## 6 Molecular Mechanism of NMOSD
### Models of NMOSD Study  
The clinical features of NMOSD and ON were evaluated in some of the studies. Animal models of NMOSD developed by introducing the NMO-IgG antibody into mice with experimental autoimmune encephalomyelitisis displayed a few features of NMOSD, such as inflammation and loss of AQP4 and GFAP [169; 170; 171]. Injecting AQP4-IgG can stimulate pathogenicity [172]. In another study, NMO-IgG and complement factors were injected directly into the murine brain, stimulating pathology around the injection site [173]. In such methods of generating animal models of NMOSD, the doses of NMO-IgG to be injected determine the pathology and the dosages need to be clarified further. [PERSON] et al. developed an NMOSD animal model by delivering NMO-IgG and complement factors into the brain intravenously after reversing the BBB opening using a microbubble-enhanced low-frequency ultrasound (MELFUS). The mice generated lesions of NMOSD with ON and myelitis. The mice also showed optic nerve and spinal cord inflammation associated with loss of AQP4 and GFAP expression in the brain, spinal cord, and optic nerve [174].  
Many of the experimental animal models of NMOSD were established by passive administration of AQP4-IgG and T cells to rodent models using various methods [175]. AQP4 is an important drug target in NMOSD pathogenesis, and the assembly of AQP4 in OAPs is prudent for AQP4-IgG pathogenicity [176]. NMO animal models were developed in animals with pre-existing experimental autoimmune encephalomyelitisits (EAE) with similar CNS inflammation and BBB injury pathogenicity.  
Myelin basic protein immunization induced EAE in rats. Other possible methods of generating NMO models involve intraperitoneal injections of AQP4-IG that have been purified from the NMO patient serum and retrobulbar administration of recombinant-AQP4-IgG in rats with EAE [170], intrathecal administration of AQP4-IgG at the altanto-occipital joint [177], continuous intraventricular perfusion of AQP4-IgG by osmotic-mini pump in mice for 7 days [38; 178], continuous intracerebral infusion of AQP4-IgG and human complement factors [173], and continuous intracranial infusion of AQP4-IgG into the optic chiasm [38]. Though the development of animal models for NMOSD pathogenesis has improved in recent years, some issues still need to be rectified, such as developing a model without spontaneous AQP4 autoimmunity, possibly due to the different neuron-to-astrocyte ratios in rodents and differential AQP4 expression and availability [175].  
In addition to animal models, culture models of CNS tissues and retinal cultures were also developed to understand NMO mechanisms. The expression of AQP4 was reduced after AQP4-IgG exposure in retinal culture [179]. Ex vivo spinal cord slice cultures were made by culturing vibratome-cut spinal cord slices from adult mice on transwell porous supports. The spinal cord cultures exposed to AQP4-IgG and the complement for 1 to 3 days showed significant loss of GFAP, AQP4, and myelin. The NMO pathology in these cultures was enhanced while incubated with macrophages, neutrophils, and inflammatory factors such as IL-6, TNF-\(\alpha\), IL-1\(\beta\), and interferon-\(\gamma\) (IFN-\(\gamma\)) [170]. Likewise, several other cultures were established to study NMOSD pathology, including the cultures of optic nerves, hippocampal slices, and cerebellar slice cultures [180].  
[PERSON] and his team developed a new model of NMOSD by prolonged infusion of AQP4-IgG into the CSF of live rats. The brain, spinal cord, and optic nerves alter astrocytes, myelin, and loss of AQP4 expression in the rats' optic nerves and spinal cord. Reduced expression of myelin binding protein was observed in the grey and white matter and certain motor deficits were observed. However, the NMO rat model showed exclusions such as the absence of immune cell infiltration, microglial activation, and complement deposition in the CNS [178].