Glucose, an essential substrate for brain oxidative metabolism, is transported across the blood brain barrier and then into glia and neurons. Glucose transport into neurons is by the most efficient facilitative glucose transporter [Glut 3] isoform (Km 1.8 mM). Previously, we reported an age-dependent pretranslational increase in murine brain Glut 3 expression with a peak noted between 14 and 21d of age (Khan et al, Ped. Res. 39:91A, 1996). Employing nuclear run-ons we presently determined a two-fold transcriptional increase in Glut 3 in the 21d versus the 1d mouse brain. Primer extension studies established a single transcriptional start site at 305 bp 5'- to the ATG translational start codon in this gene with a TATA-less promoter. Mobility shift and supershift assays with nuclear extracts from the 1d (n=3) and 21d(n=3) mouse brains demonstrated Sp1 and Sp3 binding to the -137 to -130 bp region of the Glut 3 gene with no significant age-dependent change in the Sp1 and Sp3 DNA-binding. In contrast, cyclic AMP response element binding protein(CREB) bound to the -187 to -180 bp region of the gene with a two-fold increase in the DNA-binding noted at 21d versus 1d (p < 0.05). Western blot analysis of the nuclear extracts from the 1d (n=3) and 21d (n=3) mouse brains revealed no change in Sp1 and Sp3 levels, but a two-fold increase in the CREB protein concentrations (p < 0.05). In-vitro functional analysis of the two identified cis-elements ascribed a role in Glut 3 gene transcription.We conclude that CREB regulates neuronal Glut 3 gene expression and may be involved in its developmental regulation. We speculate that neurotransmitters which evoke a neuronal cAMP response may increase the synthesis of Glut 3, thereby enhancing glucose transport necessary for fueling the process of neurotransmission in the maturing brain.
