Matrix protein mutations contribute to inefficient induction of apoptosis leading to persistent infection of human neural cells by vesicular stomatitis virus

In a model system to study factors involved in the establishment of a persistent viral infection that may lead to neurodegenerative diseases, Indiana and New Jersey variants of vesicular stomatitis virus (VSV) with different capacities to infect and persist in human neural cells were studied. Indiana matrix (M) protein mutants and the wild-type New Jersey strain persisted in the human neural cell line H4 for at least 120 days. The Indiana wild-type virus (HR) and a non-M mutant (TP6), both unable to persist, induced apoptosis more strongly than all the other variants tested, as indicated by higher levels of DNA fragmentation and caspase-3-like activity. Transfection of H4 cells with mRNA coding for the VSV M protein confirmed the importance of this protein in the induction of apoptosis. Furthermore, the pan-caspase inhibitor ZVAD-fmk maintained cell survival to about 80%, whereas inhibition of caspase-8, caspase-9, or both only partially protected the cells against death, consistent with the fact that anti-apoptotic molecules from the Bcl-2 family also protect cells from death only partially. These results suggest that VSV activates many pathways of cell death and that an inefficient induction of caspase-3-related apoptosis participates in the establishment of a persistent infection of human neural cells by less virulent VSV variants.