Effects of Cardiac Myosin Isoform Variation on Myofilament Function and Crossbridge Kinetics in Transgenic Rabbits

Department of Medicine, Cardiology Unit, Fletcher Allen Health Care, Burlington, VT 05401, USA.
Circulation Heart Failure (Impact Factor: 5.89). 07/2009; 2(4):334-41. DOI: 10.1161/CIRCHEARTFAILURE.108.802298
Source: PubMed


The left ventricles of both rabbits and humans express predominantly beta-myosin heavy chain (MHC). Transgenic (TG) rabbits expressing 40% alpha-MHC are protected against tachycardia-induced cardiomyopathy, but the normal amount of alpha-MHC expressed in humans is only 5% to 7% and its functional importance is questionable. This study was undertaken to identify a myofilament-based mechanism underlying tachycardia-induced cardiomyopathy protection and to extrapolate the impact of MHC isoform variation on myofilament function in human hearts.
Papillary muscle strips from TG rabbits expressing 40% (TG40) and 15% alpha-MHC (TG15) and from nontransgenic (NTG) controls expressing approximately 100% beta-MHC (NTG40 and NTG15) were demembranated and calcium activated. Myofilament tension and calcium sensitivity were similar in TGs and respective NTGs. Force-clamp measurements revealed approximately 50% higher power production in TG40 versus NTG40 (P<0.001) and approximately 20% higher power in TG15 versus NTG15 (P<0.05). A characteristic of acto-myosin crossbridge kinetics, the "dip" frequency, was significantly higher in TG40 versus NTG40 (0.70+/-0.04 versus 0.39+/-0.09 Hz, P<0.01) but not in TG15 versus NTG15. The calculated crossbridge time-on was also significantly shorter in TG40 (102.3+/-14.2 ms) versus NTG40 (175.7+/-19.7 ms) but not in TG15 versus NTG15.
The incorporation of 40% alpha-MHC leads to greater myofilament power production and more rapid crossbridge cycling, which facilitate ejection and relengthening during short cycle intervals, and thus protect against tachycardia-induced cardiomyopathy. Our results suggest, however, that, even when compared with the virtual absence of alpha-MHC in the failing heart, the 5% to 7% alpha-MHC content of the normal human heart has little if any functional significance.

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Available from: Jeanne James, Oct 01, 2014
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    • "Experiments in the laser trap probing the molecular performance of cardiac myosin heavy chain (MHC), for example, have shown that myosin t on is significantly shorter in the α-MHC isoform compared to β-MHC (Palmiter et al. 1999). This result is important because, when α-MHC is inadequately expressed or absent as occurs in human heart failure, one consequence is an inability for the heart to perform effectively at high frequencies (Herron and McDonald 2002; Suzuki et al. 2009). As another example, some point mutations in MHC lead to shorter t on compared to that of non-mutant myosin leading to greater velocities of shortening and ultimately to a hypertrophic cardiomyopathy (Debold et al. 2007; Palmiter et al. 2000; Tyska et al. 2000; Yamashita et al. 2000). "
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    ABSTRACT: Muscle force arises as the result of many myosin molecules, each producing a force discrete in magnitude and in time duration. In previous work we have developed a computer model and a mathematical model of many myosin molecules acting as an ensemble and demonstrated that the time duration over which myosin produces force at the molecular level (referred to here as "time-on") gives rise to specific visco-elastic properties at the whole muscle level. That model of the mechanical consequences of myosin-actin interaction predicted well the C-process of small length perturbation analysis and demonstrated that the characteristic frequency 2πc provided a measure of the myosin off-rate, which is equal to the reciprocal of the mean time-on. In this study, we develop a mathematical hypothesis that a strain-dependence of the myosin off-rate at the single molecule level can result in a negative viscous modulus like that observed at low frequencies, i.e., the B-process. We demonstrate here that a simple monotonic strain-dependency of the myosin off-rate cannot account for the observed B-process. However, a frequency-dependent strain-dependency, as may occur when visco-elastic properties of the myosin head are introduced, can explain the observed negative viscous modulus. These findings suggest that visco-elastic properties of myosin constitute the specific molecular mechanisms that underlie the frequency-dependent performance of many oscillatory muscles such as insect flight muscle and mammalian cardiac muscle.
    Advances in Experimental Medicine and Biology 01/2010; 682:57-75. DOI:10.1007/978-1-4419-6366-6_4 · 1.96 Impact Factor
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    ABSTRACT: The myosin heavy chain (MHC) isoforms, alpha- and beta-MHC, are expressed in developmental- and chamber-specific patterns. Healthy human ventricle contains approximately 2-10% alpha-MHC and these levels are reduced even further in the failing ventricle. While down-regulation of alpha-MHC in failing myocardium is considered compensatory, we previously demonstrated that persistent transgenic (TG) alpha-MHC expression in the cardiomyocytes is cardioprotective in rabbits with tachycardia-induced cardiomyopathy (TIC). We sought to determine if this benefit extends to other types of experimental heart failure and focused on two models relevant to human heart failure: myocardial infarction (MI) and left ventricular pressure overload. TG and nontransgenic rabbits underwent either coronary artery ligation at 8 months or aortic banding at 10 days of age. The effects of alpha-MHC expression were assessed at molecular, histological and organ levels. In the MI experiments, we unexpectedly found modest functional advantages to alpha-MHC expression. In contrast, despite subtle benefits in TG rabbits subjected to aortic banding, cardiac function was minimally affected. We conclude that the benefits of persistent alpha-MHC expression depend upon the mechanism of heart failure. Importantly, in none of the scenarios studied did we find any detrimental effects associated with persistent alpha-MHC expression. Thus manipulation of MHC composition may be beneficial in certain types of heart failure and does not appear to compromise heart function in others. Future considerations of myosin isoform manipulation as a therapeutic strategy should consider the underlying etiology of cardiac dysfunction.
    Journal of Molecular and Cellular Cardiology 10/2009; 48(5):999-1006. DOI:10.1016/j.yjmcc.2009.10.013 · 4.66 Impact Factor
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    ABSTRACT: Current inotropic therapies used to increase cardiac contractility of the failing heart center on increasing the amount of calcium available for contraction, but their long-term use is associated with increased mortality due to fatal arrhythmias. Thus, there is a need to develop and explore novel inotropic therapies that can act via calcium-independent mechanisms. The purpose of this study was to determine whether fast alpha-myosin molecular motor gene transfer can confer calcium-independent positive inotropy in slow beta-myosin-dominant rabbit and human failing ventricular myocytes. To this end, we generated a recombinant adenovirus (AdMYH6) to deliver the full-length human alpha-myosin gene to adult rabbit and human cardiac myocytes in vitro. Fast alpha-myosin motor expression was determined by Western blotting and immunocytochemical analysis and confocal imaging. In experiments using electrically stimulated myocytes from ischemic failing hearts, AdMYH6 increased the contractile amplitude of failing human [23.9+/-7.8 nm (n=10) vs. AdMYH6 amplitude 78.4+/-16.5 nm (n=6)] and rabbit myocytes. The intracellular calcium transient amplitude was not altered. Control experiments included the use of a green fluorescent protein or a beta-myosin heavy chain adenovirus. Our data provide evidence for a novel form of calcium-independent positive inotropy in failing cardiac myocytes by fast alpha-myosin motor protein gene transfer.
    The FASEB Journal 10/2009; 24(2):415-24. DOI:10.1096/fj.09-140566 · 5.04 Impact Factor
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