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Neuromyelitis optica (NMO) is an inflammatory autoimmune demyelinating disease of the central nervous system (CNS) which in autoantibodies produced by patients with NMO (NMO-IgG) recognize a glial water channel protein, Aquaporin-4 (AQP4) expressed as two major isoforms, M1- and M23-AQP4, in which the plasma membrane form orthogonal arrays of particles (OAPs). AQP4-M23 is the OAP-forming isoform, whereas AQP4-M1 alone is unable to form OAPs. The function of AQP4 organization into OAPs in normal physiology is unknown; however, alteration in OAP assemblies is reported for several CNS pathological states. In this study, we demonstrate that in the CNS, NMO-IgG is able to pull down both M1- and M23-AQP4 but experiments performed using cells selectively transfected with M1- or M23-AQP4 and native tissues show NMO-IgG epitope to be intrinsic in AQP4 assemblies into OAPs. Other OAP-forming water-channel proteins, such as the lens Aquaporin-0 and the insect Aquaporin-cic, were not recognized by NMO-IgG, indicating an epitope characteristic of AQP4-OAPs. Finally, water transport measurements show that NMO-IgG treatment does not significantly affect AQP4 function. In conclusion, our results suggest for the first time that OAP assemblies are required for NMO-IgG to recognize AQP4.
Nicchia et al. (Thu,) studied this question.