α-Synuclein is differentially expressed in mitochondria from different rat brain regions and dose-dependently down-regulates complex I activity

α-Synuclein (α-Syn) abnormality and mitochondrial deficiency are two major changes in the brain of patients with Parkinson's disease (PD). A link between α-Syn and mitochondria in PD has been demonstrated by a recent study showing that accumulation of α-Syn in the mitochondria from the PD-vulnerable brain regions was associated with decreased complex I activity of these mitochondria. In this study, we examined the normal expressions of α-Syn in mitochondria from different regions of the rat brain. We showed that α-Syn was highly expressed in the mitochondria in olfactory bulb, hippocampus, striatum, and thalamus, where the cytosolic α-Syn was also rich. However, the cerebral cortex and cerebellum were two exceptions, which contained rich cytosolic α-Syn but very low or even undetectable levels of mitochondrial α-Syn. The close quantitative association between mitochondrial and cytosolic α-Syn in most brain regions, suggests that the concentration of cytosolic α-Syn may determine the amount of α-Syn in mitochondria. This is partially supported by the in vitro experiment showing that incubation of α-Syn with endogenous α-Syn-undetectable cerebellar mitochondria caused a dose-dependent transport of α-Syn to the mitochondria. Moreover, we found that the inhibitory effect of α-Syn on complex I activity of mitochondrial respiratory chain was also dose-dependent. These results suggest that α-Syn in mitochondria is differentially expressed in different brain regions and the background levels of mitochondrial α-Syn may be a potential factor affecting mitochondrial function and predisposing some neurons to degeneration.