Astrocytic EAAT-2 Upregulation Supports Neuro-Axonal Homeostasis in Chronic Inflammatory CNS Lesions

Neuronal loss in multiple sclerosis (MS) is a key factor in the progression of disability. Axonal degeneration within MS lesions is central to progressive MS pathology, yet its mechanisms are not fully understood. Excess extracellular l-glutamate (GLUT) is neurotoxic due to overstimulation of GLUT receptors. Excitotoxicity, driven by GLUT, is a suspected contributor to several central nervous system (CNS) diseases. Astrocytic glutamate transporters EAAT-1 and EAAT-2 play crucial roles in managing extracellular GLUT, yet their role in chronic MS remains underexplored. Using the animal model experimental autoimmune encephalomyelitis (EAE) and advanced microscopy in live anesthetized mice, we examined GLUT-mediated neurotoxicity and protective mechanisms in chronic CNS lesions. By employing genetically encoded fluorescent biosensors, we visualized extracellular GLUT levels and axonal Ca²⁺ dynamics within the live mouse brain during EAE. In acute EAE, extracellular GLUT levels were high, while in chronic lesions, normalized GLUT levels together with sustained EAAT-2 upregulation were consistent with a protective astrocytic response. EAAT-2 inhibition in chronic lesions led to higher intra-axonal Ca²⁺ levels and morphological changes consistent with neuroaxonal injury, indicating that EAAT-2 upregulation may counter excitotoxicity and support neuroprotection. Post-mortem MS brain tissue analysis provided complementary evidence, showing elevated EAAT-1 and EAAT-2 expression in chronic MS lesions with expression patterns varying according to lesion type and cellularity. This study highlights astrocytic mechanisms that may contribute to neuroaxonal protection in MS tissue and suggests targeting GLUT transporters as a promising strategy to limit neurodegeneration in progressive MS.