Pituitary

Zebrafish Gonadotropins and Their Receptors: II. Cloning and Characterization of Zebrafish Follicle-Stimulating Hormone and Luteinizing Hormone Subunits - Their Spatial-Temporal Expression Patterns and Receptor Specificity

Abstract
Gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), play critical roles in vertebrate reproduction. In the present study, we cloned and characterized zebrafish FSH (fshb), LH (lhb) and GTH (cga) subunits. Compared with the molecules of other teleosts, the cysteine residues and potential glycosylation sites are fully conserved in zebrafish Lhb and Cga, but not in Fshb whose cysteines exhibit unique distribution. Interestingly, in addition to the pituitary, fshb, lhb and cga were also expressed in some extra-pituitary tissues, particularly the gonads and brain. In situ hybridization showed that zebrafish fshb and lhb were expressed in two distinct populations of gonadotrophs in the pituitary. Real-time RT-PCR analysis revealed that all the three subunits increased expression before ovulation (1:00 to 4:00) when the germinal vesicles in the full-grown follicles were migrating towards the periphery, but the levels dropped at 7:00 when ovulation occurred. Recombinant zebrafish FSH (zfFSH) and LH (zfLH) were produced in the Chinese hamster ovary (CHO) cells and their effects on the cognate receptors (zebrafish Fshr and Lhr) tested. Interestingly, zfFSH specifically activated zebrafish Fshr expressed together with a cAMP-responsive reporter gene in the CHO cells, whereas zfLH could stimulate both Fshr and Lhr. In conclusion, the present study systematically investigated gonadotropins in the zebrafish in terms of their structure, spatial-temporal expression patterns and receptor specificity. These results, together with the availability of recombinant zfFSH and zfLH, provide a solid foundation for further studies on the physiological relevance of FSH and LH in the zebrafish, one of the top biological models in vertebrates.

Key words: Pituitary • Follicle-stimulating hormone • Follicular development • Luteinizing hormone

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