Mitochondrial Import and Accumulation of α-Synuclein Impair Complex I in Human Dopaminergic Neuronal Cultures and Parkinson Disease Brain

Abstract

␣-Synuclein, a protein implicated in the pathogenesis of Parkinson disease (PD), is thought to affect mitochondrial functions, although the mechanisms of its action remain unclear. In this study we show that the N-terminal 32 amino acids of human ␣-synuclein contain cryptic mitochondrial targeting signal, which is important for mitochondrial targeting of ␣-synuclein. Mitochondrial imported ␣-synuclein is predominantly associated with the inner membrane. Accumulation of wild-type ␣-synuclein in the mitochondria of human dopaminergic neurons caused reduced mitochondrial complex I activity and increased production of reactive oxygen species. However, these defects occurred at an early time point in dopaminergic neurons expressing familial ␣-synuclein with A53T mutation as compared with wild-type ␣-synuclein. Importantly, ␣-synuclein that lacks mitochondrial targeting signal failed to target to the mitochondria and showed no detectable effect on complex I function. The PD relevance of these results was investigated using mitochondria of substantia nigra, striatum, and cerebellum of postmortem late-onset PD and normal human brains. Results showed the constitutive presence of ϳ14-kDa ␣-synuclein in the mitochondria of all three brain regions of normal subjects. Mitochondria of PD-vulnerable substantia nigra and striatum but not cerebellum from PD subjects showed significant accumulation of ␣-synuclein and decreased complex I activity. Analysis of mitochondria from PD brain and ␣-synuclein expressing dopaminergic neuronal cultures using blue native gel electrophoresis and immunocapture technique showed the association of ␣-synuclein with complex I. These results provide evidence that mitochondrial accumulated ␣-synuclein may interact with complex I and interfere with its functions. Parkinson disease (PD)2 is associated with the degeneration of dopaminergic neurons in the substantia nigra pars compacta.

Revisiting oxidative stress and mitochondrial dysfunction in the pathogenesis of Parkinson disease—resemblance to the effect of amphetamine drugs of abuse

“…As described before, oxidative damage to alphaͲsyn affects its aggregation Ͳ an effect that partially explains the cellular toxicity of the protein. AlphaͲsyn contains an NͲterminal mitochondrialͲtargeting sequence [181], and cytosol acidification or alphaͲsyn overexpression can lead the protein to localize to mitochondria [182,183], resulting in complex I impairment and increased ROS production [181], increased protein tyrosine nitration and decreased mitochondrial transmembrane potential [184]. In addition, it was shown that human embryonic kidney cells that overexpress alphaͲsyn, show enhanced susceptibility to cell death and have lower ATP levels compared to control cells [183].…”

Mitochondrial translocation of α-synuclein is promoted by intracellular acidification

“…While this manuscript was under revision, two papers [69,70] reported the import of synuclein into mitochondria and its effect on mitochondrial physiology. Under our conditions, however, we are able to detect synuclein only on the outer mitochondrial membrane.…”

Methionine sulfoxide reductase A protects dopaminergic cells from Parkinson's disease-related insults

“…Although α-synuclein is normally considered a cytosolic protein, a recent study revealed that the protein is also imported into mitochondria [56], suggesting that MsrA-mediated α-synuclein repair may occur in either compartment. Another unresolved question is whether MsrA carries out its neuroprotective activity preferentially in neurons or glia in the mixed midbrain cultures, given that the lentiviruses used in this study infect both cell types.…”