Disturbed activation of endoplasmic reticulum stress transducers by familial Alzheimer's disease-linked presenilin-1 mutations

Rapid, transient induction of ER stress in the liver and kidney after acute exposure to heavy metal: evidence from transgenic sensor mice

Endoplasmic reticulum (ER) stress-responsive alkaline phosphatase (ES-TRAP) serves as a sensitive indicator for ER stress. In response to heavy metals including cadmium, nickel and cobalt, hepatocytes and renal tubular cells expressing ES-TRAP exhibited ER stress and decreased ES-TRAP activity. In ES-TRAP transgenic mice, acute exposure to cadmium showed rapid, transient decreases in the activity of serum ES-TRAP. It was inversely correlated with the induction of endogenous ER stress markers in the liver and kidney. Our result provides first evidence for the acute, reversible induction of ER stress in vivo after exposure to heavy metal.

Role of caspase-12 in amyloid beta-peptide-induced toxicity in organotypic hippocampal slices cultured for long periods

Amyloid beta (Abeta) toxicity has been implicated in cell death in the hippocampus, but its specific mechanisms are poorly understood. In this study, Abeta-induced cell death was investigated in organotypic hippocampal slice cultures (OHCs) that were cultured for various periods in vitro. There were no obvious histological differences among slices cultured for 3 to 7 weeks in vitro. Although there was little neurotoxicity after treatment with Abeta25-35 in OHCs cultured for relatively shorter periods (3-5 weeks), age-dependent cell death was evident in OHCs cultured for relatively longer periods (6-7 weeks) after exposure to Abeta25-35. In OHCs cultured for 7 weeks, S-allyl-L-cysteine (SAC), a component of aged garlic extract, protected the cells in areas CA1 and CA3 and the dentate gyrus from Abeta25-35-induced toxicity. The immunoreactivity of cleaved caspase-12 was increased whereas that of glucose-regulated protein 78 was not altered after exposure to Abeta25-35. The increases in the cleaved caspase-12 were also reversed by simultaneously applied SAC. These results suggest that OHCs cultured for relatively longer periods are more susceptible to Abeta-induced toxicity and that the Abeta-induced cell death involves caspase-12-dependent pathways. It is also suggested that SAC is able to protect against the Abeta-induced neuronal cell death through the inhibition of the caspase-12-dependent pathway.

A role for presenilin in post-stress regulation: effects of presenilin mutations on Ca2+ currents in Drosophila

It has been shown that presenilin is involved in maintaining Ca2+ homeostasis in neurons, including regulating endoplasmic reticulum (ER) Ca2+ storage. From studies of primary cultures and cell lines, however, its role in stress-induced responses is still controversial. In the present study we analyzed the effects of presenilin mutations on membrane currents and synaptic functions in response to stress using an in vivo preparation. We examined voltage-gated K+ and Ca2+ currents at the Drosophila larval neuromuscular junction (NMJ) with voltage-clamp recordings. Our data showed that both currents were generally unaffected by loss-of-function or Alzheimer's disease (AD) -associated presenilin mutations under normal or stress conditions induced by heat shock (HS) or ER stress. In larvae expressing the mutant presenilins, prolonged Ca2+ tail current, reflecting slower deactivation kinetics of Ca2+ channels, was observed 1 day after stress treatments were terminated. It was further demonstrated that the L-type Ca2+ channel was specifically affected under these conditions. Moreover, synaptic plasticity at the NMJ was reduced in larvae expressing the mutant presenilins. At the behavioral level, memory in adult flies was impaired in the presenilin mutants 1 day after HS. The results show that presenilin function is important during the poststress period and its impairment contributes to memory dysfunction observed during adaptation to normal conditions after stress. Our findings suggest a new stress-related mechanism by which presenilin may be implicated in the neuropathology of AD.

Endoplasmic reticulum chaperones inhibit the production of amyloid-beta peptides

Abeta (amyloid-beta peptides) generated by proteolysis of APP (beta-amyloid precursor protein), play an important role in the pathogenesis of AD (Alzheimer's disease). ER (endoplasmic reticulum) chaperones, such as GRP78 (glucose-regulated protein 78), make a major contribution to protein quality control in the ER. In the present study, we examined the effect of overexpression of various ER chaperones on the production of Abeta in cultured cells, which produce a mutant type of APP (APPsw). Overexpression of GRP78 or inhibition of its basal expression, decreased and increased respectively the level of Abeta40 and Abeta42 in conditioned medium. Co-expression of GRP78's co-chaperones ERdj3 or ERdj4 stimulated this inhibitory effect of GRP78. In the case of the other ER chaperones, overexpression of some (150 kDa oxygen-regulated protein and calnexin) but not others (GRP94 and calreticulin) suppressed the production of Abeta. These results indicate that certain ER chaperones are effective suppressors of Abeta production and that non-toxic inducers of ER chaperones may be therapeutically beneficial for AD treatment. GRP78 was co-immunoprecipitated with APP and overexpression of GRP78 inhibited the maturation of APP, suggesting that GRP78 binds directly to APP and inhibits its maturation, resulting in suppression of the proteolysis of APP. On the other hand, overproduction of APPsw or addition of synthetic Abeta42 caused up-regulation of the mRNA of various ER chaperones in cells. Furthermore, in the cortex and hippocampus of transgenic mice expressing APPsw, the mRNA of some ER chaperones was up-regulated in comparison with wild-type mice. We consider that this up-regulation is a cellular protective response against Abeta.

Beta-amyloid treatment of two complementary P301L tau-expressing Alzheimer's disease models reveals similar deregulated cellular processes

Alzheimer's disease (AD) is characterized by Abeta peptide-containing plaques and tau-containing neurofibrillary tangles (NFTs). Both pathologies have been combined by crossing Abeta plaque-forming APP mutant mice with NFT-forming P301L tau mutant mice or by stereotaxically injecting beta-amyloid peptide 1-42 (Abeta42) into brains of P301L tau mutant mice. In cell culture, Abeta42 induces filamentous tau aggregates. To understand which processes are disrupted by Abeta42 in the presence of tau aggregates, we applied comparative proteomics to Abeta42-treated P301L tau-expressing neuroblastoma cells and the amygdala of P301L tau transgenic mice stereotaxically injected with Abeta42. Remarkably, a significant fraction of proteins altered in both systems belonged to the same functional categories, i.e. stress response and metabolism. We also identified model-specific effects of Abeta42 treatment such as differences in cell signaling proteins in the cellular model and of cytoskeletal and synapse associated proteins in the amygdala. By Western blotting (WB) and immunohistochemistry (IHC), we were able to show that 72% of the tested candidates were altered in human AD brain with a major emphasis on stress-related unfolded protein responsive candidates. These data highlight these processes as potentially important initiators in the Abeta42-mediated pathogenic cascade in AD and further support the role of unfolded proteins in the course of AD.

Toxicity mediated by soluble oligomers of beta-amyloid(1-42) on cholinergic SN56.B5.G4 cells

Alzheimer's disease (AD) is characterized by cholinergic dysfunction and progressive basal forebrain cell loss which has been assumed to be as a result of the extensive accumulation of beta-amyloid (Abeta). In addition to Abeta fibrillar assemblies, there are pre-fibrillar forms that have been shown to be neurotoxic, although their role in cholinergic degeneration is still not known. Using the cholinergic cell line SN56.B5.G4, we investigated the effect of different Abeta(1-42) aggregates on cell viability. In our model, only soluble oligomeric but not fibrillar Abeta(1-42) forms induced toxicity in cholinergic cells. To determine whether the neurotoxicity of oligomeric Abeta(1-42) was caused by its oxidative potential, we performed microarray analysis of SN56.B5.G4 cells treated either with oligomeric Abeta(1-42) or H(2)O(2). We showed that genes affected by Abeta(1-42) differed from those affected by non-specific oxidative stress. Many of the genes affected by Abeta(1-42) were present in the endoplasmic reticulum (ER), Golgi apparatus and/or otherwise involved in protein modification and degradation (chaperones, ATF6), indicating a possible role for ER-mediated stress in Abeta-mediated toxicity. Moreover, a number of genes, which are known to be involved in AD (clusterin, Slc18a3), were identified. This study provides important leads for the understanding of oligomeric Abeta(1-42) toxicity in cholinergic cells, which may account in part for cholinergic degeneration in AD.

Novel potential of tunicamycin as an activator of the aryl hydrocarbon receptor -- dioxin responsive element signaling pathway

Tunicamycin is a well-known inhibitor of protein glycosylation and used as an inducer of endoplasmic reticulum (ER) stress. We found that tunicamycin induced expression of cytochrome P450 1A1 in a dose-dependent manner. Like dioxin, the transcriptional induction was associated with dose-dependent activation of the dioxin responsive element (DRE). This effect was independent of inhibition of protein glycosylation or induction of ER stress. Pharmacological and genetic inhibition of the aryl hydrocarbon receptor (AhR) significantly attenuated activation of DRE by tunicamycin. These results elucidated the novel potential of tunicamycin as an activator of the AhR -- DRE signaling pathway.

Real-time detection and continuous monitoring of ER stress in vitro and in vivo by ES-TRAP: evidence for systemic, transient ER stress during endotoxemia

Activity of secreted alkaline phosphatase (SEAP) produced by transfected cells is rapidly down-regulated by endoplasmic reticulum (ER) stress independent of transcriptional regulation. This phenomenon was observed in a wide range of cell types triggered by various ER stress inducers. The magnitude of the decrease in SEAP was proportional to the extent of ER stress and inversely correlated with the induction of endogenous ER stress markers grp78 and grp94. In contrast to SEAP, activity of secreted luciferase was less susceptible to ER stress. The decrease in SEAP activity by ER stress was caused by abnormal post-translational modification, accelerated degradation and reduced secretion of SEAP protein. In transgenic mice constitutively producing SEAP, systemic induction of ER stress led to reduction in serum SEAP. In these mice, administration with lipopolysaccharide caused rapid, transient decrease in serum SEAP activity, and it was correlated with up-regulation of grp78 in several organs including the spleen, lung, kidney, liver and heart. These results elucidated for the first time a possible involvement of transient, systemic ER stress in endotoxemia and provided evidence for usefulness of ER stress responsive alkaline phosphatase for real-time monitoring of ER stress in vitro and in vivo.

Beta-cell deficit due to increased apoptosis in the human islet amyloid polypeptide transgenic (HIP) rat recapitulates the metabolic defects present in type 2 diabetes

Type 2 diabetes is characterized by defects in insulin secretion and action and is preceded by impaired fasting glucose (IFG). The islet anatomy in IFG and type 2 diabetes reveals an approximately 50 and 65% deficit in beta-cell mass, with increased beta-cell apoptosis and islet amyloid derived from islet amyloid polypeptide (IAPP). Defects in insulin action include both hepatic and extrahepatic insulin resistance. The relationship between changes in beta-cell mass, beta-cell function, and insulin action leading to type 2 diabetes are unresolved, in part because it is not possible to measure beta-cell mass in vivo, and most available animal models do not recapitulate the islet pathology in type 2 diabetes. We evaluated the HIP rat, a human IAPP transgenic rat model that develops islet pathology comparable to humans with type 2 diabetes, at age 2 months (nondiabetic), 5 months (with IFG), and 10 months (with diabetes) to prospectively examine the relationship between changes in islet morphology versus insulin secretion and action. We report that increased beta-cell apoptosis and impaired first-phase insulin secretion precede the development of IFG, which coincides with an approximately 50% defect in beta-cell mass and onset of hepatic insulin resistance. Diabetes was characterized by approximately 70% deficit in beta-cell mass, progressive hepatic and extrahepatic insulin resistance, and hyperglucagonemia. We conclude that IAPP-induced beta-cell apoptosis causes defects in insulin secretion and beta-cell mass that lead first to hepatic insulin resistance and IFG and then to extrahepatic insulin resistance, hyperglucagonemia, and diabetes. We conclude that a specific beta-cell defect can recapitulate the metabolic phenotype of type 2 diabetes and note that insulin resistance in type 2 diabetes may at least in part be secondary to beta-cell failure.

A novel polymorphism in SEL1L confers susceptibility to Alzheimer's disease

Alzheimer's disease (AD) is considered to be a conformational disease arising from the accumulation of misfolded and unfolded proteins in the endoplasmic reticulum (ER). SEL1L is a component of the ER stress degradation system, which serves to remove unfolded proteins by retrograde degradation using the ubiquitin-proteosome system. In order to identify genetic variations possibly involved in the disease, we analysed the entire SEL1L gene sequence in Italian sporadic AD patients. Here we report on the identification of a new polymorphism within the SEL1L intron 3 (IVS3-88 A>G), which contains potential binding sites for transcription factors involved in ER-induced stress. Our statistical analysis shows a possible role of the novel polymorphism as independent susceptibility factor of Alzheimer's dementia.

Endoplasmic Reticulum Stress Features Are Prominent in Alzheimer Disease but Not in Prion Diseases In Vivo

Prion diseases and Alzheimer disease (AD) share a variety of clinical and neuropathologic features (e.g. progressive dementia, accumulation of abnormally folded proteins in diseased tissue, and pronounced neuronal loss) as well as pathogenic mechanisms like generation of oxidative stress molecules and complement activation. Recently, it was suggested that neuronal death in AD may have its origin in the endoplasmic reticulum (ER). Cellular stress conditions can interfere with protein folding and subsequently cause accumulation of unfolded or misfolded proteins in the ER lumen. The ER responds to this by the activation of adaptive pathways, which are termed unfolded protein response (UPR). The UPR transducer PERK, which launches the most immediate response to ER stress (i.e. the transient attenuation of mRNA translation), and the downstream effector of PERK, eIF2alpha, were shown to be activated in AD. We demonstrate that neither in sporadic nor in infectiously acquired or inherited human prion diseases can the activated forms of PERK and eIF2alpha be detected, except when concomitant neurofibrillary pathology is present; whereas the distribution of phosphorylated PERK correlates with abnormally phosphorylated tau in AD. In brains of scrapie-affected mice and mice infected with sporadic or variant Creutzfeldt-Jakob disease, activated PERK is only very faintly expressed. The lack of prominent activation of the PERK-eIF2alpha pathway in prion diseases suggests that, in contrast to AD, ER stress does not play a crucial role in neuronal death in prion disorders.

Suppressive effects of 4-phenylbutyrate on the aggregation of Pael receptors and endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is defined as an accumulation of unfolded proteins in the endoplasmic reticulum. 4-phenylbutyrate (4-PBA) has been demonstrated to promote the normal trafficking of the DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) mutant from the ER to the plasma membrane and to restore activity. We have reported that 4-PBA protected against cerebral ischemic injury and ER stress-induced neuronal cell death. In this study, we revealed that 4-PBA possesses chemical chaperone activity in vitro, which prevents the aggregation of denatured alpha-lactalbumin and bovine serum albumin (BSA). Furthermore, we investigated the effects of 4-PBA on the accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R) pathologically relevant to the loss of dopaminergic neurons in autosomal recessive juvenile parkinsonism (AR-JP). Interestingly, 4-PBA restored the normal expression of Pael-R protein and suppressed ER stress induced by the overexpression of Pael-R. In addition, we showed that 4-PBA attenuated the activation of ER stress-induced signal transduction pathways and subsequent neuronal cell death. Moreover, 4-PBA restored the viability of yeasts that fail to induce an ER stress response under ER stress conditions. These results suggest that 4-PBA suppresses ER stress by directly reducing the amount of misfolded protein, including Pael-R accumulated in the ER.

Beta-amyloid peptides induces neuronal apoptosis via a mechanism independent of unfolded protein responses

Accumulation of beta-amyloid (Abeta) peptides in senile plaques is one of the pathological hallmarks in Alzheimer's disease (AD), which can trigger apoptosis. We have previously demonstrated that Abeta triggered calcium release from the ER. Depletion of ER Ca(2+) ions has been reported leading to unfolded protein responses (UPR). While hypothesis has been made about UPR and neurodegeneration in AD, little is known about the effects of extracellular accumulation of Abeta on UPR. We have shown previously that activation of PKR in Abeta-triggered apoptosis. Since UPR can trigger PKR, our study aims to elucidate whether extracellular accumulation of Abeta peptides induce UPR in cultured neurons. Our results showed that Abeta could not trigger UPR signalings including phosphorylation of PERK, alternative cleavage of xbp-1 mRNA and induction of transcription of xbp-1 and Gadd153. Taken together, our results suggest that extracellular accumulation of Abeta peptides induce apoptosis via a mechanism independent of UPR.

Modeling of tumor necrosis factor receptor superfamily 1A mutants associated with tumor necrosis factor receptor–associated periodic syndrome indicates misfolding consistent with abnormal function

OBJECTIVE

To investigate the effect of mutations in the tumor necrosis factor receptor superfamily 1A (TNFRSF1A) gene on the conformation and behavior of the TNFRSF1A protein. Mutations in TNFRSF1A cause the autosomal-dominant, autoinflammatory TNFR-associated periodic syndrome (TRAPS).

METHODS

The expression of recombinant TNFRSF1A was compared in SK-HEp-1 endothelial cells and HEK 293 epithelial cells stably transfected with full-length R347A or Deltasig constructs of wild-type or TRAPS-associated mutant TNFRSF1A. TNF binding was assessed in HEK 293 cell lines expressing R347A wild-type or mutant TNFRSF1A. Homology modeling of the 3-dimensional structure of the ectodomains of wild-type and mutant TNFRSF1A was performed.

RESULTS

TRAPS-associated mutant and wild-type TNFRSF1A behaved differently and had different localization properties within the cell, as a direct result of mutations in the ectodomains of TNFRSF1A. From a structural perspective, mutants with a predicted structure similar to that of the wild-type protein (e.g., R92Q) behaved similarly to wild-type TNFRSF1A, whereas forms of TNFRSF1A with mutations predicted to drastically destabilize the protein structure (e.g., cysteine mutations) showed defects in cell surface expression and TNF binding.

CONCLUSION

The results obtained from the in vitro experiments, in combination with the modeled structures, indicate that the phenotype and clinical differences between different TRAPS-associated mutants of TNFRSF1A result from different conformations of the TNFRSF1A ectodomains.

Protein folding in the endoplasmic reticulum and the unfolded protein response

In all eukaryotic cells, the endoplasmic reticulum (ER) is an intracellular organelle where folding and assembly occurs for proteins destined to the extracellular space, plasma membrane, and the exo/endocytic compartments (Kaufman 1999). As a protein-folding compartment, the ER is exquisitely sensitive to alterations in homeostasis, and provides stringent quality control systems to ensure that only correctly folded proteins transit to the Golgi and unfolded or misfolded proteins are retained and ultimately degraded. A number of biochemical and physiological stimuli, such as perturbation in calcium homeostasis or redox status, elevated secretory protein synthesis, expression of misfolded proteins, sugar/glucose deprivation, altered glycosylation, and overloading of cholesterol can disrupt ER homeostasis, impose stress to the ER, and subsequently lead to accumulation of unfolded or misfolded proteins in the ER lumen. The ER has evolved highly specific signaling pathways called the unfolded protein response (UPR) to cope with the accumulation of unfolded or misfolded proteins. Elucidation of the molecular mechanisms by which accumulation of unfolded proteins in the ER transmits a signal to the cytoplasm and nucleus has led to major new insights into the diverse cellular and physiological processes that are regulated by the UPR. This chapter summarizes how cells respond to the accumulation of unfolded proteins in the cell and the relevance of these signaling pathways to human physiology and disease.

Cellular abnormalities linked to endoplasmic reticulum dysfunction in cerebrovascular disease—therapeutic potential

Unfolded proteins accumulate in the lumen of the endoplasmic reticulum (ER) as part of the cellular response to cerebral hypoxia/ischemia and also to the overexpression of the mutant genes responsible for familial forms of degenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyothrophic lateral sclerosis, and Huntington's disease, as well as other disorders that are caused by an expanded CAG repeat. This accumulation arises from an imbalance between the load of proteins that need to be folded and processed in the ER lumen and the ER folding/processing capacity. To withstand such potentially lethal conditions, stress responses are activated that includes the shutdown of translation to reduce the ER work load and the activation of the expression of genes coding for proteins involved in the folding and processing reactions, to increase folding/processing capacity. In transient cerebral ischemia, ER stress-induced suppression of protein synthesis is believed to be too severe to permit sufficient activation of the genetic arm of the ER stress response. Mutations associated with Alzheimer's disease down-regulate the ER stress response and make cells more vulnerable to conditions associated with ER stress. When the functioning of the ER is severely impaired and affected cells can no longer withstand these stressful conditions, programmed cell death is induced, including a mitochondria-driven apoptotic pathway. Raising the resistance of cells to conditions that interfere with ER functions and activating the degradation and refolding of unfolded proteins accumulated in the ER lumen are possible strategies for blocking the pathological process leading to cell death at an early stage.

The unfolded protein response in vanishing white matter disease

Leukoencephalopathy with vanishing white matter (VWM) is an autosomal-recessive disorder in which febrile infections may provoke major neurologic deterioration. Characteristic pathologic findings include cystic white matter degeneration, foamy oligodendrocytes, dysmorphic astrocytes and oligodendrocytes, oligodendrocytosis, and apoptotic losses of oligodendrocytes. VWM is caused by mutations in eukaryotic initiation factor (eIF) 2B (eIF2B). eIF2B plays an important role in the regulation of protein synthesis. Mutant eIF2B may impair the ability of cells to regulate protein synthesis in response to stress and perhaps even under normal conditions. An overload of misfolded proteins in the endoplasmic reticulum activates the unfolded protein response (UPR), a compensatory mechanism that inhibits synthesis of new proteins and induces both prosurvival and proapoptotic signals. We have studied the activation of the UPR in VWM through the immunohistochemical expression of its upstream components PERK and phosphorylated eIF2alpha (eIF2alphaP) and combined immunohistochemical and Western blot analysis of the downstream effector proteins activating transcription factor-4 (ATF4) and C/EBP homologous protein (CHOP) in 4 VWM brains and 3 age-matched controls. We demonstrate activation of the UPR in glia of patients with VWM. Our findings may point to a possible explanation for the dysmorphic glia, the increased numbers of oligodendrocytes, and the apoptotic loss of oligodendrocytes in VWM.

Endoplasmic reticulum stress response and neurodegeneration

The endoplasmic reticulum (ER) is a subcellular compartment playing a central role in calcium storage and signaling. Disturbances of ER calcium homeostasis constitute a severe form of stress interfering with central functions of this structure including the folding and processing of newly synthesized membrane and secretory proteins. Blocking the folding and processing reactions results in the accumulation of unfolded proteins forming potentially toxic aggregates. To restore ER functioning, specific stress responses are activated one of which is the unfolded protein response (UPR). UPR is characterized by a shutdown of global protein synthesis and activation of expression of genes coding for ER-resident proteins that are involved in the folding and processing reactions. ER calcium homeostasis is therefore inevitably associated with major cellular functions, including gene transcription and translation. ER calcium homeostasis und ER functions are believed to be impaired in various degenerative diseases of the brain including Alzheimer's, Parkinson's and Huntington's disease, and amyotrophic lateral sclerosis. ER functioning has also been shown to be disturbed in acute pathological states of the brain such as ischemia and trauma, which have been identified as risk factors for the development of degenerative diseases. This implies that there are common underlying pathomechanisms. This review will summarize new observations suggesting that impairment of ER functioning may be a common denominator of pathological processes resulting in neuronal cell injury in acute disorders and degenerative diseases of the brain.

PS1 knockin mice with the Japanese I213T mutation: Effects on exploratory activity, motor coordination, and spatial learning

Knockin (KI) mice with a PS1/I213T mutation were compared to wild-type controls on the SHIRPA primary screening battery and for exploratory activity, motor coordination, and spatial learning. By comparison to non-transgenic controls, PS1/I213T KI mice had retarded acquisition of place learning in the Morris water maze without being impaired in the probe trial and in the visible platform subtest. PS1/I213T KI mice were more likely to display whole-body startle to an auditory stimulus and a tighter grip on a horizontal grid. PS1/I213T KI mice also had fewer enclosed arm entries in the elevated plus-maze, but did not differ from controls in open-field, photocell actimeter, and T-maze spontaneous alternation tests. No intergroup difference was seen in three motor coordination tests. The dissociation between hidden and visible platform versions in the water maze is consistent with the hypothesis that elevated Abeta42 concentrations cause cognitive disturbances.

Misfolded proteins, endoplasmic reticulum stress and neurodegeneration

The accumulation of misfolded proteins (e.g. mutant or damaged proteins) triggers cellular stress responses that protect cells against the toxic buildup of such proteins. However, prolonged stress due to the buildup of these toxic proteins induces specific death pathways. Dissecting these pathways should be valuable in understanding the pathogenesis of, and ultimately in designing therapy for, neurodegenerative diseases that feature misfolded proteins.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Phosphorylation of the alpha-subunit of the eukaryotic initiation factor-2 (eIF2alpha) reduces protein synthesis and enhances apoptosis in response to proteasome inhibition

Protein ubiquitination and subsequent degradation by the proteasome are important mechanisms regulating cell cycle, growth and differentiation, and apoptosis. Recent studies in cancer therapy suggest that drugs that disrupt the ubiquitin/proteasome pathway induce apoptosis and sensitize malignant cells and tumors to conventional chemotherapy. In this study we addressed the role of phosphorylation of the alpha-subunit eukaryotic initiation factor-2 (eIF2), and its attendant regulation of gene expression, in the cellular stress response to proteasome inhibition. Phosphorylation of eIF2alpha in mouse embryo fibroblast (MEF) cells subjected to proteasome inhibition leads to a significant reduction in protein synthesis, concomitant with induced expression of the bZIP transcription regulator, ATF4, and its target gene CHOP/GADD153. The primary eIF2alpha kinase activated by exposure of these fibroblast cells to proteasome inhibition is GCN2 (EIF2AK4), which has a central role in the recognition of cytoplasmic stress signals. Endoplasmic reticulum (ER) stress is not effectively induced in MEF cells subjected to proteasome inhibition, with minimal activation of the ER stress sensory proteins, eIF2alpha kinase PEK (PERK/EIF2AK3), IRE1 protein kinase and the transcription regulator ATF6 following up to 6 h of proteasome inhibitor treatment. Loss of eIF2alpha phosphorylation thwarts caspase activation and delays apoptosis. Central to this pro-apoptotic function of eIF2alpha kinases during proteasome inhibition is the transcriptional regulator CHOP, as deletion of CHOP in MEF cells impedes apoptosis. We conclude that eIF2alpha kinases are integral to cellular stress pathways induced by proteasome inhibitors, and may be central to the efficacy of anticancer drugs that target the ubiquitin/proteasome pathway.

Induction of neuronal death by ER stress in Alzheimer's disease

Recent studies have suggested that neuronal death in Alzheimer's disease (AD) or ischemia could arise from dysfunction of the endoplasmic reticulum (ER). Inhibition of protein glycosylation, perturbation of calcium homeostasis, and reduction of disulfide bonds provoke accumulation of unfolded protein in the ER, and are called 'ER stress'. Normal cells respond to ER stress by increasing transcription of genes encoding ER-resident chaperones such as GRP78/BiP, to facilitate protein folding or by suppressing the mRNA translation to synthesize proteins. These systems are termed the unfolded protein response (UPR). Familial Alzheimer's disease-linked presenilin-1 (PS1) mutation downregulates the unfolded protein response and leads to vulnerability to ER stress. The mechanisms by which mutant PS1 affects the ER stress response are attributed to the inhibited activation of ER stress transducers such as IRE1, PERK and ATF6. On the other hand, in sporadic Alzheimer's disease (sAD), we found the aberrant splicing isoform (PS2V), generated by exon 5 skipping of the Presenilin-2 (PS2) gene transcript, responsible for induction of high mobility group A1a protein (HMGA1a). The PS2V also downregulates the signaling pathway of the UPR, in a similar fashion to that reported for mutants of PS1 linked to familial AD. It was clarified what molecules related to cell death are activated in the case of AD and we discovered that caspase-4 plays a key role in ER stress-induced apoptosis. Caspase-4 also seems to act upstream of the beta-amyloid-induced ER stress pathway, suggesting that activation of caspase-4 might mediate neuronal cell death in AD.

Valproate protects cells from ER stress-induced lipid accumulation and apoptosis by inhibiting glycogen synthase kinase-3

A wide range of agents and conditions are known to disrupt the ability of the endoplasmic reticulum (ER) to fold proteins properly, resulting in the onset of ER dysfunction/stress. We and others have shown that ER stress can induce intracellular lipid accumulation through the activation of the sterol responsive element binding proteins (SREBPs) and initiate programmed cell death by activation of caspases. It has been suggested that ER stress-induced lipid accumulation and cell death play a role in the pathogenesis of disorders including Alzheimer's disease, Parkinson's disease, type-1 diabetes mellitus and hepatic steatosis. Here we show that exposure of HepG2 cells to the branch chain fatty acid, valproate, increases cellular resistance to ER stress-induced dysfunction. Two distinctly different potential mechanisms for this protective effect were investigated. We show that exposure to valproate increases the expression of chaperones that assist in the folding of proteins in the ER including GRP78/BiP, GRP94, PDI and calreticulin as well as the cytosolic chaperone, HSP70. However, exposure of HepG2 cells to valproate does not decrease the apparent ER stress response in cells challenged with tunicamycin, A23187 or glucosamine, suggesting that valproate-conferred protection occurs downstream of ER dysfunction. Finally, we demonstrate that valproate directly inhibits the glycogen synthase kinases (GSK)-3alpha/beta. The ability of lithium, another inhibitor of GSK3alpha/beta to protect cells from ER stress-induced lipid accumulation suggests that GSK3 plays a central role in signaling downstream effects of ER stress. Strategies to protect cells from agents/conditions that induce ER stress may have potential in the treatment of the growing number of diseases and disorders linked to ER dysfunction.

JAB1 participates in unfolded protein responses by association and dissociation with IRE1

Recent papers have reported that neuronal death in patients with Alzheimer's disease, Parkinson's disease, and cerebral ischemia has its origin in the endoplasmic reticulum (ER). IRE1alpha is one of the ER stress transducers that detect the accumulation of unfolded proteins in the ER. IRE1alpha mediates two major cellular responses, which are the unfolded protein response (UPR), a defensive response, and apoptosis that leads to cell death. However, little is known about the regulatory mechanisms that select between the UPR and apoptosis. We identified Jun activation domain-binding protein-1 (JAB1) as a molecule that interacts with IRE1alpha using a yeast two-hybrid system. We demonstrated that JAB1 binds to IRE1alpha in the absence of stress, but that binding is decreased by ER stress inducers. Moreover, mutant JAB1 down-regulates the UPR signaling pathway through tight binding with IRE1alpha. These results suggested that JAB1 may act as a key molecule in selecting the UPR or cell death by association and dissociation with IRE1alpha.

Fibroblasts from FAD-linked presenilin 1 mutations display a normal unfolded protein response but overproduce Abeta42 in response to tunicamycin

Many patients affected by early onset familial Alzheimer's disease (FAD), carry mutations in the presenilin 1 (PS1) gene. Since it has been suggested that FAD-linked PS1 mutations impair the unfolded protein response (UPR) due to endoplasmic reticulum (ER) stress, we analyzed the UPR and amyloid beta-protein processing in fibroblasts bearing various PS1 mutations. Neither in normal conditions nor after induction of ER stress with DTT or tunicamycin were the mRNA levels of UPR-responsive genes (BiP and PDI) significantly different in control and FAD fibroblasts. DTT, which blocked APP transport to the Golgi, caused a 30% decrease of secreted Abeta42 in wild type and PS1 mutant fibroblasts. In contrast, tunicamycin, which allowed exit of APP from the ER, increased secreted Abeta42 only in PS1 mutant fibroblasts. Our findings suggest that, although the UPR is active in fibroblasts from FAD patients, mutant PS1 may selectively increase Abeta42 secretion when N-glycosylation is impaired.

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.

An RNA-dependent protein kinase is involved in tunicamycin-induced apoptosis and Alzheimer's disease

Various types of stress, such as disruption of calcium homeostasis, inhibition of protein glycosylation and reduction of disulfide bonds, result in accumulation of misfolded proteins in the endoplasmic reticulum (ER). The initial cellular response involves removal of such proteins by the ER, but excessive and/or long-term stress results in apoptosis. In this study, we used a randomized ribozyme library and ER stress-mediated apoptosis (tunicamycin-induced apoptosis) in SK-N-SH human neuroblastoma cells as a selective phenotype to identify factors involved in this process. We identified a double-stranded RNA-dependent protein kinase (PKR) as one of the participants in this process. The level of nuclear PKR was elevated, but the level of cytoplasmic PKR barely changed in tunicamycin-treated SK-N-SH cells. Furthermore, tunicamycin also raised levels of phosphorylated PKR in the nucleus. We also detected the accumulation of phosphorylated PKR in the nuclei of autopsied brain tissues in Alzheimer's disease. Thus, PKR might play a role in ER stress-induced apoptosis and in Alzheimer's disease.

ER signaling in unfolded protein response

Abnormally folded proteins are susceptible to aggregation and accumulation in cells, ultimately leading to cell death. To protect cells against such dangers, expression of various genes including molecular chaperones can be induced and ER-associated protein degradation (ERAD) activated in response to the accumulation of unfolded protein in the endoplasmic reticulum (ER). This is known as the unfolded protein response (UPR). ERAD requires retrograde transport of unfolded proteins from the ER back to the cytosol via the translocon for degradation by the ubiquitin-proteasome system. Hrd1p is a UPR-induced ER membrane protein that acts as a ubiquitin ligase (E3) in the ERAD system. Hrd3p interacts with and stabilizes Hrd1p. We have isolated and identified human homologs (HRD1 and SEL1/HRD3) of Saccharomyces cerevisiae Hrd1p and Hrd3p. Human HRD1 and SEL1 were up-regulated in response to ER stress and overexpression of human IRE1 and ATF6, which are ER stress-sensor molecules in the ER. HEK293T cells overexpressing HRD1 showed resistance to ER stress-induced cell death. These results suggest that HRD1 and SEL1 are up-regulated by the UPR and contribute to protection against the ER stress-induced cell death by degrading unfolded proteins accumulated in the ER.

RIPped out by presenilin-dependent gamma-secretase

Presenilins (PS) constitute a fascinating family of proteins that play crucial roles in several major signalling processes involved in key cellular functions and are also closely associated with dysfunction in Alzheimer's disease (AD). Presenilin-dependent intramembrane cleavage of transmembrane proteins such as amyloid beta precursor protein (AbetaPP) and Notch resides in a high-molecular-weight gamma-secretase protein complex, of which at least five core components have now been identified. Remarkably, it has now become evident that presenilin-dependent gamma-secretase activity extends beyond its role in AbetaPP and Notch cleavages to have a generic role in the regulated intramembranous cleavage of certain membrane proteins. Actually, a new picture is emerging in which cells can relay signals from the extracellular space to their interior through presenilin-dependent proteolysis within the membrane-spanning regions of type 1 integral membrane proteins to generate potential transcriptionally active intracellular fragments. This review deals with the complex biology of presenilins and focuses more specifically on recent developments regarding the composition, assembly and regulation of the gamma-secretase protein complex, its substrates and its implications for cellular signalling.

Activation signal of nuclear factor-kappa B in response to endoplasmic reticulum stress is transduced via IRE1 and tumor necrosis factor receptor-associated factor 2

Conditions that perturb the function of the endoplasmic reticulum (ER) lead to an accumulation of proteins and subsequent induction of several responses, such as an increased expression of ER-resident chaperones involved in protein folding and activation of c-jun N-terminal kinase (JNK). These responses are mediated by a transmembrane kinase/ribonuclease, IRE1, which transduces the signal from the ER lumen to the cytosol. Although nuclear transcription factor-kappaB (NF-kappaB) is also activated by ER stress, whether this response depends on IRE1 is unknown. In this study, we show that IRE1 is involved in the activation of NF-kappaB induced by ER stress. NF-kappaB was activated by ER stress-inducing agents, thapsigargin and tunicamycin. The activation was inhibited by a dominant-negative IRE1. In addition, a dominant-negative TRAF2 also suppressed the activation of NF-kappaB by ER stress. These results suggest that ER stress-induced NF-kappaB activation is also mediated by the IRE1-TRAF2 pathway, as well as JNK activation.

MDG1/ERdj4, an ER-resident DnaJ family member, suppresses cell death induced by ER stress

BACKGROUND

Alterations in homeostasis after various cellular stresses, which prevent protein folding and cause an accumulation of misfolding or malfolding proteins in the endoplasmic reticulum (ER), have the potential to induce cellular damage, and are therefore a type of 'ER stress.' To understand the molecular events or cascades underlying the ER stress response regulated by gene transcription and mediated by stress transducers, it is crucial to identify the molecules induced during ER stress and to analyse the roles of these genes.

RESULTS

We identified MDG1/ERdj4, a member of the DnaJ protein family, as an inducible gene during ER stress. MDG1/ERdj4 contains the J domain, which is essential for interacting with Hsp70s, at the N-terminal portion and just at the back of the transmembrane domain. Its trypsin digestion and glycosylation of a chimeric protein composed of MDG1/ERdj4 fused with the extracellular domain of the amyloid precursor protein at its C-terminus, showed that its C-terminal portion containing the J domain could be orientated to the ER lumen. Over-expression of it inhibited the cell death induced by ER stress. In contrast, its mutants with the J domain deleted showed no protective effects against cell death.

CONCLUSIONS

MDG1/ERdj4 may play roles in stabilizing GRP78/BiP binding to unfolded substrate proteins in a J domain-dependent manner and prevent the accumulation of unfolded proteins in the ER, consequently protecting cells from ER stress.

Lead-induced endoplasmic reticulum (ER) stress responses in the nervous system

Lead (Pb) poisoning continues to be a significant health risk because of its pervasiveness in the environment, its known neurotoxic effects in children, and potential endogenous exposure from Pb deposited in bone. New information about mechanisms by which Pb enters cells and its organelle targets within cells are briefly reviewed. Toxic effects of Pb on the endoplasmic reticulum (ER) are considered in detail, based on recent evidence that Pb induces the expression of the gene for 78-kD glucose-regulated protein (GRP78) and other ER stress genes. GRP78 is a molecular chaperone that binds transiently to proteins traversing through the ER and facilitates their folding, assembly, and transport. Models are presented for the induction of ER stress by Pb in astrocytes, the major cell type of the central nervous system, in which Pb accumulates. A key feature of the models is disruption of GRP78 function by direct Pb binding. Possible pathways by which Pb-bound GRP78 stimulates the unfolded protein response (UPR) in the ER are discussed, specifically transduction by IRE1/ATF6 and/or IRE1/JNK. The effect of Pb binding to GRP78 in the ER is expected to be a key component for understanding mechanisms of Pb-induced ER stress gene expression.

Mitochondrial localization of mutant superoxide dismutase 1 triggers caspase-dependent cell death in a cellular model of familial amyotrophic lateral sclerosis

The mutations in superoxide dismutase 1 (SOD1) cause approximately 20% of familial amyotrophic lateral sclerosis cases. A toxic gain of function has been considered to be the cause of the disease, but its molecular mechanism remains uncertain. To determine whether the subcellular localization of mutant SOD1 is crucial to mutant SOD1-mediated cell death, we produced neuronal cell models with accumulation of SOD1 in each subcellular fraction/organelle, such as the cytosol, nucleus, endoplasmic reticulum, and mitochondria. We showed that the localization of mutant SOD1 in the mitochondria triggered the release of mitochondrial cytochrome c followed by the activation of caspase cascade and induced neuronal cell death without cytoplasmic mutant SOD1 aggregate formation. Nuclear and endoplasmic reticulum localization of mutant SOD1 did not induce cell death. These results suggest that the localization of mutant SOD1 in the mitochondria is critical in the pathogenesis of mutant SOD1-associated familial amyotrophic lateral sclerosis.

Human HRD1 protects against ER stress-induced apoptosis through ER-associated degradation

Stresses that impair the function of the endoplasmic reticulum (ER) lead to an accumulation of unfolded protein in the ER. Under these conditions, the expression of a variety of genes involved in preventing the accumulation of the unfolded proteins is induced. Yeast Hrd1p is an ER stress-inducible ER membrane protein that acts as a ubiquitin ligase (E3) with a RING finger motif and plays a role in the ubiquitination of proteins in the ER. We report here the identification and characterization of a human homolog to yeast Hrd1p. The predicted structures are highly conserved from yeast to humans. Indeed, human HRD1 was localized to the ER and ubiquitinated its substrates. Furthermore, it was found that human HRD1 was up-regulated by ER stress via IRE1 and ATF6, which are ER stress transducers. Interestingly, 293 cells stably expressing wild-type HRD1, but not the C329S mutant, afforded resistance to ER stress-induced apoptosis. These results suggest that the production of HRD1 is up-regulated to protect against ER stress-induced apoptosis by degrading unfolded proteins accumulated in the ER.

Involvement of Gadd153 in the pathogenic action of presenilin-1 mutations

Mutations in the presenilin-1 (PS1) gene cause early onset familial Alzheimer's disease (FAD) by a mechanism believed to involve perturbed endoplasmic reticulum (ER) function and altered proteolytic processing of the amyloid precursor protein. We investigated the molecular mechanisms underlying cell death and ER dysfunction in cultured cells and knock-in mice expressing FAD PS1 mutations. We report that PS1 mutations cause a marked increase in basal protein levels of the pro-apoptotic transcription factor Gadd153. PS1 mutations increase Gadd153 protein translation without affecting mRNA levels, while decreasing levels of the anti-apoptotic protein Bcl-2. Moreover, an exaggerated Gadd153 response to stress induced by ER stress agents was observed in PS1 mutant cells. Cell death in response to ER stress is enhanced by PS1 mutations, and this endangering effect is attenuated by anti-sense-mediated suppression of Gadd153 production. An abnormality in the translational regulation of Gadd153 may sensitize cells to the detrimental effects of ER stress and contribute to the pathogenic actions of PS1 mutations in FAD.

FAD-linked presenilin-1 mutants impede translation regulation under ER stress

FAD mutations in presenilin-1 (PS1) cause attenuation of the induction of the endoplasmic reticulum (ER)-resident chaperone GRP78/BiP under ER stress, due to disturbed function of IRE1, the sensor for accumulation of unfolded protein in the ER lumen. PERK, an ER-resident transmembrane protein kinase, is also a sensor for the unfolded protein response (UPR), causing phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) to inhibit translation initiation. Here, we report that the FAD mutant PS1 disturbs the UPR by attenuating both the activation of PERK and the phosphorylation of eIF2alpha. Consistent with the results of a disturbed UPR, inhibition of protein synthesis under ER stress was impaired in cells expressing PS1 mutants. These results suggest that mutant PS1 impedes general translational attenuation regulated by PERK and eIF2alpha, resulting in an increased load of newly synthesized proteins into the ER and subsequently increasing vulnerability to ER stress.

Gamma-secretase: substrates and inhibitors

The amyloid beta-protein (Abeta) deposited in Alzheimer's disease (AD), the most common form of dementia in the elderly, is a secreted proteolytic product of the amyloid beta-protein precursor (APP). Generation of Abeta from the APP requires two sequential proteolytic events, beta-secretase cleavage to generate the amino terminus, followed by gamma-secretase cleavage to generate the carboxyl terminus. Because this process is a central event in the pathogenesis of AD, gamma-secretase is believed to be an excellent therapeutic target. Gamma-secretase activity has been demonstrated to be membrane-associated, with the cleavage site primarily determined by the location of the substrate with respect to the membrane. It has also been shown that this unusual proteolytic activity not only occurs for APP, but also for proteins involved in morphogenic processes or cell proliferation and differentiation such as Notch and ErbB4. Thus far, all gamma-secretase substrates are involved in some form of nuclear signaling. These recent findings have important implications for the development of pharmacological interventions that target gamma-secretase.

The endoplasmic reticulum as an integrating signalling organelle: from neuronal signalling to neuronal death

The endoplasmic reticulum is one of the largest intracellular organelles represented by continuous network of cisternae and tubules, which occupies the substantial part of neuronal somatas and extends into finest neuronal processes. The endoplasmic reticulum controls protein synthesis as well as their post-translational processing, and generates variety of nucleus-targeted signals through Ca(2+)-binding chaperones. The normal functioning of the endoplasmic reticulum signalling cascades requires high concentrations of free calcium ions within the endoplasmic reticulum lumen ([Ca(2+)](L)), and severe alterations in [Ca(2+)](L) trigger endoplasmic reticulum stress response, manifested by either unfolded protein response (UPR) or endoplasmic reticulum overload response (EOR). At the same time, the endoplasmic reticulum is critically involved in fast neuronal signalling, by producing local or global cytosolic calcium signals via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). Both CICR and IICR are important for synaptic transmission and synaptic plasticity. Several special techniques allowing real-time [Ca(2+)](L) monitoring were developed recently. Video-imaging of [Ca(2+)](L) in neurones demonstrates that physiological signalling triggers minor decreases in overall intraluminal Ca(2+) concentration due to strong activation of Ca(2+) uptake, which prevents severe [Ca(2+)](L) alterations. The endoplasmic reticulum lumen also serves as a "tunnel" which allows rapid transport of Ca(2+) ions within highly polarised nerve cells. Fluctuations of intraluminal free Ca(2+) concentration represent a universal mechanism, which integrates physiological cellular signalling with protein synthesis and processing. In pathological conditions, fluctuations in [Ca(2+)](L) may initiate either adaptive or fatal stress responses.