|M.Sc Student||Haj Muhammad Majd|
|Subject||GnRH Induces Histone Modifications to Tegulate Expression|
of the Gonadotropin ALPHA-Subunit Gene
|Department||Department of Biology||Supervisor||Professor Philippa Melamed|
|Full Thesis text|
The expression of the gonadotropin α subunit gene (αGSU) is regulated by gonadotropin-releasing hormone (GnRH), through various MAPK pathways. Previous studies in our lab indicated that GnRH also targets the chromatin at this gene inducing various histone modifications. These include histone methylation, acetylation and phosphorylation, all of which have been previously implicated to play a role in yeast and mammalian transcriptional regulation.
In the absence of GnRH, the murine gonadotrope αT3-1 cells express the αGSU. In this basal state, the αGSU promoter has a low nucleosome occupancy which allows for the binding of RNA polymerase II and various gene specific transcription factors, facilitating high levels of basal gene expression. However, GnRH upregulates its expression, and ChIP studies demonstrate that there is a decrease in H3 levels at the -1 and nucleosomes upstream of the αGSU proximal promoter and downstream of the TSS respectively. The H3 depletion is likely mediated through menin, which recruits the MLL methyltransferase complex to the αGSU promoter to catalyze H3K4me3. This modification was shown to have a role in nucleosomal occupancy and play a major role in regulating the αGSU gene expression. Our findings demonstrate a novel interaction between SF-1 and menin, which was also found to be involved in regulating H3 depletion, indicating that menin is likely recruited to the promoter of the αGSU gene through its interaction with SF-1.
ChIP studies indicated that GnRH induces H3 acetylation and H3 phosphorylation. H3S10 phosphorylation was shown to be catalyzed by JNK, but is not required for GnRH-induced upregulation of the αGSU gene expression nor for H3 acetylation. Conversely, H3S28 phosphorylation, catalyzed by MSK1, is essential for GnRH-induced upregulation and may be required for downstream H3K27 acetylation at the nucleosome. Together, these results show that GnRH induced gonadotropin αGSU gene expression involves reducing nucleosomal occupancy, likely through H3K4me3, and also requires H3S28p, which may crosstalk with H3K27 acetylation. The fact that the phosphorylation increases on the nucleosome immediately down-stream of the transcriptional start site, indicates a likely role in transcriptional elongation. Our data demonstrate for the first time, a novel function of the H3S28p mark in the upregulation of the αGSU gene expression in response to MAPK pathways activation in the context of hormone signaling.