Human parainfluenza virus type 2 V protein inhibits and antagonizes tetherin

Hazara orthonairovirus nucleoprotein facilitates viral cell-to-cell spread by modulating tight junction protein, claudin-1

Background

Tight junctions act as a barrier that prevents invasion of pathogens through epithelial cells. This study aims to elucidate the correlation between tight junctions and nairoviruses using Hazara orthonairovirus (HAZV) as a surrogate model for Crimean-Congo hemorrhagic fever virus.

Methods

mRNA, total protein, and cell surface protein levels of tight junction proteins were examined by quantitative real-time reverse transcription polymerase chain reaction, immunoblot and flow cytometry, respectively. HAZV growth was measured by plaque assay. Immunofluorescence assay was used to examine viral cell-to-cell spread. The interaction between HAZV nucleoprotein and claudin-1 was analyzed by immunoprecipitation.

Results

HAZV infection induced mRNA of several tight junction proteins, especially claudin-1. HAZV infection also induced cell surface expression of claudin-1 protein. Claudin-1 overexpression inhibited the growth of HAZV by blocking its cell-to-cell spread. In contrast, HAZV nucleoprotein completely inhibited HAZV-induced cell surface expression of claudin-1, and this inhibition required interaction between HAZV nucleoprotein and claudin-1.

Conclusion

HAZV nucleoprotein was shown to bind to claudin-1 to negatively regulate its cell surface expression, and so can promote cell-to-cell spread of HAZV. This is the first presentation of a possible mechanism behind how nairoviruses counteract tight junction barrier function.

Evasion of Host Antiviral Innate Immunity by Paramyxovirus Accessory Proteins

For efficient replication, viruses have developed multiple strategies to evade host antiviral innate immunity. Paramyxoviruses are a large family of enveloped RNA viruses that comprises diverse human and animal pathogens which jeopardize global public health and the economy. The accessory proteins expressed from the P gene by RNA editing or overlapping open reading frames (ORFs) are major viral immune evasion factors antagonizing type I interferon (IFN-I) production and other antiviral innate immune responses. However, the antagonistic mechanisms against antiviral innate immunity by accessory proteins differ among viruses. Here, we summarize the current understandings of immune evasion mechanisms by paramyxovirus accessory proteins, specifically how accessory proteins directly or indirectly target the adaptors in the antiviral innate immune signaling pathway to facilitate virus replication. Additionally, some cellular responses, which are also involved in viral replication, will be briefly summarized.

Human Parainfluenza Virus Type 2 V Protein Modulates Iron Homeostasis

Intracellular iron concentration is tightly controlled for cell viability. It is known to affect the growth of several viruses, but the molecular mechanisms are not well understood. We found that iron chelators inhibit growth of human parainfluenza virus type 2 (hPIV-2). Furthermore, infection with hPIV-2 alters ferritin localization from granules to a homogenous distribution within cytoplasm of iron-stimulated cells. The V protein of hPIV-2 interacts with ferritin heavy chain 1 (FTH1), a ferritin subunit. It also binds to nuclear receptor coactivator 4 (NCOA4), which mediates autophagic degradation of ferritin, so-called ferritinophagy. V protein consequently interferes with interaction between FTH1 and NCOA4. hPIV-2 growth is inhibited in FTH1 knockdown cell line where severe hPIV-2-induced apoptosis is shown. In contrast, NCOA4 knockdown results in the promotion of hPIV-2 growth and limited apoptosis. Our data collectively suggest that hPIV-2 V protein inhibits FTH1-NCOA4 interaction and subsequent ferritinophagy. This iron homeostasis modulation allows infected cells to avoid apoptotic cell death, resulting in effective growth of hPIV-2.IMPORTANCE hPIV-2 V protein interferes with interaction between FTH1 and NCOA4 and inhibits NCOA4-mediated ferritin degradation, leading to the inhibition of iron release to the cytoplasm. This iron homeostasis modulation allows infected cells to avoid apoptotic cell death, resulting in effective growth of hPIV-2.

Inhibition of Cavin3 Degradation by the Human Parainfluenza Virus Type 2 V Protein Is Important for Efficient Viral Growth

Cavin proteins have important roles in the formation of caveolae in lipid raft microdomains. Pulse-chase experiments of cells infected with human parainfluenza virus type 2 (hPIV-2) showed decreased proteasomal degradation of Cavin3. Overexpression of hPIV-2 V protein alone was sufficient to inhibit Cavin3 degradation. Immunoprecipitation analysis revealed that V protein bound to Cavin3. Trp residues within C-terminal region of V protein, as well as the N-terminal region of Cavin3, are important for V–Cavin3 interaction. Cavin3 knockdown suppressed hPIV-2 growth without affecting its entry, replication, transcription, or translation. Higher amounts of Cavin3 were observed in V protein-overexpressing cells than in control cells in lipid raft microdomains. Our data collectively suggest that hPIV-2 V protein binds to and stabilizes Cavin3, which in turn facilitates assembly and budding of hPIV-2 in lipid raft microdomains.

BST2/Tetherin Overexpression Modulates Morbillivirus Glycoprotein Production to Inhibit Cell-Cell Fusion

The measles virus (MeV), a member of the genus Morbillivirus, is an established pathogen of humans. A key feature of morbilliviruses is their ability to spread by virus-cell and cell-cell fusion. The latter process, which leads to syncytia formation in vitro and in vivo, is driven by the viral fusion (F) and haemagglutinin (H) glycoproteins. In this study, we demonstrate that MeV glycoproteins are sensitive to inhibition by bone marrow stromal antigen 2 (BST2/Tetherin/CD317) proteins. BST2 overexpression causes a large reduction in MeV syncytia expansion. Using quantitative cell-cell fusion assays, immunolabeling, and biochemistry we further demonstrate that ectopically expressed BST2 directly inhibits MeV cell-cell fusion. This restriction is mediated by the targeting of the MeV H glycoprotein, but not other MeV proteins. Using truncation mutants, we further establish that the C-terminal glycosyl-phosphatidylinositol (GPI) anchor of BST2 is required for the restriction of MeV replication in vitro and cell-cell fusion. By extending our study to the ruminant morbillivirus peste des petits ruminants virus (PPRV) and its natural host, sheep, we also confirm this is a broad and cross-species specific phenotype.

Profilin2 is required for filamentous actin formation induced by human parainfluenza virus type 2

We previously reported that human parainfluenza virus type 2 (hPIV-2) promoted RhoA activation and subsequent filamentous actin (F-actin) formation. Actin-binding proteins, such as profilin and cofilin, are involved in the regulation of F-actin formation by RhoA signaling. In the present study, we identified profilin2 as a key molecule that is involved in hPIV-2-induced F-actin formation. Immunoprecipitation assays demonstrated that hPIV-2 V protein binds to profilin2 but not to profilin1. Mutation of Trp residues within C-terminal region of V protein abolished the binding capacity to profilin2. Depletion of profilin2 resulted in the inhibition of hPIV-2-induced F-actin formation and the suppression of hPIV-2 growth. Overexpression of wild type V but not Trp-mutated V protein reduced the quantity of actin co-immunoprecipitated with profilin2. Taken together, these results suggest that hPIV-2 V protein promotes F-actin formation by affecting actin-profilin2 interaction through its binding to profilin2.

CRISPR-mediated activation of endogenous BST-2/tetherin expression inhibits wild-type HIV-1 production

The CRISPR technology not only can knock out target genes by using the RNA-guided Cas9 nuclease but also can activate their expression when a nuclease-deficient Cas9 (dCas9) is employed. Using the latter function, we here show the effect of the CRISPR-mediated pinpoint activation of endogenous expression of BST-2 (also known as tetherin), a virus restriction factor with a broad antiviral spectrum. Single-guide RNA (sgRNA) sequences targeting the BST-2 promoter were selected by promoter assays. Potential sgRNAs and dCas9 fused to the VP64 transactivation domain, along with an accessory transcriptional activator complex, were introduced into cells by lentiviral transduction. Increased expression of BST-2 mRNA in transduced cells was confirmed by real-time RT-PCR. Cells in which BST-2 expression was highly enhanced showed the effective inhibition of HIV-1 production and replication even in the presence of the viral antagonist Vpu against BST-2. These findings confirm that the physiological stoichiometry between host restriction factors and viral antagonists may determine the outcome of the battle with viruses.

Nucleocytoplasmic shuttling of the human parainfluenza virus type 2 phosphoprotein

Human parainfluenza virus type 2 phosphoprotein (P) is an essential component of viral polymerase. The P gene encodes both P and accessory V proteins by a specific gene editing mechanism. Therefore, the N-terminal 164 amino acids of P protein are common to V protein. Interestingly, while P protein is located in the cytoplasm, V protein is found mainly in the nucleus. Using deletion mutants, we show the presence of a nuclear localization signal (NLS) in the P/V common domain, and a nuclear export signal (NES) in the C-terminal P specific region. The NLS region makes a complex with importin α5 or 7. In the presence of leptomycin B, P protein is retained in the nucleus, indicating that it contains a CRM1-dependent NES. We identified the NLS (⁶⁵PVKPRRKK⁷²) and the NES (²²⁵IIELLKGLDL²³⁴) using β-galactosidase fusion proteins. Moreover, nucleocytoplasmic shuttling of P protein appears to be important for efficient viral polymerase activity.

Limiting Respiratory Viral Infection by Targeting Antiviral and Immunological Functions of BST-2/Tetherin: Knowledge and Gaps

Recent findings regarding the cellular biology and immunology of BST-2 (also known as tetherin) indicate that its function could be exploited as a universal replication inhibitor of enveloped respiratory viruses (e.g., influenza, respiratory syncytial virus, etc.). BST-2 inhibits viral replication by preventing virus budding from the plasma membrane and by inducing an antiviral state in cells adjacent to infection via unique inflammatory signaling mechanisms. This review presents the first comprehensive summary of what is currently known about BST-2 anti-viral function against respiratory viruses, how these viruses construct countermeasures to antagonize BST-2, and how BST-2 function might be targeted to develop therapies to treat respiratory virus infections. The authors address the current gaps in knowledge, including the need for mechanistic understanding of BST-2 antagonism by respiratory viruses, that should be bridged to achieve that goal.

Tetherin antagonism by V proteins is a common trait among the genus Rubulavirus

Tetherin (BST-2/CD317/HM1.24) is an anti-viral factor that restricts the budding of several enveloped viruses. Most of these viruses have evolved to encode tetherin antagonists. Our previous study demonstrated that the growth of human parainfluenza virus type 2 (hPIV-2), a member of the genus Rubulavirus in the family Paramyxoviridae, was inhibited by tetherin, and its V protein decreases the amount of cell surface tetherin by the interaction. In the present study, we investigated whether tetherin inhibits the growth of other rubulaviruses including PIV-5, mumps virus (MuV), simian virus 41, and hPIV-4, and whether their V proteins act as tetherin antagonists. Plaque assay demonstrated that the growth of PIV-5 and MuV was inhibited by tetherin. Flow cytometry and immunoblot analyses revealed that the infection of PIV-5 and MuV caused reduction of cell surface tetherin without affecting total amount of tetherin. Immunoprecipitation analysis showed that all V proteins of rubulaviruses tested bound to tetherin. These results suggest that tetherin antagonism by V proteins is common among the genus Rubulavirus.

Human parainfluenza virus type 2 V protein inhibits induction of tetherin

Tetherin is an anti-viral factor that restricts viral budding through tethering virions to the cell surface. The human parainfluenza virus type 2 (hPIV-2) V protein decreases cell surface tetherin in HeLa cells, which constitutively express endogenous tetherin. However, the role of the hPIV-2 V protein in tetherin induction remains unclear. Here, we examined whether hPIV-2 infection itself induces tetherin in HEK293 cells that have no basal expression of tetherin. Unlike influenza A virus (IAV) infection, hPIV-2 infection induced neither tetherin mRNA nor protein expression. In contrast, robust tetherin induction was observed by infection of rPIV-2s carrying V mutants, in which either three Trp residues (W178H/W182E/W192A) or Cys residues (C209/211/214A) that are important for decreasing cell surface tetherin are mutated. hPIV-2 infection also inhibited the induction of tetherin expression by IFN-α and IAV infection. Furthermore, hPIV-2 V protein but not P and V_{W178H/W182E/W192A} suppressed tetherin induction. Our data collectively suggest that the hPIV-2 V protein inhibits tetherin expression induced by several external stimuli.

Graf1 Controls the Growth of Human Parainfluenza Virus Type 2 through Inactivation of RhoA Signaling

Rho GTPases are involved in a variety of cellular activities and are regulated by guanine nucleotide exchange factors and GTPase-activating proteins (GAPs). We found that the activation of Rho GTPases by lysophosphatidic acid promotes the growth of human parainfluenza virus type 2 (hPIV-2). Furthermore, hPIV-2 infection causes activation of RhoA, a Rho GTPase. We hypothesized that Graf1 (also known as ARHGAP26), a GAP, regulates hPIV-2 growth by controlling RhoA signaling. Immunofluorescence analysis showed that hPIV-2 infection altered Graf1 localization from a homogenous distribution within the cytoplasm to granules. Graf1 colocalized with hPIV-2 P, NP, and L proteins. Graf1 interacts with P and V proteins via their N-terminal common region, and the C-terminal Src homology 3 domain-containing region of Graf1 is important for these interactions. In HEK293 cells constitutively expressing Graf1, hPIV-2 growth was inhibited, and RhoA activation was not observed during hPIV-2 infection. In contrast, Graf1 knockdown restored hPIV-2 growth and RhoA activation. Overexpression of hPIV-2 P and V proteins enhanced hPIV-2-induced RhoA activation. These results collectively suggested that hPIV-2 P and V proteins enhanced hPIV-2 growth by binding to Graf1 and that Graf1 inhibits hPIV-2 growth through RhoA inactivation.

IMPORTANCE

Robust growth of hPIV-2 requires Rho activation. hPIV-2 infection causes RhoA activation, which is suppressed by Graf1. Graf1 colocalizes with viral RNP (vRNP) in hPIV-2-infected cells. We found that Graf1 interacts with hPIV-2 P and V proteins. We also identified regions in these proteins which are important for this interaction. hPIV-2 P and V proteins enhanced the hPIV-2 growth via binding to Graf1, while Graf1 inhibited hPIV-2 growth through RhoA inactivation.

Monoclonal Antibody to Bone Marrow Stromal Cell Antigen 2 Protein of Swine

The bone marrow stromal cell antigen 2 (BST-2) protein was identified as a novel virus restriction factor that potently restricts the replication and egress of enveloped viruses. In this study, we generated monoclonal antibodies (MAbs) against porcine BST-2 encoding 34-112 aa of porcine BST-2, which was cloned and inserted into the prokaryotic expression vector pCold-I to construct a recombinant plasmid pCold-pBST-2. The recombinant porcine BST-2 protein (rpBST-2 protein) was induced by isopropyl-β-D-thiogalactoside in Escherichia coli BL21 (DE3). Then, BALB/c mice were immunized with the purified rpBST-2 protein to prepare MAbs of BST-2. After subcloning, one strain of hybridoma cells named 1B2 secreting porcine BST-2 protein monoclonal antibody (MAb) was obtained. Indirect immunofluorescence assay and western blot analysis showed that the MAb was specifically reacted with the overexpressed porcine BST-2 protein in Vero cells. The specific MAb of porcine BST-2 provides a valuable tool for further studies of BST-2 to restrict virus infection.