Abstract
Various acute and chronic brain diseases result in disturbed expression of the glial glutamate transporters, GLAST/EAAT-1 and GLT-1/EAAT-2, and subsequent secondary neuronal cell death. The idea that glutamate-induced brain damage can be prevented by restoring glutamate homeostasis in the injured brain, focussed previous efforts on identifying the network controlling astrocytic glutamate transport. Since most of this work was performed with rat astrocytes, we now sought to compare the transcriptional regulation of the GLAST/EAAT-1 gene in rat and man. Reporter gene assay demonstrated that the human GLAST/EAAT-1 promoter comprises the 2.3 kb region immediately flanking the 5'-end of the human GLAST/EAAT-1 gene. Cloning of the previously unknown promoter of rat GLAST/EAAT-1 gene demonstrated maximal reporter gene activity with a sequence comprising the 1.5 kb region flanking the 5'-end of the gene as well as non-coding exon 1, and intron 1-2. Although the promoter regions from both species lacked sequence homology, they contained numerous identical consensus motifs. In human promoter constructs, dbcAMP, PACAP, EGF, and TGFα, which represent potent stimulators of endogenous GLAST/EAAT-1 expression, only further increased reporter gene activity in the presence of the GLAST/EAAT-1 3'-UTR. By contrast, the rat GLAST/EAAT-1 3'-UTR only mediated the stimulatory increases of dbcAMP. Moreover, the GLAST/EAAT-1 3'-UTR repressed constitutive GLAST/EAAT-1 expression in man, but enhanced GLAST/EAAT-1 transcription in rat. Together, our findings suggest the existence of close functional similarities of the GLAST/EAAT-1 promoter regions in man and rat and further point to a species-specific function of the GLAST/EAAT-1 3'-UTR in constitutive and regulated GLAST/EAAT-1 expression.