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Mutations in Human Cardiac Troponin I That Are Associated with Restrictive Cardiomyopathy Affect Basal ATPase Activity and the Calcium Sensitivity of Force Development

Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, Florida 33136, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 10/2005; 280(35):30909-15. DOI: 10.1074/jbc.M500287200
Source: PubMed

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.

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    • "RCM myofilaments isolated from transgenic animals are more sensitive to Ca2+ and show more force at lower Ca2+ concentration.[19] This is consistent with experiments done in vitro on assembled human and mice RCM myofibers.[39],[72],[73] The myofilament hypersensitivity to Ca2+ is a common feature that RCM shares with HCM. "
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    • "The maximal tension was diminished for all mutants, except R192H. The L144Q, R145W, and K178E mutants showed a significant increase in the basal force levels compared to WT [41]. Other studies using these mutants also produced a large leftward shift in the Ca2+ sensitivity of force development in exchanged cardiac skinned fibers [40]. "
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    BioMed Research International 06/2010; 2010:350706. DOI:10.1155/2010/350706 · 2.71 Impact Factor
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    • "The Ca 2+ -activation profiles are, therefore, affected to a greater extent by the RCM-causing mutations than by the HCM-causing mutations. Similar Ca 2+ -sensitizing effects of five RCMcausing mutations of troponin I (except Asp190Gly) have also been reported [59]. Examinations of troponin I fragments (TnI 129–210 ) have shown that the Lys178Glu mutation decreases the inhibitory activity on actin–tropomyosin without changing the maximum inhibition level and decreases the EC 50 of the neutralizing action of troponin C [58]. "
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    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.
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