CNS region-specific regulation of glial glutamate transporter expression

The neuronal cell death associated with certain neurodegenerative disorders as well as acute brain injuries is in part due to the reduced expression of glial glutamate transporters and the subsequent accumulation of toxic extracellular glutamate concentrations. Extracellular factors previously found to potently stimulate the expression of the glial glutamate transporters, GLT-1/EAAT2 and GLAST/EAAT1, in astroglial cultures of rat cerebral hemispheres are PACAP, TGF alpha, and EGF. In the present study, we sought to determine whether similar stimulatory influences apply for astroglia from other areas of the central nervous system (CNS). Immunoblot and real-time RT-PCR analysis of striatal astroglial cultures maintained for 72 h with PACAP, TGF alpha, or EGF revealed a prominent increase in GLT-1 and GLAST expression. In apparent contrast, all factors completely failed to affect GLT-1 and GLAST expression in astroglial cultures from the cerebellum, mesencephalon, and spinal cord between 36 h and 7 days. This failure was not due to the absence of functional recognition or transduction machineries for the extracellular factors as suggested by the additional observations that cerebellar, mesencephalic and spinal cord glia were capable of responding to stimulation with PACAP, TGF alpha, or EGF for 10 min with activation of CREB. Moreover, dibutyryl cyclic AMP (dbcAMP) potently promoted GLT-1 and/or GLAST expression in mesencephalic, cerebellar and spinal cord glia, further indicating that extracellular factors regulate glial glutamate transporter expression throughout the CNS. Together these findings identify PACAP, TGF alpha and EGF as potent regulators of glutamate transporter expression in striatal glia. In addition, these findings provide evidence for a CNS region-specific regulation of glial glutamate transport.