Gα _12/i3 regulate epiboly by inhibiting E-cadherin activity and modulating the actin cytoskeleton

Epiboly spreads and thins the blastoderm over the yolk cell during zebrafish gastrulation, and involves coordinated movements of several cell layers. Although recent studies have begun to elucidate the processes that underlie these epibolic movements, the cellular and molecular mechanisms involved remain to be fully defined. Here, we show that gastrulae with altered Gα _12/13 signaling display delayed epibolic movement of the deep cells, abnormal movement of dorsal forerunner cells, and dissociation of cells from the blastoderm, phenocopying e-cadherin mutants. Biochemical and genetic studies indicate that Gα _12/13 regulate epi- boly, in part by associating with the cytoplasmic terminus of E-cadherin, and thereby inhibiting E-cadherin activity and cell adhesion. Furthermore, we demonstrate that Gα _12/13 modulate epibolic movements of the enveloping layer by regulating actin cytoskeleton organization through a RhoGEF/Rho-dependent pathway. These results provide the first in vivo evidence that Gα _12/13 regulate epiboly through two distinct mechanisms: limiting E-cadherin activity and modulating the organization of the actin cytoskeleton.