Isolation and functional comparison of bovine cardiac troponin T isoforms

Mechanoenergetic alterations during the transition from cardiac hypertrophy to failure in Dahl salt-sensitive rats

BACKGROUND

The time course and mechanisms of altered mechanoenergetics and depressed cross-bridge cycling in hypertrophied and failing myocardium are uncertain.

METHODS AND RESULTS

We studied mechanoenergetics in Dahl salt-sensitive (DS) rats fed high-salt diet (HS) for 6 (HS-6) and 12 (HS-12) weeks to produce compensated hypertrophy and failure. The slope of the end-systolic pressure-volume relation (E'max) was similar in HS-6 and low-salt controls (LS-6), but reduced in HS-12 compared with controls (LS-12). Efficiency [1/slope of oxygen consumption (&f1;O2)-pressure-volume area (PVA) relation] was similar in HS-6 and LS-6 but higher in HS-12 versus LS-12 (59+/-16% versus 44+/-7%, P<0.05). Economy [1/slope of the force-time integral (FTI)-&f1;O2 relation] was similar in HS-6 and LS-6 but higher in HS-12 versus LS-12 (218+/-123 versus 74+/-39x10(3) g. s. mL O2-1. g; P<0.05). Compared with controls, myofibrillar ATPase activity was reduced by 24% in HS-6 and 44% in HS-12. V3 Isomyosin was increased in HS-6 (40+/-12% versus 9+/-8%; P<0.05) and further increased in HS-12 (76+/-10% versus 22+/-18%; P<0.05). Hypothyroid LS-12 rats had 100% V3 isomyosin, yet efficiency, economy, and ATPase values were intermediate between LS-12 and HS-12. HS-12 rats demonstrated increased troponin T3 isoform (17+/-2 versus 23+/-2%, P<0.05). There were no changes in troponin I or tropomyosin isoforms. However, the proportion of phosphorylated troponin T was reduced in HS-12 versus LS-12 hearts (P<.001).

CONCLUSIONS

In DS rats, the transition to failure is associated with depressed E'max and increased efficiency and economy. These findings are linked to myofibrillar ATPase activity and suggest that mechanisms other than isomyosin switching are important determinants of ventricular energetics. A troponin T isoform switch is one potential mechanism.

Ca2+-dependent muscle dysfunction caused by mutation of the Caenorhabditis elegans troponin T-1 gene

We have investigated the functions of troponin T (CeTnT-1) in Caenorhabditis elegans embryonic body wall muscle. TnT tethers troponin I (TnI) and troponin C (TnC) to the thin filament via tropomyosin (Tm), and TnT/Tm regulates the activation and inhibition of myosin-actin interaction in response to changes in intracellular [Ca2+]. Loss of CeTnT-1 function causes aberrant muscle trembling and tearing of muscle cells from their exoskeletal attachment sites (Myers, C.D., P.-Y. Goh, T. StC. Allen, E.A. Bucher, and T. Bogaert. 1996. J. Cell Biol. 132:1061-1077). We hypothesized that muscle tearing is a consequence of excessive force generation resulting from defective tethering of Tn complex proteins. Biochemical studies suggest that such defective tethering would result in either (a) Ca2+-independent activation, due to lack of Tn complex binding and consequent lack of inhibition, or (b) delayed reestablishment of TnI/TnC binding to the thin filament after Ca2+ activation and consequent abnormal duration of force. Analyses of animals doubly mutant for CeTnT-1 and for genes required for Ca2+ signaling support that CeTnT-1 phenotypes are dependent on Ca2+ signaling, thus supporting the second model and providing new in vivo evidence that full inhibition of thin filaments in low [Ca2+] does not require TnT.

Developmentally regulated, alternative RNA splicing-generated pectoral muscle-specific troponin T isoforms and role of the NH2-terminal hypervariable region in the tolerance to acidosis

The structure-function relationship of the alternative RNA splicing-generated NH2-terminal variable region of troponin T (TnT) is essential for understanding the physiological significance of developmental or muscle-specific TnT isoforms. Representing the hypervariable nature of the NH2-terminal region, a repeating transition metal-binding sequence (H(E/A)EAH)4-7 (Tx) has been found in chicken fast skeletal muscle TnT. In the present study, the developmentally regulated pectoral muscle-specific expression of this novel TnT isoform has been characterized. It was found that the variable amino terminus determined the isoelectric points of the TnT isoforms expressed, and the adult muscle-specific inclusion of the Tx sequence resulted in pectoralis TnTs, which were significantly more acidic in their NH2-terminal segment versus gastrocnemius TnTs. Experiments testing the effect of pH on TnT interaction with troponin I and tropomyosin indicated that although the interaction of acidic TnT isoforms with troponin I was less sensitive to the decrease of pH than the basic TnTs, the binding affinity of acidic TnT isoforms with tropomyosin was minimally affected by the decreased pH in contrast to basic TnT isoforms. Given that the majority of adult skeletal muscles express basic fast TnT isoforms, the switching between acidic and basic TnT isoforms may play a role in the functional adaptation of muscle to acidosis.

Conformational modulation of troponin T by configuration of the NH2-terminal variable region and functional effects

Troponin T (TnT) is an essential element in the thin filament-based regulatory system of striated muscle. Alternative mRNA splicing generates multiple TnT isoforms with primary structural differences in the NH2-terminal region. The functional significance of this hypervariable NH2-terminal domain and the developmental or muscle type-specific TnT isoforms is not fully understood. We have analyzed chicken breast muscle TnT containing a metal-binding cluster [H(E/A)EAH]4-7 (Tx) in the NH2-terminal region to demonstrate potential effects of the NH2-terminal structure on the conformation of TnT [Ogut, O., and Jin, J.-P. (1996) Biochemistry 35, 16581-16590]. Using specific antibody epitope analysis on this metal-binding TnT model, this study revealed that the binding of Zn2+ to the NH2-terminal region of chicken breast muscle TnT induces extensive conformational changes in the whole protein as demonstrated by a significant decrease in binding avidity of a polyclonal anti-TnT serum which recognizes multiple epitopes on the TnT molecule. This NH2-terminal configuration-based effect is not restricted to the metal ion interaction, whereas the binding of anti-NH2 terminus monoclonal antibodies to TnT induced similar changes. Protein-binding assays have shown that the NH2-terminal variability-induced conformational changes can alter TnT's binding affinity for tropomyosin and troponin I. The results suggest a functional modulation of TnT through the configuration of the NH2-terminal domain, and this novel mechanism may mediate the physiological significance of the TnT isoform regulation.

Three alternatively spliced mouse slow skeletal muscle troponin T isoforms: conserved primary structure and regulated expression during postnatal development

We have cloned and sequenced full-length cDNAs encoding mouse slow skeletal muscle troponin T (sTnT). Alternative mRNA splicing-generated two high Mr isoforms and one low Mr sTnT isoform differing in the NH2-terminal primary structure have been identified by Western blotting, reverse transcription-polymerase chain reaction and cDNA cloning/expression analyses. Together with a 5'-alternative exon that was also found in human sTnT encoding an 11-amino-acid acidic segment, the results revealed a novel alternative splicing pathway to include or exclude a three-base segment to generate additional sTnT isoforms with NH2-terminal charge variations. Overriding the phylogenetic divergence, primary structure of sTnT is better conserved between mammalian and avian species than that of cardiac, fast and skeletal muscle TnTs from one species. Western blots demonstrate four expression patterns of sTnT during postnatal skeletal muscle development: (1) a decrease to a non-detectable level in mouse masseter, (2) an increase to become the sole TnT in sheep masseter, (3) an increase of the total level as well as the proportion of the low Mr isoform in sheep diaphragm and, (4) no significant change in total level or high/low Mr isoform ratio in sheep gastrocnemius. The highly conserved primary structure and fiber type-specific and developmentally regulated expression of sTnT indicate a physiological importance of this under-studied member of the TnT gene family.

Troponin I phosphorylation and myofilament calcium sensitivity during decompensated cardiac hypertrophy

We have measured myocyte cell shortening, troponin-I (Tn-I) phosphorylation, Ca2+ dependence of actomyosin adenosinetriphosphatase (ATPase) activity, adenosine 3',5'-cyclic monophosphate (cAMP) levels, and myofibrillar isoform expression in the spontaneously hypertensive rat (SHR) during decompensated cardiac hypertrophy (76 wk old) and in age-matched Wistar-Kyoto rat (WKY) controls. The decreased inotropic response to beta-adrenergic stimulation previously observed in myocytes from 26-wk-old SHR was further reduced at 76 wk of age. In response to beta-adrenergic stimulation, Tn-I phosphorylation was greater in the 76-wk-old SHR than in the WKY, although cAMP-dependent protein kinase A (PKA)-dependent Tn-I phosphorylation in the SHR did not increase with progression from compensated (26 wk) to decompensated (76 wk) hypertrophy. We also observed a dissociation between the increased PKA-dependent Tn-I phosphorylation and decreased cAMP levels in the 76-wk-old SHR versus WKY during beta-adrenergic stimulation. Baseline Tn-I phosphorylation was significantly reduced in 76-wk-old SHR versus WKY and was associated with decreased basal cAMP levels and increased Ca2+ sensitivity of actomyosin ATPase activity. The change in myofilament Ca2+ sensitivity during beta-adrenergic stimulation in the 76-wk-old SHR (0.65 pCa units) was over twofold greater than in the 76-wk-old WKY (0.30 pCa units). We also determined whether embryonic troponin T isoforms were reexpressed in decompensated hypertrophy and observed significant reexpression of the embryonic cardiac troponin T isoforms in the 76-wk-old SHR. The significant decrease in Ca2+ sensitivity with beta-adrenergic stimulation in 76-wk-old SHR may contribute to the severely impaired inotropic response during decompensated hypertrophy in the SHR.

Primary structure and developmental acidic to basic transition of 13 alternatively spliced mouse fast skeletal muscle troponin T isoforms

Large samples of original cDNAs encoding neonatal and adult mouse fast skeletal muscle troponin T (fTnT) have been isolated and characterized. The results demonstrate expression relationships of 8 alternatively spliced exons of the fTnT gene and reveal the primary structure of as many as 13 fTnT isoforms that diverge into acidic and basic classes due to differential mRNA splicing in the N-terminal variable region. In the C-terminal variable region encoded by the mutually exclusive exons 16 and 17, the splicing pathway and structure of exon 16 appears to be adult fTnT-specific, suggesting an adaptation to the functional demands of mature fast skeletal muscle. The cloned cDNAs were expressed in E. coli as standards to identify a high M(r) to low M(r), acidic to basic fTnT isoform transition in postnatal developing skeletal muscles. Different from the developmental cardiac TnT switch generated by alternative splicing of a single exon, the fTnT isoform transition is an additive effect of alternative splicing of multiple N-terminal-coding exons, especially exons 4, 8 and fetal that are expressed at higher frequencies in the neonatal than in the adult muscle. The developmental fTnT isoform primary structure transition in both N- and C-terminal variable regions suggest a physiological importance of the apparently complex TnT isoform expression.

Cooperative effect of calcium binding to adjacent troponin molecules on the thin filament-myosin subfragment 1 MgATPase rate

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca2+ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca2+ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca2+, implying a cooperative effect of bound Ca2+ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca2+ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca2+ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca2+ binding, implying a true, not just an apparent, increase in Ca2+ affinity. MgATPase activation by Ca2+ was more cooperative than could be explained by cooperativeness of overall Ca2+ binding, the discrepancy between Ca2+ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca2+-mediated regulation of cross-bridge kinetics in muscle fibers.

Slow skeletal troponin I gene transfer, expression, and myofilament incorporation enhances adult cardiac myocyte contractile function

The functional significance of the developmental transition from slow skeletal troponin I (ssTnI) to cardiac TnI (cTnI) isoform expression in cardiac myocytes remains unclear. We show here the effects of adenovirus-mediated ssTnI gene transfer on myofilament structure and function in adult cardiac myocytes in primary culture. Gene transfer resulted in the rapid, uniform, and nearly complete replacement of endogenous cTnI with the ssTnI isoform with no detected changes in sarcomeric ultrastructure, or in the isoforms and stoichiometry of other myofilament proteins compared with control myocytes over 7 days in primary culture. In functional studies on permeabilized single cardiac myocytes, the threshold for Ca2+-activated contraction was significantly lowered in adult cardiac myocytes expressing ssTnI relative to control values. The tension-Ca2+ relationship was unchanged from controls in primary cultures of cardiac myocytes treated with adenovirus containing the adult cardiac troponin T (TnT) or cTnI cDNAs. These results indicate that changes in Ca2+ activation of tension in ssTnI-expressing cardiac myocytes were isoform-specific, and not due to nonspecific functional changes resulting from overexpression of a myofilament protein. Further, Ca2+-activated tension development was enhanced in cardiac myocytes expressing ssTnI compared with control values under conditions mimicking the acidosis found during myocardial ischemia. These results show that ssTnI enhances contractile sensitivity to Ca2+ activation under physiological and acidic pH conditions in adult rat cardiac myocytes, and demonstrate the utility of adenovirus vectors for rapid and efficient genetic modification of the cardiac myofilament for structure/function studies in cardiac myocytes.

Molecular mechanisms regulating the myofilament response to Ca2+: implications of mutations causal for familial hypertrophic cardiomyopathy

In this chapter we consider a current perception of the molecular mechanisms controlling myofilament activation with emphasis on alterations that may occur in familial hypertrophic cardiomyopathy (FHC). FHC is a sarcomeric disease (100) with an autosomal dominant pattern of heritability (27, 51). There is a substantial body of evidence implicating missense mutations in the beta-MHC gene as causal for the development of this disease. Recently, mutations in genes of two thin filament regulatory proteins, cardiac troponin T(cTnT) and alpha-tropomyosin (alpha-Tm), have also been linked to FHC. The commonality among the functional consequences of these mutations remains an important question. This review discusses how these pathological mutations may impact the activation process by disrupting critical structure function relations in both the thick and thin filaments.

Expression, zinc-affinity purification, and characterization of a novel metal-binding cluster in troponin T: metal-stabilized alpha-helical structure and effects of the NH2-terminal variable region on the conformation of intact troponin T and its association with tropomyosin

A repeating metal-binding (Cu2+ > Ni2+ > Zn2+ approximately Co2+) sequence [HE/AEAH]4 (Tx) has been recently identified in the NH2-terminal variable region of troponin T (TnT) isoforms specifically expressed in the breast but not leg muscles of the avian orders of Galliformes and Craciformes [Jin, J.-P., & Smillie, L. B. (1994) FEBS Lett. 341, 135-140]. In the present study, two expression plasmids were constructed to produce chicken TnT1 NH2-terminal fragments of 47 (N47) or 165 (N165) amino acids containing the Tx metal-binding cluster. The recombinant protein/peptide was expressed in Escherichia coli BL21(DE3)pLysS and purified by a highly effective Zn(2+)-affinity chromatography method. Amino acid analyses, NH2-terminal peptide sequencing, mass spectrometry and immunological identification confirmed the authenticity of the genetically engineered TnT fragments. In the presence of 2,2,2-trifluoroethanol, transition metals had significant effects on the secondary structure of TnT fragment N47, as shown by circular dichroism. N165 in non-denaturing buffer demonstrated alpha-helical content comparable to previous data from rabbit fast skeletal TnT fragment T1. Zn(2+)-binding avidity of the metal-binding TnT and its fragments demonstrated tertiary relationships between the NH2-terminal variable region and the COOH-terminal segment of the intact TnT protein. Solid-phase protein-binding assays established that Zn(2+)-binding to the Tx cluster induces epitopic structure changes in this NH2-terminal segment, further affecting other epitopic structures of intact TnT as well as the function of TnT's tropomyosin binding-sites. The results demonstrate that metal ion-binding to the Tx cluster reconfigures the overall conformation of TnT through structural relationships between the NH2-terminal variable region and other domains of the intact TnT molecule. Accordingly, the developmental and/or muscle type specific NH2-terminal structure of TnT isoforms may modulate the Ca(2+)-activation of muscle contraction.

Cardiac troponin T isoform expression correlates with pathophysiological descriptors in patients who underwent corrective surgery for congenital heart disease

BACKGROUND

This study examined cardiac troponin T (cTnT) isoform expression in patients who had undergone surgery at Duke University Medical Center (Durham, NC) between December 1, 1993, and January 31, 1995, to correct congenital heart defects. The human heart expresses four cTnT isoforms (cTnT1 through cTnT4) whose sequence differences result from combinatorial alternative splicing of two exons. We have previously shown that cTnT4 is expressed at higher levels in severely failing hearts from transplant patients. In this study, we tested the hypothesis that congenital heart defects that have a more negative effect on myocardial function increase cTnT4 expression. We used the presence or absence of drug treatment for heart failure or congested circulation before surgery and the duration of inotropic support after corrective surgery as indicators of the pathophysiological state of the heart just before surgery.

METHODS AND RESULTS

Right atrial appendage tissue was collected from 34 patients, 6 days to 35 years old (median age, 3.4 months). The amounts of the cTnT1 through cTnT4 isoforms, measured as a percentage of total cTnT, were determined from Western blots probed with MAb13-11, a cTnT-specific monoclonal antibody. We found that cTnT4 expression correlated positively with the duration of inotropic support and was higher in patients who received drug treatment before surgery than in those who did not. Furthermore, we found that the percent of cTnT4 was significantly higher in hearts with congenital defects that caused congestive failure than in hearts with tetralogy of Fallot.

CONCLUSIONS

These findings suggest that in patients with congenital cardiac defects, cTnT4 expression is modulated by heart failure and is increased in hearts that are more hemodynamically stressed.

Opposite effects of myosin subfragment 1 on binding of cardiac troponin and tropomyosin to the thin filament

To better understand the regulation of striated muscle contraction, the effects of myosin subfragment 1 (S-1) on the actin binding of cardiac troponin and tropomyosin were investigated. Troponin's affinity for actin-tropomyosin was 4-fold stronger in the absence than in the presence of myosin S-1. CaCl2 had no effect on troponin binding to the thin filament in the presence of myosin S-1. The binding curve was weakly cooperative, implying interactions between adjacent troponin molecules. Myosin S-1 increased (40-200-fold) the affinity of tropomyosin for the thin filament, an effect opposite to the effect of myosin on troponin. This effect was highly cooperative and occurred in the presence of ADP or in the absence of nucleotide. Myosin altered the effect of ionic conditions on tropomyosin-actin binding, consistent with tropomyosin binding to a different site on F-actin in the presence of myosin. The results indicate that troponin-tropomyosin and strongly binding myosin cross-bridges do not compete for an F-actin binding site. Although repositioning of troponin-tropomyosin on the actin filament may be sterically required for tight myosin-actin binding, a myosin-induced conformational change in actin provides a better explanation for the complex effects of myosin on thin filament assembly.

Expression of cDNAs encoding mouse cardiac troponin T isoforms: characterization of a large sample of independent clones

We have isolated 52 mouse cardiac troponin-T-encoding cDNA clones (TnT) by specific antibody screening of a lambda ZAPII expression library. Sequencing data from the large sample of independent cDNAs demonstrated relationships among the expression of four alternatively-spliced exons of the cardiac TnT gene, producing seven classes of cDNAs encoding four protein isoforms differing in two variable regions. In the N-terminal variable region and next to the embryonic-specific exon 4, an alternatively spliced exon 3a was identified in 20% of the adult isoforms. The alternatively spliced exon 12, corresponding to a central variable region between the two functional domains of TnT, was found in approx. 79% of the 52 mouse cardiac TnT cDNAs with a single base mutation completely abolishing the splicing at an internal acceptor site. Three novel alternative splicing acceptor sites in the 5'-untranslated portion of exon 2 have been identified with different frequencies.

Functional significance of alterations in cardiac contractile protein isoforms

Multiple closely related, yet distinct, isoforms exist for each of the cardiac contractile proteins. The isoform composition of the heart changes in response to developmental and physiologic cues. This paper reviews the molecular basis for cardiac contractile protein isoform diversity and the functional consequences of isoform shifts.

NH2-terminal truncation of skeletal muscle troponin T does not alter the Ca2+ sensitivity of thin filament assembly

To investigate how Ca2+ binding to troponin C regulates muscle contraction, the Ca(2+)-sensitive properties of thin filament assembly were studied as the tropomyosin binding, NH2-terminal region of troponin T was progressively shortened. Troponin complexes were prepared that contained skeletal muscle troponin C, troponin I, and either intact troponin T (TnT) (residues 1-259) or fragment TnT-(70-259), TnT-(151-259), or TnT-(159-259). In the absence of Ca2+ their respective affinities for pyrene-labeled tropomyosin were 2.3 x 10(7) M-1, 1.2 x 10(7) M-1, 1.9 x 10(5) M-1, and 1.9 x 10(5) M-1. Ca2+ had only a small effect on these affinities: 1.1 x 10(7) M-1 for whole troponin, 2 x 10(5) M-1 for troponin-(151-259), and 2.8 x 10(5) M-1 for troponin-(159-259). Forms of troponin that bound weakly to tropomyosin in the absence of actin increased the actin affinity of tropomyosin only 2-3-fold, even in the absence of Ca2+; weak binding of troponin to tropomyosin correlated with weak effects on tropomyosin-actin binding. In contrast, whole troponin had an approximately 500-fold effect on tropomyosin binding to actin, regardless of whether Ca2+ was present. The small effect of Ca2+ on the energetics of thin filament assembly is not attributable to the amino-terminal region of troponin T. The results suggest that Ca2+ causes the interaction between actin and the globular region of troponin to switch between two energetically similar states.

Molecular cloning of human cardiac troponin T isoforms: expression in developing and failing heart

Troponin T, which links the troponin complex to tropomyosin, is found as multiple isoforms in the hearts of many animal species. Changes in isoform composition have been correlated with variation in myofilament sensitivity to calcium. In order to determine the origin of diversity of the cardiac troponin T (cTnT) isoforms indicated by existing protein data, we have determined the sequences and patterns of expression of mRNAs encoding troponin T in fetal and adult heart and those present in adult heart in end-stage failure. Three main regions of alternative splicing within the cTnT coding region were identified using reverse-transcriptase polymerase chain reaction (RT-PCR). Alternatively spliced RNAs are developmentally regulated and some of the fetal forms are expressed in adult failing heart. The molecular structure of the spliced regions was determined from cloned cDNAs and RT-PCR products. In the 5' region of the mRNA, isoforms are generated by the inclusion or exclusion of 15-, 3- and 27-nucleotide (nt) sequences and by the inclusion or exclusion of a separate 3-nt sequence. In the 3' region of the mRNA, alternative splicing involves a 9-nt sequence which can be present in full, in part or not at all. A further splicing site was identified in the central region involving a 234-nt sequence and resulting in rare but detectable mRNAs. This work demonstrates the complexity of cTnT RNA composition in human heart and provides the information necessary to address the function of cTnT isoforms in contraction.

Characterisation of fast, slow and cardiac muscle tropomyosins from salmonid fish

Tropomyosin (TM) has been isolated from the cardiac muscle, and fast and slow trunk (myotomal) muscles of the mature salmonid fish Atlantic salmon (Salmo salar) and rainbow trout (Salmo gairdneri). When examined electrophoretically, isoforms of TM were detected which were specific, and exclusive, to each type of muscle. Cardiac and fast muscles contained single and distinct isoforms, while slow muscle contained two distinct isoforms, closely related in terms of apparent M(r), and pI. There was no detectable difference between the same TM type from either salmon or trout. On a variety of gel systems, the cardiac and slow isoforms migrated in close proximity to each other and to rabbit alpha-TM. The fast isoform comigrated with rabbit beta-TM. In developing salmon fry, a more acidic (unphosphorylated) variant of TM was present in addition to, and of similar M(r) to, the fast adult isoform. This TM declined in steady-state level during maturation and was virtually undetected in adult muscle. All of the isolated TMs contained little or no covalently bound phosphate and were blocked at the N-terminus. The amino acids released by carboxypeptidase A, when ordered to give maximal similarity to other muscle TMs, were consistent with the following sequences: fast (LDNALNDMTSI) and cardiac (LDHALNDMTSL). The C-terminal region of the slow TM contained His but was heterogeneous. In viscosity measurements, performed as a function of increasing protein concentration, at low ionic strength (t = 5 degrees C, pH 7.00), fast TM exhibited the highest relative viscosity values. Lower and equivalent levels of polymerisation occurred with the cardiac and slow TMs. Polymerisation of all three isoforms was temperature-dependent, with cardiac TM being least sensitive and fast TM being most sensitive. Determination of the complete coding sequence of adult fast TM confirmed the findings of the carboxypeptidase analysis, but the remainder of the sequence more closely resembled alpha-type TMs than beta-type TMs. Overall, salmon fast TM contains 20 (mostly conservative) substitutions compared to rabbit striated muscle alpha-TM and 40 (mostly conservative) substitutions compared to rabbit striated muscle beta-TM. This demonstrates that electrophoretic mobility is not, in all instances, a suitable method to assess the isomorphic nature of striated muscle TMs.

Diminished Ca2+ sensitivity of skinned cardiac muscle contractility coincident with troponin T-band shifts in the diabetic rat

We have measured the apparent Ca2+ sensitivities of force development in skinned cardiac trabeculae at different sarcome lengths together with shifts in troponin (Tn) T subunits on specimens from the same hearts and drawn insights into the pathogenesis of myocardial dysfunction in the diabetic rat. The Ca(2+)-force relations were measured at a long (2.4-microns) and a short (1.9-microns) sarcomere length. In disease, compared with the control condition, the apparent Ca2+ sensitivity was greatly diminished at a sarcomere length of 1.9 microns but not affected at all at the long length (2.4 microns). We also examined the alterations in contractile regulatory proteins TnT and TnI by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blots. The TnI band was largely unperturbed, but major changes were discerned in TnT. The normal rat heart indicated two major bands (TnT1 and TnT2) and a faint third band (TnT3); in the diabetic rat heart, there was a significant shift in intensity from TnT1 to TnT3. Since myosin isozyme shifts also accompany diabetes in the rat, we used a prototypical hypothyroid rat as well to evaluate the myosin influence in the length-induced effects on Ca2+ sensitivity. Myosin shifts during hypothyroidism were unaccompanied by significant changes in TnT, and there were also no length-dependent modifications in Ca2+ sensitivity. The findings raise the possibility that diabetic Ca(2+)-sensitivity changes in the myocardium are coupled with TnT alterations. A plausible explanation is offered whereby these TnT alterations modify the length dependence of Ca2+ sensitivity.

Anti-peptide monoclonal antibody imaging of a common binding domain involved in muscle regulation

Multiple-component regulatory protein systems function through a generalized mechanism where a single regulatory protein or ligand binds to a variety of receptors to modulate specific functions in a physiologically sensitive context. Muscle contraction is regulated by the interaction of actin with troponin I (TnI) or myosin in a Ca(2+)-sensitive manner. Actin utilizes a single binding domain (residues 1-28) to bind to residues 104-115 of TnI (Van Eyk JE, Sönnichsen FD, Sykes BD, Hodges RS, 1991, In: Rüegg JC, ed, Peptides as probes in muscle research, Heidelberg, Germany: Springer-Verlag, pp 15-31) and to myosin subfragment 1 (S1, an enzymatic fragment of myosin containing both the actin and ATP binding sites) (Van Eyk JE, Hodges RS, 1991, Biochemistry 30:11676-11682) in a Ca(2+)-sensitive manner. We have utilized an anti-TnI peptide (104-115) monoclonal antibody, Mab B4, that binds specifically to TnI, to image the common binding domain of actin and thus mimic the activity of actin including activation of the S1 ATPase activity and TnI-mediated regulation of the S1 ATPase. Mab B4 has also been utilized to identify a receptor binding domain on myosin (residues 633-644) that is recognized by actin. Interestingly, Mab B4 binds to the native protein receptors TnI and S1 with relative affinities of 100- and 25,000-fold higher than the binding affinity to the 12-residue peptide immunogen. Thus, anti-peptide monoclonal antibodies prepared against a receptor binding domain can mimic the ligand binding domain and be utilized as a powerful tool for the detailed analysis of complex multiple-component regulatory systems.

Molecular basis of human cardiac troponin T isoforms expressed in the developing, adult, and failing heart

Cardiac troponin T (cTnT), a protein essential for calcium-regulated myofibrillar ATPase activity, is expressed in the human heart as four isoforms (cTnT1 through cTnT4, numbered in the order of decreasing molecular size). The expression of these isoforms at the protein level has previously been found by us to differ in the normal and failing adult and fetal human heart. In the present study, we have cloned and sequenced four full-length cDNAs corresponding to the four native cTnT protein isoforms and have expressed these cDNAs in an in vitro transcription and translation system. The cDNAs differ by the variable inclusion of a 15- and a 30-nt exon in the 5' half of the coding region. These cDNAs yielded proteins that comigrate with the native isoforms, cTnT1 through cTnT4. Polyclonal antisera, raised against a synthetic peptide corresponding to the 10-residue peptide encoded by the 30-nt exon, reacted with the two human isoforms largest in molecular size (cTnT1 and cTnT2) and the two largest cTnT isoforms of the rabbit and rat. The isoforms cTnT1 and cTnT2, containing either both peptides encoded by the 30- and 15-nt exons or the peptide encoded by the 30-nt exon alone, are expressed in the fetal heart, with cTnT2 being expressed at a very low level. cTnT4, lacking both of these sequences, is expressed in the fetal heart and is reexpressed in the failing adult heart, whereas cTnT3, containing the 5-residue peptide, is the dominant isoform in the adult heart.(ABSTRACT TRUNCATED AT 250 WORDS)

A direct regulatory role for troponin T and a dual role for troponin C in the Ca2+ regulation of muscle contraction

Troponin (Tn), containing three subunits: Ca2+ binding (TnC), inhibitory (TnI), and tropomyosin binding (TnT), plays a crucial role in the Ca2+ regulation of vertebrate striated muscle contraction. These three subunits function by interacting with each other and with the other thin filament proteins. Previous studies suggested that the primary role of TnT is to anchor the TnI.TnC complex to the thin filament, primarily through its interactions with TnI and tropomyosin. We propose here a new role for TnT. Our results indicate that, when TnT is combined with the TnI.TnC complex, there is an activation of actomyosin ATPase that is Ca(2+)-dependent. To determine whether the latter results from a direct effect of TnC on TnT or indirectly from an effect of TnC on TnI which is transmitted to TnT, we prepared a deletion mutant (deletion of residues 1-57) of TnI, TnId57 (Sheng et al. (1992) J. Biol. Chem. 267, 25407-25413), which interacts with TnC but not TnT. Both wild type (TnI.TnC.TnT) and mutant (TnId57.TnC.TnT) Tn complexes demonstrated equivalent activity in the Ca2+ regulation of actomyosin-S1 ATPase activity. Similarly, both TnI and TnId57 could equally reconstitute TnI-depleted skinned muscle fibers. Therefore, since TnId57 does not interact with TnT, these results suggest that TnT reconstitutes native Ca2+ sensitivity via direct interaction with TnC. Thus Ca2+ binding to TnC would have a dual role: 1) release of the ATPase inhibition by TnI and 2) activation of the ATPase through interaction with TnT.

Role of regulatory proteins (troponin-tropomyosin) in pathologic states

In vertebrate striated muscle, troponin-tropomyosin is responsible, in part, not only for transducing the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction when calcium is absent. The regulatory troponin (Tn) complex displays several molecular and calcium binding variations in cardiac muscles of different species and undergoes genetic changes with development and in various pathologic states. Extensive reviews on the role of tropomyosin (Tm) and Tn in the regulation of striated muscle contraction have been published describing the molecular mechanisms involved in contractile protein regulation. In our studies, we have found an increase in Mg2+ ATPase activity in cardiac myofibrils from dystrophic hamsters and in rats with chronic coronary artery narrowing. The abnormalities in myofibrillar ATPase activity from cardiomyopathic hamsters were largely corrected by recombining the preparations with a TnTm complex isolated from normal hamsters indicating that the TnTm may play a major role in altered myocardial function. We have also observed down regulation of Ca2+ Mg2+ ATPase of myofibrils from hypertrophic guinea pig hearts, myocardial infarcted rats and diabetic-hypertensive rat hearts. In myosin from diabetic rats, this abnormality was substantially corrected by adding troponin-tropomyosin complex from control hearts. All of these disease models are associated with decreased ATPase activities of pure myosin and in the case of rat and hamster models, shifts of myosin heavy chain from alpha to beta predominate. In summary, there are three main troponin subunit components which might alter myofibrillar function however, very few direct links of molecular alterations in the regulatory proteins to physiologic and pathologic function have been demonstrated so far.

C2C12 cells: biophysical, biochemical, and immunocytochemical properties

We examined the myofibril biochemical, structural, and biophysical properties of C2C12, a mouse skeletal muscle cell line (American Type Culture Collection), to assess whether force development and the sensitivity of the myofilaments to calcium could be measured in C2C12 myotubes and whether a cardiac contractile protein, troponin T, is expressed and incorporated into C2C12 myofibrils. When myoblasts fused and differentiated into myotubes, expression of myofilament proteins was initiated. Multiple cardiac and skeletal muscle troponin T isoforms were coexpressed. Cardiac troponin T expression increased and then decreased with time. Fluorescence immunocytochemistry demonstrated incorporation of cardiac troponin T isoforms into the myofibrils. At the time of the biophysical studies, mean myotube diameter was 12 microns (range 5-25 microns), and mean length was 290 microns (range 130-520 microns). The estimated maximum force developed by chemically skinned myotubes at 6-7 days poststarvation, 0.88 +/- 0.12 microN (mean +/- 95% confidence interval, n = 5), was significantly less (P < 0.05) than that at 10-13 days poststarvation, 1.12 +/- 0.12 microN (n = 7). The force-pCa relation yielded a Hill coefficient of 2.9 +/- 0.6 (n = 7) and half-maximal activation at pCa of 5.77 +/- 0.20. The demonstration that the biophysical properties of C2C12 cells can be measured and that cardiac and skeletal muscle troponin T isoforms are incorporated and colocalized into myofibrils suggest that these cells could be a useful model to assess the effects of exogenous native and mutated cardiac and skeletal contractile protein isoforms on myofilament function.

Assembly of functional skeletal muscle troponin complex in Escherichia coli

The production of multi-subunit proteins of eukaryotic origin in Escherichia coli usually relies on the different subunits being expressed individually and the protein being reassembled in vitro. Here we describe the construction and characterization of plasmids capable of coexpressing the three subunits of chicken skeletal muscle troponin complex in E. coli. We demonstrate that the troponin subunits assembled in the cytoplasm of E. coli cell are fully functional. The troponin complex was purified to homogeneity in high yields. When reconstituted into actin filaments, the complex assembled in vivo was capable of regulating the myosin ATPase with a calcium dependence that was identical to the complex reconstituted in vitro. These results demonstrate that the coexpression of the subunits of a protein complex can prevent the accumulation of denatured proteins in inclusion granules.

The effect of troponin C substitution on the Ca(2+)-sensitive ATPase activity of vertebrate and invertebrate myofibrils by troponin Cs with various numbers of Ca(2+)-binding sites

The effect of four different classes of troponin C with different numbers of Ca(2+)-binding sites was investigated on the Ca(2+)-activation profiles of the ATPase of troponin C-depleted myofibrils prepared from vertebrate fast skeletal (rabbit), vertebrate cardiac (bovine) and invertebrate crustacean tail striated (crayfish, lobster) muscles. Troponin C from vertebrate sources [fast skeletal (rabbit, chicken) with four Ca(2+)-binding sites, and cardiac (bovine, chicken) with three Ca(2+)-binding sites] activated the Ca(2+)-ATPase of troponin C-depleted myofibrils from the vertebrate skeletal or cardiac muscles, but did not activate the invertebrate troponin C-depleted crustacean myofibrils. On the other hand, two kinds of crustacean (crayfish, lobster) troponin C with two Ca(2+)-binding sites activated only crustacean troponin C-depleted myofibrils. One invertebrate molluscan (Akazara scallop) troponin C with one Ca(2+)-binding site did not activate the Ca(2+)-ATPase of the troponin C-depleted myofibrils from either vertebrate or crustacean striated muscles. The results obtained from the four kinds of combinations of troponin C and troponin C-depleted myofibrils from vertebrate skeletal and cardiac muscles demonstrated that the characteristic cooperativity of the Ca(2+)-activation profiles of both vertebrate skeletal and cardiac myofibrils was determined by the skeletal or cardiac origin of troponin C molecules, irrespective of the animal species, and the Ca(2+)-affinity of the myofibrillar ATPase was related to the skeletal or cardiac origin of both the troponin C and myofibrils. These findings indicated that each of the four classes of troponin C has its own characteristic Ca(2+)-activation profile for each kind of myofibril examined in the present study.

Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T

FPLC has been employed to prepare the phosphorylated and unphosphorylated forms of rabbit striated muscle alpha alpha-tropomyosin (TM), and the major isoform of rabbit fast-skeletal-muscle troponin-T (Tn-T2f) and corresponding chymotryptic fragment T1 (residues 1-158), in order to investigate the effects which these in vivo modifications have on thin filament function. In all instances, no significance could be attributed to the presence of a phosphate moiety on acetyl serine 1 of Tn-T (or fragment T1). As expected, fragment T1 increased the relative viscosities of solutions of unphosphorylated alpha alpha-TM, but this induction was noticeably lower for phosphorylated alpha alpha-TM. In affinity chromatography experiments, fragment T1 bound equally well to either form of alpha alpha-TM, but the interaction between fragment T2 (residues 159-259) and phosphorylated alpha alpha-TM was strengthened relative to the control. In the presence of alpha alpha-TM (unphosphorylated), fragment T1 was found to down regulate the actin-activated myosin-S1 MgATPase activity, indicating that this portion of Tn-T possesses modulatory properties. Under the same conditions, less inhibition was observed with phosphorylated alpha alpha-TM. When the two different forms of alpha alpha-TM were reconstituted into a complete regulatory system, the activation of myosin-S1 was double for those thin filaments containing the phosphorylated molecule. Dephosphorylation of the phospho alpha alpha-TM reduced the rates to control values. In ATPase Ca2+ titrations, these systems exhibited no difference in the co-operativity of activation and little or no difference in the pCa2+ 1/2 value. Developmentally linked changes in the steady-state phosphorylation of alpha alpha-TM could be a mechanism to increase the activating propensity of thin filaments, by modifying the functional properties of the T1 section of Tn-T.

Isolation and characterization of human fast skeletal beta troponin T cDNA: comparative sequence analysis of isoforms and insight into the evolution of members of a multigene family

A cDNA encoding human fast skeletal beta troponin T (beta TnTf) has been isolated and characterized from a fetal skeletal muscle library. The cDNA insert is 1,000 bp in length and contains the entire coding region of 777 bp and 5' and 3' untranslated (UT) segments of 12 and 211 bp, respectively. The 3' UT segment shows the predicted stem-loop structure typical of eukaryotic mRNAs. The cDNA-derived amino acid sequence is the first available sequence for human beta TnTf protein. It is encoded by a single-copy gene that is expressed in a tissue-specific manner in fetal and adult fast skeletal muscles. Although the human beta TnTf represents the major fetal isoform, the sequence information indicates that this cDNA and the coded protein are quite distinct from the fetal and neonatal TnTf isoforms reported in other mammalian fetal muscles. The hydropathy plot indicates that human beta TnTf is highly hydrophilic along its entire length. The protein has an extremely high degree of predicted alpha-helical content involving the entire molecule except the carboxy-terminal 30 residues. Comparative sequence analysis reveals that the human beta TnTf shares a high level of sequence similarity in the coding region with other vertebrate TnTf and considerably reduced similarity with slow skeletal and cardiac TnT cDNAs. The TnT isoforms have a large central region consisting of amino acid residues 46-204 which shows a high sequence conservation both at the nucleotide and amino acid levels. This conserved region is flanked by the variable carboxy-terminal and an extremely variable amino-terminal segment. The tropomyosin-binding peptide of TnT, which is represented by amino acid residues 47-151 and also includes a part of troponin I binding region, is an important domain of this central segment. It is suggested that this conserved segment is encoded by an ancestral gene. The variable regions of vertebrate striated TnT isoforms reflect the subsequent addition and modification of genomic sequences to give rise to members of the TnT multigene family.

Molecular basis of cardiac troponin T isoform heterogeneity in rabbit heart

In the rabbit heart, multiple isoforms of cardiac troponin T (cTnT1 through cTnT5, from largest in size to smallest), a protein essential for calcium-regulated myofibrillar ATPase activity, have been identified, and a correlation has been found between these isoforms and myofilament sensitivity to calcium. We have sought to establish the molecular basis of this diversity. Restriction-digest analysis of genomic DNA has indicated that the rabbit cTnT gene is a single-copy gene. cTnT cDNA clones were isolated from cDNA libraries, yielding a consensus sequence for the protein. Newborn rabbit heart cDNAs, obtained using the reverse-transcriptase polymerase chain reaction (RT-PCR), were amplified using primers derived from this cDNA. Three full-length cDNAs that differed by the inclusion or exclusion of three short nucleotide sequences within the cDNAs were obtained. Amplification in the 5' half of the cDNAs confirmed that multiple cTnT products arose because of the variable inclusion of an 18- and a 30-nt sequence. The 30-nt sequence has homology with previously described alternatively spliced exons in rat and chicken cTnT, whereas the 18-nt sequence has not been described previously. RT-PCR in the 3' half of the cDNAs confirmed an additional region of heterogeneity: the presence, in part or in full, or absence of a 9-nt region, which matches the alternatively spliced exon 12 described for rat cTnT. In vitro transcription and translation of four cDNA clones containing both the 18- and 30-nt sequences, the 30-nt sequence, the 18-nt sequence, or neither generated protein isoforms that comigrated with cTnT1, cTnT2, cTnT3, and cTnT4, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Forced expression and assembly of rat cardiac troponin T isoforms in cultured muscle and nonmuscle cells

Cardiac troponin T (cTnT), a tropomyosin (TM)-binding subunit of the troponin complex, undergoes a developmentally regulated isoform switch from embryonic form to adult form in the rat heart. To investigate the in vivo assembly of cTnT isoforms, we transiently transfected cDNA clones of either rat cTnT isoform into nonmuscle CHO cells and chick embryo myogenic (CEM) cells. As determined by Western blotting, both isoforms can be expressed in CHO and CEM cells. The expressed proteins had the same mobility as native rat cTnT proteins on SDS polyacrylamide gels and were recognized by anti-TnT antibodies. Conventional and confocal microscopy of transfected cells, double-labelled with antibodies against cTnT and against TM, revealed that neither isoform appears to associate with the nonmuscle TM in CHO cells, although both are able to colocalize with muscle TM-containing microfilament bundles in the myogenic CEM cells. There was no appreciable cTnT isoform-related difference in association with TM, suggesting that the functional significance of isoform variability in rat cTnT does not correspond to an assembly advantage for the maturing cardiac thin filament. To help determine whether cTnT nonassembly in CHO environment is primarily due to the nonmuscle nature of the endogenous TM, or if it involves the absence of other factors specific to muscle, we have isolated several stably-transfected clones of skeletal beta TM-expressing CHO cells which incorporate this muscle TM onto stress fibres. When either isoform of cTnT was transiently expressed in these beta TM-CHO cells, the strictly filamentous beta TM staining pattern was no longer observed. Instead, beta TM codistributed with cTnT in brightly staining aggregates not associated with the intact stress fibres. This suggests that both isoforms of cTnT are interacting with the beta TM in the nonmuscle environment and that other muscle-specific proteins may indeed be required for stable assembly of cTnT onto microfilaments. It also suggests that the interaction between cTnT and muscle TM is stronger than that between muscle TM and nonmuscle microfilaments.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Complete nucleotide sequence and structural organization of rat cardiac troponin T gene. A single gene generates embryonic and adult isoforms via developmentally regulated alternative splicing

We have previously demonstrated that rat cardiac troponin T (TnT) is expressed as two different isoforms during development, the larger, more acidic embryonic isoform and the smaller, more basic adult isoform, which appear to be generated from a common transcript of the cardiac TnT gene by alternative RNA splicing. In this study, Southern blot analysis confirmed the existence of a single copy of cardiac TnT gene in the rat genome. For investigation of the molecular mechanism of isoform switch and the control of this gene expression in myocardial development, several overlapping genomic clones were isolated from a rat genomic library. Complete nucleotide sequences were determined from these genomic clones and revealed a 19,186 base-pair DNA fragment containing 16 exons of rat cardiac TnT gene. Its DNA sequence and exon organization appeared to differ from that of the rat fast skeletal muscle TnT gene or chicken cardiac TnT gene. Comparison of genomic and cDNA clones also confirmed that the cardiac TnT isoform switching was due to the inclusion or exclusion of exon 4 during RNA processing. Sequence analysis allowed us to further identify the other alternatively spliced exon containing only nine nucleotides in size (exon 12). The inclusion and complete or partial exclusion of this exon may be responsible for generating three classes of mRNAs detected by our cDNA clones. The functional significance of this variation in TnT isoforms remained unknown, but its splicing pattern did not appear to link to the developmental changes. The 5' upstream structure was very similar to that in chicken cardiac TnT gene but differed from that in the rat fast skeletal muscle TnT gene, suggesting a similar regulatory mechanism for mammalian and avian cardiac TnT expression.

Effect of removal and reconstitution of troponins C and I on the Ca(2+)-activated tension development of single glycerinated rabbit skeletal muscle fibers

The effect of troponin T treatment on the Ca(2+)-activated tension of single glycerinated rabbit skeletal muscle fibers was examined. The tension of the fiber was completely desensitized to Ca2+ by incubation in a solution containing an excessive amount of troponin T and reached a level of about 70% of the maximum tension of the control fiber. SDS/PAGE showed that most of troponins C and I was removed from the fiber by troponin T treatment. During the course of troponin T treatment, the cooperativity of Ca2+ activation (Hill coefficient) was decreased while pCa at half-maximal Ca(2+)-sensitive tension (pK) increased. Using the 26-K fragment of troponin T, the study indicated that the removal of troponins C and I was due to the replacement of the troponin C.I.T complex in the myofibrils of the fiber with the added troponin T. The troponin-T-treated fiber was again sensitized to Ca2+ by the addition of troponin C.I. The removal of troponin C by treatment with trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid did not change the minimum tension of the fiber, from which troponin C.I was partially removed by troponin T treatment, but it decreased the height of maximum tension with a concomitant decrease in the Hill coefficient as well as a decrease in pK. The above findings suggested that pK is determined by the balance between two opposite actions through troponins C and I, while the extent of cooperativity of Ca2+ activation seemed to be related mainly to the content of troponin C.

Alterations in myofibrillar function and protein profiles after complete global ischemia in rat hearts

We studied changes in myofibrillar function and protein profiles after complete global ischemia with anoxia in rat hearts. Hearts were exposed to global ischemia and anoxia (CGI) for 30 or 60 minutes at 37 degrees C, and myofibrils were prepared for measurement of Ca(2+)-dependent Mg(2+)-ATPase activity at pH 7.0 and 6.5. Hearts incubated in cold saline (1 +/- 1 degrees C) and nonincubated hearts served as controls. Maximum ATPase activity was unchanged at pH 7.0 and pH 6.5 in myofibrils from hearts treated with 30 or 60 minutes of CGI. At pH 7.0, the Hill coefficient, which is an index of cooperative interactions among thin-filament proteins, was unchanged after 30 minutes of CGI but was significantly increased after 60 minutes of CGI. A similar trend for increased cooperativity was observed when myofibrillar ATPase activity was measured at pH 6.5 in myofibrils from rat hearts made ischemic for 30 or 60 minutes. Both 30 and 60 minutes of CGI resulted in increased pCa50 values (half-maximally activating free [Ca2+]) at pH 7.0 and pH 6.5. Densitometric analysis of myofibrillar proteins separated with sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that troponin I and troponin T were degraded during 60 minutes of CGI. Two new protein bands appearing in ischemia-treated myofibrils were identified as partially degraded troponin I and troponin T with Western blots. The troponin I fragment could be phosphorylated by cAMP-dependent protein kinase. In addition, we observed phosphorylation of a protein band that corresponded to myosin light chain-2 in myofibrils from CGI-treated hearts. These results suggest that degradation of thin-filament proteins may contribute to the changes in cooperativity of Ca2+ regulation of ATPase activity observed in the myofibrils from rat hearts exposed to CGI.

Troponin T isoform expression in the normal and failing human left ventricle: a correlation with myofibrillar ATPase activity

The expression of troponin T, a thin filament regulatory protein, was examined in normal and failing left ventricles. The samples were obtained from the hearts of patients with severe heart failure who were undergoing cardiac transplantation, and from normal adult hearts that could not be used for transplantation. Western blots of the myofibrillar proteins demonstrated two isoforms, troponin T 1 (TnT1) and troponin T 2 (TnT2). TnT2 is expressed at significantly higher levels in failing hearts (p less than 0.004). Western blots of two-dimension SDS-PAGE gels resolved two dominant spots of TnT1 and of TnT2 and several minor troponin T species. Alkaline phosphatase treatment markedly decreased the sizes of the two acidic spots while increasing the two more basic spots by a comparable amount. Myofibrillar ATPase activity had an inverse and negative linear relationship (r = 0.7, p less than 0.02) with the myofibrillar percentage of total troponin T comprised of TnT2. In that heart failure in these transplant patients had multiple bases, we propose that rather than a cause of heart failure, the disease-associated changes in troponin T isoform expression are an adaptation to abnormal myocardial function.

Responsiveness of the myofilaments to Ca2+ in human heart failure: implications for Ca2+ and force regulation

Myofilament calcium sensitivity and maximal calcium-activated force are fundamental properties of the contractile proteins in the heart. We examined these properties in normal human right-ventricular trabeculae carneae obtained from hearts of brain-dead patients with no known cardiac disease, and from patients with end-stage heart failure undergoing cardiac transplantation. There were no differences in calcium-activation of the control and myopathic muscles from chemically-skinned trabeculae or from intact tetanized preparations. We then tested the effect of DPI 201-106 (4-[3-(4-diphenylmethyl-1-piperazinyl)-2-hydroxypropoxy]-1H-indole - carbonitrile), a new inotropic agent, in both preparations. In myopathic muscles, 1 microM DPI sensitized the myofilaments to Ca2+, as evidenced by a significant shift of the [Ca2+]-force relationship towards lower [Ca2+], in both skinned and intact preparations. On the other hand, the same concentration of DPI did not affect the calcium-activation in control muscles in both preparations. We also found that the twitch [Ca2+]-force relationship, which has been used as an indication of myofilament sensitivity, was dissociated from the steady-state [Ca2+]-force relationship, and was shifted along the [Ca2+] axis by modulation in the time-course of the twitch and [Ca2+]i, and not by the sensitivity of the myofilaments to Ca2+. Protein kinase C stimulation differentially altered the responsiveness of the myofilaments to Ca2+ in normal and myopathic muscle fibers. We propose that even though calcium activation and maximal calcium-activated force are unaltered in myopathic hearts there are changes in thin filament regulation in myopathic hearts that result in altered responses to agents that directly act on the thin filaments, and that the potential for force development is similar in normal and myopathic human hearts.

Replacement of three troponin components with cardiac troponin components within single glycerinated skeletal muscle fibers

The tension of single glycerinated rabbit skeletal muscle fiber was desensitized to a Ca(2+)-concentration after treatment with an excessive amount of bovine cardiac troponin T and reached a level of about 70% of the maximum tension of the untreated fiber. A SDS-gel electrophoretic examination indicated that troponin C.I.T complex in the fiber was replaced with the added cardiac troponin T. The Ca(2+)-sensitivity of the tension of the troponin T-treated fiber was then recovered by the addition of bovine cardiac troponins I and C. The rabbit skeletal muscle fiber thus hybridized with bovine cardiac troponin C.I.T showed the same cooperativity of Ca(2+)-activation as the cardiac muscle.

Force-pCa relation and troponin T isoforms of rabbit myocardium

We have previously reported the existence of at least four troponin T isoforms in rabbit ventricular muscle and described the changes in their distribution with development. In this report we test whether the proportions of the troponin T isoforms are related to the sensitivity of the myofilaments to calcium. We measured the force-pCa relations in 12 detergent-skinned ventricular strands of cardiac muscle from newborn (2-5-day-old) rabbits. We determined from each strand the amount of each troponin T isoform relative to the total amount of troponin T by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometric scans of Western blots probed with a cardiac-specific troponin T monoclonal antibody, MAb 13-11. To assess the presence of different relative amounts of cardiac and slow skeletal troponin I among the strands, we determined the amount of cardiac troponin I relative to tropomyosin. We determined the Hill coefficient and the pCa for half-maximal force, pCa50, for each strand. pCa50 was related directly to the relative amount of troponin T2 (pslope = 0.037). Our results do not indicate a relation between the Hill coefficient and troponin T2. We also did not find a relation between pCa50 and the cardiac troponin I/tropomyosin ratio, which suggests that the correlation between pCa50 and troponin T2 was not a result of changes in the relative amounts of cardiac and slow skeletal muscle troponin I. Our findings indicate that a relation exists between the force-pCa characteristics of rabbit myocardium and the troponin T isoforms that it expresses, suggesting a role for troponin T in modulating the sensitivity of cardiac myofilaments to calcium.

Identification and functional significance of troponin I isoforms in neonatal rat heart myofibrils

We investigated the mechanism(s) responsible for differences in the effects of acidic pH on Ca2+ activation of the activity of adult and neonatal rat heart myofilaments. Studies on preparations of myofilaments reconstituted with adult troponin-tropomyosin (Tn-Tm) and either adult or neonatal thick filaments indicated that the difference in effect of acidic pH is related to differences in Tn-Tm and not other myofilament proteins. Immunoblotting analysis showed that development of the rat heart myofibrils is associated with isoform switching from slow skeletal TnI to cardiac TnI and from a slow mobility isoform of TnT (TnT1) to a faster Mr isoform (TnT2. Expression of slow skeletal TnI was associated with a relative insensitivity of myofilament Ca2+ activation to deactivation by acidic pH. Moreover, the effect of acidic pH on Ca2+ activation of ATPase activity of soleus myofibrils, which contain cardiac TnC and slow skeletal TnI, was essentially the same as the effect of acidic pH on rat cardiac myofibrils in the early neonatal period. Neonatal myofilaments also contained a relative abundance of a set of polypeptides copurifying with the thin filaments. We have identified these proteins as histones. The relative amount of histones among a variety of preparations from different species was not correlated with the pH sensitivity of myofibrillar Ca2+ activation. Shifts in TnT isoforms among these species were also not correlated with an altered response to acidic pH. Our data provide evidence in support of the hypothesis that the relative insensitivity of neonatal myofilament activity to acidic pH is due to the presence of slow skeletal TnI in the thin-filament regulatory complex.

Troponin T isoform expression in humans. A comparison among normal and failing adult heart, fetal heart, and adult and fetal skeletal muscle

The expression of troponin (Tn) T, a thin-filament regulatory protein, was examined in left ventricular myocardium from normal and from failing adult human hearts. The differences in isoform expression between normal and failing myocardium led us to examine the ontogenic expression of TnT in human striated muscle. Left ventricular samples were obtained from patients with severe heart failure undergoing cardiac transplantation and normal adult organ donors. Fetal muscle was obtained from aborted fetuses after 14-15 weeks of gestation, and adult skeletal muscle was obtained from surgical biopsies. Western blots of normal and failing adult heart proteins demonstrated that two isoforms, TnT1 and TnT2, are expressed in different amounts, with TnT2 being significantly greater in failing hearts (p less than 0.004). Western blots of two-dimensional gels of these proteins resolved two predominant spots of both TnT1 and TnT2 and several minor TnT species. Alkaline phosphatase treatment converted the two major spots of each isoform into the single more basic spots. A comparison of the ATPase activities and the TnT2 percentage of total TnT in individual failing and normal adult hearts demonstrated an inverse and negative relation (r = 0.7, p less than 0.02). In the fetal heart, four TnT isoforms were found, two of which had the same electrophoretic mobilities as the adult cardiac isoforms TnT1 and TnT2. Fetal skeletal muscle expressed two of the four fetal cardiac TnT isoforms, one of which comigrated with adult cardiac TnT1. These cardiac isoforms were expressed in low abundance in fetal skeletal muscle relative to seven fast skeletal muscle TnT isoforms. No cardiac isoforms were present in adult skeletal muscle. Because many etiologies caused heart failure in the transplant patients, we propose that the disease-associated increased expression of the TnT isoform TnT2 is an adaptation to the heart failure state and a partial recapitulation of the fetal expression of cardiac TnT isoforms.

Troponin T- and troponin I-like proteins in bovine vascular smooth muscle

We have tested the hypothesis whether proteins with biochemical and immunochemical properties similar to those of troponin T (TnT) and troponin I (TnI) are expressed in bovine vascular smooth muscle (SM). Three monoclonal anti-TnT antibodies (TT-1, TT-2, and RV-C2) specific for the two isoforms of TnT present in the bovine cardiac muscle and two monoclonal antibodies (TI-1 and TI-5) reacting with cardiac TnI were used in this study. Anti-TnT antibodies were found to be unreactive with 1) skeletal and nonmuscle isoforms of glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme that shares some structural homologies with skeletal TnT, and 2) calponin, a TnT-like calmodulin/tropomyosin binding protein with some antigenic properties in common with TnT. When tested on SM extracts from aorta or coronary arteries by Western blotting, the anti-TnT antibodies were able to react exclusively with one or two polypeptides whose electrophoretic mobility corresponds to the cardiac TnT subunits. Similarly, anti-TnI antibodies specifically recognized a component in the aortic or coronary SM extracts with electrophoretic properties identical to the cardiac TnI. Immunofluorescence analysis performed on the vascular SM cells of bovine aorta, coronary arteries, and intramural branches of coronary vessels confirmed the existence of cardiac troponin immunoreactivity in these tissues. In addition, differences in the distribution of cardiac TnT- and TnI-like proteins were evidenced in nonvascular and vascular SM cells. This study shows for the first time that polypeptides with some structural properties in common with cardiac TnT and TnI can be found in the vascular SM system.

Alternative splicing is an efficient mechanism for the generation of protein diversity: contractile protein genes as a model system

Alternative splicing has emerged in recent years as a widespread device for regulating gene expression and generating protein diversity. Its analysis has provided some mechanistic understanding of this form of gene regulation and, in addition, has provided new insights into some fundamental aspects of splicing. This mode of regulation is particularly prevalent in muscle cells, where genes such as troponin T are able to generate up to 64 different isoforms from a single transcriptional unit. Alternative splicing has the potential to raise the coding capacity of the small multigene families that code for the contractile proteins so that several million structurally different sarcomeres can be generated. The mammalian alpha-tropomyosin gene has proved particularly useful for the analysis of the mechanisms involved in this type of regulation. In particular, the mutually exclusive splicing of exons 2 and 3 has provided answers about the processes involved in the three main regulatory steps: (a) establishment of mutually exclusive behavior; (b) the elements involved in setting up the default pattern of splicing, and (c) the switch from the default to the regulated splicing pattern in some cell types.