from Nature, 430 (2004)

Introduction

Although different TLRs are activated by distinct agonists, their cytoplasmic domains share the ability to transduct PAMP-generated signals to the IkB kinase (IKK) and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK), and p38. These effectos elicity target gene induction by modulating subcellular distribution, promoter recruitment, and transcriptional activity of specific transcription factors.

In the signalling that occurs in an innate immune responde, two central events are the activaiton of  and interferon regulatory factor 3 (IRF3). NFkB pathway controls the production of pro-inflammatory molecules such as IL-1 and TNF whereas the IRF3 pathway leads to the production of type I itnerferons (IFNalpha and beta).

TLRs possess the Toll-interluekin 1 (IL-1) receptor (TIR) domain in their cytoplasmic region. MyD88, a TIR domain-containing protein, associates with TLRs and acts as an adapter that recruits IL-1 receptor-associated kinase (IRAK) and tumor necrosis factor (TNF) receptor-assocaited factor 6 (TRAF6) to the TLRs. T

Broadly, TLRs can activate two branches of downstream signaling pathways; MyD88-dependent and MyD88-independent pathways: MyD88 is a common downstream adaptor molecule for all mammalian TLRs.The signaling pathway via MyD88 leads to activation of NF-kB transcription factors and c-jun NH2 terminal kinase (Jnk) mitogen-activated protein kinases (MAPKs). MyD88 is an immedate downstream adaptor molecule recruited by activated TLRs through their TIR domain. MyD88 in turn recruits IRAK-4 and induces IRAK-4 induced phosphorylation and degradation of IRAK-1. IRAK-1 associates with TRAF6 leading to the activation of MAP kinases and IKK complex resulting in the activation of AP-1 and NF-B transcription factor, respectively.

TLR3 and TLR4 activate MyD88 independent signaling pathway mediated throuigh TIR domain containing adaptor inducing IFNbeta (TRIF) leading to the expressionf type I interferon and IFN-inducible genes. The activaiton of TRIF pathway also leads to the delayed activaiton of NF-kB. TRIF interacts with TBK1 and RIP1.

The Adaptor Proteins Associated with TLR signalling

Four adaptor proteins MyD88 (myeloid differentiation factor 88), MAL/TIRAP (MyD88-adaptor-like/TIR-associated protein), TRIF (Toll-receptor-associated activator of interferon) and TRAM (Toll-receptor-associated molecule) transduce signals from all of the TIR (Toll/interleukin-1 receptor (Il-1R) homolgous region) domains, activating protein kinases and then transcription factors that cause inflammatory effects. The function of a fifth adaptor, SARM (sterile alpha and HEAT/Armadillo motif protein), has yet to be defined.

• MyD88: All the TLRs, with the exception of TLR3, hand off their signal to an adapter protein called MyD88. TLR4 also requires to other proteins – Tram and Trif- to relay the signal, whereas TLR3 relies on Trif alone. Signalling through TLRs occurs through the sequential recruitment of adaptor molecule MyD88 and the serine/threeonine kinase IRAK, and subseqeuntly activates mitogen-activated protein (MAP) kinases and the nuclear factor NK-[kappa]B. These signalling cascades increase the expression of many pro-inflammatory genes. 

Microbial inducers like LPS, lipopeptides, lipoteichoic acid, flagellin and mymethylated DNA provoke the bulk of host innate immune responses and are sensed by as few as six of the TLRs. Moreover, the signals that emanate from these 6 TLRs can be abolished at least in large part and perhaps completely by disrupting only 2 of 5 known TIR motif adaptor proteins (MyD88 and TRIF) that transduce siganls from TLRs. So the innate immune siganl traverses a narrow strait. Thereafter, it grows in complexity. 

Stimulation of TLRs triggers the association of MyD88 which in turn recruits IRAK4, thereby allowing the association of IRAK1. IRAK4 then induces the phosphorylation of IRAK1. TRAF6 is also recruited to the receptor complex, by associating with phosphorylated IRAK1. Phosphorylated IRAK1 and TRAF6 then dissociate from the receptor and form a complex with TAK1, TAB1 and TAB2 at the plasma membrane which induces the phosphoryaltion of TAB2 and TAK1. IRAK1 is degraded at the plasma membrane and the remaining complex (consisting of TRAF6, TAK1, TAB1 and TAB2) translocates to the cytosol, where it associates with the ubiquitin ligases UBC13 and UEV1A. Theis leads to the ubiquitylation of TRAF6, which indues the activation of TAK1. TAK1, in turn, phosorylates both mitogen-activated protein (AMP) kinases and the IKK complex, which consists of IKKalpha, IKKbeta and IKKgamma. The IKK complex then poshorylates Ikappabetta which leads to its ubiquitylation and subsequent degredation. This allows NFkB to translocate to the nucleus and induce expression of its target genes.

TLR-2 Signalling:

TLR2 signaling has been shown to completely depend on MyD88, unlike with TLR4.

TLR-4 Signalling:

When any one of the several described TLR ligands binds to its TLR, one or several adapter molecules are recruited to propagate the signal via interaction of Toll-IL-1 resistance (TIR) domains present on the TLR and its adapters. The adapter protein myeloid differentiation protein-88 (MyD88) seems to be involved in signaling from most TLRs, with the exception of TLR3, and TLR4 has been shown to utilize all four described TIR-containing adapters; MyD88 and MAL (MyD88-adapter-like, also known as TIRAP) seem to act in pair to active the MyD88 dependent pathway resulting in Nf-kB activation, whereas TIR-domain-containing adapter protein inducing interferon-beta (TRIF; also known as TICAM1) and TRIF-related adapter molecule (TRAM) in pair activate the interferon pathway.

LPS Signalling through TLR4: LPS-induced TLR4 activates two downstream pathways; the MyD88-dependent pathway that leads to the production of proinflammatory cytokines with quick activation of NF-kB and MAPK and the MyD88-independent pathway associated with activation of IRF-3, subsequent induction of IFN-beta, and maturation of DCs, with delayed activation of NF-kB and MAPK. Thus LPS induces the activation of NF-kB through both MyD88- and TRIF-dependent pathways.

–MyD88 dependent: After activation with LPS, an intracellular adaptor molecule, MyD88 is recruited to the cytoplasmic tail of TLR4, providing a platform for the IL-1 receptor-associated kinase (IRAK). IRAK subsequently undergores phosphorylation and interacts with TNF receptor-associated factor 6 (TRAF6), a downstream transducer required for NF-kB activation.

–MyD88 independent: In addition to the commonly used MyD88-dependent signalling pathways, LPS can activate NF-kB and mitogen-activated protein kinases through a Toll-Il-1 receptor (TIR) domain-containing adapter protein (TIRAP).