Sources of calcium and the involvement of calmodulin during steroidogenesis and oocyte maturation in follicles of Rana pipiens

Abstract

In the amphibian ovarian follicle, progesterone production is thought to induce maturation of the enclosed oocyte. Intracellular mechanisms regulating these events in the somatic and germ cells are incompletely understood. However, calcium appears to play a role in the production and action of progesterone. Experiments using calcium antagonists were carried out to delineate the role of extra- and intracellular calcium during in vitro stimulation of follicular steroidogenesis and oocyte maturation. Calcium-free medium, verapamil, and La3+ were used to block Ca2+ influx and inhibited follicular progesterone accumulation in response to frog pituitary homogenate (FPH) or exogenous cAMP + IBMX. Progesterone accumulation was not impaired under identical conditions when pregnenolone was added to cultured follicles. TMB-8, an inhibitor of intracellular Ca2+ mobilization, partially inhibited progesterone levels stimulated by FPH at low doses but not higher doses of the inhibitor. However, TMB-8 inhibited FPH-induced oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner, as well as maturation due to exogenous progesterone or La3+. Calmodulin antagonists, W-7, R24571, and trifluoperazine, were used to assess the involvement of calmodulin in the responses of these two cell types. All three antagonists inhibited progesterone accumulation induced by FPH with the apparent order of potency being R24571 greater than W-7 greater than TFP. W-7 inhibited cAMP-induced progesterone elevation, but had no effect on conversion of pregnenolone to progesterone. Of these three calmodulin antagonists, only R24571 exhibited a dramatic ability to inhibit GVBD induced by exogenous progesterone and was associated with morphologic alterations in the oocytes. These data suggest that Ca2+, acting through calmodulin at some specific step(s) distal to cAMP elevation and prior to pregnenolone formation, is involved in FPH-induced progesterone accumulation, apparently with the participation of both extracellular and intracellular pools of Ca2+. In the oocyte, mobilization of Ca2+ from intracellular stores appears to be of primary importance to maturation while extracellular Ca2+ is not. These data provide further evidence that Ca2+ mediates the hormonally provoked responses in both cell types in the intact follicle, but that the source of Ca2+ may differ. Using intact follicles it seems apparent that exploiting this difference with selective inhibitors provides a means for differential modulation and functional uncoupling of these cells with regard to steroidogenesis and steroid action.

Insights into Eyestalk Ablation Mechanism to Induce Ovarian Maturation in the Black Tiger Shrimp

“…Calnexin, similar to calrecticulin, is one of the calcium binding chaperones [45] involved in oogenesis in mammalian cells [46] by regulating GnRH receptor in human [16]. Calmodulin is a calcium binding protein involved in the regulation of steroidogenesis and oocyte maturation in rat [47], in amphibian [48], [49], in fish [50], and in crustacean [51] by activating calmodulin kinase [52] leading to the release of gonadotropins which in turn activate folliculogenesis, estradiol and progesterone production. Then, high level of progesterone synthesized and released by follicular cell is postulated to induce oocyte maturation through progesterone-mediated oocyte maturation pathway resulting in activation of maturation-promoting factor (MPF) consisting of cell division cycle2 and cyclinB.…”

Participation of IP3R, RyR and L-type Ca2+channel in the nuclear maturation ofRhinella arenarumoocytes

“…It has been proposed that the intracellular increase in Ca 2+ in amphibian oocytes is a consequence of progesterone action and that calcium is the most necessary second message for meiosis resumption (Kleis-San Francisco et al, 1987;Schorderet-Slatkine et al, 1984).…”

Ovarian Transcriptome Analysis of Vitellogenic and Non-Vitellogenic Female Banana Shrimp (Fenneropenaeus merguiensis)

“…The calcium-calmodulin system participates in the gonadotropic regulation of granulosa cell steroidogenesis independently of cellular differentiation [ 39 ], consistent with findings showing an involvement of calmodulin in ovarian steroid production in teleost fish, Atlantic croaker ( Micropogonias undulates ) [ 40 ] and crustaceans [ 41 ]. More evidence of calmodulin’s effect on oocyte maturation was found in Xenopus , which shows no significant response to calmodulin injection [ 42 ], and in Rana pipiens [ 43 ]. In summary, all of these data suggest that various DEGs are possibly crucial in the regulating of ovarian development and maturation in shrimp.…”