Heart--Hypertrophy

The Mexican axolotl (Ambystoma mexicanum) carries a cardiac lethal mutation
resulting in mutant embryos with no heartbeat. This homozygous recessive gene results
in tropomyosin deficiency and absence of organized myofibrils. Co-culturing mutant
hearts with bioactive RNA, termed myofibril-inducing RNA (MIR), from normal axolotl
embryonic anterior endoderm causes the mutant hearts to beat. It is hypothesized that the
secondary structure of the MIR binds a specific protein(s) and this is required to
synthesize tropomyosin and form organized myofibrils. In this study mutant hearts are
co-cultured with human fetal and adult heart total RNA to assess rescue of the mutant
hearts. Results show that both human fetal and adult heart total RNA rescue the mutant
condition in a manner similar to the MIR. Thus, the MIR human functional homologs
induce events leading to normal heart differentiation and function. This finding may help
people with heart muscle damage regain normal heart function again.

The Mexican axolotl, Ambystoma mexicanum, possesses a naturally-occurring
lethal mutation, designated gene "c", for cardiac non-function. Hearts form but fail to
beat, lack organized myofibrils, and are deficient in tropomyosin. Treatment with a noncoding
RNA MIR (Myofibril-Inducing RNA) rescues this defect in organ culture.
Rescued mutant hearts have restored tropomyosin, form organized myofibrils, and beat
vigorously. Studies to elucidate the mechanism of MIR heart rescue are underway.
Current evidence suggests that MIR acts by binding with at least two proteins. The yeast
three-hybrid system is being used to screen an axolotl eDNA library for these two
proteins and other possible MIR-binding candidates. This is a method utilizing two
hybrid proteins and a hybrid RNA. An interaction between these three components will
activate the expression of reporter genes, whose activity is assayed through phenotypical
and biochemical methods. In this study, the protocol for yeast three-hybrid technology is
being established for analyzing the MIR in the Mexican axolotl, cardiac mutant animal
model.