A SPECIFIC SYNTHETIC RNA PROMOTES CARDIAC MYOFIBRILLOGENESIS IN THE MEXICAN AXOLOTL

Ambystoma mexicanum is an intriguing animal model for studying heart d evelopment because in carries a mutation in gene (c) under bar. Hearts of homozygous recessive (c/c) mutant embryos do not contain organized myofibrils and fail to beat. However, the defect can be corrected by organ-culturing the mutant heart in the presence of RNA from anterior endoderm or RNA from endoderm mesoderm-conditioned medium. We construc ted a cDNA library from total conditioned medium RNA in a pcDNAII expr ession vector. We screened the cDNA library by an organ culture bioass ay and isolated a single clone (Cl#4), the synthetic RNA from which co rrects the heart defect by promoting myofibrillogenesis. The insert si ze of the active clone is 166 nt in length with a unique nucleotide se quence. The anti-sense RNA from Cl#4 using SP6 RNA polymerase failed t o rescue mutant hearts. The ability of this small RNA to correct the m utant heart defect suggests that the RNA probably does not act as an m RNA. While the precise mechanism of action is not yet known, on the ba sis of our studies to date it is very clear that the sense strand of C l#4 RNA has the ability to promote myofibrillogenesis and rescue the m utant hearts both in vivo and in vivo. (C) 1996 Academic Press.