Activation of inositol 1,4,5-trisphosphate receptors induces transient changes in cell shape of fertilized Xenopus eggs.

Injection of inositol 1,4,5-trisphosphate (InsP3) into fertilized Xenopus eggs induced transient changes in cell shape. The region around the injected site contracted during the first 2 min, followed by swelling. These changes which initiated at the injected site extended toward the opposite side. Injection of adenophostin B, a potent InsP3 receptor agonist, also induced similar morphological changes, which suggested that InsP3 receptor activation, and not the action of InsP3 metabolites, is responsible for these changes. To determine whether these changes correlate to InsP3 receptor-mediated calcium release, we examined the morphological changes and those in intracellular free calcium concentrations ([Ca2+]i). A calcium wave was observed to precede the propagation of changes in cell shape by about 2 min. The extent of propagation of cell shape changes varied with the eggs but consistently depended on the extent of the calcium wave propagation. Changes in cell shape were inhibited in eggs injected with the calcium chelator, BAPTA, indicating that calcium released from the InsP3-sensitive calcium store is required for cell shape changes. During the cell shape changes, the contracted region was strongly stained with rhodamine-phalloidin, which suggests that structural changes of actin filaments are involved in the cortical changes. We propose that spatiotemporally controlled elevation of intracellular calcium induces successive cortical cytoskeletal changes that are responsible for changes in cell shape. These observations provide insight into the potency of InsP3/calcium signaling in the regulation of cortical cytoskeleton.