Gangliosides of the starfish Aphelasterias japonica, evidence for a new linkage between two N-glycolylneuraminic acid residues through the hydroxy group of the glycolic acid residue

Sialic acid and biology of life: An introduction

Sialic acids are important molecule with high structural diversity. They are known to occur in higher animals such as Echinoderms, Hemichordata, Cephalochorda, and Vertebrata and also in other animals such as Platyhelminthes, Cephalopoda, and Crustaceae. Plants are known to lack sialic acid. But they are reported to occur in viruses, bacteria, protozoa, and fungi. Deaminated neuraminic acid although occurs in vertebrates and bacteria, is reported to occur in abundance in the lower vertebrates. Sialic acids are mostly located in terminal ends of glycoproteins and glycolipids, capsular and tissue polysialic acids, bacterial lipooligosaccharides/polysaccharides, and in different forms that dictate their role in biology. Sialic acid play important roles in human physiology of cell-cell interaction, communication, cell-cell signaling, carbohydrate-protein interactions, cellular aggregation, development processes, immune reactions, reproduction, and in neurobiology and human diseases in enabling the infection process by bacteria and virus, tumor growth and metastasis, microbiome biology, and pathology. It enables molecular mimicry in pathogens that allows them to escape host immune responses. Recently sialic acid has found role in therapeutics. In this chapter we have highlighted the (i) diversity of sialic acid, (ii) their occurrence in the diverse life forms, (iii) sialylation and disease, and (iv) sialic acid and therapeutics.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Sequential One-Pot Multienzyme Chemoenzymatic Synthesis of Glycosphingolipid Glycans

Glycosphingolipids are a diverse family of biologically important glycolipids. In addition to variations on the lipid component, more than 300 glycosphingolipid glycans have been characterized. These glycans are directly involved in various molecular recognition events. Several naturally occurring sialic acid forms have been found in sialic acid-containing glycosphingolipids, namely gangliosides. However, ganglioside glycans containing less common sialic acid forms are currently not available. Herein, highly effective one-pot multienzyme (OPME) systems are used in sequential for high-yield and cost-effective production of glycosphingolipid glycans, including those containing different sialic acid forms such as N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), 2-keto-3-deoxy-d-glycero-d-galacto-nononic acid (Kdn), and 8-O-methyl-N-acetylneuraminic acid (Neu5Ac8OMe). A library of 64 structurally distinct glycosphingolipid glycans belonging to ganglio-series, lacto-/neolacto-series, and globo-/isoglobo-series glycosphingolipid glycans is constructed. These glycans are essential standards and invaluable probes for bioassays and biomedical studies.

Chemoenzymatic synthesis of C8-modified sialic acids and related α2-3- and α2-6-linked sialosides

Naturally occurring 8-O-methylated sialic acids, including 8-O-methyl-N-acetylneuraminic acid and 8-O-methyl-N-glycolylneuraminic acid, along with 8-O-methyl-2-keto-3-deoxy-d-glycero-d-galacto-nonulosonic acid (Kdn8Me) and 8-deoxy-Kdn were synthesized from corresponding 5-O-modified six-carbon monosaccharides and pyruvate using a sialic acid aldolase cloned from Pasteurella multocida strain P-1059 (PmNanA). In addition, α2-3- and α2-6-linked sialyltrisaccharides containing Neu5Ac8Me and Kdn8Deoxy were also synthesized using a one-pot multienzyme approach. The strategy reported here provides an efficient approach to produce glycans containing various C8-modified sialic acids for biological evaluations.

Isolation and structure of hematoside-type ganglioside from the starfish Linckia laevigata

A hematoside-type ganglioside, LLG-1 (1), has been obtained from the polar lipid fraction of the chloroform/methanol extract of the starfish Linckia laevigata. The structure of the ganglioside has been determined on the basis of chemical and spectroscopic evidence as 1-O-[N-glycolyl-alpha-D-neuraminosyl-(2-->3)-beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranosyl]-ceramide. The ceramide moiety was composed of heterogeneous 2-hydroxy fatty acid and phytosphingosine units. This is the first report on the isolation and structure elucidation of naked hematoside-type ganglioside from echinoderms.

[Structure and biological activity of glycosphingolipids from starfish and feather stars]

Glycosphingolipids (GSLs) are contained in a various cell membranes and have recently been implicated in many physiologic functions. They are classified based on their sugar moieties into ceramides, cerebrosides, sulfatides, ceramide-oligohexosides, globosides, and gangliosides. A number of GSLs have been obtained from marine invertebrates such as echinoderms, poriferans, and mollusks and have unique biological activities. During the course of our search for biologically active GSLs from echinoderms, we conducted the isolation and structural elucidation of GSLs from starfish and feather stars and found numerous GSLs, some of which have unique structures. In particular, gangliosides from feather stars were unique in that the sialic acids bind to inositol-phosphoceramide. We also found that the GSLs from starfish and feather stars possess neuritogenic activity toward the rat pheochromocytoma cell line PC12, antihyperglycemic effects against type 2 diabetic BKS. Cg-m+/+Leprdb/J (db/db) mice, and antiosteoporosis effects toward the osteoporosis model mice (OVX mice). These biological activities are thought to be related to dementia, osteoporosis, and diabetes, which are becoming social problems, and are expected to become the seeds of preventive or therapeutic drugs for these illness.

Isolation and structure of monomethylated GM3-type ganglioside molecular species from the starfish Luidia maculata

Two monomethylated GM(3)-Type ganglioside molecular species, 1 and 2, have been obtained from the polar lipid fraction of the chloroform/methanol extract of the starfish Luidia maculata. The structures of these gangliosides have been determined on the basis of chemical and spectroscopic evidence as 1-O-[8-O-methyl-(N-acetyl-alpha-D-neuraminosyl)-(2-->3)-beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranosyl]-ceramide (1) and 1-O-[8-O-methyl-(N-glycolyl-alpha-D-neuraminosyl)-(2-->3)-beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranosyl]-ceramide (2). The ceramide moieties were composed of heterogeneous unsubstituted fatty acid, 2-hydroxy fatty acid, sphingosine and phytosphingosine units. Compound 1, designated as LMG-3, represents new ganglioside molecular species. Compound 2 was a known ganglioside molecular species.

Occurrence and tissue distribution of c-series gangliosides in the common squid Todarodes pacificus

We have recently demonstrated that the common squid Todarodes pacificus express acidic lipids that were reactive with a monoclonal antibody A2B5. In the present study, two A2B5-reactive acidic lipids were isolated from squid hepatopancreatic tissue and characterized for their structures by methods including glycolipid overlay analysis, product analysis after sialidase treatment, and electrospray ionization-mass spectrometry (ESI-MS). Accordingly, the two acidic lipid were identified as GT3 and GQ1c, respectively. Another A2B5-reactive acidic lipid in the tissue was tentatively assigned to GT2 based upon its reactivity to A2B5 and chromatographic mobility on thin-layer chromatography. The composition and concentration of c-series gangliosides significantly differed among squid tissues (i.e. hepatopancreas, cerebral ganglion, eye lens, and mantle tissue). Interestingly, the percentages of c-series gangliosides within total gangliosides of hepatopancreas and cerebral ganglion were even higher than that of cod fish brain, which is known to be highly enriched with this ganglioside species. These findings strongly support the hypothesis that c-series gangliosides in squid tissues are not derived from ganglioside-containing food intake, but biosynthesized in a tissue-specific manner.

Cloning and expression of a membrane-bound CMP-N-acetylneuraminic acid hydroxylase from the starfish Asterias rubens

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is synthesized by the action of CMP-Neu5Ac hydroxylase. The enzyme from various mammals has been purified, characterized and sequenced by cDNA cloning. Although functional sequence motifs can be postulated from comparisons with several enzymes, no global homologies to any other proteins have been found. The unusual characteristics of this hydroxylase raise questions about its evolution. As echinoderms are phylogenetically the oldest organisms possessing Neu5Gc, they represent a starting point for investigations on the origin of this enzyme. Despite many similarities with its mammalian counterpart, CMP-Neu5Ac hydroxylase from the starfish A. rubens exhibits fundamental differences, most notably its association with a membrane and a requirement for high ionic strength. In order to shed light on the structural basis for these differences, the primary structure of CMP-Neu5Ac hydroxylase from A. rubens has been determined by PCR and cDNA-cloning techniques, using initial sequence information from the mouse enzyme. The complete assembled cDNA contained an ORF coding for a protein of 653 amino acids with a molecular mass of 75 kDa. The deduced amino-acid sequence exhibited a high degree of homology with the mammalian enzyme, although the C-terminus was some 60 residues longer. This extension consists of a terminal hydrophobic region, which may mediate membrane-binding, and a preceding hydrophilic sequence which probably serves as a hinge or linker. The identity of the ORF was confirmed by expression of active CMP-Neu5Ac hydroxylase in E. coli at low temperatures.

Occurrence of gangliosides in the common squid and pacific octopus among protostomia

Acidic lipids from tissues of the common squid Todarodes pacificus and the pacific octopus Octopus vulgaris were characterized. Hepatopancreatic tissues of both animals had complex compositions of resorcinol-positive acidic lipids, many of which became reactive with cholera toxin B subunit and anti-G(M1) antibody after in situ treatment with sialidase on TLC. One of the major acidic lipids in squid tissue was isolated and examined for its structure. This acidic lipid was identified to be the ganglioside G(D1a) based upon the susceptibility to sialidases of different substrate specificity, characterization of reaction products, and electrospray ionization-mass spectrometry of the lipid. Hepatopancreatic tissues of squid and octopus also contained acidic lipids that reacted with A2B5, a monoclonal antibody specific to c-series gangliosides. Cerebral ganglia of both animals expressed resorcinol-positive acidic lipids, though their compositional patterns differed from the hepatopancreatic tissues. N-Acetylneuraminic acid was identified as the main species in lipid-bound sialic acid in both tissues. The contents of lipid-bound sialic acid in cerebral ganglia were significantly lower than those of hepatopancreatic tissues in both animals. The present study presents the first evidence for the occurrence of gangliosides in protostomia.

Cytidine monophosphate-N-acetylneuraminate hydroxylase in the starfish Asterias rubens and other echinoderms

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is synthesised by an NADH-dependent hydroxylase which acts on CMP-N-acetylneuraminic acid (CMP-Neu5Ac). Although Neu5Gc is the predominant sialic acid in many echinoderms, little is known about the hydroxylase from organisms of this phylum. We show here that in contrast to the mammalian enzyme, the hydroxylase from various echinoderms is predominantly membrane-bound and exhibits optimal activity in the presence of 100 mM NaCl. A detailed characterisation of the hydroxylase from echinoderms was performed using fractionated gonads of the starfish Asterias rubens. Solubilisation using detergents led to an inactivation of the hydroxylase. However, the solubilised enzyme was reactivated by the addition of cytochrome b5 reductase together with the amphiphilic or soluble form of cytochrome b5. Although these latter proteins were only available from a mammalian source, the high affinity of the hydroxylase for cytochrome b5 suggests that, as with the mammalian enzyme, these electron carriers participate in the catalytic cycle of the hydroxylase from A. rubens in vivo. The relevance of these results to the interaction between cytochrome b5 and the hydroxylase is discussed.

Structural characterization of neutral glycosphingolipids from Trypanosoma cruzi

The major neutral glycosphingolipids from Trypanosoma cruzi ceramide mono- and dihexosides (CMH and CDH, respectively) were analysed after chromatographic purification using 1H 500 MHz nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. The ceramide monohexoside fraction (CMH) contained both glucosyl- and galactosylceramides. After peracetylation, the CMH fraction was separated into 2 subfractions, CMH-COH and CMH-Cn, containing either hydroxy fatty acids or n-fatty acids. In the CMH-COH fraction glucose and galactose were present in a ratio of 2:1, whereas this ratio was 1:1 in the CMH-Cn fraction. The CDH fraction was identified as lactosylceramide with sphingosine as the long chain base and 16:0, 18:0, and 24:0, 24:4 fatty acids as major components.

Nature and biosynthesis of sialic acids in the starfish Asterias rubens. Identification of sialo-oligomers and detection of S-adenosyl-L-methionine: N-acylneuraminate 8-O-methyltransferase and CMP-N-acetylneuraminate monooxygenase activities

Mass spectrometric and NMR spectroscopic analyses of bound sialic acids from the starfish Asterias rubens revealed the presence of N-acetylneuraminic acid (4%), N-acetyl-8-O-methylneuraminic acid (12%), N-acetyl-9-O-acetyl-8-O-methylneuraminic acid (less than 1%), N-glycoloylneuraminic acid (19%), N-glycoloyl-8-O-methylneuraminic acid (47%), and N-glycoloyl-9-O-acetyl-8-O-methylneuraminic acid (18%). Analysis of sialo-oligomeric material, obtained after mild acid hydrolysis, demonstrated that N-glycoloyl-8-O-methylneuraminic acid can occur as di- and tri-oligomers, linked through the anomeric center and the N-glycoloyl moiety, Neu5Gc8Me-alpha(2----O5)-Neu5Gc8Me and Neu5Gc8Me-alpha(2----O5)-Neu5Gc8Me-alpha (2----O5)-Neu5Gc8Me. Studies on the biosynthesis of N-acyl-8-O-methylneuraminic acid in A rubens, using the tracer S-adenosyl-L-[methyl-14C]methionine, showed that N-acylneuraminate 8-O-methyltransferase activity was present predominantly in the membrane fraction. CMP-N-acetylneuraminic acid monooxygenase activity was found in the soluble protein fraction, in agreement with investigations on the corresponding vertebrate enzyme.