Activation of Toll-like receptors nucleates assembly of the MyDDosome signaling hub.

Infection and tissue damage induces assembly of supramolecular organizing centres (SMOCs)), such as the Toll-like receptor (TLR) MyDDosome, to co-ordinate inflammatory signaling. SMOC assembly is thought to drive digital all-or-none responses, yet TLR activation by diverse microbes induces anything from mild to severe inflammation. Using single-molecule imaging of TLR4-MyDDosome signaling in living macrophages, we find that MyDDosomes assemble within minutes of TLR4 stimulation. TLR4/MD2 activation leads only to formation of TLR4/MD2 heterotetramers, but not oligomers, suggesting a stoichiometric mismatch between activated receptors and MyDDosomes. The strength of TLR4 signalling depends not only on the number and size of MyDDosomes formed but also how quickly these structures assemble. Activated TLR4, therefore, acts transiently nucleating assembly of MyDDosomes, a process that is uncoupled from receptor activation. These data explain how the oncogenic mutation of MyD88 (L265P) assembles MyDDosomes in the absence of receptor activation to cause constitutive activation of pro-survival NF-κB signalling.

Impact of mutations in Toll-like receptor pathway genes on esophageal carcinogenesis.

Esophageal adenocarcinoma (EAC) develops in an inflammatory microenvironment with reduced microbial diversity, but mechanisms for these influences remain poorly characterized. We hypothesized that mutations targeting the Toll-like receptor (TLR) pathway could disrupt innate immune signaling and promote a microenvironment that favors tumorigenesis. Through interrogating whole genome sequencing data from 171 EAC patients, we showed that non-synonymous mutations collectively affect the TLR pathway in 25/171 (14.6%, PathScan p = 8.7x10-5) tumors. TLR mutant cases were associated with more proximal tumors and metastatic disease, indicating possible clinical significance of these mutations. Only rare mutations were identified in adjacent Barrett's esophagus samples. We validated our findings in an external EAC dataset with non-synonymous TLR pathway mutations in 33/149 (22.1%, PathScan p = 0.05) tumors, and in other solid tumor types exposed to microbiomes in the COSMIC database (10,318 samples), including uterine endometrioid carcinoma (188/320, 58.8%), cutaneous melanoma (377/988, 38.2%), colorectal adenocarcinoma (402/1519, 26.5%), and stomach adenocarcinoma (151/579, 26.1%). TLR4 was the most frequently mutated gene with eleven mutations in 10/171 (5.8%) of EAC tumors. The TLR4 mutants E439G, S570I, F703C and R787H were confirmed to have impaired reactivity to bacterial lipopolysaccharide with marked reductions in signaling by luciferase reporter assays. Overall, our findings show that TLR pathway genes are recurrently mutated in EAC, and TLR4 mutations have decreased responsiveness to bacterial lipopolysaccharide and may play a role in disease pathogenesis in a subset of patients.

Three-tier regulation of cell number plasticity by neurotrophins and Tolls in Drosophila.

Cell number plasticity is coupled to circuitry in the nervous system, adjusting cell mass to functional requirements. In mammals, this is achieved by neurotrophin (NT) ligands, which promote cell survival via their Trk and p75NTR receptors and cell death via p75NTR and Sortilin. Drosophila NTs (DNTs) bind Toll receptors instead to promote neuronal survival, but whether they can also regulate cell death is unknown. In this study, we show that DNTs and Tolls can switch from promoting cell survival to death in the central nervous system (CNS) via a three-tier mechanism. First, DNT cleavage patterns result in alternative signaling outcomes. Second, different Tolls can preferentially promote cell survival or death. Third, distinct adaptors downstream of Tolls can drive either apoptosis or cell survival. Toll-6 promotes cell survival via MyD88-NF-κB and cell death via Wek-Sarm-JNK. The distribution of adaptors changes in space and time and may segregate to distinct neural circuits. This novel mechanism for CNS cell plasticity may operate in wider contexts.

Structure of the Toll/Interleukin-1 Receptor (TIR) Domain of the B-cell Adaptor That Links Phosphoinositide Metabolism with the Negative Regulation of the Toll-like Receptor (TLR) Signalosome.

Ligand binding to Toll-like receptors (TLRs) results in dimerization of their cytosolic Toll/interleukin-1 receptor (TIR) domains and recruitment of post-receptor signal transducers into a complex signalosome. TLR activation leads to the production of transcription factors and pro-inflammatory molecules and the activation of phosphoinositide 3-kinases (PI3K) in a process that requires the multimodular B-cell adaptor for phosphoinositide 3-kinase (BCAP). BCAP has a sequence previously proposed as a "cryptic" TIR domain. Here, we present the structure of the N-terminal region of human BCAP and show that it possesses a canonical TIR fold. Dimeric BCAP associates with the TIR domains of TLR2/4 and MAL/TIRAP, suggesting that it is recruited to the TLR signalosome by multitypic TIR-TIR interactions. BCAP also interacts with the p85 subunit of PI3K and phospholipase Cγ, enzymes that deplete plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2), and these interactions provide a molecular explanation for BCAP-mediated down-regulation of inflammatory signaling.

The TLR signaling adaptor TRAM interacts with TRAF6 to mediate activation of the inflammatory response by TLR4.

TLRs act as sentinels in professional immune cells to detect and initiate the innate immune response to pathogen challenge. TLR4 is a widely expressed TLR, responsible for initiating potent immune responses to LPS. TRAM acts to bridge TLR4 with TRIF, orchestrating the inflammatory response to pathogen challenge. We have identified a putative TRAF6-binding motif in TRAM that could mediate a novel signaling function for TRAM in TLR4 signaling. TRAM and TRAF6 association was confirmed by immunoprecipitation of endogenous, ectopically expressed and recombinant proteins, which was ablated upon mutation of a key Glu residue in TRAM (TRAM E183A). TRAF6 and TRAM were observed colocalizing using confocal microscopy following ectopic expression in cells and the ability of TRAM and TRAM E183A to activate luciferase-linked reporter assays was determined in HEK293 and TRAF6-deficient cells. Importantly, TRAM-deficient macrophages reconstituted with TRAM E183A display significantly reduced inflammatory TNF-α, IL-6, and RANTES protein production compared with WT TRAM. These results demonstrate a novel role for TRAM in TLR4-mediated signaling in regulating inflammatory responses via its interaction with TRAF6, distinct from its role as a bridging adaptor between TLR4 and TRIF.

An alanine-to-proline mutation in the BB-loop of TLR3 Toll/IL-1R domain switches signalling adaptor specificity from TRIF to MyD88.

A functionally important proline residue is highly conserved in the cytosolic Toll/IL-1R signaling domains of human TLRs. The antiviral Toll, TLR3, is unusual because it has alanine instead of proline at this position and is the only human TLR that associates directly with the adaptor molecule TIR domain-containing adaptor inducing IFN-ß (TRIF) rather than MyD88. In this article, we report that a mutant TLR3 that substitutes the BB-loop alanine for proline (A795P) enhances NF-κB activation but is incapable of mediating TRIF-dependent IFN response factor 3 responses. Wild-type and A795P TLR3 associate constitutively with both TRIF and MyD88, and activation induces additional binding of TRIF to the wild-type and of MyD88 to the A795P mutant receptors, respectively. In addition, activation of A795P, but not wild-type TLR3, leads to the recruitment of TRAF6, a downstream signal transducer of the MyD88-dependent pathway. These results show that adaptor specificity can be conferred by minimal determinants of the Toll/IL-1R domain.

Crystal structure of Toll-like receptor adaptor MAL/TIRAP reveals the molecular basis for signal transduction and disease protection.

Initiation of the innate immune response requires agonist recognition by pathogen-recognition receptors such as the Toll-like receptors (TLRs). Toll/interleukin-1 receptor (TIR) domain-containing adaptors are critical in orchestrating the signal transduction pathways after TLR and interleukin-1 receptor activation. Myeloid differentiation primary response gene 88 (MyD88) adaptor-like (MAL)/TIR domain-containing adaptor protein (TIRAP) is involved in bridging MyD88 to TLR2 and TLR4 in response to bacterial infection. Genetic studies have associated a number of unique single-nucleotide polymorphisms in MAL with protection against invasive microbial infection, but a molecular understanding has been hampered by a lack of structural information. The present study describes the crystal structure of MAL TIR domain. Significant structural differences exist in the overall fold of MAL compared with other TIR domain structures: A sequence motif comprising a ß-strand in other TIR domains instead corresponds to a long loop, placing the functionally important "BB loop" proline motif in a unique surface position in MAL. The structure suggests possible dimerization and MyD88-interacting interfaces, and we confirm the key interface residues by coimmunoprecipitation using site-directed mutants. Jointly, our results provide a molecular and structural basis for the role of MAL in TLR signaling and disease protection.

MyD88 adapter-like (Mal)/TIRAP interaction with TRAF6 is critical for TLR2- and TLR4-mediated NF-kappaB proinflammatory responses.

Toll/interleukin-1 (TIR)receptor-containing adapters are critical in orchestrating the different signal transduction pathways following Toll-like receptor (TLR) activation. MyD88 adapter-like (Mal), also termed TIRAP, is involved in bridging MyD88 to the receptor complex for TLR-2 and TLR4 signaling in response to bacterial infection. We have previously reported an interaction between Mal and tumor necrosis factor receptor-associated factor 6 (TRAF6) via a TRAF6-binding motif, the disruption of which inhibited TLR-mediated NF-kappaB-luciferase reporter activity. Given the recent report of intracellular TRAM localization promoting sequential signaling in TLR4 responses, we further characterized Mal interaction with TRAF6, the cellular localization, and the outcomes of disrupting this association on TLR inflammatory responses. We found that Mal and TRAF6 directly interact in response to TLR2 and TLR4 stimulation, although membrane localization is not necessary to facilitate interaction. Critically, reconstitution of murine Mal-deficient macrophages with MalE190A, containing a mutation within the TRAF6-binding motif, fails to reconstitute the proinflammatory response to TLR2 and TLR4 ligands compared with wild type Mal. Furthermore, Mal interaction with TRAF6 mediates Ser phosphorylation of the p65 subunit of NF-kappaB and thus controls transcriptional activation but not nuclear translocation of NF-kappaB. This study characterizes the novel role for Mal in facilitating the direct recruitment of TRAF6 to the plasma membrane, which is necessary for TLR2- and TLR4-induced transactivation of NF-kappaB and regulation of the subsequent pro-inflammatory response.