A Mechanistic Understanding of Pyroptosis: The Fiery Death Triggered by Invasive Infection

Abstract

Immune cells and skin and mucosal epithelial cells recognize invasive microbes and other signs of danger to sound alarms that recruit responder cells and initiate an immediate "innate" immune response. An especially powerful alarm is triggered by cytosolic sensors of invasive infection that assemble into multimolecular complexes, called inflammasomes, that activate the inflammatory caspases, leading to maturation and secretion of proinflammatory cytokines and pyroptosis, an inflammatory death of the infected cell. Work in the past year has defined the molecular basis of pyroptosis. Activated inflammatory caspases cleave Gasdermin D (GSDMD), a cytosolic protein in immune antigen-presenting cells and epithelia. Cleavage separates the autoinhibitory C-terminal fragment from the active N-terminal fragment, which moves to the cell membrane, binds to lipids on the inside of the cell membrane, and oligomerizes to form membrane pores that disrupt cell membrane integrity, causing death and leakage of small molecules, including the proinflammatory cytokines and GSDMD itself. GSDMD also binds to cardiolipin on bacterial membranes and kills the very bacteria that activate the inflammasome. GSDMD belongs to a family of poorly studied gasdermins, expressed in the skin and mucosa, which can also form membrane pores. Spontaneous mutations that disrupt the binding of the N- and C-terminal domains of other gasdermins are associated with alopecia and asthma. Here, we review recent studies that identified the roles of the inflammasome, inflammatory caspases, and GSDMD in pyroptosis and highlight some of the outstanding questions about their roles in innate immunity, control of infection, and sepsis.

Keywords: Gasdermin; Inflammasome; Inflammatory caspases; Pore-forming protein; Pyroptosis; Sepsis.

Fig. 1

Model of canonical inflammasome activation and inflammatory cell death. NLR proteins (NLRP1, NLRP3, NAIP/NLRC4), pyrin and ALR proteins (AIM2 and IFI16) sense danger signs and initiate assembly of canonical inflammasomes. NLRP1b is activated by the metalloprotease activity of Bacillus anthracis lethal toxin (LT). The NLRP3 inflammasome is activated by a broad spectrum of stimuli, such as ATP, microbial toxins, and crystals, by an unknown mechanism. NAIPs recognize and directly bind to bacterial flagellin or bacterial type III secretion system components to initiate the NLRC4-dependent inflammasome. Pyrin detects the inactivation of Rho GTPases by bacterial toxins. AIM2 and IFI16 sense DNA via their HIN domain. The canonical inflammasome complexes form a platform to activate caspase-1. Activated caspase-1 processes proinflammatory cytokines (pro-IL-1β, pro-IL-18) and cleaves GSDMD, releasing its N-terminal fragment GSDMD-NT, which forms pores in the plasma membrane of the infected cell, resulting in pyroptotic cell death during which bacteria, inflammatory cytokines caspase-1, and GSDMD-NT are released. GSDMD-NT also directly kills intracellular and cell-free bacteria. It is not known whether GSDMD-NT permeabilizes the phagosome membrane and accesses bacteria that replicate within phagosomes.

Fig. 2

Cytosolic LPS activates noncanonical inflammasome activation and pyroptosis. LPS, from intracellular gram− bacteria or from outer membrane vesicles produced by cell-free gram− bacteria that are taken up by cells, binds with high affinity to caspase-4/5/11, causing self-assembly of the noncanonical inflammasome. These caspases are then activated and trigger pyroptosis by cleaving GSDMD. Active caspase-4/5/11 do not directly cleave the proinflammatory cytokine precursors, but cause them to be processed indirectly by activating the NLRP3 inflammasome and caspase-1. The NLRP3 inflammasome and caspase-1 may be activated by ion fluxes caused by damage to the plasma membrane.

Fig. 3

GSDMD-NT forms pores in liposomes. Shown are representative negative staining electron microscopy images of side views (A, indicated by arrows) and top-down views (B) of GSDMD-NT pores formed in phosphatidyl serine-containing liposomes. The inner and outer diameters of GSDMD-NT pores (red dotted lines) are approximately 15 and 32 nm, respectively. Scale bars, 20 nm. Figure is taken from Liu, X., Zhang, Z., Ruan, J., Pan, Y., Magupalli, V. G., Wu, H., et al. (2016). Inflammasome-activated gasdermin D causes pyroptosis by forming membrane pores. Nature, 535, 153.

Fig. 4

Structure of GSDMD modeled based on the crystal structure of GSDMA3. The structural model of GSDMD was generated based on the structure of GSDMA3 (PDB: 5B5R) described in Ding et al. (2016) using the Swiss Model server. Orange, GSDMD-NT; green, GSDMD-CT.

Fig. 5

Phylogenetic relationship of the human and mouse gasdermin superfamily. Phylogenetic tree showing the inferred evolutionary relationships of human and mouse gasdermin superfamily members generated using MUSCLE for amino acid sequence alignment and “maximum likelihood” method for phylogeny. Scale bar indicates the number of substitutions per amino acid.