Early Hypo-Osmolar Stress Regulates Astrocyte Reactivity after Brain Injury: "new Insights into Glial Response to Edema".

Astrocytes are among the first cellular responders to central nervous system injury, yet the mechanisms governing their earliest responses remain incompletely understood. Here, we investigated astrocyte dynamics during the first hours after focal cortical injury induced by cortical devascularization in rats. We observed a rapid and spatially restricted increase in glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQP4) immunoreactivity surrounding the ischemic core as early as 1.5-3.5 h post-lesion, in association with blood-brain barrier disruption and edema-related changes. Within the injury core, astrocytes displayed differential GFAP detection by monoclonal and polyclonal antibodies, together with the appearance of lower-molecular-weight GFAP fragments both in vivo and after oxygen-glucose deprivation in vitro, suggesting GFAP cleavage in severely damaged astrocytes. At the chromatin level, astrocytes proximal to the lesion exhibited reduced histone H3 acetylation, particularly histone 3 acetylation at lysine 9 (H3K9ac), a phenomenon recapitulated in cultured astrocytes exposed to hypo-osmolar stress. This reduction was transient, reversible upon recovery, and prevented by histone deacetylase (HDAC) inhibition. Functionally, hypo-osmolar stress conditioned astrocyte responses to subsequent stimuli, attenuating nuclear factor kappa B (NF-κB) activation and complement 3 (C3) induction after lipopolysaccharide exposure while enhancing proliferative capacity during recovery. Together, these findings identify edema-associated osmotic stress as an early regulator of astrocyte epigenetic state and functional plasticity, suggesting that astrocytes exposed to edema are primed to adopt distinct responses that may contribute to tissue repair and scar formation following brain injury.