Role of extracellular vesicles in early synaptic dysfunction in AD

Background Synaptic dysfunction is an early mechanism in Alzheimer’s Disease (AD) which involves progressively larger areas of the brain over time. However, how it starts and propagates is unknown. We hypothesised that large extracellular vesicles (EVs) released by microglia exposed to and carrying Aβ42 (Aβ-EVs) may be responsible for these early events in AD. Method Combining optical manipulation with time lapse imaging we placed single Aβ-EVs and ctrl-EVs (released by microglia not exposed to Aβ) on axons or dendrites of RFP-positive cultured neuron to test both the dynamics of their interaction with neurons and their effects on the synapse. Then, EVs were stereotaxically injected into the mouse entorhinal cortex (EC), one of the most vulnerable regions in AD, and long-term potentiation (LTP) was measured in the EC and in its main target region, the dentate gyrus of the hippocampus (DG), through field potential extracellular recordings in cortico-hippocampal brain slices. Finally, taking advantage of high-resolution accurate-mass spectrometry SWATH™-MS, we investigated the molecular differences between ctrl-EVs and Aβ-EVs. Result Aβ-EVs were able to move along the axonal surface more efficiently than ctrl-EVs and predominantly in an anterograde direction. They locally altered dendritic spine morphology in vitro and impaired LTP in vivo. One hour after injection in the EC, LTP was impaired in the EC of brains injected with Aβ-EVs but not ctrl-EVs. However, 24h after Aβ-EV injection, LTP was impaired also in the DG, indicating a spreading of synaptic dysfunction between the two connected regions. Results were reproduced using large EVs released by microglia exposed to naturally secreted Aβ oligomers, whereas neither Aβ alone nor inflammatory EVs devoid of Aβ were able to propagate LTP impairment. Importantly, when Aβ-EV motility was limited by annexin-V coating, no propagation of LTP deficit occurred along the EC-DG circuit. Proteomic analysis provided insights into the possible molecular mechanisms underneath Aβ-EV extracellular motion at the neuronal surface. Conclusion Our data provide evidence of the involvement of microglial EVs in early synaptic dysfunction, through a new mechanism - extracellular motion at the neuronal surface - not implying EV internalization, paving the way for novel therapeutic strategies.