Left Ventricular Cardiomyocytes Research Articles

OP2113, a new drug for chronic hypoxia-induced pulmonary hypertension treatment in rat.

Pulmonary hypertension (PH) is a cardiovascular disease characterised by an increase in pulmonary arterial (PA) resistance leading to right ventricular (RV) failure. Reactive oxygen species (ROS) play a major role in PH. OP2113 is a drug with beneficial effects on cardiac injuries that targets mitochondrial ROS. The aim of the study was to address the in vivo therapeutic effect of OP2113 in PH. PH was induced by 3weeks of chronic hypoxia (CH-PH) in rats treated with OP2113 or its vehicle via subcutaneous osmotic mini-pumps. Haemodynamic parameters and both PA and heart remodelling were assessed. Reactivity was quantified in PA rings and in RV or left ventricular (LV) cardiomyocytes. Oxidative stress was detected by electron paramagnetic resonance and western blotting. Mitochondrial mass and respiration were measured by western blotting and oxygraphy, respectively. In CH-PH rats, OP2113 reduced the mean PA pressure, PA remodelling, PA hyperreactivity in response to 5-HT, the contraction slowdown in RV and LV and increased the mitochondrial mass in RV. Interestingly, OP2113 had no effect on haemodynamic parameters, both PA and RV wall thickness and PA reactivity, in control rats. Whereas oxidative stress was evidenced by an increase in protein carbonylation in CH-PH, this was not affected by OP2113. Our study provides evidence for a selective protective effect of OP2113 in vivo on alterations in both PA and RV from CH-PH rats without side effects in control rats.

Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte.

Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening. Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch. We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening. Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

Combined lineage tracing and scRNA-seq reveals unexpected first heart field predominance of human iPSC differentiation.

During mammalian development, the left and right ventricles arise from early populations of cardiac progenitors known as the first and second heart fields, respectively. While these populations have been extensively studied in non-human model systems, their identification and study in vivo human tissues have been limited due to the ethical and technical limitations of accessing gastrulation stage human embryos. Human induced pluripotent stem cells (hiPSCs) present an exciting alternative for modeling early human embryogenesis due to their well-established ability to differentiate into all embryonic germ layers. Here, we describe the development of a TBX5/MYL2 lineage tracing reporter system that allows for the identification of FHF- progenitors and their descendants including left ventricular cardiomyocytes. Furthermore, using single cell RNA sequencing (scRNA-seq) with oligonucleotide-based sample multiplexing, we extensively profiled differentiating hiPSCs across 12 timepoints in two independent iPSC lines. Surprisingly, our reporter system and scRNA-seq analysis revealed a predominance of FHF differentiation using the small molecule Wnt-based 2D differentiation protocol. We compared this data with existing murine and 3D cardiac organoid scRNA-seq data and confirmed the dominance of left ventricular cardiomyocytes (>90%) in our hiPSC-derived progeny. Together, our work provides the scientific community with a powerful new genetic lineage tracing approach as well as a single cell transcriptomic atlas of hiPSCs undergoing cardiac differentiation.

Thyroid hormone maintains cardiac dyadic structures by regulating junctophilin-2 localization to ryanodine clusters

Overt and subclinical hypothyroidism are associated with increased risk of cardiovascular disease and mortality. In pre-clinical studies, supplementation with triiodo-L-thyronine (T3) has been shown to regulate cardiomyocyte T-tubule (TT) organization, and thereby maintain ryanodine receptor-2 and L-type Ca 2+ channel (RyR-LTCC) clusters in dyadic structures necessary for efficient excitation-contraction (EC) coupling. We hypothesized that the TT-sarcoplasmic reticulum (SR)-spanning protein Junctophilin-2 (Jph) is regulated by T3. Thyroid hormone (TH) deficiency was induced in 12 adult female Sprague-Dawley rats by oral propyl-thiouracil (PTU; 0.025%) treatment for 8 weeks. Rats were then randomized to continued PTU treatment or with T3 (10 μg/kg/day) supplementation for 2 weeks (PTU+T3). Untreated rats served as euthyroid (EU) controls. Thyroid status was confirmed by plasma T3 and T4 levels. Left ventricular cardiomyocytes (CM) were isolated by collagenase digestion for single molecule imaging of Jph2, RyR2 and LTCC proteins using 2D and 3D STORM (stochastic optical reconstruction microscopy; Oxford Nanoimager). DBSCAN algorithms for cluster analysis (Web-based programs developed by LumeVR) measured cluster size and number, nearest neighbor cluster distances, and protein co-localizations. Calcium sparks in live CMs loaded with Fluo-4 were captured by line-scanning laser microscopy (Zeiss 980) to assess spontaneous SR Ca 2+ leak. STORM imaging of normal CMs showed that ~60% of RyR clusters consisted of 10-50 RyR localizations (locs) with 100-200 Jph molecules co-localized within 210 nm radius of the cluster centroid. Although Jph locs per cluster increased proportionately with increased cluster size, PTU cells had markedly less Jph co-localized with RyR at all cluster sizes compared to EU and PTU+T3 cells. The reduction of associated Jph was most evident in super-clusters (>250 RyR locs). 3D image analysis showed nearest neighbor distances between RyR clusters that were significantly increased in the PTU CMs vs EU (749±146 nm vs 640±140 nm), which were normalized with T3-treatment (616±60 nm). Numbers of RyR clusters without associated Jph locs were significantly higher in PTU cells, and reduced with T3. RyR Ca 2+ leak as measured by spontaneous Ca 2+ sparks was increased in PTU CMs, but decreased significantly to control values in T3-treated cells. This study supports a regulatory role of T3 in maintaining TT-SR dyad structures, in part by increasing Jph co-localization with RyR clusters, thus promoting efficient interaction of RyR with LTCC channels for effective EC-coupling and cardiac output. Study funded by NIH R15HL15406 and by resources of the NYIT Imaging Center. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Long term in vivo Dlk1 gene transfer protects against myocardial fibrosis-induced cardiac dysfunction

Introduction: Cardiac fibrosis can independently contribute to the impairment of cardiac function and development of heart failure. Early activation of pro-fibrotic genetic pathways has been suggested to occur well before adverse ventricular remodeling occurs in fibrosis as it progresses to heart failure. Delta-like homolog 1 (Dlk1), a paternally imprinted gene that encodes a transmembrane protein that contains multiple epidermal growth factor repeats, has been shown to play a critical role as a regulator of cell differentiation, neuronal differentiation, bone differentiation, and hematopoiesis. We hypothesize that Dlk-1 could regulate myocardial fibrosis via inhibition of fibroblast-myofibroblast conversion and therefore restrain cardiac fibrosis progression. Methods: Wildtype (WT) and Dlk1 knockout (Dlk1-/-) mice were subjected to myocardial infarction (MI) by permanent ligation of the left anterior descending artery to induce heart failure. A recombinant adeno-associated virus 9 (AAV9) was used to deliver Dlk1 into the myocardium in vivo. Cardiac function and hemodynamic changes were measured by echocardiography and left ventricular catheterization. Immunohistochemistry and confocal microscopy were used to determine the expression of pro-fibrotic markers. Total RNA was prepared from left ventricular (LV) tissue or cardiomyocytes and fibroblasts from Dlk1-/- and WT hearts. Masson trichrome staining was used to determine abnormal collagen deposition in the heart. Result: Myocardial fibrosis was demonstrated by increased expression of alpha-smooth muscle actin (αSMA), collagen, fibronectin, connective tissue growth factor, along with a substantial increase in the deposition of collagen as evident from Masson's trichrome staining. We observed downregulated Dlk-1 mRNA and protein expression in ischemic tissues from human patients and infarcted pigs’ hearts. The reduced levels of Dlk1 were associated with increased mRNA expression of pro-fibrotic molecules, collagen1a, LOX and αSMA in the scar area. Echocardiographic analysis revealed substantial LV chamber dilatation and reduced fractional shortening (FS %) in Dlk1 null mice as compared to WT, whereas the mice treated with AAV9-Dlk1 for 8-10 weeks showed a significant improvement in cardiac function. Consistently, Dlk1-deficient mice exhibited impairment in heart function, cardiac fibrosis and myocyte hypertrophy and increased expression of atrial natriuretic peptide, and B-type natriuretic peptide, while AAV9-Dlk1-treated mice significantly reduced cardiac remodeling and myocardial fibrosis. Conclusion: Restoration of Dlk1 in the heart following MI-induced heart failure improved cardiac function and attenuated myocardial fibrosis. The findings from the current study suggest a critical role for Dlk1 in myocardial fibrosis-induced cardiac remodeling, indicating that Dlk1 gene therapy may be a viable therapeutic approach for heart failure. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Comparing the effects of chemical Ca2+ dyes and R-GECO on contractility and Ca2+ transients in adult and human iPSC cardiomyocytes

We compared commonly used BAPTA-derived chemical Ca2+ dyes (fura2, Fluo-4, and Rhod-2) with a newer genetically encoded indicator (R-GECO) in single cell models of the heart. We assessed their performance and effects on cardiomyocyte contractility, determining fluorescent signal-to-noise ratios and sarcomere shortening in primary ventricular myocytes from adult mouse and guinea pig, and in human iPSC-derived cardiomyocytes. Chemical Ca2+ dyes displayed dose-dependent contractile impairment in all cell types, and we observed a negative correlation between contraction and fluorescence signal-to-noise ratio, particularly for fura2 and Fluo-4. R-GECO had no effect on sarcomere shortening. BAPTA-based dyes, but not R-GECO, inhibited in vitro acto-myosin ATPase activity. The presence of fura2 accentuated or diminished changes in contractility and Ca2+ handling caused by small molecule modulators of contractility and intracellular ionic homeostasis (mavacamten, levosimendan, and flecainide), but this was not observed when using R-GECO in adult guinea pig left ventricular cardiomyocytes. Ca2+ handling studies are necessary for cardiotoxicity assessments of small molecules intended for clinical use. Caution should be exercised when interpreting small molecule studies assessing contractile effects and Ca2+ transients derived from BAPTA-like chemical Ca2+ dyes in cellular assays, a common platform for cardiac toxicology testing and mechanistic investigation of cardiac disease physiology and treatment.

The Antidepressant Paroxetine Reduces the Cardiac Sodium Current.

A considerable amount of literature has been published on antidepressants and cardiac ion channel dysfunction. The antidepressant paroxetine has been associated with Brugada syndrome and long QT syndrome, albeit on the basis of conflicting findings. The cardiac voltage-gated sodium channel (NaV1.5) is related to both of these syndromes, suggesting that paroxetine may have an effect on this channel. In the present study, we therefore carried out patch clamp experiments to examine the effect of paroxetine on human NaV1.5 channels stably expressed in human embryonic kidney 293 (HEK-293) cells as well as on action potentials of isolated rabbit left ventricular cardiomyocytes. Additionally, computer simulations were conducted to test the functional effects of the experimentally observed paroxetine-induced changes in the NaV1.5 current. We found that paroxetine led to a decrease in peak NaV1.5 current in a concentration-dependent manner with an IC50 of 6.8 ± 1.1 µM. In addition, paroxetine caused a significant hyperpolarizing shift in the steady-state inactivation of the NaV1.5 current as well as a significant increase in its rate of inactivation. Paroxetine (3 µM) affected the action potential of the left ventricular cardiomyocytes, significantly decreasing its maximum upstroke velocity and amplitude, both of which are mainly regulated by the NaV1.5 current. Our computer simulations demonstrated that paroxetine substantially reduces the fast sodium current of human left ventricular cardiomyocytes, thereby slowing conduction and reducing excitability in strands of cells, in particular if conduction and excitability are already inhibited by a loss-of-function mutation in the NaV1.5 encoding SCN5A gene. In conclusion, paroxetine acts as an inhibitor of NaV1.5 channels, which may enhance the effects of loss-of-function mutations in SCN5A.

Liraglutide provides cardioprotection through the recovery of mitochondrial dysfunction and oxidative stress in aging hearts.

Glucagon-like peptide-1 receptor (GLP-1R) agonists improve cardiovascular dysfunction via the pleiotropic effects behind their receptor action. However, it is unknown whether they have a cardioprotective action in the hearts of the elderly. Therefore, we examined the effects of GLP-1R agonist liraglutide treatment (LG, 4weeks) on the systemic parameters of aged rats (24-month-old) compared to those of adult rats (6-month-old) such as electrocardiograms (ECGs) and systolic and diastolic blood pressure (SBP and DBP). At the cellular level, the action potential (AP) parameters, ionic currents, and Ca2+ regulation were examined in freshly isolated ventricular cardiomyocytes. The LG treatment of aged rats significantly ameliorated the prolongation of QRS duration and increased both SBP and DBP together with recovery in plasma oxidant and antioxidant statuses. The prolonged AP durations and depolarized membrane potentials of the isolated cardiomyocytes from the aged rats were normalized via recoveries in K+ channel currents with LG treatment. The alterations in Ca2+ regulation including leaky-ryanodine receptors (RyR2) could be also ameliorated via recoveries in Na+/Ca2+ exchanger currents with this treatment. A direct LG treatment of isolated aged rat cardiomyocytes could recover the depolarized mitochondrial membrane potential, the increase in both reactive oxygen and nitrogen species (ROS and RNS), and the cytosolic Na+ level, although the Na+ channel currents were not affected by aging. Interestingly, LG treatment of aged rat cardiomyocytes provided a significant inhibition of activated sodium-glucose co-transporter-2 (SGLT2) and recoveries in the depressed insulin receptor substrate 1 (IRS1) and increased protein kinase G (PKG). The recovery in the ratio of phospho-endothelial nitric oxide (pNOS3) level to NOS3 protein level in LG-treated cardiomyocytes implies the involvement of LG-associated inhibition of oxidative stress-induced injury via IRS1-eNOS-PKG pathway in the aging heart. Overall, our data, for the first time, provide important information on the direct cardioprotective effects of GLP-1R agonism with LG in the hearts of aged rats through an examination of recoveries in mitochondrial dysfunction, and both levels of ROS and RNS in left ventricular cardiomyocytes.

Abstract 12178: Extracellular Vesicle-Encapsulated AAVs for Therapeutic Gene Delivery to the Heart

Background: Adeno-associated virus (AAV) has emerged as one of the best tools for cardiac gene delivery due to its cardiotropism, long-term expression, and safety. However, a significant challenge to its successful clinical use are pre-existing neutralizing antibodies (NAbs), which bind to free AAVs, prevent efficient gene transduction and reduce or negate therapeutic effects. Here we describe extracellular vesicle-encapsulated AAVs (EV-AAVs), secreted naturally by AAV-producing cells, as a superior cardiac gene delivery vector that delivers more genes and offers higher NAb resistance. Method: We developed a two-step density-gradient ultracentrifugation method to isolate highly purified EV-AAVs. Results: Using cardiotropic AAV serotypes 6 and 9 and several reporter constructs, we demonstrated that EV-AAVs deliver significantly higher quantities of genes than AAVs in the presence of NAb, both to human left ventricular and hiPSC cardiomyocytes in vitro, and to mouse hearts in vivo. Intramyocardial delivery of EV-AAV9-SERCA2a to infarcted hearts in pre-immunized mice significantly improved ejection fraction and fractional shortening compared to AAV9-SERCA2a delivery. These data validated NAb evasion by and therapeutic efficacy of EV-AAV9 vectors. Interestingly, in vivo trafficking studies in mouse hearts showed significantly higher expression of EV-AAV9-delivered genes in cardiomyocytes compared to non-cardiomyocytes, even with comparable cellular uptake. Using cellular sub-fraction analyses and pH-sensitive dyes, we discovered that EV-AAVs were internalized into acidic endosomal compartments of cardiomyocytes for releasing and acidifying AAVs for their nuclear uptake. Conclusion: Together, using six different in vitro and in vivo model systems, we demonstrate significantly higher potency and therapeutic efficacy of EV-AAV vectors compared to free AAVs. These results establish the potential of EV-AAV vectors as a gene delivery tool to treat heart failure. 1

Abstract 13709: Cardioprotective Mechanisms of FTO-Regulated m 6 A in Heart Failure

Introduction: Post-transcriptional modifications of mRNA play an important role in cardiac remodeling. We recently reported an aberrant increase in global N6-methyladenosine (m 6 A) in failing hearts associated with reduction in expression of m 6 A eraser, fat and obesity-associated protein (FTO). Overexpression of FTO increased m 6 A, and improved cardiomyocyte and cardiac function in both in vitro and in vivo models for myocardial infarction (MI). Here, we investigated the mRNA targets of FTO and mechanism of FTO-dependent cardiac protein expression in failing hearts. Hypothesis: We hypothesized that FTO mediates cardioprotection via demethylation of selective mRNA transcripts in the nucleus and cytoplasm of cardiomyocytes. Methods and Results: To identify subcellular localization of FTO-mRNA interaction, we examined FTO expression in human left ventricular cardiomyocytes under hypoxia in vitro and in mouse and human failing hearts in vivo . FTO was detected in both nucleus and cytoplasm of healthy cardiomyocytes and tissues, but not in hypoxic cardiomyocytes and MI mouse and human heart tissues. To reveal functional targets of FTO, we performed m 6 A RNA sequencing of nuclear and cytoplasmic RNA in cultured human cardiomyocytes. We identified distinct sets of RNA transcripts in nucleus and cytoplasm of cardiomyocytes involved in cardiac contraction, angiogenesis, cardiomyogenesis, hypermethylated by FTO. Methylated RNA immunoprecipitation experiments revealed that SERCA2a (involved in cardiomyocytes contraction) and FSTL1 (pro-angiogenic) mRNAs directly bind to FTO. Loss of FTO resulted in decreased nuclear export of SERCA2a and FSTL1 mRNAs. Our stable isotope labeling (SILAC) experiment confirmed that FTO regulates de-novo synthesis of SERCA2A and FSTL1 proteins. Conclusion: We conclude that FTO selectively targets the cardioprotective transcripts, regulates their nuclear export and protein expression. Our study will reveal FTO binding sites on cardiac transcripts and a novel mechanism of FTO-dependent protein expression in cardiomyocytes.

Sarco/endoplasmic reticulum calcium ATPase activity is unchanged despite increased myofilament calcium sensitivity in Zucker type 2 diabetic fatty rat heart

Systolic and diastolic dysfunction in diabetes have frequently been associated with abnormal calcium (Ca2+) regulation. However, there is emerging evidence that Ca2+ mishandling alone is insufficient to fully explain diabetic heart dysfunction, with focus shifting to the properties of the myofilament proteins. Our aim was to examine the effects of diabetes on myofilament Ca2+ sensitivity and Ca2+ handling in left ventricular tissues isolated from the same type 2 diabetic rat hearts. We measured the force-pCa relationship in skinned left ventricular cardiomyocytes isolated from 20-week-old type 2 diabetic and non-diabetic rats. Myofilament Ca2+ sensitivity was greater in the diabetic relative to non-diabetic cardiomyocytes, and this corresponded with lower phosphorylation of cardiac troponin I (cTnI) at ser23/24 in the diabetic left ventricular tissues. Protein expression of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), phosphorylation of phospholamban (PLB) at Ser16, and SERCA/PLB ratio were lower in the diabetic left ventricular tissues. However, the maximum SERCA Ca2+ uptake rate was not different between the diabetic and non-diabetic myocardium. Our data suggest that impaired contractility in the diabetic heart is not caused by SERCA Ca2+ mishandling. This study highlights the important role of the cardiac myofilament and provides new insight on the pathophysiology of diabetic heart dysfunction.

Regulation of APD and force by Na+/Ca2+ exchanger in hiPSC-cardiomyocytes

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): BMBF Introduction Human induced pluripotent stem cell-derived cardiomyocytes (HiPSC-CM) are an emerging, powerful tool to study human cardiac physiology, pharmacology and toxicology, to model cardiovascular diseases or even to use for cardiac repair. Understanding the similarities and differences between hiPSC-CM and adult human cardiomyocytes is critical for their use. Here we focus on sodium calcium exchanger (NCX) who plays a crucial role in the Ca2+-homeostasis in the mammalian heart. Importantly, alterations in NCX expression in human heart are associated with various cardiac pathologies such as heart failure or arrhythmias. In order to investigate whether hiPSC-CM could serve as model for adult human heart NCX we measured the properties of NCX in hiPSC-CM and human ventricular tissue. Rat ventricular tissue was used for comparison. Methods HiPSC-CM were differentiated from a healthy iPSC line and dissociated from engineered heart tissue (EHT). Adult human and rat cardiomyocytes were digested from ventricular samples. We measured NCX current by the whole-cell patch clamp technique at 37 °C. Standard sharp microelectrodes were used to record action potentials (AP). Contractile force in human and rat ventricular samples was measured isometrically. A video-optical contractility test system was used to measure force in EHT. SEA0400 (10 µM) was used to block NCX. Results NCX currents could be measured in every hiPSC-CM. The NCX current densities in hiPSC-CM were larger than in human ventricular cardiomyocytes (3.2±0.2 pA/pF n=28 vs. 1.3±0.2 pA/pF n=15, p<0.05), but lower than reported for rat left ventricular cardiomyocytes using the same protocol. SEA0400 shortened APD90 markedly in EHT (264.1±24.9 ms to 191±31.6 ms, n=4) and to a lesser extent in rat ventricular tissue (54.4±3.9 ms to 48.9±4.2 ms, n=7). Shortening in human left ventricular preparations was tiny (320±22.1 ms to 305.5±20.3 ms, n=6) and not different from time-matched controls (TMC). Resting membrane potential, action potential amplitude and upstroke velocity were not affected neither in EHT nor in left ventricular preparations (rat and human). Force was significantly increased by NCX block in rat ventricle (by 31±5.4%, n=18) and EHT (by 20.8±3.9%, n=4), but in human left ventricular preparations there was only a tendency to attenuate spontaneous run-down (-3.7±4.3% n=8 with SEA vs. -6.2±3.7% n=12 in TMC). Conclusion HiPSC-CM possess NCX in the physiological range. HiPSC-CM show NCX-effects on APD and force as predicted from rat ventricle and in full accordance with cardiac physiology. Lack of NCX effect in human adult ventricles that had been already reported previously needs further investigations.

More evident roles of nNOS for the regulation of Ca2+‐sensitivity and SERCA activity in the right than the left ventricular cardiomyocytes of rats

It is known that neuronal NOS (nNOS, NOS1) regulates the intracellular Ca2+ ([Ca²⁺]i) and contractility of cardiomyocytes, which was investigated only in the left ventricular myocytes (LVCM) while not in the right ventricle (RVCM). In the paced cardiomyocytes, we have recently reported more relaxed diastolic sarcomere length (SLD) and smaller shortening (∆SL) while the amplitude of Ca²⁺ transient (∆[Ca²⁺]i) were not different (Jeon et al., 2021). Here we compared the expression and the functional roles of nNOS in the RVCMs and LVCMs from Sprague‐Dawley rats. The total expression and the phosphorylated Ser1417 ratio of nNOS were not different between RVCM and LVCM. However, the treatment with nNOS‐specific inhibitor, S‐methyl‐L‐thiocitrulline (SMTC, 100 nM), induced more prominent augmentation of ∆SL along with faster contraction while paradoxical faster relaxation and decreased ∆[Ca²⁺]i in RVCM. The hysteresis curve between ∆SL and ∆[Ca²⁺]i­ showed more prominent leftward shift, i.e. sensitization of the myofilaments in RVCM by SMTC. Consistently, the speed of contraction became faster, and the longer SLD of RVCM became similar to that of RVCM by the treatment with SMTC. Intriguingly, the SMTC treatment accelerated the Ca2+ removal by SERCA in RVCM, which appears to be consistent with the decreased ∆[Ca²⁺]i and faster relaxation. As for the decreased myofilament calcium sensitivity through sGC‐cGMP‐PKG pathwy, altered phosphorylation of troponin I (TnI) was suggested. In fact, the SMTC treatment decreased the TnI phosphorylation in RVCMs, while not LVCMs. Immunoblotting revealed higher expression of nNOSβ and its phosphorylated form in the myofilament fraction from RVCM than LVCM. In summary, the nNOS‐dependent longer SLD and the paradoxical negative inotropic effect despite the inhibitory effect on SERCA might be mediated by the myofilament‐associated nNOSβ decreasing Ca2+‐sensitivity through TnI phosphorylation. Interestingly, we also found that the treatment with omecamtiv mecarbil, a recently proposed cardiac myosin activator, induced more prominent inotropic effect in RVCM, which might imply a larger functional reservoir for the RV contraction.

Metabolically driven maturation of human-induced-pluripotent-stem-cell-derived cardiac microtissues on microfluidic chips.

The immature physiology of cardiomyocytes derived from human induced pluripotent stem cells (hiPSCs) limits their utility for drug screening and disease modelling. Here we show that suitable combinations of mechanical stimuli and metabolic cues can enhance the maturation of hiPSC-derived cardiomyocytes, and that the maturation-inducing cues have phenotype-dependent effects on the cells' action-potential morphology and calcium handling. By using microfluidic chips that enhanced the alignment and extracellular-matrix production of cardiac microtissues derived from genetically distinct sources of hiPSC-derived cardiomyocytes, we identified fatty-acid-enriched maturation media that improved the cells' mitochondrial structure and calcium handling, and observed divergent cell-source-dependent effects on action-potential duration (APD). Specifically, in the presence of maturation media, tissues with abnormally prolonged APDs exhibited shorter APDs, and tissues with aberrantly short APDs displayed prolonged APDs. Regardless of cell source, tissue maturation reduced variabilities in spontaneous beat rate and in APD, and led to converging cell phenotypes (with APDs within the 300-450 ms range characteristic of human left ventricular cardiomyocytes) that improved the modelling of the effects of pro-arrhythmic drugs on cardiac tissue.

Effects of Atrial Fibrillation on the Human Ventricle.

Rationale:Atrial fibrillation (AF) and heart failure often coexist, but their interaction is poorly understood. Clinical data indicate that the arrhythmic component of AF may contribute to left ventricular (LV) dysfunction.Objective:This study investigates the effects and molecular mechanisms of AF on the human LV.Methods and Results:Ventricular myocardium from patients with aortic stenosis and preserved LV function with sinus rhythm or rate-controlled AF was studied. LV myocardium from patients with sinus rhythm and patients with AF showed no differences in fibrosis. In functional studies, systolic Ca2+ transient amplitude of LV cardiomyocytes was reduced in patients with AF, while diastolic Ca2+ levels and Ca2+ transient kinetics were not statistically different. These results were confirmed in LV cardiomyocytes from nonfailing donors with sinus rhythm or AF. Moreover, normofrequent AF was simulated in vitro using arrhythmic or rhythmic pacing (both at 60 bpm). After 24 hours of AF-simulation, human LV cardiomyocytes from nonfailing donors showed an impaired Ca2+ transient amplitude. For a standardized investigation of AF-simulation, human iPSC-cardiomyocytes were tested. Seven days of AF-simulation caused reduced systolic Ca2+ transient amplitude and sarcoplasmic reticulum Ca2+ load likely because of an increased diastolic sarcoplasmic reticulum Ca2+ leak. Moreover, cytosolic Na+ concentration was elevated and action potential duration was prolonged after AF-simulation. We detected an increased late Na+ current as a potential trigger for the detrimentally altered Ca2+/Na+-interplay. Mechanistically, reactive oxygen species were higher in the LV of patients with AF. CaMKII (Ca2+/calmodulin-dependent protein kinase IIδc) was found to be more oxidized at Met281/282 in the LV of patients with AF leading to an increased CaMKII activity and consequent increased RyR2 phosphorylation. CaMKII inhibition and ROS scavenging ameliorated impaired systolic Ca2+ handling after AF-simulation.Conclusions:AF causes distinct functional and molecular remodeling of the human LV. This translational study provides the first mechanistic characterization and the potential negative impact of AF in the absence of tachycardia on the human ventricle.

Ingenuity pathway analysis of the human cardiac cell Atlas identifies differences between right and left ventricular cardiomyocytes.

Pharmaceuticals for left ventricular (LV) dysfunction do not have similar success in right ventricular (RV) failure, which may reflect biological differences between the ventricles. In this study, we performed Ingenuity Pathway Analysis of the Human Cell Atlas to understand how the transcriptomic signatures of the RV and LV differ.

Abstract 12001: Atrial Fibrillation Induces Ventricular Dysfunction via Altered Ca 2+ Handling and Oxidative CaMKII Activation in the Human Heart

Introduction: The effects of atrial fibrillation (AF) on left ventricular (LV) function are poorly understood. Clinical data indicate that the arrhythmic component of AF may contribute to LV dysfunction. This study investigated the effects of AF on the human LV. Methods and Results: LV myocardium from patients with preserved LV function (EF>50%) with sinus rhythm (SR, n=31) or rate-controlled AF (n=24) obtained during surgery for aortic stenosis was studied. Clinical data were mostly balanced without changes in LV function and remodeling. LV myocardium from SR and AF patients showed no changes in fibrosis. In functional studies, systolic Ca 2+ transient amplitude of human LV cardiomyocytes (CM) was reduced in AF patients, while diastolic Ca 2+ levels and Ca 2+ transient kinetics were unaltered. These results were confirmed in LV CM from non-failing donors with AF (n=4) vs. SR (n=8). In addition, normofrequent AF was simulated in vitro using arrhythmic or rhythmic electrical culture pacing (both at 60 bpm). After 24h of AF-simulation, human LV CM from non-failing donors (n=9) showed also an impaired Ca 2+ transient amplitude. For a standardized chronic AF-simulation, human iPSC-CM (n=22 differentiations/5 individuals) were tested. 7 days of AF-simulation caused reduced systolic Ca 2+ transient amplitude and sarcoplasmic reticulum Ca 2+ load, which could be explained by an increased diastolic Ca 2+ leak. Mechanistically, oxidative stress markers were higher in the LV of AF patients. Ca 2+ /calmodulin-dependent protein kinase IIδ (CaMKII) was found to be more oxidized at Met281/282 in the LV of AF patients leading to an increased CaMKII activity. Thus, RyR2 phosphorylation was elevated, which likely underlies the depression of LV Ca 2+ handling in AF. In translational contractility experiments of human ventricular trabecula from patients with heart failure (n=8), in vitro AF-simulation for 8h worsened systolic and diastolic contractile function. Conclusions: Here we could firstly show that AF impairs human LV function via increased oxidative stress and CaMKII activity in different patient populations and models. Thus, this translational study demonstrates the detrimental effects and mechanisms of AF in the absence of tachycardia on the human LV.

Abstract 13709: TRPV4 Enhances Ca 2+ Transients and Promotes Arrhythmic Ca 2+ Signaling Following Hypoosmotic Stress and Ischemia-Reperfusion

Introduction: Transient Receptor Potential Vanilloid 4 (TRPV4) is an osmotically activated Ca 2+ channel which enhances cardiomyocyte Ca 2+ cycling and may contribute to arrhythmic Ca 2+ signals in hypoosmotic stress and Ischemia-Reperfusion (I/R). Hypothesis: TRPV4 overexpression enhances Ca 2+ transient (CaT) amplitude and arrhythmic Ca 2+ events in left ventricular (LV) cardiomyocytes exposed to hypoosmotic stress and Langendorff hearts following I/R. Methods: Mouse LV cardiomyocytes from TRPV4 Overexpressors (OEX) and non-transgenic controls (CTL) were exposed to hypoosmotic stress (250 mOsm, 30min). CaT amplitude (paced 0.5 Hz) and arrhythmic Ca 2+ events (prolonged kinetics, spontaneous CaT, Ca 2+ waves) were monitored via confocal imaging (fluo-4, 100Hz). I/R (45min global ischemia, 30min reperfusion) experiments were conducted in OEX and CTL Langendorff hearts expressing the GCaMP6f Ca 2+ sensor (OEX-GCaMP, CTL-GCaMP). CaT and arrhythmic Ca 2+ signals (Ca 2+ waves, alternans, Ca 2+ overload) were monitored in the LV myocardium. Experiments were conducted in the absence and presence of TRPV4 inhibition by HC067047 (HC, 1uM). Results: Incidence of arrhythmic Ca 2+ signals during minutes 0-15 of hypoosmotic stress was increased in OEX cardiomyocytes vs CTL (78 vs 20%, P<0.05, N=9, 5) and prevented in OEX by HC (11%, P<0.05, N=9). Arrhythmia score was increased in OEX vs CTL (1.3±0.7 vs. 0.1±0.1, P<0.05, N=9, 5) and decreased in OEX by HC (0.2±0.2, P<0.05, N=9). Following hypoosmotic stress, CaT amplitude was elevated in OEX vs CTL (F/F 0 = 3.6±0.4 vs 2.1±0.4, P<0.05, N=7, 4) and this effect in OEX was prevented by HC (F/F 0 =2.4±0.4, P<0.05, N=8). Langendorff OEX-GCaMP hearts subjected to I/R demonstrated increased systolic Ca 2+ (F/F 0 =1.8±0.09 vs 1.3±0.1, P<0.05, N=5, 6) during minutes 10-30 of reperfusion, which was attenuated by HC (F/F 0 =1.4±0.08, P<0.05, N=4). OEX-GCaMP hearts were more likely to demonstrate arrhythmic Ca 2+ signals during minutes 15-30 of reperfusion than CTL-GCaMP (27/487 vs 0/415 cells, p<0.05 N=5, 6) and this effect in OEX-GCaMP was prevented by HC (0/633 cells, p<0.05, N=4). Conclusions: TRPV4 activation enhances Ca 2+ transients and promotes arrhythmic Ca 2+ signals, and may provide a novel anti-arrhythmic target in I/R.

Long-Term PDE-5A Inhibition Improves Myofilament Function in Left and Right Ventricular Cardiomyocytes through Partially Different Mechanisms in Diabetic Rat Hearts.

Heart failure with preserved ejection fraction (HFpEF) and right ventricular (RV) dysfunction are frequent complications of diabetic cardiomyopathy. Here we aimed to characterize RV and left ventricular (LV) remodeling and its prevention by vardenafil (a long-acting phosphodiesterase-5A (PDE-5A) inhibitor) administration in a diabetic HFpEF model. Zucker Diabetic Fatty (ZDF) and control, ZDF Lean (Lean) male rats received 10 mg/kg vardenafil (ZDF + Vard; Lean + Vard) per os, on a daily basis for a period of 25 weeks. In vitro force measurements, biochemical and histochemical assays were employed to assess cardiomyocyte function and signaling. Vardenafil treatment increased cyclic guanosine monophosphate (cGMP) levels and decreased 3-nitrotyrosine (3-NT) levels in the left and right ventricles of ZDF animals, but not in Lean animals. Cardiomyocyte passive tension (Fpassive) was higher in LV and RV cardiomyocytes of ZDF rats than in those receiving preventive vardenafil treatment. Levels of overall titin phosphorylation did not differ in the four experimental groups. Maximal Ca2+-activated force (Fmax) of LV and RV cardiomyocytes were preserved in ZDF animals. Ca2+-sensitivity of isometric force production (pCa50) was significantly higher in LV (but not in RV) cardiomyocytes of ZDF rats than in their counterparts in the Lean or Lean + Vard groups. In accordance, the phosphorylation levels of cardiac troponin I (cTnI) and myosin binding protein-C (cMyBP-C) were lower in LV (but not in RV) cardiomyocytes of ZDF animals than in their counterparts of the Lean or Lean + Vard groups. Vardenafil treatment normalized pCa50 values in LV cardiomyocytes, and it decreased pCa50 below control levels in RV cardiomyocytes in the ZDF + Vard group. Our data illustrate partially overlapping myofilament protein alterations for LV and RV cardiomyocytes in diabetic rat hearts upon long-term PDE-5A inhibition. While uniform patterns in cGMP, 3-NT and Fpassive levels predict identical effects of vardenafil therapy for the diastolic function in both ventricles, the uneven cTnI, cMyBP-C phosphorylation levels and pCa50 values implicate different responses for the systolic function.

Glucagon-like peptide-1 receptor agonist treatment of high carbohydrate intake-induced metabolic syndrome provides pleiotropic effects on cardiac dysfunction through alleviations in electrical and intracellular Ca2+ abnormalities and mitochondrial dysfunction.

The pleiotropic effects of glucagon-like peptide-1 receptor (GLP-1R) agonists on the heart have been recognised in obese or diabetic patients. However, little is known regarding the molecular mechanisms of these agonists in cardioprotective actions under metabolic disturbances. We evaluated the effects of GLP-1R agonist liraglutide treatment on left ventricular cardiomyocytes from high-carbohydrate induced metabolic syndrome rats (MetS rats), characterised with insulin resistance and cardiac dysfunction with a long-QT. Liraglutide (0.3mg/kg for 4weeks) treatment of MetS rats significantly reversed long-QT, through a shortening the prolonged action potential duration and recovering inhibited K+ -currents. We also determined a significant recovery in the leaky sarcoplasmic reticulum (SR) and high cytosolic Ca2+ -level, which are confirmed with a full recovery in activated Na+ /Ca2+ -exchanger currents (INCX ). Moreover, the liraglutide treatment significantly reversed the depolarised mitochondrial membrane potential (MMP), increased production of oxidant markers, and cellular acidification together with the depressed ATP production. Our light microscopy analysis of isolated cardiomyocytes showed marked recoveries in the liraglutide-treated MetS group such as marked reverses in highly dilated T-tubules and SR-mitochondria junctions. Moreover, we determined a significant increase in depressed GLUT4 protein level in liraglutide-treated MetS group, possibly associated with recovery in casein kinase 2α. Overall, the study demonstrated a molecular mechanism of liraglutide-induced cardioprotection in MetS rats, at most, via its pleiotropic effects, such as alleviation in the electrical abnormalities, Ca2+ -homeostasis, and mitochondrial dysfunction in ventricular cardiomyocytes.