Fluorescence Microscopy Journal Club

Three-dimensional Ca2+ imaging advances understanding of astrocyte biology

by Bindocci E, Savtchouk I, Liaudet N, Becker D, Carriero G, Volterra A.

Science, 2017, 356, 6339, p. 11

In this paper, Bindocci et al. shine light on the role of astrocytes in brain physiology focusing on synaptic and vascular functions, and deciphering the astrocyte’s activity through three-dimensional two-photon calcium imaging. Studies so far have monitored only small portions of total astrocytes, contained in a single one-dimensional (1D) line or 2D plane. Given the highly 3D nature of astrocytes and their relations with neighboring vascular and neuronal elements, only a small fraction of the entire cell volume can be imaged in a single 2 photon plane as precisely reported in this study, leading to an underestimate of the activity of the cells that the authors are addressing by the development of a new approach.

This paper details and validates the approach consisting of a full-cell 3D approach to study astrocyte activity and thus biology by combining high speed imaging (AOD and resonant scanner), fast focusing z piezo element and high-sensitivity GaAsPs detectors available on their Bruker Ultima system, as well as new analytical methods to perfom fast volumetric acquisition and analysis of full-cell astrocyte activity. By combining different scientific approaches, such as genetic manipulation, optical monitoring through two-photon microscopy and pharmacology, the group was able to demonstrate for the first time the molecular mechanisms at the basis of amphetamine-mediated dopamine extracellular release, which determines the psychomotor stimulation and the behavioral effects already observed in mammals.

Their findings highlight the role of astrocytes in synaptic and vascular functions in the brain. In response to neuronal activity, astrocytes show intracellular Ca2+ elevations that result in downstream effects on synaptic transmission and plasticity. Astrocytic Ca2+ elevations also trigger vascular responses that may be involved in the control of cerebral blood flow. They studied Ca2+ dynamics in entire astrocyte volumes, including during axon-astrocyte interactions. In both awake mice and brain slices, they found that Ca2+ activity in an individual astrocyte is scattered throughout the cell, largely compartmented within each region, and preponderantly local within regions. Processes and endfeet displayed frequent fast activity, whereas the soma was infrequently active.