Late-life Rapamycin Treatment Enhances Cardiomyocyte Relaxation Kinetics and Reduces Myocardial Stiffness.

Diastolic dysfunction is a key feature of the aging heart. We have shown that late-life treatment with mTOR inhibitor, rapamycin, reverses age-related diastolic dysfunction in mice but the molecular mechanisms of the reversal remain unclear. To dissect the mechanisms by which rapamycin improves diastolic function in old mice, we examined the effects of rapamycin treatment at the levels of single cardiomyocyte, myofibril and multicellular cardiac muscle. Compared to young cardiomyocytes, isolated cardiomyocytes from old control mice exhibited prolonged time to 90% relaxation (RT 90 ) and time to 90% Ca 2+ transient decay (DT 90 ), indicating slower relaxation kinetics and calcium reuptake with age. Late-life rapamycin treatment for 10 weeks completely normalized RT 90 and partially normalized DT 90 , suggesting improved Ca 2+ handling contributes partially to the rapamycin-induced improved cardiomyocyte relaxation. In addition, rapamycin treatment in old mice enhanced the kinetics of sarcomere shortening and Ca 2+ transient increase in old control cardiomyocytes. Myofibrils from old rapamycin-treated mice displayed increased rate of the fast, exponential decay phase of relaxation compared to old controls. The improved myofibrillar kinetics were accompanied by an increase in MyBP-C phosphorylation at S282 following rapamycin treatment. We also showed that late-life rapamycin treatment normalized the age-related increase in passive stiffness of demembranated cardiac trabeculae through a mechanism independent of titin isoform shift. In summary, our results showed that rapamycin treatment normalizes the age-related impairments in cardiomyocyte relaxation, which works conjointly with reduced myocardial stiffness to reverse age-related diastolic dysfunction.

Measuring risks and vulnerability of tourism to the COVID-19 crisis in the context of extreme uncertainty: The case of the Balearic Islands.

The COVID-19 crisis is dramatically affecting the world economy and, particularly, the tourism sector. In the context of extreme uncertainty, the use of probabilistic forecasting models is especially suitable. We use Monte Carlo simulations to evaluate the outcomes of four possible tourism demand recovery scenarios in the Balearic Islands, which are further used to measure the risks and vulnerability of Balearic economy to the COVID-19 crisis. Our results show that fear of contagion and loss of income in tourism emitting countries will result in a maximum 89% drop in arrivals in the Balearic Islands in 2020.Given that most tourism-related occupations are not highly skilled and are characterized by lower salaries, there are greater risks of loss of welfare, especially for women, who are a major share of the tourism labour force.The model shows important differences among minimum, average and maximum estimates for tourism sector production in 2021, reflecting considerable uncertainty regarding the speed of the sector's recovery. The results serve as a basis to prepare a range of policies to reduce destination vulnerability under different crisis outcomes.

Rapamycin persistently improves cardiac function in aged, male and female mice, even following cessation of treatment.

Even in healthy aging, cardiac morbidity and mortality increase with age in both mice and humans. These effects include a decline in diastolic function, left ventricular hypertrophy, metabolic substrate shifts, and alterations in the cardiac proteome. Previous work from our laboratory indicated that short-term (10-week) treatment with rapamycin, an mTORC1 inhibitor, improved measures of these age-related changes. In this report, we demonstrate that the rapamycin-dependent improvement of diastolic function is highly persistent, while decreases in both cardiac hypertrophy and passive stiffness are substantially persistent 8 weeks after cessation of an 8-week treatment of rapamycin in both male and female 22- to 24-month-old C57BL/6NIA mice. The proteomic and metabolomic abundance changes that occur after 8 weeks of rapamycin treatment have varying persistence after 8 further weeks without the drug. However, rapamycin did lead to a persistent increase in abundance of electron transport chain (ETC) complex components, most of which belonged to Complex I. Although ETC protein abundance and Complex I activity were each differentially affected in males and females, the ratio of Complex I activity to Complex I protein abundance was equally and persistently reduced after rapamycin treatment in both sexes. Thus, rapamycin treatment in the aged mice persistently improved diastolic function and myocardial stiffness, persistently altered the cardiac proteome in the absence of persistent metabolic changes, and led to persistent alterations in mitochondrial respiratory chain activity. These observations suggest that an optimal translational regimen for rapamycin therapy that promotes enhancement of healthspan may involve intermittent short-term treatments.

Novel Adult-Onset Systolic Cardiomyopathy Due to MYH7 E848G Mutation in Patient-Derived Induced Pluripotent Stem Cells.

A novel myosin heavy chain 7 mutation (E848G) identified in a familial cardiomyopathy was studied in patient-specific induced pluripotent stem cell-derived cardiomyocytes. The cardiomyopathic human induced pluripotent stem cell-derived cardiomyocytes exhibited reduced contractile function as single cells and engineered heart tissues, and genome-edited isogenic cells confirmed the pathogenic nature of the E848G mutation. Reduced contractility may result from impaired interaction between myosin heavy chain 7 and cardiac myosin binding protein C.

Mechanical Stress Conditioning and Electrical Stimulation Promote Contractility and Force Maturation of Induced Pluripotent Stem Cell-Derived Human Cardiac Tissue.

BACKGROUND: Tissue engineering enables the generation of functional human cardiac tissue with cells derived in vitro in combination with biocompatible materials. Human-induced pluripotent stem cell-derived cardiomyocytes provide a cell source for cardiac tissue engineering; however, their immaturity limits their potential applications. Here we sought to study the effect of mechanical conditioning and electric pacing on the maturation of human-induced pluripotent stem cell-derived cardiac tissues. METHODS: Cardiomyocytes derived from human-induced pluripotent stem cells were used to generate collagen-based bioengineered human cardiac tissue. Engineered tissue constructs were subjected to different mechanical stress and electric pacing conditions. RESULTS: The engineered human myocardium exhibits Frank-Starling-type force-length relationships. After 2 weeks of static stress conditioning, the engineered myocardium demonstrated increases in contractility (0.63±0.10 mN/mm2 vs 0.055±0.009 mN/mm2 for no stress), tensile stiffness, construct alignment, and cell size. Stress conditioning also increased SERCA2 (Sarco/Endoplasmic Reticulum Calcium ATPase 2) expression, which correlated with a less negative force-frequency relationship. When electric pacing was combined with static stress conditioning, the tissues showed an additional increase in force production (1.34±0.19 mN/mm2), with no change in construct alignment or cell size, suggesting maturation of excitation-contraction coupling. Supporting this notion, we found expression of RYR2 (Ryanodine Receptor 2) and SERCA2 further increased by combined static stress and electric stimulation. CONCLUSIONS: These studies demonstrate that electric pacing and mechanical stimulation promote maturation of the structural, mechanical, and force generation properties of human-induced pluripotent stem cell-derived cardiac tissues.

Stromal Cells in Dense Collagen Promote Cardiomyocyte and Microvascular Patterning in Engineered Human Heart Tissue.

Cardiac tissue engineering is a strategy to replace damaged contractile tissue and model cardiac diseases to discover therapies. Current cardiac and vascular engineering approaches independently create aligned contractile tissue or perfusable vasculature, but a combined vascularized cardiac tissue remains to be achieved. Here, we sought to incorporate a patterned microvasculature into engineered heart tissue, which balances the competing demands from cardiomyocytes to contract the matrix versus the vascular lumens that need structural support. Low-density collagen hydrogels (1.25 mg/mL) permit human embryonic stem cell-derived cardiomyocytes (hESC-CMs) to form a dense contractile tissue but cannot support a patterned microvasculature. Conversely, high collagen concentrations (density ≥6 mg/mL) support a patterned microvasculature, but the hESC-CMs lack cell-cell contact, limiting their electrical communication, structural maturation, and tissue-level contractile function. When cocultured with matrix remodeling stromal cells, however, hESC-CMs structurally mature and form anisotropic constructs in high-density collagen. Remodeling requires the stromal cells to be in proximity with hESC-CMs. In addition, cocultured cardiac constructs in dense collagen generate measurable active contractions (on the order of 0.1 mN/mm(2)) and can be paced up to 2 Hz. Patterned microvascular networks in these high-density cocultured cardiac constructs remain patent through 2 weeks of culture, and hESC-CMs show electrical synchronization. The ability to maintain microstructural control within engineered heart tissue enables generation of more complex features, such as cellular alignment and a vasculature. Successful incorporation of these features paves the way for the use of large scale engineered tissues for myocardial regeneration and cardiac disease modeling.

MMP-10 Regulates Collagenolytic Activity of Alternatively Activated Resident Macrophages.

Matrix metalloproteinase-10 (MMP-10) is expressed by macrophages and epithelium in response to injury, but its functions in wound repair are unknown. We observed increased collagen deposition and skin stiffness in Mmp10(-/-) wounds, with no difference in collagen expression or reepithelialization. Increased collagen deposition in Mmp10(-/-) wounds was accompanied by less collagenolytic activity and reduced expression of specific metallocollagenases, particularly MMP-8 and MMP-13, where MMP-13 was the key collagenase. Ablation and adoptive transfer approaches and cell-based models demonstrated that the MMP-10-dependent collagenolytic activity was a product of alternatively activated (M2) resident macrophages. These data demonstrate a critical role for macrophage MMP-10 in controlling the tissue remodeling activity of macrophages and moderating scar formation during wound repair.

Mechanical Stress Promotes Maturation of Human Myocardium From Pluripotent Stem Cell-Derived Progenitors.

Recent advances in pluripotent stem cell biology and directed differentiation have identified a population of human cardiovascular progenitors that give rise to cardiomyocytes, smooth muscle, and endothelial cells. Because the heart develops from progenitors in 3D under constant mechanical load, we sought to test the effects of a 3D microenvironment and mechanical stress on differentiation and maturation of human cardiovascular progenitors into myocardial tissue. Progenitors were derived from embryonic stem cells, cast into collagen hydrogels, and left unstressed or subjected to static or cyclic mechanical stress. Compared to 2D culture, the unstressed 3D environment increased cardiomyocyte numbers and decreased smooth muscle numbers. Additionally, 3D culture suppressed smooth muscle α-actin content, suggesting diminished cell maturation. Cyclic stress-conditioning increased expression of several cardiac markers, including ß-myosin heavy chain and cardiac troponin T, and the tissue showed enhanced calcium dynamics and force production. There was no effect of mechanical loading on cardiomyocyte or smooth muscle specification. Thus, 3D growth conditions favor cardiac differentiation from cardiovascular progenitors, whereas 2D conditions promote smooth muscle differentiation. Mechanical loading promotes cardiomyocyte structural and functional maturation. Culture in 3-D facilitates understanding how cues such as mechanical stress affect the differentiation and morphogenesis of distinct cardiovascular cell populations into organized, functional human cardiovascular tissue. Stem Cells 2015;33:2148-2157.

2-Deoxy adenosine triphosphate improves contraction in human end-stage heart failure.

We are developing a novel treatment for heart failure by increasing myocardial 2 deoxy-ATP (dATP). Our studies in rodent models have shown that substitution of dATP for adenosine triphosphate (ATP) as the energy substrate in vitro or elevation of dATP in vivo increases myocardial contraction and that small increases in the native dATP pool of heart muscle are sufficient to improve cardiac function. Here we report, for the first time, the effect of dATP on human adult cardiac muscle contraction. We measured the contractile properties of chemically-demembranated multicellular ventricular wall preparations and isolated myofibrils from human subjects with end-stage heart failure. Isometric force was increased at both saturating and physiologic Ca(2+) concentrations with dATP compared to ATP. This resulted in an increase in the Ca(2+) sensitivity of force (pCa50) by 0.06 pCa units. The rate of force redevelopment (ktr) in demembranated wall muscle was also increased, as was the rate of contractile activation (kACT) in isolated myofibrils, indicating increased cross-bridge binding and cycling compared with ATP in failing human myocardium. These data suggest that dATP could increase dP/dT and end systolic pressure in failing human myocardium. Importantly, even though the magnitude and rate of force development were increased, there was no increase in the time to 50% and 90% myofibril relaxation. These data, along with our previous studies in rodent models, show the promise of elevating myocardial dATP to enhance contraction and restore cardiac pump function. These data also support further pre-clinical evaluation of this new approach for treating heart failure.

Thin filament incorporation of an engineered cardiac troponin C variant (L48Q) enhances contractility in intact cardiomyocytes from healthy and infarcted hearts.

Many current pharmaceutical therapies for systolic heart failure target intracellular [Ca(2+)] ([Ca(2+)]i) metabolism, or cardiac troponin C (cTnC) on thin filaments, and can have significant side-effects, including arrhythmias or adverse effects on diastolic function. In this study, we tested the feasibility of directly increasing the Ca(2+) binding properties of cTnC to enhance contraction independent of [Ca(2+)]i in intact cardiomyocytes from healthy and myocardial infarcted (MI) hearts. Specifically, cardiac thin filament activation was enhanced through adenovirus-mediated over-expression of a cardiac troponin C (cTnC) variant designed to have increased Ca(2+) binding affinity conferred by single amino acid substitution (L48Q). In skinned cardiac trabeculae and myofibrils we and others have shown that substitution of L48Q cTnC for native cTnC increases Ca(2+) sensitivity of force and the maximal rate of force development. Here we introduced L48Q cTnC into myofilaments of intact cardiomyocytes via adeno-viral transduction to deliver cDNA for the mutant or wild type (WT) cTnC protein. Using video-microscopy to monitor cell contraction, relaxation, and intracellular Ca(2+) transients (Fura-2), we report that incorporation of L48Q cTnC significantly increased contractility of cardiomyocytes from healthy and MI hearts without adversely affecting Ca(2+) transient properties or relaxation. The improvements in contractility from L48Q cTnC expression are likely the result of enhanced contractile efficiency, as intracellular Ca(2+) transient amplitudes were not affected. Expression and incorporation of L48Q cTnC into myofilaments was confirmed by Western blot analysis of myofibrils from transduced cardiomyocytes, which indicated replacement of 18±2% of native cTnC with L48Q cTnC. These experiments demonstrate the feasibility of directly targeting cardiac thin filament proteins to enhance cardiomyocyte contractility that is impaired following MI.

uPARAP function in cutaneous wound repair.

Optimal skin wound healing relies on tight balance between collagen synthesis and degradation in new tissue formation and remodeling phases. The endocytic receptor uPARAP regulates collagen uptake and intracellular degradation. In this study we examined cutaneous wound repair response of uPARAP null (uPARAP-/-) mice. Full thickness wounds were created on dorsal surface of uPARAP-/- or their wildtype littermates. Wound healing evaluation was done by macroscopic observation, histology, gene transcription and biochemical analysis at specific intervals. We found that absence of uPARAP delayed re-epithelialization during wound closure, and altered stiffness of the scar tissue. Despite the absence of the uPARAP-mediated intracellular pathway for collagen degradation, there was no difference in total collagen content of the wounds in uPARAP-/- compared to wildtype mice. This suggests in the absence of uPARAP, a compensatory feedback mechanism functions to keep net collagen in balance.

N-terminal phosphorylation of cardiac troponin-I reduces length-dependent calcium sensitivity of contraction in cardiac muscle.

Protein kinase A (PKA) phosphorylation of myofibrillar proteins constitutes an important pathway for ß-adrenergic modulation of cardiac contractility. In myofilaments PKA targets troponin I (cTnI), myosin binding protein-C (cMyBP-C) and titin. We studied how this affects the sarcomere length (SL) dependence of force-pCa relations in demembranated cardiac muscle. To distinguish cTnI from cMyBP-C/titin phosphorylation effects on the force-pCa relationship, endogenous troponin (Tn) was exchanged in rat ventricular trabeculae with either wild-type (WT) Tn, non-phosphorylatable cTnI (S23/24A) Tn or phosphomimetic cTnI (S23/24D) Tn. PKA cannot phosphorylate either cTnI S23/24 variant, leaving cMyBP-C/titin as PKA targets. Force was measured at 2.3 and 2.0 µm SL. Decreasing SL reduced maximal force (F(max)) and Ca(2+) sensitivity of force (pCa(50)) similarly with WT and S23/24A trabeculae. PKA treatment of WT and S23/24A trabeculae reduced pCa(50) at 2.3 but not at 2.0 µm SL, thus eliminating the SL dependence of pCa(50). In contrast, S23/24D trabeculae reduced pCa(50) at both SL values, primarily at 2.3 µm, also eliminating SL dependence of pCa(50). Subsequent PKA treatment moderately reduced pCa(50) at both SLs. At each SL, F(max) was unaffected by either Tn exchange and/or PKA treatment. Low-angle X-ray diffraction was performed to determine whether pCa(50) shifts were associated with changes in myofilament spacing (d(1,0)) or thick-thin filament interaction. PKA increased d(1,0) slightly under all conditions. The ratios of the integrated intensities of the equatorial X-ray reflections (I(1,1)/I(1,0)) indicate that PKA treatment increased crossbridge proximity to thin filaments under all conditions. The results suggest that phosphorylation by PKA of either cTnI or cMyBP-C/titin independently reduces the pCa(50) preferentially at long SL, possibly through reduced availability of thin filament binding sites (cTnI) or altered crossbridge recruitment (cMyBP-C/titin). Preferential reduction of pCa(50) at long SL may not reduce cardiac output during periods of high metabolic demand because of increased intracellular Ca(2+) during ß-adrenergic stimulation.

Enhanced Ca2+ binding of cardiac troponin reduces sarcomere length dependence of contractile activation independently of strong crossbridges.

Calcium sensitivity of the force-pCa relationship depends strongly on sarcomere length (SL) in cardiac muscle and is considered to be the cellular basis of the Frank-Starling law of the heart. SL dependence may involve changes in myofilament lattice spacing and/or myosin crossbridge orientation to increase probability of binding to actin at longer SLs. We used the L48Q cardiac troponin C (cTnC) variant, which has enhanced Ca(2+) binding affinity, to test the hypotheses that the intrinsic properties of cTnC are important in determining 1) thin filament binding site availability and responsiveness to crossbridge activation and 2) SL dependence of force in cardiac muscle. Trabeculae containing L48Q cTnC-cTn lost SL dependence of the Ca(2+) sensitivity of force. This occurred despite maintaining the typical SL-dependent changes in maximal force (F(max)). Osmotic compression of preparations at SL 2.0 µm with 3% dextran increased F(max) but not pCa(50) in L48Q cTnC-cTn exchanged trabeculae, whereas wild-type (WT)-cTnC-cTn exchanged trabeculae exhibited increases in both F(max) and pCa(50). Furthermore, crossbridge inhibition with 2,3-butanedione monoxime at SL 2.3 µm decreased F(max) and pCa(50) in WT cTnC-cTn trabeculae to levels measured at SL 2.0 µm, whereas only F(max) was decreased with L48Q cTnC-cTn. Overall, these results suggest that L48Q cTnC confers reduced crossbridge dependence of thin filament activation in cardiac muscle and that changes in the Ca(2+) sensitivity of force in response to changes in SL are at least partially dependent on properties of thin filament troponin.

Transcription factor CHF1/Hey2 regulates EC coupling and heart failure in mice through regulation of FKBP12.6.

Heart failure is a leading cause of morbidity and mortality in Western society. The cardiovascular transcription factor CHF1/Hey2 has been linked to experimental heart failure in mice, but the mechanisms by which it regulates myocardial function remain incompletely understood. The objective of this study was to determine how CHF1/Hey2 affects development of heart failure through examination of contractility in a myocardial knockout mouse model. We generated myocardial-specific knockout mice. At baseline, cardiac function was normal, but, after aortic banding, the conditional knockout mice demonstrated a greater increase in ventricular weight-to-body weight ratio compared with control mice (5.526 vs. 4.664 mg/g) and a significantly decreased ejection fraction (47.8 vs. 72.0% control). Isolated cardiac myocytes from these mice showed decreased calcium transients and fractional shortening after electrical stimulation. To determine the molecular basis for these alterations in excitation-contraction coupling, we first measured total sarcoplasmic reticulum calcium stores and calcium-dependent force generation in isolated muscle fibers, which were normal, suggesting a defect in calcium cycling. Analysis of gene expression demonstrated normal expression of most genes known to be involved in myocardial calcium cycling, with the exception of the ryanodine receptor binding protein FKBP12.6, which was expressed at increased levels in the conditional knockout hearts. Treatment of the isolated knockout myocytes with FK506, which inhibits the association of FKBP12.6 with the ryanodine receptor, restored contractile function. These findings demonstrate that conditional deletion of CHF1/Hey2 in the myocardium leads to abnormalities in calcium handling mediated by FKBP12.6 that predispose to pressure overload-induced heart failure.

Growth of engineered human myocardium with mechanical loading and vascular coculture.

RATIONALE: The developing heart requires both mechanical load and vascularization to reach its proper size, yet the regulation of human heart growth by these processes is poorly understood. OBJECTIVE: We seek to elucidate the responses of immature human myocardium to mechanical load and vascularization using tissue engineering approaches. METHODS AND RESULTS: Using human embryonic stem cell and human induced pluripotent stem cell-derived cardiomyocytes in a 3-dimensional collagen matrix, we show that uniaxial mechanical stress conditioning promotes 2-fold increases in cardiomyocyte and matrix fiber alignment and enhances myofibrillogenesis and sarcomeric banding. Furthermore, cyclic stress conditioning markedly increases cardiomyocyte hypertrophy (2.2-fold) and proliferation rates (21%) versus unconditioned constructs. Addition of endothelial cells enhances cardiomyocyte proliferation under all stress conditions (14% to 19%), and addition of stromal supporting cells enhances formation of vessel-like structures by ≈10-fold. Furthermore, these optimized human cardiac tissue constructs generate Starling curves, increasing their active force in response to increased resting length. When transplanted onto hearts of athymic rats, the human myocardium survives and forms grafts closely apposed to host myocardium. The grafts contain human microvessels that are perfused by the host coronary circulation. CONCLUSIONS: Our results indicate that both mechanical load and vascular cell coculture control cardiomyocyte proliferation, and that mechanical load further controls the hypertrophy and architecture of engineered human myocardium. Such constructs may be useful for studying human cardiac development as well as for regenerative therapy.

Effects of low-level α-myosin heavy chain expression on contractile kinetics in porcine myocardium.

Myosin heavy chain (MHC) isoforms are principal determinants of work capacity in mammalian ventricular myocardium. The ventricles of large mammals including humans normally express ∼10% α-MHC on a predominantly ß-MHC background, while in failing human ventricles α-MHC is virtually eliminated, suggesting that low-level α-MHC expression in normal myocardium can accelerate the kinetics of contraction and augment systolic function. To test this hypothesis in a model similar to human myocardium we determined composite rate constants of cross-bridge attachment (f(app)) and detachment (g(app)) in porcine myocardium expressing either 100% α-MHC or 100% ß-MHC in order to predict the MHC isoform-specific effect on twitch kinetics. Right atrial (∼100% α-MHC) and left ventricular (∼100% ß-MHC) tissue was used to measure myosin ATPase activity, isometric force, and the rate constant of force redevelopment (k(tr)) in solutions of varying Ca(2+) concentration. The rate of ATP utilization and k(tr) were approximately ninefold higher in atrial compared with ventricular myocardium, while tension cost was approximately eightfold greater in atrial myocardium. From these values, we calculated f(app) to be ∼10-fold higher in α- compared with ß-MHC, while g(app) was 8-fold higher in α-MHC. Mathematical modeling of an isometric twitch using these rate constants predicts that the expression of 10% α-MHC increases the maximal rate of rise of force (dF/dt(max)) by 92% compared with 0% α-MHC. These results suggest that low-level expression of α-MHC significantly accelerates myocardial twitch kinetics, thereby enhancing systolic function in large mammalian myocardium.

Determination of rate constants for turnover of myosin isoforms in rat myocardium: implications for in vivo contractile kinetics.

The ventricles of small mammals express mostly alpha-myosin heavy chain (alpha-MHC), a fast isoform, whereas the ventricles of large mammals, including humans, express approximately 10% alpha-MHC on a predominately beta-MHC (slow isoform) background. In failing human ventricles, the amount of alpha-MHC is dramatically reduced, leading to the hypothesis that even small amounts of alpha-MHC on a predominately beta-MHC background confer significantly higher rates of force development in healthy ventricles. To test this hypothesis, it is necessary to determine the fundamental rate constants of cross-bridge attachment (f(app)) and detachment (g(app)) for myosins composed of 100% alpha-MHC or beta-MHC, which can then be used to calculate twitch time courses for muscles expressing variable ratios of MHC isoforms. In the present study, rat skinned trabeculae expressing either 100% alpha-MHC or 100% beta-MHC were used to measure ATPase activity, isometric force, and the rate constant of force redevelopment (k(tr)) in solutions of varying Ca(2+) concentrations. The rate of ATP utilization was approximately 2.5-fold higher in preparations expressing 100% alpha-MHC compared with those expressing only beta-MHC, whereas k(tr) was 2-fold faster in the alpha-MHC myocardium. From these variables, we calculated f(app) to be approximately threefold higher for alpha-MHC than beta-MHC and g(app) to be twofold higher in alpha-MHC. Mathematical modeling of isometric twitches predicted that small increases in alpha-MHC significantly increased the rate of force development. These results suggest that low-level expression of alpha-MHC has significant effects on contraction kinetics.

Contribution of the myosin binding protein C motif to functional effects in permeabilized rat trabeculae.

Myosin binding protein C (MyBP-C) is a thick-filament protein that limits cross-bridge cycling rates and reduces myocyte power output. To investigate mechanisms by which MyBP-C affects contraction, we assessed effects of recombinant N-terminal domains of cardiac MyBP-C (cMyBP-C) on contractile properties of permeabilized rat cardiac trabeculae. Here, we show that N-terminal fragments of cMyBP-C that contained the first three immunoglobulin domains of cMyBP-C (i.e., C0, C1, and C2) plus the unique linker sequence termed the MyBP-C "motif" or "m-domain" increased Ca(2+) sensitivity of tension and increased rates of tension redevelopment (i.e., k(tr)) at submaximal levels of Ca(2+). At concentrations > or =20 microM, recombinant proteins also activated force in the absence of Ca(2+) and inhibited maximum Ca(2+)-activated force. Recombinant proteins that lacked the combination of C1 and the motif did not affect contractile properties. These results suggest that the C1 domain plus the motif constitute a functional unit of MyBP-C that can activate the thin filament.

Comparing guiding track requirements for myosin- and kinesin-powered molecular shuttles.

The design of nanoscale transport systems utilizing motor proteins as engines has advanced rapidly. Here, actin/myosin- and microtubule/kinesin-based molecular shuttles are compared with respect to their requirements for track designs. To this end, the trajectory persistence length of actin filaments gliding on myosin-coated surfaces has been experimentally determined to be equal to 8.8 +/- 2 microm. This measurement complements an earlier determination of the trajectory persistence length of microtubules gliding on kinesin-coated surfaces and enables a comparison of the accessible track designs for kinesin and myosin motor-powered systems. Despite the 200-fold smaller stiffness of actin filaments compared to that of microtubules, the dimensions of myosin tracks for actin filaments have to be quite similar to the dimensions of kinesin tracks for microtubules (radii larger than 200 nm and widths smaller than 0.9 microm compared to 600 nm and 19 microm). The difference in gliding speed is shown to require additional consideration in the design of track modules.

Myosin S2 is not required for effects of myosin binding protein-C on motility.

The unique myosin binding protein-c "motif" near the N-terminus of myosin binding protein-C (MyBP-C) binds myosin S2. Previous studies demonstrated that recombinant proteins containing the motif and flanking regions (e.g., C1C2) affect thin filament movement in motility assays using heavy meromyosin (S1 plus S2) as the molecular motor. To determine if S2 is required for these effects we investigated whether C1C2 affects motility in assays using only myosin S1 as the motor protein. Results demonstrate that effects of C1C2 are comparable in both systems and suggest that the MyBP-C motif affects motility through direct interactions with actin and/or myosin S1.