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 over-expression 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.Significance statement The accumulation of the pathogenic proteins α-synuclein and tau drives prion-like trans-neuronal propagation and underlies distinct neurodegenerative diseases, such as Parkinson's and Alzheimer's disease. Using a synaptic culture model of protein propagation, we identified a shared mechanism of synaptic dysfunction caused by these otherwise distinct proteins. In our models, propagated α-synuclein or tau disrupt protein degradation pathways, leading to their accumulation. These accumulated proteins promote excessive microtubule assembly and sequester the key endocytic protein dynamin, eventually impairing synaptic vesicle endocytosis. This cascade results in synaptic dysfunction that could compromise neurocircuit operations critical for brain functions. Our clarification of these mechanistic steps will improve our understanding of the synaptic pathophysiology underlying diverse symptoms of distinct neurodegenerative diseases.