Ectopic expression of tropomyosin promotes myofibrillogenesis in mutant axolotl hearts

Expression of tropomyosin protein, an essential component of the thin filam ent, has been found to be drastically reduced in cardiac mutant hearts of t he Mexican axolotl (Ambystoma mexicanum) with no formation of sarcomeric my o fibrils. Therefore, this naturally occurring cardiac mutation is an appro priate model to examine the effects of delivering tropomyosin protein or tr opomyosin cDNA into the deficient tissue. In this study, we describe the re placement of tropomyosin by using a cationic liposome transfection techniqu e applied to whole hearts in vitro. When mouse alpha-tropomyosin cDNA under the control of a cardiac-specific alpha-myosin heavy chain promoter was tr ansfected into the mutant hearts, tropomyosin expression was enhanced resul ting in the formation of well-organized sarcomeric myofibrils. Transfection of a beta-tropomyosin construct under control of the same promoter did not result in enhanced organization of the myofibrils. Transfection of a beta- galactosidase reporter gene did not result in the formation of organized my ofibrils or increased tropomyosin expression. These results demonstrate the importance of alpha-tropomyosin to the phenotype of this mutation and to n ormal myofibril formation. Moreover, we have shown that a crucial contracti le protein can be ectopically expressed in cardiac muscle that is deficient in this protein, with the resulting formation of organized sarcomeres. Dev . Dyn. 1998;213:412-420. (C) 1998 Wiley-Liss, Inc.