Reorganization of the actin cytoskeleton in the protruding lamellae of human fibroblasts.

To investigate the mechanisms of protrusion in vertebrate cells, the primary event in cell motility, human fibroblasts were treated with neomycin, an inhibitor of the phosphatidylinositol cycle, to induce protrusion. Changes in cell motility and the cytoskeleton were examined by video, fluorescence, scanning electron, and confocal microscopy and by cytofluorometry. Protrusion in neomycin-treated human fibroblasts is correlated with a transient overall decrease in F-actin followed by an increase in F-actin at the leading edge of the protruding lamella. In growing lamellae, F-actin is organized in a marginal band at the leading edge. Although actin is present in the lamella behind the leading edge, very little of it is F-actin. Scanning electron microscopy of detergent-extracted cells reveals a band of dense filaments at the leading edge, corresponding to the marginal band of F-actin seen in fluorescently labeled cells, and a sparse population of short, fragmented filaments, in the rest of the lamella. Gelsolin is colocalized with F-actin in the marginal band and is also present in the lamella where F-actin is largely absent. The data support the hypothesis that the protrusion is initiated by the breakdown of cortical actin filaments, possibly mediated by gelsolin, whereas expansion of the protrusion requires de novo polymerization of actin filaments at the leading edge. Copyright 2001 Wiley-Liss, Inc.