Expression cloning of a GM3-specific alpha-2,8-sialyltransferase (GD3 synthase)

Glycosphingolipids in neurodegeneration - Molecular mechanisms, cellular roles, and therapeutic perspectives

Neurodegenerative diseases, including Alzheimer's (AD), Parkinson's (PD), Huntington's (HD), and amyotrophic lateral sclerosis (ALS), are characterized by progressive neuronal loss and pose significant global health challenges. Glycosphingolipids (GSLs), critical components of neuronal membranes, regulate signal transduction, membrane organization, neuroinflammation, and lipid raft functionality. This review explores GSL roles in neural development, differentiation, and neurogenesis, along with their dysregulation in neurodegenerative diseases. Aberrations in GSL metabolism drive key pathological features such as protein aggregation, neuroinflammation, and impaired signaling. Specific GSLs, such as GM1, GD3, and GM3, influence amyloid-beta aggregation in AD, α-synuclein stability in PD, and mutant huntingtin toxicity in HD. Therapeutic strategies targeting GSL metabolism, such as GM1 supplementation and enzyme modulation, have demonstrated potential to mitigate disease progression. Further studies using advanced lipidomics and glycomics may support biomarker identification and therapeutic advancements. This work aims to highlight the translational potential of GSL research for diagnosing and managing devastating neurodegenerative conditions.

Genetic risk variants implicate impaired maintenance and repair of periodontal tissues as causal for periodontitis-A synthesis of recent findings

Periodontitis is a complex inflammatory disease in which the host genome, in conjunction with extrinsic factors, determines susceptibility and progression. Genetic predisposition is the strongest risk factor in the first decades of life. As people age, chronic exposure to the periodontal microbiome puts a strain on the proper maintenance of barrier function. This review summarizes our current knowledge on genetic risk factors implicated in periodontitis, derived (i) from hypothesis-free systematic whole genome-profiling studies (genome-wide association studies [GWAS] and quantitative trait loci [QTL] mapping studies), and independently validated through further unbiased approaches; (ii) from monogenic and oligogenic forms of periodontitis; and (iii) from syndromic forms of periodontitis. The genes include, but are not limited to, SIGLEC5, PLG, ROBO2, ABCA1, PF4, and CTSC. Notably, CTSC and PLG gene mutations were also identified in non-syndromic and syndromic forms of prepubertal and early-onset periodontitis. The functions of the identified genes in this review suggest that the pathways affected by the periodontitis-associated gene variants converge in functions involved in the maintenance and repair of structural integrity of the periodontal tissues. Particularly, these genes play a role in the healing of inflamed and ulcerated periodontal tissues, including roles in fibrinolysis, extrusion of cellular debris, extracellular matrix remodeling and angiogenesis. Syndromes that include periodontitis in their phenotype indicate that neutrophils play an important role in the regulation of inflammation in the periodontium. The established genetic susceptibility genes therefore collectively provide new insights into the molecular mechanisms and plausible causal factors underlying periodontitis.

The Bifunctional Effects of Lactoferrin (LFcinB11) in Inhibiting Neural Cell Adhesive Molecule (NCAM) Polysialylation and the Release of Neutrophil Extracellular Traps (NETs)

The expression of polysialic acid (polySia) on the neuronal cell adhesion molecule (NCAM) is called NCAM-polysialylation, which is strongly related to the migration and invasion of tumor cells and aggressive clinical status. Thus, it is important to select a proper drug to block tumor cell migration during clinical treatment. In this study, we proposed that lactoferrin (LFcinB11) may be a better candidate for inhibiting NCAM polysialylation when compared with CMP and low-molecular-weight heparin (LMWH), which were determined based on our NMR studies. Furthermore, neutrophil extracellular traps (NETs) represent the most dramatic stage in the cell death process, and the release of NETs is related to the pathogenesis of autoimmune and inflammatory disorders, with proposed involvement in glomerulonephritis, chronic lung disease, sepsis, and vascular disorders. In this study, the molecular mechanisms involved in the inhibition of NET release using LFcinB11 as an inhibitor were also determined. Based on these results, LFcinB11 is proposed as being a bifunctional inhibitor for inhibiting both NCAM polysialylation and the release of NETs.

GD3 ganglioside is a promising therapeutic target for glioma patients

Glioblastoma is the most frequent and aggressive primary brain tumor in adults. Currently, no curative treatment is available. Despite first-line treatment composed by the association of surgery, radiotherapy, and chemotherapy, relapse remains inevitable in a median delay of 6 to 10 months. Improving patient management and developing new therapeutic strategies are therefore a critical medical need in neuro-oncology. Gangliosides are sialic acid-containing glycosphingolipids, the most abundant in the nervous system, representing attractive therapeutic targets. The ganglioside GD3 is highly expressed in neuroectoderm-derived tumors such as melanoma and neuroblastoma, but also in gliomas. Moreover, interesting results, including our own, have reported the involvement of GD3 in the stemness of glioblastoma cells. In this review, we will first describe the characteristics of the ganglioside GD3 and its enzyme, the GD3 synthase (GD3S), including their biosynthesis and metabolism. Then, we will detail their expression and role in gliomas. Finally, we will summarize the current knowledge regarding the therapeutic development opportunities against GD3 and GD3S.

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.

Signaling domains of cancer-associated glycolipids

Immunotherapy of malignant cancers is now becoming one of representative approaches to overcome cancers. To construct strategies for immunotherapy, presence of tumor-specific antigens should be a major promise. A number of cancer specific- or cancer-associated antigens have been reported based on various experimental sets and various animal systems. The most reasonable strategy to define tumor-specific antigens might be "autologous typing" performed by Old's group, proposing three classes of tumor-antigens recognized by host immune systems of cancer patients. Namely, class 1, individual antigens that is present only in the patient's sample analyzed; class 2, shared antigens that can be found only in some group of cancers in some patients, but not in normal cells and tissues; class 3, universal antigens that are present in some cancers but also in normal cells and tissues with different densities. Sen Hakomori reported there were novel carbohydrates in cancers that could not be detected in normal cells mainly by biochemical approaches. Consequently, many of class 2 cancer-specific antigens have been revealed to be carbohydrate antigens, and been used for cancer diagnosis and treatment. Not only as cancer markers, but roles of those cancer-associated carbohydrates have also been recognized as functional molecules in cancer cells. In particular, roles of complex carbohydrates in the regulation of cell signaling on the cell surface microdomains, glycolipid-enriched microdomain (GEM)/rafts have been reported by Hakomori and many other researchers including us. The processes and present status of these studies on cancer-associated glycolipids were summarized.

Role of GD3 Synthase ST8Sia I in Cancers

Simple Summary

The carbohydrate moiety of cell surface glycolipids is modified in cancers of neuro–ectoderm origin, leading to the expression of more complex structures with two or more sialic acid residues. These alterations result from the upregulation of the ST8SIA1 gene that encodes GD3 synthase, the enzyme controlling the biosynthesis of complex gangliosides, and are usually associated with a more aggressive phenotype and a poor outcome for patients, making GD3 synthase an interesting target for cancer therapy. This review reports our general knowledge of GD3 synthase gene expression and regulation, its role in both epithelial–mesenchymal transition (EMT) and cancer progression, and the different approaches targeting GD3S expression in cancers.

Abstract

GD3 synthase controls the biosynthesis of complex gangliosides, bearing two or more sialic acid residues. Disialylated gangliosides GD3 and GD2 are tumor-associated carbohydrate antigens (TACA) in neuro–ectoderm-derived cancers, and are directly involved in cell malignant properties, i.e., migration, invasion, stemness, and epithelial–mesenchymal transition. Since GD3 and GD2 levels are directly linked to GD3 synthase expression and activity, targeting GD3 synthase appears to be a promising strategy through which to interfere with ganglioside-associated malignant properties. We review here the current knowledge on GD3 synthase expression and regulation in cancers, and the consequences of complex ganglioside expression on cancer cell signaling and properties, highlighting the relationships between GD3 synthase expression and epithelial–mesenchymal transition and stemness. Different strategies were used to modulate GD3 synthase expression in cancer cells in vitro and in animal models, such as inhibitors or siRNA/lncRNA, which efficiently reduced cancer cell malignant properties and the proportion of GD2 positive cancer stem cells, which are associated with high metastatic properties, resistance to therapy, and cancer relapse. These data show the relevance of targeting GD3 synthase in association with conventional therapies, to decrease the number of cancer stem cells in tumors.

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.

BACH1 Binding Links the Genetic Risk for Severe Periodontitis with ST8SIA1

Genome-wide association studies identified various loci associated with periodontal diseases, but assigning causal alleles remains difficult. Likewise, the generation of biological meaning underlying a statistical association has been challenging. Here, we characterized the genetic association at the gene ST8SIA1 that increases the risk for severe periodontitis in smokers. We used CRISPR/dCas9 activation and RNA-sequencing to identify genetic interaction partners of ST8SIA1 and to determine its function in the cell. We used reporter gene assays to identify regulatory elements at the associated single-nucleotide polymorphisms (SNPs) and to determine effect directions and allele-specific changes of enhancer activity. Antibody electrophoretic mobility shift assays proved allele-specific transcription factor binding at the putative causal SNPs. We found the reported periodontitis risk gene ABCA1 as the top upregulated gene following ST8SIA1 activation. Gene set enrichment analysis showed highest effects on integrin cell surface interactions (area under the curve [AUC] = 0.85; q = 4.9 × 10-6) and cell cycle regulation (AUC = 0.89; q = 1.6 × 10-5). We identified 2 associated repressor elements in the introns of ST8SIA1 that bind the transcriptional repressor BACH1. The putative causative variant rs2012722 decreased BACH1 binding by 40%. We also pinpointed ST8SIA1 as the target gene of the association. ST8SIA1 inhibits cell adhesion with extracellular matrix proteins, integrins, and cell cycle, as well as enhances apoptosis. Likewise, tobacco smoke reportedly results in an inhibition of cell adhesion and a decrease in integrin-positive cells and cell growth. We conclude that impaired ST8SIA1 repression, independently caused by reduced BACH1 binding at the effect T allele, as well as by tobacco smoke, contributes to higher ST8SIA1 levels, and in smokers who carry the effect T allele, both factors would be additive with damaging effects on the gingival barrier integrity. The activity of ST8SIA1 is also linked with the periodontitis risk gene ABCA1.

Sialic Acids and Their Influence on Human NK Cell Function

Sialic acids are sugars with a nine-carbon backbone, present on the surface of all cells in humans, including immune cells and their target cells, with various functions. Natural Killer (NK) cells are cells of the innate immune system, capable of killing virus-infected and tumor cells. Sialic acids can influence the interaction of NK cells with potential targets in several ways. Different NK cell receptors can bind sialic acids, leading to NK cell inhibition or activation. Moreover, NK cells have sialic acids on their surface, which can regulate receptor abundance and activity. This review is focused on how sialic acids on NK cells and their target cells are involved in NK cell function.

Sialylation of Human Natural Killer (NK) Cells is Regulated by IL-2

Sialic acids are terminal sugars on the cell surface that are found on all cell types including immune cells like natural killer (NK) cells. The attachment of sialic acids to different glycan structures is catalyzed by sialyltransferases in the Golgi. However, the expression pattern of sialyltransferases in NK cells and their expression after activation has not yet been analyzed. Therefore, the present study determines which sialyltransferases are expressed in human NK cells and if activation with IL-2 changes the sialylation of NK cells. The expression of sialyltransferases was analyzed in the three human NK cell lines NK-92, NKL, KHYG-1 and primary NK cells. NK-92 cells were cultured in the absence or presence of IL-2, and changes in the sialyltransferase expression were measured by qPCR. Furthermore, specific sialylation was investigated by flow cytometry. In addition, polySia and NCAM were measured by Western blot analyses. IL-2 leads to a reduced expression of ST8SIA1, ST6GAL1 and ST3GAL1. α-2,3-Sialylation remained unchanged, while α-2,6-sialylation was increased after IL-2 stimulation. Moreover, an increase in the amount of NCAM and polySia was observed in IL-2-activated NK cells, whereas GD3 ganglioside was decreased. In this study, all sialyltransferases that were expressed in NK cells could be identified. IL-2 regulates the expression of some sialyltransferases and leads to changes in the sialylation of NK cells.

Novel Molecular Mechanisms of Gangliosides in the Nervous System Elucidated by Genetic Engineering

Acidic glycosphingolipids, i.e., gangliosides, are predominantly and consistently expressed in nervous tissues of vertebrates at high levels. Therefore, they are considered to be involved in the development and function of nervous systems. Recent studies involving genetic engineering of glycosyltransferase genes have revealed novel aspects of the roles of gangliosides in the regulation of nervous tissues. In this review, novel findings regarding ganglioside functions and their modes of action elucidated mainly by studies of gene knockout mice are summarized. In particular, the roles of gangliosides in the regulation of lipid rafts to maintain the integrity of nervous systems are reported with a focus on the roles in the regulation of neuro-inflammation and neurodegeneration via complement systems. In addition, recent advances in studies of congenital neurological disorders due to genetic mutations of ganglioside synthase genes and also in the techniques for the analysis of ganglioside functions are introduced.

Molecular Interactions of the Polysialytransferase Domain (PSTD) in ST8Sia IV with CMP-Sialic Acid and Polysialic Acid Required for Polysialylation of the Neural Cell Adhesion Molecule Proteins: An NMR Study

Polysialic acid (polySia) is an unusual glycan that posttranslational modifies neural cell adhesion molecule (NCAM) proteins in mammalian cells. The up-regulated expression of polySia-NCAM is associated with tumor progression in many metastatic human cancers and in neurocognitive processes. Two members of the ST8Sia family of α2,8-polysialyltransferases (polySTs), ST8Sia II (STX) and ST8Sia IV (PST) both catalyze synthesis of polySia when activated cytidine monophosphate(CMP)-Sialic acid (CMP-Sia) is translocate into the lumen of the Golgi apparatus. Two key polybasic domains in the polySTs, the polybasic region (PBR) and the polysialyltransferase domain (PSTD) areessential forpolysialylation of the NCAM proteins. However, the precise molecular details to describe the interactions required for polysialylation remain unknown. In this study, we hypothesize that PSTD interacts with both CMP-Sia and polySia to catalyze polysialylation of the NCAM proteins. To test this hypothesis, we synthesized a 35-amino acid-PSTD peptide derived from the ST8Sia IV gene sequence and used it to study its interaction with CMP-Sia, and polySia. Our results showed for the PSTD-CMP-Sia interaction, the largest chemical-shift perturbations (CSP) were in amino acid residues V251 to A254 in the short H1 helix, located near the N-terminus of PSTD. However, larger CSP values for the PSTD-polySia interaction were observed in amino acid residues R259 to T270 in the long H2 helix. These differences suggest that CMP-Sia preferentially binds to the domain between the short H1 helix and the longer H2 helix. In contrast, polySia was principally bound to the long H2 helix of PSTD. For the PSTD-polySia interaction, a significant decrease in peak intensity was observed in the 20 amino acid residues located between the N-and C-termini of the long H2 helix in PSTD, suggesting a slower motion in these residues when polySia bound to PSTD. Specific features of the interactions between PSTD-CMP-Sia, and PSTD-polySia were further confirmed by comparing their 800 MHz-derived HSQC spectra with that of PSTD-Sia, PSTD-TriSia (DP 3) and PSTD-polySia. Based on the interactions between PSTD-CMP-Sia, PSTD-polySia, PBR-NCAM and PSTD-PBR, these findingsprovide a greater understanding of the molecular mechanisms underlying polySia-NCAM polysialylation, and thus provides a new perspective for translational pharmacological applications and development by targeting the two polysialyltransferases.

B4GALNT1 induces angiogenesis, anchorage independence growth and motility, and promotes tumorigenesis in melanoma by induction of ganglioside GM2/GD2

β-1,4-N-Acetyl-Galactosaminyltransferase 1 (B4GALNT1) encodes the key enzyme B4GALNT1 to generate gangliosides GM2/GD2. GM2/GD2 gangliosides are surface glycolipids mainly found on brain neurons as well as peripheral nerves and skin melanocytes and are reported to exacerbate the malignant potential of melanomas. In order to elucidate the mechanism, we performed functional analyses of B4GALNT1-overexpressing cells. We analyzed ganglioside pattern on four melanoma and two neuroblastoma cell lines by high performance liquid chromatography (HPLC). We overexpressed B4GALNT1 in GM2/GD2-negative human melanoma cell line (SH4) and confirmed production of GM2/GD2 by HPLC. They showed higher anchorage independence growth (AIG) in colony formation assay, and exhibited augmented motility. In vitro, cell proliferation was not affected by GM2/GD2 expression. In vivo, GM2/GD2-positive SH4 clones showed significantly higher tumorigenesis in NOD/Scid/IL2Rγ-null mice, and immunostaining of mouse CD31 revealed that GM2/GD2 induced remarkable angiogenesis. No differences were seen in melanoma stem cell and Epithelial-Mesenchymal Transition markers between GM2/GD2-positive and -negative SH4 cells. We therefore concluded that B4GALNT1, and consequently GM2/GD2, enhanced tumorigenesis via induction of angiogenesis, AIG, and cell motility. RNA-Seq suggested periostin as a potential key factor for angiogenesis and AIG. These findings may lead to development of novel therapy for refractory melanoma.

A Possible Modulation Mechanism of Intramolecular and Intermolecular Interactions for NCAM Polysialylation and Cell Migration

Polysialic acid (polySia) is a novel glycan that posttranslationally modifies neural cell adhesion molecules (NCAMs) in mammalian cells. Up-regulation of polySia-NCAM expression or NCAM polysialylation is associated with tumor cell migration and progression in many metastatic cancers and neurocognition. It has been known that two highly homologous mammalian polysialyltransferases (polySTs), ST8Sia II (STX) and ST8Sia IV (PST), can catalyze polysialylation of NCAM, and two polybasic domains, polybasic region (PBR) and polysialyltransferase domain (PSTD) in polySTs play key roles in affecting polyST activity or NCAM polysialylation. However, the molecular mechanisms of NCAM polysialylation and cell migration are still not entirely clear. In this minireview, the recent research results about the intermolecular interactions between the PBR and NCAM, the PSTD and cytidine monophosphate-sialic acid (CMP-Sia), the PSTD and polySia, and as well as the intramolecular interaction between the PBR and the PSTD within the polyST, are summarized. Based on these cooperative interactions, we have built a novel model of NCAM polysialylation and cell migration mechanisms, which may be helpful to design and develop new polysialyltransferase inhibitors.

Functional roles of ST8SIA3-mediated sialylation of striatal dopamine D2 and adenosine A2A receptors

Sialic acids are typically added to the end of glycoconjugates by sialyltransferases. Among the six ST8 α-N-acetyl-neuraminide α-2,8-sialyltransferases (ST8SIA) existing in adult brains, ST8SIA2 is a schizophrenia-associated gene. However, the in vivo substrates and physiological functions of most sialyltransferases are currently unknown. The ST8SIA3 is enriched in the striatum. Here, we showed that ablation of St8sia3 in mice (St8sia3-KO) led to fewer disialylated and trisialylated terminal glycotopes in the striatum of St8sia3-KO mice. Moreover, the apparent sizes of several striatum-enriched G-protein-coupled receptors (GPCRs) (including the adenosine A_2A receptor (A_2AR) and dopamine D₁/D₂ receptors (D₁R and D₂R)) were smaller in St8sia3-KO mice than in WT mice. A sialidase treatment removed the differences in the sizes of these molecules between St8sia3-KO and WT mice, confirming the involvement of sialylation. Expression of ST8SIA3 in the striatum of St8sia3-KO mice using adeno-associated viruses normalized the sizes of these proteins, demonstrating a direct role of ST8SIA3. The lack of ST8SIA3-mediated sialylation altered the distribution of these proteins in lipid rafts and the interaction between D₂R and A_2AR. Locomotor activity assays revealed altered pharmacological responses of St8sia3-KO mice to drugs targeting these receptors and verified that a greater population of D₂R formed heteromers with A_2AR in the striatum of St8sia3-KO mice. Since the A_2AR-D₂R heteromer is an important drug target for several basal ganglia diseases (such as schizophrenia and Parkinson's disease), the present study not only reveals a crucial role for ST8SIA3 in striatal functions but also provides a new drug target for basal ganglia-related diseases.

Ganglioside GD3 synthase (GD3S), a novel cancer drug target

Gangliosides are a class of important glycosphingolipids containing sialic acid that are widely distributed on the outer surface of cells and are abundantly distributed in brain tissue. Disialoganglioside with three glycosyl groups (GD3) and disialoganglioside with two glycosyl groups (GD2) are markedly increased in pathological conditions such as cancers and neurodegenerative diseases. GD3 and GD2 were found to play important roles in cancers by mediating cell proliferation, migration, invasion, adhesion, angiogenesis and in preventing immunosuppression of tumors. GD3 synthase (GD3S) is the regulatory enzyme of GD3 and GD2 synthesis, and is important in tumorigenesis and the development of cancers. The study of GD3S as a drug target may be of great significance for the discovery of new drugs for cancer treatment. This review will describe the gangliosides and their roles in physiological and pathological conditions; the roles of GD3 and GD2 in cancers; the expression, functions and mechanisms of GD3S, and its potential as a drug target in cancers.

Gangliosides: Structures, Biosynthesis, Analysis, and Roles in Cancer

Gangliosides are acidic glycosphingolipids containing one or more sialic acid residues. They are essential compounds at the outer leaflet of the plasma membrane, where they interact with phospholipids, cholesterol, and transmembrane proteins, forming lipid rafts. They are involved in cell adhesion, proliferation, and recognition processes, as well as in the modulation of signal transduction pathways. These functions are mainly governed by the glycan moiety, and changes in the structures of gangliosides occur under pathological conditions, particularly in neuro-ectoderm-derived cancers. With the progress in mass spectrometry analysis of gangliosides, their role in cancer progression can be now investigated in more detail. In this review we summarize the current knowledge on the biosynthesis of gangliosides and their role in cancers, together with the recent development of cancer immunotherapy targeting gangliosides.

Ganglioside GM3 Mediates Glucose-Induced Suppression of IGF-1 Receptor-Rac1 Activation to Inhibit Keratinocyte Motility

Activation of insulin-like growth factor-1 (IGF-1) receptor (IGF1R) signaling induces keratinocyte migration, but little is known about its regulation, including in diabetic wounds. GM3, a lipid raft ganglioside synthesized by GM3 synthase (GM3S), regulates receptor signaling. In diabetic mice, knockout or topically applied nanoconstruct-mediated knockdown of GM3S promotes wound edge IGF1R phosphorylation and re-epithelialization. Through modulating GM3 expression, we explored the role of GM3 in regulating human keratinocyte IGF1R signaling. Increases in GM3 and GM3S expression, including by exposure to high glucose, inhibit keratinocyte migration and IGF-1-induced chemotaxis in association with inhibition of IGF1R phosphorylation, suppression of Rac1 signaling, and activation of RhoA signaling. In contrast, GM3 depletion accelerates cell migration; increases cell velocity, displacement, and persistence; and activates IGF1R-Rac1 signaling. These data implicate GM3 in mediating glucose-induced suppression of IGF1R-Rac1 signaling. Furthermore, our findings provide evidence of a pivotal role for GM3-induced insulin resistance in impairing keratinocyte migration and reinforce the previously published studies in diabetic mice supporting GM3-depleting strategies as an approach for accelerating the healing of human diabetic wounds.

Neurophysiological and immunohistochemical studies of IgG anti-GM1 monoclonal antibody on neuromuscular transmission: effects in rat neuromuscular junctions

Guillain-Barré syndrome, which is a variant of acute inflammatory neuropathy, is associated with anti-GM1 antibodies and causes ataxia. We investigated the effects of IgG anti-GM1 monoclonal antibody (IgG anti-GM1 mAb) on spontaneous muscle action potentials in a rat spinal cord-muscle co-culture system and the localization of IgG anti-GM1 mAb binding in the rat hemi-diaphragm. The frequency of spontaneous muscle action potentials in innervated muscle cells was acutely inhibited by IgG anti-GM1 mAb. When cultures were pretreated with GM2 synthase antisense oligodeoxynucleotide, IgG anti-GM1 mAb failed to inhibit spontaneous muscle action potentials, demonstrating the importance of the GM1 epitope in the action of IgG anti-GM1 mAb. Immunohistochemistry of rat hemi-diaphragm showed that IgG anti-GM1 mAb binding overlapped with neurofilament 200 (NF200) antibodies staining, but not α-bungarotoxin (α-BuTx) staining, demonstrating that IgG anti-GM1 mAb was localized at the presynaptic nerve terminal. IgG anti-GM1 mAb binding overlapped with syntaxin antibody and S-100 antibody in the nerve terminal. After collagenase treatment, IgG anti-GM1 mAb and NF200 antibodies did not show staining, but α-BuTx selectively stained the hemi-diaphragm. IgG anti-GM1 mAb binds to the presynaptic nerve terminal of neuromuscular junctions. Therefore, we suggest that the inhibitory effect of IgG anti-GM1 mAb on spontaneous muscle action potentials is related to the GM1 epitope in presynaptic motor nerve terminals at the NMJs.

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.

Accumulation of unusual gangliosides G(Q3) and G(P3) in breast cancer cells expressing the G(D3) synthase

Glycosphingolipids from the ganglio-series are usually classified in four series according to the presence of 0 to 3 sialic acid residues linked to lactosylceramide. The transfer of sialic acid is catalyzed in the Golgi apparatus by specific sialyltransferases that show high specificity toward glycolipid substrates. ST8Sia I (EC 2.4.99.8, SAT-II, SIAT 8a) is the key enzyme controlling the biosynthesis of b- and c-series gangliosides. ST8Sia I is expressed at early developmental stages whereas in adult human tissues, ST8Sia I transcripts are essentially detected in brain. ST8Sia I together with b- and c-series gangliosides are also over-expressed in neuroectoderm-derived malignant tumors such as melanoma, glioblastoma, neuroblastoma and in estrogen receptor (ER) negative breast cancer, where they play a role in cell proliferation, migration, adhesion and angiogenesis. We have stably expressed ST8Sia I in MCF-7 breast cancer cells and analyzed the glycosphingolipid composition of wild type (WT) and GD3S+ clones. As shown by mass spectrometry, MCF-7 expressed a complex pattern of neutral and sialylated glycosphingolipids from globo- and ganglio-series. WT MCF-7 cells exhibited classical monosialylated gangliosides including G(M3), G(M2), and G(M1a). In parallel, the expression of ST8Sia I in MCF-7 GD3S+ clones resulted in a dramatic change in ganglioside composition, with the expression of b- and c-series gangliosides as well as unusual tetra- and pentasialylated lactosylceramide derivatives G(Q3) (II(3)Neu5Ac(4)-Gg(2)Cer) and G(P3) (II(3)Neu5Ac(5)-Gg(2)Cer). This indicates that ST8Sia I is able to act as an oligosialyltransferase in a cellular context.

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.

Triptolide downregulates human GD3 synthase (hST8Sia I) gene expression in SK-MEL-2 human melanoma cells

In this study, we have shown that gene expression of human GD3 synthase (hST8Sia I) is suppressed by triptolide (TPL) in human melanoma SK-MEL-2 cells. To elucidate the mechanism underlying the downregulation of hST8Sia I gene expression in TPL-treated SK-MEL-2 cells, we characterized the TPL-inducible promoter region within the hST8Sia I gene using luciferase constructs carrying 5'-deletions of the hST8Sia I promoter. Functional analysis of the 5'-flanking region of the hST8Sia I gene demonstrated that the -1146 to -646 region, which contains putative binding sites for transcription factors c-Ets-1, CREB, AP-1 and NF-κB, functions as the TPL-inducible promoter of hST8Sia I in SK-MEL-2 cells. Site-directed mutagenesis and ChIP analysis indicated that the NF-κB binding site at -731 to -722 is crucial for TPL-induced suppression of hST8Sia I in SK-MEL-2 cells. This suggests that TPL induces down-regulation of hST8Sia I gene expression through NF-κB activation in human melanoma cells.

Tumour-associated carbohydrate antigens in breast cancer

Glycosylation changes that occur in cancer often lead to the expression of tumour-associated carbohydrate antigens. In breast cancer, these antigens are usually associated with a poor prognosis and a reduced overall survival. Cellular models have shown the implication of these antigens in cell adhesion, migration, proliferation and tumour growth. The present review summarizes our current knowledge of glycosylation changes (structures, biosynthesis and occurrence) in breast cancer cell lines and primary tumours, and the consequences on disease progression and aggressiveness. The therapeutic strategies attempted to target tumour-associated carbohydrate antigens in breast cancer are also discussed.

Suppression of epidermal growth factor receptor signaling by protein kinase C-alpha activation requires CD82, caveolin-1, and ganglioside

Activation of protein kinase C (PKC)-alpha decreases normal and neoplastic cell proliferation by inhibiting epidermal growth factor receptor (EGFR)-related signaling. The molecular interactions upstream to PKC-alpha that influence its suppression of EGFR, however, are poorly understood. We have found that caveolin-1, tetraspanin CD82, and ganglioside GM3 enable the association of EGFR with PKC-alpha, ultimately leading to inhibition of EGFR signaling. GM3- and CD82-induced inhibition of EGFR signaling requires PKC-alpha translocation and serine/threonine phosphorylation, which eventually triggers EGFR Thr654 phosphorylation and receptor internalization. Within this ordered complex of signaling molecules, the ability of CD82 to associate with PKC-alpha requires the presence of caveolin-1, whereas the interaction of caveolin-1 or PKC-alpha with EGFR requires the presence of CD82 and ganglioside GM3. Disruption of the membrane with methyl-beta-cyclodextrin dissociates the EGFR/GM3/caveolin-1/CD82/PKC-alpha complex and prevents the inhibitory effect of PKC-alpha on EGFR phosphorylation, suggesting that caveolin-1, CD82, and ganglioside interact with EGFR and PKC-alpha within intact cholesterol-enriched membrane microdomains. Given the role of these membrane molecules in suppressing EGFR signaling, up-regulation of GM3, caveolin-1, and CD82 function may be an effective adjunctive therapy for treating epithelial cell malignancies.

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.

Expression of the human CMP-NeuAc:GM3 alpha2,8-sialyltransferase (GD3 synthase) gene through the NF-kappaB activation in human melanoma SK-MEL-2 cells

To elucidate the mechanism underlying the regulation of human GD3 synthase gene expression in human melanoma SK-MEL-2 cells, we identified the promoter region of the human GD3 synthase gene. The 5'-rapid amplification of cDNA end (5'-RACE) using mRNA prepared from SK-MEL-2 cells revealed the presence of multiple transcription start sites of human GD3 synthase gene. Promoter analyses of the 5'-flanking region of the human GD3 synthase gene using luciferase gene reporter system showed the strong promoter activity in SK-MEL-2 cells. Deletion study revealed that the region as the core promoter from -1146 to -646 (A of the translational start ATG as position +1) was indispensable for endogenous expression of human GD3 synthase gene. This region lacks apparent TATA and CAAT boxes but contains putative binding sites for transcription factors c-Ets-1, CREB, AP-1 and NF-kappaB. Electrophoretic mobility shift assays using specific competitors, chromatin immunoprecipitation assay and site-directed mutagenesis demonstrated that only NF-kappaB element in this region is required for the promoter activity in SK-MEL-2 cells. These results indicate that NF-kappaB plays an essential role in the transcriptional activity of human GD3 synthase gene essential for GD3 synthesis in SK-MEL-2 cells.

Molecular cloning and expression of alpha2,8-sialyltransferase (ST8Sia I, GD3 Synthase) in Xenopus

GD3, a minor ganglioside in most normal tissues, is involved in important biological events and its expression could increase in pathological conditions. Organism integrity requires a tight balance between the anabolic and catabolic processes, thus it is important to control the intracellular expression of those "key" enzymes, which act at the "branching point" of ganglioside metabolism; one of these is the GD3-synthase (ST8Sia I). In this paper, we report the sequences of two ST8Sia I mRNAs found in Xenopus laevis and their genomic organization; the canonical form resulted constituted of 5 exons and 4 introns, while the "short" mRNA lacks of the exon 2. The expression of the two ST8Sia I mRNAs during embryo development and their tissue distribution in adult animals showed the single or simultaneous presence of the two forms. Experiments of in vitro expression and evaluation of enzymatic activity of the two hypothetical proteins turned out to be ST8Sia I. In the end, considering the growing interest toward the specie Xenopus tropicalis, due to its diploid genome that render it more suitable for genetic studies, we also cloned X. tropicalis ST8Sia I.

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.

GD3 synthase gene found expressed in dental epithelium and shown to regulate cell proliferation

GD3 synthase is one of the key enzymes involved with ganglioside synthesis, and its activity regulates the main profile of ganglioside expression. We analyzed the expression of the GD3 synthase gene in laser-dissected teeth germs using RT-PCR. The GD3 synthase gene was found expressed in brain, thymus, and tooth germ tissues, however, not in liver or skin specimens. Further, it was highly expressed during the early stage of tooth germ development (embryonic day 14.5), especially in dental epithelia, which gradually reduced in the molar site until postnatal day 7, whereas it was not in dental mesenchyme tissues. In addition, dental epithelial cells transiently transfected with the GD3 synthase gene showed enhanced proliferation. These results indicate that the GD3 synthase gene may be involved in early tooth development, particularly in the proliferation of dental epithelium.

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.

Overexpression of GD3 synthase induces apoptosis of vascular endothelial ECV304 cells through downregulation of Bcl-2

The disialoganglioside GD3 plays a major role in proliferation, differentiation, and apoptosis. It has been reported that ganglioside GD3 can induce apoptosis through bcl-2 mediated mitochondrial pathway. However, the relationship between ganglioside GD3 and B-cell/CLL lymphoma 2 (Bcl-2) is not fully understood. In this study, we have demonstrated that the downregulation of Bcl-2 by overexpression of CMP-NeuAc:GM3 alpha-2,8-sialyltransferase (GD3 synthase) results in an accelerated apoptosis in vascular endothelial cells (ECV304), as evidenced by DNA fragmentation and caspase-3 activation. In addition, phosphorylation of AKT and cyclic-AMP responsive element binding protein (CREB) was reduced by GD3 synthase overexpression. Moreover, the activation of CREB as a transcriptional factor was also inhibited, as evidenced by electrophoretic mobility shift assay. Therefore, we conclude that GD3 synthase has an apoptotic effect on ECV304 cells through downregulation of Bcl-2 expression via dephosphorylation of AKT and CREB.

Further investigation of the epitope recognized by the new monoclonal antibody 2C9

OBJECTIVE

We established a new monoclonal antibody (2C9) that reacted with prostate tissue. The immunohistochemical reactivity of this antibody is similar to anti-prostate-specific membrane antigen (PSMA). Herein, we report the antigenic determinant of 2C9 antibody.

METHODS

The reactivity of the antibody was characterized by immunohistochemical staining and the antigen target was characterized by amino acid sequencing after immuno-affinity purification from an LNCaP cell lysate and cloning of a cDNA using a mammalian expression cDNA cloning system.

RESULTS

The amino acid and nucleotide sequences for the antigen molecule recognized with 2C9 monoclonal antibody demonstrated identity with PSMA.

CONCLUSION

The target molecule of the 2C9 monoclonal antibody is PSMA, pointing to future diagnostic and therapeutic applications.

Ganglioside GM3 inhibits matrix metalloproteinase-9 activation and disrupts its association with integrin

Gangliosides GM3 and GT1b both inhibit epithelial cell adhesion and migration on fibronectin. GT1b binds to integrin alpha5beta1 and blocks the integrin-fibronectin interaction; GM3 does not interact with integrin, and its effect is poorly understood. We evaluated the effects of endogenous modulation of GM3 expression on epithelial cell motility on several matrices and the mechanism of these effects. Endogenous accumulation of GM3 decreased cell migration on fibronectin, types I, IV, and VII collagen matrices; depletion of GM3 dramatically increased cell migration, regardless of matrix. GM3 overexpression and depletion in vitro correlated inversely with the expression and activity of matrix metalloproteinase-9; consistently, the cell migration stimulated by GM3 depletion is reversed by inhibition of matrix metalloproteinase-9 activity. Accumulation and depletion of GM3 in epithelial cells grown on fibronectin also correlated inversely with epidermal growth factor receptor and mitogen activated protein kinase phosphorylation and with Jun expression. Ganglioside depletion facilitated the co-immunoprecipitation of matrix metal-loproteinase-9 and integrin alpha5beta1, while endogenous accumulation of GM3, but not GT1b, blocked the co-immunoprecipitation. These data suggest modulation of epidermal growth factor receptor signaling and dissociation of integrin/matrix metalloproteinase-9 as mechanisms for the GM3-induced effects on matrix metalloproteinase-9 function.

GD3 ganglioside and apoptosis

Lipid and glycolipid mediators are important messengers of the adaptive responses to stress, including apoptosis. In mammalian cells, the intracellular accumulation of ganglioside GD3, an acidic glycosphingolipid, contributes to mitochondrial damage, a crucial event during the apoptopic program. GD3 is a minor ganglioside in most normal tissues. Its expression increases during development and in pathological conditions such as cancer and neurodegenerative disorders. Intriguingly, GD3 can mediate additional biological events such as cell proliferation and differentiation. These diverse and opposing effects indicate that tightly regulated mechanisms, including 9-O-acetylation, control GD3 function, by affecting intracellular levels, localization and structure of GD3, and eventually dictate biological outcomes and cell fate decisions.

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.

Molecular cloning and characterization of UDP-GlcNAc:lactosylceramide beta 1,3-N-acetylglucosaminyltransferase (beta 3Gn-T5), an essential enzyme for the expression of HNK-1 and Lewis X epitopes on glycolipids

A new member of the UDP-N-acetylglucosamine:beta-galactose beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T) family having the beta3Gn-T motifs was cloned from rat and human cDNA libraries and named beta3Gn-T5 based on its position in a phylogenetic tree. We concluded that beta3Gn-T5 is the most feasible candidate for lactotriaosylceramide (Lc(3)Cer) synthase, an important enzyme which plays a key role in the synthesis of lacto- or neolacto-series carbohydrate chains on glycolipids. beta3Gn-T5 exhibited strong activity to transfer GlcNAc to glycolipid substrates, such as lactosylceramide (LacCer) and neolactotetraosylceramide (nLc(4)Cer; paragloboside), resulting in the synthesis of Lc(3)Cer and neolactopentaosylceramide (nLc(5)Cer), respectively. A marked decrease in LacCer and increase in nLc(4)Cer was detected in Namalwa cells stably expressing beta3Gn-T5. This indicated that beta3Gn-T5 exerted activity to synthesize Lc(3)Cer and decrease LacCer, followed by conversion to nLc(4)Cer via endogenous galactosylation. The following four findings further supported that beta3Gn-T5 is Lc(3)Cer synthase. 1) The beta3Gn-T5 transcript levels in various cells were consistent with the activity levels of Lc(3)Cer synthase in those cells. 2) The beta3Gn-T5 transcript was presented in various tissues and cultured cells. 3) The beta3Gn-T5 expression was up-regulated by stimulation with retinoic acid and down-regulated with 12-O-tetradecanoylphorbol-13-acetate in HL-60 cells. 4) The changes in beta3Gn-T5 transcript levels during the rat brain development were determined. Points 2, 3, and 4 were consistent with the Lc(3)Cer synthase activity reported previously.

Conserved cysteines in the sialyltransferase sialylmotifs form an essential disulfide bond

The sialyltransferase gene family is comprised of 16 cloned enzymes. All members contain two conserved protein domains, termed the S- and L-sialylmotifs, that participate in substrate binding. Of only six invariant amino acids, two are cysteines, with one found in each sialylmotif. Although the recombinant soluble form of ST6Gal I has six cysteines, quantitative analysis indicated the presence of only one disulfide linkage, and thiol reducing agents dithiothreitol and beta-mercaptoethanol inactivated the enzyme. Analysis of site-directed mutants showed that alanine or serine mutants of invariant Cys(181) or Cys(332) exhibit no detectable activity, either by direct assay or by staining of the transfected cells with Sambucus nigra agglutinin, which recognizes the product NeuAcalpha2,6Galbeta1,4GlcNAc on glycoproteins. In contrast, alanine mutations of charged residues adjacent to either cysteine showed little or no effect on enzyme activity. Immunofluorescence microscopy showed that although the wild type sialyltransferase is properly localized in the Golgi apparatus, the inactive cysteine mutants are retained in the endoplasmic reticulum. The results suggest that the invariant cysteine residues in the L- and S-sialylmotifs participate in the formation of an intradisulfide linkage that is essential for proper conformation and activity of ST6Gal I.

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.

Recognition of cell surface GD3 by monoclonal antibody anti-6C2 in rheumatoid arthritis synovial fluid: expression on human T cells with transendothelial migratory activity

OBJECTIVE

We have previously reported that the anti-6C2 monoclonal antibody (mAb) defines a subset of human CD4+ memory T cells. The present study sought to determine the nature of the 6C2 molecule and the function associated with 6C2+ T cells, and to examine whether this T cell subset is involved in the pathophysiology of rheumatoid arthritis (RA).

METHODS

Cytofluorographic analysis was performed for identification of T cell surface molecules displaying a distribution similar to that of the 6C2 molecule. T cells in the synovial fluid of RA patients were examined for expression of the 6C2 molecule. Transendothelial migratory activity was assessed by assay using monolayers of human endothelial cells. Specific reactivity of the anti-6C2 mAb was determined by immunoblotting on gangliosides separated by thin-layer chromatography, and flow cytometric analysis of the cells transfected with complementary DNA (cDNA) was performed for determination of the glycosyltransferases involved in biosynthesis of the gangliosides.

RESULTS

On human peripheral T cells, the 6C2 molecule was distributed, by and large, in a pattern similar to that of CDw60, or O-acetyl-GD3. The majority (>70%) of synovial fluid T cells from patients with RA were found to be 6C2 positive, and those 6C2+ T cells exhibited a transendothelial migratory capacity that was inhibited by pretreatment of T cells with anti-6C2 mAb. Moreover, treatment of T cells with neuraminidase resulted in a loss of 6C2 expression as well as a reduction in the transendothelial migratory activity. Anti-6C2 mAb reacted specifically with GD3, but not with O-acetyl-GD3. The reactivity of anti-6C2 mAb was induced on the cell surface only by transfection with cDNA for GD3 synthase.

CONCLUSION

The 6C2 molecule is a disialoganglioside, GD3, and is present on a subset of T cells with transendothelial migratory capacity. The 6C2/GD3 molecules, as well as 6C2/GD3+ T cells, appear to play a role in T cell migration and in the inflammation of RA.

GD3 ganglioside directly targets mitochondria in a bcl-2-controlled fashion

Lipid and glycolipid diffusible mediators are involved in the intracellular progression and amplification of apoptotic signals. GD3 ganglioside is rapidly synthesized from accumulated ceramide after the clustering of death-inducing receptors and triggers apoptosis. Here we show that GD3 induces dissipation of DeltaPsim and swelling of isolated mitochondria, which results in the mitochondrial release of cytochrome c, apoptosis inducing factor, and caspase 9. Soluble factors released from GD3-treated mitochondria are sufficient to trigger DNA fragmentation in isolated nuclei. All these effects can be blocked by cyclosporin A, suggesting that GD3 is acting at the level of the permeability transition pore complex. We found that endogenous GD3 accumulates within mitochondria of cells undergoing apoptosis after ceramide exposure. Accordingly, suppression of GD3 synthase (ST8) expression in intact cells substantially prevents ceramide-induced DeltaPsim dissipation, indicating that endogenously synthesized GD3 induces mitochondrial changes in vivo. Finally, enforced expression of bcl-2 significantly prevents GD3-induced mitochondrial changes, caspase 9 activation, and apoptosis. These results show that mitochondria are a key destination for apoptogenic GD3 ganglioside along the lipid pathway to programmed cell death and indicate that relevant GD3 targets are under bcl-2 control.

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.

Molecular cloning of brain-specific GD1alpha synthase (ST6GalNAc V) containing CAG/Glutamine repeats

A novel member of the mouse CMP-NeuAc: beta-N-acetylgalactosaminide alpha2,6-sialyltransferase (ST6GalNAc) subfamily, designated ST6GalNAc V, was identified by BLAST analysis of expressed sequence tags. The sequence of the longest cDNA clone of ST6GalNAc V encoded a type II membrane protein with 8 amino acids comprising the cytoplasmic domain, 21 amino acids comprising the transmembrane region, and 306 amino acids comprising the catalytic domain. The predicted amino acid sequence showed homology to the previously cloned ST6GalNAc III and IV, with common amino acid sequences in sialyl motifs L and S among these three enzymes. Eleven CAG repeats were found in the stem region. A fusion protein with protein A and extracts from L cells transfected with ST6GalNAc V in a expression vector showed enzyme activity of alpha2,6-sialyltransferase almost exclusively for GM1b, but not toward glycoproteins. Sialidase treatment and thin layer chromatography immunostaining revealed that the product was GD1alpha. Northern blotting revealed that three transcripts of the gene were expressed specifically in brain tissues. It is concluded that this enzyme is involved in the synthesis of GD1alpha in the nervous tissues, and the CAG repeats may have implications in neurodegenerative diseases.