Increased Expression of α-Synuclein Impairs Trafficking of the Essential SNARE Protein Synaptobrevin-2.

α-Synuclein is a presynaptic protein that has been extensively studied for its role in Parkinson's disease. Multiplication and missense variants in the gene encoding α-synuclein are causes of familial PD, and aggregated α-synuclein is a major component of Lewy bodies and Lewy neurites, the histopathological hallmark of the disease. However, the native function of α-synuclein remains unclear, though several lines of evidence suggest it modulates presynaptic function. α-Synuclein binds the vesicular SNARE protein synaptobrevin-2/VAMP2, and increased levels of α-synuclein potentiate SNARE complex levels without impacting exocytic rate. Given the essential role of synaptobrevin-2 in synaptic function we sought to examine how increased expression of α-synuclein, at levels observed in familial synucleinopathies with SNCA triplication, impacts the trafficking of this essential SNARE protein. Modest (approximately 2-fold) overexpression of α-synuclein in cultured murine hippocampal neurons caused a slowing in the endocytic retrieval of synaptobrevin-2, without impacting either the targeting of the protein to synapses or altering endocytosis. We also show that α-synuclein is not working antagonistically with the synaptobrevin-2 chaperone synaptophysin, and likely acts at least partially via direct binding to synaptobrevin-2. Finally, increased expression of α-synuclein induces synaptic depression upon repeated neuronal depolarisation, in a manner requiring binding to synaptobrevin-2. We reveal α-synuclein as a modulator of synaptobrevin-2 endocytic trafficking and highlight perturbed SNARE protein trafficking as a potential contributor to synaptic dysfunction and neuronal health in Parkinson's disease.