Abstract
Many mutations in genes encoding proteins such as Parkin, PTEN-induced putative kinase 1 (PINK1), protein deglycase DJ-1 (DJ-1 or PARK7), leucine-rich repeat kinase 2 (LRRK2), and α-synuclein have been linked to familial forms of Parkinson's disease (PD). The consequences of these mutations, such as altered mitochondrial function and pathological protein aggregation, are starting to be better understood. However, little is known about the mechanisms explaining why alterations in such diverse cellular processes lead to the selective loss of dopamine (DA) neurons in the substantia nigra (SNc) in the brain of individuals with PD. Recent work has shown that one of the reasons for the high vulnerability of SNc DA neurons is their high basal rate of mitochondrial oxidative phosphorylation (OXPHOS), resulting from their highly complex axonal arborization. Here, we examined whether axonal growth and basal mitochondrial function are altered in SNc DA neurons from Parkin-, Pink1-, or DJ-1-KO mice. We provide evidence for increased basal OXPHOS in Parkin-KO DA neurons and for reduced survival of DA neurons that have a complex axonal arbor. The surviving smaller neurons exhibited reduced vulnerability to the DA neurotoxin and mitochondrial complex I inhibitor MPP+, and this reduction was associated with reduced expression of the DA transporter. Finally, we found that glial cells play a role in the reduced resilience of DA neurons in these mice and that WT Parkin overexpression rescues this phenotype. Our results provide critical insights into the complex relationship between mitochondrial function, axonal growth, and genetic risk factors for PD.
Highlights
Many mutations in genes encoding proteins such as Parkin, PTEN-induced putative kinase 1 (PINK1), protein deglycase DJ-1 (DJ-1 or PARK7), leucine-rich repeat kinase 2 (LRRK2), and ␣-synuclein have been linked to familial forms of Parkinson’s disease (PD)
Many mutations in genes coding for proteins such as the E3 ubiquitin ligase Parkin, PTEN-induced putative kinase 1 (Pink1), protein deglycase DJ-1, leucine-rich repeat kinase 2 (LRRK2), and ␣-synuclein have been linked to familial forms of the disease
Since the discovery of the first human mutations causing familial forms of Parkinson’s disease, extensive work has been performed to identify the mechanisms by which loss- or gainof-function of gene products, including Parkin, Pink1, DJ-1, LRRK2, and ␣-synuclein, trigger the disease process
Summary
Experiments were performed using a mouse primary culture system, including DA neurons microdissected from the SNc and VTA of newborn Parkin-, Pink1-, or DJ-1–KO mice. If this were the case, a prediction is that the increase in OCR should not be accompanied by an increase in ATP production and could even be accompanied by a decrease in ATP To test this hypothesis, we measured ATP content in each culture and discovered a significant reduction of 44% in ATP levels in Parkin-KO mouse SNc cultures, with no change in Pink1-KO or DJ-1–KO mouse cultures (Fig. 2, G–I). We measured ATP content in each culture and discovered a significant reduction of 44% in ATP levels in Parkin-KO mouse SNc cultures, with no change in Pink1-KO or DJ-1–KO mouse cultures (Fig. 2, G–I) Such a possible uncoupling between OCR and ATP production appears to be exacerbated in Parkin-KO mouse VTA DA neuron cultures, perhaps because more of these neurons survived to this impairment, compatible with their higher resilience in response to oxidative stress [9]. We found that fewer cells in Parkin-KO mouse cortical glial cultures after 7 this completely reverted this reduced survival phenotype
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