Elevated intracellular calcium blocks programmed neuronal death

Impact of cochlear ablation on calretinin and synaptophysin in the gerbil anteroventral cochlear nucleus before the hearing onset

Mammalian auditory system undergoes many structural and functional modifications during postnatal development, which are dependent on the relationship between auditory nerve fibers and their nuclei. In the present study, the cochlea of Meriones unguiculatus was ablated unilaterally on postnatal day 5 or 9 (P5 or P9), before the onset of hearing. Histochemical analysis of synaptophysin (SYN) and calretinin (CR) in anterior anteroventral cochlear nucleus (AVCN-A) was performed to analyze whether unilateral cochlea ablation induces changes in the auditory terminal endings and somata of spherical bushy cells (SBCs). During the period of postnatal development, CR-labeling was evident in somata of SBCs and in auditory nerve terminals. SYN was most apparent in puncta encircled cell bodies, progressing with age. Cochlear removal at P5 induced a decrease in CR-labeling in SBCs somata 6 h and 48 h post-lesion; whereas, ablation at P9 increased the somatic CR-labeling in the lesioned AVCN-A after 24 and 48 h post-lesion. The SYN-labeled synaptic puncta were remarkably reduced in the AVCN-A of P5- and P9-cochlea-ablated gerbils with stronger effects in P5 animals (a 50% reduction after 48 h). Interestingly, a significant increase in the SYN-immunolabeled puncta was found after 48 h compared to 24 h in the lesioned AVCN-A of P9 gerbils, indicating reactive synaptogenesis. Our study shows, that following the destruction of the cochlea at different postnatal periods, the CR- and SYN-labeling are differentially influenced in the AVCN-A, which in turn coincides with different critical developmental periods before the onset of hearing.

Otilonium and pinaverium trigger mitochondrial-mediated apoptosis in rat embryo cortical neurons in vitro

In the frame of a repositioning programme with cholinergic medicines in clinical use searching for neuroprotective properties, we surprisingly found that spasmolytic antimuscarinics otilonium and pinaverium exhibited neurotoxic effects in neuronal cultures. We decided to characterize such unexpected action in primary cultures of rat embryo cortical neurons. Neurotoxicity was time- and concentration-dependent, exhibiting approximate EC₅₀ values of 5 μM for both drugs. Seven antimuscarinic drugs endowed with a quaternary ammonium, and another 10 drugs with different cholinergic activities, carrying in their molecule a ternary ammonium did not exhibit neurotoxicity. Both drugs caused a concentration-dependent blockade of whole-cell inward currents through voltage-activated calcium channels (VACCs). Consistent with this, they also blocked the K⁺-elicited [Ca²⁺]_c transients. Neither antioxidant catalase, glutathione, n-acetylcysteine, nor melatonin protected against neurotoxicity of otilonium or pinaverium. However cyclosporine A, a blocker of the mitochondrial permeability transition pore, prevented the neurotoxic effects of otilonium and pinaverium monitored as the fraction of cells undergoing apoptosis. Furthermore, the caspase-9 and caspase-3 inhibitor Ac-LEHD-CHO mitigated the apoptotic neuronal death of both drugs by around 50%. Data are compatible with the hypothesis that otilonium and pinaverium elicit neuronal death by activating the intrinsic mitochondrial-mediated signaling pathway of apoptosis. This may have its origin in the mitigation of Ca²⁺ entry and the uncoupling of the Ca²⁺-dependent generation of mitochondrial bioenergetics, thus causing the opening of the mitochondrial mPTP to elicit apoptotic neuronal death.

REVIEW ■ : Physical Injury of Neurons: Important Roles for Sodium and Chloride Ions

There is growing evidence that ions other than Ca2+ play important roles in the deterioration of neuronal elements in both gray and white matter after physical injury. This review features information gathered with a tissue culture model of dendrite transection regarding the contributions of Na+ and CI- to ultrastructural damage and neuronal death. This information and the results of other in vitro investigations of physical and ischemic/excitotoxic injuries indicate that elevation of internal Na+ is an early event that may contribute significantly to neuronal injury through effects on Na+-driven transport mechanisms. Proposed deleterious consequences include cytoplasmic acidification, reduced mitochondrial energy production, and elevation of intracellular Ca2+ and extracellular excitatory amino acids to toxic levels. Prevention of Na+ entry into neurons after injury has been found to limit ultrastructural damage, prevent death, and preserve electrophysiological function. Although the role of CI- in neuronal injury is less well defined, there is also evidence that elevation of intracellular CI- contributes to structural damage, particularly to the smooth endoplasmic reticulum. In terventions that limit Na+- and CI--mediated damage to injured neurons may have utility in neurosurgery and as acute phase treatments for nervous system trauma and other pathological states. NEURO SCIENTIST 3:89-101, 1997

Neuroprotective profile of pyridothiazepines with blocking activity of the mitochondrial Na(+)/Ca(2+) exchanger

The mitochondrial Na(+)/Ca(2+) exchanger plays an important role in the control of cytosolic Ca(2+) cycling in excitable cells, essential for the regulation of a plethora of Ca(2+)-dependent physio-pathological events, such as apoptosis in the presence of a Ca(2+) overload. There are very few pharmacological tools available to study both physiological and pathological implications of the mitochondrial Na(+)/Ca(2+) exchanger, where the benzothiazepine CGP37157 is the best-known ligand, used since the 1980s. However, it is not an efficient blocker and lacks of selectivity, as also blocks several other cellular Ca(2+) transporters. Moreover, CGP37157 is a very lipophilic drug, showing very poor water solubility, what has hindered its therapeutic use. Attempting to improve its pharmacokinetic profile as well as its potency and selectivity, we herein describe the synthesis of new CGP37157 analogs, where the benzene-fused ring has been replaced by a pyridine. On top of a better water solubility and lower log P value, some of these new pyridothiazepine derivatives also presented a higher capacity to regulate the mitochondrial Ca(2+) clearance, while keeping the neuroprotective properties presented in the head compound CGP37157.

Identification of Bax-voltage-dependent anion channel 1 complexes in digitonin-solubilized cerebellar granule neurons

Mitochondrial outer membrane Bax oligomers are critical for cytochrome c release, but the role of resident mitochondrial proteins in this process remains unclear. Membrane-associated Bax has primarily been studied using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) as the solubilizing agent, as it does not induce conformational artifacts, although recent evidence indicates it may have other artifactual effects. The objective of this study was to investigate digitonin as an alternative detergent to assess Bax oligomeric state, and possible interaction with voltage-dependent anion channel (VDAC)1 in cerebellar granule neurons. VDAC1 co-immunoprecipitated with Bax in digitonin extracts from healthy and apoptotic neurons. Two-dimensional blue native-SDS-PAGE revealed five Bax and VDAC1 oligomers having similar masses from 120 to 500 kDa. The levels of two VDAC1 oligomers in Bax 1D1 immunodepleted extracts negatively correlated with levels of co-precipitated VDAC1, indicating the co-precipitated VDAC1 was derived from these oligomers. Immunodepletion with the 6A7 antibody modestly reduced the levels of Bax oligomers from apoptotic but not healthy neurons. A sixth 170 kDa oligomer containing exclusively 6A7 Bax and no VDAC1 was identified after apoptosis induction. CHAPS failed to solubilize VDAC1, and additionally yielded no distinct oligomers. We conclude that digitonin is a potentially useful detergent preserving Bax-VDAC1 interactions that may be disrupted with CHAPS.

Transgenic overexpression of active calcineurin in beta-cells results in decreased beta-cell mass and hyperglycemia

BACKGROUND

Glucose modulates beta-cell mass and function through an initial depolarization and Ca(2+) influx, which then triggers a number of growth regulating signaling pathways. One of the most important downstream effectors in Ca(2+) signaling is the calcium/Calmodulin activated serine threonine phosphatase, calcineurin. Recent evidence suggests that calcineurin/NFAT is essential for beta-cell proliferation, and that in its absence loss of beta-cells results in diabetes. We hypothesized that in contrast, activation of calcineurin might result in expansion of beta-cell mass and resistance to diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the role of activation of calcineurin signaling in the regulation of pancreatic beta-cell mass and proliferation, we created mice that expressed a constitutively active form of calcineurin under the insulin gene promoter (caCn(RIP)). To our surprise, these mice exhibited glucose intolerance. In vitro studies demonstrated that while the second phase of Insulin secretion is enhanced, the overall insulin secretory response was conserved. Islet morphometric studies demonstrated decreased beta-cell mass suggesting that this was a major component responsible for altered Insulin secretion and glucose intolerance in caCn(RIP) mice. The reduced beta-cell mass was accompanied by decreased proliferation and enhanced apoptosis.

CONCLUSIONS

Our studies identify calcineurin as an important factor in controlling glucose homeostasis and indicate that chronic depolarization leading to increased calcineurin activity may contribute, along with other genetic and environmental factors, to beta-cell dysfunction and diabetes.

A PBPK model for evaluating the impact of aldehyde dehydrogenase polymorphisms on comparative rat and human nasal tissue acetaldehyde dosimetry

Acetaldehyde is an important intermediate in the chemical synthesis and normal oxidative metabolism of several industrially important compounds, including ethanol, ethyl acetate, and vinyl acetate. Chronic inhalation of acetaldehyde leads to degeneration of the olfactory and respiratory epithelium in rats at concentrations > 50 ppm (90 day exposure) and respiratory and olfactory nasal tumors at concentrations > or = 750 ppm, the lowest concentration tested in the 2-yr chronic bioassay. Differences in the anatomy and biochemistry of the rodent and human nose, including polymorphisms in human high-affinity acetaldehyde dehydrogenase (ALDH2), are important considerations for interspecies extrapolations in the risk assessment of acetaldehyde. A physiologically based pharmacokinetic model of rat and human nasal tissues was constructed for acetaldehyde to support a dosimetry-based risk assessment for acetaldehyde (Dorman et al., 2008). The rodent model was developed using published metabolic constants and calibrated using upper-respiratory-tract acetaldehyde extraction data. The human nasal model incorporates previously published tissue volumes, blood flows, and acetaldehyde metabolic constants. ALDH2 polymorphisms were represented in the human model as reduced rates of acetaldehyde metabolism. Steady-state dorsal olfactory epithelial tissue acetaldehyde concentrations in the rat were predicted to be 409, 6287, and 12,634 microM at noncytotoxic (50 ppm), and cytotoxic/tumorigenic exposure concentrations (750 and 1500 ppm), respectively. The human equivalent concentration (HEC) of the rat no-observed-adverse-effect level (NOAEL) of 50 ppm, based on steady-state acetaldehyde concentrations from continual exposures, was 67 ppm. Respiratory and olfactory epithelial tissue acetaldehyde and H(+) (pH) concentrations were largely linear functions of exposure in both species. The impact of presumed ALDH2 polymorphisms on human olfactory tissue concentrations was negligible; the high-affinity, low-capacity ALDH2 does not contribute significantly to acetaldehyde metabolism in the nasal tissues. The human equivalent acetaldehyde concentration for homozygous low activity was 66 ppm, 1.5% lower than for the homozygous full activity phenotype. The rat and human acetaldehyde PBPK models developed here can also be used as a bridge between acetaldehyde dose-response and mode-of-action data as well as between similar databases for other acetaldehyde-producing nasal toxicants.

Why photoreceptors die (and why they don't)

Light can kill the photoreceptors of the eye, not only very bright direct sunlight, but more moderate illumination if the light is present continuously. Recent experiments show that rod apoptosis can be triggered by strong and constant activation of transduction, and that death can be prevented if transduction is inhibited even though the eye is illuminated. Vitamin A deficiency and genetically inherited diseases, such as some forms of retinitis pigmentosa and Leber congenital amaurosis, appear to kill like this: transduction is activated at a high rate and continuously, and this causes the rods to die. Why does transduction kill? Our best guess is that continuous activation produces a prolonged lowering of the Ca(2+) concentration, which is also thought to kill neurons in tissue culture and during the development of the nervous system. To prevent death in constant light, rods have evolved protective mechanisms including modulation of channels and ion transport to keep the Ca(2+) from going too low. Prolonged light exposure also causes migration of transduction proteins from one part of the cell to another and a reversible shortening of the rod outer segments, the part of the cell that contains the pigment rhodopsin. All of these mechanisms are at work in the normal eye to reduce transduction and prevent the Ca(2+) concentration from dropping too low for too long a time. That most of us retain our vision our entire lives is a testament to their effectiveness.

Calcium-dependent expression of TNF-α in neural cells is mediated by the calcineurin/NFAT pathway

We report induction of TNF-alpha via the calcium/calcineurin/NFAT pathway in PC12 neural cells. In PC12, expression of TNF-alpha mRNA, protein and TNF-alpha gene promoter activity was induced by co-stimulation with phorbol ester and either calcium ionophore A23187 or the L-type Voltage Gated Calcium Channel agonist Bay K 8644. Pre-treatment with calcineurin inhibitors CsA or FK506 inhibited the dominant calcium-dependent component of this induction, limiting it to the level achieved with phorbol ester alone. Promoter activation by Bay was abolished by nifedipine, a specific inhibitor of L-type Voltage Gated Calcium Channels. Exogenous NFAT protein transactivated the TNF-alpha promoter, and the peptide VIVIT-a specific inhibitor of calcineurin/NFAT binding-blocked calcium-inducible transactivation of the TNF-alpha promoter. Given proposed functions of TNF-alpha in spatial learning, memory and the pathogenesis of neurodegenerative diseases, the data presented suggest an important role for calcineurin/NFAT signaling in these key neurological processes.

Depolarization of neural cells induces transcription of the Down syndrome critical region 1 isoform 4 via a calcineurin/nuclear factor of activated T cells-dependent pathway

In this study we showed that the transcriptional regulation of Down syndrome critical region isoform 4 (DSCR1.4) is mediated by the calcineurin/nuclear factor of activated T cells (NFAT) pathway in neural cells. Stimuli that elicit an increase in the intracellular concentrations of calcium, such as membrane depolarization, induced de novo transcription of DSCR1.4, with mRNA expression peaking after 4 h and then declining. Action via the physiologically relevant L-type calcium channel was confirmed by blockade with nifedipine and verapamil. This calcium-dependent transcription of DSCR1.4 was inhibited by the calcineurin inhibitors cyclosporin A and FK506. Deletional analysis showed that the calcium- and calcineurin-dependent activation is mediated by the promoter region between nucleotides -350 and -166, a region that contains putative NFAT-binding motifs. Exogenous NFATc2 potently augmented the DSCR1.4 promoter transcriptional activity, and the involvement of endogenous NFAT signaling pathway in DSCR1.4 transcription was confirmed by the suppression of depolarization-inducible promoter activity with the NFAT inhibitor peptide VIVIT. Exogenous overexpression of DSCR1 protein (calcipressin 1) resulted in the inhibition of the transcription of DSCR1.4 and NFAT-dependent signaling. These findings suggest that calcineurin-dependent induction of DSCR1.4 product may represent an important auto-regulatory mechanism for the homeostatic control of NFAT signaling in neural cells.

PI3K/Akt-dependent regulation of the transcription factor myocyte enhancer factor-2 in insulin-like growth factor-1- and membrane depolarization-mediated survival of cerebellar granule neurons

Survival signals such as insulin-like growth factor-1 (IGF-1) or membrane depolarization convey their neuronal protective effects through the activation of signaling networks and nuclear factors. In cerebellar granule neurons, IGF-1 mediates survival primarily through the PI3K/Akt pathway. The function of the transcription factor myocyte enhancer factor-2 (MEF2) is required for mediating membrane depolarization-dependent neuronal survival. However, whether PI3K/Akt regulates MEF2 and the role of MEF2 in IGF-1-mediated survival of neurons are unknown. In addition, the contribution of the PI3K/Akt pathway in membrane depolarization-induced neuronal survival remains undefined. We show here that the PI3K/Akt pathway promotes the survival of cerebellar granule neurons derived from Long-Evans rats following IGF-1 stimulation or membrane depolarization through regulation of MEF2 activity. IGF-1 stimulated the gene transactivation activity of MEF2 and its DNA binding potential. Moreover, regulation of MEF2 function by IGF-1 was dependent on the activity of the PI3K/Akt signaling pathway. Blocking MEF2 function reduced IGF-1-induced survival of cerebellar granule neurons. Membrane depolarization stimulated phosphorylation of Akt in cerebellar granule neurons. Blocking of the PI3K/Akt pathway with either a pharmacological inhibitor of PI3K, LY294002, or dominant negative mutants of PI3K and Akt inhibited the membrane depolarization-induced increase in MEF2 transactivation as well as its DNA binding activity and reduced neuronal survival. Together, these findings provide clear evidence to support an important role of the PI3K/Akt pathway in the regulation of nuclear survival factor MEF2 upon either IGF-1 stimulation or membrane depolarization, thus placing MEF2 as a novel downstream effector of the PI3K/Akt pathway in neurons.

Constitutive opsin signaling: night blindness or retinal degeneration?

A subset of genetic mutations in photoreceptor-specific genes results in abnormally prolonged activation of transducin-mediated photosignaling in rod cells. In humans and animal models, these mutations cause visual dysfunctions ranging from a mild stationary night blindness to severe, early-onset retinal degeneration. There are mechanistic differences between mutations causing night blindness and those causing retinal degeneration. Here, we hypothesize that mutations causing continuous activation of the visual cascade as the result, for example, of the inability of the photoreceptor to regenerate rhodopsin, lead to retinal degeneration; those mutations that can terminate signaling, even if only partially and intermittently, slow the rate of degeneration sufficiently to give rise to stationary night blindness. Furthermore, we hypothesize that a prolonged decrease in intracellular calcium concentration resulting from persistent activation is responsible for triggering apoptotic rod-cell death.

Regulation of cell death: the calcium-apoptosis link

To live or to die? This crucial question eloquently reflects the dual role of Ca2+ in living organisms--survival factor or ruthless killer. It has long been known that Ca2+ signals govern a host of vital cell functions and so are necessary for cell survival. However, more recently it has become clear that cellular Ca2+ overload, or perturbation of intracellular Ca2+ compartmentalization, can cause cytotoxicity and trigger either apoptotic or necrotic cell death.

Gene microarrays in hippocampal aging: statistical profiling identifies novel processes correlated with cognitive impairment

Gene expression microarrays provide a powerful new tool for studying complex processes such as brain aging. However, inferences from microarray data are often hindered by multiple comparisons, small sample sizes, and uncertain relationships to functional endpoints. Here we sought gene expression correlates of aging-dependent cognitive decline, using statistical profiling of gene microarrays in well powered groups of young, mid-aged, and aged rats (n = 10 per group). Animals were trained on two memory tasks, and the hippocampal CA1 region of each was analyzed on an individual microarray (one chip per animal). Aging- and cognition-related genes were identified by testing each gene by ANOVA (for aging effects) and then by Pearson's test (correlating expression with memory). Genes identified by this algorithm were associated with several phenomena known to be aging-dependent, including inflammation, oxidative stress, altered protein processing, and decreased mitochondrial function, but also with multiple processes not previously linked to functional brain aging. These novel processes included downregulated early response signaling, biosynthesis and activity-regulated synaptogenesis, and upregulated myelin turnover, cholesterol synthesis, lipid and monoamine metabolism, iron utilization, structural reorganization, and intracellular Ca2+ release pathways. Multiple transcriptional regulators and cytokines also were identified. Although most gene expression changes began by mid-life, cognition was not clearly impaired until late life. Collectively, these results suggest a new integrative model of brain aging in which genomic alterations in early adulthood initiate interacting cascades of decreased signaling and synaptic plasticity in neurons, extracellular changes, and increased myelin turnover-fueled inflammation in glia that cumulatively induce aging-related cognitive impairment.

Neuronal cell death, nerve growth factor and neurotrophic models: 50 years on

Viktor Hamburger has just died at the age of 100. It is 50 years since he and Rita Levi-Montalcini laid the foundations for the study of naturally occurring cell death and of neurotrophic factors in the nervous system. In a period of less than 10 years, from 1949 to 1958, Hamburger and Levi-Montalcini made the following seminal discoveries: that neuron cell death occurs in dorsal root ganglia, sympathetic ganglia and the cervical column of motoneurons; that the predictions arising from this observation, namely that survival is dependent on the supply of a trophic factor, could be substantiated by studying the effects of a sarcoma on the proliferation of ganglionic processes both in vivo and in vitro; and that the proliferation of these processes could be used as an assay system to isolate the factor. This work provides a short review mostly of the early history of this subject in the context of the Hamburger/Levi-Montalcini paradigm. This acts as an introduction to a consideration of models that have been proposed to account for how the different sources of growth factors provide for the survival of neurons during development. It is suggested that what has been called the 'social-control' model provides the most parsimonious quantitative description of the contribution of trophic factors to neuronal survival, a concept for which we are in debt to Viktor Hamburger and Rita Levi-Montalcini.

Intraischemic but not postischemic hypothermia prevents non-selective hippocampal downregulation of AMPA and NMDA receptor gene expression after global ischemia

Hypothermia may afford histological neuroprotection induced by ischemia by preventing aberrant Ca2+ influx through NMDA (N-methyl-D-aspartic acid) or Ca2+-permeable AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid) receptors. Expression of hippocampal GluR1A, GluR2B, GluR3C and NMDAR1 (NR1) subunits was investigated by in situ hybridization at 1 and 7 days after 10-min transient global ischemia in the presence and absence of intraischemic or postischemic brain hypothermia (30 degrees C). At 1 day, normothermic ischemia markedly suppressed the expression of GluR1A, GluR2B, and GluR3C receptor mRNAs to a similar degree in the vulnerable CA1. Less vulnerable CA3a-c subregions were also acutely downregulated. NR1 mRNA expression was reduced in CA1 but to a lesser extent than AMPA mRNAs. At 7 days after normothermic ischemia, a time of marked CA1 cell loss, all three AMPA transcripts were nearly absent in CA1 while a percentage (33.9+/-7.2%) of NR1 mRNA remained. Intraischemic hypothermia fully blocked the damage and non-selective mRNA downregulations at 1 and 7 days. By contrast, postischemic hypothermia postponed neurodegeneration but only partially rescued the expression of AMPA and NR1 mRNAs at 7 days and not at 1 day after the insult. Therefore, hippocampal AMPA receptor mRNAs decline at a relatively similar rate after normothermic global ischemia and cellular neuroprotection by intraischemic hypothermia occurred independently of altered subunit composition of AMPA receptors. Since decreases persist within resistant neurons under the postischemic condition, AMPA receptor-mediated Ca2+ currents probably do not contribute to selective vulnerability.

In vitro inhibition of antirecoverin immunoglobulin-mediated death of mammalian photoreceptor cells

Cancer-associated retinopathy (CAR) is a blinding disease, which can be mediated by autoimmune reactions with a specific calcium-binding retinal protein, recoverin. A number of recent studies demonstrate that agents that mobilize intracellular calcium can protect neurons from apoptotic death induced by a variety of insults. In this study, we investigated the effect of one such agent, potassium, on the survival of mammalian rod photoreceptors exposed to antirecoverin IgG. Primary cell cultures of rat retinal neurons were grown in a chemically defined medium, and cells were exposed to antirecoverin IgG for 72 hr in various concentrations of potassium and the surviving cells counted. Rod photoreceptors were quantitated using antirhodopsin immunofluorescence microscopy, and total cell numbers were determined by 4',6-diamidino-2-phenylindole (DAPI) staining of nuclei. Apoptosis was evaluated by TdT-mediated biotin-dUTP nick-end labeling (TUNEL), cell death-detection ELISA, and DNA laddering. The present study shows that elevated extracellular K+ ([K+](o)) protects retinal neurons from antirecoverin antibody-mediated cell death. The protective effects of ([K+](o)) were shown to be time- and dose-dependent. The inhibition of antirecoverin IgG-mediated death of photoreceptors by elevated ([K+](o)) suggests that the mobilization of internal calcium stores rescues the cells by interfering with apoptotic signal transduction pathways. These data also suggest that the death of photoreceptor cells occurring in CAR possibly can be prevented by reagents and/or environmental changes that mobilize intracellular calcium.

Treatment of aged rat sensory neurons in short-term, serum-free culture with nerve growth factor reverses the effect of aging on neurite outgrowth, calcium currents, and neuronal survival

Impaired NGF production and release has been documented in aged animals, suggesting that decreased NGF receptor stimulation may be one factor contributing to neuronal dysfunction with aging. Other studies have suggested that aging may be associated with impaired intracellular responses to NGF. Because aging-associated neuronal dysfunction contributes to morbidity and mortality in the geriatric population, it is important to determine whether the effects of aging on sensory neuron function and survival are reversible. In the present study, we observed significantly decreased neurite outgrowth and neuronal survival in short-term cultures (0-96 h) of dorsal root ganglion (DRG) neurons from aged (>22 months) Fisher 344 x Brown Norway F1 hybrid rats, compared to young (4-6 month) and middle-aged (14 month) animals. From 24 to 96 h in culture, diminished survival of aged neurons appeared to be due to an increased rate of apoptotic cell death. DRG neurons from aged animals also exhibited significantly decreased whole cell, high-threshold voltage-dependent calcium currents, with a larger proportion of L-type current, compared to youthful and middle-aged animals. Treatment of aged DRG neurons with NGF restored neurite outgrowth, neuronal survival and calcium current amplitude and subtype distribution to those observed in youthful DRG neurons.

Zn(2+): a novel ionic mediator of neural injury in brain disease

Zn(2+) is the second most prevalent trace element in the body and is present in particularly large concentrations in the mammalian brain. Although Zn(2+) is a cofactor for many enzymes in all tissues, a unique feature of brain Zn(2+) is its vesicular localization in presynaptic terminals, where its release is dependent on neural activity. Although the physiological significance of synaptic Zn(2+) release is little understood, it probably plays a modulatory role in synaptic transmission. Furthermore, several lines of evidence support the idea that, upon excessive synaptic Zn(2+) release, its accumulation in postsynaptic neurons contributes to the selective neuronal loss that is associated with certain acute conditions, including epilepsy and transient global ischaemia. More speculatively, Zn(2+) dis-homeostasis might also contribute to some degenerative conditions, including Alzheimer's disease. Further elucidation of the pathological actions of Zn(2+) in the brain should result in new therapeutic approaches to these conditions.

Distribution of glutamate and preproenkephalin messenger RNAs following transient focal cerebral ischemia

Middle cerebral artery occlusion may result in increased activation of N-methyl-D-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type receptors by glutamate and lead to neuronal cell death. To characterize molecular events that precede cell death following transient focal ischemia, in situ hybridization histochemistry was used to measure levels of glutamate receptor subunit 1 (GluR1), GluR2, GluR3, N-methyl-D-aspartate receptor subunit 1 (NR1) and preproenkephalin messenger RNAs in adult rats at various recirculation times (1.5, 3 and 24 h) following a 90-min period of middle cerebral artery occlusion. At 1.5 and 3 h recirculation, autoradiography showed pronounced but differential decreases in AMPA, NR1 and preproenkephalin messenger RNA expression throughout the infarcted ipsilateral striatum. Non-uniform patterns of in situ hybridization grains emerged such that many striatal neurons were depleted of AMPA and preproenkephalin messenger RNAs, while others retained control levels. In cortical regions destined to undergo infarction, GluR2 and NR1 messenger RNAs were preferentially reduced relative to the contralateral side (to 75+/-8.5% and 66+/-4.5%, respectively); GluR1, GluR3 and preproenkephalin messenger RNAs were unaltered. At 24 h recirculation, depletion of striatal and cortical messenger RNAs became less selective. GluR3 and preproenkephalin messenger RNAs were up-regulated in ipsilateral spared regions of the striatum, and GluR1 and GluR2 messenger RNAs increased bilaterally in the cingulate cortex and in selective nuclei of the amygdala. Histological cell death or neurodegeneration was not detected in areas of reduced glutamate and preproenkephalin messenger RNA expression in either the ipsilateral striatum or cortex before 24 h. These findings suggest that complex and long-lasting decreases in messenger RNA expression occur prior to significant cell loss in regions destined to undergo infarction. Increased formation of Ca2+-permeable AMPA receptor assemblies may occur in "unspared" and "spared" regions via different mechanisms and contribute to alterations in post-ischemic synaptic activity. The possibility arises that there may be altered relationships between glutamatergic and enkephalin synapses, since the dorsolateral striatum, where preproenkephalin messenger RNA expression is acutely reduced, receives innervation by the affected ipsilateral cortical region.

Neuronal activity-dependent cell survival mediated by transcription factor MEF2

During mammalian development, electrical activity promotes the calcium-dependent survival of neurons that have made appropriate synaptic connections. However, the mechanisms by which calcium mediates neuronal survival during development are not well characterized. A transcription-dependent mechanism was identified by which calcium influx into neurons promoted cell survival. The transcription factor MEF2 was selectively expressed in newly generated postmitotic neurons and was required for the survival of these neurons. Calcium influx into cerebellar granule neurons led to activation of p38 mitogen-activated protein kinase-dependent phosphorylation and activation of MEF2. Once activated, MEF2 regulated neuronal survival by stimulating MEF2-dependent gene transcription. These findings demonstrate that MEF2 is a calcium-regulated transcription factor and define a function for MEF2 during nervous system development that is distinct from previously well-characterized functions of MEF2 during muscle differentiation.

A molecular mechanism of aluminium-induced Alzheimer's disease?

An abundance of research has continued to link aluminium (Al) with Alzheimer's disease (AD) (Strong et al., J. Toxicol. Environ. Health 48 (1996) 599; Savory et al., J. Toxicol. Environ. Health 48 (1996) 615). Animals loaded with Al develop both symptoms and brain lesions that are similar to those found in AD. However, these animal models of Al intoxication are not representative of human exposure to Al. They have not addressed the significance of a truly chronic exposure to Al. If Al is a cause of AD it is effective at the level of our everyday exposure to the metal and AD will be one possible outcome of the life-long presence of a low, though burgeoning, brain Al burden. Individual susceptibility to AD will be as much to do with differences in brain physiology as with changes in our everyday exposure to the metal. There will be a chemical response and indeed biochemical/physiological response in the brain to Al. The question is whether brain Al homeostasis could impact upon brain function. In reviewing the recent literature covering the neurotoxicity of Al and, in particular, of the known and probable mechanisms involved in brain Al homeostasis I have identified a mechanism through which a truly chronic exposure to Al would bring about subtle and persistent changes in neurotransmission which, in time, could instigate the cascade of events known collectively as AD. This mechanism involves the potentiation of the activities of neurotransmitters by the action of Al-ATP at adenosine 5'-triphosphate (ATP) receptors in the brain.

YoshixolTR inhibits B16 melanoma cell growth in vivo and induces apoptosis-like (quantum thermodynamic) cell death

In this report, antitumor effects of YoshixolTR in vivo and in vitro were investigated in B16 melanoma cells. For in vivo experiments, the present study shows a dramatic inhibition of tumor growth of B16 melanoma transplanted on the leg or intraperitoneal cavity after treatment with YoshixolTR intraperitoneally. A proliferation of B16 cells in vitro was inhibited by YoshixolTR in a dose-and time-dependent manner. YoshixolTR induced apoptosis-like cell death in histological observations (phase-contrast, scanning and transmission electron microscopy), DNA fragmentation, and a smaller increase in lactate dehydrogenase (LDH) as a marker of cell leakage. Immunohistochemical investigation of cytoskeletal components, such as actin and tubulin, showed a cell wall disruption of B16 melanoma cells and a nuclear extrusion after the treatment with YoshixolTR. Treatment with YoshixolTR in vitro showed an arrest at the G0/G1 stage of the cell cycle, followed by a flow cytometric measurement. As a possible physiological mechanism of YoshixolTR on B16 melanoma cells, intracellular Ca++ was measured with Fura-2 technique. An adequate concentration of YoshixolTR, which induces apoptosis-like cell death, showed a decrease in intracellular free Ca++ concentration. In conclusion, YoshixolTR has an antitumor potency with a new biological mechanism of cell growth, proliferation, and differentiation, including cellular signalling pathways, and is a new candidate for an ideal chemotherapeutic agent against malignant tumors.

Metabotropic glutamate receptor subtypes independently modulate neuronal intracellular calcium

Metabotropic glutamate receptors (mGluRs) modulate several G-protein-related signal transduction pathways including intracellular calcium (iCa(2+)) that control both neuronal development and demise. As an initial investigation, we characterized the ability of specific mGluR subtypes to modulate iCa(2+) by using Fura-2 microfluorometry in primary hippocampal neurons. Activation rather than inhibition of the metabotropic system with the group I and group II mGluR agonist 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), the specific group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG), and the specific group II agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG-I) increased iCa(2+) with increasing concentrations. In contrast, the group III mGluR agonist, L(+)-2-amino-4-phosphonobutyric acid (L-AP4) produced no significant increase in iCa(2+). Through the pharmacological modulation of individual mGluR subtypes, we further examined the role of iCa(2+) release by the mGluR system. Release of iCa(2+) by both 1S,3R-ACPD and LCCG-I was prevented only through the administration of the antagonists (2S)-alpha-ethylglutamic acid (EGlu; mGluR2 and mGluR3) and (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-IV; mGluR2), suggesting that the mGluR2 subtype was responsible for the release of iCa(2+). As a control, the group I antagonists, L(+)-2-amino-3-phosphonopropionic acid (L-AP3) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), prevented DHPG release of iCa(2+) but were ineffective against iCa(2+) release by 1S,3R-ACPD. Although extracellular calcium influx did not significantly contribute to the release of iCa(2+) by the mGluR system, pharmacological inhibition of calcium-induced calcium-release-sensitive calcium pools played a critical role in the release of iCa(2+). Further characterization of the cellular calcium pools modulated by the mGluR subtypes may provide greater insight into the mechanisms that mediate neuronal function.

Selective reduction of GluR2 protein in adult hippocampal CA3 neurons following status epilepticus but prior to cell loss

Kainic acid (KA) induces status epilepticus and delayed neurodegeneration of CA3 hippocampal neurons. Downregulation of glutamate receptor 2 (GluR2) subunit mRNA [the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) subunit that limits Ca2+ permeability] is thought to a play role in this neurodegeneration, possibly by increased formation of Ca2+ permeable AMPA receptors. The present study examined early hippocampal decreases in GluR2 mRNA and protein following kainate-induced status epilepticus and correlated expression changes with the appearance of dead or dying cells by several histological procedures. At 12 h, in situ hybridization followed by emulsion dipping showed nonuniform decreases in GluR2 mRNA hybridization grains overlying morphologically healthy-appearing CA3 neurons. GluR1 and N-methyl-D-aspartate receptor mRNAs were unchanged. At 12-16 h, when little argyrophilia or cells with some features of apoptosis were detected by silver impregnation or electron microscopy, single immunohistochemistry with GluR2 and GluR2/3 subunit-specific antibodies demonstrated a pattern of decreased GluR2 receptor protein within CA3 neurons that appeared to predict a pattern of damage, similar to the mRNA observations. Double immunolabeling showed that GluR2 immunofluorescence was depleted and that GluR1 immunofluorescence was sustained in clusters of the same CA3 neurons. Quantitation of Western blots showed increased GluR1:GluR2 ratios in CA3 but not in CA1 or dentate gyrus subfields. Findings indicate that the GluR1:GluR2 protein ratio is increased in a population of CA3 neurons prior to significant cell loss. Data are consistent with the "GluR2 hypothesis" that reduced expression of GluR2 subunits will increase formation of AMPA receptors permeable to Ca2+ and predict vulnerability to a particular subset of pyramidal neurons following status epilepticus.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Opiate-mediated inhibition of calcium signaling is decreased in dorsal root ganglion neurons from the diabetic BB/W rat

The effect of diabetes mellitus on opiate-mediated inhibition of calcium current density (I(D Ca) [pA pF-1]) and cytosolic calcium response ([Ca2+]i nM) to depolarization with elevated KCl and capsaicin was assessed. Experiments were performed on isolated, acutely dissociated dorsal root ganglion (DRG) neurons from diabetic, BioBreeding/Worcester (BB/W) rats and age-matched control animals. Sciatic nerve conduction velocity was significantly decreased in diabetic animals compared to controls. Mean I(DCa) and [Ca2+]i responses to capsaicin and elevated KCl recorded in DRGs from diabetic animals were significantly larger than those recorded in DRG neurons from controls. In neurons from diabetic animals, the opiate agonist dynorphin A (Dyn A; 1, 3, and 5 microM) had significantly less inhibitory effect on I(D Ca) and KCl-induced [Ca2+]i responses compared to controls. Omega-conotoxin GVIA (omega-CgTX; 10 microM) and pertussis toxin (PTX; 250 ng ml-1) abolished Dyn A-mediated inhibition of I(DCa) and [Ca2+]i in control and diabetic neurons, suggesting that Dyn A modulated predominantly N-type calcium channels coupled to opiate receptors via PTX-sensitive (Gi/o) inhibitory G proteins. These results suggest that opiate-mediated regulation of PTX-sensitive, G protein-coupled calcium channels is diminished in diabetes and that this correlates with impaired regulation of cytosolic calcium.

Beta-amyloid peptide blocks the fast-inactivating K+ current in rat hippocampal neurons

Deposition of beta-amyloid peptide (A beta) in senile plaques is a hallmark of Alzheimer disease neuropathology. Chronic exposure of neuronal cultures to synthetic A beta is directly toxic, or enhances neuronal susceptibility to excitotoxins. Exposure to A beta may cause a loss of cellular calcium homeostasis, but the mechanism by which this occurs is uncertain. In this work, the acute response of rat hippocampal neurons to applications of synthetic A beta was measured using whole-cell voltage-clamp techniques. Pulse application of A beta caused a reversible voltage-dependent decrease in membrane conductance. A beta selectively blocked the voltage-gated fast-inactivating K+ current, with an estimated KI < 10 microM. A beta also blocked the delayed rectifying current, but only at the highest concentration tested. The response was independent of aggregation state or peptide length. The dynamic response of the fast-inactivating current to a voltage jump was consistent with a model whereby A beta binds reversibly to closed channels and prevents their opening. Blockage of fast-inactivating K+ channels by A beta could lead to prolonged cell depolarization, thereby increasing Ca2+ influx.