Novel two‐step Ca²⁺ increase and its mechanisms and functions at fertilization in oocytes of the annelidan worm Pseudopotamilla occelata

Abstract

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 twostep Ca 2+ increase in normally fertilized oocytes. The first Ca 2+ 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 Ca 2+ increase was then followed by a larger Ca 2+ increase starting from the whole oocyte cortex and spreading inwardly to the center. The first localized Ca 2+ increase at fertilization was suppressed by the phospholipase C inhibitor U73122, and a similar Ca 2+ change was induced by inositol 1,4,5-trisphosphate (IP 3 2+ increase induces the formation of large cytoplasmic protrusion, which helps the fertilizing sperm to enter the oocyte, whereas the following global Ca 2+ increase is a prerequisite for the retraction of the cytoplasmic protrusion and the resumption of meiosis from MI.

Intracellular calcium signaling in the fertilized eggs of Annelida

“…In C. pergamentaceus and P. occelata, fertilization takes place at metaphase I of the meiotic division. The first Ca 2+ response in these eggs is restricted to a localized Ca 2+ increase at the cortex of the oocyte that doesn't propagate to the entire oocyte [34,62]. Then, the global Ca 2+ increases follow with the repetitive Ca 2+ oscillations as was reported in C. pergamentaceus [34] .…”

“…The Ca 2+ increase at fertilization in Annelida has been studied in C. pergamentaceus, Urechis caupo, and Pseudopotamilla occelata [34,53,[60][61][62]. In C. pergamentaceus and P. occelata, fertilization takes place at metaphase I of the meiotic division.…”

“…Then, the protrusion from the egg surface developed toward the spermatozoon though the peri-vitelline space (Fig. 3A) [62]. When the tip of the cortical protrusion reached the spermatozoon on the egg envelope, the spermatozoon was incorporated into the protrusion complex.…”

“…The maturation inducing hormone for P. occelata is still unknown, but the oocyte maturation can be induced within 20 min by the treatment with 8 bromo cyclic AMP (100 microM) in high pH sea water (pH 9.0) [62]. After that, germinal vesicle breakdown can take place in natural seawater within 1 h nearly in a synchronous manner.…”

“…3) [62]. Briefly, two distinct Ca 2+ increases with different characteristics took place independently of each other in the fertilized eggs of P. occelata.…”