Calcium Signaling and Reactive Oxygen Species in Mitochondria

Measuring Calcium and ROS by Genetically Encoded Protein Sensors and Fluorescent Dyes

Oxidative modifications of cellular building blocks such as proteins, lipids, and DNA have a major impact on cell behavior, fate, and clinical outcome. Reactive oxygen species (ROS) are important factors that influence these redox processes. Calcium ion (Ca²⁺) dynamics and signals are also essential regulators of key cellular processes. Therefore, the combined and precise monitoring of ROS and Ca²⁺ in single cells, with a high spatial and temporal resolution and in physiological environments, is essential to better understand their functional impact. Here, we describe protocols to detect one of the most prominent ROS (hydrogen peroxide, H₂O₂) using genetically encoded protein sensors and fluorescent dyes. We also provide guidelines on how to simultaneously detect Ca²⁺ and H₂O₂ and how to examine the influence of Ca²⁺ signals on cellular ROS production and vice versa.

Pharmacological Modulation of Mitochondrial Ca2+ Content Regulates Sarcoplasmic Reticulum Ca2+ Release via Oxidation of the Ryanodine Receptor by Mitochondria-Derived Reactive Oxygen Species

In a physiological setting, mitochondria increase oxidative phosphorylation during periods of stress to meet increased metabolic demand. This in part is mediated via enhanced mitochondrial Ca2+ uptake, an important regulator of cellular ATP homeostasis. In a pathophysiological setting pharmacological modulation of mitochondrial Ca2+ uptake or retention has been suggested as a therapeutic strategy to improve metabolic homeostasis or attenuate Ca2+-dependent arrhythmias in cardiac disease states. To explore the consequences of mitochondrial Ca2+ accumulation, we tested the effects of kaempferol, an activator of mitochondrial Ca2+ uniporter (MCU), CGP-37157, an inhibitor of mitochondrial Na+/Ca2+ exchanger, and MCU inhibitor Ru360 in rat ventricular myocytes (VMs) from control rats and rats with hypertrophy induced by thoracic aortic banding (TAB). In periodically paced VMs under β-adrenergic stimulation, treatment with kaempferol (10 μmol/L) or CGP-37157 (1 μmol/L) enhanced mitochondrial Ca2+ accumulation monitored by mitochondrial-targeted Ca2+ biosensor mtRCamp1h. Experiments with mitochondrial membrane potential-sensitive dye TMRM revealed this was accompanied by depolarization of the mitochondrial matrix. Using redox-sensitive OMM-HyPer and ERroGFP_iE biosensors, we found treatment with kaempferol or CGP-37157 increased the levels of reactive oxygen species (ROS) in mitochondria and the sarcoplasmic reticulum (SR), respectively. Confocal Ca2+ imaging showed that accelerated Ca2+ accumulation reduced Ca2+ transient amplitude and promoted generation of spontaneous Ca2+ waves in VMs paced under ISO, suggestive of abnormally high activity of the SR Ca2+ release channel ryanodine receptor (RyR). Western blot analyses showed increased RyR oxidation after treatment with kaempferol or CGP-37157 vs. controls. Furthermore, in freshly isolated TAB VMs, confocal Ca2+ imaging demonstrated that enhancement of mitochondrial Ca2+ accumulation further perturbed global Ca2+ handling, increasing the number of cells exhibiting spontaneous Ca2+ waves, shortening RyR refractoriness and decreasing SR Ca2+ content. In ex vivo optically mapped TAB hearts, kaempferol exacerbated proarrhythmic phenotype. On the contrary, incubation of cells with MCU inhibitor Ru360 (2 μmol/L, 30 min) normalized RyR oxidation state, improved intracellular Ca2+ homeostasis and reduced triggered activity in ex vivo TAB hearts. These findings suggest facilitation of mitochondrial Ca2+ uptake in cardiac disease can exacerbate proarrhythmic disturbances in Ca2+ homeostasis via ROS and enhanced activity of oxidized RyRs, while strategies to reduce mitochondrial Ca2+ accumulation can be protective.

Targeting mitochondrial dysfunction and oxidative stress in heart failure: Challenges and opportunities

Mitochondrial dysfunction characterized by impaired bioenergetics, oxidative stress and aldehydic load is a hallmark of heart failure. Recently, different research groups have provided evidence that selective activation of mitochondrial detoxifying systems that counteract excessive accumulation of ROS, RNS and reactive aldehydes is sufficient to stop cardiac degeneration upon chronic stress, such as heart failure. Therefore, pharmacological and non-pharmacological approaches targeting mitochondria detoxification may play a critical role in the prevention or treatment of heart failure. In this review we discuss the most recent findings on the central role of mitochondrial dysfunction, oxidative stress and aldehydic load in heart failure, highlighting the most recent preclinical and clinical studies using mitochondria-targeted molecules and exercise training as effective tools against heart failure.

Direct Cardiac Actions of Sodium Glucose Cotransporter 2 Inhibitors Target Pathogenic Mechanisms Underlying Heart Failure in Diabetic Patients

Sodium glucose cotransporter 2 inhibitors (SGLT2i) are the first antidiabetic compounds that effectively reduce heart failure hospitalization and cardiovascular death in type 2 diabetics. Being explicitly designed to inhibit SGLT2 in the kidney, SGLT2i have lately been investigated for their off-target cardiac actions. Here, we review the direct effects of SGLT2i Empagliflozin (Empa), Dapagliflozin (Dapa), and Canagliflozin (Cana) on various cardiac cell types and cardiac function, and how these may contribute to the cardiovascular benefits observed in large clinical trials. SGLT2i impaired the Na⁺/H⁺ exchanger 1 (NHE-1), reduced cytosolic [Ca²⁺] and [Na⁺] and increased mitochondrial [Ca²⁺] in healthy cardiomyocytes. Empa, one of the best studied SGLT2i, maintained cell viability and ATP content following hypoxia/reoxygenation in cardiomyocytes and endothelial cells. SGLT2i recovered vasoreactivity of hyperglycemic and TNF-α-stimulated aortic rings and of hyperglycemic endothelial cells. Anti-inflammatory actions of Cana in IL-1β-treated HUVEC and of Dapa in LPS-treated cardiofibroblast were mediated by AMPK activation. In isolated mouse hearts, Empa and Cana, but not Dapa, induced vasodilation. In ischemia-reperfusion studies of the isolated heart, Empa delayed contracture development during ischemia and increased mitochondrial respiration post-ischemia. Direct cardiac effects of SGLT2i target well-known drivers of diabetes and heart failure (elevated cardiac cytosolic [Ca²⁺] and [Na⁺], activated NHE-1, elevated inflammation, impaired vasorelaxation, and reduced AMPK activity). These cardiac effects may contribute to the large beneficial clinical effects of these antidiabetic drugs.

Calcium signals between the ryanodine receptor- and mitochondria critically regulate the effects of arsenite on mitochondrial superoxide formation and on the ensuing survival vs apoptotic signaling

A low concentration of arsenite (6 h), selectively stimulating the intraluminal crosstalk between the inositol-1, 4, 5-triphosphate receptor and the ryanodine receptor (RyR), increased the mitochondrial transport of RyR-derived Ca²⁺ through the mitochondrial Ca²⁺ uniporter. This event was characterized in intact and permeabilized cells, and was shown to be critical for mitochondrial superoxide (mitoO₂^.-) formation. Inhibition of mitochondrial Ca²⁺ accumulation therefore prevented the effects of arsenite, in both the mitochondrial (e.g., cardiolipin oxidation) and extramitochondrial (e.g., DNA single- strand breakage) compartments, and suppressed the Nrf2/GSH survival signaling. The effects of arsenite on Ca²⁺ homeostasis and mitoO₂^.- formation were reversible, as determined after an additional 10 h incubation in fresh culture medium and by measuring long-term viability. A 16 h continuous exposure to arsenite instead produced a sustained increase in the cytosolic and mitochondrial Ca²⁺ concentrations, a further increased mitoO₂^.- formation and mitochondrial permeability transition. These events, followed by delayed apoptosis (48 h), were sensitive to treatments/manipulations preventing mitochondrial Ca²⁺ accumulation. Interestingly, cells remained viable under conditions in which the deregulated Ca²⁺ homeostasis was not accompanied by mitoO₂^.-formation.

In conclusion, we report that the fraction of Ca²⁺ taken up by the mitochondria in response to arsenite derives from the RyR. Mitochondrial Ca²⁺ appears critical for mitoO₂^.- formation and for the triggering of both the cytoprotective and apoptotic signaling. The effects of arsenite were reversible, whereas its prolonged exposure caused a sustained increase in mitochondrial Ca²⁺ and mitoO₂^.- formation, and the prevalence of the apoptotic vs survival signaling.

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Endothelin receptor A and p66Shc regulate spontaneous Ca2+ oscillations in smooth muscle cells controlling renal arterial spontaneous motion

Adaptor protein p66Shc is overexpressed in smooth muscle cells of renal resistance vessels of hypertensive salt-sensitive rats and is involved in the regulation of renal vascular tone. We applied 2-photon laser scanning fluorescence microscopy to analyze spontaneous dynamic fluctuations in intracellular calcium concentrations ([Ca²⁺]_i) in smooth muscle cells embedded in the walls of freshly isolated renal resistance arteries. The amplitude, number of events, and frequency of spontaneous [Ca²⁺]_i oscillations triggered by endogenously released endothelin-1 were recorded in smooth muscle cells of the renal arteries. Endothelin receptor A antagonist BQ123 dramatically reduced the amplitude and frequency of spontaneous Ca²⁺ events, producing marked inhibition of renal vessels spontaneous motion. Spontaneous Ca²⁺ fluctuations in smooth muscle cells of p66Shc knockout (p66ShcKO) rats had significantly higher amplitude than in control rats. The frequency of spontaneous [Ca²⁺]_i oscillations did not change in p66ShcKO rats, suggesting that p66Shc expression did not affect endothelin-1 release from resident endothelial cells. Acute application of endothelin-1 revealed significantly elevated production of the total [Ca²⁺]_i in p66ShcKO rats. Spontaneous cytosolic Ca²⁺ oscillations in smooth muscle cells of renal vessels mediate their spontaneous motion via the endothelin-1/endothelin receptor A pathway. p66Shc decreases the amplitude of individual changes in [Ca²⁺]_i, which mitigates the spontaneous motion of renal vessels.-Palygin, O., Miller, B. S., Nishijima, Y., Zhang, D. X., Staruschenko, A., Sorokin, A. Endothelin receptor A and p66Shc regulate spontaneous Ca²⁺ oscillations in smooth muscle cells controlling renal arterial spontaneous motion.

Calcium in the Pathophysiology of Atrial Fibrillation and Heart Failure

Atrial fibrillation (AF) is commonly associated with heart failure. A bidirectional relationship exists between the two-AF exacerbates heart failure causing a significant increase in heart failure symptoms, admissions to hospital and cardiovascular death, while pathological remodeling of the atria as a result of heart failure increases the risk of AF. A comprehensive understanding of the pathophysiology of AF is essential if we are to break this vicious circle. In this review, the latest evidence will be presented showing a fundamental role for calcium in both the induction and maintenance of AF. After outlining atrial electrophysiology and calcium handling, the role of calcium-dependent afterdepolarizations and atrial repolarization alternans in triggering AF will be considered. The atrial response to rapid stimulation will be discussed, including the short-term protection from calcium overload in the form of calcium signaling silencing and the eventual progression to diastolic calcium leak causing afterdepolarizations and the development of an electrical substrate that perpetuates AF. The role of calcium in the bidirectional relationship between heart failure and AF will then be covered. The effects of heart failure on atrial calcium handling that promote AF will be reviewed, including effects on both atrial myocytes and the pulmonary veins, before the aspects of AF which exacerbate heart failure are discussed. Finally, the limitations of human and animal studies will be explored allowing contextualization of what are sometimes discordant results.

Targeted HFpEF therapy based on matchmaking of human and animal models

The diversity in clinical phenotypes and poor understanding of the underlying pathophysiology of heart failure with preserved ejection fraction (HFpEF) is the main reason why no effective treatments have been found yet. Targeted, instead of one size fits all, treatment seems the only promising approach for treating HFpEF. To be able to design a targeted, phenotype-specific HFpEF treatment, the matrix relating clinical phenotypes and underlying pathophysiological mechanisms has to be clarified. This review discusses the opportunities for additional evaluation of the underlying pathophysiological processes, e.g., to evaluate biological phenotypes on top of clinical routine, to guide us toward a phenotype-specific HFpEF treatment. Moreover, a translational approach with matchmaking of animal models to biological HFpEF phenotypes will be a valuable step to test the effectiveness of novel, targeted interventions in HFpEF. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/personalized-medicine-in-hfpef/ .

Molecular Mechanisms Involved in Oxidative Stress-Associated Liver Injury Induced by Chinese Herbal Medicine: An Experimental Evidence-Based Literature Review and Network Pharmacology Study

Oxidative stress, defined as a disequilibrium between pro-oxidants and antioxidants, can result in histopathological lesions with a broad spectrum, ranging from asymptomatic hepatitis to hepatocellular carcinoma in an orchestrated manner. Although cells are equipped with sophisticated strategies to maintain the redox biology under normal conditions, the abundance of redox-sensitive xenobiotics, such as medicinal ingredients originated from herbs or animals, can dramatically invoke oxidative stress. Growing evidence has documented that the hepatotoxicity can be triggered by traditional Chinese medicine (TCM) during treating various diseases. Meanwhile, TCM-dependent hepatic disorder represents a strong correlation with oxidative stress, especially the persistent accumulation of intracellular reactive oxygen species. Of note, since TCM-derived compounds with their modulated targets are greatly diversified among themselves, it is complicated to elaborate the potential pathological mechanism. In this regard, data mining approaches, including network pharmacology and bioinformatics enrichment analysis have been utilized to scientifically disclose the underlying pathogenesis. Herein, top 10 principal TCM-modulated targets for oxidative hepatotoxicity including superoxide dismutases (SOD), malondialdehyde (MDA), glutathione (GSH), reactive oxygen species (ROS), glutathione peroxidase (GPx), Bax, caspase-3, Bcl-2, nuclear factor (erythroid-derived 2)-like 2 (Nrf2), and nitric oxide (NO) have been identified. Furthermore, hepatic metabolic dysregulation may be the predominant pathological mechanism involved in TCM-induced hepatotoxic impairment.

Influence of wideband visible light with an padding red component on the functional state of mice embryos and embryonic stem cells

It is known that visible light, including sunlight and laboratory lighting, adversely affect the development of embryos in vitro. In with article we present a technology for the synthesis of composite screens, capable to photoconvert UV and a part of the blue spectrum into red light with the maximum ~630 nm. It is established that the application of such transformed light with an evident red component raises the chances of embryos to survive and protects embryonic stem cells. To create photoconversion screens, the CdZn/Se quantum dots were obtained, the average size being about 7 nm. When the quantum dots are excited by electromagnetic waves of the UV and blue spectral range, photoluminescence is observed. The average photon energy for photoluminescence is of the order of 2 eV. On the basis of CdZn/Se quantum dots and methylphenylsiloxane polymer, light-transforming composite screens were made. In case of the light-transforming composite screen, the UV component disappeared from the energy spectrum, and the intensity of the blue region of the spectrum was reduced. On the contrary, in the red region (λ_max = 630 nm) one can see a little more than two-fold increase of intensity. It is shown that when exposed to 2-cell embryos by transformed light, the proportion of normally developing embryos increases by 20%, the number of dead embryos decreases twice, and number of dead and apoptotic cells was lower in blastocysts, what's decreased by 70%, as compared to the control group. When blastocysts are transferred to the feeder substrate, colonies of embryonic stem cells are formed. Cells obtained from blastocysts irradiated with transformed visible light are in a normal state in 90% of cases and did not change expression levels, biochemistry and morphology for at least 20 passages. It is assumed that the data obtained can be used for the design of systems of efficient cultivation of embryonic cells for tissue engineering and cell therapy.

Myocyte [Na+]i Dysregulation in Heart Failure and Diabetic Cardiomyopathy

By controlling the function of various sarcolemmal and mitochondrial ion transporters, intracellular Na⁺ concentration ([Na⁺]_i) regulates Ca²⁺ cycling, electrical activity, the matching of energy supply and demand, and oxidative stress in cardiac myocytes. Thus, maintenance of myocyte Na⁺ homeostasis is vital for preserving the electrical and contractile activity of the heart. [Na⁺]_i is set by the balance between the passive Na⁺ entry through numerous pathways and the pumping of Na⁺ out of the cell by the Na⁺/K⁺-ATPase. This equilibrium is perturbed in heart failure, resulting in higher [Na⁺]_i. More recent studies have revealed that [Na⁺]_i is also increased in myocytes from diabetic hearts. Elevated [Na⁺]_i causes oxidative stress and augments the sarcoplasmic reticulum Ca²⁺ leak, thus amplifying the risk for arrhythmias and promoting heart dysfunction. This mini-review compares and contrasts the alterations in Na⁺ extrusion and/or Na⁺ uptake that underlie the [Na⁺]_i increase in heart failure and diabetes, with a particular emphasis on the emerging role of Na⁺ - glucose cotransporters in the diabetic heart.

Bioenergetic Feedback between Heart Cell Contractile Machinery and Mitochondrial 3D Deformations

In the heart, mitochondria are arranged in pairs sandwiched between the contractile machinery, which is the major ATP consumer. Thus, in response to the contraction-relaxation cycle of the cell, the mitochondrial membrane should deform accordingly. Membrane deformations in isolated ATP synthesis or in isolated mitochondria affect ATP production. However, it is unknown whether physiological deformation of the mitochondrial membrane in response to the contraction-relaxation cycle can act as a bioenergetic signaling mechanism between ATP demand to supply. We used both experimental and computational tools to reveal whether bioenergetic feedback exists between heart cell contractile machinery and mitochondrial three-dimensional (3D) deformations. We measured the mitochondrial 3D deformation in contracting rabbit cardiac myocytes and used published data on rat cardiac myocytes. These measurements were an input to a novel biophysics model that includes a description of ionic molecules on the mitochondrial membrane, Ca²⁺ cycling, and mitochondrial membrane stress. As is the case for rat cardiomyocytes, in rabbit cardiomyocytes, the mitochondrial length contracted and expanded with a similar dynamic as the sarcomere length. In contrast, the mitochondrial width expanded and then contracted with a similar dynamic as the mitochondrial length. Differences in the extent of deformation and fractional deformation between the width- and thick-axes were quantified and interpreted as the degree anisotropy between those respective axes. Finally, the model predicts that significant bioenergetic feedback between heart cell contractile machinery and mitochondrial 3D deformations does exist in unloaded rabbit and rat cells. However, this feedback is not a dominant mechanism in ATP supply to demand matching.