Lamellar Structures of MUC2-Rich Mucin: A Potential Role in Governing the Barrier and Lubricating Functions of Intestinal Mucus

Mucus is a ubiquitous feature of mammalian wet epithelial surfaces, where it lubricates and forms a selective barrier that excludes a range of particulates, including pathogens, while hosting a diverse commensal microflora. The major polymeric component of mucus is mucin, a large glycoprotein formed by several MUC gene products, with MUC2 expression dominating intestinal mucus. A satisfactory answer to the question of how these molecules build a dynamic structure capable of playing such a complex role has yet to be found, as recent reports of distinct layers of chemically identical mucin in the colon and anomalously rapid transport of nanoparticles through mucus have emphasized. Here we use atomic force microscopy (AFM) to image a MUC2-rich mucus fraction isolated from pig jejunum. In the freshly isolated mucin fraction, we find direct evidence for trigonally linked structures, and their assembly into lamellar networks with a distribution of pore sizes from 20 to 200 nm. The networks are two-dimensional, with little interaction between lamellae. The existence of persistent cross-links between individual mucin polypeptides is consistent with a non-self-interacting lamellar model for intestinal mucus structure, rather than a physically entangled polymer network. We only observe collapsed entangled structures in purified mucin that has been stored in nonphysiological conditions

Sodium alginate decreases the permeability of intestinal mucus

In the small intestine the nature of the environment leads to a highly heterogeneous mucus layer primarily composed of the MUC2 mucin. We set out to investigate whether the soluble dietary fibre sodium alginate could alter the permeability of the mucus layer. The alginate was shown to freely diffuse into the mucus and to have minimal effect on the bulk rheology when added at concentrations below 0.1%. Despite this lack of interaction between the mucin and alginate, the addition of alginate had a marked effect on the diffusion of 500 nm probe particles, which decreased as a function of increasing alginate concentration. Finally, we passed a protein stabilised emulsion through a simulation of oral, gastric and small intestinal digestion. We subsequently showed that the addition of 0.1% alginate to porcine intestinal mucus decreased the diffusion of fluorescently labelled lipid present in the emulsion digesta. This reduction may be sufficient to reduce problems associated with high rates of lipid absorption such as hyperlipidaemia

Transport of Particles in Intestinal Mucus under Simulated Infant and Adult Physiological Conditions: Impact of Mucus Structure and Extracellular DNA

The final boundary between digested food and the cells that take up nutrients in the small intestine is a protective layer of mucus. In this work, the microstructural organization and permeability of the intestinal mucus have been determined under conditions simulating those of infant and adult human small intestines. As a model, we used the mucus from the proximal (jejunal) small intestines of piglets and adult pigs. Confocal microscopy of both unfixed and fixed mucosal tissue showed mucus lining the entire jejunal epithelium. The mucus contained DNA from shed epithelial cells at different stages of degradation, with higher amounts of DNA found in the adult pig. The pig mucus comprised a coherent network of mucin and DNA with higher viscosity than the more heterogeneous piglet mucus, which resulted in increased permeability of the latter to 500-nm and 1-µm latex beads. Multiple-particle tracking experiments revealed that diffusion of the probe particles was considerably enhanced after treating mucus with DNase. The fraction of diffusive 500-nm probe particles increased in the pig mucus from 0.6% to 64% and in the piglet mucus from ca. 30% to 77% after the treatment. This suggests that extracellular DNA can significantly contribute to the microrheology and barrier properties of the intestinal mucus layer. To our knowledge, this is the first time that the structure and permeability of the small intestinal mucus have been compared between different age groups and the contribution of extracellular DNA highlighted. The results help to define rules governing colloidal transport in the developing small intestine. These are required for engineering orally administered pharmaceutical preparations with improved delivery, as well as for fabricating novel foods with enhanced nutritional quality or for controlled calorie uptake

Standardization of in vitro digestibility and DIAAS method based on the static INFOGEST protocol

Background: The FAO recommends the digestible indispensable amino acid score (DIAAS) as the measure for protein quality, for which the true ileal digestibility needs to be assessed in humans or pigs. However, due to high costs and ethical concerns, the FAO strongly encourages as well the development of validated in vitro methods, which complement the in vivo experiments. Method: Recently, an in vitro workflow, based on the validated static INFOGEST protocol, was developed and compared towards in vivo data. In parallel to the validation with in vivo data, the repeatability and reproducibility of the in vitro protocol were tested in an international ring trial (RT) with the aim to establish an international ISO standard method within the International Dairy Federation (IDF). Five different dairy products (skim milk powder, whole milk powder, whey protein isolate, yoghurt, and cheese) were analyzed in 32 different laboratories from 18 different countries, across 4 continents. Results: in vitro protein digestibilities based on Nitrogen, free R-NH2, and total amino acids as well as DIAAS values were calculated and compared to in vivo data, where available. Conclusion: The in vitro method is suited for quantification of digestibility and will be further implemented to other food matricesinfo:eu-repo/semantics/publishedVersio

Colloidal transport in the intestinal mucus

The last boundary between ingested food and the gastrointestinal (GI) tract mucosa is the mucus barrier. This highly complex viscoelastic medium has evolved to provide a robust barrier that can trap and immobilise potentially hazardous particulates such as bacteria but still allow the passage of nutrients to the epithelial surfaces (1,2). These conflicting properties are particularly important in the small intestine where the mucus layer is thinnest and the majority of nutrients absorption takes place. However, the rules governing this selective barrier function, particularly in relation to transport of particulates, remain unknown. Recent studies show that surface properties of microparticles can largely impact on colloidal transport in the intestinal mucus (3). The physico-chemical characteristics of the absorption of nutrients depend on the structure of post-digestion food particles which, in turn, is determined by the original food structure and the kinetics and pattern of its enzymatic breakdown in the GI tract. Detailed characterisation of both, the colloidal aspects of digestion and the transport of post-digestion food particles in the intestinal mucus is required for rational design of foods with controlled behaviour in the GI tract, improving their nutritional quality and maintaining human health, thus overcoming diet-related health problems. Here, we summarise our recent studies on the effect the structure of intestinal mucus on the rate of colloidal transport in the mucus

Colloidal transport in the intestinal mucus

The last boundary between ingested food and the gastrointestinal (GI) tract mucosa is the mucus barrier. This highly complex viscoelastic medium has evolved to provide a robust barrier that can trap and immobilise potentially hazardous particulates such as bacteria but still allow the passage of nutrients to the epithelial surfaces (1,2). These conflicting properties are particularly important in the small intestine where the mucus layer is thinnest and the majority of nutrients absorption takes place. However, the rules governing this selective barrier function, particularly in relation to transport of particulates, remain unknown. Recent studies show that surface properties of microparticles can largely impact on colloidal transport in the intestinal mucus (3).The physico-chemical characteristics of the absorption of nutrients depend on the structure of post-digestion food particles which, in turn, is determined by the original food structure and the kinetics and pattern of its enzymatic breakdown in the GI tract. Detailed characterisation of both, the colloidal aspects of digestion and the transport of post-digestion food particles in the intestinal mucus is required for rational design of foods with controlled behaviour in the GI tract, improving their nutritional quality and maintaining human health, thus overcoming diet-related health problems. Here, we summarise our recent studies on the effect the structure of intestinal mucus on the rate of colloidal transport in the mucus

Analysis of the Factors Affecting Static In Vitro Pepsinolysis of Food Proteins

In this meta-analysis, we collected 58 publications spanning the last seven decades that reported static in vitro protein gastric digestion results. A number of descriptors of the pepsinolysis process were extracted, including protein type; pepsin activity and concentration; protein concentration; pH; additives; protein form (e.g., ‘native’, ‘emulsion’, ‘gel’, etc.); molecular weight of the protein; treatment; temperature; and half-times (HT) of protein digestion. After careful analysis and the application of statistical techniques and regression models, several general conclusions could be extracted from the data. The protein form to digest the fastest was ‘emulsion’. The rate of pepsinolysis in the emulsion was largely independent of the protein type, whereas the gastric digestion of the native protein in the solution was strongly dependent on the protein type. The pepsinolysis was shown to be strongly dependent on the structural components of the proteins digested—specifically, β-sheet-inhibited and amino acid, leucine, methionine, and proline-promoted digestion. Interestingly, we found that additives included in the digestion mix to alter protein hydrolysis had, in general, a negligible effect in comparison to the clear importance of the protein form or additional treatment. Overall, the findings allowed for the targeted creation of foods for fast or slow protein digestion, depending on the nutritional needs

Colloidal transport in the intestinal mucus

The last boundary between ingested food and the gastrointestinal (GI) tract mucosa is the mucus barrier. This highly complex viscoelastic medium has evolved to provide a robust barrier that can trap and immobilise potentially hazardous particulates such as bacteria but still allow the passage of nutrients to the epithelial surfaces (1,2). These conflicting properties are particularly important in the small intestine where the mucus layer is thinnest and the majority of nutrients absorption takes place. However, the rules governing this selective barrier function, particularly in relation to transport of particulates, remain unknown. Recent studies show that surface properties of microparticles can largely impact on colloidal transport in the intestinal mucus (3). The physico-chemical characteristics of the absorption of nutrients depend on the structure of post-digestion food particles which, in turn, is determined by the original food structure and the kinetics and pattern of its enzymatic breakdown in the GI tract. Detailed characterisation of both, the colloidal aspects of digestion and the transport of post-digestion food particles in the intestinal mucus is required for rational design of foods with controlled behaviour in the GI tract, improving their nutritional quality and maintaining human health, thus overcoming diet-related health problems. Here, we summarise our recent studies on the effect the structure of intestinal mucus on the rate of colloidal transport in the mucus

Colloidal transport in the intestinal mucus

The last boundary between ingested food and the gastrointestinal (GI) tract mucosa is the mucus barrier. This highly complex viscoelastic medium has evolved to provide a robust barrier that can trap and immobilise potentially hazardous particulates such as bacteria but still allow the passage of nutrients to the epithelial surfaces (1,2). These conflicting properties are particularly important in the small intestine where the mucus layer is thinnest and the majority of nutrients absorption takes place. However, the rules governing this selective barrier function, particularly in relation to transport of particulates, remain unknown. Recent studies show that surface properties of microparticles can largely impact on colloidal transport in the intestinal mucus (3).The physico-chemical characteristics of the absorption of nutrients depend on the structure of post-digestion food particles which, in turn, is determined by the original food structure and the kinetics and pattern of its enzymatic breakdown in the GI tract. Detailed characterisation of both, the colloidal aspects of digestion and the transport of post-digestion food particles in the intestinal mucus is required for rational design of foods with controlled behaviour in the GI tract, improving their nutritional quality and maintaining human health, thus overcoming diet-related health problems. Here, we summarise our recent studies on the effect the structure of intestinal mucus on the rate of colloidal transport in the mucus