Sialyltransferase Regulates Nervous System Function in Drosophila

Abstract

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect nervous system’s development and function. While the complexity of glycosylation and the functional redundancy among sialyltransferases provide obstacles for revealing biological roles of sialylation in mammals, Drosophila possesses a sole vertebrate-type sialyltransferase, DSiaT, with significant homology to its mammalian counterparts, suggesting that Drosophila could be a suitable model to investigate the function of sialylation. To explore this possibility and investigate the role of sialylation in Drosophila, we inactivated DSiaT in vivo by gene targeting and analyzed phenotypes of DSiaT mutants using a combination of behavioural, immunolabeling, electrophysiological and pharmacological approaches. Our experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. We found that DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis and defects of neuromuscular junctions. Our results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, we showed that sialyltransferase activity is required for DSiaT function in vivo, which suggests that DSiaT mutant phenotypes result from a defect in sialylation of N-glycans. This work provided the first evidence that sialylation has an important biological function in protostomes, while also revealing a novel, nervous system-specific function of α2,6 sialylation. Thus, our data shed light on one of the most ancient functions of sialic acids in metazoan organisms and suggest a possibility that this function is evolutionarily conserved between flies and mammals.

Sugar-free frosting, a homolog of SAD kinase, drives neural-specific glycan expression in the Drosophila embryo

“…drives the predominance of paucimannose structures in the wildtype Drosophila glycan profile by converting NM3N2 glycans to M3N2, thereby eliminating the precursor pool for hybrid or complex structures (Leonard et al, 2006). Limited expression of branching and terminal glycosyltransferases, such as GlcNAcT2, GlcNAcT4, GalT, GalNAcT or SiaT, also restrict the capacity for generating complex structures (Haines and Irvine, 2005;Koles et al, 2004;Repnikova et al, 2010;Sarkar et al, 2006). HRP epitopes represent less than 1% of all N-linked glycans in the Drosophila embryo (Aoki et al, 2007).…”

Neuronal glycosylation differentials in normal, injured and chondroitinase-treated environments

“…Electrical signaling in neurons, skeletal muscle cells and cardiomycetes is modulated by the sialic acid content of particular isoforms of ion channels [35]. Altered or aberrant sialic acid expression could impact neuron polarization [35], which may be consistent with altered excitability of neurons post-injury [33]. Exposure of α(2,6)-linked sialic acid and binding to SNA-I has been observed on apoptotic and necrotic cells [36], and α(2,6)-sialylation has been identified as blocking binding to galectins, hence functioning as a biological 'off switch' [31].…”

Molecular Phylogeny and Functional Genomics of β-Galactoside α2,6-Sialyltransferases That Explain Ubiquitous Expression of st6gal1 Gene in Amniotes

“…essential for the regulation of nervous system function (36). Moreover, this Drosophila protein exhibits notable preferred enzymatic activity toward LacdiNAc substrates over LacNAc termini in in vitro assays (31), despite the fact that no evidence for the presence of LacdiNAc or LacNAc could be established in vivo (34).…”

“…DSiaT is detected almost exclusively in central nervous system (CNS) neurons in the embryonic stage 17, in the optic lobe of third instar larva, and in adult head (35). Targeted disruption of the DSiaT gene results in a neurological phenotype, suggesting that DSiaT modulates the nervous system function of voltage-gated sodium channel (36). Because the mammalian st6gal2 gene is detected mainly in CNS as well, it has been suggested that ST6Gal II might have conserved an ancestral function, whereas ST6Gal I would have developed new functions in vertebrates.…”