Our long-term goals are to understand the embryonic origin of the endocrine gonadotropin releasing hormone (GnRH) cells in the hypothalamus, and the mechanisms controlling the differentiation and migration of these cells. Kallmann Syndrome in humans, which is characterized by lack of GnRH cells in the hypothalamus and by loss of a sense of smell, has been used as evidence supporting a common origin for the GnRH cells of the hypothalamus and the olfactory sensory neurons. However, this link is likely only a consequence of the disruption in the migration of GnRH cells to the hypothalamus along the olfactory nerve. Consistent with this interpretation we have shown, in zebrafish, that mutant embryos lacking their pituitary also lack hypothalamic GnRH cells, suggesting that the GnRH cells arise from the anterior pituitary placode, not the olfactory placode as previously proposed. We will test the hypotheses that 1) the hypothalamic GnRH cells arise from precursors in the anterior pituitary placode and 2) migration of the hypothalamic GnRH cells is dependent upon anosminl and fibroblast growth factor receptor-1 gene function, 2 genes which when mutant cause Kallmann Syndrome phenotypes. We will test the first hypotheses by: labeling single cells within anterior pituitary placode domain and following the differentiation of the resulting clones of cells, and creating transgenic fish expressing Green Fluorescent Protein in the migrating GnRH cells to visualize them in living embryos. We will test the second hypothesis by: blocking the function of anosminl and fibroblast growth factor receptor-1 genes in the developing embryo using RNA interference technologies and examining effects on development of the olfactory system, anterior pituitary and hypothalamus. Understanding factors controlling the origin, differentiation and migration of the hypothalamic GnRH cells will allow us to make specific diagnoses based on developmental defects observed early in human development, thus alerting clinicians to the possibility of GnRH deficits later in life.
