Native gastrointestinal mucus: Critical features and techniques for studying interactions with drugs, drug carriers, and bacteria

Abstract

Gastrointestinal mucus plays essential roles in modulating interactions between intestinal lumen contents, including orally delivered drug carriers and the gut microbiome, and underlying epithelial and immune tissues and cells. This review is focused on the properties of and methods for studying native gastrointestinal mucus and its interactions with intestinal lumen contents, including drug delivery systems, drugs, and bacteria. The properties of gastrointestinal mucus important to consider in its analysis are first presented, followed by a discussion of different experimental setups used to study gastrointestinal mucus. Applications of native intestinal mucus are then described, including experimental methods used to study mucus as a barrier to drug delivery and interactions with intestinal lumen contents that impact barrier properties. Given the significance of the microbiota in health and disease, its impact on drug delivery and drug metabolism, and the use of probiotics and microbe-based delivery systems, analysis of interactions of bacteria with native intestinal mucus is then reviewed. Specifically, bacteria adhesion to, motility within, and degradation of mucus is discussed. Literature noted is focused largely on applications of native intestinal mucus models as opposed to isolated mucins or reconstituted mucin gels.

Keywords: Bacteria-mucin interactions; Fluorescence recovery after photobleaching; Mucosal drug delivery; Mucus barrier properties; Mucus composition; Mucus structure; Multiple particle tracking; Native intestinal mucus.

Figure 1.

MUC2 mucins are heavily glycosylated in PTS domains, imparting a “bottle brush” type structure. O-glycosylation starts with the addition of GalNAc from uridine diphospho (UDP)-GalNAc to a Serine (Ser) or a Threonine (Thr), creating the Tn antigen. Tn antigen can be modified with Gal, GlcNAc, or sialic acid to create the core 1 and 3 structure and sialyl Tn antigen, respectively. Core 1 and core 3 structures can be further extended to create core 2 and core 4 structures, whereas sialyl Tn antigen cannot be extended. Core 1 to 4 structures are usually further modified and/or branched by glycosyltransferases. Terminal ends of the glycan chains are often capped with fucose or sialic acid, forming significant antigen structures such as ABO blood group determinants and Lewis antigens. ppGalNAcTs = polypeptidyl GalNAc transferases; C1GalT1 = core 1 β1,3-galactosyltransferase; C2GnTs = core 2 β1,6 N-acetylglucosaminyltransferases; C2GnT2 = core 3 β1,3 N-acetylglucosaminyltransferase; ST6GalNAc = α2,6 sialyltransferase. Reproduced from Bergstrom and Xia [44].

Figure 2.

Schematic diagram illustrating the domain structure of MUC2 and the process of forming the mesh-like MUC2 network. Adapted from Johansson et al. [53].

Figure 3.

Mucus organization along the gastrointestinal tract. The stomach has two layers of mucus comprised of MUC5AC mucins. The small intestinal mucus is a single-layer hydrogel consisting of MUC2 mucins, whereas mucus in the colon has two layers: an outer loose layer and an inner dense layer. Both layers of the colonic mucus are comprised of MUC2 mucins. Cells secrete factors such as antimicrobial peptides and deleted in malignant brain tumors 1 (DMBT1) into overlying mucus. Reproduced from Johansson et al. [16]

Figure 4.

(A) Simplified schematic showing the procedure of probing native porcine intestinal mucus barrier properties using MPT. (B) Representative trajectories of amine-, carboxylate-, and polyethylene glycol (PEG)-modified fluorescent polystyrene nanoparticles were used to probe the barrier properties of native pig intestinal mucus upon exposure to surfactants (carboxymethylcellulose (CMC) and Tween 80) relative to exposure to maleate buffer (MB) as control. The trajectory data can be used to calculate effective diffusivity as an indicator of mucus barrier properties. Adapted from Lock et al. [66].