Aquaporin-4 is the main channel through which water enters and leaves the CNS

The target molecule of the neuromyelitis optica (NMO) antibody, NMO-immunoglobulin G (NMO-IgG), is aquaporin-4. This molecule occurs in particles and clusters that reside on the end-feet of the astrocytes which surround the blood vessels, the tight junctions of which constitute the blood-brain barrier. The astrocyte end-feet also encircle axons at the nodes of Ranvier. Mayo Clinic scientists suspect that damage to astrocyte end-feet by the NMO-IgG autoantibody contributes to myelin damage and also compromises the blood-brain barrier.