MOLECULAR ENDOCRINOLOGY OF OOCYTE GROWTH AND MATURATION IN FISH

Pituitary gonadotropins (GTHs) are of primary importance in triggering oocyte growth and maturation. However, the actions of GTHs are not di rect, but are mediated by the ovarian production of steroidal mediator s of oocyte growth (estradiol-17beta) and maturation (maturation-induc ing hormone, MIH; 17alpha,20beta-dihydroxy-4-pregnen-3-one, 17alpha,20 beta-DP in salmonid fishes; 17alpha,20beta,21-trihydroxy-4-pregnen-3-o ne, 20beta-S in sciaenid fishes). It is established that production of estradiol-17beta and 17alpha,20beta-DP by salmonid ovarian follicles occurs via the interaction of two cell layers, the thecal and granulos a cell layers (two-cell type model). A distinct shift in the salmonid steroidogenesis from estradiol-17beta to 17alpha,20beta-DP occurs in t he ovarian follicle layer immediately prior to oocyte maturation. It i s possible that this shift is a consequence of dramatic changes in the expression of the genes encoding various steroidogenic enzymes. As an initial step to address this question, we have isolated and character ized the cDNAs encoding a number of ovarian steroidogenic enzymes incl uding the rainbow trout cholesterol side-chain cleavage cytochrome P-4 50, 3beta-hydroxysteroid dehydrogenase (HSD), 17alpha-hydroxylase/17,2 0 lyase cytochrome P-450, aromatase cytochrome P-450 cDNAs as well as the pig 20beta-HSD cDNA. Estradiol-17beta stimulates the hepatic synth esis and secretion of a yolk precursor, vitellogenin. Vitellogenin is then transported to the ovary where it is selectively taken up into th e oocyte by a receptor-mediated process involving specific cell-surfac e receptors. Estradiol-17beta was also shown to induce the synthesis o f egg membrane proteins in the liver. The maturation-inducing action o f 17alpha,20beta-DP and 20beta-S is through the binding to the oocyte plasma membrane. This initial MIH-surface interaction is followed by t he formation of the major mediator of MIH, maturation-promoting factor (MPF). We have purified MPF from mature oocytes of carp. Carp MPF con sists of two components: the homolog of the cdc2+ gene product of fiss ion yeast (p34cdc2) and cyclin B. The cdc2 kinase protein is present i n immature oocytes as well as in oocytes induced to mature by 17alpha, 20beta-DP treatment, while cyclin B proteins can be detected only in m ature oocytes. Addition of bacterially expressed goldfish cyclin B to the extracts of immature goldfish oocytes induced MPF activation. Thes e results suggest that the appearance of cyclin B protein is a crucial step for 17alpha,20beta-DP-induced oocyte maturation in fish.