Early- and delayed-afterdepolarizations as cellular promoter of ventricular fibrillation in hypertrophic cardiomyopathy

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): In Silico project

Background

Hypertrophic cardiomyopathy (HCM) represents the commonest inherited cardiac disease, with a prevalence of 1/500 in the general population. The most devastating consequence of HCM is sudden cardiac death (SCD) due to ventricular fibrillation, particularly common in children and young adults. The positive correlation between the extent of late gadolinium enhancement (LGE, reflecting myocardial fibrosis) and the arrhythmic risk suggests that ventricular arrhythmias are held to originate from the fibrotic regions, by a mechanism of electrical re-entry. However, recent data suggest that enhanced cellular automaticity (i.e.  early- or delayed-afterdepolarizations, EADs or DADs-) may be clinically more relevant in promoting ventricular arrhythmias in patients.

Purpose

Aiming to better understand the cellular and molecular mechanisms of arrhythmogenesis in HCM and to establish a reliable arrhythmic risk stratification in patients, we performed a translational and retrospective study in 61 HCM patients who underwent surgical myectomy, by combining a clinical follow-up study with in vitro assessments of cellular arrhythmogenicity.

Methods

We retrospectively studied 61 HCM patients who underwent surgical interventricular-septum myectomy to relieve refractory obstruction-related symptoms. At the time of surgery, fresh ventricular tissue was collected and used to isolate single ventricular cardiomyocytes (CMs), which were used for patch-clamp measurements and Ca2+ imaging experiments to assess the occurrence of EADs and DADs. Patients were followed up for a median time of 8 years and the occurrence of non-sustained ventricular tachycardia (NSVT) or life-threatening arrhythmic events (LAE, including sustained VT and ventricular fibrillation, VF) was monitored. Moreover, data from ECG and cardiac magnetic-resonance studies were collected.

Results

EADs occurred in CMs from 36% of patients and were associated with prolonged action potential duration while DADs occurred in 24% of patients and correlated with abnormalities of intracellular Ca2+ handling. During follow up, NSVT events occurred in 19/61 patients while LAE occurred in 4/61 patients. Their combined occurrence (37%) strongly correlated with the presence of DADs in cardiomyocytes. Patients with NSVT/LAE were more likely to show specific "pro-arrhythmic" pathological ECG-patterns. Among patients with LGE, the presence of DADs in cells behaved as a necessary pre-requisite for NSVT/LAE, as none of the patients with evidence of fibrosis but negative for DADs had arrhythmic events.

Conclusions

The presence of pro-arrhythmic changes appears to be necessary for arrhythmia generation in HCM and seems to be related with specific alterations at ECG level, that might be used as clinical arrhythmia predictors in HCM patients. Fibrosis per se is not a major predictor of arrhythmias in HCM but may contribute to generate sustained arrhythmias in the presence of substantial cellular triggers (DADs).

Electrophysiological remodeling in primary versus secondary cardiac hypertrophy: a study in human cardiomyocytes

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministero dell'università e della ricerca

Electrophysiological remodeling in primary versus secondary cardiac hypertrophy: a study in human cardiomyocytes

Introduction

Myocardial hypertrophy is an adaptive condition to hemodynamic stress, characterized by the increase in size of the cardiomyocytes and in the cardiac mass. It involves more frequently the left ventricle (LV) and leads to an enhanced risk of arrhythmias; it’s characterized by the increase walls thickness and by a large remodelling of the cardiac tissue in terms of fibrosis, impaired calcium handling and ion current balance. The LV hypertrophy could be induced by genetic abnormalities, such as in hypertrophic cardiomyopathy (HCM) or it could be secondary to other pathological conditions, like aortic stenosis (SAO-HT).

Purpose

In this work we will compare the mechanisms that mainly determine primitive (HCM samples) or secondary (to aortic stenosis: SAO-HT) LV hypertrophy to assess common points and disagreements.

Methods

Human septal specimens from HCM, SAO-HT and control (CTRL) surgical patients were collected, tissue was in part rapidly frozen, in part processed to isolate responsive single cells or trabeculae. We performed patch clamp experiments to measured action potential duration (APD) and calcium current (ICaL). To study the Ca2+ handling we performed imaging analysis; isolated cardiomyocytes were loaded with a Ca2+ sensitive dye (CAL530) by incubating cells for 30 min, then we evaluate the calcium transients at different stimulation frequencies.

Results

We evaluated APD, ICaL, and Ca2+ handling in the 3 groups: HCM, SAO-HT and CTRL cardiomyocytes. Both pathological groups show to be depolarized, to have a prolonged APD, diastolic Ca2+ concentration is higher and the kinetics are slower than in CTRL cardiomyocytes. HCM cardiomyocytes has longer APD than SAO-HT and higher diastolic Ca2+ concentration, HCM also shows longer kinetics of the decay phase of the Ca2+ transient compare to SAO-HT. The ICaL is comparable between CTRL and SAO-HT, it is higher in HCM and the inactivation kinetics are slower.

Discussion and Conclusion

We evaluate the electrophysiological differences among cardiomyocytes derived from a secondary or primary myocardial hypertrophy and from non-hypertrophic myocardia. Alterations in the two groups of pathological samples are similar. Both shows slowed Ca2+ transient kinetics, that slow down the relaxation machinery, and increase diastolic Ca2+ concentration, that could cause arrhythmic events (DAD). The increase in diastolic Ca2+ could also be responsible for the hyperactivation of the CAMK-II that has among its target ICaL channel and the Na2+ channel that are hyperactivated causing the prolongation of APD. The functional alterations observed in the hypertrophic cardiomyocytes could be considered not as disease specific response but a mechanism of adaptation to the hypertrophy and to diastolic dysfunction, those are more serious where the hypertrophy is genetic determined.

Extracellular stiffness as a determinant of cardiac dysfunction in duchenne muscular distrophy: a study on human iPSC derived cardiomyocytes

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): Fondazione Intesa San Paolo

Introduction

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive degeneration of striated muscles; in addition to skeletal muscle impairment, DMD is also characterized by progressive myocardial disfunction. The low translational value of animal models and the low availability of human samples make DMD hard to investigate; induced pluripotent stem cells (iPSCs) represent a novel tool to model this disease, preserving the genetic heritage of the patient, including the pathogenic mutation causing dystrophy.

Aim

Our aim is to characterize cardiomyocytes differentiated from iPSCs (iPSC-CMs) derived from healthy donors (CTRL) and DMD patients, to identify the pathophysiological mechanisms of DMD-related cardiomyopathy.

Materials and Methods

Cardiomyocytes are differentiated from IPSCs obtained by reprogramming isolated mononucleated blood cells from healthy donors and DMD patients. IPSC-CMs are cultured until day 60, 75 or 90 post-differentiation after plating on nanostructured substrates with two different stiffness levels: PEG-substrates, with lower rigidity, mimicking healthy extracellular tissue, and DEG-substrates, with greater rigidity, that mimic the presence of myocardial fibrosis. Through imaging techniques, we evaluated calcium handling and action potentials (AP) on DMD and CTRL iPSC-CMs by using specific fluorescent dyes for Ca2+ (CAL630) and membrane voltage (Fluovolt). Cells were stimulated at different pacing rates.

Results

The calcium transient amplitude of CTRL-iPSC-CMs became larger during maturation. This adaptation did not occur in DMD lines, showing a deficit calcium release due to poor maturation of the sarcoplasmic reticulum (SR). AP duration was shorter in the DMD line at d75 but at d90 we observed no differences when compared with the CTRL line. CTRL iPSC-CMs showed a marked ability to adapt to different substrate stiffnesses. Indeed, the calcium transient amplitude was larger and its kinetics faster when cells were grown on the rigid DEG substrates rather than on PEG plates. In the DMD line, however, no differences were observed between the substrates.

Conclusions

Our results highlight a scarce ability of DMD iPSC-CM to adapt to different substrate stiffness, resulting in mechanical and electrical impairment, especially in the presence of stiffer substrates. This might explain why cardiac impairment is usually absent in the early stages of DMD, when cardiac structural changes are still absent. However, the electrophysiological and mechanical impairment of DMD hearts may precipitate rapidly when extracellular stiffness starts to increase due to development of cardiac fibrosis.

Autopsy and genetic characterization of juvenile sudden cardiac arrest and death: the ToRSADE experience

Funding Acknowledgements

Type of funding sources: Public Institution(s). Main funding source(s): FAS-Salute 2014, regione Toscana

Background

Sudden cardiac arrest (SCA) or death (SCD) in young people represents a dramatic event, often leading to severe neurologic outcomes or death. However, the incidence of this phenomenon remains largely unknown, since the recording of cases, and consequently the identification of the underlying causes, requires a multi-professional and disciplinary approach, including genetic counselling.

Purpose

In this study, we aimed to monitor and investigate SCA and SCD in young people (≤50 years) recorded in the ToRSADE© registry comparing clinical and molecular data.

Methods and Results

A total of 22 blood samples were analyzed; 14 were collected from dead patients during autopsy and 8 from resuscitated patients after cardiac arrest. Next Generation Sequencing (NGS) analysis revealed 38% of total cases with Likely Pathogenetic (LP) variants associated to Cardiomyopathy (CM) or Channelopathy, and 61% with Variant of Uncertain Significance (VUS). In three cases, NGS confirmed autopsy and histology findings: the p.(Leu466Phe) variant in SCN5A associated with Brugada Syndrome (1a), the p.(Glu173del) in TNNT2 for Hypertrophic CM (1b) and the p. (Asn480Lysfs*20) in PKP2 for Arrhythmogenic Cardiomyopathy (AC) (1c).

Conclusions

Creation of the ToRSADE© registry allowed implementation of a blood repository for molecular and genetic analysis. Genetic analysis combined with clinical information and post-mortem evaluation constitute a multi-disciplinary approach to juvenile SCD and SCA, while providing medical and genetic assistance to families, public awareness especially among youths and athletes, as well as up-to-date research on the underlying mechanisms of cardiomyopathies.

Optogenetic manipulation of cardiac electrical dynamics using sub-threshold illumination: dissecting the role of cardiac alternans in terminating rapid rhythms

Funding Acknowledgements

Type of funding sources: None.

Cardiac action potential (AP) shape and propagation are regulated by several key dynamic factors such as ions channel recovery and intracellular Ca2+-cycling. Experimental methods for manipulating AP electrical dynamics commonly use ion channel inhibitors that lack spatial and temporal specificity. In this work, we propose a novel approach based on optogenetics to manipulate cardiac electrical activity employing a light-modulated depolarizing current with intensities that are too low to elicit APs (sub-threshold illumination) but are sufficient to fine-tune AP electrical dynamics. We investigated the effects of sub-threshold illumination in isolated cardiomyocytes and whole hearts by using transgenic mice constitutively expressing a light-gated ion channel (channelrhodopsins-2, ChR2). We find that ChR2-mediated depolarizing current prolongs APs and reduces conduction velocity (CV) in a space-selective and reversible manner. Sub-threshold manipulation also affects the dynamics of cardiac electrical activity, increasing the magnitude of cardiac alternans. We use an optical system that uses real-time feedback control to generate re-entrant circuits with user-defined cycle lengths to explore the role that cardiac alternans plays in spontaneous termination of ventricular tachycardias (VTs). We find that VT stability significantly decreases during sub-threshold illumination primarily due to an increase in the amplitude of electrical oscillations, which implies that cardiac alternans may be beneficial in the context of self-termination of VT.

Cellular determinants of arrhythmic rysk in hypertrophic cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM) is the commonest inherited cardiac disease, with a prevalence of 1/500 in the general population. The most devastating consequence of HCM is sudden cardiac death (SCD) due to ventricular fibrillation, particularly common in children and young adults (age <30 years). The positive correlation between the extent of late gadolinium enhancement (LGE, reflecting myocardial fibrosis) and the arrhythmic risk in HCM suggests that ventricular arrhythmias are held to originate from the fibrotic regions, by a mechanism of electrical re-entry. However, recent data suggest that enhanced cellular automaticity (i.e. early- or delayed-afterdepolarizations, EADs or DADs-), rather than macro-reentry, may be clinically relevant in promoting ventricular arrhythmias in patients.

Purpose

Aiming to better understand the cellular and molecular mechanisms of arrhythmogenesis in HCM and to establish a reliable arrhythmic risk stratification in patients, we performed a translational study in HCM patients who underwent surgical myectomy, by combining a clinical follow-up study with in vitro assessments of cellular arrhythmogenicity in ventricular cardiomyocytes.

Methods

We retrospectively studied 61 HCM patients who underwent surgical interventricular-septum myectomy to relieve refractory obstruction-related symptoms. At the time of surgery, fresh ventricular tissue was collected and used to isolate single ventricular cardiomyocytes (CMs), which were used for patch-clamp measurements to assess the occurrence of EADs and DADs. Patients were followed up for a median time of 8 years and the occurrence of non-sustained ventricular tachycardia (NSVT) or life-threatening arrhythmic events (LAE) was monitored. Moreover, data from ECG and contract cardiac magnetic-resonance studies were collected.

Results

EADs occurred in CMs from 36% of patients and were associated with prolonged action potential duration. DADs occurred in 24% of patients and were associated with abnormalities of CM intracellular Ca2+ handling. The occurrence of NSVT/ LAE in patients was strongly associated with the presence of DADs in cardiomyocytes but not with the presence of EADs. Patients with NSVT/LAE were more likely to show specific “pro-arrhythmic” pathological ECG-patterns. Among patients with LGE, the presence of DADs in cells behaved as a necessary pre-requisite for NSVT/LAE, as none of the patients with evidence of fibrosis who were negative for DADs had arrhythmic events.

Conclusions

The presence of pro-arrhythmic changes appears to be necessary for arrhythmia generation in HCM and seems to be related with specific alterations at ECG level, that might be used as clinical arrhythmia predictors in HCM patients. Fibrosis per se is not a major predictor of arrhythmias in HCM but may contribute to generate sustained arrhythmias in the presence of substantial cellular triggers (DADs).

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): EU Horizon 2020. Grant number 777204 (silico FCM).

Characterization and modelling of the abnormal electrophysiological response under beta-adrenergic stimulation in hypertrophic cardiomyopathy

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): British Heart Foundation

Introduction

Hypertrophic Cardiomyopathy (HCM) is the most common inheritable heart pathology and the main cause of sudden cardiac death in young adults. HCM patients often present an enhanced arrhythmogenicity that can lead to lethal arrhythmias, especially during exercise. Recent studies have shown an abnormal response of HCM myocytes to β-adrenergic stimulation (β-ARS), with prolongation of their action potential duration (APD). The mechanisms underlying this aberrant response to sympathetic stimulation remain unknown.

Purpose

To investigate the key ionic mechanisms underlying the HCM abnormal response to β-ARS using human-based experimental and computational methodologies.

Methods

Experimental ionic currents, action potential and calcium transient were recorded in human adult cardiomyocytes from control and HCM patients. Isoproterenol (10-7 mol/L) was used to elicit β-ARS. Whole-cell ruptured patch voltage clamp experiments were conducted to characterise L-type calcium and potassium currents, with recordings performed before and after 3 min of drug exposure. The latest models of human ventricular electrophysiology and beta-adrenergic receptor signalling were integrated and calibrated using the human measured data. Simulations under isoproterenol were performed to quantify the effects of β-ARS on the action potential and calcium transient. The role of the main ion currents affected by β-ARS and by HCM remodelling was evaluated.

Results

In vitro, isoproterenol shortened APD (-16 ± 3%) in control, while prolonging APD in HCM myocytes (+23 ± 8%). Analysis of the measured data indicated two possible mechanisms contributing to APD prolongation in HCM myocytes. Firstly, a protracted L-type calcium current, presenting slower inactivation kinetics in HCM compared to control. The relative increase of potassium currents under β-ARS was also lower in HCM myocytes. The developed in silico models of β-ARS replicated the behaviour observed in the experimental data, based on slower L-type calcium current inactivation kinetics and a smaller increase of potassium currents in HCM. In absence of β-ARS, simulated HCM cardiomyocytes exhibited prolonged APD compared to control (525 ± 88 vs 281 ± 56 ms, p < 0.001). Under β-ARS, APD in control was reduced (-16.46%), whereas APD was prolonged in HCM (+11.63%). Further analysis showed that the reduction of the potassium currents increment under β-ARS was the main cause of the APD prolongation in HCM myocytes, with L-type calcium inactivation minimally contributing to APD prolongation.

Conclusions

In this study we assessed the effects of β-ARS on ion currents and APD in control and HCM myocytes. Our modelling results suggest that the increase of potassium repolarising currents under β-ARS is greatly reduced in HCM cardiomyocytes, being the main mechanism underlying their APD prolongation. This APD prolongation may have severe consequences in HCM patients, increasing the risk of exercise-induced arrhythmias. Abstract Figure.

Arrhythmia susceptibility in a rat model of acute atrial dilation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, associated with an increased risk of stroke and heart failure. Acute AF occurs in response to sudden increases of atrial hemodynamic load, leading to atrial stretch. The mechanisms of stretch-induced AF were investigated in large mammals with controversial results. We optimized an approach to monitor rat atrial electrical activity using a red-shifted voltage sensitive dye (VSD). The methodology includes cauterization of the main ventricular coronary arteries, allowing improved atrial staining by the VSD and appropriate atrial perfusion for long experiments. Next, we developed a rat model of acute biatrial dilation (ABD) through the insertion of latex balloons into both atria, which could be inflated with controlled volumes. A chronic model of atrial dilation (spontaneous hypertensive rats; SHR) was used for comparison. ABD was performed on atria from healthy Wistar-Kyoto (WKY) rats (WKY-ABD). The atria were characterized in terms of arrhythmias susceptibility, action potential duration and conduction velocity. The occurrence of arrhythmias in WKY-ABD was significantly higher compared to non-dilated WKY atria. In WKY-ABD we found a reduction of conduction velocity, similar to that observed in SHR atria, while action potential duration was unchanged. Low-dose caffeine was used to introduce a drop of CV in WKY atria (WKY-caff), quantitatively similar to the one observed after ABD, but no increased arrhythmia susceptibility was observed with caffeine only. In conclusion, CV decrease is not sufficient to promote arrhythmias; enlargement of atrial surface is essential to create a substrate for acute reentry-based arrhythmias.

P1243Comparison of long-term clinical course and outcome of MYBPC3 - versus MYH7 - related hypertrophic cardiomyopathy

Introduction

The presence of sarcomere mutations is a powerful predictor of heart failure-related outcomes in Hypertrophic Cardiomyopathy (HCM). However, whether the prevalence of left ventricular (LV) dysfunction differs in patients with mutations in the two most prevalent HCM-associated genes (i.e. MYBPC3 and MYH7) is unclear.

Purpose

To ascertain lifetime trends in prevalence of LV dysfunction in HCM associated with pathogenic or likely-pathogenic MYBPC3 versus MYH7 mutations.

Methods

Clinical and instrumental records of 402 HCM patients with MYBPC3 (N=251) or MYH7 (N=151) mutations were retrospectively reviewed. Presence of systolic dysfunction (ejection fraction [EF] <50%) and diastolic dysfunction (Grade II and III) were assessed for each patient. In vitro analysis of septal myectomy samples was performed to further compare electro-mechanic properties of MYBC3 and MYH7 patients.

Results

Patients were diagnosed at a mean age of 39±17 years and 63% were men. At first evaluation MYBPC3-HCM patients were less frequently obstructive (15% vs 26% in MYH7; p=0.005) and had lower LVEF (61±11% vs 64±9%; p=0.01). Prevalence of diastolic dysfunction increased with age and was lowest in MYBPC3 patients <40 years at diagnosis (19.5% vs 35.4% in MYH7, p=0.043). At a mean follow-up (FU) of 13±11 years, patients developed comparable left atrium enlargement (MYBPC3 52±29 ml/m2 vs 41±18 at baseline, p<0.001; MYH7 54±25ml/m2 vs 45±22, p=0.003). Prevalence of diastolic dysfunction was also similar. MYBPC3 patients had lower LVEF at final evaluation (61±11% vs 64±9% in MYH7, p=0.01) with greater prevalence of overt systolic dysfunction (EF<50%, MYBPC3 vs MYH7: 15% vs 5%, OR: 2.3 95% CI: 1.2–5.8, p=0.013).

No significant differences were observed in terms of NYHA class change, atrial fibrillation, stroke, heart failure, appropriate ICD intervention or cardiovascular death. However, prevalence of NSVT was higher for MYBPC3 (39% vs 14% in MYH7, p<0.0001). At Cox multivariable analysis independent predictors of systolic dysfunction at follow-up were MYBPC3 positive status (HR 2.53 95% CI: 1.09–5.82, p=0.029) and age at initial evaluation (HR 1.03 95% CI 1.00–1.06, p=0.027).

In vitro cross-sectional evaluation of myocardial samples taken during septal myectomy at different ages showed a decline in contraction-relaxation properties after age 40 in MYPBC3 carriers, but preserved function in MYH7 patients (Figure).

Kinetic of myosin cross-bridges

Conclusions

In HCM patients, mutations in the MYBPC3 gene and early diagnosis are associated with slowly progressing systolic impairment leading to overt dysfunction in 15% compared to 5% in MYH7-HCM. However, outcome was similar in the two subsets. These differences in lifetime myocardial performance between the two most common HCM-associated genes suggest diverse pathways of disease progression, potentially amenable to requiring different molecular approaches.

Real-time optical manipulation of cardiac conduction in intact hearts

Key points

Although optogenetics has clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies lack the capability to react acutely to ongoing cardiac wave dynamics.

Here, we developed an all‐optical platform to monitor and control electrical activity in real‐time.

The methodology was applied to restore normal electrical activity after atrioventricular block and to manipulate the intraventricular propagation of the electrical wavefront.

The closed‐loop approach was also applied to simulate a re‐entrant circuit across the ventricle.

The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time all‐optical stimulation can control cardiac rhythm in normal and abnormal conditions.

Abstract

Optogenetics has provided new insights in cardiovascular research, leading to new methods for cardiac pacing, resynchronization therapy and cardioversion. Although these interventions have clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies do not take into account cardiac wave dynamics in real time. Here, we developed an all‐optical platform complemented by integrated, newly developed software to monitor and control electrical activity in intact mouse hearts. The system combined a wide‐field mesoscope with a digital projector for optogenetic activation. Cardiac functionality could be manipulated either in free‐run mode with submillisecond temporal resolution or in a closed‐loop fashion: a tailored hardware and software platform allowed real‐time intervention capable of reacting within 2 ms. The methodology was applied to restore normal electrical activity after atrioventricular block, by triggering the ventricle in response to optically mapped atrial activity with appropriate timing. Real‐time intraventricular manipulation of the propagating electrical wavefront was also demonstrated, opening the prospect for real‐time resynchronization therapy and cardiac defibrillation. Furthermore, the closed‐loop approach was applied to simulate a re‐entrant circuit across the ventricle demonstrating the capability of our system to manipulate heart conduction with high versatility even in arrhythmogenic conditions. The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time optically based stimulation can control cardiac rhythm in normal and abnormal conditions, promising a new approach for the investigation of the (patho)physiology of the heart.

Although optogenetics has clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies lack the capability to react acutely to ongoing cardiac wave dynamics.

Here, we developed an all‐optical platform to monitor and control electrical activity in real‐time.

The methodology was applied to restore normal electrical activity after atrioventricular block and to manipulate the intraventricular propagation of the electrical wavefront.

The closed‐loop approach was also applied to simulate a re‐entrant circuit across the ventricle.

The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time all‐optical stimulation can control cardiac rhythm in normal and abnormal conditions.

Novel insights on the relationship between T-tubular defects and contractile dysfunction in a mouse model of hypertrophic cardiomyopathy

Abnormalities of cardiomyocyte Ca^2 + homeostasis and excitation–contraction (E–C) coupling are early events in the pathogenesis of hypertrophic cardiomyopathy (HCM) and concomitant determinants of the diastolic dysfunction and arrhythmias typical of the disease. T-tubule remodelling has been reported to occur in HCM but little is known about its role in the E–C coupling alterations of HCM. Here, the role of T-tubule remodelling in the electro-mechanical dysfunction associated to HCM is investigated in the Δ160E cTnT mouse model that expresses a clinically-relevant HCM mutation. Contractile function of intact ventricular trabeculae is assessed in Δ160E mice and wild-type siblings. As compared with wild-type, Δ160E trabeculae show prolonged kinetics of force development and relaxation, blunted force-frequency response with reduced active tension at high stimulation frequency, and increased occurrence of spontaneous contractions. Consistently, prolonged Ca^2 + transient in terms of rise and duration are also observed in Δ160E trabeculae and isolated cardiomyocytes. Confocal imaging in cells isolated from Δ160E mice reveals significant, though modest, remodelling of T-tubular architecture. A two-photon random access microscope is employed to dissect the spatio-temporal relationship between T-tubular electrical activity and local Ca^2 + release in isolated cardiomyocytes. In Δ160E cardiomyocytes, a significant number of T-tubules (> 20%) fails to propagate action potentials, with consequent delay of local Ca^2 + release. At variance with wild-type, we also observe significantly increased variability of local Ca^2 + transient rise as well as higher Ca^2 +-spark frequency. Although T-tubule structural remodelling in Δ160E myocytes is modest, T-tubule functional defects determine non-homogeneous Ca^2 + release and delayed myofilament activation that significantly contribute to mechanical dysfunction.

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Contraction and Ca^2 + transient kinetics are impaired in myocardial preparations from mice carrying the cardiac troponin T ∆ 160E mutation.

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T-tubules architecture is mildly altered in ∆160E cardiomyocytes.

•

20% of T-tubules fail to propagate action potential and produce delay of local Ca^2 + rise.

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Higher spatio-temporal variability of local Ca^2 + rise and increased Ca^2 + sparks frequency are found in ∆160E cardiomyocytes.

The transverse-axial tubular system of cardiomyocytes

A characteristic histological feature of striated muscle cells is the presence of deep invaginations of the plasma membrane (sarcolemma), most commonly referred to as T-tubules or the transverse-axial tubular system (TATS). TATS mediates the rapid spread of the electrical signal (action potential) to the cell core triggering Ca(2+) release from the sarcoplasmic reticulum, ultimately inducing myofilament contraction (excitation-contraction coupling). T-tubules, first described in vertebrate skeletal muscle cells, have also been recognized for a long time in mammalian cardiac ventricular myocytes, with a structure and a function that in recent years have been shown to be far more complex and pivotal for cardiac function than initially thought. Renewed interest in T-tubule function stems from the loss and disorganization of T-tubules found in a number of pathological conditions including human heart failure (HF) and dilated and hypertrophic cardiomyopathies, as well as in animal models of HF, chronic ischemia and atrial fibrillation. Disease-related remodeling of the TATS leads to asynchronous and inhomogeneous Ca(2+)-release, due to the presence of orphan ryanodine receptors that have lost their coupling with the dihydropyridine receptors and are either not activated or activated with a delay. Here, we review the physiology of the TATS, focusing first on the relationship between function and structure, and then describing T-tubular remodeling and its reversal in disease settings and following effective therapeutic approaches.

Neutral glycolipids and gangliosides of concanavalin A-selected SV3T3 revertant cells and of normal and SV40-transformed Balb/c 3T3 cells

The structural analysis of neutral glycolipids and gangliosides of the SV40 transformed Balb/c3T3 cells (SV3T3 cells) and concanavalin A-selected SV3T3 revertant cells, both compared with untransformed Balb/c3T3 cells, has shown: (i) a content of neutral glycolipids in revertant cells near to that found in the untransformed parental cells; (ii) a similar decrease of the higher gangliosides in transformed and revertant cells; (iii) a content of ganglioside GM3 in revertant cells much higher than that found in both SV3T3 and untransformed Balb/3T3 cells. The possible role of ganglioside GM3 in growth control is discussed.