Targeting Bid for mitoprotection - Bid crystallization, new mechanisms and inhibitory compounds

Mitochondrial demise is a key feature of the progressive neuronal death in age-related neurodegenerative diseases such as Alzheimer’s disease or Parkinson’s disease and after acute brain injury. Oxidative stress or impaired calcium homeostasis converge at mitochondria where the permeabilization of the mitochondrial outer membrane (MOMP) are considered as ´point of no return´ in the cell’s commitment to die. The pro-apoptotic Bcl-2 family protein Bid has been identified as a key regulator of mitochondrial injury and mitochondrial pathways of neuronal cell death. Upon its transactivation to mitochondria, Bid induces mitochondrial fragmentation and MOMP thereby accelerating the production of reactive oxygen species (ROS) and the release of death promoting factors into the cytosol that trigger the final steps of intrinsic cell death. The pivotal role of Bid in neuronal cell death has been further shown in models of cerebral ischemia and traumatic brain injury in vivo. Since it has been shown that inhibition of Bid maintains mitochondrial integrity and function thereby preventing mitochondrial cell death, Bid serves as a target for mitoprotection and neuronal cell survival. However, the precise mechanisms how Bid triggers mitochondrial impairment and disruption are still unknown and have to be explored to achieve efficient strategies for inhibition of Bid-mediated cell death pathways. Therefore, the present study was aimed to elucidate the mechanisms of Bid-dependent mitochondrial demise and addressed the associated Bid-protein interactions. Furthermore, the thesis focused on Bid crystallization campaigns and the development of Bid-inhibiting compounds with high impact for novel therapeutic perspectives in neurological diseases. Bid-mediated cell death pathways were mainly addressed in immortalized mouse hippocampal HT-22 neurons exposed to toxic glutamate concentrations that induce oxidative stress through glutathione depletion. The findings were additionally confirmed in a model of tBid-induced toxicity by over-expressing tBid in the HT-22 cell line and in vitro studies using recombinant proteins. Overall, the present thesis highlights the pro-apoptotic protein Bid as a promising druggable target for mitoprotection and neuronal survival and indicates recombinant Bid constructs as suitable for Bid crystal structure analysis. Furthermore, the data gained new insights in the mechanisms of Bid-dependent mitochondrial injury and the involvement of VDAC1 in mitochondrial apoptosis, connecting the controversial hypothesis of MOMP initiated by Bid and Bcl-2 family proteins or VDAC-mediated mPTP, respectively. Targeting the Bid-VDAC1 interaction serves as a promising therapeutic approach aiming at preserved mitochondrial function and neuronal survival with high relevance for the treatment of neurological disorders, where oxidative stress and mitochondrial demise are prominent.