A myosin activator improves actin assembly and sarcomere function of human-induced pluripotent stem cell-derived cardiomyocytes with a troponin T point mutation.

We have investigated cardiac myocytes derived from human-induced pluripotent stem cells (iPSC-CMs) from two normal control and two family members expressing a mutant cardiac troponin T (cTnT-R173W) linked to dilated cardiomyopathy (DCM). cTnT is a regulatory protein of the sarcomeric thin filament. The loss of this basic charge, which is strategically located to control tension, has consequences leading to progressive DCM. iPSC-CMs serve as a valuable platform for understanding clinically relevant mutations in sarcomeric proteins; however, there are important questions to be addressed with regard to myocyte adaptation that we model here by plating iPSC-CMs on softer substrates (100 kPa) to create a more physiologic environment during recovery and maturation of iPSC-CMs after thawing from cryopreservation. During the first week of culture of the iPSC-CMs, we have determined structural and functional characteristics as well as actin assembly dynamics. Shortening, actin content, and actin assembly dynamics were depressed in CMs from the severely affected mutant at 1 wk of culture, but by 2 wk differences were less apparent. Sarcomeric troponin and myosin isoform composition were fetal/neonatal. Furthermore, the troponin complex, reconstituted with wild-type cTnT or recombinant cTnT-R173W, depressed the entry of cross-bridges into the force-generating state, which can be reversed by the myosin activator omecamtiv mecarbil. Therapeutic doses of this drug increased both contractility and the content of F-actin in the mutant iPSC-CMs. Collectively, our data suggest the use of a myosin activation reagent to restore function within patient-specific iPSC-CMs may aid in understanding and treating this familial DCM.

Mouse model of a familial hypertrophic cardiomyopathy mutation in alpha-tropomyosin manifests cardiac dysfunction.

To investigate the functional consequences of a tropomyosin (TM) mutation associated with familial hypertrophic cardiomyopathy (FHC), we generated transgenic mice that express mutant alpha-TM in the adult heart. The missense mutation, which results in the substitution of asparagine for aspartic acid at amino acid position 175, occurs in a troponin T binding region of TM. S1 nuclease mapping and Western blot analyses demonstrate that increased expression of the alpha-TM 175 transgene in different lines causes a concomitant decrease in levels of endogenous alpha-TM mRNA and protein expression. In vivo physiological analyses show a severe impairment of both contractility and relaxation in hearts of the FHC mice, with a significant change in left ventricular fractional shortening. Myofilaments that contain alpha-TM 175 demonstrate an increased activation of the thin filament through enhanced Ca2+ sensitivity of steady-state force. Histological analyses show patchy areas of mild ventricular myocyte disorganization and hypertrophy, with occasional thrombi formation in the left atria. Thus, the FHC alpha-TM transgenic mouse can serve as a model system for the examination of pathological and physiological alterations imparted through aberrant TM isoforms.

Phosphorylation of troponin I by protein kinase A accelerates relaxation and crossbridge cycle kinetics in mouse ventricular muscle.

Phosphorylation of cardiac myofibrils by cAMP-dependent protein kinase (PKA) can increase the intrinsic rate of myofibrillar relaxation, which may contribute to the shortening of the cardiac twitch during beta-adrenoceptor stimulation. However, it is not known whether the acceleration of myofibrillar relaxation is due to phosphorylation of troponin I (TnI) or of myosin binding protein-C (MyBP-C). To distinguish between these possibilities, we used transgenic mice that overexpress the nonphosphorylatable, slow skeletal isoform of TnI in the myocardium and do not express the normal, phosphorylatable cardiac TNI: The intrinsic rate of relaxation of myofibrils from wild-type and transgenic mice was measured using flash photolysis of diazo-2 to rapidly decrease the [Ca(2+)] within skinned muscles from the mouse ventricles. Incubation with PKA nearly doubled the intrinsic rate of myofibrillar relaxation in muscles from wild-type mice (relaxation half-time fell from approximately 150 to approximately 90 ms at 22 degrees C) but had no effect on the relaxation rate of muscles from the transgenic mice. In parallel studies with intact muscles, we assessed crossbridge kinetics indirectly by determining f(min) (the frequency for minimum dynamic stiffness) during tetanic contractions. Stimulation of beta-adrenoceptors with isoproterenol increased f(min) from 1.9 to 3.1 Hz in muscles from wild-type mice but had no effect on f(min) in muscles from transgenic mice. We conclude that the acceleration of myofibrillar relaxation rate by PKA is due to phosphorylation of TnI, rather than MyBP-C, and that this may be due, at least in part, to faster crossbridge cycle kinetics.

Isolation of cardiac myofibrils and myosin light chains with in vivo levels of light chain phosphorylation.

Conditions are described for the preparation of functional myofibrils and myosin light chains from freeze-clamped beating hearts with the state of light chain phosphorylation chemically 'frozen' during the extraction procedure. Myofibrils were shown to be functionally intact by measurement of Ca2+ binding and ATPase activity. Highly purified cardiac myosin light chains could be routinely isolated from myofibrillar preparations using ethanol fractionation together with ion-exchange chromatography. Analysis of light chains for covalent phosphate indicated that basal levels of phosphorylation of the 18--20 000 dalton light chain of myosin in rabbit hearts beating in situ or in a perfusion apparatus were 0.3--0.4 mol/mol. Covalent phosphate content of the light chain fraction did not change during perfusion of hearts with 10 microM epinephrine.

Effects of ionic strength on calcium binding to rabbit skeletal myofibrils, thin filaments and myosin.

Calcium binding by rabbit skeletal myosin, thin filaments and myofibrils was measured in solutions with and without 2 mM MgATP and with ionic strengths adjusted with KCl to 0.05, 0.10 and 0.14 M. Free Mg2+ was held constant at 1 mM, pH at 7.0 and temperature at 25 degrees C. In the presence of MgATP, the relation between free Ca2+ and myofibrillar bound calcium shifted to the left as ionic strength was decreased from 0.14 to 0.05 M. In the absence of MgAPT, myofibrillar calcium binding was enhanced over a wide range of free Ca2+ concentration, but calcium binding was no longer a function of ionic strength. Similarly, calcium binding by thin filaments and myosin was unaffected by changes in ionic strength from 0.05 to 0.14 M. In view of evidence that cross-bridge connections between thick and thin filaments increase as ionic strength decreases, our results suggest that these connections enhance myofibrillar calcium binding. These results thus confirm previous data of Bremel and Weber (Bremel, R. D. and Weber, A. (1972) Nature New Biol. 238, 97-101) who first showed that nucleotide-free cross-bridge connections enhance thin filament calcium binding.

Vanadate and phosphate ions reduce tension and increase cross-bridge kinetics in chemically skinned heart muscle.

Tension development, immediate stiffness and ATPase of chemically skinned myocardial strips were measured in solutions with varying concentrations of phosphate (Pi) or vanadate (predominantly H2VO4 at pH 7) ion. Vanadate and Pi decreased stiffness in proportion to tension. The results show that, like Pi, vanadate accelerates the turnover rate of cross-bridges, but is effective at about 1/500 the concentration required for the Pi effect. Both Pi and vanadate increased the energy cost of isometric tension maintenance (that is, the ratio of ATPase to tension) and increased the velocity of delayed tension development following quick stretch of the chemically skinned myocardial strips. The results also show that changes in the rate of rise of delayed tension during stretch activation probably reflect changes in the kinetics of the biochemical cycle of the cross-bridges.

Ca2+, calmodulin and cyclic AMP-dependent modulation of actin-myosin interactions in aorta.

Incubation of bovine aortic native actomyosin with cyclic AMP and bovine aortic cyclic AMP-dependent protein kinase produced a rightward shift in the relation between free Ca2+ and both superprecipitation and actomyosin ATPase activity. The relation between free Ca2+ and phosphorylation of myosin light chains was also shifted to the right. The concentration of free Ca2+ required for half-maximal activation of both ATPase activity and myosin light chain phosphorylation was approximately 1.0 microM for control actomyosin and 2.5 microM for actomyosin incubated with cyclic AMP-protein kinase. Neither basal nor maximal activities were significantly affected by incubation with cyclic AMP-protein kinase. Addition of e microM calmodulin to cyclic AMP-protein kinase-treated actomyosin relieved inhibition of both superprecipitation and myosin light chain phosphorylation. These findings suggest that cyclic AMP-protein kinase-mediated inhibition of actin-myosin interactions in vascular smooth muscle involve a shift in the Ca2+ sensitivity of the system. This shift probably involves Ca2+-calmodulin interactions and the control of phosphorylation of the myosin light chains.

Cardiac sarcomeric function, small G-protein signaling, and heart failure.

Molecular signaling that induces cardiac hypertrophy and dilated cardiomyopathy and the transition to decompensation is complex and poorly understood. Extrinsic hemodynamic stresses such as hypertension as well as intrinsic stresses such as genetic defects in sarcomeric proteins and cytoskeletal proteins trigger the process. Both stresses lead to similar outcomes of altered contractility and eventually heart failure. Activation of G-protein coupled receptors initiates cascades of signaling pathways, which promote cardiac hypertrophy by phosphorylation of transcriptional factors and changes in gene expression. Stimulation of these signaling molecules also activates a variety of kinases and phosphatases that induce altered phosphorylation of myofilament proteins. In this review, we focused on these functional effects of small G-protein, Ras and Rho, signaling pathways that reside within the cytoplasm downstream of membrane receptors and upstream of the transcriptional factors. It has been demonstrated that phosphorylation of myofilament proteins alter mechano-energetics of myofilament and contractile function of the heart. Therefore, understanding the role of low molecular weight G-proteins in both cardiac and vascular biology has become particularly important in view of the development of specific inhibitors of effectors of small G-proteins such as p38 MAP kinase and Rho-dependent kinase.

Ischaemia-induced changes in canine cardiac sarcoplasmic reticulum.

Effects of myocardial ischaemia on sarcoplasmic reticulum (SR) of dog hearts were investigated. Regional ischaemia was produced by occlusion of the left circumflex artery, and a microsomal fraction enriched in vesicles of SR was isolated from subendocardium (Endo) and subepicardium (Epi) of control and ischaemic areas of the heart. No significant changes occurred in ischaemic Epi. A loss of in vitro activities (ie calcium transport and ATPase) was found for SR from ischaemic Endo which paralleled the changes in the histology of the tissue. At 5 min of coronary occlusion, Ca2+ binding and Ca2+-ATPase activities of SR from ischaemic Endo were normal. A decrease in the activities of SR was first evident at 15 min after the occlusion, decreased further at about 30 min and remained at that level at 60 min of ischaemia. The maximal rate of Ca2+ uptake did not parallel the Ca2+-binding and Ca2+-ATPase activities. The degree of cAMP-dependent phosphorylation by endogenous and exogenous protein kinase was not different between SR from control and ischaemic areas. A participatory role of SR in the ischaemic impairment of left ventricular systolic and diastolic performance is discussed.

NMR analysis of cardiac troponin C-troponin I complexes: effects of phosphorylation.

Phosphorylation of the cardiac specific amino-terminus of troponin I has been demonstrated to reduce the Ca2+ affinity of the cardiac troponin C regulatory site. Recombinant N-terminal cardiac troponin I proteins, cardiac troponin I(33-80), cardiac troponin I(1-80), cardiac troponin I(1-80)DD and cardiac troponin I(1-80)pp, phosphorylated by protein kinase A, were used to form stable binary complexes with recombinant cardiac troponin C. Cardiac troponin I(1-80)DD, having phosphorylated Ser residues mutated to Asp, provided a stable mimetic of the phosphorylated state. In all complexes, the N-terminal domain of cardiac troponin I primarily makes contact with the C-terminal domain of cardiac troponin C. The nonphosphorylated cardiac specific amino-terminus, cardiac troponin I(1-80), was found to make additional interactions with the N-terminal domain of cardiac troponin C.

The effects of modifiers on enzyme catalysis: a non-classical nearest neighbor approach.

We present a nearest neighbor lattice model of the effects of modifiers on two-state enzyme catalysis of the reaction S in equilibrium with p. We do not in general make the assumptions of the classical approach to cooperative catalysis that yield (1) adsorption isotherms of the same form as those for the corresponding equilibrium system and (2) a rate of the catalyzed reaction proportional to the number of occupied catalytic sites. Closed form results are obtained for two approximations, the Bragg-Williams and the quasi-chemical. The latter requires (1), but is exact for several simple cases, including the concerted model, under this condition. Under (1) it is found that an interaction between modifier and catalytic sites, whether attractive or repulsive, increases the magnitudes of the slopes of the adsorption isotherms but that interactions between identical sites (catalytic or modifier) increase these magnitudes if attractive and decrease them if repulsive. Thus, the former interaction allows for phase transitions if sufficiently attractive or repulsive, but the latter only if sufficiently attractive. Herein also lies the explanation for why the concerted model displays only "positive cooperativity". It is further seen that it is not possible to classify a modifier as an activator or inhibitor of the catalyzed reaction solely on the basis of the sign of the interaction energy between catalytic and modifier sites. For a given energy, the rate of the reaction may increase or decrease in response to the modifier, or it may respond biphasically. Similarly, the rate may respond biphasically to the activities of s or p, leading to instabilities. Thus, possibilities of multiple nonequilibrium stationary states or spatio-temporal patterns are raised.

Regulation of Ca(2+)-signaling in cardiac myofilaments.

The contraction and relaxation of heart muscle cells is associated with a transient change in the intracellular level of free Ca2+. This transient reflects a release from the sarcoplasmic reticulum of Ca2+ ions that turn on the reaction of myosin cross-bridges of the thick filaments with actin of the thin filaments. Without this transient no contraction occurs. The first part of this paper is concerned with the molecular processes by which Ca2+ signals the actin-myosin reaction. It will show that the activation in reality involves release of the thin filaments from a prevailing inhibited state. The inhibited state depends on the activity of the troponin (TN) complex and tropomyosin (TM); disinhibition requires Ca(2+)-binding to TN. The second part of the paper describes evidence that the signaling process is itself modulated by the mechanical and chemical state of the myofilaments in the short term and by altered gene expression in the long term.

Troponin C - troponin I interactions and molecular signalling in cardiac myofilaments.

This chapter describes a current perception of the molecular interactions regulating myofilament activity in heart cells. The focus is on the interaction between troponin-C (TnC), the Ca(2+)-receptor and troponin I (TnI), an inhibitory protein. It is this interaction that appears to form a molecular switch that turns on the thin filament. It will be seen that control of the actin-myosin reaction is not only through Ca(2+)-binding to TnC, but also through steric, cooperative and allosteric processes involving all of the main myofilament proteins-actin, myosin, tropomyosin (Tm), troponin T (TnT), TnC, and TnI. The process is modulated by covalent and non-covalent mechanisms. The process is altered in diverse myopathies and pathologies of the heart and is a target for pharmacological manipulation by a new class of inotropic agents, the "Ca(2+)-sensitizers".

Phosphate and vanadate reduce the efficiency of the chemo-mechanical energy transformation in cardiac muscle.

Trabecular preparations from the hog heart right ventricle were "skinned" by treatment with Lubrol WX and glycerol. Ca++ activated isometric contractions were gradedly relaxed by inorganic phosphate (Pi) in the millimolar range or vanadate (Vi) in the micromolar range while tension cost (ATP split/force generated) was increased by a factor of 1.75. From measurements of force, ATPase activity, immediate stiffness and stretch activation, evidence is provided that the mechanical deactivation and the increase in tension cost may result from an acceleration of the myosin cross-bridge cycle, due to a direct interference of Pi and Vi with the chemomechanical energy transformation at the contractile proteins. The possible significance of such a mechanism in cardiac failure or muscle fatigue is discussed.

A novel and personalized rehabilitation program for obese kidney transplant recipients.

INTRODUCTION: Physical rehabilitation programs for kidney transplant recipients are not routinely personalized to patients' physical and emotional health, which could result in a potentially limited health impact, shorter-term participation, and an overall low success rate. MATERIALS AND METHODS: We conducted an internal review board-approved randomized prospective study involving a 12-month supervised multidisciplinary rehabilitation program (GH method) initiated after kidney transplantation in obese recipients (body mass index >30). The new method incorporates 3 major components: physical exercise, behavioral interventions, and nutritional guidance. We compared 9 patients who underwent supervised rehabilitation with 8 patients who underwent standard care. Patients were followed up after the start of the intervention, and multiple assessments were performed. RESULTS: The adherence to training and follow-up was 100% in the intervention group, compared with 25% at 12 months in the control group. There was a trend for a higher glomerular filtration rate in the intervention group compared with the control group (55.5 ± 18.6 mL/min/1.73 m(2) vs 38.8 ± 18.9 mL/min/1.73 m(2), P = .06). The quality of life (SF-36) mean score improved more in the intervention group compared with the control group (583 ± 13 vs 436 ± 22, P = .008). There was a significantly higher employment rate in the intervention group, 77.7% at 12 months compared with 12.5% in the control group (P = .02). CONCLUSIONS: Our preliminary results suggest that this comprehensive approach to physical rehabilitation can improve adherence, kidney function, quality of life, and employment rate for obese patients after kidney transplantation.