The Pore-Forming Protein Gasdermin D Regulates Interleukin-1 Secretion from Living Macrophages

Abstract

The interleukin-1 (IL-1) family cytokines are cytosolic proteins that exhibit inflammatory activity upon release into the extracellular space. These factors are released following various cell death processes, with pyroptosis being a common mechanism. Recently, it was recognized that phagocytes can achieve a state of hyperactivation, which is defined by their ability to secrete IL-1 while retaining viability, yet it is unclear how IL-1 can be secreted from living cells. Herein, we report that the pyroptosis regulator gasdermin D (GSDMD) was necessary for IL-1β secretion from living macrophages that have been exposed to inflammasome activators, such as bacteria and their products or host-derived oxidized lipids. Cell- and liposome-based assays demonstrated that GSDMD pores were required for IL-1β transport across an intact lipid bilayer. These findings identify a non-pyroptotic function for GSDMD, and raise the possibility that GSDMD pores represent conduits for the secretion of cytosolic cytokines under conditions of cell hyperactivation.

Figure 1. GSDMD is required for IL-1β release when lysis is prevented

(A, F) WT and Gsdmd^−/− iBMDMs were primed with LPS for 3 hours (or not), and then treated nigericin for 2 hours (A) or Flatox (PA+LFn-Fla) for 2 hours (F). Stimulations contained 0 mM Glycine or 5 mM Glycine. LDH present in the extracellular media was then quantified. (B, G) WT and Gsdmd^−/− iBMDMs were primed with LPS for 3 hours (or not), and then treated with nigericin for 2 hours or Flatox (2 μg/ml PA and 0.5 μg/ml LFn-Fla) for 2 hours. Stimulations contained 0 mM Glycine or 5 mM Glycine. IL-1β release was monitored by ELISA. (C, H) WT and Gsdmd^−/− iBMDMs were primed with LPS for 3 hours (or not), and then treated with nigericin for 2 hours or Flatox (PA+LFn-Fla) for 2 hours. Stimulations contained 0 mM Glycine or 5 mM Glycine. PI (5 μM) was added to assay membrane permeability over time. (D, E) Immunoblot analysis of cell-associated (D) or extracellular (E) IL-1β in Gsdmd^−/− iBMDMs after 3 hours of LPS priming and 30 min of nigericin treatment. Stimulations contained 0 mM Glycine or 5 mM Glycine. (I, J) Immunoblot analysis of cell-associated cleaved (I) or extracellular (J) IL-1β in Gsdmd^−/− iBMDMs after 3 hours of LPS priming and 2 hours of Flatox treatment at 2 μg/ml PA and 0.5 μg/ml LFn-Fla. Data with error bars are represented as mean ± SEM. Each panel is a representative experiment of at least 3 repeats. **** p<0.0001 as determined by two-way ANOVA with Tukey’s multicomparison correction See also Figure S1.

Figure 2. GSDMD regulates IL-1β release in response to bacteria and oxidized lipids that hyperactivate macrophages

(A) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with SA113 ΔoatA bacteria at an MOI of 10 and 30 for 12 hours. LDH present in the extracellular media was then quantified. (B) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with SA113 ΔoatA bacteria at an MOI of 10 and 30 for 12 hours. IL-1β release was monitored by ELISA. (C) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with SA113 ΔoatA bacteria at an MOI of 10 and 30 for 12 hours with PI (5 μM) in the culture media during infection. Pore formation was assessed after 12 hours by quantifying PI fluorescence intensity. (D) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then infected with SA113 ΔoatA for 12 hours at an MOI 30 for 12 hours. IL-1β release was monitored by ELISA. (E) Immunoblot analysis of cleaved IL-1β in the extracellular media of WT and Gsdmd^−/− iBMDMs after 4 hours of LPS priming or unprimed, and then infected with SA113 ΔoatA for 12 hours at an MOI of 10 and 30. (F) Immunoblot analysis of cleaved IL-1β in lysates of WT and Gsdmd^−/− iBMDMs after 4 hours of LPS priming or unprimed, and then infected with SA113 ΔoatA for 12 hours at an MOI of 10 and 30. (G) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with PGN for 6 hours. LDH present in the extracellular media was then quantified. (H) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with PGN for 6 hours. IL-1β release was monitored by ELISA. (I) Immunoblot analysis of cleaved IL-1β in the extracellular media of WT and Gsdmd^−/− iBMDMs after 4 hours of LPS priming or unprimed, and then treated with PGN for 6 hours. (J) Immunoblot analysis of cleaved IL-1β in lysates of Gsdmd^−/− iBMDMs after 4 hours of LPS priming or unprimed, and then treated with PGN for 6 hours. (K) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with PGPC or POVPC for 6 hours. LDH present in the extracellular media was then quantified. (L) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with PGPC or POVPC for 6 hours. IL-1β release was monitored by ELISA. (M) WT and Gsdmd^−/− iBMDMs were primed with LPS for 4 hours (or not), and then treated with PGPC or POVPC for 6 hours with PI (5 μM) in the culture media. Pore formation was assessed after 6 hours by quantifying PI fluorescence intensity. (N) Immunoblot analysis of cleaved IL-1β of cell culture supernatants from WT and Gsdmd^−/− iBMDMs after 4 hours of LPS priming (or not), and then challenged with PGPC or POVPC for 6 hours. (O) Immunoblot analysis of cell-associated cleaved IL-1β in Gsdmd^−/− iBMDMs after 4 hours of LPS priming or unprimed, and then challenged with PGPC or POVPC for 6 hours. Data with error bars are represented as mean ± SEM. Each panel is a representative experiment of at least 3 repeats. See also Figure S2.

Figure 3. Hyperactive stimuli induce inflammasome assembly within living macrophages

(A) Confocal imaging in the red and green channels of live cell ASC-citrine expressing BMDMs primed with LPS for 3 hours, with second stimulations of nigericin, 5 mM ATP, 50 μg/ml PGN, MOI 30 of SA113 ΔoatA, POVPC, or PGPC for 16–22 hours. Red signal corresponds to MitoTracker Red CMXRos. Green signal corresponds to ASC-citrine fusion protein diffusely cytosolic or oligomerized into inflammasome specks. (B) Enumeration of cells with visible ASC specks that also contain polarized mitochondria as a sign of viability. (n > 25 speck containing cells per condition) (C) Enumeration of cells with visible ASC specks that also phagocytose zymosan particles as a sign of viability. (n > 28 speck containing cells per condition) See also Figure S3

Figure 4. Cleaved GSDMD acts as a conduit for the release of cytosolic or encapsulated IL-1 family members

(A) EGFP signal was monitored by flow cytometry in 293T cells stably expressing IL-1β-tdTomato after electroporation with GSDMD alleles tagged with EGFP. (B) 7-AAD signal was monitored by flow cytometry in 293T cells stably expressing IL-1β-tdTomato after electroporation with GSDMD alleles tagged with EGFP. (C) tdTomato signal was monitored by flow cytometry in 293T cells stably expressing IL-1β-tdTomato after electroporation with GSDMD alleles tagged with EGFP. (D) Representative histogram of tdTomato fluorescence overlays for 0 and 7 hours post electroporation with GSDMD alleles tagged with EGFP. (E–G) EM micrographs of PC:PS unextruded liposomes treated with recombinant GSDMD alone (E) caspase-11 alone (F) or with combined treatment (G). Scale bar equals 50nm (H) Immunoblot analysis of IL-1β present within liposomes (pellet), or supernatants after ultracentrifugation of liposomes that were untreated, treated with caspase-11, or treated with caspase-11 and GSDMD for 30 and 120 minutes. (I) Densitometry quantification of western blot band density for IL-1β release from liposomes. (J) Immunoblot analysis of IL-18 present within liposomes (pellet), or supernatants after ultracentrifugation of liposomes that were untreated, treated with caspase-11, or treated with caspase-11 and GSDMD for 30 and 120 minutes. (K) Densitometry quantification of western band density for IL-18 release from liposomes. Data with error bars are represented as mean ± SEM. Each panel is a representative experiment of at least 3 repeats. See also Figure S4