Mutations in human cardiac troponin I that are associated with restrictive cardiomyopathy affect basal ATPase activity and the calcium sensitivity of force development.
ABSTRACT Human cardiac Troponin I (cTnI) is the first sarcomeric protein for which mutations have been associated with restrictive cardiomyopathy. To determine whether five mutations in cTnI (L144Q, R145W, A171T, K178E, and R192H) associated with restrictive cardiomyopathy were distinguishable from hypertrophic cardiomyopathy-causing mutations in cTnI, actomyosin ATPase activity and skinned fiber studies were carried out. All five mutations investigated showed an increase in the Ca2+ sensitivity of force development compared with wild-type cTnI. The two mutations with the worst clinical phenotype (K178E and R192H) both showed large increases in Ca2+ sensitivity (deltapCa50 = 0.47 and 0.36, respectively). Although at least one of these mutations is not in the known inhibitory regions of cTnI, all of the mutations investigated caused a decrease in the ability of cTnI to inhibit actomyosin ATPase activity. Mixtures of wild-type and mutant cTnI showed that cTnI mutants could be classified into three different groups: dominant (L144Q, A171T and R192H), equivalent (K178E), or weaker (R145W) than wild-type cTnI in actomyosin ATPase assays in the absence of Ca2+. Although most of the mutants were able to activate actomyosin ATPase similarly to wild-type cTnI, L144Q had significantly lower maximal ATPase activities than any of the other mutants or wild-type cTnI. Three mutants (L144Q, R145W, and K178E) were unable to fully relax contraction in the absence of Ca2+. The inability of the five cTnI mutations investigated to fully inhibit ATPase activity/force development and the generally larger increases in Ca2+ sensitivity than observed for most hypertrophic cardiomyopathy mutations would likely lead to severe diastolic dysfunction and may be the major physiological factors responsible for causing the restrictive cardiomyopathy phenotype in some of the genetically affected individuals.
- SourceAvailable from: Sachio Morimoto[Show abstract] [Hide abstract]
ABSTRACT: Study of the molecular biology of the calcium regulation of muscle contraction was initiated by Professor Ebashi's discovery of a protein factor that sensitized actomyosin to calcium ions. This protein factor was separated into two proteins: tropomyosin and a novel protein named troponin. Troponin is a Ca(2+)-receptive protein for the Ca(2+)-regulation of muscle contraction and, in association with tropomyosin, sensitizes actomyosin to Ca(2+). Troponin forms an ordered regulatory complex with tropomyosin in the thin filament. Several regulatory properties of troponin, which is composed of three different components, troponins C, I, and T, are discussed in this article. Genetic studies have revealed that many mutations of genes for troponin components, especially troponins T and I, are involved in the three types of inherited cardiomyopathy. Results of functional analyses indicate that changes in the Ca(2+)-sensitivity caused by troponin mutations are the critical functional consequences leading to these disorders. Recent results of this pathophysiological aspect of troponin are also discussed.Biochemical and Biophysical Research Communications 05/2008; 369(1):62-73. DOI:10.1016/j.bbrc.2007.11.187 · 2.28 Impact Factor
Article: Troponin and cardiomyopathy[Show abstract] [Hide abstract]
ABSTRACT: The troponin complex was discovered over thirty years ago and since then much insight has been gained into how this complex senses fluctuating levels of Ca(2+) and transmits this signal to the myofilament. Advances in genetics methods have allowed identification of mutations that lead to the phenotypically distinct cardiomyopathies: hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM) and dilated cardiomyopathy (DCM). This review serves to highlight key in vivo studies of mutation effects that have followed many years of functional studies and discusses how these mutations alter energetics and promote the characteristic remodeling associated with cardiomyopathic diseases. Studies have been performed that examine alterations in signaling and genomic methods have been employed to isolate upregulated proteins, however these processes are complex as there are multiple roads to hypertrophy or dilation associated with genetic cardiomyopathies. This review suggests future directions to explore in the troponin field that would heighten our understanding of the complex regulation of cardiac muscle contraction.Biochemical and Biophysical Research Communications 05/2008; 369(1):74-81. DOI:10.1016/j.bbrc.2007.12.081 · 2.28 Impact Factor
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ABSTRACT: Striated muscle myosin is a multidomain ATP-dependent molecular motor. Alterations to various domains affect the chemomechanical properties of the motor, and they are associated with skeletal and cardiac myopathies. The myosin transducer domain is located near the nucleotide-binding site. Here, we helped define the role of the transducer by using an integrative approach to study how Drosophila melanogaster transducer mutations D45 and Mhc(5) affect myosin function and skeletal and cardiac muscle structure and performance. We found D45 (A261T) myosin has depressed ATPase activity and in vitro actin motility, whereas Mhc(5) (G200D) myosin has these properties enhanced. Depressed D45 myosin activity protects against age-associated dysfunction in metabolically demanding skeletal muscles. In contrast, enhanced Mhc(5) myosin function allows normal skeletal myofibril assembly, but it induces degradation of the myofibrillar apparatus, probably as a result of contractile disinhibition. Analysis of beating hearts demonstrates depressed motor function evokes a dilatory response, similar to that seen with vertebrate dilated cardiomyopathy myosin mutations, and it disrupts contractile rhythmicity. Enhanced myosin performance generates a phenotype apparently analogous to that of human restrictive cardiomyopathy, possibly indicating myosin-based origins for the disease. The D45 and Mhc(5) mutations illustrate the transducer's role in influencing the chemomechanical properties of myosin and produce unique pathologies in distinct muscles. Our data suggest Drosophila is a valuable system for identifying and modeling mutations analogous to those associated with specific human muscle disorders.Molecular biology of the cell 03/2008; 19(2):553-62. DOI:10.1091/mbc.E07-09-0890 · 5.98 Impact Factor