Rabies virus infection-induced apoptosis
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Induction of apoptosis by rabies virus (RV) has been reported to be associated with the expression of the glycoprotein (G), but inversely correlated with pathogenicity. However, the importance of apoptosis in the cause of death of infected individuals is still not clear. The objective of this study was to understand the importance of apoptosis in RV infection and the role of rabies virus glycoprotein (RVG) in the induction of apoptosis. Furthermore, the apoptotic pathway by which RV induces apoptosis was delineated. To this end, wild-type (wt), laboratory-adapted (attenuated), as well as recombinant RVs with replacement of only the G gene were used to infect mice intracerebrally. Results indicate that attenuated RV and recombinant RVs expressing the G from attenuated viruses expressed a higher level of the G and induced more apoptosis in mice than recombinant RV expressing the G from wt or pathogenic RV, demonstrating that it is the G that determines the level of G expression and, consequently, the induction of apoptosis. Likewise, recombinant viruses expressing the G from wt or pathogenic RV are more pathogenic in mice than those expressing G from attenuated RV, confirming the inverse correlation between RV pathogenicity and the induction of apoptosis. To investigate the mechanism by which induction of apoptosis attenuates viral pathogenicity, mice were infected with wt or attenuated virus intramuscularly. It was found that low doses of attenuated RV induced apoptosis in the spinal cord and failed to spread to the brain or produce neurological disease. On the other hand, apoptosis was not observed in the spinal cord of mice infected with the same doses of wt RV and the virus spread to various parts of the brain and induced fatal neurologic disease. To determine the pathways by which CVS-B2C induces apoptosis, cells were infected with CVS-B2C. Caspase activity and expression of several apoptotic proteins were analyzed. Caspase-8 and caspase-3 were activated in B2C infected cells. In addition, AIF was up-regulated and translocated to the nucleus. Overall, these results suggest that attenuated RVs, by inducing glycoprotein-mediated apoptosis, limit the spread of the virus in the CNS via caspase-dependent and caspase-independent pathways.