Jekyll/UDP-glucose dehydrogenase is required for cardiac valve, semicircular canal, and cartilage development in zebrafish

Cardiac valve formation is a complex process that involves cell signaling events between the myocardial and endocardial layers of the heart across an elaborate extracellular matrix. These signals lead to dramatic morphogenetic movements and transdifferentiation of the endocardial cells at chamber boundaries. In this thesis, I analyze the most severe valve mutation identified to date in zebrafish, jekyll. I show, through examination of an endothelially-restricted GFP transgene, that heart valve initiation is deficient in jekyll mutants. Through positional cloning, I demonstrate that the jekyll mutation disrupts a vertebrate homologue of Drosophila Sugarless, an enzyme required for heparan sulfate, chondroitin sulfate and hyaluronic acid production. In Drosophila , Sugarless function has been shown to be important for both Wg and Fgf signal transduction. I show that atrioventricular border cells never differentiate from their neighbors in jekyll mutants, suggesting that Jekyll is required in a cell signaling event that establishes a boundary between the atrium and ventricle. jekyll mutants also exhibit defects in semicircular canal formation and in alcian staining for sulfated proteoglycans in the developing arches and fins. Despite these defects in alcian staining, many molecular aspects of chondrocyte differentiation occur normally. However, proper morphogenesis of the arches fails to occur and the morphogenetic defects present greatly resemble those found in knypek/glypican and pipetail/wnt5a mutants. This suggests that in the branchial arches, Jekyll may function in the Wnt5a pathway and act to modify the heparan sulfate proteoglycan, Glypican. In the case of the heart, Jekyll activity does not seem to be required for Fgf or Wnt signalling but rather may function in the Neuregulin signalling pathway. I have begun experiments to address this possibility and those are detailed herewith.