Requirements for CEACAMs and cholesterol during murine coronavirus cell entry

Previous reports have documented that cholesterol supplementations increase cytopathic effects in tissue culture and also intensify in vivo pathogenicities during infection by the enveloped coronavirus murine hepatitis virus (MHV). To move toward a mechanistic understanding of these phenomena, we used growth media enriched with methyl-beta-cyclodextrin or cholesterol to reduce or elevate cellular membrane sterols, respectively. Cholesterol depletions reduced plaque development 2- to 20-fold, depending on the infecting MHV strain, while supplementations increased susceptibility 2- to 10-fold. These various cholesterol levels had no effect on the binding of viral spike (S) proteins to cellular carcinoembryonic antigen-related cell adhesion molecule (CEACAM) receptors, rather they correlated directly with S-protein-mediated membrane fusion activities. We considered whether cholesterol was indirectly involved in membrane fusion by condensing CEACAMs into "lipid raft" membrane microdomains, thereby creating opportunities for simultaneous binding of multiple S proteins that subsequently cooperate in the receptor-triggered membrane fusion process. However, the vast majority of CEACAMs were solubilized by cold Triton X-100 (TX-100), indicating their absence from lipid rafts. Furthermore, engineered CEACAMs appended to glycosylphosphatidylinositol anchors partitioned with TX-100-resistant lipid rafts, but cells bearing these raft-associated CEACAMs were not hypersensitive to MHV infection. These findings argued against the importance of cholesterol-dependent CEACAM localizations into membrane microdomains for MHV entry, instead suggesting that cholesterol had a more direct role. Indeed, we found that cholesterol was required even for those rare S-mediated fusions taking place in the absence of CEACAMs. We conclude that cholesterol is an essential membrane fusion cofactor that can act with or without CEACAMs to promote MHV entry.