Human Chorionic Gonadotropin Mediated Generation of Reactive Oxygen Species Is Sufficient to Induce Meiotic Exit but Not Apoptosis in Rat Oocytes

Abstract

Generation of reactive oxygen species (ROS) is associated with final stages of follicular development and ovulation in mammals. The human chorionic gonadotropin (hCG) mimics the action of luteinizing hormone and triggers follicular development and ovulation. However, it remains unclear whether hCG induces generation of ROS, if yes, whether hCG-mediated increased level of ROS could induce meiotic exit and/or apoptosis in rat oocytes. For this purpose, cumulus–oocyte complexes (COCs) were collected from ovary of experimental rats injected with 20 IU pregnant mare's serum gonadotropin for 48 h followed by 20 IU hCG for 0, 7, 14, and 21 h. The morphological changes in COCs, meiotic status of oocyte, total ROS, hydrogen peroxide (H2O2), inducible nitric oxide synthase (iNOS), nitric oxide (NO), Bax, Bcl-2, cytochrome c, telomerase reverse transcriptase (TERT) expression levels, and DNA fragmentation were analyzed in COCs. Our data suggest that hCG surge increased total ROS as well as H2O2 levels but decreased iNOS expression and total NO level in oocytes. The hCG-mediated increased level of ROS was sufficient to induce meiotic cell cycle resumption in majority of oocytes as evidenced by meiotic exit from diplotene as well as metaphase-II (M-II) arrest and their meiotic status. However, increase of ROS level due to hCG surge was not sufficient to trigger Bax and cytochrome c expression levels and DNA fragmentation in COCs. In addition, increased TERT activity was observed in oocytes collected 21 h post-hCG surge showing onset of oocyte aging. Taken together, these results suggest that hCG induces generation of ROS sufficient to trigger meiotic exit from diplotene, as well as M-II arrest, but not good enough to induce apoptosis in rat oocytes.

Metabolic depletion of synaptosomal enzymes linked with neurotoxicity and ovarian dysfunction by phenolic antioxidants of Croton zambsicus leaves in rats exposed to chronic mixture of anthropogenic toxicant

“…Thus, augmentation of the body's antioxidant status through dietary means or ethno-pharmacotherapy could be a practical approach through which oxidative stress-induced neuroreproductive disorder is controlled. However, the inhibition of oxidative stress by 400 mg/kg phenolic C-ZAMB leaves in the rat brain and ovary homogenates are in line with former studies on herbal phytochemicals, which was alluded to their antioxidant properties [ 48 ].…”

Inability to Maintain Metaphase-II Arrest due to Increase of Reactive Oxygen Species in Rat Eggs

“…The elevated levels of ROS as well as Ca ++ trig-ger maturation promoting factor (MPF) destabilization and thereby exit from diplotene as well as M-II arrest in vitro [2,3,27,28]. Similar to in vitro studies, we postulated that a moderate increase in ROS as well as Ca ++ levels may drive EM-II and thereby abortive SEA in vivo too [6][7][8]. To test this hypothesis, the present study was designed to investigate whether increases in ROS as well as Ca ++ levels could induce MPF destabilization and thereby EM-II under in vivo conditions using rats as an experimental animal model.…”

“…Normally, fertilizing spermatozoa triggers meiotic exit from M-II arrest and thereby egg activation process in mammals [5]. Surprisingly, ovulated eggs in several mammalian species do not wait for fertilization and quickly undergo spontaneous exit from metaphase-II arrest (EM-II) by initiating the extrusion of second polar body (PB-II), a first morphological feature of spontaneous egg activation (SEA) under in vitro culture conditions [3,[5][6][7][8]. Although extrusion of PB-II starts quickly in these eggs, it never gets completed and pronucleus is also not formed [6,7].…”