Transient increases in intracellular calcium concentration [Ca2+]i communicate with specific signaling pathways to trigger diverse cellular processes. In some cases, the 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. However, the relationship between the coding of the [Ca2+]i signal and selective recruitment of different Ca2+-dependent processes 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. Cortical granule (CG) exocytosis occurred after a single [Ca2+]i transient, whereas four stimuli caused reinitiation of the cell cycle, and 24 stimulated formation of a pronucleus. Egg activation was 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 protein synthesis. Twenty-four pulses further stimulated the expression of some proteins but reduced the expression of others. Thus calcium signaling does not act as a simple on/off switch to initiate egg development, and transient number can be used to differentially regulate and temporally coordinate different Ca2+-dependent processes. -- EA
Dev. Biol. 250, 280 (2002).
