Journal
JOURNAL OF CELL SCIENCE
Volume 116, Issue 2, Pages 367-376Publisher
COMPANY OF BIOLOGISTS LTD
DOI: 10.1242/jcs.00221
Keywords
intracellular Ca2+; fertilization; Ca2+ channels; serotonin; Ins(1,4,5)P-3
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Oocytes of the marine bivalve Mactra chinensis are spawned and arrested at the germinal vesicle stage (first meiotic prophase) until fertilization, without undergoing a process called oocyte maturation. As is the case of other animals, a fertilized oocyte of the bivalve displays increases in intracellular free Ca2+. We have clarified here the spatiotemporal patterns and sources of the intracellular Ca2+ changes at fertilization. Shortly after insemination, increased, Ca2+ simultaneously appeared at the whole cortical region of the oocyte and spread inwardly to the center, attaining the maximal Ca2+ levels throughout the oocyte, including the cytoplasm and nucleus. The initial maximal Ca2+ peak was followed by a submaximal plateau phase of cytoplasmic and nuclear Ca2+ elevations, which persisted for several minutes. The nuclear envelope began to break down shortly before the termination of the plateau phase. These sperm-induced Ca2+ changes were inhibited by suppression of the influx of external Ca2+ from seawater but not by disturbance of the release of internal Ca2+ from inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3]-sensitive stores, suggesting that the increased Ca2+ is from an external source. In contrast to the situation observed at fertilization, an oocyte artificially stimulated with serotonin (5-hydroxytryptamine, 5-HT) displayed repetitive Ca2+ transients, each of which started from one cortical region and propagated across the oocyte as a Ca2+ wave. The 5-HT-induced Ca2+ transients persisted even in the absence of external Ca2+. Experiments with caged Ins(1,4,5)P-3 revealed that Ca2+ release from Ins(1,4,5)P-3-sensitive stores is another pathway that is sufficient to trigger meiosis reinitiation from the first prophase. These results demonstrate that Mactra oocytes can potentially use two different Ca2+-mobilizing pathways: Ca2+ influx producing a centripetal Ca2+ wave from the whole cortex and Ca2+ release from Ins(1,4,5)P3-sensitive stores producing a point-source propagating Ca2+ wave. However, it seems likely that the Ca2+ influx pathway is predominantly activated at fertilization.
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