NMO-IgG Present in Neuromyelitis Optica Patients Sera Affects AQP4 Expression and Water Permeability of Astrocytes Plasma Membrane

Abstract

NMO-IgG autoantibody selectively binds to Aquaporin-4 (AQP4), the most abundant water channel in the central nervous system, and is now considered an useful serum biomarker of neuromyelitis optica (NMO). A series of clinical and pathological observations suggest that NMO-IgG may play a central role in NMO physiopathology. In the current study we evaluated, in well-differentiated astrocytes cultures, the consequences of NMO-IgG binding on the expression pattern of AQP4 and on plasma membrane water permeability. To avoid or to facilitate AQP4 down-regulation cells were exposed to inactivated sera in two different situations (1 hour at 4°C or 12 hours at 37°C). AQP4 expression was detected by immunofluorescence studies, using a polyclonal anti-AQP4 or a human anti-IgG antibody and the water permeability coefficient was evaluated by a videomicroscopy technique. Our results showed that at low temperatures, cell exposure to either control or NMO-IgG sera does not affect neither AQP4 expression nor plasma membrane water permeability, indicating that the simple binding of NMO-IgG does not affect the water channel´s activity. However, at 37 °C, long-term exposure to NMO-IgG induced a loss of human IgG signal from the plasma membrane along with M1-AQP4 isoform removal and a significant reduction of water permeability. These results suggest that binding of NMO-IgG to cell membranes expressing AQP4 is a specific mechanism that may account for at least part of the pathogenic process.