Drac1 and Crumbs participate in amnioserosa morphogenesis during dorsal closure in Drosophila.
2002
Dorsal closure of the Drosophila embryo involves morphological
changes in two epithelia, the epidermis and the amnioserosa, and is a popular
system for studying the regulation of epithelial morphogenesis. We previously
implicated the small GTPase Drac1 in the assembly of an actomyosin contractile
apparatus, contributing to cell shape change in the epidermis during dorsal
closure. We now present evidence that Drac1 and Crumbs, a determinant of
epithelial polarity, are involved in setting up an actomyosin contractile
apparatus that drives amnioserosa morphogenesis by inducing apical cell
constriction. Expression of constitutively active Drac1 causes excessive
constriction of amnioserosa cells and contraction of the tissue, whereas
expression of dominant-negative Drac1 impairs amnioserosa morphogenesis. These
Drac1 transgenes may be acting through their effects on the amnioserosa
cytoskeleton, as constitutively active Drac1 causes increased staining for
F-actin and myosin, whereas dominant-negative Drac1 reduces F-actin levels.
Overexpression of Crumbs causes premature cell constriction in the
amnioserosa, and dorsal closure defects are seen in embryos homozygous for
hypomorphic crumbs alleles. The ability of constitutively active
Drac1 to cause contraction of the amnioserosa is impaired in a crumbs
mutant background. We propose that amnioserosa morphogenesis is a useful
system for studying the regulation of epithelial morphogenesis by Drac1.
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