Regulation of Ca2+ signaling by acute hypoxia and acidosis in cardiomyocytes derived from human induced pluripotent stem cells

Loss-of-function W4645R mutation in the RyR2-caffeine binding site: implications for synchrony and arrhythmogenesis

AIMS

Previous studies have identified RyR2 W4645R mutation, located in the caffeine-binding site, to associate with CPVT1 pathology. Caffeine binding to its site is thought to displace the carboxyl-terminal domain to Ca2+-binding, allowing the tryptophan residue (W4645) to regulate Ca2+ sensitivity of RyR2. To gain insights into regulation of RyR2 Ca2+-binding and its interaction with caffeine-binding site, we introduced W4645R-RyR2 point mutation via CRISPR/Cas9 gene-editing in human induced pluripotent stem cell-derived cardiomyocytes (hiPSCCMs) and characterized their Ca2+-signaling phenotype compared to WT hiPSCCMs.

METHODS AND RESULTS

W4645R-RyR2 cardiomyocytes had: (1) no significant change in ICa magnitude or voltage-dependence; (2) slightly reduced CICR; (3) altered relaxation kinetics of Ca2+-transients with no change in isoproterenol sensitivity; (4) complete loss of caffeine-triggered Ca2+ release; (5) larger SR Ca2+ leak resulting in 40 % lower SR Ca2+ content, as determined by myocytes' response to 4-CmC; (6) lower incidence of calcium sparks and asynchronous spontaneous SR Ca2+ releases.

CONCLUSIONS

W4645R-RyR2 mutation induces loss of caffeine-triggered SR Ca2+ release and enhances SR Ca2+ leak that underlie asynchronous spontaneous Ca2+ releases, triggering arrhythmia and impairing cardiac function.

Amelioration of obsessive-compulsive disorder by intracellular acidification of cortical neurons with a proton pump inhibitor

Obsessive-compulsive disorder (OCD) is a highly prevalent neuropsychiatric disorder poorly controlled with pharmacological treatment because of the wide variation in symptom patterns. We analysed real-world data on adverse self-reports and insurance claims to identify a novel therapeutic target for OCD. We found that dopamine D2 receptor (D2R) agonists increased the incidence of OCD-like symptoms, which were suppressed by the concomitant use of proton pump inhibitors (PPIs). Further, OCD-like repetitive and habitual behaviours were observed in mice repeatedly injected with a D2R agonist, quinpirole. However, these abnormalities were suppressed by short-term PPI treatment. In quinpirole-treated mice, PPI inhibited pyramidal neuron hyperactivity in the lateral orbitofrontal cortex, a region where the P-type proton pump gene Atp4a is abundantly expressed. In primary cultured cortical neurons, short-term PPI treatment lowered intracellular pH and decreased firing activity, which was mimicked by Atp4a knockdown. Our findings show that inhibition of P-type proton pumps may be a novel therapeutic strategy for OCD.

Ahf-Caltide, a Novel Polypeptide Derived from Calpastatin, Protects against Oxidative Stress Injury by Stabilizing the Expression of CaV1.2 Calcium Channel

Reperfusion after ischemia would cause massive myocardial injury, which leads to oxidative stress (OS). Calcium homeostasis imbalance plays an essential role in myocardial OS injury. CaV1.2 calcium channel mediates calcium influx into cardiomyocytes, and its activity is modulated by a region of calpastatin (CAST) domain L, CSL54-64. In this study, the effect of Ahf-caltide, derived from CSL54-64, on myocardial OS injury was investigated. Ahf-caltide decreased the levels of LDH, MDA and ROS and increased heart rate, coronary flow, cell survival and SOD activity during OS. In addition, Ahf-caltide permeated into H9c2 cells and increased CaV1.2, CaVβ2 and CAST levels by inhibiting protein degradation. At different Ca2+ concentrations (25 nM, 10 μM, 1 mM), the binding of CSL to the IQ motif in the C terminus of the CaV1.2 channel was increased in a H2O2 concentration-dependent manner. CSL54-64 was predicted to be responsible for the binding of CSL to CaV1.2. In conclusion, Ahf-caltide exerted a cardioprotective effect on myocardial OS injury by stabilizing CaV1.2 protein expression. Our study, for the first time, proposed that restoring calcium homeostasis by targeting the CaV1.2 calcium channel and its regulating factor CAST could be a novel treatment for myocardial OS injury.

Electrophysiological Changes of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes during Acute Hypoxia and Reoxygenation

Ischemic heart disease is the most common cardiovascular disease and a major burden for healthcare worldwide. However, its pathophysiology is still not fully understood, and human-based models for disease mechanisms and treatments are needed. Here, we used human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to model acute ischemia-reperfusion in our novel cell culture assembly. The assembly enables exchange of oxygen partial pressure for the cells within minutes, mimicking acute ischemic event. In this study, hypoxia was induced using 0% O₂ gas for three hours and reoxygenation with 19% O₂ gas for 24 hours in serum- and glucose-free medium. According to electrophysiological recordings, hypoxia decreased the hiPSC-CM-beating frequency and field potential (FP) amplitude. Furthermore, FP depolarization time and propagation slowed down. Most of the electrophysiological changes reverted during reoxygenation. However, immunocytochemical staining of the hypoxic and reoxygenation samples showed that morphological changes and changes in the sarcomere structure did not revert during reoxygenation but further deteriorated. qPCR results showed no significant differences in apoptosis or stress-related genes or in the expression of glycolytic genes. In conclusion, the hiPSC-CMs reproduced many characteristic changes of adult CMs during ischemia and reperfusion, indicating their usefulness as a human-based model of acute cardiac ischemia-reperfusion.

New insights in ferroptosis: Potential therapeutic targets for the treatment of ischemic stroke

Stroke is a common disease in clinical practice, which seriously endangers people's physical and mental health. The neurovascular unit (NVU) plays a key role in the occurrence and development of ischemic stroke. Different from other classical types of cell death such as apoptosis, necrosis, autophagy, and pyroptosis, ferroptosis is an iron-dependent lipid peroxidation-driven new form of cell death. Interestingly, the function of NVU and stroke development can be regulated by activating or inhibiting ferroptosis. This review systematically describes the NVU in ischemic stroke, provides a comprehensive overview of the regulatory mechanisms and key regulators of ferroptosis, and uncovers the role of ferroptosis in the NVU and the progression of ischemic stroke. We further discuss the latest progress in the intervention of ferroptosis as a therapeutic target for ischemic stroke and summarize the research progress and regulatory mechanism of ferroptosis inhibitors on stroke. In conclusion, ferroptosis, as a new form of cell death, plays a key role in ischemic stroke and is expected to become a new therapeutic target for this disease.

Oxygen Sensor of the Heart

How oxygen is sensed by the heart and what mechanisms mediate its sensing remain poorly understood. Since recent reports show that low PO₂ levels are detected by the cardiomyocytes in a few seconds, the rapid and short applications of low levels of oxygen (acute hypoxia), that avoid multiple effects of chronic hypoxia may be used to probe the oxygen sensing pathway of the heart. Here we explore the oxygen sensing pathway, focusing primarily on cellular surface membrane proteins that are first exposed to low PO₂. Such studies suggest that acute hypoxia primarily targets the cardiac calcium channels, where either the channel itself or moieties closely associated with it, for instance, heme-oxygenase-2 (HO-2) interacting through kinase phosphorylation, signals the α-subunit of the channel as to the altered levels of PO₂. Amino acids 1572-1651, the CaMKII phosphorylation sites (S1487 and S1545), CaM-binding site (I1624, Q1625) and Ser1928 of the carboxyl tail of the α-subunit appear to be critical residues that sense oxygen. Future studies in HO-2 knockout mice or CRISPR/Cas9 gene-edited hiPSC-CMs that reduce CaM-binding affinity are likely to provide deeper insights in the O₂-sensinsing mechanisms.

Cardiac Ischemia On-a-Chip: Antiarrhythmic Effect of Levosimendan on Ischemic Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Ischemic heart disease (IHD) is one of the leading causes of mortality worldwide. Preserving functionality and preventing arrhythmias of the heart are key principles in the management of patients with IHD. Levosimendan, a unique calcium (Ca2+) enhancer with inotropic activity, has been introduced into clinical usage for heart failure treatment. Human-induced pluripotent cell-derived cardiomyocytes (hiPSC-CMs) offer an opportunity to better understand the pathophysiological mechanisms of the disease as well as to serve as a platform for drug screening. Here, we developed an in vitro IHD model using hiPSC-CMs in hypoxic conditions and defined the effects of the subsequent hypoxic stress on CMs functionality. Furthermore, the effect of levosimendan on hiPSC-CMs functionality was evaluated during and after hypoxic stress. The morphology, contractile, Ca2+-handling, and gene expression properties of hiPSC-CMs were investigated in response to hypoxia. Hypoxia resulted in significant cardiac arrhythmia and decreased Ca2+ transient amplitude. In addition, disorganization of sarcomere structure was observed after hypoxia induction. Interestingly, levosimendan presented significant antiarrhythmic properties, as the arrhythmia was abolished or markedly reduced with levosimendan treatment either during or after the hypoxic stress. Moreover, levosimendan presented significant protection from the sarcomere alterations induced by hypoxia. In conclusion, this chip model appears to be a suitable preclinical representation of IHD. With this hypoxia platform, detailed knowledge of the disease pathophysiology can be obtained. The antiarrhythmic effect of levosimendan was clearly observed, suggesting a possible new clinical use for the drug.

Mutation in RyR2-FKBP Binding site alters Ca2+ signaling modestly but increases "arrhythmogenesis" in human stem cells derived cardiomyocytes

AIMS

To gain insights into FKBP regulation of cardiac ryanodine receptor (RyR2) and Ca²⁺ signaling, we introduced the point mutation (N771D-RyR2) corresponding to skeletal muscle mutation (N760D-RyR1) associated with central core disease (CCD) via CRISPR/Cas9 gene-editing in the RyR2 FKBP binding site expressed in human induced pluripotent stem cell-derived cardiomyocytes (hiPSCCMs). Patients inflicted with CCD and other hereditary skeletal muscle diseases often show higher incidence of atrial or ventricular arrhythmias.

METHODS AND RESULTS

Ca²⁺ imaging of voltage-clamped N771D-RyR2 mutant compared to WT hiPSCCMs showed: (1) ∼30% suppressed I_Ca with no significant changes in the gating kinetics of I_Ca; (2) 29% lower SR Ca²⁺ content and 33% lower RyR2 Ca²⁺ leak; (3) higher CICR gain and 30-35% increased efficiency of I_Ca-triggered Ca^2±release; (4) higher incidence of aberrant SR Ca²⁺ releases, DADs, and Ca²⁺ sparks; (5) no change in fractional Ca²⁺-release, action potential morphology, sensitivity to isoproterenol, and sarcomeric FKBP-binding pattern.

CONCLUSIONS

The more frequent spontaneous Ca²⁺ releases and longer Ca²⁺ sparks underlie the increased incidence of DADs and cellular arrhythmogenesis of N771D-RyR2 mutant. The smaller RyR2 Ca^2±leak and SR content result from suppressed I_Cathat is compensated by higher CICR gain.

Human induced pluripotent stem cell-based platform for modeling cardiac ischemia

Ischemic heart disease is a major cause of death worldwide, and the only available therapy to salvage the tissue is reperfusion, which can initially cause further damage. Many therapeutics that have been promising in animal models have failed in human trials. Thus, functional human based cardiac ischemia models are required. In this study, a human induced pluripotent stem cell derived-cardiomyocyte (hiPSC-CM)-based platform for modeling ischemia-reperfusion was developed utilizing a system enabling precise control over oxygen concentration and real-time monitoring of the oxygen dynamics as well as iPS-CM functionality. In addition, morphology and expression of hypoxia-related genes and proteins were evaluated as hiPSC-CM response to 8 or 24 h hypoxia and 24 h reoxygenation. During hypoxia, initial decrease in hiPSC-CM beating frequency was observed, after which the CMs adapted to the conditions and the beating frequency gradually increased already before reoxygenation. During reoxygenation, the beating frequency typically first surpassed the baseline before settling down to the values close the baseline. Furthermore, slowing on the field potential propagation throughout the hiPSC-CM sheet as well as increase in depolarization time and decrease in overall field potential duration were observed during hypoxia. These changes were reversed during reoxygenation. Disorganization of sarcomere structures was observed after hypoxia and reoxygenation, supported by decrease in the expression of sarcomeric proteins. Furthermore, increase in the expression of gene encoding glucose transporter 1 was observed. These findings indicate, that despite their immature phenotype, hiPSC-CMs can be utilized in modeling ischemia-reperfusion injury.

Arrhythmia Mechanisms in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

ABSTRACT

Despite major efforts by clinicians and researchers, cardiac arrhythmia remains a leading cause of morbidity and mortality in the world. Experimental work has relied on combining high-throughput strategies with standard molecular and electrophysiological studies, which are, to a great extent, based on the use of animal models. Because this poses major challenges for translation, the progress in the development of novel antiarrhythmic agents and clinical care has been mostly disappointing. Recently, the advent of human induced pluripotent stem cell-derived cardiomyocytes has opened new avenues for both basic cardiac research and drug discovery; now, there is an unlimited source of cardiomyocytes of human origin, both from healthy individuals and patients with cardiac diseases. Understanding arrhythmic mechanisms is one of the main use cases of human induced pluripotent stem cell-derived cardiomyocytes, in addition to pharmacological cardiotoxicity and efficacy testing, in vitro disease modeling, developing patient-specific models and personalized drugs, and regenerative medicine. Here, we review the advances that the human induced pluripotent stem cell-derived-based modeling systems have brought so far regarding the understanding of both arrhythmogenic triggers and substrates, while also briefly speculating about the possibilities in the future.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

Beyond Family: Modeling Non-hereditary Heart Diseases With Human Pluripotent Stem Cell-Derived Cardiomyocytes

Non-genetic cardiac pathologies develop as an aftermath of extracellular stress-conditions. Nevertheless, the response to pathological stimuli depends deeply on intracellular factors such as physiological state and complex genetic backgrounds. Without a thorough characterization of their in vitro phenotype, modeling of maladaptive hypertrophy, ischemia and reperfusion injury or diabetes in human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has been more challenging than hereditary diseases with defined molecular causes. In past years, greater insights into hPSC-CM in vitro physiology and advancements in technological solutions and culture protocols have generated cell types displaying stress-responsive phenotypes reminiscent of in vivo pathological events, unlocking their application as a reductionist model of human cardiomyocytes, if not the adult human myocardium. Here, we provide an overview of the available literature of pathology models for cardiac non-genetic conditions employing healthy (or asymptomatic) hPSC-CMs. In terms of numbers of published articles, these models are significantly lagging behind monogenic diseases, which misrepresents the incidence of heart disease causes in the human population.

Role of mitochondrial Ca2+ homeostasis in cardiac muscles

Recent discoveries of the molecular identity of mitochondrial Ca2+ influx/efflux mechanisms have placed mitochondrial Ca2+ transport at center stage in views of cellular regulation in various cell-types/tissues. Indeed, mitochondria in cardiac muscles also possess the molecular components for efficient uptake and extraction of Ca2+. Over the last several years, multiple groups have taken advantage of newly available molecular information about these proteins and applied genetic tools to delineate the precise mechanisms for mitochondrial Ca2+ handling in cardiomyocytes and its contribution to excitation-contraction/metabolism coupling in the heart. Though mitochondrial Ca2+ has been proposed as one of the most crucial secondary messengers in controlling a cardiomyocyte's life and death, the detailed mechanisms of how mitochondrial Ca2+ regulates physiological mitochondrial and cellular functions in cardiac muscles, and how disorders of this mechanism lead to cardiac diseases remain unclear. In this review, we summarize the current controversies and discrepancies regarding cardiac mitochondrial Ca2+ signaling that remain in the field to provide a platform for future discussions and experiments to help close this gap.