After recognition of microbial products by members of the Toll-like receptor (TLR) family, the adaptor molecule MYD88 initiates intracellular signalling cascades that result in the activation of host defence mechanisms. However, recent studies have shown a MYD88-independent signalling pathway downstream of TLR3 and TLR4 that is regulated by the adaptor molecule TRIF (also known as TICAM1) and leads to the activation of interferon regulatory factor 3 (IRF3), as well as signals that sustain activation of nuclear factor-κB (NF-κB). Now, three independent studies have identified TRAM (also known as TICAM2) as an adaptor molecule that is a key link in the TRIF-dependent TLR4, but not TLR3, signalling cascade.
Previously identified TLR adaptor molecules contain Toll–interleukin 1 receptor (TIR) domains and each of the three groups identified TRAM using database searches for new TIR-domain-containing molecules. Oshiumi et al. and Fitzgerald et al. used a yeast two-hybrid assay and/or co-immunoprecipitation studies to show that TRAM interacts directly with both TRIF and TLR4, but not TLR3, implicating TRAM as a specific component of the TLR4-signalling cascade.
To determine the role of TRAM in TLR4 signalling, all three groups used the same approach — analysing the effect of ectopic overexpression of TRAM — and all observed activation of IRF3 and NF-κB, as well as activation of the promoter for the gene encoding interferon-β (IFN-β).
Oshiumi et al. and Fitzgerald et al. went on to generate a series of TRAM mutants, which they used to show that TRAM is crucial for IRF3 and NF-κB activation after ligation of TLR4 by lipopolysaccharide (LPS), but not TLR3 triggering with double-stranded RNA. Further evidence for the importance of TRAM in the TLR4-signalling pathway was provided by their demonstration that marked reduction of the level of TRAM by small interfering RNA impaired IRF3 and NF-κB activation in response to TLR4 triggering, but not TLR3 ligation.
Yamamoto et al. examined the physiological role of TRAM in the TLR-signalling cascade by generating Tram-knockout mice. Tram-deficient cells responded normally to signalling through TLR3; however the production of pro-inflammatory cytokines and IFN-β was substantially impaired after LPS triggering of TLR4, as was the activation of B cells. These functional defects coincided with inefficient activation of signalling molecules downstream of TRAM, including IRF3, and an inability to sustain NF-κB activation. By contrast, activation of the MYD88-signalling pathway was intact in the TRAM-deficient cells. So, in vivo, TRAM is essential for the MYD88-independent signalling cascade after LPS ligation of TLR4. This cascade induces B-cell activation and the production of IFN-β; however, TRAM and MYD88 signals are required for the secretion of pro-inflammatory cytokines in response to LPS.
These studies provide invaluable insight into the TRIF-regulated MYD88-independent response generated after triggering of both TLR3 and TLR4. Ligation of these molecules induces distinct host responses and this specificity might be a result of TLR4 using the adaptor molecule TRAM to initiate signalling through TRIF, whereas TLR3 signals directly to TRIF.
References
ORIGINAL RESEARCH PAPERS
Yamamoto, M. et al. TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway. Nature Immunol. 13 October 2003 (DOI:10.1038/ni986)
Fitzgerald, K. A. et al. LPS–TLR4 signaling to IRF-3/7 and NF-κB involves the Toll adaptors TRAM and TRIF. J. Exp. Med. 198, 1043–1055 (2003)
Oshiumi, H. et al. TICAM-2: a bridging adaptor recruiting to Toll-like receptor 4 TICAM-1 that induces interferon-β J. Biol. Chem. 30 September 2003 (DOI: 10.1074/jbc.M305820200)
FURTHER READING
Bell, E. TLR signaling. Nature Rev. Immunol. 3, 692 (2003)
Yeh, W. C. & Chen, N. J. Immunology: another toll road. Nature 424, 726–737 (2003)
Rights and permissions
About this article
Cite this article
Honey, K. New link in the chain. Nat Rev Immunol 3, 846 (2003). https://doi.org/10.1038/nri1237
Issue Date:
DOI: https://doi.org/10.1038/nri1237