Identification of the mucin core protein by cell-free translation of messenger RNA from bovine submaxillary glands

Hydrogel-based immunoregulation of macrophages for tissue repair and regeneration

The rational design of hydrogel materials to modulate the immune microenvironment has emerged as a pivotal approach in expediting tissue repair and regeneration. Within the immune microenvironment, an array of immune cells exists, with macrophages gaining prominence in the field of tissue repair and regeneration due to their roles in cytokine regulation to promote regeneration, maintain tissue homeostasis, and facilitate repair. Macrophages can be categorized into two types: classically activated M1 (pro-inflammatory) and alternatively activated M2 (anti-inflammatory and pro-repair). By regulating the physical and chemical properties of hydrogels, the phenotypic transformation and cell behavior of macrophages can be effectively controlled, thereby promoting tissue regeneration and repair. A full understanding of the interaction between hydrogels and macrophages can provide new ideas and methods for future tissue engineering and clinical treatment. Therefore, this paper reviews the effects of hydrogel components, hardness, pore size, and surface morphology on cell behaviors such as macrophage proliferation, migration, and phenotypic polarization, and explores the application of hydrogels based on macrophage immune regulation in skin, bone, cartilage, and nerve tissue repair. Finally, the challenges and future prospects of macrophage-based immunomodulatory hydrogels are discussed.

Probing Relative Humidity Impact on Biological Protein Bovine Serum Albumin and Bovine Submaxillary Gland Mucin by Using Contact Resonance Atomic Force Microscopy

In biomaterials, a substantial amount of research has been placed on the mechanical properties of biomolecules and their interactions with body fluids. Bovine serum albumin (BSA) is a widely studied model protein, while bovine submaxillary gland mucin (BSM) is another cow-derived protein frequently employed in research. Films were examined with contact resonance atomic force microscopy (CR-AFM), and the results showed that the mechanical characteristics of the films were affected by the relative humidity. We quantitatively analyze the viscoelasticity of these proteins after they have been subjected to humidity by measuring the resonance frequency and quality factor. The findings indicate that prolonged humidity exposure has a different effect on the mechanical properties of BSA and BSM films. The results show that after exposure to humidity, the resonance peaks of BSA shift to the left, indicating stiffness, while those of BSM shift to the right, indicating hydration. Moreover, BSM's hydration is caused by relative humidity, leading to a constant increase in resonance frequency and material softness. Contrarily, BSA showed a decrease in contact resonance frequency due to ongoing strain-induced deformation, indicating increased material stiffness. The findings have significance for the design and development of biomaterials for a variety of applications, such as the delivery of drugs, the engineering of tissue, and the development of biosensors. Our research demonstrates that CR-AFM has the potential to become a non-invasive and sensitive method that can be used to characterize the mechanical characteristics of biomolecules and their interactions with bodily fluids.

Early osteoimmunomodulation by mucin hydrogels augments the healing and revascularization of rat critical-size calvarial bone defects

The design principle of osteogenic bone grafts has shifted from immunological inertness to limiting foreign body response to combined osteoimmunomodulatory activity to promote high-quality endogenous bone regeneration. Recently developed immunomodulatory mucin hydrogels have been shown to elicit very low complement activation and suppress macrophage release and activation after implantation in vivo. However, their immunoregulatory activity has not yet been studied in the context of tissue repair. Herein, we synthesized mucin-monetite composite materials and investigated their early osteoimmunomodulation using a critical-size rat bone defect model. We demonstrated that the composites can polarize macrophages towards the M2 phenotype at weeks 1 and 2. The early osteoimmunomodulation enhanced early osteogenesis and angiogenesis and ultimately promoted fracture healing and engraftment (revascularization of the host vasculature) at weeks 6 and 12. Overall, we demonstrated the applicability of mucin-based immunomodulatory biomaterials to enhance tissue repair in tissue engineering and regenerative medicine.

Covalently Crosslinked Pig Gastric Mucin Hydrogels Prepared by Radical-based Chain-Growth and Thiol-ene Mechanisms

Mucin, a high molecular mass hydrophilic glycoprotein, is the main component of mucus that coats every wet epithelium in animals. It is thus intrinsically biocompatible, and with its protein backbone and the o-glycosidic bound oligosaccharides, it contains a plethora of functional groups which can be used for further chemical modifications. In this study we introduce and compare chain-growth and step-growth (thiol-ene) free-radical crosslinked hydrogels prepared from commercially available pig gastric mucin (PGM) as cost-efficient and easily accessible alternative to the more broadly applied bovine submaxillary gland mucin (BSM). For this, PGM was functionalized with photoreactive acrylate groups or allylether-moieties, respectively. Whereas homopolymerization of acrylate functionalized polymers was performed, for thiol-ene crosslinking, the allylether functionalized PGM was cross-linked with thiol-functionalized hyaluronic acid (HA). Morphology, mechanical properties, and cell compatibility of both kinds of PGM hydrogels are characterized and compared. Furthermore, the biocompatibility of these hydrogels could be evaluated in cell culture experiments. This article is protected by copyright. All rights reserved.

Improved online LC-MS/MS identification of O-glycosites by EThcD fragmentation, chemoenzymatic reaction, and SPE enrichment

O-glycosylation is a highly diverse and complex form of protein post-translational modification. Mucin-type O-glycosylation is initiated by the transfer of N-acetyl-galactosamine (GalNAc) to the hydroxyl group of serine, threonine and tyrosine residues through catalysis by a family of glycosyltransferases, the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (E.C. 2.4.1.41) that are conserved across metazoans. In the last decade, structural characterization of glycosylation has substantially advanced due to the development of analytical methods and advances in mass spectrometry. However, O-glycosite mapping remains challenging since mucin-type O-glycans are densely packed, often protecting proteins from cleavage by proteases. Adding to the complexity is the fact that a given glycosite can be modified by different glycans, resulting in an array of glycoforms rising from one glycosite. In this study, we investigated conditions of solid phase extraction (SPE) enrichment, protease digestion, and Electron-transfer/Higher Energy Collision Dissociation (EThcD) fragmentation to optimize identification of O-glycosites in densely glycosylated proteins. Our results revealed that anion-exchange stationary phase is sufficient for glycopeptide enrichment; however, the use of a hydrophobic-containing sorbent was detrimental to the binding of polar-hydrophilic glycopeptides. Different proteases can be employed for enhancing coverage of O-glycosites, while derivatization of negatively charged amino acids or sialic acids would enhance the identification of a short O-glycopeptides. Using a longer than normal electron transfer dissociation (ETD) reaction time, we obtained enhanced coverage of peptide bonds that facilitated the localization of O-glycosites. O-glycosite mapping strategy via proteases, cut-off filtration and solid-phase chemoenzymatic processing. Glycopeptides are enriched by SPE column, followed by release of N-glycans, collection of higher MW O-glycopeptides via MW cut-off filter, O-glycopeptide release via O-protease, and finally detected by LC-MS/MS using EThcD.

Genetically Engineered Mucoadhesive Spider Silk

Mucoadhesion is defined as the adhesion of a material to the mucus gel covering the mucous membranes. The mechanisms controlling mucoadhesion include nonspecific electrostatic interactions and specific interactions between the materials and the mucins, the heavily glycosylated proteins that form the mucus gel. Mucoadhesive materials can be used to develop mucosal wound dressings and noninvasive transmucosal drug delivery systems. Spider silk, which is strong, biocompatible, biodegradable, nontoxic, and lightweight would serve as an excellent base for the development of such materials. Here, we investigated two variants of the partial spider silk protein 4RepCT genetically engineered in order to functionalize them with mucoadhesive properties. The pLys-4RepCT variant was functionalized with six cationically charged lysines, aiming to provide nonspecific adhesion from electrostatic interactions with the anionically charged mucins, while the hGal3-4RepCT variant was genetically fused with the Human Galectin-3 Carbohydrate Recognition Domain which specifically binds the mucin glycans Galβ1-3GlcNAc and Galβ1-4GlcNAc. First, we demonstrated that coatings, fibers, meshes, and foams can be readily made from both silk variants. Measured by the adsorption of both bovine submaxillary mucin and pig gastric mucin, the newly produced silk materials showed enhanced mucin binding properties compared with materials of wild-type (4RepCT) silk. Moreover, we showed that pLys-4RepCT silk coatings bind mucins through electrostatic interactions, while hGal3-4RepCT silk coatings bind mucins through specific glycan-protein interactions. We envision that the two new mucoadhesive silk variants pLys-4RepCT and hGal3-4RepCT, alone or combined with other biofunctional silk proteins, constitute useful new building blocks for a range of silk protein-based materials for mucosal treatments.

Mucins as multifunctional building blocks of biomaterials

Mucins glycoproteins are emerging as a multifunctional building block for biomaterials with diverse applications in chemistry and biomedicine.

An improved lectin-based method for the detection of mucin-type O-glycans in biological samples

Mucins and mucin-type glycoproteins, collectively referred to as mucin-type O-glycans, are implicated in many important biological functions and pathological conditions, including malignancy. Presently, there is no reliable method to measure the total mucin-type O-glycans of a sample, which may contain one or more of these macromolecules of unknown structures. We report the development of an improved microassay that is based on the binding of lectins to the unique and constant GalNAc-Ser/Thr structural feature of mucin-type O-glycans. Since the sugar-amino acid linkage in the mucin-type O-glycans is invariably cryptic, we first chemically removed the heterogeneous peripheral and core saccharides of model glycoconjugates before examining for their interactions using an enzyme-linked lectin assay (ELLA). Desialylation of the model glycoconjugates led to maximal binding of the lectins but additional treatments such as Smith degradation did not result in increased binding. Of the lectins tested for their ability to probe the desialylated O-glycans, jacalin showed the highest sensitivity followed by champedak galactose binding (CGB) lectin and Vicia villosa agglutinin. Further improvement in the sensitivity of ELLA was achieved by using microtiter plates that were pre-coated with the CGB lectin, which increased the specificity of the assay to mucin-type O-glycans. Finally, the applicability of the developed sandwich ELLA to crude samples was demonstrated by estimating trace quantities of the mucin-type O-glycans in the human serum.

Effect of hydration on structural and thermodynamic properties of pig gastric and bovine submaxillary gland mucins

One of the essential functions of mucous gel is protection of tissues against dehydration. The effect of hydration on the structural and thermodynamic properties of pig gastric mucin (PGM) and bovine submaxillary gland mucin (BSM) have been studied using atomic force microscopy (AFM), sorption, and differential scanning calorimetry (DSC). The analysis of sorption isotherms shows the higher water sorption capacity of PGM compared to BSM at RH levels lower than about 78%. The value of the hydration enthalpy at zero water content at 25 °C for both biopolymers is about -20 kJ/mol. Glass transitions of BSM and PGM occur at RH levels between 60 and 70% for both mucins. AFM indicates the presence of a dumbbell structure as well as a fiber-like structure in PGM samples. The experimental volume of the dry dumbbell molecule obtained by AFM is 3140 ± 340 nm(3). Using DSC data, the amount of nonfreezing water was calculated to be about 0.51 g/g of PGM. The phase diagram of PGM demonstrates two regions of different Tg: dependent and independent of hydration levels. In particular, at mucin concentrations from 0 to 67 wt %, the glass transition occurs at a constant temperature of about -15 °C. At higher concentrations of mucin, Tg is increasing with increasing mucin concentrations.

Enhanced bioavailability of polyaromatic hydrocarbons in the form of mucin complexes

Increasing exposure of biological systems to large amounts of polycyclic aromatic hydrocarbons is of great public concern. Organisms have an array of biological defense mechanisms, and it is believed that mucosal gel (which covers the respiratory system, the gastrointestinal tract, etc.) provides an effective chemical shield against a range of toxic materials. However, in this work, we demonstrate, for the first time, that, upon complexation of polyaromatic hydrocarbons with mucins, enhanced bioavailability and, therefore, toxicity are obtained. This work was aimed to demonstrate how complexation of various highly hydrophobic polycyclic aromatic hydrocarbons with representative mucin glycoprotein could lead to the formation of previously undescribed materials, which exhibit increased toxicity versus pristine polycyclic aromatic hydrocarbons. In the present work, we show that a representative mucin glycoprotein, bovine submaxillary mucin, has impressive and unprecedented capabilities of binding and solubilizing water-insoluble materials in physiological solution. The complexes formed between the mucin and a series of polycyclic aromatic hydrocarbons were comprehensively characterized, and their toxicity was evaluated by both in vivo and in vitro assays. In addition, the bioavailability and membrane-penetration capabilities were tested using an internalization assay. Our results provide, for the first time, evidence of an unknown route by which hydrophobic materials may achieve higher bioavailability, penetrating some of the biological defense systems, in the form of water-soluble complexes with mucosal proteins.

Mucin complexes of nanomaterials: first biochemical encounter

In recent years, the exposure of biological systems to various nanomaterials has become an issue of great public concern. Although living organisms have arrays of biological defense mechanisms against exposure to exogenous compounds, the biochemical mechanisms allowing various nanomaterials to enter the body are not well understood. A unique example of a typical mucosal glycoprotein capable of binding and solubilizing nanomaterials in physiological solution is provided, suggesting a possible route for entry into biological systems.

The lectin domain of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 1 is involved in O-glycosylation of a polypeptide with multiple acceptor sites

Mucin type O-glycosylation begins with the transfer of GalNAc to serine and threonine residues on proteins by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminlytransferases. These enzymes all contain a lectin-like (QXW)(3) repeat sequence at the C terminus that consists of three tandem repeats (alpha, beta, and gamma). The putative lectin domain of one of the most ubiquitous isozymes, GalNAc-T1, is reportedly not functional. In this report, we have reevaluated the role of the GalNAc-T1 lectin domain. Deletion of the lectin domain resulted in a complete loss of enzymatic activity. We also found that GalNAc-T1 has two activities distinguished by their sensitivities to inhibition with free GalNAc; one activity is sensitive, and the other is resistant. In our experiments, the former activity is represented by the O-glycosylation of apomucin, an acceptor that contains multiple glycosylation sites, and the latter is represented by synthetic peptides that contain a single glycosylation site. Site-directed mutagenesis of the lectin domain selectively reduced the former activity and identified Asp(444) in the alpha repeat as the most important site for GalNAc recognition. A further reduction of the GalNAc-inhibitable activity was observed when both Asp(444) and the corresponding aspartate residues in the beta and the gamma repeats were mutated. This suggests a cooperative involvement of each repeat unit in the glycosylation of polypeptides with multiple acceptor sites.

Characterization of the rabbit homolog of human MUC1 glycoprotein isolated from bladder by affinity chromatography on immobilized jacalin

The urinary bladder is lined by transitional epithelium, the glycocalyx on the luminal surface has interesting properties and is implicated in protective functions. Glycoconjugates are major components of the glycocalyx, but their biochemical nature is not well understood. Previous studies on rabbit bladder indicated the presence of significant levels of sialoglycoproteins compared to glycosaminoglycans in the epithelium. In this study, rabbit explant cultures were radiolabeled by precursor sugars or amino acids and a major lectin-reactive glycoprotein of rabbit bladder mucosa was isolated by affinity chromatography on jacalin-agarose. The radiolabeled glycoprotein was purified to homogeneity by a second cycle on the lectin column, followed by gel filtration and density gradient centrifugation. The average molecular mass of the glycoprotein was estimated to be 245 kDa and 210 kDa by gel filtration and SDS-PAGE, respectively. Its buoyant density was 1.40 g/ml, suggesting a carbohydrate content of approximately 50%. The percent distribution of glucosamine-derived tritium label in sialic acid, galactosamine, and glucosamine was 30, 52, and 18, respectively. The glycoprotein consisted entirely of small sialylated and neutral oligosaccharides O-glycosidically linked to serine and threonine residues. The same glycoprotein could be immunoprecipitated with an antibody against the carboxy terminal 17 amino acid peptide of human MUC1 mucin glycoprotein. This suggests that this mucin glycoprotein is the rabbit homolog of MUC1 glycoprotein, which has been previously established to be a component of human bladder urothelium and has been purified from human urine and biochemically characterized.

Identification of the glycosidically bound sialic acid in mucin glycoproteins that reacts as "free sialic acid" in the Warren assay

A widely employed colorimetric assay for sialic acids based on periodate oxidation followed by reaction with thiobarbituric acid depends on the formation of a hexos-5-uluronic acid product, the pre-chromogen, by the periodate cleavage of the C6-C7, C7-C8, and C8-C9 bonds in free sialic acid. Glycosidically bound sialic acids are not expected to react in the assay since cleavage cannot occur between C6-C7 to yield the pre-chromogen. However, several investigators have reported the detection of a positive reaction by certain sialoglycoconjugates. In this study, it was found that various mucins but not other classes of sialoglycoconjugates or asialomucins exhibited this phenomenon. Of the mucins tested, ovine submaxillary mucin showed the maximum reactivity followed by the bovine and porcine counterparts. The disaccharide Neu5Acalpha2-->6 GalNAc(OH) released from mucins by alkaline borohydride treatment also reacted, albeit weakly compared to the native mucins, but other sialyl saccharides including 6'-sialyllactose and 6'-sialyl N -acetyllactosamine did not react. The positive reaction of the submaxillary mucins is not due to the presence of 3-deoxy-d-glycero-d-galacto-2-nonulosonic acid (KDN), a minor component in submaxillary mucins, or the release of sialic acid by the acidic condition of the assay. It is demonstrated that sialyl residues linked alpha2-->6 to unsubstituted N -acetylgalactosamine (sialyl Tn antigen structure) in mucin glycoproteins is responsible for the positive reaction. Apparently, periodate oxidation of the N -acetylgalactosamine residue leads to the release of sialic acid from the Neu5Acalpha2-->6 GalNAc linked to serine/threonine by an acid-catalyzed beta-elimination reaction. The findings provide a basis for the development of a chemical method to estimate sialyl Tn epitopes associated with cancer cells.

Biosynthesis of mucin derived from a 60-kDa precursor protein in the human stomach

We studied the biosynthesis of mucin in the human stomach using an anti-mucin core peptide monoclonal antibody, 3G12. Human stomach mucosa was labeled with [35S]methionine, and chased for 3 h. An approximately 60-kDa subunit of human gastric mucin precursor protein was detected in the intracellular product. Under nonreducing conditions, dimer, trimer, and tetramer mucin precursor protein (120, 180, 240 kDa) were detected. Treatment with tunicamycin or endo-beta-N-acetylglucosaminidase H had no effect on the 60-kDa subunit and its oligomers. Extracellular products contained only the high molecular weight mucin, and the secretion was not affected by tunicamycin. By treatment with monensin or brefeldin A, the mature mucin was not secreted extracellularly. These findings suggested that a 60-kDa subunit of the mucin precursor protein was biosynthesized into mature mucin after oligomerization to tetramers, and that neither the oligomerization nor the intracellular transport of the mucin in the human stomach was associated with N-glycosylation.

Characterization of a unique mucin-like glycoprotein secreted by a human endometrial adenocarcinoma cell line (Ishikawa)

A human endometrial adenocarcinoma cell line (Ishikawa) has been shown to incorporate [3H]glucosamine and to secrete a radiolabeled high molecular weight compound which is excluded from a Sepharose CL-2B column. The excluded material was resistant to hyaluronidase, chondroitinase ABC, and heparinase. These findings rule out the possibility of this material being a proteoglycan. The susceptibility of this material to digestion with pronase, neuraminidase, and alkaline borohydride treatment strongly suggests that the excluded material is an O-glycosidic glycoprotein. The glycoprotein secreted by Ishikawa cells (ICGP) did not react immunologically with antibodies against either lactoferrin or fibronectin, but did react with an antibody made against tracheal mucin. Conversely, immunoblot analysis revealed that an antibody made against ICGP did not recognize hyaluronic acid, chondroitin, heparin, nasal turbinate mucin, bovine submaxillary gland mucin, lactoferrin, or fibronectin, but did recognize tracheal mucin. Analysis of ICGP amino acid and carbohydrate composition showed that it is rich in serine, threonine, glutamic acid, aspartic acid, and N-acetylneuraminic acid. In this respect, ICGP differs from other mucins, even though it is immunologically similar to respiratory mucin; hence we may consider ICGP to be a mucin-like glycoprotein. Secretion of ICGP can be modulated by Ca(2+)-ionophore and other mucus secretagogues, such as platelet activating factor, carbachol, and monocyte/macrophage mucus secretagogue, all mediators of lung inflammation. Ishikawa cells and anti-ICGP antibody may be used in studies on in vitro regulation of mucin-like glycoprotein synthesis and secretion in the respiratory tract as well as in the endometrium.

Biophysical approaches to salivary mucin structure, conformation and dynamics

Our understanding of the origins of the physical and biochemical properties of mucous glycoproteins is incomplete and not with out controversy. Recent molecular biological and biophysical studies revealing the architecture and solution structure and dynamics of a series of salivary mucins, invaluable toward resolving many of these questions, are discussed. Mucins are very large, structurally heterogeneous, and highly expanded molecules with the carbohydrate playing a key role in maintaining the extended mucin conformation.

Purification of mucin glycoproteins by density gradient centrifugation in cesium trifluoroacetate

A procedure for the rapid isolation of mucin glycoprotein by density gradient centrifugation in cesium trifluoroacetate (CsTFA) is described. The separation of mixtures of rat tracheobronchial mucin, DNA, hyaluronic acid, and bovine serum albumin in CsTFA gradients was superior to that in cesium bromide gradients. Inclusion of guanidinium chloride or urea in the gradient had no influence on the separation obtained. The mucins isolated from sputum samples of cystic fibrosis patients by this procedure are largely free of nucleic acid, nonglycosylated proteins, and glycosaminoglycans. The results of the use of CsTFA gradient centrifugation for the isolation of mucin from extracts of bovine submaxillary gland are also presented. The CsTFA method is particularly suitable for the high-yield isolation of mucin from individual samples which are available in limited quantities.

Isolation and partial characterization of serine- and threonine-rich porcine gastric mucus glycopeptides

1. Two subfractions from purified porcine gastric mucus glycopeptide were found to separate from each other by cesium chloride equilibrium centrifugation. The highest density fraction and two lower density fractions separated were designated VHD, HD and LD, respectively. A comparative study of these components was made. 2. The high and low density fractions, HD and LD, appeared almost the same or identical, while VHD differed completely from either of them in the following respects: (1) VHD exhibited strong alcian blue binding activity. (2) 57% of VHD bound to the DEAE-Toyopearl column equilibrated with 0.2 M NaCl. (3) VHD eluted from the Sephacryl S-400 column as a lower molecular subunit. (4) One third of the sialic acid as a minor component in VHD was constituted by N-glycolylneuraminic acid. (5) Carbohydrate composition showed typical mucus glycoprotein with slightly higher fucose content. (6) Amino acid compositions of the anionic components prepared from VHD showed the highest Ser/Thr ratio, 1.92 compared to 0.46 for LD and 0.62 for HD. (7) Oligosaccharide released from VHD by alkaline-sodium borohydride treatment was larger than that from HD or LD. 3. The above results indicate the minor component, VHD, separated from the major components, to be a quite similar but not identical component to the so-called sulfated mucus glycoprotein reported previously [Slomiany et al. (1972) J. biol. Chem. 247, 5062-5070].

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

Characterization of the major and minor mucus glycoproteins from bovine submandibular gland

Two mucins were isolated from bovine submandibular glands and termed major and minor on a quantitative basis. The major mucin representing over 80% of the total glycoprotein fraction contained 37% of its dry weight as protein in contrast to 62% for the minor mucin. Differences in the amino acid composition reflected the higher proportion of typically non-glycosylated peptide in the minor mucin. The molar ratio of N-acetylgalactosamine to serine plus threonine was 0.82 in major and 0.65 in minor mucins, indicating a lower degree of substitution of potential glycosylation sites in the minor mucin. Differences in the carbohydrate composition were found largely related to the sialic acids, with higher relative amounts of N-glycoloylneuraminic acid in the minor mucin. In addition, the proportion of di-O-acetylated sialic acids was higher in the major mucin. The rate of sialidase action on the two mucins could be correlated with the content of N-glycoloylneuraminic acid in each glycoprotein. There was no difference in the type of oligosaccharide found in each mucin and the differences in relative proportions reflected the monosaccharide composition for the two mucins. Gel filtration on Sepharose CL 2B showed a lower molecular weight distribution for the minor in contrast to the major mucin which was partially excluded. Density gradient centrifugation reflected this variation. SDS-PAGE demonstrated a regular banding pattern for the major mucin with a lowest subunit size of 1.8 x 10(5) Da and aggregates in excess of 10(6) Da, while the minor mucin ranged from 3.0 x 10(5) to 10(6) Da. The chemical composition of the isolated mucins was compared with previous histochemical analysis of mucin distribution in bovine submandibular glands and indicates a possible cellular location for each mucin.

Identity of mucin's "118-kDa link protein" with fibronectin fragment

Human and rat intestinal mucin was purified by equilibrium density gradient centrifugation and Sepharose 2B chromatography according to M. Mantle, D. Mantle, and A. Allen (1981, Biochem. J. 195, 277-285) and analyzed using mucin, DNA, and fibronectin-specific antibodies in dot-blot, ELISA, and Western blotting. The 118-kDa component of the mucins and the 118-kDa fragment of fibronectin from the same source displayed affinity for concanavalin A and immunoreacted with fibronectin antibodies. The amino acid and carbohydrate compositions of the 118-kDa peptide electroeluted by gel electrophoresis of mucin and fibronectin preparations were identical within each pair of glycopeptides and closely resembled the "link protein component" of human and rat intestinal mucin preparations of R. E. F. Fahim, R. D. Specian, G. G. Forstner, and J. F. Forstner (1987, Biochem. J. 243, 631-640) and M. Mantle and G. Stewart (1989, Biochem. J. 259, 631-640). We therefore conclude that the "link protein" claimed to be an integral part of mucus glycoproteins in actuality is the 118-kDa fragment of fibronectin.

Multiple apomucin translation products from human respiratory mucosa mRNA

Poly(A)-rich RNA was purified from a pool of five human tracheobronchial mucosa. After in vitro translation in a reticulocyte lysate and immunoprecipitation of the translated products, using either a polyclonal antiserum or a monoclonal antibody to deglycosylated respiratory mucin peptides, the products were characterized by SDS/PAGE. The respiratory mucin precursors migrated as a very large smear from almost the top of the resolving polyacrylamide gel to an area corresponding to a molecular mass of about 100 kDa. After hybridization with mucin cDNA probe TH 29 described by Crepin et al. [Crepin, M., Porchet, N., Aubert, J. P. & Degand, P. (1990) Biorheology 27, 471-484] respiratory mucin mRNAs also appeared polydisperse. Although degradation or incomplete translation of high-molecular-mass mRNA cannot be entirely ruled out, these results suggest that human respiratory apomucins consist of a family of peptides which share some common epitopes. This possibility is in agreement with (a) the diversity of mucin precursors observed previously with pulse/chase experiments performed with explants of human respiratory mucosa and (b) the polydispersity of secreted respiratory mucins observed by electron microscopy.

Viscoelastic properties of solutions of ovine submaxillary mucin

The linear viscoelastic and rheological properties of high molecular weight ovine submaxillary mucin (OSM) solution have been investigated in terms of the Newtonian steady-flow viscosity [eta(gamma)], the complex oscillatory viscosity [eta*(omega)], and the storage and loss shear moduli [G'(omega) and G"(omega)]. It was observed that tau(gamma), eta*(omega), and G'(omega) are always higher when OSM is dissolved in 0.1M NaCl than when at the same concentration in 6M GdnHCl. This is consistent with previous observations that submaxillary mucins self-associate in 0.1M NaCl to form large aggregates, which are disrupted in 6M GdnHCl. As the OSM concentration increases, the appearance of a plateau shear modulus indicates the formation of a gel network in both solvents. The results suggest gelation involves specific intermolecular interactions, perhaps due to hydrophobic forces between interdigitated oligosaccharide side chains. The viscoelastic behavior of OSM solution at high concentration is thus similar to that reported in the literature for porcine gastric mucin (PGM). However, the OSM gels are mechanically weaker, having moduli that are an order of magnitude lower than those for PGM gels of comparable concentration. The oligosaccharide side chains of OSM consist of only 1-2 sugar units compared to 10-15 for PGM, but it appears that this is sufficient to allow for intermolecular interaction and the formation of weak gels.

Colorimetric determination of N-acetylhexosamine-terminating O-glycosidically linked saccharides in mucins and glycoproteins

A sensitive colorimetric assay for detecting mucins and glycoproteins rich in O-glycosidically linked saccharides is reported. The method combines the susceptibility of N-acetylgalactosamine terminating O-glycosidically linked saccharides to beta-elimination with the Morgan-Elson reaction for N-acetylhexosamines with free reducing ends. All mucin and mucin-type glycoproteins but none of the serum-type glycoproteins tested resulted in characteristic color production. All mucins tested gave linear responses in the range 5 to 200 micrograms and the assay was also adapted to the microscale involving the use of 96-well microtiter plates. The microassay in which the volumes of samples and reagents are scaled down 2.5-fold was particularly useful in monitoring of mucins, in the presence of other glycoconjugates, in large numbers of samples obtained during fractionation procedures. Cesium chloride, cesium bromide, potassium thiocyanate, and various detergents do not interfere with the colorimetric determination. Guanidine hydrochloride, cesium trifluoroacetate, and beta-mercaptoethanol decreased color by 30 to 45%; however, the interference was not serious to prevent the use of the method for detection of mucins in their presence. The use of the method for the specific detection of mucin during fractionation by gel filtration and density gradient centrifugation of cystic fibrosis sputum samples is demonstrated.

Purification and characterization of subunits of a high molecular weight human salivary mucin

A high molecular-weight mucin was purified from human submandibular-sublingual saliva. The purity of the mucin preparation was demonstrated by the absence of other salivary proteins, by antibody reactivity and by gel electrophoresis. After reduction with mercaptoethanol a putative link component with approximate Mr 150,000 and a glycoprotein component of higher Mr could be detected by gel electrophoresis. These subunits were subsequently purified and they showed distinct differences in their amino acid compositions, demonstrating that the mucin consisted of two different subunits. The link had a number of similarities with the link component of intestinal mucin and a parotid agglutinin and has previously been shown to cross-react with antiserum to link component from intestinal mucin. Salivary and intestinal mucins may therefore have similar subunit structure.

The complexity of mucins

Mucins represent the main components of gel-like secretions, or mucus, secreted by mucosae or some exocrine glands. These high-molecular-weight glycoproteins are characterized by the large number of carbohydrate chains O-glycosidically linked to the peptide. The determination of mucin molecular weight and conformation has been controversial for several reasons: 1) the methods used to solubilize mucus and to purify mucins are different and 2) the molecules have a strong tendency to aggregate or to bind to other molecules (peptides or lipids). Recently, electron microscopy has shown the filamentous shape of most mucins and their polydisperse character which, in some secretions, might correspond to a polymorphism of the peptide part of these molecules. The recent development of high pressure liquid chromatography and high-resolution proton NMR spectroscopy has allowed major progress in the structural study of mucin carbohydrate chains. These chains may have from 1 to about 20 sugars and bear different antigenic determinants, such as A, B, H, I, i, X, Y or Cad antigens. In some mucins, such as human respiratory mucins, the carbohydrate chain diversity is remarkable, which raises many questions. Mucins are molecules located at the interface between mucosae and the external environment. The carbohydrate chain diversity might allow many interactions between mucins and microorganisms and play a major role in the colonization or the defense of mucosae.

Biochemistry and lectin binding properties of mammalian salivary mucous glycoproteins

The molecules responsible for the highly viscous properties of mucus are secretory glycoproteins referred to as mucins. Salivary mucins are characterized by a high sugar to protein ratio and are of a broad range of molecular weight from 7 x 10(4) to millions. With a few exceptions, they contain up to 30% of hexosamine (galactosamine and glucosamine), 8-33% of sialic acid, trace to 15% of galactose or fucose and little or no mannose. The size of carbohydrate side chains of these glycoproteins ranges from one to about fifteen units of sugar. These carbohydrate side chains are usually O-glycosidically linked through N-acetylgalactosamine to a peptidyl serine or threonine. In some instances, ester sulfate groups, mainly on N-acetylglucosamine, are also a structural feature. In many of these glycoproteins, the saccharide sequence is the same as that which determines the specificity of blood groups. Carbohydrate sequence analysis shows that salivary mucins exhibit considerable polydispersity, great diversity and remarkable structural flexibility not only among animal species but also within the same mucin molecule. Based on their lectin-binding ability, they can be used for purification of lectins, and lectins coupled to resin may be useful for the isolation of mucin-type glycoproteins. The epithelial mucous secretions modulate oral microbial flora; many secretory components serve as lectin-receptors for the attachment of microbes. The judicious use of lectins with widely differing binding characteristics has already been valuable in the in situ localization of salivary glycoproteins, in elucidating structural details, recording sugar density within a given tissue section, and defining host-parasite interactions. It is hoped that their use, together with monoclonal antibody (158) and tissue culture techniques (159, 160) will further clarify the roles of individual secretory mucous glycoproteins in health and disease.

Deglycosylation studies on tracheal mucin glycoproteins

Following several model experiments, conditions were developed for optimal deglycosylation of tracheal mucin glycoproteins. Exposure of rigorously dried material to trifluoromethanesulfonic acid at 0 degree C for up to 8 h results in cleavage of essentially all fucose, galactose, and N-acetylglucosamine, about 80% of the N-acetylneuraminic acid (NeuNAc), and a variable amount of N-acetylgalactosamine (GalNAc), the sugar involved in linkage to protein. Residual N-acetylneuraminic acid is sialidase susceptible and apparently in disaccharide units, presumably NeuNAc2----GalNAc. The remaining N-acetylgalactosamine is mostly present as monosaccharides, and a few Gal beta 1----3GalNAc alpha units are also present; both are cleaved by appropriate enzymatic treatment. The saccharide-free proteins obtained from either human or canine mucin glycoproteins have molecular weights of about 100,000 and require chaotropic agents or detergents for effective solubilization.