Prion Protein Accumulation in Lipid Rafts of Mouse Aging Brain

Abstract

The cellular form of the prion protein (PrPC) 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 PrPC. In old mice, this change favors PrPC accumulation in detergent-resistant membranes, particularly in hippocampi. To confirm the relationship between lipid content changes and PrPC translocation into detergent-resistant membranes (DRMs), we looked at PrPC 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 PrPC 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.

Therapeutic Molecules and Endogenous Ligands Regulate the Interaction between Brain Cellular Prion Protein (PrPC) and Metabotropic Glutamate Receptor 5 (mGluR5)

“…Pretreatment of PrP C -and Myc-mGluR5-expressing cell cultures with M␤CD destroyed lipid rafts and prevented an A␤o-dependent modulation of the PrP C -Myc-mGluR5 interaction. This is in accordance with the fact that A␤o effects are proposed to occur in lipid raft-like domains, where PrP C and mGluR5 receptors are located (31)(32)(33)(34). is of high biological relevance because of findings of previous studies that demonstrated the reversal of A␤o-induced effects in cell-based toxicity assays by the mGluR5-specific antagonist MTEP (19,30).…”

“…C -mGluR5 Interaction Requires Intact Lipid Rafts-A␤o effects are proposed to occur in lipid raft-like domains to which PrP C and mGluR5 are known to localize (31)(32)(33). We show that disruption of lipid rafts by pretreatment of PrP C and Myc-mGluR5 coexpressing HEK293 cells with methyl-␤-cyclodextrin (M␤CD) pre- 's t test).…”

“…When A␤o/PrP C complexes form, they trigger AD pathophysiology by interacting with mGluR5 (30). Both PrP C and mGluR5 receptors are located in lipid raft-like domains, and these are hypothesized to be the key location of A␤o-triggered induction of synaptotoxicity (31)(32)(33)(34). Consistent with this finding, Renner et al (35) revealed a PrP C -and mGluR5-dependent binding of A␤o to synapses using live single particle tracking of labeled A␤o in hippocampal neurons.…”

“…Pharmacological strategies targeting the PrP CmGluR5 interaction would largely benefit from a better understanding of the interaction between PrP C and mGluR5. Human PrP C is a 209-residue glycoprotein anchored into the membrane of lipid rafts by a glycosylphosphatidylinositol anchor (32,55). It contains two potential glycosylation sites at residues Asn-181 and Asn-197, respectively.…”

The Upsides and Downsides of Organelle Interconnectivity

“…Ltc1 is a sterol transporter required for the formation of sterol lipid-enriched vacuolar membrane domains during stress (Murley and Nunnari, 2016). The exact function of such domains is not known but is interesting in the context of PQC as sterols can promote the formation of lipid rafts, which have been shown to be associated with misfolded and aggregating disease proteins (Lewis and Hooper 2011; Agostini et al, 2013). …”