Characterization and Role of Secretogranin-II/Secretoneurin in Zebrafish Reproduction
Creators: name: Mitchell, Kimberly
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2018
Subjects: subject: Zebrafish
subject: Reproduction
subject: FOS: Medical biotechnology
subject: Neuropeptide
subject: Secretoneurin
subject: Secretogranin

description: At the hypothalamo-pituitary interface, multiple neurotransmitters and neuropeptides interact to control luteinizing hormone and follicle stimulating hormone release from gonadotrophs. The luteinizing hormone surge is essential for fertility as it triggers ovulation in females and sperm release in males. While it is well-established that gonadotropin-releasing hormone and/or kisspeptin are required for pulsatile and surge release of luteinizing hormone in mammalian species, their essentiality is challenged by studies showing knockouts in zebrafish and medaka do not block reproduction. In goldfish, secretoneurin-a, a neuropeptide derived from secretogranin-IIa processing, stimulates luteinizing hormone release in vivo and from dispersed pituitary cells in vitro. Secretoneurin does not bind to the human gonadotropin releasing hormone receptor and can enhance luteinizing hormone release when applied directly to mouse LbetaT2 cells. Our study indicates the presence of secretogranin-IIa and secretogranin-IIb mRNA in specific regions of the zebrafish brain and pituitary that have been implicated in the control of reproductive processes. I also observed that secretogranin-II knockout disrupts normal morphology of the pectoral fins by reducing the number of breeding tubercule rows, breeding tubercule width and breeding tubercule cluster length which could potentially lead to a reduced spawning success. Knocking out the secretogranin-IIa and secretogranin-IIb genes using TALENs in zebrafish indicates that these genes are required for optimal reproduction. Rates of oviposition for double secretogranin-II knockout females are 6% and 11% when crossed with double secretogranin-II knockout and wild-type males, respectively, compared to 62% in virgin wild-type pairings. Comprehensive video analysis demonstrates that secretogranin-II knockout reduces all stereotypical male courtship behaviours. Severe reductions in the expression of gonadotropin releasing hormone 3 in the hypothalamus and luteinizing hormone in the pituitary suggest that secretogranin-II-derived peptides drive the gonadotropin releasing hormone-luteinizing hormone control system. Spawning success is rescued in double secretogranin-II knockouts following one injection of synthetic secretoneurin-a in which it increases from 11% to 30% thereby supporting the hypothesis that secretoneurin-a is a key reproductive hormone. However, embryo survival rate of secretoneurin-a injected double secretogranin-II knockout was lower than saline-injected wild-type pairings. Injection of human chorionic gonadotropin, a hormone that mimics the action of luteinizing hormone on binding and activating the luteinizing hormone receptor in fish, increased double secretogranin-II knockout spawning success to 38%, thus comparing favourably to 47% in saline-injected wild-type controls. My data provides support that secretogranin-II is required for optimal reproductive functions although the precise mechanisms behind this reduced capacity in zebrafish lacking the secretogranin-II genes remain to be investigated. Moreover, the high conservation of secretoneurin from lamprey to human suggests a broader importance of this emerging peptide family.