Cardiac function and modulation of sarcomeric function by length

Functional control of myosin motors in the cardiac cycle

Contraction of the heart is driven by cyclical interactions between myosin and actin filaments powered by ATP hydrolysis. The modular structure of heart muscle and the organ-level synchrony of the heartbeat ensure tight reciprocal coupling between this myosin ATPase cycle and the macroscopic cardiac cycle. The myosin motors respond to the cyclical activation of the actin and myosin filaments to drive the pressure changes that control the inflow and outflow valves of the heart chambers. Opening and closing of the valves in turn switches the myosin motors between roughly isometric and roughly isotonic contraction modes. Peak filament stress in the heart is much smaller than in fully activated skeletal muscle, although the myosin filaments in the two muscle types have the same number of myosin motors. Calculations indicate that only ~5% of the myosin motors in the heart are needed to generate peak systolic pressure, although many more motors are needed to drive ejection. Tight regulation of the number of active motors is essential for the efficient functioning of the healthy heart - this control is commonly disrupted by gene variants associated with inherited heart disease, and its restoration might be a useful end point in the development of novel therapies.

Cooperative mechanisms underlie differences in myocardial contractile dynamics between large and small mammals

Ca2+ binding to troponin C (TnC) and myosin cross-bridge binding to actin act in a synergistic cooperative manner to modulate myocardial contraction and relaxation. The responsiveness of the myocardial thin filament to the activating effects of Ca2+ and myosin cross-bridge binding has been well-characterized in small mammals (e.g., mice). Given the nearly 10-fold difference in resting heart rates and twitch kinetics between small and large mammals, it is unlikely that the cooperative mechanisms underlying thin filament activation are identical in these two species. To test this idea, we measured the Ca2+ dependencies of steady-state force and the rate constant of force redevelopment (ktr) in murine and porcine permeabilized ventricular myocardium. While murine myocardium exhibited a steep activation-dependence of ktr, the activation-dependent profile of ktr was significantly reduced in porcine ventricular myocardium. Further insight was attained by examining force-pCa and ktr-pCa relationships. In the murine myocardium, the pCa50 for ktr was right-shifted compared with the pCa50 for force, meaning that increases in steady-state force occurred well before increases in the rate of force redevelopment were observed. In the porcine myocardium, we observed a tighter coupling of the force-pCa and ktr-pCa relationships, as evidenced by near-maximal rates of force redevelopment at low levels of Ca2+ activation. These results demonstrate that the molecular mechanisms underlying the cooperative activation of force are a dynamic property of the mammalian heart, involving, at least in part, the species- and tissue-specific expression of cardiac myosin heavy chain isoforms.

A highly versatile biopolymer-based platform for the maturation of human pluripotent stem cell-derived cardiomyocytes enables functional analysis in vitro and 3D printing of heart patches

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) represent a valuable tool for in vitro modeling of the cardiac niche and possess great potential in tissue engineering applications. However, conventional polystyrene-based cell culture substrates have adverse effects on cardiomyocytes in vitro due to the stress applied by a stiff substrate on contractile cells. Ultra-high viscosity alginates offer a unique versatility as tunable substrates for cardiac cell cultures due to their biocompatibility, flexible biofunctionalization, and stability. In this work, we analyzed the effect of alginate substrates on hPSC-CM maturity and functionality. Alginate substrates in high-throughput compatible culture formats fostered a more mature gene expression and enabled the simultaneous assessment of chronotropic and inotropic effects upon beta-adrenergic stimulation. Furthermore, we produced 3D-printed alginate scaffolds with differing mechanical properties and plated hPSC-CMs on the surface of these to create Heart Patches for tissue engineering applications. These exhibited synchronous macro-contractions in concert with more mature gene expression patterns and extensive intracellular alignment of sarcomeric structures. In conclusion, the combination of biofunctionalized alginates and human cardiomyocytes represents a valuable tool for both in vitro modeling and regenerative medicine, due to its beneficial effects on cardiomyocyte physiology, the possibility to analyze cardiac contractility, and its applicability as Heart Patches.

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Revealing the nanometric structural changes in myocardial infarction models by time-lapse intravital imaging

In the development of bioinspired nanomaterials for therapeutic applications, it is very important to validate the design of nanomaterials in the disease models. Therefore, it is desirable to visualize the change of the cells in the diseased site at the nanoscale. Heart diseases often start with structural, morphological, and functional alterations of cardiomyocyte components at the subcellular level. Here, we developed straightforward technique for long-term real-time intravital imaging of contracting hearts without the need of cardiac pacing and complex post processing images to understand the subcellular structural and dynamic changes in the myocardial infarction model. A two-photon microscope synchronized with electrocardiogram signals was used for long-term in vivo imaging of a contracting heart with subcellular resolution. We found that the structural and dynamic behaviors of organelles in cardiomyocytes closely correlated with heart function. In the myocardial infarction model, sarcomere shortening decreased from ∼15% (healthy) to ∼8% (diseased) as a result of impaired cardiac function, whereas the distances between sarcomeres increased by 100 nm (from 2.11 to 2.21 μm) in the diastolic state. In addition, T-tubule system regularity analysis revealed that T-tubule structures that were initially highly organized underwent significant remodeling. Morphological remodeling and changes in dynamic activity at the subcellular level are essential to maintain heart function after infarction in a heart disease model.

Frank-Starling mechanism, fluid responsiveness, and length-dependent activation: Unravelling the multiscale behaviors with an in silico analysis

The Frank-Starling mechanism is a fundamental regulatory property which underlies the cardiac output adaptation to venous filling. Length-dependent activation is generally assumed to be the cellular origin of this mechanism. At the heart scale, it is commonly admitted that an increase in preload (ventricular filling) leads to an increased cellular force and an increased volume of ejected blood. This explanation also forms the basis for vascular filling therapy. It is actually difficult to unravel the exact nature of the relationship between length-dependent activation and the Frank-Starling mechanism, as three different scales (cellular, ventricular and cardiovascular) are involved. Mathematical models are powerful tools to overcome these limitations. In this study, we use a multiscale model of the cardiovascular system to untangle the three concepts (length-dependent activation, Frank-Starling, and vascular filling). We first show that length-dependent activation is required to observe both the Frank-Starling mechanism and a positive response to high vascular fillings. Our results reveal a dynamical length dependent activation-driven response to changes in preload, which involves interactions between the cellular, ventricular and cardiovascular levels and thus highlights fundamentally multiscale behaviors. We show however that the cellular force increase is not enough to explain the cardiac response to rapid changes in preload. We also show that the absence of fluid responsiveness is not related to a saturating Frank-Starling effect. As it is challenging to study those multiscale phenomena experimentally, this computational approach contributes to a more comprehensive knowledge of the sophisticated length-dependent properties of cardiac muscle.

The regulatory light chain mediates inactivation of myosin motors during active shortening of cardiac muscle

The normal function of heart muscle depends on its ability to contract more strongly at longer length. Increased venous filling stretches relaxed heart muscle cells, triggering a stronger contraction in the next beat- the Frank-Starling relation. Conversely, heart muscle cells are inactivated when they shorten during ejection, accelerating relaxation to facilitate refilling before the next beat. Although both effects are essential for the efficient function of the heart, the underlying mechanisms were unknown. Using bifunctional fluorescent probes on the regulatory light chain of the myosin motor we show that its N-terminal domain may be captured in the folded OFF state of the myosin dimer at the end of the working-stroke of the actin-attached motor, whilst its C-terminal domain joins the OFF state only after motor detachment from actin. We propose that sequential folding of myosin motors onto the filament backbone may be responsible for shortening-induced de-activation in the heart.

Micro- and nanoscale biophysical cues for cardiovascular disease therapy

After cardiovascular injury, numerous pathological processes adversely impact the homeostatic function of cardiomyocyte, macrophage, fibroblast, endothelial cell, and vascular smooth muscle cell populations. Subsequent malfunctioning of these cells may further contribute to cardiovascular disease onset and progression. By modulating cellular responses after injury, it is possible to create local environments that promote wound healing and tissue repair mechanisms. The extracellular matrix continuously provides these mechanosensitive cell types with physical cues spanning the micro- and nanoscale to influence behaviors such as adhesion, morphology, and phenotype. It is therefore becoming increasingly compelling to harness these cell-substrate interactions to elicit more native cell behaviors that impede cardiovascular disease progression and enhance regenerative potential. This review discusses recent in vitro and preclinical work that have demonstrated the therapeutic implications of micro- and nanoscale biophysical cues on cell types adversely affected in cardiovascular diseases - cardiomyocytes, macrophages, fibroblasts, endothelial cells, and vascular smooth muscle cells.

Multiplexing physical stimulation on single human induced pluripotent stem cell-derived cardiomyocytes for phenotype modulation

Traditional in vitro bioengineering approaches whereby only individual biophysical cues are manipulated at any one time are highly inefficient, falling short when recapitulating the complexity of the cardiac environment. Multiple biophysical cues are present in the native myocardial niche and are essential during development, as well as in maintenance of adult cardiomyocyte (CM) phenotype in both health and disease. This study establishes a novel biofabrication workflow to study and manipulate hiPSC-CMs and to understand how these cells respond to a multiplexed biophysical environment, namely 3D shape and substrate stiffness, at a single cell level. Silicon masters were fabricated and developed to generate inverse patterns of the desired 3D shapes in bas relief, which then were used to mold the designed microwell arrays into a hydrogel. Polyacrylamide (PAAm) was modified with the incorporation of acrylic acid to provide a carboxylic group conjugation site for adhesion motifs, without compromising capacity to modulate stiffness. In this manner, two individual parameters can be finely tuned independently within the hydrogel: the shape of the 3D microwell and its stiffness. The design allows the platform to isolate single hiPSC-CMs to study solely biophysical cues in the absence of cell-cell physical interaction. Under physiologic-like physical conditions (3D shape resembling that of adult CM and 9.83 kPa substrate stiffness that mimics muscle stiffness), isolated single hiPSC-CMs exhibit increased Cx-43 density, cell membrane stiffness and calcium transient amplitude; co-expression of the subpopulation-related MYL2-MYL7 proteins; and higher anisotropism than cells in pathologic-like conditions (flat surface and 112 kPa substrate stiffness). This demonstrates that supplying a physiologic or pathologic microenvironment to an isolated single hiPSC-CM in the absence of any physical cell-to-cell communication in this biofabricated platform leads to a significantly different set of cellular features, thus presenting a differential phenotype. Importantly, this demonstrates the high plasticity of hiPSC-CMs even in isolation. The ability of multiple biophysical cues to significantly influence isolated single hiPSC-CM phenotype and functionality highlights the importance of fine-tuning such cues for specific applications. This has the potential to produce more fit-for-purpose hiPSC-CMs. Further understanding of human cardiac development is enabled by the robust, versatile and reproducible biofabrication techniques applied here. We envision that this system could be easily applied to other tissues and cell types where the influence of cellular shape and stiffness of the surrounding environment is hypothesized to play an important role in physiology.

cMyBP-C phosphorylation modulates the time-dependent slowing of unloaded shortening in murine skinned myocardium

In myocardium, phosphorylation of cardiac myosin-binding protein-C (cMyBP-C) is thought to modulate the cooperative activation of the thin filament by binding to myosin and/or actin, thereby regulating the probability of cross-bridge binding to actin. At low levels of Ca2+ activation, unloaded shortening velocity (Vo) in permeabilized cardiac muscle is comprised of an initial high-velocity phase and a subsequent low-velocity phase. The velocities in these phases scale with the level of activation, culminating in a single high-velocity phase (Vmax) at saturating Ca2+. To test the idea that cMyBP-C phosphorylation contributes to the activation dependence of Vo, we measured Vo before and following treatment with protein kinase A (PKA) in skinned trabecula isolated from mice expressing either wild-type cMyBP-C (tWT), nonphosphorylatable cMyBP-C (t3SA), or phosphomimetic cMyBP-C (t3SD). During maximal Ca2+ activation, Vmax was monophasic and not significantly different between the three groups. Although biphasic shortening was observed in all three groups at half-maximal activation under control conditions, the high- and low-velocity phases were faster in the t3SD myocardium compared with values obtained in either tWT or t3SA myocardium. Treatment with PKA significantly accelerated both the high- and low-velocity phases in tWT myocardium but had no effect on Vo in either the t3SD or t3SA myocardium. These results can be explained in terms of a model in which the level of cMyBP-C phosphorylation modulates the extent and rate of cooperative spread of myosin binding to actin.

Ventricular contraction and relaxation rates during muscle metaboreflex activation in heart failure: are they coupled?

NEW FINDINGS

What is the central question of this study? Does the muscle metaboreflex affect the ratio of left ventricular contraction/relaxation rates and does heart failure impact this relationship. What is the main finding and its importance? The effect of muscle metaboreflex activation on the ventricular relaxation rate was significantly attenuated in heart failure. Heart failure attenuates the exercise and muscle metaboreflex-induced changes in the contraction/relaxation ratio. In heart failure, the reduced ability to raise cardiac output during muscle metaboreflex activation may not solely be due to attenuation of ventricular contraction but also alterations in ventricular relaxation and diastolic function.

ABSTRACT

The relationship between contraction and relaxation rates of the left ventricle varies with exercise. In in vitro models, this ratio was shown to be relatively unaltered by changes in sarcomere length, frequency of stimulation, and β-adrenergic stimulation. We investigated whether the ratio of contraction to relaxation rate is maintained in the whole heart during exercise and muscle metaboreflex activation and whether heart failure alters these relationships. We observed that in healthy subjects the ratio of contraction to relaxation increases from rest to exercise as a result of a higher increase in contraction relative to relaxation. During muscle metaboreflex activation the ratio of contraction to relaxation is significantly reduced towards 1.0 due to a large increase in relaxation rate matching contraction rate. In heart failure, contraction and relaxation rates are significantly reduced, and increases during exercise are attenuated. A significant increase in the ratio was observed from rest to exercise although baseline ratio values were significantly reduced close to 1.0 when compared to healthy subjects. There was no significant change observed between exercise and muscle metaboreflex activation nor was the ratio during muscle metaboreflex activation significantly different between heart failure and control. We conclude that heart failure reduces the muscle metaboreflex gain and contraction and relaxation rates. Furthermore, we observed that the ratio of the contraction and relaxation rates during muscle metaboreflex activation is not significantly different between control and heart failure, but significant changes in the ratio in healthy subjects due to increased relaxation rate were abolished in heart failure.

The use of Ca-transient to evaluate Ca2+ utilization by myofilaments in living cardiac muscle

The kinetics of Ca2+ interaction with myofilaments is an important determinant of the preload-dependent effects on myocardial contractility (the Frank-Starling Mechanism). However, the direct evaluation of this interaction in intact tissue is limited. To overcome this issue, the method of difference curve was proposed, which implements the subtraction of the referent Ca-transient (measured in non-stretched muscle) from the Ca-transients measured at different preloads. This method was tested on the cardiac trabeculae of healthy (CONT) and monocrotaline-treated rats (MCT), subjected to force-length protocol with simultaneous measurement of isometric twitch and Ca-transient. The difference curve had two components, C2 and C3, which are distinct in their directions and, as hypothesized, may reflect mainly the kinetics of Ca2+ utilization by and release from myofilaments, respectively. Both the components were quantitatively evaluated by their amplitude, integral magnitude and time-to-peak. The C3 component in either CONT or MCT was significantly higher in its amplitude/integral magnitude vs the C2 component, at any preload (P < .05). The time-to-peak value was preload-dependent only for the C3 component. There were tight relationships between the above characteristics of C2/C3 components and the characteristics of isometric tension (peak value, time-to-peak and the maximal rates of rise/decline) in CONT and MCT muscles. The C3 component was highly consistent with tension relaxation (Ca2+ release from myofilaments), but the C2 component was partially consistent with tension development (Ca2+ utilization by myofilaments). The novel method of the analysis of Ca-transients can be utilized for indirect evaluation of Ca2+ interaction with myofilaments in healthy and diseased myocardium.

Hypertrophic and Dilated Cardiomyopathy-Associated Troponin T Mutations R130C and ΔK210 Oppositely Affect Length-Dependent Calcium Sensitivity of Force Generation

Length-dependent activation of calcium-dependent myocardial force generation provides the basis for the Frank-Starling mechanism. To directly compare the effects of mutations associated with hypertrophic cardiomyopathy and dilated cardiomyopathy, the native troponin complex in skinned trabecular fibers of guinea pigs was exchanged with recombinant heterotrimeric, human, cardiac troponin complexes containing different human cardiac troponin T subunits (hcTnT): hypertrophic cardiomyopathy-associated hcTnT^R130C, dilated cardiomyopathy-associated hcTnT^ΔK210 or the wild type hcTnT (hcTnT^WT) serving as control. Force-calcium relations of exchanged fibers were explored at short fiber length defined as 110% of slack length (L₀) and long fiber length defined as 125% of L₀ (1.25 L₀). At short fiber length (1.1 L₀), calcium sensitivity of force generation expressed by −log [Ca²⁺] required for half-maximum force generation (pCa₅₀) was highest for the hypertrophic cardiomyopathy-associated mutation R130C (5.657 ± 0.019), intermediate for the wild type control (5.580 ± 0.028) and lowest for the dilated cardiomyopathy-associated mutation ΔK210 (5.325 ± 0.038). Lengthening fibers from 1.1 L₀ to 1.25 L₀ increased calcium sensitivity in fibers containing hcTnT^R130C (delta-pCa₅₀ = +0.030 ± 0.010), did not alter calcium sensitivity in the wild type control (delta-pCa₅₀ = −0.001 ± 0.010), and decreased calcium sensitivity in fibers containing hcTnT^ΔK210 (delta-pCa₅₀ = −0.034 ± 0.013). Length-dependent activation indicated by the delta-pCa₅₀ was highly significantly (P < 0.001) different between the two mutations. We hypothesize that primary effects of mutations on length-dependent activation contribute to the development of the diverging phenotypes in hypertrophic and dilated cardiomyopathy.

The role of pacing rate in the modulation of mechano-induced immediate and delayed changes in the force and Ca-transient of cardiac muscle

Myocardial function is tuned by dynamic changes in length and load via mechano-calcium feedback. This regulation may be significantly affected by heart rhythm. We evaluated the mechano-induced modulation of contractility and Ca-transient (CaT) in the rat myocardium subjected to twitch-by-twitch shortening-re-lengthening (↓-↑) trains of different lengths (N = 1 … 720 cycles) at low (1 Hz) and near-physiological (3.5 Hz) pacing rates. Force/CaT characteristics were evaluated in the first post-train isometric twitch (immediate effect) and during slow changes (delayed maximal elevation/decrease) and compared with those of the pre-train twitch. The immediate inotropic effect was positive for N = 30 … 720 and negative for N = 1 … 20, while the delayed effect was always positive. The immediate and delayed inotropic effects were significantly higher at 3.5-Hz vs 1-Hz (P < 0.05). The prominent inotropism was accompanied by much smaller changes in the CaT diastolic level/amplitude. The shortening-re-lengthening train induced oscillations of the slow change in force at 3.5-Hz (always) and at 1-Hz (∼50% of muscles), which were dependent of the train length and independent of the pacing rate. We suggest that twitch-by-twitch shortening-re-lengthening of cardiac muscle decreases Ca²⁺ buffering by troponin C and elevates Ca²⁺ loading of the sarcoplasmic reticulum (SR); the latter cumulatively depends on the train length. A high pacing rate intensifies the cumulative transient shift in the SR Ca²⁺ loading, augmenting the post-train inotropic response and prolonging its recovery to the pre-train level. The pacing-dependent mechano-induced inotropic effects remain to be elucidated in the myocardium with impaired Ca handling.

A Novel Fluorescent Reporter System Identifies Laminin-511/521 as Potent Regulators of Cardiomyocyte Maturation

Pluripotent stem cell-derived cardiomyocytes (PSC-CMs) hold great promise for disease modeling and drug discovery. However, PSC-CMs exhibit immature phenotypes in culture, and the lack of maturity limits their broad applications. While physical and functional analyses are generally used to determine the status of cardiomyocyte maturation, they could be time-consuming and often present challenges in comparing maturation-enhancing strategies. Therefore, there is a demand for a method to assess cardiomyocyte maturation rapidly and reproducibly. In this study, we found that Myomesin-2 (Myom2), encoding M-protein, is upregulated postnatally, and based on this, we targeted TagRFP to the Myom2 locus in mouse embryonic stem cells. Myom2-RFP⁺ PSC-CMs exhibited more mature phenotypes than RFP^- cells in morphology, function and transcriptionally, conductive to sarcomere shortening assays. Using this system, we screened extracellular matrices (ECMs) and identified laminin-511/521 as potent enhancers of cardiomyocyte maturation. Together, we developed and characterized a novel fluorescent reporter system for the assessment of cardiomyocyte maturation and identified potent maturation-enhancing ECMs through this simple and rapid assay. This system is expected to facilitate use of PSC-CMs in a variety of scientific and medical investigations.

Tissue-engineered human embryonic stem cell-containing cardiac patches: evaluating recellularization of decellularized matrix

Decellularized cardiac extracellular matrix scaffolds with preserved composition and architecture can be used in tissue engineering to reproduce the complex cardiac extracellular matrix. However, evaluating the extent of cardiomyocyte repopulation of decellularized cardiac extracellular matrix scaffolds after recellularization attempts is challenging. Here, we describe a unique combination of biochemical, biomechanical, histological, and physiological parameters for quantifying recellularization efficiency of tissue-engineered cardiac patches compared with native cardiac tissue. Human embryonic stem cell-derived cardiomyocytes were seeded into rat heart atrial and ventricular decellularized cardiac extracellular matrix patches. Confocal and atomic force microscopy showed cell integration within the extracellular matrix basement membrane that was accompanied by restoration of native cardiac tissue passive mechanical properties. Multi-electrode array and immunostaining (connexin 43) were used to determine synchronous field potentials with electrical coupling. Myoglobin content (~60%) and sarcomere length measurement (>45% vs 2D culture) were used to evaluate cardiomyocyte maturation of integrated cells. The combination of these techniques allowed us to demonstrate that as cellularization efficiency improves, cardiomyocytes mature and synchronize electrical activity, and tissue mechanical/biochemical properties improve toward those of native tissue.

Length-Dependent Activation of Contractility and Ca-Transient Kinetics in Auxotonically Contracting Isolated Rat Ventricular Cardiomyocytes

Length-dependent activation (LDA) of contraction is an important mechanism of proper myocardial function that is often blunted in diseases accompanied by deficient contractility and impaired calcium homeostasis. We evaluated how the extent of LDA is related to the decreased force in healthy rat myocardium under negative inotropic conditions that affect the calcium cycle. The length-dependent effects on auxotonic twitch and Ca-transient were compared in isolated rat ventricular cardiomyocytes at room temperature (“25C”) and near-physiological temperature (“35C”) in normal Tyrode and at 25°C with thapsigargin-depleted sarcoplasmic reticulum (“25C + Thap”). At the slack length, a similar negative inotropy in “35C” and “25C + Thap” was accompanied by totally different changes in Ca-transient amplitude, time-to-peak, and time-to-decline from peak to 50% amplitude. End-systolic/end-diastolic tension-sarcomere length relationships were obtained for each individual cell, and the ratio of their slopes, the dimensionless Frank-Starling Gain index, was 2.32 ± 0.16, 1.78 ± 0.09, and 1.37 ± 0.06 in “25C,” “35C” and “25C + Thap,” respectively (mean ± S.E.M.). Ca-transient diastolic level and amplitude did not differ between “25C” and “35C” at any SL, but in “35C” it developed and declined significantly faster. In contrast, thapsigargin-induced depletion of SERCA2a significantly attenuated and retarded Ca-transient. The relative amount of Ca²⁺ utilized by troponin C, evaluated by the integral magnitude of a short-lived component of Ca-transient decline (“bump”), increased by ~25% per each 0.05 μm increase in SL in all groups. The kinetics of the Ca-TnC dissociation, evaluated by the bump time-to-peak, was significantly faster in “35C” and slower in “25C + Thap” vs. “25C” (respectively, 63.7 ± 5.3 and 253.6 ± 8.3% of the value in “25C,” mean ± S.E.M.). In conclusion, a similar inotropic effect can be observed in rat ventricular myocardium under totally different kinetics of free cytosolic calcium. The extent of LDA is not determined by actual peak systolic tension but is regulated by the level of peak systolic calcium and the kinetics of Ca-transient decline which, in turn, are governed by Ca-TnC dissociation and Ca²⁺ reuptake by the sarcoplasmic reticulum. Altogether, these findings constitute new evidence about the role of the length-dependent modulation of Ca²⁺ homeostasis in the mechanisms of calcium regulation of contraction and mechano-calcium feedback in the myocardium.

Matrix-guided control of mitochondrial function in cardiac myocytes

In ventricular myocardium, extracellular matrix (ECM) remodeling is a hallmark of physiological and pathological growth, coincident with metabolic rewiring of cardiac myocytes. However, the direct impact of the biochemical and mechanical properties of the ECM on the metabolic function of cardiac myocytes is mostly unknown. Furthermore, understanding the impact of distinct biomaterials on cardiac myocyte metabolism is critical for engineering physiologically-relevant models of healthy and diseased myocardium. For these reasons, we systematically measured morphological and metabolic responses of neonatal rat ventricular myocytes cultured on fibronectin- or gelatin-coated polydimethylsiloxane (PDMS) of three elastic moduli and gelatin hydrogels with four elastic moduli. On all substrates, total protein content, cell morphology, and the ratio of mitochondrial DNA to nuclear DNA were preserved. Cytotoxicity was low on all substrates, although slightly higher on PDMS compared to gelatin hydrogels. We also quantified oxygen consumption rates and extracellular acidification rates using a Seahorse extracellular flux analyzer. Our data indicate that several metrics associated with baseline glycolysis and baseline and maximum mitochondrial function are enhanced when cardiac myocytes are cultured on gelatin hydrogels compared to all PDMS substrates, irrespective of substrate rigidity. These results yield new insights into how mechanical and biochemical cues provided by the ECM impact mitochondrial function in cardiac myocytes. STATEMENT OF SIGNIFICANCE: Cardiac development and disease are associated with remodeling of the extracellular matrix coincident with metabolic rewiring of cardiac myocytes. However, little is known about the direct impact of the biochemical and mechanical properties of the extracellular matrix on the metabolic function of cardiac myocytes. In this study, oxygen consumption rates were measured in neonatal rat ventricular myocytes maintained on several commonly-used biomaterial substrates to reveal new relationships between the extracellular matrix and cardiac myocyte metabolism. Several mitochondrial parameters were enhanced on gelatin hydrogels compared to synthetic PDMS substrates. These data are important for comprehensively understanding matrix-regulation of cardiac myocyte physiology. Additionally, these data should be considered when selecting scaffolds for engineering in vitro cardiac tissue models.

3D printed micro-scale force gauge arrays to improve human cardiac tissue maturation and enable high throughput drug testing

Human induced pluripotent stem cell - derived cardiomyocytes (iPSC-CMs) are regarded as a promising cell source for establishing in-vitro personalized cardiac tissue models and developing therapeutics. However, analyzing cardiac force and drug response using mature human iPSC-CMs in a high-throughput format still remains a great challenge. Here we describe a rapid light-based 3D printing system for fabricating micro-scale force gauge arrays suitable for 24-well and 96-well plates that enable scalable tissue formation and measurement of cardiac force generation in human iPSC-CMs. We demonstrate consistent tissue band formation around the force gauge pillars with aligned sarcomeres. Among the different maturation treatment protocols we explored, 3D aligned cultures on force gauge arrays with in-culture pacing produced the highest expression of mature cardiac marker genes. We further demonstrated the utility of these micro-tissues to develop significantly increased contractile forces in response to treatment with isoproterenol, levosimendan, and omecamtiv mecarbil. Overall, this new 3D printing system allows for high flexibility in force gauge design and can be optimized to achieve miniaturization and promote cardiac tissue maturation with great potential for high-throughput in-vitro drug screening applications. STATEMENT OF SIGNIFICANCE: The application of iPSC-derived cardiac tissues in translatable drug screening is currently limited by the challenges in forming mature cardiac tissue and analyzing cardiac forces in a high-throughput format. We demonstrate the use of a rapid light-based 3D printing system to build a micro-scale force gauge array that enables scalable cardiac tissue formation from iPSC-CMs and measurement of contractile force development. With the capability to provide great flexibility over force gauge design as well as optimization to achieve miniaturization, our 3D printing system serves as a promising tool to build cardiac tissues for high-throughput in-vitro drug screening applications.

Myocardial Dysfunction after Severe Food Restriction Is Linked to Changes in the Calcium-Handling Properties in Rats

Severe food restriction (FR) impairs cardiac performance, although the causative mechanisms remain elusive. Since proteins associated with calcium handling may contribute to cardiac dysfunction, this study aimed to evaluate whether severe FR results in alterations in the expression and activity of Ca²⁺-handling proteins that contribute to impaired myocardial performance. Male 60-day-old Wistar–Kyoto rats were fed a control or restricted diet (50% reduction in the food consumed by the control group) for 90 days. Body weight, body fat pads, adiposity index, as well as the weights of the soleus muscle and lung, were obtained. Cardiac remodeling was assessed by morphological measures. The myocardial contractile performance was analyzed in isolated papillary muscles during the administration of extracellular Ca²⁺ and in the absence or presence of a sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a) specific blocker. The expression of Ca²⁺-handling regulatory proteins was analyzed via Western Blot. Severe FR resulted in a 50% decrease in body weight and adiposity measures. Cardiac morphometry was substantially altered, as heart weights were nearly twofold lower in FR rats. Papillary muscles isolated from FR hearts displayed mechanical dysfunction, including decreased developed tension and reduced contractility and relaxation. The administration of a SERCA2a blocker led to further decrements in contractile function in FR hearts, suggesting impaired SERCA2a activity. Moreover, the FR rats presented a lower expression of L-type Ca²⁺ channels. Therefore, myocardial dysfunction induced by severe food restriction is associated with changes in the calcium-handling properties in rats.

The lack of slow force response in failing rat myocardium: role of stretch-induced modulation of Ca-TnC kinetics

The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20-30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca2+ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca2+ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca2+ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca2+ utilized by TnC was indirectly evaluated using the methods of Ca2+ transient "bump" and "difference curve." The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca2+ transient "bump" and "difference curve." The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca2+ transient decay, evaluated by the methods of "bump" and "difference curve." We conclude that the intracellular mechanisms of Ca2+ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca-TnC interaction.

Lost in translation: Interpreting cardiac muscle mechanics data in clinical practice

Current inotropic therapies improve systolic function in heart failure patients but also elicit undesirable side effects such as arrhythmias and increased intracellular Ca2+ transients. In order to maintain myocyte Ca2+ homeostasis, the increased cytosolic Ca2+ needs to be actively transported back to sarcoplasmic reticulum leading to depleted ATP reserves. Thus, an emerging approach is to design sarcomere-based treatments to correct impaired contractility via a direct and allosteric modulation of myosin's intrinsic force-generating behavior -a concept that potentially avoids the "off-target" effects. To achieve this goal, various biophysical approaches are utilized to investigate the mechanistic impact of sarcomeric modulators but information derived from diverse approaches is not fully integrated into therapeutic applications. This is in part due to the lack of information that provides a coherent connecting link between biophysical data to in vivo function. Hence, our ability to clearly discern the drug-mediated impact on whole-heart function is diminished. Reducing this translational barrier can significantly accelerate clinical progress related to sarcomere-based therapies by optimizing drug-dosing and treatment duration protocols based on information obtained from biophysical studies. Therefore, we attempt to link biophysical mechanical measurements obtained in isolated cardiac muscle and in vivo contractile function.

Force-Dependent Recruitment from the Myosin Off State Contributes to Length-Dependent Activation

Cardiac muscle develops more force when it is activated at longer lengths. The concentration of Ca²⁺ required to develop half-maximal force also decreases. These effects are known as length-dependent activation and are thought to play critical roles in the Frank-Starling relationship and cardiovascular homeostasis. The molecular mechanisms underpinning length-dependent activation remain unclear, but recent experiments suggest that they may include recruitment of myosin heads from the off (sometimes called super-relaxed) state. This manuscript presents a mathematical model of muscle contraction that was developed to investigate this hypothesis. Myosin heads in the model transitioned between an off state (that could not interact with actin), an on state (that could bind to actin), and a single attached state. Simulations were fitted to experimental data using multidimensional parameter optimization. Statistical analysis showed that a model in which the rate of the off-to-on transition increased linearly with force reproduced the length-dependent behavior of chemically permeabilized myocardium better than a model with a constant off-to-on transition rate (F-test, p < 0.001). This result suggests that the thick-filament transitions are modulated by force. Additional calculations showed that the model incorporating a mechanosensitive thick filament could also reproduce twitch responses measured in a trabecula stretched to different lengths. A final set of simulations was then used to test the model. These calculations predicted how reducing passive stiffness would impact the length dependence of the calcium sensitivity of contractile force. The prediction (a 60% reduction in ΔpCa₅₀) mimicked the 58% reduction in ΔpCa₅₀ in myocardium from rats that expressed a giant isoform of titin and had low resting tension. Together, these computational results suggest that force-dependent recruitment of myosin heads from the thick-filament off state contributes to length-dependent activation and the Frank-Starling relationship.

Simultaneous imaging of local calcium and single sarcomere length in rat neonatal cardiomyocytes using yellow Cameleon-Nano140

In cardiac muscle, contraction is triggered by sarcolemmal depolarization, resulting in an intracellular Ca(2+) transient, binding of Ca(2+) to troponin, and subsequent cross-bridge formation (excitation-contraction [EC] coupling). Here, we develop a novel experimental system for simultaneous nano-imaging of intracellular Ca(2+) dynamics and single sarcomere length (SL) in rat neonatal cardiomyocytes. We achieve this by expressing a fluorescence resonance energy transfer (FRET)-based Ca(2+) sensor yellow Cameleon-Nano (YC-Nano) fused to α-actinin in order to localize to the Z disks. We find that, among four different YC-Nanos, α-actinin-YC-Nano140 is best suited for high-precision analysis of EC coupling and α-actinin-YC-Nano140 enables quantitative analyses of intracellular calcium transients and sarcomere dynamics at low and high temperatures, during spontaneous beating and with electrical stimulation. We use this tool to show that calcium transients are synchronized along the length of a myofibril. However, the averaging of SL along myofibrils causes a marked underestimate (∼50%) of the magnitude of displacement because of the different timing of individual SL changes, regardless of the absence or presence of positive inotropy (via β-adrenergic stimulation or enhanced actomyosin interaction). Finally, we find that β-adrenergic stimulation with 50 nM isoproterenol accelerated Ca(2+) dynamics, in association with an approximately twofold increase in sarcomere lengthening velocity. We conclude that our experimental system has a broad range of potential applications for the unveiling molecular mechanisms of EC coupling in cardiomyocytes at the single sarcomere level.

Dose-Dependent Effects of the Myosin Activator Omecamtiv Mecarbil on Cross-Bridge Behavior and Force Generation in Failing Human Myocardium

BACKGROUND

Omecamtiv mecarbil (OM) enhances systolic function in vivo by directly binding the myosin cross-bridges (XBs) in the sarcomere. However, the mechanistic details governing OM-induced modulation of XB behavior in failing human myocardium are unclear.

METHODS AND RESULTS

The effects of OM on steady state and dynamic XB behavior were measured in chemically skinned myocardial preparations isolated from human donor and heart failure (HF) left ventricle. HF myocardium exhibited impaired contractile function as evidenced by reduced maximal force, magnitude of XB recruitment (P_df), and a slowed rate of XB detachment (k_rel) at submaximal Ca²⁺ activations. Ca²⁺ sensitivity of force generation (pCa₅₀) was higher in HF myocardium when compared with donor myocardium, both prior to and after OM incubations. OM incubation (0.5 and 1.0 μmol/L) enhanced force generation at submaximal Ca²⁺ activations in a dose-dependent manner. Notably, OM induced a slowing in k_rel with 1.0 μmol/L OM but not with 0.5 μmol/L OM in HF myocardium. Additionally, OM exerted other differential effects on XB behavior in HF myocardium as evidenced by a greater enhancement in P_df and slowing in the time course of cooperative XB recruitment (T_rec), which collectively prolonged achievement of peak force development (T_pk), compared with donor myocardium.

CONCLUSIONS

Our findings demonstrate that OM augments force generation but also prolongs the time course of XB transitions to force-bearing states in remodeled HF myocardium, which may extend the systolic ejection time in vivo. Optimal OM dosing is critical for eliciting enhanced systolic function without excessive prolongation of systolic ejection time, which may compromise diastolic filling.

Exercise And Heart Failure: Advancing Knowledge And Improving Care

Exercise limitation is the hallmark of heart failure, and an increasing degree of intolerance is associated with poor prognosis. Objective evaluation of functional class (e.g., cardiopulmonary exercise testing) is essential for adequate prognostication in patients with advanced heart failure and for implementing an appropriate exercise training program. A graded exercise program has been shown to be beneficial in patients with heart failure and has become an essential component of comprehensive cardiac rehabilitation in these patients. An exercise program tailored to the patient's preferences, possibilities, and physiologic reserve has the greatest chance of being successful. Despite being safe, effective, and a guideline-recommended treatment to improve quality of life, exercise training remains grossly underutilized. Patient, physician, insurance and practice barriers need to be addressed to improve this quality gap.

The contractile adaption to preload depends on the amount of afterload

Aims

The Frank–Starling mechanism (rapid response (RR)) and the secondary slow response (SR) are known to contribute to increases contractile performance. The contractility of the heart muscle is influenced by pre‐load and after‐load. Because of the effect of pre‐load vs. after‐load on these mechanisms in not completely understood, we studied the effect in isolated muscle strips.

Methods and results

Progressive stretch lead to an increase in shortening/force development under isotonic (only pre‐load) and isometric conditions (pre‐ and after‐load). Muscle length with maximal function was reached earlier under isotonic (L_{max‐isotonic}) compared with isometric conditions (L_{max‐isometric}) in nonfailing rabbit, in human atrial and in failing ventricular muscles. Also, SR after stretch from slack to L_{max‐isotonic} was comparable under isotonic and isometric conditions (human: isotonic 10 ± 4%, isometric 10 ± 4%). Moreover, a switch from isotonic to isometric conditions at L_{max‐isometric} showed no SR proving independence of after‐load. To further analyse the degree of SR on the total contractile performance at higher pre‐load muscles were stretched from slack to 98% L_{max‐isometric} under isotonic conditions. Thereby, the SR was 60 ± 9% in rabbit and 51 ± 14% in human muscle strips.

Conclusions

This work shows that the acute contractile response largely depends on the degree and type of mechanical load. Increased filling of the heart elevates pre‐load and prolongs the isotonic part of contraction. The reduction in shortening at higher levels of pre‐load is thereby partially compensated by the pre‐load‐induced SR. After‐load shifts the contractile curve to a better ‘myofilament function’ by probably influencing thin fibers and calcium sensitivity, but has no effect on the SR.

A model of cardiac contraction based on novel measurements of tension development in human cardiomyocytes

Experimental data from human cardiac myocytes at body temperature is crucial for a quantitative understanding of clinically relevant cardiac function and development of whole-organ computational models. However, such experimental data is currently very limited. Specifically, important measurements to characterize changes in tension development in human cardiomyocytes that occur with perturbations in cell length are not available. To address this deficiency, in this study we present an experimental data set collected from skinned human cardiac myocytes, including the passive and viscoelastic properties of isolated myocytes, the steady-state force calcium relationship at different sarcomere lengths, and changes in tension following a rapid increase or decrease in length, and after constant velocity shortening. This data set is, to our knowledge, the first characterization of length and velocity-dependence of tension generation in human skinned cardiac myocytes at body temperature. We use this data to develop a computational model of contraction and passive viscoelasticity in human myocytes. Our model includes troponin C kinetics, tropomyosin kinetics, a three-state crossbridge model that accounts for the distortion of crossbridges, and the cellular viscoelastic response. Each component is parametrized using our experimental data collected in human cardiomyocytes at body temperature. Furthermore we are able to confirm that properties of length-dependent activation at 37°C are similar to other species, with a shift in calcium sensitivity and increase in maximum tension. We revise our model of tension generation in the skinned isolated myocyte to replicate reported tension traces generated in intact muscle during isometric tension, to provide a model of human tension generation for multi-scale simulations. This process requires changes to calcium sensitivity, cooperativity, and crossbridge transition rates. We apply this model within multi-scale simulations of biventricular cardiac function and further refine the parametrization within the whole organ context, based on obtaining a healthy ejection fraction. This process reveals that crossbridge cycling rates differ between skinned myocytes and intact myocytes.

Aberrant developmental titin splicing and dysregulated sarcomere length in Thymosin β4 knockout mice

Sarcomere assembly is a highly orchestrated and dynamic process which adapts, during perinatal development, to accommodate growth of the heart. Sarcomeric components, including titin, undergo an isoform transition to adjust ventricular filling. Many sarcomeric genes have been implicated in congenital cardiomyopathies, such that understanding developmental sarcomere transitions will inform the aetiology and treatment. We sought to determine whether Thymosin β4 (Tβ4), a peptide that regulates the availability of actin monomers for polymerization in non-muscle cells, plays a role in sarcomere assembly during cardiac morphogenesis and influences adult cardiac function. In Tβ4 null mice, immunofluorescence-based sarcomere analyses revealed shortened thin filament, sarcomere and titin spring length in cardiomyocytes, associated with precocious up-regulation of the short titin isoforms during the postnatal splicing transition. By magnetic resonance imaging, this manifested as diminished stroke volume and limited contractile reserve in adult mice. Extrapolating to an in vitro cardiomyocyte model, the altered postnatal splicing was corrected with addition of synthetic Tβ4, whereby normal sarcomere length was restored. Our data suggest that Tβ4 is required for setting correct sarcomere length and for appropriate splicing of titin, not only in the heart but also in skeletal muscle. Distinguishing between thin filament extension and titin splicing as the primary defect is challenging, as these events are intimately linked. The regulation of titin splicing is a previously unrecognised role of Tβ4 and gives preliminary insight into a mechanism by which titin isoforms may be manipulated to correct cardiac dysfunction.

The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function

KEY POINTS

β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.

Isolated neonatal rat papillary muscles: a new model to translate neonatal rat myocyte signaling into contractile mechanics

Isolated cardiac tissue allows investigators to study mechanisms underlying normal and pathological conditions, which would otherwise be difficult or impossible to perform in vivo. Cultured neonatal rat ventricular cardiac myocytes (NRVM) are widely used to study signaling and growth mechanisms in the heart, primarily due to the versatility, economy, and convenience of this in vitro model. However, the lack of a well‐defined longitudinal cellular axis greatly hampers the ability to measure contractile function in these cells, and therefore to associate signaling with mechanical function. In these methods, we demonstrate that this limitation can be overcome by using papillary muscles isolated from neonatal rat hearts. In the methods we describe procedures for isolation of right ventricular papillary muscles from 3‐day‐old neonatal rats and effects of mechanical and humoral stimuli on contraction and relaxation properties of these tissues.

Revisiting Frank-Starling: regulatory light chain phosphorylation alters the rate of force redevelopment (ktr ) in a length-dependent fashion

Key points

Regulatory light chain (RLC) phosphorylation has been shown to alter the ability of muscle to produce force and power during shortening and to alter the rate of force redevelopment (k_tr) at submaximal [Ca²⁺].

Increasing RLC phosphorylation ∼50% from the in vivo level in maximally [Ca²⁺]‐activated cardiac trabecula accelerates k_tr.

Decreasing RLC phosphorylation to ∼70% of the in vivo control level slows k_tr and reduces force generation.

k_tr is dependent on sarcomere length in the physiological range 1.85–1.94 μm and RLC phosphorylation modulates this response.

We demonstrate that Frank–Starling is evident at maximal [Ca²⁺] activation and therefore does not necessarily require length‐dependent change in [Ca²⁺]‐sensitivity of thin filament activation.

The stretch response is modulated by changes in RLC phosphorylation, pinpointing RLC phosphorylation as a modulator of the Frank–Starling law in the heart.

These data provide an explanation for slowed systolic function in the intact heart in response to RLC phosphorylation reduction.

Abstract

Force and power in cardiac muscle have a known dependence on phosphorylation of the myosin‐associated regulatory light chain (RLC). We explore the effect of RLC phosphorylation on the ability of cardiac preparations to redevelop force (k_tr) in maximally activating [Ca²⁺]. Activation was achieved by rapidly increasing the temperature (temperature‐jump of 0.5–20ºC) of permeabilized trabeculae over a physiological range of sarcomere lengths (1.85–1.94 μm). The trabeculae were subjected to shortening ramps over a range of velocities and the extent of RLC phosphorylation was varied. The latter was achieved using an RLC‐exchange technique, which avoids changes in the phosphorylation level of other proteins. The results show that increasing RLC phosphorylation by 50% accelerates k_tr by ∼50%, irrespective of the sarcomere length, whereas decreasing phosphorylation by 30% slows k_tr by ∼50%, relative to the k_tr obtained for in vivo phosphorylation. Clearly, phosphorylation affects the magnitude of k_tr following step shortening or ramp shortening. Using a two‐state model, we explore the effect of RLC phosphorylation on the kinetics of force development, which proposes that phosphorylation affects the kinetics of both attachment and detachment of cross‐bridges. In summary, RLC phosphorylation affects the rate and extent of force redevelopment. These findings were obtained in maximally activated muscle at saturating [Ca²⁺] and are not explained by changes in the Ca²⁺‐sensitivity of acto‐myosin interactions. The length‐dependence of the rate of force redevelopment, together with the modulation by the state of RLC phosphorylation, suggests that these effects play a role in the Frank–Starling law of the heart.

Regulatory light chain (RLC) phosphorylation has been shown to alter the ability of muscle to produce force and power during shortening and to alter the rate of force redevelopment (k_tr) at submaximal [Ca²⁺].

Increasing RLC phosphorylation ∼50% from the in vivo level in maximally [Ca²⁺]‐activated cardiac trabecula accelerates k_tr.

Decreasing RLC phosphorylation to ∼70% of the in vivo control level slows k_tr and reduces force generation.

k_tr is dependent on sarcomere length in the physiological range 1.85–1.94 μm and RLC phosphorylation modulates this response.

We demonstrate that Frank–Starling is evident at maximal [Ca²⁺] activation and therefore does not necessarily require length‐dependent change in [Ca²⁺]‐sensitivity of thin filament activation.

The stretch response is modulated by changes in RLC phosphorylation, pinpointing RLC phosphorylation as a modulator of the Frank–Starling law in the heart.

These data provide an explanation for slowed systolic function in the intact heart in response to RLC phosphorylation reduction.

The α11 integrin mediates fibroblast–extracellular matrix–cardiomyocyte interactions in health and disease

Excessive cardiac interstitial fibrosis impairs normal cardiac function. We have shown that the α11β1 (α11) integrin mediates fibrotic responses to glycated collagen in rat myocardium by a pathway involving transforming growth factor-β. Little is known of the role of the α11 integrin in the developing mammalian heart. Therefore, we examined the impact of deletion of the α11 integrin in wild-type mice and in mice treated with streptozotocin (STZ) to elucidate the role of the α11 integrin in normal cardiac homeostasis and in the pathogenesis of diabetes-related fibrosis. As anticipated, cardiac fibrosis was reduced in α11 integrin knockout mice (α11−/−; C57BL/6 background) treated with STZ compared with STZ-treated wild-type mice ( P < 0.05). Unexpectedly, diastolic function was impaired in both vehicle and STZ-treated α11−/− mice, as shown by the decreased minimum rate of pressure change and prolonged time constant of relaxation in association with increased end-diastolic pressure (all P < 0.05 compared with wild-type mice). Accordingly, we examined the phenotype of untreated α11−/− mice, which demonstrated a reduced cardiomyocyte cross-sectional cell area and myofibril thickness (all P < 0.05 compared with wild-type mice) and impaired myofibril arrangement. Immunostaining for desmin and connexin 43 showed abnormal intermediate filament organization at intercalated disks and impaired gap-junction development. Overall, deletion of the α11 integrin attenuates cardiac fibrosis in the mammalian mouse heart and reduces ECM formation as a result of diabetes. Furthermore, α11 integrin deletion impairs cardiac function and alters cardiomyocyte morphology. These findings shed further light on the poorly understood interaction between the fibroblast–cardiomyocyte and the ECM.

Insights into length-dependent regulation of cardiac cross-bridge cycling kinetics in human myocardium

Cross-bridge cycling kinetics play an essential role in the heart's ability to contract and relax. The rate of tension redevelopment (ktr) slows down as a muscle length is increased in intact human myocardium. We set out to determine the effect of rapid length step changes and protein kinase A (PKA) and protein kinase C-βII (PKC-βII) inhibitors on the ktr in ultra-thin non-failing and failing human right ventricular trabeculae. After stabilizing the muscle either at L90 (90% of optimal length) or at Lopt (optimal length), we rapidly changed the length to either Lopt or L90 and measured ktr. We report that length-dependent changes in ktr occur very rapidly (in the order of seconds or faster) in both non-failing and failing muscles and that the length at which a muscle had been stabilized prior to the length change does not significantly affect ktr. In addition, at L90 and at Lopt, PKA and PKC-βII inhibitors did not significantly change ktr. Our results reveal that length-dependent regulation of cross-bridge cycling kinetics predominantly occurs rapidly and involves the intrinsic properties of the myofilament rather than post-translational modifications that are known to occur in the cardiac muscle as a result of a change in muscle/sarcomere length.

Cardiac Myosin Binding Protein-C Phosphorylation Modulates Myofilament Length-Dependent Activation

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is an important regulator of contractile function, however, its contributions to length-dependent changes in cross-bridge (XB) kinetics is unknown. Therefore, we performed mechanical experiments to quantify contractile function in detergent-skinned ventricular preparations isolated from wild-type (WT) hearts, and hearts expressing non-phosphorylatable cMyBP-C [Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)], at sarcomere length (SL) 1.9 μm or 2.1μm, prior and following protein kinase A (PKA) treatment. Steady-state force generation measurements revealed a blunting in the length-dependent increase in myofilament Ca(2+)-sensitivity of force generation (pCa50) following an increase in SL in 3SA skinned myocardium compared to WT skinned myocardium. Dynamic XB behavior was assessed at submaximal Ca(2+)-activations by imposing an acute rapid stretch of 2% of initial muscle length, and measuring both the magnitudes and rates of resultant phases of force decay due to strain-induced XB detachment and delayed force rise due to recruitment of additional XBs with increased SL (i.e., stretch activation). The magnitude (P2) and rate of XB detachment (k rel) following stretch was significantly reduced in 3SA skinned myocardium compared to WT skinned myocardium at short and long SL, and prior to and following PKA treatment. Furthermore, the length-dependent acceleration of k rel due to decreased SL that was observed in WT skinned myocardium was abolished in 3SA skinned myocardium. PKA treatment accelerated the rate of XB recruitment (k df) following stretch at both SL's in WT but not in 3SA skinned myocardium. The amplitude of the enhancement in force generation above initial pre-stretch steady-state levels (P3) was not different between WT and 3SA skinned myocardium at any condition measured. However, the magnitude of the entire delayed force phase which can dip below initial pre-stretch steady-state levels (Pdf) was significantly lower in 3SA skinned myocardium under all conditions, in part due to a reduced magnitude of XB detachment (P2) in 3SA skinned myocardium compared to WT skinned myocardium. These findings demonstrate that cMyBP-C phospho-ablation regulates SL- and PKA-mediated effects on XB kinetics in the myocardium, which would be expected to contribute to the regulation of the Frank-Starling mechanism.

Biomechanics of Cardiac Function

The heart pumps blood to maintain circulation and ensure the delivery of oxygenated blood to all the organs of the body. Mechanics play a critical role in governing and regulating heart function under both normal and pathological conditions. Biological processes and mechanical stress are coupled together in regulating myocyte function and extracellular matrix structure thus controlling heart function. Here, we offer a brief introduction to the biomechanics of left ventricular function and then summarize recent progress in the study of the effects of mechanical stress on ventricular wall remodeling and cardiac function as well as the effects of wall mechanical properties on cardiac function in normal and dysfunctional hearts. Various mechanical models to determine wall stress and cardiac function in normal and diseased hearts with both systolic and diastolic dysfunction are discussed. The results of these studies have enhanced our understanding of the biomechanical mechanism in the development and remodeling of normal and dysfunctional hearts. Biomechanics provide a tool to understand the mechanism of left ventricular remodeling in diastolic and systolic dysfunction and guidance in designing and developing new treatments.

A defined synthetic substrate for serum-free culture of human stem cell derived cardiomyocytes with improved functional maturity identified using combinatorial materials microarrays

Cardiomyocytes from human stem cells have applications in regenerative medicine and can provide models for heart disease and toxicity screening. Soluble components of the culture system such as growth factors within serum and insoluble components such as the substrate on which cells adhere to are important variables controlling the biological activity of cells. Using a combinatorial materials approach we develop a synthetic, chemically defined cellular niche for the support of functional cardiomyocytes derived from human embryonic stem cells (hESC-CMs) in a serum-free fully defined culture system. Almost 700 polymers were synthesized and evaluated for their utility as growth substrates. From this group, 20 polymers were identified that supported cardiomyocyte adhesion and spreading. The most promising 3 polymers were scaled up for extended culture of hESC-CMs for 15 days and were characterized using patch clamp electrophysiology and myofibril analysis to find that functional and structural phenotype was maintained on these synthetic substrates without the need for coating with extracellular matrix protein. In addition, we found that hESC-CMs cultured on a co-polymer of isobornyl methacrylate and tert-butylamino-ethyl methacrylate exhibited significantly longer sarcomeres relative to gelatin control. The potential utility of increased structural integrity was demonstrated in an in vitro toxicity assay that found an increase in detection sensitivity of myofibril disruption by the anti-cancer drug doxorubicin at a concentration of 0.05 µM in cardiomyocytes cultured on the co-polymer compared to 0.5 µM on gelatin. The chemical moieties identified in this large-scale screen provide chemically defined conditions for the culture and manipulation of hESC-CMs, as well as a framework for the rational design of superior biomaterials.

Length-dependent changes in contractile dynamics are blunted due to cardiac myosin binding protein-C ablation

Enhanced cardiac contractile function with increased sarcomere length (SL) is, in part, mediated by a decrease in the radial distance between myosin heads and actin. The radial disposition of myosin heads relative to actin is modulated by cardiac myosin binding protein-C (cMyBP-C), suggesting that cMyBP-C contributes to the length-dependent activation (LDA) in the myocardium. However, the precise roles of cMyBP-C in modulating cardiac LDA are unclear. To determine the impact of cMyBP-C on LDA, we measured isometric force, myofilament Ca²⁺-sensitivity (pCa₅₀) and length-dependent changes in kinetic parameters of cross-bridge (XB) relaxation (k_rel), and recruitment (k_df) due to rapid stretch, as well as the rate of force redevelopment (k_tr) in response to a large slack-restretch maneuver in skinned ventricular multicellular preparations isolated from the hearts of wild-type (WT) and cMyBP-C knockout (KO) mice, at SL's 1.9 μm or 2.1 μm. Our results show that maximal force was not significantly different between KO and WT preparations but length-dependent increase in pCa₅₀ was attenuated in the KO preparations. pCa₅₀ was not significantly different between WT and KO preparations at long SL (5.82 ± 0.02 in WT vs. 5.87 ± 0.02 in KO), whereas pCa₅₀ was significantly different between WT and KO preparations at short SL (5.71 ± 0.02 in WT vs. 5.80 ± 0.01 in KO; p < 0.05). The k_tr, measured at half-maximal Ca²⁺-activation, was significantly accelerated at short SL in WT preparations (8.74 ± 0.56 s⁻¹ at 1.9 μm vs. 5.71 ± 0.40 s⁻¹ at 2.1 μm, p < 0.05). Furthermore, k_rel and k_df were accelerated by 32% and 50%, respectively at short SL in WT preparations. In contrast, k_tr was not altered by changes in SL in KO preparations (8.03 ± 0.54 s⁻¹ at 1.9 μm vs. 8.90 ± 0.37 s⁻¹ at 2.1 μm). Similarly, KO preparations did not exhibit length-dependent changes in k_rel and k_df. Collectively, our data implicate cMyBP-C as an important regulator of LDA via its impact on dynamic XB behavior due to changes in SL.

Tri-modal regulation of cardiac muscle relaxation; intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics

Cardiac muscle relaxation is an essential step in the cardiac cycle. Even when the contraction of the heart is normal and forceful, a relaxation phase that is too slow will limit proper filling of the ventricles. Relaxation is too often thought of as a mere passive process that follows contraction. However, many decades of advancements in our understanding of cardiac muscle relaxation have shown it is a highly complex and well-regulated process. In this review, we will discuss three distinct events that can limit the rate of cardiac muscle relaxation: the rate of intracellular calcium decline, the rate of thin-filament de-activation, and the rate of cross-bridge cycling. Each of these processes are directly impacted by a plethora of molecular events. In addition, these three processes interact with each other, further complicating our understanding of relaxation. Each of these processes is continuously modulated by the need to couple bodily oxygen demand to cardiac output by the major cardiac physiological regulators. Length-dependent activation, frequency-dependent activation, and beta-adrenergic regulation all directly and indirectly modulate calcium decline, thin-filament deactivation, and cross-bridge kinetics. We hope to convey our conclusion that cardiac muscle relaxation is a process of intricate checks and balances, and should not be thought of as a single rate-limiting step that is regulated at a single protein level. Cardiac muscle relaxation is a system level property that requires fundamental integration of three governing systems: intracellular calcium decline, thin filament deactivation, and cross-bridge cycling kinetics.

Cardiomyocyte subdomain contractility arising from microenvironmental stiffness and topography

Cellular structure and function are interdependent. To understand this relationship in beating heart cells, individual neonatal rat ventricular myocytes (NRVMs) were analyzed one and 3 days after plating when cultured on different stiffness (100, 400 kPa) and surface structures (flat or [Formula: see text] high, [Formula: see text] diameter, microposts spaced [Formula: see text] apart) manufactured from polydimethylsiloxane. Myofibril structure seen by immunohistochemistry was organized in three dimensions when NRVMs were attached to microposts. On day three, paxillin distribution near the post serving as cellular anchorage was quantified on both soft posts (12.04 % of total voxel count) and stiff posts (8.16 %). Living NRVMs were analyzed using line scans for sarcomeric shortening and shortening velocity, and traction force microscopy for surface stress and surface tension. One day after plating, NRVMs shortened more on soft posts ([Formula: see text] at [Formula: see text]) compared to either soft flat ([Formula: see text] at [Formula: see text]), stiff posts ([Formula: see text] at [Formula: see text]) or stiff flat ([Formula: see text] at [Formula: see text]). NRVMs have decreased shortening and shortening velocity on soft posts ([Formula: see text] at [Formula: see text]) compared to soft flat ([Formula: see text] at [Formula: see text]) substrates. The surface stress and surface tension increased over time for both soft post ([Formula: see text] and [Formula: see text] to [Formula: see text] and [Formula: see text]) and flat ([Formula: see text] and [Formula: see text] to [Formula: see text] and [Formula: see text]) substrates. Paxillin displacement during contraction on day three was significantly greater in NRVMs attached to soft posts [Formula: see text] compared to flat [Formula: see text] substrates. The volume and time creating four-dimensional data, interpreted by structural engineering theory, demonstrate subdomain structure is maintained by the counterbalance between the external load acting upon and the internal forces generated by the cardiomyocyte. These findings provide further insight into localized regulation of cellular mechanical function.

Computational modeling of Takotsubo cardiomyopathy: effect of spatially varying β-adrenergic stimulation in the rat left ventricle

In Takotsubo cardiomyopathy, the left ventricle shows apical ballooning combined with basal hypercontractility. Both clinical observations in humans and recent experimental work on isolated rat ventricular myocytes suggest the dominant mechanisms of this syndrome are related to acute catecholamine overload. However, relating observed differences in single cells to the capacity of such alterations to result in the extreme changes in ventricular shape seen in Takotsubo syndrome is difficult. By using a computational model of the rat left ventricle, we investigate which mechanisms can give rise to the typical shape of the ventricle observed in this syndrome. Three potential dominant mechanisms related to effects of β-adrenergic stimulation were considered: apical-basal variation of calcium transients due to differences in L-type and sarco(endo)plasmic reticulum Ca²⁺-ATPase activation, apical-basal variation of calcium sensitivity due to differences in troponin I phosphorylation, and apical-basal variation in maximal active tension due to, e.g., the negative inotropic effects of p38 MAPK. Furthermore, we investigated the interaction of these spatial variations in the presence of a failing Frank-Starling mechanism. We conclude that a large portion of the apex needs to be affected by severe changes in calcium regulation or contractile function to result in apical ballooning, and smooth linear variation from apex to base is unlikely to result in the typical ventricular shape observed in this syndrome. A failing Frank-Starling mechanism significantly increases apical ballooning at end systole and may be an important additional factor underpinning Takotsubo syndrome.

The contribution of cardiac myosin binding protein-c Ser282 phosphorylation to the rate of force generation and in vivo cardiac contractility

Cardiac myosin binding protein-C phosphorylation plays an important role in modulating cardiac muscle function and accelerating contraction. It has been proposed that Ser282 phosphorylation may serve as a critical molecular switch that regulates the phosphorylation of neighbouring Ser273 and Ser302 residues, and thereby govern myofilament contractile acceleration in response to protein kinase A (PKA). Therefore, to determine the regulatory roles of Ser282 we generated a transgenic (TG) mouse model expressing cardiac myosin binding protein-C with a non-phosphorylatable Ser282 (i.e. serine to alanine substitution, TG(S282A)). Myofibrils isolated from TG(S282A) hearts displayed robust PKA-mediated phosphorylation of Ser273 and Ser302, and the increase in phosphorylation was identical to TG wild-type (TG(WT)) controls. No signs of pathological cardiac hypertrophy were detected in TG(S282A) hearts by either histological examination of cardiac sections or echocardiography. Baseline fractional shortening, ejection fraction, isovolumic relaxation time, rate of pressure development and rate of relaxation (τ) were unaltered in TG(S282A) mice. However, the increase in cardiac contractility as well as the acceleration of pressure development observed in response to β-adrenergic stimulation was attenuated in TG(S282A) mice. In agreement with our in vivo data, in vitro force measurements revealed that PKA-mediated acceleration of cross-bridge kinetics in TG(S282A) myocardium was significantly attenuated compared to TG(WT) myocardium. Taken together, our data suggest that while Ser282 phosphorylation does not regulate the phosphorylation of neighbouring Ser residues and basal cardiac function, full acceleration of cross-bridge kinetics and left ventricular pressure development cannot be achieved in its absence.

Cardiac dysfunction is associated with altered sarcomere ultrastructure in intrauterine growth restriction

OBJECTIVE

The purpose of this study was to assess whether abnormal cardiac function in human fetuses with intrauterine growth restriction (IUGR) is associated with ultrastructural differences in the cardiomyocyte sarcomere.

STUDY DESIGN

Nine severe early-onset IUGR fetuses and 9 normally grown fetuses (appropriate growth for gestational age) who died in the perinatal period were included prospectively. Cardiac function was assessed by echocardiography and levels of B-type natriuretic peptide and troponin-I. Heart sections were imaged by second harmonic generation microscopy, which allowed unstained visualization of cardiomyocyte's sarcomere length.

RESULTS

Echocardiographic and biochemical markers showed signs of severe cardiac dysfunction in IUGR fetuses. Second harmonic generation microscopy demonstrated a significantly shorter sarcomere length in IUGR as compared with appropriate growth for gestational age fetuses.

CONCLUSION

IUGR is associated with changes in the cardiomyocyte contractile machinery in the form of shorter sarcomere length, which could help to explain the cardiac dysfunction previously documented in IUGR.

Sarcomere length nanometry in rat neonatal cardiomyocytes expressed with α-actinin-AcGFP in Z discs

Nanometry is widely used in biological sciences to analyze the movement of molecules or molecular assemblies in cells and in vivo. In cardiac muscle, a change in sarcomere length (SL) by a mere ∼100 nm causes a substantial change in contractility, indicating the need for the simultaneous measurement of SL and intracellular Ca(2+) concentration ([Ca(2+)]i) in cardiomyocytes at high spatial and temporal resolution. To accurately analyze the motion of individual sarcomeres with nanometer precision during excitation-contraction coupling, we applied nanometry techniques to primary-cultured rat neonatal cardiomyocytes. First, we developed an experimental system for simultaneous nanoscale analysis of single sarcomere dynamics and [Ca(2+)]i changes via the expression of AcGFP in Z discs. We found that the averaging of the lengths of sarcomeres along the myocyte, a method generally used in today's myocardial research, caused marked underestimation of sarcomere lengthening speed because of the superpositioning of different timings for lengthening between sequentially connected sarcomeres. Then, we found that after treatment with ionomycin, neonatal myocytes exhibited spontaneous sarcomeric oscillations (cell-SPOCs) at partial activation with blockage of sarcoplasmic reticulum functions, and the waveform properties were indistinguishable from those obtained in electric field stimulation. The myosin activator omecamtiv mecarbil markedly enhanced Z-disc displacement during cell-SPOC. Finally, we interpreted the present experimental findings in the framework of our mathematical model of SPOCs. The present experimental system has a broad range of application possibilities for unveiling single sarcomere dynamics during excitation-contraction coupling in cardiomyocytes under various settings.

Maladaptive modifications in myofilament proteins and triggers in the progression to heart failure and sudden death

In this review, we address the following question: Are modifications at the level of sarcomeric proteins in acquired heart failure early inducers of altered cardiac dynamics and signaling leading to remodeling and progression to decompensation? There is no doubt that most inherited cardiomyopathies are caused by mutations in proteins of the sarcomere. We think this linkage indicates that early changes at the level of the sarcomeres in acquired cardiac disorders may be significant in triggering the progression to failure. We consider evidence that there are rate-limiting mechanisms downstream of the trigger event of Ca(2+) binding to troponin C, which control cardiac dynamics. We discuss new perspectives on how modifications in these mechanisms may be of relevance to redox signaling in diastolic heart failure, to angiotensin II signaling via β-arrestin, and to remodeling related to altered structural rigidity of tropomyosin. We think that these new perspectives provide a rationale for future studies directed at a more thorough understanding of the question driving our review.

Sildenafil improves exercise hemodynamics in Fontan patients

BACKGROUND

Patients with Fontan circulation have reduced exercise capacity. The absence of a presystemic pump may limit flow through the pulmonary circulation, restricting ventricular filling and cardiac output. We evaluated exercise hemodynamics and the effect of sildenafil on exercise hemodynamics in Fontan patients.

METHODS AND RESULTS

Ten Fontan patients (6 men, 20±4 years) underwent cardiac magnetic resonance imaging at rest and during supine bicycle exercise before and after sildenafil. Systemic ventricular volumes were obtained at rest and during low- (34±15 W), moderate- (69±29 W), and high-intensity (97±36 W) exercise using an ungated, free-breathing cardiac magnetic resonance sequence and analyzed correcting for cardiac phase and respiratory translation. Radial and pulmonary artery pressures and cGMP were measured. Before sildenafil, cardiac index increased throughout exercise (4.0±0.9, 5.9±1.1, 7.0±1.6, 7.4±1.7 L/(min·m(2)); P<0.0001) with 106±49% increase in heart rate. Stroke volume index (P=0.015) and end-diastolic volume index (P=0.001) decreased during exercise. End-systolic volume index remained unchanged (P=0.8). Total pulmonary resistance index (P=0.005) increased, whereas systemic vascular resistance index decreased during exercise (P<0.0001). Sildenafil increased cardiac index (P<0.0001) and stroke volume index (P=0.003), especially at high-intensity exercise (interaction P=0.004 and P=0.003, respectively). Systemic vascular resistance index was reduced (P<0.0001-interaction P=0.1), whereas total pulmonary resistance index was reduced at rest and reduced further during exercise (P=0.008-interaction P=0.029). cGMP remained unchanged before sildenafil (P=0.9), whereas it increased significantly after sildenafil (P=0.019).

CONCLUSIONS

In Fontan patients, sildenafil improved cardiac index during exercise with a decrease in total pulmonary resistance index and an increase in stroke volume index. This implies that pulmonary vasculature represents a physiological limitation, which can be attenuated by sildenafil, the clinical significance of which warrants further study.

Earning stripes: myosin binding protein-C interactions with actin

Myosin binding protein-C (MyBP-C) was first discovered as an impurity during the purification of myosin from skeletal muscle. However, soon after its discovery, MyBP-C was also shown to bind actin. While the unique functional implications for a protein that could cross-link thick and thin filaments together were immediately recognized, most early research nonetheless focused on interactions of MyBP-C with the thick filament. This was in part because interactions of MyBP-C with the thick filament could adequately explain most (but not all) effects of MyBP-C on actomyosin interactions and in part because the specificity of actin binding was uncertain. However, numerous recent studies have now established that MyBP-C can indeed bind to actin through multiple binding sites, some of which are highly specific. Many of these interactions involve critical regulatory domains of MyBP-C that are also reported to interact with myosin. Here we review current evidence supporting MyBP-C interactions with actin and discuss these findings in terms of their ability to account for the functional effects of MyBP-C. We conclude that the influence of MyBP-C on muscle contraction can be explained equally well by interactions with actin as by interactions with myosin. However, because data showing that MyBP-C binds to either myosin or actin has come almost exclusively from in vitro biochemical studies, the challenge for future studies is to define which binding partner(s) MyBP-C interacts with in vivo.

Depressed Frank-Starling mechanism in the left ventricular muscle of the knock-in mouse model of dilated cardiomyopathy with troponin T deletion mutation ΔK210

It has been reported that the Frank-Starling mechanism is coordinately regulated in cardiac muscle via thin filament "on-off" equilibrium and titin-based lattice spacing changes. In the present study, we tested the hypothesis that the deletion mutation ΔK210 in the cardiac troponin T gene shifts the equilibrium toward the "off" state and accordingly attenuate the sarcomere length (SL) dependence of active force production, via reduced cross-bridge formation. Confocal imaging in isolated hearts revealed that the cardiomyocytes were enlarged, especially in the longitudinal direction, in ΔK210 hearts, with striation patterns similar to those in wild type (WT) hearts, suggesting that the number of sarcomeres is increased in cardiomyocytes but the sarcomere length remains unaltered. For analysis of the SL dependence of active force, skinned muscle preparations were obtained from the left ventricle of WT and knock-in (ΔK210) mice. An increase in SL from 1.90 to 2.20μm shifted the mid-point (pCa50) of the force-pCa curve leftward by ~0.21pCa units in WT preparations. In ΔK210 muscles, Ca(2+) sensitivity was lower by ~0.37pCa units, and the SL-dependent shift of pCa50, i.e., ΔpCa50, was less pronounced (~0.11pCa units), with and without protein kinase A treatment. The rate of active force redevelopment was lower in ΔK210 preparations than in WT preparations, showing blunted thin filament cooperative activation. An increase in thin filament cooperative activation upon an increase in the fraction of strongly bound cross-bridges by MgADP increased ΔpCa50 to ~0.21pCa units. The depressed Frank-Starling mechanism in ΔK210 hearts is the result of a reduction in thin filament cooperative activation.