TLR4 Signalling

R e s e a r ch highlights

innate immunity TLR4 signalling

Toll-like receptor 4 (TLR4) is unique plasma membrane. Now, a study from sufficient to allow TRAM to localize among TLRs in its ability to activate Ruslan Medzhitov’s laboratory shows specifically to endosomes. These 20 two distinct signalling pathways that the two signalling pathways amino acids constitute a bipartite — one pathway is activated by the are induced sequentially and that localization domain — consisting of adaptors TIRAP (Toll/interleukin-1- the TRAM–TRIF pathway is only a myristoylation motif (the first 7 receptor (TIR)-domain-containing operational from early endosomes amino acids) and a polybasic domain adaptor protein) and MyD88, following endocytosis of TLR4. — that is commonly found in other which leads to the induction of The authors found it puzzling proteins that shuttle between the pro‑inflammatory cytokines, and that TLR4 is the only known TLR plasma membrane and endosomes. the second pathway is activated by capable of inducing the production Mutational analysis showed that the the adaptors TRIF (TIR-domain- of type I interferons from the plasma myristoylation motif is necessary containing adaptor protein inducing membrane so they decided to take for endosomal localization but interferon‑β) and TRAM (TRIF- a closer look at TLR4 signalling. both parts of the bipartite motif related adaptor molecule), which First, they assessed the subcellular are required for plasma-membrane leads to the induction of type I inter- localization of tagged TLR4 and found targeting. A TRAM transgene of ferons. Until now, it had been believed that it localized to both the plasma which the protein product resided that these two signalling pathways membrane and endosomal vesicles. specifically on endosomes retained were activated simultaneously at the They found that lipopolysaccharide- the ability to signal the production of induced TLR4 internalization was type I interferons. disrupted in the presence of a specific This study supports a model inhibitor of dynamin — a GTPase whereby ligand engagement of TLR4 that regulates the endocytosis of at the plasma membrane induces the ‘pinched-off’ invaginations of the TIRAP–MyD88 pathway. TLR4 is plasma membrane. Disruption of then endocytosed into endosomes to endocytosis abolished the TRAM– engage the TRAM–TRIF pathway. TRIF-dependent phosphorylation of These results also confirm that interferon-regulatory factor 3 (IRF3), TRAM, similar to TIRAP, is a ‘sorting’ a component of the pathway that leads adaptor that defines the location of to the production of type I interferons, subcellular TLR signalling, and that whereas TIRAP–MyD88-dependent receptors that induce the production signalling proceeded normally. of type I interferons all signal from Next, a deletional analysis of intracellular compartments. TRAM showed that the first 20 amino Elaine Bell

acids constituted the minimum signal ORIGINAL RESEARCH PAPER Kagan, J. C. et al. for TRAM localization to both the TRAM couples endocytosis of Toll-like receptor 4 plasma membrane and endosomes, to the induction of interferon-β. Nature Immunol. 24 February 2008 (doi:10.1038/ni1569) but the first 7 amino acids were SUPERSTOCK