Glucocorticoid receptor activation following elevated oocyte cortisol content is associated with zygote activation, early embryo cell division, and IGF system gene responses in rainbow trout

Abstract

Increasedin ovocortisol content of rainbow trout oocytes from ∼3.5 to ∼5.0 ng.oocyte−1before fertilization enhances the growth of embryos and juveniles and changes the long-term expression pattern of IGF-related genes. This study used embryos reared from oocytes enriched with cortisol and the glucocorticoid receptor (GR) antagonist, RU486, to determine whether the growth-promoting actions of cortisol involve GR protein activation and modulation ofgrexpression. Whole-mountin situimmunohistofluorescence studies of zygotes showed that enhanced oocyte cortisol increased the immunofluorescent GR signal and activated the relocation of GR from a general distribution throughout the cytoplasm to an accumulation in the peri-nuclear cytoplasm.In ovocortisol treatment increased the number of embryonic cells within 48-h post-fertilization, and RU486 partially suppressed this cortisol stimulation of cell duplication. In addition, there was complex interplay between the expression ofgrandigfsystem-related genes spatiotemporally in the different treatment groups, suggesting a role for GR in the regulation of the expression of development. Taken together, these findings indicate an essential role for GR in the regulation of epigenomic events in very early embryos that promoted the long-term growth effects of the embryos and juvenile fish. Moreover, the pretreatment of the oocyte with RU486 had a significant suppressive effect on the maternal mRNA transcript number ofgrandigfsystem-related genes in oocytes and very early stage embryos, suggesting an action of antagonist on the stability of the maternal transcriptome.