Transgenic overexpression of ribonucleotide reductase improves cardiac performance

Abstract

We previously demonstrated that cardiac myosin can use 2-deoxy-ATP (dATP) as an energy substrate, that it enhances contraction and relaxation with minimal effect on calcium-handling properties in vitro, and that contractile enhancement occurs with only minor elevation of cellular [dATP]. Here, we report the effect of chronically enhanced dATP concentration on cardiac function using a transgenic mouse that overexpresses the enzyme ribonucleotide reductase (TgRR), which catalyzes the rate-limiting step in de novo deoxyribonucleotide biosynthesis. Hearts from TgRR mice had elevated left ventricular systolic function compared with wild-type (WT) mice, both in vivo and in vitro, without signs of hypertrophy or altered diastolic function. Isolated cardiomyocytes from TgRR mice had enhanced contraction and relaxation, with no change in Ca 2+ transients, suggesting targeted improvement of myofilament function. TgRR hearts had normal ATP and only slightly decreased phosphocreatine levels by 31 P NMR spectroscopy, and they maintained rate responsiveness to dobutamine challenge. These data demonstrate long-term (at least 5-mo) elevation of cardiac [dATP] results in sustained elevation of basal left ventricular performance, with maintained β-adrenergic responsiveness and energetic reserves. Combined with results from previous studies, we conclude that this occurs primarily via enhanced myofilament activation and contraction, with similar or faster ability to relax. The data are sufficiently compelling to consider elevated cardiac [dATP] as a therapeutic option to treat systolic dysfunction.metabolism | inotropy | cross-bridge cycling A wide variety of cardiac pathologies such as myocardial infarct, dilated cardiomyopathy, and congestive heart failure involve reduced systolic function of ventricles that often results from altered ATP-mediated actin-myosin (cross-bridge) cycling (1-4). The schematic shown in Fig. 1 outlines the basic components of this chemomechanical cycle that are critical to understand how cardiomyocytes use energy to develop tension and shortening: (i) ATP bound to detached myosin is hydrolyzed to ADP and inorganic phosphate (Pi); (ii) myosin binds to actin and undergoes the power-stroke associated with Pi release, tension development, and shortening; (iii) ADP is released from myosin; and (iv) ATP binds and precipitates myosin detachment from actin.We (5-8) and others (9-17) have reported that striated muscle myosin can use most naturally occurring nucleotides to support cross-bridge cycling and contraction to varying degrees. Although most are not as effective as ATP, we found that 2-deoxy-ATP (dATP) is more effective than ATP as a substrate for contraction of demembranated cardiac muscle, augmenting both force and shortening at all levels of Ca 2+ -mediated contractile activation (5-8). Our detailed biochemical and mechanical analysis suggests that dATP increases the rates of both myosin binding and product release (steps 2 and 3 in Fig. 1) (6). More recently, we reported that adenoviral overexpres...

Cell-based delivery of dATP via gap junctions enhances cardiac contractility

“…This notion is well supported by a number of prior studies showing functional gap junction coupling with fibroblasts[45–48]. In agreement with our previous reports[15,16], this experiment further supports the absence of deleterious effects of dATP on the relaxation kinetics of cardiomyocytes and supports the idea that dATP may represent a way to increase systolic function without impairing diastolic performance.…”

“…Our previous work has shown that R1R2 overexpression results in increased contractility in neonatal and mature adult rat cardiomyocytes[15,16]. We first verified that overexpression of R1R2 results in similar increases in contraction magnitude and velocity in hESC-CMs.…”

“…Importantly, these effects were noted in the absence of any increase in maximal [Ca 2+ ] i , suggesting that dATP exerts its effects on contractility primarily by acting on the highly cooperative myofilament. Furthermore, we recently reported that transgenic mice overexpressing R1R2 maintain a hypercontractile state without detrimental effects on metabolism or adverse remodeling[16]. Taken together, these findings suggest that dATP may represent a next-generation therapy for the treatment of heart failure.…”

“…We[8–16] and others[17,18] have extensively studied the ability of 2-deoxyadenosine triphosphate (dATP), a novel inotropic agent, to enhance cardiac contractility by acting as an energy substrate for striated muscle. These studies have demonstrated that, compared to ATP, dATP increases both the force and the rate of shortening at all levels of Ca 2+ activation in demembranated cardiac trabeculae[11].…”

Muscle Fatigue from the Perspective of a Single Crossbridge

“…Motivated by this goal, Longyear et al 51 recently demonstrated that replacing ATP with the ATP analog, 2-deoxy-ATP, can significantly attenuate the depressive effects of acidosis on filament velocity and Ca 2+ sensitivity 51 in the in vitro motility assay. Thus, this may represent a promising agent not only to potentially treat chronic heart failure, as is being explored, 62 but also, based on our results, to help attenuate skeletal muscle fatigue. In addition, College of Sports Medicine and permission has been granted for this version to appear in e-Publications@Marquette.…”