Vesicle accumulation and exocytosis at sites of plasma membrane disruption.

Plasma membrane disruptions are resealed by an active molecular mechanism thought to be composed, in part, of kinesin, CaM kinase, snap-25, and synaptobrevin. We have used HRP to mark the cytoplasmic site of a mechanically induced plasma membrane disruption. Transmission electron microscopy revealed that vesicles of a variety of sizes rapidly (s) accumulate in large numbers within the cytoplasm surrounding the disruption site and that microvilli-like surface projections overlie this region. Scanning electron microscopy confirmed that tufts of microvilli rapidly appear on wounded cells. Three assays, employing the membrane specific dye FM1-43, provide quantitative evidence that disruption induces Ca(2+)-dependent exocytosis involving one or more of the endosomal/lysosomal compartments. Confocal microscopy revealed the presence in wounded cells of cortical domains that were strikingly depleted of FM dye fluorescence, suggesting that a local bolus of exocytosis is induced by wounding rather than global exocytosis. Finally, flow cytometry recorded a disruption-induced increase in cell forward scatter, suggesting that cell size increases after injury. These results provide the first direct support for the hypothesis that one or more internal membrane compartments accumulate at the disruption site and fuse there with the plasma membrane, resulting in the local addition of membrane to the surface of the mechanically wounded cell.