Sialic acids on B cells are crucial for their survival and provide protection against apoptosis.

Sialic acids (Sias) on the B cell membrane are involved in cell migration, in the control of the complement system and, as sialic acid-binding immunoglobulin-like lectin (Siglec) ligands, in the regulation of cellular signaling. We studied the role of sialoglycans on B cells in a mouse model with B cell-specific deletion of cytidine monophosphate sialic acid synthase (CMAS), the enzyme essential for the synthesis of sialoglycans. Surprisingly, these mice showed a severe B cell deficiency in secondary lymphoid organs. Additional depletion of the complement factor C3 rescued the phenotype only marginally, demonstrating a complement-independent mechanism. The B cell survival receptor BAFF receptor was not up-regulated, and levels of activated caspase 3 and processed caspase 8 were high in B cells of Cmas-deficient mice, indicating ongoing apoptosis. Overexpressed Bcl-2 could not rescue this phenotype, pointing to extrinsic apoptosis. These results show that sialoglycans on the B cell surface are crucial for B cell survival by counteracting several death-inducing pathways.

Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS.

BACKGROUND: The etiology of steroid-resistant nephrotic syndrome, which manifests as FSGS, is not completely understood. Aberrant glycosylation is an often underestimated factor for pathologic processes, and structural changes in the glomerular endothelial glycocalyx have been correlated with models of nephrotic syndrome. Glycans are frequently capped by sialic acid (Sia), and sialylation's crucial role for kidney function is well known. Human podocytes are highly sialylated; however, sialylation's role in podocyte homeostasis remains unclear. METHODS: We generated a podocyte-specific sialylation-deficient mouse model (PCmas-/- ) by targeting CMP-Sia synthetase, and used histologic and ultrastructural analysis to decipher the phenotype. We applied CRISPR/Cas9 technology to generate immortalized sialylation-deficient podocytes (asialo-podocytes) for functional studies. RESULTS: Progressive loss of sialylation in PCmas-/- mice resulted in onset of proteinuria around postnatal day 28, accompanied by foot process effacement and loss of slit diaphragms. Podocyte injury led to severe glomerular defects, including expanded capillary lumen, mesangial hypercellularity, synechiae formation, and podocyte loss. In vivo, loss of sialylation resulted in mislocalization of slit diaphragm components, whereas podocalyxin localization was preserved. In vitro, asialo-podocytes were viable, able to proliferate and differentiate, but showed impaired adhesion to collagen IV. CONCLUSIONS: Loss of cell-surface sialylation in mice resulted in disturbance of podocyte homeostasis and FSGS development. Impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Our data support the notion that loss of sialylation might be part of the complex process causing FSGS. Sialylation, such as through a Sia supplementation therapy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.

Regulation and function of endothelial glycocalyx layer in vascular diseases.

In the vascular system, the endothelial surface layer (ESL) as the inner surface of blood vessels affects mechanotransduction, vascular permeability, rheology, thrombogenesis, and leukocyte adhesion. It creates barriers between endothelial cells and blood and neighbouring cells. The glycocalyx, composed of glycoconjugates and proteoglycans, is an integral component of the ESL and a key element in inter- and intracellular communication and tissue homeostasis. In pathophysiological conditions (atherosclerosis, infection, ischemia/reperfusion injury, diabetes, trauma and acute lung injury) glycocalyx-degrading factors, i.e. reactive oxygen and nitrogen species, matrix metalloproteinases, heparanase and sialidases, damage the ESL, thereby impairing endothelial functions. This leads to increased capillary permeability, leucocyte-endothelium interactions, thrombosis and vascular inflammation, the latter further driving glycocalyx destruction. The present review highlights current knowledge on the vasculoprotective role of the ESL, with specific emphasis on its remodelling in inflammatory vascular diseases and discusses its potential as a novel therapeutic target to treat vascular pathologies.

Deficits in sialylation impair podocyte maturation.

The role of sialylation in kidney biology is not fully understood. The synthesis of sialoglycoconjugates, which form the outermost structures of animal cells, requires CMP-sialic acid, which is a product of the nuclear enzyme CMAS. We used a knock-in strategy to create a mouse with point mutations in the canonical nuclear localization signal of CMAS, which relocated the enzyme to the cytoplasm of transfected cells without affecting its activity. Although insufficient to prevent nuclear entry in mice, the mutation led to a drastically reduced concentration of nuclear-expressed enzyme. Mice homozygous for the mutation died from kidney failure within 72 hours after birth. The Cmas(nls) mouse exhibited podocyte foot process effacement, absence of slit diaphragms, and massive proteinuria, recapitulating features of nephrin-knockout mice and of patients with Finnish-type congenital nephrotic syndrome. Although the Cmas(nls) mouse displayed normal sialylation in all organs including kidney, a critical shortage of CMP-sialic acid prevented sialylation of nephrin and podocalyxin in the maturing podocyte where it is required during the formation of foot processes. Accordingly, the sialylation defects progressed with time and paralleled the morphologic changes. In summary, sialylation is critical during the development of the glomerular filtration barrier and required for the proper function of nephrin. Whether altered sialylation impairs nephrin function in human disease requires further study.

Identification and biochemical characterization of two functional CMP-sialic acid synthetases in Danio rerio.

Sialic acids (Sia) form the nonreducing end of the bulk of cell surface-expressed glycoconjugates. They are, therefore, major elements in intercellular communication processes. The addition of Sia to glycoconjugates requires metabolic activation to CMP-Sia, catalyzed by CMP-Sia synthetase (CMAS). This highly conserved enzyme is located in the cell nucleus in all vertebrates investigated to date, but its nuclear function remains elusive. Here, we describe the identification and characterization of two Cmas enzymes in Danio rerio (dreCmas), one of which is exclusively localized in the cytosol. We show that the two cmas genes most likely originated from the third whole genome duplication, which occurred at the base of teleost radiation. cmas paralogues were maintained in fishes of the Otocephala clade, whereas one copy got subsequently lost in Euteleostei (e.g. rainbow trout). In zebrafish, the two genes exhibited a distinct spatial expression pattern. The products of these genes (dreCmas1 and dreCmas2) diverged not only with respect to subcellular localization but also in substrate specificity. Nuclear dreCmas1 favored N-acetylneuraminic acid, whereas the cytosolic dreCmas2 showed highest affinity for 5-deamino-neuraminic acid. The subcellular localization was confirmed for the endogenous enzymes in fractionated zebrafish lysates. Nuclear entry of dreCmas1 was mediated by a bipartite nuclear localization signal, which seemed irrelevant for other enzymatic functions. With the current demonstration that in zebrafish two subfunctionalized cmas paralogues co-exist, we introduce a novel and unique model to detail the roles that CMAS has in the nucleus and in the sialylation pathways of animal cells.

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 A 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.

Chemoenzymatic synthesis of CMP-N-acetyl-7-fluoro-7-deoxy-neuraminic acid.

7-Fluoro sialic acid was prepared and activated as cytidine monophosphate (CMP) ester. The synthesis started with d-glucose, which was efficiently converted into N-acetyl-4-fluoro-4-deoxy-d-mannosamine. Aldolase catalyzed transformation yielded the corresponding fluorinated sialic acid which was activated as CMP ester using three different synthetases in the presence as well as in the absence of pyrophosphatase which possesses inhibitory properties. Finally, conditions were optimized to perform a one-pot reaction starting from fluorinated mannosamine, which yielded the 7-fluoro-7-deoxy-CMP-sialic acid by incubation with three enzymes.

The rainbow trout CMP-sialic acid synthetase utilises a nuclear localization signal different from that identified in the mouse enzyme.

The terminal sugar sialic acid (Sia) plays a pivotal role in cell-cell interaction and recognition. A prerequisite for the biosynthesis of sialoglycoconjugates is the activation of Sia to cytidine monophosphate-Sia (CMP-Sia), by CMP-Sia synthetases (CMP-Sia-syn). CMP-Sia-syn are conserved from bacteria to man, and have been found to reside in the nucleus of all vertebrate species analysed to date. We previously cloned the CMP-Sia-syn from rainbow trout (rt) and identified three clusters of basic amino acids (BC) that might act as nuclear localization signals (NLS). Here, we utilised chimeric proteins and rt CMP-Sia-syn mutants in which putative NLS sequences were deleted, to identify the nuclear transport signal. Divergent from the mouse enzyme, where the crucial NLS is part of the enzyme's active site, in the rt CMP-Sia-syn the NLS and active site are disparate. The crucial NLS in the fish enzyme is bipartite and the functionality depends on a free N-terminus. Comparative analysis of all putative rt NLS in mouse and fish cells identified a second inferior motif (rtBC5-6), which was functional only in fish cells suggesting some differences in transport mechanism or folding variabilities in fish. Moreover, based on computational analyses of putative CMP-Sia-syn from distant deuterostomian organisms it was concluded that CMP-Sia-syn nuclear localization is a relatively recent invention, originating in echinoderms. In summary, our data describing structural differences in the NLS of vertebrate CMP-Sia-syn, and the independence of Sia activation from the subcellular localization of the enzyme, provide supporting evidence that nuclear localization is linked to a second yet unknown function.

Direct detection of ligand binding to Sepharose-immobilised protein using saturation transfer double difference (STDD) NMR spectroscopy.

We report an easy and direct application of 'Saturation Transfer Double Difference' (STDD) NMR spectroscopy to identify ligands that bind to a Sepharose-immobilised target protein. The model protein, cytidine 5'-monophosphate sialic acid (CMP-Sia) synthetase, was expressed as a Strep-Tag II fusion protein and immobilised on Strep-Tactin Sepharose. STD NMR experiments of the protein-enriched Sepharose matrix in the presence of a binding ligand (cytidine 5'-triphosphate, CTP) and a non-binding ligand (alpha/beta-glucose) clearly show that CTP binds to the immobilised enzyme, whereas glucose has no affinity. This approach has three major advantages: (a) only low quantities of protein are required, (b) no specialised NMR technology or the application of additional data analysis by non-routine methods is required, and (c) easy multiple use of the immobilised protein is available.

A 1H STD NMR spectroscopic investigation of sialylnucleoside mimetics as probes of CMP-Kdn synthetase.

CMP-Kdn synthetase catalyses the reaction of sialic acids (Sia) and CTP to the corresponding activated sugar nucleotide CMP-Sia and pyrophosphate PP( i ). Saturation Transfer Difference (STD) NMR spectroscopy has been employed to investigate the sub-structural requirements of the enzyme's binding domain. Sialylnucleoside mimetics, where the sialic acid moiety has been replaced by a carboxyl group and a hydrophobic moiety, have been used in NMR experiments, to probe the tolerance of the CMP-Kdn synthetase to such replacements. From our data it would appear that unlike another sialylnucleotide-recognising protein, the CMP-Neu5Ac transport protein, either a phosphate group or other functional groups on the sialic acid framework may play important roles in recognition by the synthetase.

Probing a CMP-Kdn synthetase by 1H, 31P, and STD NMR spectroscopy.

CMP-Kdn synthetase catalyses the reaction of sialic acids (Sia) and cytidine-5'-triphosphate (CTP) to the corresponding activated sugar nucleotide CMP-Sia and pyrophosphate PP(i). STD NMR experiments of a recombinant nucleotide cytidine-5'-monophosphate-3-deoxy-d-glycero-d-galacto-nonulosonic acid synthetase (CMP-Kdn synthetase) were performed to map the binding epitope of the substrate CTP and the product CMP-Neu5Ac. The STD NMR analysis clearly shows that the anomeric proton of the ribose moiety of both investigated compounds is in close proximity to the protein surface and is likely to play a key role in the binding process. The relative rates of the enzyme reaction, derived from (1)H NMR signal integrals, show that Kdn is activated at a rate 2.5 and 3.1 faster than Neu5Ac and Neu5Gc, respectively. Furthermore, proton-decoupled (31)P NMR spectroscopy was successfully used to follow the enzyme reaction and clearly confirmed the appearance of CMP-Sia and the inorganic pyrophosphate by-product.

Development of a simple and efficient method for assaying cytidine monophosphate sialic acid synthetase activity using an enzymatic reduced nicotinamide adenine dinucleotide/oxidized nicotinamide adenine dinucleotide converting system.

A new reliable method to assay the activity of cytidine monophosphate sialic acid (CMP-Sia) synthetase (CSS) has been developed. The activation of sialic acids (Sia) to CMP-Sia is a prerequisite for the de novo synthesis of sialoglycoconjugates. In vertebrates, CSS has been cloned from human, mouse, and rainbow trout, and the crystal structure has been resolved for the mouse enzyme. The mouse and rainbow trout enzyme have been compared with respect to substrate specificity, demonstrating that the mouse enzyme exhibits a pronounced specificity for N-acetylneuraminic acid (Neu5Ac), while the rainbow trout CSS is equally active with either of three Sia species, Neu5Ac, N-glycolylneuraminic acid (Neu5Gc), and deaminoneuraminic acid (KDN). However, molecular details that explain the pronounced substrate specificities are unknown. Understanding the catalytic mechanisms of these enzymes is of major importance, since CSSs play crucial roles in cellular sialylation patterns and thus are potential drug targets in a number of pathophysiological situations. The availability of the cDNAs and the obtained structural data enable rational approaches; however, these efforts are limited by the lack of a reliable high-throughput assay system. Here we describe a new assay system that allows product quantification in a reduced nicotinamide adenine dinucleotide (NADH)-dependent color reaction. The activation reaction catalyzed by CSS, CTP+Sia-->CMP-Sia+pyrophosphate, was evaluated by a consumption of Sia, which corresponds to that of NADH on the following two successive reactions: (i) Sia-->pyruvate+ManNAc (or Man), catalyzed by a sialic acid lyase (SAL), and (ii) pyruvate+NADH-->lactate+oxidized nicotinamide adenine dinucleotide (NAD+), catalyzed by a lactate dehydrogenase (LDH). Consumption of NADH can be photometrically monitored on a microtiter plate reader for a number of test samples at the same time. Furthermore, based on the quantification of CSS used in the SAL/LDH assay, relative activities toward Sia derivatives have been obtained. The preference of mouse CSS toward Neu5Ac and the ability of the rainbow trout enzyme to activate both KDN and Neu5Ac were confirmed. Thus, this simple and time-saving method is suitable for a systematic comparison of enzyme activity of structurally mutated enzymes based on the relative specific activity.

CMP-sialic acid synthetase of the nucleus.

Sialic acids of cell surface glycoconjugates play a pivotal role in the structure and function of animal cells and in some bacterial pathogens. The pattern of cell surface sialylation is species specific, and, in the animal, highly regulated during embryonic development. A prerequisite for the synthesis of sialylated glycoconjugates is the availability of the activated sugar-nucleotide cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc), which provides the substrate for sialyltransferases. Trials to purify the enzymatic activity responsible for the synthesis of CMP-NeuAc from different animal sources demonstrated that the major localisation of the enzyme is the cell nucleus. These earlier findings were confirmed when the murine CMP-NeuAc synthetase was cloned and the subcellular transport of recombinant epitope tagged forms visualised by indirect immunofluorescence. Today, the primary sequence elements that direct murine CMP-NeuAc synthetase into the cell nucleus are known, however, information regarding the physiological relevance of the nuclear destination is still not available. With this article, we provide a detailed review on earlier and recent findings that identified and confirmed the unusual subcellular localisation of the CMP-NeuAc synthetase. In addition, we take the advantage to discuss most recent developments towards understanding structure--function relations of this enzyme.

Structure and function of vertebrate CMP-sialic acid synthetases.

Activation of sugars into nucleotide sugars is critical for their entry into biosynthetic pathways. In eukaryotic cells, the activation of the acidic nine-carbon sugar sialic acid to CMP-sialic acid takes place in the cell nucleus, whereas all other nucleotide sugars are made in the cytoplasm. Molecular cloning of vertebrate CMP-sialic acid synthetases confirmed the nuclear localization and introduced new molecular tools for directly exploring the functional mechanisms of the enzymes, as well as the physiological relevance of their nuclear transport. Although major advances have been made in understanding structure-function relationships and defining elements involved in the nuclear transport, the riddle surrounding the physiological relevance of nuclear localization awaits resolution.

The crystal structure of murine CMP-5-N-acetylneuraminic acid synthetase.

Sialic acids are activated by CMP-5-N-acetylneuraminic acid synthetase prior to their transfer onto oligo- or polysaccharides. Here, we present the crystal structure of the N-terminal catalytically active domain of the murine 5-N-acetylneuraminic acid synthetase in complex with the reaction product. In contrast to the previously solved structure of 5-N-acetylneuraminic acid synthetase from Neisseria meningitidis and the related CMP-KDO-synthetase of Escherichia coli, the murine enzyme is a tetramer, which was observed with the active sites closed. In this conformation a loop is shifted by 6A towards the active site and thus an essential arginine residue can participate in catalysis. Furthermore, a network of intermolecular salt-bridges and hydrogen bonds in the dimer as well as hydrophobic interfaces between two dimers indicate a cooperative behaviour of the enzyme. In addition, a complex regulation of the enzyme activity is proposed that includes phosphorylation and dephosphorylation.