Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

The Molecular Heterogeneity of Store-Operated Ca(2+) Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca(2+)-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

mtROS Induced via TLR-2-SOCE Signaling Plays Proapoptotic and Bactericidal Role in Mycobacterium fortuitum-Infected Head Kidney Macrophages of Clarias gariepinus

The mechanisms underlying Mycobacterium fortuitum-induced mycobacteriosis remain unexplored. Using head kidney macrophages (HKM) from catfish (Clarias gariepinus), we report that Ca²⁺ surge across mitochondrial-Ca²⁺ uniporter (MICU), and consequent mitochondrial ROS (mtROS) production, is imperative for mycobactericidal activity. Inhibition of mtROS alleviated HKM apoptosis and enhanced bacterial survival. Based on RNA interference (RNAi) and inhibitor studies, we demonstrate that the Toll-like receptor (TLR)-2–endoplasmic reticulum (ER) stress–store-operated calcium entry (SOCE) axis is instrumental for activating the mt-Ca²⁺/mtROS cascade in M. fortuitum-infected HKM. Additionally, pharmacological inhibition of mtROS attenuated the expression of CHOP, STIM1, and Orai1, which suggests a positive feedback loop between ER-stress-induced SOCE and mtROS production. Elevated tumor necrosis factor alpha (TNF-α) levels and caspase-8 activity were observed in HKM consequent to M. fortuitum infection, and our results implicate that mtROS is crucial in activating the TNF-mediated caspase-8 activation. Our results for the first time demonstrate mitochondria as an innate immune signaling center regulating mycobacteriosis in fish. We conclude that M. fortuitum-induced persistent SOCE signaling leads to mtROS production, which in turn activates the TNF-α/caspase-8 axis culminating in HKM apoptosis and bacterial clearance.

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.

The Mitochondrial Ca2+ uniporter is a central regulator of interorganellar Ca2+ transfer and NFAT activation

Mitochondrial Ca²⁺ uptake tailors the strength of stimulation of plasma membrane phospholipase C–coupled receptors to that of cellular bioenergetics. However, how Ca²⁺ uptake by the mitochondrial Ca²⁺ uniporter (MCU) shapes receptor-evoked interorganellar Ca²⁺ signaling is unknown. Here, we used CRISPR/Cas9 gene knockout, subcellular Ca²⁺ imaging, and mathematical modeling to show that MCU is a universal regulator of intracellular Ca²⁺ signaling across mammalian cell types. MCU activity sustains cytosolic Ca²⁺ signaling by preventing Ca²⁺-dependent inactivation of store-operated Ca²⁺ release–activated Ca²⁺ channels and by inhibiting Ca²⁺ extrusion. Paradoxically, MCU knockout (MCU-KO) enhanced cytosolic Ca²⁺ responses to store depletion. Physiological agonist stimulation in MCU-KO cells led to enhanced frequency of cytosolic Ca²⁺ oscillations, endoplasmic reticulum Ca²⁺ refilling, nuclear translocation of nuclear factor for activated T cells transcription factors, and cell proliferation, without altering inositol-1,4,5-trisphosphate receptor activity. Our data show that MCU has dual counterbalancing functions at the cytosol–mitochondria interface, whereby the cell-specific MCU-dependent cytosolic Ca²⁺ clearance and buffering capacity of mitochondria reciprocally regulate interorganellar Ca²⁺ transfer and nuclear factor for activated T cells nuclear translocation during receptor-evoked signaling. These findings highlight the critical dual function of the MCU not only in the acute Ca²⁺ buffering by mitochondria but also in shaping endoplasmic reticulum and cytosolic Ca²⁺ signals that regulate cellular transcription and function.

Near-UV Light Induced ROS Production Initiates Spatial Ca2+ Spiking to Fire NFATc3 Translocation

Ca2+-dependent gene regulation controls several functions to determine the fate of the cells. Proteins of the nuclear factor of activated T-cells (NFAT) family are Ca2+ sensitive transcription factors that control the cell growth, proliferation and insulin secretion in β-cells. Translocation of NFAT proteins to the nucleus occurs in a sequence of events that starts with activating calmodulin-dependent phosphatase calcineurin in a Ca2+-dependent manner, which dephosphorylates the NFAT proteins and leads to their translocation to the nucleus. Here, we examined the role of IP3-generating agonists and near-UV light in the induction of NFATc3 migration to the nucleus in the pancreatic β-cell line INS-1. Our results show that IP3 generation yields cytosolic Ca2+ rise and NFATc3 translocation. Moreover, near-UV light exposure generates reactive oxygen species (ROS), resulting in cytosolic Ca2+ spiking via the L-type Ca2+ channel and triggers NFATc3 translocation to the nucleus. Using the mitochondria as a Ca2+ buffering tool, we showed that ROS-induced cytosolic Ca2+ spiking, not the ROS themselves, was the triggering mechanism of nuclear import of NFATc3. Collectively, this study reveals the mechanism of near-UV light induced NFATc3 migration.

Cellular and mitochondrial calcium communication in obstructive lung disorders

Calcium (Ca²⁺) signalling is well known to dictate cellular functioning and fate. In recent years, the accumulation of Ca²⁺ in the mitochondria has emerged as an important factor in Chronic Respiratory Diseases (CRD) such as Asthma and Chronic Obstructive Pulmonary Disease (COPD). Various reports underline an aberrant increase in the intracellular Ca²⁺, leading to mitochondrial ROS generation, and further activation of the apoptotic pathway in these diseases. Mitochondria contribute to Ca²⁺ buffering which in turn regulates mitochondrial metabolism and ATP production. Disruption of this Ca²⁺ balance leads to impaired cellular processes like apoptosis or necrosis and thus contributes to the pathophysiology of airway diseases. This review highlights the key role of cytoplasmic and mitochondrial Ca²⁺ signalling in regulating CRD, such as asthma and COPD. A better understanding of the dysregulation of mitochondrial Ca²⁺ homeostasis in these diseases could provide cues for the development of advanced therapeutic interventions in these diseases.

Dynamic Control of Mitochondrial Ca2+ Levels as a Survival Strategy of Cancer Cells

Cancer cells have increased energy requirements due to their enhanced proliferation activity. This energy demand is, among others, met by mitochondrial ATP production. Since the second messenger Ca²⁺ maintains the activity of Krebs cycle dehydrogenases that fuel mitochondrial respiration, proper mitochondrial Ca²⁺ uptake is crucial for a cancer cell survival. However, a mitochondrial Ca²⁺ overload induces mitochondrial dysfunction and, ultimately, apoptotic cell death. Because of the vital importance of balancing mitochondrial Ca²⁺ levels, a highly sophisticated machinery of multiple proteins manages mitochondrial Ca²⁺ homeostasis. Notably, mitochondria sequester Ca²⁺ preferentially at the interaction sites between mitochondria and the endoplasmic reticulum (ER), the largest internal Ca²⁺ store, thus, pointing to mitochondrial-associated membranes (MAMs) as crucial hubs between cancer prosperity and cell death. To investigate potential regulatory mechanisms of the mitochondrial Ca²⁺ uptake routes in cancer cells, we modulated mitochondria–ER tethering and the expression of UCP2 and analyzed mitochondrial Ca²⁺ homeostasis under the various conditions. Hence, the expression of contributors to mitochondrial Ca²⁺ regulation machinery was quantified by qRT-PCR. We further used data from The Cancer Genome Atlas (TCGA) to correlate these in vitro findings with expression patterns in human breast invasive cancer and human prostate adenocarcinoma. ER-mitochondrial linkage was found to support a mitochondrial Ca²⁺ uptake route dependent on uncoupling protein 2 (UCP2) in cancer cells. Notably, combined overexpression of Rab32, a protein kinase A-anchoring protein fostering the ER-mitochondrial tethering, and UCP2 caused a significant drop in cancer cells' viability. Artificially enhanced ER-mitochondrial tethering further initiated a sudden decline in the expression of UCP2, probably as an adaptive response to avoid mitochondrial Ca²⁺ overload. Besides, TCGA analysis revealed an inverse expression correlation between proteins stabilizing mitochondrial-ER linkage and UCP2 in tissues of human breast invasive cancer and prostate adenocarcinoma. Based on these results, we assume that cancer cells successfully manage mitochondrial Ca²⁺ uptake to stimulate Ca²⁺-dependent mitochondrial metabolism while avoiding Ca²⁺-triggered cell death by fine-tuning ER-mitochondrial tethering and the expression of UCP2 in an inversed manner. Disruption of this equilibrium yields cancer cell death and may serve as a treatment strategy to specifically kill cancer cells.

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

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

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

Ca2+ signaling in T lymphocytes: the interplay of the endoplasmic reticulum, mitochondria, membrane potential, and CRAC channels on transcription factor activation

T cell receptor stimulation initiates a cascade of reactions that cause an increase in intracellular calcium (Ca²⁺) concentration mediated through inositol 1,4,5-trisphosphate (IP₃). To understand the basic mechanisms by which the immune response in T cells is activated, it is useful to understand the signaling pathways that contain important targets for drugs in a quantitative fashion. A computational model helps us to understand how the selected elements in the pathways interact with each other, and which component plays the crucial role in systems. We have developed a mathematical model to explore the mechanism for controlling transcription factor activity, which regulates gene expression, by the modulation of calcium signaling triggered during T cell activation. The model simulates the activation and modulation of Ca²⁺ release-activated Ca²⁺ (CRAC) channels by mitochondrial dynamics and depletion of endoplasmic reticulum (ER) store, and also includes membrane potential in T-cells. The model simulates the experimental finding that increases in Ca²⁺ current enhances the activation of transcription factors and the Ca²⁺ influx through CRAC is also essential for the NFAT and NFκB activation. The model also suggests that plasma membrane Ca²⁺-ATPase (PMCA) controls a majority of the extrusion of Ca²⁺ and modulates the activation of CRAC channels. Furthermore, the model simulations explain how the complex interaction of the endoplasmic reticulum, membrane potential, mitochondria, and ion channels such as CRAC channels control T cell activation.

The Puzzling Role of Neuron-Specific PMCA Isoforms in the Aging Process

The aging process is a physiological phenomenon associated with progressive changes in metabolism, genes expression, and cellular resistance to stress. In neurons, one of the hallmarks of senescence is a disturbance of calcium homeostasis that may have far-reaching detrimental consequences on neuronal physiology and function. Among several proteins involved in calcium handling, plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive calcium detector controlling calcium homeostasis. PMCA exists in four main isoforms and PMCA2 and PMCA3 are highly expressed in the brain. The overall effects of impaired calcium extrusion due to age-dependent decline of PMCA function seem to accumulate with age, increasing the susceptibility to neurotoxic insults. To analyze the PMCA role in neuronal cells, we have developed stable transfected differentiated PC12 lines with down-regulated PMCA2 or PMCA3 isoforms to mimic age-related changes. The resting Ca²⁺ increased in both PMCA-deficient lines affecting the expression of several Ca²⁺-associated proteins, i.e., sarco/endoplasmic Ca²⁺-ATPase (SERCA), calmodulin, calcineurin, GAP43, CCR5, IP₃Rs, and certain types of voltage-gated Ca²⁺ channels (VGCCs). Functional studies also demonstrated profound changes in intracellular pH regulation and mitochondrial metabolism. Moreover, modification of PMCAs membrane composition triggered some adaptive processes to counterbalance calcium overload, but the reduction of PMCA2 appeared to be more detrimental to the cells than PMCA3.

Dynamic Mitochondrial Migratory Features Associated with Calcium Responses during T Cell Antigen Recognition

A T cell clone is able to distinguish Ags in the form of peptide-MHC complexes with high specificity and sensitivity; however, how subtle differences in peptide-MHC structures translate to distinct T cell effector functions remains unknown. We hypothesized that mitochondrial positioning and associated calcium responses play an important role in T cell Ag recognition. We engineered a microfluidic system to precisely manipulate and synchronize a large number of cell-cell pairing events, which provided simultaneous real-time signaling imaging and organelle tracking with temporal precision. In addition, we developed image-derived metrics to quantify calcium response and mitochondria movement. Using myelin proteolipid altered peptide ligands and a hybridoma T cell line derived from a mouse model of experimental autoimmune encephalomyelitis, we observed that Ag potency modulates calcium response at the single-cell level. We further developed a partial least squares regression model, which highlighted mitochondrial positioning as a strong predictor of calcium response. The model revealed T cell mitochondria sharply alter direction within minutes following exposure to agonist peptide Ag, changing from accumulation at the immunological synapse to retrograde movement toward the distal end of the T cell body. By quantifying mitochondria movement as a highly dynamic process with rapidly changing phases, our result reconciles conflicting prior reports of mitochondria positioning during T cell Ag recognition. We envision applying this pipeline of methodology to study cell interactions between other immune cell types to reveal important signaling phenomenon that is inaccessible because of data-limited experimental design.

Enhanced inter-compartmental Ca2+ flux modulates mitochondrial metabolism and apoptotic threshold during aging

Background

Senescence is characterized by a gradual decline in cellular functions, including changes in energy homeostasis and decreased proliferation activity. As cellular power plants, contributors to signal transduction, sources of reactive oxygen species (ROS) and executors of programmed cell death, mitochondria are in a unique position to affect aging-associated processes of cellular decline. Notably, metabolic activation of mitochondria is tightly linked to Ca²⁺ due to the Ca²⁺ -dependency of several enzymes in the Krebs cycle, however, overload of mitochondria with Ca²⁺ triggers cell death pathways. Consequently, a machinery of proteins tightly controls mitochondrial Ca²⁺ homeostasis as well as the exchange of Ca²⁺ between the different cellular compartments, including Ca²⁺ flux between mitochondria and the endoplasmic reticulum (ER).

Methods

In this study, we investigated age-related changes in mitochondrial Ca²⁺ homeostasis, mitochondrial-ER linkage and the activity of the main ROS production site, the mitochondrial respiration chain, in an in vitro aging model based on porcine aortic endothelial cells (PAECs), using high-resolution live cell imaging, proteomics and various molecular biological methods.

Results

We describe that in aged endothelial cells, increased ER-mitochondrial Ca²⁺ crosstalk occurs due to enhanced ER-mitochondrial tethering. The close functional inter-organelle linkage increases mitochondrial Ca²⁺ uptake and thereby the activity of the mitochondrial respiration, but also makes senescent cells more vulnerable to mitochondrial Ca²⁺-overload-induced cell death. Moreover, we identified the senolytic properties of the polyphenol resveratrol, triggering cell death via mitochondrial Ca²⁺ overload exclusively in senescent cells.

Conclusion

By unveiling aging-related changes in the inter-organelle tethering and Ca²⁺ communications we have advanced the understanding of endothelial aging and highlighted a potential basis to develop drugs specifically targeting senescent cells.

Mitochondrial Ca2+ signaling

Mitochondrial Ca²⁺ regulation is crucial for bioenergetics and cellular signaling. The mechanisms controlling mitochondrial calcium homeostasis have been recently unraveled with the discovery of mitochondrial inner membrane proteins that regulate mitochondrial Ca²⁺ uptake and extrusion. Mitochondrial Ca²⁺ uptake depends on a large complex of proteins centered around the Ca²⁺ channel protein, mitochondrial Ca²⁺ uniporter (MCU) in close interactions with several regulatory subunits (MCUb, EMRE, MICU1, MICU2). Mitochondrial Ca²⁺ extrusion is mainly mediated by the mitochondrial Na⁺/Ca²⁺/Li⁺ exchanger (NCLX). Here, we review the major players of mitochondrial Ca²⁺ homeostasis and their physiological functions.

New insights into the role of mitochondrial calcium homeostasis in cell migration

Mitochondria are dynamic organelles involved in numerous physiological functions. Beyond their function in ATP production, mitochondria regulate cell death, reactive oxygen species (ROS) generation, immunity and metabolism. Mitochondria also play a key role in the buffering of cytosolic calcium, and calcium transported into the matrix regulates mitochondrial metabolism. Recently, the identification of the mitochondrial calcium uniporter (MCU) and associated regulators has allowed the characterization of new physiological roles for calcium in both mitochondrial and cellular homeostasis. Indeed, recent work has highlighted the importance of mitochondrial calcium homeostasis in regulating cell migration. Cell migration is a property common to all metazoans and is critical to embryogenesis, cancer progression, wound-healing and immune surveillance. Previous work has established that cytoplasmic calcium is a key regulator of cell migration, as oscillations in cytosolic calcium activate cytoskeletal remodelling, actin contraction and focal adhesion (FA) turnover necessary for cell movement. Recent work using animal models and in cellulo experiments to genetically modulate MCU and partners have shed new light on the role of mitochondrial calcium dynamics in cytoskeletal remodelling through the modulation of ATP and ROS production, as well as intracellular calcium signalling. This review focuses on MCU and its regulators in cell migration during physiological and pathophysiological processes including development and cancer. We also present hypotheses to explain the molecular mechanisms by which MCU may regulate mitochondrial dynamics and motility to drive cell migration.

Targeting Mitochondria to Counteract Age-Related Cellular Dysfunction

Senescence is related to the loss of cellular homeostasis and functions, which leads to a progressive decline in physiological ability and to aging-associated diseases. Since mitochondria are essential to energy supply, cell differentiation, cell cycle control, intracellular signaling and Ca²⁺ sequestration, fine-tuning mitochondrial activity appropriately, is a tightrope walk during aging. For instance, the mitochondrial oxidative phosphorylation (OXPHOS) ensures a supply of adenosine triphosphate (ATP), but is also the main source of potentially harmful levels of reactive oxygen species (ROS). Moreover, mitochondrial function is strongly linked to mitochondrial Ca²⁺ homeostasis and mitochondrial shape, which undergo various alterations during aging. Since mitochondria play such a critical role in an organism’s process of aging, they also offer promising targets for manipulation of senescent cellular functions. Accordingly, interventions delaying the onset of age-associated disorders involve the manipulation of mitochondrial function, including caloric restriction (CR) or exercise, as well as drugs, such as metformin, aspirin, and polyphenols. In this review, we discuss mitochondria’s role in and impact on cellular aging and their potential to serve as a target for therapeutic interventions against age-related cellular dysfunction.

The effects of mitochondrial inhibitors on Ca2+ signalling and electrical conductances required for pacemaking in interstitial cells of Cajal in the mouse small intestine

Interstitial cells of Cajal (ICC-MY) are pacemakers that generate and propagate electrical slow waves in gastrointestinal (GI) muscles. Slow waves appear to be generated by the release of Ca²⁺ from intracellular stores and activation of Ca²⁺-activated Cl^- channels (Ano1). Conduction of slow waves to smooth muscle cells coordinates rhythmic contractions. Mitochondrial Ca²⁺ handling is currently thought to be critical for ICC pacemaking. Protonophores, inhibitors of the electron transport chain (FCCP, CCCP or antimycin) or mitochondrial Na⁺/Ca²⁺ exchange blockers inhibited slow waves in several GI muscles. Here we utilized Ca²⁺ imaging of ICC in small intestinal muscles in situ to determine the effects of mitochondrial drugs on Ca²⁺ transients in ICC. Muscles were obtained from mice expressing a genetically encoded Ca²⁺ indicator (GCaMP3) in ICC. FCCP, CCCP, antimycin, a uniporter blocker, Ru360, and a mitochondrial Na⁺/Ca²⁺ exchange inhibitor, CGP-37157 inhibited Ca²⁺ transients in ICC-MY. Effects were not due to depletion of ATP, as oligomycin did not affect Ca²⁺ transients. Patch-clamp experiments were performed to test the effects of the mitochondrial drugs on key pacemaker conductances, Ano1 and T-type Ca²⁺ (Ca_V3.2), in HEK293 cells. Antimycin blocked Ano1 and reduced Ca_V3.2 currents. CCCP blocked Ca_V3.2 current but did not affect Ano1 current. Ano1 and Cav3.2 currents were inhibited by CGP-37157. Inhibitory effects of mitochondrial drugs on slow waves and Ca²⁺ signalling in ICC can be explained by direct antagonism of key pacemaker conductances in ICC that generate and propagate slow waves. A direct obligatory role for mitochondria in pacemaker activity is therefore questionable.

Sirolimus induces depletion of intracellular calcium stores and mitochondrial dysfunction in pancreatic beta cells. Sci Rep. 2017;7:15823. [PMID: 29158477 PMCID: PMC5696524 DOI: 10.1038/s41598-017-15283-y]

Sirolimus (rapamycin) is an immunosuppressive drug used in transplantation. One of its major side effects is the increased risk of diabetes mellitus; however, the exact mechanisms underlying such association have not been elucidated. Here we show that sirolimus impairs glucose-stimulated insulin secretion both in human and murine pancreatic islets and in clonal β cells in a dose- and time-dependent manner. Importantly, we demonstrate that sirolimus markedly depletes calcium (Ca²⁺) content in the endoplasmic reticulum and significantly decreases glucose-stimulated mitochondrial Ca²⁺ uptake. Crucially, the reduced mitochondrial Ca²⁺ uptake is mirrored by a significant impairment in mitochondrial respiration. Taken together, our findings indicate that sirolimus causes depletion of intracellular Ca²⁺ stores and alters mitochondrial fitness, eventually leading to decreased insulin release. Our results provide a novel molecular mechanism underlying the increased incidence of diabetes mellitus in patients treated with this drug.

Mitochondria sustain store-operated currents in colon cancer cells but not in normal colonic cells: reversal by non-steroidal anti-inflammatory drugs

Tumor cells undergo a critical remodeling of intracellular Ca²⁺ homeostasis that contribute to important cancer hallmarks. Store-operated Ca²⁺ entry (SOCE), a Ca²⁺ entry pathway modulated by mitochondria, is dramatically enhanced in colon cancer cells. In addition, most cancer cells display the Warburg effect, a metabolic switch from mitochondrial metabolism to glycolysis that provides survival advantages. Accordingly, we investigated mitochondria control of store-operated currents (SOCs) in two cell lines previously selected for representing human normal colonic cells and colon cancer cells. We found that, in normal cells, mitochondria are important for SOCs activity but they are unable to prevent current inactivation. In contrast, in colon cancer cells, mitochondria are dispensable for SOCs activation but are able to prevent the slow, Ca²⁺-dependent inactivation of SOCs. This effect is associated to increased ability of tumor cell mitochondria to take up Ca²⁺ due to increased mitochondrial potential (ΔΨ) linked to the Warburg effect. Consistently with this view, selected non-steroidal anti-inflammatory drugs (NSAIDs) depolarize mitochondria, inhibit mitochondrial Ca²⁺ uptake and promote SOC inactivation, leading to inhibition of both SOCE and cancer cell proliferation. Thus, mitochondria sustain store-operated currents in colon cancer cells but not in normal colonic cells and this effect is counteracted by selected NSAIDs providing a mechanism for cancer chemoprevention.

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.

GPR55 agonist lysophosphatidylinositol and lysophosphatidylcholine inhibit endothelial cell hyperpolarization via GPR-independent suppression of Na+-Ca2+ exchanger and endoplasmic reticulum Ca2+ refilling

Lysophosphatidylinositol (LPI) and lysophosphatidylcholine (LPC) are lipid signaling molecules that induce endothelium-dependent vasodilation. In addition, LPC suppresses acetylcholine (Ach)-induced responses. We aimed to determine the influence of LPC and LPI on hyperpolarizing responses in vitro and in situ endothelial cells (EC) and identify the underlying mechanisms. Using patch-clamp method, we show that LPI and LPC inhibit EC hyperpolarization to histamine and suppress Na⁺/Ca²⁺ exchanged (NCX) currents in a concentration-dependent manner. The inhibition is non-mode-specific and unaffected by intracellular GDPβS infusion and tempol, a superoxide dismutase mimetic. In excised mouse aorta, LPI strongly inhibits the sustained and the peak endothelial hyperpolarization induced by Ach, but not by SKA-31, an opener of Ca²⁺-dependent K⁺ channels of intermediate and small conductance. The hyperpolarizing responses to consecutive histamine applications are strongly reduced by NCX inhibition. In a Ca²⁺-re-addition protocol, bepridil, a NCX inhibitor, and KB-R7943, a blocker of reversed NCX, inhibit the hyperpolarizing responses to Ca²⁺-re-addition following Ca²⁺ stores depletion. These finding indicate that LPC and LPI inhibit endothelial hyperpolarization to Ach and histamine independently of G-protein coupled receptors and superoxide anions. Reversed NCX is critical for ER Ca²⁺ refilling in EC. The inhibition of NCX by LPI and LPC underlies diminished endothelium-dependent responses and endothelial dysfunction accompanied by increased levels of these lipids in the blood.

The Role of Mitochondria in the Activation/Maintenance of SOCE: Store-Operated Ca2+ Entry and Mitochondria

Mitochondria extensively modify virtually all cellular Ca²⁺ transport processes, and store-operated Ca²⁺ entry (SOCE) is no exception to this rule. The interaction between SOCE and mitochondria is complex and reciprocal, substantially altering and, ultimately, fine-tuning both capacitative Ca²⁺ influx and mitochondrial function. Mitochondria, owing to their considerable Ca²⁺ accumulation ability, extensively buffer the cytosolic Ca²⁺ in their vicinity. In turn, the accumulated ion is released back into the neighboring cytosol during net Ca²⁺ efflux. Since store depletion itself and the successive SOCE are both Ca²⁺-regulated phenomena, mitochondrial Ca²⁺ handling may have wide-ranging effects on capacitative Ca²⁺ influx at any given time. In addition, mitochondria may also produce or consume soluble factors known to affect store-operated channels. On the other hand, Ca²⁺ entering the cell during SOCE is sensed by mitochondria, and the ensuing mitochondrial Ca²⁺ uptake boosts mitochondrial energy metabolism and, if Ca²⁺ overload occurs, may even lead to apoptosis or cell death. In several cell types, mitochondria seem to be sterically excluded from the confined space that forms between the plasma membrane (PM) and endoplasmic reticulum (ER) during SOCE. This implies that high-Ca²⁺ microdomains comparable to those observed between the ER and mitochondria do not form here. In the following chapter, the above aspects of the many-sided SOCE-mitochondrion interplay will be discussed in greater detail.

Intact mitochondrial Ca2+ uniport is essential for agonist-induced activation of endothelial nitric oxide synthase (eNOS)

Mitochondrial Ca²⁺ uptake regulates diverse endothelial cell functions and has also been related to nitric oxide (NO^•) production. However, it is not entirely clear if the organelles support or counteract NO^• biosynthesis by taking up Ca²⁺. The objective of this study was to verify whether or not mitochondrial Ca²⁺ uptake influences Ca²⁺-triggered NO^• generation by endothelial NO^• synthase (eNOS) in an immortalized endothelial cell line (EA.hy926), respective primary human umbilical vein endothelial cells (HUVECs) and eNOS-RFP (red fluorescent protein) expressing human embryonic kidney (HEK293) cells. We used novel genetically encoded fluorescent NO^• probes, the geNOps, and Ca²⁺ sensors to monitor single cell NO^• and Ca²⁺ dynamics upon cell treatment with ATP, an inositol 1,4,5-trisphosphate (IP₃)-generating agonist. Mitochondrial Ca²⁺ uptake was specifically manipulated by siRNA-mediated knock-down of recently identified key components of the mitochondrial Ca²⁺ uniporter machinery. In endothelial cells and the eNOS-RFP expressing HEK293 cells we show that reduced mitochondrial Ca²⁺ uptake upon the knock-down of the mitochondrial calcium uniporter (MCU) protein and the essential MCU regulator (EMRE) yield considerable attenuation of the Ca²⁺-triggered NO^• increase independently of global cytosolic Ca²⁺ signals. The knock-down of mitochondrial calcium uptake 1 (MICU1), a gatekeeper of the MCU, increased both mitochondrial Ca²⁺ sequestration and Ca²⁺-induced NO^• signals. The positive correlation between mitochondrial Ca²⁺ elevation and NO^• production was independent of eNOS phosphorylation at serine¹¹⁷⁷. Our findings emphasize that manipulating mitochondrial Ca²⁺ uptake may represent a novel strategy to control eNOS-mediated NO^• production.

Tissue Specificity: SOCE: Implications for Ca2+ Handling in Endothelial Cells

Many cellular functions of the vascular endothelium are regulated by fine-tuned global and local, microdomain-confined changes of cytosolic free Ca²⁺ ([Ca²⁺]_i). Vasoactive agonist-induced stimulation of vascular endothelial cells (VECs) typically induces Ca²⁺ release through IP₃ receptor Ca²⁺ release channels embedded in the membrane of the endoplasmic reticulum (ER) Ca²⁺ store, followed by Ca²⁺ entry from the extracellular space elicited by Ca²⁺ store depletion and referred to as capacitative or store-operated Ca²⁺ entry (SOCE). In vascular endothelial cells, SOCE is graded with the degree of store depletion and controlled locally in the subcellular microdomain where depletion occurs. SOCE provides distinct Ca²⁺ signals that selectively control specific endothelial functions: in calf pulmonary artery endothelial cells, the SOCE Ca²⁺ signal drives nitric oxide (an endothelium-derived relaxing factor of the vascular smooth muscle) production and controls activation and nuclear translocation of the transcription factor NFAT. Both cellular events are not affected by Ca²⁺ signals of comparable magnitude arising directly from Ca²⁺ release from intracellular stores, clearly indicating that SOCE regulates specific Ca²⁺-dependent cellular tasks by a unique and exclusive mechanism. This review discusses the mechanisms of intracellular Ca²⁺ regulation in vascular endothelial cells and the role of store-operated Ca²⁺ entry for endothelium-dependent smooth muscle relaxation and nitric oxide signaling, endothelial oxidative stress response, and excitation-transcription coupling in the vascular endothelium.

The Role of Mitochondria in the Activation/Maintenance of SOCE: The Contribution of Mitochondrial Ca2+ Uptake, Mitochondrial Motility, and Location to Store-Operated Ca2+ Entry

In most cell types, the depletion of internal Ca²⁺ stores triggers the activation of Ca²⁺ entry. This crucial phenomenon is known since the 1980s and referred to as store-operated Ca²⁺ entry (SOCE). With the discoveries of the stromal-interacting molecules (STIMs) and the Ca²⁺-permeable Orai channels as the long-awaited molecular constituents of SOCE, the role of mitochondria in controlling the activity of this particular Ca²⁺ entry pathway is kind of buried in oblivion. However, the capability of mitochondria to locally sequester Ca²⁺ at sites of Ca²⁺ release and entry was initially supposed to rule SOCE by facilitating the Ca²⁺ depletion of the endoplasmic reticulum and removing entering Ca²⁺ from the Ca²⁺-inhibitable channels, respectively. Moreover, the central role of these organelles in controlling the cellular energy metabolism has been linked to the activity of SOCE. Nevertheless, the exact molecular mechanisms by which mitochondria actually determine SOCE are still pretty obscure. In this essay we describe the complexity of the mitochondrial Ca²⁺ uptake machinery and its regulation, molecular components, and properties, which open new ways for scrutinizing the contribution of mitochondria to SOCE. Moreover, data concerning the variability of the morphology and cellular distribution of mitochondria as putative determinants of SOCE activation, maintenance, and termination are summarized.

Arsenite-induced endoplasmic reticulum-dependent apoptosis through disturbance of calcium homeostasis in HBE cell line

Calcium (Ca²⁺ ) is a ubiquitous cell signal responsible for multiple fundamental cellular functions, including apoptosis. Whether the homeostasis of Ca²⁺ is involved in arsenite-induced apoptosis remains unclear. In this study, we observed that arsenite significantly elevated the intracellular Ca²⁺ concentration in a dose- and time-dependent manner. By using the Ca²⁺ -ATPase inhibitor, thapsigargin, and the inositol 1,4,5- trisphosphate receptors (IP3Rs) inhibitor, heparin, we further confirmed that the disturbance of endoplasmic reticulum (ER) Ca²⁺ homeostasis caused Ca²⁺ overload in the cells. Moreover, loss of ER Ca²⁺ homeostasis also led to ER stress, mitochondrial dysfunction, and NF-κB activation. Importantly, pretreatment of cells with heparin remarkably attenuated the elevated cell apoptosis induced by arsenite, but inhibition of ER Ca²⁺ uptake with thapsigargin exacerbated arsenite-induced cell damage significantly. Together, we demonstrated for the first time that arsenite disturbed the Ca²⁺ homeostasis in ER, which subsequently led to ER stress, mitochondrial dysfunction, and NF-κB nuclear translocation, and thus consequently triggering cell apoptosis. Our findings indicate regulation of disrupted Ca²⁺ homeostasis in ER may be a potential strategy for prevention of arsenite toxicity. © 2015 Wiley Periodicals, Inc. Environ Toxicol 32: 197-216, 2017.

Mitochondrial Ca2+ uptake controls actin cytoskeleton dynamics during cell migration

Intracellular Ca²⁺ signaling regulates cell migration by acting on cytoskeleton architecture, cell directionality and focal adhesions dynamics. In migrating cells, cytosolic Ca²⁺ pool and Ca²⁺ pulses are described as key components of these effects. Whereas the role of the mitochondrial calcium homeostasis and the Mitochondria Cacium Uniporter (MCU) in cell migration were recently highlighted in vivo using the zebrafish model, their implication in actin cystokeleton dynamics and cell migration in mammals is not totally characterized. Here, we show that mcu silencing in two human cell lines compromises their migration capacities. This phenotype is characterized by actin cytoskeleton stiffness, a cell polarization loss and an impairment of the focal adhesion proteins dynamics. At the molecular level, these effects appear to be mediated by the reduction of the ER and cytosolic Ca²⁺ pools, which leads to a decrease in Rho-GTPases, RhoA and Rac1, and Ca²⁺-dependent Calpain activites, but seem to be independent of intracellular ATP levels. Together, this study highlights the fundamental and evolutionary conserved role of the mitochondrial Ca²⁺ homeostasis in cytoskeleton dynamics and cell migration.

Resveratrol Specifically Kills Cancer Cells by a Devastating Increase in the Ca2+ Coupling Between the Greatly Tethered Endoplasmic Reticulum and Mitochondria

Background/Aims

Resveratrol and its derivate piceatannol are known to induce cancer cell-specific cell death. While multiple mechanisms of actions have been described including the inhibition of ATP synthase, changes in mitochondrial membrane potential and ROS levels, the exact mechanisms of cancer specificity of these polyphenols remain unclear. This paper is designed to reveal the molecular basis of the cancer-specific initiation of cell death by resveratrol and piceatannol.

Methods

The two cancer cell lines EA.hy926 and HeLa, and somatic short-term cultured HUVEC were used. Cell viability and caspase 3/7 activity were tested. Mitochondrial, cytosolic and endoplasmic reticulum Ca²⁺ as well as cytosolic and mitochondrial ATP levels were measured using single cell fluorescence microscopy and respective genetically-encoded sensors. Mitochondria-ER junctions were analyzed applying super-resolution SIM and ImageJ-based image analysis.

Results

Resveratrol and piceatannol selectively trigger death in cancer but not somatic cells. Hence, these polyphenols strongly enhanced mitochondrial Ca²⁺ uptake in cancer exclusively. Resveratrol and piceatannol predominantly affect mitochondrial but not cytosolic ATP content that yields in a reduced SERCA activity. Decreased SERCA activity and the strongly enriched tethering of the ER and mitochondria in cancer cells result in an enhanced MCU/Letm1-dependent mitochondrial Ca²⁺ uptake upon intracellular Ca²⁺ release exclusively in cancer cells. Accordingly, resveratrol/piceatannol-induced cancer cell death could be prevented by siRNA-mediated knock-down of MCU and Letm1.

Conclusions

Because their greatly enriched ER-mitochondria tethering, cancer cells are highly susceptible for resveratrol/piceatannol-induced reduction of SERCA activity to yield mitochondrial Ca²⁺ overload and subsequent cancer cell death.

Store-Operated Calcium Channels

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and secretion to tissue and organ development and the immune response. SOCs are activated by the depletion of Ca(2+) from the endoplasmic reticulum (ER), triggered physiologically through stimulation of a diverse set of surface receptors. Over 15 years after the first characterization of SOCs through electrophysiology, the identification of the STIM proteins as ER Ca(2+) sensors and the Orai proteins as store-operated channels has enabled rapid progress in understanding the unique mechanism of store-operate calcium entry (SOCE). Depletion of Ca(2+) from the ER causes STIM to accumulate at ER-plasma membrane (PM) junctions where it traps and activates Orai channels diffusing in the closely apposed PM. Mutagenesis studies combined with recent structural insights about STIM and Orai proteins are now beginning to reveal the molecular underpinnings of these choreographic events. This review describes the major experimental advances underlying our current understanding of how ER Ca(2+) depletion is coupled to the activation of SOCs. Particular emphasis is placed on the molecular mechanisms of STIM and Orai activation, Orai channel properties, modulation of STIM and Orai function, pharmacological inhibitors of SOCE, and the functions of STIM and Orai in physiology and disease.

Generation of Red-Shifted Cameleons for Imaging Ca²⁺ Dynamics of the Endoplasmic Reticulum

Cameleons are sophisticated genetically encoded fluorescent probes that allow quantifying cellular Ca²⁺ signals. The probes are based on Förster resonance energy transfer (FRET) between terminally located fluorescent proteins (FPs), which move together upon binding of Ca²⁺ to the central calmodulin myosin light chain kinase M13 domain. Most of the available cameleons consist of cyan and yellow FPs (CFP and YFP) as the FRET pair. However, red-shifted versions with green and orange or red FPs (GFP, OFP, RFP) have some advantages such as less phototoxicity and minimal spectral overlay with autofluorescence of cells and fura-2, a prominent chemical Ca²⁺ indicator. While GFP/OFP- or GFP/RFP-based cameleons have been successfully used to study cytosolic and mitochondrial Ca²⁺ signals, red-shifted cameleons to visualize Ca²⁺ dynamics of the endoplasmic reticulum (ER) have not been developed so far. In this study, we generated and tested several ER targeted red-shifted cameleons. Our results show that GFP/OFP-based cameleons due to miss-targeting and their high Ca²⁺ binding affinity are inappropriate to record ER Ca²⁺ signals. However, ER targeted GFP/RFP-based probes were suitable to sense ER Ca²⁺ in a reliable manner. With this study we increased the palette of cameleons for visualizing Ca²⁺ dynamics within the main intracellular Ca²⁺ store.

Mitochondrial Ca²⁺ uniporter is critical for store-operated Ca²⁺ entry-dependent breast cancer cell migration

Metastasis of cancer cells is a complicated multistep process requiring extensive and continuous cytosolic calcium modulation. Mitochondrial Ca(2+) uniporter (MCU), a regulator of mitochondrial Ca(2+) uptake, has been implicated in energy metabolism and various cellular signaling processes. However, whether MCU contributes to cancer cell migration has not been established. Here we examined the expression of MCU mRNA in the Oncomine database and found that MCU is correlated to metastasis and invasive breast cancer. MCU inhibition by ruthenium red (RuR) or MCU silencing by siRNA abolished serum-induced migration in MDA-MB-231 breast cancer cells and reduced serum- or thapsigargin (TG)-induced store-operated Ca2+ entry (SOCE). Serum-induced migrations in MDA-MB-231 cells were blocked by SOCE inhibitors. Our results demonstrate that MCU plays a critical role in breast cancer cell migration by regulating SOCE.

Rapid recycling of Ca2+ between IP3-sensitive stores and lysosomes

Inositol 1,4,5-trisphosphate (IP₃) evokes release of Ca²⁺ from the endoplasmic reticulum (ER), but the resulting Ca²⁺ signals are shaped by interactions with additional intracellular organelles. Bafilomycin A₁, which prevents lysosomal Ca²⁺ uptake by inhibiting H⁺ pumping into lysosomes, increased the amplitude of the initial Ca²⁺ signals evoked by carbachol in human embryonic kidney (HEK) cells. Carbachol alone and carbachol in combination with parathyroid hormone (PTH) evoke Ca²⁺ release from distinct IP₃-sensitive Ca²⁺ stores in HEK cells stably expressing human type 1 PTH receptors. Bafilomycin A₁ similarly exaggerated the Ca²⁺ signals evoked by carbachol or carbachol with PTH, indicating that Ca²⁺ released from distinct IP₃-sensitive Ca²⁺ stores is sequestered by lysosomes. The Ca²⁺ signals resulting from store-operated Ca²⁺ entry, whether evoked by thapsigargin or carbachol, were unaffected by bafilomycin A₁. Using Gd³⁺ (1 mM) to inhibit both Ca²⁺ entry and Ca²⁺ extrusion, HEK cells were repetitively stimulated with carbachol to assess the effectiveness of Ca²⁺ recycling to the ER after IP₃-evoked Ca²⁺ release. Blocking lysosomal Ca²⁺ uptake with bafilomycin A₁ increased the amplitude of each carbachol-evoked Ca²⁺ signal without affecting the rate of Ca²⁺ recycling to the ER. This suggests that Ca²⁺ accumulated by lysosomes is rapidly returned to the ER. We conclude that lysosomes rapidly, reversibly and selectively accumulate the Ca²⁺ released by IP₃ receptors residing within distinct Ca²⁺ stores, but not the Ca²⁺ entering cells via receptor-regulated, store-operated Ca²⁺ entry pathways.

N-Arachidonoyl glycine suppresses Na⁺/Ca²⁺ exchanger-mediated Ca²⁺ entry into endothelial cells and activates BK(Ca) channels independently of GPCRs

Background and Purpose

N-arachidonoyl glycine (NAGly) is a lipoamino acid with vasorelaxant properties. We aimed to explore the mechanisms of NAGly's action on unstimulated and agonist-stimulated endothelial cells.

Experimental Approach

The effects of NAGly on endothelial electrical signalling were studied in combination with vascular reactivity.

Key Results

In EA.hy926 cells, the sustained hyperpolarization to histamine was inhibited by the non-selective Na⁺/Ca²⁺ exchanger (NCX) inhibitor bepridil and by an inhibitor of reversed mode NCX, KB-R7943. In cells dialysed with Cs⁺-based Na⁺-containing solution, the outwardly rectifying current with typical characteristics of NCX was augmented following histamine exposure, further increased upon external Na⁺ withdrawal and inhibited by bepridil. NAGly (0.3–30 μM) suppressed NCX currents in a URB597- and guanosine 5′-O-(2-thiodiphosphate) (GDPβS)-insensitive manner, [Ca²⁺]_i elevation evoked by Na⁺ removal and the hyperpolarization to histamine. In rat aorta, NAGly opposed the endothelial hyperpolarization and relaxation response to ACh. In unstimulated EA.hy926 cells, NAGly potentiated the whole-cell current attributable to large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in a GDPβS-insensitive, paxilline-sensitive manner and produced a sustained hyperpolarization. In cell-free inside-out patches, NAGly stimulated single BK_Ca channel activity.

Conclusion and Implications

Our data showed that NCX is a Ca²⁺ entry pathway in endothelial cells and that NAGly is a potent G-protein-independent modulator of endothelial electrical signalling and has a dual effect on endothelial electrical responses. In agonist pre-stimulated cells, NAGly opposes hyperpolarization and relaxation via inhibition of NCX-mediated Ca²⁺ entry, while in unstimulated cells, it promotes hyperpolarization via receptor-independent activation of BK_Ca channels.

Mitochondrial calcium handling within the interstitial cells of Cajal

The interstitial cells of Cajal (ICC) drive rhythmic pacemaking contractions in the gastrointestinal system. The ICC generate pacemaking signals by membrane depolarizations associated with the release of intracellular calcium (Ca(2+)) in the endoplasmic reticulum (ER) through inositol-trisphosphate (IP3) receptors (IP3R) and uptake by mitochondria (MT). This Ca(2+) dynamic is hypothesized to generate pacemaking signals by calibrating ER Ca(2+) store depletions and membrane depolarization with ER store-operated Ca(2+) entry mechanisms. Using a biophysically based spatio-temporal model of integrated Ca(2+) transport in the ICC, we determined the feasibility of ER depletion timescale correspondence with experimentally observed pacemaking frequencies while considering the impact of IP3R Ca(2+) release and MT uptake on bulk cytosolic Ca(2+) levels because persistent elevations of free intracellular Ca(2+) are toxic to the cell. MT densities and distributions are varied in the model geometry to observe MT influence on free cytosolic Ca(2+) and the resulting frequencies of ER Ca(2+) store depletions, as well as the sarco-endoplasmic reticulum Ca(2+) ATP-ase (SERCA) and IP3 agonist concentrations. Our simulations show that high MT densities observed in the ICC are more relevant to ER establishing Ca(2+) depletion frequencies than protection of the cytosol from elevated free Ca(2+), whereas the SERCA pump is more relevant to containing cytosolic Ca(2+) elevations. Our results further suggest that the level of IP3 agonist stimulating ER Ca(2+) release, subsequent MT uptake, and eventual activation of ER store-operated Ca(2+) entry may determine frequencies of rhythmic pacemaking exhibited by the ICC across species and tissue types.

IP3-mediated STIM1 oligomerization requires intact mitochondrial Ca2+ uptake

Mitochondria contribute to cell signaling by controlling store-operated Ca²⁺ entry (SOCE). SOCE is activated by Ca²⁺ release from the endoplasmic reticulum (ER), whereupon stromal interacting molecule 1 (STIM1) forms oligomers, redistributes to ER–plasma-membrane junctions and opens plasma membrane Ca²⁺ channels. The mechanisms by which mitochondria interfere with the complex process of SOCE are insufficiently clarified. In this study, we used an shRNA approach to investigate the direct involvement of mitochondrial Ca²⁺ buffering in SOCE. We demonstrate that knockdown of either of two proteins that are essential for mitochondrial Ca²⁺ uptake, the mitochondrial calcium uniporter (MCU) or uncoupling protein 2 (UCP2), results in decelerated STIM1 oligomerization and impaired SOCE following cell stimulation with an inositol-1,4,5-trisphosphate (IP₃)-generating agonist. Upon artificially augmented cytosolic Ca²⁺ buffering or ER Ca²⁺ depletion by sarcoplasmic or endoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitors, STIM1 oligomerization did not rely on intact mitochondrial Ca²⁺ uptake. However, MCU-dependent mitochondrial sequestration of Ca²⁺ entering through the SOCE pathway was essential to prevent slow deactivation of SOCE. Our findings show a stimulus-specific contribution of mitochondrial Ca²⁺ uptake to the SOCE machinery, likely through a role in shaping cytosolic Ca²⁺ micro-domains.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Organelle communication: signaling crossroads between homeostasis and disease

Cellular organelles do not function as isolated or static units, but rather form dynamic contacts between one another that can be modulated according to cellular needs. The physical interfaces between organelles are important for Ca2+ and lipid homeostasis, and serve as platforms for the control of many essential functions including metabolism, signaling, organelle integrity and execution of the apoptotic program. Emerging evidence also highlights the importance of organelle communication in disorders such as Alzheimer's disease, pulmonary arterial hypertension, cancer, skeletal and cardiac muscle dysfunction. Here, we provide an overview of the current literature on organelle communication and the link to human pathologies.

Separate Ca2+ sources are buffered by distinct Ca2+ handling systems in aplysia neuroendocrine cells

Although the contribution of Ca(2+) buffering systems can vary between neuronal types and cellular compartments, it is unknown whether distinct Ca(2+) sources within a neuron have different buffers. As individual Ca(2+) sources can have separate functions, we propose that each is handled by unique systems. Using Aplysia californica bag cell neurons, which initiate reproduction through an afterdischarge involving multiple Ca(2+)-dependent processes, we investigated the role of endoplasmic reticulum (ER) and mitochondrial sequestration, as well as extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, to the clearance of voltage-gated Ca(2+) influx, Ca(2+)-induced Ca(2+)-release (CICR), and store-operated Ca(2+) influx. Cultured bag cell neurons were filled with the Ca(2+) indicator, fura-PE3, to image Ca(2+) under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca(2+) influx followed by EGTA-sensitive CICR from the mitochondria. A compartment model of Ca(2+) indicated the effect of EGTA on CICR was due to buffering of released mitochondrial Ca(2+) rather than uptake competition. Removal of voltage-gated Ca(2+) influx was dominated by the mitochondria and PMCA, with no contribution from the Na(+)/Ca(2+) exchanger or sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA). In contrast, CICR recovery was slowed by eliminating the Na(+)/Ca(2+) exchanger and PMCA. Last, store-operated influx, evoked by ER depletion, was removed by the SERCA and depended on the mitochondrial membrane potential. Our results demonstrate that distinct buffering systems are dedicated to particular Ca(2+) sources. In general, this may represent a means to differentially regulate Ca(2+)-dependent processes, and for Aplysia, influence how reproductive behavior is triggered.

Silencing of the Charcot-Marie-Tooth disease-associated gene GDAP1 induces abnormal mitochondrial distribution and affects Ca2+ homeostasis by reducing store-operated Ca2+ entry

GDAP1 is an outer mitochondrial membrane protein that acts as a regulator of mitochondrial dynamics. Mutations of the GDAP1 gene cause Charcot-Marie-Tooth (CMT) neuropathy. We show that GDAP1 interacts with the vesicle-organelle trafficking proteins RAB6B and caytaxin, which suggests that GDAP1 may participate in the mitochondrial movement within the cell. GDAP1 silencing in the SH-SY5Y cell line induces abnormal distribution of the mitochondrial network, reduces the contact between mitochondria and endoplasmic reticulum (ER) and alters the mobilization of mitochondria towards plasma membrane upon depletion of ER-Ca(2+) stores. GDAP1 silencing does not affect mitochondrial Ca(2+) uptake, ER-Ca(2+), or Ca(2+) flow from ER to mitochondria, but reduces Ca(2+) inflow through store-operated Ca(2+) entry (SOCE) following mobilization of ER-Ca(2+) and SOCE-driven Ca(2+) entry in mitochondria. Our studies suggest that the pathophysiology of GDAP1-related CMT neuropathies may be associated with abnormal distribution and movement of mitochondria throughout cytoskeleton towards the ER and subplasmalemmal microdomains, resulting in a decrease in SOCE activity and impaired SOCE-driven Ca(2+) uptake in mitochondria.

The endocannabinoid N-arachidonoyl glycine (NAGly) inhibits store-operated Ca2+ entry by preventing STIM1-Orai1 interaction

The endocannabiniod anandamide (AEA) and its derivate N-arachidonoyl glycine (NAGly) have a broad spectrum of physiological effects, which are induced by both binding to receptors and receptor-independent modulations of ion channels and transporters. The impact of AEA and NAGly on store-operated Ca(2+) entry (SOCE), a ubiquitous Ca(2+) entry pathway regulating many cellular functions, is unknown. Here we show that NAGly, but not AEA reversibly hinders SOCE in a time- and concentration-dependent manner. The inhibitory effect of NAGly on SOCE was found in the human endothelial cell line EA.hy926, the rat pancreatic β-cell line INS-1 832/13, and the rat basophilic leukemia cell line RBL-2H3. NAGly diminished SOCE independently from the mode of Ca(2+) depletion of the endoplasmic reticulum, whereas it had no effect on Ca(2+) entry through L-type voltage-gated Ca(2+) channels. Enhanced Ca(2+) entry was effectively hampered by NAGly in cells overexpressing the key molecular constituents of SOCE, stromal interacting molecule 1 (STIM1) and the pore-forming subunit of SOCE channels, Orai1. Fluorescence microscopy revealed that NAGly did not affect STIM1 oligomerization, STIM1 clustering, or the colocalization of STIM1 with Orai1, which were induced by Ca(2+) depletion of the endoplasmic reticulum. In contrast, independently from its slow depolarizing effect on mitochondria, NAGly instantly and strongly diminished the interaction of STIM1 with Orai1, indicating that NAGly inhibits SOCE primarily by uncoupling STIM1 from Orai1. In summary, our findings revealed the STIM1-Orai1-mediated SOCE machinery as a molecular target of NAGly, which might have many implications in cell physiology.

Spatiotemporal correlations between cytosolic and mitochondrial Ca(2+) signals using a novel red-shifted mitochondrial targeted cameleon

The transfer of Ca²⁺ from the cytosol into the lumen of mitochondria is a crucial process that impacts cell signaling in multiple ways. Cytosolic Ca²⁺ ([Ca²⁺]_cyto) can be excellently quantified with the ratiometric Ca²⁺ probe fura-2, while genetically encoded Förster resonance energy transfer (FRET)-based fluorescent Ca²⁺ sensors, the cameleons, are efficiently used to specifically measure Ca²⁺ within organelles. However, because of a significant overlap of the fura-2 emission with the spectra of the cyan and yellow fluorescent protein of most of the existing cameleons, the measurement of fura-2 and cameleons within one given cell is a complex task. In this study, we introduce a novel approach to simultaneously assess [Ca²⁺]_cyto and mitochondrial Ca²⁺ ([Ca²⁺]_mito) signals at the single cell level. In order to eliminate the spectral overlap we developed a novel red-shifted cameleon, D1GO-Cam, in which the green and orange fluorescent proteins were used as the FRET pair. This ratiometric Ca²⁺ probe could be successfully targeted to mitochondria and was suitable to be used simultaneously with fura-2 to correlate [Ca²⁺]_cyto and [Ca²⁺]_mito within same individual cells. Our data indicate that depending on the kinetics of [Ca²⁺]_cyto rises there is a significant lag between onset of [Ca²⁺]_cyto and [Ca²⁺]_mito signals, pointing to a certain threshold of [Ca²⁺]_cyto necessary to activate mitochondrial Ca²⁺ uptake. The temporal correlation between [Ca²⁺]_mito and [Ca²⁺]_cyto as well as the efficiency of the transfer of Ca²⁺ from the cytosol into mitochondria varies between different cell types. Moreover, slow mitochondrial Ca²⁺ extrusion and a desensitization of mitochondrial Ca²⁺ uptake cause a clear difference in patterns of mitochondrial and cytosolic Ca²⁺ oscillations of pancreatic beta-cells in response to D-glucose.

Endothelial mitochondria--less respiration, more integration

Lining the inner surface of the circulatory system, the vascular endothelium accomplishes a vast variety of specialized functions. Even slight alterations of these functions are implicated in the development of certain cardiovascular diseases that represent major causes of morbidity and mortality in developed countries. Endothelial mitochondria are essential to the functional integrity of the endothelial cell as they integrate a wide range of cellular processes including Ca²⁺ handling, redox signaling and apoptosis, all of which are closely interrelated. Growing evidence supports the notion that impairment of mitochondrial signaling in the endothelium is an early event and a causative factor in the development of diseases such as atherosclerosis or diabetic complications. In this review, we want to outline the significance of mitochondria in both physiology and pathology of the vascular endothelium.

Inhibition of autophagy rescues palmitic acid-induced necroptosis of endothelial cells

Accumulation of palmitic acid (PA) in cells from nonadipose tissues is known to induce lipotoxicity resulting in cellular dysfunction and death. The exact molecular pathways of PA-induced cell death are still mysterious. Here, we show that PA triggers autophagy, which did not counteract but in contrast promoted endothelial cell death. The PA-induced cell death was predominantly necrotic as indicated by annexin V and propidium iodide (PI) staining, absence of caspase activity, low levels of DNA hypoploidy, and an early ATP depletion. In addition PA induced a strong elevation of mRNA levels of ubiquitin carboxyl-terminal hydrolase (CYLD), a known mediator of necroptosis. Moreover, siRNA-mediated knockdown of CYLD significantly antagonized PA-induced necrosis of endothelial cells. In contrast, inhibition and knockdown of receptor interacting protein kinase 1 (RIPK1) had no effect on PA-induced necrosis, indicating the induction of a CYLD-dependent but RIPK1-independent cell death pathway. PA was recognized as a strong and early inducer of autophagy. The inhibition of autophagy by both pharmacological inhibitors and genetic knockdown of the autophagy-specific genes, vacuolar protein sorting 34 (VPS34), and autophagy-related protein 7 (ATG7), could rescue the PA-induced death of endothelial cells. Moreover, the initiation of autophagy and cell death by PA was reduced in endothelial cells loaded with the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-(acetoxymethyl) ester (BAPTA-AM), indicating that Ca(2+) triggers the fatal signaling of PA. In summary, we introduce an unexpected mechanism of lipotoxicity in endothelial cells and provide several novel strategies to counteract the lipotoxic signaling of PA.