Fertilization and InsP3-induced Ca2+ release stimulate a persistent increase in the rate of degradation of cyclin B1 specifically in mature mouse oocytes.
Vertebrate oocytes proceed through meiosis I before undergoing a cytostatic factor (CSF)-mediated arrest at metaphase of meiosis II. Exit from MII arrest is stimulated by a sperm-induced increase in intracellular Ca2+. This increase in Ca2+ results in the destruction of cyclin B1, the regulatory subunit of cdk1 that leads to inactivation of maturation promoting factor (MPF) and egg activation. Progression through meiosis I also involves cyclin B1 destruction, but it is not known whether Ca2+ can activate the destruction machinery during MI. We have investigated Ca2+ -induced cyclin destruction in MI and MII by using a cyclin B1-GFP fusion protein and measurement of intracellular Ca2+. We find no evidence for a role for Ca2+ in MI since oocytes progress through MI in the absence of detectable Ca2+ transients. Furthermore, Ca2+ increases induced by photorelease of InsP3 stimulate a persistent destruction of cyclin B1-GFP in MII but not MI stage oocytes. In addition to a steady decrease in cyclin B1-GFP fluorescence, the increase in Ca2+ stimulated a transient decrease in fluorescence in both MI and MII stage oocytes. Similar transient decreases in fluorescence imposed on a more persistent fluorescence decrease were detected in cyclin-GFP-injected eggs undergoing fertilization-induced Ca2+ oscillations. The transient decreases in fluorescence were not a result of cyclin B1 destruction since transients persisted in the presence of a proteasome inhibitor and were detected in controls injected with eGFP and in untreated oocytes. We conclude that increases in cytosolic Ca2+ induce transient changes in autofluorescence and that the pattern of cyclin B1 degradation at fertilization is not stepwise but exponential. Furthermore, this Ca2+ -induced increase in degradation of cyclin B1 requires factors specific to mature oocytes, and that to overcome arrest at MII, Ca2+ acts to release the CSF-mediated brake on cyclin B1 destruction.
Marangos
Petros
P
Department of Physiology, University College London, WC1E 6BT, UK.
Carroll
John
J
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D019926:Cyclin B / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D056744:Cyclin B1; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D005260:Female; D005306:Fertilization / Q000187:drug effects / Q000502:physiology*; D049452:Green Fluorescent Proteins; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism / Q000494:pharmacology*; D007976:Leupeptins / Q000494:pharmacology; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008297:Male; D008540:Meiosis; D000090204:Mesothelin; D051379:Mice; D008815:Mice, Inbred Strains; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013084:Sperm-Ovum Interactions
C534700:Ccnb1 protein, mouse; D019926:Cyclin B; D056744:Cyclin B1; D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D008164:Luminescent Proteins; C000715528:Msln protein, mouse; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D049452:Green Fluorescent Proteins; D015544:Inositol 1,4,5-Trisphosphate; D017231:Proto-Oncogene Proteins c-mos; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000090204:Mesothelin; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde; D002118:Calcium