Date of Award

1-1-2009

Degree Name

Master of Science

Department

Molecular Cellular and Systemic Physiology

First Advisor

Narayan, Prema

Abstract

Luteinizing hormone (LH), one of the two gonadotropin hormones released from the pituitary gland, binds its receptor (LHR) in the gonads to initiate steroid hormone production, as well as gametogenesis and ovulation. Mutations of amino acid sequence within the receptor can render it either inactive or constitutively active. All activating mutations result in male-limited precocious puberty. Males afflicted with this condition undergo puberty around 4 years of age, with elevated testosterone levels and premature skeletal development. In order to better understand how chronic ligand-mediated activation of the LHR affects gonadal development and function, a mouse model expressing a yoked hormone-receptor (YHR) complex, engineered by covalently linking the hormone human chorionic gonadotropin to the rat LHR, has been studied. YHR+ males have prepubertally elevated testosterone and decreased gonadotropin levels. Histological evaluation of the testes of these animals show significantly smaller seminiferous tubules and Leydig cell clusters. Finally, testis gene expression analysis revealed a significant decrease in the relative mRNA expression of three Leydig cell specific genes. Based on these results, it was hypothesized that premature activation of the LHR impairs postnatal Leydig cell development. In the testis there are two morphologically and developmentally distinct populations of Leydig cells, the fetal and the adult. The first objective of this study was to quantify the populations of cells in the adult Leydig cell lineage in both the YHR+ and the WT controls. Real-time RT-PCR, for markers of the immature and adult Leydig cell populations, as well as Leydig cell quantification, suggested a delay in adult Leydig cell development. Interestingly, there was a significant increase in the fetal Leydig cell population in the YHR+ mice. The second objective was to determine if the decrease in the adult population is due to either a decrease in proliferation or an increase in apoptosis in the YHR+ animal. There was not a difference in apoptosis between the WT and the YHR+ at any age examined, however, there was a decrease in progenitor Leydig cell proliferation in the YHR+ animals at 2 weeks of age. The final objective was to determine if elevated neonatal testosterone levels impairs the development of the adult Leydig cell population. Seven-day old WT pups were subjected to testosterone supplementation via subdermal implant. Quantification of the total Leydig cell population revealed a significant decrease in the number of adult Leydig cells in the testosterone-treated group similar to that seen in the YHR+ animal. Taken together, these data suggest that elevated neonatal testosterone levels resulting from premature LHR activation inhibits the proliferation of progenitor Leydig cells, resulting in fewer adult Leydig cells in the YHR+ animals.

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