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

A Biotinylated Glycosylphosphatidylinositol (GPI) as the Universal Platform To Access GPI-Anchored Protein Analogues

A glycosylphosphatidylinositol (GPI) derivative with biotin linked to its mannose III 6-O-position was prepared by a convergent strategy. This biotinylated GPI was demonstrated to bind avidinated proteins readily through biotin-avidin interaction and, therefore, can serve as a universal platform to access various biologically significant GPI-anchored protein analogues.

Synthesis of a Bifunctionalized Glycosylphosphatidylinositol (GPI) Anchor Useful for the Study of GPI Biology

A new, bifunctional glycosylphosphatidylinositol (GPI) derivative containing the highly conserved core structure of all natural GPI anchors with a photoactivable diazirine in the lipid chain and clickable alkynes in the glycan was synthesized by a convergent [3+2] glycosylation strategy with late stage protecting group manipulation and regioselective phosphorylation. The challenges of this synthesis were due to the presence of several distinctive functional groups in the synthetic target, which complicated the protection tactics, in addition to the inherent difficulties associated with GPI synthesis. This bifunctional GPI derivative can cross-react with molecules in proximity upon photoactivation and be subsequently labeled with other molecular tags via click reaction. Therefore, it should be a valuable probe for biological studies of GPIs, such as analysis of GPI-interacting membrane proteins, and gaining insights into their functional mechanisms.

Diversity-Oriented Synthesis of Glycosylphosphatidylinositol Probes Based on an Orthogonally Protected Pentasaccharide

Two glycosylphosphatidylinositol (GPI) derivatives having an alkynyl group at different positions were derived from the same orthogonally protected pentasaccharide that in turn was assembled by a convergent [3+2] glycosylation strategy. The resultant alkynylated GPIs are useful biological probes and are suitable for further modification by click reaction to obtain other GPI probes. The pentasaccharide is a versatile platform for the synthesis of various uniquely functionalized GPI probes.

Profiling Glycosylphosphatidylinositol (GPI)-Interacting Proteins in the Cell Membrane Using a Bifunctional GPI Analogue as the Probe

Glycosylphosphatidylinositol (GPI) anchorage of cell surface proteins to the membrane is biologically important and ubiquitous in eukaryotes. However, GPIs do not contain long enough lipids to span the entire membrane bilayer. To transduce binding signals, GPIs must interact with other membrane components, but such interactions are difficult to define. Here, a new method was developed to explore GPI-interacting membrane proteins in live cell with a bifunctional analogue of the glucosaminylphosphatidylinositol motif conserved in all GPIs as a probe. This probe contained a diazirine functionality in the lipid and an alkynyl group on the glucosamine residue to respectively facilitate the cross-linkage of GPI-binding membrane proteins with the probe upon photoactivation and then the installation of biotin to the cross-linked proteins via a click reaction for affinity-based protein isolation and analysis. Profiling the proteins pulled down from the Hela cells revealed 94 unique and 18 overrepresented proteins compared to the control, and most of them are membrane proteins and many are GPI-related. The results have proved not only the concept of using the new bifunctional GPI probe to investigate GPI-binding membrane proteins but also the important role of inositol in the biological functions of GPI anchors and GPI-anchored proteins.

Membrane interaction of off-pathway prion oligomers and lipid-induced on-pathway intermediates during prion conversion: A clue for neurotoxicity

Soluble oligomers of prion proteins (PrP), produced during amyloid aggregation, have emerged as the primary neurotoxic species, instead of the fibrillar end-products, in transmissible spongiform encephalopathies. However, whether the membrane is among their direct targets, that mediate the downstream adverse effects, remains a question of debate. Recently, questions arise from the formation of membrane-active oligomeric species generated during the β-aggregation pathway, either in solution, or in lipid environment. In the present study, we characterized membrane interaction of off-pathway oligomers from recombinant prion protein generated along the amyloid aggregation and compared to lipid-induced intermediates produced during lipid-accelerated fibrillation. Using calcein-leakage assay, we show that the soluble prion oligomers are the most potent in producing leakage with negatively charged vesicles. Binding affinities, conformational states, mode of action of the different PrP assemblies were determined by thioflavin T binding-static light scattering experiments on DOPC/DOPS vesicles, as well as by FTIR-ATR spectroscopy and specular neutron reflectivity onto the corresponding supported lipid bilayers. Our results indicate that the off-pathway PrP oligomers interact with lipid membrane via a distinct mechanism, compared to the inserted lipid-induced intermediates. Thus, separate neurotoxic mechanisms could exist following the puzzling intermediates generated in the different cell compartments. These results not only reveal an important regulation of lipid membrane on PrP behavior but may also provide clues for designing stage-specific and prion-targeted therapy.

Sialylated glycosylphosphatidylinositols suppress the production of toxic amyloid-β oligomers

The production of amyloid-β (Aβ) is a key factor driving pathogenesis in Alzheimer's disease (AD). Increasing concentrations of soluble Aβ oligomers within the brain lead to synapse degeneration and the progressive dementia characteristic of AD. Since Aβ exists in both disease-relevant (toxic) and non-toxic forms, the factors that affected the release of toxic Aβ were studied in a cell model. 7PA2 cells expressing the human amyloid precursor protein released Aβ oligomers that caused synapse damage when incubated with cultured neurones. These Aβ oligomers had similar potency to soluble Aβ oligomers derived from the brains of Alzheimer's patients. Although the conditioned media from 7PA2 cells treated with the cellular prion protein (PrP^C) contained Aβ, it did not cause synapse damage. The loss of toxicity was associated with a reduction in Aβ oligomers and an increase in Aβ monomers. The suppression of toxic Aβ release was dependent on the glycosylphosphatidylinositol (GPI) anchor attached to PrP^C, and treatment of cells with specific GPIs alone reduced the production of toxic Aβ. The efficacy of GPIs was structure-dependent and the presence of sialic acid was critical. The conditioned medium from GPI-treated cells protected neurones against Aβ oligomer-induced synapse damage; neuroprotection was mediated by Aβ monomers. These studies support the hypothesis that the ratio of Aβ monomers to Aβ oligomers is a critical factor that regulates synapse damage.

The Biological Function of the Prion Protein: A Cell Surface Scaffold of Signaling Modules

The prion glycoprotein (PrP^C) is mostly located at the cell surface, tethered to the plasma membrane through a glycosyl-phosphatydil inositol (GPI) anchor. Misfolding of PrP^C is associated with the transmissible spongiform encephalopathies (TSEs), whereas its normal conformer serves as a receptor for oligomers of the β-amyloid peptide, which play a major role in the pathogenesis of Alzheimer’s Disease (AD). PrP^C is highly expressed in both the nervous and immune systems, as well as in other organs, but its functions are controversial. Extensive experimental work disclosed multiple physiological roles of PrP^C at the molecular, cellular and systemic levels, affecting the homeostasis of copper, neuroprotection, stem cell renewal and memory mechanisms, among others. Often each such process has been heralded as the bona fide function of PrP^C, despite restricted attention paid to a selected phenotypic trait, associated with either modulation of gene expression or to the engagement of PrP^C with a single ligand. In contrast, the GPI-anchored prion protein was shown to bind several extracellular and transmembrane ligands, which are required to endow that protein with the ability to play various roles in transmembrane signal transduction. In addition, differing sets of those ligands are available in cell type- and context-dependent scenarios. To account for such properties, we proposed that PrP^C serves as a dynamic platform for the assembly of signaling modules at the cell surface, with widespread consequences for both physiology and behavior. The current review advances the hypothesis that the biological function of the prion protein is that of a cell surface scaffold protein, based on the striking similarities of its functional properties with those of scaffold proteins involved in the organization of intracellular signal transduction pathways. Those properties are: the ability to recruit spatially restricted sets of binding molecules involved in specific signaling; mediation of the crosstalk of signaling pathways; reciprocal allosteric regulation with binding partners; compartmentalized responses; dependence of signaling properties upon posttranslational modification; and stoichiometric requirements and/or oligomerization-dependent impact on signaling. The scaffold concept may contribute to novel approaches to the development of effective treatments to hitherto incurable neurodegenerative diseases, through informed modulation of prion protein-ligand interactions.

Sialic Acid within the Glycosylphosphatidylinositol Anchor Targets the Cellular Prion Protein to Synapses

Although the cellular prion protein (PrP(C)) is concentrated at synapses, the factors that target PrP(C) to synapses are not understood. Here we demonstrate that exogenous PrP(C) was rapidly targeted to synapses in recipient neurons derived from Prnp knock-out((0/0)) mice. The targeting of PrP(C) to synapses was dependent upon both neuronal cholesterol concentrations and the lipid and glycan composition of its glycosylphosphatidylinositol (GPI) anchor. Thus, the removal of either an acyl chain or sialic acid from the GPI anchor reduced the targeting of PrP(C) to synapses. Isolated GPIs (derived from PrP(C)) were also targeted to synapses, as was IgG conjugated to these GPIs. The removal of sialic acid from GPIs prevented the targeting of either the isolated GPIs or the IgG-GPI conjugate to synapses. Competition studies showed that pretreatment with sialylated GPIs prevented the targeting of PrP(C) to synapses. These results are consistent with the hypothesis that the sialylated GPI anchor attached to PrP(C) acts as a synapse homing signal.

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.

Monoacylated Cellular Prion Proteins Reduce Amyloid-β-Induced Activation of Cytoplasmic Phospholipase A2 and Synapse Damage

Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) and the loss of synapses. Aggregation of the cellular prion protein (PrP^C) by Aβ oligomers induced synapse damage in cultured neurons. PrP^C is attached to membranes via a glycosylphosphatidylinositol (GPI) anchor, the composition of which affects protein targeting and cell signaling. Monoacylated PrP^C incorporated into neurons bound “natural Aβ”, sequestering Aβ outside lipid rafts and preventing its accumulation at synapses. The presence of monoacylated PrP^C reduced the Aβ-induced activation of cytoplasmic phospholipase A₂ (cPLA₂) and Aβ-induced synapse damage. This protective effect was stimulus specific, as treated neurons remained sensitive to α-synuclein, a protein associated with synapse damage in Parkinson’s disease. In synaptosomes, the aggregation of PrP^C by Aβ oligomers triggered the formation of a signaling complex containing the cPLA₂.a process, disrupted by monoacylated PrP^C. We propose that monoacylated PrP^C acts as a molecular sponge, binding Aβ oligomers at the neuronal perikarya without activating cPLA₂ or triggering synapse damage.

α-Synuclein-induced synapse damage in cultured neurons is mediated by cholesterol-sensitive activation of cytoplasmic phospholipase A2

The accumulation of aggregated forms of the α-synuclein (αSN) is associated with the pathogenesis of Parkinson's disease (PD) and Dementia with Lewy Bodies. The loss of synapses is an important event in the pathogenesis of these diseases. Here we show that aggregated recombinant human αSN, but not βSN, triggered synapse damage in cultured neurons as measured by the loss of synaptic proteins. Pre-treatment with the selective cytoplasmic phospholipase A2 (cPLA2) inhibitors AACOCF3 and MAFP protected neurons against αSN-induced synapse damage. Synapse damage was associated with the αSN-induced activation of synaptic cPLA2 and the production of prostaglandin E2. The activation of cPLA2 is the first step in the generation of platelet-activating factor (PAF) and PAF receptor antagonists (ginkgolide B or Hexa-PAF) also protect neurons against αSN-induced synapse damage. αSN-induced synapse damage was also reduced in neurons pre-treated with the cholesterol synthesis inhibitor (squalestatin). These results are consistent with the hypothesis that αSN triggered synapse damage via hyperactivation of cPLA2. They also indicate that αSN-induced activation of cPLA2 is influenced by the cholesterol content of membranes. Inhibitors of this pathway that can cross the blood brain barrier may protect against the synapse damage seen during PD.

The GPI-anchoring of PrP: implications in sorting and pathogenesis

The cellular prion protein (PrP(C)) is an N-glycosylated GPI-anchored protein usually present in lipid rafts with numerous putative functions. When it changes its conformation to a pathological isoform (then referred to as PrP(Sc)), it is an essential part of the prion, the agent causing fatal and transmissible neurodegenerative prion diseases. There is growing evidence that toxicity and neuronal damage on the one hand and propagation/infectivity on the other hand are two distinct processes of the disease and that the GPI-anchor attachment of PrP(C) and PrP(Sc) plays an important role in protein localization and in neurotoxicity. Here we review how the signal sequence of the GPI-anchor matters in PrP(C) localization, how an altered cellular localization of PrP(C) or differences in GPI-anchor composition can affect prion infection, and we discuss through which mechanisms changes on the anchorage of PrP(C) can modify the disease process.

Prion protein accumulation in lipid rafts of mouse aging brain

The cellular form of the prion protein (PrP^C) is a normal constituent of neuronal cell membranes. The protein misfolding causes rare neurodegenerative disorders known as transmissible spongiform encephalopathies or prion diseases. These maladies can be sporadic, genetic or infectious. Sporadic prion diseases are the most common form mainly affecting aging people. In this work, we investigate the biochemical environment in which sporadic prion diseases may develop, focusing our attention on the cell membrane of neurons in the aging brain. It is well established that with aging the ratio between the most abundant lipid components of rafts undergoes a major change: while cholesterol decreases, sphingomyelin content rises. Our results indicate that the aging process modifies the compartmentalization of PrP^C. In old mice, this change favors PrP^C accumulation in detergent-resistant membranes, particularly in hippocampi. To confirm the relationship between lipid content changes and PrP^C translocation into detergent-resistant membranes (DRMs), we looked at PrP^C compartmentalization in hippocampi from acid sphingomyelinase (ASM) knockout (KO) mice and synaptosomes enriched in sphingomyelin. In the presence of high sphingomyelin content, we observed a significant increase of PrP^C in DRMS. This process is not due to higher levels of total protein and it could, in turn, favor the onset of sporadic prion diseases during aging as it increases the PrP intermolecular contacts into lipid rafts. We observed that lowering sphingomyelin in scrapie-infected cells by using fumonisin B1 led to a 50% decrease in protease-resistant PrP formation. This may suggest an involvement of PrP lipid environment in prion formation and consequently it may play a role in the onset or development of sporadic forms of prion diseases.

A critical survey of methods to detect plasma membrane rafts

The plasma membrane is still one of the enigmatic cellular structures. Although the microscopic structure is getting clearer, not much is known about the organization at the nanometre level. Experimental difficulties have precluded unambiguous approaches, making the current picture rather fuzzy. In consequence, a variety of different membrane models has been proposed over the years, on the basis of different experimental strategies. Recent data obtained via high-resolution single-molecule microscopy shed new light on the existing hypotheses. We thus think it is a good time for reviewing the consistency of the existing models with the new data. In this paper, we summarize the available models in ten propositions, each of which is discussed critically with respect to the applied technologies and the strengths and weaknesses of the approaches. Our aim is to provide the reader with a sound basis for his own assessment. We close this chapter by exposing our picture of the membrane organization at the nanoscale.

Clustering of sialylated glycosylphosphatidylinositol anchors mediates PrP-induced activation of cytoplasmic phospholipase A 2 and synapse damage

Precisely how the accumulation of PrP (Sc) causes the neuronal degeneration that leads to the clinical symptoms of prion diseases is poorly understood. Our recent paper showed that the clustering of specific glycosylphosphatidylinositol (GPI) anchors attached to PrP proteins triggered synapse damage in cultured neurons. First, we demonstrated that small, soluble PrP (Sc) oligomers caused synapse damage via a GPI-dependent process. Our hypothesis, that the clustering of specific GPIs caused synapse damage, was supported by observations that cross-linkage of PrP (C), either chemically or by monoclonal antibodies, also triggered synapse damage. Synapse damage was preceded by an increase in the cholesterol content of synapses and activation of cytoplasmic phospholipase A 2 (cPLA 2). The presence of a terminal sialic acid moiety, a rare modification of mammalian GPI anchors, was essential in the activation of cPLA 2 and synapse damage induced by cross-linked PrP (C). We conclude that the sialic acid modifies local membrane microenvironments (rafts) surrounding clustered PrP molecules resulting in aberrant activation of cPLA 2 and synapse damage. A recent observation, that toxic amyloid-β assemblies cross-link PrP (C), suggests that synapse damage in prion and Alzheimer diseases is mediated via a common molecular mechanism, and raises the possibility that the pharmacological modification of GPI anchors might constitute a novel therapeutic approach to these diseases.

Amyloid-β-induced synapse damage is mediated via cross-linkage of cellular prion proteins

The cellular prion protein (PrP(C)), which is highly expressed at synapses, was identified as a receptor for the amyloid-β (Aβ) oligomers that are associated with dementia in Alzheimer disease. Here, we report that Aβ oligomers secreted by 7PA2 cells caused synapse damage in cultured neurons via a PrP(C)-dependent process. Exogenous PrP(C) added to Prnp knock-out((0/0)) neurons was targeted to synapses and significantly increased Aβ-induced synapse damage. In contrast, the synapse damage induced by a phospholipase A(2)-activating peptide was independent of PrP(C). In Prnp wild-type((+/+)) neurons Aβ oligomers activated synaptic cytoplasmic phospholipase A(2) (cPLA(2)). In these cells, the addition of Aβ oligomers triggered the translocation of cPLA(2) in synapses to cholesterol dense membranes (lipid rafts) where it formed a complex also containing Aβ and PrP(C). In contrast, the addition of Aβ to Prnp((0/0)) neurons did not activate synaptic cPLA(2), which remained in the cytoplasm and was not associated with Aβ. Filtration assays and non-denaturing gels demonstrated that Aβ oligomers cross-link PrP(C). We propose that it is the cross-linkage of PrP(C) by Aβ oligomers that triggers abnormal activation of cPLA(2) and synapse damage. This hypothesis was supported by our observation that monoclonal antibody mediated cross-linkage of PrP(C) also activated synaptic cPLA(2) and caused synapse damage.

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.