Paracellular permeability and tight junction regulation in gut health and disease.

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Intestinal Epithelial Digestive, Transport, and Barrier Protein Expression Is Increased in Environmental Enteric Dysfunction.

Environmental enteric dysfunction (EED) is characterized by malabsorption and diarrhea that result in irreversible deficits in physical and intellectual growth. We sought to define the expression of transport and tight junction proteins by quantitative analysis of duodenal biopsies from patients with EED. Biopsies from Pakistani children with confirmed EED diagnoses were compared to those from age-matched North American healthy controls, patients with celiac disease, and patients with nonceliac disease with villous atrophy or intraepithelial lymphocytosis. Expression of brush border digestive and transport proteins and paracellular (tight junction) proteins was assessed by quantitative multiplex immunofluorescence microscopy. EED was characterized by partial villous atrophy and marked intraepithelial lymphocytosis. Epithelial proliferation and enteroendocrine, tuft, and Paneth cell numbers were unchanged, but there was significant goblet cell expansion in EED biopsies. Expression of proteins involved in nutrient and water absorption and that of the basolateral Cl- transport protein NKCC1 were also increased in EED. Finally, the barrier-forming tight junction protein claudin-4 (CLDN4) was significantly upregulated in EED, particularly within villous enterocytes. In contrast, expression of CFTR, CLDN2, CLDN15, JAM-A, occludin, ZO-1, and E-cadherin was unchanged. Upregulation of a barrier-forming tight junction protein and brush border and basolateral membrane proteins that support nutrient and water transport in EED is paradoxical, as their increased expression would be expected to be correlated with increased intestinal barrier function and enhanced absorption, respectively. These data suggest that EED activates adaptive intestinal epithelial responses to enhance nutrient absorption but that these changes are insufficient to restore health.

Tight junction channel regulation by interclaudin interference.

Tight junctions form selectively permeable seals across the paracellular space. Both barrier function and selective permeability have been attributed to members of the claudin protein family, which can be categorized as pore-forming or barrier-forming. Here, we show that claudin-4, a prototypic barrier-forming claudin, reduces paracellular permeability by a previously unrecognized mechanism. Claudin-4 knockout or overexpression has minimal effects on tight junction permeability in the absence of pore-forming claudins. However, claudin-4 selectively inhibits flux across cation channels formed by claudins 2 or 15. Claudin-4-induced loss of claudin channel function is accompanied by reduced anchoring and subsequent endocytosis of pore-forming claudins. Analyses in nonepithelial cells show that claudin-4, which is incapable of independent polymerization, disrupts polymeric strands and higher order meshworks formed by claudins 2, 7, 15, and 19. This process of interclaudin interference, in which one claudin disrupts higher order structures and channels formed by a different claudin, represents a previously unrecognized mechanism of barrier regulation.