Regulation of the Mitochondrial Permeability Transition Pore: Role of Outer Membrane and of Translocator Protein (Peripheral Benzodiazepine Receptor)
Year: 2013
Type: dissertation
Abstract: The mitochondrial permeability transition (PT) is a sudden increase in the permeability of the inner mitochondrial membrane (IMM) to solutes with molecular masses up to 1,500 Da. This process is due to opening of a voltage- and Ca2+-dependent, cyclosporin (Cs) A-sensitive, high-conductance channel called the permeability transition pore (PTP). Its involvement in pathological states and in the loss of cell viability is widely recognized, but its molecular identity remains elusive. The long standing idea that the PTP may form at inner-outer membrane contact sites and that it may be constituted by the adenine nucleotide translocator (ANT) in the IMM and voltage dependent anion channel (VDAC) in the outer mitochondrial membrane (OMM) has not been confirmed by genetic ablation of these proteins. Yet, the PT can be regulated by proteins that interact with the OMM such as hexokinase, and by ligands of the OMM translocator protein of 18 kDa (TSPO, formerly known as peripheral benzodiazepine receptor). TSPO is highly conserved throughout evolution and was first identified as a binding site for benzodiazepines in tissues that lack GABA receptors, the clinical target of benzodiazepines in the central nervous system. Moreover, TSPO together with VDAC and ANT was included in the early models of PTP and as of today remains the only putative PTP component whose role has not been addressed by genetic inactivation. Thus, we first investigated the role of OMM in PTP regulation; and then created mice where Tspo gene could be inactivated. We demonstrate that (i) PT is exclusively an IMM event, as it occurs in mitoplasts (mitochondria stripped of outer membranes); (ii) in the presence of specific, high affinity TSPO-ligands, such as PK11195, Ro5-4864, FGIN1-27 and protoporphyrin (PP) IX the PT is observed in rat liver mitochondria, but not in mitoplasts; and (iii) upon photooxidative stress (achieved by photoirradiation after treatment with PP IX-like dicarboxylic porphyrins), low doses of light inactivate the PT both in mitochondria and in mitoplasts while high light doses activate the pore in mitochondria only. Conditional elimination of TSPO in mouse livers abolished high affinity binding of the TSPO ligand PK11195, but it did not prevent: (i) CsA-sensitive PTP formation; (ii) PT induction by TSPO ligands PK11195, Ro5-4864 and PP IX; (iii) PT induction by photooxidative stress. The latter findings are unequivocal demonstration that the ability of ‘specific’ TSPO ligands to regulate PTP is due to off-target effects, possibly at the FoF1 ATP synthase.
Author: Justina Šileikytė
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2276257613