Loss of Guanylyl Cyclase C (GCC) Signaling Leads to Dysfunctional Intestinal Barrier

Guanylyl Cyclase C (GCC) signaling via uroguanylin (UGN) and guanylin activation is a critical mediator of intestinal fluid homeostasis, intestinal cell proliferation/apoptosis, and tumorigenesis. As a mechanism for some of these effects, we hypothesized that GCC signaling mediates regulation of intestinal barrier function.Paracellular permeability of intestinal segments was assessed in wild type (WT) and GCC deficient (GCC-/-) mice with and without lipopolysaccharide (LPS) challenge, as well as in UGN deficient (UGN-/-) mice. IFNγ and myosin light chain kinase (MLCK) levels were determined by real time PCR. Expression of tight junction proteins (TJPs), phosphorylation of myosin II regulatory light chain (MLC), and STAT1 activation were examined in intestinal epithelial cells (IECs) and intestinal mucosa. The permeability of Caco-2 and HT-29 IEC monolayers, grown on Transwell filters was determined in the absence and presence of GCC RNA interference (RNAi). We found that intestinal permeability was increased in GCC-/- and UGN-/- mice compared to WT, accompanied by increased IFNγ levels, MLCK and STAT1 activation in IECs. LPS challenge promotes greater IFNγ and STAT1 activation in IECs of GCC-/- mice compared to WT mice. Claudin-2 and JAM-A expression were reduced in GCC deficient intestine; the level of phosphorylated MLC in IECs was significantly increased in GCC-/- and UGN-/- mice compared to WT. GCC knockdown induced MLC phosphorylation, increased permeability in IEC monolayers under basal conditions, and enhanced TNFα and IFNγ-induced monolayer hyperpermeability.GCC signaling plays a protective role in the integrity of the intestinal mucosal barrier by regulating MLCK activation and TJ disassembly. GCC signaling activation may therefore represent a novel mechanism in maintaining the small bowel barrier in response to injury

Preventing and Treating Colic

Colic is a common and distressing functional gastrointestinal disorder during infancy. It is a behavioral phenomenon in infants aged 1–4 months involving prolonged inconsolable crying and agitated status with multifactorial etiology. Colic can be considered as a benign, self-limited process because the baby normally grows and feeds even with transient irritable mood. Nevertheless, infantile colic is a common difficulty causing anxiety during parenthood and a recurrent reason for them to seek medical help, especially if it is the first child. The causes of colic can be classified as non-gastrointestinal or gastrointestinal. The former includes altered feeding techniques, modified child-parent relationship, immaturity of central nervous system, behavioral etiology, and maternal smoking or nicotine replacement therapy. Instead, the latter involves inadequate production of lactase enzyme, cow’s milk protein intolerance, alteration of intestinal microbiota, gastrointestinal immaturity, or inflammation which causes intestinal hyperperistalsis due to increase in serotonin secretion and motilin receptor expression. Probiotics may play a crucial part in the manipulation of the microbiota. Probiotic administration is likely to maintain intestinal homeostasis through the modulation of permeability and peristalsis, influencing the gut-brain axis and inhibiting hypersensitivity. This is a decisive field in the development of preventive and therapeutic strategies for infantile colic. However, further studies are needed for each specific formulation in order to better characterize pharmacodynamic and pharmacokinetic properties and to evaluate their application as a possible preventive strategy if administered early during infancy against the later development of pain-related FGIDs

Proteases and the gut barrier

Serine proteases, cysteine proteases, aspartic proteases and matrix metalloproteinases play an essential role in extracellular matrix remodeling and turnover through their proteolytic action on collagens, proteoglycans, fibronectin, elastin and laminin. Proteases can also act on chemokines, receptors and anti-microbial peptides, often potentiating their activity. The intestinal mucosa is the largest interface between the external environment and the tissues of the human body and is constantly exposed to proteolytic enzymes from many sources, including bacteria in the intestinal lumen, fibroblasts and immune cells in the lamina propria and enterocytes. Controlled proteolytic activity is crucial for the maintenance of gut immune homeostasis, for normal tissue turnover and for the integrity of the gut barrier. However, in intestinal immune-mediated disorders, pro-inflammatory cytokines induce the up-regulation of proteases, which become the end-stage effectors of mucosal damage by destroying the epithelium and basement membrane integrity and degrading the extracellular matrix of the lamina propria to produce ulcers. Protease-mediated barrier disruption in turn results in increased amounts of antigen crossing into the lamina propria, driving further immune responses and sustaining the inflammatory process