Synapse loss is an early phenomenon in several neurodegenerative disorders such as Alzheimer’s disease (AD) and strongly correlates with cognitive decline. It can occur via neuron-autonomous responses, or via active removal by microglia and astrocytes.We recently showed that astrocytes contain ingested synapses around plaques in human post-mortem AD brains, and cultured astrocytes show increased ingestion of synapses isolated from AD tissue (Tzioras, Cell.Rep.Medicine2023). It is unclear however if synapse engulfment is a protective event removing dysfunctional synapses or a detrimental phenomenon removing healthy ones. The removal of functional synapses would exacerbate disease progression, either by directing damaging connections or by reducing the availability synapses able to compensate the pathological synapse loss.Here, we address whether astrocytes ingest functionally active synapses in living brain slices using multiphoton fluorescence imaging. To model the response of healthy tissue to toxic Abeta species, we challenge organotypic mouse brain slices with Abeta-immunodepleted (8pM Abeta42) or mock-immunodepleted (90pM) AD brain homogenate. To determine if synapses with physiological or aberrant activity are preferentially phagocytosed, we express the green fluorescent GCaMP indicator in pyramidal neurons, while astrocytes are marked with a blue fluorescent reporter. We will then monitor the phagocytosis rate at 24h of synapses that maintain a physiological response, or else displaying aberrant hyper- or hypo-activity following AD homogenate challenge.Our results will elucidate if synapse phagocytosis by astrocytes has a protective or detrimental effect on healthy synapses. We will also develop a brain slice model for testing and modulating phagocytosis mechanisms in AD.