Beyond inflammasomes: emerging function of gasdermins during apoptosis and NETosis

Abstract

Programmed cell death is a key mechanism involved in several biological processes ranging from development and homeostasis to immunity, where it promotes the removal of stressed, damaged, malignant or infected cells. Abnormalities in the pathways leading to initiation of cell death or removal of dead cells are consequently associated with a range of human diseases including infections, autoinflammatory disease, neurodegenerative disease and cancer. Apoptosis, pyroptosis and NETosis are three well-studied modes of cell death that were traditionally believed to be independent of one another, but emerging evidence indicates that there is extensive cross-talk between them, and that all three pathways can converge onto the activation of the same cell death effector-the pore-forming protein Gasdermin D (GSDMD). In this review, we highlight recent advances in gasdermin research, with a particular focus on the role of gasdermins in pyroptosis, NETosis and apoptosis, as well as cell type-specific consequences of gasdermin activation. In addition, we discuss controversies surrounding a related gasdermin family protein, Gasdermin E (GSDME), in mediating pyroptosis and secondary necrosis following apoptosis, chemotherapy and inflammasome activation.

Keywords: NETosis; apoptosis; gasdermin; inflammasome; pyroptosis.

Figure 1. Canonical and non‐canonical inflammasome activation in neutrophils

Neutrophils express several inflammasome‐forming PRR including NLRC4, NLRP3, AIM2 and caspase‐11. Activation of canonical inflammasome selectively triggers IL‐1β maturation without accompanying cell lysis. IL‐1β secretion in living neutrophils requires the pore‐forming protein GSDMD. Upon cytoplasmic Gram‐negative bacterial infection, caspase‐11 triggers robust GSDMD cleavage. GSDMD^NT targets plasma membrane and nuclear membrane to elicit neutrophil extracellular traps (NETs). Caspase‐11‐driven GSDMD pores promotes potassium efflux and activation of the NLRP3 inflammasome.

Figure 2. GSDMD promotes spontaneous neutrophil cell death and NET extrusion

In ageing neutrophils, release of neutrophil elastase (NE) from specific neutrophil granules cleaves and activates GSDMD, resulting in neutrophil cell death. Upon treatment with classical NETosis activators (e.g. PMA), reactive oxygen species (ROS) promote the release of NE from the granules to cytosol in an ill‐defined manner. NE cleaves and activates GSDMD, leading to nuclear and plasma membrane rupture and neutrophil cell lysis by NETosis.

Figure 3. GSDMD is a novel effector protein in the extrinsic apoptosis pathway

In TNF‐stimulated cells, loss or inhibition of IAP and TAK1 function promotes assembly of a caspase‐8‐activating platform called TNF Complex IIb (also commonly referred as the ripoptosome). Active caspase‐8 cleaves GSDMD at D276, leading to pyroptosis. Caspase‐8‐driven GSDMD activation, or caspase‐3/7‐dependent pannexin‐1 activation promotes potassium efflux and NLRP3 assembly. NLRP3‐dependent caspase‐1 activation cleaves GSDMD to further drive pyroptosis. Probenecid, spironolactone and trovafloxacin are pannexin‐1 channel inhibitors.