Persistent DNA methylation and downregulation of homeostatic genes in astrocytes after pilocarpine-induced status epilepticus: Implications for epileptogenesis

Abstract

Epilepsy is a debilitating neurological disorder characterized by recurrent seizures, affecting millions of patients worldwide. Retrospective studies in Temporal lobe epilepsy (TLE) patients have shown a high incidence of an initial precipitating event (IPE) in early childhood followed by a silent period where epileptogenesis occurs to end up in chronic epilepsy. Epileptogenesis, the process through which a normal brain undergoes structural and functional changes leading to epilepsy, remains an enigmatic phenomenon. We hypothesized that epigenetics may be involved in epileptogenesis and specifically astrocytes could be affected by pathological remodeling. To study this process, we used three approaches: The lithium-pilocarpine model of TLE in rats, primary astroglial cultures exposed to epileptogenic DAMP named HMGB1, and brain tissue samples resected from TLE patients with drug-resistant epilepsy. We found that the IPE achieved by lithium- pilocarpine treatment (127/30 mg/kg IP) induced the hypermethylation of astrocytes at 7-, 21-, and 35 days post-IPE, indicating persistent epigenetic alterations in astrocytes during the epileptogenic period. In addition, we observed the downregulation of homeostatic astroglial genes AQP4; glutamine synthase (GS), and Kir4.1 with increased proinflammatory genes (C3, MAFG) and DNA methyl transferases (DNMT) expression. These alterations were mimicked in primary astrocyte cultures exposed to the epileptogenic HMGB1 (500 ng/ml; 18 hours) that also induced the hypermethylation of homeostatic astroglial genes. Astrocytes from TLE patients brains showed reactive astrogliosis, increased DNA methylation, and downregulation of homeostatic genes Kir4.1 and GS. These findings show that astrocytes are pathologically altered during the epileptogenic period, combining the proinflammatory gain of function with the loss of homeostatic profile. This may sustain the long-term alterations underlining epileptogenesis.