Parkin-deficient mice are not more sensitive to 6-hydroxydopamine or methamphetamine neurotoxicity

Abstract

Background: Autosomal recessive juvenile parkinsonism (AR-JP) is caused by mutations in the parkin gene which encodes an E3 ubiquitin-protein ligase. Parkin is thought to be critical for protecting dopaminergic neurons from toxic insults by targeting misfolded or oxidatively damaged proteins for proteasomal degradation. Surprisingly, mice with targeted deletions of parkin do not recapitulate robust behavioral or pathological signs of parkinsonism. Since Parkin is thought to protect against neurotoxic insults, we hypothesized that the reason Parkin-deficient mice do not develop parkinsonism is because they are not exposed to appropriate environmental triggers. To test this possibility, we challenged Parkin-deficient mice with neurotoxic regimens of either methamphetamine (METH) or 6-hydroxydopamine (6-OHDA). Because Parkin function has been linked to many of the pathways involved in METH and 6-OHDA toxicity, we predicted that Parkin-deficient mice would be more sensitive to the neurotoxic effects of these agents.

Results: We found no signs consistent with oxidative stress, ubiquitin dysfunction, or degeneration of striatal dopamine neuron terminals in aged Parkin-deficient mice. Moreover, results from behavioral, neurochemical, and immunoblot analyses indicate that Parkin-deficient mice are not more sensitive to dopaminergic neurotoxicity following treatment with METH or 6-OHDA.

Conclusion: Our results suggest that the absence of a robust parkinsonian phenotype in Parkin-deficient mice is not due to the lack of exposure to environmental triggers with mechanisms of action similar to METH or 6-OHDA. Nevertheless, Parkin-deficient mice could be more sensitive to other neurotoxins, such as rotenone or MPTP, which have different mechanisms of action; therefore, identifying conditions that precipitate parkinsonism specifically in Parkin-deficient mice would increase the utility of this model and could provide insight into the mechanism of AR-JP. Alternatively, it remains possible that the absence of parkinsonism in Parkin-deficient mice could reflect fundamental differences between the function of human and mouse Parkin, or the existence of a redundant E3 ubiquitin-protein ligase in mouse that is not found in humans. Therefore, additional studies are necessary to understand why Parkin-deficient mice do not display robust signs of parkinsonism.

Figure 1

Parkin deficient-mice are not more sensitive to 6-OHDA toxicity. A. WT mice (filled square symbols) and KO mice (open circle symbols) treated with 4 μg 6-OHDA or 8 μg 6-OHDA displayed similar contralateral, rotational behavior (expressed as net negative rotations) following APO-treatment. B. Ipsilateral rotational behavior (expressed as net positive rotations) following AMPH treatment was also indistinguishable between WT and KO mice. C. There was no difference between WT and KO mice in striatal dopamine (DA) levels (expressed as ng/mg protein) on the intact, uninjected side versus the side treated with SAL, 4 μg 6-OHDA, or 8 μg 6-OHDA. The numbers of mice used for each genotype are indicated in A; these same mice were also analyzed in B and C. Data are expressed as mean ± SEM.

Figure 2

Parkin-deficient mice are not more sensitive to METH toxicity. The hyperthermic response to METH treatment at A. 2.5 mg/kg and B. 5.0 mg/kg was indistinguishable between WT (filled square symbols) and KO (open circle symbols) mice. Body temperature (°C) was measured before METH treatment (time = 0 hr) and every hour thereafter. C. There was no difference between WT and KO mice in the depletion of striatal dopamine (DA) levels (expressed as ng/mg protein) following METH treatment. D. There was also no difference in the reduction of striatal DAT levels following METH treatment (data are expressed in arbitrary units after normalization to WT levels). The numbers of mice used for each genotype are indicated in the figure. Data are expressed as mean ± SEM.