Endoplasmic reticulum reorganizations and Ca²⁺ signaling in maturing and fertilized oocytes of marine protostome worms: the roles of MAPKs and MPF

Before a proper Ca²⁺ response is produced at fertilization,oocytes typically undergo a maturation process during which their endoplasmic reticulum (ER) is restructured. In marine protostome worms belonging to the phylum Nemertea, the ER of maturing oocytes forms numerous distinct clusters that are about 5 μm in diameter. After fertilization, mature oocytes with such aggregates generate a normal series of Ca²⁺ oscillations and eventually disassemble their ER clusters at around the time that the oscillations cease. Immature oocytes, however, lack prominent ER clusters and fail to exhibit repetitive Ca²⁺ oscillations upon insemination,collectively suggesting that cell cycle-related changes in ER structure may play a role in Ca²⁺ signaling. To assess the effects of meiotic regulators on the morphology of the ER and the type of Ca²⁺response that is produced at fertilization, nemertean oocytes were treated with pharmacological modulators of mitogen-activated protein kinases (MAPKs)or maturation-promoting factor (MPF) prior to confocal microscopic analyses. Based on such imaging studies and correlative assays of kinase activities,MAPKs of the ERK1/2 type (extracellular signal regulated kinases 1/2) do not seem to be essential for either structural reorganizations of the ER or repetitive Ca²⁺ signaling at fertilization. Conversely, MPF levels appear to modulate both ER structure and the capacity to produce normal Ca²⁺ oscillations. The significance of these findings is discussed with respect to other reports on ER structure, MPF cycling and Ca²⁺signaling in oocytes of deuterostome animals.