Toll-like Receptor Signaling

Abstract

The innate immune response in vertebrates is the first line of defense against invading microorganisms. The main players in innate immunity are phagocytes such as neutrophils, macrophages, and dendritic cells. These cells can discriminate between pathogens and self by utilizing signals from the Toll-like receptors (TLRs) 1The abbreviations used are: TLR, Toll-like receptor; IL, interleukin; IL-1R, interleukin-1 receptor; TIR, Toll/IL-1R; LRR, leucine-rich repeat; IRAK, IL-1RI-associated protein kinase; TAK, transforming growth factor (TGF)-β-activated kinase; TAB1, TAK1-binding protein 1; TNF, tumor necrosis factor; TRAF, TNF receptor-associated factor; IKK, IκB kinase kinase; MAP, mitogen-activated protein; DD, death domain; LPS, lipopolysaccharide; siRNA, small interfering RNA; PI, phosphatidylinositol; IFN, interferon; DN, dominant-negative form.1The abbreviations used are: TLR, Toll-like receptor; IL, interleukin; IL-1R, interleukin-1 receptor; TIR, Toll/IL-1R; LRR, leucine-rich repeat; IRAK, IL-1RI-associated protein kinase; TAK, transforming growth factor (TGF)-β-activated kinase; TAB1, TAK1-binding protein 1; TNF, tumor necrosis factor; TRAF, TNF receptor-associated factor; IKK, IκB kinase kinase; MAP, mitogen-activated protein; DD, death domain; LPS, lipopolysaccharide; siRNA, small interfering RNA; PI, phosphatidylinositol; IFN, interferon; DN, dominant-negative form. (1Akira S. Takeda K. Kaisho T. Nat. Immunol. 2001; 2: 675-680Crossref PubMed Scopus (3866) Google Scholar, 2Aderem A. Ulevitch R.J. Nature. 2000; 406: 782-787Crossref PubMed Scopus (2589) Google Scholar, 3Janeway Jr., C.A. Medzhitov R. Annu. Rev. Immunol. 2002; 20: 197-216Crossref PubMed Scopus (5994) Google Scholar, 4Akira S. Curr. Opin. Immunol. 2003; 15: 5-11Crossref PubMed Scopus (469) Google Scholar). TLRs recognize conserved motifs predominantly found in microorganisms but not in vertebrates. Stimulation of TLRs causes an immediate defensive response, including the production of an array of antimicrobial peptides and cytokines. Accumulating evidence has shown that individual TLRs can activate overlapping as well as distinct signaling pathways, ultimately giving rise to distinct biological effects. Here, I will review our current understanding of the TLR signaling pathways. Toll was initially identified in insects as a receptor essential for dorsoventral polarity during embryogenesis. Subsequent studies revealed that it also plays an essential role in insects in the innate immune response against fungal infection (5Lemaitre B. Nicolas E. Michaut L. Reichhart J.M. Hoffmann J.A. Cell. 1996; 86: 973-983Abstract Full Text Full Text PDF PubMed Scopus (2930) Google Scholar). Mammalian homologues of Toll were subsequently identified through expressed sequence tag and genomic sequence data base searches. To date, 10 members of the TLR family have been identified in mammals. TLRs are members of a larger superfamily of interleukin-1 receptors (IL-1Rs) that share significant homology in their cytoplasmic regions. In particular, TLRs and members of the IL-1R family share a conserved stretch of ∼200 amino acids in their cytoplasmic region known as the Toll/IL-1R (TIR) domain. The region of homology in the TIR motif is confined to three conserved boxes that contain amino acids crucial for signaling. In contrast, the extracellular regions are quite diverse. The extracellular portion of the TLRs contains a leucine-rich repeat (LRR) motif whereas that of the IL-1Rs contains three immunoglobulin domains. The LRR domains consist of varying numbers of repeats, each 24–29 amino acids in length, containing the motif XXLXLXX and other conserved leucines. It is thought that these LRR domains are directly involved in the recognition of a variety of pathogens. The major ligands recognized by individual TLRs are summarized in Fig. 1. The IL-1R and TLR family signal via shared downstream signaling molecules (6O'Neill L.A. Curr. Top. Microbiol. Immunol. 2002; 270: 47-61Crossref PubMed Scopus (248) Google Scholar). They include the adaptor molecule MyD88, IL-1RI-associated protein kinases (IRAKs), the transforming growth factor (TGF)-β-activated kinase (TAK1), TAK1-binding protein 1 (TAB1) and 2 (TAB2), and the tumor necrosis factor receptor-associated factor 6 (TRAF6). The generally accepted scenario of the IL-1/TLR signaling pathway is shown in Fig. 2. Triggering of the IL-1R or TLR causes the adaptor protein MyD88 to be recruited to the receptor complex, which in turn promotes association with the IL-1R-associated kinases IRAK4 and IRAK1. During the formation of this complex, IRAK4 is activated, leading to the hyperphosphorylation of IRAK-1, which then induces the interaction of TRAF6 with the complex. The association of IRAK-4·IRAK-1·TRAF6 causes some conformational change in one or more of these factors, leading to their disengagement from the receptor complex. The IRAK-4·IRAK-1·TRAF6 complex then interacts at the membrane with another preformed complex consisting of TAK1, TAB1, and TAB2. This interaction induces phosphorylation of TAB2 and TAK1, which then translocate together with TRAF6 and TAB1 to the cytosol. TAK1 is subsequently activated in the cytoplasm, leading to the activation of IKK. Inactive IKK sequesters NF-κB in the cytoplasm, but activation leads to phosphorylation and degradation of IκB and consequent release of NF-κB. Activation of TAK1 also results in the activation of MAP kinases and c-Jun NH2-terminal kinase (JNK). I will discuss these molecules and their interactions in more detail below. MyD88 —MyD88 was originally isolated as a myeloid differentiation primary response gene that is rapidly induced upon IL-6-stimulated differentiation of M1 myeloleukemic cells into macrophages (7Lord K.A. Hoffman-Liebermann B. Liebermann D.A. Oncogene. 1990; 5: 1095-1097PubMed Google Scholar). MyD88 consists of an N-terminal death domain (DD) separated from its C-terminal TIR domain by a short linker sequence. MyD88 was subsequently cloned as an adapter molecule that functions to recruit IRAK to the IL-1 receptor complex following IL-1 stimulation (8Wesche H. Henzel W.J. Shillinglaw W. Li S. Cao Z. Immunity. 1997; 7: 837-847Abstract Full Text Full Text PDF PubMed Scopus (907) Google Scholar, 9Muzio M. Ni J. Feng P. Dixit V.M. Science. 1997; 278: 1612-1615Crossref PubMed Scopus (967) Google Scholar). The association between MyD88 and IRAK is mediated through a DD-DD interaction. MyD88 also forms homodimers through DD-DD and TIR-TIR domain interactions and exists as a dimer when recruited to the receptor complex. When the C-terminal TIR domain of MyD88 is expressed by itself, it acts as a dominant-negative inhibitor of TLR4 and IL-1R signaling by preventing IRAK association with the receptors. Thus, MyD88 functions as an adapter linking IL-1R/TLRs with downstream signaling molecules harboring DD. IRAK Family—Four different IRAKs (IRAK-1, IRAK-2, IRAK-M, and IRAK-4) have been identified in mammals (9Muzio M. Ni J. Feng P. Dixit V.M. Science. 1997; 278: 1612-1615Crossref PubMed Scopus (967) Google Scholar, 10Janssens S. Beyaert R. Mol. Cell. 2003; 11: 293-302Abstract Full Text Full Text PDF PubMed Scopus (466) Google Scholar, 11Cao Z. Henzel W.J. Gao X. Science. 1996; 271: 1128-1131Crossref PubMed Scopus (763) Google Scholar, 12Wesche H. Gao X. Li X. Kirschning C.J. Stark G.R. Cao Z. J. Biol. Chem. 1999; 274: 19403-19410Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar, 13Li S. Strelow A. Fontana E.J. Wesche H. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 5567-5572Crossref PubMed Scopus (526) Google Scholar, 14Suzuki N. Suzuki S. Duncan G.S. Millar D.G. Wada T. Mirtsos C. Takada H. Wakeham A. Itie A. Li S. Penninger J.M. Wesche H. Ohashi P.S. Mak T.W. Yeh W.C. Nature. 2002; 416: 750-756Crossref PubMed Scopus (647) Google Scholar). The expression patterns of these members differ; IRAK-1 and IRAK-4 are expressed in all tissues, IRAK-2 has a narrower cellular distribution, and IRAK-M expression is mainly restricted to cells of a myeloid origin. All IRAKs contain an N-terminal DD and a central Ser/Thr kinase domain (KD). Although the kinase activity of IRAK-1 increases strongly following IL-1 stimulation, IRAK-1 kinase activity is not required for its signaling function, because overexpression of a kinase-defective mutant of IRAK-1 is observed to strongly induce NF-κB activation in cells otherwise deficient for IRAK-1. Upon stimulation, IRAK-1 is recruited to the receptor through a homophilic interaction with the DD of MyD88. MyD88 also binds to IRAK-4 and thereby facilitates IRAK-4 phosphorylation of critical residue(s) in the kinase activation loop of IRAK-1, triggering IRAK-1's own kinase activity. Once activated, IRAK-1 likely autophosphorylates residues in its N terminus. TRAF6 is also recruited to the receptor complex via interaction with IRAK-1. Three TRAF6 binding motifs (Pro-X-Glu-X-X-aromatic/acidic residue) are found in IRAK-1, as well as one in IRAK-M and two in IRAK-2 (15Ye H. Arron J.R. Lamothe B. Cirilli M. Kobayashi T. Shevde N.K. Segal D. Dzivenu O.K. Vologodskaia M. Yim M. Du K. Singh S. Pike J.W. Darnay B.G. Choi Y. Wu H. Nature. 2002; 418: 443-447Crossref PubMed Scopus (519) Google Scholar). However, in contrast to IRAK-1 and IRAK-4, IRAK-2 and IRAK-M do not possess any detectable kinase activity. This is presumably because they have an asparagine and serine residue, respectively, in their kinase domains in place of an aspartate residue shown to be critical for the kinase activity of other IRAKs. IRAK-1-deficient mice and cell lines showed diminished cytokine production in response to IL-1 and LPS; nevertheless some response remained, suggesting that IRAK-2 or IRAK-M might compensate to some extent for the lack of IRAK-1 (16Kanakaraj P. Schafer P.H. Cavender D.E. Wu Y. Ngo K. Grealish P.F. Wadsworth S.A. Peterson P.A. Siekierka J.J. Harris C.A. Fung-Leung W.P. J. Exp. Med. 1998; 187: 2073-2079Crossref PubMed Scopus (176) Goo

Keywords

Affiliated Institutions

• The University of Osaka JP

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