CHI3L1 and neuromyelitis optica: Consistent with NMO histopathological findings, we observed a marked loss of AQP4 expression due to internalization and degradation (1, 17, 18, 21), along with a significant increase in CHI3L1 expression at the AQP4-IgG injection site within the striatum compared with Ctrl-IgG injection within 7 days after injection (Supplemental Figure 1, C and D).