Sperm-induced Ca²⁺ oscillations in mouse oocytes and eggs can be mimicked by photolysis of caged inositol 1,4,5-trisphosphate: evidence to support a continuous low level production of inositol 1,4,5-trisphosphate during mammalian fertilization

摘要

During mouse fertilization the spermatozoon induces a series of low-frequency long-lasting Ca²⁺ oscillations. It is generally accepted that these oscillations are due to Ca²⁺ release through the inositol 1,4,5-trisphosphate (InsP₃) receptor. However, InsP₃ microinjection does not mimic sperm-induced Ca²⁺ oscillations, leading to the suggestion that the spermatozoon causes Ca²⁺ release by sensitizing the InsP₃ receptor to basal levels of InsP₃. This contradicts recent evidence that the spermatozoon triggers Ca²⁺ oscillations by introducing a phospholipase C or else an activator of phospholipase C. Here we show for the first time that sperm-induced Ca²⁺ oscillations may be mimicked by the photolysis of caged InsP₃ in both mouse metaphase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes that had displayed spontaneous Ca²⁺ oscillations, gave long-lasting Ca²⁺ oscillations when fertilized or when caged InsP₃ was photolyzed. In contrast, oocytes that had shown no spontaneous Ca²⁺ oscillations did not generate many oscillations when fertilized or following photolysis of caged InsP₃. Fertilization in eggs was most closely mimicked when InsP₃ was uncaged at relatively low amounts for extended periods. Here we observed an initial Ca²⁺ transient with superimposed spikes, followed by a series of single transients with a low frequency; all characteristics of the Ca²⁺ changes at fertilization. We therefore show that InsP₃ can mimic the distinctive pattern of Ca²⁺ release in mammalian eggs at fertilization. It is proposed that a sperm Ca²⁺-releasing factor operates by generating a continuous small amount of InsP₃ over an extended period of time, consistent with the evidence for the involvement of a phospholipase C.