Targeting TLR4 Signaling by TLR4 TIR-derived Decoy Peptides: Identification of the TLR4 TIR Dimerization Interface

摘要

Agonist-induced dimerization of TLR4 TIR domains initiates intracellular signaling. Therefore, identification of the TLR4 TIR dimerization interface is one key to the rational design of therapeutics that block TLR4 signaling. A library of cell-permeating “decoy peptides,” each of which represents a non-fragmented patch of the TLR4 TIR surface, was designed such that the peptides entirely encompass the TLR4 TIR surface. Each peptide was synthesized in tandem with a cell-permeating Antennapedia homeodomain sequence and tested for the ability to inhibit early cytokine mRNA expression and MAPK activation in LPS-stimulated primary murine macrophages. Five peptides, 4R1, 4R3, 4BB, 4R9, and 4αE, potently inhibited all manifestations of TLR4, but not TLR2 signaling. When tested for their ability to bind directly to TLR4 TIR by FRET using time-resolved fluorescence spectroscopy, Bodipy-TMR-X (BTX)-labeled 4R1, 4BB, and 4αE quenched fluorescence of TLR4-Cerulean (Cer) expressed in HeLa or HEK293T cells, while 4R3 was partially active and 4R9 was least active. These findings suggest that the area between BB loop of TLR4 and its fifth helical region mediates TLR4 TIR dimerization. Moreover, our data provide direct evidence for the utility of the “decoy peptide approach,” in which peptides representing various surface-exposed segments of a protein are initially probed for the ability to inhibit protein function and then their specific targets are identified by FRET, to define recognition sites in signaling proteins that may be targeted therapeutically to disrupt functional transient protein interactions.