Glycomic analysis of N-linked carbohydrate epitopes from CD24 of mouse brain

From mechanism to therapy: the journey of CD24 in cancer

CD24 is a glycosylphosphatidylinositol-anchored protein that is expressed in a wide range of tissues and cell types. It is involved in a variety of physiological and pathological processes, including cell adhesion, migration, differentiation, and apoptosis. Additionally, CD24 has been studied extensively in the context of cancer, where it has been found to play a role in tumor growth, invasion, and metastasis. In recent years, there has been growing interest in CD24 as a potential therapeutic target for cancer treatment. This review summarizes the current knowledge of CD24, including its structure, function, and its role in cancer. Finally, we provide insights into potential clinical application of CD24 and discuss possible approaches for the development of targeted cancer therapies.

Protein glycosylation and glycoinformatics for novel biomarker discovery in neurodegenerative diseases

Glycosylation is a common post-translational modification of brain proteins including cell surface adhesion molecules, synaptic proteins, receptors and channels, as well as intracellular proteins, with implications in brain development and functions. Using advanced state-of-the-art glycomics and glycoproteomics technologies in conjunction with glycoinformatics resources, characteristic glycosylation profiles in brain tissues are increasingly reported in the literature and growing evidence shows deregulation of glycosylation in central nervous system disorders, including aging associated neurodegenerative diseases. Glycan signatures characteristic of brain tissue are also frequently described in cerebrospinal fluid due to its enrichment in brain-derived molecules. A detailed structural analysis of brain and cerebrospinal fluid glycans collected in publications in healthy and neurodegenerative conditions was undertaken and data was compiled to create a browsable dedicated set in the GlyConnect database of glycoproteins (https://glyconnect.expasy.org/brain). The shared molecular composition of cerebrospinal fluid with brain enhances the likelihood of novel glycobiomarker discovery for neurodegeneration, which may aid in unveiling disease mechanisms, therefore, providing with novel therapeutic targets as well as diagnostic and progression monitoring tools.

Precise O-Glycosylation Site Localization of CD24Fc by LC-MS Workflows

CD24Fc is a homodimeric recombinant Fc-fusion protein comprised of human CD24 connected to immunoglobulin G1 (IgG1) Fc fragment. CD24 is heavily glycosylated, and its biological function is considered mainly mediated by its glycosylation. Identification of the O-glycosylation sites would facilitate an in-depth understanding of the functional role of O-glycans in CD24. However, the presence of clustered mucin-type O-glycans together with N-glycans makes the determination of O-glycosylation sites and abundance very challenging. In this study, two sets of liquid chromatography-mass spectrometry (LC-MS) workflows were developed for the comprehensive characterization of O-glycosylation in CD24: (1) Fractionation and collision-induced dissociation (CID) workflow involving multienzyme digestion, fractionation, OpeRATOR/SialEXO digestion, and CID analysis; (2) Direct OpeRATOR/SialEXO digestion followed by electron-transfer/higher-energy collision dissociation (EThcD) analysis. The precise O-glycosylation sites were identified in CD24 for the first time, and the site occupancy was assessed. A total of 12 O-glycosylation sites were identified. Seven glycosylation sites were identified by both workflows, and five additional sites were identified only by the EThcD workflow. The predominant O-glycosylation site in CD24 was Thr25 followed by Thr15. The CID workflow provided an overall relative quantitation of O-glycoforms at the CD24 level and site localization for singly O-glycosylated peptides. The EThcD workflow directly identified glycosylation sites by tandem mass spectrometry (MS/MS) for singly, doubly, and triply O-glycosylated peptides. Together, both workflows validated each other's results and can be applied to a complex mucin-type O-glycosylation site analysis of other glycoproteins and Fc-fusion therapeutics.

Towards Mapping of the Human Brain N-Glycome with Standardized Graphitic Carbon Chromatography

The brain N-glycome is known to be crucial for many biological functions, including its involvement in neuronal diseases. Although large structural studies of brain N-glycans were recently carried out, a comprehensive isomer-specific structural analysis has still not been achieved, as indicated by the recent discovery of novel structures with galactosylated bisecting GlcNAc. Here, we present a detailed, isomer-specific analysis of the human brain N-glycome based on standardized porous graphitic carbon (PGC)-LC-MS/MS. To achieve this goal, we biosynthesized glycans with substitutions typically occurring in the brain N-glycome and acquired their normalized retention times. Comparison of these values with the standardized retention times of neutral and desialylated N-glycan fractions of the human brain led to unambiguous isomer specific assignment of most major peaks. Profound differences in the glycan structures between naturally neutral and desialylated glycans were found. The neutral and sialylated N-glycans derive from diverging biosynthetic pathways and are biosynthetically finished end products, rather than just partially processed intermediates. The focus on structural glycomics defined the structure of human brain N-glycans, amongst these are HNK-1 containing glycans, a bisecting sialyl-lactose and structures with fucose and N-acetylgalactosamine on the same arm, the so-called LDNF epitope often associated with parasitic worms.

Bisecting Lewis X in Hybrid-Type N-Glycans of Human Brain Revealed by Deep Structural Glycomics

The importance of protein glycosylation in the biomedical field requires methods that not only quantitate structures by their monosaccharide composition, but also resolve and identify the many isomers expressed by mammalian cells. The art of unambiguous identification of isomeric structures in complex mixtures, however, did not yet catch up with the fast pace of advance of high-throughput glycomics. Here, we present a strategy for deducing structures with the help of a deci-minute accurate retention time library for porous graphitic carbon chromatography with mass spectrometric detection. We implemented the concept for the fundamental N-glycan type consisting of five hexoses, four N-acetylhexosamines and one fucose residue. Nearly all of the 40 biosynthetized isomers occupied unique elution positions. This result demonstrates the unique isomer selectivity of porous graphitic carbon. With the help of a rather tightly spaced grid of isotope-labeled internal N-glycan, standard retention times were transposed to a standard chromatogram. Application of this approach to animal and human brain N-glycans immediately identified the majority of structures as being of the bisected type. Most notably, it exposed hybrid-type glycans with galactosylated and even Lewis X containing bisected N-acetylglucosamine, which have not yet been discovered in a natural source. Thus, the time grid approach implemented herein facilitated discovery of the still missing pieces of the N-glycome in our most noble organ and suggests itself—in conjunction with collision induced dissociation—as a starting point for the overdue development of isomer-specific deep structural glycomics.

Novel insights into the function of CD24: A driving force in cancer

CD24 is a highly glycosylated protein with a small protein core that is linked to the plasma membrane via a glycosyl-phosphatidylinositol anchor. CD24 is primarily expressed by immune cells but is often overexpressed in human tumors. In cancer, CD24 is a regulator of cell migration, invasion and proliferation. Its expression is associated with poor prognosis and it is used as cancer stemness marker. Recently, CD24 on tumor cells was identified as a phagocytic inhibitor ("do not eat me" signal) having a suppressive role in tumor immunity via binding to Siglec-10 on macrophages. This finding is reminiscent of the demonstration that soluble CD24-Fc can dampen the immune system in autoimmune disease. In the present review, we summarize recent progress on the role of the CD24-Siglec-10 binding axis at the interface between tumor cells and the immune system, and the role of CD24 genetic polymorphisms in cancer. We describe the specific function of cytoplasmic CD24 and discuss the presence of CD24 on tumor-released extracellular vesicles. Finally, we evaluate the potential of CD24-based immunotherapy.

Molecular Mechanism of Tumor Cell Immune Escape Mediated by CD24/Siglec-10

Tumor immune escape is an important part of tumorigenesis and development. Tumor cells can develop a variety of immunosuppressive mechanisms to combat tumor immunity. Exploring tumor cells that escape immune surveillance through the molecular mechanism of related immunosuppression in-depth is helpful to develop the treatment strategies of targeted tumor immune escape. The latest studies show that CD24 on the surface of tumor cells interacts with Siglec-10 on the surface of immune cells to promote the immune escape of tumor cells. It is necessary to comment on the molecular mechanism of inhibiting the activation of immune cells through the interaction between CD24 on tumor cells and Siglec-10 on immune cells, and a treatment strategy of tumors through targeting CD24 on the surface of tumor cells or Siglec-10 on immune cells.

Enhanced Neuronal Survival and Neurite Outgrowth Triggered by Novel Small Organic Compounds Mimicking the LewisX Glycan

Glycosylation fine-tunes signal transduction of adhesion molecules during neural development and supports synaptic plasticity and repair after injury in the adult nervous system. One abundantly expressed neural glycan is LewisX (LeX). Although it is known that its expression starts at the formation of the neural tube during the second embryonic week in the mouse and peaks during the first postnatal week, its functional relevance is only rudimentarily understood. To gain better insights into the functions of this glycan, we identified small organic compounds that mimic structurally and functionally this glycan glycosidically linked to several neural adhesion molecules. Mimetic compounds were identified by competitive enzyme-linked immunosorbent assay (ELISA) using the LeX-specific monoclonal antibodies L5 and SSEA-1 for screening a library of small organic molecules. In this assay, antibody binding to substrate-coated LeX glycomimetic peptide is measured in the presence of compounds, allowing identification of molecules that inhibit antibody binding and thereby mimic LeX. Gossypol, orlistat, ursolic acid, folic acid, and tosufloxacin inhibited antibody binding in a concentration-dependent manner. With the aim to functionally characterize the molecular consequences of the compounds' actions, we here present evidence that, at nM concentrations, the mimetic compounds enhance neurite outgrowth and promote neuronal survival of cultured mouse cerebellar granule cells via, notably, distinct signal transduction pathways. These findings raise hopes that these LeX mimetics will be powerful tools for further studying the functions of LeX and its effects in acute and chronic nervous system disease models. It is worth mentioning in this context that the LeX compounds investigated in the present study have been clinically approved for different therapies.

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.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

Expression of CD24 and Siglec-10 in first trimester placenta: implications for immune tolerance at the fetal-maternal interface

During pregnancy, the fetal-maternal interface establishes immune tolerance between the fetus and the mother. CD24, a mucin-like glycoprotein expressed at the surface of hematopoietic cells and diverse tumor cells, is known to interact with the sialic acid-binding immunoglobulin-type lectins (Siglecs). This interaction was assessed as a candidate complex for the immune suppression response in the placenta. CD24 was affinity purified from term placenta and characterized by SDS-PAGE, Western blot and ELISA. Binding of recombinant Siglecs to placental CD24 was evaluated by ELISA. The expression of CD24 and Siglec-10 in first trimester placental tissues was investigated by immunohistochemistry and immunofluorescence. Placental CD24 had an apparent molecular weight of 30-70 kDa consistent with its high degree of N- and O-linked glycosylation. EDTA-sensitive CD24-Siglec-10 interaction via the terminal sialic acid glycan residues of CD24 was observed. CD24 did not interact with Siglec-3 or Siglec-5. During the first trimester, and already in gestational week (GA) 8, CD24 showed high expression in villous and extravillous cytotrophoblasts. There was also a mild expression in stromal cells, while syncytiotrophoblasts were negative. Co-localization of CD24 with Siglec-10 was observed in endometrial glands and in first trimester decidual cells in close vicinity to extracellular trophoblasts. This study is the first to demonstrate the early presence of CD24 in the placenta cytotrophoblast layers, placental bed and maternal uterine glands. The presence of the CD24-Siglec-10 in these regions of fetal-maternal interactions suggests a possible role in mediating immune tolerance at the fetal-maternal interface.

The CD24 surface antigen in neural development and disease

A cell's surface molecular signature enables its reciprocal interactions with the associated microenvironments in development, tissue homeostasis and pathological processes. The CD24 surface antigen (heat-stable antigen, nectadrin; small cell lung cancer antigen cluster-4) represents a prime example of a neural surface molecule that has long been known, but whose diverse molecular functions in intercellular communication we have only begun to unravel. Here, we briefly summarize the molecular fundamentals of CD24 structure and provide a comprehensive review of CD24 expression and functional studies in mammalian neural developmental systems and disease models (rodent, human). Striving for an integrated view of the intracellular signaling processes involved, we discuss the most pertinent routes of CD24-mediated signaling pathways and functional networks in neurobiology (neural migration, neurite extension, neurogenesis) and pathology (tumorigenesis, multiple sclerosis).

Mass Spectrometry in Pharmacokinetic Studies of a Synthetic Compound for Spinal Cord Injury Treatment

The studies of drugs that could constitute a palliative to spinal cord injury (SCI) are a continuous and increasing demand in biomedicine field from developed societies. Recently we described the chemical synthesis and antiglioma activity of synthetic glycosides. A synthetic sulfated glycolipid (here IG20) has shown chemical stability, solubility in polar solvents, and high inhibitory capacity over glioma growth. We have used mass spectrometry (MS) to monitor IG20 (m/z = 550.3) in cells and tissues of the central nervous system (CNS) that are involved in SCI recovery. IG20 was detected by MS in serum and homogenates from CNS tissue of rats, though in the latter a previous deproteinization step was required. The pharmacokinetic parameters of serum clearance at 24 h and half-life at 4 h were determined for synthetic glycoside in the adult rat using MS. A local administration of the drug near of spinal lesion site is proposed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth 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 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Expression of glycogenes in differentiating human NT2N neurons. Downregulation of fucosyltransferase 9 leads to decreased Lewis(x) levels and impaired neurite outgrowth

BACKGROUND

Several glycan structures are functionally relevant in biological events associated with differentiation and regeneration which occur in the central nervous system. Here we have analysed the glycogene expression and glycosylation patterns during human NT2N neuron differentiation. We have further studied the impact of downregulating fucosyltransferase 9 (FUT9) on neurite outgrowth.

METHODS

The expression of glycogenes in human NT2N neurons differentiating from teratocarcinoma NTERA-2/cl.D1 cells has been analysed using the GlycoV4 GeneChip expression microarray. Changes in glycosylation have been monitored by immunoblot, immunofluorescence microscopy, HPLC and MALDI-TOF MS. Peptide mass fingerprinting and immunoprecipitation have been used for protein identification. FUT9 was downregulated using silencing RNA.

RESULTS AND CONCLUSIONS

One hundred twelve mRNA transcripts showed statistically significant up-regulation, including the genes coding for proteins involved in the synthesis of the Lewis(x) motif (FUT9), polysialic acid (ST8SIA2 and ST8SIA4) and HNK-1 (B3GAT2). Accordingly, increased levels of the corresponding carbohydrate epitopes have been observed. The Lewis(x) structure was found in a carrier glycoprotein that was identified as the CRA-a isoform of human neural cell adhesion molecule 1. Downregulation of FUT9 caused significant decreases in the levels of Lewis(x), as well as GAP-43, a marker of neurite outgrowth. Concomitantly, a reduction in neurite formation and outgrowth has been observed that was reversed by FUT9 overexpression.

GENERAL SIGNIFICANCE

These results provided information about the regulation of glycogenes during neuron differentiation and they showed that the Lewis(x) motif plays a functional role in neurite outgrowth from human neurons.

Clinical value of CD24 expression in retinoblastoma

Background. The expression of CD24 has been detected in a wide variety of human malignancies. Downregulation of CD24 inhibits proliferation and induces apoptosis in tumor cells, whereas its upregulation increases tumor growth and metastasis. However, no data on CD24 protein levels in retinoblastoma are available, and the mechanism of CD24 involvement in retinoblastoma progress has not been elucidated. The aim of this study was to explore the expression profile of CD24 in the retinoblastoma tumor samples and to correlate with clinicopathological parameters. Methods. Immunohistochemistry was performed for CD24 on the archival paraffin sections of retinoblastoma and correlated with clinicopathological features. Western blotting was performed to confirm immunoreactivity results. Results. CD24 immunoreactivity was observed in 72.0% (36/50) of the retinoblastoma specimens. Among the 35 low-risk tumors, CD24 was expressed in 62.9% (22/35) tumors and among the 15 high-risk tumors, CD24 was expressed in 93.3% (14/15) tumors. High-risk tumors showed significantly increased expression of CD24 compared to tumors with low-risk (P < 0.05). Conclusions. This is the first correlation between CD24 expression and histopathology in human retinoblastoma. Our study showed increased expression of CD24 in high risk tumors compared to low risk tumors. Further functional studies are required to explore the role of CD24 in retinoblastoma.

Glycoproteomics-based identification of cancer biomarkers

Protein glycosylation is one of the most common posttranslational modifications in mammalian cells. It is involved in many biological pathways and molecular functions and is well suited for proteomics-based disease investigations. Aberrant protein glycosylation may be associated with disease processes. Specific glycoforms of glycoproteins may serve as potential biomarkers for the early detection of disease or as biomarkers for the evaluation of therapeutic efficacy for treatment of cancer, diabetes, and other diseases. Recent technological developments, including lectin affinity chromatography and mass spectrometry, have provided researchers the ability to obtain detailed information concerning protein glycosylation. These in-depth investigations, including profiling and quantifying glycoprotein expression, as well as comprehensive glycan structural analyses may provide important information leading to the development of disease-related biomarkers. This paper describes methodologies for the detection of cancer-related glycoprotein and glycan structural alterations and briefly summarizes several current cancer-related findings.

EUROCarbDB: An open-access platform for glycoinformatics

The EUROCarbDB project is a design study for a technical framework, which provides sophisticated, freely accessible, open-source informatics tools and databases to support glycobiology and glycomic research. EUROCarbDB is a relational database containing glycan structures, their biological context and, when available, primary and interpreted analytical data from high-performance liquid chromatography, mass spectrometry and nuclear magnetic resonance experiments. Database content can be accessed via a web-based user interface. The database is complemented by a suite of glycoinformatics tools, specifically designed to assist the elucidation and submission of glycan structure and experimental data when used in conjunction with contemporary carbohydrate research workflows. All software tools and source code are licensed under the terms of the Lesser General Public License, and publicly contributed structures and data are freely accessible. The public test version of the web interface to the EUROCarbDB can be found at http://www.ebi.ac.uk/eurocarb.

Sialoside-based pattern recognitions discriminating infections from tissue injuries

Recognition of pathogens-associated molecular patterns (PAMPs) by Toll-like receptors (TLRs), NOD-like receptors (NLRs) and RIG-I-like receptors (RLR) plays a critical role in protecting host against pathogens. In addition, TLR and NLR also recognize danger-associated molecular patterns (DAMPs) to initiate limited innate immune responses. While innate immune response to DAMPs may be important for tissue repairs and wound healing, it is normally well controlled to avoid autoimmune destruction. Recent data support a role for sialoside-based pattern recognition by members of the Siglec family to attenuate innate immunity. In particular, since CD24-Siglec 10/G interaction selectively dampens host response to DAMPs but not PAMPs, this sialoside-based pattern recognition may serve as a foundation to discriminate PAMPs from DAMPs.

Nanoelectrospray-MS( n ) of native and permethylated glycans

The profound biological relevance of protein and lipid glycosylation has made glycomics (i.e., the comprehensive study of all glycans in a cell or organism), an indispensable field of research in the life sciences. Consequently, numerous strategies have been developed for a high-throughput analysis of complex glycan mixtures, with mass spectrometry (MS) playing a key role. In particular, nanoelectrospray ionization (ESI-) MS( n ), employing multiple cycles of isolation and fragmentation of native or derivatized precursor ions, is recognized as a highly valuable tool in this context, as it allows, at least in part, structural characterization of glycans without prior fractionation. This chapter describes suitable work flows for this purpose and illustrates both advantages and limitations for this type of analysis. Furthermore, the use of newly developed software tools for data handling is outlined.

Molecular and clinical dissection of CD24 antibody specificity by a comprehensive comparative analysis

CD24 is a small, highly glycosylated cell surface protein that is linked to the membrane through a glycosyl-phosphatidylinositol anchor. It is overexpressed in many human carcinomas and its expression is linked to bad prognosis. Lately, lack or low expression of CD24 was used to identify tumor stem cells resulting in conflicting data on the usefulness of this marker. In many immunohistochemical studies, the mAb SN3b was used but the epitope and specificity of this antibody have never been thoroughly investigated. In other studies based mainly on cytofluorographic analysis, the mAb ML-5 was applied. In this study, we compared the epitope of mAb SN3b to the CD24 mAbs SWA-11 and ML-5 that both bind to the core protein of CD24. Using tissue microarrays and affinity-purified CD24 glycoforms, we observed only a partial overlap of SN3b and SWA11 reactivity. The mAb SN3b recognizes sialic acid most likely on O-linked glycans that can occur independently of the CD24 protein backbone. The SN3b epitope was not related to common sialylated cancer-associated glycan structures. Both SN3b epitope positive or negative CD24 glycoforms supported the binding of P-selectin and Siglec-5. In breast cancer, the SN3b reactivity was associated with bad prognosis, whereas SWA11 was not. In renal cell cancer, the SN3b epitope was completely absent but SWA11 reactivity was a prognostic factor. Our results shed new light on the tumorbiological role of CD24 and resolve discrepancies in the literature related to the use of different CD24 mAbs.

CD24: from A to Z

As a testament to the importance of CD24, researchers with diverse interests, including adaptive immunity, inflammation, autoimmune diseases and cancer, have encountered CD24. CD24 is overexpressed in many cancers and appears oncogenic. In the adaptive immune response, CD24 is a redundant costimulatory molecule in costimulation-rich lymphoid organs but is essential in selected target organs tested, such as brain and skin. More recent studies suggest it may have a role in discriminating danger and pathogen-associated molecular patterns by dendritic cells. The biology of CD24 is intriguing but poorly understood. Here we summarize the major findings associated with CD24 to stimulate new ideas for further research that may reveal the underlying link among the diverse processes mediated by CD24.

A multi-method approach toward de novo glycan characterization: a Man-5 case study

Regulatory agencies' expectations for biotherapeutic approval are becoming more stringent with regard to product characterization, where minor species as low as 0.1% of a given profile are typically identified. The mission of this manuscript is to demonstrate a multi-method approach toward de novo glycan characterization and quantitation, including minor species at or approaching the 0.1% benchmark. Recently, unexpected isomers of the Man(5)GlcNAc(2) (M(5)) were reported (Prien JM, Ashline DJ, Lapadula AJ, Zhang H, Reinhold VN. 2009. The high mannose glycans from bovine ribonuclease B isomer characterization by ion trap mass spectrometry (MS). J Am Soc Mass Spectrom. 20:539-556). In the current study, quantitative analysis of these isomers found in commercial M(5) standard demonstrated that they are in low abundance (<1% of the total) and therefore an exemplary "litmus test" for minor species characterization. A simple workflow devised around three core well-established analytical procedures: (1) fluorescence derivatization; (2) online rapid resolution reversed-phase separation coupled with negative-mode sequential mass spectrometry (RRRP-(-)-MS(n)); and (3) permethylation derivatization with nanospray sequential mass spectrometry (NSI-MS(n)) provides comprehensive glycan structural determination. All methods have limitations; however, a multi-method workflow is an at-line stopgap/solution which mitigates each method's individual shortcoming(s) providing greater opportunity for more comprehensive characterization. This manuscript is the first to demonstrate quantitative chromatographic separation of the M(5) isomers and the use of a commercially available stable isotope variant of 2-aminobenzoic acid to detect and chromatographically resolve multiple M(5) isomers in bovine ribonuclease B. With this multi-method approach, we have the capabilities to comprehensively characterize a biotherapeutic's glycan array in a de novo manner, including structural isomers at >/=0.1% of the total chromatographic peak area.

Glycomics and disease markers

Glycomics is the comprehensive study of all glycans expressed in biological systems. The biosynthesis of glycan relies on a number of highly competitive processes involving glycosyl transferases. Glycosylation is therefore highly sensitive to the biochemical environment and has been implicated in many diseases including cancer. Recently, interest in profiling the glycome has increased owing to the potential of glycans for disease markers. In this regard, mass spectrometry is emerging as a powerful technique for profiling the glycome. Global glycan profiling of human serum based on mass spectrometry has already led to several potentially promising markers for several types of cancer and diseases.

O-glycosylation pattern of CD24 from mouse brain

The cell adhesion molecule CD24 is a highly glycosylated glycoprotein that plays important roles in the central nervous system, the immune system and in tumor biology. Since CD24 comprises only a short protein core of approximately 30 amino acids and low conservation among species, it has been proposed that the functions of CD24 are mediated by its glycosylation pattern. Our present study provides evidence that interaction of CD24 with the cell adhesion molecule L1 is mediated by O-linked glycans carrying alpha2,3-linked sialic acid. Furthermore, de-N-glycosylated CD24 was shown to promote or inhibit neurite outgrowth of cerebellar neurons or dorsal root ganglion neurons, respectively, to the same extent as untreated CD24. Therefore, this study is focused on the structural elucidation of the chemically released, permethylated CD24 O-glycans by electrospray ionization ion trap mass spectrometry. Our analyses revealed the occurrence of a diverse mixture of mucin-type and O-mannosyl glycans carrying, in part, functionally relevant epitopes, such as 3-linked sialic acid, disialyl motifs, Le(X), sialyl-Le(X) or HNK-1 units. Hence, our data provide the basis for further studies on the contribution of carbohydrate determinants to CD24-mediated biological activities.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.