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Fertilization differently affects the levels of cyclin B1 and MPF activity in maturing and metaphase II mouse oocytes

A Ajduk, Department of Embryology, University of Warsaw, Warsaw, Poland
M Ciemerych, Department of Cytology, University of Warsaw, Warsaw, Poland
V Nixon, Department of Anatomy and Developmental Biology, University College London, London, United Kingdom
K Swann, Department of Obstetrics and Gynaecology, Cardiff University, Cardiff, United Kingdom
M Maleszewski, Department of Embryology, University of Warsaw, Warsaw, Poland

Correspondence: Anna Ajduk, Email: aajduk{at}biol.uw.edu.pl

Abstract

Fertilization affects levels of cyclin B1 and MPF activity in maturing and metaphase II mouse oocytes in two distinct ways. In metaphase II oocytes it leads to a Ca2+ dependent, continuous degradation of cyclin B1 and inactivation of CDK1/cyclin B1 complex (MPF). In this paper we show that neither mono- nor polyspermic fertilization of prometaphase I and metaphase I oocytes triggered degradation of cyclin B1. However, polyspermic fertilization of prometaphase I oocytes led to a transient decrease in MPF activity that lasted for 2 hrs. The inactivation of MPF in polyspermic prometaphase I oocytes did not depend on the fertilization-induced increase in the cytoplasmic concentration of free Ca2+ ions, but was caused, at least in part, by dephosphorylation of CDK1 at threonine 161. We found that polyspermic fertilization did not affect glutathione levels in prometaphase I oocytes, and concluded that the decrease in MPF activity and dephosphorylation of CDK1 at Thr161 in polyspermic prometaphase I oocytes were not caused by a change in the redox status of the cell induced by an introduction of excessive amount of sperm protamines. Instead we propose that inactivation of MPF activity in polyspermic maturing oocytes is caused by a change in nucleo-cytoplasmic ratio that leads to a 'titration' of kinases and phosphatases responsible for keeping MPF in an active state. This idea is supported by the finding that oocytes fused with thymocytes rather than spermatozoa also showed a transient decrease in MPF activity.