Astrocytes functionally integrate multiple synapses via specialized leaflet domains

Astrocyte Ca 2+ dynamics control synaptic circuits and behavior, yet the underlying biology remains poorly understood. By combining volumetric high-resolution electron microscopy and two-photon Ca 2+ 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 IP 3 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 Ca 2+ events that were synaptically induced, IP 3 R1-mediated, and often displayed separate originations merging into large, long-lasting Ca 2+ elevations. Using combined axon-leaflet Ca 2+ imaging, we show that these complex events reflect integration of incoming inputs from different neurons. The astrocyte leaflet organization may thus coordinate, via Ca 2+ signals, multiple synapses and circuits active at different spatiotemporal scales, executing computations distinct from neurons.