Spatiotemporal characteristics and mechanisms of intracellular Ca 2+ increases at fertilization in eggs of jellyfish (Phylum Cnidaria, Class Hydrozoa)

Abstract

We have clarified, for the first time, the spatiotemporal patterns of intracellular Ca 2+ increases at fertilization and the Ca 2+-mobilizing mechanisms in eggs of hydrozoan jellyfish, which belong to the evolutionarily old diploblastic phylum, Cnidaria. An initial Ca 2+ increase just after fertilization took the form of a Ca 2+ wave starting from one cortical region of the egg and propagating to its antipode in all of four hydrozoan species tested: Cytaeis uchidae, Cladonema pacificum, Clytia sp., and Gonionema vertens. The initiation site of the Ca 2+ wave was restricted to the animal pole, which is known to be the only area of sperm–egg fusion in hydrozoan eggs, and the wave propagating velocity was estimated to be 4.2–5.9 μm/s. After a Ca 2+ peak had been attained by the initial Ca 2+ wave, the elevated Ca 2+ gradually declined and returned nearly to the resting value at 7–10 min following fertilization. Injection of inositol 1,4,5-trisphosphate (IP 3), an agonist of IP 3 receptors (IP 3R), was highly effective in inducing a Ca 2+ increase in unfertilized eggs; IP 3 at a final intracellular concentration of 12–60 nM produced a fully propagating Ca 2+ wave equivalent to that observed at fertilization. In contrast, a higher concentration of cyclic ADP-ribose (cADPR), an agonist of ryanodine receptors (RyR), only generated a localized Ca 2+ increase that did not propagate in the egg. In addition, caffeine, another stimulator of RyR, was completely without effect. Sperm-induced Ca 2+ increases in Gonionema eggs were severely affected by preinjection of heparin, an inhibitor of Ca 2+ release from IP 3R. These results strongly suggest that there is a well-developed IP 3R-, but not RyR-mediated Ca 2+ release mechanism in hydrozoan eggs and that the former system primarily functions at fertilization. Our present data also demonstrate that the spatial characteristics and mechanisms of Ca 2+ increases at fertilization in hydrozoan eggs resemble those reported in higher triploblastic animals.