Listeria monocytogenes and Shigella flexneri Activate the NLRP1B Inflammasome

Abstract

Activation of the innate immune receptor NLRP1B leads to the formation of an inflammasome, which induces autoproteolytic processing of pro-caspase-1, and ultimately to the release of inflammatory cytokines and to the execution of pyroptosis. One of the signals to which NLRP1B responds is metabolic stress that occurs in cells deprived of glucose or treated with metabolic inhibitors. NLRP1B might therefore sense microbial infection, as intracellular pathogens such as Listeria monocytogenes and Shigella flexneri cause metabolic stress as a result of nutrient scavenging and host cell damage. Here we addressed whether these pathogens activate the NLRP1B inflammasome. We found that Listeria infection activated the NLRP1B inflammasome in a reconstituted fibroblast model. Activation of NLRP1B by Listeria was diminished in an NLRP1B mutant shown previously to be defective at detecting energy stress and was dependent on the expression of listeriolysin O (LLO), a protein required for vacuolar escape. Infections of either Listeria or Shigella activated NLRP1B in the RAW264.7 murine macrophage line, which expresses endogenous NLRP1B. We conclude that NLRP1B senses cellular infection by distinct invasive pathogens.KEYWORDS Listeria monocytogenes, NLRP1B, Shigella, caspase-1, inflammasome N LRP1B is a pattern recognition receptor that forms a multiprotein complex termed an inflammasome after it detects an activating signal (1). The NLRP1B inflammasome complex consists of multiple copies of NLRP1B and pro-caspase-1. The assembly of the complex leads to autoproteolysis of pro-caspase-1 and, consequently, to processing of inflammatory cytokines interleukin-1␤ (IL-1␤) and IL-18 and to a type of cell death called pyroptosis (2, 3). NLRP1B has four domains (2) (Fig. 1A). The N-terminal NACHT domain (a domain present in NAIP, CIITA, HET-E, and TP-1) is a nucleotide-binding domain that selfassociates. The leucine-rich repeat (LRR) domain is involved in autoinhibition (4, 5), and the function to find domain (FIIND) undergoes autoproteolytic processing, which facilitates inflammasome assembly (6-8). The C-terminal caspase-activating and recruitment domain (CARD) binds the CARD of pro-caspase-1 (4).Anthrax lethal toxin is the only known direct activator of murine NLRP1B (1). The proteolytic component of the toxin cleaves NLRP1B near its N terminus; this cleavage is sufficient to relieve autoinhibition and allow oligomerization (9-11). Depletion of intracellular ATP is another activator of NLRP1B but one that probably triggers inflammasome assembly indirectly (12). The N-terminal region of NLRP1B is not cleaved after depletion of ATP, and the FIIND of NLRP1B facilitated the detection of this signal instead (5). Thus, activation of NLRP1B occurs through at least two distinct mechanisms.The intracellular parasite Toxoplasma gondii is also detected by NLRP1B (13,14), although the direct signal has not been determined. It is possible that Toxoplasma infection causes a reduction in cytosolic ATP. Notably, the parasite cannot synthesiz...

Checks and Balances between Autophagy and Inflammasomes during Infection

“…In addition, bacterial DNA released upon cytosolic lysis of L. monocytogenes activates the AIM2 inflammasome [54,55]. More recently, L. monocytogenes was proposed to activate the NLRP1B inflammasome, via causing energy stress in fibroblasts and macrophages [56]. It is thought that L. monocytogenes maintains a low-level of inflammasome activation in order to establish a successful infection.…”

The NLRP1 and CARD8 inflammasomes

“…60 RAW 264.7 cells were derived from BALB/c mice, which express NLRP1B allele 1 and do not express NLRP1A. 16,61 Okondo et al 60 15 Surprisingly, CARD8, and not hNLRP1, was found to mediate VbP-induced pyroptosis in these cells, 15 identifying CARD8 for the first time as an inflammasome-forming PRR.…”

“…Mogridge and co-workers, noting the link between infectious disease and host metabolism, investigated the relationship between energy stress and NLRP1B activation. 61,70,71 Liao et al 70 T gondii and DPP8/9 inhibitors, 2DG plus sodium azide activated NLRP1B without direct N-terminal cleavage. 70,71 Although the molecular details leading to NLRP1B activation remain largely unknown, the authors speculated that 2DG plus sodium azide activates NLRP1B via depletion of cytosolic ATP.…”

“…Recently, the Mogridge lab reported that high doses of 2DG, Listeria monocytogenes and S flexneri activate the NLRP1B inflammasome in RAW 264.7 cells. 61 Like 2DG, L monocytogenes and S flexneri reduced cytosolic ATP levels, and the authors thus speculated that these stimuli may all activate NLRP1B via ATP depletion.…”

Vying for the control of inflammasomes: The cytosolic frontier of enteric bacterial pathogen–host interactions

“…AIM2 expression requires type I IFN signalling, consistent with which, L. monocytogenes ‐induced inflammasome activation is severely reduced in type I IFN‐receptor deficient ( Ifnar −/− ) macrophages. L. monocytogenes is a poor activator of NLRC4 (Sauer et al, ; Warren et al, ; Sauer et al, ), but can activate NLRP6, caspase‐11 (Hara et al, ) and NLRP1B (Neiman‐Zenevich et al, ) in mouse macrophages, and NLRP3 in mouse bone‐marrow derived DCs (Clark, Schmidt, McDermott, & Lenz, ). Most L. monocytogenes strains turn down flagellin expression at 37°C and thus evade detection by TLR5 and the NAIP5‐NLRC4 pathway (Theisen & Sauer, ).…”