Disease severity, arrhythmogenesis, and fibrosis are related to longer action potentials in tetralogy of Fallot

BACKGROUND

Arrhythmias may originate from surgically unaffected right ventricular (RV) regions in patients with tetralogy of Fallot (TOF). We aimed to investigate action potential (AP) remodelling and arrhythmia susceptibility in RV myocardium of patients with repaired and with unrepaired TOF, identify possible correlations with clinical phenotype and myocardial fibrosis, and compare findings with data from patients with atrial septal defect (ASD), a less severe congenital heart disease.

METHODS

Intracellular AP were recorded ex vivo in RV outflow tract samples from 22 TOF and three ASD patients. Arrhythmias were provoked by superfusion with solutions containing reduced potassium and barium chloride, or isoprenaline. Myocardial fibrosis was quantified histologically and associations between clinical phenotype, AP shape, tissue arrhythmia propensity, and fibrosis were examined.

RESULTS

Electrophysiological abnormalities (arrhythmias, AP duration [APD] alternans, impaired APD shortening at increased stimulation frequencies) were generally present in TOF tissue, even from infants, but rare or absent in ASD samples. More severely diseased and acyanotic patients, pronounced tissue susceptibility to arrhythmogenesis, and greater fibrosis extent were associated with longer APD. In contrast, APD was shorter in tissue from patients with pre-operative cyanosis. Increased fibrosis and repaired-TOF status were linked to tissue arrhythmia inducibility.

CONCLUSIONS

Functional and structural tissue remodelling may explain arrhythmic activity in TOF patients, even at a very young age. Surprisingly, clinical acyanosis appears to be associated with more severe arrhythmogenic remodelling. Further research into the clinical drivers of structural and electrical myocardial alterations, and the relation between them, is needed to identify predictive factors for patients at risk.

Piezo1 stretch-activated channel activity differs between murine bone marrow-derived and cardiac tissue-resident macrophages

Macrophages (MΦ) play pivotal roles in tissue homeostasis and repair. Their mechanical environment has been identified as a key modulator of various cell functions, and MΦ mechanosensitivity is likely to be critical - in particular in a rhythmically contracting organ such as the heart. Cultured MΦ, differentiated in vitro from bone marrow (MΦBM), form a popular research model. This study explores the activity of mechanosensitive ion channels (MSC) in murine MΦBM and compares it to MSC activity in MΦ enzymatically isolated from cardiac tissue (tissue-resident MΦ; MΦTR). We show that MΦBM and MΦTR have stretch-induced currents, indicating the presence of functional MSC in their plasma membrane. The current profiles in MΦBM and in MΦTR show characteristics of cation non-selective MSC such as Piezo1 or transient receptor potential channels. While Piezo1 ion channel activity is detectable in the plasma membrane of MΦBM using the patch-clamp technique, or by measuring cytosolic calcium concentration upon perfusion with the Piezo1 channel agonist Yoda1, no Piezo1 channel activity was observed in MΦTR. The selective transient receptor potential vanilloid 4 (TRPV4) channel agonist GSK1016790A induces calcium entry in MΦTR and in MΦBM. In MΦ isolated from left-ventricular scar tissue 28 days after cryoablation, stretch-induced current characteristics are not significantly different compared to non-injured control tissue, even though scarred ventricular tissue is expected to be mechanically remodelled and to contain an altered composition of pre-existing cardiac and circulation-recruited MΦ. Our data suggest that the in vitro differentiation protocols used to obtain MΦBM generate cells that differ from MΦ recruited from the circulation during tissue repair in vivo. Further investigations are needed to explore MSC identity in lineage-traced MΦ in scar tissue, and to compare mechanosensitivity of circulating monocytes with that of MΦBM. KEY POINTS: Bone marrow-derived (MΦBM) and tissue resident (MΦTR) macrophages have stretch-induced currents, indicating expression of functional mechanosensitive channels (MSC) in their plasma membrane. Stretch-activated current profiles show characteristics of cation non-selective MSC; and mRNA coding for MSC, including Piezo1 and TRPV4, is expressed in murine MΦBM and in MΦTR. Calcium entry upon pharmacological activation of TRPV4 confirms functionality of the channel in MΦTR and in MΦBM. Piezo1 ion channel activity is detected in the plasma membrane of MΦBM but not in MΦTR, suggesting that MΦBM may not be a good model to study the mechanotransduction of MΦTR. Stretch-induced currents, Piezo1 mRNA expression and response to pharmacological activation are not significantly changed in cardiac MΦ 28 days after cryoinjury compared to sham operated mice.

Longer and better lives for patients with atrial fibrillation: the 9th AFNET/EHRA consensus conference

AIMS

Recent trial data demonstrate beneficial effects of active rhythm management in patients with atrial fibrillation (AF) and support the concept that a low arrhythmia burden is associated with a low risk of AF-related complications. The aim of this document is to summarize the key outcomes of the 9th AFNET/EHRA Consensus Conference of the Atrial Fibrillation NETwork (AFNET) and the European Heart Rhythm Association (EHRA).

METHODS AND RESULTS

Eighty-three international experts met in Münster for 2 days in September 2023. Key findings are as follows: (i) Active rhythm management should be part of the default initial treatment for all suitable patients with AF. (ii) Patients with device-detected AF have a low burden of AF and a low risk of stroke. Anticoagulation prevents some strokes and also increases major but non-lethal bleeding. (iii) More research is needed to improve stroke risk prediction in patients with AF, especially in those with a low AF burden. Biomolecules, genetics, and imaging can support this. (iv) The presence of AF should trigger systematic workup and comprehensive treatment of concomitant cardiovascular conditions. (v) Machine learning algorithms have been used to improve detection or likely development of AF. Cooperation between clinicians and data scientists is needed to leverage the potential of data science applications for patients with AF.

CONCLUSIONS

Patients with AF and a low arrhythmia burden have a lower risk of stroke and other cardiovascular events than those with a high arrhythmia burden. Combining active rhythm control, anticoagulation, rate control, and therapy of concomitant cardiovascular conditions can improve the lives of patients with AF.

Age-related structural and functional changes of the intracardiac nervous system

BACKGROUND

Although aging is known to be associated with an increased incidence of both atrial and ventricular arrhythmias, there is limited knowledge about how Schwann cells (SC) and the intracardiac nervous system (iCNS) remodel with age. Here we investigate the differences in cardiac SC, parasympathetic nerve fibers, and muscarinic acetylcholine receptor M2 (M2R) expression in young and old mice. Additionally, we examine age-related changes in cardiac responses to sympathomimetic and parasympathomimetic drugs.

METHODS AND RESULTS

Lower SC density, lower SC proliferation and fewer parasympathetic nerve fibers were observed in cardiac and, as a control sciatic nerves from old (20-24 months) compared to young mice (2-3 months). In old mice, chondroitin sulfate proteoglycan 4 (CSPG4) was increased in sciatic but not cardiac nerves. Expression of M2R was lower in ventricular myocardium and ventricular conduction system from old mice compared to young mice, while no significant difference was seen in M2R expression in sino-atrial or atrio-ventricular node pacemaker tissue. Heart rate was slower and PQ intervals were longer in Langendorff-perfused hearts from old mice. Ventricular tachycardia and fibrillation were more frequently observed in response to carbachol administration in hearts from old mice versus those from young mice.

CONCLUSIONS

On the background of reduced presence of SC and parasympathetic nerve fibers, and of lower M2R expression in ventricular cardiomyocytes and conduction system of aged hearts, the propensity of ventricular arrhythmogenesis upon parasympathomimetic drug application is increased. Whether this is caused by an increase in heterogeneity of iCNS structure and function remains to be elucidated.

Meta-Analysis of Mechano-Sensitive Ion Channels in Human Hearts: Chamber- and Disease-Preferential mRNA Expression

The cardiac cell mechanical environment changes on a beat-by-beat basis as well as in the course of various cardiac diseases. Cells sense and respond to mechanical cues via specialized mechano-sensors initiating adaptive signaling cascades. With the aim of revealing new candidates underlying mechano-transduction relevant to cardiac diseases, we investigated mechano-sensitive ion channels (MSC) in human hearts for their chamber- and disease-preferential mRNA expression. Based on a meta-analysis of RNA sequencing studies, we compared the mRNA expression levels of MSC in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and from patients in sinus rhythm (underlying diseases: heart failure, coronary artery disease, heart valve disease) or with atrial fibrillation. Our results suggest that a number of MSC genes are expressed chamber preferentially, e.g., CHRNE in the atria (compared to the ventricles), TRPV4 in the right atrium (compared to the left atrium), CACNA1B and KCNMB1 in the left atrium (compared to the right atrium), as well as KCNK2 and KCNJ2 in ventricles (compared to the atria). Furthermore, 15 MSC genes are differentially expressed in cardiac disease, out of which SCN9A (lower expressed in heart failure compared to donor tissue) and KCNQ5 (lower expressed in atrial fibrillation compared to sinus rhythm) show a more than twofold difference, indicative of possible functional relevance. Thus, we provide an overview of cardiac MSC mRNA expression in the four cardiac chambers from patients with different cardiac diseases. We suggest that the observed differences in MSC mRNA expression may identify candidates involved in altered mechano-transduction in the respective diseases.

Early diagnosis and better rhythm management to improve outcomes in patients with atrial fibrillation: the 8th AFNET/EHRA consensus conference

Despite marked progress in the management of atrial fibrillation (AF), detecting AF remains difficult and AF-related complications cause unacceptable morbidity and mortality even on optimal current therapy. This document summarizes the key outcomes of the 8th AFNET/EHRA Consensus Conference of the Atrial Fibrillation NETwork (AFNET) and the European Heart Rhythm Association (EHRA). Eighty-three international experts met in Hamburg for 2 days in October 2021. Results of the interdisciplinary, hybrid discussions in breakout groups and the plenary based on recently published and unpublished observations are summarized in this consensus paper to support improved care for patients with AF by guiding prevention, individualized management, and research strategies. The main outcomes are (i) new evidence supports a simple, scalable, and pragmatic population-based AF screening pathway; (ii) rhythm management is evolving from therapy aimed at improving symptoms to an integrated domain in the prevention of AF-related outcomes, especially in patients with recently diagnosed AF; (iii) improved characterization of atrial cardiomyopathy may help to identify patients in need for therapy; (iv) standardized assessment of cognitive function in patients with AF could lead to improvement in patient outcomes; and (v) artificial intelligence (AI) can support all of the above aims, but requires advanced interdisciplinary knowledge and collaboration as well as a better medico-legal framework. Implementation of new evidence-based approaches to AF screening and rhythm management can improve outcomes in patients with AF. Additional benefits are possible with further efforts to identify and target atrial cardiomyopathy and cognitive impairment, which can be facilitated by AI.

Common Structural Pattern for Flecainide Binding in Atrial-Selective Kv1.5 and Nav1.5 Channels: A Computational Approach

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Its treatment includes antiarrhythmic drugs (AADs) to modulate the function of cardiac ion channels. However, AADs have been limited by proarrhythmic effects, non-cardiovascular toxicities as well as often modest antiarrhythmic efficacy. Theoretical models showed that a combined blockade of Na_v1.5 (and its current, I_Na) and K_v1.5 (and its current, I_Kur) ion channels yield a synergistic anti-arrhythmic effect without alterations in ventricles. We focused on K_v1.5 and Na_v1.5 to search for structural similarities in their binding site (BS) for flecainide (a common blocker and widely prescribed AAD) as a first step for prospective rational multi-target directed ligand (MTDL) design strategies. We present a computational workflow for a flecainide BS comparison in a flecainide-K_v1.5 docking model and a solved structure of the flecainide-Na_v1.5 complex. The workflow includes docking, molecular dynamics, BS characterization and pattern matching. We identified a common structural pattern in flecainide BS for these channels. The latter belongs to the central cavity and consists of a hydrophobic patch and a polar region, involving residues from the S6 helix and P-loop. Since the rational MTDL design for AF is still incipient, our findings could advance multi-target atrial-selective strategies for AF treatment.

Diminished PLK2 Induces Cardiac Fibrosis and Promotes Atrial Fibrillation

[Figure: see text].

Dynamic risk assessment to improve quality of care in patients with atrial fibrillation: the 7th AFNET/EHRA Consensus Conference

AIMS

The risk of developing atrial fibrillation (AF) and its complications continues to increase, despite good progress in preventing AF-related strokes.

METHODS AND RESULTS

This article summarizes the outcomes of the 7th Consensus Conference of the Atrial Fibrillation NETwork (AFNET) and the European Heart Rhythm Association (EHRA) held in Lisbon in March 2019. Sixty-five international AF specialists met to present new data and find consensus on pressing issues in AF prevention, management and future research to improve care for patients with AF and prevent AF-related complications. This article is the main outcome of an interactive, iterative discussion between breakout specialist groups and the meeting plenary. AF patients have dynamic risk profiles requiring repeated assessment and risk-based therapy stratification to optimize quality of care. Interrogation of deeply phenotyped datasets with outcomes will lead to a better understanding of the cardiac and systemic effects of AF, interacting with comorbidities and predisposing factors, enabling stratified therapy. New proposals include an algorithm for the acute management of patients with AF and heart failure, a call for a refined, data-driven assessment of stroke risk, suggestions for anticoagulation use in special populations, and a call for rhythm control therapy selection based on risk of AF recurrence.

CONCLUSION

The remaining morbidity and mortality in patients with AF needs better characterization. Likely drivers of the remaining AF-related problems are AF burden, potentially treatable by rhythm control therapy, and concomitant conditions, potentially treatable by treating these conditions. Identifying the drivers of AF-related complications holds promise for stratified therapy.

Feasibility of digital atrial fibrillation screening in an elderly population : The Ulm heart rhythm weeks

AIMS

Atrial fibrillation (AF) screening in risk populations has the potential to prevent strokes. The authors tested the feasibility of a digital program with initial photoplethysmographic (PPG) self-screening and cardiologist-attended electrocardiographic (ECG) confirmation of screen-positive cases.

METHODS

Inhabitants of the city of Ulm aged ≥ 65 years were invited to participate. After digital consent, participants were given access to a smartphone application for 14 days of self-screening (two recordings per day recommended). Screen-positive participants without known AF were invited to present to a cardiologist for AF confirmation with a 14-day ECG event recorder. PPG recordings were first analyzed by algorithm using a combination of linear and non-linear methods. The quality of pathological (classified by algorithm) PPG and all ECG recordings were checked by a telecare service. Primary outcomes included adherence to the screening protocol defined as the proportion of participants performing at least 14 PPG recordings (or until documentation of absolute arrhythmia) and the proportion of pathological PPG and all ECG recordings rejected by the telecare center.

RESULTS

A total of 215 participants registered. Of these, 204 (95%) performed at least one recording and 169 (79%) reached the performance target of two sufficient measurements per day; 75 PPG recordings were automatically classified as pathological by algorithm; 14 (19%) were rejected by the telecare service due to poor quality. Of the 12 participants with a suspected first diagnosis of AF, five visited a cardiologist as part of the study. Of 1090 ECG recordings obtained, 390 (36%) were qualified as non-diagnostic. AF was confirmed in three cases.

CONCLUSIONS

A digital AF screening program with initial self-screening and referral of screen-positive cases to a cardiologist-attended ECG-confirmation service is feasible with meaningful results in an elderly risk population. However, the availability of the target population of persons > 65 years of age for such a digital screening program appears to be limited despite extensive public relations activities.

Prognostic markers of all-cause mortality in patients with atrial fibrillation: data from the prospective long-term registry of the German Atrial Fibrillation NETwork (AFNET)

AIMS

Atrial fibrillation (AF) is associated with a high risk of cardiovascular and non-cardiovascular death, even on anticoagulation. It is controversial, which conditions-including concomitant diseases and AF itself-contribute to this mortality. To further clarify these questions, major determinants of long-term mortality and their contribution to death were quantified in an unselected cohort of AF patients.

METHODS AND RESULTS

We established a large nationwide registry comprising 8833 AF-patients with a median follow-up of 6.5 years (45 345 patient-years) and central adjudication of adverse events. Baseline characteristics of the patients were evaluated as predictors of mortality using Cox regression and C-indices for determination of predictive power. Annualized mortality was highest in the first year (6.2%) and remained high thereafter (5.2% in men and 5.5% in women). Thirty-eight percent of all deaths were cardiovascular, mainly due to heart failure or sudden death. Sex-specific age was the strongest predictor of mortality, followed by concomitant cardiovascular and non-cardiovascular conditions. These factors accounted for 25% of the total mortality beyond age and sex and for 84% of the mortality differences between AF types. Thus, the electrical phenotype of the disease at baseline contributed only marginally to prediction of mortality.

CONCLUSION

Mortality is high in AF patients and arises primarily from heart failure, peripheral artery disease, chronic obstructive lung disease, chronic kidney disease, and diabetes mellitus, which, therefore, should be targeted to lower mortality. Parameters related to the electrical manifestation of AF did not have an independent impact on long-term mortality in our representative cohort.

Heterogeneity and Remodeling of Ion Currents in Cultured Right Atrial Fibroblasts From Patients With Sinus Rhythm or Atrial Fibrillation

Cardiac fibroblasts express multiple voltage-dependent ion channels. Even though fibroblasts do not generate action potentials, they may influence cardiac electrophysiology by electrical coupling via gap junctions with cardiomyocytes, and through fibrosis. Here, we investigate the electrophysiological phenotype of cultured fibroblasts from right atrial appendage tissue of patients with sinus rhythm (SR) or atrial fibrillation (AF). Using the patch-clamp technique in whole-cell mode, we observed steady-state outward currents exhibiting either no rectification or inward and/or outward rectification. The distributions of current patterns between fibroblasts from SR and AF patients were not significantly different. In response to depolarizing voltage pulses, we measured transient outward currents with fast and slow activation kinetics, an outward background current, and an inward current with a potential-dependence resembling that of L-type Ca²⁺ channels. In cell-attached patch-clamp mode, large amplitude, paxilline-sensitive single channel openings were found in ≈65% of SR and ∼38% of AF fibroblasts, suggesting the presence of “big conductance Ca²⁺-activated K⁺ (BK_Ca)” channels. The open probability of BK_Ca was significantly lower in AF than in SR fibroblasts. When cultured in the presence of paxilline, the shape of fibroblasts became wider and less spindle-like. Our data confirm previous findings on cardiac fibroblast electrophysiology and extend them by illustrating differential channel expression in human atrial fibroblasts from SR and AF tissue.

Piezo1 and BKCa channels in human atrial fibroblasts: interplay and remodelling in atrial fibrillation

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministry of Science, Research and Arts Baden-Württemberg (MWK-BW Sonderlinie Medizin)

Atrial Fibrillation (AF) is an arrhythmia of increasing prevalence. One of the important indicators for AF is sustained atrial dilatation, highlighting the importance of mechanical overload in the pathophysiology of AF. The mechanisms by which atrial cells, including fibroblasts, sense and react to such changing mechanical forces, are not fully elucidated. Here, we characterise stretch-activated ion channels (SAC) in human atrial fibroblasts and changes in their expression and activity associated with AF.

Using primary cultures of human atrial fibroblasts, isolated from patients in sinus rhythm or with sustained AF, we combine electrophysiological, molecular and pharmacological tools to identify SAC. Two electrophysiological SAC-signatures were detected, indicative of cation-nonselective and potassium-selective channels. Using siRNA-mediated knockdown, we identified the nonselective SAC as Piezo1. Biophysical properties of the potassium-selective channel and its pharmacology indicated presence of ‘big potassium channels’, BKCa. In cells from AF patients, Piezo1 activity and mRNA expression levels were higher than in cells from sinus rhythm patients, while BKCa activity (but not expression) was downregulated. Both Piezo1-knockdown and removal of extracellular calcium from the patch pipette resulted in a significant reduction of stretch-induced BKCa current. No co-immunoprecipitation of Piezo1 and BKCa was detected.

Human atrial fibroblasts express functional Piezo1 and BKCa channels. While Piezo1 is directly stretch-activated, the increase in BKCa activity during mechanical stimulation appears to be mainly secondary to calcium influx via SAC such as Piezo1. During sustained AF, Piezo1 is increased, while BKCa activity is reduced, highlighting differential regulation of both channels. Our data show the presence and activity of Piezo1 and BKCa in human atrial fibroblasts and suggest an interplay between the two in the absence of direct physical interactions.

The lectin LecA sensitizes the human stretch-activated channel TREK-1 but not Piezo1 and binds selectively to cardiac non-myocytes

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): This research was supported by the European Research Council (Advanced Grant CardioNECT, Project ID: #323099, PK) and a research grant from the Ministry of Science, Research and Arts Baden-Württemberg (MWK-BW Sonderlinie Medizin, #3091311631).

Mechanical stimuli are detected and transduced by cellular mechano-sensors, including stretch-activated ion channels (SAC). SAC are activated by stretch and changes in membrane curvature but their precise role in the heart is unclear.

The lectin LecA is a virulence factor of Pseudomonas aeruginosa and essential for bacterial cell invasion by inducing membrane curvature. We investigate whether LecA modulates SAC activity, namely TREK-1 and Piezo1 in human embryonic kidney (HEK) cells.

Confocal microscopy and electron tomography were used to follow binding dynamics of LecA, and the ion channel activity was recorded using the patch-clamp technique. Additionally, freshly isolated cardiac cells were used for studies into cell type dependency of LecA binding.

LecA binds within seconds to cell surface. Local plasma membrane invaginations are detected by 17 min of LecA exposure. LecA sensitizes TREK-1, but not Piezo1, to voltage and mechanical stimulation. In freshly isolated cardiac cells, LecA binds to non-myocytes, but not to cardiomyocytes from mouse, rabbit, pig, and human.

Our results suggest that LecA may serve as a pharmacological tool to study cardiac SAC in a cell type-preferential manner. Abstract Figure. Graphical abstract

Piezo1 and BKCa channels in human atrial fibroblasts: Interplay and remodelling in atrial fibrillation

AIMS

Atrial Fibrillation (AF) is an arrhythmia of increasing prevalence in the aging populations of developed countries. One of the important indicators of AF is sustained atrial dilatation, highlighting the importance of mechanical overload in the pathophysiology of AF. The mechanisms by which atrial cells, including fibroblasts, sense and react to changing mechanical forces, are not fully elucidated. Here, we characterise stretch-activated ion channels (SAC) in human atrial fibroblasts and changes in SAC- presence and activity associated with AF.

METHODS AND RESULTS

Using primary cultures of human atrial fibroblasts, isolated from patients in sinus rhythm or sustained AF, we combine electrophysiological, molecular and pharmacological tools to identify SAC. Two electrophysiological SAC- signatures were detected, indicative of cation-nonselective and potassium-selective channels. Using siRNA-mediated knockdown, we identified the cation-nonselective SAC as Piezo1. Biophysical properties of the potassium-selective channel, its sensitivity to calcium, paxilline or iberiotoxin (blockers), and NS11021 (activator), indicated presence of calcium-dependent 'big potassium channels' (BK_Ca). In cells from AF patients, Piezo1 activity and mRNA expression levels were higher than in cells from sinus rhythm patients, while BK_Ca activity (but not expression) was downregulated. Both Piezo1-knockdown and removal of extracellular calcium from the patch pipette resulted in a significant reduction of BK_Ca current during stretch. No co-immunoprecipitation of Piezo1 and BK_Ca was detected.

CONCLUSIONS

Human atrial fibroblasts contain at least two types of ion channels that are activated during stretch: Piezo1 and BK_Ca. While Piezo1 is directly stretch-activated, the increase in BK_Ca activity during mechanical stimulation appears to be mainly secondary to calcium influx via SAC such as Piezo1. During sustained AF, Piezo1 is increased, while BK_Ca activity is reduced, highlighting differential regulation of both channels. Our data support the presence and interplay of Piezo1 and BK_Ca in human atrial fibroblasts in the absence of physical links between the two channel proteins.

Novel insights into the electrophysiology of murine cardiac macrophages: relevance of voltage-gated potassium channels

AIMS

Macrophages (MΦ), known for immunological roles such as phagocytosis and antigen presentation, have been found to electrotonically couple to cardiomyocytes (CM) of the atrio-ventricular node via Cx43, affecting cardiac conduction in isolated mouse hearts. Here, we characterise passive and active electrophysiological properties of murine cardiac resident MΦ, and model their potential electrophysiological relevance for CM.

METHODS AND RESULTS

We combined classic electrophysiological approaches with 3 D florescence imaging, RNA-sequencing, pharmacological interventions and computer simulations. We used Cx3cr1eYFP/+ mice wherein cardiac MΦ were fluorescently labelled. FACS-purified fluorescent MΦ from mouse hearts were studied by whole-cell patch-clamp. MΦ electrophysiological properties include: membrane resistance 2.2 ± 0.1 GΩ (all data mean±SEM), capacitance 18.3 ± 0.1 pF, resting membrane potential -39.6 ± 0.3 mV, and several voltage-activated, outward or inwardly-rectifying potassium currents. Using ion channel blockers (barium, TEA, 4-AP, margatoxin, XEN-D0103, DIDS), flow cytometry, immuno-staining and RNA-sequencing, we identified Kv1.3, Kv1.5 and Kir2.1 as channels contributing to observed ion currents. MΦ displayed four patterns for outward and two for inward-rectifier potassium currents. Additionally, MΦ showed surface expression of Cx43, a prerequisite for homo- and/or heterotypic electrotonic coupling. Experimental results fed into development of an original computational model to describe cardiac MΦ electrophysiology. Computer simulations to quantitatively assess plausible effects of MΦ on electrotonically coupled CM showed that MΦ can depolarise resting CM, shorten early and prolong late action potential duration, with effects depending on coupling strength and individual MΦ electrophysiological properties, in particular resting membrane potential and presence/absence of Kir2.1.

CONCLUSIONS

Our results provide a first electrophysiological characterisation of cardiac resident MΦ, and a computational model to quantitatively explore their relevance in the heterocellular heart. Future work will be focussed at distinguishing electrophysiological effects of MΦ-CM coupling on both cell types during steady-state and in patho-physiological remodelling, when immune cells change their phenotype, proliferate, and/or invade from external sources.

TRANSLATIONAL PERSPECTIVE

Cardiac tissue contains resident macrophages (MΦ) which, beyond immunological and housekeeping roles, have been found to electrotonically couple via connexins to cardiomyocytes (CM), stabilising atrio-ventricular conduction at high excitation rates. Here, we characterise structure and electrophysiological function of murine cardiac MΦ and provide a computational model to quantitatively probe the potential relevance of MΦ-CM coupling for cardiac electrophysiology. We find that MΦ are unlikely to have major electrophysiological effects in normal tissue, where they would hasten early and slow late CM-repolarisation. Further work will address potential arrhythmogenicity of MΦ in patho-physiologically remodelled tissue containing elevated MΦ-numbers, incl. non-resident recruited cells.

Small Conductance Ca2 +-Activated K+ (SK) Channel mRNA Expression in Human Atrial and Ventricular Tissue: Comparison Between Donor, Atrial Fibrillation and Heart Failure Tissue

In search of more efficacious and safe pharmacological treatments for atrial fibrillation (AF), atria-selective antiarrhythmic agents have been promoted that target ion channels principally expressed in the atria. This concept allows one to engage antiarrhythmic effects in atria, but spares the ventricles from potentially proarrhythmic side effects. It has been suggested that cardiac small conductance Ca²⁺-activated K⁺ (SK) channels may represent an atria-selective target in mammals including humans. However, there are conflicting data concerning the expression of SK channels in different stages of AF, and recent findings suggest that SK channels are upregulated in ventricular myocardium when patients develop heart failure. To address this issue, RNA-sequencing was performed to compare expression levels of three SK channels (KCNN1, KCNN2, and KCNN3) in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and patients with cardiac disease in sinus rhythm or with AF. In addition, for control purposes expression levels of several genes known to be either chamber-selective or differentially expressed in AF and heart failure were determined. In atria, as compared to ventricle from transplant donor hearts, we confirmed higher expression of KCNN1 and KCNA5, and lower expression of KCNJ2, whereas KCNN2 and KCNN3 were statistically not differentially expressed. Overall expression of KCNN1 was low compared to KCNN2 and KCNN3. Comparing atrial tissue from patients with AF to sinus rhythm samples we saw downregulation of KCNN2 in AF, as previously reported. When comparing ventricular tissue from heart failure patients to non-diseased samples, we found significantly increased ventricular expression of KCNN3 in heart failure, as previously published. The other channels showed no significant difference in expression in either disease. Our results add weight to the view that SK channels are not likely to be an atria-selective target, especially in failing human hearts, and modulators of these channels may prove to have less utility in treating AF than hoped. Whether targeting SK1 holds potential remains to be elucidated.

Pathophysiological Changes in the Enteric Nervous System of Rotenone-Exposed Mice as Early Radiological Markers for Parkinson's Disease

Parkinson's disease (PD) is known to involve the peripheral nervous system (PNS) and the enteric nervous system (ENS). Functional changes in PNS and ENS appear early in the course of the disease and are responsible for some of the non-motor symptoms observed in PD patients like constipation, that can precede the appearance of motor symptoms by years. Here we analyzed the effect of the pesticide rotenone, a mitochondrial Complex I inhibitor, on the function and neuronal composition of the ENS by measuring intestinal contractility in a tissue bath and by analyzing related protein expression. Our results show that rotenone changes the normal physiological response of the intestine to carbachol, dopamine and electric field stimulation (EFS). Changes in the reaction to EFS seem to be related to the reduction in the cholinergic input but also related to the noradrenergic input, as suggested by the non-adrenergic non-cholinergic (NANC) reaction to the EFS in rotenone-exposed mice. The magnitude and direction of these alterations varies between intestinal regions and exposure times and is associated with an early up-regulation of dopaminergic, cholinergic and adrenergic receptors and an irregular reduction in the amount of enteric neurons in rotenone-exposed mice. The early appearance of these alterations, that start occurring before the substantia nigra is affected in this mouse model, suggests that these alterations could be also observed in patients before the onset of motor symptoms and makes them ideal potential candidates to be used as radiological markers for the detection of Parkinson's disease in its early stages.

Piezo1 Channels Contribute to the Regulation of Human Atrial Fibroblast Mechanical Properties and Matrix Stiffness Sensing

The mechanical environment of cardiac cells changes continuously and undergoes major alterations during diseases. Most cardiac diseases, including atrial fibrillation, are accompanied by fibrosis which can impair both electrical and mechanical function of the heart. A key characteristic of fibrotic tissue is excessive accumulation of extracellular matrix, leading to increased tissue stiffness. Cells are known to respond to changes in their mechanical environment, but the molecular mechanisms underlying this ability are incompletely understood. We used cell culture systems and hydrogels with tunable stiffness, combined with advanced biophysical and imaging techniques, to elucidate the roles of the stretch-activated channel Piezo1 in human atrial fibroblast mechano-sensing. Changing the expression level of Piezo1 revealed that this mechano-sensor contributes to the organization of the cytoskeleton, affecting mechanical properties of human embryonic kidney cells and human atrial fibroblasts. Our results suggest that this response is independent of Piezo1-mediated ion conduction at the plasma membrane, and mediated in part by components of the integrin pathway. Further, we show that Piezo1 is instrumental for fibroblast adaptation to changes in matrix stiffness, and that Piezo1-induced cell stiffening is transmitted in a paracrine manner to other cells by a signaling mechanism requiring interleukin-6. Piezo1 may be a new candidate for targeted interference with cardiac fibroblast function.

No impact of sex and age on beta-adrenoceptor-mediated inotropy in human right atrial trabeculae

AIM

There is an increasing awareness of the impact of age and sex on cardiovascular diseases (CVDs). Differences in physiology are suspected. Beta-adrenoceptors (beta-ARs) are an important drug target in CVD and potential differences might have significant impact on the treatment of many patients. To investigate whether age and sex affects beta-AR function, we analysed a large data set on beta-AR-induced inotropy in human atrial trabeculae.

METHODS

We performed multivariable analysis of individual atrial contractility data from trabeculae obtained during heart surgery of patients in sinus rhythm (535 trabeculae from 165 patients). Noradrenaline or adrenaline were used in the presence of the beta₂ -selective antagonist (ICI 118 551, 50 nmol/L) or the beta₁ -selective antagonist (CGP 20712A, 300 nmol/L) to stimulate beta₁ -AR or beta₂ -AR respectively. Agonist concentration required to achieve half-maximum inotropic effects (EC₅₀ ) was taken as a measure of beta-AR sensitivity.

RESULTS

Impact of clinical variables was modelled using multivariable mixed model regression. As previously reported, chronic treatment with beta-blockers sensitized beta-AR. However, there was no significant interaction between basal force, maximum force and beta-AR sensitivity when age and sex were modelled continuously. In addition, there was no statistically significant effect of body mass index or diabetes on atrial contractility.

CONCLUSION

Our large, multivariable analysis shows that neither age nor sex affects beta-AR-mediated inotropy or catecholamine sensitivity in human atrial trabeculae. These findings may have important clinical implications because beta-ARs, as a common drug target in CVD and heart failure, do not behave differently in women and men across age decades.

PLK2 is a novel regulator of osteopontin-driven fibrosis and diastolic dysfunction in permanent atrial fibrillation

Background and aim

Atrial fibrillation (AF) is frequently accompanied by cardiac fibrosis and diastolic heart failure. Due to the heterogeneous nature and complexity of fibrosis, the knowledge of the underlying pathomechanisms is limited. Thus, effective antifibrotic pharmacotherapy is missing. The objective of this study was to decipher the role of polo-like kinase 2 (PLK2) in the pathogenesis of cardiac fibrosis and left ventricular diastolic dysfunction. We put particular emphasis on the identification of profibrotic downstream targets of PLK2, which can serve as therapeutic targets.

Methods and results

This study was based on human atrial tissue biopsies and peripheral blood samples, a PLK2 knockout mouse model, a canine tachy-pacing model and specific pharmacological interventions on cardiac fibroblasts. In human atrial AF tissue samples, PLK2 was 50% downregulated by hypoxia-induced promoter methylation compared to sinus rhythm (SR) control. Confirmatory analysis of a canine tachy-pacing model showed PLK2 downregulation exclusively in the atria but not in the ventricles. Specific pharmacological inhibition as well as genetic deletion of PLK2 led to a striking myofibroblast phenotype. Discovery proteomics revealed that the global knockout of PLK2 resulted in de novo secretion of the inflammatory cytokine osteopontin (OPN) in cardiac fibroblasts and concomitant ventricular fibrosis in the PLK2 knockout mouse model. An ELISA analysis of peripheral blood samples of AF patients with electrophysiologically proven fibrosis, confirmed significantly increased OPN plasma concentrations compared to SR and non-fibrosis AF controls. Consequently, echocardiography on PLK2 KO mice revealed left ventricular diastolic dysfunction, tachycardia and fibrosis-typical surface ECG anomalies (PQ and QRS prolongation). Mechanistically, we identified the ERK1/2 signaling pathway as the molecular link between reduced expression of PLK2 and elevated osteopontin transcription. In a reverse translational attempt, we successfully tested the capability of 5-amino-salicylic acid (5-ASA) to inhibit osteopontin transcription and to reverse a TGF-β-induced myofibroblast phenotype in vitro. Currently the long-term administration of 5-ASA is tested in PLK2 knockout mice to evaluate the therapeutic potential to prevent cardiac fibrosis and diastolic heart failure development.

Conclusion and clinical impact

We identified PLK2 as an epigenetically regulated kinase involved in the pathophysiology of fibrosis in AF. PLK2 knockout mice can serve as a model of diastolic heart failure wherein OPN is a promising therapeutic target. Our results strengthen the current hypothesis that atrial fibrillation is not only an ion channel disease but a complex systemic disorder. Restoration of physiological PLK2 expression and blockade of osteopontin release with 5-ASA may constitute valuable new drug targets for the prevention and treatment of fibrosis and diastolic heart failure in AF.

Funding Acknowledgement

Type of funding source: Public Institution(s). Main funding source(s): Faculty of Medicine, Carl Gustav Carus, Dresden, “MeDDrive Start” Grant

Repurposing mesalazine against cardiac fibrosis in vitro

Cardiovascular diseases are exacerbated and driven by cardiac fibrosis. TGFβ induces fibroblast activation and differentiation into myofibroblasts that secrete excessive extracellular matrix proteins leading to stiffening of the heart, concomitant cardiac dysfunction, and arrhythmias. However, effective pharmacotherapy for preventing or reversing cardiac fibrosis is presently unavailable. Therefore, drug repurposing could be a cost- and time-saving approach to discover antifibrotic interventions. The aim of this study was to investigate the antifibrotic potential of mesalazine in a cardiac fibroblast stress model. TGFβ was used to induce a profibrotic phenotype in a human cardiac fibroblast cell line. After induction, cells were treated with mesalazine or solvent control. Fibroblast proliferation, key fibrosis protein expression, extracellular collagen deposition, and mechanical properties were subsequently determined. In response to TGFβ treatment, fibroblasts underwent a profound phenoconversion towards myofibroblasts, determined by the expression of fibrillary αSMA. Mesalazine reduced differentiation nearly by half and diminished fibroblast proliferation by a third. Additionally, TGFβ led to increased cell stiffness and adhesion, which were reversed by mesalazine treatment. Collagen 1 expression and deposition—key drivers of fibrosis—were significantly increased upon TGFβ stimulation and reduced to control levels by mesalazine. SMAD2/3 and ERK1/2 phosphorylation, along with reduced nuclear NFκB translocation, were identified as potential modes of action. The current study provides experimental pre-clinical evidence for antifibrotic effects of mesalazine in an in vitro model of cardiac fibrosis. Furthermore, it sheds light on possible mechanisms of action and suggests further investigation in experimental and clinical settings.

Atrial Myocyte NLRP3/CaMKII Nexus Forms a Substrate for Postoperative Atrial Fibrillation

RATIONALE

Postoperative atrial fibrillation (POAF) is a common and troublesome complication of cardiac surgery. POAF is generally believed to occur when postoperative triggers act on a preexisting vulnerable substrate, but the underlying cellular and molecular mechanisms are largely unknown.

OBJECTIVE

To identify cellular POAF mechanisms in right atrial samples from patients without a history of atrial fibrillation undergoing open-heart surgery.

METHODS AND RESULTS

Multicellular action potentials, membrane ion-currents (perforated patch-clamp), or simultaneous membrane-current (ruptured patch-clamp) and [Ca²⁺]_i-recordings in atrial cardiomyocytes, along with protein-expression levels in tissue homogenates or cardiomyocytes, were assessed in 265 atrial samples from patients without or with POAF. No indices of electrical, profibrotic, or connexin remodeling were noted in POAF, but Ca²⁺-transient amplitude was smaller, although spontaneous sarcoplasmic reticulum (SR) Ca²⁺-release events and L-type Ca²⁺-current alternans occurred more frequently. CaMKII (Ca²⁺/calmodulin-dependent protein kinase-II) protein-expression, CaMKII-dependent phosphorylation of the cardiac RyR2 (ryanodine-receptor channel type-2), and RyR2 single-channel open-probability were significantly increased in POAF. SR Ca²⁺-content was unchanged in POAF despite greater SR Ca²⁺-leak, with a trend towards increased SR Ca²⁺-ATPase activity. Patients with POAF also showed stronger expression of activated components of the NLRP3 (NACHT, LRR, and PYD domains-containing protein-3)-inflammasome system in atrial whole-tissue homogenates and cardiomyocytes. Acute application of interleukin-1β caused NLRP3-signaling activation and CaMKII-dependent RyR2/phospholamban hyperphosphorylation in an immortalized mouse atrial cardiomyocyte cell-line (HL-1-cardiomyocytes) and enhanced spontaneous SR Ca²⁺-release events in both POAF cardiomyocytes and HL-1-cardiomyocytes. Computational modeling showed that RyR2 dysfunction and increased SR Ca²⁺-uptake are sufficient to reproduce the Ca²⁺-handling phenotype and indicated an increased risk of proarrhythmic delayed afterdepolarizations in POAF subjects in response to interleukin-1β.

CONCLUSIONS

Preexisting Ca²⁺-handling abnormalities and activation of NLRP3-inflammasome/CaMKII signaling are evident in atrial cardiomyocytes from patients who subsequently develop POAF. These molecular substrates sensitize cardiomyocytes to spontaneous Ca²⁺-releases and arrhythmogenic afterdepolarizations, particularly upon exposure to inflammatory mediators. Our data reveal a potential cellular and molecular substrate for this important clinical problem.

Modeling atrial fibrosis in vitro-Generation and characterization of a novel human atrial fibroblast cell line

Atrial fibrillation (AF) is regularly accompanied by cardiac fibrosis and concomitant heart failure. Due to the heterogeneous nature and complexity of fibrosis, the knowledge about the underlying mechanisms is limited, which prevents effective pharmacotherapy. A deeper understanding of cardiac fibroblasts is essential to meet this need. We previously described phenotypic and functional differences between atrial fibroblasts from patients in sinus rhythm and with AF. Herein, we established and characterized a novel human atrial fibroblast line, which displays typical fibroblast morphology and function comparable to primary cells but with improved proliferation capacity and low spontaneous myofibroblast differentiation. These traits make our model suitable for the study of fibrosis mechanisms and for drug screening aimed at developing effective antifibrotic pharmacotherapy.

The Lectin LecA Sensitizes the Human Stretch-Activated Channel TREK-1 but Not Piezo1 and Binds Selectively to Cardiac Non-myocytes

The healthy heart adapts continuously to a complex set of dynamically changing mechanical conditions. The mechanical environment is altered by, and contributes to, multiple cardiac diseases. Mechanical stimuli are detected and transduced by cellular mechano-sensors, including stretch-activated ion channels (SAC). The precise role of SAC in the heart is unclear, in part because there are few SAC-specific pharmacological modulators. That said, most SAC can be activated by inducers of membrane curvature. The lectin LecA is a virulence factor of Pseudomonas aeruginosa and essential for P. aeruginosa-induced membrane curvature, resulting in formation of endocytic structures and bacterial cell invasion. We investigate whether LecA modulates SAC activity. TREK-1 and Piezo1 have been selected, as they are widely expressed in the body, including cardiac tissue, and they are “canonical representatives” for the potassium selective and the cation non-selective SAC families, respectively. Live cell confocal microscopy and electron tomographic imaging were used to follow binding dynamics of LecA, and to track changes in cell morphology and membrane topology in human embryonic kidney (HEK) cells and in giant unilamellar vesicles (GUV). HEK cells were further transfected with human TREK-1 or Piezo1 constructs, and ion channel activity was recorded using the patch-clamp technique. Finally, freshly isolated cardiac cells were used for studies into cell type dependency of LecA binding. LecA (500 nM) binds within seconds to the surface of HEK cells, with highest concentration at cell-cell contact sites. Local membrane invaginations are detected in the presence of LecA, both in the plasma membrane of cells (by 17 min of LecA exposure) as well as in GUV. In HEK cells, LecA sensitizes TREK-1, but not Piezo1, to voltage and mechanical stimulation. In freshly isolated cardiac cells, LecA binds to non-myocytes, but not to ventricular or atrial cardiomyocytes. This cell type specific lack of binding is observed across cardiomyocytes from mouse, rabbit, pig, and human. Our results suggest that LecA may serve as a pharmacological tool to study SAC in a cell type-preferential manner. This could aid tissue-based research into the roles of SAC in cardiac non-myocytes.

Human Atrial Fibroblast Adaptation to Heterogeneities in Substrate Stiffness

Fibrosis is associated with aging and many cardiac pathologies. It is characterized both by myofibroblast differentiation and by excessive accumulation of extracellular matrix proteins. Fibrosis-related tissue remodeling results in significant changes in tissue structure and function, including passive mechanical properties. This research area has gained significant momentum with the recent development of new tools and approaches to better characterize and understand the ability of cells to sense and respond to their biophysical environment. We use a novel hydrogel, termed CyPhyGel, to provide an advanced in vitro model of remodeling-related changes in tissue stiffness. Based on light-controlled dimerization of a Cyanobacterial Phytochrome, it enables contactless and reversible tuning of hydrogel mechanical properties with high spatial and temporal resolution. Human primary atrial fibroblasts were cultured on CyPhyGels. After 4 days of culturing on stiff (~4.6 kPa) or soft (~2.7 kPa) CyPhyGels, we analyzed fibroblast cell area and stiffness. Cells grown on the softer substrate were smaller and softer, compared to cells grown on the stiffer substrate. This difference was absent when both soft and stiff growth substrates were combined in a single CyPhyGel, with the resulting cell areas being similar to those on homogeneously stiff gels and cell stiffnesses being similar to those on homogeneously soft substrates. Using CyPhyGels to mimic tissue stiffness heterogeneities in vitro, our results confirm the ability of cardiac fibroblasts to adapt to their mechanical environment, and suggest the presence of a paracrine mechanism that tunes fibroblast structural and functional properties associated with mechanically induced phenotype conversion toward myofibroblasts. In the context of regionally increased tissue stiffness, such as upon scarring or in diffuse fibrosis, such a mechanism could help to prevent abrupt changes in cell properties at the border zone between normal and diseased tissue. The light-tunable mechanical properties of CyPhyGels and their suitability for studying human primary cardiac cells make them an attractive model system for cardiac mechanobiology research. Further investigations will explore the interactions between biophysical and soluble factors in the response of cardiac fibroblasts to spatially and temporally heterogeneous mechanical cues.

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

A novel isoform of myosin 18A (Myo18Aγ) is an essential sarcomeric protein in mouse heart

Whereas myosin 18B (Myo18B) is known to be a critical sarcomeric protein, the function of myosin 18A (Myo18A) is unclear, although it has been implicated in cell motility and Golgi shape. Here, we show that homozygous deletion (homozygous tm1a, tm1b, or tm1d alleles) of Myo18a in mouse is embryonic lethal. Reminiscent of Myo18b, Myo18a was highly expressed in the embryo heart, and cardiac-restricted Myo18a deletion in mice was embryonic lethal. Surprisingly, using Western blot analysis, we were unable to detect the known isoforms of Myo18A, Myo18Aα and Myo18Aβ, in mouse heart using a custom C-terminal antibody. However, alternative anti-Myo18A antibodies detected a larger than expected protein, and RNA-Seq analysis indicated that a novel Myo18A transcript is expressed in mouse ventricular myocytes (and human heart). Cloning and sequencing revealed that this cardiac isoform, denoted Myo18Aγ, lacks the PDZ-containing N terminus of Myo18Aα but includes an alternative N-terminal extension and a long serine-rich C terminus. EGFP-tagged Myo18Aγ expressed in ventricular myocytes localized to the level of A-bands in sarcomeres, and Myo18a knockout embryos at day 10.5 exhibited disorganized sarcomeres with wavy thick filaments. We additionally generated myeloid-restricted Myo18a knockout mice to investigate the role of Myo18A in nonmuscle cells, exemplified by macrophages, which express more Myo18Aβ than Myo18Aα, but no defects in cell shape, motility, or Golgi shape were detected. In summary, we have identified a previously unrecognized sarcomere component, a large novel isoform (denoted Myo18Aγ) of Myo18A. Thus, both members of class XVIII myosins are critical components of cardiac sarcomeres.

Atrial-like Engineered Heart Tissue: An In Vitro Model of the Human Atrium

Cardiomyocytes (CMs) generated from human induced pluripotent stem cells (hiPSCs) are under investigation for their suitability as human models in preclinical drug development. Antiarrhythmic drug development focuses on atrial biology for the treatment of atrial fibrillation. Here we used recent retinoic acid-based protocols to generate atrial CMs from hiPSCs and establish right atrial engineered heart tissue (RA-EHT) as a 3D model of human atrium. EHT from standard protocol-derived hiPSC-CMs (Ctrl-EHT) and intact human muscle strips served as comparators. RA-EHT exhibited higher mRNA and protein concentrations of atrial-selective markers, faster contraction kinetics, lower force generation, shorter action potential duration, and higher repolarization fraction than Ctrl-EHTs. In addition, RA-EHTs but not Ctrl-EHTs responded to pharmacological manipulation of atrial-selective potassium currents. RA- and Ctrl-EHTs’ behavior reflected differences between human atrial and ventricular muscle preparations. Taken together, RA-EHT is a model of human atrium that may be useful in preclinical drug screening.

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Retinoic acid induced differentiation of hiPSCs into atrial-like myocytes

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3D engineered heart tissue format favored atrial specificity compared with 2D culture

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Atrial-like engineered heart tissue can be used as a model of human atrium

Profibrotic, Electrical, and Calcium-Handling Remodeling of the Atria in Heart Failure Patients With and Without Atrial Fibrillation

Atrial fibrillation (AF) and heart failure (HF) are common cardiovascular diseases that often co-exist. Animal models have suggested complex AF-promoting atrial structural, electrical, and Ca²⁺-handling remodeling in the setting of HF, but data in human samples are scarce, particularly regarding Ca²⁺-handling remodeling. Here, we evaluated atrial remodeling in patients with severe left ventricular (LV) dysfunction (HFrEF), long-standing persistent ('chronic') AF (cAF) or both (HFrEF-cAF), and sinus rhythm controls with normal LV function (Ctl) using western blot in right-atrial tissue, sharp-electrode action potential (AP) measurements in atrial trabeculae and voltage-clamp experiments in isolated right-atrial cardiomyocytes. Compared to Ctl, expression of profibrotic markers (collagen-1a, fibronectin, periostin) was higher in HFrEF and HFrEF-cAF patients, indicative of structural remodeling. Connexin-43 expression was reduced in HFrEF patients, but not HFrEF-cAF patients. AP characteristics were unchanged in HFrEF, but showed classical indices of electrical remodeling in cAF and HFrEF-cAF (prolonged AP duration at 20% and shorter AP duration at 50% and 90% repolarization). L-type Ca²⁺ current (I_Ca,L) was significantly reduced in HFrEF, cAF and HFrEF-cAF, without changes in voltage-dependence. Potentially proarrhythmic spontaneous transient-inward currents were significantly more frequent in HFrEF and HFrEF-cAF compared to Ctl, likely resulting from increased sarcoplasmic reticulum (SR) Ca²⁺ load (integrated caffeine-induced current) in HFrEF and increased ryanodine-receptor (RyR2) single-channel open probability in HFrEF and HFrEF-cAF. Although expression and phosphorylation of the SR Ca²⁺-ATPase type-2a (SERCA2a) regulator phospholamban were unchanged in HFrEF and HFrEF-cAF patients, protein levels of SERCA2a were increased in HFrEF-cAF and sarcolipin expression was decreased in both HFrEF and HFrEF-cAF, likely increasing SR Ca²⁺ uptake and load. RyR2 protein levels were decreased in HFrEF and HFrEF-cAF patients, but junctin levels were higher in HFrEF and relative Ser2814-RyR2 phosphorylation levels were increased in HFrEF-cAF, both potentially contributing to the greater RyR2 open probability. These novel insights into the molecular substrate for atrial arrhythmias in HF-patients position Ca²⁺-handling abnormalities as a likely trigger of AF in HF patients, which subsequently produces electrical remodeling that promotes the maintenance of the arrhythmia. Our new findings may have important implications for the development of novel treatment options for AF in the context of HF.

Sex differences in cardiac electrophysiology

Women have a longer QT interval than men, which appears to evolve after puberty suggesting that sex hormones have an influence on cardiac electrophysiology. Sex hormones do in fact regulate cardiac ion channels via genomic and nongenomic pathways. Women are at greater risk for life-threatening arrhythmias under conditions that prolong the QT interval. In addition, women exhibit greater sensitivity to QT interval–prolonging drugs. Female sex has also an impact on propensity to cardiovascular disease, including atrial fibrillation. However, ex vivo recorded atrial action potentials (APs) from female and male patients in atrial fibrillation did not exhibit significant differences in shape, except that APs from women had slower upstroke velocity. It is concluded that sex-related differences should be taken into account not only in the clinics, but also in basic research.

Predictors of sinus rhythm after electrical cardioversion of atrial fibrillation: results from a data mining project on the Flec-SL trial data set

Aims

Data mining is the computational process to obtain information from a data set and transform it for further use. Herein, through data mining with supportive statistical analyses, we identified and consolidated variables of the Flecainide Short-Long (Flec-SL-AFNET 3) trial dataset that are associated with the primary outcome of the trial, recurrence of persistent atrial fibrillation (AF) or death.

Methods and results

The 'Ranking Instances by Maximizing the Area under the ROC Curve' (RIMARC) algorithm was applied to build a classifier that can predict the primary outcome by using variables in the Flec-SL dataset. The primary outcome was time to persistent AF or death. The RIMARC algorithm calculated the predictive weights of each variable in the Flec-SL dataset for the primary outcome. Among the initial 21 parameters, 6 variables were identified by the RIMARC algorithm. In univariate Cox regression analysis of these variables, increased heart rate during AF and successful pharmacological conversion (PC) to sinus rhythm (SR) were found to be significant predictors. Multivariate Cox regression analysis revealed successful PC as the single relevant predictor of SR maintenance. The primary outcome risk was 3.14 times (95% CI:1.7-5.81) lower in those who had successful PC to SR than those who needed electrical cardioversion.

Conclusions

Pharmacological conversion of persistent AF with flecainide without the need for electrical cardioversion is a powerful and independent predictor of maintenance of SR. A strategy of flecainide pretreatment for 48 h prior to planned electrical cardioversion may be a useful planning of a strategy of long-term rhythm control.

Inverse remodelling of K2P3.1 K+ channel expression and action potential duration in left ventricular dysfunction and atrial fibrillation: implications for patient-specific antiarrhythmic drug therapy

Aims

Atrial fibrillation (AF) prevalence increases with advanced stages of left ventricular (LV) dysfunction. Remote proarrhythmic effects of ventricular dysfunction on atrial electrophysiology remain incompletely understood. We hypothesized that repolarizing K2P3.1 K+ channels, previously implicated in AF pathophysiology, may contribute to shaping the atrial action potential (AP), forming a specific electrical substrate with LV dysfunction that might represent a target for personalized antiarrhythmic therapy.

Methods and results

A total of 175 patients exhibiting different stages of LV dysfunction were included. Ion channel expression was quantified by real-time polymerase chain reaction and Western blot. Membrane currents and APs were recorded from atrial cardiomyocytes using the patch-clamp technique. Severely reduced LV function was associated with decreased atrial K2P3.1 expression in sinus rhythm patients. In contrast, chronic (c)AF resulted in increased K2P3.1 levels, but paroxysmal (p)AF was not linked to significant K2P3.1 remodelling. LV dysfunction-related suppression of K2P3.1 currents prolonged atrial AP duration (APD) compared with patients with preserved LV function. In individuals with concomitant LV dysfunction and cAF, APD was determined by LV dysfunction-associated prolongation and by cAF-dependent shortening, respectively, consistent with changes in K2P3.1 abundance. K2P3.1 inhibition attenuated APD shortening in cAF patients irrespective of LV function, whereas in pAF subjects with severely reduced LV function, K2P3.1 blockade resulted in disproportionately high APD prolongation.

Conclusion

LV dysfunction is associated with reduction of atrial K2P3.1 channel expression, while cAF leads to increased K2P3.1 abundance. Differential remodelling of K2P3.1 and APD provides a basis for patient-tailored antiarrhythmic strategies.

Modeling Short QT Syndrome Using Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

BACKGROUND

Short QT syndrome (SQTS), a disorder associated with characteristic ECG QT-segment abbreviation, predisposes affected patients to sudden cardiac death. Despite some progress in assessing the organ-level pathophysiology and genetic changes of the disorder, the understanding of the human cellular phenotype and discovering of an optimal therapy has lagged because of a lack of appropriate human cellular models of the disorder. The objective of this study was to establish a cellular model of SQTS using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).

METHODS AND RESULTS

This study recruited 1 patient with short QT syndrome type 1 carrying a mutation (N588K) in KCNH2 as well as 2 healthy control subjects. We generated hiPSCs from their skin fibroblasts, and differentiated hiPSCs into cardiomyocytes (hiPSC-CMs) for physiological and pharmacological studies. The hiPSC-CMs from the patient showed increased rapidly activating delayed rectifier potassium channel current (IKr) density and shortened action potential duration compared with healthy control hiPSC-CMs. Furthermore, they demonstrated abnormal calcium transients and rhythmic activities. Carbachol increased the arrhythmic events in SQTS but not in control cells. Gene and protein expression profiling showed increased KCNH2 expression in SQTS cells. Quinidine but not sotalol or metoprolol prolonged the action potential duration and abolished arrhythmic activity induced by carbachol.

CONCLUSIONS

Patient-specific hiPSC-CMs are able to recapitulate single-cell phenotype features of SQTS and provide novel opportunities to further elucidate the cellular disease mechanism and test drug effects.