Slower calcium handling balances faster cross-bridge cycling in human MYBPC3 HCM

Background: The pathogenesis of MYBPC3-associated hypertrophic cardiomyopathy (HCM) is still unresolved. In our HCM patient cohort, a large and well-characterized population carrying the MYBPC3:c772G>A variant (p.Glu258Lys, E258K) provides the unique opportunity to study the basic mechanisms of MYBPC3-HCM with a comprehensive translational approach. Methods: We collected clinical and genetic data from 93 HCM patients carrying the MYBPC3:c772G>A variant. Functional perturbations were investigated using different biophysical techniques in left ventricular samples from 4 patients who underwent myectomy for refractory outflow obstruction, compared with samples from non-failing non-hypertrophic surgical patients and healthy donors. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and engineered heart tissues (EHTs) were also investigated. Results: Haplotype analysis revealed MYBPC3:c772G>A as a founder mutation in Tuscany. In ventricular myocardium, the mutation leads to reduced cMyBP-C (cardiac myosin binding protein-C) expression, supporting haploinsufficiency as the main primary disease mechanism. Mechanical studies in single myofibrils and permeabilized muscle strips highlighted faster cross-bridge cycling, and higher energy cost of tension generation. A novel approach based on tissue clearing and advanced optical microscopy supported the idea that the sarcomere energetics dysfunction is intrinsically related with the reduction in cMyBP-C. Studies in single cardiomyocytes (native and hiPSC-derived), intact trabeculae and hiPSC-EHTs revealed prolonged action potentials, slower Ca2+ transients and preserved twitch duration, suggesting that the slower excitation-contraction coupling counterbalanced the faster sarcomere kinetics. This conclusion was strengthened by in silico simulations. Conclusions: HCM-related MYBPC3:c772G>A mutation invariably impairs sarcomere energetics and cross-bridge cycling. Compensatory electrophysiological changes (eg, reduced potassium channel expression) appear to preserve twitch contraction parameters, but may expose patients to greater arrhythmic propensity and disease progression. Therapeutic approaches correcting the primary sarcomeric defects may prevent secondary cardiomyocyte remodeling

Optical Investigation of Action Potential and Calcium Handling Maturation of hiPSC-Cardiomyocytes on Biomimetic Substrates

Cardiomyocytes from human induced pluripotent stem cells (hiPSC-CMs) are the most promising human source with preserved genetic background of healthy individuals or patients. This study aimed to establish a systematic procedure for exploring development of hiPSC-CM functional output to predict genetic cardiomyopathy outcomes and identify molecular targets for therapy. Biomimetic substrates with microtopography and physiological stiffness can overcome the immaturity of hiPSC-CM function. We have developed a custom-made apparatus for simultaneous optical measurements of hiPSC-CM action potential and calcium transients to correlate these parameters at specific time points (day 60, 75 and 90 post differentiation) and under inotropic interventions. In later-stages, single hiPSC-CMs revealed prolonged action potential duration, increased calcium transient amplitude and shorter duration that closely resembled those of human adult cardiomyocytes from fresh ventricular tissue of patients. Thus, the major contribution of sarcoplasmic reticulum and positive inotropic response to β-adrenergic stimulation are time-dependent events underlying excitation contraction coupling (ECC) maturation of hiPSC-CM; biomimetic substrates can promote calcium-handling regulation towards adult-like kinetics. Simultaneous optical recordings of long-term cultured hiPSC-CMs on biomimetic substrates favor high-throughput electrophysiological analysis aimed at testing (mechanistic hypothesis on) disease progression and pharmacological interventions in patient-derived hiPSC-CMs

R4496C RyR2 mutation impairs atrial and ventricular contractility

Ryanodine receptor (RyR2) is the major Ca(2+) channel of the cardiac sarcoplasmic reticulum (SR) and plays a crucial role in the generation of myocardial force. Changes in RyR2 gating properties and resulting increases in its open probability (Po) are associated with Ca(2+) leakage from the SR and arrhythmias; however, the effects of RyR2 dysfunction on myocardial contractility are unknown. Here, we investigated the possibility that a RyR2 mutation associated with catecholaminergic polymorphic ventricular tachycardia, R4496C, affects the contractile function of atrial and ventricular myocardium. We measured isometric twitch tension in left ventricular and atrial trabeculae from wild-type mice and heterozygous transgenic mice carrying the R4496C RyR2 mutation and found that twitch force was comparable under baseline conditions (30°C, 2 mM [Ca(2+)]o, 1 Hz). However, the positive inotropic responses to high stimulation frequency, 0.1 µM isoproterenol, and 5 mM [Ca(2+)]o were decreased in R4496C trabeculae, as was post-rest potentiation. We investigated the mechanisms underlying inotropic insufficiency in R4496C muscles in single ventricular myocytes. Under baseline conditions, the amplitude of the Ca(2+) transient was normal, despite the reduced SR Ca(2+) content. Under inotropic challenge, however, R4496C myocytes were unable to boost the amplitude of Ca(2+) transients because they are incapable of properly increasing the amount of Ca(2+) stored in the SR because of a larger SR Ca(2+) leakage. Recovery of force in response to premature stimuli was faster in R4496C myocardium, despite the unchanged rates of recovery of L-type Ca(2+) channel current (ICa-L) and SR Ca(2+) content in single myocytes. A faster recovery from inactivation of the mutant R4496C channels could explain this behavior. In conclusion, changes in RyR2 channel gating associated with the R4496C mutation could be directly responsible for the alterations in both ventricular and atrial contractility. The increased RyR2 Po and fractional Ca(2+) release from the SR induced by the R4496C mutation preserves baseline contractility despite a slight decrease in SR Ca(2+) content, but cannot compensate for the inability to increase SR Ca(2+) content during inotropic challenge

Cell and Myofibril Contractile Properties of hiPSC-Derived Cardiomyocytes from a Patient with a MYH7 Mutation Associated with Familial Cardiomyopathy

Myosin heavy chain 7 (MYH7) mutations are associated with familial cardiomyopathies (FCM) and result in a high rate of sudden cardiac death. Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have recently shown promise as a model for studying FCM. We identified a cohort with familial cardiomyopathy (FCM) associated with a MYH7 mutation (E848G) and middle-age onset of systolic dysfunction and arrhythmias. hiPSC-CMs from patient affected (FCM-CMs) and non-affected (WT-CMs) individuals were generated from skin fibroblasts. Here we report, for the first time, contractile properties of isolated myofibrils from these cultured hiPSC-CMs for comparison using cultured cells and 3D engineered cardiac tissue (3D-ECT) constructs. Isolated myofibrils were obtained from differentiation day 20 hiPSC-CMs that were replated onto fibronectin-coated nanopatterned cover slides and matured in culture for an additional 60 days to obtain elongated and aligned myofibrils. This procedure produced hiPSC-CMs that were usually > 100+ µm in length. hiPSC- FCM-CMs and WT-CMs were harvested and skinned in a rigor solution containing 1% Triton and contractile properties of single or small bundles of myofibrils were measured in a custom built apparatus with rapid solution switching capabilities. During maximal calcium activation FCM-CM myofibrils produced approximately half the amount of force of WT-CM myofibrils, but preliminary data suggests no differences in the kinetics of force development or relaxation. This compares well with 50 day cardiomyocytes plated on nano-patterned surfaces or seeded into 3D-ECT constructs, where shortening and force (respectively) of FCM-CMs was much less than for WT-CMs, with no difference in calcium transient amplitudes. We speculate this early stage contractile deficit may contribute to disease development and conclude hiPSC-FCM-CMs can be a viable model for mechanical studies of cardiomyopathies in vitro

MYBPC3-c.772G>A mutation results in haploinsufficiency and altered myosin cycling kinetics in a patient induced stem cell derived cardiomyocyte model of hypertrophic cardiomyopathy

Approximately 40% of hypertrophic cardiomyopathy (HCM) mutations are linked to the sarcomere protein cardiac myosin binding protein-C (cMyBP-C). These mutations are either classified as missense mutations or truncation mutations. One mutation whose nature has been inconsistently reported in the literature is the MYBPC3-c.772G > A mutation. Using patient-derived human induced pluripotent stem cells differentiated to cardiomyocytes (hiPSC-CMs), we have performed a mechanistic study of the structure-function relationship for this MYBPC3-c.772G > A mutation versus a mutation corrected, isogenic cell line. Our results confirm that this mutation leads to exon skipping and mRNA truncation that ultimately suggests ∼20% less cMyBP-C protein (i.e., haploinsufficiency). This, in turn, results in increased myosin recruitment and accelerated myofibril cycling kinetics. Our mechanistic studies suggest that faster ADP release from myosin is a primary cause of accelerated myofibril cross-bridge cycling due to this mutation. Additionally, the reduction in force generating heads expected from faster ADP release during isometric contractions is outweighed by a cMyBP-C phosphorylation mediated increase in myosin recruitment that leads to a net increase of myofibril force, primarily at submaximal calcium activations. These results match well with our previous report on contractile properties from myectomy samples of the patients from whom the hiPSC-CMs were generated, demonstrating that these cell lines are a good model to study this pathological mutation and extends our understanding of the mechanisms of altered contractile properties of this HCM MYBPC3-c.772G > A mutation