Recent studies have suggested that cell surface mucins contribute to the maintenance of the mucosal barrier integrity by preventing adhesion of foreign debris, cells, or pathogens onto the mucosal surface epithelia (Argueso et al., 2006; Imbert et al., 2006; Blalock et al., 2007; Corrales et al., 2011). The large protein core, dense O-glycosylation, charge repulsion, and hydration results in steric hindrance, making the cell surface mucins relatively rigid structures (Gipson and Inatomi, 1998; Cone, 2009; McGuckin et al., 2011; Petrou and Crouzier, 2018), which remain unaltered despite movement and shearing at the mucosal surfaces. Protruding considerably further from the cell surface (e.g., MUC1 can extend >200 nm), the cell surface mucins are thought to prevent adhesion of the secreted mucus layer directly to the epithelial cell surface. Sumiyoshi et al. demonstrated that the anti-adhesive character of mucin O-glycans at the apical surface of corneal epithelial cells was caused by the repulsive negative charge interactions between secreted and cell surface mucins (Sumiyoshi et al., 2008). In addition, the mucins anchored to the cell surface have the ability to bind to glycan ligands, such as galectin-3, which can generate molecular matrices and facilitate mucin assembly, reinforcing the physical barrier on the epithelial cell surface (Argueso et al., 2009; Gipson et al., 2014).

Cell surface mucins regulate the signal transduction through both extracellular and cytoplasmic domains. It has been suggested that cell surface mucins sense the environment through their extracellular domain and signal through their intracellular domain (Singh and Hollingsworth, 2006). Several cell surface mucins have epidermal growth factor (EGF)-like motifs in their extracellular domain adjacent to the cell membrane. These EGF-like motifs interact with a wide range of receptors, prompting cell proliferation following epithelial injury in the normal tissue, and in cancer progression to metastasis (Singh and Hollingsworth, 2006). MUC4 has at least two EGF-like domains, which can activate ErbB2 and ErbB3 receptors responsible for epithelial cell proliferation and apoptosis (Singh and Hollingsworth, 2006). MUC1 and MUC13 can regulate chemokine secretion from intestinal epithelial cells. The cytoplasmic domain of MUC1 contains multiple phosphorylated tyrosine residues that can activate intracellular signalling pathways, such as the ERK1/2 pathway in airway epithelial cells (Wang et al., 2003). It can also interact with β-catenin, catenin p120, ER-α, p53, and nuclear factor kappa B (NF-ĸB) to convey specialised signalling in response to conditions at the cell surface, such as binding with pathogens and changes in pH (Singh and Hollingsworth, 2006). MUC13 can enhance NF-ĸB activation and prevent cell death, which can advance tumour formation and progression (Sheng et al., 2017).

Once secreted by goblet cells, submucosal gland cells, or serous cells, gel-forming mucins form a highly hydrated mucus gel and contribute to the lubrication of the epithelial cell surfaces. The hydrophilic nature of the mucins is thought to reduce shear stress at the epithelial surface: for instance, peristalsis and movement of stool through the intestine (McGuckin et al., 2011; Petrou and Crouzier, 2018). This polymeric network of secreted mucins acts similarly to a gel filtration system. Ex vivo measurements using fluorescent probes have shown that large molecules such as pathogens cannot pass through, while smaller molecules like antimicrobial peptides can easily penetrate the mucus gel (Hasnain et al., 2010; Gustafsson et al., 2012).

In the intestine, the mucus layer, mainly composed of MUC2, forms an inner adherent layer and an outer loose layer. The inner adherent layer is “sterile,” composed of MUC2 multimers that are presumably tightly packed to provide protection from the commensal flora (Hansson and Johansson, 2010; Gustafsson et al., 2012). Hansson et al. hypothesised that MUC2 multimers are organised as sheets that interact with the epithelial cell layer and cell surface mucins; however, X-ray crystal structure of MUC2 multimerisation module suggests that non-covalent and covalent interactions form a lateral network (Javitt et al., 2019). The outer mucus layer is exposed to proteases and bacteria, which enables it to become less dense. This is also important for maintaining homeostasis, as the faecal material in the intestine generates mechanical stress (McGuckin et al., 2011). The respiratory mucus layer is more complex. It comprises two gel-forming mucins, MUC5AC and MUC5B, which only form homo-multimers in the lung. Although MUC5B expression was mainly thought to be restricted to submucosal glands, recent data show that the distal airway superficial epithelium is the predominant site for MUC5B expression, while MUC5AC expression is concentrated in the proximal airways (Bonser and Erle, 2017; Hancock et al., 2018; Hughes et al., 2019; Okuda et al., 2019). Once secreted into the airway lumen, these mucins can non-covalently cross-link to form a physical barrier that can be easily moved by the cilia (McGuckin et al., 2011; Bonser and Erle, 2017).

The secreted gel-forming mucins can trap allergens and debris to facilitate their clearance from the mucosal surface (Rubin, 2002). This role is assisted by the incredible diversity of the carbohydrate side chains, which enhances the possibility of pathogens binding to the mucus (Thornton and Sheehan, 2004). While it has been reported that hydrophilic contaminants are easily repulsed by the mucus gel, weakly polar contaminants are trapped in the mucus gel (Sharma, 1993). Contaminants, including pathogens and allergens, are then eliminated along with the mucus (Gipson and Argueso, 2003; Dartt and Masli, 2014). In a patient with congenital loss of MUC5B, Costain et al. (2022) highlight the key role of mucins and demonstrate impaired mucociliary clearance and increased inflammatory macrophage infiltrate in sputum (Costain et al. 2022).

MUC1 has been suggested to play an anti-inflammatory role during Pseudomonas aeruginosa respiratory infection, as Muc1^−/− mice are more susceptible to infection in a repetitive Pseudomonas infection model ( Table 2 ) (Lu et al., 2006). Colonisation is associated with stronger immune responses (Lu et al., 2006), including increases in tumor necrosis factor alpha (TNFα) and interleukin (IL)-8 in bronchoalveolar lavage fluids compared with wild-type (WT) mice (Umehara et al., 2012). Deficiency in Muc1 also predisposed mice to infection with the gastrointestinal pathogen C. jejuni and the gastric pathogen H. pylori ( Table 2 ) (McAuley et al., 2007; McGuckin et al., 2007). In cases of acute infection with C. jejuni, Muc1^−/− mice rapidly develop systemic infection, suggesting that Muc1 limits this pathogen penetration through the mucosal barrier, and Muc1 also modulates the epithelial cell response to a bacterial genotoxin (McAuley et al., 2007). Similarly, Muc1^−/− mice showed a five times higher density of infection (increased colony forming units) as early as 1 day after oral gavage with H. pylori compared with WT mice (Sheng et al., 2020). Furthermore, more severe chronic inflammation was observed in Muc1^−/− mice after exposure to H. pylori, demonstrating that this cell surface mucin can modulate the inflammatory response to chronic infection (McGuckin et al., 2007). Muc1^−/− mice also develop severe pathology in response to influenza A virus infection ( Table 2 ) (McAuley et al., 2017). In the absence of Muc1, the kinetics of the infection are altered. Animals reach maximal influenza A viral load earlier than WT mice and also display enhanced inflammatory response to the infection (McAuley et al., 2017). Similarly, a higher viral titre was detected in Muc1^−/− mice compared to WT mice in response to intranasal inoculation of murine adenovirus type 1 (MAV-1) (McAuley et al., 2017), suggesting that the Muc1 may protect against MAV-1 respiratory infections (Nguyen et al., 2011). MUC1 has been shown to have an anti-inflammatory response during respiratory syncytial viral infection in vitro (Li et al., 2010).

Muc3 (the murine orthologue of human MUC17) and Muc13 are two of the most abundant cell surface mucins in the normal intestinal tract. Interestingly, Muc3 may play a role in wound healing in acute chemical-induced [dextran sodium sulphate (DSS)] colitis: intrarectal administration of recombinant cysteine-rich domains of Muc3 accelerated cell migration and reduced apoptosis in the distal colon (Ho et al., 2006). Corroborating this anti-inflammatory effect, we have demonstrated that Muc13-deficient mice have increased susceptibility to DSS-induced colitis, increased local inflammatory cytokine production, and increased epithelial cell apoptosis (Sheng et al., 2011). Overall, there is strong evidence supporting a critical role of cell surface mucins in protection against inflammation by modulating growth and inhibiting apoptosis of epithelial cells during wounding and repair.

Transgenic animals lacking secreted gel-forming mucins have also demonstrated their anti-inflammatory effects. Muc5ac^−/− mice have higher H. pylori colonisation densities compared with WT animals at 16 weeks post-infection, along with a significant reduction in gastric Tnfα and Il-17a ( Table 2 ) (Muthupalani et al., 2019). Furthermore, H. pylori-infected Muc5ac^−/− mice had significantly lowered gastric corpus mucous metaplasia at 16 weeks post-infection (wpi) and 32 wpi compared with WT mice. Our work has shown that de novo intestinal goblet cell expression of Muc5ac in the intestine is critical in the protection against Trichuris nematode. Muc5ac but not Muc2 reduced nematode ATP levels and was responsible for the expulsion of the nematode. These studies demonstrate a protective role for Muc5ac in inhibiting pathogen-associated inflammatory pathology. Significantly greater inflammation and fibrosis by bleomycin were developed in Muc5ac^−/− lungs compared to WT animals (Cho et al., 2021). Airway respiratory syncytial viral (RSV) replication was higher in Muc5ac^−/− than in Muc5ac^+/+ during early infection. RSV-caused pulmonary epithelial death, bronchial smooth muscle thickening, and syncytia formation were more severe in Muc5ac^−/− compared to WT mice (Cho et al., 2021).

Muc5b, but not Muc5ac, was shown to be critical in trapping and clearing microbial pathogens through mucociliary clearance in the lung (Roy et al., 2014; Hancock et al., 2018). Muc5b ^−/− mice have a significantly dysfunctional inflammatory response, with an increase in neutrophils and eosinophils but an absence of lymphocytes. There was an increased bacterial accumulation, including Staphylococcus aureus, in the lung of Muc5b ^−/− , which, combined with the lack of lymphocytes, leads to increased mortality (Roy et al., 2014).

MUC2/Muc2 is the main secreted intestinal mucin expressed and secreted by all enteric goblet cells. While acute infection leads to goblet cell hyperplasia and increased Muc2, chronic inflammation in the intestine is associated with goblet cell depletion. Muc2^−/− mice are more susceptible to Salmonella typhimurium infection, with increased mortality rates, higher pathogen burdens, and developing significantly higher barrier disruption compared with WT animals ( Table 2 ) (Zarepour et al., 2013). Similarly, Muc2^−/− mice show rapid loss of weight and up to 90% higher mortality in response to Citrobacter rodentium, a murine attaching–effacing (A/E) pathogen related to diarrheagenic A/E. coli ( Table 2 ) (Bergstrom et al., 2010). We have shown that expulsion of the Trichuris worms from the intestine was significantly delayed in Muc2-deficient mice compared with WT mice ( Table 2 ) (Hasnain et al., 2010).