The role of intracellular Na+ and mitochondria in buffering of kainate-induced intracellular free Ca2+ changes in rat forebrain neurones

Molecular Mechanisms in Hippocampus Involved on Object Recognition Memory Consolidation and Reconsolidation

Acquired information is stabilized into long-term memory through a process known as consolidation. Though, after consolidation, when stored information is retrieved they can be again susceptible, allowing modification, updating and strengthening and to be re-stabilized they need a new process referred to as memory reconsolidation. However, the molecular mechanisms of recognition memory consolidation and reconsolidation are not fully understood. Also, considering that the study of the link between synaptic proteins is key to understanding of memory processes, we investigated, in male Wistar rats, molecular mechanisms in the hippocampus involved on object recognition memory (ORM) consolidation and reconsolidation. We verified that the blockade of AMPA receptors (AMPAr) and L-VDCCs calcium channels impaired ORM consolidation and reconsolidation when administered into CA1 immediately after sample phase or reactivation phase and that these impairments were blocked by the administration of AMPAr agonist and of neurotrophin BDNF. Also, the blockade of CaMKII impaired ORM consolidation when administered 3 h after sample phase but had no effect on ORM reconsolidation and its effect was blocked by the administration of BDNF, but not of AMPAr agonist. So, this study provides new evidence of the molecular mechanisms involved on the consolidation and reconsolidation of ORM, demonstrating that AMPAr and L-VDCCs are necessary for the consolidation and reconsolidation of ORM while CaMKII is necessary only for the consolidation and also that there is a link between BDNF and AMPAr, L-VDCCs and CaMKII as well as a link between AMPAr and L-VDCCs on ORM consolidation and reconsolidation.

Natural Product Inspired Novel Indole based Chiral Scaffold Kills Human Malaria Parasites via Ionic Imbalance Mediated Cell Death

Natural products offer an abundant source of diverse novel scaffolds that inspires development of next generation anti-malarials. With this vision, a library of scaffolds inspired by natural biologically active alkaloids was synthesized from chiral bicyclic lactams with steps/scaffold ratio of 1.7:1. On evaluation of library of scaffolds for their growth inhibitory effect against malaria parasite we found one scaffold with IC50 in low micro molar range. It inhibited parasite growth via disruption of Na+ homeostasis. P-type ATPase, PfATP4 is responsible for maintaining parasite Na+ homeostasis and is a good target for anti-malarials. Molecular docking with our scaffold showed that it fits well in the binding pocket of PfATP4. Moreover, inhibition of Na+-dependent ATPase activity by our potent scaffold suggests that it targets parasite by inhibiting PfATP4, leading to ionic imbalance. However how ionic imbalance attributes to parasite's death is unclear. We show that ionic imbalance caused by scaffold 7 induces autophagy that leads to onset of apoptosis in the parasite evident by the loss of mitochondrial membrane potential (ΔΨm) and DNA degradation. Our study provides a novel strategy for drug discovery and an insight into the molecular mechanism of ionic imbalance mediated death in malaria parasite.

Carisbamate blockade of T-type voltage-gated calcium channels

OBJECTIVES

Carisbamate (CRS) is a novel monocarbamate compound that possesses antiseizure and neuroprotective properties. However, the mechanisms underlying these actions remain unclear. Here, we tested both direct and indirect effects of CRS on several cellular systems that regulate intracellular calcium concentration [Ca²⁺ ]_i .

METHODS

We used a combination of cellular electrophysiologic techniques, as well as cell viability, Store Overload-Induced Calcium Release (SOICR), and mitochondrial functional assays to determine whether CRS might affect [Ca²⁺ ]_i levels through actions on the endoplasmic reticulum (ER), mitochondria, and/or T-type voltage-gated Ca²⁺ channels.

RESULTS

In CA3 pyramidal neurons, kainic acid induced significant elevations in [Ca²⁺ ]_i and long-lasting neuronal hyperexcitability, both of which were reversed in a dose-dependent manner by CRS. Similarly, CRS suppressed spontaneous rhythmic epileptiform activity in hippocampal slices exposed to zero-Mg²⁺ or 4-aminopyridine. Treatment with CRS also protected murine hippocampal HT-22 cells against excitotoxic injury with glutamate, and this was accompanied by a reduction in [Ca²⁺ ]_i . Neither kainic acid nor CRS alone altered the mitochondrial membrane potential (ΔΨ) in intact, acutely isolated mitochondria. In addition, CRS did not affect mitochondrial respiratory chain activity, Ca²⁺ -induced mitochondrial permeability transition, and Ca²⁺ release from the ER. However, CRS significantly decreased Ca²⁺ flux in human embryonic kidney tsA-201 cells transfected with Ca_v 3.1 (voltage-dependent T-type Ca²⁺ ) channels.

SIGNIFICANCE

Our data indicate that the neuroprotective and antiseizure activity of CRS likely results in part from decreased [Ca²⁺ ]_i accumulation through blockade of T-type Ca²⁺ channels.

Correlation between afferent rearrangements and behavioral deficits after local excitotoxic insult in the mammalian vestibule: a rat model of vertigo symptoms

Damage to inner ear afferent terminals is believed to result in many auditory and vestibular dysfunctions. The sequence of afferent injuries and repair, as well as their correlation with vertigo symptoms, remains poorly documented. In particular, information on the changes that take place at the primary vestibular endings during the first hours following a selective insult is lacking. In the present study, we combined histological analysis with behavioral assessments of vestibular function in a rat model of unilateral vestibular excitotoxic insult. Excitotoxicity resulted in an immediate but transient alteration of the balance function that was resolved within a week. Concomitantly, vestibular primary afferents underwent a sequence of structural changes followed by spontaneous repair. Within the first two hours after the insult, a first phase of pronounced vestibular dysfunction coincided with extensive swelling of afferent terminals. In the next 24 h, a second phase of significant but incomplete reduction of the vestibular dysfunction was accompanied by a resorption of swollen terminals and fiber retraction. Eventually, within 1 week, a third phase of complete balance restoration occurred. The slow and progressive withdrawal of the balance dysfunction correlated with full reconstitution of nerve terminals. Competitive re-innervation by afferent and efferent terminals that mimicked developmental synaptogenesis resulted in full re-afferentation of the sensory epithelia. By deciphering the sequence of structural alterations that occur in the vestibule during selective excitotoxic impairment, this study offers new understanding of how a vestibular insult develops in the vestibule and how it governs the heterogeneity of vertigo symptoms.

Summary: Early sequence of afferent injury and repair in vestibular sensory epithelium that correlates with balance disorders and functional restoration is detailed in a rodent model of excitotoxicity.

The effect of aging-associated impaired mitochondrial status on kainate-evoked hippocampal gamma oscillations

Oscillations in hippocampal neuronal networks in the gamma frequency band have been implicated in various cognitive tasks and we showed previously that aging reduces the power of such oscillations. Here, using submerged hippocampal slices allowing simultaneous electrophysiological recordings and imaging, we studied the correlation between the kainate-evoked gamma oscillation and mitochondrial activity, as monitored by rhodamine 123. We show that the initiation of kainate-evoked gamma oscillations induces mitochondrial depolarization, indicating a metabolic response. Aging had an opposite effect on these parameters: while depressing the gamma oscillation strength, it increases mitochondrial depolarization. Also, in the aged neurons, kainate induced significantly larger Ca2+ signals. In younger slices, acute mitochondrial depolarization induced by low concentrations of mitochondrial protonophores strongly, but reversibly, inhibits gamma oscillations. These data indicating that the complex network activity required by the maintenance of gamma activity is susceptible to changes and modulations in mitochondrial status.

Molecular identity and functional properties of the mitochondrial Na+/Ca2+ exchanger

The mitochondrial membrane potential that powers the generation of ATP also facilitates mitochondrial Ca(2+) shuttling. This process is fundamental to a wide range of cellular activities, as it regulates ATP production, shapes cytosolic and endoplasmic recticulum Ca(2+) signaling, and determines cell fate. Mitochondrial Ca(2+) transport is mediated primarily by two major transporters: a Ca(2+) uniporter that mediates Ca(2+) uptake and a Na(+)/Ca(2+) exchanger that subsequently extrudes mitochondrial Ca(2+). In this minireview, we focus on the specific role of the mitochondrial Na(+)/Ca(2+) exchanger and describe its ion exchange mechanism, regulation by ions, and putative partner proteins. We discuss the recent molecular identification of the mitochondrial exchanger and how its activity is linked to physiological and pathophysiological processes.

AP-1 inhibitory peptides attenuate in vitro cortical neuronal cell death induced by kainic acid

This study has assessed the neuroprotective efficacy of five AP-1 inhibitory peptides in an in vitro excitotoxicity model. The five AP-1 inhibitory peptides and controls of the JNK inhibitor peptide (JNKI-1D-TAT) and TAT cell-penetrating-peptide were administered to primary cortical neuronal cultures prior to kainic acid exposure. All five AP-1 inhibitory peptides and JNKI-1D-TAT provided significant neuroprotection from kainic acid induced neuronal cell death. Kainic acid exposure induced caspase and calpain activation in neuronal cultures, with caspase-induced cleavage of α-fodrin reduced by administration of the AP-1 inhibitory peptides. Sequence analysis of the AP-1 inhibitory peptides did not reveal the presence of any secondary structures; however two peptides shared 66% amino-acid sequence homology. As a result, truncated sequences were designed and synthesised to identify the active region of the peptides. All truncated peptides were significantly neuroprotective following kainic acid and glutamate exposure. We have shown for the first time the neuroprotective efficacy of full-length and truncated AP-1 inhibitory peptides in kainic acid and glutamate neuronal excitotoxicity models. The identification of therapeutic targets, such as the AP-1 complex, is an important step for the development of pharmaceuticals to reduce neuronal loss in disorders with a prevalence of excitotoxic cell death such as epilepsy, cerebral ischaemia, and traumatic brain injury.

Positive AMPA receptor modulation rapidly stimulates BDNF release and increases dendritic mRNA translation

Brain-derived neurotrophic factor (BDNF) stimulates local dendritic mRNA translation and is involved in formation and consolidation of memory. 2H,3H,6aH-pyrrolidino[2'',1''-3',2']1,3-oxazino[6',5'-5,4]-benzo[e]1,4-dioxan-10-one (CX614), one of the best-studied positive AMPA receptor modulators (also known as ampakines), increases BDNF mRNA and protein and facilitates long-term potentiation (LTP) induction. Several other ampakines also improve performance in various behavioral and learning tasks. Since local dendritic protein synthesis has been implicated in LTP stabilization and in memory consolidation, this study investigated whether CX614 could influence synaptic plasticity by upregulating dendritic protein translation. CX614 treatment of primary neuronal cultures and acute hippocampal slices rapidly activated the translation machinery and increased local dendritic protein synthesis. CX614-induced activation of translation was blocked by K252a [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester], CNQX, APV, and TTX, and was inhibited in the presence of an extracellular BDNF scavenger, TrkB-Fc. The acute effect of CX614 on translation was mediated by increased BDNF release as demonstrated with a BDNF scavenging assay using TrkB-Fc during CX614 treatment of cultured primary neurons and was blocked by nifedipine, ryanodine, and lack of extracellular Ca(2+) in acute hippocampal slices. Finally, CX614, like BDNF, rapidly increased dendritic translation of an exogenous translation reporter. Together, our results demonstrate that positive modulation of AMPA receptors rapidly stimulates dendritic translation, an effect mediated by BDNF secretion and TrkB receptor activation. They also suggest that increased BDNF secretion and stimulation of local protein synthesis contribute to the effects of ampakines on synaptic plasticity.

Ion transporters and ischemic mitochondrial dysfunction

Ischemia-induced ionic imbalance leads to the activation of numerous events including mitochondrial dysfunction and eventual cell death. Dysregulation of mitochondrial Ca(2+) (Ca(2+)(m)) plays a critical role in cell damage under pathological conditions including traumatic brain injury and stroke. High Ca(2+)(m) levels can induce the persistent opening of the mitochondrial permeability transition pore and trigger mitochondrial membrane depolarization, Ca(2+) release, cessation of oxidative phosphorylation, matrix swelling and eventually outer membrane rupture with release of cytochrome c and other apoptogenic proteins. Thus, the dysregulation of mitochondrial Ca(2+) homeostasis is now recognized to play a crucial role in triggering mitochondrial dysfunction and subsequent apoptosis. Recent studies show that some secondary active transport proteins, such as Na(+)-dependent chloride transporter and Na(+)/Ca(2+) exchanger, contribute to ischemia-induced dissipation of ion homeostasis including Ca(2+)(m).

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.

Localized loss of Ca2+ homeostasis in neuronal dendrites is a downstream consequence of metabolic compromise during extended NMDA exposures

Excessive Ca(2+) loading is central to most hypotheses of excitotoxic neuronal damage. We examined dendritic Ca(2+) signals in single CA1 neurons, injected with fluorescent indicators, after extended exposures to a low concentration of NMDA (5 microM). As shown previously, NMDA produces an initial transient Ca(2+) elevation of several micromolar, followed by recovery to submicromolar levels. Then after a delay of approximately 20-40 min, a large Ca(2+) elevation appears in apical dendrites and propagates to the soma. We show here that this large delayed Ca(2+) increase is required for ultimate loss of membrane integrity. However, transient removal of extracellular Ca(2+) for varying epochs before and after NMDA exposure does not delay the propagation of these events. In contrast to compound Ca(2+) elevations, intracellular Na(+) elevations are monophasic and were promptly reversed by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. MK-801 applied after the transient Ca(2+) elevations blocked the delayed propagating Ca(2+) increase. Even if applied after the propagating response was visualized, MK-801 restored resting Ca(2+) levels. Propagating Ca(2+) increases in dendrites were delayed or prevented by (1) reducing extracellular Na(+), (2) injecting ATP together with the Ca(2+) indicator, or (3) provision of exogenous pyruvate. These results show that extended NMDA exposure initiates degenerative signaling generally in apical dendrites. Although very high Ca(2+) levels can report the progression of these responses, Ca(2+) itself may not be required for the propagation of degenerative signaling along dendrites. In contrast, metabolic consequences of sustained Na(+) elevations may lead to failure of ionic homeostasis in dendrites and precede Ca(2+)-dependent cellular compromise.

Heat shock protein 90beta1 is essential for polyunsaturated fatty acid-induced mitochondrial Ca2+ efflux

Nonesterified fatty acids may influence mitochondrial function by alterations in gene expression, metabolism, and/or mitochondrial Ca(2+) ([Ca(2+)](m)) homeostasis. We have previously reported that polyunsaturated fatty acids induce Ca(2+) efflux from mitochondria, an action that may deplete [Ca(2+)](m) and thus contribute to nonesterified fatty acid-responsive mitochondrial dysfunction. Here we show that the chaperone protein heat shock protein 90 beta1 (hsp90beta1) is required for polyunsaturated fatty acid-induced mitochondrial Ca(2+) efflux (PIMCE). Retinoic acid induced differentiation of human teratocarcinoma NT2 cells in association with attenuation of PIMCE. Proteomic analysis of mitochondrial proteins revealed that hsp90beta1, among other proteins, was reduced in retinoic acid-differentiated cells. Blockade of PIMCE in NT2 cells by 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin, a known inhibitor of the chaperone activity of hsp90, and hsp90beta1 RNA interference demonstrated that hsp90beta1 is essential for PIMCE. We also show localization of hsp90beta1 in mitochondria by Western blot and immunofluorescence. Distinctive effects of inhibitors binding to the N or C terminus of hsp90 on PIMCE in isolated mitochondria suggested that the C terminus of hsp90beta1 plays a critical role in PIMCE.

Role of mitochondria in kainate-induced fast Ca2+ transients in cultured spinal motor neurons

Motor neuron death in amyotrophic lateral sclerosis (ALS) has been linked to selective vulnerability towards AMPA receptor-mediated excitotoxicity. We investigated intracellular mechanisms leading to impairment of motor neuron Ca2+ homeostasis with near physiological AMPA receptor activation. Using fast solution exchange on patch-clamped cultured neurons, kainate (KA) was applied for 2s. This induced a transient increase in the cytosolic Ca2+ concentration ([Ca2+]c) for seconds. Inhibition of the mitochondrial uniporter by RU-360 abolished the decay of the Ca2+ transient and caused immediate [Ca2+]c overload. Repetitive short KA stimulation caused a slowing of the decay of the Ca2+ transient and a gradual increase in peak and baseline [Ca2+]c in motor neurons, but not in other neurons, indicating saturation of the mitochondrial buffer. Furthermore, mitochondrial density was lower in motor neurons and, in a network of neurons with physiological synaptic AMPA receptor input, RU-360 acutely induced an increase in Ca2+ transients. We conclude that motor neurons have an insufficient mitochondrial capacity to buffer large Ca2+ elevations which is partly due to a reduced mitochondrial density per volume compared to non-motor neurons. This may exert deleterious effects in motor neuron disease where mitochondrial function is thought to be compromised.

Normal brain ageing: models and mechanisms

Normal ageing is associated with a degree of decline in a number of cognitive functions. Apart from the issues raised by the current attempts to expand the lifespan, understanding the mechanisms and the detailed metabolic interactions involved in the process of normal neuronal ageing continues to be a challenge. One model, supported by a significant amount of experimental evidence, views the cellular ageing as a metabolic state characterized by an altered function of the metabolic triad: mitochondria-reactive oxygen species (ROS)-intracellular Ca2+. The perturbation in the relationship between the members of this metabolic triad generate a state of decreased homeostatic reserve, in which the aged neurons could maintain adequate function during normal activity, as demonstrated by the fact that normal ageing is not associated with widespread neuronal loss, but become increasingly vulnerable to the effects of excessive metabolic loads, usually associated with trauma, ischaemia or neurodegenerative processes. This review will concentrate on some of the evidence showing altered mitochondrial function with ageing and also discuss some of the functional consequences that would result from such events, such as alterations in mitochondrial Ca2+ homeostasis, ATP production and generation of ROS.

Decreased neuronal death in Na+/H+ exchanger isoform 1-null mice after in vitro and in vivo ischemia

Na+/H+ exchanger isoform 1 (NHE1) is a major acid extrusion mechanism after intracellular acidosis. We hypothesized that stimulation of NHE1 after cerebral ischemia contributes to the disruption of Na+ homeostasis and neuronal death. In the present study, expression of NHE1 was detected in cultured mouse cortical neurons. Three hours of oxygen and glucose deprivation (OGD) followed by 21 h of reoxygenation (REOX) led to 68 +/- 10% cell death. Inhibition of NHE1 with the potent inhibitor cariporide (HOE 642) or genetic ablation of NHE1 reduced OGD-induced cell death by approximately 40-50% (p < 0.05). In NHE1(+/+) neurons, OGD caused a twofold increase in [Na+]i, and 60 min REOX triggered a sevenfold increase. Genetic ablation of NHE1 or HOE 642 treatment had no effects on the OGD-mediated initial Na+(i) rise but reduced the second phase of Na+(i) rise by approximately 40-50%. In addition, 60 min REOX evoked a 1.5-fold increase in [Ca2+]i in NHE1(+/+) neurons, which was abolished by inhibition of either NHE1 or reverse-mode operation of Na+/Ca2+ exchange. OGD/REOX-mediated mitochondrial Ca2+ accumulation and cytochrome c release were attenuated by inhibition of NHE1 activity. In an in vivo focal ischemic model, 2 h of left middle cerebral artery occlusion followed by 24 h of reperfusion induced 84.8 +/- 8.0 mm3 infarction in NHE1(+/+) mice. NHE1(+/+) mice treated with HOE 642 or NHE1 heterozygous mice exhibited a approximately 33% decrease in infarct size (p < 0.05). These results imply that NHE1 activity disrupts Na+ and Ca2+ homeostasis and contributes to ischemic neuronal damage.

Inhibition of Na+/H+ exchanger isoform 1 attenuates mitochondrial cytochrome C release in cortical neurons following in vitro ischemia

Na+/H+ exchanger isoform 1 (NHE1) is a major acid extrusion mechanism following intracellular acidosis. We hypothesized that stimulation of NHE1 after cerebral ischemia contributes to disruption of Na+ homeostasis and neuronal death. In the present study, expression of NHE1 was detected in cultured mouse cortical neurons. Oxygen and glucose deprivation (OGD) for 3 hours followed by 21 hours of reoxygenation (REOX) led to 68 +/- 10% cell death. Inhibition of NHE1 with the potent inhibitor HOE 642 or genetic ablation of NHE1 reduced OGD-induced cell death by approximately 40% to 50% (p < 0.05). In NHE1 +/+ neurons, OGD/REOX triggered significant increases in Na+ and Ca(i)2+. Genetic ablation of NHE1 and HOE 642 treatment reduced the rise of Na(i)+ by approximately 40% to 50% and abolished the OGD/REOX-mediated Ca2+ accumulation. Moreover, mitochondrial cytochrome C release was significantly attenuated by inhibition of NHE1 activity. These results imply that NHE1 activity disrupts Na+ and Ca2+ homeostasis and contributes to ischemic neuronal damage.

Simultaneous detection of intracellular free calcium and zinc using fura-2FF and FluoZin-3

Elevation of intracellular free zinc ([Zn2+]i) probably contributes to cell death in injury paradigms involving calcium deregulation and oxidative stress such as glutamate excitotoxicity. However, it is difficult to monitor both ions simultaneously in live cells. Here we present a new method using fluorescence microscopy and the ion sensitive indicators fura-2FF and FluoZin-3 to monitor both [Ca2+]i and [Zn2+]i in primary cortical neurons. We show that the new single wavelength dye FluoZin-3 responds robustly to small zinc loads, is insensitive to high Ca2+ or Mg2+, and is relatively unaffected by low pH or oxidants. The ratiometric indicator fura-2FF is sensitive to both Ca2+ and Zn2+. However, in conditions analogous to excitotoxic glutamate exposure where [Ca2+]i is high relative to [Zn2+]i, we found that fura-2FF responds mostly to [Ca2+]i but is relatively unaffected by low [Zn2+]i. Moreover, fura-2FF ratio changes caused by high [Ca2+]i or high [Zn2+]i could be distinguished because each ion produces a different spectral response. Finally, dual dye experiments showed that FluoZin-3 and fura-2FF respond robustly to [Zn2+]i and [Ca2+]j, respectively, in the same neurons during intense glutamate exposure. These studies provide a novel method for the simultaneous detection of both calcium and zinc in cells.

Prolonged positive modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors induces calpain-mediated PSD-95/Dlg/ZO-1 protein degradation and AMPA receptor down-regulation in cultured hippocampal slices

Prolonged exposure of cultured hippocampal slices to CX614 [2H,3H,6aH-pyrrolidino[2'',1''-3',2']1,3-oxazino[6',5'-5,4]-benzo[e]1,4-dioxan 10-one], a positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAr) modulator, decreases receptor response to synaptic stimulation, an effect that could reflect reduced receptor expression. The present study investigates this down-regulation and its underlying mechanisms using cultured rat hippocampal slices. Chronic treatment with CX614 gradually reduced levels of glutamate receptor (GluR)1 and GluR2/3 AMPAr subunits and of their anchoring proteins synapse-associated protein 97 (SAP97) and glutamate receptor interacting protein 1 (GRIP1) through 48 h. Decline in SAP97 and GRIP1 levels was associated with increased abundance of lower molecular weight bands, suggesting degradation of these proteins. CX614 effects were partially reversible after drug removal. GluR1 and GluR2/3 down-regulation and their slow recovery were associated with similar changes in SAP97 and GRIP1 levels. Treatment with CX614 for 48 h significantly reduced AMPAr mRNA levels in hippocampus, whereas 8-h exposure did not. Blockade of ionotropic glutamate receptors prevented CX614-induced decrease in AMPAr subunits and mRNA, with regional selectivity, although an AMPAr blocker was more efficacious than an N-methyl-D-aspartate receptor blocker. Blockade of calpain activity reduced CX614-induced degradation of SAP97 and GRIP1 and prevented decreases in AMPAr subunit but not mRNA levels. Treatment with CX614 alone or in combination with glutamate receptor blockers or calpain inhibitor III did not modify lactate dehydrogenase release into culture medium, implying the absence of cell toxicity. We conclude that CX614-induced AMPAr protein loss is primarily mediated by AMPAr activation and involves calpain-dependent proteolysis of SAP97 and GRIP1. CX614-induced suppression of AMPAr gene expression is, however, calpain-independent, and all these effects are not associated with cell damage.

Involvement of mitochondrial Na+-Ca2+ exchange in intracellular Ca2+ increase induced by ATP in PC12 cells

The involvement of mitochondrial Na+-Ca2+ exchange in Ca2+ responses to ATP was examined in rat pheochromocytoma (PC) 12 cells. Intracellular Ca2+ ([Ca2+]i) and Na+ concentrations ([Na+]i) were measured using fura-2 and SBFI, respectively. ATP caused concentration-dependent increases in [Ca2+]i and [Na+]i. High concentrations of ATP elicited a Ca2+ transient followed by a slow recovery of [Ca2+]i (a sustained phase) in 77% of PC12 cells. The sustained phase of Ca2+ response appeared only when the peak Ca2+ transient exceeded 500 nM. FCCP, a protonophore, greatly enhanced Ca2+ responses to ATP only in cells with the sustained phase but not without this phase. The sustained phase was decreased by clonazepam and CGP37157, mitochondrial Na+-Ca2+ exchange inhibitors, and extracellular Na+ removal but not by cyclosporin A, an inhibitor of permeability transition pores. The reintroduction of Na+ 3.5 min after ATP stimulation in the absence of Na+ caused Na+ concentration-dependent increases in [Ca2+]i and [Na+]i. The increase in [Na+]i was correlated with that in [Ca2+]i. FCCP caused a great increase in [Ca2+]i 4.5 min after ATP stimulation in the absence of extracellular Na+ but not in its presence, indicating that mitochondria retain Ca2+ in the absence of Na+. These results suggest that ATP causes a large increase in [Ca2+]i which was sequestered in mitochondria and that the sustained phase of Ca2+ response to ATP are mainly due to the release of mitochondrial Ca2+ through Na+-Ca2+ exchangers in PC12 cells.

Calcium mobilization from mitochondria in synaptic transmitter release

Mitochondria can rapidly accumulate and release Ca²⁺ upon cell stimulation. A paper by Yang and coworkers in this issue reports an unusual form of synaptic potentiation, dependent on Ca²⁺ release from mitochondria through the Na⁺/Ca²⁺ exchanger and triggered by Na⁺ entry through voltage-gated channels (Yang et al., 2003).

Effects of commercial antiglaucoma drugs to glutamate-induced [Ca2+)]i increase in cultured neuroblastoma cells

Over releasing of glutamate and cellular calcium influx always results in neuronal death. In the present study, we investigated various commercial antiglaucoma drugs including timolol (0.58 microM to 58 microM), betaxolol (1.62 microM to 162 microM), carteolol (6.8 microM to 680 microM), pilocarpine (4.08 microM to 408 microM), latanoprost (0.01 microM to 1.1 microM), dorzolamide (6.16 microM to 616 microM), brinzolamide (2.6 microM to 260 microM), brimonidine (0.68 microM to 68 microM), dipivefrin (0.28 microM to 28 microM) and preservative benzalkonium chloride on their effects to inhibit glutamate-induced intracellular free Ca(2+) ([Ca(2+)](i)) increase in cultured N1E-115 neuroblastoma cells. These drugs were diluted from original concentrations to 1/100, 1/1000 and 1/10000. The [Ca(2+)](i) mobility was studied after loading with fura-2-AM and analyzed by spectrofluorometry. It was found that betaxolol, dipivefrin and brimonidine have remarkable effects not only to inhibit the glutamate-induced [Ca(2+)](i) increase but also to decrease the basal [Ca(2+)](i). In the case of other drugs, only high concentration of timolol (58 microM) exhibited significant effect to completely prevent glutamate-induced [Ca(2+)](i) increase. Moreover, benzalkonium chloride did not exhibit any inhibitive effect. These results indicate that betaxolol, dipivefrin and brimonidine may have neuroprotective effects to inhibit the glutamate-induced over Ca(2+) influx damage.

Role of mitochondria in Ca(2+) oscillations and shape of Ca(2+) signals in pancreatic acinar cells

We studied the role of mitochondria in Ca(2+) signals in fura-2 loaded exocrine pancreatic acinar cells. Mitochondrial depolarization in response to carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone or rotenone (assessed by confocal microscopy using rhodamine-123) induced a partial but statistically significant reduction in the decay of Ca(2+) signals under different experimental conditions. Spreading of Ca(2+) waves evoked by the pancreatic secretagogue cholecystokinin cholecystokinin octapeptide was accelerated by mitochondrial inhibitors, whereas the cytosolic Ca(2+) concentration ([Ca(2+)](i)) oscillations in response to physiological levels of this hormone were suppressed by rotenone and carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone. Oligomycin, an inhibitor of mitochondrial ATP synthase, did no affect either propagation of calcium waves nor [Ca(2+)](i) oscillations. Individual mitochondria of rhod-2 loaded acinar cells showed heterogeneous matrix Ca(2+) concentration increases in response to oscillatory and maximal levels of cholecystokinin octapeptide. On the other hand, using Ba(2+) for unequivocal study of capacitative calcium entry we found that mitochondrial inhibitors did not affect this process. Our results show that although the role of mitochondria as a Ca(2+) clearing system in exocrine cells is quantitatively secondary, they play an essential role in the spatial propagation of Ca(2+) waves and in the development of [Ca(2+)](i) oscillations.

Repolarization of the plasma membrane shapes NMDA-induced cytosolic [Ca2+ ] transients

After inactivation of NMDA receptors, restoration of basal cytosolic [Ca2+] ([Ca2+]c) is delayed. This may be caused by Ca2+ influx via reverse Na/Ca exchange or voltage-gated Ca2+ channels, and/or by Ca2+ efflux from internal stores. Monitoring of [Na+]c, [Ca2+]c, and plasma membrane potential in cultured cerebellar granule cells showed that repolarization of the plasma membrane and inactivation of voltage-gated Ca channels plays the most critical role in restoration of low [Ca2+]c following NMDA receptor inactivation. During NMDA receptor activation, however, an Na-dependent mechanism enhanced NMDA-induced elevation in [Ca2+]c. This mechanism did not involve Na,K-ATPase activation by Na+, because it operated even when Na,K-ATPase was inhibited.

Resistance to NMDA toxicity correlates with appearance of nuclear inclusions, behavioural deficits and changes in calcium homeostasis in mice transgenic for exon 1 of the huntington gene

Transgenic Huntington's disease (HD) mice, expressing exon 1 of the human HD gene (lines R6/1 and R6/2), are totally resistant to striatal lesions caused by the NMDA receptor agonist quinolinic acid (QA). Here we show that this resistance develops gradually over time in both R6/1 and R6/2 mice, and that it occurred earlier in R6/2 (CAG-155) than in R6/1 (CAG-115) mice. The development of the resistance coincided with the appearance of nuclear inclusions and with the onset of motor deficits. In the HD mice, hippocampal neurons were also resistant to QA, especially in the CA1 region. Importantly, there was no change in susceptibility to QA in transgenic mice with a normal CAG repeat (CAG-18). R6/1 mice were also resistant to NMDA-, but not to AMPA-induced striatal damage. Interestingly, QA-induced current and calcium influx in striatal R6/2 neurons were not decreased. However, R6/2 neurons had a better capacity to handle cytoplasmic calcium ([Ca2+]c) overload following QA and could avoid [Ca2+]c deregulation and cell lysis. In addition, basal [Ca2+]c levels were increased five-fold in striatal R6/2 neurons. This might cause an adaptation of R6 neurons to excitotoxic stress resulting in an up-regulation of defense mechanisms, including an increased capacity to handle [Ca2+]c overload. However, the increased level of basal [Ca2+]c in the HD mice might also disturb intracellular signalling in striatal neurons and thereby cause neuronal dysfunction and behavioural deficits.

Spontaneous changes in mitochondrial membrane potential in cultured neurons

Using the mitochondrial membrane potential (DeltaPsi(m))-sensitive fluorescent dyes 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1) and tetramethylrhodamine methyl ester (TMRM), we have observed spontaneous changes in the DeltaPsi(m) of cultured forebrain neurons. These fluctuations in DeltaPsi(m) appear to represent partial, transient depolarizations of individual mitochondria. The frequency of these DeltaPsi(m) fluctuations can be significantly lowered by exposure to a photo-induced oxidant burden, an ATP synthase inhibitor, or a glutamate-induced sodium load, without changing overall JC-1 fluorescence intensity. These spontaneous fluctuations in JC-1 signal were not inhibited by altering plasma membrane activity with tetrodotoxin or MK-801 or by blocking the mitochondrial permeability transition pore (PTP) with cyclosporin A. Neurons loaded with TMRM showed similar, low-amplitude, spontaneous fluctuations in DeltaPsi(m). We hypothesize that these DeltaPsi(m) fluctuations are dependent on the proper functioning of the mitochondria and reflect mitochondria alternating between the active and inactive states of oxidative phosphorylation.

Spontaneous changes in mitochondrial membrane potential in cultured neurons

Using the mitochondrial membrane potential (DeltaPsi(m))-sensitive fluorescent dyes 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1) and tetramethylrhodamine methyl ester (TMRM), we have observed spontaneous changes in the DeltaPsi(m) of cultured forebrain neurons. These fluctuations in DeltaPsi(m) appear to represent partial, transient depolarizations of individual mitochondria. The frequency of these DeltaPsi(m) fluctuations can be significantly lowered by exposure to a photo-induced oxidant burden, an ATP synthase inhibitor, or a glutamate-induced sodium load, without changing overall JC-1 fluorescence intensity. These spontaneous fluctuations in JC-1 signal were not inhibited by altering plasma membrane activity with tetrodotoxin or MK-801 or by blocking the mitochondrial permeability transition pore (PTP) with cyclosporin A. Neurons loaded with TMRM showed similar, low-amplitude, spontaneous fluctuations in DeltaPsi(m). We hypothesize that these DeltaPsi(m) fluctuations are dependent on the proper functioning of the mitochondria and reflect mitochondria alternating between the active and inactive states of oxidative phosphorylation.

Mitochondria control ampa/kainate receptor-induced cytoplasmic calcium deregulation in rat cerebellar granule cells

Although mitochondria mediate the delayed failure of cytoplasmic Ca(2+) homeostasis [delayed Ca(2+) deregulation (DCD)] in rat cerebellar granule cells resulting from chronic activation of NMDA receptors, their role in AMPA/KA-induced DCD remains to be established. The mitochondrial ATP synthase inhibitor oligomycin protected cells against KA- but not NMDA-evoked DCD. In contrast to NMDA-evoked DCD, no additional protection was afforded by the further addition of rotenone. The effects of KA on cytoplasmic Ca(2+) homeostasis, including the protection afforded by oligomycin, could be reproduced by veratridine. KA exposure induced a partial mitochondrial depolarization that was enhanced by oligomycin, indicating ATP synthase reversal. The nonglycolytic substrates pyruvate and lactate were unable to maintain Ca(2+) homeostasis in the presence of KA. In contrast to NMDA, KA exposure did not cause mitochondrial Ca(2+) loading. The data indicate that Na(+) entry via noninactivating AMPA/KA receptors or voltage-activated Na(+) channels compromises mitochondrial function sufficiently to cause ATP synthase reversal. Oligomycin may protect by preventing the consequent mitochondrial drain of cytoplasmic ATP.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Mitochondria control ampa/kainate receptor-induced cytoplasmic calcium deregulation in rat cerebellar granule cells

Although mitochondria mediate the delayed failure of cytoplasmic Ca(2+) homeostasis [delayed Ca(2+) deregulation (DCD)] in rat cerebellar granule cells resulting from chronic activation of NMDA receptors, their role in AMPA/KA-induced DCD remains to be established. The mitochondrial ATP synthase inhibitor oligomycin protected cells against KA- but not NMDA-evoked DCD. In contrast to NMDA-evoked DCD, no additional protection was afforded by the further addition of rotenone. The effects of KA on cytoplasmic Ca(2+) homeostasis, including the protection afforded by oligomycin, could be reproduced by veratridine. KA exposure induced a partial mitochondrial depolarization that was enhanced by oligomycin, indicating ATP synthase reversal. The nonglycolytic substrates pyruvate and lactate were unable to maintain Ca(2+) homeostasis in the presence of KA. In contrast to NMDA, KA exposure did not cause mitochondrial Ca(2+) loading. The data indicate that Na(+) entry via noninactivating AMPA/KA receptors or voltage-activated Na(+) channels compromises mitochondrial function sufficiently to cause ATP synthase reversal. Oligomycin may protect by preventing the consequent mitochondrial drain of cytoplasmic ATP.

Stimulation-evoked increases in cytosolic [Ca(2+)] in mouse motor nerve terminals are limited by mitochondrial uptake and are temperature-dependent

Increases in cytosolic [Ca(2+)] evoked by trains of action potentials (20-100 Hz) were recorded from mouse and lizard motor nerve terminals filled with a low-affinity fluorescent indicator, Oregon Green BAPTA 5N. In mouse terminals at near-physiological temperatures (30-38 degrees C), trains of action potentials at 25-100 Hz elicited increases in cytosolic [Ca(2+)] that stabilized at plateau levels that increased with stimulation frequency. Depolarization of mitochondria with carbonylcyanide m-chlorophenylhydrazone (CCCP) or antimycin A1 caused cytosolic [Ca(2+)] to rise to much higher levels during stimulation. Thus, mitochondrial Ca(2+) uptake contributes importantly to limiting the rise of cytosolic [Ca(2+)] during repetitive stimulation. In mouse terminals, the stimulation-induced increase in cytosolic [Ca(2+)] was highly temperature-dependent over the range 18-38 degrees C, with greater increases at lower temperatures. At the lower temperatures, application of CCCP continued to depolarize mitochondria but produced a much smaller increase in the cytosolic [Ca(2+)] transient evoked by repetitive stimulation. This result suggests that the larger amplitude of the stimulation-induced cytosolic [Ca(2+)] transient at lower temperatures was attributable in part to reduced mitochondrial Ca(2+) uptake. In contrast, the stimulation-induced increases in cytosolic [Ca(2+)] measured in lizard motor terminals showed little or no temperature-dependence over the range 18-33 degrees C.

Kainate-induced K+ efflux and plasma membrane depolarization in cultured cerebellar granule cells

It is becoming increasingly evident that activation of ionotropic glutamate receptors leads to significant changes in cytosolic [K+] ([K+]c), a major determinant of the plasma membrane (PM) potential, Em. Since Em affects fluxes of key cations, such as Ca2+, it is important to precisely quantify [K+]c and Em in neurons exposed to glutamate receptor agonists. Here we studied the relationships between [K+]c and Em in primary cultures of cerebellar granule cells, and found that kainate elicits a rapid drop in [K+]c below 10 mM. Using patch electrodes containing 10 or 150 mM K+, we determined that kainate depolarizes the PM to -2 or -28 mV, respectively. Therefore, the actual PM depolarization elicited by kainate is much larger than that routinely measured with K+-rich electrodes.

Pharmacological investigation of mitochondrial ca(2+) transport in central neurons: studies with CGP-37157, an inhibitor of the mitochondrial Na(+)-Ca(2+) exchanger

Mitochondria buffer large changes in [Ca(2+)](i)following an excitotoxic glutamate stimulus. Mitochondrial sequestration of [Ca(2+)](i)can beneficially stimulate oxidative metabolism and ATP production. However, Ca(2+)overload may have deleterious effects on mitochondrial function and cell survival, particularly Ca(2+)-dependent production of reactive oxygen species (ROS) by the mitochondria. We recently demonstrated that the mitochondrial Na(+)-Ca(2+)exchanger in neurons is selectively inhibited by CGP-37157, a benzothiazepine analogue of diltiazem. In the present series of experiments we investigated the effects of CGP-37157 on mitochondrial functions regulated by Ca(2+). Our data showed that 25 microM CGP-37157 quenches DCF fluorescence similar to 100 microM glutamate and this effect was enhanced when the two stimuli were applied together. CGP-37157 did not increase ROS generation and did not alter glutamate or 3mM hydrogen-peroxide-induced increases in ROS as measured by DHE fluorescence. CGP-37157 induces a slight decrease in intracellular pH, much less than that of glutamate. In addition, CGP-37157 does not enhance intracellular acidification induced by glutamate. Although it is possible that CGP-37157 can enhance mitochondrial respiration both by blocking Ca(2+)cycling and by elevating intramitochondrial Ca(2+), we did not observe any changes in ATP levels or toxicity either in the presence or absence of glutamate. Finally, mitochondrial Ca(2+)uptake during an excitotoxic glutamate stimulus was only slightly enhanced by inhibition of mitochondrial Ca(2+)efflux. Thus, although CGP-37157 alters mitochondrial Ca(2+)efflux in neurons, the inhibition of Na(+)-Ca(2+)exchange does not profoundly alter glutamate-mediated changes in mitochondrial function or mitochondrial Ca(2+)content.

Stimulation-evoked increases in cytosolic [Ca(2+)] in mouse motor nerve terminals are limited by mitochondrial uptake and are temperature-dependent

Increases in cytosolic [Ca(2+)] evoked by trains of action potentials (20-100 Hz) were recorded from mouse and lizard motor nerve terminals filled with a low-affinity fluorescent indicator, Oregon Green BAPTA 5N. In mouse terminals at near-physiological temperatures (30-38 degrees C), trains of action potentials at 25-100 Hz elicited increases in cytosolic [Ca(2+)] that stabilized at plateau levels that increased with stimulation frequency. Depolarization of mitochondria with carbonylcyanide m-chlorophenylhydrazone (CCCP) or antimycin A1 caused cytosolic [Ca(2+)] to rise to much higher levels during stimulation. Thus, mitochondrial Ca(2+) uptake contributes importantly to limiting the rise of cytosolic [Ca(2+)] during repetitive stimulation. In mouse terminals, the stimulation-induced increase in cytosolic [Ca(2+)] was highly temperature-dependent over the range 18-38 degrees C, with greater increases at lower temperatures. At the lower temperatures, application of CCCP continued to depolarize mitochondria but produced a much smaller increase in the cytosolic [Ca(2+)] transient evoked by repetitive stimulation. This result suggests that the larger amplitude of the stimulation-induced cytosolic [Ca(2+)] transient at lower temperatures was attributable in part to reduced mitochondrial Ca(2+) uptake. In contrast, the stimulation-induced increases in cytosolic [Ca(2+)] measured in lizard motor terminals showed little or no temperature-dependence over the range 18-33 degrees C.

Role played by sodium in activity-dependent secretion of neurotrophins - revisited

In previous experiments, a causal relationship between sodium influx and secretion of nerve growth factor (NGF) was deduced from the observation that the sodium substitute N-methyl-D-glucamine (NMDG) abolished any activity-mediated NGF secretion that depends on intact internal calcium stores. However, all available experimental evidence speaks against sodium-mediated calcium mobilization from these stores under physiological conditions. We now report that rapid sodium influx initiated by monensin or ouabain did not induce brain-derived neurotrophic factor (BDNF) secretion from either native hippocampal slices or BDNF-transduced hippocampal neuronal cultures. Additionally, we found marked differences between the replacement of sodium by NMDG and sucrose on the one hand, and choline and lithium on the other. Replacement of 100% (and as little as 10%) sodium by NMDG or sucrose not only blocked the activity-mediated neurotrophin secretion, but itself led to a rapid and substantial increase of neurotrophin secretion. In contrast, the replacement of sodium (10% and 100%) by lithium and choline did not result in a release of neurotrophins, and only 100% replacement blocked the activity-mediated neurotrophin secretion. We conclude that the blocking effects of NMDG and sucrose on neurotrophin secretion do not reflect the sodium replacement, but instead represent an independent blocking effect. These differences were also reflected in part by electrophysiological investigations in individually patched hippocampal neurons. The importance of the present observations lies not only in the reevaluation of the involvement of sodium in activity-dependent neurotrophin secretion, but also in the demonstration that sodium replacement may initiate 'side effects' that are unrelated to sodium replacement.

Mitochondrial membrane potential and the permeability transition in excitotoxicity

Acute neuronal injury caused by activation of glutamate receptors in neurons, or excitotoxicity, can be triggered by the activation of N-methyl-D-aspartate receptors and the entry of large amounts of Ca2+. Recent studies have suggested that mitochondria have a critical role in the excitotoxicity injury mechanism. Mitochondria accumulate large amounts of Ca2+ following glutamate stimulation, and also generate reactive oxygen species. Moreover, the prevention of mitochondrial Ca2+ accumulation protects neurons from injury. The target for the actions of Ca2+ in the mitochondrial matrix has not yet been established. The permeability transition pore has the characteristics of a mechanism that is well suited to mediate neuronal injury. However, evidence for activation of the permeability transition pore in intact neurons is rather indirect, and these data suffer from some ambiguities that make it difficult to conclude that permeability transition is a critical contributor to mitochondrially mediated neuronal injury.

Ca(2+)- and metabolism-related changes of mitochondrial potential in voltage-clamped CA1 pyramidal neurons in situ

In hippocampal slices from rats, dialysis with rhodamine-123 (Rh-123) and/or fura-2 via the patch electrode allowed monitoring of mitochondrial potential (DeltaPsi) changes and intracellular Ca(2+) ([Ca(2+)](i)) of CA1 pyramidal neurons. Plasmalemmal depolarization to 0 mV caused a mean [Ca(2+)](i) rise of 300 nM and increased Rh-123 fluorescence signal (RFS) by </=50% of control. The evoked RFS, indicating depolarization of DeltaPsi, and the [Ca(2+)](i) transient were abolished by Ca(2+)-free superfusate or exposure of Ni(2+)/Cd(2+). Simultaneous measurements of RFS and [Ca(2+)](i) showed that the kinetics of both the Ca(2+) rise and recovery were considerably faster than those of the DeltaPsi depolarization. The plasmalemmal Ca(2+)/H(+) pump blocker eosin-B potentiated the peak of the depolarization-induced RFS and delayed recovery of both the RFS and [Ca(2+)](i) transient. Thus the DeltaPsi depolarization due to plasmalemmal depolarization is related to mitochondrial Ca(2+) sequestration secondary to Ca(2+) influx through voltage-gated Ca(2+) channels. CN(-) elevated [Ca(2+)](i) by <50 nM but increased RFS by 221% as a result of extensive depolarization of DeltaPsi. Oligomycin decreased RFS by 52% without affecting [Ca(2+)](i). In the presence of oligomycin, CN(-) and p-trifluoromethoxy-phenylhydrazone (FCCP) elevated [Ca(2+)](i) by <50 nM and increased RFS by 285 and 290%, respectively. Accordingly, the metabolism-related DeltaPsi changes are independent of [Ca(2+)](i). Imaging techniques revealed that evoked [Ca(2+)](i) rises are distributed uniformly over the soma and primary dendrites, whereas corresponding changes in RFS occur more localized in subregions within the soma. The results show that microfluorometric measurement of the relation between mitochondrial function and intracellular Ca(2+) is feasible in whole cell recorded mammalian neurons in situ.

The local control of cytosolic Ca2+ as a propagator of CNS communication--integration of mitochondrial transport mechanisms and cellular responses

Ca2+ signals propagate in wave form along individual cells of the central nervous system (CNS) and through networks of connected cells of neuronal and multiple glial cell types. In order for wave fronts to convey information, signaling mechanisms are required that allow waves to propagate reproducibly and without decrement in signal strength over long distances. CNS Ca2+ waves are under specific integrated local control, made possible by interactions at local subcellular microdomains between endoplasmic reticulum and mitochondria. Active mitochondria located near the mouth of inositol trisphosphate receptor (InsP3R) channel clusters in glia take up Ca2+, which may prevent a buildup of Ca2+ around the InsP3R channel, thereby decreasing the rate of Ca2+-induced receptor inactivation, and prolonging channel open time. Mitochondria may amplify InsP3-dependent Ca2+ signals by a transient permeability transition in response to Ca2+ uptake into the mitochondrion. Other evidence suggests privileged access into mitochondria for Ca2+ entering neurons by glutamatergic receptor channels. This enables specific signal modulation as the Ca2+ wave is propagated into neurons, such that mitochondria located close to glutamate channels can prolong the neuronal cytosolic response time by successive uptake and release of Ca2+. Disruption of mitochondrial function deregulates the ability of CNS-derived cells to undergo normal Ca2+ signaling and wave propagation.

AMPA exposures induce mitochondrial Ca(2+) overload and ROS generation in spinal motor neurons in vitro

The reason for the selective vulnerability of motor neurons in amyotrophic lateral sclerosis (ALS) is primarily unknown. A possible factor is the expression by motor neurons of Ca(2+)-permeable AMPA/kainate channels, which may permit rapid Ca(2+) influx in response to synaptic receptor activation. However, other subpopulations of central neurons, most notably forebrain GABAergic interneurons, consistently express large numbers of these channels but do not degenerate in ALS. Indeed, when subjected to identical excitotoxic exposures, motor neurons were more susceptible than GABAergic neurons to AMPA/kainate receptor-mediated neurotoxicity. Microfluorimetric studies were performed to examine the basis for the difference in vulnerability. First, AMPA or kainate exposures appeared to trigger substantial mitochondrial Ca(2+) loading in motor neurons, as indicated by a sharp increase in intracellular Ca(2+) after addition of the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenyl hydrazone (FCCP) after the agonist exposure. The same exposures caused little mitochondrial Ca(2+) accumulation in GABAergic cortical neurons. Subsequent experiments examined other measures of mitochondrial function to compare sequelae of AMPA/kainate receptor activation between these populations. Brief exposure to either AMPA or kainate caused mitochondrial depolarization, assessed using tetramethylrhodamine ethylester, and reactive oxygen species (ROS) generation, assessed using hydroethidine, in motor neurons. However, these effects were only seen in the GABAergic neurons after exposure to the nondesensitizing AMPA receptor agonist kainate. Finally, addition of either antioxidants or toxins (FCCP or CN(-)) that block mitochondrial Ca(2+) uptake attenuated AMPA/kainate receptor-mediated motor neuron injury, suggesting that the mitochondrial Ca(2+) uptake and consequent ROS generation are central to the injury process.

Mitochondrial calcium accumulation following activation of vanilloid (VR1) receptors by capsaicin in dorsal root ganglion neurons

Stimulation of the vanilloid (capsaicin) receptor (VR1), currently viewed as a molecular integrator of chemical and physical noxious stimuli, evoked intracellular Ca2+ transients in a capsaicin-sensitive subpopulation of rat dorsal root ganglion neurons. These were comprised of an initial fast rise (seconds) followed by a long-lasting intracellular Ca2+ recovery (tens of minutes). The rate of intracellular Ca2+ recovery was dependent on the magnitude of intracellular Ca2+ transients. Opening of voltage-operated Ca2+ channels in the same neurons by K+ depolarization evoked intracellular Ca2+ elevation of a similar amplitude and rate of rise; however, the recovery of intracellular Ca2+ to the prestimulated level was significantly faster. A mitochondrial uncoupler (10 microM carbonyl cyanide m-chlorophenylhydrasone) was used to reveal the role of mitochondria in intracellular Ca2+ buffering. Carbonyl cyanide m-chlorophenylhydrasone-evoked elevation in intracellular Ca2+ was greater in neurons previously stimulated with capsaicin compared with KCl. Neither extracellular Ca2+ nor ATP depletion influenced significantly the carbonyl cyanide m-chlorophenylhydrasone-sensitive intracellular Ca2+ elevation in neurons loaded with Ca2+ via vanilloid 1 receptor stimulation. The effects of carbonyl cyanide m-chlorophenylhydrasone suggest that the amount of Ca2+ buffered by mitochondria is greater when extracellular Ca2+ enters the neuron via the vanilloid 1 receptor channel than via voltage-operated Ca2+ channels. The long duration of intracellular Ca2+ decline in neurons stimulated with capsaicin, which depends on the amount of Ca2+ buffered by mitochondria, may reflect a specific mechanism of Ca2+ buffering following activation the pain receptor VR1.

AMPA exposures induce mitochondrial Ca(2+) overload and ROS generation in spinal motor neurons in vitro

The reason for the selective vulnerability of motor neurons in amyotrophic lateral sclerosis (ALS) is primarily unknown. A possible factor is the expression by motor neurons of Ca(2+)-permeable AMPA/kainate channels, which may permit rapid Ca(2+) influx in response to synaptic receptor activation. However, other subpopulations of central neurons, most notably forebrain GABAergic interneurons, consistently express large numbers of these channels but do not degenerate in ALS. Indeed, when subjected to identical excitotoxic exposures, motor neurons were more susceptible than GABAergic neurons to AMPA/kainate receptor-mediated neurotoxicity. Microfluorimetric studies were performed to examine the basis for the difference in vulnerability. First, AMPA or kainate exposures appeared to trigger substantial mitochondrial Ca(2+) loading in motor neurons, as indicated by a sharp increase in intracellular Ca(2+) after addition of the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenyl hydrazone (FCCP) after the agonist exposure. The same exposures caused little mitochondrial Ca(2+) accumulation in GABAergic cortical neurons. Subsequent experiments examined other measures of mitochondrial function to compare sequelae of AMPA/kainate receptor activation between these populations. Brief exposure to either AMPA or kainate caused mitochondrial depolarization, assessed using tetramethylrhodamine ethylester, and reactive oxygen species (ROS) generation, assessed using hydroethidine, in motor neurons. However, these effects were only seen in the GABAergic neurons after exposure to the nondesensitizing AMPA receptor agonist kainate. Finally, addition of either antioxidants or toxins (FCCP or CN(-)) that block mitochondrial Ca(2+) uptake attenuated AMPA/kainate receptor-mediated motor neuron injury, suggesting that the mitochondrial Ca(2+) uptake and consequent ROS generation are central to the injury process.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Altered mitochondrial oxidative phosphorylation in hippocampal slices of kainate-treated rats

Mitochondria provide the main neuronal energy supply and are important organelles for the sequestration of intracellular Ca2+. This indicates a possible important role for mitochondria in modulating neuronal excitability in normal function as well as in disease. Therefore, we have investigated mitochondrial oxidative phosphorylation in the kainate model of epilepsy. We measured the oxygen consumption of single 400-micron rat hippocampal slices applying high resolution respirometry and determined mitochondrial NAD(P)H autofluorescence signal changes in single slices by laser-excited fluorescence spectroscopy. We observed an about 2-fold higher (p<0.001) basal glucose oxidation rate in slices from kainate-treated animals. This increased endogenous energy consumption was found to be unrelated to spontaneous activity since it was not sensitive to the inhibitors of the sodium-potassium ATPase ouabain and of the mitochondrial adenine nucleotide translocator atractyloside. This finding suggested an increased mitochondrial energy turnover in kainate-induced epilepsy. Furthermore, the uncoupler-stimulated oxygen consumption of the slices was approximately 1.3-fold higher (p<0.01) in the kainate model. In accordance with the respirometric data, fluorescence spectroscopy showed decreased reduction levels of the mitochondrial NAD-system in glucose oxidizing slices from kainate-treated rats. The preincubation of epileptic hippocampal slices with either BAPTA AM, ruthenium red or TPP+ increased the atractyloside sensitivity of glucose oxidation to about 1.4-fold (p<0.01). These observations indicate that the increased mitochondrial energy turnover in hippocampal slices from kainate-treated rats is most possibly caused by futile Ca2+-cycling.

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.