AQP4 and neuromyelitis optica: It would take more than two centuries for more light to be shed on the pathogenesis of NMO, when a role for antibodies was suggested based on histopathological findings by Lucchinetti and colleagues in 1999 [57] and when finally aquaporin-4, the most abundant water channel in the CNS, was revealed as target antigen in the majority of patients with NMO by Lennon and colleagues in 2004/2005 [2, 47].