Journal of Molecular and Cellular Cardiology

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) hold great promise for cardiac regenerative medicine and disease modeling. However, hiPSC-CMs generated through conventional in vitro differentiation exhibit immature, fetal-like phenotypes. While in vivo bioincubation in neonatal rodent hearts promotes hiPSC-CM maturation toward adult-like phenotypes, studies in large animal models remain limited, particularly with detailed morphological characterization. In this study, we investigated bioincubation of fluorescently labeled hiPSC-CMs in both neonatal rat and pig hearts. Human iPSCs were differentiated into cardiomyocytes expressing GFP or RFP reporters and subsequently injected intramyocardially into neonatal rats (GFP-labeled) and pigs (RFP-labeled). After 4-8 weeks of bioincubation, fluorescent hiPSC-CMs were isolated using large-particle fluorescence-activated cell sorting (COPAS), which preserves cellular morphology of adult-like cardiomyocytes. Immunostaining for cardiac troponin T revealed well-organized sarcomeric structures in multinucleated hiPSC-CMs. Bioincubated hiPSC-CMs displayed rod-shaped morphology with binucleation, characteristic features of mature adult cardiomyocytes. Quantitative analysis demonstrated that bioincubated hiPSC-CMs from rat hearts exhibited sarcomere length and cell circularity comparable to native rat adult cardiomyocytes, though with higher intra-cellular variability in sarcomere organization. Histological examination confirmed successful engraftment of RFP-positive hiPSC-CMs within pig myocardium, with engrafted cells also displaying mature adult-like features. These findings provide critical proof-of-concept data for bioincubation in large animal models and support further investigation for disease modeling, drug screening, and regenerative cell therapies. SIGNIFICANCE STATEMENTHuman induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer tremendous potential for cardiac disease modeling and regenerative therapies, but their clinical application is limited by their immature characteristics. Here we show that in vivo bioincubation in neonatal rat hearts enables hiPSC-CMs to achieve structural maturity, exhibiting features of adult cardiomyocytes, including organized sarcomeres, rod-shaped morphology, and multinucleation. We further provided proof-of-concept evidence for engraftment in neonatal pig hearts for maturation, supporting feasibility in large animal models. The use of large-particle cell sorting enables recovery of intact, adult-sized cardiomyocytes for subsequent analysis. These findings establish a practical and scalable platform for generating structurally mature human cardiomyocytes through in vivo bioincubation.

BackgroundAlternative splicing is a critical determinant of protein diversity in the heart, where it drives the postnatal functional maturation of cardiomyocytes and specifies the ion-channel and calcium-handling isoforms required for mature contractile function; dysregulated splicing programs have in turn been implicated in cardiomyopathies and arrhythmias. However, the splicing regulators that control cardiomyocyte calcium handling remain largely unknown. ObjectiveWe systematically screened 276 splicing factor genes in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to identify regulators of calcium handling, and we selected SF3B2, a core U2 snRNP spliceosome component, for detailed follow-up based on complex-level enrichment and prior identification by Murphy et al. 2021 [1]. MethodsA high-throughput siRNA screen targeting 276 splicing factor genes was performed in hiPSC-CMs using 384-well calcium transient imaging. SF3B2 knockdown was followed by deep bulk RNA-seq analysis (n = 3 per group) for differential gene expression with DESeq2 and alternative splicing quantification with rMATS. RNA immunoprecipitation sequencing (RIP-seq) using an epitope-tagged SF3B2 construct was performed to identify direct mRNA binding targets of SF3B2. ResultsThe screen identified multiple U2 snRNP components, including SF3A2, SF3B1, and SF3B4--as regulators of calcium transient duration. SF3B2 was previously identified as a regulator of contractility in this screen [1], and the enrichment of its complex partners above the significance threshold in the current CTD75 analysis supported its selection for follow-up characterization. SF3B2 knockdown resulted in 2,683 differentially expressed genes (adjusted p < 0.05), with downregulated genes enriched in cell cycle and DNA replication pathways. Alternative splicing analysis revealed significant changes across all five rMATS event types in 36 genes within the cardiac muscle cell action potential involved in contraction gene ontology term (GO:0086002), including calcium channel (CACNA1C, CACNA1D, CACNA2D1), potassium channel (KCNH2, KCNQ1), and sodium channel (SCN5A) genes, although the large number of affected genes is consistent with broad spliceosomal perturbation and a formal enrichment test would be needed to determine whether cardiac action potential genes are preferentially affected. Integration of RIP-seq data (597 SF3B2-enriched transcripts, FDR < 0.05) with splicing analysis identified CACNA2D1, which encodes an auxiliary subunit of L-type calcium channels, as both directly bound by SF3B2 and alternatively spliced upon knockdown. ConclusionThese findings identify SF3B2 as a regulator of cardiomyocyte calcium handling and suggest that SF3B2-dependent missplicing of CACNA2D1 may link core spliceosome function to the splicing programs underlying cardiomyocyte functional maturation. Significance StatementThis study provides the first systematic functional screen of splicing factors in cardiomyocyte calcium handling and identifies SF3B2, a U2 snRNP subunit, as a regulator of cardiac ion channel splicing. Transcriptomic, splicing, and RNA binding data converge on CACNA2D1, identifying a route by which a core splicing factor shapes cardiac contractility, extending cardiac splicing regulation beyond the accessory RBPs studied to date.

The actin cytoskeleton of the cardiomyocyte is organized into two structurally and functionally distinct filament networks. The sarcomeric thin filaments, built primarily from cardiac -actin (CA), generate contractile force, whereas a separate subsarcolemmal cytoplasmic network, built from {beta}-actin (Actb gene) and {gamma}-actin (Actg1 gene), lies beneath the sarcolemma and is comparatively understudied in the mature heart. We hypothesized that this cytoplasmic actin network is required for sarcolemmal membrane integrity, signal transduction, and mechanosensing in the adult cardiomyocyte, and we generated cardiomyocyte-specific Actb and Actg1 double-gene deleted mice, using loxP (fl)-targeted Actbfl/fl and Actg1fl/fl alleles combined with an MHC (Myh6) promoter-driven Cre-recombinase transgene, to test this directly. Deletion of cytoplasmic {beta}-actin and {gamma}-actin from cardiomyocytes (Actb/g1fl/fl-Myh6-Cre mice) drove compensatory upregulation of a {gamma}-interferon like stress response with compensatory upregulation of skeletal -actin (SKA) and smooth muscle -actin (SMA) protein in adult cardiomyocytes, without altering baseline cardiac structure or function. To test if lost {beta}-actin and {gamma}-actin in the mouse heart impacts sarcolemmal stability, we crossed Actb/g1fl/fl-Myh6-Cremice onto the dystrophin-deficient mdx background, which is characterized by a fragile sarcolemma. Unexpectedly, hearts from Actb/g1fl/fl-Myh6- Cre; mdx mice showed greater membrane stability than mdx hearts alone with intact Actb/Actg1. We also observed that Actb/g1fl/fl-Myh6-Cremice subjected to chronic pressure overload by transverse aortic constriction (TAC) were protected and developed less cardiac hypertrophy, had better preserved systolic function, and improved survival. Together, these data indicate induction of the cytoplasmic {beta}-actin/{gamma}-actin network in the heart during disease stimulation is maladaptive and weakens the sarcolemma, and 2 downstream mechanisms are considered that could mediate this effect.

BackgroundCardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. Metabolic dysregulation of cancer cells extends beyond the tumor microenvironment and increases the risk for cardiovascular diseases. One common somatic mutation in cancer cells affects isocitrate dehydrogenase (IDH) 1 and 2, which catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate in the cytosol and mitochondria, respectively. IDH1 and 2 mutations cause the production of the oncometabolite D-2-hydroxyglutarate (D2-HG), which allosterically inhibits -ketoglutarate dehydrogenase (-KGDH) and is associated with reduced cardiac contractile function. MethodsWe combined stable isotope tracer studies with computational modeling to investigate the fundamental role of IDH isoforms in cardiac adaptation under oncometabolic stress. ResultsWe uncovered an unexpected cardiac phenotype that expands the role of IDH1 in the heart beyond oxidative metabolism. We quantified the stable isotopomer distributions from glucose and glutamine in perfused working rat hearts and isolated adult ventricular cardiomyocytes using mass spectrometry-based metabolomics. Our analysis revealed that defective mitochondrial metabolism causes the redirection of carbon flux from oxidative towards reductive pathways. Reductive carboxylation of -KGDH increases glutamine uptake and glutamine-derived citrate formation in working rat heart perfusions and cultured adult mouse ventricular cardiomyocytes. To identify which IDH isoform is responsible for redirecting carbon flux, we developed knockout models of IDH1, IDH2, and IDH3 in adult mouse ventricular cardiomyocytes. Loss of IDH1 expression impaired the reductive formation of citrate and caused functional defects in cardiomyocytes. Lastly, epigenetic analyses of histone marks revealed that IDH1 induces widespread alterations in histone acetylation and tri-methylation. ConclusionOur results highlight a novel role for IDH1 in cardiac metabolism and transcriptional control of metabolic adaptation to tumor-mediated stress and provide evidence that reductive-citrate formation may induce epigenetic modifications in the heart.

Coronavirus disease 2019 (COVID-19), caused by SARS-CoV-2, is associated with significant cardiovascular complications, including myocardial injury and long-term cardiac dysfunction. Individuals with Down syndrome (trisomy 21) exhibit increased susceptibility to severe COVID-19 outcomes, yet the cardiomyocyte-intrinsic mechanisms underlying this vulnerability remain poorly understood. To investigate genotype-specific responses to SARS-CoV-2 infection, we generated induced pluripotent stem cell-derived cardiomyocytes from individuals with trisomy 21 and their euploid, sex-matched biological relatives. Cardiomyocytes were inoculated with SARS-CoV-2, and viral susceptibility was assessed by immunofluorescence. Bulk RNA sequencing was performed under baseline and infected conditions to define transcriptional programs associated with viral response. Trisomy 21 iPSC-CMs exhibited increased susceptibility to SARS-CoV-2 infection, with greater viral protein expression and a higher proportion of infected cardiomyocytes compared to controls. Baseline transcriptomic analysis revealed no significant differences in canonical viral entry factors including ACE2 and TMPRSS2, suggesting that differential susceptibility is not driven by entry receptor availability. Following infection, both trisomy 21 and euploid control groups activated conserved antiviral pathways; however, trisomy 21 cardiomyocytes displayed a markedly amplified transcriptional response, with substantially greater numbers of differentially expressed genes. Upregulated pathways included interferon signaling, NF-{kappa}B activation, cytokine and chemokine signaling, and innate immune responses, while downregulated pathways were enriched for cardiomyocyte structural integrity, calcium handling, and metabolic processes. Notably, inflammatory and cytokine-related transcripts were significantly more elevated in trisomy 21 cells, consistent with an exaggerated immune response. These findings provide mechanistic insight into the increased cardiovascular risk observed in individuals with Down syndrome and highlight dysregulated immune signaling as a potential therapeutic target in this high-risk population.

Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous condition with incompletely defined myocardial mechanisms. Here, using a two-hit murine model of cardiometabolic HFpEF induced by high-fat diet and endothelial nitric oxide synthase inhibition, we define a mitochondrial metabolic phenotype characterized by substrate inflexibility, redox stress, and S-nitrosylation remodeling. While global proteomic changes were modest, metabolomic profiling revealed accumulation of tricarboxylic acid cycle intermediates, increased dicarboxylic acids, and altered redox-associated metabolites, consistent with inefficient oxidative metabolism and mitochondrial redox imbalance in this experimental setting. S-nitrosylation proteomics demonstrated a highly organized and bidirectional remodeling pattern affecting proteins involved in fatty acid/lipid metabolism, carbohydrate metabolism, mitochondrial energy metabolism, amino acid and organic acid metabolism, nucleotide/cofactor metabolism, and redox defense. Beta-hydroxybutyrate (BHB), an alternative mitochondrial substrate, improved basal and ATP-linked respiration, reduced selected TCA-cycle intermediates, lowered mitochondrial reactive oxygen species and the NADH/NAD+ ratio, partially restored the GSH/GSSG ratio, and improved diastolic and functional phenotypes without altering preserved ejection fraction. Together, these findings define a redox-sensitive mitochondrial metabolic state in the HFD/L-NAME model and identify ketone supplementation as a partial metabolic rescue strategy in this context. At the same time, they highlight an important limitation of murine HFpEF models: such models do not faithfully reproduce the metabolic phenotype of human HFpEF and should therefore be interpreted as experimental systems rather than human disease equivalents.

RationaleSCN1B encodes the {beta}-subunits of the main cardiac voltage-gated sodium channel, NaV1.5. Variants are linked to cardiac conduction disease, often with concealed phenotypes. Whether {beta}1-subunits regulate conduction through nanoscale intercalated disc (ID) structures, e.g. perinexi, and ephaptic coupling remains unresolved. ObjectiveTest whether Scn1b haploinsufficiency induces latent conduction abnormalities that are unmasked by perturbations in extracellular nanodomains. Methods and ResultsAdult Scn1b+/- mice and wild-type (WT) littermates underwent multiscale phenotyping (qRT-PCR, Western blot, patch clamp, transmission electron microscopy (TEM), ex vivo optical mapping, in vivo ECG). Scn1b+/- hearts showed [~]50% reductions in Scn1b mRNA and {beta}1 protein without changes in canonical conduction proteins. Peak sodium current, baseline conduction velocity ex vivo, and baseline QRS duration in vivo were unchanged. However, TEM revealed increased baseline perinexal width in Scn1b+/- hearts. Osmotic expansion of the perinexus with mannitol slowed conduction to a greater extent in Scn1b+/- hearts and prolonged QRS duration in vivo. In contrast, perinexal narrowing with dextran 2MDa selectively increased conduction velocity in Scn1b+/- hearts. ConclusionsScn1b haploinsufficiency preserves baseline excitability and conduction but structurally remodels the ID at the nanoscale, increasing sensitivity to extracellular nanodomain perturbations. These data support a structural role for {beta}1-subunits in ephaptic coupling, and that conduction is maintained over a range of perinexal widths with pathological conduction slowing occurring beyond a critical width. Importantly, osmotic stress unmasks a concealed conduction phenotype, identifying extracellular nanodomain stability as a potential therapeutic target to mitigate arrhythmia risk in SCN1B-associated disease.

Klf9 is a cardiac-enriched transcription factor of the Kruppel-like factor (Klf) family. Klf9 levels decrease during cardiac hypertrophy; however, no studies have examined its transcriptional targets or role in the progression of hypertrophy. Here, we report genome-wide differential Klf9 occupancy during cardiac hypertrophy, with a predominant enrichment at the metabolic gene promoters. Further, using conditional Klf9 knock-in mice subjected to pressure overload for 1 or 2 weeks, we show that restoring Klf9 expression initially inhibits hypertrophy but later leads to early-onset heart failure. We conclude that a decrease in Klf9 is required for metabolic adaptations that support the development of compensatory hypertrophy.

AimsWhile the muscle-enriched microRNA-378a (miR-378a) has been implicated in cardiac hypertrophy and stress responses, its role in maintaining cardiomyocyte metabolic homeostasis, mitochondrial function, and angiogenic paracrine signaling under physiological and post-injury conditions remains unclear. This study addresses these gaps by examining the molecular and functional consequences of miR-378a deficiency in murine heart and human cardiomyocytes. Methods and ResultsCardiac structure and function were analyzed in miR-378a-deficient (miR-378a-/-) and wild-type (miR-378a+/+) mice at 12 weeks and 17 months of age, revealing that miR-378a loss promoted myocardial fibrosis, altered IGF1R-AKT signaling, and impaired cardiac performance, with age-dependent effects. Integrated transcriptomic and proteomic analyses in miR-378a-/- and control mice, as well as in human iPSC-derived cardiomyocytes (hiPSC-CM) of both genotypes, revealed deregulated pathways related to translation, metabolism, and cardiomyopathy-associated signaling. In hiPSC-CM, miR-378a knockout (KO) impaired mitochondrial respiration, disrupted mitochondrial morphology, and reduced mitochondrial DNA content, accompanied by altered mitophagy and biogenesis. KO cells also showed increased glucose uptake but reduced glycogen storage, accompanied by changes in key metabolic regulators, and displayed diminished angiogenic potential. Finally, hiPSC-CM overexpressing miR-378a were delivered in a mouse model of acute myocardial infarction, but overexpression did not further enhance their therapeutic effect. ConclusionsThis study broadens our understanding of miR-378as physiological role in murine hearts and human cardiomyocytes, demonstrating its impact on contractility, mitochondrial integrity, glucose metabolism, and angiogenic paracrine signaling. However, overexpression of miR-378a in hiPSC-CM offers limited additional benefit in cell therapy for acute myocardial infarction.

Background Ischemic heart failure (IHF) has been shown to impair contractility and disrupt sarcomere function in the left ventricle. Left ventricular failure can cause left atrial dysfunction, which is associated with a greater risk of patient mortality. Despite this, the biochemical and biomechanical characteristics of the left atrium in IHF remain obscure. Methods Myocardial mechanical properties were measured using permeabilized muscle isolated from the left ventricle (LV) and left atrium (LA) of donors and patients with IHF. Tissue homogenates from these samples were used to measure titin and myosin isoforms as well as the phosphorylation of sarcomeric regulatory proteins. Histology was used to quantify fibrosis in the patients' left ventricle and left atrium. Results Length-dependent changes in Ca2+-sensitivity were blunted in LV myocardium from patients with IHF. LA myocardium did not show robust length-dependence of Ca2+-dependent force. The calcium sensitivity of both LA and LV myocardium was increased in IHF. The maximum force generated by LV but not LA myocardium was decreased in IHF. LA myocardial samples exhibited faster contractile kinetics than LV samples, irrespective of disease. Troponin I phosphorylation decreased in both chambers with IHF. Conclusions Left atrial IHF myocardium maintained contractile force and displayed increases in calcium sensitivity, which may allow for increased LA contraction under pathological conditions. The increases in calcium sensitivity observed in ischemic myocardium of both chambers are likely driven by decreased phosphorylation of troponin I, which alters thin filament regulation. Conversely, thick filament properties of the left ventricle, such as thick filament protein isoforms and phosphorylation of myosin binding protein-C, displayed chamber-specific differences independent of disease state. These biochemical changes may explain the chamber-specific differences in kinetics and length-dependent properties. Collectively, these biophysical and biochemical data suggest LA remodeling in IHF may assist in increasing LV end-diastolic volume to maintain adequate cardiac output.

BackgroundCatecholamines play a central role in cardiac performance, coordinating myocardial contractility, conduction, metabolism, and electrophysiological stability. In the heart, their actions have been attributed primarily to sympathetic nerve terminals and circulating adrenal catecholamines. The discovery of an intrinsic non-neuronal cholinergic system within cardiomyocytes challenges this neurocentric paradigm and raises the possibility that cardiomyocytes also possess an intrinsic catecholaminergic programme. Here, we investigated whether cardiomyocytes possess an intrinsic catecholaminergic programme and its contribution to cardiomyocyte homeostasis and stress responses. MethodsWe investigated catecholamine biosynthesis and handling in human induced pluripotent stem cell-derived cardiomyocytes, adult mouse cardiomyocytes, H9C2 cells, rat ventricular tissue, and Langendorff-perfused mouse hearts. Protein expression of catecholamine biosynthetic enzymes and vesicular monoamine transporters was assessed by immunoblotting and immunohistochemistry, while vesicular monoamine uptake was evaluated using fluorescent false neurotransmitters. Functional consequences of catecholamine biosynthesis inhibition were examined using pharmacological approaches, assessing cell viability, apoptosis, organelle homeostasis, metabolic signalling, and cardiac electrophysiology. ResultsTyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine {beta}-hydroxylase, and vesicular monoamine transporters were detected in cardiomyocytes across models. Expression of catecholamine biosynthetic enzymes increased following ischaemia-reperfusion injury in rat heart tissue (TH p=0.008, AADC p=0.031, DBH p=0.008). Pharmacological inhibition of catecholamine biosynthesis caused dose-dependent reductions in cardiomyocyte viability (p<0.0001), increased apoptosis, organelle stress, and mitochondrial dysfunction, with greater effects under oxidative stress. Mechanistically, catecholamine depletion suppressed mTORC1 signalling and activated LKB1-AMPK-ULK1 pathways. In Langendorff-perfused hearts, tyrosine hydroxylase inhibition induced ventricular arrhythmias in 5 of 6 hearts, including sustained ventricular tachycardia, polymorphic ventricular tachycardia, and ventricular fibrillation. ConclusionsThese findings identify cardiomyocytes as previously unrecognised catecholamine-competent cells expressing intrinsic machinery for catecholamine biosynthesis and vesicular handling. Disruption of this pathway compromises metabolic and organelle homeostasis, activates energy-stress and autophagy-related signalling, and promotes malignant ventricular arrhythmias. Intrinsic cardiomyocyte catecholamine biology therefore represents a non-neuronal regulatory axis essential for myocardial resilience and electrical stability, with potential relevance to ischaemic injury and stress-induced dysfunction.

Cardiometabolic diseases associated with obesity are continuously increasing worldwide. Yet, current therapeutic strategies remain insufficient to improve patient outcomes. The beta3-adrenergic receptor ({beta}3AR) promotes lipolysis in adipose tissue (AT) and thermogenesis specifically in brown adipose tissue (BAT). In mice, BAT activation also improves systemic metabolism and limits cardiometabolic damage. While BAT is limited in humans (e.g., with ageing and obesity), {beta}3AR activation induces beige adipocytes within white adipose depots with similar thermogenic properties. To study the role of adipocyte {beta}3AR in the regulation of cardiac metabolism and remodeling, mice with/without adipocyte-specific {beta}3AR genetic deletion were fed a high-fat-sucrose (HF-S) diet and treated with the selective {beta}3AR agonist CL316,243 (CL). The metabolic and cardiac protection following {beta}3AR activation was abrogated upon {beta}3AR deletion in adipocytes, together with the beiging of the epididymal (visceral) AT, highlighting a critical role of adipose {beta}3AR signalling in mediating these benefits. Multi-omic analysis of AT and cardiac samples identified CL-induced secreted mediators of the crosstalk between AT and the heart. Therefore, adipocyte {beta}3AR is critical for the adipose-cardiac communication and supports the therapeutic potential of targeting {beta}3AR for the management of obesity-related cardiometabolic diseases.

BackgroundMyocardial ischemia drives adverse cardiac remodeling, metabolic inflexibility, and progression to heart failure. Mitochondrial dysfunction and impaired substrate utilization contribute to cardiomyocyte death and fibrosis, particularly with aging. Humanin (HNG), a mitochondria-derived peptide, has been shown to reduce acute ischemic injury, but its role in chronic ischemia and cardiac remodeling remains unknown. MethodsWe investigated the effects of HNG treatment in young and aged murine models of myocardial ischemia without reperfusion. Cardiac function and structure were assessed by echocardiography and molecular markers of remodeling. Myocardial metabolism was interrogated using targeted metabolomics, gene expression, substrate uptake assays, and metabolic flux analyses. Mechanistic studies examined glucose transporter trafficking and protein-protein interactions. ResultsHNG treatment improved cardiac function and significantly attenuated adverse remodeling in both young and old mice. HNG treatment induced marked metabolic reprogramming characterized by reduced myocardial fatty acid content, downregulation of fatty acid uptake and oxidation pathways, and decreased oxidative stress. Importantly, these changes were accompanied by enhanced glucose oxidation, increased tricarboxylic acid cycle flux, improved coupling of glycolysis to mitochondrial oxidation, and increased ATP production. Time-course studies demonstrated that increased glucose oxidation preceded reductions in fatty acid oxidation, indicating a primary role for glucose metabolism in HNG-mediated cardioprotection. Mechanistically, we identified vesicle-associated membrane protein 7 (VAMP7) as a novel binding partner of HNG, and that this interaction is required for GLUT4 translocation to the plasma membrane and HNG-induced ATP generation. ConclusionsHNG protects the ischemic heart by promoting metabolic reprogramming that shifts substrate utilization from fatty acids to glucose and limiting maladaptive remodeling. These findings identify HNG as a novel regulator of cardiac metabolism and a potential therapeutic strategy for ischemic heart failure. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=65 SRC="FIGDIR/small/732776v1_ufig1.gif" ALT="Figure 1"> View larger version (21K): org.highwire.dtl.DTLVardef@1929de8org.highwire.dtl.DTLVardef@bcf254org.highwire.dtl.DTLVardef@c99b70org.highwire.dtl.DTLVardef@1fc08bb_HPS_FORMAT_FIGEXP M_FIG C_FIG What are the clinical implications?Heart failure (HF) is a major global health concern, affecting over 6.7 million adults in the United States alone, with projections to exceed 11 million by 2050. Myocardial infarction (MI) is a leading cause of HF. Despite substantial advances in acute MI care, survivors remain at high risk for adverse cardiac remodeling and chronic HF, especially in the elderly. We report here that treatment with a potent analog of Humanin (HN), an endogenous mitochondria-associated peptide, decreases infarct size, decreases fibrosis and improves cardiac function following cardiac ischemia induced by permanent ligation of coronary artery in both young and aged mice. These effects are associated with changes in cardiac metabolism, oxidative stress, and remodeling. HN and analogs have been shown to be beneficial in many age-related diseases. The endogenous origin of Humanin, its favorable safety profile in preclinical studies and its pleiotropic effects support targeting HNG as a promising therapeutic strategy for ischemic heart disease and post-myocardial infarction heart failure in humans.

Infantile cardiomyopathies are rare, life-threatening disorders for which genetic diagnosis has been accelerated by next-generation sequencing approaches, including gene panel, exome, and genome sequencing. However, determining the functional consequences of identified variants remains a major challenge. Variants in SMYD1, a striated muscle-specific lysine methyltransferase critical for cardiac development and mitochondrial function, have only recently been linked to human cardiomyopathy. Here, we functionally characterize a homozygous SMYD1 variant (c.302A>G; p.Asn101Ser) identified in a patient with severe early-onset cardiomyopathy requiring cardiac transplantation. Structural modeling predicts that the N101S substitution perturbs a highly conserved residue near the cofactor binding pocket within SMYD1s catalytic domain, disrupting local interactions and modestly destabilizing the protein. Consistent with these predictions, in vitro studies demonstrate that the N101S variant impairs mitochondrial respiratory capacity in myocytes. Quantification of SMYD1 protein levels in patient cardiac tissue revealed increased SMYD1 abundance, suggesting that the N101S variant results in functional impairment rather than protein instability and may trigger compensatory upregulation of SMYD1 expression. Together, these findings support a hypomorphic mechanism in which the N101S variant disrupts SMYD1 activity, leading to mitochondrial dysfunction and cardiomyopathy. This study provides mechanistic insight into SMYD1-associated cardiomyopathy and highlights the importance of integrating genetic, structural, and functional analyses to establish the pathogenicity of rare variants. New & NoteworthyThis study provides the first functional characterization of the cardiomyopathy-associated SMYD1 N101S variant identified in a child with severe infantile cardiomyopathy. Structural modeling predicts reduced protein stability, while cellular assays demonstrate impaired mitochondrial respiratory function, supporting a hypomorphic effect. These findings establish a mechanistic link between SMYD1 dysfunction and infantile cardiomyopathy and highlight the importance of integrating genomic and functional approaches in rare cardiovascular disease.

Approximately half of the adult sinoatrial node (SAN) consists of non-myocyte populations, indicating that cardiac pacemaking depends on interactions within a multicellular tissue rather than on pacemaker cardiomyocytes alone. Among these, an S100B-associated cell population has been implicated in pacemaker function, yet its identity and physiological roles remain poorly understood. These cells are rare and dispersed throughout the small, structurally complex SAN, making them difficult to observe repeatedly while preserving the native multicellular environment. Here, we established a dissociated multicellular culture of adult mouse SAN tissue on soft collagen-gelatin hydrogels that retains spontaneous electrical activity and permits longitudinal live imaging of S100B-associated cells. Using an S100B-EGFP+ reporter, we identified at least six reproducible morphological and behavioral phenotypes, including migration, proliferation, phagocytic behavior, and spontaneous self-organization into three-dimensional clusters. Cultures remained spontaneously electrically active for more than 10 days in vitro, with peak activity around day 10. This multicellular culture model bridges the gap between intact SAN preparations and isolated-cell cultures, allowing repeated observation of rare S100B-associated cells within a spontaneously active multicellular environment. HighlightsO_LIThe platform enables longitudinal live imaging of rare S100B-associated cells within an diverse multicellular SAN culture. C_LIO_LILive imaging reveals at least six reproducible morphological and behavioral phenotypes of S100B-associated cells. C_LIO_LIDissociated multicellular SAN cultures remain spontaneously electrically active for more than 10 days in vitro. C_LI

BackgroundRare heterozygous loss-of-function (LoF) variants in SVIL, encoding the Z-disk and costameric protein supervillin, have recently been identified as a cause of hypertrophic cardiomyopathy (HCM). Although supervillin is implicated in actin-dependent mechanotransduction, the mechanisms linking SVIL deficiency to cardiomyopathy remain poorly understood. Homozygous LoF cause a novel skeletal Myofibrillar Myopathy-10 (MFM-10) while heterozygous LoF cause HCM without skeletal myopathy. In this study we use a human model system to disentangle the LoF pathomechanism of the scaffolding protein supervillin in cardiomyocytes and its clinical implications. MethodsUsing CRISPR/Cas-9 we engineered a representative pathogenic LoF variant Q255X into an isogenic induced pluripotent stem cell (iPSC) line creating the heterozygous SVILQ255X/+ and homozygous SVILQ255X/Q255X cell lines. These lines were differentiated into iPSC-derived cardiomyocytes (iPSC-CMs) and cellular phenotypes were assessed using bulk RNA-sequencing, LC-MS proteomics, electrophysiological and calcium handling analyses, contractility measurements, sarcomere organization analysis, Seahorse metabolic flux assay, and pharmacological intervention with mavacamten. ResultsThe Q255X variant resulted in SVIL haploinsufficiency at both RNA and protein levels with no evidence of a truncated protein. Compared with isogenic controls, SVILQ255X/+ iPSC-CMs demonstrated action potential shortening, calcium transient elongation, sarcomeric disorganization and hypertrophy, and impaired mitochondrial respiration. Multi-omic analyses of SVILQ255X/+ iPSC-CMs showed a profile of cellular stress and inflammation, hypertrophic and pro-fibrotic signalling, and a pseudohypoxic state driven by decreased respiration and a HIF-induced glycolytic shift. These abnormalities were not present in SVILQ255X/Q255X cardiomyocytes, consistent with a relatively limited cardiac phenotype reported in homozygous variant carriers. Mavacamten improved sarcomeric disorganization and hypertrophy in SVILQ255X/+ cells but did not rescue energetic compromise. ConclusionsPathogenic heterozygous SVIL LoF produces a distinct cellular phenotype characterized by impaired mechanotransduction, mitochondrial dysfunction, and maladaptive metabolic remodelling that promotes hypertrophic and pro-fibrotic signalling. These findings define a mechanistic basis for SVIL-associated cardiomyopathy and identify metabolic dysfunction as a potential therapeutic target beyond sarcomere-directed therapy. Clinical PerspectiveO_ST_ABSWhat Is New?C_ST_ABSO_LISVIL haploinsufficiency causes HCM through a mechanism distinct from canonical sarcomeric disease, characterized by impaired mechanotransduction, mitochondrial dysfunction, and pseudohypoxia-driven metabolic remodeling. C_LIO_LIHeterozygous SVIL loss of function produces a substantially more severe cardiomyocyte phenotype than homozygous loss of function, providing a mechanistic explanation for the predominance of cardiac disease in heterozygous variant carriers. C_LIO_LIMavacamten improves sarcomeric organization but does not restore impaired mitochondrial respiration, demonstrating that energetic dysfunction persists despite sarcomere-directed therapy. C_LI What Are the Clinical Implications?O_LIOur findings give functional evidence to support SVIL as a clinically relevant HCM disease gene and its inclusion in clinical genetic testing panels. C_LIO_LIThese findings establish SVIL-associated cardiomyopathy as a mechanistically distinct form of HCM and offer insight into the pathomechanism of Z-disk and costameric HCM C_LIO_LIThe persistence of mitochondrial dysfunction despite myosin inhibition suggests that drugs targeting mitochondrial bioenergetics may be a therapeutic strategy in patients with SVIL-associated cardiomyopathy. C_LI

Background: Cardiometabolic diseases frequently involve concurrent cardiovascular and hepatic dysfunction, yet the conserved molecular mechanisms underlying these systemic responses remain poorly defined. Objectives: To identify conserved molecular responses across complementary manifestations of cardiometabolic stress and determine whether integrated multi-organ analyses reveal therapeutically actionable targets for heart failure. Methods: Cardiac functional phenotyping, hepatic injury profiling, and bulk RNA sequencing were performed across three complementary mouse models representing distinct manifestations of cardiometabolic stress: high-fat diet plus L-NAME (HFD+LN)-induced heart failure with preserved ejection fraction (HFpEF; cardiovascular disease), Western diet (WD)-induced obesity (systemic metabolic stress), and choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD)-induced steatotic liver disease (hepatic metabolic stress). Comparative transcriptomic analyses distinguished organ-specific responses from conserved molecular signatures. Results: Each model produced distinct systemic, hepatic, and cardiac phenotypes accompanied by divergent transcriptional responses within individual organs. Cross-model and cross-organ integration identified a limited set of conserved molecular responses to cardiometabolic stress, with Serpine1, encoding plasminogen activator inhibitor-1 (PAI-1), emerging as a highly conserved candidate that exhibited preferential induction in the heart. Pharmacologic inhibition of PAI-1 significantly improved cardiac function and attenuated adverse remodeling in established HFpEF, whereas hepatic pathology was comparatively less affected, indicating differential organ-specific dependence on this pathway. Conclusions: Integrated analyses across complementary manifestations of cardiometabolic stress identified conserved molecular signatures that transcend individual disease models and organs. These findings establish a comparative framework for discovering cardiovascular therapeutic targets and identify PAI-1 as a promising mediator of cardiac remodeling in cardiometabolic disease.

Impaired contractility and reduced myocardial energetic reserve underlie heart failure with reduced ejection fraction. Catecholaminergic inotropes such as dobutamine are used to augment cardiac output. However, dobutamine increases Ca{superscript 2} cycling, raising ATP demand and worsening energetic stress. The myotrope CK-138 increases contractility by directly activating myosin, sparing the added energetic cost of Ca{superscript 2} handling. This study compares CK-138 and dobutamine with respect to the relationship between contractile performance and myocardial energetic state, including high-energy phosphate balance, energetic efficiency, and substrate-specific metabolic fluxes. Isolated rat hearts were perfused with escalating concentrations of CK-138 or dobutamine. Contractility was assessed by measuring left ventricular pressure and rate-pressure product. Myocardial energetics were analyzed using 31P-NMR, and metabolic fluxes by 13C NMR and mass spectrometry. Unlike dobutamine, CK-138 increased LV contractility without increasing heart rate or LV end-diastolic pressure. CK-138 preserved ATP and phosphocreatine levels, maintaining a stable phosphocreatine-to-ATP ratio and free energy of ATP hydrolysis, whereas dobutamine progressively depleted both. At comparable workload, dobutamine exhibited higher glycolytic flux and lactate production, indicating greater reliance on glycolysis relative to mitochondrial oxidative metabolism, whereas CK-138 exhibited a 13% higher rate of ATP synthesis and [~]50% lower anaplerotic flux, consistent with preserved mitochondrial efficiency. In conclusion, CK-138 enhances cardiac contractility while preserving myocardial energetic state and substrate utilization. Unlike dobutamine, which depletes ATP reserves and shifts metabolism toward glycolysis, CK-138 maintains ATP homeostasis and supports oxidative metabolism. These findings support cardiac myosin activators, including CK-138 and omecamtiv mecarbil, as a mechanistically distinct class of energy-efficient inotropes.

Precise quantification of myofilament protein phosphorylation is essential for understanding the regulation of cardiac contractility in health and disease. Although Phos-tag SDS-PAGE is widely used to resolve phosphorylated protein isoforms, its reproducibility and quantitative reliability are often limited by variability in the key experimental factors, including gel composition, electrophoretic conditions, protein loading, and sample preparation. Here, we present a standardized manganese (Mn2+)-Phos-tag SDS-PAGE workflow optimized for cardiac myofilament proteins, using myosin regulatory light chain 2 (MLC2) and cardiac troponin I (cTnI) as model targets. We systematically evaluated critical parameters - including Mn2+ and Phos-tag concentrations, acrylamide composition, electrophoretic regime, buffer chemistry, protein loading, and EDTA-mediated transfer - to define conditions that maximize phospho-species resolution while preserving quantitative fidelity. We further demonstrate that electrophoresis rate, sample loading, and extraction strategy significantly influence band morphology, signal intensity, and the apparent distribution of phospho-species. As a use case scenario, we compared Trichloroacetic acid (TCA) extracted mouse left ventricular homogenates with myofibrils prepared using a widely adopted Triton-X-100 tissue-demembranization protocol. Myofibril preparation was associated with profound MLC2 dephosphorylation at the earliest stages of preparation, whereas cTnI exhibited a marked reduction in higher-order, low-stoichiometry phosphoforms. Further evaluation of Myosin-binding protein C (MyBP-C) showed progressive loss of phosphorylation over the course of 24 hours. We submit that TCA-extracted heart standards in combination with Phos-tag gels can provide valuable quality control for the phosphorylation status of myofibril preparations, and that inclusion of a high-affinity PP2A and PP1 phosphatase inhibitor like okadaic acid may benefit future myofibril mechanics studies.

The human heart is characterized by aberrant lipid accumulation and remodeling during periods of stress and disease, yet spatially resolved lipidomic profiling of the human heart remains unreported. Here, we employ matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to map lipid distributions across five anatomical regions of healthy human donor hearts (left and right atrium, left and right ventricle, and interventricular septum). Cryosections of donor hearts from healthy subjects were thaw-mounted onto indium tin oxide slides, coated with 2,5-dihydroxyacetophenone, and analyzed on a Bruker timsTOF fleX mass spectrometer in positive and negative ionization modes (50-1850 m/z; 20 m raster). Serial sections were stained with hematoxylin and eosin to enable histological co-registration with lipid distributions. We identified 150 unique lipid species with acyl-chain resolution across four glycerophospholipid classes -- phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylserine (PS) -- with abundances and spatial distributions differing significantly between regions. H&E co-registration further resolved epicardial and myocardial compartments, enabling tissue-specific lipid mapping. These findings establish the first spatially resolved lipidomic atlas of the human heart and provide a framework for identifying region-specific lipid biomarkers of cardiovascular disease.