As a critical factor in the hypothalamic-pituitary-gonad axis of vertebrates, gonadotropin-releasing hormone (Gnrh) is well-known in gonadotropin release, gonadal development, maturation, and reproduction. As a fish specific form of Gnrh, Gnrh3 was considered to be an important regulator in the reproduction of teleost. However, recent studies demonstrated that knockout of Gnrh3 had no obvious effects on the reproduction in adult zebrafish.

Using gene editing technology, a group led by Prof. HU Wei from Institute of Hydrobiology (IHB) of Chinese Academy of Sciences, generated a new line of gnrh3-null zebrafish by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated system (CRISPR/Cas9). The study was recently published in the journal Endocrinology. 

Adult gnrh3-null zebrafish showed a significant male-biased sex ratio. Primordial germ cell (PGC) numbers in gnrh3-null zebrafish embryos were less than those in wild-type embryos after the dome stage during early embryonic development. 

Through immunofluorescence and confocal analysis technology, they found there was no proliferation signals of PGCs in adult gnrh3-null zebrafish embryos. The migration of PGCs was normal but no significant proliferation in mutant embryos observed with time-lapse imaging.  

The decreased PGC number in gnrh3-null zebrafish could be partially rescued by gnrh3 overexpression with gnrh3 mRNA microinjection and crossed with homozygous gnrh3 overexpression transgenic line. 

The research group provided evidence in vivo that Gnrh3 regulates PGC proliferation through a mitogen-activated protein kinase (MARK) signaling pathway. At the key period of sex differentiation, the expression of male sex markers was up-regulated significantly, and a female sex marker was down-regulated significantly in gnrh3-null zebrafish.  

Their results indicated that Gnrh3 plays an important role in early sex differentiation by regulating the proliferation of PGCs through a MAPK-dependent path in the zebrafish model system.  

This study uncovered a new function of Gnrh3 in PGC proliferation and sex differentiation in any species for the first time.

PGC distribution in different lines. A, Tg(piwil1: egfp-UTRnanos3)^ihb327Tg. B, the homozygous mutant in the offspring of the outcross between gnrh3^-/^- and A lines. C, the injection of gnrh3 mRNA in gnrh3^-/^-  embryos. D, the homozygous mutant in the offspring of the outcross between B and over express gnrh3 lines. (Image by IHB)