Rho guanine nucleotide exchange factor xNET1 implicated in gastrulation movements during Xenopus development

During Xenopus development, embryonic cells dramatically change their shape and position. Rho family small GTPases, such as RhoA, Rac, and Cdc42, play important roles in this process. These GTPases are generally activated by guanine nucleotide exchange factors (GEFs); however, the roles of RhoGEFs in Xenopus development have not yet been elucidated. We therefore searched for RhoGEF genes in our Xenopus EST database, and we identified several genes expressed during embryogenesis. Among them, we focused on one gene, designated xNET1. It is similar to mammalian NET1, a RhoA-specific GEF. An in vitro binding assay revealed that xNET1 bound to RhoA, but not to Rac or Cdc42. In addition, transient expression of xNET1 activated endogenous RhoA. These results indicated that xNET1 is a GEF for RhoA. Epitope-tagged xNET1 was localized mainly to the nucleus, and the localization was regulated by nuclear localization signals in the N-terminal region of xNET1. Overexpression of either wild-type or a mutant form of xNET1 severely inhibited gastrulation movements. We demonstrated that xNET1 was co-immunoprecipitated with the Dishevelled protein, which is an essential signaling component in the non-canonical Wnt pathway. This pathway has been shown to activate RhoA and regulate gastrulation movements. We propose that xNET1 or a similar RhoGEF may mediate Dishevelled signaling to RhoA in the Wnt pathway.