Molecular identification, tissue distribution and subcellular localization of mST3GalV/GM3 synthase

Putting a cap on the glycome: Dissecting human sialyltransferase functions

Human glycans are capped with sialic acids and these nine-carbon sugars mediate many of the biological functions and interactions of glycans. Structurally diverse sialic acid caps mark human cells as self and they form the ligands for the Siglec immune receptors and other glycan-binding proteins. Sialic acids enable host interactions with the human microbiome and many human pathogens utilize sialic acids to infect host cells. Alterations in sialic acid-carrying glycans, sialoglycans, can be found in every major human disease including inflammatory conditions and cancer. Twenty sialyltransferase family members in the Golgi apparatus of human cells transfer sialic acids to distinct glycans and glycoconjugates. Sialyltransferases catalyze specific reactions to form unique sialoglycans or they have shared functions where multiple family members generate the same sialoglycan product. Moreover, some sialyltransferases compete for the same glycan substrate, but create different sialic acid caps. The redundant and competing functions make it difficult to understand the individual roles of the human sialyltransferases in biology and to reveal the specific contributions to pathobiological processes. Recent insights hint towards the existence of biosynthetic rules formed by the individual functions of sialyltransferases, their interactions, and cues from the local Golgi environment that coordinate sialoglycan biosynthesis. In this review, we discuss the current structural and functional understanding of the human sialyltransferase family and we review recent technological advances that enable the dissection of individual sialyltransferase activities.

Glyco-engineered MDCK cells display preferred receptors of H3N2 influenza absent in eggs used for vaccines

Evolution of human H3N2 influenza viruses driven by immune selection has narrowed the receptor specificity of the hemagglutinin (HA) to a restricted subset of human-type (Neu5Acα2-6 Gal) glycan receptors that have extended poly-LacNAc (Galβ1-4GlcNAc) repeats. This altered specificity has presented challenges for hemagglutination assays, growth in laboratory hosts, and vaccine production in eggs. To assess the impact of extended glycan receptors on virus binding, infection, and growth, we have engineered N-glycan extended (NExt) cell lines by overexpressing β3-Ν-acetylglucosaminyltransferase 2 in MDCK, SIAT, and hCK cell lines. Of these, SIAT-NExt cells exhibit markedly increased binding of H3 HAs and susceptibility to infection by recent H3N2 virus strains, but without impacting final virus titers. Glycome analysis of these cell lines and allantoic and amniotic egg membranes provide insights into the importance of extended glycan receptors for growth of recent H3N2 viruses and relevance to their production for cell- and egg-based vaccines.

Diversity of sialic acids and sialoglycoproteins in gametes and at fertilization

Sialic acids are a family of 9-carbon monosaccharides with particular physicochemical properties. They modulate the biological functions of the molecules that carry them and are involved in several steps of the reproductive process. Sialoglycoproteins participate in the balance between species recognition and specificity, and the mechanisms of these aspects remain an issue in gametes formation and binding in metazoan reproduction. Sialoglycoproteins form a specific coat at the gametes surface and specific polysialylated chains are present on marine species oocytes. Spermatozoa are submitted to critical sialic acid changes in the female reproductive tract facilitating their migration, their survival through the modulation of the female innate immune response, and the final oocyte-binding event. To decipher the role of sialic acids in gametes and at fertilization, the dynamical changes of enzymes involved in their synthesis and removal have to be further considered.

Computational analysis of the structure, glycosylation and CMP binding of human ST3GAL sialyltransferases

Sialyltransferases (STs) are the fundamental enzymes which are related to many biological processes such as cell signalling, cellular recognition, cell-cell and host-pathogen interactions and metastasis of cancer. All STs catalyse the terminal sialic acid addition from CMP donor to the glycan units. ST3GAL family is one of the most important STs and divided into the six subfamily in mouse and humans which are ST3Gal I, ST3Gal II, ST3Gal III, ST3Gal IV, ST3Gal V, and ST3Gal VI. The members of the ST3GAL family transfer sialic acid to the terminal galactose residues of glycochains through an α2,3-linkage. There are many reports on the ST3GAL function in mammals but, there is a paucity of information about structure of human ST3GAL family. Herein, we investigated the structure, glycosylation and CMP binding site of human ST3GAL family using computational methods. We found for the first time N-glycosylation positions in ST3Gal IV and VI, mucin type glycosylation in ST3Gal III and O-GlcNAcylation in ST3Gal V and their relation with sialylmotifs. In addition, we predicted CMP binding positions of human ST3GAL enzyme family on three-dimensional structure using molecular docking and first demonstrated the sialylmotifs relation with the CMP binding positions in ST3Gal III-VI subfamilies.

Biology of GM3 Ganglioside

Since the successful molecular cloning in 1998 of GM3 synthase (GM3S, ST3GAL5), the enzyme responsible for initiating biosynthesis of all complex gangliosides, the efforts of our research group have been focused on clarifying the physiological and pathological implications of gangliosides, particularly GM3. We have identified isoforms of GM3S proteins having distinctive lengths of N-terminal cytoplasmic tails, and found that these cytoplasmic tails define subcellular localization, stability, and in vivo activity of GM3S isoforms. Our studies of the molecular pathogenesis of type 2 diabetes, focused on interaction between insulin receptor and GM3 in membrane microdomains, led to a novel concept: type 2 diabetes and certain other lifestyle-related diseases are membrane microdomain disorders resulting from aberrant expression of gangliosides. This concept has enhanced our understanding of the pathophysiological roles of GM3 and related gangliosides in various diseases involving chronic inflammation, such as insulin resistance, leptin resistance, and T-cell function and immune disorders (e.g., allergic asthma). We also demonstrated an essential role of GM3 in murine and human auditory systems; a common pathological feature of GM3S deficiency is deafness. This is the first direct link reported between gangliosides and auditory functions.

Lipid metabolic perturbation is an early-onset phenotype in adult spinster mutants: a Drosophila model for lysosomal storage disorders

Intracellular accumulation of lipids and swollen dysfunctional lysosomes are linked to several neurodegenerative diseases, including lysosomal storage disorders (LSD). Detailed characterization of lipid metabolic changes in relation to the onset and progression of neurodegeneration is currently missing. We systematically analyzed lipid perturbations in spinster (spin) mutants, a Drosophila model of LSD-like neurodegeneration. Our results highlight an imbalance in brain ceramide and sphingosine in the early stages of neurodegeneration, preceding the accumulation of endomembranous structures, manifestation of altered behavior, and buildup of lipofuscin. Manipulating levels of ceramidase and altering these lipids in spin mutants allowed us to conclude that ceramide homeostasis is the driving force in disease progression and is integral to spin function in the adult nervous system. We identified 29 novel physical interaction partners of Spin and focused on the lipid carrier protein, Lipophorin (Lpp). A subset of Lpp and Spin colocalize in the brain and within organs specialized for lipid metabolism (fat bodies and oenocytes). Reduced Lpp protein was observed in spin mutant tissues. Finally, increased levels of lipid metabolites produced by oenocytes in spin mutants allude to a functional interaction between Spin and Lpp, underscoring the systemic nature of lipid perturbation in LSD.

The origin and function of platelet glycosyltransferases

Platelets are megakaryocyte subfragments that participate in hemostatic and host defense reactions and deliver pro- and antiangiogenic factors throughout the vascular system. Although they are anucleated cells that lack a complex secretory apparatus with distinct Golgi/endoplasmic reticulum compartments, past studies have shown that platelets have glycosyltransferase activities. In the present study, we show that members of 3 distinct glycosyltransferase families are found within and on the surface of platelets. Immunocytology and flow cytometry results indicated that megakaryocytes package these Golgi-derived glycosyltransferases into vesicles that are sent via proplatelets to nascent platelets, where they accumulate. These glycosyltransferases are active, and intact platelets glycosylate large exogenous substrates. Furthermore, we show that activation of platelets results in the release of soluble glycosyltransferase activities and that platelets contain sufficient levels of sugar nucleotides for detection of glycosylation of exogenously added substrates. Therefore, the results of the present study show that blood platelets are a rich source of both glycosyltransferases and donor sugar substrates that can be released to function in the extracellular space. This platelet-glycosylation machinery offers a pathway to a simple glycoengineering strategy improving storage of platelets and may serve hitherto unknown biologic functions.

Roles of gangliosides in mouse embryogenesis and embryonic stem cell differentiation

Gangliosides have been suggested to play important roles in various functions such as adhesion, cell differentiation, growth control, and signaling. Mouse follicular development, ovulation, and luteinization during the estrous cycle are regulated by several hormones and cell-cell interactions. In addition, spermatogenesis in seminiferous tubules of adult testes is also regulated by several hormones, including follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and cell-cell interactions. The regulation of these processes by hormones and cell-cell interactions provides evidence for the importance of surface membrane components, including gangliosides. During preimplantation embryo development, a mammalian embryo undergoes a series of cleavage divisions whereby a zygote is converted into a blastocyst that is sufficiently competent to be implanted in the ma ternal uterus and continue its development. Mouse embryonic stem (mES) cells are pluripotent cells derived from mouse embryo, specifically, from the inner cell mass of blastocysts. Differentiated neuronal cells are derived from mES cells through the formation of embryonic bodies (EBs). EBs recapitulate many aspects of lineage-specific differentiation and temporal and spatial gene expression patterns during early embryogenesis. Previous studies on ganglioside expression during mouse embryonic development (including during in vitro fertilization, ovulation, spermatogenesis, and embryogenesis) reported that gangliosides were expressed in both undifferentiated and differentiated (or differentiating) mES cells. In this review, we summarize some of the advances in our understanding of the functional roles of gangliosides during the stages of mouse embryonic development, including ovulation, spermatogenesis, and embryogenesis, focusing on undifferentiated and differentiated mES cells (neuronal cells).

Ganglioside GM3 and its biological functions

Metabolism, topology, and possible mechanisms for regulation of the ganglioside GM3 content in the cell are reviewed. Under consideration are biological functions of GM3, such as involvement in cell differentiation, proliferation, oncogenesis, and apoptosis.

Human GM3 synthase: a new mRNA variant encodes an NH2-terminal extended form of the protein

All human GM3 synthase mRNA variants until now identified predict a protein of 362 amino acids having substrate activity highly restricted to lactosylceramide. In this report we describe the identification of a new GM3 synthase transcript containing an additional translation start codon, located upstream and in-frame with that up to now considered unique translation initiation site in the human GM3 synthase gene. In vitro expression studies showed that the new transcript produces a longer form of human GM3 synthase, that is efficiently translocated into the microsomal lumen and glycosylated. Moreover, stable cDNA transfection into mammalian cells gives rise to a threefold increase of GM3 synthase activity, associated to a broader substrate specificity. Although this transcript has been initially identified in the human placenta, RT-PCR analyses verified the expression of an identical mRNA also in undifferentiated HL60 cells, but not in the monocytic lineage. Altogether, these results are the first demonstration of the existence of a new isoform of human GM3 synthase, which could play an important role during HL60 cell differentiation. The functional relevance of the existence of two isoforms of GM3 synthase is also discussed.

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Gene expression profiling reveals multiple novel intrinsic and extrinsic factors associated with axonal regeneration failure

In contrast to the regeneration-competent peripheral nervous system (PNS), lesions of nerve tracts within the central nervous system (CNS) lead to chronically impaired neuronal connections. We have analysed changes in gene expression patterns occurring as a consequence of postcommissural fornix transection at a time when spontaneous axonal growth has ceased at the lesion site. This was done in order to describe both extrinsic and intrinsic determinants of regeneration failure. Using a genomic approach we have identified a number of so far undetected factors such as bamacan and semaphorin 6B, which relate to chronic axonal growth arrest and therefore are promising candidates for lesion-induced axonal growth inhibitors. In addition, we observed that within the subiculum, where the fornix axons originate, neuronal Oct-6 was induced and NG2 was down-regulated, indicating that axotomized neurons as well as glial cells react at the level of gene expression to remote axotomy.

Role of multiple drug resistance protein 1 in neutral but not acidic glycosphingolipid biosynthesis

Transfection studies have implicated the multiple drug resistance pump, MDR1, as a glucosyl ceramide translocase within the Golgi complex (Lala, P., Ito, S., and Lingwood, C. A. (2000) J. Biol. Chem. 275, 6246-6251). We now show that MDR1 inhibitors, cyclosporin A or ketoconazole, inhibit neutral glycosphingolipid biosynthesis in 11 of 12 cell lines tested. The exception, HeLa cells, do not express MDR1. Microsomal lactosyl ceramide and globotriaosyl ceramide synthesis from endogenous or exogenously added liposomal glucosyl ceramide was inhibited by cyclosporin A, consistent with a direct role for MDR1/glucosyl ceramide translocase activity in their synthesis. In contrast, cellular ganglioside synthesis in the same cells, was unaffected by MDR1 inhibition, suggesting neutral and acid glycosphingolipids are synthesized from distinct precursor glycosphingolipid pools. Metabolic labeling in wild type and knock-out (MDR1a, 1b, MRP1) mouse fibroblasts showed the same loss of neutral glycosphingolipid (glucosyl ceramide, lactosyl ceramide) but not ganglioside (GM3) synthesis, confirming the proposed role for MDR1 translocase activity. Cryo-immunoelectron microscopy showed MDR1 was predominantly intracellular, largely in rab6-containing Golgi vesicles and Golgi cisternae, the site of glycosphingolipid synthesis. These studies identify MDR1 as the major glucosyl ceramide flippase required for neutral glycosphingolipid anabolism and demonstrate a previously unappreciated dichotomy between neutral and acid glycosphingolipid synthesis.

Biosynthesis and degradation of mammalian glycosphingolipids

Glycolipids are a large and heterogeneous family of sphingolipids that form complex patterns on eukaryotic cell surfaces. This molecular diversity is generated by only a few enzymes and is a paradigm of naturally occurring combinatorial synthesis. We report on the biosynthetic principles leading to this large molecular diversity and focus on sialic acid-containing glycolipids of the ganglio-series. These glycolipids are particularly concentrated in the plasma membrane of neuronal cells. Their de novo synthesis starts with the formation of the membrane anchor, ceramide, at the endoplasmic reticulum (ER) and is continued by glycosyltransferases of the Golgi complex. Recent findings from genetically engineered mice are discussed. The constitutive degradation of glycosphingolipids (GSLs) occurs in the acidic compartments, the endosomes and the lysosomes. Here, water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues from glycolipids. For glycolipid substrates with short oligosaccharide chains, the additional presence of membrane-active sphingolipid activator proteins (SAPs) is required. A considerable part of our current knowledge about glycolipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within this pathway. A new post-translational modification is the attachment of glycolipids to proteins of the human skin.

Region-specific and epileptogenic-dependent expression of six subtypes of alpha2,3-sialyltransferase in the adult mouse brain

Sialylated glycoconjugates play important roles in various biological functions. The structures are also observed in brains and it has been proposed that sialylation may affect neural plasticity. To clarify the effects of sialylation in the brain, particular neurons that exhibit sialylation should first be determined. Using in situ hybridization, we performed systematic surveys of the localization of mRNAs encoding the six alpha2,3-sialyltransferases (ST3Gal I-VI) in the adult mouse brain with or without physiological stimulation. First, striking region-specific patterns of expression were observed: While ST3Gal II, III, and V mRNAs were in neuronal cells throughout the brain, ST3Gal I, IV, and VI mRNAs were in restricted brain regions. Next, to assess whether the expression of the six mRNAs can be regulated, we examined the effect of kindling epileptogenesis on the six mRNA levels. Of the six subtypes, upregulation in the ST3Gal IV level in the thalamus was most pronounced; the number of ST3Gal IV-expressing neurons in the anterior thalamic nuclei increased from 2% to 21% in a time-dependent manner during epileptogenesis. Western blot analysis evaluated the increase of the end-products in the thalamus. These findings provide a molecular basis to clarify when and where sialylated glycoconjugates function accompanied by neural plasticity.

Ectopic localizations of Golgi glycosyltransferases

Glycosyltransferases involved in N- and O-glycan chain elongation and termination are localized in the Golgi apparatus. Early evidence in support of this rule was based on fractionation techniques and was corroborated by numerous immunocytochemical studies. Usually these studies were confined to cultured cell lines exhibiting little differentiation features, such as HeLa cells. However, localization studies conducted in primary cell cultures (e.g., human umbilical vein endothelial cells), cells obtained ex vivo (e.g., sperm cells), and tissue sections (e.g., intestinal, renal, or hepatic tissue) often reveal ectopic localizations of glycosyltransferases usually at post-Golgi sites, including the plasma membrane. Hence, extracellular cues resulting from specific adhesion sites may influence post-Golgi trafficking routes, which may be reflected by ectopic localization of Golgi enzymes.

Differential distribution of ganglioside GM3 in seminiferous tubule and epididymis of adult rats

Gangliosides are ubiquitous membrane components in mammalian cells and are suggested to play important roles in various functions such as cell-cell interaction, adhesion, cell differentiation, growth control and signaling. Among all ganglio-series gangliosides, GM3 has the simplest carbohydrate structure, and has been shown as a major ganglioside in male reproductive system. To study GM3 distribution in the seminiferous tubule and epididymis, frozen sections were stained with specific monoclonal antibody (MAb) against ganglioside GM3. In the seminiferous tubule of testis, pachytene spermatocytes and spermatids expressed ganglioside GM3, but not in spermatogonia and sertoli cells. Spermatogonia and sertoli cells near the basement membrane were negatively reacted to anti-GM3. In the epididymis, GM3 was expressed only in some interstitial cells. Taken together, these results suggest that the expression of ganglioside GM3 in rat seminiferous tubule and epididymis is spatio-temporally regulated during spermatogenesis.

A ganglioside-specific sialyltransferase localizes to axons and non-Golgi structures in neurons

To investigate the tissue distribution and subcellular localization of ST3GalV (CMP-NeuAc:lactosylceramide alpha2,3 sialyltransferase/GM3 synthase) in the adult mouse, we generated two antisera against mouse ST3GalV that were designated CS2 (directed against amino acids K227-I272) and CS14 (directed against amino acids D308-H359). We previously reported that CS2 antiserum stains medial and trans-Golgi cisternae in all cell types investigated. In neural tissue, however, CS14 antiserum reveals a subpopulation of ST3GalV with a subcellular distribution complementary to CS2 antiserum. CS14 antiserum strongly stains axons in cortical, cerebellar, brainstem, and spinal cord tissue sections. The subcellular localization of neuronal ST3GalV is maintained in primary cultures of rat hippocampal neurons and in PC12 cells. In PC12 cells, ST3GalV localization evolves during NGF-induced differentiation such that a pool of enzyme leaves the Golgi for a distal compartment in conjunction with neurite outgrowth. In PC12 cells transfected with an epitope-tagged form of ST3GalV, staining for the epitope tag coincides with expression of endogenous enzyme. The non-Golgi pool of ST3GalV does not colocalize with markers for the trans-Golgi network, endosome, or synaptic vesicles, nor is it detected on the cell surface. Distinct subpopulations of ST3GalV imply that ganglioside synthesis can occur outside of the Golgi or, alternatively, that a portion of the total ST3GalV pool subserves a nonenzymatic function. Significantly fewer transfected cells were found in PC12 cultures treated with plasmid encoding ST3GalV than in cultures treated with control plasmid, indicating that the expression of ST3GalV in excess of endogenous levels results in either cell death or a decreased rate of cell division.

Developmental patterns of mST3GalV mRNA expression in the mouse: in situ hybridization using DIG-labeled RNA probes

mST3GalV synthesizes ganglioside GM3, the precursor for simple and complex a- and b- series gangliosides, and the expression and regulation of mST3GalV (CMP-NeuAc: lactosylceramide alpha2,3-sialyltransferase) activity is central to the production of almost all gangliosides, a class of glycosphingolipids implicated in variety of cellular processes such as transmembrane signaling, synaptic transmission, specialized membrane domain formation and cell-cell interactions. To understand the developmental expression of mST3GalV in mice, we investigated the spatial and temporal expression of mST3GalV mRNA during the mouse embryogenesis [embryonic (E) days; E9, E11, E13, E15] by in situ hybridization with digoxigenin-labeled RNA probes. All tissues from E9 and E11 were positive for mST3GalV mRNA. On E13, mST3GalV mRNA was expressed in various neural and non-neural tissues. In contrast to these, on E15, the telencephalon and liver produced a strong expression of mST3Gal V which was a quite similar to that of E13. In this stage, mST3GalV mRNA was also expressed in some non-neural tissues. These data indicate that mST3GalV is differently expressed at developmental stages of embryo, and this may be importantly related with regulation of organogenesis in mice.