Aquaporin-4 Mz Isoform:Brain Expression,Supramolecular Assembly and Neuromyelitis Optica Antibody Binding

水通道蛋白4（AQP4）表达于大脑和脊髓的星形胶质细胞。2种主要的AQP4亚型,M1和M23,均有表达,具有不同的翻译起始点。研究表明,一种较新的亚型Mz表达于大鼠,其翻译起始点位于M1的翻译起始点上游126 bp。通过C端标记的抗AQP4抗体SDS和非变性胶免疫印迹技术,大鼠大脑中的Mz被检测到为39 kDa大小的条带。Mz因其提前终止密码子,在人类和小鼠大脑中并不表达。通过单粒子追踪和非变性胶电泳技术检测,发现大鼠Mz可形成正交粒子阵列（OAPs）。本文发现,Mz与M1类似,在细胞浆膜内迅速扩散,并不形成OAPs。但是,当与M23共表达时,Mz通过与M23形成异四聚体可与OAPs关联。意外的是,Mz表达的细胞极弱地与视神经脊髓炎自身抗体（NMO-IgG）结合,小于M1表达细胞的5倍。切割分析提示,Met-1上游的31~41残基参与NMO-IgG与Mz之间较弱的结合。总之,Mz AQP4在大鼠中低量表达,但不表达于人和小鼠的大脑;自身无法形成OAPs,除非与M23 AQP4形成异四聚体;因AQP4/NMO-IgG结合点的N端功能。

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. 2010 Wiley-Liss,Inc