Synthetic sulfated glucuronosyl paragloboside (SGPG) and its use for the detection of autoimmune peripheral neuropathies

Synthesis and conformational analysis of vicinally branched trisaccharide β-d-Galf-(1 → 2)-[β-d-Galf-(1 → 3)-]-α-Galp from Cryptococcus neoformans galactoxylomannan

The synthesis of a vicinally branched trisaccharide composed of two d-galactofuranoside residues attached viaβ-(1 → 2)- and β-(1 → 3)-linkages to the α-d-galactopyranoside unit has been performed for the first time. The reported trisaccharide represents the galactoxylomannan moiety first described in 2017, which is the capsular polysaccharide of the opportunistic fungal pathogen Cryptococcus neoformans responsible for life-threatening infections in immunocompromised patients. The NMR-data reported here for the synthetic model trisaccharide are in good agreement with the previously assessed structure of galactoxylomannan and are useful for structural analysis of related polysaccharides. The target trisaccharide as well as the constituent disaccharides were analyzed by a combination of computational and NMR methods to demonstrate good convergence of the theoretical and experimental results. The results suggest that the furanoside ring conformation may strongly depend on the aglycon structure. The reported conformational tendencies are important for further analysis of carbohydrate-protein interaction, which is critical for the host response toward C. neoformans infection.

Chemoenzymatic Synthesis of O-Mannose Glycans Containing Sulfated or Nonsulfated HNK-1 Epitope

The human natural killer-1 (HNK-1) epitope is a unique sulfated trisaccharide sequence presented on O- and N-glycans of various glycoproteins and on glycolipids. It is overexpressed in the nervous system and plays crucial roles in nerve regeneration, synaptic plasticity, and neuronal diseases. However, the investigation of functional roles of HNK-1 in a more complex glycan context at the molecular level remains a big challenge due to lack of access to related structurally well-defined complex glycans. Herein, we describe a highly efficient chemoenzymatic approach for the first collective synthesis of HNK-1-bearing O-mannose glycans with different branching patterns, and for their nonsulfated counterparts. The successful strategy relies on both chemical glycosylation of a trisaccharide lactone donor for the introduction of sulfated HNK-1 branch and substrate promiscuities of bacterial glycosyltransferases that can tolerate sulfated substrates for enzymatic diversification. Glycan microarray analysis with the resulting complex synthetic glycans demonstrated their recognition by two HNK-1-specific antibodies including anti-HNK-1/N-CAM (CD57) and Cat-315, which provided further evidence for the recognition epitopes of these antibodies and the essential roles of the sulfate group for HNK-1 glycan-antibody recognition.

Glycoreplica peptides to investigate molecular mechanisms of immune-mediated physiological versus pathological conditions

Investigation of the role of saccharides and glycoconjugates in mechanisms of immune-mediated physiological and pathological conditions is a hot topic. In fact, in many autoimmune diseases cross-reactivity between sugar moieties exposed on exogenous pathogens and self-molecules has long been hinted. Several peptides have been reported as mimetics of glycans specifically interacting with sugar-binding antibodies. The seek for these glycoreplica peptides is instrumental in characterizing antigen mimicry pathways and their involvement in triggering autoimmunity. Therefore, peptides mimicking glycan-protein interactions are valuable molecular tools to overcome the difficulties of oligosaccharide preparations. The clinical impact of peptide-based probes for autoimmune diseases diagnosis and follow-up is emerging only recently as just the tip of the iceberg of an overlooked potential. Here we provide a brief overview of the relevance of the structural and functional aspects of peptide probes and their mimicry effect in autoimmunity mechanisms for promising applications in diagnostics and therapeutics.

Structure-Activity Relationship Studies, SPR Affinity Characterization, and Conformational Analysis of Peptides That Mimic the HNK-1 Carbohydrate Epitope

The design of molecules that mimic biologically relevant glycans is a significant goal for understanding important biological processes and may lead to new therapeutic and diagnostic agents. In this study we focused our attention on the trisaccharide human natural killer cell-1 (HNK-1), considered the antigenic determinant of myelin-associated glycoprotein and the target of clinically relevant auto-antibodies in autoimmune neurological disorders such as IgM monoclonal gammopathy and demyelinating polyneuropathy. We describe a structure-activity relationship study based on surface plasmon resonance binding affinities aimed at the optimization of a peptide that mimics the HNK-1 minimal epitope. We developed a cyclic heptapeptide that shows an affinity of 1.09×10^-7  m for a commercial anti-HNK1 mouse monoclonal antibody. Detailed conformational analysis gave possible explanations for the good affinity displayed by this novel analogue, which was subsequently used as an immunological probe. However, preliminary screening indicates that patients' sera do not specifically recognize this peptide, showing that murine monoclonal antibodies cannot be used as a guide to select immunological probes for the detection of clinically relevant human auto-antibodies.

Synthesis and molecular recognition studies of the HNK-1 trisaccharide and related oligosaccharides. The specificity of monoclonal anti-HNK-1 antibodies as assessed by surface plasmon resonance and STD NMR

The human natural killer cell carbohydrate, HNK-1, plays function-conducive roles in peripheral nerve regeneration and synaptic plasticity. It is also the target of autoantibodies in polyneuropathies. It is thus important to synthesize structurally related HNK-1 carbohydrates for optimizing its function-conducive roles, and for diagnosis and neutralization of autoantibodies in the fatal Guillain-Barré syndrome. As a first step toward these goals, we have synthesized several HNK-1 carbohydrate derivatives to assess the specificity of monoclonal HNK-1 antibodies from rodents: 2-aminoethyl glycosides of selectively O-sulfated trisaccharide corresponding to the HNK-1 antigen, its nonsulfated analogue, and modified structures containing 3-O-fucosyl or 6-O-sulfo substituents in the N-acetylglucosamine residues. These were converted, together with several related oligosaccharides, into biotin-tagged probes to analyze the precise carbohydrate specificity of two anti-HNK-1 antibodies by surface plasmon resonance that revealed a crucial role of the glucuronic acid in antibody binding. The contribution of the different oligosaccharide moieties in the interaction was shown by saturation transfer difference (STD) NMR of the complex consisting of the HNK-1 pentasaccharide and the HNK-1 412 antibody.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

The role of sulfoglucuronosyl glycosphingolipids in the pathogenesis of monoclonal IgM paraproteinemia and peripheral neuropathy

In IgM paraproteinemia and peripheral neuropathy, IgM M-protein secretion by B cells leads to a T helper cell response, suggesting that it is antibody-mediated autoimmune disease involving carbohydrate epitopes in myelin sheaths. An immune response against sulfoglucuronosyl glycosphingolipids (SGGLs) is presumed to participate in demyelination or axonal degeneration in the peripheral nervous system (PNS). SGGLs contain a 3-sulfoglucuronic acid residue that interacts with anti-myelin-associated glycoprotein (MAG) and the monoclonal antibody anti-HNK-1. Immunization of animals with sulfoglucuronosyl paragloboside (SGPG) induced anti-SGPG antibodies and sensory neuropathy, which closely resembles the human disease. These animal models might help to understand the disease mechanism and lead to more specific therapeutic strategies. In an in vitro study, destruction or malfunction of the blood-nerve barrier (BNB) was found, resulting in the leakage of circulating antibodies into the PNS parenchyma, which may be considered as the initial key step for development of disease.

Total synthesis and biological evaluation of the cytotoxic resin glycosides ipomoeassin A-F and analogues

A multitasking C-silylation strategy using the readily available compound 26 as a surrogate for cinnamic acid represents the key design element of a total synthesis of all known members of the ipomoeassin family of resin glyosides. This protecting group maneuver allows the unsaturated acids decorating the glucose subunit of the targets to be attached at an early phase of the synthesis, prevents their participation in the ruthenium-catalyzed ring-closing metathesis (RCM) used to form the macrocyclic ring, and protects them against reduction during the hydrogenation of the resulting cycloalkene over Wilkinson's catalyst. As the C-silyl group can be concomitantly removed with the O-TBS substituent using tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) in acetonitrile, no separate protecting group manipulations were necessary in the final stages, thus contributing to a favorable overall "economy of steps". In addition to the naturally occurring ipomoeassins, a small set of synthetic analogues has also been prepared by "diverted total synthesis". The cytotoxicity of these compounds was assayed with two different cancer cell lines. The recorded data confirm previous findings that the acylation- and oxygenation pattern of these amphiphilic glycoconjugates is highly correlated with their biological activity profile. Ipomoeassin F turned out to be the most promising member of the series, showing IC(50) values in the low nanomolar range.