MUC5B is a major gel-forming, oligomeric mucin from human salivary gland, respiratory tract and endocervix: identification of glycoforms and C-terminal cleavage

Gastric MUC5AC and MUC6 are large oligomeric mucins that differ in size, glycosylation and tissue distribution

Gastric MUC5AC and MUC6 mucins were studied using polyclonal antibodies. Immunohistochemistry showed MUC5AC to originate from the surface epithelium, whereas MUC6 was produced by the glands. Mucins from the surface epithelium or glands of corpus and antrum were purified using CsCl/4M guanidinium chloride density-gradient centrifugation. MUC5AC appeared as two distinct populations at 1.4 and 1.3 g/ml, whereas MUC6, which was enriched in the gland tissue, appeared at 1.45 g/ml. Reactivity with antibodies against the Le(b) structure (where Le represents the Lewis antigen) followed the MUC5AC distribution, whereas antibodies against the Le(y) structure and reactivity with the GlcNAc-selective Solanum tuberosum lectin coincided with MUC6, suggesting that the two mucins are glycosylated differently. Rate-zonal centrifugation of whole mucins and reduced subunits showed that both gastric MUC5AC and MUC6 are oligomeric glycoproteins composed of disulphide-bond linked subunits and that oligomeric MUC5AC was apparently smaller than MUC6. A heterogeneous population of 'low-density' MUC5AC mucins, which were smaller than the 'high-density' ones both before and after reduction, reacted with an antibody against a variable number tandem repeat sequence within MUC5AC, suggesting that they represent precursor forms of this mucin. Following ion-exchange HPLC, both MUC5AC and MUC6 appeared as several distinct populations, probably corresponding to 'glycoforms' of the mucins, the most highly charged of which were found in the gland tissue.

Expression of respiratory mucins in fatal status asthmaticus and mild asthma

AIMS

The airways of patients with asthma are characterized by chronic inflammatory changes comprising mainly T-cells and eosinophils, and airway remodelling with goblet cell metaplasia and submucosal gland hyperplasia. Mucus hypersecretion is often a marked feature, particularly in status asthmaticus. The matrix of airway sputum consists of high molecular glycoproteins and mucins. In this study, the expression and distribution of the major gel-forming mucins MUC5AC and MUC5B were studied in fatal status asthmaticus tissues and bronchial biopsies of mild asthmatic patients. The effect of inhaled corticosteroids on the expression of these mucins was also investigated.

METHODS AND RESULTS

Polyclonal antibodies specific for MUC5AC and MUC5B, and a monoclonal antibody for MUC5B were used to stain lung tissues and airway mucosal biopsies obtained from patients who died of status asthmaticus (n=5) and from mild asthmatics (n=4), respectively. Immunohistochemistry for MUC5AC revealed abundant staining of goblet cells situated in the epithelial surface lining and glandular ducts of tissues from patients with fatal asthma. MUC5B immunoreactivity was restricted to mucous cells of submucosal glands and to epithelial cells. In mild asthmatics, large amounts of MUC5B, but not MUC5AC, positive extracellular mucus was found in the airway lumen as plugs, adjacent to the epithelial lining and in the necks of glandular secretory ducts of mild asthmatics. The distribution of MUC5AC and MUC5B in bronchial biopsies of mild asthmatics was similar before and after inhaled steroid treatment.

CONCLUSIONS

The expression of MUC5AC and MUC5B shares a similar distribution to normal airways in different states of asthma. The distribution is not affected by topical corticosteroid therapy.

Concentrated solutions of salivary MUC5B mucin do not replicate the gel-forming properties of saliva

We have developed a new approach to study the molecular organization of salivary mucus and salivary mucins using confocal fluorescence recovery after photobleaching (confocal-FRAP). MUC5B mucin, its reduced subunit and T-domains were prepared from saliva and fluorescently labelled. The translational self-diffusion coefficients were determined up to 3.6 mg/ml by confocal-FRAP. The results suggest that, in solutions of purified MUC5B mucin, at concentrations at which the hydrodynamic domains overlap, the intermolecular interactions are predominantly due to dynamic entanglements, and there was no evidence of specific self-association of MUC5B mucin, or of its subunits, or T-domains. The analysis of the salivary mucus gel also showed no specific interactions with the purified MUC5B components, but it was much less permeable than expected from its MUC5B content. The saliva was completely permeable to microspheres of 207 nm diameter, but showed size-dependent effects on the diffusion of larger microspheres (499 nm and 711 nm diameter). From these analyses the salivary mucus was shown to be both permeable and dynamic, and with the characteristics of a semi-dilute transient network at physiological concentration. Comparison of the results from saliva and purified MUC5B mucin solutions showed that the network properties of saliva were equivalent to a solution of purified MUC5B mucin of 10-20 times higher concentration. This showed that saliva has additional structure and organization not present in the purified MUC5B mucin and suggests there are other interactions and/or components within saliva that combine with MUC5B to produce its complete properties.

Expression of MUC5AC and MUC5B mucins in normal and cystic fibrosis lung

Hypersecretion of airway mucus is a characteristic feature of chronic airway diseases like cystic fibrosis (CF) and leads via impairment of the muco-ciliary clearance and bacterial superinfection to respiratory failure. The major components of the mucus matrix forming family of mucins in the airways are MUC5AC and MUC5B. To investigate the expression of these glycoproteins in CF, immunohistochemistry was carried out on trachea, bronchi and peripheral lung obtained from CF patients and compared to normal lung tissues. MUC5AC immunohistochemistry demonstrated signals in goblet cells of the epithelial lining. Also, goblet cells inside glandular secretory ducts revealed MUC5AC-positive staining. In comparison to those from normal subjects, CF sections were characterized by inflammatory changes and goblet cell hyperplasia, resulting in increased numbers of MUC5AC-positive cells. Immunohistochemical staining for MUC5B showed abundant staining of submucosal glands and epithelial goblet cells. Inside the glands, the immunoreactivity was restricted to glandular mucous cells. MUC5AC and MUC5B are expressed in the same histological pattern in CF compared to normal tissues with an increase of MUC5AC-positive cells due to goblet cell hyper- and metaplasia.

Heterogeneity of airways mucus: variations in the amounts and glycoforms of the major oligomeric mucins MUC5AC and MUC5B

Respiratory mucus contains a mixture of gel-forming mucins but the functional significance of these different mucin species is unknown. To help gain a better understanding of mucus in airways we therefore need to ascertain the concentration of each of the gel-forming mucins within respiratory secretions. Thus the aim of this study was to determine the amounts of specific gel-forming mucins directly from solubilized secretions of the airways and purified mucin preparations. We investigated the feasibility of using direct-binding ELISA employing mucin-specific antisera but were unable to obtain reliable data owing to interference with the immobilization of the mucins on the assay surface by 6 M urea and high levels of non-mucin proteins. We therefore developed an alternative approach based on quantitative Western blotting after agarose-gel electrophoresis, which was not subject to these problems. Here we demonstrate that this procedure provides reliable and reproducible data and have employed it to determine the amounts of the MUC2, MUC5AC and MUC5B mucins in saline-induced sputa from healthy airways and spontaneous sputa from asthmatic airways. Additionally we have used this procedure to analyse these glycoproteins in mucin preparations purified from cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD) mucus. Our findings indicate that MUC5AC and MUC5B are the major oligomeric mucins and that airways mucus contains variable amounts of these glycoproteins. By contrast, the MUC2 mucin comprised, at most, only 2.5% of the weight of the gel-forming mucins, indicating that MUC2 is a minor component in sputum. Finally, we show that the amounts and glycosylated variants of the MUC5AC and MUC5B mucins can be altered significantly in diseased airways with, for instance, an increase in the low-charge form of the MUC5B mucin in CF and COPD mucus.

Self-association of mucin

Aggregation phenomena in aqueous solutions of purified human tracheobronchial mucin have been studied by rheological methods, steady-state fluorescence, quasielastic light scattering, and spin probe techniques. At temperatures below 30 degrees C and concentrations above 15 mg/mL and in the absence of chaotropic agents, mucin solutions are viscoelastic gels. A gel-sol transition is observed at temperatures above 30 degrees C that is manifested by the diminishing storage modulus and a loss tangent above unity throughout the studied frequency range of the oscillatory shear. No decline in the mucin molecular weight is observed by size-exclusion chromatography above 30 degrees C in the absence of redox agents or proteolytic enzymes. Aggregation of hydrophobic protein segments of the mucin chains at 37 degrees C is indicated by QELS experiments. The decreasing polarity of the microenvironment of pyrene solubilized into mucin solutions at temperatures above 30 degrees C, concomitant with the gel-sol transition, shows the hydrophobicity of the formed aggregates. ESR spectra of the fatty acid spin probe, 16-doxylstearic acid indicate that the aggregate-aqueous interface becomes more developed at elevated temperatures.

Mucoactive drugs for asthma and COPD: any place in therapy?

Airway mucus hypersecretion is a clinical and pathophysiological feature of a number of severe respiratory conditions, including asthma and chronic obstructive pulmonary disease (COPD). The importance of mucus hypersecretion to the morbidity and mortality of asthma is acknowledged, whereas in COPD it appears to affect only certain groups of patients, particularly the elderly and those prone to chest infections. Treatment with compounds that alter mucus is perceived as a therapeutic option, in particular in continental Europe, and numerous compounds have been developed and are available for clinical use worldwide. However, acceptance (or otherwise) of these drugs in guidelines for management of asthma or COPD has been hampered by lack of information from well designed clinical trials. In addition, the mechanism of action of most of these drugs is unknown and is it likely that any beneficial effects are due to activities other than, or in addition to, effects on mucus. Current information indicates that the most effective use of mucolytic drugs is long-term therapy for reduction of exacerbations of COPD. Cost-effective treatment would be in patients with poor lung function who have frequent or prolonged exacerbations or are repeatedly admitted to hospital.

N-terminal cleavage of the salivary MUC5B mucin. Analogy with the Van Willebrand propolypeptide?

Sequence similarities between the oligomeric mucins (MUC2, MUC5AC, MUC5B) and the von Willebrand factor suggest that they may be assembled in a similar way. After oligomerization, a fragment corresponding to the D1 and D2 domains is released from the von Willebrand factor. This cleavage does not appear to occur in pig submaxillary mucin, the only mammalian mucin in which this cleavage has been examined thus far, but whether other oligomeric mucins undergo N terminus proteolysis is not known. Antibodies recognizing the D1, D2, D3, and the first Cys domains in MUC5B were established and used to investigate to what extent proteolytic cleavage occurs within the N-terminal part of salivary MUC5B. The antibodies against the D1 and D2 domains identified a polypeptide corresponding in size to a MUC5B fragment generated by cleavage within the D' domain analogously with the von Willebrand factor propolypeptide. The antibodies did not recognize the main mucin population, suggesting that the major part of salivary MUC5B is subjected to this cleavage. An antibody recognizing the D3 domain was used to reveal a second cleavage site in the "soluble" but not in the "insoluble" MUC5B fraction: the first structural difference observed between soluble and insoluble salivary MUC5B. The identification of these cleavage events shows that the N-terminal sites for MUC5B oligomerization are present in the D3 domain and/or in domains located C-terminal to this part of the molecule.

Mucinases and sialidases: their role in the pathogenesis of sexually transmitted infections in the female genital tract

BACKGROUND

Mucinases and sialidases contribute to the process of invasion and colonisation in many conditions and infections of the female reproductive tract by degrading the protective cervical mucus. The role of hydrolytic enzymes in the pathogenesis of sexually transmitted diseases and their effect on cervical mucus are discussed in this review.

METHODS

Articles were searched for using the keywords "sialidase," "mucinase," "protease," and "sexually transmitted infections." As well as review and other articles held by our group, searches were conducted using PubMed, Grateful Med, and the University of Bath search engine, BIDS.

RESULTS

Numerous publications were found describing the production of hydrolytic enzymes in sexually transmitted diseases. Because the number of publications exceeded the restrictions imposed on the size of the review, the authors selected and discussed those which they considered of the most relevance to sexually transmitted infections.

Characterization of human mucin 5B gene expression in airway epithelium and the genomic clone of the amino-terminal and 5'-flanking region

Human mucin (MUC) 5B gene expression in human airway epithelium was studied in both tissue sections and cultures of tracheobronchial epithelial (TBE) cells. In situ hybridization demonstrated that MUC5B message was expressed mainly in the mucous cells of submucosal glands of normal human airway tissues. Nevertheless, an elevated MUC5B message level could be seen in surface goblet cells from patients with airway diseases and inflammation. Regardless of the airway tissue sources, MUC5B message was regulated by all-trans-retinoic acid (RA) and culture conditions in both primary and passage-1 cultures of TBE cells. MUC5B message, to a lesser extent, was also found in the immortalized epithelial cell line HBE1, but not in BEAS-2B cells. To elucidate the molecular mechanism of MUC5B gene expression, a genomic clone was obtained and sequenced for the amino terminal and the 5'-flanking region of MUC5B gene. A luciferase reporter construct containing 4,169 base pairs of the 5'-flanking region of MUC5B gene demonstrated a cell type-specific basal promoter activity in transfection studies. Both RA and the air-liquid interface culture condition further enhanced this promoter activity. These results suggest that the 5'-flanking region of MUC5B gene contains cis-elements that are potentially involved in the regulation of MUC5B gene expression.

Mucin gene expression in intestinal epithelial cells in Crohn's disease

BACKGROUND

Crohn's disease (CD) is a chronic relapsing inflammatory bowel disease of unknown origin. It is characterised by chronic mucosal ulcerations which affect any part of the intestine but most commonly are found in the ileum and proximal colon.

AIMS

Studies were undertaken to provide information regarding cell specific expression of mucin genes in the ileum of patients with CD.

PATIENTS AND METHODS

Expression of mucin genes was analysed in the ileal mucosa of patients with CD and controls by in situ hybridisation and immunohistochemistry.

RESULTS

In healthy ileal mucosa, patients with CD showed a pattern identical to normal controls with main expression of MUC2 and MUC3, lesser expression of MUC1 and MUC4, and no expression of MUC5AC, MUC5B, MUC6, or MUC7. In the involved mucosa, the pattern was somewhat comparable although heterogeneous to that observed in healthy ileal mucosa. Importantly, a particular mucin gene expression pattern was observed in ileal mucosa close to the ulcer margins in ulcer associated cell lineage, with the appearance of MUC5AC and MUC6 mRNAs and peptides, which are normally restricted to the stomach (MUC5AC and MUC6) and duodenum (MUC6), and disappearance of MUC2.

CONCLUSIONS

Our results suggest that gel forming mucins (more particularly MUC5AC and MUC6) may have a role in epithelial wound healing after mucosal injury in inflammatory bowel diseases in addition to mucosal protection.

Differential regulation of airway mucin gene expression and mucin secretion by extracellular nucleotide triphosphates

The effects of extracellular nucleotide triphosphates on the stimulation of mucin production by airway epithelial cells were examined. The order of potency in stimulating mucin secretion in primary cultures of human tracheobronchial epithelial cells is: uridine 5'-triphosphate (UTP) approximately equal to adenosine 5'-triphosphate (ATP) approximately equal to ATP-gamma-S > uridine 5'-diphosphate approximately equal to adenosine 5'-diphosphate > alpha,beta-methylene ATP >> adenosine. However, only UTP can increase mucin gene (MUC5AC, MUC5B) expression; ATP and other analogues have no stimulatory effect. The stimulation of MUC5AC and MUC5B expression by UTP is time- and dose-dependent. A similar effect on the elevation of mucous cell population in mouse airway epithelium can be demonstrated in vivo by an intratracheal instillation of UTP-saline solution. The stimulatory effect of UTP or ATP on mucin secretion was inhibited by pertussis toxin, U73122, and Calphostin C, but not by PD98059, suggesting a G-protein/ phospholipase (PL) C/protein kinase (PK) C-dependent and mitogen-activated protein kinase (MAPK)-independent signaling pathway. However, the stimulatory effect of UTP on mucin gene expression was sensitive to pertussis toxin and PD98059, but not to Calphostin C and U73122, suggesting a G-protein/MAPK-dependent and PLC/PKC-independent signaling pathway. These findings are the first demonstration that UTP, a pyrimidine nucleotide triphosphate, can enhance both mucin secretion and mucin gene expression through different signaling pathways.

Distribution of MUC1 in the normal human oral cavity is localized to the ducts of minor salivary glands

The change in expression of MUC1 from health to disease forms the basis of its use as a potential disease marker. Previous attempts at isolating MUC1 from normal, healthy human oral mucosa have, however, drawn conflicting conclusions as to its presence. Furthermore, when MUC1 was detected in the oral glycocalyx, it was not clear which cells were synthesising it. We examined human oral glycocalyx using pooled buccal smears from 50 normal individuals. Following isopycnic density centrifugation and membrane extraction with octyl glucoside and saponin, MUC1 was detected with the polyclonal antibody CT1. Immunohistochemistry using antibodies CT1 and BC2 was performed on sections from eight labial, seven palatal, four buccal, three retromolar pad, three dorsum of tongue and two ventral surface of tongue biopsies. In-situ hybridisation using MUC1 and cytoplasmic tail oligoprobes on sections from four palatal, seven labial and two retromolar pad biopsies was also carried out. MUC1 mRNA could only be detected in the minor salivary mucous glands. MUC1 has already been identified in the ducts of normal parotid and submandibular gland, and our findings demonstrate a similar distribution in minor salivary glands. We conclude that when present in the normal oral glycocalyx, the only oral source of MUC1 is from cell membranes of the minor salivary glands.

Specific secretion of gel-forming mucins and TFF peptides in HT-29 cells of mucin-secreting phenotype

Trefoil factor family (TFF) peptides are typical secretory products of mucin-producing cells, e.g. of the gastrointestinal tract. Here, the expression and secretion of mucins and TFF peptides was studied in the HT-29 cell line throughout cellular growth and differentiation in relation to a mucin-secreting (HT-29 MTX) or an enterocyte-like (HT-29 G(-)) phenotype. mRNAs of several MUC and TFF genes were expressed in both cell subpopulations. However, for most MUC and TFF genes, the expression appeared strongly induced with the differentiation into the mucin-secreting phenotype. On the other hand, TFF2 was specifically expressed in the mucin-secreting HT-29 MTX cells. The differentiation of HT-29 MTX cells into the mucin-secreting phenotype was characterised by secretion of the gel-forming mucins MUC2, MUC5AC, and MUC5B, however, according to a different pattern in the course of differentiation. A significant amount of TFF1 and TFF3 was secreted after differentiation, also according to a different pattern, whereas TFF2 was only faintly detected. Secretagogues, known to induce the secretion of mucus, increased the secretion of all three TFF peptides. In contrast, neither a secretory mucin nor a TFF peptide was found in the culture medium of HT-29 G(-) cells. Overlay assays indicated that HT-29 MTX mucins bound to secretory peptides of HT-29 MTX cells with relative molecular mass similar to TFF peptides. TFF1 and TFF3 were specifically localised in the mucus layer of HT-29 MTX cells by confocal microscopy. Finally, the secretion of TFF peptides and mucins appears as a co-ordinated process which only occurs after differentiation into goblet cell-like phenotype.

Regulation of mucous differentiation and mucin gene expression in the tracheobronchial epithelium

The goal of our studies is to elucidate mechanisms that control and modulate mucous differentiation and mucin gene expression in the conducting airways. We used cultures of normal human tracheobronchial epithelial (NHTBE) cells that were shown to secrete two major airway mucins, namely MUC5AC and MUC5B as well as several other secretory products. Mucous differentiation and expression of MUC2, MUC5AC, MUC5B and MUC7, but not MUCi, MUC4, and MUC8 mucin genes, were shown to be retinoic acid- (RA) or retinol-dependent. We found that RA control of mucin genes was mediated by the retinoid acid receptors RAR alpha and, to a lesser extent, by RAR gamma. Our studies also showed that other important bioregulators such as thyroid hormone (T3) and epidermal growth factor (EGF) modulate basal expression of mucin genes, interacting with RA in a concentration-dependent manner. T3, which binds to thyroid receptors (TRs) belonging to the same superfamily of steroid hormone nuclear receptors as the RARs, inhibits mucin gene expression, particularly MUC5AC. One possible mechanism of this T3 effect is downregulation of RAR proteins, which are critical for mucin gene expression. However, we also found that T3 inhibits MUC5AC transcription.EGF, which had previously been shown to stimulate mucin expression and mucin secretion in cultured rat tracheal epithelial (RTE) cells, inhibited mucin secretion in human bronchial epithelial cell cultures. This effect was EGF concentration- and time-dependent and was progressively abolished by increasing the RA concentration. Subsequent studies suggested that the inhibitory effects of high concentrations of EGF may result from selective reduction of MUC5AC expression. These studies thus point to potentially important species differences in the mechanisms regulating mucous production, and they also confirm previous findings indicating differential regulation of MUC5AC and MUC5B gene expression.

Mucinous bronchioloalveolar carcinomas display a specific pattern of mucin gene expression among primary lung adenocarcinomas

Lung adenocarcinomas are heterogeneous clinically and histologically. Expression of the mucin genes was analyzed as a molecular marker of glandular cytodifferentiation in primary lung adenocarcinomas. Expression was correlated with histopathologic subtypes of World Health Organization classification with the aim of investigating the histogenesis of primary lung adenocarcinomas. Thirty-four primary lung adenocarcinomas were examined by in situ hybridization for mucin gene expression (MUC1-4, MUC5AC, MUC5B, MUC6-7) and by immunohistochemistry for MUC5AC and MUC5B apomucin expression. Mucinous bronchioloalveolar carcinoma (BAC) had a homogeneous pattern of mucin gene expression different from those of other types of lung adenocarcinoma, involving secreted mucins (MUC5AC, MUC5B, and MUC6) and membrane-bound mucins (MUC1, MUC3, and MUC4). Non-BAC adenocarcinoma and mucinous BAC aberrantly expressed mucin genes MUC3, and MUC3 and MUC6, respectively, which are undetectable in normal fetal and adult lung. Our results show the particular phenotype of mucin gene expression in mucinous type of BACs and the heterogeneous expression of respiratory and nonrespiratory mucins in the other types. This finding supports the theory of a common progenitor cell with the potential of multicellular differentiation. From a practical point of view, the aberrant expression of MUC3 and MUC6 could serve as a diagnostic marker in the management of the mucinous type of bronchioloalveolar carcinomas. HUM PATHOL 32:274-281.

Regulation of mucin gene expression in human tracheobronchial epithelial cells by thyroid hormone

We reported previously that the expression of the gene encoding MUC5AC mucin in human airway epithelial cells is controlled by retinoic acid via the retinoic acid receptor (RAR)-alpha and that 3,3',5-tri-iodothyronine (T(3)) inhibits the expression of MUC5AC. The purpose of the present study was to identify mechanisms mediating the effect of T(3). T(3) has been shown to inhibit gene expression via several mechanisms, either by enhancing or repressing the transcription of target genes or by the regulation of post-transcriptional events. Results showed that T(3) strongly inhibited MUC5AC-driven luciferase activity in normal human tracheobronchial epithelial cells that had been transiently transfected with a MUC5AC-luciferase reporter construct; however, it did not affect MUC5AC mRNA stability. These results indicate that T(3) suppresses MUC5AC expression at the transcriptional level. An analysis of deletion constructs showed that deletion of the region downstream of 3 kb resulted in markedly decreased levels of MUC5AC transcription in the absence of T(3) (i.e. under control conditions) as well as a loss of responsiveness to the inhibitory effects of T(3). This suggests that this region might contain elements important for the activation as well as the repression of MUC5AC transcription. To determine whether T(3) modulates retinoic-acid-dependent MUC5AC transcription via an alteration in the abundance of retinoid receptor proteins, we examined the type and abundance of these receptors in nuclear extracts of airway epithelial cells grown in the presence or absence of T(3). Western blots showed that T(3) markedly decreased several types of retinoid receptor while not affecting T(3) receptor proteins. Consistent with this finding were gel-shift assays revealing a decrease in RAR-retinoic acid response element complexes obtained from T(3)-treated cells. We propose that T(3) might inhibit retinoid-dependent MUC5AC expression by decreasing retinoid receptor levels and thereby decreasing the transcriptional activation of this gene for mucins.

Expression and localization of immunoreactive-sialomucin complex (Muc4) in salivary glands

Sialomucin Complex (SMC; Muc4) is a heterodimeric glycoprotein consisting of two subunits, the mucin component ASGP-1 and the transmembrane subunit ASGP-2. Northern blot and immunoblot analyses demonstrated the presence of SMC/Muc4 in submaxillary, sublingual and parotid salivary glands of the rat. Immunocytochemical staining of SMC using monoclonal antisera raised against ASGP-2 and glycosylated ASGP-1 on paraffin-embedded sections of parotid, submaxillary and sublingual tissues was performed to examine the localization of the mucin in the major rat salivary glands. Histological and immunocytochemical staining of cell markers showed that the salivary glands consisted of varying numbers of serous and mucous acini which are drained by ducts. Parotid glands were composed almost entirely of serous acini, sublingual glands were mainly mucous in composition and a mixture of serous and mucous acini were present in submaxillary glands. Since immunoreactive (ir)-SMC was specifically localized to the serous cells, staining was most abundant in parotid glands, intermediate levels in submaxillary glands and least in sublingual glands. Ir-SMC in sublingual glands was localized to caps of cells around mucous acini, known as serous demilunes, which are also present in submaxillary glands. Immunocytochemical staining of SMC in human parotid glands was localized to epithelial cells of serous acini and ducts. However, the staining pattern of epithelial cells was heterogeneous, with ir-SMC present in some acinar and ductal epithelial cells but not in others. This report provides a map of normal ir-SMC/Muc4 distribution in parotid, submaxillary and sublingual glands which can be used for the study of SMC/Muc4 expression in salivary gland tumors.

Mucin biosynthesis and secretion in tracheal epithelial cells in primary culture

Density-gradient centrifugation of bovine tracheal epithelial cell extracts revealed a 'high-density' (1.48 g/ml) sialic-acid-rich population as well as a 'low-density' (1.42 g/ml) one that reacted more strongly with a periodate-Schiff (PAS) assay. The sialic-acid-rich mucins were oligomeric molecules containing disulphide- bond-linked subunits and large glycosylated domains, whereas the PAS-reactive component seemed to be smaller and 'monomeric'. Only the 'high-density' population was secreted from cells cultured for 5 days on plastic or a collagen type 1, Matrigel or Vitrogen substrate. Release was less from cells grown on plastic than from those on a substrate and the amount was unaffected by increasing the thickness of the collagen layer. For cells grown on collagen, the amount of the sialic-acid-rich mucin increased over 10 days, whereas the PAS-reactive component was largely absent after 24 h, which was consistent with an initial release of stored PAS-reactive molecules and synthesis of the sialic-acid-rich mucins de novo. Both [(3)H]proline and [(35)S]sulphate were poorly incorporated into mucins detected with the chemical assays but molecules with a higher buoyant density than that of either of the previously identified species were labelled with [(35)S]sulphate. The [(35)S]sulphate-labelled material yielded large trypsin-resistant fragments and contained O-linked glycans but was not affected by digestion with chondroitin ABC lyase or heparan sulphate lyase, suggesting that it is a mucin rather than a proteoglycan. [(35)S]Sulphate is thus a poor marker for the major oligomeric mucins produced by bovine tracheal epithelial cells but the radiolabel is incorporated into a heavily labelled mucin-like component.

O-glycan variability of egg-jelly mucins from Xenopus laevis: characterization of four phenotypes that differ by the terminal glycosylation of their mucins

Eggs from Xenopus laevis are surrounded by several layers of jelly that are needed for proper fertilization. Jelly coat is composed of high-molecular-mass glycoconjugates to which are bound many globular proteins. O-glycans released from the jelly coat of X. laevis have been partially described in previous studies. In this study, we compared the glycosylation pattern of the egg jelly coat isolated from six specimens of X. laevis. The O-glycans were released from jelly coats by alkali/borohydride treatment. Structural characterization was performed through a combination of one- and two-dimensional (1)H-NMR and methylation analysis. This allowed the description of a new family of sulphated O-glycans present in jelly coats of all X. laevis. However, the jelly O-glycans showed a low extent of polymorphism between specimens. This intra-specific variability was restricted to the terminal substitution of O-linked oligosaccharides. The differential expression of two glycosyltransferase [an alpha-(1-->4) galactosyltransferase and an alpha-(1-->3) fucosyltransferase] activities resulted in the characterization of four phenotypes of X. laevis. Furthermore, electrophoretic analysis suggested that the high-molecular-mass fraction of jelly coat was mostly composed of mucin-type glycoproteins. Blot analysis with lectins confirmed that the glycan variability was borne by these mucin-type components. However, fertilization assays suggested that the glycan polymorphism had no repercussion on egg fertilizability.

Polymorphism of human mucin genes in chest disease: possible significance of MUC2

Most of the genes that encode epithelial mucins are highly polymorphic due to variations in the length of domains of tandemly repeated (TR) coding sequence, the part of the apomucin that is heavily glycosylated. We report here for the first time a difference in the distribution of MUC TR length alleles in chest disease. We examined the distribution of the length alleles of those MUC genes whose expression we have confirmed in the bronchial tree in an age- and sex-matched series of 50 pairs of atopic patients with and without asthma. There was no significant difference in the distribution of alleles of MUC1, MUC4, MUC5AC, and MUC5B. MUC2, however, showed a highly significant difference in distribution. The atopic, nonasthmatic individuals showed an allele distribution that was very different from all our other patient and control groups, this group showing a longer mean allele length. The observations suggest that longer MUC2 alleles may help protect atopic individuals from developing asthma, though the effect may be due to a linked gene. The biological significance of this variation with respect to susceptibility to asthma will merit further investigation, and it will also be important to substantiate this finding on an independent data set.

Macromolecular organization of saliva: identification of 'insoluble' MUC5B assemblies and non-mucin proteins in the gel phase

Stimulated human submandibular/sublingual (HSMSL) and whole saliva were separated into sol and gel phases and mucins were isolated by density-gradient centrifugation in CsCl/4M guanidinium chloride. MUC5B and MUC7 were identified using anti-peptide antisera raised against sequences within the MUC5B and MUC7 apoproteins respectively. MUC7 was found mainly in the sol phase of both HSMSL and whole saliva, but some MUC7 was consistently present in the gel phase, suggesting that this mucin may interact with the salivary gel matrix. In HSMSL saliva, MUC5B was found in the gel phase; however, most of the material was 'insoluble' in guanidinium chloride and was only brought into solution by reduction. In whole saliva, the MUC5B mucin was present both in the sol and gel phases although some material was again 'insoluble'. Rate-zonal centrifugation of whole saliva showed that MUC5B mucins in the sol phase were smaller than those in the gel phase, suggesting differences in oligomerization and/or degradation. Antibodies against IgA, secretory component, lysozyme and lactoferrin were used to study the distribution of non-gel-forming proteins in the different phases of saliva. The majority of these proteins was found in the sol phase of both HSMSL and whole saliva. However, a significant fraction was present in the gel phase of whole saliva, suggesting a post-secretory interaction with the salivary gel matrix. A monoclonal antibody against a parotid salivary agglutinin was used to show that this protein is present mainly in the gel phase of both whole saliva and parotid secretion.

Evolution of the large secreted gel-forming mucins

Mucins, the major component of mucus, contain tandemly repeated sequences that differ from one mucin to another. Considerable advances have been made in recent years in our knowledge of mucin genes. The availability of the complete genomic and cDNA sequences of MUC5B, one of the four human mucin genes clustered on chromosome 11, provides an exemplary model for studying the molecular evolution of large mucins. The emerging picture is one of expansion of mucin genes by gene duplications, followed by internal repeat expansion that strictly preserves frameshift. Computational and phylogenetic analyses have permitted the proposal of an evolutionary history of the four human mucin genes located on chromosome 11 from an ancestor gene common to the human von Willebrand factor gene and the suggestion of a model for the evolution of the repeat coding portion of the MUC5B gene from a hypothetical ancestral minigene. The characterization of MUC5B, a member of the large secreted gel-forming mucin family, offers a new model for the comparative study of the structure-function relationship within this important family.

Characterization of mucins from cultured normal human tracheobronchial epithelial cells

Early-passage normal human tracheobronchial epithelial (NHTBE) cells grown in air-liquid interface cultures in medium containing retinoids differentiate into a mucociliary epithelium over a 2- to 3-wk period and express increasing mRNA levels of the airway mucin genes MUC5AC and MUC5B as the cultures age; the levels of MUC2 mRNA were very low throughout the study. Using specific antibodies to MUC5AC and MUC5B mucins, we noted a gradual increase in these two mucins in the intracellular and apically secreted pools as a function of time. A low level of MUC2 mucin was detected, which did not change with time. The intracellular and apically secreted mucins isolated from day 14 and day 21 cultures by density gradient centrifugation were similar in density to those previously isolated from human respiratory mucus secretions. The sedimentation rate of the apically secreted mucins indicated that they were highly oligomerized, polydisperse macromolecules similar to those previously documented from in vivo secretions. In contrast, the cell-associated mucins from the cultured NHTBE cells were much smaller, possibly only monomers and dimers. Anion-exchange chromatography detected no differences in charge density between the reduced and carboxymethylated cell-associated and secreted forms of the MUC5AC and MUC5B mucins. The MUC5AC mucin was of similar charge density to its in vivo counterpart; however, MUC5B was more homogeneous than that found in vivo. Finally, evidence is presented for an intracellular NH(2)-terminal cleavage of the MUC5B mucins. These studies indicate that the mucins produced by cultured NHTBE cells are similar to those found in human airways, suggesting that this cell culture model is suited for studies of respiratory mucin biosynthesis, processing, and assembly.

Physical characterization of the MUC5AC mucin: a highly oligomeric glycoprotein whether isolated from cell culture or in vivo from respiratory mucous secretions

We have isolated the high-M(r) mucins from growth medium of the early stage of an HT-29 cell culture by gel chromatography and isopycnic density gradient centrifugation. The mucins (buoyant density 1.34-1.44 g/ml) were reactive with an anti-peptide antiserum (MAN-5ACI) raised against a sequence from within the MUC5AC mucin. Similar antisera raised against the MUC2 and MUC5B mucins were not reactive. The MUC5AC reduced-mucin subunits exhibited a homogeneous charge distribution on anion-exchange chromatography, but appeared as two bands, one major and one more minor, after agarose gel electrophoresis. The unreduced mucins had an average M(r) in excess of 40 MDa and were visualized in the electron microscope as large, fine filamentous threads (many microns in length) that after reduction were greatly reduced in size (number average length 570 nm). Agarose gel electrophoresis of unreduced MUC5AC mucins identified a major band just entering the gel with evidence of a 'ladder' of faster-migrating minor bands. Partial reduction of the mucins increased the proportion of the faster bands and at least 16 could be discriminated. M(r) measurements showed that these bands differed by single monomer units. The mucins behaved as very stiff extended structures in solution and this characteristic might explain the poor separation of different-sized oligomers in sedimentation-rate experiments. The cell-culture mucin preparation had similar characteristics of charge and buoyant density to MUC5AC mucins from respiratory secretions in vivo. In addition the MUC5AC mucin from respiratory tract secretions exhibited similar behaviour, reduced and unreduced on agarose gel electrophoresis, indicating that the mucin has a similar molecular phenotype in vivo and in vitro.

Preliminary study pointing out a significant alteration in the biochemical composition of MUC2 in colorectal mucinous carcinoma

OBJECTIVES

In this study, we characterized colonic MUC2 mucin from a mucinous carcinoma cell line and tried to find out carcinoma-associated alterations by comparing the results with those obtained from its benign phenotype previously.

DESIGN AND METHODS

The molecular size distribution of the extracted molecules and their reactivity with two different MUC2 polypeptide antibodies indicated the presence of precursor and mature forms of the mucin in both cell lines. Isopycnic density gradient centrifugation gave good resolution of mature and precursor forms of MUC2 as assessed by agarose gel electrophoresis. Using this approach, we compared the different forms of MUC2 between benign and malign colonic cells.

RESULTS

In the comparison, we detected some aberrant glycosylated MUC2 molecules in mucinous carcinoma cell line. Agarose gel electrophoretic analysis of the low-density fractions indicated that these molecules are more charged than precursors, however, they are smaller and/or less glycosylated than mature MUC2 molecules.

CONCLUSION

The identification of unusual partially glycosylated forms of the major colonic mucin MUC2 is novel and unexpected. Implication of defective processes in the post translational modification/ processing of MUC2 opens a new field in the cancer mucin biology.

Identification of MUC5B, MUC5AC and small amounts of MUC2 mucins in cystic fibrosis airway secretions

To investigate the genetic identities of the mucins secreted in cystic fibrosis (CF) airways, sputum was collected from five individuals. Samples were separated into gel and sol phases by high-speed centrifugation and the gel phase was extracted in 6 M guanidinium chloride. The 'insoluble' residue remaining after extraction of the gel phase was brought into solution by reduction/alkylation. Density-gradient centrifugation in CsCl revealed polydisperse distributions of sialic acid-containing mucins in the gel phase, insoluble residue and sol phase fractions and the degree of variation between the different individuals was low. Antibodies recognizing MUC5AC and MUC5B identified these mucins in each of the fractions. MUC2, however, was present only as a component of the insoluble residue from the gel which accounted for less than 4% by mass of the total mucins. MUC5B and MUC5AC from the gel phase were large oligomeric species composed of disulphide-bond linked subunits and MUC5B was present as two populations with different charge densities which are likely to correspond to MUC5B 'glycoforms'. The sol phase contained, in addition to MUC5AC and MUC5B, mainly smaller mucins which did not react with the antisera and which were probably degraded. MUC5AC appeared to be enriched in the sol, suggesting that this mucin may be more susceptible to proteolytic degradation than MUC5B. The mucins present in sputum remained broadly similar during acute exacerbation and following antibiotic treatment, although the relative amount of an acidic MUC5B glycoform was decreased during infection.

Physical characterization of a low-charge glycoform of the MUC5B mucin comprising the gel-phase of an asthmatic respiratory mucous plug

We have previously noted that sequential extraction of an asthmatic mucous exudate with 6 M guanidinium chloride yielded a fraction of the mucins that were most resistant to solubilization and of high Mr [Sheehan, Richardson, Fung, Howard and Thornton (1995) Am. J. Respir. Cell Mol. Biol. 13, 748-756]. Here we show that this mucin fraction is dominated (at least 96% of the total) by the low-charge glycoform of the MUC5B gene product. Seen in the electron microscope the mucins appeared mainly as compact 'island' structures composed of linear threads often emanating from globular 'nodes' rather than the discrete linear threads more typical of mucins that we have previously described. The effect of reducing agents was as expected for other gel-forming mucins, i.e. reduced subunits or monomers of Mr 3x10(6)) were produced within 15 min of treatment. Kinetic experiments on the cleavage of the intact mucins with the proteinase trypsin indicated two clear regimes of fragmentation. An initial rapid cleavage generated mucins ranging from Mr=4x10(6) to 30x10(6) that in the electron microscope appeared as polydisperse threads (500-3000 nm in length), similar to normal and other respiratory mucins that we have previously characterized. A subsequent slower fragmentation over many hours yielded a major fragment of Mr 3x10(6) and length 200-600 nm, very similar in size and Mr to the subunits obtained by reduction. The results suggest that the MUC5B mucin is assembled, first into polydisperse linear threads, which are then linked together via a protein-mediated process. This might involve part of the mucin polypeptide or an as yet unidentified protein(s). The high proteinase susceptibility of the linkage suggests that it might be a point of control for mucin size and thus mucus rheology.