Microglial large extracellular vesicles propagate early synaptic dysfunction in Alzheimer's disease

: Synaptic dysfunction is an early mechanism in Alzheimer's disease that involves progressively larger areas of the brain over time. However, how it starts and propagates is unknown. Here we show that Aβ released by microglia in association with large extracellular vesicles (Aβ-EVs) alters dendritic spine morphology in vitro, at the site of neuron interaction, and impairs synaptic plasticity both in vitro and in vivo in the entorhinal cortex-dentate gyrus circuitry. 1 h after Aβ-EV injection into the mouse entorhinal cortex, long-term potentiation (LTP) was impaired in the entorhinal cortex but not in the dentate gyrus, its main target region, while 24 h later it was impaired also in the dentate gyrus, revealing a spreading of LTP deficit between the two regions. Similar results were obtained upon injection of EVs carrying Aβ naturally secreted by CHO7PA2 cells, while neither Aβ42alone nor inflammatory EVs devoid of Aβ were able to propagate LTP impairment. Using optical tweezers combined to time-lapse imaging to study Aβ-EV-neuron interaction, we show that Aβ-EVs move anterogradely at the axon surface and that their motion can be blocked through annexin-V coating. Importantly, when Aβ-EV motility was inhibited, no propagation of LTP deficit occurred along the entorhinal-hippocampal circuit, implicating large EV motion at the neuron surface in the spreading of LTP impairment. Our data indicate the involvement of large microglial EVs in the rise and propagation of early synaptic dysfunction in Alzheimer's disease, and suggest a new mechanism controlling the diffusion of large EVs and their pathogenic signals in the brain parenchyma, paving the way for novel therapeutic strategies to delay the disease.