusion of Cdc25 appears to depend on the serine/threonine phosphorylation of Cdc25 and the presence of Rad24/14-3-3 protein, since amino acid substitutions of the nine possible phosphorylation sites of Cdc25 with Ala (9A) or deletion of the rad24 gene abolished nuclear exclusion induced by Vpr. Interestingly, Vpr is still able to promote Cdc25 nuclear export in mutants defective in the checkpoints (rad3 and chk1/cds1), the kinases that are normally required for Cdc25 phosphorylation and nuclear exclusion of Cdc25, suggesting that others kinase(s) might modulate phosphorylation of Cdc25 for the Vpr-induced G(2) arrest. We report here that this kinase is Srk1. Deletion of the srk1 gene blocks the nuclear exclusion of Cdc25 caused by Vpr. Overexpression of srk1 induces cell elongation, an indication of cell cycle G(2) delay, in a similar fashion to Vpr; however, no additive effect of cell elongation was observed when srk1 and vpr were coexpressed, indicating Srk1 and Vpr are likely affecting the cell cycle G(2)/M transition through the same cellular pathway. Immunoprecipitation further shows that Vpr and Srk1 are part of the same protein complex. Consistent with our findings in fission yeast, depletion of the MK2 gene, a human homologue of Srk1, either by small interfering RNA or an MK2 inhibitor suppresses Vpr-induced cell cycle G(2) arrest in mammalian cells. Collectively, our data suggest that Vpr induces cell cycle G(2) arrest at least in part through a Srk1/MK2-mediated mechanism.

Localization of HIV-1 Vpr to the nuclear envelope: impact on Vpr functions and virus replication in macrophages

Background

HIV-1 Vpr is a dynamic protein that primarily localizes in the nucleus, but a significant fraction is concentrated at the nuclear envelope (NE), supporting an interaction between Vpr and components of the nuclear pore complex, including the nucleoporin hCG1. In the present study, we have explored the contribution of Vpr accumulation at the NE to the Vpr functions, including G2-arrest and pro-apoptotic activities, and virus replication in primary macrophages.

Results

In order to define the functional role of Vpr localization at the NE, we have characterized a set of single-point Vpr mutants, and selected two new mutants with substitutions within the first α-helix of the protein, Vpr-L23F and Vpr-K27M, that failed to associate with hCG1, but were still able to interact with other known relevant host partners of Vpr. In mammalian cells, these mutants failed to localize at the NE resulting in a diffuse nucleocytoplasmic distribution both in HeLa cells and in primary human monocyte-derived macrophages. Other mutants with substitutions in the first α-helix (Vpr-A30L and Vpr-F34I) were similarly distributed between the nucleus and cytoplasm, demonstrating that this helix contains the determinants required for localization of Vpr at the NE. All these mutations also impaired the Vpr-mediated G2-arrest of the cell cycle and the subsequent cell death induction, indicating a functional link between these activities and the Vpr accumulation at the NE. However, this localization is not sufficient, since mutations within the C-terminal basic region of Vpr (Vpr-R80A and Vpr-R90K), disrupted the G2-arrest and apoptotic activities without altering NE localization. Finally, the replication of the Vpr-L23F and Vpr-K27M hCG1-binding deficient mutant viruses was also affected in primary macrophages from some but not all donors.

Conclusion

These results indicate that the targeting of Vpr to the nuclear pore complex may constitute an early step toward Vpr-induced G2-arrest and subsequent apoptosis; they also suggest that Vpr targeting to the nuclear pore complex is not absolutely required, but can improve HIV-1 replication in macrophages.

ROS production by adrenodoxin does not cause apoptosis in fission yeast

We previously showed that production of reactive oxygen species (ROS) caused by overexpression of the mitochondrial electron transfer protein adrenodoxin (Adx) induces apoptosis in mammalian cells. In the fission yeast Schizosaccharomyces pombe, ROS are also produced in cells that undergo an apoptotic-like cell death, but it is not yet clear whether they are actually causative for this phenomenon or whether they are merely produced as a by-product. Therefore, the purpose of this study was to trigger mitochondrial ROS production in fission yeast by overexpression of either wildtype Adx (Adx-WT) or of several activated Adx mutants and to investigate its consequences. It was found that strong expression of either Adx-WT or Adx-S112W did not produce any ROS, while Adx-D113Y caused a twofold and Adx1-108 a threefold increase in ROS formation as compared to basal levels. However, no typical apoptotic markers or decreased viability could be observed in these strains. Since we previously observed that an increase in mitochondrial ROS formation of about 60% above basal levels is sufficient to strongly induce apoptosis in mammalian cells, we conclude that S. pombe is either very robust to mitochondrial ROS production or does not undergo apoptotic cell death in response to mitochondrial ROS at all.

Anti-vpr activities of heat shock protein 27

HIV-1 Vpr plays a pivotal role in viral pathogenesis and is preferentially targeted by the host immune system. In this report, we demonstrate that a small heat shock protein, HSP27, exhibits Vpr-specific antiviral activity, as its expression is specifically responsive to vpr gene expression and increased levels of HSP27 inhibit Vpr-induced cell cycle G2 arrest and cell killing. We further show that overexpression of HSP27 reduces viral replication in T-lymphocytes in a Vpr-dependent manner. Mechanistically, Vpr triggers HSP27 expression through heat shock factor (HSF) 1, but inhibits prolonged expression of HSP27 under heat-shock conditions. Together, these data suggest a potential dynamic and antagonistic interaction between HIV-1 Vpr and a host cell HSP27, suggesting that HSP27 may contribute to cellular intrinsic immunity against HIV infection.

Anti-vpr activities of heat shock protein 27

HIV-1 Vpr plays a pivotal role in viral pathogenesis and is preferentially targeted by the host immune system. In this report, we demonstrate that a small heat shock protein, HSP27, exhibits Vpr-specific antiviral activity, as its expression is specifically responsive to vpr gene expression and increased levels of HSP27 inhibit Vpr-induced cell cycle G2 arrest and cell killing. We further show that overexpression of HSP27 reduces viral replication in T-lymphocytes in a Vpr-dependent manner. Mechanistically, Vpr triggers HSP27 expression through heat shock factor (HSF) 1, but inhibits prolonged expression of HSP27 under heat-shock conditions. Together, these data suggest a potential dynamic and antagonistic interaction between HIV-1 Vpr and a host cell HSP27, suggesting that HSP27 may contribute to cellular intrinsic immunity against HIV infection.

HIV-1 Vpr-induced apoptosis is cell cycle dependent and requires Bax but not ANT

The HIV-1 accessory protein viral protein R (Vpr) causes G₂ arrest and apoptosis in infected cells. We previously identified the DNA damage–signaling protein ATR as the cellular factor that mediates Vpr-induced G₂ arrest and apoptosis. Here, we examine the mechanism of induction of apoptosis by Vpr and how it relates to induction of G₂ arrest. We find that entry into G₂ is a requirement for Vpr to induce apoptosis. We investigated the role of the mitochondrial permeability transition pore by knockdown of its essential component, the adenine nucleotide translocator. We found that Vpr-induced apoptosis was unaffected by knockdown of ANT. Instead, apoptosis is triggered through a different mitochondrial pore protein, Bax. In support of the idea that checkpoint activation and apoptosis induction are functionally linked, we show that Bax activation by Vpr was ablated when ATR or GADD45α was knocked down. Certain mutants of Vpr, such as R77Q and I74A, identified in long-term nonprogressors, have been proposed to inefficiently induce apoptosis while activating the G₂ checkpoint in a normal manner. We tested the in vitro phenotypes of these mutants and found that their abilities to induce apoptosis and G₂ arrest are indistinguishable from those of HIV-1_NL4–3vpr, providing additional support to the idea that G₂ arrest and apoptosis induction are mechanistically linked.

HIV-1 encodes a small gene known as vpr (viral protein regulatory) whose product is a 96–amino acid protein. HIV-1 infects cells of the immune system, such as CD4-positive lymphocytes. When cells become infected with HIV-1, two deleterious effects result from expression of the vpr gene. One effect of vpr is to manipulate the cell cycle by blocking the cells in G₂ (the phase of the cell cycle immediately preceding mitosis). Thus, cells infected with HIV-1 cease to proliferate, due to the action of vpr. A second effect of vpr is the induction of cell death by a process known as apoptosis or programmed cell death. When cells die by apoptosis, they do so following activation of a cellular set of genes and proteins whose primary function is to inactivate various cellular functions that are needed in order to maintain cellular viability. In this study, Andersen et al. demonstrate that the above two effects of vpr are linked. In particular, the authors show that the blockade in cell proliferation in G₂ is a requirement toward the onset of programmed cell death. Programmed cell death can be accomplished by a number of cellular proteins known as “executioners.” Various executioner proteins reside on the mitochondrial membranes and may trigger release of factors from the mitochondria, which in turn will precipitate the onset of apoptosis. In this work Anderson et al. identify the mitochondrial protein, Bax, as the key executioner of apoptosis in the context of HIV-1 vpr. The authors' findings provide important mechanistic understanding of how the vpr gene contributes to HIV-1–induced cell death.

Role of HIV-1 Vpr-induced apoptosis on the release of mitochondrial lysyl-tRNA synthetase

Mitochondrial lysyl-tRNA synthetase (LysRS) is thought to be involved in the specific packaging of tRNA(3)(Lys) into HIV-1 viral particles. The HIV-1 auxiliary viral protein Vpr is an apoptogenic protein that affects the integrity of the mitochondrial membrane and has also been reported to interact with LysRS. In the present study, we show that HIV-1 Vpr expressed in E. coli and purified to homogeneity does not interact specifically with LysRS and does not impact its aminoacylation activity. However, we also show that the mitochondrial localization of LysRS in HeLa cells is altered after addition of Vpr in the culture medium. These results suggest that HIV-1 Vpr fulfills an essential role in the process of packaging of mitochondrial LysRS.

Antagonistic interaction of HIV-1 Vpr with Hsf-mediated cellular heat shock response and Hsp16 in fission yeast (Schizosaccharomyces pombe)

Background

Expression of the HIV-1 vpr gene in human and fission yeast cells displays multiple highly conserved activities, which include induction of cell cycle G2 arrest and cell death. We have previously characterized a yeast heat shock protein 16 (Hsp16) that suppresses the Vpr activities when it is overproduced in fission yeast. Similar suppressive effects were observed when the fission yeast hsp16 gene was overexpressed in human cells or in the context of viral infection. In this study, we further characterized molecular actions underlying the suppressive effect of Hsp16 on the Vpr activities.

Results

We show that the suppressive effect of Hsp16 on Vpr-dependent viral replication in proliferating T-lymphocytes is mediated through its C-terminal end. In addition, we show that Hsp16 inhibits viral infection in macrophages in a dose-dependent manner. Mechanistically, Hsp16 suppresses Vpr activities in a way that resembles the cellular heat shock response. In particular, Hsp16 activation is mediated by a heat shock factor (Hsf)-dependent mechanism. Interestingly, vpr gene expression elicits a moderate increase of endogenous Hsp16 but prevents its elevation when cells are grown under heat shock conditions that normally stimulate Hsp16 production. Similar responsive to Vpr elevation of Hsp and counteraction of this elevation by Vpr were also observed in our parallel mammalian studies. Since Hsf-mediated elevation of small Hsps occurs in all eukaryotes, this finding suggests that the anti-Vpr activity of Hsps is a conserved feature of these proteins.

Conclusion

These data suggest that fission yeast could be used as a model to further delineate the potential dynamic and antagonistic interactions between HIV-1 Vpr and cellular heat shock responses involving Hsps.

Construction of doxycyline-dependent mini-HIV-1 variants for the development of a virotherapy against leukemias

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk type of blood-cell cancer. We describe the improvement of a candidate therapeutic virus for virotherapy of leukemic cells. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. To improve the safety of such a virus, we constructed an HIV-1 variant that replicates exclusively in the presence of the nontoxic effector doxycycline (dox). This was achieved by replacement of the viral TAR-Tat system for transcriptional activation by the Escherichia coli-derived Tet system for inducible gene expression. This HIV-rtTA virus replicates in a strictly dox-dependent manner. In this virus, additional deletions and/or inactivating mutations were introduced in the genes for accessory proteins. These proteins are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. These minimized HIV-rtTA variants contain up to 7 deletions/inactivating mutations (TAR, Tat, vif, vpR, vpU, nef and U3) and replicate efficiently in the leukemic SupT1 T cell line, but do not replicate in normal peripheral blood mononuclear cells. These virus variants are also able to efficiently remove leukemic cells from a mixed culture with untransformed cells. The therapeutic viruses use CD4 and CXCR4 for cell entry and could potentially be used against CXCR4 expressing malignancies such as T-lymphoblastic leukemia/lymphoma, NK leukemia and some myeloid leukemias.

Human immunodeficiency virus type 1 Vpr induces DNA replication stress in vitro and in vivo

The human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) causes cell cycle arrest in G2. Vpr-expressing cells display the hallmarks of certain forms of DNA damage, specifically activation of the ataxia telangiectasia mutated and Rad3-related kinase, ATR. However, evidence that Vpr function is relevant in vivo or in the context of viral infection is still lacking. In the present study, we demonstrate that HIV-1 infection of primary, human CD4+ lymphocytes causes G2 arrest in a Vpr-dependent manner and that this response requires ATR, as shown by RNA interference. The event leading to ATR activation in CD4+ lymphocytes is the accumulation of replication protein A in nuclear foci, an indication that Vpr likely induces stalling of replication forks. Primary macrophages are refractory to ATR activation by Vpr, a finding that is consistent with the lack of detectable ATR, Rad17, and Chk1 protein expression in these nondividing cells. These observations begin to explain the remarkable resilience of macrophages to HIV-1-induced cytopathicity. To study the in vivo consequences of Vpr function, we isolated CD4+ lymphocytes from HIV-1-infected individuals and interrogated the cell cycle status of anti-p24Gag-immunoreactive cells. We report that infected cells in vivo display an aberrant cell cycle profile whereby a majority of cells have a 4N DNA content, consistent with the onset of G2 arrest.

Human immunodeficiency virus type 1 (HIV-1) Vpr-regulated cell death: insights into mechanism

The destruction of CD4(+) T cells and eventual induction of immunodeficiency is a hallmark of the human immunodeficiency virus type 1 infection (HIV-1). However, the mechanism of this destruction remains unresolved. Several auxiliary proteins have been proposed to play a role in this aspect of HIV pathogenesis including a 14 kDa protein named viral protein R (Vpr). Vpr has been implicated in the regulation of various cellular functions including apoptosis, cell cycle arrest, differentiation, and immune suppression. However, the mechanism(s) involved in Vpr-mediated apoptosis remains unresolved, and several proposed mechanisms for these effects are under investigation. In this review, we discuss the possibility that some of these proposed pathways might converge to modulate Vpr's behavior. Further, we also discuss caveats and future directions for investigation of the interesting biology of this HIV accessory gene.