Scott, Hilary Anne (2017-08). Regulation of Sialylation in the Drosophila Nervous System. Doctoral Dissertation.
Sialylation is a common post-translational modification in animal cells, yet its molecular and cellular regulation is poorly understood. It is involved in many vital functions in vertebrates, while perturbations in sialylation have been implicated in severe human pathologies such as mental impairment, epilepsy and heart failure. However, functional studies of sialylation have been hampered due to the pleiotropic effects on the organism as well as the complexity of the sialylation pathway in vertebrates. To overcome these problems, we use Drosophila as a model organism to reveal mechanisms of neural sialylation. Our experiments characterized key genes in Drosophila sialylation pathway, including sialyltransferase (DSiaT) and CMP-sialic acid synthetase (CSAS), and shed light on significant conservation between these genes and their mammalian counterparts. We found that DSiaT and CSAS function within a unique pathway that modulates neural transmission and relies on interactions between neurons and glia. Using a series of cell specific rescue studies, we have found that CSAS functions in glial cells, while DSiaT function is required in neurons. Analyses of the expression of these genes supported these conclusions, while revealing that CSAS and DSiaT are expressed in different types of cells in a non-overlapping manner. Our results demonstrate that sialylation is coordinated between glial cells and neurons to modulate neural transmission suggesting a role in glia-neuron coupling. Additionally, we found that DSiaT can operate cell non-autonomously to regulate neural excitability. Ectopic expression showed DSiaT can bypass CSAS taking advantage of previously unknown sialyl-donors to modify glycoproteins in non-canonical sialylation. Collectively, this work provides novel mechanistic insights into the role of sialic acids in regulation of neural excitability in Drosophila. Our results also suggest that these mechanisms can be evolutionarily conserved in mammals, which potentially unveils a novel paradigm relevant for regulation of the nervous system in normal and pathological conditions in humans.