1. Laboratory of Cell and Developmental Biology, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
2. Department of Animal Resource Production, United Graduate School of Agricultural Science, Gifu University, Gifu 501-1193, Japan

Correspondence to: Keita Ohsumi¹ Correspondence and requests for materials should be addressed to K.O. (Email: kohsumi@bio.titech.ac.jp).

Abstract

At fertilization, an increase of cytosolic calcium ions (Ca²⁺) triggers various activation responses in animal eggs^{1, 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.

1. Laboratory of Cell and Developmental Biology, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
2. Department of Animal Resource Production, United Graduate School of Agricultural Science, Gifu University, Gifu 501-1193, Japan

Correspondence to: Keita Ohsumi¹ Correspondence and requests for materials should be addressed to K.O. (Email: kohsumi@bio.titech.ac.jp).

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