By Sarah Choi
Cell death is required in growth. While cell survival and proliferation are undoubtedly important for development, cell death and its regulation also play a fundamental role in monitoring and controlling cell populations. Research efforts have therefore endeavoured to elucidate the specific mechanisms involved in cell death.
Cell death occurs in an ever-increasing number of characterized forms. Pyroptosis is one of the forms of regulated cell death (RCD), along with apoptosis and necroptosis. Highly inflammatory and lytic, pyroptosis is also characterized by caspase-1 dependence, nuclear condensation (but not nuclear fragmentation), cell swelling and plasma membrane rupture. It can be triggered by microbial infection, pathogens, and proinflammatory signals from stimuli such as stroke and cancer (Bergsbaken, Fink and Cookson, 2009). Research into the mechanisms involved has revealed that initiation of pyroptosis occurs via four pathways: the inflammasome-dependent canonical pathway; inflammasome-dependent non-canonical pathway; inflammasome-independent caspase-3 mediated pathway; and inflammasome-independent granzyme-mediated pathway. Of these mechanisms, the inflammasome-dependent canonical pathway is the most well-understood.
Inflammasomes are multimeric signalling complexes that function in the cytosol as innate immune system sensors, detecting cytosolic abnormalities, and inducing inflammatory responses (Guo, Callaway and Ting, 2015). In the inflammasome-dependent canonical pathway, these are activated by pathogens and endogenous damage, sensed by proteins such as NOD-like receptors NLRP1b, NLRC4, and NLRP3. The inflammasomes then recruit and activate caspase-1, leading to cleavage and maturation of the proinflammatory cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18) from their inactive precursors. The mass release of these cytokines, as well as alarmins and other danger-associated molecular patterns, leads to an influx of immune cells, promoting an inflammatory microenvironment (Man, Karki and Kanneganti, 2017; Tan et al., 2021).
The main executioner proteins in pyroptosis are members of the gasdermin family. In particular, gasdermin D (GSDMD) has been shown to drive pore formation in the plasma membrane. Alongside the cytokine release, active caspase-1 and caspase-11 on inflammasomes activate GSDMD by proteolytic cleavage. The cytotoxic N-terminals of activated GSDMD then oligomerize, travel to the membrane, and induce pore formation. This allows passage of ions and water molecules, upsetting membrane potentials, disrupting osmotic balance and ultimately leading to cell swelling and formation of pyroptotic bodies. This then results in plasma membrane rupture, and there has been emerging evidence that the transmembrane protein nerve injury-induced protein 1 (NINJ1) is a key molecule involved in this rupture (Kayagaki et al., 2020). This being said, NINJ1 does not seem to be specific to pyroptosis. Following membrane rupture, the osmotic lysis of affected cells was thought to mediate the release of active cytokines. However, pore formation, caspase 1-independent lysosome exocytosis, and microvesicle shedding have also been proposed as potential mechanisms for cytokine secretion (Bergsbaken, Fink and Cookson, 2009).
Lipopolysaccharides (LPS) are glycolipids in the outer membrane of Gram-negative bacteria. The non-canonical inflammasome pathway of pyroptosis is activated by these bacteria, via direct binding of intracellular lipopolysaccharide (LPS) to proinflammatory caspase-4, caspase-5, and (murine) caspase-11. This causes caspase oligomerization and activation, and GSDMD cleavage at Asp276. As with the canonical pathway, activated GSDMD then forms pores. The non-canonical inflammasome also activates NLRP3 (Tan et al., 2021), allowing IL-1β and IL-18 maturation.
The inflammasome-independent caspase-3 mediated pathway has been observed in epithelial cells. Upon TNF-α stimulation, activated caspase-3 mediates gasdermin E (GSDME) cleavage at specific residues. Pyroptosis is therefore induced without inflammasome involvement. Nonetheless, GSDME can also activate the canonical inflammasome. In natural killer cells and cytotoxic T cells, gasdermins are cleaved and activated via the granzyme-mediated pathway (Tan et al., 2021). Pyroptosis mediated by these serine proteases has been demonstrated in cancer cells, prompting efforts to induce pyroptosis as part of cancer treatment.
There is still much to learn about pyroptosis and inflammasomes. Specific details in certain pathways remain the subject of controversies, and new data on processes upstream and downstream of caspase-1 may add to or modify the existing understanding of the molecular mechanisms involved. This knowledge will aid identification of potential therapeutic targets within the pyroptotic pathways and allow development of novel treatments for numerous diseases, including cancer, autoimmune diseases and neurological injuries.
Bergsbaken, T., Fink, S. L. and Cookson, B. T. (2009) ‘Pyroptosis: Host cell death and inflammation’, Nature Reviews Microbiology. NIH Public Access, pp. 99–109. doi: 10.1038/nrmicro2070.
Guo, H., Callaway, J. B. and Ting, J. P. Y. (2015) ‘Inflammasomes: Mechanism of action, role in disease, and therapeutics’, Nature Medicine. Nature Publishing Group, pp. 677–687. doi: 10.1038/nm.3893.
Kayagaki, N. et al. (2020) NINJ1 mediates plasma membrane rupture during lytic cell death. doi: 10.21203/RS.3.RS-62714/V1.
Man, S. M., Karki, R. and Kanneganti, T. D. (2017) ‘Molecular mechanisms and functions of pyroptosis, inflammatory caspases and inflammasomes in infectious diseases’, Immunological Reviews. Blackwell Publishing Ltd, pp. 61–75. doi: 10.1111/imr.12534.
Tan, Y. et al. (2021) ‘Pyroptosis: a new paradigm of cell death for fighting against cancer’, Journal of Experimental and Clinical Cancer Research. BioMed Central Ltd, p. 153. doi: 10.1186/s13046-021-01959-x.