SAT-406 Deletion of Musashi in Gonadotropes Leads to Increased GnRHR Protein Levels and Gonadotrope Dysfunction

Abstract

Leptin is a critical mediator of metabolic regulation of the hypothalamic-pituitary gonadal (HPG) axis. We have previously shown that leptin is responsible for the optimal expression of GnRHR, a rate-limiting component of the reproductive process. Female mice lacking leptin receptors (Lepr-null) specifically on gonadotropes are sub-fertile. Their reduced GnRHR proteins and normal Gnrhr mRNA levels suggest leptin’s actions on gonadotropes are post-transcriptional. A clue about a candidate translational regulator is seen in our studies showing that Lepr-null gonadotropes have increased expression of Musashi (MSI), which has binding sites within the Gnrhr 3’ UTR. Furthermore, leptin reduced MSI expression specifically in gonadotropes and increased GnRHR expression. We hypothesized that MSI may repress Gnrhr mRNA translation and that leptin alleviates this repression. To determine the effects of MSI on the HPG axis, we developed a gonadotrope-Msi1/2-null mouse line and compared mutant females (MUT) to their littermate controls (CTL) on the morning of diestrus, when the pituitary GnRHR protein levels should reach a peak. The levels of GnRHR proteins (measured by EIA) are significantly increased (1.6X) in the pituitary of the mutant females (CTL: 1.355 ± 0.11 ng/ml vs MUT: 2.202 ± 0.16 ng/ml, p=.0006), with no change in mRNA. The gonadotrope-Msi1/2-null females are subfertile, with litter sizes of 3 ± 0.4 pups, with the first litter at around day 40 and an average of 41 day delay between litters. To understand the downstream effect of the MSI knockout on gonadotropin levels, we measured pituitary and serum LH and FSH protein levels (Luminex EIA) and mRNA (qPCR). Serum FSH levels are decreased by >50% in mutant females (CTL: 0.947 ± 0.14 ng/mL vs MUT: 0.406 ± 0.05 ng/ml, p=.0049), but FSH stores and Fsh mRNAs are unchanged. Additionally, we observed a 2.2X increase in the pituitary LH protein content in mutant females (CTL: 0.328 ± 0.11 ng/ml vs MUT: 0.729 ± 0.09 ng/ml, p=.0174), but no changes in serum LH or Lh mRNA levels. These studies thus show that, as a repressor of GnRHR translation, Musashi can also regulate expression of gonadotropins. In the gonadotrope Lepr-null model, we hypothesized that leptin signals were needed to de-repress Musashi actions and allow GnRHR translation. In contrast, the gonadotrope-MSI-null mice over-express GnRHR, which confirms this role for MSI as a GnRHR regulator. We propose that the increased expression of GnRHR in the gonadotrope MSI-null animals causes the increased LH content, and disrupted FSH secretion, resulting in a higher serum LH:FSH ratio and subfertility. Collectively these data suggest that MSI regulation is necessary for optimal fertility in the adult female mouse. Future studies will determine the impact of loss of MSI on gonadotrope function throughout the estrous cycle.