Dynamic Behavior of the Cadherin-Based Cell–Cell Adhesion System duringDrosophilaGastrulation

Abstract

DuringDrosophilagastrulation, morphogenesis occurs as a series of cell shape changes and cell movements which probably involve adhesive interactions between cells. In the present study, we examined the dynamic aspects of cadherin-based cell–cell adhesion in the morphogenetic events to assess its contribution to morphogenesis.DE- andDN-cadherin show complementary expression patterns in the presumptive ectoderm and mesoderm at the mRNA level. We found that switching of cadherin expression from theDE- to theDN-type in the mesodermal germ layer occurred downstream of the mesoderm-determination genestwistandsnail.However, examination of their protein expression patterns showed that considerable amounts ofDE-cadherin remained on the surfaces of mesodermal cells during invagination, whileDN-cadherin did not appear on the cell surfaces at this stage. Further immunocytochemical analysis of the localizations ofDE-cadherin and its associated proteins Armadillo (β-catenin) andDα-catenin revealed dynamic changes in their distributions which were accompanied by changes in cell morphology in the neuroectoderm and mesoderm. Simultaneously, adherens junctions (AJs), based on the cadherin–catenin system, were shown to change their location, size, and morphology. These dynamic aspects of cadherin-based cell–cell adhesion appeared to be associated with the following: (1) initial establishment of the blastoderm epithelium, (2) acquisition of cell motility in the neuroectoderm, (3) cell sheet folding, and (4) epithelial to mesenchymal conversion of the mesoderm. These observations suggest that the behavior of theDE-cadherin–catenin adhesion system may be regulated in a stepwise manner during gastrulation to perform successive cell-morphology conversions. Moreover, the processes responsible for loss of epithelial cell polarity and elimination of preexistingDE-cadherin-based epithelial junctions during early mesodermal morphogenesis are discussed.