SUMMARY Diarrhea is a host response to enteric pathogens, but its impact on pathogenesis remains poorly defined. By infecting mice with the attaching and effacing bacteria Citrobacter rodentium, we defined the mechanisms and contributions of diarrhea and intestinal barrier loss to host defense. Increased permeability occurred within 2 days of infection and coincided with IL-22-dependent upregulation of the epithelial tight junction protein claudin-2. Permeability increases were limited to small molecules, as expected for the paracellular water and Na+ channel formed by claudin-2. Relative to wildtype, claudin-2-deficient mice experienced severe disease, including increased mucosal colonization by C. rodentium, prolonged pathogen shedding, exaggerated cytokine responses, and greater tissue injury. Conversely, transgenic claudin-2 overexpression reduced disease severity. Chemically-induced osmotic diarrhea reduced colitis severity and C. rodentium burden in claudin-2 deficient, but not transgenic, mice, demonstrating that claudin-2-mediated protection is the result of enhanced water efflux. Thus, IL-22-induced claudin-2 upregulation drives diarrhea and pathogen clearance.

Tight junctions create a paracellular barrier that is essential for survival of complex organisms. In many cases tight junctions define separate, generally sterile, tissue compartments. In the skin and gut, tight junctions must also seal the paracellular space to prevent microbiota from accessing the internal milieu. This is a relatively simple task in the integument, where an absolute barrier is effective. However, intestinal epithelial tight junctions are charged with the far more complex task of supporting paracellular transport of water, ions, and nutrients while providing a barrier to microbial translocation. The delicate nature of this balance, which is disrupted in disease, makes the intestine a unique organ in which to explore the complexities of tight junction permeability and barrier regulation. Here we review recent progress in understanding the molecular determinants of barrier function and events responsible for regulation, and dysregulation, of tight junction permeability.

Dynamic maintenance of cell polarity is essential for development and physiology. Here we combine experiments and modeling to elucidate mechanisms that maintain cortical polarity in the C. elegans zygote. We show that polarity is dynamically stabilized by two coupled cross-inhibitory feedback loops: one involves the oligomeric scaffold PAR-3 and the kinase PAR-1; the other involves CDC-42 and its putative GAP CHIN-1. PAR-3 and CDC-42 are both required locally to recruit PAR-6/PKC-3, which inhibits PAR-1 (shown previously) and inhibits local growth/accumulation of CHIN-1 clusters. Conversely, PAR-1 inhibits local accumulation of PAR-3 oligomers, while CHIN-1 inhibits CDC-42 (shown previously), such that either PAR-1 or CHIN-1 can prevent recruitment of PAR-6/PKC-3, but loss of both causes complete loss of polarity. Ultrasensitive dependence of CHIN-1 cluster growth on PAR-6/PKC-3 endows this core circuit with bistable dynamics; while transport of CHIN-1 clusters by cortical flow can stabilize the AP boundary against diffusive spread of PAR-6/PKC-3.

Intestinal damage in malnutrition constitutes a threat to the survival of many thousands of children globally. We studied children in Lusaka, Zambia, with severe acute malnutrition (SAM) and persistent diarrhea using endoscopy, biopsy and analysis of markers and protective proteins in blood and intestinal secretions. We carried out parallel investigations in apparently healthy adults, and analyzed biomarkers only in apparently healthy children. Villus height and crypt depth did not differ in children with SAM and adult controls, but epithelial surface was reduced in children with SAM (median 445, interquartile range (IQR) 388, 562 μm per 100 μm muscularis mucosae) compared to adults (578, IQR 465,709; P = 0.004). Histological lesions and disruptions of claudin-4 and E-cadherin were most pronounced in children with SAM. Circulating lipopolysaccharide, a marker of bacterial translocation, was higher in malnourished children (251, IQR 110,460 EU/ml) than in healthy children (51, IQR 0,111; P = 0.0001). Other translocation markers showed similar patterns. Anti-Deamidated Gliadin Peptide IgG concentrations, although within the normal range, were higher in children with SAM (median 2.7 U/ml, IQR 1.5–8.6) than in adults (1.6, 1.4–2.1; P = 0.005), and were inversely correlated with villus height (ρ = − 0.79, n = 13, P = 0.001). Malnutrition enteropathy is associated with intestinal barrier failure and immune dysregulation.