Inositol 1,4,5-trisphosphate microinjection activates sea urchin eggs

Abstract

When a sperm activates a sea urchin egg at fertilization, it triggers the release of calcium from a store within the egg by a hitherto unknown mechanism1,2. The ensuing transient increase in cytoplasmic calcium concentration is the stimulus for the beginning of embryonic development3,4. Here we have activated sea urchin (Lytechinus pictus) eggs by microinjecting inositol 1,4,5-trisphosphate (InsP3), a substance which Berridge5 has suggested may be an intracellular messenger whose probable function is to release calcium from intracellular stores6–9. It is thought that InsP3 is produced from plasma membrane phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) via enzymatic hydrolysis by a phospholipase C10. Recently, it has been demonstrated that there is a substantial increase in the turnover of polyphosphoinositides in the sea urchin egg at fertilization11. We now present preliminary evidence that there is a calcium-stimulated breakdown of PtdIns(4,5)P2 in the egg cortex. We believe that, just as microinjecting InsP3 stimulates an increase in cytoplasmic calcium, the production of InsP3 at fertilization may be responsible for the release of calcium from the egg's intracellular store.