Astrocyte aquaporin mediates a tonic water efflux maintaining brain homeostasis

Brain water homeostasis provides not only physical protection, but also determines the diffusion of chemical molecules key for information processing and metabolic stability. As a major type of glial cell in the brain parenchyma, astrocytes are the dominant cell type expressing aquaporin water channels. However, how astrocyte aquaporin contributes to brain water homeostasis remains to be understood. We report here that astrocyte aquaporin 4 (AQP4) mediates a tonic water efflux in basal conditions. Acute inhibition of astrocyte AQP4 leads to intracellular water accumulation as optically resolved by fluorescence-translated imaging in acute brain slices, and in vivo by fiberphotometry in moving mice. We then show that the tonic aquaporin water efflux maintains astrocyte volume equilibrium, astrocyte and neuron Ca2+ signaling, and extracellular space remodeling during optogenetically induced cortical spreading depression. Using diffusion- weighted magnetic resonance imaging (DW-MRI), we observed that in vivo inhibition of AQP4 water efflux heterogeneously disturbs brain water homeostasis in a region-dependent manner. Our data suggest that astrocyte aquaporin, though bidirectional in nature, mediates a tonic water outflow to sustain cellular and environmental equilibrium in brain parenchyma. ### Competing Interest Statement The authors have declared no competing interest.