Mitochondrial regulation of sarcoplasmic reticulum Ca2+ content in vascular smooth muscle cells

Cellular communication among smooth muscle cells: The role of membrane potential via connexins

Communication via action potentials among neurons has been extensively studied. However, effective communication without action potentials is ubiquitous in biological systems, yet it has received much less attention in comparison. Multi-cellular communication among smooth muscles is crucial for regulating blood flow, for example. Understanding the mechanism of this non-action potential communication is critical in many cases, like synchronization of cellular activity, under normal and pathological conditions. In this paper, we employ a multi-scale asymptotic method to derive a macroscopic homogenized bidomain model from the microscopic electro-neutral (EN) model. This is achieved by considering different diffusion coefficients and incorporating nonlinear interface conditions. Subsequently, the homogenized macroscopic model is used to investigate communication in multi-cellular tissues. Our computational simulations reveal that the membrane potential of syncytia, formed by interconnected cells via connexins, plays a crucial role in propagating oscillations from one region to another, providing an effective means for fast cellular communication. Statement of Significance: In this study, we investigated cellular communication and ion transport in vascular smooth muscle cells, shedding light on their mechanisms under normal and abnormal conditions. Our research highlights the potential of mathematical models in understanding complex biological systems. We developed effective macroscale electro-neutral bi-domain ion transport models and examined their behavior in response to different stimuli. Our findings revealed the crucial role of connexinmediated membrane potential changes and demonstrated the effectiveness of cellular communication through syncytium membranes. Despite some limitations, our study provides valuable insights into these processes and emphasizes the importance of mathematical modeling in unraveling the complexities of cellular communication and ion transport.

RyR2 Serine-2030 PKA Site Governs Ca2+ Release Termination and Ca2+ Alternans

BACKGROUND

PKA (protein kinase A)-mediated phosphorylation of cardiac RyR2 (ryanodine receptor 2) has been extensively studied for decades, but the physiological significance of PKA phosphorylation of RyR2 remains poorly understood. Recent determination of high-resolution 3-dimensional structure of RyR2 in complex with CaM (calmodulin) reveals that the major PKA phosphorylation site in RyR2, serine-2030 (S2030), is located within a structural pathway of CaM-dependent inactivation of RyR2. This novel structural insight points to a possible role of PKA phosphorylation of RyR2 in CaM-dependent inactivation of RyR2, which underlies the termination of Ca²⁺ release and induction of cardiac Ca²⁺ alternans.

METHODS

We performed single-cell endoplasmic reticulum Ca²⁺ imaging to assess the impact of S2030 mutations on Ca²⁺ release termination in human embryonic kidney 293 cells. Here we determined the role of the PKA site RyR2-S2030 in a physiological setting, we generated a novel mouse model harboring the S2030L mutation and carried out confocal Ca²⁺ imaging.

RESULTS

We found that mutations, S2030D, S2030G, S2030L, S2030V, and S2030W reduced the endoplasmic reticulum luminal Ca²⁺ level at which Ca²⁺ release terminates (the termination threshold), whereas S2030P and S2030R increased the termination threshold. S2030A and S2030T had no significant impact on release termination. Furthermore, CaM-wild-type increased, whereas Ca²⁺ binding deficient CaM mutant (CaM-M [a loss-of-function CaM mutation with all 4 EF-hand motifs mutated]), PKA, and Ca²⁺/CaMKII (CaM-dependent protein kinase II) reduced the termination threshold. The S2030L mutation abolished the actions of CaM-wild-type, CaM-M, and PKA, but not CaMKII, in Ca²⁺ release termination. Moreover, we showed that isoproterenol and CaM-M suppressed pacing-induced Ca²⁺ alternans and accelerated Ca²⁺ transient recovery in intact working hearts, whereas CaM-wild-type exerted an opposite effect. The impact of isoproterenol was partially and fully reversed by the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide and the CaMKII inhibitor N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide individually and together, respectively. S2030L abolished the impact of CaM-wild-type, CaM-M, and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide-sensitive component, but not the N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide-sensitive component, of isoproterenol.

Essential role of the mitochondrial Na+/Ca2+ exchanger NCLX in mediating PDE2-dependent neuronal survival and learning

Impaired phosphodiesterase (PDE) function and mitochondrial Ca2+ (i.e., [Ca2+]m) lead to multiple health syndromes by an unknown pathway. Here, we fluorescently monitor robust [Ca2+]m efflux mediated by the mitochondrial Na+/Ca2+ exchanger NCLX in hippocampal neurons sequentially evoked by caffeine and depolarization. Surprisingly, neuronal depolarization-induced Ca2+ transients alone fail to evoke strong [Ca2+]m efflux in wild-type (WT) neurons. However, pre-treatment with the selective PDE2 inhibitor Bay 60-7550 effectively rescues [Ca2+]m efflux similarly to caffeine. Moreover, PDE2 acts by diminishing mitochondrial cAMP, thus promoting NCLX phosphorylation at its PKA site. We find that the protection of neurons against excitotoxic insults, conferred by PDE2 inhibition in WT neurons, is NCLX dependent. Finally, the administration of Bay 60-7550 enhances new object recognition in WT, but not in NCLX knockout (KO), mice. Our results identify a link between PDE and [Ca2+]m signaling that may provide effective therapy for cognitive and ischemic syndromes.

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

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

In cardiomyocytes, to carry out cell contraction, the distribution, morphology, and dynamic interaction of different cellular organelles are tightly regulated. For instance, the repetitive close apposition between junctional sarcoplasmic reticulum (jSR) and specialized sarcolemma invaginations, called transverse-tubules (TTs), is essential for an efficient excitation-contraction coupling (ECC). Upon an action potential, Ca²⁺ microdomains, generated in synchrony at the interface between TTs and jSR, underlie the prompt increase in cytosolic Ca²⁺ concentration, ultimately responsible for cell contraction during systole. This process requires a considerable amount of energy and the active participation of mitochondria, which encompass ∼30% of the cell volume and represent the major source of ATP in the heart. Importantly, in adult cardiomyocytes, mitochondria are distributed in a highly orderly fashion and strategically juxtaposed with SR. By taking advantage of the vicinity to Ca²⁺ releasing sites, they take up Ca²⁺ and modulate ATP synthesis according to the specific cardiac workload. Interestingly, with respect to SR, a biased, polarized positioning of mitochondrial Ca²⁺ uptake/efflux machineries has been reported, hinting the importance of a strictly regulated mitochondrial Ca²⁺ handling for heart activity. This notion, however, has been questioned by the observation that, in some mouse models, the deficiency of specific molecules, modulating mitochondrial Ca²⁺ dynamics, triggers non-obvious cardiac phenotypes. This review will briefly summarize the physiological significance of SR-mitochondria apposition in cardiomyocytes, as well as the pathological consequences of an altered organelle communication, focusing on Ca²⁺ signaling. We will discuss ongoing debates and propose future research directions.

Loss of MCU prevents mitochondrial fusion in G1-S phase and blocks cell cycle progression and proliferation

The role of the mitochondrial Ca²⁺ uniporter (MCU) in physiologic cell proliferation remains to be defined. Here, we demonstrated that the MCU was required to match mitochondrial function to metabolic demands during the cell cycle. During the G₁-S transition (the cycle phase with the highest mitochondrial ATP output), mitochondrial fusion, oxygen consumption, and Ca²⁺ uptake increased in wild-type cells but not in cells lacking MCU. In proliferating wild-type control cells, the addition of the growth factors promoted the activation of the Ca²⁺/calmodulin-dependent kinase II (CaMKII) and the phosphorylation of the mitochondrial fission factor Drp1 at Ser⁶¹⁶ The lack of the MCU was associated with baseline activation of CaMKII, mitochondrial fragmentation due to increased Drp1 phosphorylation, and impaired mitochondrial respiration and glycolysis. The mitochondrial fission/fusion ratio and proliferation in MCU-deficient cells recovered after MCU restoration or inhibition of mitochondrial fragmentation or of CaMKII in the cytosol. Our data highlight a key function for the MCU in mitochondrial adaptation to the metabolic demands during cell cycle progression. Cytosolic CaMKII and the MCU participate in a regulatory circuit, whereby mitochondrial Ca²⁺ uptake affects cell proliferation through Drp1.

Spatial Separation of Mitochondrial Calcium Uptake and Extrusion for Energy-Efficient Mitochondrial Calcium Signaling in the Heart

Mitochondrial Ca2+ elevations enhance ATP production, but uptake must be balanced by efflux to avoid overload. Uptake is mediated by the mitochondrial Ca2+ uniporter channel complex (MCUC), and extrusion is controlled largely by the Na+/Ca2+ exchanger (NCLX), both driven electrogenically by the inner membrane potential (ΔΨm). MCUC forms hotspots at the cardiac mitochondria-junctional SR (jSR) association to locally receive Ca2+ signals; however, the distribution of NCLX is unknown. Our fractionation-based assays reveal that extensively jSR-associated mitochondrial segments contain a minor portion of NCLX and lack Na+-dependent Ca2+ extrusion. This pattern is retained upon in vivo NCLX overexpression, suggesting extensive targeting to non-jSR-associated submitochondrial domains and functional relevance. In cells with non-polarized MCUC distribution, upon NCLX overexpression the same given increase in matrix Ca2+ expends more ΔΨm. Thus, cardiac mitochondrial Ca2+ uptake and extrusion are reciprocally polarized, likely to optimize the energy efficiency of local calcium signaling in the beating heart.

Droplet digital PCR shows the D-Loop to be an error prone locus for mitochondrial DNA copy number determination

Absolute quantification of mitochondrial DNA copy number (mCN) provides important insights in many fields of research including cancer, cardiovascular and reproductive health. Droplet digital PCR (ddPCR) natively reports absolute copy number, and we have developed a single-dye, multiplex assay to measure rat mCN that is accurate, precise and affordable. We demonstrate simple methods to optimize this assay and to determine nuclear reference pseudogene copy number to extend the range of mCN that can be measured with this assay. We evaluated two commonly used mitochondrial DNA reference loci to determine mCN, the ND1 gene and the D-Loop. Harnessing the absolute measures of ddPCR, we found that the D-Loop amplifies with a copy number of ~1.0–1.5 relative to other sites on the mitochondrial genome. This anomalous copy number varied significantly between rats and tissues (aorta, brain, heart, liver, soleus muscle). We advocate for avoiding the D-Loop as a mitochondrial reference in future studies of mCN. Further, we report a novel approach to quantifying immunolabelled mitochondrial DNA that provides single-cell estimates of mCN that closely agree with the population analyses by ddPCR. The combination of these assays represents a cost-effective and powerful suite of tools to study mCN.

CaMKII (Ca2+/Calmodulin-Dependent Kinase II) in Mitochondria of Smooth Muscle Cells Controls Mitochondrial Mobility, Migration, and Neointima Formation

OBJECTIVE

The main objective of this study is to define the mechanisms by which mitochondria control vascular smooth muscle cell (VSMC) migration and impact neointimal hyperplasia.

APPROACH AND RESULTS

The multifunctional CaMKII (Ca²⁺/calmodulin-dependent kinase II) in the mitochondrial matrix of VSMC drove a feed-forward circuit with the mitochondrial Ca²⁺ uniporter (MCU) to promote matrix Ca²⁺ influx. MCU was necessary for the activation of mitochondrial CaMKII (mtCaMKII), whereas mtCaMKII phosphorylated MCU at the regulatory site S92 that promotes Ca²⁺ entry. mtCaMKII was necessary and sufficient for platelet-derived growth factor-induced mitochondrial Ca²⁺ uptake. This effect was dependent on MCU. mtCaMKII and MCU inhibition abrogated VSMC migration and mitochondrial translocation to the leading edge. Overexpression of wild-type MCU, but not MCU S92A, mutant in MCU^-/- VSMC rescued migration and mitochondrial mobility. Inhibition of microtubule, but not of actin assembly, blocked mitochondrial mobility. The outer mitochondrial membrane GTPase Miro-1 promotes mitochondrial mobility via microtubule transport but arrests it in subcellular domains of high Ca²⁺ concentrations. In Miro-1^-/- VSMC, mitochondrial mobility and VSMC migration were abolished, and overexpression of mtCaMKII or a CaMKII inhibitory peptide in mitochondria (mtCaMKIIN) had no effect. Consistently, inhibition of mtCaMKII increased and prolonged cytosolic Ca²⁺ transients. mtCaMKII inhibition diminished phosphorylation of focal adhesion kinase and myosin light chain, leading to reduced focal adhesion turnover and cytoskeletal remodeling. In a transgenic model of selective mitochondrial CaMKII inhibition in VSMC, neointimal hyperplasia was significantly reduced after vascular injury.

CONCLUSIONS

These findings identify mitochondrial CaMKII as a key regulator of mitochondrial Ca²⁺ uptake via MCU, thereby controlling mitochondrial translocation and VSMC migration after vascular injury.

Crosslink between calcium and sodium signalling

NEW FINDINGS

What is the topic of this review? This paper overviews the links between Ca2+ and Na+ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na+ and Ca2+ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na+ -Ca2+ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca2+ and Na+ signalling for co-ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed.

ABSTRACT

Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca2+ and Na+ is tightly linked through several molecular pathways that generate Ca2+ and Na+ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca2+ release with generation of Na+ and Ca2+ currents. The plasmalemmal Na+ -Ca2+ exchanger (NCX) flickers between forward and reverse mode to co-ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na+ -Ca2+ exchanger (NCLX) mediate Ca2+ entry into and release from this organelle and couple cytosolic Ca2+ and Na+ fluctuations with cellular energetics. Cellular Ca2+ and Na+ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.

TNFα decreases mitochondrial movement in human airway smooth muscle

In airway smooth muscle (ASM) cells, excitation-contraction coupling is accomplished via a cascade of events that connect an elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) with cross-bridge attachment and ATP-consuming mechanical work. Excitation-energy coupling is mediated by linkage of the elevation of [Ca²⁺]_cyt to an increase in mitochondrial Ca²⁺ concentration, which in turn stimulates ATP production. Proximity of mitochondria to the sarcoplasmic reticulum (SR) and plasma membrane is thought to be an important mechanism to facilitate mitochondrial Ca²⁺ uptake. In this regard, mitochondrial movement in ASM cells may be key in establishing proximity. Mitochondria also move where ATP or Ca²⁺ buffering is needed. Mitochondrial movement is mediated through interactions with the Miro-Milton molecular complex, which couples mitochondria to kinesin motors at microtubules. We examined mitochondrial movement in human ASM cells and hypothesized that, at basal [Ca²⁺]_cyt levels, mitochondrial movement is necessary to establish proximity of mitochondria to the SR and that, during the transient increase in [Ca²⁺]_cyt induced by agonist stimulation, mitochondrial movement is reduced, thereby promoting transient mitochondrial Ca²⁺ uptake. We further hypothesized that airway inflammation disrupts basal mitochondrial movement via a reduction in Miro and Milton expression, thereby disrupting the ability of mitochondria to establish proximity to the SR and, thus, reducing transient mitochondrial Ca²⁺ uptake during agonist activation. The reduced proximity of mitochondria to the SR may affect establishment of transient "hot spots" of higher [Ca²⁺]_cyt at the sites of SR Ca²⁺ release that are necessary for mitochondrial Ca²⁺ uptake via the mitochondrial Ca²⁺ uniporter.

Mitochondria control store-operated Ca2+ entry through Na+ and redox signals

Mitochondria exert important control over plasma membrane (PM) Orai1 channels mediating store-operated Ca²⁺ entry (SOCE). Although the sensing of endoplasmic reticulum (ER) Ca²⁺ stores by STIM proteins and coupling to Orai1 channels is well understood, how mitochondria communicate with Orai1 channels to regulate SOCE activation remains elusive. Here, we reveal that SOCE is accompanied by a rise in cytosolic Na⁺ that is critical in activating the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) causing enhanced mitochondrial Na⁺ uptake and Ca²⁺ efflux. Omission of extracellular Na⁺ prevents the cytosolic Na⁺ rise, inhibits NCLX activity, and impairs SOCE and Orai1 channel current. We show further that SOCE activates a mitochondrial redox transient which is dependent on NCLX and is required for preventing Orai1 inactivation through oxidation of a critical cysteine (Cys195) in the third transmembrane helix of Orai1. We show that mitochondrial targeting of catalase is sufficient to rescue redox transients, SOCE, and Orai1 currents in NCLX-deficient cells. Our findings identify a hitherto unknown NCLX-mediated pathway that coordinates Na⁺ and Ca²⁺ signals to effect mitochondrial redox control over SOCE.

Calumenin contributes to ER-Ca2+ homeostasis in bronchial epithelial cells expressing WT and F508del mutated CFTR and to F508del-CFTR retention

Cystic Fibrosis (CF) is the most frequent fatal genetic disease in Caucasian populations. Mutations in the chloride channel CF Transmembrane Conductance Regulator (CFTR) gene are responsible for functional defects of the protein and multiple associated dysregulations. The most common mutation in patients with CF, F508del-CFTR, causes defective CFTR protein folding. Thus minimal levels of the receptor are expressed at the cell surface as the mutated CFTR is retained in the endoplasmic reticulum (ER) where it correlates with defective calcium (Ca²⁺) homeostasis. In this study, we discovered that the Ca²⁺ binding protein Calumenin (CALU) is a key regulator in the maintenance of ER-Ca²⁺ calcium homeostasis in both wild type and F508del-CFTR expressing cells. Calumenin modulates SERCA pump activity without drastically affecting ER-Ca²⁺ concentration. In addition, reducing Calumenin expression in CF cells results in a partial restoration of CFTR activity, highlighting a potential function of Calumenin in CFTR maturation. These findings demonstrate a pivotal role for Calumenin in CF cells, providing insights into how modulation of Calumenin expression or activity may be used as a potential therapeutic tool to correct defects in F508del-CFTR.

Characterization of the ER-Targeted Low Affinity Ca(2+) Probe D4ER

Calcium ion (Ca²⁺) is a ubiquitous intracellular messenger and changes in its concentration impact on nearly every aspect of cell life. Endoplasmic reticulum (ER) represents the major intracellular Ca²⁺ store and the free Ca²⁺ concentration ([Ca²⁺]) within its lumen ([Ca²⁺]_ER) can reach levels higher than 1 mM. Several genetically-encoded ER-targeted Ca²⁺ sensors have been developed over the last years. However, most of them are non-ratiometric and, thus, their signal is difficult to calibrate in live cells and is affected by shifts in the focal plane and artifactual movements of the sample. On the other hand, existing ratiometric Ca²⁺ probes are plagued by different drawbacks, such as a double dissociation constant (K_d) for Ca²⁺, low dynamic range, and an affinity for the cation that is too high for the levels of [Ca²⁺] in the ER lumen. Here, we report the characterization of a recently generated ER-targeted, Förster resonance energy transfer (FRET)-based, Cameleon probe, named D4ER, characterized by suitable Ca²⁺ affinity and dynamic range for monitoring [Ca²⁺] variations within the ER. As an example, resting [Ca²⁺]_ER have been evaluated in a known paradigm of altered ER Ca²⁺ homeostasis, i.e., in cells expressing a mutated form of the familial Alzheimer’s Disease-linked protein Presenilin 2 (PS2). The lower Ca²⁺ affinity of the D4ER probe, compared to that of the previously generated D1ER, allowed the detection of a conspicuous, more clear-cut, reduction in ER Ca²⁺ content in cells expressing mutated PS2, compared to controls.

Measurement of Calcium Fluctuations Within the Sarcoplasmic Reticulum of Cultured Smooth Muscle Cells Using FRET-based Confocal Imaging

Maintenance of steady-state calcium (Ca(2+)) levels in the sarcoplasmic reticulum (SR) of vascular smooth muscle cells (VSMCs) is vital to their overall health. A significant portion of intracellular Ca(2+) content is found within the SR stores in VSMCs. As the only intracellular organelle with a close association to the surrounding extracellular space through plasma membrane-SR junctions, the SR can be considered to constitute the first line of response to any irregularity in Ca(2+) transients, or stress experienced by the cell. Among its many functions, one of the most important is its role in the transmission of Ca(2+)-regulated signals throughout the cell to induce further cell-wide reactions downstream. The more common use of cytoplasmic Ca(2+) indicators in this regard is overall insufficient for research into the highly dynamic changes to the intraluminal SR Ca(2+) store that have yet to be fully characterized. Here, we provide a detailed protocol for the direct and clear measurement of luminal SR Ca(2+). This tool is useful for investigation into the nuanced changes in SR Ca(2+) that have significant subsequent effects on the normal function and health of the cell. Fluctuations in SR Ca(2+) content specifically can provide us with a significant amount of information pertaining to cellular responses to disease or stress conditions experienced by the cell. In this method, a modified version of a SR-targeted Ca(2+) indicator, D1SR, is used to detect Ca(2+) fluctuations in response to the introduction of agents to cultured rat aortic smooth muscle cells (SMCs). Following incubation with the D1SR indicator, confocal fluorescence microscopy and fluorescence resonance energy transfer (FRET)-based imaging are used to directly observe changes to intraluminal SR Ca(2+) levels under control conditions and with the addition of agonist agents that function to induce intracellular Ca(2+) movement.

TRP-Na(+)/Ca(2+) Exchanger Coupling

Na(+)/Ca(2+) exchangers (NCXs) have traditionally been viewed principally as a means of Ca(2+) removal from non-excitable cells. However there has recently been increasing interest in the operation of NCXs in reverse mode acting as a means of eliciting Ca(2+) entry into these cells. Reverse mode exchange requires a significant change in the normal resting transmembrane ion gradients and membrane potential, which has been suggested to occur principally via the coupling of NCXs to localised Na(+) entry through non-selective cation channels such as canonical transient receptor potential (TRPC) channels. Here we review evidence for functional or physical coupling of NCXs to non-selective cation channels, and how this affects NCX activity in non-excitable cells. In particular we focus on the potential role of nanojunctions, where the close apposition of plasma and intracellular membranes may help create the conditions needed for the generation of localised rises in Na(+) concentration that would be required to trigger reverse mode exchange.

Mitochondrial Ca²⁺ handling is crucial for generation of rhythmical Ca²⁺ waves in vascular interstitial cells from rabbit portal vein

Vasomotion is the rhythmical changes in vascular tone of various blood vessels. It was proposed that in rabbit portal vein (RPV) the spontaneous contractile activity is driven by vascular interstitial cells (VICs), since RPV VICs generate rhythmical changes in intracellular Ca(2+) concentration ([Ca(2+)]i) associated with membrane depolarisation in these cells. In this work, using confocal imaging in Fluo-3 loaded RPV VICs we studied if generation of rhythmical [Ca(2+)]i changes is affected when Ca(2+) handling by mitochondria is compromised. We also visualised mitochondria in VICs using Mito Tracker Green fluorescent dye. Our results showed that freshly dispersed RPV VICs generated rhythmical [Ca(2+)]i oscillations with a frequency of 0.2-0.01 Hz. Imaging of VICs stained with Mito Tracker Green revealed abundant mitochondria in these cells with a higher density of the organelles in sub-plasmalemmar region compared to the central region of the cell. Oligomycin, an ATP synthase inhibitor, did not affect the amplitude and frequency of rhythmical [Ca(2+)]i oscillations. In contrast, two uncoupling agents, carbonylcyanide-3-chlorophenylhydrazone (CCCP) and carbonylcyanide-4-trifluoromethoxyphenylhydrazone (FCCP) effectively abolished rhythmical [Ca(2+)]i changes with simultaneous increase in basal [Ca(2+)]i in RPV VICs. These data suggest that in RPV VICs mitochondrial Ca(2+) handling is important for the generation of rhythmical [Ca(2+)]i changes which underlie the spontaneous rhythmical contractile activity in this vessel.

SERCA and PMCA pumps contribute to the deregulation of Ca2+ homeostasis in human CF epithelial cells

Cystic Fibrosis (CF) disease is caused by mutations in the CFTR gene (CF transmembrane conductance regulator). F508 deletion is the most represented mutation, and F508del-CFTR is absent of plasma membrane and accumulates into the endoplasmic reticulum (ER) compartment. Using specific Ca2+ genetics cameleon probes, we showed in the human bronchial CF epithelial cell line CFBE that ER Ca2+ concentration was strongly increased compared to non-CF (16HBE) cells, and normalized by the F508del-CFTR corrector agent, VX-809. We also showed that ER F508del-CFTR retention increases SERCA (Sarcoplasmic/Reticulum Ca2+ ATPase) pump activity whereas PMCA (Plasma Membrane Ca2+ ATPase) activities were reduced in these CF cells compared to corrected CF cells (VX-809) and non-CF cells. We are showing for the first time CFTR/SERCA and CFTR/PMCA interactions that are modulated in CF cells and could explain part of Ca2+ homeostasis deregulation due to mislocalization of F508del-CFTR. Using ER or mitochondria genetics Ca2+ probes, we are showing that ER Ca2+ content, mitochondrial Ca2+ uptake, SERCA and PMCA pump, activities are strongly affected by the localization of F508del-CFTR protein.

Inositol 1,4,5 trisphosphate receptor 1 is a key player of human myoblast differentiation

Cytosolic Ca(2+) signals are fundamental for the early and late steps of myoblast differentiation and are, as in many cells, generated by Ca(2+) release from internal stores as well as by plasma membrane Ca(2+) entry. Our recent studies identified the store-operated Ca(2+) channels, Orai1 and TRPC1&C4, as crucial for the early steps of human myogenesis and for the late fusion events. In the present work, we assessed the role of the inositol-1,4,5 tris-phosphate receptor (IP3R) type 1 during human myoblast differentiation. We demonstrated, using siRNA strategy that IP3R1 is required for the expression of muscle-specific transcription factors such as myogenin and MEF2 (myocyte enhancer factor 2), and for the formation of myotubes. The knockdown of IP3R1 strongly reduced endogenous spontaneous Ca(2+) transients, and attenuated store-operated Ca(2+) entry. As well, two Ca(2+)-dependent key enzymes of muscle differentiation, NFAT and CamKII are down-regulated upon siIP3R1 treatment. On the contrary, the overexpression of IP3R1 accelerated myoblasts differentiation. These findings identify Ca(2+) release mediated by IP3R1 as an essential mechanism during the early steps of myoblast differentiation.

Redox regulation of store-operated Ca2+ entry

SIGNIFICANCE

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling mechanism triggered by Ca2+ depletion of the endoplasmic reticulum (ER) and by a variety of cellular stresses. Reactive oxygen species (ROS) are often concomitantly produced in response to these stresses, however, the relationship between redox signaling and SOCE is not completely understood. Various cardiovascular, neurological, and immune diseases are associated with alterations in both Ca2+ signaling and ROS production, and thus understanding this relationship has therapeutic implications.

RECENT ADVANCES

Several reactive cysteine modifications in stromal interaction molecule (STIM) and Orai proteins comprising the core SOCE machinery were recently shown to modulate SOCE in a redox-dependent manner. Moreover, STIM1 and Orai1 expression levels may reciprocally regulate and be affected by responses to oxidative stress. ER proteins involved in oxidative protein folding have gained increased recognition as important sources of ROS, and the recent discovery of their accumulation in contact sites between the ER and mitochondria provides a further link between ROS production and intracellular Ca2+ handling.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Future research should aim to establish the complete set of SOCE controlling molecules, to determine their redox-sensitive residues, and to understand how intracellular Ca2+ stores dynamically respond to different types of stress. Mapping the precise nature and functional consequence of key redox-sensitive components of the pre- and post-translational control of SOCE machinery and of proteins regulating ER calcium content will be pivotal in advancing our understanding of the complex cross-talk between redox and Ca2+ signaling.

The destiny of Ca(2+) released by mitochondria

Mitochondrial Ca²⁺ is known to regulate diverse cellular functions, for example energy production and cell death, by modulating mitochondrial dehydrogenases, inducing production of reactive oxygen species, and opening mitochondrial permeability transition pores. In addition to the action of Ca²⁺ within mitochondria, Ca²⁺ released from mitochondria is also important in a variety of cellular functions. In the last 5 years, the molecules responsible for mitochondrial Ca²⁺ dynamics have been identified: a mitochondrial Ca²⁺ uniporter (MCU), a mitochondrial Na⁺–Ca²⁺ exchanger (NCLX), and a candidate for a mitochondrial H⁺–Ca²⁺ exchanger (Letm1). In this review, we focus on the mitochondrial Ca²⁺ release system, and discuss its physiological and pathophysiological significance. Accumulating evidence suggests that the mitochondrial Ca²⁺ release system is not only crucial in maintaining mitochondrial Ca²⁺ homeostasis but also participates in the Ca²⁺ crosstalk between mitochondria and the plasma membrane and between mitochondria and the endoplasmic/sarcoplasmic reticulum.

Pancreatic β-cell Na+ channels control global Ca2+ signaling and oxidative metabolism by inducing Na+ and Ca2+ responses that are propagated into mitochondria

Communication between the plasma membrane and mitochondria is essential for initiating the Ca(2+) and metabolic signals required for secretion in β cells. Although voltage-dependent Na(+) channels are abundantly expressed in β cells and activated by glucose, their role in communicating with mitochondria is unresolved. Here, we combined fluorescent Na(+), Ca(2+), and ATP imaging, electrophysiological analysis with tetrodotoxin (TTX)-dependent block of the Na(+) channel, and molecular manipulation of mitochondrial Ca(2+) transporters to study the communication between Na(+) channels and mitochondria. We show that TTX inhibits glucose-dependent depolarization and blocks cytosolic Na(+) and Ca(2+) responses and their propagation into mitochondria. TTX-sensitive mitochondrial Ca(2+) influx was largely blocked by knockdown of the mitochondrial Ca(2+) uniporter (MCU) expression. Knockdown of the mitochondrial Na(+)/Ca(2+) exchanger (NCLX) and Na(+) dose response analysis demonstrated that NCLX mediates the mitochondrial Na(+) influx and is tuned to sense the TTX-sensitive cytosolic Na(+) responses. Finally, TTX blocked glucose-dependent mitochondrial Ca(2+) rise, mitochondrial metabolic activity, and ATP production. Our results show that communication of the Na(+) channels with mitochondria shape both global Ca(2+) and metabolism signals linked to insulin secretion in β cells.- Nita, I. I., Hershfinkel, M., Kantor, C., Rutter, G. A., Lewis, E. C., Sekler, I. Pancreatic β-cell Na(+) channels control global Ca(2+) signaling and oxidative metabolism by inducing Na(+) and Ca(2+) responses that are propagated into mitochondria.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Calmodulin modulates the termination threshold for cardiac ryanodine receptor-mediated Ca2+ release

RyR2 (cardiac ryanodine receptor)-mediated Ca2+ release in cardiomyocytes terminates when the sarcoplasmic reticulum Ca2+ content depletes to a threshold level, known as the termination threshold. Despite its importance, little is known about the mechanism that regulates the termination threshold. CaM (calmodulin), by inhibiting RyR2, has been implicated in Ca2+-release termination, but whether CaM modulates the termination threshold is unknown. To this end, we monitored the endoplasmic reticulum Ca2+ dynamics in RyR2-expressing HEK (human embryonic kidney)-293 cells transfected with WT (wild-type) CaM or mutants. We found that WT CaM or CaM mutations which abolish Ca2+ binding to the N-lobe (N-terminal lobe) of CaM increased the termination threshold (i.e. facilitated termination), but had no effect on the activation threshold at which spontaneous Ca2+ release occurs. On the other hand, CaM mutations that diminish Ca2+ binding to both the N-lobe and C-lobe (C-terminal lobe), or the C-lobe only, decreased the termination threshold (i.e. delayed termination) with a similar activation threshold. Furthermore, deletion of residues 3583-3603 or point mutations (W3587A/L3591D/F3603A, W3587A, or L3591D) in the CaM-binding domain of RyR2 that are known to abolish or retain CaM binding all reduced the termination threshold without having a significant impact on the activation threshold. Interestingly, the RyR2-F3603A mutation affected both the activation and termination threshold. Collectively, these data indicate that CaM facilitates the termination of Ca2+ release by increasing the termination threshold, and that this action of CaM depends on Ca2+ binding to the C-lobe, but not to the N-lobe, of CaM. The results of the present study also suggest that the CaM-binding domain of RyR2 is an important determinant of Ca2+-release termination and activation.

Mitochondrial dynamics in the heart as a novel therapeutic target for cardioprotection

Traditionally, mitochondria have been regarded solely as energy generators for cells; however, accumulating data have demonstrated that these complex organelles play a variety of roles within the cardiomyocyte that extend beyond this classic function. Mitochondrial dynamics involves mitochondrial movements and morphologic alterations by tethering, fusion, and fission, which depend on cellular energy requirements and metabolic status. Many studies have indicated that mitochondrial dynamics may be a fundamental component of the maintenance of normal cellular homeostasis and cardiac function. Mitochondrial dynamics is controlled by the protein machinery responsible for mitochondrial fusion and fission, but cardiomyocytes are densely packed as part of an intricate cytoarchitecture for efficient and imbalanced contraction; thus, mitochondrial dynamics in the adult heart are restricted and occur more slowly than in other organs. Cardiac mitochondrial dynamics is important for cardiac physiology in diseased conditions such as ischemia-reperfusion (IR) injury. Changes in mitochondrial morphology through modulation of the expression of proteins regulating mitochondrial dynamics demonstrates the beneficial effects on cardiac performance after IR injury. Thus, accurately defining the roles of mitochondrial dynamics in the adult heart can guide the identification and development of novel therapeutic targets for cardioprotection. Further studies should be performed to establish the exact mechanisms of mitochondrial dynamics.

From structure to function: mitochondrial morphology, motion and shaping in vascular smooth muscle

The diversity of mitochondrial arrangements, which arise from the organelle being static or moving, or fusing and dividing in a dynamically reshaping network, is only beginning to be appreciated. While significant progress has been made in understanding the proteins that reorganise mitochondria, the physiological significance of the various arrangements is poorly understood. The lack of understanding may occur partly because mitochondrial morphology is studied most often in cultured cells. The simple anatomy of cultured cells presents an attractive model for visualizing mitochondrial behaviour but contrasts with the complexity of native cells in which elaborate mitochondrial movements and morphologies may not occur. Mitochondrial changes may take place in native cells (in response to stress and proliferation), but over a slow time-course and the cellular function contributed is unclear. To determine the role mitochondrial arrangements play in cell function, a crucial first step is characterisation of the interactions among mitochondrial components. Three aspects of mitochondrial behaviour are described in this review: (1) morphology, (2) motion and (3) rapid shape changes. The proposed physiological roles to which various mitochondrial arrangements contribute and difficulties in interpreting some of the physiological conclusions are also outlined.

Waves of calcium depletion in the sarcoplasmic reticulum of vascular smooth muscle cells: an inside view of spatiotemporal Ca2+ regulation

Agonist-stimulated smooth muscle Ca²⁺ waves regulate blood vessel tone and vasomotion. Previous studies employing cytoplasmic Ca²⁺ indicators revealed that these Ca²⁺ waves were stimulated by a combination of inositol 1,4,5-trisphosphate- and Ca²⁺-induced Ca²⁺ release from the endo/sarcoplasmic reticulum. Herein, we present the first report of endothelin-1 stimulated waves of Ca²⁺ depletion from the sarcoplasmic reticulum of vascular smooth muscle cells using a calsequestrin-targeted Ca²⁺ indicator. Our findings confirm that these waves are due to regenerative Ca²⁺-induced Ca²⁺ release by the receptors for inositol 1,4,5-trisphosphate. Our main new finding is a transient elevation in SR luminal Ca²⁺ concentration ([Ca²⁺]_SR) both at the site of wave initiation, just before regenerative Ca²⁺ release commences, and at the advancing wave front, during propagation. This strongly suggests a role for [Ca²⁺]_SR in the activation of inositol 1,4,5-trisphosphate receptors during agonist-induced calcium waves. In addition, quantitative analysis of the gradual decrease in the velocity of the depletion wave, observed in the absence of external Ca²⁺, indicates continuity of the lumen of the sarcoplasmic reticulum network. Finally, our observation that the depletion wave was arrested by the nuclear envelope may have implications for selective Ca²⁺ signalling.

Pan-junctional sarcoplasmic reticulum in vascular smooth muscle: nanospace Ca2+ transport for site- and function-specific Ca2+ signalling

This review focuses on how smooth muscle sarcoplasmic reticulum (SR), the major releasable Ca(2+) store in these cells, performs its many functions by communicating with the plasma membrane (PM) and other organelles across cytoplasmic nanospaces, defined by membrane-membrane junctions less than 50 nm across. In spite of accumulating evidence in favour of the view that cytoplasmic nanospaces are a prerequisite for effective control of diverse cellular functions, our current understanding of how smooth muscle cells accomplish site- and function-specific Ca(2+) signalling remains in its infancy. We first present evidence in support of the view that effective Ca(2+) signalling depends on the restricted diffusion of Ca(2+) within cytoplasmic nanospaces. We then develop an evidence-based model of the smooth muscle SR - the 'pan-junctional SR' model - that incorporates a network of tubules and quilts that are capable of auto-regulating their Ca(2+) content and determining junctional [Ca(2+)]i through loading and unloading at membrane-membrane nanojunctions. Thereby, we provide a novel working hypothesis in order to inform future investigation into the control of a variety of cellular functions by local Ca(2+) signals at junctional nanospaces, from contraction and energy metabolism to nuclear transcription. Based on the current literature, we discuss the molecular mechanisms whereby the SR mediates these multiple functions through the interaction of ion channels and pumps embedded in apposing membranes within inter-organellar junctions. We finally highlight the fact that although most current hypotheses are qualitatively supported by experimental data, solid quantitative simulations are seriously lacking. Considering that at physiological concentrations the number of calcium ions in a typical junctional nanospace between the PM and SR is of the order of 1, ion concentration variability plays a major role as the currency of information transfer and stochastic quantitative modelling will be required to both test and develop working hypotheses.

Mitochondria Na(+)-Ca (2+) exchange in cardiomyocytes and lymphocytes

Mitochondria Na(+)-Ca(2+) exchange (NCX(mit)) was first discovered by Carafoli et al. in 1974. Thereafter, the mechanisms and roles of NCX(mit) have been extensively studied. We review NCX(mit) in cardiomyocytes and lymphocytes by presenting our recent studies on it. Studies of NCX(mit) in rat ventricular cells demonstrated that NCX(mit) is voltage dependent and electrogenic. A targeted knockdown and knockout of NCLX in HL-1 cardiomyocytes and B lymphocytes, respectively, significantly reduced the NCX(mit) activity, indicating that NCLX is a major component of NCX(mit) in these cells. The store-operated Ca(2+) entry was greatly attenuated in NCLX knockout lymphocytes, suggesting that substantial amount of Ca(2+) enters into mitochondria and is released to cytosol via NCX(mit). NCX(mit) or NCLX has pivotal roles in Ca(2+) handling in mitochondria and cytoplasm.

Molecular identity and functional properties of the mitochondrial Na+/Ca2+ exchanger

The mitochondrial membrane potential that powers the generation of ATP also facilitates mitochondrial Ca(2+) shuttling. This process is fundamental to a wide range of cellular activities, as it regulates ATP production, shapes cytosolic and endoplasmic recticulum Ca(2+) signaling, and determines cell fate. Mitochondrial Ca(2+) transport is mediated primarily by two major transporters: a Ca(2+) uniporter that mediates Ca(2+) uptake and a Na(+)/Ca(2+) exchanger that subsequently extrudes mitochondrial Ca(2+). In this minireview, we focus on the specific role of the mitochondrial Na(+)/Ca(2+) exchanger and describe its ion exchange mechanism, regulation by ions, and putative partner proteins. We discuss the recent molecular identification of the mitochondrial exchanger and how its activity is linked to physiological and pathophysiological processes.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Abnormal termination of Ca2+ release is a common defect of RyR2 mutations associated with cardiomyopathies

RATIONALE

Naturally occurring mutations in the cardiac ryanodine receptor (RyR2) have been associated with both cardiac arrhythmias and cardiomyopathies. It is clear that delayed afterdepolarization resulting from abnormal activation of sarcoplasmic reticulum Ca2+ release is the primary cause of RyR2-associated cardiac arrhythmias. However, the mechanism underlying RyR2-associated cardiomyopathies is completely unknown.

OBJECTIVE

In the present study, we investigate the role of the NH2-terminal region of RyR2 in and the impact of a number of cardiomyopathy-associated RyR2 mutations on the termination of Ca2+ release.

METHODS AND RESULTS

The 35-residue exon-3 region of RyR2 is associated with dilated cardiomyopathy. Single-cell luminal Ca2+ imaging revealed that the deletion of the first 305 NH2-terminal residues encompassing exon-3 or the deletion of exon-3 itself markedly reduced the luminal Ca2+ threshold at which Ca2+ release terminates and increased the fractional Ca2+ release. Single-cell cytosolic Ca2+ imaging also showed that both RyR2 deletions enhanced the amplitude of store overload-induced Ca2+ transients in HEK293 cells or HL-1 cardiac cells. Furthermore, the RyR2 NH2-terminal mutations, A77V, R176Q/T2504M, R420W, and L433P, which are associated with arrhythmogenic right ventricular displasia type 2, also reduced the threshold for Ca2+ release termination and increased fractional release. The RyR2 A1107M mutation associated with hypertrophic cardiomyopathy had the opposite action (i.e., increased the threshold for Ca2+ release termination and reduced fractional release).

CONCLUSIONS

These results provide the first evidence that the NH2-terminal region of RyR2 is an important determinant of Ca2+ release termination, and that abnormal fractional Ca2+ release attributable to aberrant termination of Ca2+ release is a common defect in RyR2-associated cardiomyopathies.

The role of cytoplasmic nanospaces in smooth muscle cell Ca2+ signalling

We address the importance of cytoplasmic nanospaces in Ca(2+) transport and signalling in smooth muscle cells and how quantitative modelling can shed significant light on the understanding of signalling mechanisms. Increasingly more convincing evidence supports the view that these nanospaces--nanometre-scale spaces between organellar membranes, hosting cell signalling machinery--are key to Ca(2+) signalling as much as Ca(2+) transporters and Ca(2+) storing organelles. Our research suggests that the origin of certain diseases is to be sought in the disruption of the proper functioning of cytoplasmic nanospaces. We begin with a historical perspective on the study of smooth muscle cell plasma membrane-sarcoplasmic reticulum nanospaces, including experimental evidence of their role in the generation of asynchronous Ca(2+) waves. We then summarize how stochastic modelling approaches have aided and guided our understanding of two basic functional steps leading to healthy smooth muscle cell contraction. We furthermore outline how more sophisticated and realistic quantitative stochastic modelling is now being employed not only to deepen our understanding but also to aid in the hypothesis generation for further experimental investigation.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Pivotal role of mitochondrial Na⁺₋Ca²⁺ exchange in antigen receptor mediated Ca²⁺ signalling in DT40 and A20 B lymphocytes

Cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) increases upon activation of antigen-receptor in lymphocytes. Mitochondria have been suggested to regulate the [Ca(2+)](i) response, but the molecular mechanisms and the roles are poorly understood. To clarify them, we carried out a combination study of mathematical simulations and knockout or knockdown of NCLX, a gene candidate for the mitochondrial Na(+)-Ca(2+) exchanger (NCX(mit)), in B lymphocytes. A mathematical model of Ca(2+) dynamics in B lymphocytes demonstrated that NCX(mit) inhibition reduces basal Ca(2+) content of endoplasmic reticulum (ER) and suppresses B-cell antigen receptor (BCR)-mediated [Ca(2+)](i) rise. The predictions were validated in DT40 B lymphocytes of heterozygous NCLX knockout (NCLX(+/-)). In NCLX(+/-) cells, mitochondrial Ca(2+) efflux via NCX(mit) was strongly decelerated, suggesting NCLX is a gene responsible for NCX(mit) in B lymphocytes. Consistent with the predictions, ER Ca(2+) content declined and [Ca(2+)](i) hardly rose upon BCR activation in NCLX(+/-) cells. ER Ca(2+) uptake was reduced to ∼58% of the wild-type (WT), while it was comparable to WT when mitochondrial respiration was disturbed. Essentially the same results were obtained by a pharmacological inhibition or knockdown of NCLX by siRNA in A20 B lymphocytes. Unexpectedly, ER Ca(2+) leak was augmented and co-localization of mitochondria with ER was lower in NCLX(+/-) and NCLX silenced cells. Taken together, we concluded that NCLX is a key Ca(2+) provider to ER, and that NCLX-mediated Ca(2+) recycling between mitochondria and ER is pivotal in B cell responses to antigen.

Differential mitochondrial calcium responses in different cell types detected with a mitochondrial calcium fluorescent indicator, mito-GCaMP2

Mitochondrial calcium plays a crucial role in mitochondrial metabolism, cell calcium handling, and cell death. However, some mechanisms concerning mitochondrial calcium regulation are still unknown, especially how mitochondrial calcium couples with cytosolic calcium. In this work, we constructed a novel mitochondrial calcium fluorescent indicator (mito-GCaMP2) by genetic manipulation. Mito-GCaMP2 was imported into mitochondria with high efficiency and the fluorescent signals co-localized with that of tetramethyl rhodamine methyl ester, a mitochondrial membrane potential indicator. The mitochondrial inhibitors specifically decreased the signals of mito-GCaMP2. The apparent K(d) of mito-GCaMP2 was 195.0 nmol/L at pH 8.0 in adult rat cardiomyocytes. Furthermore, we observed that mito-GCaMP2 preferred the alkaline pH surrounding of mitochondria. In HeLa cells, we found that mitochondrial calcium ([Ca(2+)](mito)) responded to the changes of cytosolic calcium ([Ca(2+)](cyto)) induced by histamine or thapasigargin. Moreover, external Ca(2+) (100 μmol/L) directly induced an increase of [Ca(2+)](mito) in permeabilized HeLa cells. However, in rat cardiomyocytes [Ca(2+)](mito) did not respond to cytosolic calcium transients stimulated by electric pacing or caffeine. In permeabilized cardiomyocytes, 600 nmol/L free Ca(2+) repeatedly increased the fluorescent signals of mito-GCaMP2, which excluded the possibility that mito-GCaMP2 lost its function in cardiomyocytes mitochondria. These results showed that the response of mitochondrial calcium is diverse in different cell lineages and suggested that mitochondria in cardiomyocytes may have a special defense mechanism to control calcium flux.

Uncoupling protein 3 (UCP3) modulates the activity of Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) by decreasing mitochondrial ATP production

The uncoupling proteins UCP2 and UCP3 have been postulated to catalyze Ca(2+) entry across the inner membrane of mitochondria, but this proposal is disputed, and other, unrelated proteins have since been identified as the mitochondrial Ca(2+) uniporter. To clarify the role of UCPs in mitochondrial Ca(2+) handling, we down-regulated the expression of the only uncoupling protein of HeLa cells, UCP3, and measured Ca(2+) and ATP levels in the cytosol and in organelles with genetically encoded probes. UCP3 silencing did not alter mitochondrial Ca(2+) uptake in permeabilized cells. In intact cells, however, UCP3 depletion increased mitochondrial ATP production and strongly reduced the cytosolic and mitochondrial Ca(2+) elevations evoked by histamine. The reduced Ca(2+) elevations were due to inhibition of store-operated Ca(2+) entry and reduced depletion of endoplasmic reticulum (ER) Ca(2+) stores. UCP3 depletion accelerated the ER Ca(2+) refilling kinetics, indicating that the activity of sarco/endoplasmic reticulum Ca(2+) (SERCA) pumps was increased. Accordingly, SERCA inhibitors reversed the effects of UCP3 depletion on cytosolic, ER, and mitochondrial Ca(2+) responses. Our results indicate that UCP3 is not a mitochondrial Ca(2+) uniporter and that it instead negatively modulates the activity of SERCA by limiting mitochondrial ATP production. The effects of UCP3 on mitochondrial Ca(2+) thus reflect metabolic alterations that impact on cellular Ca(2+) homeostasis. The sensitivity of SERCA to mitochondrial ATP production suggests that mitochondria control the local ATP availability at ER Ca(2+) uptake and release sites.

Calcium signaling dysfunction in heart disease

In the heart, Ca(2+) is crucial for the regulation of contraction and intracellular signaling, processes, which are vital to the functioning of the healthy heart. Ca(2+) -activated signaling pathways must function against a background of large, rapid, and tightly regulated changes in intracellular free Ca(2+) concentrations during each contraction and relaxation cycle. This review highlights a number of proteins that regulate signaling Ca(2+) in both normal and pathological conditions including cardiac hypertrophy and heart failure, and discusses how these pathways are not regulated by the marked elevation in free intracellular calcium ([Ca(2+) ](i)) during contraction but require smaller sustained increases in Ca(2+) concentration. In addition, we present published evidence that the pool of Ca(2+) that regulates signaling is compartmentalized into distinct cellular microdomains and is thus distinct from that regulating contraction.

Temporary sequestration of potassium by mitochondria in astrocytes

Increases in extracellular potassium concentration ([K(+)](o)), which can occur during neuronal activity and under pathological conditions such as ischemia, lead to a variety of potentially detrimental effects on neuronal function. Although astrocytes are known to contribute to the clearance of excess K(+)(o), the mechanisms are not fully understood. We examined the potential role of mitochondria in sequestering K(+) in astrocytes. Astrocytes were loaded with the fluorescent K(+) indicator PBFI and release of K(+) from mitochondria into the cytoplasm was examined after uncoupling the mitochondrial membrane potential with carbonyl cyanide m-chlorophenylhydrazone (CCCP). Under the experimental conditions employed, transient applications of elevated [K(+)](o) led to increases in K(+) within mitochondria, as assessed by increases in the magnitudes of cytoplasmic [K(+)] ([K(+)](i)) transients evoked by brief exposures to CCCP. When mitochondrial K(+) sequestration was impaired by prolonged application of CCCP, there was a robust increase in [K(+)](i) upon exposure to elevated [K(+)](o). Blockade of plasmalemmal K(+) uptake routes by ouabain, Ba(2+), or a mixture of voltage-activated K(+) channel inhibitors reduced K(+) uptake into mitochondria. Also, reductions in mitochondrial K(+) uptake occurred in the presence of mito-K(ATP) channel inhibitors. Rises in [K(+)](i) evoked by brief applications of CCCP following exposure to high [K(+)](o) were also reduced by gap junction blockers and in astrocytes isolated from connexin43-null mice, suggesting that connexins also play a role in K(+) uptake into astrocyte mitochondria. We conclude that mitochondria play a key role in K(+)(o) handling by astrocytes.

The interdependence of endothelin-1 and calcium: a review

The 21-amino-acid peptide ET-1 (endothelin-1) regulates a diverse array of physiological processes, including vasoconstriction, angiogenesis, nociception and cell proliferation. Most of the effects of ET-1 are associated with an increase in intracellular calcium concentration. The calcium influx and mobilization pathways activated by ET-1, however, vary immensely. The present review begins with the basics of calcium signalling and investigates the different ways intracellular calcium concentration can increase in response to a stimulus. The focus then shifts to ET-1, and discusses how ET receptors mobilize calcium. We also examine how disease alters calcium-dependent responses to ET-1 by discussing changes to ET-1-mediated calcium signalling in hypertension, as there is significant interest in the role of ET-1 in this important disease. A list of unanswered questions regarding ET-mediated calcium signals are also presented, as well as perspectives for future research of calcium mobilization by ET-1.

Mitochondria control functional CaV1.2 expression in smooth muscle cells of cerebral arteries

RATIONALE

Physiological functions of mitochondria in contractile arterial myocytes are poorly understood. Mitochondria can uptake calcium (Ca(2+)), but intracellular Ca(2+) signals that regulate mitochondrial Ca(2+) concentration ([Ca(2+)](mito)) and physiological functions of changes in [Ca(2+)](mito) in arterial myocytes are unclear.

OBJECTIVE

To identify Ca(2+) signals that regulate [Ca(2+)](mito), examine the significance of changes in [Ca(2+)](mito), and test the hypothesis that [Ca(2+)](mito) controls functional ion channel transcription in myocytes of resistance-size cerebral arteries.

METHODS AND RESULTS

Endothelin (ET)-1 activated Ca(2+) waves and elevated global Ca(2+) concentration ([Ca(2+)](i)) via inositol 1,4,5-trisphosphate receptor (IP(3)R) activation. IP(3)R-mediated sarcoplasmic reticulum (SR) Ca(2+) release increased [Ca(2+)](mito) and induced mitochondrial depolarization, which stimulated mitochondrial reactive oxygen species (mitoROS) generation that elevated cytosolic ROS. In contrast, a global [Ca(2+)](i) elevation did not alter [Ca(2+)](mito), mitochondrial potential, or mitoROS generation. ET-1 stimulated nuclear translocation of nuclear factor (NF)-kappaB p50 subunit and ET-1-induced IP(3)R-mediated mitoROS elevated NF-kappaB-dependent transcriptional activity. ET-1 elevated voltage-dependent Ca(2+) (Ca(V)1.2) channel expression, leading to an increase in both pressure (myogenic tone)- and depolarization-induced vasoconstriction. Baseline Ca(V)1.2 expression and the ET-1-induced elevation in Ca(V)1.2 expression were both reduced by IP(3)R inhibition, mitochondrial electron transport chain block, antioxidant treatment, and NF-kappaB subunit knockdown, leading to vasodilation.

CONCLUSIONS

IP(3)R-mediated SR Ca(2+) release elevates [Ca(2+)](mito), which induces mitoROS generation. MitoROS activate NF-kappaB, which stimulates Ca(V)1.2 channel transcription. Thus, mitochondria sense IP(3)R-mediated SR Ca(2+) release to control NF-kappaB-dependent Ca(V)1.2 channel expression in arterial myocytes, thereby modulating arterial contractility.

Endothelial mitochondria and heart disease

The endothelium is vital to the proper functioning in the heart, in particular due to its production of nitric oxide (NO) which regulates vascular tone. Damage to the endothelium contributes to the development of atherosclerosis, and hence to possible myocardial infarction and subsequent heart failure. Like most cells, endothelial cells contain mitochondria, despite their having relatively little dependence on oxidative phosphorylation for ATP production. However, endothelial mitochondria are centrally involved in maintaining the fine regulatory balance between mitochondrial calcium concentration, reactive oxygen species (ROS) production, and NO. This raises the question of whether damage to endothelial mitochondria would have repercussions in terms of the development of heart disease. In fact, increasingly nuanced techniques enabling restricted transgenic expression of antioxidant proteins in mice has demonstrated that mitochondrial ROS do contribute to endothelial damage. New pharmaceutical approaches designed to target protective molecules such as ROS scavengers to the mitochondria promise to be effective in preventing heart disease. As well as protecting cardiomyocytes, these drugs may have the added benefit of preventing damage to the endothelial mitochondria. However, much remains to be done in understanding the contribution that mitochondria make to endothelial function.

Coupling of mitochondria to store-operated Ca(2+)-signaling sustains constitutive activation of protein kinase B/Akt and augments survival of malignant melanoma cells

Mitochondria are emerging as a major hub for cellular Ca(2+)-signaling, though their contribution to Ca(2+)-driven growth- and survival-promoting events in cancer is poorly understood. Here employing flow cytometry to monitor mitochondrial and cytosolic Ca(2+), we assessed trans-mitochondrial Ca(2+)-transport and store-operated Ca(2+)-influx (store-operated channels (SOC)) in malignant vs. non-malignant B16BL6 melanoma clones. Remarkably, mitochondrial Ca(2+)-fluxes measured in whole cells or in isolated mitochondria were accelerated in the malignant clones compared to their non-malignant counterpart clones. This coincided with enhanced SOC-mediated Ca(2+)-influx and high levels of constitutively active protein kinase B/Akt (PKB). Interruption of trans-mitochondrial Ca(2+)-transport in the malignant cells with an antagonist of the mitochondrial Na(+)/Ca(2+) exchanger, CGP-37157, abolsihed SOC-mediated Ca(2+)-influx, inactivated PKB, retarded cell growth and increased vulnerability to apoptosis. Similarly, direct SOC blockade by silencing Stim1 inhibited PKB, indicating that the crosstalk between SOC and mitochondria is essential to preserve PKB in constitutively active state. Finally, the retraction of mitochondria from sub-plasmalemmal micro-domains triggered by Fis1 over-expression inhibited SOC-coupled trans-mitochondrial Ca(2+)-flux, Ca(2+)-entry via SOC and PKB activity. Taken together, our data show that in the malignant melanoma cells, the functional and spatial relationship of up-regulated mitochondrial Ca(2+)-transport to the SOC sustains the robust Ca(2+)-responses and down-stream signaling critical for apoptosis-resistance and proliferation.

The ups and downs of mitochondrial calcium signalling in the heart

Regulation of intramitochondrial free calcium ([Ca2+]m) is critical in both physiological and pathological functioning of the heart. The full extent and importance of the role of [Ca2+]m is becoming apparent as evidenced by the increasing interest and work in this area over the last two decades. However, controversies remain, such as the existence of beat-to-beat mitochondrial Ca2+ transients; the role of [Ca2+]m in modulating whole-cell Ca2+ signalling; whether or not an increase in [Ca2+]m is essential to couple ATP supply and demand; and the role of [Ca2+]m in cell death by both necrosis and apoptosis, especially in formation of the mitochondrial permeability transition pore. The role of [Ca2+]m in heart failure is an area that has also recently been highlighted. [Ca2+]m can now be measured reasonably specifically in intact cells and hearts thanks to developments in fluorescent indicators and targeted proteins and more sensitive imaging technology. This has revealed interactions of the mitochondrial Ca2+ transporters with those of the sarcolemma and sarcoplasmic reticulum, and has gone a long way to bringing the mitochondrial Ca2+ transporters to the forefront of cardiac research. Mitochondrial Ca2+ uptake occurs via the ruthenium red sensitive Ca2+ uniporter (mCU), and efflux via an Na+/Ca2+ exchanger (mNCX). The purification and cloning of the transporters, and development of more specific inhibitors, would produce a step-change in our understanding of the role of these apparently critical but still elusive proteins. In this article we will summarise the key physiological roles of [Ca2+]m in ATP production and cell Ca2+ signalling in both adult and neonatal hearts, as well as highlighting some of the controversies in these areas. We will also briefly discuss recent ideas on the interactions of nitric oxide with [Ca2+]m.

NCLX is an essential component of mitochondrial Na+/Ca2+ exchange

Mitochondrial Ca(2+) efflux is linked to numerous cellular activities and pathophysiological processes. Although it is established that an Na(+)-dependent mechanism mediates mitochondrial Ca(2+) efflux, the molecular identity of this transporter has remained elusive. Here we show that the Na(+)/Ca(2+) exchanger NCLX is enriched in mitochondria, where it is localized to the cristae. Employing Ca(2+) and Na(+) fluorescent imaging, we demonstrate that mitochondrial Na(+)-dependent Ca(2+) efflux is enhanced upon overexpression of NCLX, is reduced by silencing of NCLX expression by siRNA, and is fully rescued by the concomitant expression of heterologous NCLX. NCLX-mediated mitochondrial Ca(2+) transport was inhibited, moreover, by CGP-37157 and exhibited Li(+) dependence, both hallmarks of mitochondrial Na(+)-dependent Ca(2+) efflux. Finally, NCLX-mediated mitochondrial Ca(2+) exchange is blocked in cells expressing a catalytically inactive NCLX mutant. Taken together, our results converge to the conclusion that NCLX is the long-sought mitochondrial Na(+)/Ca(2+) exchanger.