Identification of Residues in the BST-2 TM Domain Important for Antagonism by HIV-1 Vpu Using a Gain-of-Function Approach

The HIV-1 Vpu protein enhances the release of viral particles from the cell-surface in a cell-type specific manner. In the absence of Vpu, nascent virions remain tethered to the cell-surface in restricted cell-types. Recently, the human host factor BST-2/CD317/tetherin was found to be responsible for the inhibition of virus release. It was also reported that HIV-1 Vpu can target human BST-2 but is unable to interfere with the function of murine or simian BST-2. We performed a gain-of-function study to determine which of the differences between human and rhesus BST-2 account for the differential sensitivity to Vpu. We transferred human BST-2 sequences into rhesus BST-2 and assessed the resulting chimeras for inhibition of HIV-1 virus release and sensitivity to Vpu. We found that rhesus BST-2 carrying the transmembrane (TM) domain of human BST-2 is susceptible to HIV-1 Vpu. Finally, a single-amino-acid change in the rhesus BST-2 TM domain was sufficient to confer Vpu sensitivity

The formation of cysteine-linked dimers of BST-2/tetherin is important for inhibition of HIV-1 virus release but not for sensitivity to Vpu

<p>Abstract</p> <p>Background</p> <p>The Human Immunodeficiency virus type 1 (HIV-1) Vpu protein enhances virus release from infected cells and induces proteasomal degradation of CD4. Recent work identified BST-2/CD317 as a host factor that inhibits HIV-1 virus release in a Vpu sensitive manner. A current working model proposes that BST-2 inhibits virus release by tethering viral particles to the cell surface thereby triggering their subsequent endocytosis.</p> <p>Results</p> <p>Here we defined structural properties of BST-2 required for inhibition of virus release and for sensitivity to Vpu. We found that BST-2 is modified by N-linked glycosylation at two sites in the extracellular domain. However, N-linked glycosylation was not important for inhibition of HIV-1 virus release nor did it affect surface expression or sensitivity to Vpu. Rodent BST-2 was previously found to form cysteine-linked dimers. Analysis of single, double, or triple cysteine mutants revealed that any one of three cysteine residues present in the BST-2 extracellular domain was sufficient for BST-2 dimerization, for inhibition of virus release, and sensitivity to Vpu. In contrast, BST-2 lacking all three cysteines in its ectodomain was unable to inhibit release of wild type or Vpu-deficient HIV-1 virions. This defect was not caused by a gross defect in BST-2 trafficking as the mutant protein was expressed at the cell surface of transfected 293T cells and was down-modulated by Vpu similar to wild type BST-2.</p> <p>Conclusion</p> <p>While BST-2 glycosylation was functionally irrelevant, formation of cysteine-linked dimers appeared to be important for inhibition of virus release. However lack of dimerization did not prevent surface expression or Vpu sensitivity of BST-2, suggesting Vpu sensitivity and inhibition of virus release are separable properties of BST-2.</p

The Great Escape: Viral Strategies to Counter BST-2/Tetherin

The interferon-induced BST-2 protein has the unique ability to restrict the egress of HIV-1, Kaposi's sarcoma–associated herpesvirus (KSHV), Ebola virus, and other enveloped viruses. The observation that virions remain attached to the surface of BST-2-expressing cells led to the renaming of BST-2 as “tetherin”. However, viral proteins such as HIV-1 Vpu, simian immunodeficiency virus Nef, and KSHV K5 counteract BST-2, thereby allowing mature virions to readily escape from infected cells. Since the anti-viral function of BST-2 was discovered, there has been an explosion of research into several aspects of this intriguing interplay between host and virus. This review focuses on recent work addressing the molecular mechanisms involved in BST-2 restriction of viral egress and the species-specific countermeasures employed by various viruses

Vpu Antagonizes BST-2–Mediated Restriction of HIV-1 Release via β-TrCP and Endo-Lysosomal Trafficking

The interferon-induced transmembrane protein BST-2/CD317 (tetherin) restricts the release of diverse enveloped viruses from infected cells. The HIV-1 accessory protein Vpu antagonizes this restriction by an unknown mechanism that likely involves the down-regulation of BST-2 from the cell surface. Here, we show that the optimal removal of BST-2 from the plasma membrane by Vpu requires the cellular protein β-TrCP, a substrate adaptor for a multi-subunit SCF E3 ubiquitin ligase complex and a known Vpu-interacting protein. β-TrCP is also required for the optimal enhancement of virion-release by Vpu. Mutations in the DSGxxS β-TrCP binding-motif of Vpu impair both the down-regulation of BST-2 and the enhancement of virion-release. Such mutations also confer dominant-negative activity, consistent with a model in which Vpu links BST-2 to β-TrCP. Optimal down-regulation of BST-2 from the cell surface by Vpu also requires the endocytic clathrin adaptor AP-2, although the rate of endocytosis is not increased; these data suggest that Vpu induces post-endocytic membrane trafficking events whose net effect is the removal of BST-2 from the cell surface. In addition to its marked effect on cell-surface levels, Vpu modestly decreases the total cellular levels of BST-2. The decreases in cell-surface and intracellular BST-2 are inhibited by bafilomycin A1, an inhibitor of endosomal acidification; these data suggest that Vpu induces late endosomal targeting and partial degradation of BST-2 in lysosomes. The Vpu-mediated decrease in surface expression is associated with reduced co-localization of BST-2 and the virion protein Gag along the plasma membrane. Together, the data support a model in which Vpu co-opts the β-TrCP/SCF E3 ubiquitin ligase complex to induce endosomal trafficking events that remove BST-2 from its site of action as a virion-tethering factor

Epigenetic Regulation of BST-2 Expression Levels and the Effect on HIV-1 Pathogenesis

HIV-1 must overcome host antiviral restriction factors for efficient replication. We hypothesized that elevated levels of bone marrow stromal cell antigen 2 (BST-2), a potent host restriction factor that interferes with HIV-1 particle release in some human cells and is antagonized by the viral protein Vpu, may associate with viral control. Using cryopreserved samples, from HIV-1 seronegative and seropositive Black women, we measured in vitro expression levels of BST-2 mRNA using a real-time PCR assay and protein levels were validated by Western blotting. The expression level of BST-2 showed an association with viral control within two independent cohorts of Black HIV infected females (r=-0.53, p=0.015, [n =21]; and r=-0.62, p=0.0006, [n=28]). DNA methylation was identified as a mechanism regulating BST-2 levels, where increased BST-2 methylation results in lower expression levels and associates with worse HIV disease outcome. We further demonstrate the ability to regulate BST-2 levels using a DNA hypomethylation drug. Our results suggest BST-2 as a factor for potential therapeutic intervention against HIV and other diseases known to involve BST-2

Bone marrow stromal antigen 2 (BST-2) genetic variants influence expression levels and disease outcome in HIV-1 chronically infected patients

BACKGROUND: Bone marrow stromal antigen 2 (BST-2) also known as Tetherin (CD317/HM1.24), is a host restriction factor that blocks the release of HIV-1 virions from infected cells. Previous studies reported that BST-2 genetic variants or single nucleotide polymorphims (SNPs) have a preventative role during HIV-1 infection. However, the influence of BST-2 SNPs on expression levels remains unknown. In this study, we investigated the influence of BST-2 SNPs on expression levels and disease outcome in HIV-1 subtype C chronically infected antiretroviral therapy naïve individuals. RESULTS: We quantified BST-2 mRNA levels in peripheral blood mononuclear cells (PBMCs), determined BST-2 protein expression on the surface of CD4+ T cells using flow cytometry and genotyped two intronic single nucleotide polymorphisms (SNPs) rs919267 and rs919266 together with one SNP rs9576 located in the 3' untranslated region (UTR) of bst-2 gene using TaqMan assays from HIV-1 uninfected and infected participants. Subsequently, we determined the ability of plasma antibody levels to mediate antibody-dependent cellular phagocytosis (ADCP) using gp120 consensus C and p24 subtype B/C protein. Fc receptor-mediated NK cell degranulation was evaluated as a surrogate for ADCC activity using plasma from HIV-1 positive participants. BST-2 mRNA expression levels in PBMCs and protein levels on CD4+ T cells were lower in HIV-1 infected compared to uninfected participants (p = 0.075 and p < 0.001, respectively). rs919267CT (p = 0.042) and rs919267TT (p = 0.045) were associated with lower BST-2 mRNA expression levels compared to rs919267CC in HIV-1 uninfected participants. In HIV-1 infected participants, rs919267CT associated with lower CD4 counts, (p = 0.003), gp120-IgG1 (p = 0.040), gp120-IgG3 (p = 0.016) levels but higher viral loads (p = 0.001) while rs919267TT was associated with lower BST-2 mRNA levels (p = 0.046), CD4 counts (p = 0.001), gp120-IgG1 levels (p = 0.033) but higher plasma viral loads (p = 0.007). Conversely, rs9576CA was associated with higher BST-2 mRNA expression levels (p = 0.027), CD4 counts (p = 0.079), gp120-IgG1 (p = 0.009), gp120-IgG3 (p = 0.039) levels but with lower viral loads (p = 0.037). CONCLUSION: Our findings show that bst-2 SNPs mediate BST-2 expression and disease outcome, correlate with gp120-IgG1, gp120-IgG3 levels but not p24-IgG levels, ADCC and ADCP activity

Structural Basis for the Antiviral Activity of BST-2/Tetherin and Its Viral Antagonism

The interferon-inducible host restriction factor bone marrow stromal antigen 2 (BST-2/tetherin) blocks the release of HIV-1 and other enveloped viruses. In turn, these viruses have evolved specific antagonists to counteract this host antiviral molecule, such as the HIV-1 protein Vpu. BST-2 is a type II transmembrane protein with an unusual topology consisting of an N-terminal cytoplasmic tail (CT) followed by a single transmembrane (TM) domain, a coiled-coil extracellular (EC) domain, and a glycosylphosphatidylinositol (GPI) anchor at the C terminus. We and others showed that BST-2 restricts enveloped virus release by bridging the host and virion membranes with its two opposing membrane anchors and that deletion of either one completely abrogates antiviral activity. The EC domain also shows conserved structural properties that are required for antiviral function. It contains several destabilizing amino acids that confer the molecule with conformational flexibility to sustain the protein’s function as a virion tether, and three conserved cysteine residues that mediate homodimerization of BST-2, as well as acting as a molecular ruler that separates the membrane anchors. Conversely, the efficient release of virions is promoted by the HIV-1 Vpu protein and other viral antagonists. Our group and others provided evidence from mutational analyses indicating that Vpu antagonism of BST-2-mediated viral restriction requires a highly specific interaction of their mutual TM domains. This interpretation is further supported and expanded by the findings of the latest structural modeling studies showing that critical amino acids in a conserved helical face of these TM domains are required for Vpu–BST-2 interaction and antagonism. In this review, we summarize the current advances in our understanding of the structural basis for BST-2 antiviral function as well as BST-2-specific viral antagonism

Ebolavirus Replication and Tetherin/BST-2

Ebolavirus (EBOV) is an enveloped, non-segmented, negative-stranded RNA virus, which consists of five species: Zaire ebolavirus, Sudan ebolavirus, Tai Forest ebolavirus, Bundibugyo ebolavirus, and Reston ebolavirus. EBOV causes a lethal hemorrhagic fever in both humans and non-human primates. The EBOV RNA genome encodes seven viral proteins: NP, VP35, VP40, GP, VP30, VP24, and L. VP40 is a matrix protein and is essential for virus assembly and release from host cells. Expression of VP40 in mammalian cells is sufficient to generate extracellular virus-like particles, which resemble authentic virions. Tetherin/BST-2, which was identified as an effective cellular factor that prevents human immunodeficiency virus-1 release in the absence of viral accessory protein Vpu, has been reported to inhibit ZEBOV VP40-induced VLP release. Tetherin/BST-2 appears to inhibit virus release by physically tethering viral particles to the cell surface via its N-terminal transmembrane domain and C-terminal glycosylphosphatidylinositol anchor. Replication of ZEBOV is not inhibited by tetherin/BST-2 expression, although tetherin/BST-2 was expected to inhibit EBOV release as well as VLP release. Recently, it was reported that viral glycoprotein of EBOV, GP, antagonizes the antiviral effect of tetherin/BST-2. However, the mechanism by which GP antagonizes the antiviral activity of tetherin/BST-2 and whether GP of the other EBOV species function as antagonists of tetherin/BST-2 remain unclear

Antagonism of BST-2/Tetherin is a conserved function of the Env glycoprotein of primary HIV-2 isolates

Although HIV-2 does not encode a vpu gene, the ability to antagonize bone marrow stromal antigen 2 (BST-2) is conserved in some HIV-2 isolates, where it is controlled by the Env glycoprotein. We previously reported that a single-amino-acid difference between the laboratory-adapted ROD10 and ROD14 Envs controlled the enhancement of virus release (referred to here as Vpu-like) activity. Here, we investigated how conserved the Vpu-like activity is in primary HIV-2 isolates. We found that half of the 34 tested primary HIV-2 Env isolates obtained from 7 different patients enhanced virus release. Interestingly, most HIV-2 patients harbored a mixed population of viruses containing or lacking Vpu-like activity. Vpu-like activity and Envelope functionality varied significantly among Env isolates; however, there was no direct correlation between these two functions, suggesting they evolved independently. In comparing the Env sequences from one HIV-2 patient, we found that similar to the ROD10/ROD14 Envs, a single-amino-acid change (T568I) in the ectodomain of the TM subunit was sufficient to confer Vpu-like activity to an inactive Env variant. Surprisingly, however, absence of Vpu-like activity was not correlated with absence of BST-2 interaction. Taken together, our data suggest that maintaining the ability to antagonize BST-2 is of functional relevance not only to HIV-1 but also to HIV-2 as well. Our data show that as with Vpu, binding of HIV-2 Env to BST-2 is important but not sufficient for antagonism. Finally, as observed previously, the Vpu-like activity in HIV-2 Env can be controlled by single-residue changes in the TM subunit.info:eu-repo/semantics/publishedVersio