Effects of induced Na+/Ca2+ exchanger overexpression on the spatial distribution of L-type Ca2+ channels and junctophilin-2 in pressure-overloaded hearts

FGF13 deficiency ameliorates calcium signaling abnormality in heart failure by regulating microtubule stability

Calcium signaling abnormality in cardiomyocytes, as a key mechanism, is closely associated with developing heart failure. Fibroblast growth factor 13 (FGF13) demonstrates important regulatory roles in the heart, but its association with cardiac calcium signaling in heart failure remains unknown. This study aimed to investigate the role and mechanism of FGF13 on calcium mishandling in heart failure. Mice underwent transaortic constriction to establish a heart failure model, which showed decreased ejection fraction, fractional shortening, and contractility. FGF13 deficiency alleviated cardiac dysfunction. Heart failure reduces calcium transients in cardiomyocytes, which were alleviated by FGF13 deficiency. Meanwhile, FGF13 deficiency restored decreased Cav1.2 and Serca2α expression and activity in heart failure. Furthermore, FGF13 interacted with microtubules in the heart, and FGF13 deficiency inhibited the increase of microtubule stability during heart failure. Finally, in isoproterenol-stimulated FGF13 knockdown neonatal rat ventricular myocytes (NRVMs), wildtype FGF13 overexpression, but not FGF13 mutant, which lost the binding site of microtubules, promoted calcium transient abnormality aggravation and Cav1.2 downregulation compared with FGF13 knockdown group. Generally, FGF13 deficiency improves abnormal calcium signaling by inhibiting the increased microtubule stability in heart failure, indicating the important role of FGF13 in cardiac calcium homeostasis and providing new avenues for heart failure prevention and treatment.

Decrease in Ca2+ Concentration in Quail Cardiomyocytes Is Faster than That in Rat Cardiomyocytes

Mammals and birds have quicker heart rates compared to other species. Mammalian cardiomyocytes have T-tubule membranes that facilitate rapid changes in Ca2+ concentrations. In contrast, bird cardiomyocytes do not possess T-tubule membranes, which raises the question of how birds achieve fast heartbeats. In this study, we compared the changes in Ca2+ concentration in cardiomyocytes isolated from adult quails and rats to elucidate the mechanism resulting in rapid heart rates in birds. Cardiomyocytes isolated from quails were significantly narrower than those isolated from rats. When Ca2+ concentration changes in the entire cardiomyocytes were measured using Fura-2 acetoxymethyl ester (AM), the time to peak was statistically longer in quails than in rats. In contrast, the decay time was markedly shorter in quails than in rats. As a result, the total time of Ca2+ concentration change was shorter in quails than in rats. A spatiotemporal analysis of Ca2+ concentration changes in quail cardiomyocytes showed that the decrease in Ca2+ concentration was faster in the center of the cell than near the cell membrane. These results suggest that avian cardiomyocytes achieve rapid changes in Ca2+ concentration by increasing the Ca2+ removal capacity in the central part of the cell compared to mammalian cardiomyocytes.

Exercise Training Reverses Lipotoxicity-induced Cardiomyopathy by Inhibiting HMGCS2

PURPOSE

This study aimed to determine the effect of exercise training on preventing lipotoxic cardiomyopathy and to investigate the role of the 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2) and miR-344g-5p in cardiomyocytes.

METHODS

Male C57BL/6 mice were fed a 60% high-fat diet (HFD) for 12 wk then began swimming exercise or remained sedentary for 8 wk. Thereafter, cardiac function was assessed by echocardiography, and heart tissue and plasma were collected for further measurements. The molecular mechanism of exercise was investigated after treating Hmgcs2 siRNA in palmitate-induced neonatal mouse cardiomyocytes.

RESULTS

HFD induced myocardial hypertrophy and fibrosis and reduced coronary reserve and cardiac function. HMGCS2 levels increased, but junctophilin-2 (JPH2) levels decreased in HFD mice hearts. Such effects were attenuated by swimming exercise. Mechanistically, Hmgcs2 silencing prevented apoptosis and caspase-3 cleavage and elevated the expression of JPH2 in palmitate-stimulated cardiomyocytes. In addition, exercise promoted miR-344g-5p expression in HFD hearts. The overexpression of miR-344g-5p by chemical mimic reduced HMGCS2, apoptosis, and caspase-3 cleavage and elevated JPH2 expression in palmitate-induced cardiomyocytes.

CONCLUSION

Our results suggest that exercise limits lipid metabolic disorder, cardiac hypertrophy, and fibrosis and aids in the prevention of lipotoxic cardiomyopathy. Exercise-mediated cardioprotection by upregulating miR-344g-5p, which targets Hmgcs2 mRNA, prohibits HMGCS2 upregulation and thus lipotoxicity.

Effect of BIN1 on cardiac dysfunction and malignant arrhythmias

Heart failure (HF) is the end-stage syndrome for most cardiac diseases, and the 5-year morbidity and mortality of HF remain high. Malignant arrhythmia is the main cause of sudden death in the progression of HF. Recently, bridging integrator 1 (BIN1) was discovered as a regulator of transverse tubule function and calcium signalling in cardiomyocytes. BIN1 downregulation is linked to abnormal cardiac contraction, and it increases the possibility of malignant arrhythmias preceding HF. Because of the detectability of cardiac BIN1 in peripheral blood, BIN1 may serve as a predictor of HF and may be useful in therapy development. However, the mechanism of BIN1 downregulation in HF and how BIN1 regulates normal cardiac function under physiological conditions remain unclear. In this review, recent progress in the biological studies of BIN1-related cardiomyocytes and the effect of cardiac dysfunction and malignant arrhythmia will be discussed.

Elimination of fukutin reveals cellular and molecular pathomechanisms in muscular dystrophy-associated heart failure

Heart failure is the major cause of death for muscular dystrophy patients, however, the molecular pathomechanism remains unknown. Here, we show the detailed molecular pathogenesis of muscular dystrophy-associated cardiomyopathy in mice lacking the fukutin gene (Fktn), the causative gene for Fukuyama muscular dystrophy. Although cardiac Fktn elimination markedly reduced α-dystroglycan glycosylation and dystrophin-glycoprotein complex proteins in sarcolemma at all developmental stages, cardiac dysfunction was observed only in later adulthood, suggesting that membrane fragility is not the sole etiology of cardiac dysfunction. During young adulthood, Fktn-deficient mice were vulnerable to pathological hypertrophic stress with downregulation of Akt and the MEF2-histone deacetylase axis. Acute Fktn elimination caused severe cardiac dysfunction and accelerated mortality with myocyte contractile dysfunction and disordered Golgi-microtubule networks, which were ameliorated with colchicine treatment. These data reveal fukutin is crucial for maintaining myocyte physiology to prevent heart failure, and thus, the results may lead to strategies for therapeutic intervention.

Mutations in Ftkn cause Fukuyama muscular dystrophy, and heart failure is the main cause of death in thes patients. Here the authors show that acute elimination of Fktn in adult mice causes early mortality, and this is associated with myocyte dysfunction, with disorganised Golg-microtubule networks, and that the pathology can be ameliorated with colchicine treatment.

MG53 is dispensable for T-tubule maturation but critical for maintaining T-tubule integrity following cardiac stress

The cardiac transverse (T)-tubule membrane system is the safeguard for cardiac function and undergoes dramatic remodeling in response to cardiac stress. However, the mechanism by which cardiomyocytes repair damaged T-tubule network remains unclear. In the present study, we tested the hypothesis that MG53, a muscle-specific membrane repair protein, antagonizes T-tubule damage to protect against maladaptive remodeling and thereby loss of excitation-contraction coupling and cardiac function. Using MG53-knockout (MG53-KO) mice, we first established that deficiency of MG53 had no impact on maturation of the T-tubule network in developing hearts. Additionally, MG53 ablation did not influence T-tubule integrity in unstressed adult hearts as late as 10months of age. Following left ventricular pressure overload-induced cardiac stress, MG53 protein levels were increased by approximately three-fold in wild-type mice, indicating that pathological stress induces a significant upregulation of MG53. MG53-deficient mice had worsened T-tubule disruption and pronounced dysregulation of Ca²⁺ handling properties, including decreased Ca²⁺ transient amplitude and prolonged time to peak and decay. Moreover, MG53 deficiency exacerbated cardiac hypertrophy and dysfunction and decreased survival following cardiac stress. Our data suggest MG53 is not required for T-tubule development and maintenance in normal physiology. However, MG53 is essential to preserve T-tubule integrity and thereby Ca²⁺ handling properties and cardiac function under pathological cardiac stress.

Heterogeneity of transverse-axial tubule system in mouse atria: Remodeling in atrial-specific Na+-Ca2+ exchanger knockout mice

Transverse-axial tubules (TATs) are commonly assumed to be sparse or absent in atrial myocytes from small animals. Atrial myocytes from rats, cats and rabbits lack TATs, which results in a characteristic "V"-shaped Ca release pattern in confocal line-scan recordings due to the delayed rise of Ca in the center of the cell. To examine TAT expression in isolated mouse atrial myocytes, we loaded them with the membrane dye Di-4-ANEPPS to label TATs. We found that >80% of atrial myocytes had identifiable TATs. Atria from male mice had a higher TAT density than female mice, and TAT density correlated with cell width. Using the fluorescent Ca indicator Fluo-4-AM and confocal imaging, we found that wild type (WT) mouse atrial myocytes generate near-synchronous Ca transients, in contrast to the "V"-shaped pattern typically reported in other small animals such as rat. In atrial-specific Na-Ca exchanger (NCX) knockout (KO) mice, which develop sinus node dysfunction and atrial hypertrophy with dilation, we found a substantial loss of atrial TATs in isolated atrial myocytes. There was a greater loss of transverse tubules compared to axial tubules, resulting in a dominance of axial tubules. Consistent with the overall loss of TATs, NCX KO atrial myocytes displayed a "V"-shaped Ca transient with slower and reduced central (CT) Ca release and uptake in comparison to subsarcolemmal (SS) Ca release. We compared chemically detubulated (DT) WT cells to KO, and found similar slowing of CT Ca release and uptake. However, SS Ca transients in the WT DT cells had faster uptake kinetics than KO cells, consistent with the presence of NCX and normal sarcolemmal Ca efflux in the WT DT cells. We conclude that the remodeling of NCX KO atrial myocytes is accompanied by a loss of TATs leading to abnormal Ca release and uptake that could impact atrial contractility and rhythm.