The molecular identity of the mitochondrial Ca2+ sequestration system

Neuroprotective effects of dantrolene in neurodegenerative disease: Role of inhibition of pathological inflammation

Neurodegenerative diseases (NDs) refer to a group of diseases in which slow, continuous cell death is the main pathogenic event in the nervous system. Most NDs are characterized by cognitive dysfunction or progressive motor dysfunction. Treatments of NDs mainly target alleviating symptoms, and most NDs do not have disease-modifying drugs. The pathogenesis of NDs involves inflammation and apoptosis mediated by mitochondrial dysfunction. Dantrolene, approved by the US Food and Drug Administration, acts as a RyRs antagonist for the treatment of malignant hyperthermia, spasticity, neuroleptic syndrome, ecstasy intoxication and exertional heat stroke with tolerable side effects. Recently, dantrolene has also shown therapeutic effects in some NDs. Its neuroprotective mechanisms include the reduction of excitotoxicity, apoptosis and neuroinflammation. In summary, dantrolene can be considered as a potential therapeutic candidate for NDs.

Mitochondrial morphology in the mouse adrenal cortex: Influence of chronic psychosocial stress

Mitochondria within the adrenal cortex play a key role in synthesizing steroid hormones. The adrenal cortex is organized in three functionally specialized zones (glomerulosa, fasciculata, and reticularis) that produce different classes of steroid hormones in response to various stimuli, including psychosocial stress. Given that the functions and morphology of mitochondria are dynamically related and respond to stress, we applied transmission electron microscopy (TEM) to examine potential differences in mitochondrial morphology under basal and chronic psychosocial stress conditions. We used the chronic subordinate colony housing (CSC) paradigm, a murine model of chronic psychosocial stress. Our findings quantitatively define how mitochondrial morphology differs among each of the three adrenal cortex zones under basal conditions, and show that chronic psychosocial stress mainly affected mitochondria in the zona glomerulosa, shifting their morphology towards the more typical glucocorticoid-producing zona fasciculata mitochondrial phenotype. Analysis of adrenocortical lipid droplets that provide cholesterol for steroidogenesis showed that chronic psychosocial stress altered lipid droplet diameter, without affecting droplet number or inter-organellar mitochondria-lipid droplet interactions. Together, our findings support the hypothesis that each adrenal cortex layer is characterized by morphologically distinct mitochondria and that this adrenal zone-specific mitochondrial morphology is sensitive to environmental stimuli, including chronic psychosocial stressors. Further research is needed to define the role of these stress-induced changes in mitochondrial morphology, particularly in the zona glomerulosa, on stress resilience and related behaviors.

Calcium-Mediated Calpain Activation and Microtubule Dissociation in Cell Model of Hereditary Sensory Neuropathy Type-1 Expressing V144D SPTLC1 Mutation

Hereditary sensory neuropathy type 1A (HSN1A) is an autosomal, dominantly inherited peripheral neuropathy caused by mutations in serine palmitoyl transferase long chain 1 (SPTLC1), involved in the de novo synthesis of sphingolipids. We have previously reported calcium imbalance, as well as mitochondrial and ER stress in both HSN1 patient lymphoblasts and a transiently transfected cell model. In this study, we investigated the role of the Ca2+-activated protease calpain in destabilizing the cell cytoskeleton, by examining calpain activity in SH-SY5Y cells overexpressing the V144D mutant and changes in microtubule-associated proteins (MAP). Intramitochondrial Ca2+ was found to be significantly depleted and cytoplasmic Ca2+ increased in the V144D mutant. Subsequently, calpain and proteasome activity were increased and calpain substrates, microtubule associated proteins MAP2, and tau were significantly reduced in the microtubule fraction of the mutant. Significant changes were also found in motor proteins dynein and KIF2A detected in the microtubule fraction of cells overexpressing the V144D mutation. There was also a reduction in anterograde and retrograde mitochondrial transport velocities in the V144D mutant. These findings strongly implicate cytoskeletal aberration caused by Ca2+ dysregulation and subsequent loss of microtubule transport functions as the cause of axonal dying back that is characteristic of HSN1.

Total Matrix Ca2+ Modulates Ca2+ Efflux via the Ca2+/H+ Exchanger in Cardiac Mitochondria

Mitochondrial Ca²⁺ handling is accomplished by balancing Ca²⁺ uptake, primarily via the Ru360-sensitive mitochondrial calcium uniporter (MCU), Ca²⁺ buffering in the matrix and Ca²⁺ efflux mainly via Ca²⁺ ion exchangers, such as the Na⁺/Ca²⁺ exchanger (NCLX) and the Ca²⁺/H⁺ exchanger (CHE). The mechanism of CHE in cardiac mitochondria is not well-understood and its contribution to matrix Ca²⁺ regulation is thought to be negligible, despite higher expression of the putative CHE protein, LETM1, compared to hepatic mitochondria. In this study, Ca²⁺ efflux via the CHE was investigated in isolated rat cardiac mitochondria and permeabilized H9c2 cells. Mitochondria were exposed to (a) increasing matrix Ca²⁺ load via repetitive application of a finite CaCl₂ bolus to the external medium and (b) change in the pH gradient across the inner mitochondrial membrane (IMM). Ca²⁺ efflux at different matrix Ca²⁺ loads was revealed by inhibiting Ca²⁺ uptake or reuptake with Ru360 after increasing number of CaCl₂ boluses. In Na⁺-free experimental buffer and with Ca²⁺ uptake inhibited, the rate of Ca²⁺ efflux and steady-state free matrix Ca²⁺ [mCa²⁺]_ss increased as the number of administered CaCl₂ boluses increased. ADP and cyclosporine A (CsA), which are known to increase Ca²⁺ buffering while maintaining a constant [mCa²⁺]_ss, decreased the rate of Ca²⁺ efflux via the CHE, with a significantly greater decrease in the presence of ADP. ADP also increased Ca²⁺ buffering rate and decreased [mCa²⁺]_ss. A change in the pH of the external medium to a more acidic value from 7.15 to 6.8∼6.9 caused a twofold increase in the Ca²⁺ efflux rate, while an alkaline change in pH from 7.15 to 7.4∼7.5 did not change the Ca²⁺ efflux rate. In addition, CHE activation was associated with membrane depolarization. Targeted transient knockdown of LETM1 in permeabilized H9c2 cells modulated Ca²⁺ efflux. The results indicate that Ca²⁺ efflux via the CHE in cardiac mitochondria is modulated by acidic buffer pH and by total matrix Ca²⁺. A mechanism is proposed whereby activation of CHE is sensitive to changes in both the matrix Ca²⁺ buffering system and the matrix free Ca²⁺ concentration.

The Emerging Role of RHOT1/Miro1 in the Pathogenesis of Parkinson's Disease

The expected increase in prevalence of Parkinson's disease (PD) as the most common neurodegenerative movement disorder over the next years underscores the need for a better understanding of the underlying molecular pathogenesis. Here, first insights provided by genetics over the last two decades, such as dysfunction of molecular and organellar quality control, are described. The mechanisms involved relate to impaired intracellular calcium homeostasis and mitochondrial dynamics, which are tightly linked to the cross talk between the endoplasmic reticulum (ER) and mitochondria. A number of proteins related to monogenic forms of PD have been mapped to these pathways, i.e., PINK1, Parkin, LRRK2, and α-synuclein. Recently, Miro1 was identified as an important player, as several studies linked Miro1 to mitochondrial quality control by PINK1/Parkin-mediated mitophagy and mitochondrial transport. Moreover, Miro1 is an important regulator of mitochondria-ER contact sites (MERCs), where it acts as a sensor for cytosolic calcium levels. The involvement of Miro1 in the pathogenesis of PD was recently confirmed by genetic evidence based on the first PD patients with heterozygous mutations in RHOT1/Miro1. Patient-based cellular models from RHOT1/Miro1 mutation carriers showed impaired calcium homeostasis, structural alterations of MERCs, and increased mitochondrial clearance. To account for the emerging role of Miro1, we present a comprehensive overview focusing on the role of this protein in PD-related neurodegeneration and highlighting new developments in our understanding of Miro1, which provide new avenues for neuroprotective therapies for PD patients.

Regulation of cellular senescence by eukaryotic members of the FAH superfamily - A role in calcium homeostasis?

Fumarylacetoacetate hydrolase (FAH) superfamily members are commonly expressed in the prokaryotic kingdom, where they take part in the committing steps of degradation pathways of complex carbon sources. Besides FAH itself, the only described FAH superfamily members in the eukaryotic kingdom are fumarylacetoacetate hydrolase domain containing proteins (FAHD) 1 and 2, that have been a focus of recent work in aging research. Here, we provide a review of current knowledge on FAHD proteins. Of those, FAHD1 has recently been described as a regulator of mitochondrial function and senescence, in the context of mitochondrial dysfunction associated senescence (MiDAS). This work further describes data based on bioinformatics analysis, 3D structure comparison and sequence alignment, that suggests a putative role of FAHD proteins as calcium binding proteins.

Targeting apoptosis and autophagy following spinal cord injury: Therapeutic approaches to polyphenols and candidate phytochemicals

Spinal cord injury (SCI) is a neurological disorder associated with the loss of sensory and motor function. Understanding the precise dysregulated signaling pathways, especially apoptosis and autophagy following SCI, is of vital importance in developing innovative therapeutic targets and treatments. The present study lies in the fact that it reveals the precise dysregulated signaling mediators of apoptotic and autophagic pathways following SCI and also examines the effects of polyphenols and other candidate phytochemicals. It provides new insights to develop new treatments for post-SCI complications. Accordingly, a comprehensive review was conducted using electronic databases including, Scopus, Web of Science, PubMed, and Medline, along with the authors' expertise in apoptosis and autophagy as well as their knowledge about the effects of polyphenols and other phytochemicals on SCI pathogenesis. The primary mechanical injury to spinal cord is followed by a secondary cascade of apoptosis and autophagy that play critical roles during SCI. In terms of pharmacological mechanisms, caspases, Bax/Bcl-2, TNF-α, and JAK/STAT in apoptosis along with LC3 and Beclin-1 in autophagy have shown a close interconnection with the inflammatory pathways mainly glutamatergic, PI3K/Akt/mTOR, ERK/MAPK, and other cross-linked mediators. Besides, apoptotic pathways have been shown to regulate autophagy mediators and vice versa. Prevailing evidence has highlighted the importance of modulating these signaling mediators/pathways by polyphenols and other candidate phytochemicals post-SCI. The present review provides dysregulated signaling mediators and therapeutic targets of apoptotic and autophagic pathways following SCI, focusing on the modulatory effects of polyphenols and other potential phytochemical candidates.

Arginase II Contributes to the Ca2+/CaMKII/eNOS Axis by Regulating Ca2+ Concentration Between the Cytosol and Mitochondria in a p32-Dependent Manner

Background

Arginase II activity contributes to reciprocal regulation of endothelial nitric oxide synthase (eNOS). We tested the hypotheses that arginase II activity participates in the regulation of Ca²⁺/Ca²⁺/calmodulin‐dependent kinase II/eNOS activation, and this process is dependent on mitochondrial p32.

Methods and Results

Downregulation of arginase II increased the concentration of cytosolic Ca²⁺ ([Ca²⁺]c) and decreased mitochondrial Ca²⁺ ([Ca²⁺]m) in microscopic and fluorescence‐activated cell sorting analyses, resulting in augmented eNOS Ser1177 phosphorylation and decreased eNOS Thr495 phosphorylation through Ca²⁺/Ca²⁺/calmodulin‐dependent kinase II. These changes were observed in human umbilical vein endothelial cells treated with small interfering RNA against p32 (sip32). Using matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, fluorescence immunoassay, and ion chromatography, inhibition of arginase II reduced the amount of spermine, a binding molecule, and the release of Ca²⁺ from p32. In addition, arginase II gene knockdown using small interfering RNA and knockout arginase II‐null mice resulted in reduced p32 protein level. In the aortas of wild‐type mice, small interfering RNA against p32 induced eNOS Ser1177 phosphorylation and enhanced NO‐dependent vasorelaxation. Arginase activity, p32 protein expression, spermine amount, and [Ca²⁺]m were increased in the aortas from apolipoprotein E (ApoE^−/−) mice fed a high‐cholesterol diet, and intravenous administration of small interfering RNA against p32 restored Ca²⁺/Ca²⁺/calmodulin‐dependent kinase II‐dependent eNOS Ser1177 phosphorylation and improved endothelial dysfunction. The effects of arginase II downregulation were not associated with elevated NO production when tested in aortic endothelia from eNOS knockout mice.

Conclusions

These data demonstrate a novel function of arginase II in regulation of Ca²⁺‐dependent eNOS phosphorylation. This novel mechanism drives arginase activation, mitochondrial dysfunction, endothelial dysfunction, and atherogenesis.

Cyclosporin A Increases Mitochondrial Buffering of Calcium: An Additional Mechanism in Delaying Mitochondrial Permeability Transition Pore Opening

Regulation of mitochondrial free Ca²⁺ is critically important for cellular homeostasis. An increase in mitochondrial matrix free Ca²⁺ concentration ([Ca²⁺]_m) predisposes mitochondria to opening of the permeability transition pore (mPTP). Opening of the pore can be delayed by cyclosporin A (CsA), possibly by inhibiting cyclophilin D (Cyp D), a key regulator of mPTP. Here, we report on a novel mechanism by which CsA delays mPTP opening by enhanced sequestration of matrix free Ca²⁺. Cardiac-isolated mitochondria were challenged with repetitive CaCl₂ boluses under Na⁺-free buffer conditions with and without CsA. CsA significantly delayed mPTP opening primarily by promoting matrix Ca²⁺ sequestration, leading to sustained basal [Ca²⁺]_m levels for an extended period. The preservation of basal [Ca²⁺]_m during the CaCl₂ pulse challenge was associated with normalized NADH, matrix pH (pH_m), and mitochondrial membrane potential (ΔΨ_m). Notably, we found that in PO₄³⁻ (P_i)-free buffer condition, the CsA-mediated buffering of [Ca²⁺]_m was abrogated, and mitochondrial bioenergetics variables were concurrently compromised. In the presence of CsA, addition of P_i just before pore opening in the P_i-depleted condition reinstated the Ca²⁺ buffering system and rescued mitochondria from mPTP opening. This study shows that CsA promotes P_i-dependent mitochondrial Ca²⁺ sequestration to delay mPTP opening and, concomitantly, maintains mitochondrial function.

Emerging molecular therapeutic targets for spinal cord injury

Introduction: Spinal cord injury (SCI) is a complicated and devastating neurological disorder. Patients with SCI usually have dramatically reduced quality of life. In recent years, numerous studies have reported advances in understanding the pathophysiology of SCI and developing preclinical therapeutic strategies for SCI, including various molecular therapies, and yet there is still no cure. Areas covered: After SCI, tissue damage, responses and repair involve interactions among many cellular components, including neurons, axons, glia, leukocytes, and other cells. Accordingly, numerous cellular genes and molecules have become therapeutic targets for neural tissue repair, circuit reconstruction, and behavioral restoration. Here, we review the major recent advances in biological and molecular strategies to enhance neuroprotection, axon regeneration, remyelination, neuroplasticity and functional recovery in preclinical studies of SCI. Expert opinion: Researchers have made tremendous progress in identifying individual and combined molecular therapies in animal studies. It is very important to identify additional highly effective treatments for early neuroprotective intervention and for functionally meaningful axon regeneration and neuronal reconnections. Because multiple mechanisms contribute to the functional loss after SCI, combining the most promising approaches that target different pathophysiological and molecular mechanisms should exhibit synergistic actions for maximal functional restoration. [Databases searched: PubMed; inclusive dates: 6/27/2019].

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.

Dead space ventilation promotes alveolar hypocapnia reducing surfactant secretion by altering mitochondrial function

BACKGROUND

In acute respiratory distress syndrome (ARDS), pulmonary perfusion failure increases physiologic dead space ventilation (V_D/V_T), leading to a decline of the alveolar CO₂ concentration [CO₂]_iA. Although it has been shown that alveolar hypocapnia contributes to formation of atelectasis and surfactant depletion, a typical complication in ARDS, the underlying mechanism has not been elucidated so far.

METHODS

In isolated perfused rat lungs, cytosolic or mitochondrial Ca²⁺ concentrations ([Ca²⁺]_cyt or [Ca²⁺]_mito, respectively) of alveolar epithelial cells (AECs), surfactant secretion and the projected area of alveoli were quantified by real-time fluorescence or bright-field imaging (n=3-7 per group). In ventilated White New Zealand rabbits, the left pulmonary artery was ligated and the size of subpleural alveoli was measured by intravital microscopy (n=4 per group). Surfactant secretion was determined in the bronchoalveolar lavage (BAL) by western blot.

RESULTS

Low [CO₂]_iA decreased [Ca²⁺]_cyt and increased [Ca²⁺]_mito in AECs, leading to reduction of Ca²⁺-dependent surfactant secretion, and alveolar ventilation in situ. Mitochondrial inhibition by ruthenium red or rotenone blocked these responses indicating that mitochondria are key players in CO₂ sensing. Furthermore, ligature of the pulmonary artery of rabbits decreased alveolar ventilation, surfactant secretion and lung compliance in vivo. Addition of 5% CO₂ to the inspiratory gas inhibited these responses.

CONCLUSIONS

Accordingly, we provide evidence that alveolar hypocapnia leads to a Ca²⁺ shift from the cytosol into mitochondria. The subsequent decline of [Ca²⁺]_cyt reduces surfactant secretion and thus regional ventilation in lung regions with high V_D/V_T. Additionally, the regional hypoventilation provoked by perfusion failure can be inhibited by inspiratory CO₂ application.

In vitro and in vivo studies of the ALS-FTLD protein CHCHD10 reveal novel mitochondrial topology and protein interactions

Mutations in coiled-coil-helix-coiled-coil-helix-domain containing 10 (CHCHD10), a mitochondrial twin CX9C protein whose function is still unknown, cause myopathy, motor neuron disease, frontotemporal dementia, and Parkinson's disease. Here, we investigate CHCHD10 topology and its protein interactome, as well as the effects of CHCHD10 depletion or expression of disease-associated mutations in wild-type cells. We find that CHCHD10 associates with membranes in the mitochondrial intermembrane space, where it interacts with a closely related protein, CHCHD2. Furthermore, both CHCHD10 and CHCHD2 interact with p32/GC1QR, a protein with various intra and extra-mitochondrial functions. CHCHD10 and CHCHD2 have short half-lives, suggesting regulatory rather than structural functions. Cell lines with CHCHD10 knockdown do not display bioenergetic defects, but, unexpectedly, accumulate excessive intramitochondrial iron. In mice, CHCHD10 is expressed in many tissues, most abundantly in heart, skeletal muscle, liver, and in specific CNS regions, notably the dopaminergic neurons of the substantia nigra and spinal cord neurons, which is consistent with the pathology associated with CHCHD10 mutations. Homozygote CHCHD10 knockout mice are viable, have no gross phenotypes, no bioenergetic defects or ultrastructural mitochondrial abnormalities in brain, heart or skeletal muscle, indicating that functional redundancy or compensatory mechanisms for CHCHD10 loss occur in vivo. Instead, cells expressing S59L or R15L mutant versions of CHCHD10, but not WT, have impaired mitochondrial energy metabolism. Taken together, the evidence obtained from our in vitro and in vivo studies suggest that CHCHD10 mutants cause disease through a gain of toxic function mechanism, rather than a loss of function.

Electrophysiological Characterization of Calcium-Permeable Channels Using Planar Lipid Bilayer

Numerous researchers tried to identify the key players of calcium signaling in mitochondria using molecular and cell biology techniques for more than five decades. However, only an integrated approach involving also electrophysiological techniques has finally allowed to define the components of the protein complex responsible for the uptake of this ion into mitochondria.Here we describe the protocol used for the electrophysiological characterization of the mitochondrial calcium uniporter (MCU) complex: the following outline indicates step-by-step the setup of planar lipid bilayer experiments.

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.

Metabolite accumulation in VLCAD deficiency markedly disrupts mitochondrial bioenergetics and Ca2+ homeostasis in the heart

We studied the effects of the major long-chain fatty acids accumulating in very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, namely cis-5-tetradecenoic acid (Cis-5) and myristic acid (Myr), on important mitochondrial functions in isolated mitochondria from cardiac fibers and cardiomyocytes of juvenile rats. Cis-5 and Myr at pathological concentrations markedly reduced mitochondrial membrane potential (ΔΨ_m ), matrix NAD(P)H pool, Ca²⁺ retention capacity, ADP- (state 3) and carbonyl cyanide 3-chlorophenyl hydrazine-stimulated (uncoupled) respiration, and ATP generation. By contrast, these fatty acids increased resting (state 4) respiration (uncoupling effect) with the involvement of the adenine nucleotide translocator because carboxyatractyloside significantly attenuated the increased state 4 respiration provoked by Cis-5 and Myr. Furthermore, the classical inhibitors of mitochondrial permeability transition (MPT) pore cyclosporin A plus ADP, as well as the Ca²⁺ uptake blocker ruthenium red, fully prevented the Cis-5- and Myr-induced decrease in ΔΨ_m in Ca²⁺ -loaded mitochondria, suggesting, respectively, the induction of MPT pore opening and the contribution of Ca²⁺ toward these effects. The findings of the present study indicate that the major long-chain fatty acids that accumulate in VLCAD deficiency disrupt mitochondrial bioenergetics and Ca²⁺ homeostasis, acting as uncouplers and metabolic inhibitors of oxidative phosphorylation, as well as inducers of MPT pore opening, in the heart at pathological relevant concentrations. It is therefore presumed that a disturbance of bioenergetics and Ca²⁺ homeostasis may contribute to the cardiac manifestations observed in VLCAD deficiency.

Mitochondrial-Based Therapeutics for the Treatment of Spinal Cord Injury: Mitochondrial Biogenesis as a Potential Pharmacological Target

Spinal cord injury (SCI) is characterized by an initial trauma followed by a progressive cascade of damage referred to as secondary injury. A hallmark of secondary injury is vascular disruption leading to vasoconstriction and decreased oxygen delivery, which directly reduces the ability of mitochondria to maintain homeostasis and leads to loss of ATP-dependent cellular functions, calcium overload, excitotoxicity, and oxidative stress, further exacerbating injury. Restoration of mitochondria dysfunction during the acute phases of secondary injury after SCI represents a potentially effective therapeutic strategy. This review discusses the past and present pharmacological options for the treatment of SCI as well as current research on mitochondria-targeted approaches. Increased antioxidant activity, inhibition of the mitochondrial permeability transition, alternate energy sources, and manipulation of mitochondrial morphology are among the strategies under investigation. Unfortunately, many of these tactics address single aspects of mitochondrial dysfunction, ultimately proving largely ineffective. Therefore, this review also examines the unexplored therapeutic efficacy of pharmacological enhancement of mitochondrial biogenesis, which has the potential to more comprehensively improve mitochondrial function after SCI.

TrpA1 activation in peripheral sensory neurons underlies the ionic basis of pain hypersensitivity in response to vinca alkaloids

Chemotherapy induced peripheral neuropathy (CIPN), a side effect of many anti-cancer drugs including the vinca alkaloids, is characterized by a severe pain syndrome that compromises treatment in many patients. Currently there are no effective treatments for this pain syndrome except for the reduction of anti-cancer drug dose. Existing data supports the model that the pain associated with CIPN is the result of anti-cancer drugs augmenting the function of the peripheral sensory nociceptors but the cellular mechanisms underlying the effects of anti-cancer drugs on sensory neuron function are not well described. Studies from animal models have suggested a number of disease etiologies including mitotoxicity, axonal degeneration, immune signaling, and reduced sensory innervations but these outcomes are the result of prolonged treatment paradigms and do not necessarily represent the early formative events associated with CIPN. Here we show that acute exposure to vinca alkaloids results in an immediate pain syndrome in both flies and mice. Furthermore, we demonstrate that exposure of isolated sensory neurons to vinca alkaloids results in the generation of an inward sodium current capable of depolarizing these neurons to threshold resulting in neuronal firing. These neuronal effects of vinca alkaloids require the transient receptor potential ankyrin-1 (TrpA1) channel, and the hypersensitization to painful stimuli in response to the acute exposure to vinca alkaloids is reduced in TrpA1 mutant flies and mice. These findings demonstrate the direct excitation of sensory neurons by CIPN-causing chemotherapy drugs, and identify TrpA1 as an important target during the pathogenesis of CIPN.

IDH3 mediates apoptosis of alveolar epithelial cells type 2 due to mitochondrial Ca2+ uptake during hypocapnia

In adult respiratory distress syndrome (ARDS) pulmonary perfusion failure increases physiologic dead-space (V_D/V_T) correlating with mortality. High V_D/V_T results in alveolar hypocapnia, which has been demonstrated to cause edema formation, atelectasis, and surfactant depletion, evoked, at least in part, by apoptosis of alveolar epithelial cells (AEC). However, the mechanism underlying the hypocapnia-induced AEC apoptosis is unknown. Here, using fluorescent live-cell imaging of cultured AEC type 2 we could show that in terms of CO₂ sensing the tricarboxylic acid cycle enzyme isocitrate dehydrogenase (IDH) 3 seems to be an important player because hypocapnia resulted independently from pH in an elevation of IDH3 activity and subsequently in an increase of NADH, the substrate of the respiratory chain. As a consequence, the mitochondrial transmembrane potential (ΔΨ) rose causing a Ca²⁺ shift from cytosol into mitochondria, whereas the IDH3 knockdown inhibited these responses. Furthermore, the hypocapnia-induced mitochondrial Ca²⁺ uptake resulted in reactive oxygen species (ROS) production, and both the mitochondrial Ca²⁺ uptake and ROS production induced apoptosis. Accordingly, we provide evidence that in AEC type 2 hypocapnia induces elevation of IDH3 activity leading to apoptosis. This finding might give new insight into the pathogenesis of ARDS and may help to develop novel strategies to reduce tissue injury in ARDS.

Mitochondrial permeability transition in cardiac ischemia-reperfusion: whether cyclophilin D is a viable target for cardioprotection?

Growing number of studies provide strong evidence that the mitochondrial permeability transition pore (PTP), a non-selective channel in the inner mitochondrial membrane, is involved in the pathogenesis of cardiac ischemia-reperfusion and can be targeted to attenuate reperfusion-induced damage to the myocardium. The molecular identity of the PTP remains unknown and cyclophilin D is the only protein commonly accepted as a major regulator of the PTP opening. Therefore, cyclophilin D is an attractive target for pharmacological or genetic therapies to reduce ischemia-reperfusion injury in various animal models and humans. Most animal studies demonstrated cardioprotective effects of PTP inhibition; however, a recent large clinical trial conducted by international groups demonstrated that cyclosporine A, a cyclophilin D inhibitor, failed to protect the heart in patients with myocardial infarction. These studies, among others, raise the question of whether cyclophilin D, which plays an important physiological role in the regulation of cell metabolism and mitochondrial bioenergetics, is a viable target for cardioprotection. This review discusses previous studies to provide comprehensive information on the physiological role of cyclophilin D as well as PTP opening in the cell that can be taken into consideration for the development of new PTP inhibitors.

Mutant TDP-43 does not impair mitochondrial bioenergetics in vitro and in vivo

Background

Mitochondrial dysfunction has been linked to the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Functional studies of mitochondrial bioenergetics have focused mostly on superoxide dismutase 1 (SOD1) mutants, and showed that mutant human SOD1 impairs mitochondrial oxidative phosphorylation, calcium homeostasis, and dynamics. However, recent reports have indicated that alterations in transactivation response element DNA-binding protein 43 (TDP-43) can also lead to defects of mitochondrial morphology and dynamics. Furthermore, it was proposed that TDP-43 mutations cause oxidative phosphorylation impairment associated with respiratory chain defects and that these effects were caused by mitochondrial localization of the mutant protein. Here, we investigated the presence of bioenergetic defects in the brain of transgenic mice expressing human mutant TDP-43 (TDP-43^A315T mice), patient derived fibroblasts, and human cells expressing mutant forms of TDP-43.

Methods

In the brain of TDP-43^A315T mice, TDP-43 mutant fibroblasts, and cells expressing mutant TDP-43, we tested several bioenergetics parameters, including mitochondrial respiration, ATP synthesis, and calcium handling. Differences between mutant and control samples were evaluated by student t-test or by ANOVA, followed by Bonferroni correction, when more than two groups were compared. Mitochondrial localization of TDP-43 was investigated by immunocytochemistry in fibroblasts and by subcellular fractionation and western blot of mitochondrial fractions in mouse brain.

Results

We did not observe defects in any of the mitochondrial bioenergetic functions that were tested in TDP-43 mutants. We detected a small amount of TDP-43^A315T peripherally associated with brain mitochondria. However, there was no correlation between TDP-43 associated with mitochondria and respiratory chain dysfunction. In addition, we observed increased calcium uptake in mitochondria from TDP-43^A315T mouse brain and cells expressing A315T mutant TDP-43.

Conclusions

While alterations of mitochondrial morphology and dynamics in TDP-43 mutant neurons are well established, the present study did not demonstrate oxidative phosphorylation defects in TDP-43 mutants, in vitro and in vivo. On the other hand, the increase in mitochondrial calcium uptake in A315T TDP-43 mutants was an intriguing finding, which needs to be investigated further to understand its mechanisms and potential pathogenic implications.

Synaptic proteome changes in the hypothalamus of mother rats

To establish synaptic proteome changes associated with motherhood, we isolated synaptosome fractions from the hypothalamus of mother rats and non-maternal control females at the 11th postpartum day. Proteomic analysis by two-dimensional differential gel electrophoresis combined with mass spectrometric protein identification established 26 significant proteins, 7 increasing and 19 decreasing protein levels in the dams. The altered proteins are mainly involved in energy homeostasis, protein folding, and metabolic processes suggesting the involvement of these cellular processes in maternal adaptations. The decrease in a significantly altered protein, complement component 1q subcomponent-binding protein (C1qbp) was validated with Western blotting. Furthermore, immunohistochemistry showed its presence in hypothalamic fibers and terminals in agreement with its presence in synaptosomes. We also found the expression of C1qbp in different hypothalamic nuclei including the preoptic area and the paraventricular hypothalamic nucleus at the protein and at the mRNA level using immunohistochemistry and in situ hybridization histochemistry, respectively. Bioinformatical network analysis revealed that cytokines, growth factors, and protein kinases are common regulators, which indicates a complex regulation of the proteome change in mothers. The results suggest that maternal responsiveness is associated with synaptic proteins level changes in the hypothalamus, and that growth factors and cytokines may govern these alterations.

BIOLOGICAL SIGNIFICANCE

The period of motherhood is accompanied with several behavioral, neuroendocrine, emotional and metabolic adaptations in the brain. Although it is established that various hypothalamic networks participate in the maternal adaptations of the rodent brain, our knowledge on the molecular background of these alterations remains seriously limited. In the present study, we first determined that the functional alterations of the maternal brain can be detected at the level of the synaptic proteome in the hypothalamus. Independent confirmation of synaptic localization, and also the established decrease in the level of C1qbp protein suggest the validity of the data. Common regulators of altered proteins belonging to the growth factor and cytokine family suggest that the synaptic adaptation is governed by these extracellular signals and future studies should focus on their specific roles. Our study was also the first to describe the expression pattern of C1qbp in the hypothalamus, a protein potentially involved in mitochondrial and neuroimmunological regulations of synaptic plasticity. Its presence in the preoptic area responsible for maternal behaviors and also in the paraventricular hypothalamic and arcuate nuclei regulating hormonal levels suggests that the same proteins may be involved in different aspects of maternal adaptations. The conclusions of the present work contribute to establishing the molecular alterations that determine different maternal adaptations in the brain. Since maternal changes are models of neuronal plasticity in all social interactions, the reported results can affect a wide field of molecular and behavioral neuroscience.

Mitochondrial Ca(2+) uptake in skeletal muscle health and disease

Muscle uses Ca²⁺ as a messenger to control contraction and relies on ATP to maintain the intracellular Ca²⁺ homeostasis. Mitochondria are the major sub-cellular organelle of ATP production. With a negative inner membrane potential, mitochondria take up Ca²⁺ from their surroundings, a process called mitochondrial Ca²⁺ uptake. Under physiological conditions, Ca²⁺ uptake into mitochondria promotes ATP production. Excessive uptake causes mitochondrial Ca²⁺ overload, which activates downstream adverse responses leading to cell dysfunction. Moreover, mitochondrial Ca²⁺ uptake could shape spatio-temporal patterns of intracellular Ca²⁺ signaling. Malfunction of mitochondrial Ca²⁺ uptake is implicated in muscle degeneration. Unlike non-excitable cells, mitochondria in muscle cells experience dramatic changes of intracellular Ca²⁺ levels. Besides the sudden elevation of Ca²⁺ level induced by action potentials, Ca²⁺ transients in muscle cells can be as short as a few milliseconds during a single twitch or as long as minutes during tetanic contraction, which raises the question whether mitochondrial Ca²⁺ uptake is fast and big enough to shape intracellular Ca²⁺ signaling during excitation-contraction coupling and creates technical challenges for quantification of the dynamic changes of Ca²⁺ inside mitochondria. This review focuses on characterization of mitochondrial Ca²⁺ uptake in skeletal muscle and its role in muscle physiology and diseases.

Disturbance of mitochondrial functions provoked by the major long-chain 3-hydroxylated fatty acids accumulating in MTP and LCHAD deficiencies in skeletal muscle

The pathogenesis of the muscular symptoms and recurrent rhabdomyolysis that are commonly manifested in patients with mitochondrial trifunctional protein (MTP) and long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiencies is still unknown. In this study we investigated the effects of the major long-chain monocarboxylic 3-hydroxylated fatty acids (LCHFA) accumulating in these disorders, namely 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, on important mitochondrial functions in rat skeletal muscle mitochondria. 3HTA and 3HPA markedly increased resting (state 4) and decreased ADP-stimulated (state 3) and CCCP-stimulated (uncoupled) respiration. 3HPA provoked similar effects in permeabilized skeletal muscle fibers, validating the results obtained in purified mitochondria. Furthermore, 3HTA and 3HPA markedly diminished mitochondrial membrane potential, NAD(P)H content and Ca(2+) retention capacity in Ca(2+)-loaded mitochondria. Mitochondrial permeability transition (mPT) induction probably underlie these effects since they were totally prevented by cyclosporin A and ADP. In contrast, the dicarboxylic analogue of 3HTA did not alter the tested parameters. Our data strongly indicate that 3HTA and 3HPA behave as metabolic inhibitors, uncouplers of oxidative phosphorylation and mPT inducers in skeletal muscle. It is proposed that these pathomechanisms disrupting mitochondrial homeostasis may be involved in the muscle alterations characteristic of MTP and LCHAD deficiencies.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Mitochondria, calcium homeostasis and calcium signaling

Са2+ is a very important and versatile intracellular signal which controls numerous biochemical and physiological (pathophysiological) processes in the cell. Good evidence exists that mitochondria are sensors, decoders and regulators of calcium signaling. Precise regulation of calcium signaling in the cell involves numerous molecular targets, which induce and decode changes of Са2+ concentrations in the cell (pumps, channels, Са2+-binding proteins, Са2+-dependent enzymes, localized in the cytoplasm and organelles). Mitochondrial Са2+ uniporter accumulates excess of Са2+ in mitochondria, while Na+/Са2+- and H+/Са2+-antiporters extrude Са2+ in the cytoplasm. Mitochondrial Са2+ overloading results in formation of mitochondria permeability transition pores which play an important role in cell death under many pathological conditions. Mitochondria regulate Са2+ homeostasis and control important cellular functions such as metabolism, proliferation, survival. Identification of cellular and mitochondrial Ca2+ transporters and understanding their functional mechanisms open up new prospects for their using as therapeutic targets

Deregulation of mitochondrial functions provoked by long-chain fatty acid accumulating in long-chain 3-hydroxyacyl-CoA dehydrogenase and mitochondrial permeability transition deficiencies in rat heart--mitochondrial permeability transition pore opening as a potential contributing pathomechanism of cardiac alterations in these disorders

Mitochondrial trifunctional protein and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiencies are fatty acid oxidation disorders biochemically characterized by tissue accumulation of long-chain fatty acids and derivatives, including the monocarboxylic long-chain 3-hydroxy fatty acids (LCHFAs) 3-hydroxytetradecanoic acid (3HTA) and 3-hydroxypalmitic acid (3HPA). Patients commonly present severe cardiomyopathy for which the pathogenesis is still poorly established. We investigated the effects of 3HTA and 3HPA, the major metabolites accumulating in these disorders, on important parameters of mitochondrial homeostasis in Ca(2+) -loaded heart mitochondria. 3HTA and 3HPA significantly decreased mitochondrial membrane potential, the matrix NAD(P)H pool and Ca(2+) retention capacity, and also induced mitochondrial swelling. These fatty acids also provoked a marked decrease of ATP production reflecting severe energy dysfunction. Furthermore, 3HTA-induced mitochondrial alterations were completely prevented by the classical mitochondrial permeability transition (mPT) inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca(2+) uptake blocker, indicating that LCHFAs induced Ca(2+)-dependent mPT pore opening. Milder effects only achieved at higher doses of LCHFAs were observed in brain mitochondria, implying a higher vulnerability of heart to these fatty acids. By contrast, 3HTA and docosanoic acids did not change mitochondrial homeostasis, indicating selective effects for monocarboxylic LCHFAs. The present data indicate that the major LCHFAs accumulating in mitochondrial trifunctional protein and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiencies induce mPT pore opening, compromising Ca(2+) homeostasis and oxidative phosphorylation more intensely in the heart. It is proposed that these pathomechanisms may contribute at least in part to the severe cardiac alterations characteristic of patients affected by these diseases.

In vitro analyses of mitochondrial ATP/phosphate carriers from Arabidopsis thaliana revealed unexpected Ca(2+)-effects

BACKGROUND

Adenine nucleotide/phosphate carriers (APCs) from mammals and yeast are commonly known to adapt the mitochondrial adenine nucleotide pool in accordance to cellular demands. They catalyze adenine nucleotide--particularly ATP-Mg--and phosphate exchange and their activity is regulated by calcium. Our current knowledge about corresponding proteins from plants is comparably limited. Recently, the three putative APCs from Arabidopsis thaliana were shown to restore the specific growth phenotype of APC yeast loss-of-function mutants and to interact with calcium via their N-terminal EF--hand motifs in vitro. In this study, we performed biochemical characterization of all three APC isoforms from A. thaliana to gain further insights into their functional properties.

RESULTS

Recombinant plant APCs were functionally reconstituted into liposomes and their biochemical characteristics were determined by transport measurements using radiolabeled substrates. All three plant APCs were capable of ATP, ADP and phosphate exchange, however, high preference for ATP-Mg, as shown for orthologous carriers, was not detectable. By contrast, the obtained data suggest that in the liposomal system the plant APCs rather favor ATP-Ca as substrate. Moreover, investigation of a representative mutant APC protein revealed that the observed calcium effects on ATP transport did not primarily/essentially involve Ca(2+)-binding to the EF-hand motifs in the N-terminal domain of the carrier.

CONCLUSION

Biochemical characteristics suggest that plant APCs can mediate net transport of adenine nucleotides and hence, like their pendants from animals and yeast, might be involved in the alteration of the mitochondrial adenine nucleotide pool. Although, ATP-Ca was identified as an apparent import substrate of plant APCs in vitro it is arguable whether ATP-Ca formation and thus the corresponding transport can take place in vivo.

Mitochondrial Ca²⁺ and membrane potential, an alternative pathway for Interleukin 6 to regulate CD4 cell effector function

IL-6 plays an important role in determining the fate of effector CD4 cells and the cytokines that these cells produce. Here we identify a novel molecular mechanism by which IL-6 regulates CD4 cell effector function. We show that IL-6-dependent signal facilitates the formation of mitochondrial respiratory chain supercomplexes to sustain high mitochondrial membrane potential late during activation of CD4 cells. Mitochondrial hyperpolarization caused by IL-6 is uncoupled from the production of ATP by oxidative phosphorylation. However, it is a mechanism to raise the levels of mitochondrial Ca²⁺ late during activation of CD4 cells. Increased levels of mitochondrial Ca²⁺ in the presence of IL-6 are used to prolong Il4 and Il21 expression in effector CD4 cells. Thus, the effect of IL-6 on mitochondrial membrane potential and mitochondrial Ca²⁺ is an alternative pathway by which IL-6 regulates effector function of CD4 cells and it could contribute to the pathogenesis of inflammatory diseases.

DOI:http://dx.doi.org/10.7554/eLife.06376.001

Inflammation is a normal part of the body's response to an infection or injury and it helps to start the healing process. However, if left unchecked, inflammation itself can damage tissues, and diseases such as rheumatoid arthritis are the result of uncontrolled inflammation.

Certain immune cells release molecules that can either trigger or suppress inflammation. Interleukin 6 is an example of a ‘pro-inflammatory’ molecule, which regulates the activity of groups of immune cells collectively known as ‘CD4 cells’. People who are overweight or obese have higher levels of interleukin 6 than people of a healthy weight. Obesity and other metabolic conditions have been linked to problems with structures called mitochondria, which make a molecule called ATP that provides cells with the energy they need to survive. But it is not known if interleukin 6 can affect the activity of mitochondria inside CD4 cells.

Now, Yang et al. have discovered that interleukin 6 can affect the mitochondria inside CD4 cells and, in doing so, have identified a new way that interleukin 6 can regulate these cells' activity. Experiments involving immune cells from mice revealed that interleukin 6 triggers a cascade of signaling events that aid the formation of so-called ‘mitochondrial respiratory chain supercomplexes’ in CD4 cells. These are groups of proteins that work together in the membranes of mitochondria and are vital for the activity of these structures. The formation of these supercomplexes maintains a large voltage difference across the membrane of the mitochondria that occurs during the later stages of CD4 cell activation.

Yang et al. found that this voltage difference was not linked to the production of ATP, but that it did raise the levels of calcium ions inside the mitochondria. Further experiments revealed that these increased levels of calcium ions prolong the production of other pro-inflammatory molecules in the CD4 cells.

Following the discovery of a new pathway that regulates the activity of CD4 cells, the next challenge is to see if the parts of this pathway could be targeted with drugs to help treat inflammatory diseases such as rheumatoid arthritis. Moreover, because interleukin 6 plays an active role in other diseases such as cancer, further studies of this new pathway may help explain how this molecule encourages cancers to progress and/or spread around the body.

DOI:http://dx.doi.org/10.7554/eLife.06376.002

Uncoupling, metabolic inhibition and induction of mitochondrial permeability transition in rat liver mitochondria caused by the major long-chain hydroxyl monocarboxylic fatty acids accumulating in LCHAD deficiency

Patients with long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiency commonly present liver dysfunction whose pathogenesis is unknown. We studied the effects of long-chain 3-hydroxylated fatty acids (LCHFA) that accumulate in LCHAD deficiency on liver bioenergetics using mitochondrial preparations from young rats. We provide strong evidence that 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, the monocarboxylic acids that are found at the highest tissue concentrations in this disorder, act as metabolic inhibitors and uncouplers of oxidative phosphorylation. These conclusions are based on the findings that these fatty acids decreased ADP-stimulated (state 3) and uncoupled respiration, mitochondrial membrane potential and NAD(P)H content, and, in contrast, increased resting (state 4) respiration. We also verified that 3HTA and 3HPA markedly reduced Ca2+ retention capacity and induced swelling in Ca2+-loaded mitochondria. These effects were mediated by mitochondrial permeability transition (MPT) induction since they were totally prevented by the classical MPT inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca2+ uptake blocker. Taken together, our data demonstrate that the major monocarboxylic LCHFA accumulating in LCHAD deficiency disrupt energy mitochondrial homeostasis in the liver. It is proposed that this pathomechanism may explain at least in part the hepatic alterations characteristic of the affected patients.

Physiological and pathological roles of the mitochondrial permeability transition pore in the heart

Prolonged mitochondrial permeability transition pore (MPTP) opening results in mitochondrial energetic dysfunction, organelle swelling, rupture, and typically a type of necrotic cell death. However, acute opening of the MPTP has a critical physiologic role in regulating mitochondrial Ca(2+) handling and metabolism. Despite the physiological and pathological roles that the MPTP orchestrates, the proteins that comprise the pore itself remain an area of ongoing investigation. Here, we will discuss the molecular composition of the MPTP and its role in regulating cardiac physiology and disease. A better understanding of MPTP structure and function will likely suggest novel cardioprotective therapeutic approaches.

Astrocytes and mitochondria from adrenoleukodystrophy protein (ABCD1)-deficient mice reveal that the adrenoleukodystrophy-associated very long-chain fatty acids target several cellular energy-dependent functions

X-linked adrenoleukodystrophy (X-ALD) is a severe neurodegenerative disorder resulting from defective ABCD1 transport protein. ABCD1 mediates peroxisomal uptake of free very-long-chain fatty acids (VLCFA) as well as their CoA-esters. Consequently, VLCFA accumulate in patients' plasma and tissues, which is considered as pathogenic X-ALD triggering factor. Clinical symptoms are mostly manifested in neural tissues and adrenal gland. Here, we investigate astrocytes from wild-type control and a genetic X-ALD mouse model (Abcd1-knockout), exposed to supraphysiological VLCFA (C22:0, C24:0 and C26:0) concentrations. They exhibit multiple impairments of energy metabolism. Furthermore, brain mitochondria from Abcd1(-/-) mice and wild-type control respond similarly to VLCFA with increased ROS generation, impaired oxidative ATP synthesis and diminished Ca(2+) uptake capacity, suggesting that a defective ABCD1 exerts no adaptive pressure on mitochondria. In contrast, astrocytes from Abcd1(-/-) mice respond more sensitively to VLCFA than wild-type control astrocytes. Moreover, long-term application of VLCFA induces high ROS generation, and strong in situ depolarization of mitochondria, and, in Abcd1(-/-) astrocytes, severely diminishes the capability to revert oxidized pyridine nucleotides to NAD(P)H. In addition, observed differences in responses of mitochondria and astrocytes to the hydrocarbon chain length of VLCFA suggest that detrimental VLCFA activities in astrocytes involve defective cellular functions other than mitochondria. In summary, we clearly demonstrate that VLCFA increase the vulnerability of Abcd1(-/-) astrocytes.

The ER mitochondria calcium cycle and ER stress response as therapeutic targets in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of upper and lower motor neurons. Although the etiology remains unclear, disturbances in calcium homoeostasis and protein folding are essential features of neurodegeneration in this disorder. Here, we review recent research findings on the interaction between endoplasmic reticulum (ER) and mitochondria, and its effect on calcium signaling and oxidative stress. We further provide insights into studies, providing evidence that structures of the ER mitochondria calcium cycle serve as a promising targets for therapeutic approaches for treatment of ALS.

Ethylmalonic acid induces permeability transition in isolated brain mitochondria

Predominant accumulation of ethylmalonic acid (EMA) in tissues and biological fluids is a characteristic of patients affected by short chain acyl-CoA dehydrogenase deficiency and ethylmalonic encephalopathy. Neurological abnormalities are frequently found in these disorders, but the mechanisms underlying the brain injury are still obscure. Since hyperlacticacidemia is also found in many affected patients indicating a mitochondrial dysfunction; in the present work, we evaluated the in vitro and ex vivo effects of EMA plus Ca(2+) on mitochondrial integrity and redox balance in succinate-supported brain organelles. We verified that the evaluated parameters were disturbed only when EMA was associated with exogenous micromolar Ca(2+) concentrations. Thus, we found that this short chain organic acid plus Ca(2+) dissipated the membrane potential and provoked mitochondrial swelling, as well as impaired the mitochondrial Ca(2+) retention capacity, resulting in a rapid Ca(2+) release and decreased NAD(P)H matrix content. In contrast, EMA was not able to stimulate mitochondrial hydrogen peroxide generation. We also observed that all these effects were prevented by the mitochondrial Ca(2+) uptake inhibitor ruthenium red and the mitochondrial permeability transition (MPT) inhibitors cyclosporin A (CsA) and ADP. Furthermore, mitochondria isolated from rat brains after in vivo intrastriatal administration of EMA was more susceptible to Ca(2+)-induced swelling, which was fully prevented by CsA and ADP. Finally, EMA significantly decreased striatal slice viability, which was attenuated by CsA. The data strongly indicate that EMA reduced the mitochondrial threshold for Ca(2+)-induced MPT reinforcing the role of this cation in EMA-induced disruption of mitochondrial bioenergetics. It is, therefore, presumed that EMA acting synergistically with Ca(2+) compromises mitochondrial energy homeostasis in the central nervous system that may explain at least in part the neurologic alterations presented by patients with abnormal levels of this organic acid.

Role of intracellular calcium in proteasome inhibitor-induced endoplasmic reticulum stress, autophagy, and cell death

PURPOSE

Proteasome inhibition induces endoplasmic reticulum (ER) stress and compensatory autophagy to relieve ER stress. Disturbance of intracellular calcium homeostasis can lead to ER stress and alter the autophagy process. It has been suggested that inhibition of the proteasome disrupts intracellular calcium homeostasis. However, it is unknown if intracellular calcium affects proteasome inhibitor-induced ER stress and autophagy.

METHODS

Human colon cancer HCT116 Bax positive and negative cell lines were treated with MG132, a proteasome inhibitor. BAPTA-AM, a cell permeable free calcium chelator, was used to modulate intracellular calcium levels. Autophagy and cell death were determined by fluorescence microscopy and immunoblot analysis.

RESULTS

MG132 increased intracellular calcium levels in HCT116 cells, which was suppressed by BAPTA-AM. MG132 suppressed proteasome activity independent of Bax and intracellular calcium levels in HCT116 cells. BAPTA-AM inhibited MG132-induced cellular vacuolization and ER stress, but not apoptosis. MG132 induced autophagy with normal autophagosome-lysosome fusion. BAPTA-AM seemed not to affect autophagosome-lysosome fusion in MG132-treated cells but further enhanced MG132-induced LC3-II levels and GFP-LC3 puncta formation, which was likely via impaired lysosome function.

CONCLUSIONS

Blocking intracellular calcium by BAPTA-AM relieved MG132-induced ER stress, but it was unable to rescue MG132-induced apoptosis, which was likely due to impaired autophagic degradation.

Membrane associated complexes in calcium dynamics modelling

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

Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore

This review focuses on the role of cyclophilin D (CypD) as a prominent mediator of the mitochondrial permeability transition pore (MPTP) and subsequent effects on cardiovascular physiology and pathology. Although a great number of reviews have been written on the MPTP and its effects on cell death, we focus on the biology surrounding CypD itself and the non-cell death physiologic functions of the MPTP. A greater understanding of the physiologic functions of the MPTP and its regulation by CypD will likely suggest novel therapeutic approaches for cardiovascular disease, both dependent and independent of programmed necrotic cell death mechanisms.  

Extra-matrix Mg2+ limits Ca2+ uptake and modulates Ca2+ uptake-independent respiration and redox state in cardiac isolated mitochondria

Cardiac mitochondrial matrix (m) free Ca(2+) ([Ca(2+)]m) increases primarily by Ca(2+) uptake through the Ca(2+) uniporter (CU). Ca(2+) uptake via the CU is attenuated by extra-matrix (e) Mg(2+) ([Mg(2+)]e). How [Ca(2+)]m is dynamically modulated by interacting physiological levels of [Ca(2+)]e and [Mg(2+)]e and how this interaction alters bioenergetics are not well understood. We postulated that as [Mg(2+)]e modulates Ca(2+) uptake via the CU, it also alters bioenergetics in a matrix Ca(2+)-induced and matrix Ca(2+)-independent manner. To test this, we measured changes in [Ca(2+)]e, [Ca(2+)]m, [Mg(2+)]e and [Mg(2+)]m spectrofluorometrically in guinea pig cardiac mitochondria in response to added CaCl2 (0-0.6 mM; 1 mM EGTA buffer) with/without added MgCl2 (0-2 mM). In parallel, we assessed effects of added CaCl2 and MgCl2 on NADH, membrane potential (ΔΨm), and respiration. We found that ≥0.125 mM MgCl2 significantly attenuated CU-mediated Ca(2+) uptake and [Ca(2+)]m. Incremental [Mg(2+)]e did not reduce initial Ca(2+)uptake but attenuated the subsequent slower Ca(2+) uptake, so that [Ca(2+)]m remained unaltered over time. Adding CaCl2 without MgCl2 to attain a [Ca(2+)]m from 46 to 221 nM enhanced state 3 NADH oxidation and increased respiration by 15 %; up to 868 nM [Ca(2+)]m did not additionally enhance NADH oxidation or respiration. Adding MgCl2 did not increase [Mg(2+)]m but it altered bioenergetics by its direct effect to decrease Ca(2+) uptake. However, at a given [Ca(2+)]m, state 3 respiration was incrementally attenuated, and state 4 respiration enhanced, by higher [Mg(2+)]e. Thus, [Mg(2+)]e without a change in [Mg(2+)]m can modulate bioenergetics independently of CU-mediated Ca(2+) transport.

Long-term effects of ionising radiation on the brain: cause for concern?

There is no clear evidence proving or disproving that ionising radiation is causally linked with neurodegenerative diseases such as Parkinson's and Alzheimer's. However, it is known that high doses of ionising radiation to the head (20-50 Gy) lead to severe learning and memory impairment which is characteristical for Alzheimer's. The cumulative doses of ionising radiation to the Western population are accruing, mostly due to the explosive growth of medical imaging procedures. Children are in particular prone to ionising radiation as the molecular processes within the brain are not completely finished. Furthermore, they have a long lifespan under risk. We wish to open a debate if such low doses of radiation exposure may lead to delayed long-term cognitive and other defects, albeit at a lower frequency than those observed during application of high doses. Further, we want to sensitise the society towards the risks of ionising radiation. To achieve these aims, we will recapitulate the known symptoms of Parkinson's and Alzheimer's on the molecular level and incorporate data of mainly low- and moderate-ionising radiation (<5 Gy). Thus, we want to highlight in general the potential similarities of both the neurodegenerative and radiation-induced pathways. We will propose a mechanistic model for radiation-induced neurodegeneration pointing out mitochondria as a key element. This includes effects of oxidative stress and neuroinflammation-all fundamental players of neurodegenerative diseases.

Acitretin affects bioenergetics of liver mitochondria and promotes mitochondrial permeability transition: potential mechanisms of hepatotoxicity

Acitretin is a synthetic retinoid used for severe extensive psoriasis and it has been shown to be an effective and a safe therapeutic drug for other diseases including cancer when used in combination with other agents. However, cases of acitretin-associated liver injury have been documented, but the possible mechanisms of acitretin-associated hepatotoxicity and apoptosis are not entirely clarified. This study reports that mitochondrial dysfunctions may play an important role in liver injury and apoptosis induced by this retinoid. Acitretin (5-20 μM) impaired mitochondrial phosphorylation efficiency as demonstrated by the decrease in the state 3 respiration and ATP levels, and by the increase in the lag phase of ADP phosphorylation cycle, without affecting the membrane potential. Acitretin induced Ca(2+)-mediated mitochondrial permeability transition (MPT) and decreased the adenine nucleotide translocase (ANT) content. Acitretin-induced MPT was not prevented by thiol group protecting and antioxidant agents, excluding the involvement of oxidative stress mechanisms. However, MPT was prevented by ANT ligands ATP, ADP, tamoxifen and 4-hydroxytamoxifen, implying that the MPT induction by acitretin is mediated by the ANT. ANT plays a major role in promoting apoptosis and ATP synthesis, and it is still considered as a structural component of the pore with a regulatory role in MPT formation. Therefore, our results, including the decrease in the state 3 respiration and the increase in the lag phase of phosphorylation cycle, the ATP depletion and the induction of Ca(2+)-mediated MPT, indicate that acitretin-associated liver toxicity and apoptosis is possibly related with mitochondrial dysfunctions due to interactions with the ANT. Additionally, the combination of acitretin with other drugs, such as antiestrogens, which are able to inhibit the MPT, may contribute to decrease the toxicity induced by acitretin.

Platelet-based coagulation: different populations, different functions

Platelets in a thrombus interact with (anti)coagulation factors and support blood coagulation. In the concept of cell-based control of coagulation, three different roles of platelets can be distinguished: control of thrombin generation, support of fibrin formation, and regulation of fibrin clot retraction. Here, we postulate that different populations of platelets with distinct surface properties are involved in these coagulant functions. Platelets with elevated Ca(2+) and exposed phosphatidylserine control thrombin and fibrin generation, while platelets with activated α(IIb) β(3) regulate clot retraction. We review how coagulation factor binding depends on the platelet activation state. Furthermore, we discuss the ligands, platelet receptors and downstream intracellular signaling pathways implicated in these coagulant functions. These insights lead to an adapted model of platelet-based coagulation.

Dynamic buffering of mitochondrial Ca2+ during Ca2+ uptake and Na+-induced Ca2+ release

In cardiac mitochondria, matrix free Ca(2+) ([Ca(2+)]m) is primarily regulated by Ca(2+) uptake and release via the Ca(2+) uniporter (CU) and Na(+)/Ca(2+) exchanger (NCE) as well as by Ca(2+) buffering. Although experimental and computational studies on the CU and NCE dynamics exist, it is not well understood how matrix Ca(2+) buffering affects these dynamics under various Ca(2+) uptake and release conditions, and whether this influences the stoichiometry of the NCE. To elucidate the role of matrix Ca(2+) buffering on the uptake and release of Ca(2+), we monitored Ca(2+) dynamics in isolated mitochondria by measuring both the extra-matrix free [Ca(2+)] ([Ca(2+)]e) and [Ca(2+)]m. A detailed protocol was developed and freshly isolated mitochondria from guinea pig hearts were exposed to five different [CaCl2] followed by ruthenium red and six different [NaCl]. By using the fluorescent probe indo-1, [Ca(2+)]e and [Ca(2+)]m were spectrofluorometrically quantified, and the stoichiometry of the NCE was determined. In addition, we measured NADH, membrane potential, matrix volume and matrix pH to monitor Ca(2+)-induced changes in mitochondrial bioenergetics. Our [Ca(2+)]e and [Ca(2+)]m measurements demonstrate that Ca(2+) uptake and release do not show reciprocal Ca(2+) dynamics in the extra-matrix and matrix compartments. This salient finding is likely caused by a dynamic Ca(2+) buffering system in the matrix compartment. The Na(+)- induced Ca(2+) release demonstrates an electrogenic exchange via the NCE by excluding an electroneutral exchange. Mitochondrial bioenergetics were only transiently affected by Ca(2+) uptake in the presence of large amounts of CaCl2, but not by Na(+)- induced Ca(2+) release.

Piperphentonamine (PPTA) attenuated cerebral ischemia-induced memory deficits via neuroprotection associated with anti-apoptotic activity

The calcium sensitizers levosimendan and piperphentonamine hydrochloride (PPTA) are used as cardiovascular drugs for treatment of heart failure. Given that levosimendan has been reported to exhibit a neuroprotective profile in a model of traumatic brain injury, it was interesting to know whether PPTA, a new calcium sensitizer recently developed in China, exerts a similar effect. The objective of this study was to determine whether PPTA exhibited neuroprotective effects and whether these properties were associated with memory. Four-vessel occlusion (4-VO) was used to induce global cerebral ischemia/reperfusion injury in rats treated with or without PPTA (5, 10 mg/kg, i.p., 2 h after the onset of reperfusion and then once a day for 15 consecutive days). Memory was measured using the step-through passive avoidance test. Neurochemical changes were examined in rat PC12 cells treated with oxygen-glucose deprivation (OGD) for 4 h followed by reoxygenation (OGD-R) for 24 h, in the absence or presence of PPTA. In vehicle-treated animals, 4-VO for 10 min produced memory deficits, as demonstrated by decreased retention in step-through passive avoidance, and massive neuron loss in the hippocampal CA1 subregion. These effects were attenuated by PPTA. The results were consistent with those observed in PC12 cells. PPTA treatment increased cell viability, as indicated by MTT assay, inhibited apoptosis, and decreased extracellular lactate dehydrogenase levels in Na(2)S(2)O(4)-treated PC12 cells. These results provide novel demonstration for the ability of PPTA to attenuate cerebral ischemia-induced memory deficits via neuroprotection in the hippocampus. The neuroprotective effect of PPTA appears to be associated with its anti-apoptotic activity. PPTA has the therapeutic potential for ischemic stroke.

Calcium signatures and signaling in cytosol and organelles of tobacco cells induced by plant defense elicitors

Calcium signatures induced by two elicitors of plant defense reactions, namely cryptogein and oligogalacturonides, were monitored at the subcellular level, using apoaequorin-transformed Nicotiana tabacum var Xanthi cells, in which the apoaequorin calcium sensor was targeted either to cytosol, mitochondria or chloroplasts. Our study showed that both elicitors induced specific Ca(2+) signatures in each compartment, with the most striking difference relying on duration. Common properties also emerged from the analysis of Ca(2+) signatures: both elicitors induced a biphasic cytosolic [Ca(2+)] elevation together with a single mitochondrial [Ca(2+)] elevation concomitant with the first cytosolic [Ca(2+)] peak. In addition, both elicitors induced a chloroplastic [Ca(2+)] elevation peaking later in comparison to cytosolic [Ca(2+)] elevation. In cryptogein-treated cells, pharmacological studies indicated that IP(3) should play an important role in Ca(2+) signaling contrarily to cADPR or nitric oxide, which have limited or no effect on [Ca(2+)] variations. Our data also showed that, depending on [Ca(2+)] fluxes at the plasma membrane, cryptogein triggered a mitochondrial respiration increase and affected excess energy dissipation mechanisms in chloroplasts. Altogether the results indicate that cryptogein profoundly impacted cell functions at many levels, including organelles.