A systematic study of site-specific GalNAc-type O-glycosylation modulating proprotein convertase processing

Host Cell Proteases Involved in Human Respiratory Viral Infections and Their Inhibitors: A Review

Viral tropism is most commonly linked to receptor use, but host cell protease use can be a notable factor in susceptibility to infection. Here we review the use of host cell proteases by human viruses, focusing on those with primarily respiratory tropism, particularly SARS-CoV-2. We first describe the various classes of proteases present in the respiratory tract, as well as elsewhere in the body, and incorporate the targeting of these proteases as therapeutic drugs for use in humans. Host cell proteases are also linked to the systemic spread of viruses and play important roles outside of the respiratory tract; therefore, we address how proteases affect viruses across the spectrum of infections that can occur in humans, intending to understand the extrapulmonary spread of SARS-CoV-2.

Glycosylation of FGF/FGFR: An underrated sweet code regulating cellular signaling programs

Fibroblast growth factors (FGFs) and their receptors (FGFRs) constitute plasma-membrane localized signaling hubs that transmit signals from the extracellular environment to the cell interior, governing pivotal cellular processes like motility, metabolism, differentiation, division and death. FGF/FGFR signaling is critical for human body development and homeostasis; dysregulation of FGF/FGFR units is observed in numerous developmental diseases and in about 10% of human cancers. Glycosylation is a highly abundant posttranslational modification that is critical for physiological and pathological functions of the cell. Glycosylation is also very common within FGF/FGFR signaling hubs. Vast majority of FGFs (15 out of 22 members) are N-glycosylated and few FGFs are O-glycosylated. Glycosylation is even more abundant within FGFRs; all FGFRs are heavily N-glycosylated in numerous positions within their extracellular domains. A growing number of studies points on the multiple roles of glycosylation in fine-tuning FGF/FGFR signaling. Glycosylation modifies secretion of FGFs, determines their stability and affects interaction with FGFRs and co-receptors. Glycosylation of FGFRs determines their intracellular sorting, constitutes autoinhibitory mechanism within FGFRs and adjusts FGF and co-receptor recognition. Sugar chains attached to FGFs and FGFRs constitute also a form of code that is differentially decrypted by extracellular lectins, galectins, which transform FGF/FGFR signaling at multiple levels. This review focuses on the identified functions of glycosylation within FGFs and FGFRs and discusses their relevance for the cell physiology in health and disease.

Loss of the glycosyltransferase Galnt11 affects vitamin D homeostasis and bone composition

O-glycosylation is a conserved posttranslational modification that impacts many aspects of organismal viability and function. Recent studies examining the glycosyltransferase Galnt11 demonstrated that it glycosylates the endocytic receptor megalin in the kidneys, enabling proper binding and reabsorption of ligands, including vitamin D-binding protein (DBP). Galnt11-deficient mice were unable to properly reabsorb DBP from the urine. Vitamin D plays an essential role in mineral homeostasis and its deficiency is associated with bone diseases such as rickets, osteomalacia, and osteoporosis. We therefore set out to examine the effects of the loss of Galnt11 on vitamin D homeostasis and bone composition. We found significantly decreased levels of serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, consistent with decreased reabsorption of DBP. This was accompanied by a significant reduction in blood calcium levels and a physiologic increase in parathyroid hormone (PTH) in Galnt11-deficient mice. Bones in Galnt11-deficient mice were smaller and displayed a decrease in cortical bone accompanied by an increase in trabecular bone and an increase in a marker of bone formation, consistent with PTH-mediated effects on bone. These results support a unified model for the role of Galnt11 in bone and mineral homeostasis, wherein loss of Galnt11 leads to decreased reabsorption of DBP by megalin, resulting in a cascade of disrupted mineral and bone homeostasis including decreased circulating vitamin D and calcium levels, a physiological increase in PTH, an overall loss of cortical bone, and an increase in trabecular bone. Our study elucidates how defects in O-glycosylation can influence vitamin D and mineral homeostasis and the integrity of the skeletal system.

Highly efficient identification of nucleocytoplasmic O-glycosylation by the TurboID-based proximity labeling method in living cells

Glycosylation is a ubiquitous posttranslational modification and plays an important role in many processes, such as protein stability, folding, processing, and trafficking. Among glycosylation types, O-glycosylation is difficult to analyze due to the complex glycan composition, low abundance and lack of glycosidases to remove the O-glycans. Many methods have been applied to analyze the O-glycosylation of membrane glycoproteins and secreted glycoproteins since the synthesis of O-glycosylation occurred in the Golgi apparatus. In recent years, some O-glycosylation has been reported in the nucleus. In this work, we present a proximity labeling strategy based on TurboID by combining core 1 β1-3 galactosyltransferase (C1GalT1), which has been reported in the nucleus, to characterize nucleocytoplasmic O-glycosylation in living HeLa cells. The O-glycosylated protein C1GalT1 was biotinylated by the proximity labeling method in living HeLa cells overexpressing C1GalT1 fused by TurboID and enriched by streptavidin-coated beads. Following digestion with trypsin and mass spectrometry analysis, 68 high-confidence and 298 putative O-glycosylated sites were identified on 366 peptides mapped to 267 proteins. These results indicated that the proximity labeling method is a highly efficient technique to identify O-glycosylation. Furthermore, the finding of abundant O-glycosylation from nucleocytoplasmic proteins indicates a new pathway of O-glycosylation synthesis in cells.

A high-sensitivity ELISA for detection of human FGF18 in culture supernatants from tumor cell lines

Fibroblast growth factor 18 (FGF18) is elevated in several human cancers, such as gastrointestinal and ovarian cancers, and stimulates the proliferation of tumor cells. This suggests that FGF18 may be a promising candidate biomarker in cancer patients. However, the lack of a high-sensitivity enzyme-linked immunosorbent assay (ELISA) does not permit testing of this possibility. In this study, we generated monoclonal antibodies against human FGF18 and developed a high-sensitivity ELISA to measure human FGF18 at concentrations as low as 10 pg/mL. Of the eight tumor cell lines investigated, we detected human FGF18 in culture supernatants from four tumor cell lines, including HeLa, OVCAR-3, BxPC-3, and SW620 cells, albeit the production levels were relatively low in the latter two cell lines. Moreover, the in-house ELISA could detect murine FGF18 in sera from mice overexpressing murine Fgf18 in hepatocytes, although the sensitivity in detecting murine FGF18 was relatively low. This FGF18 ELISA could be a valuable tool to validate FGF18 as a potential biomarker for cancer patients and to test the contribution of FGF18 for various disease models invivo and in vitro.

SARS-CoV-2 Syncytium under the Radar: Molecular Insights of the Spike-Induced Syncytia and Potential Strategies to Limit SARS-CoV-2 Replication

SARS-CoV-2 infection induces non-physiological syncytia when its spike fusogenic protein on the surface of the host cells interacts with the ACE2 receptor on adjacent cells. Spike-induced syncytia are beneficial for virus replication, transmission, and immune evasion, and contribute to the progression of COVID-19. In this review, we highlight the properties of viral fusion proteins, mainly the SARS-CoV-2 spike, and the involvement of the host factors in the fusion process. We also highlight the possible use of anti-fusogenic factors as an antiviral for the development of therapeutics against newly emerging SARS-CoV-2 variants and how the fusogenic property of the spike could be exploited for biomedical applications.

Whole Exome-Wide Association Identifies Rare Variants in GALNT9 Associated with Middle Eastern Papillary Thyroid Carcinoma Risk

Papillary thyroid carcinoma (PTC) is the commonest thyroid cancer. The majority of inherited causes of PTC remain elusive. However, understanding the genetic underpinnings and origins remains a challenging endeavor. An exome-wide association study was performed to identify rare germline variants in coding regions associated with PTC risk in the Middle Eastern population. By analyzing exome-sequencing data from 249 PTC patients (cases) and 1395 individuals without any known cancer (controls), GALNT9 emerged as being strongly associated with rare inactivating variants (RIVs) (4/249 cases vs. 1/1395 controls, OR = 22.75, p = 5.09 × 10-5). Furthermore, three genes, TRIM40, ARHGAP23, and SOX4, were enriched for rare damaging variants (RDVs) at the exome-wide threshold (p < 2.5 × 10-6). An additional seven genes (VARS1, ZBED9, PRRC2A, VWA7, TRIM31, TRIM40, and COL8A2) were associated with a Middle Eastern PTC risk based on the sequence kernel association test (SKAT). This study underscores the potential of GALNT9 and other implicated genes in PTC predisposition, illuminating the need for large collaborations and innovative approaches to understand the genetic heterogeneity of PTC predisposition.

O-Linked Sialoglycans Modulate the Proteolysis of SARS-CoV-2 Spike and Likely Contribute to the Mutational Trajectory in Variants of Concern

The emergence of a polybasic cleavage motif for the protease furin in SARS-CoV-2 spike has been established as a major factor for human viral transmission. The region N-terminal to that motif is extensively mutated in variants of concern (VOCs). Besides furin, spikes from these variants appear to rely on other proteases for maturation, including TMPRSS2. Glycans near the cleavage site have raised questions about proteolytic processing and the consequences of variant-borne mutations. Here, we identify that sialic acid-containing O-linked glycans on Thr678 of SARS-CoV-2 spike influence furin and TMPRSS2 cleavage and posit O-linked glycosylation as a likely driving force for the emergence of VOC mutations. We provide direct evidence that the glycosyltransferase GalNAc-T1 primes glycosylation at Thr678 in the living cell, an event that is suppressed by mutations in the VOCs Alpha, Delta, and Omicron. We found that the sole incorporation of N-acetylgalactosamine did not impact furin activity in synthetic O-glycopeptides, but the presence of sialic acid reduced the furin rate by up to 65%. Similarly, O-glycosylation with a sialylated trisaccharide had a negative impact on TMPRSS2 cleavage. With a chemistry-centered approach, we substantiate O-glycosylation as a major determinant of spike maturation and propose disruption of O-glycosylation as a substantial driving force for VOC evolution.

In silico analysis of mutations near S1/S2 cleavage site in SARS-CoV-2 spike protein reveals increased propensity of glycosylation in Omicron strain

Cleavage of the severe respiratory syndrome coronavirus-2 (SARS-CoV-2) spike protein has been demonstrated to contribute to viral-cell fusion and syncytia formation. Studies have shown that variants of concern (VOC) and variants of interest (VOI) show differing membrane fusion capacity. Mutations near cleavage motifs, such as the S1/S2 and S2' sites, may alter interactions with host proteases and, thus, the potential for fusion. The biochemical basis for the differences in interactions with host proteases for the VOC/VOI spike proteins has not yet been explored. Using sequence and structure-based bioinformatics, mutations near the VOC/VOI spike protein cleavage sites were inspected for their structural effects. All mutations found at the S1/S2 sites were predicted to increase affinity to the furin protease but not TMPRSS2. Mutations at the spike residue P681 in several strains, such P681R in the Delta strain, resulted in the disruption of a proline-directed kinase phosphorylation motif at the S1/S2 site, which may lessen the impact of phosphorylation for these variants. However, the unique N679K mutation in the Omicron strain was found to increase the propensity for O-linked glycosylation at the S1/S2 cleavage site, which may prevent recognition by proteases. Such glycosylation in the Omicron strain may hinder entry at the cell surface and, thus, decrease syncytia formation and induce cell entry through the endocytic pathway as has been shown in previous studies. Further experimental work is needed to confirm the effect of mutations and posttranslational modifications on SARS-CoV-2 spike protein cleavage sites.

Reg-1α, a New Substrate of Calpain-2 Depending on Its Glycosylation Status

Reg-1α/lithostathine, a protein mainly associated with the digestive system, was previously shown to be overexpressed in the pre-clinical stages of Alzheimer’s disease. In vitro, the glycosylated protein was reported to form fibrils at physiological pH following the proteolytic action of trypsin. However, the nature of the protease able to act in the central nervous system is unknown. In the present study, we showed that Reg-1α can be cleaved in vitro by calpain-2, the calcium activated neutral protease, overexpressed in neurodegenerative diseases. Using chemical crosslinking experiments, we found that the two proteins can interact with each other. Identification of the cleavage site using mass spectrometry, between Gln⁴ and Thr⁵, was found in agreement with the in silico prediction of the calpain cleavage site, in a position different from the one reported for trypsin, i.e., Arg¹¹-Ile¹² peptide bond. We showed that the cleavage was impeded by the presence of the neighboring glycosylation of Thr⁵. Moreover, in vitro studies using electron microscopy showed that calpain-cleaved protein does not form fibrils as observed after trypsin cleavage. Collectively, our results show that calpain-2 cleaves Reg-1α in vitro, and that this action is not associated with fibril formation.

Why All the Fury over Furin?

Analysis of the SARS-CoV-2 sequence revealed a multibasic furin cleavage site at the S1/S2 boundary of the spike protein distinguishing this virus from SARS-CoV. Furin, the best-characterized member of the mammalian proprotein convertases, is an ubiquitously expressed single pass type 1 transmembrane protein. Cleavage of SARS-CoV-2 spike protein by furin promotes viral entry into lung cells. While furin knockout is embryonically lethal, its knockout in differentiated somatic cells is not, thus furin provides an exciting therapeutic target for viral pathogens including SARS-CoV-2 and bacterial infections. Several peptide-based and small-molecule inhibitors of furin have been recently reported, and select cocrystal structures have been solved, paving the way for further optimization and selection of clinical candidates. This perspective highlights furin structure, substrates, recent inhibitors, and crystal structures with emphasis on furin's role in SARS-CoV-2 infection, where the current data strongly suggest its inhibition as a promising therapeutic intervention for SARS-CoV-2.

Developmental O-glycan profile analysis shows pentasaccharide mucin-type O-glycans are linked with pupation of Tribolium castaneum

Eukaryotic cells can decorate their proteins with carbohydrate structures or glycans, significantly affecting the properties and activities of these proteins. Despite the importance of protein glycosylation in numerous biological processes, our knowledge of this modification in insects is far from complete. While N-glycosylation is the most studied, the study of O-glycans in insects is still very fragmentary and these studies are limited to a specific developmental stage or a specific tissue. In this article, matrix-assisted laser desorption/ionization (MALDI)-Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) technology was used to analyze the O-glycan profile for the different developmental stages of egg, larva, pupa, and adult of the red flour beetle Tribolium castaneum, an important insect model and pest worldwide. The results on the O-glycan profile showed that the mucin-type glycans dominate the O-glycome of the red flour beetle. Interestingly, some of the more complex mucin-type O-glycans, such as a tetra- (O-GalNAcGalGlcAGalNAc) and pentasaccharide O-glycan (O-GalNAc(GalGlcA)GalNAcGlcA), were highly abundant during the pupa stage, the intermediate stage between larval and adult stage in holometabolous insects, demonstrating that insect metamorphosis is accompanied with a change in the insect O-glycan profile. Together with the N-glycan profile, the current data are a foundation to better understand the role of protein glycosylation in the development of insects.

Furin cleavage of the SARS-CoV-2 spike is modulated by O-glycosylation

The SARS-CoV-2 coronavirus responsible for the global pandemic contains a novel furin cleavage site in the spike protein (S) that increases viral infectivity and syncytia formation in cells. Here, we show that O-glycosylation near the furin cleavage site is mediated by members of the GALNT enzyme family, resulting in decreased furin cleavage and decreased syncytia formation. Moreover, we show that O-glycosylation is dependent on the novel proline at position 681 (P681). Mutations of P681 seen in the highly transmissible alpha and delta variants abrogate O-glycosylation, increase furin cleavage, and increase syncytia formation. Finally, we show that GALNT family members capable of glycosylating S are expressed in human respiratory cells that are targets for SARS-CoV-2 infection. Our results suggest that host O-glycosylation may influence viral infectivity/tropism by modulating furin cleavage of S and provide mechanistic insight into the role of the P681 mutations found in the highly transmissible alpha and delta variants.

The glycosylation in SARS-CoV-2 and its receptor ACE2

Coronavirus disease 2019 (COVID-19), a highly infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected more than 235 million individuals and led to more than 4.8 million deaths worldwide as of October 5 2021. Cryo-electron microscopy and topology show that the SARS-CoV-2 genome encodes lots of highly glycosylated proteins, such as spike (S), envelope (E), membrane (M), and ORF3a proteins, which are responsible for host recognition, penetration, binding, recycling and pathogenesis. Here we reviewed the detections, substrates, biological functions of the glycosylation in SARS-CoV-2 proteins as well as the human receptor ACE2, and also summarized the approved and undergoing SARS-CoV-2 therapeutics associated with glycosylation. This review may not only broad the understanding of viral glycobiology, but also provide key clues for the development of new preventive and therapeutic methodologies against SARS-CoV-2 and its variants.

Global functions of O-glycosylation: promises and challenges in O-glycobiology

Mucin type O-glycosylation is one of the most diverse types of glycosylation, playing essential roles in tissue development and homeostasis. In complex organisms, O-GalNAc glycans comprise a substantial proportion of the glycocalyx, with defined functions in hemostatic, gastrointestinal, and respiratory systems. Furthermore, O-GalNAc glycans are important players in host-microbe interactions, and changes in O-glycan composition are associated with certain diseases and metabolic conditions, which in some instances can be used for diagnosis or therapeutic intervention. Breakthroughs in O-glycobiology have gone hand in hand with the development of new technologies, such as advancements in mass spectrometry, as well as facilitation of genetic engineering in mammalian cell lines. High-throughput O-glycoproteomics have enabled us to draw a comprehensive map of O-glycosylation, and mining this information has supported the definition and confirmation of functions related to site-specific O-glycans. This includes protection from proteolytic cleavage, as well as modulation of binding affinity or receptor function. Yet, there is still much to discover, and among the important next challenges will be to define the context-dependent functions of O-glycans in different stages of cellular differentiation, cellular metabolism, host-microbiome interactions, and in disease. In this review, we present the achievements and the promises in O-GalNAc glycobiology driven by technological advances in analytical methods, genetic engineering, and systems biology.

Sclerostin Directly Stimulates Osteocyte Synthesis of Fibroblast Growth Factor-23

Osteocyte produced fibroblast growth factor 23 (FGF23) is the key regulator of serum phosphate (Pi) homeostasis. The interplay between parathyroid hormone (PTH), FGF23 and other proteins that regulate FGF23 production and serum Pi levels is complex and incompletely characterised. Evidence suggests that the protein product of the SOST gene, sclerostin (SCL), also a PTH target and also produced by osteocytes, plays a role in FGF23 expression, however the mechanism for this effect is unclear. Part of the problem of understanding the interplay of these mediators is the complex multi-organ system that achieves Pi homeostasis in vivo. In the current study, we sought to address this using a cell line model of the osteocyte, IDG-SW3, known to express FGF23 at both the mRNA and protein levels. In cultures of differentiated IDG-SW3 cells, both PTH_1-34 and recombinant human (rh) SCL remarkably induced Fgf23 mRNA expression dose-dependently within 3 h. Both rhPTH_1-34 and rhSCL also strongly induced C-terminal FGF23 protein secretion. Secreted intact FGF23 levels remained unchanged, consistent with constitutive post-translational cleavage of FGF23 in this cell model. Both rhPTH_1-34 and rhSCL treatments significantly suppressed mRNA levels of Phex, Dmp1 and Enpp1 mRNA, encoding putative negative regulators of FGF23 levels, and induced Galnt3 mRNA expression, encoding N-acetylgalactosaminyl-transferase 3 (GalNAc-T3), which protects FGF23 from furin-like proprotein convertase-mediated cleavage. The effect of both rhPTH_1-34 and rhSCL was antagonised by pre-treatment with the NF-κβ signalling inhibitors, BAY11 and TPCK. RhSCL also stimulated FGF23 mRNA expression in ex vivo cultures of human bone. These findings provide evidence for the direct regulation of FGF23 expression by sclerostin. Locally expressed sclerostin via the induction of FGF23 in osteocytes thus has the potential to contribute to the regulation of Pi homeostasis.

Glycosylation Regulates N-Terminal Proteolysis and Activity of the Chemokine CCL14

Chemokines are secreted proteins that regulate leukocyte migration during inflammatory responses by signaling through chemokine receptors. Full length CC chemokine ligand 14, CCL14(1-74), is a weak agonist for the chemokine receptor CCR1, but its activity is substantially enhanced upon proteolytic cleavage to CCL14(9-74). CCL14 is O-glycosylated at Ser7, adjacent to the site of proteolytic activation. To determine whether glycosylation regulates the activity of CCL14, we used native chemical ligation to prepare four homogeneously glycosylated variants of CCL14(1-74). Each protein was assembled from three synthetic peptide fragments in "one-pot" using two sequential ligation reactions. We show that while glycosylation of CCL14(1-74) did not affect CCR1 binding affinity or potency of activation, sialylated variants of CCL14(1-74) exhibited reduced activity after treatment with plasmin compared to nonsialylated forms. These data indicate that glycosylation may influence the biological activity of CCL14 by regulating its conversion from the full-length to the truncated, activated form.

RNAi-Mediated Silencing of Pgants Shows Core 1 O-Glycans Are Required for Pupation in Tribolium castaneum

Protein glycosylation is one of the most common and most important post-translational modifications. Despite the growing knowledge on N-glycosylation, the research on O-glycosylation is lagging behind. This study investigates the importance of O-glycosylation in the post-embryonic development of insects using the red flour beetle, Tribolium castaneum, as a model. We identified 28 O-glycosylation-related genes (OGRGs) in the genome of the red flour beetle. 14 OGRGs were selected for functional analysis based on their involvement in the initial attachment of the carbohydrate in the different O-glycosylation pathways or the further elongation of the most abundant O-glycans and, in addition, showing severe RNAi-induced phenotypes in Drosophila melanogaster. The expression profile of these OGRGs was mapped throughout the developmental stages of the insect and in the different tissues of the pupa and adult. Subsequently, these genes were silenced using RNA interference (RNAi) to analyze their role in development. A broad spectrum of phenotypes was observed: from subtle effects and disrupted wing formation when silencing the genes involved in O-mannosylation, to blockage of pupation and high mortality after silencing of the genes involved in O-GalNAc and core 1 O-glycan (O-GalNAc-Gal) synthesis. RNAi experiments were also performed to assess the effects of blocking multiple pathways of O-glycosylation. However, the observed phenotypes induced by multiple RNAi were similar to those of the single gene RNAi experiments. The silencing of OGRGs often resulted in high mortality and wing phenotypes, indicating the importance of O-glycosylation for the survival of the insect and the formation of wings during the post-embryonic development of T. castaneum.

Furin cleavage of the SARS-CoV-2 spike is modulated by O-glycosylation

The SARS-CoV-2 coronavirus responsible for the global pandemic contains a unique furin cleavage site in the spike protein (S) that increases viral infectivity and syncytia formation. Here, we show that O-glycosylation near the furin cleavage site is mediated by specific members of the GALNT enzyme family and is dependent on the novel proline at position 681 (P681). We further demonstrate that O-glycosylation of S decreases furin cleavage. Finally, we show that GALNT family members capable of glycosylating S are expressed in human respiratory cells that are targets for SARS-CoV-2 infection. Our results suggest that O-glycosylation may influence viral infectivity/tropism by modulating furin cleavage of S and provide mechanistic insight into the potential role of P681 mutations in the recently identified, highly transmissible B.1.1.7 variant.

N- and O-Glycosylation of the SARS-CoV-2 Spike Protein

Covid-19 pandemic outbreak is the reason of the current world health crisis. The development of effective antiviral compounds and vaccines requires detailed descriptive studies of SARS-CoV-2 proteins. The SARS-CoV-2 spike (S) protein mediates virion binding to the human cells through its interaction with the ACE2 cell surface receptor and is one of the prime immunization targets. A functional virion is composed of three S1 and three S2 subunits created by furin cleavage of the spike protein at R682, a polybasic cleavage site that differs from the SARS-CoV spike protein of 2002. By analysis of the protein produced in HEK293 cells, we observe that the spike is O-glycosylated on a threonine (T678) near the furin cleavage site occupied by core-1 and core-2 structures. In addition, we have identified eight additional O-glycopeptides on the spike glycoprotein and confirmed that the spike protein is heavily N-glycosylated. Our recently developed liquid chromatography-mass spectrometry methodology allowed us to identify LacdiNAc structural motifs on all occupied N-glycopeptides and polyLacNAc structures on six glycopeptides of the spike protein. In conclusion, our study substantially expands the current knowledge of the spike protein's glycosylation and enables the investigation of the influence of O-glycosylation on its proteolytic activation.

Mucin-Type O-GalNAc Glycosylation in Health and Disease

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

Global view of human protein glycosylation pathways and functions

Glycosylation is the most abundant and diverse form of post-translational modification of proteins that is common to all eukaryotic cells. Enzymatic glycosylation of proteins involves a complex metabolic network and different types of glycosylation pathways that orchestrate enormous amplification of the proteome in producing diversity of proteoforms and its biological functions. The tremendous structural diversity of glycans attached to proteins poses analytical challenges that limit exploration of specific functions of glycosylation. Major advances in quantitative transcriptomics, proteomics and nuclease-based gene editing are now opening new global ways to explore protein glycosylation through analysing and targeting enzymes involved in glycosylation processes. In silico models predicting cellular glycosylation capacities and glycosylation outcomes are emerging, and refined maps of the glycosylation pathways facilitate genetic approaches to address functions of the vast glycoproteome. These approaches apply commonly available cell biology tools, and we predict that use of (single-cell) transcriptomics, genetic screens, genetic engineering of cellular glycosylation capacities and custom design of glycoprotein therapeutics are advancements that will ignite wider integration of glycosylation in general cell biology.

An atlas of O-linked glycosylation on peptide hormones reveals diverse biological roles

Peptide hormones and neuropeptides encompass a large class of bioactive peptides that regulate physiological processes like anxiety, blood glucose, appetite, inflammation and blood pressure. Here, we execute a focused discovery strategy to provide an extensive map of O-glycans on peptide hormones. We find that almost one third of the 279 classified peptide hormones carry O-glycans. Many of the identified O-glycosites are conserved and are predicted to serve roles in proprotein processing, receptor interaction, biodistribution and biostability. We demonstrate that O-glycans positioned within the receptor binding motifs of members of the neuropeptide Y and glucagon families modulate receptor activation properties and substantially extend peptide half-lives. Our study highlights the importance of O-glycosylation in the biology of peptide hormones, and our map of O-glycosites in this large class of biomolecules serves as a discovery platform for an important class of molecules with potential opportunities for drug designs.

N and O glycosylation of the SARS-CoV-2 spike protein

Covid-19 pandemic outbreak is the reason of the current world health crisis. The development of effective antiviral compounds and vaccines requires detailed descriptive studies of the SARS-CoV-2 proteins. The SARS-CoV-2 spike (S) protein mediates virion binding to the human cells through its interaction with the ACE2 cell surface receptor and is one of the prime immunization targets. A functional virion is composed of three S1 and three S2 subunits created by furin cleavage of the spike protein at R682, a polybasic cleavage sites that differs from the SARS-CoV spike protein of 2002. We observe that the spike protein is O-glycosylated on a threonine (T678) near the furin cleavage site occupied by core-1 and core-2 structures. In addition, we have identified eight additional O-glycopeptides on the spike glycoprotein and we confirmed that the spike protein is heavily N-glycosylated. Our recently developed LC-MS/MS methodology allowed us to identify LacdiNAc structural motifs on all occupied N-glycopeptides and polyLacNAc structures on six glycopeptides of the spike protein. In conclusion, our study substantially expands the current knowledge of the spike proteins glycosylation and enables the investigation of the influence of the O-glycosylation on its proteolytic activation.

Comparison of Severe Acute Respiratory Syndrome Coronavirus 2 Spike Protein Binding to ACE2 Receptors from Human, Pets, Farm Animals, and Putative Intermediate Hosts

The emergence of a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), resulted in a pandemic. Here, we used X-ray structures of human ACE2 bound to the receptor-binding domain (RBD) of the spike protein (S) from SARS-CoV-2 to predict its binding to ACE2 proteins from different animals, including pets, farm animals, and putative intermediate hosts of SARS-CoV-2. Comparing the interaction sites of ACE2 proteins known to serve or not serve as receptors allows the definition of residues important for binding. From the 20 amino acids in ACE2 that contact S, up to 7 can be replaced and ACE2 can still function as the SARS-CoV-2 receptor. These variable amino acids are clustered at certain positions, mostly at the periphery of the binding site, while changes of the invariable residues prevent S binding or infection of the respective animal. Some ACE2 proteins even tolerate the loss or acquisition of N-glycosylation sites located near the S interface. Of note, pigs and dogs, which are not infected or are not effectively infected and have only a few changes in the binding site, exhibit relatively low levels of ACE2 in the respiratory tract. Comparison of the RBD of S of SARS-CoV-2 with that from bat coronavirus strain RaTG13 (Bat-CoV-RaTG13) and pangolin coronavirus (Pangolin-CoV) strain hCoV-19/pangolin/Guangdong/1/2019 revealed that the latter contains only one substitution, whereas Bat-CoV-RaTG13 exhibits five. However, ACE2 of pangolin exhibits seven changes relative to human ACE2, and a similar number of substitutions is present in ACE2 of bats, raccoon dogs, and civets, suggesting that SARS-CoV-2 may not be especially adapted to ACE2 of any of its putative intermediate hosts. These analyses provide new insight into the receptor usage and animal source/origin of SARS-CoV-2.IMPORTANCE SARS-CoV-2 is threatening people worldwide, and there are no drugs or vaccines available to mitigate its spread. The origin of the virus is still unclear, and whether pets and livestock can be infected and transmit SARS-CoV-2 are important and unknown scientific questions. Effective binding to the host receptor ACE2 is the first prerequisite for infection of cells and determines the host range. Our analysis provides a framework for the prediction of potential hosts of SARS-CoV-2. We found that ACE2 from species known to support SARS-CoV-2 infection tolerate many amino acid changes, indicating that the species barrier might be low. Exceptions are dogs and especially pigs, which revealed relatively low ACE2 expression levels in the respiratory tract. Monitoring of animals is necessary to prevent the generation of a new coronavirus reservoir. Finally, our analysis also showed that SARS-CoV-2 may not be specifically adapted to any of its putative intermediate hosts.

O-glycan initiation directs distinct biological pathways and controls epithelial differentiation

Post-translational modifications (PTMs) greatly expand the function and potential for regulation of protein activity, and O-glycosylation is among the most abundant and diverse PTMs. Initiation of O-GalNAc glycosylation is regulated by 20 distinct GalNAc-transferases (GalNAc-Ts), and deficiencies in individual GalNAc-Ts are associated with human disease, causing subtle but distinct phenotypes in model organisms. Here, we generate a set of isogenic keratinocyte cell lines lacking either of the three dominant and differentially expressed GalNAc-Ts. Through the ability of keratinocytes to form epithelia, we investigate the phenotypic consequences of the loss of individual GalNAc-Ts. Moreover, we probe the cellular responses through global transcriptomic, differential glycoproteomic, and differential phosphoproteomic analyses. We demonstrate that loss of individual GalNAc-T isoforms causes distinct epithelial phenotypes through their effect on specific biological pathways; GalNAc-T1 targets are associated with components of the endomembrane system, GalNAc-T2 targets with cell-ECM adhesion, and GalNAc-T3 targets with epithelial differentiation. Thus, GalNAc-T isoforms serve specific roles during human epithelial tissue formation.

G Protein-Coupled Receptors in the Sweet Spot: Glycosylation and other Post-translational Modifications

Post-translational modifications (PTMs) are a fundamental phenomenon across all classes of life and several hundred different types have been identified. PTMs contribute widely to the biological functions of proteins and greatly increase their diversity. One important class of proteins regulated by PTMs, is the cell surface expressed G protein-coupled receptors (GPCRs). While most PTMs have been shown to exert distinct biological functions, we are only beginning to approach the complexity that the potential interplay between different PTMs may have on biological functions and their regulation. Importantly, PTMs and their potential interplay represent an appealing mechanism for cell and tissue specific regulation of GPCR function and may partially contribute to functional selectivity of some GPCRs. In this review we highlight examples of PTMs located in GPCR extracellular domains, with special focus on glycosylation and the potential interplay with other close-by PTMs such as tyrosine sulfation, proteolytic cleavage, and phosphorylation.

Galnt11 regulates kidney function by glycosylating the endocytosis receptor megalin to modulate ligand binding

Chronic kidney disease (CKD) affects more than 20 million Americans and ∼10% of the population worldwide. Genome-wide association studies (GWAS) of kidney functional decline have identified genes associated with CKD, but the precise mechanisms by which they influence kidney function remained largely unexplored. Here, we examine the role of 1 GWAS-identified gene by creating mice deficient for Galnt11, which encodes a member of the enzyme family that initiates protein O-glycosylation, an essential posttranslational modification known to influence protein function and stability. We find that Galnt11-deficient mice display low-molecular-weight proteinuria and have specific defects in proximal tubule-mediated resorption of vitamin D binding protein, α₁-microglobulin, and retinol binding protein. Moreover, we identify the endocytic receptor megalin (LRP2) as a direct target of Galnt11 in vivo. Megalin in Galnt11-deficient mice displays reduced ligand binding and undergoes age-related loss within the kidney. Differential mass spectrometry revealed specific sites of Galnt11-mediated glycosylation within mouse kidney megalin/LRP2 that are known to be involved in ligand binding, suggesting that O-glycosylation directly influences the ability to bind ligands. In support of this, recombinant megalin containing these sites displayed reduced albumin binding in cells deficient for Galnt11 Our results provide insight into the association between GALNT11 and CKD, and identify a role for Galnt11 in proper kidney function.

A novel cell-based sensor detecting the activity of individual basic proprotein convertases

The basic proprotein convertases (PCs) furin, PC1/3, PC2, PC5/6, PACE4, PC4, and PC7 are promising drug targets for human diseases. However, developing selective inhibitors remains challenging due to overlapping substrate recognition motifs and limited structural information. Classical drug screening approaches for basic PC inhibitors involve homogeneous biochemical assays using soluble recombinant enzymes combined with fluorogenic substrate peptides that may not accurately recapitulate the complex cellular context of the basic PC-substrate interaction. Herein we report basic PC sensor (BPCS), a novel cell-based molecular sensor that allows rapid screening of candidate inhibitors and their selectivity toward individual basic PCs within mammalian cells. BPCS consists of Gaussia luciferase linked to a sortilin-1 membrane anchor via a cleavage motif that allows efficient release of luciferase specifically if individual basic PCs are provided in the same membrane. Screening of selected candidate peptidomimetic inhibitors revealed that BPCS can readily distinguish between general and selective PC inhibitors in a high-throughput screening format. The robust and cost-effective assay format of BPCS makes it suitable to identify novel specific small-molecule inhibitors against basic PCs for therapeutic application. Its cell-based nature will allow screening for drug targets in addition to the catalytically active mature enzyme, including maturation, transport, and cellular factors that modulate the enzyme's activity. This broadened 'target range' will enhance the likelihood to identify novel small-molecule compounds that inhibit basic PCs in a direct or indirect manner and represents a conceptual advantage.

Discovery of O-glycans on atrial natriuretic peptide (ANP) that affect both its proteolytic degradation and potency at its cognate receptor

Atrial natriuretic peptide (ANP) is a peptide hormone that in response to atrial stretch is secreted from atrial myocytes into the circulation, where it stimulates vasodilatation and natriuresis. ANP is an important biomarker of heart failure where low plasma concentrations exclude cardiac dysfunction. ANP is a member of the natriuretic peptide (NP) family, which also includes the B-type natriuretic peptide (BNP) and the C-type natriuretic peptide. The proforms of these hormones undergo processing to mature peptides, and for proBNP, this process has previously been demonstrated to be regulated by O-glycosylation. It has been suggested that proANP also may undergo post-translational modifications. Here, we conducted a targeted O-glycoproteomics approach to characterize O-glycans on NPs and demonstrate that all NP members can carry O-glycans. We identified four O-glycosites in proANP in the porcine heart, and surprisingly, two of these were located on the mature bioactive ANP itself. We found that one of these glycans is located within a conserved sequence motif of the receptor-binding region, suggesting that O-glycans may serve a function beyond intracellular processing and maturation. We also identified an O-glycoform of proANP naturally occurring in human circulation. We demonstrated that site-specific O-glycosylation shields bioactive ANP from proteolytic degradation and modifies potency at its cognate receptor in vitro Furthermore, we showed that ANP O-glycosylation attenuates acute renal and cardiovascular ANP actions in vivo The discovery of novel glycosylated ANP proteoforms reported here significantly improves our understanding of cardiac endocrinology and provides important insight into the etiology of heart failure.

Polypeptide GalNAc-Ts: from redundancy to specificity

Mucin-type O-glycosylation is a post-translational modification (PTM) that is predicted to occur in more than the 80% of the proteins that pass through the Golgi apparatus. This PTM is initiated by a family of polypeptide GalNAc-transferases (GalNAc-Ts) that modify Ser and Thr residues of proteins through the addition of a GalNAc moiety. These enzymes are type II membrane proteins that consist of a Golgi luminal catalytic domain connected by a flexible linker to a ricin type lectin domain. Together, both domains account for the different glycosylation preferences observed among isoenzymes. Although it is well accepted that most of the family members share some degree of redundancy toward their protein and glycoprotein substrates, it has been recently found that several GalNAc-Ts also possess activity toward specific targets. Despite the high similarity between isoenzymes, structural differences have recently been reported that are key to understanding the molecular basis of both their redundancy and specificity. The present review focuses on the molecular aspects of the protein substrate recognition and the different glycosylation preferences of these enzymes, which in turn will serve as a roadmap to the rational design of specific modulators of mucin-type O-glycosylation.

N-Linked Glycosylation-Dependent and -Independent Mechanisms Regulating CTRP12 Cleavage, Secretion, and Stability

C1q/TNF-related protein 12 (CTRP12) is a secreted regulator of glucose and lipid metabolism. It circulates in plasma as a full-length protein or as a cleaved isoform generated by furin/PCSK3 cleavage. These isoforms preferentially activate different signaling pathways, and their ratio in plasma is altered in obesity and diabetes. Here, we show that three conserved asparagine residues (Asn-39, Asn-287, and Asn-297) play important roles in modulating CTRP12 cleavage, secretion, and stability. Mass spectrometry analysis provided direct evidence of Asn-39 glycosylation. When N-linked glycosylation was inhibited by tunicamycin or abolished by the N39Q, N39A, or T41A mutation, CTRP12 cleavage was enhanced. Complex-type N-glycans on CTRP12 blocked cleavage by the Golgi-localized furin. In N-acetylglucosaminyltransferase I (GnTI)-deficient cells that could not form hybrid and complex-type N-glycans in the Golgi, CTRP12 cleavage was enhanced, and re-expressing GnTI reduced cleavage. Replacing the nonglycosylated Asn-297 with glutamine or alanine also increased CTRP12 cleavage. Both Asn-39 and Asn-297 contributed independently to CTRP12 cleavage: maximum cleavage was observed in the double mutant. In addition, CTRP12 cleavage was abolished in furin-deficient cells and restored by furin re-expression. Replacing the nonglycosylated Asn-287 with glutamine or alanine resulted in protein misfolding and aggregation, leading to retention in the endoplasmic reticulum. Cycloheximide chase analyses indicated reduced protein stability for N39Q, T41A, and N297Q mutants. Lastly, we show that increasing the flux through the hexosamine biosynthesis pathway by exogenous glucosamine, known to disrupt protein glycosylation, also promoted CTRP12 cleavage. Combined, these data highlight glycosylation-dependent and -independent mechanisms regulating CTRP12 cleavage, secretion, and protein stability.

Highly Efficient Identification of O-GalNAc Glycosylation by an Acid-Assisted Glycoform Simplification Approach

Compared with N-linked glycosylation, the analysis of O-GalNAc glycosylation is extremely challenging due to the high structure diversity of glycans and lack of glycosidases to release O-GalNAc glycans. In this work, a glycoform simplification strategy by combining HILIC enrichment with chemical de-sialylation to characterize O-GalNAc glycosylation of human serum is presented. This method is first validated by using the bovine fetuin as the test sample. It is found that more than 90% of the sialic acid residues can be removed from bovine fetuin by the acid-assisted de-sialylation method, which significantly simplifies the glycan structure and improves identification sensitivity. Indeed, the number of identified peptide backbones increases nearly one fold when this strategy is used. This method is further applied to analyze the human serum sample, where 185 O-GalNAc modified peptide sequences corresponding to 94 proteins with high confidence (FDR (false detection rate) <1%) are identified. This straight forward strategy can significantly reduce the variations of glycan structures, and is applicable to analysis of other biological samples with high complexity.

TAILS N-terminomics and proteomics reveal complex regulation of proteolytic cleavage by O-glycosylation

Proteolytic processing is an irreversible post-translational modification functioning as a ubiquitous regulator of cellular activity. Protease activity is tightly regulated via control of gene expression, enzyme and substrate compartmentalization, zymogen activation, enzyme inactivation, and substrate availability. Emerging evidence suggests that proteolysis can also be regulated by substrate glycosylation and that glycosylation of individual sites on a substrate can decrease or, in rare cases, increase its sensitivity to proteolysis. Here, we investigated the relationship between site-specific, mucin-type (or GalNAc-type) O-glycosylation and proteolytic cleavage of extracellular proteins. Using in silico analysis, we found that O-glycosylation and cleavage sites are significantly associated with each other. We then used a positional proteomic strategy, terminal amine isotopic labeling of substrates (TAILS), to map the in vivo cleavage sites in HepG2 SimpleCells with and without one of the key initiating GalNAc transferases, GalNAc-T2, and after treatment with exogenous matrix metalloproteinase 9 (MMP9) or neutrophil elastase. Surprisingly, we found that loss of GalNAc-T2 not only increased cleavage, but also decreased cleavage across a broad range of other substrates, including key regulators of the protease network. We also found altered processing of several central regulators of lipid homeostasis, including apolipoprotein B and the phospholipid transfer protein, providing new clues to the previously reported link between GALNT2 and lipid homeostasis. In summary, we show that loss of GalNAc-T2 O-glycosylation leads to a general decrease in cleavage and that GalNAc-T2 O-glycosylation affects key regulators of the cellular proteolytic network, including multiple members of the serpin family.

Fine-Tuning Limited Proteolysis: A Major Role for Regulated Site-Specific O-Glycosylation

Limited proteolytic processing is an essential and ubiquitous post-translational modification (PTM) affecting secreted proteins; failure to regulate the process is often associated with disease. Glycosylation is also a ubiquitous protein PTM and site-specific O-glycosylation in close proximity to sites of proteolysis can regulate and direct the activity of proprotein convertases, a disintegrin and metalloproteinases (ADAMs), and metalloproteinases affecting the activation or inactivation of many classes of proteins, including G-protein-coupled receptors (GPCRs). Here, we summarize the emerging data that suggest O-glycosylation to be a key regulator of limited proteolysis, and highlight the potential for crosstalk between multiple PTMs.

N-acetylgalactosaminyltransferases in cancer

Aberrant mucin-type O-glycosylation by glycosyltransferases is a well-described hallmark of many cancers and is also associated with additional non-cancerous developmental and metabolic disorders. The current review focuses on N-acetylgalactosaminyltransferase genes (GALNTs) and proteins (GalNAcTs) to illustrate their importance in cancer biology. Aberrant O-glycosylation by GalNAcTs activates a wide range of proteins that carry out interactions of sessile and motile cells affecting organogenesis, responses to agonists and stimulating hyperproliferation and metastatisation of neoplastic cells. As genome-wide analyses have provided abundant clues regarding under- or over-expressed genes that characterize different types of cancers, GALNTs and their transferase products have attracted attention by being unexpected actors in neoplastic contexts. We intend to review the current knowledge on GALNTs and their encoded transferases in cancer and suggest what could be the significance of such information in cancer pathogenesis and management.

Expanding the chondroitin glycoproteome of Caenorhabditis elegans

Chondroitin sulfate proteoglycans (CSPGs) are important structural components of connective tissues in essentially all metazoan organisms. In vertebrates, CSPGs are involved also in more specialized processes such as neurogenesis and growth factor signaling. In invertebrates, however, knowledge of CSPGs core proteins and proteoglycan-related functions is relatively limited, even for Caenorhabditis elegans. This nematode produces large amounts of non-sulfated chondroitin in addition to low-sulfated chondroitin sulfate chains. So far, only nine core proteins (CPGs) have been identified, some of which have been shown to be involved in extracellular matrix formation. We recently introduced a protocol to characterize proteoglycan core proteins by identifying CS-glycopeptides with a combination of biochemical enrichment, enzymatic digestion, and nano-scale liquid chromatography MS/MS analysis. Here, we have used this protocol to map the chondroitin glycoproteome in C. elegans, resulting in the identification of 15 novel CPG proteins in addition to the nine previously established. Three of the newly identified CPGs displayed homology to vertebrate proteins. Bioinformatics analysis of the primary protein sequences revealed that the CPG proteins altogether contained 19 unique functional domains, including Kunitz and endostatin domains, suggesting direct involvement in protease inhibition and axonal migration, respectively. The analysis of the core protein domain organization revealed that all chondroitin attachment sites are located in unstructured regions. Our results suggest that CPGs display a much greater functional and structural heterogeneity than previously appreciated and indicate that specialized proteoglycan-mediated functions evolved early in metazoan evolution.

Recent advances in methods for the analysis of protein o-glycosylation at proteome level

O-Glycosylation, which refers to the glycosylation of the hydroxyl group of side chains of Serine/Threonine/Tyrosine residues, is one of the most common post-translational modifications. Compared with N-linked glycosylation, O-glycosylation is less explored because of its complex structure and relatively low abundance. Recently, O-glycosylation has drawn more and more attention for its various functions in many sophisticated biological processes. To obtain a deep understanding of O-glycosylation, many efforts have been devoted to develop effective strategies to analyze the two most abundant types of O-glycosylation, i.e. O-N-acetylgalactosamine and O-N-acetylglucosamine glycosylation. In this review, we summarize the proteomics workflows to analyze these two types of O-glycosylation. For the large-scale analysis of mucin-type glycosylation, the glycan simplification strategies including the ''SimpleCell'' technology were introduced. A variety of enrichment methods including lectin affinity chromatography, hydrophilic interaction chromatography, hydrazide chemistry, and chemoenzymatic method were introduced for the proteomics analysis of O-N-acetylgalactosamine and O-N-acetylglucosamine glycosylation.

Alternative cleavage of the bone morphogenetic protein (BMP), Gbb, produces ligands with distinct developmental functions and receptor preferences

The family of TGF-β and bone morphogenetic protein (BMP) signaling proteins has numerous developmental and physiological roles. They are made as proprotein dimers and then cleaved by proprotein convertases to release the C-terminal domain as an active ligand dimer. Multiple proteolytic processing sites in Glass bottom boat (Gbb), the Drosophila BMP7 ortholog, can produce distinct ligand forms. Cleavage at the S1 or atypical S0 site in Gbb produces Gbb15, the conventional small BMP ligand, whereas NS site cleavage produces a larger Gbb38 ligand. We hypothesized that the Gbb prodomain is involved not only in regulating the production of specific ligands but also their signaling output. We found that blocking NS cleavage increased association of the full-length prodomain with Gbb15, resulting in a concomitant decrease in signaling activity. Moreover, NS cleavage was required in vivo for Gbb-Decapentaplegic (Dpp) heterodimer-mediated wing vein patterning but not for Gbb15-Dpp heterodimer activity in cell culture. Gbb NS cleavage was also required for viability through its regulation of pupal ecdysis in a type II receptor Wishful thinking (Wit)-dependent manner. In fact, Gbb38-mediated signaling exhibits a preference for Wit over the other type II receptor Punt. Finally, we discovered that Gbb38 is produced when processing at the S1/S0 site is blocked by O-linked glycosylation in third instar larvae. Our findings demonstrate that BMP prodomain cleavage ensures that the mature ligand is not inhibited by the prodomain. Furthermore, alternative processing of BMP proproteins produces ligands that signal through different receptors and exhibit specific developmental functions.

Detection of Biosynthetic Precursors, Discovery of Glycosylated Forms, and Homeostasis of Calcitonin in Human Cancer Cells

The peptide hormone calcitonin is intimately connected with human cancer development and proliferation. Its biosynthesis is reasoned to proceed via glycine-, α-hydroxyglycine-, glycyllysine-, and glycyllysyllysine-extended precursors; however, as a result of the limitations of current analytical methods, until now, there has been no procedure capable of detecting these individual species in cell or tissue samples. Therefore, their presence and dynamics in cancer had not been established. Here, we report the first methodology for the separation, detection, and quantification of calcitonin and each of its precursors in human cancer cells. We also report the discovery and characterization of O-glycosylated calcitonin and its analogous biosynthetic precursors. Through direct and simultaneous analysis of the glycosylated and nonglycosylated species, we interrogate the hormone biosynthesis. This shows that the cellular calcitonin level is maintained to mitigate effects of biosynthetic enzyme inhibitors that substantially change the proportions of calcitonin-related species released into the culture medium.

Systematic identification of the protein substrates of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase-T1/T2/T3 using a human proteome microarray

O-GalNAc glycosylation is the initial step of the mucin-type O-glycosylation. In humans, it is catalyzed by a family of 20 homologous UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts). So far, there is very limited information on their protein substrate specificities. In this study, we developed an on-chip ppGalNAc-Ts assay that could rapidly and systematically identify the protein substrates of each ppGalNAc-T. In detail, we utilized a human proteome microarray as the protein substrates and UDP-GalNAz as the nucleotide sugar donor for click chemistry detection. From a total of 16 368 human proteins, we identified 570 potential substrates of ppGalNAc-T1, T2, and T3. Among them, 128 substrates were overlapped, while the rest were isoform specific. Further cluster analysis of these substrates showed that the substrates of ppGalNAc-T1 had a closer phylogenetic relationship with that of ppGalNAc-T3 compared with ppGalNAc-T2, which was consistent with the topology of the phylogenetic tree of these ppGalNAc-Ts. Taken together, our microarray-based enzymatic assay comprehensively reveals the substrate profile of the ppGalNAc-T1, T2, and T3, which not only provides a plausible explanation for their partial functional redundancy as reported, but clearly implies some specialized roles of each enzyme in different biological processes.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article 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 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Mechanistic insights into ectodomain shedding: susceptibility of CADM1 adhesion molecule is determined by alternative splicing and O-glycosylation

Ectodomain shedding (shedding) is a post-translational modification, which liberates the extracellular domain of membrane proteins through juxtamembrane processing executed mainly by the ADAM (a disintegrin and metalloprotease) family of metalloproteases. Because shedding alters characteristics of cells in a rapid and irreversible manner, it should be strictly regulated. However, the molecular mechanisms determining membrane protein susceptibility to shedding (shedding susceptibility) are largely unknown. Here we report that alternative splicing can give rise to both shedding-susceptible and shedding-resistant CADM1 (cell adhesion molecule 1) variant proteins. We further show that O-glycans adjacent to the shedding cleavage site interfere with CADM1 shedding, and the only 33-bp alternative exon confers shedding susceptibility to CADM1 by inserting five non-glycosylatable amino acids between interfering O-glycans and the shedding cleavage site. These results demonstrate that shedding susceptibility of membrane protein can be determined at two different levels of its biosynthesis pathway, alternative splicing and O-glycosylation.

Inhibitor of ppGalNAc-T3-mediated O-glycosylation blocks cancer cell invasiveness and lowers FGF23 levels

Small molecule inhibitors of site-specific O-glycosylation by the polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) family are currently unavailable but hold promise as therapeutics, especially if selective against individual ppGalNAc-T isozymes. To identify a compound targeting the ppGalNAc-T3 isozyme, we screened libraries to find compounds that act on a cell-based fluorescence sensor of ppGalNAc-T3 but not on a sensor of ppGalNAc-T2. This identified a hit that subsequent in vitro analysis showed directly binds and inhibits purified ppGalNAc-T3 with no detectable activity against either ppGalNAc-T2 or ppGalNAc-T6. Remarkably, the inhibitor was active in two medically relevant contexts. In cell culture, it opposed increased cancer cell invasiveness driven by upregulated ppGalNAc-T3 suggesting the inhibitor might be anti-metastatic. In cells and mice, it blocked ppGalNAc-T3-mediated glycan-masking of FGF23 thereby increasing its cleavage, a possible treatment of chronic kidney disease. These findings establish a pharmacological approach for the ppGalNAc-transferase family and suggest that targeting specific ppGalNAc-transferases will yield new therapeutics.

Site-specific O-Glycosylation by Polypeptide N-Acetylgalactosaminyltransferase 2 (GalNAc-transferase T2) Co-regulates β1-Adrenergic Receptor N-terminal Cleavage

The β1-adrenergic receptor (β1AR) is a G protein-coupled receptor (GPCR) and the predominant adrenergic receptor subtype in the heart, where it mediates cardiac contractility and the force of contraction. Although it is the most important target for β-adrenergic antagonists, such as β-blockers, relatively little is yet known about its regulation. We have shown previously that β1AR undergoes constitutive and regulated N-terminal cleavage participating in receptor down-regulation and, moreover, that the receptor is modified by O-glycosylation. Here we demonstrate that the polypeptide GalNAc-transferase 2 (GalNAc-T2) specifically O-glycosylates β1AR at five residues in the extracellular N terminus, including the Ser-49 residue at the location of the common S49G single-nucleotide polymorphism. Using in vitro O-glycosylation and proteolytic cleavage assays, a cell line deficient in O-glycosylation, GalNAc-T-edited cell line model systems, and a GalNAc-T2 knock-out rat model, we show that GalNAc-T2 co-regulates the metalloproteinase-mediated limited proteolysis of β1AR. Furthermore, we demonstrate that impaired O-glycosylation and enhanced proteolysis lead to attenuated receptor signaling, because the maximal response elicited by the βAR agonist isoproterenol and its potency in a cAMP accumulation assay were decreased in HEK293 cells lacking GalNAc-T2. Our findings reveal, for the first time, a GPCR as a target for co-regulatory functions of site-specific O-glycosylation mediated by a unique GalNAc-T isoform. The results provide a new level of β1AR regulation that may open up possibilities for new therapeutic strategies for cardiovascular diseases.

Characterizing the O-glycosylation landscape of human plasma, platelets, and endothelial cells

The hemostatic system comprises platelet aggregation, coagulation, and fibrinolysis, and is critical to the maintenance of vascular integrity. Multiple studies indicate that glycans play important roles in the hemostatic system; however, most investigations have focused on N-glycans because of the complexity of O-glycan analysis. Here we performed the first systematic analysis of native-O-glycosylation using lectin affinity chromatography coupled to liquid chromatography mass spectrometry (LC-MS)/MS to determine the precise location of O-glycans in human plasma, platelets, and endothelial cells, which coordinately regulate hemostasis. We identified the hitherto largest O-glycoproteome from native tissue with a total of 649 glycoproteins and 1123 nonambiguous O-glycosites, demonstrating that O-glycosylation is a ubiquitous modification of extracellular proteins. Investigation of the general properties of O-glycosylation established that it is a heterogeneous modification, frequently occurring at low density within disordered regions in a cell-dependent manner. Using an unbiased screen to identify associations between O-glycosites and protein annotations we found that O-glycans were over-represented close (± 15 amino acids) to tandem repeat regions, protease cleavage sites, within propeptides, and located on a select group of protein domains. The importance of O-glycosites in proximity to proteolytic cleavage sites was further supported by in vitro peptide assays demonstrating that proteolysis of key hemostatic proteins can be inhibited by the presence of O-glycans. Collectively, these data illustrate the global properties of native O-glycosylation and provide the requisite roadmap for future biomarker and structure-function studies.

MiR30-GALNT1/2 Axis-Mediated Glycosylation Contributes to the Increased Secretion of Inactive Human Prohormone for Brain Natriuretic Peptide (proBNP) From Failing Hearts

BACKGROUND

Recent studies have shown that plasma levels of the biologically inactive prohormone for brain natriuretic peptide (proBNP) are increased in patients with heart failure. This can contribute to a reduction in the effectiveness of circulating BNP and exacerbate heart failure progression. The precise mechanisms governing the increase in proBNP remain unclear, however.

METHODS AND RESULTS

We used our recently developed, highly sensitive human proBNP assay system to investigate the mechanisms underlying the increase in plasma proBNP levels. We divided 53 consecutive patients hospitalized with heart failure into 2 groups based on their aortic plasma levels of immunoreactive BNP. Patients with higher levels exhibited more severe heart failure, a higher proportion of proBNP among the immunoreactive BNP forms secreted from failing hearts, and a weaker effect of BNP as estimated from the ratio of plasma cyclic guanosine monophosphate levels to log-transformed plasma BNP levels. Glycosylation at threonines 48 and 71 of human proBNP contributed to the increased secretion of proBNP by attenuating its processing, and GalNAc-transferase (GALNT) 1 and 2 mediated the glycosylation-regulated increase in cardiac human proBNP secretion. Cardiac GALNT1 and 2 expression was suppressed by microRNA (miR)-30, which is abundantly expressed in the myocardium of healthy hearts, but is suppressed in failing hearts.

CONCLUSIONS

We have elucidated a novel miR-30-GALNT1/2 axis whose dysregulation increases the proportion of inactive proBNP secreted by the heart and impairs the compensatory actions of BNP during the progression of heart failure.

Effects of Glycosylation on the Enzymatic Activity and Mechanisms of Proteases

Posttranslational modifications are an important feature of most proteases in higher organisms, such as the conversion of inactive zymogens into active proteases. To date, little information is available on the role of glycosylation and functional implications for secreted proteases. Besides a stabilizing effect and protection against proteolysis, several proteases show a significant influence of glycosylation on the catalytic activity. Glycans can alter the substrate recognition, the specificity and binding affinity, as well as the turnover rates. However, there is currently no known general pattern, since glycosylation can have both stimulating and inhibiting effects on activity. Thus, a comparative analysis of individual cases with sufficient enzyme kinetic and structural data is a first approach to describe mechanistic principles that govern the effects of glycosylation on the function of proteases. The understanding of glycan functions becomes highly significant in proteomic and glycomic studies, which demonstrated that cancer-associated proteases, such as kallikrein-related peptidase 3, exhibit strongly altered glycosylation patterns in pathological cases. Such findings can contribute to a variety of future biomedical applications.

Unraveling the Molecular Complexity of O-Glycosylated Endogenous (N-Terminal) pro-B-Type Natriuretic Peptide Forms in Blood Plasma of Patients with Severe Heart Failure

BACKGROUND

Currently, N-terminal pro-B-type natriuretic peptide (NT-proBNP) and its physiologically active counterpart, BNP, are most frequently used as biomarkers for diagnosis, prognosis, and disease monitoring of heart failure (HF). Commercial NT-proBNP and BNP immunoassays cross-react to varying degrees with unprocessed proBNP, which is also found in the circulation. ProBNP processing and immunoassay response are related to O-linked glycosylation of NT-proBNP and proBNP. There is a clear and urgent need to identify the glycosylation sites in the endogenously circulating peptides requested by the community to gain further insights into the different naturally occurring forms.

METHODS

The glycosylation sites of (NT-) proBNP (NT-proBNP and/or proBNP) were characterized in leftovers of heparinized plasma samples of severe HF patients (NT-proBNP: >10000 ng/L) by using tandem immunoaffinity purification, sequential exoglycosidase treatment for glycan trimming, β-elimination and Michael addition chemistry, as well as high-resolution nano-flow liquid chromatography electrospray multistage mass spectrometry.

RESULTS

We describe 9 distinct glycosylation sites on circulating (NT-) proBNP in HF patients. Differentially glycosylated variants were detected based on highly accurate mass determination and multistage mass spectrometry. Remarkably, for each of the identified proteolytic glycopeptides, a nonglycosylated form also was detectable.

CONCLUSIONS

Our results directly demonstrate for the first time a rather complex distribution of the endogenously circulating glycoforms by mass spectrometric analysis in HF patients, and show 9 glycosites in human (NT-) proBNP. This information may also have an impact on commercial immunoassays applying antibodies specific for the central region of (NT-) proBNP, which detect mostly nonglycosylated forms.

O-Glycosylation of a Secretory Granule Membrane Enzyme Is Essential for Its Endocytic Trafficking

Peptidylglycine α-amidating monooxygenase (PAM) (EC 1.14.17.3) catalyzes peptide amidation, a crucial post-translational modification, through the sequential actions of its monooxygenase (peptidylglycine α-hydroxylating monooxygenase) and lyase (peptidyl-α-hydroxyglycine α-amidating lyase (PAL)) domains. Alternative splicing generates two different regions that connect the protease-resistant catalytic domains. Inclusion of exon 16 introduces a pair of Lys residues, providing a site for controlled endoproteolytic cleavage of PAM and the separation of soluble peptidylglycine α-hydroxylating monooxygenase from membrane-associated PAL. Exon 16 also includes two O-glycosylation sites. PAM-1 lacking both glycosylation sites (PAM-1/OSX; where OSX is O-glycan-depleted mutant of PAM-1) was stably expressed in AtT-20 corticotrope tumor cells. In PAM-1/OSX, a cleavage site for furin-like convertases was exposed, generating a shorter form of membrane-associated PAL. The endocytic trafficking of PAM-1/OSX differed dramatically from that of PAM-1. A soluble fragment of the cytosolic domain of PAM-1 was produced in the endocytic pathway and entered the nucleus; very little soluble fragment of the cytosolic domain was produced from PAM-1/OSX. Internalized PAM-1/OSX was rapidly degraded; unlike PAM-1, very little internalized PAM-1/OSX was detected in multivesicular bodies. Blue native PAGE analysis identified high molecular weight complexes containing PAM-1; the ability of PAM-1/OSX to form similar complexes was markedly diminished. By promoting the formation of high molecular weight complexes, O-glycans may facilitate the recycling of PAM-1 through the endocytic compartment.