Astrocytes functionally integrate multiple synapses via specialized leaflet domains

Astrocyte Ca2+ dynamics control synaptic circuits and behavior, yet the underlying biology remains poorly understood. By combining volumetric high-resolution electron microscopy and two-photon Ca2+ imaging, we characterize astrocyte leaflets that interface with synapses. These convoluted structures with ≤250 nm diameter originate from astrocytic shafts or cell bodies, contain minuscule endoplasmic reticulum saccules expressing IP3 receptors but not mitochondria, and are often interconnected via gap junctions forming domains with cytosolic continuity. Leaflets enwrap 90% of synapses in clusters and only 10% individually. By fast imaging of astrocyte peripheral microvolumes, we identify leaflet-specific Ca2+ events that were synaptically induced, IP3R1-mediated, and often displayed separate originations merging into large, long-lasting Ca2+ elevations. Using combined axon-leaflet Ca2+ imaging, we show that these complex events reflect integration of incoming inputs from different neurons. The astrocyte leaflet organization may thus coordinate, via Ca2+ signals, multiple synapses and circuits active at different spatiotemporal scales, executing computations distinct from neurons.