Gonadotropin-releasing hormone (GnRH) is essential for the regulation of reproductive function. GnRH neurons sit at the top of the HPG axis and mediate the effects of a number of cues on reproduction. For example, fasting has been shown in both experimental paradigms and clinical observations to decrease reproductive function via an inhibition of GnRH release. In addition, some models ascribe a role for a permissive level of nutritional growth to trigger the onset of puberty. It is unclear how GnRH neurons respond to nutritional signals. Previous studies from our laboratory indicated that GnRH neurons respond to treatment with IGF-1 with an IGF-1 receptor dependent increase in GnRH mRNA expression that is mediated by a Ras signaling cascade. Erkl/2 activation of AP-1 activates human GnRH promoter activity and is thought to activate the mouse GnRH (mGnRH) promoter, albeit via a non-classical AP-1 element. A conditional KO mouse lacking the insulin receptor in the brain resulted in hypothalamic infertility suggesting a critical in vivo role for the insulin receptors in the brain for the regulation of reproduction. Preliminary results from transfections indicate that insulin treatment increased GnRH promoter activity in a GnRH neuronal cell line nearly 3-fold when compared to insulin's effects on a minimal GnRH promoter and that this response was blocked by a MEK inhibitor. In a fasted mouse paradigm, insulin treatment induced an activation of mouse GnRH promoter activity. These results, in total, suggest that insulin plays a regulatory role in GnRH gene expression in vitro and in vivo. I propose to identify the elements of the molecular signaling pathway for insulin and IGF-1 using classic pharmacological and binding studies. The regulation of the mGnRH promoter by insulin and IGF-1 will be mapped to identify cis-regulatory elements important for promoter responsiveness to insulin and/or IGF-1. In addition, I propose to create conditional knock out mice in which insulin receptor or IGF-1 receptor is deleted only in GnRH neurons. I will also construct a mouse in which c-Jun, one of the downstream signaling molecules for IGF-1, and perhaps insulin, signaling is deleted in GnRH neurons. These mice should prove invaluable for confirming a potential direct effect of insulin or IGF-1 receptor signaling at the level of the GnRH neuron.
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