AQP4 and neuromyelitis optica: Since ependymal cells harbour low levels of tight junctions, NMO-IgG present in the CSF could then easily reach and bind AQP4 expressed at the basolateral membrane of ependymal cells and sub-ventricular astrocytes.51 Astrocytes might thus be first targeted and fragilized by NMO-IgG penetrating parenchyma through the ependymal interface, leading to a blood–brain barrier (BBB) weakening from the inside.