Uncoupling of ATP-depletion and cell death in human dopaminergic neurons

The mitochondrial inhibitor 1-methyl-4-phenylpyridinium (MPP+) is the toxicologically relevant metabolite of 1-methyl-4-phenyltetrahydropyridine (MPTP), which causes relatively selective degeneration of dopaminergic neurons in the substantia nigra. Dopaminergic LUHMES cells were used to investigate whether ATP-depletion can be uncoupled from cell death as a downstream event in these fully post-mitotic human neurons. Biochemical assays indicated that in the homogeneously differentiated cell cultures, MPP+ was taken up by the dopamine transporter (DAT). MPP+ then triggered oxidative stress and caspase activation, as well as ATP-depletion followed by cell death. Enhanced survival of the neurons in the presence of agents interfering with mitochondrial pathology, such as the fission inhibitor Mdivi-1 or a Bax channel blocker suggested a pivotal role of mitochondria in this model. However, these compounds did not prevent cellular ATP-depletion. To further investigate whether cells could be rescued despite respiratory chain inhibition by MPP+, we have chosen a diverse set of pharmacological inhibitors well-known to interfere with MPP+ toxicity. The antioxidant ascorbate, the iron chelator desferoxamine, the stress kinase inhibitor CEP1347, and different caspase inhibitors reduced cell death, but allowed ATP-depletion in protected cells. None of these compounds interfered with MPP+ accumulation in the cells. These findings suggest that ATP-depletion, as the initial mitochondrial effect of MPP+, requires further downstream processes to result in neuronal death. These processes may form self-enhancing signaling loops, that aggravate an initial energetic impairment and eventually determine cell fate.