Cycling of intracellular free calcium and intracellular pH in Xenopus embryos: a possible role in the control of the cell cycle

The scope of this commentary is to propose the incorporation of recent data, obtained from Xenopus eggs and embryos, into the models of cell cycle regulation by MPF, a universal M-phase Promoting Factor operating in most, if not all, mitotic cells, from yeast to human. These new data are: (1) the cycling activity of MPF in Xenopus eggs is temporally and functionally related to the cycling activity of intracellular pH (pHi) (Grandin and Charbonneau, 1990a); (2) cell division in Xenopus embryos is accompanied by oscillations of the intracellular free calcium activity ([Ca²⁺]₁) (Grandin and Charbonneau, 1991). There is now definitive evidence that one of the molecular components of the cell cycle, the ‘master oscillator’ (or cytoplasmic clock), is represented by MPF (Masui and Markert, 1971) and its correlated cdc2 kinase activity and cyclin level (see, for instance, Draetta and Beach, 1989; Murray, 1989; Murray and Kirschner, 1989). In addition, a variety of different systems have revealed a direct implication of [Ca²⁺]₁ variations in mitotic events (reviewed by Berridge and Irvine, 1989; Hepler, 1989). Although Xenopus embryonic cells have become, in addition to yeast, one of the most important systems for studying the molecular biology of the cell division cycle, almost no attention has been paid to the possible involvement of ionic messengers, particularly Ca²⁺, in the control of mitosis in Xenopus embryos. This lack of attention to the possible role of [Ca²⁺]₁ variations in the cell division cycle of Xenopus embryos was due to their repeatedly noted absence. However, the recent demonstration of Ca²⁺ oscillations occurring with a periodicity equal to that of the cell division cycle in Xenopus embryos (Grandin and Charbonneau, 1991) now offers an opportunity of re-evaluating the already proposed models of cell division.