The role of cardiolipin in the regulation of mitochondria-dependent apoptosis

Mitochondria are known as the powerhouse of the cell due to their central role in energy generation and as the site of key metabolic pathways. Over the past 15 years, it has become unequivocally clear that most pro-apoptotic stimuli require a mitochondria-dependent step, involving the permeabilisation of the mitochondrial outer membrane to apoptogenic factors, such as cytochrome c and Smac/DIABLO. The release of these factors into the cytosol is tightly regulated by proteins of the Bcl-2 family and results in the activation of the caspase cascade, leading to cell death. This event is considered as a point of no return in the apoptotic pathway and is often inhibited in cancer. Cardiolipin (CL) is a mitochondria-specific phospholipid that contains four acyl-chains. CL has been implicated in many of the mitochondria-dependent steps that lead to the release of apoptogenic factors including interaction with the Bcl-2 family protein tBid, Bax-dependant mitochondrial outer membrane permeabilization and cytochrome c release. Despite this growing body of evidence, the mechanism by which CL and its fatty acyl chain composition regulate mitochondrial apoptotic pathways remains unresolved, mostly due to the lack of cellular model. Tafazzin is a mitochondrial enzyme, which is mutated in Barth syndrome (BTHS) and is involved in the maturation process of CL. In BTHS, loss of tafazzin activity results in a decrease in mature CL, making it a good model to investigate the role of CL in apoptosis. Using BTHS patients-derived lymphoblastoid cells and HeLa cells in which tafazzin was stably knocked-down using RNA interference, this study provides the first evidence that mature CL are required for an efficient extrinsic apoptotic pathway in type II cells. Further investigation of the impaired apoptotic pathway revealed that the major block is in the activation of caspase-8. In this work, mature CL was identified as a crucial component of a mitochondrial platform required for caspase-8 translocation, oligomerization and activation following Fas signalling in type II cells. These results support a model in which once the first cleavage of procaspase-8 occurs at the DISC, the p43/p10 heteromer product translocates and inserts into the mitochondrial membrane in a CL-dependant manner. In the mitochondria, caspase-8 further oligomerizes and auto-cleaves to adopt its fully active form p18/p10. Additionally, it is shown here that mature CL is required for the physiological association of full-length Bid, the major caspase-8 cleavage substrate, with mitochondria. Thus, Bid is directly available for active caspase-8 on the mitochondrial surface where it cleaves into tBid, which in turn inserts into the mitochondrial outer membrane and induces cytochrome c release. Therefore, by tethering full-length Bid on mitochondria and by providing an activation site for caspase-8 following Fas signalling, CL brings together both the enzyme and its substrate and provides a platform from which the mitochondrial phase of apoptosis is launched. In summary, the data presented in this thesis provide the first evidence that mature CL participates in a new mitochondrial associated platform, called the “mitosome”, required for the activation of caspase-8 in type II cells.