Development of the Endocrine Hypothalamus

Gonadotropin releasing hormone (GnRH) neurons are required to establish and maintain the reproductive axis. The immediate progenitors of these neurons are found in the olfactory placode (OP) from which they migrate into the CNS along the olfactory nerve. We have found that cGnRHI mRNA is expressed prior to OP formation, in the rostral neuroectoderm. This region is incorporated into the neural folds and designated the anterior neural ridge (ANR). The ANR gives rise to both olfactory and hypophyseal placodes; adjacent tissue gives rise to the nasal epithelium. The presumptive olfactory and nasal progenitors merge as the face undergoes morphogenesis, and invaginate to form the conchae of the nose, the superior aspect of which is the olfactory epithelium. The mechanisms of differentiation of the GnRH neurons prior to placode formation remain unknown. To understand the origins of these critical neurons we propose three sets of experiments. (1) We will collect normative data on the temporal and spatial expression of cGnRHI or mGnRH mRNA and peptide in chick and mouse embryo, respectively, from formation of the ANR through the GnRH migratory phase. In the chick, we will investigate whether GnRH+ cells express transcription factors of the ANR and its derivatives. (2) We will test the hypothesis that GnRH neurons become specified and committed to their unique lineage in a manner analogous to their presumptive olfactory neighbors (or the alternative, that GnRH neurons' differentiation pathway is different from olfactory sensory cells). The ANR (or facial ectoderm) is placed in culture at selected embryonic stages, with or without adjacent inductive tissues. If GnRH neurons develop in vitro without inductive tissues, then the GnRH progenitors are said to be specified. Once specified, GnRH neurons are committed to their lineage if, when transplanted to a new inductive environment (e.g., forming otic placode), the tissue retains the ability to generate GnRH neurons and NOT take on the otic phenotype. For these experiments chick/quail chimeras are used. Finally, we ask, what are the molecular mechanisms leading to GnRH neuronal differentiation? To this end we will suppress the activity of selected transcription factors using dominant negative forms of transcription factors, of their upstream regulators and/or by small interfering RNA. In toto, these experiments will establish a hierarchy of developmental steps for GnRH neurons to identify critical junctures in the establishment of this essential population.