Role of calcium in the activation of erp72 and heme oxygenase-1 expression on depletion of endoplasmic reticulum calcium stores in rat neuronal cell culture

Protein disulfide isomerase plays a crucial role in mediating chemically-induced, glutathione depletion-associated hepatocyte injury in vitro and in vivo

Recently we have shown that protein disulfide isomerase (PDI or PDIA1) is involved in mediating chemically-induced, glutathione (GSH) depletion-associated ferroptotic cell death through NOS activation (dimerization) and NO accumulation. The present study aims to determine the role of PDI in mediating chemically-induced hepatocyte injury in vitro and in vivo and whether PDI inhibitors can effectively protect against chemically-induced hepatocyte injury. We show that during the development of erastin-induced ferroptotic cell death, accumulation of cellular NO, ROS and lipid-ROS follows a sequential order, i.e., cellular NO accumulation first, followed by accumulation of cellular ROS, and lastly cellular lipid-ROS. Cellular NO, ROS and lipid-ROS each play a crucial role in mediating erastin-induced ferroptosis in cultured hepatocytes. In addition, it is shown that PDI is an important upstream mediator of erastin-induced ferroptosis through PDI-mediated conversion of NOS monomer to its dimer, which then leads to accumulation of cellular NO, ROS and lipid-ROS, and ultimately ferroptotic cell death. Genetic manipulation of PDI expression or pharmacological inhibition of PDI function each can effectively abrogate erastin-induced ferroptosis. Lastly, evidence is presented to show that PDI is also involved in mediating acetaminophen-induced liver injury in vivo using both wild-type C57BL/6J mice and hepatocyte-specific PDI conditional knockout (PDIfl/fl Alb-cre) mice. Together, our work demonstrates that PDI is an important upstream mediator of chemically-induced, GSH depletion-associated hepatocyte ferroptosis, and inhibition of PDI can effectively prevent this injury.

PDIA4: The basic characteristics, functions and its potential connection with cancer

Disulfide bond formation is catalyzed by the protein disulfide Isomerases (PDI) family. This is a critical step in protein folding which occurs within the endoplasmic reticulum. PDIA4, as a member of the PDI family, can cause the adjustment of αIIβ 3 affinities which activate platelet and promote thrombosis formation. Endoplasmic reticulum response is triggered by accumulation of abnormal folding proteins concomitant with increasing PDIA4 expression. Besides, current researches indicate that activated platelets and ERS response affect tumor progression. And PDIA4, as previous reported, also participates in tumor progression by affecting cell apoptosis and DNA repair machinery without specific mechanisms revealed.Therefore, PDI inhibitor might possess great potential value in against tumor progression. In this review, we summarize information on PDIA4 including its the basic characteristics and its implication on tumor.

Differential Effects of Antiretroviral Drugs on Neurons In Vitro: Roles for Oxidative Stress and Integrated Stress Response

Mounting evidence suggests that antiretroviral drugs may contribute to the persistence of HIV-associated neurocognitive disorders (HAND), which impact 30%-50% of HIV-infected patients in the post-antiretroviral era. We previously reported that two first generation HIV protease inhibitors, ritonavir and saquinavir, induced oxidative stress, with subsequent neuronal death in vitro, which was reversed by augmentation of the endogenous antioxidant response by monomethyl fumarate. We herein determined whether two newer-generation PIs, darunavir and lopinavir, were deleterious to neurons in vitro. Further, we expanded our assessment to include three integrase strand transfer inhibitors, raltegravir, dolutegravir, and elvitegravir. We found that only lopinavir and elvitegravir were neurotoxic to primary rat neuroglial cultures as determined by the loss of microtubule-associated protein 2 (MAP2). Intriguingly, lopinavir but not elvitegravir led to oxidative stress and induced the endogenous antioxidant response (EAR). Furthermore, neurotoxicity of lopinavir was blocked by pharmacological augmentation of the endogenous antioxidant heme oxygenase-1 (HO-1), expanding our previous finding that protease inhibitor-induced neurotoxicity was mediated by oxidative stress. Conversely, elvitegravir but not lopinavir led to increased eIF2α phosphorylation, indicating the activation of a common adaptive pathway termed the integrated stress response (ISR), and elvitegravir-mediated neurotoxicity was partially alleviated by the ISR inhibitor trans-ISRIB, suggesting ISR as a promoter of elvitegravir-associated neurotoxicity. Overall, we found that neurotoxicity was induced only by a subset of protease inhibitors and integrase strand transfer inhibitors, providing evidence for class- and drug-specific neurotoxic effects of antiretroviral drugs. Future in vivo studies will be critical to confirm the neurotoxicity profiles of these drugs for incorporation of these findings into patient management. The EAR and ISR pathways are potential access points for the development of adjunctive therapies to complement antiretroviral therapies and limit their contribution to HAND persistence.

Micelles of zinc protoporphyrin conjugated to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer for imaging and light-induced antitumor effects in vivo

We synthesized N-(2-hydroxypropyl)methacrylamide polymer conjugated with zinc protoporphyrin (HPMA-ZnPP) and evaluated its application for tumor detection by imaging and treatment by light exposure using in mouse sarcoma model. To characterize HPMA-ZnPP micelle, we measured its micellar size, surface charge, stability, photochemical, biochemical properties and tissue distribution. In vivo anti-tumor effect and fluorescence imaging were carried out to validate the tumor selective accumulation and therapeutic effect by inducing singlet oxygen by light exposure. HPMA-ZnPP was highly water soluble and formed micelles spontaneously having hydrophobic clustered head group of ZnPP, in aqueous solution, with a hydrodynamic diameter of 82.8±41.8 nm and zeta-potential of +1.12 mV. HPMA-ZnPP had a long plasma half-life and effectively and selectively accumulated in tumors. Although HPMA-ZnPP alone had no toxicity in S-180 tumor-bearing mice, light-irradiation significantly suppressed tumor growth in vivo, similar to the cytotoxicity to HeLa cells in vitro upon endoscopic light-irradiation. HPMA-ZnPP can visualize tumors by fluorescence after i.v. injection, which suggests that this micelle may be useful for both tumor imaging and therapy. Here we describe preparation of a new fluorescence nanoprobe that is useful for simultaneous tumor imaging and treatment, and application to fluorescence endoscopy is now at visible distance.

Endoplasmic reticulum stress inducers provide protection against 6-hydroxydopamine-induced cytotoxicity

6-Hydroxydopamine (6-OHDA) is a neurotoxin used to establish experimental models of Parkinson's disease. Exposure to 6-OHDA results in cell death associated with oxidative stress. Pretreatments with sublethal oxidative stress and some pharmacological drugs have been shown to exert preconditioning effects on cytotoxicity caused by 6-OHDA. In this study, we investigated whether endoplasmic reticulum (ER) stress exerts preconditioning effects on 6-OHDA-induced cytotoxicity in human neuroblastoma SH-SY5Y cells. Pretreatment with ER stress inducers, thapsigargin (Tg) and tunicamycin (Tm), promoted GRP78 mRNA induction and ATF4 translation, which are ER stress markers, under our experimental conditions and protected against the cytotoxicity. The protective effect of Tg was more potent than that of Tm. We also found that Tg induced the expression of the antioxidant gene heme oxygenase-1 (HO-1) in a dose-dependent manner, whereas Tm had a weak effect on HO-1 induction. Flow cytometric analysis revealed that reactive oxygen species generated by 6-OHDA were more effectively suppressed in cells pretreated with Tg than with Tm. Therefore, it is likely that Tg enhances antioxidative defenses in SH-SY5Y cells compared with Tm. Because actinomycin D inhibited HO-1 induction by Tg, the induction of HO-1 may be regulated at the transcriptional level. Moreover, the specific eIF2α phosphatase inhibitor salubrinal augmented Tg-induced HO-1 expression. Therefore, the downstream signaling pathway of eIF2α might be involved in Tg-induced HO-1 expression. On the other hand, the reporter assay revealed that Tg stimulated the antioxidant response element (ARE) that is located in regulatory regions of antioxidant genes such as HO-1. Taken together, our data suggest that preconditioning effects induced by Tg mediate an adaptive response to 6-OHDA-induced cytotoxicity via phosphorylation of eIF2α and activation of the ARE.

Membrane depolarization induces calcium-dependent upregulation of Hsp70 and Hmox-1 in skeletal muscle cells

Heat shock proteins (HSPs) are a conserved family of cytoprotective polypeptides, synthesized by cells in response to stress. Hsp70 and heme oxygenase 1 (Hmox-1) are induced by a variety of cellular stressors in skeletal muscle, playing a role in long-term adaptations and muscle fibers regeneration. Though HSPs expression after exercise has been intensely investigated, the molecular mechanisms concerning Hsp70 and Hmox-1 induction are poorly understood. The aim of this work was to investigate the involvement of calcium in Hsp70 and Hmox-1 expression upon depolarization of skeletal muscle cells. We observed that depolarization of myotubes increased both mRNA levels and protein expression for Hsp70 and Hmox-1. Stimulation in the presence of intracellular calcium chelator BAPTA-AM resulted in a complete inhibition of Hsp70-induced expression. It is known that inositol-1,4,5-trisphophate (IP(3))-mediated slow Ca(2+) transients, evoked by membrane depolarization, are involved in the regulation of gene expression. Here we demonstrated that inhibition of IP(3)-dependent calcium signals decreased both Hsp70 mRNA induction and Hsp70 and Hmox-1 protein expression. Inhibitors of calcium-dependent protein kinase C also abolished Hsp70 mRNA induction. Our results provide evidence that membrane depolarization increases Hsp70 and Hmox-1 expression in cultured skeletal muscle cells, which the effect is critically dependent on Ca(2+) released from IP(3)-sensitive intracellular stores and that it involves PKC as an upstream effector in Hsp70 mRNA-induced expression.

Methamphetamine induces dopamine D1 receptor-dependent endoplasmic reticulum stress-related molecular events in the rat striatum

Methamphetamine (METH) is an illicit toxic psychostimulant which is widely abused. Its toxic effects depend on the release of excessive levels of dopamine (DA) that activates striatal DA receptors. Inhibition of DA-mediated neurotransmission by the DA D1 receptor antagonist, SCH23390, protects against METH-induced neuronal apoptosis. The initial purpose of the present study was to investigate, using microarray analyses, the influence of SCH23390 on transcriptional responses in the rat striatum caused by a single METH injection at 2 and 4 hours after drug administration. We identified 545 out of a total of 22,227 genes as METH-responsive. These include genes which are involved in apoptotic pathways, endoplasmic reticulum (ER) stress, and in transcription regulation, among others. Of these, a total of 172 genes showed SCH23390-induced inhibition of METH-mediated changes. Among these SCH23390-responsive genes were several genes that are regulated during ER stress, namely ATF3, HSP27, Hmox1, HSP40, and CHOP/Gadd153. The secondary goal of the study was to investigate the role of DA D1 receptor stimulation on the expression of genes that participate in ER stress-mediated molecular events. We thus used quantitative PCR to confirm changes in the METH-responsive ER genes identified by the microarray analyses. We also measured the expression of these genes and of ATF4, ATF6, BiP/GRP78, and of GADD34 over a more extended time course. SCH23390 attenuated or blocked METH-induced increases in the expression of the majority of these genes. Western blot analysis revealed METH-induced increases in the expression of the antioxidant protein, Hmox1, which lasted for about 24 hours after the METH injection. Additionally, METH caused DA D1 receptor-dependent transit of the Hmox1 regulator protein, Nrf2, from cytosolic into nuclear fractions where the protein exerts its regulatory functions. When taken together, these findings indicate that SCH23390 can provide protection against neuronal apoptosis by inhibiting METH-mediated DA D1 receptor-mediated ER stress in the rat striatum. Our data also suggest that METH-induced toxicity might be a useful model to dissect molecular mechanisms involved in ER stress-dependent events in the rodent brain.

Induction of heme oxygenase-1 and heat shock protein 70 in rat hepatocytes: the role of calcium signaling

Stress response genes including heat shock proteins are induced under a variety of conditions to confer cellular protection. This study investigated the role of calcium signaling in the induction of two stress response genes, heme oxygenase-1/hsp32 and hsp70, in isolated rat hepatocytes. Both genes were induced by cellular glutathione depletion. This induction could be inhibited by BAPTA-AM. Culturing in a calcium-free medium prevented the induction of hsp70 gene expression after glutathione depletion without affecting heme oxygenase-1 gene expression. Thapsigargin increased the gene expression of heme oxygenase-1 but not that of hsp70. Thapsigargin-induced heme oxygenase-1 induction was completely inhibited by BAPTA-AM. Incubation with the Ca²⁺-ionophore A23187 augmented heme oxygenase-1 (two-fold) and hsp70 (5.2-fold) mRNA levels. Our data suggests a significant role of Ca²⁺-dependent pathways in the induction of the two stress genes. An increase in the cytoplasmic Ca²⁺ activity seems to play a key role in the cascade of signaling leading to the induction of the two genes. However, the source of Ca²⁺ that fluxes into the cytoplasm seems to be different. Our data provides evidence for a compartmentalization of calcium fluxes, i.e. the Ca²⁺ flux from intracellular stores (e.g. the endoplasmic reticulum) plays a major role in the induction of heme oxygenase-1. By contrast, Ca²⁺ flux from the extracellular medium seems to be a mechanism initiating the cellular signaling cascade leading to hsp70 gene induction.

Heme Oxygenase-1 Protects Gastric Mucosal Cells against Non-steroidal Anti-inflammatory Drugs

Gastric mucosal cell death by non-steroidal anti-inflammatory drugs (NSAIDs) is suggested to be involved in NSAID-induced gastric lesions. Therefore, cellular factors that suppress this cell death are important for protection of the gastric mucosa from NSAIDs. Heme oxygenase-1 (HO-1) is up-regulated by various stressors and protects cells against stressors. Here, we have examined up-regulation of HO-1 by NSAIDs and the contribution of HO-1 to the protection of gastric mucosal cells against NSAIDs both in vitro and in vivo. In cultured gastric mucosal cells, all NSAIDs tested up-regulated HO-1. In rats, orally administered indomethacin up-regulated HO-1, induced apoptosis, and produced lesions at gastric mucosa. An inhibitor of HO-stimulated NSAID-induced apoptosis in vitro and in vivo and also stimulated NSAID-produced gastric lesions, suggesting that NSAID-induced up-regulation of HO-1 protects the gastric mucosa from NSAID-induced gastric lesions by inhibiting NSAID-induced apoptosis. Indomethacin activated the HO-1 promoter and caused nuclear accumulation of NF-E2-related factor 2 (Nrf2), a transcription factor for the HO-1 gene. Examination of phosphorylation of p38 mitogen-activated protein kinase (MAPK) and experiments with its inhibitor strongly suggest that the nuclear accumulation of Nrf2 and resulting up-regulation of HO-1 by NSAIDs is mediated through NSAID-dependent activation (phosphorylation) of p38 MAPK. This is the first report showing the protective role of HO-1 against irritant-induced gastric lesions.

Heme oxygenase-1 induction by dieldrin in dopaminergic cells

We investigated the transcriptional events and signaling pathways involved in the induction of heme oxygenase-1 (HO-1) by dieldrin, an environmental risk factor of Parkinson's disease, in a dopaminergic neuronal cells (SN4741). Dieldrin exposure caused dose-dependent and time-dependent induction of heme oxygenase activity and HO-1 protein expression. Deletional and mutational analyses showed that the 5' distal enhancers, E1 and E2, mediate dieldrin-induced HO-1 gene transcription, and the AP-1 DNA binding sites in the E2 enhancer are critical for E2-mediated HO-1 gene activation. Furthermore, both the p38 and JNK mitogen-activated protein kinase pathways are utilized for HO-1 transcriptional activation by dieldrin. HO-1 inhibitor, ZnPP IX reduced the expression of HO-1 but enhanced the cytotoxicity induced by dieldrin.

Endoplasmic reticulum stress stimulates heme oxygenase-1 gene expression in vascular smooth muscle. Role in cell survival

Heme oxygenase-1 (HO-1) is a cytoprotective protein that catalyzes the degradation of heme to biliverdin, iron, and carbon monoxide (CO). In the present study, we found that endoplasmic reticulum (ER) stress induced by a variety of experimental agents stimulated a time- and concentration-dependent increase in HO-1 mRNA and protein in vascular smooth muscle cells (SMC). The induction of HO-1 by ER stress was blocked by actinomycin D or cycloheximide and was independent of any changes in HO-1 mRNA stability. Luciferase reporter assays indicated that ER stress stimulated HO-1 promoter activity via the antioxidant response element. Moreover, ER stress induced the nuclear import of Nrf2 and the binding of Nrf2 to the HO-1 antioxidant response element. Interestingly, ER stress stimulated SMC apoptosis, as demonstrated by annexin V binding, caspase-3 activation, and DNA laddering. The induction of apoptosis by ER stress was potentiated by HO inhibition, whereas it was prevented by addition of HO substrate. In addition, exposure of SMC to exogenously administered CO inhibited ER stress-mediated apoptosis, and this was associated with a decrease in the expression of the proapoptotic protein, GADD153. In contrast, the other HO-1 products failed to block apoptosis or GADD153 expression during ER stress. These results demonstrated that ER stress is an inducer of HO-1 gene expression in vascular SMC and that HO-1-derived CO acts in an autocrine fashion to inhibit SMC apoptosis. The capacity of ER stress to stimulate the HO-1/CO system provides a novel mechanism by which this organelle regulates cell survival.

Endoplasmic reticulum: a primary target in various acute disorders and degenerative diseases of the brain

Changes in neuronal calcium activity in the various subcellular compartments have divergent effects on affected cells. In the cytoplasm and mitochondria, where calcium activity is normally low, a prolonged excessive rise in free calcium levels is believed to be toxic, in the endoplasmic reticulum (ER), in contrast, calcium activity is relatively high and severe stress is caused by a depletion of ER calcium stores. Besides its role in cellular calcium signaling, the ER is the site where membrane and secretory proteins are folded and processed. These calcium-dependent processes are fundamental to normal cell functioning. Under conditions of ER dysfunction unfolded proteins accumulate in the ER lumen, a signal responsible for activation of the unfolded protein response (UPR) and the ER-associated degradation (ERAD). UPR is characterized by activation of two ER-resident kinases, PKR-like ER kinase (PERK) and IRE1. PERK induces phosphorylation of the eukaryotic initiation factor (eIF2alpha), resulting in a shut-down of translation at the initiation step. This stress response is needed to block new synthesis of proteins that cannot be correctly folded, and thus to protect cells from the effect of unfolded proteins which tend to form toxic aggregates. IRE1, on the other hand, is turned after activation into an endonuclease that cuts out a sequence of 26 bases from the coding region of xbp1 mRNA. Processed xbp1 mRNA is translated into the respective protein, an active transcription factor specific for ER stress genes such as grp78. In acute disorders and degenerative diseases, the ER calcium pool is a primary target of toxic metabolites or intermediates, such as oxygen free radicals, produced during the pathological process. Affected neurons need to activate the entire UPR to cope with the severe form of stress induced by ER dysfunction. This stress response is however hindered under conditions where protein synthesis is suppressed to such an extent that processed xbp1 mRNA is not translated into the processed XBP1 protein (XBP1(proc)). Furthermore, activation of ERAD is important for the degradation of unfolded proteins through the ubiquitin/proteasomal pathway, which is impaired in acute disorders and degenerative diseases, resulting in further ER stress. ER functioning is thus impaired in two different ways: first by the direct action of toxic intermediates, produced in the course of the pathological process, hindering vital ER reactions, and second by the inability of cells to fully activate UPR and ERAD, leaving them unable to withstand the severe form of stress induced by ER dysfunction.

Shutdown of translation: lethal or protective? Unfolded protein response versus apoptosis

Shutdown of translation is a highly conserved response of cells to a severe form of metabolic, thermal, or physical stress. After the metabolic stress induced by transient cerebral ischemia, translational recovery is observed only in cells that withstand the transient interruption of blood supply, implying that restoration of translation critically determines the final outcome. On the other hand, apoptosis is believed to play a role in ischemia-induced cell death. Apoptosis is an active process that is blocked by agents known to suppress protein synthesis. Thus, the question arises whether stress-induced suppression of protein synthesis is protective or toxic for the affected cells. Accepting the notion that endoplasmic reticulum (ER) dysfunction is the mechanism underlying shutdown of translation after transient cerebral ischemia, an attempt may be made to try to solve the protein synthesis paradox by understanding the role of protein synthesis suppression in conditions associated with ER dysfunction. Endoplasmic reticulum dysfunction-induced accumulation of unfolded proteins in the ER lumen is the trigger of two signal transduction pathways: PKR-like ER kinase-induced shutdown of translation to suppress new synthesis of proteins that cannot be correctly folded, and IRE1-induced expression of ER stress genes, a protein synthesis-dependent pathway needed to restore ER functions. Together these comprise the unfolded protein response. They are also induced after transient ischemia, implying a dual effect of protein synthesis suppression, a protective and a pathologic effect during early and prolonged reperfusion.

Transient cerebral ischemia activates processing of xbp1 messenger RNA indicative of endoplasmic reticulum stress

Cells respond to conditions associated with endoplasmic reticulum (ER) dysfunction with activation of the unfolded protein response, characterized by a shutdown of translation and induction of the expression of genes coding for ER stress proteins. The genetic response is based on IRE1-induced processing of xbp1 messenger RNA (mRNA), resulting in synthesis of new XBP1proc protein that functions as a potent transcription factor for ER stress genes. xbp1 processing in models of transient global and focal cerebral ischemia was studied. A marked increase in processed xbp1 mRNA levels during reperfusion was observed, most pronounced (about 35-fold) after 1-h occlusion of the right middle cerebral artery. The rise in processed xbp1 mRNA was not paralleled by a similar increase in XBP1proc protein levels because transient ischemia induces severe suppression of translation. As a result, mRNA levels of genes coding for ER stress proteins were only slightly increased, whereas mRNA levels of heat-shock protein 70 rose about 550-fold. Under conditions associated with ER dysfunction, cells require activation of the entire ER stress-induced signal transduction pathway, to cope with this severe form of stress. After transient cerebral ischemia, however, the block of translation may prevent synthesis of new XBP1proc protein and thus hinder recovery from ischemia-induced ER dysfunction.

Expression of endoplasmic reticulum stress proteins during skeletal muscle disuse atrophy

Disuse atrophy of skeletal muscle leads to an upregulation of genes encoding sarcoplasmic reticulum (SR) calcium-handling proteins. Because many of the proteins that are induced with endoplasmic reticulum (ER) stress are ER calcium-handling proteins, we sought to determine whether soleus muscle atrophy was associated with a prototypical ER stress response. Seven days of rat hindlimb unloading did not alter expression of ubiquitous ER stress proteins such as Grp78, calreticulin, and CHOP/GADD-153, nor other proteins that have been shown to be activated by ER stressors such as vinculin, the type I D-myo-inositol 1,4,5-trisphosphate receptor, or protein kinase R, a eukaryotic initiation factor 2 alpha kinase. On the other hand, expression of heme oxygenase-1 (HO-1), an antioxidant ER stress protein, was significantly increased 2.2-fold. In addition, unloading led to an increase in calsequestrin, the muscle-specific SR calcium-binding protein, at both the mRNA (68%) and protein (24%) levels. Although disuse atrophy is associated with a significant remodeling of muscle-specific proteins controlling SR calcium flux, it is not characterized by a prototypical ER stress response. However, the upregulation of HO-1 may indicate ER adaptation to oxidative stress during muscle unloading.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Effect of nitric oxide on endoplasmic reticulum calcium homeostasis, protein synthesis and energy metabolism

It has been suggested that nitric oxide (NO) may contribute to ischemia-induced cell injury. However, the mechanisms underlying NO toxicity have not yet been fully elucidated. In the present study, we investigated the effect of NO on the level of endoplasmic reticulum (ER) calcium stores, on ER Ca2+ pump activity, on protein synthesis, on concentrations of high-energy phosphates, and on gadd153 mRNA levels. Primary neuronal cells were exposed to the NO-donor (+/-)-S-Nitroso-N-acetylpenicillamine (SNAP) for 1 h, 2 h, 6 h or 24 h. The level of ER calcium stores was evaluated by measuring the increase in cytoplasmic calcium activity induced by exposing cells to thapsigargin, an irreversible inhibitor of ER Ca(2+)-ATPase; the activity of ER Ca(2+)-ATPase was determined by measuring a phosphorylated intermediate; SNAP-induced changes in gadd153 expression were evaluated by quantitative PCR; SNAP-induced changes in protein synthesis were investigated by measuring the incorporation of L-[4,5-3H]leucine into proteins, and changes in the levels of ATP, ADP, AMP were measured by HPLC. Exposing cells to SNAP for 1 h to 2 h induced a marked depletion of ER calcium stores through an inhibition of ER Ca(2+)-ATPase (to 58% of control), and a concentration-dependent suppression of protein synthesis which was reversed in the presence of hemoglobin, suggesting NO-related effects. ATP levels and adenylate energy charge were significantly decreased only when cells were exposed to the highest SNAP concentration for 6 h or 24 h, excluding significant effects of NO on the energy state of cells in the acute state, i.e. when ER calcium stores were already completely depleted and protein synthesis severely suppressed. In light of the regulatory role of ER calcium homeostasis in the control of protein synthesis, the results imply that the suppression of protein synthesis resulted from NO-induced inhibition of ER Ca(2+)-ATPase and depletion of ER calcium stores, and that NO-induced disturbances of energy metabolism are secondary to the effect of NO on ER calcium homeostasis. It is, therefore, concluded that ER calcium stores are a primary target of NO-toxicity.

Disturbances of the functioning of endoplasmic reticulum: a key mechanism underlying neuronal cell injury?

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.