Seminar: Miranda Marvel (Ph.D. Candidate, MEES Program)
Title: The roles of Gonadotropin-releasing hormone 2 (Gnrh2) in feeding and reproduction in zebrafish: a potential mediator between the two processes.
Speaker: Miranda Marvel (Ph.D. Candidate, MEES Program)
Abstract: The neuropeptide in the brain of vertebrates most well-known for controlling reproduction is called the Gonadotropin-releasing hormone (GNRH). There are three isoforms of GNRH, named for their location and function in the brain. GNRH1 is found in the hypothalamus and projects to the median eminence (in mammals) or pituitary (in teleosts), and thus termed hypophysiotropic, and is the main stimulator of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which circulate to the gonads and stimulate steroidogenesis and gametogenesis. The teleost-specific Gnrh3 is found in the olfactory bulb terminal nerve (OB-TN), and is thought to have neuromodulatory roles. GNRH2 is found in the midbrain tegmentum and despite being the most evolutionarily conserved and ubiquitous, is the least studied form among the three. The lack of studies on Gnrh2 is most likely due to the absence of a functional GNRH2 protein in the common murine research models. Zebrafish, another common vertebrate model for genetic manipulation and biological studies, possesses Gnrh2, unlike murine models, and is therefore an ideal organism to study this neuropeptide. Through a combination of gene knockout/knockdown, neuroanatomical imaging, molecular tools, and functional assays, the roles of Gnrh2 in zebrafish were comprehensively explored. Characterization of the Gnrh2 knockout (gnrh2-/-) zebrafish lines demonstrated decreased lhb expression and compromised oocyte quality, increased feeding and growth, and, most strikingly, major inhibitions in spawning and oocyte maturation after long-term fasting. Interestingly, Gnrh2 was unable to compensate for Gnrh3 loss, as the double knockout (gnrh2-/-;gnrh3-/-) fish were fertile and displayed normal reproduction. Functional and neuroanatomical assays confirmed that Gnrh2 exhibits plasticity under fasting conditions, and projects more neurons to Lh and Fsh gonadotropes, whereas Gnrh3 is inhibited by the same conditions. Through transcriptomics and targeted qPCR, several novel differentially expressed reproductive factors were discovered in the Gnrh2 and Gnrh3 single and double knockout lines, potentially playing a part in the regulation of reproductive processes in zebrafish. In addition, thorough examination of Gnrh2 neuronal projections under normal feeding conditions identified projections innervating melatonin cells in the pineal gland, neurons of the feeding factor, Agouti-related peptide 1 (Agrp1) in the mediobasal hypothalamus, and neurons of the reproduction regulator, Gonadotropin-inhibitory hormone (Gnih), in the hypothalamus. Gnrh2 was found to be either inhibitory or stimulatory to gnih expression, but able to potently downregulate agrp1expression. It is likely that Gnrh2 exerts its reproductive/feeding effects through these three factors. For the first time in vertebrates, using tg(Gnrh2:eGFP) transgenic fish to map neuronal projections throughout the brain, a previously unstudied population of Gnrh2 was identified in the olfactory region. Social studies combined with neuroanatomical studies implicate these neurons in transducing pheromonal cues, and thus affecting reproductive behavior. As a result of these studies, it was found that overall, under normal conditions, Gnrh2 most likely has roles in stimulating lhb expression, maintaining optimal oocyte quality, transducing pheromonal cues from the olfactory bulb, and regulating energy expenditure by reducing feeding behavior through the downregulation of agrp1, and potentially indirectly through stimulating melatonin secretion. Under fasting conditions, Gnrh2 most likely exhibits plasticity to become the main hypophysiotropic stimulator of gonadotropin secretion. Gnrh2 therefore appears to be an important upstream factor mediating feeding and reproductive processes in vertebrates.