AQP4 and neuromyelitis optica: A target of NMO-IgG is a water channel, aquaporin-4 (AQP4) [2], and binding of NMO-IgG to AQP4 expressed in astrocytic end-feet followed by complement-dependent and cell-mediated disruption of astrocytes causes demyelination in NMO [3], [4], [5], [6], [7].