PARKIN is not required to sustain OXPHOS function in adult mammalian tissues

Loss-of-function variants in the PRKN gene encoding the ubiquitin E3 ligase PARKIN cause autosomal recessive early-onset Parkinson’s disease (PD). Extensive in vitro and in vivo studies have reported that PARKIN is involved in multiple pathways of mitochondrial quality control, including mitochondrial degradation and biogenesis. However, these findings are surrounded by substantial controversy due to conflicting experimental data. In addition, the existing PARKIN-deficient mouse models have failed to faithfully recapitulate PD phenotypes. Therefore, we have investigated the mitochondrial role of PARKIN during ageing and in response to stress by employing a series of conditional Parkin knockout mice. We report that PARKIN loss does not affect oxidative phosphorylation (OXPHOS) capacity and mitochondrial DNA (mtDNA) levels in the brain, heart, and skeletal muscle of aged mice. We also demonstrate that PARKIN deficiency does not exacerbate the brain defects and the pro-inflammatory phenotype observed in mice carrying high levels of mtDNA mutations. To rule out compensatory mechanisms activated during embryonic development of Parkin-deficient mice, we generated a mouse model where loss of PARKIN was induced in adult dopaminergic (DA) neurons. Surprisingly, also these mice did not show motor impairment or neurodegeneration, and no major transcriptional changes were found in isolated midbrain DA neurons. Finally, we report a patient with compound heterozygous PRKN pathogenic variants that lacks PARKIN and has developed PD. The PARKIN deficiency did not impair OXPHOS activities or induce mitochondrial pathology in skeletal muscle from the patient. Altogether, our results argue that PARKIN is dispensable for OXPHOS function in adult mammalian tissues. To investigate transcriptional responses induced by PARKIN loss in adult DA neurons we used a strategy to isolate fluorophore-labelled midbrain DA neurons from adult mice. We generated iParkinDA (genotype: Parkin loxP/loxP; +/DatCreERT2) mice allowing the disruption of Parkin by tamoxifen injection at the age of 5-7 weeks. To investigate the efficiency of the inducible knockout system, we used a reporter allele expressing mitochondrially targeted YFP preceded by a loxP-flanked stop cassette (mitoYFP). We also generated control mice containing the mitoYFP allele. We isolated DA neurons for bulk RNAseq analysis at 5 and 40 weeks after tamoxifen injection from both iParkin and control mice. For each time point, we collected both mitoYFP-positive (DA neurons) and mitoYFP-negative cells (mixed cells).