MicroRNA-25/93 induction by Vpu as a mechanism for counteracting MARCH1-restriction on HIV-1 infectivity in macrophages

IMPORTANCE

In order to efficiently produce infectious viral particles, HIV must counter several restrictions exerted by host cell antiviral proteins. MARCH1 is a member of the MARCH protein family that restricts HIV infection by limiting the incorporation of viral envelope glycoproteins into nascent virions. Here, we identified two regulatory RNAs, microRNAs-25 and -93, induced by the HIV-1 accessory protein Vpu, that downregulate MARCH1 mRNA. We also show that Vpu induces these cellular microRNAs in macrophages by hijacking the cellular β-catenin pathway. The notion that HIV-1 has evolved a mechanism to counteract MARCH1 restriction on viral infectivity underlines the importance of MARCH1 in the host antiviral response.

Near full-length HIV sequencing in multiple tissues collected postmortem reveals shared clonal expansions across distinct reservoirs during ART

HIV persists in tissues during antiretroviral therapy (ART), but the relative contribution of different anatomical compartments to the viral reservoir in humans remains unknown. We performed an extensive characterization of HIV reservoirs in two men who donated their bodies to HIV cure research and who had been on suppressive ART for years. HIV DNA is detected in all tissues, with large variations across anatomical compartments and between participants. Intact HIV genomes represent 2% and 25% of all proviruses in the two participants and are mainly detected in secondary lymphoid organs, with the spleen and mediastinal lymph nodes harboring intact viral genomes in both individuals. Multiple copies of identical HIV genomes are found in all tissues, indicating that clonal expansions are common in anatomical sites. The majority (>85%) of these expanded clones are shared across multiple tissues. These findings suggest that infected cells expand, migrate, and possibly circulate between anatomical sites.

Generation of functional human T cell development in NOD/SCID/IL2rγnull humanized mice without using fetal tissue: Application as a model of HIV infection and persistence

Humanization of mice with functional T cells currently relies on co-implantation of hematopoietic stem cells from fetal liver and autologous fetal thymic tissue (so-called BLT mouse model). Here, we show that NOD/SCID/IL2rγ^null mice humanized with cord blood- derived CD34⁺ cells and implanted with allogeneic pediatric thymic tissues excised during cardiac surgeries (CCST) represent an alternative to BLT mice. CCST mice displayed a strong immune reconstitution, with functional T cells originating from CD34⁺ progenitor cells. They were equally susceptible to mucosal or intraperitoneal HIV infection and had significantly higher HIV-specific T cell responses. Antiretroviral therapy (ART) robustly suppressed viremia and reduced the frequencies of cells carrying integrated HIV DNA. As in BLT mice, we observed a complete viral rebound following ART interruption, suggesting the presence of HIV reservoirs. In conclusion, CCST mice represent a practical alternative to BLT mice, broadening the use of humanized mice for research.

SKI-1/S1P Facilitates SARS-CoV-2 Spike Induced Cell-to-Cell Fusion via Activation of SREBP-2 and Metalloproteases, Whereas PCSK9 Enhances the Degradation of ACE2

Proprotein convertases activate various envelope glycoproteins and participate in cellular entry of many viruses. We recently showed that the convertase furin is critical for the infectivity of SARS-CoV-2, which requires cleavage of its spike protein (S) at two sites: S1/S2 and S2'. This study investigates the implication of the two cholesterol-regulating convertases SKI-1 and PCSK9 in SARS-CoV-2 entry. The assays used were cell-to-cell fusion in HeLa cells and pseudoparticle entry into Calu-3 cells. SKI-1 increased cell-to-cell fusion by enhancing the activation of SREBP-2, whereas PCSK9 reduced cell-to-cell fusion by promoting the cellular degradation of ACE2. SKI-1 activity led to enhanced S2' formation, which was attributed to increased metalloprotease activity as a response to enhanced cholesterol levels via activated SREBP-2. However, high metalloprotease activity resulted in the shedding of S2' into a new C-terminal fragment (S2″), leading to reduced cell-to-cell fusion. Indeed, S-mutants that increase S2″ formation abolished S2' and cell-to-cell fusion, as well as pseudoparticle entry, indicating that the formation of S2″ prevents SARS-CoV-2 cell-to-cell fusion and entry. We next demonstrated that PCSK9 enhanced the cellular degradation of ACE2, thereby reducing cell-to-cell fusion. However, different from the LDLR, a canonical target of PCSK9, the C-terminal CHRD domain of PCSK9 is dispensable for the PCSK9-induced degradation of ACE2. Molecular modeling suggested the binding of ACE2 to the Pro/Catalytic domains of mature PCSK9. Thus, both cholesterol-regulating convertases SKI-1 and PCSK9 can modulate SARS-CoV-2 entry via two independent mechanisms.

Donating One's Body to HIV Cure Research Through Canadian Medical Assistance in Dying: A Case Study

BACKGROUND

Finding a cure for HIV is challenged by persisting reservoirs, the mapping of which necessitates invasive procedures. Inviting people with HIV (PWHIV) at the end of life to donate body specimens post-mortem through research autopsies is a novel approach, raising ethical concerns.

OBJECTIVE

This case study aims to explore the motivations, barriers, and facilitators of a terminally-ill Canadian PWHIV who requested medical assistance in dying (MAID) and expressed interest in donating his body for HIV cure research.

CASE PRESENTATION

An in-depth 3-hour and semi-structured interview was conducted with the participant. The interview transcription was thematically coded to identify motivations and perceived barriers and facilitators to participate in end-of-life HIV cure research. Our analysis identified six themes. Two themes expressed motivations: Collaboration in progress in health and science, seeing cure research as collaboration with professionals; and Opportunity to learn more, mostly about science and health. One theme expressed a barrier: Losing interest in or identification with long-term care research matters, especially those related to the management of long-term care. Three themes expressed by facilitators: Receiving information from professionals one trusts and knows, especially clinical and research teams; Perceiving research procedures as simple, useful, and embedded in care, perceiving clinical, educational, and interpersonal benefits that surpass costs of participation; and Perceiving research as one last way to contribute, that is, feeling useful or give back.

CONCLUSION

Several circumstances facilitated the patient's participation: being a single man, having time to participate, having no strong religious belief, and valuing clear, direct communication. His motivations to participate in HIV cure research were altruistic, and also an experience of working with clinical and research teams. Finally, this perspective highlights HIV cure research participant candidates' need for education about research procedures.

MiRNA-103 downmodulates CCR5 expression reducing HIV-1 entry and impacting latency establishment in CD4+ T cells

Activated-to-memory transitioning CD4⁺ T cells display elevated expression of the HIV-1 co-receptor CCR5 and are more prone to HIV-1 latent infection. Here, we show that p53-regulated miRNA-103 downmodulates CCR5 levels in CD4⁺ T lymphocytes. We reveal that miRNA-103 mimics, as well as Nutlin-3, an inhibitor of Mdm2-mediated p53 degradation, decrease CCR5-dependent HIV-1 infection. Using a dual-reporter virus, we subsequently validate that in transitioning CD4⁺ T cells, Nutlin-3 treatment decreases the frequency of both productively and latently infected cells via upregulation of miRNA-103. Importantly, we provide evidence that CD4⁺ T cells from HIV-1 elite controllers express less CCR5 than those from antiretroviral therapy-naïve progressors, an effect linked to a significant increase in miRNA-103 levels. By contributing to the control of CCR5 expression in CD4⁺ T cells, miRNA-103 is likely to play a key role in countering the establishment of latent HIV-1 reservoirs in vivo.

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p53-regulated miRNA-103 reduces CCR5 in primary CD4⁺ T cells

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HIV-1 latent infection-prone activated-to-memory transitioning T cells reduce p53

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Stabilizing p53 reduces CCR5 in CD4⁺ T cells via upregulation of miRNA-103

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Decreased CCR5 in CD4⁺ T cells is associated with an increase in miRNA-103 in vivo

Willingness of Older Canadians with HIV to Participate in HIV Cure Research Near and After the End of Life: A Mixed-Method Study

HIV cure research requires interrogating latent HIV reservoirs in deep tissues, which necessitates autopsies to avoid risks to participants. An HIV autopsy biobank would facilitate this research, but such research raises ethical issues and requires participant engagement. This study explores the willingness to participate in HIV cure research at the end of life. Participants include Canadians with HIV [people with HIV (PWHIV)] aged 55 years or older. Following a mixed-method study design, all participants completed a phone or online survey, and a subset of participants participated in in-depth phone or videoconference interviews. We produced descriptive statistics of quantitative data and a thematic analysis of qualitative data. Barriers and facilitators were categorized under domains of the Theoretical Domains Framework. From April 2020 to August 2021, 37 participants completed the survey (mean age = 69.9 years old; mean duration of HIV infection = 28.5 years), including 15 interviewed participants. About three quarters of participants indicated being willing to participate in hypothetical medical studies toward the end of life (n = 30; 81.1%), in HIV biobanking (n = 30; 81.1%), and in a research autopsy (n = 28; 75.7%) to advance HIV cure research, mainly for altruistic benefits. The main perceived risks had to do with physical pain and confidentiality. Barriers and facilitators were distributed across five domains: social/professional role and identity, environmental context and resources, social influences, beliefs about consequences, and capabilities. Participants wanted more information about study objectives and procedures, possible accommodations with their last will, and rationale for studies or financial interests funding studies. Our results indicate that older PWHIV would be willing to participate in HIV cure research toward the end of life, HIV biobanking, and research autopsy. However, a dialogue should be initiated to inform participants thoroughly about HIV cure studies, address concerns, and accommodate their needs and preferences. Additional work is required, likely through increased community engagement, to address educational needs.

IL-17A reprograms intestinal epithelial cells to facilitate HIV-1 replication and outgrowth in CD4+ T cells

The crosstalk between intestinal epithelial cells (IECs) and Th17-polarized CD4⁺ T cells is critical for mucosal homeostasis, with HIV-1 causing significant alterations in people living with HIV (PLWH) despite antiretroviral therapy (ART). In a model of IEC and T cell co-cultures, we investigated the effects of IL-17A, the Th17 hallmark cytokine, on IEC ability to promote de novo HIV infection and viral reservoir reactivation. Our results demonstrate that IL-17A acts in synergy with TNF to boost IEC production of CCL20, a Th17-attractant chemokine, and promote HIV trans-infection of CD4⁺ T cells and viral outgrowth from reservoir cells of ART-treated PLWH. Importantly, the Illumina RNA-sequencing revealed an IL-17A-mediated pro-inflammatory and pro-viral molecular signature, including a decreased expression of type I interferon (IFN-I)-induced HIV restriction factors. These findings point to the deleterious features of IL-17A and raise awareness for caution when designing therapies aimed at restoring the paucity of mucosal Th17 cells in ART-treated PLWH.

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IL-17A acts in synergy with TNF to enhance CCL20 production in IEC exposed to HIV

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IL-17A/TNF-activated IEC efficiently promote HIV trans-infection of CD4⁺ T cells

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IL-17A reprograms IEC to boost HIV outgrowth from CD4⁺ T cells of ART-treated PLWH

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IL-17A decreases the expression of IFN-I-induced HIV restriction factors in IEC

L-Carnitine Tartrate Downregulates the ACE2 Receptor and Limits SARS-CoV-2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been responsible for one of the worst pandemics in modern history. Several prevention and treatment strategies have been designed and evaluated in recent months either through the repurposing of existing treatments or the development of new drugs and vaccines. In this study, we show that L-carnitine tartrate supplementation in humans and rodents led to significant decreases of key host dependency factors, notably angiotensin-converting enzyme 2 (ACE2), transmembrane protease serine 2 (TMPRSS2), and Furin, which are responsible for viral attachment, viral spike S-protein cleavage, and priming for viral fusion and entry. Interestingly, pre-treatment of Calu-3, human lung epithelial cells, with L-carnitine tartrate led to a significant and dose-dependent inhibition of the infection by SARS-CoV-2. Infection inhibition coincided with a significant decrease in ACE2 mRNA expression levels. These data suggest that L-carnitine tartrate should be tested with appropriate trials in humans for the possibility to limit SARS-CoV-2 infection.

Flt3L-Mediated Expansion of Plasmacytoid Dendritic Cells Suppresses HIV Infection in Humanized Mice

Plasmacytoid dendritic cells (plasmacytoid DC, pDC) are major IFN-I producers and have been shown to be affected by HIV through ill-defined mechanisms. In this study, we directly assess the role of pDC in early infection, evaluating whether modulating their abundance can alter viral replication. First, HIV infection of humanized mice induces systemic depletion of pDC, and in the presence of soluble FMS-like tyrosine kinase 3 ligand (Flt3L), pDC levels remain elevated. Flt3L significantly delays the onset of viremia and reduces viral replication via a process that is dependent on pDC and mediated through an enhanced early IFN-I response. pDC from Flt3L-treated mice are more prone to express IFN-α following TLR7 stimulation, but this propensity is gradually decreased during infection. In conclusion, maintaining pDC levels and function is key to effective early viral control, and in this context, these findings provide practical insights for anti-HIV strategies and vaccine design.

USP18 is a significant driver of memory CD4 T-cell reduced viability caused by type I IFN signaling during primary HIV-1 infection

The loss of Memory CD4 T-cells (Mem) is a major hallmark of HIV-1 immuno-pathogenesis and occurs early during the first months of primary infection. A lot of effort has been put into understanding the molecular mechanisms behind this loss, yet they still have not been fully identified. In this study, we unveil the unreported role of USP18 in the deleterious effects of sustained type I IFN signaling on Mem, including HIV-1-specific CD4 T-cells. We find that interfering with IFN-I signaling pathway in infected patients, notably by targeting the interferon-stimulated gene USP18, resulted in reduced PTEN expression similar to those observed in uninfected control donors. We show that AKT activation in response to cytokine treatment, T-cell receptor (TcR) triggering, as well as HIV-1 Gag stimulation was significantly improved in infected patients when PTEN or USP18 were inhibited. Finally, our data demonstrate that higher USP18 in Mem from infected patients prevent proper cell survival and long-lasting maintenance in an AKT-dependent manner. Altogether, we establish a direct role for type I IFN/USP18 signaling in the maintenance of total and virus-specific Mem and provide a new mechanism for the reduced survival of these populations during primary HIV-1 infection.

In this study, we expend our knowledge of how type I interferons (IFN-I) leads to memory CD4 T-cell defective survival by unveiling the molecular mechanism behind such impairments, placing USP18 at its center. Our data further deciphers the specific USP18-related mechanism that is responsible for such impairments by implicating AKT inhibition in a PTEN-dependent manner. Our findings also point to a potential use of neutralizing anti-interferon α/β receptor antibodies to rescue the defective memory CD4 T-cell survival during HIV-1 infection, even in HIV-1 specific CD4 T-cell. To conclude, our findings provide the characterization of the molecular pathway leading to disturbances caused by sustained IFN-I signaling which occurs early during primary HIV-1 infection, complementing current knowledge which placed sustained IFN-I signaling as detrimental to the host during this infection.

Activation of the ILT7 receptor and plasmacytoid dendritic cell responses are governed by structurally-distinct BST2 determinants

Type I interferons (IFN-I) are key innate immune effectors predominantly produced by activated plasmacytoid dendritic cells (pDCs). By modulating immune responses at their foundation, IFNs can widely reshape immunity to control infectious diseases and malignancies. Nevertheless, their biological activities can also be detrimental to surrounding healthy cells, as prolonged IFN-I signaling is associated with excessive inflammation and immune dysfunction. The interaction of the human pDC receptor immunoglobulin-like transcript 7 (ILT7) with its IFN-I-regulated ligand, bone marrow stromal cell antigen 2 (BST2) plays a key role in controlling the IFN-I amounts produced by pDCs in response to Toll-like receptor (TLR) activation. However, the structural determinants and molecular features of BST2 that govern ILT7 engagement and activation are largely undefined. Using two functional assays to measure BST2-stimulated ILT7 activation as well as biophysical studies, here we identified two structurally-distinct regions of the BST2 ectodomain that play divergent roles during ILT7 activation. We found that although the coiled-coil region contains a newly defined ILT7-binding surface, the N-terminal region appears to suppress ILT7 activation. We further show that a stable BST2 homodimer binds to ILT7, but post-binding events associated with the unique BST2 coiled-coil plasticity are required to trigger receptor signaling. Hence, BST2 with an unstable or a rigid coiled-coil fails to activate ILT7, whereas substitutions in its N-terminal region enhance activation. Importantly, the biological relevance of these newly defined domains of BST2 is underscored by the identification of substitutions having opposing potentials to activate ILT7 in pathological malignant conditions.

HIV-induced neuroinflammation: impact of PAR1 and PAR2 processing by Furin

HIV-associated neurocognitive disorders (HAND) is a syndrome defined by neurocognitive deficits that are driven by viral neurotoxins, cytokines, free radicals, and proteases expressed in the brain. This neurological disease has also been linked to activation of Protease-Activated Receptors 1 and 2 (PAR1,2). These receptors are highly expressed in the central nervous system and are upregulated in HAND. Secretory basic-amino-acid-specific Proprotein Convertases (PCs), which cleave precursor proteins at basic residues, are also induced in HAND. They are vital for many biological processes including HIV-1 entry into cells. The cytoprotective role of Furin, PC5, and PACE4 has been linked to the presence of a potential PC-cleavage site R₄₁XXXXR₄₆↓ in PAR1. Furthermore, Furin binds PAR1 and both are trapped in the trans-Golgi-network (TGN) as inactive proteins, likely due to the intermediary trafficking role of phospho-Furin acidic cluster sorting protein 1 (PACS1). Nothing is known about PAR2 and its possible recognition by PCs at its putative R₃₁XXXXR₃₆↓ processing site. The present study implicates PACS1 in the retrograde trafficking of PAR1 to the TGN and demonstrates that the cytosolic extreme C-terminal tail of PAR1 contains an acidic phosphorylatable PACS1-sensitive domain. We further show the requirement of Asn₄₇ in PAR1 for its Furin-dependent TGN localization. Our data revealed that Furin is the only convertase that efficiently cleaves PAR2 at Arg₃₆↓. N-glycosylation of PAR2 at Asn₃₀ reduces the efficacy, but enhances selectivity of the Furin cleavage. Finally, in co-cultures comprised of human neuroblastoma SK-N-SH cells (stably expressing PAR1/2 and/or Furin) and HIV-1-infected primary macrophages, we demonstrate that the expression of Furin enhances neuronal cell viability in the context of PAR1- or PAR2-induced neuronal cytotoxicity. The present study provides insights into early stages of HIV-1 induced neuronal injury and the protective role of Furin in neurons co-expressing PAR1 and/or PAR2, as observed in HAND.

HIV-1 Vpr hijacks EDD-DYRK2-DDB1DCAF1 to disrupt centrosome homeostasis

Viruses exploit the host cell machinery for their own profit. To evade innate immune sensing and promote viral replication, HIV type 1 (HIV-1) subverts DNA repair regulatory proteins and induces G2/M arrest. The preintegration complex of HIV-1 is known to traffic along microtubules and accumulate near the microtubule-organizing center. The centrosome is the major microtubule-organizing center in most eukaryotic cells, but precisely how HIV-1 impinges on centrosome biology remains poorly understood. We report here that the HIV-1 accessory protein viral protein R (Vpr) localized to the centrosome through binding to DCAF1, forming a complex with the ubiquitin ligase EDD-DYRK2-DDB1DCAF1 and Cep78, a resident centrosomal protein previously shown to inhibit EDD-DYRK2-DDB1DCAF1 Vpr did not affect ubiquitination of Cep78. Rather, it enhanced ubiquitination of an EDD-DYRK2-DDB1DCAF1 substrate, CP110, leading to its degradation, an effect that could be overcome by Cep78 expression. The down-regulation of CP110 and elongation of centrioles provoked by Vpr were independent of G2/M arrest. Infection of T lymphocytes with HIV-1, but not with HIV-1 lacking Vpr, promoted CP110 degradation and centriole elongation. Elongated centrioles recruited more γ-tubulin to the centrosome, resulting in increased microtubule nucleation. Our results suggest that Vpr is targeted to the centrosome where it hijacks a ubiquitin ligase, disrupting organelle homeostasis, which may contribute to HIV-1 pathogenesis.

Regulation of CD4 Receptor and HIV-1 Entry by MicroRNAs-221 and -222 during Differentiation of THP-1 Cells

Human immunodeficiency virus type-1 (HIV-1) infection of monocyte/macrophages is modulated by the levels of entry receptors cluster of differentiation 4 (CD4) and C-C chemokine receptor type 5 (CCR5), as well as by host antiviral restriction factors, which mediate several post-entry blocks. We recently identified two microRNAs, miR-221 and miR-222, which limit HIV-1 entry during infection of monocyte-derived macrophages (MDMs) by down-regulating CD4 expression. Interestingly, CD4 is also down-regulated during the differentiation of monocytes into macrophages. In this study, we compared microRNA expression profiles in primary monocytes and macrophages by RNAseq and found that miR-221/miR-222 are enhanced in macrophages. We took advantage of the monocytic THP-1 cell line that, once differentiated, is poorly susceptible to HIV-1. Accordingly, we found that CD4 levels are very low in THP-1 differentiated cells and that this down-regulation of the virus receptor is the result of miR-221/miR-222 up-regulation during differentiation. We thus established a THP-1 cell line stably expressing a modified CD4 (THP-1-CD4^R) that is not modulated by miR-221/miR-222. We show that in contrast to parental THP-1, this line is productively infected by HIV-1 following differentiation, sustaining efficient HIV-1 CD4-dependent replication and spread. This new THP-1-CD4^R cell line represents a useful tool for the study of HIV-1-macrophage interactions particularly in contexts where spreading of viral infection is necessary.

Conserved residues within the HIV-1 Vpu transmembrane-proximal hinge region modulate BST2 binding and antagonism

Background

BST2 inhibits HIV-1 release by tethering nascent virions to the surface of infected cells. HIV-1 Vpu overcomes this restriction by removing BST2 from viral budding sites via BST2 intracellular trapping and sequestration, surface downregulation and/or displacement mechanisms. Vpu is composed of a short luminal tail, a transmembrane domain (TMD) and a cytoplasmic hinge region that is followed by two helices. BST2 counteraction relies on the ability of Vpu to physically bind BST2 through TMD interactions and recruit the clathrin-dependent trafficking machinery via a canonical acidic di-leucine signalling motif within the helix-2 of Vpu. The highly conserved Vpu transmembrane-proximal hinge region encompasses residues that resemble an acidic leucine-based trafficking motif, whose functional roles are currently ill-defined. In this study, we investigated the contribution of these residues towards Vpu-mediated BST2 antagonism.

Results

We show that while these conserved residues have no intrinsic activity on the cellular distribution of Vpu in the absence of BST2, they regulate the ability of Vpu to bind to BST2 and, consequently, govern both BST2-dependent trafficking properties of the protein as well as its co-localization with BST2. Moreover, these residues, particularly a glutamic acid residue positioned immediately following the TMD, are a determinant not only for efficient targeting of BST2, but also binding and degradation of CD4, another host membrane protein targeted by Vpu. Mechanistically, our data are consistent with a role of these residues in the maintenance of the Vpu TMD conformational configuration such that interactions with membrane-associated host targets are favoured.

Conclusions

Altogether, this work demonstrates an important regulatory role of the transmembrane-proximal Vpu hinge region residues towards enabling the protein to efficiently engage its target host proteins. Thus, this highly conserved, cytosolic Vpu hinge region may represent an attractive target for the development of anti-Vpu inhibitors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0345-6) contains supplementary material, which is available to authorized users.

International AIDS Society global scientific strategy: towards an HIV cure 2016

Antiretroviral therapy is not curative. Given the challenges in providing lifelong therapy to a global population of more than 35 million people living with HIV, there is intense interest in developing a cure for HIV infection. The International AIDS Society convened a group of international experts to develop a scientific strategy for research towards an HIV cure. This Perspective summarizes the group's strategy.

Attacking the Supply Lines: HIV-1 Restricts Alanine Uptake to Prevent T Cell Activation

HIV commonly escapes host antiviral immunity by downregulating cell-surface immunoreceptors. In a recent issue of Cell Host & Microbe, Matheson et al. (2015) systematically examined how HIV-1 infection remodels the T cell surface and identified serine carriers SERINC3/5 and alanine transporter SNAT1 as targets of HIV-1 Nef and Vpu, respectively.

Assessing the Innate Sensing of HIV-1 Infected CD4+ T Cells by Plasmacytoid Dendritic Cells Using an Ex vivo Co-culture System

HIV-1 innate sensing requires direct contact of infected CD4+ T cells with plasmacytoid dendritic cells (pDCs). In order to study this process, the protocols described here use freshly isolated human peripheral blood mononuclear cells (PBMCs) or plasmacytoid dendritic cells (pDCs) to sense infections in either T cell line (MT4) or heterologous primary CD4+ T cells. In order to ensure proper sensing, it is essential that PBMC are isolated immediately after blood collection and that optimal percentage of infected T cells are used. Furthermore, multi-parametric flow cytometric staining can be used to confirm that PBMC samples contain the different cell lineages at physiological ratios. A number of controls can also be included to evaluate viability and functionality of pDCs. These include, the presence of specific surface markers, assessing cellular responses to known agonist of Toll-Like Receptors (TLR) pathways, and confirming a lack of spontaneous type-I interferon (IFN) production. In this system, freshly isolated PBMCs or pDCs are co-cultured with HIV-1 infected cells in 96 well plates for 18-22 hr. Supernatants from these co-cultures are then used to determine the levels of bioactive type-I IFNs by monitoring the activation of the ISGF3 pathway in HEK-Blue IFN-α/β cells. Prior and during co-culture conditions, target cells can be subjected to flow cytometric analysis to determine a number of parameters, including the percentage of infected cells, levels of specific surface markers, and differential killing of infected cells. Although, these protocols were initially developed to follow type-I IFN production, they could potentially be used to study other imuno-modulatory molecules released from pDCs and to gain further insight into the molecular mechanisms governing HIV-1 innate sensing.

Vpu Exploits the Cross-Talk between BST2 and the ILT7 Receptor to Suppress Anti-HIV-1 Responses by Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) constitute a major source of type-I interferon (IFN-I) production during acute HIV infection. Their activation results primarily from TLR7-mediated sensing of HIV-infected cells. However, the interactions between HIV-infected T cells and pDCs that modulate this sensing process remain poorly understood. BST2/Tetherin is a restriction factor that inhibits HIV release by cross-linking virions onto infected cell surface. BST2 was also shown to engage the ILT7 pDC-specific inhibitory receptor and repress TLR7/9-mediated IFN-I production by activated pDCs. Here, we show that Vpu, the HIV-1 antagonist of BST2, suppresses TLR7-mediated IFN-I production by pDC through a mechanism that relies on the interaction of BST2 on HIV-producing cells with ILT7. Even though Vpu downregulates surface BST2 as a mean to counteract the restriction on HIV-1 release, we also find that the viral protein re-locates remaining BST2 molecules outside viral assembly sites where they are free to bind and activate ILT7 upon cell-to-cell contact. This study shows that through a targeted regulation of surface BST2, Vpu promotes HIV-1 release and limits pDC antiviral responses upon sensing of infected cells. This mechanism of innate immune evasion is likely to be important for an efficient early viral dissemination during acute infection.

Plasmacytoid dendritic cells (pDCs) produce large quantities of type I interferon (IFN-I) upon stimulation by many viruses, including HIV. Their activation is very effective following cell contacts with HIV-1-infected CD4+ T cells. We investigated whether HIV-1 could regulate the antiviral responses of pDCs triggered upon sensing of infected cells. We show that HIV-1 suppresses the levels of IFN-I produced by pDCs through a process that requires expression of the Vpu accessory protein in virus-producing cells. A well-described role of Vpu is to promote efficient HIV-1 production by counteracting BST2, a host factor that entraps nascent viral particle at the cell surface. Apart from its antiviral activity, BST2 was reported to inhibit IFN-I production by pDCs through binding and activation of the ILT7 pDC-specific inhibitory receptor. Our results reveal that through a highly sophisticated targeted regulation of BST2 levels at the surface of infected cells, Vpu promotes HIV-1 release and limits IFN-I production by pDCs via the negative signaling exerted by the BST2-ILT7 pair. Overall, this study sheds light on a novel Vpu-BST2 interaction that allows HIV-1 to escape pDC antiviral responses. This modulation of pDC antiviral response by HIV Vpu may facilitate the initial viral expansion during acute infection.

miRNA-1236 inhibits HIV-1 infection of monocytes by repressing translation of cellular factor VprBP

Primary monocytes are refractory to HIV-1 infection and become permissive upon differentiation into monocyte-derived dendritic cells (MDDCs) or macrophages. Multiple mechanisms have been proposed to interpret HIV-1 restriction in monocytes. Human cellular miRNAs can modulate HIV-1 infection by targeting either conserved regions of the HIV-1 genome or host gene transcripts. We have recently reported that the translation of host protein pur-alpha is repressed by abundant cellular miRNAs to inhibit HIV-1 infection in monocytes. Here, we report that the transcript of another cellular factor, VprBP [Vpr (HIV-1)-binding protein], was repressed by cellular miRNA-1236, which contributes to HIV-1 restriction in monocytes. Transfection of miR-1236 inhibitors enhanced translation of VprBP in monocytes and significantly promoted viral infection; exogenous input of synthesized miR-1236 mimics into MDDCs suppressed translation of VprBP, and, accordingly, significantly impaired viral infection. Our data emphasize the role of miRNA in modulating differentiation-dependent susceptibility of the host cell to HIV-1 infection. Understanding the modulation of HIV-1 infection by cellular miRNAs may provide key small RNAs or the identification of new important protein targets regulated by miRNAs for the development of antiviral strategies.

Defining the interactions and role of DCAF1/VPRBP in the DDB1-cullin4A E3 ubiquitin ligase complex engaged by HIV-1 Vpr to induce a G2 cell cycle arrest

HIV viral protein R (Vpr) induces a cell cycle arrest at the G2/M phase by activating the ATR DNA damage/replication stress signalling pathway through engagement of the DDB1-CUL4A-DCAF1 E3 ubiquitin ligase via a direct binding to the substrate specificity receptor DCAF1. Since no high resolution structures of the DDB1-DCAF1-Vpr substrate recognition module currently exist, we used a mutagenesis approach to better define motifs in DCAF1 that are crucial for Vpr and DDB1 binding. Herein, we show that the minimal domain of DCAF1 that retained the ability to bind Vpr and DDB1 was mapped to residues 1041 to 1393 (DCAF1 WD). Mutagenic analyses identified an α-helical H-box motif and F/YxxF/Y motifs located in the N-terminal domain of DCAF1 WD that are involved in exclusive binding to DDB1. While we could not identify elements specifically involved in Vpr binding, overall, the mutagenesis data suggest that the predicted β-propeller conformation of DCAF1 is likely to be critical for Vpr association. Importantly, we provide evidence that binding of Vpr to DCAF1 appears to modulate the formation of a DDB1/DCAF1 complex. Lastly, we show that expression of DCAF1 WD in the absence of endogenous DCAF1 was not sufficient to enable Vpr-mediated G2 arrest activity. Overall, our results reveal that Vpr and DDB1 binding on DCAF1 can be genetically separated and further suggest that DCAF1 contains determinants in addition to the Vpr and DDB1 minimal binding domain, which are required for Vpr to enable the induction of a G2 arrest.

HIV Nef and Vpu protect HIV-infected CD4+ T cells from antibody-mediated cell lysis through down-modulation of CD4 and BST2

BACKGROUND

HIV proteins Nef and Vpu down-modulate various host factors to evade immune defenses. Indeed, the CD4 receptor is down-regulated by Nef and Vpu, whereas virion-tethering BST2 is depleted by Vpu. Antibody-dependent cell-mediated cytotoxicity (ADCC) is increasingly recognized as a potentially powerful anti-HIV response. Given that epitopes which are specific for ADCC-competent anti-HIV antibodies are transitionally exposed upon CD4-mediated HIV entry, we investigated whether by depleting CD4 and BST2, HIV could negatively affect ADCC function.

RESULTS

Using anti-envelope (Env) Abs A32 and 2G12 to trigger ADCC activity, we find that interactions between CD4 and Env within infected cells expose ADCC-targeted epitopes on cell-surface Env molecules, marking infected T cells for lysis by immune cells. We also provide evidence to show that by cross-linking nascent virions at the plasma membrane, hence increasing cell-surface Env density, BST2 further enhances the efficiency of this antiviral process. The heightened susceptibility of T cells infected with a virus lacking Nef and Vpu to ADCC was recapitulated when plasmas from HIV-infected patients were used as an alternative source of Abs.

CONCLUSIONS

Our data unveil a mechanism by which HIV Nef and Vpu function synergistically to protect infected cells from ADCC and promote viral persistence. These findings also renew the potential practical relevance of ADCC function in vivo.

The human immunodeficiency virus type 1 Vpr protein upregulates PVR via activation of the ATR-mediated DNA damage response pathway

Viral infection may induce the cell-surface expression of PVR (CD155) that, upon recognition by its cognate activating DNAM-1 receptor present on cytotoxic lymphocytes, may promote antiviral immune responses. Here we show that expression of the human immunodeficiency virus type 1 (HIV-1) Vpr protein in Jurkat T cells increases cell-surface and total PVR levels. Analysis of mutated Vpr variants indicated that Vpr uses the same protein surfaces, and hence probably the same mechanisms, to upregulate PVR and arrest the cell cycle in the G2 phase. Moreover, we found that PVR upregulation by Vpr relied on the ability of the protein to activate the ATR kinase that triggers the DNA damage response pathway and G2 arrest. Finally, we showed that Vpr contributes to PVR up-modulation in HIV-infected CD4(+) T lymphocytes and inhibits the PVR downregulating activity of the viral Nef protein.

Dual mechanisms of translation initiation of the full-length HIV-1 mRNA contribute to gag synthesis

The precursor group-specific antigen (pr55^Gag) is central to HIV-1 assembly. Its expression alone is sufficient to assemble into virus-like particles. It also selects the genomic RNA for encapsidation and is involved in several important virus-host interactions for viral assembly and restriction, making its synthesis essential for aspects of viral replication. Here, we show that the initiation of translation of the HIV-1 genomic RNA is mediated through both a cap-dependent and an internal ribosome entry site (IRES)-mediated mechanisms. In support of this notion, pr55^Gag synthesis was maintained at 70% when cap-dependent translation initiation was blocked by the expression of eIF4G- and PABP targeting viral proteases in two in vitro systems and in HIV-1-expressing cells directly infected with poliovirus. While our data reveal that IRES-dependent translation of the viral genomic RNA ensures pr55^Gag expression, the synthesis of other HIV-1 proteins, including that of pr160^Gag/Pol, Vpr and Tat is suppressed early during progressive poliovirus infection. The data presented herein implies that the unspliced HIV-1 genomic RNA utilizes both cap-dependent and IRES-dependent translation initiation to supply pr55^Gag for virus assembly and production.

Major histocompatibility complex class-II molecules promote targeting of human immunodeficiency virus type 1 virions in late endosomes by enhancing internalization of nascent particles from the plasma membrane

Productive assembly of human immunodeficiency virus type 1 (HIV-1) takes place, primarily, at the plasma membrane. However, depending on the cell types, a significant proportion of nascent virus particles are internalized and routed to late endosomes. We previously reported that expression of human leucocyte antigen (HLA)-DR promoted a redistribution of Gag in late endosomes and an increased detection of mature virions in these compartments in HeLa and human embryonic kidney 293T model cell lines. Although this redistribution of Gag resulted in a marked decrease of HIV-1 release, the underlying mechanism remained undefined. Here, we provide evidence that expression of HLA-DR at the cell surface induces a redistribution of mature Gag products into late endosomes by enhancing nascent HIV-1 particle internalization from the plasma membrane through a process that relies on the presence of intact HLA-DR α and β-chain cytosolic tails. These findings raise the possibility that major histocompatibility complex class-II molecules might influence endocytic events at the plasma membrane and as a result promote endocytosis of progeny HIV-1 particles.

Naturally-occurring genetic variants in human DC-SIGN increase HIV-1 capture, cell-transfer and risk of mother-to-child transmission

BACKGROUND

Mother-to-child transmission (MTCT) is the main cause of HIV-1 infection in children worldwide. Dendritic cell-specific ICAM-3 grabbing-nonintegrin (DC-SIGN, also known as CD209) is an HIV-1 receptor that enhances its transmission to T cells and is expressed on placental macrophages.

METHODS AND FINDINGS

We have investigated the association between DC-SIGN genetic variants and risk of MTCT of HIV-1 among Zimbabwean infants and characterized the impact of the associated mutations on DC-SIGN expression and interaction with HIV-1. DC-SIGN promoter (p-336C and p-201A) and exon 4 (198Q and 242V) variants were all significantly associated with increased risk of intrauterine (IU) HIV-1 infection. Promoter variants decreased DC-SIGN expression both in vitro and in placental CD163(+) macrophages (Hofbauer cells) of HIV-1 unexposed infants but not of HIV-1 exposed infants. The exon 4 protein-modifying mutations increased HIV-1 capture and transmission to T cells in vitro.

CONCLUSION

This study provides compelling evidence to support an important role of DC-SIGN in IU HIV-1 infection.

HIV-1 Vpu disarms natural killer cells

Natural killer (NK)-cell killing of virus-infected cells is regulated in part by the engagement of activation and coactivation receptors. In this issue of Cell Host & Microbe, Shah et al. (2010) demonstrate that HIV-1 protects infected cells from NK-cell-mediated killing by hindering NK-cell degranulation through downmodulation of NTB-A coactivation receptor ligands by the Vpu accessory protein.

Modulation of HIV-1-host interaction: role of the Vpu accessory protein

Viral protein U (Vpu) is a type 1 membrane-associated accessory protein that is unique to human immunodeficiency virus type 1 (HIV-1) and a subset of related simian immunodeficiency virus (SIV). The Vpu protein encoded by HIV-1 is associated with two primary functions during the viral life cycle. First, it contributes to HIV-1-induced CD4 receptor downregulation by mediating the proteasomal degradation of newly synthesized CD4 molecules in the endoplasmic reticulum (ER). Second, it enhances the release of progeny virions from infected cells by antagonizing Tetherin, an interferon (IFN)-regulated host restriction factor that directly cross-links virions on host cell-surface. This review will mostly focus on recent advances on the role of Vpu in CD4 downregulation and Tetherin antagonism and will discuss how these two functions may have impacted primate immunodeficiency virus cross-species transmission and the emergence of pandemic strain of HIV-1.

Hepatitis C virus core protein induces neuroimmune activation and potentiates Human Immunodeficiency Virus-1 neurotoxicity

Background

Hepatitis C virus (HCV) genomes and proteins are present in human brain tissues although the impact of HIV/HCV co-infection on neuropathogenesis remains unclear. Herein, we investigate HCV infectivity and effects on neuronal survival and neuroinflammation in conjunction with HIV infection.

Methodology

Human microglia, astrocyte and neuron cultures were infected with cell culture-derived HCV or exposed to HCV core protein with or without HIV-1 infection or HIV-1 Viral Protein R (Vpr) exposure. Host immune gene expression and cell viability were measured. Patch-clamp studies of human neurons were performed in the presence or absence of HCV core protein. Neurobehavioral performance and neuropathology were examined in HIV-1 Vpr-transgenic mice in which stereotaxic intrastriatal implants of HCV core protein were performed.

Principal Findings

HCV-encoded RNA as well as HCV core and non-structural 3 (NS3) proteins were detectable in human microglia and astrocytes infected with HCV. HCV core protein exposure induced expression of pro-inflammatory cytokines including interleukin-1β, interleukin-6 and tumor necrosis factor-α in microglia (p<0.05) but not in astrocytes while increased chemokine (e.g. CXCL10 and interleukin-8) expression was observed in both microglia and astrocytes (p<0.05). HCV core protein modulated neuronal membrane currents and reduced both β-III-tubulin and lipidated LC3-II expression (p<0.05). Neurons exposed to supernatants from HCV core-activated microglia exhibited reduced β-III-tubulin expression (p<0.05). HCV core protein neurotoxicity and interleukin-6 induction were potentiated by HIV-1 Vpr protein (p<0.05). HIV-1 Vpr transgenic mice implanted with HCV core protein showed gliosis, reduced neuronal counts together with diminished LC3 immunoreactivity. HCV core-implanted animals displayed neurobehavioral deficits at days 7 and 14 post-implantation (p<0.05).

Conclusions

HCV core protein exposure caused neuronal injury through suppression of neuronal autophagy in addition to neuroimmune activation. The additive neurotoxic effects of HCV- and HIV-encoded proteins highlight extrahepatic mechanisms by which HCV infection worsens the disease course of HIV infection.

Formation of mobile chromatin-associated nuclear foci containing HIV-1 Vpr and VPRBP is critical for the induction of G2 cell cycle arrest

HIV-1 Viral protein R (Vpr) induces a cell cycle arrest at the G2/M phase by activating the ATR DNA damage/stress checkpoint. Recently, we and several other groups showed that Vpr performs this activity by recruiting the DDB1-CUL4A (VPRBP) E3 ubiquitin ligase. While recruitment of this E3 ubiquitin ligase complex has been shown to be required for G2 arrest, the subcellular compartment where this complex forms and functionally acts is unknown. Herein, using immunofluorescence and confocal microscopy, we show that Vpr forms nuclear foci in several cell types including HeLa cells and primary CD4+ T-lymphocytes. These nuclear foci contain VPRBP and partially overlap with DNA repair foci components such as γ-H2AX, 53BP1 and RPA32. While treatment with the non-specific ATR inhibitor caffeine or depletion of VPRBP by siRNA did not inhibit formation of Vpr nuclear foci, mutations in the C-terminal domain of Vpr and cytoplasmic sequestration of Vpr by overexpression of Gag-Pol resulted in impaired formation of these nuclear structures and defective G2 arrest. Consistently, we observed that G2 arrest-competent sooty mangabey Vpr could form these foci but not its G2 arrest-defective paralog Vpx, suggesting that formation of Vpr nuclear foci represents a critical early event in the induction of G2 arrest. Indeed, we found that Vpr could associate to chromatin via its C-terminal domain and that it could form a complex with VPRBP on chromatin. Finally, analysis of Vpr nuclear foci by time-lapse microscopy showed that they were highly mobile and stable structures. Overall, our results suggest that Vpr recruits the DDB1-CUL4A (VPRBP) E3 ligase to these nuclear foci and uses these mobile structures to target a chromatin-bound cellular substrate for ubiquitination in order to induce DNA damage/replication stress, ultimately leading to ATR activation and G2 cell cycle arrest.

HIV-1, the causative agent of AIDS, encodes several proteins termed accessory, which play a critical role in viral pathogenesis. One of these accessory proteins, viral protein R (Vpr), has been found to block normal cell division. This impairment of cell division by Vpr is thought to increase viral replication and to trigger immune cell death. However, how Vpr is able to block cell growth remains unknown. We and other investigators recently showed that Vpr was performing this activity by interacting with a cellular protein complex involved in ubiquitination. Ubiquitination is characterized by the conjugation of a small protein called ubiquitin to various other proteins to regulate their degradation or activities. In this report, we demonstrate that Vpr forms mobile punctuate structures called foci on the DNA of host cells. We also show that formation of these foci by Vpr is required to block cell division. We propose that Vpr recruits the ubiquitination complex to these nuclear foci and uses these mobile structures to target a DNA-bound cellular protein for degradation, resulting in the activation of a host cell response leading to a cell division block. Identification of the unknown cellular factor targeted by Vpr will contribute to the understanding of the role of Vpr during HIV infection and AIDS pathogenesis.

Antagonism of tetherin restriction of HIV-1 release by Vpu involves binding and sequestration of the restriction factor in a perinuclear compartment

The Vpu accessory protein promotes HIV-1 release by counteracting Tetherin/BST-2, an interferon-regulated restriction factor, which retains virions at the cell-surface. Recent reports proposed β-TrCP-dependent proteasomal and/or endo-lysosomal degradation of Tetherin as potential mechanisms by which Vpu could down-regulate Tetherin cell-surface expression and antagonize this restriction. In all of these studies, Tetherin degradation did not, however, entirely account for Vpu anti-Tetherin activity. Here, we show that Vpu can promote HIV-1 release without detectably affecting Tetherin steady-state levels or turnover, suggesting that Tetherin degradation may not be necessary and/or sufficient for Vpu anti-Tetherin activity. Even though Vpu did not enhance Tetherin internalization from the plasma membrane (PM), it did significantly slow-down the overall transport of the protein towards the cell-surface. Accordingly, Vpu expression caused a specific removal of cell-surface Tetherin and a re-localization of the residual pool of Tetherin in a perinuclear compartment that co-stained with the TGN marker TGN46 and Vpu itself. This re-localization of Tetherin was also observed with a Vpu mutant unable to recruit β-TrCP, suggesting that this activity is taking place independently from β-TrCP-mediated trafficking and/or degradation processes. We also show that Vpu co-immunoprecipitates with Tetherin and that this interaction involves the transmembrane domains of the two proteins. Importantly, this association was found to be critical for reducing cell-surface Tetherin expression, re-localizing the restriction factor in the TGN and promoting HIV-1 release. Overall, our results suggest that association of Vpu to Tetherin affects the outward trafficking and/or recycling of the restriction factor from the TGN and as a result promotes its sequestration away from the PM where productive HIV-1 assembly takes place. This mechanism of antagonism that results in TGN trapping is likely to be augmented by β-TrCP-dependent degradation, underlining the need for complementary and perhaps synergistic strategies to effectively counteract the powerful restrictive effects of human Tetherin.

Restriction factors are cellular proteins that interfere with the multiplication and transmission of viruses and are therefore important components of natural immunity. Tetherin (also known as BST-2) is a recently identified restriction factor that traps viruses at the cell-surface, preventing their release and thus infection of other cells. Viruses have, however, developed means to counteract this restriction factor. Viral protein U (Vpu) is an accessory protein encoded by HIV-1, the causative agent of AIDS. Vpu antagonizes Tetherin and consequently promotes the release of HIV-1 particles. A series of recent reports proposed that Vpu would induce the degradation of this restriction factor in order to overcome its anti-viral activity. Here, we report that Vpu is able to enhance HIV-1 release in absence of Tetherin degradation. Instead, we found that Vpu interacts with Tetherin and interferes with the transport of the restriction factor towards the cell-surface. This would lead to re-localization of Tetherin in an intracellular organelle called the trans-Golgi network, resulting in insufficient levels of Tetherin at the cell-surface to trap progeny viruses. This mechanism of antagonism that results in TGN trapping could be augmented by the induction of degradation to effectively counteract the powerful restrictive effects of human Tetherin. Further characterization of this mechanism will improve our understanding of host antiviral defenses as well as provide new targets for the development of novel anti-HIV drugs.

HIV-1 Vpr-mediated G2 arrest involves the DDB1-CUL4AVPRBP E3 ubiquitin ligase

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) has been shown to cause G2 cell cycle arrest in human cells by inducing ATR-mediated inactivation of p34cdc2, but factors directly engaged in this process remain unknown. We used tandem affinity purification to isolate native Vpr complexes. We found that damaged DNA binding protein 1 (DDB1), viral protein R binding protein (VPRBP), and cullin 4A (CUL4A)--components of a CUL4A E3 ubiquitin ligase complex, DDB1-CUL4A(VPRBP)--were able to associate with Vpr. Depletion of VPRBP by small interfering RNA impaired Vpr-mediated induction of G2 arrest. Importantly, VPRBP knockdown alone did not affect normal cell cycle progression or activation of ATR checkpoints, suggesting that the involvement of VPRBP in G2 arrest was specific to Vpr. Moreover, leucine/isoleucine-rich domain Vpr mutants impaired in their ability to interact with VPRBP and DDB1 also produced strongly attenuated G2 arrest. In contrast, G2 arrest-defective C-terminal Vpr mutants were found to maintain their ability to associate with these proteins, suggesting that the interaction of Vpr with the DDB1-VPRBP complex is necessary but not sufficient to block cell cycle progression. Overall, these results point toward a model in which Vpr could act as a connector between the DDB1-CUL4A(VPRBP) E3 ubiquitin ligase complex and an unknown cellular factor whose proteolysis or modulation of activity through ubiquitination would activate ATR-mediated checkpoint signaling and induce G2 arrest.

HIV-1 Vpr causes neuronal apoptosis and in vivo neurodegeneration

Despite the introduction of highly active antiretroviral therapy, dementia caused by human immunodeficiency virus-1 (HIV-1) infection remains a devastating and common neurological disorder. Although the mechanisms governing neurodegeneration during HIV-1 infection remain uncertain, the HIV-1 accessory protein, viral protein R (Vpr), has been proposed as a neurotoxic protein. Herein, we report that Vpr protein and transcript were present in the brains of HIV-infected persons. Moreover, soluble Vpr caused neuronal apoptosis, involving cytochrome c extravasation, p53 induction, and activation of caspase-9 while exerting a depressive effect on whole-cell currents in neurons (p < 0.05), which was inhibited by iberiotoxin. Vpr-activated glial cells secreted neurotoxins in a concentration-dependent manner (p < 0.001). Transgenic (Tg) mice expressing Vpr in brain monocytoid cells displayed the transgene principally in the basal ganglia (p < 0.05) and cerebral cortex (p < 0.01) compared with hindbrain expression. Vpr was released from cultured transgenic macrophages, which was cytotoxic to neurons and was blocked by anti-Vpr antibody (p < 0.05). Neuronal injury was observed in Tg animals compared with wild-type littermates, chiefly affecting GAD65 (p < 0.01) and vesicular acetylcholine transferase (p < 0.001) immunopositive neuronal populations in the basal ganglia. There was also a loss of subcortical synaptophysin (p < 0.001) immunoreactivity as well as an increase in activated caspase-3, which was accompanied by a hyperexcitable neurobehavioral phenotype (p < 0.05). Thus, HIV-1 Vpr caused neuronal death through convergent pathogenic mechanisms with ensuing in vivo neurodegeneration, yielding new insights into the mechanisms by which HIV-1 injures the nervous system.

Association between disruption of CD4 receptor dimerization and increased human immunodeficiency virus type 1 entry

Background

Human immunodeficiency virus (HIV) enters target cells by a membrane fusion process that involves a series of sequential interactions between its envelope glycoproteins, the CD4 receptor and CXCR4/CCR5 coreceptors. CD4 molecules are expressed at the cell surface of lymphocytes and monocytes mainly as monomers, but basal levels of CD4 dimers are also present at the cell surface of these cells. Previous evidence indicates that the membrane distal and proximal extracellular domains of CD4, respectively D1 and D4, are involved in receptor dimerization.

Results

Here, we have used A201 cell lines expressing two CD4 mutants, CD4-E91K, E92K (D1 mutant) and CD4-Q344E (D4 mutant), harboring dimerization defects to analyze the role of CD4 dimerization in HIV-1 entry. Using entry assays based on β-lactamase-Vpr or luciferase reporter activities, as well as virus encoding envelope glycoproteins derived from primary or laboratory-adapted strains, we obtained evidence suggesting an association between disruption of CD4 dimerization and increased viral entry efficiency.

Conclusion

Taken together, our results suggest that monomeric forms of CD4 are preferentially used by HIV-1 to gain entry into target cells, thus implying that the dimer/monomer ratio at the cell surface of HIV-1 target cells may modulate the efficiency of HIV-1 entry.

Contribution of the C-terminal tri-lysine regions of human immunodeficiency virus type 1 integrase for efficient reverse transcription and viral DNA nuclear import

Background

In addition to mediating the integration process, HIV-1 integrase (IN) has also been implicated in different steps during viral life cycle including reverse transcription and viral DNA nuclear import. Although the karyophilic property of HIV-1 IN has been well demonstrated using a variety of experimental approaches, the definition of domain(s) and/or motif(s) within the protein that mediate viral DNA nuclear import and its mechanism are still disputed and controversial. In this study, we performed mutagenic analyses to investigate the contribution of different regions in the C-terminal domain of HIV-1 IN to protein nuclear localization as well as their effects on virus infection.

Results

Our analysis showed that replacing lysine residues in two highly conserved tri-lysine regions, which are located within previously described Region C (²³⁵WKGPAKLLWKGEGAVV) and sequence Q (²¹¹KELQKQITK) in the C-terminal domain of HIV-1 IN, impaired protein nuclear accumulation, while mutations for RK_263,4 had no significant effect. Analysis of their effects on viral infection in a VSV-G pseudotyped RT/IN trans-complemented HIV-1 single cycle replication system revealed that all three C-terminal mutant viruses (KK215,9AA, KK240,4AE and RK263,4AA) exhibited more severe defect of induction of β-Gal positive cells and luciferase activity than an IN class 1 mutant D64E in HeLa-CD4-CCR5-β-Gal cells, and in dividing as well as non-dividing C8166 T cells, suggesting that some viral defects are occurring prior to viral integration. Furthermore, by analyzing viral DNA synthesis and the nucleus-associated viral DNA level, the results clearly showed that, although all three C-terminal mutants inhibited viral reverse transcription to different extents, the KK240,4AE mutant exhibited most profound effect on this step, whereas KK215,9AA significantly impaired viral DNA nuclear import. In addition, our analysis could not detect viral DNA integration in each C-terminal mutant infection, even though they displayed various low levels of nucleus-associated viral DNA, suggesting that these C-terminal mutants also impaired viral DNA integration ability.

Conclusion

All of these results indicate that, in addition to being involved in HIV-1 reverse transcription and integration, the C-terminal tri-lysine regions of IN also contribute to efficient viral DNA nuclear import during the early stage of HIV-1 replication.

Analysis of HIV-1 Vpr determinants responsible for cell growth arrest in Saccharomyces cerevisiae

Background

The HIV-1 genome encodes a well-conserved accessory gene product, Vpr, that serves multiple functions in the retroviral life cycle, including the enhancement of viral replication in nondividing macrophages, the induction of G2 cell-cycle arrest, and the modulation of HIV-1-induced apoptosis. We previously reported the genetic selection of a panel of di-tryptophan (W)-containing peptides capable of interacting with HIV-1 Vpr and inhibiting its cytostatic activity in Saccharomyces cerevisiae (Yao, X.-J., J. Lemay, N. Rougeau, M. Clément, S. Kurtz, P. Belhumeur, and E. A. Cohen, J. Biol. Chem. v. 277, p. 48816–48826, 2002). In this study, we performed a mutagenic analysis of Vpr to identify sequence and/or structural determinants implicated in the interaction with di-W-containing peptides and assessed the effect of mutations on Vpr-induced cytostatic activity in S. cerevisiae.

Results

Our data clearly shows that integrity of N-terminal α-helix I (17–33) and α-helix III (53–83) is crucial for Vpr interaction with di-W-containing peptides as well as for the protein-induced cytostatic effect in budding yeast. Interestingly, several Vpr mutants, mainly in the N- and C-terminal domains, which were previously reported to be defective for cell-cycle arrest or apoptosis in human cells, still displayed a cytostatic activity in S. cerevisiae and remained sensitive to the inhibitory effect of di-W-containing peptides.

Conclusions

Vpr-induced growth arrest in budding yeast can be effectively inhibited by GST-fused di-W peptide through a specific interaction of di-W peptide with Vpr functional domain, which includes α-helix I (17–33) and α-helix III (53–83). Furthermore, the mechanism(s) underlying Vpr-induced cytostatic effect in budding yeast are likely to be distinct from those implicated in cell-cycle alteration and apoptosis in human cells.