Abstract
Inflammatory responses play a key role in many neural pathologies, with localized signaling from the non-immune cells making critical contributions. The NLRP3 inflammasome is an important component of innate immune signaling and can link neural insult to chronic inflammation. The NLRP3 inflammasome requires two stages to contribute: priming and activation. The priming stage involves upregulation of inflammasome components while the activation stage results in the assembly and activation of the inflammasome complex. The priming step can be rate limiting and can connect insult to chronic inflammation, but our knowledge of the signals that regulate NLRP3 inflammasome priming in sterile inflammation is limited. This study examined the link between mechanical strain and inflammasome priming in neural systems. Transient non-ischemic elevation of intraocular pressure increased mRNA for inflammasome components IL-1β, NLRP3, ASC, and CASP1 in rat and mouse retinas. The elevation was greater 1 day after the insult, with the rise in IL-1β most pronounced. The P2X7 receptor was implicated in the mechanosensitive priming of IL-1β mRNA in vivo, as the antagonist Brilliant Blue G (BBG) blocked the increased expression, the agonist BzATP mimicked the pressure-dependent rise in IL-1β, and the rise was absent in P2X7 knockout mice. In vitro measurements from optic nerve head astrocytes demonstrated an increased expression of IL-1β following stretch or swelling. This increase in IL-1β was eliminated by degradation of extracellular ATP with apyrase, or by the block of pannexin hemichannels with carbenoxolone, probenecid, or 10panx1 peptide. The rise in IL-1β expression was also blocked by P2X7 receptor antagonists BBG, A839977 or A740003. The rise in IL-1β was prevented by blocking transcription factor NFκB with Bay 11-7082, while the swelling-dependent fall in NFκB inhibitor IκB-α was reduced by A839977 and in P2X7 knockout mice. In summary, mechanical trauma to the retina primed NLRP3 inflammasome components, but only if there was ATP release through pannexin hemichannels, and autostimulation of the P2X7 receptor. As the P2X7 receptor can also trigger stage two of inflammasome assembly and activation, the P2X7 receptor may have a central role in linking mechanical strain to neuroinflammation.
Highlights
Mechanical trauma can induce complex pathological changes to neural tissue via inflammation (Corps et al, 2015; Heppner et al, 2015)
RNA was extracted from the retina of treated and contralateral control eyes soon after return of intraocular pressure (IOP) to baseline, and Quantitative Polymerase Chain Reaction (qPCR) was used to compare expression of genes associated with the NLRP3 inflammasome
There was a significant elevation in the expression of mRNA for IL-1β, NLRP3, Interleukin-1 converting enzyme/caspase1 gene (CASP1) and Apoptosis-Associated Speck-Like Protein Containing CARD (ASC), but not in expression of cytokine IL-18 (Figure 1A)
Summary
Mechanical trauma can induce complex pathological changes to neural tissue via inflammation (Corps et al, 2015; Heppner et al, 2015). In the first stage, referred to as the priming step, expression of inflammasome components such as pro-IL-1β and NLRP3 is increased at the transcriptional and translational level (Mariathasan et al, 2006; Patel et al, 2017). Inflammasome components are assembled and activated, turning on caspase 1 which subsequently catalyzes the maturation of cytokines IL-1β and IL-18 (Stutz et al, 2009) This later step has been linked to efflux of K+ through the P2X7 purinergic receptor (Mariathasan et al, 2006; Petrilli et al, 2007; Bernier, 2012; Karmakar et al, 2016), even for activation associated with lysosomal rupture (Muñoz-Planillo et al, 2013), and can be mimicked by the K+ ionophore nigericin (Perregaux and Gabel, 1994). Standard models attribute priming to microbial molecules or other toll-like receptor agonists that are rarely detected in sterile neural environments
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