Phospholipase A2 activity during the replication cycle of the flavivirus West Nile virus.

Positive-sense RNA virus intracellular replication is intimately associated with membrane platforms that are derived from host organelles and comprised of distinct lipid composition. For flaviviruses, such as West Nile virus strain Kunjin virus (WNVKUN) we have observed that these membrane platforms are derived from the endoplasmic reticulum and are rich in (at least) cholesterol. To extend these studies and identify the cellular lipids critical for WNVKUN replication we utilized a whole cell lipidomics approach and revealed an elevation in phospholipase A2 (PLA2) activity to produce lyso-phosphatidylcholine (lyso-PChol). We observed that the PLA2 enzyme family is activated in WNVKUN-infected cells and the generated lyso-PChol lipid moieties are sequestered to the subcellular sites of viral replication. The requirement for lyso-PChol was confirmed using chemical inhibition of PLA2, where WNVKUN replication and production of infectious virus was duly affected in the presence of the inhibitors. Importantly, we could rescue chemical-induced inhibition with the exogenous addition of lyso-PChol species. Additionally, electron microscopy results indicate that lyso-PChol appears to contribute to the formation of the WNVKUN membranous replication complex (RC); particularly affecting the morphology and membrane curvature of vesicles comprising the RC. These results extend our current understanding of how flaviviruses manipulate lipid homeostasis to favour their own intracellular replication.

A Putative Lipid-Associating Motif in the West Nile Virus NS4A Protein Is Required for Efficient Virus Replication.

Flavivirus replication is intimately associated with re-organized cellular membranes. These virus-induced changes in membrane architecture form three distinct membranous "organelles" that have specific functions during the flavivirus life cycle. One of these structures is the replication complex in which the flaviviral RNA is replicated to produce progeny genomes. We have previously observed that this process is strictly dependent on cellular cholesterol. In this study we have identified a putative cholesterol recognition/interaction amino acid consensus (CRAC) motif within the West Nile virus strain Kunjin virus (WNVKUN) NS4A protein. Site-directed mutagenesis of this motif within a WNVKUN infectious clone severely attenuated virus replication and the capacity of the mutant viruses to form the replication complex. Replication of the mutant viruses also displayed reduced co-localization with cellular markers recruited to replication sites during wild-type virus replication. In addition, we observed that the mutant viruses were significantly impaired in their ability to remodel cytoplasmic membranes. However, after extensive analysis we are unable to conclusively reveal a role for the CRAC motif in direct cholesterol binding to NS4A, suggesting additional complex lipid-protein and protein-protein interactions. We believe this study highlights the crucial role for this region within NS4A protein in recruitment of cellular and viral proteins to specialized subdomains on membrane platforms to promote efficient virus replication.

The Host Protein Reticulon 3.1A Is Utilized by Flaviviruses to Facilitate Membrane Remodelling.

Flaviviruses are enveloped, positive-sensed single-stranded RNA viruses that remodel host membranes, incorporating both viral and host factors facilitating viral replication. In this study, we identified a key role for the membrane-bending host protein Reticulon 3.1 (RTN3.1A) during the replication cycle of three flaviviruses: West Nile virus (WNV), Dengue virus (DENV), and Zika virus (ZIKV). We observed that, during infection, RTN3.1A is redistributed and recruited to the viral replication complex, a recruitment facilitated via the WNV NS4A protein, however, not DENV or ZIKV NS4A. Critically, small interfering RNA (siRNA)-mediated knockdown of RTN3.1A expression attenuated WNV, DENV, and ZIKV replication and severely affected the stability and abundance of the NS4A protein, coinciding with a significant alternation and reduction of viral membrane structures in the endoplasmic reticulum. These observations identified a crucial role of RTN3.1A for the viral remodelling of host membranes during efficient flavivirus replication and the stabilization of viral proteins within the endoplasmic reticulum.

Potent cationic inhibitors of West Nile virus NS2B/NS3 protease with serum stability, cell permeability and antiviral activity.

West Nile virus (WNV) has spread rapidly around the globe, efficiently crossing species from migrating birds into humans and other mammals. The viral protease NS2B-NS3 is important for WNV replication and recognizes dibasic substrate sequences common to other flaviviral proteases but different from most mammalian proteases. Potent inhibitors of WNV protease with antiviral activity have been elusive to date. We report the smallest and most potent inhibitors known for this enzyme, cationic tripeptides with nonpeptidic caps at the N-terminus and aldehyde at the C-terminus. One of these, compound 3 ( Ki = 9 nM) is stable in serum (>90% intact after 3 h, 37 degrees C), cell permeable, and shows antiviral activity (IC 50 1.6 microM) without cytotoxicity (IC 50 >400 microM), thereby validating the approach of inhibiting WNV protease to suppress WNV replication.