Immunocytochemical studies of the interactions of cadherins and catenins in the early Xenopus embryo

Linkage of cadherins to the cytoskeleton is crucial for their adhesive func tion. Since alpha- and beta-catenin play a key role in this linkage, these proteins are possible targets for processes that control cell-cell adhesion . To achieve a better understanding of the regulation of cell-cell adhesion in embryonic morphogenesis, we used immunohistology to investigate how in Xenopus blastomeres catenins respond to disturbances in the expression of m aternal cadherins, Overexpression of myc-tagged maternal cadherin leads to a proportionate increase of the level of beta-catenin. The two proteins col ocalize in the endoplasmic reticulum, in cytoplasmic vesicles, and along th e cell membrane, indicating that the beta-catenin binds to overexpressed ca dherin early in its passage to the plasma membrane. Expression of cadherin is essential for the stable presence of beta-catenin, as depletion from mat ernal cadherin mRNA leads to a complete loss of beta-catenin from the blast omeres. alpha-catenin behaves differently Overexpression of cadherin leaves the amount and localization of alpha-catenin largely unaffected, and addit ional cadherin inserts itself into the membrane without a proportionate ris e in the level of membrane-bound alpha-catenin, However, cadherin mRNA depl etion leads to a redistribution of alpha-catenin from the membrane to the c ytoplasm, Thus, cadherin is required to localize alpha-catenin to the membr ane, but the amount of alpha-catenin along the membrane seems to be restric ted to a certain level which cannot be exceeded. The relevance of these obs ervations for the regulation of cadherin-mediated cell adhesion in the Xeno pus embryo is discussed. Additionally, we demonstrate that plakoglobin, lik e beta-catenin an armadillo repeat protein, shows neither accumulation afte r overexpression nor colocalization with the overexpressed cadherin, Dev Dy n 1999;215:155-169. (C) 1999 Wiley-Liss, Inc.