Cdc42 GTPase and Rac1 GTPase act downstream of p120 catenin and require GTP exchange during gastrulation of zebrafish mesoderm

We investigated the roles of p120 catenin, Cdc42, Rac1, and RhoA GTPases in regulating migration of presomitic mesoderm cells in zebrafish embryos. p120 catenin has dual roles: It binds the intracellular and juxtamembrane region of cadherins to stabilize cadherin-mediated adhesion with the aid of RhoA GTPase, and it activates Cdc42 GTPase and Rac1 GTPase in the cytosol to initiate cell motility. During gastrulation of zebrafish embryos, knockdown of the synthesis of zygotic p120 catenin?δ1 mRNAs with a splice-site morpholino caused lateral widening and anterior-posterior shortening of the presomitic mesoderm and somites and a shortened anterior-posterior axis: these phenotypes indicate a cell-migration defect. Co-injection of low amounts of wild-type Cdc42 or wild-type Rac1 or dominant-negative RhoA mRNAs, but not constitutively-active Cdc42 mRNA, rescued these p120 catenin δ1-depleted embryos. These downstream small GTPases require appropriate spatiotemporal stimulation or cycling of GTP to guide mesodermal cell migration. A delicate balance of Rho GTPases and p120 catenin underlies normal development.