Regional localization of the intestinal mucin gene MUC3 to chromosome 7q22

Structure, Function and Gene Expression of Epithelial Mucins

In this review the main characteristics, i.e., structure, function and gene expression, of the different mucins are discussed. Mucin-type molecules consist of a core protein moiety (apomucin) where a number of carbohydrate chains are attached to serines and threonines by glycosidic bonds. O-linked carbohydrates form up to 80% of the molecule and the length of the glucidic side chains varies from one to more than 20 residues. At least eight mucin-like genes have been isolated so far, and the main characteristic is the presence of a central domain composed of a variable number of “tandem repeats”. The sequence homology of the central domain among the different members of the mucin-type family is limited, indicating that this internal domain is unique for each mucin. Thanks to the integrated results of genetic, immunological and biochemical studies, it is now possible to identify eight apomucin genes, namely MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7. MUC1 is the best characterized mucin and it is expressed on the apical surface of most polarized epithelial cells. The MUC1 gene has been cloned and sequenced. The MUC2 gene encodes a typical secretory gel-forming mucin which represents the predominant form in human intestinal and colon tissues. Another intestinal mucin is MUC3. The MUC4, MUC5AC and MUC5B genes have been isolated from a bronchial tissue cDNA library. The MUC4 and MUC5AC genes are mainly expressed in the respiratory tract, in gastric and reproductive mucosa, while MUC5B is highly detectable only in the bronchial glands. The MUC6 gene is expressed by gastric tissue and, recently, MUC7 has been cloned and sequenced using a salivary cDNA library.

Mucins: Structural diversity, biosynthesis, its role in pathogenesis and as possible therapeutic targets

Mucins are the main structural components of mucus that create a selective protective barrier for epithelial surface and also execute wide range of other physiological functions. Mucins can be classified into two types, namely secreted mucins and membrane bounded mucins. Alterations in mucin expression or glycosylation and mislocalization have been seen in various types of pathological conditions such as cancers, inflammatory bowel disease and ocular disease, which highlight the importance of mucin in maintaining homeostasis. Hence mucins can be used as attractive target for therapeutic intervention. In this review, we discuss in detail about the structural diversity of mucins; their biosynthesis; its role in pathogenesis; regulation and as possible therapeutic targets.

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.

Current status of mucins in the diagnosis and therapy of cancer

Mucins are the most abundant high molecular weight glycoproteins in mucus. Their nature and glycosylation content dictates the biochemical and biophysical properties of viscoelastic secretions, pointing out an important role in diverse biological functions, such as differentiation, cell adhesions, immune responses, and cell signaling. Mucins are expressed in tubular organs by specialized epithelial cells in the body. Their aberrant expression is well documented in a variety of inflammatory or malignant diseases. From a prognosis point of view, their expression and alterations in glycosylation are associated with the development and progression of malignant diseases. Therefore, mucins can be used as valuable markers to distinguish between normal and disease conditions. Indeed, this alteration in glycosylation patterns generates several epitopes in the oligosaccharide side chains that can be used as diagnostic and/or prognostic markers. Furthermore, these characteristic tumor-associated epitopes are extensively used as appropriate immunotargets of malignant epithelial cells. Therefore, in an effort to detect and treat cancer at the earliest stage possible, mucins are analyzed as potential markers of disease for diagnosis, progression, and for therapeutic purposes. In this review, we focused on the current status of the distribution of mucins in normal and pathologic conditions and their clinical use both in cancer diagnosis and therapeutics treatments.

MUC1 and the MUCs: A Family of Human Mucins with Impact in Cancer Biology

Mucins represent a family of glycoproteins characterized by repeat domains and a dense O-glycosylation. During the last two decades, the gene and peptide structures of various mucins as well as their glycosylation states were partly elucidated. Characteristic tumor-associated alterations of the expression patterns and glycosylation profiles were observed in biochemical, immunochemical, and histological studies and are discussed in the light of efforts to use the most prominent member in this family, MUC1, as a tumor target in anti-tumor strategies. Within this context the present review, focusing on MUC1, describes recent work on the regulation of mucin biosynthesis by cytokines and hormones, the role of mucins in cell adhesion, and their interaction with the immune system. Important aspects of clinical diagnostics based on mucin antigens are discussed, including the application of tumor serum assays and the significance of numerous studies revealing correlations between the expression of peptide cores or mucin-associated carbohydrates and clinicopathological parameters like tumor progression and prognosis.

Architecture of the large membrane-bound mucins

Epithelial membrane-bound mucins are high molecular mass glycoproteins that may be also secreted or released into the extracellular environment. The genomic and multi-domain organizations of human large epithelial membrane-bound mucins are reviewed here with the purpose to clarify the literature on the subject with the help of mouse sequences. This family of complex molecules contains at least MUC3A, MUC12, MUC17, all organized in a cluster of genes, MUC4 and likely MUC16. In addition, we discuss the splicing events reported for these mucins with an emphasis on the human mucin MUC4.

Mucin overproduction in chronic inflammatory lung disease

Mucus overproduction and hypersecretion are commonly observed in chronic inflammatory lung disease. Mucins are gel-forming glycoproteins that can be stimulated by a variety of mediators. The present review addresses the mechanisms involved in the upregulation of secreted mucins. Mucin induction by neutrophil elastase, bacteria, cytokines, growth factors, smoke and cystic fibrosis transmembrane conductance regulator malfunction are also discussed.

Distinct evolution of the human carcinoma-associated transmembrane mucins, MUC1, MUC4 AND MUC16

The MUC family of mucins consists of secreted and membrane-bound forms. Overexpression of the membrane-bound family members, MUC1 (CA15-3), MUC4 and MUC16 (CA125), is found in diverse human carcinomas. However, despite being classified in the same family, little is known about the genetic origins of the carcinoma-associated mucins. The present results show that MUC1 homologs are restricted to mammalian species. MUC1 has no sequence similarity with the other membrane-bound mucins, except for the presence of a sea urchin sperm protein-enterokinase-agrin (SEA) domain. The results indicate that the MUC1 SEA domain originated from heparin sulfate proteoglycan of basement membrane (HSPG2; perlecan), an inducer of tumor cell growth. MUC4 has no SEA domain, but does have (i) a NIDO domain that evolved from an ancestor common to nidogen, and (ii) AMOP and VWD domains that originated from an ancestor common to the Sushi-domain containing protein. MUC16 contains multiple SEA domains that are found in a chicken gene and were subsequently repeated through duplication events. The SEA domains in MUC16 appear to have evolved from agrin before the divergence of birds and mammals. These findings indicate that MUC1, MUC4 and MUC16 evolved from distinct ancestors and that the membrane-bound mucins consist of different subgroups based on their genetic backgrounds.

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.

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.

The MUC family: an obituary

Mucins are glycoproteins that are common on the surfaces of many epithelial cells; they are deemed to mediate many interactions between these cells and their milieu. Several of these mucins form the mucus layer that is found in many hollow organs. The biophysical properties of mucins are related to their extensive O-linked glycosylation rather than directly to their polypeptide sequences. Despite the frequent absence of sequence homology, many human genes encoding mucins have been named MUC followed by a number, unjustly suggesting the existence of one large gene family. In this article, it is suggested that the mucin genes be renamed according to their sequence homologies.

Assessments of clonal composition of colorectal adenomas by FISH analysis of chromosomes 1, 7, 13 and 20

Chromosome banding analysis has shown that numerical aberrations, in particular gains of chromosomes 7, 13 and 20, are common in colorectal adenomas but cannot provide reliable information on the size of the abnormal clones in vivo. We examined interphase nuclei from 70 colorectal adenomas, of which 64 had been previously karyotyped, using fluorescence in situ hybridization (FISH) with probes for the pericentromeric regions of chromosomes 1, 7, 13 and 20. Gain of chromosome 7 was seen in 34% of the analyzed adenomas, +13 was seen in 44% and trisomy 20 was found in 32% of the adenomas, verifying that the trisomies are in vivo phenomena. The median proportion of cells with trisomy was larger than 50%. A comparison with the G-banding analysis showed a good correlation between the results yielded by the 2 methods. Based on the clonal size and karyotypic findings, a likely order of events during clonal evolution could be ascribed to each case. More than 1 numerical aberration was detected by FISH analysis in 16 adenomas. In 6 adenomas, a clone with only trisomy 7 was present alongside a clone with additional gain(s) of chromosomes 13 and/or 20. Seven cases had gain of chromosome 13 and/or gain of chromosome 20 in the largest clone, suggesting that a clone with either of these changes was present before the changes in chromosome 7 copy number took place. On the basis of the results of this combined meta- and interphase cytogenetic study, we conclude that gains of chromosomes 7, 13 and 20 are common in colorectal adenomas and that the trisomies usually are present in a large proportion of the cells. They seem to be primary chromosome aberrations in some adenomas, whereas in others they arise secondarily as part of the clonal evolution. Although the first gain usually is of chromosome 7, it is evident that it is the end result of the chromosomal aberrations, not the exact sequence in which they occur, that determines the pathogenetic consequences.

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

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

Multiple transcripts of MUC3: evidence for two genes, MUC3A and MUC3B

The MUC3 gene encodes a transmembrane mucin-type glycoprotein. A number of consistent single nucleotide changes were observed in different MUC3 cDNAs from a single individual, suggesting the presence of at least three different transcripts per individual. This transcript heterogeneity is due both to the existence of a second copy of the MUC3 gene and to allelic changes. Sequencing of the second MUC3 shows that it has the same C-terminal domain and intron/exon structure as the previously described MUC3. The tandem repeat domain has the same amino acid consensus sequence but shows more substitutions. The unique exonic sequences range from 94 to 100% identity at the nucleotide level and correspondingly few amino acid changes have been identified. The introns show around 95% identity. We propose to name this second gene MUC3B, MUC3A being reserved for the first MUC3 gene. MUC3B, like MUC3A, is expressed in intestine and Caco-2 cells.

The central domain of bovine submaxillary mucin consists of over 50 tandem repeats of 329 amino acids. Chromosomal localization of the BSM1 gene and relations to ovine and porcine counterparts

We previously elucidated five distinct protein domains (I-V) for bovine submaxillary mucin, which is encoded by two genes, BSM1 and BSM2. Using Southern blot analysis, genomic cloning and sequencing of the BSM1 gene, we now show that the central domain (V) consists of approximately 55 tandem repeats of 329 amino acids and that domains III-V are encoded by a 58.4-kb exon, the largest exon known for all genes to date. The BSM1 gene was mapped by fluorescence in situ hybridization to the proximal half of chromosome 5 at bands q2. 2-q2.3. The amino-acid sequence of six tandem repeats (two full and four partial) were found to have only 92-94% identities. We propose that the variability in the amino-acid sequences of the mucin tandem repeat is important for generating the combinatorial library of saccharides that are necessary for the protective function of mucins. The deduced peptide sequences of the central domain match those determined from the purified bovine submaxillary mucin and also show 68-94% identity to published peptide sequences of ovine submaxillary mucin. This indicates that the core protein of ovine submaxillary mucin is closely related to that of bovine submaxillary mucin and contains similar tandem repeats in the central domain. In contrast, the central domain of porcine submaxillary mucin is reported to consist of 81-amino-acid tandem repeats. However, both bovine submaxillary mucin and porcine submaxillary mucin contain similar N-terminal and C-terminal domains and the corresponding genes are in the conserved linkage regions of the respective genomes.

Mucin gene expression during differentiation of human airway epithelia in vitro. Muc4 and muc5b are strongly induced

Mucus hypersecretion is characteristic of chronic airway diseases. However, regulatory mechanisms are poorly understood. Human airway epithelial cells grown on permeable supports at the air-liquid interface (ALI) develop a mucociliary morphology resembling that found in vivo. Such cultures provide a model for studying secretory cell lineage, differentiation, and function, and may provide insight regarding events leading to mucus hypersecretion. The mucin gene expression profile of well-differentiated human airway epithelial cells in culture has not yet been established. We compared expression of all the currently described mucin genes in poorly differentiated (conventional cultures on plastic) and well-differentiated (ALI) human nasal and bronchial epithelial cells. Differentiation-dependent upregulation of MUC3, MUC5AC, MUC5B, and MUC6 messenger RNA (mRNA) was demonstrated using reverse transcriptase-polymerase chain reaction (RT-PCR). Northern blot analysis showed a similar increase for MUC4 and demonstrated that induction of MUC4 and MUC5B expression depended on retinoic acid. MUC1, MUC2, MUC7, and MUC8 mRNAs were also detected by RT-PCR, but these genes did not appear to be strongly regulated as a function of differentiation. Mucin gene expression was similar in bronchial and nasal cells. Thus, mucociliary differentiation of human airway epithelia in vitro entails upregulation of several mucin genes.

Abnormalities in mucin gene expression in Crohn's disease

Alterations in the structure and/or quantity of mucins could alter the barrier function of mucus and play a role in initiating and maintaining mucosal inflammation in Crohn's disease. To investigate the hypothesis of a mucin gene defect in Crohn's disease, we analyzed the expression of the different mucin genes in the ileal mucosa of patients with Crohn's disease and controls. mRNA expression levels were assessed by a quantitative dot blot analysis and compared (i) between healthy and involved ileal mucosa of patients with Crohn's disease and (ii) between healthy mucosa of patients with Crohn's disease and controls. Expression of the different mucin genes was heterogeneous among controls and patients with Crohn's disease, except for MUC6 in controls. Nevertheless, MUC1 mRNA expression was significantly decreased in the involved ileal mucosa of patients with Crohn's disease when compared to the healthy mucosa (p = 0.02). Moreover, the expression levels of MUC3, MUC4, and MUC5B were significantly lower in both healthy and involved ileal mucosa of patients with Crohn's disease compared to controls (p < or = 0.05). The decrease of expression levels of some mucin genes (more particularly MUC3, MUC4, and MUC5B) in both healthy and involved ileal mucosa suggests a primary or very early mucosal defect of these genes in CD.

Abnormalities in mucin gene expression in Crohn's disease

Alterations in the structure and/or quantity of mucins could alter the barrier function of mucus and play a role in initiating and maintaining mucosal inflammation in Crohn's disease. To investigate the hypothesis of a mucin gene defect in Crohn's disease, we analyzed the expression of the different mucin genes in the ileal mucosa of patients with Crohn's disease and controls. mRNA expression levels were assessed by a quantitative dot blot analysis and compared (i) between healthy and involved ileal mucosa of patients with Crohn's disease and (ii) between healthy mucosa of patients with Crohn's disease and controls. Expression of the different mucin genes was heterogeneous among controls and patients with Crohn's disease, except for MUC6 in controls. Nevertheless, MUC1 mRNA expression was significantly decreased in the involved ileal mucosa of patients with Crohn's disease when compared to the healthy mucosa (p = 0.02). Moreover, the expression levels of MUC3, MUC4, and MUC5B were significantly lower in both healthy and involved ileal mucosa of patients with Crohn's disease compared to controls (p < or = 0.05). The decrease of expression levels of some mucin genes (more particularly MUC3, MUC4, and MUC5B) in both healthy and involved ileal mucosa suggests a primary or very early mucosal defect of these genes in CD.

Cloning and characterization of mouse intestinal MUC3 mucin: 3' sequence contains epidermal-growth-factor-like domains

Mucin glycoproteins are a heterogeneous family of high-molecular-mass, heavily glycosylated proteins differentially expressed in epithelial tissue of the gastrointestinal, reproductive and respiratory tracts. We report here the cloning of a mouse caecal mucin (MCM). Amino acid analysis of purified MCM revealed a high content of serine (10.8%) and threonine (25.1%). Antibodies against deglycosylated MCM were prepared for immunohistochemical analysis and for screening a mouse caecal cDNA library. Immunohistochemical analysis showed strong staining of goblet cells and patchy staining of surface columnar cells in the duodenum, small intestine, caecum, colon and rectum. Screening of a mouse caecal cDNA library yielded clones containing tandem repeats of 18 bp with two predominant peptide sequences of TTTADV and TTTVVV. The tandem repeat domain is followed by 1137 bp of non-repetitive sequence and 521 bp of 3' untranslated sequence prior to the poly(A) tail. Two cysteine-rich regions lie within the 3' non-repetitive domain. The arrangement of the cysteines within these regions corresponds to epidermal growth factor-like domains. Following the second cysteine-rich region is a stretch of 19 hydrophobic amino acids which may act as a transmembrane domain or allow for interaction with hydrophobic molecules. Northern blot analysis indicates the mRNA is approximately 13.5 kb with greatest expression in the caecum and lesser amounts in the colon and small intestine. No MCM message is found in mouse stomach, trachea, lung, kidney, oesophagus or pancreas. In situ hybridization studies show that MCM message is expressed at the tips of villi in the intestine and in the upper crypts and surface cells of the caecum and colon. Chromosomal analysis assigns this gene to mouse chromosome 5 in a region of conserved linkage with human chromosome 7, the location of the human MUC3 gene. We conclude that we have identified a mouse caecal mucin which represents the mouse homologue of human MUC3. The mouse MUC3 cDNA sequence suggests that it is a novel non-polymerizing mucin which may participate in membrane or intermolecular interactions through its 3' non-repetitive region.

MUC3 human intestinal mucin. Analysis of gene structure, the carboxyl terminus, and a novel upstream repetitive region

MUC3 is a large mucin glycoprotein expressed by the human intestine and gall bladder. In this manuscript, we present details of the deduced protein structure of MUC3. The MUC3 carboxyl-terminal domain is 617 residues in length, including 511 residues of a non-repetitive mucin-like domain (27% Thr, 22% Ser, and 11% Pro) and a 106-residue Cys-rich domain with homology to the epidermal growth factor (EGF) -like structural motifs found in many proteins. The region of MUC3 located upstream of the previously described 51-base pair (bp) tandem repeats, which encode a major Ser and Thr-rich domain, consists of a second type of repetitive structure with an imperfect periodicity of approximately 1125 bp. This domain is also mucin-like and appears to be considerably larger than 2000 residues (6000 bp). The MUC3 gene itself is large and complex. Using pulse field gel electrophoresis and blot analysis, the smallest fragment found that contained all human genomic DNA hybridizing to the 51-bp tandem repeat probe was 200 kilobases with restriction enzyme SwaI. Both PvuII and PstI produced two sets of hybridizing fragments that were hypervariable within the human population with a pattern suggestive of both a variation in the number of tandem repeats (VNTR) and sequence polymorphism. These fragments varied independently of each other, but no genetic recombination was detected in a study of 40 human families. Thus, the MUC3 gene encodes a very large glycoprotein with a structure very different from that of any mucin currently described.

MUC7 gene expression and genetic polymorphism

This study examined differential expression of several mucin genes in the human submandibular gland and trachea, MUC7 tissue and species specificity, and MUC7 genetic polymorphism. Mucin gene expression examined by RT-PCR indicated that MUC1, MUC4 and MUC7 are expressed in the human submandibular gland, while MUC1, MUC2, MUC4, MUC5 and MUC7 are expressed in the human trachea. Northern blot analysis confirmed the expression of MUC7 in the human trachea and MUC4 in the human submandibular gland. Northern blot analysis also demonstrated that MUC7 is not expressed in the submandibular/sublingual gland complexes of hamster, mouse and rat. Southern blot analysis suggested the presence of a MUC7 homologue in monkey genomic DNA. Genetic polymorphism studies of MUC7 by PCR and Southern blot analysis revealed the presence of a limited variable number of tandem repeats (VNTR) polymorphism.

Mucin glycoproteins in neoplasia

Mucins are high molecular weight glycoproteins that are heavily glycosylated with many oligosaccharide side chains linked O-glycosidically to the protein backbone. With the recent application of molecular biological methods, the structures of apomucins and regulation of mucin genes are beginning to be understood. At least nine human mucin genes have been identified to date. Although a complete protein sequence is known for only three human mucins (MUC1, MUC2, and MUC7), common motifs have been identified in many mucins. The pattern of tissue and cell-specific expression of these mucin genes are emerging, suggesting a distinct role for each member of this diverse mucin gene family. In epithelial cancers, many of the phenotypic markers for pre-malignant and malignant cells have been found on the carbohydrate and peptide moieties of mucin glycoproteins. The expression of carbohydrate antigens appears to be due to modification of peripheral carbohydrate structures and the exposure of inner core region carbohydrates. The expression of some of the sialylated carbohydrate antigens appears to correlate with poor prognosis and increased metastatic potential in some cancers. The exposure of peptide backbone structures of mucin glycoproteins in malignancies appears to be due to abnormal glycosylation during biosynthesis. Dysregulation of tissue and cell-specific expression of mucin genes also occurs in epithelial cancers. At present, the role of mucin glycoproteins in various stages of epithelial cell carcinogenesis (including the preneoplastic state and metastasis), in cancer diagnosis and immunotherapy is under investigation.

Biochemical composition of human saliva in relation to other mucosal fluids

This paper describes several salivary components and their distribution in other mucosal secretions. Histatins are polypeptides which possess exceptional anti-fungal and anti-bacterial activities, but are nevertheless present only in saliva. Proline-rich proteins (PRPs) are members of a closely related family, of which the acidic PRPs are found solely in saliva, whereas the basic PRPs are also found in other secretions. Mucins are a group of glycoproteins that contribute to the visco-elastic character of the mucosal secretions. Despite the similarities in their structure and behavior, mucins have distinct tissue distributions and amino acid sequences. Other salivary proteins are present in one or more mucosal secretions. Lysozyme is an example of a component belonging to an ancient self-defense system, whereas secretory immunoglobulin A (sIgA) is the secreted part of a sophisticated adaptive immune system. Cystatins are closely related proteins which belong to a multigene family. Alpha-Amylase is a component that is believed to play a specific role in digestion, but is nevertheless present in several body fluids. Kallikrein and albumin are components of blood plasma. But whereas albumin diffuses into the different mucosal secretions, kallikrein is secreted specifically by the mucosal glands. The presence of these proteins specifically in saliva, or their distribution in other mucosal secretions as well, may provide important clues with respect to the physiology of those proteins in the oral cavity.

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.

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 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.