Mucins – Potential Regulators of Cell Functions Volume Title: Gel-Forming and Soluble Mucins

Mucus, a viscous colloid gel, is an important element of cell defense developed in the course of evolution by live systems. Mucins, the main components of mucus, are glycoproteins characterized by specific structure and functions. Two subfamilies of the large mucin superfamily have been identified: secreted mucin glycoproteins and membrane-bound mucins. The secreted mucins are further subdivided into two groups: insoluble gel-forming mucins and soluble mucins. All gel-forming mucins share several features, such as specific domain structures, glycosylation patterns and biosynthetic pathways that differ from those of the membrane-bound mucins. Several classifications of the mucin glycoproteins have been proposed, but no one is universal. Further studies of the mucins are needed for development of an appropriate classification system.

Two main subfamilies of the mucin glycoproteins have been identified: secreted and membrane-bound. The secreted mucins can be further divided into insoluble gel-forming mucins, including MUC2, MUC5AC, MUC5B, MUC6 and MUC19, and soluble mucins, including MUC7, MUC8 and MUC9. Evolutionary studies showed that the gel-forming mucins are more ancient than the membrane-bound mucins. The evolutionary separation of these two subfamilies is partially reflected in the chromosomal localization of the genes encoding each of mucins. The differences between secreted and membrane-bound mucins are also reflected in the composition of their structural domains, in biosynthesis of their precursors and in posttranslational modifications. Despite some differences, the common features of mucin glycoproteins, such as the structure of the mucin specific domain with its tandem repeats and associated functions, relate them to the same protein family.

The group of secreted mucins has eight members: five genes encode the gelforming mucins (MUC2, MUC5AC, MUC5B, MUC6 and MUC19) and three genes encode the soluble mucins (MUC7, MUC8 and MUC9). Gel-forming mucins share structural and evolutionary features with von Willibrand Factor. Soluble mucins differ from gel-forming mucins in that the former all do not have the von Willibrand Factor specific domains while mucin specific tandem repeat-containing domain is a common feature of all secreted mucins. Genes encoding the soluble mucins are located at different chromosomes, whereas all gel-forming mucin genes except MUC19 are clustered on chromosome locus 11p15.5 and the MUC19 gene is located on chromosome 12. Structural aspects and functions of the secreted mucins are discussed.

The MUC2 glycoprotein is one of the most extensively studied secreted gelforming mucins. The gene encoding this protein has been cloned, sequenced and analyzed both structurally and functionally. The exon/intron composition of the MUC2 gene has been established, and its transcription investigated. The transcriptional activity of the gene is controlled by promoter regulatory elements and by the epigenetic mechanism. Biosynthesis of the MUC2 protein precursor and its processing and maturation associated with posttranslational modifications, such as N- and Oglycosylation, sulfation, sialylation and posttranslational proteolysis, have been thoroughly studied. Expression of MUC2 gene has been analyzed in different organs, tissues and cells under physiological conditions, including embryogenesis and fetal development, and in different forms of pathology. The molecular biology aspects of the MUC2 glycoprotein functioning, as well as its biosynthesis, posttranslational modifications and expression are discussed

The MUC5AC mucin plays an essential role in homeostasis of respiratory, gastrointestinal and urogenital tracts under physiological conditions and fulfills various important functions in human pathology. This chapter describes the molecular structure of the MUC5AC gene and the mechanisms that regulate its activity. It analyzes the biochemical and biophysical properties of the MUC5AC glycoprotein, its biosynthesis and posttranslational modifications, as well as its expression in the respiratory, gastrointestinal and male and female urogenital organs under physiological and pathological conditions. The roles of the MUC5AC mucin in eye physiology and pathology and in breast cancer are also discussed.

The MUC5B mucin plays an important role in the homeostasis of respiratory, gastrointestinal and female reproductive tracts under physiological conditions and fulfills various important functions in human pathology. In this chapter, the molecular structure of the MUC5B gene and the mechanisms that regulate its activity are considered. Biochemical and biophysical properties of the MUC5B glycoprotein, its biosynthesis and posttranslational modifications as well as expression in the respiratory, gastrointestinal and female reproductive organs under physiological and pathological conditions are analyzed. The impact of MUC5B mucin in physiology and pathology of respiratory, gastrointestinal and female reproductive tracts and of such organs as eye, middle ear, breast and thyroid is discussed.

The MUC6 mucin plays an essential role mainly in homeostasis of the gastrointestinal tract, and in pathogenesis of many diseases associated with the gastrointestinal, respiratory, and male and female urogenital systems. The structure of the MUC6 gene and the mechanisms that regulate its activity are analyzed in this chapter. The biochemical and biophysical properties of the MUC6 glycoprotein, its biosynthesis, posttranslational modifications, and expression in various cells and tissues are discussed.

The mucin MUC19 is a recently discovered member of the gel-forming mucin family. At present, limited information is available regarding its structure and functions. This chapter covers several issues: the evolutionary connection between MUC19 and other gel-forming mucin genes, domain structure of the MUC19 mucin glycoprotein, and specific features of the mouse Smgc/Muc19 mRNA transcription and splicing. Some recent data regarding human MUC19 and mouse Muc19 expression in embryonic development and in adult tissues are presented, and the possible role of MUC19 in physiology of the eye and middle ear are discussed.

The MUC7 gene belongs to a group of genes encoding soluble mucin glycoproteins. This chapter describes the structure of the MUC7 gene and its protein product; the mechanisms responsible for regulation of its expression; important functions associated with the individual domains; and the role of posttranslational modifications of the MUC7 apomucin in expression of the functions embedded in its polypeptide structure.

Glycoprotein MUC8, one of the less studied mucins, belongs to the group of soluble mucins. Its cDNA is only partially cloned. Like other mucins, MUC8 has a tandem repeat containing domain, but in contrast to other mucins, which have, as a rule, only one type of repeats, the MUC8 mucin has two types of repeats: one of 13 amino acids and another one containing 41 amino acid residues. The MUC8 gene is localized on human chromosome 12q24.3. MUC8 mucin appears to serve a ciliated cell marker. Regulation of MUC8 gene expression has been studied and some regulatory pathways have been identified. MUC8 mucin plays a major role in the airway mucosa and its over-expression and hypersecretion are associated with airway inflammatory diseases.

The MUC9/OGP gene encodes the oviduct-specific secreted glycoprotein, MUC9/OGP. This glycoprotein is a member of two protein superfamilies: the mucin protein superfamily and the glycosyl hydrolase 18 one. It contains 11 exons and 10 introns. Although MUC9/OGP genes cloned from different mammals display the same structure, species-specific features have been observed for each gene. The mechanisms of transcriptional regulation of the MUC9/OGP gene have been investigated. Comparative studies of the cloned 5’-flanking promoter sequences showed that the homologous MUC9/OGP genes demonstrate conservation of transcription factor-specific cis-elements, but their individual promoters bear species-specific features that determine species-specific expression of a given MUC9/OGP gene. The biosynthetic pathway leading to production of the mature MUC9/OGP glycoprotein has been identified. Overt and covert functional potentials of the MUC9/OGP mucin are discussed.

Multifunctionality is one of the ubiquitous properties of biological macromolecules and is also a feature of the gel-forming and soluble secreted mucin glycoproteins. These mucin molecules participate in various basic cell processes in both physiological and pathological conditions. Their functions appear to be important both for embryonic/fetal development and for adult cells and tissues. They perform such functions as lubrication and hydration of mucosal surfaces, defending them against mechanical and chemical harm and infection. The gel-forming mucins have the potential to modify and catalyze organic chemical reactions. The secreted glycoproteins are active participants in innate immunity. Depending on cell type and type of neoplasm, gel-forming mucins function as tumor suppressors or tumor promoters. The secreted mucins possess antibacterial, antifungal and antiviral activities. In addition, the important processes of biological reproduction are also regulated by mucins, in particular by the soluble MUC9/OGP mucin glycoprotein. This chapter presents the data on the multifunctional potentials of the secreted mucins as regulators of cell functions.

Functions that potentially can be performed by secreted mucins have been analyzed by bioinformatics using the Motifscan and Eukaryotic Linear Motif (ELM) tools and STRING 9.0 interaction network. A number of potentially important motifs were discovered and possible functional partners of the secreted mucin glycoproteins were predicted. A new role of galactin-3 in physiological transformation of the intestinal mucus gel layers is proposed.