Sheet-Like Remodeling of the Transverse Tubular System in Human Heart Failure Impairs Excitation-Contraction Coupling and Functional Recovery by Mechanical Unloading

Remodelling of T-Tubules and Associated Calcium Handling Dysfunction in Heart Failure: Mechanisms and Therapeutic Insights

In cardiomyocytes, transverse tubules (T-tubules) are sarcolemmal invaginations that facilitate excitation-contraction coupling and diastolic function. The clinical significance of T-tubules has become evident in that their remodelling is recognised as a hallmark feature of heart failure (HF) and a key contributor to disrupted Ca2+ homeostasis, compromised cardiac function, and arrhythmogenesis. Further investigations have revealed that T-tubule remodelling is particularly pronounced in HF with reduced ejection fraction (HFrEF), but not in HF with preserved ejection fraction, implying that T-tubule remodelling may play a crucial pathophysiologic role in HFrEF. While research on the functional importance of T-tubules is ongoing, T-tubule remodelling has been found to be reversible. That finding has triggered a surge in studies aimed at identifying specific therapeutic approaches for HFrEF. This review discusses the functional importance of T-tubules and their microdomains, the pathophysiology of T-tubule remodelling, and the potential mechanisms of current HFrEF therapeutic approaches in reversing T-tubule alterations. We also highlight discrepancies regarding the roles of T-tubule proteins in the recovery process across studies to offer valuable insights for future research.

Nanoscale organization of cardiac calcium channels is dependent on thyroid hormone status

Thyroid hormone dysfunction is frequently observed in patients with chronic illnesses including heart failure, which increases the risk of adverse events. This study examined the effects of thyroid hormones (THs) on cardiac transverse-tubule (TT) integrity, Ca2+ sparks, and nanoscale organization of ion channels in excitation-contraction (EC) coupling, including L-type calcium channel (CaV1.2), ryanodine receptor type 2 (RyR2), and junctophilin-2 (Jph2). TH deficiency was established in adult female rats by propyl-thiouracil (PTU) ingestion for 8 wk; followed by randomization to continued PTU without or with oral triiodo-l-thyronine (T3; 10 µg/kg/day) for an additional 2 wk (PTU + T3). Confocal microscopy of isolated cardiomyocytes (CMs) showed significant misalignment of TTs and increased Ca2+ sparks in thyroid-deficient CMs. Density-based spatial clustering of applications with noise (DBSCAN) analysis of stochastic optical reconstruction microscopy (STORM) images showed decreased (P < 0.0001) RyR2 cluster number per cell area in PTU CMs compared with euthyroid (EU) control myocytes, and this was normalized by T3 treatment. CaV1.2 channels and Jph2 localized within a 210 nm radius of the RyR2 clusters were significantly reduced in PTU myocytes, and these values were increased with T3 treatment. A significant percentage of the RyR2 clusters in the PTU myocytes had neither CaV1.2 nor Jph2, suggesting fewer functional clusters in EC coupling. Nearest neighbor distances between RyR2 clusters were greater (P < 0.001) in PTU cells compared with EU- and T3-treated CMs that correspond to disarray of TTs at the sarcomere z-discs. These results support a regulatory role of T3 in the nanoscale organization of RyR2 clusters and colocalization of CaV1.2 and Jph2 in optimizing EC coupling.NEW & NOTEWORTHY Thyroid hormone (TH) dysfunction exacerbates preexisting heart conditions leading to an increased risk of premature morbidity/mortality. Triiodo-l-thyronine (T3) optimizes cardiac excitation-contraction (EC) coupling by maintaining myocardial T-tubule (TT) structures and organization of calcium ion channels. Single-molecule localization microscopy shows T3 effects on the clustering of ryanodine receptors (RyR2) with colocalization of L-type calcium channels (CaV1.2) and junctophilin-2 (Jph2) at TT-SR structures. Heart disease with subclinical hypothyroidism/low T3 syndrome may benefit from TH treatment.

Using a Failing Human Ventricular Cardiomyocyte Model to Re-Evaluate Ca2+ Cycling, Voltage Dependence, and Spark Characteristics

Previous studies have observed alterations in excitation-contraction (EC) coupling during end-stage heart failure that include action potential and calcium (Ca2+) transient prolongation and a reduction of the Ca2+ transient amplitude. Underlying these phenomena are the downregulation of potassium (K+) currents, downregulation of the sarcoplasmic reticulum Ca2+ ATPase (SERCA), increase Ca2+ sensitivity of the ryanodine receptor, and the upregulation of the sodium-calcium (Na=-Ca2+) exchanger. However, in human heart failure (HF), debate continues about the relative contributions of the changes in calcium handling vs. the changes in the membrane currents. To understand the consequences of the above changes, they are incorporated into a computational human ventricular myocyte HF model that can explore the contributions of the spontaneous Ca2+ release from the sarcoplasmic reticulum (SR). The reduction of transient outward K+ current (Ito) is the main membrane current contributor to the decrease in RyR2 open probability and L-type calcium channel (LCC) density which emphasizes its importance to phase 1 of the action potential (AP) shape and duration (APD). During current-clamp conditions, RyR2 hyperphosphorylation exhibits the least amount of Ca2+ release from the SR into the cytosol and SR Ca2+ fractional release during a dynamic slow-rapid-slow (0.5-2.5-0.5 Hz) pacing, but it displays the most abundant and more lasting Ca2+ sparks two-fold longer than a normal cell. On the other hand, under voltage-clamp conditions, HF by decreased SERCA and upregulated INCX show the least SR Ca2+ uptake and EC coupling gain, as compared to HF by hyperphosphorylated RyR2s. Overall, this study demonstrates that the (a) combined effect of SERCA and NCX, and the (b) RyR2 dysfunction, along with the downregulation of the cardiomyocyte's potassium currents, could substantially contribute to Ca2+ mishandling at the spark level that leads to heart failure.

Restoring Atrial T-Tubules Augments Systolic Ca Upon Recovery From Heart Failure

BACKGROUND

Transverse (t)-tubules drive the rapid and synchronous Ca2+ rise in cardiac myocytes. The virtual complete atrial t-tubule loss in heart failure (HF) decreases Ca2+ release. It is unknown if or how atrial t-tubules can be restored and how this affects systolic Ca2+.

METHODS

HF was induced in sheep by rapid ventricular pacing and recovered following termination of rapid pacing. Serial block-face scanning electron microscopy and confocal imaging were used to study t-tubule ultrastructure. Function was assessed using patch clamp, Ca2+, and confocal imaging. Candidate proteins involved in atrial t-tubule recovery were identified by western blot and expressed in rat neonatal ventricular myocytes to determine if they altered t-tubule structure.

RESULTS

Atrial t-tubules were lost in HF but reappeared following recovery from HF. Recovered t-tubules were disordered, adopting distinct morphologies with increased t-tubule length and branching. T-tubule disorder was associated with mitochondrial disorder. Recovered t-tubules were functional, triggering Ca2+ release in the cell interior. Systolic Ca2+, ICa-L, sarcoplasmic reticulum Ca2+ content, and sarcoendoplasmic reticulum Ca2+ ATPase function were restored following recovery from HF. Confocal microscopy showed fragmentation of ryanodine receptor staining and movement away from the z-line in HF, which was reversed following recovery from HF. Acute detubulation, to remove recovered t-tubules, confirmed their key role in restoration of the systolic Ca2+ transient, the rate of Ca2+ removal, and the peak L-type Ca2+ current. The abundance of telethonin and myotubularin decreased during HF and increased during recovery. Transfection with these proteins altered the density and structure of tubules in neonatal myocytes. Myotubularin had a greater effect, increasing tubule length and branching, replicating that seen in the recovery atria.

CONCLUSIONS

We show that recovery from HF restores atrial t-tubules, and this promotes recovery of ICa-L, sarcoplasmic reticulum Ca2+ content, and systolic Ca2+. We demonstrate an important role for myotubularin in t-tubule restoration. Our findings reveal a new and viable therapeutic strategy.

Quantitative 3D electron microscopy characterization of mitochondrial structure, mitophagy, and organelle interactions in murine atrial fibrillation

Atrial fibrillation (AF) is the most common clinical arrhythmia, however there is limited understanding of its pathophysiology including the cellular and ultrastructural changes rendered by the irregular rhythm, which limits pharmacological therapy development. Prior work has demonstrated the importance of reactive oxygen species (ROS) and mitochondrial dysfunction in the development of AF. Mitochondrial structure, interactions with other organelles such as sarcoplasmic reticulum (SR) and T-tubules (TT), and degradation of dysfunctional mitochondria via mitophagy are important processes to understand ultrastructural changes due to AF. However, most analysis of mitochondrial structure and interactome in AF has been limited to two-dimensional (2D) modalities such as transmission electron microscopy (EM), which does not fully visualize the morphological evolution of the mitochondria during mitophagy. Herein, we utilize focused ion beam-scanning electron microscopy (FIB-SEM) and perform reconstruction of three-dimensional (3D) EM from murine left atrial samples and measure the interactions of mitochondria with SR and TT. We developed a novel 3D quantitative analysis of FIB-SEM in a murine model of AF to quantify mitophagy stage, mitophagosome size in cardiomyocytes, and mitochondrial structural remodeling when compared with control mice. We show that in our murine model of spontaneous and continuous AF due to persistent late sodium current, left atrial cardiomyocytes have heterogenous mitochondria, with a significant number which are enlarged with increased elongation and structural complexity. Mitophagosomes in AF cardiomyocytes are located at Z-lines where they neighbor large, elongated mitochondria. Mitochondria in AF cardiomyocytes show increased organelle interaction, with 5X greater contact area with SR and are 4X as likely to interact with TT when compared to control. We show that mitophagy in AF cardiomyocytes involves 2.5X larger mitophagosomes that carry increased organelle contents. In conclusion, when oxidative stress overcomes compensatory mechanisms, mitophagy in AF faces a challenge of degrading bulky complex mitochondria, which may result in increased SR and TT contacts, perhaps allowing for mitochondrial Ca2+ maintenance and antioxidant production.

Learnings from the 2024 Utah Cardiac Recovery Symposium: A Roadmap for the Field of Myocardial Recovery

The 12th annual Utah Cardiac Recovery Symposium (U-CARS) in 2024 continued its mission to advance cardiac recovery by uniting experts across various fields. The symposium featured key presentations on cutting-edge topics such as CRISPR gene editing for heart failure, guideline-directed medical therapy for heart failure (HF) with improved/recovered ejection fraction (HFimpEF), the role of extracorporeal cardiopulmonary resuscitation (ECPR) in treating cardiac arrest, and others. Discussions explored genetic and metabolic contributions to HF, emphasized the importance of maintaining pharmacotherapy in HFimpEF to prevent relapse, and identified future research directions including refining ECPR protocols, optimizing patient selection, and leveraging genetic insights to enhance therapeutic strategies.

Overexpression of ATP5F1A in Cardiomyocytes Promotes Cardiac Reverse Remodeling

BACKGROUND

The mechanism of cardiac reverse remodeling (CRR) mediated by the left ventricular assist device remains unclear. This study aims to identify the specific cell type responsible for CRR and develop the therapeutic target that promotes CRR.

METHODS

The nuclei were extracted from the left ventricular tissue of 4 normal controls, 4 CRR patients, and 4 no cardiac reverse remodeling patients and then subjected to single-nucleus RNA sequencing for identifying key cell types responsible for CRR. Gene overexpression in transverse aortic constriction and dilated cardiomyopathy heart failure mouse model (C57BL/6J background) and pathological staining were performed to validate the results of single-nucleus RNA sequencing.

RESULTS

Ten cell types were identified among 126 156 nuclei. Cardiomyocytes in CRR patients expressed higher levels of ATP5F1A than the other 2 groups. The macrophages in CRR patients expressed more anti-inflammatory genes and functioned in angiogenesis. Endothelial cells that elevated in no cardiac reverse remodeling patients were involved in the inflammatory response. Echocardiography showed that overexpressing ATP5F1A through cardiomyocyte-specific adeno-associated virus 9 demonstrated an ability to improve heart function and morphology. Pathological staining showed that overexpressing ATP5F1A could reduce fibrosis and cardiomyocyte size in the heart failure mouse model.

CONCLUSIONS

The present results of single-nucleus RNA sequencing and heart failure mouse model indicated that ATP5F1A could mediate CRR and supported the development of therapeutics for overexpressing ATP5F1A in promoting CRR.

Integrating molecular and clinical variables to predict myocardial recovery

Mechanical unloading and circulatory support with left ventricular assist devices (LVADs) mediate significant myocardial improvement in a subset of advanced heart failure (HF) patients. The clinical and biological phenomena associated with cardiac recovery are under intensive investigation. Left ventricular (LV) apical tissue, alongside clinical data, were collected from HF patients at the time of LVAD implantation (n=208). RNA was isolated and mRNA transcripts were identified through RNA sequencing and confirmed with RT-qPCR. To our knowledge this is the first study to combine transcriptomic and clinical data to derive predictors of myocardial recovery. We used a bioinformatic approach to integrate 59 clinical variables and 22,373 mRNA transcripts at the time of LVAD implantation for the prediction of post-LVAD myocardial recovery defined as LV ejection fraction (LVEF) ≥40% and LV end-diastolic diameter (LVEDD) ≤5.9cm, as well as functional and structural LV improvement independently by using LVEF and LVEDD as continuous variables, respectively. To substantiate the predicted variables, we used a multi-model approach with logistic and linear regressions. Combining RNA and clinical data resulted in a gradient boosted model with 80 features achieving an AUC of 0.731±0.15 for predicting myocardial recovery. Variables associated with myocardial recovery from a clinical standpoint included HF duration, pre-LVAD LVEF, LVEDD, and HF pharmacologic therapy, and LRRN4CL (ligand binding and programmed cell death) from a biological standpoint. Our findings could have diagnostic, prognostic, and therapeutic implications for advanced HF patients, and inform the care of the broader HF population.

Restoration of calcium release synchrony: A novel target for heart failure and ventricular arrhythmia

Myocardial calcium (Ca2+) signaling plays a crucial role in contractile function and membrane electrophysiology. An abnormal myocardial Ca2+ transient is linked to heart failure and ventricular arrhythmias. At the subcellular level, the synchronous release of Ca2+ sparks from sarcoplasmic Ca2+ release units determines the configuration and amplitude of the global Ca2+ transient. This narrative review evaluates the role of aberrant Ca2+ release synchrony in the pathophysiology of cardiomyopathies and ventricular arrhythmias. The potential therapeutic benefits of restoration of Ca2+ release synchrony in heart failure and ventricular arrhythmias are also discussed.

Contractility measurements for cardiotoxicity screening with ventricular myocardial slices of pigs

AIMS

Cardiotoxicity is one major reason why drugs do not enter or are withdrawn from the market. Thus, approaches are required to predict cardiotoxicity with high specificity and sensitivity. Ideally, such methods should be performed within intact cardiac tissue with high relevance for humans and detect acute and chronic side effects on electrophysiological behaviour, contractility, and tissue structure in an unbiased manner. Herein, we evaluate healthy pig myocardial slices and biomimetic cultivation setups (BMCS) as a new cardiotoxicity screening approach.

METHODS AND RESULTS

Pig left ventricular samples were cut into slices and spanned into BMCS with continuous electrical pacing and online force recording. Automated stimulation protocols were established to determine the force-frequency relationship (FFR), frequency dependence of contraction duration, effective refractory period (ERP), and pacing threshold. Slices generated 1.3 ± 0.14 mN/mm2 force at 0.5 Hz electrical pacing and showed a positive FFR and a shortening of contraction duration with increasing pacing rates. Approximately 62% of slices were able to contract for at least 6 days while showing stable ERP, contraction duration-frequency relationship, and preserved cardiac structure confirmed by confocal imaging and X-ray diffraction analysis. We used specific blockers of the most important cardiac ion channels to determine which analysis parameters are influenced. To validate our approach, we tested five drug candidates selected from the Comprehensive in vitro Proarrhythmia Assay list as well as acetylsalicylic acid and DMSO as controls in a blinded manner in three independent laboratories. We were able to detect all arrhythmic drugs and their respective mode of action on cardiac tissue including inhibition of Na+, Ca2+, and hERG channels as well as Na+/Ca2+ exchanger.

CONCLUSION

We systematically evaluate this approach for cardiotoxicity screening, which is of high relevance for humans and can be upscaled to medium-throughput screening. Thus, our approach will improve the predictive value and efficiency of preclinical cardiotoxicity screening.

The ryanodine receptor microdomain in cardiomyocytes

The ryanodine receptor type 2 (RyR) is a key player in Ca2+ handling during excitation-contraction coupling. During each heartbeat, RyR channels are responsible for linking the action potential with the contractile machinery of the cardiomyocyte by releasing Ca2+ from the sarcoplasmic reticulum. RyR function is fine-tuned by associated signalling molecules, arrangement in clusters and subcellular localization. These parameters together define RyR function within microdomains and are subject to disease remodelling. This review describes the latest findings on RyR microdomain organization, the alterations with disease which result in increased subcellular heterogeneity and emergence of microdomains with enhanced arrhythmogenic potential, and presents novel technologies that guide future research to study and target RyR channels within specific microdomains.

Ryanodine Receptor Staining Identifies Viable Cardiomyocytes in Human and Rabbit Cardiac Tissue Slices

In terms of preserving multicellularity and myocardial function in vitro, the cultivation of beating myocardial slices is an emerging technique in basic and translational cardiac research. It can be used, for example, for drug screening or to study pathomechanisms. Here, we describe staining for viable cardiomyocytes based on the immunofluorescence of ryanodine receptors (RyRs) in human and rabbit myocardial slices. Biomimetic chambers were used for culture and measurements of contractile force. Fixable fluorophore-conjugated dextran, entering cells with a permeable membrane, was used for death staining. RyRs, nuclei and the extracellular matrix, including the t-system, were additionally stained and analyzed by confocal microscopy and image processing. We found the mutual exclusion of the RyR and dextran signals in cultivated slices. T-System density and nucleus size were reduced in RyR-negative/dextran-positive myocytes. The fraction of RyR-positive myocytes and pixels correlated with the contractile force. In RyR-positive/dextran-positive myocytes, we found irregular RyR clusters and SERCA distribution patterns, confirmed by an altered power spectrum. We conclude that RyR immunofluorescence indicates viable cardiomyocytes in vibratome-cut myocardial slices, facilitating the detection and differential structural analysis of living vs. dead or dying myocytes. We suggest the loss of sarcoplasmic reticulum integrity as an early event during cardiomyocyte death.

Risk Prediction Models and Novel Prognostic Factors for Heart Failure with Preserved Ejection Fraction: A Systematic and Comprehensive Review

BACKGROUND

Patients with heart failure with preserved ejection fraction (HFpEF) have large individual differences, unclear risk stratification, and imperfect treatment plans. Risk prediction models are helpful for the dynamic assessment of patients' prognostic risk and early intensive therapy of high-risk patients. The purpose of this study is to systematically summarize the existing risk prediction models and novel prognostic factors for HFpEF, to provide a reference for the construction of convenient and efficient HFpEF risk prediction models.

METHODS

Studies on risk prediction models and prognostic factors for HFpEF were systematically searched in relevant databases including PubMed and Embase. The retrieval time was from inception to February 1, 2023. The Quality in Prognosis Studies (QUIPS) tool was used to assess the risk of bias in included studies. The predictive value of risk prediction models for end outcomes was evaluated by sensitivity, specificity, the area under the curve, C-statistic, C-index, etc. In the literature screening process, potential novel prognostic factors with high value were explored.

RESULTS

A total of 21 eligible HFpEF risk prediction models and 22 relevant studies were included. Except for 2 studies with a high risk of bias and 2 studies with a moderate risk of bias, other studies that proposed risk prediction models had a low risk of bias overall. Potential novel prognostic factors for HFpEF were classified and described in terms of demographic characteristics (age, sex, and race), lifestyle (physical activity, body mass index, weight change, and smoking history), laboratory tests (biomarkers), physical inspection (blood pressure, electrocardiogram, imaging examination), and comorbidities.

CONCLUSION

It is of great significance to explore the potential novel prognostic factors of HFpEF and build a more convenient and efficient risk prediction model for improving the overall prognosis of patients. This review can provide a substantial reference for further research.

The interplay between cardiac dyads and mitochondria regulated the calcium handling in cardiomyocytes

Calcium mishandling and mitochondrial dysfunction have been increasingly recognized as significant factors involved in the progression procedure of cardiomyopathy. Ca2+ mishandling could cause calcium-triggered arrhythmias, which could enhance force development and ATP consumption. Mitochondrial disorganization and dysfunction in cardiomyopathy could disturb the balance of energy catabolic and anabolic procedure. Close spatial localization and arrangement of structural among T-tubule, sarcoplasmic reticulum, mitochondria are important for Ca2+ handling. So that, we illustrate the regulating network between calcium handling and mitochondrial homeostasis, as well as its intracellular mechanisms in this review, which would be worthy to develop novel therapeutic strategy and restore the function of injured cardiomyocytes.

Direct proteomic and high-resolution microscopy biopsy analysis identifies distinct ventricular fates in severe aortic stenosis

The incidence of aortic valve stenosis (AS), the most common reason for aortic valve replacement (AVR), increases with population ageing. While untreated AS is associated with high mortality, different hemodynamic subtypes range from normal left-ventricular function to severe heart failure. However, the molecular nature underlying four different AS subclasses, suggesting vastly different myocardial fates, is unknown. Here, we used direct proteomic analysis of small left-ventricular biopsies to identify unique protein expression profiles and subtype-specific AS mechanisms. Left-ventricular endomyocardial biopsies were harvested from patients during transcatheter AVR, and inclusion criteria were based on echocardiographic diagnosis of severe AS and guideline-defined AS-subtype classification: 1) normal ejection fraction (EF)/high-gradient; 2) low EF/high-gradient; 3) low EF/low-gradient; and 4) paradoxical low-flow/low-gradient AS. Samples from non-failing donor hearts served as control. We analyzed 25 individual left-ventricular biopsies by data-independent acquisition mass spectrometry (DIA-MS), and 26 biopsies by histomorphology and cardiomyocytes by STimulated Emission Depletion (STED) superresolution microscopy. Notably, DIA-MS reliably detected 2273 proteins throughout each individual left-ventricular biopsy, of which 160 proteins showed significant abundance changes between AS-subtype and non-failing samples including the cardiac ryanodine receptor (RyR2). Hierarchical clustering segregated unique proteotypes that identified three hemodynamic AS-subtypes. Additionally, distinct proteotypes were linked with AS-subtype specific differences in cardiomyocyte hypertrophy. Furthermore, superresolution microscopy of immunolabeled biopsy sections showed subcellular RyR2-cluster fragmentation and disruption of the functionally important association with transverse tubules, which occurred specifically in patients with systolic dysfunction and may hence contribute to depressed left-ventricular function in AS.

Dyssynchronous Heart Failure: A Clinical Review

PURPOSE OF THE REVIEW

Dyssynchrony occurs when portions of the cardiac chambers contract in an uncoordinated fashion. Ventricular dyssynchrony primarily impacts the left ventricle and may result in heart failure. This entity is recognized as a major contributor to the development and progression of heart failure. A hallmark of dyssynchronous heart failure (HF_d) is left ventricular recovery after dyssynchrony is corrected. This review discusses the current understanding of pathophysiology of HF_d and provides clinical examples and current techniques for treatment.

RECENT FINDINGS

Data show that HF_d responds poorly to medical therapy. Cardiac resynchronization therapy (CRT) in the form of conventional biventricular pacing (BVP) is of proven benefit in HF_d, but is limited by a significant non-responder rate. Recently, conduction system pacing (His bundle or left bundle branch area pacing) has also shown promise in correcting HF_d. HF_d should be recognized as a distinct etiology of heart failure; HF_d responds best to CRT.

Serial block face scanning electron microscopy reveals region-dependent remodelling of transverse tubules post-myocardial infarction

The highly organized transverse tubule (t-tubule) network facilitates cardiac excitation-contraction coupling and synchronous cardiac myocyte contraction. In cardiac failure secondary to myocardial infarction (MI), changes in the structure and organization of t-tubules result in impaired cardiac contractility. However, there is still little knowledge on the regional variation of t-tubule remodelling in cardiac failure post-MI. Here, we investigate post-MI t-tubule remodelling in infarct border and remote regions, using serial block face scanning electron microscopy (SBF-SEM) applied to a translationally relevant sheep ischaemia reperfusion MI model and matched controls. We performed minimally invasive coronary angioplasty of the left anterior descending artery, followed by reperfusion after 90 min to establish the MI model. Left ventricular tissues obtained from control and MI hearts eight weeks post-MI were imaged using SBF-SEM. Image analysis generated three-dimensional reconstructions of the t-tubular network in control, MI border and remote regions. Quantitative analysis revealed that the MI border region was characterized by t-tubule depletion and fragmentation, dilation of surviving t-tubules and t-tubule elongation. This study highlights region-dependent remodelling of the tubular network post-MI and may provide novel localized therapeutic targets aimed at preservation or restoration of the t-tubules to manage cardiac contractility post-MI. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Tubulator: an automated approach to analysis of t-tubule and dyadic organization in cardiomyocytes

During cardiac disease, t-tubules and dyads are remodelled and disrupted within cardiomyocytes, thereby reducing cardiac performance. Given the pathological implications of such dyadic remodelling, robust and versatile tools for characterizing these sub-cellular structures are needed. While analysis programs for continuous and regular structures such as rodent ventricular t-tubules are available, at least in two dimensions, these approaches are less appropriate for assessment of more irregular structures, such as dyadic proteins and non-rodent t-tubules. Here, we demonstrate versatile, easy-to-use software that performs such analyses. This software, called Tubulator, enables automated analysis of t-tubules and dyadic proteins alike, in both tissue sections and isolated myocytes. The program measures densities of subcellular structures and proteins in individual cells, quantifies their distribution into transversely and longitudinally oriented elements, and supports detailed co-localization analyses. Importantly, Tubulator provides tools for three-dimensional assessment and rendering of image stacks, extending examinations from the single plane to the whole-myocyte level. To provide insight into the consequences of dyadic organization for synchrony of Ca2+ handling, Tubulator also creates 'distance maps', by calculating the distance from all cytosolic positions to the nearest t-tubule and/or dyad. In conclusion, this freely accessible program provides detailed automated analysis of the three-dimensional nature of dyadic and t-tubular structures. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Successful conservative management of left ventricular assist device candidates

AIMS

Clinical trials comparing LVADs vs. conservative therapy were performed before the availability of novel medications or used suboptimal medical therapy. This study aimed to report that long-term stabilization of patients entering a left ventricular assist device (LVAD) programme is possible with the use of aggressive conservative therapy. This is important because the excellent clinical stabilization provided by LVADs comes at the expense of significant complications.

METHODS AND RESULTS

This study was a single-centre prospective evaluation of consecutive patients with advanced heart failure (HF) fulfilling criteria for LVAD implantation based on clinical and echocardiographic characteristics, cardiopulmonary exercise test, and right heart catheterization results. Their initial therapy included inotropes, thiamine, beta-blockers, digoxin, spironolactone, hydralazine, and nitrates followed by the introduction of novel HF therapies. Coronary revascularization and cardiac resynchronization therapy were performed when indicated, and all patients were closely followed at our outpatient clinic. During the study period, 28 patients were considered suitable for LVAD implantation (mean age 63 ± 10.8 years, 92% men, 78% ischaemic, median HF duration 4 years). Clinical stabilization was achieved and maintained in 21 patients (median follow-up 20 months, range 9-38 months). Compared with baseline evaluation, cardiac index increased from 2.05 (1.73-2.28) to 2.88 (2.63-3.55) L/min/m² , left ventricular end-diastolic diameter decreased from 65.5 (62.4-66) to 58.3 (53.8-62.5) mm, and maximal oxygen consumption increased from 10.1 (9.2-11.3) to 16.1 (15.3-19) mL/kg/min. Three patients died and only four ultimately required LVAD implantation.

CONCLUSIONS

Notwithstanding the small size of our cohort, our results suggest that LVAD implantation could be safely deferred in the majority of LVAD candidates.

Modeling Calcium Cycling in the Heart: Progress, Pitfalls, and Challenges

Intracellular calcium (Ca) cycling in the heart plays key roles in excitation-contraction coupling and arrhythmogenesis. In cardiac myocytes, the Ca release channels, i.e., the ryanodine receptors (RyRs), are clustered in the sarcoplasmic reticulum membrane, forming Ca release units (CRUs). The RyRs in a CRU act collectively to give rise to discrete Ca release events, called Ca sparks. A cell contains hundreds to thousands of CRUs, diffusively coupled via Ca to form a CRU network. A rich spectrum of spatiotemporal Ca dynamics is observed in cardiac myocytes, including Ca sparks, spark clusters, mini-waves, persistent whole-cell waves, and oscillations. Models of different temporal and spatial scales have been developed to investigate these dynamics. Due to the complexities of the CRU network and the spatiotemporal Ca dynamics, it is challenging to model the Ca cycling dynamics in the cardiac system, particularly at the tissue sales. In this article, we review the progress of modeling of Ca cycling in cardiac systems from single RyRs to the tissue scale, the pros and cons of the current models and different modeling approaches, and the challenges to be tackled in the future.

Biology of myocardial recovery in advanced heart failure with long-term mechanical support

Cardiac remodeling is an adaptive, compensatory biological process following an initial insult to the myocardium that gradually becomes maladaptive and causes clinical deterioration and chronic heart failure (HF). This biological process involves several pathophysiological adaptations at the genetic, molecular, cellular, and tissue levels. A growing body of clinical and translational investigations demonstrated that cardiac remodeling and chronic HF does not invariably result in a static, end-stage phenotype but can be at least partially reversed. One of the paradigms which shed some additional light on the breadth and limits of myocardial elasticity and plasticity is long term mechanical circulatory support (MCS) in advanced HF pediatric and adult patients. MCS by providing (a) ventricular mechanical unloading and (b) effective hemodynamic support to the periphery results in functional, structural, cellular and molecular changes, known as cardiac reverse remodeling. Herein, we analyze and synthesize the advances in our understanding of the biology of MCS-mediated reverse remodeling and myocardial recovery. The MCS investigational setting offers access to human tissue, providing an unparalleled opportunity in cardiovascular medicine to perform in-depth characterizations of myocardial biology and the associated molecular, cellular, and structural recovery signatures. These human tissue findings have triggered and effectively fueled a "bedside to bench and back" approach through a variety of knockout, inhibition or overexpression mechanistic investigations in vitro and in vivo using small animal models. These follow-up translational and basic science studies leveraging human tissue findings have unveiled mechanistic myocardial recovery pathways which are currently undergoing further testing for potential therapeutic drug development. Essentially, the field is advancing by extending the lessons learned from the MCS cardiac recovery investigational setting to develop therapies applicable to the greater, not end-stage, HF population. This review article focuses on the biological aspects of the MCS-mediated myocardial recovery and together with its companion review article, focused on the clinical aspects, they aim to provide a useful framework for clinicians and investigators.

Novel Targets for a Combination of Mechanical Unloading with Pharmacotherapy in Advanced Heart Failure

LVAD therapy is an effective rescue in acute and especially chronic cardiac failure. In several scenarios, it provides a platform for regeneration and sustained myocardial recovery. While unloading seems to be a key element, pharmacotherapy may provide powerful tools to enhance effective cardiac regeneration. The synergy between LVAD support and medical agents may ensure satisfying outcomes on cardiomyocyte recovery followed by improved quality and quantity of patient life. This review summarizes the previous and contemporary strategies for combining LVAD with pharmacotherapy and proposes new therapeutic targets. Regulation of metabolic pathways, enhancing mitochondrial biogenesis and function, immunomodulating treatment, and stem-cell therapies represent therapeutic areas that require further experimental and clinical studies on their effectiveness in combination with mechanical unloading.

LVAD as a Bridge to Remission from Advanced Heart Failure: Current Data and Opportunities for Improvement

Left ventricular assist devices (LVADs) are an established treatment modality for advanced heart failure (HF). It has been shown that through volume and pressure unloading they can lead to significant functional and structural cardiac improvement, allowing LVAD support withdrawal in a subset of patients. In the first part of this review, we discuss the historical background, current evidence on the incidence and assessment of LVAD-mediated cardiac recovery, and out-comes including quality of life after LVAD support withdrawal. In the second part, we discuss current and future opportunities to promote LVAD-mediated reverse remodeling and improve our pathophysiological understanding of HF and recovery for the benefit of the greater HF population.

Long-Term Cultivation of Human Atrial Myocardium

Organotypic culture of human ventricular myocardium is emerging in basic and translational cardiac research. However, few institutions have access to human ventricular tissue, whereas atrial tissue is more commonly available and important for studying atrial physiology. This study presents a method for long-term cultivation of beating human atrial myocardium. After written informed consent, tissues from the right-atrial appendage were obtained from patients with sinus rhythm undergoing open heart surgery with cardiopulmonary bypass. Trabeculae (pectinate muscles) prepared from the samples were installed into cultivation chambers at 37°C with a diastolic preload of 500 μN. After 2 days with 0.5 Hz pacing, stimulation frequency was set to 1 Hz. Contractile force was monitored continuously. Beta-adrenergic response, refractory period (RP) and maximum captured frequency (f_max) were assessed periodically. After cultivation, viability and electromechanical function were investigated, as well as the expression of several genes important for intracellular Ca²⁺ cycling and electrophysiology. Tissue microstructure was analyzed by confocal microscopy. We cultivated 19 constantly beating trabeculae from 8 patient samples for 12 days and 4 trabeculae from 3 specimen for 21 days. Functional parameters were compared directly after installation (0 d) with those after 12 d in culture. Contraction force was 384 ± 69 μN at 0 d and 255 ± 90 μN at 12 d (p = 0.8, n = 22), RP 480 ± 97 ms and 408 ± 78 ms (p = 0.3, n = 9), f_max 3.0 ± 0.5 Hz and 3.8 ± 0.5 Hz (p = 0.18, n = 9), respectively. Application of 100 nM isoprenaline to 11 trabeculae at 7 d increased contraction force from 168 ± 35 μN to 361 ± 60 μN (p < 0.01), f_max from 6.4 ± 0.6 Hz to 8.5 ± 0.4 Hz (p < 0.01) and lowered RP from 319 ± 22 ms to 223 ± 15 ms. CACNA1c (L-type Ca²⁺ channel subunit) and GJA1 (connexin-43) mRNA expressions were not significantly altered at 12 d vs 0 d, while ATP2A (SERCA) and KCNJ4 (Kir2.3) were downregulated, and KCNJ2 (Kir2.1) was upregulated. Simultaneous Ca²⁺ imaging and force recording showed preserved excitation-contraction coupling in cultivated trabeculae. Confocal microscopy indicated preserved cardiomyocyte structure, unaltered amounts of extracellular matrix and gap junctions. MTT assays confirmed viability at 12 d. We established a workflow that allows for stable cultivation and functional analysis of beating human atrial myocardium for up to 3 weeks. This method may lead to novel insights into the physiology and pathophysiology of human atrial myocardium.

Cardiac Transverse Tubules in Physiology and Heart Failure

In mammalian cardiac myocytes, the plasma membrane includes the surface sarcolemma but also a network of membrane invaginations called transverse (t-) tubules. These structures carry the action potential deep into the cell interior, allowing efficient triggering of Ca2+ release and initiation of contraction. Once thought to serve as rather static enablers of excitation-contraction coupling, recent work has provided a newfound appreciation of the plasticity of the t-tubule network's structure and function. Indeed, t-tubules are now understood to support dynamic regulation of the heartbeat across a range of timescales, during all stages of life, in both health and disease. This review article aims to summarize these concepts, with consideration given to emerging t-tubule regulators and their targeting in future therapies.

Nanoscale Organisation of Ryanodine Receptors and Junctophilin-2 in the Failing Human Heart

The disrupted organisation of the ryanodine receptors (RyR) and junctophilin (JPH) is thought to underpin the transverse tubule (t-tubule) remodelling in a failing heart. Here, we assessed the nanoscale organisation of these two key proteins in the failing human heart. Recently, an advanced feature of the t-tubule remodelling identified large flattened t-tubules called t-sheets, that were several microns wide. Previously, we reported that in the failing heart, the dilated t-tubules up to ~1 μm wide had increased collagen, and we hypothesised that the t-sheets would also be associated with collagen deposits. Direct stochastic optical reconstruction microscopy (dSTORM), confocal microscopy, and western blotting were used to evaluate the cellular distribution of excitation-contraction structures in the cardiac myocytes from patients with idiopathic dilated cardiomyopathy (IDCM) compared to myocytes from the non-failing (NF) human heart. The dSTORM imaging of RyR and JPH found no difference in the colocalisation between IDCM and NF myocytes, but there was a higher colocalisation at the t-tubule and sarcolemma compared to the corbular regions. Western blots revealed no change in the JPH expression but did identify a ~50% downregulation of RyR (p = 0.02). The dSTORM imaging revealed a trend for the smaller t-tubular RyR clusters (~24%) and reduced the t-tubular RyR cluster density (~35%) that resulted in a 50% reduction of t-tubular RyR tetramers in the IDCM myocytes (p < 0.01). Confocal microscopy identified the t-sheets in all the IDCM hearts examined and found that they are associated with the reticular collagen fibres within the lumen. However, the size and density of the RyR clusters were similar in the myocyte regions associated with t-sheets and t-tubules. T-tubule remodelling is associated with a reduced RyR expression that may contribute to the reduced excitation-contraction coupling in the failing human heart.

Circulating and Myocardial Cytokines Predict Cardiac Structural and Functional Improvement in Patients With Heart Failure Undergoing Mechanical Circulatory Support

Background Recent prospective multicenter data from patients with advanced heart failure demonstrated that left ventricular assist device (LVAD) support combined with standard heart failure medications, induced significant cardiac structural and functional improvement, leading to high rates of LVAD weaning in selected patients. We investigated whether preintervention myocardial and systemic inflammatory burden could help identify the subset of patients with advanced heart failure prone to LVAD-mediated cardiac improvement to guide patient selection, treatment, and monitoring. Methods and Results Ninety-three patients requiring durable LVAD were prospectively enrolled. Myocardial tissue and blood were acquired during LVAD implantation, for measurement of inflammatory markers. Cardiac structural and functional improvement was prospectively assessed via serial echocardiography. Eleven percent of the patients showed significant reverse remodeling following LVAD support (ie, responders). Circulating tumor necrosis factor alpha, interleukin (IL)-4, IL-5, IL-6, IL-7, IL-13, and interferon gamma were lower in responders, compared with nonresponders (P<0.05, all comparisons). The myocardial tissue signal transducer and activator of transcription-3, an inflammatory response regulator, was less activated in responders (P=0.037). Guided by our tissue studies and a multivariable dichotomous regression analysis, we identified that low levels of circulating interferon gamma (odds ratio [OR], 0.06; 95% CI, 0.01-0.35) and tumor necrosis factor alpha (OR, 0.05; 95% CI, 0.00-0.43), independently predict cardiac improvement, creating a 2-cytokine model effectively predicting responders (area under the curve, 0.903; P<0.0001). Conclusions Baseline myocardial and systemic inflammatory burden inversely correlates with cardiac improvement following LVAD support. A circulating 2-cytokine model predicting significant reverse remodeling was identified, warranting further investigation as a practical preintervention tool in identifying patients prone to LVAD-mediated cardiac improvement and device weaning.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Electrophysiological Remodeling: Cardiac T-Tubules and ß-Adrenoceptors

Beta-adrenoceptors (βAR) are often viewed as archetypal G-protein coupled receptors. Over the past fifteen years, investigations in cardiovascular biology have provided remarkable insights into this receptor family. These studies have shifted pharmacological dogma, from one which centralized the receptor to a new focus on structural micro-domains such as caveolae and t-tubules. Important studies have examined, separately, the structural compartmentation of ion channels and βAR. Despite links being assumed, relatively few studies have specifically examined the direct link between structural remodeling and electrical remodeling with a focus on βAR. In this review, we will examine the nature of receptor and ion channel dysfunction on a substrate of cardiomyocyte microdomain remodeling, as well as the likely ramifications for cardiac electrophysiology. We will then discuss the advances in methodologies in this area with a specific focus on super-resolution microscopy, fluorescent imaging, and new approaches involving microdomain specific, polymer-based agonists. The advent of powerful computational modelling approaches has allowed the science to shift from purely empirical work, and may allow future investigations based on prediction. Issues such as the cross-reactivity of receptors and cellular heterogeneity will also be discussed. Finally, we will speculate as to the potential developments within this field over the next ten years.

Dysferlin links excitation-contraction coupling to structure and maintenance of the cardiac transverse-axial tubule system

AIMS

The multi-C2 domain protein dysferlin localizes to the T-Tubule system of skeletal and heart muscles. In skeletal muscle, dysferlin is known to play a role in membrane repair and in T-tubule biogenesis and maintenance. Dysferlin deficiency manifests as muscular dystrophy of proximal and distal muscles. Cardiomyopathies have been also reported, and some dysferlinopathy mouse models develop cardiac dysfunction under stress. Generally, the role and functional relevance of dysferlin in the heart is not clear. The aim of this study was to analyse the effect of dysferlin deficiency on the transverse-axial tubule system (TATS) structure and on Ca2+ homeostasis in the heart.

METHODS AND RESULTS

We studied dysferlin localization in rat and mouse cardiomyocytes by immunofluorescence microscopy. In dysferlin-deficient ventricular mouse cardiomyocytes, we analysed the TATS by live staining and assessed Ca2+ handling by patch-clamp experiments and measurement of Ca2+ transients and Ca2+ sparks. We found increasing co-localization of dysferlin with the L-type Ca2+-channel during TATS development and show that dysferlin deficiency leads to pathological loss of transversal and increase in longitudinal elements (axialization). We detected reduced L-type Ca2+-current (ICa,L) in cardiomyocytes from dysferlin-deficient mice and increased frequency of spontaneous sarcoplasmic reticulum Ca2+ release events resulting in pro-arrhythmic contractions. Moreover, cardiomyocytes from dysferlin-deficient mice showed an impaired response to β-adrenergic receptor stimulation.

CONCLUSIONS

Dysferlin is required for TATS biogenesis and maintenance in the heart by controlling the ratio of transversal and axial membrane elements. Absence of dysferlin leads to defects in Ca2+ homeostasis which may contribute to contractile heart dysfunction in dysferlinopathy patients.

Triiodothyronine maintains cardiac transverse-tubule structure and function

Subclinical hypothyroidism and low T3 syndrome are commonly associated with an increased risk of cardiovascular disease (CVD) and mortality. We examined effects of T3 on T-tubule (TT) structures, Ca²⁺ mobilization and contractility, and clustering of dyadic proteins. Thyroid hormone (TH) deficiency was induced in adult female rats by propyl-thiouracil (PTU; 0.025%) treatment for 8 weeks. Rats were then randomized to continued PTU or triiodo-L-thyronine (T3; 10 μg/kg/d) treatment for 2 weeks (PTU + T3). After in vivo echocardiographic and hemodynamic recordings, cardiomyocytes (CM) were isolated to record Ca²⁺ transients and contractility. TT organization was assessed by confocal microscopy, and STORM images were captured to measure ryanodine receptor (RyR2) cluster number and size, and L-type Ca²⁺ channel (LTCC, Ca_v1.2) co-localization. Expressed genes including two integral TT proteins, junctophilin-2 (Jph-2) and bridging integrator-1 (BIN1), were analyzed in left ventricular (LV) tissues and cultured CM using qPCR and RNA sequencing. The T3 dosage used normalized serum T3, and reversed adverse effects of TH deficiency on in vivo measures of cardiac function. Recordings of isolated CM indicated that T3 increased rates of Ca²⁺ release and re-uptake, resulting in increased velocities of sarcomere shortening and re-lengthening. TT periodicity was significantly decreased, with reduced transverse tubules but increased longitudinal tubules in TH-deficient CMs and LV tissue, and these structures were normalized by T3 treatment. Analysis of STORM data of PTU myocytes showed decreased RyR2 cluster numbers and RyR localizations within each cluster without significant changes in Ca_v1.2 localizations within RyR clusters. T3 treatment normalized RyR2 cluster size and number. qPCR and RNAseq analyses of LV and cultured CM showed that Jph2 expression was T3-responsive, and its increase with treatment may explain improved TT organization and RyR-LTCC coupling.

Remodeling of t-system and proteins underlying excitation-contraction coupling in aging versus failing human heart

It is well established that the aging heart progressively remodels towards a senescent phenotype, but alterations of cellular microstructure and their differences to chronic heart failure (HF) associated remodeling remain ill-defined. Here, we show that the transverse tubular system (t-system) and proteins underlying excitation-contraction coupling in cardiomyocytes are characteristically remodeled with age. We shed light on mechanisms of this remodeling and identified similarities and differences to chronic HF. Using left ventricular myocardium from donors and HF patients with ages between 19 and 75 years, we established a library of 3D reconstructions of the t-system as well as ryanodine receptor (RyR) and junctophilin 2 (JPH2) clusters. Aging was characterized by t-system alterations and sarcolemmal dissociation of RyR clusters. This remodeling was less pronounced than in HF and accompanied by major alterations of JPH2 arrangement. Our study indicates that targeting sarcolemmal association of JPH2 might ameliorate age-associated deficiencies of heart function.

Reverse Remodeling With Left Ventricular Assist Devices

This review provides a comprehensive overview of the past 25+ years of research into the development of left ventricular assist device (LVAD) to improve clinical outcomes in patients with severe end-stage heart failure and basic insights gained into the biology of heart failure gleaned from studies of hearts and myocardium of patients undergoing LVAD support. Clinical aspects of contemporary LVAD therapy, including evolving device technology, overall mortality, and complications, are reviewed. We explain the hemodynamic effects of LVAD support and how these lead to ventricular unloading. This includes a detailed review of the structural, cellular, and molecular aspects of LVAD-associated reverse remodeling. Synergisms between LVAD support and medical therapies for heart failure related to reverse remodeling, remission, and recovery are discussed within the context of both clinical outcomes and fundamental effects on myocardial biology. The incidence, clinical implications and factors most likely to be associated with improved ventricular function and remission of the heart failure are reviewed. Finally, we discuss recognized impediments to achieving myocardial recovery in the vast majority of LVAD-supported hearts and their implications for future research aimed at improving the overall rates of recovery.

Weaning from ventricular assist device support after recovery from left ventricular failure with or without secondary right ventricular failure

Although complete myocardial recovery after ventricular assist device (VAD) implantation is rather seldom, systematic search for recovery is worthwhile because for recovered patients weaning from VADs is feasible and can provide survival benefits with long-term freedom from heart failure (HF) recurrence, even if a chronic cardiomyopathy was the primary cause for the drug-refractory HF necessitating left ventricular (LVAD) or biventricular support (as bridge-to-transplantation or definitive therapy) and even if recovery remains incomplete. LVAD patients explanted for myoacardial recovery compared to those transplanted from LVAD support showed similar survival rates and a significant proportion of explanted patients can achieve cardiac and physical functional capacities that are within the normal range of healthy controls. In apparently sufficiently recovered patients, a major challenge remains still the pre-explant prediction of the weaning success which is meanwhile reliably possible for experienced clinicians. In weaning candidates, the combined use of certain echocardiography and right heart catheterization parameters recorded before VAD explantation can predict post-weaning cardiac stability with good accuracy. However, in the absence of standardization or binding recommendations, the protocols for assessment of native cardiac improvement and also the weaning criteria differ widely among centers. Currently there are still only few larger studies on myocardial recovery assessment after VAD implantation. Therefore, the weaning practice relies mostly on small case series, local practice patterns, and case reports, and the existing knowledge, as well as the partially differing recommendations which are based mainly on expert opinions, need to be periodically systematised. Addressing these shortcomings, our review aims to summarize the evidence and expert opinion on the evaluation of cardiac recovery during mechanical ventricular support by paying special attention to the reliability of the methods and parameters used for assessment of myocardial recovery and the challenges met in both evaluation of recovery and weaning decision making.

Altered calcium handling in cardiomyocytes from arginine-glycine amidinotransferase-knockout mice is rescued by creatine

The creatine kinase system facilitates energy transfer between mitochondria and the major ATPases in the heart. Creatine-deficient mice, which lack arginine-glycine amidinotransferase (AGAT) to synthesize creatine and homoarginine, exhibit reduced cardiac contractility. We studied how the absence of a functional CK system influences calcium handling in isolated cardiomyocytes from AGAT-knockouts and wild-type littermates as well as in AGAT-knockout mice receiving lifelong creatine supplementation via the food. Using a combination of whole cell patch clamp and fluorescence microscopy, we demonstrate that the L-type calcium channel (LTCC) current amplitude and voltage range of activation were significantly lower in AGAT-knockout compared with wild-type littermates. Additionally, the inactivation of LTCC and the calcium transient decay were significantly slower. According to our modeling results, these changes can be reproduced by reducing three parameters in knockout mice when compared with wild-type: LTCC conductance, the exchange constant of Ca2+ transfer between subspace and cytosol, and SERCA activity. Because tissue expression of LTCC and SERCA protein were not significantly different between genotypes, this suggests the involvement of posttranslational regulatory mechanisms or structural reorganization. The AGAT-knockout phenotype of calcium handling was fully reversed by dietary creatine supplementation throughout life. Our results indicate reduced calcium cycling in cardiomyocytes from AGAT-knockouts and suggest that the creatine kinase system is important for the development of calcium handling in the heart.NEW & NOTEWORTHY Creatine-deficient mice lacking arginine-glycine amidinotransferase exhibit compromised cardiac function. Here, we show that this is at least partially due to an overall slowing of calcium dynamics. Calcium influx into the cytosol via the L-type calcium current (LTCC) is diminished, and the rate of the sarcoendoplasmic reticulum calcium ATPase (SERCA) pumping calcium back into the sarcoplasmic reticulum is slower. The expression of LTCC and SERCA did not change, suggesting that the changes are regulatory.

Severe T-System Remodeling in Pediatric Viral Myocarditis

Chronic heart failure (HF) in adults causes remodeling of the cardiomyocyte transverse tubular system (t-system), which contributes to disease progression by impairing excitation-contraction (EC) coupling. However, it is unknown if t-system remodeling occurs in pediatric heart failure. This study investigated the t-system in pediatric viral myocarditis. The t-system and integrity of EC coupling junctions (co-localization of L-type Ca²⁺ channels with ryanodine receptors and junctophilin-2) were analyzed by 3D confocal microscopy in left-ventricular (LV) samples from 5 children with myocarditis (age 14 ± 3 months), undergoing ventricular assist device (VAD) implantation, and 5 children with atrioventricular septum defect (AVSD, age 17 ± 3 months), undergoing corrective surgery. LV ejection fraction (EF) was 58.4 ± 2.3% in AVSD and 12.2 ± 2.4% in acute myocarditis. Cardiomyocytes from myocarditis samples showed increased t-tubule distance (1.27 ± 0.05 μm, n = 34 cells) and dilation of t-tubules (volume-length ratio: 0.64 ± 0.02 μm²) when compared with AVSD (0.90 ± 0.02 μm, p < 0.001; 0.52 ± 0.02 μm², n = 61, p < 0.01). Intriguingly, 4 out of 5 myocarditis samples exhibited sheet-like t-tubules (t-sheets), a characteristic feature of adult chronic heart failure. The fraction of extracellular matrix was slightly higher in myocarditis (26.6 ± 1.4%) than in AVSD samples (24.4 ± 0.8%, p < 0.05). In one case of myocarditis, a second biopsy was taken and analyzed at VAD explantation after extensive cardiac recovery (EF from 7 to 56%) and clinical remission. When compared with pre-VAD, t-tubule distance and density were unchanged, as well as volume-length ratio (0.67 ± 0.04 μm² vs. 0.72 ± 0.05 μm², p = 0.5), reflecting extant t-sheets. However, junctophilin-2 cluster density was considerably higher (0.12 ± 0.02 μm⁻³ vs. 0.05 ± 0.01 μm⁻³, n = 9/10, p < 0.001), approaching values of AVSD (0.13 ± 0.05 μm⁻³, n = 56), and the measure of intact EC coupling junctions showed a distinct increase (20.2 ± 5.0% vs. 6.8 ± 2.2%, p < 0.001). Severe t-system loss and remodeling to t-sheets can occur in acute HF in young children, resembling the structural changes of chronically failing adult hearts. T-system remodeling might contribute to cardiac dysfunction in viral myocarditis. Although t-system recovery remains elusive, recovery of EC coupling junctions may be possible and deserves further investigation.

The architecture and function of cardiac dyads

Cardiac excitation-contraction (EC) coupling, which links plasma membrane depolarization to activation of cardiomyocyte contraction, occurs at dyads, the nanoscopic microdomains formed by apposition of transverse (T)-tubules and junctional sarcoplasmic reticulum (jSR). In a dyadic junction, EC coupling occurs through Ca2+-induced Ca2+ release. Membrane depolarization opens voltage-gated L-type Ca2+ channels (LTCCs) in the T-tubule. The resulting influx of extracellular Ca2+ into the dyadic cleft opens Ca2+ release channels known as ryanodine receptors (RYRs) in the jSR, leading to the rapid increase in cytosolic Ca2+ that triggers sarcomere contraction. The efficacy of LTCC-RYR communication greatly affects a myriad of downstream intracellular signaling events, and it is controlled by many factors, including T-tubule and jSR structure, spatial distribution of ion channels, and regulatory proteins that closely regulate the activities of channels within dyads. Alterations in dyad architecture and/or channel activity are seen in many types of heart disease. This review will focus on the current knowledge regarding cardiac dyad structure and function, their alterations in heart failure, and new approaches to study the composition and function of dyads.

Nanoscale regulation of L-type calcium channels differentiates between ischemic and dilated cardiomyopathies

BACKGROUND

Subcellular localization and function of L-type calcium channels (LTCCs) play an important role in regulating contraction of cardiomyocytes. Understanding how this is affected by the disruption of transverse tubules during heart failure could lead to new insights into the disease.

METHODS

Cardiomyocytes were isolated from healthy donor hearts, as well as from patients with cardiomyopathies and with left ventricular assist devices. Scanning ion conductance and confocal microscopy was used to study membrane structures in the cells. Super-resolution scanning patch-clamp was used to examine LTCC function in different microdomains. Computational modeling predicted the impact of these changes to arrhythmogenesis at the whole-heart level.

FINDINGS

We showed that loss of structural organization in failing myocytes leads to re-distribution of functional LTCCs from the T-tubules to the sarcolemma. In ischemic cardiomyopathy, the increased LTCC open probability in the T-tubules depends on the phosphorylation by protein kinase A, whereas in dilated cardiomyopathy, the increased LTCC opening probability in the sarcolemma results from enhanced phosphorylation by calcium-calmodulin kinase II. LVAD implantation corrected LTCCs pathophysiological activity, although it did not improve their distribution. Using computational modeling in a 3D anatomically-realistic human ventricular model, we showed how LTCC location and activity can trigger heart rhythm disorders of different severity.

INTERPRETATION

Our findings demonstrate that LTCC redistribution and function differentiate between disease aetiologies. The subcellular changes observed in specific microdomains could be the consequence of the action of distinct protein kinases.

FUNDING

This work was supported by NIH grant (ROI-HL 126802 to NT-JG) and British Heart Foundation (grant RG/17/13/33173 to JG, project grant PG/16/17/32069 to RAC). Funders had no role in study design, data collection, data analysis, interpretation, writing of the report.

Nexilin Is Necessary for Maintaining the Transverse-Axial Tubular System in Adult Cardiomyocytes

BACKGROUND

NEXN (nexilin) is a protein of the junctional membrane complex required for development of cardiac T-tubules. Global and cardiomyocyte-specific loss of Nexn in mice leads to a rapidly progressive dilated cardiomyopathy and premature death. Therefore, little is known as to the role of NEXN in adult cardiomyocytes. Transverse-axial tubular system remodeling are well-known features in heart failure. Although NEXN is required during development for T-tubule formation, its role, if any, in mature T-tubules remains to be addressed.

METHODS

Nexn inducible adult cardiomyocyte-specific KO mice were generated. Comprehensive morphological and functional analyses were performed. Heart samples (n>3) were analyzed by molecular, biochemical, and electron microscopy analyses. Isolated single adult cardiomyocytes were analyzed by confocal microscopy, and myocyte shortening/re-lengthening and Ca2+ transient studies were conducted.

RESULTS

Inducible cardiomyocyte-specific loss of Nexn in adult mice resulted in a dilated cardiomyopathy with reduced cardiac function (13% reduction in percentage fractional shortening; P<0.05). In vivo and in vitro analyses of adult mouse heart samples revealed that NEXN was essential for optimal contraction and calcium handling and was required for maintenance of T-tubule network organization (transverse tubular component in Nexn inducible adult cardiomyocyte-specific KO mice reduced by 40% with respect to controls, P<0.05).

CONCLUSIONS

Results here reported reveal NEXN to be a pivotal component of adult junctional membrane complexes required for maintenance of transverse-axial tubular architecture. These results demonstrate that NEXN plays an essential role in the adult cardiomyocyte and give further understanding of pathological mechanisms responsible for cardiomyopathy in patients carrying mutations in the NEXN gene.

The Degree of t-System Remodeling Predicts Negative Force-Frequency Relationship and Prolonged Relaxation Time in Failing Human Myocardium

The normally positive cardiac force-frequency relationship (FFR) becomes flat or negative in chronic heart failure (HF). Here we explored if remodeling of the cardiomyocyte transverse tubular system (t-system) is associated with alterations in FFR and contractile kinetics in failing human myocardium. Left-ventricular myocardial slices from 13 failing human hearts were mounted into a biomimetic culture setup. Maximum twitch force (F), 90% contraction duration (CD₉₀), time to peak force (TTP) and time to relaxation (TTR) were determined at 37°C and 0.2–2 Hz pacing frequency. F_1Hz/F₀._5Hz and F_2Hz/F₀._5Hz served as measures of FFR, intracellular cardiomyocyte t-tubule distance (ΔTT) as measure of t-system remodeling. Protein levels of SERCA2, NCX1, and PLB were quantified by immunoblotting. F_1Hz/F₀._5Hz (R² = 0.82) and F_2Hz/F₀._5Hz (R² = 0.5) correlated negatively with ΔTT, i.e., samples with severe t-system loss exhibited a negative FFR and reduced myocardial wall tension at high pacing rates. PLB levels also predicted F_1Hz/F₀._5Hz, but to a lesser degree (R² = 0.49), whereas NCX1 was not correlated (R² = 0.02). CD₉₀ correlated positively with ΔTT (R² = 0.39) and negatively with SERCA2/PLB (R² = 0.42), indicating that both the t-system and SERCA activity are important for contraction kinetics. Surprisingly, ΔTT was not associated with TTP (R² = 0) but rather with TTR (R² = 0.5). This became even more pronounced when interaction with NCX1 expression was added to the model (R² = 0.79), suggesting that t-system loss impairs myocardial relaxation especially when NCX1 expression is low. The degree of t-system remodeling predicts FFR inversion and contraction slowing in failing human myocardium. Moreover, together with NCX, the t-system may be important for myocardial relaxation.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

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.

High-density lipoprotein-mediated cardioprotection in heart failure

The prevalence of heart failure (HF), including reduced ejection fraction (HFrEF) and preserved ejection fraction (HFpEF), has increased significantly worldwide. However, the prognosis and treatment of HF are still not good. Recent studies have demonstrated that high-density lipoprotein (HDL) plays an important role in cardiac repair during HF. The exact role and mechanism of HDL in the regulation of HF remain unexplained. Here, we discuss recent findings regarding HDL in the progression of HF, such as the regulation of excitation-contraction coupling, energy homeostasis, inflammation, neurohormone activation, and microvascular dysfunction. The effects of HDL on the regulation of cardiac-related cells, such as endothelial cells (ECs), cardiomyocytes (CMs), and on cardiac resident immune cell dysfunction in HF are also explained. An in-depth understanding of HDL function in the heart may provide new strategies for the prevention and treatment of HF.

Modeling Depolarization Delay, Sodium Currents, and Electrical Potentials in Cardiac Transverse Tubules

T-tubules are invaginations of the lateral membrane of striated muscle cells that provide a large surface for ion channels and signaling proteins, thereby supporting excitation–contraction coupling. T-tubules are often remodeled in heart failure. To better understand the electrical behavior of T-tubules of cardiac cells in health and disease, this study addresses two largely unanswered questions regarding their electrical properties: (1) the delay of T-tubular membrane depolarization and (2) the effects of T-tubular sodium current on T-tubular potentials. Here, we present an elementary computational model to determine the delay in depolarization of deep T-tubular membrane segments as the narrow T-tubular lumen provides resistance against the extracellular current. We compare healthy tubules to tubules with constrictions and diseased tubules from mouse and human, and conclude that constrictions greatly delay T-tubular depolarization, while diseased T-tubules depolarize faster than healthy ones due to tubule widening. Increasing the tubule length non-linearly delays the depolarization. We moreover model the effect of T-tubular sodium current on intraluminal T-tubular potentials. We observe that extracellular potentials become negative during the sodium current transient (up to −40 mV in constricted T-tubules), which feedbacks on sodium channel function (self-attenuation) in a manner resembling ephaptic effects that have been described for intercalated discs where opposing membranes are very close together. The intraluminal potential and sodium current self-attenuation however greatly depend on sodium current conductance. These results show that (1) the changes in passive electrical properties of remodeled T-tubules cannot explain the excitation–contraction coupling defects in diseased cells; and (2) the sodium current may modulate intraluminal potentials. Such extracellular potentials might also affect excitation–contraction coupling and macroscopic conduction.

Adverse transverse-tubule remodeling in a rat model of heart failure is attenuated with low-dose triiodothyronine treatment

Abstract

Pre-clinical animal studies have shown that triiodothyronine (T3) replacement therapy improves cardiac contractile function after myocardial infarction (MI). We hypothesized that T3 treatment could prevent adverse post-infarction cardiomyocyte remodeling by maintaining transverse-tubule (TT) structures, thus improving calcium dynamics and contractility.

Methods

Myocardial infarction (MI) or sham surgeries were performed on female Sprague-Dawley rats (aged 12 wks), followed by treatment with T3 (5μg/kg/d) or vehicle in drinking water for 16 wks (n = 10–11/group). After in vivo echocardiographic and hemodynamic analyses, left ventricular myocytes were isolated by collagenase digestion and simultaneous calcium and contractile transients in single cardiomyocytes were recorded using IonOptix imaging. Live cardiomyocytes were stained with AlexaFluor-488 conjugated wheat germ agglutinin (WGA-488) or di-8-ANEPPS, and multiple z-stack images per cell were captured by confocal microscopy for analysis of TT organization. RTqPCR and immunoblot approaches determined expression of TT proteins.

Results

Echocardiography and in vivo hemodynamic measurements showed significant improvements in systolic and diastolic function in T3- vs vehicle-treated MI rats. Isolated cardiomyocyte analysis showed significant dysfunction in measurements of myocyte relengthening in MI hearts, and improvements with T3 treatment: max relengthening velocity (Vmax, um/s), 2.984 ± 1.410 vs 1.593 ± 0.325, p < 0.05 and time to Vmax (sec), 0.233 ± 0.037 vs 0.314 ± 0.019, p < 0.001; MI + T3 vs MI + Veh, respectively. Time to peak contraction was shortened by T3 treatment (0.161 ± 0.021 vs 0.197 ± 0.011 s., p < 0.01; MI + T3 vs MI + Veh, respectively). Analysis of TT periodicity of WGA- or ANEPPS-stained cardiomyocytes indicated significant TT disorganization in MI myocytes and improvement with T3 treatment (transverse-oriented tubules (TE%): 9.07 ± 0.39 sham, 6.94 ± 0.67 MI + Veh and 8.99 ± 0.38 MI + T3; sham vs MI + Veh, p < 0.001; MI + Veh vs MI + T3, p < 0.01). Quantitative RT-PCR showed that reduced expression of BIN1 (Bridging integrator-1), Jph2 (junctophilin-2), RyR2 (ryanodine receptor) and Ca_v1.2 (L-type calcium channel) in the failing myocardium were increased by T3 and immunoblot analysis further supporting a potential T3 effect on the TT-associated proteins, BIN1 and Jph2.

In conclusion, low dose T3 treatment initiated immediately after myocardial infarction attenuated adverse TT remodeling, improved calcium dynamics and contractility, thus supporting the potential therapeutic utility of T3 treatment in heart failure.

Hyperactive ryanodine receptors in human heart failure and ischaemic cardiomyopathy reside outside of couplons

Aims

In ventricular myocytes from humans and large mammals, the transverse and axial tubular system (TATS) network is less extensive than in rodents with consequently a greater proportion of ryanodine receptors (RyRs) not coupled to this membrane system. TATS remodelling in heart failure (HF) and after myocardial infarction (MI) increases the fraction of non-coupled RyRs. Here we investigate whether this remodelling alters the activity of coupled and non-coupled RyR sub-populations through changes in local signalling. We study myocytes from patients with end-stage HF, compared with non-failing (non-HF), and myocytes from pigs with MI and reduced left ventricular (LV) function, compared with sham intervention (SHAM).

Methods and results

Single LV myocytes for functional studies were isolated according to standard protocols. Immunofluorescent staining visualized organization of TATS and RyRs. Ca²⁺ was measured by confocal imaging (fluo-4 as indicator) and using whole-cell patch-clamp (37°C). Spontaneous Ca²⁺ release events, Ca²⁺ sparks, as a readout for RyR activity were recorded during a 15 s period following conditioning stimulation at 2 Hz. Sparks were assigned to cell regions categorized as coupled or non-coupled sites according to a previously developed method. Human HF myocytes had more non-coupled sites and these had more spontaneous activity than in non-HF. Hyperactivity of these non-coupled RyRs was reduced by Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibition. Myocytes from MI pigs had similar changes compared with SHAM controls as seen in human HF myocytes. As well as by CaMKII inhibition, in MI, the increased activity of non-coupled sites was inhibited by mitochondrial reactive oxygen species (mito-ROS) scavenging. Under adrenergic stimulation, Ca²⁺ waves were more frequent and originated at non-coupled sites, generating larger Na⁺/Ca²⁺ exchange currents in MI than in SHAM. Inhibition of CaMKII or mito-ROS scavenging reduced spontaneous Ca²⁺ waves, and improved excitation–contraction coupling.

Conclusions

In HF and after MI, RyR microdomain re-organization enhances spontaneous Ca²⁺ release at non-coupled sites in a manner dependent on CaMKII activation and mito-ROS production. This specific modulation generates a substrate for arrhythmia that appears to be responsive to selective pharmacologic modulation.