Extracellular α-synuclein levels are regulated by neuronal activity

Background: alpha-Synuclein is a presynaptic protein abundant in the cytoplasmic compartment of neurons, whereas its presence in the extracellular space has also been observed under physiological conditions. Extracellular alpha-synuclein has pathological significance, exhibiting cellular toxicity and impairment of synaptic transmission. Notably, misfolded alpha-synuclein drives the cell-to-cell propagation of pathology via the extracellular space. However, the primary mechanism that regulates the extracellular levels of alpha-synuclein remains to be determined. Methods: Using several mechanistically distinct methods to modulate neuronal/synaptic activities in primary neuronal culture and in vivo microdialysis, we examined the involvement of neuronal/synaptic activities on alpha-synuclein release. Results: We demonstrate here that physiological release of endogenous alpha-synuclein highly depends on intrinsic neuronal activities. Elevating neuronal activity rapidly increased, while blocking activity decreased, alpha-synuclein release. In vivo microdialysis experiments in freely moving mice revealed that similar to 70% of extracellular alpha-synuclein arises from neuronal activity-dependent pathway. Selective modulation of glutamatergic neurotransmission altered extracellular alpha-synuclein levels, implicating this specific neuronal network in the mechanism of activity-dependent release of alpha-synuclein. While neuronal activity tightly regulated alpha-synuclein release, elevated synaptic vesicle exocytosis per se was capable to elicit alpha-synuclein release. We also found that extracellular alpha-synuclein exists as high molecular weight species. Conclusions: The present study uncovers a novel regulatory pathway associated with alpha-synuclein release, whose dysregulation might affect various pathological actions of extracellular alpha-synuclein including its trans-synaptic propagation.