Rasagiline and selegiline suppress calcium efflux from mitochondria by PK11195-induced opening of mitochondrial permeability transition pore: a novel anti-apoptotic function for neuroprotection

Rasagiline and selegiline, inhibitors of type B monoamine oxidase (MAO-B), protect neurons from cell death in cellular and animal models. Suppression of mitochondrial membrane permeabilization and subsequent activation of apoptosis cascade, and induction of anti-apoptotic, pro-survival genes are proposed to contribute the anti-apoptotic function. Rasagiline suppresses neurotoxin- and oxidative stress-induced membrane permeabilization in isolated mitochondria, but the mechanism has been not fully clarified. In this paper, regulation of the mitochondrial permeability transition pore by rasagiline and selegiline was examined in apoptosis induced by PK11195, a ligand of the outer membrane translocator protein 18 kDa (TSPO) in SH-SY5Y cells. The pore opening was quantitatively measured using a simultaneous monitoring system for calcium (Ca²⁺) and superoxide (O₂ ⁻) (Ishibashi et al. in Biochem Biophys Res Commun 344:571–580, 2006). The association of the pore opening with Ca²⁺ efflux and ROS increase was proved by the inhibition of Bcl-2 overexpression and cyclosporine A treatment. Potency to release Ca²⁺ was correlated with the cytotoxicity of TSPO antagonists, PK11195, FGIN-1-27 and protoporphyrin IX, whereas a TSPO agonist, 4-chloro-diazepamine, did not significantly increase Ca²⁺ or cause cell death. Rasagiline and selegiline inhibited mitochondrial Ca²⁺ efflux through the mitochondrial permeability transition pore dose dependently. Ca²⁺ efflux was confirmed as the initial signal in mitochondrial apoptotic cascade, and the suppression of Ca²⁺ efflux may account for the neuroprotective function of rasagiline and selegiline. The quantitative measurement of Ca²⁺ efflux can be applied to determine anti-apoptotic activity of neuroprotective compounds. The role of mitochondrial Ca²⁺ release in neuronal death and also in neuroprotection by MAO-B inhibitors is discussed.