The changes in glycosylation after partial hepatectomy enhance collagen binding of vitronectin in plasma

Development of a novel lectin-based gold nanoparticle point-of-care immunoassay for rapid diagnosis of patients with severe Dengue infection

Rapid diagnosis of patients with severe Dengue infection can be useful for the efficient clinical management of cases caused by the Dengue virus. Lateral Flow Immunoassay (LFIA) have been broadly used for rapid Dengue diagnosis, because of their quick readouts with the human eye, simplicity of use, and affordability. Despite the availability of several commercial Dengue point-of-care assays, none has shown to be successful in discriminating between severe and nonsevere forms of Dengue infection. In the current study, for the first time, a novel lectin-based point-of-care assay for the early detection of patients with severe Dengue infection with gold-adorned sheets as detection labels is being reported. In this assay, Dengue severity was diagnosed by detecting the glycosylation profile of vitronectin, a known Dengue severity marker. Two lectins were employed namely DSA (Datura stramonium) and MAA (Maackia amurensis) that can recognize specific glycans like galactose Gal-(1-4) GlcNAc and sialic acid in an (α2-3) linkage, which displayed high sensitivity and high specificity, i.e. 90% and 85% for DSA and 90.91% and 95% for MAA. The new assay has a detection limit of 5 µg µl-1 and enables the quick (30 min) and sensitive detection of severe Dengue cases. The reported point-of-care immunoassay exhibits considerable promise for early identification of patients with Dengue severity.

Regulatory properties of vitronectin and its glycosylation in collagen fibril formation and collagen-degrading enzyme cathepsin K activity

Vitronectin (VN) is a glycoprotein found in extracellular matrix and blood. Collagen, a major extracellular matrix component in mammals, is degraded by cathepsin K (CatK), which is essential for bone resorption under acidic conditions. The relationship between VN and cathepsins has been unclear. We discovered that VN promoted collagen fibril formation and inhibited CatK activity, and observed its activation in vitro. VN accelerated collagen fibril formation at neutral pH. Collagen fibers formed with VN were in close contact with each other and appeared as scattered flat masses in scanning electron microscopy images. VN formed collagen fibers with high acid solubility and significantly inhibited CatK; the IC₅₀ was 8.1–16.6 nM and competitive, almost the same as those of human and porcine VNs. VN inhibited the autoprocessing of inactive pro-CatK from active CatK. DeN-glycosylation of VN attenuated the inhibitory effects of CatK and its autoprocessing by VN, but had little effect on acid solubilization of collagen and VN degradation via CatK. CatK inhibition is an attractive treatment approach for osteoporosis and osteoarthritis. These findings suggest that glycosylated VN is a potential biological candidate for CatK inhibition and may help to understand the molecular mechanisms of tissue re-modeling.

Deciphering the Roles of N-Glycans on Collagen-Platelet Interactions

Collagen is a potent agonist for platelet activation, presenting itself as a key contributor to coagulation via interactions with platelet glycoproteins. The fine details dictating platelet-collagen interactions are poorly understood. In particular, glycosylation could be a key determinant in the platelet-collagen interaction. Here, we report an affinity purification coupled to a mass spectrometry-based approach to elucidate the function of N-glycans in dictating platelet-collagen interactions. By integrative proteomic and glycoproteomic analysis of collagen-platelet interactive proteins with N-glycan manipulation, we demonstrate that the interaction of platelet adhesive receptors with collagen is highly N-glycan regulated, with glycans on many receptors playing positive roles in collagen binding, with glycans on other platelet glycoproteins exhibiting inhibitory roles on the binding to collagen. Our results significantly enhance our understanding of the details of glycans influencing the platelet-collagen interaction.

Vitronectins produced by human cirrhotic liver and CCl4-treated rats differ in their glycosylation pattern and tissue remodeling activity

Liver cirrhosis (LC) is a disease characterized by pathological accumulation and alteration of extracellular matrix (ECM) proteins; the interaction between two such proteins, collagen and vitronectin (VN), is considered to be the key to controlling ECM remodeling in liver cirrhosis. If it is possible to control the modification of oligosaccharides on VN, it may be possible to retard progression of liver cirrhosis. In this study, we examined the relationship between changes in VN glycosylation and activity related to the remodeling of hepatic tissue in human LC and a rat model of LC generated using carbon tetrachloride (CCl₄). Plasma concentrations of VN in human LC declined to approximately two‐thirds that in normal plasma, but the ratio of active VN, which has collagen‐binding activities, increased 2.8 times in LC plasma. In contrast, purified LC‐VN exhibited similar binding activities toward type I, IV, and V collagens to those of normal VN. Lectin reactivities and carbohydrate analyses of LC‐VN revealed that branching, fucosylation, and sialylation of N‐glycans were higher than those of normal VN. On the other hand, the plasma level of rat CCl₄‐VN increased and the ratio of active molecules to collagen in plasma decreased. Increased fucosylation of LC‐VN was not detected in carbohydrates of CCl₄‐VN. The changes in rat VN due to CCl₄ treatment did not correspond to the changes in plasma levels of human VN caused by LC, the ratio of active molecules, or carbohydrate composition, thereby indicating that CCl₄‐treated rats are not an appropriate model for studying VNs in human LC. Glycosidase treatment of VNs supported the hypothesis that the collagen‐binding activity of VN is modulated by alterations of glycosylation during LC, which may contribute to (a) the matrix incorporation of VN and (b) tissue fibrosis.

Vitronectin deficiency attenuates hepatic fibrosis in a non-alcoholic steatohepatitis-induced mouse model

Vitronectin (VN), an extracellular matrix protein, is a promising immune biomarker of non-alcoholic steatohepatitis (NASH); however, its precise function remains unclear. This study investigated how VN deficiency contributes to the development of NASH. Towards this aim, wild-type (WT) and VN-/- mice were fed with a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD) for 6 and 10 weeks to induce NASH, and the livers were isolated. In WT mice fed with CDAHFD for 6 and 10 weeks, the expression of Vn mRNA and protein was up-regulated compared with that in mice fed with the MF control diet, indicating that VN is regulated in NASH condition. VN-/- mice showed decreased picrosirius red staining in the liver area and Col1a2 mRNA expression levels, compared with WT mice, indicating that the severity of hepatic fibrosis is attenuated in the CDAHFD-fed VN-/- mice. In addition, VN deficiency did not affect the area of lipid droplets in haematoxylin-eosin staining and the mRNA expression levels of fatty acid synthases, Srebp, Acc and Fas in the CDAHFD-fed mice. Moreover, VN deficiency decreased the inflammation score and the mRNA expression levels of Cd11b and F4/80, macrophage markers, as well as Tnf-α and Il-1β, inflammatory cytokines in the CDAHFD-fed mice. Furthermore, VN deficiency decreased the protein and mRNA expression levels of α-smooth muscle actin in the CDAHFD-fed mice, suggesting that VN deficiency inhibits the activation of hepatic stellate cells (HSCs). Our findings indicate that VN contributes to the development of fibrosis in the NASH model mice via modulation of the inflammatory reaction and activation of HSCs.

Vitronectin (Vn) glycosylation patterned by lectin affinity assays-A potent glycoproteomic tool to discriminate plasma Vn from cancer ascites Vn

Changes in glycosylation have been associated with human cancer, but their complexity poses an analytical challenge. Ovarian cancer is a major cause of death in women because of an often late diagnosis. At least one-third of patients presents ascites fluid at diagnosis, and almost all have ascites at recurrence. Vitronectin (Vn) is a multifunctional glycoprotein that is suggested to be implicated in ovarian cancer metastasis and is found within ascites. The present study evaluated the potential of using lectin affinity for characterizing the glycosylation pattern of Vn. Human Vn was purified from 1 sample of ovarian cancer ascites or a pool of plasma samples. Consistent findings were observed with both dot blot and lectin array assays. Based on a panel of 40 lectins, the lectin array revealed discriminant patterns of lectin binding to Vn glycans. Interestingly, almost all the highlighted interactions were found to be higher with Vn from ascites relative to the plasma counterpart. Also, the lectin array was able to discriminate profiles of lectin interactions (ConA, SNA-I, PHA-E, PHA-L) between Vn samples that were not evident using dot blot, indicating its high sensitivity. The model of ConA binding during thermal unfolding of Vn confirmed the higher accessibility of mannosylated glycans in Vn from ascites as monitored by turbidimetry. Thus, this study demonstrated the usefulness of lectins and the lectin array as a glycoproteomic tool for high throughput and sensitive analysis of glycosylation patterns. Our data provide novel insights concerning Vn glycosylation patterns in clinical specimens, paving the way for further investigations regarding their functional impact and clinical interest.

gFinder: A Web-Based Bioinformatics Tool for the Analysis of N-Glycopeptides

Glycoproteins influence numerous indispensable biological functions, and changes in protein glycosylation have been observed in various diseases. The identification and characterization of glycoprotein and glycosylation sites by mass spectrometry (MS) remain challenging tasks, and great efforts have been devoted to the development of proteome informatics tools that facilitate the MS analysis of glycans and glycopeptides. Here we report on the development of gFinder, a web-based bioinformatics tool that analyzes mixtures of native N-glycopeptides that have been profiled by tandem MS. gFinder not only enables the simultaneous integration of collision-induced dissociation (CID) and high-energy collisional dissociation (HCD) fragmentation but also merges the spectra for high-throughput analysis. These merged spectra expedite the identification of both glycans and N-glycopeptide backbones in tandem MS data using the glycan database and a proteomic search tool (e.g., Mascot). These data can be used to simultaneously characterize peptide backbone sequences and possible N-glycan structures using assigned scores. gFinder also provides many convenient functions that make it easy to perform manual calculations while viewing the spectrum on-screen. We used gFinder to detect an additional protein (Q8N9B8) that was missed from the previously published data set containing N-linked glycosylation. For N-glycan analysis, we used the GlycomeDB glycan structure database, which integrates the structural and taxonomic data from all of the major carbohydrate databases available in the public domain. Thus, gFinder is a convenient, high-throughput analytical tool for interpreting the tandem mass spectra of N-glycopeptides, which can then be used for identification of potential missing proteins having glycans. gFinder is available publicly at http://gFinder.proteomix.org/ .

Sialylation of vitronectin regulates stress fiber formation and cell spreading of dermal fibroblasts via a heparin-binding site

Vitronectin (VN) plays an important role in tissue regeneration. We previously reported that VN from partial hepatectomized (PH) rats results in a decrease of sialylation of VN and de-sialylation of VN decreases the cell spreading of hepatic stellate cells. In this study, we analyzed the mechanism how sialylation of VN regulates the properties of mouse primary cultured dermal fibroblasts (MDF) and a dermal fibroblast cell line, Swiss 3T3 cells. At first, we confirmed that VN from PH rats or de-sialylated VN also decreased cell spreading in MDF and Swiss 3T3 cells. The de-sialylation suppressed stress fiber formation in Swiss 3T3 cells. Next, we analyzed the effect of the de-sialylation of VN on stress fiber formation in Swiss 3T3 cells. RGD peptide, an inhibitor for a cell binding site of VN, did not affect the cell attachment of Swiss 3T3 cells on untreated VN but significantly decreased it on de-sialylated VN, suggesting that the de-sialylation attenuates the binding activity of an RGD-independent binding site in VN. To analyze a candidate RGD-independent binding site, an inhibition experiment of stress fiber formation for a heparin binding site was performed. The addition of heparin and treatment of cells with heparinase decreased stress fiber formation in Swiss 3T3 cells. Furthermore, de-sialylation increased the binding activity of VN to heparin, as detected by surface plasmon resonance (SPR). These results demonstrate that sialylation of VN glycans regulates stress fiber formation and cell spreading of dermal fibroblast cells via a heparin binding site.

Autoactivation of pancreatic trypsinogen is controlled by carbohydrate-specific interaction

Activation of bovine pancreatic trypsinogen (BPTG) by trypsin (BPT) was found to be inhibited by d GalN/GalNAc at pH 5.5, the pH of secretory granules in the pancreas. Binding studies with biotinylated sugar-polymers indicated that BPTG and BPT bind to α-GalNAc, α-Man, and α-Gal better at pH 5.5 than at pH 7.5. Ultraviolet-difference spectra indicated that BPTG binding to α-GalNAc differs substantially from BPTG binding to other sugars. The N-α-benzoyl-d,l-arginine-p-nitroanilide hydrochloride-hydrolyzing activity of BPT was only slightly affected by these sugars. The results indicate that the binding of GalNAc - containing glycoconjugates protects BPTG from autoactivation, and this may be a self-defense mechanism against intrapancreatic activation.

MRM validation of targeted nonglycosylated peptides from N-glycoprotein biomarkers using direct trypsin digestion of undepleted human plasma

A rapid, simple, and reproducible MRM-based validation method for serological glycoprotein biomarkers in clinical use was developed by targeting the nonglycosylated tryptic peptides adjacent to N-glycosylation sites. Since changes in protein glycosylation are known to be associated with a variety of diseases, glycoproteins have been major targets in biomarker discovery. We previously found that nonglycosylated tryptic peptides adjacent to N-glycosylation sites differed in concentration between normal and hepatocellular carcinoma (HCC) plasma due to differences in steric hindrance of the glycan moiety in N-glycoproteins to tryptic digestion (Lee et al., 2011). To increase the feasibility and applicability of clinical validation of biomarker candidates (nonglycosylated tryptic peptides), we developed a method to effectively monitor nonglycosylated tryptic peptides from a large number of plasma samples and to reduce the total analysis time with maximizing the effect of steric hindrance by the glycans during digestion of glycoproteins. The AUC values of targeted nonglycosylated tryptic peptides were excellent (0.955 for GQYCYELDEK, 0.880 for FEDGVLDPDYPR and 0.907 for TEDTIFLR), indicating that these could be effective biomarkers for hepatocellular carcinoma. This method provides the necessary throughput required to validate glycoprotein biomarkers, as well as quantitative accuracy for human plasma analysis, and should be amenable to clinical use.

BIOLOGICAL SIGNIFICANCE

Difficulties in verifying and validating putative protein biomarkers are often caused by complex sample preparation procedures required to determine their concentrations in a large number of plasma samples. To solve the difficulties, we developed MRM-based protein biomarker assays that greatly reduce complex, time-consuming, and less reproducible sample pretreatment steps in plasma for clinical implementation. First, we used undepleted human plasma samples without any enrichment procedures. Using nanoLC/MS/MS, we targeted nonglycosylated tryptic peptides adjacent to N-linked glycosylation sites in N-linked glycoprotein biomarkers, which could be detected in human plasma samples without depleting highly abundant proteins. Second, human plasma proteins were digested with trypsin without reduction and alkylation procedures to minimize sample preparation. Third, trypsin digestion times were shortened so as to obtain reproducible results with maximization of the steric hindrance effect of the glycans during enzyme digestion. Finally, this rapid and simple sample preparation method was applied to validate targeted nonglycosylated tryptic peptides as liver cancer biomarker candidates for diagnosis in 40 normal and 41 hepatocellular carcinoma (HCC) human plasma samples. This strategy provided the necessary throughput required to monitor protein biomarkers, as well as quantitative accuracy in human plasma analysis. From biomarker discovery to clinical implementation, our method will provide a biomarker study platform that is suitable for clinical deployment, and can be applied to high-throughput approaches.

Glycosylation in a mammalian expression system is critical for the production of functionally active leukocyte immunoglobulin-like receptor A3 protein

The leukocyte immunoglobulin-like receptor (LILR) A3 is a member of the highly homologous activating and inhibitory receptors expressed on leukocytes. LILRA3 is a soluble receptor of unknown functions but is predicted to act as a broad antagonist to other membrane-bound LILRs. Functions of LILRA3 are unclear primarily because of the lack of high quality functional recombinant protein and insufficient knowledge regarding its ligand(s). Here, we expressed and characterized recombinant LILRA3 (rLILRA3) proteins produced in 293T cells, Escherichia coli, and Pichia pastoris. We found that the purified rLILRA3 produced in the mammalian system was the same size as a 70-kDa native macrophage LILRA3. This is 20 kDa larger than the calculated size, suggesting significant post-translational modifications. In contrast, rLILRA3 produced in E. coli was similar in size to the unprocessed protein, but yeast-produced protein was 2-4 times larger than the unprocessed protein. Treatment with peptide-N-glycosidase F reduced the size of the mammalian cell- and yeast-produced rLILRA3 to 50 kDa, suggesting that most modifications are due to glycosylation. Consistent with this, mass spectrometric analysis of the mammalian rLILRA3 revealed canonical N-glycosylation at the predicted Asn(140), Asn(281), Asn(302), Asn(341), and Asn(431) sites. Functionally, only mammalian cell-expressed rLILRA3 bound onto the surface of monocytes with high affinity, and importantly, only this significantly abrogated LPS-induced TNFα production by monocytes. Binding to monocytes was partially blocked by β-lactose, indicating that optimally glycosylated LILRA3 might be critical for ligand binding and function. Overall, our data demonstrated for the first time that LILRA3 is a potential new anti-inflammatory protein, and optimal glycosylation is required for its functions.

Vitronectin--master controller or micromanager?

The concept that the mammalian glycoprotein vitronectin acts as a biological 'glue' and key controller of mammalian tissue repair and remodelling activity is emerging from nearly 50 years of experimental in vitro and in vivo data. Unexpectedly, the vitronectin-knockout (VN-KO) mouse was found to be viable and to have largely normal phenotype. However, diligent observation revealed that the VN-KO animal exhibits delayed coagulation and poor wound healing. This is interpreted to indicate that VN occupies a role in the earliest events of thrombogenesis and tissue repair. VN is the foundation upon which the thrombus grows in an organised structure. In addition to sealing the wound, the thrombus also serves to protect the underlying tissue from oxidation, is a reservoir of mitogens and tissue repair mediators, and provides a provisional scaffold for the repairing tissue. In the absence of VN (e.g., VN-KO animal), this cascade is disrupted before it begins. A wide variety of biologically active species associate with VN. Although initial studies were focused on mitogens, other classes of bioactives (e.g., glycosaminoglycans and metalloproteinases) are now also known to specifically interact with VN. Although some interactions are transient, others are long-lived and often result in multi-protein complexes. Multi-protein complexes provide several advantages: prolonging molecular interactions, sustaining local concentrations, facilitating co-stimulation of cell surface receptors and thereby enhancing cellular/biological responses. We contend that these, or equivalent, multi-protein complexes facilitate VN polyfunctionality in vivo. It is also likely that many of the species demonstrated to associate with VN in vitro, also associate with VN in vivo in similar multi-protein complexes. Thus, the predominant biological function of VN is that of a master controller of the extracellular environment; informing, and possibly instructing cells 'where' to behave, 'when' to behave and 'how' to behave (i.e., appropriately for the current circumstance).

Survival signals of hepatic stellate cells in liver regeneration are regulated by glycosylation changes in rat vitronectin, especially decreased sialylation

The extracellular matrix (ECM) molecules play important roles in many biological and pathological processes. During tissue remodeling, the ECM molecules that are glycosylated are different from those of normal tissue owing to changes in the expression of many proteins that are responsible for glycan synthesis. Vitronectin (VN) is a major ECM molecule that recognizes integrin on hepatic stellate cells (HSCs). The present study attempted to elucidate how changes in VN glycans modulate the survival of HSCs, which play a critical role in liver regeneration. Plasma VN was purified from partially hepatectomized (PH) and sham-operated (SH) rats at 24 h after operation and non-operated (NO) rats. Adhesion of rat HSCs (rHSCs), together with phosphorylation of focal adhesion kinase, in PH-VN was decreased to one-half of that in NO- or SH-VN. Spreading of rHSCs on desialylated NO-VN was decreased to one-half of that of control VN, indicating the importance of sialylation of VN for activation of HSCs. Liquid chromatography/multiple-stage mass spectrometry analysis of Glu-C glycopeptides of each VN determined the site-specific glycosylation. In addition to the major biantennary complex-type N-glycans, hybrid-type N-glycans were site-specifically present at Asn(167). Highly sialylated O-glycans were found to be present in the Thr(110)-Thr(124) region. In PH-VN, the disialyl O-glycans and complex-type N-glycans were decreased while core-fucosylated N-glycans were increased. In addition, immunodetection after two-dimensional PAGE indicated the presence of hyper- and hyposialylated molecules in each VN and showed that hypersialylation was markedly attenuated in PH-VN. This study proposes that the alteration of VN glycosylation modulates the substrate adhesion to rat HSCs, which is responsible for matrix restructuring.

Glycosylation and ligand-binding activities of rat plasma fibronectin during liver regeneration after partial hepatectomy

Fibronectin (FN) is a multifunctional glycoprotein present in the extracellular matrix (ECM) and plasma. We previously reported that the glycosylation and ligand-binding of vitronectin (VN) change markedly after partial hepatectomy (PH). Here we show the changes of FN during liver regeneration. The yields of purified sham-operated (SH-) and PH-FN were higher than that of non-operated (NO)-FN, while binding activities of FNs to ECM ligands were changed only slightly by hepatectomy. The carbohydrate concentration of PH-FN decreased to 66% of that of NO- and SH-FN. By using LC/MS(n), eight kinds of complex-type N-glycan structures were found to be present in all FNs, and bi- and trisialobiantennary glycans were the major structures. Fucosylation was markedly increased, while O-acetylation of sialic acid was found to be decreased in PH-FN. The alterations in glycosylation and biological activities of FN after PH are different from those of VN, suggesting that these glycoproteins play different biological functions in tissue remodeling.

Fe2+-catalyzed non-enzymatic glycosylation alters collagen conformation during AGE-collagen formation in vitro

Advanced glycation end-products (AGEs) are one of the major factors of hyperglycemia related complications for diabetic patients. We studied the formation of AGEs in type I collagen after Fe2+-catalyzed non-enzymatic glycosylation in vitro. Type I collagen isolated from rat tail tendon was incubated with glucose and increasing concentrations of iron ions Fe2+. After 4 weeks incubation, cytotoxity of AGEs was indicated by the cytotoxity assay of primary human umbilical vein endothelial cells and primary human monocytes cultured with glycosylated collagen AGEs. Fourier transform infrared spectroscopy analysis revealed that structural changes of functional groups in glycosylated collagen are accelerated by the catalyst Fe2+. Using two-dimensional Fourier-transform infrared correlation spectroscopy analyses, for the first time, we demonstrated that the order of structural changes of these functional groups is -CH->Amide I>Amide II>Amide III>nu(C=O) the carboxylic group of Asn, Gln or polyproline amino acid residue in the course of AGE-collagen formation. Knowing the positions of these functional groups in collagen, this order of changes indicates that during glycation of collagen, the structure of the main chain residues in collagen changed first, and then the side chain changed gradually, which may lead to more carboxylic groups exposed to glucose for further formation of AGE-collagen irreversibly. The findings presented may support the design of new therapeutic strategies to prevent or slow down the Fe2+-catalyzed glycosylation of collagen and other matrix proteins.

Glycoproteins of drusen and drusen-like lesions

Drusen are a marker of age-related macular degeneration (AMD). Lesions similar to drusen, both in histology and their clinical appearance, are also seen in choroidal tumours, chronic inflammatory and degenerative conditions of the eye, and in mesangiocapillary glomerulonephritis type II (MCGN-II). This study aims to compare the saccharide composition of these drusen-like lesions in the various ocular pathological groups and in MCGN-II. Formalin fixed and paraffin wax embedded tissue from 21 eyes was studied. The histological diagnoses included AMD, retinal detachment, phthisis bulbi following failed retinal detachment surgery, malignant melanoma, long-standing uveitis, glaucoma and MCGN II. Glycosylation was examined using a panel of twenty biotinylated lectins and an avidin-peroxidase DAB-cobalt revealing system, with and without neuraminidase pre-treatment. High mannose, bi/tri-nonbisected and bisected complex N-glycan, N-acetyl glucosaminyl, galactosyl and sialyl residues were found to be expressed by drusen, while treatment with neuraminidase exposed subterminal N-acetyl galactosamine and galactosyl residues. Similar binding patterns were found in the various pathological groups studied. As there was no significant difference in the lectin-binding pattern in drusen in different pathologies, a common pathogenesis or at least a final common pathway for the elaboration of carbohydrate components of drusen is suggested.

Changes in glycosylation of vitronectin modulate multimerization and collagen binding during liver regeneration

Elucidating the mechanisms and factors regulating multimerization is biologically important in order to modulate the biological activities of functional proteins, especially adhesive proteins in the extracellular matrix (ECM). Vitronectin (VN) is a multifunctional glycoprotein present in plasma and ECM. Linkage of cellular adhesion and fibrinolysis by VN plays an essential role during tissue remodeling. Our previous study determined that the collagen-binding activity of VN was markedly enhanced with the decreased glycosylation during liver regeneration. This study demonstrated how alternations of glycans modulate the biological activity of VN. Human and rat VNs were used because of their similarities in structure and activities. The binding affinity of human VN to immobilized collagen was shown to be higher at pH 4.5 than at 7.5, at 37 degrees C than at 4 degrees C. Sedimentation velocity studies indicated that the greater the multimerization of human VN, the better it bound to collagen. The results indicate that the collagen binding of VN was modulated through its multimerization. Stepwise trimming of glycan with various exoglycosidases increased both the multimer size and the collagen binding of human VN, indicating that they are modulated by changes in glycosylation. The multimer sizes of VN purified from plasma of partially hepatectomized (PH) rats and sham-operated (SH) rats increased by about 45 and 31%, respectively, compared with those of nonoperated (NO) rats. In accordance with this, PH-VN exhibited remarkably enhanced collagen binding than SH-VN and NO-VN on surface plasmon resonance. In the PH rat sera, the multimer VN was increased in both amount and size compared with those in SH- and NO-sera. The results demonstrate that glycan alterations during tissue remodeling induce increased multimerization state to enhance the biological activity of VN.

Identification of disialic acid-containing glycoproteins in mouse serum: a novel modification of immunoglobulin light chains, vitronectin, and plasminogen

Serum glycoproteins are involved in various biologic activities, such as the removal of exogenous antigens, fibrinolysis, and metal transport. Some of them are also useful markers of inflammation and disease. Although the amount of sialic acid increases following inflammation, little attention has been paid to the presence of linkage-specific epitopes in serum, especially the alpha2,8-linkage. In a previous study, we demonstrated that four components in mouse serum contain alpha2,8-linked disialic acid (diSia), based on immunoreactivity with monoclonal antibody 2-4B, which is specific to N-glycolylneuraminic acid (Neu5Gc)alpha2-->(8Neu5Gc alpha2-->)(n-1), n > or = 2 [Yasukawa et al., (2005) Glycobiology, 15, 827-837]. In this study, we purified three components, 30-, 70-, and 120-kDa gp, and identified them as an immunoglobulin (Ig) light chain, vitronectin, and plasminogen, respectively, using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy analyses. Modifications of these proteins with alpha2,8-linked diSia were chemically confirmed by fluorometric C7/C9 analyses and mild acid hydrolysates-fluorometric anion-exchange chromatography analyses. We also demonstrated that the IgG, IgM, and IgE light chains are commonly modified with alpha2,8-linked diSia. In addition, both mouse and rat vitronectin contained diSia, and the amount of disialylation in vitronectin dramatically decreased after hepatectomy. These results indicate that a novel diSia modification of serum glycoproteins is biologically important for immunologic events and fibrinolysis.

Novel Carbohydrate-binding Activity of Pancreatic Trypsins to N-Linked Glycans of Glycoproteins

How glycosylation affects the reactivity of proteins to trypsin is not well understood. Bovine and porcine pancreatic trypsins were discovered to bind to alpha-Man, Neu5Acalpha2,6Galbeta1,4Glc, and alpha-galactose sequences by binding studies with biotinylated sugar-polymers. Quantitative kinetic studies supported that phenylmethylsulfonyl fluoride (PMSF)-treated trypsin binds to glycolipid analogues possessing alpha-Man or alpha-NeuAc but not to those possessing beta-galactose or beta-GlcNAc residue. Enzyme-linked immunosorbent assay (ELISA) showed that trypsin binds to six kinds of biotinylated glycoproteins possessing high mannose-type and complex-type N-glycans but not to bovine submaxillary mucin, which possesses only O-glycans. Further, the binding of trypsin to glycoproteins was differentially changed by treatments with sequential exoglycosidases, endoglycosidase H, or N-glycosidase F. Quantitative kinetic studies indicated that PMSF-treated trypsin binds with bovine thyroglobulin with the affinity constant of 10(10) m(-1), which was the highest among the glycoproteins examined, and that alpha-galactosidase treatment decreased it to 10(5) m(-1). PMSF-treated trypsin bound to other glycoproteins, including ovomucoid, a trypsin inhibitor, with the affinity constants of 10(8)-10(5) mol(-1) and were markedly changed by glycosidase treatments in manners consistent with the sugar-binding specificities suggested by ELISA. Thus, the binding site for glycans was shown to be distinct from the catalytic site, allowing trypsin to function as an uncompetitive activator in the hydrolysis of a synthetic peptide substrate. Correspondingly the carbohydrate-binding activities of trypsin were unaffected by treatment with PMSF or soybean trypsin inhibitor. The results indicate the presence of an allosteric regulatory site on trypsin that sugar-specifically interacts with glycoproteins in addition to the proteolytic catalytic site.

Screening for and purification of novel self-aggregatable lectins reveal a new functional lectin group in the bark of leguminous trees

A solubility-insolubility transition assay was used to screen the bark and stems of seven leguminous trees and plants for self-aggregatable lectins. Novel lectins were found in two trees, Robinia pseudoacacia and Wisteria floribunda, but not in the leguminous plants. The Robinia lectin was isolated from coexisting lectin by combined affinity chromatographies on various sugar adsorbents. The purified lectins proved to be differently glycosylated glycoproteins. One lectin exhibited the remarkable characteristics of self-aggregatable lectins: localization in the bark of legume trees, self-aggregation dissociated by N-acetylglucosamine/mannose, and coexistence with N-acetylgalactosamine/galactose-specific lectins, which are potential endogenous receptors. Self-aggregatable lectins are a functional lectin group that can link enhanced photosynthesis to dissociation of glycoproteins.

Porcine pancreatic alpha-amylase shows binding activity toward N-linked oligosaccharides of glycoproteins

Porcine pancreatic alpha-amylase was shown by interaction analyses using a resonance mirror detector and alpha-amylase-immobilized Sepharose to bind with glycoproteins possessing N-glycans but not O-linked mucin-type glycans. Direct binding of three types of N-glycans to the alpha-amylase was demonstrated by surface plasmon resonance. Binding with biotin-polymer sugar probes revealed that the alpha-amylase has affinity to alpha-mannose, alpha-N-acetylneuraminic acid, and beta-N-acetyllactosamine, which are components of N-glycans. The binding of glycoproteins or carbohydrates enhanced the enzyme activity, indicating that the recognition site for N-glycans is different from its catalytic site. The binding activity was unique to porcine pancreatic alpha-amylase and was not observed for alpha-amylase from saliva, wheat, and fungus.