Our results indicate that the degree of expression of G protein in RC-HL AZD2014 strain-infected cells is comparable to that in R(G 242/255/268) strain-infected cells (Fig. 4). This supports the observation that RC-HL and R(G 242/255/268) strains do not differ in their apoptosis-inducing abilities.
Rabies virus G protein is known to play an important role in cell-to-cell spread. Dietzschold et al. demonstrated that an amino acid substitution at position 333 in G protein (Arg to Ile or Gln), which is known to attenuate viral pathogenicity (7), impaired the efficient cell-to-cell spread of parental CVS-11 and ERA strains both in vivo and in vitro (13). Furthermore, Faber et al. indicated that a single amino acid substitution at position 194 in G protein (Asn to Lys) increased both the viral pathogenicity and the efficiency of cell-to-cell spread (24). In this study, we also showed that three amino acids at positions 242, 255 and 268 in G protein, which are related to the different pathogenicities of the Nishigahara and RC-HL strains (18), determine the efficiency of cell-to-cell spread (Fig. 6). The fact that different determinants of pathogenicity in G protein equally affect cell-to-cell spread of the rabies virus strongly suggests that the efficiency of cell-to-cell spread is generally an important factor for pathogenicity of rabies virus. The molecular mechanism by which G protein determines
the efficiency of cell-to-cell spread of rabies virus remains unclear. Since a variety of Sclareol amino acid residues in G protein are involved in the cell-to-cell spread of virus as buy PD0332991 mentioned above, multiple mechanisms might determine the efficiency of cell-to-cell spread. Although the mechanism by which amino acid substitutions at positions 242, 255 and 268 in G protein affect cell-to-cell spread remains to be elucidated, the finding that the apoptosis-inducing abilities of RC-HL and R (G 242/255/268) strains are almost identical in NA cells strongly suggested that apoptosis is not involved in the inefficient spread of RC-HL infection in NA cells (Figs 3a and 6).
Previous studies have demonstrated that internalization of rabies virus into cells and pH-dependent membrane fusion, which are also controlled by the G protein, are important factors for viral pathogenicity (13, 24, 25). However, we found no clear difference between the efficiencies of internalization of the RC-HL and R(G 242/255/268) strains (Fig. 5b). Also, we have previously demonstrated that the pH threshold of membrane fusion activity of the RC-HL strain is identical to the threshold of the pathogenic R(G 164–303) strain, which has amino acid residues from the Nishigahara strain at positions from 164 to 303 in the G protein in the genetic background of the RC-HL strain (pH 6.1) (26). This result strongly suggests that the pH threshold of the R(G 242/255/268) strain is also not different from the pH threshold of the RC-HL strain.