Injection of myo-inositol reverses the effects of lithium on sea urchin blastomeres

Lithium is known to cause sea urchin blastomeres destined to give rise to epithelium rather than to differentiate into gut or skeleton. While it has been proposed that lithium alters development by interfering with the inositol-tris phosphate-protein kinase C (IP₃-PKC) signaling pathway, the mechanism of action of lithium in sea urchins has remained elusive. Here we describe experiments that examine the hypothesis that lithium exerts its effect on sea urchin embryos via the IP₃-PKC pathway. We make use of methods developed to isolate epithelial precursor cells from the animal hemisphere of cleavage 16-cell stage embryos. Pairs of cells were isolated and one of each pair was injected with either myo-inositol or its inactive isomer, epi-inositol. Rhodamine dextran was co-injected as a lineage tracer to follow the fate of injected cells. We demonstrate that injected myo-inositol, but not epi-inositol, can reverse the effects of lithium on sea urchin blastomeres. This is direct evidence that lithium affects the IP₃-PKC pathway in sea urchins, and that this pathway plays an important role in cell fate determination.