The core polypeptide of cystic fibrosis tracheal mucin contains a tandem repeat structure. Evidence for a common mucin in airway and gastrointestinal tissue

Precise integration of inducible transcriptional elements (PrIITE) enables absolute control of gene expression

Tetracycline-based inducible systems provide powerful methods for functional studies where gene expression can be controlled. However, the lack of tight control of the inducible system, leading to leakiness and adverse effects caused by undesirable tetracycline dosage requirements, has proven to be a limitation. Here, we report that the combined use of genome editing tools and last generation Tet-On systems can resolve these issues. Our principle is based on precise integration of inducible transcriptional elements (coined PrIITE) targeted to: (i) exons of an endogenous gene of interest (GOI) and (ii) a safe harbor locus. Using PrIITE cells harboring a GFP reporter or CDX2 transcription factor, we demonstrate discrete inducibility of gene expression with complete abrogation of leakiness. CDX2 PrIITE cells generated by this approach uncovered novel CDX2 downstream effector genes. Our results provide a strategy for characterization of dose-dependent effector functions of essential genes that require absence of endogenous gene expression.

Cellular and molecular biology of airway mucins

Airway mucus constitutes a thin layer of airway surface liquid with component macromolecules that covers the luminal surface of the respiratory tract. The major function of mucus is to protect the lungs through mucociliary clearance of inhaled foreign particles and noxious chemicals. Mucus is comprised of water, ions, mucin glycoproteins, and a variety of other macromolecules, some of which possess anti-microbial, anti-protease, and anti-oxidant activities. Mucins comprise the major protein component of mucus and exist as secreted and cell-associated glycoproteins. Secreted, gel-forming mucins are mainly responsible for the viscoelastic property of mucus, which is crucial for effective mucociliary clearance. Cell-associated mucins shield the epithelial surface from pathogens through their extracellular domains and regulate intracellular signaling through their cytoplasmic regions. However, neither the exact structures of mucin glycoproteins, nor the manner through which their expression is regulated, are completely understood. This chapter reviews what is currently known about the cellular and molecular properties of airway mucins.

Regulation of mucin genes in chronic inflammatory airway diseases

In this review, we summarize work over the past 15 years on mucin gene expression and regulation in the lung, as well as how mucin gene expression is altered in chronic lung diseases. This field owes a great debt to Carol Basbaum for her pioneering work in dissecting signaling pathways regulating mucin gene expression and for her tremendous energy in promoting the importance of understanding the basic pathogenic mechanisms that drive mucus overproduction in cystic fibrosis, chronic obstructive pulmonary disease, and asthma.

In vivo effect of wood smoke on the expression of two mucin genes in rat airways

A short-term, time-dependent smoke exposure of rats in a nose-only chamber to burning wood and 24-h recovery time revealed inflammation of the airways with varying degrees of injury from loss of cilia, degeneration of epithelium, and squamous metaplasia to submucosal edema. These histological changes were reflected in variable expression of the secretory Muc5AC and low expression of membrane-associated Muc4 mucin genes. 20-min smoke exposure in extended recovery experiments showed marked disorder of tracheal epithelium for up to 72 h of recovery with a return to normal by 7 days. Gene expressions were elevated at 24 and 48 h of recovery. 30-min smoke exposure showed a more severe degeneration of the epithelium and a longer recovery time. Muc5AC expression decreased after 72 h of recovery, while there was upregulation of Muc4 gene from 48 through 96 h. Because Muc4 upregulation and histological results correlate and it has reportedly been associated with epithelium renewal, Muc4 gene may be a useful marker for the regeneration of tracheal epithelium.

Characterization of mucins in human middle ear and Eustachian tube

Mucins are important glycoproteins in the mucociliary transport system of the middle ear and Eustachian tube. Little is known about mucin expression within this system under physiological and pathological conditions. This study demonstrated the expression of MUC5B, MUC5AC, MUC4, and MUC1 in the human Eustachian tube, whereas only MUC5B mucin expression was demonstrated in noninflamed middle ears. MUC5B and MUC4 mucin genes were upregulated 4.2- and 6-fold, respectively, in middle ears with chronic otitis media (COM) or mucoid otitis media (MOM). This upregulation of mucin genes was accompanied by an increase of MUC5B- and MUC4-producing cells in the middle ear mucosa. Electron microscopy of the secretions from COM and MOM showed the presence of chainlike polymeric mucin. These data indicate that the epithelium of the middle ear and Eustachian tube expresses distinct mucin profiles and that MUC5B and MUC4 mucins are highly produced and secreted in the diseased middle ear. These mucins may form thick mucous effusion in the middle ear cavity and compromise the function of the middle ear.

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.

Developmental mucin gene expression in the human respiratory tract

The epithelial surface of the respiratory tract is coated with a protective film of mucus secreted by epithelial goblet and submucosal gland cells. Histology of the airway mucosa and composition of secretions during the second trimester of fetal life are known to differ from the normal adult in that these secretions show similarities with those of hypersecretory disorders. To provide information regarding cell-specific expression of mucin genes and their relation to developmental patterns of epithelial cytodifferentiation, we studied the expression of eight different mucin genes (MUC1-MUC4, MUC5AC, MUC5B, MUC6, MUC7) in human embryonic and fetal respiratory tract using in situ hybridization. These investigations demonstrated that MUC4 is the earliest gene expressed in the foregut at 6.5 wk, followed by MUC1 and MUC2 from 9. 5 wk of gestation in trachea, bronchi, epithelial tubules, and terminal sacs before epithelial cytodifferentiation. In contrast, MUC5AC, MUC5B, and MUC7 are expressed at later gestational ages concomitant with epithelial cytodifferentiation. During this developmental stage, MUC1 and MUC4 mRNAs are located in goblet and ciliated cells, whereas MUC2 mRNAs are located in basal and goblet cells. MUC5AC expression is confined to goblet cells. In the submucosal glands, MUC2 mRNAs are located in both mucous and serous cells, whereas MUC5B and MUC7 mRNAs are expressed in mucous and in serous cells, respectively. These data suggest distinct developmental roles for MUC1, MUC2, MUC4, MUC5AC, MUC5B, and MUC7 in the elongation, branching, and epithelial cytodifferentiation of the respiratory tract during ontogenesis. Distinct patterns of mucin gene expression are also likely to play an important role in regulating appropriate epithelial cell proliferation and cytodifferentiation in adult airway mucosa as it is indicated by aberrant expression in hypersecretory disorders.

A novel Dictyostelium cell surface protein important for both cell adhesion and cell sorting

A mutant of Dictyostelium that is aberrant in the process of tip formation (dtfA-: defective in tip formation A) has been isolated by gene tagging. The dtfA gene is predicted to encode a protein of 163 kDa. There are no extensive sequence homologies between DTFA and previously identified proteins, but four short N-terminal sequence motifs show partial homology to repeats found in mammalian mucins. Immunofluorescence reveals a lattice-like arrangement of DTFA protein at the cell surface. When developing on a bacterial lawn, cells of the mutant strain (dtfA- cells) aggregate to form tight mounds, but development then becomes arrested. When developed in the absence of nutrients, a fraction of dtfA- cells complete development, but there is a long delay at the tight mound stage and the culminants that eventually form are aberrant. In such dtfA- mounds the prestalk cells fail to move to the apex on cue and so tip formation is delayed. dtfA- cells also show a conditional defect in early development, in that they are unable to aggregate when plated at low density. In addition dtfA- cells do not agglomerate efficiently when shaken in suspension. In combination, these results suggest that DTFA may form part of a cell-cell adhesion system that is needed both for optimal aggregation and for efficient cell sorting during multicellular development. The DTFA protein also appears to be important during cell growth, because cytokinesis is defective and the actin cytoskeleton aberrant in growing dtfA- cells.

Smoke-induced inhalation injury: effects of retinoic acid and antisense oligodeoxynucleotide on stability and differentiated state of the mucociliary epithelium

Rabbit tracheal explants, exposed to burning pine wood smoke, were cultured in a chemically defined medium with and without retinoic acid (+/- RA). Exposures of 15-20 minute led to RA-independent degeneration of the mucociliary epithelial sheath. In 10 minute exposures tissue integrity was retained, but epithelial morphology changed from normal pseudostratified columnar to the flattened appearance typical of the squamous phenotype. Despite the dramatic shift in morphology, explants exhibited normal RA-dependent mucin gene expression characteristic of the mucociliary phenotype. Furthermore, electron micrographs showed continued presence of both secretory granules and cilia. RA(+) cultures also showed a normal pattern of adherent epithelial cells. In RA(-) cultures, however, there were prominent intercellular spaces indicating an RA dependence for maintaining adhesive contacts following smoke exposure. An 18-mer mucin antisense oligomer that suppressed mucin gene expression also unexpectedly blocked the smoke induced metaplasia in RA(+) cultures, but the sense oligomer had no effect.

Expression of mucin antigens in human cancers and its relationship with malignancy potential

Mucins are high molecular weight glycoproteins having oligosaccharides attached to the apomucin protein backbone by O-glycosidic linkages. Biochemical studies on the structures and the organ specificities of several mucin core proteins (MUC1-MUC7) have been reported during the past several years. In the present study of pancreas and intrahepatic bile duct tumors, MUC1 mucin (membrane bound mucin detected by monoclonal antibody, DF3) was highly expressed in invasive ductal carcinomas of the pancreas (IDC) and invasive cholangiocarcinomas of the liver (ICC), which show invasive growth and a poor prognosis, but it was rarely expressed in intraductal papillary mucinous tumors of the pancreas (IPMT) and bile duct cystadenocarcinomas of the liver (BDCC), which show a favorable prognosis. In contrast, MUC2 mucin (intestinal type secretory mucin detected by polyclonal antibody, anti-MRP) was rarely expressed in IDC and ICC, whereas it was highly expressed in IPMT and BDCC. The results suggest that the differences in the expression of MUC1 and MUC2 mucins are a useful prognostic indicator of malignancy potential in the neoplasms of the pancreas and intrahepatic bile duct. Moreover, the expression of MUC1 and MUC2 mucins was a useful indicator of the malignancy potential of tumors derived from other organs, such as the ampulla of Vater, stomach and breast. In another study on the expression of several MUC1 mucin antigens with different patterns of glycosylation, sialylated-MUC1 mucin detected by monoclonal antibody, MY.1E12, was found to be expressed in all the invasive carcinomas (IDC and ICC) but was not frequently seen in the non-invasive type tumors (IPMT and BDCC), although the other types of MUC1 mucins did not show such contrast between the invasive and non-invasive type tumors. The results suggest that sialylation of MUC1 mucin is associated with invasive growth of neoplasms. In contrast, our study of the expression of MUC2 mRNA (transcript of intestinal type mucin) and MUC5AC mRNA (transcript of gastric type mucin) by in situ hybridization in the tumors of the pancreas and intrahepatic bile duct found that the non-invasive type tumors (IPMT and BDCC) synthesize MUC2 mRNA and MUC5AC mRNA, whereas most of the invasive carcinomas (IDC and ICC) do not. Furthermore, patients positive for MUC2 mRNA or MUC5AC mRNA expression in the tumors showed significantly better survival than the patients with no expression. The production of MUC2 or MUC5AC, an abundant extracellular intestinal or gastric type secretory mucin with high viscosity may be correlated, by a majority of the non-invasive type tumors, with the expansive growth of the tumors that display lower levels of invasion and metastasis.

Expression of MUC1 and MUC2 mucins in epithelial ovarian tumours

This is the first study to describe the association between expression of MUC1 and MUC2 mucins and prognosis in ovarian cancer. Paraffin sections of epithelial ovarian tumours (n = 182: 29 benign, 21 low malignant potential, and 132 invasive tumours) were analysed immunohistochemically for expression of MUC1 and MUC2 mucin core proteins. Most benign, low malignant potential, and invasive tumours showed high MUC1 expression in the cytoplasm. Low cytoplasmic expression of MUC1 was a predictor for good prognosis, particularly within stage III tumours. A minority of benign epithelial tumours, but most low malignant potential and invasive non-mucinous tumours, showed high MUC1 expression on the cell membrane. High apical MUC1 reactivity was associated with non-mucinous tumours. Low expression of MUC1 in the apical membrane was associated with early stage and good outcome for invasive tumours. Most benign and low malignant potential tumours, but only a minority of invasive tumours, showed MUC2 expression. MUC2 was found in non-mucinous as well as in mucinous tumours. The presence of MUC2 was inversely associated with high tumour grade but was not associated with altered survival. These results support experimental evidence that MUC1 influences the metastatic ability of ovarian cancer.

Identification and characterization of the MUC2 (human intestinal mucin) gene 5'-flanking region: promoter activity in cultured cells

The initiation point for MUC2 gene transcription is located within a 7000-base GC-rich region of the mucin gene cluster found on chromosome 11p15.5. The promoter activity of the 5'-flanking region of the MUC2 gene was examined following its cloning into the luciferase-producing pGL2-Basic reporter vector. A short segment comprising bases -91 to -73 relative to the start of transcription was found to be important for basal promoter activity in all cell lines tested. Electrophoretic mobility shift assays demonstrated nuclear protein binding to this region, which contains the consensus CACCC motif (5'-GCCACACCC). This element has been shown to be functionally important in several promoters that are active in diverse cell types. Competition experiments using an Sp1 oligonucleotide and antibody supershift experiments indicated that both Sp1 and other Sp1 family members bind to this element. Inclusion of the region between bases -228 and -171 in pGL2-Basic constructs increased normalized luciferase reporter activity by almost 3-fold in C1a cells, which produce relatively high levels of MUC2 mRNA. Significantly lower levels of normalized luciferase activity resulted when the same construct was transfected into cultured cell lines that express low or undetectable levels of MUC2, suggesting a possible role for this region in conferring cell-type specificity of expression. We also demonstrate, using actinomycin D, that the MUC2 mRNA is long-lived, at least in cultured cells. Moreover, no evidence was found that the MUC2 mRNA turned over more rapidly in LS174T cells, which produce relatively low levels of MUC2 mRNA, as compared with C1a cells, which produce high levels of mRNA. Thus a long mRNA half-life appears to be an important mechanism involved in achieving elevated levels of MUC2 mRNA.

The carboxyl-terminal sequence of the human secretory mucin, MUC6. Analysis Of the primary amino acid sequence

The distribution of MUC6 suggests that its primary function is protection of vulnerable epithelial surfaces from damaging effects of constant exposure to a wide range of endogenous caustic or proteolytic agents. A combination of genomic, cDNA. and 3' rapid amplification of cDNA ends techniques was used to isolate the carboxyl-terminal end of MUC6. The 3' nontandem repeat region contained 1083 base pairs of coding sequence (361 amino acids) followed by 632 base pairs of 3'-untranslated region. The coding sequence consists of two distinct regions; region 1 contained the initial 270 amino acids (62% Ser-Thr-Pro with no Cys residues), and region 2 contained the COOH-terminal 91 amino acids (22% Ser-Thr-Pro with 12% Cys). Although region 1 had no homology to any sequences in GenBank, region 2 had approximately 25% amino acid homology to the COOH-terminal regions of human mucins MUC2, -5, and -5B and von Willebrand factor. The shortness of region 2 would leave little of the peptide backbone exposed to a potentially hostile environment. Antibody studies suggest that MUC6 in its native form exists as a disulfide-bonded multimer. The conservation of the 11 cysteine positions in region 2 suggests the importance of this short region to mucin polymerization.

MUC5AC, but not MUC2, is a prominent mucin in respiratory secretions

Airway mucus was collected from healthy and chronic bronchitic subjects. The chronic bronchitic sputum was separated into gel and sol phase by centrifugation and mucins were isolated using isopycnic density-gradient centrifugation in CsCl. The presence of the MUC5AC and MUC2 mucins was investigated with antisera raised against synthetic peptides with sequences from the respective apoproteins. The gel and sol phase of chronic bronchitic sputum as well as healthy respiratory secretions were shown to contain MUC5AC whereas the MUC2 mucin could not be detected. Rate-zonal centrifugation showed that the MUC5AC mucin was large, polydisperse in size and that reduction yielded subunits. Ion-exchange HPLC revealed the presence of two subunit populations in all secretions, the MUC5AC subunits always being the more acidic. MUC5AC is thus the first large, subunit-based, gel-forming respiratory mucin identified and this glycoprotein is biochemically distinct from at least one other population of large, gel-forming mucins also composed of subunits but lacking a genetic identity.

Identification of a surface glycoprotein on African green monkey kidney cells as a receptor for hepatitis A virus

Very little is known about the mechanism of cell entry of hepatitis A virus (HAV), and the identification of cellular receptors for this picornavirus has been elusive. Here we describe the molecular cloning of a cellular receptor for HAV using protective monoclonal antibodies raised against susceptible African green monkey kidney (AGMK) cells as probes. Monoclonal antibodies 190/4, 235/4 and 263/6, which reacted against similar epitopes, specifically protected AGMK cells against HAV infection by blocking the binding of HAV. Expression cloning and nucleotide sequence analysis of the cDNA coding for epitope 190/4 revealed a novel mucin-like class I integral membrane glycoprotein of 451 amino acids, the HAV cellular receptor 1 (HAVcr-1). Immunofluorescence analysis indicated that mouse Ltk- cells transfected with HAVcr-1 cDNA gained limited susceptibility to HAV infection, which was blocked by treatment with monoclonal antibody 190/4. Our results demonstrate that the HAVcr-1 polypeptide is an attachment receptor for HAV and strongly suggest that it is also a functional receptor which mediates HAV infection. This report constitutes the first identification of a cellular receptor for HAV.

Cloning and sequence homology of a rat UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase

A UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (polypeptide GalNAc transferase) cDNA was amplified from rat sublingual, submandibular and parotid glands, brain, skeletal muscle, and liver, using the polymerase chain reaction (PCR) and sequences derived from bovine polypeptide GalNAc transferase-Type 1 (polypeptide GalNAc transferase-T1). The transcripts encoding the rat sublingual gland and bovine enzymes were 91% identical in nucleotide sequence, except in their 5' and 3' untranslated regions. The enzymes encoded by the rat and bovine cDNAs were 559 amino acids in length and were virtually identical (98% amino acid sequence identity and 99.5% homologous overall). Northern blot analysis indicates that the polypeptide GalNAc transferase-T1 transcripts are expressed in many tissues but at widely differing levels. Although the amino acid sequence of polypeptide GalNAc transferase-T1 is conserved among mammals, the pattern of tissue expression varies between rats and humans. For example, the steady-state level of polypeptide GalNAc transferase-T1 transcript is quite low in lung relative to other rat tissues, whereas high expression of this transcript is detected in human lung. Therefore, we surmise that isoforms of polypeptide GalNAc transferase must exist and that isoforms are expressed in a tissue-dependent fashion. Searches of the GenBank database have revealed homologous sequences for several isoforms derived from several human tissues. In addition, hypothetical proteins from C. elegans also display strong homology; evidence suggests six ancestral isoforms of polypeptide GalNAc transferases may exist in C. elegans.

MUC2 mucin gene expression in the nose and maxillary sinus

PURPOSE

The MUC2 gene encodes the core protein of a mucin expressed in the intestine and lower airway. The purpose of this study is to examine if the MUC2 gene is expressed in the nose and maxillary sinus.

MATERIALS AND METHODS

A Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) were performed. For the Northern blot analysis, RNAs were extracted from maxillary mucosae and nasal polyps of patients with chronic sinusitis and from the inferior turbinates of a nasal allergy patient. For RT-PCR, RNAs were extracted from 10 patients with chronic sinusitis, 4 patients with allergic rhinitis, 2 patients with hypertrophic rhinitis, and 6 volunteers with normal nasal mucosa.

RESULTS

Hybridization of the Northern blot with SMUC41 (a part of MUC2) cDNA probe showed clear bands in 2 of the 3 samples. In RT-PCR, the first round of amplification (35 cycles) failed to give any bands, but the additional 30 cycles with internal primers gave bands in 6 of 22 samples.

CONCLUSIONS

These results indicate that MUC2 mucin gene is expressed in the nose and paranasal sinus. This two-round RT-PCR method will be useful for analysis of MUC2 mucin gene expression using relatively small amount of RNA.

Retinoic acid-regulated cellular differentiation and mucin gene expression in isolated rabbit tracheal-epithelial cells in culture

Rabbit tracheal epithelial cells were cultured in a serum-free and hormone-supplemented medium with and without retinoic acid. The cells showed time-dependent mucin gene expression when cultured in the medium with retinoic acid. In the absence of retinoic acid, however, mucin mRNA was barely detectable in the cells. When retinoic acid was added back to the medium, the mucin message was prominent again. Actinomycin D and cycloheximide did not inhibit mucin gene expression. The mucin message was slightly elevated by cAMP agonists. A mucin antisense oligomer inhibited the retinoic acid-induced mucin mRNA expression and secretion, thus offering an alternate approach in the management of mucus hypersecretion in upper airway respiratory diseases such as chronic bronchitis, asthma, and cystic fibrosis.

Mucins: structure, function, and associations with malignancy

Mucins are a family of high molecular weight, highly glycosylated glycoproteins found in the apical cell membrane of human epithelial cells from the mammary gland, salivary gland, digestive tract, respiratory tract, kidney, bladder, prostate, uterus and rete testis. Increased synthesis of the core protein and alterations in the carbohydrates attached to these glycoproteins are believed to play important roles in the function and proliferation of tumour cells. Aberrant glycosylation leads not only to the production of novel carbohydrate structures, but also to the exposure of the core peptide. These novel epitopes may be candidates for diagnosis or therapy, by using either synthetic mucin fragments as vaccines, or monoclonal antibody-based reagents which detect these structures.

Characterization of the human mucin gene MUC5AC: a consensus cysteine-rich domain for 11p15 mucin genes?

To date five human mucin cDNAs (MUC2, 5A, 5B, 5C and 6) mapped to 11p15.3-15.5, so it appears that this chromosome region might contain several distinct gene loci for mucins. Three of these cDNAs, MUC5A, B and C, were cloned in our laboratory and previously published. A common number, 5, was recommended by the Human Gene Mapping Nomenclature Committee to designate them because of their common provenance from human tracheobronchial mucosa. In order to define whether they are products of the same gene locus or distinct loci, we describe in this paper physical mapping of these cDNAs using the strategy of analysis of CpG islands by pulse-field gel electrophoresis. The data suggest that MUC5A and MUC5C are part of the same gene (called MUC5AC) which is distinct from MUC5B. In the second part of this work, complete sequences of the inserts corresponding to previously described (JER47, JER58) and novel (JER62, JUL32, MAR2, MAR10 and MAR11) cDNAs of the so-called MUC5AC gene are presented and analysed. The data show that in this mucin gene, the tandem repeat domain is interrupted several times with a subdomain encoding a 130 amino acid cysteine-rich peptide in which the TR3A and TR3B peptides previously isolated by Rose et al. [Rose, Kaufman and Martin (1989) J. Biol. Chem., 264, 8193-8199] from airway mucins are found. A consensus peptide sequence for these subdomains involving invariant positions of most of the cysteines is proposed. The consensus nucleotide sequence of this subdomain is also found in the MUC2 gene and in the MUC5B gene, two other mucin genes mapped to 11p15. The functional significance for secreted mucins of these cysteine-rich subdomains and the modular organization of mucin peptides are discussed.

Structure, biosynthesis, and function of salivary mucins

The glandular secretions of the oral cavity lining the underlying buccal mucosa are highly specialized fluids which provide lubrication, prevent mechanical damage, protect efficiently against viral and bacterial infections, and promote the clearance of external pollutants. This mucus blanket contains large glycoproteins termed mucins which contribute greatly to the viscoelastic nature of saliva and affect its complex physiological activity. The protein core of mucins consists of repetitive sequences, rich in O-glycosylated serine and threonine, and containing many helix-breaking proline residues. These features account for the extended, somewhat rigid structure of the molecule, a high hydrodynamic volume, its high buoyant density, and high viscosity. The oligosaccharide moiety of salivary mucins accounts for up to 85% of their weight. The oligosaccharide side chains exhibit an astonishing structural diversity. The isolation, composition, structure, molecular characteristics, and functional relevance of salivary mucins and their constituents is discussed in relation to recent advancements in biochemistry and molecular biology.

Localization of mucin (MUC2 and MUC3) messenger RNA and peptide expression in human normal intestine and colon cancer

BACKGROUND/AIMS

Several studies have reported Northern blot data showing that mucin is expressed in a tissue-specific manner. To determine whether expression is limited to specific cell types within these tissues requires histological analysis.

METHODS

Both immunocytochemistry and in situ hybridization were used to identify cell types expressing the MUC2 and MUC3 mucins in the human small intestine, colon, and colon carcinoma.

RESULTS

In the normal small intestine and colon, an antibody recognizing the MUC2 apomucin stained goblet cells. In contrast, an antibody recognizing the MUC3 apomucin stained both goblet and absorptive cells. Consistent with this, in situ hybridization showed MUC2 messenger RNA (mRNA) only in goblet cells and MUC3 mRNA in both goblet and absorptive cells. In several samples of moderately well-differentiated colon cancer, MUC2 and MUC3 showed distinct patterns of expression, but the expression level of each was reduced compared with levels in normal tissue; there was considerable tumor-to-tumor and cell-to-cell variability using both mucin antibodies and complementary DNA probes.

CONCLUSIONS

Individual mucin genes have distinct patterns of expression within mucin-producing tissues, suggesting that the various mucin gene products play distinct functional roles.

Differential apomucin expression in normal and neoplastic human gastrointestinal tissues

BACKGROUND/AIMS

The cloning of genes encoding human mucins is the basis for the study of their normal tissue distribution and the alterations associated with cancer. The aim of this study was to determine the normal and tumor tissue expression of MUC1, MUC2, MUC5B, and MUC5C.

METHODS

The reactivity of apomucin-specific antibodies with fresh normal and tumor tissues was analyzed using immunohistochemical techniques.

RESULTS

Anti-MUC1 antibodies reacted with most glandular epithelia. Anti-MUC2 antibody was mainly reactive with intestinal goblet cells and cervical mucous cells. Anti-MUC5B was reactive with a wide range of epithelial tissues whereas anti-MUC5C was reactive with stomach, trachea, and endocervix. Double-labeling experiments showed coexpression of MUC1/MUC2 and MUC2/MUC5C in colonic tissue. Multiple apomucins were detected in colon cancers, but no relationship to histochemical mucus stains was observed.

CONCLUSIONS

It is concluded that (1) each apomucin shows a distinct tissue expression pattern; (2) multiple apomucins are present in a single tissue and at the single cell level; and (3) altered apomucin expression takes place in pathological colonic tissue.

The role of airway mucus in pulmonary toxicology

Airway mucus is a complex airway secretion whose primary function as part of the mucociliary transport mechanism is to to serve as renewable and transportable barrier against inhaled particulates and toxic agents. The rheologic properties necessary for this function are imparted by glycoproteins, or mucins. Some respiratory disease states, e.g., asthma, cystic fibrosis, and bronchitis, are characterized by quantitative and qualitative changes in mucus biosynthesis that contribute to pulmonary pathology. Similar alterations in various aspects of mucin biochemistry and biophysics, leading to mucus hypersecretion and altered mucus rheology, result from inhalation of certain air pollutants, such as ozone, sulfur dioxide, nitrogen dioxide, and cigarette smoke. The consequences of these pollutant-induced alterations in mucus biology are discussed in the context of pulmonary pathophysiology and toxicology.

Effect of retinoic acid on mucin gene expression in rat airways in vitro

Ultrastructural examination of rat tracheal explants at various times of culture in a serum-free and hormone-supplemented medium containing retinoic acid showed that the cytological characteristics of the epithelium were well preserved for at least 192 h. Hybridization analyses for mucin core protein mRNA in the explants were performed with a 30-base oligonucleotide probe, the design of which was based on the tandem repeat sequence of the rat intestine mucin core protein. The probe reacted with total RNA prepared from trachea, intestine and colon, but not with total RNA obtained from liver or alveolar region of the lung. Type-I keratin expression was observed in the explant grown at different periods of time in a medium with and without retinoic acid. The hybridization probe gave a prominent reaction with RNA preparations obtained from tracheal explants incubated for as long as 192 h in a medium containing retinoic acid. In the absence of retinoic acid, however, the mucin message was evident at the 24 h time point but thereafter decreased to barely detectable levels. When retinoic acid was added at 96 h to the latter cultures, the mucin mRNA was prominent again after additional incubation for 24 and 48 h. Northern-blot analyses of tracheal RNA showed a diffuse band at approx. 7.5 kb. Addition of a variety of chemical and pharmacological agents to explants cultured in the presence of retinoic acid had no dramatic induction or inhibitory effects on the mucin mRNA. Only the steroid prednisolone had a reproducible inhibitory effect.

Antigenic similarities between respiratory and reproductive tract mucins: heterogeneity of mucin expression by human endocervix and endometrium

OBJECTIVES

To determine whether common epitopes are shared by respiratory and reproductive tract mucins and to compare the expression of cross-reactive mucin subtypes in human endocervix and endometrium.

DESIGN

An immunohistochemical study of mucin expression using a panel of monoclonal and polyclonal antimucin antibodies and timed endocervical and endometrial biopsies.

SETTING

University of Oklahoma Health Sciences Center, a tertiary care referral center.

PATIENTS

Twenty-eight women who underwent laparoscopy, laparotomy, or hysterectomy.

MAIN OUTCOME MEASURES

The expression of human tracheal mucin subspecies (types I to V) in endocervix (n = 3) and endometria (n = 25).

RESULTS

Of the five mucin subspecies, type I mucin was localized to the squamous epithelium of endocervix and both glands and stroma of endometrium. Both tissues failed to react with type II mucin. Type III mucin was localized to differentiated cells of the squamous epithelium of endocervix and the glandular endometrium. Type IV mucin was specific to endometrium and was localized both in endometrial glands and stroma with no reactivity with endocervix. Type V mucin was expressed in both cervical and endometrial stroma and glands.

CONCLUSIONS

Human respiratory and reproductive tract mucins share common peptide and carbohydrate epitopes. Human endocervix and endometria express a unique pattern of mucin antigens. Because of their restricted specificity, these monoclonal antibodies could provide new tools to investigate normal and aberrant expression of reproductive tract mucin subtypes in tissues and secretions.

Differential expression of the human mucin genes MUC1 to MUC5 in relation to growth and differentiation of different mucus-secreting HT-29 cell subpopulations

Mucin expression was analysed, in relation to cell growth, in parental HT-29 cells and in two populations of mucus-secreting HT-29 cells selected by adaptation to methotrexate (HT29-MTX) or 5-fluorouracil (HT29-FU). These two populations express mature mucins that differ in their immunoreactivity to antibodies against gastric (HT29-MTX) or colonic mucins (HT29-FU). In the parental population, at late confluency, only very few cells produce mucins or the MUC1 glycoprotein, this being consistent with the low level of expression of the mRNAs corresponding to the MUC1 to MUC5C mucin genes. In the HT29-MTX and HT29-FU populations, the appearance of mucus droplets, as shown by histochemistry and immunofluorescence, starts a few days after confluency, progressively involving a greater proportion of cells and reaching a steady state at late confluency. The MUC1 glycoprotein appears earlier, already being detectable in preconfluent cells. Its distribution is restricted to the apical surface of the cells and is distinct from that of the mucus droplets. In both populations the growth-related levels of MUC1 mRNA are concordant with the apparent levels of expression of the MUC1 glycoprotein. The levels of MUC2, MUC3, MUC4 and MUC5C mRNAs differ from one population to another and, within each population, according to the stage of the culture. The highest levels of MUC2 and MUC4 mRNAs are found in the HT29-FU cells, whereas the highest levels of MUC3 and MUC5C are found in the HT29-MTX cells, suggesting that the differences observed in the mature mucins expressed by either population may be related to which MUC genes are expressed. In both populations significant or even high levels of MUC mRNAs are already present in early cultures, i.e. at a stage when the mature mucins are not yet detectable, suggesting that mucin maturation is a later event.

Molecular cloning and sequencing of a canine tracheobronchial mucin cDNA containing a cysteine-rich domain

To date the complete sequence of only one mammalian mucin cDNA, MUC1, has been reported, although several mucin proteins have been partially characterized. Here we report the nucleotide sequence of a canine tracheal mucin cDNA containing two potential translation initiation codons, one translation termination codon and a poly(A) tail. A lambda gt11 cDNA library prepared from canine tracheal epithelial cells was screened with polyclonal anti-apo-canine tracheal mucin antibodies with the aim of obtaining the deduced amino acid sequence of the mucin core protein. Antibody-positive clones containing overlapping inserts of various lengths were purified and used for nucleotide sequencing. Based on the sequencing data, synthetic oligonucleotide primers were constructed and both ends (5' and 3') of the cDNA were determined. The complete sequence was 3.7 kb and included an open reading frame with coding capacity for 1118 aa, two translation initiation ATG codons in context with Kozak consensus sequences, one polyadenylylation site, and a poly(A) stretch. The protein was rich in Thr, Pro, Ser, Gly, and Ala and poor in Tyr, Phe, and Trp. Although tandem repeats of amino acids were absent in the deduced canine tracheal mucin sequence, motifs TPTPTP and TTTTPV appeared 13 and 19 times, respectively. The C-terminal region contained a Cys-rich domain (although a few Cys residues were also present in the middle of the protein) as has been reported for bovine submaxillary mucin, porcine submaxillary mucin, rat intestinal mucin, human intestinal mucin, and frog skin mucin. This suggested that a broad group of mucins contain such a Cys-rich domain whose functional significance is yet to be understood. Three potential N-glycosylation sites were present in canine tracheal mucin and the amino acid sequence showed homology with both human tracheal and intestinal mucins. The N-terminal domain showed more flexibility (probably due to a high number of Pro residues in this region) when analyzed by the University of Wisconsin Genetics Computer Group program package to determine the predicted secondary structure. Evaluation of the transcripts using the canine mucin cDNA as a probe indicated a polydisperse message with total RNA.

Monoclonal antibodies reacting with the MUC2 mucin core protein

This study sought to produce monoclonal antibodies (MAbs) which reacted with the MUC2 core protein. Two MAbs [3A2 (IgG1) and 4F1 (IgM)] were produced by immunising female BALB/c mice with gel-formed mucin from the LS174T colon cancer cell line followed by a KLH conjugate of a 29 amino acid synthetic peptide whose sequence was derived from the variable number of tandem repeats (VNTR) region of a MUC2 cDNA clone. The MAbs reacted with synthetic MUC2 VNTR peptides but not synthetic MUC1 or MUC3 VNTR peptides, and showed specific reactivity in Western blotting with a high molecular weight protein produced by the LS174T colon carcinoma cell line. The use of shorter peptides indicated that the minimum peptide epitopes for these MAbs were different. Mab 3A2 reacted with amino acids 5-19 of the MUC2 VNTR by inhibition ELISA but not by direct ELISA, while 4F1 reacted with this peptide in both assays. Furthermore, 4F1 reacted in direct ELISA when a larger (29 amino acid) MUC2-derived peptide was coated onto the assay plate by incubating in carbonate buffer or by drying the peptide onto the assay plate, while 3A2 only reacted when this peptide was coated in carbonate buffer. The different specificity of the MAbs was also illustrated by the reactivity of 4F1 but not 3A2 with partially deglycosylated cystic fibrosis mucin. Immunohistochemical analysis with these MAbs revealed a strong reactivity with lung, gastric and colon tumours relative to normal tissue, with some breast and ovarian tumours also reacting. Both MAbs stained some normal goblet cells in the perinuclear region but not the mucin droplet or secreted mucin, indicating a reaction with immature (poorly glycosylated) mucin in the endoplasmic reticulum and/or golgi, but not with mature (fully glycosylated) mucin. In contrast, tumours showed strong diffuse cytoplasmic staining. 4F1 also showed weak apical cytoplasmic staining in some goblet cells and stained some tumours which showed no reactivity with 3A2. These antibodies should prove useful in the study of MUC2 structure and function, and in the diagnosis of some tumours.

Detection of the MUC2 apomucin tandem repeat with a mouse monoclonal antibody

BACKGROUND

The MUC2 intestinal mucin gene contains tandem repeats of 23 amino acid length that are rich in threonine.

METHODS

Mouse monoclonal antibody LDQ10 was raised against chemically deglycosylated mucin isolated from LS174T colon cancer nude mouse xenografts.

RESULTS

LDQ10 reacts with deglycosylated colon cancer mucin and with a synthetic peptide encompassing the MUC2 tandem repeat sequence. In immunohistochemical assays, strong reactivity with goblet cells in colon, small bowel, and stomach is observed; weaker reactivity with mucin-producing cells in other epithelial tissues is shown. The epitope recognized by LDQ10 is localized in the rough endoplasmic reticulum of normal colonic goblet cells. LDQ10 also shows strong reactivity with colorectal and stomach cancers and weaker reactivity with pancreas, breast, and bladder cancers.

CONCLUSIONS

Antibody LDQ10 detects a peptide epitope of MUC2 that becomes cryptic on glycosylation. Altered synthesis of the MUC2 apomucin takes place in a variety of epithelial cancers.

Molecular cloning of the carboxy terminus of a canine tracheobronchial mucin

A cDNA library constructed from canine tracheal mRNA was screened with polyclonal antiserum specific to canine tracheal apomucin (CTM-A). Eight antibody reactive clones were isolated and purified to clonality. One of the clones, designated pCTM-A, had a 1.7 kb insert and included a single open reading frame with a poly (A)+ tail. The amino acid composition of the encoded protein was consistent with that expected for CTM-A. The fusion protein produced by cloning the 1.7 kb insert in the pMALc expression vector reacted with the purified anti-apomucin CTM-A antibody. Also, polyclonal antibodies raised to the purified protein product encoded by pCTM-A reacted with deglycosylated CTM-A confirming that this clone does indeed code for apomucin CTM-A. This is the first report of a cDNA encoding the C-terminus of a canine tracheal mucin.

Mucin genes and the proteins they encode: structure, diversity, and regulation

Mucins are the structural components of the mucus gels that protect the respiratory, gastrointestinal, and reproductive tracts. These polydisperse glycoproteins (250,000 to 20,000,000 D) are approximately 80% carbohydrate on a mass basis and have a high intrinsic viscosity due to their large size and extreme hydrophilicity. Mucin oligosaccharides, the structures responsible for this hydrophilicity, are heterogeneous in size and structure but are chiefly O-linked, i.e., they initiate from N-acetylgalactosamine residues attached to threonine and serine residues of the polypeptide backbone. Our understanding of the structure of mucins has advanced rapidly in the last few years with the isolation and sequencing of cDNA clones that encode mucin polypeptide backbones. All currently well-characterized mucins have been found to contain extended arrays of tandemly repeated peptides rich in potential O-glycosylation sites. Less is known about the unique sequences that flank the tandem repeat arrays of secretory mucins, but currently available information indicates that these flanking regions contain cysteine-rich stretches that participate in mucin oligomer formation. Thus, secretory mucins appear to consist of oligomers containing heavily glycosylated domains flanked by unique sequences required for polymerization. Progress has also been made in characterizing the genes that encode mucins. At least four human mucin genes are known at present, although many others may remain to be discovered. Moreover, much work remains before we gain an understanding of the mechanisms involved in the expression of mucin genes and their tissue-specific regulation.

Structural features of the low-molecular-mass human salivary mucin

The low-molecular-mass human salivary mucin has at least two isoforms, MG2a and MG2b, that differ primarily in their sialic acid and fucose content. In this study, we characterize further these isoforms, particularly their peptide moieties. Trypsin digests of MG2a and MG2b yielded high- and low-molecular-mass glycopeptides following gel filtration on Sephacryl S-300. The larger glycopeptides from MG2a and MG2b had similar amino acid compositions and identical N-terminal sequences, suggesting common structural features between their peptides. An oligonucleotide probe generated from the amino acid sequence of the smaller glycopeptide from MG2a was employed in Northern-blot analysis. This probe specifically hybridized to two mRNA species from human submandibular and sublingual glands. A cDNA clone selected from a human submandibular gland cDNA expression library with antibody generated against deglycosylated MG2a also hybridized to these two mRNA species. In both cases, the larger mRNA was polydisperse, and the hybridization signal was more intense in the sublingual gland. In addition, the N-terminal amino acid sequence of the larger glycopeptide was found to be part of one of the selected MG2 cDNA clones.

In vitro effects of drugs on production of mucins in rabbit tracheal epithelial cells expressing mucin gene: a model system for studying upper airway respiratory diseases

Rabbit tracheal epithelial cells, cultured on collagen-coated dishes in serum-free and hormone-supplemented medium, were found to incorporate [3H]glucosamine into high-molecular-weight components that were secreted in the medium. The chemical analysis of the secreted products resulted in a profile that resembled that of mucous glycoproteins (mucins). When examined by dot blot analysis, the total RNA isolated from these cells hybridized to an antisense 30-mer oligonucleotide corresponding to a rat intestine mucin peptide sequence, indicating that mucin gene was expressed in these cell lines. Lung and liver tissues of rabbit did not express this gene. Transmission electron microscopy exhibited secretory granules in these cells. The incorporation of [3H]glucosamine into mucins was inhibited by three aryl-N-acetyl-galactosaminides and a chemical carcinogen, N-nitroso-N-ethyl urea, whereas 5-azacytidine enhanced the proliferation of cells as well as the radiolabeling of mucins. Parasympathetic agent (pilocarpine), cholinergic antagonist (atropine), and beta-adrenergic agonist (isoproterenol) alone have little effect on the secretion of mucins. The cholinergic agonist, methacholine, was found to increase the production of mucins and addition of atropine to the medium before methacholine blocked this stimulation. Histamine was found to stimulate mucin production in these cells.

MUC1 mucin mRNA expression in cultured human nasal and bronchial epithelial cells

The MUC1 mucin mRNA, for which the cDNA was previously cloned from human breast and pancreatic tissues, was found to be expressed in nasal and bronchial epithelial cell primary cultures from cystic fibrotic, atopic, and normal individuals. Sequence analysis of cDNA clones from the CF/T43 cystic fibrosis nasal epithelial cell line revealed only insignificant differences in the 3' untranslated region of the mRNA when compared with the pancreas and breast mucin cDNAs.

Mucin-type glycoproteins

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

MUC-2 human small intestinal mucin gene structure. Repeated arrays and polymorphism

MUC-2, the first described intestinal mucin gene, has become important as a prototype for secreted mucins in several organ systems. However, little is known about its protein backbone structure and hence its role in diseases such as colon cancer, ulcerative colitis, and cystic fibrosis, which are known to have mucin abnormalities. Studies in this manuscript show that MUC-2 contains two distinct regions with a high degree of internal homology, but the two regions bear no significant homology to each other. Region 1 consists mostly of 48-bp repeats which are interrupted in places by 21-24-bp segments. Several of these interrupting sequences show similarity to each other, creating larger composite repeat units. Region 1 has no length polymorphisms. Region 2 is composed of 69-bp tandem repeats arranged in an uninterrupted array of up to 115 individual units. Southern analysis of genomic DNA samples using TaqI and HinfI reveals both length and sequence polymorphisms which occur within region 2. The sequence polymorphisms have different ethnic distributions, while the length polymorphisms are due to variable numbers of tandem repeats.