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Transient activation of calcineurin is essential to initiate embryonic development in Xenopus laevis

Abstract

At fertilization, an increase of cytosolic calcium ions (Ca2+) triggers various activation responses in animal eggs1,2. 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 documented3, 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 contraction4 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.

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Figure 1: Calcineurin is transiently activated in CSF extracts immediately after Ca 2+ addition.
Figure 2: Calcineurin activity is required for MII exit in response to a Ca 2+ increase in CSF extracts.
Figure 3: Calcineurin activity is required for the activation contraction, but not for cortical granule exocytosis in activated eggs.
Figure 4: Persistence of calcineurin activity after activation inhibits the growth of sperm asters and the coming together of the male and female pronuclei.

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Acknowledgements

We thank S. Mochida and T. Hunt for sharing their unpublished findings with us and all the laboratory members for discussions. We thank M. J. Lohka for critical reading of the manuscript. This work was supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan to K.O. and T.K.

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Correspondence to Keita Ohsumi.

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Nishiyama, T., Yoshizaki, N., Kishimoto, T. et al. Transient activation of calcineurin is essential to initiate embryonic development in Xenopus laevis. Nature 449, 341–345 (2007). https://doi.org/10.1038/nature06136

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