Troponin I

Progressive troponin I loss impairs cardiac relaxation and causes heart failure in mice

Cardiac troponin I (TnI) knockout mice exhibit a phenotype of sudden death at 17–18 days after birth due to a progressive loss of TnI. The objective of this study was to gain insight into the physiological consequences of TnI depletion and the cause of death in these mice. Cardiac function was monitored serially between 12 and 17 days of age by using high-resolution ultrasonic imaging and Doppler echocardiography. Two-dimensional B-mode and anatomical M-mode imaging and Doppler echocardiography were performed using a high-frequency (∼20–45 MHz) ultrasound imaging system on homozygous cardiac TnI mutant mice (cTnI−/−) and wild-type littermates. On day 12, cTnI−/−mice were indistinguishable from wild-type mice in terms of heart rate, atrial and LV (LV) chamber dimensions, LV posterior wall thickness, and body weight. By days 16 through 17, wild-type mice showed up to a 40% increase in chamber dimensions due to normal growth, whereas cTnI−/−mice showed increases in atrial dimensions of up to 97% but decreases in ventricular dimensions of up to 70%. Mitral Doppler analysis revealed prolonged isovolumic relaxation time and pronounced inversion of the mitral E/A ratio (early ventricular filling wave-to-late atrial contraction filling wave) only in cTnI−/−mice indicative of impaired LV relaxation. cTnI−/−mouse hearts showed clear signs of failure on day 17, characterized by >50% declines in cardiac output, ejection fraction, and fractional shortening. B-mode echocardiography showed a profoundly narrowed tube-like LV and enlarged atria at this time. Our data are consistent with TnI deficiency causing impaired LV relaxation, which leads to diastolic heart failure in this model.

Myofibril MgATPase activities and energy metabolism in cardiomyopathic mice with diastolic dysfunction

To study the genomic physiology of cardiac myofibril proteins in the heart, we have successfully created a cardiac troponin I (cTnI; a myofibril protein) gene knockout mouse model using gene targeting techniques. The phenotype of the cTnI gene knockout mouse is a cardiomyopathy with diastolic dysfunction resulting in sudden death in neonates. In the present studies, energy metabolism was analyzed in myocardial cells from cTnI-null hearts. Myofibril MgATPase activities were determined in myocardial cells from either wild-type or cTnI mutant mouse hearts. Furthermore, the quantity and quality of the mitochondria in wild-type and cTnI mutant animals were counted and analyzed. Our results demonstrate that damaged relaxation and increased Ca(2+)-independent force production in cTnI-null hearts is in part related to the increased myofibril MgATPase activities accompanied by an increase in mitochondria quantity and mitochondrial ATPase activities. These data indicate that cardiomyopathies with diastolic dysfunction are different from cardiomyopathies caused by systolic dysfunction. The former involves the damage of cardiac relaxation due to increased MgATPase activities and increased Ca(2+)-independent force production inside of myofilaments, while the latter involves the damage of systolic contraction due to decreased MgATPase activities and decreased force production.