DOMINANT-NEGATIVE EXPRESSION OF A CYTOPLASMICALLY DELETED MUTANT OF XB U-CADHERIN DISTURBS MESODERM MIGRATION DURING GASTRULATION IN XENOPUS-LAEVIS/

XB/U-cadherin is a maternal Xenopus cadherin which mediates interblast omere adhesion in early embryogenesis. In order to explore its role in gastrulation, we expressed a cytoplasmic deletion mutant of XB/U-cadh erin (XB Delta c38) under the control of the CMV promoter in Xenopus e mbryos. This truncated XB-cadherin fails to form complexes with cateni ns and does not mediate cell-cell aggregation as shown by transfection of mouse Ltk(-) cells. Injections of the deletion for XB/U-cadherin i nto the dorsal-marginal region of four cell stage embryos resulted in a dominant negative expression of the cadherin mutant after MET. Two d ifferent phenotypes were observed in a dose dependent manner: high dos es (125-250 pg DNA) led to severe distortions of the gastrulation move ment. Involution of the mesoderm was impaired, posterior mesoderm migr ated laterally around the blastopore and formed two bands of axial tis sue. Low doses (up to 50 pg DNA) resulted in embryos of a posteriorize d phenotype with disorganized neural structures. Both phenotypes could be rescued by coinjection of cDNA constructs containing wild-type XB/ U-cadherin. Injections of constructs encoding a XB/U-cadherin protein truncated both in its extracellular and cytoplasmic domains yielded no rmal phenotypes. These results suggest that a proper function of XB/U- cadherin is essential for mesoderm movements during gastrulation.