Altered expression of glutamate transporters under hypoxic conditions in vitro

Regulation of extracellular excitotoxins by glial and neuronal glutamate tr ansporters is critical to maintain synaptic terminal integrity. Factors int erfering with the normal functioning of these transporters might be involve d in neurodegeneration, Among them, recent studies have shown that hypoxia alters glutamate transporter function; however, it is unclear if hypoxia ha s an effect on the expression of glutamate transporters and which intracell ular signaling pathways are involved. The C6 rat glial and GT1-7 mouse neur onal cell lines were exposed to hypoxic conditions (5% CO2, 95% N-2) and le vels of glutamate transporter mRNA were determined by ribonuclease protecti on assay. After 21 hr, there was a 100% increase in levels of rat excitator y amino acid transporter 3 (EAAT3) mRNA in C6 cells and a 600% increase in levels of murine EAAT2 mRNA in GT1-7 cells. There was a similar increase in mRNA levels after hypoxia in C6 cells transfected with human EAAT2, wherea s reoxygenation normalized the expression levels of glutamate transporters. Although the expression of EAATs was associated with increased immunoreact ivity by Western blot, functioning of the transporters was decreased as evi denced by D-aspartate uptake. Finally, although the protein kinase C stimul ator phorbol-12-myristate-13-acetate enhanced EAAT2 mRNA levels after hypox ia, protein kinase C inhibitor bisindolylmaleimide I had the opposite effec t. Taken together, this study suggests that the hypoxia is capable of upreg ulating levels of EAATs via a protein kinase C-dependent compensatory mecha nism. This increased expression is not sufficient to overcome the decreased functioning of the EAATs associated with decreased ATP production and mito chondrial dysfunction. J. Neurosci. Res. 64:193-202, 2001. (C) 2001 Wiley-L iss, Inc.