Preventing Site-Specific Calpain Proteolysis of Junctophilin-2 Protects Against Stress-Induced Excitation-Contraction Uncoupling and Heart Failure Development.

BACKGROUND: Excitation-contraction (E-C) coupling processes become disrupted in heart failure (HF), resulting in abnormal Ca2+ homeostasis, maladaptive structural and transcriptional remodeling, and cardiac dysfunction. Junctophilin-2 (JP2) is an essential component of the E-C coupling apparatus but becomes site-specifically cleaved by calpain, leading to disruption of E-C coupling, plasmalemmal transverse tubule degeneration, abnormal Ca2+ homeostasis, and HF. However, it is not clear whether preventing site-specific calpain cleavage of JP2 is sufficient to protect the heart against stress-induced pathological cardiac remodeling in vivo. METHODS: Calpain-resistant JP2 knock-in mice (JP2CR) were generated by deleting the primary JP2 calpain cleavage site. Stress-dependent JP2 cleavage was assessed through in vitro cleavage assays and in isolated cardiomyocytes treated with 1 µmol/L isoproterenol by immunofluorescence. Cardiac outcomes were assessed in wild-type and JP2CR mice 5 weeks after transverse aortic constriction compared with sham surgery using echocardiography, histology, and RNA-sequencing methods. E-C coupling efficiency was measured by in situ confocal microscopy. E-C coupling proteins were evaluated by calpain assays and Western blotting. The effectiveness of adeno-associated virus gene therapy with JP2CR, JP2, or green fluorescent protein to slow HF progression was evaluated in mice with established cardiac dysfunction. RESULTS: JP2 proteolysis by calpain and in response to transverse aortic constriction and isoproterenol was blocked in JP2CR cardiomyocytes. JP2CR hearts are more resistant to pressure-overload stress, having significantly improved Ca2+ homeostasis and transverse tubule organization with significantly attenuated cardiac dysfunction, hypertrophy, lung edema, fibrosis, and gene expression changes relative to wild-type mice. JP2CR preserves the integrity of calpain-sensitive E-C coupling-related proteins, including ryanodine receptor 2, CaV1.2, and sarcoplasmic reticulum calcium ATPase 2a, by attenuating transverse aortic constriction-induced increases in calpain activity. Furthermore, JP2CR gene therapy after the onset of cardiac dysfunction was found to be effective at slowing the progression of HF and superior to wild-type JP2. CONCLUSIONS: The data presented here demonstrate that preserving JP2-dependent E-C coupling by prohibiting the site-specific calpain cleavage of JP2 offers multifaceted beneficial effects, conferring cardiac protection against stress-induced proteolysis, hypertrophy, and HF. Our data also indicate that specifically targeting the primary calpain cleavage site of JP2 by gene therapy approaches holds great therapeutic potential as a novel precision medicine for treating HF.

IRS2 Signaling Protects Against Stress-Induced Arrhythmia by Maintaining Ca2+ Homeostasis.

BACKGROUND: The docking protein IRS2 (insulin receptor substrate protein-2) is an important mediator of insulin signaling and may also regulate other signaling pathways. Murine hearts with cardiomyocyte-restricted deletion of IRS2 (cIRS2-KO) are more susceptible to pressure overload-induced cardiac dysfunction, implying a critical protective role of IRS2 in cardiac adaptation to stress through mechanisms that are not fully understood. There is limited evidence regarding the function of IRS2 beyond metabolic homeostasis regulation, particularly in the context of cardiac disease. METHODS: A retrospective analysis of an electronic medical record database was conducted to identify patients with IRS2 variants and assess their risk of cardiac arrhythmias. Arrhythmia susceptibility was examined in cIRS2-KO mice. The underlying mechanisms were investigated using confocal calcium imaging of ex vivo whole hearts and isolated cardiomyocytes to assess calcium handling, Western blotting to analyze the involved signaling pathways, and pharmacological and genetic interventions to rescue arrhythmias in cIRS2-KO mice. RESULTS: The retrospective analysis identified patients with IRS2 variants of uncertain significance with a potential association to an increased risk of cardiac arrhythmias compared with matched controls. cIRS2-KO hearts were found to be prone to catecholamine-sensitive ventricular tachycardia and reperfusion ventricular tachycardia. Confocal calcium imaging of ex vivo whole hearts and single isolated cardiomyocytes from cIRS2-KO hearts revealed decreased Ca²+ transient amplitudes, increased spontaneous Ca²+ sparks, and reduced sarcoplasmic reticulum Ca²+ content during sympathetic stress, indicating sarcoplasmic reticulum dysfunction. We identified that overactivation of the AKT1/NOS3 (nitric oxide synthase 3)/CaMKII (Ca2+/calmodulin-dependent protein kinase II)/RyR2 (type 2 ryanodine receptor) signaling pathway led to calcium mishandling and catecholamine-sensitive ventricular tachycardia in cIRS2-KO hearts. Pharmacological AKT inhibition or genetic stabilization of RyR2 rescued catecholamine-sensitive ventricular tachycardia in cIRS2-KO mice. CONCLUSIONS: Cardiac IRS2 inhibits sympathetic stress-induced AKT/NOS3/CaMKII/RyR2 overactivation and calcium-dependent arrhythmogenesis. This novel IRS2 signaling axis, essential for maintaining cardiac calcium homeostasis under stress, presents a promising target for developing new antiarrhythmic therapies.

CIB2 Is a Novel Endogenous Repressor of Atrial Remodeling.

BACKGROUND: Atrial fibrillation (AF) is a highly prevalent condition that can cause or exacerbate heart failure, is an important risk factor for stroke, and is associated with pronounced morbidity and death. Genes uniquely expressed in the atria are known to be essential for maintaining atrial structure and function. Atrial tissue remodeling contributes to arrhythmia recurrence and maintenance. However, the mechanism underlying atrial remodeling remains poorly understood. This study was designed to investigate whether other uncharacterized atrial specific genes play important roles in atrial physiology and arrhythmogenesis. METHODS: RNA-sequencing analysis was used to identify atrial myocyte specific and angiotensin II-responsive genes. Genetically modified, cardiomyocyte-specific mouse models (knockout and overexpression) were generated. In vivo and in vitro electrophysiological, histology, and biochemical analyses were performed to determine the consequences of CIB2 (calcium and integrin binding family member 2 protein) gain and loss of function in the atrium. RESULTS: Using RNA-sequencing analysis, we identified CIB2 as an atrial-enriched protein that is significantly downregulated in the left atria of patients with AF and mouse models of AF from angiotensin II infusion or pressure overload. Using cardiomyocyte-specific Cib2 knockout (Cib2-/-) and atrial myocyte-specific Cib2-overexpressing mouse models, we found that loss of Cib2 enhances AF occurrence, prolongs AF duration, and correlates with a significant increase in atrial fibrosis under stress. Conversely, Cib2 overexpression mitigates AF occurrence and atrial fibrosis triggered by angiotensin II stress. Mechanistically, we revealed that CIB2 competes with and inhibits CIB1-mediated calcineurin activation, thereby negating stress-induced structural remodeling and AF. CONCLUSIONS: Our data suggest that CIB2 represents a novel endogenous and atrial-enriched regulator that protects against atrial remodeling and AF under stress conditions. Therefore, CIB2 may represent a new potential target for treating AF.

Gene Therapy With the N-Terminus of Junctophilin-2 Improves Heart Failure in Mice.

BACKGROUND: Transcriptional remodeling is known to contribute to heart failure (HF). Targeting stress-dependent gene expression mechanisms may represent a clinically relevant gene therapy option. We recently uncovered a salutary mechanism in the heart whereby JP2 (junctophilin-2), an essential component of the excitation-contraction coupling apparatus, is site-specifically cleaved and releases an N-terminal fragment (JP2NT [N-terminal fragment of JP2]) that translocates into the nucleus and functions as a transcriptional repressor of HF-related genes. This study aims to determine whether JP2NT can be leveraged by gene therapy techniques for attenuating HF progression in a preclinical pressure overload model. METHODS: We intraventricularly injected adeno-associated virus (AAV) (2/9) vectors expressing eGFP (enhanced green fluorescent protein), JP2NT, or DNA-binding deficient JP2NT (JP2NTΔbNLS/ARR) into neonatal mice and induced cardiac stress by transaortic constriction (TAC) 9 weeks later. We also treated mice with established moderate HF from TAC stress with either AAV-JP2NT or AAV-eGFP. RNA-sequencing analysis was used to reveal changes in hypertrophic and HF-related gene transcription by JP2NT gene therapy after TAC. Echocardiography, confocal imaging, and histology were performed to evaluate heart function and pathological myocardial remodeling following stress. RESULTS: Mice preinjected with AAV-JP2NT exhibited ameliorated cardiac remodeling following TAC. The JP2NT DNA-binding domain is required for cardioprotection as its deletion within the AAV-JP2NT vector prevented improvement in TAC-induced cardiac dysfunction. Functional and histological data suggest that JP2NT gene therapy after the onset of cardiac dysfunction is effective at slowing the progression of HF. RNA-sequencing analysis further revealed a broad reversal of hypertrophic and HF-related gene transcription by JP2NT overexpression after TAC. CONCLUSIONS: Our prevention- and intervention-based approaches here demonstrated that AAV-mediated delivery of JP2NT into the myocardium can attenuate stress-induced transcriptional remodeling and the development of HF when administered either before or after cardiac stress initiation. Our data indicate that JP2NT gene therapy holds great potential as a novel therapeutic for treating hypertrophy and HF.

Polymer Photocatalysts Containing Segregated π-Conjugation Units with Electron-Trap Activity for Efficient Natural-light-driven Bacterial Inactivation.

Development of highly efficient and metal-free photocatalysts for bacterial inactivation under natural light is a major challenge in photocatalytic antibiosis. Herein, we developed an acidizing solvent-thermal approach for inserting a non-conjugated ethylenediamine segment into the conjugated planes of 3,4,9,10-perylene tetracarboxylic anhydride to generate a photocatalyst containing segregated π-conjugation units (EDA-PTCDA). Under natural light, EDA-PTCDA achieved 99.9 % inactivation of Escherichia coli and Staphylococcus aureus (60 and 45 min), which is the highest efficiency among all the natural light antibacterial reports. The difference in the surface potential and excited charge density corroborated the possibility of a built-in electron-trap effect of the non-conjugated segments of EDA-PTCDA, thus forming a highly active EDA-PTDA/bacteria interface. In addition, EDA-PTCDA exhibited negligible toxicity and damage to normal tissue cells. This catalyst provides a new opportunity for photocatalytic antibiosis under natural light conditions.

Impaired Regulatory Volume Decrease and Characterization of Underlying Volume-Activated Currents in Cystic Fibrosis Human Cholangiocyte Cell Line.

The volume-activated chloride channel (VACC) serves vital cellular functions in secretion and cell volume regulation via regulatory volume decrease (RVD) in various epithelia. Previously, we have shown that RVD in primary CF mouse cholangiocytes is impaired. Thus, the effect of CFTR defect on VACC and RVD in CF human immortalized cholangiocyte cell (HBDC) was examined in comparison with those in normal HBDC by using cell volume measurement and whole-cell patch clamp techniques, respectively. The CF HBDC had an impaired RVD, which was not further inhibited by removing the extracellular calcium or administering BAPTA-AM, NPPB, or DIDS. When exposed to a hypotonic solution, CF HBDC exhibited large, outwardly rectified currents with time-dependent inactivation at a positive potential. The amplitude of the outward currents was about three times that of the inward currents. The amplitude and reversal potential of VACC was dependent on chloride concentration. The VACC was significantly inhibited by replacing chloride with gluconate, glutamate, sucrose, or acetate in the hypotonic solution as well as by an administration of NPPB or tamoxifen, classical VACC inhibitors. Surprisingly, the VACC amplitude is greater in CF HBDC than in normal HBDC, suggesting that the channel density or open probability of VACC is increased, thus CFTR may have inhibitory effects on VACC. On the contrary, the amplitude of the volume-activated potassium current is lower in CF HBDC, suggesting the potassium channel density or open probability is decreased in CF cholangiocytes and/or CFTR may have regulatory effects on volume-activated potassium current. In conclusion, RVD is impaired in CF human cholangiocytes. The VACC of CF human cholangiocytes has similar electrophysiological characteristics as that of normal cholangiocytes but its activity is augmented in CF cholangiocytes, while volume-activated potassium current is decreased in CF human cholangiocytes, providing a fundamental underlying pathophysiologic mechanism for the impaired RVD in CF cholangiocytes.

Levofloxacin exerts broad-spectrum anticancer activity via regulation of THBS1, LAPTM5, SRD5A3, MFAP5 and P4HA1.

One cost-effective way for identifying novel cancer therapeutics is in the repositioning of available drugs for which current therapies are inadequate. Levofloxacin prevents DNA duplication in bacteria by inhibiting the activity of DNA helicase. As eukaryotic cells have similar intracellular biologic characteristics as prokaryotic cells, we speculate that antibiotics inhibiting DNA duplication in bacteria may also affect the survival of cancer cells. Here we report that levofloxacin significantly inhibited the proliferation and clone formation of cancer cells and xenograft tumor growth through cell cycle arrest at G2/M and by enhancing apoptosis. Levofloxacin significantly altered gene expression in a direction favoring anticancer activity. THBS1 and LAPTM5 were dose-dependently upregulated whereas SRD5A3, MFAP5 and P4HA1 were downregulated. Pathway analysis revealed that levofloxacin significantly regulated canonical oncogenic pathways. Specific network enrichment included a MAPK/apoptosis/cytokine-cytokine receptor interaction pathway network that associates with cell growth, differentiation, cell death, angiogenesis and development and repair processes and a bladder cancer/P53 signaling pathway network mediating the inhibition of angiogenesis and metastasis. THBS1 overlapped in 16 of the 22 enriched apoptotic pathways and the 2 pathways in the bladder cancer/P53 signaling pathway network. P4HA1 enriched in 7 of the top 10 molecular functions regulated by differential downregulated genes. Our results indicate that levofloxacin has broad-spectrum anticancer activity with the potential to benefit cancer patients already treated or requiring prophylaxis for an infectious syndrome. The efficacy we find with levofloxacin may provide insight into the discovery and the design of novel less toxic anticancer drugs.

Atrial-paced, exercise-similar heart rate envelope induces myocardial protection from ischaemic injury.

AIMS: The study investigates the role and mechanisms of clinically translatable exercise heart rate (HR) envelope effects, without dyssynchrony, on myocardial ischaemia tolerance compared to standard preconditioning methods. Since the magnitude and duration of exercise HR acceleration are tightly correlated with beneficial cardiac outcomes, it is hypothesized that a paced exercise-similar HR envelope, delivered in a maximally physiologic way that avoids the toxic effects of chamber dyssynchrony, may be more than simply a readout, but rather also a significant trigger of myocardial conditioning and stress resistance. METHODS AND RESULTS: For 8 days over 2 weeks, sedated mice were atrial-paced once daily via an oesophageal electrode to deliver an exercise-similar HR pattern with preserved atrioventricular and interventricular synchrony. Effects on cardiac calcium handling, protein expression/modification, and tolerance to ischaemia-reperfusion (IR) injury were assessed and compared to those in sham-paced mice and to the effects of exercise and ischaemic preconditioning (IPC). The paced cohort displayed improved myocardial IR injury tolerance vs. sham controls with an effect size similar to that afforded by treadmill exercise or IPC. Hearts from paced mice displayed changes in Ca2+ handling, coupled with changes in phosphorylation of calcium/calmodulin protein kinase II, phospholamban and ryanodine receptor channel, and transcriptional remodelling associated with a cardioprotective paradigm. CONCLUSIONS: The HR pattern of exercise, delivered by atrial pacing that preserves intracardiac synchrony, induces cardiac conditioning and enhances ischaemic stress resistance. This identifies the HR pattern as a signal for conditioning and suggests the potential to repurpose atrial pacing for cardioprotection.

Calpain-2 specifically cleaves Junctophilin-2 at the same site as Calpain-1 but with less efficacy.

Calpain proteolysis contributes to the pathogenesis of heart failure but the calpain isoforms responsible and their substrate specificities have not been rigorously defined. One substrate, Junctophilin-2 (JP2), is essential for maintaining junctional cardiac dyads and excitation-contraction coupling. We previously demonstrated that mouse JP2 is cleaved by calpain-1 (CAPN1) between Arginine 565 (R565) and Threonine 566 (T566). Recently, calpain-2 (CAPN2) was reported to cleave JP2 at a novel site between Glycine 482 (G482) and Threonine 483 (T483). We aimed to directly compare the contributions of each calpain isoform, their Ca2+ sensitivity, and their cleavage site selection for JP2. We find CAPN1, CAPN2 and their requisite CAPNS1 regulatory subunit are induced by pressure overload stress that is concurrent with JP2 cleavage. Using in vitro calpain cleavage assays, we demonstrate that CAPN1 and CAPN2 cleave JP2 into similar 75 kD N-terminal (JP2NT) and 25 kD C-terminal fragments (JP2CT) with CAPNS1 co-expression enhancing proteolysis. Deletion mutagenesis shows both CAPN1 and CAPN2 require R565/T566 but not G482/T483. When heterologously expressed, the JP2CT peptide corresponding to R565/T566 cleavage approximates the 25 kD species found during cardiac stress while the C-terminal peptide from potential cleavage at G482/T483 produces a 35 kD product. Similar results were obtained for human JP2. Finally, we show that CAPN1 has higher Ca2+ sensitivity and cleavage efficacy than CAPN2 on JP2 and other cardiac substrates including cTnT, cTnI and ß2-spectrin. We conclude that CAPN2 cleaves JP2 at the same functionally conserved R565/T566 site as CAPN1 but with less efficacy and suggest heart failure may be targeted through specific inhibition of CAPN1.

Integrin ß1D Deficiency-Mediated RyR2 Dysfunction Contributes to Catecholamine-Sensitive Ventricular Tachycardia in Arrhythmogenic Right Ventricular Cardiomyopathy.

BACKGROUND: Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a hereditary heart disease characterized by fatty infiltration, life-threatening arrhythmias, and increased risk of sudden cardiac death. The guideline for management of ARVC in patients is to improve quality of life by reducing arrhythmic symptoms and to prevent sudden cardiac death. However, the mechanism underlying ARVC-associated cardiac arrhythmias remains poorly understood. METHODS: Using protein mass spectrometry analyses, we identified that integrin ß1 is downregulated in ARVC hearts without changes to Ca2+-handling proteins. As adult cardiomyocytes express only the ß1D isoform, we generated a cardiac specific ß1D knockout mouse model and performed functional imaging and biochemical analyses to determine the consequences of integrin ß1D loss on function in the heart in vivo and in vitro. RESULTS: Integrin ß1D deficiency and RyR2 Ser-2030 hyperphosphorylation were detected by Western blotting in left ventricular tissues from patients with ARVC but not in patients with ischemic or hypertrophic cardiomyopathy. Using lipid bilayer patch clamp single channel recordings, we found that purified integrin ß1D protein could stabilize RyR2 function by decreasing RyR2 open probability, mean open time, and increasing mean close time. Also, ß1D knockout mice exhibited normal cardiac function and morphology but presented with catecholamine-sensitive polymorphic ventricular tachycardia, consistent with increased RyR2 Ser-2030 phosphorylation and aberrant Ca2+ handling in ß1D knockout cardiomyocytes. Mechanistically, we revealed that loss of DSP (desmoplakin) induces integrin ß1D deficiency in ARVC mediated through an ERK1/2 (extracellular signal-regulated kinase 1 and 2)-fibronectin-ubiquitin/lysosome pathway. CONCLUSIONS: Our data suggest that integrin ß1D deficiency represents a novel mechanism underlying the increased risk of ventricular arrhythmias in patients with ARVC.

Cardiomyocyte PKA Ablation Enhances Basal Contractility While Eliminates Cardiac ß-Adrenergic Response Without Adverse Effects on the Heart.

RATIONALE: PKA (Protein Kinase A) is a major mediator of ß-AR (ß-adrenergic) regulation of cardiac function, but other mediators have also been suggested. Reduced PKA basal activity and activation are linked to cardiac diseases. However, how complete loss of PKA activity impacts on cardiac physiology and if it causes cardiac dysfunction have never been determined. OBJECTIVES: We set to determine how the heart adapts to the loss of cardiomyocyte PKA activity and if it elicits cardiac abnormalities. METHODS AND RESULTS: (1) Cardiac PKA activity was almost completely inhibited by expressing a PKA inhibitor peptide in cardiomyocytes (cPKAi) in mice; (2) cPKAi reduced basal phosphorylation of 2 myofilament proteins (TnI [troponin I] and cardiac myosin binding protein C), and one longitudinal SR (sarcoplasmic reticulum) protein (PLB [phospholamban]) but not of the sarcolemmal proteins (Cav1.2 α1c and PLM [phospholemman]), dyadic protein RyR2, and nuclear protein CREB (cAMP response element binding protein) at their PKA phosphorylation sites; (3) cPKAi increased the expression of CaMKII (Ca2+/calmodulin-dependent kinase II), the Cav1.2 ß subunits and current, but decreased CaMKII phosphorylation and CaMKII-mediated phosphorylation of PLB and RyR2; (4) These changes resulted in significantly enhanced myofilament Ca2+ sensitivity, prolonged contraction, slowed relaxation but increased myocyte Ca2+ transient and contraction amplitudes; (5) Isoproterenol-induced PKA and CaMKII activation and their phosphorylation of proteins were prevented by cPKAi; (6) cPKAi abolished the increases of heart rate, and cardiac and myocyte contractility by a ß-AR agonist (isoproterenol), showing an important role of PKA and a minimal role of PKA-independent ß-AR signaling in acute cardiac regulation; (7) cPKAi mice have partial exercise capability probably by enhancing vascular constriction and ventricular filling during ß-AR stimulation; and (8) cPKAi mice did not show any cardiac functional or structural abnormalities during the 1-year study period. CONCLUSIONS: PKA activity suppression induces a unique Ca2+ handling phenotype, eliminates ß-AR regulation of heart rates and cardiac contractility but does not cause cardiac abnormalities.

Stress Signaling JNK2 Crosstalk With CaMKII Underlies Enhanced Atrial Arrhythmogenesis.

RATIONALE: Atrial fibrillation (AF) is the most common arrhythmia, and advanced age is an inevitable and predominant AF risk factor. However, the mechanisms that couple aging and AF propensity remain unclear, making targeted therapeutic interventions unattainable. OBJECTIVE: To explore the functional role of an important stress response JNK (c-Jun N-terminal kinase) in sarcoplasmic reticulum Ca2+ handling and consequently Ca2+-mediated atrial arrhythmias. METHODS AND RESULTS: We used a series of cutting-edge electrophysiological and molecular techniques, exploited the power of transgenic mouse models to detail the molecular mechanism, and verified its clinical applicability in parallel studies on donor human hearts. We discovered that significantly increased activity of the stress response kinase JNK2 (JNK isoform 2) in the aged atria is involved in arrhythmic remodeling. The JNK-driven atrial proarrhythmic mechanism is supported by a pathway linking JNK, CaMKII (Ca2+/calmodulin-dependent kinase II), and sarcoplasmic reticulum Ca2+ release RyR2 (ryanodine receptor) channels. JNK2 activates CaMKII, a critical proarrhythmic molecule in cardiac muscle. In turn, activated CaMKII upregulates diastolic sarcoplasmic reticulum Ca2+ leak mediated by RyR2 channels. This leads to aberrant intracellular Ca2+ waves and enhanced AF propensity. In contrast, this mechanism is absent in young atria. In JNK challenged animal models, this is eliminated by JNK2 ablation or CaMKII inhibition. CONCLUSIONS: We have identified JNK2-driven CaMKII activation as a novel mode of kinase crosstalk and a causal factor in atrial arrhythmic remodeling, making JNK2 a compelling new therapeutic target for AF prevention and treatment.

Analysis of Cardiac Myocyte Maturation Using CASAAV, a Platform for Rapid Dissection of Cardiac Myocyte Gene Function In Vivo.

RATIONALE: Loss-of-function studies in cardiac myocytes (CMs) are currently limited by the need for appropriate conditional knockout alleles. The factors that regulate CM maturation are poorly understood. Previous studies on CM maturation have been confounded by heart dysfunction caused by whole organ gene inactivation. OBJECTIVE: To develop a new technical platform to rapidly characterize cell-autonomous gene function in postnatal murine CMs and apply it to identify genes that regulate transverse tubules (T-tubules), a hallmark of mature CMs. METHODS AND RESULTS: We developed CRISPR/Cas9/AAV9-based somatic mutagenesis, a platform in which AAV9 delivers tandem guide RNAs targeting a gene of interest and cardiac troponin-T promoter-driven Cre to RosaCas9GFP/Cas9GFP neonatal mice. When directed against junctophilin-2 (Jph2), a gene previously implicated in T-tubule maturation, we achieved efficient, rapid, and CM-specific JPH2 depletion. High-dose AAV9 ablated JPH2 in 64% CMs and caused lethal heart failure, whereas low-dose AAV9 ablated JPH2 in 22% CMs and preserved normal heart function. In the context of preserved heart function, CMs lacking JPH2 developed T-tubules that were nearly morphologically normal, indicating that JPH2 does not have a major, cell-autonomous role in T-tubule maturation. However, in hearts with severe dysfunction, both adeno-associated virus-transduced and nontransduced CMs exhibited T-tubule disruption, which was more severe in the transduced subset. These data indicate that cardiac dysfunction disrupts T-tubule structure and that JPH2 protects T-tubules in this context. We then used CRISPR/Cas9/AAV9-based somatic mutagenesis to screen 8 additional genes for required, cell-autonomous roles in T-tubule formation. We identified RYR2 (Ryanodine Receptor-2) as a novel, cell-autonomously required T-tubule maturation factor. CONCLUSIONS: CRISPR/Cas9/AAV9-based somatic mutagenesis is a powerful tool to study cell-autonomous gene functions. Genetic mosaics are invaluable to accurately define cell-autonomous gene function. JPH2 has a minor role in normal T-tubule maturation but is required to stabilize T-tubules in the failing heart. RYR2 is a novel T-tubule maturation factor.

Transient activation of PKC results in long-lasting detrimental effects on systolic [Ca2+]i in cardiomyocytes by altering actin cytoskeletal dynamics and T-tubule integrity.

AIMS: Protein kinase C (PKC) isozymes contribute to the development of heart failure through dysregulation of Ca2+ handling properties and disruption of contractile function in cardiomyocytes. However, the mechanisms by which PKC activation leads to Ca2+ dysfunction are incompletely understood. METHODS AND RESULTS: Shortly upon ventricular pressure overload in mice, we detected transient PKC activation that was associated with pulsed actin cytoskeletal rearrangement. In cultured cardiomyocytes, transient activation of PKC promoted long-term deleterious effects on the integrity of the transverse (T)- tubule system, resulting in a significant decrease in the amplitude and increase in the rising kinetics of Ca2+ transients. Treatment with a PKCα/ß inhibitor restored the synchronization of Ca2+ transients and maintained T-tubule integrity in cultured cardiomyocytes. Supporting these data, PKCα/ß inhibition protected against T-tubule remodeling and cardiac dysfunction in a mouse model of pressure overload-induced heart failure. Mechanistically, transient activation of PKC resulted in biphasic actin cytoskeletal rearrangement, consistent with in vivo observations in the pressure overloaded mouse model. Transient inhibition of actin polymerization or depolymerization resulted in severe T-tubule damage, recapitulating the T-tubule damage induced by PKC activation. Moreover, inhibition of stretch activated channels (SAC) protected against T-tubule remodeling and E-C coupling dysfunction induced by transient PKC activation and actin cytoskeletal rearrangement. CONCLUSIONS: These data identify a key mechanistic link between transient PKC activation and long-term Ca2+ handling defects through PKC-induced actin cytoskeletal rearrangement and resultant T-tubule damage.

CaMKII determines mitochondrial stress responses in heart.

Myocardial cell death is initiated by excessive mitochondrial Ca(2+) entry causing Ca(2+) overload, mitochondrial permeability transition pore (mPTP) opening and dissipation of the mitochondrial inner membrane potential (ΔΨm). However, the signalling pathways that control mitochondrial Ca(2+) entry through the inner membrane mitochondrial Ca(2+) uniporter (MCU) are not known. The multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is activated in ischaemia reperfusion, myocardial infarction and neurohumoral injury, common causes of myocardial death and heart failure; these findings suggest that CaMKII could couple disease stress to mitochondrial injury. Here we show that CaMKII promotes mPTP opening and myocardial death by increasing MCU current (I(MCU)). Mitochondrial-targeted CaMKII inhibitory protein or cyclosporin A, an mPTP antagonist with clinical efficacy in ischaemia reperfusion injury, equivalently prevent mPTP opening, ΔΨm deterioration and diminish mitochondrial disruption and programmed cell death in response to ischaemia reperfusion injury. Mice with myocardial and mitochondrial-targeted CaMKII inhibition have reduced I(MCU) and are resistant to ischaemia reperfusion injury, myocardial infarction and neurohumoral injury, suggesting that pathological actions of CaMKII are substantially mediated by increasing I(MCU). Our findings identify CaMKII activity as a central mechanism for mitochondrial Ca(2+) entry in myocardial cell death, and indicate that mitochondrial-targeted CaMKII inhibition could prevent or reduce myocardial death and heart failure in response to common experimental forms of pathophysiological stress.

Inhibition of MCU forces extramitochondrial adaptations governing physiological and pathological stress responses in heart.

Myocardial mitochondrial Ca(2+) entry enables physiological stress responses but in excess promotes injury and death. However, tissue-specific in vivo systems for testing the role of mitochondrial Ca(2+) are lacking. We developed a mouse model with myocardial delimited transgenic expression of a dominant negative (DN) form of the mitochondrial Ca(2+) uniporter (MCU). DN-MCU mice lack MCU-mediated mitochondrial Ca(2+) entry in myocardium, but, surprisingly, isolated perfused hearts exhibited higher O2 consumption rates (OCR) and impaired pacing induced mechanical performance compared with wild-type (WT) littermate controls. In contrast, OCR in DN-MCU-permeabilized myocardial fibers or isolated mitochondria in low Ca(2+) were not increased compared with WT, suggesting that DN-MCU expression increased OCR by enhanced energetic demands related to extramitochondrial Ca(2+) homeostasis. Consistent with this, we found that DN-MCU ventricular cardiomyocytes exhibited elevated cytoplasmic [Ca(2+)] that was partially reversed by ATP dialysis, suggesting that metabolic defects arising from loss of MCU function impaired physiological intracellular Ca(2+) homeostasis. Mitochondrial Ca(2+) overload is thought to dissipate the inner mitochondrial membrane potential (ΔΨm) and enhance formation of reactive oxygen species (ROS) as a consequence of ischemia-reperfusion injury. Our data show that DN-MCU hearts had preserved ΔΨm and reduced ROS during ischemia reperfusion but were not protected from myocardial death compared with WT. Taken together, our findings show that chronic myocardial MCU inhibition leads to previously unanticipated compensatory changes that affect cytoplasmic Ca(2+) homeostasis, reprogram transcription, increase OCR, reduce performance, and prevent anticipated therapeutic responses to ischemia-reperfusion injury.

MG53 is dispensable for T-tubule maturation but critical for maintaining T-tubule integrity following cardiac stress.

The cardiac transverse (T)-tubule membrane system is the safeguard for cardiac function and undergoes dramatic remodeling in response to cardiac stress. However, the mechanism by which cardiomyocytes repair damaged T-tubule network remains unclear. In the present study, we tested the hypothesis that MG53, a muscle-specific membrane repair protein, antagonizes T-tubule damage to protect against maladaptive remodeling and thereby loss of excitation-contraction coupling and cardiac function. Using MG53-knockout (MG53-KO) mice, we first established that deficiency of MG53 had no impact on maturation of the T-tubule network in developing hearts. Additionally, MG53 ablation did not influence T-tubule integrity in unstressed adult hearts as late as 10months of age. Following left ventricular pressure overload-induced cardiac stress, MG53 protein levels were increased by approximately three-fold in wild-type mice, indicating that pathological stress induces a significant upregulation of MG53. MG53-deficient mice had worsened T-tubule disruption and pronounced dysregulation of Ca2+ handling properties, including decreased Ca2+ transient amplitude and prolonged time to peak and decay. Moreover, MG53 deficiency exacerbated cardiac hypertrophy and dysfunction and decreased survival following cardiac stress. Our data suggest MG53 is not required for T-tubule development and maintenance in normal physiology. However, MG53 is essential to preserve T-tubule integrity and thereby Ca2+ handling properties and cardiac function under pathological cardiac stress.

Critical Roles of STAT3 in ß-Adrenergic Functions in the Heart.

BACKGROUND: ß-Adrenergic receptors (ßARs) play paradoxical roles in the heart. On one hand, ßARs augment cardiac performance to fulfill the physiological demands, but on the other hand, prolonged activations of ßARs exert deleterious effects that result in heart failure. The signal transducer and activator of transcription 3 (STAT3) plays a dynamic role in integrating multiple cytokine signaling pathways in a number of tissues. Altered activation of STAT3 has been observed in failing hearts in both human patients and animal models. Our objective is to determine the potential regulatory roles of STAT3 in cardiac ßAR-mediated signaling and function. METHODS AND RESULTS: We observed that STAT3 can be directly activated in cardiomyocytes by ß-adrenergic agonists. To follow up this finding, we analyzed ßAR function in cardiomyocyte-restricted STAT3 knockouts and discovered that the conditional loss of STAT3 in cardiomyocytes markedly reduced the cardiac contractile response to acute ßAR stimulation, and caused disengagement of calcium coupling and muscle contraction. Under chronic ß-adrenergic stimulation, Stat3cKO hearts exhibited pronounced cardiomyocyte hypertrophy, cell death, and subsequent cardiac fibrosis. Biochemical and genetic data supported that Gαs and Src kinases are required for ßAR-mediated activation of STAT3. Finally, we demonstrated that STAT3 transcriptionally regulates several key components of ßAR pathway, including ß1AR, protein kinase A, and T-type Ca(2+) channels. CONCLUSIONS: Our data demonstrate for the first time that STAT3 has a fundamental role in ßAR signaling and functions in the heart. STAT3 serves as a critical transcriptional regulator for ßAR-mediated cardiac stress adaption, pathological remodeling, and heart failure.

Overexpression of junctophilin-2 does not enhance baseline function but attenuates heart failure development after cardiac stress.

Heart failure is accompanied by a loss of the orderly disposition of transverse (T)-tubules and a decrease of their associations with the junctional sarcoplasmic reticulum (jSR). Junctophilin-2 (JP2) is a structural protein responsible for jSR/T-tubule docking. Animal models of cardiac stresses demonstrate that down-regulation of JP2 contributes to T-tubule disorganization, loss of excitation-contraction coupling, and heart failure development. Our objective was to determine whether JP2 overexpression attenuates stress-induced T-tubule disorganization and protects against heart failure progression. We therefore generated transgenic mice with cardiac-specific JP2 overexpression (JP2-OE). Baseline cardiac function and Ca(2+) handling properties were similar between JP2-OE and control mice. However, JP2-OE mice displayed a significant increase in the junctional coupling area between T-tubules and the SR and an elevated expression of the Na(+)/Ca(2+) exchanger, although other excitation-contraction coupling protein levels were not significantly changed. Despite similar cardiac function at baseline, overexpression of JP2 provided significantly protective benefits after pressure overload. This was accompanied by a decreased percentage of surviving mice that developed heart failure, as well as preservation of T-tubule network integrity in both the left and right ventricles. Taken together, these data suggest that strategies to maintain JP2 levels can prevent the progression from hypertrophy to heart failure.

Cholesterol is required for maintaining T-tubule integrity and intercellular connections at intercalated discs in cardiomyocytes.

BACKGROUNDS: Low serum cholesterol levels are associated with cardiac arrhythmias and poor prognosis in patients with chronic heart failure. However, the underlying mechanisms by which decreases in cholesterol content lead to cardiac dysfunction remain unclear. Multiple studies have implicated damage to cardiac transverse (T)-tubules as a key mediator of excitation-contraction (E-C) coupling dysfunction and heart failure. Since the T-tubule membrane system is enriched in cholesterol, we hypothesized that depletion of membrane cholesterol promotes T-tubule remodeling and E-C coupling dysfunction. METHODS AND RESULTS: We first examined the impact of membrane cholesterol depletion on T-tubule architecture by treating isolated C57BL/6 murine cardiomyocytes with methyl-ß-cyclodextrin (MßCD). T-tubule structural integrity was progressively decreased by MßCD in a concentration- and time-dependent manner. Membrane cholesterol depletion also promoted a severe decrease in the amplitude of Ca(2+) transients and an increase in Ca(2+) release dyssynchrony as well as a significant increase in the frequency of spontaneous Ca(2+) sparks. Reintroduction of cholesterol restored T-tubule integrity and partially restored Ca(2+) handling properties in acutely-treated myocytes and slowed T-tubule deterioration in response to chronic MßCD exposure. Studies were extended to determine the impact of membrane cholesterol depletion on T-tubule structure in intact hearts. In addition to T-tubule remodeling, Langendorff perfusion of MßCD resulted in rapid and severe disruption of the intercellular connections between cardiomyocytes, in particular at intercalated disc regions in intact hearts. CONCLUSIONS: These data provide the first evidence that cholesterol plays a critical role in maintaining cardiac T-tubule structure as well as the integrity of intercalated discs.