Ca²⁺ oscillations at fertilization in mammals are regulated by the formation of pronuclei Available to Purchase

In mammals, the sperm triggers a series of cytosolic Ca²⁺oscillations that continue for ∼4 hours, stopping close to the time of pronucleus formation. Ca²⁺ transients are also seen in fertilized embryos during the first mitotic division. The mechanism that controls this pattern of sperm-induced Ca²⁺ signalling is not known. Previous studies suggest two possible mechanisms: first, regulation of Ca²⁺oscillations by M-phase kinases; and second, regulation by the presence or absence of an intact nucleus. We describe experiments in mouse oocytes that differentiate between these mechanisms. We find that Ca²⁺oscillations continue after Cdk1-cyclin B1 activity falls at the time of polar body extrusion and after MAP kinase has been inhibited with UO126. This suggests that M-phase kinases are not necessary for continued Ca²⁺oscillations. A role for pronucleus formation in regulating Ca²⁺signalling is demonstrated in experiments where pronucleus formation is inhibited by microinjection of a lectin, WGA, without affecting the normal inactivation of the M-phase kinases. In oocytes with no pronuclei but with low M-phase kinase activity, sperm-induced Ca²⁺ oscillations persist for nearly 10 hours. Furthermore, a dominant negative importin β that inhibits nuclear transport, also prevents pronucleus formation and causes Ca²⁺ oscillations that continue for nearly 12 hours. During mitosis, fluorescent tracers that mark nuclear envelope breakdown and the subsequent reformation of nuclei in the newly formed two-cell embryo establish that Ca²⁺ oscillations are generated only in the absence of a patent nuclear membrane. We conclude by suggesting a model where nuclear sequestration and release of a Ca²⁺-releasing activity contributes to the temporal organization of Ca²⁺ transients in meiosis and mitosis in mice.