Increased cross-bridge cycling rate in stunned myocardium

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

Myofilament dysfunction contributes to impaired myocardial contraction in the infarct border zone

After myocardial infarction, a poorly contracting nonischemic border zone forms adjacent to the infarct. The cause of border zone dysfunction is unclear. The goal of this study was to determine the myofilament mechanisms involved in postinfarction border zone dysfunction. Two weeks after anteroapical infarction of sheep hearts, we studied in vitro isometric and isotonic contractions of demembranated myocardium from the infarct border zone and a zone remote from the infarct. Maximal force development (Fmax) of the border zone myocardium was reduced by 31 ± 2% versus the remote zone myocardium (n = 6/group, P < 0.0001). Decreased border zone Fmax was not due to a reduced content of contractile material, as assessed histologically, and from myosin content. Furthermore, decreased border zone Fmax did not involve altered cross-bridge kinetics, as assessed by muscle shortening velocity and force development kinetics. Decreased border zone Fmax was associated with decreased cross-bridge formation, as assessed from muscle stiffness in the absence of ATP where cross-bridge formation should be maximized (rigor stiffness was reduced 34 ± 6%, n = 5, P = 0.011 vs. the remote zone). Furthermore, the border zone myocardium had significantly reduced phosphorylation of myosin essential light chain (ELC; 41 ± 10%, n = 4, P < 0.05). However, for animals treated with doxycycline, an inhibitor of matrix metalloproteinases, rigor stiffness and ELC phosphorylation were not reduced in the border zone myocardium, suggesting that doxycycline had a protective effect. In conclusion, myofilament dysfunction contributes to postinfarction border zone dysfunction, myofilament dysfunction involves impaired cross-bridge formation and decreased ELC phosphorylation, and matrix metalloproteinase inhibition may be beneficial for limiting postinfarct border zone dysfunction.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

The C terminus of cardiac troponin I stabilizes the Ca2+-activated state of tropomyosin on actin filaments

RATIONALE

Ca(2+) control of troponin-tropomyosin position on actin regulates cardiac muscle contraction. The inhibitory subunit of troponin, cardiac troponin (cTn)I is primarily responsible for maintaining a tropomyosin conformation that prevents crossbridge cycling. Despite extensive characterization of cTnI, the precise role of its C-terminal domain (residues 193 to 210) is unclear. Mutations within this region are associated with restrictive cardiomyopathy, and C-terminal deletion of cTnI, in some species, has been associated with myocardial stunning.

OBJECTIVE

We sought to investigate the effect of a cTnI deletion-removal of 17 amino acids from the C terminus- on the structure of troponin-regulated tropomyosin bound to actin.

METHODS AND RESULTS

A truncated form of human cTnI (cTnI(1-192)) was expressed and reconstituted with troponin C and troponin T to form a mutant troponin. Using electron microscopy and 3D image reconstruction, we show that the mutant troponin perturbs the positional equilibrium dynamics of tropomyosin in the presence of Ca(2+). Specifically, it biases tropomyosin position toward an "enhanced C-state" that exposes more of the myosin-binding site on actin than found with wild-type troponin.

CONCLUSIONS

In addition to its well-established role of promoting the so-called "blocked-state" or "B-state," cTnI participates in proper stabilization of tropomyosin in the "Ca(2+)-activated state" or "C-state." The last 17 amino acids perform this stabilizing role. The data are consistent with a "fly-casting" model in which the mobile C terminus of cTnI ensures proper conformational switching of troponin-tropomyosin. Loss of actin-sensing function within this domain, by pathological proteolysis or cardiomyopathic mutation, may be sufficient to perturb tropomyosin conformation.

Increased cross-bridge cycling kinetics after exchange of C-terminal truncated troponin I in skinned rat cardiac muscle

The precise mechanism of cardiac troponin I (cTnI) proteolysis in myocardial stunning is not fully understood. Accordingly, we determined the effect of cTnI C terminus truncation on chemo-mechanical transduction in isolated skinned rat trabeculae. Recombinant troponin complex (cTn), containing either mouse cTnI-(1-193) or human cTnI-(1-192) was exchanged into skinned cardiac trabeculae; Western blot analysis confirmed that 60-70% of the endogenous cTn was replaced by recombinant Tn. Incorporation of truncated cTnI induced significant reductions ( approximately 50%) in maximum force and cooperative activation as well as increases ( approximately 50%) in myofilament Ca(2+) sensitivity and tension cost. Similar results were obtained with either mouse or human truncated cTn. Presence of truncated cTnI increased maximum actin-activated S1 ATPase activity as well as its Ca(2+) sensitivity in vitro. Partial exchange (50%) for truncated cTnI resulted in similar reductions in maximum force and cooperativity; tension cost was increased in proportion to truncated cTnI content. In vitro, to determine the molecular mechanism responsible for the enhanced myofilament Ca(2+) sensitivity, we measured Ca(2+) binding to cTn as reported using a fluorescent probe. Incorporation of truncated cTnI did not affect Ca(2+) binding affinity to cTn alone. However, when cTn was incorporated into thin filaments, cTnI truncation induced a significant increase in Ca(2+) binding affinity to cTn. We conclude that cTnI truncation induces depressed myofilament function. Decreased cardiac function after ischemia/reperfusion injury may directly result, in part, from proteolytic degradation of cTnI, resulting in alterations in cross-bridge cycling kinetics.

Doppler strain imaging closely reflects myocardial energetic status in acute progressive ischemia and indicates energetic recovery after reperfusion

BACKGROUND

Capitalizing on mechanoenergetic coupling, we investigated whether strain echocardiography can noninvasively estimate the ratio of adenosine triphosphate (ATP) to adenosine diphosphate (ADP), a marker of energetic status during acute myocardial ischemia and reperfusion.

METHODS

Twenty-eight pigs were divided into 7 groups (1 baseline, 4 ischemic, and 2 reperfusion). Ischemia was induced by left anterior descending coronary artery occlusion. Longitudinal systolic lengthening (SL) and postsystolic shortening (PSS) strain were measured by echocardiography. The ATP/ADP ratio was obtained from myocardial biopsies in the ischemic and control regions.

RESULTS

SL and PSS strain and the ATP/ADP ratio progressively decreased (P < .05) with increased duration (12, 40, 120, and 200 minutes) of ischemia. A mathematical formula (ATP/ADP = -0.97 + 0.25 x PSS strain + 0.20 x SL strain) estimated best the ATP/ADP ratio (r = 0.94, P < .05). Reperfusion after 12 but not after 120 minutes of ischemia significantly improved the ATP/ADP ratio and decreased SL and PSS strain.

CONCLUSIONS

Strain echocardiography closely reflected changes and enabled the noninvasive estimation of the ATP/ADP ratio. A higher ATP/ADP ratio is associated with functional improvement after reperfusion.

Impaired Diastolic Function After Exchange of Endogenous Troponin I With C-Terminal Truncated Troponin I in Human Cardiac Muscle

The specific and selective proteolysis of cardiac troponin I (cTnI) has been proposed to play a key role in human ischemic myocardial disease, including stunning and acute pressure overload. In this study, the functional implications of cTnI proteolysis were investigated in human cardiac tissue for the first time. The predominant human cTnI degradation product (cTnI 1–192 ) and full-length cTnI were expressed in Escherichia coli , purified, reconstituted with the other cardiac troponin subunits, troponin T and C, and subsequently exchanged into human cardiac myofibrils and permeabilized cardiomyocytes isolated from healthy donor hearts. Maximal isometric force and kinetic parameters were measured in myofibrils, using rapid solution switching, whereas force development was measured in single cardiomyocytes at various calcium concentrations, at sarcomere lengths of 1.9 and 2.2 μm, and after treatment with the catalytic subunit of protein kinase A (PKA) to mimic β-adrenergic stimulation. One-dimensional gel electrophoresis, Western immunoblotting, and 3D imaging revealed that approximately 50% of endogenous cTnI had been homogeneously replaced by cTnI 1–192 in both myofibrils and cardiomyocytes. Maximal tension was not affected, whereas the rates of force activation and redevelopment as well as relaxation kinetics were slowed down. Ca 2+ sensitivity of the contractile apparatus was increased in preparations containing cTnI 1–192 (pCa 50 : 5.73±0.03 versus 5.52±0.03 for cTnI 1–192 and full-length cTnI, respectively). The sarcomere length dependency of force development and the desensitizing effect of PKA were preserved in cTnI 1–192 -exchanged cardiomyocytes. These results indicate that degradation of cTnI in human myocardium may impair diastolic function, whereas systolic function is largely preserved.