The role of polysaccharide modifications during Drosophila development

Sammanfattning: Glucosides attached to proteins and lipids have been shown to modulate the activity of signal transduction pathways regulating growth and patterning processes during animal development. In particular, O-linked glycans such as glycosaminoglycans (GAGs) have been implicated in modulation of growth factor signaling pathways. GAGs are unbranched polysaccharide chains composed of repeating disaccharide units that are covalently attached to various core proteins in the ER/Golgi. In Drosophila, genetic analyses have identified several genes required for the biosynthesis of GAGs, which have been demonstrated to modulate signaling of the Wnt/Wg, Hh, TGF-?/Dpp and FGF pathways. Moreover, genes involved in the O-glycosylation of the Notch receptor have been shown to be essential for the modulation of Notch specificity. This thesis is focused on the identification of genes involved in the synthesis and modification of O-linked saccharides and the characterization of their role in signaling events during Drosophila development. We have identified slalom (sll), a Drosophila gene essential for GAG biosynthesis. Molecular characterization of sll mutants suggests that sll encodes a putative transmembrane transporter. We show that sll is required for the transfer of adenosine 3’-phosphate 5’-phosphosulfate (PAPS) from the cytosol into the Golgi lumen. PAPS provides the sulfate donor substrate essential for the sulfation of macromolecules such as GAGs by sulfotransferases in the Golgi. Our observations suggest that lack of GAG sulfation is responsible for the defects in Wg, Hh and Dpp signaling observed in sll mutants. We also demonstrate that sll-dependent sulfation of an as yet unknown factor is essential for the establishment of the dorsal-ventral axis of the Drosophila embryo. In addition, we have identified and functionally characterized another gene in Drosophila, named CG7440. CG7440 encodes a putative ?1,3-galactosyltransferase that may be involved in O-glycan biosynthesis. We show that RNAi-mediated down-regulation of CG7440 expression in the embryo leads to morphogenetic defects during development of the intestinal tract. When CG7440 expression is down-regulated by RNAi in the developing wing primordia, Wg-dependent patterning and growth of the wings is compromised. We show that CG7440 function is not required for HSPG synthesis. Thus, our results suggest that non-GAG-O-glycans may play a role in Wg signaling. Taken together, our data underscore the important function of glycans in the modulation of growth factor signaling pathways during development.