Structural Basis of Astrocytic Ca <sup>2 </sup> Signals at Tripartite Synapses

Abstract

According to the concept of the ‘tripartite synapse’, synaptic communication is a result of dynamic signaling between pre- and postsynaptic structures as well as perisynaptic astrocytic processes. Recent studies have shown that astrocytic Ca2 signals can be fast and local, supporting the possibility that astrocytes are involved in actively regulating neural circuits at the level of single synapses. However, the anatomical basis of such specific signaling remains unclear, owing to technical difficulties in resolving the spongiform domain of astrocytes where most tripartite synapses are located. Using 3D-STED microscopy in living organotypic brain slices, we could unambiguously resolve the spongiform domain and reveal new aspects of its morphological architecture. We observed a reticular meshwork of nodes and shafts that featured rings of re-connecting astrocytic processes. The majority of dendritic spines were in contact with nodes, correlating in size with them. FRAP experiments and Ca2 imaging showed that individual nodes were biochemically compartmentalized and hosted highly localized spontaneous Ca2 transients. Mapping these Ca2 signals onto STED images of nodes and spines confirmed that they were associated with individual synapses. Our study reveals the nanoscale anatomical organization of astrocytes in living brain slices, identifying nodes as the functional astrocytic component of tripartite synapses, which may provide the anatomical basis for synapse-specific communication between neurons and astrocytes.