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

Recent studies have demonstrated that the expression ofthe glial glutamate transporters GLT‐1 (glutamate transporter 1) and GLAST(glutamate aspartate transporter) is regulated both in vivo and in vitro. Forexample, co‐culturing with neurons, treatment with N 6 ,2′‐ O ‐dibutyryladenosine3′:5′‐cyclic monophosphate (dbcAMP), and treatment with epidermalgrowth factor all increase the steady‐state levels of GLT‐1 and GLAST proteinin astrocyte cultures. These changes in protein expression are correlated withincreased mRNA levels. In the present study, the degradation of GLT‐1 andGLAST mRNAs was examined in control and dbcAMP‐treated astrocyte culturesafter inhibiting transcription with actinomycin D. Although one would predictthat inhibition of transcription would cause a decrease in GLT‐1 and GLASTmRNAs and that this decrease would depend on the rate of mRNA degradation, thelevels of GLT‐1 and GLAST mRNAs did not decrease even after 24 h of treatmentwith actinomycin D. Withdrawal of dbcAMP caused the levels of GLT‐1 and GLASTmRNAs to fall to basal levels within 24 h, but this degradation was blocked ifactinomycin D was added at the time of dbcAMP withdrawal. Importantly,actinomycin D did not block the degradation of c‐fos mRNA also induced bydbcAMP in these cultures. Inhibition of translation with cycloheximide did notstabilize GLT‐1 but partially attenuated the degradation of GLAST mRNA.Although the mechanism of this effect remains to be defined, these studiessuggest that GLT‐1 and GLAST mRNAs belong to a select class of inducible mRNAsstabilized by inhibitors of transcription. The possible relevance of thesedata to astrocyte differentiation is briefly discussed.