Cellular and Biochemical Events in Toll-like Receptor Signaling

Abstract

In multicellular organisms, communication between cells relies on transmitting information across membrane barriers. Different cell types interrogate particular aspects of their surrounding environment through protein receptors that span membranes and upon ligand binding, trigger enzymatic signaling cascades that culminate in the activation of one or more transcription factors. Information transmission is bidirectional, as individual cells must be able to sense unique aspects of their surroundings, relay their specialized knowledge with others, and receive the collective knowledge of surrounding cells and tissues. This two-way communication is particularly important in the innate immune system, where potentially infectious organisms must be readily detected and identified, and their presence communicated to other cells in the vicinity. Because of the rapid generation time of microorganisms, delays between any of these steps - detection, information processing or information transmission - can make the difference between successful control of infection and pathogen outgrowth. For this reason, the receptors that identify potential pathogens must be able to detect pathogens wherever they are found, be exquisitely sensitive, and initiate a robust response. At the same time, the inflammatory response to infection is itself damaging. This requires that the same receptors are tightly controlled, both by modulating their sensitivity and by rapidly turning off responses through negative feedback pathways. Here, I show that the toll/interleukin-1 receptor domain-containing adaptor protein (TIRAP) plays a critical role in controlling the sensitivity of toll-like receptor (TLR) signaling. First, TIRAP controls the assembly of the myddosome, a protein complex that activates signal transduction, from both the plasma membrane and within endosomes of macrophages. Though TIRAP's role at the cell surface was previously described, its endosomal function was previously unknown. Second, TIRAP is an important target for negative regulation. After stimulation with the TLR4 ligand lipopolysaccharide (LPS), macrophages induce a state known as endotoxin tolerance, in which they are refractory for additional LPS stimulation. Many mechanisms for endotoxin tolerance have been proposed, but here I show that TIRAP is degraded in endotoxin tolerance, and that the mechanism of TIRAP degradation also has implications for viral/bacterial superinfection.