[Show abstract][Hide abstract] ABSTRACT: Cardiomyopathies are diseases that primarily affect the myocardium, leading to serious cardiac dysfunction and heart failure. Out of the three major categories of cardiomyopathies (hypertrophic, dilated and restrictive), restrictive cardiomyopathy (RCM) is less common and also the least studied. However, the prognosis for RCM is poor as some patients dying in their childhood. The molecular mechanisms behind the disease development and progression are not very clear and the treatment of RCM is very difficult and often ineffective. In this article, we reviewed the recent progress in RCM research from the clinical studies and the translational studies done on diseased transgenic animal models. This will help for a better understanding of the mechanisms underlying the etiology and development of RCM and for the design of better treatments for the disease.
[Show abstract][Hide abstract] ABSTRACT: Troponin T (TnT) and troponin I (TnI) are two evolutionarily and functionally linked subunits of the troponin complex that regulates striated muscle contraction. We previously reported a single amino acid substitution in the highly conserved TnT-binding helix of cardiac TnI (cTnI) in wild turkey hearts in concurrence with an abnormally spliced myopathic cardiac TnT (cTnT) (Biesiadecki, B. J., Schneider, K. L., Yu, Z. B., Chong, S. M., and Jin, J. P. (2004) J. Biol. Chem. 279, 13825-13832). To investigate the functional effect of this cTnI mutation and its potential value in compensating for the cTnT abnormality, we developed transgenic mice expressing the mutant cTnI (K118C) in the heart with or without the deletion of the endogenous cTnI gene to mimic the homozygote and heterozygote of wild turkeys. Double and triple transgenic mice were created by crossing the cTnI-K118C lines with transgenic mice overexpressing the myopathic cTnT (exon 7 deletion). Functional studies of ex vivo working hearts found that cTnI-K118C alone had a dominantly negative effect on diastolic function and blunted the inotropic responses of cardiac muscle to beta-adrenergic stimuli without abolishing the protein kinase A-dependent phosphorylation of cTnI. When co-expressed with the cTnT mutation, cTnI-K118C corrected the significant depression of systolic function caused by cTnT exon 7 deletion, and the co-existence of exon 7-deleted cTnT minimized the diastolic abnormality of cTnI-K118C. Characterization of this naturally selected pair of mutually rescuing mutations demonstrated that TnI-TnT interaction is a critical link in the Ca(2+) signaling and beta-adrenergic regulation in cardiac muscle, suggesting a potential target for the treatment of troponin cardiomyopathies and heart failure.
[Show abstract][Hide abstract] ABSTRACT: The highly organized contractile machinery in skeletal and cardiac muscles requires an assembly of myofilament proteins with stringent stoichiometry. To understand the maintenance of myofilament protein stoichiometry under dynamic protein synthesis and catabolism in muscle cells, we investigated the equilibrium of troponin I (TnI) in mouse cardiac muscle during developmental isoform switching and in under- and over-expression models. Compared with the course of developmental TnI isoform switching in normal hearts, the postnatal presence of slow skeletal muscle TnI lasted significantly longer in the hearts of cardiac TnI (cTnI) knockout (cTnI-KO) mice, in which the diminished synthesis was compensated by prolonging the life of myofilamental TnI. Transgenic postnatal expression of an N-terminal truncated cTnI (cTnI-ND) using alpha-myosin heavy chain promoter effectively rescued the lethality of cTnI-KO mice and shortened the postnatal presence of slow TnI in cardiac muscle. cTnI-KO mice rescued with different levels of cTnI-ND over-expression exhibited similar levels of myocardial TnI comparable to that in wild type hearts, demonstrating that excessive synthesis would not increase TnI stoichiometry in the myofilaments. Consistently, haploid under-expression of cTnI in heterozygote cTnI-KO mice was sufficient to sustain the normal level of myocardial cTnI, indicating that cTnI is synthesized in excess in wild type cardiomyocytes. Altogether, these observations suggest that under wide ranges of protein synthesis and turnover, myofilament incorporation determines the stoichiometry of troponin subunits in muscle cells.
Archives of Biochemistry and Biophysics 06/2009; 487(1):36-41. DOI:10.1016/j.abb.2009.05.001 · 3.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transgenic mice were generated to express a restrictive cardiomyopathy (RCM) human cardiac troponin I (cTnI) R192H mutation in the heart (cTnI(193His) mice). The objective of this study was to assess cardiac function during the development of diastolic dysfunction and to gain insight into the pathophysiological impact of the RCM cTnI mutation. Cardiac function and pathophysiological changes were monitored in cTnI193His mice and wild-type littermates for a period of 12 mo. It progressed gradually from abnormal relaxation to diastolic dysfunction characterized with high-resolution echocardiography by a reversed E-to-A ratio, increased deceleration time, and prolonged isovolumetric relaxation time. At the age of 12 mo, cardiac output in cTnI(193His) mice was significantly declined, and some transgenic mice showed congestive heart failure. The negative impact of cTnI193His on ventricular contraction and relaxation was further demonstrated in isolated mouse working heart preparations. The main morphological change in cTnI193His myocytes was shortened cell length. Dobutamine stimulation increased heart rate in cTnI193His mice but did not improve CO. The cTnI193His mice had a phenotype similar to that in human RCM patients carrying the cTnI mutation characterized morphologically by enlarged atria and restricted ventricles and functionally by diastolic dysfunction and diastolic heart failure. The results demonstrate a critical role of the COOH-terminal domain of cTnI in the diastolic function of cardiac muscle.
[Show abstract][Hide abstract] ABSTRACT: Cardiomyopathies are diseases of heart muscle that are associated with cardiac dysfunction. Molecular genetic studies performed to date have demonstrated that the damage or mutations in several sarcomeric contractile protein genes are associated with the development of the diseases. In this review, cardiac troponin I, one of the sarcomeric thin filament protein, will be discussed regarding its role in cardiac function, its deficiency-related diastolic dysfunction, and the mutation of this protein-mediated restrictive cardiomyopathy.