Related Experiment Videos
Common interaction surfaces of the toll-like receptor 4 cytoplasmic domain stimulate multiple nuclear targets
Tapani Ronni1, Vishal Agarwal, Michael Haykinson
1Howard Hughes Medical Institute and Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, Los Angeles, California 90095, USA.
Abstract:
Toll-like receptor 4 (TLR4) mediates the host response to lipopolysaccharide (LPS) by promoting the activation of pro- and anti-inflammatory cytokine genes. To activate each gene, numerous signal transduction pathways are required. The adaptor proteins MyD88 and TIRAP contribute to the activation of several and possibly all pathways via direct interactions with TLR4's Toll/interleukin-1 receptor (IL-1R) (TIR) domain. However, additional adaptors that are required for the activation of specific subsets of pathways may exist, which could contribute to the differential regulation of target genes. Furthermore, it remains unknown whether direct interactions that have been reported between TIR domains and other proteins are required for TLR4 signaling. To address these issues, we systematically mutated the TLR4 TIR domain in the context of a CD4/TLR4 fusion protein. Several exposed residues defining at least two structural surfaces were required in macrophages for activation of the proinflammatory IL-12 p40 and anti-inflammatory IL-10 promoters, as well as promoters dependent on individual transcription factors. Interestingly, the same residues were required by all promoters tested, suggesting that the signaling pathways diverge downstream of the adaptors. The mutant phenotypes provide a framework for future studies of TLR4 signaling, as the interaction supported by each critical surface residue will need to be defined.
Insights
Toll-like receptor 4 (TLR4) signaling involves crucial interactions within its Toll/interleukin-1 receptor (TIR) domain. Mutations revealed specific residues essential for activating both pro- and anti-inflammatory cytokine genes, indicating downstream pathway divergence.
Area of Science:
- Immunology
- Molecular Biology
- Cell Signaling
Background:
- Toll-like receptor 4 (TLR4) is key in host defense against lipopolysaccharide (LPS).
- TLR4 activation requires complex signal transduction pathways involving adaptor proteins like MyD88 and TIRAP.
- The precise mechanisms and adaptor requirements for differential gene activation downstream of TLR4 remain incompletely understood.
Purpose of the Study:
- To systematically investigate the role of the TLR4 Toll/interleukin-1 receptor (TIR) domain in mediating downstream signaling.
- To identify specific structural surfaces and residues within the TLR4 TIR domain critical for activating pro- and anti-inflammatory gene promoters.
- To elucidate whether TLR4 signaling pathways diverge downstream of adaptor protein interactions.
Main Methods:
- Systematic mutagenesis of the TLR4 TIR domain within a CD4/TLR4 fusion protein construct.
- Assessment of mutant TLR4 function in macrophages by measuring the activation of IL-12 p40 and IL-10 promoters.
- Analysis of promoter activation dependent on individual transcription factors to dissect signaling pathways.
Main Results:
- Several exposed residues within the TLR4 TIR domain, defining at least two distinct structural surfaces, were essential for activating both pro-inflammatory (IL-12 p40) and anti-inflammatory (IL-10) gene promoters.
- The same critical residues were required for the activation of all tested promoters, irrespective of their inflammatory or transcription factor dependency.
- These findings suggest that the divergence of TLR4-mediated signaling pathways occurs downstream of the identified critical residues and adaptor interactions.
Conclusions:
- The TLR4 TIR domain contains critical structural surfaces and residues that are indispensable for initiating downstream signaling cascades.
- The conserved requirement of these residues across different promoters implies that TLR4 signaling pathways converge at the TIR domain level before diverging.
- The generated mutant phenotypes provide a valuable resource for future research aimed at defining specific protein interactions and downstream effectors in TLR4 signal transduction.