Elucidating structural and functional mechanisms of pyroptosis

Summary Pyroptosis is a most “explosive” and immunogenic form of lytic cell death involving spillage of cellular contents, and has been defined as cell death mediated by the gasdermin (GSDM) family of proteins. This breakthrough understanding was brought about in 2015 when gasdermin D (GSDMD) was identified as a downstream effector of inflammasomes, which are supramolecular complexes that activate inflammatory caspases (-1, -4 and -5 in human and -1 and -11 in mouse). GSDMD gets cleaved by caspases to generate an N-terminal fragment (GSDMD-NT) and a C- terminal fragment (GSDMD-CT), and GSDMD-NT was shown to mediate pyroptosis, as well as the release of IL-1 family cytokines, which are processed by caspase-1 to the mature form. The GSDM family comprises six members in humans (GSDMA, GSDMB, GSDMC, GSDMD, GSDME/DFNA5, and DFNB59). In 2021, Ninjurin1 (NINJ1), a member of the NINJ family comprised of 2 transmembrane proteins in mammals, was shown to act downstream of GSDMD pore formation to induce the plasma membrane rupture required for full release of damage- associated molecular patterns (DAMP) such as lactate dehydrogenase (LDH). We and others found that upon cleavage by inflammatory caspases, GSDMD-NT specifically binds to acidic lipids, and exhibits strong membrane-disrupting cytotoxicity in mammalian cells by forming pores on membranes during pyroptosis and in vitro. Other GSDMs, upon cleavage by appropriate proteases, also form transmembrane pores. Here we propose to elucidate the structural mechanism of pore formation by the GSDM family, and of membrane disruption by NINJ1. Understanding how GSDMs are regulated and exert their pore forming activity, and how NINJ1 oligomerizes to induce plasma membrane rupture will not only provide new insights on pyroptosis, but also afford new therapeutic strategies for treating inflammasome-related and pyroptosis-related diseases.