Induction of Antibodies Directed Against Branched Core O-Mannosyl Glycopeptides-Selectivity Complimentary to the ConA Lectin

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.

The mucin-selective protease StcE enables molecular and functional analysis of human cancer-associated mucins

Mucin domains are densely O-glycosylated modular protein domains that are found in a wide variety of cell surface and secreted proteins. Mucin-domain glycoproteins are known to be key players in a host of human diseases, especially cancer, wherein mucin expression and glycosylation patterns are altered. Mucin biology has been difficult to study at the molecular level, in part, because methods to manipulate and structurally characterize mucin domains are lacking. Here, we demonstrate that secreted protease of C1 esterase inhibitor (StcE), a bacterial protease from Escherichia coli, cleaves mucin domains by recognizing a discrete peptide- and glycan-based motif. We exploited StcE's unique properties to improve sequence coverage, glycosite mapping, and glycoform analysis of recombinant human mucins by mass spectrometry. We also found that StcE digests cancer-associated mucins from cultured cells and from ascites fluid derived from patients with ovarian cancer. Finally, using StcE, we discovered that sialic acid-binding Ig-type lectin-7 (Siglec-7), a glycoimmune checkpoint receptor, selectively binds sialomucins as biological ligands, whereas the related receptor Siglec-9 does not. Mucin-selective proteolysis, as exemplified by StcE, is therefore a powerful tool for the study of mucin domain structure and function.

Chemoenzymatic Assembly of Mammalian O-Mannose Glycans

O-Mannose glycans account up to 30 % of total O-glycans in the brain. Previous synthesis and functional studies have only focused on the core M3 O-mannose glycans of α-dystroglycan, which are a causative factor for various muscular diseases. In this study, a highly efficient chemoenzymatic strategy was developed that enabled the first collective synthesis of 63 core M1 and core M2 O-mannose glycans. This chemoenzymatic strategy features the gram-scale chemical synthesis of five judiciously designed core structures, and the diversity-oriented modification of the core structures with three enzyme modules to provide 58 complex O-mannose glycans in a linear sequence that does not exceed four steps. The binding profiles of synthetic O-mannose glycans with a panel of lectins, antibodies, and brain proteins were also explored by using a printed O-mannose glycan array.

Facile Chemoenzymatic Synthesis of O-Mannosyl Glycans

O Mannosylation is a vital protein modification involved in brain and muscle development whereas the biological relevance of O-mannosyl glycans has remained largely unknown owing to the lack of structurally defined glycoforms. An efficient scaffold synthesis/enzymatic extension (SSEE) strategy was developed to prepare such structures by combining gram-scale convergent chemical syntheses of three scaffolds and strictly controlled sequential enzymatic extension catalyzed by glycosyltransferases. In total, 45 O-mannosyl glycans were obtained, covering the majority of identified mammalian structures. Subsequent glycan microarray analysis revealed fine specificities of glycan-binding proteins and specific antisera.