The machinery of local Ca2+ signalling between sarco-endoplasmic reticulum and mitochondria

Evidence of mitochondrial dysfunction in ALS and methods for measuring in model systems

Metabolic dysfunction is a hallmark of multiple amyotrophic lateral sclerosis (ALS) models with a majority of ALS patients exhibiting hypermetabolism. The central sites of metabolism in the cell are mitochondria, capable of utilising a multitude of cellular substrates in an array of ATP-generating reactions. With reactive oxygen species (ROS) production occurring during some of these reactions, mitochondria can contribute considerably to oxidative stress. Mitochondria are also very dynamic organelles, interacting with other organelles, undergoing fusion/fission in response to changing metabolic states and being turned over by the cell regularly. Disruptions to many of these mitochondrial functions and processes have been reported in ALS models, largely indicating compromised mitochondrial function, increased ROS production by mitochondria, disrupted interactions with the endoplasmic reticulum and reduced turnover. This chapter summarises methods routinely used to assess mitochondria in ALS models and the alterations that have been reported in these models.

Spatiotemporal Distribution and Function of Mitochondria in Oocytes

Mitochondria are energy provider organelles in eukaryotic cells that contain their own specific genome. This review addresses structural and functional properties of mitochondria, focusing on recent discoveries about the changes in quality and number of mitochondria per cell during oocyte development. We highlight how oocyte mitochondria exhibit stage-specific morphology and characteristics at different stages of development, in sharp contrast to the elongated mitochondria present in somatic cells. We then evaluate the latest transcriptomic data to elucidate the complex functions of mitochondria during oocyte maturation and the impact of mitochondria on oocyte development. Finally, we describe the methodological progress of mitochondrial replacement therapy to rescue oocytes with developmental disorders or mitochondrial diseases, hoping to provide a guiding reference to future clinical applications.

Modulation of mitochondrial permeability transition pore opening by Myricetin and prediction of its-drug-like potential using in silico approach

Myricetin has been demonstrated to have multiple biological functions with promising research and development prospects. This study investigated the effect of myricetin on liver mitochondrial membrane permeability transition pores and its inhibitory potential on proteins that are important in the apoptotic process in silico. Mitochondrial swelling was assessed as changes in absorbance under succinate-energized conditions. Cytochrome c release, mitochondrial-lipid peroxidation, caspase 3 and 9 expressions, as well as calcium ATPase, were assessed. Pharmacokinetic properties of myricetin were predicted through the SwissADME server while the binding affinity of myricetin toward the proteins was computed using the AutodockVina Screening tool. The conformational stability of protein-ligand interactions was evaluated using molecular dynamics simulations analysis through the iMODS server. Myricetin inhibited the opening of the mitochondrial permeability transition pore and also reversed the increase in mitochondrial lipid peroxidation caused by calcium and other toxicants. Myricetin also caused a reduction in the expression of caspase 3 and 9 as well as calcium ATPase activity. The molecular docking results revealed that myricetin had a considerable binding affinity to the pocket site of caspase 3 and 9 as well as calcium ATPase. Myricetin showed a good drug-likeness based on the predicted pharmacokinetic properties as revealed by low CYP 450 inhibitory promiscuity and relatively low toxicity. It could therefore be suggested that myricetin could be useful in the management of diseases where too many apoptosis occur characterized by excessive tissue wastage such as neurodegenerative conditions and could as well play a role in protecting the physicochemical properties of membrane bilayers from free radical-induced severe cellular damage.

Therapeutic Effects of Mesenchymal Stromal Cells Require Mitochondrial Transfer and Quality Control

Due to their beneficial effects in an array of diseases, Mesenchymal Stromal Cells (MSCs) have been the focus of intense preclinical research and clinical implementation for decades. MSCs have multilineage differentiation capacity, support hematopoiesis, secrete pro-regenerative factors and exert immunoregulatory functions promoting homeostasis and the resolution of injury/inflammation. The main effects of MSCs include modulation of immune cells (macrophages, neutrophils, and lymphocytes), secretion of antimicrobial peptides, and transfer of mitochondria (Mt) to injured cells. These actions can be enhanced by priming (i.e., licensing) MSCs prior to exposure to deleterious microenvironments. Preclinical evidence suggests that MSCs can exert therapeutic effects in a variety of pathological states, including cardiac, respiratory, hepatic, renal, and neurological diseases. One of the key emerging beneficial actions of MSCs is the improvement of mitochondrial functions in the injured tissues by enhancing mitochondrial quality control (MQC). Recent advances in the understanding of cellular MQC, including mitochondrial biogenesis, mitophagy, fission, and fusion, helped uncover how MSCs enhance these processes. Specifically, MSCs have been suggested to regulate peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1α)-dependent biogenesis, Parkin-dependent mitophagy, and Mitofusins (Mfn1/2) or Dynamin Related Protein-1 (Drp1)-mediated fission/fusion. In addition, previous studies also verified mitochondrial transfer from MSCs through tunneling nanotubes and via microvesicular transport. Combined, these effects improve mitochondrial functions, thereby contributing to the resolution of injury and inflammation. Thus, uncovering how MSCs affect MQC opens new therapeutic avenues for organ injury, and the transplantation of MSC-derived mitochondria to injured tissues might represent an attractive new therapeutic approach.

Endoplasmic reticulum in oocytes: spatiotemporal distribution and function

ENDOPLASMIC RETICULUM IN OOCYTES

The storage and release of calcium ions (Ca2 +) in oocyte maturation and fertilization are particularly noteworthy features of the endoplasmic reticulum (ER). The ER is the largest organelle in the cell composed of rough ER, smooth ER, and nuclear envelope, and is the main site of protein synthesis, transport and folding, and lipid and steroid synthesis. An appropriate calcium signaling response can initiate oocyte development and embryogenesis, and the ER is the central link that initiates calcium signaling. The transition from immature oocytes to zygotes also requires many coordinated organelle reorganizations and changes. Therefore, the purpose of this review is to generalize information on the function, structure, interaction with other organelles, and spatiotemporal localization of the ER in mammalian oocytes. Mechanisms related to maintaining ER homeostasis have been extensively studied in recent years. Resolving ER stress through the unfolded protein response (UPR) is one of them. We combined the clinical problems caused by the ER in in vitro maturation (IVM), and the mechanisms of ER have been identified by single-cell RNA-seq. This article systematically reviews the functions of ER and provides a reference for assisted reproductive technology (ART) research.

The mitochondrial Ca2+ uniporter channel synergizes with fluid shear stress to induce mitochondrial Ca2+ oscillations

The mitochondrial calcium (Ca²⁺) uniporter (MCU) channel is responsible for mitochondrial Ca²⁺ influx. Its expression was found to be upregulated in endothelial cells (ECs) under cardiovascular disease conditions. Since the role of MCU in regulating cytosolic Ca²⁺ homeostasis in ECs exposed to shear stress (SS) is unknown, we studied mitochondrial Ca²⁺ dynamics (that is known to decode cytosolic Ca²⁺ signaling) in sheared ECs. To understand cause-and-effect, we ectopically expressed MCU in ECs. A higher percentage of MCU-transduced ECs exhibited mitochondrial Ca²⁺ transients/oscillations, and at higher frequency, under SS compared to sheared control ECs. Transients/oscillations correlated with mitochondrial reactive oxygen species (mROS) flashes and mitochondrial membrane potential (ΔΨ_m) flickers, and depended on activation of the mechanosensitive Piezo1 channel and the endothelial nitric oxide synthase (eNOS). A positive feedback loop composed of mitochondrial Ca²⁺ uptake/mROS flashes/ΔΨ_m flickers and endoplasmic reticulum Ca²⁺ release, in association with Piezo1 and eNOS, provided insights into the mechanism by which SS, under conditions of high MCU activity, may shape vascular EC energetics and function.

Emerging role of mitochondria in response to HBV infection

Hepatitis B is a major global health problem that potentially life‐threatening liver infection caused by the hepatitis B virus (HBV), which can lead to death due to liver cirrhosis and hepatocellular carcinoma (HCC). A considerable of research has demonstrated that mitochondrial dysfunction exists in patients with HBV infection, indicating that there is clinical relation between HBV infection and mitochondrial alterations. To explore the complex interplay between the functions of mitochondria and HBV infection in greater depth, we systematically summarized these mitochondrial alterations due to HBV infection in recent years. The liver is the central organ of metabolism that is a mitochondria‐rich tissue and represents strong defense and regeneration capabilities in the body. Infested cells and their microenvironment must upregulate energy production for proliferation, growth, and effector functions to restrain the damage imposed by HBV. The changes in metabolic pathways caused by HBV infection are nothing more than those in the cytoplasm and mitochondria. Thus, this article brings into focus the effects of novel reprogramming of inner and outer mitochondria on HBV infection and then derives novel insights and new approaches for HBV diagnosis and therapy.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

The 40-Hz White Light-Emitting Diode (LED) Improves the Structure-Function of the Brain Mitochondrial KATP Channel and Respiratory Chain Activities in Amyloid Beta Toxicity

It has been described that using noninvasive exposure to 40-Hz white light LED reduces amyloid-beta, a peptide thought to initiate neurotoxic events in Alzheimer's disease (AD). However, the mechanisms remain to be identified. Since AD impairs mitochondrial potassium channels and respiratory chain activity, the objectives of the current study were to determine the effect of 40-Hz white light LED on structure-function of mitoK_ATP channel and brain mitochondrial respiratory chain activity, production of reactive oxygen species (ROS), and ΔΨ_m in AD. Single mitoK_ATP channel was considered using a channel incorporated into the bilayer lipid membrane and expression of mitoK_ATP-Kir6.1 subunit as a pore-forming subunit of the channel was determined using a western blot analysis in Aβ1-42 toxicity and light-treated rats. Our results indicated a severe decrease in mito-K_ATP channel permeation and Kir6.1 subunit expression coming from the Aβ1-42-induced neurotoxicity. Furthermore, we found that Aβ1-42-induced neurotoxicity decreased activities of complexes I and IV and increased ROS production and ΔΨ_m. Surprisingly, light therapy increased channel permeation and mitoK_ATP-Kir6.1 subunit expression. Noninvasive 40-Hz white light LED treatment also increased activities of complexes I and IV and decreased ROS production and ΔΨ_m up to ~ 70%. Here, we report that brain mito-K_ATP channel and respiratory chain are, at least in part, novel targets of 40-Hz white light LED therapy in AD.

Intranasal insulin improves the structure-function of the brain mitochondrial ATP-sensitive Ca2+ activated potassium channel and respiratory chain activities under diabetic conditions

Although it is well established that diabetes impairs mitochondrial respiratory chain activity, little is known of the effects of intranasal insulin (INI) on the mitochondrial respiratory chain and structure-function of mitoBKCa channel in diabetes. We have investigated this mechanism in an STZ-induced early type 2 diabetic model. Single ATP-sensitive mitoBKCa channel activity was considered in diabetic and INI-treated rats using a channel incorporated into the bilayer lipid membrane. Because mitoBKCa channels have been involved in mitochondrial respiratory chain activity, a study was undertaken to investigate whether the NADH, complexes I and IV, mitochondrial ROS production, and ΔΨm are altered in an early diabetic model. In this work, we provide evidence for a significant decrease in channel open probability and conductance in diabetic rats. Evidence has been shown that BKCa channel β2 subunits induce a left shift in the BKCa channel voltage dependent curve in low Ca2+ conditions,; our results indicated a significant decrease in mitoBKCa β2 subunits using Western blot analysis. Importantly, INI treatment improved mitoBKCa channel behaviors and β2 subunits expression up to ~70%. We found that early diabetes decreased activities of complex I and IV and increased NADH, ROS production, and ΔΨm. Surprisingly, INI modified the mitochondrial respiratory chain, ROS production, and ΔΨm up to ~70%. Our results thus demonstrate an INI improvement in respiratory chain activity and ROS production in brain mitochondrial preparations coming from the STZ early diabetic rat model, an effect potentially linked to INI improvement in mitoBKCa channel activity and channel β2 subunit expression.

Molecular nature and physiological role of the mitochondrial calcium uniporter channel

Calcium (Ca2+) signaling is critical for cell function and cell survival. Mitochondria play a major role in regulating the intracellular Ca2+ concentration ([Ca2+]i). Mitochondrial Ca2+ uptake is an important determinant of cell fate and governs respiration, mitophagy/autophagy, and the mitochondrial pathway of apoptosis. Mitochondrial Ca2+ uptake occurs via the mitochondrial Ca2+ uniporter (MCU) complex. This review summarizes the present knowledge on the function of MCU complex, regulation of MCU channel, and the role of MCU in Ca2+ homeostasis and human disease pathogenesis. The channel core consists of four MCU subunits and essential MCU regulators (EMRE). Regulatory proteins that interact with them include mitochondrial Ca2+ uptake 1/2 (MICU1/2), MCU dominant-negative β-subunit (MCUb), MCU regulator 1 (MCUR1), and solute carrier 25A23 (SLC25A23). In addition to these proteins, cardiolipin, a mitochondrial membrane-specific phospholipid, has been shown to interact with the channel core. The dynamic interplay between the core and regulatory proteins modulates MCU channel activity after sensing local changes in [Ca2+]i, reactive oxygen species, and other environmental factors. Here, we highlight the structural details of the human MCU heteromeric assemblies and their known roles in regulating mitochondrial Ca2+ homeostasis. MCU dysfunction has been shown to alter mitochondrial Ca2+ dynamics, in turn eliciting cell apoptosis. Changes in mitochondrial Ca2+ uptake have been implicated in pathological conditions affecting multiple organs, including the heart, skeletal muscle, and brain. However, our structural and functional knowledge of this vital protein complex remains incomplete, and understanding the precise role for MCU-mediated mitochondrial Ca2+ signaling in disease requires further research efforts.

Stem cell plasticity and regenerative potential regulation through Ca2+-mediated mitochondrial nuclear crosstalk

The multi-lineage differentiation potential is one of the prominent mechanisms through which stem cells can repair damaged tissues. The regenerative potential of stem cells is the manifestation of several changes at the structural and molecular levels in stem cells that are regulated through intricate mitochondrial-nuclear interactions maintained by Ca²⁺ ion signaling. Despite the exhilarating evidences strengthening the versatile and indispensible role of Ca²⁺ in regulating mitochondrial-nuclear interactions, the extensive details of signaling mechanisms remains largely unexplored. In this review we have discussed the effect of Ca²⁺ ion mediated mitochondrial-nuclear interactions participating in stem plasticity and its regenerative potential.

Dopamine-induced interactions of female mouse hypothalamic proteins with progestin receptor-A in the absence of hormone

Neural progestin receptors (PR) function in reproduction, neural development, neuroprotection, learning, memory and the anxiety response. In the absence of progestins, PR can be activated by dopamine (DA) in the rodent hypothalamus to elicit female sexual behaviour. The present study investigated mechanisms of DA activation of PR by testing the hypothesis that proteins from DA-treated hypothalami interact with PR in the absence of progestins. Ovariectomised, oestradiol-primed mice were infused with a D1-receptor agonist, SKF38393 (SKF), into the third ventricle 30 minutes prior to death. Proteins from SKF-treated hypothalami were pulled-down with glutathione S-transferase-tagged mouse PR-A or PR-B and the interactomes were analysed by mass spectrometry. The largest functional group to interact with PR-A in a DA-dependent manner was synaptic proteins. To test the hypothesis that DA activation of PR regulates synaptic proteins, we developed oestradiol-induced PR-expressing hypothalamic-like neurones derived from human-induced pluripotent stem cells (hiPSCs). Similar to progesterone (P4), SKF treatment of hiPSCs increased synapsin1/2 expression. This SKF-dependent effect was blocked by the PR antagonist RU486, suggesting that PR are necessary for this DA-induced increase. The second largest DA-dependent PR-A protein interactome comprised metabolic regulators involved in glucose metabolism, lipid synthesis and mitochondrial energy production. Interestingly, hypothalamic proteins interacted with PR-A, but not PR-B, in an SKF-dependent manner, suggesting that DA promotes the interaction of multiple hypothalamic proteins with PR-A. These in vivo and in vitro results indicate novel mechanisms by which DA can differentially activate PR isoforms in the absence of P4 and provide a better understanding of ligand-independent PR activation in reproductive, metabolic and mental health disorders in women.

Acute Induction of Translocon-Mediated Ca2+ Leak Protects Cardiomyocytes Against Ischemia/Reperfusion Injury

During myocardial infarction, dysregulation of Ca²⁺ homeostasis between the reticulum, mitochondria, and cytosol occurs in cardiomyocytes and leads to cell death. Ca²⁺ leak channels are thought to be key regulators of the reticular Ca²⁺ homeostasis and cell survival. The present study aimed to determine whether a particular reticular Ca²⁺ leak channel, the translocon, also known as translocation channel, could be a relevant target against ischemia/reperfusion-mediated heart injury. To achieve this objective, we first used an intramyocardial adenoviral strategy to express biosensors in order to assess Ca²⁺ variations in freshly isolated adult mouse cardiomyocytes to show that translocon is a functional reticular Ca²⁺ leak channel. Interestingly, translocon activation by puromycin mobilized a ryanodine receptor (RyR)-independent reticular Ca²⁺ pool and did not affect the excitation–concentration coupling. Second, puromycin pretreatment decreased mitochondrial Ca²⁺ content and slowed down the mitochondrial permeability transition pore (mPTP) opening and the rate of cytosolic Ca²⁺ increase during hypoxia. Finally, this translocon pre-activation also protected cardiomyocytes after in vitro hypoxia reoxygenation and reduced infarct size in mice submitted to in vivo ischemia-reperfusion. Altogether, our report emphasizes the role of translocon in cardioprotection and highlights a new paradigm in cardioprotection by functionally uncoupling the RyR-dependent Ca²⁺ stores and translocon-dependent Ca²⁺ stores.

Molecular Mechanisms of ER Stress and UPR in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease involving aggregation of misfolded proteins inside the neuron causing prolonged cellular stress. The neuropathological hallmarks of AD include the formation of senile plaques and neurofibrillary tangles in specific brain regions that lead to synaptic loss and neuronal death. The exact mechanism of neuron dysfunction in AD remains obscure. In recent years, endoplasmic reticulum (ER) dysfunction has been implicated in neuronal degeneration seen in AD. Apart from AD, many other diseases also involve misfolded proteins aggregations in the ER, a condition referred to as ER stress. The response of the cell to ER stress is to activate a group of signaling pathways called unfolded protein response (UPR) that stimulates a particular transcriptional program to restore ER function and ensure cell survival. ER stress also involves the generation of reactive oxygen species (ROS) that, together with mitochondrial ROS and decreased effectiveness of antioxidant mechanisms, producing a condition of chronic oxidative stress. The unfolded proteins may not always produce a response that leads to the restoration of cellular functions, but they may also lead to inflammation by a set of different pathways with deleterious consequences. In this review, we extensively discuss the role of ER stress and how to target it using different pharmacological approaches in AD development and onset.

Blockade of MCU-Mediated Ca2+ Uptake Perturbs Lipid Metabolism via PP4-Dependent AMPK Dephosphorylation

Mitochondrial Ca²⁺ uniporter (MCU)-mediated Ca²⁺ uptake promotes the buildup of reducing equivalents that fuel oxidative phosphorylation for cellular metabolism. Although MCU modulates mitochondrial bioenergetics, its function in energy homeostasis in vivo remains elusive. Here we demonstrate that deletion of the Mcu gene in mouse liver (MCU^Δhep) and in Danio rerio by CRISPR/Cas9 inhibits mitochondrial Ca²⁺ (_mCa²⁺) uptake, delays cytosolic Ca²⁺ (_cCa²⁺) clearance, reduces oxidative phosphorylation, and leads to increased lipid accumulation. Elevated hepatic lipids in MCU^Δhep were a direct result of extramitochondrial Ca²⁺-dependent protein phosphatase-4 (PP4) activity, which dephosphorylates AMPK. Loss of AMPK recapitulates hepatic lipid accumulation without changes in MCU-mediated Ca²⁺ uptake. Furthermore, reconstitution of active AMPK, or PP4 knockdown, enhances lipid clearance in MCU^Δhep hepatocytes. Conversely, gain-of-function MCU promotes rapid _mCa²⁺ uptake, decreases PP4 levels, and reduces hepatic lipid accumulation. Thus, our work uncovers an MCU/PP4/AMPK molecular cascade that links Ca²⁺ dynamics to hepatic lipid metabolism.

Endoplasmic Reticulum Stress and Mitochondrial Function in Airway Smooth Muscle

Inflammatory airway diseases such as asthma affect more than 300 million people world-wide. Inflammation triggers pathophysiology via such as tumor necrosis factor α (TNFα) and interleukins (e.g., IL-13). Hypercontraction of airway smooth muscle (ASM) and ASM cell proliferation are major contributors to the exaggerated airway narrowing that occurs during agonist stimulation. An emergent theme in this context is the role of inflammation-induced endoplasmic reticulum (ER) stress and altered mitochondrial function including an increase in the formation of reactive oxygen species (ROS). This may establish a vicious cycle as excess ROS generation leads to further ER stress. Yet, it is unclear whether inflammation-induced ROS is the major mechanism leading to ER stress or the consequence of ER stress. In various diseases, inflammation leads to an increase in mitochondrial fission (fragmentation), associated with reduced levels of mitochondrial fusion proteins, such as mitofusin 2 (Mfn2). Mitochondrial fragmentation may be a homeostatic response since it is generally coupled with mitochondrial biogenesis and mitochondrial volume density thereby reducing demand on individual mitochondrion. ER stress is triggered by the accumulation of unfolded proteins, which induces a homeostatic response to alter protein balance via effects on protein synthesis and degradation. In addition, the ER stress response promotes protein folding via increased expression of molecular chaperone proteins. Reduced Mfn2 and altered mitochondrial dynamics may not only be downstream to ER stress but also upstream such that a reduction in Mfn2 triggers further ER stress. In this review, we summarize the current understanding of the link between inflammation-induced ER stress and mitochondrial function and the role played in the pathophysiology of inflammatory airway diseases.

Upregulation of the Sarco-Endoplasmic Reticulum Calcium ATPase 1 Truncated Isoform Plays a Pathogenic Role in Alzheimer's Disease

Dysregulation of the Endoplasmic Reticulum (ER) Ca²⁺ homeostasis and subsequent ER stress activation occur in Alzheimer Disease (AD). We studied the contribution of the human truncated isoform of the sarco-endoplasmic reticulum Ca²⁺ ATPase 1 (S1T) to AD. We examined S1T expression in human AD-affected brains and its functional consequences in cellular and transgenic mice AD models. S1T expression is increased in sporadic AD brains and correlates with amyloid β (Aβ) and ER stress chaperone protein levels. Increased S1T expression was also observed in human neuroblastoma cells expressing Swedish-mutated β-amyloid precursor protein (βAPP) or treated with Aβ oligomers. Lentiviral overexpression of S1T enhances in return the production of APP C-terminal fragments and Aβ through specific increases of β-secretase expression and activity, and triggers neuroinflammation. We describe a molecular interplay between S1T-dependent ER Ca²⁺ leak, ER stress and βAPP-derived fragments that could contribute to AD setting and/or progression.

Modeling the role of endoplasmic reticulum-mitochondria microdomains in calcium dynamics

Upon inositol trisphosphate (IP₃) stimulation of non-excitable cells, including vascular endothelial cells, calcium (Ca²⁺) shuttling between the endoplasmic reticulum (ER) and mitochondria, facilitated by complexes called Mitochondria-Associated ER Membranes (MAMs), is known to play an important role in the occurrence of cytosolic Ca²⁺ concentration ([Ca²⁺]_Cyt) oscillations. A mathematical compartmental closed-cell model of Ca²⁺ dynamics was developed that accounts for ER-mitochondria Ca²⁺ microdomains as the µd compartment (besides the cytosol, ER and mitochondria), Ca²⁺ influx to/efflux from each compartment and Ca²⁺ buffering. Varying the distribution of functional receptors in MAMs vs. the rest of ER/mitochondrial membranes, a parameter called the channel connectivity coefficient (to the µd), allowed for generation of [Ca²⁺]_Cytoscillations driven by distinct mechanisms at various levels of IP₃ stimulation. Oscillations could be initiated by the transient opening of IP₃ receptors facing either the cytosol or the µd, and subsequent refilling of the respective compartment by Ca²⁺ efflux from the ER and/or the mitochondria. Only under conditions where the µd became the oscillation-driving compartment, silencing the Mitochondrial Ca²⁺ Uniporter led to oscillation inhibition. Thus, the model predicts that alternative mechanisms can yield [Ca²⁺]_Cyt oscillations in non-excitable cells, and, under certain conditions, the ER-mitochondria µd can play a regulatory role.

Hydrogen peroxide depolarizes mitochondria and inhibits IP3-evoked Ca2+ release in the endothelium of intact arteries

•

H₂O₂ is produced by several cell processes including mitochondria and may act as an intracellular messenger and cell-cell signalling molecule.

•

Spontaneous local Ca²⁺ signals and IP₃-evoked Ca²⁺ increases were inhibited by H₂O₂.

•

H₂O₂ suppression of IP₃-evoked Ca²⁺ signalling may be mediated by mitochondria via a decrease in the mitochondrial membrane potential.

•

H₂O₂-induced mitochondrial depolarization and inhibition of IP₃-evoked Ca²⁺ release, may protect mitochondria from Ca²⁺ overload during IP₃-linked Ca²⁺ signals.

Hydrogen peroxide (H₂O₂) is a mitochondrial-derived reactive oxygen species (ROS) that regulates vascular signalling transduction, vasocontraction and vasodilation. Although the physiological role of ROS in endothelial cells is acknowledged, the mechanisms underlying H₂O₂ regulation of signalling in native, fully-differentiated endothelial cells is unresolved. In the present study, the effects of H₂O₂ on Ca²⁺ signalling were investigated in the endothelium of intact rat mesenteric arteries. Spontaneous local Ca²⁺ signals and acetylcholine evoked Ca²⁺ increases were inhibited by H₂O₂. H₂O₂ inhibition of acetylcholine-evoked Ca²⁺ signals was reversed by catalase. H₂O₂ exerts its inhibition on the IP₃ receptor as Ca²⁺ release evoked by photolysis of caged IP₃ was supressed by H₂O₂. H₂O₂ suppression of IP₃-evoked Ca²⁺ signalling may be mediated by mitochondria. H₂O₂ depolarized mitochondria membrane potential. Acetylcholine-evoked Ca²⁺ release was inhibited by depolarisation of the mitochondrial membrane potential by the uncoupler carbonyl cyanide 3-chlorophenylhydrazone (CCCP) or complex 1 inhibitor, rotenone. We propose that the suppression of IP₃-evoked Ca²⁺ release by H₂O₂ arises from the decrease in mitochondrial membrane potential. These results suggest that mitochondria may protect themselves against Ca²⁺ overload during IP₃-linked Ca²⁺ signals by a H₂O₂ mediated negative feedback depolarization of the organelle and inhibition of IP₃-evoked Ca²⁺ release.

Molecular docking analysis of apigenin and quercetin from ethylacetate fraction of Adansonia digitata with malaria-associated calcium transport protein: An in silico approach

Background

The investigation and knowledge of calcium handling mechanisms in the plasmodium has been considered as a potential biological target against malaria.

Objective

This study deals with the evaluation of inhibitory activity of secondary metabolites of ethylacetate partitioned-fraction of Adansonia digitata stem bark extract on malaria-associated protein using in silico docking studies.

Materials and methods

Molecular docking and virtual screening was performed to understand the mechanism of ligand binding and to identify potent calcium transporter inhibitors. The stem bark extracts of A. digitata contains rich sources of phytochemicals. The secondary metabolites were determined by HPLC-DAD and HRGC-MS analysis. The major chemical constituent present in the ethylacetate partitioned-fraction of A. digitata stem bark extract were examined for their antiplasmodial activity and were also involved in docking study.

Results

The secondary metabolites, quercetin and apigenin inhibited the formation of β-hematin. The results showed that all the selected compounds in the A. digitata showed binding energy ranging between -6.5 kcal/mol and -7.1 kcal/mol. Among the two chemical constituents, apigenin has the highest docking score along with the highest number of hydrogen bonds formed when compared to quercetin. Analysis of the results suggests that apigenin and quercetin could act as an anti-malaria agent.

Conclusion

Molecular docking analysis could lead to further development of potent calcium transporter inhibitors for the prevention and treatment of malaria and related conditions.

Intrafibrillar and perinuclear mitochondrial heterogeneity in adult cardiac myocytes

Mitochondria are involved in multiple cellular functions, in addition to their core role in energy metabolism. Mitochondria localized in different cellular locations may have different morphology, Ca²⁺ handling and biochemical properties and may interact differently with other intracellular structures, causing functional specificity. However, most prior studies have utilized isolated mitochondria, removed from their intracellular environment. Mitochondria in cardiac ventricular myocytes are highly organized, with a majority squeezed between the myofilaments in longitudinal chains (intrafibrillar mitochondria, IFM). There is another population of perinuclear mitochondria (PNM) around and between the two nuclei typical in myocytes. Here, we take advantage of live myocyte imaging to test for quantitative morphological and functional differences between IFM and PNM with respect to calcium fluxes, membrane potential, sensitivity to oxidative stress, shape and dynamics. Our findings show higher mitochondrial Ca²⁺ uptake and oxidative stress sensitivity for IFM vs. PNM, which may relate to higher local energy demand supporting the contractile machinery. In contrast to IFM which are remarkably static, PNM are relatively mobile, appear to participate readily in fission/fusion dynamics and appear to play a central role in mitochondrial genesis and turnover. We conclude that while IFM may be physiologically tuned to support local myofilament energy demands, PNM may be more critical in mitochondrial turnover and regulation of nuclear function and import/export. Thus, important functional differences are present in intrafibrillar vs. perinuclear mitochondrial subpopulations.

CD95/Fas and metastatic disease: What does not kill you makes you stronger

CD95 (also known as Fas) is the prototype of death receptors; however, evidence suggests that this receptor mainly implements non-apoptotic signaling pathways such as NF-κB, MAPK, and PI3K that are involved in cell migration, differentiation, survival, and cytokine secretion. At least two different forms of CD95 L exist. The multi-aggregated transmembrane ligand (m-CD95 L) is cleaved by metalloproteases to release a homotrimeric soluble ligand (s-CD95 L). Unlike m-CD95 L, the interaction between s-CD95 L and its receptor CD95 fails to trigger apoptosis, but instead promotes calcium-dependent cell migration, which contributes to the accumulation of inflammatory Th17 cells in damaged organs of lupus patients and favors cancer cell invasiveness. Novel inhibitors targeting the pro-inflammatory roles of CD95/CD95 L may provide attractive therapeutic options for patients with chronic inflammatory disorders or cancer. This review discusses the roles of the CD95/CD95 L pair in cell migration and metastasis.

Presence of aggregates of smooth endoplasmic reticulum in MII oocytes affects oocyte competence: molecular-based evidence

STUDY QUESTION

Does the presence of aggregates of smooth endoplasmic reticulum (SERa) impact the transcriptome of human metaphase II (MII) oocytes?.

SUMMARY ANSWER

The presence of SERa alters the molecular status of human metaphase II oocytes.

WHAT IS KNOWN ALREADY

Oocytes presenting SERa are considered dysmorphic. Oocytes with SERa (SERa+) have been associated with reduced embryological outcome and increased risk of congenital anomalies, although some authors have reported that SERa+ oocytes can lead to healthy newborns. The question of whether or not SERa+ oocytes should be discarded is still open for debate, and no experimental information about the effect of the presence of SERa on the oocyte molecular status is available.

STUDY DESIGN, SIZE, DURATION

This study included 28 women, aged <38 years, without any ovarian pathology, and undergoing IVF treatment. Supernumerary MII oocytes with no sign of morphological alterations as well as SERa+ oocytes were donated after written informed consent. A total of 31 oocytes without SERa (SERa-) and 24 SERa+ oocytes were analyzed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Pools of 8-10 oocytes for both group were prepared. Total RNA was extracted from each pool, amplified, labeled and hybridized on oligonucleotide microarrays. Analyses were performed by R software using the limma package.

MAIN RESULTS AND THE ROLE OF CHANCE

The expression profiles of SERa+ oocytes significantly differed from those of SERa- oocytes in 488 probe sets corresponding to 102 down-regulated and 283 up-regulated unique transcripts. Gene Ontology analysis by DAVID bioinformatics disclosed that genes involved in three main biological processes were significantly down-regulated in SERa+ oocytes respective to SERa- oocytes: (i) cell and mitotic/meiotic nuclear division, spindle assembly, chromosome partition and G2/M transition of mitotic cell cycle; (ii) organization of cytoskeleton and microtubules; and (iii) mitochondrial structure and activity. Among the transcripts up-regulated in SERa+ oocytes, the most significantly (P = 0.002) enriched GO term was 'GoLoco motif', including the RAP1GAP, GPSM3 and GPSM1 genes.

LARGE SCALE DATA

Raw microarray data are accessible through GEO Series accession number GSE106222 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE106222).

LIMITATIONS, REASONS FOR CAUTION

Data validation in a larger cohort of samples would be beneficial, although we applied stringent criteria for gene selection (fold-change >3 or <1/3 and FDR < 0.1). Surveys on clinical outcomes, malformation rates and follow-up of babies born after transfer of embryos from SERa+ oocytes are necessary.

WIDER IMPLICATIONS OF THE FINDINGS

We provide information on the molecular status of SERa+ oocytes, highlighting possible associations between presence of SERa, altered oocyte physiology and reduced developmental competence. Our study may offer further information that can assist embryologists to make decisions on whether, and with what possible implications, SERa+ oocytes should be used. We believe that the presence of SERa should be still a 'red flag' in IVF practices and that the decision to inseminate SERa+ oocytes should be discussed on a case-by-case basis.

STUDY FUNDING/COMPETING INTEREST(s)

This study was partially supported by Ferring Pharmaceuticals. The authors have no conflicts of interest to declare.

GRKs and β-Arrestins: "Gatekeepers" of Mitochondrial Function in the Failing Heart

Mitochondrial regulation of energy production, calcium homeostasis, and cell death are critical for cardiac function. Accordingly, the structural and functional abnormalities of these organelles (mitochondrial dysfunction) contribute to developing cardiovascular diseases and heart failure. Therefore the preservation of mitochondrial integrity is essential for cardiac cell survival. Mitochondrial function is regulated by several proteins, including GRK2 and β-arrestins which act in a GPCR independent manner to orchestrate intracellular signaling associated with key mitochondrial processes. It is now ascertained that GRK2 is able to recover mitochondrial function in response to insults. β-arrestins affect several intracellular signaling pathways within the cell which in turn are involved in the regulation of mitochondrial function, but a direct regulation of mitochondria needs further investigations. In this review, we discuss the recent acquisitions on the role of GRK2 and β-arrestins in the regulation of mitochondrial function.

Mitochondrial ATP production provides long-range control of endothelial inositol trisphosphate-evoked calcium signaling

Endothelial cells are reported to be glycolytic and to minimally rely on mitochondria for ATP generation. Rather than providing energy, mitochondria in endothelial cells may act as signaling organelles that control cytosolic Ca2+ signaling or modify reactive oxygen species (ROS). To control Ca2+ signaling, these organelles are often observed close to influx and release sites and may be tethered near Ca2+ transporters. In this study, we used high-resolution, wide-field fluorescence imaging to investigate the regulation of Ca2+ signaling by mitochondria in large numbers of endothelial cells (∼50 per field) in intact arteries from rats. We observed that mitochondria were mostly spherical or short-rod structures and were distributed widely throughout the cytoplasm. The density of these organelles did not increase near contact sites with smooth muscle cells. However, local inositol trisphosphate (IP3)-mediated Ca2+ signaling predominated near these contact sites and required polarized mitochondria. Of note, mitochondrial control of Ca2+ signals occurred even when mitochondria were far from Ca2+ release sites. Indeed, the endothelial mitochondria were mobile and moved throughout the cytoplasm. Mitochondrial control of Ca2+ signaling was mediated by ATP production, which, when reduced by mitochondrial depolarization or ATP synthase inhibition, eliminated local IP3-mediated Ca2+ release events. ROS buffering did not significantly alter local Ca2+ release events. These results highlight the importance of mitochondrial ATP production in providing long-range control of endothelial signaling via IP3-evoked local Ca2+ release in intact endothelium.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Astrocytic metabolic switch is a novel etiology for Cocaine and HIV-1 Tat-mediated neurotoxicity

Calcium (Ca2+) dynamics and oxidative signaling control mitochondrial bioenergetics in the central nervous system, where astrocytes are a major energy source for neurons. Cocaine use exacerbates HIV-associated neurocognitive disorders, but little is known about disruptions in astrocyte metabolism in this context. Our data show that the HIV protein Tat and cocaine induce a metabolic switch from glucose to fatty acid oxidation in astrocytes, thereby limiting lactate transport to neurons. Mechanistic analyses revealed increased Mitochondrial Ca2+ Uniporter (MCU)-mediated Ca2+ uptake in astrocytes exposed to Tat and cocaine due to oxidation of MCU. Since our data suggest that mitochondrial oxidation is dependent in part on MCU-mediated Ca2+ uptake, we targeted MCU to restore glycolysis in astrocytes to normalize extracellular lactate levels. Knocking down MCU in astrocytes prior to Tat and cocaine exposure prevented metabolic switching and protected neurons. These findings identify a novel molecular mechanism underlying neuropathogenesis in HIV and cocaine use.

Calcium controls the formation of vacuoles from mitochondria to regulate microspore development in wheat

Potassium antimonite was used to investigate the localisation of calcium in developing wheat anthers to examine the relationship between Ca²⁺ and pollen development. During anther development, calcium precipitate formation increased in anther wall cells prior to microspore mother cell meiosis and appeared in microspores, suggesting the presence of a calcium influx from anther wall cells into the locule. Initially, the precipitates in microspore cytoplasm primarily accumulated in the mitochondria and destroyed their inner membranes (cisterns) to become small vacuoles, which expanded and fused, ultimately becoming a large vacuole during microspore vacuolisation. After microspore division and large vacuole decomposition, many calcium precipitates again accumulated in the small vacuoles, indicating that calcium from the large vacuole moved back into the cytoplasm of bicellular pollen.

Mitochondrial Ca2+-activated K+ channels and their role in cell life and death pathways

Ca²⁺-activated K⁺ channels (K_Ca) are expressed at the plasma membrane and in cellular organelles. Expression of all K_Ca channel subtypes (BK, IK and SK) has been detected at the inner mitochondrial membrane of several cell types. Primary functions of these mitochondrial K_Ca channels include the regulation of mitochondrial ROS production, maintenance of the mitochondrial membrane potential and preservation of mitochondrial calcium homeostasis. These channels are therefore thought to contribute to cellular protection against oxidative stress through mitochondrial mechanisms of preconditioning. In this review, we summarize the current knowledge on mitochondrial KCa channels, and their role in mitochondrial function in relation to cell death and survival pathways. More specifically, we systematically discuss studies on the role of these mitochondrial K_Ca channels in pharmacological preconditioning, and according protective effects on ischemic insults to the brain and the heart.

The mitochondrial phosphate carrier: Role in oxidative metabolism, calcium handling and mitochondrial disease

The mitochondrial phosphate carrier (PiC) is a mitochondrial solute carrier protein, which is encoded by SLC25A3 in humans. PiC delivers phosphate, a key substrate of oxidative phosphorylation, across the inner mitochondrial membrane. This transport activity is also relevant for allowing effective mitochondrial calcium handling. Furthermore, PiC has also been described to affect cell survival mechanisms via interactions with cyclophilin D and the viral mitochondrial-localized inhibitor of apoptosis (vMIA). The significance of PiC has been supported by the recent discovery of a fatal human condition associated with PiC mutations. Here, we present first the early studies that lead to the discovery and molecular characterization of the PiC, then discuss the very recently developed mouse models for PiC and pathological mutations in the human SLC25A3 gene.

Proteomic analysis of mitochondrial-associated ER membranes (MAM) during RNA virus infection reveals dynamic changes in protein and organelle trafficking

RIG-I pathway signaling of innate immunity against RNA virus infection is organized between the ER and mitochondria on a subdomain of the ER called the mitochondrial-associated ER membrane (MAM). The RIG-I adaptor protein MAVS transmits downstream signaling of antiviral immunity, with signaling complexes assembling on the MAM in association with mitochondria and peroxisomes. To identify components that regulate MAVS signalosome assembly on the MAM, we characterized the proteome of MAM, ER, and cytosol from cells infected with either chronic (hepatitis C) or acute (Sendai) RNA virus infections, as well as mock-infected cells. Comparative analysis of protein trafficking dynamics during both chronic and acute viral infection reveals differential protein profiles in the MAM during RIG-I pathway activation. We identified proteins and biochemical pathways recruited into and out of the MAM in both chronic and acute RNA viral infections, representing proteins that drive immunity and/or regulate viral replication. In addition, by using this comparative proteomics approach, we identified 3 new MAVS-interacting proteins, RAB1B, VTN, and LONP1, and defined LONP1 as a positive regulator of the RIG-I pathway. Our proteomic analysis also reveals a dynamic cross-talk between subcellular compartments during both acute and chronic RNA virus infection, and demonstrates the importance of the MAM as a central platform that coordinates innate immune signaling to initiate immunity against RNA virus infection.

Dysregulation of calcium homeostasis in muscular dystrophies

Muscular dystrophies are a group of diseases characterised by the primary wasting of skeletal muscle, which compromises patient mobility and in the most severe cases originate a complete paralysis and premature death. Existing evidence implicates calcium dysregulation as an underlying crucial event in the pathophysiology of several muscular dystrophies, such as dystrophinopathies, calpainopathies or myotonic dystrophy among others. Duchenne muscular dystrophy is the most frequent myopathy in childhood, and calpainopathy or LGMD2A is the most common form of limb-girdle muscular dystrophy, whereas myotonic dystrophy is the most frequent inherited muscle disease worldwide. In this review, we summarise recent advances in our understanding of calcium ion cycling through the sarcolemma, the sarcoplasmic reticulum and mitochondria, and its involvement in the pathogenesis of these dystrophies. We also discuss some of the clinical implications of recent findings regarding Ca2+ handling as well as novel approaches to treat muscular dystrophies targeting Ca2+ regulatory proteins.

Effect of α, β momorcharin on viability, caspase activity, cytochrome c release and on cytosolic calcium levels in different cancer cell lines

A multitude of plants have been used extensively for the treatment of cancers throughout the world. The protein, α, β momorcharin has been extracted from the plant Momordica charantia (MC), and it possesses anti-cancer and anti-HIV properties similar to the crude water and methanol soluble extract of the plant. This study investigated the anti-cancer effects and the cellular mechanisms of action of α, β momocharin (200-800 μM) on 1321N1, Gos-3, U87-MG, Sk Mel, Corl-23 and Weri Rb-1 cancer cell lines compared to normal healthy L6 muscle cell line measuring cell viability using MTT assay kit, Caspase-3 and 9 activities, cytochrome c release and intracellular free calcium concentrations [Ca(2+)]i. The results show that α, β momorcharin can evoke significant dose-dependent (P < 0.05; Student's t test) decreases in the viability (increases in cell death) of 1321N1, Gos-3, U87-MG, Sk Mel, Corl-23 and Weri Rb-1 cancer cell lines compared to healthy L6 muscle cell line and untreated glioma cells. α, β momorcharin (800 μM) also evoked significant (P < 0.05) increases in caspase-3 and 9 activities and cytochrome c release. Similarly, α, β momorcharin elicited significant (P < 0.05) time-dependent elevation in [Ca(2+)]i in all five glioma cell lines compared to untreated cells. Together, the results have demonstrated that α, β momorcharin can exert its anti-cancer effect on different cancer cell lines by intracellular processes involving an insult to the mitochondria resulting in cellular calcium over loading, apoptosis, cytochrome release and subsequently, cell death.

Membrane associated complexes in calcium dynamics modelling

Mitochondria not only govern energy production, but are also involved in crucial cellular signalling processes. They are one of the most important organelles determining the Ca(2+) regulatory pathway in the cell. Several mathematical models explaining these mechanisms were constructed, but only few of them describe interplay between calcium concentrations in endoplasmic reticulum (ER), cytoplasm and mitochondria. Experiments measuring calcium concentrations in mitochondria and ER suggested the existence of cytosolic microdomains with locally elevated calcium concentration in the nearest vicinity of the outer mitochondrial membrane. These intermediate physical connections between ER and mitochondria are called MAM (mitochondria-associated ER membrane) complexes. We propose a model with a direct calcium flow from ER to mitochondria, which may be justified by the existence of MAMs, and perform detailed numerical analysis of the effect of this flow on the type and shape of calcium oscillations. The model is partially based on the Marhl et al model. We have numerically found that the stable oscillations exist for a considerable set of parameter values. However, for some parameter sets the oscillations disappear and the trajectories of the model tend to a steady state with very high calcium level in mitochondria. This can be interpreted as an early step in an apoptotic pathway.

Mitochondria as a source of reactive oxygen and nitrogen species: from molecular mechanisms to human health

Mitochondrially generated reactive oxygen species are involved in a myriad of signaling and damaging pathways in different tissues. In addition, mitochondria are an important target of reactive oxygen and nitrogen species. Here, we discuss basic mechanisms of mitochondrial oxidant generation and removal and the main factors affecting mitochondrial redox balance. We also discuss the interaction between mitochondrial reactive oxygen and nitrogen species, and the involvement of these oxidants in mitochondrial diseases, cancer, neurological, and cardiovascular disorders.

Pharmacological approaches to the treatment of oxidative stress-induced cardiovascular dysfunctions

Cardiovascular diseases are a growing major global health problem. Our understanding of the mechanisms of pathophysiology of cardiovascular diseases has been gaining significant advances and a wealth of knowledge implicates oxidative stress as a key causative agent. However, to date, most efforts to treat heart failure using conventional antioxidant therapies have been less than encouraging. With increasing incidences of cardiovascular disease in young as well as in aging populations, and the problem of long-term diminishing efficacy of conventional therapeutics, the need for new treatments has never been greater. In this review, a variety of therapeutic targets and compounds applied to treat cardiovascular diseases via inhibition of oxidative stress are presented.

Effect of potential-dependent potassium uptake on calcium accumulation in rat brain mitochondria

The effect of potential-dependent potassium uptake at 0-120 mM K+ on matrix Ca2+ accumulation in rat brain mitochondria was studied. An increase in oxygen consumption and proton extrusion rates as well as increase in matrix pH with increase in K+ content in the medium was observed due to K+ uptake into the mitochondria. The accumulation of Ca2+ was shown to depend on K+ concentration in the medium. At K+ concentration ≤30 mM, Ca2+ uptake is decreased due to K+-induced membrane depolarization, whereas at higher K+ concentrations, up to 120 mM K+, Ca2+ uptake is increased in spite of membrane depolarization caused by matrix alkalization due to K+ uptake. Mitochondrial K+(ATP)-channel blockers (glibenclamide and 5-hydroxydecanoic acid) diminish K+ uptake as well as K+-induced depolarization and matrix alkalization, which results in attenuation of the potassium-induced effects on matrix Ca2+ uptake, i.e. increase in Ca2+ uptake at low K+ content in the medium due to the smaller membrane depolarization and decrease in Ca2+ uptake at high potassium concentrations because of restricted rise in matrix pH. The results show the importance of potential-dependent potassium uptake, and especially the K+(ATP) channel, in the regulation of calcium accumulation in rat brain mitochondria.

Dynamics of matrix-free Ca2+ in cardiac mitochondria: two components of Ca2+ uptake and role of phosphate buffering

Mitochondrial Ca(2+) uptake is thought to provide an important signal to increase energy production to meet demand but, in excess, can also trigger cell death. The mechanisms defining the relationship between total Ca(2+) uptake, changes in mitochondrial matrix free Ca(2+), and the activation of the mitochondrial permeability transition pore (PTP) are not well understood. We quantitatively measure changes in [Ca(2+)](out) and [Ca(2+)](mito) during Ca(2+) uptake in isolated cardiac mitochondria and identify two components of Ca(2+) influx. [Ca(2+)](mito) recordings revealed that the first, MCU(mode1), required at least 1 µM Ru360 to be completely inhibited, and responded to small Ca(2+) additions in the range of 0.1 to 2 µM with rapid and large changes in [Ca(2+)](mito). The second component, MCU(mode2), was blocked by 100 nM Ru360 and was responsible for the bulk of total Ca(2+) uptake for large Ca(2+) additions in the range of 2 to 10 µM; however, it had little effect on steady-state [Ca(2+)](mito). MCU(mode1) mediates changes in [Ca(2+)](mito) of 10s of μM, even in the presence of 100 nM Ru360, indicating that there is a finite degree of Ca(2+) buffering in the matrix associated with this pathway. In contrast, the much higher Ca(2+) loads evoked by MCU(mode2) activate a secondary dynamic Ca(2+) buffering system consistent with calcium-phosphate complex formation. Increasing P(i) potentiated [Ca(2+)](mito) increases via MCU(mode1) but suppressed [Ca(2+)](mito) changes via MCU(mode2). The results suggest that the role of MCU(mode1) might be to modulate oxidative phosphorylation in response to intracellular Ca(2+) signaling, whereas MCU(mode2) and the dynamic high-capacity Ca(2+) buffering system constitute a Ca(2+) sink function. Interestingly, the trigger for PTP activation is unlikely to be [Ca(2+)](mito) itself but rather a downstream byproduct of total mitochondrial Ca(2+) loading.

Measuring local gradients of intramitochondrial [Ca(2+)] in cardiac myocytes during sarcoplasmic reticulum Ca(2+) release

RATIONALE

Mitochondrial [Ca(2+)] ([Ca(2+)](mito)) regulates mitochondrial energy production, provides transient Ca(2+) buffering under stress, and can be involved in cell death. Mitochondria are near the sarcoplasmic reticulum (SR) in cardiac myocytes, and evidence for crosstalk exists. However, quantitative measurements of [Ca(2+)](mito) are limited, and spatial [Ca(2+)](mito) gradients have not been directly measured.

OBJECTIVE

To directly measure local [Ca(2+)](mito) during normal SR Ca release in intact myocytes, and evaluate potential subsarcomeric spatial [Ca(2+)](mito) gradients.

METHODS AND RESULTS

Using the mitochondrially targeted inverse pericam indicator Mitycam, calibrated in situ, we directly measured [Ca(2+)](mito) during SR Ca(2+) release in intact rabbit ventricular myocytes by confocal microscopy. During steady state pacing, Δ[Ca(2+)](mito) amplitude was 29±3 nmol/L, rising rapidly (similar to cytosolic free [Ca(2+)]) but declining much more slowly. Taking advantage of the structural periodicity of cardiac sarcomeres, we found that [Ca(2+)](mito) near SR Ca(2+) release sites (Z-line) versus mid-sarcomere (M-line) reached a high peak amplitude (37±4 versus 26±4 nmol/L, respectively P<0.05) which occurred earlier in time. This difference was attributed to ends of mitochondria being physically closer to SR Ca(2+) release sites, because the mitochondrial Ca(2+) uniporter was homogeneously distributed, and elevated [Ca(2+)] applied laterally did not produce longitudinal [Ca(2+)](mito) gradients.

CONCLUSIONS

We developed methods to measure spatiotemporal [Ca(2+)](mito) gradients quantitatively during excitation-contraction coupling. The amplitude and kinetics of [Ca(2+)](mito) transients differ significantly from those in the cytosol and are respectively higher and faster near the Z-line versus M-line. This approach will help clarify SR-mitochondrial Ca(2+) signaling.

High Ca2+ load promotes hydrogen peroxide generation via activation of α-glycerophosphate dehydrogenase in brain mitochondria

H(2)O(2) generation associated with α-glycerophosphate (α-GP) oxidation was addressed in guinea pig brain mitochondria challenged with high Ca(2+) load (10 μM). Exposure to 10 μM Ca(2+) induced an abrupt 2.5-fold increase in H(2)O(2) release compared to that measured in the presence of a physiological cytosolic Ca(2+) concentration (100 nM) from mitochondria respiring on 5 mM α-GP in the presence of ADP (2 mM). The Ca(2+)-induced stimulation of H(2)O(2) generation was reversible and unaltered by the uniporter blocker Ru 360, indicating that it did not require Ca(2+) uptake into mitochondria. Enhanced H(2)O(2) generation by Ca(2+) was also observed in the absence of ADP when mitochondria exhibited permeability transition pore opening with a decrease in the NAD(P)H level, dissipation of membrane potential, and mitochondrial swelling. Furthermore, mitochondria treated with the pore-forming peptide alamethicin also responded with an elevated H(2)O(2) generation to a challenge with 10 μM Ca(2+). Ca(2+)-induced promotion of H(2)O(2) formation was further enhanced by the complex III inhibitor myxothiazol. With 20 mM α-GP concentration, stimulation of H(2)O(2) formation by Ca(2+) was detected only in the presence, not in the absence, of ADP. It is concluded that α-glycerophosphate dehydrogenase, which is accessible to and could be activated by a rise in the level of cytosolic Ca(2+), makes a major contribution to Ca(2+)-stimulated H(2)O(2) generation. This work highlights a unique high-Ca(2+)-stimulated reactive oxygen species-forming mechanism in association with oxidation of α-GP, which is largely independent of the bioenergetic state and can proceed even in damaged, functionally incompetent mitochondria.

Mitochondria at the interface between danger signaling and metabolism: role of unfolded protein responses in chronic inflammation

Inflammatory bowel diseases (IBDs), like many other chronic diseases, feature multiple cellular stress responses including endoplasmic reticulum (ER) unfolded protein response (UPR). Maintaining protein homeostasis is indispensable for cell survival and, consequently, distinct signaling pathways have evolved to transmit organelle stress. While the ER UPR, aiming to restore ER homeostasis after challenges to ER function, has been extensively studied in the context of chronic diseases, only recently the related mitochondrial UPR (mtUPR), induced by disturbances of mitochondrial proteostasis, has drawn some attention. ER and mitochondria are in close contact and interact physically and functionally. Accumulating data have placed mitochondria at the center of diverse cellular functions and suggest mitochondria as integrators of signaling pathways such as autophagy and inflammation. Consequently, it is likely that mitochondrial stress and ER stress cannot be regarded separately and that mitochondrial stress, as well as ER stress, participates in the pathology of IBD. Protein homeostasis is particularly sensitive toward infections, oxidative stress, and energy deficiency. Thus, environmental disturbances impacting organelle function lead to the concerted activation of distinct UPRs. The metabolic status might therefore serve as an innate mechanism to sense the epithelial environment, including luminal-derived and host-derived factors. This review highlights mtUPR and its interrelation with ER UPR, focuses on recent studies identifying mitochondria as integrators of cellular danger signaling, and, furthermore, illustrates the importance ER UPR and mitochondrial dysfunction in IBD.

Mitochondrial- and endoplasmic reticulum-associated oxidative stress in Alzheimer's disease: from pathogenesis to biomarkers

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, affecting several million of people worldwide. Pathological changes in the AD brain include the presence of amyloid plaques, neurofibrillary tangles, loss of neurons and synapses, and oxidative damage. These changes strongly associate with mitochondrial dysfunction and stress of the endoplasmic reticulum (ER). Mitochondrial dysfunction is intimately linked to the production of reactive oxygen species (ROS) and mitochondrial-driven apoptosis, which appear to be aggravated in the brain of AD patients. Concomitantly, mitochondria are closely associated with ER, and the deleterious crosstalk between both organelles has been shown to be involved in neuronal degeneration in AD. Stimuli that enhance expression of normal and/or folding-defective proteins activate an adaptive unfolded protein response (UPR) that, if unresolved, can cause apoptotic cell death. ER stress also induces the generation of ROS that, together with mitochondrial ROS and decreased activity of several antioxidant defenses, promotes chronic oxidative stress. In this paper we discuss the critical role of mitochondrial and ER dysfunction in oxidative injury in AD cellular and animal models, as well as in biological fluids from AD patients. Progress in developing peripheral and cerebrospinal fluid biomarkers related to oxidative stress will also be summarized.

Intracellular and plasma membrane-initiated pathways involved in the [Ca2+]i elevations induced by iodothyronines (T3 and T2) in pituitary GH3 cells

The role of 3,5,3'-triiodo-l-thyronine (T3) and its metabolite 3,5-diiodo-l-thyronine (T2) in modulating the intracellular Ca(2+) concentration ([Ca(2+)](i)) and endogenous nitric oxide (NO) synthesis was evaluated in pituitary GH(3) cells in the absence or presence of extracellular Ca(2+). When applied in Ca(2+)-free solution, T2 and T3 increased [Ca(2+)](i), in a dose-dependent way, and NO levels. Inhibition of neuronal NO synthase by N(G)-nitro-l-arginine methyl ester and l-n(5)-(1-iminoethyl)ornithine hydrochloride significantly reduced the [Ca(2+)](i) increase induced by T2 and T3. However, while depletion of inositol trisphosphate-dependent Ca(2+) stores did not interfere with the T2- and T3-induced [Ca(2+)](i) increases, the inhibition of phosphatidylinositol 3-kinase by LY-294002 and the dominant negative form of Akt mutated at the ATP binding site prevented these effects. Furthermore, the mitochondrial protonophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone prevented the increases in both [Ca(2+)](i) and NO elicited by T2 or T3. Interestingly, rotenone blocked the early [Ca(2+)](i) increases elicited by T2 and T3, while antimycin prevented only that elicited by T3. Inhibition of mitochondrial Na(+)/Ca(2+) exchanger by CGP37157 significantly reduced the [Ca(2+)](i) increases induced by T2 and T3. In the presence of extracellular calcium (1.2 mM), under carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, T2 and T3 increased both [Ca(2+)](i) and intracellular Na(+) concentration; nimodipine reduced the [Ca(2+)](i) increases elicited by T2 and T3, but inhibition of NO synthase and blockade of the Na(+)/H(+) pump by 5-(N-ethyl-N-isopropyl)amiloride prevented only that elicited by T3; and CB-DMB, bisindolylmaleimide, and LY-294002 (inhibitors of the Na(+)/Ca(2+) exchanger, PKC, and phosphatidylinositol 3-kinase, respectively) failed to modify the T2- and T3-induced effects. Collectively, the present results suggest that T2 and T3 exert short-term nongenomic effects on intracellular calcium and NO by modulating plasma membrane and mitochondrial pathways that differ between these iodothyronines.

Calsequestrin (CASQ1) rescues function and structure of calcium release units in skeletal muscles of CASQ1-null mice

Amplitude of Ca(2+) transients, ultrastructure of Ca(2+) release units, and molecular composition of sarcoplasmic reticulum (SR) are altered in fast-twitch skeletal muscles of calsequestrin-1 (CASQ1)-null mice. To determine whether such changes are directly caused by CASQ1 ablation or are instead the result of adaptive mechanisms, here we assessed ability of CASQ1 in rescuing the null phenotype. In vivo reintroduction of CASQ1 was carried out by cDNA electro transfer in flexor digitorum brevis muscle of the mouse. Exogenous CASQ1 was found to be correctly targeted to the junctional SR (jSR), as judged by immunofluorescence and confocal microscopy; terminal cisternae (TC) lumen was filled with electron dense material and its width was significantly increased, as judged by electron microscopy; peak amplitude of Ca(2+) transients was significantly increased compared with null muscle fibers transfected only with green fluorescent protein (control); and finally, transfected fibers were able to sustain cytosolic Ca(2+) concentration during prolonged tetanic stimulation. Only the expression of TC proteins, such as calsequestrin 2, sarcalumenin, and triadin, was not rescued as judged by Western blot. Thus our results support the view that CASQ1 plays a key role in both Ca(2+) homeostasis and TC structure.

The hepatitis B virus X protein elevates cytosolic calcium signals by modulating mitochondrial calcium uptake

Chronic hepatitis B virus (HBV) infections are associated with the development of hepatocellular carcinoma (HCC). The HBV X protein (HBx) is thought to play an important role in the development of HBV-associated HCC. One fundamental HBx function is elevation of cytosolic calcium signals; this HBx activity has been linked to HBx stimulation of cell proliferation and transcription pathways, as well as HBV replication. Exactly how HBx elevates cytosolic calcium signals is not clear. The studies described here show that HBx stimulates calcium entry into cells, resulting in an increased plateau level of inositol 1,4,5-triphosphate (IP3)-linked calcium signals. This increased calcium plateau can be inhibited by blocking mitochondrial calcium uptake and store-operated calcium entry (SOCE). Blocking SOCE also reduced HBV replication. Finally, these studies also demonstrate that there is increased mitochondrial calcium uptake in HBx-expressing cells. Cumulatively, these studies suggest that HBx can increase mitochondrial calcium uptake and promote increased SOCE to sustain higher cytosolic calcium and stimulate HBV replication.