title: Novel Ca2+ increases in the maturing oocytes of starfish during the germinal vesicle breakdown.
abstract: It has been known that the intracellular Ca(2+) level transiently rises at the specific stages of mitosis such as the moment of nuclear envelope breakdown and at the metaphase-anaphase transition. Comparable intracellular Ca(2+) increases may also take place during meiosis, as was intermittently reported in mouse, Xenopus, and starfish oocytes. In a majority of starfish species, the maturing oocytes display an intracellular Ca(2+) increase within few minutes after the addition of the maturation hormone, 1-methyladenine (1-MA). Although starfish oocytes at meiosis also manifest a Ca(2+) increase at the time of polar body extrusion, a similar Ca(2+) increase has never been observed during the envelope breakdown of the nucleus (germinal vesicle, GV). Here, we report, for the first time, the existence of an additional Ca(2+) response in the maturing oocytes of Asterina pectinifera at the time of GV breakdown. In contrast to the immediate early Ca(2+) response to 1-MA, which is independent of external Ca(2+) and takes a form of intracellular Ca(2+) wave traveling three times as fast as that in the fertilized eggs, this late stage Ca(2+) response comprised a train of numerous spikes representing Ca(2+) influx. These Ca(2+) spikes coinciding with GV breakdown were mostly eliminated when the GV was removed from the oocytes prior to the addition of 1-MA, suggesting that the Ca(2+) spikes are rather a consequence of the GV breakdown. In support of the idea that these Ca(2+) spikes play a physiological role, the oocytes matured in calcium-free seawater had a higher rate of cleavage failure 2h after the fertilization in natural seawater. Specific inhibitors of L-type Ca(2+) channels, verapamil and diltiazem, severely suppressed the amplitude of the individual Ca(2+) spikes, but not their frequencies. On the other hand, latrunculin-A (LAT-A), which promotes net depolymerization of the actin cytoskeleton, had a dual effect on this late Ca(2+) response. When added immediately after the hormone-dependent period, LAT-A inhibited the occurrence (frequency) of the spikes in a dose-dependent manner, but the amplitude of the prevailing Ca(2+) spikes itself was rather significantly increased. These results suggest that the cortical actin cytoskeleton and some nuclear factors may play a role in regulating ion channel activities during this stage of meiotic progression.
authors: Biology and Evolution of Marine Organisms, Stazione Zoologica Anton Dohrn, Napoli, Italy.
authors: Nunzia
authors: N
authors: Limatola
authors: Biology and Evolution of Marine Organisms, Stazione Zoologica Anton Dohrn, Napoli, Italy. Electronic address: chun@szn.it.
authors: Jong T
authors: JT
authors: Chun
authors: Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Japan.
authors: Keiichiro
authors: K
authors: Kyozuka
authors: Biology and Evolution of Marine Organisms, Stazione Zoologica Anton Dohrn, Napoli, Italy.
authors: Luigia
authors: L
authors: Santella
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002467:Cell Nucleus / Q000378:metabolism*; D009865:Oocytes / Q000166:cytology* / Q000378:metabolism*; D013215:Starfish / Q000166:cytology*
chemical_list: D002118:Calcium
keywords: Actin; Calcium; Cell cycle; Meiosis; Oocyte; Starfish