AQP4 and neuromyelitis optica: OAP-formation is also of central importance in NMO pathogenesis, with NMO-Immunoglobulin G (IgG) antibodies having higher affinity towards AQP4 in OAPs than as individual tetramers and OAP-dependent clustering of NMO-IgG antibodies leading to enhanced complement-dependent cytotoxicity [55,56].