Astrocytic Interleukin‐33 Deficiency Reduces Glial Fibrillary Acidic Protein Expression and Exacerbates Microglial Activation and Neuronal Damage in a Chemically Induced Inflamed Frontal Cortex

Astrocytes, the most abundant glial cells in the CNS, play a crucial role in supporting neurons and respond to injury or disease through astrogliosis, a process marked by cellular hypertrophy and increased glial fibrillary acidic protein (GFAP) expression. Interleukin‐33 (IL‐33) was originally identified as an alarmin and is known to be produced by astrocytes and oligodendrocytes in the CNS. Recently, we reported its role in regulating oligodendrocyte differentiation. However, its role in astrocytes remains less defined. In a demyelinating mouse model induced by gliotoxin cuprizone (CPZ), IL‐33 was previously shown to be reduced in oligodendrocytes within the corpus callosum. In this study, we found that lipopolysaccharide (LPS) stimulation enhanced nuclear IL‐33 expression and GFAP production in cortical astrocytes. Using lentiviral‐mediated IL‐33 knockdown (IL33KD) and overexpression (IL33oe), we demonstrated that IL‐33 positively regulates GFAP expression. Interestingly, we observed an increase in nuclear IL‐33‐expressing GFAP + cortical astrocytes in CPZ‐treated mice. In contrast, CPZ‐induced GFAP upregulation in cortical astrocytes was abolished in IL‐33 knockout ( il33 KO ) mice. Furthermore, chronic CPZ feeding in il33 KO mice led to increased microgliosis and neuronal damage within the frontal cortex, as well as abnormal anxiety‐like behaviors. Collectively, these results indicate that elevated nuclear IL‐33 in astrocytes under inflammatory conditions is critical for GFAP upregulation and astrogliosis. Loss of IL‐33 disrupts astrocyte neuroprotective functions and glial reactivity in the frontal cortex, contributing to behavioral abnormalities under a demyelinating insult.