Structural basis of astrocytic Ca2+ signals at tripartite synapses

Misa Arizono,Jillian Stobart,Aude Panatier,Luigi Bellocchio,Julie Angibaud,Stéphane H R Oliet,Katsuhiko Mikoshiba,U Valentin Nägerl,Bruno Weber,Mirelle J T Ter Veer,V V G Krishna Inavalli,Florian Levet,Thomas Pfeiffer,Giovanni Marsicano

doi:10.1038/s41467-020-15648-4

Abstract

Astrocytic Ca2+ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. However, the anatomical basis of such specific signaling remains unclear, owing to difficulties in resolving the spongiform domain of astrocytes where most tripartite synapses are located. Using 3D-STED microscopy in living organotypic brain slices, we imaged the spongiform domain of astrocytes and observed a reticular meshwork of nodes and shafts that often formed loop-like structures. These anatomical features were also observed in acute hippocampal slices and in barrel cortex in vivo. The majority of dendritic spines were contacted by nodes and their sizes were correlated. FRAP experiments and Ca2+ imaging showed that nodes were biochemical compartments and Ca2+ microdomains. Mapping astrocytic Ca2+ signals onto STED images of nodes and dendritic spines showed they were associated with individual synapses. Here, we report on the nanoscale organization of astrocytes, identifying nodes as a functional astrocytic component of tripartite synapses that may enable synapse-specific communication between neurons and astrocytes.

Highlights

Astrocytic Ca2+ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses
We labeled the astrocytes with a cytosolic fluorescent marker protein (ZsGreen) in living organotypic mouse brain slices
Our study brings to light several new and striking aspects of the micro-anatomical organization and Ca2+ activity of astrocytes, sharpening our understanding of the tripartite synapse, which is a major concept in the neuron-glia field[3]

Summary

Introduction

Astrocytic Ca2+ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. Using 3D-STED microscopy in living organotypic brain slices, we imaged the spongiform domain of astrocytes and observed a reticular meshwork of nodes and shafts that often formed loop-like structures. These anatomical features were observed in acute hippocampal slices and in barrel cortex in vivo. The concept of the tripartite synapse posits that the flow of information at synapses is a result of dynamic signaling between pre- and post-synaptic neurons as well as astrocytes[1] It assigns astrocytes an active and pivotal role in information processing in the brain, beyond their multiple homeostatic functions. Astrocytes have an extremely intricate morphology, which could provide the structural basis for compartmentalized intracellular signaling[18]

Methods

Results

Conclusion