Electrical remodeling in a transgenic mouse model of alpha1B-adrenergic receptor overexpression

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.

Alpha blockade potentiates CPVT therapy in calsequestrin-mutant mice

BACKGROUND

Spontaneous calcium release evoking delayed afterdepolarization is believed to cause catecholaminergic polymorphic ventricular tachycardia (CPVT), a lethal human arrhythmia provoked by exercise or emotional stress. β-Adrenergic blockers are the drug of choice, but fail to achieve complete arrhythmia control in some patients. These individuals often require flecainide, device implantation, and/or sympathetic denervation.

OBJECTIVE

To optimize the arrhythmia therapy by pharmacological inhibition of the sympathetic nervous system in the homozygous calsequestrin knockout (CASQ2(Δ/Δ)) mouse model of CPVT2.

METHODS

A heart telemetry device was implanted for continuous electrocardiographic recording at rest and during provocation testing. Calcium transients and abnormal calcium release were studied in cardiomyocytes isolated from adult mice. Adrenergic receptor expression was determined by using Western blotting and confocal microscopy.

RESULTS

Adult CASQ2(Δ/Δ) mice suffer from complex ventricular arrhythmia at rest and ventricular tachycardia during treadmill exercise and after epinephrine injection. β-Adrenergic blockers, propranolol and metoprolol, attenuated arrhythmia at rest but not after stress. Reserpine had no efficacy in controlling arrhythmia. Agents with α-blocking activity, phentolamine or labetalol, abolished both exercise- and epinephrine-induced arrhythmia. In contrast, injection of α-adrenergic agonist phenylephrine reproducibly provoked ventricular tachycardia. Isolated cardiomyocytes from CASQ2(Δ/Δ) mice had delayed calcium release waves upon exposure to sympathetic agonists, which were abolished by phentolamine. Hearts of calsequestrin-mutant mice expressed more α1-adrenergic receptor than did wild type control mice (P < .05).

CONCLUSION

We identified a contribution of the α-adrenergic pathway to the pathogenesis of catecholamine-induced arrhythmia. α-Blockade emerges as an effective therapy in the murine model of CPVT2 and should be tried in humans resistant to β-blockers.

Noradrenaline stimulates cell proliferation by suppressing potassium channels via G(i/o) -protein-coupled α(1B) -adrenoceptors in human osteoblasts

BACKGROUND AND PURPOSE

Recent studies demonstrated that the sympathetic nervous system regulates bone metabolism via β(2) -adrenoceptors. Although α-adrenoceptors are also expressed in osteogenic cells, their functions in bone metabolism have been less studied. We previously demonstrated that noradrenaline suppressed potassium currents via α(1B) -adrenoceptors in the human osteoblast SaM-1 cell line. The aim of this study was to investigate the signal transduction pathway and the physiological role of noradrenaline in human osteoblasts in more detail.

EXPERIMENTAL APPROACH

To investigate signal transduction through α(1B) -adrenoceptors, we used whole-cell patch clamp recording and Ca fluorescence imaging. Potassium channels regulate membrane potential and cell proliferation activity in non-excitable cells, so we evaluated cell proliferation activity by BrdU incorporation and WST assay.

KEY RESULTS

In SaM-1 cells, bath-applied noradrenaline elevated intracellular Ca(2+) concentration and this effect was abolished by both chloroethylclonidine, an α(1B) -adrenoceptor antagonist, and U73122, a PLC inhibitor. However, the inhibitory effect of noradrenaline on whole-cell current was unaffected by U73122. In contrast, in cells pretreated with either Pertussis toxin, a G(i/o) -protein-coupled receptor inhibitor, or gallein, a Gβγ-protein inhibitor, the inhibitory effect of noradrenaline on whole-cell current was significantly suppressed. Noradrenaline-induced enhancement of cell proliferation was inhibited by CsCl, a non-selective potassium channel blocker, gallein and H89, a PKA inhibitor, but not by U73122.

CONCLUSIONS AND IMPLICATIONS

Noradrenaline facilitated cell proliferation by regulation of potassium currents in human osteoblasts via G(i/o) -protein-coupled α(1B) -adrenoceptors, not via coupling to Gq-proteins.

Multistep ion channel remodeling and lethal arrhythmia precede heart failure in a mouse model of inherited dilated cardiomyopathy

BACKGROUND

Patients with inherited dilated cardiomyopathy (DCM) frequently die with severe heart failure (HF) or die suddenly with arrhythmias, although these symptoms are not always observed at birth. It remains unclear how and when HF and arrhythmogenic changes develop in these DCM mutation carriers. In order to address this issue, properties of the myocardium and underlying gene expressions were studied using a knock-in mouse model of human inherited DCM caused by a deletion mutation ΔK210 in cardiac troponinT.

METHODOLOGY/PRINCIPAL FINDINGS

By 1 month, DCM mice had already enlarged hearts, but showed no symptoms of HF and a much lower mortality than at 2 months or later. At around 2 months, some would die suddenly with no clear symptoms of HF, whereas at 3 months, many of the survivors showed evident symptoms of HF. In isolated left ventricular myocardium (LV) from 2 month-mice, spontaneous activity frequently occurred and action potential duration (APD) was prolonged. Transient outward (I(to)) and ultrarapid delayed rectifier K(+) (I(Kur)) currents were significantly reduced in DCM myocytes. Correspondingly, down-regulation of Kv4.2, Kv1.5 and KChIP2 was evident in mRNA and protein levels. In LVs at 3-months, more frequent spontaneous activity, greater prolongation of APD and further down-regulation in above K(+) channels were observed. At 1 month, in contrast, infrequent spontaneous activity and down-regulation of Kv4.2, but not Kv1.5 or KChIP2, were observed.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that at least three steps of electrical remodeling occur in the hearts of DCM model mice, and that the combined down-regulation of Kv4.2, Kv1.5 and KChIP2 prior to the onset of HF may play an important role in the premature sudden death in this DCM model. DCM mice at 1 month or before, on the contrary, are associated with low risk of death in spite of inborn disorder and enlarged heart.

Cardiac and neuroprotection regulated by α(1)-adrenergic receptor subtypes

Sympathetic nervous system regulation by the α(1)-adrenergic receptor (AR) subtypes (α(1A), α(1B), α(1D)) is complex, whereby chronic activity can be either detrimental or protective for both heart and brain function. This review will summarize the evidence that this dual regulation can be mediated through the different α(1)-AR subtypes in the context of cardiac hypertrophy, heart failure, apoptosis, ischemic preconditioning, neurogenesis, locomotion, neurodegeneration, cognition, neuroplasticity, depression, anxiety, epilepsy, and mental illness.

Upregulation of ventricular potassium channels by chronic tamoxifen treatment

AIMS

Tamoxifen is a selective oestrogen receptor modulator widely used in the prevention and treatment of breast cancer. Women receiving long-term tamoxifen therapy do not experience cardiac arrhythmias although acute perfusion of tamoxifen has been shown to inhibit cardiac K(+) currents. This observation suggests that chronic tamoxifen treatment does not negatively modulate cardiac K(+) currents. Therefore, we investigated the chronic effects of tamoxifen on K(+) currents and channels in mouse and guinea pig ventricles.

METHODS AND RESULTS

Female mice and guinea pigs were treated with placebo or tamoxifen pellets for 60 days. Voltage-clamp experiments showed that the density of the Ca²(+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)), the steady-state (I(ss)), and the inward rectifier (I(K1)) K(+) currents were increased in tamoxifen-treated mice ventricle. Western blot analysis revealed that protein expression of the underlying K(+) channels Kv4.3 (I(to)), Kv1.5 (I(Kur)), Kv2.1 (I(ss)), and Kir2.1 (I(K1)) were significantly higher in the ventricle of tamoxifen-treated mice. Protein expression of the K(+) channel subunits encoding I(Kr) and I(Ks) (ERG1, KCNQ1, and KCNE1) was also increased in tamoxifen-treated guinea pig ventricle.

CONCLUSION

Conditions with high oestrogen levels are associated with reduced K(+) currents. Thus, conceivably, tamoxifen might prevent the inhibitory effects of oestrogen on K(+) channels by blocking the oestrogen receptors, which would explain the reported increase in K(+) currents. These findings could contribute to explain the absence of cardiac arrhythmia with long-term tamoxifen therapy.

Molecular remodeling of ion channels, exchangers and pumps in atrial and ventricular myocytes in ischemic cardiomyopathy

Existing molecular knowledge base of cardiovascular diseases is rudimentary because of lack of specific attribution to cell type and function. The aim of this study was to investigate cell-specific molecular remodeling in human atrial and ventricular myocytes associated with ischemic cardiomyopathy. Our strategy combines two technological innovations, laser-capture microdissection of identified cardiac cells in selected anatomical regions of the heart and splice microarray of a narrow catalog of the functionally most important genes regulating ion homeostasis. We focused on expression of a principal family of genes coding for ion channels, exchangers and pumps (CE&P genes) that are involved in electrical, mechanical and signaling functions of the heart and constitute the most utilized drug targets. We found that (1) CE&P genes remodel in a cell-specific manner: ischemic cardiomyopathy affected 63 CE&P genes in ventricular myocytes and 12 essentially different genes in atrial myocytes. (2) Only few of the identified CE&P genes were previously linked to human cardiac disfunctions. (3) The ischemia-affected CE&P genes include nuclear chloride channels, adrenoceptors, cyclic nucleotide-gated channels, auxiliary subunits of Na(+), K(+) and Ca(2+) channels, and cell-surface CE&Ps. (4) In both atrial and ventricular myocytes ischemic cardiomyopathy reduced expression of CACNG7 and induced overexpression of FXYD1, the gene crucial for Na(+) and K(+) homeostasis. Thus, our cell-specific molecular profiling defined new landmarks for correct molecular modeling of ischemic cardiomyopathy and development of underlying targeted therapies.

Ventricular K+ currents are reduced in mice with elevated levels of serum TNFalpha

In the present study mice were treated with tumor necrosis factor alpha (TNFalpha) for 6 weeks to determine if chronic TNFalpha treatment could produce serum levels of TNFalpha similar to what has been observed in disease states (heart failure, HIV) and to determine if these levels of TNFalpha alter ventricular K(+) currents. Mice chronically treated with TNFalpha and sham treated mice were utilized for experiments. Serum levels were measured with a Searchlight protein array. Patch-clamp techniques, real-time PCR and Western blot analysis were used to study K(+) current densities and K(+) channel expression. Results showed that serum concentrations of TNFalpha were significantly higher in TNFalpha treated mice compared to controls (control: 9.5+/-1.5 pg/ml, TNFalpha: 27.4+/-5.0 pg/ml; p<0.05) and comparable to serum TNFalpha levels observed in heart failure and HIV models. In ventricular myocytes from TNFalpha treated mice the outward K(+) currents I(to) and I(Kur) were significantly reduced (at +30 mV: I(to): control: 45.0+/-2.9 pA/pF, TNFalpha: 34.5+/-2.9 pA/pF; p<0.05; I(Kur): control 34.1+/-2.7 pA/pF, TNFalpha: 25.0+/-2.2 pA/pF; p<0.05). Expression studies revealed that ventricular mRNA and protein expression for the channels underlying I(to) and I(Kur) did not differ between the two groups. However, the recovery from inactivation for I(Kur) was significantly longer in TNFalpha treated mice. Overall, this study shows that pathologically relevant levels of serum TNFalpha modulate K(+) currents in mouse ventricle. These findings could help to explain the role of TNFalpha in the pathogenesis of cardiac arrhythmia.