Molecular mechanisms regulating oocyte maturation in sciaenid fishes
MetadataShow full item record
The overall aim of this research was to study the molecular mechanisms regulating oocyte maturation in sciaenid fishes using two models, the Atlantic croaker and the spotted seatrout. The steroid receptor mediating oocyte maturation in these species, as well as the signal transduction pathways altered by binding of the maturation inducing steroid 17,20β,21-trihydroxy-4-pregene-3-one (20β-S) to the oocyte were investigated. Finally, a cDNA of the gene encoding the membrane progestin receptor, mPRα, was isolated from Atlantic croaker ovary and hormonal regulation of its mRNA and protein expression was examined. The 20β-S receptor in seatrout oocytes was shown to be a novel steroid hormone receptor located in oocyte membranes that was directly coupled to a pertussis toxin sensitive G-protein. Activation of this protein was found to be necessary for oocyte maturation in Atlantic croaker and seatrout. Support for a role for the novel G-protein coupled progestin binding receptor, mPRα, in spotted seatrout oocyte maturation was presented. Multiple signal transduction pathways were activated by 20β-S binding to its receptor in Atlantic croaker oocyte membranes. Treatment of oocyte membranes with 20β-S significantly decreased adenylyl cyclase activity within 1 minute of exposure, although inhibition of cAMP-dependent protein kinase activity was not sufficient to induce oocyte maturation in the absence of steroid. Activation of phosphatidylinositol-3-kinase and Akt was necessary for 20β-S-induced oocyte maturation in Atlantic croaker and data suggested that phosphodiesterase 3 and phosphodiesterase 4 assist in maintaining oocyte meiotic arrest. The mitogen activated protein kinase pathway in Atlantic croaker oocytes was activated within 1 hour of 20β-S treatment; however inhibition of this pathway had no effect on steroid-mediated oocyte maturation. Finally, a cDNA of the membrane progestin receptor, mPRα, was isolated from Atlantic croaker ovary. mPRα mRNA was expressed in all tissues, however its protein distribution was limited to ovary, testis, intestine, heart, gill, brain and olfactory epithelium. Treatment of Atlantic croaker ovarian follicles with gonadotropin, but not MIS, upregulated mPRα protein and inhibition of steroidogenesis had no effect on this hCG-mediated effect or on GVBD. These findings support previous studies in seatrout and support the hypothesis that mPRα is a mediator of steroid-mediated oocyte maturation.