Imbalanced mitochondrial dynamics in human and mouse PD brains

Mitochondrial dysfunction is a major pathogenic mechanism in Parkinson's disease (PD). Emerging studies have shown that dysregulation in mitochondrial dynamics (fission/fusion/movement) has a major negative impact on mitochondria - both morphologically and functionally. Partial genetic deletion and pharmacological inhibition of the mitochondrial fission dynamin-related protein 1 (Drp1) have been demonstrated to be beneficial in experimental models of PD. However, the expression of DRP1 (and other fission and fusion genes/proteins) has not been investigated in the brains of Parkinson's patients. Without these data, the question remains whether targeting DRP1 is a valid therapeutic target for PD. To address this gap of knowledge, first, we used post-mortem substantia nigra specimens of Parkinson's patients and controls. Significant increases in the levels of both DNM1L , which encodes DRP1, as well as the DRP1 protein were detected in Parkinson's patients. Immunostaining revealed increased DRP1 expression in dopamine (DA) neurons, astrocytes, and microglia. In addition to DRP1, the levels of other fission and fusion genes/proteins were also altered in Parkinson's patients. To complement these human studies and given the significant role of α-synuclein in PD pathogenesis, we performed time-course studies (3-, 6- and 12-month) using transgenic mice overexpressing human wild-type SNCA under the mouse Thy-1 promoter. As early as 6 months old, we detected an upregulation of Dnm1l and Drp1 in the nigral DA neurons of the SNCA mice as compared to their WT littermates. Furthermore, these mutant animals exhibited more Drp1 phosphorylation at serine 616, which promotes its translocation to mitochondria to induce fragmentation. Together, this study shows an upregulation of DRP1/Drp1 and alterations in other fission/fusion proteins in both human and mouse PD brains, leading to a pro-fission phenotype, providing additional evidence that blocking mitochondrial fission or promoting fusion is a potential therapeutic strategy for PD.