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. Unexpectedly, cortactin-deficient cells showed little changes in overall cell morphology and growth. Ultrastructural analyses and live-cell imaging studies revealed unimpaired lamellipodial architecture, Rac-induced protrusion, and actin network turnover, although actin assembly rates in the lamellipodium were modestly increased. In contrast, platelet-derived growth factor-induced actin reorganization and Rac activation were impaired in cortactin null cells. In addition, cortactin deficiency caused reduction of Cdc42 activity and defects in random and directed cell migration. Reduced migration of cortactin null cells could be restored, at least in part, by active Rac and Cdc42 variants. Finally, cortactin removal did not affect the efficiency of receptor-mediated endocytosis. Together, we conclude that cortactin is fully dispensable for Arp2/3-complex activation during lamellipodia protrusion or clathrin pit endocytosis. Furthermore, we propose that cortactin promotes cell migration indirectly, through contributing to activation of selected Rho-GTPases. INTRODUCTIONCell migration is a complex process requiring the coordinated activities of multiple cellular machines, driving actin polymerization, actin-myosin II-based force generation, and coupling to the extracellular matrix. However, the relative contribution of each of these machines to the different steps in the motility cycle is just beginning to emerge. Irrespective of the complexity of coordination of these activities, it is commonly agreed that protrusion at the cell front is initiated by localized actin polymerization, to form structures such as lamellipodia or ruffles (Small et al., 2002;Pollard and Borisy, 2003).The best characterized factors driving the nucleation of actin filaments in vertebrate cells are the Arp2/3-complex and formins (Pollard, 2007). Arp2/3-complex activity is considered essential for processes as diverse as lamellipodium protrusion, actin assembly during clathrin-mediated endocytosis, podosome formation, and different types of hostpathogen interaction (Goley and Welch, 2006;Linder, 2007).Activators of Arp2/3-complex are termed nucleation promoting factors (NPFs) and are roughly subdivided into class I and II composed of classical members Wiskott-Aldrich syndrome protein (WASP)/WASP family Verprolin-homologous protein (WAVE) proteins and cortactin, respectively (Welch and Mullins, 2002). This distinction is derived, at least in part, from their mode of interaction with actin: the WH2 domains of class I and tande...

Cortactin is required for endothelial barrier function and leukocyte recruitment in vivo.

In osteoclasts (OCs) podosomes are organized in a belt, a feature critical for bone resorption. Although microtubules (MTs) promote the formation and stability of the belt, the MT and/or podosome molecules that mediate the interaction of the two systems are not identified. Because the growing "plus" ends of MTs point toward the podosome belt, plus-end tracking proteins (؉TIPs) might regulate podosome patterning. Among the ؉TIPs, EB1 increased as OCs matured and was enriched in the podosome belt, and EB1-positive MTs targeted podosomes. Suppression of MT dynamic instability, displacement of EB1 from MT ends, or EB1 depletion resulted in the loss of the podosome belt. We identified cortactin as an Src-dependent interacting partner of EB1. Cortactin-deficient OCs presented a defective MT targeting to, and patterning of, podosomes and reduced bone resorption. Suppression of MT dynamic instability or EB1 depletion increased cortactin phosphorylation, decreasing its acetylation and affecting its interaction with EB1. Thus, dynamic MTs and podosomes interact to control bone resorption.