Novel two-step Ca2+ increase and its mechanisms and functions at fertilization in oocytes of the annelidan worm Pseudopotamilla occelata.
Mature oocytes of the annelidan worm Pseudopotamilla occelata have a wide perivitelline space between the oocyte surface and the vitelline envelope and are arrested at the first metaphase (MI). We found a novel two-step Ca2+ increase in normally fertilized oocytes. The first Ca2+ increase originated at a cortex situated underneath a fertilizing sperm on the vitelline envelope, but failed to propagate beyond the center of the oocyte. The first localized Ca2+ increase was then followed by a larger Ca2+ increase starting from the whole oocyte cortex and spreading inwardly to the center. The first localized Ca2+ increase at fertilization was suppressed by the phospholipase C inhibitor U73122, and a similar Ca2+ change was induced by inositol 1,4,5-trisphosphate (IP3). On the other hand, the second global Ca2+ increase in fertilized oocytes was blocked by removal of external Ca2+ or the voltage-gated Ca2+ channel blocker D-600, and a similar Ca2+ change could be mimicked by addition of excess K+ only when external Ca2+ was present. These results suggest that the first localized Ca2+ increase and the second global Ca2+ increase at fertilization are regulated by Ca2+ release from IP3-sensitive stores and Ca2+ influx via voltage-gated Ca2+ channels, respectively. Our data also demonstrated that the localized Ca2+ increase induces the formation of large cytoplasmic protrusion, which helps the fertilizing sperm to enter the oocyte, whereas the following global Ca2+ increase is a prerequisite for the retraction of the cytoplasmic protrusion and the resumption of meiosis from MI.
Nakano
Takeshi
T
Department of Biology, Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan. t-nakano@mail.tains.tohoku.ac.jp
Kyozuka
Keiichiro
K
Deguchi
Ryusaku
R
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D005260:Female; D005306:Fertilization / Q000502:physiology*; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology; D008297:Male; D008540:Meiosis / Q000502:physiology; D009865:Oocytes / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D011077:Polychaeta / Q000201:enzymology / Q000378:metabolism*
D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium