Abstract
Methyl beta-cyclodextrin (MβCD) treatment of herpes simplex virus 1 (HSV-1) reduced envelope cholesterol levels and inhibited viral entry and infectivity in several cell types, regardless of the dependence of entry on endocytosis or low pH. Viral protein composition was similar in MβCD-treated and untreated virions, and ultrastructural analysis by electron microscopy revealed that cholesterol removal did not grossly affect virion structure or integrity. Removal of envelope cholesterol greatly reduced virion fusion activity as measured by fusion-from-without, suggesting that virion cholesterol is critical for the step of membrane fusion. MβCD-treatment of HSV-1 did not reduce viral attachment to the cells nor endocytic uptake of HSV-1 from the cell surface. The pre-fusion form of gB present in the HSV-1 envelope undergoes conformational changes in response to mildly acidic pH. These gB changes occurred independently of envelope cholesterol. Removal of cholesterol compromised virion stability as measured by recovery of infectivity following cycles of freeze-thaw. Taken together, the data suggest that HSV-1 envelope cholesterol is important for viral entry and infectivity due to a critical role in membrane fusion.
Highlights
Herpesviruses are ubiquitous pathogens that cause significant morbidity and mortality worldwide
We use MBCD-treated virions to demonstrate that cholesterol is critical for herpes simplex virus type 1 (HSV-1) membrane fusion using a FFWO approach
HSV-1 propagated on cells containing the cholesterol precursor desmosterol was previously shown to be cholesterol-free, yet infectious (Wudiri and Nicola, 2017)
Summary
Herpesviruses are ubiquitous pathogens that cause significant morbidity and mortality worldwide. The human herpes simplex viruses cause cold sores and sexually transmitted infections. Entry of herpes simplex virus type 1 (HSV-1) is initiated by attachment of virions to cell surface glycosaminoglycans (Shukla and Spear, 2001). All enveloped viruses must fuse with host cell membranes to initiate entry and infection (Barrow et al, 2013). In cell types such as human epithelial cells, attached HSV-1 particles undergo endocytosis followed by fusion of the viral envelope with an endosomal membrane (Nicola et al, 2003). In other cell types, such as the model Vero cell line, fusion occurs immediately following attachment at the cell surface in a pH-neutral manner (Wittels and Spear, 1991). HSV-1 membrane fusion requires a complex of envelope glycoproteins gD, gH-gL and the core fusion protein gB (Campadelli-Fiume et al, 2012; Krummenacher et al, 2013; Weed and Nicola, 2017)
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