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Research Article |
1 Department of Molecular Biology and Biochemistry, Simon Fraser University,
8888 University Drive, Burnaby, BC, V5A 1S6, Canada
2 Drosophila Neurobiology, Institute of Molecular and Cell Biology, 30 Medical
Drive, Singapore 117609, Republic of Singapore
3 Glaxo-IMCB Laboratories, Institute of Molecular and Cell Biology, 30 Medical
Drive, Singapore 117609, Republic of Singapore
4 Department of Neurochemistry, Institute of Neurology, 1 Wakefield St., London
WC1 1PJ, UK
Present address: MRC Centre for Developmental Neurobiology, King's College
London SE1 1UL, UK
* Author for correspondence (e-mail: nharden{at}sfu.ca )
Accepted 21 February 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.
Key words: Drosophila, Rac, Small GTPase, Crumbs, Amnioserosa, Dorsal Closure, Cytoskeleton, Morphogenesis
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