The mechanisms underlying ICa heterogeneity across murine left ventricle

PGC-1α4 Interacts with REST to Upregulate Neuronal Genes and Augment Energy Consumption in Developing Cardiomyocytes

Transcriptional coactivator PGC-1α is a main regulator of cardiac energy metabolism. In addition to canonical PGC-1α1, other PGC-1α isoforms have been found to exert specific biological functions in a variety of tissues. We investigated the expression patterns and the biological effects of the non-canonical isoforms in the heart. We used RNA sequencing data to identify the expression patterns of PGC-1α isoforms in the heart. To evaluate the biological effects of the alternative isoform expression, we generated a transgenic mouse with cardiac-specific overexpression of PGC-1α4 and analysed the cardiac phenotype with a wide spectrum of physiological and biophysical tools. Our results show that non-canonical isoforms are expressed in the heart, and that the main variant PGC-1α4 is induced by β-adrenergic signalling in adult cardiomyocytes. Cardiomyocyte specific PGC-1α4 overexpression in mice relieves the RE1-Silencing Transcription factor (REST)-mediated suppression of neuronal genes during foetal heart development. The resulting de-repression of REST target genes induces a cardiac phenotype with increased cellular energy consumption, resulting in postnatal dilated cardiomyopathy. These results propose a new concept for actions of the PGC-1α protein family where activation of the Pgc-1α gene, through its isoforms, induces a phenotype with concurrent supply and demand for cellular energy. These data highlight the biological roles of the different PGC-1α isoforms, which should be considered when future therapies are developed.

Regulation of cardiac ion channels by transcription factors: Looking for new opportunities of druggable targets for the treatment of arrhythmias

Cardiac electrical activity is governed by different ion channels that generate action potentials. Acquired or inherited abnormalities in the expression and/or function of ion channels usually result in electrophysiological changes that can cause cardiac arrhythmias. Transcription factors (TFs) control gene transcription by binding to specific DNA sequences adjacent to target genes. Linkage analysis, candidate-gene screening within families, and genome-wide association studies have linked rare and common genetic variants in the genes encoding TFs with genetically-determined cardiac arrhythmias. Besides its critical role in cardiac development, recent data demonstrated that they control cardiac electrical activity through the direct regulation of the expression and function of cardiac ion channels in adult hearts. This narrative review summarizes some studies showing functional data on regulation of the main human atrial and ventricular Na⁺, Ca²⁺, and K⁺ channels by cardiac TFs such as Pitx2c, Tbx20, Tbx5, Zfhx3, among others. The results have improved our understanding of the mechanisms regulating cardiac electrical activity and may open new avenues for therapeutic interventions in cardiac acquired or inherited arrhythmias through the identification of TFs as potential drug targets. Even though TFs have for a long time been considered as 'undruggable' targets, advances in structural biology have led to the identification of unique pockets in TFs amenable to be targeted with small-molecule drugs or peptides that are emerging as novel therapeutic drugs.

PGC-1α deficiency reveals sex-specific links between cardiac energy metabolism and EC-coupling during development of heart failure in mice

Aims

Biological sex has fundamental effects on mammalian heart physiology and pathogenesis. While it has been established that female sex is a protective factor against most cardiovascular diseases (CVDs), this beneficial effect may involve pathways associated with cardiac energy metabolism. Our aim was to elucidate the role of transcriptional coactivator PGC-1α in sex dimorphism of heart failure (HF) development.

Methods and results

Here, we show that mice deficient in cardiac expression of the peroxisome proliferator-activated receptor gamma (PPAR-γ) coactivator-1α (PGC-1α) develop dilated HF associated with changes in aerobic and anaerobic metabolism, calcium handling, cell structure, electrophysiology, as well as gene expression. These cardiac changes occur in both sexes, but female mice develop an earlier and more severe structural and functional phenotype associated with dyssynchronous local calcium release resulting from disruption of t-tubular structures of the cardiomyocytes.

Conclusions

These data reveal that the integrity of the subcellular Ca²⁺ release and uptake machinery is dependent on energy metabolism and that female hearts are more prone to suffer from contractile dysfunction in conditions with compromised production of cellular energy. Furthermore, these findings suggest that PGC-1α is a central mediator of sex-specific differences in heart function and CVD susceptibility.

High doses of digoxin increase the myocardial nuclear factor-kB and CaV1.2 channels in healthy mice. A possible mechanism of digitalis toxicity

BACKGROUND

Toxic effects of digoxin may occur with normal therapeutic serum level. However, the underlying mechanisms are not fully understood. Nuclear factor kappa-B (NF-kB) is an important transcription factor in most organ systems and is often implicated in the harmful effects of cardiac injury. NF-kB promotes inflammatory responses, mediates adverse cardiac remodeling and has a function correlation with calcium. The voltage-gated L-type calcium channel CaV1.2 mediates the influx of Ca+2 into the cell in response to membrane depolarization. Our aim was to characterize the role of NF-kB during digoxin toxicity and to assess its correlation with Cav 1.2 in healthy mice in vivo.

METHODS

To address these questions, digoxin was administered in doses of 0.1, 1 or 5 mg/kg orally daily for seven days to the animals. Serum digoxin, serum calcium, atrial and ventricular calcium levels were measured. We, also, looked for NF-kB and CaV1.2 channel expression in cardiac muscle of mice.

RESULTS

Digoxin at a dose of 0.1 mg/kg did not enhance serum, atrial, and ventricular Ca+2 levels, but were increased when digoxin dose of 1 and 5 mg/kg were administered. Histologically, myocardial necrosis and cellular infiltration on day 7 were significantly more severe in the 5 mg/kg/day digoxin group. Immunohistochemical studies showed more expression of both NF-kB and CaV1.2 in 1 and 5 mg/kg/day digoxin groups.

CONCLUSIONS

These data suggest that NF-kB may be responsible for digoxin toxicity, at least partially via modulation of CaV1.2 and intracellular calcium homeostasis in the myocardium.

Contractile heterogeneity in ventricular myocardium

The transmural heterogeneity of the contractility in ventricular muscle has not been well-studied. Here, we investigated the calcium transient and sarcomere contraction/relaxation in the endocardial (Endo) and epicardial (Epi) myocytes. Endo and Epi myocytes were isolated from C57/BL6 mice by Langendorff perfusion. Ca²⁺ transient and sarcomere contraction/relaxation were recorded simultaneously at different stimulation frequencies using a dual excitation fluorescence photomultiplier system. We found that the Endo myocytes have higher baseline diastolic calcium, significantly larger calcium transient and stronger sarcomere shortening than Epi myocytes. However, both the rising and decline phases for calcium transient and sarcomere shortening were slower in Endo than in Epi myocytes. When simulation frequency was increased from 1 to 3 Hz, a greater percent increase in the diastole calcium level, Ca²⁺ transient and sarcomere shortening amplitude has been observed in the Endo myocytes. Accordingly, the frequency-dependent acceleration in the decay rate of calcium transient and sarcomere relaxation was more profound in the Endo than in Epi myocytes. Western blot analysis showed that CaMKII activity was significantly higher in Epi than in Endo myocardium before stimulation. However, this transmural heterogeneity was reversed by rapid pacing. CaMKII inhibition by KN93 diminished the frequency-dependent alterations of Ca²⁺ transient and sarcomere contraction. Our results suggest that the contractility of ventricular myocytes is heterogeneous. The Endo-myocardium is the major force generating layer in the heart, both at slow and fast heart rate, and the transmural heterogeneity of CaMKII activation plays an important role in the frequency-dependent alterations.

Structural Immaturity of Human iPSC-Derived Cardiomyocytes: In Silico Investigation of Effects on Function and Disease Modeling

Background: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have emerged as a promising experimental tool for translational heart research and drug development. However, their usability as a human adult cardiomyocyte model is limited by their functional immaturity. Our aim is to analyse quantitatively those characteristics and how they differ from adult CMs.

Methods and Results: We have developed a novel in silico model with all essential functional electrophysiology and calcium handling features of hiPSC-CMs. Importantly, the virtual cell recapitulates the immature intracellular ion dynamics that are characteristic for hiPSC-CMs, as quantified based our in vitro imaging data. The strong “calcium clock” is a source for a dual function of excitation-contraction coupling in hiPSC-CMs: action potential and calcium transient morphology vary substantially depending on the activation sequence of underlying ionic currents and fluxes that is altered in spontaneous vs. paced mode. Furthermore, parallel simulations with hiPSC-CM and adult cardiomyocyte models demonstrate the central differences. Results indicate that hiPSC-CMs translate poorly the disease specific phenotypes of Brugada syndrome, long QT Syndrome and catecholaminergic polymorphic ventricular tachycardia, showing less robustness and greater tendency for arrhythmic events than adult CMs. Based on a comparative sensitivity analysis, hiPSC-CMs share some features with adult CMs, but are still functionally closer to prenatal CMs than adult CMs. A database analysis of 3000 hiPSC-CM model variants suggests that hiPSC-CMs recapitulate poorly fundamental physiological properties of adult CMs. Single modifications do not appear to solve this problem, which is mostly contributed by the immaturity of intracellular calcium handling.

Conclusion: Our data indicates that translation of findings from hiPSC-CMs to human disease should be made with great caution. Furthermore, we established a mathematical platform that can be used to improve the translation from hiPSC-CMs to human, and to quantitatively evaluate hiPSC-CMs development toward more general and valuable model for human cardiac diseases.

Peroxisome proliferator-activated receptor-γ coactivator 1 α1 induces a cardiac excitation-contraction coupling phenotype without metabolic remodelling

KEY POINTS

Transcriptional co-activator PGC-1α1 has been shown to regulate energy metabolism and to mediate metabolic adaptations in pathological and physiological cardiac hypertrophy but other functional implications of PGC-1α1 expression are not known. Transgenic PGC-1α1 overexpression within the physiological range in mouse heart induces purposive changes in contractile properties, electrophysiology and calcium signalling but does not induce substantial metabolic remodelling. The phenotype of the PGC-1α1 transgenic mouse heart recapitulates most of the functional modifications usually associated with the exercise-induced heart phenotype, but does not protect the heart against load-induced pathological hypertrophy. Transcriptional effects of PGC-1α1 show clear dose-dependence with diverse changes in genes in circadian clock, heat shock, excitability, calcium signalling and contraction pathways at low overexpression levels, while metabolic genes are recruited at much higher PGC-1α1 expression levels. These results imply that the physiological role of PGC-1α1 is to promote a beneficial excitation-contraction coupling phenotype in the heart.

ABSTRACT

The transcriptional coactivator PGC-1α1 has been identified as a central factor mediating metabolic adaptations of the heart. However, to what extent physiological changes in PGC-1α1 expression levels actually contribute to the functional adaptation of the heart is still mostly unresolved. The aim of this study was to characterize the transcriptional and functional effects of physiologically relevant, moderate PGC-1α1 expression in the heart. In vivo and ex vivo physiological analysis shows that expression of PGC-1α1 within a physiological range in mouse heart does not induce the expected metabolic alterations, but instead induces a unique excitation-contraction (EC) coupling phenotype recapitulating features typically seen in physiological hypertrophy. Transcriptional screening of PGC-1α1 overexpressing mouse heart and myocyte cultures with higher, acute adenovirus-induced PGC-1α1 expression, highlights PGC-1α1 as a transcriptional coactivator with a number of binding partners in various pathways (such as heat shock factors and the circadian clock) through which it acts as a pleiotropic transcriptional regulator in the heart, to both augment and repress the expression of its target genes in a dose-dependent fashion. At low levels of overexpression PGC-1α1 elicits a diverse transcriptional response altering the expression state of circadian clock, heat shock, excitability, calcium signalling and contraction pathways, while metabolic targets of PGC-1α1 are recruited at higher PGC-1α1 expression levels. Together these findings demonstrate that PGC-1α1 elicits a dual effect on cardiac transcription and phenotype. Further, our results imply that the physiological role of PGC-1α1 is to promote a beneficial EC coupling phenotype in the heart.

Vascular Endothelial Growth Factor-B Induces a Distinct Electrophysiological Phenotype in Mouse Heart

Vascular endothelial growth factor B (VEGF-B) is a potent mediator of vascular, metabolic, growth, and stress responses in the heart, but the effects on cardiac muscle and cardiomyocyte function are not known. The purpose of this study was to assess the effects of VEGF-B on the energy metabolism, contractile, and electrophysiological properties of mouse cardiac muscle and cardiac muscle cells. In vivo and ex vivo analysis of cardiac-specific VEGF-B TG mice indicated that the contractile function of the TG hearts was normal. Neither the oxidative metabolism of isolated TG cardiomyocytes nor their energy substrate preference showed any difference to WT cardiomyocytes. Similarly, myocyte Ca²⁺ signaling showed only minor changes compared to WT myocytes. However, VEGF-B overexpression induced a distinct electrophysiological phenotype characterized by ECG changes such as an increase in QRSp time and decreases in S and R amplitudes. At the level of isolated TG cardiomyocytes, these changes were accompanied with decreased action potential upstroke velocity and increased duration (APD_60–70). These changes were partly caused by downregulation of sodium current (I_Na) due to reduced expression of Na_v1.5. Furthermore, TG myocytes had alterations in voltage-gated K⁺ currents, namely decreased density of transient outward current (I_to) and total K⁺ current (I_peak). At the level of transcription, these were accompanied by downregulation of K_v channel-interacting protein 2 (Kcnip2), a known modulatory subunit for K_v4.2/3 channel. Cardiac VEGF-B overexpression induces a distinct electrophysiological phenotype including remodeling of cardiomyocyte ion currents, which in turn induce changes in action potential waveform and ECG.

Transmural gradients in ion channel and auxiliary subunit expression

Evolution has acted to shape the action potential in different regions of the heart in order to produce a maximally stable and efficient pump. This has been achieved by creating regional differences in ion channel expression levels within the heart as well as differences between equivalent cardiac tissues in different species. These region- and species-dependent differences in channel expression are established by regulatory evolution, evolution of the regulatory mechanisms that control channel expression levels. Ion channel auxiliary subunits are obvious targets for regulatory evolution, in order to change channel expression levels and/or modify channel function. This review focuses on the transmural gradients of ion channel expression in the heart and the role that regulation of auxiliary subunit expression plays in generating and shaping these gradients.

Distinctive profile of IsomiR expression and novel microRNAs in rat heart left ventricle

MicroRNAs (miRNAs) are single-stranded non-coding RNAs that negatively regulate target gene expression through mRNA cleavage or translational repression. There is mounting evidence that they play critical roles in heart disease. The expression of known miRNAs in the heart has been studied at length by microarray and quantitative PCR but it is becoming evident that microRNA isoforms (isomiRs) are potentially physiologically important. It is well known that left ventricular (patho)physiology is influenced by transmural heterogeneity of cardiomyocyte phenotype, and this likely reflects underlying heterogeneity of gene expression. Given the significant role of miRNAs in regulating gene expression, knowledge of how the miRNA profile varies across the ventricular wall will be crucial to better understand the mechanisms governing transmural physiological heterogeneity. To determinine miRNA/isomiR expression profiles in the rat heart we investigated tissue from different locations across the left ventricular wall using deep sequencing. We detected significant quantities of 145 known rat miRNAs and 68 potential novel orthologs of known miRNAs, in mature, mature* and isomiR formation. Many isomiRs were detected at a higher frequency than their canonical sequence in miRBase and have different predicted targets. The most common miR-133a isomiR was more effective at targeting a construct containing a sequence from the gelsolin gene than was canonical miR-133a, as determined by dual-fluorescence assay. We identified a novel rat miR-1 homolog from a second miR-1 gene; and a novel rat miRNA similar to miR-676. We also cloned and sequenced the rat miR-486 gene which is not in miRBase (v18). Signalling pathways predicted to be targeted by the most highly detected miRNAs include Ubiquitin-mediated Proteolysis, Mitogen-Activated Protein Kinase, Regulation of Actin Cytoskeleton, Wnt signalling, Calcium Signalling, Gap junctions and Arrhythmogenic Right Ventricular Cardiomyopathy. Most miRNAs are not expressed in a gradient across the ventricular wall, with exceptions including miR-10b, miR-21, miR-99b and miR-486.

N-acetylcysteine prevents electrical remodeling and attenuates cellular hypertrophy in epicardial myocytes of rats with ascending aortic stenosis

Pressure overload is associated with cardiac hypertrophy and electrical remodeling. Here, we investigate the effects of the antioxidant N-acetylcysteine (NAC) on the cellular cardiac electrophysiology of female Sprague-Dawley rats with ascending aortic stenosis (AS). Rats were treated with NAC (1 g/kg body weight) or control solution 1 week before the intervention and in the week following AS or sham operation. Seven days after the operation, blood pressure and left ventricular pressure were measured before the heart was excised. Single cells were isolated from epicardial and endocardial layers of the left ventricular free wall and investigated using the whole-cell patch-clamp technique. Systolic blood pressure and left ventricular peak pressure were not significantly altered in the NAC group. NAC reduced the increase (p < 0.001) in the relative left ventricular weight (p < 0.05) as well as the increase (p < 0.001) in cell capacitance in epicardial (p < 0.05), but not in endocardial myocytes of AS animals. The L-type Ca(2+) current (I (CaL)) was significantly increased by AS in epicardial (+19 % at 0 mV, p < 0.01) but not in endocardial myocytes. NAC completely prevented this increase in I (CaL) (p < 0.01). The current density of the transient outward K(+) current (I (to)) was not affected by AS or NAC. Action potential duration to 90 % repolarization was significantly prolonged in epicardial (p < 0.01) as well as in endocardial (p < 0.001) cells of AS animals. NAC prevented the AP prolongation in epicardial myocytes only (p < 0.05). We conclude that reducing oxidative stress in pressure overload can prevent electrical remodeling and ameliorate hypertrophy in epicardial but not in endocardial myocytes.

Analysis of Na(+)/Ca (2+) exchanger (NCX) function and current in murine cardiac myocytes during heart failure

Na(+)/Ca(2+) exchanger (NCX) plays important roles in cardiac electrical activity and calcium homeostasis. NCX current (I(NCX)) shows transmural gradient across left ventricle in many species. Previous studies demonstrated that NCX expression was increased and transmural gradient of I(NCX) was disrupted in failing heart, but the mechanisms underlying I(NCX) remodeling still remain unknown. In present study, we used patch clamp technique to record I(NCX) from subepicardial (EPI) myocytes and subendocardial (ENDO) myocytes isolated from sham operation (SO) mice and heart failure (HF) mice. Our results showed that I(NCX) was higher in normal EPI cells compared with that in ENDO, whatever for forward mode or reverse mode. In HF group, I(NCX) was significantly up-regulated, but EPI-ENDO difference was disrupted because of a more increase of I(NCX) in ENDO myocytes. In order to explore the molecular mechanism underlying remodeling of I(NCX) in failing heart, we detected the protein expression of NCX1 and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) by Western blot. We found that CaMKII activity was dramatically enhanced and parallel with the expression of NCX1 in failing heart. Our study demonstrated that transmural gradient of I(NCX) existed in murine left ventricle, and increased activity of CaMKII should account for I(NCX) remodeling in failing heart.