Significance of gray matter brain lesions in multiple sclerosis and neuromyelitis optica.

Multiple sclerosis (MS) and neuromyelitis optica (NMO) are the two main autoimmune diseases of the CNS. In patients with NMO, the target antigen is aquaporin-4 (AQP4), the most abundant water channel protein in the CNS. AQP4 is mainly expressed on astrocytic endfoot processes at the blood-brain barrier and in subpial and subendymal regions. MS and NMO are distinct diseases, but they have some common clinical features: both have long been considered autoimmune diseases that primarily affect the white matter (WM). However, because WM demyelination by itself cannot explain the full extent of the clinical disabilities, including cognitive decline in patients with MS and NMO, renewed interest in gray matter (GM) pathology in MS and NMO is emerging. Important hallmarks of WM and GM lesions in MS and NMO may differentially influence neuronal degeneration and demyelination in the brain and spinal cord, given different detrimental effects, including cytokine diffusion, disruption of water homeostasis associated with or without AQP4 (the target antigen in NMO) dynamics, or other unidentified mechanisms. An increase in knowledge of the structure of GM and WM lesions in MS and NMO will result in more targeted therapeutic approaches to these two diseases.