Protein O-Mannosylation in the Murine Brain: Occurrence of Mono-O-Mannosyl Glycans and Identification of New Substrates

Protein O-mannosylation: one sugar, several pathways, many functions

Recent research has unveiled numerous important functions of protein glycosylation in development, homeostasis, and diseases. A type of glycosylation taking the center stage is protein O-mannosylation, a posttranslational modification conserved in a wide range of organisms, from yeast to humans. In animals, protein O-mannosylation plays a crucial role in the nervous system, whereas protein O-mannosylation defects cause severe neurological abnormalities and congenital muscular dystrophies. However, the molecular and cellular mechanisms underlying protein O-mannosylation functions and biosynthesis remain not well understood. This review outlines recent studies on protein O-mannosylation while focusing on the functions in the nervous system, summarizes the current knowledge about protein O-mannosylation biosynthesis, and discusses the pathologies associated with protein O-mannosylation defects. The evolutionary perspective revealed by studies in the Drosophila model system are also highlighted. Finally, the review touches upon important knowledge gaps in the field and discusses critical questions for future research on the molecular and cellular mechanisms associated with protein O-mannosylation functions.

Anti-glycan monoclonal antibodies: Basic research and clinical applications

Anti-glycan monoclonal antibodies have important applications in human health and basic research. Therapeutic antibodies that recognize cancer- or pathogen-associated glycans have been investigated in numerous clinical trials, resulting in two FDA-approved biopharmaceuticals. Anti-glycan antibodies are also utilized to diagnose, prognosticate, and monitor disease progression, as well as to study the biological roles and expression of glycans. High-quality anti-glycan mAbs are still in limited supply, highlighting the need for new technologies for anti-glycan antibody discovery. This review discusses anti-glycan monoclonal antibodies with applications to basic research, diagnostics, and therapeutics, focusing on recent advances in mAbs targeting cancer- and infectious disease-associated glycans.

Transcriptomic Analysis of Glycosylation and Neuroregulatory Pathways in Rodent Models in Response to Psychedelic Molecules

The potential for psychedelic molecules in impacting cognitive flexibility has long been supported and acknowledged across scientific reports. In the current study, an approach leveraging knowledge-based gene-set information analysis has been adopted to explore the potential impact of psychedelic molecules on both glycosylation, (a post-translational modifications (PTM)) and on neuro-regulatory pathways. Though limitations and restrictions rise from the scarcity of publicly available 'omics' data, targeted analysis enabled us to identify a number of key glycogenes (Hexb, Hs6st2, Col9a2, B3gat2, Mgat5, Bgn) involved the structural organization of extracellular matrix and neuroprotective factors (Kl, Pomc, Oxt, Gal, Avp, Cartpt) which play vital roles in neuron protection, development as well as synaptic stability. In response to psychedelic molecules, we found that these genes and associated pathways are transcriptional altered in rodent models. The approach used indicates the potential to exploit existing datasets for hypothesis generation and testing for the molecular processes which play a role in the physiological response to psychedelic molecule effects. These reported findings, which focused on alterations in glycogenes and neuro-regulatory factors may provide a novel range of biomarkers to track the beneficial, as well as potential toxicological effects of psychedelic molecules.

Analysis of the expression and distribution of protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 in the normal adult mouse brain

Introduction

Protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) is crucial for the elongation of O-mannosyl glycans. Mutations in POMGNT1 cause muscle-eye-brain (MEB) disease, one of the main features of which is anatomical aberrations in the brain. A growing number of studies have shown that defects in POMGNT1 affect neuronal migration and distribution, disrupt basement membranes, and misalign Cajal-Retzius cells. Several studies have examined the distribution and expression of POMGNT1 in the fetal or neonatal brain for neurodevelopmental studies in the mouse or human brain. However, little is known about the neuroanatomical distribution and expression of POMGNT1 in the normal adult mouse brain.

Methods

We analyzed the expression of POMGNT1 mRNA and protein in the brains of various neuroanatomical regions and spinal cords by western blotting and RT-qPCR. We also detected the distribution profile of POMGnT1 in normal adult mouse brains by immunohistochemistry and double-immunofluorescence.

Results

In the present study, we found that POMGNT1-positive cells were widely distributed in various regions of the brain, with high levels of expression in the cerebral cortex and hippocampus. In terms of cell type, POMGNT1 was predominantly expressed in neurons and was mainly enriched in glutamatergic neurons; to a lesser extent, it was expressed in glial cells. At the subcellular level, POMGNT1 was mainly co-localized with the Golgi apparatus, but expression in the endoplasmic reticulum and mitochondria could not be excluded.

Discussion

The present study suggests that POMGNT1, although widely expressed in various brain regions, may has some regional and cellular specificity, and the outcomes of this study provide a new laboratory basis for revealing the possible involvement of POMGNT1 in normal physiological functions of the brain from a morphological perspective.

Structure, function, and pathology of Neurexin-3

Neurexin-3 is primarily localized in the presynaptic membrane and forms complexes with various ligands located in the postsynaptic membrane. Neurexin-3 has important roles in synapse development and synapse functions. Neurexin-3 mediates excitatory presynaptic differentiation by interacting with leucine-rich-repeat transmembrane neuronal proteins. Meanwhile, neurexin-3 modulates the expression of presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors and γ-aminobutyric acid A receptors by interacting with neuroligins at excitatory and inhibitory synapses. Numerous studies have documented the potential contribution of neurexin-3 to neurodegenerative and neuropsychiatric disorders, such as Alzheimer's disease, addiction behaviors, and other diseases, which raises hopes that understanding the mechanisms of neurexin-3 may hold the key to developing new strategies for related illnesses. This review comprehensively covers the literature to provide current knowledge of the structure, function, and clinical role of neurexin-3.

Protein O-mannosylation across kingdoms and related diseases: From glycobiology to glycopathology

The post-translational glycosylation of proteins by O-linked α-mannose is conserved from bacteria to humans. Due to advances in high-throughput mass spectrometry-based approaches, a variety of glycoproteins are identified to be O-mannosylated. Various proteins with O-mannosylation are involved in biological processes, providing essential necessity for proper growth and development. In this review, we summarize the process and regulation of O-mannosylation. The multi-step O-mannosylation procedures are quite dynamic and complex, especially when considering the structural and functional inspection of the involved enzymes. The widely studied O-mannosylated proteins in human include α-Dystroglycan (α-DG), cadherins, protocadherins, and plexin, and their aberrant O-mannosylation are associated with many diseases. In addition, O-mannosylation also contributes to diverse functions in lower eukaryotes and prokaryotes. Finally, we present the relationship between O-mannosylation and gut microbiota (GM), and elucidate that O-mannosylation in microbiome is of great importance in the dynamic balance of GM. Our study provides an overview of the processes of O-mannosylation in mammalian cells and other organisms, and also associated regulated enzymes and biological functions, which could contribute to the understanding of newly discovered O-mannosylated glycoproteins.

Mammalian brain glycoproteins exhibit diminished glycan complexity compared to other tissues

Glycosylation is essential to brain development and function, but prior studies have often been limited to a single analytical technique and excluded region- and sex-specific analyses. Here, using several methodologies, we analyze Asn-linked and Ser/Thr/Tyr-linked protein glycosylation between brain regions and sexes in mice. Brain N-glycans are less complex in sequence and variety compared to other tissues, consisting predominantly of high-mannose and fucosylated/bisected structures. Most brain O-glycans are unbranched, sialylated O-GalNAc and O-mannose structures. A consistent pattern is observed between regions, and sex differences are minimal compared to those in plasma. Brain glycans correlate with RNA expression of their synthetic enzymes, and analysis of glycosylation genes in humans show a global downregulation in the brain compared to other tissues. We hypothesize that this restricted repertoire of protein glycans arises from their tight regulation in the brain. These results provide a roadmap for future studies of glycosylation in neurodevelopment and disease.

Evaluation of a quality improvement bundle aimed to reduce opioid prescriptions after Cesarean delivery: an interrupted time series study

Purpose

To evaluate whether opioid prescriptions at discharge after Cesarean delivery decreased following implementation of a quality improvement bundle.

Methods

A quality improvement bundle was instituted at Mount Sinai Hospital in Toronto. Interventions included opioid prescribing instructions in resident orientation, nursing and patient education, and standard electronic prescriptions. We used an interrupted time series study design and included patients who had a Cesarean delivery six months pre intervention and six months post intervention. Primary outcome data (opioids prescribed at discharge in morphine milliequivalents [MME]), were aggregated (averaged) by calendar week and analyzed using interrupted time series. Secondary outcomes were assessed using bivariate methods and included opioid use for breakthrough pain in hospital, and amount of opioids prescribed by prescriber specialty and training level.

Results

We included 2,578 women in our analysis. Based on the segmented regression analysis, prescribed opioids decreased from 97.6 MME in 2018 to 35.8 MME in 2019 (difference in means, − 61.7; 95% confidence interval [CI], − 72.2 to − 51.3; P < 0.001), and this decrease was sustained over the study period. Post intervention, there were no visits to our postnatal assessment clinic for inadequate pain control.

Conclusion

A quality improvement bundle was associated with a marked and sustained decrease in discharge prescriptions of opioids post Cesarean delivery at a large Canadian tertiary academic hospital.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12630-021-02143-7.

Protein mannosylation as a diagnostic and prognostic biomarker of lupus nephritis: an unusual glycan-neoepitope in Systemic Lupus Erythematosus

OBJECTIVES

Changes in protein glycosylation are a hallmark of immune-mediated diseases. Glycans are master regulators of the inflammatory response being important molecules for the discrimination between "self"/"non-self". We here explored whether lupus nephritis (LN) exhibit an altered cellular glycosylation aiming to identify a unique glycosignature that characterizes LN pathogenesis.

METHODS

A comprehensive tissue glycomics characterization was performed in a cohort of human biopsy-proven LN clinical samples from SLE patients. A combination of advanced tissue mass spectrometry imaging (MSI); in situ glyco-characterization and ex vivo glycophenotyping of human kidney biopsies were performed to structurally map the N-glycans repertoire in LN samples.

RESULTS

LN revealed a unique glycan signature characterized by an increased abundance and spatial distribution of unusual mannose-enriched glycans that are typically found in lower microorganisms; this glycosignature was specific of LN as it was not observed in other kidney diseases. Exposure of mannosylated glycans in LN was found to occur at the cell surface of kidney cells promoting an increased recognition by specific glycans-recognizing receptors, expressed by immune cells. This abnormal glyco-signature of LN was demonstrated to be due to a deficient complex N-glycosylation and a proficient O-mannosylation pathway. Moreover, levels of mannosylation detected in kidney biopsies from LN patients at diagnosis were demonstrated to predict the development of chronic kidney disease (CKD) with 93% of specificity.

CONCLUSIONS

Cellular mannosylation is a marker of LN, predicting the development of CKD, and thus constituting a potential glycobiomarker to be included in the diagnostic and prognostic algorithm of LN.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Multifaceted antibodies development against synthetic α-dystroglycan mucin glycopeptide as promising tools for dystroglycanopathies diagnostic

Dystroglycanopathies are diseases characterized by progressive muscular degeneration and impairment of patient's quality of life. They are associated with altered glycosylation of the dystrophin-glycoprotein (DGC) complex components, such as α-dystroglycan (α-DG), fundamental in the structural and functional stability of the muscle fiber. The diagnosis of dystroglycanopathies is currently based on the observation of clinical manifestations, muscle biopsies and enzymatic measures, and the available monoclonal antibodies are not specific for the dystrophic hypoglycosylated muscle condition. Thus, modified α-DG mucins have been considered potential targets for the development of new diagnostic strategies toward these diseases. In this context, this work describes the synthesis of the hypoglycosylated α-DG mimetic glycopeptide NHAc-Gly-Pro-Thr-Val-Thr[αMan]-Ile-Arg-Gly-BSA (1) as a potential tool for the development of novel antibodies applicable to dystroglycanopathies diagnosis. Glycopeptide 1 was used for the development of polyclonal antibodies and recombinant monoclonal antibodies by Phage Display technology. Accordingly, polyclonal antibodies were reactive to glycopeptide 1, which enables the application of anti-glycopeptide 1 antibodies in immune reactive assays targeting hypoglycosylated α-DG. Regarding monoclonal antibodies, for the first time variable heavy (VH) and variable light (VL) immunoglobulin domains were selected by Phage Display, identified by NGS and described by in silico analysis. The best-characterized VH and VL domains were cloned, expressed in E. coli Shuffle T7 cells, and used to construct a single chain fragment variable that recognized the Glycopeptide 1 (GpαDG1 scFv). Molecular modelling of glycopeptide 1 and GpαDG1 scFv suggested that their interaction occurs through hydrogen bonds and hydrophobic contacts involving amino acids from scFv (I51, Y33, S229, Y235, and P233) and R8 and α-mannose from Glycopeptide 1.

Proteomics, Glycomics, and Glycoproteomics of Matrisome Molecules

The most straightforward applications of proteomics database searching involve intracellular proteins. Although intracellular gene products number in the thousands, their well-defined post-translational modifications (PTMs) makes database searching practical. By contrast, cell surface and extracellular matrisome proteins pass through the secretory pathway where many become glycosylated, modulating their physicochemical properties, adhesive interactions, and diversifying their functions. Although matrisome proteins number only a few hundred, their high degree of complex glycosylation multiplies the number of theoretical proteoforms by orders of magnitude. Given that extracellular networks that mediate cell-cell and cell-pathogen interactions in physiology depend on glycosylation, it is important to characterize the proteomes, glycomes, and glycoproteomes of matrisome molecules that exist in a given biological context. In this review, we summarize proteomics approaches for characterizing matrisome molecules, with an emphasis on applications to brain diseases. We demonstrate the availability of methods that should greatly increase the availability of information on matrisome molecular structure associated with health and disease.

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.

GlcNAc6ST3 is a keratan sulfate sulfotransferase for the protein-tyrosine phosphatase PTPRZ in the adult brain

Keratan sulfate (KS) is a carbohydrate side chain covalently attached to extracellular proteoglycans. KS is composed of disaccharide units of 6-sulfated N-acetylglucosamine (GlcNAc) and galactose. We have previously shown that GlcNAc-6-O-sulfotransferase (GlcNAc6ST) 1 encoded by Chst2 is an enzyme necessary for the synthesis of GlcNAc-6-sulfated KS chains that are required for neuronal plasticity in the visual cortex of the mouse brain during the critical period, but not in adulthood. Here, we show that GlcNAc-6-sulfated KS recognized by the R-10G anti-KS antibody, of which the minimum epitope structure is Galß1-4GlcNAc(6S)ß1-3Galß1-4GlcNAc(6S), distributes diffusely in neuropils and presents densely in close proximity to the perineuronal region of the perineuronal net (PNN)-positive neurons in the adult visual cortex. Surprisingly, GlcNAc6ST3, which was discovered as an intestinal GlcNAc6ST encoded by Chst5, is a major brain KS sulfotransferase expressed in oligodendrocytes in adulthood. Moreover, we identified an isoform of the protein-tyrosine phosphatase PTPRZ as a R-10G-reactive KS proteoglycan. These results indicate that GlcNAc6ST3 may play a role in synthesis of a component of PNN in the adult brain, and that the KS-modified isoform of PTPRZ encoded by Ptprz1 could be an extracellular molecule associated with PNNs.

The Human Brain Proteome Project: Biological and Technological Challenges

Brain proteomics has become a method of choice that allows zooming-in where neuropathophysiological alterations are taking place, detecting protein mediators that might eventually be measured in cerebrospinal fluid (CSF) as potential neuropathologically derived biomarkers. Following this hypothesis, mass spectrometry-based neuroproteomics has emerged as a powerful approach to profile neural proteomes derived from brain structures and CSF in order to map the extensive protein catalog of the human brain. This chapter provides a historical perspective on the Human Brain Proteome Project (HBPP), some recommendation to the experimental design in neuroproteomic projects, and a brief description of relevant technological and computational innovations that are emerging in the neurobiology field thanks to the proteomics community. Importantly, this chapter highlights recent discoveries from the biology- and disease-oriented branch of the HBPP (B/D-HBPP) focused on spatiotemporal proteomic characterizations of mouse models of neurodegenerative diseases, elucidation of proteostatic networks in different types of dementia, the characterization of unresolved clinical phenotypes, and the discovery of novel biomarker candidates in CSF.

Analysis of Mammalian O-Glycopeptides-We Have Made a Good Start, but There is a Long Way to Go

Glycosylation is perhaps the most common post-translational modification. Recently there has been growing interest in cataloging the glycan structures, glycoproteins, and specific sites modified and deciphering the biological functions of glycosylation. Although the results are piling up for N-glycosylation, O-glycosylation is seriously trailing behind. In our review we reiterate the difficulties researchers have to overcome in order to characterize O-glycosylation. We describe how an ingenious cell engineering method delivered exciting results, and what could we gain from "wild-type" samples. Although we refer to the biological role(s) of O-glycosylation, we do not provide a complete inventory on this topic.

Discovery of an O-mannosylation pathway selectively serving cadherins and protocadherins

The cadherin (cdh) superfamily of adhesion molecules carry O-linked mannose (O-Man) glycans at highly conserved sites localized to specific β-strands of their extracellular cdh (EC) domains. These O-Man glycans do not appear to be elongated like O-Man glycans found on α-dystroglycan (α-DG), and we recently demonstrated that initiation of cdh/protocadherin (pcdh) O-Man glycosylation is not dependent on the evolutionary conserved POMT1/POMT2 enzymes that initiate O-Man glycosylation on α-DG. Here, we used a CRISPR/Cas9 genetic dissection strategy combined with sensitive and quantitative O-Man glycoproteomics to identify a homologous family of four putative protein O-mannosyltransferases encoded by the TMTC1-4 genes, which were found to be imperative for cdh and pcdh O-Man glycosylation. KO of all four TMTC genes in HEK293 cells resulted in specific loss of cdh and pcdh O-Man glycosylation, whereas combined KO of TMTC1 and TMTC3 resulted in selective loss of O-Man glycans on specific β-strands of EC domains, suggesting that each isoenzyme serves a different function. In addition, O-Man glycosylation of IPT/TIG domains of plexins and hepatocyte growth factor receptor was not affected in TMTC KO cells, suggesting the existence of yet another O-Man glycosylation machinery. Our study demonstrates that regulation of O-mannosylation in higher eukaryotes is more complex than envisioned, and the discovery of the functions of TMTCs provide insight into cobblestone lissencephaly caused by deficiency in TMTC3.

Recent advancements in understanding mammalian O-mannosylation

The post-translational glycosylation of select proteins by O-linked mannose (O-mannose or O-man) is a conserved modification from yeast to humans and has been shown to be necessary for proper development and growth. The most well studied O-mannosylated mammalian protein is α-dystroglycan (α-DG). Hypoglycosylation of α-DG results in varying severities of congenital muscular dystrophies, cancer progression and metastasis, and inhibited entry and infection of certain arenaviruses. Defects in the gene products responsible for post-translational modification of α-DG, primarily glycosyltransferases, are the basis for these diseases. The multitude of clinical phenotypes resulting from defective O-mannosylation highlights the biomedical significance of this unique modification. Elucidation of the various O-mannose biosynthetic pathways is imperative to understanding a broad range of human diseases and for the development of novel therapeutics. In this review, we will focus on recent discoveries delineating the various enzymes, structures and functions associated with O-mannose-initiated glycoproteins. Additionally, we discuss current gaps in our knowledge of mammalian O-mannosylation, discuss the evolution of this pathway, and illustrate the utility and limitations of model systems to study functions of O-mannosylation.

Mammalian O-mannosylation of cadherins and plexins is independent of protein O-mannosyltransferases 1 and 2

Protein O-mannosylation is found in yeast and metazoans, and a family of conserved orthologous protein O-mannosyltransferases is believed to initiate this important post-translational modification. We recently discovered that the cadherin superfamily carries O-linked mannose (O-Man) glycans at highly conserved residues in specific extracellular cadherin domains, and it was suggested that the function of E-cadherin was dependent on the O-Man glycans. Deficiencies in enzymes catalyzing O-Man biosynthesis, including the two human protein O-mannosyltransferases, POMT1 and POMT2, underlie a subgroup of congenital muscular dystrophies designated α-dystroglycanopathies, because deficient O-Man glycosylation of α-dystroglycan disrupts laminin interaction with α-dystroglycan and the extracellular matrix. To explore the functions of O-Man glycans on cadherins and protocadherins, we used a combinatorial gene-editing strategy in multiple cell lines to evaluate the role of the two POMTs initiating O-Man glycosylation and the major enzyme elongating O-Man glycans, the protein O-mannose β-1,2-N-acetylglucosaminyltransferase, POMGnT1. Surprisingly, O-mannosylation of cadherins and protocadherins does not require POMT1 and/or POMT2 in contrast to α-dystroglycan, and moreover, the O-Man glycans on cadherins are not elongated. Thus, the classical and evolutionarily conserved POMT O-mannosylation pathway is essentially dedicated to α-dystroglycan and a few other proteins, whereas a novel O-mannosylation process in mammalian cells is predicted to serve the large cadherin superfamily and other proteins.

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

Mammalian protein O-mannosylation, initiated by attachment of α-mannopyranose to Ser or Thr residues, comprise a group of post-translational modifications (PTMs) involved in muscle and brain development. Recent advances in glycoproteomics methodology and the "SimpleCell" strategy have enabled rapid identification of glycoproteins and specific glycosylation sites. Despite the enormous progress made, the biological impact of the mammalian O-mannosyl glycoproteome remains largely unknown to date. Tools are still needed to investigate the structure, role, and abundance of O-mannosyl glycans. Although O-mannosyl branching has been shown to be of relevance in integrin-dependent cell migration, and also plays a role in demyelinating diseases, such as multiple sclerosis, a broader understanding of the biological roles of branched O-mannosyl glycans is lacking in part due to the paucity of detection tools. In this work, a glycopeptide vaccine construct was synthesized and used to generate antibodies against branched O-mannosyl glycans. Glycopeptide microarray screening revealed high selectivity of the induced antibodies for branched glycan core structures presented on different peptide backbones, with no cross-reactivity observed with related linear glycans. For comparison, microarray screening of the mannose-binding lectin concanavalin A (ConA), which is commonly used in glycoproteomics workflows to enrich tryptic O-mannosyl peptides, showed that the ConA lectin did not recognize branched O-mannosyl glycans. The binding preference of ConA for short linear O-mannosyl glycans was rationalized in terms of molecular structure using crystallographic data augmented by molecular modeling. The contrast between the ConA binding specificity and that of the new antibodies indicates a novel role for the antibodies in studies of protein O-mannosylation.