The ryanodine receptors Ca2+ release channels: cellular redox sensors?

Novel tumor therapy strategies targeting endoplasmic reticulum-mitochondria signal pathways

Organelles form tight connections through membrane contact sites, thereby cooperating to regulate homeostasis and cell function. Among them, the contact between endoplasmic reticulum (ER), the main intracellular calcium storage organelles, and mitochondria has been recognized for decades, and its main roles in the ion and lipid transport, ROS signaling, membrane dynamic changes and cellular metabolism are basically determined. At present, many tumor chemotherapeutic drugs rely on ER-mitochondrial calcium signal to function, but the mechanism of targeting resident molecules at the mitochondria-associated endoplasmic reticulum membranes (MAM) to sensitize traditional chemotherapy and the new tumor therapeutic targets identified based on the signal pathways on the MAM have not been thoroughly discussed. In this review, we highlight the key roles of various signaling pathways at the ER-mitochondria contact site in tumorigenesis and focus on novel anticancer therapy strategies targeting potential targets at this contact site.

Ryanodine Receptor Mediated Calcium Release Contributes to Ferroptosis Induced in Primary Hippocampal Neurons by GPX4 Inhibition

Ferroptosis, a newly described form of regulated cell death, is characterized by the iron-dependent accumulation of lipid peroxides, glutathione depletion, mitochondrial alterations, and enhanced lipoxygenase activity. Inhibition of glutathione peroxidase 4 (GPX4), a key intracellular antioxidant regulator, promotes ferroptosis in different cell types. Scant information is available on GPX4-induced ferroptosis in hippocampal neurons. Moreover, the role of calcium (Ca2+) signaling in ferroptosis remains elusive. Here, we report that RSL3, a selective inhibitor of GPX4, caused dendritic damage, lipid peroxidation, and induced cell death in rat primary hippocampal neurons. Previous incubation with the ferroptosis inhibitors deferoxamine or ferrostatin-1 reduced these effects. Likewise, preincubation with micromolar concentrations of ryanodine, which prevent Ca2+ release mediated by Ryanodine Receptor (RyR) channels, partially protected against RSL3-induced cell death. Incubation with RSL3 for 24 h suppressed the cytoplasmic Ca2+ concentration increase induced by the RyR agonist caffeine or by the SERCA inhibitor thapsigargin and reduced hippocampal RyR2 protein content. The present results add to the current understanding of ferroptosis-induced neuronal cell death in the hippocampus and provide new information both on the role of RyR-mediated Ca2+ signals on this process and on the effects of GPX4 inhibition on endoplasmic reticulum calcium content.

A bivalent remipede toxin promotes calcium release via ryanodine receptor activation

Multivalent ligands of ion channels have proven to be both very rare and highly valuable in yielding unique insights into channel structure and pharmacology. Here, we describe a bivalent peptide from the venom of Xibalbanus tulumensis, a troglobitic arthropod from the enigmatic class Remipedia, that causes persistent calcium release by activation of ion channels involved in muscle contraction. The high-resolution solution structure of φ-Xibalbin3-Xt3a reveals a tandem repeat arrangement of inhibitor-cysteine knot (ICK) domains previously only found in spider venoms. The individual repeats of Xt3a share sequence similarity with a family of scorpion toxins that target ryanodine receptors (RyR). Single-channel electrophysiology and quantification of released Ca²⁺ stores within skinned muscle fibers confirm Xt3a as a bivalent RyR modulator. Our results reveal convergent evolution of RyR targeting toxins in remipede and scorpion venoms, while the tandem-ICK repeat architecture is an evolutionary innovation that is convergent with toxins from spider venoms.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

H2O2/Ca2+/Zn2+ Complex Can Be Considered a “Collaborative Sensor” of the Mitochondrial Capacity?

In order to maintain a state of well-being, the cell needs a functional control center that allows it to respond to changes in the internal and surrounding environments and, at the same time, carry out the necessary metabolic functions. In this review, we identify the mitochondrion as such an “agora”, in which three main messengers are able to collaborate and activate adaptive response mechanisms. Such response generators, which we have identified as H₂O₂, Ca²⁺, and Zn²⁺, are capable of “reading” the environment and talking to each other in cooperation with the mitochondrion. In this manner, these messengers exchange information and generate a holistic response of the whole cell, dependent on its functional state. In this review, to corroborate this claim, we analyzed the role these actors, which in the review we call “sensors”, play in the regulation of skeletal muscle contractile capacities chosen as a model of crosstalk between Ca²⁺, Zn²⁺, and H₂O₂.

Modulations of Cardiac Functions and Pathogenesis by Reactive Oxygen Species and Natural Antioxidants

Homeostasis in the level of reactive oxygen species (ROS) in cardiac myocytes plays a critical role in regulating their physiological functions. Disturbance of balance between generation and removal of ROS is a major cause of cardiac myocyte remodeling, dysfunction, and failure. Cardiac myocytes possess several ROS-producing pathways, such as mitochondrial electron transport chain, NADPH oxidases, and nitric oxide synthases, and have endogenous antioxidation mechanisms. Cardiac Ca²⁺-signaling toolkit proteins, as well as mitochondrial functions, are largely modulated by ROS under physiological and pathological conditions, thereby producing alterations in contraction, membrane conductivity, cell metabolism and cell growth and death. Mechanical stresses under hypertension, post-myocardial infarction, heart failure, and valve diseases are the main causes for stress-induced cardiac remodeling and functional failure, which are associated with ROS-induced pathogenesis. Experimental evidence demonstrates that many cardioprotective natural antioxidants, enriched in foods or herbs, exert beneficial effects on cardiac functions (Ca²⁺ signal, contractility and rhythm), myocytes remodeling, inflammation and death in pathological hearts. The review may provide knowledge and insight into the modulation of cardiac pathogenesis by ROS and natural antioxidants.

Characterisation of a copper/zinc superoxide dismutase from Pieris rapae and its role in protecting against oxidative stress induced by chlorantraniliprole

Insecticide exposure typically leads to abnormally high levels of reactive oxygen species (ROS) and oxidative damage in insects. Superoxide dismutases (SODs) are potent antioxidant enzymes for ROS scavenging that are essential to protect insects against insecticide-induced oxidative injury. The small white butterfly, Pieris rapae, is an economically important lepidopteran pest of cruciferous crops, and the anthranilic diamide insecticide chlorantraniliprole is widely used to control this organism. However, whether chlorantraniliprole causes oxidative stress, and whether SODs are involved in ROS scavenging, remains unclear in P. rapae. In this study, an intracellular copper/zinc SOD (designated PrSOD1) gene was identified and characterised in P. rapae. The gene consists of four exons and three introns, and the PrSOD1 protein encoded by the gene has typical highly conserved features of CuZnSODs, including two signature motifs and seven Cu/Zn-interacting residues. Transcription of PrSOD1 was highest in the larval fat body and at the fifth-instar larval stage. Recombinant PrSOD1 protein expressed in Escherichia coli displayed antioxidant activity and high thermal and pH stability, confirming that PrSOD1 encodes a functional enzyme. Exposure to three sublethal doses of chlorantraniliprole for 6, 12 or 24 h resulted in significantly increased malondialdehyde concentration in P. rapae larvae, indicating insecticide-induced oxidative stress. Furthermore, both PrSOD1 transcription levels and CuZnSOD activity were quickly (6 and 12 h, respectively) upregulated in larvae subjected to chlorantraniliprole, strongly suggesting that PrSOD1 plays an important role in protecting against oxidative damage and possibly chlorantraniliprole tolerance in P. rapae.

Role of mitochondrial calcium in hypochlorite induced oxidative damage of cells

Hypochlorite (HOCl) is one of the most important mediators of inflammatory processes. Recent evidence demonstrates that changes in intracellular calcium pool play a significant role in the damaging effects of hypochlorite and other oxidants. Mitochondria are shown to be one of the intracellular targets of hypochlorite. But little is known about the mitochondrial calcium pool changes in HOCl-induced mitochondrial dysfunction. Using isolated rat liver mitochondria, we showed the oxidative damage of mitochondria (GSH oxidation and mixed protein-glutathione formation without membrane lipid peroxidation) and alterations in the mitochondrial functional parameters (decrease of respiratory activity and efficiency of oxidative phosphorylation, NADH and FADH coenzyme levels, and membrane potential) under hypochlorite action (50-300 μM). Simultaneously, the mitochondrial calcium release and swelling were demonstrated. In the presence of EGTA, the damaging effects of HOCl were less pronounced, reflecting direct involvement of mitochondrial Ca²⁺ in mechanisms of oxidant-induced injury. Furthermore, exposure of HeLa cells to hypochlorite resulted in a considerable increase in cytoplasmic calcium concentrations and a decrease in mitochondrial ones. Applying specific inhibitors of calcium transfer systems, we demonstrated that mitochondria play a key role in the redistribution of cytoplasmic Ca²⁺ ions under hypochlorite action and act as mediators of calcium release from the endoplasmic reticulum into the cytoplasm.

Pyridine nucleotide regulation of hepatic endoplasmic reticulum calcium uptake

Pyridine nucleotides serve an array of intracellular metabolic functions such as, to name a few, shuttling electrons in enzymatic reactions, safeguarding the redox state against reactive oxygen species, cytochrome P450 (CYP) enzyme detoxification pathways and, relevant to this study, the regulation of ion fluxes. In particular, the maintenance of a steep calcium gradient between the cytosol and endoplasmic reticulum (ER), without which apoptosis ensues, is achieved by an elaborate combination of energy–requiring ER membrane pumps and efflux channels. In liver microsomes, net calcium uptake was inhibited by physiological concentrations of NADP. In the presence of 1 mmol/L NADP, calcium uptake was attenuated by nearly 80%, additionally, this inhibitory effect was blunted by concomitant addition of NADPH. No other nicotinamide containing compounds ‐save a slight inhibition by NAADP‐hindered calcium uptake; thus, only oxidized pyridine nucleotides, or related compounds with a phosphate moiety, had an imposing effect. Moreover, the NADP inhibition was evident even after selectively blocking ER calcium efflux channels. Given the fundamental role of endoplasmic calcium homeostasis, it is plausible that changes in cytosolic NADP concentration, for example, during anabolic processes, could regulate net ER calcium uptake.

Calcium & ROS: Two orchestra directors for the requiem of death

Tumor necrosis factor alpha (TNF) triggers regulated necrosis of mycobacterium-infected macrophages through of mitochondrial reactive oxygen species (mitoROS) production in a RIPK1/3-dependent manner. To explain that, Roca and colleagues describe an inter-orgallenar circuit which involves the lysosomal ceramide production, mitoROS, BAX activation and RyR Ca²⁺ efflux from the endoplasmic reticulum into the mitochondrion.

Estradiol-Mediated Axogenesis of Hypothalamic Neurons Requires ERK1/2 and Ryanodine Receptors-Dependent Intracellular Ca2+ Rise in Male Rats

17β-estradiol (E2) induces axonal growth through extracellular signal-regulated kinase 1 and 2 (ERK1/2)-MAPK cascade in hypothalamic neurons of male rat embryos in vitro, but the mechanism that initiates these events is poorly understood. This study reports the intracellular Ca²⁺ increase that participates in the activation of ERK1/2 and axogenesis induced by E2. Hypothalamic neuron cultures were established from 16-day-old male rat embryos and fed with astroglia-conditioned media for 48 h. E2-induced ERK phosphorylation was completely abolished by a ryanodine receptor (RyR) inhibitor (ryanodine) and partially attenuated by an L-type voltage-gated Ca²⁺ channel (L-VGCC) blocker (nifedipine), an inositol-1,4,5-trisphosphate receptor (IP₃R) inhibitor (2-APB), and a phospholipase C (PLC) inhibitor (U-73122). We also conducted Ca²⁺ imaging recording using primary cultured neurons. The results show that E2 rapidly induces an increase in cytosolic Ca²⁺, which often occurs in repetitive Ca²⁺ oscillations. This response was not observed in the absence of extracellular Ca²⁺ or with inhibitory ryanodine and was markedly reduced by nifedipine. E2-induced axonal growth was completely inhibited by ryanodine. In summary, the results suggest that Ca²⁺ mobilization from extracellular space as well as from the endoplasmic reticulum is necessary for E2-induced ERK1/2 activation and axogenesis. Understanding the mechanisms of brain estrogenic actions might contribute to develop novel estrogen-based therapies for neurodegenerative diseases.

Dual action of L-Lactate on the activity of NR2B-containing NMDA receptors: from potentiation to neuroprotection

L-Lactate is a positive modulator of NMDAR-mediated signaling resulting in plasticity gene induction and memory consolidation. However, L-Lactate is also able to protect neurons against excito-toxic NMDAR activity, an indication of a mitigating action of L-Lactate on NMDA signaling. In this study, we provide experimental evidence that resolves this apparent paradox. Transient co-application of glutamate/glycine (1 μM/100 μM; 2 min) in primary cultures of mouse cortical neurons triggers a NMDA-dependent Ca²⁺ signal positively modulated by L-Lactate (10 mM) or DTT (1 mM) but decreased by Pyruvate (10 mM). This L-Lactate and DTT-induced potentiation is blocked by Ifenprodil (2 μM), a specific blocker of NMDARs containing NR2B sub-units. In contrast, co-application of glutamate/glycine (1 mM/100 μM; 2 min) elicits a NMDAR-dependent excitotoxic death in 49% of neurons. L-Lactate and Pyruvate significantly reduce this rate of cell death processes (respectively to 23% and 9%) while DTT has no effect (54% of neuronal death). This L-Lactate-induced neuroprotection is blocked by carbenoxolone and glibenclamide, respectively blockers of pannexins and K_ATP. In conclusion, our results show that L-Lactate is involved in two distinct and independent pathways defined as NMDAR-mediated potentiation pathway (or NADH pathway) and a neuroprotective pathway (or Pyruvate/ATP pathway), the prevalence of each one depending on the strength of the glutamatergic stimulus.

Ferritin and neuromelanin "quantum dot" array structures in dopamine neurons of the substantia nigra pars compacta and norepinephrine neurons of the locus coeruleus

In this review, the author shows that ferritin has documented quantum dot material properties that have been reported in numerous independent studies, and can enable quantum mechanical electron transport over substantial distances. In addition, neuromelanin is a pi-conjugated polymer, and quantum dot/pi-conjugated polymer combinations have been reported in numerous independent studies to facilitate electron transport for solar photovoltaic and other applications. Both ferritin and neuromelanin are present in large quantities in the dopamine neurons of the substantia nigra pars compactaand the norepinephrine neurons of the locus coeruleus. The unique structure of subgroups of these neurons that have a large number of axon branches and synapses may have evolved to take advantage of this electron transport mechanism, if it is present, such as to coordinate conscious action, or for other purposes. Independent clinical and laboratory studies are also reviewed that corroborate this theory of coordinated action in these neuron groups. Research to validate the theory using charge transport measurements, materials characterization, existing fluorescent probe material and reaction time testing is proposed.

Isoliquiritigenin attenuates glutamate-induced mitochondrial fission via calcineurin-mediated Drp1 dephosphorylation in HT22 hippocampal neuron cells

Numerous studies suggest that glutamate toxicity is a major contributor to neuronal dysfunction and death in several neurodegenerative diseases. In our previous study, isoliquiritigenin (ISL) isolated from Glycyrrhiza uralensis showed neuroprotective effects against neuronal cell death mediated by intracellular reactive oxygen species (ROS) generation and loss of mitochondrial membrane potential. However, the mechanisms by which ISL protects against glutamate-induced oxidative stress are unknown. In the present study, we focused on the cellular and molecular mechanisms underlying the inhibition of ROS production and induction of mitochondrial dysfunction by ISL in glutamate-stimulated HT22 mouse hippocampal neuron cells. The results revealed that ISL inhibited glutamate-induced mitochondrial ROS production and decline of glutathione levels and ATP generation in HT22 cells. Interestingly, we discovered that ISL prevents glutamate-induced mitochondrial fission by inhibiting the dephosphorylation of Drp1 at the serine 637 residue, which is a regulatory factor of mitochondrial dynamics, and both a S637D mutation of Drp1, which resulted in a phosphorylation-mimetic form of Drp1 at Ser637, and mitochondria-targeted antioxidant Mito-TEMPO inhibited glutamate-induced mitochondrial fission. Furthermore, ISL also prevented the increase of intracellular calcium accompanied by activation of calcineurin, which is a key regulator of dephosphorylation of Drp1 (Ser637), in glutamate-treated HT22 cells. Taken together, our results demonstrated that ISL protects against glutamate-induced mitochondrial fission by inhibiting the increase of mitochondrial ROS and intracellular calcium, which are accompanied by dephosphorylation of Drp1 (Ser637), and consequently attenuates glutamate-induced neuronal cell death. Therefore, these findings suggest that ISL exhibits the potential for protection against glutamate toxicity. These results may contribute to the development of new drugs and novel strategies for the treatment of neurodegenerative disorders related to glutamate toxicity.

Mitochondrial junctions with cellular organelles: Ca2+ signalling perspective

Cellular organelles form multiple junctional complexes with one another and the emerging research area dealing with such structures and their functions is undergoing explosive growth. A new research journal named “Contact” has been recently established to facilitate the development of this research field. The current consensus is to define an organellar junction by the maximal distance between the participating organelles; and the gap of 30 nm or less is considered appropriate for classifying such structures as junctions or membrane contact sites. Ideally, the organellar junction should have a functional significance, i.e. facilitate transfer of calcium, sterols, phospholipids, iron and possibly other substances between the organelles (Carrasco and Meyer in Annu Rev Biochem 80:973–1000, 2011; Csordas et al. in Trends Cell Biol 28:523–540, 2018; Phillips and Voeltz in Nat Rev Mol Cell Biol 17:69–82, 2016; Prinz in J Cell Biol 205:759–769, 2014). It is also important to note that the junction is not just a result of a random organelle collision but have active and specific formation, stabilisation and disassembly mechanisms. The nature of these mechanisms and their role in physiology/pathophysiology are the main focus of an emerging research field. In this review, we will briefly describe junctional complexes formed by cellular organelles and then focus on the junctional complexes that are formed by mitochondria with other organelles and the role of these complexes in regulating Ca²⁺ signalling.

Endoplasmic Reticulum-Mitochondrial Contactology: Structure and Signaling Functions

Interorganellar contacts are increasingly recognized as central to the control of cellular behavior. These contacts, which typically involve a small fraction of the endomembrane surface, are local communication hubs that resemble synapses. We propose the term contactology to denote the analysis of interorganellar contacts. Endoplasmic reticulum (ER) contacts with mitochondria were recognized several decades ago; major roles in ion and lipid transfer, signaling, and membrane dynamics have been established, while others continue to emerge. The functional diversity of ER-mitochondrial (ER-mito) contacts is mirrored in their structural heterogeneity, with subspecialization likely supported by multiple, different linker-forming protein structures. The nanoscale size of the contacts has made studying their structure, function, and dynamics difficult. This review focuses on the structure of the ER-mito contacts, methods for studying them, and the roles of contacts in Ca²⁺ and reactive oxygen species (ROS) signaling.

Excitation-Contraction Coupling Alterations in Myopathies

During the complex series of events leading to muscle contraction, the initial electric signal coming from motor neurons is transformed into an increase in calcium concentration that triggers sliding of myofibrils. This process, referred to as excitation-contraction coupling, is reliant upon the calcium-release complex, which is restricted spatially to a sub-compartment of muscle cells ("the triad") and regulated precisely. Any dysfunction in the calcium-release complex leads to muscle impairment and myopathy. Various causes can lead to alterations in excitation-contraction coupling and to muscle diseases. The latter are reviewed and classified into four categories: (i) mutation in a protein of the calcium-release complex; (ii) alteration in triad structure; (iii) modification of regulation of channels; (iv) modification in calcium stores within the muscle. Current knowledge of the pathophysiologic mechanisms in each category is described and discussed.

Unraveling the Burden of Iron in Neurodegeneration: Intersections with Amyloid Beta Peptide Pathology

Iron overload is a hallmark of many neurodegenerative processes such as Alzheimer's, Parkinson's, and Huntington's diseases. Unbound iron accumulated as a consequence of brain aging is highly reactive with water and oxygen and produces reactive oxygen species (ROS) or free radicals. ROS are toxic compounds able to damage cell membranes, DNA, and mitochondria. Which are the mechanisms involved in neuronal iron homeostasis and in neuronal response to iron-induced oxidative stress constitutes a cutting-edge topic in metalloneurobiology. Increasing our knowledge about the underlying mechanisms that operate in iron accumulation and their consequences would shed light on the comprehension of the molecular events that participate in the pathophysiology of the abovementioned neurodegenerative diseases. In this review, current evidences about iron accumulation in the brain, the signaling mechanisms triggered by metal overload, as well as the interaction between amyloid β (Aβ) and iron, will be summarized.

Perspectives and challenges of antioxidant therapy for atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia associated with significant morbidity and mortality. The mechanisms underlying the pathogenesis of AF are poorly understood, although electrophysiological remodeling has been described as an important initiating step. There is growing evidence that oxidative stress is involved in the pathogenesis of AF. Many known triggers of oxidative stress, such as age, diabetes, smoking, and inflammation, are linked with an increased risk of arrhythmia. Numerous preclinical studies and clinical trials reported the importance of antioxidant therapy in the prevention of AF, using vitamins C and E, polyunsaturated fatty acids, statins, or nitric oxide donors. The aim of our work is to give a current overview and analysis of opportunities, challenges, and benefits of antioxidant therapy in AF.

Reactive Oxygen Species: Physiological and Physiopathological Effects on Synaptic Plasticity

In the mammalian central nervous system, reactive oxygen species (ROS) generation is counterbalanced by antioxidant defenses. When large amounts of ROS accumulate, antioxidant mechanisms become overwhelmed and oxidative cellular stress may occur. Therefore, ROS are typically characterized as toxic molecules, oxidizing membrane lipids, changing the conformation of proteins, damaging nucleic acids, and causing deficits in synaptic plasticity. High ROS concentrations are associated with a decline in cognitive functions, as observed in some neurodegenerative disorders and age-dependent decay of neuroplasticity. Nevertheless, controlled ROS production provides the optimal redox state for the activation of transductional pathways involved in synaptic changes. Since ROS may regulate neuronal activity and elicit negative effects at the same time, the distinction between beneficial and deleterious consequences is unclear. In this regard, this review assesses current research and describes the main sources of ROS in neurons, specifying their involvement in synaptic plasticity and distinguishing between physiological and pathological processes implicated.

The chemical biology of HNO signaling

Nitroxyl (HNO) is a simple molecule with significant potential as a pharmacological agent. For example, its use in the possible treatment of heart failure has received recent attention due to its unique therapeutic properties. Recent progress has been made on the elucidation of the mechanisms associated with its biological signaling. Importantly, the biochemical mechanisms described for HNO bioactivity are consistent with its unique and novel chemical properties/reactivity. To date, much of the biology of HNO can be associated with interactions and modification of important regulatory thiol proteins. Herein will be provided a description of HNO chemistry and how this chemistry translates to some of its reported biological effects.

Reversible redox modifications of ryanodine receptor ameliorate ventricular arrhythmias in the ischemic-reperfused heart

Previous results from our laboratory showed that phosphorylation of ryanodine receptor 2 (RyR2) by Ca(2+) calmodulin-dependent kinase II (CaMKII) was a critical but not the unique event responsible for the production of reperfusion-induced arrhythmogenesis, suggesting the existence of other mechanisms cooperating in an additive way to produce these rhythm alterations. Oxidative stress is a prominent feature of ischemia/reperfusion injury. Both CaMKII and RyR2 are proteins susceptible to alteration by redox modifications. This study was designed to elucidate whether CaMKII and RyR2 redox changes occur during reperfusion and whether these changes are involved in the genesis of arrhythmias. Langendorff-perfused hearts from rats or transgenic mice with genetic ablation of CaMKII phosphorylation site on RyR2 (S2814A) were subjected to ischemia-reperfusion in the presence or absence of a free radical scavenger (mercaptopropionylglycine, MPG) or inhibitors of NADPH oxidase and nitric oxide synthase. Left ventricular contractile parameters and monophasic action potentials were recorded. Oxidation and phosphorylation of CaMKII and RyR2 were assessed. Increased oxidation of CaMKII during reperfusion had no consequences on the level of RyR2 phosphorylation. Avoiding the reperfusion-induced thiol oxidation of RyR2 with MPG produced a reduction in the number of arrhythmias and did not modify the contractile recovery. Conversely, selective prevention of S-nitrosylation and S-glutathionylation of RyR2 was associated with higher numbers of arrhythmias and impaired contractility. In S2814A mice, treatment with MPG further reduced the incidence of arrhythmias. Taken together, the results suggest that redox modification of RyR2 synergistically with CaMKII phosphorylation modulates reperfusion arrhythmias.

Oxidative stress and ca(2+) release events in mouse cardiomyocytes

Cellular oxidative stress, associated with a variety of common cardiac diseases, is well recognized to affect the function of several key proteins involved in Ca(2+) signaling and excitation-contraction coupling, which are known to be exquisitely sensitive to reactive oxygen species. These include the Ca(2+) release channels of the sarcoplasmic reticulum (ryanodine receptors or RyR2s) and the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Oxidation of RyR2s was found to increase the open probability of the channel, whereas CaMKII can be activated independent of Ca(2+) through oxidation. Here, we investigated how oxidative stress affects RyR2 function and SR Ca(2+) signaling in situ, by analyzing Ca(2+) sparks in permeabilized mouse cardiomyocytes under a broad range of oxidative conditions. The results show that with increasing oxidative stress Ca(2+) spark duration is prolonged. In addition, long and very long-lasting (up to hundreds of milliseconds) localized Ca(2+) release events started to appear, eventually leading to sarcoplasmic reticulum (SR) Ca(2+) depletion. These changes of release duration could be prevented by the CaMKII inhibitor KN93 and did not occur in mice lacking the CaMKII-specific S2814 phosphorylation site on RyR2. The appearance of long-lasting Ca(2+) release events was paralleled by an increase of RyR2 oxidation, but also by RyR-S2814 phosphorylation, and by CaMKII oxidation. Our results suggest that in a strongly oxidative environment oxidation-dependent activation of CaMKII leads to RyR2 phosphorylation and thereby contributes to the massive prolongation of SR Ca(2+) release events.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

Calcium and ROS: A mutual interplay

Calcium is an important second messenger involved in intra- and extracellular signaling cascades and plays an essential role in cell life and death decisions. The Ca²⁺ signaling network works in many different ways to regulate cellular processes that function over a wide dynamic range due to the action of buffers, pumps and exchangers on the plasma membrane as well as in internal stores. Calcium signaling pathways interact with other cellular signaling systems such as reactive oxygen species (ROS). Although initially considered to be potentially detrimental byproducts of aerobic metabolism, it is now clear that ROS generated in sub-toxic levels by different intracellular systems act as signaling molecules involved in various cellular processes including growth and cell death. Increasing evidence suggests a mutual interplay between calcium and ROS signaling systems which seems to have important implications for fine tuning cellular signaling networks. However, dysfunction in either of the systems might affect the other system thus potentiating harmful effects which might contribute to the pathogenesis of various disorders.

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Calcium and ROS act as signaling molecules inside the cell and their pathways can interact.

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The mutual interplay of calcium and ROS is required for the fine tuning of signaling.

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Failure in the interplay results in dysfunction and pathologies.

Protein S-nitrosylation in preconditioning and postconditioning

The coronary artery disease is a leading cause of death and morbidity worldwide. This disease has a complex pathophysiology that includes multiple mechanisms. Among these is the oxidative/nitrosative stress. Paradoxically, oxidative/nitrosative signaling plays a major role in cardioprotection against ischemia/reperfusion injury. In this context, the gas transmitter nitric oxide may act through several mechanisms, such as guanylyl cyclase activation and via S-nitrosylation of proteins. The latter is a covalent modification of a protein cysteine thiol by a nitric oxide-group that generates an S-nitrosothiol. Here, we report data showing that nitric oxide and S-nitrosylation of proteins play a pivotal role not only in preconditioning but also in postconditioning cardioprotection.

ROS Production via P2Y1-PKC-NOX2 Is Triggered by Extracellular ATP after Electrical Stimulation of Skeletal Muscle Cells

During exercise, skeletal muscle produces reactive oxygen species (ROS) via NADPH oxidase (NOX2) while inducing cellular adaptations associated with contractile activity. The signals involved in this mechanism are still a matter of study. ATP is released from skeletal muscle during electrical stimulation and can autocrinely signal through purinergic receptors; we searched for an influence of this signal in ROS production. The aim of this work was to characterize ROS production induced by electrical stimulation and extracellular ATP. ROS production was measured using two alternative probes; chloromethyl-2,7- dichlorodihydrofluorescein diacetate or electroporation to express the hydrogen peroxide-sensitive protein Hyper. Electrical stimulation (ES) triggered a transient ROS increase in muscle fibers which was mimicked by extracellular ATP and was prevented by both carbenoxolone and suramin; antagonists of pannexin channel and purinergic receptors respectively. In addition, transient ROS increase was prevented by apyrase, an ecto-nucleotidase. MRS2365, a P2Y₁ receptor agonist, induced a large signal while UTPyS (P2Y₂ agonist) elicited a much smaller signal, similar to the one seen when using ATP plus MRS2179, an antagonist of P2Y₁. Protein kinase C (PKC) inhibitors also blocked ES-induced ROS production. Our results indicate that physiological levels of electrical stimulation induce ROS production in skeletal muscle cells through release of extracellular ATP and activation of P2Y1 receptors. Use of selective NOX2 and PKC inhibitors suggests that ROS production induced by ES or extracellular ATP is mediated by NOX2 activated by PKC.

Role of Cys³⁶⁰² in the function and regulation of the cardiac ryanodine receptor

The cardiac Ca²⁺ release channel [ryanodine receptor type 2 (RyR2)] is modulated by thiol reactive agents, but the molecular basis of RyR2 modulation by thiol reagents is poorly understood. Cys³⁶³⁵ in the skeletal muscle RyR1 is one of the most hyper-reactive thiols and is important for the redox and calmodulin (CaM) regulation of the RyR1 channel. However, little is known about the role of the corresponding cysteine residue in RyR2 (Cys³⁶⁰²) in the function and regulation of the RyR2 channel. In the present study, we assessed the impact of mutating Cys³⁶⁰² (C³⁶⁰²A) on store overload-induced Ca²⁺ release (SOICR) and the regulation of RyR2 by thiol reagents and CaM. We found that the C³⁶⁰²A mutation suppressed SOICR by raising the activation threshold and delayed the termination of Ca²⁺ release by reducing the termination threshold. As a result, C³⁶⁰²A markedly increased the fractional Ca²⁺ release. Furthermore, the C³⁶⁰²A mutation diminished the inhibitory effect of N-ethylmaleimide on Ca²⁺ release, but it had no effect on the stimulatory action of 4,4'-dithiodipyridine (DTDP) on Ca²⁺ release. In addition, Cys³⁶⁰² mutations (C³⁶⁰²A or C³⁶⁰²R) did not abolish the effect of CaM on Ca²⁺-release termination. Therefore, RyR2-Cys³⁶⁰² is a major site mediating the action of thiol alkylating agent N-ethylmaleimide, but not the action of the oxidant DTDP. Our data also indicate that residue Cys³⁶⁰² plays an important role in the activation and termination of Ca²⁺ release, but it is not essential for CaM regulation of RyR2.

Nitric oxide signals postovulatory aging-induced abortive spontaneous egg activation in rats

OBJECTIVE

The aim of this study was to determine whether an increase of intracellular nitric oxide (NO) level signals postovulatory aging-induced abortive spontaneous egg activation (SEA) in rats.

METHODS

Freshly ovulated eggs (arrested at metaphase-II stage; M-II) were cultured in vitro for 3 hours to induce postovulatory egg aging. The morphological changes, inducible nitric oxide synthase (iNOS) expression, NO, cytosolic free Ca(2+), 3',5' cyclic guanosine monophosphate (cGMP), cell division cycle 25B (Cdc25B) and Wee1 levels, specific phosphorylation (pThr-14/Tyr-15) as well as total cyclin-dependent kinases-1 (Cdk1) (PSTAIRE) levels were analyzed.

RESULTS

Postovulatory aging induced generation of NO possibly through an iNOS-mediated pathway. The increase in NO level was associated with augmented cytosolic free Ca(2+) as well as cGMP levels in aged eggs. A significant increase in Wee1 level and decrease of Cdc25B level were observed in aged eggs. An accumulation of phosphorylated Cdk1 (pThr-14/Tyr-15) level was observed in aged eggs, while total Cdk1 (PSTAIR) level remained unchanged.

CONCLUSION

Our study demonstrates that generation of NO through an iNOS-mediated pathway increases cytosolic free Ca2+and cGMP levels. High levels of these signal molecules trigger the accumulation of phosphorylated Cdk1 in aged eggs. Thus, NO signals the accumulation of phosphorylated Cdk1 and induces postovulatory aging-induced abortive SEA in the rat.

Genetically encoded reactive oxygen species (ROS) and redox indicators

Redox processes are increasingly being recognized as key elements in the regulation of cellular signaling cascades. They are frequently encountered at the frontier between physiological functions and pathological events. The biological relevance of intracellular redox changes depends on the subcellular origin, the spatio-temporal distribution and the redox couple involved. Thus, a key task in the elucidation of the role of redox reactions is the specific and quantitative measurement of redox conditions with high spatio-temporal resolution. Unfortunately, until recently, our ability to perform such measurements was limited by the lack of adequate technology. Over the last 10 years, promising imaging tools have been developed from fluorescent proteins. Genetically encoded reactive oxygen species (ROS) and redox indicators (GERRIs) have the potential to allow real-time and pseudo-quantitative monitoring of specific ROS and thiol redox state in subcellular compartments or live organisms. Redox-sensitive yellow fluorescent proteins (rxYFP family), redox-sensitive green fluorescent proteins (roGFP family), HyPer (a probe designed to measure H2 O2 ), circularly permuted YFP and others have been used in several models and sufficient information has been collected to highlight their main characteristics. This review is intended to be a tour guide of the main types of GERRIs, their origins, properties, advantages and pitfalls.

Effects of advanced glycation end products on calcium handling in cardiomyocytes

BACKGROUND AND AIMS

Advanced glycation end products (AGEs) accumulate in diabetes and the engagement of receptor for AGE (RAGE) by AGEs contributes to the pathogenesis of diabetic cardiomyopathy. This study aims to investigate the effects of AGE/RAGE on ryanodine receptor (RyR) activity and Ca(2+) handling in cardiomyocytes to elucidate the possible mechanism underlying cardiac dysfunction in diabetic cardiomypathy.

METHODS AND RESULTS

Confocal imaging Ca(2+) spark, the elementary Ca(2+) release event reflecting RyR activity in intact cell, as well as SR Ca(2+) content and systolic Ca(2+) transient were performed in cultured neonatal rat ventricular myocytes. The results show that 50 mg/ml AGE increased the frequency of Ca(2+) sparks by 160%, while 150 mg/ml AGE increased it by 53%. AGE decreased the amplitude, width and duration of Ca(2+) sparks. Blocking RAGE with anti-RAGE IgG completely abolished the alteration of Ca(2+) sparks. The SR Ca(2+) content indicated by the amplitude (ΔF/F0) of 20 mM caffeine-elicited Ca(2+) transient was significantly decreased by 150 mg/ml AGE. In parallel, the amplitude of systolic Ca(2+) transient evoked by 1 Hz-field stimulation was remarkably decreased by 150 mg/ml AGE. The anti-RAGE antibody completely restored the impaired SR load and systolic Ca(2+) transient.

CONCLUSION

AGE/RAGE signal enhanced Ca(2+) spark-mediated SR Ca(2+) leak, causing partial depletion of SR Ca(2+) content and consequently decreasing systolic Ca(2+) transient, which may contribute to contractile dysfunction in diabetic cardiomyopathy.

Ryanodine receptor-mediated Ca(2+) release underlies iron-induced mitochondrial fission and stimulates mitochondrial Ca(2+) uptake in primary hippocampal neurons

Mounting evidence indicates that iron accumulation impairs brain function. We have reported previously that addition of sub-lethal concentrations of iron to primary hippocampal neurons produces Ca²⁺ signals and promotes cytoplasmic generation of reactive oxygen species. These Ca²⁺ signals, which emerge within seconds after iron addition, arise mostly from Ca²⁺ release through the redox-sensitive ryanodine receptor (RyR) channels present in the endoplasmic reticulum. We have reported also that addition of synaptotoxic amyloid-β oligomers to primary hippocampal neurons stimulates RyR-mediated Ca²⁺ release, generating long-lasting Ca²⁺ signals that activate Ca²⁺-sensitive cellular effectors and promote the disruption of the mitochondrial network. Here, we describe that 24 h incubation of primary hippocampal neurons with iron enhanced agonist-induced RyR-mediated Ca²⁺ release and promoted mitochondrial network fragmentation in 43% of neurons, a response significantly prevented by RyR inhibition and by the antioxidant agent N-acetyl-L-cysteine. Stimulation of RyR-mediated Ca²⁺ release by a RyR agonist promoted mitochondrial Ca²⁺ uptake in control neurons and in iron-treated neurons that displayed non-fragmented mitochondria, but not in neurons with fragmented mitochondria. Yet, the global cytoplasmic Ca²⁺ increase induced by the Ca²⁺ ionophore ionomycin prompted significant mitochondrial Ca²⁺ uptake in neurons with fragmented mitochondria, indicating that fragmentation did not prevent mitochondrial Ca²⁺ uptake but presumably decreased the functional coupling between RyR-mediated Ca²⁺ release and the mitochondrial Ca²⁺ uniporter. Taken together, our results indicate that stimulation of redox-sensitive RyR-mediated Ca²⁺ release by iron causes significant neuronal mitochondrial fragmentation, which presumably contributes to the impairment of neuronal function produced by iron accumulation.

Polydatin protects cardiac function against burn injury by inhibiting sarcoplasmic reticulum Ca2+ leak by reducing oxidative modification of ryanodine receptors

Our recent studies demonstrate that burn trauma induces leaky sarcoplasmic reticulum (SR) in heart due to excessively active ryanodine receptor (RyR) function. SR Ca(2+) leak causes partial depletion of SR Ca(2+) content and disturbances in intracellular Ca(2+) homeostasis, resulting in the pathogenesis of burn-generated cardiac dysfunction. This study investigated the role of polydatin, a resveratrol glucoside, in preventing SR leak and its therapeutic effect against burn-generated cardiac dysfunction. We found that polydatin treatment improved cardiac function impaired by burn injury of 30% of total body surface area. Parallel to the alterations in cardiac function, polydatin significantly increased the defective systolic Ca(2+) transient and contractility in burn-traumatized cardiomyocytes. Burn injury increased the occurrence of Ca(2+) sparks. The enhancement of Ca(2+) spark-mediated SR leak caused partial depletion of SR Ca(2+) content in burn-traumatized cardiomyocytes. Furthermore, we found that the content of free thiols (the number of reduced cysteines) in RyR2 in cardiomyocytes determined by the monobromobimane fluorescence of RyR2 was decreased markedly in burn-traumatized hearts. Polydatin treatment decreased intracellular reactive oxygen species levels and restored the amount of free thiols in RyR2 in burns. Concomitantly, polydatin corrected Ca(2+) spark-mediated SR leak and restored SR Ca(2+) load. The systolic Ca(2+) transient and cellular contractility were significantly increased by polydatin treatment. Taken together, the present findings provide the first evidence demonstrating that polydatin prevents enhanced Ca(2+) spark-mediated SR leak by reducing oxidative stress in RyR2 in burn-traumatized heart, leading to protection of cardiac function against burn injury.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

ROS regulation of microdomain Ca(2+) signalling at the dyads

Reactive oxygen species (ROS) are emerging as centre-stage players in cardiac functional regulation. ROS and Ca(2+) signals converge at dyads, the structural and functional units of cardiac excitation-contraction coupling. These two prominent signalling systems are intertwined with ROS modulation of the entire Ca(2+)-signalling network, and vice versa. While constitutively generated homoeostatic ROS are important in setting the redox potential of the intracellular milieu, dynamic signalling ROS shape microdomain and global Ca(2+) signals on both the beat-to-beat and greater time scales. However, ROS effects are complex and subtle, characterized by multiphasic and bidirectional Ca(2+) responses; and sustained oxidative stress may lead to compromised contractility and arrhythmogenicity. These new understandings should be leveraged to harness ROS for their beneficial roles while avoiding deleterious effects in the heart.

β amyloid peptide plaques fail to alter evoked neuronal calcium signals in APP/PS1 Alzheimer's disease mice

Alzheimer's disease (AD) is a multifactorial disorder of unknown etiology. Mechanistically, beta amyloid peptides (Aβ) and elevated Ca(2+) have been implicated as proximal and likely interactive features of the disease process. We tested the hypothesis that proximity to Aβ plaque might exacerbate activity-dependent neuronal Ca(2+) signaling in hippocampal pyramidal neurons from APPSWE/PS1M146V mice. Using combined approaches of whole cell patch clamp recording and 2-photon imaging of neuronal Ca(2+) signals with thioflavin-S plaque labeling in hippocampal slices, we found no correlation between thioflavin-S labeled Aβ plaque proximity and Ca(2+) responses triggered by ryanodine receptor (RyR) activation or action potentials in either dendrites or somata of AD mice, regardless of age. Baseline and RyR-stimulated spontaneous excitatory postsynaptic potentials also showed little difference in relation to Aβ plaque proximity. Consistent with previous studies, RyR-evoked Ca(2+) release in APPSWE/PS1M146V mice was greater than in nontransgenic controls. Within the soma, RyR-evoked Ca(2+) release was elevated in older APPSWE/PS1M146V mice compared with younger APPSWE/PS1M146V mice, but was still independent of plaque proximity. The results indicate that early Ca(2+) signaling disruptions can become yet more severe with age through mechanisms independent of Aβ plaques, suggesting that alternative pathogenic mechanisms might contribute to AD-associated dysfunction.

Nitric oxide-induced calcium release: activation of type 1 ryanodine receptor by endogenous nitric oxide

Ryanodine receptors (RyRs), located in the sarcoplasmic/endoplasmic reticulum (SR/ER) membrane, are required for intracellular Ca2+ release that is involved in a wide range of cellular functions. In addition to Ca2+-induced Ca2+ release in cardiac cells and voltage-induced Ca2+ release in skeletal muscle cells, we recently identified another mode of intracellular Ca2+ mobilization mediated by RyR, i.e., nitric oxide-induced Ca2+ release (NICR), in cerebellar Purkinje cells. NICR is evoked by neuronal activity, is dependent on S-nitrosylation of type 1 RyR (RyR1) and is involved in the induction of long-term potentiation (LTP) of cerebellar synapses. In this addendum, we examined whether peroxynitrite, which is produced by the reaction of nitric oxide with superoxide, may also have an effect on the Ca2+ release via RyR1 and the cerebellar LTP. We found that scavengers of peroxynitrite have no significant effect either on the Ca2+ release via RyR1 or on the cerebellar LTP. We also found that an application of a high concentration of peroxynitrite does not reproduce neuronal activity-dependent Ca2+ release in Purkinje cells. These results support that NICR is induced by endogenous nitric oxide produced by neuronal activity through S-nitrosylation of RyR1.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

HNO signaling mechanisms

Due to recent discoveries of important and novel biological activity, nitroxyl (HNO) has become a molecule of significant interest. Although it has been used in the past as a treatment for alcoholism, it is currently being touted as a treatment for heart failure. It is becoming increasingly clear that many of the biological actions of HNO can be attributed to its ability to react with specific thiol- and, possibly, heme-proteins. Herein is discussed the chemistry of HNO with likely biological targets. A particular focus is given to targets associated with the pharmacological utility of HNO as a cardiovascular agent and for the treatment of alcoholism.