Suppression of TLR Signaling by Targeting TIR domain-Containing Proteins

Abstract

Toll-like receptors (TLRs) recognize molecules specific to pathogens and endogenous danger signals. Binding of agonists to the ectodomain of the receptor initiates TLR activation and is followed by the association of receptor cytosolic Toll/Interleukin-1 receptor (TIR) domains with TIR domains of adapter proteins leading to the assembly of signaling cascade of protein kinases that ultimately trigger the activation of transcription factors and expression of genes involved in the immune response. Excessive activation of TIR-domain mediated signaling has been implicated in inflammatory diseases (e.g. rheumatoid arthritis, systemic lupus erythematosus, colitis) as well as in the development of cancer. Targeting receptor-adapter interactions represents a potential strategy for the therapeutic TLR/IL-1R-specific inhibition due to the unique interacting domains involved. Peptide and protein-domain binding TLR inhibitors originating from the interacting surfaces of TIR-domain containing proteins can bind to the site on their target interacting protein thereby preventing the assembly of the functional signaling complex. Here we review protein-domain, peptide and peptidomimetic inhibitors targeting TIR-domain mediated interactions and their application demonstrated on in vitro and in vivo models. Recent structural data and elucidation of the molecular mechanisms of TIR-domain mediated signaling enabled the development of peptide inhibitors from TIR domains of TLRs and adapters, MyD88 intermediary domain as well as improved protein inhibitors based on TIR domain dimerization, mimicking bacterial TIR-domain containing immunosuppressors (TCPs) which we discuss with challenges concerning the delivery and specificity of inhibitors targeting TLR adapters.

Toll-like Receptors in the Vascular System: Sensing the Dangers Within

“…The cytosolic TIR domain mediates TLR signaling and is the common structural feature between TLRs and TLR adapter proteins . In the early stages of TLR signaling, there are multiple interactions of TIR domains of TLRs and their adapters that mediate adapter recruitment, assembly, and stabilization of TLR signaling complex (Fekonja et al, 2012a). Interruption of these interactions has been accomplished using decoy peptides derived from TIR domains, TIRAP/MAL, and TRAM and resulted in inhibition of TLR signaling in vitro and in vivo (Jeyaseelan et al, 2005;Couture et al, 2012;Piao et al, 2013b).…”

“…Intense interest in the BB loop of TIR domain has guided research efforts in the development of inhibitory peptides (Toshchakov et al, 2007(Toshchakov et al, , 2011Piao et al, 2013a). The BB loop peptides, however, lack specificity for a specific TLR or TLR-adapter complex because they bind to proteins containing TIR domains with variable affinity (Toshchakov et al, 2007(Toshchakov et al, , 2011Fekonja et al, 2012a;Piao et al, 2013a). Other inhibitory peptides including INT peptides (derived from the N terminus of the intermediary domain of MyD88) and VIPER peptides (derived from vaccinia virus protein A46) have been also used.…”

“…Peptidomimetics have been developed with a goal to increase stability, internalization, pharmacological properties, and receptor sensitivity (Fekonja et al, 2012a). The BB loop of TIR domain was also the starting point for the development of these peptidomimetics.…”

“…To counteract this problem, protein inhibitors with greater inhibitory potency have been used. Mutated forms of TLR adapters have been shown to inhibit TLR signaling as dominant negative (dn) mutants (Fekonja et al, 2012a). A dominant-negative form of MyD88 was used in cultures of cells isolated from atherosclerotic plaques and reduced the production of cytokines and inflammatory mediators (Monaco et al, 2009).…”

Contributions of Unique Intracellular Domains to Switchlike Biosensing by Toll-like Receptor 4

“…Consequently, modulation of TLR signaling is a promising therapeutic strategy (8,9). However, despite substantial progress in the identification of intracellular mediators of TLR signaling (10) and the structural solution of several TLR-ligand complexes (11), the mechanisms by which the initiation of signaling is regulated in the absence of ligand and promoted upon formation of a signaling complex remain poorly understood (4).…”