Common Mechanism Underlying Synaptic Dysfunction Caused by Preformed Fibril-Induced Accumulation of α-Synuclein or Tau in a Culture Propagation Model

In sporadic neurodegenerative diseases, the endogenous proteins α-synuclein in Parkinson's disease and tau in Alzheimer's disease undergo pathogenic prion-like propagation over many years, accumulating in both soluble and insoluble forms in neurons including synapses, where they impair synaptic transmission and potentially cause various neuronal symptoms. To investigate the functional outcome of such synaptic accumulation, we induced accumulation of endogenous proteins in murine and human synapses by incubating mouse (of either sex) neuronal cultures with pathogenic preformed fibrils (pffs). Two weeks after treatment with human α-synuclein or tau pff, the respective endogenous proteins accumulated in neurons including presynaptic terminals, where we also observed tubulin accumulation, suggesting microtubule over-assembly. These were not associated with mRNA upregulation and were prevented by pharmacological stimulation of autophagy. Both pffs caused accumulation of p62 in cell bodies, suggesting compromised protein degradation. pHluorin imaging in synapses indicated a marked prolongation of vesicular endocytic time, which was rescued by pharmacological depolymerization of microtubules or by the overexpression of full-length dynamin 1. Since dynamin is a high-affinity binding partner of microtubules as well as an endocytic key molecule, over-assembled microtubules can sequester dynamin, thereby inhibiting endocytosis. We conclude that pff-induced accumulation of α-synuclein or tau in presynaptic terminals can disrupt vesicle endocytosis through a common mechanism. Since endocytosis-dependent vesicle recycling is critical for maintaining neurotransmitter release, its disruption can affect the neurocircuitry operations involved, thereby causing diverse symptoms associated with neurodegenerative diseases. Thus, our data suggest a common molecular mechanism underlying synaptic dysfunctions associated with Parkinson's and Alzheimer's diseases.