Advances in the biology and chemistry of sialic acids

Efficient metabolic oligosaccharide engineering of glycoproteins by UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) knock-down

Improving the accessibility and functions of therapeutic and diagnostic glycoproteins is one of the major goals of glycobiotechnology. Here we present that stable knock-down of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme in the sialic acid biosynthetic pathway, dramatically increases incorporation of N-acetylmannosamine analogues into glycoproteins of HEK293 cells. By means of these GNE-deficient cells highly sialylated glycoproteins can efficiently be decorated with reactive functional groups, which can be employed in bioorthogonal functionalization strategies for fluorescence labelling or biotinylation.

Features and applications of bacterial sialidases

Sialidases, or neuraminidases (EC 3.2.1.18), belong to a class of glycosyl hydrolases that release terminal N-acylneuraminate residues from the glycans of glycoproteins, glycolipids, and polysaccharides. In bacteria, sialidases can be used to scavenge sialic acids as a nutrient from various sialylated substrates or to recognize sialic acids exposed on the surface of the host cell. Despite the fact that bacterial sialidases share many structural features, their biochemical properties, especially their linkage and substrate specificities, vary widely. Bacterial sialidases can catalyze the hydrolysis of terminal sialic acids linked by the α(2,3)-, α(2,6)-, or α(2,8)-linkage to a diverse range of substrates. In addition, some of these enzymes can catalyze the transfer of sialic acids from sialoglycans to asialoglycoconjugates via a transglycosylation reaction mechanism. Thus, some bacterial sialidases have been applied to synthesize complex sialyloligosaccharides through chemoenzymatic approaches and to analyze the glycan structure. In this review article, the biochemical features of bacterial sialidases and their potential applications in regioselective hydrolysis reactions as well as sialylation by transglycosylation for the synthesis of sialylated complex glycans are discussed.

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.

PmST2: a novel Pasteurella multocida glycolipid α2-3-sialyltransferase

Pasteurella multocida (Pm) is a multi-species pathogen that causes diseases in animals and humans. Sialyltransferase activity has been detected in multiple Pm strains and sialylation has been shown to be important for the pathogenesis of Pm. Three putative sialyltransferase genes have been identified in Pm genomic strain Pm70. We have reported previously that a Pm0188 gene homolog in Pm strain P-1059 (ATCC 15742) encodes a multifunctional sialyltransferase (PmST1). We demonstrate here that while PmST1 prefers to use oligosaccharides as acceptors, PmST2 encoded by the Pm0508 gene homolog in the same Pm strain is a novel glycolipid α2-3-sialyltransferase that prefers to use lactosyl lipids as acceptor substrates. PmST2 and PmST1 thus complement each other for an efficient synthesis of α2-3-linked sialosides with or without lipid portion. In addition, β1-4-linked galactosyl lipids are better PmST2 substrates than β1-3-linked galactosyl lipids. PmST2 has been used successfully in the preparative scale synthesis of sialyllactosyl sphingosine (lyso-GM3), which is an important glycolipid and an intermediate for synthesizing more complex glycolipids such as gangliosides.

High-throughput neuraminidase substrate specificity study of human and avian influenza A viruses

Despite the importance of neuraminidase (NA) activity in effective infection by influenza A viruses, limited information exists about the differences of substrate preferences of viral neuraminidases from different hosts or from different strains. Using a high-throughput screening format and a library of twenty α2-3- or α2-6-linked para-nitrophenol-tagged sialylgalactosides, substrate specificity of NAs on thirty-seven strains of human and avian influenza A viruses was studied using intact viral particles. Neuraminidases of all viruses tested cleaved both α2-3- and α2-6-linked sialosides but preferred α2-3-linked ones and the activity was dependent on the terminal sialic acid structure. In contrast to NAs of other subtypes of influenza A viruses which did not cleave 2-keto-3-deoxy-d-glycero-d-galacto-nonulosonic acid (Kdn) or 5-deoxy Kdn (5d-Kdn), NAs of all N7 subtype viruses tested had noticeable hydrolytic activities on α2-3-linked sialosides containing Kdn or 5d-Kdn. Additionally, group 1 NAs showed efficient activity in cleaving N-azidoacetylneuraminic acid from α2-3-linked sialoside.

Emerging role of alpha2,6-sialic acid as a negative regulator of galectin binding and function

Galectins are β-galactoside-binding lectins that regulate diverse cell behaviors, including adhesion, migration, proliferation, and apoptosis. Galectins can be expressed both intracellularly and extracellularly, and extracellular galectins mediate their effects by associating with cell-surface oligosaccharides. Despite intensive current interest in galectins, strikingly few studies have focused on a key enzyme that acts to inhibit galectin signaling, namely β-galactoside α2,6-sialyltransferase (ST6Gal-I). ST6Gal-I adds an α2,6-linked sialic acid to the terminal galactose of N-linked glycans, and this modification blocks galectin binding to β-galactosides. This minireview summarizes the evidence suggesting that ST6Gal-I activity serves as an "off switch" for galectin function.

Chemoenzymatic synthesis of α2-3-sialylated carbohydrate epitopes

Sialic acids are common terminal carbohydrates on cell surface. Together with internal carbohydrate structures, they play important roles in many physiological and pathological processes. In order to obtain α2-3-sialylated oligosaccharides, a highly efficient one-pot three-enzyme synthetic approach was applied. The P. multocida α2-3-sialyltransferase (PmST1) involved in the synthesis was a multifunctional enzyme with extremely flexible donor and acceptor substrate specificities. Sialyltransferase acceptors, including type 1 structure (Galβ1-3GlcNAcβProN(3)), type 2 structures (Galβ1-4GlcNAcβProN(3) and 6-sulfo-Galβ1-4GlcNAcβProN(3)), type 4 structure (Galβ1-3GalNAcβProN(3)), type 3 or core 1 structure (Galβ1-3GalNAcαProN(3)) and human milk oligosaccharide or lipooligosaccharide lacto-N-tetraose (LNT) (Galβ1-3GlcNAcβ1-3Galβ1-4GlcβProN(3)), were chemically synthesized. They were then used in one-pot three-enzyme reactions with sialic acid precursor ManNAc or ManNGc, to synthesize a library of natural occurring α2-3-linked sialosides with different internal sugar units. The sialylated oligosaccharides obtained are valuable probes for their biological studies.

Sialic acid transport contributes to pneumococcal colonization

Streptococcus pneumoniae is a major cause of pneumonia and meningitis. Airway colonization is a necessary precursor to disease, but little is known about how the bacteria establish and maintain colonization. Carbohydrates are required as a carbon source for pneumococcal growth and, therefore, for colonization. Free carbohydrates are not readily available in the naso-oropharynx; however, N- and O-linked glycans are common in the airway. Sialic acid is the most common terminal modification on N- and O-linked glycans and is likely encountered frequently by S. pneumoniae in the airway. Here we demonstrate that sialic acid supports pneumococcal growth when provided as a sole carbon source. Growth on sialic acid requires import into the bacterium. Three genetic regions have been proposed to encode pneumococcal sialic acid transporters: one sodium solute symporter and two ATP binding cassette (ABC) transporters. Data demonstrate that one of these, satABC, is required for transport of sialic acid. A satABC mutant displayed significantly reduced growth on both sialic acid and the human glycoprotein alpha-1. The importance of satABC for growth on human glycoprotein suggests that sialic acid transport may be important in vivo. Indeed, the satABC mutant was significantly reduced in colonization of the murine upper respiratory tract. This work demonstrates that S. pneumoniae is able to use sialic acid as a sole carbon source and that utilization of sialic acid is likely important during pneumococcal colonization.

Functional assays for the molecular chaperone cosmc

Mucin type O-glycosylation involves sequential actions of several glycosyltransferases in the Golgi apparatus. Among those enzymes, a single gene product termed core 1 beta3-galactosyltransferase (T-synthase) in vertebrates is the key enzyme that converts the precursor Tn antigen GalNAcalpha1-Ser/Thr to the core 1 structure, Galbeta1-3GalNAcalpha1-Ser/Thr, also known as T antigen. This represents the most common structure within typical O-glycans of membrane and secreted glycoproteins. Formation of the active T-synthase requires that it interacts with Core 1 beta3Gal-T Specific Molecular Chaperone (Cosmc), which is a specific molecular chaperone in the endoplasmic reticulum (ER). T-synthase activity is commonly measured by its ability to transfer [3H]Gal from UDP-[3H]Gal to an artificial acceptor GalNAcalpha-1-O-phenyl to form [3H]Galbeta1-3GalNAcalpha-1-O-phenyl, which can then be isolated and quantified. Because the primary function of Cosmc is to form active T-synthase, the activity of Cosmc is assessed indirectly by its ability to promote formation of active T-synthase when it is coexpressed with T-synthase in cells lacking functional Cosmc. Such cells include insect cells, which constitutively lack Cosmc, and Cosmc-deficient mammalian cell lines. Cosmc is encoded by the X-linked Cosmc gene (Xq24 in human, Xc3 in mice), thus, acquired mutations in Cosmc, which have been observed in several human diseases, such as Tn syndrome and cancers, cause a loss of T-synthase, and expression of the Tn antigen. The methods described here allow the functional activities of such mutated Cosmc (mCosmc) to be measured and compared to wild-type (wtCosmc).

Quantum dots with phenylboronic acid tags for specific labeling of sialic acids on living cells

Sialic acids with a nine-carbon backbone are commonly found at the terminal position of the glycans structures on cell membranes. The unique distribution and ubiquitous existence of sialic acid on the cell membrane make them important mediators in various biological and pathological processes. We report a new class of imaging probes based on semiconductor quantum dots with small molecular phenylboronic acid tags for highly specific and efficient labeling of sialic acid on living cells. Our results have shown that the use of these probes enables one-step labeling and continuous tracking of the cell surface sialic acid moieties without any pretreatment of living cells. The one-step procedure with fast binding kinetics and the biocompatibility of these probes make it an ideal noninvasive technology for living cell imaging. We also find that the labeled sialic acids undergo quick internalization shortly after surface binding via endocytosis and eventually distribute in the perinuclear region. This distribution pattern is consistent with the notion that sialylated glycoproteins are populated on cell membranes and recycled through the vesicular exocytotic and endocytic pathways. The superior photostability and brightness of quantum dots enable quantitative analysis of the diffusion dynamics of sialic acids, which has been a significant challenge for glycan imaging.

Cellular uptake of ribonuclease A relies on anionic glycans

Bovine pancreatic ribonuclease (RNase A) can enter human cells, even though it lacks a cognate cell-surface receptor protein. Here, we report on the biochemical basis for its cellular uptake. Analyses in vitro and in cellulo revealed that RNase A interacts tightly with abundant cell-surface proteoglycans containing glycosaminoglycans, such as heparan sulfate and chondroitin sulfate, as well as with sialic acid-containing glycoproteins. The uptake of RNase A correlates with cell anionicity, as quantified by measuring electrophoretic mobility. The cellular binding and uptake of RNase A contrast with those of Onconase, an amphibian homologue that does not interact tightly with anionic cell-surface glycans. As anionic glycans are especially abundant on human tumor cells, our data predicate utility for mammalian ribonucleases as cancer chemotherapeutic agents.

Current trends in the structure-activity relationships of sialyltransferases

Sialyltransferases (STs) represent an important group of enzymes that transfer N-acetylneuraminic acid (Neu5Ac) from cytidine monophosphate-Neu5Ac to various acceptor substrates. In higher animals, sialylated oligosaccharide structures play crucial roles in many biological processes but also in diseases, notably in microbial infection and cancer. Cell surface sialic acids have also been found in a few microorganisms, mainly pathogenic bacteria, and their presence is often associated with virulence. STs are distributed into five different families in the CAZy database (http://www.cazy.org/). On the basis of crystallographic data available for three ST families and fold recognition analysis for the two other families, STs can be grouped into two structural superfamilies that represent variations of the canonical glycosyltransferase (GT-A and GT-B) folds. These two superfamilies differ in the nature of their active site residues, notably the catalytic base (a histidine or an aspartate residue). The observed structural and functional differences strongly suggest that these two structural superfamilies have evolved independently.

Cloning and characterization of a viral α2-3-sialyltransferase (vST3Gal-I) for the synthesis of sialyl Lewisx

Sialyl Lewis(x) (SLe(x), Siaα2-3Galβ1-4(Fucα1-3)GlcNAcβOR) is an important sialic acid-containing carbohydrate epitope involved in many biological processes such as inflammation and cancer metastasis. In the biosynthetic process of SLe(x), α2-3-sialyltransferase-catalyzed sialylation generally proceeds prior to α1-3-fucosyltransferase-catalyzed fucosylation. For the chemoenzymatic synthesis of SLe(x) containing different sialic acid forms, however, it would be more efficient if diverse sialic acid forms are transferred in the last step to the fucosylated substrate Lewis(x) (Le(x)). An α2-3-sialyltransferase obtained from myxoma virus-infected European rabbit kidney RK13 cells (viral α2-3-sialyltransferase (vST3Gal-I)) was reported to be able to tolerate fucosylated substrate Le(x). Nevertheless, the substrate specificity of the enzyme was only determined using partially purified protein from extracts of cells infected with myxoma virus. Herein we demonstrate that a previously reported multifunctional bacterial enzyme Pasteurella multocida sialyltransferase 1 (PmST1) can also use Le(x) as an acceptor substrate, although at a much lower efficiency compared to nonfucosylated acceptor. In addition, N-terminal 30-amino-acid truncated vST3Gal-I has been successfully cloned and expressed in Escherichia coli Origami™ B(DE3) cells as a fusion protein with an N-terminal maltose binding protein (MBP) and a C-terminal His(6)-tag (MBP-Δ30vST3Gal-I-His(6)). The viral protein has been purified to homogeneity and characterized biochemically. The enzyme is active in a broad pH range varying from 5.0 to 9.0. It does not require a divalent metal for its α2-3-sialyltransferase activity. It has been used in one-pot multienzyme sialylation of Le(x) for the synthesis of SLe(x) containing different sialic acid forms with good yields.

Novel mechanism for the generation of human xeno-autoantibodies against the nonhuman sialic acid N-glycolylneuraminic acid

The nonhuman sialic acid N-glycolylneuraminic acid (Neu5Gc) is metabolically incorporated into human tissues from certain mammalian-derived foods, and this occurs in the face of an anti-Neu5Gc “xeno-autoantibody” response. Given evidence that this process contributes to chronic inflammation in some diseases, it is important to understand when and how these antibodies are generated in humans. We show here that human anti-Neu5Gc antibodies appear during infancy and correlate with weaning and exposure to dietary Neu5Gc. However, dietary Neu5Gc alone cannot elicit anti-Neu5Gc antibodies in mice with a humanlike Neu5Gc deficiency. Other postnatally appearing anti-carbohydrate antibodies are likely induced by bacteria expressing these epitopes; however, no microbe is known to synthesize Neu5Gc. Here, we show that trace exogenous Neu5Gc can be incorporated into cell surface lipooligosaccharides (LOS) of nontypeable Haemophilus influenzae (NTHi), a human-specific commensal/pathogen. Indeed, infant anti-Neu5Gc antibodies appear coincident with antibodies against NTHi. Furthermore, NTHi that express Neu5Gc-containing LOS induce anti-Neu5Gc antibodies in Neu5Gc-deficient mice, without added adjuvant. Finally, Neu5Gc from baby food is taken up and expressed by NTHi. As the flora residing in the nasopharynx of infants can be in contact with ingested food, we propose a novel model for how NTHi and dietary Neu5Gc cooperate to generate anti-Neu5Gc antibodies in humans.

Oxidative deamination of amino sugars: recent advances

An overview of the oxidative deamination of N-acetylneuraminic acid derivatives (Neu5Ac) leading to the formation of ketodeoxynonulosonic acid (KDN), its stereoisomers and glycosides is presented. A brief historical introduction to the deamination is given, followed by a description of recent advances in reaction conditions, which have allowed application of the process to Neu5Ac thioglycosides, and that have enabled the range of nucleophiles incorporated in the course of the reaction to be extended beyond the original acetate and azide. Recent advances resulting in derivatization of the Neu55Ac 4-position concomitant with replacement of the acetamido group, via the presumed intermediacy of a vinyl diazonium ion, are then described. The literature on the mechanism of the deamination reaction is next considered leading to the presentation of an overall mechanistic framework that accounts for all observations to date. Finally, the application of the deamination reaction to complex Neu5Ac-based oligosaccharides and other aminosugars is presented.