Microglial large extracellular vesicles propagate early synaptic dysfunction in Alzheimer's disease by moving at the axon surface

Gabrielli et al. show that microglial extracellular vesicles carrying amyloid-beta (A beta-EVs) move along axons and propagate synaptic dysfunction in the mouse brain in vitro. However, inhibiting A beta-EV motion restricts the spread of synaptic dysfunction, unveiling a potential new strategy to delay Alzheimer's disease progression. 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 amyloid-beta released by microglia in association with large extracellular vesicles (A beta-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. One hour after A beta-EV injection into the mouse entorhinal cortex, long-term potentiation was impaired in the entorhinal cortex but not in the dentate gyrus, its main target region, while 24 h later it was also impaired in the dentate gyrus, revealing a spreading of long-term potentiation deficit between the two regions. Similar results were obtained upon injection of extracellular vesicles carrying A beta naturally secreted by CHO7PA2 cells, while neither A beta(42) alone nor inflammatory extracellular vesicles devoid of A beta were able to propagate long-term potentiation impairment. Using optical tweezers combined to time-lapse imaging to study A beta-EV-neuron interaction, we show that A beta-EVs move anterogradely at the axon surface and that their motion can be blocked through annexin-V coating. Importantly, when A beta-EV motility was inhibited, no propagation of long-term potentiation deficit occurred along the entorhinal-hippocampal circuit, implicating large extracellular vesicle motion at the neuron surface in the spreading of long-term potentiation impairment. Our data indicate the involvement of large microglial extracellular vesicles in the rise and propagation of early synaptic dysfunction in Alzheimer's disease and suggest a new mechanism controlling the diffusion of large extracellular vesicles and their pathogenic signals in the brain parenchyma, paving the way for novel therapeutic strategies to delay the disease.