Mitochondrial DNA depletion in respiratory chain‐deficient Parkinson disease neurons

Parkinson disease (PD) is a progressive movement disorder characterized by tremor, rigidity, bradykinesia, and postural instability that affects about 1% of those aged > 65 years.1 The pathological hallmarks of PD are selective loss of dopaminergic neurons and the presence of Lewy bodies in the substantia nigra (SN).2 Mitochondrial dysfunction has emerged as a potential mechanism in PD. Soon after the detrimental effects of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine on motor function were described and the inhibitory action of the toxin against respiratory chain complex I (CI) was unraveled,3 isolated CI deficiency was discovered in homogenates from postmortem SN samples of PD patients.4 The importance of this finding was emphasized when familial PD cases were found to harbor mutations in proteins involved in the removal of damaged mitochondria or the scavenging of reactive oxygen species that are predominantly generated by the electron transport chain.5 A study aiming to elucidate the molecular underpinnings of mitochondrial dysfunction in idiopathic PD (IPD) patients at the single‐cell level identified an accumulation of respiratory chain complex IV (CIV)‐deficient SN neurons with large somatic mtDNA deletions.6 Interestingly, the primary risk factor for PD development is ageing, which is itself correlated with the presence of CIV‐deficient neurons at similar levels to patients with IPD,6, 7 suggesting that age‐related damage accumulation could contribute to neuronal demise in IPD.6, 7 However, in the investigated IPD patients,6 CIV‐negative SN neurons constituted <3% of the total number of analyzed cells, a fraction well below the detection limit in traditional homogenate analysis. In comparison, the postmortem results from Schapira and colleagues4 imply an impairment of CI that is of sufficient magnitude to be detected in SN homogenates. Due to the lack of a robust histochemical method for the assessment of CI activity in single neurons, it has remained elusive whether mtDNA damage also underlies CI deficiency in IPD. In this study, we aimed to determine the relative incidence of respiratory chain abnormalities and the molecular mechanisms underlying the loss of respiratory chain complexes in individual dopaminergic neurons from IPD patients. To establish the extent and relationship of these deficiencies in single neurons rather than SN homogenate, we employed quantitative quadruple immunofluorescence as an indicator of respiratory chain complex function at single‐cell resolution.8 In combination with laser‐capture microdissection (LCM) and real‐time polymerase chain reaction (PCR) analysis, this approach revealed altered levels of factors responsible for crosstalk between mitochondrial and nuclear compartments as a cause of respiratory chain dysfunction in IPD.