Ectopic expression of tropomyosin promotes myofibrillogenesis in mutant axolotl hearts

Expression of tropomyosin protein, an essential component of the thin filament, has been found to be drastically reduced in cardiac mutant heart s of the Mexican axolotl (Ambystoma mexicanum) with no formation of sarcoroetic myofibrils. Therefore, this naturally occurring cardiac mutation is an appropriate model to examine the effects of delivering tropomyosin protein or tropomyosin cDNA into the deficient tissue. In this study, we describe the replacement of tropomyosin by using a cationic liposome transfection technique applied to whole hearts in vitro. When mouse α-tropomyosin cDNA under the control of a cardiac-specific α-myosin heavy chain promoter was transfected into the mutant hearts, tropomyosin expression was enhanced resulting in the formation of well-organized sarcomeric myofibrils. Transfection of a β-tropomyosin construct under control of the same promoter did not result in enhanced organization of the myofibrils. Transfection of a β-galactosidase reporter gene did not result in the formation of organized myofibrils or increased tropomyosin expression. These results demonstrate the importance of α-tropomyosin to the phenotype of this mutation and to normal myofibril formation. Moreover, we have shown that a crucial contractile protein can be ectopically expressed in cardiac muscle that is deficient in this protein, with the resulting formation of organized sarcomeres.