Tolerable glycometabolic stress boosts cancer cell resilience through altered N-glycosylation and Notch signaling activation

Chronic metabolic stress paradoxically elicits pro-tumorigenic signals that facilitate cancer stem cell (CSC) development. Therefore, elucidating the metabolic sensing and signaling mechanisms governing cancer cell stemness can provide insights into ameliorating cancer relapse and therapeutic resistance. Here, we provide convincing evidence that chronic metabolic stress triggered by hyaluronan production augments CSC-like traits and chemoresistance by partially impairing nucleotide sugar metabolism, dolichol lipid-linked oligosaccharide (LLO) biosynthesis and N-glycan assembly. Notably, preconditioning with either low-dose tunicamycin or 2-deoxy-D-glucose, which partially interferes with LLO biosynthesis, reproduced the promoting effects of hyaluronan production on CSCs. Multi-omics revealed characteristic changes in N-glycan profiles and Notch signaling activation in cancer cells exposed to mild glycometabolic stress. Restoration of N-glycan assembly with glucosamine and mannose supplementation and Notch signaling blockade attenuated CSC-like properties and further enhanced the therapeutic efficacy of cisplatin. Therefore, our findings uncover a novel mechanism by which tolerable glycometabolic stress boosts cancer cell resilience through altered N-glycosylation and Notch signaling activation.

Quantitative label-free site-specific glycoproteomic analysis of the milk fat globule membrane protein in human colostrum and mature milk

Human milk fat globule membrane (MFGM) proteins, which are N-glycosylated, play essential roles in neonatal development and physiological health. However, the profiles and landscape changes in the site-specific N-glycosylation of human MFGM proteins during lactation remain unclear. Therefore, in this study, based on an intact glycopeptide-centred strategy, 2617 unique site-specific N-glycans of 221 MFGM glycoproteins in human colostrum and 986 unique site-specific N-glycans of 200 MFGM glycoproteins in mature milk were characterised and quantified using label-free glycoproteomics. With milk maturation, 33 site-specific N-glycans on 10 N-glycoproteins increased significantly, and 113 site-specific N-glycans on 25 N-glycoproteins decreased significantly. Moreover, human MFGM glycoproteins with core-α1,6-fucosylated structures and Lewis and sialylated branching structures play a role in the biological processes of antigen processing and presentation. This study reveals the dynamic changes in human MFGM protein N-glycosylation patterns during lactation. Meanwhile, the study deepens our understanding of site-specific N-glycosylation of human MFGM glycoproteins. The results of the study provide a background reference for the development of infant formulas.

Characterization and comparison site-specific N-glycosylation profiling of milk fat globule membrane proteome in donkey and human colostrum and mature milk

Milk fat globule membrane (MFGM) proteins are highly glycosylated and involved in various biological processes within the body. However, information on site-specific N-glycosylation of MFGM glycoproteins in donkey and human milk remains limited. This study aimed to map the most comprehensive site-specific N-glycosylation fingerprinting of donkey and human MFGM glycoproteins using a site-specific glycoproteomics strategy. We identified 1,360, 457, 2,617, and 986 site-specific N-glycans from 296, 77, 214, and 196 N-glycoproteins in donkey colostrum (DC), donkey mature milk (DM), human colostrum (HC), and human mature milk (HM), respectively. Bioinformatics was used to describe the structure-activity relationships of DC, DM, HC, and HM MFGM N-glycoproteins. The results revealed differences in the molecular composition of donkey and human MFGM N-glycoproteins and the dynamic changes to site-specific N-glycosylation of donkey and human MFGM glycoproteins during lactation, deepening our understanding of the composition of donkey and human MFGM N-glycoproteins and their potential physiological roles.

The dynamic brain N-glycome

The attachment of carbohydrates to other macromolecules, such as proteins or lipids, is an important regulatory mechanism termed glycosylation. One subtype of protein glycosylation is asparagine-linked glycosylation (N-glycosylation) which plays a key role in the development and normal functioning of the vertebrate brain. To better understand the role of N-glycans in neurobiology, it's imperative we analyse not only the functional roles of individual structures, but also the collective impact of large-scale changes in the brain N-glycome. The systematic study of the brain N-glycome is still in its infancy and data are relatively scarce. Nevertheless, the prevailing view has been that the neuroglycome is inherently restricted with limited capacity for variation. The development of improved methods for N-glycomics analysis of brain tissue has facilitated comprehensive characterisation of the complete brain N-glycome under various experimental conditions on a larger scale. Consequently, accumulating data suggest that it's more dynamic than previously recognised and that, within a general framework, it has a given capacity to change in response to both intrinsic and extrinsic stimuli. Here, we provide an overview of the many factors that can alter the brain N-glycome, including neurodevelopment, ageing, diet, stress, neuroinflammation, injury, and disease. Given this emerging evidence, we propose that the neuroglycome has a hitherto underappreciated plasticity and we discuss the therapeutic implications of this regarding the possible reversal of pathological changes via interventions. We also briefly review the merits and limitations of N-glycomics as an analytical method before reflecting on some of the outstanding questions in the field.

Minimal epitope for Mannitou IgM on paucimannose-carrying glycoproteins

Paucimannosidic glycans are restricted to the core structure [Man1-3GlcNAc2Fuc0-1] of N-glycans and are rarely found in mammalian tissues. Yet, especially [Man2-3GlcNAc2Fuc1] have been found significantly upregulated in tumors, including in colorectal and liver cancer. Mannitou IgM is a murine monoclonal antibody that was previously shown to recognise Man3GlcNAc2 with an almost exclusive selectivity. Here, we have sought the definition of the minimal glycan epitope of Mannitou IgM, initiated by screening on a newly designed paucimannosidic glycan microarray. Among the best binders were Man3GlcNAc2 and its α1,6 core-fucosylated variant, Man3GlcNAc2Fuc1. Unexpectedly and in contrast to earlier findings, Man5GlcNAc2-type structures bind equally well and a large tolerance was observed for substitutions on the α1,6 arm. It was confirmed that any substitution on the single α1,3-linked mannose completely abolishes binding. Surface plasmon resonance for kinetic measurements of Mannitou IgM binding, either directly on the glycans or as presented on omega-1 and kappa-5 soluble egg antigens from the helminth parasite Schistosoma mansoni, showed submicromolar affinities. To characterize the epitope in greater and atomic detail, saturation transfer difference nuclear magnetic resonance spectroscopy was performed with the Mannitou antigen-binding fragment. The STD-NMR data demonstrated the strongest interactions with the aliphatic protons H1 and H2 of the α1-3-linked mannose, and weaker imprints on its H3, H4 and H5 protons. In conclusion, Mannitou IgM binding requires a non-substituted α1,3-linked mannose branch of paucimannose also on proteins, making it a highly specific tool for the distinction of concurrent human tumor-associated carbohydrate antigens.

Integrated Glycoproteomics Identifies a Role of N-Glycosylation and Galectin-1 on Myogenesis and Muscle Development

Many cell surface and secreted proteins are modified by the covalent addition of glycans that play an important role in the development of multicellular organisms. These glycan modifications enable communication between cells and the extracellular matrix via interactions with specific glycan-binding lectins and the regulation of receptor-mediated signaling. Aberrant protein glycosylation has been associated with the development of several muscular diseases, suggesting essential glycan- and lectin-mediated functions in myogenesis and muscle development, but our molecular understanding of the precise glycans, catalytic enzymes, and lectins involved remains only partially understood. Here, we quantified dynamic remodeling of the membrane-associated proteome during a time-course of myogenesis in cell culture. We observed wide-spread changes in the abundance of several important lectins and enzymes facilitating glycan biosynthesis. Glycomics-based quantification of released N-linked glycans confirmed remodeling of the glycome consistent with the regulation of glycosyltransferases and glycosidases responsible for their formation including a previously unknown digalactose-to-sialic acid switch supporting a functional role of these glycoepitopes in myogenesis. Furthermore, dynamic quantitative glycoproteomic analysis with multiplexed stable isotope labeling and analysis of enriched glycopeptides with multiple fragmentation approaches identified glycoproteins modified by these regulated glycans including several integrins and growth factor receptors. Myogenesis was also associated with the regulation of several lectins, most notably the upregulation of galectin-1 (LGALS1). CRISPR/Cas9-mediated deletion of Lgals1 inhibited differentiation and myotube formation, suggesting an early functional role of galectin-1 in the myogenic program. Importantly, similar changes in N-glycosylation and the upregulation of galectin-1 during postnatal skeletal muscle development were observed in mice. Treatment of new-born mice with recombinant adeno-associated viruses to overexpress galectin-1 in the musculature resulted in enhanced muscle mass. Our data form a valuable resource to further understand the glycobiology of myogenesis and will aid the development of intervention strategies to promote healthy muscle development or regeneration.

Post-natal developmental changes in the composition of the rat neocortical N-glycome

Asparagine-linked glycosylation (N-glycosylation) plays a key role in many neurodevelopmental processes, including neural cell adhesion, neurite outgrowth and axon targeting. However, little is known about the dynamics of N-glycosylation during brain development and, in particular, how the N-glycome of the developing neocortex differs from that of the adult. The aim of this study, therefore, was to perform a thorough characterization of N-glycosylation in both the adult and neonatal rat neocortex in order to gain insights into the types of changes occurring in the N-glycome during neurodevelopment. To this end, we used hydrophilic interaction ultraperformance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry to compare the adult neocortical N-glycome with that of 24- and 48-h neonates. We report that the abundance of complex N-glycans is significantly lower in adults compared with neonates. Furthermore, the proportion of charged complex N-glycans is also greatly reduced. This decrease in the abundance of complex N-glycans is offset by a corresponding increase in the proportion of truncated and, to a lesser extent, hybrid N-glycans. Lastly, we report that although the proportion of oligomannose N-glycans remains constant at around 24%, the distribution of high-mannose subtypes shifts from predominantly large subtypes in neonates to smaller subtypes in the adult. In summary, our findings indicate that N-glycan synthesis in the rat neocortex is fundamentally different in neonates compared with adults with a general shift occurring from large, sialylated N-glycans towards smaller, neutral structures as neonates develop into adults, coupled with a parallel shift towards smaller oligomannose structures.

N-Glycan profiling of lung adenocarcinoma in patients at different stages of disease

Lung adenocarcinoma (LAC) is the most common form of lung cancer that increases in non-smokers at younger age. Altered protein glycosylation is one of the hallmarks of malignancy, its role in cancer progression is still poorly understood. In this study, we report mass spectrometric (MS) analysis of N-glycans released from fresh or defrosted tissue specimens from 24 patients with LAC. Comparison of cancerous versus adjacent healthy tissues revealed substantial differences in N-glycan profiles associated with disease. The significant increase in paucimannose and high-mannose glycans with 6-9 mannose residues and decline in the sialylated complex biantenary core fucosylated glycan with composition NeuAcGal₂GlcNAc₂Man₃GlcNAc₂Fuc were general features of tumors. In addition, 42 new N-glycan compositions were detected in cancerous tissues. The prominent changes in advanced disease stages were mostly observed in core fucosylated N-glycans with additional fucose (Fuc) residue/s and enhanced branching with non-galactosylated N-acetyl-glucosamine (GlcNAc) units. Both of these monosaccharide types were linked preferably on the 6-antenna. Importantly, as compared with noncancerous tissues, a number of these significant changes were clearly detectable early on in stage I. Application of N-glycan data obtained from tissues was next assessed and validated for evaluation of small sized biopsies obtained via bronchoscopy. In summary, observed alterations and data of newly detected N-glycans expand knowledge about the glycosylation in LAC and may contribute to research in more tailored therapies. Moreover, the results demonstrate effectiveness of the presented approach for utility in rapid discrimination of cancerous from healthy lung tissues.

High-resolution longitudinal N- and O-glycoprofiling of human monocyte-to-macrophage transition

Protein glycosylation impacts the development and function of innate immune cells. The glycophenotypes and the glycan remodelling associated with the maturation of macrophages from monocytic precursor populations remain incompletely described. Herein, label-free porous graphitised carbon–liquid chromatography–tandem mass spectrometry (PGC-LC-MS/MS) was employed to profile with high resolution the N- and O-glycome associated with human monocyte-to-macrophage transition. Primary blood-derived CD14+ monocytes were differentiated ex vivo in the absence of strong anti- and proinflammatory stimuli using a conventional 7-day granulocyte-macrophage colony-stimulating factor differentiation protocol with longitudinal sampling. Morphology and protein expression monitored by light microscopy and proteomics validated the maturation process. Glycomics demonstrated that monocytes and macrophages display similar N-glycome profiles, comprising predominantly paucimannosidic (Man1-3GlcNAc2Fuc0–1, 22.1–30.8%), oligomannosidic (Man5-9GlcNAc2, 29.8–35.7%) and α2,3/6-sialylated complex-type N-glycans with variable core fucosylation (27.6–39.1%). Glycopeptide analysis validated conjugation of these glycans to human proteins, while quantitative proteomics monitored the glycoenzyme expression levels during macrophage differentiation. Significant interperson glycome variations were observed suggesting a considerable physiology-dependent or heritable heterogeneity of CD14+ monocytes. Only few N-glycome changes correlated with the monocyte-to-macrophage transition across donors including decreased core fucosylation and reduced expression of mannose-terminating (paucimannosidic-/oligomannosidic-type) N-glycans in macrophages, while lectin flow cytometry indicated that more dramatic cell surface glycan remodelling occurs during maturation. The less heterogeneous core 1-rich O-glycome showed a minor decrease in core 2-type O-glycosylation but otherwise remained unchanged with macrophage maturation. This high-resolution glycome map underpinning normal monocyte-to-macrophage transition, the most detailed to date, aids our understanding of the molecular makeup pertaining to two vital innate immune cell types and forms an important reference for future glycoimmunological studies.

Increased Expression of Immature Mannose-Containing Glycoproteins and Sialic Acid in Aged Mouse Brains

Aging represents the accumulation of changes in an individual over time, encompassing physical, psychological, and social changes. Posttranslational modifications of proteins such as glycosylation, including sialylation or glycation, are proposed to be involved in this process, since they modulate a variety of molecular and cellular functions. In this study, we analyzed selected posttranslational modifications and the respective proteins on which they occur in young and old mouse brains. The expression of neural cell adhesion molecule (NCAM), receptor for advanced glycation endproducts (RAGE), as well as the carbohydrate-epitopes paucimannose and high-mannose, polysialic acid, and O-GlcNAc were examined. We demonstrated that mannose-containing glycans increased on glycoproteins in aged mouse brains and identified synapsin-1 as one major carrier of paucimannose in aged brains. In addition, we found an accumulation of so-called advanced glycation endproducts, which are generated by non-enzymatic reactions and interfere with protein function. Furthermore, we analyzed the expression of sialic acid and found also an increase during aging.

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Paucimannosidic glycoepitopes inhibit tumorigenic processes in glioblastoma multiforme

Glioblastoma multiforme is an aggressive cancer type with poor patient outcomes. Interestingly, we reported previously a novel association between the little studied paucimannosidic N-linked glycoepitope and glioblastoma. Paucimannose has only recently been detected in vertebrates where it exhibits a very restricted tumor-specific expression. Herein, we demonstrate for the first time a very high protein paucimannosylation in human grade IV glioblastoma and U-87MG and U-138MG glioblastoma cells. Furthermore, we revealed the involvement of paucimannosidic epitopes in tumorigenic processes including cell proliferation, migration, invasion and adhesion. Finally, we identified AHNAK which is discussed as a tumor suppressor as the first paucimannose-carrying protein in glioblastoma and show the involvement of AHNAK in the observed paucimannose-dependent effects. This study is the first to provide evidence of a protective role of paucimannosylation in glioblastoma, a relationship that with further in vivo support may have far reaching benefits for patients suffering from this often fatal disease.

Human disease glycomics: technology advances enabling protein glycosylation analysis - part 2

INTRODUCTION

The changes in glycan structures have been attributed to disease states for several decades. The surface glycosylation pattern is a signature of physiological state of a cell. In this review we provide a link between observed substructural glycan changes and a range of diseases. Areas covered: We highlight biologically relevant glycan substructure expression in cancer, inflammation, neuronal diseases and diabetes. Furthermore, the alterations in antibody glycosylation in a disease context are described. Expert commentary: Advances in technologies, as described in Part 1 of this review have now enabled the characterization of specific glycan structural markers of a range of disease states. The requirement of including glycomics in cross-disciplinary omics studies, such as genomics, proteomics, epigenomics, transcriptomics and metabolomics towards a systems glycobiology approach to understanding disease mechanisms and management are highlighted.

N-glycan maturation mutants in Lotus japonicus for basic and applied glycoprotein research

Studies of protein N-glycosylation are important for answering fundamental questions on the diverse functions of glycoproteins in plant growth and development. Here we generated and characterised a comprehensive collection of Lotus japonicusLORE1 insertion mutants, each lacking the activity of one of the 12 enzymes required for normal N-glycan maturation in the glycosylation machinery. The inactivation of the individual genes resulted in altered N-glycan patterns as documented using mass spectrometry and glycan-recognising antibodies, indicating successful identification of null mutations in the target glyco-genes. For example, both mass spectrometry and immunoblotting experiments suggest that proteins derived from the α1,3-fucosyltransferase (Lj3fuct) mutant completely lacked α1,3-core fucosylation. Mass spectrometry also suggested that the Lotus japonicus convicilin 2 was one of the main glycoproteins undergoing differential expression/N-glycosylation in the mutants. Demonstrating the functional importance of glycosylation, reduced growth and seed production phenotypes were observed for the mutant plants lacking functional mannosidase I, N-acetylglucosaminyltransferase I, and α1,3-fucosyltransferase, even though the relative protein composition and abundance appeared unaffected. The strength of our N-glycosylation mutant platform is the broad spectrum of resulting glycoprotein profiles and altered physiological phenotypes that can be produced from single, double, triple and quadruple mutants. This platform will serve as a valuable tool for elucidating the functional role of protein N-glycosylation in plants. Furthermore, this technology can be used to generate stable plant mutant lines for biopharmaceutical production of glycoproteins displaying relative homogeneous and mammalian-like N-glycosylation features.

Mycobacterium tuberculosis Infection Manipulates the Glycosylation Machinery and the N-Glycoproteome of Human Macrophages and Their Microparticles

Tuberculosis (TB) remains a prevalent and lethal infectious disease. The glycobiology associated with Mycobacterium tuberculosis infection of frontline alveolar macrophages is still unresolved. Herein, we investigated the regulation of protein N-glycosylation in human macrophages and their secreted microparticles (MPs) used for intercellular communication upon M. tb infection. LC-MS/MS-based proteomics and glycomics were performed to monitor the regulation of glycosylation enzymes and receptors and the N-glycome in in vitro-differentiated macrophages and in isolated MPs upon M. tb infection. Infection promoted a dramatic regulation of the macrophage proteome. Most notably, significant infection-dependent down-regulation (4-26 fold) of 11 lysosomal exoglycosidases, e.g., β-galactosidase, β-hexosaminidases and α-/β-mannosidases, was observed. Relative weak infection-driven transcriptional regulation of these exoglycosidases and a stronger augmentation of the extracellular hexosaminidase activity demonstrated that the lysosome-centric changes may originate predominantly from infection-induced secretion of the lysosomal content. The macrophages showed heterogeneous N-glycan profiles and displayed significant up-regulation of complex-type glycosylation and concomitant down-regulation of paucimannosylation upon infection. Complementary intact N-glycopeptide analysis supported a subcellular-specific manipulation of the glycosylation machinery and altered glycosylation patterns of lysosomal N-glycoproteins within infected macrophages. Interestingly, the corresponding macrophage-derived MPs displayed unique N-glycome and proteome signatures supporting a preferential packaging from plasma membranes. The MPs were devoid of infection-dependent N-glycosylation signatures, but interestingly displayed increased levels of the glyco-initiating oligosaccharyltransferase complex and associated α-glucosidases that correlated with increased formation, N-glycan precursor levels and N-glycan density of infected MPs. In conclusion, this system-wide study provides new insight into the host- and pathogen-driven N-glycoproteome manipulation of macrophages in TB.

Functional roles of glycoconjugates in the maintenance of stemness and differentiation process of neural stem cells

Neural stem cells (NSCs) possess a high proliferative potential and capacity for self-renewal with retention of multipotency to differentiate into brain-forming cells. NSCs have gained a considerable attention because of their potential application in treatment strategies on the basis of transplantation for neurodegenerative disorders and nerve injuries. Although several signaling pathways have been reportedly involved in the fate determination process of NSCs, the molecular mechanisms underlying the maintenance of neural cell stemness and differentiation process remain largely unknown. Glycoconjugates expressed in the NSC niche in the brain offer markers of NSCs; moreover, they serve as cell regulators, which are actively involved in the modulation of signal transduction. The glycans function on NCS surfaces by recruiting growth factor receptors to specific microdomains as components of glycolipids, thereby mediating the ligand-receptor interactions both indirectly and directly as components of proteoglycans and interacting with specific lectin-type receptors as components of ligand glycoproteins. In this review, we outline current knowledge of the possible functional mechanisms of glycoconjugates to determine cell fates, which are associated with their expression pattern and structural characteristic features.

Maturing Glycoproteomics Technologies Provide Unique Structural Insights into the N-glycoproteome and Its Regulation in Health and Disease

The glycoproteome remains severely understudied because of significant analytical challenges associated with glycoproteomics, the system-wide analysis of intact glycopeptides. This review introduces important structural aspects of protein N-glycosylation and summarizes the latest technological developments and applications in LC-MS/MS-based qualitative and quantitative N-glycoproteomics. These maturing technologies provide unique structural insights into the N-glycoproteome and its synthesis and regulation by complementing existing methods in glycoscience. Modern glycoproteomics is now sufficiently mature to initiate efforts to capture the molecular complexity displayed by the N-glycoproteome, opening exciting opportunities to increase our understanding of the functional roles of protein N-glycosylation in human health and disease.

Emerging roles of protein mannosylation in inflammation and infection

Proteins are frequently modified by complex carbohydrates (glycans) that play central roles in maintaining the structural and functional integrity of cells and tissues in humans and lower organisms. Mannose forms an essential building block of protein glycosylation, and its functional involvement as components of larger and diverse α-mannosidic glycoepitopes in important intra- and intercellular glycoimmunological processes is gaining recognition. With a focus on the mannose-rich asparagine (N-linked) glycosylation type, this review summarises the increasing volume of literature covering human and non-human protein mannosylation, including their structures, biosynthesis and spatiotemporal expression. The review also covers their known interactions with specialised host and microbial mannose-recognising C-type lectin receptors (mrCLRs) and antibodies (mrAbs) during inflammation and pathogen infection. Advances in molecular mapping technologies have recently revealed novel immuno-centric mannose-terminating truncated N-glycans, termed paucimannosylation, on human proteins. The cellular presentation of α-mannosidic glycoepitopes on N-glycoproteins appears tightly regulated; α-mannose determinants are relative rare glycoepitopes in physiological extracellular environments, but may be actively secreted or leaked from cells to transmit potent signals when required. Simultaneously, our understanding of the molecular basis on the recognition of mannosidic epitopes by mrCLRs including DC-SIGN, mannose receptor, mannose binding lectin and mrAb is rapidly advancing, together with the functional implications of these interactions in facilitating an effective immune response during physiological and pathophysiological conditions. Ultimately, deciphering these complex mannose-based receptor-ligand interactions at the detailed molecular level will significantly advance our understanding of immunological disorders and infectious diseases, promoting the development of future therapeutics to improve patient clinical outcomes.

Complementary LC-MS/MS-Based N-Glycan, N-Glycopeptide, and Intact N-Glycoprotein Profiling Reveals Unconventional Asn71-Glycosylation of Human Neutrophil Cathepsin G

Neutrophil cathepsin G (nCG) is a central serine protease in the human innate immune system, but the importance of its N-glycosylation remains largely undescribed. To facilitate such investigations, we here use complementary LC-MS/MS-based N-glycan, N-glycopeptide, and intact glycoprotein profiling to accurately establish the micro- and macro-heterogeneity of nCG from healthy individuals. The fully occupied Asn71 carried unconventional N-glycosylation consisting of truncated chitobiose core (GlcNAcβ: 55.2%; Fucα1,6GlcNAcβ: 22.7%), paucimannosidic N-glycans (Manβ1,4GlcNAcβ1,4GlcNAcβ: 10.6%; Manβ1,4GlcNAcβ1,4(Fucα1,6)GlcNAcβ: 7.9%; Manα1,6Manβ1,4GlcNAcβ1,4GlcNAcβ: 3.7%, trace level of Manα1,6Manβ1,4GlcNAcβ1,4(Fucα1,6)GlcNAcβ), and trace levels of monoantennary α2,6- and α2,3-sialylated complex N-glycans. High-resolution/mass accuracy LC-MS profiling of intact nCG confirmed the Asn71-glycoprofile and identified two C-terminal truncation variants at Arg243 (57.8%) and Ser244 (42.2%), both displaying oxidation of solvent-accessible Met152. Asn71 appeared proximal (~19 Å) to the active site of nCG, but due to the truncated nature of Asn71-glycans (~5-17 Å) we questioned their direct modulation of the proteolytic activity of the protein. This work highlights the continued requirement of using complementary technologies to accurately profile even relatively simple glycoproteins and illustrates important challenges associated with the analysis of unconventional protein N-glycosylation. Importantly, this study now facilitates investigation of the functional role of nCG Asn71-glycosylation.

In-depth N-glycome profiling of paired colorectal cancer and non-tumorigenic tissues reveals cancer-, stage- and EGFR-specific protein N-glycosylation

Glycomics may assist in uncovering the structure-function relationships of protein glycosylation and identify glycoprotein markers in colorectal cancer (CRC) research. Herein, we performed label-free quantitative glycomics on a carbon-liquid chromatography-tandem mass spectrometry-based analytical platform to accurately profile the N-glycosylation changes associated with CRC malignancy. N-Glycome profiling was performed on isolated membrane proteomes of paired tumorigenic and adjacent non-tumorigenic colon tissues from a cohort of five males (62.6 ± 13.1 y.o.) suffering from colorectal adenocarcinoma. The CRC tissues were typed according to their epidermal growth factor receptor (EGFR) status by western blotting and immunohistochemistry. Detailed N-glycan characterization and relative quantitation identified an extensive structural heterogeneity with a total of 91 N-glycans. CRC-specific N-glycosylation phenotypes were observed including an overrepresentation of high mannose, hybrid and paucimannosidic type N-glycans and an under-representation of complex N-glycans (P < 0.05). Sialylation, in particular α2,6-sialylation, was significantly higher in CRC tumors relative to non-tumorigenic tissues, whereas α2,3-sialylation was down-regulated (P < 0.05). CRC stage-specific N-glycosylation was detected by high α2,3-sialylation and low bisecting β1,4-GlcNAcylation and Lewis-type fucosylation in mid-late relative to early stage CRC. Interestingly, a novel link between the EGFR status and the N-glycosylation was identified using hierarchical clustering of the N-glycome profiles. EGFR-specific N-glycan signatures included high bisecting β1,4-GlcNAcylation and low α2,3-sialylation (both P < 0.05) relative to EGFR-negative CRC tissues. This is the first study to correlate CRC stage and EGFR status with specific N-glycan features, thus advancing our understanding of the mechanisms causing the biomolecular deregulation associated with CRC.