Facile synthesis of C5-azido derivatives of thiosialosides and 2,3-dehydro-5-N-acetylneuraminic acid (DANA)

Neuraminic acid (Neu5Ac, also known as sialic acid) is an important monosaccharide found in glycoproteins and glycolipids which plays a vital role in regulation of physiological functions and pathological conditions. The study of sialoglycans has benefitted from the development of glycomimetic probes and inhibitors for proteins and enzymes that interact with and modify neuraminic acid in glycan chains. Methods to access sialoside intermediates with high yield are needed to facilitate the design of new targets. Here, we report the synthesis of C5-azido thiosialosides using a mild method to deprotect the C5-acetamido functional group followed by the use of a diazo-transfer reagent. We examined two diazo-transfer strategies and compared their yields and tolerance of acetate protecting groups. The same methods and comparisons were also performed for the 2,3-dehydro-5-N-acetylneuraminic acid (DANA) scaffold which is commonly used to generate inhibitors of neuraminidase (sialidase) enzymes. We found that C5-azido derivatives of both thiosialosides and DANA could be produced in five or six steps with yields up to 76 % and 83 %, respectively. Diazo-transfer reagents compared in this study were trifluoromethanesulfonyl azide (TfN3) and imidazole-1-sulfonyl azide (ImzSO2N3). We found that both reagents were compatible with this method and showed comparable yields. Finally, we show that C5-azido derivatives can help to avoid O, N-acyl protecting group migration which was observed in C5-NHAc analogs.

Surface acoustic wave (SAW) real-time interaction analysis of influenza A virus hemagglutinins with sialylated neoglycolipids

Real-time interaction analysis of H1 hemagglutinin from influenza A H1N1 (A/New York/18/2009) and H7 hemagglutinin from influenza A H7N7 (A/Netherlands/219/03) with sialylated neoglycolipids (neoGLs) was performed using the surface acoustic wave (SAW) technology. The produced neoGLs carried phosphatidylethanolamine (PE) as lipid anchor and terminally sialylated lactose (Lc2, Galβ1-4Glc) or neolactotetraose (nLc4, Galβ1-4GlcNAcβ1-3Galβ1-4Glc) harbouring an N-acetylneuraminic acid (Neu5Ac). Using α2-6-sialylated neoGLs, H1 and H7 exhibited marginal attachment towards II6Neu5Ac-Lc2-PE, whereas Sambucus nigra lectin (SNL) exhibited strong binding and Maackia amurensis lectin (MAL) was negative in accordance with their known binding preference towards a distal Neu5Acα2-6Gal- and Neu5Acα2-3Gal-residue, respectively. H1 revealed significant binding towards IV6Neu5Ac-nLc4-PE when compared to weak interaction of H7, while SNL showed strong and MAL no attachment corresponding to their interaction specificities. Additional controls of MAL and SNL with α2-3-sialylated II3Neu5Ac-Lc2-PE and IV3Neu5Ac-nLc4-PE underscored the reliability of the SAW technology. Pre-exposure of model membranes spiked with α2-6-sialylated neoGLs to Vibrio cholerae neuraminidase substantially reduced the binding of the hemagglutinins and the SNL reference. Collectively, the SAW technology is capable of accurate measuring binding features of hemagglutinins towards neoGL-spiked lipid bilayers, which can be easily loaded to the functionalized biosensor gold surface thereby simulating biological membranes and suggesting promising clinical application for influenza virus research.

A GntR-type transcriptional repressor controls sialic acid utilization in Bifidobacterium breve UCC2003

Bifidobacterium breve strains are numerically prevalent among the gut microbiota of healthy, breast-fed infants. The metabolism of sialic acid, a ubiquitous monosaccharide in the infant and adult gut, by B. breve UCC2003 is dependent on a large gene cluster, designated the nan/nag cluster. This study describes the transcriptional regulation of the nan/nag cluster and thus sialic acid metabolism in B. breve UCC2003. Insertion mutagenesis and transcriptome analysis revealed that the nan/nag cluster is regulated by a GntR family transcriptional repressor, designated NanR. Crude cell extract of Escherichia coli EC101 in which the nanR gene had been cloned and overexpressed was shown to bind to two promoter regions within this cluster, each of which containing an imperfect inverted repeat that is believed to act as the NanR operator sequence. Formation of the DNA-NanR complex is prevented in the presence of sialic acid, which we had previously shown to induce transcription of this gene cluster.

Potential deleterious role of anti-Neu5Gc antibodies in xenotransplantation

Human beings do not synthesize the glycolyl form of the sialic acid (Neu5Gc) and only express the acetylated form of the sugar, whereas a diet-based intake of Neu5Gc provokes a natural immunization and production of anti-Neu5Gc antibodies in human serum. However, Neu5Gc is expressed on mammal glycoproteins and glycolipids in most organs and cells. We review here the relevance of Neu5Gc and anti-Neu5Gc antibodies in the context of xenotransplantation and the use of animal-derived molecules and products, as well as the possible consequences of a long-term exposure to anti-Neu5Gc antibodies in recipients of xenografts. In addition, the importance of an accurate estimation of the anti-Neu5Gc response following xenotransplantation and the future contribution of knockout animals mimicking the human situation are also assessed.

Fluorescent mimetics of CMP-Neu5Ac are highly potent, cell-permeable polarization probes of eukaryotic and bacterial sialyltransferases and inhibit cellular sialylation

Oligosaccharides of the glycolipids and glycoproteins at the outer membranes of human cells carry terminal neuraminic acids, which are responsible for recognition events and adhesion of cells, bacteria, and virus particles. The synthesis of neuraminic acid containing glycosides is accomplished by intracellular sialyl transferases. Therefore, the chemical manipulation of cellular sialylation could be very important to interfere with cancer development, inflammations, and infections. The development and applications of the first nanomolar fluorescent inhibitors of sialyl transferases are described herein. The obtained carbohydrate-nucleotide mimetics were found to bind all four commercially available and tested eukaryotic and bacterial sialyl transferases in a fluorescence polarization assay. Moreover, it was observed that the anionic mimetics intruded rapidly and efficiently into cells in vesicles and translocated to cellular organelles surrounding the nucleus of CHO cells. The new compounds inhibit cellular sialylation in two cell lines and open new perspectives for investigations of cellular sialylation.

Characterization of the sialic acid synthase from Aliivibrio salmonicida suggests a novel pathway for bacterial synthesis of 7-O-acetylated sialic acids

Resolving the enzymatic pathways leading to sialic acids (Sias) in bacteria are vitally important for understanding their roles in pathogenesis and for subsequent development of tools to combat infections. A detailed characterization of the involved enzymes is also essential due to the highly applicable properties of Sias, i.e., as used in a wide range of medical applications and human nutrition. Bacterial strains that produce Sias display them mainly on their cell surface to mimic animal cells thereby evading the host's immune system. Despite several studies, little is known about the virulence mechanisms of the fish pathogen Aliivibrio salmonicida. The genome of A. salmonicida LFI1238 contains a gene cluster homologous to the Escherichia coli neuraminic acid (Neu) gene cluster involved in biosynthesis of Sias found in the E. coli capsule. This cluster is probably responsible for the biosynthesis of Neu found in A. salmonicida. In this work, we have produced and characterized the sialic acid (Sia) synthase NeuB1, the key enzyme in the pathway. The Sia synthase is an enzyme producing N-acetylneuraminic acid by the condensation of N-acetylmannosamine and phosphoenolpyruvate. Genome content, kinetic data obtained, together with structural considerations, have led us to the prediction that the substrate for NeuB1 from A. salmonicida, E. coli and Streptococcus agalactiae among others, is 4-O-acetyl-N-acetylmannosamine. This means that the product of its enzymatic reaction is 7-O-acetyl-N-acetylneuraminic acid. We propose a pathway for production of this Sia in A. salmonicida, and present evidence for the presence of diacetylated Neu in the bacterium.

Ganglioside composition and histology of a spontaneous metastatic brain tumour in the VM mouse

Glycosphingolipid abnormalities have long been implicated in tumour malignancy and metastasis. Gangliosides are a family of sialic acid-containing glycosphingolipids that modulate cell-cell and cell-matrix interactions. Histology and ganglioside composition were examined in a natural brain tumour of the VM mouse strain. The tumour is distinguished from other metastatic tumour models because it arose spontaneously and metastasizes to several organs including brain and spinal cord after subcutaneous inoculation of tumour tissue in the flank. By electron microscopy, the tumour consisted of cells (15 to 20 microm in diameter) that had slightly indented nuclei and scant cytoplasm. The presence of smooth membranes with an absence of junctional complexes was a characteristic ultrastructural feature. No positive immunostaining was found for glial or neuronal markers. The total ganglioside sialic acid content of the subcutaneously grown tumour was low (12.6 +/- 0.9 microg per 100 mg dry wt, n = 6 separate tumours) and about 70% of this was in the form of N-glycolylneuraminic acid. In contrast, the ganglioside content of the cultured VM tumour cells was high (248.4 +/- 4.4 microg, n = 3) and consisted almost exclusively of N-acetylneuraminic acid. The ganglioside pattern of the tumour grown subcutaneously was complex, while GM3, GM2, GM1, and GD1a were the major gangliosides in the cultured tumour cells. This tumour will be a useful natural model for evaluating the role of gangliosides and other glycolipids in tumour cell invasion and metastasis.

Differential expression of sialic acid on porcine organs during the maturation process

Sialylated structures play important roles in cell communication, and change in a regulated manner during development and differentiation. In this work, we report the main glycosidic modifications that occur during the maturation of porcine tissues, involving the sialylation process as determined with lectins. Sialic acids were identified at several levels in a broad range of cell types of nervous, respiratory, genitourinary and lymphoid origin. Nevertheless, the most contrasting was the type of glycosidic linkage between 5-N-acetyl-neuraminic acid (Neu5Ac) and galactose (Gal) expressed in central nervous system (CNS). Newborn CNS abundantly expressed Neu5Acalpha2,3Gal, but weakly or scarcely expressed Neu5Acalpha2,6Gal/GalNAc. Maturation of CNS induced drastic changes in sialic acid expression. These changes include decrease or complete loss of NeuAcalpha2,3Gal residues, mainly in olfactory structures and brain cortex, which were replaced by their isomers Neu5Acalpha2,6Gal/GalNAc. In the brain cortex and cerebellum, the increase of Neu5Acalpha2,6Gal/GalNAc molecules was paralleled by an increase of 5-N-acetyl-9-O-acetyl-neuraminic acid (Neu5,9Ac2). In addition, terminal Gal and N-acetyl-D-galactosamine (GalNAc) residues also increased their expression in adult CNS tissues, but this was more significant in structures forming the encephalic trunk. Our results show that sialylation of porcine CNS is finely modulated throughout the maturation process.