Degradation of Glial Glutamate Transporter mRNAs Is Selectively Blockedby Inhibition of Cellular Transcription

Recent studies have demonstrated that the expression of the glial glutamate transporters GLT-1 (glutamate transporter 1) and GLAST (glutamate aspartate transporter) is regulated both in vivo and in vitro. For example, co-culturing with neurons, treatment with N:(6), 2'-O:-dibutyryladenosine 3':5'-cyclic monophosphate (dbcAMP), and treatment with epidermal growth factor all increase the steady-state levels of GLT-1 and GLAST protein in astrocyte cultures. These changes in protein expression are correlated with increased mRNA levels. In the present study, the degradation of GLT-1 and GLAST mRNAs was examined in control and dbcAMP-treated astrocyte cultures after inhibiting transcription with actinomycin D. Although one would predict that inhibition of transcription would cause a decrease in GLT-1 and GLAST mRNAs and that this decrease would depend on the rate of mRNA degradation, the levels of GLT-1 and GLAST mRNAs did not decrease even after 24 h of treatment with actinomycin D. Withdrawal of dbcAMP caused the levels of GLT-1 and GLAST mRNAs to fall to basal levels within 24 h, but this degradation was blocked if actinomycin D was added at the time of dbcAMP withdrawal. Importantly, actinomycin D did not block the degradation of c-fos mRNA also induced by dbcAMP in these cultures. Inhibition of translation with cycloheximide did not stabilize GLT-1 but partially attenuated the degradation of GLAST mRNA. Although the mechanism of this effect remains to be defined, these studies suggest that GLT-1 and GLAST mRNAs belong to a select class of inducible mRNAs stabilized by inhibitors of transcription. The possible relevance of these data to astrocyte differentiation is briefly discussed.