Transient activation of calcineurin is essential to initiate embryonic development in Xenopus laevis.
At fertilization, an increase of cytosolic calcium ions (Ca2+) triggers various activation responses in animal eggs. In vertebrates, these responses include exit from metaphase arrest in meiosis II (MII exit) and cortical remodelling initiated by cortical granule exocytosis. Although the essential requirement of Ca2+/calmodulin-dependent protein kinase II for inducing MII exit has been documented, a role of the Ca2+/calmodulin-dependent protein phosphatase calcineurin in egg activation has not been investigated. Here we show, using cell-free extracts from unfertilized eggs of Xenopus laevis, that calcineurin is transiently activated immediately after Ca2+ addition to a concentration that induces MII exit. When calcineurin activation is inhibited, cyclin-dependent kinase 1 (Cdk1) inactivation by means of cyclin B degradation is prevented and sperm chromatin incubated in the extracts remains condensed. Similarly, if calcineurin is inhibited in intact eggs, MII exit on egg activation is prevented. In addition, the activation contraction in the cortex is suppressed whereas cortical granule exocytosis occurs. We further demonstrate that, when a high level of calcineurin activity is maintained after activation, growth of sperm asters is prevented in egg extracts and, consistently, migration of male and female pronuclei towards each other is hindered in fertilized eggs. Thus, both activation and the subsequent inactivation of calcineurin in fertilized eggs are crucial for the commencement of vertebrate embryonic development.
Nishiyama
Tomoko
T
Laboratory of Cell and Developmental Biology, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan.
Yoshizaki
Norio
N
Kishimoto
Takeo
T
Ohsumi
Keita
K
D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism; D019703:Calcineurin / Q000378:metabolism*; D002122:Calcium Chloride / Q000494:pharmacology; D002457:Cell Extracts / Q000737:chemistry; D019926:Cyclin B / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D005089:Exocytosis; D005260:Female; D008297:Male; D008540:Meiosis / Q000187:drug effects; D019319:Okadaic Acid / Q000494:pharmacology; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D013094:Spermatozoa / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D014982:Xenopus laevis / Q000196:embryology* / Q000378:metabolism*
D002457:Cell Extracts; D019926:Cyclin B; D019319:Okadaic Acid; D016203:CDC2 Protein Kinase; D019703:Calcineurin; D002122:Calcium Chloride