Neobiosynthesis of glycosphingolipids by plasma membrane-associated glycosyltransferases

Exploring New Frontiers: Alternative Breast Cancer Treatments Through Glycocalyx Research

Breast cancer (BC) treatment is developing toward more precise and personalized care through the approval of different comprehensive approaches. Clinical practice emphasizes significant patient-to-patient variability in treatment response among patients, even those with similar clinical and biological profiles. Recent studies have demonstrated that the glycocalyx is an essential organelle that plays an important role in many cellular processes and can be a promising target for treatment. The glycocalyx of cancer cells is a key component influencing the interaction between the tumor and the immune system. Glycan modifications attached to glycoproteins and glycolipids are a common characteristic of the transition to malignancy. We review how the specific structure and function of the glycocalyx are regulated at the molecular level, contribute to immune evasion, and can be overcome by using both traditional drugs and combination therapies, as well as drugs not previously used in standard cancer treatments, to address treatment resistance associated with glycocalyx alterations. Trial Registration: ClinicalTrials.gov identifier: NCT00770354, NCT00925548, NCT01731587, NCT00088413, NCT00179309, NCT00986609, NCT05812326, NCT04020575, NCT05239143, NCT01279603, and NCT03562637.

GM1 structural requirements to mediate neuronal functions

Since the 1980s, it has been known that the administration of ganglioside GM1 to cultured cells induced or enhanced neuronal differentiation. GM1 mechanism of action relies on its direct interaction and subsequent activation of the membrane tyrosine kinase receptor, TrkA, which naturally serves as NGF receptor. This process is mediated by the sole oligosaccharide portion of GM1, the pentasaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal-(1-4)-β-Glc. Here we detailed the minimum structural requirements of the oligosaccharide portion of GM1 for mediating the TrkA dependent neuritogenic processing. By in vitro and in silico biochemical approaches, we demonstrated that the minimal portion of GM1 required for the TrkA activation is the inner core of the ganglioside's oligosaccharide β-Gal-(1-3)-β-GalNAc-(1-4)-[α-Neu5Ac-(2-3)]-β-Gal. The addition of a sialic acid residue at position 3 of the outer galactose of the GM1 oligosaccharide, which forms the oligosaccharide of GD1a, prevented the interaction with TrkA and the resulting neuritogenesis. On the contrary, the addition of a fucose residue at position 2 of the outer galactose, forming the Fucosyl-GM1 oligosaccharide, did not prevent the TrkA-mediated neuritogenesis.

Multi-dimensional role of gangliosides in modulating cancer hallmarks and their prospects in targeted cancer therapy

Gangliosides are glycosphingolipids with prevalence in nervous tissue and their involvement in certain neuronal diseases have been widely known. Interestingly, many recent studies highlighted their importance in the development and progression of various cancers through orchestration of multiple attributes of tumorigenesis, i.e., promoting migration, invasion, escaping the host immune system, and influencing other cancer hallmarks. Therefore, the multidimensional role of gangliosides in different cancers has established them as potential cancer targets. However, the tremendous structural complexity and functional heterogeneity are the major challenges in ganglioside research. Moreover, despite numerous immunotherapeutic attempts to target different gangliosides, it has failed to yield consistent results in clinical trials owing to their poor immunogenicity, a broad range of cross-reactivity, severe side effects, lack of uniform expression as well as heterogeneity. The recent identification of selective O-acetylated ganglioside expression in cancer tissues, but not in normal tissues, has strengthened their potential as a better and specific target for treating cancer patients. It was further supported by reduced cross-reactivity and side effects in clinical trials, although poor immunogenicity remains a major concern. Therefore, in addition to characterization and identification of the biological importance of O-acetylated gangliosides, their specific and efficient targeting in cancer through engineered antibodies is an emerging area of glycobiology research. This review highlights the modulatory effect of select gangliosides on different hallmarks of cancer and presents the overall development of ganglioside targeted immunotherapies along with recent progress. Here, we have also discussed its potential for future modifications aimed towards improvement in ganglioside-based cancer therapies.

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.

Deciphering the Action of Neuraminidase in Glioblastoma Models

Glioblastoma (GBM) contains cancer stem cells (CSC) that are resistant to treatment. GBM CSC expresses glycolipids recognized by the A2B5 antibody. A2B5, induced by the enzyme ST8 alpha-N-acetyl-neuraminide alpha-2,8-sialyl transferase 3 (ST8Sia3), plays a crucial role in the proliferation, migration, clonogenicity and tumorigenesis of GBM CSC. Our aim was to characterize the resulting effects of neuraminidase that removes A2B5 in order to target GBM CSC. To this end, we set up a GBM organotypic slice model; quantified A2B5 expression by flow cytometry in U87-MG, U87-ST8Sia3 and GBM CSC lines, treated or not by neuraminidase; performed RNAseq and DNA methylation profiling; and analyzed the ganglioside expression by liquid chromatography-mass spectrometry in these cell lines, treated or not with neuraminidase. Results demonstrated that neuraminidase decreased A2B5 expression, tumor size and regrowth after surgical removal in the organotypic slice model but did not induce a distinct transcriptomic or epigenetic signature in GBM CSC lines. RNAseq analysis revealed that OLIG2, CHI3L1, TIMP3, TNFAIP2, and TNFAIP6 transcripts were significantly overexpressed in U87-ST8Sia3 compared to U87-MG. RT-qPCR confirmed these results and demonstrated that neuraminidase decreased gene expression in GBM CSC lines. Moreover, neuraminidase drastically reduced ganglioside expression in GBM CSC lines. Neuraminidase, by its pleiotropic action, is an attractive local treatment against GBM.

Alkali-labile gangliosides

The structure and properties of a group of gangliosides modified by mild alkaline treatment are discussed. We will present the occurrence and the structure of gangliosides carrying the N-acetyneuraminic acid O-acetylated in position 9, the Neu5,9Ac₂, and of gangliosides carrying a sialic acid that forms a lactone ring. Starting from biochemical data we will discuss the possible biochemical role played by these gangliosides in the processes of cell signaling and maintenance of brain functions.

Ganglioside Synthesis by Plasma Membrane-Associated Sialyltransferase in Macrophages

Gangliosides are constituents of the mammalian cell membranes and participate in the inflammatory response. However, little is known about the presence and enzymatic activity of ganglioside sialyltransferases at the cell surface of macrophages, one of the most important immune cells involved in the innate inflammatory process. In the present study, using biochemical and fluorescent microscopy approaches, we found that endogenous ST8Sia-I is present at the plasma membrane (ecto-ST8Sia-I) of murine macrophage RAW264.7 cells. Moreover, ecto-ST8Sia-I can synthetize GD3 ganglioside at the cell surface in lipopolysaccharide (LPS)-stimulated macrophages even when LPS-stimulated macrophages reduced the total ST8Sia-I expression levels. Besides, cotreatment of LPS with an inhibitor of nitric oxide (NO) synthase recovered the ecto-ST8Sia-I expression, suggesting that NO production is involved in the reduction of ST8Sia-I expression. The diminution of ST8Sia-I expression in LPS-stimulated macrophages correlated with a reduction of GD3 and GM1 gangliosides and with an increment of GD1a. Taken together, the data supports the presence and activity of sialyltransferases at the plasma membrane of RAW264.7 cells. The variations of ecto-ST8Sia-I and ganglioside levels in stimulated macrophages constitutes a promissory pathway to further explore the physiological role of this and others ganglioside metabolism-related enzymes at the cell surface during the immune response.

Curcumin Analogue C1 Promotes Hex and Gal Recruitment to the Plasma Membrane via mTORC1-Independent TFEB Activation

The monocarbonyl analogue of curcumin (1E,4E)-1,5-Bis(2-methoxyphenyl)penta-1,4-dien-3-one (C1) has been used as a specific activator of the master gene transcription factor EB (TFEB) to correlate the activation of this nuclear factor with the increased activity of lysosomal glycohydrolases and their recruitment to the cell surface. The presence of active lysosomal glycohydrolases associated with the lipid microdomains has been extensively demonstrated, and their role in glycosphingolipid (GSL) remodeling in both physiological and pathological conditions, such as neurodegenerative disorders, has been suggested. Here, we demonstrate that Jurkat cell stimulation elicits TFEB nuclear translocation and an increase of both the expression of hexosaminidase subunit beta (HEXB), hexosaminidase subunit alpha (HEXA), and galactosidase beta 1 (GLB1) genes, and the recruitment of β-hexosaminidase (Hex, EC 3.2.1.52) and β-galactosidase (Gal, EC 3.2.1.23) on lipid microdomains. Treatment of Jurkat cells with the curcumin analogue C1 also resulted in an increase of both lysosomal glycohydrolase activity and their targeting to the cell surface. Similar effects of C1 on lysosomal glycohydrolase expression and their recruitment to lipid microdomains was observed by treating the SH-SY5Y neuroblastoma cell line; the effects of C1 treatment were abolished by TFEB silencing. Together, these results clearly demonstrate the existence of a direct link between TFEB nuclear translocation and the transport of Hex and Gal from lysosomes to the plasma membrane.

Integrin-mediated cell migration is blocked by inhibitors of human neuraminidase

Integrins are critical receptors in cell migration and adhesion. A number of mechanisms are known to regulate the function of integrins, including phosphorylation, conformational change, and cytoskeletal anchoring. We investigated whether native neuraminidase (Neu, or sialidase) enzymes which modify glycolipids could play a role in regulating integrin-mediated cell migration. Using a scratch assay, we found that exogenously added Neu3 and Neu4 activity altered rates of cell migration. We observed that Neu4 increased the rate of migration in two cell lines (HeLa, A549); while Neu3 only increased migration in HeLa cells. A bacterial neuraminidase was able to increase the rate of migration in HeLa, but not in A549 cells. Treatment of cells with complex gangliosides (GM1, GD1a, GD1b, and GT1b) resulted in decreased cell migration rates, while LacCer was able to increase rates of migration in both lines. Importantly, our results show that treatment of cells with inhibitors of native Neu enzymes had a dramatic effect on the rates of cell migration. The most potent compound tested targeted the human Neu4 isoenzyme, and was able to substantially reduce the rate of cell migration. We found that the lateral mobility of integrins was reduced by treatment of cells with Neu3, suggesting that Neu3 enzyme activity resulted in changes to integrin-co-receptor or integrin-cytoskeleton interactions. Finally, our results support the hypothesis that inhibitors of human Neu can be used to investigate mechanisms of cell migration and for the development of anti-adhesive therapies.

Dysregulated Expression of Glycolipids in Tumor Cells: From Negative Modulator of Anti-tumor Immunity to Promising Targets for Developing Therapeutic Agents

Glycolipids are complex molecules consisting of a ceramide lipid moiety linked to a glycan chain of variable length and structure. Among these are found the gangliosides, which are sialylated glycolipids ubiquitously distributed on the outer layer of vertebrate plasma membranes. Changes in the expression of certain species of gangliosides have been described to occur during cell proliferation, differentiation, and ontogenesis. However, the aberrant and elevated expression of gangliosides has been also observed in different types of cancer cells, thereby promoting tumor survival. Moreover, gangliosides are actively released from the membrane of tumor cells, having a strong impact on impairing anti-tumor immunity. Beyond the undesirable effects of gangliosides in cancer cells, a substantial number of cancer immunotherapies have been developed in recent years that have used gangliosides as the main target. This has resulted in successful immune cell- or antibody-responses against glycolipids, with promising results having been obtained in clinical trials. In this review, we provide a general overview on the metabolism of glycolipids, both in normal and tumor cells, as well as examining glycolipid-mediated immune modulation and the main successes achieved in immunotherapies using gangliosides as molecular targets.

Role of plasma-membrane-bound sialidase NEU3 in clathrin-mediated endocytosis

Gangliosides are sialic acid-containing glycosphingolipids mainly expressed at the outer leaflet of the plasma membrane. Sialidase NEU3 is a key enzyme in the catabolism of gangliosides with its up-regulation having been observed in human cancer cells. In the case of CME (clathrin-mediated endocytosis), although this has been widely studied, the role of NEU3 and gangliosides in this cellular process has not yet been established. In the present study, we found an increased internalization of Tf (transferrin), the archetypical cargo for CME, in cells expressing complex gangliosides with high levels of sialylation. The ectopic expression of NEU3 led to a drastic decrease in Tf endocytosis, suggesting the participation of gangliosides in this process. However, the reduction in Tf endocytosis caused by NEU3 was still observed in glycosphingolipid-depleted cells, indicating that NEU3 could operate in a way that is independent of its action on gangliosides. Additionally, internalization of α2-macroglobulin and low-density lipoprotein, other typical ligands in CME, was also decreased in NEU3-expressing cells. In contrast, internalization of cholera toxin β-subunit, which is endocytosed by both clathrin-dependent and clathrin-independent mechanisms, remained unaltered. Kinetic assays revealed that NEU3 caused a reduction in the sorting of endocytosed Tf to early and recycling endosomes, with the Tf binding at the cell surface being also reduced. NEU3-expressing cells showed an altered subcellular distribution of clathrin adaptor AP-2 (adaptor protein 2), but did not reveal any changes in the membrane distribution of clathrin, PtdIns(4,5)P2 or caveolin-1. Overall, these results suggest a specific and novel role of NEU3 in CME.

Critical role of evolutionarily conserved glycosylation at Asn211 in the intracellular trafficking and activity of sialyltransferase ST3Gal-II

ST3Gal-II is largely responsible for ganglioside terminal α2,3-sialylation in mammals. We demonstrated that ST3Gal-II mainly distributes in proximal Golgi compartments and that the inhibition of N-glycosylation and oligosaccharide trimming is critical for its enzymatic activity and intracellular distribution.

Glycosylation of glycolipids in cancer: basis for development of novel therapeutic approaches

Altered networks of gene regulation underlie many pathologies, including cancer. There are several proteins in cancer cells that are turned either on or off, which dramatically alters the metabolism and the overall activity of the cell, with the complex machinery of enzymes involved in the metabolism of glycolipids not being an exception. The aberrant glycosylation of glycolipids on the surface of the majority of cancer cells, associated with increasing evidence about the functional role of these molecules in a number of cellular physiological pathways, has received considerable attention as a convenient immunotherapeutic target for cancer treatment. This has resulted in the development of a substantial number of passive and active immunotherapies, which have shown promising results in clinical trials. More recently, antibodies to glycolipids have also emerged as an attractive tool for the targeted delivery of cytotoxic agents, thereby providing a rationale for future therapeutic interventions in cancer. This review first summarizes the cellular and molecular bases involved in the metabolic pathway and expression of glycolipids, both in normal and tumor cells, paying particular attention to sialosylated glycolipids (gangliosides). The current strategies in the battle against cancer in which glycolipids are key players are then described.

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

Cell surface associated glycohydrolases in normal and Gaucher disease fibroblasts

Gaucher disease (GD) is the most common lysosomal disorder and is caused by an inherited autosomal recessive deficiency in β-glucocerebrosidase. This enzyme, like other glycohydrolases involved in glycosphingolipid (GSL) metabolism, is present in both plasma membrane (PM) and intracellular fractions. We analyzed the activities of CBE-sensitive β-glucosidase (GBA1) and AMP-DNM-sensitive β-glucosidase (GBA2) in total cell lysates and PM of human fibroblast cell lines from control (normal) subjects and from patients with GD clinical types 1, 2, and 3. GBA1 activities in both total lysate and PM of GD fibroblasts were low, and their relative percentages were similar to those of control cells. In contrast, GBA2 activities were higher in GD cells than in control cells, and the degree of increase differed among the three GD types. The increase of GBA2 enzyme activity was correlated with increased expression of GBA2 protein as evaluated by QRT-PCR. Activities of β-galactosidase and β-hexosaminidase in PM were significantly higher for GD cells than for control cells and also showed significant differences among the three GD types, suggesting the occurrence of cross-talk among the enzymes involved in GSL metabolism. Our findings indicate that the profiles of glycohydrolase activities in PM may provide a valuable tool to refine the classification of GD into distinct clinical types.

Metabolic pathways and intracellular trafficking of gangliosides

Gangliosides constitute a large and heterogeneous family of acidic glycosphingolipids that contain one or more sialic acid residues and are expressed in nearly all vertebrate cells. Their de novo synthesis starts at the endoplasmic reticulum and is continued by a combination of glycosyltransferase activities at the Golgi complex, followed by vesicular delivery to the plasma membrane. At the cell surface, gangliosides participate in a variety of physiological as well as pathological processes. The cloning of genes for most of the glycosyltransferases responsible for ganglioside biosynthesis has produced a better understanding of the cellular and molecular basis of the ganglioside metabolism. In addition, the ability to delete groups of glycosphingolipid structures in mice has been enormously important in determining their physiological roles. Recently, a number of enzymes for ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane, which might contribute to modulate local glycolipid composition, and consequently, the cell function.

Lipoarabinomannan and related glycoconjugates: structure, biogenesis and role in Mycobacterium tuberculosis physiology and host-pathogen interaction

Approximately one third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. This bacterium has an unusual lipid-rich cell wall containing a vast repertoire of antigens, providing a hydrophobic impermeable barrier against chemical drugs, thus representing an attractive target for vaccine and drug development. Apart from the mycolyl-arabinogalactan-peptidoglycan complex, mycobacteria possess several immunomodulatory constituents, notably lipomannan and lipoarabinomannan. The availability of whole-genome sequences of M. tuberculosis and related bacilli over the past decade has led to the identification and functional characterization of various enzymes and the potential drug targets involved in the biosynthesis of these glycoconjugates. Both lipomannan and lipoarabinomannan possess highly variable chemical structures, which interact with different receptors of the immune system during host-pathogen interactions, such as Toll-like receptors-2 and C-type lectins. Recently, the availability of mutants defective in the synthesis of these glycoconjugates in mycobacteria and the closely related bacterium, Corynebacterium glutamicum, has paved the way for host-pathogen interaction studies, as well as, providing attenuated strains of mycobacteria for the development of new vaccine candidates. This review provides a comprehensive account of the structure, biosynthesis and immunomodulatory properties of these important glycoconjugates.

Trans-activity of plasma membrane-associated ganglioside sialyltransferase in mammalian cells

Gangliosides are acidic glycosphingolipids that contain sialic acid residues and are expressed in nearly all vertebrate cells. They are synthesized at the Golgi complex by a combination of glycosyltransferase activities followed by vesicular delivery to the plasma membrane, where they participate in a variety of physiological as well as pathological processes. Recently, a number of enzymes of ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane. In particular, it was observed that CMP-NeuAc:GM3 sialyltransferase (Sial-T2) is able to sialylate GM3 at the plasma membrane (cis-catalytic activity). In this work, we demonstrated that plasma membrane-integrated ecto-Sial-T2 also displays a trans-catalytic activity at the cell surface of epithelial and melanoma cells. By using a highly sensitive enzyme-linked immunosorbent assay combined with confocal fluorescence microscopy, we observed that ecto-Sial-T2 was able to sialylate hydrophobically or covalently immobilized GM3 onto a solid surface. More interestingly, we observed that ecto-Sial-T2 was able to sialylate GM3 exposed on the membrane of neighboring cells by using both the exogenous and endogenous donor substrate (CMP-N-acetylneuraminic acid) available at the extracellular milieu. In addition, the trans-activity of ecto-Sial-T2 was considerably reduced when the expression of the acceptor substrate was inhibited by using a specific inhibitor of biosynthesis of glycolipids, indicating the lipidic nature of the acceptor. Our findings provide the first direct evidence that an ecto-sialyltransferase is able to trans-sialylate substrates exposed in the plasma membrane from mammalian cells, which represents a novel insight into the molecular events that regulate the local glycosphingolipid composition.

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.

Organization of the synthesis of glycolipid oligosaccharides in the Golgi complex

Glycolipids constitute a complex family of amphipathic molecules structurally characterized by a hydrophilic mono- or oligo-saccharide moiety linked to a hydrophobic ceramide moiety. Due to their asymmetric distribution in cell membranes, exposing the saccharide moiety to the extracytoplasmic side of the cell, glycolipids participate in a variety of cell-cell and cell-ligand interactions. Here we summarize aspects of the cell biology of the stepwise synthesis of the saccharide moiety in the Golgi complex of cells from vertebrates. In particular we refer to the participant glycosyltransferases, with emphasis on their trafficking along the secretory pathway, their retention and organization in the Golgi complex membranes and their dependence on the Golgi complex ultra structural organization for proper function.

Remodeling of sphingolipids by plasma membrane associated enzymes

The sphingolipid plasma membrane content and pattern is the result of several processes, among which the main, in term of quantity, are: neo-biosynthesis in endoplasmic reticulum and Golgi apparatus, membrane turnover with final catabolism in lysosomes and membrane shedding. In addition to this, past and recent data suggest that the head group of sphingolipids can be opportunely modified at the plasma membrane level, probably inside specific membrane lipid domains, by the action of enzymes involved in the sphingolipids metabolism, working directly at the cell surface. The number of membrane enzymes, hydrolases and transferases, acting on membrane sphingolipids is growing very rapidly. In this report we describe some properties of these enzymes.