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Calcium influx-mediated signaling is required for complete mouse egg activation

Yi-Liang Miao, Paula Stein, Wendy N. Jefferson, Elizabeth Padilla-Banks and Carmen J. Williams
Proceedings of the National Academy of Sciences of the United States of America
Vol. 109, No. 11 (March 13, 2012), pp. 4169-4174
Stable URL: http://www.jstor.org/stable/41507114
Page Count: 6
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Calcium influx-mediated signaling is required for complete mouse egg activation
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Abstract

Mammalian fertilization is accompanied by oscillations in egg cytoplasmic calcium (Ca²⁻) concentrations that are critical for completion of egg activation. These oscillations are initiated by Ca²⁺ release from inositol 1,4,5-trisphosphate (IP₃)-sensitive intracellular stores. We tested the hypothesis that Ca²⁺ influx across the plasma membrane was a requisite component of egg activation signaling, and not simply a Ca²⁺ source for store repletion. Using intracytoplasmic sperm injection (ICSI) and standard in vitro fertilization (IVF), we found that Ca²⁺ influx was not required to initiate resumption of meiosis II. However, even if multiple oscillations in intracellular Ca²⁺ occurred, in the absence of Ca²⁺ influx, the fertilized eggs failed to emit the second polar body, resulting in formation of three pronuclei. Additional experiments using the Ca²⁺ chelator, BAPTA/AM, demonstrated that Ca²⁺ influx is sufficient to support polar body emission and pronucleus formation after only a single sperm-induced Ca²⁺ transient, whereas BAPTA/AM-treated ICSI or fertilized eggs cultured in Ca²⁺-free medium remained arrested in metaphase II. Inhibition of store-operated Ca²⁺ entry had no effect on ICSI-induced egg activation, so Ca²⁺ influx through alternative channels must participate in egg activation signaling. Ca²⁺ influx appears to be upstream of CaMKIIγ in the absence of extracellular Ca²⁺. These results suggest that Ca²⁺ influx at fertilization not only maintains Ca²⁺ oscillations by replenishing Ca²⁺ stores, but also activates critical signaling pathways upstream of CaMKIIγ that are required for second polar body emission.

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