Abstract
Bacterium-induced diarrhea results in 2 to 2.5 million deaths in the world each year. The mechanism needs to be further understood. Staphylococcus aureus infection has a close relation with diarrhea; its cell wall component peptidoglycan (PGN) has strong biological activity on immune cells and possibly plays a role in S. aureus-induced diarrhea. The present study showed that oral PGN-induced diarrhea in mice in a dose-dependent manner. Intestinal epithelial cells absorbed PGN via the intracellular pathway. Intestinal mast cells were activated after PGN gavage. Toll-like receptor (TLR)2 expression was detected in mast cells in the intestine as well as in the murine mast cell line p815 cells. Blocking TLR2 or nucleotide-binding oligomerization domain (NOD)1 with related antibodies or RNA interference abolished PGN-induced p815 cell activation. The mast cell mediator histamine and serotonin had synergistic effects in PGN-induced diarrhea. In summary, oral PGN can induce diarrhea in mice, and TLR2 and NOD1 mediate the PGN-induced mast cell activation that plays a critical role in diarrhea induction. Blockade of TLR2 or NOD1 or treating mice with a mast cell stabilizer can efficiently inhibit PGN-induced-diarrhea, providing potential therapeutic significance.
Diarrhea is a condition characterized by frequent watery or loose stools. Viral infection- and bacterial toxin-induced diarrhea are the most frequent cases, which result in 2 to 2.5 million deaths annually worldwide,1 mainly due to severe dehydration and electrolyte imbalance.2 Chronic diarrhea occurs in chronic intestinal diseases such as chronic infection, Crohn’s disease, and ulcerative colitis,3,4 which causes malnutrition and chronic intestinal tissue damage, etc. The mechanisms of diarrhea have been well discussed, such as in osmotic diarrhea or secretory diarrhea. Bacterial toxin-induced diarrhea is categorized as secretory diarrhea.5,6 In fact, intestinal epithelial cells are thought to be tolerant to commensal bacterial toxins and components7,8; however, how these microbial products break down the established “tolerance” and induce diarrhea and other inflammatory reactions remains unclear.
Mast cells are strategically localized at the host-environment interfaces such as the skin, the airway, and the intestine.9 The proinflammatory cytokine, tumor necrosis factor-α, which is presynthesized and stored in mast cells, is capable of killing invaded microbes to prevent infection.10 Mast cells thus are regarded as a critical component of host defense against bacterial infections. However, besides tumor necrosis factor-α, mast cells also release histamine, serotonin, and an array of other mediators in response to bacterial stimulation.11 Histamine and serotonin-induced diarrhea has been well documented.12,13 Bacterial-derived histamine is one of the factors causing diarrhea following ingestion of infected fish,14 with histamine release from food hypersensitivity reactions being well described.15 However, the mechanism of diarrhea caused by microbial product-induced mast cell mediator release remains largely unknown.
Staphylococcus aureus (S. aureus) is one of the pathogens associated with infectious diarrhea.16 Peptidoglycan (PGN) is a cell wall component of almost all gram-positive (including S. aureus) and gram-negative bacteria that has strong immune activity. It is capable of modifying the functions of some immune cells17 and may play a critical role in bacterial diarrhea. Mast cells express Toll-like receptors (TLRs) including TLR2 and can respond to the stimulation of PGN to release chemical mediators leading to inflammatory reactions in tissue.18,19
The hypothesis underlying the present study is that microbial products activate mast cells in the intestine to release mediators such as histamine and serotonin to induce diarrhea. The objectives of this study involved testing i) whether oral PGN induces diarrhea in mice, ii) whether TLR2 and nucleotide-binding oligomerization domain (NOD)1 mediate PGN-induced mast cell activation in the intestine, and iii) whether mast cell activation is required in PGN-induced diarrhea. The results show that PGN induces mouse diarrhea via mast cell activation that can be inhibited by mast cell stabilizers. PGN-induced mast cell activation can be inhibited by anti-TLR2 antibody, TLR2 RNA interference, or NOD1 RNA interference.
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