AQP4 and neuromyelitis optica: Previously characterized as “optical-spinal” MS, the identification of pathogenic antibodies to the astrocytic water channel aquaporin-4 (AQP4)40 heralded a recategorization of the disorder as an independent entity.41,42 Specific antibody and B-cell depleting therapies have been shown to be efficacious and AQP4 antibodies have been shown to predate the development of NMO.