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

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.

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.

The AKV murine leukemia virus is restricted and hypermutated by mouse APOBEC3

APOBEC3 proteins are potent restriction factors against retroviral infection in primates. This restriction is accompanied by hypermutations in the retroviral genome that are attributable to the cytidine deaminase activity of the APOBEC3 proteins. Studies of nucleotide sequence diversity among endogenous gammaretroviruses suggest that the evolution of endogenous retroelements could have been shaped by the mutagenic cytidine deaminase activity of APOBEC3. In mice, however, APOBEC3 appears to restrict exogenous murine retroviruses in the absence of detectable levels of deamination. AKV is an endogenous retrovirus that is involved in causing a high incidence of thymic lymphoma in AKR mice. A comparative analysis of several mouse strains revealed a relatively low level of APOBEC3 expression in AKR mice. Here we show that endogenous mouse APOBEC3 restricts AKV infection and that this restriction likely reflects polymorphisms affecting APOBEC3 abundance rather than differences in the APOBEC3 isoforms expressed. We also observe that restriction of AKV by APOBEC3 is accompanied by G-->A hypermutations in the viral genome. Our findings demonstrate that APOBEC3 acts as a restriction factor in rodents affecting the strain tropism of AKV, and they provide good support for the proposal that APOBEC3-mediated hypermutation contributed to the evolution of endogenous rodent retroviral genomes.

Mechanisms underlying glycosylation-mediated loss of ecotropic receptor function in murine MDTF cells and implications for receptor evolution

A Mus dunni tail fibroblast (MDTF) cell line is highly resistant to infection by ecotropic Moloney murine leukemia virus (Mo-MLV). The cationic amino acid transporter type 1 (CAT1) paralogues of murine NIH 3T3 and MDTF cells (mCAT1 and dCAT1, respectively) contain two conserved N-linked glycosylation sites in the third extracellular loop (ECL3, the putative Mo-MLV binding site). Glycosylation of dCAT1 inhibits Mo-MLV infection, but that of mCAT1 does not. Compared with mCAT1, dCAT1 possesses an Ile-to-Val substitution at position 214 and a Gly insertion at position 236 in the ECL3. To determine the residues responsible for the loss of dCAT1 receptor function, mutants of mCAT1 were constructed. The mCAT1/insG receptor (with a Gly residue inserted at mCAT1 position 236) had greatly reduced Mo-MLV receptor function compared with mCAT1. Treatment of mCAT1/insG-expressing cells with tunicamycin, an N-linked glycosylation inhibitor, increased the transduction titre. In addition, the reduced susceptibility to Mo-MLV observed with mCAT1/insG-expressing cells correlated with impaired binding of Mo-MLV. These results show that a single amino acid insertion confers mCAT1 receptor properties on dCAT1 and provide an important insight into the co-evolution of virus–host interactions.

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.

Phosphatidylserine treatment relieves the block to retrovirus infection of cells expressing glycosylated virus receptors

Background

A major determinant of retrovirus host range is the presence or absence of appropriate cell-surface receptors required for virus entry. Often orthologs of functional receptors are present in a wide range of species, but amino acid differences can render these receptors non-functional. In some cases amino acid differences result in additional N-linked glycosylation that blocks virus infection. The latter block to retrovirus infection can be overcome by treatment of cells with compounds such as tunicamycin, which prevent the addition of N-linked oligosaccharides.

Results

We have discovered that treatment of cells with liposomes composed of phosphatidylserine (PS) can also overcome the block to infection mediated by N-linked glycosylation. Importantly, this effect occurs without apparent change in the glycosylation state of the receptors for these viruses. This effect occurs with delayed kinetics compared to previous results showing enhancement of virus infection by PS treatment of cells expressing functional virus receptors.

Conclusion

We have demonstrated that PS treatment can relieve the block to retrovirus infection of cells expressing retroviral receptors that have been rendered non-functional by glycosylation. These findings have important implications for the current model describing inhibition of virus entry by receptor glycosylation.