Abstract
Enteroviruses invade the host by crossing the intestinal mucosa, which is lined by polarized epithelium. A number of enteroviruses, including echoviruses (EV) and group B coxsackieviruses (CVB), initiate infection by attaching to decay-accelerating factor (DAF), a molecule that is highly expressed on the apical surface of polarized epithelial cells. We previously observed that entry of DAF-binding CVB3 into polarized intestinal epithelial cells occurs by an unusual endocytic mechanism that requires caveolin but does not involve clathrin or dynamin. Here we examined the entry of a DAF-binding echovirus, EV7. We found that drugs, small interfering RNAs (siRNAs), and dominant negative mutants that target factors required for clathrin-mediated endocytosis, including clathrin and dynamin, inhibited both EV7 infection and internalization of virions from the cell surface. Once virus had entered the cell, it colocalized with markers of early endosomes (EEA1) and then late endosomes (LAMP-2). Inhibition of endosomal maturation-with siRNAs or dominant negative mutants targeting Rab5 and Rab7-inhibited infection and prevented release of viral RNA into the cell. These results indicate that EV7 is internalized by clathrin-mediated endocytosis and then moves to early and late endosomes before releasing its RNA. Trafficking through endosomes is known to be important for viruses that depend on low pH or endosomal cathepsin proteases to complete the entry process. However, we found that EV7 infection required neither low pH nor cathepsins. Importance: The results demonstrate that echovirus 7 (EV7), after binding to decay-accelerating factor (DAF) on the cell surface, enters cells by clathrin-mediated endocytosis; this entry mechanism differs markedly from that of another DAF-binding enterovirus, coxsackievirus B3 (CVB3). Thus, after attachment to the same cell surface receptor, these closely related viruses enter the same cells by different mechanisms. The cellular cues required for release of viral RNA from the enterovirus capsid ("uncoating") remain poorly defined. We found that EV7 moved to late endosomes and that release of RNA depended on endosomal maturation; nonetheless, EV7 did not depend on the endosomal factors implicated in uncoating and entry by other viruses. The results suggest either that an unidentified endosomal factor is essential for uncoating of EV7 or that trafficking through the endosome is an essential step in a pathway that leads to another intracellular organelle where uncoating is completed.