The role of DE-cadherin during cellularization, germ layer formation and early neurogenesis in the Drosophila embryo.

The Drosophila E-cadherin homolog, DE-cadherin, is expressed and required in all epithelial tissues throughout embryogenesis. Due to a strong maternal component of DE-cadherin, its early function during embryogenesis has remained elusive. The expression of a dominant negative DE-cadherin construct (UAS-DE-cad(ex)) using maternally active driver lines allowed us to analyze the requirements for DE-cadherin during this early phase of development. Maternally expressed DE-cad(ex) result in phenotype with variable expressivity. Most severely affected embryos have abnormalities in epithelialization of the blastoderm, resulting in loss of the blastodermal cells' apico-basal polarity and monolayered structure. Another phenotypic class forms a rather normal blastoderm, but shows abnormalities in proliferation and morphogenetic movements during gastrulation and neurulation. Mitosis of the mesoderm occurs prematurely before invagination, and proliferation in the ectoderm, normally a highly ordered process, occurs in a random pattern. Mitotic spindles of ectodermal cells, normally aligned horizontally, frequently occurred vertically or at an oblique angle. This finding further supports recent findings indicating that, in the wild-type ectoderm, the zonula adherens is required for the horizontal orientation of mitotic spindles. Proliferation defects in DE-cad(ex)-expressing embryos are accompanied by the loss of epithelial structure of ectoderm and neuroectoderm. These germ layers form irregular double or triple layers of rounded cells that lack zonula adherens. In the multilayered neuroectoderm, epidermal precursors, neuroblasts and ganglion mother cells occurred intermingled, attesting to the pivotal role of DE-cadherin in delamination and polarized division of neuroblasts. By contrast, the overall number and spacing of neuroblasts was grossly normal, indicating that DE-cadherin-mediated adhesion is less important for cell-cell interaction controlling the ratio of epidermal vs. neural progenitors.