Factors determining the density of AQP4 water channel molecules at the brain–blood interface

Perivascular endfeet of astrocytes are enriched with aquaporin-4 (AQP4)—a water channel that is critically involved in water transport at the brain–blood interface and that recently was identified as a key molecule in a system for waste clearance. The factors that determine the size of the perivascular AQP4 pool remain to be identified. Here we show that the size of this pool differs considerably between brain regions, roughly mirroring regional differences in Aqp4 mRNA copy numbers. We demonstrate that a targeted deletion of α-syntrophin—a member of the dystrophin complex responsible for AQP4 anchoring—removes a substantial and fairly constant proportion (79–94 %) of the perivascular AQP4 pool across the central nervous system (CNS). Quantitative immunogold analyses of AQP4 and α-syntrophin in perivascular membranes indicate that there is a fixed stoichiometry between these two molecules. Both molecules occur at higher densities in endfoot membrane domains facing pericytes than in endfoot membrane domains facing endothelial cells. Our data suggest that irrespective of region, endfoot targeting of α-syntrophin is the single most important factor determining the size of the perivascular AQP4 pool and hence the capacity for water transport at the brain–blood interface.