In vivo models of mucin biosynthesis and function

Abstract

The secreted mucus layer that lines and protects epithelial cells is conserved across diverse species. While the exact composition of this protective layer varies between organisms, certain elements are conserved, including proteins that are heavily decorated with N-acetylgalactosamine-based sugars linked to serines or threonines (O-linked glycosylation). These heavily O-glycosylated proteins, known as mucins, exist in many forms and are able to form hydrated gel-like structures that coat epithelial surfaces. In vivo studies in diverse organisms have highlighted the importance of both the mucin proteins as well as their constituent O-glycans in the protection and health of internal epithelia. Here, we summarize in vivo approaches that have shed light on the synthesis and function of these essential components of mucus.

Keywords: Colon; Drosophila; Muc2; Muc5ac; Muc5b; Mucins; Mucus; O-glycosylation; Salivary gland; Secretion; Secretory granules; Small intestine.

Figure 1.. Drosophila and human mucins.

Shown are Drosophila and human mucins. Human mucins fall into 3 categories: transmembrane, secreted gel-forming and secreted non-gel forming. Both Drosophila and human mucins have highly O-glycosylated PTS domains as well as domains that are involved in multimer formation. Sgs1, salivary gland secretion protein 1; Sgs3, salivary gland secretion protein 3; Muc26B, mucin 26B; PTS, proline-threonine-serine domain; CBD, chitin-binding domain. TM, transmembrane domain; VWC, Von Willebrand C-domain; NIDO, nidogen homology region; TIL, trypsin inhibitor-like cysteine-rich domain; VWD, Von Willebrand D-domain; AMOP, the adhesion-associated domain; C8, conserved cysteine-rich domain; CysD, domain rich in cysteine residues; SEA, sea urchin sperm protein, enterokinase and agrin domain; CK, cysteine knot domain.

Figure 2.. Mucin-type O-glycosylation.

The enzymes responsible for the synthesis of mucin-type O-glycans and the common core structures are shown. S/T=serine or threonine.

Figure 3.. Biosynthesis and packaging of mucins within the Drosophila salivary glands.

(A) A Drosophila third instar larval salivary gland (SG) is shown. An enlarged single secretory cell with secretory granules (green) containing mucins is shown. (B) Secretory mucin formation and secretion are regulated by the hormone 20-hydroxyecdysone (20E) during 3^rd instar larval development. Mucin gene expression and protein synthesis are induced by a small 20E pulse during the early 3^rd instar larval stage. Mucins are then packaged into small secretory granules (green dots), which fuse to each other to form mature granules. During the late 3^rd instar larval stage, a large pulse of 20E induces the secretion of secretory granule contents. AEL, after egg lay. (C) Model of Tango1 forming a docking site between the ER and Golgi to facilitate mucin movement between compartments and secretory granule formation. Tango1 forms ring structures that mediate the formation of COPII rings, which serve as docking sites for the cis Golgi to assist secretory mucin movement from the ER to the Golgi. Secretory granules bud from the trans-Golgi side of these structures. (D) Loss of PGANT9 results in abnormal secretory granules with an irregular, shard-like appearance, suggesting that O-glycosylation of mucin influences the maturation and morphology of secretory granules.

Figure 4.. The protective mucus layer plays essential roles in health and homeostasis in the Drosophila and mammalian digestive systems.

(A). The peritrophic membrane (PM) of the Drosophila larval midgut contains highly O-glycosylated mucins and protects the epithelium from mechanical and microbial damage. Disruption/loss of this protective mucus layer results in epithelial cell damage, activation of the host defense response, and induction of specific signaling cascades that lead to increased cell proliferation and disruption of progenitor cell niche. B. The mucus lining of the mammalian colon, which is comprised of the highly O-glycosylated mucin Muc2, exists as 2 layers: an outer layer that is rich in commensal bacteria and a tightly-packed inner layer that is attached to the epithelium and devoid of bacteria. Disruption/loss of this mucus lining in the mammalian colon results in aberrant intestinal morphology, increased intestinal permeability, intestinal inflammation, increased cell proliferation and spontaneous colorectal cancer.