Molecular cloning and expression of a fifth type of alpha2,8-sialyltransferase (ST8Sia V). Its substrate specificity is similar to that of SAT-V/III, which synthesize GD1c, GT1a, GQ1b and GT3

Identification of a buried β-strand as a novel disease-related motif in the human polysialyltransferases

The polysialyltransferases ST8SIA2 and ST8SIA4 and their product, polysialic acid (polySia), are known to be related to cancers and mental disorders. ST8SIA2 and ST8SIA4 have conserved amino acid (AA) sequence motifs essential for the synthesis of the polySia structures on the neural cell adhesion molecule. To search for a new motif in the polysialyltransferases, we adopted the in silico Individual Meta Random Forest program that can predict disease-related AA substitutions. The Individual Meta Random Forest program predicted a new eight-amino-acids sequence motif consisting of highly pathogenic AA residues, thus designated as the pathogenic (P) motif. A series of alanine point mutation experiments in the pathogenic motif (P motif) showed that most P motif mutants lost the polysialylation activity without changing the proper enzyme expression levels or localization in the Golgi. In addition, we evaluated the enzyme stability of the P motif mutants using newly established calculations of mutation energy, demonstrating that the subtle change of the conformational energy regulates the activity. In the AlphaFold2 model, we found that the P motif was a buried β-strand underneath the known surface motifs unique to ST8SIA2 and ST8SIA4. Taken together, the P motif is a novel buried β-strand that regulates the full activity of polysialyltransferases from the inside of the molecule.

The Ying and Yang of Ganglioside Function in Cancer

The plethora of information about the expression of cancer cell-associated gangliosides, their role(s) in signal transduction, and their potential usefulness in the development of cancer treatments makes this an appropriate time to review these enigmatic glycosphingolipids. Evidence, reflecting the work of many, indicates that (1) expression of specific gangliosides, not generally found in high concentrations in most normal human cells, can be linked to certain types of cancer. (2) Gangliosides can affect the ability of cells to interact either directly or indirectly with growth factor receptors, thereby changing such things as a cell's mobility, rate of proliferation, and metastatic ability. (3) Anti-ganglioside antibodies have been tested, with some success, as potential treatments for certain cancers. (4) Cancer-associated gangliosides shed into the circulation can (a) affect immune cell responsiveness either positively or negatively, (b) be considered as diagnostic markers, and (c) be used to look for recurrence. (5) Cancer registries enable investigators to evaluate data from sufficient numbers of patients to obtain information about potential therapies. Despite advances that have been made, a discussion of possible approaches to identifying additional treatment strategies to inhibit metastasis, responsible for the majority of deaths of cancer patients, as well as for treating therapy-resistant tumors, is included.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

The α2,8-sialyltransferase 6 (St8sia6) localizes in the ER and enhances the anchorage-independent cell growth in cancer

Sialylation, the final stage of post-translational modification of proteins, is achieved in the Golgi apparatus and is related to the malignant phenotype of cancer. Disialylation of ganglioside (GD3) by St8sia1 and polysialylation by St8sia2 and 4 have been shown to be related to malignant phenotypes; however, di/oligosialylation by St8sia6 is still unknown. In this study, we analyzed the malignant phenotype of St8sia6 and found that upregulation of St8sia6 in melanoma B16 cells increased anchorage-independent cell growth, which was not due to sialic acid cleavage by a sialidase. Moreover, unlike other sialyltransferases, St8sia6 localized to the endoplasmic reticulum (ER). We found that the localization to the Golgi apparatus could be regulated by swapping experiments using St8sia2; however, the malignant phenotype did not change. These data demonstrate that the enhancement of anchorage-independent cell growth by St8sia6 is not due to its localization of ER, but is due to the expression of the protein itself.

Analysis of biochemical features of ST8 α-N-acetyl-neuraminide α2,8-sialyltransferase (St8sia) 5 isoforms

Gangliosides are important components of the membrane and are involved in many biological activities. St8sia5 is an α2,8-sialyltransferase involved in ganglioside synthesis, and has three isoforms. In this study, we analyzed the features of three isoforms, St8sia5-S, -M, and -L that had not been analyzed, and found that only St8sia5-L was localized in the Golgi, while the majority of St8sia5-M and -S were localized in the ER. The localization of Golgi of St8sia5 depended on the stem region. In addition, the incorporation of exogenous GD3 was upregulated only in St8sia5-L expressing cells. Taken together, the localization of St8sia5 is important for the activity of the enzyme.

The Distinct Roles of Sialyltransferases in Cancer Biology and Onco-Immunology

Aberrant glycosylation is a key feature of malignant transformation. Hypersialylation, the enhanced expression of sialic acid-terminated glycoconjugates on the cell surface, has been linked to immune evasion and metastatic spread, eventually by interaction with sialoglycan-binding lectins, including Siglecs and selectins. The biosynthesis of tumor-associated sialoglycans involves sialyltransferases, which are differentially expressed in cancer cells. In this review article, we provide an overview of the twenty human sialyltransferases and their roles in cancer biology and immunity. A better understanding of the individual contribution of select sialyltransferases to the tumor sialome may lead to more personalized strategies for the treatment of cancer.

Data mining of the cancer-related lncRNAs GO terms and KEGG pathways by using mRMR method

LncRNAs plays an important role in the regulation of gene expression. Identification of cancer-related lncRNAs GO terms and KEGG pathways is great helpful for revealing cancer-related functional biological processes. Therefore, in this study, we proposed a computational method to identify novel cancer-related lncRNAs GO terms and KEGG pathways. By using existing lncRNA database and Max-relevance Min-redundancy (mRMR) method, GO terms and KEGG pathways were evaluated based on their importance on distinguishing cancer-related and non-cancer-related lncRNAs. Finally, GO terms and KEGG pathways with high importance were presented and analyzed. Our literature reviewing showed that the top 10 ranked GO terms and pathways were really related to interpretable tumorigenesis according to recent publications.

Emerging concepts of ganglioside metabolism

Gangliosides (GGs) are sialic acid-containing glycosphingolipids (GSLs) and major membrane components enriched on cellular surfaces. Biosynthesis of mammalian GGs starts at the cytosolic leaflet of endoplasmic reticulum (ER) membranes with the formation of their hydrophobic ceramide anchors. After intracellular ceramide transfer to Golgi and trans-Golgi network (TGN) membranes, anabolism of GGs, as well as of other GSLs, is catalyzed by membrane-spanning glycosyltransferases (GTs) along the secretory pathway. Combined activity of only a few promiscuous GTs allows for the formation of cell-type-specific glycolipid patterns. Following an exocytotic vesicle flow to the cellular plasma membranes, GGs can be modified by metabolic reactions at or near the cellular surface. For degradation, GGs are endocytosed to reach late endosomes and lysosomes. Whereas membrane-spanning enzymes of the secretory pathway catalyze GSL and GG formation, a cooperation of soluble glycosidases, lipases and lipid-binding cofactors, namely the sphingolipid activator proteins (SAPs), act as the main players of GG and GSL catabolism at intralysosomal luminal vesicles (ILVs).

Advanced mass spectrometry and chemical analyses reveal the presence of terminal disialyl motif on mouse B-cell glycoproteins

The occurrence of a terminal disialyl motif on mammalian O-glycans is increasingly being identified through recent mass spectrometry (MS)-based glycomic profiling. In most cases, it is carried on simple core 1 structures in which both the galactose and N-acetyl galactosamine can be disialylated. In contrast, a disialyl motif on N-glycans is less readily revealed by MS mapping, since additional MS/MS analysis is required to determine the distribution of the various sialic acids on typically multisialylated complex type N-glycans. In our MS-based glycomic screening, we found that a mouse B lymphoma cell line, BCL1, ranks among those that have the highest amount of disialyl motif on its O-glycans, including those carried on CD45. More intriguingly, detailed chemical and MS/MS analyses unambiguously showed that the Neu5Gcα2-8Neu5Gc disialyl motif is also present on the N-glycans and that it can be carried on the termini of polylactosaminoglycan chains, which can be further sulfated on the proximal GlcNAc, occurring alongside other monosialylated sulfated LacNAc termini. Upon silencing the expression of mouse α2,8-sialyltransferase VI (ST8Sia VI), the overall disialyl content decreases significantly, but more so for that on the N-glycans than the O-glycans. ST8Sia VI was further shown to be the most significantly upregulated ST8Sia during plasma cell differentiation, which coincides with increasing content of the disialyl motif. Increasing terminal disialylation without leading to polysialylation may thus have important biological consequences awaiting further investigation. Likewise, the expression of mono- and disialylated sulfated LacNAc may constitute novel recognition codes modulating B-cell activation and differentiation.

Effects of amino acid substitutions in the sialylmotifs on molecular expression and enzymatic activities of α2,8-sialyltransferases ST8Sia-I and ST8Sia-VI

Mouse sialyltransferases are grouped into four families according to the type of carbohydrate linkage they synthesize: β-galactoside α2,3-sialyltransferases (ST3Gal-I-VI), β-galactoside α2,6-sialyltransferases (ST6Gal-I and ST6Gal-II), N-acetylgalactosamine α2,6-sialyltransferases (ST6GalNAc-I-VI) and α2,8-sialyltransferases (ST8Sia-I-VI). These sialyltransferases feature a type II transmembrane topology and contain highly conserved motifs termed sialylmotifs L, S, III and VS. Sialylmotifs L and S are involved in substrate binding, whereas sialylmotifs III and VS are involved in catalytic activity. In addition to the conventional sialylmotifs, family and subfamily specific sequence motifs have been proposed. In this study, we analyzed the properties and functions of sialylmotifs in characterizing the enzymatic activity of mouse ST8Sia-I and ST8Sia-VI, both of which are α2,8-sialyltransferases involved in the synthesis of either ganglioside GD3 or disialic acid structures on O-glycans, respectively. The ST8Sia-VI-based chimeric enzymes, whose sialylmotif L sequences were replaced with those of ST8Sia-I and ST8Sia-IV (polysialic acid synthetase), were still active toward O-glycans. However, ST8Sia-VI-based chimeric enzymes lost expression or activity when their sialylmotif L sequences were replaced with those of ST3Gal-I and ST6GalNAc-II, suggesting the existence of an ST8Sia family specific motif in the sialylmotif L. The ST8Sia-I- and ST8Sia-VI-based chimeric enzymes lost enzymatic activity when their sialylmotif S sequences were interchanged. Amino acid substitutions in the sialylmotif S of ST8Sia-I and ST8Sia-VI also affected the enzymatic activity in many cases, indicating the crucial and functional importance of the sialylmotif S in substrate binding, which determines the substrate specificity of sialyltransferase.

Expression of glycogenes in differentiating human NT2N neurons. Downregulation of fucosyltransferase 9 leads to decreased Lewis(x) levels and impaired neurite outgrowth

BACKGROUND

Several glycan structures are functionally relevant in biological events associated with differentiation and regeneration which occur in the central nervous system. Here we have analysed the glycogene expression and glycosylation patterns during human NT2N neuron differentiation. We have further studied the impact of downregulating fucosyltransferase 9 (FUT9) on neurite outgrowth.

METHODS

The expression of glycogenes in human NT2N neurons differentiating from teratocarcinoma NTERA-2/cl.D1 cells has been analysed using the GlycoV4 GeneChip expression microarray. Changes in glycosylation have been monitored by immunoblot, immunofluorescence microscopy, HPLC and MALDI-TOF MS. Peptide mass fingerprinting and immunoprecipitation have been used for protein identification. FUT9 was downregulated using silencing RNA.

RESULTS AND CONCLUSIONS

One hundred twelve mRNA transcripts showed statistically significant up-regulation, including the genes coding for proteins involved in the synthesis of the Lewis(x) motif (FUT9), polysialic acid (ST8SIA2 and ST8SIA4) and HNK-1 (B3GAT2). Accordingly, increased levels of the corresponding carbohydrate epitopes have been observed. The Lewis(x) structure was found in a carrier glycoprotein that was identified as the CRA-a isoform of human neural cell adhesion molecule 1. Downregulation of FUT9 caused significant decreases in the levels of Lewis(x), as well as GAP-43, a marker of neurite outgrowth. Concomitantly, a reduction in neurite formation and outgrowth has been observed that was reversed by FUT9 overexpression.

GENERAL SIGNIFICANCE

These results provided information about the regulation of glycogenes during neuron differentiation and they showed that the Lewis(x) motif plays a functional role in neurite outgrowth from human neurons.

The role of milk sialyllactose in intestinal bacterial colonization

Milk oligosaccharides influence the composition of intestinal microbiota and thereby mucosal inflammation. Some of the major milk oligosaccharides are α2,3-sialyllactose (3SL) and α2,6-sialyllactose, which are mainly produced by the sialyltransferases ST3GAL4 and ST6GAL1, respectively. Recently, we showed that mice fed milk deficient in 3SL were more resistant to dextran sulfate sodium-induced colitis. By contrast, the exposure to milk containing or deficient in 3SL had no impact on the development of mucosal leukocyte populations. Milk 3SL mainly affected the colonization of the intestine by clostridial cluster IV bacteria.

Disulphide linkage in mouse ST6Gal-I: determination of linkage positions and mutant analysis

All cloned sialyltransferases from vertebrates are classified into four subfamilies and are characterized as having type II transmembrane topology. The catalytic domain has highly conserved motifs known as sialylmotifs. Besides sialylmotifs, each family has several unique conserved cysteine (Cys) residues mainly in the catalytic domain. The number and loci of conserved amino acids, however, differ with each subfamily, suggesting that the conserved Cys-residues and/or disulphide linkages they make may contribute to linkage specificity. Using Matrix Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF)-mass spectrometry, the present study performed disulphide linkage analysis on soluble mouse ST6Gal-I, which has six Cys-residues. Results confirmed that there were no free Cys-residues, and all six residues contributed to disulphide linkage formation, C(139)-C(403), C(181)-C(332) and C(350)-C(361). Study of single amino acid-substituted mutants revealed that the disulphide linkage C(181)-C(332) was necessary for molecular expression of the enzyme, and that the disulphide linkage C(350)-C(361) was necessary for enzyme activity. The remaining disulphide linkage C(139)-C(403) was not necessary for enzyme expression or for activity, including substrate specificity. Crystallographic study of pig ST3Gal I has recently been reported. Interestingly, the loci of disulphide linkages in ST6Gal-I differ from those in ST3Gal I, suggesting that the linkage specificity of sialyltransferase may results from significant structural differences, including the loci of disulphide linkages.

Cellular and molecular biology of glycosphingolipid glycosylation

Brain tissue is characterized by its high glycosphingolipid content, particularly those containing sialic acid (gangliosides). As a result of this observation, brain tissue was a focus for studies leading to the characterization of the enzymes participating in ganglioside biosynthesis, and their participation in driving the compositional changes that occur in glycolipid expression during brain development. Later on, this focus shifted to the study of cellular aspects of the synthesis, which lead to the identification of the site of synthesis in the neuronal soma and their axonal transport toward the periphery. In this review article, we will focus in subcellular aspects of the biosynthesis of glycosphingolipid oligosaccharides, particularly the mechanisms underlying the trafficking of glycosphingolipid glycosyltransferases from the endoplasmic reticulum to the Golgi, those that promote their retention in the Golgi and those that participate in their topological organization as part of the complex membrane bound machinery for the synthesis of glycosphingolipids.

The cytoplasmic tail of GM3 synthase defines its subcellular localization, stability, and in vivo activity

GM3 synthase (SAT-I) is the primary glycosyltransferase responsible for the biosynthesis of ganglio-series gangliosides. In this study, we identify three isoforms of mouse SAT-I proteins, named M1-SAT-I, M2-SAT-I, and M3-SAT-I, which possess distinct lengths in their NH(2)-terminal cytoplasmic tails. These isoforms are produced by leaky scanning from mRNA variants of mSAT-Ia and mSAT-Ib. M2-SAT-I and M3-SAT-I were found to be localized in the Golgi apparatus, as expected, whereas M1-SAT-I was exclusively found in the endoplasmic reticulum (ER). Specific multiple arginines (R) arranged in an R-based motif, RRXXXXR necessary for ER targeting, were found in the cytoplasmic tail of M1-SAT-I, and in vivo GM3 biosynthesis by M1-SAT-I was very low because of restricted transport to the Golgi apparatus. In addition, M1-SAT-I and M3-SAT-I had a long half-life relative to M2-SAT-I. This is the first report demonstrating the presence of an ER-targeting R-based motif in the cytoplasmic tail of a protein in the mammalian glycosyltransferase family of enzymes. The system, which produces SAT-I isoforms having distinct characteristics, is likely to be of critical importance for the regulation of GM3 biosynthesis under various pathological and physiological conditions.

Evolutionary history of the alpha2,8-sialyltransferase (ST8Sia) gene family: tandem duplications in early deuterostomes explain most of the diversity found in the vertebrate ST8Sia genes

BACKGROUND

The animal sialyltransferases, which catalyze the transfer of sialic acid to the glycan moiety of glycoconjugates, are subdivided into four families: ST3Gal, ST6Gal, ST6GalNAc and ST8Sia, based on acceptor sugar specificity and glycosidic linkage formed. Despite low overall sequence identity between each sialyltransferase family, all sialyltransferases share four conserved peptide motifs (L, S, III and VS) that serve as hallmarks for the identification of the sialyltransferases. Currently, twenty subfamilies have been described in mammals and birds. Examples of the four sialyltransferase families have also been found in invertebrates. Focusing on the ST8Sia family, we investigated the origin of the three groups of alpha2,8-sialyltransferases demonstrated in vertebrates to carry out poly-, oligo- and mono-alpha2,8-sialylation.

RESULTS

We identified in the genome of invertebrate deuterostomes, orthologs to the common ancestor for each of the three vertebrate ST8Sia groups and a set of novel genes named ST8Sia EX, not found in vertebrates. All these ST8Sia sequences share a new conserved family-motif, named "C-term" that is involved in protein folding, via an intramolecular disulfide bridge. Interestingly, sequences from Branchiostoma floridae orthologous to the common ancestor of polysialyltransferases possess a polysialyltransferase domain (PSTD) and those orthologous to the common ancestor of oligosialyltransferases possess a new ST8Sia III-specific motif similar to the PSTD. In osteichthyans, we have identified two new subfamilies. In addition, we describe the expression profile of ST8Sia genes in Danio rerio.

CONCLUSION

Polysialylation appeared early in the deuterostome lineage. The recent release of several deuterostome genome databases and paralogons combined with synteny analysis allowed us to obtain insight into events at the gene level that led to the diversification of the ST8Sia genes, with their corresponding enzymatic activities, in both invertebrates and vertebrates. The initial expansion and subsequent divergence of the ST8Sia genes resulted as a consequence of a series of ancient duplications and translocations in the invertebrate genome long before the emergence of vertebrates. A second subset of ST8sia genes in the vertebrate genome arose from whole genome duplication (WGD) R1 and R2. Subsequent selective ST8Sia gene loss is responsible for the characteristic ST8Sia gene expression pattern observed today in individual species.

Cloning and transcriptional regulation of genes responsible for synthesis of gangliosides

Ganglioside synthases are glycosyltransferases involved in the biosynthesis of glycoconjugates. A number of ganglioside synthase genes have been cloned and characterized. They are classified into different families of glycosyltransferases based on similarities of their amino acid sequences. Tissue-specific expression of these genes has been analyzed by hybridization using cDNA fragments. Enzymatic characterization with the expressed recombinant enzymes showed these enzymes differ in their donor and acceptor substrate specificities and other biochemical parameters. In vitro enzymatic analysis also showed that one linkage can be synthesized by multiple enzymes and one enzyme may be responsible for synthesis of multiple gangliosides. Following the cloning of the ganglioside synthase genes, the promoters of the key synthase genes in the ganglioside biosynthetic pathway have been cloned and analyzed. All of the promoters are TATA-less, lacking a CCAAT box but containing GC-rich boxes, characteristic of the house-keeping genes, although transcription of ganglioside synthase genes is subject to complex developmental and tissue-specific regulation. A set of cis-acting elements and transcription factors, including Sp1, AP2, and CREB, function in the proximal promoters. Negative-regulatory regions have also been defined in most of the promoters. We present here an overview of these genes and their transcriptional regulation.

Developmental changes of glycosphingolipids and expression of glycogenes in mouse brains

Glycosphingolipids (GSLs) and their sialic acid-containing derivatives, gangliosides, are important cellular components and are abundant in the nervous system. They are known to undergo dramatic changes during brain development. However, knowledge on the mechanisms underlying their qualitative and qualitative changes is still fragmentary. In this investigation, we have provided a detailed study on the developmental changes of the expression patterns of GSLs, GM3, GM1, GD3, GD1a, GD2, GD1b, GT1b, GQ1b, A2B5 antigens (c-series gangliosides such as GT3 and GQ1c), Chol-1alpha (GT1aalpha and GQ1balpha), glucosylceramide, galactosylceramide (O1 antigen), sulfatide (O4 antigen), stage-specific embryonic antigen-1 (Lewis x) glycolipids, and human natural killer-1 glycolipid (sulfoglucuronosyl paragloboside) in developing mouse brains [embryonic day 12 (E12) to adult]. In E12-E14 brains, GD3 was a predominant ganglioside. After E16, the concentrations of GD3 and GM3 markedly decreased, and the concentrations of a-series gangliosides, such as GD1a, increased. GT3, glucosylceramide, and stage-specific embryonic antigen-1 were expressed in embryonic brains. Human natural killer-1 glycolipid was expressed transiently in embryonic brains. On the other hand, Chol-1alpha, galactosylceramide, and sulfatide were exclusively found after birth. To provide a better understanding of the metabolic basis for these changes, we analyzed glycogene expression patterns in the developing brains and found that GSL expression is regulated primarily by glycosyltransferases, and not by glycosidases. In parallel studies using primary neural precursor cells in culture as a tool for studying developmental events, dramatic changes in ganglioside and glycosyltransferase gene expression were also detected in neurons induced to differentiate from neural precursor cells, including the expression of GD3, followed by up-regulation of complex a- and b-series gangliosides. These changes in cell culture systems resemble that occurring in brain. We conclude that the dramatic changes in GSL pattern and content can serve as useful markers in neural development and that these changes are regulated primarily at the level of glycosyltransferase gene expression.

Molecular basis for polysialylation: a novel polybasic polysialyltransferase domain (PSTD) of 32 amino acids unique to the alpha 2,8-polysialyltransferases is essential for polysialylation

To determine the molecular basis of eukaryotic polysialylation, the function of a structurally unique polybasic motif of 32 amino acids (pI approximately 12) in the polysialyltransferases (polySTs), ST8Sia II (STX and ST8Sia IV (PST) was investigated. This motif, designated the "polysialyltransferase domain" (PSTD), is immediately upstream of the sialylmotif S (SM-S). PolyST activity was lost in COS-1 mutants in which the entire PSTD in ST8Sia IV was deleted, or in mutants in which 10 and 15 amino acids in either the N- or C- terminus of PSTD were deleted. Site-directed mutagenesis showed that Ile(275), Lys(276) and Arg(277) in the C-terminus of PSTD in ST8Sia IV, which is contiguous with the N-terminus of sialylmotif-S, were essential for polysialylation. Arg(252) in the N-terminus segment of the PSTD was also required, as was the overall positive charge. Thus, multiple domains in the polySTs can influence their activity. Immunofluorescent microscopy showed that the mutated proteins were folded correctly, based on their Golgi localization. The structural distinctness of the conserved PSTD in the polySTs, and its absence in the mono- oligoSTs, suggests that it is a "polymerization domain" that distinguishes a polyST from a monosialyltransferases. We postulate that the electrostatic interaction between the polybasic PSTD and the polyanionic polySia chains may function to tether nascent polySia chains to the enzyme, thus facilitating the processive addition of new Sia residues to the non-reducing end of the growing chain. In accord with this hypothesis, the polyanion heparin was shown to inhibit recombinant human ST8Sia II and ST8Sia IV at 10 microM.

Developmental changes in the expression of glycogenes and the content of N-glycans in the mouse cerebral cortex

Biosynthesis of N-glycans varies significantly among tissues and is strictly regulated spatially and temporally within the tissue. The strict molecular mechanisms that are responsible for control of N-glycan synthesis remain largely unknown. We developed complementary deoxyribonucleic acid (cDNA) macroarray system and analyzed gene expression levels of more than 140 glycosyltransferases and glycosidases in the cerebral cortex from developing and adult mice. We also analyzed the relative amounts of major N-glycans present in the cerebral cortex and examined how the synthesis of N-glycans might be regulated through the expression of these genes. We demonstrated that the content of N-linked oligosaccharides dramatically changed during the course of brain development. Some of these changes could not be explained by alterations in the expression of the corresponding genes. For example, the amount of core fucosylated sugar chains in the early embryonic brain and the expression level of fucosyltransferase VIII, the only gene known to be responsible for core fucosylation, did not change proportionately. This result suggests that post-transcriptional regulation of this gene plays an important role in regulating its enzymatic activity. On the other hand, the amount of beta1,3-galactose residue-containing sugar chains increased postnatally following an increase in the level of beta1,3-galactosyltransferase messenger ribonucleic acid (mRNA). Furthermore, the amount of sugar chains with an outer fucose residue, containing LewisX-BA-2, correlated well with the expression of fusocyltransferase IX mRNA. These findings add to our understanding of the molecular mechanisms responsible for the regulation of N-glycan biosynthesis in the cerebral cortex.

Chronic ethanol consumption down-regulates CMP-NeuAc:GM3 α2,8-sialyltransferase (ST8Sia-1) gene in the rat brain

Alcoholics have an increase in sialic acid-deficient glycoconjugates such as carbohydrate-deficient transferrin, sialic acid-deficient gangliosides and free sialic acids. The elevated presence of these asialoconjugates could be a consequence of alcohol-mediated impaired sialylation rate or due to increased desialylation rate. Chronic ethanol-induced brain abnormalities and behavioral changes could be mediated through these asialogangliosides. We have therefore determined the level of brain CMP-NeuAc:GM(3) alpha2,8-sialyltransferase (ST8Sia-1) and Gal-beta1,3GalNAc alpha2,3-sialyltransferase (ST3Gal-11) messenger RNA (mRNA) and correlated with the activity of these key enzymes in male Wistar rats as a function of increasing dietary concentration of ethanol after 8 weeks of feeding. The relative level of brain synaptosomal ST8Sia-1 and ST3Gal-11 mRNA were determined by real-time quantitative polymerase chain reaction (RT-PCR). We compared the observed ST8Sia-1 gene expression with its enzymatic activity in the synaptosomal membrane fraction isolated from the rat brain in the ethanol and pair-fed control groups. The results showed that the relative level of brain ST8Sia-1 mRNA expression was down-regulated by 13% (p<0.05) in 10.6%, by 40% (p<0.01) in 20.8% and by 57% (p<0.01) in the 36% ethanol-calorie groups, compared to the control (0% ethanol-calorie) group. In addition, ethanol at 36% dietary calories caused a significant 61% (p<0.01) decrease in the brain synaptosomal ST8Sia-1 activity compared to the control group. However, ethanol (10.6, 20.8 or 36% level) did not significantly affect the relative level of brain ST3Gal-11 mRNA as compared to the control (0% ethanol-calorie) group. Thus, our findings imply that chronic ethanol exposure preferentially down-regulates brain ST8Sia-1 mRNA accompanied by a concomitant decrease in its activity in a dose-dependent manner. Therefore, the selective loss of 2,8-sialic acid residues from gangliosides might contribute towards the appearance of asialogangliosides and related brain-abnormalities associated with ethanol abuse.

Molecular cloning and expression of a human hST8Sia VI (alpha2,8-sialyltransferase) responsible for the synthesis of the diSia motif on O-glycosylproteins

Based on BLAST analysis of the human and mouse genome databases using the human CMP sialic acid; alpha2,8-sialyltransferase cDNA (hST8Sia I; EC 2.4.99.8), a putative sialyltransferase gene, was identified on human chromosome 10. The genomic organization was found to be similar to that of hST8Sia I and hST8Sia V. Transcriptional expression analysis showed that the newly identified gene was constitutively expressed at low levels in various human tissues and cell lines. We have isolated a full-length cDNA clone from the breast cancer cell line MCF-7 that encoded a type II membrane protein of 398 amino acid residues with the conserved motifs of sialyltransferases. We have established a mammary cell line (MDA-MB-231) stably transfected with the full-length hST8Sia VI and the analysis of sialylated carbohydrate structures expressed at the cell surface clearly indicated the disappearance of Neu5Acalpha2-3-sialylated structures. The transient expression of a truncated soluble form of the enzyme in either COS-7 cells or insect Sf-9 cells led to the production of an active enzyme in which substrate specificity was determined. Detailed substrate specificity analysis of the hST8Sia VI recombinant enzyme in vitro, revealed that this enzyme required the trisaccharide Neu5Acalpha2-3Galbeta1-3GalNAc (where Neu5Ac is N-acetylneuraminic acid and GalNAc is N-acetylgalactosamine) to generate diSia (disialic acid) motifs specifically on O-glycans.

Gene expression profiling of mouse postnatal cerebellar development using oligonucleotide microarrays designed to detect differences in glycoconjugate expression

Differences in gene expression patterns between adult and postnatal day 7 (P7) mouse cerebellum, at the peak of granule neuron migration, were analyzed by hybridization to the GLYCOv2 glycogene array. This custom designed oligonucleotide array focuses on glycosyl transferases, carbohydrate-binding proteins, proteoglycans and related genes, and 173 genes were identified as being differentially expressed with statistical confidence. Expression levels for 11 of these genes were compared by RT-PCR, and their differential expression between P7 and adult cerebellum confirmed. Within the group of genes showing differential expression, the sialyltransferases (SiaTs) and GalNAc-Ts that were elevated at P7 prefer glycoprotein substrates, whilst the SiaTs and GalNAc-Ts that were elevated in the adult preferentially modify glycolipids, consistent with a role for gangliosides in maintaining neuronal function in the adult. Also within this group, three proteoglycans--versican, bamacan and glypican-2--were elevated at P7, along with growth factor midkine, which is known to bind to multiple types of proteoglycans, and fibroblast growth factor receptor 1, whose activity is known to be influenced by heparan sulfate proteoglycans. Two sulfotransferases that can modify the extent of proteoglycan sulfation were also differentially regulated, and may modify the interaction of a subset of proteoglycans with their binding partners during cerebellar development. Bamacan, glypican-2 and midkine were shown to be expressed in different cell types, and their roles in cerebellar development during granule neuron migration and maturation are discussed.

A focused microarray approach to functional glycomics: transcriptional regulation of the glycome

Glycosylation is the most common posttranslational modification of proteins, yet genes relevant to the synthesis of glycan structures and function are incompletely represented and poorly annotated on the commercially available arrays. To fill the need for expression analysis of such genes, we employed the Affymetrix technology to develop a focused and highly annotated glycogene-chip representing human and murine glycogenes, including glycosyltransferases, nucleotide sugar transporters, glycosidases, proteoglycans, and glycan-binding proteins. In this report, the array has been used to generate glycogene-expression profiles of nine murine tissues. Global analysis with a hierarchical clustering algorithm reveals that expression profiles in immune tissues (thymus [THY], spleen [SPL], lymph node, and bone marrow [BM]) are more closely related, relative to those of nonimmune tissues (kidney [KID], liver [LIV], brain [BRN], and testes [TES]). Of the biosynthetic enzymes, those responsible for synthesis of the core regions of N- and O-linked oligosaccharides are ubiquitously expressed, whereas glycosyltransferases that elaborate terminal structures are expressed in a highly tissue-specific manner, accounting for tissue and ultimately cell-type-specific glycosylation. Comparison of gene expression profiles with matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) profiling of N-linked oligosaccharides suggested that the alpha1-3 fucosyltransferase 9, Fut9, is the enzyme responsible for terminal fucosylation in KID and BRN, a finding validated by analysis of Fut9 knockout mice. Two families of glycan-binding proteins, C-type lectins and Siglecs, are predominately expressed in the immune tissues, consistent with their emerging functions in both innate and acquired immunity. The glycogene chip reported in this study is available to the scientific community through the Consortium for Functional Glycomics (CFG) (http://www.functionalglycomics.org).

Inflammation-dependent changes in α2,3-, α2,6-, and α2,8-sialic acid glycotopes on serum glycoproteins in mice

The expression of acute-phase serum proteins increases in response to inflammatory stimuli. Most of these proteins are glycoproteins that often contain sialic acids (Sia). It is unknown, however, how the expression of Sia in these glycoproteins changes during inflammation. This study demonstrates changes in the alpha2,3-, alpha2,6-, and alpha2,8-Sia glycotopes on serum glycoproteins in response to turpentine oil-induced inflammation, based on lectin- and immunoblot analyses by using sialyl linkage-specific lectins, Maackia amurensis for the alpha2,3-Sia glycotope and Sambucus sieboldiana for the alpha2,6-Sia glycotopes, and monoclonal antibody 2-4B (mAb.2-4B) recognizing the di- and oligomers of the alpha2,8-Neu5Gc residue. There was an increase in a limited number of sialoglycoproteins containing the alpha2,3-, alpha2,6-, or alpha2,8-Sia glycotopes. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the expression profiles of mRNAs for the known sialyltransferases in mouse liver during inflammation indicated the up-regulated expression of beta-galactoside alpha2,3-sialyltransferases (ST3Gal I and ST3Gal III) and beta-N-acetylgalactosaminide alpha2,6-sialyltransferase (ST6GalNAc VI) as well as beta-galactoside alpha2,6-sialyltransferase (ST6Gal I) mRNAs. Notably, ST3Gal I and III and ST6GalNAc VI are involved in the synthesis of the alpha2,3- and alpha2,6-Sia glycotopes on O-glycan chains and possibly on gangliosides, whereas ST6Gal I is specific for N-glycan chains. These results provide evidence for the inflammation-induced expression of sialyl glycotopes in serum glycoproteins. We demonstrated that inflammation significantly increased the expression of an unknown 32-kDa glycoprotein containing the alpha2,8-Sia glycotope. The mechanism for the increase in glycoprotein in inflamed mouse serum remains to be examined, as mRNA expression for all of the alpha2,8-sialyltransferases (ST8Sia I-VI) was unchanged during inflammation.

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.

Characterization of the second type of human beta-galactoside alpha 2,6-sialyltransferase (ST6Gal II), which sialylates Galbeta 1,4GlcNAc structures on oligosaccharides preferentially. Genomic analysis of human sialyltransferase genes

A novel member of the human beta-galactoside alpha2,6-sialyltransferase (ST6Gal) family, designated ST6Gal II, was identified by BLAST analysis of expressed sequence tags and genomic sequences. The sequence of ST6Gal II encoded a protein of 529 amino acids, and it showed 48.9% amino acid sequence identity with human ST6Gal I. Recombinant ST6Gal II exhibited alpha2,6-sialyltransferase activity toward oligosaccharides that have the Galbeta1,4GlcNAc sequence at the nonreducing end of their carbohydrate groups, but it exhibited relatively low and no activities toward some glycoproteins and glycolipids, respectively. It is concluded that ST6Gal II is an oligosaccharide-specific enzyme compared with ST6Gal I, which exhibits broad substrate specificities, and is mainly involved in the synthesis of sialyloligosaccharides. The expression of the ST6Gal II gene was significantly detected by reverse transcription PCR in small intestine, colon, and fetal brain, whereas the ST6Gal I gene was ubiquitously expressed, and its expression levels were much higher than those of the ST6Gal II gene. The ST6Gal I gene was also expressed in all tumors examined, but no expression was observed for the ST6Gal II gene in these tumors. The ST6Gal II gene is located on chromosome 2 (2q11.2-q12.1), and it spans over 85 kb of human genomic DNA consisting of at least eight exons and shares a similar genomic structure with the ST6Gal I gene. In this paper, we have shown that ST6Gal I, which has been known as the sole member of the ST6Gal family, also has the counterpart enzyme (ST6Gal II) like other sialyltransferases.

Neuronal differentiation-dependent expression of the disialic acid epitope on CD166 and its involvement in neurite formation in Neuro2A cells

We previously demonstrated that alpha2,8-linked disialic acid (diSia) residues occur in several glycoproteins of mammalian brains (Sato, C., Fukuoka, H., Ohta, K., Matsuda, T., Koshino, R., Kobayashi, K., Troy, F. A., II, and Kitajima, K. (2000) J. Biol. Chem. 275, 15422-15431). The role of the diSia epitope on these glycoproteins is not known, whereas the importance of the diSia epitope on glycolipids is well documented in neurite formation. In this study, we demonstrated that the diSia epitope (Neu5Acalpha2 --> 8Neu5Acalpha2 --> 3Gal) on glycoproteins, but not on glycolipids, is involved in neurite formation in a mouse neuroblastoma cell line, Neuro2A, based on the following lines of evidence. First, the amount of diSia epitope on glycoproteins increased during retinoic acid-induced neurite formation. Second, retinoic acid treatment primarily increased the diSia epitope on a 100-kDa glycoprotein. We identified this protein as CD166 (SC1), an immunoglobulin superfamily cell adhesion molecule involved in neurite extension. Third, a monoclonal antibody against the diSia epitope specifically inhibited neurite formation. We also demonstrated that alpha2,8-sialyltransferase III mRNA expression increased 1.7-fold after the induction of neurite formation, suggesting that alpha2,8-sialyltransferase III is responsible for formation of the diSia epitope on CD166.

Molecular cloning and expression of a sixth type of alpha 2,8-sialyltransferase (ST8Sia VI) that sialylates O-glycans

A novel member of the mouse alpha2,8-sialyltransferase (ST8Sia) family, designated ST8Sia VI, was identified by BLAST analysis of expressed sequence tags. The sequence of ST8Sia VI encodes a protein of 398 amino acids and shows 42.0 and 38.3% amino acid sequence identities to mouse alpha2,8-sialyltransferases ST8Sia I (GD3 synthase) and ST8Sia V (GD1c, GT1a, GQ1b, and GT3 synthases), respectively. The recombinant soluble form of ST8Sia VI expressed in COS-7 cells exhibited alpha2,8-sialyltransferase activity toward both glycolipids and glycoproteins that have the NeuAcalpha2,3(6)Gal sequence at the nonreducing end of their carbohydrate groups. This enzyme formed NeuAcalpha2,8NeuAc structures, but not oligosialic or polysialic acid structures. Analysis of the fetuin sialylated by ST8Sia VI indicated that ST8Sia VI prefers O-glycans to N-glycans as acceptor substrates. Substrate specificities and kinetic properties also showed that ST8Sia VI prefers O-glycans to glycolipids as acceptor substrates. ST8Sia VI also exhibited activity toward oligosaccharides such as sialyllactose and sialyllactosamine, and the structure of the minimal acceptor substrate for ST8Sia VI was determined as the NeuAcalpha2,3(6)Gal sequence. The expression of the ST8Sia VI gene was ubiquitous, and the highest expression was observed in kidney, with three major transcripts of 8.2, 3.8, and 2.7 kb. This is the first report of a mammalian alpha2,8-sialyltransferase that sialylates O-glycans preferentially.

Sialyltransferases in cancer

It has long been known that cancer cells often express more heavily sialylated glycans on their surface and that this feature sometimes correlates with invasion. It is now well established that specific sialylated structures, such as the Thomsen-Friedenreich-related antigens, the sialyl Lewis antigens, the sialyl alpha2-6 lactosaminyl structure, the polysialic acid or some gangliosides, can mediate cellular interactions and are altered in cancer cells. This review summarizes the current knowledge on the cancer-associated alterations in sialyltransferase expression which are often at the basis of the deranged expression of sialylated structures.

Genomic structure of human GM3 synthase gene (hST3Gal V) and identification of mRNA isoforms in the 5'-untranslated region

GM3 synthase, which transfers CMP-NeuAc with an alpha2,3-linkage to a galactose residue of lactosylceramide, plays a key role in the biosynthesis of all complex gangliosides. In this study, cDNA and genomic clones encoding human GM3 synthase (hST3Gal V) were isolated, and the structural organization of the gene was determined. The hST3Gal V cDNA was identical in the coding region with cDNA that has been cloned previously from the HL-60 cells but different in the 5'-untranslated region (UTR). The hST3Gal V gene consisted of nine exons, which span approximately 44 kb, with exons ranging in size from 112 to 1242 bp. The coding region was located in exons 4-9, and all exon-intron boundaries except the acceptor site of intron 1 followed the GT-AG rule. The expression of this gene was highly restricted in both human fetal and adult tissues. By comparison of the nucleotide sequences of the genomic DNA with cDNA sequences including 5'-RACE products, we identified four isoforms (types 1-4) of the hST3Gal V mRNA that differ only in the 5'-UTR. Structural analysis of these isoforms suggests that mRNA isoforms of hST3Gal V are produced by a combination of alternative splicing and alternative promoter utilization.

The human sialyltransferase family

The human genome encodes probably more than 20 different sialyltransferases involved in the biosynthesis of sialylated glycoproteins and glycolipids but to date only 15 different human sialyltransferase cDNAs have been cloned and characterized. Each of the sialyltransferase genes is differentially expressed in a tissue-, cell type-, and stage-specific manner to regulate the sialylation pattern of cells. These enzymes differ in their substrate specificity, tissue distribution and various biochemical parameters. However, enzymatic analysis conducted in vitro with recombinant enzyme revealed that one linkage can be synthesized by multiple enzymes. We present here an overview of these human genes and enzymes, the regulation of their occurrence and their involvement in several physiological and pathological processes.

Unique disulfide bond structures found in ST8Sia IV polysialyltransferase are required for its activity

NCAM polysialylation plays a critical role in neuronal development and regeneration. Polysialylation of the neural cell adhesion molecule (NCAM) is catalyzed by two polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST), which contain sialylmotifs L and S conserved in all members of the sialyltransferases. The members of the ST8Sia gene family, including ST8Sia II and ST8Sia IV are unique in having three cysteines in sialylmotif L, one cysteine in sialylmotif S, and one cysteine at the COOH terminus. However, structural information, including how disulfide bonds are formed, has not been determined for any of the sialyltransferases. To obtain insight into the structure/function of ST8Sia IV, we expressed human ST8Sia IV in insect cells, Trichoplusia ni, and found that the enzyme produced in the insect cells catalyzes NCAM polysialylation, although it cannot polysialylate itself ("autopolysialylation"). We also found that ST8Sia IV does not form a dimer through disulfide bonds. By using the same enzyme preparation and performing mass spectrometric analysis, we found that the first cysteine in sialylmotif L and the cysteine in sialylmotif S form a disulfide bridge, whereas the second cysteine in sialylmotif L and the cysteine at the COOH terminus form a second disulfide bridge. Site-directed mutagenesis demonstrated that mutation at cysteine residues involved in the disulfide bridges completely inactivated the enzyme. Moreover, changes in the position of the COOH-terminal cysteine abolished its activity. By contrast, the addition of green fluorescence protein at the COOH terminus of ST8Sia IV did not render the enzyme inactive. These results combined indicate that the sterical structure formed by intramolecular disulfide bonds, which bring the sialylmotifs and the COOH terminus within close proximity, is critical for the catalytic activity of ST8Sia IV.

Identification and characterization of three novel beta 1,3-N-acetylglucosaminyltransferases structurally related to the beta 1,3-galactosyltransferase family

We have isolated three types of cDNAs encoding novel beta1,3-N-acetylglucosaminyltransferases (designated beta3Gn-T2, -T3, and -T4) from human gastric mucosa and the neuroblastoma cell line SK-N-MC. These enzymes are predicted to be type 2 transmembrane proteins of 397, 372, and 378 amino acids, respectively. They share motifs conserved among members of the beta1,3-galactosyltransferase family and a beta1,3-N-acetylglucosaminyltransferase (designated beta3Gn-T1), but show no structural similarity to another type of beta1,3-N-acetylglucosaminyltransferase (iGnT). Each of the enzymes expressed by insect cells as a secreted protein fused to the FLAG peptide showed beta1,3-N-acetylglucosaminyltransferase activity for type 2 oligosaccharides but not beta1,3-galactosyltransferase activity. These enzymes exhibited different substrate specificity. Transfection of Namalwa KJM-1 cells with beta3Gn-T2, -T3, or -T4 cDNA led to an increase in poly-N-acetyllactosamines recognized by an anti-i-antigen antibody or specific lectins. The expression profiles of these beta3Gn-Ts were different among 35 human tissues. beta3Gn-T2 was ubiquitously expressed, whereas expression of beta3Gn-T3 and -T4 was relatively restricted. beta3Gn-T3 was expressed in colon, jejunum, stomach, esophagus, placenta, and trachea. beta3Gn-T4 was mainly expressed in brain. These results have revealed that several beta1,3-N-acetylglucosaminyltransferases form a family with structural similarity to the beta1,3-galactosyltransferase family. Considering the differences in substrate specificity and distribution, each beta1,3-N-acetylglucosaminyltransferase may play different roles.

Differential biosynthesis of polysialic acid on neural cell adhesion molecule (NCAM) and oligosaccharide acceptors by three distinct alpha 2,8-sialyltransferases, ST8Sia IV (PST), ST8Sia II (STX), and ST8Sia III

Polysialylated neural cell adhesion molecule (NCAM) is thought to play a critical role in neural development. Polysialylation of NCAM was shown to be achieved by two alpha2,8-polysialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), which are moderately related to another alpha2,8-sialyltransferase, ST8Sia III. Here we describe that all three alpha2,8-sialyltransferases can utilize oligosaccharides as acceptors but differ in the efficiency of adding polysialic acid on NCAM. First, we found that ST8Sia III can form polysialic acid on the enzyme itself (autopolysialylation) but not on NCAM. These discoveries prompted us to determine if ST8Sia IV and ST8Sia II share the property of ST8Sia III in utilizing low molecular weight oligosaccharides as acceptors. By using a newly established method, we found that ST8Sia IV, ST8Sia II, and ST8Sia III all add oligosialic and polysialic acid on various sialylated N-acetyllactosaminyl oligosaccharides, including NCAM N-glycans, fetuin N-glycans, synthetic sialylated N-acetyllactosamines, and on alpha(2)-HS-glycoprotein. Our results also showed that monosialyl and disialyl N-acetyllactosamines can serve equally as an acceptor, suggesting that no initial addition of alpha2,8-sialic acid is necessary for the action of polysialyltransferases. Polysialylation of NCAM by ST8Sia IV and ST8Sia II is much more efficient than polysialylation of N-glycans isolated from NCAM. Moreover, ST8Sia IV and ST8Sia II catalyze polysialylation of NCAM much more efficiently than ST8Sia III. These results suggest that no specific acceptor recognition is involved in polysialylation of low molecular weight sialylated oligosaccharides, whereas the enzymes exhibit pronounced acceptor specificities if glycoproteins are used as acceptors.

Redefined substrate specificity of ST6GalNAc II: a second candidate sialyl-Tn synthase

The acceptor substrate specificities of ST6GalNAc I and II, which act on the synthesis of O-linked oligosaccharides, were reexamined using ovine submaxillary mucin, [Ala-Thr(GalNAc)-Ala]n polymer (n = 7-11). It has been suggested that only ST6GalNAc I can synthesize carbohydrate structures of sialyl-Tn-antigen; i.e., NeuAc alpha2-6GalNAc-O-Thr/Ser [Kurosawa et al., J. Biol. Chem. 269, 19048-19053 (1994)] based on the result that ST6GalNAc I, not ST6GalNAc II, exhibited activity toward asialoagalacto-fetuin. In this study, we present evidence that both ST6GalNAc I and II exhibit activity toward asialo-OSM (ovine submaxillary mucin) and [Ala-Thr(GalNAc)-Ala]n polymer (n = 7-11) which have only the GalNAc-O-Thr/Ser-structures. These results strongly indicate that not only ST6GalNAc I but also II are candidates for sialyl-Tn synthases.

A novel glycosyltransferase with a polyglutamine repeat; a new candidate for GD1alpha synthase (ST6GalNAc V)(1)

The fifth type GalNAcalpha2,6-sialyltransferase (mST6GalNAc V) was cloned from a mouse brain cDNA library. mST6GalNAc V exhibited type II transmembrane topology containing a polyglutamine repeat, which showed 42.6% and 44.8% identity to mouse ST6GalNAc III and IV, respectively. Northern blot analysis revealed that the mST6GalNAc V gene was specifically expressed in forebrain and cerebellum. mST6GalNAc V exhibited GD1alpha synthetic activity from GM1b the same as mST6GalNAc III and IV. The activity ratio of GM1b toward fetuin and the expression pattern were completely different among the three ST6GalNAcs. Interestingly, the polyglutamine repeat number was different from that of inbred mice. We report the first glycosyltransferase with a polymorphic polyglutamine repeat.

Multi-enzyme kinetic analysis of glycolipid biosynthesis

Gangliosides are acidic glycosphingolipids synthesized sequentially by a series of glycosyltransferases acting in parallel biosynthetic pathways. While most glycosyltransferases are highly specific, some, however, may catalyze equivalent steps in each pathway using different gangliosides as substrates (e.g. N-acetylgalactosaminyltransferase, sialyltransferase-IV). A multi-enzyme kinetic analysis was developed on the condition that serial enzymatic reactions operate below substrate saturation. A multi-enzyme kinetic analysis enabled a simultaneous calculation of the Vmax/Km value of each enzyme derived from the equilibrium concentration of the respective substrate. Substrate concentrations [S] were determined by radioactive labelling of gangliosides in intact cells with the precursor sugars [14C]galactose and [14C]glucosamine, followed by high-performance thin-layer chromatography and autoradiography of the radiolabelled glycolipids. On the basis of Michaelis-Menten kinetics, Vmax/Km values were derived from [S] by a system of linear equations. The procedure was used to analyze the development of the glycolipid composition during differentiation of rat gliomaxmurine neuroblastoma (NG108-15) cells. The Vmax/Km values calculated by multi-enzyme kinetic analysis were consistent with the kinetic data obtained with solubilized enzymes. Application of multi-enzyme kinetic analysis to published data on the correlation of enzyme activities with ganglioside levels in various cell lines and tissues indicated the validity of this method for analysis of the glycolipid biosynthesis, in particular, of its initial steps. On the basis of the kinetic analysis, it is suggested that the cell lines can be divided into two groups with respect to the substrate pools of GM3 used by sialyltransferase-II and N-acetylgalactosaminyltransferase-I. The first group encompasses the majority of the neuroblastoma cell lines and the embryonic rat brain where the two enzymes share a common pool of GM3. In the second group, the two enzymes do not compete for the same pool of GM3, indicating a different subcellular localization of CMP-NeuAc:GM3 alpha2-8-sialyltransferase and UDP-N-acetylgalactosaminyl:GM3 N-acetylgalactosaminyltransferase. In this study, the theory of a multi-enzyme kinetic analysis is discussed and its application to analysis of the glycolipid biosynthesis in neuroblastoma cells is demonstrated. A multi-enzyme kinetic analysis can be applied to other biosynthetic pathways and provides the advantage of analyzing kinetic data with intact cells or tissue samples.

Developmental patterns of GalBeta1,3(4)GlcNAc alpha2,3-sialyltransferase (ST3Gal III) expression in the mouse: in situ hybridization using DIG-labeled RNA probes

Sialic acids are key determinants for biological processes, such as cell-cell interaction and differentiation. Sialyltransferases contribute to the diversity in carbohydrate structure through their attachment of sialic acid in various terminal positions on glycolipid and glycoprotein (N-linked and O-linked) carbohydrate groups. Galbeta 1,3(4)GlcNAc alpha2,3-sialyltransferase (ST3Gal III) is involved in the biosynthesis of sLe(x)and sLe(a) known as selectin ligands and tumor-associated carbohydrate structures. The appearance and differential distribution of ST3Gal III mRNA during mice embryogenesis [embryonic (E) days; E9, E11, E13, E15] were investigated by in situ hybridization with digoxigenin-labeled RNA probes coupled with alkaline phosphatase detection. On E9, all tissues were positive for ST3Gal III mRNA expression, whereas ST3Gal III mRNA on E11 was not detected throughout all tissues. On E13, ST3Gal III mRNA was expressed in different manner in various tissues. In this stage, ST3Gal III mRNA was positive only in the liver, pancreas and bladder. On E15, specific signal for ST3Gal III was detected in the liver, lung and forebrain. These results indicate that ST3GAI III is differently expressed at developmental stages of mice embryo, and this may be importantly related with regulation of organogenesis in mice.

Molecular cloning and functional expression of two members of mouse NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2,6-sialyltransferase family, ST6GalNAc III and IV

Two cDNA clones encoding NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2, 6-sialyltransferase have been isolated from mouse brain cDNA libraries. One of the cDNA clones is a homologue of previously reported rat ST6GalNAc III according to the amino acid sequence identity (94.4%) and the substrate specificity of the expressed recombinant enzyme, while the other cDNA clone includes an open reading frame coding for 302 amino acids. The deduced amino acid sequence is not identical to those of other cloned mouse sialyltransferases, although it shows the highest sequence similarity with mouse ST6GalNAc III (43.0%). The expressed soluble recombinant enzyme exhibited activity toward NeuAcalpha2, 3Galbeta1, 3GalNAc, fetuin, and GM1b, while no significant activity was detected toward Galbeta1,3GalNAc or asialofetuin, or the other glycoprotein substrates tested. The sialidase sensitivity of the 14C-sialylated residue of fetuin, which was sialylated by this enzyme with CMP-[14C]NeuAc, was the same as that of ST6GalNAc III. These results indicate that the expressed enzyme is a new type of GalNAcalpha2,6-sialyltransferase, which requires sialic acid residues linked to Galbeta1,3GalNAc residues for its activity; therefore, we designated it mouse ST6GalNAc IV. Although the substrate specificity of this enzyme is similar to that of ST6GalNAc III, ST6GalNAc IV prefers O-glycans to glycolipids. Glycolipids, however, are better substrates for ST6GalNAc III.

Polysialic acid, a unique glycan that is developmentally regulated by two polysialyltransferases, PST and STX, in the central nervous system: from biosynthesis to function

Polysialic acid is a developmentally regulated carbohydrate composed of a linear homopolymer of alpha-2,8-linked sialic acid residues. This unique glycan is mainly attached to the neural cell adhesion molecule (N-CAM) and implicated in many morphogenic events of the neural cells by modulating the adhesive property of N-CAM. Recently, the cDNA that encodes polysialyltransferase, which is responsible for the polysialylation of N-CAM, was successfully cloned from three mammalian species. This review focuses on the molecular cloning of human polysialyltransferase, designated PST. It then describes the number of enzymes actually required for the polysialylation of N-CAM using an in vitro polysialyltransferase assay. Comparisons between PST and another polysialyltransferase, sialyltransferase X (STX), are made and it is demonstrated that both enzymes can independently form polysialic acid in vitro, but that during neural development they coordinately but distinctly synthesize polysialic acid on N-CAM. The role of polysialic acid in the central nervous system is also discussed. Finally, evidence that the two polysialyltransferases, PST and STX, apparently have distinct roles in the development of neural cells is provided by using a neurite outgrowth assay.

Mutation of the sialyltransferase S-sialylmotif alters the kinetics of the donor and acceptor substrates

Protein sequence analysis of the cloned sialyltransferase gene family has revealed the presence of two conserved protein motifs in the middle of the lumenal catalytic domain, termed L-sialylmotif and S-sialylmotif. In our previous study (Datta, A. K., and Paulson, J. C. (1995) J. Biol. Chem. 270, 1497-1500) the larger L-sialylmotif of ST6Gal I was analyzed by site-directed mutagenesis, which provided evidence that it participates in the binding of the CMP-NeuAc, a common donor substrate for all the sialyltransferases. However, none of the mutants tested in this motif had any significant effect on their binding affinities toward the acceptor substrate asialo alpha1-acid glycoprotein. In this study, we have investigated the role of the S-sialylmotif of the same enzyme ST6Gal I. In total, nine mutants have been constructed by changing the conserved amino acids of this motif to mostly alanine by site-directed mutagenesis. Kinetic analysis for the mutants which retained sialyltransferase activity showed that the mutations in the S-sialylmotif caused a change of Km values for both the donor and the acceptor substrates. Our results indicated that this motif participates in the binding of both the substrates. A sequence homology search also supported this finding, which showed that the downstream amino acid sequence of the S-sialylmotif is conserved for each subgroup of this enzyme family, indicating its association with the acceptor substrate.