PrP-grafted antibodies bind certain amyloid β-protein aggregates, but do not prevent toxicity

Prion Protein: The Molecule of Many Forms and Faces

Cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored protein most abundantly found in the outer membrane of neurons. Due to structural characteristics (a flexible tail and structured core), PrP^C interacts with a wide range of partners. Although PrP^C has been proposed to be involved in many physiological functions, only peripheral nerve myelination homeostasis has been confirmed as a bona fide function thus far. PrP^C misfolding causes prion diseases and PrP^C has been shown to mediate β-rich oligomer-induced neurotoxicity in Alzheimer’s and Parkinson’s disease as well as neuroprotection in ischemia. Upon proteolytic cleavage, PrP^C is transformed into released and attached forms of PrP that can, depending on the contained structural characteristics of PrP^C, display protective or toxic properties. In this review, we will outline prion protein and prion protein fragment properties as well as overview their involvement with interacting partners and signal pathways in myelination, neuroprotection and neurodegenerative diseases.

On the role of the cellular prion protein in the uptake and signaling of pathological aggregates in neurodegenerative diseases

Neurodegenerative disorders are associated with intra- or extra-cellular deposition of aggregates of misfolded insoluble proteins. These deposits composed of tau, amyloid-β or α-synuclein spread from cell to cell, in a prion-like manner. Novel evidence suggests that the circulating soluble oligomeric species of these misfolded proteins could play a major role in pathology, while insoluble aggregates would represent their protective less toxic counterparts. Recent convincing data support the proposition that the cellular prion protein, PrP^C, act as a toxicity-inducing receptor for amyloid-β oligomers. As a consequence, several studies focused their investigations to the role played by PrP^C in binding other protein aggregates, such as tau and α-synuclein, for its possible common role in mediating toxic signalling. The biological relevance of PrP^C as key ligand and potential mediator of toxicity for multiple proteinaceous aggregated species, prions or PrP^Sc included, could lead to relevant therapeutic implications. Here we describe the structure of PrP^C and the proposed interplay with its pathological counterpart PrP^Sc and then we recapitulate the most recent findings regarding the role of PrP^C in the interaction with aggregated forms of other neurodegeneration-associated proteins.

The Cellular Prion Protein Increases the Uptake and Toxicity of TDP-43 Fibrils

Cytoplasmic aggregation of the primarily nuclear TAR DNA-binding protein 43 (TDP-43) affects neurons in most amyotrophic lateral sclerosis (ALS) and approximately half of frontotemporal lobar degeneration (FTLD) cases. The cellular prion protein, PrP^C, has been recognized as a common receptor and downstream effector of circulating neurotoxic species of several proteins involved in neurodegeneration. Here, capitalizing on our recently adapted TDP-43 real time quaking induced reaction, we set reproducible protocols to obtain standardized preparations of recombinant TDP-43 fibrils. We then exploited two different cellular systems (human SH-SY5Y and mouse N2a neuroblastoma cells) engineered to express low or high PrP^C levels to investigate the link between PrP^C expression on the cell surface and the internalization of TDP-43 fibrils. Fibril uptake was increased in cells overexpressing either human or mouse prion protein. Increased internalization was associated with detrimental consequences in all PrP-overexpressing cell lines but was milder in cells expressing the human form of the prion protein. As described for other amyloids, treatment with TDP-43 fibrils induced a reduction in the accumulation of the misfolded form of PrP^C, PrP^Sc, in cells chronically infected with prions. Our results expand the list of misfolded proteins whose uptake and detrimental effects are mediated by PrP^C, which encompass almost all pathological amyloids involved in neurodegeneration.

Transcription Factor-Based Strategies to Generate Neural Cell Types from Human Pluripotent Stem Cells

In the last years, the use of pluripotent stem cells in studies of human biology has grown exponentially. These cells represent an infinite source for differentiation into several human cell types facilitating the investigation on biological processes, functionality of cells, or diseases mechanisms in relevant human models. In the neurobiology field, pluripotent stem cells have been extensively used to generate the main neuronal and glial cells of the brain. Traditionally, protocols following developmental cues have been applied to pluripotent stem cells to drive differentiation toward different cell lineages; however, these protocols give rise to populations with mixed identities. Interestingly, new protocols applying overexpression of lineage-specific transcription factors (TFs) have emerged and facilitated the generation of highly pure populations of specific subtypes of neurons and glial cells in an easy, reproducible, and rapid manner. In this study, we review protocols based on this strategy to generate excitatory, inhibitory, dopaminergic, and motor neurons as well as astrocytes, oligodendrocytes, and microglia. In addition, we will discuss the main applications for cells generated with these protocols, including disease modeling, drug screening, and mechanistic studies. Finally, we will discuss the advantages and disadvantages of TF-based protocols and present our view of the future in this field.

Structural details of amyloid β oligomers in complex with human prion protein as revealed by solid-state MAS NMR spectroscopy

Human PrP (huPrP) is a high-affinity receptor for oligomeric amyloid β (Aβ) protein aggregates. Binding of Aβ oligomers to membrane-anchored huPrP has been suggested to trigger neurotoxic cell signaling in Alzheimer’s disease, while an N-terminal soluble fragment of huPrP can sequester Aβ oligomers and reduce their toxicity. Synthetic oligomeric Aβ species are known to be heterogeneous, dynamic, and transient, rendering their structural investigation particularly challenging. Here, using huPrP to preserve Aβ oligomers by coprecipitating them into large heteroassemblies, we investigated the conformations of Aβ(1–42) oligomers and huPrP in the complex by solid-state MAS NMR spectroscopy. The disordered N-terminal region of huPrP becomes immobilized in the complex and therefore visible in dipolar spectra without adopting chemical shifts characteristic of a regular secondary structure. Most of the well-defined C-terminal part of huPrP is part of the rigid complex, and solid-state NMR spectra suggest a loss in regular secondary structure in the two C-terminal α-helices. For Aβ(1–42) oligomers in complex with huPrP, secondary chemical shifts reveal substantial β-strand content. Importantly, not all Aβ(1–42) molecules within the complex have identical conformations. Comparison with the chemical shifts of synthetic Aβ fibrils suggests that the Aβ oligomer preparation represents a heterogeneous mixture of β-strand-rich assemblies, of which some have the potential to evolve and elongate into different fibril polymorphs, reflecting a general propensity of Aβ to adopt variable β-strand-rich conformers. Taken together, our results reveal structural changes in huPrP upon binding to Aβ oligomers that suggest a role of the C terminus of huPrP in cell signaling. Trapping Aβ(1–42) oligomers by binding to huPrP has proved to be a useful tool for studying the structure of these highly heterogeneous β-strand-rich assemblies.

Amyloid Oligomers: A Joint Experimental/Computational Perspective on Alzheimer's Disease, Parkinson's Disease, Type II Diabetes, and Amyotrophic Lateral Sclerosis

Protein misfolding and aggregation is observed in many amyloidogenic diseases affecting either the central nervous system or a variety of peripheral tissues. Structural and dynamic characterization of all species along the pathways from monomers to fibrils is challenging by experimental and computational means because they involve intrinsically disordered proteins in most diseases. Yet understanding how amyloid species become toxic is the challenge in developing a treatment for these diseases. Here we review what computer, in vitro, in vivo, and pharmacological experiments tell us about the accumulation and deposition of the oligomers of the (Aβ, tau), α-synuclein, IAPP, and superoxide dismutase 1 proteins, which have been the mainstream concept underlying Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes (T2D), and amyotrophic lateral sclerosis (ALS) research, respectively, for many years.

The Prion-like Properties of Amyloid-beta Peptide and Tau: Is there Any Risk of Transmitting Alzheimer's Disease During Neurosurgical Interventions?

Recent studies have recognized similarities between the peptides involved in the neuropathology of Alzheimer's disease and prions. The Tau protein and the Amyloid β peptide represent the theoretical pillars of Alzheimer's disease development. It is probable that there is a shared mechanism for the transmission of these substances and the prion diseases development; this presumption is based on the presentation of several cases of individuals without risk factors who developed dementia decades after a neurosurgical procedure. This article aims to present the role of Aβ and Tau, which underlie the pathophysiologic mechanisms involved in the AD and their similarities with the prion diseases infective mechanisms by means of the presentation of the available evidence at molecular (in-vitro), animal, and human levels that support the controversy on whether these diseases might be transmitted in neurosurgical interventions, which may constitute a wide public health issue.

Apolipoprotein J in Alzheimer's Disease: Shedding Light on Its Role with Cell Signaling Pathway Perspective and Possible Therapeutic Approaches

Apolipoprotein J (ApoJ), or clusterin, is one of the main apolipoproteins in the brain. It is synthesized and released from astrocytes in a healthy brain, and its expression increases in neurodegenerative disorders. Genetic evidence has suggested an association between ApoJ polymorphism and the risk of Alzheimer's disease (AD)-it is now considered the third main genetic risk factor for late-onset AD. However, the role of ApoJ overexpression in the state of disorder, toxicity, or protection is not yet clear. Since ApoJ plays different roles in AD, we review the function of ApoJ using different cell signaling pathways in AD and outline its paradoxical roles in AD. ApoJ helps in amyloid-beta (Aβ) clearance. Vice versa, ApoJ gene knock-out causes fibrillary Aβ reduction and prevents Aβ-induced neuron cell death. Understanding ApoJ, through various cellular signaling pathways, creates a new perspective on AD's cellular principles. The overall message is that ApoJ can be a valuable tool in controlling AD.

A human relevant mixture of persistent organic pollutants (POPs) and perfluorooctane sulfonic acid (PFOS) enhance nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells

Disruption of neurite outgrowth is a marker for neurotoxicity. Persistent organic pollutants (POPs) are potential developmental neurotoxicants. We investigated their effect on neurite outgrowth in PC12 rat pheochromocytoma cells, in absence or presence of nerve growth factor (NGF), an inducer of neuronal differentiation. Cells were exposed for 72 h to a defined mixture of POPs with chemical composition and concentrations based on blood levels in the Scandinavian population. We also evaluated perfluorooctane sulfonic acid (PFOS) alone, the most abundant compound in the POP mixture. Only higher concentrations of POP mixture reduced tetrazolium salt (MTT) conversion. High-content analysis showed a decrease in cell number, but no changes for nuclear and mitochondrial cellular health parameters. Robust glutathione levels were observed in NGF-differentiated cells. Live imaging, using the IncuCyte ZOOM platform indicated ongoing cell proliferation over time, but slower in presence of NGF. The pollutants did not inhibit neuritogenesis, but rather increased NGF-induced neurite length. PFOS induced neurite outgrowth to about 50 % of the level seen with the POP mixture. Neither the POP mixture nor PFOS affected neurite length in the absence of NGF. Our observations indicate that realistic complex mixtures of environmental pollutants can affect neuronal connectivity via NGF-induced neurite outgrowth.

The role of the cellular prion protein in the uptake and toxic signaling of pathological neurodegenerative aggregates

Neurodegenerative disorders are invariably associated with intra- or extra-cellular deposition of aggregates composed of misfolded insoluble proteins. These deposits composed of tau, amyloid-β or α-synuclein spread from cell to cell, in a prion-like manner. Emerging evidence suggests that the circulating soluble species of these misfolded proteins (usually referred as oligomers) could play a major role in pathology, while insoluble aggregates would represent their protective less toxic counterparts. Convincing data support the hypothesis that the cellular prion protein, PrP^C, act as a toxicity-transducing receptor for amyloid-β oligomers. As a consequence, several studies extended investigations to the role played by PrP^C in binding aggregates of proteins other than Aβ, such as tau and α-synuclein, for its possible common role in mediating toxic signaling. A better characterization of the biological relevance of PrP^C as key ligand and potential mediator of toxicity for multiple proteinaceous aggregated species, prions or PrP^Sc included, would bring relevant therapeutic implications. Here we will first describe the structure of the prion protein and the hypothesized interplay with its pathological counterpart PrP^Sc and then we will recapitulate the most relevant discoveries regarding the role of PrP^C in the interaction with aggregated forms of other neurodegeneration-associated proteins.

Transgenic Overexpression of the Disordered Prion Protein N1 Fragment in Mice Does Not Protect Against Neurodegenerative Diseases Due to Impaired ER Translocation

The structurally disordered N-terminal half of the prion protein (PrP^C) is constitutively released into the extracellular space by an endogenous proteolytic cleavage event. Once liberated, this N1 fragment acts neuroprotective in ischemic conditions and interferes with toxic peptides associated with neurodegenerative diseases, such as amyloid-beta (Aβ) in Alzheimer’s disease. Since analog protective effects of N1 in prion diseases, such as Creutzfeldt-Jakob disease, have not been studied, and given that the protease releasing N1 has not been identified to date, we have generated and characterized transgenic mice overexpressing N1 (TgN1). Upon intracerebral inoculation of TgN1 mice with prions, no protective effects were observed at the levels of survival, clinical course, neuropathological, or molecular assessment. Likewise, primary neurons of these mice did not show protection against Aβ toxicity. Our biochemical and morphological analyses revealed that this lack of protective effects is seemingly due to an impaired ER translocation of the disordered N1 resulting in its cytosolic retention with an uncleaved signal peptide. Thus, TgN1 mice represent the first animal model to prove the inefficient ER translocation of intrinsically disordered domains (IDD). In contrast to earlier studies, our data challenge roles of cytoplasmic N1 as a cell penetrating peptide or as a potent “anti-prion” agent. Lastly, our study highlights both the importance of structured domains in the nascent chain for proteins to be translocated and aspects to be considered when devising novel N1-based therapeutic approaches against neurodegenerative diseases.

PrP is a central player in toxicity mediated by soluble aggregates of neurodegeneration-causing proteins

Neurodegenerative diseases are an enormous public health problem, affecting tens of millions of people worldwide. Nearly all of these diseases are characterized by oligomerization and fibrillization of neuronal proteins, and there is great interest in therapeutic targeting of these aggregates. Here, we show that soluble aggregates of α-synuclein and tau bind to plate-immobilized PrP in vitro and on mouse cortical neurons, and that this binding requires at least one of the same N-terminal sites at which soluble Aβ aggregates bind. Moreover, soluble aggregates of tau, α-synuclein and Aβ cause both functional (impairment of LTP) and structural (neuritic dystrophy) compromise and these deficits are absent when PrP is ablated, knocked-down, or when neurons are pre-treated with anti-PrP blocking antibodies. Using an all-human experimental paradigm involving: (1) isogenic iPSC-derived neurons expressing or lacking PRNP, and (2) aqueous extracts from brains of individuals who died with Alzheimer's disease, dementia with Lewy bodies, and Pick's disease, we demonstrate that Aβ, α-synuclein and tau are toxic to neurons in a manner that requires PrPC. These results indicate that PrP is likely to play an important role in a variety of late-life neurodegenerative diseases and that therapeutic targeting of PrP, rather than individual disease proteins, may have more benefit for conditions which involve the aggregation of more than one protein.

Neuronal apolipoprotein E4 increases cell death and phosphorylated tau release in alzheimer disease

OBJECTIVE

The apolipoprotein E (APOE) E4 isoform is the strongest genetic risk factor for sporadic Alzheimer disease (AD). Although APOE is predominantly expressed by astrocytes in the central nervous system, neuronal expression of APOE is of increasing interest in age-related cognitive impairment, neurological injury, and neurodegeneration. Here, we show that endogenous expression of E4 in stem-cell-derived neurons predisposes them to injury and promotes the release of phosphorylated tau.

METHODS

Induced pluripotent stem cells from 2 unrelated AD patients carrying the E4 allele were corrected to the E3/E3 genotype with the CRISPR/Cas9 system and differentiated into pure cultures of forebrain excitatory neurons without contamination from other cells types.

RESULTS

Compared to unedited E4 neurons, E3 neurons were less susceptible to ionomycin-induced cytotoxicity. Biochemically, E4 cells exhibited increased tau phosphorylation and ERK1/2 phosphoactivation. Moreover, E4 neurons released increased amounts of phosphorylated tau extracellularly in an isoform-dependent manner by a heparin sulfate proteoglycan-dependent mechanism.

INTERPRETATION

Our results demonstrate that endogenous expression of E4 by stem-cell-derived forebrain excitatory neurons predisposes neurons to calcium dysregulation and ultimately cell death. This change is associated with increased cellular tau phosphorylation and markedly enhanced release of phosphorylated tau. Importantly, these effects are independent of glial APOE. These findings suggest that E4 accelerates spreading of tau pathology and neuron death in part by neuron-specific, glia-independent mechanisms. Ann Neurol 2019;85:726-739.