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How chronic pressure overload affects the Purkinje fibers of the ventricular peripheral conduction system (PCS) is not known. Here, we used a connexin (Cx)40 knockout/enhanced green fluorescent protein knockin transgenic mouse model to specifically label the PCS. We hypothesized that the subendocardially located PCS would remodel after chronic pressure overload and therefore analyzed cell size, markers of hypertrophy, and PCS-specific Cx and ion channel expression patterns. Left ventricular hypertrophy with preserved systolic function was induced by 30 days of surgical transaortic constriction. After transaortic constriction, we observed that PCS cardiomyocytes hypertrophied by 23% ( P < 0.05) and that microdissected PCS tissue exhibited upregulated markers of hypertrophy. PCS cardiomyocytes showed a 98% increase in the number of Cx40-positive gap junction particles, with an associated twofold increase in gene expression ( P < 0.05). We also identified a 50% reduction in Cx43 gap junction particles located at the interface between PCS cardiomyocytes and the working cardiomyocyte. In addition, we measured a fourfold increase of an ion channel, hyperpolarization-activated cyclic nucleotide-gated channel (HCN)4, throughout the PCS ( P < 0.05). As a direct consequence of PCS remodeling, we found that pressure-overloaded hearts exhibited marked changes in ventricular activation patterns during normal sinus rhythm. These novel findings characterize PCS cardiomyocyte remodeling after chronic pressure overload. We identified significant hypertrophic growth accompanied by modified expression of Cx40, Cx43, and HCN4 within PCS cardiomyocytes. We found that a functional outcome of these changes is a failure of the PCS to activate the ventricular myocardium normally. Our findings provide a proof of concept that pressure overload induces specific cellular changes, not just within the working myocardium but also within the specialized PCS.

International audience; The level of expression of ion channels has been demonstrated to vary over a threefold to fourfold range from neuron to neuron, although the expression of distinct channels may be strongly correlated in the same neurons. We demonstrate that variability and covariation also apply to the biophysical properties of ion channels. We show that, in rat substantia nigra pars compacta dopaminergic neurons, the voltage dependences of the A-type (I(A)) and H-type (I(H)) currents exhibit a high degree of cell-to-cell variability, although they are strongly correlated in these cells. Our data also demonstrate that this cell-to-cell covariability of voltage dependences is sensitive to cytosolic cAMP and calcium levels. Finally, using dynamic clamp, we demonstrate that covarying I(A) and I(H) voltage dependences increases the dynamic range of rebound firing while covarying their amplitudes has a homeostatic effect on rebound firing. We propose that the covariation of voltage dependences of ion channels represents a flexible and energy-efficient way of tuning firing in neurons.

International audience; AIMS: Cardiac mineralocorticoid receptor (MR) activation triggers adverse cardiovascular events that could be efficiently prevented by mineralocorticoid antagonists. To gain insights into the pathophysiological role of MR function, we established embryonic stem (ES) cell lines from blastocysts of transgenic mice overexpressing the human MR driven by its proximal P1 or distal P2 promoter and presenting with cardiomyopathy, tachycardia, and arrhythmia. Cardiomyocyte differentiation allowed us to investigate the molecular mechanisms contributing to MR-mediated cardiac dysfunction. METHODS AND RESULTS: During cardiac differentiation, wild-type (WT) and recombinant ES cell cultures and excised beating patches expressed endogenous MR along with cardiac gene markers. The two-fold increase in MR protein detected in P1.hMR and P2.hMR cardiomyocytes led to a parallel increase in the spontaneous beating frequency of hMR-overexpressing cardiomyocytes compared with WT. The MR-mediated chronotropic effect was ligand-independent, could be partially repressed by spironolactone, and was accompanied by a significant two- to four-fold increase in mRNA and protein levels of the pacemaker channel HCN1, generating depolarizing If currents, thus revealing a potential new MR target. This was associated with modification in the expression of HCN4, the inward-rectifier potassium channel Kir2.1, and the L-type voltage-dependent calcium channel Cav1.2. CONCLUSION: We demonstrate that the amplification of MR signalling in ES-derived cardiomyocytes has a major impact on cardiomyocyte contractile properties through an important remodelling of ion channel expression, contributing to arrhythmias. Our results highlight the prominent role of MR function in cardiac physiology and support the benefit of MR antagonists in the management of cardiac dysfunctions.

International audience; AIMS: Cardiac mineralocorticoid receptor (MR) activation triggers adverse cardiovascular events that could be efficiently prevented by mineralocorticoid antagonists. To gain insights into the pathophysiological role of MR function, we established embryonic stem (ES) cell lines from blastocysts of transgenic mice overexpressing the human MR driven by its proximal P1 or distal P2 promoter and presenting with cardiomyopathy, tachycardia, and arrhythmia. Cardiomyocyte differentiation allowed us to investigate the molecular mechanisms contributing to MR-mediated cardiac dysfunction. METHODS AND RESULTS: During cardiac differentiation, wild-type (WT) and recombinant ES cell cultures and excised beating patches expressed endogenous MR along with cardiac gene markers. The two-fold increase in MR protein detected in P1.hMR and P2.hMR cardiomyocytes led to a parallel increase in the spontaneous beating frequency of hMR-overexpressing cardiomyocytes compared with WT. The MR-mediated chronotropic effect was ligand-independent, could be partially repressed by spironolactone, and was accompanied by a significant two- to four-fold increase in mRNA and protein levels of the pacemaker channel HCN1, generating depolarizing If currents, thus revealing a potential new MR target. This was associated with modification in the expression of HCN4, the inward-rectifier potassium channel Kir2.1, and the L-type voltage-dependent calcium channel Cav1.2. CONCLUSION: We demonstrate that the amplification of MR signalling in ES-derived cardiomyocytes has a major impact on cardiomyocyte contractile properties through an important remodelling of ion channel expression, contributing to arrhythmias. Our results highlight the prominent role of MR function in cardiac physiology and support the benefit of MR antagonists in the management of cardiac dysfunctions.

International audience; AIMS: Cardiac mineralocorticoid receptor (MR) activation triggers adverse cardiovascular events that could be efficiently prevented by mineralocorticoid antagonists. To gain insights into the pathophysiological role of MR function, we established embryonic stem (ES) cell lines from blastocysts of transgenic mice overexpressing the human MR driven by its proximal P1 or distal P2 promoter and presenting with cardiomyopathy, tachycardia, and arrhythmia. Cardiomyocyte differentiation allowed us to investigate the molecular mechanisms contributing to MR-mediated cardiac dysfunction. METHODS AND RESULTS: During cardiac differentiation, wild-type (WT) and recombinant ES cell cultures and excised beating patches expressed endogenous MR along with cardiac gene markers. The two-fold increase in MR protein detected in P1.hMR and P2.hMR cardiomyocytes led to a parallel increase in the spontaneous beating frequency of hMR-overexpressing cardiomyocytes compared with WT. The MR-mediated chronotropic effect was ligand-independent, could be partially repressed by spironolactone, and was accompanied by a significant two- to four-fold increase in mRNA and protein levels of the pacemaker channel HCN1, generating depolarizing If currents, thus revealing a potential new MR target. This was associated with modification in the expression of HCN4, the inward-rectifier potassium channel Kir2.1, and the L-type voltage-dependent calcium channel Cav1.2. CONCLUSION: We demonstrate that the amplification of MR signalling in ES-derived cardiomyocytes has a major impact on cardiomyocyte contractile properties through an important remodelling of ion channel expression, contributing to arrhythmias. Our results highlight the prominent role of MR function in cardiac physiology and support the benefit of MR antagonists in the management of cardiac dysfunctions.