Cell cycle-dependent Ca2+ oscillations in mouse embryos are regulated by nuclear targeting of PLCζ

During the first cell cycle Ca²⁺ oscillations are regulated in a cell cycle-dependent manner, such that the oscillations are unique to M phase. How the Ca²⁺ oscillations are regulated with such cell cycle stage-dependency is unknown, despite their importance for egg activation and embryo development. We recently identified a novel, sperm-specific phospholipase C (PLCzeta; PLCζ) that triggers Ca²⁺ oscillations similar to those caused by sperm. We show that PLCζ-induced Ca²⁺ oscillations also occur exclusively during M phase. The cell cycle-dependency can be explained by PLCζ's localisation to the pronuclei, which depends specifically upon a nuclear localisation signal sequence. Preventing pronuclear localisation of PLCζ by mutation of the nuclear localisation signal, or by inhibiting pronuclear formation/import, can prolong Ca²⁺ oscillations or allow them to occur during interphase. These data suggest a novel mechanism for regulating a PLC through nuclear sequestration and may explain the cell cycle-dependent regulation of Ca²⁺ oscillations following fertilisation.