MUC7 gene expression and genetic polymorphism

Pathological Implications of Mucin Signaling in Metastasis

The dynamic mucosal layer provides a selective protective barrier for the epithelial cells lining the body cavities. Diverse human malignancies exploit their intrinsic role to protect and repair epithelia for promoting growth and survival. Aberrant expression of mucin has been known to be associated with poor prognosis of many cancers. However, the emergence of new paradigms in the study of metastasis recognizes the involvement of MUC1, MUC4, MUC5AC, MUC5B, and MUC16 during metastasis initiation and progression. Hence mucins can be used as an attractive target in future diagnostic and therapeutic strategies. In this review, we discuss in detail about mucin family and its domains and the role of different mucins in regulating cancer progression and metastasis. In addition, we briefly discuss insights into mucins as a therapeutic agent.

Human and Nonhuman Primate Lineage-Specific Footprints in the Salivary Proteome

Proteins in saliva are needed for preprocessing food in the mouth, maintenance of tooth mineralization, and protection from microbial pathogens. Novel insights into human lineage-specific functions of salivary proteins and clues to their involvement in human disease can be gained through evolutionary studies, as recently shown for salivary amylase AMY1 and salivary agglutinin DMBT1/gp340. However, the entirety of proteins in saliva, the salivary proteome, has not yet been investigated from an evolutionary perspective. Here, we compared the proteomes of human saliva and the saliva of our closest extant evolutionary relatives, chimpanzees and gorillas, using macaques as an outgroup, with the aim to uncover features in saliva protein composition that are unique to each species. We found that humans produce a waterier saliva, containing less than half total protein than great apes and Old World monkeys. For all major salivary proteins in humans, we could identify counterparts in chimpanzee and gorilla saliva. However, we discovered unique protein profiles in saliva of humans that were distinct from those of nonhuman primates. These findings open up the possibility that dietary differences and pathogenic pressures may have shaped a distinct salivary proteome in the human lineage.

Archaic Hominin Introgression in Africa Contributes to Functional Salivary MUC7 Genetic Variation

One of the most abundant proteins in human saliva, mucin-7, is encoded by the MUC7 gene, which harbors copy number variable subexonic repeats (PTS-repeats) that affect the size and glycosylation potential of this protein. We recently documented the adaptive evolution of MUC7 subexonic copy number variation among primates. Yet, the evolution of MUC7 genetic variation in humans remained unexplored. Here, we found that PTS-repeat copy number variation has evolved recurrently in the human lineage, thereby generating multiple haplotypic backgrounds carrying five or six PTS-repeat copy number alleles. Contrary to previous studies, we found no associations between the copy number of PTS-repeats and protection against asthma. Instead, we revealed a significant association of MUC7 haplotypic variation with the composition of the oral microbiome. Furthermore, based on in-depth simulations, we conclude that a divergent MUC7 haplotype likely originated in an unknown African hominin population and introgressed into ancestors of modern Africans.

Physicochemical properties of mucus and their impact on transmucosal drug delivery

Mucus is a selective barrier to particles and molecules, preventing penetration to the epithelial surface of mucosal tissues. Significant advances in transmucosal drug delivery have recently been made and have emphasized that an understanding of the basic structure, viscoelastic properties, and interactions of mucus is of great value in the design of efficient drug delivery systems. Mucins, the primary non-aqueous component of mucus, are polymers carrying a complex and heterogeneous structure with domains that undergo a variety of molecular interactions, such as hydrophilic/hydrophobic, hydrogen bonds and electrostatic interactions. These properties are directly relevant to the numerous mucin-associated diseases, as well as delivering drugs across the mucus barrier. Therefore, in this review we discuss regional differences in mucus composition, mucus physicochemical properties, such as pore size, viscoelasticity, pH, and ionic strength. These factors are also discussed with respect to changes in mucus properties as a function of disease state. Collectively, the review seeks to provide a state of the art roadmap for researchers who must contend with this critical barrier to drug delivery.

Genetically engineered mucin mouse models for inflammation and cancer

Mucins are heavily O-glycosylated proteins primarily produced by glandular and ductal epithelial cells, either in membrane-tethered or secretory forms, for providing lubrication and protection from various exogenous and endogenous insults. However, recent studies have linked their aberrant overexpression with infection, inflammation, and cancer that underscores their importance in tissue homeostasis. In this review, we present current status of the existing mouse models that have been developed to gain insights into the functional role(s) of mucins under physiological and pathological conditions. Knockout mouse models for membrane-associated (Muc1 and Muc16) and secretory mucins (Muc2) have helped us to elucidate the role of mucins in providing effective and protective barrier functions against pathological threats, participation in disease progression, and improved our understanding of mucin interaction with biotic and abiotic environmental components. Emphasis is also given to available transgenic mouse models (MUC1 and MUC7), which has been exploited to understand the context-dependent regulation and therapeutic potential of human mucins during inflammation and cancer.

Recent evolution of the salivary mucin MUC7

Genomic structural variants constitute the majority of variable base pairs in primate genomes and affect gene function in multiple ways. While whole gene duplications and deletions are relatively well-studied, the biology of subexonic (i.e., within coding exon sequences), copy number variation remains elusive. The salivary MUC7 gene provides an opportunity for studying such variation, as it harbors copy number variable subexonic repeat sequences that encode for densely O-glycosylated domains (PTS-repeats) with microbe-binding properties. To understand the evolution of this gene, we analyzed mammalian and primate genomes within a comparative framework. Our analyses revealed that (i) MUC7 has emerged in the placental mammal ancestor and rapidly gained multiple sites for O-glycosylation; (ii) MUC7 has retained its extracellular activity in saliva in placental mammals; (iii) the anti-fungal domain of the protein was remodified under positive selection in the primate lineage; and (iv) MUC7 PTS-repeats have evolved recurrently and under adaptive constraints. Our results establish MUC7 as a major player in salivary adaptation, likely as a response to diverse pathogenic exposure in primates. On a broader scale, our study highlights variable subexonic repeats as a primary source for modular evolutionary innovation that lead to rapid functional adaptation.

Exploring the role and diversity of mucins in health and disease with special insight into non-communicable diseases

Mucins are major glycoprotein components of the mucus that coats the surfaces of cells lining the respiratory, digestive, gastrointestinal and urogenital tracts. They function to protect epithelial cells from infection, dehydration and physical or chemical injury, as well as to aid the passage of materials through a tract i.e., lubrication. They are also implicated in the pathogenesis of benign and malignant diseases of secretory epithelial cells. In Human there are two types of mucins, membrane-bound and secreted that are originated from mucous producing goblet cells localized in the epithelial cell layer or in mucous producing glands and encoded by MUC gene. Mucins belong to a heterogeneous family of high molecular weight proteins composed of a long peptidic chain with a large number of tandem repeats that form the so-called mucin domain. The molecular weight is generally high, ranging between 0.2 and 10 million Dalton and all mucins contain one or more domains which are highly glycosylated. The size and number of repeats vary between mucins and the genetic polymorphism represents number of repeats (VNTR polymorphisms), which means the size of individual mucins can differ substantially between individuals which can be used as markers. In human it is only MUC1 and MUC7 that have mucin domains with less than 40% serine and threonine which in turn could reduce number of PTS domains. Mucins can be considered as powerful two-edged sword, as its normal function protects from unwanted substances and organisms at an arm's length while, malfunction of mucus may be an important factor in human diseases. In this review we have unearthed the current status of different mucin proteins in understanding its role and function in various non-communicable diseases in human with special reference to its organ specific locations. The findings described in this review may be of direct relevance to the major research area in biomedicine with reference to mucin and mucin associated diseases.

Expression and localization of trefoil factor family genes in rat submandibular glands

The trefoil factor (TFF) family, which comprises TFF1, TFF2 and TFF3, plays an essential role in epithelial regeneration within the gastrointestinal tract. All three TFFs are present in human saliva; TFF3 is the predominant trefoil peptide. Little is known about the expression and tissue distribution of TFFs in rats, which are commonly used as a model system for human studies. We investigated the localization of the TFF genes that encode secretory peptides in rat submandibular glands (SMG). All three TFFs were expressed in rat SMG, although their location varied. Substantial amounts of TFF1 were detected only in the cytoplasm of epithelial cells in the SMG granular convoluted tubules (GCT), while TFF2 and TFF3 were widely distributed in the cytoplasm of epithelial cells of intercalated ducts (ID), striated ducts (SD) and interlobular ducts (ILD). The three TFFs also were detected especially in the lumens of the SD and ILD. Semi-quantitative RT-PCR and in situ hybridization experiments confirmed TFF1, TFF2 and TFF3 mRNA expressions in the SMG. Greater expression of TFF peptides and mRNA was observed in male rats than in females. The broad expression of TFFs in rat SMG cells and lumens suggests that TFFs function in this organ by their secretion into the duct lumens. We also found differences in TFF expression profiles between rat and human SMG; therefore, caution should be exercised when using rats as a model for human TFF studies.

Tear film mucins: front line defenders of the ocular surface; comparison with airway and gastrointestinal tract mucins

The ocular surface including the cornea and conjunctiva and its overlying tear film are the first tissues of the eye to interact with the external environment. The tear film is complex containing multiple layers secreted by different glands and tissues. Each layer contains specific molecules and proteins that not only maintain the health of the cells on the ocular surface by providing nourishment and removal of waste products but also protect these cells from environment. A major protective mechanism that the corneal and conjunctival cells have developed is secretion of the innermost layer of the tear film, the mucous layer. Both the cornea and conjunctiva express membrane spanning mucins, whereas the conjunctiva also produces soluble mucins. The mucins present in the tear film serve to maintain the hydration of the ocular surface and to provide lubrication and anti-adhesive properties between the cells of the ocular surface and conjunctiva during the blink. A third function is to contribute to the epithelial barrier to prevent pathogens from binding to the ocular surface. This review will focus on the different types of mucins produced by the corneal and conjunctival epithelia. Also included in this review will be a presentation of the structure of mucins, regulation of mucin production, role of mucins in ocular surface diseases, and the differences in mucin production by the ocular surface, airways and gastrointestinal tract.

A variant form of the human deleted in malignant brain tumor 1 (DMBT1) gene shows increased expression in inflammatory bowel diseases and interacts with dimeric trefoil factor 3 (TFF3)

The protein deleted in malignant brain tumors (DMBT1) and the trefoil factor (TFF) proteins have all been proposed to have roles in epithelial cell growth and cell differentiation and shown to be up regulated in inflammatory bowel diseases. A panel of monoclonal antibodies was raised against human DMBT1^gp340. Analysis of lung washings and colon tissue extracts by Western blotting in the unreduced state, two antibodies (Hyb213-1 and Hyb213-6) reacted with a double band of 290 kDa in lung lavage. Hyb213-6, in addition, reacted against a double band of 270 kDa in colon extract while Hyb213-1 showed no reaction. Hyb213-6 showed strong cytoplasmic staining in epithelial cells of both the small and large intestine whereas no staining was seen with Hyb213-1. The number of DMBT1^gp340 positive epithelial cells, stained with Hyb213-6, was significantly up regulated in inflammatory colon tissue sections from patients with ulcerative colitis (p<0.0001) and Crohn’s disease (p = 0.006) compared to normal colon tissue. Immunohistochemical analysis of trefoil factor TFF1, 2 and 3 showed that TFF1 and 3 localized to goblet cells in both normal colon tissue and in tissue from patients with ulcerative colitis or Crohn’s disease. No staining for TFF2 was seen in goblet cells in normal colon tissue whereas the majority of tissue sections in ulcerative colitis and Crohn’s disease showed sparse and scattered TFF2 positive goblet cells. DMBT1 and TFF proteins did therefore not co-localize in the same cells but localized in adjacent cells in the colon. The interaction between DMBT1^gp340 and trefoil TFFs proteins was investigated using an ELISA assay. DMBT1^gp340 bound to solid-phase bound recombinant dimeric TFF3 in a calcium dependent manner (p<0.0001) but did not bind to recombinant forms of monomeric TFF3, TFF2 or glycosylated TFF2. This implies a role for DMBT1 and TFF3 together in inflammatory bowel disease.

Genetic polymorphism of MUC7 in individuals with aggressive or chronic periodontitis

Individuals with periodontitis exhibit differential expression of mucin-glycoprotein-2 (MG2), a protein encoded by the MUC7 gene. It is well known that MG2 exerts bactericidal activity as well as exhibiting genetic polymorphism involving a variable number of tandem repeats (VNTR). In the present study, we assessed the distribution of allelic variants of the MUC7 gene in 22 individuals with aggressive periodonitis, 68 with chronic periodonitis, and 87 without periodonitis. Oral mucosal cells were collected, the DNA was extracted, and specific primers were used to amplify the region encoding the MUC7 tandem repeats (TRs). Polymerase chain reaction products were subjected to electrophoresis and analyzed on polyacrylamide gels stained with silver nitrate. Although the percentage distribution of homozygosity (6-6TR) and heterozygosity (5-6TR) showed variation among the groups, the observed differences were not statistically significant (P > 0.05; Fisher's Exact Test). The present results indicate that the expression of different numbers of TRs in this salivary mucin in the oral environment does not interfere with the etiopathogenesis of aggressive or chronic periodontitis.

MUC7 polymorphisms are associated with a decreased risk of a diagnosis of asthma in an African American population

PURPOSE

Mucin glycoproteins contribute to lung pathophysiology in asthma. The protein backbone of mucin glycoproteins is encoded by specific MUC genes, which exhibit a high degree of polymorphisms that generate a variable number of tandem repeat (VNTR) domains. MUC7 typically encodes for 6 VNTRs, each with 23 amino acids. In a northern European cohort, a polymorphism encoding MUC7*5 (5-VNTR) is in 100% linkage disequilibrium with the single nucleotide polymorphism rs9982010 and associated with a decreased risk of being asthmatic and having better lung function. African Americans have a 5- to 10-fold increase in incidence of asthma relative to whites, who are believed to be partially associated with higher genetic susceptibility. Occurrence of the rs9982010 and MUC7 allelic frequencies was evaluated in inner-city African Americans to test their association with a diagnosis of asthma.

METHODS

Genomic DNA, collected from a cohort of African American asthmatic subjects, was used to detect the MUC7 VNTR polymorphisms and to analyze the rs9982010 single nucleotide polymorphism.

RESULTS

A logistic regression analysis showed that the MUC7*5-VNTR allele decreased the likelihood of a diagnosis of asthma (odds ratio, 0.173 [95% confidence interval, 0.041-0.737]; P < 0.018) and is not in a strong linkage disequilibrium with the rs9982010 (r = 0.03; odds ratio, 66; confidence interval, 5.913-736.72). A novel MUC7*4-VNTR polymorphism, identified in an African American nonasthmatic individual, was linked to a structural rearrangement of the VNTR domain.

CONCLUSIONS

These data extend the association of MUC7*5 allelic polymorphisms and asthma to inner-city African Americans.

Regulation of Human MUC7 Mucin Gene Expression by Cigarette Smoke Extract or Cigarette Smoke and Pseudomonas aeruginosa Lipopolysaccharide in Human Airway Epithelial Cells and in MUC7 Transgenic Mice

Objective:

The human MUC7 gene encodes a low-molecular-weight mucin glycoprotein that functions in lubrication/protection of epithelial surfaces of the oral cavity and respiratory tract. This study was designed to evaluate the effect of cigarette smoke extract (CSE), cigarette smoke (CS), and Pseudomonas aeruginosa lipopolysaccharide (LPS), either alone or in the combination, on MUC7 expression in vitro and in vivo.

Materials and Methods:

qRT-PCR was used to determine the levels of mucin gene transcription in the human lung carcinoma cell line NCI-H292 (in vitro) and MUC7 transgenic mouse tissues (in vivo). ELISA was used to assess mucin glycoprotein levels in the cell line, and immunohistochemistry to assess mucins in lung and trachea sections.

Results:

In vitro treatment of cells with LPS (10 (µg/ml) or CSE (0.5, 1, 2.5 and 5%) alone, resulted in a statistically significant increase of MUC7 transcripts only with 1%CSE (3.2-fold). The combined CSE/LPS treatment resulted in a synergistic increase of MUC7 with 0.5%CSE/LPS (4.4 fold). MUC7 glycoprotein levels increased only minimally, the highest increase was seen with the 0.5%CSE/LPS combination treatment (1.3-fold). In vivo exposure of MUC7 transgenic mice to CS, LPS or CS/LPS combination resulted in significant increase in MUC7 transcripts only with LPS treatment (in both trachea and lung). Immunohistochemistry indicated variable increase in MUC7 glycoprotein with CS and LPS treatment, both in the trachea and lungs, but CS/LPS exposure appeared to yield the highest increase.

Conclusion:

In vitro, CSE and a combination of CSE/LPS treatment upregulated MUC7 gene transcription. In vivo, LPS upregulated MUC7 transcription, and a combination of CS/LPS appeared to increase MUC7 glycoprotein.

Mucin-bacterial interactions in the human oral cavity and digestive tract

Mucins are a family of heavily glycosylated proteins that are the major organic components of the mucus layer, the protective layer covering the epithelial cells in many human and animal organs, including the entire gastro-intestinal tract. Microbes that can associate with mucins benefit from this interaction since they can get available nutrients, experience physico-chemical protection and adhere, resulting in increased residence time. Mucin-degrading microorganisms, which often are found in consortia, have not been extensively characterized as mucins are high molecular weight glycoproteins that are hard to study because of their size, complexity and heterogeneity. The purpose of this review is to discuss how advances in mucus and mucin research, and insight in the microbial ecology promoted our understanding of mucin degradation. Recent insight is presented in mucin structure and organization, the microorganisms known to use mucin as growth substrate, with a specific attention on Akkermansia muciniphila, and the molecular basis of microbial mucin degradation owing to availability of genome sequences.

Distribution of mucins and antimicrobial substances lysozyme and lactoferrin in the laryngeal subglottic region

The subglottic region of the larynx is of high clinical relevance with regard to infections and malignancies. Little is known about the distribution of mucins and antimicrobial substances in this area. In this study, we have investigated the mucin distribution in the normal subglottis of the larynx. Moreover, we analysed the expression of lysozyme and lactoferrin in this area. Therefore, the subglottic region of 34 larynges was investigated immunohistochemically with different antibodies to mucins and antimicrobial substances. The epithelium reacted positive with antibodies to mucins MUC1 (34/34), 5AC (26/34), 5B (10/34), 7 (8/34), 8 (10/34) and 16 (19/34); submucosal glands were positive to mucins MUC1 (34/34), 5B (10/34), 7 (8/34), and 16 (19/34); high columnar epithelial cells and serous parts of subepithelial seromucous glands were also positive for lysozyme (34/34) and lactoferrin (34/34). The results show that human subglottic epithelium and subepithelial submucosal glands produce a broad spectrum of mucins that is almost comparable with that in other areas of the respiratory tract. We hypothesize that the mucin diversity of the subglottis has an impact on positive functional consequences during vocal production and antimicrobial defence. This antimicrobial defence is supported by synthesis and secretion of antimicrobial substances such as lysozyme and lactoferrin. Moreover, knowledge of the observed distribution pattern of mucins in the subglottis can be a useful tool for a classification of subglottic laryngeal carcinomas.

Structure and function of the polymeric mucins in airways mucus

The airways mucus gel performs a critical function in defending the respiratory tract against pathogenic and environmental challenges. In normal physiology, the secreted mucins, in particular the polymeric mucins MUC5AC and MUC5B, provide the organizing framework of the airways mucus gel and are major contributors to its rheological properties. However, overproduction of mucins is an important factor in the morbidity and mortality of chronic airways disease (e.g., asthma, cystic fibrosis, and chronic obstructive pulmonary disease). The roles of these enormous, multifunctional, O-linked glycoproteins in health and disease are discussed.

MUC7 haplotype analysis: results from a longitudinal birth cohort support protective effect of the MUC7*5 allele on respiratory function

The mucin MUC7 is a glycoprotein that plays a role in bacterial clearance and has candidacidal activity. There are two common allelic forms with 5 or 6 tandem repeats (TR) of a 23 amino acid motif within the highly glycosylated (mucin) domain. The MUC7*5 allele has previously been shown to be less prevalent in patients with asthma, suggesting a protective role in respiratory function. Here we report the characterisation of other frequent genetic variation within and in the vicinity of the gene MUC7. A total of 26 polymorphisms were identified of which 5 are located in transcribed regions. A subset of 8 polymorphisms was selected to represent the major haplotypes, and allelic association was studied in individuals of Northern European ancestry, including known asthmatics. There was low haplotype diversity and strong association between each of the loci, and the MUC7*5 allele-carrying haplotype remained the one most strongly associated with asthma. Five of these polymorphisms have also been tested in the 1946 longitudinal birth cohort, for whom developmental, environmental and respiratory health data are available. We show that the haplotype carrying MUC7*5 is associated with higher FEV1 at 53 years, reduced age-related decline of FEV1, and also reduced incidence of wheeze.

Functional analysis of human MUC7 mucin gene 5'-flanking region in lung epithelial cells

The human MUC7 gene encodes a low-molecular-mass mucin glycoprotein that functions in modulation of microbial flora in the oral cavity and respiratory tracts. MUC7 gene expression is tissue- and cell-specific, with dominant expression in salivary gland acinar cells. To begin to understand the molecular mechanisms responsible for controlling MUC7 gene expression, we analyzed the promoter activity of MUC7 5'-flanking region in a human lung epithelial cell line A549. We demonstrated that MUC7 gene is expressed constitutively in this cell line and is upregulated by TNF-alpha stimulation. The promoter activities of a 2,762-bp fragment of the human genomic DNA (-2,732/+30 bp) and its deletion series, subcloned into a luciferase reporter vector, were characterized at the basal level and under stimulation by TNF-alpha. The results indicated that the minimal functional MUC7 promoter is in the region of -138/+30 bp. This region also revealed the greatest increase in the promoter activity upon TNF-alpha stimulation. Two putative AP1-binding elements and one NF-kappaB-binding element were identified within the proximal promoter. Further analyses demonstrated that mutations of these elements dramatically reduced specific DNA-protein binding ability and reporter gene expression. AP1 elements played an essential role in the constitutive expression, while the NF-kappaB element was crucially important in the response to TNF-alpha stimulation, demonstrating that TNF-alpha activates MUC7 transcription via NF-kappaB signaling pathway.

Modulation of MUC7 mucin expression by exogenous factors in airway cells in vitro and in vivo

The human MUC7 gene encodes a low-molecular-mass mucin that participates in the maintenance of healthy epithelium in the oral cavity, and possibly in respiratory tracts, by promoting the clearance of various bacteria. We examined whether MUC7 gene is expressed in primary normal human tracheobronchial epithelial cells and whether the expression is modulated by exogenous factors. By assessing MUC7 transcripts, we found that the MUC7 gene was induced by culturing the normal human tracheobronchial epithelial cells at the air-liquid interface, in which the cells were well differentiated. When the cells were treated with a panel of cytokines (IL-1beta, IL-4, IL-13, and TNF-alpha), epidermal growth factor, or a bacterial product (Pseudomonas aeruginosa lipopolysaccharide [LPS]), MUC7 transcripts and glycoprotein products were increased 1.7- to 3.2-fold. The effect of LPS on MUC7 gene expression was also studied in the airway tissues of MUC7 gene transgenic mice. In the in vitro cultured trachea and lung explants, the LPS-treated tissues showed over 2-fold increased levels of MUC7 mRNA compared with the untreated specimens. These results were confirmed by in vivo studies using the lungs and tracheas harvested from the transgenic mice irritated by LPS through the tracheal instillation. By immunohistochemistry, MUC7 glycoprotein was localized in tracheal submucosa within the serous cells. Upon LPS stimulation, the overexpressed MUC7 remains confined to the serous glands. In the lungs, MUC7 seems to be expressed within the respiratory epithelium at the level of the bronchioles. Upon stimulation with LPS, it seems to be overexpressed within the same cells and within the stromal tissue.

Respiratory tract mucin genes and mucin glycoproteins in health and disease

This review focuses on the role and regulation of mucin glycoproteins (mucins) in airway health and disease. Mucins are highly glycosylated macromolecules (> or =50% carbohydrate, wt/wt). MUC protein backbones are characterized by numerous tandem repeats that contain proline and are high in serine and/or threonine residues, the sites of O-glycosylation. Secretory and membrane-tethered mucins contribute to mucociliary defense, an innate immune defense system that protects the airways against pathogens and environmental toxins. Inflammatory/immune response mediators and the overproduction of mucus characterize chronic airway diseases: asthma, chronic obstructive pulmonary diseases (COPD), or cystic fibrosis (CF). Specific inflammatory/immune response mediators can activate mucin gene regulation and airway remodeling, including goblet cell hyperplasia (GCH). These processes sustain airway mucin overproduction and contribute to airway obstruction by mucus and therefore to the high morbidity and mortality associated with these diseases. Importantly, mucin overproduction and GCH, although linked, are not synonymous and may follow from different signaling and gene regulatory pathways. In section i, structure, expression, and localization of the 18 human MUC genes and MUC gene products having tandem repeat domains and the specificity and application of MUC-specific antibodies that identify mucin gene products in airway tissues, cells, and secretions are overviewed. Mucin overproduction in chronic airway diseases and secretory cell metaplasia in animal model systems are reviewed in section ii and addressed in disease-specific subsections on asthma, COPD, and CF. Information on regulation of mucin genes by inflammatory/immune response mediators is summarized in section iii. In section iv, deficiencies in understanding the functional roles of mucins at the molecular level are identified as areas for further investigations that will impact on airway health and disease. The underlying premise is that understanding the pathways and processes that lead to mucus overproduction in specific airway diseases will allow circumvention or amelioration of these processes.

Regulation of mucin expression: mechanistic aspects and implications for cancer and inflammatory diseases

Mucins are large multifunctional glycoproteins whose primary functions are to protect and lubricate the surfaces of epithelial tissues lining ducts and lumens within the human body. Several lines of evidence also support the involvement of mucins in more complex biological processes such as epithelial cell renewal and differentiation, cell signaling, and cell adhesion. Recent studies have uncovered the role of select mucins in the pathogenesis of cancer, underscoring the importance of a detailed knowledge about mucin biology. Under normal physiological conditions, the production of mucins is optimally maintained by a host of elaborate and coordinated regulatory mechanisms, thereby affording a well-defined pattern of tissue-, time-, and developmental state-specific distribution. However, mucin homeostasis may be disrupted by the action of environmental and/or intrinsic factors that affect cellular integrity. This results in an altered cell behavior that often culminates into a variety of pathological conditions. Deregulated mucin production has indeed been associated with numerous types of cancers and inflammatory disorders. It is, therefore, crucial to comprehend the underlying basis of molecular mechanisms controlling mucin production in order to design and implement adequate therapeutic strategies for combating these diseases. Herein, we discuss some physiologically relevant regulatory aspects of mucin production, with a particular emphasis on aberrations that pertain to pathological situations. Our views of the achievements, the conceptual and technical limitations, as well as the future challenges associated with studies of mucin regulation are exposed.

Rhesus monkey gastric mucins: oligomeric structure, glycoforms and Helicobacter pylori binding

Mucins isolated from the stomach of Rhesus monkey are oligomeric glycoproteins with a similar mass, density, glycoform profile and tissue localization as human MUC5AC and MUC6. Antibodies raised against the human mucins recognize those from monkey, which thus appear to be orthologous to those from human beings. Rhesus monkey muc5ac and muc6 are produced by the gastric-surface epithelium and glands respectively, and occur as three distinct glycoforms. The mucins are substituted with the histo blood-group antigens B, Le(a) (Lewis a), Le(b), Le(x), Le(y), H-type-2, the Tn-antigen, the T-antigen, the sialyl-Le(x) and sialyl-Le(a) structures, and the expression of these determinants varies between individuals. At neutral pH, Helicobacter pylori strains expressing BabA (blood-group antigen-binding adhesin) bind Rhesus monkey gastric mucins via the Le(b) or H-type-1 structures, apparently on muc5ac, as well as on a smaller putative mucin, and binding is inhibited by Le(b) or H-type-1 conjugates. A SabA (sialic acid-binding adhesin)-positive H. pylori mutant binds to sialyl-Le(x)-positive mucins to a smaller extent compared with the BabA-positive strains. At acidic pH, the microbe binds to mucins substituted by sialylated structures such as sialyl-Le(x) and sialylated type-2 core, and this binding is inhibited by DNA and dextran sulphate. Thus mucin- H. pylori binding occurs via at least three different mechanisms: (1) BabA-dependent binding to Le(b) and related structures, (2) SabA-dependent binding to sialyl-Le(x) and (3) binding through a charge-mediated mechanism to sialylated structures at low pH values.

Detection of circulating MUC7-positive cells by reverse transcription-polymerase chain reaction in bladder cancer patients

BACKGROUND

To determine whether MUC7 gene expression can be used as a bladder cancer marker in peripheral blood.

METHODS

Nested reverse transcription-polymerase chain reaction (RT-PCR) was performed on four types of bladder cancer cell lines (RT4, T24, EJ-1 and TCC) and the peripheral blood of 38 (31 superficial disease and seven invasive disease) bladder cancer patients and 18 subjects with urinary tract infections or other non-malignant conditions to determine the expression of MUC7.

RESULTS

No MUC7 gene expression was detected in control subjects. MUC7-positive cells were detected in all bladder cancer cell lines and in 18 of 38 (47.4%) peripheral blood samples of bladder cancer patients. Based on the tumor stage, MUC7 was detected in 11 of 29 (37.9%) patients with superficial disease (Ta and T1) and in seven of nine (77.7%) invasive disease patients (>/=T2). There was a significant difference between superficial and invasive disease (P = 0.042). Based on tumor grade, we could not detect MUC7 in five patients with grade 1, in five of 15 patients (33.3%) with grade 2 and in 13 of 18 patients (72.2%) with grade 3. There was a significant difference between grades 1 and 3 (P = 0.007) and grades 2 and 3 (P = 0.025).

CONCLUSIONS

These results suggest that MUC7 is a highly specific marker for bladder cancer and may be a useful method for the molecular staging and management of bladder cancer.

Diagnostic value of MUC4 immunostaining in distinguishing epithelial mesothelioma and lung adenocarcinoma

The distinction between pleural malignant mesothelioma and pleural infiltration by adenocarcinomas has complex therapeutic and medicolegal implications. Although the panel of adenocarcinoma-associated antibodies and one or two mesothelioma markers is useful in this purpose, most of these antibodies are not totally specific. We determined the diagnostic value of MUC4 immunostaining in this issue. MUC4 gene expression was also studied by in situ hybridization and RT-PCR. MUC4 is a membrane-bound mucin that has been suggested to be implicated in malignant progression in humans and rats. The MUC4 gene is expressed in various normal epithelial tissues of endodermic origin and carcinomas. In the respiratory tract, MUC4 transcripts have been detected in normal respiratory epithelium and lung carcinomas. MUC4 protein was expressed in 32 of 35 (91.4%) lung adenocarcinomas on paraffin-embedded tissue. None of the 41 malignant mesotheliomas nor the 32 cases of benign mesothelial cells expressed MUC4 at the protein and mRNA levels. We conclude that MUC4 is a very specific (100%) and sensitive (91.4%) marker of lung adenocarcinomas on paraffin-embedded tissue that could be useful in diagnostic practice in the distinction between malignant mesothelioma and adenocarcinoma.

Molecular cloning, genomic structure, and expression analysis of MUC20, a novel mucin protein, up-regulated in injured kidney

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis in the world. Here, we identify a cDNA encoding a novel mucin protein, shown previously to be up-regulated in IgAN patients, from a human kidney cDNA library. This protein contains a mucin tandem repeat of 19 amino acids consisting of many threonine, serine, and proline residues and likely to be extensively O-glycosylated; thus, this gene was classified in the mucin family and named MUC20. The human MUC20 gene contains at least four exons and is localized close to MUC4 on chromosome 3q29. We found variations in repeat numbers in the mucin tandem domain, suggesting polymorphism of this region. Northern blot and reverse transcription-PCR analyses revealed that human MUC20 mRNA was expressed most highly in kidney and moderately in placenta, colon, lung, prostate, and liver. Immunohistochemical analysis of human kidney revealed that MUC20 protein was localized in the proximal tubules. Immunoblotting analysis of MUC20 proteins produced in Madin-Darby canine kidney and HEK293 cells indicated the localization of MUC20 protein in a membrane fraction and extensive posttranslational modification. Immunoelectron microscopy of MUC20-producing Madin-Darby canine kidney cells demonstrated that MUC20 protein was localized on the plasma membrane. Expression of MUC20 mRNA in a human kidney cell line was up-regulated by tumor necrosis factor-alpha, phorbol 12-myristate 13-acetate, or lipopolysaccharide. Two species of MUC20 mRNA (hMUC20-L and hMUC20-S), resulting from alternative transcription, were identified in human tissue, whereas only one variant was observed in mouse tissues. Mouse MUC20 mRNA was expressed in the epithelial cells of proximal tubules, and the expression increased dramatically with the progression of lupus nephritis in the kidney of MRL/MpJ-lpr/lpr mice. Moreover, the expression of mouse MUC20 was augmented in renal tissues acutely injured by cisplatin or unilateral ureteral obstruction. These characteristics suggest that the production of MUC20 is correlated with development and progression of IgAN and other renal injuries.

MUC7 expression in the human lacrimal gland and conjunctiva

PURPOSE

Several mucins including MUC1, MUC2, MUC4, and MUC5AC have been identified at the ocular surface and in tears. The lacrimal gland, however, is not generally considered a source of ocular mucin. Because the lacrimal glands are similar to the salivary glands, we hypothesized that the lacrimal gland would express MUC7, a distinctive salivary mucin. We report the presence of MUC7 RNA and protein in normal human lacrimal glands as determined by reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and Western blot analysis.

METHODS

RNA from lacrimal glands and conjunctivae was isolated and subjected to RT-PCR with primers specific for MUC7. The identity of the PCR products was confirmed by sequencing. In situ hybridization with PCR product-based riboprobes was used to locate MUC7 transcripts in the lacrimal gland. MUC7 protein was detected by Western blot analysis.

RESULTS

Of six normal human lacrimal glands from which relatively intact mRNA could be extracted, four expressed MUC7. Hybridization with an antisense riboprobe for MUC7 indicates the presence of MUC7 transcripts in the cytoplasm of acinar cells. Western blot analysis confirms expression of the protein in the lacrimal gland. The presence of MUC1, MUC4, and MUC5B was also demonstrated by RT-PCR in lacrimal gland tissue. MUC7 transcripts and protein were also detected in normal human conjunctivae.

CONCLUSIONS

The mucin profile of the lacrimal gland resembles that of the salivary gland. Both RNA and protein corresponding to MUC7 are present in the normal human lacrimal gland. Reverse transcription-polymerase chain reaction indicates that other transcripts of MUC1, MUC4, and MUC5B are present as well. Ocular MUC7 is also produced by the conjunctival mucosa. The lacrimal gland, therefore, contributes not only to the aqueous component of tears but also, in concert with the conjunctiva, may contribute to the total pool of ocular surface mucins.

Interaction of human salivary mucin MG2, its recombinant N-terminal region and a synthetic peptide with Actinobacillus actinomycetemcomitans

The antimicrobial properties of human salivary mucin MG2 against the periodontal pathogen, Actinobacillus actinomycetemcomitans (A. actinomycetemcomitans), were investigated using purified MG2, rNMUC7 (a recombinant polypeptide containing residue 1-144 of MG2) and synthetic peptides PEP1 (residue 1-17) and PEP2 (residue 47-63). MG2 and rNMUC7 bound to A. actinomycetemcomitans strains SUNY75, SUNY465, SUNY523, 652 and JP2 in a liquid phase binding assay. The bactericidal activities of rNMUC7, PEP1 and PEP2 against A. actinomycetemcomitans SUNY523 were examined in a colony forming unit killing assay. The LD50 for rNMUC7 was 9 microM, for PEP2 was 20 microM and PEP1 did not exhibit bactericidal activity. The primary structure of these polypeptides was analyzed and a direct relationship between net positive charge and bactericidal activity was found. Screening of saliva samples from 60 individuals on Western blots probed with an anti-MG2 antibody against PEP2 revealed that a 20 kDa MG2 fragment was present in 66% of subjects and that this fragment was not present in glandular secretions. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of tryptic peptides derived from the 20 kDa fragment confirmed that this fragment contained a portion of the amino terminal region of MG2. The present study showed that the N-terminal region of MG2 and a subdomain within this region are microbicidal against A. actinomycetemcomitans and that a 20 kDa fragment of MG2 occurs in whole saliva. This suggests that cleavage of MG2 in vivo may produce fragments with microbicidal properties and that this may represent a novel mechanism of host defense.

Structural characterisation of cysteines in a bacterial-binding motif of human salivary mucin MG2

Human salivary mucin MG2 is a 180 kDa glycoprotein secreted by submandibular/sublingual and minor salivary glands. Secreted MG2 contains a domain with the only two cysteines (Cys(45) and Cys(50)) present in the polypeptide backbone; in native and recombinant MG2 this domain is involved in mucin binding to oral microbes. As the reduction and alkylation of MG2 has been shown to abolish binding, the present study was undertaken to determine whether the cysteine residues exist in the dithiol or disulphide form. Electrophoretic analysis under reducing and non-reducing conditions showed that intermolecular disulphide bonds do not occur between MG2 molecules. The same incorporation of radiolabelled iodoacetamide into MG2 was obtained with or without prior reduction. When radiolabelled alkylated MG2 was digested with Endoproteinase Lys-C and the derived peptides were separated by reversed-phase high-performance liquid chromatography (RP-HPLC), radioactivity was found in two fractions. Mass spectral analyses of these fractions showed the presence of peptides Cys-Leu-His-Lys and Arg-Cys-Arg-Pro-Lys, both containing carboxymethylated cysteines. These results show that the cysteines in the structural motif associated with bacterial binding exist in the dithiol form, and suggest the potential use of cysteine-containing peptides as agents to modify interactions of MG2 with microbes and oral surfaces.

Expression of membrane-associated mucins MUC1 and MUC4 in major human salivary glands

Mucins are high molecular weight glycoproteins secreted by salivary glands and epithelial cells lining the digestive, respiratory, and reproductive tracts. These glycoproteins, encoded in at least 13 distinct human genes, can be subdivided into gel-forming and membrane-associated forms. The gel-forming mucin MUC5B is secreted by mucous acinar cells in major and minor salivary glands, but little is known about the expression pattern of membrane-associated mucins. In this study, RT-PCR and Northern blotting demonstrated the presence of transcripts for MUC1 and MUC4 in both parotid and submandibular glands, and in situ hybridization localized these transcripts to epithelial cells lining striated and excretory ducts and in some serous acinar cells. The same cellular distribution was observed by immunohistochemistry. Soluble forms of both mucins were detected in parotid secretion after immunoprecipitation with mucin-specific antibodies. These studies have shown that membrane-associated mucins are produced in both parotid and submandibular glands and that they are expressed in different cell types than gel-forming mucins. Although the function of these mucins in the oral cavity remains to be elucidated, it is possible that they both contribute to the epithelial protective mucin layer and act as receptors initiating one or more intracellular signal transduction pathways.

Mucin apoprotein expression in COPD

Mucins, which are complex glycoproteins that provide the viscoelastic properties of mucus that are essential for the protection of the airways, are characterized by a variable-number tandem repeats (VNTR) region that may undergo alternate splicing during transcription. Such transcripts may yield multiple proteins via diverse post-translational modifications involving glycosylation (within each VNTR). Fifteen distinct mucin genes have been identified, with several mapping to chromosomal clusters (ie, 7q22 and 11p15.5), possibly having evolved by gene duplication. The deduced protein sequences can be subdivided into both membrane-associated mucins and secreted mucins. Membrane-associated mucins consist of cytoplasmic, transmembrane, and extracellular domains. The membrane-associated mucins MUC1, MUC4, and MUC11 have been localized to the lung. In addition to VNTRs, secreted mucins possess repeated cysteine-rich D-domains (which are important in polymerization). Secreted mucins that are localized to the lung include MUC2 (in cells with and without secretory granules), MUC5AC (in surface and submucosal mucous cells), MUC5B and MUC8 (in submucosal mucous cells), and MUC7 (in submucosal serous cells). Currently, little is known about the regulation of mucins in COPD patients. Recent studies with acrolein and cigarette smoke have suggested that MUC5AC is inducible (accompanied by epidermal growth factor [EGF] ligand formation and the activation of EGF receptor-dependent pathways), whereas MUC5B is constitutively expressed (increasing through gland enlargement). Similarly, little is known about the genetic determinants that control mucus hypersecretion, but preliminary findings in animal models suggest that intrastrain differences in acrolein-induced mucin formation are amenable to genetic analysis. As our understanding of the functional genomics of mucin biology increases, further clinical targets and therapeutic strategies are likely to emerge.

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.

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.

Heterogeneity of high-molecular-weight human salivary mucins

The existence of high-molecular-weight glycoproteins in saliva and salivary secretions has been recognized for nearly 30 years. These proteins, called mucins, are essential for oral health and perform many diverse functions in the oral cavity. Mucins have been intensively studied, and much has been learned about their biochemical properties and their interactions with oral micro-organisms and other salivary proteins. In the past several years, the major high-molecular-weight mucin in salivary secretions has been identified as MUC5B, one of a family of 11 human mucin gene products expressed in tissue-specific patterns in the gastrointestinal, respiratory, and reproductive tracts. MUC5B is one of four gel-forming mucins which exist as multimeric proteins with molecular weights greater than 20-40 million daltons. The heavily glycosylated mucin multimers form viscous layers which protect underlying epithelial surfaces from microbial, mechanical, and chemical assault. Another class of mucin molecules, the membrane-bound mucins, is structurally and functionally distinct from the gel-forming mucins. These proteins do not form multimers and can exist as both secreted and membrane-bound forms, with the latter anchored to epithelial cell membranes through a short membrane-spanning domain. In the present work, we show that two of the membrane-bound mucins, MUC1 and MUC4, are expressed in all major human salivary glands as well as in buccal epithelial cells. While the functions of these mucins in the oral environment are not understood, it is possible that they form a structural framework on the cell surface which not only is cytoprotective, but also may serve as a scaffold upon which MUC5B, and possibly other salivary proteins, assemble.

Immunoquantification of human salivary mucins MG1 and MG2 in stimulated whole saliva: factors influencing mucin levels

While more and more is known about the structure and function of human salivary mucins, there is relatively little information on quantification of these glycoconjugates in whole saliva and on factors influencing their secretion. The goal of the present work was to develop capture ELISAs that would allow for rapid, inexpensive, and reliable measurement of the salivary mucins MG1 and MG2, and to use these immunological procedures to investigate the significance of age, gender, flow rate, and protein concentration on mucin levels in whole saliva. Previously, we described a rabbit polyclonal antibody against MG1 (Troxler et al., 1995) and a rabbit polyclonal peptide antibody against an epitope in the N-terminal region of MG2 (Liu et al., 1999) which were used to develop the capture ELISAs. We verified the accuracy and specificity of these assays by showing correct measurement of known quantities of purified MG1 or MG2 added to whole saliva and lack of cross-reactivity between mucins and heterologous antisera on Western blots or in ELISAs. Whole saliva was collected from 60 subjects under conditions of masticatory stimulation, flow rates were recorded, and mucin concentrations were determined. The results showed that the mean concentration of MG1 and MG2 was 23.3 +/- 14.6 mg% and 13.3 +/- 11.6 mg%, respectively, and that mucins constitute approximately 16% of the total protein in whole saliva. No significant correlations were found between mucin levels and age or flow rate; however, a significant correlation was found between MG2 levels and total protein concentration. Furthermore, there were statistically significant gender differences in flow rate and MG1 levels, but not in MG2 levels. The availability of these immunoassays for quantification of MG1 and MG2 will help to elucidate the role of mucin in oral health and disease.

From normal respiratory mucosa to epidermoid carcinoma: expression of human mucin genes

Mucous cells in the respiratory tract contribute to the maintenance of the normal epithelial cell population via mechanisms of cell proliferation and differentiation. Mucous cell hyperplasia often occurs as a basic response to injury in the tracheobronchial epithelium. These cells are also thought to be involved in the histogenesis of epidermoid metaplasia. A typical biochemical feature of these cells is mucus secretion. Aberrant glycosylation or under-glycosylation of mucins is well known in cancer; however, the specific role played by mucin genes is at present unclear. To provide information regarding the expression of these genes in squamous metaplasia and squamous cell carcinoma, we analyzed and compared the expression of MUC1-MUC7 genes by in situ hybridization in control respiratory mucosa and lesions associated with neoplasia (hyperplasia, metaplasia and dysplasia) and squamous cell carcinomas. MUC4 was expressed independently of mucus secretion since it was expressed weakly by basal cells and probably by ciliated cells as well as collecting ducts, epidermoid metaplasia with complete squamous cell differentiation, and most of epidermoid carcinomas even well differentiated and keratinized. In squamous metaplasia and dysplasia, MUC4 gene expression was diffuse and less intense than in normal epithelium. MUC5AC was overexpressed in dysplasia as well as in mucous cell and basal cell hyperplasia and undetectable when squamous differentiation was achieved.

Isolation of human salivary mucin MG2 by a novel method and characterization of its interactions with oral bacteria

Human salivary mucin MG2 was purified from submandibular/sublingual gland secretion by ultrafiltration and sequential gel filtration chromatography on Sephadex G-200, Superose 6 (prepgrade), and Superose 6. This method differs from earlier procedures in that all steps are performed in the presence of 4 M guanidine hydrochloride and do not involve covalent modification of the mucin molecule. Electrophoretic analyses and Western blotting showed that purified MG2 did not contain detectable levels of other salivary proteins. Amino acid analysis showed that the composition of purified MG2 was in excellent agreement with the deduced sequence of MG2 apomucin encoded in the MUC7 gene. The yield of purified MG2 was 10-15 mg from 750 ml of salivary secretion. Binding of purified MG2 to Streptococcus mutans in vitro was not significantly affected by reductive methylation, but was nearly abolished by reduction and alkylation. These data identified a functional determinant for mucin-bacterial interactions in the N-terminal region where the only two cysteines (Cys45 and Cys50) in the MG2 apomucin occur. Additionally, purified MG2 bound to four strains of oral Streptococci, indicating that the binding is not dependent on complexing with other salivary proteins, such as secretory immunoglobulin A. The purification procedure described in this work will facilitate investigation of the role of MG2 in the oral environment.

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.

In-situ hybridization localized MUC7 mucin gene expression to the mucous acinar cells of human and MUC7-transgenic mouse salivary glands

MUC7 gene that encodes the low molecular weight human salivary mucin MG2, was previously shown to be expressed in tissue-specific manner in normal human salivary glands and in salivary glands of transgenic mice carrying the MUC7 transgene. The purpose of this study was to examine the cell specificity of MUC7 expression in human and transgenic mice salivary glands. To localize the MUC7 transcripts, we used in-situ hybridization in combination with Tyramide Signal Amplification procedure. The results clearly showed that in both the human and transgenic mice salivary gland tissue sections, MUC7 transcripts were localized only in mucous acinar cells; no signals were found in serous acinar cells or any other cell types present in these salivary glands.

Sequence of a second gene encoding bovine submaxillary mucin: implication for mucin heterogeneity and cloning

Secreted epithelial mucins are extremely large and heterogeneous glycoproteins. We report the 5 kilobase DNA sequence of a second gene, BSM2, which encodes bovine submaxillary mucin. The determined nucleotide and deduced amino acid sequences of BSM2 are 95.2% and 92. 2% identical, respectively, to those of the previously described BSM1 gene isolated from the same cow. Further, the five predicted protein domains of the two genes are 100%, 94%, 93%, 77%, and 88% identical. Based on the above results, we propose that expression of multiple homologous core proteins from a single animal is a factor in generating diversity of saccharides in mucins and in providing resistance of the molecules to proteolysis. In addition, this work raises several important issues in mucin cloning such as assembling sequences from seemingly overlapping clones and deducing consensus sequences for nearly identical tandem repeats.

MUC4 is a major component of salivary mucin MG1 secreted by the human submandibular gland

High molecular weight salivary mucin (MG1) is an important component of saliva, contributing to the lubricative and tissue-protective functions of this biological fluid. We have shown previously that the human mucin gene MUC5B is expressed at high levels in sublingual gland and is a significant constituent of MG1. Since many epithelia express multiple mucin genes, it seemed likely that MG1 in salivary secretions is also a heterogeneous mixture of mucin gene products. The aim of this study was to determine whether MUC4, a mucin shown in Northern blotting experiments to be expressed in salivary glands, was a significant protein component of MG1 in salivary secretions. Two cDNA clones containing MUC4 tandem repeats were isolated from a human submandibular gland cDNA library. In addition, recombinant MUC4 produced in a bacterial expression system cross-reacted with an antibody directed against deglycosylated MG1. This shows conclusively that human salivary mucin MG1 contains both MUC5B and MUC4 gene products suggesting that each mucin may perform distinct functions in the oral cavity.

Tissue-specific expression of human salivary mucin gene, MUC7, in transgenic mice

The MUC7 gene encodes the protein core of the low molecular weight human salivary mucin (MG2, mucin glycoprotein 2) and is expressed in a tissue-specific manner in salivary glands. The purpose of this study was to examine MUC7 expression by transgenic mouse technology. A 16 kb DNA fragment, containing the MUC7 gene (10 kb) and 3 kb of the upstream and 3 kb of the downstream sequences, was used to generate transgenic mice. We have identified five transgenic founder mice which were propagated as individual transgenic lines and analysed. Tissues of transgenic offspring from each line were analysed by RT-PCR to determine the sites of the MUC7 expression. The results indicated that only line 3 and line 5 expressed the MUC7 gene in salivary glands. The level of MUC7 expression in selected tissues was then determined by northern blot analyses. The results showed that line 3 mice contained high levels of MUC7 transcripts in the sublingual glands of both males and females and indicated low levels of MUC7 transcripts in the submandibular glands of females. No MUC7 expression was detected in this line by northern blot analysis in any other tissue tested. On the other hand, no expression of MUC7 was detected in any tissues of line 5 mice examined by northern blot analysis. A Southern blot analysis of human and mouse genomic DNA demonstrated multiple copies of the MUC7 transgene in line 3 and a single copy in line 5. Collectively, these results indicate that the regulatory sequences required for the tissue-specific expression of MUC7 are within the MUC7 transgene. However, the sequences necessary for expression comparable to that of MUC7 in human salivary glands may be missing from this construct. Western blot analysis of protein extracts from different tissues of transgenic mice line 3 showed that MUC7 gene product was produced in the submandibular-sublingual gland complex of both male and female mice and not in the other tissues examined.