Characterization of bbtTICAM from amphioxus suggests the emergence of a MyD88-independent pathway in basal chordates

Abstract

The MyD88-independent pathway, one of the two crucial TLR signaling routes, is thought to be a vertebrate innovation. However, a novel Toll/interleukin-1 receptor (TIR) adaptor, designated bbtTICAM, which was identified in the basal chordate amphioxus, links this pathway to invertebrates. The protein architecture of bbtTICAM is similar to that of vertebrate TICAM1 (TIR-containing adaptor molecule-1, also known as TRIF), while phylogenetic analysis based on the TIR domain indicated that bbtTICAM is the oldest ortholog of vertebrate TICAM1 and TICAM2 (TIR-containing adaptor molecule-2, also known as TRAM). Similar to human TICAM1, bbtTICAM activates NF-κB in a MyD88-independent manner by interacting with receptor interacting protein (RIP) via its RHIM motif. Such activation requires bbtTICAM to form homodimers in endosomes, and it may be negatively regulated by amphioxus SARM (sterile α and armadillo motif-containing protein) and TRAF2. However, bbtTICAM did not induce the production of type I interferon. Thus, our study not only presents the ancestral features of vertebrate TICAM1 and TI-CAM2, but also reveals the evolutionary origin of the MyD88-independent pathway from basal chordates, which will aid in understanding the development of the vertebrate TLR network.

The Archaic Roles of the Amphioxus NF-κB/IκB Complex in Innate Immune Responses

“…6C, 6E). Because the TLR signaling pathway have been well characterized in amphioxus, the bbtIkB protein was also coexpressed with amphioxus MyD88 or TICAM (31,32), and the activation of NF-kB mediated by bbtMyD88 or bbtTICAM was inhibited by bbtIkB in a dose-dependent manner (Fig. 6F).…”

“…In our previous studies of amphioxus TLRs and related signaling molecules, we provided compelling evidence for a functional intracellular TLR-NF-kB signaling cascade (25,31,32,34). Amphioxus expresses $48 TLRs and .40 TIR-containing adaptors, which suggests an expanded repertoire of adaptors and intermediate signal transducers in its TLR signaling pathways.…”

Novel Toll/IL-1 Receptor Homologous Region Adaptors Act as Negative Regulators in Amphioxus TLR Signaling

“…2C-E). Because MyD88-dependent and TICAM-dependent pathways have been demonstrated in amphioxus (18,19), we further used luciferase assays to test whether bbtTIRA, bbtTIRB, and bbtTIRC are involved in amphioxus MyD88-and TICAM-dependent signaling. Results showed that bbtTIRA specifically inhibited the bbtTICAM-dependent activation of NF-kB (Fig.…”

“…Amphioxus MyD88 was shown to mediate the activation of NFkB through its middle and death domains, whereas TICAM was shown to mediate the most primitive TRIF-dependent activation of NF-kB (18)(19)(20). Amphioxus SARM plays inhibitory roles in both MyD88-and TICAM-dependent pathways by interacting with MyD88, TRAF6, and TICAM (19,21). Recently, ubiquitination was shown to be essential in regulating the activation of amphioxus NF-kB (22,23).…”

“…Among these adaptors, amphioxus MyD88, TRAM-like [also known as amphioxus TIR domain-containing adaptor molecule (TICAM)], and SARM have been well studied. Amphioxus MyD88 was shown to mediate the activation of NFkB through its middle and death domains, whereas TICAM was shown to mediate the most primitive TRIF-dependent activation of NF-kB (18)(19)(20). Amphioxus SARM plays inhibitory roles in both MyD88-and TICAM-dependent pathways by interacting with MyD88, TRAF6, and TICAM (19,21).…”