Mechanisms of neuronal cell death: diverse roles of calcium in the various subcellular compartments

CDK5 Targeting as a Therapy for Recovering Neurovascular Unit Integrity in Alzheimer's Disease

The neurovascular unit (NVU) is responsible for synchronizing the energetic demand, vasodynamic changes, and neurochemical and electrical function of the brain through a closed and interdependent interaction of cell components conforming to brain tissue. In this review, we will focus on cyclin-dependent kinase 5 (CDK5) as a molecular pivot, which plays a crucial role in the healthy function of neurons, astrocytes, and the endothelium and is implicated in the cross-talk of cellular adhesion signaling, ion transmission, and cytoskeletal remodeling, thus allowing the individual and interconnected homeostasis of cerebral parenchyma. Then, we discuss how CDK5 overactivation affects the integrity of the NVU in Alzheimer's disease (AD) and cognitive impairment; we emphasize how CDK5 is involved in the excitotoxicity spreading of glutamate and Ca2+ imbalance under acute and chronic injury. Additionally, we present pharmacological and gene therapy strategies for producing partial depletion of CDK5 activity on neurons, astrocytes, or endothelium to recover neuroplasticity and neurotransmission, suggesting that the NVU should be the targeted tissue unit in protective strategies. Finally, we conclude that CDK5 could be effective due to its intervention on astrocytes by its end feet on the endothelium and neurons, acting as an intermediary cell between systemic and central communication in the brain. This review provides integrated guidance regarding the pathogenesis of and potential repair strategies for AD.

Prion peptide-mediated calcium level alteration governs neuronal cell damage through AMPK-autophagy flux

Background

The distinctive molecular structure of the prion protein, PrPsc, is established only in mammals with infectious prion diseases. Prion protein characterizes either the transmissible pathogen itself or a primary constituent of the disease. Our report suggested that prion protein-mediated neuronal cell death is triggered by the autophagy flux. However, the alteration of intracellular calcium levels, AMPK activity in prion models has not been described. This study is focused on the effect of the changes in intracellular calcium levels on AMPK/autophagy flux pathway and PrP (106–126)-induced neurotoxicity.

Methods

Western blot and Immunocytochemistry was used to detect AMPK and autophagy-related protein expression. Flow cytometry and a TdT-mediated biotin-16-dUTP nick-end labeling (TUNEL) assay were used to detect the percentage of apoptotic cells. Calcium measurement was employed using fluo-4 by confocal microscope.

Results

We examined the effect of calcium homeostasis alterations induced by human prion peptide on the autophagy flux in neuronal cells. Treatment with human prion peptide increased the intracellular calcium concentration and induced cell death in primary neurons as well as in a neuronal cell line. Using pharmacological inhibitors, we showed that the L-type calcium channel is involved in the cellular entry of calcium ions. Inhibition of calcium uptake prevented autophagic cell death and reduction in AMP-activated protein kinase (AMPK) activity induced by human prion peptide.

Conclusion

Our data demonstrated that prion peptide-mediated calcium inflow plays a pivotal role in prion peptide-induced autophagic cell death, and reduction in AMPK activity in neurons. Altogether, our results suggest that calcium influx might play a critical role in neurodegenerative diseases, including prion diseases.

Inhibition of Autophagy by Captopril Attenuates Prion Peptide-Mediated Neuronal Apoptosis via AMPK Activation

Accumulation of prion protein (PrPc) into a protease-resistant form (PrPsc) in the brains of humans and animals affects the central nervous system. PrPsc occurs only in mammals with transmissible prion diseases. Prion protein refers to either the infectious pathogen itself or the main component of the pathogen. Recent studies suggest that autophagy is one of the major functions that keep cells alive and which has a protective effect against neurodegeneration. In this study, we investigated whether the anti-hypertensive drug, captopril, could attenuate prion peptide PrP (106-126)-induced calcium alteration-mediated neurotoxicity. Treatment with captopril increased both LC3-II (microtubule-associated protein 1A/1B-light chain 3-II) and p62 protein levels, indicating autophagy flux inhibition. Electron microscopy confirmed the occurrence of autophagic flux inhibition in neuronal cells treated with captopril. Captopril attenuated PrP (106-126)-induced neuronal cell death via AMPK activation and autophagy inhibition. Compound C suppressed AMPK activation as well as the neuroprotective effects of captopril. Thus, these data showed that an anti-hypertensive drug has a protective effect against prion-mediated neuronal cell death via autophagy inhibition and AMPK activation, and also suggest that anti-hypertensive drugs may be effective therapeutic agents against neurodegenerative disorders, including prion diseases.

Direct Imaging of Hippocampal Epileptiform Calcium Motifs Following Kainic Acid Administration in Freely Behaving Mice

Prolonged exposure to abnormally high calcium concentrations is thought to be a core mechanism underlying hippocampal damage in epileptic patients; however, no prior study has characterized calcium activity during seizures in the live, intact hippocampus. We have directly investigated this possibility by combining whole-brain electroencephalographic (EEG) measurements with microendoscopic calcium imaging of pyramidal cells in the CA1 hippocampal region of freely behaving mice treated with the pro-convulsant kainic acid (KA). We observed that KA administration led to systematic patterns of epileptiform calcium activity: a series of large-scale, intensifying flashes of increased calcium fluorescence concurrent with a cluster of low-amplitude EEG waveforms. This was accompanied by a steady increase in cellular calcium levels (>5 fold increase relative to the baseline), followed by an intense spreading calcium wave characterized by a 218% increase in global mean intensity of calcium fluorescence (n = 8, range [114–349%], p < 10⁻⁴; t-test). The wave had no consistent EEG phenotype and occurred before the onset of motor convulsions. Similar changes in calcium activity were also observed in animals treated with 2 different proconvulsant agents, N-methyl-D-aspartate (NMDA) and pentylenetetrazol (PTZ), suggesting the measured changes in calcium dynamics are a signature of seizure activity rather than a KA-specific pathology. Additionally, despite reducing the behavioral severity of KA-induced seizures, the anticonvulsant drug valproate (VA, 300 mg/kg) did not modify the observed abnormalities in calcium dynamics. These results confirm the presence of pathological calcium activity preceding convulsive motor seizures and support calcium as a candidate signaling molecule in a pathway connecting seizures to subsequent cellular damage. Integrating in vivo calcium imaging with traditional assessment of seizures could potentially increase translatability of pharmacological intervention, leading to novel drug screening paradigms and therapeutics designed to target and abolish abnormal patterns of both electrical and calcium excitation.

Elevated concentrations of circulating cytokines and correlations with nerve conduction velocity in human peripheral nerves

The purpose of the current study was to quantify the potential relationship between various cytokines and peripheral nerve function in humans, in-vivo. Measures of nerve conduction velocity (NCV) were examined prior to and following the induction of a cytokine spike. A significant negative correlation was found between the change in IL-1ra and the change in NCV at 24h post-exercise (r=-0.65, p=0.02) while a significant positive correlation was found between the change in IL-6 and the change in NCV at 2h post-exercise (r=0.61, p=0.048). It may be possible that different cytokines induce a unique neural influence at elevated concentrations.

Reticulon Protein-1C: A New Hope in the Treatment of Different Neuronal Diseases

Reticulons (RTNs) are a group of membrane proteins localized on the ER and known to regulate ER structure and functions. Several studies have suggested that RTNs are involved in different important cellular functions such as changes in calcium homeostasis, ER-stress-mediated cell death, and autophagy. RTNs have been demonstrated to exert a cancer specific proapoptotic function via the interaction or the modulation of specific proteins. Reticulons have also been implicated in different signaling pathways which are at the basis of the pathogenesis of several neurodegenerative diseases. In this paper we discuss the accumulating evidence identifying RTN-1C protein as a promising target in the treatment of different pathologies such as cancer or neurodegenerative disorders.

Endoplasmic reticulum stress involved in heart and liver injury in iron-loaded rats

1. Iron overload contributes to the pathogenesis of various diseases and directly induces tissue injury. In the present study, we investigated the relationship between heart and liver injury induced by iron overload and cellular endoplasmic reticulum (ER) stress to explore the molecular mechanism of iron overload-induced cellular injury. 2. Iron overload in rats was generated by intraperitoneal injection of iron-dextran chronically (30 mg/kg per day for 9 weeks) or acutely (300 mg/kg once). Tissue injury was assessed by determining serum lactate dehydrogenase (LDH), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activity, as well as malondialdehyde (MDA) content in the heart and liver. The ER stress response was analysed by expression of glucose-response protein 78 (GRP78) and activation of caspase 12. 3. In chronic iron-loaded rats, iron levels in the heart and liver were higher, by approximately 2- and 7.8-fold, respectively (P < 0.01), compared with control. Serum LDH, ALT and AST activity, as well as MDA content, GRP78 expression and caspase 12 activity in the heart and liver, were upregulated in chronically iron-loaded rats. In acute iron-loaded rats, iron content in the heart and liver was 51% and 63% higher than in controls (both P < 0.01). Serum LDH, ALT and AST activity, MDA content in the heart and liver and levels of ER stress markers were all increased in acute iron-loaded rats. N-Acetylcysteine (150 mg/kg, s.c.) lowered the levels of these parameters in acute iron-loaded rats. 4. The results of the present study indicate that ER stress may play an important role in iron-induced tissue injury and that reactive oxygen species may mediate the ER stress response in the pathogenesis of iron-overload cellular injury.

Parkin deficiency disrupts calcium homeostasis by modulating phospholipase C signalling

Mutations in the E3 ubiquitin ligase parkin cause early-onset, autosomal-recessive juvenile parkinsonism (AJRP), presumably as a result of a lack of function that alters the level, activity, aggregation or localization of its substrates. Recently, we have reported that phospholipase Cgamma1 is a substrate for parkin. In this article, we show that parkin mutants and siRNA parkin knockdown cells possess enhanced levels of phospholipase Cgamma1 phosphorylation, basal phosphoinositide hydrolysis and intracellular Ca2+ concentration. The protein levels of Ca2+-regulated protein kinase Calpha were decreased in AJRP parkin mutant cells. Neomycin and dantrolene both decreased the intracellular Ca2+ levels in parkin mutants in comparison with those seen in wild-type parkin cells, suggesting that the differences were a consequence of altered phospholipase C activity. The protection of wild-type parkin against 6-hydroxydopamine (6OHDA) toxicity was also established in ARJP mutants on pretreatment with dantrolene, implying that a balancing Ca2+ release from ryanodine-sensitive stores decreases the toxic effects of 6OHDA. Our findings suggest that parkin is an important factor for maintaining Ca2+ homeostasis and that parkin deficiency leads to a phospholipase C-dependent increase in intracellular Ca2+ levels, which make cells more vulnerable to neurotoxins, such as 6OHDA.

The ER and ageing II: calcium homeostasis

Increase in intracellular Ca(2+) concentration occurs by Ca(2+) influx through the plasma membrane and by Ca(2+) release from intracellular stores. The ER is the most important Ca(2+) store. Its stress, characterized by the impairment of Ca(2+) homeostasis and by the accumulation of misfolded proteins, can be induced by different factors. In turn, it induces defense mechanisms such as unfolded protein response, and when it is severe and prolonged, activation of the apoptotic pathway. Damage to the ER, impairment of its function, and a decreased level of its Ca(2+)-handling proteins might all play a role in physiological ageing by handicapping the ER stress response. Thus, healthy ageing is accompanied by subtle alterations of Ca(2+) homeostasis and signaling, including alterations in the ER Ca(2+) load and release. The expression and/or function of ryanodine receptors, IP3 receptors, and SERCA Ca(2+) pumps located in the ER membrane, and Ca(2+)-binding proteins within ER lumen all seem to be affected in aged cells. Data are presented on age-dependent, tissue-specific changes in ER-related Ca(2+) homeostasis in skeletal, cardiac and smooth muscles, as well as in the nervous and immune systems. Disturbances of Ca(2+) homeostasis and of signaling are potential targets for intervention in aged humans.

Subcellular stress response and induction of molecular chaperones and folding proteins after transient global ischemia in rats

Brain ischemia induces the toxic accumulation of unfolded proteins in vulnerable neurons. This cellular event can trigger the unfolded protein response (UPR) and activate the expression of a number of genes involved in pro-survival pathways. One of the pro-survival pathways involves the sequestration and elimination of misfolded and aggregated proteins. Recent evidence suggests that the endoplasmic reticulum (ER), mitochondria, and cytoplasm respond individually to the accumulation of unfolded proteins by induction of organelle specific molecular chaperones and folding enzymes. This study utilized a rat model of transient (15 min) global ischemia (2-vessel occlusion) to investigate the regional and temporal induction of some of these key stress proteins after ischemia. Electron microscopy demonstrated that visible protein aggregates accumulated predominately in the cytoplasm. We used in situ hybridization (forebrain structures) and western blot (hippocampus) analysis to measure changes in expression of heat shock protein 70 (HSP70 cytoplasmic), HSP60 (mitochondrial), ER luminal proteins glucose response proteins GRP78 and GRP94, protein disulphide isomerase (PDI), homocysteine-inducible, endoplasmic reticulum stress-inducible protein (HERP), and calnexin. Induction of mRNA for HSP70 occurred earlier (beginning at 30 min) and at a higher level relative to the delayed (4-24 h) and more moderate induction of mRNAs for mitochondrial matrix HSP60 and the ER lumen HERP, GRP78, GRP94, calnexin and PDI. Increases in hippocampal proteins were observed at 4 h (HSP70) and 24 h (HSP60, GRP78, GRP94) after reperfusion. These results demonstrate that after a transient ischemic insult, the subcellular responses to the accumulation of unfolded proteins varies between cellular compartments and are most prevalent in the cytoplasm and, to a lesser degree, in the mitochondrial matrix and ER lumen.

The effects of dantrolene on hypoxic-ischemic injury in the neonatal rat brain

BACKGROUND

The pathophysiology of brain damage from hypoxia or ischemia has been ascribed to various mechanisms and cascades. Intracellular calcium overload and a calcium excitotoxic cascade have been implicated. It has been suggested that disturbances of endoplasmic reticulum calcium homeostasis are involved in the induction of neuronal cell injury. Two types of intracellular Ca2+-release channels, involving the ryanodyne receptor and the inositol (1,4,5)-triphosphate receptor, are essential for Ca2+ signaling in cells. Dantrolene, which is used for the treatment of malignant hyperthermia syndrome, has been reported to inhibit Ca2+ release through ryanodyne receptors from the endoplasmic reticulum into the cytosol. We designed this study to investigate the neuroprotective effects of dantrolene on hypoxic-ischemic brain damage in the neonatal rat brain.

METHODS

Seven-day-old Sprague-Dawley rats were assigned into two groups; control group (n = 69) and dantrolene group (n = 60). Dimethyl sulfoxide was administered intracerebroventricularly in the control group, and dantrolene in dimethyl sulfoxide was similarly administered to the dantrolene group, before hypoxic-ischemic brain injury (HII). HII was induced by the ligation of the common carotid artery under isoflurane anesthesia, followed by exposure to about 2.5 h of hypoxia (oxygen concentration was maintained at 7%-8%). 1H magnetic resonance spectroscopy was performed 1 day after HII. This noninvasive method evaluated apoptotic processes in the brain after HII. Morphologic score analyses and the calculated percentage of infarct areas after 2,3,5-triphenyltetrazolium chloride staining 14 days after HII were also used to evaluate the effects of dantrolene on HII. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) staining was performed 1 day after HII using 24 more rats.

RESULTS

The lipid/creatine ratios in the right hemispheres in the dantrolene group 1 day after HII were significantly lower than those of the control group (P < 0.05). There was no significant difference between the two groups in the N-acetylaspartate/creatine ratios. The gross morphologic scores were lower in the dantrolene group than in the control group (P < 0.05), and infarct area (%) after 2,3,5-triphenyltetrazolium chloride staining was less in the dantrolene group than in the control group (P < 0.05) 14 days after HII. Further work with 24 rats showed no significant difference, however, in the number of TUNEL positive cells on the two groups.

CONCLUSIONS

Our results show that dantrolene, administered intracerebroventricularly before HII, had a neuroprotective effect in HII model of the neonatal rat brain.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Cerebral ischemia/stroke and small ubiquitin-like modifier (SUMO) conjugation--a new target for therapeutic intervention?

Transient cerebral ischemia/stroke activates various post-translational protein modifications such as phosphorylation and ubiquitin conjugation that are believed to play a major role in the pathological process triggered by an interruption of blood supply and culminating in cell death. A new system of post-translational protein modification has been identified, termed as small ubiquitin-like modifier (SUMO) conjugation. Like ubiquitin, SUMO is conjugated to the lysine residue of target proteins in a complex process. This review summarizes observations from recent experiments focusing on the effect of cerebral ischemia on SUMO conjugation. Transient global and focal cerebral ischemia both induced a rapid, dramatic and long-lasting rise in levels of SUMO2/3 conjugation. After transient focal cerebral ischemia, SUMO conjugation was particularly prominent in neurons located at the border of the ischemic territory where SUMO-conjugated proteins translocated to the nucleus. Many SUMO conjugation target proteins are transcription factors and sumoylation has been shown to have a major impact on the activity, stability, and cellular localization of target proteins. The rise in levels of SUMO-conjugated proteins is therefore likely to have a major effect on the fate of post-ischemic neurons. The sumoylation process could provide an exciting new target for therapeutic intervention.

Quinolinate-induced rat striatal excitotoxicity impairs endoplasmic reticulum Ca2+-ATPase function

Excessive activation of NMDA glutamate receptors and the resulting loss of intracellular Ca(2+) homeostasis may be lethal (excitotoxic) to neurons. Such excitotoxicity can be induced in vivo by intrastriatal infusion of quinolinate, as this substance selectively activates NMDA receptors. The aim of the present research was to investigate whether the in vivo treatment of striatal tissue with quinolinate would lead to an early impairment of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) activity or mitochondrial Ca(2+) sequestration, two intracellular mechanisms involved in Ca(2+) homeostasis and signaling. Sodium quinolinate was infused intrastriatally into adult rats, and 6 h later the brains were removed and the corpora striata dissected. At this time point, striatal sections stained with Fluoro-Jade, a cellular marker of cell death, showed initial signs of neuronal degeneration. In addition, SERCA activity decreased 39% in relation to the activity observed in the control striata. A corresponding decrease of the same magnitude in (45)Ca(2+) uptake by striatal microsomes was also found in the treated striata. Western blot analysis did not indicate any decrease in SERCA levels in striatal tissue after quinolinate infusion. Mitochondrial Ca(2+) sequestration was still preserved in quinolinate-treated striatal tissue when the assay was carried out in the presence of physiological concentrations of ATP and Mg(2+). These results suggest that impairment of the SERCA function may be an early event in excitotoxicity.

Response to neonatal anesthesia: effect of sex on anatomical and behavioral outcome

Numerous studies have documented the consequences of exposure to anesthesia in models of term and post-term infants, evaluating the incidence of cell loss, physiological alterations and cognitive dysfunction. However, surprisingly few studies have investigated the effect of anesthetic exposure on outcomes in newborn rodents, the developmental equivalent of premature human infants. This is critical given that one out of every eight babies born in the United States is premature, with an increased prevalence of surgical procedures required in these individuals. Also, no studies have investigated if the genetic sex of the individual influences the response to neonatal anesthesia. Using the newborn rat as the developmental equivalent of the premature human, we documented the effect of a single bout of exposure to either the inhalant isoflurane or the injectable barbiturate phenobarbital on hippocampal anatomy, hippocampal dependent behavioral performance and normal developmental endpoints in male and female rats. While both forms of anesthesia led to significant decrements in cognitive abilities, along with a significant reduction in volume and neuron number in the hippocampus in adulthood, the decrements were significantly greater in males than in females. Interestingly, the deleterious effects of anesthesia were manifest on developmental measures including surface righting and forelimb grasp, but were not evident on basic physiological parameters including body weight or suckling. These findings point to the hazardous effects of exposure to anesthesia on the developing CNS and the particular sensitivity of males to deficits.

Trifluoperazine protects brain plasma membrane Ca(2+)-ATPase from oxidative damaging

In the central nervous system (CNS), a number of different pathological processes such as necrosis, Parkinson's and Alzheimer's diseases are related to disturbance in calcium homeostasis associated with oxidative stress. Here we compare the susceptibility of rat brain plasma membrane Ca(2+)-ATPase (PMCA) and sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) isoforms to in vitro oxidative stress, and investigate a putative role of trifluoperazine (TFP), an antipsychotic drug that is also a powerful inhibitor of Ca(2+)-transporter proteins, in protecting these enzymes. It is shown that, in rat brain, PMCA is very sensitive to the damage induced by preincubation with Fe(2+)-ascorbate, or Fe(2+)-ascorbate plus H2O2, while SERCA is resistant. Inhibition of PMCA activity promoted by Fe(2+)/ascorbate medium is fully prevented by the presence of microM concentrations of either butylated hydroxytoluene (BHT) or TFP, but only partially protected, or reversed, by dithiothreitol (DTT), pointing to some protein cysteine(s) as one of the main targets for a lipid peroxidation-dependent damaging mechanism. However, when 0.5-1 mM H2O2 is added together with Fe(2+)/ascorbate, both BHT and TFP only partially prevent ATPase activity inhibition, and DTT does not confer any protection, suggesting two possible additional mechanisms involving both lipid peroxidation and direct damage to PMCA at amino acid residues other than cysteines. A possible use of micromolar concentrations of TFP as a direct antioxidant protector for PMCA under oxidative stress conditions is discussed.

Synthetic bastadins modify the activity of ryanodine receptors in cultured cerebellar granule cells

Although the interactions of several natural bastadins with the RyR1 isoform of the ryanodine receptor in sarcoplasmic reticulum has been described, their structure-dependent interference with the RyR2 isoform, mainly expressed in cardiac muscle and brain neurons, has not been studied. In this work, we examined calcium transients induced by natural bastadin 10 and several synthetic bastadins in cultured cerebellar granule cells known to contain RyR2. The fluorescent calcium indicator fluo-3 and confocal microscopy were used to evaluate changes in the intracellular Ca(2+) concentration (Ca(i)), and the involvement of ryanodine receptors was assessed using pharmacological tools. Our results demonstrate that apart from the inactive BAST218F6 (a bisdebromo analogue of bastadin 10), synthetic bastadin 5, and synthetic analogues BAST217B, BAST240 and BAST268 (at concentrations >20 microM) increased Ca(i) in a concentration-dependent, ryanodine- and FK-506-sensitive way, with a potency significantly exceeding that of 20 mM caffeine. Moreover, the same active bastadins at a concentration of 5 muM in the presence of ryanodine prevented a thapsigargin-induced increase in Ca(i). These results indicate that bastadins, acting in a structure-dependent manner, modify the activity of RyR2 in primary neuronal culture and provide new information about structure-related pharmacological properties of bastadins.

Inhalational anesthetics as neuroprotectants or chemical preconditioning agents in ischemic brain

This review will focus on inhalational anesthetic neuroprotection during cerebral ischemia and inhalational anesthetic preconditioning before ischemic brain injury. The limitations and challenges of past and current research in this area will be addressed before reviewing experimental and clinical studies evaluating the effects of inhalational anesthetics before and during cerebral ischemia. Mechanisms underlying volatile anesthetic neuroprotection and preconditioning will also be examined. Lastly, future directions for inhalational anesthetics and ischemic brain injury will be briefly discussed.

Temporospatial coupling of networked synaptic activation of AMPA-type glutamate receptor channels and calcium transients in cultured motoneurons

AMPA-type glutamate receptor (GluR) channels provide fast excitatory synaptic transmission in the CNS, but mediate also cytotoxic insults. It could be shown that AMPA-type GluR channel-mediated chronic excitotoxicity leads to an increased intracellular calcium concentration and plays an important role in neurodegenerative diseases like for example amyotrophic lateral sclerosis (ALS). As calcium is an important mediator of various processes in the cell and calcium signals have to be very precise in the temporospatial resolution, excessive intracellular calcium increases can seriously impair cell function. It is still unclear if AMPA-type receptors can directly interact with the intracellular calcium homeostasis or if other mechanisms are involved in this process. The objective of this study was therefore to investigate the calcium homeostasis in rat motoneurons under physiological stimulation of AMPA-type GluR channels using calcium imaging techniques and patch-clamp recordings simultaneously. It was found that spontaneous excitatory postsynaptic currents of cultured motoneurons did not elicit significant intracellular calcium transients. Large intracellular calcium transients occurred only when preceding fast sodium currents were observed. Pharmacological experiments showed that activation of AMPA-type GluR channels during synaptic transmission has a great functional impact on the calcium homeostasis in motoneurons as all kinds of activity was completely blocked by application of the selective kainate- and AMPA-type GluR channel blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Furthermore we suggest from our experiments that calcium transients of several hundred milliseconds' duration result from release of calcium from the endoplasmic reticulum via activation of ryanodine receptors (calcium-induced calcium release, CICR). Our results help to understand the regulatory function of AMPA-type GluR channels in the intracellular calcium homeostasis which is known to be disturbed in neurodegenerative diseases.

Recombinant human TNFalpha induces concentration-dependent and reversible alterations in the electrophysiological properties of axons in mammalian spinal cord

Increased expression of the proinflammatory cytokine tumor necrosis factor-alpha (TNFalpha) and its soluble receptors is evident within the central nervous system (CNS) following traumatic brain injury and spinal cord injury. TNFalpha is integral to the acute inflammatory cascade that follows neurotrauma and has been shown to have both beneficial and detrimental properties. We examined the effects of varying concentrations (1-5000 ng/mL) of recombinant human TNFalpha (rhTNFalpha) on select electrophysiological properties of excised guinea pig spinal cord tissue. Pulsed electrical stimuli (0.33 Hz) were delivered to strips of isolated ventral white matter in a double sucrose gap chamber. Recordings were made of the compound action potential (CAP) and membrane potential before, during, and after bathing the tissue with rhTNFalpha for 30 min. Increasing concentrations of rhTNFalpha yielded progressively greater reductions in amplitude of the CAP that were temporally associated with depolarization of the resting compound membrane potential. These effects were largely reversed on washout of rhTNFalpha and were not present when heat-denatured rhTNFalpha was introduced. The results provide evidence that elevated concentrations of TNFalpha induce reversible depolarization of the compound membrane potential and reduction in CAP amplitude, sometimes to the point of extinction of the CAP, suggestive of impaired axonal conduction. These observations point to a new mechanism of immune-mediated central conduction deficit. Cytokine-induced alterations in membrane properties and axonal conduction may contribute to neurological deficits following CNS injury by compounding trauma-induced myelinopathy and axonopathy.

Increased neuronal nuclear calcium influx in neonatal seizures

We hypothesized that neonatal seizures lead to increased Ca(2+) influx (nCa(2+)I) in neuronal nuclei of newborn rats and that such increase is nitric-oxide mediated. Neuronal nuclear (45)Ca(2+) influx (nCa(2+)I) was measured in neuronal nuclei of 25 10-day-old male rat-pups newborn brains. They were divided into five groups (n = 5/group). (I) control; (II) hypoxia without seizures; (III) hypoxia with seizures; (IV) kainate, 2 mg/kg intraperitoneal (i.p.)-induced seizures and (V) 7-nitroindazole (7-NINA), 1 mg/kg i.p. pretreated, kainate-induced seizures. nCa(2+)I was significantly (P < 0.05) increased following hypoxia or seizures (hypoxic- or kainate-induced). Post-hypoxic seizures further enhanced nCa(2+)I increase induced by hypoxia (P < 0.05). 7-NINA abated the nCa(2+)I increase induced by kainate. We conclude that (1) kainate or hypoxia-induced seizures in newborn rats modify the neuronal nuclear membrane function, resulting in increased nCa(2+)I, (2) seizures exacerbate the hypoxia-induced increased nCa(2+)I incurred after hypoxia and (3) intranuclear calcium surges during kainate-induced neonatal seizures are nitric oxide-mediated.

Cytochrome c binds to inositol (1,4,5) trisphosphate and ryanodine receptors in vivo after transient brain ischemia in gerbils

Previously we have shown that the biphasic efflux of mitochondrial protein cytochrome c to cytoplasm is one of the important events of the delayed postichemic neuronal death. We concluded that early and transient appearance of cytochrome c in cytoplasm of cells recovering after ischemia was decisive for initiation of the pathological signaling cascade leading to neuronal death, but the precise mechanism remained unknown. In vitro cytochrome c was identified as a messenger that coordinates mitochondrial-endoplasmatic reticulum interactions that drive apoptosis. Here we show that in vivo cytochrome c interacts with inositol (1,4,5) trisphosphate receptor type 1 in gerbil hippocampus subjected to transient brain ischemia and short reperfusion. Moreover, cytochrome c binds also to ryanodine receptor type 2, the role of which in postischemic neuronal death is suggested. The complexes could be coimmunoprecipitated by antibodies against any of the two proteins. Our data verified that the mechanism observed in vitro applies to the pathological in vivo situation.

Disruption of endoplasmic reticulum calcium stores is involved in neuronal death induced by glycolysis inhibition in cultured hippocampal neurons

Disturbances in neuronal calcium homeostasis have been implicated in a variety of neuropathological conditions, including cerebral ischemia, hypoglycemia, and epilepsy, and possibly constitute part of the cell death process associated with chronic neurodegenerative disorders. We investigated if endoplasmic reticulum (ER) calcium stores participate in neuronal death triggered by moderate glycolysis inhibition induced by iodoacetate, an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, in cultured hippocampal neurons. Results show that exposure to iodoacetate leads to a slow partial decrease in cell survival, which is significantly prevented in the absence of Ca(2+) or in the presence of the calcium chelator BAPTA-AM. Treatment with caffeine and a low (1 microM) concentration of ryanodine, which activates the ryanodine receptor (RyR), exacerbates neuronal death, whereas dantrolene and 25 microM ryanodine, which antagonizes RyR, prevents damage. Xestospongin C (XeC), an antagonist of the inositol-3-phosphate (IP(3)) receptor (IP(3)R) also prevents neuronal damage. Inhibitors of the ER calcium ATPase (sarcoendoplasmic reticulum Ca(2+) ATPase; SERCA) have no effect. The decrease in ATP levels induced by iodoacetate is potentiated by caffeine and prevented by dantrolene. Although only a slight increase in glutamate extracellular levels is observed 3.5 hr after iodoacetate exposure, the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, MK-801, efficiently prevents neuronal damage. Taken together, the data suggest that neuronal death induced during moderate glycolysis inhibition involves calcium influx through NMDA receptors and calcium release from intracellular ER stores. These results might be relevant to the understanding the mechanisms involved in neuronal damage related to aging and chronic neurodegenerative diseases, which have been associated with decreased glucose metabolism.

Neurotoxic calcium transfer from endoplasmic reticulum to mitochondria is regulated by cyclin-dependent kinase 5-dependent phosphorylation of tau

Increased levels of mitochondrial-free calcium have been associated with several cell-death paradigms, such as excitotoxicity and ceramide-mediated neuronal death. In the latter, calcium is transferred from the endoplasmic reticulum to mitochondria by a mechanism that is only partly understood. We show here that CDK5 (cyclin-dependent kinase 5) plays a role. Free calcium levels in the endoplasmic reticulum and mitochondria were measured with fluorescent markers in C2-ceramide-treated primary cultures of mesencephalic neurons and differentiated pheochromocytoma PC12 cells. Calcium levels decreased in the endoplasmic reticulum as they increased in mitochondria. Both changes were blocked by the pharmacological and molecular CDK5 inhibitors roscovitine and a dominant-negative form of CDK5. Although the kinase did not mediate the transfer of calcium per se, which required the proapoptotic Bcl-2 family protein t-Bid (the truncated form of Bid), it facilitated the transfer by inducing the clustering of endoplasmic reticulum and mitochondria around the centrosome where they formed close contacts, as shown by immunocytochemistry and electron microscopy. Organelle clustering resulted from CDK5-dependent phosphorylation of the microtubule-associated protein tau on threonine 231. This caused its release from microtubules into the soluble fraction of cellular proteins, which appears to favor retrograde transport of the organelles. Mutation of threonine 231 to alanine, so that tau could not be phosphorylated at this site, prevented the ceramide-induced release of tau from microtubules, organelle clustering, the increase in mitochondrial-free calcium levels, and neuronal death, demonstrating the importance of the CDK5-dependent signaling cascade in this calcium-dependent cell-death mechanism.

Extracellular signal-regulated kinase as an inducer of non-apoptotic neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase, which has been implicated in signaling numerous biological functions ranging from embryonic development to memory formation. Recent reports, including ours, indicate that ERK plays a central role in promoting neuronal degeneration in various neuronal systems including neurodegenerative diseases. Mechanisms involved in ERK-induced neuronal degeneration are beginning to emerge. In this review, we summarize evidence suggesting ERK to be a predominant inducer of a non-apoptotic mode of neuronal death. Further, we discuss the mechanisms and the putative molecular inter-players associated with ERK-mediated neuronal death.

Na+-dependent sources of intra-axonal Ca2+ release in rat optic nerve during in vitro chemical ischemia

The contribution of intracellular stores to axonal Ca2+ overload during chemical ischemia in vitro was examined by confocal microscopy. Ca2+ accumulation was measured by fluo-4 dextran (low-affinity dye, KD approximately 4 microM) or by Oregon Green 488 BAPTA-1 dextran (highaffinity dye, KD approximately 450 nM). Axonal Na+ was measured using CoroNa Green. Ischemia in CSF containing 2 mM Ca2+ caused an approximately 3.5-fold increase in fluo-4 emission after 30 min, indicating a large axonal Ca2+ rise well into the micromolar range. Axonal Na+ accumulation was enhanced by veratridine and reduced, but not abolished, by TTX. Ischemia in Ca2+-free (plus BAPTA) perfusate resulted in a smaller but consistent Ca2+ increase monitored by Oregon Green 488 BAPTA-1, indicating release from intracellular sources. This release was eliminated in large part when Na+ influx was reduced by replacement with N-methyl-D-glucamine (NMDG+; even in depolarizing high K+ perfusate), Li+, or by the application of TTX and significantly increased by veratridine. Intracellular release also was reduced significantly by neomycin or 1-(6-[(17beta-methoxyestra-1,3,5 [10]-trien-17-yl) amino] hexyl)-1H-pyrrole-2,5-dione (U73122 [GenBank]) (phospholipase C inhibitors), heparin [inositol trisphosphate (IP3) receptor blocker], or 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157; mitochondrial Na+/Ca2+ exchange inhibitor) as well as ryanodine. Combining CGP37157 with U73122 [GenBank] or heparin decreased the response more than either agent alone and significantly improved electrophysiological recovery. Our conclusion is that intra-axonal Ca2+ release during ischemia in rat optic nerve is mainly dependent on Na+ influx. This Na+ accumulation stimulates three distinct intra-axonal sources of Ca2+: (1) the mitochondrial Na+/Ca2+ exchanger driven in the Na+ import/Ca2+ export mode, (2) positive modulation of ryanodine receptors, and (3) promotion of IP3 generation by phospholipase C.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

PERK is responsible for the increased phosphorylation of eIF2alpha and the severe inhibition of protein synthesis after transient global brain ischemia

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons that is due to inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2). To address the role of the eIF2alpha kinase RNA-dependent protein kinase-like endoplasmic reticulum kinase (PERK) in the reperfused brain, transgenic mice with a targeted disruption of the Perk gene were subjected to 20 min of forebrain ischemia followed by 10 min of reperfusion. In wild-type mice, phosphorylated eIF2alpha was detected in the non-ischemic brain and its levels were elevated threefold after 10 min of reperfusion. Conversely, there was no phosphorylated eIF2alpha detected in the non-ischemic transgenic mice and there was no sizeable rise in phosphorylated eIF2alpha levels in the forebrain after ischemia and reperfusion. Moreover, there was a substantial rescue of protein translation in the reperfused transgenic mice. Neither group showed any change in total eIF2alpha, phosphorylated eukaryotic elongation factor 2 or total eukaryotic elongation factor 2 levels. These data demonstrate that PERK is responsible for the large increase in phosphorylated eIF2alpha and the suppression of translation early in reperfusion after transient global brain ischemia.

Intracellular calcium disturbances induced by arsenic and its methylated derivatives in relation to genomic damage and apoptosis induction

Arsenic and its methylated derivatives are contaminants of air, water, and food and are known as toxicants and carcinogens. Arsenic compounds are also being used as cancer chemotherapeutic agents. In humans, inorganic arsenic is metabolically methylated to mono-, di-, and trimethylated forms. Recent findings suggest that the methylation reactions represent a toxification rather than a detoxification pathway. In recent years, the correlation between arsenic exposure, cytotoxicity and genotoxicity, mutagenicity, and tumor promotion has been established, as well as the association of arsenic exposure with perturbation of physiologic processes, generation of reactive oxygen species, DNA damage, and apoptosis induction. Trivalent forms of arsenic have been found to induce apoptosis in several cellular systems with involvement of membrane-bound cell death receptors, activation of caspases, release of calcium stores, and changes of the intracellular glutathione level. It is well known that calcium ion deregulation plays a critical role in apoptotic cell death. A calcium increase in the nuclei might lead to toxic effects in the cell. In this review, we highlight the relationship between induced disturbances of calcium homeostasis, genomic damage, and apoptotic cell death caused by arsenic and its organic derivatives.

Calcium signaling and neuronal vulnerability to ischemia in the striatum

Neurons express extremely different sensitivity to ischemic insults. The neuronal vulnerability is region-specific and the striatum is among the most susceptible areas to ischemic damage. Projecting GABAergic medium-sized neurons are very sensitive to energy metabolism impairment, whereas interneurons are selectively spared. However, the reasons for this differential vulnerability are largely unknown. Calcium ions (Ca2+) are important intracellular messengers enabling several physiological processes. However, excessive Ca2+ influx from the extracellular space or release from internal stores can elevate Ca2+ to levels that exceed the capacity of single neurons to appropriately buffer such overload. This capacity also appears to be a peculiar feature of single neuronal subtypes. This review will provide a brief survey of the ionic basis underlying the differential responses to in vitro ischemia of distinct striatal neuronal subtypes, mainly focusing on the role of Ca2+. The potential relevance of these findings in the development of therapeutic strategies for acute stroke will be discussed.

Subtype-Specific and ER Lumenal Environment-Dependent Regulation of Inositol 1,4,5-Trisphosphate Receptor Type 1 by ERp44

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are intracellular channel proteins that mediate Ca(2+) release from the endoplasmic reticulum (ER) and are involved in many biological processes and diseases. IP(3)Rs are differentially regulated by a variety of cytosolic proteins, but their regulation by ER lumenal protein(s) remains largely unexplored. In this study, we found that ERp44, an ER lumenal protein of the thioredoxin family, directly interacts with the third lumenal loop of IP(3)R type 1 (IP(3)R1) and that the interaction is dependent on pH, Ca(2+) concentration, and redox state: the presence of free cysteine residues in the loop is required. Ca(2+)-imaging experiments and single-channel recording of IP(3)R1 activity with a planar lipid bilayer system demonstrated that IP(3)R1 is directly inhibited by ERp44. Thus, ERp44 senses the environment in the ER lumen and modulates IP(3)R1 activity accordingly, which should in turn contribute to regulating both intralumenal conditions and the complex patterns of cytosolic Ca(2+) concentrations.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Endoplasmic reticulum Ca(2+) homeostasis and neuronal death

The endoplasmic reticulum (ER) is a universal signalling organelle, which regulates a wide range of neuronal functional responses. Calcium release from the ER underlies various forms of intracellular Ca(2+) signalling by either amplifying Ca(2+) entry through voltage-gated Ca(2+) channels by Ca(2+)-induced Ca(2+) release (CICR) or by producing local or global cytosolic calcium fluctuations following stimulation of metabotropic receptors through inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The ER Ca(2+) store emerges as a single interconnected pool, thus allowing for a long-range Ca(2+) signalling via intra-ER tunnels. The fluctuations of intra-ER free Ca(2+) concentration regulate the activity of numerous ER resident proteins responsible for post-translational protein folding and modification. Disruption of ER Ca(2+) homeostasis results in the developing of ER stress response, which in turn controls neuronal survival. Altered ER Ca(2+) handling may be involved in pathogenesis of various neurodegenerative diseases including brain ischemia and Alzheimer dementia.