Canine distemper virus N protein induces autophagy to facilitate viral replication

Canine distemper virus (CDV) contains 6 transcription-units (N-P-M-F-H-L) and causes fatal disease in Canidae. Although autophagy plays roles in degradation and activation of immune response against pathogens, viruses have developed strategies to evade autophagy. Induction of autophagy in members of Paramyxoviridae family has been confirmed. Therefore, this study evaluated effect of autophagy mechanisms on CDV infection. The study confirmed onset of autophagy in CDV infected Vero cells by elevated autophagy associated proteins (LC3-II) with progression of CDV infection. Vero cells were treated with rapamycin to induce autophagy and Wortmannin and Chloroquine to inhibit autophagy. Autophagy induction resulted in increased viral RNA titer while inhibition of autophagy resulted in decreased viral RNA titer. Onset of autophagy in Vero cells infected with infectious and UV-inactivated CDV implicated structural proteins of CDV in virus induced autophagy. Further analysis using flag-tagged proteins, revealed that flag-nucleoprotein had the most significant increase in LC3-II level. Flag-nucleoprotein further promoted degradation of p62 protein. These results suggested that CDV nucleoprotein induced complete autophagic flux. The mammalian target of rapamycin (mTOR), a regulator of autophagy, was detected in low levels in Vero cells expressing flag-nucleoprotein, suggesting that CDV nucleoprotein induced autophagy by inhibiting mTOR pathway to facilitate viral replication. 

(LML)