Elevated Astrocytic NFAT5 of the hippocampus increases epilepsy susceptibility in hypoxic-ischemic mice.

Objective: Hypoxic-ischemic brain damage (HIBD) is a leading cause of neonatal mortality, resulting in brain injury and persistent seizures that can last into the late neonatal period and beyond. Effective treatments and interventions for infants affected by hypoxia-ischemia remain lacking. Clinical investigations have indicated an elevation of nuclear factor of activated T cells 5 (NFAT5) in whole blood from umbilical cords of severely affected HIBD infants with epilepsy. Experimental research has demonstrated that NFAT5 has ambivalent effects on neuroprotection and neurologic damage. However, the mechanistic role of NFAT5 in HIBD remains unclear. This investigation aims to further clarify the role of NFAT5 in epilepsy following HIBD insult.

Methods: We created a neonatal HIBD mouse model through left common carotid artery occlusion. By specifically knocking down astrocytic NFAT5 and its downstream molecule, Nedd4-2, using hippocampal delivery of adeno-associated virus 5-driven targeted shRNA, we investigated the role of astrocytic NFAT5 in epilepsy susceptibility in HIBD mice. This was assessed through electroencephalographic recordings, behavioral observations in vivo, and whole-cell recordings of hippocampal neuronal activity. In vitro, we evaluated the effects of astrocytic NFAT5 alteration on Kir4.1 expression and I_Kir4.1 in both brain slices from HIBD mice and cultured astrocytes treated with oxygen-glucose deprivation/reoxygenation.

Results: Hypoxia-ischemia-induced upregulation of hippocampal NFAT5 occurs in astrocytes rather than in neurons. This upregulation leads to increased expression of the ubiquitin ligase Nedd4-2, resulting in excessive degradation of Kir4.1 in astrocytes. Consequently, astrocytic function in buffering extracellular K⁺ is impaired, causing depolarization of the resting potential and enhanced neuronal discharge. This disruption ultimately affects local neural network balance and increases susceptibility to epilepsy. In contrast, inhibiting or knocking down astrocytic NFAT5 almost completely reverses these effects.

Significance: Our findings suggest that manipulating the NFAT5-Nedd4-2-Kir4.1 axis in astrocytes could provide a potential therapeutic strategy for the epileptic complications of HIBD.