Consequences of NMO-IgG binding to aquaporin-4 in neuromyelitis optica

Abstract

Hinson et al. (1) suggested a unique mechanism for the pathogenesis of neuromyelitis optica (NMO), a neuroinflammatory disease caused by binding of an autoantibody (NMO-IgG) to aquaporin-4 (AQP4) on astrocyte plasma membranes. AQP4 is expressed as long (called M1) and short (called M23) isoforms, the latter forming orthogonal arrays of particles (OAPs). Hinson et al. (1) reported that M1-AQP4 is rapidly internalized in cells whereas M23-AQP4 resists internalization, and that NMO-IgG inhibits AQP4 water permeability. They propose that NMO pathogenesis involves NMO-IgG–induced internalization of M1-AQP4 and direct inhibition of AQP4 water permeability, resulting in increased OAP size, enhanced complement-dependent cytotoxicity, and tissue swelling. Although their mechanism is unique and interesting, data from our laboratory and others challenge each of their major findings and hence the validity of their model.