Wild mouse species differ from one another and from the various inbred laboratory mouse strains in their susceptibility to the mouse gammaretroviruses and retrovirus-induced diseases. These differences are due to variations in specific host genes, and we have been engaged in an ongoing effort to identify and characterize several mouse genes involved in virus resistance. Our major focus has been on cell surface virus receptors and these studies focus on identifying viral and cell receptor determinants responsible for virus binding, entry, and receptor mediated cytopathicity. One series of experiments focuses on two unusual variants of the ecotropic gammaretroviruses that are cytopathic in M. dunni cells and also have altered host range. These phenotypes are due to different amino acid substitutions at the same site in viral envelope gene of the two viruses. This substitution alters one of the 3 amino acids that form the cell surface receptor binding site. The fact that these 2 viruses cause cytopathic effects in a cell line with a variant receptor gene suggests that the virus-receptor interaction mediates cytopathicity. This conclusion was confirmed by the observation that cytopathicity due to virus infection is seen in stable transfectants expressing this variant receptor but not in transfectants expressing the prototypical receptor. Because evidence also suggests that cytopathicity is affected by inhibitors of glycosylation, we are currently evaluating the role of glycosylation in virus infectivity and cytopathicity. We have now demonstrated that receptor glycosylation modulates virus entry through both naturally occuring variants of the ecotropic receptor. We are currently looking at the effect of virus envelope glycosylation on the efficiency of virus entry.? ? In another series of experiments, we have been characterizing virus susceptibility differences identified in a panel of cells derived from evolutionarily divergent wild mouse species. These cells show novel patterns of resistance not found among laboratory strains. One of these resistance phenotypes was identified in the African pygmy mouse, Mus minutoides which is completely resistant to the ecotropic gammaretrovirus found in common inbred strains, although it is susceptible to laboratory virus strains such as Moloney leukemia virus. We have shown that the block to virus replication is post entry and targets the viral capsid. These cells are being evaluated for the presence of a novel allele at Fv1, a gene known to be responsible for capsid-mediated resistance to other subsets of gammaretroviruses in laboratory mice. ? ? A second susceptibility variant in wild mouse strains was identified in the Asian mouse, Mus pahari. These mice are resistant to the polytropic gammaretroviruses and susceptible to the xenotropic viruses, a pattern that is the opposite of that found in laboratory mice. We have now cloned and characterized the pahari cellular receptor that mediates entry of both xenotropic and polytropic viruses. We analyzed chimeras and mutants of the different receptors to show that 2 of the 4 extracellular loops of this membrane protein contain the determinants for virus entry. We identified two critical amino acids, one in each loop, that independently mediate entry of xenotropic viruses. We are currently screening additional mutants to identify determinants for polytropic virus entry. Analysis of the provirus content of the various wild mouse species suggests that the phenotypic variants of mouse XPR1 likely arose in conjunction with exposure to gammaretrovirus infections and co-evolutionary adaptations in the virus envelope.
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