Aim: 

Neuromyelitis optica (NMO) is an inflammatory autoimmune demyelinating disease of the central nervous system. Recent studies have demonstrated that the autoantibody produced by NMO patiens (NMO-IgG) recognizes aquaporin-4 (AQP4) a water channel protein expressed in glial cells plasma membrane in several independent pools corresponding to orthogonal arrays of particle (OAPs) of different size.

Methods: 

Immunofluorescence, Immunoprecipitation and Western blot analysis were conducted together to analyze the specificity of NMO IgG for AQP4. TIRF-M was used for water permeability measurements and FRAP for the analysis of AQP4 lateral diffusion in the plasma membrane.

Results: 

Results obtained showed that NMO-IgG specifically recognized AQP4 and not other AQPs, with a higher affinity for the perivascular astrocytes in the brain. AQP4 was efficiently immunoprecipitated by NMO-IgG, but its detection after SDS-PAGE and Western blot was only achievable using anti-AQP4 antibodies, indicating that the NMO-IgG epitope was not preserved after SDS treatment. Interestingly, we showed that the recognized epitope is directly associated to the organization of AQP4 in OAPs, being tetramers formed by the longer M1 isoform not labelled by NMO-IgG. Astrocyte water permeability measurements showed no alteration in AQP4-mediated water transport after treatment with NMO-IgG indicating that the pathogenic effect of these autoantibodies is not due to a block of the AQP4 water permeation pore. FRAP experiments, performed to study the effect of NMO-IgG on AQP4 lateral mobility, indicate that NMO-IgG did not determine changes in AQP4 structural organization such as a disassembly of AQP4-OAPs.

Conclusion: 

NMO-IgG recognize an extracellular and tridimensional epitope associated with AQP4 organization in OAPs. Moreover, NMO-IgG binding does not alter AQP4 lateral diffusion in the plasma membrane as well as its water permeability.