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Soluble sperm extract specifically recapitulates the initial phase of the Ca²⁺ response in the fertilized oocyte of P. occelata following a G-protein/ PLCβ signaling pathway
- Nakano, Takeshi, Kyozuka, Keiichiro
- Zygote 2015 v.23 no.6 pp. 821-835
- G-protein coupled receptors, calcium, calcium channels, eggs, inositols, meiosis, metaphase, oocytes, phospholipases, signal transduction, spermatozoa
- Matured oocytes of the annelidan worm Pseudopotamilla occelata are fertilized at the first metaphase of the meiotic division. During the activation by fertilizing spermatozoa, the mature oocyte shows a two-step intracellular Ca²⁺ increase. Whereas the first Ca²⁺ increase is localized and appears to utilize the inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ stores, the second Ca²⁺ increase is global and involves Ca²⁺ influx via voltage-gated Ca²⁺ channels on the entire surface of the oocyte. To study how sperm trigger the Ca²⁺ increases during fertilization, we prepared soluble sperm extract (SE) and examined its ability to induce Ca²⁺ increases in the oocyte. The SE could evoke a Ca²⁺ increase in the oocyte when it was added to the medium, but not when it was delivered by microinjection. However, the second-step Ca²⁺ increase leading to the resumption of meiosis did not follow in these eggs. Local application of SE induced a non-propagating Ca²⁺ increase and formed a cytoplasmic protrusion that was similar to that created by the fertilizing sperm at the first stage of the Ca²⁺ response, important for sperm incorporation into the oocyte. Our results suggest that the fertilizing spermatozoon may trigger the first-step Ca²⁺ increase before it fuses with the oocyte in a pathway that involves the G-protein-coupled receptor and phospholipase C. Thus, the first phase of the Ca²⁺ response in the fertilized egg of this species is independent of the second phase of the Ca²⁺ increase for egg activation.