Selective vulnerability of dopaminergic neurons in Parkinson's disease connects PRKN and differential expression of CHCHD2 and GPNMB.

The mechanism(s) causing selective vulnerability of dopaminergic neurons in Parkinson's disease (PD) remain largely elusive. To improve our understanding of mitochondrial involvement and related pathways suggested to play a role in this selective vulnerability, we used tyrosine hydroxylase (TH)-mCherry reporter-induced pluripotent stem cells generated by CRISPR/Cas9. We sorted neurons into pure TH-positive and TH-negative neurons upon differentiation into a dopaminergic neuron-containing cell culture. We characterized mitochondrial function in both dopaminergic and non-dopaminergic neurons from PD patients and controls and identified differentially expressed genes between patients and controls in both cell populations. Dopaminergic neurons had a lower mitochondrial membrane potential than non-dopaminergic neurons. Furthermore, ATP levels were lower in PRKN mutation carriers than controls, and mitochondrial mass was reduced in PRKN mutation carriers only in the TH-positive but not in TH-negative neurons. Importantly, in PRKN mutation carriers, we demonstrated elevated levels of dopamine, which can serve as a significant source of toxic, oxidized dopamine. Using unbiased RNA sequencing, we detected increased levels of CHCHD2 and decreased expression of GPNMB in TH-positive neurons from Parkin mutation carriers compared to healthy controls. This suggests a possible interaction of these three PD genes in response to a dopaminergic neuron-specific increase in oxidative stress, which further leads to the selective vulnerability of dopaminergic neurons.