## 6 Molecular Mechanism of NMOSD  
### AQP4 Dependent Pathology  
AQP4 is a bidirectional transmembrane protein highly present in the astrocyte end feet of the BBB and the ependymal cells of the CNS [60]. AQP4 has been found to be necessary for the functioning of the BBB and important for maintaining CNS water homeostasis [60]. BBB disruption is the hallmark pathological sign of NMOSD, resulting in BBB breakdown and an influx of serum AQP4 IgG and complement factors into the CNS compartment, followed by the entry of B cells into the CNS and the formation of lesions [102]. Many histological studies in NMOSD patients have revealed the loss of AQP4 and CNS lesions, with or without astrocyte loss [103]. The presence of the circulating serum autoanti against the AQP4 water channel (AQP4-IgG) induces neuroinflammation by breaching the BBB, releasing proinflammatory cytokines into the CNS, and producing acute CNS attacks, CNS pathologies, area postrema syndrome, astrocyte inactivation, and astrocyte loss through complement inactivation, BBB disruption, and neural injury [103; 104; 105]. Besides the astrocytes, the retinal muler cells are highly packed with AQP4 and inwardly rectifying potassium (K\({}_{\text{ir}}\)4.1) channels [106]. The binding of AQP4-ab with AQP4 in the astrocyte initiates immune complement cascade signals, which results in the infiltration of granulocytes, lymphocytes, and eosinophils, leading to astrocyte and oligodendrocytes injury and demyelination, myelin loss, neurodegeneration [5], and thickened hyalinization blood [35]. The high expression of AQP4 in the periventricular zone and particularly in the area postrema make these regions more prone to NMOSD pathology [51]. The international consensus diagnostic criteria for NMOSD suggest that AQP4-IgG plays a major causative role in NMOSD. AQP4-IgG seropositive individuals showed six affected regions in the CNS, including the optic nerves, spinal cord, area postrema, brain stem, and cerebrum. The spinal cord and optic nerves are affected in AQP4-IgG seronegative individuals [17]. The astrocyte-specific, glial fibrillary acidic protein (GFAP) necessary for maintaining the structural integrity of the BBB was significantly higher in the CSF of NMOSD patients [107]. A cohort study in North American patients with meningeal encephalomyelitis revealsed that GFAP autoantibodies could act as the serum biomarker for CNS autoimmune diseases and potentially initiate complement-mediated immune responses and cause astrocyte damage [108; 109]. Aquaporins are the transmembrane proteins essential for water transport and homeostasis in the CNS. Among the thirteen aquaporins in mammals, AQP1, AQP4, and AQP9 are in the brain, especially AQP4, which is present in the CNS-blood and CNS-CSF interfaces in the subarachnoid, subependymal, and pericapillary spaces [110; 111]. The AQP4 protein is also expressed more in the retina, optic nerves, and optic tract [112]. AQP4 exists as two dominant isoforms, M1 and M23, due to the extra 22 amino acids in the intracellular N terminus of M1 isoforms. AQP4 monomers consist of six transmembrane helices and two helices with asparagine-proline-alanine motifs that form the pore for transporting water [113]. Both the AQP4 isoforms are highly selective, allowing only the water molecules and excluding other ions or molecules. AQP4 involves potassium reuptake, which is essential for maintaining BBB integrity, synaptic plasticity, and spatial memory [114; 115]. The absence of AQP4 in the AQP4 \({}^{-/-}\) mice showed hyperpermeability of the BBB [7]. A study on autoimmune encephalitisals AQP4 \({}^{-/-}\) mice revealed reduced demyelination, CNS inflammation, motor dysfunction, and cytokine production [116]. The antibodies of AQP4 (AQP4-ab), also known as NMO-IgG, are the prominent serum markers of autoimmune NMO. AQP4-mediated complement pathways and internalization of AQP4 are important reasons for NMO disease [117].