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

Paucimannose-Rich N-glycosylation of Spatiotemporally Regulated Human Neutrophil Elastase Modulates Its Immune Functions

Human neutrophil elastase (HNE) is an important N-glycosylated serine protease in the innate immune system, but the structure and immune-modulating functions of HNE N-glycosylation remain undescribed. Herein, LC-MS/MS-based glycan, glycopeptide and glycoprotein profiling were utilized to first determine the heterogeneous N-glycosylation of HNE purified from neutrophil lysates and then from isolated neutrophil granules of healthy individuals. The spatiotemporal expression of HNE during neutrophil activation and the biological importance of its N-glycosylation were also investigated using immunoblotting, cell surface capture, native MS, receptor interaction, protease inhibition, and bacteria growth assays. Site-specific HNE glycoprofiling demonstrated that unusual paucimannosidic N-glycans, particularly Manα1,6Manβ1,4GlcNAcβ1,4(Fucα1,6)GlcNAcβ, predominantly occupied Asn124 and Asn173. The equally unusual core fucosylated monoantenna complex-type N-sialoglycans also decorated these two fully occupied sites. In contrast, the mostly unoccupied Asn88 carried nonfucosylated paucimannosidic N-glycans probably resulting from low glycosylation site solvent accessibility. Asn185 was not glycosylated. Subcellular- and site-specific glycoprofiling showed highly uniform N-glycosylation of HNE residing in distinct neutrophil compartments. Stimulation-induced cell surface mobilization demonstrated a spatiotemporal regulation, but not cell surface-specific glycosylation signatures, of HNE in activated human neutrophils. The three glycosylation sites of HNE were located distal to the active site indicating glycan functions other than interference with HNE enzyme activity. Functionally, the paucimannosidic HNE glycoforms displayed preferential binding to human mannose binding lectin compared with the HNE sialoglycoforms, suggesting a glycoform-dependent involvement of HNE in complement activation. The heavily N-glycosylated HNE protease inhibitor, α1-antitrypsin, displayed concentration-dependent complex formation and preferred glycoform-glycoform interactions with HNE. Finally, both enzymatically active HNE and isolated HNE N-glycans demonstrated low micromolar concentration-dependent growth inhibition of clinically-relevant Pseudomonas aeruginosa, suggesting some bacteriostatic activity is conferred by the HNE N-glycans. Taken together, these observations support that the unusual HNE N-glycosylation, here reported for the first time, is involved in modulating multiple immune functions central to inflammation and infection.

Glycoconjugates from extracellular vesicles: Structures, functions and emerging potential as cancer biomarkers

Extracellular vesicles (EVs) are released by virtually all cells, carry cellular molecules to the extracellular environment, and may interact with other cells. They are found in body fluids, therefore, constituting useful target sources for the identification of disease biomarkers, for example, in cancer. EVs originate from the plasma membrane or from multivesicular endosomes. They have the same topology as the plasma membrane and are rich in glycoconjugates, displaying specific glycosignatures. Surface glycoconjugates play important roles in EVs biogenesis and in their interaction with other cells. Changes in glycosylation constitute a hallmark of different types of cancer, therefore, the study of glycoconjugates and glycosignatures of EVs appear as promising candidates to identify novel cancer biomarkers and to increase the specificity and sensitivity of the existing clinical biomarkers, many of which are glycosylated.

A standardized method for lectin microarray-based tissue glycome mapping

The significance of glycomic profiling has been highlighted by recent findings that structural changes of glycans are observed in many diseases, including cancer. Therefore, glycomic profiling of the whole body (glycome mapping) under different physiopathological states may contribute to the discovery of reliable biomarkers with disease-specific alterations. To achieve this, standardization of high-throughput and in-depth analysis of tissue glycome mapping is needed. However, this is a great challenge due to the lack of analytical methodology for glycans on small amounts of endogenous glycoproteins. Here, we established a standardized method of lectin-assisted tissue glycome mapping. Formalin-fixed, paraffin-embedded tissue sections were prepared from brain, liver, kidney, spleen, and testis of two C57BL/6J mice. In total, 190 size-adjusted fragments with different morphology were serially collected from each tissue by laser microdissection and subjected to lectin microarray analysis. The results and subsequent histochemical analysis with selected lectins were highly consistent with previous reports of mass spectrometry-based N- and/or O-glycome analyses and histochemistry. This is the first report to look at both N- and O-glycome profiles of various regions within tissue sections of five different organs. This simple and reproducible mapping approach is also applicable to various disease model mice to facilitate disease-related biomarker discovery.

Reduced paucimannosidic N-glycan formation by suppression of a specific β-hexosaminidase from Nicotiana benthamiana

Plants are attractive hosts for the production of recombinant glycoproteins for therapeutic use. Recent advances in glyco-engineering facilitate the elimination of nonmammalian-type glycosylation and introduction of missing pathways for customized N-glycan formation. However, some therapeutically relevant recombinant glycoproteins exhibit unwanted truncated (paucimannosidic) N-glycans that lack GlcNAc residues at the nonreducing terminal end. These paucimannosidic N-glycans increase product heterogeneity and may affect the biological function of the recombinant drugs. Here, we identified two enzymes, β-hexosaminidases (HEXOs) that account for the formation of paucimannosidic N-glycans in Nicotiana benthamiana, a widely used expression host for recombinant proteins. Subcellular localization studies showed that HEXO1 is a vacuolar protein and HEXO3 is mainly located at the plasma membrane in N. benthamiana leaf epidermal cells. Both enzymes are functional and can complement the corresponding HEXO-deficient Arabidopsis thaliana mutants. In planta expression of HEXO3 demonstrated that core α1,3-fucose enhances the trimming of GlcNAc residues from the Fc domain of human IgG. Finally, using RNA interference, we show that suppression of HEXO3 expression can be applied to increase the amounts of complex N-glycans on plant-produced human α1-antitrypsin.

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.

Identifying N-Glycan Biomarkers in Colorectal Cancer by Mass Spectrometry

Colorectal cancer (CRC) is one of the most prevalent cancers worldwide. Delineating biological markers (biomarkers) for early detection, when treatment is most effective, is key to prevention and long-term survival of patients. Development of reliable biomarkers requires an increased understanding of the CRC biology and the underlying molecular and cellular mechanisms of the disease. With recent advances in new technologies and approaches, tremendous efforts have been put in proteomics and genomics fields to deliver detailed analysis of the two major biomolecules, genes and proteins, to gain a more complete understanding of cellular systems at both genomic and proteomic levels, allowing a mechanistic understanding of the human diseases, including cancer, and opening avenues for identification of novel gene and protein based prognostic and therapeutic markers. Although the importance of glycosylation in modulating protein function has long been appreciated, glycan analysis has been complicated by the diversity of the glycan structures and the large number of potential glycosylation combinations. Driven by recent technological advances, LC-MS/MS based glycomics is gaining momentum in cancer research and holds considerable potential to deliver new glycan-based markers. In our laboratory, we investigated alterations in N-glycosylation associated with CRC malignancy in a panel of CRC cell lines and CRC patient tissues. In an initial study, LC-MS/MS-based N-glycomics were utilized to map the N-glycome landscape associated with a panel of CRC cell lines (LIM1215, LIM1899, and LIM2405). These studies were subsequently extended to paired tumor and nontumorigenic CRC tissues to validate the findings in the cell line. Our studies in both CRC cell lines and tissues identified a strong representation of high mannose and α2,6-linked sialylated complex N-glycans, which corroborate findings from previous studies in CRC and other cancers. In addition, certain unique glycan determinants such as bisecting β1,4-GlcNAcylation and α2,3-sialylation, identified in the metastatic (LIM1215) and aggressive (LIM2405) CRC cell lines, respectively, were shown to be associated with epidermal growth factor receptor (EGFR) expression status. In this Account, we will describe the mass spectrometry based N-glycomics approach utilized in our laboratory to accurately profile the cell- and tissue-specific N-glycomes associated with CRC. We will highlight altered N-glycosylation observed by our studies, consistent with findings from other cancer studies, and discuss how the observed alterations can provide insights into CRC pathogenesis, opening new avenues to identify novel disease-associated glycan markers.

Glycomics and glycoproteomics of membrane proteins and cell-surface receptors: Present trends and future opportunities

Membrane proteins mediate cell-cell interactions and adhesion, the transfer of ions and metabolites, and the transmission of signals from the extracellular environment to the cell interior. The extracellular domains of most cell membrane proteins are glycosylated, often at multiple sites. There is a growing awareness that glycosylation impacts the structure, interaction, and function of membrane proteins. The application of glycoproteomics and glycomics methods to membrane proteins has great potential. However, challenges also arise from the unique physical properties of membrane proteins. Successful analytical workflows must be developed and disseminated to advance functional glycoproteomics and glycomics studies of membrane proteins. This review explores the opportunities and challenges related to glycomic and glycoproteomic analysis of membrane proteins, including discussion of sample preparation, enrichment, and MS/MS analyses, with a focus on recent successful workflows for analysis of N- and O-linked glycosylation of mammalian membrane proteins.

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.

Recombinant human heterodimeric IL-15 complex displays extensive and reproducible N- and O-linked glycosylation

Human interleukin 15 (IL-15) circulates in blood as a stable molecular complex with the soluble IL-15 receptor alpha (sIL-15Rα). This heterodimeric IL-15:sIL-15Rα complex (hetIL-15) shows therapeutic potential by promoting the growth, mobilization and activation of lymphocytes and is currently evaluated in clinical trials. Favorable pharmacokinetic properties are associated with the heterodimeric formation and the glycosylation of hetIL-15, which, however, remains largely uncharacterized. We report the site-specific N- and O-glycosylation of two clinically relevant large-scale preparations of HEK293-derived recombinant human hetIL-15. Intact IL-15 and sIL-15Rα and derived glycans and glycopeptides were separately profiled using multiple LC-MS/MS strategies. IL-15 Asn79 and sIL-15Rα Asn107 carried the same repertoire of biosynthetically-related N-glycans covering mostly α1-6-core-fucosylated and β-GlcNAc-terminating complex-type structures. The two potential IL-15 N-glycosylation sites (Asn71 and Asn112) located at the IL-2 receptor interface were unoccupied. Mass analysis of intact IL-15 confirmed its N-glycosylation and suggested that Asn79-glycosylation partially prevents Asn77-deamidation. IL-15 contained no O-glycans, whereas sIL-15Rα was heavily O-glycosylated with partially sialylated core 1 and 2-type mono- to hexasaccharides on Thr2, Thr81, Thr86, Thr156, Ser158, and Ser160. The sialoglycans displayed α2-3- and α2-6-NeuAc-type sialylation. Non-human, potentially immunogenic glycoepitopes (e.g. N-glycolylneuraminic acid and α-galactosylation) were not displayed by hetIL-15. Highly reproducible glycosylation of IL-15 and sIL-15Rα of two batches of hetIL-15 demonstrated consistent manufacturing and purification. In conclusion, we document the heterogeneous and reproducible N- and O-glycosylation of large-scale preparations of the therapeutic candidate hetIL-15. Site-specific mapping of these molecular features is important to evaluate the consistent large-scale production and clinical efficacy of hetIL-15.

Identification of IgM as a contaminant in lectin-FLISA assays for HCC detection

Liver disease, in the form of hepatocellular carcinoma (HCC) accounts for > 700,000 deaths worldwide. A major reason for this is late diagnosis of HCC. The currently used biomarker, serum alpha-fetoprotein (AFP) is elevated in 40-60% of those with HCC and other markers that can either compliment or replace AFP are desired. Our previous work has identified a number of proteins that contain altered glycans in HCC. Specifically, these altered glycans were increased levels of core and outer arm fucosylation. To determine the clinical usefulness of those identified glycoproteins, a plate based assay was developed that allowed for the detection of fucosylated glycoforms. While this method was applicable to a number of independent patient sets, it was unable to specifically detect fucosylated glycoforms in many patient samples. That is, some material was present in serum that led to non-specific signal in the lectin- fluorescence -linked immunosorbent assay (lectin-FLISA). To address this issue, a systematic process was undertaken to identify the material. This material was found to be increased levels of lectin reactive IgM. Removal of both IgG and IgM using a multi-step protein A/G incubation and filtration step removed the contaminating signal and allowed for the analysis of specific protein glycoforms. This assay was subsequently used on two sample sets, one that was shown previously to be unable to be tested via a lectin FLISA and in a larger independent sample set. The clinical usefulness of this assay in the early detection of HCC is discussed.

Enzymes for N-Glycan Branching and Their Genetic and Nongenetic Regulation in Cancer

N-glycan, a fundamental and versatile protein modification in mammals, plays critical roles in various physiological and pathological events including cancer progression. The formation of N-glycan branches catalyzed by specific N-acetylglucosaminyltransferases [GnT-III, GnT-IVs, GnT-V, GnT-IX (Vb)] and a fucosyltransferase, Fut8, provides functionally diverse N-glycosylated proteins. Aberrations of these branches are often found in cancer cells and are profoundly involved in cancer growth, invasion and metastasis. In this review, we focus on the GlcNAc and fucose branches of N-glycans and describe how their expression is dysregulated in cancer by genetic and nongenetic mechanisms including epigenetics and nucleotide sugar metabolisms. We also survey the roles that these N-glycans play in cancer progression and therapeutics. Finally, we discuss possible applications of our knowledge on basic glycobiology to the development of medicine and biomarkers for cancer therapy.

Generation of a monoclonal antibody recognizing the CEACAM glycan structure and inhibiting adhesion using cancer tissue-originated spheroid as an antigen

Spheroids cultured directly from tumours can better reflect in vivo tumour characteristics than two-dimensional monolayer culture or three-dimensional culture of established cell lines. In this study, we generated antibodies by directly immunizing mice with primary-cultured living spheroids from human colorectal cancer. We performed phenotypic screening via recognition of the surface of the spheroids and inhibition of their adhesion to extracellular matrices to identify a monoclonal antibody, clone 5G2. The antibody inhibited cell migration in two-dimensional culture and promoted cell detachment. Western blotting and immunohistochemistry detected the 5G2 signal in many colorectal cancer spheroids, as well as patient tumours, but failed to detect in various cell lines examined. We found that 5G2 recognized the Le^a and Le^c on N-glycan, and their major carrier proteins were CEACAM5 and CEACAM6. Pre-incubation of the spheroids with 5G2 impaired translocation of integrin β4 from the lateral membrane to the contact interface between the extracellular matrix when embedded in it. As we successfully obtained a functional antibody, which antigen was glycan structures and lost in cell lines, cancer tissue-originated spheroids can be a useful antigen for generating novel anti-cancer antibodies.

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.

Glycomic Approaches for the Discovery of Targets in Gastrointestinal Cancer

Gastrointestinal (GI) cancer is the most common group of malignancies and many of its types are among the most deadly. Various glycoconjugates have been used in clinical practice as serum biomarker for several GI tumors, however, with limited diagnose application. Despite the good accessibility by endoscopy of many GI organs, the lack of reliable serum biomarkers often leads to late diagnosis of malignancy and consequently low 5-year survival rates. Recent advances in analytical techniques have provided novel glycoproteomic and glycomic data and generated functional information and putative biomarker targets in oncology. Glycosylation alterations have been demonstrated in a series of glycoconjugates (glycoproteins, proteoglycans, and glycosphingolipids) that are involved in cancer cell adhesion, signaling, invasion, and metastasis formation. In this review, we present an overview on the major glycosylation alterations in GI cancer and the current serological biomarkers used in the clinical oncology setting. We further describe recent glycomic studies in GI cancer, namely gastric, colorectal, and pancreatic cancer. Moreover, we discuss the role of glycosylation as a modulator of the function of several key players in cancer cell biology. Finally, we address several state-of-the-art techniques currently applied in this field, such as glycomic and glycoproteomic analyses, the application of glycoengineered cell line models, microarray and proximity ligation assay, and imaging mass spectrometry, and provide an outlook to future perspectives and clinical applications.

Mass Spectrometry-Based N-Glycomics of Colorectal Cancer

Colorectal cancer (CRC) is one of the most prevalent cancers worldwide. An increased molecular understanding of the CRC pathology is warranted to gain insights into the underlying molecular and cellular mechanisms of the disease. Altered protein glycosylation patterns are associated with most diseases including malignant transformation. Recent advances in mass spectrometry and bioinformatics have accelerated glycomics research and present a new paradigm for cancer biomarker discovery. Mass spectrometry (MS)-based glycoproteomics and glycomics, therefore, hold considerable promise to improve the discovery of novel biomarkers with utility in disease diagnosis and therapy. This review focuses on the emerging field of glycomics to present a comprehensive review of advances in technologies and their application in studies aimed at discovering novel glycan-based biomarkers. We will also discuss some of the challenges associated with using glycans as biomarkers.