Normal reproductive function requires the precise orchestration of hormonal regulation at the hypothalamic, pituitary, and gonadal levels. Within the anterior pituitary, the gonadotrope serves as the control center for integration of hormonal signals, producing luteinizing hormone (LH) and follicle-stimulating hormone to regulate reproduction. This cell responds to pulses of gonadotropin-releasing hormone (GnRH) from the hypothalamus through the GnRH receptor, to endocrine and autocrine activin and follistatin through activin receptors, and to steroid hormone feedback from the gonads. In Research Project I,. Our focus will be the cellular and molecular mechanisms of GnRH and activin regulation of LH and GnRH receptor gene expression in the gonadotrope. Our model system is the mouse, due to the facile manipulation s of the genome attainable by transgenic technology and the ability to exploit our immortal mouse pituitary gonadotrope cell lines that express both LH subunits, activin B, follistatin, and activin, GnRH, and steroid receptors. In the first two aims, these cell lines will be used to investigate the molecular basis of hormonal regulation in the gonadotrope. We have demonstrated that constant GnRH or its second messengers will down regulate, and short-term or pulsatile GnRH will induce, the LHBeta subunit gene in the LbetaT cells.
In Specific Aim 1, we will determine the signaling pathways and their transcriptional targets for long-term repression, and short-term and pulsatile induction of the LHBeta gene, and the mechanism for induction of the Alpha-subunit gene by GnRH. Activin sensitizes the gonadotrope to GnRH by inducing the GnRH receptor mRNA, while follistatin counteracts this response. We have mapped the sequences responsive to activin/follistatin regulation in the mouse GnRH receptor gene and demonstrated that activin treatment induces, and follistatin represses, the activity of a nuclear DNA-binding protein.
In Specific Aim 2 we will investigate the induction of the GnRH receptor gene and the repression of the alpha-subunit gene by activin. The mouse alpha-subunit gene is regulated by GnRH in alpha T3 cells through an Ets binding site.
In Specific Aim 3, we will address the role of MAPKinase signaling through Ets proteins in GnRH action in transgenic mice. In addition, we will investigate the physiological role of activin signaling exclusively in the gonadotrope in transgenic mice.
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