Astrocyte subtype-specific alterations in the dentate gyrus of individuals with mesial temporal lobe epilepsy.

Abstract

Objective: Epilepsy affects approximately 50 million people worldwide and, although primarily attributed to neuronal dysfunction, increasing evidence highlights a critical role of glial cells, particularly astrocytes, in the pathophysiological mechanisms. Mesial temporal lobe epilepsy (MTLE), the most common form of drug-resistant epilepsy, is frequently associated with hippocampal sclerosis (HS) and pronounced astrogliosis. Given the limited efficacy of current anti-seizure medication (ASM) and the side effects of surgical hippocampus removal, there is a need for more specific and effective therapies that potentially address non-neuronal mechanisms. Astrocytes, with their inherent heterogeneity, are key candidates for such approaches due to their role in regulating and maintaining neuronal activity.

Methods: To investigate subtype-specific astrocyte alterations in MTLE, we performed immunofluorescence analyses of human dentate gyrus (DG) tissue from MTLE patients (HS1, HS2, and noHS according to the International League Against Epilepsy (ILAE) consensus classification of HS in temporal lobe epilepsy) and postmortem controls. We analyzed the expression and spatial distribution of selected functionally relevant astrocytic proteins, including glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), excitatory amino acid transporter 2 (EAAT2), and aquaporin-4 (AQP4).

Results: In control tissue, astrocyte subtypes, characterized by combinatorial expression of GFAP, GS, EAAT2, and AQP4, displayed distinct, layer-specific protein expression profiles across the DG compartments. While the subtype identities were largely preserved in MTLE, localization and expression levels of GFAP, EAAT2, and AQP4 were dramatically altered, suggesting functional deficits in glutamate transport and water homeostasis. Since GS expression was unaffected by MTLE, it served as a proxy to quantify the number of astrocytes. In contrast to existing reports, we found that astrocyte numbers did not differ between control and MTLE patients.

Significance: Our findings demonstrate that astrocyte reactivity in MTLE is not uniform but occurs in a subtype- and region-specific manner. This highlights astrocyte heterogeneity as an important feature of MTLE pathology and underscores the need to consider astrocyte diversity in understanding disease mechanisms and developing precision medicine-based therapeutic strategies.

Plain language summary: Epilepsy is usually studied as a disease of nerve cells, but support cells, called astrocytes, are also involved. We studied brain tissue from patients with a common form of drug-resistant epilepsy and found that different groups of astrocytes showed distinct changes in proteins that help control brain activity and water movement in the brain. Although the overall number of astrocytes did not change compared with postmortem control patients, the cells displayed strong signs of disease-related reactivity. Our findings highlight astrocytes as potential targets for future epilepsy treatments.