REGULATION OF C-CADHERIN FUNCTION DURING ACTIVIN INDUCED MORPHOGENESIS OF XENOPUS ANIMAL CAPS

Treatment of Xenopus animal pole tissue with activin results in the in duction of mesodermal cell types and a dramatic elongation of the tiss ue. The morphogenetic movements involved in the elongation appear simi lar to those in normal gastrulation, which is driven by cell rearrange ment and cell intercalations. We have used this system to explore the potential regulation of cell-cell adhesion and cadherin function durin g morphogenesis. Quantitative blastomere aggregation assays revealed t hat activininduction reduced the calcium-dependent adhesion between bl astomeres. Activin-induced blastomeres formed smaller aggregates, and a greater proportion of the population remained as single cells compar ed to uninduced blastomeres. The aggregation was mediated by C-cadheri n because C-cadherin was present in the blastomeres during the aggrega tion assay, and monoclonal antibodies against C-cadherin inhibited the calcium-dependent aggregation of blastomeres. E-cadherin was not dete ctable until after the completion of the assay and, therefore, does no t explain the adhesive differences between induced and uninduced blast omeres. L cells stably expressing C-cadherin (LC cells) were used to d emonstrate that C-cadherin activity was specifically altered after act ivin induction. Blastomeres induced with activin bound fewer LC cells than uninduced blastomeres. L cells not expressing C-cadherin did not adhere to blastomeres. The changes in C-cadherin-mediated adhesion occ urred without detectable changes in the steady-state levels of C-cadhe rin or the amount of C-cadherin present on the surface of the cell. Im munoprecipitation of C-cadherin and its associated catenins revealed t hat the ratio of C-cadherin and the catenins was not altered by activi n induction. These results demonstrate that activin decreases the adhe sive function of existing C-cadherin molecules on the surface of blast omeres and suggest that decreased cadherin mediated cell-cell adhesion is associated with increased morphogenetic movement.