Ca2+ activation of troponin C-extracted vertebrate striated fast-twitch muscle fibers

Phenotypical transitions and Ca2+activation properties in human muscle fibers: effects of a 60-day bed rest and countermeasures

Muscle biopsies were taken from soleus and vastus lateralis before and after a 60-day bed rest (BR) to examine expression changes in the regulatory proteins of the thin filament and in contractile function. Twenty-four women separated in three groups were submitted to BR or a combined protocol of resistance and aerobic exercises during BR or received a supplementation of amino acids during BR. Ca2+-tension relationships were established in single skinned fibers identified by their myosin heavy chain and troponin C isoform expressions. Expression patterns of regulatory proteins were analyzed on muscle pieces. For both muscles, BR produced similar decreases in slow and fast fiber diameters but larger decreases in P0maximal forces in slow than in fast fibers. Specific forces were decreased in slow soleus and vastus fibers, which displayed a reduction in Ca2+affinity. These changes were accompanied by slow-to-fast transitions in regulatory proteins, with troponins C and T appearing as sensitive markers of unloading. Exercises prevented the changes in fiber diameters and forces and counteracted most of the slow-to-fast transitions. The nutrition program had a morphological beneficial effect on slow fibers. However, these fibers still presented decreases in specific P0after BR. Phenotypical transitions due to BR were not prevented by amino acids. Finally, in vastus lateralis muscle, BR induced a decrease in O-glycosylation level that was prevented by exercise and attenuated by nutrition. In conclusion, this study has addressed for the first time in women the respective efficiencies of two countermeasures associated with BR on muscle properties and regulatory protein expression.

Thin filament activation probed by fluorescence of N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole-labeled troponin I incorporated into skinned fibers of rabbit psoas muscle

A method is described for the exchange of native troponin of single rabbit psoas muscle fibers for externally applied troponin complexes without detectable impairment of functional properties of the skinned fibers. This approach is used to exchange native troponin for rabbit skeletal troponin with a fluorescent label (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1, 3-diazole, IANBD) on Cys(133) of the troponin I subunit. IANBD-labeled troponin I has previously been used in solution studies as an indicator for the state of activation of reconstituted actin filaments (. Proc. Natl. Acad. Sci. USA. 77:7209-7213). In the skinned fibers, the fluorescence of this probe is unaffected when cross-bridges in their weak binding states attach to actin filaments but decreases either upon the addition of Ca(2+) or when cross-bridges in their strong binding states attach to actin. Maximum reduction is observed when Ca(2+) is raised to saturating concentrations. Additional attachment of cross-bridges in strong binding states gives no further reduction of fluorescence. Attachment of cross-bridges in strong binding states alone (low Ca(2+) concentration) gives only about half of the maximum reduction seen with the addition of calcium. This illustrates that fluorescence of IANBD-labeled troponin I can be used to evaluate thin filament activation, as previously introduced for solution studies. In addition, at nonsaturating Ca(2+) concentrations IANBD fluorescence can be used for straightforward classification of states of the myosin head as weak binding (nonactivating) and strong binding (activating), irrespective of ionic strength or other experimental conditions. Furthermore, the approach presented here not only can be used as a means of exchanging native skeletal troponin and its subunits for a variety of fluorescently labeled or mutant troponin subunits, but also allows the exchange of native skeletal troponin for cardiac troponin.

Ca2+ regulation of mechanical properties of striated muscle. Mechanistic studies using extraction and replacement of regulatory proteins

Extraction of regulatory proteins from thick and thin filaments of vertebrate striated muscle has proven to be an important approach in elucidating roles of these proteins in regulating contraction and in probing specific mechanisms of activation. For some proteins, such as LC2 and C protein, extraction has been fundamental in demonstrating the importance of these proteins in modulating contraction and the kinetics of cross-bridge interaction. For other proteins, such as TnC and troponin, extraction has provided significant insight into the importance of thin-filament intermolecular cooperativity in modulating Ca2+ sensitivity of the contractile process. A combination of extraction and readdition has provided a means of introducing mutated or derivatized proteins into fibers to accomplish a variety of experimental objectives. The use of this approach is likely to grow with the need to test the functional consequences of site-specific mutations as part of studies directed to mechanisms of regulation or altered regulation in heart and skeletal muscles under normal and pathophysiological conditions. Such studies are likely to include extraction in combination with other probes of function such as flash photolysis of reaction substrates or products within the cross-bridge interaction cycle. Although extraction is a powerful approach and is likely to be extended to proteins not discussed in this review, an essential element of experimental design in studies such as these is that appropriate control experiments be done to verify that observed effects of the extraction protocol are specifically attributable to the protein that is removed.

The role of troponin C in the length dependence of Ca(2+)-sensitive force of mammalian skeletal and cardiac muscles

1. Skinned fibre preparations of right ventricular trabeculae, psoas and soleus muscles from hamster and rabbit were activated by Ca2+ and the length dependencies of their pCa (-log [Ca2+])-force relationships were compared. 2. Ca2+ sensitivity of the myocardium was higher at 2.2-2.4 microns than that at 1.7-1.9 microns. The length dependence was at least twofold greater in cardiac muscle than in fast skeletal fibres at identical temperatures and salt concentrations. Slow-twitch fibres gave a response similar to that in the myocardium. 3. The effect of the troponin C (TnC) phenotype on the length dependence of Ca2+ sensitivity was measured on both fast skeletal fibres and cardiac muscle with TnC exchange in situ. The length-induced increase in Ca2+ sensitivity was found to be greater in the presence of cardiac TnC than with fast skeletal TnC. Thus the results indicate that a certain domain of TnC is specialized in this length function, and that this domain is different in the two phenotypes. 4. The possibility that the enhanced length dependence of Ca2+ sensitivity after cardiac TnC reconstitution was attributable to reduced TnC binding was excluded when the length dependence of partially extracted fast fibres was reduced to one-half the normal value after a 50% deletion of the native TnC. 5. Two recombinant forms of cardiac TnC (kindly provided by Dr John Putkey, Houston, TX, USA) were used next, to investigate the roles of two specific domains in TnC in the control of length dependence of Ca2+ sensitivity and in the contraction-relaxation switching of cardiac muscle: 6. Using mutant CBM1 [corrected], in which site 1 was modified such as to bind the 4th Ca2+ ion, as in skeletal TnC, the length-induced Ca2+ sensitivity in cardiac muscle was suppressed. The effect was intermediate between cardiac and skeletal TnCs under the same conditions. The pSr (-log [Sr2+])-force relationship of cardiac muscle was also measured. In the presence of the mutant, skinned trabeculae manifest pSr-activation curves identical to those of fast fibres. This indicates that the metal ion binding properties of site 1 in TnC modulate the regulatory action of site 2. 7. Using mutant CBM2A, in which site 2 was inactivated, the activation of cardiac muscle by both Ca2+ and Sr2+ ions was completely blocked. This is the expected result, since both regulatory sites were now inactive, regulatory site 1 being normally inactive in cardiac muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Co-operative activation of skeletal muscle thin filaments by rigor crossbridges. The effect of troponin C extraction

When Ca2+ binds to troponin C (TnC), all 26 troponin-tropomyosin (Tn-Tm) complexes of a regulatory strand change in concert from the inactive to the active configuration. To see if the complexes respond similarly when they are activated by rigor crossbridges in the absence of Ca2+, we determined the slope (ns) of the bell-shaped pS/tension (pS = -log [MgATP], where S = MgATP2-) relationship between pS 5, where the tension is maximal, and pS 2.3, where fibers are fully relaxed. In control skinned rabbit psoas fibers the ns value is greater than 4; it progressively decreases with TnC extraction. This decrease in ns with TnC extraction is analogous to the decrease in the slope (Hill coefficient) of the pCa/tension (pCa = -log [Ca2+]) relationship with extraction. Complete TnC extraction reduces the maximum substrate-induced tension by only 25%; in contrast, it reduces the maximum Ca2+ induced tension to zero. The effects of TnC extraction on the slope of the pS/tension curve are explained by the assumptions that (1) extracted Tn-Tm complexes no longer change in concert with their neighbors but change independently of them, and (2) co-operative signals cannot cross extracted Tn-Tm complexes. The ns value, therefore, like the nH, is a direct function of the number of contiguous, intact, Tn-Tm complexes in a stretch of a regulatory strand. To describe qualitatively the bi-phasic pS/tension relationship, the mono-phasic pCa/tension relationship, and the effects of TnC extraction on them, we introduce a version of the concerted-transition formalism which includes two activating ligands, Ca2+ and rigor crossbridges.

Modification of temperature dependence of myofilament Ca sensitivity by troponin C replacement

The Ca sensitivity of chemically skinned right ventricular trabeculae from the rat heart was determined at 22 and 8 degrees C. Endogenous troponin C (TnC) was then extracted with EDTA and replaced with either bovine cardiac TnC or rabbit fast-twitch skeletal TnC. The temperature dependence of myofilament Ca sensitivity was then reevaluated. Cooling native cardiac tissue from 22 to 8 degrees C reduced the pCa (-log10 [Ca2+]), generating half-maximal tension (K1/2) from 5.20 +/- 0.07 to 4.89 +/- 0.08 (SD, n = 14), and also reduced maximum Ca-activated force to 33 +/- 6% of its value at 22 degrees C. After extraction of endogenous TnC and reconstitution with cardiac TnC, cooling from 22 to 8 degrees C caused a similar shift in mean K1/2 from 4.93 +/- 0.08 to 4.69 +/- 0.06 (n = 7). When skeletal TnC was reconstituted into TnC-extracted ventricular fibers, cooling from 22 to 8 degrees C led to a much smaller mean shift in K1/2 from 4.88 +/- 0.07 to 4.78 +/- 0.04 (n = 7). The results show that the magnitude of the cooling-induced shift in myofilament Ca sensitivity observed in the native state (or after reconstitution with cardiac TnC) is significantly reduced if the fiber is reconstituted with skeletal TnC (P less than 0.001). This indicates that the temperature dependence of myofilament Ca sensitivity of cardiac muscle can be modified by incorporation of skeletal TnC. Thus Ca binding to TnC plays an important role in determining the temperature dependence of myofilament Ca sensitivity.

Characterization of the Ca2+-switch in skeletal and cardiac muscles

To determine the significance of the global structure of the regulatory proteins in the mechanism of the Ca2+-switch in cardiac and skeletal muscle contractions, the properties of a family of Ca2+-binding proteins with 4 or 3 EF-hand motifs have been studied with desensitized skinned fiber preparations. Proteins with 4 EF hands (such as troponins C - TnCs) are dumb-bell shaped, those with 3 EF hands (parvalbumin) being ellipsoidal. The number of active sites varied between four and two. We find that the ability to anchor in the fiber is limited to proteins with 4 EF hands and, at least, two active Ca2+-binding sites, one each in the N- and C-termini. The results suggest that the dumb-bell shaped global structure is critical for the switching action in muscular contraction, and a trigger site in the N-terminus and a structural site in the C-terminus need to be active in order to regulate contractility.

Down-regulation of fast-twitch skeletal muscle fiber with cardiac troponin-C and recombinant mutants. Structure/function studies with site-directed mutagenesis

Structure/function relationships in troponin C are studied with vertebrate fast-twitch fibers by exchanging the skeletal troponin C with two bacterially synthesized recombinant proteins designed by site-directed mutagenesis of cardiac troponin C. One mutant (CBM1) contained an additional active site, by deleting Val-28 and converting Leu-29, Gly-30, Ala-31 and Glu-32 to Asp, Ala, Asp and Gly, respectively, in the normally inactive trigger site 1 in the N-terminus. In another mutant (CBM2A), the normally active site 2 was inactivated by conversion of Asp-65 to Ala. The fibers were found to be down-regulated with recombinant cardiac troponin C (CTnC3), as with tissue-cardiac-troponin-C. With mutants, in one case (CBM1) the regulation was unmodified despite Ca2+ coordination by all sites. In contrast, regulation was found to be completely blocked with the mutant (CBM2A) where both trigger sites were inactive. The results provide the first indication that structural specification of the entire EF-hand motif of site 1, and not just Ca2+ coordination, is needed to operate fully the Ca2+ switch in fast-twitch fibers.

Molecular basis for the influence of muscle length on myocardial performance

According to Starling's law of the heart, the force of contraction during the ejection of blood is a function of the end-diastolic volume. To seek the molecular explanation of this effect, a study was made of the effects of length on Ca2+ sensitivity during tension development by isolated demembranated cardiac muscle in which the cardiac form of troponin C was substituted with skeletal troponin C. The results of troponin C exchange were compared at sarcomere lengths of 1.9 and 2.4 micrometers. Enhancement of the myocardial performance at the stretched length was greatly suppressed with the skeletal troponin C compared with the cardiac troponin C. Thus the troponin C subunit of the troponin complex that regulates the activation of actin filaments has intrinsic molecular properties that influence the length-induced autoregulation of myocardial performance and may be a basis for Starling's law of the heart.

Evidence for novel 30,000-50,000Mr cofactor in the activation of muscle

A new approach is described for reconstituting a fully desensitized skeletal muscle fiber to restore its contractility. These studies revealed a novel regulatory cofactor, 30-50,000Mr by filtration (26-55kDa by SDS PAGE). It was shown to be critical for the Ca2+-activation in the physiological milieu. The cofactor was present in skeletal and cardiac muscles as well as in brain, but not in kidney and liver. The cofactor may be a second Ca2+ switch in a dual-regulation scheme for vertebrate muscle, or could provide an essential link in the cross-bridge cycle beyond activation.