The US3 Protein of Pseudorabies Virus Drives Viral Passage across the Basement Membrane in Porcine Respiratory Mucosa Explants.

Passage of the basement membrane (BM), which forms a barrier between the epithelium and the underlying lamina propria, represents an important step in the early pathogenesis of different alphaherpesviruses. Rho GTPase signaling plays an important role in transmigration of cells across the BM during physiological and pathological processes. We reported earlier that the US3 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) interferes with Rho GTPase signaling and causes a reorganization of the host cell cytoskeleton, which as a consequence, enhances viral cell-to-cell spread in epithelial cell cultures. Here, using an ex vivo system of porcine nasal respiratory mucosa explants that allows to study PRV invasion through the BM, we found that a PRV strain that lacks US3 expression (ΔUS3 PRV) showed a reduced spread in mucosal epithelium and was virtually unable to breach the BM, in contrast to isogenic wild-type (WT) or US3 rescue PRV strains. Interestingly, addition of IPA3, an inhibitor of p21-activated kinases that blocks the effects of US3 on the cytoskeleton, suppressed the ability of WT PRV to spread across the BM. In addition, artificial suppression of RhoA signaling using CPC3 (cell-permeable C3 transferase) to mimic the effects of US3 on Rho GTPase signaling, significantly increased passage of ΔUS3 PRV through the BM, whereas it did not significantly affect BM passage of WT or US3 rescue PRV. In conclusion, these data indicate that US3 plays an important role in PRV mucosal invasion across the BM, which involves its interference with Rho GTPase signaling. This is the first report describing an alphaherpesvirus protein that drives viral BM passage. IMPORTANCE: Many viruses, including alphaherpesviruses, primarily replicate in epithelial cells of surface mucosae, such as the respiratory mucosa. Some of these viruses breach the basement membrane underlying these epithelial cells to reach underlying connective tissue and blood vessels and invade the host. Hence, epithelial spread and basement membrane passage represent crucial but still poorly understood early steps in (alphaherpes)virus pathogenesis. Here, using ex vivo porcine respiratory mucosa explants, we show that the conserved US3 protein of the porcine alphaherpesvirus pseudorabies virus (PRV) is critical for passage of PRV across the basement membrane and contributes to efficient viral epithelial spread. In addition, we show that US3-mediated viral epithelial spread and passage across the basement membrane depend at least in part on the ability of this viral protein to modulate cellular Rho GTPase signaling. This is the first report that identifies an alphaherpesvirus protein that drives viral basement membrane passage.