Excitation-Contraction Coupling Time is More Sensitive in Evaluating Cardiac Systolic Function

Background： Pressure overload-induced myocardial hypertrophy is a key step leading to heart failure. Previous cellular and animal studies demonstrated that deteriorated excitation-contraction coupling occurs as early as the compensated stage of hypertrophy before the global decrease in left ventricular ejection fraction （LVEF）. This study was to evaluate the cardiac electromechanical coupling time in evaluating cardiac systolic function in the early stage of heart failure. Methods： Twenty-six patients with Stage B heart failure （SBHF） and 31 healthy controls （CONs） were enrolled in this study. M-mode echocardiography was performed to measure LVEF. Tissue Doppler imaging （TDI） combined with electrocardiography （ECG） was used to measure cardiac electromechanical coupling time. Results： There was no significant difference in LVEF between SBHF patients and CONs （64.23 ± 8.91% vs. 64.52 ± 5.90%; P= 0.886）. However, all four electromechanical coupling time courses （Qsb： onset of Q wave on ECG to beginning of S wave on TDI, Qst： onset of Q wave on ECG to top of S wave on TDI, Rsb： top of R wave on ECG to beginning orS wave on TDI, and Rst： top of R wave on ECG to top orS wave on TDI） of SBHF patients were significantly longer than those of CONs （Qsb： 119.19 ± 35.68 ms vs. 80.30 ± 14.81 ms, P 〈 0.001 ; Qst： 165.42 ± 60.93 ms vs. 129.04 ± 16.97 ms, P = 0.006; Rsb： 82.43 ± 33.66 ms vs. 48.30 ± 15.18 ms, P 〈 0.001; and Rst： 122.37 ± 36.66 ins vs. 93.25 ± 16.72 ms, P = 0.001 ）, and the Qsb, Rsb, and Rst time showed a significantly higher sensitivity than LVEF （Rst： P =0.032; Rsb： P = 0.003; and Qsb： P = 0.004）. Conclusions： The cardiac electromechanical coupling time is more sensitive than LVEF in evaluating cardiac systolic function.

Mechanisms underlying the impaired contractility of diabetic cardiomyopathy

Cardiac dysfunction is a well-known consequence of diabetes,with sustained hyperglycaemia leading to the development of a cardiomyopathy that is independent of cardiovascular disease or hypertension.Animal models of diabetes are commonly used to study the pathophysiology of diabetic cardiomyopathy,with the hope that increased knowledge will lead ultimately to better therapeutic strategies being developed.At physiological temperature,left ventricular trabeculae isolated from the streptozotocin rat model of type 1 diabetes showed decreased stress and prolonged relaxation,but with no evidence that decreased contractility was a result of altered myocardial Ca2+handling.Although sarcoplasmic reticulum(SR)Ca2+reuptake appeared slower in diabetic trabeculae,it was offset by an increase in actionpotential duration,thereby maintaining SR Ca2+content and favouring increased contraction force.Frequency analysis of t-tubule distribution by confocal imaging of ventricular tissue labeled with wheat germ agglutinin or ryanodine receptor antibodies showed a reduced T-power for diabetic tissue,but the differences were minor in comparison to other models of heart failure.The contractile dysfunction appeared to be the result of disrupted F-actin in conjunction with the increased typeⅠcollagen,with decreased myofilament Ca2+sensitivity contributing to the slowed relaxation.

On the footsteps of Triadin and its role in skeletal muscle

Calcium is a crucial element for striated muscle function. As such, myoplasmic free Ca2+ concentration is delicately regulated through the concerted action of multiple Ca2+ pathways that relay excitation of the plasma membrane to the intracellular contractile machinery. In skeletal muscle, one of these major Ca2+ pathways is Ca2+ release from intracellular Ca2+ stores through type-1 ryanodine receptor/Ca2+ release channels (RyR1), which positions RyR1 in a strategic cross point to regulate Ca2+ homeostasis. This major Ca2+ traff ic point appears to be highly sensitive to the intracellular environment, which senses through a plethora of chemical and protein-protein interactions. Among these modulators, perhaps one of the most elusive is Triadin, a musclespecif ic protein that is involved in many crucial aspect of muscle function. This family of proteins mediates complex interactions with various Ca2+ modulators and seems poised to be a relevant modulator of Ca2+ signaling in cardiac and skeletal muscles. The purpose of this review is to examine the most recent evidence and current understanding of the role of Triadin in muscle function, in general, with particular emphasis on its contribution to Ca2+ homeostasis.

The structural biology of ryanodine receptors

Ryanodine receptors are ion channels that allow for the release of Ca2+ from the endoplasmic or sarcoplasmic reticulum.They are expressed in many different cell types but are best known for their predominance in skeletal and cardiac myocytes,where they are directly involved in excitation-contraction coupling.With molecular weights exceeding 2 MDa,Ryanodine Receptors are the largest ion channels known to date and present major challenges for structural biology.Since their discovery in the 1980s,significant progress has been made in understanding their behaviour through multiple structural methods.Cryo-electron microscopy reconstructions of intact channels depict a mushroom-shaped structure with a large cytoplasmic region that pre-sents many binding sites for regulatory molecules.This region undergoes significant motions during opening and closing of the channel,demonstrating that the Ryanodine Receptor is a bona fide allosteric protein.High-resolution structures through X-ray crystallography and NMR currently cover～11% of the entire protein.The combination of high-and low-resolution methods allows us to build pseudo-atomic models.Here we present an overview of the electron microscopy,NMR,and crystallographic analyses of this membrane protein giant.

Role of FK506-binding protein in Ca^(2+) spark regulation

The elementary Ca^2＋ release events, Ca2＋ sparks, has been found for a quarter of century. However, the molecular regulation of the spark generator, the ryanodine receptor （RyR） on the sarcoplasmic reticulum, remains obscure. Although each subunit of the RyR homotetramer has a site for FKS06-binding protein （FKBP）, the role of FKBPs in modifying RyR Ca^2＋ sparks has been debated for long. One of the reasons behind the controversy is that most previous studies detect spontaneous sparks, where the mixture with out-of-focus events and local wavelets prevents an accurate characterization of Ca^2＋ sparks. In the pre- sent study, we detected Ca^2＋ sparks triggered by single L-type Ca^2＋ channels （LCCs） under loose-seal patch clamp conditions in FKS06-treated or FKBPI2.6 knockout cardiomyocytes. We found that FKBP dissociation both by FKS06 and by rapamycin decreased the Ca^2＋ spark amplitude in ventricular cardiomyocytes. This change was neither due to decreased releasable Ca^2＋ in the sarcoplasmic reticulum, nor explained by changed RyR sensitivity. Actually FKS06 increased the LCC-RyR coupling probability and curtailed the latency for an LCC to trigger a RyR Ca^2＋ spark. FKBP12.6 knockout had similar effects as FKS06/rapamycin treatment, indicating that the decreased spark amplitude was attributable to the dissociation of FKBP12.6 rather than FKBP12. We also explained how decreased amplitude of spontaneous sparks after FKBP dissociation sometimes appears to be increased or unchanged due to inappropriate data processing. Our results provided firm evidence that without the inter-RyR coordination by functional FKBP12.6, the RyR recruitment during a Ca^2＋ spark would be compromised despite the sensitization of individual RyRs.

Abnormal expression of miR-331 leads to impaired heart function

MicroRNAs (miRNAs) play important roles in maintaining normal heart function. Abnormal expression of miR-331 has been observed in the hearts of patients with atrial fibrillation and Marfan syndrome. However, whether miR-331 regulates cardiac function under physiological and pathological conditions still remains unknown. In the present study, we investigated the function and underlying mechanisms of miR-331 in a pressure overload-induced heart failure model and miR-331 transgenic rat model. First, we found that the expression of miR-331-3p exhibited a 1.7-fold increase in hypertrophy compared with that in the sham group (P<0.01), yet the expression of miR-331-5p remained unchanged. Furthermore, overexpression of miR-331 in cardiomyocytes and defective excitation-contraction (E-C) coupling efficiency were observed. Luciferase assays showed that miR-331-3p suppressed JPH2 expression by binding to the coding region of JPH2 mRNA. Finally, in the miR-331 transgenic rat model, JPH2 expression was suppressed at both the mRNA and protein levels in vivo, which resulted in impairment of both the E-C coupling efficiency of cardiomyocytes and systolic function of the heart. This finding mechanistically linked miR-331 to JPH2 downregulation and suggested an important role for the abnormal expression of miR-331 leading to the dysfunction of E-C coupling in heart failure.