Alpha-Herpesvirus Infection Induces the Formation of Nuclear Actin Filaments

Abstract

Herpesviruses are large double-stranded DNA viruses that replicate in the nuclei of infected cells. Spatial control of viral replication and assembly in the host nucleus is achieved by the establishment of nuclear compartments that serve to concentrate viral and host factors. How these compartments are established and maintained remains poorly understood. Pseudorabies virus (PRV) is an alpha-herpesvirus often used to study herpesvirus invasion and spread in the nervous system. Here, we report that PRV and herpes simplex virus type 1 infection of neurons results in formation of actin filaments in the nucleus. Filamentous actin is not found in the nucleus of uninfected cells. Nuclear actin filaments appear physically associated with the viral capsids, as shown by serial block-face scanning electron micropscopy and confocal microscopy. Using a green fluorescent protein-tagged viral capsid protein (VP26), we show that nuclear actin filaments form prior to capsid assembly and are required for the efficient formation of viral capsid assembly sites. We find that actin polymerization dynamics (e.g., treadmilling) are not necessary for the formation of these sites. Green fluorescent protein-VP26 foci co-localize with the actin motor myosin V, suggesting that viral capsids travel along nuclear actin filaments using myosin-based directed transport. Viral transcription, but not viral DNA replication, is required for actin filament formation. The finding that infection, by either PRV or herpes simplex virus type 1, results in formation of nuclear actin filaments in neurons, and that PRV infection of an epithelial cell line results in a similar phenotype is evidence that F-actin plays a conserved role in herpesvirus assembly. Our results suggest a mechanism by which assembly domains are organized within infected cells and provide insight into how the viral infectious cycle and host actin cytoskeleton are integrated to promote the infection process.

Actin and Rho GTPases in herpesvirus biology

“…Feierbach et al found that actin filaments are formed in the nucleus of cells infected with PRV or HSV-1. Because these actin filaments associated with viral capsids during capsid assembly and were similar to the ones seen by SBFSEM [33], the authors concluded that intranuclear filaments are composed of actin. The function of nuclear actin during infection is currently unknown.…”

“…The actin cytoskeleton and associated proteins are also implicated in herpesvirus capsid assembly and nuclear egress [32,33,41]. Capsid assembly and genome packaging occurs in the nucleus in higher order structures that might depend on F-actin for their formation [33].…”

“…Herpesviruses induce the formation of actin filaments in the nucleus Using serial-section block-face scanning electron microscopy (SBFSEM), which enables the acquisition of volume electron-microscopic images, Feierbach et al found that PRV capsids are associated with filamentous structures in the nuclei of infected neurons and epithelial cells [33]. Similar filaments associate with HSV-1 and HSV-2 nucleocapsids in infected central and peripheral neurons and glial cell nuclei [34].…”

“…The function of nuclear actin during infection is currently unknown. The formation of higher order capsid assemblies is dependent on actin [32,33], but the addition of actin-depolymerizing drugs does not affect viral titers [35]. These seemingly conflicting results might be owing to differences in cell type or might simply reflect that higher order capsid structures are not essential for efficient viral assembly.…”

“…Do nuclear actin filaments assist in intranuclear transport of herpes capsids (Figure 2)? Although a mechanism for capsid movement in the nucleus is still unknown, PRV capsids co-localize with the actin motor myosin Va in the nucleus [33]. Class V myosins are among the most thoroughly studied forms of unconventional myosins and considerable evidence supports their role in the transport of organelles and vesicles [36].…”