Rabies Virus (RV) Glycoprotein Expression Levels Are Not Critical for Pathogenicity of RV

Previous comparisons of different rabies virus ( RV) strains suggested an inverse relationship between pathogenicity and the amount of glycoprotein produced in infected cells. In order to provide more insight into this relationship, we pursued an experimental approach that allowed us to alter the glycoprotein expression level without altering the glycoprotein sequence, thereby eliminating the contribution of amino acid changes to differences in viral virulence. To this end, we constructed an infectious clone of the highly pathogenic rabies virus strain CVS-N2c and replaced its cognate glycoprotein gene with synthetic versions in which silent mutations were introduced to replace wild-type codons with the most or least frequently used synonymous codons. A recombinant N2c variant containing the fully codon-optimized G gene and three variants carrying a partially codon-deoptimized G gene were recovered on mouse neuroblastoma cells and shown to express 2-to 3-fold more and less glycoprotein, respectively, than wild-type N2c. Pathogenicity studies in mice revealed the WT-N2c virus to be the most pathogenic strain. Variants containing partially codon-deoptimized glycoprotein genes or the codon-optimized gene were less pathogenic than WT-N2c but still caused significant mortality. We conclude that the expression level of the glycoprotein gene does have an impact on pathogenicity but is not a dominant factor that determines pathogenicity. Thus, strategies such as changes in codon usage that aim solely at altering the expression level of the glycoprotein gene do not suffice to render a pathogenic rabies virus apathogenic and are not a viable and safe approach for attenuation of a pathogenic strain.