The biology of mucus: Composition, synthesis and organization

In this review we discuss mucus, the viscoelastic secretion from goblet or mucous producing cells that lines the epithelial surfaces of all organs exposed to the external world. Mucus is a complex aqueous fluid that owes its viscoelastic, lubricating and hydration properties to the glycoprotein mucin combined with electrolytes, lipids and other smaller proteins. Electron microscopy of mucosal surfaces reveals a highly convoluted surface with a network of fibers and pores of varying sizes. The major structural and functional component, mucin is a complex glycoprotein coded by about 20 mucin genes which produce a protein backbone having multiple tandem repeats of Serine, Threonine (ST repeats) where oligosaccharides are covalently O-linked. The N- and C-terminals of this apoprotein contain other domains with little or no glycosylation but rich in cysteines leading to dimerization and further multimerization via SS bonds. The synthesis of this complex protein starts in the endoplasmic reticulum with the formation of the apoprotein and is further modified via glycosylation in the cis and medial Golgi and packaged into mucin granules via Ca²⁺ bridging of the negative charges on the oligosaccharide brush in the trans Golgi. The mucin granules fuse with the plasma membrane of the secretory cells and following activation by signaling molecules release Ca²⁺ and undergo a dramatic change in volume due to hydration of the highly negatively charged polymer brush leading to exocytosis from the cells and forming the mucus layer. The rheological properties of mucus and its active component mucin and its mucoadhesivity are briefly discussed in light of their importance to mucosal drug delivery.