α-synuclein and synapsin III cooperatively regulate synaptic function in dopamine neurons

The main neuropathological features of Parkinson's disease are dopaminergic nigrostriatal neuron degeneration, and intraneuronal and intraneuritic proteinaceous inclusions named Lewy bodies and Lewy neurites, respectively, which mainly contain alpha-synuclein (alpha-syn, also known as SNCA). The neuronal phosphoprotein synapsin III (also known as SYN3), is a pivotal regulator of dopamine neuron synaptic function. Here, we show that alpha-syn interacts with and modulates synapsin III. The absence of alpha-syn causes a selective increase and redistribution of synapsin III, and changes the organization of synaptic vesicle pools in dopamine neurons. In alpha-syn-null mice, the alterations of synapsin III induce an increased locomotor response to the stimulation of synapsin-dependent dopamine overflow, despite this, these mice show decreased basal and depolarization-dependent striatal dopamine release. Of note, synapsin III seems to be involved in alpha-syn aggregation, which also coaxes its increase and redistribution. Furthermore, synapsin III accumulates in the caudate and putamen of individuals with Parkinson's disease. These findings support a reciprocal modulatory interaction of alpha-syn and synapsin III in the regulation of dopamine neuron synaptic function.