Adult Rat Cardiomyocytes Research Articles

Regulation of Junctophilin‐2 Expression by Triiodothyronine in Adult Rat Cardiomyocytes

IntroductionCardiac junctophilin‐2 (Jph2) plays a central role in myocardial contractility by anchoring transverse tubules (TT) to the junctional sarcoplasmic reticulum (jSR), essential for excitation‐contraction (EC) coupling and calcium signaling. Decreases in the expression levels of Jph2 have been observed in a variety of pathological conditions including hypertrophic cardiomyopathy, arrhythmias, and progression of disease in failing hearts. Studies have shown that T3 regulates SR Ca2+ ATPase (SERCa2), phospholamban (PLN), L‐type Ca2+ channels and ryanodine receptors (RyR2), that are responsible for regulating intracellular Ca2+ and maintaining normal systolic and diastolic function. We previously reported that T3 improves cardiac contractile function after induction of myocardial infarction (MI) by maintaining TT network integrity and by regulating expression of myofibrillar and Ca2+ handling proteins. We hypothesize that T3 regulates Jph2 expression and its localization within the jSR‐TT complex to maintain functional TT structures and thus normal Ca2+ transients and contractility.MethodsThyroid deficiency was induced in female Sprague Dawley rats (~220g) by ingestion of 0.025% PTU (propylthiouracil) in drinking water. Control (EU) rats had access to untreated water. After 8 weeks, a subgroup of PTU treated rats received T3 (10ug/kg/d) in PTU‐containing water for an additional 2 weeks. RNA was extracted from left ventricular (LV) tissue to measure Jph2, SERCa2, PLN, LTCC and RyR2 mRNA by RT‐qPCR. RNAseq analysis interrogated 15,000 genes. LV myocytes were isolated by collagenase digestion and cultured with and without T3 (10−7 M). Cells were harvested at 24, 48, and 72 hrs for RT‐qPCR analysis of Jph2, SERCa2, and Bridging Integrator‐1 (Bin1). Isolated myocytes from all study groups were immunolabeled with RyR2, Cav1.2 and Jph2 for nanoimaging using STORM.ResultsRT‐PCR analysis showed significant upregulation of Jph2 expression compared to thyroid deficiency (1.404±0.73 vs 0.322±0.17 fold change). RNAseq revealed 567 differentially expressed genes with 377 genes up‐regulated and 190 down‐regulated in T3‐treated hearts compared to TH deficient hearts. Results showed up‐regulation of Jph2 (p<0.001) and SERCa2 by >1.5‐fold (p<0.00001). Cultured EU and PTU cardiomyocytes similarly showed significantly (p<0.05) increased Jph2 expression in T3 treatment vs non‐treated cells at 48 and 72 hr time points. STORM imaging showed increased clustering of LTCC (Cav1.2) within the RyR/Cav1.2 clusters of TT‐jSR complexes in the T3‐treated vs TH deficient cardiomyocytes (0.53±0.13 vs 0.36±0.03).ConclusionsT3 regulates expression of Jph2 which in turn is a critical regulator of the jSR‐TT organization in cardiomyocytes allowing for optimal calcium transients and contractile function.

Augmenting Vacuolar H+-ATPase Function Prevents Cardiomyocytes from Lipid-Overload Induced Dysfunction.

The diabetic heart is characterized by a shift in substrate utilization from glucose to lipids, which may ultimately lead to contractile dysfunction. This substrate shift is facilitated by increased translocation of lipid transporter CD36 (SR-B2) from endosomes to the sarcolemma resulting in increased lipid uptake. We previously showed that endosomal retention of CD36 is dependent on the proper functioning of vacuolar H+-ATPase (v-ATPase). Excess lipids trigger CD36 translocation through inhibition of v-ATPase function. Conversely, in yeast, glucose availability is known to enhance v-ATPase function, allowing us to hypothesize that glucose availability, via v-ATPase, may internalize CD36 and restore contractile function in lipid-overloaded cardiomyocytes. Increased glucose availability was achieved through (a) high glucose (25 mM) addition to the culture medium or (b) adenoviral overexpression of protein kinase-D1 (a kinase mediating GLUT4 translocation). In HL-1 cardiomyocytes, adult rat and human cardiomyocytes cultured under high-lipid conditions, each treatment stimulated v-ATPase re-assembly, endosomal acidification, endosomal CD36 retention and prevented myocellular lipid accumulation. Additionally, these treatments preserved insulin-stimulated GLUT4 translocation and glucose uptake as well as contractile force. The present findings reveal v-ATPase functions as a key regulator of cardiomyocyte substrate preference and as a novel potential treatment approach for the diabetic heart.

ROS-Dependent Regulation of SOCS in Adult Rat Cardiomyocytes Preconditioned with Diazoxide

ROS-Dependent Regulation of SOCS in Adult Rat Cardiomyocytes Preconditioned with Diazoxide

Characterization of the Cardiac Myosin Inhibitor CK-3773274: a Potential Therapeutic Approach for Hypertrophic Cardiomyopathy

Hypercontractility of the cardiac sarcomere may be essential for the underling pathological hypertrophy and fibrosis in genetic hypertrophic cardiomyopathies. Here, we characterize the small molecule, CK-3773274, as a novel cardiac myosin inhibitor that decreases contractility in vitro and in vivo. In bovine cardiac myofibrils, CK-3773274 decreased myosin ATPase activity in a concentration-dependent fashion (IC50:1.26 μM). CK-3773274 selectively targets cardiac myosin, as it reduced myosin ATPase activity in the absence of other sarcomere proteins, including actin, troponin, and tropomyosin. Importantly, CK-3773274 did not inhibit the ATPase activity of smooth muscle myosin. In transient kinetic studies, CK-3773274 substantially slowed the rate of actin-activated phosphate release, likely stabilizing myosin conformations that bind weakly to actin. Binding of CK-3773274 to cardiac myosin is mutually exclusive with the nonspecific myosin II inhibitor blebbistatin, suggesting they bind to the same or overlapping sites. Consistent with its biochemical phenotype, CK-3773274 (10 μM) reduced fractional shortening (FS) by 84% in electrically paced, isolated adult rat cardiomyocytes relative to control without any effect on the calcium (Ca2+) transient. The effect of CK-3773274 on cardiac contractility in vivo was assessed in healthy male Sprague Dawley (SD) rats using single oral doses ranging from 0.5 to 4 mg/kg. Left ventricular dimensions and FS were determined by echocardiography at select time points over a 24-hour period. One hour after dose administration, CK-3773274 significantly reduced FS in a dose-related fashion by 20% to 70% relative to vehicle treatment without any changes to heart rate. In conclusion, CK-3773274 is a novel, small molecule, cardiac myosin inhibitor that reduces cardiac contractility in vitro and in vivo. Cardiac myosin inhibition may be a viable approach to treat the underlying hypercontractility of the cardiac sarcomere in hypertrophic cardiomyopathies.

Generation of Ventricular-Like HiPSC-Derived Cardiomyocytes and High-Quality Cell Preparations for Calcium Handling Characterization.

Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) provide a valuable human source for studying the basic science of calcium (Ca2+) handling and signaling pathways as well as high-throughput drug screening and toxicity assays. Herein, we provide a detailed description of the methodologies used to generate high-quality iPSC-CMs that can consistently reproduce molecular and functional characteristics across different cell lines. Additionally, a method is described to reliably assess their functional characterization through the evaluation of Ca2+ handling properties. Low oxygen (O2) conditions, lactate selection, and prolonged time in culture produce high-purity and high-quality ventricular-like cardiomyocytes. Similar to isolated adult rat cardiomyocytes (ARCMs), 3-month-old iPSC-CMs exhibit higher Ca2+ amplitude, faster rate of Ca2+ reuptake (decay-tau), and a positive lusitropic response to β-adrenergic stimulation compared to day 30 iPSC-CMs. The strategy is technically simple, cost-effective, and reproducible. It provides a robust platform to model cardiac disease and for the large-scale drug screening to target Ca2+ handling proteins.

Pharmacological Preconditioning Using Diazoxide Regulates Store-Operated Ca2 + Channels in Adult Rat Cardiomyocytes.

Voltage-dependent Ca2+ channels and store-operated Ca2+ channels (SOCs) are the major routes of Ca2+ entry into mammalian cells. Previously, we reported that pharmacological preconditioning (PPC) leads to a decrease in the amplitude of L-type calcium channel current in the heart. In this study, we examined PPC-associated changes in SOC function. We measured adult cardiomyocyte membrane currents using the whole-cell patch-clamp technique, and we evaluated reactive oxygen species (ROS) production and intracellular Ca2+ levels in cardiomyocytes using fluorescent probes. Diazoxide (Dzx) and thapsigargin (Tg) were used to induce PPC and to deplete internal stores of Ca2+, respectively. Ca2+ store depletion generated inward currents with strong rectification, which were suppressed by the SOC blocker GSK-7975-A. These currents were completely abolished by PPC, an effect that could be countered with 5-hydroxydecanoate (5-HD; a selective mitochondrial ATP-sensitive K+ channel blocker), an intracellular mitochondrial energizing solution, or Ni2+ [a blocker of sodium–calcium exchanger (NCX)]. Buffering of ROS and intracellular Ca2+ also prevented PPC effects on SOC currents. Refilling of intracellular stores was largely suppressed by PPC, as determined by measuring intracellular Ca2+ with a fluorescent Ca2+ indicator. These results indicate that influx of Ca2+ through SOCs is inhibited by their ROS and Ca2+-dependent inactivation during PPC and that NCX is a likely source of PPC-inactivating Ca2+. We further showed that NCX associates with Orai1. Down-regulation of SOCs by PPC may play a role in cardioprotection following ischemia–reperfusion.

Short-term angiotensin II treatment regulates cardiac nanomechanics via microtubule modifications.

Mechanical properties of single myocytes contribute to the whole heart performance, but the measurement of mechanics in living cells at high resolution with minimal force interaction remains challenging. Angiotensin II (AngII) is a peptide hormone that regulates a number of physiological functions, including heart performance. It has also been shown to contribute to cell mechanics by inducing cell stiffening. Using non-contact high-resolution Scanning Ion Conductance Microscopy (SICM), we determine simultaneously cell topography and membrane transverse Young's modulus (YM) by a constant pressure application through a nanopipette. While applying pressure, the vertical position is recorded and a deformation map is generated from which YM can be calculated and corrected for the uneven geometry. High resolution of this method also allows studying specific membrane subdomains, such as Z-grooves and crests. We found that short-term AngII treatment reduces the transversal YM in isolated adult rat cardiomyocytes acting via an AT1 receptor. Blocking either a TGF-β1 receptor or Rho kinase abolishes this effect. Analysis of the cytoskeleton showed that AngII depletes microtubules by decreasing long-lived detyrosinated and acetylated microtubule populations. Interestingly, in the failing cardiomyocytes, which are stiffer than controls, the short-term AngII treatment also reduces the YM, thus normalizing the mechanical state of cells. This suggests that the short-term softening effect of AngII on cardiac cells is opposite to the well-characterized long-term hypertrophic effect. In conclusion, we generate a precise nanoscale indication map of location-specific transverse cortical YM within the cell and this can substantially advance our understanding of cellular mechanics in a physiological environment, for example in isolated cardiac myocytes.

Specific Upregulation of TRPC1 and TRPC5 Channels by Mineralocorticoid Pathway in Adult Rat Ventricular Cardiomyocytes.

Whereas cardiac TRPC (transient receptor potential canonical) channels and the associated store-operated Ca2+ entry (SOCE) are abnormally elevated during cardiac hypertrophy and heart failure, the mechanism of this upregulation is not fully elucidated but might be related to the activation of the mineralocorticoid pathway. Using a combination of biochemical, Ca2+ imaging, and electrophysiological techniques, we determined the effect of 24-h aldosterone treatment on the TRPCs/Orai-dependent SOCE in adult rat ventricular cardiomyocytes (ARVMs). The 24-h aldosterone treatment (from 100 nM to 1 µM) enhanced depletion-induced Ca2+ entry in ARVMs, as assessed by a faster reduction of Fura-2 fluorescence decay upon the addition of Mn2+ and increased Fluo-4/AM fluorescence following Ca2+ store depletion. These effects were prevented by co-treatment with a specific mineralocorticoid receptor (MR) antagonist, RU-28318, and they are associated with the enhanced depletion-induced N-[4-[3,5-Bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl]-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2)-sensitive macroscopic current recorded by patch-clamp experiments. Molecular screening by qRT-PCR and Western blot showed a specific upregulation of TRPC1, TRPC5, and STIM1 expression at the messenger RNA (mRNA) and protein levels upon 24-h aldosterone treatment of ARVMs, corroborated by immunostaining. Our study provides evidence that the mineralocorticoid pathway specifically promotes TRPC1/TRPC5-mediated SOCE in adult rat cardiomyocytes.

Metabolic Remodeling Promotes Cardiac Hypertrophy by Directing Glucose to Aspartate Biosynthesis.

Hypertrophied hearts switch from mainly using fatty acids (FAs) to an increased reliance on glucose for energy production. It has been shown that preserving FA oxidation (FAO) prevents the pathological shift of substrate preference, preserves cardiac function and energetics, and reduces cardiomyocyte hypertrophy during cardiac stresses. However, it remains elusive whether substrate metabolism regulates cardiomyocyte hypertrophy directly or via a secondary effect of improving cardiac energetics. The goal of this study was to determine the mechanisms of how preservation of FAO prevents the hypertrophic growth of cardiomyocytes. We cultured adult rat cardiomyocytes in a medium containing glucose and mixed-chain FAs and induced pathological hypertrophy by phenylephrine. Phenylephrine-induced hypertrophy was associated with increased glucose consumption and higher intracellular aspartate levels, resulting in increased synthesis of nucleotides, RNA, and proteins. These changes could be prevented by increasing FAO via deletion of ACC2 (acetyl-CoA-carboxylase 2) in phenylephrine-stimulated cardiomyocytes and in pressure overload-induced cardiac hypertrophy in vivo. Furthermore, aspartate supplementation was sufficient to reverse the antihypertrophic effect of ACC2 deletion demonstrating a causal role of elevated aspartate level in cardiomyocyte hypertrophy. 15N and 13C stable isotope tracing revealed that glucose but not glutamine contributed to increased biosynthesis of aspartate, which supplied nitrogen for nucleotide synthesis during cardiomyocyte hypertrophy. Our data show that increased glucose consumption is required to support aspartate synthesis that drives the increase of biomass during cardiac hypertrophy. Preservation of FAO prevents the shift of metabolic flux into the anabolic pathway and maintains catabolic metabolism for energy production, thus preventing cardiac hypertrophy and improving myocardial energetics.

Neuregulin-1 triggers GLUT4 translocation and enhances glucose uptake independently of insulin receptor substrate and ErbB3 in neonatal rat cardiomyocytes

During stress conditions such as pressure overload and acute ischemia, the myocardial endothelium releases neuregulin-1β (NRG-1), which acts as a cardioprotective factor and supports recovery of the heart. Recently, we demonstrated that recombinant human (rh)NRG-1 enhances glucose uptake in neonatal rat ventricular myocytes via the ErbB2/ErbB4 heterodimer and PI3Kα. The present study aimed to further elucidate the mechanism whereby rhNRG-1 activates glucose uptake in comparison to the well-established insulin and to extend the findings to adult models. Combinations of rhNRG-1 with increasing doses of insulin did not yield any additive effect on glucose uptake measured as 3H-deoxy-d-glucose incorporation, indicating that the mechanisms of the two stimuli are similar. In c-Myc-GLUT4-mCherry-transfected neonatal rat cardiomyocytes, rhNRG-1 increased sarcolemmal GLUT4 by 16-fold, similar to insulin. In contrast to insulin, rhNRG-1 did not phosphorylate IRS-1 at Tyr612, indicating that IRS-1 is not implicated in the signal transmission. Treatment of neonatal rats with rhNRG-1 induced a signaling response comparable with that observed in vitro, including increased ErbB4-pTyr1284, Akt-pThr308 and Erk1/2-pThr202/Tyr204. In contrast, in adult cardiomyocytes rhNRG-1 only increased the phosphorylation of Erk1/2 without having any significant effect on Akt and AS160 phosphorylation and glucose uptake, suggesting that rhNRG-1 function in neonatal cardiomyocytes differs from that in adult cardiomyocytes. In conclusion, our results show that similar to insulin, rhNRG-1 can induce glucose uptake by activating the PI3Kα-Akt-AS160 pathway and GLUT4 translocation. Unlike insulin, the rhNRG-1-induced effect is not mediated by IRS proteins and is observed in neonatal, but not in adult rat cardiomyocytes.

Disturbed cardiac mitochondrial and cytosolic calcium handling in a metabolic risk-related rat model of heart failure with preserved ejection fraction.

AimCalcium ions play a pivotal role in matching energy supply and demand in cardiac muscle. Mitochondrial calcium concentration is lower in animal models of heart failure with reduced ejection fraction (HFrEF), but limited information is available about mitochondrial calcium handling in heart failure with preserved ejection fraction (HFpEF).MethodsWe assessed mitochondrial Ca2+ handling in intact cardiomyocytes from Zucker/fatty Spontaneously hypertensive F1 hybrid (ZSF1)‐lean (control) and ZSF1‐obese rats, a metabolic risk‐related model of HFpEF. A mitochondrially targeted Ca2+ indicator (MitoCam) was expressed in cultured adult rat cardiomyocytes. Cytosolic and mitochondrial Ca2+ transients were measured at different stimulation frequencies. Mitochondrial respiration and swelling, and expression of key proteins were determined ex vivo.ResultsAt rest, mitochondrial Ca2+ concentration in ZSF1‐obese was larger than in ZSF1‐lean. The diastolic and systolic mitochondrial Ca2+ concentrations increased with stimulation frequency, but the steady‐state levels were larger in ZSF1‐obese. The half‐widths of the contractile responses, the resting cytosolic Ca2+ concentration and the decay half‐times of the cytosolic Ca2+ transients were higher in ZSF1‐obese, likely because of a lower SERCA2a/phospholamban ratio. Mitochondrial respiration was lower, particularly with nicotinamide adenine dinucleotide (NADH) (complex I) substrates, and mitochondrial swelling was larger in ZSF1‐obese.ConclusionThe free mitochondrial calcium concentration is higher in HFpEF owing to alterations in mitochondrial and cytosolic Ca2+ handling. This coupling between cytosolic and mitochondrial Ca2+ levels may compensate for myocardial ATP supply in vivo under conditions of mild mitochondrial dysfunction. However, if mitochondrial Ca2+ concentration is sustainedly increased, it might trigger mitochondrial permeability transition pore opening.

Comparative study on isolation and mitochondrial function of adult mouse and rat cardiomyocytes

BackgroundCultured adult mouse and rat cardiomyocytes are the best and low-cost cell model for cardiac cellular physiology, pathology, drug toxicity screening, and intervention. The functions of mouse cardiomyocytes decline faster than rat cardiomyocytes in culture conditions. However, little is known about the difference of mitochondrial function between cultured mouse and rat myocytes. Methods and resultsA large number of adult mouse and rat cardiomyocytes were comparative isolated using a simple perfusion system. Cardiomyocytes mitochondrial functions were measured after 2 h, 1 day, 2 days, 3 days, and 4 days culture by monitoring mitoflashes. We found that the mitochondrial function of mouse myocytes was remarkedly declined on the third day. Then, we focused on the third day cultured mouse and rat myocytes, comparatively analyzing the respiration function and superoxide generation stimulated by pyruvate/malate/ADP and the mitochondrial permeability transition pore (mPTP) opening induction. Mouse myocytes showed lower respiration and mitoflash activity, but without the change of maximum uncoupled respiration when compared with rat myocytes. Although the response to superoxide production stimulated by respiration substrates was slower than rat myocytes, the basal superoxide generation is faster than the rat. The faster mitochondrial reactive oxygen species (ROS) generation of mouse myocytes upon laser stimulation triggered the faster mPTP opening compared with the rat. Finally, antioxidant MitoTEMPO pretreatment preserved the mitochondrial function of mouse myocytes on the third day. ConclusionsThe mitochondrial function and stability are different between cultured mouse and rat cardiac myocytes beyond 3 days even though they both belong to Muridae. Mitochondrial ROS impairs the mitochondrial functions of mouse cardiomyocytes on the third day. Suppressing superoxide maintained the mitochondrial function of mouse myocytes on the third day.

Resveratrol prevents palmitic-acid-induced cardiomyocyte contractile impairment

Long-chain saturated fatty acids, especially palmitic acid (PA), contribute to cardiomyocyte lipotoxicity. This study tests the effects of PA on adult rat cardiomyocyte contractile function and proteins associated with calcium regulating cardiomyocyte contraction and relaxation. Adult rat cardiomyocytes were pretreated with resveratrol (Resv) and then treated with PA. For the reversal study, cardiomyocytes were incubated with PA prior to treatment with Resv. Cardiomyocyte contractility, ratio of rod- to round-shaped cardiomyocytes, and Hoechst staining were used to measure functional and morphological changes in cardiomyocytes. Protein expression of sarco-endoplasmic reticulum ATPase 2a (SERCA2a), native phospholamban (PLB) and phosphorylated PLB (pPLB ser16 and pPLB thr17), and troponin I (TnI) and phosphorylated TnI (pTnI) were measured. SERCA2a activity was also measured. Our results show that PA (200 μM) decreased the rate of cardiomyocyte relaxation, reduced the number of rod-shaped cardiomyocytes, and increased the number of cells with condensed nuclei; pre-treating cardiomyocytes with Resv significantly prevented these changes. Post-treatment with Resv did not reverse morphological changes induced by PA. Protein expression levels of SERCA2a, PLB, pPLBs, TnI, and pTnI were unchanged by PA or Resv. SERCA2a activity assay showed that Vmax and Iono ratio were increased with PA and pre-treatment with Resv prevented this increase. In conclusion, our results show that Resv protect cardiomyocytes from contractile dysfunction induced by PA.

Abstract 214: Effect of Matrix Stiffness on Adult Cardiomyocytes Using Dynamic, Tunable, and Reversible Magnetorheological PDMS Substrates

The physical properties of the extracellular matrix (ECM), including stiffness and viscoelasticity, play a key role in the development of hypertensive and hypertrophic cardiac diseases at the cellular level, affecting the morphology and function of heart tissue. The ECM is a dynamic structure that is continuously changing in both normal and diseased tissue. In an extension of prior studies examining how substrates of fixed stiffness affect cardiac cells, we introduce a novel experimental model that allows exploration of cardiomyocyte functional alterations in response to rapid and reversible matrix stiffness modulation. We fabricated a biocompatible magneto-rheological elastomer (MRE) culture substrate by adding iron particles to a PDMS elastomer. In the presence of an applied magnetic field, this substrate stiffens reversibly and nearly instantaneously from 10 kPa to 50 kPa: the range observed in normal and diseased myocardium, respectively. Adult cardiomyocytes isolated from non-failing human hearts (unused organ donors) or adult rats were cultured for 24 - 48 hours on the tunable MREs. Cardiomyocytes cultured on a stiffened MRE (~50 kPa) exhibited time-dependent reductions in cell shortening and peak Ca 2+ transient amplitude compared with cardiomyocytes cultured on a soft MRE (~10 kPa). Rates of cardiomyocyte shortening and re-lengthening were also slowed by culture on 50 kPa substrates. Using super-resolution imaging, we found that cardiomyocytes cultured on a stiffened MRE exhibited an increased microtubule network density and demonstrated increased cellular stiffness and viscoelasticity, as measured by nanoindentation. These studies indicate that adult human and rat cardiomyocytes are acutely sensitive to changes in extracellular stiffness within the pathophysiological range and tend to adapt their own stiffness to match that of their surroundings via dynamic changes in microtubule architecture. These mechanisms may contribute to the regulation of mechanical homogeneity within the myocardium. Our method provides a unique in vitro platform for studying mechanosensing, mechanotransduction and mechanical memory in isolated adult cardiomyocytes.

Abstract 902: Leveraging Natural Cardiomyocyte Variability to Investigate Downregulation of β-1 Adrenoceptors Following Dobutamine Treatment

In heart failure, the most consistent effect on the myocardial beta-adrenergic pathway is a downregulation of β-1 adrenoceptors (β1AR). This loss of receptors impairs the ability of the heart to respond to increases in cardiac demand, and can lead to decreased myocyte function even at rest. As a model of the process that leads to downregulation of β1AR, we measured intracellular Ca 2+ transients and unloaded sarcomere shortening in electrically stimulated isolated adult rat cardiomyocytes undergoing 30 minutes of exposure to 40 nM dobutamine. These experiments were facilitated by a custom device containing micro-patterned wells that orient and immobilize the cardiomyocytes, allowing for rapid and consistent measurements over an extended time period. After functional measurements, cells are picked up individually with a computerized micro-aspirator and subjected to single-cell RT-qPCR to quantify abundance of transcripts of interest. In this way, the physiological responses of individual cells to β1AR stimulus can be correlated to gene expression levels. We found a surprising diversity in the single-cell responses to dobutamine exposure. Although average cardiomyocyte contractility increased significantly, several apparently healthy cells registered no change or even a decrease in sarcomere shortening in the presence of dobutamine. Gene expression was quantified in cardiomyocytes representing a spectrum of sensitivities to dobutamine (n = 7). The transcript for β1AR was significantly downregulated in all cells after 30 minutes, indicating consistent adaptation to chronic stimulus. However, abundance of the gene encoding PKA was negatively correlated with dobutamine-triggered augmentation of Ca 2+ release (p-value: 0.0201). In other words, PKA expression was reduced in cells that experienced potent increases in Ca 2+ release after dobutamine treatment. These data suggest the presence of gene regulatory circuits that modulate individual components of the β1AR pathway independent of overall β1AR abundance. These circuits may be critical to restoring β1AR sensitivity in the failing heart.

Abstract 332: Pharmacologic Characterization of the Cardiac Myosin Inhibitor, CK-3773274: A Potential Therapeutic Approach for Hypertrophic Cardiomyopathy

Hypercontractility of the cardiac sarcomere appears to underlie pathological hypertrophy and fibrosis in select genetic hypertrophic cardiomyopathies. Here, we characterize the small molecule, CK-3773274, as a novel cardiac myosin inhibitor that decreases contractility in vitro and in vivo . In bovine cardiac myofibrils, CK-3773274 decreased myosin ATPase activity in a concentration-dependent fashion (IC 50 :1.26 μM). CK-3773274 specifically inhibited myosin activity, as it reduced myosin ATPase activity in a concentration-dependent manner in the absence of other sarcomere proteins, including actin, troponin, and tropomyosin. CK-3773274 (10 μM) reduced fractional shortening by 84% in electrically paced, isolated adult rat cardiomyocytes relative to control without any effect on the calcium transient. The effect of CK-3773274 on cardiac contractility in vivo was assessed in healthy male Sprague Dawley (SD) rats using single oral doses ranging from 0.5 to 4 mg/kg. Fractional shortening (FS) and left ventricular dimensions were determined by echocardiography at select time points over a 24-hour period. One hour after dose administration, CK-3773274 significantly reduced fractional shortening in a dose-related fashion by 20-70% relative to vehicle treatment (FS %: vehicle: 47.9± 1%; 0.5 mg/kg: 39 ± 2%; 4 mg/kg: 15 ± 4%; mean ±SEM, p<0.05 vehicle vs. all doses) without any changes to heart rate. Lastly, the effect of CK-3773274 was evaluated by echocardiography in healthy beagle dogs. Left ventricular ejection fraction (LVEF) was evaluated following single oral doses ranging from 0.75-3 mg/kg over a 48 hour period. 2 hours after dosing, CK-3773274 decreased LVEF in a dose-related fashion by approximately 15-50% relative to vehicle treatment (LVEF vehicle: 74.6 ± 3 %; 0.75 mg/kg: 62.5 ± 3%; 2 mg/kg: 44.9± 3%; 3 mg/kg: 36.8 ± 2%; mean ±SEM, p<0.05 vehicle vs. all doses). In conclusion, CK-3773274 is a novel, small molecule, cardiac myosin inhibitor that reduces cardiac contractility in vitro and in vivo . Cardiac myosin inhibition may be a viable approach to treat the underlying hypercontractility of the cardiac sarcomere in hypertrophic cardiomyopathies.

Inhibition of Senescence-Associated Genes Rb1 and Meis2 in Adult Cardiomyocytes Results in Cell Cycle Reentry and Cardiac Repair Post-Myocardial Infarction.

BackgroundMyocardial infarction results in a large‐scale cardiomyocyte loss and heart failure due to subsequent pathological remodeling. Whereas zebrafish and neonatal mice have evident cardiomyocyte expansion following injury, adult mammalian cardiomyocytes are principally nonproliferative. Despite historical presumptions of stem cell–mediated cardiac regeneration, numerous recent studies using advanced lineage‐tracing methods demonstrated that the only source of cardiomyocyte renewal originates from the extant myocardium; thus, the augmented proliferation of preexisting adult cardiomyocytes remains a leading therapeutic approach toward cardiac regeneration. In the present study we investigate the significance of suppressing cell cycle inhibitors Rb1 and Meis2 to promote adult cardiomyocyte reentry to the cell cycle.Methods and ResultsIn vitro experiments with small interfering RNA–mediated simultaneous knockdown of Rb1 and Meis2 in both adult rat cardiomyocytes, isolated from 12‐week‐old Fischer rats, and human induced pluripotent stem cell–derived cardiomyocytes showed a significant increase in cell number, a decrease in cell size, and an increase in mononucleated cardiomyocytes. In vivo, a hydrogel‐based delivery method for small interfering RNA–mediated silencing of Rb1 and Meis2 is utilized following myocardial infarction. Immunofluorescent imaging analysis revealed a significant increase in proliferation markers 5‐ethynyl‐2′‐deoxyuridine, PH3, KI67, and Aurora B in adult cardiomyocytes as well as improved cell survivability with the additional benefit of enhanced peri‐infarct angiogenesis. Together, this intervention resulted in a reduced infarct size and improved cardiac function post–myocardial infarction.ConclusionsSilencing of senescence‐inducing pathways in adult cardiomyocytes via inhibition of Rb1 and Meis2 results in marked cardiomyocyte proliferation and increased protection of cardiac function in the setting of ischemic injury.

Divergent Effects of Resveratrol on Rat Cardiac Fibroblasts and Cardiomyocytes.

In this study, we tested the potential cardioprotective effects of the phytoalexin resveratrol (Rsv) on primary adult rat cardiac fibroblasts (CF), myofibroblasts (MF) and cardiomyocytes. Adult rat CF and cardiomyocytes were isolated from male 10-week old Sprague–Dawley rats, cultured for either 24 h (cardiomyocytes) or 48 h (CF) before treatments. To isolate MF, CF were trypsinized after 48 h in culture, seeded in fresh plates and cultured for 24 h prior to treatment. All three cells were then treated for a further 24 h with a range of Rsv doses. In CF and MF, cell proliferation, viability, apoptosis assays were performed with or without Rsv treatment for 24 h. In cardiomyocytes, cell viability and apoptosis assay were performed 24 h after treatment. In separate experiments, CF was pre-incubated with estrogen, tamoxifen and fulvestrant for 30 min prior to Rsv treatment. Rsv treatment decreased proliferation of both fibroblasts and myofibroblasts. Rsv treatment also increased the proportion of dead CF and MF in a dose dependent manner. However, treatment with Rsv did not induce cell death in adult cardiomyocytes. There was an increase in the percentage of cells with condensed nuclei with Rsv treatment in both CF and MF, but not in cardiomyocytes. Treatment with estrogen, tamoxifen and fulvestrant alone or in combination with Rsv did not have any additional effects on CF survival. Our results demonstrate that treatment with Rsv can inhibit cell proliferation and induce cell death in rat CF and MF, while not affecting cardiomyocyte survival. We also demonstrated that the induction of cell death in CF with Rsv treatment was independent of estrogen receptor alpha (ERα) signaling.

Hyperosmotic stress promotes endoplasmic reticulum stress-dependent apoptosis in adult rat cardiac myocytes.

In different pathological situations, cardiac cells undergo hyperosmotic stress and cell shrinkage. This change in cellular volume has been associated with contractile dysfunction and cell death. However, the intracellular mechanisms involved in hyperosmotic stress-induced cell death have not been investigated in depth in adult cardiac myocytes. Given that osmotic stress has been shown to promote endoplasmic reticulum stress (ERS), a recognized trigger for apoptosis, we examined whether hyperosmotic stress triggers ERS in adult cardiac myocytes and if so whether this mechanism mediates hyperosmotic stress-induced cell death. Adult rat cardiomyocytes cultured overnight in a hypertonic solution (HS) containing mannitol as the osmolite, showed increased expression of ERS markers, GRP78, CHOP and cleaved-Caspase-12, compared with myocytes in isotonic solution (IS), suggesting that hyperosmotic stress induces ERS. In addition, HS significantly reduced cell viability and increased TUNEL staining and the expression of active Caspase-3, indicative of apoptosis. These effects were prevented with the addition of the ERS inhibitor, 4-PBA, indicating that hyperosmotic stress-induced apoptosis is mediated by ERS. Hyperosmotic stress-induced apoptosis was also prevented when cells were cultured in the presence of a Ca2+-chelating agent (EGTA) or the CaMKII inhibitor (KN93), suggesting that hyperosmotic stress-induced ERS is mediated by a Ca2+ and CaMKII-dependent mechanism. Similar results were observed when hyperosmotic stress was induced using glucose as the osmolite. We conclude that hyperosmotic stress promotes ERS by a CaMKII-dependent mechanism leading to apoptosis of adult cardiomyocytes. More importantly, we demonstrate that hyperosmotic stress-triggered ERS contributes to hyperglycemia-induced cell death.

Pseudo-bipolar spindle formation and cell division in postnatal binucleated cardiomyocytes

BackgroundThe majority of adult human, mouse and rat cardiomyocytes is not diploid mononucleated. Nevertheless, the current literature on heart regeneration based on cardiomyocyte proliferation focuses mainly on the proliferation capacity of diploid mononucleated cardiomyocytes, instead of the more abundant mononucleated polyploid or binucleated cardiomyocytes. Here, we aimed at a better understanding of the process of mitosis and cell division in postnatal binucleated cardiomyocytes. Methods and resultsPostnatal rat binucleated cardiomyocytes were stimulated to re-enter the cell cycle either by fetal bovine serum or a combination of fibroblast growth factor 1 and p38 MAP kinase inhibitor. Phase-contrast videos revealed that binucleated cardiomyocytes form one metaphase plate and preferentially undergo afterwards cytokinesis failure. The maximum rate of cell division of video-recorded binucleated cardiomyocytes was around 6%. Immunofluorescence analyses of centriole number in mitotic binucleated cardiomyocytes revealed that these cells contain more than four centrioles, which can be paired as well as unpaired. In agreement with multiple and/or unpaired centrioles, multipolar spindle formation was observed in mitotic binucleated cardiomyocytes using fluorescence live imaging of tubulin-GFP. Multipoles were transient and resolved into pseudo-bipolar spindles both in case of cell division and cytokinesis failure. Notably, centrioles were in most cases unevenly distributed among daughter cells. ConclusionsOur results indicate that postnatal binucleated cardiomyocytes upon stimulation can enter mitosis, cope with their multiple and/or unpaired centrioles by forming pseudo-bipolar spindles, and divide.