MAITOTOXIN TRIGGERS THE CORTICAL REACTION AND PHOSPHATIDYLINOSITOL-4,5-BISPHOSPHATE BREAKDOWN IN AMPHIBIAN OOCYTES

Maitotoxin, a potent marine toxin extracted from peredinians, was found to mimic fertilization in Xenopus oocytes and to trigger the breakdown of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2, the precursor of inositol 1,4,5-trisphosphate], an increase of intracellular pCa and the cortical reaction, including the exocytosis of cortical granules and a wave-like propagation of contraction in the animal hemisphere. All these effects of maitotoxin required the presence of external calcium. Moreover, the toxin considerably increased Ca2+ influx in amphibian oocytes arrested at first meiotic prophase, due to the permanent activation of voltage-dependent Ca2+ channels. Nevertheless it is doubtful that maitotoxin acts primarily as a Ca2+ ionophore or at the level of Ca2+ channels. Indeed no stimulation of Ca2+ uptake was observed in metaphase-II-arrested oocytes, although maitotoxin readily triggered the breakdown of PtdIns(4,5)P2 as well as the cortical reaction in such cells. On the other hand, PtdIns(4,5)P2 breakdown was not reduced in oocytes microinjected with EGTA, although the calcium chelator prevented the oocytes from undergoing the cortical reaction. Taken together, these findings support the view that the toxin might act primarily by increasing PtdIns(4,5)P2 phosphodiesterase activity.