Background: Parkinson's disease (PD) is marked by dopaminergic neuron loss and motor deficits. Impaired autophagy and mitophagy contribute to PD, and α-synuclein (α-Syn) may worsen neurodegeneration by disrupting these pathways. This study investigates α-Syn's role in aggravating motor deficits and dopaminergic neuron loss in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice, focusing on its inhibition of autophagy and mitophagy via mechanistic target of rapamycin (mTOR).

Methods: The MPTP-induced PD mouse model was used to study α-Syn's role in PD. Motor function was assessed using the tail suspension, pole, and traction tests, while cognitive function was evaluated with the Morris water maze. α-Syn and tyrosine hydroxylase (TH) levels in the substantia nigra were measured using Western blotting and immunohistochemistry. Autophagy and mitochondrial autophagy markers were analyzed via Western blotting. SH-SY5Y cells were treated with 1-methyl-4-phenylpyridinium ion (MPP⁺) to model PD in vitro, followed by interventions α-Syn and rapamycin (Rapa). Cell viability and apoptosis were assessed using 5-ethynyl-2′-deoxyuridine (EdU) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining, while autophagy and mitochondrial autophagy markers, reactive oxygen species (ROS) levels, and mitochondrial/lysosomal activity were analyzed using fluorescence staining and co-localization with MitoTracker and LysoTracker.

Results: The tail suspension, pole, and traction tests showed that MPTP treatment significantly impaired motor function, with α-Syn further exacerbating deficits (p < 0.05). However, Rapa improved motor function. The Morris water maze test revealed increased escape latency and reduced swimming speed in the MPTP group, indicating spatial learning impairment, which worsened with α-Syn but improved with α-Syn+Rapa (p < 0.05). The spatial probe test showed decreased spatial memory in the MPTP + α-Syn group, with significant improvement in the α-Syn+Rapa group. Western blotting and immunohistochemistry showed that α-Syn enhanced MPTP-induced dopaminergic neuron degeneration and inhibited autophagy and mitophagy (p < 0.05). In vitro, α-Syn worsened SH-SY5Y cell viability and apoptosis, and inhibited autophagy and mitophagy by activating the mTOR pathway (p < 0.05).

Conclusions: α-Syn induces dopaminergic neuron degeneration and worsens PD by inhibiting autophagy and mitophagy, but Rapa can partially reverse this by restoring these processes. Targeting autophagy and mitophagy may offer a promising strategy for PD treatment.