Effects of Differences in Lipid A Structure on TLR4 Pro-Inflammatory Signaling and Inflammasome Activation

Abstract

The vertebrate immune system exists in equilibrium with the microbial world. The innate immune system recognizes pathogen-associated molecular patterns via a family of Toll-like receptors (TLR) that activate cells upon detection of potential pathogens. Because some microbes benefit their hosts, mobilizing the appropriate response, and then controlling that response is critical in the maintenance of health. TLR4 recognizes the various forms of lipid A produced by Gram-negative bacteria. Depending on the structural form of the eliciting lipid A molecule, TLR4 responses range from a highly inflammatory endotoxic response involving inflammasome and other pro-inflammatory mediators, to an inhibitory, protective response. Mounting the correct response against an offending microbe is key to maintaining health when exposed to various bacterial species. Further study of lipid A variants may pave the way to understanding how TLR4 responses are generally able to avoid chronic inflammatory damage.

Keywords: LPS; NLRP3; TLR4; inflammasome; monophosphoryl lipid A.

Figure 1

Increased NLRP3 expression via MyD88 leads to inflammasome formation and processing of IL-1β after lipid A treatment. Priming (1–3) and activation (4–7) are required for mature IL-1β secretion. (1) TLR4/MD2 binds lipid A forming endotoxin receptor dimers that signal through either MyD88 or TRIF, or both (2) to activate NF-κB. The MyD88-dependent “early” phase of NF-κB activity results in increased transcription of both nlrp3 and il1b genes. TRIF-dependent TLR4 signaling occurs after receptor-mediated endocytosis of TLR4 leading to the “late” phase of NF-κB activity, and increases il1b gene expression, but not nlrp3. Inflammasome activation occurs in response to such agonists as extracellular ATP which is sensed by the P2X₇R and leads to K⁺ loss through the pannexin-1 ion channel (4). If TLR4 signaling increases NLRP3 protein levels (5a), such as with MyD88-competent LPS or DPLA, then the activation event permits the newly expressed NLRP3 to aggregate with soluble ASC protein and pro-caspase-1, leading to caspase-1 processing to its active form and inflammasome assembly (6a). Pro-IL-1β that is associated with the inflammasome is processed to mature IL-1β and secreted into the extracellular space (7a). However, when TLR4 priming does not adequately increase NLRP3 expression (5b), as is the case with MPLA treatment, then there is little to no NLRP3 protein to direct inflammasome assembly, pro-caspase-1 does not get efficiently activated (6b), and any IL-1β that is secreted is generally in the inactive pro-IL-1β form (7b).