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Altered expression of tropomodulin in cardiomyocytes disrupts the sarcomeric structure of myofibrils
M A Sussman1, S Baqué, C S Uhm
1Department of Biochemistry and Molecular Biology and the Institute for Genetic Medicine, University of Southern California School of Medicine, Los Angeles, USA. sussman@heart.chmcc.org
Abstract:
Tropomodulin is a tropomyosin-binding protein that terminates "pointed-end" actin filament polymerization. To test the hypothesis that regulation of tropomodulin:actin filament stoichiometry is critical for maintenance of actin filament length, tropomodulin levels were altered in cells by infection with recombinant adenoviral expression vectors, which produce either sense or antisense tropomodulin mRNA. Neonatal rat cardiomyocytes were infected, and sarcomeric actin filament organization was examined. Confocal microscopy indicated that overexpression of tropomodulin protein shortened actin filaments and caused myofibril degeneration. In contrast, decreased tropomodulin content resulted in the formation of abnormally long actin filament bundles. Despite changes in myofibril structure caused by altered tropomodulin expression, total protein turnover of the cardiomyocytes was unaffected. Biochemical analyses of infected cardiomyocytes indicated that changes in actin distribution, rather than altered actin content, accounted for myofibril reorganization. Ultrastructural analysis showed thin-filament disarray and revealed the presence of leptomeres after tropomodulin overexpression. Tropomodulin-mediated effects constitute a novel mechanism to control actin filaments, and our findings demonstrate that regulated tropomodulin expression is necessary to maintain stabilized actin filament structures in cardiac muscle cells.
Insights
Regulating tropomodulin levels is crucial for maintaining actin filament length in cardiomyocytes. Altering tropomodulin affects actin organization, demonstrating its role in cardiac muscle cell structure.
Area of Science:
- Cell Biology
- Muscle Physiology
- Biochemistry
Background:
- Tropomodulin is a protein that binds tropomyosin and regulates actin filament polymerization.
- Maintaining proper actin filament length is essential for sarcomeric structure and function in muscle cells.
Purpose of the Study:
- To investigate the role of tropomodulin:actin filament stoichiometry in regulating actin filament length.
- To determine the effects of altered tropomodulin levels on sarcomeric actin organization in cardiomyocytes.
Main Methods:
- Neonatal rat cardiomyocytes were infected with recombinant adenoviral vectors to alter tropomodulin expression (sense and antisense).
- Confocal and ultrastructural microscopy were used to examine sarcomeric actin filament organization and myofibril structure.
- Biochemical analyses assessed protein turnover and actin content.
Main Results:
- Overexpression of tropomodulin shortened actin filaments, leading to myofibril degeneration and thin-filament disarray.
- Decreased tropomodulin resulted in abnormally long actin filament bundles.
- Changes in actin distribution, not content, caused myofibril reorganization; protein turnover remained unaffected.
Conclusions:
- Regulated tropomodulin expression is necessary for maintaining stabilized actin filament structures in cardiac muscle cells.
- Tropomodulin plays a critical role in controlling actin filament length and organization within the sarcomere.
- Altered tropomodulin levels lead to structural defects in cardiomyocytes, highlighting a novel regulatory mechanism for actin filaments.