Control of the direction of lamellipodia extension through changes in the balance between Rac and Rho activities.

The direction in which cells extend new motile processes, such as lamellipodia and filopodia, can be controlled by altering the geometry of extracellular matrix adhesive islands on which individual cells are cultured, thereby altering mechanical interactions between cells and the adhesive substrate [Parker (2002)]. Here we specifically investigate the intracellular molecular signals that mediate the mechanism by which cells selectively extend these processes from the corners of polygonal-shaped adhesive islands. Constitutive activation of the small GTPase Rac within cells cultured on square-shaped islands of fibronectin resulted in the elimination of preferential extension from corners. This loss of directionality was accompanied by a re-distribution of focal adhesions: the large focal adhesions normally found within the corner regions of square cells were lost and replaced by many smaller focal contacts that were distributed along the entire cell perimeter. Inhibition of the small GTPase, Rho, using C3 exoenzyme blocked lamellipodia extension entirely. However, inhibition of Rho signaling in combination with ectopic Rac activation rescued the corner localization of motile processes and focal adhesions. These results suggest that the ability of cells to sense their physical surroundings and respond by moving in a spatially oriented manner is mediated by a balance between Rho and Rac activities.