These data point to an important role of the polarized expression of astrocyte AQP4 in maintaining BBB integrity; this is further supported by the observation that BBB breakdown is a key feature of neuromyelitis optica, a neuroinflammatory disease, in which antibodies to AQP4 contribute to disease pathogenesis (Bradl et al. 2009). This evidence concerns the gene AQP4 and neuromyelitis optica.