Involvement of the small GTPases XRhoA and XRnd1 in cell adhesion and headformation in early Xenopus development

The Rho family of small GTPases regulates a variety of cellular functions, including the dynamics of the actin cytoskeleton, cell adhesion, transcript ion, cell growth and membrane trafficking. We have isolated the first Xenop us homologs of the Rho-like GTPases RhoA and Rnd1 and examined their potent ial roles in early Xenopus development. We found that Xenopus Rnd1 (XRnd1) is expressed in tissues undergoing extensive morphogenetic changes, such as marginal zone cells involuting through the blastopore, somitogenic mesoder m during somite formation and neural crest cells. XRnd1 also causes a sever e loss of cell adhesion in overexpression experiments, These data and the e xpression pattern suggest that XRnd1 regulates morphogenetic movements by m odulating cell adhesion in early embryos. Xenopus RhoA (XRhoA) is a potenti al XRnd1 antagonist, since overexpression of XRhoA increases cell adhesion in the embryo and reverses the disruption of cell adhesion caused by XRnd1. In addition to the potential roles of XRnd1 and SRhoA in the regulation of cell adhesion, we find a role for XRhoA in axis formation. When coinjected with dominant-negative BMP receptor (tBR) in the ventral side of the embry o, XRhoA causes the formation of head structures resembling the phenotype s een after coinjection of wnt inhibitors with dominant-negative BMP receptor , Since dominant-negative XRhoA is able to reduce the formation of head str uctures, we propose that XRhoA activity is essential for head formation. Th us, XRhoA may have a dual role in the embryo by regulating cell adhesion pr operties and pattern formation.