Gasdermin-D (GSDMD) in inflammasome-activated macrophages is cleaved by caspase-1 to generate N-GSDMD fragments. N-GSDMD then oligomerizes in the plasma membrane (PM) to form pores that increase membrane permeability, leading to pyroptosis and IL-1β release. In contrast, we report that although N-GSDMD is required for IL-1β secretion in NLRP3activated human and murine neutrophils, N-GSDMD does not localize to the PM or increase PM permeability or pyroptosis. Instead, biochemical and microscopy studies reveal that N-GSDMD in neutrophils predominantly associates with azurophilic granules and LC3 + autophagosomes. N-GSDMD trafficking to azurophilic granules causes leakage of neutrophil elastase into the cytosol, resulting in secondary cleavage of GSDMD to an alternatively cleaved N-GSDMD product. Genetic analyses using ATG7-deficient cells indicate that neutrophils secrete IL-1β via an autophagy-dependent mechanism. These findings reveal fundamental differences in GSDMD trafficking between neutrophils and macrophages that underlie neutrophil-specific functions during inflammasome activation.

Dysregulation of inflammatory cell death is a key driver of many inflammatory diseases. Pyroptosis, a highly inflammatory form of cell death, uses intracellularly generated pores to disrupt electrolyte homeostasis and execute cell death. Gasdermin D, the pore-forming effector protein of pyroptosis, coordinates membrane lysis and the release of highly inflammatory molecules, such as interleukin-1β, which potentiate the overactivation of the innate immune response. However, to date, there is no pharmacologic mechanism to disrupt pyroptosis. Here, we identify necrosulfonamide as a direct chemical inhibitor of gasdermin D, the pyroptotic pore-forming protein, which binds directly to gasdermin D to inhibit pyroptosis. Pharmacologic inhibition of pyroptotic cell death by necrosulfonamide is efficacious in sepsis models and suggests that gasdermin D inhibitors may be efficacious clinically in inflammatory diseases.

Canonical inflammasome activation induces a caspase-1/gasdermin D (Gsdmd) dependent lytic cell death called pyroptosis which promotes anti-microbial host defense but may contribute to sepsis. The nature of the caspase-1-dependent change in plasma membrane (PM) permeability during pyroptotic progression remains incompletely defined. We assayed propidium2+ (Pro2+) influx kinetics during NLRP3 or Pyrin inflammasome activation in murine bone marrow-derived macrophages (BMDM) as an indicator of this PM permeabilization. BMDM were characterized by rapid Pro2+ influx after initiation of NLRP3 or Pyrin inflammasomes by nigericin or C. difficile toxin B (TcdB), respectively. No Pro2+ uptake in response to nigericin or TcdB was observed in Caspase-1−/− or ASC−/− BMDM. The cytoprotectant glycine profoundly suppressed nigericin and TcdB-induced lysis but not Pro2+ influx. The absence of Gsdmd expression resulted in suppression of nigericin-stimulated Pro2+ influx and pyroptotic lysis. Extracellular La3+ and Gd3+ rapidly and reversibly blocked the induced Pro2+ influx and markedly delayed pyroptotic lysis without limiting upstream inflammasome assembly and caspase-1 activation. Thus, caspase-1 driven pyroptosis requires induction of initial pre-lytic pores in the PM that are dependent on Gsdmd expression. These PM pores also facilitated the efflux of cytosolic ATP and influx of extracellular Ca2+. Although lanthanides and Gsdmd deletion both suppressed PM pore activity and pyroptotic lysis, robust IL-1β release was observed in lanthanide-treated BMDM but not in Gsdmd-deficient cells. This suggests roles for Gsdmd in both passive IL-1β release secondary to pyroptotic lysis and in non-lytic/non-classical IL-1β export.

The inflammasomes are signaling platforms that promote the activation of inflammatory caspases such as caspases-1, -4, -5, and -11. Recent studies identified gasdermin D (GSDMD) as an effector for pyroptosis downstream of the inflammasome signaling pathways. Cleavage of GSDMD by inflammatory caspases allows its N-terminal domain to associate with membrane lipids and form pores that induce pyroptotic cell death. Despite the important role of GSDMD in pyroptosis, the molecular mechanisms of GSDMD recognition and cleavage by inflammatory caspases that trigger pyroptosis are poorly understood. Here, we demonstrate that the catalytic domains of inflammatory caspases can directly bind to both the full-length GSDMD and its cleavage site peptide, FLTD. A GSDMD-derived inhibitor, -acetyl-Phe-Leu-Thr-Asp-chloromethylketone (Ac-FLTD-CMK), inhibits GSDMD cleavage by caspases-1, -4, -5, and -11 in vitro, suppresses pyroptosis downstream of both canonical and noncanonical inflammasomes, as well as reduces IL-1β release following activation of the NLRP3 inflammasome in macrophages. By contrast, the inhibitor does not target caspase-3 or apoptotic cell death, suggesting that Ac-FLTD-CMK is a specific inhibitor for inflammatory caspases. Crystal structure of caspase-1 in complex with Ac-FLTD-CMK reveals extensive enzyme-inhibitor interactions involving both hydrogen bonds and hydrophobic contacts. Comparison with other caspase-1 structures demonstrates drastic conformational changes at the four active-site loops that assemble the catalytic groove. The present study not only contributes to our understanding of GSDMD recognition by inflammatory caspases but also reports a specific inhibitor for these caspases that can serve as a tool for investigating inflammasome signaling.

Pyroptosis is a form of cell death important in defenses against pathogens that can also result in a potent and sometimes pathological inflammatory response. During pyroptosis, GSDMD (gasdermin D), the pore-forming effector protein, is cleaved, forms oligomers, and inserts into the membranes of the cell, resulting in rapid cell death. However, the potent cell death induction caused by GSDMD has complicated our ability to understand the biology of this protein. Studies aimed at visualizing GSDMD have relied on expression of GSDMD fragments in epithelial cell lines that naturally lack GSDMD expression and also lack the proteases necessary to cleave GSDMD. In this work, we performed mutagenesis and molecular modeling to strategically place tags and fluorescent proteins within GSDMD that support native pyroptosis and facilitate live-cell imaging of pyroptotic cell death. Here, we demonstrate that these fusion proteins are cleaved by caspases-1 and -11 at Asp-276. Mutations that disrupted the predicted p30-p20 autoinhibitory interface resulted in GSDMD aggregation, supporting the oligomerizing activity of these mutations. Furthermore, we show that these novel GSDMD fusions execute inflammasome-dependent pyroptotic cell death in response to multiple stimuli and allow for visualization of the morphological changes associated with pyroptotic cell death in real time. This work therefore provides new tools that not only expand the molecular understanding of pyroptosis but also enable its direct visualization.

Pyroptosis is an inflammatory form of programmed cell death that plays important roles in immune protection against infections and in inflammatory disorders. Gasdermin D (GSDMD) is an executor of pyroptosis upon cleavage by caspases-1/4/5/11 following canonical and noncanonical inflammasome activation. GSDMD N-terminal domain assembles membrane pores to induce cytolysis, whereas its C-terminal domain inhibits cell death through intramolecular association with the N domain. The molecular mechanisms of autoinhibition for GSDMD are poorly characterized. Here we report the crystal structures of the human and murine GSDMD C-terminal domains, which differ from those of the full-length murine GSDMA3 and the human GSDMB C-terminal domain. Mutations of GSDMD C-domain residues predicted to locate at its interface with the N-domain enhanced pyroptosis. Our results suggest that GSDMDs may employ a distinct mode of intramolecular domain interaction and autoinhibition, which may be relevant to its unique role in pyroptosis downstream of inflammasome activation.

The X-linked inhibitor of apoptosis (XIAP) protein has been identified as a key genetic driver of two distinct inflammatory disorders, X-linked lymphoproliferative syndrome 2 (XLP-2) and very-early-onset inflammatory bowel disease (VEO-IBD). Molecularly, the role of XIAP mutations in the pathogenesis of these disorders is unclear. Recent work has consistently shown XIAP to be critical for signaling downstream of the Crohn's disease susceptibility protein nucleotide-binding oligomerization domain-containing 2 (NOD2); however, the reported effects of XLP-2 and VEO-IBD XIAP mutations on cell death have been inconsistent. In this manuscript, we describe a CRISPR-mediated genetic system for cells of the myeloid lineage in which XIAP alleles can be replaced with disease-associated XIAP variants expressed at endogenous levels to simultaneously study inflammation-related cell death and NOD2 signaling. We show that, consistent with previous studies, NOD2 signaling is critically dependent on the BIR2 domain of XIAP. We further used this system to reconcile the aforementioned inconsistent XIAP cell death data to show that XLP-2 and VEO-IBD XIAP mutations that exhibit a loss-of-function NOD2 phenotype also lower the threshold for inflammatory cell death. Last, we identified and studied three novel patient XIAP mutations and used this system to characterize NOD2 and cell death phenotypes driven by XIAP. The results of this work support the role of XIAP in mediating NOD2 signaling while reconciling the role of XLP-2 and VEO-IBD XIAP mutations in inflammatory cell death and provide a set of tools and framework to rapidly test newly discovered XIAP variants.

Humans are able to preferentially identify the speed which minimizes energy expenditure during running, as well as in walking. Over long distances, energy conservation during running would be particularly crucial so further investigations should focus on the mechanisms by which people are able to detect their 'optimal' running speeds.