AQP4 and neuromyelitis optica: Neuromyelitis optica spectrum disorder (NMOSD) is a rare and severe autoimmune disease of the central nervous system (CNS) driven by pathogenic antibodies mainly directed against aquaporin-4 (AQP4-Ab), which targets the AQP4 water channels of astrocytes in CNS and preferentially involves the optic nerve, spinal cord and posterior medullary area.