LOCAL PERINUCLEAR CALCIUM SIGNALS ASSOCIATED WITH MITOSIS-ENTRY IN EARLY SEA-URCHIN EMBRYOS

Using calcium-sensitive dyes together with their dextran conjugates an d confocal microscopy, we have looked for evidence of localized calciu m signaling in the region of the nucleus before entry into mitosis, us ing the sea urchin egg first mitotic cell cycle as a model. Global cal cium transients that appear to originate from the nuclear area are oft en observed just before nuclear envelope breakdown (NEB). Tn the absen ce of global increases in calcium, confocal microscopy using Calcium G reen-1 dextran indicator dye revealed localized calcium transients in the perinuclear region. We have also used a photoinactivatable calcium chelator, nitrophenyl EGTA (NP-EGTA), to test whether the chelator-in duced block of mitosis entry can be reversed after inactivation of the chelator. Cells arrested before NEB by injection of NP-EGTA resume th e cell cycle after flash photolysis of the chelator. Photolysis of che lator triggers calcium release. Treatment with caffeine to enhance cal cium-induced calcium release increases the amplitude of NEB-associated calcium transients. These results indicate that calcium increases loc al to the nucleus are required to trigger entry into mitosis. Local ca lcium transients arise in the perinuclear region and can spread from t his region into the cytoplasm. Thus, cell cycle calcium signals are ge nerated by the perinuclear mitotic machinery in early sea urchin embry os.