Activation of E2F-dependent transcription by the mouse cytomegalovirus M117 protein affects the viral host range.

Eléonore Ostermann,Albert Sickmann,Stefan Loroch,Lüder Wiebusch,Zhikang Qian,Wolfram Brune

doi:10.1371/journal.ppat.1007481

Abstract

Cytomegaloviruses (CMVs) have a highly restricted host range as they replicate only in cells of their own or closely related species. To date, the molecular mechanisms underlying the CMV host restriction remain poorly understood. However, it has been shown that mouse cytomegalovirus (MCMV) can be adapted to human cells and that adaptation goes along with adaptive mutations in several viral genes. In this study, we identify MCMV M117 as a novel host range determinant. Mutations in this gene enable the virus to cross the species barrier and replicate in human RPE-1 cells. We show that the M117 protein is expressed with early kinetics, localizes to viral replication compartments, and contributes to the inhibition of cellular DNA synthesis. Mechanistically, M117 interacts with members of the E2F transcription factor family and induces E2F target gene expression in murine and human cells. While the N-terminal part of M117 mediates E2F interaction, the C-terminal part mediates self-interaction. Both parts are required for the activation of E2F-dependent transcription. We further show that M117 is dispensable for viral replication in cultured mouse fibroblasts and endothelial cells, but is required for colonization of mouse salivary glands in vivo. Conversely, inactivation of M117 or pharmacological inhibition of E2F facilitates MCMV replication in human RPE-1 cells, whereas replacement of M117 by adenovirus E4orf6/7, a known E2F activator, prevents it. These results indicate that E2F activation is detrimental for MCMV replication in human cells. In summary, this study identifies MCMV M117 as a novel E2F activator that functions as a host range determinant by precluding MCMV replication in human cells.

Highlights

Attempts to understand the CMV host species specificity have revealed that the CMVs can enter non-permissive host cells and even express a subset of viral genes, mainly of the immediate early (IE) class, but viral DNA replication and late gene expression are inefficient or absent [4,5]
In previous work we have reported the isolation of three human cell-adapted mouse cytomegalovirus (MCMV) (MCMV/h1, MCMV/h2, and MCMV/h3) and described the mutations associated with adaptation to human RPE-1 epithelial cells [14]
We hypothesized that these mutations contributed to human cell adaptation and that M117 is a host range determinant

Summary

Introduction

As such, they rely on suitable host cells for their replication. Cytomegaloviruses (CMVs), representatives of the β-herpesvirus subfamily, are highly species-specific as they can replicate only in cells of their own or closely related host species [3]. HCMV pathogenesis cannot be studied in small animal models Instead, related viruses such as the mouse and rat cytomegaloviruses (MCMV and RCMV) are used as models to study CMV pathogenesis in their natural hosts. These viruses do not replicate in human cells. The molecular mechanisms underlying the CMV host species specificity have remained largely unknown

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Conclusion