Department of Laboratory Medicine and Pathology, Mayo Clinic, 200 First Street S.W., Rochester, MN 55905, USA.
J Autoimmun. 2013 Feb;40:21-7. doi: 10.1016/j.jaut.2012.07.008. Epub 2012 Aug 18.
The principal central nervous system (CNS) water channel, aquaporin-4 (AQP4), is confined to astrocytic and ependymal membranes and is the target of a pathogenic autoantibody, neuromyelitis optica (NMO)-IgG. This disease-specific autoantibody unifies a spectrum of relapsing CNS autoimmune inflammatory disorders of which NMO exemplifies the classic phenotype. Multiple sclerosis and other immune-mediated demyelinating disorders of the CNS lack a distinctive biomarker. Two AQP4 isoforms, M1 and M23, exist as homotetrameric and heterotetrameric intramembranous particles (IMPs). Orthogonal arrays of predominantly M23 particles (OAPs) are an ultrastructural characteristic of astrocytic membranes. We used high-titered serum from 32 AQP4-IgG-seropositive patients and 85 controls to investigate the nature and molecular location of AQP4 epitopes that bind NMO-IgG, and the influence of supramolecular structure. NMO-IgG bound to denatured AQP4 monomers (68% of cases), to native tetramers and high order arrays (90% of cases), and to AQP4 in live cell membranes (100% of cases). Disease-specific epitopes reside in extracellular loop C more than in loops A or E. IgG binding to intracellular epitopes lacks disease specificity. These observations predict greater disease sensitivity and specificity for tissue-based and cell-based serological assays employing "native" AQP4 than assays employing denatured AQP4 and fragments. NMO-IgG binds most avidly to plasma membrane surface AQP4 epitopes formed by loop interactions within tetramers and by intermolecular interactions within high order structures. The relative abundance and localization of AQP4 high order arrays in distinct CNS regions may explain the variability in clinical phenotype of NMO spectrum disorders.
主要的中枢神经系统 (CNS) 水通道，水通道蛋白-4 (AQP4)，局限于星形胶质细胞和室管膜细胞，是一种致病性自身抗体，视神经脊髓炎 (NMO)-IgG 的靶标。这种疾病特异性自身抗体统一了一系列复发性中枢自身免疫炎症性疾病，其中 NMO 代表了经典表型。多发性硬化症和其他中枢神经系统免疫介导的脱髓鞘疾病缺乏独特的生物标志物。两种 AQP4 同工型，M1 和 M23，存在于同源四聚体和异源四聚体膜内颗粒 (IMP)中。主要由 M23 颗粒组成的正交排列 (OAP) 是星形胶质细胞膜的超微结构特征。我们使用来自 32 名 AQP4-IgG 阳性患者和 85 名对照者的高滴度血清，研究与 NMO-IgG 结合的 AQP4 表位的性质和分子位置，以及超分子结构的影响。NMO-IgG 结合到变性的 AQP4 单体（68%的病例）、天然四聚体和高级排列（90%的病例）以及活细胞膜中的 AQP4（100%的病例）。疾病特异性表位位于细胞外环 C 中，而不是环 A 或 E 中。位于细胞内的 IgG 结合缺乏疾病特异性。这些观察结果预测，基于组织和细胞的血清学检测使用“天然”AQP4 比使用变性 AQP4 和片段的检测具有更高的疾病敏感性和特异性。NMO-IgG 最紧密地结合到四聚体内环相互作用和高级结构内分子间相互作用形成的质膜表面 AQP4 表位。在不同的中枢神经系统区域，AQP4 高级排列的相对丰度和定位可能解释 NMO 谱障碍的临床表型的可变性。
