The protein α-synuclein accumulates in the brain of patients with sporadic Parkinson's disease (PD), and increased gene dosage causes a severe, dominantly inherited form of PD, but we know little about the effects of synuclein that precede degeneration. α-Synuclein localizes to the nerve terminal, but the knockout has little if any effect on synaptic transmission. In contrast, we now find that the modest overexpression of α-synuclein, in the range predicted for gene multiplication and in the absence of overt toxicity, markedly inhibits neurotransmitter release. The mechanism, elucidated by direct imaging of the synaptic vesicle cycle, involves a specific reduction in size of the synaptic vesicle recycling pool. Ultrastructural analysis demonstrates reduced synaptic vesicle density at the active zone, and imaging further reveals a defect in the reclustering of synaptic vesicles after endocytosis. Increased levels of α-synuclein thus produce a specific, physiological defect in synaptic vesicle recycling that precedes detectable neuropathology.
Bibliographical noteFunding Information:
We thank E. Wallender and B. Mukherjee for technical support, K. Thorn and the UCSF Nikon Center for assistance with the imaging, M. Larsson for help with the electron microscopy, A. Frigessi for guidance in the statistical analysis, and R. Tsien for thoughtful suggestions. This work was supported by the UCSF MSTP and a predoctoral fellowship from the Hillblom Foundation (to V.M.N.), a postdoctoral fellowship from the Hillblom Foundation and a Burroughs-Wellcome Medical Scientist Fund Career Award (to K.N.), the American Heart Association (W.L.), the University of Oslo (V.B.), the Norwegian Research Council (F.A.C.), the National Institutes of Mental Health (R.A.N. and R.H.E.), the National Parkinson Foundation, and the Michael J. Fox Foundation (R.H.E.).