Alpha-synuclein overexpression protects against paraquat-induced neurodegeneration

Abstract

Alpha-synuclein is likely to play a role in neurodegenerative processes, including the degeneration of nigrostriatal dopaminergic neurons that underlies Parkinson's disease. However, the toxicological properties of alpha-synuclein remain relatively unknown. Here, the relationship between alpha-synuclein expression and neuronal injury was studied in mice exposed to the herbicide paraquat. Paraquat neurotoxicity was compared in control animals versus mice with transgenic expression of human alpha-synuclein driven by the tyrosine hydroxylase (TH) promoter. In control mice, paraquat caused both the formation of alpha-synuclein-containing intraneuronal deposits and the degeneration of nigrostriatal neurons, as demonstrated by silver staining and a reduction of the counts of TH-positive and Nissl-stained cells. Mice overexpressing alpha-synuclein, either the human wild-type or the Ala53Thr mutant form of the protein, displayed paraquat-induced protein aggregates but were completely protected against neurodegeneration. These resistant animals were also characterized by increased levels of HSP70, a chaperone protein that has been shown to counteract paraquat toxicity in other experimental models and could therefore contribute to neuroprotection in alpha-synuclein transgenic mice. The results indicate a dissociation between toxicant-induced alpha-synuclein deposition and neurodegeneration. They support a role of alpha-synuclein against toxic insults and suggest that its involvement in human neurodegenerative processes may arise not only from a gain of toxic function, as previously proposed, but also from a loss of defensive properties.

Fig. 1.

Effect of α-synuclein overexpression on paraquat-induced neurodegeneration. Silver staining of the substantia nigra pars compacta of control (A, C) and α-synuclein-overexpressing (B, D) mice injected with saline (A, B) or paraquat (C, D). Animals were treated once a week for 3 consecutive weeks and killed at 7 d after the last injection of saline or paraquat. The arrow inC indicates silver grain deposition in a degenerating neuron, and the inset represents a higher magnification image of this neuron. Sections shown in this figure were obtained from mice overexpressing human wild-type α-synuclein and corresponding controls. Similar results, however, were observed when saline or paraquat was administered to mice overexpressing human Ala53Thr mutant α-synuclein and littermate controls. Scale bar, 25 μm.

Fig. 2.

Paraquat-induced α-synuclein deposition in control and α-synuclein-overexpressing mice. Confocal images of neurons in the substantia nigra pars compacta of control (A, C) and α-synuclein-overexpressing (B, D) mice injected with saline (A, B) or paraquat (C, D). Midbrain sections were stained with an antibody against α-synuclein and counterstained with Hoechst bisbenzimide. Arrows inC and D indicate α-synuclein-positive deposits. Sections shown in this figure were obtained from mice overexpressing human Ala53Thr mutant α-synuclein and corresponding controls. Similar results, however, were observed when saline or paraquat was administered to mice overexpressing human wild-type α-synuclein and littermate controls. Scale bars, 10 μm.

Fig. 3.

Enhanced HSP70 levels in α-synuclein overexpressing mice. Western blot analysis of HSP70 and synaptophysin immunoreactivities in homogenates from the ventral mesencephalon (A) and cerebellum (B) of control animals and transgenic mice overexpressing human wild-type α-synuclein is shown. C–H, Midbrain sections from control animals (C–E) and mice overexpressing human Ala53Thr mutant α-synuclein (F–H) were immunostained with antibodies against α-synuclein (C, F) and HSP70 (D, G). Merged images (E, H) show colocalization within nigral cell bodies of overexpressing animals (H). Scale bar, 100 μm.