CASP4 inflammasome assembly

CASP4 inflammasome assembly Caspase-4 (CASP4) is an inflammatory caspase involved in the innate immune response, particularly against Gram-negative bacteria, and is activated when intracellular bacterial lipopolysaccharide (LPS) binds to the N-terminal caspase activation and recruitment domain (CARD) of CASP4 (Shi J et al., 2014; An J et al., 2019, 2022; Wang K et al., 2020). LPS can enter the host cell cytosol through several mechanisms, including endocytosis of LPS-containing outer membrane vesicles (OMVs), which are naturally secreted by Gram-negative bacteria (Wacker MA et al., 2017; Bitto NJ et al., 2018; reviewed by Barker JH & Weiss JP 2019; Page MJ et al., 2022). Additionally, LPS may be released from phagocytosed bacteria following the rupture of phagolysosomal compartments, allowing bacterial components to escape into the host cytosol. Guanylate-binding proteins (GBPs), a family of interferon-inducible, dynamin-like GTPases, localize to pathogen-containing vacuoles or directly to exposed LPS on the bacterial cell surface. There, GBPs, namely, GBP1, GBP2, GBP3 and GBP4, assemble into a supramolecular complex known as the GBP coat, which disrupt bacterial membranes and exposes LPS to the cytosol, thereby promoting CASP4 recruitment (Santos JC et al., 2020, Wandel MP et al., 2020). LPS-bound CASP4 oligomerizes and undergoes self-cleavage at specific sites, leading to the full proteolytic activity (Wang K et al., 2020; Chan AH et al., 2023).Activated CASP4 can then cleave gasdermin D (GSDMD), which is also a substrate of CASP1, CASP5, and Casp11, a murine homolog of human CASP4/CASP5 (Shi J et al., 2014, 2015; Kayagaki N et al., 2015; Zhao Y et al., 2018; Wang K et al., 2020). The resulting N-terminal fragment of GSDMD oligomerizes to form pores in the cell membrane, leading to pyroptosis in mammals (Liu X et al., 2016; Ding J et al., 2016; Sborgi L et al., 2016; Aglietti RA et al., 2016). In addition, CASP4 and CASP5 cleave pro-interleukin-18 (pro-IL-18) at aspartic acid D36 with high efficiency, producing the mature, active cytokine (Shi X et al., 2023; Exconde PM et al., 2023; Devant P et al., 2023; reviewed by Exconde PM, 2024). Structural analyses revealed that this cleavage relies on a bivalent recognition mechanism, in which pro-IL-18 binds caspase-4 through two interfaces: the protease exosite binds a hydrophobic pocket within pro-IL-18, while the active site of CASP4 engages charged residues located within and adjacent to the tetrapeptide recognition motif in the pro-domain (Shi X et al., 2023; Devant P et al., 2023). In contrast, CASP4- and CASP5-mediated cleavage of pro-IL-1β at D27 produces an inactive fragment that lacks receptor-stimulating activity (Exconde PM et al., 2023; reviewed by Exconde PM, 2024). An alternative CASP4-mediated cleavage at D116, the canonical pro-IL-1β activation site, has been observed but occurs with lower efficiency comparing to pro-IL-18 processing (Bibo-Verdugo B et al., 2020; Chan AH et al., 2023; Devant P et al., 2023).Intracellular bacterial pathogens have evolved strategies to suppress host inflammatory responses. For example, Shigella flexneri secretes the type III secretion system (T3SS) effector OspC3, which catalyzes ADP-riboxanation of CASP4, thereby inhibiting LPS-induced, CASP4-mediated pyroptosis (Li Z et al., 2021; Hou Y et al., 2023).