Effects of Sarcomere Activators and Inhibitors Targeting Myosin Cross-Bridges on Ca2+-Activation of Mature and Immature Mouse Cardiac Myofilaments.

We tested the hypothesis that isoform shifts in sarcomeres of the immature heart modify the effect of cardiac myosin-directed sarcomere inhibitors and activators. Omecamtiv mecarbil (OM) activates tension and is in clinical trials for the treatment of adult acute and chronic heart failure. Mavacamten (Mava) inhibits tension and is in clinical trials to relieve hypercontractility and outflow obstruction in advanced genetic hypertrophic cardiomyopathy (HCM), which is often linked to mutations in sarcomeric proteins. To address the effect of these agents in developing sarcomeres, we isolated heart fiber bundles, extracted membranes with Triton X-100, and measured tension developed over a range of Ca2+ concentrations with and without OM or Mava treatment. We made measurements in fiber bundles from hearts of adult nontransgenic (NTG) controls expressing cardiac troponin I (cTnI), and from hearts of transgenic (TG-ssTnI) mice expressing the fetal/neonatal form, slow skeletal troponin I (ssTnI). We also compared fibers from 7- and 14-day-old NTG mice expressing ssTnI and cTnI. These studies were repeated with 7- and 14-day-old transgenic mice (TG-cTnT-R92Q) expressing a mutant form of cardiac troponin T (cTnT) linked to HCM. OM increased Ca2+-sensitivity and decreased cooperative activation in both ssTnI- and cTnI-regulated myofilaments with a similar effect: reducing submaximal tension in immature and mature myofilaments. Although Mava decreased tension similarly in cTnI- and ssTnI-regulated myofilaments controlled either by cTnT or cTnT-R92Q, its effect involved a depressed Ca2+-sensitivity in the mature cTnT-R92 myofilaments. Our data demonstrate an influence of myosin and thin-filament associated proteins on the actions of myosin-directed agents such as OM and Mava. SIGNIFICANCE STATEMENT: The effects of myosin-targeted activators and inhibitors on Ca2+-activated tension in developing cardiac sarcomeres presented here provide novel, ex vivo evidence as to their actions in early-stage cardiac disorders. These studies advance understanding of the molecular mechanisms of these agents, which are important in preclinical studies employing sarcomere Ca2+-response as a screening approach. The data also inform the use of commonly immature cardiac myocytes generated from human-inducible pluripotent stem cells in screening for sarcomere activators and inhibitors.

Truncation of the N-terminus of cardiac troponin I initiates adaptive remodeling of the myocardial proteosome via phosphorylation of mechano-sensitive signaling pathways.

The cardiac isoform of troponin I has a unique N-terminal extension (~ 1-30 amino acids), which contributes to the modulation of cardiac contraction and relaxation. Hearts of various species including humans produce a truncated variant of cardiac troponin I (cTnI-ND) deleting the first ~ 30 amino acids as an adaption in pathophysiological conditions. In this study, we investigated the impact of cTnI-ND chronic expression in transgenic mouse hearts compared to wildtype (WT) controls (biological n = 8 in each group). We aimed to determine the global phosphorylation effects of cTnI-ND on the cardiac proteome, thereby determining the signaling pathways that have an impact on cardiac function. The samples were digested and isobarically labeled and equally mixed for relative quantification via nanoLC-MS/MS. The peptides were then enriched for phospho-peptides and bioinformatic analysis was done with Ingenuity Pathway Analysis (IPA). We found approximately 77% replacement of the endogenous intact cTnI with cTnI-ND in the transgenic mouse hearts with 1674 phospho-proteins and 2971 non-modified proteins. There were 73 significantly altered phospho-proteins; bioinformatic analysis identified the top canonical pathways as associated with integrin, protein kinase A, RhoA, and actin cytoskeleton signaling. Among the 73 phospho-proteins compared to controls cTnI-ND hearts demonstrated a significant decrease in paxillin and YAP1, which are known to play a role in cell mechano-sensing pathways. Our data indicate that cTnI-ND modifications in the sarcomere are sufficient to initiate changes in the phospho-signaling profile that may underly the chronic-adaptive response associated with cTnI cleavage in response to stressors by modifying mechano-sensitive signaling pathways.

Concomitant treatment of atrial fibrillation during mitral valve surgery.

INTRODUCTION: Surgical management of atrial fibrillation (AF) is a well-established method of preventing complications and late mortality in patients presenting with AF before mitral valve (MV) surgery. However, despite a substantial body of evidence and a Class I recommendation to apply surgical ablation (SA) concomitant to MV surgery, the utilization of SA remains low. METHODS: In this study, we sought to summarize the current trends in the SA of AF during MV surgery and update the medical community on its advantages, including perioperative mortality and morbidity, freedom from AF, as well as long-term survival and stroke rates. RESULTS: The data indicate that SA can be added with no increased risk (and perhaps a reduction in perioperative risk) and improved late survival compared to patients with AF left untreated during MV surgery. DISCUSSION: Inconsistent application of SA may be related to inaccurate perceptions regarding the complexity of the procedure itself, extended cross-clamp and bypass times with attendant increased risks, views that it is ineffective, and increased need for an early pacemaker. CONCLUSION: Education in the proper performance of SA, including careful placement of the lesions and attainment of the full transmural effect, contributes to procedure success. Propagating the safety and positive outcomes may also address the concerns.

Deletion of P21-activated kinase-1 induces age-dependent increased visceral adiposity and cardiac dysfunction in female mice.

It is known that there is an age-related progression in diastolic dysfunction, especially prevalent in postmenopausal women, who develop heart failure with preserved ejection fraction (HFpEF, EF > 50%). Mechanisms and therapies are poorly understood, but there are strong correlations between obesity and HFpEF. We have tested the hypothesis that P21-activated kinase-1 (PAK1) preserves cardiac function and adipose tissue homeostasis during aging in female mice. Previous demonstrations in male mice by our lab that PAK1 activity confers cardio-protection against different stresses formed the rationale for this hypothesis. Our studies compared young (3-6 months) and middle-aged (12-15 months) female and male PAK1 knock-out mice (PAK1-/-) and wild-type (WT) equivalent. Female WT mice exhibited increased cardiac PAK1 abundance during aging. By echocardiography, compared to young WT female mice, middle-aged WT female mice showed enlargement of the left atrium as well as thickening of posterior wall and increased left ventricular mass; however, all contraction and relaxation parameters were preserved during aging. Compared to WT controls, middle-aged PAK1-/- female mice demonstrated worsening of cardiac function involving a greater enlargement of the left atrium, ventricular hypertrophy, and diastolic dysfunction. Moreover, with aging PAK1-/- female mice, unlike male PAK1-/- mice, exhibited increased adiposity with increased accumulation of visceral adipose tissue. Our data provide evidence for the significance of PAK1 signaling as an element in the preservation of cardiac function and adipose tissue homeostasis in females during aging.

Altered coronary artery function, arteriogenesis and endothelial YAP signaling in postnatal hypertrophic cardiomyopathy.

Introduction: Hypertrophic cardiomyopathy (HCM) is a cardiovascular genetic disease caused largely by sarcomere protein mutations. Gaps in our understanding exist as to how maladaptive sarcomeric biophysical signals are transduced to intra- and extracellular compartments leading to HCM progression. To investigate early HCM progression, we focused on the onset of myofilament dysfunction during neonatal development and examined cardiac dynamics, coronary vascular structure and function, and mechano-transduction signaling in mice harboring a thin-filament HCM mutation. Methods: We studied postnatal days 7-28 (P7-P28) in transgenic (TG) TG-cTnT-R92Q and non-transgenic (NTG) mice using skinned fiber mechanics, echocardiography, biochemistry, histology, and immunohistochemistry. Results: At P7, skinned myofiber bundles exhibited an increased Ca2+-sensitivity (pCa50 TG: 5.97 ± 0.04, NTG: 5.84 ± 0.01) resulting from cTnT-R92Q expression on a background of slow skeletal (fetal) troponin I and α/ß myosin heavy chain isoform expression. Despite the transition to adult isoform expressions between P7-P14, the increased Ca2+- sensitivity persisted through P28 with no apparent differences in gross morphology among TG and NTG hearts. At P7 significant diastolic dysfunction was accompanied by coronary flow perturbation (mean diastolic velocity, TG: 222.5 ± 18.81 mm/s, NTG: 338.7 ± 28.07 mm/s) along with localized fibrosis (TG: 4.36% ± 0.44%, NTG: 2.53% ± 0.47%). Increased phosphorylation of phospholamban (PLN) was also evident indicating abnormalities in Ca2+ homeostasis. By P14 there was a decline in arteriolar cross-sectional area along with an expansion of fibrosis (TG: 9.72% ± 0.73%, NTG: 2.72% ± 0.2%). In comparing mechano-transduction signaling in the coronary arteries, we uncovered an increase in endothelial YAP expression with a decrease in its nuclear to cytosolic ratio at P14 in TG hearts, which was reversed by P28. Conclusion: We conclude that those early mechanisms that presage hypertrophic remodeling in HCM include defective biophysical signals within the sarcomere that drive diastolic dysfunction, impacting coronary flow dynamics, defective arteriogenesis and fibrosis. Changes in mechano-transduction signaling between the different cellular compartments contribute to the pathogenesis of HCM.

NH2-Terminal Cleavage of Cardiac Troponin I Signals Adaptive Response to Cardiac Stressors.

Cardiac sarcomeres express a variant of troponin I (cTnI) that contains a unique N-terminal extension of ~30 amino acids with regulatory phosphorylation sites. The extension is important in the control of myofilament response to Ca2+, which contributes to the neuro-humoral regulation of the dynamics of cardiac contraction and relaxation. Hearts of various species including humans express a stress-induced truncated variant of cardiac troponin I (cTnI-ND) missing the first ~30 amino acids and functionally mimicking the phosphorylated state of cTnI. Studies have demonstrated that upregulation of cTnI-ND potentially represents a homeostatic mechanism as well as an adaptive response in pathophysiology including ischemia/reperfusion injury, beta adrenergic maladaptive activation, and aging. We present evidence showing that cTnI-ND can modify the trigger for hypertrophic cardiomyopathy (HCM) by reducing the Ca2+ sensitivity of myofilaments from hearts with an E180G mutation in α-tropomyosin. Induction of this truncation may represent a therapeutic approach to modifying Ca2+-responses in hearts with hypercontractility or heat failure with preserved ejection fraction.