Hypoglycemia enhances the expression of prion protein and heat-shock protein 70 in a mouse neuroblastoma cell line

Effect of recurrent severe insulin-induced hypoglycemia on the cognitive function and brain oxidative status in the rats

BACKGROUND

Episodes of recurrent or severe hypoglycemia can occur in patients with diabetes mellitus, insulinoma, neonatal hypoglycemia, and medication errors. However, little is known about the short-term and long-term effects of repeated episodes of acute severe hypoglycemia on the brain, particularly in relation to hippocampal damage and cognitive dysfunction.

METHODS

Thirty-six wistar rats were randomly assigned to either the experimental or control group. The rats were exposed to severe hypoglycemia, and assessments were conducted to evaluate oxidative stress in brain tissue, cognitive function using the Morris water maze test, as well as histopathology and immunohistochemistry studies. The clinical and histopathological evaluations were conducted in the short-term and long-term.

RESULTS

The mortality rate attributed to hypoglycemia was 34%, occurring either during hypoglycemia or within 24 h after induction. Out of the 14 rats monitored for 7 to 90 days following severe/recurrent hypoglycemia, all exhibited clinical symptoms, which mostly resolved within three days after the last hypoglycemic episode, except for three rats. Despite the decrease in catalase activity in the brain, the total antioxidant capacity following severe insulin-induced hypoglycemia increased. The histopathology findings revealed that the severity of the hippocampal damage was higher compared to the brain cortex 90 days after hypoglycemia. Memory impairments with neuron loss particularly pronounced in the dentate gyrus region of the hippocampus were observed in the rats with severe hypoglycemia. Additionally, there was an increase in reactive astrocytes indicated by GFAP immunoreactivity in the brain cortex and hippocampus.

CONCLUSION

Recurrent episodes of severe hypoglycemia can lead to high mortality rates, memory impairments, and severe histopathological changes in the brain. While many histopathological and clinical changes improved after three months, it seems that the vulnerability of the hippocampus and the development of sustained changes in the hippocampus were greater and more severe compared to the brain cortex following severe and recurrent hypoglycemia. Furthermore, it does not appear that oxidative stress plays a central role in neuronal damage following severe insulin-induced hypoglycemia. Further research is necessary to assess the consequences of repeated hypoglycemic episodes on sustained damage across various brain regions.

Multifactorial Attenuation of the Murine Heat Shock Response With Age

Age-dependent perturbation of the cellular stress response affects proteostasis and other key functions relevant to cellular action and survival. Central to age-related changes in the stress response is loss of heat shock factor 1 (HSF1)-DNA binding and transactivation properties. This report elucidates how age alters different checkpoints of HSF1 activation related to posttranslational modification and protein interactions. When comparing liver extracts from middle aged (12 M) and old (24 M) mice, significant differences are found in HSF1 phosphorylation and acetylation. HSF1 protein levels and messenger RNA decline with age, but its protein levels are stress-inducible and exempt from age-dependent changes. This surprising adaptive change in the stress response has additional implications for aging and chronic physiological stress that might explain an age-dependent dichotomy of HSF1 protein levels that are low in neurodegeneration and elevated in cancer.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

Cellular stress mechanisms of prenatal maternal stress: Heat shock factors and oxidative stress

Development of the brain prenatally is affected by maternal experience and exposure. Prenatal maternal psychological stress changes brain development and results in increased risk for neuropsychiatric disorders. In this review, multiple levels of prenatal stress mechanisms (offspring brain, placenta, and maternal physiology) are discussed and their intersection with cellular stress mechanisms explicated. Heat shock factors and oxidative stress are closely related to each other and converge with the inflammation, hormones, and cellular development that have been more deeply explored as the basis of prenatal stress risk. Increasing evidence implicates cellular stress mechanisms in neuropsychiatric disorders associated with prenatal stress including affective disorders, schizophrenia, and child-onset psychiatric disorders. Heat shock factors and oxidative stress also have links with the mechanisms involved in other kinds of prenatal stress including external exposures such as environmental toxicants and internal disruptions such as preeclampsia. Integrative understanding of developmental neurobiology with these cellular and physiological mechanisms is necessary to reduce risks and promote healthy brain development.

Mechanisms of neuroprotection against ischemic insult by stress-inducible phosphoprotein-1/prion protein complex

Stress-inducible phosphoprotein 1 (STI1) acts as a neuroprotective factor in the ischemic brain and its levels are increased following ischemia. Previous work has suggested that some of these STI1 actions in a stroke model depend on the recruitment of bone marrow-derived stem cells to improve outcomes after ischemic insult. However, STI1 can directly increase neuroprotective signaling in neurons by engaging with the cellular prion protein (PrPC ) and activating α7 nicotinic acetylcholine receptors (α7nAChR). Given that α7nAChR activation has also been involved in neuroprotection in stroke, it is possible that STI1 can have direct actions on neurons to prevent deleterious consequences of ischemic insults. Here, we tested this hypothesis by exposing primary neuronal cultures to 1-h oxygen-glucose deprivation (OGD) and reperfusion and assessing signaling pathways activated by STI1/PrPC . Our results demonstrated that STI1 treatment significantly decreased apoptosis and cell death in mouse neurons submitted to OGD in a manner that was dependent on PrPC and α7nAChR, but also on the activin A receptor 1 (ALK2), which has emerged as a signaling partner of STI1. Interestingly, pharmacological inhibition of the ALK2 receptor prevented neuroprotection by STI1, while activation of ALK2 receptors by bone morphogenetic protein 4 (BMP4) either before or after OGD was effective in decreasing neuronal death induced by ischemia. We conclude that PrPC /STI1 engagement and its subsequent downstream signaling cascades involving α7nAChR as well as the ALK2 receptor may be activated in neurons by increased levels of STI1. This signaling pathway protects neurons from ischemic insults.

Dual role of cellular prion protein in normal host and Alzheimer's disease

Using PrPC-knockout cell lines, it has been shown that the inhibition of apoptosis through STI1 is mediated by PrPC-dependent SOD activation. Antioxidant PrPC may contribute to suppression of inflammasome activation. PrPC is functionally involved in copper metabolism, signal transduction, neuroprotection, and cell maturation. Recently several reports have shown that PrPC participates in trans-membrane signaling processes associated with hematopoietic stem cell replication and neuronal differentiation. In another role, PrPC also tends to function as a neurotoxic protein. Aβ oligomer, which is associated with neurodegeneration in Alzheimer's disease (AD), has also been reported to act as a ligand of PrPC. However, the physiological role of PrPC as an Aβ42-binding protein is not clear. Actually, PrPC is critical in Aβ42-mediated autophagy in neurons. PrPC shows a beneficial role in lipid rafts to promote autophagy. Further search for PrPC-interaction molecules using Prnp-/- mice and various types of Prnp-/- cell lines under various conditions may elucidate other important PrPC important functions.

Cellular prion protein directly interacts with and enhances lactate dehydrogenase expression under hypoxic conditions

Although a physiological function of the cellular prion protein (PrP(c)) is still not fully clarified, a PrP(c)-mediated neuroprotection against hypoxic/ischemic insult is intriguing. After ischemic stroke prion protein knockout mice (Prnp(0/0)) display significantly greater lesions as compared to wild-type (WT) mice. Earlier reports suggested an interaction between the glycolytic enzyme lactate dehydrogenase (LDH) and PrP(c). Since hypoxic environment enhances LDH expression levels and compels neurons to rely on lactate as an additional oxidative substrate for energy metabolism, we examined possible differences in LDH protein expression in WT and Prnp(0/0) knockout models under normoxic/hypoxic conditions in vitro and in vivo, as well as in a HEK293 cell line. While no differences are observed under normoxic conditions, LDH expression is markedly increased after 60-min and 90-min of hypoxia in WT vs. Prnp(0/0) primary cortical neurons with concurrent less hypoxia-induced damage in the former group. Likewise, cerebral ischemia significantly increases LDH levels in WT vs. Prnp(0/0) mice with accompanying smaller lesions in the WT group. HEK293 cells overexpressing PrP(c) show significantly higher LDH expression/activity following 90-min of hypoxia as compared to control cells. Moreover, a cytoplasmic co-localization of LDH and PrP(c) was recorded under both normoxic and hypoxic conditions. Interestingly, an expression of monocarboxylate transporter 1, responsible for cellular lactate uptake, increases with PrP(c)-overexpression under normoxic conditions. Our data suggest LDH as a direct PrP(c) interactor with possible physiological relevance under low oxygen conditions.

Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3

Prion protein (PrP) plays crucial roles in regulating antioxidant systems to improve cell defenses against cellular stress. Here, we show that the interactions of PrP with the excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (γ-GT), and multi-drug resistance protein 1 (MRP1) in astrocytes and the interaction between PrP and EAAT3 in neurons regulate the astroglial and neuronal metabolism of the antioxidant glutathione. Ablation of PrP in astrocytes and cerebellar neurons leads to dysregulation of EAAT3-mediated uptake of glutamate and cysteine, which are precursors for the synthesis of glutathione. In PrP-deficient astrocytes, levels of intracellular glutathione are increased, and under oxidative stress, levels of extracellular glutathione are increased, due to (i) increased glutathione release via MRP1 and (ii) reduced activity of the glutathione-degrading enzyme γ-GT. In PrP-deficient cerebellar neurons, cell death is enhanced under oxidative stress and glutamate excitotoxicity, when compared to wild-type cerebellar neurons. These results indicate a functional interplay of PrP with EAAT3, MRP1 and γ-GT in astrocytes and of PrP and EAAT3 in neurons, suggesting that these interactions play an important role in the metabolic cross-talk between astrocytes and neurons and in protection of neurons by astrocytes from oxidative and glutamate-induced cytotoxicity. Interactions of prion protein (PrP) with excitatory amino acid transporter 3 (EAAT3), γ-glutamyl transpeptidase (GGT) and multi-drug resistance protein 1 (MRP1) regulate the astroglial and neuronal metabolism of glutathione (GSH) which protects cells against the cytotoxic oxidative stress. PrP controls the release of GSH from astrocytes via MRP1 and regulates the hydrolysis of extracellular GSH by GGT as well as the neuronal and astroglial glutamate and cysteine uptake via EAAT3.

Nrf-2 regulation of prion protein expression is independent of oxidative stress

Cellular expression of host prion protein (PrP) is essential to infection with prion disease. Understanding the mechanisms that regulate prion protein expression at both the transcriptional and translational levels is therefore an important goal. The cellular prion protein has been associated with resistance to oxidative, and its expression is also increased by oxidative stress. The transcription factor Nrf-2 is associated with cellular responses to oxidative stress and is known to induce upregulation of antioxidant defense mechanisms. We have identified an Nrf-2 binding site in the prion protein promoter (Prnp) and shown that Nrf-2 downregulated PrP expression. However, this effect is independent of oxidative stress as oxidative stress can up-regulate PrP expression regardless of the level of Nrf-2 expression. Furthermore, Nrf-2 has no impact on PrP expression when cells are infected with scrapie. These findings highlight that Nrf-2 can regulate PrP expression, but that this regulation becomes uncoupled during cellular stress.

Prion protein and aging

The cellular prion protein (PrP^C) has been widely investigated ever since its conformational isoform, the prion (or PrP^Sc), was identified as the etiological agent of prion disorders. The high homology shared by the PrP^C-encoding gene among mammals, its high turnover rate and expression in every tissue strongly suggest that PrP^C may possess key physiological functions. Therefore, defining PrP^C roles, properties and fate in the physiology of mammalian cells would be fundamental to understand its pathological involvement in prion diseases. Since the incidence of these neurodegenerative disorders is enhanced in aging, understanding PrP^C functions in this life phase may be of crucial importance. Indeed, a large body of evidence suggests that PrP^C plays a neuroprotective and antioxidant role. Moreover, it has been suggested that PrP^C is involved in Alzheimer disease, another neurodegenerative pathology that develops predominantly in the aging population. In prion diseases, PrP^C function is likely lost upon protein aggregation occurring in the course of the disease. Additionally, the aging process may alter PrP^C biochemical properties, thus influencing its propensity to convert into PrP^Sc. Both phenomena may contribute to the disease development and progression. In Alzheimer disease, PrP^C has a controversial role because its presence seems to mediate β-amyloid toxicity, while its down-regulation correlates with neuronal death. The role of PrP^C in aging has been investigated from different perspectives, often leading to contrasting results. The putative protein functions in aging have been studied in relation to memory, behavior and myelin maintenance. In aging mice, PrP^C changes in subcellular localization and post-translational modifications have been explored in an attempt to relate them to different protein roles and propensity to convert into PrP^Sc. Here we provide an overview of the most relevant studies attempting to delineate PrP^C functions and fate in aging.

Short-term but not long-term hypoglycaemia enhances plasma levels and hepatic expression of HSP72 in insulin-treated rats: an effect associated with increased IL-6 levels but not with IL-10 or TNF-α

The inducible expression of the 70-kDa heat shock proteins (HSP70) is associated with homeostatically stressful situations. Stresses involving sympathetic nervous system (SNS) activation, including α1-adrenergic agonists and physical exercise, are capable of inducing HSP70 expression and release of the HSP70 inducible form, HSP72. However, whether hypoglycaemia is capable of influencing HSP70 status under a stressful situation such as insulin-induced hypoglycaemia (IIH), which also involves SNS activation, is unsettled. Hence, we decided to investigate whether the predominant signal for HSP70 expression and delivery into the blood comes from either low glucose, high insulin, or both during short-term IIH (STIIH) and long-term IIH (LTIIH). Our data indicated that low glucose level (up to 1.56 ± 0.14 mM), but not insulin, is the triggering factor responsible for a dramatic rise in HSP72 plasma concentrations (from 0.15 ± 0.01 in fed state to 0.77 ± 0.13 ng/mL during hypoglycaemic episodes). This was observed in parallel with up to 7-fold increases in interleukin-6 (IL-6) but not interleukin-10 (IL-10) or tumour necrosis factor-α (TNF-α) at STIIH. Together, the observations may suggest that HSP72 is released under hypoglycaemic conditions as a part of the homeostatic stress response, whereas at long-term, both hypoglycaemia and insulin may influence HSP72 expression in the liver, but not in kidneys. Secreted extracellular HSP72 (eHSP72) may be purely a danger signal to all the tissues of the body for the enhancement of immune and metabolic surveillance state or actively participates in glycaemic control under stressful situations.

Bovine Spongiform Encephalopathy in Japan: History and Recent Studies on Oxidative Stress in Prion Diseases

With the respect to BSE and vCJD, compliance with the following three rules should strictly be observed: (i) Identification and destruction of all clinically affected cattle; (ii) destruction of all mammalian proteins used in feeding ruminant livestock; and (iii) destruction of all high-risk tissues for use in human consumption. Scrapie in sheep has been documented in the 18th century in the United Kingdom. Through studies of brain-to-brain transmission in the same species in 1935, Cuille et al. successfully isolated the culprit protein from the sheep brain. To transmit said protein from an animal to another, intracerebral inoculation was much more efficient than intraperitoneal or oral route in certain species; i.e. the hamster and mouse. Since discovery of the more efficacious infection route, studies and development of prion research have undergone 4 developmental phases. Phase I depicted discoveries of the pathological features of Creutzfeldt-Jakob disease (CJD) and scrapie with typical lesions of spongiform encephalopathy, while Phase II revealed individual-to-individual (or cross-species) transmissions of CJD, kuru and scrapie in animals. Phases I and II suggested the possible participation of a slow virus in the infection process. In Phase III, Prusiner et al. proposed the 'prion' theory in 1982, followed by the milestone development of the transgenic or gene-targeted mouse in prion research in Phase IV. By strain-typing of prions, CJD has been classified as type 2 or 4 by Parchi et al. and Wadsworth as type-2 or -4 and type-1 or -2, respectively. Wadsworth type 1 is detected in the cerebellum, while Wadsworth type 2 was detected in the prefrontal cortex of 10% of sporadic CJD patients. In 1999, Puoti et al. have reported the co-existence of two types of PrP(res) in a same patient. These reports indicated that PrP(res)-typing is a quantitative rather than a qualitative process, and the relationship between the molecular type and the prion strain is rather complex. In fact, previous findings of Truchot have correlated type-1 distribution with synaptic deposits, and type-2 with arrangement of diffuse deposits in neurons. Although the normal function of PrP(C) has not been fully understood, recent studies have shown that PrP(C) plays a role in copper metabolism, signal transduction, neuroprotection and cell maturation. Further search of PrP(C)-interacting molecules and detailed studies using Prnp(-/-) mice and various type of Prnp(-/-) cell lines under various conditions are the prerequisites in elucidating PrP functions. In the pathogenesis of prion diseases, present results support the hypothesis that 'loss-of-function' of PrP(C) decreases resistance to oxidative stress, and 'gain-of-function' of PrP(Sc) increases oxidative stress. The mechanisms of (i) the 'loss-of-function' of PrP(C) in enhanced susceptibility to oxidative stress and (ii) the 'gain-of-function' of PrP(Sc) in generation of oxidative stress remain to be elucidated, although their mechanisms of action, at least in part, involve the decrease and increase in SOD activity, respectively.

IGF-1-induced enhancement of PRNP expression depends on the negative regulation of transcription factor FOXO3a

The conformational conversion of the cellular prion protein (PrP^C) into its β-sheet-rich scrapie isoform (PrP^Sc) causes fatal prion diseases, which are also called transmissible spongiform encephalopathies (TSEs). Recent studies suggest that the expression of PrP^C by the PRNP gene is crucial for the development of TSEs. Therefore, the identification of the exogenous and endogenous stimulating factors that regulate PRNP expression would help to understand the pathogenesis of TSEs. Here, we demonstrate that forkhead box O3a (FOXO3a) negatively regulates PRNP expression by binding to the PRNP promoter, which is negatively regulated by insulin-like growth factor 1 (IGF-1). Our results show that the IGF-1-induced enhancement of PRNP mRNA and protein levels is due to the activation of the PI3K-Akt signaling pathway. The activation of Akt then induces the phosphorylation of FOXO3a, leading to its translocation from the nucleus to the cytoplasm and preventing its binding to the PRNP promoter. Treatment with the PI3K-Akt inhibitor LY294002 induces the nuclear retention of FOXO3a, which leads to a decrease in PRNP expression. We present a new IGF-1-PI3K-Akt-FOXO3a pathway, which influences PRNP expression. The results of this work are vital for understanding the function of PrP^C and for future therapeutic approaches to human TSEs.

Silencing prion protein in MDA-MB-435 breast cancer cells leads to pleiotropic cellular responses to cytotoxic stimuli

Prion protein (PrP) is well studied for its pathogenic role in prion disease, but its potential contribution to other pathological processes is less understood. PrP is expressed in a variety of cancers and at least in pancreatic and breast cancers, its expression appears to be associated with poor prognosis. To understand the role of PrP in breast cancer cells, we knocked down PrP expression in MDA-MB-435 breast cancer cells with small interfering RNA and subjected these cells to a series of analyses. We found that PrP knockdown in these cells does not affect cell proliferation or colony formation, but significantly influences the cellular response to cytotoxic stimuli. Compared to control cells, PrP knockdown cells exhibited an increased susceptibility to serum deprivation induced apoptosis, no change to staurosporine- or paclitaxel-induced cell deaths, and a reduced susceptibility to chemotherapy drug doxorubicin-induced cell death. To understand the mechanism of unexpected role of PrP in exacerbating doxorubicin-induced cytotoxicity, we analyzed cell death related Bcl-2 family proteins. We found that PrP knockdown alters the expression of several Bcl-2 family proteins, correlating with increased resistance to doxorubicin-induced cytotoxicity. Moreover, the enhanced doxorubicin resistance is independent of DNA damage related p53 pathway, but at least partially through the ERK1/2 pathway. Together, our study revealed that silencing PrP in MDA-MB-435 breast cancer cells results in very different responses to various cytotoxic stimuli and ERK1/2 signaling pathway is involved in PrP silencing caused resistance to doxorubicin.

Cytotoxicity, anti-angiogenic, apoptotic effects and transcript profiling of a naturally occurring naphthyl butenone, guieranone A

Background

Malignant diseases are responsible of approximately 13% of all deaths each year in the world. Natural products represent a valuable source for the development of novel anticancer drugs. The present study was aimed at evaluating the cytotoxicity of a naphtyl butanone isolated from the leaves of Guiera senegalensis, guieranone A (GA).

Results

The results indicated that GA was active on 91.67% of the 12 tested cancer cell lines, the IC₅₀ values below 4 μg/ml being recorded on 83.33% of them. In addition, the IC₅₀ values obtained on human lymphoblastic leukemia CCRF-CEM (0.73 μg/ml) and its resistant subline CEM/ADR5000 (1.01 μg/ml) and on lung adenocarcinoma A549 (0.72 μg/ml) cell lines were closer or lower than that of doxorubicin. Interestingly, low cytotoxicity to normal hepatocyte, AML12 cell line was observed. GA showed anti-angiogenic activity with up to 51.9% inhibition of the growth of blood capillaries on the chorioallantoic membrane of quail embryo. Its also induced apotosis and cell cycle arrest. Ingenuity Pathway Analysis identified several pathways in CCRF-CEM cells and functional group of genes regulated upon GA treatment (P < 0.05), the Cell Cycle: G2/M DNA Damage Checkpoint Regulation and ATM Signaling pathways being amongst the four most involved functional groups.

Conclusion

The overall results of this work provide evidence of the cytotoxic potential of GA and supportive data for its possible use in cancer chemotherapy.

Increased immunohistochemical labelling for prion protein occurs in diverse neurological disorders of sheep: relevance for normal cellular PrP function

The classical prion diseases (e.g. scrapie of sheep and goats and bovine spongiform encephalopathy of cattle) are characterized by the accumulation of abnormal forms of the prion protein (PrP), usually recognized by their relative resistance to proteolysis compared with the physiological cellular forms of PrP. However, novel prion diseases have been detected in sheep, cattle and man, in which the abnormal PrP has less resistance to proteolysis than identified previously. These more subtle differences between abnormal and normal forms of PrP can be problematic in routine diagnostic tests and raise questions in respect of the range of PrP disorders. Abnormal accumulations of PrP in atypical and classical prion diseases can be recognized by immunohistochemistry. To determine whether altered PrP expression or trafficking might occur in nosological entities not previously connected with prion disease, the brains of sheep affected with diverse neurological conditions were examined for evidence of altered PrP labelling. Such altered immunolabelling was detected in association with either basic lesions or specific diseases. Some reactive glial cells and degenerate neurons found in several different recognized disorders and non-specific inflammatory processes were associated with abnormal PrP labelling, which was absent from brains of healthy, age-matched sheep. The results agree with previous indications that normal PrP function may be linked with the oxidative stress response, but the data also suggest that PrP functions are more extensive than simple protective responses against stress insults.

Neuroprotective role of PrPC against kainate-induced epileptic seizures and cell death depends on the modulation of JNK3 activation by GluR6/7-PSD-95 binding

Cellular prion protein (PrP(C)) is a glycosyl-phosphatidylinositol-anchored glycoprotein. When mutated or misfolded, the pathogenic form (PrP(SC)) induces transmissible spongiform encephalopathies. In contrast, PrP(C) has a number of physiological functions in several neural processes. Several lines of evidence implicate PrP(C) in synaptic transmission and neuroprotection since its absence results in an increase in neuronal excitability and enhanced excitotoxicity in vitro and in vivo. Furthermore, PrP(C) has been implicated in the inhibition of N-methyl-d-aspartic acid (NMDA)-mediated neurotransmission, and prion protein gene (Prnp) knockout mice show enhanced neuronal death in response to NMDA and kainate (KA). In this study, we demonstrate that neurotoxicity induced by KA in Prnp knockout mice depends on the c-Jun N-terminal kinase 3 (JNK3) pathway since Prnp(o/o)Jnk3(o/o) mice were not affected by KA. Pharmacological blockage of JNK3 activity impaired PrP(C)-dependent neurotoxicity. Furthermore, our results indicate that JNK3 activation depends on the interaction of PrP(C) with postsynaptic density 95 protein (PSD-95) and glutamate receptor 6/7 (GluR6/7). Indeed, GluR6-PSD-95 interaction after KA injections was favored by the absence of PrP(C). Finally, neurotoxicity in Prnp knockout mice was reversed by an AMPA/KA inhibitor (6,7-dinitroquinoxaline-2,3-dione) and the GluR6 antagonist NS-102. We conclude that the protection afforded by PrP(C) against KA is due to its ability to modulate GluR6/7-mediated neurotransmission and hence JNK3 activation.

Gene expression profiling to identify druggable targets in prion diseases

IMPORTANCE OF THE FIELD

Despite many recent advances in prion research, the molecular mechanisms by which prions cause neurodegeneration have not been established. In fact, the complexity and the novelty characterizing this class of disorders pose a huge challenge to drug discovery. Pharmacogenomics has recently adopted high-throughput transcriptome analyses to predict potential drug target candidates, with promising results in various fields of medicine.

AREAS COVERED IN THIS REVIEW

The present work offers an overview of the transcriptional alterations induced by prion infection in different biological systems. Hereafter, therapeutic approaches are discussed in light of the identified altered processes.

WHAT THE READER WILL GAIN

This review offers readers a detailed overview on microarray analyses, taking into account their advantages and limitations. Our work can help readers, from many research areas, to design a suitable microarray experiment.

TAKE HOME MESSAGE

So far, drugs acting on the pathways identified by microarray analysis have not been found to be effective in prion diseases therapy. An integration of gene expression profiling, proteomics and physiology should be applied to pursue this aim.

Activation and repression of prion protein expression by key regions of intron 1

Expression of the prion protein is necessary for infection with prion diseases. Altered expression levels may play an important role in susceptibility to infection. Therefore, understanding the mechanisms that regulate prion protein expression is of great importance. It was previously shown that expression of the prion protein is to some degree regulated by an alternative promoter within intron 1. Studies using GFP and luciferase reporter systems were undertaken to determine key sites for the repression and activation of expression of the prion protein driven by intron 1. We identified a region within intron 1 sufficient to drive prion protein expression. Our findings highlight two potential repressor regions. Both regions have binding sites for the known repressor Hes-1. Hes-1 overexpression caused a dramatic decrease in PrP protein expression. Additionally, we have identified Atox-1 as a transcription factor that upregulates prion protein expression. These findings clearly indicate that intron 1 plays a key role in regulation of prion protein expression levels.

Two adjacent nuclear factor-binding domains activate expression from the human PRNP promoter

Background

The transmissible spongiform encephalopathies (TSEs) comprise a group of fatal degenerative neurological diseases in humans and other mammals. After infection, the cellular prion protein isoform PrP^C is converted to the pathological PrP^SC scrapie isoform. The continued conversion of PrP^C to PrP^SC requires de novo endogenous PrP synthesis for disease progression. The human prion protein gene (PRNP) promoter was therefore investigated to identify regulatory elements that could serve as targets for therapeutic intervention.

Findings

The human prion protein gene (PRNP) promoter from position -1593 to +134 relative to the putative transcriptional start site (+1) was analyzed by transient transfection in HeLa cells. Deletions from the 5' end between positions -1593 and -232 yielded little change in activity. A further 5' deletion at position -90 resulted in a decline in activity to a level of about 30% of the full-length value. DNase I footprinting of the region between positions -259 and +2 identified two adjacent protected domains designated as prpA (-116 to -143) and prpB (-147 to -186). Internal deletions combined with mobility shift electrophoresis and methylation interference assays indicated the presence of sequence specific nuclear factor complexes that bind to the prpA and prpB domains and activate expression from the human PRNP promoter in an additive fashion.

Conclusion

Results from transient transfection, DNase I footprinting, mobility shift electrophoresis, and methylation interference experiments suggest that two DNase I protected domains designated as prpA and prpB are binding sites for as yet unidentified regulatory factors that independently activate expression from the PRNP promoter.

The effects of prion protein expression on metal metabolism

The prion protein is a glycoprotein that binds metals such as copper and manganese. When converted to a proteinase resistant isoform it is associated with prion diseases such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy. Although, the co-ordination and metal affinity of the prion protein has been well studied, the association of the protein with cellular metal metabolism has been less well investigated. We used transgenic manipulation of prion protein expression and other recombinant techniques to alter expression of known copper binding proteins to investigate the role of the prion protein in copper metabolism. We found that changing the expression of the prion protein alters proteins associated with copper uptake, storage and export from the cell. In addition, alteration in the expression of superoxide dismutases increased prion protein expression dramatically. Reducing copper in the diet decreased expression of the prion protein in the brain while increased dietary manganese dramatically increased the protein's expression. Cellular prion infection also increased the expression of metal transporting proteins and increased cellular manganese concentrations. Overall our results show a close link between cellular resistance to oxidative stress and also copper metabolism. These findings are in line with previous data suggesting that the prion protein is an antioxidant and associated with copper uptake into cells. The disturbance to copper metabolism, as a result of altered prion protein expression clearly demonstrates the important role of the prion protein in copper metabolism. The implication is that prion protein expression has a homeostatic role in copper metabolism.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Regulation of prion protein expression by noncoding regions of the Prnp gene

Expression of the cellular prion protein is necessary for the transmission and propagation of prion diseases. Increasing the level of prion protein expression decreases the incubation period for these diseases. Therefore, understanding the regulation of prion protein expression could be critical for treating or preventing these diseases. We investigated the regulation of prion protein expression by the promoter and noncoding regions of the bovine and murine Prnp genes. We determined that expression is modulated by intron 1 and exon 1. In the absence of intron1, exon 1 inhibited activity of the promoter. However, intron 1 demonstrated promoter-like activity and possessed a TATA box. In addition, we identified an alternative transcript present in the brains of cattle and mice that lacks exon 1. Taken together, these results show that intron 1 and exon 1 play a critical role in the regulation of prion protein expression. Because switching off prion protein expression has been shown to arrest prion disease, these regions present novel targets for intervention in the disease process.

Pronounced cytosolic aggregation of cellular prion protein in pancreatic beta-cells in response to hyperglycemia

Cellular prion protein (PrP(C)), an N-linked glycoprotein, is expressed in a variety of tissues, but its functions remain unclear. PrP(C) is abundantly expressed in the endocrine pancreas, which regulates blood glucose homeostasis. Therefore, we investigated whether the expression of PrP(C) was altered in islets of Langerhans in a model of spontaneous type 1 diabetes, the diabetes-prone BioBreeding (BBdp) rat and a model of beta-cell adaptation to hyperglycemia, the chronic glucose-infused Sprague Dawley rat. Pancreatic sections from animals aged 7-100 days were stained immunohistochemically and evaluated using light, fluorescence and confocal microscopy. PrP(C) was ubiquitously expressed in all four major endocrine cell types within islets. Surprisingly, cytosolic inclusions containing PrP(C) were identified exclusively in a subpopulation of insulin-producing beta-cells. The inclusions exhibited different molecular characteristics from the PrP aggregates previously described in vitro in neurons. The frequency of beta-cells with PrP(C) inclusions increased with age and was threefold greater in diabetes-prone rats than in controls at 100 days. Cytosolic PrP(C) expression in beta-cells was suppressed whereas the number and size of PrP(C) inclusions markedly increased in response to hyperglycemia during the first 2 days of continuous glucose infusion in Sprague Dawley rats. In summary, this is the first report describing in vivo cytosolic PrP(C) aggregation. These unique PrP(C) inclusions were beta-cell specific, more frequent in diabetes-prone animals, and responded to hyperglycemia in glucose-infused Sprague Dawley rats. These data suggest a potential dysfunction in beta-cells of diabetes-prone rats, and point to new avenues for the study of diabetes pathogenesis.