The microglial NLRP3 inflammasome is activated by amyotrophic lateral sclerosis proteins

<jats:title>Abstract</jats:title><jats:p>Microglial NLRP3 inflammasome activation is emerging as a key contributor to neuroinflammation during neurodegeneration. Pathogenic protein aggregates such as β‐amyloid and α‐synuclein trigger microglial NLRP3 activation, leading to caspase‐1 activation and IL‐1β secretion. Both caspase‐1 and IL‐1β contribute to disease progression in the mouse SOD1<jats:sup>G93A</jats:sup> model of amyotrophic lateral sclerosis (ALS), suggesting a role for microglial NLRP3. Prior studies, however, suggested SOD1<jats:sup>G93A</jats:sup> mice microglia do not express NLRP3, and SOD1<jats:sup>G93A</jats:sup> protein generated IL‐1β in microglia independent to NLRP3. Here, we demonstrate using <jats:italic>Nlrp3</jats:italic>‐GFP gene knock‐in mice that microglia express NLRP3 in SOD1<jats:sup>G93A</jats:sup> mice. We show that both aggregated and soluble SOD1<jats:sup>G93A</jats:sup> activates inflammasome in primary mouse microglia leading caspase‐1 and IL‐1β cleavage, ASC speck formation, and the secretion of IL‐1β in a dose‐ and time‐dependent manner. Importantly, SOD1<jats:sup>G93A</jats:sup> was unable to induce IL‐1β secretion from microglia deficient for <jats:italic>Nlrp3</jats:italic>, or pretreated with the specific NLRP3 inhibitor MCC950, confirming NLRP3 as the key inflammasome complex mediating SOD1‐induced microglial IL‐1β secretion. Microglial NLRP3 upregulation was also observed in the TDP‐43<jats:sup>Q331K</jats:sup> ALS mouse model, and TDP‐43 wild‐type and mutant proteins could also activate microglial inflammasomes in a NLRP3‐dependent manner. Mechanistically, we identified the generation of reactive oxygen species and ATP as key events required for SOD1<jats:sup>G93A</jats:sup>‐mediated NLRP3 activation. Taken together, our data demonstrate that ALS microglia express NLRP3, and that pathological ALS proteins activate the microglial NLRP3 inflammasome. NLRP3 inhibition may therefore be a potential therapeutic approach to arrest microglial neuroinflammation and ALS disease progression.</jats:p>