Characterization of simian immunodeficiency virus SIVSM/human immunodeficiency virus type 2 Vpx function in human myeloid cells

Human immunodeficiency virus type 2 (HIV-2)/simian immunodeficiency virus SIV_SM Vpx is incorporated into virion particles and is thus present during the early steps of infection, when it has been reported to influence the nuclear import of viral DNA. We recently reported that Vpx promoted the accumulation of full-length viral DNA following the infection of human monocyte-derived dendritic cells (DCs). This positive effect was exerted following the infection of DCs with cognate viruses and with retroviruses as divergent as HIV-1, feline immunodeficiency virus, and even murine leukemia virus, leading us to suggest that Vpx counteracted an antiviral restriction present in DCs. Here, we show that Vpx is required, albeit to a different extent, for the infection of all myeloid but not of lymphoid cells, including monocytes, macrophages, and monocytoid THP-1 cells that had been induced to differentiate with phorbol esters. The intracellular localization of Vpx was highly heterogeneous and cell type dependent, since Vpx localized differently in HeLa cells and DCs. Despite these differences, no clear correlation between the functionality of Vpx and its intracellular localization could be drawn. As a first insight into its function, we determined that SIV_SM/HIV-2 and SIV_RCM Vpx proteins interact with the DCAF1 adaptor of the Cul4-based E3 ubiquitin ligase complex recently described to associate with HIV-1 Vpr and HIV-2 Vpx. However, the functionality of Vpx proteins in the infection of DCs did not strictly correlate with DCAF1 binding, and knockdown experiments failed to reveal a functional role for this association in differentiated THP-1 cells. Lastly, when transferred in the context of a replication-competent viral clone, Vpx was required for replication in DCs.     

Primate lentiviral Vpx commandeers DDB1 to counteract a macrophage restriction

Primate lentiviruses encode four "accessory proteins" including Vif, Vpu, Nef, and Vpr/Vpx. Vif and Vpu counteract the antiviral effects of cellular restrictions to early and late steps in the viral replication cycle. We present evidence that the Vpx proteins of HIV-2/SIV(SM) promote virus infection by antagonizing an antiviral restriction in macrophages. Fusion of macrophages in which Vpx was essential for virus infection, with COS cells in which Vpx was dispensable for virus infection, generated heterokaryons that supported infection by wild-type SIV but not Vpx-deleted SIV. The restriction potently antagonized infection of macrophages by HIV-1, and expression of Vpx in macrophages in trans overcame the restriction to HIV-1 and SIV infection. Vpx was ubiquitylated and both ubiquitylation and the proteasome regulated the activity of Vpx. The ability of Vpx to counteract the restriction to HIV-1 and SIV infection was dependent upon the HIV-1 Vpr interacting protein, damaged DNA binding protein 1 (DDB1), and DDB1 partially substituted for Vpx when fused to Vpr. Our results indicate that macrophage harbor a potent antiviral restriction and that primate lentiviruses have evolved Vpx to counteract this restriction.     

Virological characterization of HIV-2 vpx gene mutants in various cell systems

Requirement of intrinsically disordered protein Vpx for HIV-2 replication is cell-type dependent. To define Vpx-dependent conditions, replication ability of HIV-2 vpx mutants was analyzed in various cell lines that differ in cellular type, differentiation state and/or expression level of anti-HIV-1 SAMHD1 degraded by Vpx. Induction of Vpx-sensitive anti-HIV-2 state was not always associated with SAMHD1 expression. Compared with our previous data in lymphocytic cells, growth-defectiveness of the vpx mutants in differentiated THP-1 cells, a newly established multi-cycle infection system, was considerably different. Taken together, our results suggest that Vpx plays cell-type dependent role through its undetermined structure and/or function.     

Localization of the Vpx packaging signal within the C terminus of the human immunodeficiency virus type 2 Gag precursor protein.

Viral protein X (Vpx) is a human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus accessory protein that is packaged into virions in molar amounts equivalent to Gag proteins. To delineate the processes of virus assembly that mediate Vpx packaging, we used a recombinant vaccinia virus-T7 RNA polymerase system to facilitate Gag protein expression, particle assembly, and extracellular release. HIV genes were placed under control of the bacteriophage T7 promoter and transfected into HeLa cells expressing T7 RNA polymerase. Western immunoblot analysis detected p55gag and its cleavage products p39 and p27 in purified particles derived by expression of gag and gag-pol, respectively. In trans expression of vpx with either HIV-2 gag or gag-pol gave rise to virus-like particles that contained Vpx in amounts similar to that detected in HIV-2 virus produced from productively infected T cells. Using C-terminal deletion and truncation mutants of HIV-2 Gag, we mapped the p15 coding sequence for determinants of Vpx packaging. This analysis revealed a region (residues 439 to 497) downstream of the nucleocapsid protein (NC) required for incorporation of Vpx into virions. HIV-1/HIV-2 gag chimeras were constructed to further characterize the requirements for incorporation of Vpx into virions. Chimeric HIV-1/HIV-2 Gag particles consisting of HIV-1 p17 and p24 fused in frame at the C terminus with HIV-2 p15 effectively incorporate Vpx, while chimeric HIV-2/HIV-1 Gag particles consisting of HIV-2 p17 and p27 fused in frame at the C terminus with HIV-1 p15 do not. Expression of a 68-amino-acid sequence of HIV-2 containing residues 439 to 497 fused to the coding regions of HIV-1 p17 and p24 also produced virus-like particles capable of packaging Vpx in amounts similar to that of full-length HIV-2 Gag. Sucrose gradient analysis confirmed particle association of Vpx and Gag proteins. These results demonstrate that the HIV-2 Gag precursor (p55) regulates incorporation of Vpx into virions and indicates that the packaging signal is located within residues 439 to 497.     

A non-canonical transferable signal mediates nuclear import of simian immunodeficiency virus Vpx protein

Protein transport into the nucleus is generally considered to involve specific nuclear localization signals (NLS) though it is becoming increasingly evident that efficient and well controlled import of proteins which lack a canonical NLS also occurs in cells. Vpx, a 112 amino acid protein from human immunodeficiency virus type 2 (HIV-2) and the closely related simian immunodeficiency virus (SIV) is one such protein, which does not have an identifiable canonical NLS and is yet efficiently imported to the nuclear compartment. Here we report that Vpx protein is imported to the nucleus independently of virus-encoded cofactors. When fusions of truncated versions of Vpx with full-length beta-galactosidase (beta-Gal) were tested, the region from Vpx 61 to 80 was found to be sufficient to mediate the import of the heterologous cytoplasmic protein to the nucleus. Inactivation of Vpx NLS precluded nuclear import of Vpx and reduced virus replication in non-dividing macrophage cultures, even when functional integrase and Gag matrix proteins implicated in viral nuclear import were present. Importantly, we identified and characterized a novel type of 20 amino acid transferable nuclear import signal in Vpx that is distinct from other import signals described. In addition, we show that the minimal nuclear targeting domain identified here overlaps with helical domain III (amino acid (aa) 64-82) and the structural integrity of this helical motif is critical for the nuclear import of Vpx. Taken together, these data suggest that Vpx is imported to the nucleus via a novel import pathway that is dependent on its 20 amino acid unique nuclear targeting signal, and that the nuclear import property of Vpx is critical for the optimal virus replication in non-dividing cells such as macrophages.     

Vpx and Vpr proteins of HIV-2 up-regulate the viral infectivity by a distinct mechanism in lymphocytic cells

Mutants of human immunodeficiency virus type 2 (HIV-2) carrying a frame-shift mutation in vpx, vpr, and in both genes were monitored for their growth potentials in a newly established lymphocytic cell line, HSC-F. Worthy of note, the replication of a vpx single mutant, but not vpr, was severely impaired in these cells, and that of a vpx-vpr double mutant was more damaged. Defective replication sites of the vpx single and vpx-vpr double mutants were demonstrated to be mapped, respectively, to the nuclear import of viral genome, and to both, this process and the virus assembly/release stage. While the mutational effect of vpr was small, the replication efficiency in one cycle of the vpx mutant relative to that of wild-type virus was estimated to be 10%. The growth phenotypes of the vpx, vpr, and vpx-vpr mutant viruses in HSC-F cells were essentially repeated in primary human lymphocytes. In primary human macrophages, whereas the vpx and vpx-vpr mutants did not grow at all, the vpr mutant grew equally as well as the wild-type virus. These results strongly suggested that Vpx is critical for up-regulation of HIV-2 replication in natural target cells by enhancing the genome nuclear import, and that Vpr promotes HIV-2 replication somewhat, at least in lymphocytic cells, at a very late replication phase.     

Comparative study on the structure and cytopathogenic activity of HIV Vpr/Vpx proteins

The three-dimensional (3-D) structure of human immunodeficiency virus type 2 (HIV-2) Vpr/Vpx was predicted by homology modeling based on the NMR structure of human immunodeficiency virus type 1 (HIV-1) Vpr. The three proteins similarly have three major amphipathic alpha-helices. In contrast to HIV-1 Vpr, Vpr/Vpx of HIV-2 have a long N-terminal loop and clustered prolines in the second half of the C-terminal loop. HIV-2 Vpx uniquely contains a long region between the second and third major helices, and bears several glycines in the first half of the C-terminal loop. Instead of the glycines, there is a group of hydrophilic amino acids and arginines in the corresponding regions of the two Vprs. To compare the cytopathogenic potentials of HIV-1 Vpr and HIV-2 Vpr/Vpx, we examined the production of luciferase as a marker of cell damage. We further analyzed the characteristics of cells transduced with vpr/vpx genes driven by an inducible promoter. The results obtained clearly show that structurally similar, but distinct, HIV Vpr/Vpx proteins are detrimental to target cells.     

The C-terminal proline-rich tail of human immunodeficiency virus type 2 Vpx is necessary for nuclear localization of the viral preintegration complex in nondividing cells

Human immunodeficiency virus type 2 (HIV-2), like other lentiviruses, is capable of infecting nondividing T cells and macrophages. The present work shows that in HIV-2-infected cells, Vpx is necessary for efficient nuclear import of the preintegration complex. In agreement with this finding, the subcellular localization of a GFP-Vpx fusion protein was found to be predominantly nuclear. However, deletion of the proline-rich C-terminal 11 residues of Vpx resulted in a shift of the fusion protein to the cytoplasm. Furthermore, the same deletion in the context of the provirus resulted in a decrease in nuclear import of the preintegration complex and attenuated replication in macrophages.     

Differential apoptosis effects of primate lentiviral Vpr and Vpx in mammalian cells

The growth inhibitory effects of Vpr and Vpx are species- and cell type-dependent. HIV-1, HIV-2 and SIV Vpr are primarily cytostatic in mammalian cells and HIV-1 Vpr has been reported to induce apoptosis in human cells. Our previous studies have shown that HIV-1, HIV-2 and SIV Vpr and Vpx have differential cytostatic and cytotoxic effects in the yeast cells [Zhang et al.: Virology, 230:103-112; 1997]. Here, we further examined the apoptosis function of HIV-1 Vpr in different species of mammalian cells and investigated if other primate lentiviral Vpr and Vpx exert similar functions. Our results show that none of the primate lentiviral Vpr or Vpx we tested induces apoptosis in nonhuman species of mammalian cells. However, HIV-1 Vpr, but not HIV-2 or SIV Vpr and/or Vpx, induced apoptosis in different types of human cell lines. Further, the apoptotic effect of HIV-1 Vpr can be distinguished from that of the human interferon-gamma, a known proapoptotic protein, that HIV-1 Vpr shows little to no paracrine and/or bystander effect. When coexpressed with Bcl-2 or Bcl-X(L), the apoptotic effect of HIV-1 Vpr became markedly attenuated. These results indicate that the apoptotic effect of HIV-1 Vpr is species-dependent and is intracellularly modulated by the Bcl-2 family of proteins. Our study also suggests that the proapoptotic function of HIV-1 Vpr is developmentally associated with human but not nonhuman primate species.     

Functional Analysis of the Relationship between Vpx and the Restriction Factor SAMHD1

SAMHD1 is a newly identified restriction factor that targets lentiviruses in myeloid cells and is countered by the SIV_SM/HIV-2 Vpx protein. By analyzing a large panel of Vpx mutants, we identify several residues throughout the 3-helix bundle predicted for Vpx that impair both its functionality and its ability to degrade SAMHD1. We determine that SAMHD1 is a strictly non-shuttling nuclear protein and that as expected WT Vpx localizes with it in the nucleus. However, we also identify a functional Vpx mutant with predominant cytoplasmic distribution that colocalizes with SAMHD1 in this location, suggesting that Vpx may also retain SAMHD1 in the cell cytoplasm, prior to its entry into the nucleus. Several mutations in Vpx were shown to affect the stability of Vpx, as well as Vpx:Vpx interactions. However, no strict correlation was observed between these parameters and the functionality of Vpx, implying that neither properties is absolutely required for this function and indicating that even unstable Vpx mutants may be very efficient in inducing SAMHD1 degradation. Overall, our analysis identifies several Vpx residues required for SAMHD1 degradation and points to a very efficient and plastic mechanism through which Vpx depletes this restriction factor.     

Targeting of Chrolamphenicol Acetyltransferase to Human Immunodeficiency Virus Particles via Vpr and Vpx

Vpr and Vpx are the auxiliary proteins of human immunodeficiency viruses (HIVs) selectively incorporated into mature viral particles. We showed that the bacterial chloramphenicol acetyltransferase (CAT) fused to the N-terminus of HIV-1 Vpr, HIV-2 Vpr, or HIV-2 Vpx was incorporated into mature virions in a type-selective manner. By using chimeric proteins between HIV-1 Vpr and HIV-2 Vpx, we found that the N-terminal side of these proteins was mainly important for type-selective virion incorporation. The C-terminal arginine-rich region of HIV-1 Vpr was also found to transport CAT fusion proteins into virions but without any type selectivity. Furthermore, the corresponding regions of HIV-2 Vpr and HIV-2 Vpx had no such activity. This region of HIV-1 Vpr may interact nonspecifically with viral genomic RNA. Collectively, Vpr and Vpx may provide a means to introduce foreign proteins and other molecules into HIV virions for therapeutic purposes.     

Human immunodeficiency virus type 1 modified to package Simian immunodeficiency virus Vpx efficiently infects macrophages and dendritic cells

The lentiviral accessory protein Vpx is thought to facilitate the infection of macrophages and dendritic cells by counteracting an unidentified host restriction factor. Although human immunodeficiency virus type 1 (HIV-1) does not encode Vpx, the accessory protein can be provided to monocyte-derived macrophages (MDM) and monocyte-derived dendritic cells (MDDC) in virus-like particles, dramatically enhancing their susceptibility to HIV-1. Vpx and the related accessory protein Vpr are packaged into virions through a virus-specific interaction with the p6 carboxy-terminal domain of Gag. We localized the minimal Vpx packaging motif of simian immunodeficiency virus SIVmac(239) p6 to a 10-amino-acid motif and introduced this sequence into an infectious HIV-1 provirus. The chimeric virus packaged Vpx that was provided in trans and was substantially more infectious on MDDC and MDM than the wild-type virus. We further modified the virus by introducing the Vpx coding sequence in place of nef. The resulting virus produced Vpx and replicated efficiently in MDDC and MDM. The virus also induced a potent type I interferon response in MDDC. In a coculture system, the Vpx-containing HIV-1 was more efficiently transmitted from MDDC to T cells. These findings suggest that in vivo, Vpx may facilitate transmission of the virus from dendritic cells to T cells. In addition, the chimeric virus could be used to design dendritic cell vaccines that induce an enhanced innate immune response. This approach could also be useful in the design of lentiviral vectors that transduce these relatively resistant cells.