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The Nod-like receptor protein 3 (NLRP3) inflammasome is activated by stimuli that induce perturbations in cell homeostasis, which commonly converge on cellular potassium efflux. NLRP3 has thus emerged as a sensor for ionic flux. Here, we identify forchlorfenuron (FCF) as an inflammasome activator that triggers NLRP3 signaling independently of potassium efflux. FCF triggers the rearrangement of septins, key cytoskeletal proteins that regulate mitochondrial function. We report that FCF triggered the rearrangement of SEPT2 into tubular aggregates and stimulated SEPT2-independent NLRP3 inflammasome signaling. Similar to imiquimod, FCF induced the collapse of the mitochondrial membrane potential and mitochondrial respiration. FCF thereby joins the imidazoquinolines as a structurally distinct class of molecules that triggers NLRP3 inflammasome signaling independent of potassium efflux, likely by inducing mitochondrial damage. [Display omitted] • Forchlorfenuron (FCF) activates the inflammasome in human and murine macrophages • FCF triggers the canonical NLRP3 inflammasome independently of K+ efflux • Septin-2 is dramatically rearranged by FCF but is dispensable for NLRP3 activation • FCF disrupts the mitochondrial network and ablates mitochondrial respiration NLRP3 is widely regarded as a sensor for ionic flux because most NLRP3 activators do so via potassium efflux. Holley, Emming et al. report that the septin modifier FCF induces septin-2 rearrangement and NLRP3 signaling. FCF joins the imidazoquinolines as small-molecule potassium-independent NLRP3 activators that induce mitochondrial collapse. [ABSTRACT FROM AUTHOR]

The Nod-like Receptor (NLR) apoptosis inhibitory proteins (NAIPs) are cytosolic receptors that sense cytosolic bacterial proteins. NAIP ligation induces its association with NLRC4, leading to the assembly of the NAIP/NLRC4 inflammasome, which induces the activation of the caspase-1 protease. Caspase-1 then cleaves pro-interleukin (IL)-1β, pro-IL-18, and gasdermin D and induces a form of pro-inflammatory cell death, pyroptosis. These processes culminate in host defense against bacterial infection. Here we describe methods for activating NAIP/NLRC4 inflammasome signalling in human and murine macrophages and quantifying inflammasome-induced cell death., (© 2023. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

The globally significant human pathogen group A Streptococcus (GAS) sequesters the host protease plasmin to the cell surface during invasive disease initiation. Recent evidence has shown that localized plasmin activity prevents opsonization of several bacterial species by key components of the innate immune system in vitro. Here we demonstrate that plasmin at the GAS cell surface resulted in degradation of complement factor C3b, and that plasminogen acquisition is associated with a decrease in C3b opsonization and neutrophil-mediated killing in vitro. Furthermore, the ability to acquire cell surface plasmin(ogen) correlates directly with a decrease in C3b opsonization, neutrophil phagocytosis, and increased bacterial survival in a humanized plasminogen mouse model of infection. These findings demonstrate that localized plasmin(ogen) plays an important role in facilitating GAS escape from the host innate immune response and increases bacterial virulence in the early stages of infection., (Copyright © 2013 S. Karger AG, Basel.)