ABSTRACT
As a remarkable chemical and physical defence barrier, mucins are large glycoproteins that cover epithelial surfaces of the intestine and form a mucus layer to protect epithelial cells from gut health challenge. There are two major types of mucins: membrane-bound and secreted (Williams et al., 2001; Toribara et al., 1993). In chickens, three transmembrane mucins (MUC4, MUC13 and MUC16) and four gel-forming mucins (MUC6, MUC2, MUC5ac and MUC5b) have been identified (Lang et al., 2006). In mammals, MUC2, the mucin secreted by goblet cells, is the most abundant mucin in the intestine, and its deficiency has been reported to increase BT and inflammation (Wei et al., 2012; Kim and Ho, 2010). Evolutionary studies suggest that mucins share a common ancestor, since their domain structures are well conserved in metazoans (Kimbrell and Beutler, 2001; Lang et al., 2007). All mucins (MUC) contain at least one PTS domain, a region rich in proline, threonine and serine (Jiang et al., 2013; Kim and Ho, 2010). Chicken MUC2 has been reported to be remarkably similar to human and mouse outside of the central PTS domain, but is highly divergent within this central repetitive structure (Jiang et al., 2013; Zhang et al., 2015). The highly glycosylated PTS domain, once hydrated post-secretion, allows for different electrostatic bonding and effective ‘pore’ sizes within the mucin layers that create different micro-environments for bacteria. Jiang et al. (2013) noted that alternate splicing within the PTS domain of MUC2 may lead to differences observed in mucin composition. Although the physiological implications and disease associations of MUC on various mucosal surfaces are well understood, there are still many questions as to how and why the gene architecture of this family contributes to diverse protein modifications that show diverse biological effects between metazoans in health and disease (Clark et al., 2006; Forder et al., 2012; Liu et al., 2012; Hollingsworth and Swanson, 2004). MUC2 gene expression has been used as a marker for gut health in poultry and other species (Forder et al., 2012; Chen et al., 2015).
