Phospholipase A2 Inhibitors or Platelet-activating Factor Antagonists Prevent Prion Replication

Neuroregulatory role of ginkgolides

The application of ginkgolides as a herbal remedy reaches ancient China. Over time many studies confirmed the neuroprotective effect of standard Ginkgo biloba tree extract—the only available ginkgolide source. Ginkgolides present a wide variety of neuroregulatory properties, commonly used in the therapy process of common diseases, such as Alzheimer’s, Parkinson’s, and many other CNS-related diseases and disorders. The neuroregulative properties of ginkgolides include the conditioning of neurotransmitters action, e.g., glutamate or dopamine. Besides, natural compounds induce the inhibition of platelet-activating factors (PAF). Furthermore, ginkgolides influence the inflammatory process. This review focuses on the role of ginkgolides as neurotransmitters or neuromodulators and overviews their impact on the organism at the molecular, cellular, and physiological levels. The clinical application of ginkgolides is discussed as well.

Does the tail wag the dog? How the structure of a glycosylphosphatidylinositol anchor affects prion formation

There is increasing interest in the role of the glycosylphosphatidylinositol (GPI) anchor attached to the cellular prion protein (PrP(C)). Since GPI anchors can alter protein targeting, trafficking and cell signaling, our recent study examined how the structure of the GPI anchor affected prion formation. PrP(C) containing a GPI anchor from which the sialic acid had been removed (desialylated PrP(C)) was not converted to PrP(Sc) in prion-infected neuronal cell lines and in scrapie-infected primary cortical neurons. In uninfected neurons desialylated PrP(C) was associated with greater concentrations of gangliosides and cholesterol than PrP(C). In addition, the targeting of desialylated PrP(C) to lipid rafts showed greater resistance to cholesterol depletion than PrP(C). The presence of desialylated PrP(C) caused the dissociation of cytoplasmic phospholipase A2 (cPLA2) from PrP-containing lipid rafts, reduced the activation of cPLA2 and inhibited PrP(Sc) production. We conclude that the sialic acid moiety of the GPI attached to PrP(C) modifies local membrane microenvironments that are important in PrP-mediated cell signaling and PrP(Sc) formation.

Glycosylphosphatidylinositols: More than just an anchor?

There is increasing interest in the role of glycosylphosphatidylinositol (GPI) anchors that attach some proteins to cell membranes. Far from being biologically inert, GPIs influence the targeting, intracellular trafficking and function of the attached protein. Our recent paper demonstrated the role of sialic acid on the GPI of the cellular prion protein (PrP(C)). The "prion diseases" arise following the conversion of PrP(C) to a disease-associated isoform called PrP(Sc) or "prion". Our paper showed that desialylated PrP(C) inhibited PrP(Sc) formation. Aggregated PrP(Sc) creates a signaling platform in the cell membrane incorporating and activating cytoplasmic phospholipase A2 (cPLA2), an enzyme that regulates PrP(C) trafficking and hence PrP(Sc) formation. The presence of desialylated PrP(C) caused the dissociation of cPLA2 from PrP-containing platforms, reduced the activation of cPLA2 and inhibited PrP(Sc) production. We concluded that sialic acid contained within the GPI attached to PrP(C) modifies local membrane microenvironments that are important in PrP-mediated cell signaling and PrP(Sc) formation.

Sialic Acid on the Glycosylphosphatidylinositol Anchor Regulates PrP-mediated Cell Signaling and Prion Formation

The prion diseases occur following the conversion of the cellular prion protein (PrP(C)) into disease-related isoforms (PrP(Sc)). In this study, the role of the glycosylphosphatidylinositol (GPI) anchor attached to PrP(C) in prion formation was examined using a cell painting technique. PrP(Sc) formation in two prion-infected neuronal cell lines (ScGT1 and ScN2a cells) and in scrapie-infected primary cortical neurons was increased following the introduction of PrP(C). In contrast, PrP(C) containing a GPI anchor from which the sialic acid had been removed (desialylated PrP(C)) was not converted to PrP(Sc). Furthermore, the presence of desialylated PrP(C) inhibited the production of PrP(Sc) within prion-infected cortical neurons and ScGT1 and ScN2a cells. The membrane rafts surrounding desialylated PrP(C) contained greater amounts of sialylated gangliosides and cholesterol than membrane rafts surrounding PrP(C). Desialylated PrP(C) was less sensitive to cholesterol depletion than PrP(C) and was not released from cells by treatment with glimepiride. The presence of desialylated PrP(C) in neurons caused the dissociation of cytoplasmic phospholipase A2 from PrP-containing membrane rafts and reduced the activation of cytoplasmic phospholipase A2. These findings show that the sialic acid moiety of the GPI attached to PrP(C) modifies local membrane microenvironments that are important in PrP-mediated cell signaling and PrP(Sc) formation. These results suggest that pharmacological modification of GPI glycosylation might constitute a novel therapeutic approach to prion diseases.

Synthetic and natural inhibitors of phospholipases A2: their importance for understanding and treatment of neurological disorders

Phospholipases A2 (PLA2) are a diverse group of enzymes that hydrolyze membrane phospholipids into arachidonic acid and lysophospholipids. Arachidonic acid is metabolized to eicosanoids (prostaglandins, leukotrienes, thromboxanes), and lysophospholipids are converted to platelet-activating factors. These lipid mediators play critical roles in the initiation, maintenance, and modulation of neuroinflammation and oxidative stress. Neurological disorders including excitotoxicity; traumatic nerve and brain injury; cerebral ischemia; Alzheimer's disease; Parkinson's disease; multiple sclerosis; experimental allergic encephalitis; pain; depression; bipolar disorder; schizophrenia; and autism are characterized by oxidative stress, inflammatory reactions, alterations in phospholipid metabolism, accumulation of lipid peroxides, and increased activities of brain phospholipase A2 isoforms. Several old and new synthetic inhibitors of PLA2, including fatty acid trifluoromethyl ketones; methyl arachidonyl fluorophosphonate; bromoenol lactone; indole-based inhibitors; pyrrolidine-based inhibitors; amide inhibitors, 2-oxoamides; 1,3-disubstituted propan-2-ones and polyfluoroalkyl ketones as well as phytochemical based PLA2 inhibitors including curcumin, Ginkgo biloba and Centella asiatica extracts have been discovered and used for the treatment of neurological disorders in cell culture and animal model systems. The purpose of this review is to summarize information on selective and potent synthetic inhibitors of PLA2 as well as several PLA2 inhibitors from plants, for treatment of oxidative stress and neuroinflammation associated with the pathogenesis of neurological disorders.

The role of secretory phospholipase A₂ in the central nervous system and neurological diseases

Secretory phospholipase A2 (sPLA2s) are small secreted proteins (14-18 kDa) and require submillimolar levels of Ca(2+) for liberating arachidonic acid from cell membrane lipids. In addition to the enzymatic function, sPLA2 can exert various biological responses by binding to specific receptors. Physiologically, sPLA2s play important roles on the neurotransmission in the central nervous system and the neuritogenesis in the peripheral nervous system. Pathologically, sPLA2s are involved in the neurodegenerative diseases (e.g., Alzheimer's disease) and cerebrovascular diseases (e.g., stoke). The common pathology (e.g., neuronal apoptosis) of Alzheimer's disease and stroke coexists in the mixed dementia, suggesting common pathogenic mechanisms of the two neurological diseases. Among mammalian sPLA2s, sPLA2-IB and sPLA2-IIA induce neuronal apoptosis in rat cortical neurons. The excess influx of calcium into neurons via L-type voltage-dependent Ca(2+) channels mediates the two sPLA2-induced apoptosis. The elevated concentration of intracellular calcium activates PKC, MAPK and cytosolic PLA2. Moreover, it is linked with the production of reactive oxygen species and apoptosis through activation of the superoxide producing enzyme NADPH oxidase. NADPH oxidase is involved in the neurotoxicity of amyloid β peptide, which impairs synaptic plasticity long before its deposition in the form of amyloid plaques of Alzheimer's disease. In turn, reactive oxygen species from NADPH oxidase can stimulate ERK1/2 phosphorylation and activation of cPLA2 and result in a release of arachidonic acid. sPLA2 is up-regulated in both Alzheimer's disease and cerebrovascular disease, suggesting the involvement of sPLA2 in the common pathogenic mechanisms of the two diseases. Thus, our review presents evidences for pathophysiological roles of sPLA2 in the central nervous system and neurological diseases.

Patent Highlights

Snapshot of recent key developments in the patent literature of relevance to the advancement of pharmaceutical and medical R&D

Inhibition of cytosolic Phospholipase A2 prevents prion peptide-induced neuronal damage and co-localisation with Beta III Tubulin

Background

Activation of phospholipase A₂ (PLA₂) and the subsequent metabolism of arachidonic acid (AA) to prostaglandins have been shown to play an important role in neuronal death in neurodegenerative disease. Here we report the effects of the prion peptide fragment HuPrP106-126 on the PLA₂ cascade in primary cortical neurons and translocation of cPLA₂ to neurites.

Results

Exposure of primary cortical neurons to HuPrP106-126 increased the levels of phosphorylated cPLA₂ and caused phosphorylated cPLA₂ to relocate from the cell body to the cellular neurite in a PrP-dependent manner, a previously unreported observation. HuPrP106-126 also induced significant AA release, an indicator of cPLA₂ activation; this preceded synapse damage and subsequent cellular death. The novel translocation of p-cPLA₂ postulated the potential for exposure to HuPrP106-126 to result in a re-arrangement of the cellular cytoskeleton. However p-cPLA₂ did not colocalise significantly with F-actin, intermediate filaments, or microtubule-associated proteins. Conversely, p-cPLA₂ did significantly colocalise with the cytoskeletal protein beta III tubulin. Pre-treatment with the PLA₂ inhibitor, palmitoyl trifluoromethyl ketone (PACOCF₃) reduced cPLA₂ activation, AA release and damage to the neuronal synapse. Furthermore, PACOCF₃ reduced expression of p-cPLA₂ in neurites and inhibited colocalisation with beta III tubulin, resulting in protection against PrP-induced cell death.

Conclusions

Collectively, these findings suggest that cPLA₂ plays a vital role in the action of HuPrP106-126 and that the colocalisation of p-cPLA₂ with beta III tubulin could be central to the progress of neurodegeneration caused by prion peptides. Further work is needed to define exactly how PLA₂ inhibitors protect neurons from peptide-induced toxicity and how this relates to intracellular structural changes occurring in neurodegeneration.

The cellular prion protein with a monoacylated glycosylphosphatidylinositol anchor modifies cell membranes, inhibits cell signaling and reduces prion formation

The prion diseases occur following the conversion of the cellular prion protein (PrPC) into a disease-related isoform (PrPSc). In this study a cell painting technique was used to examine the role of the glycosylphosphatidylinositol (GPI) anchor attached to PrPC in prion formation. The introduction of PrPC to infected neuronal cells increased the cholesterol content of cell membranes, increased activation of cytoplasmic phospholipase A2 (cPLA2) and increased PrPSc formation. In contrast, PrPC with a monoacylated GPI anchor did not alter the amount of cholesterol in cell membranes, was not found within lipid rafts and did not activate cPLA2. Although monoacylated PrPC remains within cells for longer than native PrPC it was not converted to PrPSc. Moreover, the presence of monoacylated PrPC displaced cPLA2 from PrPSc-containing lipid rafts, reducing the activation of cPLA2 and PrPSc formation. We conclude that acylation of the GPI anchor attached to PrPC modifies the local membrane microenvironments that control some cell signaling pathways, the trafficking of PrPC and PrPSc formation. In addition, such observations raise the possibility that the pharmacological modification of GPI anchors might constitute a novel therapeutic approach to prion diseases.

Inhibition of cPLA2 activation by Ginkgo biloba extract protects spinal cord neurons from glutamate excitotoxicity and oxidative stress-induced cell death

Ginkgo biloba extract (EGb761) has been shown to be neuroprotective; however, the mechanism by which EGb761 mediates neuroprotection remains unclear. We hypothesized that the neuroprotective effect of EGb761 is mediated by inhibition of cytosolic phospholipase A(2) (cPLA(2)), an enzyme that is known to play a key role in mediating secondary pathogenesis after acute spinal cord injury (SCI). To determine whether EGb761 neuroprotection involves the cPLA(2) pathway, we first investigated the effect of glutamate and hydrogen peroxide on cPLA(2) activation. Results showed that both insults induced an increase in the expression of phosphorylated cPLA(2) (p-cPLA(2)), a marker of cPLA(2) activation, and neuronal death in vitro. Such effects were significantly reversed by EGb761 administration. Additionally, EGb761 significantly decreased prostaglandin E(2) (PGE(2)) release, a downstream metabolite of cPLA(2). Moreover, inhibition of cPLA(2) activity with arachidonyl trifluromethyl ketone improved neuroprotection against glutamate and hydrogen peroxide-induced neuronal death, and reversed Bcl-2/Bax ratio; notably, EGb761 produced greater effects than arachidonyl trifluromethyl ketone. Finally, we showed that the extracellular signal-regulated kinase 1/2 signaling pathway is involved in EGb761's modulation of cPLA(2) phosphorylation. These results collectively suggest that the protective effect of EGb761 is mediated, at least in part, through inhibition of cPLA(2) activation, and that the extracellular signal-regulated kinase 1/2 signaling pathway may play an important role in mediating the EGb761's effect.

Monoacylated cellular prion protein modifies cell membranes, inhibits cell signaling, and reduces prion formation

Prion diseases occur following the conversion of the cellular prion protein (PrP(C)) into a disease related, protease-resistant isoform (PrP(Sc)). In these studies, a cell painting technique was used to introduce PrP(C) to prion-infected neuronal cell lines (ScGT1, ScN2a, or SMB cells). The addition of PrP(C) resulted in increased PrP(Sc) formation that was preceded by an increase in the cholesterol content of cell membranes and increased activation of cytoplasmic phospholipase A(2) (cPLA(2)). In contrast, although PrP(C) lacking one of the two acyl chains from its glycosylphosphatidylinositol (GPI) anchor (PrP(C)-G-lyso-PI) bound readily to cells, it did not alter the amount of cholesterol in cell membranes, was not found within detergent-resistant membranes (lipid rafts), and did not activate cPLA(2). It remained within cells for longer than PrP(C) with a conventional GPI anchor and was not converted to PrP(Sc). Moreover, the addition of high amounts of PrP(C)-G-lyso-PI displaced cPLA(2) from PrP(Sc)-containing lipid rafts, reduced the activation of cPLA(2), and reduced PrP(Sc) formation in all three cell lines. In addition, ScGT1 cells treated with PrP(C)-G-lyso-PI did not transmit infection following intracerebral injection to mice. We propose that that the chemical composition of the GPI anchor attached to PrP(C) modified the local membrane microenvironments that control cell signaling, the fate of PrP(C), and hence PrP(Sc) formation. In addition, our observations raise the possibility that pharmacological modification of GPI anchors might constitute a novel therapeutic approach to prion diseases.

Inhibition of phospholipase A2 increased the removal of the prion derived peptide PrP82-146 from cultured neurons

The prion diseases are characterised by the formation of the disease-associated isoform of the prion protein (PrP(Sc)) and the production of disease-related peptides. The prion derived peptide PrP82-146 bound readily to cortical neurons and was found within detergent resistant membranes that are commonly called lipid rafts. It was not found within lysosomes and the slow degradation of PrP82-146 resulted in a half-life of approximately 5 days. In cortical neurons pre-treated with phospholipase A(2) (PLA(2)) inhibitors (AACOCF(3) or MAFP) less PrP82-146 entered lipid rafts, more PrP82-146 was found within lysosomes and the half-life of PrP82-146 was reduced to 24 h. Similarly, pre-treatment of neurons with platelet-activating factor (PAF) receptor antagonists (Hexa-PAF and ginkgolide B) increased the entry of PrP82-146 into lysosomes and reduced its half-life. Furthermore, the addition of PAF reversed the effects of PLA(2) inhibitors on PrP82-146 trafficking. PAF controlled the amount of cholesterol in cell membranes and the effects of PAF receptor antagonists on the trafficking of PrP82-146 were reversed by the addition of cholesterol. We conclude that activation of PLA(2) and the production of PAF control a cholesterol-sensitive pathway that affects the cellular localisation and hence the fate of PrP82-146 in neurons.

Residues surrounding the glycosylphosphatidylinositol anchor attachment site of PrP modulate prion infection: insight from the resistance of rabbits to prion disease

Prion diseases are a group of transmissible, invariably fatal neurodegenerative diseases that affect both humans and animals. According to the protein-only hypothesis, the infectious agent is a prion (proteinaceous infectious particle) that is composed primarily of PrP(Sc), the disease-associated isoform of the cellular prion protein, PrP. PrP(Sc) arises from the conformational change of the normal, glycosylphosphatidylinositol (GPI)-anchored protein, PrP(C). The mechanism by which this process occurs, however, remains enigmatic. Rabbits are one of a small number of mammalian species reported to be resistant to prion infection. Sequence analysis of rabbit PrP revealed that its C-terminal amino acids differ from those of PrP from other mammals and may affect the anchoring of rabbit PrP through its GPI anchor. Using a cell culture model, this study investigated the effect of the rabbit PrP-specific C-terminal amino acids on the addition of the GPI anchor to PrP(C), PrP(C) localization, and PrP(Sc) formation. The incorporation of rabbit-specific C-terminal PrP residues into mouse PrP did not affect the addition of a GPI anchor or the localization of PrP. However, these residues did inhibit PrP(Sc) formation, suggesting that these rabbit-specific residues interfere with a C-terminal PrP(Sc) interaction site.

Glycosylphosphatidylinositol anchor analogues sequester cholesterol and reduce prion formation

A hallmark of prion diseases is the conversion of the host-encoded prion protein (PrP(C) where C is cellular) into an alternatively folded, disease-related isoform (PrP(Sc), where Sc is scrapie), the accumulation of which is associated with synapse degeneration and ultimately neuronal death. The formation of PrP(Sc) is dependent upon the presence of PrP(C) in specific, cholesterol-sensitive membrane microdomains, commonly called lipid rafts. PrP(C) is targeted to these lipid rafts because it is attached to membranes via a glycosylphosphatidylinositol anchor. Here, we show that treatment of prion-infected neuronal cell lines (ScN2a, ScGT1, or SMB cells) with synthetic glycosylphosphatidylinositol analogues, glucosamine-phosphatidylinositol (glucosamine-PI) or glucosamine 2-O-methyl inositol octadecyl phosphate, reduced the PrP(Sc) content of these cells in a dose-dependent manner. In addition, ScGT1 cells treated with glucosamine-PI did not transmit infection following intracerebral injection to mice. Treatment with glucosamine-PI increased the cholesterol content of ScGT1 cell membranes and reduced activation of cytoplasmic phospholipase A(2) (PLA(2)), consistent with the hypothesis that the composition of cell membranes affects key PLA(2)-dependent signaling pathways involved in PrP(Sc) formation. The effect of glucosamine-PI on PrP(Sc) formation was also reversed by the addition of platelet-activating factor. Glucosamine-PI caused the displacement of PrP(C) from lipid rafts and reduced expression of PrP(C) at the cell surface, putative sites for PrP(Sc) formation. We propose that treatment with glucosamine-PI modifies local micro-environments that control PrP(C) expression and activation of PLA(2) and subsequently inhibits PrP(Sc) formation.

Recent advances in prion chemotherapeutics

The transmissible spongiform encephalopathies are rapidly progressive and invariably fatal neurodegenerative diseases for which there are no proven efficacious treatments. Many approaches have been undertaken to find ways to prevent, halt, or reverse these prion diseases, with limited success to date. However, as both our understanding of pathogenesis and our ability to detect early disease increases, so do our potential therapeutic targets and our chances of finding effective drugs. There is increasing pressure to find effective decontaminants for blood supplies, as variant Creutzfeldt Jakob Disease (vCJD) has been shown to be transmissible by blood, and to find non-toxic preventative therapies, with ongoing cases of Bovine Spongiform Encephalopathy (BSE) and the spread of Chronic Wasting Disease (CWD). Within the realm of chemotherapeutic approaches, much research has focussed on blocking the conversion of the normal form of prion protein (PrP(c)) to its abnormal counterpart (PrP(res)). Structurally, these chemotherapeutic agents are often polyanionic or polycyclic and may directly bind PrP(c) or PrP(res), or act by redistributing, sequestering, or down-regulating PrP(c), thus preventing its conversion. There are also some polycationic compounds which proport to enhance the clearance of PrP(res). Other targets include accessory molecules such as the laminin receptor precursor which influences conversion, or cell signalling molecules which may be required for pathogenesis. Of recent interest are the possible neuroprotective effects of some drugs. Importantly, there is evidence that combining compounds may provide synergistic responses. This review provides an update on current testing methods, therapeutic targets, and promising candidates for chemical-based therapy.

Synthesis of polyfluoro ketones for selective inhibition of human phospholipase A2 enzymes

The development of selective inhibitors for individual PLA(2) enzymes is necessary in order to target PLA(2)-specific signaling pathways, but it is challenging due to the observed promiscuity of known PLA(2) inhibitors. In the current work, we present the development and application of a variety of synthetic routes to produce pentafluoro, tetrafluoro, and trifluoro derivatives of activated carbonyl groups in order to screen for selective inhibitors and characterize the chemical properties that can lead to selective inhibition. Our results demonstrate that the pentafluoroethyl ketone functionality favors selective inhibition of the GVIA iPLA(2), a very important enzyme for which specific, potent, reversible inhibitors are needed. We find that 1,1,1,2,2-pentafluoro-7-phenyl-heptan-3-one (FKGK11) is a selective inhibitor of GVIA iPLA(2) (X(I)(50) = 0.0073). Furthermore, we conclude that the introduction of an additional fluorine atom at the alpha' position of a trifluoromethyl ketone constitutes an important strategy for the development of new potent GVIA iPLA(2) inhibitors.

Docosahexaenoic and eicosapentaenoic acids increase prion formation in neuronal cells

Background

The transmissible spongiform encephalopathies, otherwise known as prion diseases, occur following the conversion of the cellular prion protein (PrP^C) to an alternatively folded, disease-associated isoform (PrP^Sc). Recent studies suggest that this conversion occurs via a cholesterol-sensitive process, as cholesterol synthesis inhibitors reduced the formation of PrP^Sc and delayed the clinical phase of scrapie infection. Since polyunsaturated fatty acids also reduced cellular cholesterol levels we tested their effects on PrP^Sc formation in three prion-infected neuronal cell lines (ScGT1, ScN2a and SMB cells).

Results

We report that treatment with docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) or the cholesterol synthesis inhibitor simvastatin reduced the amounts of free cholesterol in membrane extracts from prion-infected neuronal cells. Simvastatin reduced cholesterol production while DHA and EPA promoted the conversion of free cholesterol to cholesterol esters. Crucially, while simvastatin reduced PrP^Sc formation, both DHA and EPA significantly increased the amounts of PrP^Sc in these cells. Unlike simvastatin, the effects of DHA and EPA on PrP^Sc content were not reversed by stimulation of cholesterol synthesis with mevalonate. Treatment of ScGT1 cells with DHA and EPA also increased activation of cytoplasmic phospholipase A₂ and prostaglandin E₂ production. Finally, treatment of neuronal cells with DHA and EPA increased the amounts of PrP^C expressed at the cell surface and significantly increased the half-life of biotinylated PrP^C.

Conclusion

We report that although treatment with DHA or EPA significantly reduced the free cholesterol content of prion-infected cells they significantly increased PrP^Sc formation in three neuronal cell lines. DHA or EPA treatment of infected cells increased activation of phospholipase A₂, a key enzyme in PrP^Sc formation, and altered the trafficking of PrP^C. PrP^C expression at the cell surface, a putative site for the PrP^Sc formation, was significantly increased, and the rate at which PrP^C was degraded was reduced. Cholesterol depletion is seen as a potential therapeutic strategy for prion diseases. However, these results indicate that a greater understanding of the precise relationship between membrane cholesterol distribution, PrP^C trafficking, cell activation and PrP^Sc formation is required before cholesterol manipulation can be considered as a prion therapeutic.

Sequestration of free cholesterol in cell membranes by prions correlates with cytoplasmic phospholipase A2 activation

Background

The transmissible spongiform encephalopathies (TSEs), otherwise known as the prion diseases, occur following the conversion of the normal cellular prion protein (PrP^C) to an alternatively folded isoform (PrP^Sc). The accumulation of PrP^Sc within the brain leads to neurodegeneration through an unidentified mechanism. Since many neurodegenerative disorders including prion, Parkinson's and Alzheimer's diseases may be modified by cholesterol synthesis inhibitors, the effects of prion infection on the cholesterol balance within neuronal cells were examined.

Results

We report the novel observation that prion infection altered the membrane composition and significantly increased total cholesterol levels in two neuronal cell lines (ScGT1 and ScN2a cells). There was a significant correlation between the concentration of free cholesterol in ScGT1 cells and the amounts of PrP^Sc. This increase was entirely a result of increased amounts of free cholesterol, as prion infection reduced the amounts of cholesterol esters in cells. These effects were reproduced in primary cortical neurons by the addition of partially purified PrP^Sc, but not by PrP^C. Crucially, the effects of prion infection were not a result of increased cholesterol synthesis. Stimulating cholesterol synthesis via the addition of mevalonate, or adding exogenous cholesterol, had the opposite effect to prion infection on the cholesterol balance. It did not affect the amounts of free cholesterol within neurons; rather, it significantly increased the amounts of cholesterol esters. Immunoprecipitation studies have shown that cytoplasmic phospholipase A₂ (cPLA₂) co-precipitated with PrP^Sc in ScGT1 cells. Furthermore, prion infection greatly increased both the phosphorylation of cPLA₂ and prostaglandin E₂ production.

Conclusion

Prion infection, or the addition of PrP^Sc, increased the free cholesterol content of cells, a process that could not be replicated by the stimulation of cholesterol synthesis. The presence of PrP^Sc increased solubilisation of free cholesterol in cell membranes and affected their function. It increased activation of the PLA₂ pathway, previously implicated in PrP^Sc formation and in PrP^Sc-mediated neurotoxicity. These observations suggest that the neuropathogenesis of prion diseases results from PrP^Sc altering cholesterol-sensitive processes. Furthermore, they raise the possibility that disturbances in membrane cholesterol are major triggering events in neurodegenerative diseases.

Gene expression profile of quinacrine-cured prion-infected mouse neuronal cells

Prion diseases are transmissible fatal neurodegenerative diseases of humans and animals, characterised by the presence of an abnormal isoform (scrapie prion protein; PrP(Sc)) of the endogenous cellular prion protein (PrP(C)). The pathological mechanisms at the basis of prion diseases remain elusive, although the accumulation of PrP(Sc) has been linked to neurodegeneration. Different genomic approaches have been applied to carry out large-scale expression analysis in prion-infected brains and cell lines, in order to define factors potentially involved in pathogenesis. However, the general lack of overlap between the genes found in these studies prompted us to carry an analysis of gene expression using an alternative approach. Specifically, in order to avoid the complexities of shifting gene expression in a heterogeneous cell population, we used a single clone of GT1 cells that was de novo infected with mouse prion-infected brain homogenate and then treated with quinacrine to clear PrP(Sc). By comparing the gene expression profiles of about 15 000 genes in quinacrine-cured and not cured prion-infected GT1 cells, we investigated the influence of the presence or the absence of PrP(Sc). By real-time PCR, we confirmed that the gene encoding for laminin was down-regulated as a consequence of the elimination of PrP(Sc) by the quinacrine treatment. Thus, we speculate that this protein could be a specific candidate for further analysis of its role in prion infection and pathogenesis.

Squalestatin alters the intracellular trafficking of a neurotoxic prion peptide

Background

Neurotoxic peptides derived from the protease-resistant core of the prion protein are used to model the pathogenesis of prion diseases. The current study characterised the ingestion, internalization and intracellular trafficking of a neurotoxic peptide containing amino acids 105–132 of the murine prion protein (MoPrP105-132) in neuroblastoma cells and primary cortical neurons.

Results

Fluorescence microscopy and cell fractionation techniques showed that MoPrP105-132 co-localised with lipid raft markers (cholera toxin and caveolin-1) and trafficked intracellularly within lipid rafts. This trafficking followed a non-classical endosomal pathway delivering peptide to the Golgi and ER, avoiding classical endosomal trafficking via early endosomes to lysosomes. Fluorescence resonance energy transfer analysis demonstrated close interactions of MoPrP105-132 with cytoplasmic phospholipase A₂ (cPLA₂) and cyclo-oxygenase-1 (COX-1), enzymes implicated in the neurotoxicity of prions. Treatment with squalestatin reduced neuronal cholesterol levels and caused the redistribution of MoPrP105-132 out of lipid rafts. In squalestatin-treated cells, MoPrP105-132 was rerouted away from the Golgi/ER into degradative lysosomes. Squalestatin treatment also reduced the association between MoPrP105-132 and cPLA₂/COX-1.

Conclusion

As the observed shift in peptide trafficking was accompanied by increased cell survival these studies suggest that the neurotoxicity of this PrP peptide is dependent on trafficking to specific organelles where it activates specific signal transduction pathways.

Platelet activating factor-induced neuronal apoptosis is initiated independently of its G-protein coupled PAF receptor and is inhibited by the benzoate orsellinic acid

The bioactive lipid mediator platelet activating factor (PAF) is recognized as a key effecter of neuronal apoptosis, yet it is not clear whether its G-protein coupled receptor (PAFR) initiates or prevents PAF neurotoxicity. Using PAFR-/- and congenic wild-type mice, we show that PAF triggers caspase-3/7 activity and neuronal death in PAFR-/- but not PAFR+/+ cerebellar granule neurons. Restoring receptor expression by recombinant adenoviral infection protected cells from PAF challenge. Neuronal death was not mediated by nitric oxide or N-methyl-d-aspartate receptor signaling given that N-nitro-l-arginine methyl ester and MK-801 did not inhibit PAF-induced neuronal loss in PAFR-/- neurons. To intervene in PAFR-independent neurotoxicity, the anti-apoptotic actions of three structurally distinct PAF antagonists were compared to a panel of plant and fungal benzoic acid derivatives. We found that the PAF antagonist BN 52021 but not FR 49175 or CV 3988 inhibited PAFR-independent neurotoxicity. Orsellinic acid, a fungal-derived benzoic acid, blocked PAF-mediated neuronal apoptosis without affecting PAFR-mediated neuroprotection. These findings demonstrate that PAF can transduce apoptotic death in primary neurons independently of its G-protein coupled receptor, that PAFR activation is neuroprotective, and that orsellinic acid effectively attenuates PAFR-independent neuronal apoptosis.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Prion infection-impaired functional blocks identified by proteomics enlighten the targets and the curing pathways of an anti-prion drug

Prion-induced neurodegeneration results from multiple cellular alterations among which the accumulation of a modified form of the host protein PrP is but a hallmark. Drug treatments need understanding of underlying mechanisms. Proteomics allows getting a comprehensive view of perturbations leading to neuronal death. Heparan sulfate mimetics has proved to be efficient to clear scrapie protein in cultured cells and in animals. To investigate the mechanisms of drug attack, protein profiles of the neuronal cell line GT1 and its chronically Chandler strain infected counterpart were compared, either in steady state cultures or after a 4-day drug treatment. Differentially expressed proteins were associated into functional blocks relevant to neurodegenerative diseases. Protein structure repair and modification, proteolysis, cell shape and energy/oxidation players were affected by infection, in agreement with prion biology. Unexpectedly, novel affected blocks related to translation, nucleus structure and DNA replication were unravelled displaying commonalities with proliferative processes. The drug had a double action in infected cells by reversing protein levels back to normal in some blocks and by heightening survival functions in others. This study emphasizes the interest of a proteomic approach to unravel novel networks involved in prion infection and curing.

Interferon-gamma increases neuronal death in response to amyloid-beta1-42

Background

Alzheimer's disease is a neurodegenerative disorder characterized by a progressive cognitive impairment, the consequence of neuronal dysfunction and ultimately the death of neurons. The amyloid hypothesis proposes that neuronal damage results from the accumulation of insoluble, hydrophobic, fibrillar peptides such as amyloid-β_1-42. These peptides activate enzymes resulting in a cascade of second messengers including prostaglandins and platelet-activating factor. Apoptosis of neurons is thought to follow as a consequence of the uncontrolled release of second messengers. Biochemical, histopathological and genetic studies suggest that pro-inflammatory cytokines play a role in neurodegeneration during Alzheimer's disease. In the current study we examined the effects of interferon (IFN)-γ, tumour necrosis factor (TNF)α, interleukin (IL)-1β and IL-6 on neurons.

Methods

Primary murine cortical or cerebellar neurons, or human SH-SY5Y neuroblastoma cells, were grown in vitro. Neurons were treated with cytokines prior to incubation with different neuronal insults. Cell survival, caspase-3 activity (a measure of apoptosis) and prostaglandin production were measured. Immunoblots were used to determine the effects of cytokines on the levels of cytoplasmic phospholipase A₂ or phospholipase C γ-1.

Results

While none of the cytokines tested were directly neurotoxic, pre-treatment with IFN-γ sensitised neurons to the toxic effects of amyloid-β_1-42 or HuPrP82-146 (a neurotoxic peptide found in prion diseases). The effects of IFN-γ were seen on cortical and cerebellar neurons, and on SH-SY5Y neuroblastoma cells. However, pre-treatment with IFN-γ did not affect the sensitivity to neurons treated with staurosporine or hydrogen peroxide. Pre-treatment with IFN-γ increased the levels of cytoplasmic phospholipase A₂ in SH-SY5Y cells and increased prostaglandin E₂ production in response to amyloid-β_1-42.

Conclusion

Treatment of neuronal cells with IFN-γ increased neuronal death in response to amyloid-β_1-42 or HuPrP82-146. IFN-γ increased the levels of cytoplasmic phospholipase A₂ in cultured neuronal cells and increased expression of cytoplasmic phospholipase A₂ was associated with increased production of prostaglandin E₂ in response to amyloid-β_1-42 or HuPrP82-146. Such observations suggest that IFN-γ produced within the brain may increase neuronal loss in Alzheimer's disease.

The interaction between cytoplasmic prion protein and the hydrophobic lipid core of membrane correlates with neurotoxicity

Prion protein (PrP), normally a cell surface protein, has been detected in the cytosol of a subset of neurons. The appearance of PrP in the cytosol could result from either retro-translocation of misfolded PrP from the endoplasmic reticulum (ER) or impaired import of PrP into the ER. Transgenic mice expressing cytoplasmic PrP (cyPrP) developed neurodegeneration in cerebellar granular neurons, although no detectable pathology was observed in other brain regions. In order to understand why granular neurons in the cerebellum were most susceptible to cyPrP-induced degeneration, we investigated the subcellular localization of cyPrP. Interestingly, we found that cyPrP is membrane-bound. In transfected cells, it binds to the ER and plasma/endocytic vesicular membranes. In transgenic mice, it is associated with synaptic and microsomal membranes. Furthermore, the cerebellar neurodegeneration in transgenic mice correlates with the interaction between cyPrP and the hydrophobic lipid core of the membrane but not with either the aggregation status or the dosage of cyPrP. These results suggest that lipid membrane perturbation could be a cellular mechanism for cyPrP-induced neurotoxicity and explain the seemingly conflicting results concerning cyPrP.

Anti-apoptotic Actions of the Platelet-activating Factor Acetylhydrolase I α2 Catalytic Subunit

Platelet-activating factor (PAF) is an important mediator of cell loss following diverse pathophysiological challenges, but the manner in which PAF transduces death is not clear. Both PAF receptor-dependent and -independent pathways are implicated. In this study, we show that extracellular PAF can be internalized through PAF receptor-independent mechanisms and can initiate caspase-3-dependent apoptosis when cytosolic concentrations are elevated by approximately 15 pM/cell for 60 min. Reducing cytosolic PAF to less than 10 pM/cell terminates apoptotic signaling. By pharmacological inhibition of PAF acetylhydrolase I and II (PAF-AH) activity and down-regulation of PAF-AH I catalytic subunits by RNA interference, we show that the PAF receptor-independent death pathway is regulated by PAF-AH I and, to a lesser extent, by PAF-AH II. Moreover, the anti-apoptotic actions of PAF-AH I are subunit-specific. PAF-AH I alpha1 regulates intracellular PAF concentrations under normal physiological conditions, but expression is not sufficient to reduce an acute rise in intracellular PAF levels. PAF-AH I alpha2 expression is induced when cells are deprived of serum or exposed to apoptogenic PAF concentrations limiting the duration of pathological cytosolic PAF accumulation. To block PAF receptor-independent death pathway, we screened a panel of PAF antagonists (CV-3988, CV-6209, BN 52021, and FR 49175). BN 52021 and FR 49175 accelerated PAF hydrolysis and inhibited PAF-mediated caspase 3 activation. Both antagonists act indirectly to promote PAF-AH I alpha2 homodimer activity by reducing PAF-AH I alpha1 expression. These findings identify PAF-AH I alpha2 as a potent anti-apoptotic protein and describe a new means of pharmacologically targeting PAF-AH I to inhibit PAF-mediated cell death.