Cyclical mitochondrial deltapsiM fluctuations linked to electron transport, F0F1 ATP-synthase and mitochondrial Na+/Ca+2 exchange are reduced in Alzheimer's disease cybrids

The contribution of mitochondrial DNA alterations to aging, cancer, and neurodegeneration

Mitochondrial DNA (mtDNA) is as a double-stranded molecule existing in hundreds to thousands copies in cells depending on cell metabolism and exposure to endogenous and/or environmental stressors. The coordination of mtDNA replication and transcription regulates the pace of mitochondrial biogenesis to guarantee the minimum number of organelles per cell. mtDNA inheritance follows a maternal lineage, although bi-parental inheritance has been reported in some species and in the case of mitochondrial diseases in humans. mtDNA mutations (e.g., point mutations, deletions, copy number variations) have been identified in the setting of several human diseases. For instance, sporadic and inherited rare disorders involving the nervous system as well higher risk of developing cancer and neurodegenerative conditions, including Parkinson's and Alzheimer's disease, have been associated with polymorphic mtDNA variants. An accrual of mtDNA mutations has also been identified in several tissues and organs, including heart and muscle, of old experimental animals and humans, which may contribute to the development of aging phenotypes. The role played by mtDNA homeostasis and mtDNA quality control pathways in human health is actively investigated for the possibility of developing targeted therapeutics for a wide range of conditions.

The Alzheimer's Disease Mitochondrial Cascade Hypothesis: A Current Overview

Viable Alzheimer's disease (AD) hypotheses must account for its age-dependence; commonality; association with amyloid precursor protein, tau, and apolipoprotein E biology; connection with vascular, inflammation, and insulin signaling changes; and systemic features. Mitochondria and parameters influenced by mitochondria could link these diverse characteristics. Mitochondrial biology can initiate changes in pathways tied to AD and mediate the dysfunction that produces the clinical phenotype. For these reasons, conceptualizing a mitochondrial cascade hypothesis is a straightforward process and data accumulating over decades argue the validity of its principles. Alternative AD hypotheses may yet account for its mitochondria-related phenomena, but absent this happening a primary mitochondrial cascade hypothesis will continue to evolve and attract interest.

Exploring the multimodal role of phytochemicals in the modulation of cellular signaling pathways to combat age-related neurodegeneration

Neurodegeneration is the progressive loss of neuronal structures and functions that lead to copious disorders like Alzheimer's (AD), Parkinson's (PD), Huntington's (HD), amyotrophic lateral sclerosis (ALS), and other less recurring diseases. Aging is the prime culprit for most neurodegenerative events. Moreover, the shared pathogenic factors of many neurodegenerative processes are inflammatory responses and oxidative stress (OS). Unfortunately, it is very complicated to treat neurodegeneration and there is no effective remedy. The rapid progression of the neurodegenerative diseases that exacerbate the burden and the concurrent absence of effective treatment strategies force the researchers to investigate more therapeutic approaches that ultimately target the causative factors of the neurodegeneration. Phytochemicals have great potential to exert their neuroprotective effects by targeting various mechanisms, such as OS, neuroinflammation, abnormal protein aggregation, neurotrophic factor deficiency, disruption in mitochondrial function, and apoptosis. Therefore, this review represents the molecular mechanisms of neuroprotection by multifunctional phytochemicals to combat age-linked neurodegenerative disorders.

A Spatiotemporal Ventricular Myocyte Model Incorporating Mitochondrial Calcium Cycling

Intracellular calcium (Ca²⁺) cycling dynamics in cardiac myocytes are spatiotemporally generated by stochastic events arising from a spatially distributed network of coupled Ca²⁺ release units that interact with an intertwined mitochondrial network. In this study, we developed a spatiotemporal ventricular myocyte model that integrates mitochondria-related Ca²⁺ cycling components into our previously developed ventricular myocyte model consisting of a three-dimensional Ca²⁺ release unit network. Mathematical formulations of mitochondrial membrane potential, mitochondrial Ca²⁺ cycling, mitochondrial permeability transition pore stochastic opening and closing, intracellular reactive oxygen species signaling, and oxidized Ca²⁺/calmodulin-dependent protein kinase II signaling were incorporated into the model. We then used the model to simulate the effects of mitochondrial depolarization on mitochondrial Ca²⁺ cycling, Ca²⁺ spark frequency, and Ca²⁺ amplitude, which agree well with experimental data. We also simulated the effects of the strength of mitochondrial Ca²⁺ uniporters and their spatial localization on intracellular Ca²⁺ cycling properties, which substantially affected diastolic and systolic Ca²⁺ levels in the mitochondria but exhibited only a small effect on sarcoplasmic reticulum and cytosolic Ca²⁺ levels under normal conditions. We show that mitochondrial depolarization can cause Ca²⁺ waves and Ca²⁺ alternans, which agrees with previous experimental observations. We propose that this new, to our knowledge, spatiotemporal ventricular myocyte model, incorporating properties of mitochondrial Ca²⁺ cycling and reactive-oxygen-species-dependent signaling, will be useful for investigating the effects of mitochondria on intracellular Ca²⁺ cycling and action potential dynamics in ventricular myocytes.

Mitochondria and Mitochondrial Cascades in Alzheimer's Disease

Decades of research indicate mitochondria from Alzheimer's disease (AD) patients differ from those of non-AD individuals. Initial studies revealed structural differences, and subsequent studies showed functional deficits. Observations of structure and function changes prompted investigators to consider the consequences, significance, and causes of AD-related mitochondrial dysfunction. Currently, extensive research argues mitochondria may mediate, drive, or contribute to a variety of AD pathologies. The perceived significance of these mitochondrial changes continues to grow, and many currently believe AD mitochondrial dysfunction represents a reasonable therapeutic target. Debate continues over the origin of AD mitochondrial changes. Some argue amyloid-β (Aβ) induces AD mitochondrial dysfunction, a view that does not challenge the amyloid cascade hypothesis and that may in fact help explain that hypothesis. Alternatively, data indicate mitochondrial dysfunction exists independent of Aβ, potentially lies upstream of Aβ deposition, and suggest a primary mitochondrial cascade hypothesis that assumes mitochondrial pathology hierarchically supersedes Aβ pathology. Mitochondria, therefore, appear at least to mediate or possibly even initiate pathologic molecular cascades in AD. This review considers studies and data that inform this area of AD research.

L-OPA1 regulates mitoflash biogenesis independently from membrane fusion

Mitochondrial flashes mediated by optic atrophy 1 (OPA1) fusion protein are bioenergetic responses to stochastic drops in mitochondrial membrane potential (Δψ_m) whose origin is unclear. Using structurally distinct genetically encoded pH‐sensitive probes, we confirm that flashes are matrix alkalinization transients, thereby establishing the pH nature of these events, which we renamed “mitopHlashes”. Probes located in cristae or intermembrane space as verified by electron microscopy do not report pH changes during Δψ_m drops or respiratory chain inhibition. Opa1 ablation does not alter Δψ_m fluctuations but drastically decreases the efficiency of mitopHlash/Δψ_m coupling, which is restored by re‐expressing fusion‐deficient OPA1^K301A and preserved in cells lacking the outer‐membrane fusion proteins MFN1/2 or the OPA1 proteases OMA1 and YME1L, indicating that mitochondrial membrane fusion and OPA1 proteolytic processing are dispensable. pH/Δψ_m uncoupling occurs early during staurosporine‐induced apoptosis and is mitigated by OPA1 overexpression, suggesting that OPA1 maintains mitopHlash competence during stress conditions. We propose that OPA1 stabilizes respiratory chain supercomplexes in a conformation that enables respiring mitochondria to compensate a drop in Δψ_m by an explosive matrix pH flash.

Mitochondria, Cybrids, Aging, and Alzheimer's Disease

Mitochondrial and bioenergetic function change with advancing age and may drive aging phenotypes. Mitochondrial and bioenergetic changes are also documented in various age-related neurodegenerative diseases, including Alzheimer's disease (AD). In some instances AD mitochondrial and bioenergetic changes are reminiscent of those observed with advancing age but are greater in magnitude. Mitochondrial and bioenergetic dysfunction could, therefore, link neurodegeneration to brain aging. Interestingly, mitochondrial defects in AD patients are not brain-limited, and mitochondrial function can be linked to classic AD histologic changes including amyloid precursor protein processing to beta amyloid. Also, transferring mitochondria from AD subjects to cell lines depleted of endogenous mitochondrial DNA (mtDNA) creates cytoplasmic hybrid (cybrid) cell lines that recapitulate specific biochemical, molecular, and histologic AD features. Such findings have led to the formulation of a "mitochondrial cascade hypothesis" that places mitochondrial dysfunction at the apex of the AD pathology pyramid. Data pertinent to this premise are reviewed.

Intracellular Calcium Dysregulation: Implications for Alzheimer's Disease

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by progressive neuronal loss. AD is associated with aberrant processing of the amyloid precursor protein, which leads to the deposition of amyloid-β plaques within the brain. Together with plaques deposition, the hyperphosphorylation of the microtubules associated protein tau and the formation of intraneuronal neurofibrillary tangles are a typical neuropathological feature in AD brains. Cellular dysfunctions involving specific subcellular compartments, such as mitochondria and endoplasmic reticulum (ER), are emerging as crucial players in the pathogenesis of AD, as well as increased oxidative stress and dysregulation of calcium homeostasis. Specifically, dysregulation of intracellular calcium homeostasis has been suggested as a common proximal cause of neural dysfunction in AD. Aberrant calcium signaling has been considered a phenomenon mainly related to the dysfunction of intracellular calcium stores, which can occur in both neuronal and nonneuronal cells. This review reports the most recent findings on cellular mechanisms involved in the pathogenesis of AD, with main focus on the control of calcium homeostasis at both cytosolic and mitochondrial level.

Cytoplasmic hybrid (cybrid) cell lines as a practical model for mitochondriopathies

Cytoplasmic hybrid (cybrid) cell lines can incorporate human subject mitochondria and perpetuate its mitochondrial DNA (mtDNA)-encoded components. Since the nuclear background of different cybrid lines can be kept constant, this technique allows investigators to study the influence of mtDNA on cell function. Prior use of cybrids has elucidated the contribution of mtDNA to a variety of biochemical parameters, including electron transport chain activities, bioenergetic fluxes, and free radical production. While the interpretation of data generated from cybrid cell lines has technical limitations, cybrids have contributed valuable insight into the relationship between mtDNA and phenotype alterations. This review discusses the creation of the cybrid technique and subsequent data obtained from cybrid applications.

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The cytoplasmic hybrid (cybrid) model can be used to determine mitochondrial DNA (mtDNA) contributions to phenotypic alterations.

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Cybrids are used to study mitochondriopathies such as Parkinson’s disease and Alzheimer’s disease.

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mtDNA heteroplasmy threshold and nuclear DNA-mtDNA compatibility can be determined using cybrid models.

OPA1 promotes pH flashes that spread between contiguous mitochondria without matrix protein exchange

The chemical nature and functional significance of mitochondrial flashes associated with fluctuations in mitochondrial membrane potential is unclear. Using a ratiometric pH probe insensitive to superoxide, we show that flashes reflect matrix alkalinization transients of ∼0.4 pH units that persist in cells permeabilized in ion-free solutions and can be evoked by imposed mitochondrial depolarization. Ablation of the pro-fusion protein Optic atrophy 1 specifically abrogated pH flashes and reduced the propagation of matrix photoactivated GFP (paGFP). Ablation or invalidation of the pro-fission Dynamin-related protein 1 greatly enhanced flash propagation between contiguous mitochondria but marginally increased paGFP matrix diffusion, indicating that flashes propagate without matrix content exchange. The pH flashes were associated with synchronous depolarization and hyperpolarization events that promoted the membrane potential equilibration of juxtaposed mitochondria. We propose that flashes are energy conservation events triggered by the opening of a fusion pore between two contiguous mitochondria of different membrane potentials, propagating without matrix fusion to equilibrate the energetic state of connected mitochondria.

Mitochondrial fusion events and transient changes in matrix pH linked to membrane depolarization are found to underlie mitochondrial flashes, whose propagation may help equilibrate energy states between connected mitochondria.

Benzothiazepine CGP37157 and its isosteric 2'-methyl analogue provide neuroprotection and block cell calcium entry

Benzothiazepine CGP37157 is widely used as tool to explore the role of mitochondria in cell Ca(2+) handling, by its blocking effect of the mitochondria Na(+)/Ca(2+) exchanger. Recently, CGP37157 has shown to exhibit neuroprotective properties. In the trend to improve its neuroprotection profile, we have synthesized ITH12505, an isosteric analogue having a methyl instead of chlorine at C2' of the phenyl ring. ITH12505 has exerted neuroprotective properties similar to CGP37157 in chromaffin cells and hippocampal slices stressed with veratridine. Also, both compounds afforded neuroprotection in hippocampal slices stressed with glutamate. However, while ITH12505 elicited protection in SH-SY5Y cells stressed with oligomycin A/rotenone, CGP37157 was ineffective. In hippocampal slices subjected to oxygen/glucose deprivation plus reoxygenation, ITH12505 offered protection at 3-30 μM, while CGP37157 only protected at 30 μM. Both compounds caused blockade of Ca(2+) channels in high K(+)-depolarized SH-SY5Y cells. An in vitro experiment for assaying central nervous system penetration (PAMPA-BBB; parallel artificial membrane permeability assay for blood-brain barrier) revealed that both compounds could cross the blood-brain barrier, thus reaching their biological targets in the central nervous system. In conclusion, by causing a mild isosteric replacement in the benzothiazepine CGP37157, we have obtained ITH12505, with improved neuroprotective properties. These findings may inspire the design and synthesis of new benzothiazepines targeting mitochondrial Na(+)/Ca(2+) exchanger and L-type voltage-dependent Ca(2+) channels, having antioxidant properties.

Mitochondria and cell bioenergetics: increasingly recognized components and a possible etiologic cause of Alzheimer's disease

SIGNIFICANCE

Mitochondria and brain bioenergetics are increasingly thought to play an important role in Alzheimer's disease (AD).

RECENT ADVANCES

Data that support this view are discussed from the perspective of the amyloid cascade hypothesis, which assumes beta-amyloid perturbs mitochondrial function, and from an opposite perspective that assumes mitochondrial dysfunction promotes brain amyloidosis. A detailed review of cytoplasmic hybrid (cybrid) studies, which argue mitochondrial DNA (mtDNA) contributes to sporadic AD, is provided. Recent AD endophenotype data that further suggest an mtDNA contribution are also summarized.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Biochemical, molecular, cybrid, biomarker, and clinical data pertinent to the mitochondria-bioenergetics-AD nexus are synthesized and the mitochondrial cascade hypothesis, which represents a mitochondria-centric attempt to conceptualize sporadic AD, is discussed.

NLRP5 mediates mitochondrial function in mouse oocytes and embryos

Unraveling molecular pathways responsible for regulation of early embryonic development is crucial for our understanding of female infertility. Maternal determinants that control the transition from oocyte to embryo are crucial molecules that govern developmental competence of the newly conceived zygote. We describe a series of defects that are triggered by a disruption of maternal lethal effect gene, Nlrp5. Previous studies have shown that Nlrp5 hypomorph embryos fail to develop beyond the two-cell stage. Despite its importance in preimplantation development, the mechanism by which the embryo arrest occurs remains unclear. We confirmed that Nlrp5 mutant and wild-type females possess comparable ovarian germ pool and follicular recruitment rates. However, ovulated oocytes lacking Nlrp5 have abnormal mitochondrial localization and increased activity in order to sustain physiological ATP content. This results in an accumulation of reactive oxygen species and increased cellular stress causing mitochondrial depletion. Compromised cellular state is also accompanied by increased expression of cell death inducer Bax and depletion of cytochrome c. However, neither genetic deletion (Bax/Nlrp5 double knockout) nor mimetic interference (BH4 domain or Bax inhibitory peptide) were sufficient to alleviate embryo demise caused by depletion of Nlrp5. We therefore conclude that lack of Nlrp5 in oocytes triggers premature activation of the mitochondrial pool, causing mitochondrial damage that cannot be rescued by inactivation of Bax.

A mitochondrial etiology of Alzheimer and Parkinson disease

BACKGROUND

The genetics and pathophysiology of Alzheimer Disease (AD) and Parkinson Disease (PD) appears complex. However, mitochondrial dysfunction is a common observation in these and other neurodegenerative diseases.

SCOPE OF REVIEW

We argue that the available data on AD and PD can be incorporated into a single integrated paradigm based on mitochondrial genetics and pathophysiology.

MAJOR CONCLUSIONS

Rare chromosomal cases of AD and PD can be interpreted as affecting mitochondrial function, quality control, and mitochondrial DNA (mtDNA) integrity. mtDNA lineages, haplogroups, such haplogroup H5a which harbors the mtDNA tRNA(Gln) A8336G variant, are important risk factors for AD and PD. Somatic mtDNA mutations are elevated in AD, PD, and Down Syndrome and Dementia (DSAD) both in brains and also systemically. AD, DS, and DSAD brains also have reduced mtDNA ND6 mRNA levels, altered mtDNA copy number, and perturbed Aβ metabolism. Classical AD genetic changes incorporated into the 3XTg-AD (APP, Tau, PS1) mouse result in reduced forebrain size, life-long reduced mitochondrial respiration in 3XTg-AD males, and initially elevated respiration and complex I and IV activities in 3XTg-AD females which markedly declines with age.

GENERAL SIGNIFICANCE

Therefore, mitochondrial dysfunction provides a unifying genetic and pathophysiology explanation for AD, PD, and other neurodegenerative diseases. This article is part of a Special Issue entitled Biochemistry of Mitochondria.

Linking flickering to waves and whole-cell oscillations in a mitochondrial network model

It has been shown that transient single mitochondrial depolarizations, known as flickers, tend to occur randomly in space and time. On the other hand, many studies have shown that mitochondrial depolarization waves and whole-cell oscillations occur under oxidative stress. How single mitochondrial flickering events and whole-cell oscillations are mechanistically linked remains unclear. In this study, we developed a Markov model of the inner membrane anion channel in which reactive-oxidative-species (ROS)-induced opening of the inner membrane anion channel causes transient mitochondrial depolarizations in a single mitochondrion that occur in a nonperiodic manner, simulating flickering. We then coupled the individual mitochondria into a network, in which flickers occur randomly and sparsely when a small number of mitochondria are in the state of high superoxide production. As the number of mitochondria in the high-superoxide-production state increases, short-lived or abortive waves due to ROS-induced ROS release coexist with flickers. When the number of mitochondria in the high-superoxide-production state reaches a critical number, recurring propagating waves are observed. The origins of the waves occur randomly in space and are self-organized as a consequence of random flickering and local synchronization. We show that at this critical state, the depolarization clusters exhibit a power-law distribution, a signature of self-organized criticality. In addition, the whole-cell mitochondrial membrane potential changes from exhibiting small random fluctuations to more periodic oscillations as the superoxide production rate increases. These simulation results may provide mechanistic insight into the transition from random mitochondrial flickering to the waves and whole-cell oscillations observed in many experimental studies.

Calcium dysregulation and homeostasis of neural calcium in the molecular mechanisms of neurodegenerative diseases provide multiple targets for neuroprotection

The intracellular free calcium concentration subserves complex signaling roles in brain. Calcium cations (Ca(2+)) regulate neuronal plasticity underlying learning and memory and neuronal survival. Homo- and heterocellular control of Ca(2+) homeostasis supports brain physiology maintaining neural integrity. Ca(2+) fluxes across the plasma membrane and between intracellular organelles and compartments integrate diverse cellular functions. A vast array of checkpoints controls Ca(2+), like G protein-coupled receptors, ion channels, Ca(2+) binding proteins, transcriptional networks, and ion exchangers, in both the plasma membrane and the membranes of mitochondria and endoplasmic reticulum. Interactions between Ca(2+) and reactive oxygen species signaling coordinate signaling, which can be either beneficial or detrimental. In neurodegenerative disorders, cellular Ca(2+)-regulating systems are compromised. Oxidative stress, perturbed energy metabolism, and alterations of disease-related proteins result in Ca(2+)-dependent synaptic dysfunction, impaired plasticity, and neuronal demise. We review Ca(2+) control processes relevant for physiological and pathophysiological conditions in brain tissue. Dysregulation of Ca(2+) is decisive for brain cell death and degeneration after ischemic stroke, long-term neurodegeneration in Alzheimer's disease, Parkinson's disease, Huntington's disease, inflammatory processes, such as in multiple sclerosis, epileptic sclerosis, and leucodystrophies. Understanding the underlying molecular processes is of critical importance for the development of novel therapeutic strategies to prevent neurodegeneration and confer neuroprotection.

Systemic mitochondrial dysfunction and the etiology of Alzheimer's disease and down syndrome dementia

Increasing evidence is implicating mitochondrial dysfunction as a central factor in the etiology of Alzheimer's disease (AD). The most significant risk factor in AD is advanced age and an important neuropathological correlate of AD is the deposition of amyloid-beta peptide (Abeta40 and Abeta42) in the brain. An AD-like dementia is also common in older individuals with Down syndrome (DS), though with a much earlier onset. We have shown that somatic mitochondrial DNA (mtDNA) control region (CR) mutations accumulate with age in post-mitotic tissues including the brain and that the level of mtDNA mutations is markedly elevated in the brains of AD patients. The elevated mtDNA CR mutations in AD brains are associated with a reduction in the mtDNA copy number and in the mtDNA L-strand transcript levels. We now show that mtDNA CR mutations increase with age in control brains; that they are markedly elevated in the brains of AD and DS and dementia (DSAD) patients; and that the increased mtDNA CR mutation rate in DSAD brains is associated with reduced mtDNA copy number and L-strand transcripts. The increased mtDNA CR mutation rate is also seen in peripheral blood DNA and in lymphoblastoid cell DNAs of AD and DSAD patients, and distinctive somatic mtDNA mutations, often at high heteroplasmy levels, are seen in AD and DSAD brain and blood cells DNA. In aging, DS, and DSAD, the mtDNA mutation level is positively correlated with beta-secretase activity and mtDNA copy number is inversely correlated with insoluble Abeta40 and Abeta42 levels. Therefore, mtDNA alterations may be responsible for both age-related dementia and the associated neuropathological changes observed in AD and DSAD.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

A Na+/Ca2+ exchanger isoform, NCX1, is involved in retinal cell death after N-methyl-D-aspartate injection and ischemia-reperfusion

We investigated the expression of Na(+)/Ca(2+) exchanger (NCX) and the functional role of NCX in retinal damage by using NCX1-heterozygous deficient mice (NCX1(+/-)) and SEA0400 (2-[4-[(2,5-difluorophenyl)methoxy] phenoxy]-5-ethoxyaniline), a selective NCX inhibitor in vivo. We also examined the role of NCX in oxygen-glucose deprivation (OGD) stress with a retinal ganglion cell line (RGC-5) cell culture in vitro. The expression of NCX1 was confirmed and entirely localized in retina by immunoblotting and immunohistochemistry, respectively. NCX1(+/-) mice possessed significant protection against retinal damage induced by intravitreal injection of N-methyl-D-aspartate (NMDA). SEA0400 at 3 and 10 mg/kg significantly reduced NMDA- or high intraocular pressure-induced retinal cell damage in mice. Furthermore, SEA0400 reduced the number of TUNEL (terminal deoxynucleotidyl transferase dUTP nick-end labeling)-positive cells and the expression of phosphorylated mitogen-activated protein kinases (ERK1/2, JNK, p38) induced by NMDA injection. In RGC-5, SEA0400 at 0.3 and 1 microM significantly inhibited OGD-induced cell damage. OGD-induced cell damage was aggravated by ouabain (a Na(+),K(+)-ATPase inhibitor) at 100 microM, and this increased damage was significantly reduced by SEA0400 at 1 microM. In conclusion, these results suggest that NCX1 may play a role in retinal cell death induced by NMDA and ischemia-reperfusion.

Role of the mitochondrial sodium/calcium exchanger in neuronal physiology and in the pathogenesis of neurological diseases

In neurons, as in other excitable cells, mitochondria extrude Ca(2+) ions from their matrix in exchange with cytosolic Na(+) ions. This exchange is mediated by a specific transporter located in the inner mitochondrial membrane, the mitochondrial Na(+)/Ca(2+) exchanger (NCX(mito)). The stoichiometry of NCX(mito)-operated Na(+)/Ca(2+) exchange has been the subject of a long controversy, but evidence of an electrogenic 3 Na(+)/1 Ca(2+) exchange is increasing. Although the molecular identity of NCX(mito) is still undetermined, data obtained in our laboratory suggest that besides the long-sought and as yet unfound mitochondrial-specific NCX, the three isoforms of plasmamembrane NCX can contribute to NCX(mito) in neurons and astrocytes. NCX(mito) has a role in controlling neuronal Ca(2+) homeostasis and neuronal bioenergetics. Indeed, by cycling the Ca(2+) ions captured by mitochondria back to the cytosol, NCX(mito) determines a shoulder in neuronal [Ca(2+)](c) responses to neurotransmitters and depolarizing stimuli which may then outlast stimulus duration. This persistent NCX(mito)-dependent Ca(2+) release has a role in post-tetanic potentiation, a form of short-term synaptic plasticity. By controlling [Ca(2+)](m) NCX(mito) regulates the activity of the Ca(2+)-sensitive enzymes pyruvate-, alpha-ketoglutarate- and isocitrate-dehydrogenases and affects the activity of the respiratory chain. Convincing experimental evidence suggests that supraphysiological activation of NCX(mito) contributes to neuronal cell death in the ischemic brain and, in epileptic neurons coping with seizure-induced ion overload, reduces the ability to reestablish normal ionic homeostasis. These data suggest that NCX(mito) could represent an important target for the development of new neurological drugs.

Amyloid precursor protein and amyloid beta-peptide bind to ATP synthase and regulate its activity at the surface of neural cells

Amyloid precursor protein (APP) and amyloid beta-peptide (Abeta) have been implicated in a variety of physiological and pathological processes underlying nervous system functions. APP shares many features with adhesion molecules in that it is involved in neurite outgrowth, neuronal survival and synaptic plasticity. It is, thus, of interest to identify binding partners of APP that influence its functions. Using biochemical cross-linking techniques we have identified ATP synthase subunit alpha as a binding partner of the extracellular domain of APP and Abeta. APP and ATP synthase colocalize at the cell surface of cultured hippocampal neurons and astrocytes. ATP synthase subunit alpha reaches the cell surface via the secretory pathway and is N-glycosylated during this process. Transfection of APP-deficient neuroblastoma cells with APP results in increased surface localization of ATP synthase subunit alpha. The extracellular domain of APP and Abeta partially inhibit the extracellular generation of ATP by the ATP synthase complex. Interestingly, the binding sequence of APP and Abeta is similar in structure to the ATP synthase-binding sequence of the inhibitor of F1 (IF(1)), a naturally occurring inhibitor of the ATP synthase complex in mitochondria. In hippocampal slices, Abeta and IF(1) similarly impair both short- and long-term potentiation via a mechanism that could be suppressed by blockade of GABAergic transmission. These observations indicate that APP and Abeta regulate extracellular ATP levels in the brain, thus suggesting a novel mechanism in Abeta-mediated Alzheimer's disease pathology.

Neuronal calcium mishandling and the pathogenesis of Alzheimer's disease

Perturbed neuronal Ca(2+) homeostasis is implicated in age-related cognitive impairment and Alzheimer's disease (AD). With advancing age, neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular Ca(2+) dynamics. Toxic forms of amyloid beta-peptide (Abeta) can induce Ca(2+) influx into neurons by inducing membrane-associated oxidative stress or by forming an oligomeric pore in the membrane, thereby rendering neurons vulnerable to excitotoxicity and apoptosis. AD-causing mutations in the beta-amyloid precursor protein and presenilins can compromise these normal proteins in the plasma membrane and endoplasmic reticulum, respectively. Emerging knowledge of the actions of Ca(2+) upstream and downstream of Abeta provides opportunities to develop novel preventative and therapeutic interventions for AD.

Mitochondria in cybrids containing mtDNA from persons with mitochondriopathies

The cytoplasmic hybrid (cybrid) technique allows investigators to express selected mitochondrial DNA (mtDNA) sequences against fixed nuclear DNA (nDNA) backgrounds. Cybrids have been used to study the effects of known mtDNA mutations on mitochondrial biochemistry, mtDNA-nDNA inter-species compatibility, and mtDNA integrity in persons without mtDNA mutations defined previously. This review discusses events leading up to creation of the cybrid technique, as well as data obtained via application of the cybrid strategies listed above. Although interpreting cybrid data requires awareness of technique limitations, valuable insights into mtDNA genotype-functional phenotype relationships are suggested.

Mitochondria and neurodegeneration

Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. However, despite the evidence of morphological, biochemical and molecular abnormalities in mitochondria in various tissues of patients with neurodegenerative disorders, the question "is mitochondrial dysfunction a necessary step in neurodegeneration?" is still unanswered. In this review, we highlight some of the major neurodegenerative disorders (Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis and Huntington's disease) and discuss the role of the mitochondria in the pathogenetic cascade leading to neurodegeneration.

Cell and animal models of mtDNA biology: progress and prospects

The past two decades have witnessed an evolving understanding of the mitochondrial genome's (mtDNA) role in basic biology and disease. From the recognition that mutations in mtDNA can be responsible for human disease to recent efforts showing that mtDNA mutations accumulate over time and may be responsible for some phenotypes of aging, the field of mitochondrial genetics has greatly benefited from the creation of cell and animal models of mtDNA mutation. In this review, we critically discuss the past two decades of efforts and insights gained from cell and animal models of mtDNA mutation. We attempt to reconcile the varied and at times contradictory findings by highlighting the various methodologies employed and using human mtDNA disease as a guide to better understanding of cell and animal mtDNA models. We end with a discussion of scientific and therapeutic challenges and prospects for the future of mtDNA transfection and gene therapy.

Melatonin in Alzheimer's disease and other neurodegenerative disorders

Increased oxidative stress and mitochondrial dysfunction have been identified as common pathophysiological phenomena associated with neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). As the age-related decline in the production of melatonin may contribute to increased levels of oxidative stress in the elderly, the role of this neuroprotective agent is attracting increasing attention. Melatonin has multiple actions as a regulator of antioxidant and prooxidant enzymes, radical scavenger and antagonist of mitochondrial radical formation. The ability of melatonin and its kynuramine metabolites to interact directly with the electron transport chain by increasing the electron flow and reducing electron leakage are unique features by which melatonin is able to increase the survival of neurons under enhanced oxidative stress. Moreover, antifibrillogenic actions have been demonstrated in vitro, also in the presence of profibrillogenic apoE4 or apoE3, and in vivo, in a transgenic mouse model. Amyloid-β toxicity is antagonized by melatonin and one of its kynuramine metabolites. Cytoskeletal disorganization and protein hyperphosphorylation, as induced in several cell-line models, have been attenuated by melatonin, effects comprising stress kinase downregulation and extending to neurotrophin expression. Various experimental models of AD, PD and HD indicate the usefulness of melatonin in antagonizing disease progression and/or mitigating some of the symptoms. Melatonin secretion has been found to be altered in AD and PD. Attempts to compensate for age- and disease-dependent melatonin deficiency have shown that administration of this compound can improve sleep efficiency in AD and PD and, to some extent, cognitive function in AD patients. Exogenous melatonin has also been reported to alleviate behavioral symptoms such as sundowning. Taken together, these findings suggest that melatonin, its analogues and kynuric metabolites may have potential value in prevention and treatment of AD and other neurodegenerative disorders.