Dominant role of the MyD88-dependent signaling pathway in mediating early endotoxin-induced murine ileus.
TLR4 ligation by pathogen-associated molecular patterns, such as Gram-negative bacteria-derived LPS, triggers a nonhematopoietic cell-mediated ileus during early endotoxemia. Our objective was to investigate the quantitative contributions of the two downstream signaling pathways of TLR4, namely the adapter proteins myeloid differentiation primary response gene 88 (MyD88) and Toll-IL-1-resistance (TIR) domain-containing adaptor-inducing IFN-beta (TRIF). Six hours after intraperitoneal injection of highly purified LPS (UP-LPS, 5 mg/kg), in vivo gastrointestinal transit and intestinal muscularis gene transcripts of inflammatory mediators chemokine (C-X-C motif) ligand 10, synonymous IP-10 (CXCL10), granulomonocyte colony stimulating factor (GM-CSF, synonymous CSF-2), IL-1beta, IL-6, IL-10, and inducible NO synthase (iNOS) were assessed in mice with transgenic loss-of-function for MyD88 or TRIF. LPS-induced MyD88 and TRIF mRNA upregulation was quantified within the intestinal muscularis of TLR4-competent and TLR4-mutant mice, and MyD88 mRNA levels were additionally measured in TLR4 bone marrow chimeras. MyD88 deficiency completely protected mice from early endotoxin-induced ileus, while TRIF deficiency partially ameliorated ileus severity. LPS induction of the primary downstream signaling element MyD88 was TLR4 dependent and was derived in equal amounts from both the hematopoietic and the nonhematopoietic cells. Conversely, no induction of TRIF mRNA was detectable. Significant gene induction of all inflammatory mediators was dependent on intracellular signal transduction by MyD88, while the TRIF MyD88-independent pathway predominantly regulated the molecular levels of CXCL10. In summary, MyD88 and TRIF are nonredundant signaling pathways in early endotoxin-induced rodent ileus, but MyD88 is the essential adaptor molecule for transduction of early TLR4-induced ileus and inflammatory signaling. The dependency of ileus on individual adaptor protein pathways is also reflected in the manifestation of specific molecular inflammatory events within the intestinal muscularis externa.
Buchholz
Bettina M
BM
Dept. of Medicine/Gastroenterology; Univ. of Pittsburgh, Pennsylvania 15261, USA.
Billiar
Timothy R
TR
Bauer
Anthony J
AJ
D033942:Adaptor Proteins, Vesicular Transport / Q000378:metabolism*; D000818:Animals; D004731:Endotoxins / Q000378:metabolism*; D045823:Ileus / Q000209:etiology*; D018836:Inflammation Mediators / Q000378:metabolism; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000187:drug effects; D008070:Lipopolysaccharides / Q000494:pharmacology; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008817:Mice, Mutant Strains; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D051197:Toll-Like Receptor 4 / Q000378:metabolism*; D015533:Transcriptional Activation
D033942:Adaptor Proteins, Vesicular Transport; D004731:Endotoxins; D018836:Inflammation Mediators; D008070:Lipopolysaccharides; D053594:Myeloid Differentiation Factor 88; D051197:Toll-Like Receptor 4; C476286:Ticam1 protein, mouse; C493487:Tlr4 protein, mouse