Release from the metaphase I block in invertebrate oocytes: possible involvement of Ca2+/calmodulin-dependent kinase III.

Full grown mature oocytes of the prosobranch gastropod mollusc Patella or the bivalves Mytilus or Ruditapes provide an excellent model for studying the mechanisms which trigger cyclin degradation and exit from the M phase. They are naturally arrested in metaphase of the first maturation division and their fertilization or artificial activation rapidly results in destruction of the cyclins and completion of meiosis. In this paper, we establish the presence of Ca2+/calmodulin-dependent kinase III or eEF-2 kinase in these oocytes and describe how the protein synthesis inhibitor emetine is able to release them from the metaphase block. Using the fluorescent Ca2+ indicator dye, fluo-3, we demonstrate moreover that both fertilization or KCl-dependent activation of Ruditapes and Mytilus oocytes actually trigger a measurable transient increase in cytosolic free Ca2+ concentration. We also show that the activations triggered by these signals as well as by the ionophore A 23187 can be reversibly blocked by the calmodulin antagonists TFP (30 microM) and W7 (100 microM), while these drugs have no effect upon emetine-dependent activations. Finally, we report that the rate of protein synthesis, measured in pulse experiments, decreases at each meiotic and mitotic cleavage following fertilization of metaphase I-arrested oocytes of Mytilus. On the basis of these experiments and as a working hypothesis, we thus propose that the Ca2+ surge which activates the oocyte may inhibit protein synthesis by triggering a transient phosphorylation of eEF-2. This would result in disappearance of the putative short-lived proteins which protect cyclins from degradation during the metaphase block.