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Ventricular cardiac arrhythmia (VA) arises in acquired or congenital heart disease. Long QT syndrome type-3 (LQT3) is a congenital form of VA caused by cardiac sodium channel (INaL) SCN5A mutations that prolongs cardiac action potential (AP) and enhances INaL current. Mexiletine inhibits INaL and shortens the QT interval in LQT3 patients. Above therapeutic doses, mexiletine prolongs the cardiac AP. We explored structure-activity relationships (SAR) for AP shortening and prolongation using dynamic medicinal chemistry and AP kinetics in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Using patient-derived LQT3 and healthy hiPSC-CMs, we resolved distinct SAR for AP shortening and prolongation effects in mexiletine analogues and synthesized new analogues with enhanced potency and selectivity for INaL. This resulted in compounds with decreased AP prolongation effects, increased metabolic stability, increased INaL selectivity, and decreased avidity for the potassium channel. This study highlights using hiPSC-CMs to guide medicinal chemistry and "drug development in a dish".

Aims We prospectively isolate and characterize first and second heart field- and nodal-like cardiomyocytes using a double reporter line from human embryonic stem cells. Our double reporter line utilizes two important transcription factors in cardiac development, TBX5 and NKX2-5. TBX5 expression marks first heart field progenitors and cardiomyocytes while NKX2-5 is expressed in nearly all myocytes of the developing heart (excluding nodal cells). We address the shortcomings of prior work in the generation of heart-field specific cardiomyocytes from induced pluripotent stem cells and provide a comprehensive early developmental transcriptomic as well as electrophysiological analyses of these three populations. Methods and results Transcriptional, immunocytochemical, and functional studies support the cellular identities of isolated populations based on the expression pattern of NKX2-5 and TBX5. Importantly, bulk and single-cell RNA sequencing analyses provide evidence of unique molecular signatures of isolated first and second heart-field cardiomyocytes, as well as nodal-like cells. Extensive electrophysiological analyses reveal dominant atrial action potential phenotypes in first and second heart fields in alignment with our findings in single-cell RNA sequencing. Lastly, we identify two novel surface markers, POPDC2 and CORIN, that enables purification of cardiomyocytes and first heart field cardiomyocytes, respectively. Conclusions We describe a high yield approach for isolation and characterization of human embryonic stem cell-derived heart field specific and nodal-like cardiomyocytes. Obtaining enriched populations of these different cardiomyocyte subtypes increases the resolution of gene expression profiling during early cardiogenesis, arrhythmia modeling, and drug screening. This paves the way for the development of effective stem cell therapy to treat diseases that affect specific regions of the heart or chamber-specific congenital heart defects. Translational perspective Myocardial infarction leads to irreversible loss of cardiomyocytes and eventually heart failure. Human embryonic stem cells (hESCs) can be differentiated to cardiomyocytes and are considered a potential source of cell therapy for cardiac regeneration. However, current differentiation strategies yield a mixture of cardiomyocyte subtypes and safety concerns stemming from the use of a heterogenous population of cardiomyocytes have hindered its application. Here, we report generation of enriched heart field-specific cardiomyocytes using a hESC double reporter. Our study facilitates investigating early human cardiogenesis in vitro and generating chamber-specific cardiomyocytes to treat diseases that affect specific regions of the heart.

Myocardial infarction (MI) can lead to irreversible loss of cardiomyocytes (CMs), primarily localized to the left ventricle (LV) of the heart. The CMs of the LV are predominantly derived from first heart field (FHF) progenitors, whereas the majority of CMs within the right ventricle originate from the second heart field (SHF) during early cardiogenesis. Human embryonic stem cells (hESCs) serve as a valuable source of CMs for understanding early cardiac development and lineage commitment of CMs within these two heart fields that ultimately enable the development of more effective candidates for cell therapy. An ideal candidate may be FHF CMs that share the same ontogeny with the LV CMs that die after MI. We previously generated a double reporter hESC line that utilizes two important cardiac transcription factors, TBX5 and NKX2-5. TBX5 marks FHF progenitors and CMs, while NKX2-5 is expressed in nearly all myocytes of the developing heart. Here, we describe a step-by-step approach to efficiently generate FHF and SHF CMs using this double reporter hESC line. In addition, this approach can be applied to any non-genetically modified hESC lines to enrich FHF and SHF CMs. Obtaining enriched populations of these two CM subtypes provides a platform for downstream comparative analyses and in vitro studies to facilitate a deeper understanding of cardiovascular lineage commitment and the development of more effective candidates for cell therapy to treat diseases or defects that affect specific regions of the heart.

Ventricular arrhythmias, a leading cause of sudden cardiac death, can be triggered by cardiomyocyte early afterdepolarizations (EADs). EADs can result from an abnormal late activation of L-type Ca2+ channels (LTCCs). Current LTCC blockers (class IV antiarrhythmics), while effective at suppressing EADs, block both early and late components of ICa,L, compromising inotropy. However, computational studies have recently demonstrated that selective reduction of late ICa,L (Ca2+ influx during late phases of the action potential) is sufficient to potently suppress EADs, suggesting that effective antiarrhythmic action can be achieved without blocking the early peak ICa,L, which is essential for proper excitation–contraction coupling. We tested this new strategy using a purine analogue, roscovitine, which reduces late ICa,L with minimal effect on peak current. Scaling our investigation from a human CaV1.2 channel clone to rabbit ventricular myocytes and rat and rabbit perfused hearts, we demonstrate that (1) roscovitine selectively reduces ICa,L noninactivating component in a human CaV1.2 channel clone and in ventricular myocytes native current, (2) the pharmacological reduction of late ICa,L suppresses EADs and EATs (early after Ca2+ transients) induced by oxidative stress and hypokalemia in isolated myocytes, largely preserving cell shortening and normal Ca2+ transient, and (3) late ICa,L reduction prevents/suppresses ventricular tachycardia/fibrillation in ex vivo rabbit and rat hearts subjected to hypokalemia and/or oxidative stress. These results support the value of an antiarrhythmic strategy based on the selective reduction of late ICa,L to suppress EAD-mediated arrhythmias. Antiarrhythmic therapies based on this idea would modify the gating properties of CaV1.2 channels rather than blocking their pore, largely preserving contractility.

Cardiovascular disease is a leading cause of mortality in the world and is exacerbated by the presence of cardiac fibrosis, defined by the accumulation of non-contractile extracellular matrix proteins. Cardiac fibrosis is directly linked to cardiac dysfunction and increased risk of arrhythmia. Despite its prevalence, there is a lack of efficacious therapies for inhibiting or reversing cardiac fibrosis, largely due to the complexity of the cell types and signaling pathways involved. Ongoing research has aimed to understand the mechanisms of cardiac fibrosis and develop new therapies for treating scar formation. Major approaches include preventing the formation of scar tissue and replacing fibrous tissue with functional cardiomyocytes. While targeting the renin-angiotensin-aldosterone system is currently used as the standard line of therapy for heart failure, there has been increased interest in inhibiting the transforming growth factor-β signaling pathway due its established role in cardiac fibrosis. Significant advances in cell transplantation therapy and biomaterials engineering have also demonstrated potential in regenerating the myocardium. Novel techniques, such as cellular direct reprogramming, and molecular targets, such as non-coding RNAs and epigenetic modifiers, are uncovering novel therapeutic options targeting fibrosis. This review provides an overview of current approaches and discuss future directions for treating cardiac fibrosis.

Cardiovascular disease (CVD) is the leading cause of death worldwide. Human embryonic stem cell (hESC)-derived cardiovascular progenitors (CVPs) or cardiomyocytes (CMs) represent a promising candidate for cell-based therapies to treat CVD. Myocardial infarction leads to extensive CM death mainly within the left ventricle, which is predominantly derived from the first heart field (FHF) during embryonic development. We postulate that the generation of chamber-specific CMs may play a key role in the development of safe and efficacious regenerative therapy. As a first step, we generated and characterized a FHF-specific TBX5-TdTomato +/W hESC reporter line. We show that TBX5 + cells represent an enriched population of FHF CVPs that can give rise to CMs, endothelial, and smooth muscle cells in vitro . Interestingly, we observed that TBX5 - cells can also generate contractile CMs. Bulk RNA-sequencing analysis at different stages of development suggested that TBX5 - cells are enriched for second heart field (SHF) CMs. To enable prospective isolation of FHF and SHF CMs, we generated a double transgenic TBX5-TdTomato +/W /NKX2-5 eGFP/W hESC reporter line. We performed detailed electrophysiological, functional, and transcriptional studies to characterize the heart-field specificity of these hESC-derived CMs. Electrophysiological studies revealed that, despite the presence of atrial and ventricular action potentials (APs) in both FHF and SHF, there are significant differences in their AP duration and cycle length. In addition, both FHF and SHF CMs responded appropriately to adrenergic stimuli. Single-cell RNA sequencing analysis confirmed the absence of nodal genes within these populations and provided evidence of unique molecular signatures for isolating FHF- and SHF-like CMs. Finally, we identified CORIN as a novel cell surface marker for FHF CMs. Our studies provide a platform for investigating in vitro cardiovascular development, drug screening, and may facilitate a safe approach for cell therapy in heart disease.

Hypertension is a risk factor for sudden cardiac death caused by ventricular tachycardia and fibrillation (VT/VF). We hypothesized that, in early hypertension, the susceptibility to stress-induced VT/VF increases. We compared the susceptibility of 5- to 6-month-old male spontaneously hypertensive rats (SHR) and age/sex-matched normotensive rats (NR) to VT/VF during challenge with oxidative stress (H2 O2 ; 0.15 mmol l(-1) ). We found that only SHR hearts exhibited left ventricular fibrosis and hypertrophy. H2 O2 promoted VT in all 30 SHR but none of the NR hearts. In 33% of SHR cases, focal VT degenerated to VF within 3 s. Simultaneous voltage-calcium optical mapping of Langendorff-perfused SHR hearts revealed that H2 O2 -induced VT/VF arose spontaneously from focal activations at the base and mid left ventricular epicardium. Microelectrode recording of SHR hearts showed that VT was initiated by early afterdepolarization (EAD)-mediated triggered activity. However, despite the increased susceptibility of SHR hearts to VT/VF, patch clamped isolated SHR ventricular myocytes developed EADs and triggered activity to the same extent as NR ventricular myocytes, except with larger EAD amplitude. During the early stages of hypertension, when challenged with oxidative stress, SHR hearts showed an increased ventricular arrhythmogenicity that stems primarily from tissue remodelling (hypertrophy, fibrosis) rather than cellular electrophysiological changes. Our findings highlight the need for early hypertension treatment to minimize myocardial fibrosis, ventricular hypertrophy, and arrhythmias.

Author(s): Pezhouman, Arash; Cao, Hong; Fishbein, Michael C; Belardinelli, Luiz; Weiss, James N; Karagueuzian, Hrayr S | Abstract: BackgroundThe mechanism of Atrial Fibrillation (AF) that emerges spontaneously during acute oxidative stress is poorly defined and its drug therapy remains suboptimal. We hypothesized that oxidative activation of Ca-calmodulin dependent protein kinase (CaMKII) promotes Early Afterdepolarization-(EAD)-mediated triggered AF in aged fibrotic atria that is sensitive to late Na current (INa-L) blockade.Method and resultsHigh-resolution voltage optical mapping of the Left and Right Atrial (LA a RA) epicardial surfaces along with microelectrode recordings were performed in isolated-perfused male Fisher 344 rat hearts in Langendorff setting. Aged atria (23-24 months) manifested 10-fold increase in atrial tissue fibrosis compared to young/adult (2-4 months) atria (Pl0001. Spontaneous AF arose in 39 out of 41 of the aged atria but in 0 out of 12 young/adult hearts (Pl001) during arterial perfusion of with 0.1 mm of hydrogen peroxide (H2O2). Optical Action Potential (AP) activation maps showed that the AF was initiated by a focal mechanism in the LA suggestive of EAD-mediated triggered activity. Cellular AP recordings with glass microelectrodes from the LA epicardial sites showing focal activity confirmed optical AP recordings that the spontaneous AF was initiated by late phase 3 EAD-mediated triggered activity. Inhibition of CaMKII activity with KN-93 (1 μM) (N=6) or its downstream target, the enhanced INa-L with GS-967 (1 μM), a specific blocker of INa-L (N=6), potently suppressed the AF and prevented its initiation when perfused 15 min prior to H2O2 (n=6).ConclusionsIncreased atrial tissue fibrosis combined with acute oxidative activation of CaMK II Initiate AF by EAD-mediated triggered activity. Specific block of the INa-L with GS-967 effectively suppresses the AF. Drug therapy of oxidative AF in humans with traditional antiarrhythmic drugs remains suboptimal; suppressing INa-L offers a potential new strategy for effective suppression of oxidative human AF that remains suboptimal.

While recent advances clarified the molecular and cellular modes of action of antiarrhythmic drugs (AADs), their link to suppression of dynamical arrhythmia mechanisms remains only partially understood. The current classifications of AADs (Classes I, III, and IV) rely on blocking peak Na, K and L-type calcium currents (ICa,L), with Class II with dominant beta receptor blocking activity and Class V including drugs with diverse classes of actions. The discovery that the calcium and redox sensor, cardiac Ca/calmodulin-dependent protein kinase II (CaMKII) enhances both the late Na (INa-L) and the late ICa,L in patients at high risk of VT/VF provided a new and a rational AAD target. Pathological rise of either or both of INa-L and late ICa,L are demonstrated to promote cellular early afterdepolarizations (EADs) and EAD-mediated triggered activity that can initiate VT/VF in remodeled hearts. Selective inhibition of the INa-L without affecting their peak transients with the highly specific prototype drug, GS-967 suppresses these EAD-mediated VT/VFs. As in the case of INa-L, selective inhibition of the late ICa,L without affecting its peak with the prototype drug, roscovitine suppressed oxidative EAD-mediated VT/VF. These findings indicate that specific blockers of the late inward currents without affecting their peaks (gating modifiers), offer a new and effective AAD class action i.e., “Class VI.” The development of safe drugs with selective Class VI actions provides a rational and effective approach to treat VT/VF particularly in cardiac conditions associated with enhanced CaMKII activity such as heart failure.

Hypertension is a risk factor for sudden cardiac death caused by ventricular tachycardia and fibrillation (VT/VF). We hypothesized that during the early stages of uncontrolled hypertension, the susceptibility to otherwise benign stress promotes VT/VF. To test this hypothesis we compared the susceptibility of 5-6-month-old male spontaneously hypertensive rats (SHR) and age/gender-matched normotensive rats (NR) to VT/VF during challenge with mild oxidative stress (H2O2, 0.15 mmol/L). H2O2 initiated spontaneous VT in all 30 SHR but none of the NR hearts. In 33% of SHR cases, focal VT degenerated to VF within 3 s. Simultaneous voltage-Ca optical mapping of Langendorff-perfused SHR hearts revealed that H2O2-induced VT/VF arose spontaneously from focal activations at the base and mid left ventricular epicardium. Microelectrode recording of SHR hearts showed that VT was initiated by early afterdepolarization (EAD)-mediated triggered activity. However, despite the increased susceptibility of SHR hearts to VT/VF, patch-clamped isolated SHR ventricular myocytes developed EADs and triggered activity to the same extent as NR ventricular myocytes, except that SHR myocytes had a relatively larger EAD amplitude. Histopathological and echocardiographic studies showed that compared to NR, SHR hearts exhibited increased left ventricular fibrosis and mild hypertrophy. We conclude that during the early stages of hypertension (i.e., prior to the development of overt heart failure) SHR hearts manifest increased susceptibility to VT/VF when challenged with mild oxidative stress that appears to be related to tissue remodeling (i.e., hypertrophy, fibrosis) moreso than cellular electrophysiological remodeling. Our findings highlight the need for aggressive treatment during the stages of early hypertension to minimize myocardial fibrosis, ventricular hypertrophy, and arrhythmias.

Background— Hypokalemia is known to promote ventricular arrhythmias, especially in combination with class III antiarrhythmic drugs like dofetilide. Here, we evaluated the underlying molecular mechanisms. Methods and Results— Arrhythmias were recorded in isolated rabbit and rat hearts or patch-clamped ventricular myocytes exposed to hypokalemia (1.0–3.5 mmol/L) in the absence or presence of dofetilide (1 μmol/L). Spontaneous early afterdepolarizations (EADs) and ventricular tachycardia/fibrillation occurred in 50% of hearts at 2.7 mmol/L [K] in the absence of dofetilide and 3.3 mmol/L [K] in its presence. Pretreatment with the Ca-calmodulin kinase II (CaMKII) inhibitor KN-93, but not its inactive analogue KN-92, abolished EADs and hypokalemia-induced ventricular tachycardia/fibrillation, as did the selective late Na current ( I Na ) blocker GS-967. In intact hearts, moderate hypokalemia (2.7 mmol/L) significantly increased tissue CaMKII activity. Computer modeling revealed that EAD generation by hypokalemia (with or without dofetilide) required Na-K pump inhibition to induce intracellular Na and Ca overload with consequent CaMKII activation enhancing late I Na and the L-type Ca current. K current suppression by hypokalemia and dofetilide alone in the absence of CaMKII activation were ineffective at causing EADs. Conclusions— We conclude that Na-K pump inhibition by even moderate hypokalemia plays a critical role in promoting EAD-mediated arrhythmias by inducing a positive feedback cycle activating CaMKII and enhancing late I Na . Class III antiarrhythmic drugs like dofetilide sensitize the heart to this positive feedback loop.

Enhanced late inward Na current (INa-L) modulates action potential duration (APD) and plays a key role in the genesis of early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs) and triggered activity.The purpose of this study was to define the influence of selective block of INa-L on EAD- and DAD-mediated triggered ventricular tachycardia (VT) and ventricular fibrillation (VF) in intact hearts using (GS967), a selective and potent (IC50 = 0.13 ± 0.01 μM) blocker of INa-L.VT/VF were induced either by local aconitine injection (50 μg) in the left ventricular muscle of adult (3-4 months) male rats (N = 21) or by arterial perfusion of 0.1 mM hydrogen peroxide (H2O2) in aged male rats (24-26 months, N = 16). The left ventricular epicardial surface of the isolated-perfused hearts was optically mapped using fluorescent voltage-sensitive dye, and microelectrode recordings of action potentials were made adjacent to the aconitine injection site. The suppressive and preventive effects of GS967 (1 μM) against EAD/DAD-mediated VT/VF were then determined.Aconitine induced VT in all 13 hearts studied. Activation map (N = 6) showed that the VT was initiated by a focal activity arising from the aconitine injection site (cycle length [CL] 84 ± 12) that degenerated to VF (CL 52 ± 8 ms) within a few seconds. VF was maintained by multifocal activity with occasional incomplete reentrant wavefronts. Administration of GS967 suppressed the VT/VF in 10 of 13 hearts (P.001). Preexposure to GS967 for 15 minutes before aconitine injection prevented initiation of VT/VF in 5 of 8 additional hearts (P.02). VF reoccurred within 10 minutes on washout of GS967. Microelectrode recordings (N = 7) showed that VT/VF was initiated by EAD- and DAD-mediated triggered activity at CL of 86 ± 14 ms (NS from VT CL) that preceded the VF. GS967 shortened APD, flattened the slope of the dynamic APD restitution curve, and reduced APD dispersion from 42 ± 12 ms to 8 ± 3 ms (P.01). H2O2 perfusion in eight fibrotic aged hearts promoted EAD-mediated focal VT/VF, which was suppressed by GS967 in five hearts (P.02).The selective INa-L blocker GS967 effectively suppresses and prevents aconitine and oxidative stress-induced EADs, DADs, and focal VT/VF. Suppression of EADs, DADs, and reduction of APD dispersion make GS967 a potentially useful antiarrhythmic drug in conditions of enhanced INa-L.

L-type calcium current (ICa,L) plays a central role in counterbalancing potassium current during the plateau phase of cardiac action potential (AP). When repolarization reserve is reduced, the aberrant reactivation of ICa,L during repolarization can cause transient depolarization events called Early Afterdepolarizations (EADs), a recognized trigger of cardiac arrhythmias. Recently we demonstrated that EADs are highly sensitive to the steady-state voltage-dependent properties of ICa,L. Using the dynamic clamp technique in isolated rabbit ventricular myocytes under EAD-favoring condition (i.e. oxidative stress or hypokalemia), we found that selectively inhibiting the non-inactivating component (late) of ICa,L potently suppressed EAD-occurrence. Roscovitine, a purine analog, has been shown to reduce the late ICa,L component while leaving peak unperturbed. Here we tested the ability of this drug to suppress EADs in isolated rabbit ventricular myocytes induced by exposure to H2O2 (600µM). This oxidative stress caused EADs in 81% [95% CI: 66-95%] of the APs, prolonging AP duration (APD90) from 276 ms [97-433 ms] to 840ms [285-1967ms] (n=4). Addition of roscovitine (20µM) completely abolished EADs and reduced the APD90 to 189 ms [155-222 ms]. Moreover, roscovitine did not significantly perturb the Ca transient, implying maintenance of normal excitation-contraction coupling. We next exposed isolated aged rat hearts to 0.1 mM H2O2 to induce EADs and spontaneous ventricular tachycardia/fibrillation (VT/VF). In all tested hearts (n=4), roscovitine terminated VT/VF and led to resumption of sinus rhythm within 10 min. Our study offers additional proof of concept supporting a pharmacological strategy to target late ICa,L to prevent EAD-mediated arrhythmias, with roscovitine as an archetypal drug to guide the development of this new class of antiarrhythmics.