Function and characteristics of repetitive calcium waves associated with meiosis.

BACKGROUND: Internal calcium waves and oscillations are now recognized as universal features of cellular activation, but their exact role remains uncertain. In mammalian and ascidian eggs, a large, sperm-triggered calcium activation wave crosses the egg at fertilization, followed by a series of periodic increases in intracellular calcium concentration ([Ca2+]i). We have previously shown that, in eggs of the ascidian Phallusia mammillata, these periodic, post-activation [Ca2+]i increases are in the form of waves, the origin of which relocalizes to a pacemaker region, and that they stop seconds before the completion of meiosis.
RESULTS: We show here that the origin of the first one to four post-activation calcium waves in P. mammillata eggs transfers progressively from the site of sperm entry, usually in the animal hemisphere, towards an endoplasmic reticulum (ER)-rich contraction pole in the vegetal hemisphere, a process that takes about five minutes. Once the origin of these repetitive post-activation calcium waves has reached the contraction pole, all subsequent calcium waves originate from the domain of ER concentrated there, which acts as a pacemaker. The first few post-activation calcium waves are faster than the activation wave and, like the activation wave, they propagate homogeneously throughout the cytoplasm. Approximately five to ten minutes after fertilization, the post-activation calcium waves begin to propagate preferentially in the egg cortex. By manipulating intracellular calcium levels with caged inositol 1,4,5 trisphosphate (InsP3) and a competitive inhibitor of InsP3-induced calcium release, we show that the activation wave induced by the sperm is sufficient to induce extrusion of the first polar body, but that additional [Ca2+]i increases are necessary for completion of the second meiotic division. However, periodic calcium waves per se do not seem to be strictly necessary for the completion of meiosis, as a persistent and homogeneous increase in calcium, induced by the calcium ionophore ionomycin, is sufficient to cause second polar body formation and allow completion of meiosis on time.
CONCLUSION: These results clearly show that, in the ascidian egg, post-activation calcium waves are required to complete meiosis. They also show that following a period of progressive relocalization of the wave origin, which lasts approximately five minutes, an ER-rich domain at the contraction pole finally becomes a pacemaker from which the calcium waves originate. Once their origin becomes stably localized, the calcium waves begin to propagate preferentially around the cortex of the egg rather than throughout the egg cytoplasm.