1.C.123 The Pore-forming Gasdermin (Gasdermin) Family
Pyroptosis (cell death with inflamation) was long regarded as caspase-1-mediated monocyte death in response to certain bacterial insults. Caspase-1 is activated upon various infectious and immunological challenges through different inflammasomes. The discovery of caspase-11/4/5 function in sensing intracellular lipopolysaccharide expanded the spectrum of pyroptosis mediators and also revealed that pyroptosis is not cell type specific.
The gasdermin (GSDM) family consists of gasdermin A (GSDMA), B (GSDMB), C (GSDMC), D (GSDMD), E or DNFA5 (GSDME), and DFNB59 in humans. Expressed in the skin, gastrointestinal tract, and various immune cells, GSDMs mediate homeostasis and inflammation upon activation by caspases and unknown proteases (Xia et al. 2019).
The pyroptosis executioner, gasdermin D (GSDMD), is a substrate of both caspase-1 and caspase-11/4/5 and is in the large gasdermin family bearing membrane pore-forming activity (Shi et al. 2016). Thus, pyroptosis is defined as gasdermin-mediated programmed necrosis. These proteins are associated with various genetic diseases.
The N-terminal domain of Gasdermin-D promotes pyroptosis in response to microbial infection and danger signals. The active protein is produced by the cleavage of gasdermin-D by an inflammatory caspase, CASP1 or CASP4, in response to canonical, as well as non-canonical (such as cytosolic LPS) inflammasome activators (Shi et al. 2015; Kayagaki et al. 2015; Sborgi et al. 2016). After cleavage, the product moves to the plasma membrane where it binds to inner leaflet lipids, including monophosphorylated phosphatidylinositols, as well as phosphatidic acid and phosphatidylserine (Ding et al. 2016). Homooligomerization within the membrane generates pores of 10 - 15 nanometers (nm) (inner diameter), allowing the release of mature IL1B and triggering pyroptosis (Sborgi et al. 2016; Ding et al. 2016). It thus exhibits bactericidal activity. The N-terminal domain of Gasdermin-D, released from pyroptotic cells into the extracellular milieu rapidly binds to and kills both Gram-negative and Gram-positive bacteria, without harming neighboring mammalian cells, as it does not disrupt the plasma membrane from the outside due to lipid-binding specificity (Ding et al. 2016). It strongly binds to bacterial and mitochondrial lipids, including cardiolipin but does not bind to unphosphorylated phosphatidylinositol, phosphatidylethanolamine or phosphatidylcholine (Ding et al. 2016).
Once inserted, GSDMDNterm assembles arc-, slit-, and ring-shaped oligomers, each of which being able to form transmembrane pores. This assembly and pore forming process is independent of whether GSDMD has been cleaved by caspase-1, caspase-4, or caspase-5. Using time-lapse AFM, Mulvihill et al. 2018 monitored how GSDMDNterm assembles into arc-shaped oligomers that can transform into larger slit-shaped and finally into stable ring-shaped oligomers. The mechanism of GSDMDNterm transmembrane pore assembly is likely shared with other members of the gasdermin protein family. Granzyme A from cytotoxic lymphocytes cleaves gasdermin B (GSDMB) to trigger pyroptosis in target cells via oligomeric pore formation (Zhou et al. 2020).
The reactions catalyzed by Gasdermins is:
Solutes (in) → Solutes (out)