Academic Neurosurgery Unit, St. George's University of London, Cranmer Terrace, Tooting, London SW17 0RE, UK.
About 80% of the brain is water. This review discusses the importance of the three brain water-channel proteins (AQP1, AQP4, AQP9) in brain physiology. AQP1 is expressed in the choroid plexus and participates in forming cerebrospinal fluid. AQP4, found in astrocyte foot processes, glia limitans and ependyma, facilitates water movement into and out of the brain, accelerates astrocyte migration and alters neuronal activity. Recently, AQP4 autoantibodies were discovered in patients with neuromyelitis optica, a demyelinating disease, and are now being used to diagnose this condition. AQP9 is present in some glia and neurons, but its function is unclear. Finally, we discuss how the discovery of AQP activators and inhibitors will be the next major step in this field.