Interaction of West Nile and Kunjin viruses with cellular components during morphogenesis

Remodeling of the Actin Network Associated with the Non-Structural Protein 1 (NS1) of West Nile Virus and Formation of NS1-Containing Tunneling Nanotubes

The cellular response to the recombinant NS1 protein of West Nile virus (NS1^WNV) was studied using three different cell types: Vero E6 simian epithelial cells, SH-SY5Y human neuroblastoma cells, and U-87MG human astrocytoma cells. Cells were exposed to two different forms of NS1^WNV: (i) the exogenous secreted form, sNS1^WNV, added to the extracellular milieu; and (ii) the endogenous NS1^WNV, the intracellular form expressed in plasmid-transfected cells. The cell attachment and uptake of sNS1^WNV varied with the cell type and were only detectable in Vero E6 and SH-SY5Y cells. Addition of sNS1^WNV to the cell culture medium resulted in significant remodeling of the actin filament network in Vero E6 cells. This effect was not observed in SH-SY5Y and U-87MG cells, implying that the cellular uptake of sNS1^WNV and actin network remodeling were dependent on cell type. In the three cell types, NS1^WNV-expressing cells formed filamentous projections reminiscent of tunneling nanotubes (TNTs). These TNT-like projections were found to contain actin and NS1^WNV proteins. Interestingly, similar actin-rich, TNT-like filaments containing NS1^WNV and the viral envelope glycoprotein E^WNV were also observed in WNV-infected Vero E6 cells.

Viral and cellular determinants of West Nile virus entry and morphogenesis

The re-emergence of the Old World flavivirus – West Nile in the Western hemisphere – has spurred intense research to decipher the host-cellular and viral determinants in contributing to West Nile virus pathogenesis. The increasing understanding of the complex interactions between West Nile virus and host cells will definitely help to accelerate the development of clinically effective antiviral therapies and a vaccine. In this review article, we present a perspective on the recent advances in revealing how the host-cellular factors are engaged during the entry, morphogenesis and assembly of West Nile virus.

The Src family kinase c-Yes is required for maturation of West Nile virus particles

The role of cellular genes in West Nile virus (WNV) replication is not well understood. Examination of cellular transcripts upregulated during WNV infection revealed an increase in the expression of the src family kinase (SFK) c-Yes. WNV-infected cell lines treated with the SFK inhibitor PP2 demonstrated a 2- to 4-log decrease in viral titers, suggesting that SFK activity is required for completion of the viral replication cycle. RNA interference mediated knock-down of c-Yes, but not c-Src, and similarly reduced virus yield, specifically implicating c-Yes in WNV production. Interestingly, PP2 treatment did not reduce intracellular levels of either viral RNA or protein, suggesting that the drug does not act on the early stages of replication. However, endoglycosidase H (endoH) digestion of the viral envelope (E) glycoprotein revealed that the acquisition of endoH-resistant glycans by E, but not endogenous major histocompatibility complex class I, was reduced in PP2-treated cells, demonstrating that E specifically does not traffic beyond the endoplasmic reticulum in the absence of SFK activity. Electron microscopy further revealed that PP2-treated WNV-infected cells accumulated an increased number of virions in the ER compared to untreated cells. Therefore, we conclude that inhibition of SFK activity did not interfere with virus assembly but prevented transit of virions through the secretory pathway. These results identify c-Yes as a cellular protein that is involved in WNV assembly and egress.

Infectious entry of West Nile virus occurs through a clathrin-mediated endocytic pathway

The pathway of West Nile flavivirus early internalization events was mapped in detail in this study. Overexpression of dominant-negative mutants of Eps15 strongly inhibits West Nile virus (WNV) internalization, and pharmacological drugs that blocks clathrin also caused a marked reduction in virus entry but not caveola-dependent endocytosis inhibitory agent, filipin. Using immunocryoelectron microscopy, WNV particles were seen within clathrin-coated pits after 2 min postinfection. Double-labeling immunofluorescence assays and immunoelectron microscopy performed with anti-WNV envelope or capsid proteins and cellular markers (EEA1 and LAMP1) revealed the trafficking pathway of internalized virus particles from early endosomes to lysosomes and finally the uncoating of the virus particles. Disruption of host cell cytoskeleton (actin filaments and microtubules) with cytochalasin D and nocodazole showed significant reduction in virus infectivity. Actin filaments are shown to be essential during the initial penetration of the virus across the plasma membrane, whereas microtubules are involved in the trafficking of internalized virus from early endosomes to lysosomes for uncoating. Cells treated with lysosomotropic agents were largely resistant to infection, indicating that a low-pH-dependent step is required for WNV infection. In situ hybridization of DNA probes specific for viral RNA demonstrated the trafficking of uncoated viral RNA genomes to the endoplasmic reticulum.