N-glycolylneuraminic acid deficiency in humans

A systematic review reveals conflicting evidence for the prevalence of antibodies against the sialic acid 'xenoautoantigen' Neu5Gc in humans and the need for a standardised approach to quantification

Despite an array of hypothesised implications for health, disease, and therapeutic development, antibodies against the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) remain a subject of much debate. This systematic review of 114 publications aimed to generate a comprehensive overview of published studies in this field, addressing both the reported prevalence of anti-Neu5Gc antibodies in the human population and whether experimental variation accounts for the conflicting reports about the extent of this response. Absolute titres of anti-Neu5Gc antibodies, the reported prevalence of these antibodies, and the individual variation observed within experiments were analysed and grouped according to biological context ('inflammation', 'xenotransplantation', 'biotherapeutic use', 'cancer', and 'healthy populations'), detection method, target epitope selection, and choice of blocking agent. These analyses revealed that the experimental method had a notable impact on both the reported prevalence and absolute titres of anti-Neu5Gc antibodies in the general population, thereby limiting the ability to ascribe reported trends to genuine biological differences or the consequence of experimental design. Overall, this review highlights important knowledge gaps in the study of antibodies against this important xenoautoantigen and the need to establish a standardised method for their quantification if the extent of the importance of Neu5Gc in human health is to be fully understood.

Chimeric antigen receptor T cells targeting the GM3(Neu5Gc) ganglioside

Chimeric antigen receptor (CAR) T cell technology has ushered in a new era of immunotherapy, enabling the targeting of a broad range of surface antigens, surpassing the limitations of traditional T cell epitopes. Despite the wide range of non-protein tumor-associated antigens, the advancement in crafting CAR T cells for these targets has been limited. Owing to an evolutionary defect in the CMP-Neu5Ac hydroxylase (CMAH) that abolishes the synthesis of CMP-Neu5Gc from CMP-Neu5Ac, Neu5Gc is generally absent in human tissues. Despite this, Neu5Gc-containing antigens, including the ganglioside GM3(Neu5Gc) have consistently been observed on tumor cells across a variety of human malignancies. This restricted expression makes GM3(Neu5Gc) an appealing and highly specific target for immunotherapy. In this study, we designed and evaluated 14F7-28z CAR T cells, with a targeting unit derived from the GM3(Neu5Gc)-specific murine antibody 14F7. These cells exhibited exceptional specificity, proficiently targeting GM3(Neu5Gc)-expressing murine tumor cells in syngeneic mouse models, ranging from B cell malignancies to epithelial tumors, without compromising safety. Notably, human tumor cells enhanced with murine Cmah were effectively targeted and eliminated by the 14F7 CAR T cells. Nonetheless, despite the detectable presence of GM3(Neu5Gc) in unmodified human tumor xenografts, the levels were insufficient to trigger a tumoricidal T-cell response with the current CAR T cell configuration. Overall, our findings highlight the potential of targeting the GM3(Neu5Gc) ganglioside using CAR T cells across a variety of cancers and set the stage for the optimization of 14F7-based therapies for future human clinical application.

Low levels of endogenous cholinesterases support the choice of cows, sheep and goats for the transgenic expression of human butyrylcholinesterase in milk

Butyrylcholinesterase purified from human plasma (Hu BChE) as well as recombinant (r) Hu BChE are candidate enzymes that can protect humans from toxicity of organophosphorus compounds (OPs). Domestic animals such as cows, pigs, sheep, and goats have been used for the transgenic expression of a variety of valuable therapeutic proteins. Indeed, rHu BChE was successfully expressed in the milk of transgenic goats, but the presence of any endogenous cholinesterases (ChE) in milk would interfere with the isolation of expressed rHu BChE. The aim of this study was to determine the presence of endogenous ChEs in bovine, ovine, caprine, and porcine milk to determine the suitability of these species for the production of rHu BChE. Using acetyl- and butyryl- thiocholine as substrates, ChE activity (2-4 U/mL) was detected in pig milk only. ChE activities in milk from other animals were <0.01 U/mL and could only be detected following enrichment on procainamide-Sepharose gel. Two different methods based on measuring activity in the presence of acetylcholinesterase (AChE)- or BChE- specific inhibitors were used to estimate the proportions of AChE and BChE activities in enriched milk. Monoclonal antibodies (MAbs), against fetal bovine serum AChE that recognize AChEs from ruminants only, were also used to confirm the identity of AChEs. While bovine and ovine milk contain both AChE and BChE activities, caprine and porcine milk contain predominantly BChE activity. The presence of very low ChE activity supports the choice of cows, sheep, and goats for the transgenic expression of rHu BChE in milk.

The generation of 5-N-glycolylneuraminic acid as a consequence of high levels of reactive oxygen species

The presence of N-glycolylneuraminic acid (Neu5Gc), a non-human sialic acid in cancer patients, is currently attributed to the consumption of red meat. Excess dietary red meat has been considered a risk factor causing chronic inflammation and for the development of cancers. However, it remains unknown whether Neu5Gc can be generated via a chemical reaction rather than via a metabolic pathway in the presence of high levels of reactive oxygen species (ROS) found in the inflammatory and tumor environments. In this study, the conversion of N-acetylneuraminic acid (Neu5Ac) to Neu5Gc has been assessed in vitro under conditions mimicking the hydroxyl radical-rich humoral environment found in inflammatory and cancerous tissues. As a result, Neu5Gc has been detected via liquid chromatography-multiple reaction monitoring mass spectrometry. Furthermore, this conversion has also been found to take place in serum biomatrix containing ROS and in cancer cell cultures with induced ROS production.

Key role of a structural water molecule for the specificity of 14F7-An antitumor antibody targeting the NeuGc GM3 ganglioside

Tumor-associated glycolipids such as NeuGc GM3 are auspicious molecular targets in antineoplastic therapies and vaccine strategies. 14F7 is a monoclonal IgG1 with high clinical potential in cancer immunotherapy as it displays extraordinary specificity for NeuGc GM3, while it does not recognize the very similar, ubiquitous NeuAc GM3. Here we present the 2.3 Å crystal structure of the 14F7 antigen-binding domain (14F7 scFv) in complex with the NeuGc GM3 trisaccharide. Modeling analysis and previous mutagenesis data suggest that 14F7 may also bind to an alternative NeuGc GM3 conformation, not observed in the crystal structure. The most intriguing finding, however, was that a water molecule centrally placed in the complementarity-determining region directly mediates the specificity of 14F7 to NeuGc GM3. This has profound impact on the complexity of engineering in the binding site and provides an excellent example of the importance in understanding the water structure in antibody-antigen interactions.

Are there specific antibodies against Neu5Gc epitopes in the blood of healthy individuals?

Strong discrepancies in published data on the levels and epitope specificities of antibodies against the xenogenic N-glycolyl forms of sialoglycans (Hanganutziu-Deicher Neu5Gcɑ2-3Galβ1-4Glc and related antigens) in healthy donors prompted us to carry out a systematic study in this area using the printed glycan array and other methods. This article summarizes and discusses our published and previously unpublished data, as well as publicly available data from the Consortium for Functional Glycomics. As a result, we conclude that (1) the level of antibodies referred to as anti-Neu5Gc in healthy individuals is low; (2) there are antibodies that seem to interact with Neu5Gc-containing epitopes, but in fact they recognize internal fragments of Neu5Gc-containing glycans (without sialic acids), which served as antigens in the assays used and; (3) a population capable of interacting specifically with Neu5Gc (it does not bind the corresponding NAc analogs) does exist, but it binds the monosaccharide Neu5Gc better than the entire glycans containing it. In other words, in healthy donors, there are populations of antibodies capable of binding the Neu5Gc monosaccharide or the inner core -Galβ1-4Glc, but very few true anti-Neu5Gcɑ2-3Galβ1-4Glc antibodies, i.e., antibodies capable of specifically recognizing the entire trisaccharide.

Identification of chicken-derived scFv against N-glycolylneuraminic acid retrieved from an immune library by phage display

N- glycolylneuraminic acid (Neu5Gc) is a type of sialic acid, it can be synthesized by a range of mammals except chickens and healthy human. After entering human body, Neu5Gc in foods such as red meat and milk can cause chronic inflammation, thus promoting the development of cancer and related diseases. In this study, we identified a gene sequence of Neu5Gc-specific single-chain variable fragment (ScFv) by phage display from a primary chicken antibodies library. Then the gene sequence was used to express a 29 kDa anti-Neu5Gc ScFv protein as detection probe in competitive inhibition ELISA (IC-ELISA). The linear regression equation of the IC-ELISA was y = 23.12x+33.19 (R = 0.980), and the half-maximal inhibitory concentration (IC50) and the limit of detection (LOD) was 5.333 and 0.66 μg/mL. The mean recovery of the spiked samples was 83.04%, and the intra-assay and inter-assay coefficients of variation (CVs) were both 5.59%. The results suggested that the specific anti-Neu5Gc ScFv is a promising probe for the development of IC-ELISA and test strip in order to detect the presence of Neu5Gc in red meat, milk, and tumor tissues.

Association between Inflammation and Function of Cell Adhesion Molecules Influence on Gastrointestinal Cancer Development

Gastrointestinal cancer is highly associated with inflammatory processes inducing the release of cytokines from cancer or immune cells, including interferons, interleukins, chemokines, colony-stimulating factors, and growth factors, which promote or suppress tumor progression. Inflammatory cytokines within the tumor microenvironment promote immune cell infiltration. Infiltrating immune, and tumor-surrounding stromal cells support tumor growth, angiogenesis, metastasis, and immunosuppression through communication with inflammatory cytokines and cell adhesion molecules. Notably, infiltrating immune and tumor cells present immunosuppressive molecules, such as programmed death-ligand 1 (PD-L1) and CD80/CD86. Suppression of cytotoxic T cells promotes tumor avoidance of immune surveillance and greater malignancy. Moreover, glycosylation and sialylation of proteins hyperexpressed on the cancer cell surface have been shown to enhance immune escape and metastasis. Cytokine treatments and immune checkpoint inhibitors are widely used in clinical practice. However, the tumor microenvironment is a rapidly changing milieu involving several factors. In this review, we have provided a summary of the interactions of inflammation and cell adhesion molecules between cancer and other cell types, to improve understanding of the tumor microenvironment.

Tissue specific expression of sialic acid metabolic pathway: role in GNE myopathy

GNE myopathy is an adult-onset degenerative muscle disease that leads to extreme disability in patients. Biallelic mutations in the rate-limiting enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine-kinase (GNE) of sialic acid (SA) biosynthetic pathway, was shown to be the cause of this disease. Other genetic disorders with muscle pathology where defects in glycosylation are known. It is yet not clear why a defect in SA biosynthesis and glycosylation affect muscle cells selectively even though they are ubiquitously present in all tissues. Here we have comprehensively examined the complete SA metabolic pathway involving biosynthesis, sialylation, salvage, and catabolism. To understand the reason for tissue-specific phenotype caused by mutations in genes of this pathway, we analysed the expression of different SA pathway genes in various tissues, during the muscle tissue development and in muscle tissues from GNE myopathy patients (p.Met743Thr) using publicly available databases. We have also analysed gene co-expression networks with GNE in different tissues as well as gene interactions that are unique to muscle tissues only. The results do show a few muscle specific interactions involving ANLN, MYO16 and PRAMEF25 that could be involved in specific phenotype. Overall, our results suggest that SA biosynthetic and catabolic genes are expressed at a very low level in skeletal muscles that also display a unique gene interaction network.

Underlying Glycans Determine the Ability of Sialylated Lipooligosaccharide To Protect Nontypeable Haemophilus influenzae from Serum IgM and Complement

Nontypeable Haemophilus influenzae (NTHi) efficiently colonizes the human nasopharynx asymptomatically but also causes respiratory mucosal infections, including otitis media, sinusitis, and bronchitis. The lipooligosaccharide (LOS) on the cell surface of NTHi displays complex glycans that mimic host structures, allowing it to evade immune recognition. However, LOS glycans are also targets of host adaptive and innate responses. To aid in evasion of these responses, LOS structures exhibit interstrain heterogeneity and are also subject to phase variation, the random on/off switching of gene expression, generating intrastrain population diversity. Specific LOS modifications, including terminal sialylation of the LOS, which exploits host-derived sialic acid (Neu5Ac), can also block recognition of NTHi by bactericidal IgM and complement by mechanisms that are not fully understood. We investigated the LOS sialic acid-mediated resistance of NTHi to antibody-directed killing by serum complement. We identified specific LOS structures extending from heptose III that are targets for binding by naturally occurring bactericidal IgM in serum and are protected by sialylation of the LOS. Phase-variable galactosyltransferases encoded by lic2A and lgtC each add a galactose epitope bound by IgM that results in antibody-dependent killing via the classical pathway of complement. NTHi's survival can be influenced by the expression of phase-variable structures on the LOS that may also depend on environmental conditions, such as the availability of free sialic acid. Identification of surface structures on NTHi representing potential targets for antibody-based therapies as alternatives to antibiotic treatment would thus be valuable for this medically important pathogen.

The Symmetry of Viral Sialic Acid Binding Sites-Implications for Antiviral Strategies

Virus infections are initiated by the attachment of the viral particle to protein or carbohydrate receptors on the host cell. Sialic acid-bearing glycan structures are prominently displayed at the cell surface, and, consequently, these structures can function as receptors for a large number of diverse viruses. Structural biology research has helped to establish the molecular bases for many virus–sialic acid interactions. Due to the icosahedral 532 point group symmetry that underlies many viral capsids, the receptor binding sites are frequently arranged in a highly symmetric fashion and linked by five-fold, three-fold, or two-fold rotation axes. For the inhibition of viral attachment, one emerging strategy is based on developing multivalent sialic acid-based inhibitors that can simultaneously engage several of these binding sites, thus binding viral capsids with high avidity. In this review, we will evaluate the structures of non-enveloped virus capsid proteins bound to sialylated glycan receptors and discuss the potential of these structures for the development of potent antiviral attachment inhibitors.

Nontypeable Haemophilus influenzae Has Evolved Preferential Use of N-Acetylneuraminic Acid as a Host Adaptation

Host-adapted bacterial pathogens such as NTHi cannot survive out of their host environment and have evolved host-specific mechanisms to obtain nutrients and evade the immune response. Relatively few of these host adaptations have been characterized at the molecular level. NTHi utilizes sialic acid as a nutrient and also incorporates this sugar into LOS, which is important in biofilm formation and immune evasion. In the present study, we showed that NTHi has evolved to preferentially utilize the Neu5Ac form of sialic acid. This adaptation is due to the substrate preference of the enzyme CMP-Neu5Ac synthetase, which synthesizes the activated form of Neu5Ac for macromolecule biosynthesis. This adaptation allows NTHi to evade killing by a human antibody response against the nonhuman sialic acid Neu5Gc.

Nontypeable Haemophilus influenzae (NTHi) is a Gram-negative bacterial pathogen that is adapted exclusively to human hosts. NTHi utilizes sialic acid from the host as a carbon source and as a terminal sugar on the outer membrane glycolipid lipooligosaccharide (LOS). Sialic acid expressed on LOS is critical in NTHi biofilm formation and immune evasion. There are two major forms of sialic acids in most mammals, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), the latter of which is derived from Neu5Ac. Humans lack the enzyme to convert Neu5Ac to Neu5Gc and do not express Neu5Gc in normal tissues; instead, Neu5Gc is recognized as a foreign antigen. A recent study showed that dietary Neu5Gc can be acquired by NTHi colonizing humans and then presented on LOS, which acts as an antigen for the initial induction of anti-Neu5Gc antibodies. Here we examined Neu5Gc uptake and presentation on NTHi LOS. We show that, although Neu5Gc and Neu5Ac are utilized equally well as sole carbon sources, Neu5Gc is not incorporated efficiently into LOS. When equal amounts of Neu5Gc and Neu5Ac are provided in culture media, there is ∼4-fold more Neu5Ac incorporated into LOS, suggesting a bias in a step of the LOS biosynthetic pathway. CMP-Neu5Ac synthetase (SiaB) was shown to have ∼4,000-fold-higher catalytic efficiency for Neu5Ac than for Neu5Gc. These data suggest that NTHi has adapted preferential utilization of Neu5Ac, thus avoiding presentation of the nonhuman Neu5Gc in the bacterial cell surface. The selective pressure for this adaptation may represent the human antibody response to the Neu5Gc xenoantigen.

Identification of mesothelioma-specific sialylated epitope recognized with monoclonal antibody SKM9-2 in a mucin-like membrane protein HEG1

The anti-mesothelioma mAb SKM9-2 recognizes the sialylated protein HEG homolog 1 (HEG1). HEG1 is a 400 kDa mucin-like membrane protein found on mesothelioma. SKM9-2 can detect mesothelioma more specifically and sensitively than other antibodies against current mesothelioma markers; therefore, SKM9-2 would be likely useful for the precise detection and diagnosis of malignant mesothelioma. In the present study, we investigated the epitope of SKM9-2. We analyzed the binding of SKM9-2 to truncated HEG1 and candidate epitope-fused glycosylphosphatidylinositol-anchor proteins. The epitope of SKM9-2 was identified as an O-glycosylated region, 893-SKSPSLVSLPT-903, in HEG1. An alanine scanning assay of the epitope showed that SKM9-2 bound to a simple epitope in HEG1, and the SKxPSxVS sequence within the epitope was essential for SKM9-2 recognition. Mass spectrometry analysis and lectin binding analysis of soluble epitope peptides indicated that the SKM9-2 epitope, in which Ser⁸⁹⁷ was not glycosylated, contained two disialylated core 1 O-linked glycan-modified serine residues, Ser⁸⁹³ and Ser⁹⁰⁰. Neuraminidase treatment analysis also confirmed that the epitope in mesothelioma cells contained a similar glycan modification. The specific detection of mesothelioma with SKM9-2 can thus be performed by the recognition of sialylated glycan modification in the specific region of HEG1.

Crystal structure of an L chain optimised 14F7 anti-ganglioside Fv suggests a unique tumour-specificity through an unusual H-chain CDR3 architecture

Targeted cancer immunotherapy offers increased efficacy concomitantly with reduced side effects. One antibody with promising clinical potential is 14F7, which specifically recognises the NeuGc GM3 ganglioside. This antigen is found in the plasma membrane of a range of tumours, but is essentially absent from healthy human cells. 14F7 can discriminate NeuGc GM3 from the very similar NeuAc GM3, a common component of cell membranes. The molecular basis for this unique specificity is poorly understood. Here we designed and expressed 14F7-derived single-chain Fvs (scFvs), which retained the specificity of the parent antibody. Detailed expression and purification protocols are described as well as the synthesis of the NeuGc GM3 trisaccharide. The most successful scFv construct, which comprises an alternative variable light chain (V_LA), allowed structure determination to 2.2 Å resolution. The structure gives insights into the conformation of the important CDR H3 loop and the suspected antigen binding site. Furthermore, the presence of V_LA instead of the original V_L elucidates how this subdomain indirectly stabilises the CDR H3 loop. The current work may serve as a guideline for the efficient production of scFvs for structure determination.

Evolution of sialic acids: Implications in xenotransplant biology

All living cells are covered with a dense “sugar-coat” of carbohydrate chains (glycans) conjugated to proteins and lipids. The cell surface glycome is determined by a non-template driven process related to the collection of enzymes that assemble glycans in a sequential manner. In mammals, many of these glycans are topped with sialic acids (Sia), a large family of acidic sugars. The “Sialome” is highly diverse owing to various Sia types, linkage to underlying glycans, range of carriers, and complex spatial organization. Presented at the front of cells, Sia play a major role in immunity and recognition of “self” versus “non-self,” largely mediated by the siglecs family of Sia-binding host receptors. Albeit many mammalian pathogens have evolved to hijack this recognition system to avoid host immune attack, presenting a fascinating host-pathogen evolutionary arms race. Similarly, cancer cells exploit Sia for their own survival and propagation. As part of this ongoing fitness, humans lost the ability to synthesize the Sia type N-glycolylneuraminic acid (Neu5Gc), in contrast to other mammals. While this loss had provided an advantage against certain pathogens, humans are continuously exposed to Neu5Gc through mammalian-derived diet (eg, red meat), consequently generating a complex immune response against it. Circulating anti-Neu5Gc antibodies together with Neu5Gc on some human tissues mediate chronic inflammation “xenosialitis” that exacerbate various human diseases (eg, cancer and atherosclerosis). Similarly, Neu5Gc-containing xenografts are exposed to human anti-Neu5Gc antibodies with implications to sustainability. This review aimed to provide a glimpse into the evolution of Sia and their implications to xenotransplantation.

Hypothesis: Hypoxia induces de novo synthesis of NeuGc gangliosides in humans through CMAH domain substitute

Immunotherapy is a growing field in cancer research. A privileged tumor-associated antigen that has received much attention is N-glycolyl (NeuGc) GM3. This ganglioside is present in several types of cancer, but is almost undetectable in human healthy tissues. However, its non-hydroxylated variant, NeuAc GM3, is abundant in all mammals. Due to a deletion in the human gene encoding the key enzyme for synthesis of NeuGc, humans, in contrast to other mammals, cannot synthesize NeuGc GM3. Therefore the presence of this ganglioside in human cancer cells represents an enigma. It has been shown that hypoxic conditions trigger the expression of NeuGc gangliosides, which not only serve as attractive targets for cancer therapy, but also as diagnostic and prognostic tumor marker. Here, we confirm hypoxia-induced expression of the NeuGc GM3 ganglioside also in HeLa cells and reveal several candidate proteins, in particular GM3 synthase and subunit B of respiratory complex II (SDHB), that may be involved in the generation of NeuGc GM3 by SILAC-based proteome analysis. These findings have the potential to significantly advance our understanding of how this enigmatic tumor-associated antigen is produced in humans, and also suggest a possible mechanism of action of anti-tumor antibodies that recognize hypoxia markers, such as 14F7.

Altered expression of ganglioside GM3 molecular species and a potential regulatory role during myoblast differentiation

Gangliosides (sialic acid-containing glycosphingolipids) help regulate many important biological processes, including cell proliferation, signal transduction, and differentiation, via formation of functional microdomains in plasma membranes. The structural diversity of gangliosides arises from both the ceramide moiety and glycan portion. Recently, differing molecular species of a given ganglioside are suggested to have distinct biological properties and regulate specific and distinct biological events. Elucidation of the function of each molecular species is important and will provide new insights into ganglioside biology. Gangliosides are also suggested to be involved in skeletal muscle differentiation; however, the differential roles of ganglioside molecular species remain unclear. Here we describe striking changes in quantity and quality of gangliosides (particularly GM3) during differentiation of mouse C2C12 myoblast cells and key roles played by distinct GM3 molecular species at each step of the process.

Murine Plasma N-Glycosylation Traits Associated with Sex and Strain

Glycosylation is an abundant and important protein modification with large influence on the properties and interactions of glycoconjugates. Human plasma N-glycosylation has been the subject of frequent investigation, revealing strong associations with physiological and pathological conditions. Less well-characterized is the plasma N-glycosylation of the mouse, the most commonly used animal model for studying human diseases, particularly with regard to differences between strains and sexes. For this reason, we used MALDI-TOF(/TOF)-MS(/MS) assisted by linkage-specific derivatization of the sialic acids to comparatively analyze the plasma N-glycosylation of both male and female mice originating from BALB/c, CD57BL/6, CD-1, and Swiss Webster strains. The combined use of this analytical method and the recently developed data processing software named MassyTools allowed the relative quantification of the N-glycan species within plasma, the distinction between α2,3- and α2,6-linked N-glycolylneuraminic acids (due to respective lactonization and ethyl esterification), the detection of sialic acid O-acetylation, as well as the characterization of branching sialylation (Neu5Gcα2,3-Hex-[Neu5Gcα2,6-]HexNAc). When analyzing the glycosylation according to mouse sex, we found that female mice present a considerably higher degree of core fucosylation (2-4-fold depending on the strain), galactosylation, α2,6-linked sialylation, and larger high-mannose type glycan species compared with their male counterparts. Male mice, on the contrary, showed on average higher α2,3-linked sialylation, branching sialylation, and putative bisection. These differences together with sialic acid acetylation proved to be strain-specific as well. Interestingly, the outbred strains CD-1 and Swiss Webster displayed considerably larger interindividual variation than inbred strains BALB/c and CD57BL/6, suggesting a strong hereditable component of the observed plasma N-glycome.

Functional analysis of glyco-molecules that bind with influenza virus

Influenza A virus (IAV) recognizes terminal sialic acid of sialoglyco-conjugates on host cells through the viral envelope glycoprotein hemagglutinin (HA), followed by initiation of entry into the cells. Molecular species of sialic acid are largely divided into two moieties: N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). A receptor for IAV infection generally means Neu5Ac. Almost all equine IAVs and some human, swine, and duck IAVs bind not only to Neu5Ac but also to Neu5Gc. In nonhuman animals, Neu5Gc has been detected in swine and equine tracheas and the duck colon, which are the main replication sites of mammalian and avian IAVs. Therefore, Neu5Gc in these sites has been suggested to be a functional receptor for IAV infection. Humans cannot synthesize Neu5Gc due to a genetic defect of the Neu5Gc-synthesizing enzyme. We evaluated the receptor function of Neu5Gc in IAV infection in human cells. Our results indicated that Neu5Gc expression on the surface of human cells is not a functional receptor for IAV infection and that it has a negative effect on infectivity of IAV possessing Neu5Gc binding ability. IAV also binds to non-sialo 3-O-sulfated galactosylceramide (sulfatide). Sulfatide has been suggested to be a functional receptor for IAV infection. However, we have shown that sulfatide is not a functional receptor for IAV infection and that the binding of HA with sulfatide enhances progeny virus production. It is expected that functions of these glyco-molecules can be used in prevention and development of new drugs against IAV.

Pharmacokinetics and immunogenicity of a recombinant human butyrylcholinesterase bioscavenger in macaques following intravenous and pulmonary delivery

Recombinant (r) and native butyrylcholinesterse (BChE) are potent bioscavengers of organophosphates (OPs) such as nerve agents and pesticides and are undergoing development as antidotal treatments for OP-induced toxicity. Because of the lethal properties of such agents, regulatory approval will require extensive testing under the Animal Rule. However, human (Hu) glycoprotein biologicals, such as BChE, present a challenge for assessing immunogenicity and efficacy in heterologous animal models since any immune responses to the small species differences in amino acids or glycans between the host and biologic may alter pharmacodynamics and preclude accurate efficacy testing; possibly underestimating their potential protective value in humans. To establish accurate pharmacokinetic and efficacy data, an homologous animal model has been developed in which native and PEGylated forms of CHO-derived rMaBChE were multiply injected into homologous macaques with no induction of antibody. These now serve as controls for assessing the pharmacokinetics and immunogenicity in macaques of multiple administrations of PEGylated and unmodified human rBChE (rHuBChE) by both intravenous (IV) and pulmonary routes. The results indicate that, except for maximal concentration (Cmax), the pharmacokinetic parameters following IV injection with heterologous PEG-rHuBChE were greatly reduced even after the first injection compared with homologous PEG-rMaBChE. Anti-HuBChE antibody responses were induced in all monkeys after the second and third administrations regardless of the route of delivery; impacting rates of clearance and usually resulting in reduced endogenous MaBChE activity. These data highlight the difficulties inherent in assessing pharmacokinetics and immunogenicity in animal models, but bode well for the efficacy and safety of rHuBChE pretreatments in homologous humans.

Microfluidic Chip-LC/MS-based Glycomic Analysis Revealed Distinct N-glycan Profile of Rat Serum

The rat is an important alternative for studying human pathology owing to certain similarities to humans. Glycomic studies on rat serum have revealed that variations in the N-glycans of glycoproteins correlated with disease progression, which is consistent with the findings in human serum. Therefore, we comprehensively characterized the rat serum N-glycome using microfluidic chip-LC-ESI-QTOF MS and MS/MS techniques. In total, 282 N-glycans, including isomers, were identified. This study is the first to present comprehensive profiling of N-glycans containing O-acetylated sialic acid, among which 27 N-glycans are novel. In addition, the co-existence of N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc) in a single N-glycan ('mixed' N-glycan) was detected and represents a new type of N-glycan in rat serum. The existence of O-acetylated sialic acid is the characteristic feature of rat serum that distinguishes it from mouse and human sera. Comparisons between the rat, mouse, and human serum glycomes revealed that the rat glycome is more similar to that of human sera than to that of mouse sera. Our findings highlight the similarities between the glycomic profile of rat and human sera and provided important selection criteria for choosing an appropriate animal model for pathological and pharmacological studies.

Molecular subtyping of metastatic melanoma based on cell ganglioside metabolism profiles

BACKGROUND

In addition to alterations concerning the expression of oncogenes and onco-suppressors, melanoma is characterized by the presence of distinctive gangliosides (sialic acid carrying glycosphingolipids). Gangliosides strongly control cell surface dynamics and signaling; therefore, it could be assumed that these alterations are linked to modifications of cell behavior acquired by the tumor. On these bases, this work investigated the correlations between melanoma cell ganglioside metabolism profiles and the biological features of the tumor and the survival of patients.

METHODS

Melanoma cell lines were established from surgical specimens of AJCC stage III and IV melanoma patients. Sphingolipid analysis was carried out on melanoma cell lines and melanocytes through cell metabolic labeling employing [3-3H]sphingosine and by FACS. N-glycolyl GM3 was identified employing the 14 F7 antibody. Gene expression was assayed by Real Time PCR. Cell invasiveness was assayed through a Matrigel invasion assay; cell proliferation was determined through the soft agar assay, MTT, and [3H] thymidine incorporation. Statistical analysis was performed using XLSTAT software for melanoma hierarchical clustering based on ganglioside profile, the Kaplan-Meier method, the log-rank (Mantel-Cox) test, and the Mantel-Haenszel test for survival analysis.

RESULTS

Based on the ganglioside profiles, through a hierarchical clustering, we classified melanoma cells isolated from patients into three clusters: 1) cluster 1, characterized by high content of GM3, mainly in the form of N-glycolyl GM3, and GD3; 2) cluster 2, characterized by the appearance of complex gangliosides and by a low content of GM3; 3) cluster 3, which showed an intermediate phenotype between cluster 1 and cluster 3. Moreover, our data demonstrated that: a) a correlation could be traced between patients' survival and clusters based on ganglioside profiles, with cluster 1 showing the worst survival; b) the expression of several enzymes (sialidase NEU3, GM2 and GM1 synthases) involved in ganglioside metabolism was associated with patients' survival; c) melanoma clusters showed different malignant features such as growth in soft agar, invasiveness, expression of anti-apoptotic proteins.

CONCLUSIONS

Ganglioside profile and metabolism is strictly interconnected with melanoma aggressiveness. Therefore, the profiling of melanoma gangliosides and enzymes involved in their metabolism could represent a useful prognostic and diagnostic tool.

Erythrocytes from GGTA1/CMAH knockout pigs: implications for xenotransfusion and testing in non-human primates

BACKGROUND

Pig erythrocytes are potentially useful to solve the worldwide shortage of human blood for transfusion. Domestic pig erythrocytes, however, express antigens that are bound by human preformed antibodies. Advances in genetic engineering have made it possible to rapidly knock out the genes of multiple xenoantigens, namely galactose α1,3 galactose (aGal) and N-glycolylneuraminic acid (Neu5Gc). We have recently targeted the GGTA1 and CMAH genes with zinc finger endonucleases resulting in double knockout pigs that no longer express aGal or Neu5Gc and attract significantly fewer human antibodies. In this study, we characterized erythrocytes from domestic and genetically modified pigs, baboons, chimpanzees, and humans for binding of human and baboon natural antibody, and complement-mediated lysis.

METHODS

Distribution of anti-Neu5Gc IgG and IgM in pooled human AB serum was analyzed by ELISA. Erythrocytes from domestic pigs (Dom), aGal knockout pigs (GGTA1 KO), aGal and Neu5Gc double knockout pigs (GGTA1/CMAH KO), baboons, chimpanzees, and humans were analyzed by flow cytometry for aGal and Neu5Gc expression. In vitro comparative analysis of erythrocytes was conducted with pooled human AB serum and baboon serum. Total antibody binding was accessed by hemagglutination; complement-dependent lysis was measured by hemolytic assay; IgG or IgM binding to erythrocytes was characterized by flow cytometry.

RESULTS

The pooled human AB serum contained 0.38 μg/ml anti-Neu5Gc IgG and 0.085 μg/ml anti-Neu5Gc IgM. Both Gal and Neu5Gc were not detectable on GGTA1/CMAH KO erythrocytes. Hemagglutination of GGTA1/CMAH KO erythrocytes with human serum was 3.5-fold lower compared with GGTA1 KO erythrocytes, but 1.6-fold greater when agglutinated with baboon serum. Hemolysis of GGTA1/CMAH KO erythrocytes by human serum (25%) was reduced 9-fold compared with GGTA1 KO erythrocytes, but increased 1.64-fold by baboon serum. Human IgG binding was reduced 27-fold on GGTA1/CMAH KO erythrocytes compared with GGTA1 KO erythrocytes, but markedly increased 3-fold by baboon serum IgG. Human IgM binding was decreased 227-fold on GGTA1/CMAH KO erythrocytes compared with GGTA1 KO erythrocytes, but enhanced 5-fold by baboon serum IgM.

CONCLUSIONS

Removal of aGal and Neu5Gc antigens from pig erythrocytes significantly reduced human preformed antibody-mediated cytotoxicity but may have complicated future in vivo analysis by enhancing reactivity from baboons. The creation of the GGTA1/CMAH KO pig has provided the xenotransplantation researcher with organs and cells that attract fewer human antibodies than baboon and our closest primate relative, chimpanzee. These finding suggest that while GGTA1/CMAH KO erythrocytes may be useful for human transfusions, in vivo testing in the baboon may not provide a direct transplantation to the clinic.

N-glycolylneuraminic acid on human epithelial cells prevents entry of influenza A viruses that possess N-glycolylneuraminic acid binding ability

Some animal influenza A viruses (IAVs) bind not only to N-acetylneuraminic acid (Neu5Ac) but also to N-glycolylneuraminic acid (Neu5Gc), which has been discussed as a virus receptor. Human cells cannot synthesize Neu5Gc due to dysfunction of the CMP-Neu5Ac hydroxylase (CMAH) gene, which converts CMP-Neu5Ac to CMP-Neu5Gc. However, exogenous Neu5Gc from Neu5Gc-rich dietary sources is able to be metabolically incorporated into surfaces of tissue cells and may be related to enhancement of the infectivity and severity of IAV. Here, we investigated the receptor function of Neu5Gc on IAV infection in Neu5Gc-expressing cells by transfection of the monkey CMAH gene into human cells or by incubation with human cells in the presence of N-glycolylmannosamine. Expression of Neu5Gc on human cells clearly suppressed infectivity of IAVs that possess Neu5Gc binding ability. Furthermore, there was no difference in infectivity of a transfectant virus that included the wild-type HA gene from A/Memphis/1/1971 (H3N2), which shows no Neu5Gc binding, between parent MCF7 cells and cells stably expressing the monkey CMAH gene (CMAH-MCF7 cells). On the other hand, cell entry of the transfectant virus that included the Neu5Gc-binding HA gene with a single mutation to Tyr at position Thr155 was arrested at the stage of internalization from the plasma membrane of the CMAH-MCF7 cells. These results indicate that expression of Neu5Gc on the surface of human epithelial cells suppresses infection of IAVs that possess Neu5Gc binding ability. Neu5Gc is suggested to work as a decoy receptor of Neu5Gc-binding IAVs but not a functional receptor for IAV infection.

IMPORTANCE

Influenza A viruses (IAVs) bind to the host cell surfaces through sialic acids at the terminal of glycoconjugates. For IAV binding to sialic acids, some IAVs bind not only to N-acetylneuraminic acid (Neu5Ac) as a receptor but also to N-glycolylneuraminic acid (Neu5Gc). Neu5Gc has been discussed as a receptor of human and animal IAVs. Our results showed that Neu5Gc expression on human epithelial cells suppresses infection of IAVs that possess Neu5Gc binding ability. Neu5Gc is suggested to be a "decoy receptor" of Neu5Gc-binding IAVs but not a functional receptor for IAV infection. Human cells cannot synthesize Neu5Gc because of dysfunction of the CMP-N-acetylneuraminic acid hydroxylase gene but can exogenously and metabolically incorporate Neu5Gc from dietary sources. The expression of Neu5Gc on human epithelial cells by taking in exogenous Neu5Gc from Neu5Gc-rich dietary sources may be related to restriction of the infection of IAVs that have acquired Neu5Gc binding ability.

Glycans in immune recognition and response

Glycans at the forefront of cells facilitate immune recognition processes. Cancer cells commonly present altered cell surface glycosylation, especially manifested in the expression of sialic acid at the termini of glycolipids and glycoproteins. Although tumor-associated carbohydrate antigens (TACAs) result in expression of altered-self, most such carbohydrates do not elicit strong humoral responses. Various strategies had been devised to elicit increased immunogenicity of such TACA aiming for potent immunotherapeutic antibodies or cancer vaccines. However some carbohydrates are immunogenic in humans and hold potential for novel glycotherapies. N-Glycolylneuraminic acid (Neu5Gc) is a foreign immunogenic sugar in humans originating from the diet (e.g., red meat) and subsequently expressed on the cell surface, especially accumulating on carcinoma. Consequently, the human immune system detects this non-self carbohydrate generating a broad anti-Neu5Gc antibody response. The co-existence of Neu5Gc/anti-Neu5Gc within humans spurs chronic inflammation mediated disease, including cancer. Concurrently, anti-Neu5Gc antibodies hold potential for novel targeted therapy. αGal is another foreign immunogenic carbohydrate antigen in humans and all humans have circulating anti-Gal antibodies. This review aims to describe the immunogenicity of Neu5Gc and its implications for human diseases, highlighting differences and similarities with αGal and its potential for novel targeted theranostics.

One-pot synthesis of N-acetyl- and N-glycolylneuraminic acid capped trisaccharides and evaluation of their influenza A(H1 N1) inhibition

Human lung epithelial cells natively offer terminal N-acetylneuraminic acid (Neu5Ac) α(2→6)-linked to galactose (Gal) as binding sites for influenza virus hemagglutinin. N-Glycolylneuraminic acid (Neu5Gc) in place of Neu5Ac is known to affect hemagglutinin binding in other species. Not normally generated by humans, Neu5Gc may find its way to human cells from dietary sources. To compare their influence in influenza virus infection, six trisaccharides with Neu5Ac or Neu5Gc α(2→6) linked to Gal and with different reducing end sugar units were prepared using one-pot assembly and divergent transformation. The sugar assembly made use of an N-phthaloyl-protected sialyl imidate for chemoselective activation and α-stereoselective coupling with a thiogalactoside. Assessment of cytopathic effect showed that the Neu5Gc-capped trisaccharides inhibited the viral infection better than their Neu5Ac counterparts.

Interlaboratory study on differential analysis of protein glycosylation by mass spectrometry: the ABRF glycoprotein research multi-institutional study 2012

One of the principal goals of glycoprotein research is to correlate glycan structure and function. Such correlation is necessary in order for one to understand the mechanisms whereby glycoprotein structure elaborates the functions of myriad proteins. The accurate comparison of glycoforms and quantification of glycosites are essential steps in this direction. Mass spectrometry has emerged as a powerful analytical technique in the field of glycoprotein characterization. Its sensitivity, high dynamic range, and mass accuracy provide both quantitative and sequence/structural information. As part of the 2012 ABRF Glycoprotein Research Group study, we explored the use of mass spectrometry and ancillary methodologies to characterize the glycoforms of two sources of human prostate specific antigen (PSA). PSA is used as a tumor marker for prostate cancer, with increasing blood levels used to distinguish between normal and cancer states. The glycans on PSA are believed to be biantennary N-linked, and it has been observed that prostate cancer tissues and cell lines contain more antennae than their benign counterparts. Thus, the ability to quantify differences in glycosylation associated with cancer has the potential to positively impact the use of PSA as a biomarker. We studied standard peptide-based proteomics/glycomics methodologies, including LC-MS/MS for peptide/glycopeptide sequencing and label-free approaches for differential quantification. We performed an interlaboratory study to determine the ability of different laboratories to correctly characterize the differences between glycoforms from two different sources using mass spectrometry methods. We used clustering analysis and ancillary statistical data treatment on the data sets submitted by participating laboratories to obtain a consensus of the glycoforms and abundances. The results demonstrate the relative strengths and weaknesses of top-down glycoproteomics, bottom-up glycoproteomics, and glycomics methods.

Mutation tryptophan to leucine at position 222 of haemagglutinin could facilitate H3N2 influenza A virus infection in dogs

An avian-like H3N2 influenza A virus (IAV) has recently caused sporadic canine influenza outbreaks in China and Korea, but the molecular mechanisms involved in the interspecies transmission of H3N2 IAV from avian to canine species are not well understood. Sequence analysis showed that residue 222 in haemagglutinin (HA) is predominantly tryptophan (W) in the closely related avian H3N2 IAV, but was leucine (L) in canine H3N2 IAV. In this study, reassortant viruses rH3N2-222L (canine-like) and rH3N2-222W (avian-like) with HA mutation L222W were generated using reverse genetics to evaluate the significance of the L222W mutation on receptor binding and host tropism of H3N2 IAV. Compared with rH3N2-222W, rH3N2-222L grew more rapidly in MDCK cells and had significantly higher infectivity in primary canine tracheal epithelial cells. Tissue-binding assays demonstrated that rH3N2-222L had a preference for canine tracheal tissues rather avian tracheal tissues, whereas rH3N2-222W favoured slightly avian rather canine tracheal tissues. Glycan microarray analysis suggested both rH3N2-222L and rH3N2-222W bound preferentially to α2,3-linked sialic acids. However, the rH3N2-222W had more than twofold less binding affinity than rH3N2-222L to a set of glycans with Neu5Aca2-3Galb1-4(Fuca-)-like or Neu5Aca2-3Galb1-3(Fuca-)-like structures. These data suggest the W to L mutation at position 222 of the HA could facilitate infection of H3N2 IAV in dogs, possibly by increasing the binding affinities of the HA to specific receptors with Neu5Aca2-3Galb1-4(Fuca-) or Neu5Aca2-3Galb1-3(Fuca-)-like structures that are present in dogs.

LC-MS/MS quantification of N-acetylneuraminic acid, N-glycolylneuraminic acid and ketodeoxynonulosonic acid levels in the urine and potential relationship with dietary sialic acid intake and disease in 3- to 5-year-old children

Red meat and dairy products contain high sialic acid (Sia) levels, but the metabolic fate and health impact in children remain unknown. The aims of the present study were to quantify the levels of urinary Sia N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc) and ketodeoxynonulosonic acid (KDN) and to determine their relationship with dietary Sia intake. Spot urine samples were collected from 386 healthy children aged 3 (n 108), 4 (n 144) and 5 (n 134) years at 06.30-07.00, 11.30-12.00 and 16.30-17.00 hours. Food intake levels were recorded on the day of urine sample collection. Sia levels were quantified using LC-MS/MS with [13C3]Sia as an internal standard. We found that (1) total urinary Sia levels in healthy pre-school children ranged from 40 to 79 mmol Sia/mol creatinine; (2) urinary Sia levels were independent of age and consisted of conjugated Neu5Ac (approximately 70·8 %), free Neu5Ac (approximately 21·3 %), conjugated KDN (approximately 4·2 %) and free KDN (approximately 3·7 %); Neu5Gc was detected in the urine of only one 4-year-old girl; (3) total urinary Sia levels were highest in the morning and declined over time in 4- and 5-year-old children (P< 0·05), but not in 3-year-old children; (4) Sia intake levels at breakfast and lunch were approximately 2·5 and 0·16 mg Sia/kg body weight; and (5) there was no significant correlation between dietary Sia intake levels and urinary Sia levels. Urinary Sia levels varied with age and time of day, but did not correlate with Sia intake in 3- to 5-year-old children. The difference in urinary Sia levels in children of different age groups suggests that the metabolism and utilisation rates of dietary Sia are age dependent.

Unique gangliosides synthesized in vitro by sialyltransferases from marine bacteria and their characterization: ganglioside synthesis by bacterial sialyltransferases

On the basis of the results outlined in our previous report, bacterial sialyltransferases (ST) from marine sources were further characterized using glycosphingolipids (GSL), especially ganglio-series GSLs, based on the enzymatic characteristics and kinetic parameters obtained by Line weaver-Burk plots. Among them, GA1 and GA2 were found to be good substrates for these unique STs. Thus, new gangliosides synthesized by α2-3 and α2-6STs were structurally characterized by several analytical procedures. The ganglioside generated by the catalytic activity of α2-3ST was identified as GM1b. On the other hand, when enzyme reactions by α2-6STs were performed using substrates GA2 and GA1, very unique gangliosides were generated. The structures were identified as NeuAcα2-6GalNAcβ1-4Galβ1-4Glcβ-Cer and NeuAcα2-6Galβ1-3GalNAcβ1-4Galβ1-4Glcβ-Cer, respectively. The synthesized ganglioside NeuAcα2-6GalNAcβ1-4Galβ1-4Glcβ-Cer showed binding activity to the influenza A virus {A/Panama/2007/99 (H3N2)} at a similar level to purified sialyl(α2-3)paragloboside (S2-3PG) and sialyl(α2-6)paragloboside (S2-6PG) from mammalian sources. The evidence suggests that these STs have unique features, including substrate specificities restricted not only to lacto-series but also to ganglio-series GSLs, as well as catalytic potentials for ganglioside synthesis. This evidence demonstrates that effective in vitro ganglioside synthesis could be a valuable tool for selectively synthesizing sialic acid (Sia) modifications, thereby preparing large-scale gangliosides and permitting the exploration of unknown functions.

Natural Antibodies Against Sialoglycans

Natural antibodies, part of the innate immunity system, are produced at strictly regulated levels in normal sera without immunization and thus are part of the innate immune system. The best studied natural antibodies are those directed against blood group antigens A and B and xeno-antigens including glycolylneuraminic acid containing Hanganutziu-Deicher (HD) glycolipid. Abnormal levels of anti-glycan antibodies were found in a number of pathologies. In many cases pathological antibodies are known to bind gangliosides. The genesis of anti-glycan antibodies in healthy humans and the reasons for their changes in pathologies are poorly understood. With a growing interest in their diagnostic applications, it is important to determine the carbohydrate structures that are recognized by antibodies present in the circulation of healthy individuals. We tested a large number of healthy donors using a printed glycan array (PGA) in a microchip format. The PGA contained ~300 glycans, representing mostly normal mammalian structures of glycoproteins and glycolipids, and many of the structures presented are biologically relevant sialylated motifs. As revealed by PGA, the sera interacted with at least 70 normal human glycans. With only few exceptions, antibodies recognizing sialosides have not been identified. Moderate levels of antibodies and moderate variability were observed in the case of SiaT n and its glycolyl variant. Unexpectedly, we found minimal antibody titer directed against Neu5Gcα and the trisaccharide Neu5Gcα2-6Galβ1-4GlcNAc, although this form of neuraminic acid does not occur naturally in humans. Antibodies recognizing sialosides in unnatural β-configuration have been detected and confirmed Springer's paradigm that circulating antibodies represent a reaction against bacteria. Gram-negative bacteria contain LPS with βKDN and/or βKDO which are very close analogs of Neu5Ac that are found in β-connected form. Antibodies against the biantennary N-glycan chain, (Neu5Acα2-6Galβ1-4GlcNAcβ1-2Manα)2-3,6-Manβ1-4GlcNAcβ1-4GlcNAc were never observed and similarly we never saw antibodies directed against the SiaLe(a)/SiaLe (x) motifs. Anti-sialoglycan antibodies can be masked with gangliosides: for example, we observe about a five times higher level of anti-GD3 in purified total IgG compared to the same concentration of total Ig in the composition of native serum. For several antibodies we observed anomalous binding in diluted sera, namely, the signals towards sialylated glycans were increased in the PGA if diluted sera were used.

α1,3-galactosyltransferase deficiency in germ-free miniature pigs increases N-glycolylneuraminic acids as the xenoantigenic determinant in pig-human xenotransplantation

In this study, we examined whether Hanganutziu-Deicher (H-D) antigens are important as an immunogenic non-α1,3-galactose (Gal) epitope in pigs with a disrupted α1,3-galactosyltransferase gene. The targeting efficiency of the AO blood genotype was achieved (2.2%) in pig fibroblast cells. A total of 1800 somatic cell nuclear transfer (SCNT) embryos were transferred to 10 recipients. One recipient developed to term and naturally delivered two piglets. The α1,3-galactosyltransferase activity in lung, liver, spleen, and testis of heterozygote α1,3-galactosyltransferase gene knockout (GalT-KO) pigs was significantly decreased, whereas brain and heart showed very low decreasing levels of α1,3-galactosyltransferase activity when compared to those of control. Enzyme-linked lectinosorbent assay showed that the heterozygote GalT-KO pig had more sialylα2,6- and sialylα2,3-linked glycan than the control. Furthermore, the heart, liver, and kidney of the heterozygote GalT-KO pig had a higher N-glycolylneuraminic acid (Neu5Gc) content than the control, whereas the lung of the heterozygote GalT-KO pig had Neu5Gc content similar to the control. Collectively, the data strongly indicated that Neu5Gc is a more critical xenoantigen to overcoming the next acute immune rejection in pig to human xenotransplantation.

Metabolism of vertebrate amino sugars with N-glycolyl groups: incorporation of N-glycolylhexosamines into mammalian glycans by feeding N-glycolylgalactosamine

The outermost positions of mammalian cell-surface glycans are predominantly occupied by the sialic acids N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). To date, hydroxylation of CMP-Neu5Ac resulting in the conversion into CMP-Neu5Gc is the only known enzymatic reaction in mammals to synthesize a monosaccharide carrying an N-glycolyl group. In our accompanying paper (Bergfeld, A. K., Pearce, O. M., Diaz, S. L., Pham, T., and Varki, A. (2012) J. Biol. Chem. 287, jbc.M112.363549), we report a metabolic pathway for degradation of Neu5Gc, demonstrating that N-acetylhexosamine pathways are tolerant toward the N-glycolyl substituent of Neu5Gc breakdown products. In this study, we show that exogenously added N-glycolylgalactosamine (GalNGc) serves as a precursor for Neu5Gc de novo biosynthesis, potentially involving seven distinct mammalian enzymes. Following the GalNAc salvage pathway, UDP-GalNGc is epimerized to UDP-GlcNGc, which might compete with the endogenous UDP-GlcNAc for the sialic acid biosynthetic pathway. Using UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase-deficient cells, we confirm that conversion of GalNGc into Neu5Gc depends on this key enzyme of sialic acid biosynthesis. Furthermore, we demonstrate by mass spectrometry that the metabolic intermediates UDP-GalNGc and UDP-GlcNGc serve as substrates for assembly of most major classes of cellular glycans. We show for the first time incorporation of GalNGc and GlcNGc into chondroitin/dermatan sulfates and heparan sulfates, respectively. As demonstrated by structural analysis, N-glycolylated hexosamines were found in cellular gangliosides and incorporated into Chinese hamster ovary cell O-glycans. Remarkably, GalNAc derivatives altered the overall O-glycosylation pattern as indicated by the occurrence of novel O-glycan structures. This study demonstrates that mammalian N-acetylhexosamine pathways and glycan assembly are surprisingly tolerant toward the N-glycolyl substituent.

Metabolism of vertebrate amino sugars with N-glycolyl groups: elucidating the intracellular fate of the non-human sialic acid N-glycolylneuraminic acid

The two major mammalian sialic acids are N-acetylneuraminic acid and N-glycolylneuraminic acid (Neu5Gc). The only known biosynthetic pathway generating Neu5Gc is the conversion of CMP-N-acetylneuraminic acid into CMP-Neu5Gc, which is catalyzed by the CMP-Neu5Ac hydroxylase enzyme. Given the irreversible nature of this reaction, there must be pathways for elimination or degradation of Neu5Gc, which would allow animal cells to adjust Neu5Gc levels to their needs. Although humans are incapable of synthesizing Neu5Gc due to an inactivated CMAH gene, exogenous Neu5Gc from dietary sources can be metabolically incorporated into tissues in the face of an anti-Neu5Gc antibody response. However, the metabolic turnover of Neu5Gc, which apparently prevents human cells from continued accumulation of this immunoreactive sialic acid, has not yet been elucidated. In this study, we show that pre-loaded Neu5Gc is eliminated from human cells over time, and we propose a conceivable Neu5Gc-degrading pathway based on the well studied metabolism of N-acetylhexosamines. We demonstrate that murine tissue cytosolic extracts harbor the enzymatic machinery to sequentially convert Neu5Gc into N-glycolylmannosamine, N-glycolylglucosamine, and N-glycolylglucosamine 6-phosphate, whereupon irreversible de-N-glycolylation of the latter results in the ubiquitous metabolites glycolate and glucosamine 6-phosphate. We substantiate this finding by demonstrating activity of recombinant human enzymes in vitro and by studying the fate of radiolabeled pathway intermediates in cultured human cells, suggesting that this pathway likely occurs in vivo. Finally, we demonstrate that the proposed degradative pathway is partially reversible, showing that N-glycolylmannosamine and N-glycolylglucosamine (but not glycolate) can serve as precursors for biosynthesis of endogenous Neu5Gc.

Sialic acid utilization

Early postnatal development encounters milk as a key environmental variable and yet the sole nutrient source. One evolutionary conserved constituent of milk is sialic acid, which is generally displayed on glycoconjugates and free glycans. During early postnatal development, high sialic acid need was proposed to be unmet by the endogenous sialic acid synthetic capacity. Hence, milk sialic acid was proposed to serve as a conditional nutrient for the newborn. In the elderly, at the other end of ontogeny, decreased sialylation in the brain, saliva, and immune system is observed. Analogous to the neonatal situation, the endogenous synthetic capacity may be unable to keep up with the need in this age group. The data discussed here propose a functional dietary role of sialic acid as a building block for sialylation and beyond.

Nontypeable Haemophilus influenzae: the role of N-acetyl-5-neuraminic acid in biology

Nontypeable Haemophilus influenzae (NTHi) is an exclusive human pathogen, which has evolved a number of unique mechanisms to survive within the human environment. An important part of this is the ability of the organism to take up and incorporate sialic acid into its surface structures. This protects the organism against host adaptive and innate immune factor as well as serving as a mechanism for sustaining itself within biofilms. Recent evidence suggests that this also may be the source of the evolution of human antibodies to non-human sialic acid structures, which can lead to inflammation in the host. In very rare instances, evolution of antibodies to sialylated lipooligosaccharide (LOS) mimics of human antigens can result in autoimmune disease.

Recent advances in developing synthetic carbohydrate-based vaccines for cancer immunotherapies

Cancer cells can often be distinguished from healthy cells by the expression of unique carbohydrate sequences decorating the cell surface as a result of aberrant glycosyltransferase activity occurring within the cell; these unusual carbohydrates can be used as valuable immunological targets in modern vaccine designs to raise carbohydrate-specific antibodies. Many tumor antigens (e.g., GM2, Ley, globo H, sialyl Tn and TF) have been identified to date in a variety of cancers. Unfortunately, carbohydrates alone evoke poor immunogenicity, owing to their lack of ability in inducing T-cell-dependent immune responses. In order to enhance their immunogenicity and promote long-lasting immune responses, carbohydrates are often chemically modified to link to an immunogenic protein or peptide fragment for eliciting T-cell-dependent responses. This review will present a summary of efforts and advancements made to date on creating carbohydrate-based anticancer vaccines, and will include novel approaches to overcoming the poor immunogenicity of carbohydrate-based vaccines.

An overview of influenza A virus receptors

Influenza A viruses are spherical particles that attach to cells through bonds between hemagglutinin and specific cellular receptors. Numerous studies performed have recently revealed that Sialic acid (SA) is a crucial component of influenza A virus receptors. This brief review summarizes recent advances in our understanding of influenza A virus receptors. The introduction describes the classification of influenza A virus receptors and the review continues with a survey of the distribution of SA in different tissue and host. This is followed by research applications of influenza A virus receptors, and explanation of why receptor studies are so important on a world-wide scale.

Evolutionary glycomics: characterization of milk oligosaccharides in primates

Free oligosaccharides are abundant components of mammalian milk and have primary roles as prebiotic compounds, in immune defense, and in brain development. A mass spectrometry-based technique is applied to profile milk oligosaccharides from apes (chimpanzee, gorilla, and siamang), new world monkeys (golden lion tamarin and common marmoset), and an old world monkey (rhesus). The purpose of this study is to evaluate the patterns of primate milk oligosaccharide composition from a phylogenetic perspective to assess the extent to which the compositions of HMOs derives from ancestral primate patterns as opposed to more recent evolutionary events. Milk oligosaccharides were quantitated by nanoflow liquid chromatography on chip-based devices. The relative abundances of fucosylated and sialylated milk oligosaccharides in primates were also determined. For a systematic and comprehensive study of evolutionary patterns of milk oligosaccharides, cluster analysis of primate milk was performed using the chromatographic profile. In general, the oligosaccharides in primate milk, including humans, are more complex and exhibit greater diversity compared to the ones in nonprimate milk. A detailed comparison of the oligosaccharides across evolution revealed nonsequential developmental pattern, that is, that primate milk oligosaccharides do not necessarily cluster according to the primate phylogeny. This report represents the first comprehensive and quantitative effort to profile and elucidate the structures of free milk oligosaccharides so that they can be related to glycan function in different primates.