Background: Sudden cardiac death (SCD) is a major complication of diet-induced obesity. Whether early metabolic stress from nutrient overload per se, prior to overt obesity or related remodeling, can independently promote arrhythmias remains unclear. Fatty acids can allosterically stimulate AMP-activated protein kinase (AMPK), a key metabolic sensor that preserves myocardial function following ischemia-reperfusion (I/R). We hypothesized that AMPK signaling modulates the electrophysiological (EP) response to high-fat diet (HFD)-induced metabolic stress. Methods: Wild-type (WT) and AMPK kinase-dead (AMPK-KD) mice were placed on HFD for 8 weeks beginning at 4 weeks of age; controls remained on a normal diet (ND). This regimen led to minimal weight gain (<5g), no plasma lipid changes and preserved LV function. Arrhythmia susceptibility was assessed ex vivo using rapid pacing and I/R challenge. EP properties were evaluated via high resolution optical mapping; mechanisms were examined by western blotting, confocal, and electron microscopy. Results: HFD-fed WT hearts showed no increase in arrhythmia risk (ND WT: 0/7; HFD WT: 1/9; p>0.99) and exhibited improved post-ischemic conduction recovery (CV recovery >90%: ND WT 1/6 vs HFD WT 5/5; p=0.015), with increased phosphorylation of AMPK targets (ACC, raptor). In contrast, AMPK-KD hearts were arrhythmia-prone regardless of diet (ND KD: 4/9; HFD KD: 5/8), due to slowed (~30%, p<0.001) and aberrant conduction. These abnormalities, arising in structurally normal hearts, occurred despite Cx43 and Nav1.5 upregulation and were linked to impaired ULK1 phosphorylation and autophagic turnover, leading to unphosphorylated Cx43 accumulation at intercalated discs. AMPK-KD hearts also exhibited mitochondrial fragmentation, reduced fusion proteins, and elevated ROS under HFD, which accelerated repolarization (APD75: ND KD 46.7 ms vs HFD KD 33.8 ms; p<0.001) and promoted sustained VT (VT duration: ND KD 5.6 min vs HFD KD 18.7 min; p=0.011). Conclusions: AMPK preserves EP stability during early nutrient stress by coordinating redox balance, mitochondrial integrity, and conduction-related protein homeostasis. Its loss in aging and advanced metabolic disease may be a key link between HFD and increased SCD risk.

Background:Fibroblast growth factor homologous factor (FHF) variants associate with arrhythmias. Although FHFs are best characterized as regulators of voltage gated sodium channel (VGSC) gating, recent studies suggest broader, non-VGSC-related functions, including regulation of Cx43 gap junctions and/or hemichannels, mechanisms that have generally been understudied or disregarded. Methods:We assessed cardiac conduction and cardiomyocyte action potentials in mice with constitutive cardiac-specific Fgf13 ablation (c Fgf13 KO ) while targeting Cx43 gap junctions and hemichannels pharmacologically. Using immunostaining and biochemistry, we characterized FGF13 regulation of Cx43 abundance and subcellular distribution. With proximity labeling proteomics, we investigated novel candidate mechanisms underlying FGF13 regulation of Cx43. Results:FGF13 ablation prolonged the QRS and QT intervals on the surface electrocardiogram. Carbenoxolone, a Cx43 gap junction uncoupler, markedly prolonged the QRS duration leading to conduction system block in c Fgf13 KO but not in WT mice. Optical mapping revealed markedly decreased conduction velocity (CV) during ventricular pacing. Microscopy revealed markedly perturbed trafficking of Cx43, reduced localization in the intercalated disc, and suggested decreased membrane Cx43 but increased Cx43 hemichannels in cardiomyocytes from c Fgf13 KO mice. Resting membrane potential (RMP) was depolarized and APD50 was prolonged in c Fgf13 KO cardiomyocytes. Both were restored towards wildtype (WT) values with Gap19 (a Cx43 hemichannel inhibitor), expression of FGF13, or expression of a mutant FGF13 incapable of binding to VGSCs, emphasizing VGSC-independent regulation by FGF13. To assess the functional impact of RMP depolarization, hearts were subjected to hypokalemia, which had no effect in WT hearts but fully rescued CV in c Fgf13 KO hearts. Proteomic analyses revealed candidate roles for FGF13 in the regulation of vesicular-mediated transport. Biochemistry and immunocytochemistry showed that FGF13 ablation destabilized microtubules and reduced the expression of tubulins and MAP4, the major cardiac microtubule regulator. Conclusions:FGF13 regulates microtubule-dependent trafficking and targeting of Cx43, thereby impacting cardiac impulse propagation via VGSC-independent mechanisms.

Sudden cardiac death (SCD) is a major complication of obesity, yet it remains unclear whether early metabolic stress, prior to the onset of overt obesity or structural remodeling, can independently promote arrhythmias. In vitro studies suggest that fatty acids can allosterically stimulate AMP-activated protein kinase (AMPK), a key metabolic sensor known to preserve myocardial viability and mitochondrial function following ischemia-reperfusion (I/R) injury. We hypothesized that AMPK signaling critically modulates the electrophysiological (EP) response to high-fat diet (HFD)-induced metabolic stress. Methods:To test this, wild-type (WT) and AMPK kinase-dead (AMPK-KD) mice were subjected to an 8-week HFD regimen beginning at 4 weeks of age. Controls remained on normal diet (ND) for the same duration. Arrhythmia susceptibility was assessed ex vivo using rapid pacing and I/R challenge protocols. Changes in the EP substrate were defined by high-resolution optical action potential mapping. Underlying mechanisms were probed using western blotting, confocal and transmission electron microscopy. Results:HFD-fed wild-type (WT) hearts did not display increased arrhythmia susceptibility in response to either burst pacing or I/R challenge. On the contrary, they exhibited a paradoxical enhancement in post-ischemic EP recovery compared to ND-fed controls. This improvement was associated with increased phosphorylation of canonical AMPK targets, including acetyl-CoA carboxylase (ACC) and raptor, consistent with the activation of a cardioprotective metabolic program. In sharp contrast, AMPK-deficient (AMPK-KD) hearts demonstrated heightened vulnerability to inducible ventricular tachycardia (VT), irrespective of diet. Conduction slowing emerged as an early EP abnormality in these hearts and served as the initial substrate (or 'first hit') that promoted their increased incidence of non-sustained VT. Notably, this conduction impairment arose in conjunction with an increase (rather than decrease) in Cx43 and Nav1.5 protein expression. Mechanistically, defective conduction in AMPK-KD hearts was linked to impaired autophagic degradation of intercalated disc proteins resulting from reduced phosphorylation of ULK1, a downstream effector of AMPK. Consequently, unphosphorylated Cx43 accumulated at the intercalated disc, likely replacing phosphorylated isoforms (p-Cx43). In addition, AMPK-KD hearts exhibited swollen, fragmented mitochondria and reduced levels of mitochondrial fusion proteins. Upon HFD challenge, this vulnerable mitochondrial substrate generated excessive reactive oxygen species (ROS) coinciding with accelerated repolarization. Together, impaired conduction and action potential shortening promoted VT sustenance in HFD-fed AMPK-deficient hearts. Conclusions:Our findings identify AMPK as a key metabolic regulator that integrates redox balance, mitochondrial integrity, and protein homeostasis to preserve cardiac excitability during early nutrient overload. Loss of AMPK signaling, as occurs with aging and advanced metabolic disease, may therefore represent a pivotal mechanism linking HFD to increased SCD risk.

Background:Mutations in DSP, which encodes the protein desmoplakin, lead to cardiomyopathy with unusually high penetrance. Clinical features include ventricular tachyarrhythmias, fibro-fatty infiltration of both ventricles, and ultimately dilated cardiomyopathy. While some data have been gathered to explain the electrophysiological and contractile consequences of desmoplakin cardiomyopathy, a comprehensive mechanism linking DSP mutations to ventricular dilation and heart failure remains elusive. Methods:We use iPSC-derived engineered heart tissue (EHT) bearing a functional desmoplakin haploinsufficiency to model the heart failure phenotype that occurs in desmoplakin cardiomyopathy. Functional haploinsufficiency is secondary to a missense mutation, R451G, that results in proteolytic degradation of desmoplakin with no detectable protein. We complement functional data obtained in tissue-engineered constructs with cell biology assays in 2D cardiomyocytes to glean insights into the mechanism and mechanobiology of desmoplakin cardiomyopathy. Results:Engineered heart tissues harboring a desmoplakin insufficiency recapitulate a patient phenotype notable for hypocontractility and ventricular dilation. Surprisingly, DSP-mutant tissues exhibited a shortened resting sarcomere length that was dependent on protein kinase C activity. Concurrently, mechanical load on α-catenin was increased, suggesting a mechanism by which desmosomal insufficiency redistributes force to adherens junctions. Excessive loading on adherens junctions may act as a stimulus for avid insertion of series sarcomeres, shortening the length per sarcomere, and resulting in a contractile deficit. PKC inhibition rescues shortened sarcomere length in DSP-mutant tissues, suggesting that it could be a target for future molecular therapies. Conclusions:Our study uncovers a novel mechanism underlying systolic dysfunction in desmoplakin cardiomyopathy. We not only recapitulate the disease phenotype, but we identify sarcomere length regulation through altered force transmission at the intercalated disc as a previously-unrecognized mechanism.

Background:Loss of stromal interaction molecule 1 (STIM1) expression in smooth muscle cells protects against ischemia-reperfusion (I/R) injury. Whether and how decreased STIM1 expression in cardiomyocytes (CM) impacts cardiac remodeling in response to I/R injury remains unknown. Objective:To examine mechanisms by which decreased CM-STIM1 expression in the adult heart modulates cardiac function before and after I/R injury. Methods:8-week old mice underwent cardiotropic AAV9-mediated gene transfer of shRNA directed against STIM1 (shSTIM1). Control (Ctrl) mice underwent shRNA luciferase or PBS injections. Ctrl and shSTIM1 mice were then challenged by 30-min coronary occlusion to induce MI, in-vivo . Mechanical, structural and electrophysiological (EP) properties were compared 1-week following MI. In a second cohort of mice, the impact of CM-STIM1 knockdown per se on upstream metabolic signaling, mitochondrial ultrastructure, and electrophysiological properties were studied. Results:CM-STIM1 expression was markedly decreased in shSTIM1 vs Ctrl hearts. Challenge with in-vivo I/R injury resulted in more pronounced (p<0.0001) LV dysfunction indexed by % drop in fractional shortening in shSTIM1 (44.3%) vs Ctrl (12.2%) hearts 1-week post-MI. Similarly, post-MI structural remodeling and the extent of fibrosis were more severe in shSTIM1 vs Ctrl despite comparable infarct size (p=0.514). Consistently, shSTIM1 exhibited greater impairment in post-MI EP function including predisposition to spatially-discordant action potential alternans. To understand mechanisms underlying this differential remodeling, we examined the impact of CM-STIM1 downregulation on mitochondrial ultrastructure and regulation by metabolic signaling. Quantification of mitochondrial morphology revealed smaller, more rounded mitochondria caused by CM-STIM1 downregulation per se . Underlying these changes was a marked (by 55%, p=0.0057) increase in phosphorylated (p)DRP1 at S616 along with reduced OPA1 expression. Mitochondrial alterations were associated with significant decreases in AMPK downstream signaling with loss of phosphorylated-to-total Raptor and ACC expression in shSTIM1-vs-Ctrl hearts consistent with impaired fatty acid oxidation. These MI-independent metabolic alterations coincided with higher pro-arrhythmic vulnerability under conditions of elevated heart rate. Conclusions:Our findings reveal that decreased CM-STIM1 expression exacerbates post-MI remodeling likely by altering metabolic processes and mitochondrial network dynamics.Functionally, STIM1-dependent mitochondrial alterations impact EP function during conditions of elevated heart rate even without the confounding influence of MI.

Mitochondria play a central role in cellular energy metabolism, and their dysfunction is increasingly recognized as a critical factor in the pathogenesis of diabetes-related cardiac pathophysiology, including vulnerability to ischemic events that culminate in myocardial infarction on the one hand and ventricular arrhythmias on the other. In diabetes, hyperglycemia and altered metabolic substrates lead to excessive production of reactive oxygen species (ROS) by mitochondria, initiating a cascade of oxidative stress that damages mitochondrial DNA, proteins, and lipids. This mitochondrial injury compromises the efficiency of oxidative phosphorylation, leading to impaired ATP production. The resulting energy deficit and oxidative damage contribute to functional abnormalities in cardiac cells, placing the heart at an increased risk of electromechanical dysfunction and irreversible cell death in response to ischemic insults. While cardiac mitochondria are often considered to be relatively autonomous entities in their capacity to produce energy and ROS, their highly dynamic nature within an elaborate network of closely-coupled organelles that occupies 30-40% of the cardiomyocyte volume is fundamental to their ability to exert intricate regulation over global cardiac function. In this article, we review evidence linking the dynamic properties of the mitochondrial network to overall cardiac function and its response to injury. We then highlight select studies linking mitochondrial ultrastructural alterations driven by changes in mitochondrial fission, fusion and mitophagy in promoting cardiac ischemic injury to the diabetic heart.

Mitochondria control cardiac function by regulating adenosine triphosphate synthesis, reactive oxygen species (ROS) generation, intracellular calcium (Ca2+), and the cellular redox state. Yet, their role in the pathophysiology of arrhythmias remains massively underappreciated. In this Top Stories article, we shed light on this issue by focusing on recent advances in our understanding of mechanisms by which mitochondria act as central mediators of inherited arrhythmic disorders.

Atrial fibrillation is the most common clinical arrhythmia and may be due in part to metabolic stress. Atrial specific deletion of the master metabolic sensor, AMP-activated protein kinase (AMPK), induces atrial remodeling culminating in atrial fibrillation in mice, implicating AMPK signaling in the maintenance of atrial electrical and structural homeostasis. However, atrial substrate preference for mitochondrial oxidation and the role of AMPK in regulating atrial metabolism are unknown. Here, using LC-MS/MS methodology combined with infusions of [ 13 C 6 ]glucose and [ 13 C 4 ]β-hydroxybutyrate in conscious mice, we demonstrate that conditional deletion of atrial AMPK catalytic subunits shifts mitochondrial atrial metabolism away from fatty acid oxidation and towards pyruvate oxidation. LC-MS/MS-based quantification of acyl-CoAs demonstrated decreased atrial tissue content of long-chain fatty acyl-CoAs. Proteomic analysis revealed a broad downregulation of proteins responsible for fatty acid uptake (LPL, CD36, FABP3), acylation and oxidation. Atrial AMPK deletion reduced expression of atrial PGC1-α and downstream PGC1-α/PPARα/RXR regulated gene transcripts. In contrast, atrial [ 14 C]2-deoxyglucose uptake and GLUT1 expression increased with fasting in mice with AMPK deletion, while the expression of glycolytic enzymes exhibited heterogenous changes. Thus, these results highlight the crucial homeostatic role of AMPK in the atrium, with loss of atrial AMPK leading to downregulation of the PGC1-α/PPARα pathway and broad metabolic reprogramming with a loss of fatty acid oxidation, which may contribute to atrial remodeling and arrhythmia.

Mitochondria within a cardiomyocyte form a highly dynamic network that undergoes fusion and fission events in response to acute and chronic stressors, such as hyperglycemia and diabetes mellitus. Changes in mitochondrial architecture and morphology not only reflect their capacity for oxidative phosphorylation and ATP synthesis but also impact their subcellular localization and interaction with other organelles. The role of these ultrastructural abnormalities in modulating electrophysiological properties and excitation-contraction coupling remains largely unknown and warrants direct investigation considering the growing appreciation of the functional and structural coupling between the mitochondrial network, the calcium cycling machinery, and sarcolemmal ion channels in the cardiac myocyte. In this Methods in Molecular Biology chapter, we provide a protocol that allows for a quantitative assessment of mitochondrial shape and morphology in control and diabetic hearts that had undergone detailed electrophysiological measurements using high resolution optical action potential (AP) mapping.

We recently showed in a murine model of arrhythmogenic cardiomyopathy (ACM) that VT was driven by discrete rotors deep within the RV. Mechanisms underlying the evolution of these RV-pinned rotors, however, remained unknown considering that the initial wavebreaks that preceded VT were at the LV/RV interface.

Although impaired AMPK signaling is a hallmark of metabolic diseases (MetD), which heighten arrhythmia risk, whether and how loss of AMPK activity per se promotes adverse electrophysiological (EP) remodeling independently of confounding factors in MetD remains unknown. We investigated the EP substrate of wildtype (WT) mice on high-fat diet (HFD) relative to those on normal diet (ND) and to mice expressing the muscle-specific AMPK kinase dead isoform (AMPK-KD). After 8-weeks of HFD, mice underwent hemodynamic analysis by echocardiography and ex vivo optical action potential (AP) mapping. While HFD-fed mice did not exhibit structural or hemodynamic changes by 8-weeks, they were more prone to pacing-induced VT than their ND counterparts (57% HFD vs 11% ND). Investigation of the EP substrate revealed a marked decrease in conduction velocity (CV) in HFD vs ND-fed mice (54.9±6.7 cm/s vs 71.6±10.2 cm/s, p<0.05) with no change in AP duration (APD). Importantly, we found similar CV slowing in AMPK-KD on ND as WT on HFD. Challenging AMPK-KD mice with HFD did not reduce CV further (45.4±10.6 cm/s HFD vs 49.0±11.1 cm/s ND). In contrast, APD was decreased exclusively in AMPK-KD mice on HFD, reflecting the importance of both factors (AMPK depletion and HFD) in the modulation of myocardial repolarization. Functionally, the subset of hearts that were prone to HFD-related VT exhibited decreased CV compared to those that were protected against VT. We examined the molecular correlates of myocardial conduction, and found a paradoxical increase in Nav1.5, total and Ser368-phosphorylated Cx43 in AMPK-KD, discounting overall protein expression as a factor in CV slowing. In conclusion, impaired AMPK signaling accounts for early CV defects in HFD prior to onset of structural or mechanical remodeling. Our findings highlight AMPK as a potentially powerful anti-arrhythmic target at early stages of MetD.

The long QT syndrome (LQTS) is a primary electrical disorder of impaired ventricular repolarization.It was recognized as a genetic disorder with autosomal recessive and dominant transmission patterns by Jervell and

Background and Objective: Renewal theory is a statistical approach to model the formation and destruction of phase singularities (PS), which occur at the pivots of spiral waves. A common issue arising during observation of renewal processes is an inspection paradox, due to oversampling of longer events. The objective of this study was to characterise the effect of a potential inspection paradox on the perception of PS lifetimes in cardiac fibrillation. Methods: A multisystem, multi-modality study was performed, examining computational simulations (Aliev-Panfilov (APV) model, Courtmanche-Nattel model), experimentally acquired optical mapping Atrial and Ventricular Fibrillation (AF/VF) data, and clinically acquired human AF and VF. Distributions of all PS lifetimes across full epochs of AF, VF, or computational simulations, were compared with distributions formed from lifetimes of PS existing at 10,000 simulated commencement timepoints. Results: In all systems, an inspection paradox led towards oversampling of PS with longer lifetimes. In APV computational simulations there was a mean PS lifetime shift of +84.9% (95% CI, ± 0.3%) (p < 0.001 for observed vs overall), in Courtmanche-Nattel simulations of AF +692.9% (95% CI, ±57.7%) (p < 0.001), in optically mapped rat AF +374.6% (95% CI, ± 88.5%) (p = 0.052), in human AF mapped with basket catheters +129.2% (95% CI, ±4.1%) (p < 0.05), human AF-HD grid catheters 150.8% (95% CI, ± 9.0%) (p < 0.001), in optically mapped rat VF +171.3% (95% CI, ±15.6%) (p < 0.001), in human epicardial VF 153.5% (95% CI, ±15.7%) (p < 0.001). Conclusion: Visual inspection of phase movies has the potential to systematically oversample longer lasting PS, due to an inspection paradox. An inspection paradox is minimised by consideration of the overall distribution of PS lifetimes.

Arrhythmogenic cardiomyopathy (ACM) is an inherited disorder characterized by fibro-fatty infiltration with an increased propensity for ventricular arrhythmias and sudden death. Genetic variants in desmosomal genes are associated with ACM. Incomplete penetrance is a common feature in ACM families, complicating the understanding of how external stressors contribute towards disease development. To analyze the dual role of genetics and external stressors on ACM progression, we developed one of the first mouse models of ACM that recapitulates a human variant by introducing the murine equivalent of the human R451G variant into endogenous desmoplakin (DspR451G/+). Mice homozygous for this variant displayed embryonic lethality. While DspR451G/+ mice were viable with reduced expression of DSP, no presentable arrhythmogenic or structural phenotypes were identified at baseline. However, increased afterload resulted in reduced cardiac performance, increased chamber dilation, and accelerated progression to heart failure. In addition, following catecholaminergic challenge, DspR451G/+ mice displayed frequent and prolonged arrhythmic events. Finally, aberrant localization of connexin-43 was noted in the DspR451G/+ mice at baseline, becoming more apparent following cardiac stress via pressure overload. In summary, cardiovascular stress is a key trigger for unmasking both electrical and structural phenotypes in one of the first humanized ACM mouse models.