The cathelicidin LL AMP has been shown to electrostatically interact with human airway mucus as well as with porcine gastric mucus and bovine submaxillary mucins, which decreased its antimicrobial activity. It is still unknown how the mucus reservoir of AMPs performs under such inhibitory conditions.
Nevertheless, their role as bactericidal molecules in mucus could be explained by sudden bursts of AMP secretion that could overcome mucin binding, or the local degradation and release of the AMP by incoming pathogens.
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The co-production, co-secretion and co-localization of trefoil factors with mucus and mucins, , imply that they may also be co-regulated and potentially assist each other's functions, jointly promoting the protection and repair of the gastrointestinal mucosa. Thim et al. Moreover, in the ulcer-associated cell lineage as well as in the gastrointestinal mucosa, each TFF seems to be co-localized with a specific mucin type, hinting that TFFs could act differently when coupled with different mucins. Growth factors are also found co-localized with mucins, especially in the gut. EGF and mucins are co-localized in saliva, milk, and in the digestive system.
Thus, the mucus matrix would play a role similar to that of the extracellular matrix ECM , which mediates cellular processes through the sequestration of growth factors. Our understanding of the role of mucins in the modulation of cellular behaviors is still limited, which is why there has been a limited number of attempts at re-creating these effects in engineered systems. Early results obtained with coatings of mucin from commercial sources exposed to neutrophils, did suggest an immunomodulatory effect of mucins.
The mucin coatings were found to reduce the adhesion of neutrophils and their secretion of inflammatory markers, such as reactive oxygen species. Some inflammation was noticed after prolonged injection, which could be attributed to the relatively crude mucin extraction protocol used. Ovomucins are known to have anti-viral, anti-tumor and immune stimulating properties. The in vivo studies showed good infiltration of fibroblasts cells in the hydrogels, most likely due to the gelatin components.
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Interestingly, the subcutaneous implantation of the gel in rats favored the infiltration of alternatively activated macrophage cells that are favorable for tissue repair and did not trigger any allergic response measured by IgE levels in rat serum. Unfortunately, the study did not compare the mucin-gelatin to gelatin-only materials, which makes it difficult to isolate the contribution of ovomucins to these effects.
Although the technological potential of mucins seems clear from a functional point of view, some obstacles need to be overcome before translational work can begin. First, mucin research heavily relies on the commercial sources of mucins. The batch to batch variability, the uncertainty as to what protocols are used for purification, and the presence of impurities are challenges that need to be addressed. The development of protocols to further purify commercial mucins, protocols to produce native-like mucin on a larger scale, 35 and protocols for the quality control of the molecular , and physical properties of the mucins will be essential.
Challenges around sterilization and conservation of mucin containing products have started to be addressed, but could face more challenges if the products are to rely on un-altered protein and glycan structures of the mucins. Other strategies to produce highly-reproducible mucin materials have been suggested. For instance, one approach is to first simplify the protein or glycan sections of mucins through enzymatic treatments, and then to use this natural backbone to reconstruct the mucins or mucin-mimic in a controlled manner.
This would result in mucin mimics incorporating the unique glycan ligand diversity of native mucins. There is now a significant body of research focused on the study and engineering of the chemical and physical functionalities of mucins. Scarcer, however, are studies investigating the bioactivity of mucin-materials either in vitro or in vivo.
We found seven studies reporting on in vivo testing of mucin materials, providing only initial indications that valuable functionalities could result, but also clearly showing the very early stages of mucin materials for biomedical applications Table 2. Indeed, most of these early studies do not describe the purification protocols used to obtain mucins. Contaminants co-purified with the mucins can then lead to unwanted side effects. Two more recent experiments performed with purified mucins and implanted in rats and sheep did show encouraging results. In both cases, the investigators found the animals had a low immune reaction to the allogeneic mucin implementation.
Received 27th April , Accepted 6th July Table 1 List of the properties for which mucins have been applied.
The references to relevant landmark studies and reviews are listed for each of the properties. Property Description Ref. Barrier Mucins assemble through covalent and noncovalent intermolecular interactions into a mesh of polymers that forms a size exclusion barrier to molecules, particles, and pathogens.
Mucins typically bind particles and molecules that bear positive charges, thiol groups, hydrophobic moieties, and sugar-binding domains.
Mucins also form stable coatings with anti-fouling properties on a variety of surfaces. A constant fine-tuning of mucus rheological properties occurs in vivo , determining mucus clearance rates and barrier properties. The mucin-associated glycans can associate with water molecules via hydrogen bonding.
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The hydrated mucin molecules can entangle into networks that trap large amounts of water. Hydration of mucins is intimately linked with their rheological, tribological, and barrier properties. As discussed in the text below, mucins can directly act as ligands for cell surface receptors or sequester biologically active molecules. A All mucin molecules share the common feature of a central protein core depicted in blue decorated with regions of dense O-linked glycosylation. The terminal sugars of the glycans display an impressive diversity and can bear a negative charge, if composed of the sialic acid sugar.
B Atomic force microscopy images of mucin molecules adsorbed onto surfaces reveal a uni-dimensional molecule that can be in the hundreds of nanometers in length and only a few nanometers thick. C The mucin monomers assemble into high molar mass oligomers that can interact through intermolecular bonds and entanglement into physical hydrogels. D Electron scanning microscopy images of a mucus gel, composed of the mucin fibrillar network, supplemented with a range of soluble proteins, lipids, and salts. The scale bar is nm. The Fig. A Most surfaces are susceptible to colonization by macromolecules, cells and bacteria.
B Mucins can adsorb to form stable coatings on a number of surface chemistries. Mucin coatings can repel bacteria, proteins, and mammalian cells. C Mucin coatings are hypothesised to resemble those of PEG-based antifouling surfaces. D—E The antifouling properties of mucin coatings as shown by microscopy observation and quantification of bacteria binding. Pseudomonas aeruginosa and other bacteria possess receptors that bind the glycan and protein domains of the mucins, allowing them to overcome the repellent effect.
F The repellent effect of mucin coatings towards mammalian cells was utilized to patterned epithelial, fibroblast and myoblast cells on tissue culture polystyrene.
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D is reprinted from ref. A Three-dimensional mucin hydrogels mimic the native mucus barrier by preventing bacteria and other pathogens from accessing the surface. C Certain parasites and cancer cells secrete mucins on their surface to create a barrier and evade the immune system. The encapsulation of cells with mucins could constitute a strategy to provide chemical and biological protection to individual cells. B is adapted from ref. Comparison of aqueous solutions vial to the left or mucin solutions vial to the right of A fullerene C60 and B inorganic fullerene-like disulfide tungsten IF-WS2.
The amphiphilic properties of mucins were shown to enhance the uptake of benzo[ a ]pyrene BaP into CaCo2 epithelial cells. E Methacrylate BSM was generated to create photocrosslinkable mucin hydrogels. F The covalently cross-linked mucin hydrogels release in a sustained manner both a hydrophilic and positively charged antibiotic and a hydrophobic anticancer drug. Dextran and polyglutamic acid are control molecules showing limited interactions with the mucin hydrogel. A and B are reprinted from ref. E and F are reprinted from ref.
A Stribeck curve of pig gastric mucins dissolved in pure water lubricating smooth polydimethylsiloxane PDMS surfaces. B Boundary friction coefficient measured on cartilage explants lubricated with lubricin. C Histology images revealing the beneficial effect of a lubricin LUB:1 injection in the knee compared to control PBS in a rat model of osteoarthritis. The sections were immunostained with anti-lubricin antibody.
In addition to lubricity, mucins and lubricins are able to limit wear of surfaces in friction. In a steel on cartilage model system, the surface topography of the cartilage was analysed for the effect of wear. D The cartilage lubricated with 0. E Lubrication by 0. The whiskers denote the 1. A is reprinted with permission from ref. B and C are reprinted from ref. Mucins can illicit biological responses from bacteria and mammalian cells through several mechanisms.
Mucin-associated glycans contain ligands to carbohydrate binding surface receptors such as galectins and siglec. Domains of the protein core can also serve as ligands. Sialic acid residues have a physical role in mediating cell adhesion and shielding antigens from antibody recognition. Mucins and mucin gels can also sequester a number of bioactive molecules, perhaps mediating their activities in ways that are still not understood. Table 2 In vivo studies of mucin or mucin-based materials.
Animal model Type of mucins implanted Implanted material Observations Ref. Mouse, rat Pig gastric and salivary Co-injected with bacteria Inhibition of phagocytosis, antibody uptake and bactericidal action.