Mitochondrial calcium transport

Harnessing Mitochondrial Stress for Health and Disease: Opportunities and Challenges

Mitochondria, essential organelles orchestrating cellular metabolism, have emerged as central players in various disease pathologies. Recent research has shed light on mitohormesis, a concept proposing an adaptive response of mitochondria to minor disturbances in homeostasis, offering novel therapeutic avenues for mitochondria-related diseases. This comprehensive review explores the concept of mitohormesis, elucidating its induction mechanisms and occurrence. Intracellular molecules like reactive oxygen species (ROS), calcium, mitochondrial unfolded proteins (UPRmt), and integrated stress response (ISR), along with external factors such as hydrogen sulfide (H2S), physical stimuli, and exercise, play pivotal roles in regulating mitohormesis. Based on the available evidence, we elucidate how mitohormesis maintains mitochondrial homeostasis through mechanisms like mitochondrial quality control and mitophagy. Furthermore, the regulatory role of mitohormesis in mitochondria-related diseases is discussed. By envisioning future applications, this review underscores the significance of mitohormesis as a potential therapeutic target, paving the way for innovative interventions in disease management.

Functional pleiotropy of calcium binding protein Regucalcin in signaling and diseases

Regucalcin (Mr ∼ 33.38 kDa) is a calcium binding protein, discovered in rat liver. In humans, gene for regucalcin is located on chromosome-11 (p11.3-q11.2) consisting of seven exons and six introns. The protein differs from other calcium binding protein in the way that it lacks EF-hand motif of calcium binding domain. It is also called as Senescence Marker Protein-30 (SMP-30) as previously its weight assumes to be 30 kDa and expression of this protein decreases with aging in androgen independent manner. Among vertebrates, it is a highly conserved protein showing gene homology in Drosophila, Xenopus, fireflies and others too. It is primarily expressed in liver and kidney in addition to brain, lungs, and skeletal muscles. Regucalcin acts as a Ca²⁺ regulatory protein and controls various cellular functions in liver and other organs. It suppresses protein phosphatase, protein kinase, DNA and RNA synthesis. Published evidences suggest regucalcin to be a reliable biomarker in various disorders of liver, kidney, brain and ocular. In over expressed state, it subdues apoptosis in cloned rat hepatoma cells and also induces hyperlipidemia and osteoblastogenesis by regulating various factors. Owing to the multi-functionality of regucalcin this review is presented to elaborate its importance in order to understand its involvement in cellular signaling during various pathologies.

Mitochondrial bioenergetics and redox dysfunctions in hypercholesterolemia and atherosclerosis

In the first part of this review, we summarize basic mitochondrial bioenergetics concepts showing that mitochondria are critical regulators of cell life and death. Until a few decades ago, mitochondria were considered to play essential roles only in respiration, ATP formation, non-shivering thermogenesis and a variety of metabolic pathways. However, the concept presented by Peter Mitchell regarding coupling between electron flow and ATP synthesis through the intermediary of a H⁺ electrochemical potential leads to the recognition that the proton-motive force also regulates a series of relevant cell signalling processes, such as superoxide generation, redox balance and Ca²⁺ handling. Alterations in these processes lead to cell death and disease states. In the second part of this review, we discuss the role of mitochondrial dysfunctions in the specific context of hypercholesterolemia-induced atherosclerosis. We provide a literature analysis that indicates a decisive role of mitochondrial redox dysfunction in the development of atherosclerosis and discuss the underlying molecular mechanisms. Finally, we highlight the potential mitochondrial-targeted therapeutic strategies that are relevant for atherosclerosis.

Bright red-emitting highly reliable styryl probe with large Stokes shift for visualizing mitochondria in live cells under wash-free conditions

Bright red-emitting pyridinium cyanine based styryl probe 2 is synthesized in good yields. Probe 2 demonstrated a large Stokes' shift (Δλ ≈ 128 nm, 4227 cm^-1 in DCM) and excellent fluorescent quantum yield (ϕ_fl ≈ 0.2 - 0.7) due to strong Intra-molecular charge transfer (ICT). Probe 2 found to exhibit exceptional selectivity for cellular mitochondria in both normal (COS-7) and cancer (A549) cell lines. Probe 2 is readily applicable as a "wash-free" dye to visualize mitochondria as it does not require post-staining washing prior to imaging. Styryl probe 2 also showed an excellent biocompatibility as the calculated LC₅₀ (lethal concentration, 50%) value was > 20 μM. Probe 2 emission did not show any interferences from anionic species or other biological molecules. Probe 2 is readily excitable (λ_ex ∼460 and λ_em ∼618 nm) with the available laser (454 nm) in commercial microscopes and thus it can be a useful probe for mitochondrial tracking in live cells.

Effects of Phenelzine Administration on Mitochondrial Function, Calcium Handling, and Cytoskeletal Degradation after Experimental Traumatic Brain Injury

Traumatic brain injury (TBI) results in the production of peroxynitrite (PN), leading to oxidative damage of lipids and protein. PN-mediated lipid peroxidation (LP) results in production of reactive aldehydes 4-hydroxynonenal (4-HNE) and acrolein. The goal of these studies was to explore the hypothesis that interrupting secondary oxidative damage following a TBI via phenelzine (PZ), analdehyde scavenger, would protect against LP-mediated mitochondrial and neuronal damage. Male Sprague-Dawley rats received a severe (2.2 mm) controlled cortical impact (CCI)-TBI. PZ was administered subcutaneously (s.c.) at 15 min (10 mg/kg) and 12 h (5 mg/kg) post-injury and for the therapeutic window/delay study, PZ was administered at 1 h (10 mg/kg) and 24 h (5 mg/kg). Mitochondrial and cellular protein samples were obtained at 24 and 72 h post-injury (hpi). Administration of PZ significantly improved mitochondrial respiration at 24 and 72 h compared with vehicle-treated animals. These results demonstrate that PZ administration preserves mitochondrial bioenergetics at 24 h and that this protection is maintained out to 72 hpi. Additionally, delaying the administration still elicited significant protective effects. PZ administration also improved mitochondrial Ca2+ buffering (CB) capacity and mitochondrial membrane potential parameters compared with vehicle-treated animals at 24 h. Although PZ treatment attenuated aldehyde accumulation post-injury, the effects were insignificant. The amount of α-spectrin breakdown in cortical tissue was reduced by PZ administration at 24 h, but not at 72 hpi compared with vehicle-treated animals. In conclusion, these results indicate that acute PZ treatment successfully attenuates LP-mediated oxidative damage eliciting multiple neuroprotective effects following TBI.

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

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

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Slow activation of fast mitochondrial Ca2+ uptake by cytosolic Ca2

Mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniporter (MCU) is a tightly controlled process that sustains cell functions mainly by fine-tuning oxidative metabolism to cellular needs. The kinetics of Ca²⁺ fluxes across the mitochondrial membranes have been studied both in vitro and in vivo for many years, and the discovery of the molecular components of the MCU has further clarified that this Ca²⁺ uptake mechanism is based on a complex system subject to elaborate layers of controls. Alterations in the speed or capacity of the in-and-out pathways can have detrimental consequences for both the organelle and the cell, impairing cellular metabolism and ultimately causing cell death. Here, we report that pretreatment of deenergized mitochondria with low-micromolar Ca²⁺ concentrations for a few minutes markedly increases the speed of mitochondrial Ca²⁺ uptake upon re-addition of an oxidizable substrate. We found that this phenomenon is sensitive to alterations in the level of the MCU modulator proteins mitochondrial calcium uptake 1 (MICU1) and 2 (MICU2), and is accompanied by changes in the association of MICU1-MICU2 complexes with MCU. This increased Ca²⁺ uptake capacity, occurring under conditions mimicking those during ischemia/reperfusion in vivo, could lead to a massive amount of Ca²⁺ entering the mitochondrial matrix even at relatively low levels of cytosolic Ca²⁺ We conclude that the phenomenon uncovered here represents a potential threat of mitochondrial Ca²⁺ overload to the cell.

Synaptic Mitochondria are More Susceptible to Traumatic Brain Injury-induced Oxidative Damage and Respiratory Dysfunction than Non-synaptic Mitochondria

Traumatic brain injury (TBI) results in mitochondrial dysfunction and induction of lipid peroxidation (LP). Lipid peroxidation-derived neurotoxic aldehydes such as 4-HNE and acrolein bind to mitochondrial proteins, inducing additional oxidative damage and further exacerbating mitochondrial dysfunction and LP. Mitochondria are heterogeneous, consisting of both synaptic and non-synaptic populations. Synaptic mitochondria are reported to be more vulnerable to injury; however, this is the first study to characterize the temporal profile of synaptic and non-synaptic mitochondria following TBI, including investigation of respiratory dysfunction and oxidative damage to mitochondrial proteins between 3 and 120 h following injury. These results indicate that synaptic mitochondria are indeed the more vulnerable population, showing both more rapid and severe impairments than non-synaptic mitochondria. By 24 h, synaptic respiration is significantly impaired compared to synaptic sham, whereas non-synaptic respiration does not decline significantly until 48 h. Decreases in respiration are associated with increases in oxidative damage to synaptic and non-synaptic mitochondrial proteins at 48 h and 72 h, respectively. These results indicate that the therapeutic window for mitochondria-targeted pharmacological neuroprotectants to prevent respiratory dysfunction is shorter for the more vulnerable synaptic mitochondria than for the non-synaptic population.

Mitohormesis in exercise training

Hormesis is a process whereby exposure to a low dose of a potentially harmful stressor promotes adaptive changes to the cell that enables it to better tolerate subsequent stress. In recent years this concept has been applied specifically to the mitochondria (mitohormesis), suggesting that in response to a perturbation the mitochondria can initiate and transduce a signal to the nucleus that coordinates a transcriptional response resulting in both mitochondrial and non-mitochondrial adaptations that return and maintain cellular homeostasis. In this review we summarize the evidence that mitohormesis is a significant adaptive-response signaling pathway, and suggest that it plays a role in mediating exercise-induced adaptations. We discuss potential mitochondrial emitters of retrograde signals that may activate known exercise-sensitive transcription factors to modulate transcription responses to exercise, and draw on evidence from mitochondrial dysfunction animal models to support a role for mitohormesis in mitochondrial biogenesis. Studies directly linking mitohormesis to the exercise training response are lacking, however mounting evidence suggests numerous signals are emitted from the mitochondria during exercise and have the potential to induce a nuclear transcription response, with reactive oxygen species (ROS) being the primary candidate.

Cd(2+) extrusion by P-type Cd(2+)-ATPase of Staphylococcus aureus 17810R via energy-dependent Cd(2+)/H(+) exchange mechanism

Cd²⁺ is highly toxic to Staphylococcus aureus since it blocks dithiols in cytoplasmic 2-oxoglutarate dehydrogenase complex (ODHC) participating in energy conservation process. However, S. aureus 17810R is Cd²⁺-resistant due to possession of cadA-coded Cd²⁺ efflux system, recognized here as P-type Cd²⁺-ATPase. This Cd²⁺ pump utilizing cellular energy—ATP, ∆μ_H⁺ (electrochemical proton potential) and respiratory protons, extrudes Cd²⁺ from cytoplasm to protect dithiols in ODHC, but the mechanism of Cd²⁺ extrusion remains unknown. Here we propose that two Cd²⁺ taken up by strain 17810R via Mn²⁺ uniporter down membrane potential (∆ψ) generated during glutamate oxidation in 100 mM phosphate buffer (high P_iB) are trapped probably by high affinity sites in cytoplasmic domain of Cd²⁺-ATPase, forming SCdS. This stops Cd²⁺ transport towards dithiols in ODHC, allowing undisturbed NADH production, its oxidation and energy conservation, while ATP could change orientation of SCdS towards facing transmembrane channel. Now, increased number of P_i-dependent protons pumped electrogenically via respiratory chain and countertransported through the channel down ∆ψ, extrude two trapped cytoplasmic Cd²⁺, which move to low affinity sites, being then extruded into extracellular space via ∆ψ-dependent Cd²⁺/H⁺ exchange. In 1 mM phosphate buffer (low P_iB), external Cd²⁺ competing with decreased number of P_i-dependent protons, binds to ψ_s of Cd²⁺-ATPase channel, enters cytoplasm through the channel down ∆ψ via Cd²⁺/Cd²⁺ exchange and blocks dithiols in ODHC. However, Mg²⁺ pretreatment preventing external Cd²⁺ countertransport through the channel down ∆ψ, allowed undisturbed NADH production, its oxidation and extrusion of two cytoplasmic Cd²⁺ via Cd²⁺/H⁺ exchange, despite low P_iB.

Mitochondrial calcium and the regulation of metabolism in the heart

Consumption of adenosine triphosphate (ATP) by the heart can change dramatically as the energetic demands increase from a period of rest to strenuous activity. Mitochondrial ATP production is central to this metabolic response since the heart relies largely on oxidative phosphorylation as its source of intracellular ATP. Significant evidence has been acquired indicating that Ca(2+) plays a critical role in regulating ATP production by the mitochondria. Here the evidence that the Ca(2+) concentration in the mitochondrial matrix ([Ca(2+)]m) plays a pivotal role in regulating ATP production by the mitochondria is critically reviewed and aspects of this process that are under current active investigation are highlighted. Importantly, current quantitative information on the bidirectional Ca(2+) movement across the inner mitochondrial membrane (IMM) is examined in two parts. First, we review how Ca(2+) influx into the mitochondrial matrix depends on the mitochondrial Ca(2+) channel (i.e., the mitochondrial calcium uniporter or MCU). This discussion includes how the MCU open probability (PO) depends on the cytosolic Ca(2+) concentration ([Ca(2+)]i) and on the mitochondrial membrane potential (ΔΨm). Second, we discuss how steady-state [Ca(2+)]m is determined by the dynamic balance between this MCU-based Ca(2+) influx and mitochondrial Na(+)/Ca(2+) exchanger (NCLX) based Ca(2+) efflux. These steady-state [Ca(2+)]m levels are suggested to regulate the metabolic energy supply due to Ca(2+)-dependent regulation of mitochondrial enzymes of the tricarboxylic acid cycle (TCA), the proteins of the electron transport chain (ETC), and the F1F0 ATP synthase itself. We conclude by discussing the roles played by [Ca(2+)]m in influencing mitochondrial responses under pathological conditions. This article is part of a Special Issue entitled "Mitochondria: From BasicMitochondrial Biology to Cardiovascular Disease."

Effect of maternal protein restriction on liver metabolism in rat offspring

Consequences of gestational protein restriction (GPR) on liver metabolism in rat offspring were investigated. Pregnant dams were divided into groups: normal (NP, 17% casein) or low-protein diet (LP, 6% casein). Livers were collected from 30-day-old offspring (d30) for analysis or isolation of mitochondria. At d30, hepatic and muscle glycogen was increased in LP group. Mitochondrial swelling and oxygen uptake (recorded with a Clark-type electrode) were significantly reduced in NP female and LP pups. Thiobarbituric acid reactive substances production was lower in females (NP or LP), suggesting significant inhibition of lipid peroxidation. Measurement of mitochondrial respiration (states 3 and 4 stimulated by succinate) showed a higher ADP/O ratio in LP pups, particularly females, suggesting higher phosphorylation efficiency. In the 1st month of life, under our experimental conditions, GPR protects liver mitochondria against oxidative stress and females seem to be more resistant or more suitable for survival.

[The effect of Ca(2+)-induced opening of cyclosporine-sensitive pore on the oxygen consumption and functional state of rat liver mitochondria]

The effect of Ca(2+)-induced opening of cyclosporine-sensitive pore (mitochondrial permeability transition pore, MPTP) on the oxygen consumption and mitochondrial functional state was studied in the rat liver mitochondria. It was shown that, with the use of glutamate as oxidation substrate, in the absence of depolarization MPTP opening results in the increase of steady state respiration rate because of the activation of cyclosporine-sensitive Ca2+/H(+)-exchange and Ca2+ cycling, which was supported by the simultaneous work of MPTP and Ca(2+)-uniporter. With the aid of selective blockers, cyclosporine A and ruthenium red, it was shown that MPTP and Ca(2+)-uniporter contribute equally to the Ca(2+)-cycling and mitochondrial respiration. It was shown that bioenergetic effects of MPTP opening under steady state conditions (increase in the oxygen consumption rate under substrate oxidation without ADP, decrease in respiratory control ratio as well as the effectiveness of ATP synthesis, P/O) are close to the functional alterations, which result from the increase of endogenous proton conductance of mitochondrial membrane. Uncoupling effect of MPTP opening, by itself, had no effect on phosphorylation rate, which remains relatively stable because the fall of P/O is compensated by the activation of respiratory chain and the increase in the rate of state 3 respiration. It was concluded that under physiologically normal conditions MPTP might function as the endogenous mechanism of mild uncoupling of respiratory chain.

Disruption of oxidative phosphorylation and synaptic Na(+), K(+)-ATPase activity by pristanic acid in cerebellum of young rats

AIMS

Peroxisomal biogenesis disorders (PBD) are inherited disorders clinically manifested by neurological symptoms and brain abnormalities, in which the cerebellum is usually involved. Biochemically, patients affected by these neurodegenerative diseases accumulate branched-chain fatty acids, including pristanic acid (Prist) in the brain and other tissues.

MAIN METHODS

In the present investigation we studied the in vitro influence of Prist, at doses found in PBD, on oxidative phosphorylation, by measuring the activities of the respiratory chain complexes I-IV and ATP production, as well as on creatine kinase and synaptic Na(+), K(+)-ATPase activities in rat cerebellum.

KEY FINDINGS

Prist significantly decreased complexes I-III (65%), II (40%) and especially II-III (90%) activities, without altering the activities of complex IV of the respiratory chain and creatine kinase. Furthermore, ATP formation and synaptic Na(+), K(+)-ATPase activity were markedly inhibited (80-90%) by Prist. We also observed that this fatty acid altered mitochondrial and synaptic membrane fluidity that may have contributed to its inhibitory effects on the activities of the respiratory chain complexes and Na(+), K(+)-ATPase.

SIGNIFICANCE

Considering the importance of oxidative phosphorylation for mitochondrial homeostasis and of Na(+), K(+)-ATPase for the maintenance of cell membrane potential, the present data indicate that Prist compromises brain bioenergetics and neurotransmission in cerebellum. We postulate that these pathomechanisms may contribute to the cerebellar alterations observed in patients affected by PBD in which Prist is accumulated.

Cardiac mitochondrial network excitability: insights from computational analysis

In the heart, mitochondria form a regular lattice and function as a coordinated, nonlinear network to continuously produce ATP to meet the high-energy demand of the cardiomyocytes. Cardiac mitochondria also exhibit properties of an excitable system: electrical or chemical signals can spread within or among cells in the syncytium. The detailed mechanisms by which signals pass among individual elements (mitochondria) across the network are still not completely understood, although emerging studies suggest that network excitability might be mediated by the local diffusion and autocatalytic release of messenger molecules such as reactive oxygen species and/or Ca(2+). In this short review, we have attempted to described recent advances in the field of cardiac mitochondrial network excitability. Specifically, we have focused on how mitochondria communicate with each other through the diffusion and regeneration of messenger molecules to initiate and propagate waves or oscillations, as revealed by computational models of mitochondrial network.

DLP1-dependent mitochondrial fragmentation mediates 1-methyl-4-phenylpyridinium toxicity in neurons: implications for Parkinson's disease

Selective degeneration of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) can be modeled by the administration of the neurotoxin 1-methyl-4-phenylpyridinium (MPP(+) ). Because abnormal mitochondrial dynamics are increasingly implicated in the pathogenesis of PD, in this study, we investigated the effect of MPP(+) on mitochondrial dynamics and assessed temporal and causal relationship with other toxic effects induced by MPP(+) in neuronal cells. In SH-SY5Y cells, MPP(+) causes a rapid increase in mitochondrial fragmentation followed by a second wave of increase in mitochondrial fragmentation, along with increased DLP1 expression and mitochondrial translocation. Genetic inactivation of DLP1 completely blocks MPP(+) -induced mitochondrial fragmentation. Notably, this approach partially rescues MPP(+) -induced decline in ATP levels and ATP/ADP ratio and increased [Ca(2+) ](i) and almost completely prevents increased reactive oxygen species production, loss of mitochondrial membrane potential, enhanced autophagy and cell death, suggesting that mitochondria fragmentation is an upstream event that mediates MPP(+) -induced toxicity. On the other hand, thiol antioxidant N-acetylcysteine or glutamate receptor antagonist D-AP5 also partially alleviates MPP(+) -induced mitochondrial fragmentation, suggesting a vicious spiral of events contributes to MPP(+) -induced toxicity. We further validated our findings in primary rat midbrain dopaminergic neurons that 0.5 μm MPP(+) induced mitochondrial fragmentation only in tyrosine hydroxylase (TH)-positive dopaminergic neurons in a similar pattern to that in SH-SY5Y cells but had no effects on these mitochondrial parameters in TH-negative neurons. Overall, these findings suggest that DLP1-dependent mitochondrial fragmentation plays a crucial role in mediating MPP(+) -induced mitochondria abnormalities and cellular dysfunction and may represent a novel therapeutic target for PD.

Glucose sensing in the pancreatic beta cell: a computational systems analysis

Background

Pancreatic beta-cells respond to rising blood glucose by increasing oxidative metabolism, leading to an increased ATP/ADP ratio in the cytoplasm. This leads to a closure of K_ATP channels, depolarization of the plasma membrane, influx of calcium and the eventual secretion of insulin. Such mechanism suggests that beta-cell metabolism should have a functional regulation specific to secretion, as opposed to coupling to contraction. The goal of this work is to uncover contributions of the cytoplasmic and mitochondrial processes in this secretory coupling mechanism using mathematical modeling in a systems biology approach.

Methods

We describe a mathematical model of beta-cell sensitivity to glucose. The cytoplasmic part of the model includes equations describing glucokinase, glycolysis, pyruvate reduction, NADH and ATP production and consumption. The mitochondrial part begins with production of NADH, which is regulated by pyruvate dehydrogenase. NADH is used in the electron transport chain to establish a proton motive force, driving the F₁F₀ ATPase. Redox shuttles and mitochondrial Ca²⁺ handling were also modeled.

Results

The model correctly predicts changes in the ATP/ADP ratio, Ca²⁺ and other metabolic parameters in response to changes in substrate delivery at steady-state and during cytoplasmic Ca²⁺ oscillations. Our analysis of the model simulations suggests that the mitochondrial membrane potential should be relatively lower in beta cells compared with other cell types to permit precise mitochondrial regulation of the cytoplasmic ATP/ADP ratio. This key difference may follow from a relative reduction in respiratory activity. The model demonstrates how activity of lactate dehydrogenase, uncoupling proteins and the redox shuttles can regulate beta-cell function in concert; that independent oscillations of cytoplasmic Ca²⁺ can lead to slow coupled metabolic oscillations; and that the relatively low production rate of reactive oxygen species in beta-cells under physiological conditions is a consequence of the relatively decreased mitochondrial membrane potential.

Conclusion

This comprehensive model predicts a special role for mitochondrial control mechanisms in insulin secretion and ROS generation in the beta cell. The model can be used for testing and generating control hypotheses and will help to provide a more complete understanding of beta-cell glucose-sensing central to the physiology and pathology of pancreatic β-cells.

Downstream mechanisms triggered by mitochondrial dysfunction in the basal ganglia: from experimental models to neurodegenerative diseases

Mitochondrial dysfunctions have been implicated in the cellular processes underlying several neurodegenerative disorders affecting the basal ganglia. These include Huntington's chorea and Parkinson's disease, two highly debilitating motor disorders for which recent research has also involved gene mutation linked to mitochondrial deficits. Experimental models of basal ganglia diseases have been developed by using toxins able to disrupt mitochondrial function: these molecules act by selectively inhibiting mitochondrial respiratory complexes, uncoupling cellular respiration. This in turn leads to oxidative stress and energy deficit that trigger critical downstream mechanisms, ultimately resulting in neuronal vulnerability and loss. Here we review the molecular and cellular downstream effects triggered by mitochondrial dysfunction, and the different experimental models that are obtained by the administration of selective mitochondrial toxins or by the expression of mutant genes.

Protection against kainate neurotoxicity by ginsenosides: attenuation of convulsive behavior, mitochondrial dysfunction, and oxidative stress

We previously demonstrated that kainic acid (KA)-mediated mitochondrial oxidative stress contributed to hippocampal degeneration and that ginsenosides attenuated KA-induced neurotoxicity and neuronal degeneration. Here, we examined whether ginsenosides affected KA-induced mitochondrial dysfunction and oxidative stress in the rat hippocampus. Treatment with ginsenosides attenuated KA-induced convulsive behavior dose-dependently. KA treatment increased lipid peroxidation and protein oxidation and decreased the reduced glutathione/oxidized glutathione (GSH/GSSG) ratio to a greater degree in the mitochondrial fraction than in the hippocampal homogenate. KA treatment resulted in decreased Mn-superoxide dismutase expression and diminished the mitochondrial membrane potential. Furthermore, KA treatment increased intramitochondrial Ca(2+) and promoted ultrastructural degeneration in hippocampal mitochondria. Treatment with ginsenosides dose-dependently attenuated convulsive behavior and the KA-induced mitochondrial effects. Protection appeared to be more evident in mitochondria than in tissue homogenates. Collectively, the results suggest that ginsenosides prevent KA-induced neurotoxicity by attenuating mitochondrial oxidative stress and mitochondrial dysfunction.

Mitochondrial calcium function and dysfunction in the central nervous system

The ability of isolated brain mitochondria to accumulate, store and release calcium has been extensively characterized. Extrapolation to the intact neuron led to predictions that the in situ mitochondria would reversibly accumulate Ca(2+) when the concentration of the cation in the vicinity of the mitochondria rose above the 'set-point' at which uptake and efflux were in balance, storing Ca(2+) as a complex with phosphate, and slowly releasing the cation when plasma membrane ion pumps lowered the cytoplasmic free Ca(2+). Excessive accumulation of the cation was predicted to lead to activation of the permeability transition, with catastrophic consequences for the neuron. Each of these predictions has been confirmed with intact neurons, and there is convincing evidence for the permeability transition in cellular Ca(2+) overload associated with glutamate excitotoxicity and stroke, while the neurodegenerative disease in which possible defects in mitochondrial Ca(2+) handling have been most intensively investigated is Huntington's Disease. In this brief review evidence that mitochondrial Ca(2+) transport is relevant to neuronal survival in these conditions will be discussed.

Regulation of mitochondrial dehydrogenases by calcium ions

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.

Oxidative stress and energy crises in neuronal dysfunction

Mitochondrial dysfunction is implicated in many forms of cell death, particularly in the central nervous system. The mitochondria are required at the same time to generate adenosine 5'-triphosphate (ATP) for the cell, sequester excess cytoplasmic Ca(2+), and both produce and detoxify superoxide free radicals. The electron transport chain and proton circuit are central in keeping these three balls in the air at the same time. We have investigated the bioenergetics of the in situ mitochondria in cultured neurons exposed to pathological glutamate concentrations to model glutamate excitotoxicity and have revised the conventional view that mitochondrial calcium loading results in increased oxidative stress that damages the mitochondrion and ultimately the cell. Instead, a central role is played under these conditions by limitations in mitochondrial and cellular ATP generating capacity. Sodium and calcium entering via the N-methyl-D-aspartate receptor impose a large energetic load on cells and can use the entire respiratory capacity of the in situ mitochondria. As a result, even modest restrictions in mitochondrial capacity -- caused by low concentrations of electron transport chain inhibitors such as rotenone, as in models of Parkinson's disease; low concentrations of uncouplers, to test the so-called neuroprotective mild uncoupling hypothesis; or preexisting oxidative stress -- greatly potentiate glutamate excitotoxicity. Our findings may lead to a reevaluation of the potential for mild uncoupling to provide a neuroprotective role in aging-related neurodegenerative disorders because the deleterious consequences of restricting ATP generating capacity greatly outweigh the negligible effects on the levels of mitochondrial superoxide radicals in intact neurons.

Feedback loops shape cellular signals in space and time

Positive and negative feedback loops are common regulatory elements in biological signaling systems. We discuss core feedback motifs that have distinct roles in shaping signaling responses in space and time. We also discuss approaches to experimentally investigate feedback loops in signaling systems.

Mitochondria and calcium signaling

The kinetic properties for the uptake, storage and release of Ca2+ from isolated mitochondria accurately predict the behaviour of the organelles within the intact cell. While the steady-state cycling of Ca2+ across the inner membrane between independent uptake and efflux pathways seems at first sight to be symmetrical, the distinctive kinetics of the uniporter, which is highly dependent on external free Ca2+ concentration and the efflux pathway, whose activity is clamped over a wide range of total matrix Ca2+ by the solubility of the calcium phosphate complex provide a mechanism whereby mitochondria reversibly sequester transient elevations in cytoplasmic Ca2+. Under non-stimulated conditions, the same transport processes can regulate matrix Ca2+ concentrations and hence citric acid cycle activity.

Effects of anoxia and aglycemia on cytosolic calcium regulation in rat sensory neurons

Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca²⁺ regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca²⁺]_i by evoking Ca²⁺ release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca²⁺ content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca²⁺ uptake when neurons were depolarized but substantially delayed mitochondrial Ca²⁺ release and subsequent Ca²⁺ clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity and Ca²⁺ extrusion [probably via plasma membrane Ca²⁺-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca²⁺]_i homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca²⁺ pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca²⁺]_i, which was associated with complete ER Ca²⁺ store emptying and progressive, and eventually complete, inhibition of Ca²⁺ clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca²⁺ regulation, and this may play an important role in modulating neuronal responses to ischemia.

Kinetic model for Ca2+-induced permeability transition in energized liver mitochondria discriminates between inhibitor mechanisms

Cytotoxicity associated with pathophysiological Ca(2+) overload (e.g. in stroke) appears mediated by an event termed the mitochondrial permeability transition (mPT). We built and solved a kinetic model of the mPT in populations of isolated rat liver mitochondria that quantitatively describes Ca(2+)-induced mPT as a two-step sequence of pre-swelling induction followed by Ca(2+)-driven, positive feedback, autocatalytic propagation. The model was formulated as two differential equations, each directly related to experimental parameters (Ca(2+) flux/mitochondrial swelling). These parameters were simultaneously assessed using a spectroscopic approach to monitor multiple mitochondrial properties. The derived kinetic model correctly identifies a correlation between initial Ca(2+) concentration and delay interval prior to mPT induction. Within the model's framework, Ru-360 (a ruthenium complex) and Mg(2+) were shown to compete with the Ca(2+)-stimulated initiation phase of mPT induction, consistent with known inhibition at the phenomenological level of the Ca(2+) uniporter. The model further reveals that Mg(2+), but not Ru-360, inhibits Ca(2+)-induced effects on a downstream stage of mPT induction at a site distinct from the uniporter. The analytical approach was then applied to promethazine, an FDA-approved drug previously shown to inhibit both mPT and ischemia-reperfusion injury. Kinetic analysis revealed that promethazine delayed mPT induction in a manner qualitatively distinct from that of lower concentrations of Mg(2+). In summary, we have developed a kinetic model to aid in the quantitative characterization of mPT induction. This model is consistent with/informative about the biochemistry of several mPT inhibitors, and its success suggests that this kinetic approach can aid in the classification of agents or targets that modulate mPT induction.

The sodium-calcium exchange system

Sodium-calcium counter-transport represents one of a number of processes for transporting calcium ions across cellular membranes. The physiological importance of the exchanger is outlined and its underlying mechanism discussed in terms of a comparison of the partial reactions of Na+-Ca2+ exchange in intact cells with those of plasma membrane vesicles.

Organotellurane-promoted mitochondrial permeability transition concomitant with membrane lipid protection against oxidation

Organotelluranes exhibit potent antioxidant properties as well as the ability to react with protein thiol groups and, thereby, they are good models to study the mechanism of the mitochondrial permeability transition (MPT). We evaluated the effects of the concentration of organotelluranes, namely RT-03 and RT-04, on rat liver mitochondria. At the concentration range of 0.25-1.0 microM, organotelluranes did not cause any mitochondrial dysfunction. At the concentration range of 5-10 microM, RT-03 and RT-04 caused the Ca2+-dependent opening of the (MPT) pore, regulated by Cyclosporin A. At the concentration range of 15-30 microM the swelling was not inhibited by Cyclosporin A and in the absence of Ca2+, a significant decrease of respiratory control ratio was observed due to concomitant phosphorylation impairment and uncoupling, transmembrane potential disruption, depletion of mitochondrial reduced thiol groups, and alterations in the bilayer fluidity. Above 100 microM, the organotelluranes caused complete inhibition of respiratory chain. Over the whole studied concentration range, RT-03 and RT-04 did not induce mitochondrial oxidative stress assessed by using the reactive oxygen and nitrogen species indicator 2',7'-dichlorodihydrofluorescein diacetate. Further, the organotelluranes also exhibited protective effect against t-butyl hydroperoxide-induced oxidative stress as well as against Fe2+/citrate-induced peroxidation of mitochondrial membranes and PCPECL liposomes. These results point out that MPT pore opening can involve damage exclusively to mitochondrial membrane proteins. The exclusive antioxidant activity observed at nanomolar range is also an interesting new finding described in this work.

2-APB protects against liver ischemia-reperfusion injury by reducing cellular and mitochondrial calcium uptake

Ischemia-reperfusion (I/R) injury is a commonly encountered clinical problem in liver surgery and transplantation. The pathogenesis of I/R injury is multifactorial, but mitochondrial Ca(2+) overload plays a central role. We have previously defined a novel pathway for mitochondrial Ca(2+) handling and now further characterize this pathway and investigate a novel Ca(2+)-channel inhibitor, 2-aminoethoxydiphenyl borate (2-APB), for preventing hepatic I/R injury. The effect of 2-APB on cellular and mitochondrial Ca(2+) uptake was evaluated in vitro by using (45)Ca(2+). Subsequently, 2-APB (2 mg/kg) or vehicle was injected into the portal vein of anesthetized rats either before or following 1 h of inflow occlusion to 70% of the liver. After 3 h of reperfusion, liver injury was assessed enzymatically and histologically. Hep G2 cells transfected with green fluorescent protein-tagged cytochrome c were used to evaluate mitochondrial permeability. 2-APB dose-dependently blocked Ca(2+) uptake in isolated liver mitochondria and reduced cellular Ca(2+) accumulation in Hep G2 cells. In vivo I/R increased liver enzymes 10-fold, and 2-APB prevented this when administered pre- or postischemia. 2-APB significantly reduced cellular damage determined by hematoxylin and eosin and terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling staining of liver tissue. In vitro I/R caused a dissociation between cytochrome c and mitochondria in Hep G2 cells that was prevented by administration of 2-APB. These data further establish the role of cellular Ca(2+) uptake and subsequent mitochondrial Ca(2+) overload in I/R injury and identify 2-APB as a novel pharmacological inhibitor of liver I/R injury even when administered following a prolonged ischemic insult.

Calcium blocks formation of apoptosome by preventing nucleotide exchange in Apaf-1

Apaf-1 plays an essential role in apoptosis. In the presence of cytochrome c and dATP, Apaf-1 assembles into an oligomeric apoptosome, which is responsible for the activation of procaspase-9 and the maintenance of the enzymatic activity of the processed caspase-9. Regulation of apoptosome assembly by other cellular factors is poorly understood. Here we report that physiological concentrations of calcium ion negatively affect the assembly of apoptosome by inhibiting nucleotide exchange in the monomeric, autoinhibited Apaf-1 protein. Consequently, calcium blocks the ability of Apaf-1 to activate caspase-9. These observations suggest an important role of calcium homeostasis on the Apaf-1-dependent apoptotic pathway.

Estrogen protects neuronal cells from amyloid beta-induced apoptosis via regulation of mitochondrial proteins and function

BACKGROUND

Neurodegeneration in Alzheimer's disease is associated with increased apoptosis and parallels increased levels of amyloid beta, which can induce neuronal apoptosis. Estrogen exposure prior to neurotoxic insult of hippocampal neurons promotes neuronal defence and survival against neurodegenerative insults including amyloid beta. Although all underlying molecular mechanisms of amyloid beta neurotoxicity remain undetermined, mitochondrial dysfunction, including altered calcium homeostasis and Bcl-2 expression, are involved in neurodegenerative vulnerability.

RESULTS

In this study, we investigated the mechanism of 17beta-estradiol-induced prevention of amyloid beta-induced apoptosis of rat hippocampal neuronal cultures. Estradiol treatment prior to amyloid beta exposure significantly reduced the number of apoptotic neurons and the associated rise in resting intracellular calcium levels. Amyloid beta exposure provoked down regulation of a key antiapoptotic protein, Bcl-2, and resulted in mitochondrial translocation of Bax, a protein known to promote cell death, and subsequent release of cytochrome c. E2 pretreatment inhibited the amyloid beta-induced decrease in Bcl-2 expression, translocation of Bax to the mitochondria and subsequent release of cytochrome c. Further implicating the mitochondria as a target of estradiol action, in vivo estradiol treatment enhanced the respiratory function of whole brain mitochondria. In addition, estradiol pretreatment protected isolated mitochondria against calcium-induced loss of respiratory function.

CONCLUSION

Therefore, we propose that estradiol pretreatment protects against amyloid beta neurotoxicity by limiting mitochondrial dysfunction via activation of antiapoptotic mechanisms.

Optical and pharmacological tools to investigate the role of mitochondria during oxidative stress and neurodegeneration

Mitochondria are critical for cellular adenosine triphosphate (ATP) production; however, recent studies suggest that these organelles fulfill a much broader range of tasks. For example, they are involved in the regulation of cytosolic Ca(2+) levels, intracellular pH and apoptosis, and are the major source of reactive oxygen species (ROS). Various reactive molecules that originate from mitochondria, such as ROS, are critical in pathological events, such as ischemia, as well as in physiological events such as long-term potentiation, neuronal-vascular coupling and neuronal-glial interactions. Due to their key roles in the regulation of several cellular functions, the dysfunction of mitochondria may be critical in various brain disorders. There has been increasing interest in the development of tools that modulate mitochondrial function, and the refinement of techniques that allow for real time monitoring of mitochondria, particularly within their intact cellular environment. Innovative imaging techniques are especially powerful since they allow for mitochondrial visualization at high resolution, tracking of mitochondrial structures and optical real time monitoring of parameters of mitochondrial function. The techniques discussed include classic imaging techniques, such as rhodamine-123, the highly advanced semi-conductor nanoparticles (quantum dots), and wide field microscopy as well as high-resolution multiphoton imaging. We have highlighted the use of these techniques to study mitochondrial function in brain tissue and have included studies from our laboratories in which these techniques have been successfully applied.

Mathematical model of mitochondrial ionic homeostasis: three modes of Ca2+ transport

Mitochondria play an important role in regulation of Ca2+ homeostasis in a cell. Here we present a mathematical model of mitochondrial ion transport and use this model to analyse different modes of Ca2+ uptake by mitochondria. The model includes transport of H+, Ca2+, K+, inorganic phosphate and oxidative substrates across the inner mitochondrial membrane harboring permeability transition pore (PTP). The detailed description of ion fluxes is based on the experimental ion kinetics in isolated mitochondria. Using the model we show that the kinetics of Ca2+ uptake by mitochondria is regulated by the total amount of Ca2+ in the system and the rate of Ca2+ infusion. Varying these parameters we find three different modes of ion transport. When the total amount of Ca2+ is below 140 nmol Ca2+/mg protein, all available Ca2+ is accumulated in the matrix without activation of the PTP. Between 140 and 160 nmol Ca2+/mg protein, accumulation of Ca2+ generates periodic opening and closure of the PTP and oscillations of ion fluxes. Higher levels of Ca2+ (> 160 nmol Ca2+/mg protein) result in a permanently open PTP, membrane depolarization and loss of small ions from the matrix. We show that in the intermediate range of Ca2+ concentrations the rate of Ca2+ infusion regulates the PTP state, so that slow Ca2+ infusion does not lead to PTP opening, while fast Ca2+ infusion results in an oscillatory state.

Synaptic mitochondria are more susceptible to Ca2+overload than nonsynaptic mitochondria

Mitochondria in nerve terminals are subjected to extensive Ca2+ fluxes and high energy demands, but the extent to which the synaptic mitochondria buffer Ca2+ is unclear. In this study, we identified a difference in the Ca2+ clearance ability of nonsynaptic versus synaptic mitochondrial populations enriched from rat cerebral cortex. Mitochondria were isolated using Percoll discontinuous gradients in combination with high pressure nitrogen cell disruption. Mitochondria in the nonsynaptic fraction originate from neurons and other cell types including glia, whereas mitochondria enriched from a synaptosomal fraction are predominantly neuronal and presynaptic in origin. There were no differences in respiration or initial Ca2+ loads between nonsynaptic and synaptic mitochondrial populations. Following both bolus and infusion Ca2+ addition, nonsynaptic mitochondria were able to accumulate significantly more exogenously added Ca2+ than the synaptic mitochondria before undergoing mitochondrial permeability transition, observed as a loss in mitochondrial membrane potential and decreased Ca2+ uptake. The limited ability of synaptic mitochondria to accumulate Ca2+ could result from several factors including a primary function of ATP production to support the high energy demand of presynaptic terminals, their relative isolation in comparison with the threads or clusters of mitochondria found in the soma of neurons and glia, or the older age and increased exposure to oxidative damage of synaptic versus nonsynaptic mitochondria. By more readily undergoing permeability transition, synaptic mitochondria may initiate neuron death in response to insults that elevate synaptic levels of intracellular Ca2+, consistent with the early degeneration of distal axon segments in neurodegenerative disorders.

Ester derivatives of tournefolic acid B attenuate N-methyl-D-aspartate-mediated excitotoxicity in rat cortical neurons

The effects of tournefolic acid B (TAB) and two ester derivatives, TAB methyl ester (TABM) and TAB ethyl ester (TABE), on N-methyl-D-aspartate (NMDA)-mediated excitotoxicity and the underlying mechanisms were investigated. Treatment with 50 microM NMDA elicited neuronal death by 48.7 +/- 5.1%, coinciding with the appearance of injured morphology. TABM (50 microM) attenuated the NMDA-induced cell death by 60.9 +/- 19.7%, and to a lesser extent by TABE. The NMDA-mediated activation of calpain was not affected by TABM and TABE, as determined by the cleavage of alpha-spectrin. NMDA increased the activity of caspases 2, 3, 6, 8, and 9 and reached the maximum after 8-h treatment. TABM and TABE abrogated NMDA-induced activation of caspases 2, 3, 6, and 8 by approximately 80 to 90% and 50 to 60%, respectively, and to a higher extent for caspase 9. TABM and TABE also blocked the NMDA-mediated activation of caspase 12. Furthermore, TABM and TABE eliminated the NMDA-induced accumulation of superoxide anion (O2-*). NMDA evoked significant depolarization of mitochondria, whereas TABM elicited a mild decrease of mitochondrial membrane potential as determined by tetramethylrhodamine methyl ester perchlorate. NMDA treatment induced elevation of Ca2+ levels in cytosol, endoplasmic reticulum (ER), and mitochondria. TABM (50 microM) significantly diminished the NMDA-induced elevation of Ca2+ levels in mitochondria and ER but not cytosol. Therefore, TABM decreased mitochondrial membrane potential and attenuated the NMDA-mediated Ca2+-loading in ER and mitochondria. These events subsequently eliminated the accumulation of O2-* and blocked the activation of caspase cascade, thereby conferring their neuroprotective effects on NMDA-mediated excitotoxicity.

Calcium, mitochondria and oxidative stress in neuronal pathology. Novel aspects of an enduring theme

The interplay among reactive oxygen species (ROS) formation, elevated intracellular calcium concentration and mitochondrial demise is a recurring theme in research focusing on brain pathology, both for acute and chronic neurodegenerative states. However, causality, extent of contribution or the sequence of these events prior to cell death is not yet firmly established. Here we review the role of the alpha-ketoglutarate dehydrogenase complex as a newly identified source of mitochondrial ROS production. Furthermore, based on contemporary reports we examine novel concepts as potential mediators of neuronal injury connecting mitochondria, increased [Ca2+]c and ROS/reactive nitrogen species (RNS) formation; specifically: (a) the possibility that plasmalemmal nonselective cationic channels contribute to the latent [Ca2+]c rise in the context of glutamate-induced delayed calcium deregulation; (b) the likelihood of the involvement of the channels in the phenomenon of 'Ca2+ paradox' that might be implicated in ischemia/reperfusion injury; and (c) how ROS/RNS and mitochondrial status could influence the activity of these channels leading to loss of ionic homeostasis and cell death.