Mitochondrial dysfunction in the striatum of aged chronic mouse model of Parkinson's disease

Abstract

Mitochondrial oxidative stress and dysfunction has been implicated as a possible mechanism for the onset and progression of Parkinson-like neurodegeneration. However, long-term mitochondrial defects in chronic animal neurodegenerative models have not been demonstrated. In this study, we investigated the function of striatal mitochondria 6 weeks after the induction of a chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease (MPD). Although severe depression of mitochondrial respiration was observed immediately after acute administrations of MPTP, we failed to detect a significant mitochondrial inhibition in presence of striatal dopamine (DA) deficit 6 weeks after the chronic MPD induction in young adult mice. In contrast, when aged mice were chronically treated with MPTP and at 6 weeks post-treatment, these animals suffered an inhibition of the basal (state 4) and adenosine 5'-diphosphate-stimulated (state 3) respiration and a fall in adenosine triphosphate level in the striatal mitochondria. The aged chronic MPD also brought about a sustained diminution of striatal anti-oxidant enzyme levels including that of superoxide dismutases and cytochrome c. The mitochondrial deficits in the striatum of aged chronic MPD 6 weeks after treatment were further correlated with significant losses of striatal DA, tyrosine hydroxylase, DA uptake transporter, and with impaired movement when tested on a challenging beam. Our findings suggest that MPTP may trigger the neurodegenerative process by obstructing the mitochondrial function; however, striatal mitochondria in young animals may potentially rejuvenate, whereas mitochondrial dysfunction is sustained in the aged chronic MPD. Therefore, the aged chronic MPD may serve as a suitable investigative model for further elucidating the integral relationship between mitochondrial dysfunction and neurodegenerative disorder, and for assessing the therapeutic efficacy of mitochondrial protective agents as potential neuroprotective drugs.

Keywords: Parkinson's disease; age-related; chronic animal model; mitochondrial defects; neurodegeneration.

Figure 1

Protein expression of (A) Mn SOD, (B) Cu–Zn SOD, and (C) cytochrome c in the striatum of young control, acute MPTP, and 6-week post-chronic MPTP/probenecid-treated parkinsonian (MPD) mice. Western blot technique and densitometry analysis were used to detect and quantify the respective anti-oxidant protein bands and GAPDH. The protein contents of Mn SOD, Cu–Zn SOD, and cytochrome c were expressed as a ratio to that of GAPDH to ensure that the changes in protein levels were not due to sample loading variances. A representative image of western blot protein bands was correspondingly shown above each set of bar graph. Each data point represents mean ± SEM, N = 6 per group of animals. The levels of Mn SOD, Cu–Zn SOD, and cytochrome c in the striatum of young control, acute MPTP, and 6-week post-chronic MPD mice were not statistically different from each other.

Figure 2

Western blot analysis of (A) TH and (B) DAT expression in the striatum of aged chronic probenecid control and aged chronic MPD mice. A representative image of western blot protein bands was correspondingly shown above each set of bar graph. The protein contents of TH and DAT were expressed as a ratio to that of GAPDH and each data point represents mean ± SEM (N = 5 for chronic control and N = 8 for chronic MPD). Western blots revealed that there were significant reductions of TH and DAT content in the striatum of aged MPD 6 weeks after chronic treatment when compared with that of the control mice (*P < 0.001).

Figure 3

Protein expression of (A) Mn SOD, (B) Cu–Zn SOD, and (C) cytochrome c in the striatum of aged chronic probenecid control and aged chronic MPD mice. A representative image of western blot protein bands was correspondingly shown above each set of bar graph. The protein contents of Mn SOD, Cu–Zn SOD, and cytochrome c were expressed as a ratio to that of GAPDH and each data point represents mean ± SEM, N = 6 per group of animals. Statistical analyses revealed that the levels of Mn SOD, Cu–Zn SOD, and cytochrome c in the striatum of aged chronic MPD 6 weeks after chronic treatment were significantly lower than that of chronic control animals (*P < 0.05).