PPARγ activation rescues mitochondrial function from inhibition of complex I and loss of PINK1

Parkinson's disease has long been associated with impaired mitochondrial complex I activity, while several gene defects associated with familial Parkinson's involve defects in mitochondrial function or 'quality control' pathways, causing an imbalance between mitochondrial biogenesis and removal of dysfunctional mitochondria by autophagy. Amongst these are mutations of the gene for PTEN-induced kinase 1 (PINK1) in which mitochondrial function is abnormal. Peroxisome proliferator-activated receptor gamma (PPARγ), a nuclear receptor and ligand-dependent transcription factor, regulates pathways of inflammation, lipid and carbohydrate metabolism, antioxidant defences and mitochondrial biogenesis. We have found that inhibition of complex I in human differentiated SHSY-5Y cells by the complex I inhibitor rotenone irreversibly decrease mitochondrial mass, membrane potential and oxygen consumption, while increasing free radical generation and autophagy. Similar changes are seen in PINK1 knockdown cells, in which potential, oxygen consumption and mitochondrial mass are all decreased. In both models, all these changes were reversed by pre-treatment of the cells with the PPARγ agonist, rosiglitazone, which increased mitochondrial biogenesis, increased oxygen consumption and suppressed free radical generation and autophagy. Thus, rosiglitazone is neuroprotective in two different models of mitochondrial dysfunction associated with Parkinson's disease through a direct impact on mitochondrial function.

Unexpected low-dose toxicity of the universal solvent DMSO

Dimethyl sulfoxide (DMSO) is an important aprotic solvent that can solubilize a wide variety of otherwise poorly soluble polar and nonpolar molecules. This, coupled with its apparent low toxicity at concentrations <10%, has led to its ubiquitous use and widespread application. Here, we demonstrate that DMSO induces retinal apoptosis in vivo at low concentrations (5 μl intravitreally dosed DMSO in rat from a stock concentration of 1, 2, 4, and 8% v/v). Toxicity was confirmed in vitro in a retinal neuronal cell line, at DMSO concentrations >1% (v/v), using annexin V, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and AlamarBlue cell viability assays. DMSO concentrations >10% (v/v) have recently been reported to cause cellular toxicity through plasma membrane pore formation. Here, we show the mechanism by which low concentrations (2-4% DMSO) induce caspase-3 independent neuronal death that involves apoptosis-inducing factor (AIF) translocation from mitochondria to the nucleus and poly-(ADP-ribose)-polymerase (PARP) activation. These results highlight safety concerns of using low concentrations of DMSO as a solvent for in vivo administration and in biological assays. We recommend that methods other than DMSO are employed for solubilizing drugs but, where no alternative exists, researchers compute absolute DMSO final concentrations and include an untreated control group in addition to DMSO vehicle control to check for solvent toxicity.

Sulforaphane preconditioning of the Nrf2/HO-1 defense pathway protects the cerebral vasculature against blood-brain barrier disruption and neurological deficits in stroke

Disruption of the blood-brain barrier (BBB) and cerebral edema are the major pathogenic mechanisms leading to neurological dysfunction and death after ischemic stroke. The brain protects itself against infarction via activation of endogenous antioxidant defense mechanisms, and we here report the first evidence that sulforaphane-mediated preactivation of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream target heme oxygenase-1 (HO-1) in the cerebral vasculature protects the brain against stroke. To induce ischemic stroke, Sprague-Dawley rats were subjected to 70 min middle cerebral artery occlusion (MCAo) followed by 4, 24, or 72 h reperfusion. Nrf2 and HO-1 protein expression was upregulated in cerebral microvessels of peri-infarct regions after 4-72 h, with HO-1 preferentially associated with perivascular astrocytes rather than the cerebrovascular endothelium. In naïve rats, treatment with sulforaphane increased Nrf2 expression in cerebral microvessels after 24h. Upregulation of Nrf2 by sulforaphane treatment prior to transient MCAo (1h) was associated with increased HO-1 expression in perivascular astrocytes in peri-infarct regions and cerebral endothelium in the infarct core. BBB disruption, lesion progression, as analyzed by MRI, and neurological deficits were reduced by sulforaphane pretreatment. As sulforaphane pretreatment led to a moderate increase in peroxynitrite generation, we suggest that hormetic preconditioning underlies sulforaphane-mediated protection against stroke. In conclusion, we propose that pharmacological or dietary interventions aimed to precondition the brain via activation of the Nrf2 defense pathway in the cerebral microvasculature provide a novel therapeutic approach for preventing BBB breakdown and neurological dysfunction in stroke.

Mitochondrial quality control and communications with the nucleus are important in maintaining mitochondrial function and cell health

BACKGROUND

The maintenance of cell metabolism and homeostasis is a fundamental characteristic of living organisms. In eukaryotes, mitochondria are the cornerstone of these life supporting processes, playing leading roles in a host of core cellular functions, including energy transduction, metabolic and calcium signalling, and supporting roles in a number of biosynthetic pathways. The possession of a discrete mitochondrial genome dictates that the maintenance of mitochondrial 'fitness' requires quality control mechanisms which involve close communication with the nucleus.

SCOPE OF REVIEW

This review explores the synergistic mechanisms that control mitochondrial quality and function and ensure cellular bioenergetic homeostasis. These include antioxidant defence mechanisms that protect against oxidative damage caused by reactive oxygen species, while regulating signals transduced through such free radicals. Protein homeostasis controls import, folding, and degradation of proteins underpinned by mechanisms that regulate bioenergetic capacity through the mitochondrial unfolded protein response. Autophagic machinery is recruited for mitochondrial turnover through the process of mitophagy. Mitochondria also communicate with the nucleus to exact specific transcriptional responses through retrograde signalling pathways.

MAJOR CONCLUSIONS

The outcome of mitochondrial quality control is not only reliant on the efficient operation of the core homeostatic mechanisms but also in the effective interaction of mitochondria with other cellular components, namely the nucleus.

GENERAL SIGNIFICANCE

Understanding mitochondrial quality control and the interactions between the organelle and the nucleus will be crucial in developing therapies for the plethora of diseases in which the pathophysiology is determined by mitochondrial dysfunction. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Defective quality control mechanisms and accumulation of damaged mitochondria link Gaucher and Parkinson diseases

Mutations in the GBA gene encoding glucocerebrosidase cause Gaucher disease (GD), the most prevalent of the lysosomal storage disorders (LSDs) and increase susceptibility to Parkinson disease (PD). Clinically the two disorders can present in a similar manner with analogous pathological features, suggesting mechanistic links between the two disease states. An increasing body of evidence implicates defects in quality control pathways in both, and suggests that LSDs, as a group, can be classed as disorders of autophagy. Using a mouse model of type II neuronopathic GD, we observed global defects in cellular quality control pathways in midbrain neurons and astrocytes. Our data suggest that downregulation of autophagy, mitophagy, and the ubiquitin-proteasome system (UPS) results in accumulation of dysfunctional and fragmented mitochondria, insoluble SNCA/α-synuclein deposits and ubiquitinated proteins. These observations show that dysfunction of cellular quality control pathways lead to impaired energy and free radical homeostasis, providing new insights into the mechanisms of neurodegeneration in GD and illuminating the links between GD and PD.

Mitochondria and quality control defects in a mouse model of Gaucher disease--links to Parkinson's disease

Mutations in the glucocerebrosidase (gba) gene cause Gaucher disease (GD), the most common lysosomal storage disorder, and increase susceptibility to Parkinson's disease (PD). While the clinical and pathological features of idiopathic PD and PD related to gba (PD-GBA) mutations are very similar, cellular mechanisms underlying neurodegeneration in each are unclear. Using a mouse model of neuronopathic GD, we show that autophagic machinery and proteasomal machinery are defective in neurons and astrocytes lacking gba. Markers of neurodegeneration--p62/SQSTM1, ubiquitinated proteins, and insoluble α-synuclein--accumulate. Mitochondria were dysfunctional and fragmented, with impaired respiration, reduced respiratory chain complex activities, and a decreased potential maintained by reversal of the ATP synthase. Thus a primary lysosomal defect causes accumulation of dysfunctional mitochondria as a result of impaired autophagy and dysfunctional proteasomal pathways. These data provide conclusive evidence for mitochondrial dysfunction in GD and provide insight into the pathogenesis of PD and PD-GBA.

IF1 limits the apoptotic-signalling cascade by preventing mitochondrial remodelling

Mitochondrial structure has a central role both in energy conversion and in the regulation of cell death. We have previously shown that IF1 protects cells from necrotic cell death and supports cristae structure by promoting the oligomerisation of the F1Fo-ATPsynthase. As IF1 is upregulated in a large proportion of human cancers, we have here explored its contribution to the progression of apoptosis and report that an increased expression of IF1, relative to the F1Fo-ATPsynthase, protects cells from apoptotic death. We show that IF1 expression serves as a checkpoint for the release of Cytochrome c (Cyt c) and hence the completion of the apoptotic program. We show that the progression of apoptosis engages an amplification pathway mediated by: (i) Cyt c-dependent release of ER Ca(2+), (ii) Ca(2+)-dependent recruitment of the GTPase Dynamin-related protein 1 (Drp1), (iii) Bax insertion into the outer mitochondrial membrane and (iv) further release of Cyt c. This pathway is accelerated by suppression of IF1 and delayed by its overexpression. IF1 overexpression is associated with the preservation of mitochondrial morphology and ultrastructure, consistent with a central role for IF1 as a determinant of the inner membrane architecture and with the role of mitochondrial ultrastructure in the regulation of Cyt c release. These data suggest that IF1 is an antiapoptotic and potentially tumorigenic factor and may be a valuable predictor of responsiveness to chemotherapy.

Regulation of mitochondrial morphogenesis by annexin A6

Mitochondrial homeostasis is critical in meeting cellular energy demands, shaping calcium signals and determining susceptibility to apoptosis. Here we report a role for anxA6 in the regulation of mitochondrial morphogenesis, and show that in cells lacking anxA6 mitochondria are fragmented, respiration is impaired and mitochondrial membrane potential is reduced. In fibroblasts from AnxA6^−/− mice, mitochondrial Ca²⁺ uptake is reduced and cytosolic Ca²⁺ transients are elevated. These observations led us to investigate possible interactions between anxA6 and proteins with roles in mitochondrial fusion and fission. We found that anxA6 associates with Drp1 and that mitochondrial fragmentation in AnxA6^−/− fibroblasts was prevented by the Drp1 inhibitor mdivi-1. In normal cells elevation of intracellular Ca²⁺ disrupted the interaction between anxA6 and Drp1, displacing anxA6 to the plasma membrane and promoting mitochondrial fission. Our results suggest that anxA6 inhibits Drp1 activity, and that Ca²⁺-binding to anxA6 relieves this inhibition to permit Drp1-mediated mitochondrial fission.

Cellular and molecular mechanisms of mitochondrial function

Mitochondria are membrane bound organelles present in almost all eukaryotic cells. Responsible for orchestrating cellular energy production, they are central to the maintenance of life and the gatekeepers of cell death. Thought to have originated from symbiotic ancestors, they carry a residual genome as mtDNA encoding 13 proteins essential for respiratory chain function. Mitochondria comprise an inner and outer membrane that separate and maintain the aqueous regions, the intermembrane space and the matrix. Mitochondria contribute to many processes central to cellular function and dysfunction including calcium signalling, cell growth and differentiation, cell cycle control and cell death. Mitochondrial shape and positioning in cells is crucial and is tightly regulated by processes of fission and fusion, biogenesis and autophagy, ensuring a relatively constant mitochondrial population. Mitochondrial dysfunction is implicated in metabolic and age related disorders, neurodegenerative diseases and ischemic injury in heart and brain.

Activation of PARP by oxidative stress induced by β-amyloid: implications for Alzheimer's disease

Alzheimer's disease (AD) is a major neurodegenerative disease of old age, characterised by progressive cognitive impairment, dementia and atrophy of the central nervous system. The pathological hallmarks include the accumulation of the peptide β-amyloid (Aβ) which itself is toxic to neurons in culture. Recently, it has been discovered that Aβ activates the protein poly(ADP-ribosyl) polymerase-1 (PARP-1) specifically in astrocytes, leading indirectly to neuronal cell death. PARP-1 is a DNA repair enzyme, normally activated by single strand breaks associated with oxidative stress, which catalyses the formation of poly ADP-ribose polymers from nicotinamide adenine dinucleotide (NAD(+)). The pathological over activation of PARP-1 causes depletion of NAD(+) and leads to cell death. Here we review the relationship between AD and PARP-1, and explore the role played by astrocytes in neuronal death. AD has so far proven refractory to any effective treatment. Identification of these pathways represents a step towards a greater understanding of the pathophysiology of this devastating disease with the potential to explore novel therapeutic targets.

Mild stress of caffeine increased mtDNA content in skeletal muscle cells: the interplay between Ca2+ transients and nitric oxide

Caffeine increases mitochondrial biogenesis in myotubes by evoking Ca(2+) transients. Nitric oxide (NO) also induces mitochondrial biogenesis in skeletal muscle cells via upregulation of AMP-activated protein kinase (AMPK) activity and PGC-1α. However, the interplay and timing sequence between Ca(2+) transients and NO releases remain unclear. Herein, we tested the hypothesis that caffeine-evoked Ca(2+) transients triggered NO production to increase mtDNA in skeletal muscle cells. Ca(2+) transients were recorded with Fura-2 AM and confocal microscopy; mtDNA staining, mitochondrial membrane potential and NO level were determined using fluorescent probes PicoGreen, tetramethylrhodamine methyl ester (TMRM) and DAF-FM, respectively. In primary cultured myotubes, a subtle and moderate stress of caffeine increased mtDNA exclusively. Mitochondrial membrane potential and mtDNA were increased by 1 mM as well as 5 mM caffeine, whereas 10 mM caffeine did not change the fluorescence intensity of PicoGreen and TMRM. NO level in myocytes increased gradually following the first jump of Ca(2+) transients evoked by caffeine (5 mM) till the end of recording, when Fura-2 indicated that Ca(2+) transients recovered partly and even disappeared. Importantly, nitric oxide synthase (NOS) inhibitor (L-NAME) suppressed caffeine-induced mtDNA biogenesis, whereas NO donor (DETA-NO) increased mtDNA content. These data strongly suggest that caffeine-induced mtDNA biogenesis is dose-sensitive and dependent on a certain level of stress. Further, an increasing level of NO following Ca(2+) transients is required for caffeine-induced mtDNA biogenesis. Additionally, Ca(2+) transients, a usual and first response to caffeine, was either suppressed or attenuated by L-NAME, DETA-NO, AICAR and U0126, suggesting an inability to control [Ca(2+)](i) in these treated cells. There may be an important interplay between NO and Ca(2+) transients in intracellular signaling system involving NOS, AMPK and MEK.

G2019S leucine-rich repeat kinase 2 causes uncoupling protein-mediated mitochondrial depolarization

The G2019S leucine rich repeat kinase 2 (LRRK2) mutation is the most common genetic cause of Parkinson's disease (PD), clinically and pathologically indistinguishable from idiopathic PD. Mitochondrial abnormalities are a common feature in PD pathogenesis and we have investigated the impact of G2019S mutant LRRK2 expression on mitochondrial bioenergetics. LRRK2 protein expression was detected in fibroblasts and lymphoblasts at levels higher than those observed in the mouse brain. The presence of G2019S LRRK2 mutation did not influence LRRK2 expression in fibroblasts. However, the expression of the G2019S LRRK2 mutation in both fibroblast and neuroblastoma cells was associated with mitochondrial uncoupling. This was characterized by decreased mitochondrial membrane potential and increased oxygen utilization under basal and oligomycin-inhibited conditions. This resulted in a decrease in cellular ATP levels consistent with compromised cellular function. This uncoupling of mitochondrial oxidative phosphorylation was associated with a cell-specific increase in uncoupling protein (UCP) 2 and 4 expression. Restoration of mitochondrial membrane potential by the UCP inhibitor genipin confirmed the role of UCPs in this mechanism. The G2019S LRRK2-induced mitochondrial uncoupling and UCP4 mRNA up-regulation were LRRK2 kinase-dependent, whereas endogenous LRRK2 levels were required for constitutive UCP expression. We propose that normal mitochondrial function was deregulated by the expression of G2019S LRRK2 in a kinase-dependent mechanism that is a modification of the normal LRRK2 function, and this leads to the vulnerability of selected neuronal populations in PD.

Dopamine induced neurodegeneration in a PINK1 model of Parkinson's disease

BACKGROUND

Parkinson's disease is a common neurodegenerative disease characterised by progressive loss of dopaminergic neurons, leading to dopamine depletion in the striatum. Mutations in the PINK1 gene cause an autosomal recessive form of Parkinson's disease. Loss of PINK1 function causes mitochondrial dysfunction, increased reactive oxygen species production and calcium dysregulation, which increases susceptibility to neuronal death in Parkinson's disease. The basis of neuronal vulnerability to dopamine in Parkinson's disease is not well understood.

METHODOLOGY

We investigated the mechanism of dopamine induced cell death in transgenic PINK1 knockout mouse neurons. We show that dopamine results in mitochondrial depolarisation caused by mitochondrial permeability transition pore (mPTP) opening. Dopamine-induced mPTP opening is dependent on a complex of reactive oxygen species production and calcium signalling. Dopamine-induced mPTP opening, and dopamine-induced cell death, could be prevented by inhibition of reactive oxygen species production, by provision of respiratory chain substrates, and by alteration in calcium signalling.

CONCLUSIONS

These data demonstrate the mechanism of dopamine toxicity in PINK1 deficient neurons, and suggest potential therapeutic strategies for neuroprotection in Parkinson's disease.

Slow calcium waves and redox changes precede mitochondrial permeability transition pore opening in the intact heart during hypoxia and reoxygenation

AIMS

Opening of the mitochondrial permeability transition pore (mPTP) is an important step on the pathway towards cardiomyocyte death, defining the extent of injury following cardiac ischaemia and reperfusion. In isolated mitochondria, mPTP opening is triggered by calcium overload facilitated by oxidative stress. In isolated cells, however, it has been suggested that mPTP opening occurs before calcium overload and is stimulated by oxidative stress. Our objective was to establish the events that cause mPTP opening in the intact heart.

METHODS AND RESULTS

We performed multiphoton imaging of Langendorff-perfused mouse hearts expressing an inducible, Ca(2+)-sensitive reporter (circularly Permuted GFP and calmodulin (CaM), version 2), to examine the spatiotemporal relationship between [Ca(2+)](c), redox state, and mPTP opening in the intact heart during hypoxia and reoxygenation at sub-myocyte resolution. We found that during reperfusion, calcium waves propagated across multiple cells at 3.3 µm/s. mPTP opening caused an abrupt loss of mitochondrial membrane potential, measured using a potentiometric dye, which was invariably preceded by a rise in [Ca(2+)](c). The probability that localized [Ca(2+)](c) waves led to mPTP opening was greater early during reoxygenation. During reoxygenation, coordinated redox changes also occurred across large regions and preceded mPTP opening on average by 122 ± 38 s. Fewer [Ca(2+)] waves led to mPTP opening in the presence of mPTP inhibitor cyclosporin A or mitochondrial-targeted scavenger of reactive oxygen species, MitoQ.

CONCLUSION

These experiments define the spatiotemporal relationship between changes in [Ca(2+)](c), redox state and mPTP opening during reoxygenation in the intact heart. Tissue oxidation coincident with localized calcium waves together conspire to cause mPTP opening and subsequent cell death.

Membrane cholesterol content plays a key role in the neurotoxicity of β-amyloid: implications for Alzheimer's disease

Beta amyloid (βA) plays a central role in the pathogenesis of the most common and devastating neurodegenerative disorder, Alzheimer's disease (AD). The mechanisms of βA neurotoxicity remain controversial, but include dysregulation of calcium homeostasis and oxidative stress. A large body of data suggest that cholesterol plays a significant role in AD. In mixed cultures containing hippocampal neurons and astrocytes, we have shown that neurotoxic βA peptides (1-42 and 25-35) cause sporadic cytosolic calcium ([Ca(2+) ](c) ) signals in astrocytes but not in neurons, initiating a cascade that ends in neuronal death. We now show, using the cholesterol-sensitive fluorescent probe, Filipin, that membrane cholesterol is significantly higher in astrocytes than in neurons and mediates the selective response of astrocytes to βA. Thus, lowering [cholesterol] using mevastatin, methyl-β-cyclodextrin or filipin prevented the βA-induced [Ca(2+) ](c) signals, while increased membrane [cholesterol] increased βA-induced [Ca(2+) ](c) signals in both neurons and astrocytes. Addition of βA to lipid bilayers caused the appearance of a conductance that was significantly higher in membranes containing cholesterol. Increasing membrane [cholesterol] significantly increased βA-induced neuronal and astrocytic death. We conclude that a high membrane [cholesterol] promotes βA incorporation into membranes and increased [Ca(2+) ](c) leading to cell death.

Beta-amyloid activates PARP causing astrocytic metabolic failure and neuronal death

Alzheimer's disease is characterized by β-amyloid accumulation in the central nervous system. As β-amyloid is neurotoxic in culture, we have explored the mechanisms of toxicity in the search for therapeutic targets for Alzheimer's disease and now identify a key role for poly(ADP-ribose) polymerase in β-amyloid-induced neuronal death. Exposure of hippocampal neuronal/glial co-cultures to β-amyloid peptides activates the glial nicotinamide adenine dinucleotide phosphate oxidase, followed by predominantly neuronal cell death. β-amyloid exposure caused the progressive loss of mitochondrial membrane potential in astrocytes, accompanied by transient mitochondrial depolarizations caused by reversible openings of the mitochondrial permeability transition pore. The transients were absent in cultures from cyclophilin D knockout mice, leaving the slow depolarization available for study in isolation. β-amyloid exposure decreased both nicotinamide adenine dinucleotide fluorescence and oxygen consumption, while provision of mitochondrial substrates reversed the depolarization, suggesting that substrate supply was limiting. Poly(ADP-ribose) polymerase is activated by oxidative stress and consumes nicotinamide adenine dinucleotide, decreasing substrate availability. β-amyloid exposure caused accumulation of the poly(ADP-ribose) polymerase product, poly-ADP-ribose polymers, in astrocytes. Inhibition of either poly(ADP-ribose) polymerase or of the nicotinamide adenine dinucleotide phosphate oxidase prevented the appearance of poly-ADP-ribose polymers and the mitochondrial depolarization. Exposure of co-cultures to β-amyloid for >8 h decreased nicotinamide adenine dinucleotide and mitochondrial membrane potential and increased cell death in neurons, all of which were prevented by poly(ADP-ribose) polymerase inhibitors. Poly-ADP-ribose polymers increased with age in the brains of the TASTPM Alzheimer mouse model. We conclude that β-amyloid-induced neuronal death is mediated by poly(ADP-ribose) polymerase in response to oxidative stress generated by the astrocytic nicotinamide adenine dinucleotide phosphate oxidase.

Multiphoton imaging of the functioning kidney

Translating discoveries made in isolated renal cells and tubules to the in vivo situation requires the assessment of cellular function in intact live organs. Multiphoton imaging is a form of fluorescence microscopy that is ideally suited to working with whole tissues and organs, but adequately loading cells with fluorescence dyes in vivo remains a challenge. We found that recirculation of fluorescence dyes in the rat isolated perfused kidney (IPK) resulted in levels of intracellular loading that would be difficult to achieve in vivo. This technique allowed the imaging of tubular cell structure and function with multiphoton microscopy in an intact, functioning organ. We used this approach to follow processes in real time, including (1) relative rates of reactive oxygen species (ROS) production in different tubule types, (2) filtration and tubular uptake of low-molecular-weight dextrans and proteins, and (3) the effects of ischemia-reperfusion injury on mitochondrial function and cell structure. This study demonstrates that multiphoton microscopy of the isolated perfused kidney is a powerful technique for detailed imaging of cell structure and function in an intact organ.

Measurements of threshold of mitochondrial permeability transition pore opening in intact and permeabilized cells by flash photolysis of caged calcium

Changes in intracellular calcium concentration play a major role both in signal transduction and in cell death. In particular, mitochondrial Ca2+ overload is critically important as a determinant of irreversible cell injury. When accumulated above a threshold, matrix Ca2+ triggers opening of the mitochondrial permeability transition pore (mPTP), initiating ATP depletion and cell death via necrosis or by promoting cytochrome c release and initiating the apoptotic cascade. Measurement of mitochondrial Ca2+ uptake capacity (or the threshold for mPTP opening) is, therefore, important for understanding the mechanisms of pathophysiology in a variety of disease models and also for testing neuro- or cardioprotective drugs. We have, therefore, devised an approach that delivers Ca2+ directly to the matrix of mitochondria independently of uptake and therefore independently of potential (Δψm) that allows direct study both of the Ca2+ efflux pathway and of the specific sensitivity of mPTP to Ca2+. This is achieved using the photolytic release of Ca2+ by flash photolysis of caged Ca2+ using compounds, such as o-nitrophenyl EGTA, introduced into the cell as the acetoxymethyl (AM) ester (NP-EGTA, AM). This method can be used in both intact and permeabilized cells.

Albumin uptake in OK cells exposed to rotenone: a model for studying the effects of mitochondrial dysfunction on endocytosis in the proximal tubule?

Background

The renal proximal tubule (PT) is clinically vulnerable to mitochondrial dysfunction; sub-lethal injury can lead to the Fanconi syndrome, with elevated urinary excretion of low-molecular-weight proteins. As the mechanism that couples mitochondrial dysfunction to impaired PT low-molecular weight protein uptake is unknown, we investigated the effect of respiratory chain (RC) inhibitors on endocytosis of FITC-albumin in PT-derived OK cells.

Methods

Uptake of FITC-albumin was quantified using confocal microscopy. Cytosolic ATP levels were measured in real time using both luciferin/luciferase assays and measurements of free [Mg²⁺]. Reactive oxygen species production was measured using mitosox.

Results

RC blockade produced only a small decrease in cytosolic ATP levels and had minimal effect on FITC-albumin uptake. Inhibition of glycolysis caused a much bigger decrease in both cytosolic ATP levels and FITC-albumin endocytosis. Rotenone led to higher rates of reactive oxygen species production than other RC inhibitors. Rotenone also caused widespread structural damage on electron microscopy, which was mimicked by colchicine and prevented by taxol; consistent with inhibition of microtubule polymerisation as the underlying mechanism.

Conclusions

Endocytosis of FITC-albumin is ATP-dependent in OK cells, but the cells are very glycolytic and therefore represent a poor metabolic model of the PT. Rotenone has toxic extra-mitochondrial structural effects.

Chloride intracellular channel 1 (CLIC1): Sensor and effector during oxidative stress

Oxidative stress, characterized by overproduction of reactive oxygen species (ROS), is a major feature of several pathological states. Indeed, many cancers and neurodegenerative diseases are accompanied by altered redox balance, which results from dysregulation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. In this review, we consider the role of the intracellular chloride channel 1 (CLIC1) in microglial cells during oxidative stress. Following microglial activation, CLIC1 translocates from the cytosol to the plasma membrane where it promotes a chloride conductance. The resultant anionic current balances the excess charge extruded by the active NADPH oxidase, supporting the generation of superoxide by the enzyme. In this scenario, CLIC1 could be considered to act as both a second messenger and an executor.

The legs at odd angles (Loa) mutation in cytoplasmic dynein ameliorates mitochondrial function in SOD1G93A mouse model for motor neuron disease

Amyotrophic lateral sclerosis (ALS) is a debilitating and fatal late-onset neurodegenerative disease. Familial cases of ALS (FALS) constitute ∼10% of all ALS cases, and mutant superoxide dismutase 1 (SOD1) is found in 15–20% of FALS. SOD1 mutations confer a toxic gain of unknown function to the protein that specifically targets the motor neurons in the cortex and the spinal cord. We have previously shown that the autosomal dominant Legs at odd angles (Loa) mutation in cytoplasmic dynein heavy chain (Dync1h1) delays disease onset and extends the life span of transgenic mice harboring human mutant SOD1^G93A. In this study we provide evidence that despite the lack of direct interactions between mutant SOD1 and either mutant or wild-type cytoplasmic dynein, the Loa mutation confers significant reductions in the amount of mutant SOD1 protein in the mitochondrial matrix. Moreover, we show that the Loa mutation ameliorates defects in mitochondrial respiration and membrane potential observed in SOD1^G93A motor neuron mitochondria. These data suggest that the Loa mutation reduces the vulnerability of mitochondria to the toxic effects of mutant SOD1, leading to improved mitochondrial function in SOD1^G93A motor neurons.

Maternal diet-induced obesity alters mitochondrial activity and redox status in mouse oocytes and zygotes

The negative impact of obesity on reproductive success is well documented but the stages at which development of the conceptus is compromised and the mechanisms responsible for the developmental failure still remain unclear. Recent findings suggest that mitochondria may be a contributing factor. However to date no studies have directly addressed the consequences of maternal obesity on mitochondria in early embryogenesis.

Using an established murine model of maternal diet induced obesity and a live cell dynamic fluorescence imaging techniques coupled with molecular biology we have investigated the underlying mechanisms of obesity-induced reduced fertility. Our study is the first to show that maternal obesity prior to conception is associated with altered mitochondria in mouse oocytes and zygotes. Specifically, maternal diet-induced obesity in mice led to an increase in mitochondrial potential, mitochondrial DNA content and biogenesis. Generation of reactive oxygen species (ROS) was raised while glutathione was depleted and the redox state became more oxidised, suggestive of oxidative stress. These altered mitochondrial properties were associated with significant developmental impairment as shown by the increased number of obese mothers who failed to support blastocyst formation compared to lean dams. We propose that compromised oocyte and early embryo mitochondrial metabolism, resulting from excessive nutrient exposure prior to and during conception, may underlie poor reproductive outcomes frequently reported in obese women.

The intracellular localization and function of the ATP-sensitive K+ channel subunit Kir6.1

Our aim was to determine the subcellular localization and functional roles of the K(ATP) channel subunit Kir6.1 in intracellular membranes. Specifically, we focused on the potential role of Kir6.1 as a subunit of the mitochondrial ATP-sensitive K+ channel. Cell imaging showed that a major proportion of heterologously expressed Kir6.1-GFP and endogenously expressed Kir6.1 was distributed in the endoplasmic reticulum with little in the mitochondria or plasma membrane. We used pharmacological and molecular tools to investigate the functional significance of this distribution. The K(ATP) channel opener diazoxide increased reactive oxygen species production, and glibenclamide abolished this effect. However, in cells lacking Kir6.1 or expressing siRNA or dominant negative constructs of Kir6.1, the same effect was seen. Ca2+ handling was examined in the muscle cell line C2C12. Transfection of the dominant negative constructs of Kir6.1 significantly reduced the amplitude and rate of rise of [Ca2+]( c ) transients elicited by ATP. This study suggests that Kir6.1 is located in the endoplasmic reticulum and plays a role in modifying Ca2+ release from intracellular stores.