Learning-Associated Astrocyte Ensembles Regulate Memory Recall
The physical manifestations of memory formation and recall are fundamental questions that remain unresolved. At the cellular level, ensembles of neurons called engrams are activated by learning events and control memory recall. Astrocytes are in close proximity to neurons and engage in a range of activities that support neurotransmission and circuit plasticity. Moreover, astrocytes exhibit experience dependent plasticity; however whether specific ensembles of astrocytes participate in memory recall remains obscure. Here we show that learning events induce c-Fos expression in a subset of hippocampal astrocytes, which subsequently regulates hippocampal circuit function. Intersectional, c-Fos based labeling of these astrocyte ensembles after learning events reveals that they are closely affiliated with engram neurons, while re-activation of these astrocyte ensembles stimulates memory recall. At the molecular level, these astrocyte ensembles exhibit elevated expression of NFIA and its selective deletion from this population suppresses memory recall. Together, our studies identify learning-associated astrocyte ensembles as a new form of plasticity that is sufficient to provoke memory recall, while implicating astrocytes as a reservoir for the storage of memories. Overall design: To further examine the molecular properties of learning-associated astrocytes, we employed the Fos-Flex-Flp AAV viral injection into a dual reporter mouse line (Aldh1l1-CreER; Aldh1l1-GFP; Rosa-CAG-FSF-tdTomato), in which all astrocytes express GFP, and Flp-expressing astrocytes additionally express tdTomato. With this system, we were able to uniquely label learning-associated astrocytes (GFP+, tdTomato+) and non-learning-associated astrocytes (GFP+, tdTomato-). After tamoxifen treatment and fear conditioning, we used FACS to purify the respective populations and performed transcriptomic RNA-sequencing.