InteractiveFly: GeneBrief

Sialyltransferase: Biological Overview | References

Gene name - Sialyltransferase

Synonyms -

Cytological map position - 60D14-60D14

Function - enzyme

Keywords - mediates sialic acid transfer onto glycan termini - expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system - mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions.

Symbol - SiaT

FlyBase ID: FBgn0035050

Genetic map position - chr2R:24,691,422-24,694,386

NCBI classification - Glycosyltransferase family 29 (sialyltransferase)

Cellular location - cytoplasmic and potentially extracellular

NCBI links: EntrezGene, Nucleotide, Protein

SiaT orthologs: Biolitmine

Modification by sialylated glycans can affect protein functions, underlying mechanisms that control animal development and physiology. Sialylation relies on a dedicated pathway involving evolutionarily conserved enzymes, including CMP-sialic acid synthetase (CSAS) and sialyltransferase (SiaT) that mediate the activation of sialic acid and its transfer onto glycan termini, respectively. In Drosophila, CSAS and DSiaT genes function in the nervous system, affecting neural transmission and excitability. These genes were found to function in different cells: the function of CSAS is restricted to glia, while DSiaT functions in neurons. This partition of the sialylation pathway allows for regulation of neural functions via a glia-mediated control of neural sialylation. The sialylation genes were shown to be required for tolerance to heat and oxidative stress and for maintenance of the normal level of voltage-gated sodium channels. The results uncovered a unique bipartite sialylation pathway that mediates glia-neuron coupling and regulates neural excitability and stress tolerance (Scott, 2023).

Protein glycosylation, the most common type of posttranslational modification, plays numerous important biological roles, and regulates molecular and cell interactions in animal development, physiology, and disease. The addition of sialic acid (Sia), i.e., sialylation, has prominent effects due to its negative charge, bulky size, and terminal location of Sia on glycan chains. Essential roles of sialylated glycans in cell adhesion, cell signaling, and proliferation have been documented in many studies. Sia is intimately involved in the function of the nervous system. Mutations in genes that affect sialylation are associated with neurological symptoms in human, including intellectual disability, epilepsy, and ataxia due to defects in sialic acid synthase (N-acetylneuraminic acid synthase [NANS]), sialyltransferases (ST3GAL3 and ST3GAL5), the CMP-Sia transporter (SLC35A1), and CMP-Sia synthase (CMAS). Polysialylation (PSA) of NCAM, the neural cell adhesion molecule, one of the best studied cases of sialylation in the nervous system, is involved in the regulation of cell interactions during brain development. Non-PSA-type sialylated glycans are ubiquitously present in the vertebrate nervous system, but their functions are not well defined. Increasing evidence implicates these glycans in essential regulation of neuronal signaling. Indeed, N-glycosylation can affect voltage-gated channels in different ways, ranging from modulation of channel gating to protein trafficking, cell surface expression, and recycling/degradation. Similar effects were shown for several other glycoproteins implicated in synaptic transmission and cell excitability, including neurotransmitter receptors. Glycoprotein sialylation defects were also implicated in neurological diseases, such as Angelman syndrome and epilepsy. However, the in vivo functions of sialylation and the mechanisms that regulate this posttranslational modification in the nervous system remain poorly understood (Scott, 2023).

Drosophila has recently emerged as a model to study neural sialylation in vivo, providing advantages of the decreased complexity of the nervous system and the sialylation pathway, while also showing conservation of the main biosynthetic steps of glycosylation (Koles, 2009; Scott, 2014). The final step in sialylation is mediated by sialyltransferases, enzymes that use CMP-Sia as a sugar donor to attach Sia to glycoconjugates (see Schematic of the sialylation pathways in vertebrate and Drosophila. Unlike mammals that have 20 different sialyltransferases, Drosophila possesses a single sialyltransferase, DSiaT, that has significant homology to mammalian ST6Gal enzymes. The two penultimate steps in the biosynthetic pathway of sialylation are mediated by sialic acid synthase (also known as NANS) and CMP-sialic acid synthetase (CSAS, also known as CMAS), the enzymes that synthesize sialic acid and carry out its activation, respectively. These enzymes have been characterized in Drosophila and found to be closely related to their mammalian counterparts. In vivo analyses of DSiaT and CSAS demonstrated that Drosophila sialylation is a tightly regulated process limited to the nervous system and required for normal neural transmission. Mutations in DSiaT and CSAS phenocopy each other, resulting in similar defects in neuronal excitability, causing locomotor and heat-induced paralysis phenotypes, while showing strong interactions with voltage-gated channels (Repnikova, 2010; Islam, 2013). DSiaT was found to be expressed exclusively in neurons during development and in the adult brain (Repnikova, 2010). Intriguingly, although the expression of CSAS has not been characterized in detail, it was noted that its expression appears to be different from that of DSiaT in the embryonic ventral ganglion (Koles, 2009), suggesting a possibly unusual relationship between the functions of these genes. This study tested the hypothesis that CSAS functions in glial cells, and that the separation of DSiaT and CSAS functions between neurons and glia underlies a novel mechanism of glia-neuron coupling that regulates neuronal function via a bipartite protein sialylation (Scott, 2023).

In vertebrates, phosphorylated sialic acid is produced by N-acetylneuraminic acid synthase (Neu5Ac-9-P synthase, or NANS) from N-acetyl-mannosamine 6-phosphate (ManNAc-6-P), converted to sialic acid (Scott, 2023).

Glial cells have been recognized as key players in neural regulation. Astrocytes participate in synapse formation and synaptic pruning during development, mediate the recycling of neurotransmitters, affect neurons via Ca2+ signaling, and support a number of other essential evolutionarily conserved functions. Studies of Drosophila glia have revealed novel glial functions in vivo. Drosophila astrocytes were found to modulate dopaminergic function through neuromodulatory signaling and activity-regulated Ca2+ increase. Glial cells were also shown to protect neurons and neuroblasts from oxidative stress and promote the proliferation of neuroblasts in the developing Drosophila brain. The metabolic coupling between astrocytes and neurons, which is thought to support and modulate neuronal functions in mammals, is apparently conserved in flies. Indeed, Drosophila glial cells can secrete lactate and alanine to fuel neuronal oxidative phosphorylation. In the current work, a novel mechanism id described of glia-neuron coupling mediated by a unique compartmentalization of different steps in the sialylation pathway between glial cells and neurons in the fly nervous system. This study explored the regulation of this mechanism and demonstrate its requirement for neural functions (Scott, 2023).

Genetic Interactions Between Drosophila sialyltransferase and beta1,4-N-acetylgalactosaminyltransferase-A Genes Indicate Their Involvement in the Same Pathway

Sialylated glycans play a prominent role in the Drosophila nervous system where they are involved in the regulation of neural transmission. However, the functional pathway of sialylation in invertebrates, including Drosophila, remains largely unknown. This study used a combination of genetic and behavioral approaches to shed light on the Drosophila sialylation pathway. Genetic interactions were examined between Drosophila sialyltransferase (DSiaT) and beta1,4-N-acetylgalactosaminyltransferase (beta4GalNAcT) genes. The results indicated that beta4GalNAcTA and DSiaT cooperate within the same functional pathway that regulates neural transmission. beta4GalNAcTA is epistatic to DSiaT. These data suggest an intriguing possibility that beta4GalNAcTA may participate in the biosynthesis of sialylated glycans (Nakamura, 2012).

Functions of Sialyltransferase orthologs in other species

Glycosyltransferase ST6Gal-I promotes the epithelial to mesenchymal transition in pancreatic cancer cells

ST6Gal-I, an enzyme upregulated in numerous malignancies, adds alpha2-6-linked sialic acids to select membrane receptors, thereby modulating receptor signaling and cell phenotype. This study investigated ST6Gal-I's role in epithelial to mesenchymal transition (EMT) using the Suit2 pancreatic cancer cell line, which has low endogenous ST6Gal-I and limited metastatic potential, along with two metastatic Suit2-derived subclones, S2-013 and S2-LM7AA, which have upregulated ST6Gal-I. RNA-Seq results suggested that the metastatic subclones had greater activation of EMT-related gene networks than parental Suit2 cells, and forced overexpression of ST6Gal-I in the Suit2 line was sufficient to activate EMT pathways. Accordingly, this evaluated expression of EMT markers and cell invasiveness (a key phenotypic feature of EMT) in Suit2 cells with or without ST6Gal-I overexpression, as well as S2-013 and S2-LM7AA cells with or without ST6Gal-I knockdown. Cells with high ST6Gal-I expression displayed enrichment in mesenchymal markers (N-cadherin, slug, snail, fibronectin) and cell invasiveness, relative to ST6Gal-I-low cells. Contrarily, epithelial markers (E-cadherin, occludin) were suppressed in ST6Gal-I-high cells. To gain mechanistic insight into ST6Gal-I's role in EMT,the activity of epidermal growth factor receptor (EGFR), a known EMT driver, was examined. ST6Gal-I-high cells had greater alpha2-6 sialylation and activation of EGFR than ST6Gal-I-low cells. The EGFR inhibitor, erlotinib, neutralized ST6Gal-I-dependent differences in EGFR activation, mesenchymal marker expression, and invasiveness in Suit2 and S2-LM7AA, but not S2-013, lines. Collectively, these results advance understanding of ST6Gal-I's tumor-promoting function by highlighting a role for ST6Gal-I in EMT, which may be mediated, at least in part, by alpha2-6-sialylated EGFR (Britain, 2021).

Sialyltransferase regulates nervous system function in Drosophila

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect the development and function of the nervous system. 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, Drosophila 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, this study inactivated DSiaT in vivo by gene targeting, and phenotypes of DSiaT mutants were analyzed using a combination of behavioral, immunolabeling, electrophysiological, and pharmacological approaches. These experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions. These results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, ialyltransferase activity was shown to be 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 alpha2,6-sialylation. Thus, these 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 (Repnikova, 2019).

Functional characterization of Drosophila sialyltransferase

Sialylation is an important carbohydrate modification of glycoconjugates in the deuterostome lineage of animals. By contrast, the evidence for sialylation in protostomes has been scarce and somewhat controversial. The present study characterized a Drosophila sialyltransferase gene, thus providing experimental evidence for the presence of sialylation in protostomes. This gene encodes a functional alpha2-6-sialyltransferase (SiaT) that is closely related to the vertebrate ST6Gal sialyltransferase family, indicating an ancient evolutionary origin for this family. Characterization of recombinant, purified Drosophila SiaT revealed a novel acceptor specificity as it exhibits highest activity toward GalNAcbeta1-4GlcNAc carbohydrate structures at the non-reducing termini of oligosaccharides and glycoprotein glycans. Oligosaccharides are preferred over glycoproteins as acceptors, and no activity toward glycolipid acceptors was detected. Recombinant Drosophila SiaT expressed in cultured insect cells possesses in vivo and in vitro autosialylation activity toward beta-linked GalNAc termini of its own N-linked glycans, thus representing the first example of a sialylated insect glycoconjugate. In situ hybridization revealed that Drosophila SiaT is expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system. The identification of a SiaT in Drosophila provides a new evolutionary perspective for considering the diverse functions of sialylation and, through the powerful genetic tools available in this system, a means of elucidating functions for sialylation in protostomes (Koles, 2004).

ST6Gal-I-mediated sialylation of the epidermal growth factor receptor modulates cell mechanics and enhances invasion

Heterogeneity within the glycocalyx influences cell adhesion mechanics and signaling. However, the role of specific glycosylation subtypes in influencing cell mechanics via alterations of receptor function remains unexplored. It has been shown that the addition of sialic acid to terminal glycans impacts growth, development, and cancer progression. In addition, the sialyltransferase ST6Gal-I promotes epidermal growth factor receptor (EGFR) activity, and this study has shown EGFR is an 'allosteric mechano-organizer' of integrin tension. The impact of ST6Gal-I on cell mechanics was investigated. Using DNA-based tension gauge tether probes of variable thresholds, it was found that high ST6Gal-I activity promotes increased integrin forces and spreading in Cos-7 and OVCAR3, OVCAR5, and OV4 cancer cells. Further, employing inhibitors and function-blocking antibodies against beta1, beta3, and beta5 integrins and ST6Gal-I targets EGFR, tumor necrosis factor receptor, and Fas cell surface death receptor, it was validated that the observed phenotypes are EGFR-specific. While tension, contractility, and adhesion are extracellular-signal-regulated kinase pathway-dependent, spreading, proliferation, and invasion are phosphoinositide 3-kinase-Akt serine/threonine kinase dependent. Using total internal reflection fluorescence microscopy and flow cytometry, this study also showed that high ST6Gal-I activity leads to sustained EGFR membrane retention, making it a key regulator of cell mechanics. These findings suggest a novel sialylation-dependent mechanism orchestrating cellular mechanics and enhancing cell motility via EGFR signaling (Rao, 2022).

Systemic ST6Gal-1 Is a Pro-survival Factor for Murine Transitional B Cells

Humoral immunity depends on intrinsic B cell developmental programs guided by systemic signals that convey physiologic needs. Aberrant cues or their improper interpretation can lead to immune insufficiency or a failure of tolerance and autoimmunity. The means by which such systemic signals are conveyed remain poorly understood. Hence, further insight is essential to understanding and treating autoimmune diseases and to the development of improved vaccines. ST6Gal-1 is a sialyltransferase that constructs the alpha2,6-sialyl linkage on cell surface and extracellular glycans. The requirement for functional ST6Gal-1 in the development of humoral immunity is well documented. Canonically, ST6Gal-1 resides within the intracellular ER-Golgi secretory apparatus and participates in cell-autonomous glycosylation. However, a significant pool of extracellular ST6Gal-1 exists in circulation. This study segregated the contributions of B cell intrinsic and extrinsic ST6Gal-1 to B cell development. It was observed that B cell-intrinsic ST6Gal-1 is required for marginal zone B cell development, while B cell non-autonomous ST6Gal-1 modulates B cell development and survival at the early transitional stages of the marrow and spleen. Exposure to extracellular ST6Gal-1 ex vivo enhanced the formation of IgM-high B cells from immature precursors, and increased CD23 and IgM expression. Extrinsic sialylation by extracellular ST6Gal-1 augmented BAFF-mediated activation of the non-canonical NF-kB, p38 MAPK, and PI3K/AKT pathways, and accelerated tyrosine phosphorylation after B cell receptor stimulation. in vivo, systemic ST6Gal-1 did not influence homing of B cells to the spleen but was critical for their long-term survival and systemic IgG levels. Circulatory ST6Gal-1 levels respond to inflammation, infection, and malignancy in mammals, including humans. In turn, systemic ST6Gal-1 regulates inflammatory cell production by modifying bone marrow myeloid progenitors. These data point to an additional role of systemic ST6Gal-1 in guiding B cell development, which supports the concept that circulating ST6Gal-1 is a conveyor of systemic cues to guide the development of multiple branches of immune cells (Irons, 2018).


Search PubMed for articles about Drosophila Sialyltransferase

Britain, C. M., Bhalerao, N., Silva, A. D., Chakraborty, A., Buchsbaum, D. J., Crowley, M. R., Crossman, D. K., Edwards, Y. J. K. and Bellis, S. L. (2021). Glycosyltransferase ST6Gal-I promotes the epithelial to mesenchymal transition in pancreatic cancer cells. J Biol Chem 296: 100034. PubMed ID: 33148698

Irons, E. E. and Lau, J. T. Y. (2018). Systemic ST6Gal-1 Is a Pro-survival Factor for Murine Transitional B Cells. Front Immunol 9: 2150. PubMed ID: 30294329

Islam, R., Nakamura, M., Scott, H., Repnikova, E., Carnahan, M., Pandey, D., Caster, C., Khan, S., Zimmermann, T., Zoran, M. J. and Panin, V. M. (2013). The role of Drosophila cytidine monophosphate-sialic acid synthetase in the nervous system. J Neurosci 33(30): 12306-12315. PubMed ID: 23884937

Koles, K., Irvine, K. D. and Panin, V. M. (2004). Functional characterization of Drosophila sialyltransferase. J Biol Chem 279(6): 4346-4357. PubMed ID: 14612445

Koles, K., Repnikova, E., Pavlova, G., Korochkin, L. I. and Panin, V. M. (2009). Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry. Glycoconj J 26(3): 313-324. PubMed ID: 18568399

Nakamura, M., Pandey, D. and Panin, V. M. (2012). Genetic Interactions Between Drosophila sialyltransferase and beta1,4-N-acetylgalactosaminyltransferase-A Genes Indicate Their Involvement in the Same Pathway. G3 (Bethesda) 2(6): 653-656. PubMed ID: 22690374

Rao, T. C., Beggs, R. R., Ankenbauer, K. E., Hwang, J., Ma, V. P., Salaita, K., Bellis, S. L. and Mattheyses, A. L. (2022). ST6Gal-I-mediated sialylation of the epidermal growth factor receptor modulates cell mechanics and enhances invasion. J Biol Chem 298(4): 101726. PubMed ID: 35157848

Repnikova, E., Koles, K., Nakamura, M., Pitts, J., Li, H., Ambavane, A., Zoran, M. J. and Panin, V. M. (2010). Sialyltransferase regulates nervous system function in Drosophila. J Neurosci 30(18): 6466-6476. PubMed ID: 20445073

Scott, H. and Panin, V. M. (2014). N-glycosylation in regulation of the nervous system. Adv Neurobiol 9: 367-394. PubMed ID: 25151388

Scott, H., Novikov, B., Ugur, B., Allen, B., Mertsalov, I., Monagas-Valentin, P., Koff, M., Baas Robinson, S., Aoki, K., Veizaj, R., Lefeber, D. J., Tiemeyer, M., Bellen, H. and Panin, V. (2023). Glia-neuron coupling via a bipartite sialylation pathway promotes neural transmission and stress tolerance in Drosophila. Elife 12. PubMed ID: 36946697

Repnikova, E., Koles, K., Nakamura, M., Pitts, J., Li, H., Ambavane, A., Zoran, M. J. and Panin, V. M. (2010). Sialyltransferase regulates nervous system function in Drosophila. J Neurosci 30(18): 6466-6476. PubMed ID: 20445073

Biological Overview

date revised: 23 August 2023

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