Removal of aquaporin-4 from glial and ependymal membranes causes brain water accumulation

•Aquaporin-4 (AQP4) water channels in astrocytes regulate basal brain water content.•AQP4 is not involved in draining CSF through arachnoid villi or lymphatic vessels.•Ependymal AQP4 does not have any major effect on basal brain water content.

There is a constitutive production of water in brain. The efflux routes of this excess water remain to be identified. We used basal brain water content as a proxy for the capacity of water exit routes. Basal brain water content was increased in mice with a complete loss of aquaporin-4 (AQP4) water channels (global Aqp4−/− mice), but not in mice with a selective removal of perivascular AQP4 or in a novel mouse line with a selective deletion of ependymal AQP4 (Foxj1-Cre:Aqp4flox/flox mice). Unique for the global Aqp4−/− mice is the loss of the AQP4 pool subjacent to the pial membrane. Our data suggest that water accumulates in brain when subpial AQP4 is missing, pointing to a critical role of this pool of water channels in brain water exit.