It is not clear what signal transduction pathway leads to release of calcium from the endoplasmic reticulum in sea urchins. The bindin receptor is known to be phosphorylated on its cytoplasmic tail, which may lead to activation of a signal transduction cascade, but this is not really known. As in other eggs, it is thought that stimulation of a phospholipase C results in production of inositol triphosphate (IP3) and diacylglycerol (DAG). The IP3 is thought to stimulate release of calcium from the ER. In starfish, experiments have been performed to induce expression of an inhibitory form of phospholipase C which would in theory interfere with this pathway, preventing calcium release from the ER. By injecting mRNA for this inhibitory PLC, allowing the oocyte to translate the mRNA to make protein, and then maturing and fertilizing the oocyte, we can test for a role for a particular PLC in fertilization and egg activation.
Laurinda Jaffe, Mark Terasaki and colleagues have performed this experiment. The result is shown here using a calcium indicator dye to visualize calcium transients. The oocyte receiving the inhibitory PLC fragment (an SH2 domain fragment) is on the right.
QuickTime movie of calcium dynamics following expression of an inhibitory fragment of PLC (585K)