Determining the roles of Dock proteins in dorsal vessel development. Bridget H Biersmith, Erika R Geisbrecht. Division of Cell Biology & Biophysics, University of Missouri-Kansas City, Kansas City, MO.

   Congenital heart defects (CHD) are one of the most common forms of birth abnormalities. Nearly 1.3 million Americans are affected by CHD today and have an increased risk of cardiovascular disease and stroke. Unfortunately, the underlying causes of CHD remain unclear. To address this issue, we have used the Drosophila heart, or dorsal vessel (DV), as a model tissue as aspects of early heart development are conserved throughout evolution. Specifically, two rows of heart cells form and then migrate to the midline where they undergo cell morphogenic events, including cell migration and adhesion, to make a muscular, beating, linear tube. Using this tissue, we are able to assess the roles of the highly conserved family of Dock proteins in heart development. Dock family members are guanine nucleotide exchange factors (GEFs) capable of activating the small GTPase Rac to modulate actin cytoskeletal movements, including cell migration and myoblast fusion. The Drosophila genome encodes two closely related Dock proteins, Myoblast city (Mbc or vertebrate Dock180) and Sponge (Spg or vertebrate Dock3/4), both of which are expressed in the DV. Previous studies have shown that removal of Mbc results in a complete lack of myoblast fusion in the embryonic musculature. Our preliminary data shows that Spg fails to substitute for Mbc in this fusion process, suggesting these two proteins are not functionally redundantly in this tissue. To address the specific roles Mbc and Spg are playing in heart development, we have examined the morphological changes that occur in dorsal vessel formation using 3-D reconstruction and histological thin-sections. Consistent with the idea that Mbc is required for cytoskeletal rearrangement, we failed to observe normal cell shape changes in embryos mutant for mbc. Data will be presented that analyzes cardioblast cell shape changes in mbc, spg double mutants. Furthermore, we plan to do ultra-thin sections for EM, which will allow us to visualize adherens junctions to further determine the developmental roles of these Dock proteins in heart tube development.