A NEURON-SPECIFIC ENHANCER TARGETS EXPRESSION OF THE GONADOTROPIN-RELEASING-HORMONE GENE TO HYPOTHALAMIC NEUROSECRETORY NEURONS

The molecular mechanisms specifying gene expression in individual neur ons of the mammalian central nervous system have been difficult to stu dy due to the cellular complexity of the brain and the absence of cult ured model systems representing differentiated central nervous system neurons. We have developed clonal, differentiated, neuronal tumor cell lines of the hypothalamic GnRH-producing neurons by targeting tumorig enesis in transgenic mice. These cells (GT1 cells) provide a model sys tem for molecular studies of GnRH gene regulation. Here we present the identification and characterization of a neuron-specific enhancer res ponsible for directing expression of the rat GnRH gene in GT1 hypothal amic neurons. This approximately 300 base pair (bp) upstream region (- 1571 to -1863) confers enhancer activity to a short -173-bp GnRH promo ter or to a heterologous promoter only in GT1 cells, The enhancer is b ound by multiple GT1 nuclear proteins over its entire length, Deletion of more than 30 bp from either end dramatically reduces activity, and even large internal fragments carrying seven of the eight DNAse I-pro tected elements show decreased activity, Scanning replacement mutation s demonstrate that several of the internal elements are required for a ctivity of the enhancer, Thus, the GnRH gene is targeted to hypothalam ic neurons by a complex multicomponent enhancer that relies on the int eraction of multiple nuclear-protein binding enhancer elements.