Elongation of the N-acyl side chain of sialic acids in MDCK II cells inhibits influenza A virus infection

Single Particle Analysis of H3N2 Influenza Entry Differentiates the Impact of the Sialic Acids (Neu5Ac and Neu5Gc) on Virus Binding and Membrane Fusion

Entry of influenza A viruses (IAVs) into host cells is initiated by binding to sialic acids (Sias), their primary host cell receptor, followed by endocytosis and membrane fusion to release the viral genome into the cytoplasm of the host cell. Host tropism is affected by these entry processes, with a primary factor being receptor specificity. Sias exist in several different chemical forms, including the hydroxylated N-glycolylneuraminic acid (Neu5Gc), which is found in many hosts; however, it has not been clear how modified Sias affect viral binding and entry. Neu5Gc is commonly found in many natural influenza hosts, including pigs and horses, but not in humans or ferrets. Here, we engineered HEK293 cells to express the hydoxylase gene (CMAH) that converts Neu5Ac to Neu5Gc, or knocked out the Sia-CMP transport gene (SLC35A1), resulting in cells that express 95% Neu5Gc or minimal level of Sias, respectively. H3N2 (X-31) showed significantly reduced infectivity in Neu5Gc-rich cells compared to wild-type HEK293 (>95% Neu5Ac). To determine the effects on binding and fusion, we generated supported lipid bilayers (SLBs) derived from the plasma membranes of these cells and carried out single particle microscopy. H3N2 (X-31) exhibited decreased binding to Neu5Gc-containing SLBs, but no significant difference in H3N2 (X-31)'s fusion kinetics to either SLB type, suggesting that reduced receptor binding does not affect subsequent membrane fusion. This finding suggests that for this virus to adapt to host cells rich in Neu5Gc, only receptor affinity changes are required without further adaptation of virus fusion machinery. IMPORTANCE Influenza A virus (IAV) infections continue to threaten human health, causing over 300,000 deaths yearly. IAV infection is initiated by the binding of influenza glycoprotein hemagglutinin (HA) to host cell sialic acids (Sias) and the subsequent viral-host membrane fusion. Generally, human IAVs preferentially bind to the Sia N-acetylneuraminic acid (Neu5Ac). Yet, other mammalian hosts, including pigs, express diverse nonhuman Sias, including N-glycolylneuraminic acid (Neu5Gc). The role of Neu5Gc in human IAV infections in those hosts is not well-understood, and the variant form may play a role in incidents of cross-species transmission and emergence of new epidemic variants. Therefore, it is important to investigate how human IAVs interact with Neu5Ac and Neu5Gc. Here, we use membrane platforms that mimic the host cell surface to examine receptor binding and membrane fusion events of human IAV H3N2. Our findings improve the understanding of viral entry mechanisms that can affect host tropism and virus evolution.

Metabolic Glycoengineering: A Promising Strategy to Remodel Microenvironments for Regenerative Therapy

Cell-based regenerative therapy utilizes the differentiation potential of stem cells to rejuvenate tissues. But the dynamic fate of stem cells is calling for precise control to optimize their therapeutic efficiency. Stem cell fate is regulated by specific conditions called "microenvironments." Among the various factors in the microenvironment, the cell-surface glycan acts as a mediator of cell-matrix and cell-cell interactions and manipulates the behavior of cells. Herein, metabolic glycoengineering (MGE) is an easy but powerful technology for remodeling the structure of glycan. By presenting unnatural glycans on the surface, MGE provides us an opportunity to reshape the microenvironment and evoke desired cellular responses. In this review, we firstly focused on the determining role of glycans on cellular activity; then, we introduced how MGE influences glycosylation and subsequently affects cell fate; at last, we outlined the application of MGE in regenerative therapy, especially in the musculoskeletal system, and the future direction of MGE is discussed.

Chemical tools to track and perturb the expression of sialic acid and fucose monosaccharides

The biosynthesis of glycans is a highly conserved biological process and found in all domains of life. The expression of cell surface glycans is increasingly recognized as a target for therapeutic intervention given the role of glycans in major pathologies such as cancer and microbial infection. Herein, we summarize our contributions to the development of unnatural monosaccharide derivatives to infiltrate and alter the expression of both mammalian and bacterial glycans and their therapeutic application.

Exploiting metabolic glycoengineering to advance healthcare

Metabolic glycoengineering (MGE) is a technique for manipulating cellular metabolism to modulate glycosylation. MGE is used to increase the levels of natural glycans and, more importantly, to install non-natural monosaccharides into glycoconjugates. In this Review, we summarize the chemistry underlying MGE that has been developed over the past three decades and highlight several recent advances that have set the stage for clinical translation. In anticipation of near-term application to human healthcare, we describe emerging efforts to deploy MGE in diverse applications, ranging from the glycoengineering of biotherapeutic proteins and the diagnosis and treatment of complex diseases such as cancer to the development of new immunotherapies.

Glycoengineering of Esterase Activity through Metabolic Flux-Based Modulation of Sialic Acid

This report describes the metabolic glycoengineering (MGE) of intracellular esterase activity in human colon cancer (LS174T) and Chinese hamster ovary (CHO) cells. In silico analysis of carboxylesterases CES1 and CES2 suggested that these enzymes are modified with sialylated N-glycans, which are proposed to stabilize the active multimeric forms of these enzymes. This premise was supported by treating cells with butanolylated ManNAc to increase sialylation, which in turn increased esterase activity. By contrast, hexosamine analogues not targeted to sialic acid biosynthesis (e.g., butanoylated GlcNAc or GalNAc) had minimal impact. Measurement of mRNA and protein confirmed that esterase activity was controlled through glycosylation and not through transcription or translation. Azide-modified ManNAc analogues widely used in MGE also enhanced esterase activity and provided a way to enrich targeted glycoengineered proteins (such as CES2), thereby providing unambiguous evidence that the compounds were converted to sialosides and installed into the glycan structures of esterases as intended. Overall, this study provides a pioneering example of the modulation of intracellular enzyme activity through MGE, which expands the value of this technology from its current status as a labeling strategy and modulator of cell surface biological events.

Metabolic Oligosaccharide Engineering with Alkyne Sialic Acids Confers Neuraminidase Resistance and Inhibits Influenza Reproduction

Metabolic incorporation of azide- or alkyne-modified sialic acids into the cellular glycosylation pathway enables the study of sialoglycan expression, localization, and trafficking via bioorthogonal chemistry. Herein, we report that such modifications of the sialic acid sugar can have a profound influence on their hydrolysis by neuraminidases (sialidase). Azidoacetyl (Az)-modified sialic acids were prone to neuraminidase cleavage, whereas propargyloxycarbonyl (Poc)-modified sialic acids were largely resistant to cleavage. Because the influenza virus infection cycle depends on the hydrolysis of host-cell-surface sialic acids, influenza cell-to-cell transmission was strongly reduced in Poc sialic acid glycoengineered host cells. The use of Poc sialic acids may disturb biological processes involving neuraminidase cleavage but also provides perspective for use in applications in which sialic acid hydrolysis is not desired, such as antibody modification, viral infection, etc.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

Small Molecules Targeting Human N-Acetylmannosamine Kinase

N-Acetylmannosamine kinase (MNK) plays a key role in the biosynthesis of sialic acids and glycosylation of proteins. Sialylated glycoconjugates affect a large number of biological processes, including immune modulation and cancer transformation. In search of effective inhibitors of MNK we applied high-throughput screening of drug-like small molecules. By applying different orthogonal assays for their validation we identified four potential MNK-specific inhibitors with IC₅₀ values in the low-micromolar range. Molecular modelling of the inhibitors into the active site of MNK supports their binding to the sugar or the ATP-binding pocket of the enzyme or both. These compounds are promising for downregulation of the sialic acid content of glycoconjugates and for studying the functional contribution of sialic acids to disease development.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).

Metabolic Remodeling of Cell-Surface Sialic Acids: Principles, Applications, and Recent Advances

Cell-surface sialic acids are essential in mediating a variety of physiological and pathological processes. Sialic acid chemistry and biology remain challenging to investigate, demanding new tools for probing sialylation in living systems. The metabolic glycan labeling (MGL) strategy has emerged as an invaluable chemical biology tool that enables metabolic installation of useful functionalities into cell-surface sialoglycans by "hijacking" the sialic acid biosynthetic pathway. Here we review the principles of MGL and its applications in study and manipulation of sialic acid function, with an emphasis on recent advances.

Mouse Siglec-1 Mediates trans-Infection of Surface-bound Murine Leukemia Virus in a Sialic Acid N-Acyl Side Chain-dependent Manner

Siglec-1 (sialoadhesin, CD169) is a surface receptor on human cells that mediates trans-enhancement of HIV-1 infection through recognition of sialic acid moieties in virus membrane gangliosides. Here, we demonstrate that mouse Siglec-1, expressed on the surface of primary macrophages in an interferon-α-responsive manner, captures murine leukemia virus (MLV) particles and mediates their transfer to proliferating lymphocytes. The MLV infection of primary B-cells was markedly more efficient than that of primary T-cells. The major structural protein of MLV particles, Gag, frequently co-localized with Siglec-1, and trans-infection, primarily of surface-bound MLV particles, efficiently occurred. To explore the role of sialic acid for MLV trans-infection at a submolecular level, we analyzed the potential of six sialic acid precursor analogs to modulate the sialylated ganglioside-dependent interaction of MLV particles with Siglec-1. Biosynthetically engineered sialic acids were detected in both the glycolipid and glycoprotein fractions of MLV producer cells. MLV released from cells carrying N-acyl-modified sialic acids displayed strikingly different capacities for Siglec-1-mediated capture and trans-infection; N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl side chain modifications resulted in up to 92 and 80% reduction of virus particle capture and trans-infection, respectively, whereas N-propanoyl or N-cyclopropylcarbamyl side chains had no effect. In agreement with these functional analyses, molecular modeling indicated reduced binding affinities for non-functional N-acyl modifications. Thus, Siglec-1 is a key receptor for macrophage/lymphocyte trans-infection of surface-bound virions, and the N-acyl side chain of sialic acid is a critical determinant for the Siglec-1/MLV interaction.

Artificial and natural sialic acid precursors influence the angiogenic capacity of human umbilical vein endothelial cells

N-acetylneuraminic acid (Neu5Ac) represents the most common terminal carbohydrate residue in many mammalian glycoconjugates and is directly involved in a number of different physiological as well as pathological cellular processes. Endogenous sialic acids derive from the biosynthetic precursor molecule N-acetyl-D-mannosamine (ManNAc). Interestingly, N-acyl-analogues of D-mannosamine (ManN) can also be incorporated and converted into corresponding artificial sialic acids by eukaryotic cells. Within this study, we optimized a protocol for the chemical synthesis of various peracetylated ManN derivatives resulting in yields of approximately 100%. Correct molecular structures of the obtained products ManNAc, N-propanoyl-ManN (ManNProp) and N-butyl-ManN (ManNBut) were verified by GC-, ESI-MS- and NMR-analyses. By applying these substances to human umbilical vein endothelial cells (HUVECs), we could show that each derivative was metabolized to the corresponding N-acylneuraminic acid variant and subsequently incorporated into nascent glycoproteins. To investigate whether natural and/or artificial sialic acid precursors are able to modulate the angiogenic capacity of HUVECs, a spheroid assay was performed. By this means, an increase in total capillary length has been observed when cells incorporated N-butylneuraminic acid (Neu5But) into their glycoconjugates. In contrast, the natural precursor ManNAc inhibited the growth of capillaries. Thus, sialic acid precursors may represent useful agents to modulate blood vessel formation.

Biochemical engineering of the N-acyl side chain of sialic acids alters the kinetics of a glycosylated potassium channel Kv3.1

The sialic acid of complex N-glycans can be biochemically engineered by substituting the physiological precursor N-acetylmannosamine with non-natural N-acylmannosamines. The Kv3.1 glycoprotein, a neuronal voltage-gated potassium channel, contains sialic acid. Western blots of the Kv3.1 glycoprotein isolated from transfected B35 neuroblastoma cells incubated with N-acylmannosamines verified sialylated N-glycans attached to the Kv3.1 glycoprotein. Outward ionic currents of Kv3.1 transfected B35 cells treated with N-pentanoylmannosamine or N-propanoylmannosamine had slower activation and inactivation rates than those of untreated cells. Therefore, the N-acyl side chain of sialic acid is intimately connected with the activation and inactivation rates of this glycosylated potassium channel.

Metabolism of diazirine-modified N-acetylmannosamine analogues to photo-cross-linking sialosides

Terminal sialic acid residues often mediate the interactions of cell surface glycoconjugates. Sialic acid-dependent interactions typically exhibit rapid dissociation rates, precluding the use of traditional biological techniques for complex isolation. To stabilize these transient interactions, we employ a targeted photo-cross-linking approach in which a diazirine photo-cross-linker is incorporated into cell surface sialylated glycoconjugates through the use of metabolic oligosaccharide engineering. We describe three diazirine-modified N-acetylmannosamine (ManNAc) analogues in which the length of the linker between the pyranose ring and the diazirine was varied. These analogues were each metabolized to their respective sialic acid counterparts, which were added to both glycoproteins and glycolipids. Diazirine-modified sialic acid analogues could be incorporated into both α2-3 and α2-6 linkages. Upon exposure to UV irradiation, diazirine-modified glycoconjugates were covalently cross-linked to their interaction partners. We demonstrate that all three diazirine-modified analogues were capable of competing with endogeneous sialic acid, albeit to varying degrees. We found that larger analogues were less efficiently metabolized, yet could still function as effective cross-linkers. Notably, the addition of the diazirine substituent interferes with metabolism of ManNAc analogues to glycans other than sialosides, providing fidelity to selectively incorporate the cross-linker into sialylated molecules. These compounds are nontoxic and display only minimal growth inhibition at the concentrations required for cross-linking studies. This report provides essential information for the deployment of photo-cross-linking analogues to capture and study ephemeral, yet essential, sialic acid-mediated interactions.

Glycan analysis and influenza A virus infection of primary swine respiratory epithelial cells: the importance of NeuAc{alpha}2-6 glycans

To better understand influenza virus infection of pigs, we examined primary swine respiratory epithelial cells (SRECs, the primary target cells of influenza viruses in vivo), as a model system. Glycomic profiling of SRECs by mass spectrometry revealed a diverse range of glycans terminating in sialic acid or GalαGal. In terms of sialylation, α2-6 linkage was more abundant than α2-3, and NeuAc was more abundant than NeuGc. Virus binding and infection experiments were conducted to determine functionally important glycans for influenza virus infection, with a focus on recently emerged swine viruses. Infection of SRECs with swine and human viruses resulted in different infectivity levels. Glycan microarray analysis with a high infectivity "triple reassortant" virus ((A/Swine/MN/593/99 (H3N2)) that spread widely throughout the North American swine population and a lower infectivity human virus isolated from a single pig (A/Swine/ONT/00130/97 (H3N2)) showed that both viruses bound exclusively to glycans containing NeuAcα2-6, with strong binding to sialylated polylactosamine and sialylated N-glycans. Treatment with mannosamine precursors of sialic acid (to alter NeuAc/NeuGc abundances) and linkage-specific sialidases prior to infection indicated that the influenza viruses tested preferentially utilize NeuAcα2-6-sialylated glycans to infect SRECs. Our data indicate that NeuAcα2-6-terminated polylactosamine and sialylated N-glycans are important determinants for influenza viruses to infect SRECs. As NeuAcα2-6 polylactosamine glycans play major roles in human virus infection, the importance of these receptor components in virus infection of swine cells has implications for transmission of viruses between humans and pigs and for pigs as possible adaptation hosts of novel human influenza viruses.

Quantification of nucleotide-activated sialic acids by a combination of reduction and fluorescent labeling

Sialic acids usually represent the terminal monosaccharide of glycoconjugates and are directly involved in many biological processes. The cellular concentration of their nucleotide-activated form is one pacemaker for the highly variable sialylation of glycoconjugates. Hence, the determination of CMP-sialic acid levels is an important factor to understand the complex glycosylation machinery of cells and to standardize the production of glycotherapeutics. We have established a highly sensitive strategy to quantify the concentration of nucleotide-activated sialic acid by a combination of reduction and fluorescent labeling using the fluorophore 1,2-diamino-4,5-methylenedioxybenzene (DMB). The labeling with DMB requires free keto as well as carboxyl groups of the sialic acid molecule. Reduction of the keto group prior to the labeling process precludes the labeling of nonactivated sialic acids. Since the keto group is protected against reduction by the CMP-substitution, labeling of nucleotide-activated sialic acids is still feasible after reduction. Subsequent combination of the DMB-high-performance liquid chromatography (HPLC) application with mass spectrometric approaches, such as matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOF-MS) and electrospray-ionization (ESI)-MS, allows the unambiguous identification of both natural and modified CMP-sialic acids and localization of potential substituents. Thus, the described strategy offers a sensitive detection, identification, and quantification of nucleotide-activated sialic acid derivatives in the femtomole range without the need for nucleotide-activated standards.

Fluorination of mammalian cell surfaces via the sialic acid biosynthetic pathway

Metabolic oligosaccharide engineering has been employed to introduce fluorine-containing groups onto mammalian cell surfaces. Incubation of HeLa, Jurkat, and HL60 cells in culture with fluorinated sialic acid and mannosamine analogues resulted in cell-surface presentation of fluorinated glycans. Metabolic conversion of fluorinated precursors was detected and quantified by DMB-derivatization and HPLC ESI-MS analysis. Between 7% and 72% of total membrane-associated sialosides were fluorinated, depending on the precursor used and the cell type. Fluorination of mammalian cell surfaces provides a means for introducing a bioorthogonal surface for modulating noncovalent interactions such as those involved in cell adhesion.

Transsialidase fromTrypanosoma cruzi for Regio- and Stereoselective Synthesis of N-Acyl-Modified Sialylated Oligosaccharides and Measurement of Transfer Rates

Recombinant transsialidase from Trypanosoma cruzi (TcTS) was used for the sialylation with natural and non-natural derivatives of neuraminic acid. Neu5Ac-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe was prepared in 80 % yield. Correspondingly, the modified trisaccharide derivatives Neu5Prop-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (32 %) and Neu5Gc-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (Prop=propanoyl, Gc=glycolyl) were obtained in 60 % yield, respectively.

Engineering the sialic acid in organs of mice using N-propanoylmannosamine

Sialic acids play an important role during development, regeneration and pathogenesis. The precursor of most physiological sialic acids, such as N-acetylneuraminic acid is N-acetyl-D-mannosamine. Application of the novel N-propanoylmannosamine leads to the incorporation of the new sialic acid N-propanoylneuraminic acid into cell surface glycoconjugates. Here we analyzed the modified sialylation of several organs with N-propanoylneuraminic acid in mice. By using peracetylated N-propanoylmannosamine, we were able to replace in vivo between 1% (brain) and 68% (heart) of physiological sialic acids by N-propanoylneuraminic acid. The possibility to modify cell surfaces with engineered sialic acids in vivo offers the opportunity to target therapeutic agents to sites of high sialic acid concentration in a variety of tumors. Furthermore, we demonstrated that application of N-propanoylmannosamine leads to a decrease in the polysialylation of the neural cell adhesion molecule in vivo, which is a marker of poor prognosis for some tumors with high metastatic potential.

Investigating cellular metabolism of synthetic azidosugars with the Staudinger ligation

The structure of sialic acid on living cells can be modulated by metabolism of unnatural biosynthetic precursors. Here we investigate the conversion of a panel of azide-functionalized mannosamine and glucosamine derivatives into cell-surface sialosides. A key tool in this study is the Staudinger ligation, a highly selective reaction between modified triarylphosphines and azides that produces an amide-linked product. A preliminary study of the mechanism of this reaction, and refined conditions for its in vivo execution, are reported. The reaction provided a means to label the glycoconjugate-bound azidosugars with biochemical probes. Finally, we demonstrate that the cell-surface Staudinger ligation is compatible with hydrazone formation from metabolically introduced ketones. These two strategies provide a means to selectively modify cell-surface glycans with exogenous probes.

Chemical approaches to the investigation of cellular systems

Biochemistry in the context of a living cell or organism is complicated by many variables such as supramolecular organization, cytoplasmic viscosity, and substrate heterogeneity. While these variables are easily excluded or avoided in reconstituted systems, they must be dealt with in cellular environments. New developments have allowed researchers to begin probing the inner workings of the cell to gain new insight into cell function and metabolism. Advances in cellular imaging and in small molecule-controlled gene expression, signal transduction and cell surface modification are discussed in this review. These techniques have permitted the study of molecular components within the context of living cells.

Differential effects of unnatural sialic acids on the polysialylation of the neural cell adhesion molecule and neuronal behavior

In this study we have examined how unnatural sialic acids can alter polysialic acid expression and influence the adhesive properties of the neural cell adhesion molecule (NCAM). Unnatural sialic acids are generated by metabolic conversion of synthetic N-acyl mannosamines and are typically incorporated into cell-surface glycoconjugates. However, N-butanoylmannosamine and N-pentanoylmannosamine are effective inhibitors of polysialic acid (PSA) synthesis in stably transfected HeLa cells expressing NCAM and the polysialyltransferase STX. These cells were used as substrates to examine the effect of inhibiting PSA synthesis on the development of neurons derived from the chick dorsal root ganglion. N-butanoylmannosamine blocked polysialylation of NCAM and significantly reduced neurite outgrowth comparable with enzymatic removal of PSA by endoneuraminidases. As a result, neurite outgrowth was similar to that observed for non-polysialylated NCAM. In contrast, previous studies have shown that N-propanoyl sialic acid (SiaProp), generated from N-propanoylmannosamine, is readily accepted by polysialyltransferases and permits the extension of poly(SiaProp) on NCAM. Despite being immunologically distinct, poly(SiaProp) can promote neurite outgrowth similarly to natural polysialic acid. Thus, subtle structural differences in PSA resulting from the incorporation of SiaProp residues do not alter the antiadhesive properties of polysialylated NCAM.

Versatile biosynthetic engineering of sialic acid in living cells using synthetic sialic acid analogues

Sialic acids are critical components of many glycoconjugates involved in biologically important ligand-receptor interactions. Quantitative and structural variations of sialic acid residues can profoundly affect specific cell-cell, pathogen-cell, or drug-cell interactions, but manipulation of sialic acids in mammalian cells has been technically limited. We describe the finding of a previously unrecognized and efficient uptake and incorporation of sialic acid analogues in mammalian cells. We added 16 synthetic sialic acid analogues carrying distinct C-1, C-5, or C-9 substitutions individually to cell cultures of which 10 were readily taken up and incorporated. Uptake of C-5- and C-9-substituted sialic acids resulted in the structural modification of up to 95% of sialic acids on the cell surface. Functionally, binding of murine sialic acid-binding immunoglobulin-like lectin-2 (Siglec-2, CD22) to cells increased after N-glycolylneuraminic acid treatment, whereas 9-iodo-N-acetylneuraminic acid abolished binding. Furthermore, susceptibility to infection by the B-lymphotropic papovavirus via a sialylated receptor was markedly enhanced following pretreatment of host cells with selected sialic acid analogues including 9-iodo-N-acetylneuraminic acid. This novel experimental strategy allows for an efficient biosynthetic engineering of surface sialylation in living cells. It is versatile, extending the repertoire of modification sites at least to C-9 and enables detailed structure-function studies of sialic acid-dependent ligand-receptor interactions in their native context.

Chemical and biological strategies for engineering cell surface glycosylation

Oligosaccharides play a crucial role in many of the recognition, signaling, and adhesion events that take place at the surface of cells. Abnormalities in the synthesis or presentation of these carbohydrates can lead to misfolded and inactive proteins, as well as to several debilitating disease states. However, their diverse structures, which are the key to their function, have hampered studies by biologists and chemists alike. This review presents an overview of techniques for examining and manipulating cell surface oligosaccharides through genetic, enzymatic, and chemical strategies.

Efficient biochemical engineering of cellular sialic acids using an unphysiological sialic acid precursor in cells lacking UDP-N-acetylglucosamine 2-epimerase

Sialic acids comprise a family of terminal sugars essential for a variety of biological recognition systems. N-Propanoylmannosamine, an unphysiological sialic acid precursor, is taken up and metabolized by mammalian cells resulting in oligosaccharide-bound N-propanoylneuraminic acid. N-Propanoylmannosamine, applied to endogenously hyposialylated subclones of the myeloid leukemia HL60 and of the B-cell lymphoma BJA-B, both deficient in UDP-N-acetylglucosamine 2-epimerase, is efficiently metabolized to CMP-N-propanoylneuraminic acid resulting in up to 85% of glycoconjugate-associated sialic acids being unphysiological N-propanoylneuraminic acid. Thus, UDP-N-acetylglucosamine 2-epimerase-deficient cell lines provide an important experimental progress in engineering cells to display an almost homogeneous population of defined, structurally altered sialic acids.

Chemical glycobiology

Chemical tools have proven indispensable for studies in glycobiology. Synthetic oligosaccharides and glycoconjugates provide materials for correlating structure with function. Synthetic mimics of the complex assemblies found on cell surfaces can modulate cellular interactions and are under development as therapeutic agents. Small molecule inhibitors of carbohydrate biosynthetic and processing enzymes can block the assembly of specific oligosaccharide structures. Inhibitors of carbohydrate recognition and biosynthesis can reveal the biological functions of the carbohydrate epitope and its cognate receptors. Carbohydrate biosynthetic pathways are often amenable to interception with synthetic unnatural substrates. Such metabolic interference can block the expression of oligosaccharides or alter the structures of the sugars presented on cells. Collectively, these chemical approaches are contributing great insight into the myriad biological functions of oligosaccharides.

Biochemical engineering of the N-acyl side chain of sialic acid: biological implications

N-Acetylneuraminic acid is the most prominent sialic acid in eukaryotes. The structural diversity of sialic acid is exploited by viruses, bacteria, and toxins and by the sialoglycoproteins and sialoglycolipids involved in cell-cell recognition in their highly specific recognition and binding to cellular receptors. The physiological precursor of all sialic acids is N-acetyl D-mannosamine (ManNAc). By recent findings it could be shown that synthetic N-acyl-modified D-mannosamines can be taken up by cells and efficiently metabolized to the respective N-acyl-modified neuraminic acids in vitro and in vivo. Successfully employed D-mannosamines with modified N-acyl side chains include N-propanoyl- (ManNProp), N-butanoyl- (ManNBut)-, N-pentanoyl- (ManNPent), N-hexanoyl- (ManNHex), N-crotonoyl- (ManNCrot), N-levulinoyl- (ManNLev), N-glycolyl- (ManNGc), and N-azidoacetyl D-mannosamine (ManNAc-azido). All of these compounds are metabolized by the promiscuous sialic acid biosynthetic pathway and are incorporated into cell surface sialoglycoconjugates replacing in a cell type-specific manner 10-85% of normal sialic acids. Application of these compounds to different biological systems has revealed important and unexpected functions of the N-acyl side chain of sialic acids, including its crucial role for the interaction of different viruses with their sialylated host cell receptors. Also, treatment with ManNProp, which contains only one additional methylene group compared to the physiological precursor ManNAc, induced proliferation of astrocytes, microglia, and peripheral T-lymphocytes. Unique, chemically reactive ketone and azido groups can be introduced biosynthetically into cell surface sialoglycans using N-acyl-modified sialic acid precursors, a process offering a variety of applications including the generation of artificial cellular receptors for viral gene delivery. This group of novel sialic acid precursors enabled studies on sialic acid modifications on the surface of living cells and has improved our understanding of carbohydrate receptors in their native environment. The biochemical engineering of the side chain of sialic acid offers new tools to study its biological relevance and to exploit it as a tag for therapeutic and diagnostic applications.

Incorporation of N-propanoylneuraminic acid leads to calcium oscillations in oligodendrocytes upon the application of GABA

Sialylation of glycoproteins and glycolipids plays an important role during development, regeneration and pathogenesis. It has been shown that unnatural sialylation within glial cell cultures can have distinct effects on their proliferation and antigenic profiles. These cultures metabolize N-propanoylmannosamine (N-propanoylneuraminic acid precursor=P-NAP), a synthetic non-physiological precursor of neuraminic acid, resulting in the expression of N-propanoylneuraminic acid in glycoconjugates of their cell membranes [Schmidt, C., Stehling, P., Schnitzer, J., Reutter, W. and Horstkorte, R. (1998) J. Biol. Chem. 273, 19146-19152]. To determine whether these biochemically engineered sialic acids influence calcium concentrations in cells of the oligodendrocyte lineage, mixed glial cultures of oligodendrocytes growing on top of an astrocyte monolayer were exposed to glutamate, histamine, adrenaline, gamma-aminobutyric acid (GABA), high potassium (high K(+)) and ATP. Calcium responses in P-NAP-treated oligodendrocytes were determined by confocal microscopy with the calcium indicator fluo-3 AM, and compared with control cultures. We showed that P-NAP differentially modulated the calcium responses of individual oligodendrocytes when GABA was applied. GABA induced calcium oscillations with up to four spikes per min in 60% of oligodendrocytes when treated with P-NAP.