toll-toll-样受体样受体- -讲座讲座Toll-Like Receptor SignalingToll receptor initially discovered in Drosophila as important receptor in dorso-ventral embryonic patternToll mutants refers to the fact that these mutants could not establish a proper dorsal-ventral axisToll in German means great, apparently this was one of the words describing the scientists enthusiasm after observing the mutant fliesHoffman and colleagues showed that Toll-mutant flies susceptible to fungal infectionsMammalian homologues discovered and designated as Toll-Like Receptors (TLR)TLRs recognize specific patterns in pathogens which are not observed in mammalsToll-Like ReceptorsToll-Like ReceptorsLarge molecules found in outer membrane of Gram- negative bacteriaComprised of a lipid and saccharide componentHighly immunogenicRecognized by TLR4Can cause septic shock and lead to deathOften referred to as Endotoxin since it is not secreted but is a byproduct of bacterial lysisGreat variability among different bacterial strainsLipoPolySaccharides (LPS)TLR3, TLR7 and TLR8 detect viral nucleic acidsFound in intracellular membranes since viral nucleic acids are endogenously generatedNote the TIR domains of these TLRs face the cytosol of the cellPoly I:C is an agonist for TLR3 that mimics binding of dsRNA to TLR3Viral Nucleic Acids Cytoplasmic tails of TLRs show similarities to IL-1 receptor (TIR) Common adaptor to TLRs is MyD88 Crucial proline residue in all TLR TIR domains, except TLR3 If substituted with histidine, no signaling occurs All TLRs likely have a MyD88 pathway (TLR3 is an exception) TLR4 has a MyD88 independent pathway as wellCytoplasmicTIR domainTLRs and TIR Domain MyD88 knockout mice have no response to LPS MyD88 is essential to all inflammatory signaling pathways MyD88s, a splice variant of MyD88 down regulates the inflammatory response MyD88 interacts with TIR domain of TLR and recruits IRAK-4, IRAK-1 and TRAF-6MyD88 Adaptor4 IRAKs known, IRAK1, IRAK-2, IRAK-M and IRAK-4IRAK are serine/threonine kinasesIRAK-4 phosphorylates IRAK-1IRAK-M plays an inhibitory role in TLR signaling IRAK-4 phosphorylates IRAK-1TRAF6 is a member of the TNF receptor associated factor (TRAF) familyTRAF6 interacts with IRAK-1 and gets activatedRelease of TRAF6/IRAK-1 ensuesIRAK and TRAF6TRAF6/IRAK-1 complex associates with 3 proteinsTAK1 (TGF-B activated kinase)TAB1 (TAK1 binding proteins)TAB2 (TAK1 binding proteins)Large complex associates with membraneEventually IRAK-1 stays in membrane while TRAF6/TAK1/TAB1/TAB2 move to cytosolE2 Ligases such as Ubc13 and Uev1A join further enlarging complexIRAK and TRAF6 ReleaseTAK-1 ActivationThe enlarged complex that includesTRAF6, TAK1, TAB1, TAB2, Ubc13, Uev1A activate TAK1Activated TAK1 phosphorylates IKK complex Activated TAK1 can also phosphorylate MAP Kinases IKK complex consists of IKK, and /NEMOIB phosphorylation results in NF- B translocation to nucleusMyD88 Independent Pathway MyD88 Knock out mice do not produce inflammatory cytokines such as TNF- However with TLR4 stimulation NF-B and JNK delayed activity occurs This strongly suggests the existence of 2 pathways in TLR signaling a MyD88 dependent pathway (early) a MyD88 independent pathway (late) TLR3 stimulation also exchibits a MyD88 independent pathwayTLR4Figures obtained from:Takeda and Akira, 2004WikipediaKuby, 6th editionAcknowledgements结束结束