Pathogenicity of different rabies virus variants inversely correlates with apoptosis and rabies virus glycoprotein expression in infected primary neuron cultures

The mouse-adopted rabies virus CVS-24 has stable variants, CVS-B2c and CVS-N2c, which differ greatly in their pathogenicity for normal adult mice and in their ability to infect nonneuronal cells. The glycoprotein (G Protein), which has previously been implicated in rabies virus pathogenicity, shows substantial structural differences between these variants. Although prior studies have identified antigenic site III of the G protein as the major pathogenicity determinants, CVS-B2c and CVS-N2c do not vary at this site. The possibility that pathogenicity in inversely related to G protein expression level is suggested by the finding that CVS-B2c, the less pathogenic variant, expresses at least fourfold-higher levels of G protein than CVS-N2c in infected neurons. Although there is some difference between CVS-B2c- and CVS-N2c-infected neurons in G protein mRNA expression levels, the differential expression of G protein appears to be largely determined by post-translational mechanism that effect G protein stability. Pulse-chase experiments indicated that the G protein of CVS-B2c is degraded more slowly than that od CVS-N2c. The accumulation of G protein correlated with the induction of programmed cell death in CVS-B2c-infected neurons. The extent of apoptosis was considerably lower in CVS-N2c-infected neurons, where G protein expression was minimal. While nucleoprotein (N protein) expression levels were similar in neurons infected with either variant, the transport of N protein into neuronal process was strongly inhibited in CVS-B2c-infected cells. Thus, downregulation of G protein expression in neuronal cells evidently contributes to rabies virus pathogenesis by preventing apoptosis and the apparently associated failure of the axonal transport of N protein.