Defect in TLR5 Expression Enhances Spontaneous Visceral Hypersensitivity

Abstract

Irritable Bowel Syndrome (IBS), such as Crohn’s disease (CD), is an intestinal disease that is associated with colonic hypersensitivity and abdominal pain associated with other symptoms (bloating and bowel dysfunction). Moreover, alterations in innate immune function genes and activities have been described in IBS and CD patients. Among them, Toll-Like Receptor 5 (TLR5) is a primordial innate immune receptor, responsible for the primary defense of the organism against microbial intruders. This receptor acts as a sentinel by detecting the presence of potentially virulent invasive bacteria expressing flagellin, the microbial TLR5 ligand. Mice deficient in TLR5 (T5KO) have been described to uniformly exhibit increased levels of inflammatory markers, resulting in a “sub-clinical” inflammation. Consequently, we hypothesized that, depending on the level of intestinal inflammation, T5KO mice will spontaneously develop a visceral hypersensitivity and thus provide a practical means to study mechanisms underlying inflammation and visceral pain. Two distinct strains of T5KO mice (knockout mouse strains on the C57BL/6 background deficient in TLR5 from Drs. Akira and Flavell) were assayed for intestinal inflammation level and visceral hypersensitivity. Inflammation was monitored using clinical anatomical indicators of colitis such as spleen and colon weight, and assessed by determination of the colonic myeloperoxidase (MPO) activity and expression of systemic pro-inflammatory cytokines. In parallel, visceral hypersensitivity was assessed using a technique based on measuring electromyographic abdominal contractions induced by colorectal distension. This test is to induce, by progressive inflation of a balloon inserted into the colon, stomach cramps whose amplitude is proportional to the colon sensitivity. Mice deficient for TLR5 uniformly exhibited increased levels of inflammatory markers and greater visceral hypersensitivity in comparison to their housekeeping WT mice. Direct involvement of TLR5 in colonic hypersensitivity has been demonstrated. Such impact on colonic sensitivity could be related to its role in the modulation of the intestinal microbiota and its role on micro-colonic inflammations. Thus, approaches focused on TLR5-dependent pathway could be used to initiate new pharmacological strategies based on the use of specific molecules modulating the activity of this protein.