Ca2+ shuttling between endoplasmic reticulum and mitochondria underlying Ca2+ oscillations

Temporal multiscale modeling of biochemical regulatory networks: Calcium-regulated hepatocyte lipid and glucose metabolism

Hepatocyte lipid and glucose metabolism is regulated not only by major hormones like insulin and glucagon but also by many other factors, including calcium ions. Recently, mitochondria-associated membrane (MAM) dysfunction combined with incorrect IP3-receptor regulation has been shown to result in abnormal calcium signaling in hepatocytes. This dysfunction could further lead to hepatic metabolism pathology. However, the exact contribution of MAM dysfunction, incorrect IP3-receptor regulation and insulin resistance to the calcium-insulin-glucagon interplay is not understood yet. In this work, we analyze the role of abnormal calcium signaling and insulin dysfunction in hepatocytes by proposing a model of hepatocyte metabolic regulatory network with a detailed focus on the model construction details besides the biological aspect. In this work, we analyze the role of abnormal calcium signaling and insulin dysfunction in hepatocytes by proposing a model of hepatocyte metabolic regulatory network. We focus on the model construction details, model validation, and predictions. We describe the dynamic regulation of signaling processes by sigmoid Hill function. In particular, we study the effect of both the Hill function slope and the distance between Hill function extremes on metabolic processes in hepatocytes as a model of nonspecific insulin dysfunction. We also address the significant time difference between characteristic time of glucose hepatic processing and a typical calcium oscillation period in hepatocytes. Our modeling results show that calcium signaling dysfunction results in an abnormal increase in postprandial glucose levels, an abnormal glucose decrease in fasting, and a decreased amount of stored glycogen. An insulin dysfunction of glucose phosphorylation, glucose dephosphorylation, and glycogen breakdown also cause a noticeable effect. We also get some insight into the so-called hepatic insulin resistance paradox, confirming the hypothesis regarding indirect insulin action on hepatocytes via dysfunctional adipocyte lipolysis.

Systemic cellular migration: The forces driving the directed locomotion movement of cells

Directional motility is an essential property of cells. Despite its enormous relevance in many fundamental physiological and pathological processes, how cells control their locomotion movements remains an unresolved question. Here, we have addressed the systemic processes driving the directed locomotion of cells. Specifically, we have performed an exhaustive study analyzing the trajectories of 700 individual cells belonging to three different species (Amoeba proteus, Metamoeba leningradensis, and Amoeba borokensis) in four different scenarios: in absence of stimuli, under an electric field (galvanotaxis), in a chemotactic gradient (chemotaxis), and under simultaneous galvanotactic and chemotactic stimuli. All movements were analyzed using advanced quantitative tools. The results show that the trajectories are mainly characterized by coherent integrative responses that operate at the global cellular scale. These systemic migratory movements depend on the cooperative nonlinear interaction of most, if not all, molecular components of cells.

Ca2+ oscillation in vascular smooth muscle cells control myogenic spontaneous vasomotion and counteract post-ischemic no-reflow

Ischemic stroke produces the highest adult disability. Despite successful recanalization, no-reflow, or the futile restoration of the cerebral perfusion after ischemia, is a major cause of brain lesion expansion. However, the vascular mechanism underlying this hypoperfusion is largely unknown, and no approach is available to actively promote optimal reperfusion to treat no-reflow. Here, by combining two-photon laser scanning microscopy (2PLSM) and a mouse middle cerebral arteriolar occlusion (MCAO) model, we find myogenic vasomotion deficits correlated with post-ischemic cerebral circulation interruptions and no-reflow. Transient occlusion-induced transient loss of mitochondrial membrane potential (ΔΨm) permanently impairs mitochondria-endoplasmic reticulum (ER) contacts and abolish Ca2+ oscillation in smooth muscle cells (SMCs), the driving force of myogenic spontaneous vasomotion. Furthermore, tethering mitochondria and ER by specific overexpression of ME-Linker in SMCs restores cytosolic Ca2+ homeostasis, remotivates myogenic spontaneous vasomotion, achieves optimal reperfusion, and ameliorates neurological injury. Collectively, the maintaining of arteriolar myogenic vasomotion and mitochondria-ER contacts in SMCs, are of critical importance in preventing post-ischemic no-reflow.

An integrate-and-fire approach to Ca2+ signaling. Part II: Cumulative refractoriness

Inositol 1,4,5-trisphosphate-induced Ca2+ signaling is a second messenger system used by almost all eukaryotic cells. The agonist concentration stimulating Ca2+ signals is encoded in the frequency of a Ca2+ concentration spike sequence. When a cell is stimulated, the interspike intervals (ISIs) often show a distinct transient during which they gradually increase, a system property we refer to as cumulative refractoriness. We extend a previously published stochastic model to include the Ca2+ concentration in the intracellular Ca2+ store as a slow adaptation variable. This model can reproduce both stationary and transient statistics of experimentally observed ISI sequences. We derive approximate expressions for the mean and coefficient of variation of the stationary ISIs. We also consider the response to the onset of a constant stimulus and estimate the length of the transient and the strength of the adaptation of the ISI. We show that the adaptation sets the coefficient of variation in agreement with current ideas derived from experiments. Moreover, we explain why, despite a pronounced transient behavior, ISI correlations can be weak, as often observed in experiments. Finally, we fit our model to reproduce the transient statistics of experimentally observed ISI sequences in stimulated HEK cells. The fitted model is able to qualitatively reproduce the relationship between the stationary interval correlations and the number of transient intervals, as well as the strength of the ISI adaptation. We also find positive correlations in the experimental sequence that cannot be explained by our model.

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

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

Shaped by leaky ER: Homeostatic Ca2+ fluxes

At any moment in time, cells coordinate and balance their calcium ion (Ca²⁺) fluxes. The term ‘Ca²⁺ homeostasis’ suggests that balancing resting Ca²⁺ levels is a rather static process. However, direct ER Ca²⁺ imaging shows that resting Ca²⁺ levels are maintained by surprisingly dynamic Ca²⁺ fluxes between the ER Ca²⁺ store, the cytosol, and the extracellular space. The data show that the ER Ca²⁺ leak, continuously fed by the high-energy consuming SERCA, is a fundamental driver of resting Ca²⁺ dynamics. Based on simplistic Ca²⁺ toolkit models, we discuss how the ER Ca²⁺ leak could contribute to evolutionarily conserved Ca²⁺ phenomena such as Ca²⁺ entry, ER Ca²⁺ release, and Ca²⁺ oscillations.

Organelle transporters and inter-organelle communication as drivers of metabolic regulation and cellular homeostasis

Background

Spatial compartmentalization of metabolic pathways within membrane-separated organelles is key to the ability of eukaryotic cells to precisely regulate their biochemical functions. Membrane-bound organelles such as mitochondria, endoplasmic reticulum (ER) and lysosomes enable the concentration of metabolic precursors within optimized chemical environments, greatly accelerating the efficiency of both anabolic and catabolic reactions, enabling division of labor and optimal utilization of resources. However, metabolic compartmentalization also poses a challenge to cells because it creates spatial discontinuities that must be bridged for reaction cascades to be connected and completed. To do so, cells employ different methods to coordinate metabolic fluxes occurring in different organelles, such as membrane-localized transporters to facilitate regulated metabolite exchange between mitochondria and lysosomes, non-vesicular transport pathways via physical contact sites connecting the ER with both mitochondria and lysosomes, as well as localized regulatory signaling processes that coordinately regulate the activity of all these organelles.

Scope of review

This review covers how cells use membrane transporters, membrane contact sites, and localized signaling pathways to mediate inter-organelle communication and coordinate metabolism. We also describe how disruption of inter-organelle communication is an emerging driver in a multitude of diseases, from cancer to neurodegeneration.

Major conclusions

Effective communication among organelles is essential to cellular health and function. Identifying the major molecular players involved in mediating metabolic coordination between organelles will further our understanding of cellular metabolism in health and lead us to design better therapeutics against dysregulated metabolism in disease.

Insights Into Mitochondrial Dynamics in Chlamydial Infection

Mitochondria are intracellular organelles that are instrumental in the creation of energy, metabolism, apoptosis, and intrinsic immunity. Mitochondria exhibit an extraordinarily high degree of flexibility, and are constantly undergoing dynamic fusion and fission changes. Chlamydia is an intracellular bacterium that causes serious health problems in both humans and animals. Due to a deficiency of multiple metabolic enzymes, these pathogenic bacteria are highly dependent on their eukaryotic host cells, resulting in a close link between Chlamydia infection and host cell mitochondria. Indeed, Chlamydia increase mitochondrial fusion by inhibiting the activation of dynein-related protein 1 (DRP1), which can regulate host cell metabolism for extra energy. Additionally, Chlamydia can inhibit mitochondrial fission by blocking DRP1 oligomerization, preventing host cell apoptosis. These mechanisms are critical for maintaining a favorable environment for reproduction and growth of Chlamydia. This review discusses the molecular mechanisms of mitochondrial fusion and fission, as well as the mechanisms by which Chlamydia infection alters the mitochondrial dynamics and the prospects of limiting chlamydial development by altering mitochondrial dynamics.

Ibogaine-Mediated ROS/Antioxidant Elevation in Isolated Rat Uterus Is β-Adrenergic Receptors and KATP Channels Mediated

Ibogaine effects are mediated by cellular receptors, ATP depletion followed by ROS production and antioxidant enzyme activity elevation in a dose and time dependent manner. Since the role of K_ATP channels and β-adrenoceptors in ROS cellular circuit was established here we explored their role in ibogaine pro-antioxidant effectiveness. Single dose of ibogaine (10 mg/L i.e., 28.8 μmol/L) was applied to isolated rat uterus (spontaneous and Ca²⁺-stimulated) and contractility and antioxidant enzymes activity were monitored during 4 h. Ibogaine increased amplitude and frequency of spontaneous active uteri immediately after addition that was prevented by propranolol (β₁ and β₂ adrenoceptors selective antagonists) and glibenclamide (K_ATP sensitive channels inhibitor; only frequency) pre-treatment. In Ca²⁺-stimulated uteri, ibogaine decreased both amplitude and frequency after 4 h. Pre-treatment with propranolol abolished ibogaine induced amplitude lowering, while glibenclamide had no effect. In both types of active uterus, ibogaine induced a decrease in SOD1 and an increase in CAT activity after 2 h. In Ca²⁺-stimulated uterus, there was also a decrease of SOD2 activity after 2 h. After 4 h, SOD1 activity returned to the baseline level, but GSH-Px activity increased. Pre-treatment with both propranolol and glibenclamide abolished observed changes of antioxidant enzymes activity suggesting that ibogaine pro-antioxidative effectiveness is β-adrenergic receptors and K_ATP channels mediated.

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.

Self-Organization and Information Processing: From Basic Enzymatic Activities to Complex Adaptive Cellular Behavior

One of the main aims of current biology is to understand the origin of the molecular organization that underlies the complex dynamic architecture of cellular life. Here, we present an overview of the main sources of biomolecular order and complexity spanning from the most elementary levels of molecular activity to the emergence of cellular systemic behaviors. First, we have addressed the dissipative self-organization, the principal source of molecular order in the cell. Intensive studies over the last four decades have demonstrated that self-organization is central to understand enzyme activity under cellular conditions, functional coordination between enzymatic reactions, the emergence of dissipative metabolic networks (DMN), and molecular rhythms. The second fundamental source of order is molecular information processing. Studies on effective connectivity based on transfer entropy (TE) have made possible the quantification in bits of biomolecular information flows in DMN. This information processing enables efficient self-regulatory control of metabolism. As a consequence of both main sources of order, systemic functional structures emerge in the cell; in fact, quantitative analyses with DMN have revealed that the basic units of life display a global enzymatic structure that seems to be an essential characteristic of the systemic functional metabolism. This global metabolic structure has been verified experimentally in both prokaryotic and eukaryotic cells. Here, we also discuss how the study of systemic DMN, using Artificial Intelligence and advanced tools of Statistic Mechanics, has shown the emergence of Hopfield-like dynamics characterized by exhibiting associative memory. We have recently confirmed this thesis by testing associative conditioning behavior in individual amoeba cells. In these Pavlovian-like experiments, several hundreds of cells could learn new systemic migratory behaviors and remember them over long periods relative to their cell cycle, forgetting them later. Such associative process seems to correspond to an epigenetic memory. The cellular capacity of learning new adaptive systemic behaviors represents a fundamental evolutionary mechanism for cell adaptation.

The mitochondrial calcium homeostasis orchestra plays its symphony: Skeletal muscle is the guest of honor

Skeletal muscle mitochondria are placed in close proximity of the sarcoplasmic reticulum (SR), the main intracellular Ca²⁺ store. During muscle activity, excitation of sarcolemma and of T-tubule triggers the release of Ca²⁺ from the SR initiating myofiber contraction. The rise in cytosolic Ca²⁺ determines the opening of the mitochondrial calcium uniporter (MCU), the highly selective channel of the inner mitochondrial membrane (IMM), causing a robust increase in mitochondrial Ca²⁺ uptake. The Ca²⁺-dependent activation of TCA cycle enzymes increases the synthesis of ATP required for SERCA activity. Thus, Ca²⁺ is transported back into the SR and cytosolic [Ca²⁺] returns to resting levels eventually leading to muscle relaxation. In recent years, thanks to the molecular identification of MCU complex components, the role of mitochondrial Ca²⁺ uptake in the pathophysiology of skeletal muscle has been uncovered. In this chapter, we will introduce the reader to a general overview of mitochondrial Ca²⁺ accumulation. We will tackle the key molecular players and the cellular and pathophysiological consequences of mitochondrial Ca²⁺ dyshomeostasis. In the second part of the chapter, we will discuss novel findings on the physiological role of mitochondrial Ca²⁺ uptake in skeletal muscle. Finally, we will examine the involvement of mitochondrial Ca²⁺ signaling in muscle diseases.

Molecular nature and physiological role of the mitochondrial calcium uniporter channel

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

Calcium-dependent insulin resistance in hepatocytes: mathematical model

Hepatocyte insulin resistance is one of the early factors of developing type II diabetes. If insulin resistance is treated early, type II diabetes could be prevented. In recent years, scientists have been conducting extensive research on the underlying issues on a cellular and molecular level. It was found that the modulation of IP₃-receptors, the mitochondrial ability to form the mitochondria-associated membranes (MAMs) and the endoplasmic reticulum stress during Ca²⁺ signaling play a key role in hepatocyte being able to maintain euglycemia and provide metabolic flexibility. However, researchers cannot agree on what factor is the key one in resulting in insulin resistance. In this work, we propose a mathematical model of Ca²⁺ signaling. We included in the model all the major contributors of a proper Ca²⁺ signaling during both the fasting and the postprandial state. Our modeling results are in good agreement with available experimental data. The analysis of modeling results suggests that MAMs dysfunction alone cannot result in abnormal Ca²⁺ signaling and the wrong modulation of IP₃-receptors is a more definite reason. However, both the MAMs dysfunction and the IP₃ signaling dysregulation combined can lead to a robust Ca²⁺ signal and improper glucose release. In addition, our model results suggest a strong dependence of Ca²⁺ oscillations pattern on morphological characteristics of the ER and the mitochondria.

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.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Red fluorescent CEPIA indicators for visualization of Ca2+ dynamics in mitochondria

Mitochondrial Ca2+ dynamics are involved in the regulation of multifarious cellular processes, including intracellular Ca2+ signalling, cell metabolism and cell death. Use of mitochondria-targeted genetically encoded Ca2+ indicators has revealed intercellular and subcellular heterogeneity of mitochondrial Ca2+ dynamics, which are assumed to be determined by distinct thresholds of Ca2+ increases at each subcellular mitochondrial domain. The balance between Ca2+ influx through the mitochondrial calcium uniporter and extrusion by cation exchangers across the inner mitochondrial membrane may define the threshold; however, the precise mechanisms remain to be further explored. We here report the new red fluorescent genetically encoded Ca2+ indicators, R-CEPIA3mt and R-CEPIA4mt, which are targeted to mitochondria and their Ca2+ affinities are engineered to match the intramitochondrial Ca2+ concentrations. They enable visualization of mitochondrial Ca2+ dynamics with high spatiotemporal resolution in parallel with the use of green fluorescent probes and optogenetic tools. Thus, R-CEPIA3mt and R-CEPIA4mt are expected to be a useful tool for elucidating the mechanisms of the complex mitochondrial Ca2+ dynamics and their functions.

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

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

Cytoplasmic and Mitochondrial Calcium Signaling: A Two-Way Relationship

Intracellular Ca²⁺ signals are well organized in all cell types, and trigger a variety of vital physiological processes. The temporal and spatial characteristics of cytosolic Ca²⁺ increases are mainly governed by the fluxes of this ion across the membrane of the endoplasmic/sarcoplasmic reticulum and the plasma membrane. However, various Ca²⁺ transporters also allow for Ca²⁺ exchanges between the cytoplasm and mitochondria. Increases in mitochondrial Ca²⁺ stimulate the production of ATP, which allows the cells to cope with the increased energy demand created by the stimulus. Less widely appreciated is the fact that Ca²⁺ handling by mitochondria also shapes cytosolic Ca²⁺ signals. Indeed, the frequency, amplitude, and duration of cytosolic Ca²⁺ increases can be altered by modifying the rates of Ca²⁺ transport into, or from, mitochondria. In this review, we focus on the interplay between mitochondria and Ca²⁺ signaling, highlighting not only the consequences of cytosolic Ca²⁺ changes on mitochondrial Ca²⁺, but also how cytosolic Ca²⁺ dynamics is controlled by modifications of the Ca²⁺-handling properties and the metabolism of mitochondria.

Live-Cell Imaging of Physiologically Relevant Metal Ions Using Genetically Encoded FRET-Based Probes

Essential biochemical reactions and processes within living organisms are coupled to subcellular fluctuations of metal ions. Disturbances in cellular metal ion homeostasis are frequently associated with pathological alterations, including neurotoxicity causing neurodegeneration, as well as metabolic disorders or cancer. Considering these important aspects of the cellular metal ion homeostasis in health and disease, measurements of subcellular ion signals are of broad scientific interest. The investigation of the cellular ion homeostasis using classical biochemical methods is quite difficult, often even not feasible or requires large cell numbers. Here, we report of genetically encoded fluorescent probes that enable the visualization of metal ion dynamics within individual living cells and their organelles with high temporal and spatial resolution. Generally, these probes consist of specific ion binding domains fused to fluorescent protein(s), altering their fluorescent properties upon ion binding. This review focuses on the functionality and potential of these genetically encoded fluorescent tools which enable monitoring (sub)cellular concentrations of alkali metals such as K⁺, alkaline earth metals including Mg²⁺ and Ca²⁺, and transition metals including Cu⁺/Cu²⁺ and Zn²⁺. Moreover, we discuss possible approaches for the development and application of novel metal ion biosensors for Fe²⁺/Fe³⁺, Mn²⁺ and Na⁺.

Visualization of astrocytic intracellular Ca2+ mobilization

Astrocytes generate robust intracellular Ca²⁺ concentration changes (Ca²⁺ signals), which are assumed to regulate astrocytic functions that play crucial roles in the regulation of brain functions. One frequently used strategy for exploring the role of astrocytic Ca²⁺ signalling is the use of mice deficient in the type 2 inositol 1,4,5-trisphosphate receptor (IP₃ R2). These IP₃ R2-knockout (KO) mice are reportedly devoid of Ca²⁺ mobilization from the endoplasmic reticulum (ER) in astrocytes. However, they have shown no functional deficits in several studies, causing a heated debate as to the functional relevance of ER-mediated Ca²⁺ signalling in astrocytes. Recently, the assumption that Ca²⁺ mobilization from the ER is absent in IP₃ R2-KO astrocytes has been re-evaluated using intraorganellar Ca²⁺ imaging techniques. The new results indicated that IP₃ R2-independent Ca²⁺ release may generate Ca²⁺ nanodomains around the ER, which may help explain the absence of functional deficits in IP₃ R2-KO mice.

Coding and decoding of oscillatory Ca2+ signals

About 30 years after their first observation, Ca²⁺ oscillations are now recognised as a universal mechanism of signal transduction. These oscillations are driven by periodic cycles of release and uptake of Ca²⁺ between the cytoplasm and the endoplasmic reticulum. Their frequency often increases with the level of stimulation, which can be decoded by some molecules. However, it is becoming increasingly evident that the widespread core oscillatory mechanism is modulated in many ways, depending on the cell type and on the physiological conditions. Interplay with inositol 1,4,5-trisphosphate metabolism and with other Ca²⁺ stores as the extracellular medium or mitochondria can much affect the properties of these oscillations. In many cases, these finely tuned characteristics of Ca²⁺ oscillations impact the physiological response that is triggered by the signal. Moreover, oscillations are intrinsically irregular. This randomness can also be exploited by the cell. In this review, we discuss evidences of these additional manifestations of the versatility of Ca²⁺ signalling.

Role of Endoplasmic Reticulum-Mediated Ca2+ Signaling in Neuronal Cell Death

SIGNIFICANCE

Properly controlled intracellular Ca²⁺ dynamics is crucial for regulation of neuronal function and survival in the central nervous system. The endoplasmic reticulum (ER), a major intracellular Ca²⁺ store, plays a critical role as a source and sink for neuronal Ca²⁺. Recent Advances: Accumulating evidence indicates that disrupted ER Ca²⁺ signaling is involved in neuronal cell death under various pathological conditions, providing novel insight into neurodegenerative disease mechanisms.

CRITICAL ISSUES

We summarize current knowledge concerning the relationship between abnormal ER Ca²⁺ dynamics and neuronal cell death. We also introduce recent technical advances for probing ER intraluminal Ca²⁺ dynamics with unprecedented spatiotemporal resolution.

FUTURE DIRECTIONS

Further studies on ER Ca²⁺ signaling are expected to provide progress for unmet medical needs in neurodegenerative disease. Antioxid. Redox Signal. 29, 1147-1157.

Modeling-Based Investigation of the Effect of Noise in Cellular Systems

Noise is pervasive in cellular biology and inevitably affects the dynamics of cellular processes. Biological systems have developed regulatory mechanisms to ensure robustness with respect to noise or to take advantage of stochasticity. We review here, through a couple of selected examples, some insights on possible robustness factors and constructive roles of noise provided by computational modeling. In particular, we focus on (1) factors that likely contribute to the robustness of oscillatory processes such as the circadian clocks and the cell cycle, (2) how reliable coding/decoding of calcium-mediated signaling could be achieved in presence of noise and, in some cases, enhanced through stochastic resonance, and (3) how embryonic cell differentiation processes can exploit stochasticity to create heterogeneity in a population of identical cells.

Mitochondria-associated ER membranes (MAMs) and lysosomal storage diseases

Lysosomal storage diseases (LSDs) comprise a large group of disorders of catabolism, mostly due to deficiency of a single glycan-cleaving hydrolase. The consequent endo-lysosomal accumulation of undigested or partially digested substrates in cells of virtually all organs, including the nervous system, is diagnostic of these diseases and underlies pathogenesis. A subgroup of LSDs, the glycosphingolipidoses, are caused by deficiency of glycosidases that process/degrade sphingolipids and glycosphingolipids (GSLs). GSLs are among the lipid constituents of mammalian membranes, where they orderly distribute and, together with a plethora of membrane proteins, contribute to the formation of discrete membrane microdomains or lipid rafts. The composition of intracellular membranes enclosing organelles reflects that at the plasma membrane (PM). Organelles have the tendencies to tether to one another and to the PM at specific membrane contact sites that, owing to their lipid and protein content, resemble PM lipid rafts. The focus of this review is on the MAMs, mitochondria associated ER membranes, sites of juxtaposition between ER and mitochondria that function as biological hubs for the exchange of molecules and ions, and control the functional status of the reciprocal organelles. We will focus on the lipid components of the MAMs, and highlight how failure to digest or process the sialylated GSL, GM1 ganglioside, in lysosomes alters the lipid conformation and functional properties of the MAMs and leads to neuronal cell death and neurodegeneration.

Mitochondrial Ca2+ transport in the endothelium: regulation by ions, redox signalling and mechanical forces

Calcium (Ca²⁺) transport by mitochondria is an important component of the cell Ca²⁺ homeostasis machinery in metazoans. Ca²⁺ uptake by mitochondria is a major determinant of bioenergetics and cell fate. Mitochondrial Ca²⁺ uptake occurs via the mitochondrial Ca²⁺ uniporter (MCU) complex, an inner mitochondrial membrane protein assembly consisting of the MCU Ca²⁺ channel, as its core component, and the MCU complex regulatory/auxiliary proteins. In this review, we summarize the current knowledge on the molecular nature of the MCU complex and its regulation by intra- and extramitochondrial levels of divalent ions and reactive oxygen species (ROS). Intracellular Ca²⁺ concentration ([Ca²⁺]_i), mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) and mitochondrial ROS (mROS) are intricately coupled in regulating MCU activity. Here, we highlight the contribution of MCU activity to vascular endothelial cell (EC) function. Besides the ionic and oxidant regulation, ECs are continuously exposed to haemodynamic forces (either pulsatile or oscillatory fluid mechanical shear stresses, depending on the precise EC location within the arteries). Thus, we also propose an EC mechanotransduction-mediated regulation of MCU activity in the context of vascular physiology and atherosclerotic vascular disease.

Computational properties of mitochondria in T cell activation and fate

In this article, we review how mitochondrial Ca²⁺ transport (mitochondrial Ca²⁺ uptake and Na⁺/Ca²⁺ exchange) is involved in T cell biology, including activation and differentiation through shaping cellular Ca²⁺ signals. Based on recent observations, we propose that the Ca²⁺ crosstalk between mitochondria, endoplasmic reticulum and cytoplasm may form a proportional–integral–derivative (PID) controller. This PID mechanism (which is well known in engineering) could be responsible for computing cellular decisions. In addition, we point out the importance of analogue and digital signal processing in T cell life and implication of mitochondrial Ca²⁺ transport in this process.

Mitochondrial protein Fus1/Tusc2 in premature aging and age-related pathologies: critical roles of calcium and energy homeostasis

Decreased energy production and increased oxidative stress are considered to be major contributors to aging and aging-associated pathologies. The role of mitochondrial calcium homeostasis has also been highlighted as an important factor affecting different pathological conditions. Here, we present evidence that loss of a small mitochondrial protein Fus1 that maintains mitochondrial homeostasis results in premature aging, aging-associated pathologies, and decreased survival. We showed that Fus1KO mice develop multiple early aging signs including lordokyphosis, lack of vigor, inability to accumulate fat, reduced ability to tolerate stress, and premature death. Other prominent pathological changes included low sperm counts, compromised ability of adult stem cells to repopulate tissues, and chronic inflammation. At the molecular level, we demonstrated that mitochondria of Fus1 KO cells have low reserve respiratory capacity (the ability to produce extra energy during sudden energy demanding situations), and show significantly altered dynamics of cellular calcium response.

Our recent studies on early hearing and memory loss in Fus1 KO mice combined with the new data presented here suggest that calcium and energy homeostasis controlled by Fus1 may be at the core of its aging-regulating activities. Thus, Fus1 protein and Fus1-dependent pathways and processes may represent new tools and targets for anti-aging strategies.

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.

Crosstalk between calcium and reactive oxygen species signaling in cancer

The interplay between Ca²⁺ and reactive oxygen species (ROS) signaling pathways is well established, with reciprocal regulation occurring at a number of subcellular locations. Many Ca²⁺ channels at the cell surface and intracellular organelles, including the endoplasmic reticulum and mitochondria are regulated by redox modifications. In turn, Ca²⁺ signaling can influence the cellular generation of ROS, from sources such as NADPH oxidases and mitochondria. This relationship has been explored in great depth during the process of apoptosis, where surges of Ca²⁺ and ROS are important mediators of cell death. More recently, coordinated and localized Ca²⁺ and ROS transients appear to play a major role in a vast variety of pro-survival signaling pathways that may be crucial for both physiological and pathophysiological functions. While much work is required to firmly establish this Ca²⁺-ROS relationship in cancer, existing evidence from other disease models suggests this crosstalk is likely of significant importance in tumorigenesis. In this review, we describe the regulation of Ca²⁺ channels and transporters by oxidants and discuss the potential consequences of the ROS-Ca²⁺ interplay in tumor cells.

The Effect of Gap Junctional Coupling on the Spatiotemporal Patterns of Ca2+ Signals and the Harmonization of Ca2+-Related Cellular Responses

Calcium ions (Ca²⁺) are important mediators of a great variety of cellular activities e.g. in response to an agonist activation of a receptor. The magnitude of a cellular response is often encoded by frequency modulation of Ca²⁺ oscillations and correlated with the stimulation intensity. The stimulation intensity highly depends on the sensitivity of a cell to a certain agonist. In some cases, it is essential that neighboring cells produce a similar and synchronized response to an agonist despite their different sensitivity. In order to decipher the presumed function of Ca²⁺ waves spreading among connecting cells, a mathematical model was developed. This model allows to numerically modifying the connectivity probability between neighboring cells, the permeability of gap junctions and the individual sensitivity of cells to an agonist. Here, we show numerically that strong gap junctional coupling between neighbors ensures an equilibrated response to agonist stimulation via formation of Ca²⁺ phase waves, i.e. a less sensitive neighbor will produce the same or similar Ca²⁺ signal as its highly sensitive neighbor. The most sensitive cells within an ensemble are the wave initiator cells. The Ca²⁺ wave in the cytoplasm is driven by a sensitization wave front in the endoplasmic reticulum. The wave velocity is proportional to the cellular sensitivity and to the strength of the coupling. The waves can form different patterns including circular rings and spirals. The observed pattern depends on the strength of noise, gap junctional permeability and the connectivity probability between neighboring cells. Our simulations reveal that one highly sensitive region gradually takes the lead within the entire noisy system by generating directed circular phase waves originating from this region.

The calcium ion (Ca²⁺), a universal signaling molecule, is widely recognized to play a fundamental role in the regulation of various biological processes. Agonist–evoked Ca²⁺ signals often manifest as rhythmic changes in the cytosolic free Ca²⁺ concentration (c_cyt) called Ca²⁺ oscillations. Stimuli intensity was found to be proportional to the oscillation frequency and the evoked down-steam cellular response. Stochastic receptor expression in individual cells in a cell population inevitably leads to individually different oscillation frequencies and individually different Ca²⁺-related cellular responses. However, in many organs, the neighboring cells have to overcome their individually different sensitivity and produce a synchronized response. Gap junctions are integral membrane structures that enable the direct cytoplasmic exchange of Ca²⁺ ions and InsP₃ molecules between neighboring cells. By simulations, we were able to demonstrate how the strength of intercellular gap junctional coupling in relation to stimulus intensity can modify the spatiotemporal patterns of Ca²⁺ signals and harmonize the Ca²⁺-related cellular responses via synchronization of oscillation frequency. We demonstrate that the most sensitive cells are the wave initiator cells and that a highly sensitive region plays an important role in the determination of the Ca²⁺ phase wave direction. This sensitive region will then also progressively determine the global behavior of the entire system.

Measuring Ca2+ inside intracellular organelles with luminescent and fluorescent aequorin-based sensors

GFP-Aequorin Protein (GAP) can be used to measure [Ca²⁺] inside intracellular organelles, both by luminescence and by fluorescence. The low-affinity variant GAP3 is adequate for ratiometric imaging in the endoplasmic reticulum and Golgi apparatus, and it can be combined with conventional synthetic indicators for simultaneous measurements of cytosolic Ca²⁺. GAP is bioorthogonal as it does not have mammalian homologues, and it is robust and functionally expressed in transgenic flies and mice, where it can be used for Ca²⁺ measurements ex vivo and in vivo to explore animal models of health and disease. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Genetically Encoded Fluorescent Indicators for Organellar Calcium Imaging

Optical Ca(2+) indicators are powerful tools for investigating intracellular Ca(2+) signals in living cells. Although a variety of Ca(2+) indicators have been developed, deciphering the physiological functions and spatiotemporal dynamics of Ca(2+) in intracellular organelles remains challenging. Genetically encoded Ca(2+) indicators (GECIs) using fluorescent proteins are promising tools for organellar Ca(2+) imaging, and much effort has been devoted to their development. In this review, we first discuss the key points of organellar Ca(2+) imaging and summarize the requirements for optimal organellar Ca(2+) indicators. Then, we highlight some of the recent advances in the engineering of fluorescent GECIs targeted to specific organelles. Finally, we discuss the limitations of currently available GECIs and the requirements for advancing the research on intraorganellar Ca(2+) signaling.

CRAC channels are required for [Ca(2+)]i oscillations and c-fos gene expression after muscarinic acetylcholine receptor activation in leukemic T cells

AIMS

T lymphocytes express muscarinic acetylcholine receptors (mAChRs) involved in regulating their proliferation, differentiation and cytokine release. Activation of M1, M3 or M5 mAChRs increases the intracellular Ca(2+) concentration ([Ca(2+)]i) through inositol-1,4,5-phosphate (IP3)-mediated Ca(2+) release from endoplasmic reticulum Ca(2+) stores. In addition, T lymphocytes express Ca(2+)-release activated Ca(2+) (CRAC) channels to induce Ca(2+) influx and to regulate diverse immune functions. Our aim in the present study was to assess the role of CRAC channels during mAChR activation in the Ca(2+)-dependent transduction that contributes to the regulation of T cell function.

MAIN METHODS

Changes in [Ca(2+)]i following mAChR activation on human leukemic T cells, CCRF-CEM (CEM), were monitored using fura-2, based on the ratio of 510nm fluorescences elicited by excitation at 340nm and 380nm (R340/380).

KEY FINDINGS

We demonstrate that CEM cells express mainly M3 and M5 mAChRs, but little the M1 subtype, and that oxotremorine-M (Oxo-M), an mAChR agonist, induces an initial transient increase in [Ca(2+)]i followed by repetitive [Ca(2+)]i oscillations. Removing extracellular Ca(2+) or pharmacological blockade of CRAC channels abolished the [Ca(2+)]i oscillations without affecting the initial [Ca(2+)]i transient induced by Oxo-M. Moreover, CRAC channel blockade also suppressed Oxo-M-induced c-fos and interleukin-2 expression.

SIGNIFICANCE

These results suggest that upon M3 or M5 mAChR activation, IP3-mediated Ca(2+) release induces extracellular Ca(2+) influx through CRAC channels, which generates repetitive [Ca(2+)]i oscillations and, in turn, enhances c-fos gene expression in T lymphocytes.

Mitochondria, calcium, and tumor suppressor Fus1: At the crossroad of cancer, inflammation, and autoimmunity

Mitochondria present a unique set of key intracellular functions such as ATP synthesis, production of reactive oxygen species (ROS) and Ca²⁺ buffering. Mitochondria both encode and decode Ca²⁺ signals and these interrelated functions have a direct impact on cell signaling and metabolism.

High proliferative potential is a key energy-demanding feature shared by cancer cells and activated T lymphocytes. Switch of a metabolic state mediated by alterations in mitochondrial homeostasis plays a fundamental role in maintenance of the proliferative state. Recent studies show that tumor suppressors have the ability to affect mitochondrial homeostasis controlling both cancer and autoimmunity. Herein, we discuss established and putative mechanisms of calcium–dependent regulation of both T cell and tumor cell activities. We use the mitochondrial protein Fus1 as a case of tumor suppressor that controls immune response and tumor growth via maintenance of mitochondrial homeostasis. We focus on the regulation of mitochondrial Ca²⁺ handling as a key function of Fus1 and highlight the mechanisms of a crosstalk between Ca²⁺ accumulation and mitochondrial homeostasis. Given the important role of Ca²⁺ signaling, mitochondrial Ca²⁺ transport and ROS production in the activation of NFAT and NF-κB transcription factors, we outline the importance of Fus1 activities in this context.

Enjoy the Trip: Calcium in Mitochondria Back and Forth

In the last 5 years, most of the molecules that control mitochondrial Ca(2+) homeostasis have been finally identified. Mitochondrial Ca(2+) uptake is mediated by the Mitochondrial Calcium Uniporter (MCU) complex, a macromolecular structure that guarantees Ca(2+) accumulation inside mitochondrial matrix upon increases in cytosolic Ca(2+). Conversely, Ca(2+) release is under the control of the Na(+)/Ca(2+) exchanger, encoded by the NCLX gene, and of a H(+)/Ca(2+) antiporter, whose identity is still debated. The low affinity of the MCU complex, coupled to the activity of the efflux systems, protects cells from continuous futile cycles of Ca(2+) across the inner mitochondrial membrane and consequent massive energy dissipation. In this review, we discuss the basic principles that govern mitochondrial Ca(2+) homeostasis and the methods used to investigate the dynamics of Ca(2+) concentration within the organelles. We discuss the functional and structural role of the different molecules involved in mitochondrial Ca(2+) handling and their pathophysiological role.

Visualization of Ca2+ Filling Mechanisms upon Synaptic Inputs in the Endoplasmic Reticulum of Cerebellar Purkinje Cells

The endoplasmic reticulum (ER) plays crucial roles in intracellular Ca(2+) signaling, serving as both a source and sink of Ca(2+), and regulating a variety of physiological and pathophysiological events in neurons in the brain. However, spatiotemporal Ca(2+) dynamics within the ER in central neurons remain to be characterized. In this study, we visualized synaptic activity-dependent ER Ca(2+) dynamics in mouse cerebellar Purkinje cells (PCs) using an ER-targeted genetically encoded Ca(2+) indicator, G-CEPIA1er. We used brief parallel fiber stimulation to induce a local decrease in the ER luminal Ca(2+) concentration ([Ca(2+)]ER) in dendrites and spines. In this experimental system, the recovery of [Ca(2+)]ER takes several seconds, and recovery half-time depends on the extent of ER Ca(2+) depletion. By combining imaging analysis and numerical simulation, we show that the intraluminal diffusion of Ca(2+), rather than Ca(2+) reuptake, is the dominant mechanism for the replenishment of the local [Ca(2+)]ER depletion immediately following the stimulation. In spines, the ER filled almost simultaneously with parent dendrites, suggesting that the ER within the spine neck does not represent a significant barrier to Ca(2+) diffusion. Furthermore, we found that repetitive climbing fiber stimulation, which induces cytosolic Ca(2+) spikes in PCs, cumulatively increased [Ca(2+)]ER. These results indicate that the neuronal ER functions both as an intracellular tunnel to redistribute stored Ca(2+) within the neurons, and as a leaky integrator of Ca(2+) spike-inducing synaptic inputs.

Interplay Between Intracellular Ca(2+) Oscillations and Ca(2+)-stimulated Mitochondrial Metabolism

Oscillations of cytosolic Ca(2+) concentration are a widespread mode of signalling. Oscillatory spikes rely on repetitive exchanges of Ca(2+) between the endoplasmic reticulum (ER) and the cytosol, due to the regulation of inositol 1,4,5-trisphosphate receptors. Mitochondria also sequester and release Ca(2+), thus affecting Ca(2+) signalling. Mitochondrial Ca(2+) activates key enzymes involved in ATP synthesis. We propose a new integrative model for Ca(2+) signalling and mitochondrial metabolism in electrically non-excitable cells. The model accounts for (1) the phase relationship of the Ca(2+) changes in the cytosol, the ER and mitochondria, (2) the dynamics of mitochondrial metabolites in response to cytosolic Ca(2+) changes, and (3) the impacts of cytosol/mitochondria Ca(2+) exchanges and of mitochondrial metabolism on Ca(2+) oscillations. Simulations predict that as expected, oscillations are slowed down by decreasing the rate of Ca(2+) efflux from mitochondria, but also by decreasing the rate of Ca(2+) influx through the mitochondrial Ca(2+) uniporter (MCU). These predictions were experimentally validated by inhibiting MCU expression. Despite the highly non-linear character of Ca(2+) dynamics and mitochondrial metabolism, bioenergetics were found to be robust with respect to changes in frequency and amplitude of Ca(2+) oscillations.

Adrenergic stimulation of single rat astrocytes results in distinct temporal changes in intracellular Ca(2+) and cAMP-dependent PKA responses

During the arousal and startle response, locus coeruleus neurons, innervating practically all brain regions, release catecholamine noradrenaline, which reaches neural brain cells, including astrocytes. These glial cells respond to noradrenergic stimulation by simultaneous activation of the α- and β-adrenergic receptors (ARs) in the plasma membrane with increasing cytosolic levels of Ca(2+) and cAMP, respectively. AR-activation controls a myriad of processes in astrocytes including glucose metabolism, gliosignal vesicle homeostasis, gene transcription, cell morphology and antigen-presenting functions, all of which have distinct temporal characteristics. It is known from biochemical studies that Ca(2+) and cAMP signals in astrocytes can interact, however it is presently unclear whether the temporal properties of the two second messengers are time associated upon AR-activation. We used confocal microscopy to study AR agonist-induced intracellular changes in Ca(2+) and cAMP in single cultured cortical rat astrocytes by real-time monitoring of the Ca(2+) indicator Fluo4-AM and the fluorescence resonance energy transfer-based nanosensor A-kinase activity reporter 2 (AKAR2), which reports the activity of cAMP via its downstream effector protein kinase A (PKA). The results revealed that the activation of α1-ARs by phenylephrine triggers periodic (phasic) Ca(2+) oscillations within 10s, while the activation of β-ARs by isoprenaline leads to a ∼10-fold slower tonic rise to a plateau in cAMP/PKA activity devoid of oscillations. Thus the concomitant activation of α- and β-ARs triggers the Ca(2+) and cAMP second messenger systems in astrocytes with distinct temporal properties, which appears to be tailored to regulate downstream effectors in different time domains.

Endothelial mitochondria regulate the intracellular Ca2+ response to fluid shear stress

Shear stress is known to stimulate an intracellular free calcium concentration ([Ca(2+)]i) response in vascular endothelial cells (ECs). [Ca(2+)]i is a key second messenger for signaling that leads to vasodilation and EC survival. Although it is accepted that the shear-induced [Ca(2+)]i response is, in part, due to Ca(2+) release from the endoplasmic reticulum (ER), the role of mitochondria (second largest Ca(2+) store) is unknown. We hypothesized that the mitochondria play a role in regulating [Ca(2+)]i in sheared ECs. Cultured ECs, loaded with a Ca(2+)-sensitive fluorophore, were exposed to physiological levels of shear stress. Shear stress elicited [Ca(2+)]i transients in a percentage of cells with a fraction of them displaying oscillations. Peak magnitudes, percentage of oscillating ECs, and oscillation frequencies depended on the shear level. [Ca(2+)]i transients/oscillations were present when experiments were conducted in Ca(2+)-free solution (plus lanthanum) but absent when ECs were treated with a phospholipase C inhibitor, suggesting that the ER inositol 1,4,5-trisphosphate receptor is responsible for the [Ca(2+)]i response. Either a mitochondrial uncoupler or an electron transport chain inhibitor, but not a mitochondrial ATP synthase inhibitor, prevented the occurrence of transients and especially inhibited the oscillations. Knockdown of the mitochondrial Ca(2+) uniporter also inhibited the shear-induced [Ca(2+)]i transients/oscillations compared with controls. Hence, EC mitochondria, through Ca(2+) uptake/release, regulate the temporal profile of shear-induced ER Ca(2+) release. [Ca(2+)]i oscillation frequencies detected were within the range for activation of mechanoresponsive kinases and transcription factors, suggesting that dysfunctional EC mitochondria may contribute to cardiovascular disease by deregulating the shear-induced [Ca(2+)]i response.

Routes of Ca2+ Shuttling during Ca2+ Oscillations: FOCUS ON THE ROLE OF MITOCHONDRIAL Ca2+ HANDLING AND CYTOSOLIC Ca2+ BUFFERS

In some cell types, Ca²⁺ oscillations are strictly dependent on Ca²⁺ influx across the plasma membrane, whereas in others, oscillations also persist in the absence of Ca²⁺ influx. We observed that, in primary mesothelial cells, the plasmalemmal Ca²⁺ influx played a pivotal role. However, when the Ca²⁺ transport across the plasma membrane by the “lanthanum insulation method” was blocked prior to the induction of the serum-induced Ca²⁺ oscillations, mitochondrial Ca²⁺ transport was found to be able to substitute for the plasmalemmal Ca²⁺ exchange function, thus rendering the oscillations independent of extracellular Ca²⁺. However, in a physiological situation, the Ca²⁺-buffering capacity of mitochondria was found not to be essential for Ca²⁺ oscillations. Moreover, brief spontaneous Ca²⁺ changes were observed in the mitochondrial Ca²⁺ concentration without apparent changes in the cytosolic Ca²⁺ concentration, indicating the presence of a mitochondrial autonomous Ca²⁺ signaling mechanism. In the presence of calretinin, a Ca²⁺-buffering protein, the amplitude of cytosolic spikes during oscillations was decreased, and the amount of Ca²⁺ ions taken up by mitochondria was reduced. Thus, the increased calretinin expression observed in mesothelioma cells and in certain colon cancer might be correlated to the increased resistance of these tumor cells to proapoptotic/pronecrotic signals. We identified and characterized (experimentally and by modeling) three Ca²⁺ shuttling pathways in primary mesothelial cells during Ca²⁺ oscillations: Ca²⁺ shuttled between (i) the endoplasmic reticulum (ER) and mitochondria, (ii) the ER and the extracellular space, and (iii) the ER and cytoplasmic Ca²⁺ buffers.

Elements of the cellular metabolic structure

A large number of studies have demonstrated the existence of metabolic covalent modifications in different molecular structures, which are able to store biochemical information that is not encoded by DNA. Some of these covalent mark patterns can be transmitted across generations (epigenetic changes). Recently, the emergence of Hopfield-like attractor dynamics has been observed in self-organized enzymatic networks, which have the capacity to store functional catalytic patterns that can be correctly recovered by specific input stimuli. Hopfield-like metabolic dynamics are stable and can be maintained as a long-term biochemical memory. In addition, specific molecular information can be transferred from the functional dynamics of the metabolic networks to the enzymatic activity involved in covalent post-translational modulation, so that determined functional memory can be embedded in multiple stable molecular marks. The metabolic dynamics governed by Hopfield-type attractors (functional processes), as well as the enzymatic covalent modifications of specific molecules (structural dynamic processes) seem to represent the two stages of the dynamical memory of cellular metabolism (metabolic memory). Epigenetic processes appear to be the structural manifestation of this cellular metabolic memory. Here, a new framework for molecular information storage in the cell is presented, which is characterized by two functionally and molecularly interrelated systems: a dynamic, flexible and adaptive system (metabolic memory) and an essentially conservative system (genetic memory). The molecular information of both systems seems to coordinate the physiological development of the whole cell.

Optimal microdomain crosstalk between endoplasmic reticulum and mitochondria for Ca2+ oscillations

A Ca²⁺ signaling model is proposed to consider the crosstalk of Ca²⁺ ions between endoplasmic reticulum (ER) and mitochondria within microdomains around inositol 1, 4, 5-trisphosphate receptors (IP₃R) and the mitochondrial Ca²⁺ uniporter (MCU). Our model predicts that there is a critical IP₃R-MCU distance at which 50% of the ER-released Ca²⁺ is taken up by mitochondria and that mitochondria modulate Ca²⁺ signals differently when outside of this critical distance. This study highlights the importance of the IP₃R-MCU distance on Ca²⁺ signaling dynamics. The model predicts that when MCU are too closely associated with IP₃Rs, the enhanced mitochondrial Ca²⁺ uptake will produce an increase of cytosolic Ca²⁺ spike amplitude. Notably, the model demonstrates the existence of an optimal IP₃R-MCU distance (30–85 nm) for effective Ca²⁺ transfer and the successful generation of Ca²⁺ signals in healthy cells. We suggest that the space between the inner and outer mitochondria membranes provides a defense mechanism against occurrences of high [Ca²⁺]_Cyt. Our results also hint at a possible pathological mechanism in which abnormally high [Ca²⁺]_Cyt arises when the IP₃R-MCU distance is in excess of the optimal range.

Microscopic heat pulse-induced calcium dynamics in single WI-38 fibroblasts

Temperature-sensitive Ca²⁺ dynamics occur primarily through transient receptor potential channels, but also by means of Ca²⁺ channels and pumps on the endoplasmic reticulum membrane. As such, cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) is re-equilibrated by changes in ambient temperature. The present study investigated the effects of heat pulses (heating duration: 2 s or 150 s) on [Ca²⁺]_cyt in single WI-38 fibroblasts, which are considered as normal cells. We found that Ca²⁺ burst occurred immediately after short (2 s) heat pulse, which is similar to our previous report on HeLa cells, but with less thermosensitivity. The heat pulses originated from a focused 1455-nm infrared laser light were applied in the vicinity of cells under the optical microscope. Ca²⁺ bursts induced by the heat pulse were suppressed by treating cells with inhibitors for sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) or inositol trisphosphate receptor (IP₃R). Long (150 s) heat pulses also induced Ca²⁺ bursts after the onset of heating and immediately after re-cooling. Cells were more thermosensitive at physiological (37°C) than at room (25°C) temperature; however, at 37°C, cells were responsive at a higher temperature (ambient temperature+heat pulse). These results strongly suggest that the heat pulse-induced Ca²⁺ burst is caused by a transient imbalance in Ca²⁺ flow between SERCA and IP₃R, and offer a potential new method for thermally controlling Ca²⁺-regulated cellular functions.

Interaction between endoplasmic/sarcoplasmic reticulum stress (ER/SR stress), mitochondrial signaling and Ca(2+) regulation in airway smooth muscle (ASM)

Airway inflammation is a key aspect of diseases such as asthma. Several inflammatory cytokines (e.g., TNFα and IL-13) increase cytosolic Ca(2+) ([Ca(2+)]cyt) responses to agonist stimulation and Ca(2+) sensitivity of force generation, thereby enhancing airway smooth muscle (ASM) contractility (hyper-reactive state). Inflammation also induces ASM proliferation and remodeling (synthetic state). In normal ASM, the transient elevation of [Ca(2+)]cyt induced by agonists leads to a transient increase in mitochondrial Ca(2+) ([Ca(2+)]mito) that may be important in matching ATP production with ATP consumption. In human ASM (hASM) exposed to TNFα and IL-13, the transient increase in [Ca(2+)]mito is blunted despite enhanced [Ca(2+)]cyt responses. We also found that TNFα and IL-13 induce reactive oxidant species (ROS) formation and endoplasmic/sarcoplasmic reticulum (ER/SR) stress (unfolded protein response) in hASM. ER/SR stress in hASM is associated with disruption of mitochondrial coupling with the ER/SR membrane, which relates to reduced mitofusin 2 (Mfn2) expression. Thus, in hASM it appears that TNFα and IL-13 result in ROS formation leading to ER/SR stress, reduced Mfn2 expression, disruption of mitochondrion-ER/SR coupling, decreased mitochondrial Ca(2+) buffering, mitochondrial fragmentation, and increased cell proliferation.

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.