Probing polypeptide GalNAc-transferase isoform substrate specificities by in vitro analysis

N-acetylgalactosaminyltransferase (GalNAc)-type (mucin-type) O-glycosylation is an abundant and highly diverse modification of proteins. This type of O-glycosylation is initiated in the Golgi by a large family of up to 20 homologous polypeptide GalNAc-T isoenzymes that transfer GalNAc to Ser, Thr and possibly Tyr residues. These GalNAc residues are then further elongated by a large set of glycosyltransferases to build a variety of complex O-glycan structures. What determines O-glycan site occupancy is still poorly understood, although it is clear that the substrate specificities of individual isoenzymes and the repertoire of GalNAc-Ts in cells are key parameters. The GalNAc-T isoenzymes are differentially expressed in cells and tissues in principle allowing cells to produce unique O-glycoproteomes dependent on the specific subset of isoforms present. In vitro analysis of acceptor peptide substrate specificities using recombinant expressed GalNAc-Ts has been the method of choice for probing activities of individual isoforms, but these studies have been hampered by biological validation of actual O-glycosylation sites in proteins and number of substrate testable. Here, we present a systematic analysis of the activity of 10 human GalNAc-T isoenzymes with 195 peptide substrates covering known O-glycosylation sites and provide a comprehensive dataset for evaluating isoform-specific contributions to the O-glycoproteome.

The GalNAc-type O-Glycoproteome of CHO cells characterized by the SimpleCell strategy

The Chinese hamster ovary cell (CHO) is the major host cell factory for recombinant production of biological therapeutics primarily because of its "human-like" glycosylation features. CHO is used for production of several O-glycoprotein therapeutics including erythropoietin, coagulation factors, and chimeric receptor IgG1-Fc-fusion proteins, however, some O-glycoproteins are not produced efficiently in CHO. We have previously shown that the capacity for O-glycosylation of proteins can be one limiting parameter for production of active proteins in CHO. Although the capacity of CHO for biosynthesis of glycan structures (glycostructures) on glycoproteins are well established, our knowledge of the capacity of CHO cells for attaching GalNAc-type O-glycans to proteins (glycosites) is minimal. This type of O-glycosylation is one of the most abundant forms of glycosylation, and it is differentially regulated in cells by expression of a subset of homologous polypeptide GalNAc-transferases. Here, we have genetically engineered CHO cells to produce homogeneous truncated O-glycans, so-called SimpleCells, which enabled lectin enrichment of O-glycoproteins and characterization of the O-glycoproteome. We identified 738 O-glycoproteins (1548 O-glycosites) in cell lysates and secretomes providing the first comprehensive insight into the O-glycosylation capacity of CHO (http://glycomics.ku.dk/o-glycoproteome_db/).

Advances in mass spectrometry driven O-glycoproteomics

BACKGROUND

Global analyses of proteins and their modifications by mass spectrometry are essential tools in cell biology and biomedical research. Analyses of glycoproteins represent particular challenges and we are only at the beginnings of the glycoproteomic era. Some of the challenges have been overcome with N-glycoproteins and proteome-wide analysis of N-glycosylation sites is accomplishable today but only by sacrificing information of structures at individual glycosites. More recently advances in analysis of O-glycoproteins have been made and proteome-wide analysis of O-glycosylation sites is becoming available as well.

SCOPE OF REVIEW

Here we discuss the challenges of analysis of O-glycans and new O-glycoproteomics strategies focusing on O-GalNAc and O-Man glycoproteomes.

MAJOR CONCLUSIONS

A variety of strategies are now available for proteome-wide analysis of O-glycosylation sites enabling functional studies. However, further developments are still needed for complete analysis of glycan structures at individual sites for both N- and O-glycoproteomics strategies.

GENERAL SIGNIFICANCE

The advances in O-glycoproteomics have led to identification of new biological functions of O-glycosylation and a new understanding of the importance of where O-glycans are positioned on proteins.

Osteoblastic response to pectin nanocoating on titanium surfaces

Osseointegration of titanium implants can be improved by organic and inorganic nanocoating of the surface. The aim of our study was to evaluate the effect of organic nanocoating of titanium surface with unmodified and modified pectin Rhamnogalacturonan-Is (RG-Is) isolated from potato and apple with respect to surface properties and osteogenic response in osteoblastic cells. Nanocoatings on titanium surfaces were evaluated by scanning electron microscopy, contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy. The effect of coated RG-Is on cell adhesion, cell viability, bone matrix formation and mineralization was tested using SaOS-2 cells. Nanocoating with pectin RG-Is affected surface properties and in consequence changed the environment for cellular response. The cells cultured on surfaces coated with RG-Is from potato with high content of linear 1.4-linked galactose produced higher level of mineralized matrix compared with control surfaces and surfaces coated with RG-I with low content of linear 1.4-linked galactose. The study showed that the pectin RG-Is nanocoating not only changed chemical and physical titanium surface properties, but also specific coating with RG-Is containing high amount of galactan increased mineralized matrix formation of osteoblastic cells in vitro.

Characterization of binding epitopes of CA125 monoclonal antibodies

The most used cancer serum biomarker is the CA125 immunoassay for ovarian cancer that detects the mucin glycoprotein MUC16. Several monoclonal antibodies (mAbs) including OC125 and M11 are used in CA125 assays. However, despite considerable efforts, our knowledge of the molecular characteristics of the recognized epitopes and the role played by glycosylation has remained elusive. Here a comprehensive set of recombinant MUC16 tandem repeats (TRs) expressed in glycoengineered mammalian cells and E. coli, together with overlapping peptides, was used to probe antigen-binding epitopes. We present a complete analysis of N- and O-glycosylation sites of a MUC16 TR expressed in CHO cells and demonstrate that neither N- nor O-glycosylation appear to substantially influence binding of OC125 and M11 mAbs. A series of successive N- and C-terminal truncations of a MUC16 TR construct expressed in E. coli narrowed down the epitopes for OC125 and M11 to a segment containing parts of two consecutive SEA domains with a linker. Thus, a complete SEA domain is not required. These findings suggest that binding epitopes of mAbs OC125 and M11 are dependent on conformation but not on glycosylation. The availability of recombinant TR constructs with and without aberrant glycosylation now opens the way for vaccine studies.

Low density lipoprotein receptor class A repeats are O-glycosylated in linker regions

The low density lipoprotein receptor (LDLR) is crucial for cholesterol homeostasis and deficiency in LDLR functions cause hypercholesterolemia. LDLR is a type I transmembrane protein that requires O-glycosylation for stable expression at the cell surface. It has previously been suggested that LDLR O-glycosylation is found N-terminal to the juxtamembrane region. Recently we identified O-glycosylation sites in the linker regions between the characteristic LDLR class A repeats in several LDLR-related receptors using the "SimpleCell" O-glycoproteome shotgun strategy. Herein, we have systematically characterized O-glycosylation sites on recombinant LDLR shed from HEK293 SimpleCells and CHO wild-type cells. We find that the short linker regions between LDLR class A repeats contain an evolutionarily conserved O-glycosylation site at position -1 of the first cysteine residue of most repeats, which in wild-type CHO cells is glycosylated with the typical sialylated core 1 structure. The glycosites in linker regions of LDLR class A repeats are conserved in LDLR from man to Xenopus and found in other homologous receptors. O-Glycosylation is controlled by a large family of polypeptide GalNAc transferases. Probing into which isoform(s) contributed to glycosylation of the linker regions of the LDLR class A repeats by in vitro enzyme assays suggested a major role of GalNAc-T11. This was supported by expression of LDLR in HEK293 cells, where knock-out of the GalNAc-T11 isoform resulted in the loss of glycosylation of three of four linker regions.

Precision mapping of the human O-GalNAc glycoproteome through SimpleCell technology

Glycosylation is the most abundant and diverse posttranslational modification of proteins. While several types of glycosylation can be predicted by the protein sequence context, and substantial knowledge of these glycoproteomes is available, our knowledge of the GalNAc-type O-glycosylation is highly limited. This type of glycosylation is unique in being regulated by 20 polypeptide GalNAc-transferases attaching the initiating GalNAc monosaccharides to Ser and Thr (and likely some Tyr) residues. We have developed a genetic engineering approach using human cell lines to simplify O-glycosylation (SimpleCells) that enables proteome-wide discovery of O-glycan sites using 'bottom-up' ETD-based mass spectrometric analysis. We implemented this on 12 human cell lines from different organs, and present a first map of the human O-glycoproteome with almost 3000 glycosites in over 600 O-glycoproteins as well as an improved NetOGlyc4.0 model for prediction of O-glycosylation. The finding of unique subsets of O-glycoproteins in each cell line provides evidence that the O-glycoproteome is differentially regulated and dynamic. The greatly expanded view of the O-glycoproteome should facilitate the exploration of how site-specific O-glycosylation regulates protein function.

Precision mapping of the human O-GalNAc glycoproteome through SimpleCell technology

Glycosylation is the most abundant and diverse posttranslational modification of proteins. While several types of glycosylation can be predicted by the protein sequence context, and substantial knowledge of these glycoproteomes is available, our knowledge of the GalNAc-type O-glycosylation is highly limited. This type of glycosylation is unique in being regulated by 20 polypeptide GalNAc-transferases attaching the initiating GalNAc monosaccharides to Ser and Thr (and likely some Tyr) residues. We have developed a genetic engineering approach using human cell lines to simplify O-glycosylation (SimpleCells) that enables proteome-wide discovery of O-glycan sites using 'bottom-up' ETD-based mass spectrometric analysis. We implemented this on 12 human cell lines from different organs, and present a first map of the human O-glycoproteome with almost 3000 glycosites in over 600 O-glycoproteins as well as an improved NetOGlyc4.0 model for prediction of O-glycosylation. The finding of unique subsets of O-glycoproteins in each cell line provides evidence that the O-glycoproteome is differentially regulated and dynamic. The greatly expanded view of the O-glycoproteome should facilitate the exploration of how site-specific O-glycosylation regulates protein function.

Differential expression profiles of glycosphingolipids in human breast cancer stem cells vs. cancer non-stem cells

Previous studies demonstrated that certain glycosphingolipids (GSLs) are involved in various cell functions, such as cell growth and motility. Recent studies showed changes in GSL expression during differentiation of human embryonic stem cells; however, little is known about expression profiles of GSLs in cancer stem cells (CSCs). CSCs are a small subpopulation in cancer and are proposed as cancer-initiating cells, have been shown to be resistant to numerous chemotherapies, and may cause cancer recurrence. Here, we analyzed GSLs expressed in human breast CSCs by applying a CSC model induced through epithelial-mesenchymal transition, using mass spectrometry, TLC immunostaining, and cell staining. We found that (i) Fuc-(n)Lc4Cer and Gb3Cer were drastically reduced in CSCs, whereas GD2, GD3, GM2, and GD1a were greatly increased in CSCs; (ii) among various glycosyltransferases tested, mRNA levels for ST3GAL5, B4GALNT1, ST8SIA1, and ST3GAL2 were increased in CSCs, which could explain the increased expression of GD3, GD2, GM2, and GD1a in CSCs; (iii) the majority of GD2+ cells and GD3+ cells were detected in the CD44(hi)/CD24(lo) cell population; and (iv) knockdown of ST8SIA1 and B4GALNT1 significantly reduced the expression of GD2 and GD3 and caused a phenotype change from CSC to a non-CSC, which was detected by reduced mammosphere formation and cell motility. Our results provide insight into GSL profiles in human breast CSCs, indicate a functional role of GD2 and GD3 in CSCs, and suggest a possible novel approach in targeting human breast CSCs to interfere with cancer recurrence.

Enhanced mass spectrometric mapping of the human GalNAc-type O-glycoproteome with SimpleCells

Characterizing protein GalNAc-type O-glycosylation has long been a major challenge, and as a result, our understanding of this glycoproteome is particularly poor. Recently, we presented a novel strategy for high throughput identification of O-GalNAc glycosites using zinc finger nuclease gene-engineered "SimpleCell" lines producing homogeneous truncated O-glycosylation. Total lysates of cells were trypsinized and subjected to lectin affinity chromatography enrichment, followed by identification of GalNAc O-glycopeptides by nLC-MS/MS, with electron transfer dissociation employed to specify sites of O-glycosylation. Here, we demonstrate a substantial improvement in the SimpleCell strategy by including an additional stage of lectin affinity chromatography on secreted glycoproteins from culture media (secretome) and by incorporating pre-fractionation of affinity-enriched glycopeptides via IEF before nLC-MS/MS. We applied these improvements to three human SimpleCells studied previously, and each yielded a substantial increase in the number of O-glycoproteins and O-glycosites identified. We found that analysis of the secretome was an important independent factor for increasing identifications, suggesting that further substantial improvements can also be sought through analysis of subcellular organelle fractions. In addition, we uncovered a substantial nonoverlapping set of O-glycoproteins and O-glycosites using an alternative protease digestion (chymotrypsin). In total, the improvements led to identification of 259 glycoproteins, of which 152 (59%) were novel compared with our previous strategy using the same three cell lines. With respect to individual glycosites, we identified a total of 856 sites, of which 508 (59%) were novel compared with our previous strategy; this includes four new identifications of O-GalNAc attached to tyrosine. Furthermore, we uncovered ≈ 220 O-glycosites wherein the peptides were clearly identified, but the glycosites could not be unambiguously assigned to specific positions. The improved strategy should greatly facilitate high throughput characterization of the human GalNAc-type O-glycoproteome as well as be applicable to analysis of other O-glycoproteomes.

Glycoproteomic analysis of serum from patients with gastric precancerous lesions

Gastric cancer is preceded by a carcinogenesis pathway that includes gastritis caused by Helicobacter pylori infection, chronic atrophic gastritis that may progress to intestinal metaplasia (IM), dysplasia, and ultimately gastric carcinoma of the more common intestinal subtype. The identification of glycosylation changes in circulating serum proteins in patients with precursor lesions of gastric cancer is of high interest and represents a source of putative new biomarkers for early diagnosis and intervention. This study applies a glycoproteomic approach to identify altered glycoproteins expressing the simple mucin-type carbohydrate antigens T and STn in the serum of patients with gastritis, IM (complete and incomplete subtypes), and control healthy individuals. The immunohistochemistry analysis of the gastric mucosa of these patients showed expression of T and STn antigens in gastric lesions, with STn being expressed only in IM. The serum glycoproteomic analysis using 2D-gel electrophoresis, Western blot, and MALDI-TOF/TOF mass spectrometry led to the identification of circulating proteins carrying these altered glycans. One of the glycoproteins identified was plasminogen, a protein that has been reported to play a role in H. pylori chronic infection of the gastric mucosa and is involved in extracellular matrix modeling and degradation. Plasminogen was further characterized and showed to carry STn antigens in patients with gastritis and IM. These results provide evidence of serum proteins displaying abnormal O-glycosylation in patients with precursor lesions of gastric carcinoma and include a panel of putative targets for the non-invasive clinical diagnosis of individuals with gastritis and IM.

Toward stable genetic engineering of human O-glycosylation in plants

Glycosylation is the most abundant and complex posttranslational modification to be considered for recombinant production of therapeutic proteins. Mucin-type (N-acetylgalactosamine [GalNAc]-type) O-glycosylation is found in eumetazoan cells but absent in plants and yeast, making these cell types an obvious choice for de novo engineering of this O-glycosylation pathway. We previously showed that transient implementation of O-glycosylation capacity in plants requires introduction of the synthesis of the donor substrate UDP-GalNAc and one or more polypeptide GalNAc-transferases for incorporating GalNAc residues into proteins. Here, we have stably engineered O-glycosylation capacity in two plant cell systems, soil-grown Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum) Bright Yellow-2 suspension culture cells. Efficient GalNAc O-glycosylation of two stably coexpressed substrate O-glycoproteins was obtained, but a high degree of proline hydroxylation and hydroxyproline-linked arabinosides, on a mucin (MUC1)-derived substrate, was also observed. Addition of the prolyl 4-hydroxylase inhibitor 2,2-dipyridyl, however, effectively suppressed proline hydroxylation and arabinosylation of MUC1 in Bright Yellow-2 cells. In summary, stably engineered mammalian type O-glycosylation was established in transgenic plants, demonstrating that plants may serve as host cells for the production of recombinant O-glycoproteins. However, the present stable implementation further strengthens the notion that elimination of endogenous posttranslational modifications may be needed for the production of protein therapeutics.

Engineering mammalian mucin-type O-glycosylation in plants

Mucin-type O-glycosylation is an important post-translational modification that confers a variety of biological properties and functions to proteins. This post-translational modification has a particularly complex and differentially regulated biosynthesis rendering prediction and control of where O-glycans are attached to proteins, and which structures are formed, difficult. Because plants are devoid of GalNAc-type O-glycosylation, we have assessed requirements for establishing human GalNAc O-glycosylation de novo in plants with the aim of developing cell systems with custom-designed O-glycosylation capacity. Transient expression of a Pseudomonas aeruginosa Glc(NAc) C4-epimerase and a human polypeptide GalNAc-transferase in leaves of Nicotiana benthamiana resulted in GalNAc O-glycosylation of co-expressed human O-glycoprotein substrates. A chimeric YFP construct containing a 3.5 tandem repeat sequence of MUC1 was glycosylated with up to three and five GalNAc residues when co-expressed with GalNAc-T2 and a combination of GalNAc-T2 and GalNAc-T4, respectively, as determined by mass spectrometry. O-Glycosylation was furthermore demonstrated on a tandem repeat of MUC16 and interferon α2b. In plants, prolines in certain classes of proteins are hydroxylated and further substituted with plant-specific O-glycosylation; unsubstituted hydroxyprolines were identified in our MUC1 construct. In summary, this study demonstrates that mammalian type O-glycosylation can be established in plants and that plants may serve as a host cell for production of recombinant O-glycoproteins with custom-designed O-glycosylation. The observed hydroxyproline modifications, however, call for additional future engineering efforts.

Elucidation of the sugar recognition ability of the lectin domain of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 3 by using unnatural glycopeptide substrates

Mucin-type glycosylation [α-N-acetyl-D-galactosamine (α-GalNAc)-O-Ser/Thr] on proteins is initiated biosynthetically by 16 homologous isoforms of GalNAc-Ts (uridine diphosphate-GalNAc:polypeptide N-acetylgalactosaminyltransferases). All the GalNAc-Ts consist of a catalytic domain and a lectin domain. Previous reports of GalNAc-T assays toward peptides and α-GalNAc glycopeptides showed that the lectin domain recognized the sugar on the substrates and affected the reaction; however, the details are not clear. Here, we report a new strategy to give insight on the sugar recognition ability and the function of the GalNAc-T3 lectin domain using chemically synthesized natural-type (α-GalNAc-O-Thr) and unnatural-type [β-GalNAc-O-Thr, α-Fuc-O-Thr and β-GlcNAc-O-Thr] MUC5AC glycopeptides. GalNAc-T3 is one of isoforms expressed in various organs, its substrate specificity extensively characterized and its anomalous expression has been identified in several types of cancer (e.g. pancreas and stomach). The glycopeptides used in this study were designed based on a preliminary peptide assay with a sequence derived from the MUC5AC tandem repeat. Through GalNAc-T3 and lectin-inactivated GalNAc-T3, competition assays between the glycopeptide substrates and product analyses (MALDI-TOF MS, RP-HPLC and ETD-MS/MS), we show that the lectin domain strictly recognized GalNAc on the substrate and this specificity controlled the glycosylation pathway.

Design of a covalently bonded glycosphingolipid microarray

Glycosphingolipids (GSLs) are well known ubiquitous constituents of all eukaryotic cell membranes, yet their normal biological functions are not fully understood. As with other glycoconjugates and saccharides, solid phase display on microarrays potentially provides an effective platform for in vitro study of their functional interactions. However, with few exceptions, the most widely used microarray platforms display only the glycan moiety of GSLs, which not only ignores potential modulating effects of the lipid aglycone, but inherently limits the scope of application, excluding, for example, the major classes of plant and fungal GSLs. In this work, a prototype "universal" GSL-based covalent microarray has been designed, and preliminary evaluation of its potential utility in assaying protein-GSL binding interactions investigated. An essential step in development involved the enzymatic release of the fatty acyl moiety of the ceramide aglycone of selected mammalian GSLs with sphingolipid N-deacylase (SCDase). Derivatization of the free amino group of a typical lyso-GSL, lyso-G(M1), with a prototype linker assembled from succinimidyl-[(N-maleimidopropionamido)-diethyleneglycol] ester and 2-mercaptoethylamine, was also tested. Underivatized or linker-derivatized lyso-GSL were then immobilized on N-hydroxysuccinimide- or epoxide-activated glass microarray slides and probed with carbohydrate binding proteins of known or partially known specificities (i.e., cholera toxin B-chain; peanut agglutinin, a monoclonal antibody to sulfatide, Sulph 1; and a polyclonal antiserum reactive to asialo-G(M2)). Preliminary evaluation of the method indicated successful immobilization of the GSLs, and selective binding of test probes. The potential utility of this methodology for designing covalent microarrays that incorporate GSLs for serodiagnosis is discussed.

Mining the O-glycoproteome using zinc-finger nuclease-glycoengineered SimpleCell lines

Zinc-finger nuclease (ZFN) gene targeting is emerging as a versatile tool for engineering of multiallelic gene deficiencies. A longstanding obstacle for detailed analysis of glycoproteomes has been the extensive heterogeneities in glycan structures and attachment sites. Here we applied ZFN targeting to truncate the O-glycan elongation pathway in human cells, generating stable 'SimpleCell' lines with homogenous O-glycosylation. Three SimpleCell lines expressing only truncated GalNAcα or NeuAcα2-6GalNAcα O-glycans were produced, allowing straightforward isolation and sequencing of GalNAc O-glycopeptides from total cell lysates using lectin chromatography and nanoflow liquid chromatography-mass spectrometry (nLC-MS/MS) with electron transfer dissociation fragmentation. We identified >100 O-glycoproteins with >350 O-glycan sites (the great majority previously unidentified), including a GalNAc O-glycan linkage to a tyrosine residue. The SimpleCell method should facilitate analyses of important functions of protein glycosylation. The strategy is also applicable to other O-glycoproteomes.

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

Site-specific GalNAc-type O-glycosylation is emerging as an important co-regulator of proprotein convertase (PC) processing of proteins. PC processing is crucial in regulating many fundamental biological pathways and O-glycans in or immediately adjacent to processing sites may affect recognition and function of PCs. Thus, we previously demonstrated that deficiency in site-specific O-glycosylation in a PC site of the fibroblast growth factor, FGF23, resulted in marked reduction in secretion of active unprocessed FGF23, which cause familial tumoral calcinosis and hyperostosis hyperphosphatemia. GalNAc-type O-glycosylation is found on serine and threonine amino acids and up to 20 distinct polypeptide GalNAc transferases catalyze the first addition of GalNAc to proteins making this step the most complex and differentially regulated steps in protein glycosylation. There is no reliable prediction model for O-glycosylation especially of isolated sites, but serine and to a lesser extent threonine residues are frequently found adjacent to PC processing sites. In the present study we used in vitro enzyme assays and ex vivo cell models to systematically address the boundaries of the region within site-specific O-glycosylation affect PC processing. The results demonstrate that O-glycans within at least ±3 residues of the RXXR furin cleavage site may affect PC processing suggesting that site-specific O-glycosylation is a major co-regulator of PC processing.

Lectin domains of polypeptide GalNAc transferases exhibit glycopeptide binding specificity

UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (GalNAc-Ts) constitute a family of up to 20 transferases that initiate mucin-type O-glycosylation. The transferases are structurally composed of catalytic and lectin domains. Two modes have been identified for the selection of glycosylation sites by GalNAc-Ts: confined sequence recognition by the catalytic domain alone, and concerted recognition of acceptor sites and adjacent GalNAc-glycosylated sites by the catalytic and lectin domains, respectively. Thus far, only the catalytic domain has been shown to have peptide sequence specificity, whereas the primary function of the lectin domain is to increase affinity to previously glycosylated substrates. Whether the lectin domain also has peptide sequence selectivity has remained unclear. Using a glycopeptide array with a library of synthetic and recombinant glycopeptides based on sequences of mucins MUC1, MUC2, MUC4, MUC5AC, MUC6, and MUC7 as well as a random glycopeptide bead library, we examined the binding properties of four different lectin domains. The lectin domains of GalNAc-T1, -T2, -T3, and -T4 bound different subsets of small glycopeptides. These results indicate an additional level of complexity in the initiation step of O-glycosylation by GalNAc-Ts.

Immunologic mapping of glycomes: implications for cancer diagnosis and therapy

Cancer associated glycoconjugates are important biomarkers, as exemplified by globo-H, CA125, CA15.3 and CA27.29. However, the exact chemical structures of many such biomarkers remain unknown because of technological limitations. In this article, we propose the "immunologic mapping" of cancer glycomes based on specific immune recognition of glycan structures, which can be hypothesized theoretically, produced chemically, and examined biologically by immuno-assays. Immunologic mapping of glycans not only provides a unique perspective on cancer glycomes, but also may lead to the invention of powerful reagents for diagnosis and therapy.

The structural and functional role of myelin fast-migrating cerebrosides: pathological importance in multiple sclerosis

A family of neutral glycosphingolipids containing a 3-O-acetyl-sphingosine galactosylceramide (3-SAG) has been characterized. Seven new derivatives of galactosylceramide (GalCer), designated as fast-migrating cerebrosides (FMCs) by TLC retention factor, have been identified. The simplest compounds - FMC-1 and FMC-2 - of this series have been characterized as the 3-SAG containing nonhydroxy and hydroxy fatty acyl, respectively. The next two - FMC-3 and FMC-4 - add 6-O-acetyl-galactose and the most complex glycosphingolipids, FMC-5, -6 and -7, are 2,3,4,6-tetra-O-acetyl-3-SAG. These hydrophobic myelin lipid biomarkers coappear with GalCer during myelinogenesis and disappear along with GalCer in de- or dys-myelinating disorders. Myelin lipid antigens, including FMCs, are keys to myelin biology, opening the possibility of new and novel immune modulatory tools for treatment of autoimmune diseases including multiple sclerosis.

O-glycosylation modulates proprotein convertase activation of angiopoietin-like protein 3: possible role of polypeptide GalNAc-transferase-2 in regulation of concentrations of plasma lipids

The angiopoietin-like protein 3 (ANGPTL3) is an important inhibitor of the endothelial and lipoprotein lipases and a promising drug target. ANGPTL3 undergoes proprotein convertase processing (RAPR(224)↓TT) for activation, and the processing site contains two potential GalNAc O-glycosylation sites immediately C-terminal (TT(226)). We developed an in vivo model system in CHO ldlD cells that was used to show that O-glycosylation in the processing site blocked processing of ANGPTL3. Genome-wide SNP association studies have identified the polypeptide GalNAc-transferase gene, GALNT2, as a candidate gene for low HDL and high triglyceride blood levels. We hypothesized that the GalNAc-T2 transferase performed critical O-glycosylation of proteins involved in lipid metabolism. Screening of a panel of proteins known to affect lipid metabolism for potential sites glycosylated by GalNAc-T2 led to identification of Thr(226) adjacent to the proprotein convertase processing site in ANGPTL3. We demonstrated that GalNAc-T2 glycosylation of Thr(226) in a peptide with the RAPR(224)↓TT processing site blocks in vitro furin cleavage. The study demonstrates that ANGPTL3 activation is modulated by O-glycosylation and that this step is probably controlled by GalNAc-T2.

Random glycopeptide bead libraries for seromic biomarker discovery

Identification of disease-specific biomarkers is important to address early diagnosis and management of disease. Aberrant post-translational modifications (PTM) of proteins such as O-glycosylations (O-PTMs) are emerging as triggers of autoantibodies that can serve as sensitive biomarkers. Here we have developed a random glycopeptide bead library screening platform for detection of autoantibodies and other binding proteins. Libraries were build on biocompatible PEGA beads including a safety-catch C-terminal amide linker (SCAL) that allowed mild cleavage conditions (I(2)/NaBH(4) and TFA) for release of glycopeptides and sequence determination by ESI-Orbitrap-MS(n). As proof-of-principle, tumor -specific glycopeptide reporter epitopes were built-in into the libraries and were detected by tumor-specific monoclonal antibodies and autoantibodies from cancer patients. Sequenced and identified glycopeptides were resynthesized at the preparative scale by automated parallel peptide synthesis and printed on microarrays for validation and broader analysis with larger sets of sera. We further showed that chemical synthesis of the monosaccharide O-glycopeptide library (Tn-glycoform) could be diversified to other tumor glycoforms by on-bead enzymatic glycosylation reactions with recombinant glycosyltransferases. Hence, we have developed a high-throughput flexible platform for rapid discovery of O-glycopeptide biomarkers and the method has applicability in other types of assays such as lectin/antibody/enzyme specificity studies as well as investigation of other PTMs.

N-glycosylation increases the circulatory half-life of human growth hormone

Therapeutic use of recombinant GH typically involves daily sc injections. We examined the possibilities for prolonging the in vivo circulation of GH by introducing N-glycans. Human GH variants with a single potential N-glycosylation site (N-X-S/T) introduced by site-directed mutagenesis were expressed in HEK293 cells. In a scan of 15 different positions for N-glycosylation sites, four positions (amino acids 93, 98, 99, and 101) were efficiently utilized and did not influence GH in vitro activity. A GH variant (3N-GH) with all these sites was produced in CHOK1SV cells and contained up to three N-glycans. Two pools of 3N-GH were purified and separated according to their charge by anion-exchange chromatography. Anion-exchange HPLC revealed that the N-glycans in the two pools were very similar except for the extent of sialylation. Both 3N-GH pools circulated longer in rats than wild-type GH. The terminal half-life of 3N-GH after iv injection was 24-fold prolonged compared with wild-type GH for the pool with the most pronounced sialylation, 13-fold prolonged for the less sialylated pool, and similar to the wild-type for desialylated 3N-GH. The less sialylated 3N-GH pool exhibited a profound pharmacodynamic effect in GH-deficient rats. Over a 4-d period, a single injection of 3N-GH induced a stronger IGF-I response and a larger increase in body weight than daily injections with wild-type GH. Thus, N-glycans can prolong the in vivo circulation and enhance the pharmacodynamic effect of GH. Sialic acids seem to play a pivotal role for the properties of glycosylated GH.

Mass spectrometry of fluorocarbon-labeled glycosphingolipids

A method for generation of novel fluorocarbon derivatives of glycosphingolipids (GSLs) with high affinity for fluorocarbon phases has been developed, and their potential applications to mass spectrometry (MS)-based methodologies for glycosphingolipidomics have been investigated. Sphingolipid ceramide N-deacylase (SCDase) is used to remove the fatty acid from the ceramide moiety, after which a fluorocarbon-rich substituent (F-Tag) is incorporated at the free amine of the sphingoid. In initial trials, a neutral GSL, globotriaosylceramide (Gb(3)Cer), three purified bovine brain gangliosides, and four fungal glycosylinositol phosphorylceramides (GIPCs) were de-N-acylated, derivatized by prototype F-Tags, and recovered by solid phase extraction on fluorocarbon-derivatized silica (F-SPE). The efficacy of SCDase treatment of GIPCs was here demonstrated for the first time. Compatibility with subsequent per-N,O-methylation was established for the F-tagged Gb(3) Cer and purified gangliosides, and extensive mass spectra (MS(1) and MS(2)) consistent with all of the expected products were acquired. The potential use of F-tagged derivatives for a comprehensive MS based profiling application was then demonstrated on a crude ganglioside mixture extracted from bovine brain. Finally, a simple trial in microarray format demonstrated fixation of F-tagged G(M1) ganglioside to a fluorous glass surface, with the glycan intact and available for interaction with a fluorescent derivative of cholera toxin B chain. The methods described thus provide a new avenue for rapid GSL recovery or cleanup, potentially compatible with a variety of platforms for mass spectrometric profiling and structure analysis, as well as parallel analysis of functional interactions.

Novel myelin penta- and hexa-acetyl-galactosyl-ceramides: structural characterization and immunoreactivity in cerebrospinal fluid

Fast migrating cerebrosides (FMC) are derivatives of galactosylceramide (GalCer). The structures of the most hydrophobic FMC-5, FMC-6, and FMC-7 were determined by electrospray ionization linear ion-trap mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy complementing previous NMR spectroscopy and gas chromatography-mass spectrometry to be 3-O-acetyl-sphingosine-GalCer derivatives with galactose O-acetyl modifications. FMC-5 and FMC-6 are 3-O-acetyl-sphingosine-2,3,4,6-tetra-O-acetyl-GalCer with nonhydroxy and hydroxy-N-fatty-acids, while FMC-7 has an additional O-acetylation of the 2-hydroxy-fatty acid. The immuno-reactivity in human cerebrospinal fluid (CSF) to these acetylated glycolipids was examined in central nervous system (CNS) infectious disease, noninflammatory disorders, and multiple sclerosis (MS). Screening for lipid binding in MS and other neurological disease groups revealed that the greatest anti-hydrophobic FMC reactivity was observed in the inflammatory CNS diseases (meningitis, meningo-encephalitis, and subacute sclerosing panencephalitis). Some MS patients had increased reactivity with the hydrophobic FMCs and with glycoglycerophospholipid MfGL-II from Mycoplasma fermentans. The cross-reactivity of highly acetylated GalCer with microbial acyl-glycolipid raises the possibility that myelin-O-acetyl-cerebrosides, bacterial infection, and neurological disease are linked.

A novel xylosylphosphotransferase activity discovered in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes serious disease in immunocompromised individuals. The organism produces a distinctive polysaccharide capsule that is necessary for its virulence, a predominantly polysaccharide cell wall, and a variety of protein- and lipid-linked glycans. The glycan synthetic pathways of this pathogen are of great interest. Here we report the detection of a novel glycosylphosphotransferase activity in C. neoformans, identification of the corresponding gene, and characterization of the encoded protein. The observed activity is specific for UDP-xylose as a donor and for mannose acceptors and forms a xylose-alpha-1-phosphate-6-mannose linkage. This is the first report of a xylosylphosphotransferase activity in any system.

Immunologic glycosphingolipidomics and NKT cell development in mouse thymus

Invariant NKT cells are a hybrid cell type of Natural Killer cells and T cells, whose development is dependent on thymic positive selection mediated by double positive thymocytes through their recognition of natural ligands presented by CD1d, a nonpolymorphic, non-MHC, MHC-like antigen presenting molecule. Genetic evidence suggested that beta-glucosylceramide derived glycosphingolipids (GSLs) are natural ligands for NKT cells. N-butyldeoxygalactonojirimycin (NB-DGJ), a drug that specifically inhibits the glucosylceramide synthase, inhibits the endogenous ligands for NKT cells. Furthermore, we and others have found a beta-linked glycosphingolipid, isoglobotriaosylceramide (iGb3), is a stimulatory NKT ligand. The iGb3 synthase knockout mice have a normal NKT development and function, indicating that other ligands exist and remain to be identified. In this study, we have performed a glycosphingolipidomics study of mouse thymus, and studied mice mutants which are deficient in beta-hexosaminidase b or alpha-galactosidase A, two glycosidases that are up- and downstream agents of iGb3 turnover, respectively. Our mass spectrometry methods generated a first database for glycosphingolipids expressed in mouse thymus, which are specifically regulated by rate-limiting glycosidases. Among the identified thymic glycosphingolipids, only iGb3 is a stimulatory ligand for NKT cells, suggesting that large-scale fractionation, enrichment and characterization of minor species of glycosphingolipids are necessary for identifying additional ligands for NKT cells. Our results also provide early insights into cellular lipidomics studies, with a specific focus on the important immunological functions of glycosphingolipids.

Identification of a GH110 subfamily of alpha 1,3-galactosidases: novel enzymes for removal of the alpha 3Gal xenotransplantation antigen

In search of alpha-galactosidases with improved kinetic properties for removal of the immunodominant alpha1,3-linked galactose residues of blood group B antigens, we recently identified a novel prokaryotic family of alpha-galactosidases (CAZy GH110) with highly restricted substrate specificity and neutral pH optimum (Liu, Q. P., Sulzenbacher, G., Yuan, H., Bennett, E. P., Pietz, G., Saunders, K., Spence, J., Nudelman, E., Levery, S. B., White, T., Neveu, J. M., Lane, W. S., Bourne, Y., Olsson, M. L., Henrissat, B., and Clausen, H. (2007) Nat. Biotechnol. 25, 454-464). One member of this family from Bacteroides fragilis had exquisite substrate specificity for the branched blood group B structure Galalpha1-3(Fucalpha1-2)Gal, whereas linear oligosaccharides terminated by alpha1,3-linked galactose such as the immunodominant xenotransplantation epitope Galalpha1-3Galbeta1-4GlcNAc did not serve as substrates. Here we demonstrate the existence of two distinct subfamilies of GH110 in B. fragilis and thetaiotaomicron strains. Members of one subfamily have exclusive specificity for the branched blood group B structures, whereas members of a newly identified subfamily represent linkage specific alpha1,3-galactosidases that act equally well on both branched blood group B and linear alpha1,3Gal structures. We determined by one-dimensional (1)H NMR spectroscopy that GH110 enzymes function with an inverting mechanism, which is in striking contrast to all other known alpha-galactosidases that use a retaining mechanism. The novel GH110 subfamily offers enzymes with highly improved performance in enzymatic removal of the immunodominant alpha3Gal xenotransplantation epitope.

Sensitive detection of isoglobo and globo series tetraglycosylceramides in human thymus by ion trap mass spectrometry

Glycosphingolipids serve as ligands for receptors involved in signal transduction and immune recognition, as exemplified by isoglobotrihexosylceramide, an antigenic ligand for T cell receptors. Mechanistic studies on the regulation of isoglobotrihexosylceramide require biochemical measurement of its lysosomal precursor, isoglobotetraglycosylceramide. It remains a challenge to distinguish between complex tetraglycosylceramide glycosphingolipid isomers with the same sugar components but diverse internal linkages. Here we established a simple and sensitive method to separate globo- and isoglobotetraglycosylceramide by MS5 ion trap mass spectrometry, and report the identification of isoglobotetraglycosylceramide in a CHO cell line transfected by iGb3 synthase, as well as in human thymus.

Analysis of glycosylinositol phosphorylceramides expressed by the opportunistic mycopathogen Aspergillus fumigatus

Acidic glycosphingolipid components were extracted from the opportunistic mycopathogen Aspergillus fumigatus and identified as inositol phosphorylceramide and glycosylinositol phosphorylceramides (GIPCs). Using nuclear magnetic resonance sppectroscopy, mass spectrometry, and other techniques, the structures of six major components were elucidated as Ins-P-Cer (Af-0), Manp(alpha1-->3)Manp(alpha1-->2)Ins-P-Cer (Af-2), Manp(alpha1-->2)Manp(alpha1-->3)Manp(alpha1-->2)Ins-P-Cer (Af-3a), Manp(alpha1-->3)[Galf(beta1-->6)]Manp(alpha1-->2)-Ins-P-Cer (Af-3b), Manp(alpha1-->2)-Manp(alpha1-->3)[Galf(beta1-->6)]Manp(alpha1-->2)Ins-P-Cer (Af-4), and Manp(alpha1-->3)Manp(alpha1-->6)GlcpN(alpha1-->2)Ins-P-Cer (Af-3c) (where Ins = myo-inositol and P = phosphodiester). A minor A. fumigatus GIPC was also identified as the N-acetylated version of Af-3c (Af-3c*), which suggests that formation of the GlcNalpha1-->2Ins linkage may proceed by a two-step process, similar to the GlcNalpha1-->6Ins linkage in glycosylphosphatidylinositol (GPI) anchors (transfer of GlcNAc, followed by enzymatic de-N-acetylation). The glycosylinositol of Af-3b, which bears a distinctive branching Galf(beta1-->6) residue, is identical to that of a GIPC isolated previously from the dimorphic mycopathogen Paracoccidioides brasiliensis (designated Pb-3), but components Af-3a and Af-4 have novel structures. Overlay immunostaining of A. fumigatus GIPCs separated on thin-layer chromatograms was used to assess their reactivity against sera from a patient with aspergillosis and against a murine monoclonal antibody (MEST-1) shown previously to react with the Galf(beta1-->6) residue in Pb-3. These results are discussed in relation to pathogenicity and potential approaches to the immunodiagnosis of A. fumigatus.

A beta-1,2-xylosyltransferase from Cryptococcus neoformans defines a new family of glycosyltransferases

Cryptococcus neoformans is an opportunistic fungal pathogen characterized by a prominent polysaccharide capsule that envelops the cell. Although this capsule is dispensable for in vitro growth, its presence is essential for virulence. The capsule is primarily made of two xylose-containing polysaccharides, glucuronoxylomannan and galactoxylomannan. There are likely to be multiple xylosyltransferases (XTs) involved in capsule synthesis, and the activities of these enzymes are potentially important for cryptococcal virulence. A beta-1,2-xylosyltransferase with specificity appropriate for capsule synthesis was purified approximately 3000-fold from C. neoformans, and the corresponding gene was identified and cloned. This sequence conferred XT activity when expressed in Saccharomyces cerevisiae, which lacks endogenous XT activity. The gene, termed CXT1 for cryptococcal xylosyltransferase 1, encodes a 79-kDa type II membrane protein with an N-linked glycosylation site and two DXD motifs. These latter motifs are believed to coordinate divalent cation binding in the activity of glycosyltransferases. Site-directed mutagenesis of one DXD motif abolished Cxt1p activity, even though this activity does not depend on the addition of a divalent cation. This may indicate a novel catalytic mechanism for glycosyl transfer. Five homologs of Cxt1p were found in the genome sequence of C. neoformans and 34 within the sequences of other fungi, although none were found in other organisms. Many of the homologous proteins are similar in size to Cxt1p, and all are conserved with respect to the essential DXD motif. These proteins represent a new family of glycosyltransferases, found exclusively within the fungal kingdom.

Glycosphingolipids with extended sugar chain have specialized functions in development and behavior of Drosophila

Glycosphingolipids (GSL) are glycosylated polar lipids in cell membranes essential for development of vertebrates as well as Drosophila. Mutants that impair enzymes involved in biosynthesis of GSL sugar chains provide a means to assess the functions of the sugar chains in vivo. The Drosophila glycosyltransferases Egghead and Brainiac are responsible for the 2nd and 3rd steps of GSL sugar chain elongation. Mutants lacking these enzymes are lethal and the nature of the defects that occur has suggested that GSL might impact on signaling by the Notch and EGFR pathways. Here we report on characterization of enzymes involved in the 4th and 5th steps of GSL sugar chain elongation in vitro and explore the biological consequences of removing the enzymes involved in step 4 in vivo. Two beta4-N-Acetylgalactosyltransferase enzymes can carry out step 4 (beta4GalNAcTA and beta4GalNAcTB), and while they may have overlapping activity, the mutants produce distinct phenotypes. The beta4GalNAcTA mutant displays behavioral defects, which are also observed in viable brainiac mutants, suggesting that proper locomotion and coordination primarily depend on GSL elongation. beta4GalNAcTB mutant animal shows ventralization of ovarian follicle cells, which is caused by defective EGFR signaling between the oocyte and the dorsal follicle cells to specify dorsal fate. GSL sequentially elongated by Egh, Brn and beta4GalNAcTB in the oocyte contribute to this signaling pathway. Despite the similar enzymatic activity, we provide evidence that the two enzymes are not functionally redundant in vivo, but direct distinct developmental functions of GSL.

Characterization of novel myelin components 3-O-acetyl-sphingosine galactosylceramides by electrospray ionization Q-TOF MS and MS/CID-MS of Li+ adducts

Glycosphingolipids with R(f) values higher than those of monoglycosylceramides (MGCs) in normal phase HPTLC appear to be normal components of myelin. A series of such low polarity components, referred to as 'fast moving cerebrosides' (FMCs), have been isolated from rat brain, and two of these fractions (FMC-1 and FMC-2) were found to be novel derivatives of galactosylceramide (GalCer) exhibiting O-acetylation at the 3-hydroxy group of the sphingoid moiety, and incorporating either non-hydroxy or 2-hydroxy fatty-N-acylation (Dasgupta S, Levery SB, Hogan EL. J. Lipid Res. 2002; 43: 751-761). Similar to the parent compounds, the 3-O-acetyl-sphingoid derivatives exhibit considerable diversity with respect to fatty-N-acyl chain length, manifested by heterogeneous molecular ion (Li(+) adduct) profiles. However, a detailed analysis of the individual molecular variants ('lipoforms'), e.g. by tandem MS/CID-MS analysis, was not carried out. In addition, several other FMCs distinguished by even lower polarity (higher HPTLC R(f) values) were isolated but have remained uncharacterized. For this study, analysis of both the known and unknown FMC components was carried out by positive ion ESI-MS and MS/CID-MS of their Li(+) adducts on a Q-TOF mass spectrometer. Since a Q-TOF instrument has not yet been applied to MS of lithiated cerebrosides and FMCs, MS/CID-MS spectra of bovine brain GalCer (both types) and the previously characterized rat brain FMCs (FMC-1 and FMC-2), having 3-O-acetylation of the sphingoid, were systematically acquired and their fragmentation behavior compared. This was followed by systematic analysis of previously uncharacterized FMC fractions (FMC-3 through FMC-5/6/7). The GalCer and FMC components proved to be amenable to analysis by this technique, and the data confirm that the latter are all related 3-O-acetyl-sphingoid derivatives, with the higher R(f) components carrying additional O-acetyl modifications on the galactosyl residue, which further reduce their polarity. The utility of the technique, the structures of unknown FMCs, and their characteristic fragmentation patterns are described.

Bacterial glycosidases for the production of universal red blood cells

Enzymatic removal of blood group ABO antigens to develop universal red blood cells (RBCs) was a pioneering vision originally proposed more than 25 years ago. Although the feasibility of this approach was demonstrated in clinical trials for group B RBCs, a major obstacle in translating this technology to clinical practice has been the lack of efficient glycosidase enzymes. Here we report two bacterial glycosidase gene families that provide enzymes capable of efficient removal of A and B antigens at neutral pH with low consumption of recombinant enzymes. The crystal structure of a member of the alpha-N-acetylgalactosaminidase family reveals an unusual catalytic mechanism involving NAD+. The enzymatic conversion processes we describe hold promise for achieving the goal of producing universal RBCs, which would improve the blood supply while enhancing the safety of clinical transfusions.

Characterization of neutral and acidic glycosphingolipids from the lectin-producing mushroom, Polyporus squamosus

The polypore mushroom Polyporus squamosus is the source of a lectin that exhibits a general affinity for terminal beta-galactosides, but appears to have an extended carbohydrate-binding site with high affinity and strict specificity for the nonreducing terminal trisaccharide sequence NeuAcalpha2 --> 6Galbeta1 --> 4Glc/GlcNAc. In considering the possibility that the lectin's in vivo function could involve interaction with an endogenous glycoconjugate, it would clearly be helpful to identify candidate ligands among various classes of carbohydrate-containing materials expressed by P. squamosus. Since evidence has been accumulating that glycosphingolipids (GSLs) may serve as key ligands for some endogenous lectins in animal species, possible similar roles for fungal GSLs could be considered. For this study, total lipids were extracted from mature fruiting body of P. squamosus. Multistep fractionation yielded a major monohexosylceramide (CMH) component and three major glycosylinositol phosphorylceramides (GIPCs) from the neutral and acidic lipids, respectively. These were characterized by a variety of techniques as required, including one- and two-dimensional (1)H- and (13)C-nuclear magnetic resonance (NMR) spectroscopy; electrospray ionization-mass spectrometry (ESI-MS, tandem-MS/collision-induced decay-MS, and ion trap-MS(n)); and component and methylation linkage analysis by gas chromatography-mass spectrometry. The CMH was determined to be glucosylceramide having a typical ceramide consisting of 2-hydroxy fatty-N-acylated (4E,8E)-9-methyl-sphinga-4,8-dienine. The GIPCs were identified as Manalpha1 --> 2Ins1-P-1Cer (Ps-1), Galbeta1 --> 6Manalpha1 --> 2Ins1-P-1Cer (Ps-2), and Manalpha1 --> 3Fucalpha1 --> 2Galalpha1 --> 6Galbeta1 --> 6Manalpha1 -->2Ins1-P-1Cer (Ps-5), respectively (where Ins = myo-inositol, P = phosphodiester, and Cer = ceramide consisting mainly of long-chain 2-hydroxy and 2,3-dihydroxy fatty-N-acylated 4-hydroxy-sphinganines). Of these GSLs, Ps-2 could potentially interact with P. squamosus lectin, and further investigations will focus on determining the binding affinity, if any, of the lectin for the GIPCs isolated from this fungus.

Structure elucidation of neutral, di-, tri-, and tetraglycosylceramides from High Five cells: identification of a novel (non-arthro-series) glycosphingolipid pathway

The major neutral glycosphingolipids (GSLs) of High Five insect cells have been extracted, purified, and characterized. It was anticipated that GSLs from High Five cells would follow the arthro-series pathway, known to be expressed by both insects and nematodes at least through the common tetraglycosylceramide (Glcbeta1Cer --> Manbeta4Glcbeta1Cer [MacCer] --> GlcNAcbeta3Manbeta4Glcbeta1Cer [At(3)Cer] --> GalNAcbeta4- GlcNAcbeta3Manbeta4Glcbeta1Cer [At(4)Cer]). Surprisingly, the structures of the major neutral High Five GSLs already diverge from the arthro-series pathway at the level of the triglycosylceramide. Studies by one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) showed the structure of the predominant High Five triglycosylceramide to be Galbeta3Manbeta4Glcbeta1Cer, whereas the predominant tetraglycosylceramide was characterized as GalNAcalpha4Galbeta3Manbeta4- Glcbeta1Cer. Both of these structures are novel products for any cell or organism so far studied. The GalNAcalpha4 and Galbeta3 units are found in insect GSLs, but always as the fifth and sixth residues linked to GalNAcbeta4 in the arthro-series penta- and hexaglycosylceramide structures (At(5)Cer and At(6)Cer, respectively). The structure of the High Five tetraglycosylceramide thus requires a reversal of the usual order of action of the glycosyltransferases adding the GalNAcalpha4 and Galbeta3 residues in dipteran GSL biosynthesis and implies the existence of an insect Galbeta3-T capable of using Manbeta4Glcbeta1Cer as a substrate with high efficiency. The results demonstrate the potential appearance of unexpected glycoconjugate biosynthetic products even in widely used but unexamined systems, as well as a potential for core switching based on MacCer, as observed in mammalian cells based on the common LacCer intermediate.

Neutral glycolipids of the filamentous fungus Neurospora crassa: altered expression in plant defensin-resistant mutants

To defend themselves against fungal pathogens, plants produce numerous antifungal proteins and peptides, including defensins, some of which have been proposed to interact with fungal cell surface glycosphingolipid components. Although not known as a phytopathogen, the filamentous fungus Neurospora crassa possesses numerous genes similar to those required for plant pathogenesis identified in fungal pathogens (Galagan, J. E., et al. 2003. Nature 422: 859-868), and it has been used as a model for studying plant-phytopathogen interactions targeting fungal membrane components (Thevissen, K., et al. 2003. Peptides. 24: 1705-1712). For this study, neutral glycolipid components were extracted from wild-type and plant defensin-resistant mutant strains of N. crassa. The structures of purified components were elucidated by NMR spectroscopy and mass spectrometry. Neutral glycosphingolipids of both wild-type and mutant strains were characterized as beta-glucopyranosylceramides, but those of the mutants were found with structurally altered ceramides. Although the wild type expressed a preponderance of N-2'-hydroxy-(E)-Delta3-octadecenoate as the fatty-N-acyl component attached to the long-chain base (4E,8E)-9-methyl-4,8-sphingadienine, the mutant ceramides were found with mainly N-2'-hydroxyhexadecanoate instead. In addition, the mutant strains expressed highly increased levels of a sterol glucoside identified as ergosterol-beta-glucoside. The potential implications of these findings with respect to defensin resistance in the N. crassa mutants are discussed.

Glycosphingolipid structural analysis and glycosphingolipidomics

Sphingosines, or sphingoids, are a family of naturally occurring long-chain hydrocarbon derivatives sharing a common 1,3-dihydroxy-2-amino-backbone motif. The majority of sphingolipids, as their derivatives are collectively known, can be found in cell membranes in the form of amphiphilic conjugates, each composed of a polar head group attached to an N-acylated sphingoid, or ceramide. Glycosphingolipids (GSLs), which are the glycosides of either ceramide or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, are a structurally and functionally diverse sphingolipid subclass; GSLs are ubiquitously distributed among all eukaryotic species and are found in some bacteria. Since GSLs are secondary metabolites, direct and comprehensive analysis (metabolomics) must be considered an essential complement to genomic and proteomic approaches for establishing the structural repertoire within an organism and deducing its possible functional roles. The glycosphingolipidome clearly comprises an important and extensive subset of both the glycome and the lipidome, but the complexities of GSL structure, biosynthesis, and function form the outlines of a considerable analytical problem, especially since their structural diversity confers by extension an enormous variability with respect to physicochemical properties. This chapter covers selected developments and applications of techniques in mass spectrometric (MS) that have contributed to GSL structural analysis and glycosphingolipidomics since 1990. Sections are included on basic characteristics of ionization and fragmentation of permethylated GSLs and of lithium-adducted nonderivatized GSLs under positive-ion electrospray ionization mass spectrometry (ESI-MS) and collision-induced mass spectrometry (CID-MS) conditions; on the analysis of sulfatides, mainly using negative-ion techniques; and on selected applications of ESI-MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to emerging GSL structural, functional, and analytical issues. The latter section includes a particular focus on evolving techniques for analysis of gangliosides, GSLs containing sialic acid, as well as on characterizations of GSLs from selected nonmammalian eukaryotes, such as dipterans, nematodes, cestodes, and fungi. Additional sections focus on the issue of whether it is better to leave GSLs intact or remove the ceramide; on development and uses of thin-layer chromatography (TLC) blotting and TLC-MS techniques; and on emerging issues of high-throughput analysis, including the use of flow injection, liquid chromatography mass spectrometry (LC-MS), and capillary electrophoresis mass spectrometry (CE-MS).

Egghead and brainiac are essential for glycosphingolipid biosynthesis in vivo

The Drosophila genes, brainiac and egghead, encode glycosyltransferases predicted to act sequentially in early steps of glycosphingolipid biosynthesis, and both genes are required for development in Drosophila. egghead encodes a beta4-mannosyltransferase, and brainiac encodes a beta3-N-acetylglucosaminyltransferase predicted by in vitro analysis to control synthesis of the glycosphingolipid core structure, GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer, found widely in invertebrates but not vertebrates. In this report we present direct in vivo evidence for this hypothesis. egghead and brainiac mutants lack elongated glycosphingolipids and exhibit accumulation of the truncated precursor glycosphingolipids. Furthermore, we demonstrate that despite fundamental differences in the core structure of mammalian and Drosophila glycosphingolipids, the Drosophila egghead mutant can be rescued by introduction of the mammalian lactosylceramide glycosphingolipid biosynthetic pathway (Galbeta1-4Glcbeta1-Cer) using a human beta4-galactosyltransferase (beta4Gal-T6) transgene. Conversely, introduction of egghead in vertebrate cells (Chinese hamster ovary) resulted in near complete blockage of biosynthesis of glycosphingolipids and accumulation of Manbeta1-4Glcbeta1-Cer. The study demonstrates that glycosphingolipids are essential for development of complex organisms and suggests that the function of the Drosophila glycosphingolipids in development does not depend on the core structure.

Lysosomal glycosphingolipid recognition by NKT cells

NKT cells represent a distinct lineage of T cells that coexpress a conserved alphabeta T cell receptor (TCR) and natural killer (NK) receptors. Although the TCR of NKT cells is characteristically autoreactive to CD1d, a lipid-presenting molecule, endogenous ligands for these cells have not been identified. We show that a lysosomal glycosphingolipid of previously unknown function, isoglobotrihexosylceramide (iGb3), is recognized both by mouse and human NKT cells. Impaired generation of lysosomal iGb3 in mice lacking beta-hexosaminidase b results in severe NKT cell deficiency, suggesting that this lipid also mediates development of NKT cells in the mouse. We suggest that expression of iGb3 in peripheral tissues may be involved in controlling NKT cell responses to infections and malignancy and in autoimmunity.

UDP-glucose dehydrogenase plays multiple roles in the biology of the pathogenic fungus Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus surrounded by an elaborate polysaccharide capsule that is strictly required for its virulence in humans and other mammals. Nearly half of the sugar residues in the capsule are derived from UDP-glucuronic acid or its metabolites. To examine the role of these nucleotide sugars in C. neoformans, the gene encoding UDP-glucose dehydrogenase was disrupted. Mass spectrometry analysis of nucleotide sugar pools showed that the resulting mutant lacked both UDP-glucuronic acid and its downstream product, UDP-xylose, thus confirming the effect of the knockout and indicating that an alternate pathway for UDP-glucuronic acid production was not used. The mutant was dramatically affected by the lack of specific sugar donors, demonstrating altered cell integrity, temperature sensitivity, lack of growth in an animal model of cryptococcosis, and morphological defects. Additionally, the polysaccharide capsule could not be detected on the mutant cells, although the possibility remains that abbreviated forms of capsule components are made, possibly without proper surface display. The capsule defect is largely independent of the other observed changes, as cells that are acapsular because of mutations in other genes show lack of virulence but do not exhibit alterations in cell integrity, temperature sensitivity, or cellular morphology. All of the observed alterations were reversed by correction of the gene disruption.

CD1d-restricted T cell activation by nonlipidic small molecules

In addition to NK T cells expressing invariant Valpha14 or Valpha24 T cell receptors (TCRs), the CD1d-restricted T cell repertoire is comprised of T cells with diverse TCRs that mediate inflammation during autoimmune and infectious disease. Here we describe the isolation of human Valpha24(-) T cells that are activated by antigen and CD1d. Mass spectrometric and NMR studies revealed that the stimulatory compounds were neither peptidic nor lipidic but instead were composed of sulfur and aromatic hydrocarbon rings, corresponding to the general structure of phenyl pentamethyldihydrobenzofuran sulfonates. Studies of the molecular mechanism of T cell activation showed that a clonotypic Valpha2/Vbeta21 TCR transmitted activating signals, which were highly specific for hydroxylation and methylation patterns at the terminal structures of stimulatory compounds. These studies provide evidence for noninvariant CD1d-restricted T cells in humans and identify the complete molecular structure of a nonlipidic small molecule that activates T cells through an alphabeta TCR.

T Cell Activation by Lipopeptide Antigens

Unlike major histocompatibility proteins, which bind peptides, CD1 proteins display lipid antigens to T cells. Here, we report that CD1a presents a family of previously unknown lipopeptides from Mycobacterium tuberculosis, named didehydroxymycobactins because of their structural relation to mycobactin siderophores. T cell activation was mediated by the alphabeta T cell receptors and was specific for structure of the acyl and peptidic components of these antigens. These studies identify a means of intracellular pathogen detection and identify lipopeptides as a biochemical class of antigens for T cells, which, like conventional peptides, have a potential for marked structural diversity.

CD75s is a marker of murine CD8+ suppressor T cells

We have previously described a monoclonal antibody, 984, which specifically recognizes murine CD8(+) suppressor T cells (Ts) but not CD8(+) cytolytic T lymphocytes (CTLs). Removal of 984(+) cells abrogates the suppressive effect of CD8(+) Ts generated either in vivo or in vitro while having no effect upon CTL. In this report, the molecules recognized by 984 are identified as 2-6 sialylated neolacto series gangliosides, which are members of the newly defined CD75s cluster. We proceed to demonstrate that like 984, a separate anti-CD75s antibody (CRIS-4), recognizes primary CD8(+) Ts cells. In addition, the 2,6 sialyltransferase responsible for the synthesis of the 984 epitope is identified, allowing the manipulation and study of the regulation of this epitope. This is the first report of CD75s on murine cells and the first report that delineates lymphocyte function based upon CD75s expression. In addition to contributing to the growing body of evidence that lineage dependent gangliosides are expressed by T lymphocytes, these findings suggest that CD8(+) CD75s(+) T lymphocytes represent a functionally distinct subset of CD8(+) T cells with negative regulatory function.

Isolation and characterization of Neurospora crassa mutants resistant to antifungal plant defensins

Twenty-five Neurospora crassa mutants obtained by chemical mutagenesis were screened for increased resistance to various antifungal plant defensins. Plant defensin-resistant N. crassa mutants were further tested for their cross-resistance towards other families of structurally different antimicrobial peptides. Two N. crassa mutants, termed MUT16 and MUT24, displaying resistance towards all plant defensins tested but not to structurally different antimicrobial peptides were selected for further characterization. MUT16 and MUT24 were more resistant towards plant defensin-induced membrane permeabilization as compared to the N. crassa wild-type. Based on the previously demonstrated key role of fungal sphingolipids in the mechanism of growth inhibition by plant defensins, membrane sphingolipids of MUT16 and MUT24 were analysed. Membranes of these mutants contained structurally different glucosylceramides, novel glycosylinositolphosphorylceramides, and an altered level of steryl glucosides. Evidence is provided to link these clear differences in sphingolipid profiles of N. crassa mutants with their resistance towards different plant defensins.