Exosomal cellular prion protein drives fibrillization of amyloid beta and counteracts amyloid beta-mediated neurotoxicity.

Alzheimer's disease is a common neurodegenerative, progressive, and fatal disorder. Generation and deposition of amyloid beta (Aß) peptides associate with its pathogenesis and small soluble Aß oligomers show the most pronounced neurotoxic effects and correlate with disease initiation and progression. Recent findings showed that Aß oligomers bind to the cellular prion protein (PrP(C) ) eliciting neurotoxic effects. The role of exosomes, small extracellular vesicles of endosomal origin, in Alzheimer's disease is only poorly understood. Besides serving as disease biomarkers they may promote Aß plaque formation, decrease Aß-mediated synaptotoxicity, and enhance Aß clearance. Here, we explore how exosomal PrP(C) connects to protective functions attributed to exosomes in Alzheimer's disease. To achieve this, we generated a mouse neuroblastoma PrP(C) knockout cell line using transcription activator-like effector nucleases. Using these, as well as SH-SY5Y human neuroblastoma cells, we show that PrP(C) is highly enriched on exosomes and that exosomes bind amyloid beta via PrP(C) . Exosomes showed highest binding affinity for dimeric, pentameric, and oligomeric Aß species. Thioflavin T assays revealed that exosomal PrP(C) accelerates fibrillization of amyloid beta, thereby reducing neurotoxic effects imparted by oligomeric Aß. Our study provides further evidence for a protective role of exosomes in Aß-mediated neurodegeneration and highlights the importance of exosomal PrP(C) in molecular mechanisms of Alzheimer's disease. We show that the prion protein (PrP(C) ) on exosomes captures neurotoxic species of amyloid beta (Aß) promoting its fibrillization. Our study provides evidence for a protective role of exosomes in Alzheimer`s disease and suggests that, depending on its membrane topology, PrP(C) holds a dual function: when expressed at the neuronal surface it acts as receptor for Aß leading to neurotoxic signaling, whereas it detoxifies Aß when present on exosomes. This provides further support for key roles of PrP(C) in Alzheimer's disease.

Shedding light on prion disease.

Proteolytic processing regulates key processes in health and disease. The cellular prion protein (PrP(C)) is subject to at least 3 cleavage events, α-cleavage, ß-cleavage and shedding. In contrast to α- and ß-cleavage where there is an ongoing controversy on the identity of relevant proteases, the metalloprotease ADAM10 represents the only relevant PrP sheddase. Here we focus on the roles that ADAM10-mediated shedding of PrP(C) and its pathogenic isoform (PrP(Sc)) might play in regulating their physiological and pathogenic functions, respectively. As revealed by our recent study using conditional ADAM10 knockout mice (Altmeppen et al., 2015), shedding of PrP seems to be involved in key processes of prion diseases. These aspects and several open questions arising from them are discussed. Increased knowledge on this topic can shed new light on prion diseases and other neurodegenerative conditions as well.

The sheddase ADAM10 is a potent modulator of prion disease.

The prion protein (PrP(C)) is highly expressed in the nervous system and critically involved in prion diseases where it misfolds into pathogenic PrP(Sc). Moreover, it has been suggested as a receptor mediating neurotoxicity in common neurodegenerative proteinopathies such as Alzheimer's disease. PrP(C) is shed at the plasma membrane by the metalloprotease ADAM10, yet the impact of this on prion disease remains enigmatic. Employing conditional knockout mice, we show that depletion of ADAM10 in forebrain neurons leads to posttranslational increase of PrP(C) levels. Upon prion infection of these mice, clinical, biochemical, and morphological data reveal that lack of ADAM10 significantly reduces incubation times and increases PrP(Sc) formation. In contrast, spatiotemporal analysis indicates that absence of shedding impairs spread of prion pathology. Our data support a dual role for ADAM10-mediated shedding and highlight the role of proteolytic processing in prion disease.

High molecular mass assemblies of amyloid-ß oligomers bind prion protein in patients with Alzheimer's disease.

Alzheimer's disease is the most common form of dementia and the generation of oligomeric species of amyloid-ß is causal to the initiation and progression of it. Amyloid-ß oligomers bind to the N-terminus of plasma membrane-bound cellular prion protein (PrP(C)) initiating a series of events leading to synaptic degeneration. Composition of bound amyloid-ß oligomers, binding regions within PrP(C), binding affinities and modifiers of this interaction have been almost exclusively studied in cell culture or murine models of Alzheimer's disease and our knowledge on PrP(C)-amyloid-ß interaction in patients with Alzheimer's disease is limited regarding occurrence, binding regions in PrP(C), and size of bound amyloid-ß oligomers. Here we employed a PrP(C)-amyloid-ß binding assay and size exclusion chromatography on neuropathologically characterized Alzheimer's disease and non-demented control brains (n = 15, seven female, eight male, average age: 79.2 years for Alzheimer's disease and n = 10, three female, seven male, average age: 66.4 years for controls) to investigate amyloid-ß-PrP(C) interaction. PrP(C)-amyloid-ß binding always occurred in Alzheimer's disease brains and was never detected in non-demented controls. Neither expression level of PrP(C) nor known genetic modifiers of Alzheimer's disease, such as the PrP(C) codon 129 polymorphism, influenced this interaction. In Alzheimer's disease brains, binding of amyloid-ß to PrP(C) occurred via the PrP(C) N-terminus. For synthetic amyloid-ß42, small oligomeric species showed prominent binding to PrP(C), whereas in Alzheimer's disease brains larger protein assemblies containing amyloid-ß42 bound efficiently to PrP(C). These data confirm Alzheimer's disease specificity of binding of amyloid-ß to PrP(C) via its N-terminus in a large cohort of Alzheimer's disease/control brains. Differences in sizes of separated protein fractions between synthetic and brain-derived amyloid-ß binding to PrP(C) suggest that larger assemblies of amyloid-ß or additional non-amyloid-ß components may play a role in binding of amyloid-ß42 to PrP(C) in Alzheimer's disease.

Roles of endoproteolytic α-cleavage and shedding of the prion protein in neurodegeneration.

The cellular prion protein (PrP(C)) plays important roles in neurodegenerative diseases. First, it is the well-established substrate for the conformational conversion into its pathogenic isoform (PrP(Sc)) giving rise to progressive and fatal prion diseases. Moreover, several recent reports highlight important roles of PrP(C) in other neurodegenerative conditions such as Alzheimer's disease. Since PrP(C) is subject to proteolytic processing, here we discuss the two main cleavage events under physiological conditions, α-cleavage and shedding. We focus on how these cleavages and the resulting fragments may impact prion diseases as well as other neurodegenerative proteinopathies. Finally, we discuss the recently identified sheddase of PrP(C), namely the metalloprotease ADAM10, with regard to therapeutic potential against neurodegenerative diseases.

Proteolytic processing of the prion protein in health and disease.

A variety of physiological functions, not only restricted to the nervous system, are discussed for the cellular prion protein (PrP(C)). A prominent, non-physiological property of PrPC is the conversion into its pathogenic isoform (PrP(Sc)) during fatal, transmissible, and neurodegenerative prion diseases. The prion protein is subject to posttranslational proteolytic processing and these cleavage events have been shown i) to regulate its physiological functions, ii) to produce biologically active fragments, and iii) to potentially influence the course of prion disease. Here, we give an overview on the proteolytic processing under physiological and pathological conditions and critically review what is currently known about the three main cleavage events of the prion protein, namely α-cleavage, ß-cleavage, and ectodomain shedding. The biological relevance of resulting fragments as well as controversies regarding candidate proteases, with special emphasis on members of the A-disintegrin-and-metalloproteinase (ADAM) family, will be discussed. In addition, we make suggestions aimed at facilitating clarity and progress in this important research field. The better understanding of this issue will not only answer basic questions in prion biology but will likely impact research on other neurodegenerative diseases as well.