POLAR EFFECTS OF LITHIUM IN THE HEART OF THE ZEBRAFISH DANIO-RERIO

The molecular signalling mechanisms that are believed to govern the pa tterning of the heart early in embryonic development are not well unde rstood. We have investigated the events which occur during patterning of the vertebrate heart by exposing gastrula stage zebrafish embryos t o lithium, which is known to affect the phosphoinositol signalling pat hway. Treatment of embryos at 50% epiboly (5.25 h after fertilization at 28.5 degrees) with 0.3 M LiCl for 5-15 min, results in embryos with defects which range from mild to severe, depending on the length of t ime the embryos are exposed to lithium. In the heart, defects appear p rogressively in the inflow tract, the sinus venosus and atrium. By usi ng an antibody that recognizes an atrium-specific isoform of myosin, o ur results show that lithium treatment at gastrulation specifically af fects the atrium and sinus venosus, and has little obvious effect on t he ventricle. Defects induced by lithium differ from those induced by retinoic acid (RA) treatment of similarly staged embryos, and suggest that lithium and RA may affect the patterning signals important for es tablishment of the vertebrate heart by acting on different populations of cells or by influencing different patterning pathways.