The mechanism of evagination of imaginal discs ofDrosophila melanogaster: II. Studies on trypsin-accelerated evagination

Active shape programming drives Drosophila wing disc eversion

How complex 3D tissue shape emerges during animal development remains an important open question in biology and biophysics. Here, we discover a mechanism for 3D epithelial shape change based on active, in-plane cellular events that is analogous to inanimate "shape programmable" materials, which undergo blueprinted 3D shape transformations from in-plane gradients of spontaneous strains. We study eversion of the Drosophila wing disc pouch, when the epithelium transforms from a dome into a curved fold, quantifying 3D tissue shape changes and mapping spatial patterns of cellular behaviors on the evolving geometry using cellular topology. Using a physical model inspired by shape programming, we find that active cell rearrangements are the major contributor to pouch eversion and validate this conclusion using a knockdown of MyoVI, which reduces rearrangements and disrupts morphogenesis. This work shows that shape programming is a mechanism for animal tissue morphogenesis and suggests that patterns in nature could present design strategies for shape-programmable materials.

Mutational analysis of Stubble-stubbloid gene structure and function in Drosophila leg and bristle morphogenesis

The Stubble-stubbloid (Sb-sbd) gene is required for ecdysone-regulated epithelial morphogenesis of imaginal tissues during Drosophila metamorphosis. Mutations in Sb-sbd are associated with defects in apical cell shape changes critical for the evagination of the leg imaginal disc and with defects in assembly and extension of parallel actin bundles in growing mechanosensory bristles. The Sb-sbd gene encodes a type II transmembrane serine protease (TTSP). Here we use a Sb-sbd transgenic construct to rescue both bristle and leg morphogenesis defects in Sb-sbd mutations. Molecular characterization of Sb-sbd mutations and rescue experiments with wild-type and modified Sb-sbd transgenic constructs show that the protease domain is required for both leg and bristle functions. Truncated proteins that express the noncatalytic domains without the protease have dominant effects in bristles but not in legs. Leg morphogenesis, but not bristle growth, is sensitive to Sb-sbd overexpression. Antibody localization of the Sb-sbd protein shows apical expression in elongating legs. Sb-sbd protein is found in the base and shaft in budding bristles and then concentrates at the growing tip when bristles are elongating rapidly. We propose a model whereby Sb-sbd helps coordinate proteolytic modification of extracellular matrix attachments with cytoskeletal changes in both legs and bristles.

Genetic Modifier Screens in Drosophila Demonstrate a Role for Rho1 Signaling in Ecdysone-Triggered Imaginal Disc Morphogenesis

Drosophila adult leg development provides an ideal model system for characterizing the molecular mechanisms of hormone-triggered morphogenesis. A pulse of the steroid hormone ecdysone at the onset of metamorphosis triggers the rapid transformation of a flat leg imaginal disc into an immature adult leg, largely through coordinated changes in cell shape. In an effort to identify links between the ecdysone signal and the cytoskeletal changes required for leg morphogenesis, we performed two large-scale genetic screens for dominant enhancers of the malformed leg phenotype associated with a mutation in the ecdysoneinducible broad early gene (br1). From a screen of >750 independent deficiency and candidate mutation stocks, we identified 17 loci on the autosomes that interact strongly with br1. In a complementary screen of ∼112,000 F1 progeny of EMS-treated br1 animals, we recovered 26 mutations that enhance the br1 leg phenotype [E(br) mutations]. Rho1, stubbloid, blistered (DSRF), and cytoplasmic Tropomyosin were identified from these screens as br1-interacting genes. Our findings suggest that ecdysone exerts its effects on leg morphogenesis through a Rho1 signaling cascade, a proposal that is supported by genetic interaction studies between the E(br) mutations and mutations in the Rho1 signaling pathway. In addition, several E(br) mutations produce unexpected defects in midembryonic morphogenetic movements. Coupled with recent evidence implicating ecdysone signaling in these embryonic morphogenetic events, our results suggest that a common ecdysone-dependent, Rho1-mediated regulatory pathway controls morphogenesis during the two major transitions in the life cycle, embryogenesis and metamorphosis.

Molecular cloning of cDNA for Sarcophaga prolyl endopeptidase and characterization of the recombinant enzyme produced by an E. coli expression system

A cDNA for prolyl endopeptidase (PEP) of Sarcophaga peregrina (flesh fly) was cloned and its sequence determined. The overall amino acid sequence identity between Sarcophaga and mammalian PEPs was 53%, indicating that these enzymes are structurally very similar. Northern blot hybridization revealed that the Sarcophaga PEP gene was activated significantly at the eversion stage of imaginal disc differentiation. We obtained recombinant PEP by expressing the cDNA in Escherichia coli. The recombinant and authentic enzymes showed almost identical characteristics, in terms of substrate specificities and sensitivities to inhibitors.

Identification of substrate proteins for cathepsin L that are selectively hydrolyzed during the differentiation of imaginal discs of Sarcophaga peregrina

Previously, we showed that cathepsin L is essential for the differentiation of imaginal discs of the flesh fly [Homma, K. & Natori, S. (1994) J. Biol. Chem. 269, 15258-15264]. We have now identified imaginal disc proteins that are susceptible to digestion by cathepsin L and showed that they are selectively hydrolyzed during imaginal disc differentiation. Two of these proteins, with molecular masses of 210 and 200 kDa, were further characterized. Immunofluorescence studies suggested that they were intrinsic components of the basement membranes of various tissues. They were selectively hydrolyzed at the elongation stage of imaginal leg disc differentiation. Western blotting of embryos at various developmental stages showed that these proteins were only detected at the end of embryogenesis.

The making of a fly leg: a model for epithelial morphogenesis

Epithelial development dictates the shape of an organism. The metamorphic development of a Drosophila leg precursor into an adult leg is a well-defined example of epithelial morphogenesis that can be analyzed from the perspectives of genetics and molecular and cell biology. The steroid hormone 20-hydroxyecdysone induces and regulates the entire process. Mutants affecting Drosophila leg morphogenesis characteristically have short thick legs (the malformed phenotype) resulting from a failure to execute normal cell shape changes at a specific stage of development. Mutations that cause the malformed phenotype have already led to the identification and cloning of genes encoding transcription factors, a transmembrane serine protease presumably required for modification of the apical extracellular matrix, and components of the contractile cytoskeleton and adherens junctions. All of these products are required for the execution of normal changes in leg cell shape.

Extracellular protease production by Drosophila imaginal discs

We are investigating the role of extracellular proteases in imaginal disc eversion to understand the mechanism that controls cell rearrangements within epithelia. We have identified three cation-dependent neutral proteases released by Drosophila leg discs everting in culture. Serine protease inhibitors block disc eversion and inhibit activity of disc proteases. The pattern of extracellular proteases changes when eversion is blocked with added protease inhibitors. Changes in protease activity occur when released disc proteases are treated with trypsin. Trypsin treatment of intact imaginal discs releases protease and inhibitor activities to the medium, indicating their presence on the cell surface before release. Our results suggest that extracellular proteases are required for imaginal disc morphogenesis and are regulated by more than one mechanism.

Removal of insect basal laminae using elastase

We have used the enzyme elastase to remove the basal lamina of epithelia from two insects: the upper Malpighian tubules of Rhodnius prolixus and imaginal discs of Drosophila melanogaster. Removal of the basal lamina was confirmed using scanning and transmission electron microscopy. Use of the technique on the Malphighian tubules of Rhodnius reveals for the first time the three-dimensional organization of the circumferential folds of the basal plasma membrane. Elastase is much more effective in removing the basal lamina than are the enzymes hyaluronidase, collagenase, and chymotrypsin, either alone or in combination. Following elastase treatment, cells of the Malpighian tubules dissociate with only mild mechanical agitation into single, viable cells. Treatment with elastase removes the basal laminae of imaginal discs of Drosophila and accelerates evagination as has been previously described for trypsin. To obtain single cell preparations from elastase-treated imaginal discs, mechanical stirring in Ringer low in Ca2+ was required. In addition to its usefulness in cell isolation, elastase treatment allows examination of the effect of removal of basal laminae on the physiology and development of insect epithelia.