Knockout of tac3 genes in zebrafish shows no impairment of reproduction

Hormonal dynamics reveal a stimulatory role for secretoneurin in zebrafish ovulation

Surge release of luteinizing hormone (Lh) from the pituitary is essential for fertility as it triggers ovulation. While secretoneurin (SN) is a phylogenetically conserved secretogranin-2-derived peptide that stimulates Lh, its role in ovulation has not been established. To directly compare periovulatory changes in the classical hormones to the emerging reproductive neuropeptides SNa and SNb, simultaneous mass spectrometry measurement of 9 peptides and 5 steroids was conducted in female zebrafish. Regression analysis indicated that levels of SNa1-34 in the brain peaked when type 3 gonadotropin-releasing hormone (Gnrh3) increased (R 2 = 0.71) at the time of the Lh surge, 3.5 h before ovulation. Levels of the naturally occurring derivative SNa1-14 were highest at ovulation, while SNb1-31 was invariable. The bioactivities of SNa1-34 and SNa1-14 were investigated. After injection of SNa1-34 in females that had been isolated from males, 61% (11/18) ovulated within 6 h, which was like the effects of the Lh analog human chorionic gonadotropin (72%; 13/18 females). SNa1-34 injection induces ovulation by increasing time-dependent expression of gnrh3 in the brain, a likely direct stimulation of chorionic gonadotropin alpha (cga) and luteinizing hormone b (lhb) subunit in pituitary, and via the subsequent time-dependent increase in nuclear progesterone receptor (npr) in ovaries. In contrast, SNa1-14 exhibited far fewer effects on gene expression and did not induce ovulation. Our results support the proposal that SN is a reproductive hormone.

Mutation of spexin2 promotes feeding, somatic growth, adiposity and insulin resistance in zebrafish

Spexin2 (spx2) is a newly identified gene in vertebrates, but its biological functions remain unclear. In this study, we cloned the full-length cDNA of spx2 in zebrafish. The 288 bp open reading frame encodes a protein of 95 amino acids (aa) that contains a 14 aa mature peptide. Spx2 is highly expressed in brain and testis. Its expression was significantly down-regulated in the hypothalamus after a 1 h feeding treatment and 7 days of food deprivation. Using a zebrafish spx2^-/- mutant line, we observed a greater amount of food intake and changes in mRNA levels of feeding factors. We found that, SPX2 acts as a satiety factor that inhibits food intake by downregulating the expression of agouti related neuropeptide (agrp). Moreover, spx2 mutant fish exhibited a larger body size, excessive lipid accumulation, and insulin resistance. Taken together, our results revealed that SPX2 functions as a satiety factor involved in energy metabolic regulation in zebrafish.

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.