Role of Sialylation in the Nervous System Development of Drosophila melanogaster

The sialyltransferase family is a group of enzymes that transfer sialic acid from donor CMP-Neu5Ac onto suitable carbohydrate chains of glycoproteins and glycolipids. In vertebrates, sialylation is implicated in many physiological and pathobiological processes, including nervous and immune system development and functioning, pathogen-host interaction, cancer cell proliferation and apoptosis. However, the complexity of the sialylation pathway and limitation of genetic and in vivo approaches interferes with functional analyses in mammalian organisms. We use Drosophila because of its simplified physiology and reduced genetic redundancy to characterize the evolutionarily conserved function of sialylation and to reveal its relationship to the role of sialic acids in humans. This dissertation focuses primarily on Drosophila sialyltransferase, DSIAT, so far the only sialyltransferase described in protostomes. Gene targeting of the DSIAT endogenous locus with a DSIAT-HA tagged version uncovered its remarkably dynamic stage- and cell-specific expression. I found that the expression of DSIAT is developmentally regulated and is restricted to motor neurons and cholinergic interneurons within the central nervous system of Drosophila. To reveal the role of DSIAT in development and functioning of fly nervous system I performed characterization of neurological phenotypes of DSIAT knockout flies, also generated by gene targeting approach. I observed that DSIAT mutant larvae are sluggish and have abnormal neuromuscular junction (NMJ) morphology. Electrophysiological analysis of mutant larval NMJ showed altered evoked NMJ activity. It was also observed that DSIAT knockout adult flies are paralyzed when are exposed to higher temperatures. Longevity assays showed that DSIAT adult mutants have significantly reduced life span. I used genetic interaction analysis to identify possible sialylated targets in Drosophila and found that ?-subunit of voltage gated sodium channel is a potential sialylated protein in the fly nervous system. All these data strongly supports the hypothesis that DSIAT plays an important role for neural transmission and development in Drosophila. This research work establishes Drosophila as a useful model system to study sialylation which may shed light on related biological functions in higher organisms including humans.