Ian J. Reynolds

Glutamate-induced neuron death requires mitochondrial calcium uptake.

We have investigated the role of mitochondrial calcium buffering in excitotoxic cell death. Glutamate acts at NMDA receptors in cultured rat forebrain neurons to increase the intracellular free calcium concentration. Although concurrent inhibition of mitochondrial calcium uptake substantially enhanced this cytoplasmic calcium increase, it significantly reduced glutamate-stimulated neuronal cell death. Mitochondrial inhibition did not affect nitric oxide production or MAP kinase phosphorylation, which have been proposed to mediate excitotoxicity. These results indicate that very high levels of cytoplasmic calcium are not necessarily toxic to forebrain neurons, and that potential-driven uptake of calcium into mitochondria is required to trigger NMDA-receptor-stimulated neuronal death.

ΔΨm-Dependent and -independent production of reactive oxygen species by rat brain mitochondria

Mitochondria are widely believed to be the source of reactive oxygen species (ROS) in a number of neurodegenerative disease states. However, conditions associated with neuronal injury are accompanied by other alterations in mitochondrial physiology, including profound changes in the mitochondrial membrane potential ΔΨm. In this study we have investigated the effects of ΔΨm on ROS production by rat brain mitochondria using the fluorescent peroxidase substrates scopoletin and Amplex red. The highest rates of mitochondrial ROS generation were observed while mitochondria were respiring on the complex II substrate succinate. Under this condition, the majority of the ROS signal was derived from reverse electron transport to complex I, because it was inhibited by rotenone. This mode of ROS generation is very sensitive to depolarization of ΔΨm, and even the depolarization associated with ATP generation was sufficient to inhibit ROS production. Mitochondria respiring on the complex I substrates, glutamate and malate, produce very little ROS until complex I is inhibited with rotenone, which is also consistent with complex I being the major site of ROS generation. This mode of oxidant production is insensitive to changes in ΔΨm. With both substrates, ubiquinone‐derived ROS can be detected, but they represent a more minor component of the overall oxidant signal. These studies demonstrate that rat brain mitochondria can be effective producers of ROS. However, the optimal conditions for ROS generation require either a hyperpolarized membrane potential or a substantial level of complex I inhibition.

Vanilloid receptor expression suggests a sensory role for urinary bladder epithelial cells

Edited by Louis J. Ignarro, University of California, Los Angeles School of Medicine, Los Angeles, CA, and approved August 27, 2001 (received for review May 16, 2001)

Induction of neuronal apoptosis by thiol oxidation: putative role of intracellular zinc release.

Abstract: The membrane‐permeant oxidizing agent 2,2′‐dithiodipyridine (DTDP) can induce Zn2+ release from metalloproteins in cell‐free systems. Here, we report that brief exposure to DTDP triggers apoptotic cell death in cultured neurons, detected by the presence of both DNA laddering and asymmetric chromatin formation. Neuronal death was blocked by increased extracellular potassium levels, by tetraethylammonium, and by the broad‐spectrum cysteine protease inhibitor butoxy‐carbonyl‐aspartate‐fluoromethylketone. N,N,N′,N′‐Tetrakis‐(2‐pyridylmethyl)ethylenediamine (TPEN) and other cell‐permeant metal chelators also effectively blocked DTDP‐induced toxicity in neurons. Cell death, however, was not abolished by the NMDA receptor blocker MK‐801, by the intracellular calcium release antagonist dantrolene, or by high concentrations of ryanodine. DTDP generated increases in fluorescence signals in cultured neurons loaded with the zinc‐selective dye Newport Green. The fluorescence signals following DTDP treatment also increased in fura‐2‐ and magfura‐2‐loaded neurons. These responses were completely reversed by TPEN, consistent with a DTDP‐mediated increase in intracellular free Zn2+ concentrations. Our studies suggest that under conditions of oxidative stress, Zn2+ released from intracellular stores may contribute to the initiation of neuronal apoptosis.

Glutamate Decreases Mitochondrial Size and Movement in Primary Forebrain Neurons

Mitochondria are essential to maintain neuronal viability. In addition to the generation of ATP and maintenance of calcium homeostasis, the effective delivery of mitochondria to the appropriate location within neurons is also likely to influence their function. In this study we examined mitochondrial movement and morphology in primary cultures of rat forebrain using a mitochondrially targeted enhanced yellow fluorescent protein (mt-eYFP). Mt-eYFP-labeled mitochondria display a characteristic elongated phenotype and also move extensively. Application of glutamate to cultures results in a rapid diminution of movement and also an alteration from elongated to rounded morphology. This effect required the entry of calcium and was mediated by activation of the NMDA subtype of glutamate receptor. Treatment of cultures with an uncoupler or blocking ATP synthesis with oligomycin also stopped movement but did not alter morphology. Interestingly, application of glutamate together with the uncoupler did not prevent the changes in movement or shape but facilitated recovery after washout of the stimuli. This suggests that the critical target for calcium in this paradigm is cytosolic. These studies demonstrate that in addition to altering the bioenergetic properties of mitochondria, neurotoxins can also alter mitochondrial movement and morphology. We speculate that neurotoxin-mediated impairment of mitochondrial delivery may contribute to the injurious effects of neurotoxins.

Zinc inhibition of cellular energy production: implications for mitochondria and neurodegeneration

An increasing body of evidence suggests that high intracellular free zinc promotes neuronal death by inhibiting cellular energy production. A number of targets have been postulated, including complexes of the mitochondrial electron transport chain, components of the tricarboxylic acid cycle, and enzymes of glycolysis. Consequences of cellular zinc overload may include increased cellular reactive oxygen species (ROS) production, loss of mitochondrial membrane potential, and reduced cellular ATP levels. Additionally, zinc toxicity might involve zinc uptake by mitochondria and zinc induction of mitochondrial permeability transition. The present review discusses these processes with special emphasis on their potential involvement in brain injury.

Mitochondrial Trafficking to Synapses in Cultured Primary Cortical Neurons

Functional synapses require mitochondria to supply ATP and regulate local [Ca2+]i for neurotransmission. Mitochondria are thought to be transported to specific cellular regions of increased need such as synapses. However, little is known about how this occurs, including the spatiotemporal distribution of mitochondria relative to presynaptic and postsynaptic sites, whether mitochondria are dynamically recruited to synapses, and how synaptic activity affects these trafficking patterns. We used primary cortical neurons in culture that form synaptic connections and show spontaneous synaptic activity under normal conditions. Neurons were cotransfected with a mitochondrially targeted cyan fluorescent protein and an enhanced yellow fluorescent protein-tagged synaptophysin or postsynaptic density-95 plasmid to label presynaptic or postsynaptic structures, respectively. Fluorescence microscopy revealed longer dendritic mitochondria that occupied a greater fraction of neuronal process length than axonal mitochondria. Mitochondria were significantly more likely to be localized at synaptic sites. Although this localization was unchanged by inhibition of synaptic activity by tetrodotoxin, it increased in dendritic synapses and decreased in axonal synapses during overactivity by veratridine. Mitochondrial movement and recruitment to synapses also differed between axons and dendrites under basal conditions and when synaptic activity was altered. Additionally, we show that movement of dendritic mitochondria can be selectively impaired by glutamate and zinc. We conclude that mitochondrial trafficking to synapses is dynamic in neurons and is modulated by changes in synaptic activity. Furthermore, mitochondrial morphology and distribution may be optimized differentially to best serve the synaptic distributions in axons and dendrites. Last, selective cessation of mitochondrial movement in dendrites suggests early postsynaptic dysfunction in neuronal injury and degeneration.

Mutant huntingtin aggregates impair mitochondrial movement and trafficking in cortical neurons.

Huntingtons disease (HD) is a neurodegenerative disorder caused by a polyglutamine repeat in the huntingtin gene (Htt). Mitochondrial defects and protein aggregates are characteristic of affected neurons. Recent studies suggest that these aggregates impair cellular transport mechanisms by interacting with cytoskeletal components and molecular motors. Here, we investigated whether mutant Htt alters mitochondrial trafficking and morphology in primary cortical neurons. We demonstrate that full-length mutant Htt was more effective than N-terminal mutant Htt in blocking mitochondrial movement, an effect that correlated with its heightened expression in the cytosolic compartment. Aggregates impaired the passage of mitochondria along neuronal processes, causing mitochondria to accumulate adjacent to aggregates and become immobilized. Furthermore, mitochondrial trafficking was reduced specifically at sites of aggregates while remaining unaltered in regions lacking aggregates. We conclude that in cortical neurons, an early event in HD pathophysiology is the aberrant mobility and trafficking of mitochondria caused by cytosolic Htt aggregates.

Mitochondria accumulate Ca2+ following intense glutamate stimulation of cultured rat forebrain neurones.

1. In cultures of rat forebrain neurones, mitochondria buffer glutamate‐induced, NMDA receptor‐mediated Ca2+ influx. Here, we have used the fluorescent calcium indicator, indo‐1 AM to record [Ca2+]i from single cells. We varied either the glutamate concentration or the duration of exposure to investigate the cellular mechanisms recruited to buffer [Ca2+]i within different stimulation protocols. 2. For a 15 s stimulus, the recovery time doubled as the glutamate concentration was raised from 3 to 300 microM. Changing the duration of exposure from 15 s to 5 min increased the recovery time tenfold even when the glutamate concentration was held at 3 microM. 3. We used a selective inhibitor of the mitochondrial Na(+)‐Ca2+ exchange, CGP‐37157. When applied immediately after a 15 s, 100 microM glutamate challenge, CGP‐37157 consistently caused a rapid fall in [Ca2+]i followed by a slow rise after the drug was washed out. A similar pattern was seen with the 5 min, 3 microM glutamate stimulus. The effects of CGP‐37157 are consistent with the release of substantial mitochondrial Ca2+ stores during recovery from an intense glutamate stimulus. 4. These studies suggest that mitochondria become progressively more important for buffering glutamate‐induced Ca2+ loads as the stimulus intensity increases. The recovery of [Ca2+]i to baseline following glutamate removal is critically regulated by the release of Ca2+ from mitochondrial stores via mitochondrial Na(+)‐Ca2+ exchange. The data highlight a previously under‐appreciated role for [Na+]i in the regulation of [Ca2+]i in central neurones.

Tricyclic antidepressants block N-methyl-D-aspartate receptors: similarities to the action of zinc

1 Using the radioligand [3 H]‐MK801, we have examined drug interactions with the phencyclidine recognition site of the N‐methyl‐D‐aspartate receptor. 2 The trycyclic antidepressants desmethylimipramine and imipramine inhibited [3 H]‐MK801 binding with IC50 values of 7.4 and 22.5 μM, respectively. Other related tricyclic antidepressants and neuroleptics were also effective but less potent. 3 Desmethylimipramine, imipramine and chlorimipramine slowed the dissociation rate of [3 H]‐MK801 in a similar manner to Zn2+. Phencyclidine and related compounds had no effect on the dissociation rate of [3 H]‐MK801. 4 Desmethylimipramine, imipramine and ketamine also prevented the Ca2+ influx into cultured cortical neurones of the rat produced by N‐methyl‐D‐aspartate. 5 As the actions of tricyclic antidepressants in this system are not competitive with respect to N‐methyl‐D‐aspartate, glycine or MK‐801, and as they slow the dissociation of [3 H]‐MK801, we conclude that tricyclic antidepressants may be acting at the Zn2+ recognition site on the N‐methyl‐D‐aspartate receptor.