N-Linked glycosylation is required for XC cell-specific syncytium formation by the R peptide-containing envelope protein of ecotropic murine leukemia viruses

IDO1, FAT10, IFI6, and GILT Are Involved in the Antiretroviral Activity of γ-Interferon and IDO1 Restricts Retrovirus Infection by Autophagy Enhancement

Gamma-interferon (γ-IFN) significantly inhibits infection by replication-defective viral vectors derived from the human immunodeficiency virus type 1 (HIV-1) or murine leukemia virus (MLV) but the underlying mechanism remains unclear. Previously we reported that knockdown of γ-IFN-inducible lysosomal thiolreductase (GILT) abrogates the antiviral activity of γ-IFN in TE671 cells but not in HeLa cells, suggesting that other γ-IFN-inducible host factors are involved in its antiviral activity in HeLa cells. We identified cellular factors, the expression of which are induced by γ-IFN in HeLa cells, using a microarray, and analyzed the effects of 11 γ-IFN-induced factors on retroviral vector infection. Our results showed that the exogenous expression of FAT10, IFI6, or IDO1 significantly inhibits both HIV-1- and MLV-based vector infections. The antiviral activity of γ-IFN was decreased in HeLa cells, in which the function of IDO1, IFI6, FAT10, and GILT were simultaneously inhibited. IDO1 is an enzyme that metabolizes an essential amino acid, tryptophan. However, IDO1 did not restrict retroviral vector infection in Atg3-silencing HeLa cells, in which autophagy did not occur. This study found that IDO1, IFI6, FAT10, and GILT are involved in the antiviral activity of γ-IFN, and IDO1 inhibits retroviral infection by inducing autophagy.

Cytoplasmic R-peptide of murine leukemia virus envelope protein negatively regulates its interaction with the cell surface receptor

Cytoplasmic tails of envelope (Env) glycoproteins of many retroviruses inhibit their membrane fusion activity. The cytoplasmic 16-amino acid peptide of ecotropic murine leukemia virus (E-MLV) Env protein, called the R-peptide, also inhibits the membrane fusion activity of the Env protein. However, the molecular mechanism of the inhibition has not been elucidated yet. In this study, we found that R-peptide-containing Env protein of E-MLV binds to the cell surface receptor cationic amino acid transporter-1 (CAT-1) with weaker affinity than R-peptide-truncated Env protein. Consistent with this result, R-peptide-containing Env protein had less efficient inhibition of E-MLV vector infection than R-peptide-truncated Env protein. R-peptide truncation has been reported to induce conformational change in the surface subunit of E-MLV Env protein that interacts with the receptor. Taken together, our findings indicate that R-peptide truncation induces conformational change in the receptor-binding domain of the E-MLV Env protein and facilitates the Env-receptor interaction.

Role of Ezrin Phosphorylation in HIV-1 Replication

Host-cell expression of the ezrin protein is required for CXCR4 (X4)-tropic HIV-1 infection. Ezrin function is regulated by phosphorylation at threonine-567. This study investigates the role of ezrin phosphorylation in HIV-1 infection and virion release. We analyzed the effects of ezrin mutations involving substitution of threonine-567 by alanine (EZ-TA), a constitutively inactive mutant, or by aspartic acid (EZ-TD), which mimics phosphorylated threonine. We also investigated the effects of ezrin silencing on HIV-1 virion release using a specific siRNA. We observed that X4-tropic HIV-1 vector infection was inhibited by expression of the EZ-TA mutant but increased by expression of the EZ-TD mutant, suggesting that ezrin phosphorylation in target cells is required for efficient HIV-1 entry. Expression of a dominant-negative mutant of ezrin (EZ-N) and ezrin silencing in HIV-1 vector-producing cells significantly reduced the infectivity of released virions without affecting virion production. This result indicates that endogenous ezrin expression is required for virion infectivity. The EZ-TD but not the EZ-TA inhibited virion release from HIV-1 vector-producing cells. Taken together, these findings suggest that ezrin phosphorylation in target cells is required for efficient HIV-1 entry but inhibits virion release from HIV-1 vector-producing cells.

Susceptibility of muridae cell lines to ecotropic murine leukemia virus and the cationic amino acid transporter 1 viral receptor sequences: implications for evolution of the viral receptor

Ecotropic murine leukemia viruses (Eco-MLVs) infect mouse and rat, but not other mammalian cells, and gain access for infection through binding the cationic amino acid transporter 1 (CAT1). Glycosylation of the rat and hamster CAT1s inhibits Eco-MLV infection, and treatment of rat and hamster cells with a glycosylation inhibitor, tunicamycin, enhances Eco-MLV infection. Although the mouse CAT1 is also glycosylated, it does not inhibit Eco-MLV infection. Comparison of amino acid sequences between the rat and mouse CAT1s shows amino acid insertions in the rat protein near the Eco-MLV-binding motif. In addition to the insertion present in the rat CAT1, the hamster CAT1 has additional amino acid insertions. In contrast, tunicamycin treatment of mink and human cells does not elevate the infection, because their CAT1s do not have the Eco-MLV-binding motif. To define the evolutionary pathway of the Eco-MLV receptor, we analyzed CAT1 sequences and susceptibility to Eco-MLV infection of other several murinae animals, including the southern vole (Microtus rossiaemeridionalis), large Japanese field mouse (Apodemus speciosus), and Eurasian harvest mouse (Micromys minutus). Eco-MLV infection was enhanced by tunicamycin in these cells, and their CAT1 sequences have the insertions like the hamster CAT1. Phylogenetic analysis of mammalian CAT1s suggested that the ancestral CAT1 does not have the Eco-MLV-binding motif, like the human CAT1, and the mouse CAT1 is thought to be generated by the amino acid deletions in the third extracellular loop of CAT1.

Retrovirus entry by endocytosis and cathepsin proteases

Retroviruses include infectious agents inducing severe diseases in humans and animals. In addition, retroviruses are widely used as tools to transfer genes of interest to target cells. Understanding the entry mechanism of retroviruses contributes to developments of novel therapeutic approaches against retrovirus-induced diseases and efficient exploitation of retroviral vectors. Entry of enveloped viruses into host cell cytoplasm is achieved by fusion between the viral envelope and host cell membranes at either the cell surface or intracellular vesicles. Many animal retroviruses enter host cells through endosomes and require endosome acidification. Ecotropic murine leukemia virus entry requires cathepsin proteases activated by the endosome acidification. CD4-dependent human immunodeficiency virus (HIV) infection is thought to occur via endosomes, but endosome acidification is not necessary for the entry whereas entry of CD4-independent HIVs, which are thought to be prototypes of CD4-dependent viruses, is low pH dependent. There are several controversial results on the retroviral entry pathways. Because endocytosis and endosome acidification are complicatedly controlled by cellular mechanisms, the retrovirus entry pathways may be different in different cell lines.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Infection of XC cells by MLVs and Ebola virus is endosome-dependent but acidification-independent

Inhibitors of endosome acidification or cathepsin proteases attenuated infections mediated by envelope proteins of xenotropic murine leukemia virus-related virus (XMRV) and Ebola virus, as well as ecotropic, amphotropic, polytropic, and xenotropic murine leukemia viruses (MLVs), indicating that infections by these viruses occur through acidic endosomes and require cathepsin proteases in the susceptible cells such as TE671 cells. However, as previously shown, the endosome acidification inhibitors did not inhibit these viral infections in XC cells. It is generally accepted that the ecotropic MLV infection in XC cells occurs at the plasma membrane. Because cathepsin proteases are activated by low pH in acidic endosomes, the acidification inhibitors may inhibit the viral infections by suppressing cathepsin protease activation. The acidification inhibitors attenuated the activities of cathepsin proteases B and L in TE671 cells, but not in XC cells. Processing of cathepsin protease L was suppressed by the acidification inhibitor in NIH3T3 cells, but again not in XC cells. These results indicate that cathepsin proteases are activated without endosome acidification in XC cells. Treatment with an endocytosis inhibitor or knockdown of dynamin 2 expression by siRNAs suppressed MLV infections in all examined cells including XC cells. Furthermore, endosomal cathepsin proteases were required for these viral infections in XC cells as other susceptible cells. These results suggest that infections of XC cells by the MLVs and Ebola virus occur through endosomes and pH-independent cathepsin activation induces pH-independent infection in XC cells.

Cathepsin L is required for ecotropic murine leukemia virus infection in NIH3T3 cells

Recently it has been reported that a cathepsin B inhibitor, CA-074Me, attenuates ecotropic murine leukemia virus (Eco-MLV) infection in NIH3T3 cells, suggesting that cathepsin B is required for the Eco-MLV infection. However, cathepsin B activity was negative or extremely low in NIH3T3 cells. How did CA-074Me attenuate the Eco-MLV infection? The CA-074Me treatment of NIH3T3 cells inhibited cathepsin L activity, and a cathepsin L specific inhibitor, CLIK148, attenuated the Eco-MLV vector infection. These results indicate that the suppression of cathepsin L activity by CA-074Me induces the inhibition of Eco-MLV infection, suggesting that cathepsin L is required for the Eco-MLV infection in NIH3T3 cells. The CA-074Me treatment inhibited the Eco-MLV infection in human cells expressing the exogenous mouse ecotropic receptor and endogenous cathepsins B and L, but the CLIK148 treatment did not, showing that only the cathepsin L suppression by CLIK148 is not enough to prevent the Eco-MLV infection in cells expressing both of cathepsins B and L, and CA-074Me inhibits the Eco-MLV infection by suppressing both of cathepsins B and L. These results suggest that either cathepsin B or L is sufficient for the Eco-MLV infection.

Raft localization of CXCR4 is primarily required for X4-tropic human immunodeficiency virus type 1 infection

Human immunodeficiency virus type 1 (HIV-1) infection is initiated by successive interactions of viral envelope glycoprotein gp120 with two cellular surface proteins, CD4 and chemokine receptor. The two most common chemokine receptors that allow HIV-1 entry are the CCR5 and CXCR4. The CD4 and CCR5 are mainly localized to the particular plasma membrane microdomains, termed raft, which is rich in glycolipids and cholesterol. However, the CXCR4 is localized only partially to the raft region. Although the raft domain is suggested to participate in HIV-1 infection, its role in entry of CXCR4-tropic (X4-tropic) virus is still unclear. Here, we used a combination of CD4-independent infection system and cholesterol-depletion-inducing reagent, methyl-beta-cyclodextrin (MbetaCD), to address the requirement of raft domain in the X4-tropic virus infection. Treatment of CD4-negative, CXCR4-positive human cells with MbetaCD inhibited CD4-independent infection of the X4-tropic strains. This inhibitory effect of the cholesterol depletion was observed even when the CXCR4 was over-expressed on the target cells. Soluble CD4-induced infection was also inhibited by MbetaCD. The MbetaCD had no effect on the levels of cell surface expression of CXCR4. In contrast to these infections, MbetaCD treatment did not inhibit CD4-dependent HIV-1 infection in the wild type CD4-expressing cells. This study and previous reports showing that CD4 mutants localized to non-raft domains function as HIV-1 receptor indicate that CXCR4 clustering in the raft microdomains, rather than CD4, is the key step for the HIV-1 entry.

Ezrin, Radixin, and Moesin (ERM) proteins function as pleiotropic regulators of human immunodeficiency virus type 1 infection

Ezrin, radixin, and moesin (ERM) proteins supply functional linkage between integral membrane proteins and cytoskeleton in mammalian cells to regulate membrane protein dynamisms and cytoskeleton rearrangement. To assess potential role of the ERM proteins in HIV-1 lifecycle, we examined if suppression of ERM function in human cells expressing HIV-1 infection receptors influences HIV-1 envelope (Env)-mediated HIV-1-vector transduction and cell-cell fusion. Expression of an ezrin dominant negative mutant or knockdown of ezrin, radixin, or moesin with siRNA uniformly decreased transduction titers of HIV-1 vectors having X4-tropic Env. In contrast, transduction titers of R5-tropic Env HIV-1 vectors were decreased only by radixin knockdown: ezrin knockdown had no detectable effects and moesin knockdown rather increased transduction titer. Each of the ERM suppressions had no detectable effects on cell surface expression of CD4, CCR5, and CXCR4 or VSV-Env-mediated HIV-1 vector transductions. Finally, the individual knockdown of ERM mRNAs uniformly decreased efficiency of cell-cell fusion mediated by X4- or R5-tropic Env and HIV-1 infection receptors. These results suggest that (i) the ERM proteins function as positive regulators of infection by X4-tropic HIV-1, (ii) moesin additionally functions as a negative regulator of R5-tropic HIV-1 virus infection at the early step(s) after the membrane fusion, and (iii) receptor protein dynamisms are regulated differently in R5- and X4-tropic HIV-1 infections.

Inhibitory role of CXCR4 glycan in CD4-independent X4-tropic human immunodeficiency virus type 1 infection and its abrogation in CD4-dependent infection

CXCR4 functions as an infection receptor of X4 human immunodeficiency virus type 1 (HIV-1) . CXCR4 is glycosylated at the N-terminal extracellular region, which is important for viral envelope (Env) protein binding. We compared the effects of CXCR4 glycan on the CD4-dependent and -independent infections in human cells by X4 viruses. We found that transduction mediated by Env proteins of CD4-independent HIV-1 strains increased up to 5.5-fold in cells expressing unglycosylated CXCR4, suggesting that the CXCR4 glycan inhibits CD4-independent X4 virus infection. Co-expression of CD4 on the target cell surface or pre-incubation of virus particles with soluble CD4 abrogates the glycan-mediated inhibition of X4 virus infection, suggesting that interaction of Env protein with CD4 counteracts the inhibition. These findings indicate that it will be advantageous for X4 HIV-1 to remain CD4-dependent. A structural model that explains the glycan-mediated inhibition is discussed.

Characterization of R peptide of murine leukemia virus envelope glycoproteins in syncytium formation and entry

The C-terminal R peptide of ecotropic murine leukemia virus (MLV) envelope protein (Env) negatively controls membrane fusion activity. The R peptide cleavage during virion maturation activates its fusogenicity and is required for viral entry. We analyzed fusogenicity and transduction efficiency of mutant Env proteins of ecotropic, amphotropic, polytropic, and xenotropic MLVs. As the result, we found that the hydrophobic amino acid residues around the R peptide cleavage site are important for membrane fusion inhibition by the R peptide. In addition, we found that Env complexes with R peptide-truncated and -containing Env proteins have lower fusogenicity and transduction efficiency than those with the R-peptide-truncated Env alone, suggesting that efficient R peptide cleavage is required for efficient MLV vector transduction. The role of R peptide cleavage in amphotropic, polytropic, and xenotropic MLV infection has not been investigated. We found in this study that the R peptide cleavage is required for amphotropic, xenotropic, and polytropic MLV vector transduction, like with ecotropic MLV. The R-peptide-truncated Env proteins of the xenotropic and polytropic MLVs, however, had much lower fusogenicity than those of the ecotropic and amphotropic MLVs. These results provide valuable information for construction of efficient MLV vectors and for understanding the retroviral entry mechanism.

Determinant for the inhibition of ecotropic murine leukemia virus infection by N-linked glycosylation of the rat receptor

Ecotropic murine leukemia viruses (MLVs) recognize the third extracellular loop of the receptor, cationic amino acid transporter type 1 (CAT1). The CAT1 protein contains two conserved N-linked glycosylation sites in the third extracellular loops of the mouse, rat, and hamster receptors (mCAT1, rCAT1, and hCAT1, respectively). Glycosylation of the rCAT1 and hCAT1 receptors inhibits ecotropic MLV infection of CAT1-expressing cells, but that of the mCAT1 does not afford the cells this protection. As compared to the mCAT1 protein, the rCAT1 and hCAT1 proteins possess three and six amino acid insertions, respectively, in the third extracellular loop. To determine whether these inserted amino acids are associated with ecotropic MLV infection inhibition by glycosylation, several mutants of mCAT1 and rCAT1 receptors were constructed. Of all the mutants generated in the present study, only rCAT1 mutant 1 exhibited detectable protein expression levels. The rCAT1 mutant 1-expressing human NP2 cells were more susceptible to transduction by ecotropic MLV vectors than the wild-type rCAT1-expressing cells. Tunicamycin, an N-glycosylation inhibitor, increased transduction titer in the wild-type rCAT1-expressing cells, but did not do so in the cells expressing either the mCAT1 or rCAT1 mutation 1. An amino acid substitution in the glycosylation site of the wild-type rCAT1 conferred higher infection susceptibility, but that of the rCAT1 mutant 1 did not. As with the wild-type mCAT1 and rCAT1 proteins, the rCAT1 mutants were detected on the cell surface by immunofluorescence microscopy. Tunicamycin treatment did not affect cellular distribution of the rCAT1 mutant 1, wild-type mCAT1 or rCAT1 proteins. These results indicate that the extra amino acids in the rCAT1 (as compared to the mCAT1) are associated with inhibition of ecotropic MLV infection by the rCAT1 glycosylation.