Atrial fibrillation-associated electrical remodelling in human induced pluripotent stem cell-derived atrial cardiomyocytes: a novel pathway for antiarrhythmic therapy development

AIMS

Atrial fibrillation (AF) is associated with tachycardia-induced cellular electrophysiology alterations which promote AF chronification and treatment resistance. Development of novel antiarrhythmic therapies is hampered by the absence of scalable experimental human models that reflect AF-associated electrical remodelling. Therefore, we aimed to assess if AF-associated remodelling of cellular electrophysiology can be simulated in human atrial-like cardiomyocytes derived from induced pluripotent stem cells in the presence of retinoic acid (iPSC-aCM), and atrial-engineered human myocardium (aEHM) under short term (24 h) and chronic (7 days) tachypacing (TP).

METHODS AND RESULTS

First, 24-h electrical pacing at 3 Hz was used to investigate whether AF-associated remodelling in iPSC-aCM and aEHM would ensue. Compared to controls (24 h, 1 Hz pacing) TP-stimulated iPSC-aCM presented classical hallmarks of AF-associated remodelling: (i) decreased L-type Ca2+ current (ICa,L) and (ii) impaired activation of acetylcholine-activated inward-rectifier K+ current (IK,ACh). This resulted in action potential shortening and an absent response to the M-receptor agonist carbachol in both iPSC-aCM and aEHM subjected to TP. Accordingly, mRNA expression of the channel-subunit Kir3.4 was reduced. Selective IK,ACh blockade with tertiapin reduced basal inward-rectifier K+ current only in iPSC-aCM subjected to TP, thereby unmasking an agonist-independent constitutively active IK,ACh. To allow for long-term TP, we developed iPSC-aCM and aEHM expressing the light-gated ion-channel f-Chrimson. The same hallmarks of AF-associated remodelling were observed after optical-TP. In addition, continuous TP (7 days) led to (i) increased amplitude of inward-rectifier K+ current (IK1), (ii) hyperpolarization of the resting membrane potential, (iii) increased action potential-amplitude and upstroke velocity as well as (iv) reversibly impaired contractile function in aEHM.

CONCLUSIONS

Classical hallmarks of AF-associated remodelling were mimicked through TP of iPSC-aCM and aEHM. The use of the ultrafast f-Chrimson depolarizing ion channel allowed us to model the time-dependence of AF-associated remodelling in vitro for the first time. The observation of electrical remodelling with associated reversible contractile dysfunction offers a novel platform for human-centric discovery of antiarrhythmic therapies.

Therapeutic efficacy of AAV-mediated restoration of PKP2 in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy is a severe cardiac disorder characterized by lethal arrhythmias and sudden cardiac death, with currently no effective treatment. Plakophilin 2 (PKP2) is the most frequently affected gene. Here we show that adeno-associated virus (AAV)-mediated delivery of PKP2 in PKP2c.2013delC/WT induced pluripotent stem cell-derived cardiomyocytes restored not only cardiac PKP2 levels but also the levels of other junctional proteins, found to be decreased in response to the mutation. PKP2 restoration improved sodium conduction, indicating rescue of the arrhythmic substrate in PKP2 mutant induced pluripotent stem cell-derived cardiomyocytes. Additionally, it enhanced contractile function and normalized contraction kinetics in PKP2 mutant engineered human myocardium. Recovery of desmosomal integrity and cardiac function was corroborated in vivo, by treating heterozygous Pkp2c.1755delA knock-in mice. Long-term treatment with AAV9-PKP2 prevented cardiac dysfunction in 12-month-old Pkp2c.1755delA/WT mice, without affecting wild-type mice. These findings encourage clinical exploration of PKP2 gene therapy for patients with PKP2 haploinsufficiency.

m6A modification regulates early human cardiomyocyte lineage specification

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 813716.

Background

RNA modifications affect gene expression through the regulation of RNA metabolism. N6-methyladenosine (m6A) is the most abundant post-transcriptional modification that occurs in RNAs. Its dynamic expression is regulated by the "writer complex" (methyltransferases) and "erasers" (demethylases) and affects numerous biological functions, including mammalian embryonic stem cell (ESC) fate specification. However, the role of m6A in human cardiomyocyte (CM) lineage specification remains unclear.

Purpose

In this study, we aim to investigate the impact of m6A modification on early human cardiomyocyte differentiation, following the dynamic expression of m6A modification of human induced pluripotent stem cells (hiPSC) into cardiomyocytes (hiPSC-CMs).

Methods

hiPSCs were differentiated into hiPSC-CMs by mesodermal induction, followed by inhibition of WNT-signaling.  We collected hiPSC derivates at different stages of the differentiation protocol: hiPSCs, hiPSC-derived cardiac mesoderm cells, hiPSC-derived cardiomyocyte progenitors (hiPSC-CPCs), and mature hiPSC-CMs. Protein levels of m6A key regulators were analyzed. To systematically profile the expression of m6A modification, we subjected hiPSC derivates to m6A immunoprecipitation combined with deep sequencing (MeRIP-seq) and RNA-seq. Enrichment analyses of gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway analyses were conducted to elucidate the biological significance of differentially expressed and methylated genes.

Results

m6A distribution analysis on hiPSC derivates revealed a conserved pattern on a transcriptome-wide level: methylation sites are mainly located nearby the stop coding regions. However, we observed upregulated levels of writer proteins during the transition of hiPSC-derived cardiac mesoderm cells into hiPSC-CPCs. Interestingly, the dynamic changes in writer protein levels toward hiPSC-CPC transition were accompanied by a higher number of significantly upregulated and hyper-methylated mRNA transcripts. GO and KEGG analyses indicated hyper-methylated upregulated transcripts are enriched in muscle cell differentiation, cardiac physiology and calcium and MAPK signaling pathways regulating heart contraction.

Conclusion

For the first time, our study provides evidence that m6A modification is a mediator of early human cardiomyocyte differentiation. The role of specific writer regulators and individual m6A transcripts will be further investigated.

Truncated titin proteins and titin haploinsufficiency are targets for functional recovery in human cardiomyopathy due to TTN mutations

[Figure: see text].

Cutting-free application of CRISPR-mediated endogenous gene activation in human induced pluripotent stem cell derived cardiomyocytes and engineered human myocardium

Background

Imbalanced transcriptional networks characterize cardiomyocyte stress and result in cardiac remodelling. We hypothesize that re-establishing homeostatic gene networks in cardiomyocytes will prevent further tissue damage. To tackle this challenge, we applied CRISPR-based endogenous gene activation (CRISPRa) in vivo and in vitro.

Methods

We employ precision transcriptome editing tools based on CRISPR/Cas9 with enzymatically inactive Cas9 (dCas9) fused to transcriptional activators (VPR) to induce target gene expression by directing dCas9VPR to promoter regions by guide RNAs (gRNA).

Results

Homozygous CRISPRa hiPSC cell lines were generated by targeted integration of a CAG promoter driven dCas9VPR-T2A-tdTomato expression cassette into the AAVS1 locus by CRISPR/Cas9 editing and homology directed repair. Expression of dCas9VPR was evaluated by immunoblotting and co-expressed reporter fluorescence in spontaneously beating hiPSC-CM. We previously identified a crosstalk between WNT signalling and Krueppel-like factor 15 (KLF15) necessary for controlling cardiac homeostasis. We designed and tested 8 non-overlapping gRNAs in the –400 bp region upstream of the KLF15 transcriptional start site (TSS) and tested individual gRNA effectiveness for gene activation in HEK293T cells. Five gRNAs were identified inducing KLF15 transcript levels between 2- and 5-fold compared to non-targeted (NT) gRNA transfected cells (n=3 experiments). The single most effective gRNA was transduced by lentiviral particles into CRISPRa hiPSC-CM increasing KLF15 transcript levels to 1.5-fold compared to NT-gRNA control. Synergistic effects of 3 instead of single gRNA increased KLF15 transcript levels by 3-fold compared to controls (n≥3 experiments). We hypothesized that dCas9VPR expression could be harnessed as an additional option for gene dose titration and we generated hiPSC lines with enhanced dCas9VPR expression (v2.0). We observed up to 5-fold KLF15 gene activation when triple gRNA and v2.0 were combined (n≥4 experiments). Engineered human myocardium (EHM) was generated consisting of CRISPRa cardiomyocytes, fibroblasts and collagen and we observed similar contractility in 4-week cultured EHM suggesting innocuous dCas9VPR and gRNA expression. CRISPRa component expression was maintained over the entire culture period as evaluated by dCas9VPR immunoblotting and KLF15 transcriptional activation (1.4 fold, v1.0 CRISPRa hiPSC-CM, n≥8 tissues) indicating sustained gene activition.

Conclusions

Targeted gene activation with CRISPR/Cas9 is a precise and effective tool for transcriptional activation in hiPSC-CM. We observed titratability of gene activation by 1.) dCas9VPR expression levels and 2.) single versus multiple gRNA use. We furthermore elucidated general rules for effective gRNA targeting within the 5' TSS of genes of interest which confirmed a dependency of baseline gene activity as a limiting factor for endogenous gene activation.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): German Research Foundation (DFG) - Collaborative Research Center 1002German Center for Cardiovascular Research (DZHK)

Abstract MP217: Nuclease-deficient Cas9 Transcription Factors Restore Krueppel-like Factor 15 Expression In Stressed Mouse And Human Cardiomyocytes

Transcriptional changes in cardiomyocytes drive heart failure progression, however, precise control over endogenous gene expression remains challenging. The expression of Krueppel-like factor 15 ( KLF15 ), an evolutionary conserved nuclear and cardiomyocyte specific inhibitor of WNT/CTNNB1 signalling in the heart, is lost upon cardiac remodelling, and accompanied by aberrantly active WNT/CTNNB1 resulting in heart failure progression. We investigated KLF15 expression dynamics employing CRISPR/Cas9-based tools in mouse cardiomyocytes in vivo and in human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) under the hypothesis that re-establishment of KLF15 levels in myocardial stress conditions prevents heart failure progression. Using a mouse model expressing enzymatically inactive Cas9 (dCas9) fused to transcriptional activators (VPR) under Myh6 -promoter control, we activated Klf15 in a murine pressure overload model by transverse aortic constriction. Delivery of Klf15 gRNAs targeted to the Klf15 promoter region via AAV9 induced Klf15 expression sufficiently to re-normalize Klf15 expression to transcript levels comparable to sham surgery hearts. This was accompanied by reduced decrease of fractional shortening as well as reduced cardiomyocyte hypertrophy in stressed Klf15 re-activated hearts compared to non-trageted (NT) gRNA hearts (n=3-8 per group, echo data from 4 and 8 weeks post-surgery). We achieved titratable KLF15 activation in dCas9VPR transgenic hiPSC-CM by selection of single and multiple gRNAs (n=3-4 replicates) and used these cells to generate human engineered myocardium by combining hiPSC-CM and fibroblasts which we subjected to isometric contractions in order to induce mechanical stress, which resulted in KLF15 expressional decrease in line with our in vivo data. This transcriptional loss was rescued in CRISPR/dCas9VPR hiPSC-CM targeted to the KLF15 locus compared to controls (n=6-9/2/4 tissues per group/casting sessions/differentiations). Additionally, TGFB1 induced cardiomyocyte stress resulted in decreased KLF15 expression levels in 2D hiPSC-CM cultures which were rescued by dCas9VPR- KLF15 targeting (n=3 experiments). In conclusion, we report controllable gene activity by CRISPR/dCas9VPR to restore the loss of KLF15 in stressed mouse and human cardiomyocytes. We furthermore evaluate the potential to gain full control over gene dose titratability with these models to validate and define novel therapeutic targets for the prevention of heart failure progression.

Inhibition of aquaporin-1 prevents myocardial remodeling by blocking the transmembrane transport of hydrogen peroxide

Pathological remodeling of the myocardium has long been known to involve oxidant signaling, but strategies using systemic antioxidants have generally failed to prevent it. We sought to identify key regulators of oxidant-mediated cardiac hypertrophy amenable to targeted pharmacological therapy. Specific isoforms of the aquaporin water channels have been implicated in oxidant sensing, but their role in heart muscle is unknown. RNA sequencing from human cardiac myocytes revealed that the archetypal AQP1 is a major isoform. AQP1 expression correlates with the severity of hypertrophic remodeling in patients with aortic stenosis. The AQP1 channel was detected at the plasma membrane of human and mouse cardiac myocytes from hypertrophic hearts, where it colocalized with NADPH oxidase-2 and caveolin-3. We show that hydrogen peroxide (H₂O₂), produced extracellularly, is necessary for the hypertrophic response of isolated cardiac myocytes and that AQP1 facilitates the transmembrane transport of H₂O₂ through its water pore, resulting in activation of oxidant-sensitive kinases in cardiac myocytes. Structural analysis of the amino acid residues lining the water pore of AQP1 supports its permeation by H₂O₂ Deletion of Aqp1 or selective blockade of the AQP1 intrasubunit pore inhibited H₂O₂ transport in mouse and human cells and rescued the myocyte hypertrophy in human induced pluripotent stem cell-derived engineered heart muscle. Treatment of mice with a clinically approved AQP1 inhibitor, Bacopaside, attenuated cardiac hypertrophy. We conclude that cardiac hypertrophy is mediated by the transmembrane transport of H₂O₂ by the water channel AQP1 and that inhibitors of AQP1 represent new possibilities for treating hypertrophic cardiomyopathies.

Treatments targeting inotropy

Acute heart failure (HF) and in particular, cardiogenic shock are associated with high morbidity and mortality. A therapeutic dilemma is that the use of positive inotropic agents, such as catecholamines or phosphodiesterase-inhibitors, is associated with increased mortality. Newer drugs, such as levosimendan or omecamtiv mecarbil, target sarcomeres to improve systolic function putatively without elevating intracellular Ca2+. Although meta-analyses of smaller trials suggested that levosimendan is associated with a better outcome than dobutamine, larger comparative trials failed to confirm this observation. For omecamtiv mecarbil, Phase II clinical trials suggest a favourable haemodynamic profile in patients with acute and chronic HF, and a Phase III morbidity/mortality trial in patients with chronic HF has recently begun. Here, we review the pathophysiological basis of systolic dysfunction in patients with HF and the mechanisms through which different inotropic agents improve cardiac function. Since adenosine triphosphate and reactive oxygen species production in mitochondria are intimately linked to the processes of excitation-contraction coupling, we also discuss the impact of inotropic agents on mitochondrial bioenergetics and redox regulation. Therefore, this position paper should help identify novel targets for treatments that could not only safely improve systolic and diastolic function acutely, but potentially also myocardial structure and function over a longer-term.

Abstract 372: CRISPR-based Gene Activation for Transcriptional Reprograming of Mammalian Cardiomyocytes

Cardiac remodelling is accompanied by silencing of genes necessary for cardiac homeostasis including Krüppel-like factor 15 ( Klf15 ), an anti-hypertrophic factor. Due to a lack of tools for gene activation, we aim to adapt a CRISPR-based approach for gene expression control for mammalian cardiomyocytes. We generated a transgenic mouse model (TG) expressing an enzymatically inactive, gRNA-programmable Cas9 protein under Myh6 promoter control fused to transcriptional activators (VPR) to drive gene expression. As a proof of concept, we systemically injected mice at postnatal day P4 with AAV9 carrying a triple gRNA expression cassette containing gRNAs targeted to the Klf15 promoter region. Significant Klf15 activation was observed at P10 up to 10-fold compared to controls in three independent TG lines. This Klf15 activation was sufficient to drive target gene expression of Aldh2 and Adhfe1 (controls = WT + saline, WT + AAV9, TG + saline, n ≥ 3 per group, ANOVA and Bonferroni correction). Importantly, at the neonatal stage, the Klf15 promoter is characterized by low H3K27ac presence indicating that endogenous gene activation can enhance transcription from an epigenetically inactive locus. We demonstrated that CRISPR/Cas9 mediated loss of KLF15 expression results in impaired contractile function in a 3D model of engineering heart muscle, similar to the Klf15 -/- mouse heart phenotype. Encouraged by the evolutionary conserved KLF15 -mediated mechanisms, we aimed to generate a human induced pluripotent stem cells, which constitutively express dCas9VPR (hIPSC-dCas9VPR) for gene activation in hIPSC-derived cardiomyocytes to complement the in vivo findings upon disease. We targeted the transgene integration to the AAVS1 site by CRISPR gene editing, confirmed correct integration by PCR followed by sequencing and hIPSC-dCas9VPR were successfully differentiated into beating cardiomyocytes. To this end, we have tested gRNAs targeted to the KLF15 promoter region in HEK293 cells and observed significant KLF15 activation of up to 5.5-fold (n = 3, ANOVA and Bonferroni correction). In summary, we report transgenic mammalian tools for endogenous gene (re-) activation purposes to identify potential therapeutic targets for preventing heart failure progression.

Engineered heart tissue models from hiPSC-derived cardiomyocytes and cardiac ECM for disease modeling and drug testing applications

Cardiac tissue engineering provides unique opportunities for cardiovascular disease modeling, drug testing, and regenerative medicine applications. To recapitulate human heart tissue, we combined human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with a chitosan-enhanced extracellular-matrix (ECM) hydrogel, derived from decellularized pig hearts. Ultrastructural characterization of the ECM-derived engineered heart tissues (ECM-EHTs) revealed an anisotropic muscle structure, with embedded cardiomyocytes showing more mature properties than 2D-cultured hiPSC-CMs. Force measurements confirmed typical force-length relationships, sensitivity to extracellular calcium, and adequate ionotropic responses to contractility modulators. By combining genetically-encoded calcium and voltage indicators with laser-confocal microscopy and optical mapping, the electrophysiological and calcium-handling properties of the ECM-EHTs could be studied at the cellular and tissue resolutions. This allowed to detect drug-induced changes in contraction rate (isoproterenol, carbamylcholine), optical signal morphology (E-4031, ATX2, isoproterenol, ouabin and quinidine), cellular arrhythmogenicity (E-4031 and ouabin) and alterations in tissue conduction properties (lidocaine, carbenoxolone and quinidine). Similar assays in ECM-EHTs derived from patient-specific hiPSC-CMs recapitulated the abnormal phenotype of the long QT syndrome and catecholaminergic polymorphic ventricular tachycardia. Finally, programmed electrical stimulation and drug-induced pro-arrhythmia led to the development of reentrant arrhythmias in the ECM-EHTs. In conclusion, a novel ECM-EHT model was established, which can be subjected to high-resolution long-term serial functional phenotyping, with important implications for cardiac disease modeling, drug testing and precision medicine. STATEMENT OF SIGNIFICANCE: One of the main objectives of cardiac tissue engineering is to create an in-vitro muscle tissue surrogate of human heart tissue. To this end, we combined a chitosan-enforced cardiac-specific ECM hydrogel derived from decellularized pig hearts with human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from healthy-controls and patients with inherited cardiac disorders. We then utilized genetically-encoded calcium and voltage fluorescent indicators coupled with unique optical imaging techniques and force-measurements to study the functional properties of the generated engineered heart tissues (EHTs). These studies demonstrate the unique potential of the new model for physiological and pathophysiological studies (assessing contractility, conduction and reentrant arrhythmias), novel disease modeling strategies ("disease-in-a-dish" approach) for studying inherited arrhythmogenic disorders, and for drug testing applications (safety pharmacology).

Mechanistic role of the CREB-regulated transcription coactivator 1 in cardiac hypertrophy

The sympathetic nervous system is the main stimulator of cardiac function. While acute activation of the β-adrenoceptors exerts positive inotropic and lusitropic effects by increasing cAMP and Ca²⁺, chronically enhanced sympathetic tone with changed β-adrenergic signaling leads to alterations of gene expression and remodeling. The CREB-regulated transcription coactivator 1 (CRTC1) is activated by cAMP and Ca²⁺. In the present study, the regulation of CRTC1 in cardiomyocytes and its effect on cardiac function and growth was investigated. In cardiomyocytes, isoprenaline induced dephosphorylation, and thus activation of CRTC1, which was prevented by propranolol. Crtc1-deficient mice exhibited left ventricular dysfunction, hypertrophy and enlarged cardiomyocytes. However, isoprenaline-induced contractility of isolated trabeculae or phosphorylation of cardiac troponin I, cardiac myosin-binding protein C, phospholamban, and ryanodine receptor were not altered, suggesting that cardiac dysfunction was due to the global lack of Crtc1. The mRNA and protein levels of the Gα_q GTPase activating protein regulator of G-protein signaling 2 (RGS2) were lower in hearts of Crtc1-deficient mice. Chromatin immunoprecipitation and reporter gene assays showed stimulation of the Rgs2 promoter by CRTC1. In Crtc1-deficient cardiomyocytes, phosphorylation of the Gα_q-downstream kinase ERK was enhanced. CRTC1 content was higher in cardiac tissue from patients with aortic stenosis or hypertrophic cardiomyopathy and from two murine models mimicking these diseases. These data suggest that increased CRTC1 in maladaptive hypertrophy presents a compensatory mechanism to delay disease progression in part by enhancing Rgs2 gene transcription. Furthermore, the present study demonstrates an important role of CRTC1 in the regulation of cardiac function and growth.

Passive Stretch Induces Structural and Functional Maturation of Engineered Heart Muscle as Predicted by Computational Modeling

The ability to differentiate human pluripotent stem cells (hPSCs) into cardiomyocytes (CMs) makes them an attractive source for repairing injured myocardium, disease modeling, and drug testing. Although current differentiation protocols yield hPSC-CMs to >90% efficiency, hPSC-CMs exhibit immature characteristics. With the goal of overcoming this limitation, we tested the effects of varying passive stretch on engineered heart muscle (EHM) structural and functional maturation, guided by computational modeling. Human embryonic stem cells (hESCs, H7 line) or human induced pluripotent stem cells (IMR-90 line) were differentiated to hPSC-derived cardiomyocytes (hPSC-CMs) in vitro using a small molecule based protocol. hPSC-CMs were characterized by troponin⁺ flow cytometry as well as electrophysiological measurements. Afterwards, 1.2 × 10⁶ hPSC-CMs were mixed with 0.4 × 10⁶ human fibroblasts (IMR-90 line) (3:1 ratio) and type-I collagen. The blend was cast into custom-made 12-mm long polydimethylsiloxane reservoirs to vary nominal passive stretch of EHMs to 5, 7, or 9 mm. EHM characteristics were monitored for up to 50 days, with EHMs having a passive stretch of 7 mm giving the most consistent formation. Based on our initial macroscopic observations of EHM formation, we created a computational model that predicts the stress distribution throughout EHMs, which is a function of cellular composition, cellular ratio, and geometry. Based on this predictive modeling, we show cell alignment by immunohistochemistry and coordinated calcium waves by calcium imaging. Furthermore, coordinated calcium waves and mechanical contractions were apparent throughout entire EHMs. The stiffness and active forces of hPSC-derived EHMs are comparable with rat neonatal cardiomyocyte-derived EHMs. Three-dimensional EHMs display increased expression of mature cardiomyocyte genes including sarcomeric protein troponin-T, calcium and potassium ion channels, β-adrenergic receptors, and t-tubule protein caveolin-3. Passive stretch affects the structural and functional maturation of EHMs. Based on our predictive computational modeling, we show how to optimize cell alignment and calcium dynamics within EHMs. These findings provide a basis for the rational design of EHMs, which enables future scale-up productions for clinical use in cardiovascular tissue engineering. Stem Cells 2018;36:265-277.

Cardiac optogenetics: using light to monitor cardiac physiology

Our current understanding of cardiac excitation and its coupling to contraction is largely based on ex vivo studies utilising fluorescent organic dyes to assess cardiac action potentials and signal transduction. Recent advances in optogenetic sensors open exciting new possibilities for cardiac research and allow us to answer research questions that cannot be addressed using the classic organic dyes. Especially thrilling is the possibility to use optogenetic sensors to record parameters of cardiac excitation and contraction in vivo. In addition, optogenetics provide a high spatial resolution, as sensors can be coupled to motifs and targeted to specific cell types and subcellular domains of the heart. In this review, we will give a comprehensive overview of relevant optogenetic sensors, how they can be utilised in cardiac research and how they have been applied in cardiac research up to now.

Magnetic Resonance Imaging of Cardiac Strain Pattern Following Transplantation of Human Tissue Engineered Heart Muscles

BACKGROUND

The use of tissue engineering approaches in combination with exogenously produced cardiomyocytes offers the potential to restore contractile function after myocardial injury. However, current techniques assessing changes in global cardiac performance after such treatments are plagued by relatively low detection ability. Since the treatment is locally performed, this detection could be improved by myocardial strain imaging that measures regional contractility.

METHODS AND RESULTS

Tissue engineered heart muscles (EHMs) were generated by casting human embryonic stem cell-derived cardiomyocytes with collagen in preformed molds. EHMs were transplanted (n=12) to cover infarct and border zones of recipient rat hearts 1 month after ischemia reperfusion injury. A control group (n=10) received only sham placement of sutures without EHMs. To assess the efficacy of EHMs, magnetic resonance imaging and ultrasound-based strain imaging were performed before and 4 weeks after transplantation. In addition to strain imaging, global cardiac performance was estimated from cardiac magnetic resonance imaging. Although no significant differences were found for global changes in left ventricular ejection fraction (control -9.6±1.3% versus EHM -6.2±1.9%; P=0.17), regional myocardial strain from tagged magnetic resonance imaging was able to detect preserved systolic function in EHM-treated animals compared with control (control 4.4±1.0% versus EHM 1.0±0.6%; P=0.04). However, ultrasound-based strain failed to detect any significant change (control 2.1±3.0% versus EHM 6.3±2.9%; P=0.46).

CONCLUSIONS

This study highlights the feasibility of using cardiac strain from tagged magnetic resonance imaging to assess functional changes in rat models following localized regenerative therapies, which may not be detected by conventional measures of global systolic performance.

Abstract 90: Modulation of Cardiac Fibroblast to Myofibroblast Transition by Rho-associated Kinases ROCK1 and ROCK2

Background: Rho-associated kinases ROCK1 and ROCK2 play a critical role in the pathogenesis of myocardial fibrosis; however, their specific function in cardiac fibroblasts (CF) remains unclear. Remodelling processes in diseased hearts propels the transition of CF to a myofibroblast phenotype exemplified by increased proliferation, migration and synthesis of extracellular matrix (ECM) proteins. Therefore, we sought to investigate whether ROCK1/2 have an impact on CF characteristics in isolated cells and engineered cardiac tissue.

Methods: Neonatal wild type (WT) rat CF and cardiomyocytes (CM) were isolated and lentivirally transduced/transfected resulting in downregulation of ROCK1 and ROCK2 by 75%. In addition, WT CF were treated with 10 μM Fasudil or 3 μM H1152P for non-specific ROCK1/2 inhibition. CF gene and protein expression, morphology, proliferation, and migration were assessed. Subsequently, CF and CM were mixed within a hydrogel to engineer heart constructs with assessments of contractile function/rigidity performed by isometric force and rheological measurements, respectively.

Results: Knockdown of ROCK1 and ROCK2 and inhibition of ROCK1/2 activity altered CF morphology, disrupted cytoskeletal structures, and increased adhesion velocity. Moreover, decreased migration velocity and distance was detected, and the double knockdown and inhibition of ROCK1/2 attenuated proliferation of CF. In contraction measurements, engineered heart muscle (EHM) treated with ROCK inhibitors developed a significantly higher force of contraction per cross sectional area than control EHM. Destructive tensile strength measurement of engineered connective tissue (ECT) treated with ROCK inhibitors showed that rigidity was significantly reduced compared to control, suggesting that ROCK1/2 influence the regulation and turnover of the ECM and thus viscoelastic properties of engineered tissues. Indeed, RNA sequencing of ROCK inhibitor treated ECT showed that ROCK1/2 are involved in the regulation of ECM proteins, such as collagens, biglycan, decorin, elastin and its degrading enzyme MMP12.

Conclusion: This study demonstrates that ROCK signalling controls myofibroblast characteristics of CF via remodelling of the cytoskeleton and the ECM.

Evolutionarily conserved intercalated disc protein Tmem65 regulates cardiac conduction and connexin 43 function

Membrane proteins are crucial to heart function and development. Here we combine cationic silica-bead coating with shotgun proteomics to enrich for and identify plasma membrane-associated proteins from primary mouse neonatal and human fetal ventricular cardiomyocytes. We identify Tmem65 as a cardiac-enriched, intercalated disc protein that increases during development in both mouse and human hearts. Functional analysis of Tmem65 both in vitro using lentiviral shRNA-mediated knockdown in mouse cardiomyocytes and in vivo using morpholino-based knockdown in zebrafish show marked alterations in gap junction function and cardiac morphology. Molecular analyses suggest that Tmem65 interaction with connexin 43 (Cx43) is required for correct localization of Cx43 to the intercalated disc, since Tmem65 deletion results in marked internalization of Cx43, a shorter half-life through increased degradation, and loss of Cx43 function. Our data demonstrate that the membrane protein Tmem65 is an intercalated disc protein that interacts with and functionally regulates ventricular Cx43.

Human Engineered Heart Muscles Engraft and Survive Long Term in a Rodent Myocardial Infarction Model

RATIONALE

Tissue engineering approaches may improve survival and functional benefits from human embryonic stem cell-derived cardiomyocyte transplantation, thereby potentially preventing dilative remodeling and progression to heart failure.

OBJECTIVE

Assessment of transport stability, long-term survival, structural organization, functional benefits, and teratoma risk of engineered heart muscle (EHM) in a chronic myocardial infarction model.

METHODS AND RESULTS

We constructed EHMs from human embryonic stem cell-derived cardiomyocytes and released them for transatlantic shipping following predefined quality control criteria. Two days of shipment did not lead to adverse effects on cell viability or contractile performance of EHMs (n=3, P=0.83, P=0.87). One month after ischemia/reperfusion injury, EHMs were implanted onto immunocompromised rat hearts to simulate chronic ischemia. Bioluminescence imaging showed stable engraftment with no significant cell loss between week 2 and 12 (n=6, P=0.67), preserving ≤25% of the transplanted cells. Despite high engraftment rates and attenuated disease progression (change in ejection fraction for EHMs, -6.7±1.4% versus control, -10.9±1.5%; n>12; P=0.05), we observed no difference between EHMs containing viable and nonviable human cardiomyocytes in this chronic xenotransplantation model (n>12; P=0.41). Grafted cardiomyocytes showed enhanced sarcomere alignment and increased connexin 43 expression at 220 days after transplantation. No teratomas or tumors were found in any of the animals (n=14) used for long-term monitoring.

CONCLUSIONS

EHM transplantation led to high engraftment rates, long-term survival, and progressive maturation of human cardiomyocytes. However, cell engraftment was not correlated with functional improvements in this chronic myocardial infarction model. Most importantly, the safety of this approach was demonstrated by the lack of tumor or teratoma formation.

Lights on for HIF-1α: genetically enhanced mouse cardiomyocytes for heart tissue imaging

BACKGROUND/AIMS

The hypoxia inducible factor-1 (HIF-1) is a suitable marker for tissue oxygenation. We intended to develop cardiomyocytes (CMs) expressing the oxygen-dependent degradation domain of HIF-1α fused to the firefly luciferase (ODD-Luc) followed by proof-of-concept for its applicability in the assessment of heart muscle oxygenation.

METHODS AND RESULTS

We first generated embryonic stem cell (ESC) lines (ODD-Luc ESCs) from a Tg ROSA26 ODD-Luc/+ mouse. Subsequent CMs selection was facilitated by stable integration of an antibiotic resistance expressed under the control of the αMHC promoter. ODD-Luc ESCs showed a strong Luc-signal within 1 h of hypoxia (1% oxygen), which coincided with endogenous HIF-1α. Engineered heart muscle (EHM) constructed with ODD-Luc CMs confirmed the utility of the model to sense hypoxia, and monitor reoxygenation also in a multicellular heart muscle model. Pharmacologically induced inotropy/chronotropy under isoprenaline resulted in enhanced Luc-signal suggesting enhanced oxygen consumption, leading to notable myocardial hypoxia.

CONCLUSIONS

ODD-Luc-CMs can be used to monitor dynamic changes of cardiomyocyte oxygenation in living heart muscle samples. We provide proof-of-concept for pharmacologically induced myocardial interventions and envision applications of the developed model in drug screens and fundamental studies of ischemia/reperfusion injury.

Abstract 142: Defining the Non-Myocyte Compartment is Key for Enhanced Maturation of Human Engineered Heart Muscle

Background: Tissue engineering of heart muscle from human pluripotent stem cells holds great potential for in vitro studies, disease modeling, and cardiac replacement therapy. A number of variables may however affect maturation and function of human cardiomyocytes (CM) in tissue engineered heart muscle (EHM). Here, we hypothesized that defined non-myocyte (NM) populations support structural and functional maturation of EHM.

Methods and Results: To investigate the role of non-myocytes (NM) for heart muscle assembly in vitro we generated EHM from purified CM (93±1.5% actinin+) and a mixture of CM and NM (70/30%). Notably, only the NM-supplemented EHM generated measurable forces (0.8±0.1 mN, n=9) with anisotropically aligned cardiomyocytes. Depending on pluripotent stem cell line and differentiation protocol the NM compartment may vary considerably. To further define the influence of the NM compartment we generated EHM from HES2-derived CM with undefined NM, i.e the NM typically derived during cardiac differentiation, and defined NM (fibroblasts). Defined EHM were more mature with higher forces and lower variability between experimental series (defined: 9.8±0.9 nN/CM, undefined: 4.7±1.4 nN/CM, n=10/9), higher EC50 for calcium, and enhanced inotropic response to isoprenaline despite comparable CM:NM composition of 1:1. Increased actinin protein per CM, a reduction of MLC2V/2A double positive CM, and evidence of CM cycle withdrawal indicated enhanced ventricular maturation in defined EHM. Next, we tested whether defining cell composition and NM in iPS-derived EHM will yield a comparable functional phenotype to HES2-EHM. In agreement with the above data, defined iPS-EHM displayed advanced functional maturation with high specific forces, comparable calcium EC50, and inotropic response to isoprenaline.

Summary and Conclusions: Here we demonstrate that defining the NM compartment is essential for optimized human heart muscle formation and maturation in vitro. Moreover, our data provide (1) evidence for the applicability of EHM in modelling of heart muscle development and (2) a strong rationale for the need to define CM and NM compartments in tissue engineered myocardium to reduce variability in applications such as disease modelling.

Abstract 141: Influence of the Collagen Processing Heat Shock Protein 47 on Cardiomyocyte Homeostasis and Maturation

PURPOSE: Heat shock protein 47 (Hsp47) is a collagen-specific molecular chaperone required for maturation of collagen type 1. Little is known about the role of Hsp47 in the heart. This study aims to investigate whether Hsp47 is important for cardiomyocyte maturation and assembly into functional syncytia.

METHODS: We made use of recently developed 2D and 3D culture platforms to scrutinize the role of Hsp47 for cardiomyocyte maturation and function, i.e., (1) 2D model: culture of embryonic stem cell-derived cardiomyocytes (ESC-CM) on ECMs derived from both mouse embryonic wild type (WT)- and Hsp47 (-/-) fibroblasts and (2) 3D model: construction of engineered heart muscle (EHM) consisting of purified ESC-CMs with WT- or Hsp47 (-/-) fibroblasts. Cultures were investigated using confocal microscopy and isometric force measurements.

RESULTS: ESC-CMs showed mainly immature polygonal shaped morphology and disorganized sarcomeric structures when cultured on ECM secreted by Hsp47 (-/-) fibroblasts; this morphology was in clear contrast to the mostly anisotropic cardiomyocyte appearance in control conditions. EHM with Hsp47 (-/-) fibroblasts did not develop measurable contractile forces. In contrast, WT fibroblast-supplemented EHMs contracted regularly (WT: 0.14±0.1 mN at 2.4 mM Ca2+; n=8/group). Histological analysis of Hsp47 (-/-) EHMs showed mainly rounded and underdeveloped cardiomyocytes comparable to the cardiomyocyte phenotype observed in EHMs containing only ESC-CMs, without fibroblasts.

CONCLUSIONS: Hsp47 influences the deposition of ECM-collagen and affects cardiomyocyte morphology and functionality in our 2D and 3D culture models. Our data collectively suggest that Hsp47 may be an attractive target for the regulation of cardiac tissue homeostasis.

PDE2-mediated cAMP hydrolysis accelerates cardiac fibroblast to myofibroblast conversion and is antagonized by exogenous activation of cGMP signaling pathways

Recent studies suggest that the signal molecules cAMP and cGMP have antifibrotic effects by negatively regulating pathways associated with fibroblast to myofibroblast (MyoCF) conversion. The phosphodiesterase 2 (PDE2) has the unique property to be stimulated by cGMP, which leads to a remarkable increase in cAMP hydrolysis and thus mediates a negative cross-talk between both pathways. PDE2 has been recently investigated in cardiomyocytes; here we specifically addressed its role in fibroblast conversion and cardiac fibrosis. PDE2 is abundantly expressed in both neonatal rat cardiac fibroblasts (CFs) and cardiomyocytes. The overexpression of PDE2 in CFs strongly reduced basal and isoprenaline-induced cAMP synthesis, and this decrease was sufficient to induce MyoCF conversion even in the absence of exogenous profibrotic stimuli. Functional stress-strain experiments with fibroblast-derived engineered connective tissue (ECT) demonstrated higher stiffness in ECTs overexpressing PDE2. In regard to cGMP, neither basal nor atrial natriuretic peptide-induced cGMP levels were affected by PDE2, whereas the response to nitric oxide donor sodium nitroprusside was slightly but significantly reduced. Interestingly, despite persistently depressed cAMP levels, both cGMP-elevating stimuli were able to completely prevent the PDE2-induced MyoCF phenotype, arguing for a double-tracked mechanism. In conclusion, PDE2 accelerates CF to MyoCF conversion, which leads to greater stiffness in ECTs. Atrial natriuretic peptide- and sodium nitroprusside-mediated cGMP synthesis completely reverses PDE2-induced fibroblast conversion. Thus PDE2 may augment cardiac remodeling, but this effect can also be overcome by enhanced cGMP. The redundant role of cAMP and cGMP as antifibrotic meditators may be viewed as a protective mechanism in heart failure.

Impact of AT2 receptor deficiency on postnatal cardiovascular development

BACKGROUND

The angiotensin II receptor subtype 2 (AT2 receptor) is ubiquitously and highly expressed in early postnatal life. However, its role in postnatal cardiac development remained unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Hearts from 1, 7, 14 and 56 days old wild-type (WT) and AT2 receptor-deficient (KO) mice were extracted for histomorphometrical analysis as well as analysis of cardiac signaling and gene expression. Furthermore, heart and body weights of examined animals were recorded and echocardiographic analysis of cardiac function as well as telemetric blood pressure measurements were performed. Moreover, gene expression, sarcomere shortening and calcium transients were examined in ventricular cardiomyocytes isolated from both genotypes. KO mice exhibited an accelerated body weight gain and a reduced heart to body weight ratio as compared to WT mice in the postnatal period. However, in adult KO mice the heart to body weight ratio was significantly increased most likely due to elevated systemic blood pressure. At postnatal day 7 ventricular capillarization index and the density of α-smooth muscle cell actin-positive blood vessels were higher in KO mice as compared to WT mice but normalized during adolescence. Echocardiographic assessment of cardiac systolic function at postnatal day 7 revealed decreased contractility of KO hearts in response to beta-adrenergic stimulation. Moreover, cardiomyocytes from KO mice showed a decreased sarcomere shortening and an increased peak Ca(2+) transient in response to isoprenaline when stimulated concomitantly with angiotensin II.

CONCLUSION

The AT2 receptor affects postnatal cardiac growth possibly via reducing body weight gain and systemic blood pressure. Moreover, it moderately attenuates postnatal vascularization of the heart and modulates the beta adrenergic response of the neonatal heart. These AT2 receptor-mediated effects may be implicated in the physiological maturation process of the heart.

Abstract 237: Antioxidant and Ryanodine Receptor Stabilizing Therapy Prevent Statin-Induced Contractile Dysfunction in a Tissue-Engineered Skeletal Muscle Model

Rationale: Skeletal muscle toxicity of HMG-CoA-reductase inhibitors (statins) ranges from reversible myalgia to irreversible rhabdomyolysis. The underlying molecular mechanisms are not well defined. Tissue engineering models may help to gain insight into these clinically limiting pathologies.

Objective: Here we aimed to model a phenotype of reversible myalgia in vitro to decipher mechanisms that contribute to the early stage of statin toxicity.

Methods and Results: Engineered skeletal muscle (ESM) was generated from rat myoblasts, matrigel, and collagen. Isometrically suspended ESM developed 1.2±0.1 mN force under tetanic field stimulation (80 Hz; 200 mA; n=25). Exposure of ESM to statins for 5 days resulted in a loss of force and increased fatiguability in a concentration dependent manner. Cerivastatin was identified as the most potent statin with respect to muscle toxicity with a TC50 (=50% force reduction) of 0.02 µmol/L (n=25/group). Interestingly, at low cerivastatin concentration (0.01 µmol/L) contractile force of ESM was impaired without obvious signs for structural muscle damage (sarcomeric actin content, CK activity unchanged, n=4). Importantly, ESM dysfunction was fully reversible if challenged with TC50 statin concentrations (n=12-14/group). We reasoned that contractile dysfunction with increased fatiguability resulted from calcium leak via the ryanodine receptor. To test this hypothesis we co-administered S107 and observed a concentration-dependent inhibition of statin-induced force reduction (n=12) and calpain activitiy (a calcium-dependent protease, n=4-6). We further argued that RYR destabilization may have been caused by reactive nitrogen (RNS) and/or reactive oxygen species (ROS). Interestingly, the antioxidant N-acetyl cysteine (1 mmol/L, n=3/group), but not L-NAME (10 mmol/L; NO-synthase inhibition, n=6-12/group) prevented contractile dysfunction.

Conclusion: We utilized a novel tissue engineered skeletal muscle model to decipher mechanisms of statin-induced muscle toxicity and provide evidence for a central role of ryanodine receptor leak, possibly caused by oxidative damage. Our data suggest that antioxidant and RYR-stabilizing approaches may be useful in counteracting statin myopathy.

Abstract 238: Role of Erythropoietin Signaling in the Biology of Mouse Cardiac Progenitor Cells

Erythropoietin (Epo) was shown to improve cardiac function following ischemia reperfusion mainly via neo-angiogenesis and anti-apoptotic mechanisms. We found EpoR expression to be particularly high in adult cardiac progenitor cells (CPCs). Thus, we reasoned that Epo may play a role in the biology of these cells.

We isolated CPCs from adult C57BL/6 hearts by enzymatic digestion and filtration (pore size: 30 µm). By means of immunofluorescence microscopy (IF) and flow cytometry (FC) we analyzed EpoR expression in the CPCs. 24±3% of the investigated cardiac cells were positive for EpoR with 3±2% of these being c-kit+ and 28%±2% Sca-1+. 52% of the EpoR+ cells expressed endothelial cell markers (40±2% CD34+, 9±2% FLK1+). 42±4% expressed myocyte markers (αMHC+, cTNT+). IF revealed a progenitor-like population with immature cell morphology and proliferation potential (ki67+). Cell cycle analysis showed an enrichment of αMHC+ EpoR+ cells in S and G2 phase (49±7%, n=3) as compared to the αMHC- EpoR- population (13±3%, n=3). Moreover, we tested the effect of Epo in the biology of these CPCs in vitro. At d14 we observed a two-fold increase of GATA4+ and cTnT+ cardiac cells in the co-cultures treated with Epo (n=3). CPC cycle arrest abrogated the aforementioned effects, suggesting that Epo influences mainly CPC proliferation. Finally, we tested the potential of Epo to protect against ischemia by inducing the proliferation of these αMHC+ CPCs in vivo in a myocardial infarction (MI) model. 4 weeks post MI, echocardiography did not reveal a significant functional improvement of the Epo receiving mice (2x, 2U/g Epo i.p). Nevertheless, FC analysis of the progenitor pool showed a significant augmentation of αMHC+ and cTnT+ cells (Sham: 19±3% vs Epo 35±3%, n=5; MI: 10.6±2.3%, n=6 vs Epo 20.3±1.9%, n=8). These data suggest an activation of myogenic progenitors by Epo, despite the lack of apparent regeneration under the investigated conditions.

In conclusion, we found that EpoR is expressed in a putative cardiomyogenic progenitor cell pool in the adult heart. Epo drives their proliferation in vitro and in vivo even upon acute cardiac injury. We are currently investigating the long-term consequences of the observed progenitor cell activation in models of chronic ischemic injury.

Abstract 26: Myocardial Phosphodiesterase-2A Is Upregulated in Human and Experimental Heart Failure and Blunts Cardiac β-Adrenergic Inotropic Responsiveness

Augmented cGMP- and diminished cAMP-signaling within cardiomyocytes is characteristic for failing hearts. Cyclic nucleotide phosphodiesterases (PDEs) comprise a family of cyclic-nucleotide hydrolyzing enzymes, controlling cAMP and cGMP levels. Among them the PDE-2A isoform has the unique property to be stimulated by cGMP but primarily hydrolyzing cAMP. This appears to mediate a negative cross-talk between both signaling pathways. However, a potential role for PDE-2A in the failing heart has not been addressed yet. Here we show that PDE-2A protein levels were ∼2-fold higher in failing human hearts as well as in a large animal heart failure model from dog hearts subjected to rapid-pacing (n≥6, p<0.05). Intriguingly, PDE-2A protein levels were normal in hypertrophied hearts from patients with preserved cardiac function who underwent aortic valve replacement. Chronic beta-adrenergic stimulation by catecholamine infusions enhanced cAMP hydrolyzing activity of PDE-2A by four-fold (n≥6, p<0.05) in rat hearts in vivo and in isolated cardiomyocytes (measured by radioimmunoassay and FRET-based sensors, respectively) and correlated with blunted beta-adrenergic responsiveness. Consistent with this observation, overexpressed PDE-2A, which localized to the sarcomeric Z-line, blunted the rise in cAMP by 70% (n≥6, p<0.05) and abolished the positive inotropic effect after acute beta-adrenergic stimulation by 70% (n≥6, p<0.05) in isolated cardiomyocytes. Notably, those cardiomyocytes also showed marked protection from norepinephrine-induced hypertrophic responses, e. g. 40% less increase in cell surface area (n≥10, p<0.05). In summary, PDE-2A is markedly upregulated in human and experimental failing hearts. This may constitute an important defense mechanism during cardiac stress, by antagonizing the cAMP-mediated toxic effects. Thus, activating myocardial PDE-2A may represent a new intracellular anti-adrenergic therapeutic strategy in heart failure.

Comparative study of human-induced pluripotent stem cells derived from bone marrow cells, hair keratinocytes, and skin fibroblasts

AIMS

Induced pluripotent stem cells (iPSCs) provide a unique opportunity for the generation of patient-specific cells for use in disease modelling, drug screening, and regenerative medicine. The aim of this study was to compare human-induced pluripotent stem cells (hiPSCs) derived from different somatic cell sources regarding their generation efficiency and cardiac differentiation potential, and functionalities of cardiomyocytes.

METHODS AND RESULTS

We generated hiPSCs from hair keratinocytes, bone marrow mesenchymal stem cells (MSCs), and skin fibroblasts by using two different virus systems. We show that MSCs and fibroblasts are more easily reprogrammed than keratinocytes. This corresponds to higher methylation levels of minimal promoter regions of the OCT4 and NANOG genes in keratinocytes than in MSCs and fibroblasts. The success rate and reprogramming efficiency was significantly higher by using the STEMCCA system than the OSNL system. All analysed hiPSCs are pluripotent and show phenotypical characteristics similar to human embryonic stem cells. We studied the cardiac differentiation efficiency of generated hiPSC lines (n = 24) and found that MSC-derived hiPSCs exhibited a significantly higher efficiency to spontaneously differentiate into beating cardiomyocytes when compared with keratinocyte-, and fibroblast-derived hiPSCs. There was no significant difference in the functionalities of the cardiomyocytes derived from hiPSCs with different origins, showing the presence of pacemaker-, atrial-, ventricular- and Purkinje-like cardiomyocytes, and exhibiting rhythmic Ca2+ transients and Ca2+ sparks in hiPSC-derived cardiomyocytes. Furthermore, spontaneously and synchronously beating and force-developing engineered heart tissues were generated.

CONCLUSIONS

Human-induced pluripotent stem cells can be reprogrammed from all three somatic cell types, but with different efficiency. All analysed iPSCs can differentiate into cardiomyocytes, and the functionalities of cardiomyocytes derived from different cell origins are similar. However, MSC-derived hiPSCs revealed a higher cardiac differentiation efficiency than keratinocyte- and fibroblast-derived hiPSCs.

Krueppel-like factor 15 regulates Wnt/β-catenin transcription and controls cardiac progenitor cell fate in the postnatal heart

Wnt/β-catenin signalling controls adult heart remodelling in part via regulation of cardiac progenitor cell (CPC) differentiation. An enhanced understanding of mechanisms controlling CPC biology might facilitate the development of new therapeutic strategies in heart failure. We identified and characterized a novel cardiac interaction between Krueppel-like factor 15 and components of the Wnt/β-catenin pathway leading to inhibition of transcription. In vitro mutation, reporter assays and co-localization analyses revealed that KLF15 requires both the C-terminus, necessary for nuclear localization, and a minimal N-terminal regulatory region to inhibit transcription. In line with this, functional Klf15 knock-out mice exhibited cardiac β-catenin transcriptional activation along with functional cardiac deterioration in normal homeostasis and upon hypertrophy. We further provide in vivo and in vitro evidences for preferential endothelial lineage differentiation of CPCs upon KLF15 deletion. Via inhibition of β-catenin transcription, KLF15 controls CPC homeostasis in the adult heart similar to embryonic cardiogenesis. This knowledge may provide a tool for reactivation of this apparently dormant CPC population in the adult heart and thus be an attractive approach to enhance endogenous cardiac repair.

Increased myocardial SERCA expression in early type 2 diabetes mellitus is insulin dependent: In vivo and in vitro data

BACKGROUND

Calcium (Ca2+) handling proteins are known to play a pivotal role in the pathophysiology of cardiomyopathy. However little is known about early changes in the diabetic heart and the impact of insulin treatment (Ins).

METHODS

Zucker Diabetic Fatty rats treated with or without insulin (ZDF ± Ins, n = 13) and lean littermates (controls, n = 7) were sacrificed at the age of 19 weeks. ZDF + Ins (n = 6) were treated with insulin for the last 6 weeks of life. Gene expression of Ca2+ ATPase in the cardiac sarcoplasmatic reticulum (SERCA2a, further abbreviated as SERCA) and phospholamban (PLB) were determined by northern blotting. Ca2+ transport of the sarcoplasmatic reticulum (SR) was assessed by oxalate-facilitated 45Ca-uptake in left ventricular homogenates. In addition, isolated neonatal cardiomyocytes were stimulated in cell culture with insulin, glucose or triiodthyronine (T3, positive control). mRNA expression of SERCA and PLB were measured by Taqman PCR. Furthermore, effects of insulin treatment on force of contraction and relaxation were evaluated by cardiomyocytes grown in a three-dimensional collagen matrix (engineered heart tissue, EHT) stimulated for 5 days by insulin. By western blot phosphorylations status of Akt was determed and the influence of wortmannin.

RESULTS

SERCA levels increased in both ZDF and ZDF + Ins compared to control (control 100 ± 6.2 vs. ZDF 152 ± 26.6* vs. ZDF + Ins 212 ± 18.5*# % of control, *p < 0.05 vs. control, #p < 0.05 vs. ZDF) whereas PLB was significantly decreased in ZDF and ZDF + Ins (control 100 ± 2.8 vs. ZDF 76.3 ± 13.5* vs. ZDF + Ins 79.4 ± 12.9* % of control, *p < 0.05 vs control). The increase in the SERCA/PLB ratio in ZDF and ZDF ± Ins was accompanied by enhanced Ca2+ uptake to the SR (control 1.58 ± 0.1 vs. ZDF 1.85 ± 0.06* vs. ZDF + Ins 2.03 ± 0.1* μg/mg/min, *p < 0.05 vs. control). Interestingly, there was a significant correlation between Ca2+ uptake and SERCA2a expression. As shown by in-vitro experiments, the effect of insulin on SERCA2a mRNA expression seemed to have a direct effect on cardiomyocytes. Furthermore, long-term treatment of engineered heart tissue with insulin increased the SERCA/PLB ratio and accelerated relaxation time. Akt was significantly phosphorylated by insulin. This effect could be abolished by wortmannin.

CONCLUSION

The current data demonstrate that early type 2 diabetes is associated with an increase in the SERCA/PLB ratio and that insulin directly stimulates SERCA expression and relaxation velocity. These results underline the important role of insulin and calcium handling proteins in the cardiac adaptation process of type 2 diabetes mellitus contributing to cardiac remodeling and show the important role of PI3-kinase-Akt-SERCA2a signaling cascade.

Transforming growth factor β₁ oppositely regulates the hypertrophic and contractile response to β-adrenergic stimulation in the heart

BACKGROUND

Neuroendocrine activation and local mediators such as transforming growth factor-β₁ (TGF-β₁) contribute to the pathobiology of cardiac hypertrophy and failure, but the underlying mechanisms are incompletely understood. We aimed to characterize the functional network involving TGF-β₁, the renin-angiotensin system, and the β-adrenergic system in the heart.

METHODS

Transgenic mice overexpressing TGF-β₁ (TGF-β₁-Tg) were treated with a β-blocker, an AT₁-receptor antagonist, or a TGF-β-antagonist (sTGFβR-Fc), were morphologically characterized. Contractile function was assessed by dobutamine stress echocardiography in vivo and isolated myocytes in vitro. Functional alterations were related to regulators of cardiac energy metabolism.

RESULTS

Compared to wild-type controls, TGF-β₁-Tg mice displayed an increased heart-to-body-weight ratio involving both fibrosis and myocyte hypertrophy. TGF-β₁ overexpression increased the hypertrophic responsiveness to β-adrenergic stimulation. In contrast, the inotropic response to β-adrenergic stimulation was diminished in TGF-β₁-Tg mice, albeit unchanged basal contractility. Treatment with sTGF-βR-Fc completely prevented the cardiac phenotype in transgenic mice. Chronic β-blocker treatment also prevented hypertrophy and ANF induction by isoprenaline, and restored the inotropic response to β-adrenergic stimulation without affecting TGF-β₁ levels, whereas AT₁-receptor blockade had no effect. The impaired contractile reserve in TGF-β₁-Tg mice was accompanied by an upregulation of mitochondrial uncoupling proteins (UCPs) which was reversed by β-adrenoceptor blockade. UCP-inhibition restored the contractile response to β-adrenoceptor stimulation in vitro and in vivo. Finally, cardiac TGF-β₁ and UCP expression were elevated in heart failure in humans, and UCP--but not TGF-β₁--was downregulated by β-blocker treatment.

CONCLUSIONS

Our data support the concept that TGF-β₁ acts downstream of angiotensin II in cardiomyocytes, and furthermore, highlight the critical role of the β-adrenergic system in TGF-β₁-induced cardiac phenotype. Our data indicate for the first time, that TGF-β₁ directly influences mitochondrial energy metabolism by regulating UCP3 expression. β-blockers may act beneficially by normalizing regulatory mechanisms of cellular hypertrophy and energy metabolism.

Extracellular signal-regulated kinases 1 and 2 regulate the balance between eccentric and concentric cardiac growth

RATIONALE

An increase in cardiac afterload typically produces concentric hypertrophy characterized by an increase in cardiomyocyte width, whereas volume overload or exercise results in eccentric growth characterized by cellular elongation and addition of sarcomeres in series. The signaling pathways that control eccentric versus concentric heart growth are not well understood.

OBJECTIVE

To determine the role of extracellular signal-regulated kinase 1 and 2 (ERK1/2) in regulating the cardiac hypertrophic response.

METHODS AND RESULTS

Here, we used mice lacking all ERK1/2 protein in the heart (Erk1(-/-) Erk2(fl/fl-Cre)) and mice expressing activated mitogen-activated protein kinase kinase (Mek)1 in the heart to induce ERK1/2 signaling, as well as mechanistic experiments in cultured myocytes to assess cellular growth characteristics associated with this signaling pathway. Although genetic deletion of all ERK1/2 from the mouse heart did not block the cardiac hypertrophic response per se, meaning that the heart still increased in weight with both aging and pathological stress stimulation, it did dramatically alter how the heart grew. For example, adult myocytes from hearts of Erk1(-/-) Erk2(fl/fl-Cre) mice showed preferential eccentric growth (lengthening), whereas myocytes from Mek1 transgenic hearts showed concentric growth (width increase). Isolated adult myocytes acutely inhibited for ERK1/2 signaling by adenoviral gene transfer showed spontaneous lengthening, whereas infection with an activated Mek1 adenovirus promoted constitutive ERK1/2 signaling and increased myocyte thickness. A similar effect was observed in engineered heart tissue under cyclic stretching, where ERK1/2 inhibition led to preferential lengthening.

CONCLUSIONS

Taken together, these data demonstrate that the ERK1/2 signaling pathway uniquely regulates the balance between eccentric and concentric growth of the heart.

Hepatocyte growth factor or vascular endothelial growth factor gene transfer maximizes mesenchymal stem cell-based myocardial salvage after acute myocardial infarction

BACKGROUND

Mesenchymal stem cell (MSC)-based regenerative strategies were investigated to treat acute myocardial infarction and improve left ventricular function.

METHODS AND RESULTS

Murine AMI was induced by coronary ligation with subsequent injection of MSCs, hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), or MSCs +HGF/VEGF into the border zone. Left ventricular ejection fraction was calculated using micro-computed tomography imaging after 6 months. HGF and VEGF protein injection (with or without concomitant MSC injection) significantly and similarly improved the left ventricular ejection fraction and reduced scar size compared with the MSC group, suggesting that myocardial recovery was due to the cytokines rather than myocardial regeneration. To provide sustained paracrine effects, HGF or VEGF overexpressing MSCs were generated (MSC-HGF, MSC-VEGF). MSC-HGF and MSC-VEGF showed significantly increased in vitro proliferation and increased in vivo proliferation within the border zone. Cytokine production correlated with MSC survival. MSC-HGF- and MSC-VEGF-treated animals showed smaller scar sizes, increased peri-infarct vessel densities, and better preserved left ventricular function when compared with MSCs transfected with empty vector. Murine cardiomyocytes were exposed to hypoxic in vitro conditions. The LDH release was reduced, fewer cardiomyocytes were apoptotic, and Akt activity was increased if cardiomyocytes were maintained in conditioned medium obtained from MSC-HGF or MSC-VEGF cultures.

CONCLUSIONS

This study showed that (1) elevating the tissue levels of HGF and VEGF after acute myocardial infarction seems to be a promising reparative therapeutic approach, (2) HGF and VEGF are cardioprotective by increasing the tolerance of cardiomyocytes to ischemia, reducing cardiomyocyte apoptosis and increasing prosurvival Akt activation, and (3) MSC-HGF and MSC-VEGF are a valuable source for increased cytokine production and maximize the beneficial effect of MSC-based repair strategies.

Phosphatase inhibitor-1-deficient mice are protected from catecholamine-induced arrhythmias and myocardial hypertrophy

AIMS

Phosphatase inhibitor-1 (I-1) is a conditional amplifier of beta-adrenergic signalling downstream of protein kinase A by inhibiting type-1 phosphatases only in its PKA-phosphorylated form. I-1 is downregulated in failing hearts and thus contributes to beta-adrenergic desensitization. It is unclear whether this should be viewed as a predominantly adverse or protective response.

METHODS AND RESULTS

We generated transgenic mice with cardiac-specific I-1 overexpression (I-1-TG) and evaluated cardiac function and responses to catecholamines in mice with targeted disruption of the I-1 gene (I-1-KO). Both groups were compared with their wild-type (WT) littermates. I-1-TG developed cardiac hypertrophy and mild dysfunction which was accompanied by a substantial compensatory increase in PP1 abundance and activity, confounding cause-effect relationships. I-1-KO had normal heart structure with mildly reduced sensitivity, but unchanged maximal contractile responses to beta-adrenergic stimulation, both in vitro and in vivo. Notably, I-1-KO were partially protected from lethal catecholamine-induced arrhythmias and from hypertrophy and dilation induced by a 7 day infusion with the beta-adrenergic agonist isoprenaline. Moreover, I-1-KO exhibited a partially preserved acute beta-adrenergic response after chronic isoprenaline, which was completely absent in similarly treated WT. At the molecular level, I-1-KO showed lower steady-state phosphorylation of the cardiac ryanodine receptor/Ca(2+) release channel and the sarcoplasmic reticulum (SR) Ca(2+)-ATPase-regulating protein phospholamban. These alterations may lower the propensity for diastolic Ca(2+) release and Ca(2+) uptake and thus stabilize the SR and account for the protection.

CONCLUSION

Taken together, loss of I-1 attenuates detrimental effects of catecholamines on the heart, suggesting I-1 downregulation in heart failure as a beneficial desensitization mechanism and I-1 inhibition as a potential novel strategy for heart failure treatment.

Thyroid Hormone Regulates Developmental Titin Isoform Transitions via the Phosphatidylinositol-3-Kinase/ AKT Pathway

Titins, giant sarcomere proteins with major mechanical/signaling functions, are expressed in 2 main isoform classes in the mammalian heart: N2B (3000 kDa) and N2BA (>3200 kDa). A dramatic isoform switch occurs during cardiac development, from fetal N2BA titin (3700 kDa) expressed before birth to a mix of smaller N2BA/N2B isoforms found postnatally; adult rat hearts almost exclusively have N2B titin. The isoform switch, which can be reversed in chronic human heart failure, alters myocardial distensibility and mechanosignaling. Here we determined factors regulating this switch using, as a model system, primary cardiomyocyte cultures prepared from embryonic rats. In standard culture, the mean N2B percentage initially was 14% and increased by ≈60% within 1 week, resembling the in vivo switching. The titin isoform transition was independent of endothelin-1–induced myocyte hypertrophy and was not altered by pacing, contractile arrest, or cell stretch; however, it was modestly impaired by decreasing substrate rigidity and strongly dependent on serum components. Angiotensin II significantly promoted the transition. The mean N2B proportion in 1-week-old cultures dropped 20% to 25% in hormone-reduced medium, but addition of 3,5,3′-triiodo- l -thyronine (T3) nearly restored the proportion to that found in standard culture. This T3 effect was not prevented by bisphenol A, a specific inhibitor of the classic genomic pathway of T3 action. In contrast, the titin switch could be stalled by the phosphatidylinositol 3-kinase inhibitor LY294002, which decreased the proportion of N2B mRNA transcripts within hours and suppressed a rapid T3-induced increase in Akt phosphorylation. Also, angiotensin II, but not endothelin-1 or cell stretch, enhanced Akt phosphorylation. Thus, although matrix stiffness modulates developmental titin isoform transitions, these transitions are mainly regulated through phosphatidylinositol 3-kinase/Akt-dependent signaling triggered particularly by T3 via a rapid action pathway.