RNA FROM NORMAL ANTERIOR ENDODERM MESODERM-CONDITIONED MEDIUM STIMULATES MYOFIBRILLOGENESIS IN DEVELOPING MUTANT AXOLOTL HEARTS

In the axolotl, Ambystoma mexicanum, a recessive cardiac lethal mutati on causes an incomplete differentiation of the myocardium. Mutant hear ts do not contain sarcomeric myofibrils nor do they beat. We have prev iously shown that normal anterior endoderm, medium conditioned by endo derm, or total RNA extracted from endoderm stimulates differentiation of mutant hearts in culture as indicated by the presence of organized myofibrils and rhythmic contractions of the ''rescued'' mutant heart t ube. In this study, to get a more highly purified sample of the ''acti ve'' molecule, RNA was extracted from endoderm-conditioned medium and was assayed for its ability to promote myofibrillogenesis in mutant he arts. Mutant heart mesoderm responded to conditioned-medium RNA in a d ose-dependent manner. Proteinase K treatment of the RNA did not affect inductive activity, while digestion with RNase A completely abolished the ability to rescue mutant hearts. Confocal laser scanning microsco py of immunostained, organ-cultured hearts revealed that mutant hearts contain reduced amounts of the sarcomeric protein tropomyosin in an a morphous distribution, whereas normal and corrected mutant hearts cont ain tropomyosin primarily in organized myofibrils.