Sialobiology: Structure, Biosynthesis and Function. Sialic Acid Glycoconjugates in Health and Disease

Sialic acids (Sia) are a family of 9-carbon α-keto acid aminosugars found predominantly at the non-reducing end of oligosaccharide chains on glycoproteins and glycolipids. Since their discovery in the late 1930s, and subsequent naming by Blix, Gottschalk and Klenk (Nature. 1957; 179: 1088), Sia are now recognized as occurring ubiquitously in nature (except plants), and being involved in numerous biologically important processes. In particular, the growing awareness of the significance of Sia in human health and disease has led to an increase in research into Sia chemistry, biochemistry and cell biology. In this chapter, the structure and occurrence of Sia will be summarized, as well as aspects of Sia chemistry, biochemistry and cell biology not covered in subsequent chapters of this eBook are also presented. Throughout this, and subsequent chapters of this eBook the abbreviations and nomenclature summarised in Schauer and Varki, (In Essentials of Glycobiology 2^nd Ed, Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2009) will be used. Importantly, wherever appropriate the reader will be directed to the relevant chapters of this eBook, or extensive reviews for further detail.

Sialic acids (Sia) are involved in many biological activities and are commonly present as monosialyl residues at the non-reducing terminal end of glycoconjugates. Occasionally, polymerized structures in the form of disialic acid (diSia), oligosialic acid (oligoSia), and polysialic acid (polySia) appear in glycoconjugates. In particular, polySia is known to be a common epitope from bacteria to humans and is one of the most famous, biologically-important glycotopes in vertebrates. The biological functions of polySia, especially on neural cell adhesion molecules (NCAMs), have been well studied and an indepth body of knowledge concerning polySia has been accumulated. However, considerably less attention has been paid to glycoproteins containing di- and oligoSia groups. As the analytical methods used to detect oligo/polymerized structures have been improved, glycoproteins containing di/oligo/polySia chains have been identified with an increasing frequency in nature. In addition, more sophisticated genetic techniques have helped elucidate the underlying mechanisms of polySia-mediated activities. In this chapter, the recent advances in the study of di-, oligo- and polySia residues on glycoproteins, including their distribution, chemical properties, biosynthetic pathways, and functions are described.

Sialic acids (Sia) represent a family of nine-carbon keto-sugars with an unusual high structural diversity. However, all members are biosynthetic derivatives of either N-acetylneuraminic acid or 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid. In this chapter, we describe the biosynthesis of these two Sia precursors in vertebrates with a focus on the characteristiscs of the involved enzymes. In addition, the activation of the sugars as well as the degradation is included. Furthermore, dieseases and mouse models associated to the Sia biosynthesis pathway as well as biomedical implications are addressed.

Sialylation reactions take place in the lumen of the Golgi apparatus where sialyltransferases (STs) decorate glycan moieties of both the cell surfaces associated and secreted proteins and lipids with sialic acids (Sia) predominantly, but not exclusively employing, CMP-Neu5Ac as donor substrate. Because of its physical and chemical properties, CMP-Neu5Ac is unable to diffuse across the Golgi membrane and must be translocated from the cell cytosol into the lumen of the Golgi apparatus. Such translocation is performed by the CMP-Sia transporter, a member of an evolutionary conserved family of proteins together referred to as nucleotide sugar transporters. Although several nucleotide sugar transporters, including the CMP-Sia transporter, have been biochemically characterized over the last 30 years, the lack of a three-dimensional structure of any nucleotide sugar transporter requires alternative approaches to elucidating the structure-function relationship of this class of protein. We describe in this chapter the latest data reporting the elucidation of CMP-Sia transporter structurefunction relationship.

Sialyltransferases are a subset of glycosyltransferases catalyzing the transfer of sialic acid (Sia) residues from an activated sugar donor onto glycoconjugates. The aim of this chapter is to summarize in a comprehensive review what is known about vertebrate sialyltransferases structural features, the lessons drawn from molecular biology and the production of recombinant proteins and to explore the relationships between their primary structure and function. Insights into vertebrate sialyltransferases origin and evolution using bioinformatic approaches, screening of nucleotide databases of various animal organisms (vertebrates and invertebrates), molecular phylogeny and phylogenomic will be discussed.

Sialic acids (Sia) play major roles in glycan-mediated recognition/interaction processes, which are mediated by various intrinsic and extrinsic sialic acid-binding proteins. Cells therefore require fine-tuned mechanisms to regulate cell-surface expression of sialoglycoconjugates. In mammalian cells, there are 4 distinct sialidases, termed NEU1, NEU2, NEU3 and NEU4, involved in the removal of sialic acid residues from glycoconjugates; they play pivotal roles in diverse biological processes. In this chapter we summarize our current knowledge on mammalian sialidases.

Several pathogenic bacteria decorate their cell surface with sialoglycoconjugates that in many cases mimic host structures and serve as important virulence factors. In addition to N-acetyl neuraminic acid, the prevalent sialic acid in the humans, O-acetylated sialic acids are observed in bacteria that carry acetyl groups at position C-7, C-8 and/or C-9. The ability to modify cell surface sialo-glycoconjugates by O-acetylation depends on the presence of sialate O-acetyltransferases, an enzyme class that catalyzes the transfer of acetyl groups from acetyl Coenzyme A to hydroxyl groups of either free or CMP-activated sialic acid or particularly sialylated carbohydrate structures. On the genetic level, distinct mechanisms were observed which lead to an ‘on/off’ switch of sialate O-acetyltransferase expression and/or modification of the enzymatic activity. The resulting changes in the degree of surface O-acetylation of these bacteria can lead to a huge structural variety that make them difficult targets for the immune system. Structural and biochemical analyzes demonstrated that bacterial sialate O-acetyltransferases evolved independently on two distinct structural frameworks, the left-handed β-helix fold and the α/β-hydrolase fold.

Sialic acids (Sia) play a role in the survival of microbes within the diverse environments that both pathogenic and non-pathogenic microbes inhabit. 3-Deoxy-Dmanno- oct-2-ulosonic acid (KDO) and 2-keto-3-deoxynononic acid (KDN) are crucial for the survival of almost all bacterial species, while other Sia are a favourite target for viruses and other host adapted pathogens when interacting with host tissues that are required for survival/replication of the microbe. All pathogenic microbes, whether bacteria, viruses, parasites, or fungi, must be able to specifically interact with host cells/tissues to initiate disease. The success of these pathogens at maintaining a disease state relies on the ability of these organisms to subvert or evade the host immune responses. In this chapter we will discuss the ways in which Sia, Neu5Ac specifically, is crucial for the ability for many human pathogens to cause and maintain disease.

Free sialooligosaccharides (SOS) are only found in low concentrations in normal tissue. However, an accumulation of these compounds has been described in the case of several lysosomal storage diseases leading to severe pathological changes. In contrast, SOS are found in the blood stream as products of the catabolic pathway of glycoproteins and are excreted in their free form in urine. There are significant changes in composition and level of excreted SOS in the case of lysosomal storage diseases. The analysis of urine from patients with different types of diseases by HPLC has revealed distinct patterns of oligosaccharides depending on the type and stage of disease making it possible to differentiate between these. SOS are also important components of the milk that play an important role in the healthy development of the infant by supporting the brain development, by promoting gut health, by protecting the infant against pathogens and by stimulating the immune system. Since milk and colostrums are complex mixtures, the purification of SOS from these mactrices requires a process comprising several consecutive steps. A simple strategy for the enrichment of SOS includes ‘skimming’and ultrafiltration for the removal of lipids, proteins and larger molecules. Lactose, the major carbohydrate in milk, can then be removed by enzymatic treatment with galactosidase and subsequent chromatography on graphitised carbon. With small modifications this methodology can also be applied to larger scale and can be aligned with the processes in the Dairy industry. The same methodology can also be applied in the purification of synthetic procedures.

Gangliosides are a diverse group of sialic acid containing complex glycosphingolipids consisting of a carbohydrate chain with varying length and complexity and a lypophilic ceramide residue. The diversity arises from both the oligosaccharide and ceramide moiety. Echinoderms are the only phylum of invertebrates for which gangliosides have been reported with significant differences of ganglioside composition and structure between the individual classes (Echinoidea, Holothuroidea, Asteroidea, Ophiuroidea and Crinoidea). Both NeuAc and NeuGc are found in approximately equal distribution. The major substitutions are O-methylation and O-sulfation in position 8 and 4. Gangliosides from vertebrates are classified in four series (hematoside-series, ganglio-series, lacto-series, and globo-series) based on the structure of the carbohydrate core. Major sialic acids are NeuAc and NeuGc generally linked in α2,3 and α2,6 position of the penultimate carbohydrate residue. Acetylation of the sialic acid is the most common substitution. Gangliosides play an important role in a variety of biological processes. They are involved in the development and maintenance of the brain. The composition and expression of gangliosides changes during brain development reflecting the changing requirements at different stages. Different pathways have been suggested for the involvement of gangliosides including the physicochemical properties of the membranes, storage of calcium ions, interactions with growth factors, dendritogenesis, neuritogenesis and neural differentiation. During neurodegenerative diseases including Alzheimer, Parkinson and Huntington, the ganglioside composition changes. This leads to the disruption of vital cell function including signaling processes and ion channel formation and can ultimately lead to cell apoptosis. The ganglioside composition also changes during cancers such as melanoma, breast cancer, small cell lung cancer, cancers of the digestive tract and brain cancer. These changes have implications for the metastasis of the cancer, the suppression of the immune response, promotion of growth factors and kinases associated with these and angiogenesis. The changes have major effects on the progression and survival of the tumour. However, there are also indications that selected gangliosides can be useful for the therapy of these diseases. Some clinical trials have been carried out in the case of neurodegenerative diseases, cases of spinal injuries and cancer therapy. However, the results are as yet inconclusive. The nutritional value of gangliosides especially in infant nutrition has been widely discussed. In comparison to other lipids, proteins and carbohydrates, gangliosides are only a minor component in milk and colostrum. However, they could act as an additional source for sialic acid, ceramide and fatty acids. In addition, the intact ganglioside molecules have an effect on the immune system, the gut health and the development of the brain. The bioavailability and the fate of these glycosphingolipids in the digestive tract have been investigated. The results indicate that orally administered gangliosides are taken up by the organism in concentrations that are considered as biologically effective.

Altered cell surface sialylation is a hallmark of cancer. Investigation of clinical samples, cell lines and animal models has revealed associations between aberrant sialylation and cancer progression. Total serum and lipid bound sialic acid (Sia) levels are elevated in many types of cancer displaying a positive correlation with cancer stage and grade. The expression of α2-3 and α2-6 linked Sia and the levels of sialylated structures sialyl-Tn, sialyl Lewis a (SLea) and sialyl Lewis x (SLex) are also elevated in cancer cases. The metastatic potential of cancer cells is positively correlated to the levels of terminal galactose and N-acetylgalactosamine sialylation and the activities and mRNA expression levels of sialyltransferases and sialidases/neuraminidases are also significantly altered in cancer. The potential mechanisms via which altered sialylation is contributed to the progression of cancer have been studied. Increased Sia expression on the surface of cancer cells can enhance cell-cell repulsion and such an effect can potentially promote the dispersion of cells from primary tumor encouraging metastasis. Enhanced invasion, migration and altered binding to extracellular matrix, which are cell characteristics associated with metastatic behaviour, have also been linked to changes in sialylation. Increased resistance to apoptosis which can confer a growth advantage and the elevated expression of selectin ligands, SLea and SLex, can facilitate the transport of cancer cells throughout the blood stream and participate in the docking of cancer cells to the vascular endothelium at distant sites have also been observed in cancer cells displaying aberrant sialylation.

Chemical construction of α-glycoside of sialic acid (Sia) is a demanding subject due to the structural disadvantages of sialic acid: the carboxylate group at C1, the deoxy moiety adjacent to the anomeric center, and the glycerol branch from C6. Since Meindl and Tuppy reported the first synthesis of the glycoside of Sia in 1965, the development of stereoselective sialylation has been the subject for intensive efforts. The first part of this chapter will overview the classical methods for sialylation which override the disadvantages and introduce the cutting-edge of methods for α-selective sialylation, exemplifying the stereoselective synthesis of sialyl oligosaccharides. The later part will briefly summarize several strategies used for the design of sialylmimetics and their potential for the development of sialo-pharmaceuticals in treating various human disease states.

The functions of sialic acids (Sia) in the biology of animals are multifarious and depend especially on their type, number and localization in glycolipids and glycoproteins. So far various strategies have been developed to answer the listed questions posed in the text. For the general detection of Sia several colorimetric and fluorometric assays are available. To distinguish between the different types of Sia, chromatographic separation methods, such as, gas chromatography (GC) and high-performance liquid chromatography (HPLC) are necessary. Moreover, combination with mass spectrometry (MS) facilitates the identification of a Sia. Since Sia are also present as dimers, trimers, oligomers and polymers on glycoconjugates HPLC- and MS-based approaches were also developed for their detection and composition analysis, as well as determination of the degree of polymerization. In addition to chemical methods, biochemical tools like specific antibodies, enzymes and lectins are available to determine the type of Sia, the degree of polymerization and their linkage type and position in a glycoconjugate as well as their localization to the cell surface. Nevertheless, no method is presently available for a complete analysis. A comprehensive and detailed characterization requires a combination of different analytical methods to avoid errors in interpretation of the obtained data. This chapter summarizes the diverse analytical strategies for the analysis of Sia. Both, the advantages and disadvantages of the present methods and, in addition, the possibility to combine different methods to obtain meaningful results are described.

: Metabolic glycoengineering (MGE) refers to methodology developed over the last two decades wherein non-natural analogs of N-acetylmannosamine (ManNAc) intercept the biosynthetic pathway for sialic acid (Sia) in living cells and animals and subsequently become metabolically incorporated into sialoglycoconjugates in place of the natural sialosides. This article provides an overview of this technology by describing the chemical diversity that can be installed into cell surface sugars and can be subsequently exploited for numerous applications that include glycan labeling, glycomics, and – potentially – therapies for many disorders and diseases in which Sia play a role. Translation of metabolic glycoengineering from the laboratory to the clinic, however, faces substantial obstacles including the poor bioavailability of ManNAc analogs at both the cell and systemic levels. These issues are being addressed through the use of short chain fatty acid (SCFA)-monosaccharide hybrid molecules which, in addition to more favorable pharmacological properties, also harbor new modes of biological activity that present both pitfalls and new opportunities for burgeoning MGE technology.