Neuronal Extracellular Vesicles Carrying APOE Downregulate Filament Actin Polymerization Signaling to Inhibit Synapse Formation in Alzheimer's Disease.

Synaptic formation impairment is closely correlated with cognitive impairment in Alzheimer's disease (AD), yet the underlying mechanisms remain incompletely understood. Emerging evidence indicates that extracellular vesicles (EVs), critical mediators of intercellular communication, are implicated in the progression of AD. However, the specific mechanisms through which neuron-derived EVs contribute to synaptic formation impairment in AD remain unexplored. In this study, we characterized EVs derived from primary neurons of APP/PS1 transgenic mice (APPNEVs) and investigated their impact on synapse formation. Transmission electron microscopy, nanoparticle flow cytometry, and immunoblotting confirmed that APPNEVs and WT neuron-derived EVs (WTNEVs) had similar morphology, size, and canonical small EVs markers. We further revealed that APPNEVs significantly impaired neuronal synapse formation by downregulating synaptic proteins PSD95 and Synaptophysin (SYP), reducing total synapse number, and shifting synapse morphology toward immature states. Proteomic profiling via mass spectrometry identified APOE as a key upregulated protein in APPNEVs. Pharmacological inhibition of APOE with EZ-482 effectively prevented APPNEV-induced synaptic formation impairment, APPNEV-mediated downregulation of synaptic proteins, and the APPNEV-induced decrease in synaptic maturity. Mechanistically, APPNEVs suppressed Rac1-N-WASP-Arp2/3-mediated filament actin polymerization, a critical pathway for synaptic spine formation, which was prevented by APOE inhibition. In vivo stereotactic injection of APPNEVs into the hippocampus of WT mice further validated their detrimental effects on synaptic integrity, which were prevented by EZ-482 treatment. Collectively, these findings demonstrate that APPNEVs mediate synaptic damage via carrying APOE, providing novel insights into EV-mediated neurodegeneration in AD and highlighting APOE as a potential therapeutic target for preserving synaptic formation.