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.