The cardiac mutant Mexican axolotl is a unique animal model for evaluationof cardiac myofibrillogenesis

Hearts from cardiac mutant Mexican axolotl, Ambystoma mexicanum, do not for m organized myofibrils and fail to beat. Though previous biochemical and im munohistochemical experiments showed a possible reduction of cardiac tropom yosin it was not clear that this caused the lack of organized myofibrils in mutant hearts. We used cationic liposomes to introduce both rabbit and chi cken tropomyosin protein into whole hearts of embryonic axolotls in whole h eart organ cultures. The mutant hearts had a striking increase in the numbe r of well-organized sarcomeric myofibrils when treated with rabbit or chick en tropomyosin. FITC-labeled rabbit tropomyosin was used to examine the kin etics of incorporation of the exogenous protein into mutant hearts and conf irmed the uptake of exogenous protein by the cells of live hearts in cultur e. By 4 h of transfection, both normal and mutant hearts were found to inco rporate FITC-labeled tropomyosin into myofibrils. We also delivered an anti -tropomyosin antibody (CH 1) into normal hearts to disrupt the existing car diac myofibrils which also resulted in reduced heartbeat rates. CH1 antibod y was detected within the hearts and disorganization of the myofibrils was apparent when compared to normal controls. Introduction of a C-protein mono clonal antibody (ALD 66) did not result in a disruption of organized myofib rils. The results show clearly that chicken or rabbit tropomyosin could be incorporated by the mutant hearts and that it was sufficient to overcome th e factors causing a lack of myofibril formation in the mutant. This finding also suggests that a lack of organized myofibrils is caused primarily by e ither inadequate levels of tropomyosin or endogenous tropomyosin in mutant hearts is unsuitable for myofibril formation, which we were able to duplica te with the introduction of tropomyosin antibody. Furthermore, incorporatio n of a specific exogenous protein or antibody into normal and mutant hearts of the Mexican axolotl in whole heart organ culture offers an unique model to evaluate functional roles of contractile proteins necessary for cardiac development and differentiation. (C) 1999 Academic Press.