A Role for Nuclear F-Actin Induction in Human Cytomegalovirus Nuclear Egress.

Adrian R Wilkie,Donald M Coen,Jessica L Lawler,Michael J Imperiale

doi:10.1128/mbio.01254-16

Abstract

ABSTRACTHerpesviruses, which include important pathogens, remodel the host cell nucleus to facilitate infection. This remodeling includes the formation of structures called replication compartments (RCs) in which herpesviruses replicate their DNA. During infection with the betaherpesvirus, human cytomegalovirus (HCMV), viral DNA synthesis occurs at the periphery of RCs within the nuclear interior, after which assembled capsids must reach the inner nuclear membrane (INM) for translocation to the cytoplasm (nuclear egress). The processes that facilitate movement of HCMV capsids to the INM during nuclear egress are unknown. Although an actin-based mechanism of alphaherpesvirus capsid trafficking to the INM has been proposed, it is controversial. Here, using a fluorescently-tagged, nucleus-localized actin-binding peptide, we show that HCMV, but not herpes simplex virus 1, strongly induced nuclear actin filaments (F-actin) in human fibroblasts. Based on studies using UV inactivation and inhibitors, this induction depended on viral gene expression. Interestingly, by 24 h postinfection, nuclear F-actin formed thicker structures that appeared by super-resolution microscopy to be bundles of filaments. Later in infection, nuclear F-actin primarily localized along the RC periphery and between the RC periphery and the nuclear rim. Importantly, a drug that depolymerized nuclear F-actin caused defects in production of infectious virus, capsid accumulation in the cytoplasm, and capsid localization near the nuclear rim, without decreasing capsid accumulation in the nucleus. Thus, our results suggest that for at least one herpesvirus, nuclear F-actin promotes capsid movement to the nuclear periphery and nuclear egress. We discuss our results in terms of competing models for these processes.

Highlights

Herpesviruses, which include important pathogens, remodel the host cell nucleus to facilitate infection
One study showed that herpes simplex virus 1 (HSV-1) replication compartments (RCs) move in a manner that was antagonized by inhibitors of F-actin and myosin [24]
Nuclear F-actin was induced in LifeAct-green fluorescent protein (GFP)-NLS-expressing human foreskin fibroblasts (HFFs) infected with human cytomegalovirus (HCMV) starting around 6 h postinfection

Summary

Introduction

Herpesviruses, which include important pathogens, remodel the host cell nucleus to facilitate infection. We show that HCMV, an important human pathogen, induces actin filaments in the nuclei of infected cells and that an inhibitor of nuclear F-actin impairs nuclear egress and capsid localization toward the nuclear periphery. Another group found directed intranuclear movements of HSV-1 capsids that were antagonized by ATP depletion or by inhibitors of myosin and F-actin [26] Despite these reports, the notion that a nuclear F-actinbased mechanism facilitates herpesvirus capsid motility has recently been challenged by Bosse et al [27, 28]. The notion that a nuclear F-actinbased mechanism facilitates herpesvirus capsid motility has recently been challenged by Bosse et al [27, 28] This group was unable to visualize actin filaments in the nuclei of murine embryonic fibroblasts (MEFs) infected with PRV, HSV-1, mouse cytomegalovirus (MCMV), and murine gammaherpesvirus 68 (MHV-68) [27]. Further investigation into the importance of the nuclear cytoskeleton for herpesvirus infection is important for virology and for cell biology, as the functions of nuclear F-actin remain largely unexplored

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