Abstract
The gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are key regulators of the reproductive axis in vertebrates. Despite the high popularity of zebrafish as a model organism for studying reproductive functions, to date no transgenic zebrafish with labeled gonadotropes have been introduced. Using gonadotropin regulatory elements from tilapia, we generated two transgenic zebrafish lines with labeled gonadotropes. The tilapia and zebrafish regulatory sequences were highly divergent but several conserved elements allowed the tilapia promoters to correctly drive the transgenes in zebrafish pituitaries. FSH cells reacted to stimulation with gonadotropin releasing hormone by proliferating and showing increased transgene fluorescence, whereas estrogen exposure caused a decrease in cell number and transgene fluorescence. Transgene fluorescence reflected the expression pattern of the endogenous fshb gene. Ontogenetic expression of the transgenes followed typical patterns, with FSH cells appearing early in development, and LH cells appearing later and increasing dramatically in number with the onset of puberty. Our transgenic lines provide a powerful tool for investigating the development, anatomy, and function of the reproductive axis in lower vertebrates.
Highlights
The hypothalamic–pituitary–gonadal (HPG) axis is the master regulator of reproduction in vertebrates
Despite retention of this basic division, the varying sizes of the exons and introns result in a more compact gene in tilapia (~2.6 kb) than in zebrafish (~6 kb). This stems from elongation of the zebrafish introns as well as a long (~600 bp) 3 untranslated region (UTR) situated within the third exon of the zebrafish gene
The structure of the lhb gene of zebrafish varies from that of tilapia in its exon/intron division: whereas the cyprinid genes are comprised of three exons, in tilapia the gene is divided into four segments
Summary
The hypothalamic–pituitary–gonadal (HPG) axis is the master regulator of reproduction in vertebrates. Hypothalamic axons secrete peptides that bind to specific receptors on gonadotrope cells in the anterior pituitary and stimulate the secretion of gonadotropins (GtHs) from vesicles within these cells. The main stimulator of GtH secretion is the decapeptide gonadotropin releasing hormone (GnRH) but recently, several other hypothalamic factors have been shown to act directly on the pituitary cells to enhance GtH expression and release [1,2,3,4]. Two GtHs have been identified: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both GtHs are dimers comprised of a common α subunit and a distinct β subunit that confers their biological specificity. A LH surge is associated with ovulation in females and Leydig cell stimulation in males [5, 6]
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