Polysialic acid engineering: synthesis of polysialylated neoglycosphingolipids by using the polysialyltransferase from neuroinvasive Escherichia coli K1

Directed evolution of bacterial polysialyltransferases

Polysialyltransferases (polySTs) are glycosyltransferases that synthesize polymers of sialic acid found in vertebrates and some bacterial pathogens. Bacterial polySTs have utility in the modification of therapeutic proteins to improve serum half-life, and the potential for tissue engineering. PolySTs are membrane-associated proteins and as recombinant proteins suffer from inherently low solubility, low expression levels and poor thermal stability. To improve their physicochemical and biochemical properties, we applied a directed evolution approach using a FACS-based ultrahigh-throughput assay as a simple, robust and reliable screening method. We were able to enrich a large mutant library and, in combination with plate-based high-throughput secondary screening, we discovered mutants with increased enzymatic activity and improved stability compared to the wildtype enzyme. This work presents a powerful strategy for the screening of directed evolution libraries of bacterial polySTs to identify better catalysts for in vitro polysialylation of therapeutics.

Effect of Acceptor Chain Length and Hydrophobicity on Polymerization Kinetics of the Neisseria meningitidis Group C Polysialyltransferase

Polysialic acids (PSA) are important extracellular virulence factors of the human pathogens Neisseria meningitidis and Escherichia coli. The importance of these polysaccharides in virulence make the polysialyltransferases (PST) targets for therapeutic drugs and protein engineering to facilitate efficient vaccine production. Here, we have generated recombinant bovine nucleotide monophosphate kinase to facilitate steady state kinetic assays of the PST. We have characterized the N. meningitidis group C (NmC) PST kinetically, using substrate analogues to describe the polymerization reaction. We observed a decrease in K_m as the length of the oligo-sialic acid acceptor was increased, indicating a tighter binding of longer oligomers. In addition, we observed a biphasic relationship between k_cat and chain length, which can be attributed to a switch in the mechanism of transfer of sialic acid from distributive to processive as the chain length increased above six sialic acid units. Substitution of donor substrate with the analogue CMP-9-F-sialic acid had minimal effect on acceptor K_m, but it decreased k_cat 6-fold. We propose that this decrease in k_cat is caused by a destabilization of the transition state and/or an increase affinity of the product due to presence of the fluoro substituent. The acceptor's hydrophobicity also plays a role in catalysis. The kinetic analysis of the NmC PST with hydrophobic aglycon acceptor substrates indicated that they bind tighter and are turned over at a faster rate than the α-2,9 polysialic acid substrates lacking the hydrophobic end. This finding suggests the presence of a secondary ligand binding site that tethers the acceptor substrate to the enzyme active site.

In vitro generation of polysialylated cervical mucins by bacterial polysialyltransferases to counteract cytotoxicity of extracellular histones

Neutrophil extracellular traps (NET) are formed against pathogens. However, various diseases are directly linked to this meshwork of DNA. The cytotoxic properties of extracellular histones especially seem to be an important trigger during these diseases. Furthermore, NET accumulation on implants is discussed to result in an impaired efficiency or failure, depending on the category of implant. Interestingly, mucins have been investigated as surface coatings potentially capable of reducing neutrophil adhesion. Similarly, polysialic acid was shown to inactivate the cytotoxic properties of extracellular histones. We wanted to combine the probability to decrease the adhesion of neutrophils using mucins with the capability of sialic acid polymers to counteract histone-mediated cytotoxicity. To this end, we elongate cervical mucins using bacterial polysialyltransferases. Subsequent cell-based experiments demonstrated the activity of elongated mucins against histone-mediated cytotoxicity. Thus, polysialylated mucins may represent a novel component to coat implants or to combat diseases with exaggerated NET formation.

A glyco-gold nanoparticle based assay for α-2,8-polysialyltransferase from Neisseria meningitidis

We designed a novel strategy for sensitively detecting the activity of α-2,8-polysialyltransferase (PST) by a combination of ganglioside GD3 functionalized gold nanoparticles and inactive endosialidase. We anticipate that this new method will facilitate the search for PST inhibitors as well as for improved mutant forms of PST in directed evolution experiments.

Biochemical characterization of a polysialyltransferase from Mannheimia haemolytica A2 and comparison to other bacterial polysialyltransferases

Polysialic acids are bioactive carbohydrates found in eukaryotes and some bacterial pathogens. The bacterial polysialyltransferases (PSTs), which catalyze the synthesis of polysialic acid capsules, have previously been identified in select strains of Escherichia coli and Neisseria meningitidis and are classified in the Carbohydrate-Active enZYmes Database as glycosyltransferase family GT-38. In this study using DNA sequence analysis and functional characterization we have identified a novel polysialyltransferase from the bovine/ovine pathogen Mannheimia haemolytica A2 (PST_Mh). The enzyme was expressed in recombinant form as a soluble maltose-binding-protein fusion in parallel with the related PSTs from E. coli K1 and N. meningitidis group B in order to perform a side-by-side comparison. Biochemical properties including solubility, acceptor preference, reaction pH optima, thermostability, kinetics, and product chain length for the enzymes were compared using a synthetic fluorescent acceptor molecule. PST_Mh exhibited biochemical properties that make it an attractive candidate for chemi-enzymatic synthesis applications of polysialic acid. The activity of PST_Mh was examined on a model glycoprotein and the surface of a neuroprogenitor cell line where the results supported its development for use in applications to therapeutic protein modification and cell surface glycan remodelling to enable cell migration at implantation sites to promote wound healing. The three PSTs examined here demonstrated different properties that would each be useful to therapeutic applications.

Enzymatic engineering of polysialic acid on cells in vitro and in vivo using a purified bacterial polysialyltransferase

In vertebrates, polysialic acid (PSA) is typically added to the neural cell adhesion molecule (NCAM) in the Golgi by PST or STX polysialyltransferase. PSA promotes plasticity, and its enhanced expression by viral delivery of the PST or STX gene has been shown to promote cellular processes that are useful for repair of the injured adult nervous system. Here we demonstrate a new strategy for PSA induction on cells involving addition of a purified polysialyltransferase from Neisseria meningitidis (PST(Nm)) to the extracellular environment. In the presence of its donor substrate (CMP-Neu5Ac), PST(Nm) synthesized PSA directly on surfaces of various cell types in culture, including Chinese hamster ovary cells, chicken DF1 fibroblasts, primary rat Schwann cells, and mouse embryonic stem cells. Similarly, injection of PST(Nm) and donor in vivo was able to produce PSA in different adult brain regions, including the cerebral cortex, striatum, and spinal cord. PSA synthesis by PST(Nm) requires the presence of the donor CMP-Neu5Ac, and the product could be degraded by the PSA-specific endoneuraminidase-N. Although PST(Nm) was able to add PSA to NCAM, most of its product was attached to other cell surface proteins. Nevertheless, the PST(Nm)-induced PSA displayed the ability to attenuate cell adhesion, promote neurite outgrowth, and enhance cell migration as has been reported for endogenous PSA-NCAM. Polysialylation by PST(Nm) occurred in vivo in less than 2.5 h, persisted in tissues, and then decreased within a few weeks. Together these characteristics suggest that a PST(Nm)-based approach may provide a valuable alternative to PST gene therapy.

Stable coexpression of two human sialylation enzymes in plant suspension-cultured tobacco cells

Human CMP-N-acetylneuraminic acid (NeuAc) synthase (hCSS) and α2,6-sialyltransferase (hST) participate in the sialylation of N-linked glycans in mammalian cells. hCSS synthesizes CMP-NeuAc, which hST uses as a donor substrate to transfer NeuAc to the terminal position of N-linked glycans. In plant cells, the presence of NeuAc has not yet been substantiated and the identification of the genes involved in the sialylation of N-glycan has not been carried out. In this study, we introduced hCSS and hST genes into suspension-cultured tobacco BY2 cells to provide the machinery for the sialylation pathway in plants. hCSS and hST stably expressed in the plant cells showed activity. In addition, CMP-NeuAc produced by hCSS in the transformed plant cells functioned as a donor substrate to hST. An in vitro coupled hCSS and hST reaction resulted in the production of mammalian-type sialoglycoproteins bearing terminal NeuAc residues. Furthermore, the results of the purification of the coupled-reaction products by Sambucus sieboldian lectin column chromatography and digestion with linkage-specific neuraminidase revealed that the modified terminal residue was α2,6-linked NeuAc. Here, we demonstrate that the in vitro sialylation of N-linked glycans on mammalian proteins can be achieved using plant cell extracts stably expressing hCSS and hST, providing proof-of-principle that a sialylated human therapeutic protein can be produced in plants.

Biosynthesis and production of polysialic acids in bacteria

Polysialic acids (PA) are protective capsular sialohomopolymers present in some bacteria which can invade the mammalian host and cause lethal bacteremia and meningitis. Biosynthesis and translocation of PA to the cell surface are equivalent in different species and bacterial strains which are produced. The diversity in PA structure is derived from the PA linkages and is a consequence of the specific sialyltransferase activities. The monomer acetylation and the polymer length could be important factors in the potential virulence. In vivo PA production is affected by different physical and chemical factors. The temperature of cellular growth strictly regulates PA genesis through a molecular complex and multifactorial mechanism that operate to transcription level.

Biochemical characterization of a Neisseria meningitidis polysialyltransferase reveals novel functional motifs in bacterial sialyltransferases

The extracellular polysaccharide capsule is an essential virulence factor of Neisseria meningitidis, a leading cause of severe bacterial meningitis and sepsis. Serogroup B strains, the primary disease causing isolates in Europe and America, are encapsulated in α-2,8 polysialic acid (polySia). The capsular polymer is synthesized from activated sialic acid by action of a membrane-associated polysialyltransferase (NmB-polyST). Here we present a comprehensive characterization of NmB-polyST. Different from earlier studies, we show that membrane association is not essential for enzyme functionality. Recombinant NmB-polyST was expressed, purified and shown to synthesize long polySia chains in a non-processive manner in vitro. Subsequent structure–function analyses of NmB-polyST based on refined sequence alignments allowed the identification of two functional motifs in bacterial sialyltransferases. Both (D/E-D/E-G and HP motif) are highly conserved among different sialyltransferase families with otherwise little or no sequence identity. Their functional importance for enzyme catalysis and CMP-Neu5Ac binding was demonstrated by mutational analysis of NmB-polyST and is emphasized by structural data available for the Pasteurella multocida sialyltransferase PmST1. Together our data are the first description of conserved functional elements in the highly diverse families of bacterial (poly)sialyltransferases and thus provide an advanced basis for understanding structure–function relations and for phylogenetic sorting of these important enzymes.

Successive glycosyltransfer of sialic acid by Escherichia coli K92 polysialyltransferase in elongation of oligosialic acceptors

Escherichia coli K92 produces a capsular polysialic acid with alternating alpha2,8 alpha2,9 NeuNAc linkages. This polysaccharide is cross-reactive with the neuroinvasive pathogen Neisseria meningitidis Group C. The K92 polysialyltransferase (PST) catalyzes the synthesis of the polysialic acid with alternating linkages by the transfer of NeuNAc from CMP-NeuNAc to the nonreducing end of the growing polymer. We used a fluorescent-based high-performance liquid chromatography assay to characterize the process of chain extension. The PST elongates the acceptor GT3-FCHASE in a biphasic fashion. The initial phase polymers are characterized by accumulation of product containing 1-8 additional sialic acid residues. This phase is followed by a very rapid formation of high-molecular weight (MW) polymer as the accumulated oligosaccharides containing 8-10 sialic acids are consumed. The high-MW polymer contains 90-100 sialic acids and is sensitive to degradation by periodate and K1-5 endoneuraminidase, suggesting that the polymer contains the alternating structure. The polymerization reaction does not appear to be strictly processive, since oligosaccharides of each intermediate size were detected before accumulation of high-molecular weight polymer. Synthesis can be blocked by CMP-9-azido-NeuNAc. These results suggest that the K92 PST forms both alpha2,8 and alpha2,9 linkages in a successive and nonprocessive fashion.

Biosynthesis and Assembly of Capsular Polysaccharides inEscherichia coli

Capsules are protective structures on the surfaces of many bacteria. The remarkable structural diversity in capsular polysaccharides is illustrated by almost 80 capsular serotypes in Escherichia coli. Despite this variation, the range of strategies used for capsule biosynthesis and assembly is limited, and E. coli isolates provide critical prototypes for other bacterial species. Related pathways are also used for synthesis and export of other bacterial glycoconjugates and some enzymes/processes have counterparts in eukaryotes. In gram-negative bacteria, it is proposed that biosynthesis and translocation of capsular polysaccharides to the cell surface are temporally and spatially coupled by multiprotein complexes that span the cell envelope. These systems have an impact on both a general understanding of membrane trafficking in bacteria and on bacterial pathogenesis.

Diversity of microbial sialic acid metabolism

Sialic acids are structurally unique nine-carbon keto sugars occupying the interface between the host and commensal or pathogenic microorganisms. An important function of host sialic acid is to regulate innate immunity, and microbes have evolved various strategies for subverting this process by decorating their surfaces with sialylated oligosaccharides that mimic those of the host. These subversive strategies include a de novo synthetic pathway and at least two truncated pathways that depend on scavenging host-derived intermediates. A fourth strategy involves modification of sialidases so that instead of transferring sialic acid to water (hydrolysis), a second active site is created for binding alternative acceptors. Sialic acids also are excellent sources of carbon, nitrogen, energy, and precursors of cell wall biosynthesis. The catabolic strategies for exploiting host sialic acids as nutritional sources are as diverse as the biosynthetic mechanisms, including examples of horizontal gene transfer and multiple transport systems. Finally, as compounds coating the surfaces of virtually every vertebrate cell, sialic acids provide information about the host environment that, at least in Escherichia coli, is interpreted by the global regulator encoded by nanR. In addition to regulating the catabolism of sialic acids through the nan operon, NanR controls at least two other operons of unknown function and appears to participate in the regulation of type 1 fimbrial phase variation. Sialic acid is, therefore, a host molecule to be copied (molecular mimicry), eaten (nutrition), and interpreted (cell signaling) by diverse metabolic machinery in all major groups of mammalian pathogens and commensals.

Functional relationships of the sialyltransferases involved in expression of the polysialic acid capsules of Escherichia coli K1 and K92 and Neisseria meningitidis groups B or C

Polysialic acid (PSA) capsules are cell-associated homopolymers of alpha2,8-, alpha2,9-, or alternating alpha2,8/2,9-linked sialic acid residues that function as essential virulence factors in neuroinvasive diseases caused by certain strains of Escherichia coli and Neisseria meningitidis. PSA chains structurally identical to the bacterial alpha2,8-linked capsular polysaccharides are also synthesized by the mammalian central nervous system, where they regulate neuronal function in association with the neural cell adhesion molecule (NCAM). Despite the structural identity between bacterial and NCAM PSAs, the respective polysialyltransferases (polySTs) responsible for polymerizing sialyl residues from donor CMP-sialic acid are not homologous glycosyltransferases. To better define the mechanism of capsule biosynthesis, we established the functional interchangeability of bacterial polySTs by complementation of a polymerase-deficient E. coli K1 mutant with the polyST genes from groups B or C N. meningitidis and the control E. coli K92 polymerase gene. The biochemical and immunochemical results demonstrated that linkage specificity is dictated solely by the source of the polymerase structural gene. To determine the molecular basis for linkage specificity, we created chimeras of the K1 and K92 polySTs by overlap extension PCR. Exchanging the first 52 N-terminal amino acids of the K1 NeuS with the C terminus of the K92 homologue did not alter specificity of the resulting chimera, whereas exchanging the first 85 or reciprocally exchanging the first 100 residues did. These results demonstrated that linkage specificity is dependent on residues located between positions 53 and 85 from the N terminus. Site-directed mutagenesis of the K92 polyST N terminus indicated that no single residue alteration was sufficient to affect specificity, consistent with the proposed function of this domain in orienting the acceptor. The combined results provide the first evidence for residues critical to acceptor binding and elongation in polysialyltransferase.

Elongation of alternating alpha 2,8/2,9 polysialic acid by the Escherichia coli K92 polysialyltransferase

We have chosen E. coli K92, which produces the alternating structure alpha(2-8)neuNAc alpha(2-9)neuNAc as a model system for studying bacterial polysaccharide biosynthesis. We have shown that the polysialyltransferase encoded by the K92 neuS gene can synthesize both alpha(2-8) and alpha(2-9) neuNAc linkages in vivo by 13C-nuclear magnetic resonance analysis of polysaccharide isolated from a heterologous strain containing the K92 neuS gene. The K92 polysialyltransferase is associated with the membrane in lysates of cells harboring the neuS gene in expression vectors. Although the enzyme can transfer sialic acid to the nonreducing end of oligosaccharides with either linkage, it is unable to initiate chain synthesis without exogenously added polysialic acid. Thus, the polysialyltransferase encoded by neuS is not sufficient for de novo synthesis of polysaccharide but requires another membrane component for initiation. The acceptor specificity of this polysialyltransferase was studied using sialic acid oligosaccharides of various structures as exogenous acceptors. The enzyme can transfer to the nonreducing end of all bacteria polysialic acids, but has a definite preference for alpha(2-8) acceptors. Gangliosides containing neuNAc alpha(2-8)neuNAc are elongated, whereas monsialylated gangliosides are not. Disialylgangliosides are better acceptors than short oligosaccharides, suggesting a lipid-linked oligosaccharide may be preferred in the elongation reaction. These studies show that the K92 polysialyltransferase catalyzes an elongation reaction that involves transfer of sialic acid from CMP-sialic acid to the nonreducing end of two different acceptor substrates.

Emerging themes in medicinal glycoscience

The recognition of complex carbohydrates and glycoconjugates as mediators of important biological processes has stimulated investigation into their therapeutic potential. New approaches for the simplification of glycoconjugate synthesis are overcoming the limitations of existing methods and providing a diverse array of these biomolecules. As the accessibility of glycoconjugates increases, carbohydrate-based constructs are becoming available for analysis as medicinal agents in a wide range of therapies.

Ammonia inhibits neural cell adhesion molecule polysialylation in Chinese hamster ovary and small cell lung cancer cells

Ammonia is a major concern in biotechnology because it often limits recombinant protein production by animal cells. Conditions, such as ammonia accumulation, in large-scale production systems can parallel those that develop within fast-growing solid tumors such as small cell lung cancer (SCLC). Ammonia's specific inhibition of the sialylation of secreted glycoproteins is well documented, but it is not known how ammonia affects membrane-bound proteins, nor what role it may have on important glycosylation determinants in cancer. We therefore examined the effects of NH4Cl on polysialic acid (PolySia) in the neural cell adhesion molecule (NCAM). By using flow cytometry combined with two NCAM antibodies, one specific for the peptide backbone and another that recognizes PolySia chains, we show that ammonia causes rapid, dose-dependent, and reversible inhibition of NCAM polysialylation in Chinese hamster ovary (CHO) and SCLC NCI-N417 cells. The decrease in PolySia was accompanied by a small increase in NCAM, suggesting that the changes were specific to the oligosaccharide. Inhibition by ammonia was greater for CHO cells, with PolySia cell surface content decreasing to 10% of control after a 4-day culture with 10 mM NH4Cl, while N417 cell PolySia was reduced by only 35%. Ammonia caused a 60% decrease in the CHO cell yield from glucose, while N417 cells were barely affected, suggesting that increased resistance to ammonia by N41 7 cells is a global rather than glycosylation-specific phenomenon. The data presented show that the tumor microenvironment may be an important factor in the regulation of PolySia expression.

Developmental expression and characterization of the alpha2,8-polysialyltransferase activity in embryonic chick brain

The alpha2,8-polysialyltransferases (polySTs) from embryonic chick brain catalyze the alpha2,8-specific polysialylation of endogenous neural cell adhesion molecules (N-CAMs). This posttranslation glycosylation decreases N-CAM-dependent cell adhesion and migration. The enzymatic properties of the membrane-bound form of the polyST activity was investigated in vitro. Our results show that the polyST activity was developmentally expressed with maximum specific activity appearing about 12 days after fertilization. This time shortly precedes maximal expression of the cognate polysialylated N-CAMs. Kinetic studies showed the KMand Vmaxfor CMP-Neu5Ac were 133 microM and 0.13 microM/h, respectively, at pH 6.1, 33 degrees C. CMP-Neu5Gc was not a donor substrate. PolyST activity was increased 5- to 6-fold in the presence of 10 mM MnCl2,the preferred divalent cation, and 1 mM dithiothreitol (DTT). Heparin (3 kDa) was a noncompetitive inhibitor of polysialylation with a Kiof 9 microM. Based on the affinity of the enzyme for heparin, the polyST activity was partially purified ( approximately 30-fold) by heparin-Sepharose affinity chromatography, after differential solubilization with the zwitterionic detergent, CHAPS. DTT and chemical modification studies using the thiol-directed alkylating reagents, N-ethylmaleimide (NEM) and iodoacetamide (IAA), were used to show that at least one cysteinyl residue in the polyST was of critical importance for polysialylation, but of lesser importance for monosialylation, catalyzed by the alpha2,3-, alpha2,6-, and alpha2,8-monosialyltransferases (monoSTs). A sulfhydryl residue is implicated in chain initiation. Two important structural differences between the mono- and polySTs were revealed by sequence analyses. First, the polySTs contain heparin-like, positively charged amino acid clusters upstream of both sialylmotif L and S. Second, the polySTs contain a uniquely extended basic amino acid region (pI 11. 6-12.0) of 31 residues immediately upstream of sialylmotif S. This extended, positively charged region may function in the processive mechanism of polymerization by allowing nascent polySia chains to remain bound to the polyST during the repetitive addition of each new Sia residue to the nonreducing termini of the growing chain. The importance of these studies is that they provide new information on the enzymatic basis of polysialylation. They also reveal that sulfhydryl residues and extended basic amino acid domains are two structural features unique to polysialylation, in contrast to monosialylation. Both may be important distinguishing features between the classes of distributive (monoSTs) and processive polysialyltransferases, which have not been previously described.

Characterization of a recombinant Neisseria meningitidis alpha-2,3-sialyltransferase and its acceptor specificity

The structure and specificity of the recombinant alpha-2,3-sialyltransferase from Neisseria meninigitidis are reported. This enzyme showed an unusual acceptor specificity in that it could use alpha-terminal and beta-terminal Gal residues as acceptors. In addition (beta1-->4)-linked and (beta1-->3)-linked terminal Gal served as acceptors. These properties distinguish the bacterial enzyme from the more widely investigated mammalian equivalents. The protein was expressed as a membrane-associated protein in Escherichia coli at a level of 750 U/l (approximately 250 mg/l). The protein could be extracted with buffers containing 0.2% Triton X-100 and purified to homogeneity using immobilized-metal-affinity chromatography. Electrospray-ionization mass spectrometry of peptides obtained by cleavage with cyanogen bromide and trypsin confirmed over 95% of the deduced amino acid sequence. When used for enzymatic synthesis in coupled reactions with recombinant CMP-Neu5Ac synthetase, the alpha-2,3-sialyltransferase could sialylate fluorescent derivatives of N-acetyllactosamine with N-acetylneuraminic acid, N-propionylneuraminic acid and N-glycoloylneuraminic acid.