Divergent Roles of Autophagy in Virus Infection

Chiramel, Abhilash I.; Brady, Nathan R.; Bartenschlager, Ralf

Various studies reported the activation of autophagy upon virus infection as inferred from the increased number of autophagic vesicles, the monitoring of LC3-I to LC3-II conversion and the elevated number of LC3-positive punctae in infected cells [21 23,29,30,32,33,35] . However, considering the dynamic nature of autophagy, it is important to determine the rate of autophagic flux to gain mechanistic insight into how viral infection affects autophagic activity in cells. In fact, recent reports demonstrated that Sindbis virus and hepatitis C virus (HCV) infection increase autophagic flux [36, 37] . Infection of mouse embryonic fibroblasts (MEFs) expressing GFP-LC3 with Sindbis virus resulted in an increase in the percentage of cells containing GFP-LC3 positive dots concomitant with an increase in LC3-II abundance. In addition, infected cells contained reduced levels of p62 (an accepted marker of autophagic flux), with no significant changes of its mRNA amounts, indicating a complete autophagic response during virus infection [37] . In case of HCV infection, a study by Ke and Chen [36] demonstrated that despite unaltered levels of p62 in infected cells, LC3-II protein levels were increased upon treatment with lysosomal protease inhibitors (E64 and pepstatin A), or chloroquine (a vacuolar ATPase inhibitor) or bafilomycin A1 (an inhibitor of lysosomal acidification), [36] arguing that HCV does not block autophagy. In line with this conclusion, the authors showed that treatment of infected cells with chloroquine and bafilomycin A1 resulted in increased LC3-II levels in HCV infected cells as compared to uninfected cells, clearly indicating enhanced autophagic flux. In addition, by employing a tandem-LC3 reporter (mRFP-GFP-LC3), a tool to analyze both autophagosome and autolysosome formation, the authors found that HCV induced autophagosomes that matured into autolysosomes, thus revealing a complete autophagic response in HCV-infected cells. In contrast, an earlier study by Sir and co-workers proposed that autophagy is incomplete, and cells transfected with HCV RNA formed autophagosomes that, due to inefficient fusion with lysosomes, did not convert into autolysosomes [23] . The discrepancy between these two studies might be due to differences in the experimental approaches: HCV infection versus electroporation of HCV RNA.