Caspase-activation complex ARCosome – its regulation and signalling for cell death

Abstract

Mitochondria and the endoplasmic reticulum (ER) cooperatively orchestrate signalling pathways for cell death (apoptosis). My research focused on the ARCosome, a novel protein complex that spans the interface between the mitochondria and the ER. This complex was discovered in our laboratory and was shown to play a role in cell-death regulation. It is composed of Fission1 homologue (Fis1), a fission protein, and Bap31, an integral ER membrane protein. Caspase-8 was shown to be recruited to the complex under cell death conditions and to cleave Bap31 into the pro-apoptotic p20Bap31. However, the question of how this complex is controlled in healthy and dying cells was unknown. In my study, I show that Bcl-XL and Bcl-2 dissociate from the complex upon etoposide-induced cell death. I discovered that Bcl-2 and Bcl-XL block the recruitment of caspase-8 to the complex and inhibit apoptotic signals activated by Fis1 and Bap31. Their rescue effect was demonstrated by inhibiting Bap31 cleavage into the pro-apoptotic p20Bap31. Hence, our work reveals the changes that take place at the ARCosome for apoptosis induction and highlight in particular the release of Bcl-XL and Bcl-2 as pivotal factors to control apoptosis in this pathway. I also investigated the downstream signalling of p20Bap31-inducedcell death for which I studied complex II activation and ROS generation. Prohibitin, an assembly factor for complex II, and cyclophylin D, an inhibitor of the PT pore, were found to inhibit p20Bap31-induced cell death as well as ROS generation. Ectopic p20Bap31 expression resulted in the dissociation of respiratory chain complex II components SDHB and SDHD, as shown by the decrease of FRET efficiency when their GFP-variant fusion proteins were transfected into cells. Moreover, I showed that ARCosome activation is not only involved in apoptosis pathways, but also in IFNgamma-induced necroptosis and amyloid-β-mediated cell death in neuronal cells.