A multimolecular signaling complex including PrP^C and LRP1 is strictly dependent on lipid rafts and is essential for the function of tissue plasminogen activator

Abstract

Prion protein (PrPC) localizes stably in lipid rafts microdomains and is able to recruit downstream signal transduction pathways by the interaction with promiscuous partners. Other proteins have the ability to occasionally be recruited to these specialized membrane areas, within multimolecular complexes. Among these, we highlight the presence of the low‐density lipoprotein receptor‐related protein 1 (LRP1), which was found localized transiently in lipid rafts, suggesting a different function of this receptor that through lipid raft becomes able to activate a signal transduction pathway triggered by specific ligands, including Tissue plasminogen activator (tPA). Since it has been reported that PrPC participates in the tPA‐mediated plasminogen activation, in this study, we describe the role of lipid rafts in the recruitment and activation of downstream signal transduction pathways mediated by the interaction among tPA, PrPC and LRP1 in human neuroblastoma SK‐N‐BE2 cell line. Co‐immunoprecipitation analysis reveals a consistent association between PrPC and GM1, as well as between LRP1 and GM1, indicating the existence of a glycosphingolipid‐enriched multimolecular complex. In our cell model, knocking‐down PrPC by siRNA impairs ERK phosphorylation induced by tPA. Moreover the alteration of the lipidic milieu of lipid rafts, perturbing the physical/functional interaction between PrPC and LRP1, inhibits this response. We show that LRP1 and PrPC, following tPA stimulation, may function as a system associated with lipid rafts, involved in receptor‐mediated neuritogenic pathway. We suggest this as a multimolecular signaling complex, whose activity depends strictly on the integrity of lipid raft and is involved in the neuritogenic signaling.

Prion Protein in Stem Cells: A Lipid Raft Component Involved in the Cellular Differentiation Process

“…Since its discovery, various authors have showed that PrP C is present on the cell plasma membrane-associated with different gangliosides and cholesterol-within particular structures named "lipid raft microdomains" [21,47]. These structures represent domains of plasma membrane that contain high concentrations of cholesterol and glycosphingolipids, such as GM3, GM1, GD3, and proteins; lipid rafts can dissociate and associate rapidly, forming functional clusters in cell membranes [22,48].…”

“…These clusters, present in the outer monolayer of the plasma membrane, provide highly efficient lipid-protein modules, which operate in membrane trafficking and cell signaling [52]. Several authors theorized lipid rafts as sequestering platforms of specific proteins, thus modulating cell signaling [21,53,54].…”

“…Since its discovery, it has been well known that PrP C is present on the plasma membrane associated with the lipid raft microdomains [21], structures that represent particular sub-compartments of the plasma membrane enriched in cholesterol and glycosphingolipids, such as GM3, GM1, and GD3 [22,23]. Several studies have shown that lipid rafts are important in the refolding process of the PrP C in PrP Sc [24,25].…”

New applications of squalene synthase inhibitors: Membrane cholesterol as a therapeutic target

“…[78,79] The role of lipid rafts in the recruitment and activation of downstream signal transduction pathways, such as those mediated by the interaction of PrP c , has been studied in human neuroblastoma cell lines. [80] Alteration of the lipidic content of lipid rafts perturbs the physical/functional interaction between PrP c and other proteins, thus inhibiting downstream responses. [80] However, apart from physiological roles of PrP c , it is long known that altered, protease-resistant, prion protein (PrP sc ) is involved in the pathogenesis of neurodegenerative prion diseases.…”

“…[80] Alteration of the lipidic content of lipid rafts perturbs the physical/functional interaction between PrP c and other proteins, thus inhibiting downstream responses. [80] However, apart from physiological roles of PrP c , it is long known that altered, protease-resistant, prion protein (PrP sc ) is involved in the pathogenesis of neurodegenerative prion diseases.…”