Involvement of mitochondria in endoplasmic reticulum stress-induced apoptotic cell death pathway triggered by the prion peptide PrP(106-126)

Bifenox induces programmed cell death in bovine mammary epithelial cells by impairing calcium homeostasis, triggering ER stress, and altering the signaling cascades of PI3K/AKT and MAPK

Bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate), a nitrophenyl ether herbicide, was first introduced in the 1980s to control broadleaf weeds. As a result of its wide and frequent application in diverse agricultural settings and reports on residual traces, potential adverse effects of bifenox have been studied extensively in rat hepatocytes, bovine peripheral lymphocytes, and mice. Despite the reported risks of bifenox exposure in dairy cows, the toxicity of bifenox on bovine lactation system has not been extensively investigated. Therefore, we used bovine mammary epithelial (MAC-T) cells to study the toxic effects of bifenox on mammary glands. We found that bifenox inhibited MAC-T cells proliferation and disturbed the cell cycle, especially in the sub-G1 and G1 phases. Bifenox also disrupted the calcium homeostasis within the cell and impaired mitochondrial membrane potential. We also examined phosphoinositide 3-kinase/protein kinase B (PI3K/AKT) and mitogen-activated protein kinase (MAPK) signaling cascades. The findings indicated hyperactivation of phosphorylated protein kinase B (AKT), p70 ribosomal S6 kinase (p70S6K), S6, extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38, c-Jun N-terminal kinase (JNK), and c-Jun, as well as endoplasmic reticulum (ER) stress caused by bifenox treatment. In conclusion, based on our in vitro study employing MAC-T cells, we report that bifenox can induce damage to the bovine mammary glands, potentially impacting milk production.

Curcumin Modulates p62-Keap1-Nrf2-Mediated Autophagy in Rotenone-Induced Parkinson's Disease Mouse Models

Autophagy mediates self-digestion of abnormally aggregated proteins and organelles present in the cytoplasm. This mechanism may prove to be neuroprotective against Parkinson's disease (PD) by clearing misfolded α-synuclein (α-syn) aggregates from dopaminergic neurons. p62, an adaptor protein acts as a selective substrate for autophagy and regulates the formation as well as the degradation of protein aggregates. p62 sequesters keap1 freeing Nrf2 and consequently activating the transcription of its target genes. In the present study, we aimed to investigate the anti-parkinsonian activity of curcumin targeting primarily activation of autophagy via the Nrf2-Keap1 pathway. The mice were subcutaneously injected with rotenone (2.5 mg/kg bodyweight) and co-treated with oral administration of curcumin (80 mg/kg bodyweight) for 35 days. Following completion of dosing, motor activities, anti-oxidative potential, mitochondrial dysfunction, and various protein expressions, including Nrf2, Keap1, p62, LC3, Bcl2, Bax, and caspase 3, were assessed. The results revealed that curcumin restored the motor coordination and anti-oxidative activity while improving the mitochondrial functioning in PD mice. Autophagy was evaluated by the change in the expression of autophagic markers, p62 and LC3-II. Reduced p62 and LC3-II expressions in the rotenone mouse model of PD confirmed the compromised autophagy pathway, consequently increasing the aggregation of misfolded protein α-syn. Whereas, curcumin treatment-enhanced autophagy-mediated clearance of misfolded α-syn proteins by increasing the LC3-II expression and blocked apoptotic cascade. Curcumin administration upregulated the Nrf2 expression and normalized the Nrf2-Keap1 pathway, which justifies the improved anti-oxidative activity. Therefore, the findings reveal that curcumin is a Nrf2-inducer and is endowed with neuroprotective potential, which may prove to be a potential candidate for the anti-Parkinson's disease treatment therapy.

Calcineurin Activation by Prion Protein Induces Neurotoxicity via Mitochondrial Reactive Oxygen Species

Prion diseases are caused by PrPsc accumulation in the brain, which triggers dysfunctional mitochondrial injury and reactive oxygen species (ROS) generation in neurons. Recent studies on prion diseases suggest that endoplasmic reticulum (ER) stress induced by misfolding proteins such as misfolded prion protein results in activation of calcineurin. Calcineurin is a calcium-related protein phosphatase of type 2B that exists in copious quantities in the brain and acts as a critical nodal component in the control of cellular functions. To investigate the relationship between calcineurin and intracellular ROS, we assessed the alteration of CaN and ROS induced by prion peptide (PrP) 106-126. Human prion peptide increased mitochondrial ROS by activating calcineurin, and the inhibition of calcineurin activity protected mitochondrial function and neuronal apoptosis in neuronal cells. These results suggest that calcineurin plays a pivotal role in neuronal apoptosis by mediating mitochondrial injury and ROS in prion diseases.

The PERK-Dependent Molecular Mechanisms as a Novel Therapeutic Target for Neurodegenerative Diseases

Higher prevalence of neurodegenerative diseases is strictly connected with progressive aging of the world population. Interestingly, a broad range of age-related, neurodegenerative diseases is characterized by a common pathological mechanism-accumulation of misfolded and unfolded proteins within the cells. Under certain circumstances, such protein aggregates may evoke endoplasmic reticulum (ER) stress conditions and subsequent activation of the unfolded protein response (UPR) signaling pathways via the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dependent manner. Under mild to moderate ER stress, UPR has a pro-adaptive role. However, severe or long-termed ER stress conditions directly evoke shift of the UPR toward its pro-apoptotic branch, which is considered to be a possible cause of neurodegeneration. To this day, there is no effective cure for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), or prion disease. Currently available treatment approaches for these diseases are only symptomatic and cannot affect the disease progression. Treatment strategies, currently under detailed research, include inhibition of the PERK-dependent UPR signaling branches. The newest data have reported that the use of small-molecule inhibitors of the PERK-mediated signaling branches may contribute to the development of a novel, ground-breaking therapeutic approach for neurodegeneration. In this review, we critically describe all the aspects associated with such targeted therapy against neurodegenerative proteopathies.

Combinatory FK506 and Minocycline Treatment Alleviates Prion-Induced Neurodegenerative Events via Caspase-Mediated MAPK-NRF2 Pathway

Transcription factors play a significant role during the symptomatic onset and progression of prion diseases. We previously showed the immunomodulatory and nuclear factor of activated T cells' (NFAT) suppressive effects of an immunosuppressant, FK506, in the symptomatic stage and an antibiotic, minocycline, in the pre-symptomatic stage of prion infection in hamsters. Here we used for the first time, a combinatory FK506+minocycline treatment to test its transcriptional modulating effects in the symptomatic stage of prion infection. Our results indicate that prolonged treatment with FK506+minocycline was effective in alleviating astrogliosis and neuronal death triggered by misfolded prions. Specifically, the combinatory therapy with FK506+minocycline lowered the expression of the astrocytes activation marker GFAP and of the microglial activation marker IBA-1, subsequently reducing the level of pro-inflammatory cytokines interleukin 1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α), and increasing the levels of anti-inflammatory cytokines IL-10 and IL-27. We further found that FK506+minocycline treatment inhibited mitogen-activated protein kinase (MAPK) p38 phosphorylation, NF-kB nuclear translocation, caspase expression, and enhanced phosphorylated cAMP response element-binding protein (pCREB) and phosphorylated Bcl2-associated death promoter (pBAD) levels to reduce cognitive impairment and apoptosis. Interestingly, FK506+minocycline reduced mitochondrial fragmentation and promoted nuclear factor⁻erythroid2-related factor-2 (NRF2)-heme oxygenase 1 (HO-1) pathway to enhance survival. Taken together, our results show that a therapeutic cocktail of FK506+minocycline is an attractive candidate for prolonged use in prion diseases and we encourage its further clinical development as a possible treatment for this disease.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual's brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

p62-Keap1-NRF2-ARE Pathway: A Contentious Player for Selective Targeting of Autophagy, Oxidative Stress and Mitochondrial Dysfunction in Prion Diseases

Prion diseases are a group of fatal and debilitating neurodegenerative diseases affecting humans and animal species. The conversion of a non-pathogenic normal cellular protein (PrP^c) into an abnormal infectious, protease-resistant, pathogenic form prion protein scrapie (PrP^Sc), is considered the etiology of these diseases. PrP^Sc accumulates in the affected individual’s brain in the form of extracellular plaques. The molecular pathways leading to neuronal cell death in prion diseases are still unclear. The free radical damage, oxidative stress and mitochondrial dysfunction play a key role in the pathogenesis of the various neurodegenerative disorders including prion diseases. The brain is very sensitive to changes in the redox status. It has been demonstrated that PrP^c behaves as an antioxidant, while the neurotoxic prion peptide PrP^Sc increases hydrogen peroxide toxicity in the neuronal cultures leading to mitochondrial dysfunction and cell death. The nuclear factor erythroid 2-related factor 2 (NRF2) is an oxidative responsive pathway and a guardian of lifespan, which protect the cells from free radical stress-mediated cell death. The reduced glutathione, a major small molecule antioxidant present in all mammalian cells, and produced by several downstream target genes of NRF2, counterbalances the mitochondrial reactive oxygen species (ROS) production. In recent years, it has emerged that the ubiquitin-binding protein, p62-mediated induction of autophagy, is crucial for NRF2 activation and elimination of mitochondrial dysfunction and oxidative stress. The current review article, focuses on the role of NRF2 pathway in prion diseases to mitigate the disease progression.

ER stress and the unfolded protein response in neurodegeneration

The clinical manifestation of neurodegenerative diseases is initiated by the selective alteration in the functionality of distinct neuronal populations. The pathology of many neurodegenerative diseases includes accumulation of misfolded proteins in the brain. In physiological conditions, the proteostasis network maintains normal protein folding, trafficking and degradation; alterations in this network - particularly disturbances to the function of endoplasmic reticulum (ER) - are thought to contribute to abnormal protein aggregation. ER stress triggers a signalling reaction known as the unfolded protein response (UPR), which induces adaptive programmes that improve protein folding and promote quality control mechanisms and degradative pathways or can activate apoptosis when damage is irreversible. In this Review, we discuss the latest advances in defining the functional contribution of ER stress to brain diseases, including novel evidence that relates the UPR to synaptic function, which has implications for cognition and memory. A complex concept is emerging wherein the consequences of ER stress can differ drastically depending on the disease context and the UPR signalling pathway that is altered. Strategies to target specific components of the UPR using small molecules and gene therapy are in development, and promise interesting avenues for future interventions to delay or stop neurodegeneration.

Different Molecular Mechanisms Mediate Direct or Glia-Dependent Prion Protein Fragment 90-231 Neurotoxic Effects in Cerebellar Granule Neurons

Glia over-stimulation associates with amyloid deposition contributing to the progression of central nervous system neurodegenerative disorders. Here we analyze the molecular mechanisms mediating microglia-dependent neurotoxicity induced by prion protein (PrP)90-231, an amyloidogenic polypeptide corresponding to the protease-resistant portion of the pathological prion protein scrapie (PrP^Sc). PrP90-231 neurotoxicity is enhanced by the presence of microglia within neuronal culture, and associated to a rapid neuronal [Ca⁺⁺] _i increase. Indeed, while in "pure" cerebellar granule neuron cultures, PrP90-231 causes a delayed intracellular Ca⁺⁺ entry mediated by the activation of NMDA receptors; when neuron and glia are co-cultured, a transient increase of [Ca⁺⁺] _i occurs within seconds after treatment in both granule neurons and glial cells, then followed by a delayed and sustained [Ca⁺⁺] _i raise, associated with the induction of the expression of inducible nitric oxide synthase and phagocytic NADPH oxidase. [Ca⁺⁺] _i fast increase in neurons is dependent on the activation of multiple pathways since it is not only inhibited by the blockade of voltage-gated channel activity and NMDA receptors but also prevented by the inhibition of nitric oxide and PGE₂ release from glial cells. Thus, Ca⁺⁺ homeostasis alteration, directly induced by PrP90-231 in cerebellar granule cells, requires the activation of NMDA receptors, but is greatly enhanced by soluble molecules released by activated glia. In glia-enriched cerebellar granule cultures, the activation of inducible nitric oxide (iNOS) and NADPH oxidase represents the main mechanism of toxicity since their pharmacological inhibition prevented PrP90-231 neurotoxicity, whereas NMDA blockade by D(-)-2-amino-5-phosphonopentanoic acid is ineffective; conversely, in pure cerebellar granule cultures, NMDA blockade but not iNOS inhibition strongly reduced PrP90-231 neurotoxicity. These data indicate that amyloidogenic peptides induce neurotoxic signals via both direct neuron interaction and glia activation through different mechanisms responsible of calcium homeostasis disruption in neurons and potentiating each other: the activation of excitotoxic pathways via NMDA receptors and the release of radical species that establish an oxidative milieu.

The Role of Unfolded Protein Response and Mitogen-Activated Protein Kinase Signaling in Neurodegenerative Diseases with Special Focus on Prion Diseases

Prion diseases are neurodegenerative pathologies characterized by the accumulation of a protease-resistant form of the cellular prion protein named prion protein scrapie (PrP^Sc) in the brain. PrP^Sc accumulation in the endoplasmic reticulum (ER) result in a dysregulated calcium (Ca²⁺) homeostasis and subsequent initiation of unfolded protein response (UPR) leading to neuronal dysfunction and apoptosis. The molecular mechanisms for the transition between adaptation to ER stress and ER stress-induced apoptosis are still unclear. Mitogen-activated protein kinases (MAPKs) are serine/threonine protein kinases that rule the signaling of many extracellular stimuli from plasma membrane to the nucleus. However the identification of numerous points of cross talk between the UPR and MAPK signaling pathways may contribute to our understanding of the consequences of ER stress in prion diseases. Indeed the MAPK signaling network is known to regulate cell cycle progression and cell survival or death responses following a variety of stresses including misfolded protein response stress. In this article, we review the UPR signaling in prion diseases and discuss the triad of MAPK signaling pathways. We also describe the role played by MAPK signaling cascades in Alzheimer’s (AD) and Parkinson’s disease (PD). We will also overview the mechanisms of cell death and the role of MAPK signaling in prion disease progression and highlight potential avenues for therapeutic intervention.

Early Minocycline and Late FK506 Treatment Improves Survival and Alleviates Neuroinflammation, Neurodegeneration, and Behavioral Deficits in Prion-Infected Hamsters

Prion infections of the central nervous system (CNS) are characterized by initial reactive gliosis followed by overt neuronal death. Gliosis is likely to be caused initially by the deposition of misfolded, proteinase K-resistant, isoforms (termed PrPSc) of the normal cellular prion protein (PrPc) in the brain. Proinflammatory cytokines and chemokines released by PrPSc-activated glia and stressed neurons may also contribute directly or indirectly to the disease development by enhancing gliosis and inducing neurotoxicity. Recent studies have illustrated that early neuroinflammation activates nuclear factor of activated T cells (NFAT) in the calcineurin signaling cascade, resulting in nuclear translocation of nuclear factor kappa B (NF-κB) to promote apoptosis. Hence, useful therapeutic approaches to slow down the course of prion disease development should control early inflammatory responses to suppress NFAT signaling. Here we used a hamster model of prion diseases to test, for the first time, the neuroprotective and NFAT-suppressive effect of a second-generation semisynthetic tetracycline derivative, minocycline, versus a calcineurin inhibitor, FK506, with known NFAT suppressive activity. Our results indicate that prolonged treatment with minocycline, starting from the presymptomatic stage of prion disease was more effective than FK506 given either during the presymptomatic or symptomatic stage of prion disease. Specifically, minocycline treatment reduced the expression of the astrocyte activation marker glial fibrillary acidic protein and of the microglial activation marker ionized calcium-binding adapter molecule-1, subsequently reducing the level of proinflammatory cytokines interleukin 1β and tumor necrosis factor-α. We further found that minocycline and FK506 treatment inhibited mitogen-activated protein kinase p38 phosphorylation and NF-κB nuclear translocation in a caspase-dependent manner, and enhanced phosphorylated cyclic adenosine monophosphate response element-binding protein and phosphorylated Bcl2-associated death promoter levels to reduce cognitive impairment and apoptosis. Taken together, our results indicate that minocycline is a better choice for prolonged use in prion diseases and encourage its further clinical development as a possible treatment for this disease.

Interplay of endoplasmic reticulum stress and autophagy in neurodegenerative disorders

The common underlying feature of most neurodegenerative diseases such as Alzheimer disease (AD), prion diseases, Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS) involves accumulation of misfolded proteins leading to initiation of endoplasmic reticulum (ER) stress and stimulation of the unfolded protein response (UPR). Additionally, ER stress more recently has been implicated in the pathogenesis of HIV-associated neurocognitive disorders (HAND). Autophagy plays an essential role in the clearance of aggregated toxic proteins and degradation of the damaged organelles. There is evidence that autophagy ameliorates ER stress by eliminating accumulated misfolded proteins. Both abnormal UPR and impaired autophagy have been implicated as a causative mechanism in the development of various neurodegenerative diseases. This review highlights recent advances in the field on the role of ER stress and autophagy in AD, prion diseases, PD, ALS and HAND with the involvement of key signaling pathways in these processes and implications for future development of therapeutic strategies.

Parkin Overexpression Ameliorates PrP106-126-Induced Neurotoxicity via Enhanced Autophagy in N2a Cells

Transmissible spongiform encephalopathies (TSEs) are caused by the accumulation of the abnormal prion protein scrapie (PrP^Sc). Prion protein aggregation, misfolding, and cytotoxicity in the brain are the major causes of neuronal dysfunction and ultimate neurodegeneration in all TSEs. Parkin, an E3 ubiquitin ligase, has been studied extensively in all major protein misfolding aggregating diseases, especially Parkinson's disease and Alzheimer's disease, but the role of parkin in TSEs remains unknown. Here we investigated the role of parkin in a prion disease cell model in which neuroblastoma2a (N2a) cells were treated with prion peptide PrP106-126. We observed a gradual decrease in the soluble parkin level upon treatment with PrP106-126 in a time-dependent manner. Furthermore, endogenous parkin colocalized with FITC-tagged prion fragment106-126. Overexpression of parkin in N2a cells via transfection repressed apoptosis by enhancing autophagy. Parkin-overexpressing cells also showed reductions in apoptotic BAX translocation to the mitochondria and cytochrome c release to the cytosol, which ultimately inhibited activation of proapoptotic caspases. Taken together, our findings reveal a parkin-mediated cytoprotective mechanism against PrP106-126 toxicity, which is a novel potential therapeutic target for treating prion diseases.

Receptor-interacting protein 140 attenuates endoplasmic reticulum stress in neurons and protects against cell death

Inositol 1, 4, 5-trisphosphate receptor (IP3R)-mediated Ca(2+) release from the endoplasmic reticulum (ER) triggers many physiological responses in neurons, and when uncontrolled can cause ER stress that contributes to neurological disease. Here we show that the unfolded protein response (UPR) in neurons induces rapid translocation of nuclear receptor-interacting protein 140 (RIP140) to the cytoplasm. In the cytoplasm, RIP140 localizes to the ER by binding to the IP3R. The carboxyl-terminal RD4 domain of RIP140 interacts with the carboxyl-terminal gate-keeping domain of the IP3R. This molecular interaction disrupts the IP3R's 'head-tail' interaction, thereby suppressing channel opening and attenuating IP3R-mediated Ca(2+) release. This contributes to a rapid suppression of the ER stress response and provides protection from apoptosis in both hippocampal neurons in vitro and in an animal model of ER stress. Thus, RIP140 translocation to the cytoplasm is an early response to ER stress and provides protection against neuronal death.

Rutin alleviates prion peptide-induced cell death through inhibiting apoptotic pathway activation in dopaminergic neuronal cells

Prion disorders are progressive neurodegenerative diseases characterized by extensive neuronal loss and accumulation of the abnormal form of the scrapie prion protein (PrP). Rutin is a flavonoid that occurs naturally in plant-derived beverages and foods and is used in traditional and folkloric medicine worldwide. In the present study, we evaluated the protective effects of rutin against PrP fragment (106-126)-induced neuronal cell death. Rutin treatment blocked PrP (106-126)-mediated increases in reactive oxygen species production and nitric oxide release and helped slowing the decrease of neurotrophic factors that results from PrP accumulation. Rutin attenuated PrP (106-126)-associated mitochondrial apoptotic events by inhibiting mitochondrial permeability transition and caspase-3 activity and blocking expression of the apoptotic signals Bax and PARP. Additionally, rutin treatment significantly decreased the expression of the death receptor Fas and its ligand Fas-L. Overall, our results demonstrated that rutin protects against the neurodegenerative effects of prion accumulation by increasing production of neurotropic factors and inhibiting apoptotic pathway activation in neuronal cells. These results suggested that rutin may have clinical benefits for prion diseases and other neurodegenerative disorders.

Bax inhibitor-1-mediated inhibition of mitochondrial Ca2+ intake regulates mitochondrial permeability transition pore opening and cell death

A recently studied endoplasmic reticulum (ER) stress regulator, Bax inhibitor-1 (BI-1) plays a regulatory role in mitochondrial Ca²⁺ levels. In this study, we identified ER-resident and mitochondria-associated ER membrane (MAM)-resident populations of BI-1. ER stress increased mitochondrial Ca²⁺ to a lesser extent in BI-1–overexpressing cells (HT1080/BI-1) than in control cells, most likely as a result of impaired mitochondrial Ca²⁺ intake ability and lower basal levels of intra-ER Ca²⁺. Moreover, opening of the Ca²⁺-induced mitochondrial permeability transition pore (PTP) and cytochrome c release were regulated by BI-1. In HT1080/BI-1, the basal mitochondrial membrane potential was low and also resistant to Ca²⁺ compared with control cells. The activity of the mitochondrial membrane potential-dependent mitochondrial Ca²⁺ intake pore, the Ca²⁺ uniporter, was reduced in the presence of BI-1. This study also showed that instead of Ca²⁺, other cations including K⁺ enter the mitochondria of HT1080/BI-1 through mitochondrial Ca²⁺-dependent ion channels, providing a possible mechanism by which mitochondrial Ca²⁺ intake is reduced, leading to cell protection. We propose a model in which BI-1–mediated sequential regulation of the mitochondrial Ca²⁺ uniporter and Ca²⁺-dependent K⁺ channel opening inhibits mitochondrial Ca²⁺ intake, thereby inhibiting PTP function and leading to cell protection.

Solvent microenvironments and copper binding alters the conformation and toxicity of a prion fragment

The secondary structures of amyloidogenic proteins are largely influenced by various intra and extra cellular microenvironments and metal ions that govern cytotoxicity. The secondary structure of a prion fragment, PrP(111-126), was determined using circular dichroism (CD) spectroscopy in various microenvironments. The conformational preferences of the prion peptide fragment were examined by changing solvent conditions and pH, and by introducing external stress (sonication). These physical and chemical environments simulate various cellular components at the water-membrane interface, namely differing aqueous environments and metal chelating ions. The results show that PrP(111-126) adopts different conformations in assembled and non-assembled forms. Aging studies on the PrP(111-126) peptide fragment in aqueous buffer demonstrated a structural transition from random coil to a stable β-sheet structure. A similar, but significantly accelerated structural transition was observed upon sonication in aqueous environment. With increasing TFE concentrations, the helical content of PrP(111-126) increased persistently during the structural transition process from random coil. In aqueous SDS solution, PrP(111-126) exhibited β-sheet conformation with greater α-helical content. No significant conformational changes were observed under various pH conditions. Addition of Cu²⁺ ions inhibited the structural transition and fibril formation of the peptide in a cell free in vitro system. The fact that Cu²⁺ supplementation attenuates the fibrillar assemblies and cytotoxicity of PrP(111-126) was witnessed through structural morphology studies using AFM as well as cytotoxicity using MTT measurements. We observed negligible effects during both physical and chemical stimulation on conformation of the prion fragment in the presence of Cu²⁺ ions. The toxicity of PrP(111-126) to cultured astrocytes was reduced following the addition of Cu²⁺ ions, owing to binding affinity of copper towards histidine moiety present in the peptide.

Crosstalk between Endoplasmic Reticulum Stress and Protein Misfolding in Neurodegenerative Diseases

Under physiological conditions, the endoplasmic reticulum (ER) is a central subcellular compartment for protein quality control in the secretory pathway that prevents protein misfolding and aggregation. Instrumental in protein quality control in the ER is the unfolded protein response (UPR), which is activated upon ER stress to reestablish homeostasis through a sophisticated transcriptionally and translationally regulated signaling network. However, this response can lead to apoptosis if the stress cannot be alleviated. The presence of abnormal protein aggregates containing specific misfolded proteins is recognized as the basis of numerous human conformational disorders, including neurodegenerative diseases. Here, I will highlight the overwhelming evidence that the presence of specific aberrant proteins in Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), prion diseases, and Amyotrophic Lateral Sclerosis (ALS) is intimately associated with perturbations in the ER protein quality control machinery that become incompetent to restore protein homeostasis and shift adaptive programs toward the induction of apoptotic signaling to eliminate irreversibly damaged neurons. Increasing our understanding about the deadly crosstalk between ER dysfunction and protein misfolding in these neurodegenerative diseases may stimulate the development of novel therapeutic strategies able to support neuronal survival and ameliorate disease progression.

Endoplasmic-reticulum calcium depletion and disease

The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.

Prion protein misfolding affects calcium homeostasis and sensitizes cells to endoplasmic reticulum stress

Prion-related disorders (PrDs) are fatal neurodegenerative disorders characterized by progressive neuronal impairment as well as the accumulation of an abnormally folded and protease resistant form of the cellular prion protein, termed PrP^RES. Altered endoplasmic reticulum (ER) homeostasis is associated with the occurrence of neurodegeneration in sporadic, infectious and familial forms of PrDs. The ER operates as a major intracellular calcium store, playing a crucial role in pathological events related to neuronal dysfunction and death. Here we investigated the possible impact of PrP misfolding on ER calcium homeostasis in infectious and familial models of PrDs. Neuro2A cells chronically infected with scrapie prions showed decreased ER-calcium content that correlated with a stronger upregulation of UPR-inducible chaperones, and a higher sensitivity to ER stress-induced cell death. Overexpression of the calcium pump SERCA stimulated calcium release and increased the neurotoxicity observed after exposure of cells to brain-derived infectious PrP^RES. Furthermore, expression of PrP mutants that cause hereditary Creutzfeldt-Jakob disease or fatal familial insomnia led to accumulation of PrP^RES and their partial retention at the ER, associated with a drastic decrease of ER calcium content and higher susceptibility to ER stress. Finally, similar results were observed when a transmembrane form of PrP was expressed, which is proposed as a neurotoxic intermediate. Our results suggest that alterations in calcium homeostasis and increased susceptibility to ER stress are common pathological features of both infectious and familial PrD models.

Transcriptional analysis implicates endoplasmic reticulum stress in bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) is a fatal, transmissible, neurodegenerative disease of cattle. To date, the disease process is still poorly understood. In this study, brain tissue samples from animals naturally infected with BSE were analysed to identify differentially regulated genes using Affymetrix GeneChip Bovine Genome Arrays. A total of 230 genes were shown to be differentially regulated and many of these genes encode proteins involved in immune response, apoptosis, cell adhesion, stress response and transcription. Seventeen genes are associated with the endoplasmic reticulum (ER) and 10 of these 17 genes are involved in stress related responses including ER chaperones, Grp94 and Grp170. Western blotting analysis showed that another ER chaperone, Grp78, was up-regulated in BSE. Up-regulation of these three chaperones strongly suggests the presence of ER stress and the activation of the unfolded protein response (UPR) in BSE. The occurrence of ER stress was also supported by changes in gene expression for cytosolic proteins, such as the chaperone pair of Hsp70 and DnaJ. Many genes associated with the ubiquitin-proteasome pathway and the autophagy-lysosome system were differentially regulated, indicating that both pathways might be activated in response to ER stress. A model is presented to explain the mechanisms of prion neurotoxicity using these ER stress related responses. Clustering analysis showed that the differently regulated genes found from the naturally infected BSE cases could be used to predict the infectious status of the samples experimentally infected with BSE from the previous study and vice versa. Proof-of-principle gene expression biomarkers were found to represent BSE using 10 genes with 94% sensitivity and 87% specificity.

The oligomerization properties of prion protein are restricted to the H2H3 domain

The propensity of the prion protein (PrP) to adopt different structures is a clue to its pathological behavior. The determination of the region involved in the PrP(C) to PrP(Sc) conversion is fundamental for the understanding of the mechanisms underlying this process at the molecular level. In this paper, the polymerization of the helical H2H3 domain of ovine PrP (OvPrP) was compared to the full-length construct (using chromatography and light scattering). We show that the oligomerization patterns are identical, although the H2H3 domain has a higher polymerization rate. Furthermore, the depolymerization kinetics of purified H2H3 oligomers compared to those purified from the full-length PrP reveal that regions outside H2H3 do not significantly contribute to the oligomerization process. By combining rational mutagenesis and molecular dynamics to investigate the early stages of H2H3 oligomerization, we observe a conformationally stable beta-sheet structure that we propose as a possible nucleus for oligomerization; we also show that single point mutations in H2 and H3 present structural polymorphisms and oligomerization properties that could constitute the basis of species or strain variability.

Role of SERCA1 truncated isoform in the proapoptotic calcium transfer from ER to mitochondria during ER stress

Among the new players at the endoplasmic reticulum (ER)-mitochondria interface regulating interorganelle calcium signaling, those specifically involved during ER stress are not known at present. We report here that the truncated variant of the sarcoendoplasmic reticulum Ca(2+)-ATPase 1 (S1T) amplifies ER stress through the PERK-eIF2alpha-ATF4-CHOP pathway. S1T, which is localized in the ER-mitochondria microdomains, determines ER Ca(2+) depletion due to increased Ca(2+) leak, an increased number of ER-mitochondria contact sites, and inhibition of mitochondria movements. This leads to increased Ca(2+) transfer to mitochondria in both resting and stimulated conditions and activation of the mitochondrial apoptotic pathway. Interestingly, S1T knockdown was shown to prevent ER stress, mitochondrial Ca(2+) overload, and subsequent apoptosis. Thus, by bridging ER stress to apoptosis through increased ER-mitochondria Ca(2+) transfer, S1T acts as an essential determinant of cellular fate.