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

Higher Expression of Human Endogenous Retrovirus-K was Observed in Peripheral B Lymphocytes of Leukemia and Lymphoma Patients

Hematological malignant tumors (HMTs) are serious diseases that threaten human health and life with high mortality. Therefore, it is necessary to develop novel strategies for diagnosis and treatment. Human endogenous retroviruses (HERVs) have recently attracted increasing attention as potential targets for cancer diagnosis and therapy. In this study, we explored the association between HERV-K expression levels and HMTs development. Clinical data and peripheral blood samples were collected from 236 leukemia, 384 lymphoma patients, and 69 healthy controls. Quantitative polymerase chain reaction was used to detect the expression of HERV-K gag, pol, and env genes in peripheral blood mononuclear cells or different cell subpopulations. Differently expressed HERV-K genes were further tested by using deep sequencing method, and further analyzed with gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment. B cell- and T cell-related cytokines in patients were also detected by enzyme-linked immunosorbent assay (ELISA). The results showed that the expression levels of the HERV-K gag, pol, and env genes in patients were significantly higher than in healthy controls. There was a correlation between the expression level of HERV-K and the clinicopathological parameters of leukemia patients. HERV-K expression was increased in the B lymphocytes of leukemia and lymphoma patients, but not in the T cells or neutrophils. The GO and KEGG analyses showed that abnormal expression of the HERV-K locus in patients affected immune regulation. The analysis of cytokines proved that the B cell-related cytokines, including interleukin (IL)-1β, IL-2, IL-4, IL-6, IL-10, tumor necrosis factor (TNF)-α, and interferon-gamma, were significantly decreased in patients, while the T cell-related cytokines, including IL-3, IL-12, and TNF-β, were not significantly changed. In conclusion, HERV-K genes might participate in the occurrence and development of leukemia and lymphoma, and might be biomarkers for the detection or evaluation of leukemia and lymphoma.

Bone marrow stromal cell antigen 2: Tumor biology, signaling pathway and therapeutic targeting (Review)

Bone marrow stromal cell antigen 2 (BST2) is a type II transmembrane protein that serves critical roles in antiretroviral defense in the innate immune response. In addition, it has been suggested that BST2 is highly expressed in various types of human cancer and high BST2 expression is related to different clinicopathological parameters in cancer. The molecular mechanism underlying BST2 as a potential tumor biomarker in human solid tumors has been reported on; however, to the best of our knowledge, there has been no review published on the molecular mechanism of BST2 in human solid tumors. The present review focuses on human BST2 expression, structure and functions; the molecular mechanisms of BST2 in breast cancer, hepatocellular carcinoma, gastrointestinal tumor and other solid tumors; the therapeutic potential of BST2; and the possibility of BST2 as a potential marker. BST2 is involved in cell membrane integrity and lipid raft formation, which can activate epidermal growth factor receptor signaling pathways, providing a potential mechanistic link between BST2 and tumorigenesis. Notably, BST2 may be considered a universal tumor biomarker and a potential therapeutical target.

Mutations accumulated in the Spike of SARS-CoV-2 Omicron allow for more efficient counteraction of the restriction factor BST2/Tetherin

BST2/Tetherin is a restriction factor with broad antiviral activity against enveloped viruses, including coronaviruses. Specifically, BST2 traps nascent particles to membrane compartments, preventing their release and spread. In turn, viruses have evolved multiple mechanisms to counteract BST2. Here, we examined the interactions between BST2 and SARS-CoV-2. Our study shows that BST2 reduces SARS-CoV-2 virion release. However, the virus uses the Spike (S) protein to downregulate BST2. This requires a physical interaction between S and BST2, which routes BST2 for lysosomal degradation in a Clathtin- and ubiquitination-dependent manner. By surveying different SARS-CoV-2 variants of concern (Alpha-Omicron), we found that Omicron is more efficient at counteracting BST2, and that mutations in S account for its enhanced anti-BST2 activity. Mapping analyses revealed that several surfaces in the extracellular region of BST2 are required for an interaction with the Spike, and that the Omicron variant has changed its patterns of association with BST2 to improve its counteraction. Therefore, our study suggests that, besides enhancing receptor binding and evasion of neutralizing antibodies, mutations accumulated in the Spike afford more efficient counteraction of BST2, which highlights that BST2 antagonism is important for SARS-CoV-2 infectivity and spread.

Human BST2 inhibits rabies virus release independently of cysteine-linked dimerization and asparagine-linked glycosylation

The innate immune response is a first-line defense mechanism triggered by rabies virus (RABV). Interferon (IFN) signaling and ISG products have been shown to confer resistance to RABV at various stages of the virus's life cycle. Human tetherin, also known as bone marrow stromal cell antigen 2 (hBST2), is a multifunctional transmembrane glycoprotein induced by IFN that has been shown to effectively counteract many viruses through diverse mechanisms. Here, we demonstrate that hBST2 inhibits RABV budding by tethering new virions to the cell surface. It was observed that release of virus-like particles (VLPs) formed by RABV G (RABV-G VLPs), but not RABV M (RABV-G VLPs), were suppressed by hBST2, indicating that RABV-G has a specific effect on the hBST2-mediated restriction of RABV. The ability of hBST2 to prevent the release of RABV-G VLPs and impede RABV growth kinetics is retained even when hBST2 has mutations at dimerization and/or glycosylation sites, making hBST2 an antagonist to RABV, with multiple mechanisms possibly contributing to the hBST2-mediated suppression of RABV. Our findings expand the knowledge of host antiviral mechanisms that control RABV infection.

BST2 negatively regulates porcine reproductive and respiratory syndrome virus replication by restricting the expression of viral proteins

Porcine reproductive and respiratory syndrome virus (PRRSV) has seriously affected the viability of swine industries worldwide, and effective measures to control PRRSV are urgently required. Understanding the mechanisms of action of antiviral proteins is crucial for developing antiviral strategies. Interferon-induced bone marrow stromal cell antigen 2 (BST2) can inhibit the replication of various viruses via different pathways. However, little is known about the effects of BST2 on PRRSV. Therefore, this study aimed to evaluate whether the interferon-induced BST2 can inhibit PRRSV replication. We used western blotting and RT-qPCR techniques to analyze the effect of BST2 overexpression and knockdown on PRRSV replication. Overexpression of BST2 inhibited the replication of PRRSV, whereas knockdown of BST2 by small interfering RNA promoted PRRSV replication. Additionally, the expression of BST2 was upregulated during the early phase of PRRSV infection in porcine alveolar macrophages. Analysis of PRRSV proteins showed that BST2 restricted the expression of several non-structural viral proteins. BST2 downregulated the expression of Nsp12 through a proteasome-dependent pathway and downregulated the expression and transcription of E protein. These findings demonstrate the potential of BST2 as a critical regulator of PRRSV replication.

ATG5 selectively engages virus-tethered BST2/tetherin in an LC3C-associated pathway

Bone marrow stromal antigen 2 (BST2)/tetherin is a restriction factor that reduces HIV-1 dissemination by tethering virus at the cell surface. BST2 also acts as a sensor of HIV-1 budding, establishing a cellular antiviral state. The HIV-1 Vpu protein antagonizes BST2 antiviral functions via multiple mechanisms, including the subversion of an LC3C-associated pathway, a key cell intrinsic antimicrobial mechanism. Here, we describe the first step of this viral-induced LC3C-associated process. This process is initiated at the plasma membrane through the recognition and internalization of virus-tethered BST2 by ATG5, an autophagy protein. ATG5 and BST2 assemble as a complex, independently of the viral protein Vpu and ahead of the recruitment of the ATG protein LC3C. The conjugation of ATG5 with ATG12 is dispensable for this interaction. ATG5 recognizes cysteine-linked homodimerized BST2 and specifically engages phosphorylated BST2 tethering viruses at the plasma membrane, in an LC3C-associated pathway. We also found that this LC3C-associated pathway is used by Vpu to attenuate the inflammatory responses mediated by virion retention. Overall, we highlight that by targeting BST2 tethering viruses, ATG5 acts as a signaling scaffold to trigger an LC3C-associated pathway induced by HIV-1 infection.

Evaluation of SARS-CoV-2 ORF7a Deletions from COVID-19-Positive Individuals and Its Impact on Virus Spread in Cell Culture

The spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing the COVID-19 outbreak, posed a primary concern of public health worldwide. The most common changes in SARS-CoV-2 are single nucleotide substitutions, also reported insertions and deletions. This work investigates the presence of SARS-CoV-2 ORF7a deletions identified in COVID-19-positive individuals. Sequencing of SARS-CoV-2 complete genomes showed three different ORF7a size deletions (190-nt, 339-nt and 365-nt). Deletions were confirmed through Sanger sequencing. The ORF7a∆190 was detected in a group of five relatives with mild symptoms of COVID-19, and the ORF7a∆339 and ORF7a∆365 in a couple of co-workers. These deletions did not affect subgenomic RNAs (sgRNA) production downstream of ORF7a. Still, fragments associated with sgRNA of genes upstream of ORF7a showed a decrease in size when corresponding to samples with deletions. In silico analysis suggests that the deletions impair protein proper function; however, isolated viruses with partial deletion of ORF7a can replicate in culture cells similarly to wild-type viruses at 24 hpi, but with less infectious particles after 48 hpi. These findings on deleted ORF7a accessory protein gene, contribute to understanding SARS-CoV-2 phenotypes such as replication, immune evasion and evolutionary fitness as well insights into the role of SARS-CoV-2_ORF7a in the mechanism of virus-host interactions.

Unique Evolution of Antiviral Tetherin in Bats

Bats are recognized as important reservoirs of viruses deadly to other mammals, including humans. These infections are typically nonpathogenic in bats, raising questions about host response differences that might exist between bats and other mammals. Tetherin is a restriction factor which inhibits the release of a diverse range of viruses from host cells, including retroviruses, coronaviruses, filoviruses, and paramyxoviruses, some of which are deadly to humans and transmitted by bats. Here, we characterize the tetherin genes from 27 bat species, revealing that they have evolved under strong selective pressure, and that fruit bats and vesper bats express unique structural variants of the tetherin protein. Tetherin was widely and variably expressed across fruit bat tissue types and upregulated in spleen tissue when stimulated with Toll-like receptor agonists. The expression of two computationally predicted splice isoforms of fruit bat tetherin was verified. We identified an additional third unique splice isoform which includes a C-terminal region that is not homologous to known mammalian tetherin variants but was functionally capable of restricting the release of filoviral virus-like particles. We also report that vesper bats possess and express at least five tetherin genes, including structural variants, more than any other mammal reported to date. These findings support the hypothesis of differential antiviral gene evolution in bats relative to other mammals.

IMPORTANCE Bats are an important host of various viruses which are deadly to humans and other mammals but do not cause outward signs of illness in bats. Furthering our understanding of the unique features of the immune system of bats will shed light on how they tolerate viral infections, potentially informing novel antiviral strategies in humans and other animals. This study examines the antiviral protein tetherin, which prevents viral particles from escaping their host cell. Analysis of tetherin from 27 bat species reveals that it is under strong evolutionary pressure, and we show that multiple bat species have evolved to possess more tetherin genes than other mammals, some of which encode structurally unique tetherins capable of activity against different viral particles. These data suggest that bat tetherin plays a potentially broad and important role in the management of viral infections in bats.

Distinct Molecular Mechanisms Characterizing Pathogenesis of SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has continued for over 2 years, following the outbreak of coronavirus-19 (COVID-19) in 2019. It has resulted in enormous casualties and severe economic crises. The rapid development of vaccines and therapeutics against SARS-CoV-2 has helped slow the spread. In the meantime, various mutations in the SARS-CoV-2 have emerged to evade current vaccines and therapeutics. A better understanding of SARS-CoV-2 pathogenesis is a prerequisite for developing efficient, advanced vaccines and therapeutics. Since the outbreak of COVID-19, a tremendous amount of research has been conducted to unveil SARSCoV-2 pathogenesis, from clinical observations to biochemical analysis at the molecular level upon viral infection. In this review, we discuss the molecular mechanisms of SARS-CoV-2 propagation and pathogenesis, with an update on recent advances.

Multi-functional BST2/tetherin against HIV-1, other viruses and LINE-1

Bone marrow stromal cell antigen 2 (BST2), also known as CD317, HM1.24, or tetherin, is a type II transmembrane glycoprotein. Its expression is induced by IFN-I, and it initiates host immune responses by directly trapping enveloped HIV-1 particles onto the cell surface. This antagonistic mechanism toward the virus is attributable to the unique structure of BST2. In addition to its antiviral activity, BST2 restricts retrotransposon LINE-1 through a distinct mechanism. As counteractive measures, different viruses use a variety of proteins to neutralize the function or even stability of BST2. Interestingly, BST2 seems to have both a positive and a negative influence on immunomodulation and virus propagation. Here, we review the relationship between the structural and functional bases of BST2 in anti-HIV-1 and suppressing retrotransposon LINE-1 activation and focus on its dual features in immunomodulation and regulating virus propagation.

BST2 suppresses LINE-1 retrotransposition by reducing the promoter activity of LINE-1 5'-UTR

Endogenous retrotransposons are considered the “molecular fossils” of ancient retroviral insertions. Several studies have indicated that host factors restrict both retroviruses and retrotransposons through different mechanisms. Type 1 long interspersed elements (LINE-1 or L1) are the only active retroelements that can replicate autonomously in the human genome. A recent study reported that LINE-1 retrotransposition is potently suppressed by BST2, a host restriction factor that prevents viral release mainly by physically tethering enveloped virions (such as HIV) to the surface of producer cells. However, no endoplasmic membrane structure has been associated with LINE-1 replication, suggesting that BST2 may utilize a distinct mechanism to suppress LINE-1. In this study, we showed that BST2 is a potent LINE-1 suppressor. Further investigations suggested that BST2 reduces the promoter activity of LINE-1 5′ untranslated region (UTR) and lowers the levels of LINE-1 RNA, proteins, and events during LINE-1 retrotransposition. Surprisingly, although BST2 apparently uses different mechanisms against HIV and LINE-1, two membrane-associated domains that are essential for BST2-mediated HIV tethering also proved important for BST2-induced inhibition of LINE-1 5′ UTR. Additionally, by suppressing LINE-1, BST2 prevented LINE-1-induced genomic DNA damage and innate immune activation. Taken together, our data uncovered the mechanism of BST2-mediated LINE-1 suppression and revealed new roles of BST2 as a promoter regulator, genome stabilizer, and innate immune suppressor.

IMPORTANCE BST2 is a potent antiviral protein that suppresses the release of several enveloped viruses, mainly by tethering the envelope of newly synthesized virions and restraining them on the surface of producer cells. In mammalian cells, there are numerous DNA elements replicating through reverse transcription, among which LINE-1 is the only retroelement that can replicate autonomously. Although LINE-1 retrotransposition does not involve the participation of a membrane structure, BST2 has been reported as an efficient LINE-1 suppressor, suggesting a different mechanism for BST2-mediated LINE-1 inhibition and a new function for BST2 itself. We found that BST2 specifically represses the promoter activity of LINE-1 5′ UTR, resulting in decreased levels of LINE-1 transcription, translation, and subsequent retrotransposition. Additionally, by suppressing LINE-1 activity, BST2 maintains genome stability and regulates innate immune activation. These findings expand our understanding of BST2 and its biological significance.

Zn-Induced Interactions Between SARS-CoV-2 orf7a and BST2/Tetherin

We present in this work a first X-ray Absorption Spectroscopy study of the interactions of Zn with human BST2/tetherin and SARS-CoV-2 orf7a proteins as well as with some of their complexes. The analysis of the XANES region of the measured spectra shows that Zn binds to BST2, as well as to orf7a, thus resulting in the formation of BST2-orf7a complexes. This structural information confirms the the conjecture, recently put forward by some of the present Authors, according to which the accessory orf7a (and possibly also orf8) viral protein are capable of interfering with the BST2 antiviral activity. Our explanation for this behavior is that, when BST2 gets in contact with Zn bound to the orf7a Cys15 ligand, it has the ability of displacing the metal owing to the creation of a new disulfide bridge across the two proteins. The formation of this BST2-orf7a complex destabilizes BST2 dimerization, thus impairing the antiviral activity of the latter.

Regulation of Viral Restriction by Post-Translational Modifications

Intrinsic immunity is orchestrated by a wide range of host cellular proteins called restriction factors. They have the capacity to interfere with viral replication, and most of them are tightly regulated by interferons (IFNs). In addition, their regulation through post-translational modifications (PTMs) constitutes a major mechanism to shape their action positively or negatively. Following viral infection, restriction factor modification can be decisive. Palmitoylation of IFITM3, SUMOylation of MxA, SAMHD1 and TRIM5α or glycosylation of BST2 are some of those PTMs required for their antiviral activity. Nonetheless, for their benefit and by manipulating the PTMs machinery, viruses have evolved sophisticated mechanisms to counteract restriction factors. Indeed, many viral proteins evade restriction activity by inducing their ubiquitination and subsequent degradation. Studies on PTMs and their substrates are essential for the understanding of the antiviral defense mechanisms and provide a global vision of all possible regulations of the immune response at a given time and under specific infection conditions. Our aim was to provide an overview of current knowledge regarding the role of PTMs on restriction factors with an emphasis on their impact on viral replication.

Recombinant expression, purification, and characterization of full-length human BST-2 from Escherichia coli

HIV-1 virus release from infected cells is blocked by human BST-2, but HIV-1 Vpu efficiently antagonises BST-2 due to direct transmembrane domain interactions that occur between each protein. Targeting the interaction between these two proteins is seen as viable for HIV-1 antiviral intervention. This study describes the successful over-expression and purification of a recombinant full-length human BST-2 from inclusion bodies using affinity and anion exchange chromatography. Two milligrams of purified full-length BST-2 were produced per litre of BL21 (DE3) T7 Express® pLysY E. coli culture. Far-UV circular dichroism validated the renaturing of the recombinant protein and retention of its secondary structure. Furthermore, through ELISA, a known human BST-2 binding partner, HIV-1 Vpu, was shown to bind to the renatured and purified protein, further validating its folding. To our knowledge this is the first report of the purification of a wild-type, full-length human BST-2 from Escherichia coli.

Antiviral Activity and Adaptive Evolution of Avian Tetherins

Tetherin/BST-2 is an antiviral protein that blocks the release of enveloped viral particles by linking them to the membrane of producing cells. At first, BST-2 genes were described only in humans and other mammals. Recent work identified BST-2 orthologs in nonmammalian vertebrates, including birds. Here, we identify the BST-2 sequence in domestic chicken (Gallus gallus) for the first time and demonstrate its activity against avian sarcoma and leukosis virus (ASLV). We generated a BST-2 knockout in chicken cells and showed that BST-2 is a major determinant of an interferon-induced block of ASLV release. Ectopic expression of chicken BST-2 blocks the release of ASLV in chicken cells and of human immunodeficiency virus type 1 (HIV-1) in human cells. Using metabolic labeling and pulse-chase analysis of HIV-1 Gag proteins, we verified that chicken BST-2 blocks the virus at the release stage. Furthermore, we describe BST-2 orthologs in multiple avian species from 12 avian orders. Previously, some of these species were reported to lack BST-2, highlighting the difficulty of identifying sequences of this extremely variable gene. We analyzed BST-2 genes in the avian orders Galliformes and Passeriformes and showed that they evolve under positive selection. This indicates that avian BST-2 is involved in host-virus evolutionary arms races and suggests that BST-2 antagonists exist in some avian viruses. In summary, we show that chicken BST-2 has the potential to act as a restriction factor against ASLV. Characterizing the interaction of avian BST-2 with avian viruses is important in understanding innate antiviral defenses in birds.IMPORTANCE Birds are important hosts of viruses that have the potential to cause zoonotic infections in humans. However, only a few antiviral genes (called viral restriction factors) have been described in birds, mostly because birds lack counterparts of highly studied mammalian restriction factors. Tetherin/BST-2 is a restriction factor, originally described in humans, that blocks the release of newly formed virus particles from infected cells. Recent work identified BST-2 in nonmammalian vertebrate species, including birds. Here, we report the BST-2 sequence in domestic chicken and describe its antiviral activity against a prototypical avian retrovirus, avian sarcoma and leukosis virus (ASLV). We also identify BST-2 genes in multiple avian species and show that they evolve rapidly in birds, which is an important indication of their relevance for antiviral defense. Analysis of avian BST-2 genes will shed light on defense mechanisms against avian viral pathogens.

The viral protein U (Vpu)-interacting host protein ATP6V0C down-regulates cell-surface expression of tetherin and thereby contributes to HIV-1 release

Host proteins with antiviral activity have evolved as first-line defenses to suppress viral replication. The HIV-1 accessory protein viral protein U (Vpu) enhances release of the virus from host cells by down-regulating the cell-surface expression of the host restriction factor tetherin. However, the exact mechanism of Vpu-mediated suppression of antiviral host responses is unclear. To further understand the role of host proteins in Vpu's function, here we carried out yeast two-hybrid screening and identified the V0 subunit C of vacuolar ATPase (ATP6V0C) as a Vpu-binding protein. To examine the role of ATP6V0C in Vpu-mediated tetherin degradation and HIV-1 release, we knocked down ATP6V0C expression in HeLa cells and observed that ATP6V0C depletion impairs Vpu-mediated tetherin degradation, resulting in defective HIV-1 release. We also observed that ATP6V0C overexpression stabilizes tetherin expression. This stabilization effect was specific to ATP6V0C, as overexpression of another subunit of the vacuolar ATPase, ATP6V0C″, had no effect on tetherin expression. ATP6V0C overexpression did not stabilize CD4, another target of Vpu-mediated degradation. Immunofluorescence localization experiments revealed that the ATP6V0C-stabilized tetherin is sequestered in a CD63- and lysosome-associated membrane protein 1 (LAMP1)-positive intracellular compartment. These results indicate that the Vpu-interacting protein ATP6V0C plays a role in down-regulating cell-surface expression of tetherin and thereby contributes to HIV-1 assembly and release.

Development and Characterization of the Shortest Anti-Adhesion Peptide Analogue of B49Mod1

Inhibition of cancer cell adhesion is an effective approach to killing adherent cancer cells. B49 and its analog B49Mod1 peptides, derived from the extracellular domain (ECD) of bone marrow stromal antigen 2 (BST-2), display anti-adhesion activity on breast cancer cells. However, the minimal sequence required for this anti-adhesion activity is unknown. Here, we further characterized the anti-adhesion activity of B49Mod1. We show that the anti-adhesion activity of B49Mod1 may require cysteine-linked disulfide bond and that the peptide is susceptible to proteolytic deactivation. Using structure-activity relationship studies, we identified an 18-Mer sequence (B18) as the minimal peptide sequence mediating the anti-adhesion activity of B49Mod1. Atomistic molecular dynamic (MD) simulations reveal that B18 forms a stable complex with the ECD of BST-2 in aqueous solution. MD simulations further reveal that B18 may cause membrane defects that facilitates peptide translocation across the bilayer. Placement of four B18 chains as a transmembrane bundle results in water channel formation, indicating that B18 may impair membrane integrity and form pores. We hereby identify B18 as the minimal peptide sequence required for the anti-adhesion activity of B49Mod1 and provide atomistic insight into the interaction of B18 with BST-2 and the cell membrane.

The N-glycosylation of Equine Tetherin Affects Antiviral Activity by Regulating Its Subcellular Localization

Tetherin is an interferon-inducible type II transmembrane glycoprotein which inhibits the release of viruses, including retroviruses, through a “physical tethering” model. However, the role that the glycosylation of tetherin plays in its antiviral activity remains controversial. In this study, we found that mutation of N-glycosylation sites resulted in an attenuation of the antiviral activity of equine tetherin (eqTHN), as well as a reduction in the expression of eqTHN at the plasma membrane (PM). In addition, eqTHN N-glycosylation mutants colocalize obviously with ER, CD63, LAMP1 and endosomes, while WT eqTHN do not. Furthermore, we also found that N-glycosylation impacts the transport of eqTHN in the cell not by affecting the endocytosis, but rather by influencing the anterograde trafficking of the protein. These results suggest that the N-glycosylation of eqTHN is important for the antiviral activity of the protein through regulating its normal subcellular localization. This finding will enhance our understanding of the function of this important restriction factor.

ATP1B3 cooperates with BST-2 to promote hepatitis B virus restriction

Increasing evidence indicates ATP1B3, one of the regulatory subunits of Na⁺ /K⁺ -ATPase, is involved in numerous viral propagations, such as HIV and EV71. However, the function and mechanism of ATP1B3 on hepatitis B virus (HBV) propagation is unknown. Here, we demonstrated that ATP1B3 overexpression reduced the quantity of hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg) in supernatants of HBV expression plasmids cotransfected HepG2 cells. Correspondingly, small interfering RNA and short hairpin RNA mediated ATP1B3 silencing promoted HBsAg and HBeAg expression in the supernatants of HBV expression plasmids transfected HepG2 cells. Mechanically, we reported that ATP1B3 expression could activate nuclear factor-κB (NF-κB) pathway by inducing the expression, phosphorylation, and nuclear import of P65 for the first time. And NF-κB inhibitor (Bay11) impaired the restraint of ATP1B3 on HBV replication. This counteraction effect of Bay11 proved that ATP1B3-induced NF-κB activation was crucial for HBV restriction. Accordingly, we observed that anti-HBV factors interferon-α (IFN-α) and interleukin-6 (IL-6) production were increased in HepG2 cells after the NF-κB activation. It suggested that ATP1B3 suppressed HBsAg and HBeAg by NF-κB/IFN-α and NF-κB/IL-6 axis. Further experiments proved that ATP1B3 overexpression induced anti-HBV factor BST-2 expression by NF-κB/IFN-α axis in HepG2 cells but not HEK293T cells, and ATP1B3 silencing downregulated BST-2 messenger RNA level in HepG2 cells. As an HBV restriction factor, BST-2 cooperated with ATP1B3 to antagonize HBsAg but not HBeAg in HepG2 cells. Our work identified ATP1B3 as a novel candidate of HBV restrictor with unrevealed mechanism and we highlighted it might serve as a potential therapeutic molecule for HBV infection.

Beyond Tethering the Viral Particles: Immunomodulatory Functions of Tetherin (BST-2)

Host response to viral infection is a highly regulated process involving engagement of various host factors, cytokines, chemokines, and stimulatory signals that pave the way for an antiviral immune response. The response is manifested in terms of viral sequestration, phagocytosis, and inhibition of genome replication, and, finally, if required, lymphocyte-mediated clearance of virally infected cells. During this process, cross-talk between viral and host factors can shape disease outcomes and immunopathology. Bone marrow stromal antigen 2 (BST-2), also know as tetherin, is induced by type I interferon produced in response to viral infections, as well as in certain cancers. BST-2 has been shown to be a host restriction factor of virus multiplication through its ability to physically tether budding virions and restrict viral spread. However, BST-2 has other roles in the host antiviral response. This review focuses on the diverse functions of BST-2 and its downstream signaling pathways in regulating host immune responses.

Severe acute respiratory syndrome coronavirus spike protein counteracts BST2-mediated restriction of virus-like particle release

BST2/tetherin, an interferon‐inducible antiviral factor, can block the cellular release of various enveloped viruses. We previously reported that human coronavirus 229E (HCoV‐229E) infection can alleviate the BST2 tethering of HIV‐1 virions by downregulating cell surface BST2, suggesting that coronaviruses are capable of encoding anti‐BST2 factors. Here we report our new finding that severe acute respiratory syndrome coronavirus (SARS‐CoV) spike (S) glycoprotein, similar to Vpu, is capable of antagonizing the BST2 tethering of SARS‐CoV, HCoV‐229E, and HIV‐1 virus‐like particles via BST2 downregulation. However, unlike Vpu (which downmodulates BST2 by means of proteasomal and lysosomal degradation pathways), BST2 downregulation is apparently mediated by SARS‐CoV S through the lysosomal degradation pathway only. We found that SARS‐CoV S colocalized with both BST2 and reduced cell surface BST2, suggesting an association between SARS‐CoV S and BST2 that targets the lysosomal degradation pathway. According to one recent report, SARS‐CoV ORF7a antagonizes BST2 by interfering with BST2 glycosylation¹. Our data provide support for the proposal that SARS‐CoV and other enveloped viruses are capable of evolving supplementary anti‐BST2 factors in a manner that requires virus replication. Further experiments are required to determine whether the BST2‐mediated restriction of authentic SARS‐CoV virions is alleviated by the SARS‐CoV spike protein.

BST2/tetherin inhibits the release of various enveloped viruses.

SARS‐CoV S antagonizes the BST2 tethering of human coronavirus and HIV‐1 virus‐like particles.

SARS‐CoV S colocalizes with BST2 and reduces cell surface BST2.

SARS‐CoV S downregulates BST2 through lysosomal degradation pathway.

Upregulation of BST-2 by Type I Interferons Reduces the Capacity of Vpu To Protect HIV-1-Infected Cells from NK Cell Responses

The HIV-1 accessory protein Vpu enhances viral release by counteracting the restriction factor BST-2. Furthermore, Vpu promotes NK cell evasion by downmodulating cell surface NTB-A and PVR, known ligands of the NK cell receptors NTB-A and DNAM-1, respectively. While it has been established that Vpu's transmembrane domain (TMD) is required for the interaction and intracellular sequestration of BST-2, NTB-A, and PVR, it remains unclear how Vpu manages to target these proteins simultaneously. In this study, we show that upon upregulation, BST-2 is preferentially downregulated by Vpu over its other TMD substrates. We found that type I interferon (IFN)-mediated BST-2 upregulation greatly impairs the ability of Vpu to downregulate NTB-A and PVR. Our results suggest that occupation of Vpu by BST-2 affects its ability to downregulate other TMD substrates. Accordingly, knockdown of BST-2 increases Vpu's potency to downmodulate NTB-A and PVR in the presence of type I IFN treatment. Moreover, we show that expression of human BST-2, but not that of the macaque orthologue, decreases Vpu's capacity to downregulate NTB-A. Importantly, we show that type I IFNs efficiently sensitize HIV-1-infected cells to NTB-A- and DNAM-1-mediated direct and antibody-dependent NK cell responses. Altogether, our results reveal that type I IFNs decrease Vpu's polyfunctionality, thus reducing its capacity to protect HIV-1-infected cells from NK cell responses.IMPORTANCE The restriction factor BST-2 and the NK cell ligands NTB-A and PVR are among a growing list of membrane proteins found to be downregulated by HIV-1 Vpu. BST-2 antagonism enhances viral release, while NTB-A and PVR downmodulation contributes to NK cell evasion. However, it remains unclear how Vpu can target multiple cellular factors simultaneously. Here we provide evidence that under physiological conditions, BST-2 is preferentially targeted by Vpu over NTB-A and PVR. Specifically, we show that type I IFNs decrease Vpu's polyfunctionality by upregulating BST-2, thus reducing its capacity to protect HIV-1-infected cells from NK cell responses. This indicates that there is a hierarchy of Vpu substrates upon IFN treatment, revealing that for the virus, targeting BST-2 as part of its resistance to IFN takes precedence over evading NK cell responses. This reveals a potential weakness in HIV-1's immunoevasion mechanisms that may be exploited therapeutically to harness NK cell responses against HIV-1.

Resilience of BST-2/Tetherin structure to single amino acid substitutions

Human tetherin, also known as BST-2 or CD317, is a dimeric, extracellular membrane-bound protein that consists of N and C terminal membrane anchors connected by an extracellular domain. BST-2 is involved in binding enveloped viruses, such as HIV, and inhibiting viral release in addition to a role in NF-kB signaling. Viral tethering by tetherin can be disrupted by the interaction with Vpu in HIV-1 in addition to other viral proteins. The structural mechanism of tetherin function is not clear and the effects of human tetherin mutations identified by sequencing consortiums are not known. To address this gap in the knowledge, we used data from the Ensembl database to construct and model known human missense mutations within the ectodomain to investigate how the structure of the ectodomain influences function. From the data, we identified an island of sequence stability within the ectodomain, which corresponds to a functionally and structurally important region identified in previous biochemical and biophysical studies. Most of the modeled mutations had little effect on the structure of the dimer and the coiled-coil, suggesting that the coiled-coil compensates for changes in primary structure. Thus, many of the functional defects observed in previous studies may not be due to changes in tetherin structure, but rather, due to in changes in protein-protein interactions or in aspects of tetherin not currently understood. The lack of structural effects by mutations known to decrease function further illustrates the need for more study of the structure-function connection for this system. Finally, apparent flexibility in tetherin sequence may allow for greater anti-viral activities with a larger number of viruses by reducing specific interactions with anti-tetherin proteins, while maintaining virus restriction.

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.

Membrane partitioning of peptide aggregates: coarse-grained molecular dynamics simulations

Coarse-grained molecular dynamics (CGMD) simulation technique (MARTINI force field) is applied to monitor the aggregation of helical peptides representing the transmembrane sequence and its extension of bone marrow stromal cell antigen 2 (BST-2). One of the peptides is coupled with a protein transducing domain (PTD) of nine arginine residues (R9) at its N-terminal side as well as a peptide, pep11**, which has been shown to bind to human papilloma virus 16 (HPV16) E6 oncoprotein. A short hydrophobic stretch of the transmembrane domain (TMD) of BST-2 aggregates the fastest and inserts into a lipid membrane. An aggregate of R9-pep11** attaches to the membrane via simultaneous contact of many arginine residues. Monomers from the aggregates of the shortest of the hydrophobic TMDs dissolve into the opposing leaflet when the aggregate spans the bilayer. A 'flipping' of the individual monomeric peptides is not observed.Communicated by Ramaswamy H. Sarma.

From APOBEC to ZAP: Diverse mechanisms used by cellular restriction factors to inhibit virus infections

Antiviral restriction factors are cellular proteins that inhibit the entry, replication, or spread of viruses. These proteins are critical components of the innate immune system and function to limit the severity and host range of virus infections. Here we review the current knowledge on the mechanisms of action of several restriction factors that affect multiple viruses at distinct stages of their life cycles. For example, APOBEC3G deaminates cytosines to hypermutate reverse transcribed viral DNA; IFITM3 alters membranes to inhibit virus membrane fusion; MXA/B oligomerize on viral protein complexes to inhibit virus replication; SAMHD1 decreases dNTP intracellular concentrations to prevent reverse transcription of retrovirus genomes; tetherin prevents release of budding virions from cells; Viperin catalyzes formation of a nucleoside analogue that inhibits viral RNA polymerases; and ZAP binds virus RNAs to target them for degradation. We also discuss countermeasures employed by specific viruses against these restriction factors, and mention secondary functions of several of these factors in modulating immune responses. These important examples highlight the diverse strategies cells have evolved to combat virus infections.

Post-entry restriction factors of SIVcpz

Pandemic HIV-1, a human lentivirus, is the result of zoonotic transmission of SIV from chimpanzees (SIVcpz). How SIVcpz established spread in humans after spillover is an outstanding question. Lentiviral cross-species transmissions are exceptionally rare events. Nevertheless, the chimpanzee and the gorilla were part of the transmission chains that resulted in sustained infections that evolved into HIV-1. Although many restriction factors can repress the early stages of lentiviral replication, others target replication during the late phases. In some cases, viruses incorporate host proteins that interfere with subsequent rounds of replication. Though limited and small, HIVs and SIVs, including SIVcpz can use their genome products to modulate and escape some of these barriers and thus establish a chronic infection.

Cloning, identification, and functional analysis of bone marrow stromal cell antigen-2 from sika deer (Cervus nippon)

BST-2(tetherin/CD317/HM1.24) has been identified as a cellular antiviral factor that inhibits the release of a wide range of enveloped viruses from infected cells. Orthologs of BST-2 have been identified in several species including humans, monkeys, cows, sheep, pigs, and mice. In this study, we cloned the gene and characterized the protein of the BST-2 homolog from sika deer (Cervus nippon). cnBST-2 shares 37.8% and 74.2% identity with the BST-2 homologs from Homo sapiens and Ovis aries, respectively. The extracellular domain of cnBST-2 has two putative N-linked glycosylation sites and three potential dimerization sites. cnBST-2 was shown to be expressed on the cell surface, like human BST-2. Exogenous expression of cnBST-2 resulted in potent inhibition of HIV-1 particle release in 293T cells; however, this activity resisted antagonism by HIV-1 Vpu. Moreover, cnBST-2 was not able to activate nuclear factor-κB, in contrast to human BST-2. This study is the first report of the isolation and characterization of BST-2 from C. nippon.

Strain-Specific Antagonism of the Human H1N1 Influenza A Virus against Equine Tetherin

Tetherin/BST-2/CD317 is an interferon-induced host restriction factor that can block the budding of enveloped viruses by tethering them to the cell surface. Many viruses use certain proteins to counteract restriction by tetherin from their natural hosts, but not from other species. The influenza A virus (FLUAV) has a wide range of subtypes with different host tropisms. Human tetherin (huTHN) has been reported to restrict only specific FLUAV strains and the viral hemagglutinin (HA) and neuraminidase (NA) genes determine the sensitivity to huTHN. Whether tetherins from other hosts can block human FLUAV is still unknown. Here, we evaluate the impact of equine tetherin (eqTHN) and huTHN on the replication of A/Sichuan/1/2009 (H1N1) and A/equine/Xinjiang/1/2007 (H3N8) strains. Our results show that eqTHN had higher restriction activity towards both viruses, and its shorter cytoplasmic tail contributed to that activity. We further demonstrated that HA and NA of A/Hamburg/4/2009 (H1N1) could counteract eqTHN. Notably, our results indicate that four amino acids, 13T and 49L of HA and 32T and 80V of NA, were involved in blocking the restriction activity of eqTHN. These findings reveal interspecies restriction by eqTHN towards FLUAV, and the role of the HA and NA proteins in overcoming this restriction.

B49, a BST-2-based peptide, inhibits adhesion and growth of breast cancer cells

Bone marrow stromal antigen 2 (BST-2) also known as Tetherin has been implicated in the growth and progression of many cancers. BST-2 employs its pro-tumor effects through the formation of BST-2:BST-2 dimers which ultimately promotes cell to cell and cell to matrix adhesion, cell motility, survival, and growth. The aim of this study was to evaluate the effect of a novel BST-2-based peptide-B49 on adhesion and growth of breast cancer cells. Homotypic/heterotypic adhesion, three-dimensional spheroid formation, and anchorage-independent growth were used to assess the effect of B49 on cell adhesion and growth. Additionally, we provide evidence of the anti-tumor effect of B49 in a preclinical mouse model of breast cancer. Results show that breast cancer cell adhesion to other cancer cells or components of the tumor microenvironment were inhibited by B49. Most well-known evaluation indexes of cancer cell growth, including spheroid formation, anchorage-independent, and primary tumor growth were significantly inhibited by B49. These data affirm that i) BST-2 plays a key role in mediating breast cancer cell adhesion and growth, and ii) B49 and its analog B49Mod1 significantly inhibits BST-2-mediated cancer cell adhesion and growth. Therefore, B49 and its analogs offer a promising anti-adhesion and therapeutic lead for BST-2-dependent cancers.

High-Mannose But Not Complex-Type Glycosylation of Tetherin Is Required for Restriction of HIV-1 Release

Tetherin is an interferon-inducible antiviral protein that inhibits the release of a broad spectrum of enveloped viruses by retaining virions at the surface of infected cells. While the role of specific tetherin domains in antiviral activity is clearly established, the role of glycosylation in tetherin function is not clear. In this study, we carried out a detailed investigation of this question by using tetherin variants in which one or both sites of N-linked glycosylation were mutated (N65A, N92A, and N65,92A), and chemical inhibitors that prevent glycosylation at specific stages of oligosaccharide were added or modified. The single N-linked glycosylation mutants, N65A and N92A, efficiently inhibited the release of Vpu-defective human immunodeficiency virus type 1 (HIV-1). In contrast, the non-glycosylated double mutant, N65,92A, lost its ability to block HIV-1 release. The inability of the N65,92A mutant to inhibit HIV-1 release is associated with a lack of cell-surface expression. A role for glycosylation in cell-surface tetherin expression is supported by tunicamycin treatment, which inhibits the first step of N-linked glycosylation and impairs both cell-surface expression and antiviral activity. Inhibition of complex-type glycosylation with kifunensine, an inhibitor of the oligosaccharide processing enzyme mannosidase 1, had no effect on either the cell-surface expression or antiviral activity of tetherin. These results demonstrate that high-mannose modification of a single asparagine residue is necessary and sufficient, while complex-type glycosylation is dispensable, for cell-surface tetherin expression and antiviral activity.

Structural determinant of BST-2-mediated regulation of breast cancer cell motility: a role for cytoplasmic tail tyrosine residues

There is now irrefutable evidence that overexpression of the innate immunity protein―BST-2, in breast cancer cells is implicated in tumor growth and progression. The cellular mechanisms that control BST-2-mediated effect in tumor progression involve enhancement of cancer cell motility―migration/invasion. However, the distinct structural elements of BST-2 that mediate breast cancer cell motility remain unknown. Here, we used various motility assays and different variants of BST-2 to examine the cellular and structural mechanisms controlling BST-2-mediated cell motility. We show that BST-2 silencing in various cancer cell lines inhibits cell motility. Restoration of BST-2 expression using construct expressing wild type BST-2 rescues cell motility. Mutational analysis identifies the cytoplasmic tail of BST-2 as a novel regulator of cancer cell motility, because cell motility was significantly abrogated by substitution of the BST-2 cytoplasmic tail tyrosine residues to alanine residues. Furthermore, in a spheroid invasion model, BST-2-expressing tumor spheroids are highly invasive inside 3D Matrigel matrices. In this model, the spreading distance of BST-2-expressing spheroids was significantly higher than that of BST-2-suppressed spheroids. Collectively, our data reveal that i) BST-2-expressing breast cancer cells in spheroids are more motile than their BST-2-supressed counterparts; ii) BST-2 cytoplasmic tail regulates non-proteolytic (migration) and proteolytic (invasion) mechanisms of breast cancer cell motility; and iii) replacement of the tyrosine residues at positions 6 and 8 in the cytoplasmic tail of BST-2 with alanine residues inhibits cell motility.

FRET Analysis of the Promiscuous yet Specific Interactions of the HIV-1 Vpu Transmembrane Domain

The Vpu protein of HIV-1 functions to downregulate cell surface localization of host proteins involved in the innate immune response to viral infection. For several target proteins, including the NTB-A and PVR receptors and the host restriction factor tetherin, this antagonism is carried out via direct interactions between the transmembrane domains (TMDs) of Vpu and the target. The Vpu TMD also modulates homooligomerization of this protein, and the tetherin TMD forms homodimers. The mechanism through which a single transmembrane helix is able to recognize and interact with a wide range of select targets that do not share known interaction motifs is poorly understood. Here we use Förster resonance energy transfer to characterize the energetics of homo- and heterooligomer interactions between the Vpu TMD and several target proteins. Our data show that target TMDs compete for interaction with Vpu, and that formation of each heterooligomer has a similar dissociation constant (K_d) and free energy of association to the Vpu homooligomer. This leads to a model in which Vpu monomers, Vpu homooligomers, and Vpu-target heterooligomers coexist, and suggests that the conserved binding surface of Vpu TMD has been selected for weak binding to multiple targets.

Cooperation of the Ebola Virus Proteins VP40 and GP1,2 with BST2 To Activate NF-κB Independently of Virus-Like Particle Trapping

BST2 is a host protein with dual functions in response to viral infections: it traps newly assembled enveloped virions at the plasma membrane in infected cells, and it induces NF-κB activity, especially in the context of retroviral assembly. In this study, we examined whether Ebola virus proteins affect BST2-mediated induction of NF-κB. We found that the Ebola virus matrix protein, VP40, and envelope glycoprotein, GP, each cooperate with BST2 to induce NF-κB activity, with maximal activity when all three proteins are expressed. Unlike human immunodeficiency virus type 1 Vpu protein, which antagonizes both virion entrapment and the activation of NF-κB by BST2, Ebola virus GP does not inhibit NF-κB signaling even while it antagonizes the entrapment of virus-like particles. GP from Reston ebolavirus, a nonpathogenic species in humans, showed a phenotype similar to that of GP from Zaire ebolavirus, a highly pathogenic species, in terms of both the activation of NF-κB and the antagonism of virion entrapment. Although Ebola virus VP40 and GP both activate NF-κB independently of BST2, VP40 is the more potent activator. Activation of NF-κB by the Ebola virus proteins either alone or together with BST2 requires the canonical NF-κB signaling pathway. Mechanistically, the maximal NF-κB activation by GP, VP40, and BST2 together requires the ectodomain cysteines needed for BST2 dimerization, the putative BST2 tetramerization residue L70, and Y6 of a potential hemi-ITAM motif in BST2's cytoplasmic domain. BST2 with a glycosylphosphatidylinositol (GPI) anchor signal deletion, which is not expressed at the plasma membrane and is unable to entrap virions, activated NF-κB in concert with the Ebola virus proteins at least as effectively as wild-type BST2. Signaling by the GPI anchor mutant also depended on Y6 of BST2. Overall, our data show that activation of NF-κB by BST2 is independent of virion entrapment in the case of Ebola virus. Nonetheless, BST2 may induce or amplify proinflammatory signaling during Ebola virus infection, potentially contributing to the dysregulated cytokine response that is a hallmark of Ebola virus disease.IMPORTANCE Understanding how the host responds to viral infections informs the development of therapeutics and vaccines. We asked how proinflammatory signaling by the host protein BST2/tetherin, which is mediated by the transcription factor NF-κB, responds to Ebola virus proteins. Although the Ebola virus envelope glycoprotein (GP_1,2) antagonizes the trapping of newly formed virions at the plasma membrane by BST2, we found that it does not inhibit BST2's ability to induce NF-κB activity. This distinguishes GP_1,2 from the HIV-1 protein Vpu, the prototype BST2 antagonist, which inhibits both virion entrapment and the induction of NF-κB activity. Ebola virus GP_1,2, the Ebola virus matrix protein VP40, and BST2 are at least additive with respect to the induction of NF-κB activity. The effects of these proteins converge on an intracellular signaling pathway that depends on a protein modification termed neddylation. Better mechanistic understanding of these phenomena could provide targets for therapies that modulate the inflammatory response during Ebola virus disease.

Japanese encephalitis virus counteracts BST2 restriction via its envelope protein E

It has been well documented that BST2 restricts the release of enveloped viruses by cross-linking newly produced virions to the cell membrane. However, it is less clear whether and how BST2 inhibits the release of enveloped viruses which bud via the secretory pathway. Here, we demonstrated that BST2 restricts the release of Japanese encephalitis virus (JEV) whose budding occurs at the ER-Golgi intermediate compartment, and in turn, JEV infection downregulates BST2 expression. We further found that the JEV envelope protein E, but not other viral components, significantly downregulates BST2 with the viral protein M playing an auxiliary role in the process. Envelope protein E-mediated BST2 downregulation appears to undergo lysosomal degradation pathway. Additional study revealed that the transmembrane domain and the coiled-coil domain (CC) of BST2 are the target domains of viral protein E and that the N- and C-terminal membrane anchors and the CC domain of BST2 are essential for blocking JEV release. Our results together indicate that the release of enveloped viruses whose budding take place in an intracellular compartment can be restricted by BST2.

Cysteine-linked dimerization of BST-2 confers anoikis resistance to breast cancer cells by negating proapoptotic activities to promote tumor cell survival and growth

Almost all breast tumors express the antiviral protein BST-2 with 67%, 25% and 8.2% containing high, medium or low levels of BST-2, respectively. Breast tumor cells and tissues that contain elevated levels of BST-2 are highly aggressive. Suppression of BST-2 expression reprograms tumorigenic properties of cancer cells and diminishes cancer cell aggressiveness. Using structure/function studies, we report that dimerization of BST-2 through cysteine residues located in the BST-2 extracellular domain (ECD), leads to anoikis resistance and cell survival through proteasome-mediated degradation of BIM—a key proapoptotic factor. Importantly, BST-2 dimerization promotes tumor growth in preclinical breast cancer models in vitro and in vivo. Furthermore, we demonstrate that restoration of the ECD cysteine residues is sufficient to rescue cell survival and tumor growth via a previously unreported pathway—BST-2/GRB2/ERK/BIM/Cas3. These findings suggest that disruption of BST-2 dimerization offers a potential therapeutic approach for breast cancer.

Origins and Evolution of tetherin, an Orphan Antiviral Gene

Tetherin encodes an interferon-inducible antiviral protein that traps a broad spectrum of enveloped viruses at infected cell surfaces. Despite the absence of any clearly related gene or activity, we describe possible scenarios by which tetherin arose that exemplify how protein modularity, evolvability, and robustness can create and preserve new functions. We find that tetherin genes in various organisms exhibit no sequence similarity and share only a common architecture and location in modern genomes. Moreover, tetherin is part of a cluster of three potential sister genes encoding proteins of similar architecture, some variants of which exhibit antiviral activity while others can be endowed with antiviral activity by a simple modification. Only in slowly evolving species (e.g., coelacanths) does tetherin exhibit sequence similarity to one potential sister gene. Neofunctionalization, drift, and genetic conflict appear to have driven a near complete loss of sequence similarity among modern tetherin genes and their sister genes.

The Vpu-interacting Protein SGTA Regulates Expression of a Non-glycosylated Tetherin Species

The HIV-1 accessory protein Vpu enhances virus release by counteracting the host restriction factor tetherin. To further understand the role of host cell proteins in Vpu function, we carried out yeast two-hybrid screening and identified a previously reported Vpu-interacting host factor, small glutamine-rich tetratricopeptide repeat-containing protein (SGTA). While RNAi-mediated depletion of SGTA did not significantly affect levels of tetherin or virus release efficiency, we observed that overexpression of SGTA inhibited HIV-1 release in a Vpu- and tetherin-independent manner. Overexpression of SGTA in the presence of Vpu, but not in its absence, resulted in a marked stabilization and cytosolic relocalization of a 23-kDa, non-glycosylated tetherin species. Coimmunoprecipitation studies indicated that non-glycosylated tetherin is stabilized through the formation of a ternary SGTA/Vpu/tetherin complex. This accumulation of non-glycosylated tetherin is due to inhibition of its degradation, independent of the ER-associated degradation (ERAD) pathway. Because the SGTA-stabilized tetherin species is partially localized to the cytosol, we propose that overexpression of SGTA in the presence of Vpu blocks the translocation of tetherin across the ER membrane, resulting in cytosolic accumulation of a non-glycosylated tetherin species. Although our results do not provide support for a physiological function of SGTA in HIV-1 replication, they demonstrate that SGTA overexpression regulates tetherin expression and stability, thus providing insights into the function of SGTA in ER translocation and protein degradation.

2-thio-6-azauridine inhibits Vpu mediated BST-2 degradation

Backgroud

BST-2 is an interferon-induced host restriction factor that inhibits the release of diverse mammalian enveloped viruses from infected cells by physically trapping the newly formed virions onto the host cell surface. Human Immunodeficiency Virus-1 (HIV-1) encodes an accessory protein Vpu that antagonizes BST-2 by down-regulating BST-2 from the cell surface.

Results

Using a cell-based ELISA screening system, we have discovered a lead compound, 2-thio-6-azauridine, that restores cell surface BST-2 level in the presence of Vpu. This compound has no effect on the expression of BST-2 and Vpu, but inhibits Vpu-mediated BST-2 down-regulation and exerts no effect on Vpu-induced down-regulation of CD4 or KSHV K5 protein induced BST-2 down-regulation. 2-thio-6-azauridine suppresses HIV-1 production in a BST-2-dependent manner. Further results indicate that 2-thio-6-azauridine does not interrupt the interaction of BST-2 with Vpu and β-TrCP2, but decreases BST-2 ubiquitination.

Conclusion

Our study demonstrates the feasibility of using small molecules to target Vpu function and sensitize wild type HIV-1 to BST-2-mediated host restriction.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0247-z) contains supplementary material, which is available to authorized users.

Mutation of Glycosylation Sites in BST-2 Leads to Its Accumulation at Intracellular CD63-Positive Vesicles without Affecting Its Antiviral Activity against Multivesicular Body-Targeted HIV-1 and Hepatitis B Virus

BST-2/tetherin blocks the release of various enveloped viruses including HIV-1 with a “physical tethering” model. The detailed contribution of N-linked glycosylation to this model is controversial. Here, we confirmed that mutation of glycosylation sites exerted an effect of post-translational mis-trafficking, leading to an accumulation of BST-2 at intracellular CD63-positive vesicles. BST-2 with this phenotype potently inhibited the release of multivesicular body-targeted HIV-1 and hepatitis B virus, without affecting the co-localization of BST-2 with EEA1 and LAMP1. These results suggest that N-linked glycosylation of human BST-2 is dispensable for intracellular virion retention and imply that this recently discovered intracellular tethering function may be evolutionarily distinguished from the canonical antiviral function of BST-2 by tethering nascent virions at the cell surface.

Filamin A Is Involved in HIV-1 Vpu-mediated Evasion of Host Restriction by Modulating Tetherin Expression

Tetherin, also known as bone marrow stromal antigen 2 (BST-2), inhibits the release of a wide range of enveloped viruses, including human immunodeficiency virus, type 1 (HIV-1) by directly tethering nascent virions to the surface of infected cells. The HIV-1 accessary protein Vpu counteracts tetherin restriction via sequestration, down-regulation, and/or displacement mechanisms to remove tetherin from sites of virus budding. However, the exact mechanism of Vpu-mediated antagonism of tetherin restriction remains to be fully understood. Here we report a novel role for the actin cross-linking regulator filamin A (FLNa) in Vpu anti-tetherin activities. We demonstrate that FLNa associates with tetherin and that FLNa modulates tetherin turnover. FLNa deficiency was found to enhance cell surface and steady-state levels of tetherin expression. In contrast, we observed that overexpression of FLNa reduced tetherin expression levels both on the plasma membrane and in intracellular compartments. Although FLNb shows high amino acid sequence similarity with FLNa, we reveal that only FLNa, but not FLNb, plays an essential role in tetherin turnover. We further showed that FLNa deficiency inhibited Vpu-mediated enhancement of virus release through interfering with the activity of Vpu to down-regulate cellular tetherin. Taken together, our studies suggest that Vpu hijacks the FLNa function in the modulation of tetherin to neutralize the antiviral factor tetherin. These findings may provide novel strategies for the treatment of HIV-1 infection.

The role of BST-2/Tetherin in host protection and disease manifestation

Host cells respond to viral infections by activating immune response genes that are not only involved in inflammation, but may also predispose cells to cancerous transformation. One such gene is BST‐2, a type II transmembrane protein with a unique topology that endows it tethering and signaling potential. Through this ability to tether and signal, BST‐2 regulates host response to viral infection either by inhibiting release of nascent viral particles or in some models inhibiting viral dissemination. However, despite its antiviral functions, BST‐2 is involved in disease manifestation, a function linked to the ability of BST‐2 to promote cell‐to‐cell interaction. Therefore, modulating BST‐2 expression and/or activity has the potential to influence course of disease.

Three-Dimensional Structural Characterization of HIV-1 Tethered to Human Cells

Tetherin (BST2, CD317, or HM1.24) is a host cellular restriction factor that prevents the release of enveloped viruses by mechanically linking virions to the plasma membrane. The precise arrangement of tetherin molecules at the plasma membrane site of HIV-1 assembly, budding, and restriction is not well understood. To gain insight into the biophysical mechanism underlying tetherin-mediated restriction of HIV-1, we utilized cryo-electron tomography (cryo-ET) to directly visualize HIV-1 virus-like particles (VLPs) and virions tethered to human cells in three dimensions (3D). Rod-like densities that we refer to as tethers were seen connecting HIV-1 virions to each other and to the plasma membrane. Native immunogold labeling showed tetherin molecules located on HIV-1 VLPs and virions in positions similar to those of the densities observed by cryo-ET. The location of the tethers with respect to the ordered immature Gag lattice or mature conical core was random. However, tethers were not uniformly distributed on the viral membrane but rather formed clusters at sites of contact with the cell or other virions. Chains of tethered HIV-1 virions often were arranged in a linear fashion, primarily as single chains and, to a lesser degree, as branched chains. Distance measurements support the extended tetherin model, in which the coiled-coil ectodomains are oriented perpendicular with respect to the viral and plasma membranes.

IMPORTANCE

Tetherin is a cellular factor that restricts HIV-1 release by directly cross-linking the virus to the host cell plasma membrane. We used cryo-electron tomography to visualize HIV-1 tethered to human cells in 3D. We determined that tetherin-restricted HIV-1 virions were physically connected to each other or to the plasma membrane by filamentous tethers that resembled rods ∼15 nm in length, which is consistent with the extended tetherin model. In addition, we found the position of the tethers to be arbitrary relative to the ordered immature Gag lattice or the mature conical cores. However, when present as multiple copies, the tethers clustered at the interface between virions. Tethered HIV-1 virions were arranged in a linear fashion, with the majority as single chains. This study advances our understanding of tetherin-mediated HIV-1 restriction by defining the spatial arrangement and orientation of tetherin molecules at sites of HIV-1 restriction.

Increased BST2 expression during simian immunodeficiency virus infection is not a determinant of disease progression in rhesus monkeys

Background

Bone marrow stromal cell antigen 2 (BST2), also known as tetherin, HM1.24 or CD317 represents a type 2 integral membrane protein, which has been described to restrict the production of some enveloped viruses by inhibiting the virus release from the cell surface. This innate antiviral mechanism is counteracted by the HIV-1 viral factor Vpu, targeting BST2 for cellular degradation. Since antiviral BST2 activity has been mainly confirmed by in vitro data, we investigated its role in vivo on the disease progression using the SIV/macaque model for AIDS. We determined BST2 expression in PBMC and leukocyte subsets of uninfected and SIV-infected rhesus macaques by real-time PCR and flow cytometry and correlated it with disease progression and viral load.

Results

Compared to pre-infection levels, we found increased BST2 expression in PBMC, purified CD4⁺ lymphocytes and CD14⁺ monocytes of SIV-infected animals, which correlated with viral load. Highest BST2 levels were found in progressors and lowest levels comparable to uninfected macaques were observed in long-term non-progressors (LTNPs). During acute viremia, BST2 mRNA increased in parallel with MX1, a prototype interferon-stimulated gene. This association was maintained during the whole disease course.

Conclusion

The detected relationship between BST2 expression and viral load as well as with MX1 indicate a common regulation by the interferon response and suggest rather limited influence of BST2 in vivo on the disease outcome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0219-8) contains supplementary material, which is available to authorized users.

Severe Acute Respiratory Syndrome Coronavirus ORF7a Inhibits Bone Marrow Stromal Antigen 2 Virion Tethering through a Novel Mechanism of Glycosylation Interference

Severe acute respiratory syndrome (SARS) emerged in November 2002 as a case of atypical pneumonia in China, and the causative agent of SARS was identified to be a novel coronavirus, severe acute respiratory syndrome coronavirus (SARS-CoV). Bone marrow stromal antigen 2 (BST-2; also known as CD317 or tetherin) was initially identified to be a pre-B-cell growth promoter, but it also inhibits the release of virions of the retrovirus human immunodeficiency virus type 1 (HIV-1) by tethering budding virions to the host cell membrane. Further work has shown that BST-2 restricts the release of many other viruses, including the human coronavirus 229E (hCoV-229E), and the genomes of many of these viruses encode BST-2 antagonists to overcome BST-2 restriction. Given the previous studies on BST-2, we aimed to determine if BST-2 has the ability to restrict SARS-CoV and if the SARS-CoV genome encodes any proteins that modulate BST-2's antiviral function. Through an in vitro screen, we identified four potential BST-2 modulators encoded by the SARS-CoV genome: the papain-like protease (PLPro), nonstructural protein 1 (nsp1), ORF6, and ORF7a. As the function of ORF7a in SARS-CoV replication was previously unknown, we focused our study on ORF7a. We found that BST-2 does restrict SARS-CoV, but the loss of ORF7a leads to a much greater restriction, confirming the role of ORF7a as an inhibitor of BST-2. We further characterized the mechanism of BST-2 inhibition by ORF7a and found that ORF7a localization changes when BST-2 is overexpressed and ORF7a binds directly to BST-2. Finally, we also show that SARS-CoV ORF7a blocks the restriction activity of BST-2 by blocking the glycosylation of BST-2.

IMPORTANCE

The severe acute respiratory syndrome coronavirus (SARS-CoV) emerged from zoonotic sources in 2002 and caused over 8,000 infections and 800 deaths in 37 countries around the world. Identifying host factors that regulate SARS-CoV pathogenesis is critical to understanding how this lethal virus causes disease. We have found that BST-2 is capable of restricting SARS-CoV release from cells; however, we also identified a SARS-CoV protein that inhibits BST-2 function. We show that the SARS-CoV protein ORF7a inhibits BST-2 glycosylation, leading to a loss of BST-2's antiviral function.

Determinants in HIV-2 Env and tetherin required for functional interaction

BACKGROUND

The interferon-inducible factor BST-2/tetherin blocks the release of nascent virions from the surface of infected cells for certain enveloped virus families. The primate lentiviruses have evolved several counteracting mechanisms which, in the case of HIV-2, is a function of its Env protein. We sought to further understand the features of the Env protein and tetherin that are important for this interaction, and to evaluate the selective pressure on HIV-2 to maintain such an activity.

RESULTS

By examining Env mutants with changes in the ectodomain of the protein (virus ROD14) or the cytoplasmic tail (substitution Y707A) that render the proteins unable to counteract tetherin, we determined that an interaction between Env and tetherin is important for this activity. Furthermore, this Env-tetherin interaction required an alanine face in the tetherin ectodomain, although insertion of this domain into an artificial tetherin-like protein was not sufficient to confer sensitivity to the HIV-2 Env. The replication of virus carrying the ROD14 substitutions was significantly slower than the matched wild-type virus, but it acquired second-site mutations during passaging in the cytoplasmic tail of Env which restored the ability of the protein to both bind to and counteract tetherin.

CONCLUSIONS

These results shed light on the interaction between HIV-2 and tetherin, suggesting a physical interaction that maps to the ectodomains of both proteins and indicating a strong selection pressure to maintain an anti-tetherin activity in the HIV-2 Env.