Neuromyelitis optica (NMO)-IgG-driven organelle reorganization in human iPSC-derived astrocytes

Neuromyelitis optica (NMO) is an autoimmune disease that primarily targets astrocytes. Autoantibodies (NMO-IgG) against the water channel protein, aquaporin 4 (AQP4), are a serologic marker in NMO patients, and they are known to be responsible for the pathophysiology of the disease. In the brain, AQP4 is mainly expressed in astrocytes, especially at the end-feet, where they form the blood-brain barrier. Following the interaction between NMO-IgG and AQP4 in astrocytes, rapid AQP4 endocytosis initiates pathogenesis. However, the cellular and molecular mechanisms of astrocyte destruction by autoantibodies remain largely elusive. We established an in vitro human astrocyte model system using induced pluripotent stem cells (iPSCs) technology in combination with NMO patient-derived serum and IgG to elucidate the cellular and functional changes caused by NMO-IgG. Herein, we observed that NMO-IgG induces structural alterations in mitochondria and their association with the endoplasmic reticulum (ER) and lysosomes at the ultrastructural level, which potentially leads to impaired mitochondrial functions and dynamics. Indeed, human astrocytes display impaired mitochondrial bioenergetics and autophagy activity in the presence of NMO-IgG. We further demonstrated NMO-IgG-driven ER membrane deformation into a multilamellar structure in human astrocytes. Together, we show that NMO-IgG rearranges cellular organelles and alter their functions and that our in vitro system using human iPSCs offers previously unavailable experimental opportunities to study the pathophysiological mechanisms of NMO in human astrocytes or conduct large-scale screening for potential therapeutic compounds targeting astrocytic abnormalities in patients with NMO.