The transcription factor regulatory factor X1 increases the expression of neuronal glutamate transporter type 3

Glutamate transporters (excitatory amino acid transporters, EAAT) play an important role in maintaining extracellular glutamate homeostasis and regulating glutamate neurotransmission. However, very few studies have investigated the regulation of EAAT expression. A binding sequence for the regulatory factor X1 (RFX1) exists in the promoter region of the gene encoding for EAAT3, a neuronal EAAT, but not in the promoter regions of the genes encoding for EAAT1 and EAAT2, two glial EAATs. RFX proteins are transcription factors binding to X-boxes of DNA sequences. Although RFX proteins are necessary for the normal function of sensory neurons in Caenorhabditis elegans, their roles in the mammalian brain are not known. We showed that RFX1 increased EAAT3 expression and activity in C6 glioma cells. RFX1 binding complexes were found in the nuclear extracts of C6 cells. The activity of EAAT3 promoter as measured by luciferase reporter activity was increased by RFX1 in C6 cells and the neuron-like SH-SY5Y cells. However, RFX1 did not change the expression of EAAT2 proteins in the NRK52E cells. RFX1 proteins were expressed in the neurons of rat brain. A high expression level of RFX1 proteins was found in the neurons of cerebral cortex and Purkinje cells. Knockdown of the RFX1 expression by RFX1 antisense oligonucleotides decreased EAAT3 expression in rat cortical neurons in culture. These results suggest that RFX1 enhances the activity of EAAT3 promoter to increase the expression of EAAT3 proteins. This study provides initial evidence for the regulation of gene expression in the nervous cells by RFX1.