Keeping Count of Calcium in the Activated Egg

Science's STKE  15 Oct 2002:
Vol. 2002, Issue 154, pp. tw369-TW369
DOI: 10.1126/stke.2002.154.tw369

Transient increases in intracellular calcium concentration [Ca2+]i are channeled into specific signaling pathways to trigger diverse cellular processes. In some cases, specificity of the response depends on localization of the [Ca2+]i transient; in others, however, the [Ca2+]i transient is not spatially restricted. In cells that undergo [Ca2+]i oscillations, [Ca2+]i transient frequency, amplitude, duration, and number can regulate given cellular responses. The relationship between encoding of the [Ca2+]i signal and selective recruitment of different Ca2+-dependent processes, however, is unclear. Using electric field pulses in low [Ca2+]o medium to elicit trains of [Ca2+]i transients of constant amplitude and frequency in freshly ovulated mouse eggs, Ducibella et al. investigated the dependence of Ca2+-mediated developmental processes on transient number. The authors found that different processes were evoked by different numbers of stimuli. Cortical granule (CG) exocytosis was apparent after a single [Ca2+]i transient, whereas four stimuli were required to reinitiate the cell cycle, and 24 stimuli were required to form a pronucleus. Initiation and completion of egg activation processes were also differentially regulated. CG exocytosis, for example, occurred as a graded response to increasing transient number. Eight pulses, which caused extensive CG exocytosis and cell-cycle progression, stimulated synthesis of many proteins, as determined by two-dimensional gel electrophoresis of [35S]methionine-labeled eggs. Twenty-four pulses further stimulated the expression of some proteins, but reduced the expression of others. This study indicates that calcium signaling does not act as a simple on/off switch to initiate egg development, and that transient number can be used to differentially regulate and temporally coordinate different Ca2+-dependent processes.

T. Ducibella, D. Huneau, E. Angelichio, Z. Xu, R. M. Schultz, G. S. Kopf, R. Fissore, S. Madoux, J.-P. Ozil, Egg-to-embryo transition is driven by differential responses to Ca2+ oscillation number. Dev. Biol. 250, 280-291 (2002). [Online Journal]