A specific synthetic RNA promotes cardiac myofibrillogenesis in the Mexican axolotl.

Ambystoma mexicanum is an intriguing animal model for studying heart development because it carries a mutation in gene c. 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 constructed a cDNA library from total conditioned medium RNA in a pcDNAII expression vector. We screened the cDNA library by an organ culture bioassay and isolated a single clone (Cl#4), the synthetic RNA from which corrects the heart defect by promoting myofibrillogenesis. The insert size of the active clone is 166 nt in length with a unique nucleotide sequence. The anti-sense RNA from Cl#4 using SP6 RNA polymerase failed to rescue mutant hearts. The ability of this small RNA to correct the mutant heart defect suggests that the RNA probably does not act as an mRNA. While the precise mechanism of action is not yet known, on the basis of our studies to date it is very clear that the sense strand of Cl#4 RNA has the ability to promote myofibrillogenesis and rescue the mutant hearts both in vitro and in vivo.