Identification of a Point Mutation Impairing the Binding between Aquaporin-4 and the Neuromyelitis Optica Autoantibodies

Neuromyelitis Optica (NMO) is characterized by the presence of pathogenic auto-antibodies (NMO-IgGs) against supra-molecular assemblies of Aquaporin-4 (AQP4), known as Orthogonal Array of Particles (OAPs). NMO-IgGs have a polyclonal origin and recognize different conformational epitopes involving extracellular AQP4 loops A, C and E. Here we hypothesize a pivotal role for AQP4 trans-membrane regions (TMs) in epitope assembly. On the basis of multi-alignment analysis, mutagenesis, NMO-IgG binding and cytotoxicity assay, we have disclosed the key role of Aspartate69 (D69) of TM2 for NMO-IgG epitope assembly. Mutation of D69 to histidine severely impairsNMO-IgG binding for 85.7 % ofthe NMO patient sera here analyzed. Although BN-PAGE, TIRFM and water transport assays, indicate that OAP D69 mutant is similar in structure and function to the wild type, molecular dynamic simulations have revealed that the D69H mutation has the effect of altering the structural rearrangements of extracellular loop A. In conclusion, D69 is crucial for the spatial control of loop A, the particular molecular conformation of which enables the assembly of NMO-IgG epitopes. These findings provide additional clues for new strategies for NMO treatment and a wealth of information to better approach NMO pathogenesis.