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Toll-Like Receptor Signaling Pathways

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Science  06 Jun 2003:
Vol. 300, Issue 5625, pp. 1524-1525
DOI: 10.1126/science.1085536


Members of the Toll-like receptor (TLR) family recognize conserved microbial structures, such as bacterial lipopolysaccharide and viral double-stranded RNA, and activate signaling pathways that result in immune responses against microbial infections. All TLRs activate MyD88-dependent pathways to induce a core set of stereotyped responses, such as inflammation. However, individual TLRs can also induce immune responses that are tailored to a given microbial infection. Thus, these receptors are involved in both innate and adaptive immune responses. The mechanisms and components of these varied responses are only partly understood. Given the importance of TLRs in host defense, dissection of the pathways they activate has become an important emerging research focus. TLRs and their pathways are numerous; Science's Signal Transduction Knowledge Environment's TLR Connections Map provides an immediate, clear overview of the known components and relations of this complex system.

The TLR family is a family of receptors involved in microbial recognition by the immune system (1). TLRs recognize pathogen-associated molecular patterns, which represent conserved molecular features of a given microbial class. For example, the lipopolysaccharide of Gram-negative bacteria is a TLR4 ligand, whereas double-stranded RNA (produced during viral infections) is a TLR3 ligand. One of the central features of this system of microbial recognition is that TLRs activate signaling pathways that are critical for induction of the immune response to the given microbial challenge. TLRs link microbial recognition to the activation of antigen-presenting cells, the specialized cells involved in T lymphocyte activation and initiation of adaptive immunity. One of the major challenges receiving a great deal of attention in the field is the dissection of the signaling pathways induced by TLRs. Although some of these pathways are shared by all TLRs, it is becoming evident that there are considerable differences in the signaling pathways, and consequently the gene expression patterns, stimulated by individual TLR family members.

All TLRs activate a common signaling pathway that culminates in the activation of nuclear factor–κB (NF-κB) transcription factors, as well as the mitogen-activated protein kinases (MAPKs) extracellular signal–regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK) [see the TLR pathway (2) and Fig. 1]. The proximal events of this signaling pathway depend, in part, on a series of homophilic interactions between modular signaling domains. One such domain is the Toll/interleukin (IL)–1 receptor (TIR) domain, which is present in the cytosolic region of all TLRs, as well as the members of the IL-1 receptor family. The TIR domain governs the heterodimerization and homodimerization between TLRs and the association between TLRs and TIR domain–containing adaptors. The use of protein modules is reminiscent of other signaling pathways, such as apoptotic and mitogenic signaling pathways. Modular design of signaling pathways is common because it promotes evolvability of the pathways. However, the TIR domain is found almost exclusively in the host defense pathways in both the animal and plant kingdoms. How specificity is achieved within TIR-domain interactions remains unknown and is an area of considerable interest. All TLRs use a common adaptor protein, MyD88, which contains both a TIR domain and a death domain. When associated with a TLR, MyD88recruitsmembersoftheIL-1receptor–associated kinase (IRAK) family through death domain–death domain homophilic interactions. IRAK1 and IRAK4 are serine-threonine kinases involved in the phosphorylation and activation of tumor necrosis factor (TNF) receptor–associated factor 6 (TRAF6). In contrast, IRAK2 and IRAK-M lack kinase activity and play different roles. IRAK-M negatively regulates TLR signaling by preventing dissociation of phosphorylated IRAK1 and IRAK4 from MyD88, a necessary step for signal transduction. The function of IRAK-2 remains unknown. After phosyphorylation by IRAKs, TRAF6 forms a complex with Ubc13 and Uev1A. Collectively, these proteins form an ubiquitin-conjugating enzyme (E2) for which TRAF6 serves as the ubiquitin ligase (E3). The direct target (or targets) of the TRAF6 ubiquitination complex remain unidentified, and it is not known whether TRAF6 mediates ubiquitination of proteins other than TRAF6 itself. Biochemical evidence indicates that TRAF6 activates a MAPK kinase kinase (MAPKKK) called transforming growth factor β–activated kinase (TAK-1). Activated TAK-1, in turn, can phosphorylate MKK3 and MKK6, the kinases upstream of p38 MAPKs and JNK. In addition, TAK-1 can activate the IκBα kinase complex (IKK), which consists of the kinases IKKα and IKKβ and the scaffolding protein IKKγ. The activation of the IKK complex by TAK-1 is most likely indirect, and the identity of the kinase that is responsible for direct phosphorylation of the IKK complex remains unknown. The phosphorylation of IκBα leads to its degradation, the release of NF-κB, and the activation of NF-κB–dependent genes, such as TNF-α, IL-1, and IL-6. Genetic evidence demonstrating TAK-1 involvement in TLR signaling is still missing. In addition to JNK and p38 MAPKs, TLRs activate ERK1 and ERK2 MAPKs. The mechanism of ERK activation relies on another member of MAPKKK family known as Tpl2 (3).

Fig. 1.

TLR signal transduction pathways. Individual TLR family members induce different signaling pathways and can be grouped based on their use of the known TLR adaptors. All TLRs signal through the MyD88-dependent pathway (shaded in blue). TLR3, and most likely TLR4, can induce IFNα/β expression through the TRIF (also known as TICAM-1) pathway (shaded in red). TIRAP (also known as Mal) functions downstream of TLR2 and TLR4 (shaded in yellow). Where appropriate, the modular organization of individual signaling components is represented schematically. The role played by TAK1 in IKK activation remains unclear, so a generic MAP3K is shown downstream of TRAF6. TRAF6 is a RING domain–containing ubiquitin ligase and is ubiquitinated upon activation. For additional details, refer to the Science's Signal Transduction Knowledge Environment Connections Map of the Toll-like receptor pathway (2).

The MyD88-dependent signaling pathway described above is shared by all members of the TLR family and results in the induction of a core set of responses. However, in addition to these stereotyped inflammatory responses, TLRs can also induce appropriate effector responses to distinct types of microbial infections that they recognize. Indeed, one of the most exciting aspects of TLR signal transduction has been the demonstration that differences exist in the gene expression induced by individual TLRs. In particular, analysis of cells from mice lacking MyD88 has demonstrated that TLR3 and TLR4 are capable of inducing certain signaling pathways independent of this adaptor. Two additional TIR-containing adaptors have been identified. TIR domain–containing adaptor protein (TIRAP, also known as Mal) functions downstream of TLR2 and TLR4 but is not involved in signaling by other TLRs (4, 5). TIR domain–containing adaptor-inducing IFN-β (TRIF, also known as TICAM-1) appears to function downstream of TLR3, and possibly TLR4 (6, 7). TRIF appears to be responsible for the induction of interferon (IFN)-α and IFN-β (IFNα/β) genes by these TLRs. The induction of IFNα/β expression by TLR3 and TLR4 occurs through a MyD88-independent pathway that leads to the activation of interferon regulatory factor 3 (IRF3)—a key transcription factor responsible for the induction of IFN genes. Recently, two noncanonical IKKs—IKKϵ and TBK-1—have been shown to function downstream of TRIF and upstream of IRF3 (8, 9). These kinases are likely to be responsible for the MyD88-independent induction of NF-κB by TLR3 and TLR4 as well (Fig. 1).

The known TLR signaling components still do not explain all of the known differences in signaling between individual TLRs, indicating that additional gene products and signaling mechanisms have yet to be discovered.

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