Rac function is critical for cell migration but not required for spreading and focal adhesion formation

Abstract

Cell migration is commonly accompanied by protrusion of membrane ruffles and lamellipodia. In 2D-migration, protrusion of these thin sheets of cytoplasm is considered both relevant to exploration of new space and initiation of nascent adhesion to the substratum. Lamellipodia formation can be potently stimulated by Rho GTPases of the Rac subfamily, but also by RhoG or Cdc42. Here we describe viable fibroblast cell lines genetically deficient for Rac1 that lack detectable levels of Rac2 and -3. Rac-deficient cells were devoid of apparent lamellipodia, which could be restored by expression of either Rac subfamily member, but not by Cdc42 or RhoG. Cells deficient in Rac showed strong reduction in wound closure and random cell migration and a notable loss of sensitivity to a chemotactic gradient. Despite these defects, Rac deficient cells were able to spread, formed filopodia and established focal adhesions. Spreading in these cells was achieved by the extension of filopodia followed by the advancement of cytoplasmic veils between them. The number and size of focal adhesions as well as their intensity were mostly unaffected by genetic removal of Rac1. However, Rac deficiency increased the mobility of different components in focal adhesions, potentially explaining how Rac – although not essential - can contribute to focal adhesion assembly. Together, our data demonstrate that Rac signalling is essential for lamellipodia protrusion and for efficient cell migration, but not for spreading or filopodia formation. Our findings also suggest that Rac GTPases are crucial to the establishment or maintenance of polarity in chemotactic migration.