Immunofluorescent confocal analysis of tropomyosin in developing hearts of normal and cardiac mutant axolotls, Ambystoma mexicanum.

Tropomyosin is a major component of the thin filaments in organized myofibrils of cardiac muscle cells. A purported role for this protein is to prevent myosin thick filaments from interacting with actin thin filaments until the cell contracts. Recently, tropomyosin has been implicated in actin filament formation/stabilization as well. In the cardiac mutant axolotl, Ambystoma mexicanum, heart development is arrested. Mutant embryos form hearts that fail to beat due to a lack of organized sarcomeric myofibrils. There is also a concomitant reduction in the amount of tropomyosin in mutant heart cells. Previous studies have suggested that the addition of purified tropomyosin to glycerinated homogenates of mutant hearts causes amorphous actin to polymerize (or stabilize) into thin filaments. The current study was undertaken to examine the three-dimensional distribution of tropomyosin during myofibrillogenesis in normal hearts over the course of development from the heart-beat stage (35) through the advanced embryonic stage (41) and to investigate whether myofibrils form and/or significant quantities of tropomyosin accumulate in developing mutant hearts of comparable ages.