The Intestinal Barrier and Current Techniques for the Assessment of Gut Permeability

Abstract

The intestinal barrier is essential in human health and constitutes the interface between the outside and the internal milieu of the body. A functional intestinal barrier allows absorption of nutrients and fluids but simultaneously prevents harmful substances like toxins and bacteria from crossing the intestinal epithelium and reaching the body. An altered intestinal permeability, a sign of a perturbed barrier function, has during the last decade been associated with several chronic conditions, including diseases originating in the gastrointestinal tract but also diseases such as Alzheimer and Parkinson disease. This has led to an intensified interest from researchers with diverse backgrounds to perform functional studies of the intestinal barrier in different conditions. Intestinal permeability is defined as the passage of a solute through a simple membrane and can be measured by recording the passage of permeability markers over the epithelium via the paracellular or the transcellular route. The methodological tools to investigate the gut barrier function are rapidly expanding and new methodological approaches are being developed. Here we outline and discuss, in vivo, in vitro and ex vivo techniques and how these methods can be utilized for thorough investigation of the intestinal barrier.

Keywords: gut permeability; intestinal barrier; paracellular probes; paracellular route; techniques; transcellular route.

Figure 1

A schematic drawing of the intestinal barrier and passage routes across the epithelium. Solutes can pass the intestinal epithelium via either the (A) paracellular route (larger hydrophilic solutes); (B) transcellular route (small hydrophilic and lipophilic solutes) (C); transcellular route via aqueous pores (small hydrophilic solutes) or active carrier-mediated absorption (nutrients); or (D) endocytosis, followed by transcytosis and exocytosis (larger particles, peptides and proteins). The barrier constitutes of (1) the lumen, bacteria and antigens are degraded by biliary juices, gastric and pancreatic acids and the colonization of pathogens is inhibited by commensal bacteria producing antimicrobial substances; (2) the microclimate; unstirred water layer, glycocalyx, bacterial adhesion is prevented by mucus and IgA secretion; (3) the epithelial cells; luminal content is transported while noxious stimuli is impeded by chloride secretion and production of antimicrobial peptides (AMP), junctional complexes between the cells regulate permeability, for details see right panel; (4) the lamina propria; immunoglobulins and cytokines are secreted from cells of both the innate and acquired immunity with direct or indirect effects on permeability, interactions with the endocrine and enteric nervous system. TAMP: tight junction-associated-MARVEL proteins including occludin, tricellulin and Marvel D3; JAM: junctional adhesion molecule; MLC: myosin light chain; MLCK: MLC of myosin II kinase.

Figure 2

Overview of the different techniques used to measure intestinal barrier function. It is important to consider the aim of the study and the resources in the laboratory prior choosing the methodology. For a thorough assessment of intestinal barrier function the techniques are preferably combined, for example in vivo and ex vivo techniques can be combined with in vitro studies for a more mechanistic approach. TER: transepithelial resistance.