Glycosylphosphatidylinositol anchor analogues sequester cholesterol and reduce prion formation

Cholesterol and its reciprocal association with prion infection

Prion diseases are incurable, infectious and fatal neurodegenerative diseases that affect both humans and animals. The pathogenesis of prion disease involves the misfolding of the cellular prion protein, PrP^C, to a disease-causing conformation, PrP^Sc, in the brain. The exact mechanism of conversion of PrP^C to PrP^Sc is not clear; however, there are numerous studies supporting that this process of misfolding requires the association of PrP^C with lipid raft domains of the plasma membrane. An increase in the cellular cholesterol content with prion infection has been observed in both in vivo and in vitro studies. As cholesterol is critical for the formation of lipid rafts, on the one hand, this increase may be related to, or aiding in, the process of prion conversion. On the other hand, increased cholesterol levels may affect neuronal viability. Here, we discuss current literature on the underlying mechanisms and potential consequences of elevated neuronal cholesterol in prion infection and advancements in prion disease therapeutics targeting brain cholesterol homeostasis.

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.

Melanin or a Melanin-Like Substance Interacts with the N-Terminal Portion of Prion Protein and Inhibits Abnormal Prion Protein Formation in Prion-Infected Cells

Prion diseases are progressive fatal neurodegenerative illnesses caused by the accumulation of transmissible abnormal prion protein (PrP). To find treatments for prion diseases, we searched for substances from natural resources that inhibit abnormal PrP formation in prion-infected cells. We found that high-molecular-weight components from insect cuticle extracts reduced abnormal PrP levels. The chemical nature of these components was consistent with that of melanin. In fact, synthetic melanin produced from tyrosine or 3-hydroxy-l-tyrosine inhibited abnormal PrP formation. Melanin did not modify cellular or cell surface PrP levels, nor did it modify lipid raft or cellular cholesterol levels. Neither did it enhance autophagy or lysosomal function. Melanin was capable of interacting with PrP at two N-terminal domains. Specifically, it strongly interacted with the PrP region of amino acids 23 to 50 including a positively charged amino acid cluster and weakly interacted with the PrP octarepeat peptide region of residues 51 to 90. However, the in vitro and in vivo data were inconsistent with those of prion-infected cells. Abnormal PrP formation in protein misfolding cyclic amplification was not inhibited by melanin. Survival after prion infection was not significantly altered in albino mice or exogenously melanin-injected mice compared with that of control mice. These data suggest that melanin, a main determinant of skin color, is not likely to modify prion disease pathogenesis, even though racial differences in the incidence of human prion diseases have been reported. Thus, the findings identify an interaction between melanin and the N terminus of PrP, but the pathophysiological roles of the PrP-melanin interaction remain unclear.IMPORTANCE The N-terminal region of PrP is reportedly important for neuroprotection, neurotoxicity, and abnormal PrP formation, as this region is bound by many factors, such as metal ions, lipids, nucleic acids, antiprion compounds, and several proteins, including abnormal PrP in prion disease and the Aβ oligomer in Alzheimer's disease. In the present study, melanin, a main determinant of skin color, was newly found to interact with this N-terminal region and inhibits abnormal PrP formation in prion-infected cells. However, the data for prion infection in mice lacking melanin production suggest that melanin is not associated with the prion disease mechanism, although the incidence of prion disease is reportedly much higher in white people than in black people. Thus, the roles of the PrP-melanin interaction remain to be further elucidated, but melanin might be a useful competitive tool for evaluating the functions of other ligands at the N-terminal region.

Structure-activity analysis and antiprion mechanism of isoprenoid compounds

The prion strain-specific mechanism by which normal prion protein is converted to abnormal prion protein remains largely unknown. This study found that insect juvenile hormone III reduced abnormal prion protein levels only in cells infected with the RML prion. We conducted a structure-activity analysis using juvenile hormone III biosynthetic intermediates in the isoprenoid pathway. Both farnesol and geranylgeraniol, the most potent inhibitors of abnormal prion protein formation, behaved in an RML prion-dependent fashion. Neither of them modified cellular and cell surface prion protein levels. Events downstream of this pathway include cholesterol biosynthesis and protein prenylation. However, neither of these isoprenoid compounds modified lipid raft microdomains and cellular cholesterol levels and neither affected the representative prenylated protein expression levels of prenylation pathways. Therefore, these isoprenoid compounds are a new class of prion strain-dependent antiprion compounds. They are useful for exploring strain-specific prion biology.

Anti-prion activity found in beetle grub hemolymph of Trypoxylus dichotomus septentrionalis

No remedies for prion disease have been established, and the conversion of normal to abnormal prion protein, a key event in prion disease, is still unclear. Here we found that substances in beetle grub hemolymph, after they were browned by aging for a month or heating for hours, reduced abnormal prion protein (PrP) levels in RML prion-infected cells. Active anti-prion components in the hemolymph were resistant to protease treatment and had molecular weights larger than 100 kDa. Aminoguanidine treatment of the hemolymph abolished its anti-prion activity, suggesting that Maillard reaction products are enrolled in the activity against the RML prion. However, levels of abnormal PrP in RML prion-infected cells were not decreased by incubation with the Maillard reaction products formed by amino acids or bovine serum albumin. The anti-prion components in the hemolymph modified neither cellular or cell-surface PrP levels nor lipid raft or autophagosome levels. The anti-prion activity was not observed in cells infected with 22 L prion or Fukuoka-1 prion, suggesting the anti-prion action is prion strain-dependent. Although the active components of the hemolymph need to be further evaluated, the present findings imply that certain specific chemical structures in the hemolymph, but not chemical structures common to all Maillard reaction products, are involved in RML prion formation or turnover, without modifying normal PrP expression. The anti-prion components in the hemolymph are a new tool for elucidating strain-dependent prion biology.

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Mechanism of prion formation is still unclear in prion-infected cells.

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Browned beetle-grub hemolymph has a prion strain-dependent anti-prion activity.

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Anti-prion activity of the hemolymph is abolished by a Maillard reaction inhibitor.

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No anti-prion activity is observed in other Maillard reaction products.

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Browned hemolymph is a new tool for elucidating strain-dependent prion biology.

Efficacy and mechanism of a glycoside compound inhibiting abnormal prion protein formation in prion-infected cells: implications of interferon and phosphodiesterase 4D-interacting protein

A new type of antiprion compound, Gly-9, was found to inhibit abnormal prion protein formation in prion-infected neuroblastoma cells, in a prion strain-independent manner, when the cells were treated for more than 1 day. It reduced the intracellular prion protein level and significantly modified mRNA expression levels of genes of two types: interferon-stimulated genes were downregulated after more than 2 days of treatment, and the phosphodiesterase 4D-interacting protein gene, a gene involved in microtubule growth, was upregulated after more than 1 day of treatment. A supplement of interferon given to the cells partly restored the abnormal prion protein level but did not alter the normal prion protein level. This interferon action was independent of the Janus activated kinase-signal transducer and activator of transcription signaling pathway. Therefore, the changes in interferon-stimulated genes might be a secondary effect of Gly-9 treatment. However, gene knockdown of phosphodiesterase 4D-interacting protein restored or increased both the abnormal prion protein level and the normal prion protein level, without transcriptional alteration of the prion protein gene. It also altered the localization of abnormal prion protein accumulation in the cells, indicating that phosphodiesterase 4D-interacting protein might affect prion protein levels by altering the trafficking of prion protein-containing structures. Interferon and phosphodiesterase 4D-interacting protein had no direct mutual link, demonstrating that they regulate abnormal prion protein levels independently. Although the in vivo efficacy of Gly-9 was limited, the findings for Gly-9 provide insights into the regulation of abnormal prion protein in cells and suggest new targets for antiprion compounds.

IMPORTANCE

This report describes our study of the efficacy and potential mechanism underlying the antiprion action of a new antiprion compound with a glycoside structure in prion-infected cells, as well as the efficacy of the compound in prion-infected animals. The study revealed involvements of two factors in the compound's mechanism of action: interferon and a microtubule nucleation activator, phosphodiesterase 4D-interacting protein. In particular, phosphodiesterase 4D-interacting protein was suggested to be important in regulating the trafficking or fusion of prion protein-containing vesicles or structures in cells. The findings of the study are expected to be useful not only for the elucidation of cellular regulatory mechanisms of prion protein but also for the implication of new targets for therapeutic development.

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.

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 oligomer neurotoxicity, calcium dysregulation, and lipid rafts

Amyloid proteins constitute a chemically heterogeneous group of proteins, which share some biophysical and biological characteristics, the principal of which are the high propensity to acquire an incorrect folding and the tendency to aggregate. A number of diseases are associated with misfolding and aggregation of proteins, although only in some of them—most notably Alzheimer's disease (AD) and transmissible spongiform encephalopathies (TSEs)—a pathogenetic link with misfolded proteins is now widely recognized. Lipid rafts (LRs) have been involved in the pathophysiology of diseases associated with protein misfolding at several levels, including aggregation of misfolded proteins, amyloidogenic processing, and neurotoxicity. Among the pathogenic misfolded proteins, the AD-related protein amyloid β (Aβ) is by far the most studied protein, and a large body of evidence has been gathered on the role played by LRs in Aβ pathogenicity. However, significant amount of data has also been collected for several other amyloid proteins, so that their ability to interact with LRs can be considered an additional, shared feature characterizing the amyloid protein family. In this paper, we will review the evidence on the role of LRs in the neurotoxicity of huntingtin, α-synuclein, prion protein, and calcitonin.

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.

N-acetyl-D-glucosaminylphosphatidylinositol de-N-acetylase from Entamoeba histolytica: metal alters catalytic rates but not substrate affinity

PIG-L/GPI12 proteins are endoplasmic reticulum-resident membrane proteins involved in the second step of glycosylphosphatidylinositol anchor biosynthesis in eukaryotes. We show that the Entamoeba histolytica PIG-L protein is optimally active in the acidic pH range. The enzyme has an intrinsic low level of de-N-acetylase activity in the absence of metal and is significantly stimulated by divalent cations. Metal binding induces a large conformational change in the protein that appears to improve catalytic rates while not altering the affinity of the enzyme for its substrate.