Changes of alpha1-acid glycoprotein microheterogeneity in acute inflammation stages analyzed by isoelectric focusing using serum obtained postoperatively

Exploring Charge-Detection Mass Spectrometry on Chromatographic Time Scales

Charge-detection mass spectrometry (CDMS) enables direct measurement of the charge of an ion alongside its mass-to-charge ratio. CDMS offers unique capabilities for the analysis of samples where isotopic resolution or the separation of charge states cannot be achieved, i.e., heterogeneous macromolecules or highly complex mixtures. CDMS is usually performed using static nano-electrospray ionization-based direct infusion with acquisition times in the range of several tens of minutes to hours. Whether CDMS analysis is also attainable on shorter time scales, e.g., comparable to chromatographic peak widths, has not yet been extensively investigated. In this contribution, we probed the compatibility of CDMS with online liquid chromatography interfacing. Size exclusion chromatography was coupled to CDMS for separation and mass determination of a mixture of transferrin and β-galactosidase. Molecular masses obtained were compared to results from mass spectrometry based on ion ensembles. A relationship between the number of CDMS spectra acquired and the achievable mass accuracy was established. Both proteins were found to be confidently identified using CDMS spectra obtained from a single chromatographic run when peak widths in the range of 1.4-2.5 min, translating to 140-180 spectra per protein were achieved. After demonstration of the proof of concept, the approach was tested for the characterization of the highly complex glycoprotein α-1-acid glycoprotein and the Fc-fusion protein etanercept. With chromatographic peak widths of approximately 3 min, translating to ∼200 spectra, both proteins were successfully identified, demonstrating applicability for samples of high inherent molecular complexity.

Pharmacokinetic and Pharmacodynamic Considerations for Drugs Binding to Alpha-1-Acid Glycoprotein

According to the free drug hypothesis only the unbound drug is available to act at physiological sites of action, and as such the importance of plasma protein binding primarily resides in its impact on pharmacokinetics and pharmacodynamics. Of the major plasma proteins, alpha-1-acid glycoprotein (AAG) represents an intriguing one primarily due to the high affinity, low capacity properties of this protein. In addition, there are marked species and age differences in protein expression, homology and drug binding affinity. As such, a thorough understanding of drug binding to AAG can help aid and improve the translation of pharmacokinetic/pharmacodynamic (PK/PD) relationships from preclinical species to human as well as adults to neonates. This review provides a comprehensive overview of our current understanding of the biochemistry of AAG; endogenous function, impact of disease, utility as a biomarker, and impact on PK/PD. Experimental considerations are discussed as well as recommendations for understanding the potential impact of AAG on PK through drug discovery and early development.

Structural basis for substrate specificity of mammalian neuraminidases

The removal of sialic acid (Sia) residues from glycoconjugates in vertebrates is mediated by a family of neuraminidases (sialidases) consisting of Neu1, Neu2, Neu3 and Neu4 enzymes. The enzymes play distinct physiological roles, but their ability to discriminate between the types of linkages connecting Sia and adjacent residues and between the identity and arrangement of the underlying sugars has never been systematically studied. Here we analyzed the specificity of neuraminidases by studying the kinetics of hydrolysis of BODIPY-labeled substrates containing common mammalian sialylated oligosaccharides: 3′Sia-LacNAc, 3′SiaLac, SiaLe_x, SiaLe_a, SiaLe_c, 6′SiaLac, and 6′SiaLacNAc. We found significant differences in substrate specificity of the enzymes towards the substrates containing α2,6-linked Sia, which were readily cleaved by Neu3 and Neu1 but not by Neu4 and Neu2. The presence of a branching 2-Fuc inhibited Neu2 and Neu4, but had almost no effect on Neu1 or Neu3. The nature of the sugar residue at the reducing end, either glucose (Glc) or N-acetyl-D-glucosamine (GlcNAc) had only a minor effect on all neuraminidases, whereas core structure (1,3 or 1,4 bond between D-galactose (Gal) and GlcNAc) was found to be important for Neu4 strongly preferring β3 (core 1) to β4 (core 2) isomer. Neu3 and Neu4 were in general more active than Neu1 and Neu2, likely due to their preference for hydrophobic substrates. Neu2 and Neu3 were examined by molecular dynamics to identify favorable substrate orientations in the binding sites and interpret the differences in their specificities. Finally, using knockout mouse models, we confirmed that the substrate specificities observed in vitro were recapitulated in enzymes found in mouse brain tissues. Our data for the first time provide evidence for the characteristic substrate preferences of neuraminidases and their ability to discriminate between distinct sialoside targets.

Desialylation of surface receptors as a new dimension in cell signaling

Terminal sialic acid residues are found in abundance in glycan chains of glycoproteins and glycolipids on the surface of all live cells forming an outer layer of the cell originally known as glycocalyx. Their presence affects the molecular properties and structure of glycoconjugates, modifying their function and interactions with other molecules. Consequently, the sialylation state of glycoproteins and glycolipids has been recognized as a critical factor modulating molecular recognitions inside the cell, between the cells, between the cells and the extracellular matrix, and between the cells and certain exogenous pathogens. Until recently sialyltransferases that catalyze transfer of sialic acid residues to the glycan chains in the process of their biosynthesis were thought to be mainly responsible for the creation and maintenance of a temporal and spatial diversity of sialylated moieties. However, the growing evidence suggests that in mammalian cells, at least equally important roles belong to sialidases/neuraminidases, which are located on the cell surface and in intracellular compartments, and may either initiate the catabolism of sialoglycoconjugates or just cleave their sialic acid residues, and thereby contribute to temporal changes in their structure and functions. The current review summarizes emerging data demonstrating that mammalian neuraminidase 1, well known for its lysosomal catabolic function, is also targeted to the cell surface and assumes the previously unrecognized role as a structural and functional modulator of cellular receptors.

Recent advances in sialic acid-focused glycomics

Recent emergences of glycobiology, glycotechnology and glycomics have been clarifying enormous roles of carbohydrates in biological recognition systems. For example, cell surface carbohydrates existing as glycoconjugates (glycolipids, glycoproteins and proteoglycans) play crucial roles in cell-cell communication, cell proliferation and differentiation, tumor metastasis, inflammatory response or viral infection. In particular, sialic acids (SAs) existing as terminal residues in carbohydrate chains on cell surface are involved in signal recognition and adhesion to ligands, antibodies, enzymes and microbes. In addition, plasma free SAs and sialoglycans have shown great potential for disease biomarker discovery. Therefore, the development of efficient analytical methods for structural and functional studies of SAs and sialylglycans are very important and highly demanded. The problems of SAs and sialylglycans analysis are vanishingly small sample amount, complicated and unstable structures, and complex mixtures. Nevertheless, in the past decade, mass spectrometry in combination with chemical derivatization and modern separation methodologies has become a powerful and versatile technique for structural analysis of SAs and sialylglycans. This review summarizes these recent advances in glycomic studies on SAs and sialylglycans. Specially, derivatization and capturing of SAs and sialylglycans combined with mass spectrometry analysis are highlighted.

Where catabolism meets signalling: neuraminidase 1 as a modulator of cell receptors

Terminal sialic acid residues are found in abundance in glycan chains of glycoproteins and glycolipids on the surface of all live cells forming an outer layer of the cell originally known as glycocalyx. Their presence affects the molecular properties and structure of glycoconjugates, modifying their function and interactions with other molecules. Consequently, the sialylation state of glycoproteins and glycolipids has been recognized as a critical factor modulating molecular recognitions inside the cell, between the cells, between the cells and the extracellular matrix, and between the cells and certain exogenous pathogens. Sialyltransferases that attach sialic acid residues to the glycan chains in the process of their initial synthesis were thought to be mainly responsible for the creation and maintenance of a temporal and spatial diversity of sialylated moieties. However, the growing evidence also suggests that in mammalian cells, at least equally important roles belong to sialidases/neuraminidases, which are located on the cell surface and in intracellular compartments, and may either initiate the catabolism of sialoglycoconjugates or just cleave their sialic acid residues, and thereby contribute to temporal changes in their structure and functions. The current review summarizes emerging data demonstrating that neuraminidase 1 (NEU1), well known for its lysosomal catabolic function, can be also targeted to the cell surface and assume the previously unrecognized role as a structural and functional modulator of cellular receptors.

An improved protocol for N-glycosylation analysis of gel-separated sialylated glycoproteins by MALDI-TOF/TOF

Different glycoforms of some proteins have been identified as differential spots for certain diseases in 2-DE, indicating disease-related glycosylation changes. It is routine to determine the site-specific glycosylation of nonsialylated N-glycoproteins from a single gel spot, but some obstacles still exist in analyzing sialylated glycoproteins due to the lability and higher detection limit of acid glycans in MALDI-TOF/TOF analysis. Thus, we present an improved protocol here. Tryptic glycopeptides were separated and subjected to MALDI-TOF/TOF analysis, resulting in the identification of site-specific glycosylation of high-intensity glycopeptides. Sequential deglycosylation and desialylation were used to improve the identification of glycosylation sites and desialylated glycans. The site-specific glycosylation of large glycopeptides and low-intensity glycopeptides was deduced based on the masses of glycopeptides, deglycosylated peptides and desialylated glycans. By applying it to 2-DE separated human serum, the difference of N-glycosylation was successfully determined for α1-antitrypsin between different gel spots.

Enrichment of glycoproteins using nanoscale chelating concanavalin A monolithic capillary chromatography

Immobilized lectin chromatography can be employed for glycoprotein enrichment, but commonly used columns have limitations of yield and resolution. To improve efficiency and to make the technique applicable to minimal sample material, we have developed a nanoscale chelating Concanavalin A (Con A) monolithic capillary prepared using GMA-EDMA (glycidyl methacrylate-co-ethylene dimethacrylate) as polymeric support. Con A was immobilized on Cu(II)-charged iminodiacetic acid (IDA) regenerable sorbents by forming a IDA:Cu(II):Con A sandwich affinity structure that has high column capacity, as well as stability. When compared with conventional Con A lectin chromatography, the monolithic capillary enabled the better reproducible detection of over double the number of unique N-glycoproteins in human urine samples. Utility for analysis of minimal biological samples was confirmed by the successful elucidation of glycoprotein profiles in mouse urine samples at the microliter scale. The improved efficiency of the nanoscale monolithic capillary will impact the analysis of glycoproteins in complex biological samples, especially where only limited material may be available.

Mass spectrometric and linear discriminant analysis of N-glycans of human serum alpha-1-acid glycoprotein in cancer patients and healthy individuals

N-glycan oligosaccharides of human serum alpha(1)-acid glycoprotein (AGP) samples isolated from 43 individuals (healthy individuals and patients with lymphoma and with ovarian tumor) were analyzed by MALDI-TOF mass spectrometry and a multivariate statistical method (linear discriminant analysis, LDA). 34 different glycan structures have been identified. From the glycosylation pattern determined by mass spectrometry fucosylation and branching indices have been calculated. These parameters show only small differences between the patient groups studied, but these differences are not sufficiently large to use as a potential biomarker. LDA analysis, on the other hand shows a very good separation between the three groups (with a classification of 88%). Cross-validation indicates that the method has predictive power: Identifying cancerous vs. healthy individuals shows 96% selectivity and 93% specificity; identification of lymphoma vs. the mixed group of healthy and ovarian tumor cases is also promising (72% selectivity and 84% specificity). The pilot study presented here demonstrates that mass spectrometry combined with linear discriminant analysis (LDA) may provide valuable data for identifying and studying the pathophysiology of malignant diseases.

Exploring the Sialiome Using Titanium Dioxide Chromatography and Mass Spectrometry

Strategies for biomarker discovery increasingly focus on biofluid protein and peptide expression patterns. Post-translational modifications contribute significantly to the pattern complexity and thereby increase the likelihood of obtaining specific biomarkers for diagnostics and disease monitoring. Glycosylation is a common post-translational modification that plays a role e.g. in cell adhesion and in cell-cell and receptor-ligand interactions. Abnormal protein glycosylation has important disease associations, and the glycoproteome is therefore a target for biomarker discovery. Here we present a simple and highly selective strategy for purification of sialic acid-containing glycopeptides (the sialiome) from complex peptide mixtures. The approach utilizes a high and selective affinity of sialic acids for titanium dioxide under specific buffer conditions. In combination with mass spectrometry we used this strategy to characterize the human plasma and saliva sialiomes where 192 and 97 glycosylation sites, respectively, were identified. Furthermore we illustrate the potential of this method in biomarker discovery.

CIEF with hydrodynamic and chemical mobilization for the separation of forms of α-1-acid glycoprotein

Alpha-1-acid glycoprotein (AGP) is a protein that exists in different forms, which is due to variations in the amino acid sequence and/or in the glycosidic part of the protein. These differences confer to these forms, among other characteristics, diverse pIs. Changes in these forms of AGP have been correlated to modifications of the pathophysiological conditions of the individuals. One of the analytical techniques employed for their study has been IEF performed in slab gels. CIEF method with hydrodynamic and chemical mobilization, involving an isotachophoretic process, is developed in this work to separate up to 12 bands of forms of standard AGP, which is proposed as a more reproducible, quantitative, less sample-consuming, and more automated one than conventional IEF. The challenge of this work has been the development of a CIEF method for the separation of bands of a very acidic protein (pI range: 1.8-3.8) in a capillary. Intraday RSD values < or = 1.7% have been achieved for the relative migration time of the AGP bands to that of an internal standard. For intraday area precision, RSD (%) in the range of 2.70-22.71% for AGP zones accounting for more than 10% of total area of AGP sample has been obtained. As a proof of the potential of the methodology proposed, an AGP sample purified from a pool of sera of patients suffering from ovary cancer is analyzed by CIEF.

CZE of human alpha-1-acid glycoprotein for qualitative and quantitative comparison of samples from different pathological conditions

Alpha-1-acid glycoprotein (AGP) presents different forms, which may arise from differences in the amino acid sequence and/or in the glycosidic part of the protein. Changes in forms of AGP have been described in literature as a possible tumor marker. While most previous works have approached the study of glycopeptides and/or glycans obtained after fragmentation of the protein, in this work, a CZE method is developed to separate up to eleven peaks of intact forms of AGP. A computer program developed in our laboratory is used to select the migration parameters that make possible an accurate assignment of AGP peaks. Electropherograms of AGP samples purified from sera of cancer patients and healthy donors are qualitatively and quantitatively compared. Percentages of correct assignment of AGP peaks close to 100% are achieved by using either the migration time of each peak relative to that of the EOF marker or the effective electrophoretic mobility of the peaks. The computer program permits to select, among different hypotheses for peak allotment, that one providing the highest accuracy of assignment. In this way, some peaks with different charge-to-mass ratio and a different distribution of area percentage of AGP forms are observed when comparing samples from sick and healthy individuals. Thus, a method that permits to compare AGP forms existing in sera of individuals with different pathophysiological situations has been developed. A potential for using AGP forms analyzed by CZE as a disease marker and for using this technique for screening purposes is envisaged.

Urinary protein analysis in pre- and postoperative cancer patients

Urinary proteins from six patients with esophageal cancer and two with stomach cancer were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Analyses were performed on days-1 to 3, 5, 7, 10, 14, and 21 (or 22) after surgery. The protein patterns were scanned by densitometry and divided into nine fractions. The main proteins in the fractions (Fr.) were identified as follows: immunoglobulin G in Fr. A, Tamm-Horsfall glycoprotein (THP) in Fr. B, transferrin in Fr. C, albumin in Fr. D, alpha(1)-acid glycoprotein in Fr. E, alpha(1)-microglobulin in Fr. F, retinol binding protein in Fr. G, and beta(2)-microglobulin in Fr. I. The protein in Fr. H was not identified. The percentage of each fraction was calculated from the densitometry pattern of each lane. The percentage values were averaged among all the patients, and pre- and postoperative data were compared. The percentage of Frs. E, F, and G increased on days 1-7, and the changes in these three proteins were similar to changes in serum C-reactive protein (CRP). In particular, the percentage of Fr. G peaked within 1 day of operation, which was faster than for CRP. Conversely, other fractions decreased. These results suggest that urinary protein analysis is useful for monitoring the response to surgical stress.

Glycosylation site analysis of human alpha-1-acid glycoprotein (AGP) by capillary liquid chromatography-electrospray mass spectrometry

A new anionic surfactant (RapiGest SF) was successfully used for site-specific analysis of glycosylation in human alpha-1-acid glycoprotein (AGP). By means of this analytical approach combined with capillary HPLC-mass spectrometry (and tandem mass spectrometry), the N-linked glycosylation pattern of AGP was explored. On the basis of mass matching and MS/MS experiments ca 80 different AGP-derived glycopeptides were identified. Glycosylation shows a markedly different pattern for the various glycosylation sites. At sites I and II, triantennary complex-type oligosaccharides predominate and at sites III, IV and V, tetra-antennary complex-type oligosaccharides predominate. Sites IV and V show the presence of additional N-acetyl lactosamine (Gal-GlcNAc) units (even higher degree of branching and/or longer antennae are also present).

Glycosylation of site-specific glycans of alpha1-acid glycoprotein and alterations in acute and chronic inflammation

BACKGROUND

alpha(1)-Acid glycoprotein (AGP), an acute phase reactant, is extensively glycosylated at five Asn-linked glycosylation sites. In a number of pathophysiological states, including inflammation, rheumatoid arthritis, and cancer, alterations of Asn-linked glycans (N-glycans) have been reported. We investigated alteration of N-glycans at each of glycosylation sites of AGP in the sera of patients with acute and chronic inflammation.

METHODS

AGP purified from sera was digested with Glu-C and the liberated glycopeptides were isolated by reverse phase HPLC. N-glycans released with peptide N-glycosidase F and followed by neuraminidase treatment were analyzed by matrix-assisted laser desorption ionization-time of flight mass spectrometry.

RESULTS

Site-specific differences in branching structures were observed among N-glycosylation sites 1, 3, 4 and 5. Within the sera of patients with acute inflammation, increases in bi-antennary and decreases in tri- and tetra-antennary structures were observed, as well as increases in alpha1,3-fucosylation, at most glycosylation sites. In the sera of patients with chronic inflammation, increased rates of tri-antennary alpha1,3-fucosylation at sites 3 and 4 and tetra-antennary alpha1,3-fucosylation at sites 3, 4 and 5 were detected. Although there were no significant differences between acute and chronic sera in site directed branching structures, significant differences of alpha1,3-fucosylation were detected in tri-antennary at sites 2, 4 and 5 and in tetra-antennary at sites 3 and 4.

CONCLUSION

Little variation in the N-glycan composition of the glycosylation sites of AGP was observed among healthy individuals, while the sera of patients with acute inflammation demonstrated increased numbers of bi-antennary and alpha1,3-fucosylated N-glycan structures at each glycosylation site.

Hyperglycemia induced by glucose infusion causes hepatic oxidative stress and systemic inflammation, but not STAT3 or MAP kinase activation in liver in rats

The purpose of this study was to determine the effects of acute hyperglycemia induced by glucose infusion on oxidative stress, systemic inflammation, and several key signal intermediates in liver for the systemic inflammatory response in nonstressed rats. Rats received saline or glucose infusion (hyperglycemic clamp) for 3 hours. Rats without catheter insertion were included as an additional control for observing the effects of surgical stress. Levels of malondialdehyde (MDA) and total glutathione to assess oxidative stress were determined in liver and muscle. Proinflammatory cytokines including tumor necrosis factor (TNF), interleukin (IL)-1 and IL-6, and alpha 1 acid glycoprotein (alpha1-AG) were determined in serum. The protein content and phosphorylation of extracellular signal-regulated kinase (ERK)1/2, p38 stress-activated protein kinase (p38), and signal transducer and activator of transcription-3 (STAT-3) were examined in the liver tissue with or without IL-6 stimulation. The results showed that acute hyperglycemia significantly increased MDA release and depleted total glutathione in liver but not in muscle. Hyperglycemia also significantly elevated the production of TNF, IL-1, and alpha1-AG, but not IL-6 in serum. However, hyperglycemia for 3 hours in vivo did not activate ERK1/2, p38 and STAT3 in liver, and also did not alter the response of these signal proteins to IL-6 stimulation. These data suggest that acute (3 hours) hyperglycemia causes hepatic oxidative stress and activates a low-grade systemic inflammation but does not affect key components of the IL-6 signaling pathway in liver.

A preliminary evaluation of the functional significance of alpha-1-acid glycoprotein glycosylation on wound healing

The laying down of collagen and fibrous tissue is a key process in wound healing, however excessive collagen (and glycoprotein) deposition causes hypertrophic and keloid scars, eg after burns. Collagen synthesis is increased in these scars compared with normal healing, as is collagenase activity, which controls the degradation pathway of collagen. The processes of wound healing are inextricably linked to those of the acute-phase response (APR): alpha-1-acid glycoprotein (AGP), a plasma glycoprotein that undergoes both an increase in concentration and an alteration in its glycosylation pattern during the APR. This study determined that AGP isolated from the plasma of burns patients was of an increased concentration and altered glycosylation pattern compared with normal plasma and was capable of directly interacting with type I collagen. It also had a profound effect on both collagen fibril formation and collagenase activity, to a degree dependent upon the percentage body surface area burned. Additionally, the results obtained provided the basis for predicting the formation of hypertrophic scars.

Plasma from cancer patients featuring a characteristic protein composition mediates protection against apoptosis

By comparative proteome analysis we searched for characteristic alterations of human plasma accompanying neoplastic disease. We identified protein alterations in plasma of prostate-, lung-, and breast-cancer patients in comparison to controls, comprising elevated levels of fibrinogen gamma-chain dimer, degradation products of antiplasmin and laminin gamma-chain, and elevated levels of acute phase proteins. The latter proteins and laminin fragments have been described as anti-apoptotic factors. We raised the question whether these alterations may have any relevance for the regulation of apoptosis. In contrast to plasma derived from healthy donors, samples from prostate-, lung-, and breast-cancer patients selectively inhibited Fas- and staurosporine-induced apoptosis in Jurkat cells but remained ineffective upon UV light-induced apoptosis. These data suggested that inhibition occurred by extracellular interference with apoptosis induction. Supporting this hypothesis, we found that formation of the CD95 death-inducing signal complex was strongly inhibited in the presence of plasma from cancer patients.

Exploiting lectin affinity chromatography in clinical diagnosis

Lectin affinity chromatography (LAC) offers a tool that aids purification of cell surface glycoconjugates in sufficient quantities so that studies addressing their structural elucidation could be carried out. It has several advantages over the conventional biochemical methods, such as immunoprecipitation and/or immunoaffinity chromatography, used for the purification of various glycoconjugates. Serial LAC (SLAC) not only helps establish the identity of a glycoprotein or allows purification of a glycoprotein to homogeneity from among a mixture of glycoproteins, but it also successfully resolves the microheterogeneity in these glycoproteins, which is an otherwise impracticable problem to address. Specific cases of the altered expression and maintenance of microheterogeneity of some of the glycoproteins in pathological conditions vis a vis during normal biology are presented. The application of LAC in (i) itself, (ii) a serial fashion, and (iii) conjunction with other techniques such as two-dimensional electrophoresis, capillary electrophoresis, mass spectrometry, etc. in the diagnosis of certain pathological conditions, and the possibility of using this knowledge in designing treatments for various diseases, is discussed.