Cortactin promotes migration and platelet-derived growth factor-induced actin reorganization by signaling to Rho-GTPases.

Dynamic actin rearrangements are initiated and maintained by actin filament nucleators, including the Arp2/3-complex. This protein assembly is activated in vitro by distinct nucleation-promoting factors such as Wiskott-Aldrich syndrome protein/Scar family proteins or cortactin, but the relative in vivo functions of each of them remain controversial. Here, we report the conditional genetic disruption of murine cortactin, implicated previously in dynamic actin reorganizations driving lamellipodium protrusion and endocytosis.

Correlated light and electron microscopy of the cytoskeleton.

The cytoskeleton of cultured cells can be most easily visualized in the electron microscope by simultaneous extraction and fixation with Triton-glutaraldehyde mixtures, followed by negative staining. Actin filaments are better preserved by stabilization with phalloidin, either during or after the primary fixation step. A technique is described for the combination of this procedure with live cell microscopy.

Building the actin cytoskeleton: filopodia contribute to the construction of contractile bundles in the lamella.

Filopodia are rodlike extensions generally attributed with a guidance role in cell migration. We now show in fish fibroblasts that filopodia play a major role in generating contractile bundles in the lamella region behind the migrating front. Filopodia that developed adhesion to the substrate via paxillin containing focal complexes contributed their proximal part to stress fiber assembly, and filopodia that folded laterally contributed to the construction of contractile bundles parallel to the cell edge.

Differentially oriented populations of actin filaments generated in lamellipodia collaborate in pushing and pausing at the cell front.

Eukaryotic cells advance in phases of protrusion, pause and withdrawal. Protrusion occurs in lamellipodia, which are composed of diagonal networks of actin filaments, and withdrawal terminates with the formation of actin bundles parallel to the cell edge. Using correlated live-cell imaging and electron microscopy, we have shown that actin filaments in protruding lamellipodia subtend angles from 15-90 degrees to the front, and that transitions from protrusion to pause are associated with a proportional increase in filaments oriented more parallel to the cell edge.