The role of disulfide bonds in maintaining the gel structure of bronchial mucus

Gastropod Slime-Based Gel as an Adjustable Synthetic Model for Human Airway Mucus

Airway mucus works as a protective barrier in the human body, as it entraps pathogens that will be later cleared from the airways by ciliary transport or by coughing, thus featuring the rheological properties of a highly stretchable gel. Nonetheless, the study of these physical barrier as well as transport properties remains limited due to the restricted and invasive access to lungs and bronchi to retrieve mucus and to the poor repeatability inherent to native mucus samples. To overcome these limits, we report on a biobased synthetic mucus prepared from snail slime and multibranched thiol cross-linker, which are able to establish disulfide bonds, in analogy with the disulfide bonding of mucins, and therefore build viscoelastoplastic hydrogels. The gel macroscopic properties are tuned by modifying the cross-linker and slime concentrations and can quantitatively match those of native sputum from donors with cystic fibrosis (CF) or non-cystic fibrosis bronchiectasis (NCFB) both in the small- and large-deformation regimes. Heterogeneous regimes were locally found in the mucus model by passive microrheology, in which both diffusive and non-diffusive motion are present, similar to what is observed in sputa. The biobased synthetic approach proposed in the present study thus allows to produce, with commercially available components, a promising model to native respiratory mucus regarding both mechanical and, to a lesser extent, physicochemical aspects.

Sputum Processing Method for Lateral Flow Immunochromatographic Assays to Detect Coronaviruses

Coronavirus causes an infectious disease in various species and crosses the species barriers leading to the outbreak of zoonotic diseases. Due to the respiratory diseases are mainly caused in humans and viruses are replicated and excreted through the respiratory tract, the nasal fluid and sputum are mainly used for diagnosis. Early diagnosis of coronavirus plays an important role in preventing its spread and is essential for quarantine policies. For rapid decision and prompt triage of infected host, the immunochromatographic assay (ICA) has been widely used for point of care testing. However, when the ICA is applied to an expectorated sputum in which antigens are present, the viscosity of sputum interferes with the migration of the antigens on the test strip. To overcome this limitation, it is necessary to use a mucolytic agent without affecting the antigens. In this study, we combined known mucolytic agents to lower the viscosity of sputum and applied that to alpha and beta coronavirus, porcine epidemic diarrhea virus (PEDV) and Middle East respiratory syndrome coronavirus (MERS-CoV), respectively, spiked in sputum to find optimal pretreatment conditions. The pretreatment method using tris(2-carboxyethyl)phosphine (TCEP) and BSA was suitable for ICA diagnosis of sputum samples spiked with PEDV and MERS-CoV. This sensitive assay for the detection of coronavirus in sputum provides an useful information for the diagnosis of pathogen in low respiratory tract.

Viruses and the origin of microbiome selection and immunity

The last common metazoan ancestor (LCMA) emerged over half a billion years ago. These complex metazoans provided newly available niche space for viruses and microbes. Modern day contemporaries, such as cnidarians, suggest that the LCMA consisted of two cell layers: a basal endoderm and a mucus-secreting ectoderm, which formed a surface mucus layer (SML). Here we propose a model for the origin of metazoan immunity based on external and internal microbial selection mechanisms. In this model, the SML concentrated bacteria and their associated viruses (phage) through physical dynamics (that is, the slower flow fields near a diffusive boundary layer), which selected for mucin-binding capabilities. The concentration of phage within the SML provided the LCMA with an external microbial selective described by the bacteriophage adherence to mucus (BAM) model. In the BAM model, phage adhere to mucus protecting the metazoan host against invading, potentially pathogenic bacteria. The same fluid dynamics that concentrated phage and bacteria in the SML also concentrated eukaryotic viruses. As eukaryotic viruses competed for host intracellular niche space, those viruses that provided the LCMA with immune protection were maintained. If a resident virus became pathogenic or if a non-beneficial infection occurred, we propose that tumor necrosis factor (TNF)-mediated programmed cell death, as well as other apoptosis mechanisms, were utilized to remove virally infected cells. The ubiquity of the mucosal environment across metazoan phyla suggest that both BAM and TNF-induced apoptosis emerged during the Precambrian era and continue to drive the evolution of metazoan immunity.

A biophysical basis for mucus solids concentration as a candidate biomarker for airways disease

In human airways diseases, including cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), host defense is compromised and airways inflammation and infection often result. Mucus clearance and trapping of inhaled pathogens constitute key elements of host defense. Clearance rates are governed by mucus viscous and elastic moduli at physiological driving frequencies, whereas transport of trapped pathogens in mucus layers is governed by diffusivity. There is a clear need for simple and effective clinical biomarkers of airways disease that correlate with these properties. We tested the hypothesis that mucus solids concentration, indexed as weight percent solids (wt%), is such a biomarker. Passive microbead rheology was employed to determine both diffusive and viscoelastic properties of mucus harvested from human bronchial epithelial (HBE) cultures. Guided by sputum from healthy (1.5–2.5 wt%) and diseased (COPD, CF; 5 wt%) subjects, mucus samples were generated in vitro to mimic in vivo physiology, including intermediate range wt% to represent disease progression. Analyses of microbead datasets showed mucus diffusive properties and viscoelastic moduli scale robustly with wt%. Importantly, prominent changes in both biophysical properties arose at ∼4 wt%, consistent with a gel transition (from a more viscous-dominated solution to a more elastic-dominated gel). These findings have significant implications for: (1) penetration of cilia into the mucus layer and effectiveness of mucus transport; and (2) diffusion vs. immobilization of micro-scale particles relevant to mucus barrier properties. These data provide compelling evidence for mucus solids concentration as a baseline clinical biomarker of mucus barrier and clearance functions.

Structure and function of mucus

A glycoprotein building block is common to mammalian mucins. This structure is composed of several protein chains having the same sequence. The carbohydrate side chains, which constitute over three-quarters of the weight, coat only some two-thirds of the backbone chain. The bare protein chains are linked by disulphide bridges and can be digested away with trypsin. Either procedure rapidly solubilizes mucus and results in a structural unit of about 500 000 molecular weight. Mucus solubilizes spontaneously. The first size unit which reaches solution is about 15 X 10(6) molecular weight but continues to break down further. Mechanical agitation considerably speeds up this process. The gel-like character which is an essential feature of mucus--which cannot otherwise act as transport coupler--is thus a transient phenomenon. The problem of how such a structure can arise from the building blocks known to be available is discussed.

Chemical aspects of tracheal glycoproteins

The chemical characteristics of tracheal mucus obtained directly from the epithelial surface of the trachea indicate that the mucus from each animal source consists of a group of sulphated sialic acid-containing glycoproteins. Fractionation of the native glycoprotein from the cat by gel chromatography in the presence of urea and dithiothreitol suggests a value of about 3 X 10(6) for the molecular weights. The chief monosaccharide constituents are N-acetylneuraminic acid, N-acetylglucosamine, N-acetylgalactosamine, fucose and galactose. In the goose tracheal mucin, mannose is present (serum proteins being absent). Doubly labelled cat mucus, obtained by giving Na235SO4 and [3H]glucose simultaneously into the lumen of the trachea, is massively released by parasympathetic agents, e.g. pilocarpine. The resulting mucus has a high content of 35S and is derived largely from submucosal gland cells. Subsequent exposure to an irritant, ammonia, releases a low sulphation fraction, highly labelled with 3H, arising from goblet cells. Evidence supports the view that the overall mucus is composed of mixed secretions, chemically distinct, from different cellular synthesizing sites. Differential nervous stimulation of the various sites may cause far-reaching changes in the chemical and physical properties of the mucus by selective action on the secretion of one or more of the contributing glycoproteins.

Site of synthesis, intracellular transport and secretion of glycoprotein in exocrine cells

The site of attachment of the first sugar, N-acetylgalactosamine, to the seryl and threonyl residues of the protein chain is unknown in exocrine cells. The subsequent sugars of the carbohydrate side chains, galactose and N-acetylglucosamine alternately, and the end-group sugars, galactose, N-acetylgalactosamine and fucose, are attached in the Golgi complex. Sulphate too is attached in that structure. In the stomach, sulphate is probably transferred in the most mature cisterna of the Golgi stacks, galactose and fucose in other cisternae, suggesting a gradient in transferase activities along the stack. The possibilities of regulating the amount and relative sugar composition of the glycoproteins are discussed. The secretory product is stored in granules. Their polygonal, large and swollen appearance and complex formation by loss of bordering membranes, as observed in many kinds of glycoprotein-secreting cells ('mucous cells') might be caused by ineffective fixation techniques. Direct vascular perfusion results in a picture no different from what is found in non-mucous cells. Whether secretion is merely exocytotic, as in non-mucous cells, or whether it is accompanied by a loss of membrane and even cytoplasm needs thorough investigation, with the effects of various fixation techniques being compared.

Animal models in clinical disease

Hypersecretion of mucus is a feature of several clinical diseases and in some is associated with mucous gland hypertrophy and goblet cell increase. In a variety of species these changes have been produced by irritants, by infection, or by administration of drugs. While the end result may appear the same, differences emerge in the type and amount of glycoprotein secreted and in the amount retained within the cell. Organ culture can be used to ascertain functional activity. The use of animal models has not only established that these environmental changes cause hypertrophy but indicated some of the intracellular events associated with their development. When new types of granules appear within a cell, they appear first at the apex: a flow chart is offered of the way of which individual cells change and hence the population. The nature of glycoprotein elaborated by a secretory cell can change within hours. A Clara or serous cell can develop into a mucus-secreting or goblet cell. The pattern of reversibility is emerging.

Pharmacological approaches to discovery and development of new mucolytic agents

Airway mucus is the secretory product of the mucous cells; it is a variable mixture of water, mucous glycoproteins, low molecular weight ions, proteins, and lipids, whose physical properties are important for airway defense. The factors that contribute to the physical properties of mucus are complex, and there are a number of pharmacological strategies that can potentially serve to improve the clearability of airway mucus. Novel mucoactive approaches include strategies for mucoregulation--decreasing the abnormal volume of mucus secretion--and medications designed to improve the cough clearability of airway secretions. In vitro results suggest potential benefits from the additive effects of selected combinations of mucoactive medications. Further studies are required to confirm these findings, to perform direct assessments of mucus clearability, and to extend the observations to patients with various types of pulmonary diseases where mucoactive treatments are required.

Lectin cytochemical characterization of the N- and O-linked oligosaccharides in the human rectum

The oligosaccharides of the mucus glycoproteins of the human rectum are important for the lubricant and protective role suggested for the rectal mucus. Changes in oligosaccharide composition are observed in several colon diseases, and some of these changes could be used as diagnostic and prognostic indicators. Thus, a previous knowledge of the normal mucus glycoproteins is necessary. The aim of the present study is the characterization of the oligosaccharides of the goblet cells and enterocytes of the human rectum. For this, a battery of 15 lectins, in combination with chemical and enzymatic deglycosylation procedures, was used. Our results suggest the presence of N-acetylglucosamine (GlcNAc), Man, Glc, N-acetylneuraminic acid (Neu5Ac)(alpha2-6)- and Neu5Ac(alpha2-3)-linked, N-acetylgalactosamine (GalNAc) and Gal(beta1-3)GalNAc in the oligosaccharides of the goblet cells. Moreover, N-linked oligosaccharides specifically contained Gal(beta1-4)GlcNAc, while AAA-positive Fuc was only detected in O-linked oligosaccharides. Some of these carbohydrates were only visualized after removal of N- or O-linked oligosaccharides, suggesting a high level of approximation between the oligosaccharide chains, that render the carbohydrate inaccessible to the lectins. Differences in the labelling pattern between the goblet cells of the surface epithelium and the upper half of the crypts, and those of the lower half of the crypts suggests a maturation process for the goblet cells, which modifies the oligosaccharide composition of the secreted glycoproteins, as they ascend throughout the crypts. This maturation process includes the incorporation of new carbohydrates (GlcNAc), and the masking (Neu5Ac(alpha2-3)-linked) or unmasking (Glc and GalNAc) of others.

Physiological basis of cystic fibrosis: a historical perspective

Physiological Basis of Cystic Fibrosis: A Historical Perspective. Physiol. Rev. 79, Suppl.: S3-S22, 1999. - Cystic fibrosis made a relatively late entry into medical physiology, although references to conditions probably reflecting the disease can be traced back well into the Middle Ages. This review begins with the origins of recognition of the symptoms of this genetic disease and proceeds to briefly review the early period of basic research into its cause. It then presents the two apparently distinct faces of cystic fibrosis: 1) as that of a mucus abnormality and 2) as that of defects in electrolyte transport. It considers principal findings of the organ and cell pathophysiology as well as some of the apparent conflicts and enigmas still current in understanding the disease process. It is written from the perspective of the author, whose career spans back to much of the initial endeavors to explain this fatal mutation.

Biosynthesis of frog skin mucins: cysteine-rich shuffled modules, polydispersities and genetic polymorphism

1. Frog integumentary mucins (FIM-A.1, FIM-B.1 and FIM-C.1) consist of typical threonine-rich highly O-glycosylated (semi)repetitive domains, and cysteine-rich modules, i.e. the P-domain, the short consensus repeat and a region with high similarity to the C-terminal end of von Willebrand factor (designated here CC29-motif). 2. These modules are thought to be involved in protein-protein interactions and they have been observed in a variety of extracellular proteins. In FIMs, these modules may be involved in oligomerization processes leading to an entangled mucin network. 3. Polydispersities have been detected in FIM-B.1 and FIM-C.1 within single individuals. Multiple transcripts are probably generated by alternative splicing of a huge array of different (semi)repetitive cassettes encoding the threonine-rich domains. 4. Furthermore, genetic polymorphism is observed between different individuals, probably due to allelic variations in the number of (semi)repetitive cassettes.

Mucin-like glycoproteins in the equine embryonic capsule

The equine embryonic capsule replaces the zona pellucida and envelopes the conceptus during the second and third weeks of pregnancy. Although this capsule was described more than 100 years ago, its molecular structure has not been characterized. Here we present evidence that the glycoprotein(s) of the equine capsule resembles those of the mucin glycoprotein family. The resistance of the capsule to chemical and enzymatic solubilization was confirmed, and, as in mucins, protein constituted only 35-40% of its total dry mass. Determination of the sugar composition of the capsule using colorimetric assays and high-performance anion-exchange chromatography also showed it to have mucin-like characteristics. Gal, GalNAc, sulfated sugars, and sialic acid make up a high proportion of the capsular carbohydrate, while GlcNAc, Glc, and Man are minor components. These findings were verified using lectin histochemical staining of frozen sections of conceptuses. The results of amino acid analysis were also consistent with the proposal that the capsular glycoproteins belong to the mucin family. Removal of the covalently bound carbohydrate by beta-elimination under reducing conditions demonstrated that the capsule is O-glycosylated mainly on threonine residues. Affinity chromatography on jacalin-agarose confirmed that, like mucins, the capsular glycoproteins are heavily O-glycosylated. SDS-PAGE analysis revealed a prominent 21-kDa band, specific to the capsule, in preparations solubilized by trypsin but not by other proteases. Characterization of its constituent glycoprotein(s) should be helpful in elucidating the role of the capsule (and analogous blastocyst coverings in other species) during early pregnancy.

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.

Intestinal mucin of germ-free rats. Biochemical and electron-microscopic characterization

Purified germ-free rat intestinal mucin was found by chemical analysis to contain 25% protein, enriched in serine, threonine, and proline, 75% carbohydrate, and no nucleic acid. It was analyzed by darkfield electron microscopy and found to consist of long filamentous molecules with a maximum length of approximately 740 nm, a mean length of 456 nm, and a mean width of 7 nm. Given reasonable assumptions derived from earlier work on other well-characterized mucins, the molecular weight of the peptide, calculated by the length from electron microscopy, was 200,000, and, given the chemical composition, the molecular weight of the entire mucin molecule was calculated to be approximately 800,000.

Mucin exocytosis

Mucins produced by goblet cells of the respiratory mucosa are condensed while stored in secretory granules. Mucin condensation and its decondensation upon exocytosis can be explained by the theory of polymer gel phase transition. After the opening of a secretory pore, Ca2+ inside the granule is exchanged for extracellular Na+. Na/Ca exchange triggers a polymer gel phase transition whereby the mucin polymer matrix undergoes massive swelling and thereby changes from a condensed to a hydrated phase. Swelling of the granular content is driven by a Donnan potential and results in the release of secretory product and the formation of small mucin gels, which later anneal to each other to form the respiratory mucus. Because of the tangled rather than cross-linked topology of the mucin network, the rheologic properties of the respiratory mucus depend primarily on hydration. As mucins are polyionic, the hydration of mucus is controlled by a Donnan equilibrium. Hence, mucus hydration and rheology are determined by two factors: the quantity, chain length, and charge density of the secreted mucins, and the amount and the ionic and polyionic composition of the water transported across the respiratory mucosa.

Subunit structure of deglycosylated human and swine trachea and Cowper's gland mucin glycoproteins

The oligosaccharide chains in human and swine trachea and Cowper's gland mucin glycoproteins were completely removed in order to examine the subunit structure and properties of the polypeptide chains of these glycoproteins. The carbohydrate, which constitutes more than 70% of these glycoproteins, was removed by two treatments with trifluoromethanesulfonic acid for 3 h at 3 degrees and periodate oxidation by a modified Smith degradation. All of the sialic acid, fucose, galactose, N-acetylglucosamine and N-acetylgalactosamine present in these glycoproteins was removed by these procedures. The deglycosylated polypeptide chains were purified and characterized. The size of the monomeric forms of all three polypeptide chains were very similar. Data obtained by gel filtration, release of amino acids during hydrolysis with carboxypeptidase B and gel electrophoresis in the presence of 0.1% dodecyl sulfate showed that a major fraction from each of the three mucin glycoproteins had a molecular size of about 67 kDa. All of the deglycosylated chains had a tendency to aggregate. Digestion with carboxypeptidases showed that human and swine trachea mucin glycoproteins had identical carboxyl terminal sequences, -Val-Ala-Phe-Tyr-Leu-Lys-Arg-COOH. Cowper's gland mucin glycoprotein had a similar carboxyl terminal sequence, -Val-Ala-Tyr-Leu-Phe-Arg-Arg-COOH. The yield of amino acids after long periods of hydrolysis with carboxypeptidases showed that at least 85% of the polypeptide chains in each of the deglycosylated preparations have these sequences. These results suggested that the polypeptide chains in these deglycosylated mucin glycoprotein preparations were relatively homogeneous. The deglycosylated polypeptide chains as well as the intact mucin glycoproteins had blocked amino terminii. The purified polypeptide chains were digested with trypsin-TCPK, and S. aureus V8 protease and the resulting peptides were isolated by gel electrophoresis in the presence of 0.1% dodecyl sulfate and by HPLC. Two partial amino acid sequences from swine trachea mucin glycoprotein, two partial sequences from human trachea mucin glycoprotein and three partial sequences from Cowper's gland mucin glycoprotein were determined. The partial amino acid sequences of the peptides isolated from swine trachea mucin glycoprotein showed more than 70% sequence homology to a repeating sequence present in porcine submaxillary mucin glycoprotein. Five to eight immunoprecipitable bands with sizes ranging from about 40 kDa to 46 kDa were seen when the polypeptide chains were digested with S. aureus V8 protease. All of the bands had blocked amino terminii and differed by a constant molecular weight of about 1.5 kDa.(ABSTRACT TRUNCATED AT 400 WORDS)

Neutral and acidic human tracheobronchial mucin. Isolation and characterization of core protein

Human bronchial mucin from a patient suffering from chronic bronchitis was solubilized in aqueous solution containing sodium azide and protease inhibitors and purified by Sepharose 4B and 2B column chromatography. The mucin was further purified by cesium bromide density gradient centrifugation. Sodium dodecyl sulfate-polyacrylamide gel (7.5%) electrophoresis of this material showed high-molecular-weight mucin component(s) at the top of the gel. Chemical analysis of this preparation indicated a typical mucin profile of amino acids and carbohydrates. Ion-exchange chromatography resulted in resolution of the purified mucin into neutral and acidic fractions. Comparison of the chemical composition of these two fractions showed higher mole percentage of threonine, serine, sialic acid, and sulfate in the acidic fraction. Chemical deglycosylation of the purified mucin preparation with trifluoromethane sulfonic acid was carried out at 20 degrees C for 3 1/2 h. Sialic acid, fucose, galactose, and N-acetylglucosamine were completely removed, whereas traces of N-acetylgalactosamine were still detected. High-pressure liquid chromatography of the deglycosylated products from native, neutral, and acidic mucin preparations resulted in a principal peptide, P1, with identical amino acid composition. Cyanogen bromide (CNBr) treatment of the peptide P1 from neutral and acidic mucins and subsequent fractionation of the fragments by high-pressure liquid chromatography resulted in similar peptide profiles. The P1 peptide fraction was further subjected to high-pressure liquid chromatography in a second solvent system, which resulted in two peaks, P1a and P1b. Gel filtration of both peptides in 6 M guanidine hydrochloride indicated a single peak with molecular weight of approximately 97 kDa. The amino acid profile of the two peptides was dominated by high levels of threonine, serine, and proline, which combined accounted for nearly 39% of the total residues, and in most respects, the profile resembled that of native mucin. End-group analysis of the peptide P1a indicated a blocked N-terminus, whereas serine was found to be the N-terminal amino acid in the peptide P1b. Rabbit antibodies prepared against the peptide P1 from native tracheal mucin reacted strongly with neutral and acidic mucin as well as the mucin from human colon. Both neutral and acidic human tracheal mucins were immunologically reactive with mouse monoclonal antibody HMPFG-2, which was prepared against human mammary mucin. However, the response of this antibody to human colonic mucin was rather weak.

Deglycosylation of neutral and acidic human colonic mucin

Human colonic mucin has been isolated from normal colonic mucosa by a phenol-water extraction procedure and purified by Sepharose 2B column chromatography. The mucin was further purified by cesium bromide density gradient centrifugation. Sodium dodecyl sulfate-polyacrylamide gel (5%) electrophoresis of this material showed high-molecular-weight mucin component(s) at the top of the gel. Chemical analyses of this preparation indicated a typical mucin profile of amino acids and carbohydrates. Ion-exchange chromatography resulted in the separation of two major fractions, one being more acidic than the other. Chemical deglycosylation of the purified preparation at 20 degrees C for 3 1/2 showed loss of sialic acid, fucose, galactose, and N-acetylglucosamine, whereas traces of N-acetylgalactosamine were still detected. High-pressure liquid chromatography of the deglycosylated material resulted in the purification of a major peptide, P1, with high levels of threonine, serine, and proline, resembling, in most respects, the profile of native mucin. The molecular weight of the peptide was determined to be approximately 97 kDa and serine was the single NH2 terminus.

Effects of orally administered drugs on dynamic viscoelasticity of human nasal mucus

The effects of orally administered drugs on rheologic properties of nasal mucus were investigated in adult chronic sinusitis patients. The elastic modulus G' and the dynamic viscosity eta' of nasal mucus were determined by an oscillating sphere magnetic rheometer. Both G' and eta' values of the mucus before drug administration were much higher than optimal viscoelasticity for mucociliary transport. Norfloxacin, an antibacterial agent, reduced the G' but not the eta' of nasal mucus. Serratiopeptidase, a proteolytic enzyme, reduced eta' but did not reduce G'. S-carboxymethylcysteine, a blocked thiol derivative of cysteine, did not change either G' or eta'. L-cysteine ethyl ester hydrochloride, a sulfhydryl type of agent, reduced both G' and eta'. The results indicate that some of the orally administered mucokinetic agents can improve the abnormal rheologic properties of nasal mucus in chronic sinusitis.

Evidence for the in vivo degradation of human respiratory mucins during Pseudomonas aeruginosa infection

The comparison of distribution of glycopeptides of sputa from patients suffering from various chronic hypersecretions has already shown an increased acidity with a decreased proportion of neutral glycopeptides in the respiratory secretions of patients suffering from cystic fibrosis, as compared to those of patients with chronic bronchitis. In order to find out whether this decrease is specific to cystic fibrosis mucins or whether it is due to a degradation of mucus by Pseudomonas aeruginosa, which infects most of the sputa from patients with this disease, mucus glycopeptides from patients with different chronic bronchial disorders, infected by Pseudomonas or not, were prepared and fractionated by ion-exchange chromatography. The neutral fraction, which has never been studied in detail, was gel-filtered, and provided two fractions, one containing true mucin glycopeptides and the other containing a mixture of peptides and glycopeptides with a lower molecular mass. In the Pseudomonas-infected samples, the true mucin glycopeptide fraction was greatly diminished as compared to this same fraction in non-Pseudomonas-infected samples; this was not specific to cystic fibrosis secretions. In contrast, the glycopeptide fraction with a lower molecular mass was greatly increased in all the Pseudomonas-infected samples. Polyacrylamide gel electrophoresis of this second fraction showed unique glycopeptide bands between 40-50 kDa in the Pseudomonas-infected samples, regardless of the origin of the samples. These bands were revealed by an antibody directed against whole cystic fibrosis mucin. Infected chronic bronchitis sputa and cystic fibrosis samples without P. aeruginosa did not show these bands. These studies therefore suggest that there are P. aeruginosa-associated changes in mucins which may result from degradation of mucins.

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.

Protein components of human tracheobronchial mucin: partial characterization of a closely associated 65-kilodalton protein

A high-density mucin glycoprotein was isolated from human tracheobronchial secretions substantially free of contaminating protein, low-density glycoprotein, proteolytic enzymes, and lipid. A closely associated 65-kDa protein was discovered while investigating the effect of 2-mercaptoethanol treatment on the purified mucin glycoprotein. It has been established that the 65-kDa protein is neither alpha 1-antichymotrypsin nor human serum albumin, two proteins of similar molecular weight which are found in crude tracheobronchial secretions. This protein lacks cross-reactivity with antibodies directed against serum components and is presumably comparable to the 65-kDa protein similarly isolated from canine tracheal pouch secretions [Ringler et al. (1987) Biochemistry 26, 5322-5328]. Although both the presence of sulfhydryl groups and the ability to be reassociated with the mucin molecule have been established, it is not clear whether its association is due to direct disulfide bonding, hydrophobicity, or entrapment. It was found that 14C-methylated methemoglobin was an inappropriate substrate for measurement of proteolytic activity in mucin preparations due to inherent entrapment and clearance capabilities of mucin molecules.

Fibronectin: source of mannose in a highly purified respiratory mucin

Human bronchial mucin, solubilized in an aqueous solution of sodium azide and protease inhibitors, was purified by molecular sieve chromatography. The mucin was purified by Sepharose 4B and 2B column chromatography. Chemical analyses of this preparation showed a typical mucin profile of amino acids and carbohydrates, except for the presence of an appreciable amount of mannose. Sodium dodecyl sulfate-polyacrylamide gel (5%) electrophoresis of this material showed a high Mr glycoprotein at the top of the gel and two additional bands with mobilities of fibronectin subunits (230 and 210 kD). The fibronectin was separated from the mucin by geletin-Sepharose column chromatography, and the fibronectin eluted from the column was immunologically similar to fibronectin purified from human serum. Ion-exchange chromatography of purified mucin resulted in neutral and acidic fractions. The neutral mucin was the major component. Chemical composition of these two fractions indicated that the amount of threonine, serine, and sialic acid was higher in the acidic fraction, whereas the neutral fraction contained more proline, aspartic acid, leucine, glycine, fucose, and galactose than the acidic fraction.

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.

Characterization of human middle ear mucus glycoprotein in chronic secretory otitis media (CSOM)

Middle ear effusion was obtained from children with chronic secretory otitis media undergoing myringotomy. The effusions contained about 120 mg/ml non-dialysable solids, of which 18-31% was mucus glycoprotein. The purified mucus glycoprotein had a composition characteristic of other mucus glycoproteins. Amino acid analysis of the glycoprotein indicates a protein core consisting of glycosylated regions resistant to proteolysis and non-glycosylated regions susceptible to proteolysis. Analysis of the mucus glycoprotein by gel filtration on Sepharose 2B showed that reduction caused a decrease in hydrodynamic size and proteolysis caused a further decrease. The difference was confirmed by sedimentation coefficient and viscosity measurements. The reduced glycoprotein had an intrinsic viscosity of 0.113 ml/mg and an S0(20) of 15.2S compared to a value of 0.018 ml/mg and 9.6S for the proteolytically digested glycoprotein. These results suggest a model for this middle ear mucus glycoprotein, in which the native glycoprotein is a large molecular mass polymer maintained by disulphide bridges. These disulphide linked glycoprotein units are broken down into smaller units by proteolysis. The mucus glycoprotein could not be purified completely free from low molecular mass components. A glycoprotein, susceptible to proteolysis Mr 28,000-33,000 co-fractionates with the major high molecular mass mucus glycoprotein.

Peptides of human bronchial mucus glycoproteins. Size determination by electron microscopy and by biosynthetic experiments

Secreted human bronchial mucins, directly collected from macroscopically healthy bronchial mucosa, were prepared in the presence of six proteinase inhibitors, and analysed by electron microscopy. These mucins were similar in length distribution to molecules prepared from sputum [Slayter, Lamblin, Le Treut, Galabert, Houdret, Degand & Roussel (1984) Eur. J. Biochem. 142, 209-218], although they were a little longer, their lengths ranging up to about 1,650 nm. This length corresponds to an extended mucin peptide of about 450 kDa. In order to compare these peptide lengths with the molecular size of biosynthetic precursors, an antiserum raised against trifluoromethanesulphonic acid-treated highly glycosylated regions of human bronchial mucins was used to isolate mucin precursors synthesized in explants of human bronchial mucosa during pulse-labelling with [3H]threonine or [3H]glucosamine. A main precursor labelled with [3H]threonine and with an apparent molecular mass of about 400 kDa was detected by fluorography following SDS/polyacrylamide-gel electrophoresis. This band was observed as early as 20 min; it was more intense after a 40 min chase and had disappeared after a chase period of 280 min in unlabelled medium, presumably owing to glycosylation. Much fainter bands at about 200 kDa and between 200 and 400 kDa, also labelled with [3H]threonine, were observed mainly after a 40 min chase and had disappeared after a 280 min chase. None of these bands was labelled with [3H]glucosamine, nor did they disappear after multiple treatments with immobilized lectins. After a 280 min chase, [3H]threonine-labelled material appeared in the stacking gel, which also contained [3H]glucosamine label. The results indicate that the 200-400 kDa species are mucin precursors, whose size is comparable with that obtained by electron microscopy for respiratory mucins collected directly from the macroscopically healthy bronchial mucosa.

Structure of canine tracheobronchial mucin glycoprotein

Canine tracheal mucin glycoprotein was isolated from beagle dogs fitted with tracheal pouches. Following exclusion chromatography on Sepharose CL-4B, noncovalently associated proteins were further resolved by dissociative density gradient centrifugation in CsBr-guanidinium chloride, and the mucin was then extracted with chloroform-methanol. The delipidated high-density product obtained had a nominal molecular weight of about 10(6) and an overall composition characteristic for a mucin glycoprotein, viz., a high content of serine and threonine, about 80% carbohydrate by weight, the absence of mannose or uronic acid, measurable ester sulfate, and a Pronase-resistant domain of molecular weight (1.75-3.0) X 10(5) which contains essentially all of the saccharide residues. Noncovalently bound lipid amounted to 6-10% by weight and was primarily cholesterol and cholesteryl esters. Cleavage of disulfide bonds by performic acid oxidation resulted in the release of a protein (Mr 65,000) not otherwise resolved by sodium dodecyl sulfate gel electrophoresis or the purification scheme.

Action of trifluoromethanesulfonic acid on highly glycosylated regions of human bronchial mucins

Highly glycosylated glycopeptides were prepared from human bronchial mucus. They were heterogeneous and contained an average of 45 residues of glycosylated hydroxyamino acid per 100 amino acid residues. The kinetics of deglycosylation of these glycopeptides by trifluoromethanesulfonic acid-anisole mixtures at 25 degrees was monitored by chemical analysis and by polyacrylamide gel electrophoresis. The peripheral sugars were almost completely cleaved in 45 min with 3:2 and 2:1 CF3SO3H-anisole. A maximum of 75% of the O-linked N-acetylgalactosamine residues were released and mixtures of glycopeptides and peptides were obtained. Increasing the reaction time caused peptide bond cleavage. Rather mild conditions (1.2:1 CF3SO3H-anisole at 25 degrees for 90 min) gave limited deglycosylation of highly glycosylated bronchial glycopeptides, allowing the uncovering of GalNAc-peptide linkages and peptide regions able to induce the formation of specific antibodies in the rabbit.

Lateral nasal gland secretion in the anaesthetized dog

The effects of pharmacological and nervous stimuli on the flow of secretion from the dog lateral nasal gland following catheterization are described. Drugs were injected close-arterially into the arterial supply to the nose, or intravenously. Cholinergic agonists (pilocarpine, methacholine), given intravenously (I.V.) or intra-arterially (I.A.), and stimulation of the vidian nerve produced a copious flow of secretion which was blocked by atropine. The adrenoceptor agonists phenylephrine (alpha) and salbutamol (beta 2), given I.V. or I.A., and stimulation of the vagosympathetic nerve produced a small but consistent flow of secretion. Histamine (50 micrograms), substance P (0.1 micrograms) and prostaglandin E1 (1-5 micrograms), injected I.A., produced small flows of secretion. Bradykinin (25 ng-50 micrograms), 5-hydroxytryptamine (100 ng-50 micrograms) and vasoactive intestinal peptide (VIP) (10 ng-50 micrograms) did not cause secretion. The total protein content, the composition of secretions as revealed by sodium dodecyl sulphate-polyacrylamide agarose gel electrophoresis, and changes in [Na] and [K] in relation to flow of secretion are described. Differences in ion and protein concentrations, and in protein composition, are described for vidian nerve-induced and vagosympathetically induced secretions. Electron microscopy revealed that the gland contains serous cells in the secretory region, and ducts morphologically similar to the intercalated, striated and excretory ducts of salivary glands.

Benzo(a)pyrene hydroxylase activity in human bronchial mucus

Sputum collected from patients with respiratory diseases were examined for presence of benzo(a)pyrene hydroxylase (BPH) activity. The human bronchial mucus used in these studies had significant capability to metabolize benzo(a)pyrene. Clarification of the sputum by agents such as N-acetylcysteine or pancreatin in presence of antibiotics was found to be essential for the detection of BPH activity. In vitro incubation of the clarified human bronchial mucus with benzoflavone caused inhibition, while 7,8-dimethyl-benzanthracene induced BPH enzyme activity.

Tracheobronchial mucin receptor for Pseudomonas aeruginosa: predominance of amino sugars in binding sites

Pseudomonas aeruginosa, a common respiratory tract colonizer and pathogen, adheres to injured tracheal cells and to tracheobronchial mucin. These phenomena suggest that there are specific receptors for this organism in the respiratory tract. The receptor on injured tracheal cells contains n-acetylneuraminic acid as the principal sugar, but the structure of the receptor in mucin has not been described. Using a microtiter plate assay to study bacterial adherence to mucin, we have partially characterized the mucin receptor for P. aeruginosa. The receptor for both nonmucoid and mucoid strains is sensitive to periodate oxidation, suggesting that it is carbohydrate in nature, and the amino sugars n-acetylglucosamine and n-acetylneuraminic acid inhibited the adherence of both types of strains. Nonmucoid strains were more sensitive to inhibition by n-acetylneuraminic acid than to inhibition by n-acetylglucosamine, but the mucoid strains varied in their sensitivities to inhibition by each amino sugar. Preincubation of mucin with heat-inactivated influenza A virus (which binds to neuraminic acid) significantly reduced the adherence of P. aeruginosa. Treatment of mucin with Clostridium perfringens neuraminidase also reduced bacterial adherence significantly. Treatment of mucin with pronase did not affect adherence. Our results suggest that n-acetylglucosamine and n-acetylneuraminic acid are important constituents of the binding sites for P. aeruginosa on human tracheobronchial mucin.

Hydration kinetics of exocytosed mucins in cultured secretory cells of the rabbit trachea: a new model

Experiments have been done to test the idea that mucins undergo postexocytotic swelling. Previous work led to the hypothesis that the glycoprotein network of the mucus gel is probably held together by entanglements and low energy bonds, rather than by interchain covalent bonding. Since glycoproteins and other proteins in the mucus are polyions, it was further proposed that mucus must be capable of swelling, with its swelling properties depending on the pH and ionic strength of the medium hydrating the mucus. Experiments using oestrous cervical mucus from cows as a model confirmed this prediction. Observations on tissue cultures of respiratory secretory cells reported here show that freshly secreted mucins also undergo swelling during and after exocytosis. This evidence supports the hypothesis that the rheological properties of mucus may be physiologically regulated by hydration via control of the transepithelial movement of water, ions and soluble proteins, rather than by variations in the degree of covalent cross-linking between glycoprotein chains as proposed earlier.

Isolation and characterization of tracheobronchial mucin from a laryngectomee

A tracheobronchial mucin was isolated from the tracheobronchial secretion of a laryngectomee. It was purified by gel filtration on Sepharose CL-6B in Tris-urea buffer and rechromatography of excluded materials through the same gel matrix. It was homogeneous in 0.7% agarose-2% polyacrylamide electrophoresis under nonreducing conditions. Comparable analysis with 2-mercaptoethanol revealed at least 3 subunits. Based upon recoverable weight, the mucin was composed of 75% carbohydrate, 21% protein, and 3% sulfate. Oligosaccharides obtained by alkaline beta-elimination indicated O-glycosyl linkage to the peptide component. Marked heterogeneity of the carbohydrate side-chains was reflected in the preparation of 20 distinct oligosaccharides ranging in size from 4 to 17 residues.

Analysis of human nasal mucous glycoproteins

Human nasal turbinates were cultured in the presence of 3H-glucosamine, which is incorporated into nasal mucous glycoproteins. Nasal mucous glycoprotein was then characterized biochemically, and the effects of various neurohormones and immunologic stimulation on mucous glycoprotein release were analyzed. Fractionation of nasal mucous glycoprotein by gel filtration chromatography revealed a molecular size range of 2 to 200 X 10(5) (as judged by protein markers) but displayed a single, acidic charge, as reflected both in a narrow elution pattern from DEAE-cellulose and a sharp isoelectric focusing point of 2.6. Highly enriched nasal mucous glycoprotein preparations consisted of 80 per cent carbohydrate and 20 per cent protein (by weight) and included enzymatically cleavable carbohydrate side chains with molecular weights of 1,600 to 1,800. Thus, nasal mucous glycoproteins are a family of molecules that express uniform acidic charge characteristics and a wide range of molecular sizes. Cholinergic stimulation of atropine-inhibitable muscarinic receptors increased nasal mucous glycoprotein release in a dose-related manner, as did alpha-adrenergic stimulation. However, beta-adrenergic stimulation did not affect mucous glycoprotein release. Immunologic stimulation of nasal mast cells by either reversed anaphylaxis or antigen challenge after passive sensitization caused both histamine release and increased mucous glycoprotein release. Thus, nasal turbinates provide an accessible source of tissue for the analysis of nasal mucus secretion and mast cell degranulation and may provide a model for the study of pharmacologic approaches to the universally experienced discomfort of rhinorrhea.

Complex structure of human bronchial mucus glycoprotein

Human bronchial mucus glycoproteins or mucins were isolated from the sputum of two patients by a method avoiding reducing agents and involving water extraction and gel filtration on Sepharose CL-2B in 6 M guanidinium chloride. The chemical analysis indicated approximately 25-40% lipid. The amino acid and carbohydrate analysis differ quantitatively from that of mucins purified after prior reduction of mucus. These fractions also have a higher proportion of aspartic and glutamic acids than that of the mucins from reduced sputum. These mucins are still contaminated by small amounts of peptides but do not seem to contain disulfide-attached cross-linking protein. Human bronchial mucins have a strong tendency to form aggregates except in 6 M guanidinium chloride. Electron microscopy performed with various procedures indicates the presence of both micelles and flexible threads measuring 200-1000 nm. Delipidation removes most of the micellar forms. Thereafter mucins appear mainly as polydisperse flexible extended threads and also as aggregates. These features of bronchial mucins do not fit with the generally accepted idea of mucin subunits linked by disulfide bridges (unless they are linked end to end) and alternatively favour a model where mucin molecules behave like filaments that could easily aggregate according to the solvent system (mucin concentration, absence of dissociating conditions).

A lavage technique allowing repeated measurement of IgA antibody in mouse intestinal secretions

Mouse intestinal secretions can be readily obtained without harm to the mice by administering a lavage solution to them intragastrically followed by pilocarpine intraperitoneally. These secretions are rich in proteases but this enzyme activity can be blocked by addition of a mixture of inhibitors. Both total and specific IgA antibody could be measured in these secretions using ELISA techniques. The total IgA recovered was found to vary considerably, even in the same group of mice sampled on multiple occasions. Specific IgA anti-cholera toxin antibody was easily demonstrable in the intestinal secretions of mice fed cholera toxin but not of mice fed an irrelevant antigen. Expression of the specific IgA antibody per unit of total IgA recovered is desirable in order to correct for the variable recovery of IgA.

Human respiratory mucous glycoproteins

Biochemical characterization of human respiratory mucus has generally utilized expectorated specimens. In order to exclude extraneous contaminants in the analysis of airway glycoproteins, human airways were cultured and the mucous glycoprotein released into the supernatant analyzed. By incorporating 3H-labeled glucosamine or 14C-threonine into the media, the airways biosynthetically labeled the mucous glycoproteins (MGP), facilitating their analysis. The MGP chromatograph by gel filtration on Sepharose 2B in two fractions: one excluded from the column and one that enters the column. However, employing a gel filtration column with the ability to fractionate larger molecules, Sephacryl S-1000, it was found that MGP fractionate over a large range in molecular sizes and do not segregate into distinct fractions. The diffuse, broad peak of MGP fractionation on Sephacryl S-1000 is not affected by reduction and alkylation or by chromatography in 1 M NaCl. The fractionated MGP from Sepharose 2B were divided into larger and smaller molecular species, and their charge characteristics were determined by DEAE chromatography and preparative isoelectric focussing. MGP exhibit strong acidic charge characteristics that are uniform, as reflected in elution from DEAE and a single, sharp isoelectric focussing point. Enzymatic cleavage of the oligosaccharide side chains from MGP liberates more than 70% of the radiolabeled side chains. The side chains enzymatically cleaved from the larger and smaller molecular species of MGP are similar in size. Highly purified MGP were found to be 73% carbohydrate and 27% protein. Thus, human airways release a family of MGP that express marked heterogeneity in size but a uniform, strong acid charge and include side chains of similar size.

Polymeric structure of a high-molecular-weight glycoprotein from bovine cervical mucus

A 16 X 10(6)-Mr glycoprotein isolated from bovine oestrus cervical mucus when reduced under conditions where disulphide-bond cleavage is essentially quantitative produces chains whose Mr from light-scattering and from sedimentation and diffusion data is some 4 X 10(6)-5 X 10(6). Pronase digestion of the chains indicates that glycosylated sequences of Mr 0.3 X 10(6)-0.5 X 10(6) are interspersed with enzyme-susceptible non-glycosylated peptide sequences.