Role of GnIH and GnRH in Paracrine Control of Gametogenesis in Zebrafish (Danio rerio)
AdvisorHabibi, Hamid R.
AuthorPourmohammadi Fallah, Hamideh
Committee MemberHabibi, Hamid R.
Syme, Douglas A.
Thundathil, Jacob C,
Kastelic, John P.
Wiseman, Steve B.
germinal vesicle breakdown
human chorionic gonadotropin
zebrafish (Danio rerio)
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AbstractControl of gonadal function is multifactorial and involves a number of hypothalamic, hypophyseal, and peripheral hormones. It is well established that hypothalamic GnRH (Gonadotropin-Releasing Hormone) and GnIH (Gonadotropin-Inhibitory Hormone) are key peptides involved in the neuroendocrine control of gonadal function. However, the role of GnRH and GnIH as paracrine regulators of testicular and ovarian function has not been fully investigated. Variants of GnIH and GnRH peptides have been discovered in different vertebrates, including teleost. In addition, the presence of GnRH and GnIH receptors in extra-pituitary organs, including gonads, has been demonstrated in various species. The existence of GnRH and GnIH peptides and their receptors in the testis and ovary suggest a potential for paracrine/autocrine roles of these peptides in the control of reproduction. However, information on the physiological significance of locally produced GnRH and GnIH is rather limited, and much less is known regarding the extra-pituitary actions of these peptides. The main goal of my thesis was to study the role of zebrafish GnIH (zGnIH) and GnRH peptides, which are locally produced in the gonads, in the regulation of basal and gonadotropin hormone (GTH)-induced spermatogenesis and final oocyte maturation in zebrafish, in vitro. The findings highlight the presence of GnIH and GnRH gonadal peptides and their receptors in zebrafish (Danio rerio) and provide strong evidence for direct actions of these hormones at the level of gonads. Treatment of testicular tissue with zGnIH at the lower concentrations tested, inhibited gonadotropin-induced spermatids/spermatozoa (SPD/SPZ) production ex vivo. However, at the highest concentration, zGnIH increased the basal number of SPD/SPZ and showed a paradoxical effect. The effect of zGnIH on testosterone and haploid cell production was blocked in the presence of flutamide (FLU) androgen receptor antagonist. A number of transcripts were also measured to better understand zGnIH mechanisms of action on zebrafish spermatogenesis. Further analysis of paracrine actions of zGnIH on earlier stages of spermatogenesis demonstrated that a lower concentration of zGnIH significantly decreases caspase-3 activity and changes the proliferative activity of spermatogonia cells. It was also found that zGnIH at all concentrations tested inhibits both human chorionic gonadotropin hormone (hCG) and follicle-stimulating hormone (FSH)-induced spermatogenesis. With respect to GnRH peptides, both native isoforms (GnRH2 & GnRH3) stimulated basal spermatogenesis by increasing numbers of type A undifferentiated spermatogonia, spermatozoa, and testosterone release, and GnRH2 exerted higher relative activity than GnRH3. Also, two GnRH isoforms were found to have different effects on Fsh and hCG-induced response depending on stages of spermatogenesis and concentration of peptides. In studies regarding final oocyte maturation, treatment of the late-vitellogenic ovarian follicles with GnRH and GnIH in isolation stimulated germinal vesicle break down (GVBD) in vitro. GnRH treatment increased caspase-3 activity, while GnIH treatment showed a protective effect on the late-vitellogenic follicles. GnRH and GnIH treatments were found to have similar effects on the hCG-induced resumption of meiosis while showed different effects on caspase-3 activity. Further, the interaction of GnRH and GnIH peptides in the absence and presence of hCG provides strong support for the hypothesis that gonadal GnRH and GnIH, in addition to gonadotropins are key components involved in final oocyte maturation in zebrafish. The findings in this thesis indicate that locally produced zGnIH and GnRH are important components of the complex multifactorial system that regulates gametogenesis in zebrafish in a paracrine manner.
CitationPourmohammadi Fallah, H. (2020). Role of GnIH and GnRH in Paracrine Control of Gametogenesis in Zebrafish (Danio rerio) (Unpublished doctoral thesis). University of Calgary, Calgary, AB.
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