Isolation and functional comparison of bovine cardiac troponin T isoforms

Regression of cardiac hypertrophy in spontaneously hypertensive rats by enalapril and the expression of contractile proteins

Several experimental models involving the development of cardiac hypertrophy in adult rats are characterized by the reexpression of the fetal isoform of myosin heavy chain (V3). To determine whether a similar adult-to-fetal shift in the expression of the thin-filament proteins occurs during cardiac hypertrophy, we have examined the expression of the isoforms of myosin, tropomyosin, and troponin T in the left ventricle of young spontaneously hypertensive rats (SHR) with and without treatment using enalapril, an angiotensin converting enzyme inhibitor. Phosphorylation of tropomyosin, which is predominant in the fetal state, was also analyzed. Twelve-week-old SHR were treated with enalapril for 2, 5, 8, and 9 weeks followed by withdrawal of treatment for 9 weeks. Control SHR, without drug treatment, were weight- and age-matched. After 9 weeks of enalapril treatment, mean arterial blood pressure was reduced (from 166 +/- 11 to 89 +/- 5 mm Hg), and left ventricular weight/body weight ratio was regressed (from 2.53 +/- 0.14 to 1.96 +/- 0.05 g/kg) to normotensive levels. During the 9-week treatment period, the percent V3 decreased in SHR substantially from 35 +/- 3% to 13 +/- 1%. There was a significant correlation between the left ventricular hypertrophy and the percent V3 myosin expression in the SHR during regression (r = 0.697, p less than 0.001). However, only the adult isoforms of tropomyosin and troponin T were detected in the SHR with or without enalapril treatment, and the level of tropomyosin phosphorylation remained constant irrespective of the degree of left ventricular hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of protein kinase C activation on sarcoplasmic reticulum function and apparent myofibrillar Ca2+ sensitivity in intact and skinned muscles from normal and diseased human myocardium

Protein kinase C regulates the activity of a diverse group of cellular proteins including membrane ion channel proteins. Although protein kinase C and its substrate protein have been identified in both membrane and cytosolic fractions in the heart, the physiological role of this kinase in the regulation of cardiac function remains unknown. We examined the physiological role of protein kinase C by stimulating its activity with 12-deoxyphorbol 13 isobutyrate 20 acetate (DPBA) in human trabeculae carneae. This resulted in decreased peak isometric twitch force and peak intracellular sarcoplasmic reticulum calcium release as detected with aequorin. Furthermore, in the presence of DPBA, steady-state force-[Ca2+] relations were shifted to higher intracellular calcium concentrations, and the Hill coefficient was reduced, indicating a decrease in responsiveness of the myofilaments to calcium and a change in cooperativity among thin filament proteins, respectively. Thus, DPBA affects not only intracellular calcium concentration, but myofilament calcium interactions as well. The effect of DPBA on Ca2+ activation probably reflects phosphorylation of thin-filament regulatory proteins by protein kinase C.

Regulatory proteins in hamster cardiomyopathy

We have shown in genetic myopathic hamsters that cardiac myofibrillar ATPase regulation by calcium is altered and that there are shifts in myosin isozyme distribution (V1----V3) suggesting abnormalities in multiple components of the contractile apparatus. To focus more on the regulatory proteins (troponin and tropomyosin), individual proteins of the skeletal and cardiac actomyosin system were reconstituted under controlled conditions. In this way, myosin plus actin and troponin-tropomyosin from the normal and myopathic animals could be studied enzymatically. The proteins were isolated from the skeletal or cardiac muscle of random-bred control and cardiomyopathic hamsters (BIO 53:58) at 7 months of age. Sodium dodecyl sulfate gel electrophoretic patterns indicated differences in the troponin I and troponin C regions of myopathic skeletal muscle, but cardiac samples from control and myopathic hamsters showed similarities in their mobilities. This suggests the possibility of different cardiac isozymes in the regulatory protein complex, as reported in our previous studies of cardiac myosin in cardiomyopathy. Calcium sensitivity was markedly decreased in the actomyosin reconstituted with troponin-tropomyosin from skeletal as well as cardiac muscle from myopathic animals. In summary, our data show that the regulatory proteins in skeletal and cardiac muscle of the myopathic hamsters have decreased inhibitory action on Mg2(+)-actomyosin ATPase activity. This loss of calcium regulation along with shifts in cardiac myosin heavy chain may be partially responsible for the impaired cardiac function in the hearts of myopathic hamsters.

Changes in myofibrillar activation and troponin C Ca2+ binding associated with troponin T isoform switching in developing rabbit heart

Postnatal development of the mammalian heart is associated with changes in the population of isoforms of the thin filament proteins. We correlated the change in thin filament proteins, which occur in rabbit hearts between 5 days and 22 days of age, with changes in Ca2+ dependence of myofibrillar ATPase activity, force generation, and troponin C Ca2+ binding. The preparations derived from the 5-day-old animals exhibited a high molecular weight isoform of troponin T not found in the hearts of the 22-day-old animals. Other troponin T isoforms were also found to be present in different relative amounts. No other major differences in thin filament protein composition could be identified. Compared with the 5-day-old rabbit heart preparations, the ATPase activity of myofibrils from 22-day-old rabbit hearts exhibited a reduced Ca2+ sensitivity. The pCa50 (negative log of the half-maximal-activity free Ca2+) of the MgATPase activity was shifted by 0.15 pCa units with maturation. Maturation of the myofibrils was also associated with an increased effect of Mg2+ on pCa50. On increasing the Mg2+ from 2 to 10 mM at constant MgATP2-, the pCa50 of 5-day myofibrils was increased (shifted to the right) by 0.39 pCa units for 5-day-old rabbit hearts and 0.45 pCa units for 22-day-old rabbit hearts. Although similar changes in pCa50 of force developed by myofibrils were marginally significant, fibers from hearts of 5-day-old rabbits exhibited a greater Hill coefficient than hearts from 22-day-old rabbits (3.0 vs. 2.1). Despite the increased sensitivity of 5-day-old rabbit hearts to Ca2+, these hearts exhibited significantly less Ca2+ bound to myofibrillar troponin C than did the 22-day-old rabbit hearts. Moreover, the models that best described the Ca2+ binding data are different for the two age groups. Our data indicate that the Ca2+ activation and Ca2+ binding properties of myofibrillar troponin C are altered in developing cardiac myofibrils and that the changes in these properties may be influenced by changes in the troponin T isoforms present in the myofibril.

Immunological identification of five troponin T isoforms reveals an elaborate maturational troponin T profile in rabbit myocardium

Myocardium is generally thought to express no more than two isoforms of troponin T (TnT). We have recently reported that TnT purified from rabbit myocardium is resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into five proteins (TnT1, TnT2, TnT3, TnT4, and TnT5). In this study, these proteins are characterized immunologically and a novel elaborate maturational profile is described. Myocardium was obtained from 23 days of gestation fetal rabbits and 2-day, 6-week, 3-month, and 6-month postnatal rabbits. The major species in the adult myocardium, TnT4, was identified on sodium dodecyl sulfate-polyacrylamide gels and excised. The protein was electroeluted and purified. An amino acid microsequence of a cleaved fragment of this protein was found to be virtually identical to residues 86-99 from adult rabbit cardiac TnT. The protein, TnT4, was used to raise a polyclonal antibody. This antibody recognized all five isoforms from purified cardiac TnT, but none of the TnT isoforms from fast skeletal muscle. A monoclonal antibody, Mab JLT-12, raised against a highly conserved epitope of rabbit fast skeletal muscle, recognized all five cardiac as well as five skeletal muscle isoforms. Western blots performed on intact myocardial preparations demonstrated that TnT1, the cardiac isoform with the slowest electrophoretic mobility, was expressed prominently in the immature hearts, in addition to TnT2, TnT3, and TnT4, but TnT1 was not evident in the 3-month and 6-month postnatal hearts. The expression of TnT2 also decreased with maturation. Thus, the number of TnT isoforms expressed in the rabbit decreases with maturation.(ABSTRACT TRUNCATED AT 250 WORDS)

N-terminal amino acid sequences of three functionally different troponin T isoforms from rabbit fast skeletal muscle

The different isoforms of fast skeletal muscle troponin T (TnT) are generated by alternative splicing of several 5' exons in the fast TnT gene. In rabbit skeletal muscle this process results in three major fast TnT species, TnT1f, TnT2f and TnT3f, that differ in a region of 30 to 40 amino acid residues near the N terminus. Differential expression of these three isoforms modulates the activation of the thin filament by calcium. To establish a basis for further structure-function studies, we have sequenced the N-terminal region of these proteins. TnT2f is the fast TnT sequenced by Pearlstone et al. The larger species TnT1f contains six additional amino acid residues identical in sequence and position to those encoded by exon 4 in the rat fast skeletal muscle TnT gene. TnT3f also contains that sequence but lacks 17 amino acid residues spanning the region encoded by exons 6 and 7 of the rat gene. These three TnTs appear to be generated by discrete alternative splicing pathways, each differing by a single event. Comparison of these TnT sequences with those from chicken fast skeletal muscle and bovine heart shows that the splicing pattern resulting in the excision of exon 4 is evolutionarily conserved and leads to a more calcium-sensitive thin filament.

Developmental changes in the expression of rabbit left ventricular troponin T

We examined cardiac troponin T (TnT) isoform expression in rabbit left ventricular myocardium at three different stages of postnatal development. Using sodium dodecyl sulfate gel electrophoresis (PAGE), we resolved five isoforms: TnT1, TnT2, TnT3, TnT4, and TnT5. TnT1 had the slowest electrophoretic mobility and TnT5 the fastest. The predominant isoforms were TnT2, TnT3, and TnT4. The relative amounts of TnT2, TnT3, TnT4, and TnT5 were examined in myocardium from three age-groups: 3 days (Group 1), 21-22 days (Group 2), and 99-109 days (Group 3). The amount of TnT2 relative to the total amount of TnT (determined by the ratio of the areas under the densitometric curves) decreased significantly (p less than 0.01) with age from 42 +/- 4% in Group 1 to 25 +/- 3% in Group 3. In contrast, the relative amount of TnT4 increased with age from 23 +/- 2% in Group 1 to 33 +/- 4% in Group 3 (p less than 0.01). The relative amounts of the other two isoforms change biphasically with development: TnT3 decreased from Group 1 to Group 2 and increased from Group 2 to Group 3. TnT5, a minor isoform, increased from Group 1 to Group 2 and decreased from Group 2 to Group 3. These developmental changes in troponin T expression may account for some of the maturational changes observed in the physiological and biochemical properties of the myocardium.

Cellular basis for inotropic changes in the heart

Cardiac contraction is a highly regulated process that involves nearly every aspect of the cardiac cell function. Many steps in the process are regulated by the cell to optimize contraction, and these can often be modified to benefit the patient with heart damage. These include the action potential, calcium channels, release and uptake of cellular calcium, sensitivity of contractile proteins to calcium, and energy utilization. A dramatic expansion of our understanding of these cellular contractile and regulatory processes gives us an unprecedented opportunity to devise new ways of modifying cardiac contraction to the benefit of our patients.