Partial and complete loss of myosin binding protein H-like cause cardiac conduction defects.

A premature truncation of MYBPHL in humans and a loss of Mybphl in mice is associated with dilated cardiomyopathy, atrial and ventricular arrhythmias, and atrial enlargement. MYBPHL encodes myosin binding protein H-like (MyBP-HL). Prior work in mice indirectly identified Mybphl expression in the atria and in small puncta throughout the ventricle. Because of its genetic association with human and mouse cardiac conduction system disease, we evaluated the anatomical localization of MyBP-HL and the consequences of loss of MyBP-HL on conduction system function. Immunofluorescence microscopy of normal adult mouse ventricles identified MyBP-HL-positive ventricular cardiomyocytes that co-localized with the ventricular conduction system marker contactin-2 near the atrioventricular node and in a subset of Purkinje fibers. Mybphl heterozygous ventricles had a marked reduction of MyBP-HL-positive cells compared to controls. Lightsheet microscopy of normal perinatal day 5 mouse hearts showed enrichment of MyBP-HL-positive cells within and immediately adjacent to the contactin-2-positive ventricular conduction system, but this association was not apparent in Mybphl heterozygous hearts. Surface telemetry of Mybphl-null mice revealed atrioventricular block and atrial bigeminy, while intracardiac pacing revealed a shorter atrial relative refractory period and atrial tachycardia. Calcium transient analysis of isolated Mybphl-null atrial cardiomyocytes demonstrated an increased heterogeneity of calcium release and faster rates of calcium release compared to wild type controls. Super-resolution microscopy of Mybphl heterozygous and homozygous null atrial cardiomyocytes showed ryanodine receptor disorganization compared to wild type controls. Abnormal calcium release, shorter atrial refractory period, and atrial dilation seen in Mybphl null, but not wild type control hearts, agree with the observed atrial arrhythmias, bigeminy, and atrial tachycardia, whereas the proximity of MyBP-HL-positive cells with the ventricular conduction system provides insight into how a predominantly atrial expressed gene contributes to ventricular arrhythmias and ventricular dysfunction.

Role of t-tubule remodeling on mechanisms of abnormal calcium release during heart failure development in canine ventricle.

The goal of this work was to investigate the role of t-tubule (TT) remodeling in abnormal Ca2+ cycling in ventricular myocytes of failing dog hearts. Heart failure (HF) was induced using rapid right ventricular pacing. Extensive changes in echocardiographic parameters, including left and right ventricular dilation and systolic dysfunction, diastolic dysfunction, elevated left ventricular filling pressures, and abnormal cardiac mechanics, indicated that severe HF developed. TT loss was extensive when measured as the density of total cell volume, derived from three-dimensional confocal image analysis, and significantly increased the distances in the cell interior to closest cell membrane. Changes in Ca2+ transients indicated increases in heterogeneity of Ca2+ release along the cell length. When critical properties of Ca2+ release variability were plotted as a function of TT organization, there was a complex, nonlinear relationship between impaired calcium release and decreasing TT organization below a certain threshold of TT organization leading to increased sensitivity in Ca2+ release below a TT density threshold of 1.5%. The loss of TTs was also associated with a greater incidence of triggered Ca2+ waves during rapid pacing. Finally, virtually all of these observations were replicated by acute detubulation by formamide treatment, indicating an important role of TT remodeling in impaired Ca2+ cycling. We conclude that TT remodeling itself is a major contributor to abnormal Ca2+ cycling in HF, reducing myocardial performance. The loss of TTs is also responsible for a greater incidence of triggered Ca2+ waves that may play a role in ventricular arrhythmias arising in HF.NEW & NOTEWORTHY Three-dimensional analysis of t-tubule density showed t-tubule disruption throughout the whole myocyte in failing dog ventricle. A double-linear relationship between Ca2+ release and t-tubule density displays a steeper slope at t-tubule densities below a threshold value (∼1.5%) above which there is little effect on Ca2+ release (T-tubule reserve). T-tubule loss increases incidence of triggered Ca2+ waves. Chemically induced t-tubule disruption suggests that t-tubule loss alone is a critical component of abnormal Ca2+ cycling in heart failure.

Triggered intracellular calcium waves in dog and human left atrial myocytes from normal and failing hearts.

AIMS: Abnormal intracellular Ca2+ cycling contributes to triggered activity and arrhythmias in the heart. We investigated the properties and underlying mechanisms for systolic triggered Ca2+ waves in left atria from normal and failing dog hearts. METHODS AND RESULTS: Intracellular Ca2+ cycling was studied using confocal microscopy during rapid pacing of atrial myocytes (36 °C) isolated from normal and failing canine hearts (ventricular tachypacing model). In normal atrial myocytes (NAMs), Ca2+ waves developed during rapid pacing at rates ≥ 3.3 Hz and immediately disappeared upon cessation of pacing despite high sarcoplasmic reticulum (SR) load. In heart failure atrial myocytes (HFAMs), triggered Ca2+ waves (TCWs) developed at a higher incidence at slower rates. Because of their timing, TCW development relies upon action potential (AP)-evoked Ca2+ entry. The distribution of Ca2+ wave latencies indicated two populations of waves, with early events representing TCWs and late events representing conventional spontaneous Ca2+ waves. Latency analysis also demonstrated that TCWs arise after junctional Ca2+ release has occurred and spread to non-junctional (cell core) SR. TCWs also occurred in intact dog atrium and in myocytes from humans and pigs. ß-adrenergic stimulation increased Ca2+ release and abolished TCWs in NAMs but was ineffective in HFAMs making this a potentially effective adaptive mechanism in normals but potentially arrhythmogenic in HF. Block of Ca-calmodulin kinase II also abolished TCWs, suggesting a role in TCW formation. Pharmacological manoeuvres that increased Ca2+ release suppressed TCWs as did interventions that decreased Ca2+ release but these also severely reduced excitation-contraction coupling. CONCLUSION: TCWs develop during the atrial AP and thus could affect AP duration, producing repolarization gradients and creating a substrate for reentry, particularly in HF where they develop at slower rates and a higher incidence. TCWs may represent a mechanism for the initiation of atrial fibrillation particularly in HF.

Destination therapy: the new gold standard treatment for heart failure patients with left ventricular assist devices.

Heart failure continues to be a growing health problem, eluding large-scale improvement and treatment. Cardiac transplantation has been the gold standard treatment with high post-transplant survival rates and relatively good quality of life. However, there has been an extreme shortage of organ donations, limiting transplants to only a very small portion of patients with the condition. This led to a growing interest in alternative options for the increasing population of patients who are waitlisted or ineligible for transplantation. In recent years, ventricular assist device (VAD) technologies have advanced from pulsatile blood pumps to continuous-flow pumps that have demonstrated unprecedented post-implantation survival rates. The HeartMate II, the only commercially available, continuous flow left ventricular assist device (LVAD) in the United States and Europe, has been implanted in over 10,000 patients worldwide, setting a benchmark for biomedical modalities of advanced heart failure treatment. Thanks to the successes of contemporary LVADs, patients are able to enjoy a better lifestyle, with a significantly prolonged life span and the ability to regularly partake in physical activities. In this new biomedical generation, the usage of LVADs has begun to expand towards the treatment for a wider range of heart conditions, including earlier stages of heart failure. In fact, LVAD implantations have surpassed the number of transplants taken place annually. An increasing number of patients are considering the permanent, circulatory support with an LVAD, namely destination therapy, as a promising option for treating heart failure.