The evolution of galactose alpha2,3-sialyltransferase: Ciona intestinalis ST3GAL I/II and Takifugu rubripes ST3GAL II sialylate Galbeta1,3GalNAc structures on glycoproteins but not glycolipids

The vertebrate sialylation machinery: structure-function and molecular evolution of GT-29 sialyltransferases

Every eukaryotic cell is covered with a thick layer of complex carbohydrates with essential roles in their social life. In Deuterostoma, sialic acids present at the outermost positions of glycans of glycoconjugates are known to be key players in cellular interactions including host-pathogen interactions. Their negative charge and hydrophilic properties enable their roles in various normal and pathological states and their expression is altered in many diseases including cancers. Sialylation of glycoproteins and glycolipids is orchestrated by the regulated expression of twenty sialyltransferases in human tissues with distinct enzymatic characteristics and preferences for substrates and linkages formed. However, still very little is known on the functional organization of sialyltransferases in the Golgi apparatus and how the sialylation machinery is finely regulated to provide the ad hoc sialome to the cell. This review summarizes current knowledge on sialyltransferases, their structure-function relationships, molecular evolution, and their implications in human biology.

Marine invertebrate sialyltransferase of the sea squirt Ciona savignyi sialylated core 1 O-linked glycans

An invertebrate sialyltransferase, cST3Gal-I, identified from the sea squirt Ciona savignyi, was functionally characterized in vitro using recombinant enzyme expressed in yeast strains. cST3Gal-I was localized to the Golgi membrane when expressed in Saccharomyces cerevisiae. Enzymatic characterization for substrate specificity and kinetic property indicate that cST3Gal-I prefers O-glycans, rather than N-glycan, of asialoglycoproteins as substrates. Interestingly, C. savignyi sialyltransferase exhibited effectively Neu5Ac transfer to core 1 O-glycan, Gal β(1,3)GalNAc, compared to orthologous human glycosyltransferase. Further, it is shown that cST3Gal-I catalyzes the formation of α(2,3)-linkage, through lectin blot analysis with Maackia amurensis lectin and by linkage-specific sialidase treatments. The putative active sites of cST3Gal-I for putative acid/base catalysts and sialic acid acceptor/donor substrate bindings were also identical to the counterpart residues of a mammalian enzyme, porcine ST3Gal-I, as predicted through homologous structure modeling. These results could imply that an ancestral tunicate ST3Gal-I in C. savignyi would prefer O-glycan onto glycoproteins as its sialic acid acceptor than vertebrate enzymes.

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.

Engineering of CHO cells for the production of vertebrate recombinant sialyltransferases

Background

Sialyltransferases (SIATs) are a family of enzymes that transfer sialic acid (Sia) to glycan chains on glycoproteins, glycolipids, and oligosaccharides. They play key roles in determining cell–cell and cell-matrix interactions and are important in neuronal development, immune regulation, protein stability and clearance. Most fully characterized SIATs are of mammalian origin and these have been used for in vitro and in vivo modification of glycans. Additional versatility could be achieved by the use of animal SIATs from other species that live in much more variable environments. Our aim was to generate a panel of stable CHO cell lines expressing a range of vertebrate SIATs with different physicochemical and functional properties.

Methods

The soluble forms of various animal ST6Gal and ST3Gal enzymes were stably expressed from a Gateway-modified secretion vector in CHO cells. The secreted proteins were IMAC-purified from serum-free media. Functionality of the protein was initially assessed by lectin binding to the host CHO cells. Activity of purified proteins was determined by a number of approaches that included a phosphate-linked sialyltransferase assay, HILIC-HPLC identification of sialyllactose products and enzyme-linked lectin assay (ELLA).

Results

A range of sialyltransferase from mammals, birds and fish were stably expressed in CHO Flp-In cells. The stable cell lines expressing ST6Gal1 modify the glycans on the surface of the CHO cells as detected by fluorescently labelled lectin microscopy. The catalytic domains, as isolated by Ni Sepharose from culture media, have enzymatic activities comparable to commercial enzymes. Sialyllactoses were identified by HILIC-HPLC on incubation of the enzymes from lactose or whey permeate. The enzymes also increased SNA-I labelling of asialofetuin when incubated in a plate format.

Conclusion

Stable cell lines are available that may provide options for the in vivo sialylation of glycoproteins. Proteins are active and should display a variety of biological and physicochemical properties based on the animal source of the enzyme.

Novel Zebrafish Mono-α2,8-sialyltransferase (ST8Sia VIII): An Evolutionary Perspective of α2,8-Sialylation

The mammalian mono-α2,8-sialyltransferase ST8Sia VI has been shown to catalyze the transfer of a unique sialic acid residues onto core 1 O-glycans leading to the formation of di-sialylated O-glycosylproteins and to a lesser extent to diSia motifs onto glycolipids like GD1a. Previous studies also reported the identification of an orthologue of the ST8SIA6 gene in the zebrafish genome. Trying to get insights into the biosynthesis and function of the oligo-sialylated glycoproteins during zebrafish development, we cloned and studied this fish α2,8-sialyltransferase homologue. In situ hybridization experiments demonstrate that expression of this gene is always detectable during zebrafish development both in the central nervous system and in non-neuronal tissues. Intriguingly, using biochemical approaches and the newly developed in vitro MicroPlate Sialyltransferase Assay (MPSA), we found that the zebrafish recombinant enzyme does not synthetize diSia motifs on glycoproteins or glycolipids as the human homologue does. Using comparative genomics and molecular phylogeny approaches, we show in this work that the human ST8Sia VI orthologue has disappeared in the ray-finned fish and that the homologue described in fish correspond to a new subfamily of α2,8-sialyltransferase named ST8Sia VIII that was not maintained in Chondrichtyes and Sarcopterygii.

Phylogenetic-Derived Insights into the Evolution of Sialylation in Eukaryotes: Comprehensive Analysis of Vertebrate β-Galactoside α2,3/6-Sialyltransferases (ST3Gal and ST6Gal)

Cell surface of eukaryotic cells is covered with a wide variety of sialylated molecules involved in diverse biological processes and taking part in cell–cell interactions. Although the physiological relevance of these sialylated glycoconjugates in vertebrates begins to be deciphered, the origin and evolution of the genetic machinery implicated in their biosynthetic pathway are poorly understood. Among the variety of actors involved in the sialylation machinery, sialyltransferases are key enzymes for the biosynthesis of sialylated molecules. This review focus on β-galactoside α2,3/6-sialyltransferases belonging to the ST3Gal and ST6Gal families. We propose here an outline of the evolutionary history of these two major ST families. Comparative genomics, molecular phylogeny and structural bioinformatics provided insights into the functional innovations in sialic acid metabolism and enabled to explore how ST-gene function evolved in vertebrates.

Critical role of evolutionarily conserved glycosylation at Asn211 in the intracellular trafficking and activity of sialyltransferase ST3Gal-II

ST3Gal-II is largely responsible for ganglioside terminal α2,3-sialylation in mammals. We demonstrated that ST3Gal-II mainly distributes in proximal Golgi compartments and that the inhibition of N-glycosylation and oligosaccharide trimming is critical for its enzymatic activity and intracellular distribution.

Integrative view of α2,3-sialyltransferases (ST3Gal) molecular and functional evolution in deuterostomes: significance of lineage-specific losses

Sialyltransferases are responsible for the synthesis of a diverse range of sialoglycoconjugates predicted to be pivotal to deuterostomes’ evolution. In this work, we reconstructed the evolutionary history of the metazoan α2,3-sialyltransferases family (ST3Gal), a subset of sialyltransferases encompassing six subfamilies (ST3Gal I–ST3Gal VI) functionally characterized in mammals. Exploration of genomic and expressed sequence tag databases and search of conserved sialylmotifs led to the identification of a large data set of st3gal-related gene sequences. Molecular phylogeny and large scale sequence similarity network analysis identified four new vertebrate subfamilies called ST3Gal III-r, ST3Gal VII, ST3Gal VIII, and ST3Gal IX. To address the issue of the origin and evolutionary relationships of the st3gal-related genes, we performed comparative syntenic mapping of st3gal gene loci combined to ancestral genome reconstruction. The ten vertebrate ST3Gal subfamilies originated from genome duplication events at the base of vertebrates and are organized in three distinct and ancient groups of genes predating the early deuterostomes. Inferring st3gal gene family history identified also several lineage-specific gene losses, the significance of which was explored in a functional context. Toward this aim, spatiotemporal distribution of st3gal genes was analyzed in zebrafish and bovine tissues. In addition, molecular evolutionary analyses using specificity determining position and coevolved amino acid predictions led to the identification of amino acid residues with potential implication in functional divergence of vertebrate ST3Gal. We propose a detailed scenario of the evolutionary relationships of st3gal genes coupled to a conceptual framework of the evolution of ST3Gal functions.

Human GM3 Synthase Attenuates Taxol-Triggered Apoptosis Associated with Downregulation of Caspase-3 in Ovarian Cancer Cells

BACKGROUND

Taxol (paclitaxel) inhibits proliferation and induces apoptosis in a variety of cancer cells, but it also upregulates cytoprotective proteins and/or pathways that compromise its therapeutic efficacy.

MATERIALS AND METHOD

The roles of GM3 synthase (α2,3-sialyltransferase, ST3Gal V) in attenuating Taxol-induced apoptosis and triggering drug resistance were determined by cloning and overexpressing this enzyme in the SKOV3 human ovarian cancer cell line, treating SKOV3 and the transfectants (SKOV3/GS) with Taxol and determining apoptosis, cell survival, clonogenic ability, and caspase-3 activation.

RESULTS

In this report, we demonstrated that Taxol treatment resulted in apoptosis which was associated with caspase-3 activation. Taxol treatment upregulated the expression of human GM3 synthase, an enzyme that transfers a sialic acid to lactosylceramide. Moreover, we cloned the full-length GM3 synthase gene and showed for the first time that forced expression of GM3 synthase attenuated Taxol-induced apoptosis and increased resistance to Taxol in SKOV3 cells.

CONCLUSIONS

GM3 synthase overexpression inhibited Taxol-triggered caspase-3 activation, revealing that upregulation of GM3 synthase prevents apoptosis and hence reduces the efficacy of Taxol therapy.

Sialome analysis of the cephalochordate Branchiostoma belcheri, a key organism for vertebrate evolution

Sialic acid, a common terminal substitution of glycoconjugates, has been so far consistently identified in all vertebrates as well as in a growing number of bacterial species. It is assumed to be widely distributed among animal species of the deuterostome phylum, based on its identification in few echinoderm and all vertebrate species. However, whole sections of deuterostome, especially those intermediate species between invertebrates and vertebrates including cephalochordates, urochordates and hemichordates, are still unexplored in term of sialylation capacities. The discovery of functional sialic acid machinery in some of these species may shed new light onto the evolution of glycosylation capacities in deuterostome lineage. In a first approach, we investigated the sialylation pattern of a cephalocordate species, Branchiostoma belcheri, which occupies a strategic phylogenetic position to understand the transition of invertebrates toward vertebrates. Structural analysis of B. belcheri glycoconjugates established that this organism synthesizes large quantities of various sialic acids, some of which present rare or novel structures such as methylated sialic acids. These sialic acids were shown to be mainly associated with mono- and disialylated core 1-type O-glycans. Moreover, screening of the animal organs revealed the existence of exquisite tissue specificity in the distribution of sialic acids. Description of sialylation profiles was then correlated with the expression patterns of key enzymes involved in the biosynthesis of major forms of sialic acids, which provides the first complete overview of the sialylation patterns in cephalochordates.

Moraxella catarrhalis Lgt2, a galactosyltransferase with broad acceptor substrate specificity

The genetic basis of lipo-oligosaccharide (LOS) biosynthesis for the bacterium Moraxella catarrhalis has been elucidated and functions suggested for each of the glycosyltransferases. In this study we have expressed and characterised one of these enzymes, the putative galactosyltransferase Lgt2(B/C). The lgt2(B/C) gene was amplified from M. catarrhalis, expressed in Escherichia coli, and Lgt2(B/C) was purified. Analysis of its glycosyltransferase catalytic activity ascertained the pH and temperature optima. The donor specificity and acceptor specificity were examined and they showed that Lgt2(B/C) is a galactosyltransferase with relatively broad acceptor specificity with optimal activity in the presence of exogenous Mg(2+).

Biochemical analysis of Lgt3, a glycosyltransferase of the bacterium Moraxella catarrhalis

The lipooligosaccharide (LOS) of Moraxella catarrhalis is unusual in that it lacks heptose. The sugar linking oligosaccharide to Lipid A is a trisubstituted glucose. A single enzyme, Lgt3, is suggested to trisubstitute this core sugar. The lgt3 gene encodes two distinct domains with high similarity to glucosyltransferases of the GT-A superfamily, thus encoding a bidomain, multifunctional glucosyltransferase. To characterise Lgt3, the gene was amplified from M. catarrhalis, expressed in Escherichia coli, and purified. Analysis of its glycosyltransferase catalytic activity ascertained the pH and temperature optima for Lgt3. The donor specificity and acceptor specificity were examined. This study confirms that Lgt3 is a glucosyltransferase which catalyses addition of glucose to its cognate receptor, a terminal glucose presented as the core region of LOS.

Sialic acids as regulators of molecular and cellular interactions

The wide occurrence of sialic acids (Sia) in various chemical forms linked as monomers or polymers in an outstanding position in a multitude of complex carbohydrates of animals and microorganisms renders them as most versatile function modulators in cell biology and pathology. A survey is presented of recent advances in the study of the influences that Sias have as bulky hydrophilic and electronegatively charged monosaccharides on animal cells and on their interaction with microorganisms. Some highlights are: sialylation leads to increased anti-inflammatory activity of IgG antibodies, facilitates the escape of microorganisms from the host's immune system, and in polymeric form is involved in the regulation of embryogenesis and neuronal growth and function. The role of siglecs in immunoregulation, the dynamics of lymphocyte binding to selectins and the interactions of toxins, viruses, and other microorganisms with the host's Sia are now better understood. N-Glycolylneuraminic acid from food is antigenic in man and seems to have pathogenic potential. Sia O-acetylation mediated by various eukaryotic and prokaryotic O-acetyltransferases modulates the affinity of these monosaccharides to mammalian and microbial receptors and hinders apoptosis. The functionally versatile O-acetylated ganglioside GD3 is an onco-fetal antigen.