Role Of Water Channel Aquaporin-4 In The Diagnosis And Treatment Of Neuromyelitis Optica

Aquaporin-4(AQP4)is one of the 13 members in mammalian aquaporin family,known as the mercurial insensitive water channel initially isolated from rat lung.AQP4 was later on found to be strongly expressed in mammalian brain.There are two forms of AQP4 proteins,namely AQP4-M1 containing 323 amino acids and AQP4-M23 containing 301 amino acids that were identified to be encoded by two alternatively spliced forms of AQP4 mRNAs.AQP4 proteins are expressed extensively in the central nervous system in astriglial cells,epithelial cells lining the brain ventricles and coriod plexus.AQP4 plays important functions in brain water balance,K+ buffering,neuronal transduction and neuronal stem cell function.The M23 form of AQP4 protein was found to form orthogonal arrays of particles on cell membrane.Neoromyelitis optica(NMO)is a severe demyelinating autoimmune disease affecting optical nerves and spinal cord.Inflammatory injury of optical nerves causes severe vision lose,whereas spinal cord dysfunction can lead to muscle weakness,reduced sensation,or loss of bladder and bowel control.Brain is usually not affected.NMO was primarily though to be a subtype of multiple sclerasis(MS),but was then found belong to different neuronal inflammatory diseases responsive to distinct therapeutic strategies.Therfore it is important to distinguish NMO from MS.In 2005,Lennon et al first found that the NMO-IgG autoantibody binds to the extracellular domains of AQP4 protein in astraglial cells and AQP4-transfected cells,providing evidence that AQP4 is the autoantigen of NMO-IgG.Following studies indicated that AQP4 autoantibody was seen only in NMO patients,but not in MS and other neuroinflammatory diseases.Therefore,AQP4 autoantibody detection becomes a relialble new standard for the diagnosis and evaluation of progress of NMO.On the other hand,recent studies revealed that binding of NMO-IgG can activate complement-mediated and antibody-dependent cellular cytotoxicity,traggering the release of cytokines and infiltration of microglia and neutrophiles,resulting in neuronal inflammatory injury and clinical symptoms of NMO.These important studies indicated that blocking the binding of NMO-IgG to AQP4 may become a novel effective therapeutic strategy of NMO.In this project,we first developed a cell-based fluorescent assay for sensitive detection of AQP4 autoantibodies in the serum of NMO patients and then performed two separate studies:1.To analyze the serum AQP4 autoantibody level in Chinese NMO patients and its relationship with disease progress.The results demonstrated that the live cell-based immunofluorescence assay is a sensitive and specific method to detect AQP4 autoantibody in the serum of NMO patients and the titer of AQP4 autoantibody coincide with EDSS score of NMO patients.2.To discover natural compounds that block the binding of AQP4 and NMO-IgG In this study,we screened 40,000 small molecule fractions from 500 Chinese herbs and identified 36 positive fractions.Three fractions from Cortex Phellodendr Chinensis,Coptis chinensis Franch,Berberis julianae Schneid can completely block the binding of AQP4 and NMO-IgG at 50 pg/ml.Further studies are needed to isolate single active compounds and evaluate their in vivo therapeutic effect in animal models.The present study confirmed the significance of AQP4 autoantibody detection in the diagnosis and prognosis of NMO and explored a new strategy to identify natural small molecule blockers for the treatment of NMO.