The Absence of Parkin Does Not Promote Dopamine or Mitochondrial Dysfunction in PolgAD257A/D257AMitochondrial Mutator Mice

Abstract

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). In this study, we generated a transgenic model by crossing germline Parkin–/–mice with PolgAD257Amice, an established model of premature aging and mitochondrial stress. We hypothesized that loss of Parkin–/–in PolgAD257A/D257Amice would exacerbate mitochondrial dysfunction, leading to loss of dopamine neurons and nigral-striatal specific neurobehavioral motor dysfunction. We found that aged Parkin–/–/PolgAD257A/D257Amale and female mice exhibited severe behavioral deficits, nonspecific to the nigral-striatal pathway, with neither dopaminergic neurodegeneration nor reductions in striatal dopamine. We saw no difference in expression levels of nuclear-encoded subunits of mitochondrial markers and mitochondrial Complex I and IV activities, although we did observe substantial reductions in mitochondrial-encoded COX41I, indicating mitochondrial dysfunction as a result of PolgAD257A/D257AmtDNA mutations. Expression levels of mitophagy markers LC3I/LC3II remained unchanged between cohorts, suggesting no overt mitophagy defects. Expression levels of the parkin substrates, VDAC, NLRP3, and AIMP2 remained unchanged, suggesting no parkin dysfunction. In summary, we were unable to observe dopaminergic neurodegeneration with corresponding nigral-striatal neurobehavioral deficits, nor Parkin or mitochondrial dysfunction in Parkin–/–/PolgAD257A/D257Amice. These findings support a lack of synergism of Parkin loss on mitochondrial dysfunction in mouse models of mitochondrial deficits.SIGNIFICANCE STATEMENTProducing a mouse model of Parkinson's disease (PD) that is etiologically relevant, recapitulates clinical hallmarks, and exhibits reproducible results is crucial to understanding the underlying pathology and in developing disease-modifying therapies. Here, we show that Parkin–/–/PolgAD257A/D257Amice, a previously reported PD mouse model, fails to reproduce a Parkinsonian phenotype. We show that these mice do not display dopaminergic neurodegeneration nor nigral-striatal-dependent motor deficits. Furthermore, we report that Parkin loss does not synergize with mitochondrial dysfunction. Our results demonstrate that Parkin–/–/PolgAD257A/D257Amice are not a reliable model for PD and adds to a growing body of work demonstrating that Parkin loss does not synergize with mitochondrial dysfunction in mouse models of mitochondrial deficits.