Aquaporin-4 Mz isoform: brain expression, supramolecular assembly and neuromyelitis optica antibody binding

Water channel aquaporin-4 (AQP4) is expressed in astrocytes throughout brain and spinal cord. Two major AQP4 isoforms are expressed, M1 and M23, having different translation initiation sites. A longer isoform (Mz) has been reported in rat with translation initiation 126-bp upstream from that of M1. By immunoblot analysis of SDS and native gels probed with a C-terminus anti-AQP4 antibody, Mz was detected in rat brain as a distinct band of size ∼39 kDa. Mz was absent in human and mouse brain because of in-frame stop codons. The ability of rat Mz to form orthogonal arrays of particles (OAPs) was investigated by single particle tracking and native gel electrophoresis. We found that Mz, like M1, diffused rapidly in the cell plasma membrane and did not form OAPs. However, when co-expressed with M23, Mz associated in OAPs by forming heterotetramers with M23. Unexpectedly, Mz-expressing cells bound neuromyelitis optica autoantibodies (NMO-IgG) poorly, <5-fold compared with M1-expressing cells. Truncation analysis suggested that the poor NMO-IgG binding to Mz involves residues 31-41 upstream of Met-1. We conclude that Mz AQP4 is (a) present at low level in rat but not human or mouse brain, (b) unable to form OAPs on its own but able to associate with M23 AQP4 in heterotetramers, and (c) largely unable to bind NMO-IgG because of N-terminus effects on the structure of the AQP4/NMO-IgG binding site.