(52e) TLR4 Signals Via a Novel Allosteric Switch-Like Mechanism

Toll-like Receptor 4 (TLR4) signaling is important for activation of the innate immune system in response to pathogenic bacteria. TLR4 can respond to very low levels of endotoxin (lipopolysaccharide, LPS) providing early warning of an infection. However, improper or excessive activation of TLR4 is associated with diseases such as Chron’s disease, some types of allergy, and in acute scenarios, sepsis and death. In Chron’s disease and allergy, TLR4 is activated by self-derived ligands, leading to inflammation that can damage tissues and organs. In sepsis, systemic infection results in over-activation of TLR4 causing severe damage, organ failure, and in severe cases death. For all these reasons, understanding how TLR4 signals at a mechanistic level is essential for developing therapeutics that modulate its activity in a safe and effective fashion. However, several key aspects of TLR4 signaling remain poorly understood.

The current dominant viewpoint is that signaling through TLR4 is initiated via ligand-induced dimerization. However, TLR4 posses a unique hydrophobic region adjacent to the transmembrane domain in the cytosol, which results in oligomerization in the absence of ligand, challenging this viewpoint. We have interrogated this receptor mechanism using a combination of protein engineering, synthetic biology, and computational modeling techniques. Based on these findings, we now propose that TLR4 signaling is induced via a novel “mousetrap”-like mechanism, whereby in the absence of ligand, TLR4 is dimerized but held in a conformation that precludes signaling. Upon ligand binding, this constraint is alleviated and the receptor is able to induce signaling. This type of signaling mechanism could play an important role in the ability of TLR4 to respond quickly to very low concentrations of LPS. Understanding this mechanism could also help to engineer sensitive protein-based switches for applications in which detection of the presence of a ligand at any concentration (i.e., in a binary fashion) is most important. Moreover, elucidating this novel relationships between receptor structure and function will enable targeted development of therapeutics that modulate TLR4 signaling.