Differential Requirement of P2X7 Receptor and Intracellular K+ for Caspase-1 Activation Induced by Intracellular and Extracellular Bacteria

Case Western Reserve University, Cleveland, Ohio, United States
Journal of Biological Chemistry (Impact Factor: 4.57). 07/2007; 282(26):18810-8. DOI: 10.1074/jbc.M610762200
Source: PubMed


Interleukin-1β (IL-1β) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases.
The maturation and secretion of IL-1β are mediated by caspase-1, a protease that processes pro-IL-1β into biologically active
IL-1β. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the
adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration
of K+ potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and
intracellular K+ in IL-1β secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists
such as lipopolysaccharide or infection with extracellular bacteria Staphylococcus aureus and Escherichia coli, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through
depletion of intracellular K+ caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing
of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K+. Consistently, caspase-1 activation induced by intracellular Salmonella or Listeria was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like
receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally
in the absence of P2X7 receptor-mediated cytoplasmic K+ perturbations.

    • "Similarly, deletion of RNase L reduced IL-1b induction by 70% after priming with TNF-a followed by VSV infection (Figure 2B). ATP activates the NLRP3 inflammasome through the P2X7 ion channel, leading to potassium efflux (Franchi et al., 2007). RNase L had no effect on IL-1b induction by LPS and ATP, suggesting that RNase L is not a general regulator of inflammasome activation (Figure 2C). "
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    ABSTRACT: The NLRP3 inflammasome assembles in response to danger signals, triggering self-cleavage of procaspase-1 and production of the proinflammatory cytokine IL-1β. Although virus infection activates the NLRP3 inflammasome, the underlying events remain incompletely understood. We report that virus activation of the NLRP3 inflammasome involves the 2',5'-oligoadenylate (2-5A) synthetase(OAS)/RNase L system, a component of the interferon-induced antiviral response that senses double-stranded RNA and activates endoribonuclease RNase L to cleave viral and cellular RNAs. The absence of RNase L reduces IL-1β production in influenza A virus-infected mice. RNA cleavage products generated by RNase L enhance IL-1β production but require the presence of 2',3'-cyclic phosphorylated termini characteristic of RNase L activity. Additionally, these cleavage products stimulate NLRP3 complex formation with the DExD/H-box helicase, DHX33, and mitochondrial adaptor protein, MAVS, which are each required for effective NLRP3 inflammasome activation. Thus, RNA cleavage events catalyzed by RNase L are required for optimal inflammasome activation during viral infections. Copyright © 2015 Elsevier Inc. All rights reserved.
    No preview · Article · Mar 2015 · Cell host & microbe
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    • "In some cases, it seems as though ASC can form a very large homo-multimeric complex of its own leading to cell death. Normally, oligimerization is inhibited by a high intracellular potassium concentration, which explains why P2X7 potassium channel activation or membrane penetrating toxins cause activity (Franchi et al., 2007). ASC is able to recruit NFκB and induce the expression of IL-8 (Hasegawa et al., 2005), likely through a second messenger. "
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    ABSTRACT: Gardnerella vaginalis is a Gram-positive bacterium associated with bacterial vaginosis (BV), pelvic inflammatory disease, and preterm birth. BV is the most prevalent vaginal infection in women, characterized by the absence of normal Lactobacilli and overgrowth of G. vaginalis and other bacterial species. This study tested the hypothesis that G. vaginalis induces an inflammatory response in the human cell line, THP-1. The objectives of the study were to 1) determine whether different strains of G. vaginalis cause proinflammatory cytokinesproduction in THP-1 cells, 2) characterize intracellular pathways by which these cytokines are produced, and 3) determine molecular mechanisms involved in death of cells treated with strains of G. vaginalis. In these studies, G. vaginalis strain 14018 induced statisticaliy significant increases in the inflammasome-dependent cytokines IL-1β, IL-18, as well as TNF-α in THP-1 monocytes. This same strain of G. vaginalis also caused statistically significant cell death in THP-1 monocytes and cleavage of caspase-1 by 24 h following treatment. Knockdown of the inflammasome component, NLRP3, in THP-1 cells reduced secretion of IL-1β. Additionally, THP-1 cells stably expressing ASC with a fluorescent tag exhibited colocalization of NLRP3 with ASC in G. vaginalis-treated THP-1 cells. These studies confirmed the role of the NLRP3 inflammasome in G. vaginalis inflammation. In a strain-specific study, a statistically significant increase in THP-1 monocyte differentiation and IL-1β secretion were detected in response to G. vaginalis strains 14018 and 49145 but not strain 14019. Cytokine and inflammasome responses were similar for strains 14018 and 49145, but strain 14019 did not induce an inflammatory response in THP-1 cells. The strain-specific ability of G. vaginalis to induce inflammation could con- tribute to the variability observed clinically between women colonized with G. vaginalis. The deleterious effects of G. vaginalis observed in THP-1 monocytes were not observed for the human trophoblast cell line HTR8 when treated with any of the G. vaginalis strains. The results of these studies increase the understanding of how G. vaginal- is activates the innate immune system and suggests that a strain-dependent activation of inflammation may be involved in bacterial vaginosis and preterm birth.
    Full-text · Thesis · Dec 2014
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    • "An important difference between activation of inflammasomes within the CNS and in peripheral tissues is that high extracellular potassium activates CNS inflammasomes (Silverman et al., 2009), whereas potassium efflux is responsible for activating inflammasomes in peripheral tissues (Franchi et al., 2007). In neurons and astrocytes high extracellular potassium opens the pannexin-1 channel allowing efflux of ATP (Franchi et al., 2007). The NLRP1 inflammasome forms protein–protein interactions with the purinergic receptor P2X7 and with pannexin-1 (Silverman et al., 2009). "
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    ABSTRACT: Pattern recognition receptors (PRRs) are part of the innate immune response and were originally discovered for their role in recognizing pathogens by ligating specific pathogen associated molecular patterns (PAMPs) expressed by microbes. Now the role of PRRs in sterile inflammation is also appreciated, responding to endogenous stimuli referred to as “damage associated molecular patterns” (DAMPs) instead of PAMPs. The main families of PRRs include Toll-like receptors (TLRs), Nod-like receptors (NLRs), RIG-like receptors (RLRs), AIM2-like receptors (ALRs), and C-type lectin receptors. Broad expression of these PRRs in the CNS and the release of DAMPs in and around sites of injury suggest an important role for these receptor families in mediating post-injury inflammation. Considerable data now show that PRRs are among the first responders to CNS injury and activation of these receptors on microglia, neurons, and astrocytes triggers an innate immune response in the brain and spinal cord. Here we discuss how the various PRR families are activated and can influence injury and repair processes following CNS injury.
    Full-text · Article · Aug 2014 · Experimental Neurology
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