The role of the murine cytomegalovirus protein M28 in cross-species infection

Abstract

Cytomegaloviruses (CMVs) have co-evolved with their respective hosts for many years and are known to be strictly species specific. While CMVs can infect cells of a foreign host, they cannot replicate in them, and the underlying mechanisms of the species restriction remain poorly understood. Recently, it was shown that murine CMV (MCMV) can be adapted to human epithelial cells, a phenotype attributed to adaptive mutations in several genes. While comprehensive analysis revealed that inhibition of apoptosis, dispersion of PML-nuclear bodies, and prevention of E2F-mediated gene transcription are important for crossing the species barrier in human epithelial cells (RPE-1), the replication of these adapted viruses was still attenuated in human fibroblasts. To date, why MCMV replication is restricted in human fibroblasts had not been addressed. Preliminary studies indicated that MCMV M28 is an important factor in species specificity. The aim of this study was to investigate the function and molecular mechanism of M28 in cross-species infection in human fibroblasts. In this study, I identified M28 as a novel host range determinant important for the fibroblast adaptation. Specific introduction of missense or stop mutations in M28, in addition to others, promoted efficient replication in human fibroblasts, whereas replication of MCMV-M28stop in murine fibroblasts was not impaired. In this study I could show that M28 is expressed with early kinetics, localizes to the cytoplasm, and binds to the SHC-transforming protein 1 (SHC1). SHC1 is an essential scaffold protein of the epidermal growth factor receptor (EGFR) and other receptor tyrosine kinases. It is phosphorylated upon EGFR stimulation and activates mitogen-activated protein kinases (MAPK) and phosphoinositide-3-kinase/AKT signaling pathways, which are involved in cell cycle regulation, proliferation, and survival. During infection, M28 interacts with SHC1 and prevents its phosphorylation thus restraining further downstream signaling of MAPK/ERK and PI3K/AKT. However, in primary MEFs phosphorylation of SHC1 was not affected in the presence or absence or M28, suggesting another mechanism of action in murine fibroblasts. Moreover, a transient knockdown of SHC1 in infected MRC5 cells reduced viral titers of an MCMV mutant lacking M28. Taken together these results suggest that expression of M28 restricts viral replication in human fibroblasts by binding to SHC1 and inhibiting downstream signaling. Conversely, SHC1 functions as a pro-viral factor in MCMV cross-species infection of human fibroblasts.