ALDH2 Inhibition by Lead and Ethanol Elicits Redox Imbalance and Mitochondrial Dysfunction in SH-SY5Y Human Neuroblastoma Cell Line: Reversion by Alda-1

Lead (Pb), a common environmental contaminant, and ethanol (EtOH), a widely available drug of abuse, are well-known neurotoxicants. In vivo, experimental evidence indicates that Pb exposure affects oxidative EtOH metabolism with a high impact on living organisms. On these bases, we evaluated the consequences of combined Pb and EtOH exposure on aldehyde dehydrogenase 2 (ALDH2) functionality. In vitro exposure to 10µM Pb, 200mM EtOH, or their combination for 24h reduced ALDH2 activity and content in SH-SY5Y human neuroblastoma cells. In this scenario, we observed mitochondrial dysfunction characterized by reduced mass and membrane potential, decreased maximal respiration, and spare capacity. We also evaluated the oxidative balance in these cells finding a significant increase in reactive oxygen species (ROS) production and lipid peroxidation products under all treatments accompanied by an increase in catalase (CAT) activity and content. These data suggest that ALDH2 inhibition induces the activation of converging cytotoxic mechanisms resulting in an interplay between mitochondrial dysfunction and oxidative stress. Notably, NAD⁺ (1mM for 24h) restored ALDH2 activity in all groups, while an ALDH2 enhancer (Alda-1, 20µM for 24h) also reversed some of the deleterious effects resulting from impaired ALDH2 function. Overall, these results reveal the crucial role of this enzyme on the Pb and EtOH interaction and the potential of activators such as Alda-1 as therapeutic approaches against several conditions involving aldehydes accumulation.

How glycobiology can help us treat and beat the COVID-19 pandemic

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged during the last months of 2019, spreading throughout the world as a highly transmissible infectious illness designated as COVID-19. Vaccines have now appeared, but the challenges in producing sufficient material and distributing them around the world means that effective treatments to limit infection and improve recovery are still urgently needed. This review focuses on the relevance of different glycobiological molecules that could potentially serve as or inspire therapeutic tools during SARS-CoV-2 infection. As such, we highlight the glycobiology of the SARS-CoV-2 infection process, where glycans on viral proteins and on host glycosaminoglycans have critical roles in efficient infection. We also take notice of the glycan-binding proteins involved in the infective capacity of virus and in human defense. In addition, we critically evaluate the glycobiological contribution of candidate drugs for COVID-19 therapy such as glycans for vaccines, anti-glycan antibodies, recombinant lectins, lectin inhibitors, glycosidase inhibitors, polysaccharides, and numerous glycosides, emphasizing some opportunities to repurpose FDA-approved drugs. For the next-generation drugs suggested here, biotechnological engineering of new probes to block the SARS-CoV-2 infection might be based on the essential glycobiological insight on glycosyltransferases, glycans, glycan-binding proteins, and glycosidases related to this pathology.

Core 1 O-N-acetylgalactosamine (O-GalNAc) glycosylation in the human cell nucleus

Glycosylation is a very frequent post-translational modification in proteins, and the initiation of O-N-acetylgalactosamine (O-GalNAc) glycosylation has been recently described on relevant nuclear proteins. Here we evaluated the nuclear incorporation of a second sugar residue in the biosynthesis pathway of O-GalNAc glycans to yield the terminal core 1 glycan (C1G, Galβ3GalNAcαSer/Thr). Using confocal microscopy, enzymatic assay, affinity chromatography, and mass spectrometry, we analyzed intact cells, purified nuclei and soluble nucleoplasms to identify the essential factors for C1G biosynthesis in the cell nucleus. The enzyme C1GalT1 responsible for C1G synthesis was detected inside the nucleus, while catalytic activity of C1Gal-transferase was present in nucleoplasm and purified nuclei. In addition, C1G were detected in the nucleus inside of intact cells, and nuclear proteins exposing C1G were also identified. These evidences represent the first demonstration of core 1 O-GalNAc glycosylation of proteins in the human cell nucleus. These findings reveal a novel post-translational modification on nuclear proteins, with relevant repercussion in epigenetic and chemical biology areas.

Purified human anti-Tn and anti-T antibodies specifically recognize carcinoma tissues

Described in several epithelial cancer cells, Tn- (GalNAcα1-O-Ser/Thr) and T- (Galβ3GalNAcα1-O-Ser/Thr) antigens are examples of tumor-associated antigens. Increased expression of Tn- and T-antigens is associated with tumor invasion and metastasis, and patients with high concentration of anti-Tn and anti-T antibodies have a more benign evolution of pathology. Asialofetuin (ASF) and ovine submaxillary mucin (OSM) are two glycoproteins that expose T- and Tn-antigen, respectively. In this work, using ASF or OSM we affinity-purified anti-T and anti-Tn antibodies from normal human plasma and tested their ability to specifically recognize tumor human tissues. Whereas purified anti-T antibodies (purity degree increase of 127-fold, and 22% recovery) were mainly IgG, for purified anti-Tn antibodies (purity degree enhancement of 125-fold, and 26% yield) the IgM fraction was predominant over the IgG one. IgG2 subclass was significantly enriched in both purified antibody samples. Purified antibodies did not bind normal human tissue (0/42), although recognized malignant tissues from different origin such as colon carcinoma (11/77 by anti-Tn; 7/79 by anti-T), breast carcinoma (10/23 by anti-Tn; 7/23 by anti-T), and kidney carcinoma (45/51 by anti-Tn; 42/51 by anti-T). Our results suggest that purified human anti-Tn and anti-T antibodies have a potential as anti-tumor therapeutic agents; restoring their levels in human sera could positively affect the evolution of patients with epithelial tumor pathologies.

Biosynthesis of O-N-acetylgalactosamine glycans in the human cell nucleus

Biological functions of nuclear proteins are regulated by post-translational modifications (PTMs) that modulate gene expression and cellular physiology. However, the role of O-linked glycosylation (O-GalNAc) as a PTM of nuclear proteins in the human cell has not been previously reported. Here, we examined in detail the initiation of O-GalNAc glycan biosynthesis, representing a novel PTM of nuclear proteins in the nucleus of human cells, with an emphasis on HeLa cells. Using soluble nuclear fractions from purified nuclei, enzymatic assays, fluorescence microscopy, affinity chromatography, MS, and FRET analyses, we identified all factors required for biosynthesis of O-GalNAc glycans in nuclei: the donor substrate (UDP-GalNAc), nuclear polypeptide GalNAc -transferase activity, and a GalNAc transferase (polypeptide GalNAc-T3). Moreover, we identified O-GalNAc glycosylated proteins in the nucleus and present solid evidence for O-GalNAc glycan synthesis in this organelle. The demonstration of O-GalNAc glycosylation of nuclear proteins in mammalian cells reported here has important implications for cell and chemical biology.

Functional control of polypeptide GalNAc-transferase 3 through an acetylation site in the C-terminal lectin domain

O-GalNAc glycans are important structures in cellular homeostasis. Their biosynthesis is initiated by members of the polypeptide GalNAc-transferase (ppGalNAc-T) enzyme family. Mutations in ppGalNAc-T3 isoform cause diseases (congenital disorders of glycosylation) in humans. The K626 residue located in the C-terminal β-trefoil fold of ppGalNAc-T3 was predicted to be a site with high likelihood of acetylation by CBP/p300 acetyltransferase. We used a site-directed mutagenesis approach to evaluate the role of this acetylation site in biological properties of the enzyme. Two K626 mutants of ppGalNAc-T3 (T3K626Q and T3K626A) had GalNAc-T activities lower than that of wild-type enzyme. Direct and competitive interaction assays revealed that GalNAc recognition by the lectin domain was altered in the mutants. The presence of GlcNAc glycosides affected the interaction of the three enzymes with mucin-derived peptides. In GalNAc-T activity assays, the presence of GlcNAc glycosides significantly inhibited activity of the mutant (T3K626Q) that mimicked acetylation. Our findings, taken together, reveal the crucial role of the K626 residue in the C-terminal β-trefoil fold in biological properties of human ppGalNAc-T3. We propose that acetylated residues on ppGalNAc-T3 function as control points for enzyme activity, and high level of GlcNAc glycosides promote a synergistic regulatory mechanism, leading to a metabolically disordered state.

Experimental Guillain-Barre syndrome induced by immunization with gangliosides: Keyhole limpet hemocyanin is required for disease triggering

An experimental model of Guillain-Barré Syndrome has been established in recent years. Rabbits develop disease upon immunization with a single dose of an emulsion containing bovine brain gangliosides, KLH and complete Freund's adjuvant. Within a period of four to ten weeks after immunization, they began to produce anti-ganglioside IgG-antibodies first, and to show clinical signs of neuropathy afterwards. In addition to gangliosides, KLH is a requirement for antibody production and disease triggering. Although KLH is commonly used as an immunological carrier protein, an anti-KLH-specific immune response was necessary for induction of both events. KLH is a glycoprotein carrying most of the immunogenicity in its glycan moiety. Between 20% to 80% of anti-ganglioside IgG-antibodies present in sick rabbit sera cross-reacted with KLH, indicating that both immune responses are related. The terminal Gal-ß(1,3)-GalNAc glycan (present in gangliosides and KLH) is proposed as "key" antigenic determinant involved in inducing the anti-ganglioside immune response. These results are discussed in the context of the "binding site drift" hypothesis.

Extrinsic Functions of Lectin Domains in O-N-Acetylgalactosamine Glycan Biosynthesis

Glycan biosynthesis occurs mainly in Golgi. Molecular organization and functional regulation of this process are not well understood. We evaluated the extrinsic effect of lectin domains (β-trefoil fold) of polypeptide GalNAc-transferases (ppGalNAc-Ts) on catalytic activity of glycosyltransferases during O-GalNAc glycan biosynthesis. The presence of lectin domain T3lec or T4lec during ppGalNAc-T2 and ppGalNAc-T3 catalytic reaction had a clear inhibitory effect on GalNAc-T activity. Interaction of T3lec or T4lec with ppGalNAc-T2 catalytic domain was not mediated by carbohydrate. T3lec, but not T2lec and T4lec, had a clear activating effect on Drosophila melanogaster core 1 galactosyltransferase enzyme activity and a predominant inhibitory effect on in vivo human core 1 glycan biosynthesis. The regulatory role of the β-trefoil fold of ppGalNAc-Ts in enzymatic activity of glycosyltransferases involved in the O-glycan biosynthesis pathway, described here for the first time, helps clarify the mechanism of biosynthesis of complex biopolymers (such as glycans) that is not template-driven.

In vivo immunomodulatory effect of the lectin from edible mushroom Agaricus bisporus

Lectins are glycan-binding proteins that are resistant to digestion in the gastrointestinal tract and enter intact to blood circulation. The aim of this study was to evaluate the influence of edible mushroom Agaricus bisporus lectin (ABL) on innate and adaptive immune responses as well as its effect in two different experimental pathologies that involve the immune system. ABL inhibited in vitro nitric oxide (NO) production by mouse peritoneal macrophages in response to the pro-inflammatory stimuli lipopolysaccharides (LPS). However, it did not modify the activity of arginase, showing that while ABL downregulates M1 activation, it does not affect M2 activation. ABL also inhibited mononuclear cell proliferation in response to mitogen Con A, or in a mixed lymphocyte reaction. During the in vivo studies, oral administration of ABL to BALB/c mice induced a marked inhibition of NO production by peritoneal macrophages after LPS stimuli. The influence of ABL on tumor growth was studied in BALB/c mice receiving daily oral doses of ABL and implanted with CT26 tumor cells. ABL treatment induced significantly higher rate of tumor growth when compared with control mice. On the other hand, oral ABL administration in Wistar rats induced a marked diminution of the incidence of the disease and the severity of the clinical signs of experimental autoimmune encephalomyelitis. We can conclude that ABL has an in vivo immunomodulatory effect reducing the innate and adaptive responses. This food lectin shows potential therapeutic application on control of inflammatory autoimmune pathologies.

Individual Restriction Of Fine Specificity Variability In Anti-GM1 IgG Antibodies Associated With Guillain-Barré Syndrome

Elevated titers of serum antibodies against GM1 ganglioside are associated with a variety of autoimmune neuropathies. Much evidence indicates these autoantibodies play a primary role in the disease processes, but the mechanism for their appearance is unclear. We studied the fine specificity of anti-GM1 antibodies of the IgG isotype present in sera from patients with Guillain-Barré syndrome (GBS), using thin-layer chromatogram-immunostaining of GM1, asialo-GM1 (GA1), GD1b and GM1-derivatives with small modifications on the oligosaccharide moiety. We were able to distinguish populations of antibodies with different fine specificity. Remarkably, individual patients presented only one or two of them, and different patients had different populations. This restriction in the variability of antibody populations suggests that the appearance of the anti-GM1 antibodies is a random process involving restricted populations of lymphocytes. With the origin of disease-associated anti-GM1 antibodies as a context, this finding could provide explanation for the “host susceptibility factor” observed in GBS following enteritis with GM1 oligosaccharide-carrying strains of Campylobacter jejuni.

An acetylation site in lectin domain modulates the biological activity of polypeptide GalNAc-transferase-2

Polypeptide GalNAc-transferases (ppGalNAc-Ts) are a family of enzymes that catalyze the initiation of mucin-type O-glycosylation. All ppGalNAc-T family members contain a common (QXW)3 motif, which is present in the R-type lectin group. The acetylation site K521 is part of the QKW motif of β-trefoil in the lectin domain of ppGalNAc-T2. We used a combination of acetylation and site-directed mutagenesis approaches to examine the functional role of K521 in ppGalNAc-T2. Binding assays of non-acetylated and acetylated forms of the mutant ppGalNAc-T2K521Q to various naked and αGalNAc-glycosylated mucin peptides indicated that the degree of interaction of lectin domain with αGalNAc depends on the peptide sequence of mucin. Studies of the inhibitory effect of various carbohydrates on the interactions of ppGalNAc-T2 with MUC1αGalNAc indicate that point K521Q mutation enhance the carbohydrate specificity of lectin domain for αGalNAc. K521Q mutation resulted in an enzyme activity lower than that of the wild-type ppGalNAc-T2, similar to the acetylation of ppGalNAc-T2. We conclude that an acetylation site in the QKW motif of the lectin domain modulates carbohydrate recognition specificity and catalytic activity of ppGalNAc-T2 for partially preglycosylated acceptors and a certain naked peptide. Posttranslational modifications of ppGalNAc-Ts, such as acetylation, may play key roles in modulating the functions of the R-type lectin domains in cellular homeostasis.

Staining tumor cells with biotinylated ACL-I, a lectin isolated from the marine sponge, Axinella corrugata

Axinella corrugata lectin 1 (ACL-1) was purified from aqueous extracts of the marine sponge, Axinella corrugata. ACL-1 strongly agglutinates native rabbit erythrocytes. The hemagglutination is inhibited by N-acetyl derivatives, particularly N, N', N"-triacetylchitotriose, N-acetyl-D-glucosamine, N-acetyl-D-mannosamine and N-acetyl-D-galactosamine. We investigated the capacity of biotinylated ACL-1 to stain several transformed cell lines including breast (T-47D, MCF7), colon (HT-29), lung (H460), ovary (OVCAR-3) and bladder (T24). ACL-I may bind to both monosaccharides and oligosaccharides of tumor cells, N-acetyl-D-galactosamine, and N-acetyl-D- glucosamine glycan types. The lectins are useful, not only as markers and diagnostic parameters, but also for tissue mapping in suspicious neoplasms. In addition, they provide a better understanding of neoplasms at the cytological and molecular levels. Furthermore, the use of potential metastatic markers such as lectins is crucial for developing successful tools for therapy against cancer. We observed that biotinylated ACL-I stains tumor cells and may hold potential as a probe for identifying transformed cells and for studying glycan structures synthesized by such cells.

Novel antibodies reacting with two neighboring gangliosides are induced in rabbits immunized with bovine brain gangliosides

Immunization of rabbits with bovine brain gangliosides induced an experimental neuropathy, with clinical signs resembling Guillain-Barré syndrome. All the immunized animals developed immunoglobulin G immunoreactivity to GM1 ganglioside. In a few (4 of 27) animals, an additional anti-ganglioside antibody population showing an unusual binding behavior was detected. Enzyme-linked immunosorbent assay and thin-layer chromatography immunostaining analyses showed that the binding of these unusual antibodies required the presence of two co-localized gangliosides. Maximal interaction was observed to a mixture of GM1 and GD1b, but the antibodies also showed "density-dependent" binding to GD1b. The antibodies were purified by affinity chromatography and displayed the ability to target antigens in biological membranes (rat synaptosomes).

Catalytic and glycan-binding abilities of ppGalNAc-T2 are regulated by acetylation

Post-translational acetylation is an important molecular regulatory mechanism affecting the biological activity of proteins. Polypeptide GalNAc transferases (ppGalNAc-Ts) are a family of enzymes that catalyze initiation of mucin-type O-glycosylation. All ppGalNAc-Ts in mammals are type II transmembrane proteins having a Golgi lumenal region that contains a catalytic domain with glycosyltransferase activity, and a C-terminal R-type ("ricin-like") lectin domain. We investigated the effect of acetylation on catalytic activity of glycosyltransferase, and on fine carbohydrate-binding specificity of the R-type lectin domain of ppGalNAc-T2. Acetylation effect on ppGalNAc-T2 biological activity in vitro was studied using a purified human recombinant ppGalNAc-T2. Mass spectrometric analysis of acetylated ppGalNAc-T2 revealed seven acetylated amino acids (K103, S109, K111, K363, S373, K521, and S529); the first five are located in the catalytic domain. Specific glycosyltransferase activity of ppGalNAc-T2 was reduced 95% by acetylation. The last two amino acids, K521 and S529, are located in the lectin domain, and their acetylation results in alteration of the carbohydrate-binding ability of ppGalNAc-T2. Direct binding assays showed that acetylation of ppGalNAc-T2 enhances the recognition to αGalNAc residue of MUC1αGalNAc, while competitive assays showed that acetylation modifies the fine GalNAc-binding form of the lectin domain. Taken together, these findings clearly indicate that biological activity (catalytic capacity and glycan-binding ability) of ppGalNAc-T2 is regulated by acetylation.

Anti-GM1 IgG antibodies in Guillain-Barré syndrome: fine specificity is associated with disease severity

BACKGROUND

Clinical severity of Guillain-Barré syndrome (GBS) is highly variable, but the immunopathological reason is unknown.

OBJECTIVE

The study was designed to show which antibody parameters are associated with disease severity in GBS patients with serum anti-GM1 IgG antibodies.

METHODS

Thirty-four GBS patients with anti-GM(1) IgG antibodies were grouped into two categories according to disease severity at nadir: mild (grades 1-3 by Hughes functional scale, n=13) and severe (grades 4 and 5, n=21). Titre, affinity, fine specificity and cell binding of anti-GM(1) antibodies were obtained and compared between the two groups.

RESULTS

No differences in antibody titre (GM(1)-ELISA) or affinity were found between the two patient groups. In contrast, the severe group showed a significantly higher frequency (95%, vs 46% in the mild group, p=0.002) of specific (not cross-reacting with GD(1b)) anti-GM(1) antibodies. In addition, the severe group also exhibited a higher antibody binding titre to cellular GM(1).

CONCLUSIONS

Differences in fine specificity of antibodies are strong indications that different regions of the GM(1)-oligosaccharide are involved in antibody binding. High titres of specific anti-GM(1) antibody binding to cellular GM(1) can be explained by antigen exposure, that is, GM(1) exposes or forms mainly epitopes recognised by specific antibodies, and 'hides' those involved in binding of cross-reacting antibodies. Thus, the fine specificity of anti-GM(1) antibodies may influence disease severity by affecting antibody binding to cellular targets. Additionally, since antibody specificity studies are relatively easy to implement, fine specificity could be considered a useful predictor of disease severity.

Anti-GM1 antibodies as a model of the immune response to self-glycans

Glycans are a class of molecules with high structural variability, frequently found in the plasma membrane facing the extracellular space. Because of these characteristics, glycans are often considered as recognition molecules involved in cell social functions, and as targets of pathogenic factors. Induction of anti-glycan antibodies is one of the early events in immunological defense against bacteria that colonize the body. Because of this natural infection, antibodies recognizing a variety of bacterial glycans are found in sera of adult humans and animals. The immune response to glycans is restricted by self-tolerance, and no antibodies to self-glycans should exist in normal subjects. However, antibodies recognizing structures closely related to self-glycans do exist, and can lead to production of harmful anti-self antibodies. Normal human sera contain low-affinity anti-GM1 IgM-antibodies. Similar antibodies with higher affinity or different isotype are found in some neuropathy patients. Two hypotheses have been developed to explain the origin of disease-associated anti-GM1 antibodies. According to the "molecular mimicry" hypothesis, similarity between GM1 and Campylobacter jejuni lipopolysaccharide carrying a GM1-like glycan is the cause of Guillain-Barré syndrome associated with anti-GM1 IgG-antibodies. According to the "binding site drift" hypothesis, IgM-antibodies associated with disease originate through changes in the binding site of normally occurring anti-GM1 antibodies. We now present an "integrated" hypothesis, combining the "mimicry" and "drift" concepts, which satisfactorily explains most of the published data on anti-GM1 antibodies.

Unusual presence of anti-GM1 IgG-antibodies in a healthy individual, and their possible involvement in the origin of disease-associated anti-GM1 antibodies

Anti-GM1 antibodies of the IgG isotype are found in serum from patients with Guillain-Barré syndrome. In normal human sera, anti-GM1 IgM-antibodies are commonly present, but their IgG counterpart has not been previously demonstrated. During routine screening, we found a normal human serum with a high titer of anti-GM1 IgG-antibodies (IgG1 subclass). Affinity estimation by soluble antigen-binding inhibition indicated that they are low-affinity antibodies with IC50 values between one and two orders of magnitude higher than those of anti-GM1 IgG-antibodies from Guillain-Barré patients. Various antibody parameters remained fairly constant for 1 year, in additional serum samples taken at 4-month intervals. Such anti-GM1 IgG1-antibodies were not detected in > 100 other normal serum samples tested, indicating a very low frequency in the general population. The low affinity of these unusually present antibodies could explain the absence of disease, despite their relatively high titer. The significance of this finding in the origin of disease-associated anti-GM1 antibodies is discussed.

Fine carbohydrate recognition of Euphorbia milii lectin

Glycans are key structures involved in biological processes such as cell attachment, migration, and invasion. Information coded on cell-surface glycans is frequently deciphered by proteins, as lectins, that recognize specific carbohydrate topology. Here, we describe the fine carbohydrate specificity of Euphorbia milii lectin (EML). Competitive assays using various sugars showed that GalNAc was the strongest inhibitor, and that the hydroxyl axial position of C4 and acetamido on C2 of GalNAc are critical points of EML recognition. A hydrophobic locus adjacent to GalNAc is also an important region for EML binding. Direct binding assays of EML revealed a stereochemical requirement for a structure adjacent to terminal GalNAc, showing that GalNAc residue is a necessary but not sufficient condition for EML interaction. The capacity of EML to bind epithelial tumor cells makes it a potentially useful tool for study of some over-expressed GalNAc glycoconjugates.

Immune response to Thomsen-Friedenreich disaccharide and glycan engineering

Cancer-associated mucins show frequent alterations of their oligosaccharide chain profile, with a switch to unmask normally cryptic O-glycan backbone and core regions. Epithelial tumour cells typically show overexpression of the uncovered Gal(beta)1-3GalNAc(alpha)-O-Ser/Thr (Core 1) structure, known as the T antigen or the Thomsen-Friedenreich antigen, the oligosaccharide chain of which is called the Thomsen-Friedenreich disaccharide (TFD). T antigen expression has been associated with immunosuppression, metastasis dissemination, and the proliferation of cancer cells. Several different strategies have been used to trigger a specific immune response to TFD. Natural T antigen and synthetic TFD residues have low immunodominance. In the T antigen, flexibility of the glycosidic bond reduces the immunogenicity of the sugar residue. Enhanced rigidity should favour certain glycan conformations and thereby improve TFD immunotargeting. We propose the term 'glycan engineering' for this approach. Such engineering of TFD should reduce the flexibility of its glycan moiety and thereby enhance its stability, rigidity and immunogenicity.

The Antineoplastic Lectin of the Common Edible Mushroom (Agaricus bisporus) Has Two Binding Sites, Each Specific for a Different Configuration at a Single Epimeric Hydroxyl

The lectin from the common mushroom Agaricus bisporus, the most popular edible species in Western countries, has potent antiproliferative effects on human epithelial cancer cells, without any apparent cytotoxicity. This property confers to it an important therapeutic potential as an antineoplastic agent. The three-dimensional structure of the lectin was determined by x-ray diffraction. The protein is a tetramer with 222 symmetry, and each monomer presents a novel fold with two beta sheets connected by a helix-loop-helix motif. Selectivity was studied by examining the binding of four monosaccharides and seven disaccharides in two different crystal forms. The T-antigen disaccharide, Galbeta1-3GalNAc, mediator of the antiproliferative effects of the protein, binds at a shallow depression on the surface of the molecule. The binding of N-acetylgalactosamine overlaps with that moiety of the T antigen, but surprisingly, N-acetylglucosamine, which differs from N-acetylgalactosamine only in the configuration of epimeric hydroxyl 4, binds at a totally different site on the opposite side of the helix-loop-helix motif. The lectin thus has two distinct binding sites per monomer that recognize the different configuration of a single epimeric hydroxyl. The structure of the protein and its two carbohydrate-binding sites are described in detail in this study.

Crystallization and preliminary X-ray study of the common edible mushroom (Agaricus bisporus) lectin

The lectin from the common edible mushroom Agaricus bisporus (ABL) belongs to the group of proteins that have the property of binding the Thomsen-Friedenreich antigen (T-antigen) selectively and with high affinity, but does not show any sequence similarity to the other proteins that share this property. The ABL sequence is instead similar to those of members of the saline-soluble fungal lectins, a protein family with pesticidal properties. The presence of different isoforms has been reported. It has been found that in order to be able to grow diffraction-quality crystals of the lectin, it is essential to separate the isoforms, which was performed by preparative isoelectric focusing. Using standard procedures, it was possible to crystallize the most basic of the forms by either vapour diffusion or equilibrium dialysis, but attempts to grow crystals of the other more acidic forms were unsuccessful. The ABL crystals belong to the orthorhombic space group C222(1), with unit-cell parameters a = 93.06, b = 98.16, c = 76.38 A, and diffract to a resolution of 2.2 A on a conventional source at room temperature. It is expected that the solution of this structure will yield further valuable information on the differences in the T-antigen-binding folds and will perhaps help to clarify the details of the ligand binding to the protein.

Thomsen-Friedenreich Disaccharide Immunogenicity

Mucin-type O-glycans are upregulated and aberrantly glycosylated in many carcinomas. O-glycan Core 1 (Galbeta1-3GalNAcalpha-O-), also called Thomsen-Friedenreich disaccharide, is a cryptic structure overexpressed in cancer cells through modification of its glycosyltransferase profile. This molecule is a useful model for study of carbohydrate immunogenicity as well as a candidate for active specific immunotherapy of cancer patients. Several strategies are discussed for enhancing immune response to a particular region of carbohydrate: carbohydrate-protein conjugation, linkers, synthetic clustered sugars, chemical modifications, peptide/protein mimetics, and molecular rotation.

The origin of anti-GM1 antibodies in neuropathies: the "binding site drift" hypothesis

Elevated titers of serum antibodies against GM1-ganglioside are associated with a variety of autoimmune neuropathies. The origin of these autoantibodies is still unknown, although there is evidence that they are produced by CD5+ B-lymphocytes and that antigen mimicry is involved. Anti-GM, IgM-antibodies in the normal human immunological repertoire are low affinity antibodies that cross-react with other glycoconjugates carrying Gal beta1-3GalNAc and probably do not have GM1-mediated biological activity. Other anti-GM1 IgM-antibodies with higher affinity and/or different fine specificity are present in patients with motor syndromes. Based on our studies of structural requirement for binding, we hypothesize that disease-associated anti-GM1 antibodies originate at random by mutations affecting the binding site of naturally-occurring ones. The hypothesis is conceptually similar to the established phenomenon of "genetic drift" in species evolutionary biology and is therefore termed "binding site drift".

Influence of terminal residue on adjacent disaccharide immunogenicity

Aberrant O-glycosylation of cell surface mucin antigens is characteristic of epithelial cancer cells. For example, Thomsen-Friedenreich disaccharide (TFD) is a chemically well-defined carbohydrate antigen with a documented link to malignancy. There have been many attempts to improve immune response to carbohydrate antigens, for use in immunotherapy. As part of an alternative strategy to improve carbohydrate immunogenicity, we studied the influence of terminal benzyl (Bzl) or p-nitrophenyl (pNP) residue on immunogenicity of adjacent TFD. Mice immunized with keyhole limpets hemocyanin-TFD (KLH-TFD), KLH-TFD(alpha)Bzl, or KLH-TFD(alpha)pNP produced anti-KLH antibodies, which were analyzed by enzyme-linked immunosorbent assay (ELISA). KLH-TFD did not give significant anti-TFD antibody titer, confirming the poor immunogenicity of TFD. Immunization with KLH-TFD(alpha)Bzl and KLH-TFD(alpha)pNP raised antibody titers against TFD(alpha)Bzl and TFD(alpha)pNP, respectively. KLH-TFD(alpha)Bzl also gave higher anti-TFD antibody response, whereas KLH-TFD(alpha)pNP did not, indicating that terminal Bzl residue improves immune response to adjacent carbohydrate. Analysis of anti-TFD(alpha)Bzl or anti-TFD(alpha)pNP IgG antibodies by competitive ELISA, using carbohydrate-related antigens as inhibitors, demonstrated their high specificity to their respective antigens. Anti-TFD(alpha)pNP antibody was not inhibited by TFD, but was significantly inhibited by GalNAc(alpha)pNP. The fact that p-nitrophenol (pNPol) has more competitive ability that GalNAc indicates that terminal polar residue is the main target antigen. In contrast, anti-TFD(alpha)Bzl antibody was inhibited to a similar degree by GalNAc(alpha)Bzl and TFD, confirming the carbohydrate recognition by antibodies yielded by terminal non-polar modification of the immunogen.

Variable patterns of anti-GM(1) IgM-antibody populations defined by affinity and fine specificity in patients with motor syndromes: evidence for their random origin

Elevated titers of serum antibodies against GM(1)-ganglioside are associated with a variety of autoimmune neuropathies. Although much evidence indicates that these autoantibodies play a primary role in the disease processes, the mechanism of their appearance is unclear. Low-affinity anti-GM(1) antibodies of the IgM isotype are part of the normal human immunological repertoire. In patients with motor syndromes, we found that in addition to the usual anti-GM(1) antibodies, the sera contain IgM-antibodies that recognize GM(1) with higher affinity and/or different specificity. This latter type of antibodies was not detected in other autoimmune diseases. We studied the fine specificity of both normal and motor disease-associated antibodies using HPTLC-immunostaining of GM(1) and structurally related glycolipids, soluble antigen binding inhibition, and GM(1) affinity columns. Normal low-affinity anti-GM(1) antibodies cross-react with GA(1) and/or GD(1b). In the motor syndrome patients, different populations of antibodies characterized by their affinity and cross-reactivity were detected. Although one population is relatively common (low affinity, not cross-reacting with GA(1) and GD(1b)), there are remarkably few sera having the same set of populations. These results suggest that the appearance of the new antibody populations is a random process. When the different antibody populations were analyzed in relation to the three-dimensional structure of GM(1), a restricted area of the GM(1) oligosaccharide (the terminal Galbeta1-3GalNAc) was found to be involved in binding of normal anti-GM(1) antibodies. Patient antibodies recognize slightly different areas, including additional regions of the GM(1) molecule such as the NeuNAc residue. We hypothesize that disease-associated antibodies may originate by spontaneous mutation of normal occurring antibodies.

Time course of IgM antibodies which block anti-myelin basic protein IgG antibodies associated with development of experimental autoimmune encephalomyelitis in rabbits

Several authors have demonstrated the presence in normal sera of antibodies that inhibit binding of a variety of autoantibodies. These inhibitory or blocking antibodies are generally considered to play a role in humoral self-tolerance. We examined sera from normal rabbits and from rabbits with experimental autoimmune encephalomyelitis (EAE), in search for antibodies capable to inhibit reactivity of autoantibodies directed to myelin basic protein (MBP). Rabbits injected with bovine myelin in complete Freund's adjuvant (EAE rabbits) or with adjuvant alone (control rabbits) were bled at various intervals post-injection. Sera were subjected to chomatography on a protein A-Sepharose column, retained and nonretained fractions were collected, and ability of these fractions to block reactivity of affinity-purified anti-MBP IgG-antibodies was analyzed by immunoblot technique. Protein A nonretained fraction from control rabbits inhibited anti-MBP IgG reactivity to the same degree at all intervals tested, whereas the same fraction from EAE animals showed an increase in inhibitory activity after induction of the disease. This inhibitory activity declined with the onset of clinical symptoms, and remained low in rabbits that did not recover from the disease. In contrast, the inhibitory activity remained at maximum value in EAE rabbits with spontaneous remission of clinical symptoms. We showed that the inhibitory activity is due to IgM-antibodies, and discussed the role of these antibodies in the development of EAE.

Correlative fine specificity of several Thomsen-Friedenreich disaccharide-binding proteins with an effect on tumor cell proliferation

Epithelial cancer cells show increased cell surface expression of mucin antigens with aberrant O-glycosylation, notably type I core (Galbeta1-3GalNAcalpha), termed Thomsen-Friedenreich disaccharide (TFD), a chemically well-defined carbohydrate antigen with a proven link to malignancy. Several TFD-binding proteins influence the proliferation of cells to which they bind. We studied the fine specificity of TFD-binding proteins and its relationship with epithelial tumor cell proliferation. Competitive binding assays against asialoglycophorin showed that Agaricus bisporus lectin (ABL) and human anti-TFD monoclonal antibody (mAb) TF1 were inhibited only by TFD and its alpha-derivatives. Peanut agglutinin (PNA), mAb TF2, and mAb TF5 were also inhibited by other carbohydrates such as lacto-N-biose (Galbeta1-3GlcNAc), lactose, and (Mealpha or beta) Gal, indicating lower recognition of the axial C-4 hydroxyl group position of GalNAc from TFD, and the major relevance of the terminal Gal on interaction of these three TFD-binding proteins. In the direct glycolipid-binding assay, ABL bound mostly to alpha-anomeric TFD-bearing glycolipids, whereas PNA interacted mainly with beta-linked TFD. Of the three anti-TFD mAbs analyzed, all bound N5b (terminal beta-TFD), but only TF2 interacted with N6 (terminal alpha-TFD). These findings indicate that TFD-binding proteins that stimulate the proliferation of epithelial tumor cell lines recognize mainly a terminal beta-Gal region of beta-linked TFD, whereas ABL, which inhibits the proliferation of these tumor cells, binds mainly to subterminal GalNAc of alpha-anomeric TFD.

Novel immunogenicity of Thomsen-Friedenreich disaccharide obtained by a molecular rotation on its carrier linkage

The alpha-anomeric Galbeta1-3GalNAc, called Thomsen-Friedenreich disaccharide (TFD), is overexpressed in epithelial cancer cells by aberrant O-glycosylation. TFD is also the main ligand of Agaricus bisporus lectin (ABL), a reversible noncytotoxic inhibitor of proliferation of epithelial cell lines. In order to obtain anti-TFD antibody response with a fine carbohydrate-binding specificity similar to that of ABL, we designed an immunogen of TFD with a molecular rotation on its carrier linkage that exposes more GalNAc than Gal, since ABL recognizes GalNAc more than Gal in TFD. The synthesis was accomplished by C-6 oxidation of Gal from TFD or its alpha-benzyl derivative (BzlalphaTFD), followed by reductive amination between the C-6 aldehyde yielded and the available amine of protein. Mice immunized with TFD-KLH (keyhole limpet hemocyanin) or BzlalphaTFD-KLH produced antibodies which were then analyzed by ELISA against several target antigens. Both immunogens raised anti-KLH antibody titers; however, TFD-KLH did not raise anti-TFD antibodies showing low TFD immunogenicity. In contrast, BzlalphaTFD-KLH gave much higher anti-TFD antibody response, indicating that benzyl residue helps improve anti-carbohydrate immune response. When IgG and IgM anti-TFD antibodies were analyzed by competitive ELISA using TFD-related carbohydrates as inhibitors, a high specificity to TFD as well as an enhanced binding to GalNAc over Gal were observed. The axial C-4 hydroxyl group of GalNAc interacted with IgG anti-TFD antibody, as evidenced by the lack of inhibitory activity of GlcNAc in contrast to GalNAc. These findings indicate that the anti-TFD antibodies have fine carbohydrate-binding specificity more similar to ABL than to other TFD-binding proteins that stimulate proliferation of epithelial cell lines.

Normal human plasma contains antibodies that specifically block neuropathy-associated human anti-GM1 IgG-antibodies

Intravenous immunoglobulin (IVIg) is used in the treatment of a variety of autoimmune diseases. The blocking of disease-associated antibodies by anti-idiotype antibodies present in IVIg has been proposed as an action mechanism. Anti-GM1 antibodies have been implicated in motor neuropathies. Although IVIg is frequently applied for these diseases, the presence in IVIg or in human plasma of anti-idiotype antibodies that recognize anti-GM1 antibodies has not been clearly demonstrated. Here we present evidence that normal human plasma contains antibodies that inhibit the binding of anti-GM1 IgG-antibodies from neuropathy patients but do not inhibit anti-GM1 IgG-antibodies of rabbit origin with the same fine specificity. The significance of these findings in the course of acute and chronic neuropathies is discussed.

Structural requirements of carbohydrates to bind Agaricus bisporus lectin

Galbeta1-3GalNAc (T-disaccharide) and related molecules were assayed to describe the structural requirements of carbohydrates to bind Agaricus bisporus lectin (ABL). Results provide insight into the most relevant regions of T-disaccharide involved in the binding of ABL. It was found that monosaccharides bind ABL weakly indicating a more extended carbohydrate-binding site as compared to those involvedin the T-disaccharide specific lectins such as jacalin and peanut agglutinin. Lacto-N-biose (Galbeta1-3GlcNAc) unlike T-disaccharide, is unable to inhibit the ABL interaction, thus showing the great importance of the position of the axial C-4 hydroxyl group of GalNAc in T-disaccharide. This finding could explain the inhibitory ability of Galbeta1-6GlcNAc and lactose because C-4 and C-3 hydroxyl groups of reducing Glc, respectively, occupy a similar position as reported by conformational analysis. From the comparison of different glycolipids bearing terminal T-disaccharide bound to different linkages, it can be seen than ABL binding is even more impaired by an adjacent C-6 residual position than by the anomeric influence of T-disaccharide. Furthermore, the addition of beta-GlcNAc to the terminal T-disaccharide in C-3 position of Gal does not affect the ABL binding whereas if an anionic group such as glucuronic acid is added to C-3, the binding is partially affected. These findings demonstrate that ABL holds a particular binding nature different from that of other T-disaccharide specific lectins.

Agaricus bisporus lectin binds mainly O-glycans but also N-glycans of human IgA subclasses

The primary interaction between purified Agaricus bisporus lectin (ABL) and human IgA subclasses was studied by ABL-affinity chromatography, dot blot assay and competitive enzyme-lectin assay, considering that ABL could be an alternative tool for detection of IgA1 O-glycans. Both secretory IgA subclasses bound to ABL-Sepharose and the IgA2 subclass (which contains only N-glycans) was recovered with a high degree of purity when NH4OH was used as eluent. ABL-Ig interaction was also observed by dot blot assays using ABL-peroxidase against monoclonal IgA1 k Pan, IgA2m(1)k Gir, IgA2m(2)k Bel, secretory IgA2 and normal IgG (also contains only N-glycans). When these immunoglobulins were enzymatically treated with peptide N-glycosidase F (N-glycan hydrolysis), the ABL-IgA2 and -IgG interaction did not occur while IgA1 maintained a high degree of interaction with ABL. Also, the ABL-IgA interaction was observed by competitive enzyme-lectin assay, and when IgA1 subclass was treated with endo-alpha-N-acetylgalactosaminidase for O-glycans hydrolysis, the reactivity with ABL was very low. We conclude that the complementary use of ABL and peptide N-glycosidase F could be a useful tool to assess the O-glycosylation state of human IgA1 subclass, which is of relevant importance in the effector functions of immunoglobulins.

Differential reactivity of Agaricus bisporus lectin with human IgA subclasses in gel precipitation

The interaction between purified Agaricus bisporus lectin and several human proteins was studied using the Ouchterlony double diffusion and immunoelectrophoresis techniques. Only one precipitation line was observed with normal human serum, normal human colostrum, IgA1 myeloma serum, both serum monoclonal and secretory IgA1 and monoclonal IgD. No reaction was observed with monoclonal and secretory IgA2, IgG, IgM, alpha 2 macroglobulin or pregnancy-associated alpha 2 glycoprotein. These results were confirmed by hemagglutination inhibition assays when IgA1, IgA2 and IgD were tested. On the basis of this reactivity, ABL could be a useful tool for distinguishing and isolating human IgA subclasses.