Morphogenesis: Biophysics and Genetics of Dorsal Closure

Cell sheet morphogenesis is essential for metazoan development and homeostasis of animal form – it contributes to development, such as in gastrulation, neural tube, heart and palate formation and to homeostasis, in wound healing. Gene expression and signaling cascades coordinate and regulate the cellular machines that drive morphogenesis. Disfunction in these components causes developmental and wound healing defects that can disfigure or kill. We focus on the molecular mechanisms of cell sheet morphogenesis during dorsal closure (DC) in Drosophila melanogaster. During DC, lateral epidermal sheets advance to close a dorsal opening. We pioneered the study of DC as a model system and use an unusually diverse repertoire of interdisciplinary approaches, including live imaging, reverse and forward genetics and biophysical strategies to interrogate the mechanics and regulation of closure in wild type and mutant embryos. We found that DC is the sum of four major dynamic processes and is robust and resilient – no single force that drives closure is absolutely required. Processes that contribute to DC at the molecular, cellular and tissue scales are highly conserved in animal phylogeny making Drosophila an ideal model system for interrogating the molecular basis of morphogenesis. There remain significant gaps in our understanding of this conceptually simple, yet biologically complex cell sheet movement. To identify new “DC genes”, i.e., genes that when deleted, disrupt closure, we initiated a forward genetic, live-imaging, screen. This screen used 194 deficiency stocks (Dfs) that collectively delete 5,778 of the 5,854 genes on melanogaster's 2nd chromosome. We have begun to extend our screen to the 3rd chromosome. Remarkably, 96 Dfs caused notable and diverse defects in closure, indicating that a large number of discrete biological processes contribute to closure and are susceptible to mutational disruption. Thus far, we have identified 13 new pre-DC or DC genes that are responsible for the DC Df phenotypes. When extended to the whole fly genome our screen is projected to identify ~165 new DC genes (only ~140 DC genes were known at the start of our screen). Based on phenotype, we prioritize the DC Dfs on which to focus, identify the DC genes responsible for their Df phenotypes, then characterize how the new DC gene products contribute to closure. A priority is to understand the molecular mechanisms by which cell-cell interactions and cell-matrix based adhesion couple to the actomyosin cytoskeleton – these connections must be robust enough to transmit forces, yet malleable enough to allow the cell shape changes that define morphogenesis. Of further interest is a new effort to thermodynamically and kinetically characterize the myosin 2 motor that drives morphogenesis in DC and other developmental processes. Our goal is to assess how differential splicing that encodes myosin's motor domain contributes to morphogenesis as a fast moving, slow/efficient force holding, strain sensing, or processive motor. We are uniquely positioned to address the molecular and biophysical mechanisms that underlie the basic biology of cell sheet morphogenesis in flies, research that directly informs vertebrate development and wound healing.