Parkinson’s Onset Theory Challenged: Synaptic Dysfunction Before Neuron Death

Parkinson’s disease affects 1% to 2% of the population and is characterized by resting tremor, rigidity and bradykinesia (slowness of movement). These motor symptoms are due to the progressive loss of dopaminergic neurons in the midbrain.

The findings, which will be published Sept. 15 in Neuron, open a new avenue for therapies, the scientists said.

“We showed that dopaminergic synapses become dysfunctional before neuronal death occurs,” said lead author Dr. Dimitri Krainc, chair of neurology at Northwestern University Feinberg School of Medicine and director of the Simpson Querrey Center for Neurogenetics.

“Based on these findings, we hypothesize that targeting dysfunctional synapses before the neurons are degenerated may represent a better therapeutic strategy.”

The study investigated patient-derived midbrain neurons, which is critical because mouse and human dopamine neurons have a different physiology and findings in the mouse neurons are not translatable to humans, as highlighted in Krainc’s research recently published in Science.

Northwestern scientists found that dopaminergic synapses are not functioning correctly in various genetic forms of Parkinson’s disease. This work, together with other recent studies by Krainc’s lab, addresses one of the major gaps in the field: how different genes linked to Parkinson’s lead to degeneration of human dopaminergic neurons.

 

Neurons from PD patients with mutant parkin displayed defective recycling of synaptic vesicles, leading to accumulation of toxic oxidized dopamine that was attenuated by boosting endophilin A1 expression.

Notably, combined heterozygous parkin and homozygous PTEN-induced kinase 1 (PINK1) mutations led to earlier disease onset compared with homozygous mutant PINK1 alone, further underscoring a PINK1-independent role for parkin in contributing to disease.

Thus, this study identifies a pathway for selective activation of parkin at human dopaminergic synapses and highlights the importance of this mechanism in the pathogenesis of Parkinson’s disease.

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