Differential distribution and potential regulatory roles of estrogen receptor 2a and 2b in the pituitary of ricefield eel Monopterus albus

The expression profiles of cyp19a1, sf-1, esrs and gths in the brain-pituitary during gonadal sex differentiation in juvenile Japanese eels

Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1, sf-1, foxl2s, and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1, foxl2s, and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths, especially fshb, may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels.

Two forms of G protein-coupled estrogen receptor 1 in the ricefield eel: Expression and functional characterization in relation to ovarian follicle development

G protein-coupled estrogen receptor 1 (Gper1) mediates many rapid, non-genomic estrogenic effects in vertebrates, and plays an important reproductive role in the maintenance of oocyte meiotic arrest in teleost fishes. In the present study, two genes for Gper1, namely gper1a and gper1b, were identified in the genome of a teleost fish, the ricefield eel (Monopterus albus) through Blast and syntenic analysis. Although genes neighboring gper1b are of high synteny, ricefield eel Gper1b shares very low (around 15) percent identities with Gper1 homologues of other vertebrates. In transiently transfected HEK293T cells, both ricefield eel Gper1a and Gper1b responded to estradiol (E₂) and estradiol-BSA (E₂-BSA) challenges by activating pCRE but not pSRE luciferase reporters, which were abolished by G-15 and NF-449. The production of cAMP was also increased in HEK293T cells transfected with Gper1a or Gper1b expression construct after E₂-BSA challenge, which was also abolished by G-15. Surprisingly, both Gper1a and Gper1b showed ligand-independent effects on pCRE luciferase reporters at higher transfected doses (10 ng). RT-PCR analysis showed that the transcript of gper1a is broadly expressed in tissues of both female and male fish while the expression of gper1b in tissues demonstrates obvious sexual dimorphism, with transcripts detected in all tissues examined in female whereas they were barely detectable in some peripheral tissues of male including the testis. In the ovary, the expression of both gper1a and gper1b was detected in the oocyte but not the follicular layer, with the mRNA levels increased during vitellogenesis, peaked at the late vitellogenic stage, and decreased precipitously at the full-grown and germinal vesicle breakdown (GVBD) stages. Moreover, E₂ and E₂-BSA induced cAMP production in the in vitro incubated follicles at mid-vitellogenic stage but not the GVBD stage, and the induction could be completely abolished by G-15, a Gper1 inhibitor. Taken together, these results suggest that both Gper1a and Gper1b may play important roles in the development and maturation of ovarian follicles in ricefield eels, possibly through inhibition of oocyte meiotic resumption.

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.