Infection of XC Cells by MLVs and Ebola Virus Is Endosome-Dependent but Acidification-Independent

Abstract

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.

Directed Molecular Evolution of an Engineered Gammaretroviral Envelope Protein with Dual Receptor Use Shows Stable Maintenance of Both Receptor Specificities

“…Instead, the effect of the F183L mutation was measured in a series of APJ activity assays since variations in receptor interactions and especially internalization previously have been shown to influence viral infectivity (5,15). These APJ activity assays showed that the F183L mutation results in decreased APJ affinity, as well as a decreased ability to activate APJ upon binding.…”

“…As mentioned in the introduction, interactions with apelin-13 result in recycling of the receptor, whereas binding of apelin-36 results in degradation of the receptor (22,26). These differences in the fate of the receptor might influence viral infectivity since studies have shown the internalization of virus through specific cellular compartments to be important for infectivity (5,15). Krueger and Albritton have previously shown that a chimeric MLV Env protein expressing the somatostatin ligand changes infectivity according to the somatostatin receptor variant used for internalization because of variations in receptor internalization through different cellular compartments (15).…”

“…This leads to insertion of the fusion peptide into the membrane of the host cell and hence induces membrane fusion (2), as reviewed in references 3 and 4. Other reports have suggested a need for proteolytic cleavage of SU in endosomes to allow viral fusion (5,6). However, variations have been observed among various cell types showing MLV variants to fuse either directly at the plasma membrane or following internalization through endosomes (5).…”

“…Several studies have documented the importance of the internalization through specific cellular compartments in relation to viral infectivity (5,15). Passaging of virus on permissive cells specific for a function or feature of interest can result in the fixation of particular mutations.…”

Retrovirus Entry by Endocytosis and Cathepsin Proteases

“…Previously it had been reported that ammonium chloride inhibits ecotropic MLV infection but does not amphotropic and xenotropic MLV infections, showing that ecotropic MLV infection occurs through acidic vesicles, but amphotropic and xenotropic MLV infections do not [49,50] (Table 2). The more specific inhibitors of endosome acidification (concanamycin A and bafilomycin A-1) suppress all of ecotropic, amphotropic, polytropic, and xenotropic MLV infections [51,52]. At present, it is generally accepted that ecotropic MLV infection requires acidification, because all the studies consistently reported the suppression of ecotropic virus replication with the inhibitors of endosome acidification.…”