Isolation, chemical composition, and properties of the major mucin component of normal human tracheobronchial secretions

Surface rheological properties alter aerosol formation from mucus mimetic surfaces

The effects of surface tension and surface viscoelastic properties on the formation of aerosol droplets generated from mucus-like viscoelastic gels (mucus mimetics) during shearing with a high velocity air stream were investigated. Mucus mimetic samples were formulated with similar composition (94% water and 6% dissolved solids, consisting of mucins, proteins, and ions), surface tension (via the addition of surfactant to the mimetic surface) and bulk viscoelastic properties (via crosslinking of mucin macromolecules in the mimetic) to that of native non-diseased tracheal mucus. The surface tension of the mucus mimetic was decreased by spreading one of two surfactants, dipalmitoyl phosphatidylcholine (DPPC) or calf lung surfactant (Infasurf®), on the mimetic surface. Aerosols were generated from the mimetic surfaces during simulated coughing using an enhanced simulated cough machine (ESCM) operating under controlled environmental conditions. The size distribution of aerosol droplets generated during simulated coughing from the surfactant-coated mimetic surfaces was multimodal, while no droplets were generated from the bare mimetic surface due to its high surface viscoelastic properties and high surface tension. The concentration of aerosols generated from the DPPC-coated mimetic was higher than that of the Infasurf®-coated mimetic, even though the surface tension of the two interfaces was the same. The experimental results suggest that a balance of surface elastic behavior and surface viscous behavior is required for the generation of aerosols from the viscoelastic surfaces.

Synthetic tracheal mucus with native rheological and surface tension properties

In this study, the development of a model tracheal mucus with chemical composition and physical properties (bulk viscoelasticity and surface tension) matched to that of native tracheal mucus is described. The mucus mimetics (MMs) were formulated using components that are abundant in tracheal mucus (glycoproteins, proteins, lipids, ions, and water) at concentrations similar to those found natively. Pure solutions were unable to achieve the gel behavior observed with native mucus. The addition of a bifunctional cross-linking agent enabled control over the viscoelastic properties of the MMs by tailoring the concentration of the cross-linking agent and the duration of cross-linking. Three MM formulations with different bulk viscoelastic properties, all within the normal range for nondiseased tracheal mucus, were chosen for investigation of surfactant spreading at the air-mimetic interface. Surfactant spread quickly and completely on the least viscoelastic mimetic surface, enabling the surface tension of the mimetic to be lowered to match native tracheal mucus. However, surfactant spreading on the more viscoelastic mimetics was hindered, suggesting that the bulk properties of the mimetics dictate the range of surface properties that can be achieved.

Mucins in airway secretions from healthy and chronic bronchitic subjects

Little is known about whether the properties of respiratory mucins are altered as a result of airway irritation, but histochemical studies of respiratory tract secretory cells show a more 'acidic' staining pattern after exposure to tobacco smoke. Furthermore it has been suggested that proteoglycans are the major glycoconjugates in 'normal' respiratory secretions, whereas mucins predominate in sputum. To investigate these observations further, mucins from secretions collected from the tracheal surface of healthy non-smoking 'normal' subjects and sputum from patients with chronic bronchitis were compared. All samples contained one major mucin population after density-gradient centrifugation, and a small amount of 'denser' mucin was present in some chronic bronchitic and one of the 'normal' samples. Proteoglycans were not a major component of 'normal' secretions. The major mucin population from chronic bronchitic samples had molecular masses between 10 and 30 MDa and behaved as random coils in solution. Whole mucins from 'normal' individuals and chronic bronchitic patients were excluded from Sepharose CL-2B, whereas reduced subunits were included. Proteolysis of subunits yielded two populations of high-molecular-mass glycopeptides differing in size, suggesting the presence of two different tandem repeat regions in the mucins. Finally, mucins from patients with chronic bronchitis are less, rather than more, acidic than those from 'normal' individuals. Mucins from bronchitic sputum and 'normal' secretions are thus similar in their macromolecular properties, but differ slightly in charge density.

Translation of messenger RNA from canine tracheal epithelial cells: identification of mucin core protein

A high-molecular-weight mucin (Mr approximately 11.0 x 10(6)) was purified from canine tracheal pouch secretions. The mucin was deglycosylated by treatment with trifluoromethane sulfonic acid for 8 h at 8 degrees C and subsequently with alpha-N-acetylgalactosaminidase. These treatments almost completely removed the carbohydrate moieties. The amino acid compositions of the deglycosylated and native mucins were similar, indicating that the deglycosylation procedure used did not cause notable degradation of the protein core. Antiserum specific for deglycosylated canine tracheal mucin was produced by immunization of rabbit with the antigen. RNA was isolated from fresh canine tracheal epithelial cells by extraction with guanidine isothiocyanate/hydrochloride and further fractionated by chromatography on oligo(dT)-cellulose to yield poly(A)+ RNA. The poly(A)+ RNA was translated in a rabbit reticulocyte cell-free translation system using [35S]methionine and [3H]leucine as radiolabels. The translation products were analyzed by gel electrophoresis and fluorography before and after immunoprecipitation with the antiserum to deglycosylated mucin. A labeled product of molecular weight 72,000 was present in the immunoprecipitate. When canine liver poly(A)+ RNA was used as control, no radioactivity above background was detected in the immunoprecipitate. It is concluded that the primary translation product of the canine tracheal epithelial cells is a 72,000-D protein and the monomer subunit of the mucin is about 167,000 D. Thus, in the native state, the canine tracheal mucin consists of several associating subunits.

Bronchial mucus hypersecretion in acute quadriplegia. Macromolecular yields and glycoconjugate composition

In acute quadriplegia we have noted that about one in five patients develops unexplained production of markedly excessive and tenacious bronchial mucus. Spontaneous recovery from mucus hypersecretion usually occurs within weeks to months. Mucus samples collected from 12 patients have been found to be abnormal. Macromolecular contents of single aspirates yielded as much as 500 mg. Analytical ultracentrifuge analysis showed the mucus to contain considerable epithelial glycoprotein (GP) of typical buoyant density; its amino acid and carbohydrate compositions were characteristic of the GP from hypersecretory bronchial mucus such as in chronic bronchitis and cystic fibrosis. In five patients studied after recovery from hypersecretion, there tended to be relatively less GP. The mucus samples contained a high density glycoconjugate (GC): this had sugars of GP but also reacted positively with a monoclonal antibody to keratan sulfate. Its amino acid composition was different from that of GP: threonine was lower and glycine was higher than in GP. In mucus from one patient who died, chondroitin sulfate ABC and hyaluronic acid were identified as well. This suggests proteoglycans are involved in the pathophysiology of mucus hypersecretion. The sudden onset and spontaneous recovery of hypersecretion suggests that it is not due to gland hypertrophy. We speculate that in acute quadriplegia it is due to disturbed neuronal control of bronchial mucus gland secretion, perhaps related to initial disappearance and later reappearance of peripheral sympathetic nervous system tone.

The role of mucous glycoproteins in the rheologic properties of cystic fibrosis sputum

Cystic fibrosis (CF) is characterized by excessive amounts of thick and tenacious mucous secretions that obstruct organ ducts and passages. In the respiratory tract this is associated with chronic infection resulting in the hypersecretion of purulent sputum, which the patient finds difficult to clear. We have studied the rheologic properties of purulent sputum from six patients with CF and five patients with chronic bronchitis to assess whether CF is associated with increased sputum viscoelasticity. In addition, we have isolated the major rheologic determinants, mucous glycoproteins, from CF and chronic bronchitis sputa and, using a magnetic microrheometer, investigated the possibility that the altered properties of mucus in CF are associated with abnormalities in these glycoproteins. Creep compliance analysis indicated that the CF sputa possessed raised levels of both elasticity (p less than 0.01) and viscosity (p less than 0.01). These increases in both rheologic parameters were found to be associated with increases in the DNA content (p less than 0.01) and dry weight (p less than 0.05). Mucous glycoproteins were isolated from CF and chronic bronchitis sputum samples by gel filtration on Sepharose CL4B, followed by concentration to form 8% wt/wt gels. In the absence of other sputum components, no abnormality in the rheologic properties of CF mucin gels could be detected. However, when DNA was added, the CF gels responded with increases in both elasticity and viscosity of as much as 30% (p less than 0.05), an effect not observed in the chronic bronchitis gels. These results suggest that a subtle abnormality may exist in CF mucous glycoproteins and that this could have a role in the altered physical properties of mucous secretions in CF.

Physical properties of purified human respiratory mucus glycoproteins: effects of sodium chloride concentration on the aggregation properties and shape

The biophysical properties of purified native (nonreduced) mucus glycoproteins (mucins) isolated from lung mucus secretions of cystic fibrosis (CF) patients and subjects with normal lungs were studied using the technique of light scattering. The effects of different NaCl concentrations and 6 M guanidine hydrochloride on the molecular size of mucins, their ability to form aggregates, and their shape were investigated. Under the concentration range studied (0.05-3.5 mg/ml), in buffered 0.03 and 0.01 M NaCl, the CF mucins had higher molecular weights (12.2 x 10(6) to 17.1 x 10(6) and 9.5 x 10(6) to 10.4 x 10(6), respectively) than those observed in buffered 0.15 M NaCl (4.3 x 10(6) to 6.6 x 10(6]. These results were interpreted in terms of CF mucins self-aggregating in buffered 0.03 and 0.01 M NaCl. In contrast, in the both buffered 0.3 and 0.15 M NaCl, the normal respiratory mucins had molecular weights of 6.3 x 10(6) to 8.6 x 10(6), thus suggesting the absence of normal mucin aggregation in buffered 0.03 M NaCl. In the presence of 6 M guanidine HCl both CF and normal mucins had molecular weights of about 5 x 10(6) and showed more extended structure (i.e., larger radius of gyration) than in the presence of 0.03 or 0.15 M NaCl. Studies of the relationship of the light scattering intensity with scattering angle showed that, under the above experimental conditions studied, both CF and normal respiratory mucins were polydisperse flexible coil-shaped molecules. The increased aggregation of CF mucins observed at lower salt concentrations may alter the viscoelastic properties of CF lung mucus secretions.

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.

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.

Density gradient study of bronchial mucus aspirates from healthy volunteers (smokers and nonsmokers) and from patients with tracheostomy

Because it is difficult to obtain, little is known of bronchial mucus from the normal human airway; it has been mainly studied as sputum expectorated in chronic bronchitis with particular attention to epithelial glycoprotein. We have now applied density gradient methods to study this and other macromolecules and lipids in normal airway mucus. After lavage at bronchoscopy, mucus was aspirated from six normal volunteers, that include one light and two heavy smokers. This normal mucus has been compared with that obtained from four patients with tracheostomy because of respiratory muscle paralysis due to neurological disease. The normal aspirates contained small threads of mucus, the tracheostomy aspirates viscous blobs of jelly, a difference in physical appearance reflected in macromolecular yields, 0.3-1 mg/ml and 6-24 mg/ml respectively. On analytical ultracentrifugation normal mucus showed no discernible material in the buoyant density region typical of epithelial glycoprotein (1.5 g/ml): Virtually all the material migrated to the miniscus and was predominantly lipids and proteins. A trace amount of material recovered from a higher density region (greater than or equal to 1.6 g/ml) was found to contain both glycoprotein and proteoglycan. Aspirates from the heavy smokers contained appreciable amounts of material with typical buoyant density (approximately 1.5 g/ml) but still with features of proteoglycan. In contrast in tracheostomy aspirates epithelial glycoprotein of typical buoyant density and chemical composition accounted for up to 25% of nondialyzable material. We conclude that under normal conditions typical epithelial glycoprotein is virtually absent from airway mucus and that the glycoconjugate present has features of glycoprotein and proteoglycan.

Structural features of human tracheobronchial mucus glycoprotein

Electron microscopy of platinum-shadowed preparations of human tracheobronchial mucins showed very flexible filamentous structures that frequently occurred in an intricate random-coiled pattern of filament(s) surrounding a dense core-like domain. The filament(s) associated with cores accounted for 70-80% of the mass of the mucin preparation, the remainder being accounted for by free filaments. On aggregation, the molecules formed a large interwoven network quite different from the massive rope-like structures characteristic of sheep submaxillary mucin aggregates [Rose, Voter, Sage, Brown & Kaufman (1984) J. Biol. Chem. 259, 3167-3172]. Mild sonication resulted in extensive fragmentation of the tracheobronchial mucin molecules and yielded short filaments of various lengths, free cores and some cores associated with short filaments. Mucin glycopeptide fragments obtained by proteolytic digestion were flexible, core-free, filaments. The glycopeptides obtained by Pronase digestion were shorter than those obtained by tryptic digestion. The intricate structures of human tracheobronchial mucin differ markedly from the extended filaments reported for sheep submaxillary and human ovarian-cyst mucins but agree with the roughly spherical expanded model proposed for mucins by Creeth & Knight [(1967) Biochem. J. 105, 1135-1145] on the basis of hydrodynamic measurements.

Monoclonal antibodies as probes for unique antigens in secretory cells of mixed exocrine organs

In the past, it has been difficult to identify the secretory product and control mechanisms associated with individual cell types making up mixed exocrine organs. This report establishes the feasibility of using immunological methods to characterize both the biochemical constituents and regulatory mechanisms associated with secretory cells in the trachea. Monoclonal antibodies directed against components of tracheal mucus were produced by immunizing mice with dialyzed, desiccated secretions harvested from tracheal organ culture. An immunofluorescence assay revealed that of the total 337 hybridomas screened, 100 produced antibodies recognizing goblet cell granules; 64, gland cell granules; and 3, antigen confined to the ciliated apical surface of the epithelium. The tracheal goblet cell antibody described in this report was strongly cross-reactive with intestinal goblet cells, as well as with a subpopulation of submandibular gland cells, but not with cells of Brunner's glands or the ciliated cell apical membrane. The serous cell antibody was not cross-reactive with goblet, Brunner's gland, or submandibular cells, or the ciliated cell apical membrane. The antibody directed against the apical membrane of ciliated cells did not cross-react with gland or goblet cells or the apical membrane of epithelial cells in the duodenum. Monoclonal antibodies, therefore, represent probes by which products unique to specific cells or parts of cells in the trachea can be distinguished. The antibodies, when used in enzyme immunoassays, can be used to quantitatively monitor secretion by individual cell types under a variety of physiological and pathological conditions. They also provide the means for purification and characterization of cell-specific products by immunoaffinity chromatography.

In vitro labeling of the sialic acid moiety of glycoconjugates with carbon-14

Labeling of sialoglycoproteins with carbon-14 in vitro was performed by reacting the aldehyde groups, generated by mild periodate oxidation of the terminal sialyl groups, with 14C-labeled sodium cyanide to produce the labeled cyanohydrin derivatives (Kiliani reaction). Labeling with tritium was carried out by reduction of the aldehyde groups generated on the sialyl residues with 3H-labeled sodium borohydride following standard procedures. The behavior of both types of labeled specimens of fetuin and ovine submaxillary mucin, individually and in mixtures, was investigated by gel-filtration chromatography, gel electrophoresis, and cesium bromide gradient ultracentrifugation. The labeled sialyl residues were subjected to partial characterization: color yield with the resorcinol and thiobarbituric acid reagents, behavior on ion-exchange chromatography, and susceptibility to mild acid and enzymatic hydrolyses. In addition to these model glycoproteins, this procedure was also utilized to label the sialoglycoproteins present in human tracheobronchial secretions collected from normal subjects and patients with chronic bronchitis. The potential uses of this approach for comparative studies of normal and pathological sialoglycoconjugates available in minute amounts is described. The extension of this approach to the labeling of the galactosyl and N-acetylgalactosaminyl moieties of glycoconjugates following treatment with galactose oxidase is outlined.

Secretory activity of hamster tracheal explants and isolated tracheal epithelial cells and the effects of cystic fibrosis serum

Serum from cystic fibrosis patients has been shown by scanning electron microscopy to cause release of large quantities of mucus from the cultured tracheal rings of 3-4-month-old male Golden Syrian hamsters. In order to study this phenomenon on single cells, an epithelial (HTE) cell culture has been established from the hamster tracheal rings using the cell rescue method of Goldman and Baseman (1980a, In Vitro, 16:313). The cells were demonstrated to be epithelial by histochemical staining and immunofluorescent detection of laminin. Proteins secreted by HTE cells were partially characterized and shown to consist, at least in part, of acidic glycoproteins. The proteins were precipitated by addition of buffered alcian blue (AB) to the cell-free medium under conditions in which all of a polyanionic protein [3H]-labeled mucin, was precipitated without carrier. [14C] galactosamine-labeled AB precipitate was beta-eliminated and, after neutralization and centrifugation, the material in the supernatant was sized by chromatography on a calibrated Bio-Gel P2 column. The label eluted with a molecular weight close to a disaccharide. HTE cells pulse-labeled for 1.0 hr with [3H] leucine or [14C] galactosamine secreted increasing amounts of labeled glycoprotein during the chase. Sodium dodecylsulfate-polyacrylamide gel electrophoresis and fluorography of labeled AB precipitates revealed three major bands, two with molecular weights greater than 100 kd. Secretion was stimulated by retinoate (50% increase), but not by retinol. Exposure of HTE cells to whole sera from cystic fibrosis patients resulted in heightened secretion rates as compared to results obtained with normal sera. Heterozygote sera produced secretion rates intermediate between the two extremes.

Respiratory mucous secretions in patients with cystic fibrosis: relationship between levels of highly sulfated mucin component and severity of the disease

The tracheobronchial secretions from cystic fibrosis patients contained higher levels of protein, DNA and sialic acid than the tracheobronchial secretions from healthy donors. In contrast, the neutral hexose content in CF secretions was strikingly lower than in secretions from normal subjects. The levels of neutral hexose and sialic acid in the CF secretions were found to increase with increasing severity of the disease. The alterations in the levels of these chemical parameters in the secretions of patients with increased disease severity are as a result of increased levels of the mucin content of the secretions, especially of the highly sulfated mucin component. Since mucins are considered, to a large extent, responsible for the viscoelastic properties of the secretions, the enhanced levels of the highly sulfated mucin component in the secretions of the patients with increased disease severity, may contribute to altered rheological properties and hence decreased mucociliary transport of the secretions.

Isolation of bronchial mucins from cystic fibrosis sputum by use of citraconic anhydride

Citraconylation was used to solubilize cystic fibrosis sputum and to dissociate its mucus glycoproteins from extraneous proteins. The mucin fraction was isolated by precipitation with Cetavlon, and characterized in terms of amino acid and carbohydrate composition. The data suggest that, in determining the physical properties of glycoproteins of cystic fibrosis mucus, aggregation by noncovalent forces may be as important as (or more important than) disulfide bonds.

Binding of basic proteins to glycoproteins in human bronchial secretions

1. Secretions were aspirated from a patient with no history of pulmonary disorder. 2. Mucus glycoproteins, which exhibited blood group A activity, were separated into cetyltrimethylammonium bromide (cetavlon)- and ethanol-precipitable fractions. 3. The cetavlon-precipitable mucin was pure by analytical ultracentrifugation and upon chemical analysis had a composition typical for mucin preparations. 4. The fraction that precipitated with ethanol was found to bind tightly to proteins of a basic nature from which it could be separated by using 6 M urea. 5. This non-covalent interaction may explain the lack of precipitation of this mucus glycoprotein by cetavlon. 6. These basic proteins may be important in determining the rheological behavior of mucociliary secretions.