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

Neuronal Mitochondrial Calcium Uniporter (MCU) Deficiency Is Neuroprotective in Hyperexcitability by Modulation of Metabolic Pathways and ROS Balance

Epilepsy is one of the most common neurological disorders in the world. Common epileptic drugs generally affect ion channels or neurotransmitters and prevent the emergence of seizures. However, up to a third of the patients suffer from drug-resistant epilepsy, and there is an urgent need to develop new therapeutic strategies that go beyond acute antiepileptic (antiseizure) therapies towards therapeutics that also might have effects on chronic epilepsy comorbidities such as cognitive decline and depression. The mitochondrial calcium uniporter (MCU) mediates rapid mitochondrial Ca2+ transport through the inner mitochondrial membrane. Ca2+ influx is essential for mitochondrial functions, but longer elevations of intracellular Ca2+ levels are closely associated with seizure-induced neuronal damage, which are underlying mechanisms of cognitive decline and depression. Using neuronal-specific MCU knockout mice (MCU-/-ΔN), we demonstrate that neuronal MCU deficiency reduced hippocampal excitability in vivo. Furthermore, in vitro analyses of hippocampal glioneuronal cells reveal no change in total Ca2+ levels but differences in intracellular Ca2+ handling. MCU-/-ΔN reduces ROS production, declines metabolic fluxes, and consequently prevents glioneuronal cell death. This effect was also observed under pathological conditions, such as the low magnesium culture model of seizure-like activity or excitotoxic glutamate stimulation, whereby MCU-/-ΔN reduces ROS levels and suppresses Ca2+ overload seen in WT cells. This study highlights the importance of MCU at the interface of Ca2+ handling and metabolism as a mediator of stress-related mitochondrial dysfunction, which indicates the modulation of MCU as a potential target for future antiepileptogenic therapy.

Maternal exercise preserves offspring cardiovascular health via oxidative regulation of the ryanodine receptor

OBJECTIVE

The intrauterine environment during pregnancy is a critical factor in the development of obesity, diabetes, and cardiovascular disease in offspring. Maternal exercise prevents the detrimental effects of a maternal high fat diet on the metabolic health in adult offspring, but the effects of maternal exercise on offspring cardiovascular health have not been thoroughly investigated.

METHODS

To determine the effects of maternal exercise on offspring cardiovascular health, female mice were fed a chow (C; 21% kcal from fat) or high-fat (H; 60% kcal from fat) diet and further subdivided into sedentary (CS, HS) or wheel exercised (CW, HW) prior to pregnancy and throughout gestation. Offspring were maintained in a sedentary state and chow-fed throughout 52 weeks of age and subjected to serial echocardiography and cardiomyocyte isolation for functional and mechanistic studies.

RESULTS

High-fat fed sedentary dams (HS) produced female offspring with reduced ejection fraction (EF) compared to offspring from chow-fed dams (CS), but EF was preserved in offspring from high-fat fed exercised dams (HW) throughout 52 weeks of age. Cardiomyocytes from HW female offspring had increased kinetics, calcium cycling, and respiration compared to CS and HS offspring. HS offspring had increased oxidation of the RyR2 in cardiomyocytes coupled with increased baseline sarcomere length, resulting in RyR2 overactivity, which was negated in female HW offspring.

CONCLUSIONS

These data suggest a role for maternal exercise to protect against the detrimental effects of a maternal high-fat diet on female offspring cardiac health. Maternal exercise improved female offspring cardiomyocyte contraction, calcium cycling, respiration, RyR2 oxidation, and RyR2 activity. These data present an important, translatable role for maternal exercise to preserve cardiac health of female offspring and provide insight on mechanisms to prevent the transmission of cardiovascular diseases to subsequent generations.

Redox differences between rat neonatal and adult cardiomyocytes under hypoxia

It is generally accepted that oxidative stress plays a key role in the development of ischemia-reperfusion injury in ischemic heart disease. However, the mechanisms how reactive oxygen species trigger cellular damage are not fully understood. Our study investigates redox state and highly reactive substances within neonatal and adult cardiomyocytes under hypoxia conditions. We have found that hypoxia induced an increase in H2O2 production in adult cardiomyocytes, while neonatal cardiomyocytes experienced a decrease in H2O2 levels. This finding correlates with our observation of the difference between the electron transport chain (ETC) properties and mitochondria amount in adult and neonatal cells. We demonstrated that in adult cardiomyocytes hypoxia caused the significant increase in the ETC loading with electrons compared to normoxia. On the contrary, in neonatal cardiomyocytes ETC loading with electrons was similar under both normoxic and hypoxic conditions that could be due to ETC non-functional state and the absence of the electrons transfer to O2 under normoxia. In addition to the variations in H2O2 production, we also noted consistent pH dynamics under hypoxic conditions. Notably, the pH levels exhibited a similar decrease in both cell types, thus, acidosis is a more universal cellular response to hypoxia. We also demonstrated that the amount of mitochondria and the levels of cardiac isoforms of troponin I, troponin T, myoglobin and GAPDH were significantly higher in adult cardiomyocytes compared to neonatal ones. Remarkably, we found out that under hypoxia, the levels of cardiac isoforms of troponin T, myoglobin, and GAPDH were elevated in adult cardiomyocytes, while their level in neonatal cells remained unchanged. Obtained data contribute to the understanding of the mechanisms of neonatal cardiomyocytes' resistance to hypoxia and the ability to maintain the metabolic homeostasis in contrast to adult ones.

Potential oxidative stress related targets of mitochondria-focused therapy of PTSD

Post-traumatic stress disorder (PTSD) remains a highly prevalent, under-diagnosed, and under-treated psychiatric disorder that often deteriorates over time, and is highly comorbid with major depressive disorder, suicidality, and substance use disorder. Several biomarkers have been proposed but have yet to be implemented into clinical practice. Treatments, including selective serotonin reuptake inhibitors, are efficacious in only a small number of patients, which underscores the need to develop novel, efficient treatments. Mitochondrial dysfunction resulting from chronic oxidative stress has been linked with both altered neurotransmitter signaling and the inflammatory response. Hereinafter, we discuss mechanisms by which mitochondrial dysfunction may contribute to the development of PTSD symptoms, and how these may even increase PTSD susceptibility. We also highlight possible therapeutic targets to reduce oxidative stress to prevent or treat PTSD symptoms.

Mitochondrial Calcium Overload Plays a Causal Role in Oxidative Stress in the Failing Heart

Heart failure is a serious global health challenge, affecting more than 6.2 million people in the United States and is projected to reach over 8 million by 2030. Independent of etiology, failing hearts share common features, including defective calcium (Ca2+) handling, mitochondrial Ca2+ overload, and oxidative stress. In cardiomyocytes, Ca2+ not only regulates excitation-contraction coupling, but also mitochondrial metabolism and oxidative stress signaling, thereby controlling the function and actual destiny of the cell. Understanding the mechanisms of mitochondrial Ca2+ uptake and the molecular pathways involved in the regulation of increased mitochondrial Ca2+ influx is an ongoing challenge in order to identify novel therapeutic targets to alleviate the burden of heart failure. In this review, we discuss the mechanisms underlying altered mitochondrial Ca2+ handling in heart failure and the potential therapeutic strategies.

Kaempferol regulates the thermogenic function of adipocytes in high-fat-diet-induced obesity via the CDK6/RUNX1/UCP1 signaling pathway

Activation of adipose tissue thermogenesis is a promising strategy in the treatment of obesity and obesity-related metabolic disorders. Kaempferol (KPF) is a predominant dietary flavonoid with multiple pharmacological properties, such as anti-inflammatory and antioxidant activities. In this study, we sought to characterize the role of KPF in adipocyte thermogenesis. We demonstrated that KPF-treated mice were protected from diet-induced obesity, glucose tolerance, and insulin resistance, accompanied by markedly increased energy expenditure, ex vivo oxygen consumption of white fat, and increased expression of proteins related to adaptive thermogenesis. KPF-promoted beige cell formation is a cell-autonomous effect, since the overexpression of cyclin-dependent kinase 6 (CDK6) in preadipocytes partially reversed browning phenotypes observed in KPF-treated cells. Overall, these data implicate that KPF is involved in promoting beige cell formation by suppressing CDK6 protein expression. This study provides evidence that KPF is a promising natural product for obesity treatment by boosting energy expenditure.

The ryanodine receptor microdomain in cardiomyocytes

The ryanodine receptor type 2 (RyR) is a key player in Ca2+ handling during excitation-contraction coupling. During each heartbeat, RyR channels are responsible for linking the action potential with the contractile machinery of the cardiomyocyte by releasing Ca2+ from the sarcoplasmic reticulum. RyR function is fine-tuned by associated signalling molecules, arrangement in clusters and subcellular localization. These parameters together define RyR function within microdomains and are subject to disease remodelling. This review describes the latest findings on RyR microdomain organization, the alterations with disease which result in increased subcellular heterogeneity and emergence of microdomains with enhanced arrhythmogenic potential, and presents novel technologies that guide future research to study and target RyR channels within specific microdomains.

Mitochondrial Ca 2+ uniporter (MCU) variants form plasma-membrane channels

MCU is widely recognized as a responsible gene for encoding a pore-forming subunit of highly mitochondrial-specific and Ca 2+ -selective channel, mitochondrial Ca 2+ uniporter complex (mtCUC). Here, we report a novel short variant derived from the MCU gene (termed MCU-S) which lacks mitochondria-targeted sequence and forms a Ca 2+ - permeable channel outside of mitochondria. MCU-S was ubiquitously expressed in all cell-types/tissues, with particularly high expression in human platelets. MCU-S formed Ca 2+ channels at the plasma membrane, which exhibited similar channel properties to those observed in mtCUC. MCU-S channels at the plasma membrane served as an additional Ca 2+ influx pathway for platelet activation. Our finding is completely distinct from the originally reported MCU gene function and provides novel insights into the molecular basis of MCU variant-dependent cellular Ca 2+ handling.

Distinct Effects of Mitochondrial Na+/Ca2+ Exchanger Inhibition and Ca2+ Uniporter Activation on Ca2+ Sparks and Arrhythmogenesis in Diabetic Rats

Background Mitochondrial dysfunction contributes to the cardiac remodeling triggered by type 2 diabetes (T2D). Mitochondrial Ca2+ concentration ([Ca2+]m) modulates the oxidative state and cytosolic Ca2+ regulation. Thus, we investigated how T2D affects mitochondrial Ca2+ fluxes, the downstream consequences on myocyte function, and the effects of normalizing mitochondrial Ca2+ transport. Methods and Results We compared myocytes/hearts from transgenic rats with late-onset T2D (rats that develop late-onset T2D due to heterozygous expression of human amylin in the pancreatic β-cells [HIP] model) and their nondiabetic wild-type (WT) littermates. [Ca2+]m was significantly lower in myocytes from diabetic HIP rats compared with WT cells. Ca2+ extrusion through the mitochondrial Na+/Ca2+ exchanger (mitoNCX) was elevated in HIP versus WT myocytes, particularly at moderate and high [Ca2+]m, while mitochondrial Ca2+ uptake was diminished. Mitochondrial Na+ concentration was comparable in WT and HIP rat myocytes and remained remarkably stable while manipulating mitoNCX activity. Lower [Ca2+]m was associated with oxidative stress, increased sarcoplasmic reticulum Ca2+ leak in the form of Ca2+ sparks, and mitochondrial dysfunction in T2D hearts. MitoNCX inhibition with CGP-37157 reduced oxidative stress, Ca2+ spark frequency, and stress-induced arrhythmias in HIP rat hearts while having no significant effect in WT rats. In contrast, activation of the mitochondrial Ca2+ uniporter with SB-202190 enhanced spontaneous sarcoplasmic reticulum Ca2+ release and had no significant effect on arrhythmias in both WT and HIP rat hearts. Conclusions [Ca2+]m is reduced in myocytes from rats with T2D due to a combination of exacerbated mitochondrial Ca2+ extrusion through mitoNCX and impaired mitochondrial Ca2+ uptake. Partial mitoNCX inhibition limits sarcoplasmic reticulum Ca2+ leak and arrhythmias in T2D hearts, whereas mitochondrial Ca2+ uniporter activation does not.

The mitochondrial Ca2+ channel MCU is critical for tumor growth by supporting cell cycle progression and proliferation

Introduction: The mitochondrial uniporter (MCU) Ca²⁺ ion channel represents the primary means for Ca²⁺ uptake by mitochondria. Mitochondrial matrix Ca²⁺ plays critical roles in mitochondrial bioenergetics by impinging upon respiration, energy production and flux of biochemical intermediates through the TCA cycle. Inhibition of MCU in oncogenic cell lines results in an energetic crisis and reduced cell proliferation unless media is supplemented with nucleosides, pyruvate or α-KG. Nevertheless, the roles of MCU-mediated Ca²⁺ influx in cancer cells remain unclear, in part because of a lack of genetic models. Methods: MCU was genetically deleted in transformed murine fibroblasts for study in vitro and in vivo. Tumor formation and growth were studied in murine xenograft models. Proliferation, cell invasion, spheroid formation and cell cycle progression were measured in vitro. The effects of MCU deletion on survival and cell-death were determined by probing for live/death markers. Mitochondrial bioenergetics were studied by measuring mitochondrial matrix Ca²⁺ concentration, membrane potential, global dehydrogenase activity, respiration, ROS production and inactivating-phosphorylation of pyruvate dehydrogenase. The effects of MCU rescue on metabolism were examined by tracing of glucose and glutamine utilization for fueling of mitochondrial respiration. Results: Transformation of primary fibroblasts in vitro was associated with increased MCU expression, enhanced MCU-mediated Ca²⁺ uptake, altered mitochondrial matrix Ca²⁺ concentration responses to agonist stimulation, suppression of inactivating-phosphorylation of pyruvate dehydrogenase and a modest increase of mitochondrial respiration. Genetic MCU deletion inhibited growth of HEK293T cells and transformed fibroblasts in mouse xenograft models, associated with reduced proliferation and delayed cell-cycle progression. MCU deletion inhibited cancer stem cell-like spheroid formation and cell invasion in vitro, both predictors of metastatic potential. Surprisingly, mitochondrial matrix [Ca²⁺], membrane potential, global dehydrogenase activity, respiration and ROS production were unaffected. In contrast, MCU deletion elevated glycolysis and glutaminolysis, strongly sensitized cell proliferation to glucose and glutamine limitation, and altered agonist-induced cytoplasmic Ca²⁺ signals. Conclusion: Our results reveal a dependence of tumorigenesis on MCU, mediated by a reliance on MCU for cell metabolism and Ca²⁺ dynamics necessary for cell-cycle progression and cell proliferation.

Enhanced Mitochondria-SR Tethering Triggers Adaptive Cardiac Muscle Remodeling

BACKGROUND

Cardiac contractile function requires high energy from mitochondria, and Ca2+ from the sarcoplasmic reticulum (SR). Via local Ca2+ transfer at close mitochondria-SR contacts, cardiac excitation feedforward regulates mitochondrial ATP production to match surges in demand (excitation-bioenergetics coupling). However, pathological stresses may cause mitochondrial Ca2+ overload, excessive reactive oxygen species production and permeability transition, risking homeostatic collapse and myocyte loss. Excitation-bioenergetics coupling involves mitochondria-SR tethers but the role of tethering in cardiac physiology/pathology is debated. Endogenous tether proteins are multifunctional; therefore, nonselective targets to scrutinize interorganelle linkage. Here, we assessed the physiological/pathological relevance of selective chronic enhancement of cardiac mitochondria-SR tethering.

METHODS

We introduced to mice a cardiac muscle-specific engineered tether (linker) transgene with a fluorescent protein core and deployed 2D/3D electron microscopy, biochemical approaches, fluorescence imaging, in vivo and ex vivo cardiac performance monitoring and stress challenges to characterize the linker phenotype.

RESULTS

Expressed in the mature cardiomyocytes, the linker expanded and tightened individual mitochondria-junctional SR contacts; but also evoked a marked remodeling with large dense mitochondrial clusters that excluded dyads. Yet, excitation-bioenergetics coupling remained well-preserved, likely due to more longitudinal mitochondria-dyad contacts and nanotunnelling between mitochondria exposed to junctional SR and those sealed away from junctional SR. Remarkably, the linker decreased female vulnerability to acute massive β-adrenergic stress. It also reduced myocyte death and mitochondrial calcium-overload-associated myocardial impairment in ex vivo ischemia/reperfusion injury.

CONCLUSIONS

We propose that mitochondria-SR/endoplasmic reticulum contacts operate at a structural optimum. Although acute changes in tethering may cause dysfunction, upon chronic enhancement of contacts from early life, adaptive remodeling of the organelles shifts the system to a new, stable structural optimum. This remodeling balances the individually enhanced mitochondrion-junctional SR crosstalk and excitation-bioenergetics coupling, by increasing the connected mitochondrial pool and, presumably, Ca2+/reactive oxygen species capacity, which then improves the resilience to stresses associated with dysregulated hyperactive Ca2+ signaling.

Cardioprotective Properties of Kaempferol: A Review

Cardiac diseases, such as myocardial infarction and heart failure, have become a major clinical problem globally. The accumulating data demonstrate that bioactive compounds with antioxidant and anti-inflammatory properties have favorable effects on clinical problems. Kaempferol is a flavonoid found in various plants; it has demonstrated cardioprotective properties in numerous cardiac injury models. This review aims to collate updated information regarding the effects of kaempferol on cardiac injury. Kaempferol improves cardiac function by alleviating myocardial apoptosis, fibrosis, oxidative stress, and inflammation while preserving mitochondrial function and calcium homeostasis. However, the mechanisms of action of its cardioprotective properties remain unclear; therefore, elucidating its action could provide insight into directions for future studies.

Sexual dimorphism in bidirectional SR-mitochondria crosstalk in ventricular cardiomyocytes

Calcium transfer into the mitochondrial matrix during sarcoplasmic reticulum (SR) Ca2+ release is essential to boost energy production in ventricular cardiomyocytes (VCMs) and match increased metabolic demand. Mitochondria from female hearts exhibit lower mito-[Ca2+] and produce less reactive oxygen species (ROS) compared to males, without change in respiration capacity. We hypothesized that in female VCMs, more efficient electron transport chain (ETC) organization into supercomplexes offsets the deficit in mito-Ca2+ accumulation, thereby reducing ROS production and stress-induced intracellular Ca2+ mishandling. Experiments using mitochondria-targeted biosensors confirmed lower mito-ROS and mito-[Ca2+] in female rat VCMs challenged with β-adrenergic agonist isoproterenol compared to males. Biochemical studies revealed decreased mitochondria Ca2+ uniporter expression and increased supercomplex assembly in rat and human female ventricular tissues vs male. Importantly, western blot analysis showed higher expression levels of COX7RP, an estrogen-dependent supercomplex assembly factor in female heart tissues vs males. Furthermore, COX7RP was decreased in hearts from aged and ovariectomized female rats. COX7RP overexpression in male VCMs increased mitochondrial supercomplexes, reduced mito-ROS and spontaneous SR Ca2+ release in response to ISO. Conversely, shRNA-mediated knockdown of COX7RP in female VCMs reduced supercomplexes and increased mito-ROS, promoting intracellular Ca2+ mishandling. Compared to males, mitochondria in female VCMs exhibit higher ETC subunit incorporation into supercomplexes, supporting more efficient electron transport. Such organization coupled to lower levels of mito-[Ca2+] limits mito-ROS under stress conditions and lowers propensity to pro-arrhythmic spontaneous SR Ca2+ release. We conclude that sexual dimorphism in mito-Ca2+ handling and ETC organization may contribute to cardioprotection in healthy premenopausal females.

The mitochondrial Ca 2+ channel MCU is critical for tumor growth by supporting cell cycle progression and proliferation

The mitochondrial uniporter (MCU) Ca ²⁺ ion channel represents the primary means for Ca ²⁺ uptake into mitochondria. Here we employed in vitro and in vivo models with MCU genetically eliminated to understand how MCU contributes to tumor formation and progression. Transformation of primary fibroblasts in vitro was associated with increased MCU expression, enhanced mitochondrial Ca ²⁺ uptake, suppression of inactivating-phosphorylation of pyruvate dehydrogenase, a modest increase of basal mitochondrial respiration and a significant increase of acute Ca ²⁺ -dependent stimulation of mitochondrial respiration. Inhibition of mitochondrial Ca ²⁺ uptake by genetic deletion of MCU markedly inhibited growth of HEK293T cells and of transformed fibroblasts in mouse xenograft models. Reduced tumor growth was primarily a result of substantially reduced proliferation and fewer mitotic cells in vivo , and slower cell proliferation in vitro associated with delayed progression through S-phase of the cell cycle. MCU deletion inhibited cancer stem cell-like spheroid formation and cell invasion in vitro , both predictors of metastatic potential. Surprisingly, mitochondrial matrix Ca ²⁺ concentration, membrane potential, global dehydrogenase activity, respiration and ROS production were unchanged by genetic deletion of MCU in transformed cells. In contrast, MCU deletion elevated glycolysis and glutaminolysis, strongly sensitized cell proliferation to glucose and glutamine limitation, and altered agonist-induced cytoplasmic Ca ²⁺ signals. Our results reveal a dependence of tumorigenesis on MCU, mediated by a reliance on mitochondrial Ca ²⁺ uptake for cell metabolism and Ca ²⁺ dynamics necessary for cell-cycle progression and cell proliferation.

Targeting the mitochondrial Ca2+ uniporter complex in cardiovascular disease

Cardiovascular diseases (CVDs), the leading cause of death worldwide, share in common mitochondrial dysfunction, in specific a dysregulation of Ca²⁺ uptake dynamics through the mitochondrial Ca²⁺ uniporter (MCU) complex. In particular, Ca²⁺ uptake regulates the mitochondrial ATP production, mitochondrial dynamics, oxidative stress, and cell death. Therefore, modulating the activity of the MCU complex to regulate Ca²⁺ uptake, has been suggested as a potential therapeutic approach for the treatment of CVDs. Here, the role and implications of the MCU complex in CVDs are presented, followed by a review of the evidence for MCU complex modulation, genetically and pharmacologically. While most approaches have aimed within the MCU complex for the modulation of the Ca²⁺ pore channel, the MCU subunit, its intra- and extra- mitochondrial implications, including Ca²⁺ dynamics, oxidative stress, post-translational modifications, and its repercussions in the cardiac function, highlight that targeting the MCU complex has the translational potential for novel CVDs therapeutics.

ER stress and calcium-dependent arrhythmias

The sarcoplasmic reticulum (SR) plays the key role in cardiac function as the major source of Ca²⁺ that activates cardiomyocyte contractile machinery. Disturbances in finely-tuned SR Ca²⁺ release by SR Ca²⁺ channel ryanodine receptor (RyR2) and SR Ca²⁺ reuptake by SR Ca²⁺-ATPase (SERCa2a) not only impair contraction, but also contribute to cardiac arrhythmia trigger and reentry. Besides being the main Ca²⁺ storage organelle, SR in cardiomyocytes performs all the functions of endoplasmic reticulum (ER) in other cell types including protein synthesis, folding and degradation. In recent years ER stress has become recognized as an important contributing factor in many cardiac pathologies, including deadly ventricular arrhythmias. This brief review will therefore focus on ER stress mechanisms in the heart and how these changes can lead to pro-arrhythmic defects in SR Ca²⁺ handling machinery.

SR-Mitochondria Crosstalk Shapes Ca Signalling to Impact Pathophenotype in Disease Models Marked by Dysregulated Intracellular Ca Release

AIMS

Diastolic Ca release (DCR) from sarcoplasmic reticulum (SR) Ca release channel ryanodine receptor (RyR2) has been linked to multiple cardiac pathologies, but its exact role in shaping divergent cardiac pathologies remains unclear. We hypothesize that the SR-mitochondria interplay contributes to disease phenotypes by shaping Ca signalling.

METHODS AND RESULTS

A genetic model of catecholaminergic polymorphic ventricular tachycardia (CPVT2 model of CASQ2 knockout) and a pre-diabetic cardiomyopathy model of fructose-fed mice (FFD), both marked by DCR, are employed in this study. Mitochondria Ca (mCa) is modulated by pharmacologically targeting mitochondria Ca uniporter (MCU) or permeability transition pore (mPTP), mCa uptake, and extrusion mechanisms, respectively. An MCU activator abolished Ca waves in CPVT2 but exacerbated waves in FFD cells. Mechanistically this is ascribed to mitochondria's function as a Ca buffer or source of reactive oxygen species (mtROS) to exacerbate RyR2 functionality, respectively. Enhancing mCa uptake reduced and elevated mtROS production in CPVT2 and FFD, respectively. In CPVT2, mitochondria took up more Ca in permeabilized cells, and had higher level of mCa content in intact cells vs. FFD. Conditional ablation of MCU in the CPVT2 model caused lethality and cardiac remodelling, but reduced arrhythmias in the FFD model. In parallel, CPVT2 mitochondria also employ up-regulated mPTP-mediated Ca efflux to avoid mCa overload, as seen by elevated incidence of MitoWinks (an indicator of mPTP-mediated Ca efflux) vs. FFD. Both pharmacological and genetic inhibition of mPTP promoted mtROS production and exacerbation of myocyte Ca handling in CPVT2. Further, genetic inhibition of mPTP exacerbated arrhythmias in CPVT2.

CONCLUSION

In contrast to FFD, which is more susceptible to mtROS-dependent RyR2 leak, in CPVT2 mitochondria buffer SR-derived DCR to mitigate Ca-dependent pathological remodelling and rely on mPTP-mediated Ca efflux to avoid mCa overload. SR-mitochondria interplay contributes to the divergent pathologies by disparately shaping intracellular Ca signalling.

CLIC4 localizes to mitochondrial-associated membranes and mediates cardioprotection

Mitochondrial-associated membranes (MAMs) are known to modulate organellar and cellular functions and can subsequently affect pathophysiology including myocardial ischemia-reperfusion (IR) injury. Thus, identifying molecular targets in MAMs that regulate the outcome of IR injury will hold a key to efficient therapeutics. Here, we found chloride intracellular channel protein (CLIC4) presence in MAMs of cardiomyocytes and demonstrate its role in modulating ER and mitochondrial calcium homeostasis under physiological and pathological conditions. In a murine model, loss of CLIC4 increased myocardial infarction and substantially reduced cardiac function after IR injury. CLIC4 null cardiomyocytes showed increased apoptosis and mitochondrial dysfunction upon hypoxia-reoxygenation injury in comparison to wild-type cardiomyocytes. Overall, our results indicate that MAM-CLIC4 is a key mediator of cellular response to IR injury and therefore may have a potential implication on other pathophysiological processes.

ROS-Influenced Regulatory Cross-Talk With Wnt Signaling Pathway During Perinatal Development

Over a century ago, it was found that a rapid burst of oxygen is needed and produced by the sea urchin oocyte to activate fertilization and block polyspermy. Since then, scientific research has taken strides to establish that Reactive Oxygen Species (ROS), besides being toxic effectors of cellular damage and death, also act as molecular messengers in important developmental signaling cascades, thereby modulating them. Wnt signaling pathway is one such developmental pathway, which has significant effects on growth, proliferation, and differentiation of cells at the earliest embryonic stages of an organism, apart from being significant role-players in the instances of cellular transformation and cancer when this tightly-regulated system encounters aberrations. In this review, we discuss more about the Wnt and ROS signaling pathways, how they function, what roles they play overall in animals, and mostly about how these two major signaling systems cross paths and interplay in mediating major cellular signals and executing the predestined changes during the perinatal condition, in a systematic manner.

Ero1α-Dependent ERp44 Dissociation From RyR2 Contributes to Cardiac Arrhythmia

BACKGROUND

Oxidative stress in cardiac disease promotes proarrhythmic disturbances in Ca²⁺ homeostasis, impairing luminal Ca²⁺ regulation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the RyR2 (ryanodine receptor), and increasing channel activity. However, exact mechanisms underlying redox-mediated increase of RyR2 function in cardiac disease remain elusive. We tested whether the oxidoreductase family of proteins that dynamically regulate the oxidative environment within the SR are involved in this process.

METHODS

A rat model of hypertrophy induced by thoracic aortic banding (TAB) was used for ex vivo whole heart optical mapping and for Ca²⁺ and reactive oxygen species imaging in isolated ventricular myocytes (VMs).

RESULTS

The SR-targeted reactive oxygen species biosensor ERroGFP showed increased intra-SR oxidation in TAB VMs that was associated with increased expression of Ero1α (endoplasmic reticulum oxidoreductase 1 alpha). Pharmacological (EN460) or genetic Ero1α inhibition normalized SR redox state, increased Ca²⁺ transient amplitude and SR Ca²⁺ content, and reduced proarrhythmic spontaneous Ca²⁺ waves in TAB VMs under β-adrenergic stimulation (isoproterenol). Ero1α overexpression in Sham VMs had opposite effects. Ero1α inhibition attenuated Ca²⁺-dependent ventricular tachyarrhythmias in TAB hearts challenged with isoproterenol. Experiments in TAB VMs and human embryonic kidney 293 cells expressing human RyR2 revealed that an Ero1α-mediated increase in SR Ca²⁺-channel activity involves dissociation of intraluminal protein ERp44 (endoplasmic reticulum protein 44) from the RyR2 complex. Site-directed mutagenesis and molecular dynamics simulations demonstrated a novel redox-sensitive association of ERp44 with RyR2 mediated by intraluminal cysteine 4806. ERp44-RyR2 association in TAB VMs was restored by Ero1α inhibition, but not by reducing agent dithiothreitol, as hypo-oxidation precludes formation of covalent bond between RyR2 and ERp44.

CONCLUSIONS

A novel axis of intraluminal interaction between RyR2, ERp44, and Ero1α has been identified. Ero1α inhibition exhibits promising therapeutic potential by stabilizing RyR2-ERp44 complex, thereby reducing spontaneous Ca²⁺ release and Ca²⁺-dependent tachyarrhythmias in hypertrophic hearts, without causing hypo-oxidative stress in the SR.

Visualization of Dynamic Mitochondrial Calcium Fluxes in Isolated Cardiomyocytes

Background

Cardiomyocyte contraction requires a constant supply of ATP, which varies depending on work rate. Maintaining ATP supply is particularly important during excitation-contraction coupling, where cytosolic Ca2+ fluxes drive repeated cycles of contraction and relaxation. Ca2+ is one of the key regulators of ATP production, and its uptake into the mitochondrial matrix occurs via the mitochondrial calcium uniporter. Fluorescent indicators are commonly used for detecting cytosolic Ca2+ changes. However, visualizing mitochondrial Ca2+ fluxes using similar methods is more difficult, as the fluorophore must be permeable to both the sarcolemma and the inner mitochondrial membrane. Our aim was therefore to optimize a method using the fluorescent Ca2+ indicator Rhod-2 to visualize beat-to-beat mitochondrial calcium fluxes in rat cardiomyocytes.

Methods

Healthy, adult male Wistar rat hearts were isolated and enzymatically digested to yield rod-shaped, quiescent ventricular cardiomyocytes. The fluorescent Ca2+ indicator Rhod-2 was reduced to di-hydroRhod-2 and confocal microscopy was used to validate mitochondrial compartmentalization. Cardiomyocytes were subjected to various pharmacological interventions, including caffeine and β-adrenergic stimulation. Upon confirmation of mitochondrial Rhod-2 localization, loaded myocytes were then super-fused with 1.5 mM Ca2+ Tyrodes containing 1 μM isoproterenol and 150 μM spermine. Myocytes were externally stimulated at 0.1, 0.5 and 1 Hz and whole cell recordings of both cytosolic ([Ca2+]cyto) and mitochondrial calcium ([Ca2+]mito) transients were made.

Results

Myocytes loaded with di-hydroRhod-2 revealed a distinct mitochondrial pattern when visualized by confocal microscopy. Application of 20 mM caffeine revealed no change in fluorescence, confirming no sarcoplasmic reticulum compartmentalization. Myocytes loaded with di-hydroRhod-2 also showed a large increase in fluorescence within the mitochondria in response to β-adrenergic stimulation (P < 0.05). Beat-to-beat mitochondrial Ca2+ transients were smaller in amplitude and had a slower time to peak and maximum rate of rise relative to cytosolic calcium transients at all stimulation frequencies (P < 0.001).

Conclusion

Myocytes loaded with di-hydroRhod-2 revealed mitochondrial specific compartmentalization. Mitochondrial Ca2+ transients recorded from di-hydroRhod-2 loaded myocytes were distinct in comparison to the large and rapid Rhod-2 cytosolic transients, indicating different kinetics between [Ca2+]cyto and [Ca2+]mito transients. Overall, our results showed that di-hydroRhod-2 loading is a quick and suitable method for measuring beat-to-beat [Ca2+]mito transients in intact myocytes.

Physiological and Pathophysiological Roles of Mitochondrial Na+-Ca2+ Exchanger, NCLX, in Hearts

It has been over 10 years since SLC24A6/SLC8B1, coding the Na⁺/Ca²⁺/Li⁺ exchanger (NCLX), was identified as the gene responsible for mitochondrial Na⁺-Ca²⁺ exchange, a major Ca²⁺ efflux system in cardiac mitochondria. This molecular identification enabled us to determine structure–function relationships, as well as physiological/pathophysiological contributions, and our understandings have dramatically increased. In this review, we provide an overview of the recent achievements in relation to NCLX, focusing especially on its heart-specific characteristics, biophysical properties, and spatial distribution in cardiomyocytes, as well as in cardiac mitochondria. In addition, we discuss the roles of NCLX in cardiac functions under physiological and pathophysiological conditions—the generation of rhythmicity, the energy metabolism, the production of reactive oxygen species, and the opening of mitochondrial permeability transition pores.

Relevance of mitochondrial oxidative stress to arrhythmias: Innovative concepts to target treatments

Cardiac arrhythmia occurs frequently worldwide, and in severe cases can be fatal. Mitochondria are the power plants of cardiomyocytes. In recent studies, mitochondria under certain stimuli produced excessive reactive oxygen species (ROS), which affect the normal function of cardiomyocytes through ion channels and related proteins. Mitochondrial oxidative stress (MOS) plays a key role in diseases with multifactorial etiopathogenesis, such as arrhythmia; MOS can lead to arrhythmias such as atrial fibrillation and ventricular tachycardia. This review discusses the mechanisms of arrhythmias caused by MOS, particularly of ROS produced by mitochondria. MOS can cause arrhythmias by affecting the activities of Ca²⁺-related proteins, the mitochondrial permeability transition pore protein, connexin 43, hyperpolarization-activated cyclic nucleotide-gated potassium channel 4, and ion channels. Based on these mechanisms, we discuss possible new treatments for arrhythmia. Targeted treatments focusing on mitochondria may reduce the progression of arrhythmias, as well as the occurrence of severe arrhythmias, and may be effective for personalized disease prevention.

MCU overexpression evokes disparate dose-dependent effects on mito-ROS and spontaneous Ca2+ release in hypertrophic rat cardiomyocytes

Cardiac dysfunction in heart failure (HF) and diabetic cardiomyopathy (DCM) is associated with aberrant intracellular Ca2+ handling and impaired mitochondrial function accompanied with reduced mitochondrial calcium concentration (mito-[Ca2+]). Pharmacological or genetic facilitation of mito-Ca2+ uptake was shown to restore Ca2+ transient amplitude in DCM and HF, improving contractility. However, recent reports suggest that pharmacological enhancement of mito-Ca2+ uptake can exacerbate ryanodine receptor-mediated spontaneous sarcoplasmic reticulum (SR) Ca2+ release in ventricular myocytes (VMs) from diseased animals, increasing propensity to stress-induced ventricular tachyarrhythmia. To test whether chronic recovery of mito-[Ca2+] restores systolic Ca2+ release without adverse effects in diastole, we overexpressed mitochondrial Ca2+ uniporter (MCU) in VMs from male rat hearts with hypertrophy induced by thoracic aortic banding (TAB). Measurement of mito-[Ca2+] using genetic probe mtRCamp1h revealed that mito-[Ca2+] in TAB VMs paced at 2 Hz under β-adrenergic stimulation is lower compared with shams. Adenoviral 2.5-fold MCU overexpression in TAB VMs fully restored mito-[Ca2+]. However, it failed to improve cytosolic Ca2+ handling and reduce proarrhythmic spontaneous Ca2+ waves. Furthermore, mitochondrial-targeted genetic probes MLS-HyPer7 and OMM-HyPer revealed a significant increase in emission of reactive oxygen species (ROS) in TAB VMs with 2.5-fold MCU overexpression. Conversely, 1.5-fold MCU overexpression in TABs, that led to partial restoration of mito-[Ca2+], reduced mitochondria-derived reactive oxygen species (mito-ROS) and spontaneous Ca2+ waves. Our findings emphasize the key role of elevated mito-ROS in disease-related proarrhythmic Ca2+ mishandling. These data establish nonlinear mito-[Ca2+]/mito-ROS relationship, whereby partial restoration of mito-[Ca2+] in diseased VMs is protective, whereas further enhancement of MCU-mediated Ca2+ uptake exacerbates damaging mito-ROS emission.NEW & NOTEWORTHY Defective intracellular Ca2+ homeostasis and aberrant mitochondrial function are common features in cardiac disease. Here, we directly compared potential benefits of mito-ROS scavenging and restoration of mito-Ca2+ uptake by overexpressing MCU in ventricular myocytes from hypertrophic rat hearts. Experiments using novel mito-ROS and Ca2+ biosensors demonstrated that mito-ROS scavenging rescued both cytosolic and mito-Ca2+ homeostasis, whereas moderate and high MCU overexpression demonstrated disparate effects on mito-ROS emission, with only a moderate increase in MCU being beneficial.

In cardiac muscle cells, both adrenergic agonists and antagonists induce reactive oxygen species from NOX2 but mutually attenuate each other's effects

In cardiac muscle cells adrenergic agonists stimulate the generation of reactive oxygen species, followed by redox signaling. We postulated that the antagonists would attenuate such reactive oxygen species generation by the agonists. H9c2 cardiac myoblasts, neonatal rat cardiac myocytes, and HEK293 cells expressing β₁/β₂ adrenoceptors were stimulated with several agonists and antagonists. All the agonists and antagonists independently generated reactive oxygen species; but its generation was minimum whenever an agonists was added together with an antagonist. We monitored the Ca++ signaling in the treated cells and obtained similar results. In all treatment sets, superoxide and H₂O₂ were generated in the mitochondria and the cytosol respectively. NOX2 inhibitor gp91ds-tat blocked reactive oxygen species generation by both the agonists and the antagonists. The level of p47phox subunit of NOX2 rapidly increased upon treatment, and it translocated to the plasma membrane, confirming NOX2 activation. Inhibitor studies showed that the activation of NOX2 involves ERK, PI3K, and tyrosine kinases. Recombinant promoter-reporter assays showed that reactive oxygen species generated by both the agonists and antagonists modulated downstream gene expression. Mice injected with the β-adrenergic agonist isoproterenol and fed with the antagonist metoprolol showed a robust induction of p47phox in the heart. We conclude that both the agonism and antagonism of adrenoceptors initiate redox signaling but when added together, they mutually counteract each other's effects. Our study thus highlights the importance of reactive oxygen species in adrenoceptor agonism and antagonism with relevance to the therapeutic use of the β blockers.

Mechanisms of Physical Fatigue and its Applications in Nutritional Interventions

Physical fatigue during exercise can be defined as an impairment of physical performance. Multiple factors have been found contributing to physical fatigue, including neurotransmitter-mediated defense action, insufficient energy supply, and induction of oxidative stress. These mechanistic findings provide a sound theoretical rationale for nutritional intervention since most of these factors can be modulated by nutrient supplementation. In this review, we summarize the current evidence regarding the functional role of nutrients supplementation in managing physical performance and propose the issues that need to be addressed for better utilization of nutritional supplementation approach to improve physical performance.

Sarcoplasmic reticulum-mitochondria communication; implications for cardiac arrhythmia

Sudden cardiac death due to ventricular tachyarrhythmias remains the major cause of mortality in the world. Heart failure, diabetic cardiomyopathy, old age-related cardiac dysfunction and inherited disorders are associated with enhanced propensity to malignant cardiac arrhythmias. Both defective mitochondrial function and abnormal intracellular Ca²⁺ homeostasis have been established as the key contributing factors in the pathophysiology and arrhythmogenesis in these conditions. This article reviews current advances in understanding of bidirectional control of ryanodine receptor-mediated sarcoplasmic reticulum Ca²⁺ release and mitochondrial function, and how defects in crosstalk between these two organelles increase arrhythmic risk in cardiac disease.

Mitochondrial Dysfunction Increases Arrhythmic Triggers and Substrates; Potential Anti-arrhythmic Pharmacological Targets

Incidence of cardiac arrhythmias increases significantly with age. In order to effectively stratify arrhythmic risk in the aging population it is crucial to elucidate the relevant underlying molecular mechanisms. The changes underlying age-related electrophysiological disruption appear to be closely associated with mitochondrial dysfunction. Thus, the present review examines the mechanisms by which age-related mitochondrial dysfunction promotes arrhythmic triggers and substrate. Namely, via alterations in plasmalemmal ionic currents (both sodium and potassium), gap junctions, cellular Ca²⁺ homeostasis, and cardiac fibrosis. Stratification of patients' mitochondrial function status permits application of appropriate anti-arrhythmic therapies. Here, we discuss novel potential anti-arrhythmic pharmacological interventions that specifically target upstream mitochondrial function and hence ameliorates the need for therapies targeting downstream changes which have constituted traditional antiarrhythmic therapy.

Mitochondrial and Sarcoplasmic Reticulum Interconnection in Cardiac Arrhythmia

Ca²⁺ plays a pivotal role in mitochondrial energy production, contraction, and apoptosis. Mitochondrial Ca²⁺-targeted fluorescent probes have demonstrated that mitochondria Ca²⁺ transients are synchronized with Ca²⁺ fluxes occurring in the sarcoplasmic reticulum (SR). The presence of specialized proteins tethering SR to mitochondria ensures the local Ca²⁺ flux between these organelles. Furthermore, communication between SR and mitochondria impacts their functionality in a bidirectional manner. Mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniplex is essential for ATP production and controlled reactive oxygen species levels for proper cellular signaling. Conversely, mitochondrial ATP ensures the proper functioning of SR Ca²⁺-handling proteins, which ensures that mitochondria receive an adequate supply of Ca²⁺. Recent evidence suggests that altered SR Ca²⁺ proteins, such as ryanodine receptors and the sarco/endoplasmic reticulum Ca²⁺ ATPase pump, play an important role in maintaining proper cardiac membrane excitability, which may be initiated and potentiated when mitochondria are dysfunctional. This recognized mitochondrial role offers the opportunity to develop new therapeutic approaches aimed at preventing cardiac arrhythmias in cardiac disease.

The role of calcium homeostasis remodeling in inherited cardiac arrhythmia syndromes

Sudden cardiac death due to malignant ventricular arrhythmias remains the major cause of mortality in the postindustrial world. Defective intracellular Ca²⁺ homeostasis has been well established as a key contributing factor to the enhanced propensity for arrhythmia in acquired cardiac disease, such as heart failure or diabetic cardiomyopathy. More recent advances provide a strong basis to the emerging view that hereditary cardiac arrhythmia syndromes are accompanied by maladaptive remodeling of Ca²⁺ homeostasis which substantially increases arrhythmic risk. This brief review will focus on functional changes in elements of Ca²⁺ handling machinery in cardiomyocytes that occur secondary to genetic mutations associated with catecholaminergic polymorphic ventricular tachycardia, and long QT syndrome.

Mitochondrial Ca2+ in heart failure: Not enough or too much?

Ca²⁺ serves as a ubiquitous second messenger mediating a variety of cellular processes including electrical excitation, contraction, gene expression, secretion, cell death and others. The identification of the molecular components of the mitochondrial Ca²⁺ influx and efflux pathways has created a resurgent interest in the regulation of mitochondrial Ca²⁺ balance and its physiological and pathophysiological roles. While the pace of discovery has quickened with the availability of new cellular and animal models, many fundamental questions remain to be answered regarding the regulation and functional impact of mitochondrial Ca²⁺ in health and disease. This review highlights several experimental observations pertaining to key aspects of mitochondrial Ca²⁺ homeostasis that remain enigmatic, particularly whether mitochondrial Ca²⁺ signaling is depressed or excessive in heart failure, which will determine the optimal approach to therapeutic intervention.

Allium Flavonols: Health Benefits, Molecular Targets, and Bioavailability

Allium species are revered worldwide as vegetables, condiments, and spices as well as the therapeutic agents in traditional medicine. The bioactive compounds in alliums mainly include organosulfur compounds, polyphenols, dietary fibers, and saponins. Flavonoids, particularly flavonols from alliums, have been demonstrated to have the antioxidant, anticancer, hypolipidemic, anti-diabetic, cardioprotective, neuroprotective, and antimicrobial activities. However, flavonols are mostly characterized from onions and have not been comprehensively reviewed across different species. This article therefore focuses on flavonol profiles from different Allium species, their health effects, underlying molecular mechanisms, and bioavailability. Intriguingly, the functional health effects of flavonols were mainly ascribed to their antioxidant and anti-inflammatory activities involving a cascade of multiple signaling pathways. Although the Allium-derived flavonols offer tremendous potential in preventing chronic disease risks, in-depth studies are needed to translate their clinical application.

FUNDC1 interacts with FBXL2 to govern mitochondrial integrity and cardiac function through an IP3R3-dependent manner in obesity

Defective mitophagy is causally linked to obesity complications. Here, we identified an interaction between mitophagy protein FUNDC1 (FUN14 domain containing 1) and receptor subunit of human SCF (SKP1/cullin/F-box protein) ubiquitin ligase complex FBXL2 as a gatekeeper for mitochondrial Ca²⁺ homeostasis through degradation of IP3R3 (inositol 1,4,5-trisphosphate receptor type 3). Loss of FUNDC1 in FUNDC1^-/- mice accentuated high-fat diet-induced cardiac remodeling, functional and mitochondrial anomalies, cell death, rise in IP3R3, and Ca²⁺ overload. Mass spectrometry and co-immunoprecipitation analyses revealed an interaction between FUNDC1 and FBXL2. Truncated mutants of Fbox (Delta-F-box) disengaged FBXL2 interaction with FUNDC1. Activation or transfection of FBXL2, inhibition of IP3R3 alleviated, whereas disruption of FBXL2 localization sensitized lipotoxicity-induced cardiac damage. FUNDC1 deficiency accelerated and decelerated palmitic acid-induced degradation of FBXL2 and IP3R3, respectively. Our data suggest an essential role for interaction between FUNDC1 and FBXL2 in preserving mitochondrial Ca²⁺ homeostasis and cardiac function in obese hearts.

Mitochondrial calcium uniporter complex activation protects against calcium alternans in atrial myocytes

Cardiac alternans, defined as beat-to-beat alternations in action potential duration, cytosolic Ca transient (CaT) amplitude, and cardiac contraction is associated with atrial fibrillation (AF) and sudden cardiac death. At the cellular level, cardiac alternans is linked to abnormal intracellular calcium handling during excitation-contraction coupling. We investigated how pharmacological activation or inhibition of cytosolic Ca sequestration via mitochondrial Ca uptake and mitochondrial Ca retention affects the occurrence of pacing-induced CaT alternans in isolated rabbit atrial myocytes. Cytosolic CaTs were recorded using Fluo-4 fluorescence microscopy. Alternans was quantified as the alternans ratio (AR = 1 - CaT_small/CaT_large, where CaT_small and CaT_large are the amplitudes of the small and large CaTs of a pair of alternating CaTs). Inhibition of mitochondrial Ca sequestration via mitochondrial Ca uniporter complex (MCUC) with Ru360 enhanced the severity of CaT alternans (AR increase) and lowered the pacing frequency threshold for alternans. In contrast, stimulation of MCUC mediated mitochondrial Ca uptake with spermine-rescued alternans (AR decrease) and increased the alternans pacing threshold. Direct measurement of mitochondrial [Ca] in membrane permeabilized myocytes with Fluo-4 loaded mitochondria revealed that spermine enhanced and accelerated mitochondrial Ca uptake. Stimulation of mitochondrial Ca retention by preventing mitochondrial Ca efflux through the mitochondrial permeability transition pore with cyclosporin A also protected from alternans and increased the alternans pacing threshold. Pharmacological manipulation of MCUC activity did not affect sarcoplasmic reticulum Ca load. Our results suggest that activation of Ca sequestration by mitochondria protects from CaT alternans and could be a potential therapeutic target for cardiac alternans and AF prevention.NEW & NOTEWORTHY This study provides conclusive evidence that mitochondrial Ca uptake and retention protects from Ca alternans, whereas uptake inhibition enhances Ca alternans. The data suggest pharmacological mitochondrial Ca cycling modulation as a potential therapeutic strategy for alternans-related cardiac arrhythmia prevention.

From Orai to E-Cadherin: Subversion of Calcium Trafficking in Cancer to Drive Proliferation, Anoikis-Resistance, and Metastasis

The common currency of epithelial differentiation and homeostasis is calcium, stored primarily in the endoplasmic reticulum, rationed according to need, and replenished from the extracellular milieu via store-operated calcium entry (SOCE). This currency is disbursed by the IP3 receptor in response to diverse extracellular signals. The rate of release is governed by regulators of proliferation, autophagy, survival, and programmed cell death, the strength of the signal leading to different outcomes. Intracellular calcium acts chiefly through intermediates such as calmodulin that regulates growth factor receptors such as epidermal growth factor receptor (EGFR), actin polymerization, and adherens junction assembly and maintenance. Here we review this machinery and its role in differentiation, then consider how cancer cells subvert it to license proliferation, resist anoikis, and enable metastasis, either by modulating the level of intracellular calcium or its downstream targets or effectors such as EGFR, E-cadherin, IQGAP1, TMEM16A, CLCA2, and TRPA1. Implications are considered for the roles of E-cadherin and growth factor receptors in circulating tumor cells and metastasis. The discovery of novel, cell type-specific modulators and effectors of calcium signaling offers new possibilities for cancer chemotherapy.

Increased RyR2 activity is exacerbated by calcium leak-induced mitochondrial ROS

Cardiac disease is associated with deleterious emission of mitochondrial reactive oxygen species (mito-ROS), as well as enhanced oxidation and activity of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the ryanodine receptor (RyR2). The transfer of Ca²⁺ from the SR via RyR2 to mitochondria is thought to play a key role in matching increased metabolic demand during stress. In this study, we investigated whether augmented RyR2 activity results in self-imposed exacerbation of SR Ca²⁺ leak, via altered SR-mitochondrial Ca²⁺ transfer and elevated mito-ROS emission. Fluorescent indicators and spatially restricted genetic ROS probes revealed that both pharmacologically and genetically enhanced RyR2 activity, in ventricular myocytes from rats and catecholaminergic polymorphic ventricular tachycardia (CPVT) mice, respectively, resulted in increased ROS emission under β-adrenergic stimulation. Expression of mitochondrial Ca²⁺ probe mtRCamp1h revealed diminished net mitochondrial [Ca²⁺] with enhanced SR Ca²⁺ leak, accompanied by depolarization of the mitochondrial matrix. While this may serve as a protective mechanism to prevent mitochondrial Ca²⁺ overload, protection is not complete and enhanced mito-ROS emission resulted in oxidation of RyR2, further amplifying proarrhythmic SR Ca²⁺ release. Importantly, the effects of augmented RyR2 activity could be attenuated by mitochondrial ROS scavenging, and experiments with dominant-negative paralogs of the mitochondrial Ca²⁺ uniporter (MCU) supported the hypothesis that SR-mitochondria Ca²⁺ transfer is essential for the increase in mito-ROS. We conclude that in a process whereby leak begets leak, augmented RyR2 activity modulates mitochondrial Ca²⁺ handling, promoting mito-ROS emission and driving further channel activity in a proarrhythmic feedback cycle in the diseased heart.

Mitochondrial Dysfunction as Substrate for Arrhythmogenic Cardiomyopathy: A Search for New Disease Mechanisms

Arrhythmogenic cardiomyopathy (ACM) is a familial heart disease, associated with ventricular arrhythmias, fibrofatty replacement of the myocardial mass and an increased risk of sudden cardiac death (SCD). Malignant ventricular arrhythmias and SCD largely occur in the pre-clinical phase of the disease, before overt structural changes occur. To prevent or interfere with ACM disease progression, more insight in mechanisms related to electrical instability are needed. Currently, numerous studies are focused on the link between cardiac arrhythmias and metabolic disease. In line with that, a potential role of mitochondrial dysfunction in ACM pathology is unclear and mitochondrial biology in the ACM heart remains understudied. In this review, we explore mitochondrial dysfunction in relation to arrhythmogenesis, and postulate a link to typical hallmarks of ACM. Mitochondrial dysfunction depletes adenosine triphosphate (ATP) production and increases levels of reactive oxygen species in the heart. Both metabolic changes affect cardiac ion channel gating, electrical conduction, intracellular calcium handling, and fibrosis formation; all well-known aspects of ACM pathophysiology. ATP-mediated structural remodeling, apoptosis, and mitochondria-related alterations have already been shown in models of PKP2 dysfunction. Yet, the limited amount of experimental evidence in ACM models makes it difficult to determine whether mitochondrial dysfunction indeed precedes and/or accompanies ACM pathogenesis. Nevertheless, current experimental ACM models can be very useful in unraveling ACM-related mitochondrial biology and in testing potential therapeutic interventions.

Mitochondrial Calcium Uniporter Structure and Function in Different Types of Muscle Tissues in Health and Disease

Calcium ions (Ca²⁺) influx to mitochondrial matrix is crucial for the life of a cell. Mitochondrial calcium uniporter (mtCU) is a protein complex which consists of the pore-forming subunit (MCU) and several regulatory subunits. MtCU is the main contributor to inward Ca²⁺ currents through the inner mitochondrial membrane. Extensive investigations of mtCU involvement into normal and pathological molecular pathways started from the moment of discovery of its molecular components. A crucial role of mtCU in the control of these pathways is now recognized in both health and disease. In particular, impairments of mtCU function have been demonstrated for cardiovascular and skeletal muscle-associated pathologies. This review summarizes the current state of knowledge on mtCU structure, regulation, and function in different types of muscle tissues in health and disease.

Reactive Carbonyl Species: Diabetic Complication in the Heart and Lungs

Abnormal chemical reactions in hyperglycemia alter normal metabolic processes in diabetes, which is a key process in the production of reactive carbonyls species (RCS). Increasing the concentration of RCS may result in carbonyl/oxidative stress in both the diabetic heart and lung. Ryanodine receptors (RyRs) not only play a key role in heart contraction, including rhythmic contraction and relaxation of the heart, but they are also important for controlling the airway smooth muscle. RCS modifies RyRs, resulting in RyRs dysfunction, which is involved in important mechanisms in diabetic complications. Very little is known about the mechanistic relationship between the heart and lung in diabetes. This review highlights new findings on the pathophysiological mechanisms and discusses potential approaches to treatment for these complications.

Altered Intracellular Calcium Homeostasis and Arrhythmogenesis in the Aged Heart

Aging of the heart is associated with a blunted response to sympathetic stimulation, reduced contractility, and increased propensity for arrhythmias, with the risk of sudden cardiac death significantly increased in the elderly population. The altered cardiac structural and functional phenotype, as well as age-associated prevalent comorbidities including hypertension and atherosclerosis, predispose the heart to atrial fibrillation, heart failure, and ventricular tachyarrhythmias. At the cellular level, perturbations in mitochondrial function, excitation-contraction coupling, and calcium homeostasis contribute to this electrical and contractile dysfunction. Major determinants of cardiac contractility are the intracellular release of Ca²⁺ from the sarcoplasmic reticulum by the ryanodine receptors (RyR2), and the following sequestration of Ca²⁺ by the sarco/endoplasmic Ca²⁺-ATPase (SERCa2a). Activity of RyR2 and SERCa2a in myocytes is not only dependent on expression levels and interacting accessory proteins, but on fine-tuned regulation via post-translational modifications. In this paper, we review how aberrant changes in intracellular Ca²⁺ cycling via these proteins contributes to arrhythmogenesis in the aged heart.