Spatiotemporal mobilization of Toll/IL-1 receptor domain-containing adaptor molecule-1 in response to dsRNA

TLR3 recognizes viral dsRNA and induces antiviral immune responses. TLR3-mediated cell activation relies on Toll/IL-1R (TIR) domain-containing adaptor molecule-1 (TICAM-1, also named TIR domain-containing adaptor inducing IFN-beta or TRIF), which recruits downstream signaling molecules to activate the transcription factors IFN regulatory factor 3 (IRF-3) and NF-kappaB. The mechanisms by which TICAM-1 is activated and transmits signals remain largely unknown. In this study we show that TICAM-1 alters its distribution profile from a diffuse cytoplasmic form to a speckle-like structure in response to dsRNA. The receptor-interacting protein 1 (RIP1), a crucial signaling molecule for TICAM-1-mediated NF-kappaB activation, accumulated in the TICAM-1 speckles. In addition, NF-kappaB-activating kinase-associated protein 1 (NAP1), a downstream molecule linking TICAM-1 and the IRF-3-activating kinase TBK1 (TANK-binding kinase 1), was also recruited to the TICAM-1 speckles. Notably, a transient colocalization of TICAM-1 and TLR3 was observed before the extensive formation of the TICAM-1 speckles. Thus, the spatiotemporal mobilization of TICAM-1 in response to dsRNA and the formation of the TICAM-1 speckles containing RIP1 and NAP1 are important for the activation of the TLR3-TICAM-1 pathway.