Cdc42 and Rac family GTPases regulate mode and speed but not direction of primary fibroblast migration during platelet-derived growth factor-dependent chemotaxis

Cdc42 and Rac family GTPases are important regulators of morphology, motility, and polarity in a variety of mammalian cell types. However, comprehensive analysis of their roles in the morphological and behavioral aspects of chemotaxis within a single experimental system is still lacking. Here we demonstrate using a direct viewing chemotaxis assay that of all of the Cdc42/Rac1-related GTPases expressed in primary fibroblasts, Cdc42, Rac1, and RhoG are required for efficient migration towards platelet-derived growth factor (PDGF). During migration, Cdc42-, Rac1-, and RhoG-deficient cells show aberrant morphology characterized as cell elongation and cell body rounding, loss of lamellipodia, and formation of thick membrane extensions, respectively. Analysis of individual cell trajectories reveals that cell speed is significantly reduced, as well as persistence, but to a smaller degree, while the directional response to the gradient of PDGF is not affected. Combined knockdown of Cdc42, Rac1, and RhoG results in greater inhibition of cell speed than when each protein is knocked down alone, but the cells are still capable of migrating toward PDGF. We conclude that, Cdc42, Rac1, and RhoG function cooperatively during cell migration and that, while each GTPase is implicated in the control of morphology and cell speed, these and other Cdc42/Rac-related GTPases are not essential for the directional response toward PDGF.