A Twice Tolled Tale

Science's STKE  05 Aug 2003:
Vol. 2003, Issue 194, pp. tw303-TW303
DOI: 10.1126/stke.2003.194.tw303

Toll-like receptors (TLRs) function in innate immune responses; mammalian TLRs recognize conserved microbial patterns. However, the mechanisms whereby TLRs are activated remain unclear. In Drosophila, activation of the related Toll receptor, which is involved in development as well as in the innate immune response, depends on proteolytic cleavage of the protein Spätzle. Although Spätzle is widely believed to serve as a ligand for Toll, a direct interaction between the two proteins has never been shown (see Underhill). Weber et al. injected recombinant Spätzle proprotein or a proteolytic C-terminal fragment, C-106, into Drosophila and showed that C-106 activated a Toll pathway target gene. C-106 also activated a Toll pathway gene reporter expressed in a Drosophila cell line; activation depended on Toll expression and was inhibited by the Toll ectodomain. Injection of the proprotein into a Drosophila mutant that lacked Spätzle rescued the innate immune response. Scatchard analysis indicated that C-106 bound to a specific receptor, and expression of Toll in Cos-7 cells led to C-106 binding. Moreover, C-106 activated nuclear factor κB (a mammalian target of TLR signaling) in HEK 293 cells expressing a chimeric receptor that contained the Toll ectodomain and a TLR intracytoplasmic domain, which indicated that C-106 binding to Toll was sufficient to initiate signaling. In vitro analysis of binding of the purified proteins indicated that C-106 bound to the Toll ectodomain with a stoichiometry of one C-106 dimer to two Toll receptors. Thus Toll activation appears to involve dimerization, and Toll, unlike the mammalian TLRs, can be activated solely through an endogenous ligand.

A. N. R. Weber, S. Tauszig-Delamasure, J. A. Hoffmann, E. Lelièvre, H. Gascan, K. P. Ray, M. A. Morse, J.-L. Imler, N. J. Gay, Binding of the Drosophila cytokine Spätzle to Toll is direct and establishes signaling. Nat. Immunol. 4, 794-800 (2003). [Online Journal]

D. Underhill, Toll gets tied in a knot. Nat. Immunol. 4, 723-724 (2003). [Online Journal]