Heart failure with preserved ejection fraction (HFpEF), a major public health problem that is rising in prevalence, is associated with high morbidity and mortality and is considered to be the greatest unmet need in cardiovascular medicine today because of a general lack of effective treatments. To address this challenging syndrome, the National Heart, Lung, and Blood Institute convened a working group made up of experts in HFpEF and novel research methodologies to discuss research gaps and to prioritize research directions over the next decade. Here, we summarize the discussion of the working group, followed by key recommendations for future research priorities. There was uniform recognition that HFpEF is a highly integrated, multiorgan, systemic disorder requiring a multipronged investigative approach in both humans and animal models to improve understanding of mechanisms and treatment of HFpEF. It was recognized that advances in the understanding of basic mechanisms and the roles of inflammation, macrovascular and microvascular dysfunction, fibrosis, and tissue remodeling are needed and ideally would be obtained from (1) improved animal models, including large animal models, which incorporate the effects of aging and associated comorbid conditions; (2) repositories of deeply phenotyped physiological data and human tissue, made accessible to researchers to enhance collaboration and research advances; and (3) novel research methods that take advantage of computational advances and multiscale modeling for the analysis of complex, high-density data across multiple domains. The working group emphasized the need for interactions among basic, translational, clinical, and epidemiological scientists and across organ systems and cell types, leveraging different areas or research focus, and between research centers. A network of collaborative centers to accelerate basic, translational, and clinical research of pathobiological mechanisms and treatment strategies in HFpEF was discussed as an example of a strategy to advance research progress. This resource would facilitate comprehensive, deep phenotyping of a multicenter HFpEF patient cohort with standardized protocols and a robust biorepository. The research priorities outlined in this document are meant to stimulate scientific advances in HFpEF by providing a road map for future collaborative investigations among a diverse group of scientists across multiple domains.

We examined the influence of cross-bridge cycling kinetics on the length dependence of steady-state force and the rate of force redevelopment (k(tr)) during Ca(2+)-activation at sarcomere lengths (SL) of 2.0 and 2.3 microm in skinned rat cardiac trabeculae. Cross-bridge kinetics were altered by either replacing ATP with 2-deoxy-ATP (dATP) or by reducing [ATP]. At each SL dATP increased maximal force (F(max)) and Ca(2+)-sensitivity of force (pCa(50)) and reduced the cooperativity (n(H)) of force-pCa relations, whereas reducing [ATP] to 0.5 mM (low ATP) increased pCa(50) and n(H) without changing F(max). The difference in pCa(50) between SL 2.0 and 2.3 microm (Delta pCa(50)) was comparable between ATP and dATP, but reduced with low ATP. Maximal k(tr) was elevated by dATP and reduced by low ATP. Ca(2+)-sensitivity of k(tr) increased with both dATP and low ATP and was unaffected by altered SL under all conditions. Significantly, at equivalent levels of submaximal force k(tr) was faster at short SL or increased lattice spacing. These data demonstrate that the SL dependence of force depends on cross-bridge kinetics and that the increase of force upon SL extension occurs without increasing the rate of transitions between nonforce and force-generating cross-bridge states, suggesting SL or lattice spacing may modulate preforce cross-bridge transitions.

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Cardiotoxicity resulting from direct myocyte damage has been a known complication of cancer treatment for decades. More recently, the emergence of hypertension as a clinically significant side effect of several new agents has been recognized as adversely affecting cancer treatment outcomes. With cancer patients living longer, in part because of treatment advances, these adverse events have become increasingly important to address. However, little is known about the cardiovascular pathogenic mechanisms associated with cancer treatment and even less about how to optimally prevent and manage short- and long-term cardiovascular complications, leading to improved patient safety and clinical outcomes. To identify research priorities, allocate resources, and establish infrastructure required to address cardiotoxicity associated with cancer treatment, the National Cancer Institute (NCI) and National Heart, Lung and Blood Institute (NHLBI) sponsored a two-day workshop, "Cancer treatment-related cardiotoxicity: Understanding the current state of knowledge and future research priorities," in March 2013 in Bethesda, MD. Participants included leading oncology and cardiology researchers and health professionals, patient advocates and industry representatives, with expertise ranging from basic to clinical science. Attendees were charged with identifying research opportunities to advance the understanding of cancer treatment-related cardiotoxicity across basic and clinical science. This commentary highlights the key discussion points and overarching recommendations from that workshop.

Low angle x-ray diffraction measurements of myofilament lattice spacing (D(1,0)) and equatorial reflection intensity ratio (I(1,1)/I(1,0)) were made in relaxed skinned cardiac trabeculae from rats. We tested the hypothesis that the degree of weak cross-bridge (Xbr) binding, which has been shown to be obligatory for force generation in skeletal muscle, is modulated by changes in lattice spacing in skinned cardiac muscle. Altered weak Xbr binding was detected both by changes in I(1,1)/I(1,0) and by measurements of chord stiffness (chord K). Both measurements showed that, similar to skeletal muscle, the probability of weak Xbr binding at 170-mM ionic strength was significantly enhanced by lowering temperature to 5 degrees C. The effects of lattice spacing on weak Xbr binding were therefore determined under these conditions. Changes in D(1,0), I(1,1)/I(1,0), and chord K by osmotic compression with dextran T500 were determined at sarcomere lengths (SL) of 2.0 and 2.35 micro m. At each SL increasing [dextran] caused D(1,0) to decrease and both I(1,1)/I(1,0) and chord K to increase, indicating increased weak Xbr binding. The results suggest that in intact cardiac muscle increasing SL and decreasing lattice spacing could lead to increased force by increasing the probability of initial weak Xbr binding.

To investigate the interplay between the thin and thick filaments during calcium activation in striated muscle, we employed n-(6-aminohexyl) 5-chloro-1-napthalenesulfonamide (W7) as an inhibitor of troponin C and compared its effects with that of the myosin-specific inhibitor, 2,3-butanedione 2-monoxime (BDM). In both skeletal and cardiac fibers, W7 reversibly inhibited ATPase and tension over the full range of calcium activation between pCa 8.0 and 4.5, resulting in reduced calcium sensitivity and cooperativity of ATPase and tension activations. At maximal activation in skeletal fibers, the W7 concentrations for half-maximal inhibition (KI) were 70-80 micro M for ATPase and 20-30 micro M for tension, nearly >200-fold lower than BDM (20 mM and 5-8 mM, respectively). When W7 (50 microM) and BDM (20 mM) were combined in skeletal fibers, the ATPase and tension-pCa curves exhibited lower apparent cooperativity and maxima and higher calcium sensitivity than expected from two independent activation pathways, suggesting that the interplay between the thin and thick filaments varies with the level of activation. Significantly, the inhibition of W7 increased the ATPase/tension ratio during activation in both muscle types. W7 holds much promise as a potent and reversible inhibitor of thin filament-mediated calcium activation of skeletal and cardiac muscle contraction.

The recent determination of the myosin head atomic structure has led to a new model of muscle contraction, according to which mechanical torque is generated in the catalytic domain and amplified by the lever arm made of the regulatory domain [Fisher, A. J., Smith, C. A., Thoden, J., Smith, R., Sutoh, K., Holden, H. M. & Rayment, I. (1995) Biochemistry 34, 8960-8972]. A crucial aspect of this model is the ability of the regulatory domain to move independently of the catalytic domain. Saturation transfer-EPR measurements of mobility of these two domains in myosin filaments give strong support for this notion. The catalytic domain of the myosin head was labeled at Cys-707 with indane dione spin label; the regulatory domain was labeled at the single cysteine residue of the essential light chain and exchanged into myosin. The mobility of the regulatory domain in myosin filaments was characterized by an effective rotational correlation time ( R ) between 24 and 48 s. In contrast, the mobility of the catalytic domain was found to be R ‫؍‬ 5-9 s. This difference in mobility between the two domains existed only in the filament form of myosin. In the monomeric form, or when bound to actin, the mobility of the two domains in myosin was indistinguishable, with R ‫؍‬ 1-4 s and >1,000 s, respectively. Therefore, the observed difference in filaments cannot be ascribed to differences in local conformations of the spinlabeled sites. The most straightforward interpretation suggests a f lexible hinge between the two domains, which would have to stiffen before force could be generated.The myosin head (subfragment 1, S1) plays a central role during muscle contraction. It is thought that during the ATPase cycle S1 interacts with actin and undergoes a series of specific changes in its conformation, resulting in a directional strain on the actin filaments. The nature of these conformational changes and the mechanism by which they produce the directed force is still unclear (1, 2), although the original model of Huxley and Simmons (3) provides a valid framework. The determination of the atomic structures of S1, actin, and the low-resolution structure of the acto-S1 complex has led to a hypothesis of a structural model, in which helix movement is generated by the hydrolysis of ATP and the formation of a stereospecific acto-S1 complex generates torque within the catalytic domain, which is then amplified by the regulatory domain acting as a lever arm (4, 5). Rayment's model was preceded by the observation of shape changes in the myosin heads between various chemical states of myosin. A decrease in the hydrodynamic size of S1 upon MgADP binding and upon ATP hydrolysis was observed by Highsmith and Eden (6) and by Wakabayashi et al. (7,8). More recently, low-resolution electron microscopy reconstitution of S1-decorated actin filaments and EPR studies have indicated a rotation of the regulatory domain with respect to the catalytic domain (9-11). In these cases, the rotation was observed at the distal end of the regulatory domain on th...

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