Role of sphingomyelin in alphaherpesvirus entry.

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes disease in cattle populations worldwide. Sphingomyelin (SM) is the most abundant sphingolipid in the mammalian cell membrane, where it preferentially associates with cholesterol to form lipid raft domains. SM is a substrate for the lysosome-resident enzyme acid sphingomyelinase, which plays a role in cell membrane repair following injury. Treatment of cells with noncytotoxic concentrations of Staphylococcus aureus -derived sphingomyelinase successfully reduced cell surface-exposed sphingomyelin, but did not significantly inhibit BoHV-1 entry and infection as measured by beta-galactosidase reporter assay. Interestingly, entry of the porcine alphaherpesvirus pseudorabies virus (PRV) was inhibited by sphingomyelin-depletion of cells. Treatment of BoHV-1 particles with sphingomyelinase inhibited viral entry activity, suggesting that viral SM plays a role in BoHV-1 entry, while cellular SM does not. Treatment of cells with noncytotoxic concentrations of the functional inhibitors of host acid sphingomyelinase, imipramine and amitriptyline, which induce degradation of the cellular enzyme, did not significantly inhibit BoHV-1 entry. In contrast, inhibition of cellular acid sphingomyelinase inhibited PRV entry. Entry of the human alphaherpesvirus herpes simplex virus 1 (HSV-1) was independent of both host SM and acid sphingomyelinase, in a manner similar to BoHV-1. Together, the results suggest that among the alphaherpesviruses, there is variability in entry requirements for cellular sphingomyelin and acid sphingomyelinase activity. IMPORTANCE BoHV-1 is a ubiquitous pathogen affecting cattle populations worldwide. Infection can result in complicated, polymicrobial infections due to the immunosuppressive properties of the virus. Available vaccines limit disease severity and spread, but do not prevent infection. The financial and animal welfare ramifications of BoHV-1 are significant. In order to develop more effective prevention and treatment regimens, a more complete understanding of the initial steps in viral infection is necessary. We recently identified a low pH endocytosis pathway for BoHV-1. Here we examine the role of cellular factors responsible for membrane integrity and repair in alphaherpesviral entry. This study allows comparisons of the BoHV-1 entry pathway with those of other alphaherpesviruses (PRV and HSV-1). Lastly, this is the first report of sphingomyelin and lysosomal sphingomyelinase playing a role in the entry of a herpesvirus. The results may lead to development of more effective prevention and treatment regimens.