Type A monoamine oxidase is the target of an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, leading to apoptosis in SH-SY5Y cells.

Mitochondrial monoamine oxidase (MAO) has been considered to be involved in neuronal degeneration either by increased oxidative stress or protection with the inhibitors of type B MAO (MAO-B). In this paper, the role of type A MAO (MAO-A) in apoptosis was studied using human neuroblastoma SH-SY5Y cells, where only MAO-A is expressed. An endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, an MAO-A inhibitor, reduced membrane potential, ΔΨm, in isolated mitochondria, and induced apoptosis in the cells, which 5-hydroxytryptamine, an MAO-A substrate, prevented. In contrast, β-phenylethylamine, an MAO-B substrate, did not suppress the ΔΨm decline by N-methyl(R)salsolinol. The binding of N-methyl(R)salsolinol to mitochondria was inhibited by clorgyline, a MOA-A inhibitor, but not by (–)deprenyl, an MAO-B inhibitor. RNA interference targeting MAO-A significantly reduced the binding of N-methyl(R)salsolinol with simultaneous reduction in the MAO activity. To examine the intervention of MAO-B in the apoptotic process, human MAO-B was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even although the activity and protein of MAO increased markedly. These results demonstrate a novel function of MAO-A in the binding of neurotoxins and the induction of apoptosis, which may account for neuronal cell death in neurodegenerative disorders, including Parkinson's disease.