Abstract
NLRP3 and ASC are able to form a large multimeric complex called inflammasome in response to a number danger signals. The NLRP3 inflammasome is required for the activation of caspase-1 and subsequent maturation of pro-IL-1β into active IL-1β. Although the mechanisms regulating the formation and activity of NLRP3 inflammasome are yet not fully elucidated, data suggest that the assembly of NLRP3 inflammasome requires microtubules to induce the proximity of ASC and NLRP3. In this study we show that microfilaments (F-actin) inhibit NLRP3 inflammasome activity and interact with NLRP3 and ASC. We demonstrate that the inhibition depends on the actin polymerization state but not on the active polymerization process. In ATP- or nigericin-activated macrophages, our data further indicate that Flightless-I (FliI) and leucine-rich repeat FliI-interaction protein 2 (LRRFIP2) are required for the co-localization of NLRP3, ASC and F-actin. We also established that the ability of Ca2+ to accentuate the activity of NLRP3 inflammasome is abrogated in FliI- and LRRFIP2-knockdown macrophages, suggesting that Ca2+ signaling requires the presence of FliI and LRRFIP2. Accordingly, we observed that Ca2+/FliI-dependent severing of F-actin suppresses F-actin/FliI/LRRFIP2-dependent NLRP3 inflammasome inhibition leading to increase IL-1β production. Altogether, our results unveil a new function of F-actin in the regulation of NLRP3 inflammasome activity strengthening the importance of cytoskeleton in the regulation of inflammation.
Highlights
Inflammation fulfills important functions in host protection
While a caspase recruitment domain (ASC), NLRP3 and pro-IL-1βwere detected in the cytosolic fraction of THP-1 cells under steady-state conditions, THP-1 treatment with the inflammasome activators ATP or nigericin induced the translocation of ASC and NLRP3, but not of pro-IL-1β, into the cytoskeletal fraction (Fig. 1A,B), suggesting that the activation of NLRP3 inflammasome triggers its interaction with the cytoskeleton
These results indicate that the activation of NLRP3 inflammasome in THP-1 cells by ATP and nigericin initiates the interaction of NLRP3 and ASC with filamentous actin (F-actin)
Summary
Inflammation fulfills important functions in host protection. It participates to pathogen elimination and to the restoration of tissue homeostasis after injury. Inflammasomes are large multimeric complexes that activate caspase-1, a cystein protease responsible for the processing and subsequent secretion of IL-1βand IL-18, a closely related IL-1 family member[8,9,10]. Further data indicate that FliI is recruited to the NLRP3 inflammasome by leucine-rich repeat FliI-interaction protein 2 (LRRFIP2), an NLRP3-associated protein. (C,D) IL-1βproduction in culture supernatants of primed THP-1 cells pretreated with increasing doses of cytochalasin D (CytoD) or latrunculin B (LatB) and activated by ATP (C) and nigericin (D) for 6 h. We demonstrate that active NLRP3 inflammasome complex co-localizes and interacts with actin microfilaments and that this co-localization requires the presence of FliI and LRRFIP2. Besides controlling the intracellular co-localization of the NLRP3 inflammasome and F-actin, we further showed that FliI and LRRFIP2 repress caspase-1 activation, which could be restored through the severing of actin filaments in a Ca2+-dependent process
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