What Can N-glycomics and N-glycoproteomics of Cerebrospinal Fluid Tell Us about Alzheimer Disease?

Proteomics-large-scale studies of proteins-has over the last decade gained an enormous interest for studies aimed at revealing proteins and pathways involved in disease. To fully understand biological and pathological processes it is crucial to also include post-translational modifications in the "omics". To this end, glycomics (identification and quantification of glycans enzymatically or chemically released from proteins) and glycoproteomics (identification and quantification of peptides/proteins with the glycans still attached) is gaining interest. The study of protein glycosylation requires a workflow that involves an array of sample preparation and analysis steps that needs to be carefully considered. Herein, we briefly touch upon important steps such as sample preparation and preconcentration, glycan release, glycan derivatization and quantification and advances in mass spectrometry that today are the work-horse for glycomics and glycoproteomics studies. Several proteins related to Alzheimer disease pathogenesis have altered protein glycosylation, and recent glycomics studies have shown differences in cerebrospinal fluid as well as in brain tissue in Alzheimer disease as compared to controls. In this review, we discuss these techniques and how they have been used to shed light on Alzheimer disease and to find glycan biomarkers in cerebrospinal fluid.

The N-glycan profile in cortex and hippocampus is altered in Alzheimer disease.

Protein glycosylation is crucial for the central nervous system and brain functions, including processes that are defective in Alzheimer disease (AD) such as neurogenesis, synaptic function, and memory formation. Still, the roles of glycans in the development of AD are relatively unexplored. Glycomics studies of cerebrospinal fluid (CSF) have previously shown altered glycosylation pattern in patients with different stages of cognitive impairment, including AD, compared to healthy controls. As a consequence, we hypothesized that the glycan profile is altered in the brain of patients with AD and analyzed the asparagine-linked (N-linked) glycan profile in hippocampus and cortex in AD and control brain. Glycans were enzymatically liberated from brain glycoproteins and analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Eleven glycans showed significantly different levels in hippocampus compared to cortex in both control and AD brain. Two glycans in cortex and four in hippocampus showed different levels in AD compared to control brain. All glycans that differed between controls and AD brain had similar structures with one sialic acid, at least one fucose and a confirmed or potential bisecting N-acetylglucosamine (GlcNAc). The glycans that were altered in AD brain differed from those that were altered in AD CSF. One glycan found to be present in significantly lower levels in both hippocampus and cortex in AD compared to control contained a structurally and functionally interesting epitope that we assign as a terminal galactose decorated with fucose and sialic acid. Altogether, these studies suggest that protein glycosylation is an important component in the development of AD and warrants further studies.

Glycoproteomic Analysis of Human Urinary Exosomes.

Exosomes represent a class of secreted biological vesicles, which have recently gained attention due to their function as intertissue and interorganism transporters of genetic materials, small molecules, lipids, and proteins. Although the protein constituents of these exosomes are often glycosylated, a large-scale characterization of the glycoproteome has not yet been completed. This study identified 3144 unique glycosylation events belonging to 378 glycoproteins and 604 unique protein sites of glycosylation. With these data, we investigated the level of glycan microheterogeneity within the urinary exosomes, finding on average 5.9 glycans per site. The glycan family abundance on individual proteins showed subtle differences, providing an additional level of molecular characterization compared to the unmodified proteome. Finally, we show protein site-specific changes in regard to the common urinary glycoprotein, uromodulin. While uromodulin is an individual case, these same site-specific analyses provide a way forward for developing diagnostic glycoprotein biomarkers with urine as a noninvasive biological fluid. This study represents an important first step in understanding the functional urinary glycoproteome.

Wood hemicelluloses exert distinct biomechanical contributions to cellulose fibrillar networks.

Hemicelluloses, a family of heterogeneous polysaccharides with complex molecular structures, constitute a fundamental component of lignocellulosic biomass. However, the contribution of each hemicellulose type to the mechanical properties of secondary plant cell walls remains elusive. Here we homogeneously incorporate different combinations of extracted and purified hemicelluloses (xylans and glucomannans) from softwood and hardwood species into self-assembled networks during cellulose biosynthesis in a bacterial model, without altering the morphology and the crystallinity of the cellulose bundles. These composite hydrogels can be therefore envisioned as models of secondary plant cell walls prior to lignification. The incorporated hemicelluloses exhibit both a rigid phase having close interactions with cellulose, together with a flexible phase contributing to the multiscale architecture of the bacterial cellulose hydrogels. The wood hemicelluloses exhibit distinct biomechanical contributions, with glucomannans increasing the elastic modulus in compression, and xylans contributing to a dramatic increase of the elongation at break under tension. These diverging effects cannot be explained solely from the nature of their direct interactions with cellulose, but can be related to the distinct molecular structure of wood xylans and mannans, the multiphase architecture of the hydrogels and the aggregative effects amongst hemicellulose-coated fibrils. Our study contributes to understanding the specific roles of wood xylans and glucomannans in the biomechanical integrity of secondary cell walls in tension and compression and has significance for the development of lignocellulosic materials with controlled assembly and tailored mechanical properties.

Glycan biomarkers for Alzheimer disease correlate with T-tau and P-tau in cerebrospinal fluid in subjective cognitive impairment.

Alzheimer disease (AD) is a devastating disease and a global health problem, and current treatments are only symptomatic. A wealth of clinical studies support that the disease starts to develop decades before the first symptoms appear, emphasizing the importance of studying early changes for improving early diagnosis and guiding toward novel treatment strategies. Protein glycosylation is altered in AD but it remains to be clarified why these alterations occur and how they affect the disease development. Here, we used a glycomics approach to search for alterations in protein glycosylation in cerebrospinal fluid (CSF) in AD compared with nondemented controls. Using both matrix-assisted laser desorption ionization-time of flight and liquid chromatography-electrospray mass spectrometry, we observed an increase in N-glycans carrying bisecting N-acetylglucosamine in AD. Based on those findings, we designed an enzyme-linked multiwell plate assay to quantify N-glycans binding to the lectin Phaseolus vulgaris Erythroagglutinin (PHA-E), which is specific for N-glycans containing bisecting N-acetylglucosamine. Using this assay, we found a similar increase in CSF in AD compared with controls. Further analysis of CSF from 242 patients with subjective cognitive impairment (SCI), mild cognitive impairment (MCI), or AD dementia revealed significantly increased binding to PHA-E in MCI and AD compared to SCI. Interestingly, PHA-E binding correlated with CSF levels of phosphorylated tau and total tau and this correlation was most prominent in the SCI group (R = 0.53-0.54). This study supports a link between N-glycosylation, neurodegeneration, and tau pathology in AD and suggests that glycan biomarkers have potential to identify SCI cases at risk of developing AD.

In-Depth Compositional and Structural Characterization of N-Glycans Derived from Human Urinary Exosomes.

The study of exosomes has become increasingly popular due to their potentially important biological roles. Urine can be used as an effective source of exosomes for noninvasive investigations into the pathophysiological states of the urinary system, but first, detailed characterization of exosomal components in healthy individuals is essential. Here, we significantly extend the number of N-glycan compositions, including sulfated species, identified from urinary exosomes and determine the sialic acid linkages for many of those compositions. Capillary electrophoresis-mass spectrometry (CE-MS), matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS) were used to identify N-glycan and sulfated N-glycan compositions. Second, because the alteration of sialylation patterns has been previously implicated in various disease states, ion-exchange chromatography, microfluidic capillary electrophoresis (CE), and MALDI-MS were adopted to resolve positional isomers of sialic acids. Structures of the sialyl-linkage isomers were assigned indirectly through α2-3 sialidase treatment and sialic acid linkage-specific alkylamidation (SALSA). In total, we have identified 219 N-glycan structures that include 175 compositions, 64 sialic acid linkage isomers, 26 structural isomers, and 27 sulfated glycans.

Highly Sensitive O-Glycan Profiling for Human Serum Proteins Reveals Gender-Dependent Changes in Colorectal Cancer Patients.

A newly developed microscale protocol for profiling serum O-glycans has been validated here with multiple serum samples obtained from different cohorts of colorectal cancer patients. The simultaneous cleavage and permethylation steps in this procedure preserve the integrity of released minor O-glycans, so that 39 O-linked oligosaccharides could be reliably recorded in a profile. This is far more detected components than shown in any previous studies. The analytical results were further subjected to a battery of statistical tests. Our O-glycan compositions compare favorably with the previous results obtained with solid tumors and cancer cell lines, suggesting that smaller circulatory mucins protruding into the blood circulation may be one source of O-glycans that we observe in the serum samples. While the control vs cancer statistical comparisons generally agree with the expected glycosylation trends, the comparisons of male vs female subjects have led to some surprising results for which we do not have a ready explanation due to lack of any literature describing hormonal control of O-glycosylation. Our results thus underscore the necessity of applying new analytical technologies to clinically interesting sample sets.

Analytical Scheme Leading to Integrated High-Sensitivity Profiling of Glycosphingolipids Together with N- and O-Glycans from One Sample.

Glycoconjugates are directly or indirectly involved in many biological processes. Due to their complex structures, the structural elucidation of glycans and the exploration of their role in biological systems have been challenging. Glycan pools generated through release from glycoprotein or glycolipid mixtures can often be very complex. For the sake of procedural simplicity, many glycan profiling studies choose to concentrate on a single class of glycoconjugates. In this paper, we demonstrate it feasible to cover glycosphingolipids, N-glycans, and O-glycans isolated from the same sample. Small volumes of human blood serum and ascites fluid as well as small mouse brain tissue samples are sufficient to profile sequentially glycans from all three classes of glycoconjugates and even positively identify some mixture components through MALDI-MS and LC-ESI-MS. The results show that comprehensive glycan profiles can be obtained from the equivalent of 500-µg protein starting material or possibly less. These methodological improvements can help accelerating future glycomic comprehensive studies, especially for precious clinical samples. Graphical Abstract Outline of glycan profiling procedures.

Systemic AA amyloidosis in the red fox (Vulpes vulpes).

Amyloid A (AA) amyloidosis occurs spontaneously in many mammals and birds, but the prevalence varies considerably among different species, and even among subgroups of the same species. The Blue fox and the Gray fox seem to be resistant to the development of AA amyloidosis, while Island foxes have a high prevalence of the disease. Herein, we report on the identification of AA amyloidosis in the Red fox (Vulpes vulpes). Edman degradation and tandem MS analysis of proteolyzed amyloid protein revealed that the amyloid partly was composed of full-length SAA. Its amino acid sequence was determined and found to consist of 111 amino acid residues. Based on inter-species sequence comparisons we found four residue exchanges (Ser31, Lys63, Leu71, Lys72) between the Red and Blue fox SAAs. Lys63 seems unique to the Red fox SAA. We found no obvious explanation to how these exchanges might correlate with the reported differences in SAA amyloidogenicity. Furthermore, in contrast to fibrils from many other mammalian species, the isolated amyloid fibrils from Red fox did not seed AA amyloidosis in a mouse model.

Comprehensive Analytical Approach toward Glycomic Characterization and Profiling in Urinary Exosomes.

Exosomes are extracellular nanosized vesicles with lipid bilayers encapsulating nucleic acids and proteins, both with and without glycosylation. While exosomal nucleic acids and proteins have previously been explored to identify cancer biomarkers with some promising results, little information has been available concerning their glycoconjugate content. Exosomes were isolated from normal urine samples through multistep differential centrifugation. The isolated exosomes have an average size of 146 nm and a spherical shape, as determined by dynamic light scattering and transmission electron microscopy, respectively. N-Glycans were enzymatically released from the isolated vesicles. After being reduced and permethylated, N-glycans were measured by MALDI mass spectrometry. Paucimannosidic, high-mannose, and complex type glycans were identified and their relative abundances were determined. Some detailed structures of these glycans were revealed through liquid chromatography/tandem mass spectrometry (LC/MS-MS). The reduced N-glycans, without being permethylated, were also separated and analyzed by LC/MS-MS, and their structures were further detailed through isomeric separation on porous graphitized carbon (PGC) packed in long capillaries. Using microfractionation before LC/MS-MS, minor multiantennary N-glycans were preconcentrated as based on hydrophobicity or charge. Preconcentration of the reduced and permethylated glycans on a C18 cartridge revealed numerous large glycans, whereas fractionation of the reduced N-glycans by ion-exchange cartridges facilitated detection of sulfated glycans. After removing N-glycans from the original sample aliquot, O-glycans were chemically released from urinary exosomes and profiled, revealing some unusual structures.

Shiga-like toxin binds with high avidity to multivalent O-linked blood group P1 determinants on mucin-type fusion proteins.

The binding of Shiga-like toxin 1 (Stx1) and Shiga-like toxin 2 (Stx2) to a mucin-like fusion protein, P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b), carrying multiple copies of the blood group P1 determinant on O-glycans was investigated with western blot and the biosensor Biacore. Chinese hamster ovary K-1 (CHO-K1) cells were stably transfected with linearized plasmids encoding the PSGL-1/mIgG2b fusion protein, the pigeon α1,4-galactosyltransferase (α4Gal-T) and the core 2 ß1,6-N-acetylglucosaminyltransferase (C2GnT-I). Western blot analyses of purified PSGL-1/mIgG2b and liquid chromatography-mass spectrometry (LC-MS) of released O-glycans confirmed the presence of the P1 determinant. Western blot analysis indicated strong binding of Stx1, but not Stx2, to PSGL-1/mIgG2b. In a Biacore assay, Stx1 and Stx2 were immobilized on a dextran chip and the binding of purified PSGL-1/mIgG2b and a P(k)-albumin neoglycoprotein was analyzed. Stx1 and Stx2 bound with high avidity to both PSGL-1/mIgG2b and P(k)-albumin, while the Stx1 binding was the strongest. In summary, we have shown that the pigeon α4Gal-T can be aberrantly expressed in CHO cells together with the core 2 enzyme to generate multiple, O-linked P1 determinants on a simultaneously expressed mucin-type fusion protein. P1-decorated PSGL-1/mIgG2b bound with high avidity to both Stx1 and Stx2, and as such constitutes a potential therapeutic inhibitor of these toxins.

Avian influenza H5 hemagglutinin binds with high avidity to sialic acid on different O-linked core structures on mucin-type fusion proteins.

The interaction between P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG(2b)) fusion protein carrying multiple copies of the influenza hemagglutinin receptor Siaα2-3Gal on different O-glycan chains and recombinant human influenza H5N1 A/Vietnam/1203/04 hemagglutinin was investigated with a Biacore biosensor. The fusion protein was produced by stable cell lines in large scale cultures and purified with affinity- and gel filtration chromatography. TheC-P55 and 293-P cell lines were established by transfecting the Chinese hamster ovary (CHO)-K1 and Human embryonic kidney (HEK)-293 cell lines with plasmids encoding the PSGL-1/mIgG(2b) fusion protein, while the C-PSLex cell line was engineered by transfecting CHO-K1 cells with the plasmids encoding the core 2 ß1,6GnT-I and FUT-VII glycosyltransferases. Glycosylation was characterized by lectin Western blotting of the proteins and liquid chromatography - mass spectrometry of released non-derivatized O-glycans. Biacore experiments revealed that PSGL-1/mIgG(2b) is a good binding partner of H5. The binding curves displayed a slow dissociation indicating a multivalent binding. The H5 hemagglutinin binds with similar strength to PSGL-1/mIgG(2b) carrying mostly sialylated core 1 (clone C-P55), a mix of sialylated core 1 and sialylated lactosamine (clone 293-P) or mainly sialylated lactosamine (clone C-PSLex) O-glycans, indicating that this hemagglutinin is unable to discriminate between these structures.The potential use of the large, flexible PSGL-1/mIgG(2b) mucin-type fusion protein carrying Siaα2-3Gal as a multivalent inhibitor of influenza virus is discussed.

Mass spectrometric analysis of O-linked oligosaccharides from various recombinant expression systems.

Analysis of O-linked glycosylation is one of the main challenges during structural validation of recombinant glycoproteins. With methods available for N-linked glycosylation in regard to oligosaccharide analysis as well as glycopeptide mapping, there are still challenges for O-linked glycan analysis. Here, we present mass spectrometric methodology for O-linked oligosaccharides released by reductive ß-elimination. Using LC-MS and LC-MS(2) with graphitized carbon columns, oligosaccharides are analyzed without derivatization. This approach provides a high-throughput method for screening during clonal selection, as well as product structure verification, without impairing sequencing ability. The protocols are exemplified by analysis of glycoproteins from mammalian cell cultures (CHO cells) as well as insect cells and yeast. The data shows that the method can be successfully applied to both neutral and acidic O-linked oligosaccharides, where sialic acid, hexuronic acid, and sulfate are common substituents. Further characterization of O-glycans can be achieved using permethylation. Permethylation of O-linked oligosaccharides followed by direct infusion into the mass spectrometer provide information about oligosaccharide composition, and subsequent MS (n) experiments can be carried out to elucidate oligosaccharide structure including linkage information and sequence.

Mucin-type proteins produced in the Trichoplusia ni and Spodoptera frugiperda insect cell lines carry novel O-glycans with phosphocholine and sulfate substitutions.

The O-glycans of a recombinant mucin-type protein expressed in insect cell lines derived from Trichoplusia ni (Hi-5) and Spodoptera frugiperda (Sf9) were characterized. The P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b) fusion protein carrying 106 potential O-glycosylation sites and 6 potential N-glycosylation sites was expressed and purified from the Hi-5 and Sf9 cell culture medium using affinity chromatography and gel filtration. Liquid chromatography mass spectrometry (LC-MS) of O-glycans released from PSGL-1/mIgG2b revealed a large repertoire of structurally diverse glycans, which is in contrast to previous reports of only simple glycans. O-Glycans containing hexuronic acid (HexA, here glucuronic acid and galacturonic acid) were found to be prevalent. Also sulfate (Hi-5 and Sf9) and phosphocholine (PC; Sf9) O-glycan substitutions were detected. Western blotting confirmed the presence of O-linked PC on PSGL-1/mIG2b produced in Sf9 cells. To our knowledge, this is the first structural characterization of PC-substituted O-glycans in any species. The MS analyses revealed that Sf9 oligosaccharides consisted of short oligosaccharides (<6 residues) low in hexose (Hex) and with terminating N-acetylhexosamine (HexNAc) units, whereas Hi-5 produced a family of large O-glycans with (HexNAc-HexA-Hex) repeats and sulfate substitution on terminal residues. In both cell lines, the core N-acetylgalactosamine was preferentially non-branched, but small amounts of O-glycan cores with single fucose or hexose branches were found.

Mucin-type fusion proteins with blood group A or B determinants on defined O-glycan core chains produced in glycoengineered Chinese hamster ovary cells and their use as immunoaffinity matrices.

Assays for quantification, and methods for removal, of anti-A and anti-B antibodies are the key for the success of ABO incompatible organ transplantation programs. In order to produce tools that can be used as substrates in tests for anti-A/anti-B quantification and specificity determination or as affinity matrices in extracorporeal immunoadsorption (IA) columns, we engineered Chinese hamster ovary (CHO) cells secreting mucin-type fusion proteins carrying blood group A or B determinants on defined O-glycan core saccharide chains. Besides the P-selectin glycoprotein ligand-1/mouse immunoglobulin G(2b) (PSGL-1/mIgG(2b)) cDNA, CHO cells were transfected with plasmids encoding core 2 (ß1,6GlcNAc-T1) or core 3 (ß1,3GlcNAc-T6 and ß1,3Gal-T5) enzymes together with α1,2Fuc-T1 or α1,2Fuc-T2 and the A or B gene-encoded α1,3GalNAcT or α1,3Gal-T, respectively. Selected clones with the correct glycophenotype were expanded and cultured in shaker flasks and Wave bioreactors. Western blotting was used to characterize purified fusion protein and liquid chromatography-mass spectrometry was used to characterize the released O-glycans. Clones producing PSGL-1/mIgG(2b) carrying O-glycans with A and B determinants on type 1 (Galß3GlcNAc), type 2 (Galß4GlcNAc) and type 3 (Galß3GalNAcα) outer core saccharide chains were established. The conversion of CHO cells from exclusive inner core 1 (Galß3GalNAc) to core 3 (GlcNAcß3GalNAc) O-glycan producers was almost complete, whereas conversion to inner core 2 (GlcNAcß6GalNAc) O-glycans was incomplete as was the α2-fucosylation of the core 1 chain. Sialylation may prevent these biosynthetic steps. The clinical utility of the blood group A and B substituted mucin-type fusion proteins as substrates in enzyme-linked immunosorbent assay or as affinity matrices in IA columns is explored.