Apoptosis and pyroptosis play key physiological roles in growth, survival, homeostasis and innate immunity of all mammals. Disregulation of these pathways could lead to many diseases including autoimmune, inflammatory, atherosclerotic and malignant diseases. While apoptosis is an immunologically ?silent? form of programmed cell death executed by activation of the effector caspases 3, 6 and 7, pyroptosis is an inflammatory form of programmed cell death executed by activation of the inflammatory caspases 1, 4, and 5 in humans. These inflammatory caspases cleave the gasdermin protein, GSDMD, to generate a pore- forming N-terminal fragment that permeabilizes the plasma membrane leading, to cellular swelling, osmotic cell lysis and leakage of intracellular contents. If apoptotic cells are not scavenged, such as seen under in vitro culture conditions or conditions of impaired phagocytosis in vivo, they progress to a lytic and inflammatory phase called secondary necrosis. Secondary necrosis shares several features with pyroptosis including plasma membrane permeabilization, swelling, and lysis, and therefore it could represent a form of pyroptosis. Indeed, we discovered recently that activated caspase-3 cleaves the gasdermin-related protein DFNA5 during apoptosis to generate a necrotic DFNA5-N fragment that targets the plasma membrane to permeabilize it and induce secondary necrosis/pyroptosis. Interestingly, unlike WT cells, DFNA5-deficient cells do not roundup and swell, but extensively fragment into small apoptotic bodies suggesting that DFNA5 is a regulator of cellular disassembly during apoptosis. As cellular disassembly during apoptosis may impact clearance of dying cells by phagocytes, we propose to test the hypothesis that DFNA5 cleavage during apoptosis induces pores in the plasma membrane to release ?find me? and other phagocyte activation signals to clear apoptotic cells and to temper down disassembly of apoptotic cells into small apoptotic bodies. In this application we propose aims to gain further insight into the physiological function of DFNA5 in vivo, and its role in autoimmunity, inflammation and innate immunity. We will study the clearance of apoptotic cells in DFNA5-deficient mice and further assess the pathophysiological consequences of DFNA5 deficiency in the development of autoimmunity. We will also characterize another novel function of DFNA5 involved in the regulation of the mitochondrial apoptotic pathway and how this activity might impact cell proliferation and sensitivity to apoptotic stimuli. Finally, we will investigate the role of DFNA5 in the host innate immunity against viral pathogens, septic shock and inflammation-induced malignancy. Results from this research should have a high impact on the field and increase our understanding of the signaling pathways activated during the apoptotic program leading to cell permeabilization and their impact on tissue homeostasis, and autoimmune and inflammatory diseases.
This proposal will investigate a recently discovered function of the deafness protein DFNA5 in the regulation of apoptotic cell disassembly and progression to a terminal phase called secondary necrosis. Understanding the mechanisms of these processes will provide exciting new insights into how defects in the apoptotic program can lead to autoimmune, inflammatory and possibly malignant diseases, and may lead to development of new therapeutic interventions to treat these diseases.
