Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Toll-Like Receptor Adaptor Protein Family Members

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_608

Synonyms

Historical Background

Toll-like receptors (TLRs) play a critical role in innate immunity by providing a frontline defense mechanism against invading pathogens such as bacteria, fungi and viruses. They accomplish this by recognising evolutionarily conserved pathogen-associated molecular patterns (PAMPs) which are unique to pathogens are allow the host immune system to distinguish non-self from self. For example, TLR2 and TLR4 recognise bacterial cell wall components such as lipopeptides and lipopolysaccharide (LPS) respectively, whereas TLR3 and TLR9 recognise signature viral and bacterial nucleotide sequences (Miggin and O’Neill 2006). TLRs are also activated by sterile inflammatory mediators known as danger-associated molecular patterns (DAMPs), for example hyaluronan - an extracellular matrix fragment (Chen and Nunez 2010).

The existence of the TLRs, their adaptor proteins, as well as their functionality in the context of innate immunity commenced in 1998 and was proceeded by rapid advancements in the field thereafter. This concurred with the revival of the idea that the innate immune system and its role in inflammation were the central pivot upon which the early immune response is dictated. Each member of the TLR family senses different PAMPs and DAMPs which leads to the activation of TLR signaling, the downstream dissemination of which involves recruitment of appropriate TLR adaptor proteins to the TLR complex. The TLR adaptor proteins then couple to downstream protein kinases that ultimately lead to the activation of transcription factors such as nuclear factor-κB ( NF-κB Family) and members of the interferon (IFN)-regulatory factor (IRF) family.

A critical domain common to all five of the TLR adaptors as well as to the TLRs themselves is the Toll/interleukin-1 (IL-1) receptor (TIR) domain. This is located on the intracellular portion of all TLRs and allows binding to the reciprocal TIR domain on the adaptor's exposed surface. The IL-1 receptor also contains a TIR domain hence the existence of the TLR/IL-1R superfamily (Miggin and O’Neill 2006). Despite the TLRs having somewhat similar signal transduction pathways, there is specificity with regard to their adaptor usage (Akira and Takeda 2004) (Fig. 1). There are four activating TLR adaptor proteins myeloid differentiation factor 88 (MyD88), MyD88 adaptor-like (Mal); also known as Toll-IL-1 adaptor protein (TIRAP), TIR-domain-containing adaptor inducing IFN-β (TRIF; also known as TICAM-1) and TRIF-related adaptor molecule (TRAM; also known as TICAM-2 and TIRP). Despite the TLRs having somewhat similar signal transduction pathways, there is specificity with regard to their adaptor usage (Akira and Takeda 2004). MyD88 is the common downstream adaptor that is recruited by all TLRs, except TLR3 (Kawai and Akira 2006). Mal is required for TLR4, and to a lesser extent, TLR2 signaling (Yamamoto et al. 2002; Kenny et al. 2009). TRIF mediates TLR3 and TLR4 signaling (Fitzgerald et al. 2003). Finally, TRAM mediates TLR4 signaling exclusively (Fitzgerald et al. 2003) acting as a bridging adaptor to recruit TRIF to the TLR4 complex. In addition, an inhibitory TLR adaptor protein called sterile alpha and TIR motif-containing protein (SARM) has also been identified (Carty et al. 2006) which negatively regulates TRIF mediated signaling. TLR4 is the only TLR whose activation utilises all five TLR adaptor proteins and as a result, its signaling is split into two broad categories according to their dependency on MyD88. The ‘MyD88-dependent’ pathway utilises Mal and MyD88 and is responsible for early stage inflammation. It is used by TLR2/4/7/8 and TLR9. The ‘MyD88 independent’ pathway uses TRAM and TRIF to activate anti-viral and late-stage inflammatory responses and is used by TLR3 and TLR4. Thus, the function of the adaptor proteins is to provide specificity to TLR signaling in order to tailor the resulting cytokine profile to best defend against the infectious agent.
Toll-Like Receptor Adaptor Protein Family Members, Fig. 1

Role of TLR adaptors in TLR signaling pathways. MyD88 is the central adaptor in TLR signaling, capable of transducing signals mediated by all TLRs except TLR3. MyD88 does however negatively regulate TLR3-induced IRF3 activation. Regarding TLR1/2, TLR2/6, TLR4, and TLR5 signaling, MyD88 interacts with the TLR though their TIR domains (red). IRAK4 is then recruited to the complex which in turn recruits IRAK1 and/or IRAK2 via death domain (DD) interactions. TRAF6 is then recruited to activate TAK1, leading to subsequent activation of MAPK and  NF-κB. In the case of TLR7/8/9 signaling, the MyD88 complex can also recruit TRAF3 which activates IRF7. TRIF is the sole adaptor involved in TLR3 signaling and recruits NAP1 and TRAF3 to activate the noncanonical IKKs, TBK1, and IKKε. These in turn phosphorylate IRF3 and IRF7 causing their translocation into the nucleus where they bind to the type I IFN (IFN-α and IFN-β) gene promoter. TRIF is also required to mediate maximal TLR5-induced  NF-κB and MAPK activation by binding directly to TLR5. As part of a suspected negative feedback loop, TRIF can also mediate caspase-dependent TLR5 degradation. Mal acts as a bridge to allow TLR4 and to a lesser extent TLR2, to signal via the MyD88 pathway. Mal also negatively regulates TLR3-mediated IRF7 activation. TRAM, like Mal, is another bridging adaptor and links TLR4 to the TRIF pathway. The Mal:MyD88 and TRAM:TRIF complexes facilitate bifurcation of the TLR4 pathway to allow for the synthesis of both inflammatory cytokines and type I IFN, respectively. The fifth adaptor SARM interacts with TRIF and impairs TRIF signaling

To summarise, TLR engagement in response to a PAMP or DAMP instigates the recruitment of the relevant TLR adaptor protein(s) which provides a docking platform for downstream effector signaling molecules. This culminates in the production of pro-inflammatory cytokines, chemokines, and antiviral type I IFNs (IFN-β and IFN-α) which serve to trigger an inflammatory and/or antiviral immune response to limit the infectious agent. The remainder of the chapter will provide a more detailed discussion of the functionality of each of the TLR adaptors.

MyD88

As previously stated,  MyD88 is the common downstream adaptor that is recruited by all TLRs, except TLR3. MyD88 was first identified in 1990 as a protein that was induced during the terminal differentiation of M1D+ myeloid precursors in response to IL-6 (O’Neill and Bowie 2007). Early studies indicated that  MyD88 was a TLR adaptor molecule that functioned to recruit IL-1R-associated protein kinase (IRAK-1) to the interleukin-1 receptor (IL-1R) complex following IL-1 stimulation which resulted in the activation of  NF-κB. Human  MyD88 is 296 amino acids in length and contains three domains: a N-terminal death domain which enables interactions with the IRAKs, an interdomain, and a C-terminal TIR domain which facilitates homotypic interaction with other TIR-containing proteins (Jenkins and Mansell 2010). In 1998,  MyD88 was also implicated in TLR signaling (Medzhitov et al. 1998). In contrast to the other TLR adaptor proteins,  MyD88 also mediates signaling through the IL-1 receptor. Further elaboration on MyD88 and its role in TLR signaling can be found within this encyclopedia as a separate entry.

Mal

Mal, the second TLR adaptor to be identified, was simultaneously discovered by two independent labs in 2001 (Fitzgerald et al. 2001; Horng et al. 2001). Having observed late stage  NF-κB and Jun N-terminal kinase (JNK) activation in MyD88-deficient mice, Fitzgerald and colleagues (Fitzgerald et al. 2001) speculated that another, as yet unidentified TIR-domain-containing adaptor protein was mediating this effect. High-throughput sequencing of a human DC EST cDNA library identified Mal – a TIR-domain-containing protein, 235 amino acids in length, that was capable of activating  NF-κB (via IRAK2) and JNK as well as extracellular signal-regulated kinase (ERK) -1 and -2. Mal was shown to homodimerize and heterodimerize with  MyD88. It was also shown that a dominant-negative form of Mal inhibited TLR4 (but not IL-1R or IL-18R) mediated  NF-κB activation (Fitzgerald et al. 2001; Horng et al. 2001). It is generally accepted that Mal acts a bridging adaptor between MyD88 and TLR4/2.

Mal Localization

Mal is localized primarily to the plasma membrane, although Mal-positive, actin-negative vesicles can be found throughout the cell (Kagan and Medzhitov 2006). Mal is concentrated at the leading edge of murine embryonic fibroblasts (MEFs). In macrophages, Mal is localized to discrete regions of the plasma membrane called membrane ruffles which are biochemically similar to the leading edge of fibroblasts (Kagan and Medzhitov 2006).

Mal interacts with the TIR domain of TLR2 and TLR4. This association is facilitated by the phosphatidylinositol 4,5-bisphosphate (PIP2)-binding domain contained within the N-terminal region of Mal (Kagan and Medzhitov 2006). This allows Mal to target to PIP2-rich regions of the plasma membrane which contain high levels of TLR2 and TLR4, thus facilitating their association with Mal (O’Neill and Bowie 2007). Notably, Mal is not involved in the recruitment of MyD88 to other compartments, including endosomal compartments devoid of PIP2 (Kagan and Medzhitov 2006). Further, in Mal/MyD88 double-deficient cells, transfection of a mutant construct which incorporates a PIP2 binding site to the C-terminus of MyD88 directs MyD88 to the plasma membrane and restores lipopolysaccharide (LPS) signaling via TLR4 (Kagan and Medzhitov 2006).

Surface charge distribution models of Mal, MyD88, and TLR4 have shown that the TIR domains of TLR4 and MyD88 are electropositive – and would thus be expected to repel each other under normal circumstances. However, the TIR domain of Mal is electronegative which would facilitate binding of TLR4 to MyD88 in order to transduce TLR4 signaling (O’Neill and Bowie 2007). Moreover, molecular docking experiments have suggested that Mal binds to a homodimer of TLR4 and that Mal actually competes with TRAM for TLR4 binding (Nunez Miguel et al. 2007).

Mal and TLR4 Signaling

Ligand engagement of TLR4, e.g., binding of LPS via MD-2 and CD14, causes TLR4 dimerization and nonexclusive interaction with Mal. Docking experiments have predicted that the Mal (and TRAM) interaction surfaces on the TLR4 dimer interface are at either side of the structure rather than at the top, a region that would be sterically hindered by the membrane (Nunez Miguel et al. 2007). The TLR4 dimer:Mal complex provides a platform allowing MyD88 to bind which then facilitates the recruitment of IRAK1 and IRAK4. Tumor necrosis factor (TNF)-receptor-associated factor 6 (TRAF6) is subsequently recruited and activated via an oligomerisation/auto-ubquitination event. Activated TRAF6 then recruits transforming growth factor activated kinase 1 (TAK1) and TAK1 binding protein 2 (TAB2). This complex interacts with the inhibitor of  NF-κB kinase (IKK) complex, which consists of IKKα, IKKβ, and IKKγ (also known as NEMO), leading to the activation of  NF-κB and subsequent activation of  NF-κB-dependent genes, including the pro-inflammatory cytokines IL-1,IL-6, and TNFα (Moynagh 2008).

Mal and TLR2 Signaling

The role of Mal in TLR2 signaling is complicated by the fact that TLR2 can heterodimerize with both TLR1 and TLR6 to recognize tri- and diacylated lipopeptides, respectively. Overexpression studies have shown that Mal interacts with TLR1 and TLR2, but not TLR6 (Kenny et al. 2009). Although Mal was originally suspected to be essential for TLR2 signaling (Horng et al. 2002), more recent studies have shown that Mal plays a lesser role here when compared to TLR4 (Kenny et al. 2009). Specifically, whilst Mal is required for TLR2 signaling when exposed to low levels of Salmonella typhimurium, Mal is redundant at high concentrations of ligand or in response to high levels of S. typhimurium (Kenny et al. 2009). This suggests that the physiological role of Mal in the context of TLR2 signaling is to prime or amplify low strength bacterial signals.

Modulators of Mal Functionality

Additional levels of specificity and control are added to TLR signaling by virtue of the fact that the TLR adaptors themselves are subject to a myriad of regulatory mechanisms. Mal contains a proline, glutamic acid, serine, and threonine (PEST) domain, located at amino acids 32–72 in human Mal. PEST domains are found in short-lived proteins which undergo phosphorylation, polyubiquitination of lysine residues, and subsequent degradation via the 26S proteasome. The presence of a PEST domain in Mal would therefore suggest that it may be a target for degradation. Interestingly, suppressor of cytokine signaling 1 (SOCS-1), has been shown to inhibit LPS signaling by ubiquitinating Mal and thus targeting it for proteosomal degradation (O’Neill and Bowie 2007).The ubiquitination of Mal is facilitated by Bruton’s tyrosine kinase (Btk) – a protein which is the case of Mal, performs two important functions. Specifically, Btk induces tyrosine phosphorylation of Mal, thus potentiating TLR2/4-driven  NF-κB signaling. However, the same phosphorylation event provides a platform for the aformentioned SOCS-1 mediated ubquitination/degradation of Mal – thus serving to limit the over-activation of the inflammatory immune response (O’Neill and Bowie 2007). IRAK1 and IRAK4 have also been shown to phosphorylate Mal, thereby facilitating its TLR4-ligand-mediated ubiquitination and degradation; IRAK1 and IRAK4 inhibitors blocked this effect (Dunne et al. 2010). Mal has also been shown to interact with caspase-1, with cleavage of Mal by caspase-1 being required to modulate Mal functionality (Miggin et al. 2007; Ulrichts et al. 2010).

A number of studies have been carried out on a variant of Mal that contains a leucine at position 180 instead of a serine (O’Neill and Bowie 2007; Jenkins and Mansell 2010). It has been reported that Mal Ser180Leu does not associate with TLR2 and confers a protective phenotype in malaria and tuberculosis by inhibiting the inflammatory response (O’Neill and Bowie 2007). Other groups dispute this claim (Jenkins and Mansell 2010). Overall, the studies to date indicate an association between heterozygosity at Mal Ser180Leu and protection against multiple infections.

TRIF

Initially, Mal was thought to mediate the MyD88-independent pathway following TLR4 engagement, leading to IRF3 activation and delayed/late activation of  NF-κB. However, given that Mal was instead shown to act as a bridging adaptor in the MyD88-dependent pathway which was activated following TLR4/2 engagement, it remained unclear how TLR4 might mediate IFN-β induction (O’Neill and Bowie 2007).

In 2003, a third TLR adaptor, TRIF, was identified by two separate groups, by one employing database screening to identify novel TIR-domain containing proteins with the other employing a yeast two-hybrid screen using TLR3 as a bait (O’Neill and Bowie 2007). It was found that overexpression of TRIF, 712 amino acids in length, leads to the induction of the IFN-β promoter. In TRIF-deficient mice, whilst impaired TLR3 and TLR4 ligand induced IRF3 activation and concomitant IFN-β induction was observed, TLR2, TLR7 and TLR9 signaling was unaffected (O’Neill and Bowie 2007). Notably, TLR4 ligand induced  NF-κB activation is completely abolished in cells deficient in both MyD88 and TRIF, indicating that TRIF is essential for “MyD88-independent” TLR4 signaling (Jenkins and Mansell 2010). Further, a germline mutation in mice termed Lps2 confirmed the role of TRIF in mediating “MyD88-independent” signaling (Jenkins and Mansell 2010).

TRIF Localization

TRIF is expressed at low levels in most tissues and cells and is diffusely localized in the cytoplasm of resting cells (Tatematsu et al. 2010). When endosomal TLR3 is activated by double strand (ds) RNA, TRIF transiently colocalizes with TLR3 and then dissociates from the receptor forming speckled structures that colocalize with downstream signaling molecules (Tatematsu et al. 2010). Upon stimulation of TLR4 with LPS, TRIF is activated by endosomal TRAM, which associates with the internalized TLR4 complex (Tatematsu et al. 2010). Thus, TRIF is indirectly recruited to TLR4 via TRAM. Also, overexpression of TRIF leads to homo-oligomerization through the TIR domain and the C-terminus, forming a complex called the TRIF signalosome (Jenkins and Mansell 2010; Tatematsu et al. 2010).

TRIF and TLR3/4 Signaling

TRIF, like Mal, has consensus TRAF6-binding motifs in the N-terminal region as well as a TIR domain (O’Neill and Bowie 2007). TRIF also has a TRAF2-binding site in the N-terminal region (Tatematsu et al. 2010) and a C-terminal receptor-interacting protein (RIP) homotypic interaction motif (RHIM) domain (O’Neill and Bowie 2007). The TIR domain of TRIF is essential for binding to the TIR domain of TLR3 and to TRAM. All of these domains serve to facilitate TRIF-mediated signaling, with each domain playing a distinct role therein.

The N-terminal region of TRIF participates in IRF3/7 activation by recruiting the IRF3-activating kinases, TANK-binding kinase 1 (TBK1), and inhibitor of  NF-κB kinase ε (IKKε, also known as IKKι). NAK-associated protein 1 (NAP-1) forms part of the active kinase complex for IRF3 and serves to facilitate the association of TRIF with TBK1 and IKKε (Gauzzi et al. 2010). Upon TBK1/IKKε-mediated phosphorylation and homo/heterodimerization of the IRF3/7 complex, translocation of the phosphorylated IRF complex to the nucleus occurs. Here, it binds to both the IFN-β enhanceosome (Siednienko et al. 2010) and the IFN-stimulated response elements (ISREs) to induce the transcription of responsive genes including the type I IFN and CCL5 genes (Siednienko et al. 2011) (Fig. 1). TNF receptor-associated factor 3 (TRAF3) plays a crucial role in TLR3 signaling as various independent studies show that TRAF3 forms a complex with NAP-1 and TRIF (Fig. 1) (O’Neill and Bowie 2007).

Two separate  NF-κB activation pathways bifurcate from TRIF, and these map to distinct sites at the N- and C-termini. The binding motifs in the N-terminal region of TRIF serve to recruit TRAF6 although its role in TRIF signaling remains controversial (O’Neill and Bowie 2007). Studies suggest that the participation of TRAF6 in TRIF-mediated  NF-κB induction is cell type specific as TRAF6 is essential for  NF-κB activation in mouse embryonic fibroblasts (MEFs), whereas poly(I:C), a TLR3 ligand, induced  NF-κB activation is not impaired in TRAF6-deficient macrophages (Gauzzi et al. 2010; Sasai et al. 2010). There is a separate route to  NF-κB activation involving the RHIM domain of TRIF whereby TRIF facilitates the recruitment of both RIP1 and RIP3 through this domain. Adding credence to the importance of RIP1 and RIP3 in TRIF signaling is the fact that poly(I:C)-induced  NF-κB activation is completely blocked in RIP-1-deficient MEFs. In contrast, RIP3 has been shown to negatively regulate the TRIF–RIP1– NF-κB pathway. Studies are ongoing to further define the role of RIP1 and RIP3 in TLR signaling (O’Neill and Bowie 2007).

TRIF also mediates the induction of apoptosis through TLR3 and TLR4. This is facilitated by direct recruitment of RIP1 to the C-terminal RHIM domain of TRIF, and involves activation of a complex containing TRADD, FADD, and caspase-8 (O’Neill and Bowie 2007). This apoptotic pathway is believed to be responsible for bacterial-induced apoptosis of infected macrophages, bacterial-induced DC apoptosis and DC maturation (O’Neill and Bowie 2007; Jenkins and Mansell 2010).

TRIF and TLR5 Signaling

Although it was thought that TRIF mediated TLR3 and TLR4 signaling only, a number of recent studies have shown that TRIF also plays an important role in TLR5 signaling. Stimulation of human colonic epithelial cells with the TLR5 ligand flagellin, allows TLR5 and TRIF, but not TRAM, to interact and mediate TLR5-induced  NF-κB and mitogen-activated protein kinase (MAPK) activation in intestinal epithelial cells (IEC) (Rhee 2011). TRIF-deficient IECs stimulated with flagellin exhibit decreased inflammatory cytokine expression when compared to their wild-type counterparts (Rhee 2011). Furthermore, TRIF-deficient mice are resistant to flagellin-mediated exacerbation of colonic inflammation and dextran sulfate sodium–induced experimental colitis. Moreover, studies have shown that TRIF-induced caspase activity causes the degradation of TLR5 (Rhee 2011) indicating that TRIF can participate in the proteolytic modification of TLR functionality at the posttranslational level. These recent findings therefore suggest that TRIF plays an important role in regulating host-microbial communication via TLR5 in the gut epithelium.

TRIF and Cytosolic dsRNA Detection

A further role for TRIF in innate immune signaling, independent of the TLRs, has recently been identified whereby TRIF appears to be an essential component of a novel dsRNA sensing pathway in DCs (Zhang et al. 2011). More specifically, the RNA helicases DDX1, DDX21, and DHX36 form a complex which enables the sequestration of cytosolic dsRNA which is then followed by binding to TRIF and subsequent induction of type I IFN and inflammatory cytokine responses. It has been shown that DDX1 binds dsRNA via its helicase A domain and that DHX36 and DDX21 bind to TRIF via their HA2-DUF and PRK domains, respectively. The resulting complex triggers the innate antiviral response (Zhang et al. 2011).

Negative Regulation of TRIF

Numerous strategies exist to curtail TRIF signaling, either directly, or via inhibition of downstream signaling molecules. In terms of direct inhibition of TRIF, a number of molecules have been identified. For example, the inhibitory TLR adaptor protein SARM contains a TIR domain and serves to inhibit TRIF-mediated signaling. SARM has been shown to interact with TRIF and both the TIR and SAM domains of SARM are vital for SARM’s functionality in this regard. While the exact mechanism of inhibition has not been elucidated, it is suspected that SARM and TRIF interact via their TIR domains, thus preventing the binding of downstream molecules such as RIP1. Alternatively, the SAM domain of SARM may facilitate recruitment of an, as yet unidentified, inhibitory molecule (Carty et al. 2006; Jenkins and Mansell 2010).

Consistent with a role for TRIF in restricting viral replication through type I IFN induction, at least two viruses have been shown to contain proteins that antagonize TRIF. The vaccinia virus (VACV) encoded proteins, A46R and A52R, differentially affect TRIF signaling. More specifically, A46R interacts directly with TRIF and inhibits TRIF mediated TLR3 signaling. Notably, A46R also interacts with the other TLR adaptors and also inhibits TLR4 signaling. In contrast, A52R acts downstream of the TLR adaptors by targeting TRAF6 and IRAK2 (O’Neill and Bowie 2007). Hepatitis C (HCV) virus contains a serine protease NS3-4A that causes the proteolysis of TRIF. The cleavage of TRIF by NS3-4A inhibits both  NF-κB and IRF3 activation by TLR3, thus disabling the innate immune response to the virus (Jenkins and Mansell 2010). The above examples illustrate the importance of TRIF in mediating the anti-viral signaling pathway such that specific inhibition by VACV and HCV confers an advantage to the viruses in vivo.

TRAM

In 2003, the fourth TIR-domain-containing adaptor, TRAM, 235 amino acids in length, was identified following a bioinformatic search of the human genome database (Jenkins and Mansell 2010). It was initially thought that TRAM was involved in both TLR and IL-1R mediated  NF-κB activation, but not IFN-β induction (Bin et al. 2003). Subsequently, a definitive description of TRAM showed that it interacts with TLR4 and TRIF to regulate TLR4-mediated IRF3 and IRF7 activation (Fitzgerald et al. 2003). TRAM-deficient cells have impaired TLR4-mediated cytokine production and B cell activation, as was observed with TRIF, thus supporting the notion that in the case of TLR4, both the “MyD88-dependent” and “MyD88-independent” pathways are integral for maximal production of pro-inflammatory cytokines. It is now accepted that TRAM acts as bridging adaptor between TLR4 and TRIF in the “MyD88-independent” pathway (Jenkins and Mansell 2010).

TRAM Localization and Involvement in TLR4 Signaling

TRAM exclusively mediates TLR4 signaling. It activates the “MyD88-independent” pathway by facilitating the association of TRIF with TLR4 - similar to the way in which Mal links TLR4 and MyD88. To date, it serves no other known role in TLR signaling. Regarding localization, the N-terminal region of TRAM undergoes constitutive myristoylation, thus facilitating its association with the plasma membrane (Jenkins and Mansell 2010). Moreover, mutation of the myristoylation motif in TRAM abolishes its ability to signal (O’Neill and Bowie 2007).

A distinct requirement for TRAM signaling to occur is the phosphorylation of TRAM on serine 16 by protein kinase Cε (PKCε) (Jenkins and Mansell 2010). TRAM has also been shown to contain a bipartite sorting signal that modulates its trafficking between the plasma membrane and the endosomes. In fact, TRAM must be delivered to the endosomes in a complex with TLR4 to facilitate the activation of IRF3 (Jenkins and Mansell 2010). Thus, activation of TLR4 sequentially induces two signaling pathways from two different cellular locations. The “MyD88-dependent” pathway is induced from the plasma membrane, whereas the “MyD88-independent” pathway is induced from endosomes. Further, these findings suggest that the ability of TLRs to induce an IFN response is dependent on their intracellular localization (Jenkins and Mansell 2010).

Negative Regulation of TRAM

VACV is capable of modulating TRAM functionality. Specifically, an 11-aa-long peptide derived from A46R (termed viral inhibitor peptide of TLR4, or VIPER) has been shown to interact with TRAM (and Mal), thus inhibiting TLR4 signaling. It has been postulated that masking of the critical binding sites on Mal and TRAM specifically inhibits TLR4 signaling (Lysakova-Devine et al. 2010). Also, a splice variant of TRAM, termed TRAM adaptor with GOLD domain (TAG), has been shown to competitively bind TRAM and displace TRIF during LPS-mediated signaling, leading to decreases in RANTES cytokine production without affecting  NF-κB activation (Palsson-McDermott et al. 2009).

SARM

SARM was initially identified in 2001 as a human gene encoding an orthologue of a Drosophila melanogaster protein. Structurally, SARM is 690 amino acids in length and contains two sterile alpha motifs (SAM) domains as well as a HEAT/Armadillo repeat motif (ARM) domain (O’Neill and Bowie 2007). Both the SAM and ARM domains are known to be involved in the formation of protein complexes. It was initially shown that the Caenorhabditis elegans SARM homologue, TIR1, was important in the efficient immune response against fungal and bacterial infection (O’Neill and Bowie 2007). However, in human cells, it was also shown that unlike the other TIR-domain adaptors, overexpression of SARM failed to induce  NF-κB or activate IRF3-dependent reporter genes and in fact inhibited their activation and expression. (Carty et al. 2006). Human SARM was identified as a negative regulator of TRIF mediated signaling and was therefore the first TIR-domain-containing adaptor shown to be involved in the negative regulation of TLR signaling (Carty et al. 2006). In contrast, a later study showed that macrophages from SARM knockout mice responded normally to TLR3, TLR4, and TLR9 ligands, suggesting that mouse SARM has a redundant role in regulating macrophage responses to these TLR ligands (Jenkins and Mansell 2010). Further research must be undertaken to definitively assign a role for SARM in TLR signaling.

SARM and TLR3/4 Signaling

Although disputed, it appears that in humans, SARM inhibits TRIF-mediated TLR3 and TLR4 signaling by selectively targeting TRIF. In unstimulated cells, SARM and TRIF are weak interactors, but stimulation with LPS or poly(I:C) induces SARM protein expression and enhances the interaction between SARM and TRIF (Carty et al. 2006). SARM overexpression serves to inhibit TRIF-dependent, but not MyD88- or Mal-dependent,  NF-κB activation. SARM also inhibits poly(I:C)-mediated CCL5 and IFN-β promoter activity. The exact mechanism utilized by SARM to impair TRIF functionality requires further investigation, however it is speculated that SARM may use its TIR domain to bind TRIF and use its SAM domains to recruit an as-yet-unidentified inhibitor. Alternatively, SARM may competitively block the ability of TRIF to directly interact with downstream signal transducers such as TBK1, RIP1, and TRAF6.

Negative Regulation of TLR Signaling by TLR Adaptors

A number of recent studies have highlighted the role of the TLR adaptors themselves in the curtailment of TLR signaling. For example, MyD88 has been shown to negatively regulate TLR3-TRIF-induced corneal inflammation through a mechanism involving JNK phosphorylation, but not p38, IRF3, or  NF-κB (Johnson et al. 2008) and to inhibit TLR3-dependent IL-6 induction (Kenny et al. 2009). MyD88 has also been shown to inhibit TLR3-dependent phosphorylation of IRF3 and thus curtail TLR3-mediated IFN-β and RANTES production (Siednienko et al. 2011). Furthermore, Mal has been shown to inhibit TLR3-dependent IFN-β production through a mechanism that is distinct from MyD88 whereby Mal inhibits TLR3 ligand–mediated IRF7 activation (Siednienko et al. 2010). Mal has also been shown to inhibit TLR3-dependent IL-6 induction (Kenny et al. 2009). As already stated, SARM has been shown to inhibit TRIF-dependent TLR3 and TLR4 signaling (Carty et al. 2006).

Summary

The TLR adaptor proteins are integral modulators of TLR signaling, and consequentially, of innate and adaptive immunity. The specificity of their usage, combined with their broad downstream effects on both autocrine and paracrine immune signaling, highlights them as potential targets for therapeutic immunomodulation (O’Neill et al. 2009).

Many recent advances have been made towards a greater understanding of how TLR adaptors function in the context of innate immunity. It is evident that the TLRs and their adaptor molecules have evolved to respond appropriately to a pathogenic challenge while at the same time, retaining the ability to limit their excessive activation which could otherwise cause detrimental or deleterious damage to the host system. Indeed, many intricate mechanisms by which TLR signaling may be regulated, including adaptor sequestration, differential adaptor utilization, protein degradation, and compartmentalization, have been identified (Akira and Takeda 2010).

While TLR adaptor functionality in the context of signal initiation is well described, the mechanisms that serve to negatively regulate or control them are poorly understood. Evidence suggests that the adaptors may require tight regulation to control the immune response and thus prevent chronic inflammation. Adding credence to this hypothesis is the fact that the TLR adaptors have been shown to bifurcate between activationary and inhibitory roles in TLR signaling as seen with Mal and MyD88. Further study is therefore required to expand on the current understanding of the role played by the TLR adaptors under normal physiological conditions and whether perturbations occur during chronic inflammatory diseases.

References

  1. Akira S, Takeda K. Toll-like receptor signaling. Nat Rev Immunol. 2004;4:499–511.PubMedCrossRefGoogle Scholar
  2. Bin LH, Xu LG, Shu HB. TIRP, a novel Toll/interleukin-1 receptor (TIR) domain-containing adapter protein involved in TIR signaling. J Biol Chem. 2003;278:24526–32.PubMedCrossRefGoogle Scholar
  3. Carty M, Goodbody R, Schroder M, Stack J, Moynagh PN, Bowie AG. The human adaptor SARM negatively regulates adaptor protein TRIF-dependent Toll-like receptor signaling. Nat Immunol. 2006;7:1074–81.PubMedCrossRefGoogle Scholar
  4. Chen GY, Nunez G. Sterile inflammation: sensing and reacting to damage. Nat Rev Immunol. 2010;10:826–37.PubMedPubMedCentralCrossRefGoogle Scholar
  5. Dunne A, Carpenter S, Brikos C, Gray P, Strelow A, Wesche H, et al. IRAK1 and IRAK4 promote phosphorylation, ubiquitination, and degradation of MyD88 adaptor-like (Mal). J Biol Chem. 2010;285:18276–82.PubMedPubMedCentralCrossRefGoogle Scholar
  6. Fitzgerald KA, Palsson-McDermott EM, Bowie AG, Jefferies CA, Mansell AS, Brady G, et al. Mal (MyD88-adapter-like) is required for toll-like receptor-4 signal transduction. Nature. 2001;413:78–83.PubMedCrossRefGoogle Scholar
  7. Fitzgerald KA, Rowe DC, Barnes BJ, Caffrey DR, Visintin A, Latz E, et al. LPS-TLR4 signaling to IRF-3/7 and NF-kappaB involves the toll adapters TRAM and TRIF. J Exp Med. 2003;198:1043–55.PubMedPubMedCentralCrossRefGoogle Scholar
  8. Gauzzi C, Del Corno M, Gessani S. Dissecting TLR3 signaling in dendritic cells. Immunobiology. 2010;215:713–23.PubMedCrossRefGoogle Scholar
  9. Horng T, Barton GM, Medzhitov R. TIRAP: an adapter molecule in the toll signaling pathway. Nat Immunol. 2001;2:835–41.PubMedCrossRefGoogle Scholar
  10. Horng T, Barton GM, Flavell RA, Medzhitov R. The adaptor molecule TIRAP provides signaling specificity for toll-like receptors. Nature. 2002;420:329–33.PubMedCrossRefGoogle Scholar
  11. Jenkins KA, Mansell A. TIR-containing adaptors in toll-like receptor signaling. Cytokine. 2010;49:237–44.PubMedCrossRefGoogle Scholar
  12. Johnson AC, Li X, Pearlman E. MyD88 functions as a negative regulator of TLR3/TRIF-induced corneal inflammation by inhibiting activation of c-Jun N-terminal kinase. J Biol Chem. 2008;283:3988–96.PubMedCrossRefGoogle Scholar
  13. Kagan JC, Medzhitov R. Phosphoinositide-mediated adaptor recruitment controls toll-like receptor signaling. Cell. 2006;125:943–55.PubMedCrossRefGoogle Scholar
  14. Kawai T, Akira S. TLR signaling. Cell Death Differ. 2006;13:816–25.PubMedCrossRefGoogle Scholar
  15. Kenny EF, Talbot S, Gong M, Golenbock DT, Bryant CE, O’Neill LA. MyD88 adaptor-like is not essential for TLR2 signaling and inhibits signaling by TLR3. J Immunol. 2009;183:3642–51.PubMedCrossRefGoogle Scholar
  16. Kumar H, Kawai T, Akira S. Pathogen recognition in the innate immune response. Biochem J. 2009;420:1–16.PubMedCrossRefGoogle Scholar
  17. Lysakova-Devine T, Keogh B, Harrington B, Nagpal K, Halle A, Golenbock DT, et al. Viral inhibitory peptide of TLR4, a peptide derived from vaccinia protein A46, specifically inhibits TLR4 by directly targeting MyD88 adaptor-like and TRIF-related adaptor molecule. J Immunol. 2010;185:4261–71.PubMedCrossRefGoogle Scholar
  18. Medzhitov R, Preston-Hurlburt P, Kopp E, Stadlen A, Chen C, Ghosh S, et al. MyD88 is an adaptor protein in the hToll/IL-1 receptor family signaling pathways. Mol Cell. 1998;2:253–8.PubMedCrossRefGoogle Scholar
  19. Miggin SM, O’Neill LA. New insights into the regulation of TLR signaling. J Leukoc Biol. 2006;80:220–6.PubMedCrossRefGoogle Scholar
  20. Miggin SM, Pålsson-McDermott E, Dunne A, Jefferies C, Pinteaux E, Banahan K, et al. NF-kB activation by the Toll-IL-1 receptor domain protein MyD88 adapter-like is regulated by caspase-1. Proc Natl Acad Sci USA. 2007;104:3372–7.PubMedPubMedCentralCrossRefGoogle Scholar
  21. Moynagh PN. The Pellino family: IRAK E3 ligases with emerging roles in innate immune signaling. Trends Immunol. 2008;30:33–42.PubMedCrossRefGoogle Scholar
  22. Nunez Miguel R, Wong J, Westoll JF, Brooks HJ, O’Neill LA, Gay NJ, et al. A dimer of the toll-like receptor 4 cytoplasmic domain provides a specific scaffold for the recruitment of signaling adaptor proteins. PLoS One. 2007;2:e788.PubMedPubMedCentralCrossRefGoogle Scholar
  23. O’Neill LA, Bowie AG. The family of five: TIR-domain-containing adaptors in Toll-like receptor signaling. Nat Rev Immunol. 2007;7:353–64.PubMedCrossRefGoogle Scholar
  24. O’Neill LA, Bryant CE, Doyle SL. Therapeutic targeting of toll-like receptors for infectious and inflammatory diseases and cancer. Pharmacol Rev. 2009;61(2):177–97.PubMedPubMedCentralCrossRefGoogle Scholar
  25. Palsson-McDermott EM, Doyle SL, McGettrick AF, Hardy M, Husebye H, Banahan K, et al. TAG, a splice variant of the adaptor TRAM, negatively regulates the adaptor MyD88-independent TLR4 pathway. Nat Immunol. 2009;10:579–86.PubMedCrossRefGoogle Scholar
  26. Rhee SH. Basic and translational understandings of microbial recognition by toll-like receptors in the intestine. Neurogastroenterol Motil. 2011;17:28–34.CrossRefGoogle Scholar
  27. Sasai M, Tatematsu M, Oshiumi H, Funami K, Matsumoto M, Hatakeyama S, et al. Direct binding of TRAF2 and TRAF6 to TICAM-1/TRIF adaptor participates in activation of the toll-like receptor 3/4 pathway. Mol Immunol. 2010;47:1283–91.PubMedCrossRefGoogle Scholar
  28. Siednienko J, Halle A, Nagpal K, Golenbock DT, Miggin SM. TLR3-mediated IFN-beta gene induction is negatively regulated by the TLR adaptor MyD88 adaptor-like. Eur J Immunol. 2010;40:3150–60.PubMedCrossRefGoogle Scholar
  29. Siednienko J, Gajanayake T, Fitzgerald KA, Moynagh P, Miggin SM. Absence of MyD88 results in enhanced TLR3-dependent phosphorylation of IRF3 and increased IFN-(beta) and RANTES production. J Immunol. 2011;186:2514–22.PubMedCrossRefGoogle Scholar
  30. Tatematsu M, Ishii A, Oshiumi H, Horiuchi M, Inagaki F, Seya T, et al. A molecular mechanism for toll-IL-1 receptor domain-containing adaptor molecule-1-mediated IRF-3 activation. J Biol Chem. 2010;285:20128–36.PubMedPubMedCentralCrossRefGoogle Scholar
  31. Ulrichts P, Bovijn C, Lievens S, Beyaert R, Tavernier J, Peelman F. Caspase-1 targets the TLR adaptor Mal at a crucial TIR-domain interaction site. J Cell Sci. 2010;123:256–65.PubMedCrossRefGoogle Scholar
  32. Yamamoto M, Sato S, Hemmi H, Sanjo H, Uematsu S, Kaisho T, et al. Essential role for TIRAP in activation of the signaling cascade shared by TLR2 and TLR4. Nature. 2002;420:324–9.PubMedCrossRefGoogle Scholar
  33. Zhang Z, Kim T, Bao M, Facchinetti V, Jung SY, Ghaffari AA, et al. DDX1, DDX21, and DHX36 helicases form a complex with the adaptor molecule TRIF to sense dsRNA in dendritic cells. Immunity. 2011;34:866–78.PubMedPubMedCentralCrossRefGoogle Scholar

Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of Biology, Institute of ImmunologyNational University of Ireland MaynoothMaynoothIreland