A practical approach to N-glycan production by hydrazinolysis using hydrazine monohydrate

Hydrazinolysis is a versatile method to liberate N-linked glycans from glycoproteins. However, the method is usually performed with anhydrous hydrazine, a highly toxic and explosive chemical used in rocket fuel. Thus despite the need to produce functionally important glyco-materials, hydrazinolysis is limited to small scale (e.g., 0.2-1 mL) reactions. In the present study, we report an alternative procedure for hydrazinolysis using hydrazine monohydrate in place of anhydrous hydrazine. The developed procedure was applied to both purified glycoproteins (Taka-amylase and transferrin) and hen egg yolk protein fraction with comparable yields to the traditional method using anhydrous hydrazine. The sialyl linkage of alpha2-6disialobiantennary oligosaccharides proved to be fully stable. The developed procedure facilitated the large-scale preparation of N-linked glycans. The new method should make a substantial contribution to both small- and large-scale production of functional glycans, including therapeutically relevant human-type glycans.     

Synthesis of disaccharide neu5Gcα(2–6)galNAcα as a spacer glycoside

The first synthesis of the Neu5Gc analogue of SiaT _n disaccharide, which can be detected in breast tumors by immunochemical methods, is reported. The regioselective sialylation of (3-trifluoroacetamidopropyl)-2-azido-2-deoxy-α-D-galactopyranoside with peracetate of the methyl ester ofN-acetoxyacetyl-neuraminic acid β-ethylthioglycoside in the presence ofN-iodosuccinimide and trifluoromethanesulfonic acid (or its trimethylsilyl ester) resulted in the derivatives of α- and β-sialyl(2→6)galactosaminide in 39 and 32% yields, respectively. The catalytic hydrogenolysis of the azido group and subsequentN- andO-acetylation of the α-anomer gave the peracetate of trifluoroacetamidopropyl glycoside. Removal of the protective groups led to glycoside Neu5Gcα2→6GalNAcα-O(CH₂)₃NH₂. Using the Neu5Gc derivative with acetoxyacetyl groups at positions O9 and O4 as a donor increases the α-selectivity of sialylation to afford the α- and β-anomers in 69 and 8% yields, respectively.     

The expanding roles of the Sd/Cad carbohydrate antigen and its cognate glycosyltransferase B4GALNT2

Background

The histo-blood group antigens are carbohydrate structures present in tissues and body fluids, which contribute to the definition of the individual immunophenotype. One of these, the Sd^a antigen, is expressed on the surface of erythrocytes and in secretions of the vast majority of the Caucasians and other ethnic groups.

Scope of review

We describe the multiple and unsuspected aspects of the biology of the Sd^a antigen and its biosynthetic enzyme β1,4-N-acetylgalactosaminyltransferase 2 (B4GALNT2) in various physiological and pathological settings.

Major conclusions

The immunodominant sugar of the Sd^a antigen is a β1,4-linked N-acetylgalactosamine (GalNAc). Its cognate glycosyltransferase B4GALNT2 displays a restricted pattern of tissue expression, is regulated by unknown mechanisms - including promoter methylation, and encodes at least two different proteins, one of which with an unconventionally long cytoplasmic portion. In different settings, the Sd^a antigen plays multiple and unsuspected roles. 1) In colon cancer, its dramatic down-regulation plays a potential role in the overexpression of sialyl Lewis antigens, increasing metastasis formation. 2) It is involved in the lytic function of murine cytotoxic T lymphocytes. 3) It prevents the development of muscular dystrophy in various dystrophic murine models, when overexpressed in muscular fibers. 4) It regulates the circulating half-life of the von Willebrand factor (vWf), determining the onset of a bleeding disorder in a murine model.

General significance

The expression of the Sd^a antigen has a wide impact on the physiology and the pathology of different biological systems.     

Sialosignaling: Sialyltransferases as engines of self-fueling loops in cancer progression

Background

Glycosylation is increasingly recognized as one of the most relevant postranslational modifications. Sialic acids are negatively charged sugars which frequently terminate the carbohydrate chains of glycoproteins and glycolipids. The addition of sialic acids is mediated by sialyltransferases, a family of glycosyltransferases with a crucial role in cancer progression.

Scope of the review

To describe the phenotypic and clinical implications of altered expression of sialyltransferases and of their cognate sialylated structures in cancer. To propose a unifying model of the role of sialyltransferases and sialylated structures on cancer progression.

Major conclusions

We first discuss the biosynthesis and the role played by the major cancer-associated sialylated structures, including Thomsen–Friedenreich-associated antigens, sialyl Lewis antigens, α2,6-sialylated lactosamine, polysialic acid and gangliosides. Then, we show that altered sialyltransferase expression in cancer, consequence of genetic and epigenetic alterations, generates a flow of information toward the membrane through the biosynthesis of aberrantly sialylated molecules (inside-out signaling). In turn, the presence of aberrantly sialylated structures on cell membrane receptors generates a flow of information toward the nucleus, which can exacerbate the neoplastic phenotype (outside-in signaling). We provide examples of self-fueling loops generated by these flows of information.

General significance

Sialyltransferases have a wide impact on the biology of cancer and can be the target of innovative therapies. Our unified view provides a conceptual framework to understand the impact of altered glycosylation in cancer.     

Desulfated galactosaminoglycans are potential ligands for galectins: evidence from frontal affinity chromatography

Galectins, a group of β-galactoside-binding lectins, are involved in multiple functions through specific binding to their oligosaccharide ligands. No previous work has focused on their interaction with glycosaminoglycans (GAGs). In the present work, affinities of established members of human galectins toward a series of GAGs were investigated, using frontal affinity chromatography. Structurally-defined keratan sulfate (KS) oligosaccharides showed significant affinity to a wide range of galectins if Gal residue(s) remained unsulfated, while GlcNAc sulfation had relatively little effect. Consistently, galectins showed much higher affinity to corneal type I than cartilageous type II KS. Unexpectedly, galectin-3, -7, and -9 also exerted significant affinity to desulfated, GalNAc-containing GAGs, i.e., chondroitin and dermatan, but not at all to hyaluronan and N-acetylheparosan. These observations revealed that the integrity of 6-OH of βGalNAc is important for galectin recognition of these galactosaminoglycans, which were shown, for the first time, to be implicated as potential ligands of galectins.     

Development of an all-in-one technology for glycan profiling targeting formalin-embedded tissue sections

An ultra-sensitive method for glycan analysis targeting small tissue sections (1.5 mm in diameter) is described as an application of a recently-established lectin microarray technology. The developed system achieved a high level of detection of a tissue section consisting of approximately 500 cells for differential profiling, where both N- and O-glycans attached to a pool of glycoproteins are subjected to multiplex analysis with 43 lectins. By using an optimized protocol for differential glycan analysis, sections of adenocarcinoma (n = 28) and normal epithelia (n = 12) of the colon were analyzed in an all-in-one manner. As a result, Wisteria floribunda agglutinin (WFA) was found to clearly differentiate cancerous from normal epithelia with P < 0.0001. The obtained results correlated well with the subsequent histochemical study using biotinylated WFA. Thus, the developed technology proved to be valid for expanding the lectin microarray applications to tissue-based glycomics, and hence, should accelerate a discovery phase of glycan-related biomarkers.     

Computer‐aided design of human sialyltransferase inhibitors of hST8Sia III

Sialyltransferase (ST) upregulation and the resultant hypersialylation of tumour cell surfaces is an established hallmark of many cancers including lung, breast, ovarian, pancreatic and prostate cancer. The role of ST enzymes in tumour cell growth and metastasis, as well as links to multi‐drug resistance, has seen ST inhibition emerge as a target for potential antimetastatic cancer treatments. The most potent of these reported inhibitors are transition‐state analogues. Although there are several examples of these in the literature, many have suspected poor pharmacokinetic properties and are not readily synthetically accessible. A proposed solution to these problems is the use of a neutral carbamate or 1,2,3‐triazole linker instead of the more commonly used phosphodiester linker, and replacing the traditionally utilised cytidine nucleotide with uridine. Another issue in this area is the paucity of structural information of human ST enzymes. However, in late 2015 the structure of human ST8Sia III was reported (only the second human ST described so far), creating the opportunity for structure‐based design of selective ST8 inhibitors for the first time. Herein, molecular docking and molecular dynamics simulations with the newly published crystal structure of hST8Sia III were performed for the first time with selected ST transition state analogues. Simulations showed that these compounds could participate in many of the key interactions common with the natural donor and acceptor substrates, and reveals some key insights into the synthesis of potentially selective ST inhibitors.     

Molecular basis for polysialylation: A novel polybasic polysialyltransferase domain (PSTD) of 32 amino acids unique to the α2,8-polysialyltransferases is essential for polysialylation

To determine the molecular basis of eukaryotic polysialylation, the function of a structurally unique polybasic motif of 32 amino acids (pI∼12) in the polysialyltransferases (polySTs), ST8Sia II (STX and ST8Sia IV (PST) was investigated. This motif, designated the “polysialyltransferase domain” (PSTD), is immediately upstream of the sialylmotif S (SM-S). PolyST activity was lost in COS-1 mutants in which the entire PSTD in ST8Sia IV was deleted, or in mutants in which 10 and 15 amino acids in either the N- or C- terminus of PSTD were deleted. Site-directed mutagenesis showed that Ile₂₇₅, Lys₂₇₆ and Arg₂₇₇ in the C-terminus of PSTD in ST8Sia IV, which is contiguous with the N-terminus of sialylmotif-S, were essential for polysialylation. Arg₂₅₂ in the N-terminus segment of the PSTD was also required, as was the overall positive charge. Thus, multiple domains in the polySTs can influence their activity. Immunofluorescent microscopy showed that the mutated proteins were folded correctly, based on their Golgi localization. The structural distinctness of the conserved PSTD in the polySTs, and its absence in the mono- oligoSTs, suggests that it is a “polymerization domain” that distinguishes a polyST from a monosialyltransferases. We postulate that the electrostatic interaction between the polybasic PSTD and the polyanionic polySia chains may function to tether nascent polySia chains to the enzyme, thus facilitating the processive addition of new Sia residues to the non-reducing end of the growing chain. In accord with this hypothesis, the polyanion heparin was shown to inhibit recombinant human ST8Sia II and ST8Sia IV at 10 μM.     

A C-Terminal Phosphatase Module Conserved in Vertebrate CMP-Sialic Acid Synthetases Provides a Tetramerization Interface for the Physiologically Active Enzyme

The biosynthesis of sialic acid-containing glycoconjugates is crucial for the development of vertebrate life. Cytidine monophosphate-sialic acid synthetase (CSS) catalyzes the metabolic activation of sialic acids. In vertebrates, the enzyme is chimeric, with the N-terminal domain harboring the synthetase activity. The function of the highly conserved C-terminal domain (CSS-CT) is unknown. To shed light on its biological function, we solved the X-ray structure of murine CSS-CT to 1.9 Å resolution. CSS-CT is a stable shamrock-like tetramer that superimposes well with phosphatases of the haloacid dehalogenase superfamily. However, a region found exclusively in vertebrate CSS-CT appears to block the active-site entrance. Accordingly, no phosphatase activity was observed in vitro, which points toward a nonenzymatic function of CSS-CT. A computational three-dimensional model of full-length CSS, in combination with in vitro oligomerization studies, provides evidence that CSS-CT serves as a platform for the quaternary organization governing the kinetic properties of the physiologically active enzyme as demonstrated in kinetic studies.     

The glycosylation and characterization of the candidate Gc macrophage activating factor

The vitamin D binding protein, Gc globulin, has in recent years received some attention for its role as precursor for the extremely potent macrophage activating factor (GcMAF). An O-linked trisaccharide has been allocated to the threonine residue at position 420 in two of the three most common isoforms of Gc globulin (Gc1s and Gc1f). A substitution for a lysine residue at position 420 in Gc2 prevents this isoform from being glycosylated at that position. It has been suggested that Gc globulin subjected sequentially to sialidase and galactosidase treatment generates GcMAF in the form of Gc globulin with only a single GalNAc attached to T420. In this study we confirm the location of a linear trisaccharide on T420. Furthermore, we provide the first structural evidence of the generation of the proposed GcMAF by use of glycosidase treatment and mass spectrometry. Additionally the generated GcMAF candidate was tested for its effect on cytokine release from macrophages in human whole blood.