The role of the phosphatidylinositol cycle in mitosis in sea urchin zygotes. Lithium inhibition is overcome by myo-inositol but not by other cyclitols or sugars.

We have investigated the role of the phosphatidylinositol (PI) cycle in cellular events between fertilization and first cleavage in zygotes of the sea urchin Lytechinus pictus. The effects of lithium were studied: The lithium-induced changes due to effects on the PI cycle were reversed by myo-inositol, the next step in the cycle after the lithium block, but were not reversed by scyllo-inositol or other cyclitols or sugars. In this way we implicated the PI cycle in the formation of streak birefringence, in nuclear membrane breakdown, in onset of anaphase, and in cytokinesis. With respect to karyokinesis, mitotic apparatus (MA) structure often was altered when the PI cycle was blocked, and anaphase was blocked when the PI cycle was blocked. For all stages, the effects of 400 mM lithium were overcome by 10-100 microM myo-inositol. Excess myo-inositol potentiated the effect of lithium on MA structure (and on cytokinesis), suggesting that there is a negative feedback loop in the control of the PI cycle.