Determining if AQP4 regulates transport of solutes from the CSF to optic nerve is of particular importance in AQP4-IgG seropositive neuromyelitis optica spectrum disorder (NMOSD) where autoantibodies to AQP4 cause loss of AQP4 from optic nerve astrocytes and optic neuritis [14, 15]. Here, AQP4 is linked to optic neuritis.