Calcium transport and porton electrochemical potential gradient in mitochondria from guinea-pig cerebral cortex and rat heart

Mitochondrial calcium cycling in neuronal function and neurodegeneration

Mitochondria are essential for proper cellular function through their critical roles in ATP synthesis, reactive oxygen species production, calcium (Ca2+) buffering, and apoptotic signaling. In neurons, Ca2+ buffering is particularly important as it helps to shape Ca2+ signals and to regulate numerous Ca2+-dependent functions including neuronal excitability, synaptic transmission, gene expression, and neuronal toxicity. Over the past decade, identification of the mitochondrial Ca2+ uniporter (MCU) and other molecular components of mitochondrial Ca2+ transport has provided insight into the roles that mitochondrial Ca2+ regulation plays in neuronal function in health and disease. In this review, we discuss the many roles of mitochondrial Ca2+ uptake and release mechanisms in normal neuronal function and highlight new insights into the Ca2+-dependent mechanisms that drive mitochondrial dysfunction in neurologic diseases including epilepsy, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We also consider how targeting Ca2+ uptake and release mechanisms could facilitate the development of novel therapeutic strategies for neurological diseases.

Targeting the mitochondria in chronic respiratory diseases

Mitochondria are one of the basic essential components for eukaryotic life survival. It is also the source of respiratory ATP. Recently published studies have demonstrated that mitochondria may have more roles to play aside from energy production. There is an increasing body of evidence which suggest that mitochondrial activities involved in normal and pathological states contribute to significant impact to the lung airway morphology and epithelial function in respiratory diseases such as asthma, COPD, and lung cancer. This review summarizes the pathophysiological pathways involved in asthma, COPD, lung cancer and highlights potential treatment strategies that target the malfunctioning mitochondria in such ailments. Mitochondria are responsive to environmental stimuli such as infection, tobacco smoke, and inflammation, which are essential in the pathogenesis of respiratory diseases. They may affect mitochondrial shape, protein production and ultimately cause dysfunction. The impairment of mitochondrial function has downstream impact on the cytosolic components, calcium control, response towards oxidative stress, regulation of genes and proteins and metabolic activities. Several novel compounds and alternative medicines that target mitochondria in asthma and chronic lung diseases have been discussed here. Moreover, mitochondrial enzymes or proteins that may serve as excellent therapeutic targets in COPD are also covered. The role of mitochondria in respiratory diseases is gaining much attention and mitochondria-based treatment strategies and personalized medicine targeting the mitochondria may materialize in the near future. Nevertheless, more in-depth studies are urgently needed to validate the advantages and efficacy of drugs that affect mitochondria in pathological states.

Mitochondrial clearance of calcium facilitated by MICU2 controls insulin secretion

OBJECTIVE

Transport of Ca2+ into pancreatic β cell mitochondria facilitates nutrient-mediated insulin secretion. However, the underlying mechanism is unclear. Recent establishment of the molecular identity of the mitochondrial Ca2+ uniporter (MCU) and associated proteins allows modification of mitochondrial Ca2+ transport in intact cells. We examined the consequences of deficiency of the accessory protein MICU2 in rat and human insulin-secreting cells and mouse islets.

METHODS

siRNA silencing of Micu2 in the INS-1 832/13 and EndoC-βH1 cell lines was performed; Micu2-/- mice were also studied. Insulin secretion and mechanistic analyses utilizing live confocal imaging to assess mitochondrial function and intracellular Ca2+ dynamics were performed.

RESULTS

Silencing of Micu2 abrogated GSIS in the INS-1 832/13 and EndoC-βH1 cells. The Micu2-/- mice also displayed attenuated GSIS. Mitochondrial Ca2+ uptake declined in MICU2-deficient INS-1 832/13 and EndoC-βH1 cells in response to high glucose and high K+. MICU2 silencing in INS-1 832/13 cells, presumably through its effects on mitochondrial Ca2+ uptake, perturbed mitochondrial function illustrated by absent mitochondrial membrane hyperpolarization and lowering of the ATP/ADP ratio in response to elevated glucose. Despite the loss of mitochondrial Ca2+ uptake, cytosolic Ca2+ was lower in siMICU2-treated INS-1 832/13 cells in response to high K+. It was hypothesized that Ca2+ accumulated in the submembrane compartment in MICU2-deficient cells, resulting in desensitization of voltage-dependent Ca2+ channels, lowering total cytosolic Ca2+. Upon high K+ stimulation, MICU2-silenced cells showed higher and prolonged increases in submembrane Ca2+ levels.

CONCLUSIONS

MICU2 plays a critical role in β cell mitochondrial Ca2+ uptake. β cell mitochondria sequestered Ca2+ from the submembrane compartment, preventing desensitization of voltage-dependent Ca2+ channels and facilitating GSIS.

Distinct properties of Ca2+ efflux from brain, heart and liver mitochondria: The effects of Na+, Li+ and the mitochondrial Na+/Ca2+ exchange inhibitor CGP37157

Mitochondrial Ca²⁺ transport is essential for regulating cell bioenergetics, Ca²⁺ signaling and cell death. Mitochondria accumulate Ca²⁺ via the mitochondrial Ca²⁺ uniporter (MCU), whereas Ca²⁺ is extruded by the mitochondrial Na⁺/Ca²⁺ (mtNCX) and H⁺/Ca²⁺ exchangers. The balance between these processes is essential for preventing toxic mitochondrial Ca²⁺ overload. Recent work demonstrated that MCU activity varies significantly among tissues, likely reflecting tissue-specific Ca²⁺ signaling and energy needs. It is less clear whether this diversity in MCU activity is matched by tissue-specific diversity in mitochondrial Ca²⁺ extrusion. Here we compared properties of mitochondrial Ca²⁺ extrusion in three tissues with prominent mitochondria function: brain, heart and liver. At the transcript level, expression of the Na⁺/Ca²⁺/Li⁺ exchanger (NCLX), which has been proposed to mediate mtNCX transport, was significantly greater in liver than in brain or heart. At the functional level, Na⁺ robustly activated Ca²⁺ efflux from brain and heart mitochondria, but not from liver mitochondria. The mtNCX inhibitor CGP37157 blocked Ca²⁺ efflux from brain and heart mitochondria but had no effect in liver mitochondria. Replacement of Na⁺ with Li⁺ to test the involvement of NCLX, resulted in a slowing of mitochondrial Ca²⁺ efflux by ∼70 %. Collectively, our findings suggest that mtNCX is responsible for Ca²⁺ extrusion from the mitochondria of the brain and heart, but plays only a small, if any, role in mitochondria of the liver. They also reveal that Li⁺ is significantly less effective than Na⁺ in driving mitochondrial Ca²⁺ efflux.

Mitochondrial Calcification

One of the most fascinating aspects of mitochondria is their remarkable ability to accumulate and store large amounts of calcium in the presence of phosphate leading to mitochondrial calcification. In this paper, we briefly address the mechanisms that regulate mitochondrial calcium homeostasis followed by the extensive review on the formation and characterization of intramitochondrial calcium phosphate granules leading to mitochondrial calcification and its relevance to physiological and pathological calcifications of body tissues.

Mitochondrial bioenergetics and pulmonary dysfunction: Current progress and future directions

This review summarizes current understanding of mitochondrial bioenergetic dysfunction applicable to mechanisms of lung diseases and outlines challenges and future directions in this rapidly emerging field. Although the role of mitochondria extends beyond the term of cellular "powerhouse", energy generation remains the most fundamental function of these organelles. It is not counterintuitive to propose that intact energy supply is important for favorable cellular fate following pulmonary insult. In this review, the discussion of mitochondrial dysfunction focuses on those molecular mechanisms that alter cellular bioenergetics in the lungs: (a) inhibition of mitochondrial respiratory chain, (b) mitochondrial leak and uncoupling, (c) alteration of mitochondrial Ca²⁺ handling, (d) mitochondrial production of reactive oxygen species and self-oxidation. The discussed lung diseases were selected according to their pathological nature and relevance to pediatrics: Acute lung injury (ALI), defined as acute parenchymal lung disease associated with cellular demise and inflammation (Acute Respiratory Distress Syndrome, ARDS, Pneumonia), alveolar developmental failure (Bronchopulmonary Dysplasia, BPD or chronic lung disease in premature infants), obstructive airway diseases (Bronchial asthma) and vascular remodeling affecting pulmonary circulation (Pulmonary Hypertension, PH). The analysis highlights primary mechanisms of mitochondrial bioenergetic dysfunction contributing to the disease-specific pulmonary insufficiency and proposes potential therapeutic targets.

Brain mitochondrial calcium transport: Origins of the set-point concept and its application to physiology and pathology

The transport of calcium across the inner mitochondrial membrane plays a key role in neuronal physiology and pathology. The kinetic responses of the uniporter and efflux pathways are such that a cytosolic free calcium 'set-point' can be established - above which there is net calcium accumulation into the matrix that is reversed when plasma membrane transport lowers cytosolic calcium. Pathological activation of N-methyl-d-aspartate receptor mediated sodium and calcium entry into the neuron, as occurs in stroke and spreading depression, places severe demands on both the ATP-generating and calcium loading capacities of the neuronal mitochondria as the set-point is exceeded. Experiments that led to the concept of the set-point are reviewed.

The regulation of neuronal mitochondrial metabolism by calcium

Calcium signalling is fundamental to the function of the nervous system, in association with changes in ionic gradients across the membrane. Although restoring ionic gradients is energetically costly, a rise in intracellular Ca(2+) acts through multiple pathways to increase ATP synthesis, matching energy supply to demand. Increasing cytosolic Ca(2+) stimulates metabolite transfer across the inner mitochondrial membrane through activation of Ca(2+) -regulated mitochondrial carriers, whereas an increase in matrix Ca(2+) stimulates the citric acid cycle and ATP synthase. The aspartate-glutamate exchanger Aralar/AGC1 (Slc25a12), a component of the malate-aspartate shuttle (MAS), is stimulated by modest increases in cytosolic Ca(2+) and upregulates respiration in cortical neurons by enhancing pyruvate supply into mitochondria. Failure to increase respiration in response to small (carbachol) and moderate (K(+) -depolarization) workloads and blunted stimulation of respiration in response to high workloads (veratridine) in Aralar/AGC1 knockout neurons reflect impaired MAS activity and limited mitochondrial pyruvate supply. In response to large workloads (veratridine), acute stimulation of respiration occurs in the absence of MAS through Ca(2+) influx through the mitochondrial calcium uniporter (MCU) and a rise in matrix [Ca(2+) ]. Although the physiological importance of the MCU complex in work-induced stimulation of respiration of CNS neurons is not yet clarified, abnormal mitochondrial Ca(2+) signalling causes pathology. Indeed, loss of function mutations in MICU1, a regulator of MCU complex, are associated with neuromuscular disease. In patient-derived MICU1 deficient fibroblasts, resting matrix Ca(2+) is increased and mitochondria fragmented. Thus, the fine tuning of Ca(2+) signals plays a key role in shaping mitochondrial bioenergetics.

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.

Plasma membrane Ca2+-ATPase isoforms composition regulates cellular pH homeostasis in differentiating PC12 cells in a manner dependent on cytosolic Ca2+ elevations

Plasma membrane Ca²⁺-ATPase (PMCA) by extruding Ca²⁺ outside the cell, actively participates in the regulation of intracellular Ca²⁺ concentration. Acting as Ca²⁺/H⁺ counter-transporter, PMCA transports large quantities of protons which may affect organellar pH homeostasis. PMCA exists in four isoforms (PMCA1-4) but only PMCA2 and PMCA3, due to their unique localization and features, perform more specialized function. Using differentiated PC12 cells we assessed the role of PMCA2 and PMCA3 in the regulation of intracellular pH in steady-state conditions and during Ca²⁺ overload evoked by 59 mM KCl. We observed that manipulation in PMCA expression elevated pH_mito and pH_cyto but only in PMCA2-downregulated cells higher mitochondrial pH gradient (ΔpH) was found in steady-state conditions. Our data also demonstrated that PMCA2 or PMCA3 knock-down delayed Ca²⁺ clearance and partially attenuated cellular acidification during KCl-stimulated Ca²⁺ influx. Because SERCA and NCX modulated cellular pH response in neglectable manner, and all conditions used to inhibit PMCA prevented KCl-induced pH drop, we considered PMCA2 and PMCA3 as mainly responsible for transport of protons to intracellular milieu. In steady-state conditions, higher TMRE uptake in PMCA2-knockdown line was driven by plasma membrane potential (Ψp). Nonetheless, mitochondrial membrane potential (Ψm) in this line was dissipated during Ca²⁺ overload. Cyclosporin and bongkrekic acid prevented Ψ_m loss suggesting the involvement of Ca²⁺-driven opening of mitochondrial permeability transition pore as putative underlying mechanism. The findings presented here demonstrate a crucial role of PMCA2 and PMCA3 in regulation of cellular pH and indicate PMCA membrane composition important for preservation of electrochemical gradient.

Activity-independent release of the amyloid β-peptide from rat brain nerve terminals

Synaptic degeneration is one of the earliest hallmarks of Alzheimer disease. The molecular mechanism underlying this degeneration is not fully elucidated but one key player appears to be the synaptotoxic amyloid β-peptide (Aβ). The exact localization of the production of Aβ and the mechanisms whereby Aβ is released remain elusive. We have earlier shown that Aβ can be produced in crude synaptic vesicle fractions and it has been reported that increased synaptic activity results in increased secreted but decreased intracellular Aβ levels. Therefore, we considered whether Aβ could be produced in synaptic vesicles and/or released through the same mechanisms as neurotransmitters in synaptic vesicle exocytosis. Small amounts of Aβ were found to be produced in pure synaptic vesicle preparations. We also studied the release of glutamate and Aβ from rat cortical nerve terminals (synaptosomes). We found that large amounts of Aβ were secreted from non-stimulated synaptosomes, from which glutamate was not released. On the contrary, we could not detect any differences in Aβ release between non-stimulated synaptosomes and synaptosomes stimulated with KCl or 4-aminopyridine, whereas glutamate release was readily inducible in this system. To conclude, our results indicate that the major release mechanism of Aβ from isolated nerve terminals differs from the synaptic release of glutamate and that the activity-dependent increase of secreted Aβ, reported by several groups using intact cells, is likely dependent on post-synaptic events, trafficking and/or protein synthesis mechanisms.

Activity of the mitochondrial calcium uniporter varies greatly between tissues

The mitochondrial calcium uniporter (MCU) is a highly selective channel responsible for mitochondrial Ca²⁺ uptake. The MCU shapes cytosolic Ca²⁺ signals, controls mitochondrial ATP production, and is involved in cell death. Here, using direct patch-clamp recording from the inner mitochondrial membrane, we compare MCU activity in mouse heart, skeletal muscle, liver, kidney, and brown fat. Surprisingly, heart mitochondria shows a dramatically lower MCU current density than the other tissues studied. Similarly, in Drosophila flight muscle, MCU activity is barely detectable compared to that in other fly tissues. Because mitochondria occupy up to 40% of the cell volume in highly metabolically active heart and flight muscle, low MCU activity is likely essential to avoid cytosolic Ca²⁺ sink due to excessive mitochondrial Ca²⁺ uptake. Simultaneously, low MCU activity may also prevent mitochondrial Ca²⁺ overload in such active tissues exposed to frequent cytosolic Ca²⁺ activity.

Design and synthesis of new bifunctional sigma-1 selective ligands with antioxidant activity

Herein we report the synthesis of new bifunctional sigma-1 (σ1)-selective ligands with antioxidant activity. To achieve this goal, we combined the structure of lipoic acid, a universal antioxidant, with an appropriate sigma aminic moiety. Ligands 14 and 26 displayed high affinity and selectivity for σ1 receptors (Kiσ1 = 1.8 and 5.5 nM; Kiσ2/σ1 = 354 and 414, respectively). Compound 26 exhibited in vivo antiopioid effects on kappa opioid (KOP) receptor-mediated analgesia. In rat liver and brain mitochondria (RLM, RBM), this compound significantly reduced the swelling and the oxidation of thiol groups induced by calcium ions. Our results demonstrate that the tested compound has protective effects against oxidative stress.

Comparative phenotypic analysis of the two major splice isoforms of phosphatidylinositol phosphate kinase type Iγ in vivo

Localized production of polyphosphoinositides is critical for their signaling function. To examine the biological relevance of specific pools of phosphatidylinositol 4,5-bisphosphate we compared the consequences of genetically ablating all isoforms of phosphatidylinositol phosphate (PIP) kinase type Iγ (PIPKIγ), encoded by the gene Pip5k1c, versus ablation of a specific splice isoform, PIPKIγ_i2, with respect to three reported PIPKIγ functions. Ablation of PIPKIγ_i2 caused a neuron-specific endocytosis defect similar to that found in PIPKIγ(-/-) mice, while agonist-induced calcium signaling was reduced in PIPKIγ(-/-) cells, but was not affected in the absence of PIPKIγ_i2. A reported contribution of PIPKIγ to epithelial integrity was not evident in PIPKIγ(-/-) mice. Given that mice lacking PIPKIγ_i2 live a normal lifespan whereas PIPKIγ(-/-) mice die shortly after birth, we propose that PIPKIγ-mediated metabotropic calcium signaling may represent an essential function of PIPKIγ, whereas functions specific to the PIPKIγ_i2 splice isoform are not essential for survival.

Mitochondrial oxidative stress induced by Ca2+ and monoamines: different behaviour of liver and brain mitochondria in undergoing permeability transition

Mitochondrial permeability transition (MPT) is correlated with the opening of a nonspecific pore, the so-called transition pore, that triggers bidirectional traffic of inorganic solutes and metabolites across the mitochondrial membrane. This phenomenon is caused by supraphysiological Ca(2+) concentrations and by other compounds leading to oxidative stress, while cyclosporin A, ADP, bongkrekic acid, antioxidant agents and naturally occurring polyamines strongly inhibit it. The effects of polyamines, including the diamine agmatine, have been widely studied in several types of mitochondria. The effects of monoamines on MPT have to date, been less well-studied, even if they are involved in a variety of neurological and neuroendocrine processes. This study shows that in rat liver mitochondria (RLM), monoamines such as tyramine, serotonin and dopamine amplify the swelling induced by calcium, and increase the oxidation of thiol groups and the production of hydrogen peroxide, effects that are counteracted by the above-mentioned inhibitors. In rat brain mitochondria (RBM), the monoamines do not amplify calcium-induced swelling, even if they demonstrate increases in the extent of oxidation of thiol groups and hydrogen peroxide production. In these mitochondria, the antioxidants are not at all or scarcely effective in suppressing mitochondrial swelling. In conclusion, we hypothesize that different mechanisms induce the MPT in the two different types of mitochondria evaluated. Calcium and monoamines induce oxidative stress in RLM, which in turn appears to induce and amplify MPT. This process is not apparent in RBM, where MPT seems resistant to oxidative stress.

Distinctive characteristics and functions of multiple mitochondrial Ca2+ influx mechanisms

Intracellular Ca(2+) is vital for cell physiology. Disruption of Ca(2+) homeostasis contributes to human diseases such as heart failure, neuron-degeneration, and diabetes. To ensure an effective intracellular Ca(2+) dynamics, various Ca(2+) transport proteins localized in different cellular regions have to work in coordination. The central role of mitochondrial Ca(2+) transport mechanisms in responding to physiological Ca(2+) pulses in cytosol is to take up Ca(2+) for regulating energy production and shaping the amplitude and duration of Ca(2+) transients in various micro-domains. Since the discovery that isolated mitochondria can take up large quantities of Ca(2+) approximately 5 decades ago, extensive studies have been focused on the functional characterization and implication of ion channels that dictate Ca(2+) transport across the inner mitochondrial membrane. The mitochondrial Ca(2+) uptake sensitive to non-specific inhibitors ruthenium red and Ru360 has long been considered as the activity of mitochondrial Ca(2+) uniporter (MCU). The general consensus is that MCU is dominantly or exclusively responsible for the mitochondrial Ca(2+) influx. Since multiple Ca(2+) influx mechanisms (e.g. L-, T-, and N-type Ca(2+) channel) have their unique functions in the plasma membrane, it is plausible that mitochondrial inner membrane has more than just MCU to decode complex intracellular Ca(2+) signaling in various cell types. During the last decade, four molecular identities related to mitochondrial Ca(2+) influx mechanisms have been identified. These are mitochondrial ryanodine receptor, mitochondrial uncoupling proteins, LETM1 (Ca(2+)/H(+) exchanger), and MCU and its Ca(2+) sensing regulatory subunit MICU1. Here, we briefly review recent progress in these and other reported mitochondrial Ca(2+) influx pathways and their differences in kinetics, Ca(2+) dependence, and pharmacological characteristics. Their potential physiological and pathological implications are also discussed.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

Mitochondrial ryanodine receptors and other mitochondrial Ca2+ permeable channels

Ca(2+) channels that underlie mitochondrial Ca(2+) transport first reported decades ago have now just recently been precisely characterized electrophysiologically. Numerous data indicate that mitochondrial Ca(2+) uptake via these channels regulates multiple intracellular processes by shaping cytosolic and mitochondrial Ca(2+) transients, as well as altering the cellular metabolic and redox state. On the other hand, mitochondrial Ca(2+) overload also initiates a cascade of events that leads to cell death. Thus, characterization of mitochondrial Ca(2+) channels is central to a comprehensive understanding of cell signaling. Here, we discuss recent progresses in the biophysical and electrophysiological characterization of several distinct mitochondrial Ca(2+) channels.

Agmatine prevents the Ca(2+)-dependent induction of permeability transition in rat brain mitochondria

The arginine metabolite agmatine is able to protect brain mitochondria against the drop in energy capacity by the Ca(2+)-dependent induction of permeability transition (MPT) in rat brain mitochondria. At normal levels, the amine maintains the respiratory control index and ADP/O ratio and prevents mitochondrial colloid-osmotic swelling and any electrical potential (DeltaPsi) drop. MPT is due to oxidative stress induced by the interaction of Ca(2+) with the mitochondrial membrane, leading to the production of hydrogen peroxide and, subsequently, other reactive oxygen species (ROS) such as hydroxyl radicals. This production of ROS induces oxidation of sulfhydryl groups, in particular those of two critical cysteines, most probably located on adenine nucleotide translocase, and also oxidation of pyridine nucleotides, resulting in transition pore opening. The protective effect of agmatine is attributable to a scavenging effect on the most toxic ROS, i.e., the hydroxyl radical, thus preventing oxidative stress and consequent bioenergetic collapse.

Agmatine transport in brain mitochondria: a different mechanism from that in liver mitochondria

The diamine agmatine (AGM), exhibiting two positive charges at physiological pH, is transported into rat brain mitochondria (RBM) by an electrophoretic mechanism, requiring high membrane potential values and exhibiting a marked non-ohmic force-flux relationship. The mechanism of this transport apparently resembles that observed in rat liver mitochondria (RLM), but there are several characteristics that strongly suggest the presence of a different transporter of agmatine in RBM. In this type of mitochondria, the extent of initial binding and total accumulation is higher and lower, respectively, than that in liver; saturation kinetics and the flux-voltage relationship also exhibit different trends, whereas idazoxan and putrescine, ineffective in RLM, act as inhibitors. The characteristics of agmatine uptake in RBM lead to the conclusion that its transporter is a channel with two asymmetric energy barriers, showing some characteristics similar to those of the imidazoline receptor I(2) and the sharing with the polyamine transporter.

The function of glutamatergic synapses is not perturbed by severe knockdown of 4.1N and 4.1G expression

AMPA-type glutamate receptors mediate fast excitatory synaptic transmission in the vertebrate brain. Their surface expression at synapses between neurons is regulated in an activity-dependent and activity-independent manner. The protein machinery that regulates synaptic targeting, anchoring and turnover of AMPA receptors consists of several types of specialized scaffolding proteins. The FERM domain scaffolding proteins 4.1G and 4.1N were previously suggested to act jointly in binding and regulating synaptic trafficking of the AMPA receptor subunits GluR1 and GluR4. To determine the functions of 4.1G and 4.1N in vivo, we generated a mutant mouse line that lacks 4.1G entirely and expresses 4.1N at 22% of wild-type levels. These mice had combined 4.1G and 4.1N protein expression in the hippocampus at 12% of wild-type levels (equivalent to 8-10% of combined GluR1 and GluR4 expression levels). They show a moderate reduction in synaptosomal expression levels of the AMPA receptor subunit GluR1 at 3 weeks of age, but no change in basic glutamatergic synaptic transmission and long-term potentiation in the hippocampus. Our study indicates that 4.1G and 4.1N do not have a crucial role in glutamatergic synaptic transmission and the induction and maintenance of long-term plastic changes in synaptic efficacy.

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.

Calcium signaling in brain mitochondria: interplay of malate aspartate NADH shuttle and calcium uniporter/mitochondrial dehydrogenase pathways

Ca2+ signaling in mitochondria has been mainly attributed to Ca2+ entry to the matrix through the Ca2+ uniporter and activation of mitochondrial matrix dehydrogenases. However, mitochondria can also sense increases in cytosolic Ca2+ through a mechanism that involves the aspartate-glutamate carriers, extramitochondrial Ca2+ activation of the NADH malate-aspartate shuttle (MAS). Both pathways are linked through the shared substrate alpha-ketoglutarate (alphaKG). Here we have studied the interplay between the two pathways under conditions of Ca2+ activation. We show that alphaKG becomes limiting when Ca2+ enters in brain or heart mitochondria, but not liver mitochondria, resulting in a drop in alphaKG efflux through the oxoglutarate carrier and in a drop in MAS activity. Inhibition of alphaKG efflux and MAS activity by matrix Ca2+ in brain mitochondria was fully reversible upon Ca2+ efflux. Because of their differences in cytosolic calcium concentration requirements, the MAS and Ca2+ uniporter-mitochondrial dehydrogenase pathways are probably sequentially activated during a Ca2+ transient, and the inhibition of MAS at the center of the transient may provide an explanation for part of the increase in lactate observed in the stimulated brain in vivo.

Src-Tyrosine kinases are major agents in mitochondrial tyrosine phosphorylation

Mitochondrial tyrosine phosphorylation is emerging as an important mechanism in regulating mitochondrial function. This article, aimed at identifying which kinases are the major agents in mitochondrial tyrosine phosphorylation, shows that this role should be attributed to Src family members. Indeed, various members of this family, for example, Fgr, Fyn, Lyn, c-Src, are constitutively present in the internal structure of mitochondria as well as Csk, a key enzyme in the regulation of the activity of this family. By means of different approaches, biochemical fractioning, Western blotting and immunogold analysis "in situ" of phosphotyrosine signaling, evidence is reported on the existence of a signal transduction pathway from plasma membrane to mitochondria, resulting in increasing Src-dependent mitochondrial tyrosine phosphorylation. The activation of Src kinases at mitochondrial level is associated with the proliferative status where several mitochondrial proteins are specifically tyrosine-phosphorylated.

Mitochondrial Ca2+ uniporter blockers influence activation-induced CBF response in the rat somatosensory cortex

Effects of mitochondrial calcium signaling blockade on neural activation-induced CBF response were studied in urethane-anesthetized rats. Ruthenium red (RuR), a nonspecific inhibitor of the mitochondrial calcium uniporter (MCU), and Ru360, a highly specific inhibitor of the MCU, were delivered intravenously (i.v.) or intracerebroventricularly (i.c.v.). Baseline cerebral blood flow (CBF) and cerebral hyperemic response to whisker stimulation were measured through a thinned skull over the somatosensory cortex using laser Doppler imaging (LDI). Ruthenium red or Ru360 did not alter the baseline CBF at all doses used. However, the hyperemic response, defined as the activation area and amplitude of CBF increase in response to mechanical whisker stimulation, was significantly reduced in the presence of either RuR or Ru360 delivered i.c.v. The hyperemic response reduced significantly with a dose of 14.5 nmol RuR (i.c.v.), showing a further decrease with 29 nmol RuR (i.c.v.). A comparable decrease in the hyperemic response was observed during treatment with a relatively lower dose of 4.5 and 9 nmol Ru360 (i.c.v.). Delivered intravenously, Ru360 significantly diminished the cerebral hyperemic response at doses greater than 80 microg/kg i.v., up to a dose of 320 microg/kg i.v. However, RuR (i.v.) had an opposite effect with an enhancement in the cerebral hyperemic response at all doses studied. Ruthenium red or Ru360 had no significant effect on the cerebral reactivity to hypercapnia, indicating that altered cerebral hyperemic response to whisker stimulation was predominantly neural. We conclude that mitochondrial calcium signaling through the MCU mediates neural activation-induced CBF response in vivo.

Calcium-induced precipitate formation in brain mitochondria: composition, calcium capacity, and retention

Both isolated brain mitochondria and mitochondria in intact neurons are capable of accumulating large amounts of calcium, which leads to formation in the matrix of calcium- and phosphorus-rich precipitates, the chemical composition of which is largely unknown. Here, we have used inhibitors of the mitochondrial permeability transition (MPT) to determine how the amount and rate of mitochondrial calcium uptake relate to mitochondrial morphology, precipitate composition, and precipitate retention. Using isolated rat brain (RBM) or liver mitochondria (RLM) Ca(2+)-loaded by continuous cation infusion, precipitate composition was measured in situ in parallel with Ca(2+) uptake and mitochondrial swelling. In RBM, the endogenous MPT inhibitors adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) increased mitochondrial Ca(2+) loading capacity and facilitated formation of precipitates. In the presence of ADP, the Ca/P ratio approached 1.5, while ATP or reduced infusion rates decreased this ratio towards 1.0, indicating that precipitate chemical form varies with the conditions of loading. In both RBM and RLM, the presence of cyclosporine A in addition to ADP increased the Ca(2+) capacity and precipitate Ca/P ratio. Following MPT and/or depolarization, the release of accumulated Ca(2+) is rapid but incomplete; significant residual calcium in the form of precipitates is retained in damaged mitochondria for prolonged periods.

Cyclophilin D inactivation protects axons in experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis

Multiple sclerosis (MS) is the leading cause of neurological disability in young adults, affecting some two million people worldwide. Traditionally, MS has been considered a chronic, inflammatory disorder of the central white matter in which ensuing demyelination results in physical disability [Frohman EM, Racke MK, Raine CS (2006) N Engl J Med 354:942-955]. More recently, MS has become increasingly viewed as a neurodegenerative disorder in which neuronal loss, axonal injury, and atrophy of the CNS lead to permanent neurological and clinical disability. Although axonal pathology and loss in MS has been recognized for >100 years, very little is known about the underlying molecular mechanisms. Progressive axonal loss in MS may stem from a cascade of ionic imbalances initiated by inflammation, leading to mitochondrial dysfunction and energetic deficits that result in mitochondrial and cellular Ca2+ overload. In a murine disease model, experimental autoimmune encephalomyelitis (EAE) mice lacking cyclophilin D (CyPD), a key regulator of the mitochondrial permeability transition pore (PTP), developed EAE, but unlike WT mice, they partially recovered. Examination of the spinal cords of CyPD-knockout mice revealed a striking preservation of axons, despite a similar extent of inflammation. Furthermore, neurons prepared from CyPD-knockout animals were resistant to reactive oxygen and nitrogen species thought to mediate axonal damage in EAE and MS, and brain mitochondria lacking CyPD sequestered substantially higher levels of Ca2+. Our results directly implicate pathological activation of the mitochondrial PTP in the axonal damage occurring during MS and identify CyPD, as well as the PTP, as a potential target for MS neuroprotective therapies.

Depolarization-induced Rapid Generation of 2-Arachidonoylglycerol, an Endogenous Cannabinoid Receptor Ligand, in Rat Brain Synaptosomes

2-arachidonoylglycerol (2-AG) is an endogenous ligand for the cannabinoid receptors with a variety of potent biological activities. In this study, we first examined the effects of potassium-induced depolarization on the level of 2-AG in rat brain synaptosomes. We found that a significant amount of 2-AG was generated in the synaptosomes following depolarization. Notably, depolarization did not affect the levels of other molecular species of monoacylglycerols. Furthermore, the level of anandamide was very low and did not change markedly following depolarization. It thus appeared that the depolarization-induced accelerated generation is a unique feature of 2-AG. We obtained evidence that phospholipase C is involved in the generation of 2-AG in depolarized synaptosomes: U73122, a phospholipase C inhibitor, markedly reduced the depolarization-induced generation of 2-AG, and the level of diacylglycerol was rapidly elevated following depolarization. A significant amount of 2-AG was released from synaptosomes upon depolarization. Interestingly, treatment of the synaptosomes with SR141716A, a CB1 receptor antagonist, augmented the release of glutamate from depolarized synaptosomes. These results strongly suggest that the endogenous ligand for the cannabinoid receptors, i.e. 2-AG, generated through increased phospholipid metabolism upon depolarization, plays an important role in attenuating glutamate release from the synaptic terminals by acting on the CB1 receptor.

Defects in ordered aggregates of cardiolipin visualized by atomic force microscopy

The formation and the nature of defects in ordered aggregates of cardiolipin (tetra acyl diphosphatidylglycerol) supported on solid substrates have been investigated by atomic force microscopy (AFM). The experiments were performed on two model systems, i.e. three-dimensional liquid crystals dispersed in water and partially de-hydrated on a hydrophilic surface, and two-dimensional films of molecules self-assembled onto an isotropic hydrophobic surface. Defects were induced both by varying the preparation temperature and by treatment with specific chemicals known to modify the order parameters in natural and artificial membranes, specifically: 2,4-dinitro-phenol (DNP) and pentachloro-phenol (PCP). The effect of lipid oxidation on the nanocrystalline order was also investigated. The images obtained by AFM allow to characterize the type of defects and their local density at nanoscale level. They also provide additional information to differentiate the specific role of acyl chains and polar heads in the process of lipid self-organization.

Evidence for early endosome-like fusion of recently endocytosed synaptic vesicles

Early endosomes are well-established acceptor compartments of endocytic vesicles in many cell types. Little evidence of their existence or function has been obtained in synapses, and it is generally believed that synaptic vesicles recycle without passing through an endosomal intermediate. We show here that the early endosomal SNARE proteins are enriched in synaptic vesicles. To investigate their function in the synapse, we isolated synaptic nerve terminals (synaptosomes), stimulated them in presence of different fluorescent markers to label the recycling vesicles and used these vesicles in in vitro fusion assays. The recently endocytosed vesicles underwent homotypic fusion. They also fused with endosomes from PC12 and BHK cells. The fusion process was dependent upon NSF activity. Moreover, fusion was dependent upon the early endosomal SNAREs but not upon the SNAREs involved in exocytosis. Our results thus show that at least a fraction of the vesicles endocytosed during synaptic activity are capable of fusing with early endosomes and lend support to an involvement of endosomal intermediates during recycling of synaptic vesicles.

Isolation of mitochondria with high respiratory control from primary cultures of neurons and astrocytes using nitrogen cavitation

To study neurons or glia-specific mitochondria one needs to isolate these organelles from primary neuronal or astrocytic cell culture. This work provides novel method for isolation of functional and morphologically intact mitochondria from neurons and astrocytes in cell cultures. In the first step, mitochondria are released from cells by disruption of cell membranes using a nitrogen cavitation technique. This technique is based on rapid decompression of a cell suspension from within a pressure vessel. Mitochondria released from cell bodies are then separated from the rest of cell homogenate by Percoll gradient centrifugation. This is a relatively rapid technique that yields to very well coupled mitochondria that exhibited functional and morphological characteristics comparable to mitochondria isolated from brain tissue using common techniques. This technique thus will allow examination of mitochondria that are exclusively cell specific in origin.

Pristimerin induces caspase-dependent apoptosis in MDA-MB-231 cells via direct effects on mitochondria

Pristimerin, a naturally occurring triterpenoid, has been shown to cause cytotoxicity in several cancer cell lines. However, the mechanism for the cytotoxic effect of pristimerin was never explored. In the present study, human breast cancer MDA-MB-231 cells treated with pristimerin (1 and 3 micromol/L) showed rapid induction of apoptosis, as indicated by caspase activation, DNA fragmentation, and morphologic changes. Pretreatment of a pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD-fmk) completely prevented pristimerin-induced apoptosis. Treatment of tumor cells with pristimerin resulted in a rapid release of cytochrome c from mitochondria, which preceded caspase activation and the decrease of mitochondrial membrane potential. In addition, neither benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone nor permeability transition pore inhibitor cyclosporin A markedly prevented pristimerin-induced mitochondrial cytochrome c release. Pristimerin did not significantly alter the protein level of Bcl-2 family members (Bcl-2, Bcl-X(L), and Bax), nor did it induce Bax translocation. Moreover, Bcl-2 overexpression fails to prevent pristimerin-induced apoptosis. The generation of reactive oxygen species in MDA-MB-231 cells was also not affected by pristimerin. In a cell-free system, pristimerin induced cytochrome c release from isolated mitochondria. Taken together, these results suggested that pristimerin is a novel mitochondria-targeted compound and may be further evaluated as a chemotherapeutic agent for human cancer.

Nitrogen disruption of synaptoneurosomes: an alternative method to isolate brain mitochondria

Mitochondria are known to be localized in synaptic and non-synaptic compartments in the brain. Synaptoneurosomes, which contain high numbers of mitochondria, may act as a major contaminant of currently used isolation techniques. Currently, there is no method employed to successfully disrupt synaptoneurosomes and isolate both synaptic and non-synaptic mitochondria without structural or functional damage. A novel method is reported here for disruption of synaptoneurosomes and isolation of total brain mitochondria from synaptic and non-synaptic sources using a nitrogen decompression technique. Nitrogen gas was dissolved into crude mitochondrial preparations and maintained under constant, moderate pressure. After a short incubation, the pressure was released causing the nitrogen to come out of solution as growing bubbles, which ruptures cellular and synaptoneurosomal membranes. Mitochondria isolated using this rapid technique were bioenergetically competent and exhibited functional characteristics comparable to mitochondria isolated using traditional techniques. This nitrogen decompression technique will allow for further characterization of synaptic pools of mitochondria, which are almost exclusively neuronal in origin.

Calcium-induced Cytochrome c release from rat brain mitochondria is altered by digitonin

To determine if calcium could release Cytochrome c (Cyt c) from brain mitochondria without activating the permeability transition (mPT), brain mitochondria were prepared in two different ways. Digitonin was used to lyse synaptosomes and release synaptosomal mitochondria or a Percoll gradient was used to separate non-synaptosomal mitochondria from the synaptosomes. In gradient-purified mitochondria, low levels of added digitonin produced swelling and Cyt c release. Digitonin augmented Ca(2+)-induced Cyt c release that was insensitive to the mPT inhibitors, cyclosporin A CsA and ADP. Similarly, in mitochondria prepared with digitonin, these inhibitors also failed to prevent Ca(2+)-induced Cyt c release. Thus the mPT-independent, Ca(2+)-induced Cyt c release pathway was attributable to alteration of the permeability properties of the outer mitochondrial membrane by digitonin.

Characterization and location of Src-dependent tyrosine phosphorylation in rat brain mitochondria

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.

PKC-independent inhibition of glutamate exocytosis by arachidonic acid in rat cerebrocortical synaptosomes

In rat cerebrocortical synaptosomes, the addition of 4 beta-phorbol dibutyrate (4 beta-PDBu) and arachidonic acid enhances and decreases, respectively, the glutamate release evoked by 4-aminopyridine. Pretreatment of synaptosomes with 12-O-tetradecanoylphorbol 13-acetate (TPA) or pre-incubation with staurosporine, prevent the stimulatory effect of 4 beta-PDBu, but are without effect on the inhibitory action of arachidonic acid. Moreover, methyl arachidonate, which is not effective as a PKC activator, also strongly inhibits glutamate exocytosis. These results suggest that PKC is not involved in the inhibition of glutamate release by arachidonic acid.

Synaptic vesicle acidification and exocytosis studied with acridine orange fluorescence in rat brain synaptosomes

The acidification of synaptic vesicles (SV) in rat brain synaptosomes was studied using acridine orange (AO) as a fluorescent probe. In synaptosomal suspensions the AO fluorescence was partially quenched, indicating the presence of an acidic compartment. In permeabilized synaptosomes, the quenching was augmented by MgATP and was sensitive to concanamycin A, a specific inhibitor of the V-type H(+)-ATPase known to be present in synaptic vesicles. Some ATP-dependent acidification was also observed without permeabilization, suggesting that a fraction of synaptosomes (ca. 15%) was unsealed, irrespective of the method used to prepare the synaptosomes (sucrose or Ficoll density gradient, sedimentation or flotation). Depolarization of synaptosomes with 30 mM KCl resulted in an immediate, albeit small, rise in AO fluorescence that was prevented by the removal of Ca(2+) or by substituting NaCl for KCl. This response is consistent with depolarization-evoked release of the acidic contents of an exocytosis-competent pool of synaptic vesicles, representing ca. 5% of the total. No further AO release subsequent to the immediate phase was observed in depolarized synaptosomes, which indicates an extremely rapid reacidification. The results demonstrate that AO fluorescence is suitable for monitoring SV acidification within synaptosomes, and may be used to derive an independent estimate of the relative size of the immediately releasable SV pool. In addition, the use of AO might be advantageous for the assessment of synaptosomal integrity by comparing the ATP-dependent acidification in intact and permeabilized synaptosomes.

N-acetylcysteine elicited increase in complex I activity in synaptic mitochondria from aged mice: implications for treatment of Parkinson's disease

It has been suggested that thiolic groups are essential for complex I activity and other respiratory mitochondrial enzymes. Recent experiments showed that the thiolic antioxidant N-acetylcysteine (NAC) can protect against age-related decrease in complex I activity in mice hepatic mitochondria. The present paper shows that NAC enhances complex I activity in vitro in synaptic mitochondria isolated from old mice. The optimum NAC concentration for maximum complex I activity was 10 mM in old synaptic preparations. Our data suggest that mitochondrial thiolic groups, which are essentials to oxidative phosphorylation, are impaired by aging. Based on the finding of decreased mitochondrial complex I activity in the substantia nigra of patients with Parkinson's disease, we propose that the thiol-containing antioxidant NAC could be beneficial for treatment of the disease.

N-Acetylcysteine delays age-associated memory impairment in mice: role in synaptic mitochondria

Mitochondrial oxidative damage is implicated in brain aging and in age-related neurodegenerative diseases. Since N-acetylcysteine (NAC) has recently been shown to prevent apoptotic death in neuronal cells and protect synaptic mitochondria proteins from oxidative damage in aged mice, we have investigated whether dietary administration of this thiolic antioxidant retards age-related memory loss. At 48 weeks of age, a control female OF-1 mice group was fed standard food pellets and another group received pellets containing 0.3% (w/w) of NAC. After 23 weeks of this diet, the NAC had partially restored the memory deficit associated with aging in mice. Moreover, the lipid peroxide and protein carbonyl contents of the synaptic mitochondria were significantly decreased in the NAC-supplemented animals in comparison with their age-matched controls. The antioxidant properties and probable action on mitochondrial bioenergetic ability in the synaptic terminals may explain, at least partially, the beneficial action of NAC administration.

Changes of synaptotagmin interaction with t-SNARE proteins in vitro after calcium/calmodulin-dependent phosphorylation

The regulation of multiple phases of the life cycle of synaptic vesicles is carried out by a complex series of protein-protein interactions. According to the SNARE hypothesis the core of these interactions is a heterotrimeric complex formed by syntaxin, SNAP-25, and VAMP-synaptobrevin. Other proteins interacting with the core of the SNARE complex, such as voltage-activated calcium channels and synaptotagmin (a putative calcium sensor), are considered crucial for the calcium dependence of release and also molecular mediators of synaptic plasticity. Here the interaction of synaptotagmin with SNARE proteins was studied in immunoprecipitated native complexes, and the effects of previous phosphorylation-dephosphorylation on this interaction were analyzed. It is surprising that the interaction of synaptotagmin with syntaxin and SNAP-25 in native complexes was not found to be calcium-dependent. However, previous incubation under dephosphorylating conditions decreased the synaptotagmin-syntaxin interaction. Stimulation of Ca2+/calmodulin-dependent protein kinase II, which endogenously phosphorylates synaptotagmin in synaptic vesicles, increased the interaction of syntaxin and SNAP-25 with synaptotagmin (particularly when measured in the presence of calcium), as well as increasing the binding of the kinase itself. These results suggest that calcium decreases synaptotagmin-t-SNARE interactions after dephosphorylation and increases them after phosphorylation. Overall, these results imply a phosphorylation-dephosphorylation balance in regulation of the synaptotagmin-t-SNARE interaction and suggest a role for protein phosphorylation in the modulation of calcium sensitivity in transmitter release.

N-acetylcysteine elicited increase in cytochrome c oxidase activity in mice synaptic mitochondria

It has been suggested that thiolic groups are essential for cytochrome c oxidase (COX) activity and other respiratory mitochondrial enzymes. Recent experiments showed that the thiolic antioxidant N-acetylcysteine (NAC) can protect against age-related impairment in COX activity in mice hepatic mitochondria. The present paper shows that NAC enhances COX activity in vitro in synaptic mitochondria isolated from young and old mice. The optimum NAC concentration for maximum COX activity was 5 mM in young and 10 mM in old synaptic preparations. Our data suggest that mitochondrial thiolic groups, which are essentials to oxidative phosphorylation, are impaired by aging.

A rat brain fraction and different purified peroxidases catalyzing the formation of dopaminochrome from dopamine

Dopaminochrome formation is catalyzed by commercially available purified peroxidases (EC 1.11.1.7) such as horseradish, lacto- and myelo-peroxidase using dopamine, hydrogen peroxide or promethazine sulfoxide as substrates. A rat brain fraction (RBF) catalyzes a similar reaction and its catalytic power increases after preincubation with hydrogen peroxide/ascorbic acid. The activity of both the purified enzymes and the RBF preparation is inhibited by carnosine and characterized by excess substrate inhibition. The enzymes recognize different substrates but show the highest affinity for dopamine. The RBF fraction is strongly buffered against oxidation by compounds such as glutathione and by bioreductive enzymes such as DT-diaphorase (EC 1.6.99.2) which can use as a substrate menadione or dopaminochrome. The rat brain dopamine peroxidizing activity appeared to be mostly bound to the synaptosomal fraction. The reaction catalyzed by the purified peroxidases was followed by electron spin resonance spectroscopy and, unlike that catalyzed by RBF, was shown to produce the signal of a transient dopamine-o-semiquinone radical.

Oxidant, mitochondria and calcium: an overview

Mitochondria are active in the continuous generation of reactive oxygen species (ROS), (e.g., superoxide), thereby favouring a situation of mitochondrial oxidative stress. Under oxidative stress--for example, ischaemia-reoxygenation injury to cells--mitochondria form superoxide, which in turn is converted to hydrogen peroxide and the potent reactive species, hydroxyl radical. Alternatively, mitochondrial superoxide may react with nitric oxide to form potent oxidant peroxynitrite and as a consequence, mitochondrial function is altered. An increase in the release of calcium from mitochondria by oxidants stimulates calcium-dependent enzymes such as calcium-dependent proteases, nucleases, and phospholipases, which subsequently trigger apoptosis of the cells. In principle, calcium can leave mitochondria by different ways: by non-specific leakage through the inner membrane by "pore formation," by changes in the membrane lipid phase, by reversal of the uniport influx carrier, by the specific calcium/hydrogen (or sodium) antiport system, by channel-mediated release pathways, or by a combination of two or more of these pathways. Additionally, the release of calcium from mitochondria can also occur either by oxidation of internal nicotinamide adenine nucleotides to ADP ribose and nicotinamide or by oxidation of thiols in membrane proteins. Once calcium efflux has been triggered, a series of common pathways of apoptosis are initiated, each of which may be sufficient to destroy the cell. Apoptosis requires the active participation of cellular components, and several genes have been suggested to control apoptosis. The proto-oncogene bcl-2 suppresses apoptosis through mitochondrial effects. Overexpression of bcl-2 in the mitochondrial membrane inhibits calcium efflux, but the underlying mechanisms are not clearly known. Further studies are needed to explore the nature of the apoptosis-inducing pathways, the precise mechanisms of calcium efflux, the molecular partners of bcl-2 oncoproteins at the level of the outer-inner membrane contact sites, the molecular biology of the apoptosis-inducing factor formation and release, and the essential molecular targets of apoptosis-inducing proteases. Clarification of these issues might facilitate the understanding of mitochondrial response on cellular calcium dynamics under oxidant stress.

Chloramphenicol-induced amnesia for passive avoidance training in the day-old chick

The antibiotic chloramphenicol, an inhibitor of mitochondrial protein synthesis, was used to investigate the time-related changes in protein synthesis following passive avoidance training in the day-old chick (white leghorn-black Australorp). Retention of memory for this simple learning task is known to be prevented by an inhibitor of cytosolic protein synthesis, anisomycin, in a biphasic manner, with the first phase of sensitivity occurring up to 90 min post-training and the second phase between 4 and 5 h post-training (Freeman, Rose, & Scholey, 1995). Birds received bilateral intracranial injections of chloramphenicol (10 microl/hemisphere of a 7.4 mM solution) at various times relative to training and were tested 24 h later. This report shows that at the second phase of anisomycin susceptibility there was a chloramphenicol-sensitive period (5 h post-training) which had an onset time less than 1 h after injection. The effect of chloramphenicol appears not to be due to the mitochondria being energetically compromised since intracranial injections of an uncoupler of mitochondrial oxidative phosphorylation, 2,4-dinitrophenol (0.1 mM), did not disrupt memory formation when injected 5 h after training, even though it did cause amnesia when injected at the earlier time point of 20 min post-training. These results are discussed in the context of what is already known about memory formation in the day-old chick.