A neurotoxic prion protein fragment enhances proliferation of microglia but not astrocytes in culture

The scrapie isoform of the prion protein (PrP^Sc) induces pathological changes in the central nervous system including neurodegeneration and gliosis. A synthetic prion protein (PrP) peptide corresponding to amino acid residues 106–126 has been shown to be toxic to neurons that express PrP^C, the cellular isoform of PrP. Here we show that in mixed glial cultures PrP106–126 induces astroglial proliferation that is dependent on cellular PrP^c expression. In purified cultures of glial subtypes only microglia proliferated in response to PrP106–126. This effect was independent of PrP expression. Destruction of microglia in mixed glial cultures by L-leucine methyl ester (LLME) treatment abolished enhanced proliferation caused by PrP106–126. This proliferative effect can be restored by co-culturing LLME-treated astrocytes with microglia. Microglia therefore seem to mediate the proliferative effect exerted by PrP106–126 on astrocytes. © 1996 Wiley-Liss, Inc.     

Prion protein fragment 106–126 induces apoptotic cell death and impairment of L-type voltage-sensitive calcium channel activity in the GH3 cell line

The prion diseases are transmissible neurodegenerative pathologies characterized by the accumulation of altered forms of the prion protein (PrP), termed PrP^Sc, in the brain. Previous studies have shown that a synthetic peptide homologous to residues 106–126 of PrP (PrP 106–126) maintains many characteristics of PrP^Sc, i.e., the ability to form amyloid fibrils and to induce apoptosis in neurons. We have investigated the intracellular mechanisms involved in the cellular degeneration induced by PrP 106–126, using the GH3 cells as a model of excitable cells. When assayed in serum-deprived conditions (48 hr), PrP 106–126 (50 μM) induced cell death time-dependently, and this process showed the characteristics of the apoptosis. This effect was specific because a peptide with a scrambled sequence of PrP 106–126 was not effective. Then we performed microfluorimetric analysis of single cells to monitor intracellular calcium concentrations and showed that PrP 106–126 caused a complete blockade of the increase in the cytosolic calcium levels induced by K⁺ (40 mM) depolarization. Conversely, the scrambled peptide was ineffective. The L-type voltage-sensitive calcium channel blocker nicardipine (1 μM) also induced apoptosis in GH3 cells, suggesting that the blockade of Ca²⁺ entry through this class of calcium channels may cause GH3 apoptotic cell death. We thus analyzed, by means of electrophysiological studies, whether Prp 106–126 modulate L-type calcium channels activity and demonstrated that the apoptotic effect of PrP 106–126 was due to a dose-dependent inactivation of the L-type calcium channels. These data demonstrate that the prion protein fragment 106–126 induces a GH3 apoptotic cell death inducing a selective inhibition of the activity of the L-type voltage-sensitive calcium channels. J. Neurosci. Res. 54:341–352, 1998. © 1998 Wiley-Liss, Inc.     

Intracellular mechanisms mediating the neuronal death and astrogliosis induced by the prion protein fragment 106-126.

Prion encephalopathies include fatal diseases of the central nervous system of men and animals characterized by nerve cell loss, glial proliferation and deposition of amyloid fibrils into the brain. During these diseases a cellular glycoprotein (the prion protein, PrP(C)) is converted, through a not yet completely clear mechanism, in an altered isoform (the prion scrapie, PrP(Sc)) that accumulates within the brain tissue by virtue of its resistance to the intracellular catabolism. PrP(Sc) is believed to be responsible for the neuronal loss that is observed in the prion disease. The PrP 106-126, a synthetic peptide that has been obtained from the amyloidogenic portion of the prion protein, represents a suitable model for studying the pathogenic role of the PrP(Sc), retaining, in vitro, some characteristics of the entire protein, such as the capability to aggregate in fibrils, and the neurotoxicity. In this work we present the results we have recently obtained regarding the action of the PrP 106-126 in different cellular models. We report that the PrP 106-126 induces proliferation of cortical astrocytes, as well as degeneration of primary cultures of cortical neurons or of neuroectodermal stable cell lines (GH(3) cells). In particular, these two opposite effects are mediated by the same attitude of the peptide to interact with the L-type calcium channels: in the astrocytes, the activity of these channels seems to be activated by PrP 106-126, while, in the cortical neurons and in the GH(3) cells, the same treatment causes a blockade of these channels causing a toxic effect.     

A Prion Protein Fragment Primes Type 1 Astrocytes to Proliferation Signals from Microglia

Gliosis is a hallmark of prion disease. A neurotoxic prion peptide (PrP106-126) induces astrocyte proliferation in the presence of microglia. This peptide also directly enhances microglial proliferation in culture. We have investigated this further to understand the method by which factors released by microglia and PrP106-126 work together to enhance astrocyte proliferation. PrP106-126 in the presence of microglia specifically enhanced type 1 astrocyte proliferation but not Type 2. Astrocytes that do not express the prion protein were more sensitive to oxidative stress and the toxicity of cytosine arabinoside. In the presence of cytosine arabinoside, PrP106-126 was toxic to pure astrocyte cultures. Using conditioned medium from microglia we have shown that PrP^c-expressing astrocytes proliferate in response to factors released by microglia stimulated by granulocyte/macrophage colony-stimulating factor. This response is enhanced in the presence of PrP106-126. PrP^c-deficient astrocytes do not show this response. These results suggest that astrocytes are primed by PrP106-126 to respond more to factors released by proliferating microglia. Astrocytes may proliferate in this system to escape entering the cell suicide pathway.     

Prion protein fragment interacts with PrP-deficient cells

A fragment of the prion protein (PrP106-126) induces cell death in cultures of wild-type embryonic day (E)16 mouse cortical neurons but not cells derived from mice devoid of cellular PrP(PrP^o/o). Two common binding partners for PrP106-126 expressed in both wild-type and PrP^o/o mouse brain were isolated and their sequences determined. The two proteins were found to be α and β tubulin. Further evidence that tubulin binds PrP106-126 within cells comes from cell culture experiments. Colchicine toxicity on PrP^o/o mouse cortical cells is enhanced by PrP106-126 and taxol enhances toxicity of PrP106-126 on wild-type mouse cortical cells. Our evidence shows that a fragment of PrP can bind a cellular protein and in so doing, alters the metabolism of cells even when they do not express native PrP. This indicates that PrP106-126 is nontoxic to PrP^o/o cells, not because of an inability to interact with these cells but because of the loss of some aspect of a PrP expression-dependent phenotype. J. Neurosci. Res. 52:260–267, 1998. © 1998 Wiley-Liss, Inc.     

On the biology of prions

Summary Prions cause scrapie and Creutzfeldt-Jakob disease (CJD); these infectious pathogens are composed largely, if not entirely, of protein molecules. No prion-specific polynucleotide has been identified. Purified preparations of scrapie prions contain high titers (10^9.5 ID₅₀/ml), one protein (PrP 27-30) and amyloid rods (10–20 nm in diameter ×100–200 nm in length). Considerable evidence indicates that PrP 27-30 is required for and inseparable from scrapie infectivity. PrP 27-30 is encoded by a cellular gene and is derived from a larger protein, denoted PrP^Sc or PrP 33-35^Sc, by protease digestion. A cellular isoform, designated PrP^C or PrP 33-35^C, is encoded by the same gene as PrP^Sc and both proteins appear to be translated from the same 2.1 kb mRNA. Monoclonal antibodies to PrP 27-30, as well as antisera to PrP synthetic peptides, specifically react with both PrP^C and PrP^Sc, establishing their relatedness. PrP^C is digested by proteinase K, while PrP^Sc is converted to PrP 27-30 under the same conditions. Prion proteins are synthesized with signal peptides and are integrated into membranes. Detergent extraction of microsomal membranes isolated from scrapie-infected hamster brains solubilizes PrP^C but induces PrP^Sc to polymerize into amyloid rods. This procedure allows separation of the two prion protein isoforms and the demonstration that PrP^Sc accumulates during scrapie infection, while the level of PrP^C does not change. The prion amyloid rods generated by detergent extraction are identical morphologically, except for length, to extracellular collections of prion amyloid filaments which form plaques in scrapie- and CJD-infected brains. The prion amyloid plaques stain with antibodies to PrP 27-30 and PrP peptides. PrP 33-35^C does not accumulate in the extracellular space. Prion rods composed of PrP 27-30 can be dissociated into phospholipid vesicles with full retention of scrapie infectivity. The murine PrP gene (Prn-p) is linked to thePrn-i gene which controls the length of the scrapie incubation period. Prolonged incubation times are a cardinal feature of scrapie and CJD. While the central role of PrP^Sc in scrapie pathogenesis is well established, the chemical as well as conformational differences between PrP^C and PrP^Sc are unknown but probably arise from post-translational modifications.Supported by research grants from the National Institutes of Health (AG 02132 and NS 14069) and a Senator Jacob Javits Center of Excellence in Neuroscience (NS 22786) as well as by gifts from RJR-Nabisco, Inc. and Sherman Fairchild FoundationThis review is based upon a plenary lecture entitled Biology and Neuropathology of Prions presented at the Xth International Congress of Neuropathology, Stockholm, Sweden, September 11, 1986, and is dedicated to the memory of Peter Wilhelm Lampert (1929–1986)     

Prion protein-overexpressing cells show altered response to a neurotoxic prion protein peptide

A peptide fragment of the prion protein, PrP106–126 is toxic to neuronal cells in culture. This toxicity is dependent on neuronal expression of the prion protein (PrP^c) and also the presence of microglia. The role of expression of the PrP^c in neurotoxicity of this peptide was investigated using mice that overexpress the prion protein. Cells derived from two different strains of PrP^c-overexpressing mice were used (Tg20 and Tg35). PrP106–126 was more toxic to Tg35 cerebellar cells than wild-type or Tg20 cells. This increased toxicity required the presence of microglia. Analysis of microglia derived from wild-type and PrP^c-overexpressing cells showed that Tg35 microglia were more easily activated than wild-type microglia, were more easily stimulated to proliferate by astrocytes, and had a higher level of PrP^c expression. This may explain the increased PrP106–126 toxicity to Tg35 PrP^c-overexpressing cerebellar cells. These results suggest that the toxicity of PrP106–126 may depend on the level of expression of PrP^c by microglia as well as by neurones. J. Neurosci. Res. 54:331–340, 1998. © 1998 Wiley-Liss, Inc.     

Activation of microglial cells by PrP and β-amyloid fragments raises intracellular calcium through L-type voltage sensitive calcium channels

The prion protein (PrP) and the amyloid β (Aβ) precursor protein (APP) are two normal proteins constitutively synthesised in human brain. An altered form of PrP accumulates in Creutzfeldt–Jakob disease, while Aβ is involved in the pathogenesis of Alzheimer's disease. Synthetic fragments of both proteins, PrP106–126 and β25–35 (β25–35), have been demonstrated to induce neurodegeneration and microglia activation. This study was undertaken to compare PrP106–126 and β25–35 capability of activating human resting microglial cells. Our results show that both peptides are able to induce microglial activation and to elicit an increase in [Ca²⁺]_i levels in cells loaded with calcium-green 1. Inhibitors of L-type voltage-sensitive calcium channels (verapamil, nifedipine and diltiazem) prevented the increase in [Ca²⁺]_i concentration as observed after treatment with PrP106–126 and β25–35, thus indicating a transmembrane calcium influx through these channels. In addition, verapamil abolished the proliferative effect of both PrP106–126 and β25–35.     

Novel Compounds Identified by Structure-Based Prion Disease Drug Discovery Using In Silico Screening Delay the Progression of an Illness in Prion-Infected Mice

The accumulation of abnormal prion protein (PrP^Sc) produced by the structure conversion of PrP (PrP^C) in the brain induces prion disease. Although the conversion process of the protein is still not fully elucidated, it has been known that the intramolecular chemical bridging in the most fragile pocket of PrP, known as the “hot spot,” stabilizes the structure of PrP^C and inhibits the conversion process. Using our original structure-based drug discovery algorithm, we identified the low molecular weight compounds that predicted binding to the hot spot. NPR-130 and NPR-162 strongly bound to recombinant PrP in vitro, and fragment molecular orbital (FMO) analysis indicated that the high affinity of those candidates to the PrP is largely dependent on nonpolar interactions, such as van der Waals interactions. Those NPRs showed not only significant reduction of the PrP^Sc levels but also remarkable decrease of the number of aggresomes in persistently prion-infected cells. Intriguingly, treatment with those candidate compounds significantly prolonged the survival period of prion-infected mice and suppressed prion disease-specific pathological damage, such as vacuole degeneration, PrP^Sc accumulation, microgliosis, and astrogliosis in the brain, suggesting their possible clinical use. Our results indicate that in silico drug discovery using NUDE/DEGIMA may be widely useful to identify candidate compounds that effectively stabilize the protein.Electronic supplementary materialThe online version of this article (10.1007/s13311-020-00903-9) contains supplementary material, which is available to authorized users.     

Polyunsaturated Fatty Acids Protect Against Prion-Mediated Synapse Damage In Vitro

A loss of synapses is characteristic of the early stages of the prion diseases. Here we modelled the synapse damage that occurs in prion diseases by measuring the amount of synaptophysin, a pre-synaptic membrane protein essential for neurotransmission, in cortical or hippocampal neurones incubated with the disease associated isoform of the prion protein (PrP^Sc), or with the prion-derived peptide PrP82-146. The addition of PrP^Sc or PrP82-146 caused a dose-dependent reduction in the synaptophysin content of PrP wildtype neurones indicative of synapse damage. They did not affect the synaptophysin content of PrP null neurones. The loss of synaptophysin in PrP wildtype neurones was preceded by the accumulation of PrP82-146 within synapses. Since supplements containing polyunsaturated fatty acids (PUFA) are frequently taken for their perceived health benefits including reported amelioration of neurodegenerative conditions, the effects of some common PUFA on prion-mediated synapse damage were examined. Pre-treatment of cortical or hippocampal neurones with docosahexaenoic (DHA) or eicosapentaenoic acids (EPA) protected neurones against the loss of synaptophysin induced by PrP82-146 or PrP^Sc. This effect of DHA and EPA was selective as they did not alter the loss of synaptophysin induced by a snakevenom neurotoxin. The effects of DHA and EPA were associated with a significant reduction in the amount of FITC-PrP82-146 that accumulated within synapses. Such observations raise the possibility that supplements containing PUFA may protect against the synapse damage and cognitive loss seen during the early stages of prion diseases.     

Abnormal Activation of Microglia Accompanied with Disrupted CX3CR1/CX3CL1 Pathway in the Brains of the Hamsters Infected with Scrapie Agent 263K

Microglial cells are resident mononuclear phagocytes of the central nervous system (CNS). Active proliferation of microglia in the brain has been identified in neurodegenerative disorders, including some kinds of prion disease. However, the detailed regional distribution between microglia and PrP^Sc deposition has not been presented, and investigation of fractalkine signaling which is involved in the regulation of activation of microglia in prion disease is not well documented. In this study, the disease phenomenon of microglial accumulation in the CNS was thoroughly analyzed using a scrapie-infected experimental model. Western blots of microglia-specific markers Iba1 and CD68, immunohistochemical and immunofluorescent assays demonstrated obviously activation of microglia in almost whole brain regions in the infected animals. Under the dynamic analysis on hallmarks of activation of microglia, a time-dependent increase of Iba1 and CD68 was detected, accompanied by accumulation of PrP^Sc and progression of neurodegenerative symptoms. With serial brain sections and double staining of Iba1 and PrP^Sc, we observed that the microglia distributed around PrP^Sc deposits in 263K-infected hamsters’ brains, proposing PrP^Sc phagocytosis. Flow cytometry assays with the single-cell suspensions prepared from the cortical region of the infected brains verified an activation of microglial population. ELISA assays of the cytokines in brain homogenates revealed significant upregulations of interleukin (IL)-1β, IL-6 and TNF-α when infected. Evaluation of fractalkine signaling in the infected hamsters’ brains showed progressively downregulation of CX3CL1 during the incubation. Prion peptide PrP106-126 also disrupted fractalkine and evoked microglial activation in rat primary neuron–glia mixed cultures. Our data here demonstrate an activated status of microglia in CNS tissues of infectious prion disease, possibly through fractalkine signaling deficiency.     

Molecular biology and pathology of prion strains in sporadic human prion diseases

Prion diseases are believed to propagate by the mechanism involving self-perpetuating conformational conversion of the normal form of the prion protein, PrP^C, to the misfolded, pathogenic state, PrP^Sc. One of the most intriguing aspects of these disorders is the phenomenon of prion strains. It is believed that strain properties are fully encoded in distinct conformations of PrP^Sc. Strains are of practical relevance to human prion diseases as their diversity may explain the unusual heterogeneity of these disorders. The first insight into the molecular mechanisms underlying heterogeneity of human prion diseases was provided by the observation that two distinct disease phenotypes and their associated PrP^Sc conformers co-distribute with distinct PrP genotypes as determined by the methionine/valine polymorphism at codon 129 of the PrP gene. Subsequent studies identified six possible combinations of the three genotypes (determined by the polymorphic codon 129) and two common PrP^Sc conformers (named types 1 and 2) as the major determinants of the phenotype in sporadic human prion diseases. This scenario implies that each 129 genotype–PrP^Sc type combination would be associated with a distinct disease phenotype and prion strain. However, notable exceptions have been found. For example, two genotype–PrP^Sc type combinations are linked to the same phenotype, and conversely, the same combination was found to be associated with two distinct phenotypes. Furthermore, in some cases, PrP^Sc conformers naturally associated with distinct phenotypes appear, upon transmission, to lose their phenotype-determining strain characteristics. Currently it seems safe to assume that typical sporadic prion diseases are associated with at least six distinct prion strains. However, the intrinsic characteristics that distinguish at least four of these strains remain to be identified.     

PrP106-126 and Aβ1-42 Peptides Induce BV-2 Microglia Chemotaxis and Proliferation

Transmissible spongiform encephalopathies (TSEs) and Alzheimer's disease (AD) belong to a growing family of neurodegenerative disorders that is characterized by the generation of toxic protein aggregates in affected brains (PrP^Sc and Aβ in TSEs and AD, respectively). To better understand how protein aggregates can modulate microglial processes in these diseases, we treated BV-2 microglia with PrP^106-126 or Aβ_1-42 peptides individually at three different concentrations (25–100 μM PrP^106-12 and 2.5–10 μM Aβ_1-42) or with a mixture of PrP^106-126 and Aβ_1-42 peptides at specified concentrations for 6–24 h. BV-2 microglia chemotaxis, proliferation, and monocyte chemoattractant protein-1 and transforming growth factor-β1 (TGF-β1) secretion were measured and compared between treatments. The results demonstrate that PrP^106-126 and Aβ_1-42 peptides induce increases in all four parameters from 6 to 12 h. However, the measured indices plateaued beyond 12 h in BV-2 cells treated >50 μM PrP or >5 μM Aβ_1-42, with the exception of TGF-β1 secretion, which continued to increase gradually. Overall, the results of this study indicate that these two peptides may mutually inhibit microglial chemotaxis and proliferation simultaneously via changes induced at the protein level.     

A model for the mechanism of astrogliosis in prion disease

Astrogliosis is a hallmark of prion diseases. Finding ways of inhibiting astrocyte proliferation may be beneficial to treating these diseases. PrP106-126 a peptide fragment of the prion protein induces proliferation of astrocytes. The mechanism of its action was studied in detail. Induction of astrocyte proliferation in culture requires cytokines interleukin-1 and interleukin-6 released from microglia in the presence of PrP106-126. However, the increased release of these cytokines is insufficient without direct effects of PrP106-126 on astrocytes. PrP106-126 induces increased progression through the cell cycle to late G1 and enhances the level of both p53 and phosphorylated ERKs in astrocytes. PrP106-126-induced proliferation of astrocytes in culture can be inhibited by antibodies to cytokines or by MEK inhibitors.     

Cellular Prion Protein (PrPC) of the Neuron Cell Transformed to a PK-Resistant Protein Under Oxidative Stress, Comprising Main Mitochondrial Damage in Prion Diseases

Prion diseases characterize a category of fatal neurodegenerative diseases. Although reports have increasingly shown that oxidative stress plays an important role in the progression of prion diseases, little is known about whether oxidative stress is a cause or a consequence of a prion disease. The mechanism of prion disease development also remains unclear. The purpose of this study was to investigate three things: the possible mechanisms of neuron cell damage, the conformation of anti-protease K (PK) PrP^Sc, and the role of oxidative stress in the progression of prion diseases. The study results demonstrated that normal PrP^C transformed into a PK-resistant protein under oxidative stress in the presence of PrP106–126. Further, the protein misfolding cyclic amplification procedure may have accelerated this process. Mitochondrial damage and dysfunction in prion disease progression were also observed in this study. Our results suggested that neuron cell damage, and particularly mitochondrial damage, was induced by oxidative stress. This damage may be the initial cause of a given prion disease.     

Ionic mechanisms of action of prion protein fragment PrP(106–126) in rat basal forebrain neurons

Prion diseases are neurodegenerative disorders that are characterized by the presence of the misfolded prion protein (PrP). Neurotoxicity in these diseases may result from prion‐induced modulation of ion channel function, changes in neuronal excitability, and consequent disruption of cellular homeostasis. We therefore examined PrP effects on a suite of potassium (K⁺) conductances that govern excitability of basal forebrain neurons. Our study examined the effects of a PrP fragment [PrP(106–126), 50 nM] on rat neurons using the patch clamp technique. In this paradigm, PrP(106–126) peptide, but not the “scrambled” sequence of PrP(106–126), evoked a reduction of whole‐cell outward currents in a voltage range between –30 and +30 mV. Reduction of whole‐cell outward currents was significantly attenuated in Ca²⁺‐free external media and also in the presence of iberiotoxin, a blocker of calcium‐activated potassium conductance. PrP(106–126) application also evoked a depression of the delayed rectifier (I_K) and transient outward (I_A) potassium currents. By using single cell RT‐PCR, we identified the presence of two neuronal chemical phenotypes, GABAergic and cholinergic, in cells from which we recorded. Furthermore, cholinergic and GABAergic neurons were shown to express K_v4.2 channels. Our data establish that the central region of PrP, defined by the PrP(106–126) peptide used at nanomolar concentrations, induces a reduction of specific K⁺ channel conductances in basal forebrain neurons. These findings suggest novel links between PrP signalling partners inferred from genetic experiments, K⁺ channels, and PrP‐mediated neurotoxicity. © 2010 Wiley‐Liss, Inc.     

Using Protein Misfolding Cyclic Amplification Generates a Highly Neurotoxic PrP Dimer Causing Neurodegeneration

Under the “protein-only” hypothesis, prion-based diseases are proposed to result from an infectious agent that is an abnormal isoform of the prion protein in the scrapie form, PrP^Sc. However, since PrP^Sc is highly insoluble and easily aggregates in vivo, this view appears to be overly simplistic, implying that the presence of PrP^Sc may indirectly cause neurodegeneration through its intermediate soluble form. We generated a neurotoxic PrP dimer with partial pathogenic characteristics of PrP^Sc by protein misfolding cyclic amplification in the presence of 1-palmitoyl-2-oleoylphosphatidylglycerol consisting of recombinant hamster PrP (23–231). After intracerebral injection of the PrP dimer, wild-type hamsters developed signs of neurodegeneration. Clinical symptoms, necropsy findings, and histopathological changes were very similar to those of transmissible spongiform encephalopathies. Additional investigation showed that the toxicity is primarily related to cellular apoptosis. All results suggested that we generated a new neurotoxic form of PrP, PrP dimer, which can cause neurodegeneration. Thus, our study introduces a useful model for investigating PrP-linked neurodegenerative mechanisms.     

Excitotoxicity Through NMDA Receptors Mediates Cerebellar Granule Neuron Apoptosis Induced by Prion Protein 90-231 Fragment

Prion diseases recognize, as a unique molecular trait, the misfolding of CNS-enriched prion protein (PrP^C) into an aberrant isoform (PrP^Sc). In this work, we characterize the in vitro toxicity of amino-terminally truncated recombinant PrP fragment (amino acids 90-231, PrP90-231), on rat cerebellar granule neurons (CGN), focusing on glutamatergic receptor activation and Ca²⁺ homeostasis impairment. This recombinant fragment assumes a toxic conformation (PrP90-231^TOX) after controlled thermal denaturation (1 h at 53 °C) acquiring structural characteristics identified in PrP^Sc (enrichment in β-structures, increased hydrophobicity, partial resistance to proteinase K, and aggregation in amyloid fibrils). By annexin-V binding assay, and evaluation of the percentage of fragmented and condensed nuclei, we show that treatment with PrP90-231^TOX, used in pre-fibrillar aggregation state, induces CGN apoptosis. This effect was associated with a delayed, but sustained elevation of [Ca²⁺]_i. Both CGN apoptosis and [Ca²⁺]_i increase were not observed using PrP90-231 in PrP^C-like conformation. PrP90-231^TOX effects were significantly reduced in the presence of ionotropic glutamate receptor antagonists. In particular, CGN apoptosis and [Ca²⁺]_i increase were largely reduced, although not fully abolished, by pre-treatment with the NMDA antagonists APV and memantine, while the AMPA antagonist CNQX produced a lower, although still significant, effect. In conclusion, we report that CGN apoptosis induced by PrP90-231^TOX correlates with a sustained elevation of [Ca²⁺]_i mediated by the activation of NMDA and AMPA receptors.