A constitutively activated mutant of galphaq down-regulates EP-cadherin expression and decreases adhesion between ectodermal cells at gastrulation.

We have examined the expression and function of the heterotrimeric GTP-binding protein Gq during early Xenopus embryogenesis. Abundant XGalphaq transcripts were detected in oocytes and early embryos by Northern blot analysis. In situ hybridization revealed that these transcripts are confined to the animal hemisphere of the mature oocyte and to the presumptive ectoderm of cleaving embryos. Microinjection at the two-cell stage of alphaq and Q209Lalphaq, a constitutively activated mutant, causes a disruption in ectodermal cell adhesion at late gastrulation. Dissociation/reaggregation experiments performed on animal cap explants clearly demonstrate that the Q209Lalphaq-induced phenotype occurs after reaggregation of the explants with a time-course similar to that observed in whole embryos. RT-PCR experiments performed on the explants from Q209Lalphaq-injected embryos revealed a selective decrease in the amount of EP-cadherin mRNA. Co-injection of EP-cadherin RNA, but also E-cadherin RNA, rescued the disaggregated phenotype. These data emphasize the functional link between Gq protein-coupled signalling pathways and cadherin molecules in the ectodermal layer during the morphogenetic movements of gastrulation.