Sarcoplasmic Reticulum-Mitochondria Kissing in Cardiomyocytes: Ca2+, ATP, and Undisclosed Secrets

Myocardial transcriptomic analysis of diabetic patients with aortic stenosis: key role for mitochondrial calcium signaling

BACKGROUND

Type 2 diabetes (T2D) is a frequent comorbidity encountered in patients with severe aortic stenosis (AS), leading to an adverse left ventricular (LV) remodeling and dysfunction. Metabolic alterations have been suggested as contributors of the deleterious effect of T2D on LV remodeling and function in patients with severe AS, but so far, the underlying mechanisms remain unclear. Mitochondria play a central role in the regulation of cardiac energy metabolism.

OBJECTIVES

We aimed to explore the mitochondrial alterations associated with the deleterious effect of T2D on LV remodeling and function in patients with AS, preserved ejection fraction, and no additional heart disease.

METHODS

We combined an in-depth clinical, biological and echocardiography phenotype of patients with severe AS, with (n = 34) or without (n = 50) T2D, referred for a valve replacement, with transcriptomic and histological analyses of an intra-operative myocardial LV biopsy.

RESULTS

T2D patients had similar AS severity but displayed worse cardiac remodeling, systolic and diastolic function than non-diabetics. RNAseq analysis identified 1029 significantly differentially expressed genes. Functional enrichment analysis revealed several T2D-specific upregulated pathways despite comorbidity adjustment, gathering regulation of inflammation, extracellular matrix organization, endothelial function/angiogenesis, and adaptation to cardiac hypertrophy. Downregulated gene sets independently associated with T2D were related to mitochondrial respiratory chain organization/function and mitochondrial organization. Generation of causal networks suggested a reduced Ca2+ signaling up to the mitochondria, with the measured gene remodeling of the mitochondrial Ca2+ uniporter in favor of enhanced uptake. Histological analyses supported a greater cardiomyocyte hypertrophy and a decreased proximity between the mitochondrial VDAC porin and the reticular IP3-receptor in T2D.

CONCLUSIONS

Our data support a crucial role for mitochondrial Ca2+ signaling in T2D-induced cardiac dysfunction in severe AS patients, from a structural reticulum-mitochondria Ca2+ uncoupling to a mitochondrial gene remodeling. Thus, our findings open a new therapeutic avenue to be tested in animal models and further human cardiac biopsies in order to propose new treatments for T2D patients suffering from AS.

TRIAL REGISTRATION

URL: https://www.

CLINICALTRIALS

gov ; Unique Identifier: NCT01862237.

Cardiac lipidomic profiles in mice undergo changes from fetus to adult

AIMS

Lipids are essential cellular components with many important biological functions. Disturbed lipid biosynthesis and metabolism has been shown to cause cardiac developmental abnormality and cardiovascular diseases. In this study, we aimed to investigate the composition and the molecular profiles of lipids in mammalian hearts between embryonic and adult stages and uncover the underlying links between lipid and cardiac development and maturation.

MATERIALS AND METHODS

We collected mouse hearts at the embryonic day 11.5 (E11.5), E15.5, and the age of 2 months, 4 months and 10 months, and performed lipidomic analysis to determine the changes of the composition, molecular species, and relative abundance of cardiac lipids between embryonic and adult stages. Additionally, we also performed the electronic microscopy and RNA sequencing in both embryonic and adult mouse hearts.

KEY FINDINGS

The relative abundances of certain phospholipids and sphingolipids including cardiolipin, phosphatidylglycerol, phosphatidylethanolamine, and ceramide, are different between embryonic and adult hearts. Such lipidomic changes are accompanied with increased densities of mitochondrial membranes and elevated expression of genes related to mitochondrial formation in adult mouse hearts. We also analyzed individual molecular species of phospholipids and sphingolipids, and revealed that the composition and distribution of lipid molecular species in hearts also change with development.

SIGNIFICANCE

Our study provides not only a lipidomic view of mammalian hearts when developing from the embryonic to the adult stage, but also a potential pool of lipid indicators for cardiac cell development and maturation.

COX6A2 deficiency leads to cardiac remodeling in human pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Cardiac remodeling is the initiating factor for the development of heart failure, which can result from various cardiomyopathies. Cytochrome c oxidase subunit 6A2 (COX6A2) is one of the components of cytochrome c oxidase that drives oxidative phosphorylation. The pathogenesis of myocardial remodeling caused by COX6A2 deficiency in humans remains unclear because there are no suitable research models. In this study, we established a COX6A2-deficient human cardiac myocyte (CM) model that mimics the human COX6A2 homozygous mutation and determined the effects of COX6A2 dysfunction and its underlying mechanism.

METHODS

A human COX6A2 homozygous knockout cardiomyocyte model was established by combining CRISPR/Cas9 gene editing technology and hiPSC-directed differentiation technology. Cell model phenotypic assays were done to characterize the pathological features of the resulting COX6A2-deficient cardiomyocytes.

RESULTS

COX6A2 gene knockout did not affect the pluripotency and differentiation efficiency of hiPSCs. Myocardial cells with a COX6A2 gene knockout showed abnormal energy metabolism, increased oxidative stress levels, abnormal calcium transport activity, and decreased contractility. In addition, L-carnitine and trimetazidine significantly improved energy metabolism in the COX6A2-deficient human myocardial model.

CONCLUSIONS

We have established a COX6A2-deficient human cardiomyocyte model that exhibits abnormal energy metabolism, elevated oxidative stress levels, abnormal calcium transport, and reduced contractility. This model represents an important tool to gain insight into the mechanism of action of energy metabolism disorders resulting in myocardial remodeling, elucidate the gene-phenotype relationship of COX6A2 deficiency, and facilitate drug screening.

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.

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 Ca2+ signaling and Alzheimer's disease: Too much or too little?

Alzheimer's disease (AD) is a neurodegenerative disease, caused by poorly known pathogenic mechanisms and aggravated by delayed therapeutic intervention, that still lacks an effective cure. However, it is clear that some important neurophysiological processes are altered years before the onset of clinical symptoms, offering the possibility of identifying biological targets useful for implementation of new therapies. Of note, evidence has been provided suggesting that mitochondria, pivotal organelles in sustaining neuronal energy demand and modulating synaptic activity, are dysfunctional in AD samples. In particular, alterations in mitochondrial Ca²⁺ signaling have been proposed as causal events for neurodegeneration, although the exact outcomes and molecular mechanisms of these defects, as well as their longitudinal progression, are not always clear. Here, we discuss the importance of a correct mitochondrial Ca²⁺ handling for neuronal physiology and summarize the latest findings on dysfunctional mitochondrial Ca²⁺ pathways in AD, analysing possible consequences contributing to the neurodegeneration that characterizes the disease.

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.

Flavonoids from Hippophae rhamnoides Linn. Revert Doxorubicin-Induced Cardiotoxicity through Inhibition of Mitochondrial Dysfunction in H9c2 Cardiomyoblasts In Vitro

Doxorubicin (Dox) is one of the most frequently prescribed anti-cancer drugs. However, treatment with Dox is limited due to cumulative cardiotoxicity. 3-O-β-d-Sophorosylkaempferol-7-O-{3-O-[2(E)-2,6-dimethyl-6-hydroxyocta-2,7-dienoyl]}-α-L-rhamnoside (F-A), kaempferol 3-sophoroside 7-rhamnoside (F-B), and hippophanone (F-C) were successfully obtained by purification and separation of seabuckthorn seed residue in our previous research. This study was undertaken to investigate the protective effect of three flavonoids against Dox-induced H9c2 cell apoptosis. Cell proliferation was detected by MTT assay. 2',7'-Dichlorofluorescein diacetate (DCFH-DA) was used to determine the production of intracellular reactive oxygen species (ROS). ATP content was measured using an assay kit. Transmission electron microscopy (TEM) was used to observe changes in mitochondrial ultrastructure. The expression levels of proteins (p-JNK, JNK, p-Akt, Akt, p-P38, P38, p-ERK, ERK, p-Src, Src, Sab, IRE1α, Mfn1, Mfn2, and cleaved caspase-3) were evaluated by Western blot. Molecular docking was performed using AutoDock Vina. The three flavonoids could significantly relieve Dox-induced cardiac injury and inhibit cardiomyocyte apoptosis. The mechanisms were mainly related to the stability of mitochondrial structure and function maintained by suppressing the production of intracellular ROS, p-JNK and cleaved caspase-3, and increasing ATP contents and protein expression of mitochondrial mitofusin (Mfn1, Mfn2), Sab and p-Src. Pretreatment with flavonoids from Hippophae rhamnoides Linn. can reduce Dox-induced H9c2 cell apoptosis based on the 'JNK-Sab-Ros' signal pathway.

Skeletal and cardiac muscle calcium transport regulation in health and disease

In healthy muscle, the rapid release of calcium ions (Ca2+) with excitation-contraction (E-C) coupling, results in elevations in Ca2+ concentrations which can exceed 10-fold that of resting values. The sizable transient changes in Ca2+ concentrations are necessary for the activation of signaling pathways, which rely on Ca2+ as a second messenger, including those involved with force generation, fiber type distribution and hypertrophy. However, prolonged elevations in intracellular Ca2+ can result in the unwanted activation of Ca2+ signaling pathways that cause muscle damage, dysfunction, and disease. Muscle employs several calcium handling and calcium transport proteins that function to rapidly return Ca2+ concentrations back to resting levels following contraction. This review will detail our current understanding of calcium handling during the decay phase of intracellular calcium transients in healthy skeletal and cardiac muscle. We will also discuss how impairments in Ca2+ transport can occur and how mishandling of Ca2+ can lead to the pathogenesis and/or progression of skeletal muscle myopathies and cardiomyopathies.

Dynamics of metabolism and regulation of epigenetics during cardiomyocytes maturation

Maturation is the last step of heart growth that prepares the organ over the lifetime of the mammal for powerful, effective, and sustained pumping. Structural, gene expression, physiological, and functional specialties of cardiomyocytes describe this mechanism as the heart transits from fetus to adult phases. The main cornerstones of maturation of cardiomyocytes are reviewed and primary regulatory mechanisms are summarized to facilitate and organize these cellular activities. During embryonic development, cardiomyocytes proliferate rigorously but leave the cell cycle permanently immediately after the parturition of the child and experience terminal differentiation. The activation of a host of genes specific for the mature heart is correlated with the exit from the cell cycle. Even when exposed to mitogenic stimuli, the bulk of mature cardiomyocytes do not re-join the cell cycle. The reason for this permanent exit from the cell cycle is shown to be linked with stable switching off of the genes of the cell cycle directly involved in the G2/M transition phase and cytokinesis development. Researchers also trying to explain the molecular mechanism involved in stable inhibition of the gene and described structural changes (epigenetic and chromatin) in this mechanism. Substantial developments in the future with advances in the scientific platforms used for cardiomyocyte maturation research will broaden our understanding of this mechanism and result in better maturation of cardiomyocyte-derived pluripotent stem cells and effective treatment approaches for cardiovascular diseases.

Effects of MICU1-Mediated Mitochondrial Calcium Uptake on Energy Metabolism and Quality of Vitrified-Thawed Mouse Metaphase II Oocytes

Background: Oocyte vitrification has been widely used in the treatment of infertility and fertility preservation. However, vitrification-induced mitochondrial damage adversely affects oocyte development. Several studies have reported that mitochondrial calcium uptake protein 1 (MICU1) regulates the uptake of mitochondrial calcium by the mitochondrial calcium uniporter (MCU) and subsequently controls aerobic metabolism and oxidative stress in mitochondria, but research considering oocytes remains unreported. We evaluated whether the addition of MICU1 modulators enhances mitochondrial activity, pyruvate metabolism, and developmental competence after warming of MII oocytes. Methods: Retrieved MII oocytes of mice were classified as vitrified or control groups. After thawing, oocytes of vitrified group were cultured with or without DS16570511 (MICU1 inhibitor) and MCU-i4 (MICU1 activator) for 2 h. Results: Mitochondrial Ca2+ concentration, pyruvate dephosphorylation level, and MICU1 expression of MII oocytes were significantly increased after vitrification. These phenomena were further exacerbated by the addition of MCU-i4 and reversed by the addition of DS16570511 after warming. However, the mitochondrial membrane potential (MMP) and adenosine triphosphate (ATP) in vitrified-warmed MII oocytes drop significantly after vitrification, which was improved after MCU-i4 treatment and decreased significantly after DS16570511 treatment. The vitrification process was able to elicit a development competence reduction. After parthenogenetic activation, incubation of the thawed oocytes with MCU-i4 did not alter the cleavage and blastocyst rates. Moreover, incubation of the thawed oocytes with DS16570511 reduced the cleavage and blastocyst rates. Conclusions: MICU1-mediated increasing mitochondrial calcium uptake after vitrification of the MII oocytes promoted the pyruvate oxidation, and this process may maintain oocyte development competence by compensating for the consumption of ATP under stress state.

Long non-coding RNAs in the pathogenesis of heart failure: A literature review

Heart failure (HF) is a common cardiovascular disorder and a major cause of mortality and morbidity in older people. The mechanisms underlying HF are still not fully understood, restricting novel therapeutic target discovery and drug development. Besides, few drugs have been shown to improve the survival of HF patients. Increasing evidence suggests that long non-coding RNAs (lncRNAs) serve as a critical regulator of cardiac physiological and pathological processes, regarded as a new target of treatment for HF. lncRNAs are versatile players in the pathogenesis of HF. They can interact with chromatin, protein, RNA, or DNA, thereby modulating chromatin accessibility, gene expressions, and signaling transduction. In this review, we summarized the current knowledge on how lncRNAs involve in HF and categorized them into four aspects based on their biological functions, namely, cardiomyocyte contractility, cardiac hypertrophy, cardiac apoptosis, and myocardial fibrosis. Along with the extensive laboratory data, RNA-based therapeutics achieved great advances in recent years. These indicate that targeting lncRNAs in the treatment of HF may provide new strategies and address the unmet clinical needs.

Monoamine oxidase A and organic cation transporter 3 coordinate intracellular β1AR signaling to calibrate cardiac contractile function

We have recently identified a pool of intracellular β1 adrenergic receptors (β1ARs) at the sarcoplasmic reticulum (SR) crucial for cardiac function. Here, we aim to characterize the integrative control of intracellular catecholamine for subcellular β1AR signaling and cardiac function. Using anchored Förster resonance energy transfer (FRET) biosensors and transgenic mice, we determined the regulation of compartmentalized β1AR-PKA signaling at the SR and plasma membrane (PM) microdomains by organic cation transporter 3 (OCT3) and monoamine oxidase A (MAO-A), two critical modulators of catecholamine uptake and homeostasis. Additionally, we examined local PKA substrate phosphorylation and excitation-contraction coupling in cardiomyocyte. Cardiac-specific deletion of MAO-A (MAO-A-CKO) elevates catecholamines and cAMP levels in the myocardium, baseline cardiac function, and adrenergic responses. Both MAO-A deletion and inhibitor (MAOi) selectively enhance the local β1AR-PKA activity at the SR but not PM, and augment phosphorylation of phospholamban, Ca2+ cycling, and myocyte contractile response. Overexpression of MAO-A suppresses the SR-β1AR-PKA activity and PKA phosphorylation. However, deletion or inhibition of OCT3 by corticosterone prevents the effects induced by MAOi and MAO-A deletion in cardiomyocytes. Deletion or inhibition of OCT3 also negates the effects of MAOi and MAO-A deficiency in cardiac function and adrenergic responses in vivo. Our data show that MAO-A and OCT3 act in concert to fine-tune the intracellular SR-β1AR-PKA signaling and cardiac fight-or-flight response. We reveal a drug contraindication between anti-inflammatory corticosterone and anti-depressant MAOi in modulating adrenergic regulation in the heart, providing novel perspectives of these drugs with cardiac implications.

Mitofusin 2 positively regulates Ca2+ signaling by tethering the sarcoplasmic reticulum and mitochondria in rat aortic smooth muscle cells

Mitochondria buffer cytosolic Ca²⁺increases following Ca²⁺ influx from extracellular spaces and Ca²⁺ release from intracellular Ca²⁺ store sites under physiological circumstances. Therefore, close contact of mitochondria with the sarcoplasmic reticulum (SR) is required for maintaining Ca²⁺ homeostasis. Mitofusin 2 (Mfn2) localizes in both mitochondrial and SR membranes, and is hypothesized to optimize the distance and Ca²⁺ transfer between these organelles. However, the physiological significance of Mfn2 in vascular smooth muscle cells (VSMCs) is poorly understood. In the present study, the role of Mfn2 in the physical and functional couplings between SR and mitochondria was examined in rat aortic smooth muscle cells (rASMCs) by confocal and electron microscope imaging. When Mfn2 was knocked-down using siRNA in rASMCs, the mean distance between these organelles was extended from 16.2 to 21.6 nm. The increase in the cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) induced by 100 nM arginine vasopressin (AVP) was not affected by Mfn2 siRNA knockdown, whereas cytosolic Ca²⁺ removal was slower after Mfn2 knockdown. Following the AVP-induced [Ca²⁺]_cyt increase, mitochondrial Ca²⁺ uptake and Ca²⁺ refill into the SR were attenuated by Mfn2 knockdown. In addition, Mfn2-knockdown cells exhibited a loss of mitochondrial membrane potential (ΔΨ_mito) and lower ATP levels in mitochondria. Moreover, Mfn2 knockdown inhibited cell proliferation. In contrast, Mfn2 overexpression increased ΔΨ_mito and cell growth. This study strongly suggests that Mfn2 is responsible for SR-mitochondria Ca²⁺ signaling by tethering mitochondria to SR, thereby regulating ATP production and proliferation of VSMCs.

The RyR2-R2474S Mutation Sensitizes Cardiomyocytes and Hearts to Catecholaminergic Stress-Induced Oxidation of the Mitochondrial Glutathione Pool

Missense mutations in the cardiac ryanodine receptor type 2 (RyR2) characteristically cause catecholaminergic arrhythmias. Reminiscent of the phenotype in patients, RyR2-R2474S knockin mice develop exercise-induced ventricular tachyarrhythmias. In cardiomyocytes, increased mitochondrial matrix Ca²⁺ uptake was recently linked to non-linearly enhanced ATP synthesis with important implications for cardiac redox metabolism. We hypothesize that catecholaminergic stimulation and contractile activity amplify mitochondrial oxidation pathologically in RyR2-R2474S cardiomyocytes. To investigate this question, we generated double transgenic RyR2-R2474S mice expressing a mitochondria-restricted fluorescent biosensor to monitor the glutathione redox potential (E_GSH). Electrical field pacing-evoked RyR2-WT and RyR2-R2474S cardiomyocyte contractions resulted in a small but significant baseline E_GSH increase. Importantly, β-adrenergic stimulation resulted in excessive E_GSH oxidization of the mitochondrial matrix in RyR2-R2474S cardiomyocytes compared to baseline and RyR2-WT control. Physiologically β-adrenergic stimulation significantly increased mitochondrial E_GSH further in intact beating RyR2-R2474S but not in RyR2-WT control Langendorff perfused hearts. Finally, this catecholaminergic E_GSH increase was significantly attenuated following treatment with the RyR2 channel blocker dantrolene. Together, catecholaminergic stimulation and increased diastolic Ca²⁺ leak induce a strong, but dantrolene-inhibited mitochondrial E_GSH oxidization in RyR2-R2474S cardiomyocytes.

Ageing Causes Ultrastructural Modification to Calcium Release Units and Mitochondria in Cardiomyocytes

Ageing is associated with an increase in the incidence of heart failure, even if the existence of a real age-related cardiomyopathy remains controversial. Effective contraction and relaxation of cardiomyocytes depend on efficient production of ATP (handled by mitochondria) and on proper Ca2+ supply to myofibrils during excitation-contraction (EC) coupling (handled by Ca2+ release units, CRUs). Here, we analyzed mitochondria and CRUs in hearts of adult (4 months old) and aged (≥24 months old) mice. Analysis by confocal and electron microscopy (CM and EM, respectively) revealed an age-related loss of proper organization and disposition of both mitochondria and EC coupling units: (a) mitochondria are improperly disposed and often damaged (percentage of severely damaged mitochondria: adults 3.5 ± 1.1%; aged 16.5 ± 3.5%); (b) CRUs that are often misoriented (longitudinal) and/or misplaced from the correct position at the Z line. Immunolabeling with antibodies that mark either the SR or T-tubules indicates that in aged cardiomyocytes the sarcotubular system displays an extensive disarray. This disarray could be in part caused by the decreased expression of Cav-3 and JP-2 detected by western blot (WB), two proteins involved in formation of T-tubules and in docking SR to T-tubules in dyads. By WB analysis, we also detected increased levels of 3-NT in whole hearts homogenates of aged mice, a product of nitration of protein tyrosine residues, recognized as marker of oxidative stress. Finally, a detailed EM analysis of CRUs (formed by association of SR with T-tubules) points to ultrastructural modifications, i.e., a decrease in their frequency (adult: 5.1 ± 0.5; aged: 3.9 ± 0.4 n./50 μm2) and size (adult: 362 ± 40 nm; aged: 254 ± 60 nm). The changes in morphology and disposition of mitochondria and CRUs highlighted by our results may underlie an inefficient supply of Ca2+ ions and ATP to the contractile elements, and possibly contribute to cardiac dysfunction in ageing.

Loosening ER-Mitochondria Coupling by the Expression of the Presenilin 2 Loop Domain

Presenilin 2 (PS2), one of the three proteins in which mutations are linked to familial Alzheimer's disease (FAD), exerts different functions within the cell independently of being part of the γ-secretase complex, thus unrelated to toxic amyloid peptide formation. In particular, its enrichment in endoplasmic reticulum (ER) membrane domains close to mitochondria (i.e., mitochondria-associated membranes, MAM) enables PS2 to modulate multiple processes taking place on these signaling hubs, such as Ca²⁺ handling and lipid synthesis. Importantly, upregulated MAM function appears to be critical in AD pathogenesis. We previously showed that FAD-PS2 mutants reinforce ER-mitochondria tethering, by interfering with the activity of mitofusin 2, favoring their Ca²⁺ crosstalk. Here, we deepened the molecular mechanism underlying PS2 activity on ER-mitochondria tethering, identifying its protein loop as an essential domain to mediate the reinforced ER-mitochondria connection in FAD-PS2 models. Moreover, we introduced a novel tool, the PS2 loop domain targeted to the outer mitochondrial membrane, Mit-PS2-LOOP, that is able to counteract the activity of FAD-PS2 on organelle tethering, which possibly helps in recovering the FAD-PS2-associated cellular alterations linked to an increased organelle coupling.

Ticagrelor alleviates high-carbohydrate intake induced altered electrical activity of ventricular cardiomyocytes by regulating sarcoplasmic reticulum-mitochondria miscommunication

Metabolic syndrome (MetS) is associated with additional cardiovascular risk in mammalians while there are relationships between hyperglycemia-associated cardiovascular dysfunction and increased platelet P2Y12 receptor activation. Although P2Y12 receptor antagonist ticagrelor (Tica) plays roles in reduction of cardiovascular events, its beneficial mechanism remains poorly understood. Therefore, we aimed to clarify whether Tica can exert a direct protective effect in ventricular cardiomyocytes from high-carbohydrate diet-induced MetS rats, at least, through affecting sarcoplasmic reticulum (SR)-mitochondria (Mit) miscommunication. Tica treatment of MetS rats (150 mg/kg/day for 15 days) significantly reversed the altered parameters of action potentials by reversing sarcolemmal ionic currents carried by voltage-dependent Na⁺ and K⁺ channels, and Na⁺/Ca²⁺-exchanger in the cells, expressed P2Y12 receptors. The increased basal-cytosolic Ca²⁺ level and depressed SR Ca²⁺ load were also reversed in Tica-treated cells, at most, though recoveries in the phosphorylation levels of ryanodine receptors and phospholamban. Moreover, there were marked recoveries in Mit structure and function (including increases in both autophagosomes and fragmentations) together with recoveries in Mit proteins and the factors associated with Ca²⁺ transfer between SR-Mit. There were further significant recoveries in markers of both ER stress and oxidative stress. Taken into consideration the Tica-induced prevention of ER stress and mitochondrial dysfunction, our data provided an important document on the pleiotropic effects of Tica in the electrical activity of the cardiomyocytes from MetS rats. This protective effect seems through recoveries in SR-Mit miscommunication besides modulation of different sarcolemmal ion-channel activities, independent of P2Y12 receptor antagonism.

Metformin Reverses the Enhanced Myocardial SR/ER-Mitochondria Interaction and Impaired Complex I-Driven Respiration in Dystrophin-Deficient Mice

Besides skeletal muscle dysfunction, Duchenne muscular dystrophy (DMD) exhibits a progressive cardiomyopathy characterized by an impaired calcium (Ca²⁺) homeostasis and a mitochondrial dysfunction. Here we aimed to determine whether sarco-endoplasmic reticulum (SR/ER)–mitochondria interactions and mitochondrial function were impaired in dystrophic heart at the early stage of the pathology. For this purpose, ventricular cardiomyocytes and mitochondria were isolated from 3-month-old dystrophin-deficient mice (mdx mice). The number of contacts points between the SR/ER Ca²⁺ release channels (IP3R1) and the porine of the outer membrane of the mitochondria, VDAC1, measured using in situ proximity ligation assay, was greater in mdx cardiomyocytes. Expression levels of IP3R1 as well as the mitochondrial Ca²⁺ uniporter (MCU) and its regulated subunit, MICU1, were also increased in mdx heart. MICU2 expression was however unchanged. Furthermore, the mitochondrial Ca²⁺ uptake kinetics and the mitochondrial Ca²⁺ content were significantly increased. Meanwhile, the Ca²⁺-dependent pyruvate dehydrogenase phosphorylation was reduced, and its activity significantly increased. In Ca²⁺-free conditions, pyruvate-driven complex I respiration was decreased whereas in the presence of Ca²⁺, complex I-mediated respiration was boosted. Further, impaired complex I-mediated respiration was independent of its intrinsic activity or expression, which remains unchanged but is accompanied by an increase in mitochondrial reactive oxygen species production. Finally, mdx mice were treated with the complex I modulator metformin for 1 month. Metformin normalized the SR/ER-mitochondria interaction, decreased MICU1 expression and mitochondrial Ca²⁺ content, and enhanced complex I-driven respiration. In summary, before any sign of dilated cardiomyopathy, the DMD heart displays an aberrant SR/ER-mitochondria coupling with an increase mitochondrial Ca²⁺ homeostasis and a complex I dysfunction. Such remodeling could be reversed by metformin providing a novel therapeutic perspective in DMD.

The yin and yang of mitochondrial Ca2+ signaling in cell physiology and pathology

Mitochondria are autonomous and dynamic cellular organelles orchestrating a diverse range of cellular activities. Numerous cell-signaling pathways target these organelles and Ca²⁺ is one of the most significant. Mitochondria are able to rapidly and transiently take up Ca²⁺, thanks to the mitochondrial Ca²⁺ uniporter complex, as well as to extrude it through the Na⁺/Ca²⁺ and H⁺/Ca²⁺ exchangers. The transient accumulation of Ca²⁺ in the mitochondrial matrix impacts on mitochondrial functions and cell pathophysiology. Here we summarize the role of mitochondrial Ca²⁺ signaling in both physiological (yang) and pathological (yin) processes and the methods that can be used to investigate mitochondrial Ca²⁺ homeostasis. As an example of the pivotal role of mitochondria in pathology, we described the state of the art of mitochondrial Ca²⁺ alterations in different pathological conditions, with a special focus on Alzheimer's disease.