Herpes simplex viruses (HSV) can spread very efficiently directly from one cell to a neighboring cell, in epithelial and neuronal tissues. The property appears to involve transport of virus particles across cell junctions or contacts made between cells. Viral glycoproteins gE and gI are necessary for efficient spread of HSV between cells but are not required for entry of extracellular virus. Therefore, HSV gE and gI and homologues in other herpesviruses appear to selectively act in cell-to- cell spread and not at the cell surface during entry of extracellular viruses. This the only example of proteins which function in this manner. gE/gI has also been ascribed an IgG-Fc receptor activity, but this property appears to be unrelated to the capacity to facilitate cell-to-cell transmission. We have found that gE/gI localizes specifically to the lateal surfaces of epithelial cells, colocalizing with adherens junctions and being excluded from tight junctions, and is not present on those lateral surfaces of cells not in contact with other cells. This suggests that gE/gI has the capacity to bind to cell junction components so that accumulation at cell junctions occurs. It is my hypothesis that gE/gI binds to cellular ligands concentrated at cell junctions to promote transfer of virus between cells. The objectives of this research are to examine the molecular mechanisms by which HSV spreads from cell-to-cell. We will study the relationship between cell-cell spread and accumulation of gE/gI at cell junctions, whether gE/gI expressed in cells can saturate cell ligands and block cell-cell spread, the role of various cell junction components and cell adhesion molecules (CAMs) in this process, and attempt to identify cellular ligands of gE/gI. These studies should shed light not only on how this virus interacts with cell junctions but also provide valuable new information on the cell biology of cell junctions. The integrity of cell junctions is important to maintain a permeability barrier and also so that the normal course of differentiation and growth regulation are maintained. Loss of CAMs or CAM adaptors occurs during progression to a more malignant phenotype. CAMs or CAM adaptors, e.g. APC, DCC, and cadherins, are known to act as tumor suppressors and their loss leads to a less differentiated and more motile or metastatic phenotype. I expect that our studies of the interactions between HSV and cell junctions will have significance, providing new information on the relationship between specific CAMs and virus spread and a better understanding of how CAMs function in cells and tissues.
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