A small-molecule modulator of poly-alpha 2,8-sialic acid expression on cultured neurons and tumor cells

Steroid and bioactive molecule conjugates: Improving therapeutic approaches in disease management

Conjugates of steroids and other natural bioactive molecules (such as amino acids or carbohydrates) have proven promising compounds with diverse biological effects. This literature review summarises the importance of steroid conjugates in a broad spectrum of therapeutic applications. Steroid conjugates exhibit improved pharmacokinetic properties, improved target specificity, and reduced side effects compared to the parent compounds. This increases their clinical usefulness. Their versatility extends to drug delivery systems, enabling precise modulation of drug release kinetics and bioavailability. Moreover, steroid conjugates are vital in treating inflammatory and neurodegenerative diseases, hormonal disorders, cancer therapy, and combating microbial infections. The review presents the current state of research on steroid conjugates, highlighting the crucial role of steroid conjugates in modern medicine and their potential to revolutionise therapeutic paradigms and improve patient outcomes. Steroid compounds are excellent for developing agents with better bioavailability and are used as drug carriers or hydrogelators.

Deciphering the Cell Surface Sugar-Coating via Biochemical Pathways

Cell surface components, specifically glycans, play a significant role in several biological functions like cell structure, crosstalk between cells, and eventual target recognition of the cells for therapeutics. The dense layer of glycans, i. e., glycocalyx, could differ in taxon, species, and cell type. Glycans are coupled with lipids and proteins to form glycolipids, glycoproteins, proteoglycans, and glycosylphosphatidylinositol-anchored proteins, making their study challenging. However, understanding glycosylation at the cellular level is vital for fundamental research and advancing glycan-targeted therapy. Among different pathways, metabolic glycan labelling uses the natural metabolic processes of the cell to introduce abiotic functionality into glycan residues. The Bertozzi group pioneered metabolic oligosaccharide engineering using glycan salvage pathways to convert monosaccharides with unnatural modifications. This eventually results in the probe becoming part of the complex cellular glycan structures via click chemistry using copper. On the other hand, the boronic acid-based probe can recognise carbohydrates in a single step without any chemical modification of the surface. This review discusses the significance of glycans as biomarkers for different diseases and the necessity to evaluate them in situ within the physiological environment. The review also discusses the prospect of this field and its potential applications.

Metabolic Glycoengineering: A Promising Strategy to Remodel Microenvironments for Regenerative Therapy

Cell-based regenerative therapy utilizes the differentiation potential of stem cells to rejuvenate tissues. But the dynamic fate of stem cells is calling for precise control to optimize their therapeutic efficiency. Stem cell fate is regulated by specific conditions called "microenvironments." Among the various factors in the microenvironment, the cell-surface glycan acts as a mediator of cell-matrix and cell-cell interactions and manipulates the behavior of cells. Herein, metabolic glycoengineering (MGE) is an easy but powerful technology for remodeling the structure of glycan. By presenting unnatural glycans on the surface, MGE provides us an opportunity to reshape the microenvironment and evoke desired cellular responses. In this review, we firstly focused on the determining role of glycans on cellular activity; then, we introduced how MGE influences glycosylation and subsequently affects cell fate; at last, we outlined the application of MGE in regenerative therapy, especially in the musculoskeletal system, and the future direction of MGE is discussed.

Attenuation of Polysialic Acid Biosynthesis in Cells by the Small Molecule Inhibitor 8-Keto-sialic acid

Sialic acids are key mediators of cell function, particularly with regard to cellular interactions with the surrounding environment. Reagents that modulate the display of specific sialyl glycoforms at the cell surface would be useful biochemical tools and potentially allow for therapeutic intervention in numerous challenging chronic diseases. While multiple strategies are being explored for the control of cell surface sialosides, none that shows high selectivity between sialyltransferases or that targets a specific sialyl glycoform has yet to emerge. Here, we describe a strategy to block the formation of α2,8-linked sialic acid chains (oligo- and polysialic acid) through the use of 8-keto-sialic acid as a chain-terminating metabolic inhibitor that, if incorporated, cannot be elongated. 8-Keto-sialic acid is nontoxic at effective concentrations and serves to block polysialic acid synthesis in cancer cell lines and primary immune cells, with minimal effects on other sialyl glycoforms.

Chemical tools to track and perturb the expression of sialic acid and fucose monosaccharides

The biosynthesis of glycans is a highly conserved biological process and found in all domains of life. The expression of cell surface glycans is increasingly recognized as a target for therapeutic intervention given the role of glycans in major pathologies such as cancer and microbial infection. Herein, we summarize our contributions to the development of unnatural monosaccharide derivatives to infiltrate and alter the expression of both mammalian and bacterial glycans and their therapeutic application.

The Applications of Metabolic Glycoengineering

Mammalian cell membranes are decorated by the glycocalyx, which offer versatile means of generating biochemical signals. By manipulating the set of glycans displayed on cell surface, it is vital for gaining insight into the cellular behavior modulation and medical and biotechnological adhibition. Although genetic engineering is proven to be an effective approach for cell surface modification, the technique is only suitable for natural and genetically encoded molecules. To circumvent these limitations, non-genetic approaches are developed for modifying cell surfaces with unnatural but functional groups. Here, we review latest development of metabolic glycoengineering (MGE), which enriches the chemical functions of the cell surface and is becoming an intriguing new tool for regenerative medicine and tissue engineering. Particular emphasis of this review is placed on discussing current applications and perspectives of MGE.

Sialyltransferase Inhibitors for the Treatment of Cancer Metastasis: Current Challenges and Future Perspectives

Potent, cell-permeable, and subtype-selective sialyltransferase inhibitors represent an attractive family of substances that can potentially be used for the clinical treatment of cancer metastasis. These substances operate by specifically inhibiting sialyltransferase-mediated hypersialylation of cell surface glycoproteins or glycolipids, which then blocks the sialic acid recognition pathway and leads to deterioration of cell motility and invasion. A vast amount of evidence for the in vitro and in vivo effects of sialyltransferase inhibition or knockdown on tumor progression and tumor cell metastasis or colonization has been accumulated over the past decades. In this regard, this review comprehensively discusses the results of studies that have led to the recent discovery and development of sialyltransferase inhibitors, their potential biomedical applications in the treatment of cancer metastasis, and their current limitations and future opportunities.

Glycoengineering Human Neural and Adipose Stem Cells with Novel Thiol-Modified N-Acetylmannosamine (ManNAc) Analogs

This report describes novel thiol-modified N-acetylmannosamine (ManNAc) analogs that extend metabolic glycoengineering (MGE) applications of Ac₅ManNTGc, a non-natural monosaccharide that metabolically installs the thio-glycolyl of sialic acid into human glycoconjugates. We previously found that Ac₅ManNTGc elicited non-canonical activation of Wnt signaling in human embryoid body derived (hEBD) cells but only in the presence of a high affinity, chemically compatible scaffold. Our new analogs Ac₅ManNTProp and Ac₅ManNTBut overcome the requirement for a complementary scaffold by displaying thiol groups on longer, N-acyl linker arms, thereby presumably increasing their ability to interact and crosslink with surrounding thiols. These new analogs showed increased potency in human neural stem cells (hNSCs) and human adipose stem cells (hASCs). In the hNSCs, Ac₅ManNTProp upregulated biochemical endpoints consistent with Wnt signaling in the absence of a thiol-reactive scaffold. In the hASCs, both Ac₅ManNTProp and Ac₅ManNTBut suppressed adipogenic differentiation, with Ac₅ManNTBut providing a more potent response, and they did not interfere with differentiation to a glial lineage (Schwann cells). These results expand the horizon for using MGE in regenerative medicine by providing new tools (Ac₅ManNTProp and Ac₅ManNTBut) for manipulating human stem cells.

Sialyltransferase Inhibitors Suppress Breast Cancer Metastasis

We report the synthesis and evaluation of a series of cell-permeable and N- versus O-selective sialyltransferase inhibitors. Inhibitor design entailed the functionalization of lithocholic acid at C(3) and at the cyclopentane ring side chain. Among the series, FCW34 and FCW66 were shown to inhibit MDA-MB-231 cell migration as effectively as ST3GALIII-gene knockdown did. FCW34 was shown to inhibit tumor growth, reduce angiogenesis, and delay cancer cell metastasis in animal models. Furthermore, FCW34 inhibited vessel development and suppressed angiogenic activity in transgenic zebrafish models. Our results provide clear evidence that FCW34-induced sialyltransferase inhibition reduces cancer cell metastasis by decreasing N-glycan sialylation, thus altering the regulation of talin/integrin/FAK/paxillin and integrin/NFκB signaling pathways.

A Chemoenzymatic Synthon Strategy for Synthesizing N-Acetyl Analogues of O-Acetylated N. meningitidis W Capsular Polysaccharide Oligosaccharides

O-Acetylated sialic acid has been found in the Neisseria meningitidis serogroup W (NmW) capsular polysaccharide (CPS) and is a required structural component of clinically used NmW CPS-based polysaccharide and polysaccharide-conjugate vaccines. The role of sialic acid O-acetylation in NmW CPS, however, is not clearly understood. This is partially due to the lack of a precise control of the percentage and the location of O-acetylation which is labile and susceptible to migration. We explore chemoenzymatic synthetic strategies for preparing N-acetylated analogues of O-acetylated NmW CPS oligosaccharides which can serve as structurally stable probe mimics. Substrate specificity studies of NmW CPS polymerase (NmSiaD_W) identified 4-azido-4-deoxy-N-acetylmannosamine (ManNAc4N₃) and 6-azido-6-deoxy-N-acetylmannosamine (ManNAc6N₃) as suitable chemoenzymatic synthons for synthesizing N-acetyl analogues of NmW CPS oligosaccharides containing 7-O-acetyl-N-acetylneuraminic acid (Neu5,7Ac₂) and/or 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac₂). The synthesis was achieved by NmSiaD_W-dependent sequential one-pot multienzyme (OPME) strategy with in situ generation of the corresponding sugar nucleotides from simple monosaccharides or derivatives to form N₃-oligosaccharides which were converted to the desired NAc-oligosaccharides by an efficient one-step chemical transformation.

The Cooperative Effect between Polybasic Region (PBR) and Polysialyltransferase Domain (PSTD) within Tumor-Target Polysialyltranseferase ST8Sia II

ST8Sia II (STX) is a highly homologous mammalian polysialyltransferase (polyST), which is a validated tumor-target in the treatment of cancer metastasis reliant on tumor cell polysialylation. PolyST catalyzes the synthesis of α2,8-polysialic acid (polySia) glycans by carrying out the activated CMP-Neu5Ac (Sia) to N- and O-linked oligosaccharide chains on acceptor glycoproteins. In this review article, we summarized the recent studies about intrinsic correlation of two polybasic domains, Polysialyltransferase domain (PSTD) and Polybasic region (PBR) within ST8Sia II molecule, and suggested that the critical amino acid residues within the PSTD and PBR motifs of ST8Sia II for polysialylation of Neural cell adhesion molecules (NCAM) are related to ST8Sia II activity. In addition, the conformational changes of the PSTD domain due to point mutations in the PBR or PSTD domain verified an intramolecular interaction between the PBR and the PSTD. These findings have been incorporated into Zhou's NCAM polysialylation/cell migration model, which will provide new perspectives on drug research and development related to the tumor-target ST8Sia II.

Sialic Acids in Neurology

Sialic acid (Sia) is involved in many biological activities and commonly occurs as a monosialyl residue at the nonreducing terminal end of glycoconjugates. The loss of activity of UDP-GlcNAc2-epimerase/ManNAc kinase, which is a key enzyme in Sia biosynthesis, is lethal to the embryo, which clearly indicates the importance of Sia in embryogenesis. Occasionally, oligo/polymeric Sia structures such as disialic acid (diSia), oligosialic acid (oligoSia), and polysialic acid (polySia) occur in glycoconjugates. In particular, polySia, a well-known epitope that commonly occurs in neuroinvasive bacteria and vertebrate brains, is one of the most well-known and biologically/neurologically important glycotopes in vertebrates. The biological effects of polySia, especially on neural cell-adhesion molecules, have been well studied, and in-depth knowledge regarding polySia has been accumulated. In addition, the importance of diSia and oligoSia epitopes has been reported. In this chapter, the recent advances in the study of diSia, oligoSia, and polySia residues in glycoproteins in neurology, and their history, definition, occurrence, analytical methods, biosynthesis, and biological functions evaluated by phenotypes of gene-targeted mice, biochemical features, and related diseases are described.

Asking more from metabolic oligosaccharide engineering

Glycans form one of the four classes of biomolecules, are found in every living system and present a huge structural and functional diversity. As an illustration of this diversity, it has been reported that more than 50% of the human proteome is glycosylated and that 2% of the human genome is dedicated to glycosylation processes. Glycans are involved in many biological processes such as signalization, cell-cell or host pathogen interactions, immunity, etc. However, fundamental processes associated with glycans are not yet fully understood and the development of glycobiology is relatively recent compared to the study of genes or proteins. Approximately 25 years ago, the studies of Bertozzi's and Reutter's groups paved the way for metabolic oligosaccharide engineering (MOE), a strategy which consists in the use of modified sugar analogs which are taken up into the cells, metabolized, incorporated into glycoconjugates, and finally detected in a specific manner. This groundbreaking strategy has been widely used during the last few decades and the concomitant development of new bioorthogonal ligation reactions has allowed many advances in the field. Typically, MOE has been used to either visualize glycans or identify different classes of glycoproteins. The present review aims to highlight recent studies that lie somewhat outside of these more traditional approaches and that are pushing the boundaries of MOE applications.

Chemical and biological methods for probing the structure and functions of polysialic acids

Owing to its poly-anionic charge and large hydrodynamic volume, polysialic acid (polySia) attached to neural cell adhesion molecule regulates axon–axon and axon–substratum interactions and signalling, particularly, in the development of the central nervous system (CNS). Expression of polySia is spatiotemporally regulated by the action of two polysialyl transferases, namely ST8SiaII and ST8SiaIV. PolySia expression peaks during late embryonic and early post-natal period and maintained at a steady state in adulthood in neurogenic niche of the brain. Aberrant polySia expression is associated with neurological disorders and brain tumours. Investigations on the structure and functions, over the past four decades, have shed light on the physiology of polySia. This review focuses on the biological, biochemical, and chemical tools available for polySia engineering. Genetic knockouts, endo-neuraminidases that cleave polySia, antibodies, exogenous expression, and neuroblastoma cells have provided deep insights into the ability of polySia to guide migration of neuronal precursors in neonatal brain development, neuronal clustering, axonal pathway guidance, and axonal targeting. Advent of metabolic sialic acid engineering using ManNAc analogues has enabled reversible and dose-dependent modulation polySia in vitro and ex vivo. In vivo, ManNAc analogues readily engineer the sialoglycans in peripheral tissues, but show no effect in the brain. A recently developed carbohydrate-neuroactive hybrid strategy enables a non-invasive access to the brain in living animals across the blood–brain barrier. A combination of recent advances in CNS drugs and imaging with ManNAc analogues for polySia modulation would pave novel avenues for understanding intricacies of brain development and tackling the challenges of neurological disorders.

Chemoenzymatic synthesis of para-nitrophenol (pNP)-tagged α2-8-sialosides and high-throughput substrate specificity studies of α2-8-sialidases

para-Nitrophenol (pNP)-tagged α2-8-linked sialosides containing different sialic acid forms were chemoenzymatically synthesized using an efficient one-pot three-enzyme α2-8-sialylation system. The resulting compounds allowed high-throughput substrate specificity studies of the α2-8-sialidase activity of a recombinant human cytosolic sialidase hNEU2 and various bacterial sialidases. The sialoside substrate profiles obtained can be used to guide the selection of suitable sialidases for sialylglycan analysis and for cell and tissue surface glycan modification. They can also be used to guide sialidase inhibitor design.

Neuro-Compatible Metabolic Glycan Labeling of Primary Hippocampal Neurons in Noncontact, Sandwich-Type Neuron-Astrocyte Coculture

Glycans are intimately involved in several facets of neuronal development and neuropathology. However, the metabolic labeling of surface glycans in primary neurons is a difficult task because of the neurotoxicity of unnatural monosaccharides that are used as a metabolic precursor, hindering the progress of metabolic engineering in neuron-related fields. Therefore, in this paper, we report a neurosupportive, neuron-astrocyte coculture system that neutralizes the neurotoxic effects of unnatural monosaccharides, allowing for the long-term observation and characterization of glycans in primary neurons in vitro. Polysialic acids in neurons are selectively imaged, via the metabolic labeling of sialoglycans with peracetylated N-azidoacetyl-d-mannosamine (Ac₄ManNAz), for up to 21 DIV. Two-color labeling shows that neuronal activities, such as neurite outgrowth and recycling of membrane components, are highly dynamic and change over time during development. In addition, the insertion sites of membrane components are suggested to not be random, but be predominantly localized in developing neurites. This work provides a new research platform and also suggests advanced 3D systems for metabolic-labeling studies of glycans in primary neurons.

Heterocyclic boronic acids display sialic acid selective binding in a hypoxic tumor relevant acidic environment

A group of heterocyclic boronic acids demonstrating unusually high affinity and selectivity for sialic acids are described, with strong interactions under the weakly acidic pH conditions associated with a hypoxic tumoral microenvironment.

Boronic acids are well known for their ability to reversibly interact with the diol groups found in sugars and glycoproteins. However, they are generally indiscriminate in their binding. Herein we describe the discovery of a group of heterocyclic boronic acids demonstrating unusually high affinity and selectivity for sialic acids (SAs or N-acetylneuraminic acid), which are sugar residues that are intimately linked with tumor growth and cancer progression. Remarkably, these interactions strengthen under the weakly acidic pH conditions associated with a hypoxic tumoral microenvironment. In vitro competitive binding assays uncovered a significantly higher ability of 5-boronopicolinic acid, one of the derivatives identified in this work as a strong SA-binder, to interact with cell surface SA in comparison to a gold-standard structure, 3-propionamidophenylboronic acid, which has proven to be an efficient SA-binder in numerous reports. This structure also proved to be suitable for further chemical conjugation with a well-preserved SA-binding capability. These findings suggest an attractive alternative to other ongoing boronic acid based chemistry techniques aiming to achieve tumor-specific chemotherapies and diagnoses.

Exploring and Exploiting Acceptor Preferences of the Human Polysialyltransferases as a Basis for an Inhibitor Screen

α2,8-Linked polysialic acid (polySia) is an oncofoetal antigen with high abundance during embryonic development. It reappears in malignant tumours of neuroendocrine origin. Two polysialyltransferases (polySTs) ST8SiaII and IV are responsible for polySia biosynthesis. During development, both enzymes are essential to control polySia expression. However, in tumours ST8SiaII is the prevalent enzyme. Consequently, ST8SiaII is an attractive target for novel cancer therapeutics. A major challenge is the high structural and functional conservation of ST8SiaII and -IV. An assay system that enables differential testing of ST8SiaII and -IV would be of high value to search for specific inhibitors. Here we exploited the different modes of acceptor recognition and elongation for this purpose. With DMB-DP3 and DMB-DP12 (fluorescently labelled sialic acid oligomers with a degree of polymerisation of 3 and 12, respectively) we identified stark differences between the two enzymes. The new acceptors enabled the simple comparative testing of the polyST initial transfer rate for a series of CMP-activated and N-substituted sialic acid derivatives. Of these derivatives, the non-transferable CMP-Neu5Cyclo was found to be a new, competitive ST8SiaII inhibitor.

An optimised assay for quantitative, high-throughput analysis of polysialyltransferase activity

Optimisation of a highly sensitive cell-free high-throughput HPLC-based assay for assessment of human polysialyltransferase activity is reported.

Metabolic Remodeling of Cell-Surface Sialic Acids: Principles, Applications, and Recent Advances

Cell-surface sialic acids are essential in mediating a variety of physiological and pathological processes. Sialic acid chemistry and biology remain challenging to investigate, demanding new tools for probing sialylation in living systems. The metabolic glycan labeling (MGL) strategy has emerged as an invaluable chemical biology tool that enables metabolic installation of useful functionalities into cell-surface sialoglycans by "hijacking" the sialic acid biosynthetic pathway. Here we review the principles of MGL and its applications in study and manipulation of sialic acid function, with an emphasis on recent advances.

Metabolic glycoengineering bacteria for therapeutic, recombinant protein, and metabolite production applications

Metabolic glycoengineering is a specialization of metabolic engineering that focuses on using small molecule metabolites to manipulate biosynthetic pathways responsible for oligosaccharide and glycoconjugate production. As outlined in this article, this technique has blossomed in mammalian systems over the past three decades but has made only modest progress in prokaryotes. Nevertheless, a sufficient foundation now exists to support several important applications of metabolic glycoengineering in bacteria based on methods to preferentially direct metabolic intermediates into pathways involved in lipopolysaccharide, peptidoglycan, teichoic acid, or capsule polysaccharide production. An overview of current applications and future prospects for this technology are provided in this report.

Tissue-based metabolic labeling of polysialic acids in living primary hippocampal neurons

The posttranslational modification of neural cell-adhesion molecule (NCAM) with polysialic acid (PSA) and the spatiotemporal distribution of PSA-NCAM play an important role in the neuronal development. In this work, we developed a tissue-based strategy for metabolically incorporating an unnatural monosaccharide, peracetylated N-azidoacetyl-D-mannosamine, in the sialic acid biochemical pathway to present N-azidoacetyl sialic acid to PSA-NCAM. Although significant neurotoxicity was observed in the conventional metabolic labeling that used the dissociated neuron cells, neurotoxicity disappeared in this modified strategy, allowing for investigation of the temporal and spatial distributions of PSA in the primary hippocampal neurons. PSA-NCAM was synthesized and recycled continuously during neuronal development, and the two-color labeling showed that newly synthesized PSA-NCAMs were transported and inserted mainly to the growing neurites and not significantly to the cell body. This report suggests a reliable and cytocompatible method for in vitro analysis of glycans complementary to the conventional cell-based metabolic labeling for chemical glycobiology.

Why Is N-Glycolylneuraminic Acid Rare in the Vertebrate Brain?

The sialic acids N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) differ by a single oxygen atom and are widely found at the terminal position of glycans on vertebrate cell surfaces. In animals capable of synthesizing Neu5Gc, most tissues and cell types express both sialic acids, in proportions that vary between species. However, it has long been noted that Neu5Gc is consistently expressed at trace to absent levels in the brains of all vertebrates studied to date. Although several reports have claimed to find low levels of Neu5Gc-containing glycans in neural tissue, no study definitively excludes the possibility of contamination with glycans from non-neural cell types. This distribution of a molecule - prominently but variably expressed in extraneural tissues but very low or absent in the brain - is, to our knowledge, unique. The evolutionarily conserved brain-specific suppression of Neu5Gc may indicate that its presence is toxic to this organ; however, no studies to date have directly addressed this very interesting question. Here we provide a historical background to this issue and discuss potential mechanisms causing the suppression of Neu5Gc expression in brain tissue, as well as mechanisms by which Neu5Gc may exert the presumed toxicity. Finally, we discuss future approaches towards understanding the mechanisms and implications of this unusual finding.

Polysialic acid: biosynthesis, novel functions and applications

As an anti-adhesive, a reservoir for key biological molecules, and a modulator of signaling, polysialic acid (polySia) is critical for nervous system development and maintenance, promotes cancer metastasis, tissue regeneration and repair, and is implicated in psychiatric diseases. In this review, we focus on the biosynthesis and functions of mammalian polySia, and the use of polySia in therapeutic applications. PolySia modifies a small subset of mammalian glycoproteins, with the neural cell adhesion molecule, NCAM, serving as its major carrier. Studies show that mammalian polysialyltransferases employ a unique recognition mechanism to limit the addition of polySia to a select group of proteins. PolySia has long been considered an anti-adhesive molecule, and its impact on cell adhesion and signaling attributed directly to this property. However, recent studies have shown that polySia specifically binds neurotrophins, growth factors, and neurotransmitters and that this binding depends on chain length. This work highlights the importance of considering polySia quality and quantity, and not simply its presence or absence, as its various roles are explored. The capsular polySia of neuroinvasive bacteria allows these organisms to evade the host immune response. While this "stealth" characteristic has made meningitis vaccine development difficult, it has also made polySia a worthy replacement for polyetheylene glycol in the generation of therapeutic proteins with low immunogenicity and improved circulating half-lives. Bacterial polysialyltransferases are more promiscuous than the protein-specific mammalian enzymes, and new studies suggest that these enzymes have tremendous therapeutic potential, especially for strategies aimed at neural regeneration and tissue repair.

Pharmacological inhibition of polysialyltransferase ST8SiaII modulates tumour cell migration

Polysialic acid (polySia), an α-2,8-glycosidically linked polymer of sialic acid, is a developmentally regulated post-translational modification predominantly found on NCAM (neuronal cell adhesion molecule). Whilst high levels are expressed during development, peripheral adult organs do not express polySia-NCAM. However, tumours of neural crest-origin re-express polySia-NCAM: its occurrence correlates with aggressive and invasive disease and poor clinical prognosis in different cancer types, notably including small cell lung cancer (SCLC), pancreatic cancer and neuroblastoma. In neuronal development, polySia-NCAM biosynthesis is catalysed by two polysialyltransferases, ST8SiaII and ST8SiaIV, but it is ST8SiaII that is the prominent enzyme in tumours. The aim of this study was to determine the effect of ST8SiaII inhibition by a small molecule on tumour cell migration, utilising cytidine monophosphate (CMP) as a tool compound. Using immunoblotting we showed that CMP reduced ST8iaII-mediated polysialylation of NCAM. Utilizing a novel HPLC-based assay to quantify polysialylation of a fluorescent acceptor (DMB-DP3), we demonstrated that CMP is a competitive inhibitor of ST8SiaII (K i = 10 µM). Importantly, we have shown that CMP causes a concentration-dependent reduction in tumour cell-surface polySia expression, with an absence of toxicity. When ST8SiaII-expressing tumour cells (SH-SY5Y and C6-STX) were evaluated in 2D cell migration assays, ST8SiaII inhibition led to significant reductions in migration, while CMP had no effect on cells not expressing ST8SiaII (DLD-1 and C6-WT). The study demonstrates for the first time that a polysialyltransferase inhibitor can modulate migration in ST8SiaII-expressing tumour cells. We conclude that ST8SiaII can be considered a druggable target with the potential for interfering with a critical mechanism in tumour cell dissemination in metastatic cancers.

Withania somnifera water extract as a potential candidate for differentiation based therapy of human neuroblastomas

Neuroblastoma is an aggressive childhood disease of the sympathetic nervous system. Treatments are often ineffective and have serious side effects. Conventional therapy of neuroblastoma includes the differentiation agents. Unlike chemo-radiotherapy, differentiation therapy shows minimal side effects on normal cells, because normal non-malignant cells are already differentiated. Keeping in view the limited toxicity of Withania somnifera (Ashwagandha), the current study was aimed to investigate the efficacy of Ashwagandha water extract (ASH-WEX) for anti-proliferative potential in neuroblastoma and its underlying signalling mechanisms. ASH-WEX significantly reduced cell proliferation and induced cell differentiation as indicated by morphological changes and NF200 expression in human IMR-32 neuroblastoma cells. The induction of differentiation was accompanied by HSP70 and mortalin induction as well as pancytoplasmic translocation of the mortalin in ASH-WEX treated cells. Furthermore, the ASH-WEX treatment lead to induction of neural cell adhesion molecule (NCAM) expression and reduction in its polysialylation, thus elucidating its anti-migratory potential, which was also supported by downregulation of MMP 2 and 9 activity. ASH-WEX treatment led to cell cycle arrest at G0/G1 phase and increase in early apoptotic population. Modulation of cell cycle marker Cyclin D1, anti-apoptotic marker bcl-xl and Akt-P provide evidence that ASH-WEX may prove to be a promising phytotherapeutic intervention in neuroblatoma related malignancies.

Pleiotropic effects of the bone morphogenetic proteins on development of the enteric nervous system

Formation of the enteric nervous system (ENS) from migratory neural crest-derived cells that colonize the primordial gut involves a complex interplay among different signaling molecules. The bone morphogenetic proteins (BMPs), specifically BMP2 and BMP4, play a particularly important role in virtually every stage of gut and ENS development. BMP signaling helps to pattern both the anterior-posterior axis and the radial axis of the gut prior to colonization by migratory crest progenitor cells. BMP signaling then helps regulate the migration of enteric neural crest-derived precursors as they colonize the fetal gut and form ganglia. BMP2 and -4 promote differentiation of enteric neurons in early fetal ENS development and glia at later stages. A major role for BMP signaling in the ENS is regulation of responses to other growth factors. Thus BMP signaling first regulates neurogenesis by modulating responses to GDNF and later gliogenesis through its effects on GGF-2 responses. Furthermore, BMPs promote growth factor dependency for survival of ENS neurons (on NT-3) and glia (on GGF-2) by inducing TrkC (neurons) and ErbB3 (glia). BMP signaling limits total neuron numbers, favoring the differentiation of later born neuronal phenotypes at the expense of earlier born ones thus influencing the neuronal composition of the ENS and the glia/neuron ratio. BMP2 and -4 also promote gangliogenesis via modification of neural cell adhesion molecules and promote differentiation of the circular and then longitudinal smooth muscles. Disruption of BMP signaling leads to defects in the gut and in ENS function commensurate with these complex developmental roles.

Glycomic strategy for efficient linkage analysis of di-, oligo- and polysialic acids

Sialic acid polymers of glycoproteins and glycolipids are characterized by a high diversity in nature and are involved in distinct biological processes depending inter alia on the glycosidic linkages between the present sialic acid residues. Though suitable protocols are available for chain length and sialic acid determination, sensitive methods for linkage analysis of di-, oligo-, and polysialic acids (di/oligo/polySia) are still pending. In this study, we have established a highly sensitive glycomic strategy for this purpose which is based on permethylation of di/oligo/polySia after tagging their reducing ends with the fluorescent dye 1,2-diamino-4,5-methylenedioxybenzene (DMB). Using DMB-labeled sialic acid di/oligo/polymers glycosidic linkages could be efficiently determined and, optionally, the established working procedure can be combined with HPLC for in depth characterization of distinct di/oligo/polySia chains. Moreover, the outlined approach can be directly applied to mammalian tissue samples and linkage analysis of sialic acid polymers present in biopsy samples of neuroblastoma tissue demonstrating the usefulness of the outlined work flow to screen, for example, cancer tissue for the presence of distinct variants of di/oligo/polySia as potentially novel biomarkers. Hence, the described strategy offers a highly sensitive and efficient strategy for identification of glycosidic linkages in sialic acid di/oligo/polymers of glycoproteins and glycolipids.

Metabolism of vertebrate amino sugars with N-glycolyl groups: resistance of α2-8-linked N-glycolylneuraminic acid to enzymatic cleavage

The sialic acid (Sia) N-acetylneuraminic acid (Neu5Ac) and its hydroxylated derivative N-glycolylneuraminic acid (Neu5Gc) differ by one oxygen atom. CMP-Neu5Gc is synthesized from CMP-Neu5Ac, with Neu5Gc representing a highly variable fraction of total Sias in various tissues and among different species. The exception may be the brain, where Neu5Ac is abundant and Neu5Gc is reported to be rare. Here, we confirm this unusual pattern and its evolutionary conservation in additional samples from various species, concluding that brain Neu5Gc expression has been maintained at extremely low levels over hundreds of millions of years of vertebrate evolution. Most explanations for this pattern do not require maintaining neural Neu5Gc at such low levels. We hypothesized that resistance of α2-8-linked Neu5Gc to vertebrate sialidases is the detrimental effect requiring the relative absence of Neu5Gc from brain. This linkage is prominent in polysialic acid (polySia), a molecule with critical roles in vertebrate neural development. We show that Neu5Gc is incorporated into neural polySia and does not cause in vitro toxicity. Synthetic polymers of Neu5Ac and Neu5Gc showed that mammalian and bacterial sialidases are much less able to hydrolyze α2-8-linked Neu5Gc at the nonreducing terminus. Notably, this difference was not seen with acid-catalyzed hydrolysis of polySias. Molecular dynamics modeling indicates that differences in the three-dimensional conformation of terminal saccharides may partly explain reduced enzymatic activity. In keeping with this, polymers of N-propionylneuraminic acid are sensitive to sialidases. Resistance of Neu5Gc-containing polySia to sialidases provides a potential explanation for the rarity of Neu5Gc in the vertebrate brain.

Polysialylation of the neural cell adhesion molecule: interfering with polysialylation and migration in neuroblastoma cells

Polysialic acid represents a unique posttranslational modification of the neural cell adhesion molecule (NCAM). It is built as a homopolymer of up to 150 molecules of alpha 2-8-linked sialic acids on N-glycans of the fifth immunoglobulin-like domain of NCAM. Besides its role in cell migration and axonal growth during development, polysialic acids are closely related to tumor malignancy as they are linked to the malignant potential of several tumors, such as undifferentiated neuroblastoma. Polysialic acid expression is significantly more frequent in high-grade tumors than in low-grade tumors. It is synthesized in the Golgi apparatus by the activity of two closely related enzymes, the polysialyltransferases ST8SiaII and ST8SiaIV. Interestingly, polysialylation of tumors is not equally synthesized by both polysialyltransferases. It has been shown that especially the ST8SiaII gene is not expressed in some normal tissue, but is strongly expressed in tumor tissue. Here we summarize some knowledge on the role of polysialic acid in cell migration and tumor progression and present novel evidence that interfering with polysialylation using unnatural sialic acid precursors decreases the migration of neuroblastoma cells.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

Homeostatic regulation of NCAM polysialylation is critical for correct synaptic targeting

During development, axonal projections have a remarkable ability to innervate correct dendritic subcompartments of their target neurons and to form regular neuronal circuits. Altered axonal targeting with formation of synapses on inappropriate neurons may result in neurodevelopmental sequelae, leading to psychiatric disorders. Here we show that altering the expression level of the polysialic acid moiety, which is a developmentally regulated, posttranslational modification of the neural cell adhesion molecule NCAM, critically affects correct circuit formation. Using a chemically modified sialic acid precursor (N-propyl-D: -mannosamine), we inhibited the polysialyltransferase ST8SiaII, the principal enzyme involved in polysialylation during development, at selected developmental time-points. This treatment altered NCAM polysialylation while NCAM expression was not affected. Altered polysialylation resulted in an aberrant mossy fiber projection that formed glutamatergic terminals on pyramidal neurons of the CA1 region in organotypic slice cultures and in vivo. Electrophysiological recordings revealed that the ectopic terminals on CA1 pyramids were functional and displayed characteristics of mossy fiber synapses. Moreover, ultrastructural examination indicated a "mossy fiber synapse"-like morphology. We thus conclude that homeostatic regulation of the amount of synthesized polysialic acid at specific developmental stages is essential for correct synaptic targeting and circuit formation during hippocampal development.

Discovery of inhibitors of Leishmania β-1,2-mannosyltransferases using a click-chemistry-derived guanosine monophosphate library

Leishmania spp. are a medically important group of protozoan parasites that synthesize a novel intracellular carbohydrate reserve polymer termed mannogen. Mannogen is a soluble homopolymer of β-1,2-linked mannose residues that accumulates in the major pathogenic stages in the sandfly vector and mammalian host. While several steps in mannogen biosynthesis have been defined, none of the enzymes have been isolated or characterized. We report the development of a simple assay for the GDP-mannose–dependent β-1,2-mannosyltransferases involved in mannogen synthesis. This assay utilizes octyl α-d-mannopyranoside to prime the formation of short mannogen oligomers up to 5 mannose residues. This assay was used to screen a focussed library of 44 GMP-triazole adducts for inhibitors. Several compounds provided effective inhibition of mannogen β-1,2-mannosyltransferases in a cell-free membrane preparation. This assay and inhibitor compounds will be useful for dissecting the role of different mannosyltransferases in regulating de novo biosynthesis and elongation reactions in mannogen metabolism.

Modified GM3 gangliosides produced by metabolic oligosaccharide engineering

Metabolic oligosaccharide engineering is powerful approach to altering the structure of cellular sialosides. This method relies on culturing cells with N-acetylmannosamine (ManNAc) analogs that are metabolized to their sialic acid counterparts and added to glycoproteins and glycolipids. Here we employed two cell lines that are deficient in ManNAc biosynthesis and examined their relative abilities to metabolize a panel of ManNAc analogs to sialosides. In addition to measuring global sialoside production, we also examined biosynthesis of the sialic acid-containing glycolipid, GM3. We discovered that the two cell lines differ in their ability to discriminate among the variant forms of ManNAc. Further, our data suggest that modified forms of sialic acid may be preferentially incorporated into certain sialosides and excluded from others. Taken together, our results demonstrate that global analysis of sialoside production can obscure sialoside-specific differences. These findings have implications for downstream applications of metabolic oligosaccharide engineering, including imaging and proteomics.

Defective polysialylation and sialylation induce opposite effects on gating of the skeletal Na+ channel NaV1.4 in Chinese hamster ovary cells

Polysialic acid (polySia) is a large, negatively charged homopolymer of 2,8-linked N-acetylneuraminic acid residues resulting from remodeling and extension of protein-bound sialic acid (Sia) residues and seems to have a key role in regulating neural cell development and function. The aim of this study was to explore and compare the effects of polySia and sialylation on gating of voltage-gated sodium channels. The skeletal muscle α-subunit NaV1.4 was transiently expressed in wild-type Chinese hamster ovary (CHO) cells or in mutant CHO cells with deficits in their capacity to produce sialylated or polysialylated membrane components. Expression in both mutant cell lines resulted in larger peak current amplitudes as compared to wild-type CHO cells. Loss of Sia and polySia also resulted in significant shifts of voltage-dependent activation and steady-state inactivation, however, in opposite directions. Furthermore, only the loss of Sia had a significant effect on recovery from fast inactivation. Our data demonstrate for the first time that gating of voltage-gated sodium channels seems to be differentially regulated by polySia and Sia.

Quantification of nucleotide-activated sialic acids by a combination of reduction and fluorescent labeling

Sialic acids usually represent the terminal monosaccharide of glycoconjugates and are directly involved in many biological processes. The cellular concentration of their nucleotide-activated form is one pacemaker for the highly variable sialylation of glycoconjugates. Hence, the determination of CMP-sialic acid levels is an important factor to understand the complex glycosylation machinery of cells and to standardize the production of glycotherapeutics. We have established a highly sensitive strategy to quantify the concentration of nucleotide-activated sialic acid by a combination of reduction and fluorescent labeling using the fluorophore 1,2-diamino-4,5-methylenedioxybenzene (DMB). The labeling with DMB requires free keto as well as carboxyl groups of the sialic acid molecule. Reduction of the keto group prior to the labeling process precludes the labeling of nonactivated sialic acids. Since the keto group is protected against reduction by the CMP-substitution, labeling of nucleotide-activated sialic acids is still feasible after reduction. Subsequent combination of the DMB-high-performance liquid chromatography (HPLC) application with mass spectrometric approaches, such as matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOF-MS) and electrospray-ionization (ESI)-MS, allows the unambiguous identification of both natural and modified CMP-sialic acids and localization of potential substituents. Thus, the described strategy offers a sensitive detection, identification, and quantification of nucleotide-activated sialic acid derivatives in the femtomole range without the need for nucleotide-activated standards.

A novel sialyltransferase inhibitor AL10 suppresses invasion and metastasis of lung cancer cells by inhibiting integrin-mediated signaling

Aberrant sialylation catalyzed by sialyltransferases (STs) is frequently found in cancer cells and is associated with increased cancer metastasis. However, ST inhibitors developed till now are not applicable for clinical use because of their poor cell permeability. In this study, a novel ST inhibitor AL10 derived from the lead compound lithocholic acid identified in our previous study is synthesized and the anti-cancer effect of this compound is studied. AL10 is cell-permeable and effectively attenuates total sialylation on cell surface. This inhibitor shows no cytotoxicity but inhibits adhesion, migration, actin polymerization and invasion of alpha-2,3-ST-overexpressing A549 and CL1.5 human lung cells. Inhibition of adhesion and migration by AL10 is associated with reduced sialylation of various integrin molecules and attenuated activation of the integrin downstream signaling mediator focal adhesion kinase. More importantly, AL10 significantly suppresses experimental lung metastasis in vivo without affecting liver and kidney function of experimental animals as determined by serum biochemical assays. Taken together, AL10 is the first ST inhibitor, which exhibits potent anti-metastatic activity in vivo and may be useful for clinical cancer treatment.

Polysialic acid neural cell adhesion molecule (PSA-NCAM) is an adverse prognosis factor in glioblastoma, and regulates olig2 expression in glioma cell lines

BACKGROUND

Glioblastoma multiforme (GBM) is the most aggressive and frequent brain tumor, albeit without cure. Although patient survival is limited to one year on average, significant variability in outcome is observed. The assessment of biomarkers is needed to gain better knowledge of this type of tumor, help prognosis, design and evaluate therapies. The neurodevelopmental polysialic acid neural cell adhesion molecule (PSA-NCAM) protein is overexpressed in various cancers. Here, we studied its expression in GBM and evaluated its prognosis value for overall survival (OS) and disease free survival (DFS).

METHODS

We set up a specific and sensitive enzyme linked immunosorbent assay (ELISA) test for PSA-NCAM quantification, which correlated well with PSA-NCAM semi quantitative analysis by immunohistochemistry, and thus provides an accurate quantitative measurement of PSA-NCAM content for the 56 GBM biopsies analyzed. For statistics, the Spearman correlation coefficient was used to evaluate the consistency between the immunohistochemistry and ELISA data. Patients' survival was estimated by using the Kaplan-Meier method, and curves were compared using the log-rank test. On multivariate analysis, the effect of potential risk factors on the DFS and OS were evaluated using the cox regression proportional hazard models. The threshold for statistical significance was p = 0.05.

RESULTS

We showed that PSA-NCAM was expressed by approximately two thirds of the GBM at variable levels. On univariate analysis, PSA-NCAM content was an adverse prognosis factor for both OS (p = 0.04) and DFS (p = 0.0017). On multivariate analysis, PSA-NCAM expression was an independent negative predictor of OS (p = 0.046) and DFS (p = 0.007). Furthermore, in glioma cell lines, PSA-NCAM level expression was correlated to the one of olig2, a transcription factor required for gliomagenesis.

CONCLUSION

PSA-NCAM represents a valuable biomarker for the prognosis of GBM patients.

Advances in the biology and chemistry of sialic acids

Sialic acids are a subset of nonulosonic acids, which are nine-carbon alpha-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis, as well as large-scale E. coli cell-based production, have provided a large library of sialoside standards and derivatives in amounts sufficient for structure-activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts not only will lead to a better understanding of the biological and pathological importance of sialic acids and their diversity but also could lead to the development of therapeutics.

Chemical approaches to glycobiology

Glycans are ubiquitous components of all organisms. Efforts to elucidate glycan function and to understand how they are assembled and disassembled can reap benefits in fields ranging from bioenergy to human medicine. Significant advances in our knowledge of glycan biosynthesis and function are emerging, and chemical biology approaches are accelerating the pace of discovery. Novel strategies for assembling oligosaccharides, glycoproteins, and other glycoconjugates are providing access to critical materials for interrogating glycan function. Chemoselective reactions that facilitate the synthesis of glycan-substituted imaging agents, arrays, and materials are yielding compounds to interrogate and perturb glycan function and dysfunction. To complement these advances, small molecules are being generated that inhibit key glycan-binding proteins or biosynthetic enzymes. These examples illustrate how chemical glycobiology is providing new insight into the functional roles of glycans and new opportunities to interfere with or exploit these roles.