Characteristics of the mitochondrial and cellular uptake of MPP+, as probed by the fluorescent mimic, 4'I-MPP

The discovery that 1-methyl-4-phenylpyridinium (MPP+) selectively destroys dopaminergic neurons and causes Parkinson's disease (PD) symptoms in mammals has strengthened the environmental hypothesis of PD. The current model for the dopaminergic toxicity of MPP+ is centered on its uptake into dopaminergic neurons, accumulation into the mitochondria, inhibition of the complex-I leading to ATP depletion, increased reactive oxygen species (ROS) production, and apoptotic cell death. However, some aspects of this mechanism and the details of the cellular and mitochondrial accumulation of MPP+ are still poorly understood. The aim of this study was to characterize a structural and functional MPP+ mimic which is suitable to study the cellular distribution and mitochondrial uptake of MPP+ in live cells and use it to identify the molecular details of these processes to advance the understanding of the mechanism of the selective dopaminergic toxicity of MPP+. Here we report the characterization of the fluorescent MPP+ derivative, 1-methyl-4-(4'-iodophenyl)pyridinium (4'I-MPP+), as a suitable candidate for this purpose. Using this novel probe, we show that cytosolic/mitochondrial Ca2+ play a critical role through the sodium-calcium exchanger (NCX) in the mitochondrial and cellular accumulation of MPP+ suggesting for the first time that MPP+ and related mitochondrial toxins may also exert their toxic effects through the perturbation of Ca2+ homeostasis in dopaminergic cells. We also found that the specific mitochondrial NCX (mNCX) inhibitors protect dopaminergic cells from the MPP+ and 4'I-MPP+ toxicity, most likely through the inhibition of the mitochondrial uptake, which could potentially be exploited for the development of pharmacological agents to protect the central nervous system (CNS) dopaminergic neurons from PD-causing environmental toxins.