A Neuronal Culture System to Detect Prion Synaptotoxicity

Prion protein pathology in Ubiquilin 2 models of ALS

Mutations in UBQLN2 cause ALS and frontotemporal dementia (FTD). The pathological signature in UBQLN2 cases is deposition of highly unusual types of inclusions in the brain and spinal cord that stain positive for UBQLN2. However, what role these inclusions play in pathogenesis remains unclear. Here we show cellular prion protein (PrPC) is found in UBQLN2 inclusions in both mouse and human neuronal induced pluripotent (IPSC) models of UBQLN2 mutations, evidenced by the presence of aggregated forms of PrPC with UBQLN2 inclusions. Turnover studies indicated that the P497H UBQLN2 mutation slows PrPC protein degradation and leads to mislocalization of PrPC in the cytoplasm. Immunoprecipitation studies indicated UBQLN2 and PrPC bind together in a complex. The abnormalities in PrPC caused by UBQLN2 mutations may be relevant in disease pathogenesis.

α-Synuclein strain propagation is independent of cellular prion protein expression in a transgenic synucleinopathy mouse model

The cellular prion protein, PrPC, has been postulated to function as a receptor for α-synuclein, potentially facilitating cell-to-cell spreading and/or toxicity of α-synuclein aggregates in neurodegenerative disorders such as Parkinson's disease. Previously, we generated the "Salt (S)" and "No Salt (NS)" strains of α-synuclein aggregates that cause distinct pathological phenotypes in M83 transgenic mice overexpressing A53T-mutant human α-synuclein. To test the hypothesis that PrPC facilitates the propagation of α-synuclein aggregates, we produced M83 mice that either express or do not express PrPC. Following intracerebral inoculation with the S or NS strain, the absence of PrPC in M83 mice did not prevent disease development and had minimal influence on α-synuclein strain-specified attributes such as the extent of cerebral α-synuclein deposition, selective targeting of specific brain regions and cell types, the morphology of induced α-synuclein deposits, and the structural fingerprints of protease-resistant α-synuclein aggregates. Likewise, there were no appreciable differences in disease manifestation between PrPC-expressing and PrPC-lacking M83 mice following intraperitoneal inoculation of the S strain. Interestingly, intraperitoneal inoculation with the NS strain resulted in two distinct disease phenotypes, indicative of α-synuclein strain evolution, but this was also independent of PrPC expression. Overall, these results suggest that PrPC plays at most a minor role in the propagation, neuroinvasion, and evolution of α-synuclein strains in mice that express A53T-mutant human α-synuclein. Thus, other putative receptors or cell-to-cell propagation mechanisms may have a larger effect on the spread of α-synuclein aggregates during disease.

Simple 3D spheroid cell culture model for studies of prion infection

Mouse neuronal CAD 5 cell line effectively propagates various strains of prions. Previously, we have shown that it can also be differentiated into the cells morphologically resembling neurons. Here, we demonstrate that CAD 5 cells chronically infected with prions undergo differentiation under the same conditions. To make our model more realistic, we triggered the differentiation in the 3D culture created by gentle rocking of CAD 5 cell suspension. Spheroids formed within 1 week and were fully developed in less than 3 weeks of culture. The mature spheroids had a median size of ~300 μm and could be cultured for up to 12 weeks. Increased expression of differentiation markers GAP 43, tyrosine hydroxylase, β-III-tubulin and SNAP 25 supported the differentiated status of the spheroid cells. The majority of them were found in the G0/G1 phase of the cell cycle, which is typical for differentiated cells. Moreover, half of the PrPC on the cell membrane was N-terminally truncated, similarly as in differentiated CAD 5 adherent cells. Finally, we demonstrated that spheroids could be created from prion-infected CAD 5 cells. The presence of prions was verified by immunohistochemistry, western blot and seed amplification assay. We also confirmed that the spheroids can be infected with the prions de novo. Our 3D culture model of differentiated CAD 5 cells is low cost, easy to produce and cultivable for weeks. We foresee its possible use in the testing of anti-prion compounds and future studies of prion formation dynamics.

The importance of prion research

Over the past four decades, prion diseases have received considerable research attention owing to their potential to be transmitted within and across species as well as their consequences for human and animal health. The unprecedented nature of prions has led to the discovery of a paradigm of templated protein misfolding that underlies a diverse range of both disease-related and normal biological processes. Indeed, the "prion-like" misfolding and propagation of protein aggregates is now recognized as a common underlying disease mechanism in human neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and the prion principle has led to the development of novel diagnostic and therapeutic strategies for these illnesses. Despite these advances, research into the fundamental biology of prion diseases has declined, likely due to their rarity and the absence of an acute human health crisis. Given the past translational influence, continued research on the etiology, pathogenesis, and transmission of prion disease should remain a priority. In this review, we highlight several important "unsolved mysteries" in the prion disease research field and how solving them may be crucial for the development of effective therapeutics, preventing future outbreaks of prion disease, and understanding the pathobiology of more common human neurodegenerative disorders.

Sigma Receptor Ligands Are Potent Antiprion Compounds that Act Independently of Sigma Receptor Binding

Prion diseases are invariably fatal neurodegenerative diseases of humans and other animals for which there are no effective treatment options. Previous work from our laboratory identified phenethylpiperidines as a novel class of anti-prion compounds. While working to identify the molecular target(s) of these molecules, we unexpectedly discovered ten novel antiprion compounds based on their known ability to bind to the sigma receptors, σ1R and σ2R, which are currently being tested as therapeutic or diagnostic targets for cancer and neuropsychiatric disorders. Surprisingly, however, knockout of the respective genes encoding σ1R and σ2R (Sigmar1 and Tmem97) in prion-infected N2a cells did not alter the antiprion activity of these compounds, demonstrating that these receptors are not the direct targets responsible for the antiprion effects of their ligands. Further investigation of the most potent molecules established that they are efficacious against multiple prion strains and protect against downstream prion-mediated synaptotoxicity. While the precise details of the mechanism of action of these molecules remain to be determined, the present work forms the basis for further investigation of these compounds in preclinical studies. Given the therapeutic utility of several of the tested compounds, including rimcazole and haloperidol for neuropsychiatric conditions, (+)-pentazocine for neuropathic pain, and the ongoing clinical trials of SA 4503 and ANAVEX2-73 for ischemic stroke and Alzheimer's disease, respectively, this work has immediate implications for the treatment of human prion disease.

Strain-Specific Targeting and Destruction of Cells by Prions

Prion diseases are caused by the disease-specific self-templating infectious conformation of the host-encoded prion protein, PrPSc. Prion strains are operationally defined as a heritable phenotype of disease under controlled conditions. One of the hallmark phenotypes of prion strain diversity is tropism within and between tissues. A defining feature of prion strains is the regional distribution of PrPSc in the CNS. Additionally, in both natural and experimental prion disease, stark differences in the tropism of prions in secondary lymphoreticular system tissues occur. The mechanism underlying prion tropism is unknown; however, several possible hypotheses have been proposed. Clinical target areas are prion strain-specific populations of neurons within the CNS that are susceptible to neurodegeneration following the replication of prions past a toxic threshold. Alternatively, the switch from a replicative to toxic form of PrPSc may drive prion tropism. The normal form of the prion protein, PrPC, is required for prion formation. More recent evidence suggests that it can mediate prion and prion-like disease neurodegeneration. In vitro systems for prion formation have indicated that cellular cofactors contribute to prion formation. Since these cofactors can be strain specific, this has led to the hypothesis that the distribution of prion formation cofactors can influence prion tropism. Overall, there is evidence to support several mechanisms of prion strain tropism; however, a unified theory has yet to emerge.

Sigma receptor ligands are potent anti-prion compounds that act independently of sigma receptor binding

Prion diseases are invariably fatal neurodegenerative diseases of humans and other animals for which there are no treatment options. Previous work from our laboratory identified phenethyl piperidines as novel class of anti-prion compounds. While working to identify the molecular target(s) of these molecules, we unexpectedly discovered ten novel anti-prion compounds based on their known ability to bind to the sigma receptors, σ 1 R and 2 R, which are currently being tested as therapeutic or diagnostic targets for cancer and neuropsychiatric disorders. Surprisingly, however, knockout of the respective genes encoding σ 1 R and σ 2 R ( Sigmar1 and Tmem97 ), in prion infected N2a cells did not alter the anti-prion activity of these compounds, demonstrating that these receptors are not the direct targets responsible the anti-prion effects of their ligands. Further investigation of the most potent molecules established that they are efficacious against multiple prion strains and protect against downstream prion-mediated synaptotoxicity. While the precise details of the mechanism of action of these molecules remains to be determined, the present work forms the basis for further investigations of these compounds in pre-clinical studies. Given the therapeutic utility of several of the tested compounds, including rimcazole and haloperidol for neuropsychiatric conditions, (+)-pentazocine for neuropathic pain, and the ongoing clinical trials of SA 4503 and ANAVEX2-73 for ischemic stroke and Alzheimer's disease, respectively, this work has immediate implications for the treatment of human prion disease.

PHB2 Alleviates Neurotoxicity of Prion Peptide PrP106-126 via PINK1/Parkin-Dependent Mitophagy

Prion diseases are a group of neurodegenerative diseases characterized by mitochondrial dysfunction and neuronal death. Mitophagy is a selective form of macroautophagy that clears injured mitochondria. Prohibitin 2 (PHB2) has been identified as a novel inner membrane mitophagy receptor that mediates mitophagy. However, the role of PHB2 in prion diseases remains unclear. In this study, we isolated primary cortical neurons from rats and used the neurotoxic prion peptide PrP106-126 as a cell model for prion diseases. We examined the role of PHB2 in PrP106-126-induced mitophagy using Western blotting and immunofluorescence microscopy and assessed the function of PHB2 in PrP106-126-induced neuronal death using the cell viability assay and the TUNEL assay. The results showed that PrP106-126 induced mitochondrial morphological abnormalities and mitophagy in primary cortical neurons. PHB2 was found to be indispensable for PrP106-126-induced mitophagy and was involved in the accumulation of PINK1 and recruitment of Parkin to mitochondria in primary neurons. Additionally, PHB2 depletion exacerbated neuronal cell death induced by PrP106-126, whereas the overexpression of PHB2 alleviated PrP106-126 neuronal toxicity. Taken together, this study demonstrated that PHB2 is indispensable for PINK1/Parkin-mediated mitophagy in PrP106-126-treated neurons and protects neurons against the neurotoxicity of the prion peptide.

Temporary alteration of neuronal network communication is a protective response to redox imbalance that requires GPI-anchored prion protein

Cellular prion protein (PrP^C) protects neurons against oxidative stress damage. This role is lost upon its misfolding into insoluble prions in prion diseases, and correlated with cytoskeletal breakdown and neurophysiological deficits. Here we used mouse neuronal models to assess how PrP^C protects the neuronal cytoskeleton, and its role in network communication, from oxidative stress damage. Oxidative stress was induced extrinsically by potassium superoxide (KO₂) or intrinsically by Mito-Paraquat (MtPQ), targeting the mitochondria. In mouse neural lineage cells, KO₂ was damaging to the cytoskeleton, with cells lacking PrP^C (PrP^-/-) damaged more than wild-type (WT) cells. In hippocampal slices, KO₂ acutely inhibited neuronal communication in WT controls without damaging the cytoskeleton. This inhibition was not observed in PrP^-/- slices. Neuronal communication and the cytoskeleton of PrP^-/- slices became progressively disrupted and degenerated post-recovery, whereas the dysfunction in WT slices recovered in 5 days. This suggests that the acute inhibition of neuronal activity in WT slices in response to KO₂ was a neuroprotective role of PrP^C, which PrP^-/- slices lacked. Heterozygous expression of PrP^C was sufficient for this neuroprotection. Further, hippocampal slices from mice expressing PrP^C without its GPI anchor (PrP^GPI-/-) displayed acute inhibition of neuronal activity by KO₂. However, they failed to restore normal activity and cytoskeletal formation post-recovery. This suggests that PrP^C facilitates the depressive response to KO₂ and its GPI anchoring is required to restore KO₂-induced damages. Immuno spin-trapping showed increased radicals formed on the filamentous actin of PrP^-/- and PrP^GPI-/- slices, but not WT and PrP^+/- slices, post-recovery suggesting ongoing dysregulation of redox balance in the slices lacking GPI-anchored PrP^C. The MtPQ treatment of hippocampal slices temporarily inhibited neuronal communication independent of PrP^C expression. Overall, GPI-anchored PrP^C alters synapses and neurotransmission to protect and repair the neuronal cytoskeleton, and neuronal communication, from extrinsically induced oxidative stress damages.

Prions induce an early Arc response and a subsequent reduction in mGluR5 in the hippocampus

Synapse dysfunction and loss are central features of neurodegenerative diseases, caused in part by the accumulation of protein oligomers. Amyloid-β, tau, prion, and α-synuclein oligomers bind to the cellular prion protein (PrPC), resulting in the activation of macromolecular complexes and signaling at the post-synapse, yet the early signaling events are unclear. Here we sought to determine the early transcript and protein alterations in the hippocampus during the pre-clinical stages of prion disease. We used a transcriptomic approach focused on the early-stage, prion-infected hippocampus of male wild-type mice, and identify immediate early genes, including the synaptic activity response gene, Arc/Arg3.1, as significantly upregulated. In a longitudinal study of male, prion-infected mice, Arc/Arg-3.1 protein was increased early (40% of the incubation period), and by mid-disease (pre-clinical), phosphorylated AMPA receptors (pGluA1-S845) were increased and metabotropic glutamate receptors (mGluR5 dimers) were markedly reduced in the hippocampus. Notably, sporadic Creutzfeldt-Jakob disease (sCJD) post-mortem cortical samples also showed low levels of mGluR5 dimers. Together, these findings suggest that prions trigger an early Arc response, followed by an increase in phosphorylated GluA1 and a reduction in mGluR5 receptors.

Mechanisms of prion-induced toxicity

Prion diseases are devastating neurodegenerative diseases caused by the structural conversion of the normally benign prion protein (PrP^C) to an infectious, disease-associated, conformer, PrP^Sc. After decades of intense research, much is known about the self-templated prion conversion process, a phenomenon which is now understood to be operative in other more common neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In this review, we provide the current state of knowledge concerning a relatively poorly understood aspect of prion diseases: mechanisms of neurotoxicity. We provide an overview of proposed functions of PrP^C and its interactions with other extracellular proteins in the central nervous system, in vivo and in vitro models used to delineate signaling events downstream of prion propagation, the application of omics technologies, and the emerging appreciation of the role played by non-neuronal cell types in pathogenesis.

Recent advances in cellular models for discovering prion disease therapeutics

INTRODUCTION

Prion diseases are a group of rare and lethal rapidly progressive neurodegenerative diseases arising due to conversion of the physiological cellular prion protein into its pathological counterparts, denoted as "prions". These agents are resistant to inactivation by standard decontamination procedures and can be transmitted between individuals, consequently driving the irreversible brain damage typical of the diseases.

AREAS COVERED

Since its infancy, prion research has mainly depended on animal models for untangling the pathogenesis of the disease as well as for the drug development studies. With the advent of prion-infected cell lines, relevant animal models have been complemented by a variety of cell-based models presenting a much faster, ethically acceptable alternative.

EXPERT OPINION

To date, there are still either no effective prophylactic regimens or therapies for human prion diseases. Therefore, there is an urgent need for more relevant cellular models that best approximate in vivo models. Each cellular model presented and discussed in detail in this review has its own benefits and limitations. Once embarking in a drug screening campaign for the identification of molecules that could interfere with prion conversion and replication, one should carefully consider the ideal cellular model.

Central Residues in Prion Protein PrPC Are Crucial for Its Conversion into the Pathogenic Isoform

Conformational conversion of the cellular prion protein, PrP^C, into the amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the conversion mechanism remains to be elucidated. Here, we generated Tg(PrPΔ91-106)-8545/Prnp^0/0 mice, which overexpress mouse PrP lacking residues 91-106. We showed that none of the mice became sick after intracerebral inoculation with RML, 22L, and FK-1 prion strains nor accumulated PrP^ScΔ91-106 in their brains except for a small amount of PrP^ScΔ91-106 detected in one 22L-inoculated mouse. However, they developed disease around 85 days after inoculation with bovine spongiform encephalopathy (BSE) prions with PrP^ScΔ91-106 in their brains. These results suggest that residues 91-106 are important for PrP^C conversion into PrP^Sc in infection with RML, 22L, and FK-1 prions but not BSE prions. We then narrowed down the residues 91-106 by transducing various PrP deletional mutants into RML- and 22L-infected cells and identified that PrP mutants lacking residues 97-99 failed to convert into PrP^Sc in these cells. Our in vitro conversion assay also showed that RML, 22L, and FK-1 prions did not convert PrPΔ97-99 into PrP^ScΔ97-99, but BSE prions did. We further found that PrP mutants with proline residues at positions 97 to 99 or charged residues at positions 97 and 99 completely or almost completely lost their converting activity into PrP^Sc in RML- and 22L-infected cells. These results suggest that the structurally flexible and noncharged residues 97-99 could be important for PrP^C conversion into PrP^Sc following infection with RML, 22L, and FK-1 prions but not BSE prions.

A high-content neuron imaging assay demonstrates inhibition of prion disease-associated neurotoxicity by an anti-prion protein antibody

There is an urgent need to develop disease-modifying therapies to treat neurodegenerative diseases which pose increasing challenges to global healthcare systems. Prion diseases, although rare, provide a paradigm to study neurodegenerative dementias as similar disease mechanisms involving propagation and spread of multichain assemblies of misfolded protein ("prion-like" mechanisms) are increasingly recognised in the commoner conditions such as Alzheimer's disease. However, studies of prion disease pathogenesis in mouse models showed that prion propagation and neurotoxicity can be mechanistically uncoupled and in vitro assays confirmed that highly purified prions are indeed not directly neurotoxic. To aid development of prion disease therapeutics we have therefore developed a cell-based assay for the specific neurotoxicity seen in prion diseases rather than to simply assess inhibition of prion propagation. We applied this assay to examine an anti-prion protein mouse monoclonal antibody (ICSM18) known to potently cure prion-infected cells and to delay onset of prion disease in prion-infected mice. We demonstrate that whilst ICSM18 itself lacks inherent neurotoxicity in this assay, it potently blocks prion disease-associated neurotoxicity.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Neurons and Astrocytes Elicit Brain Region Specific Transcriptional Responses to Prion Disease in the Murine CA1 and Thalamus

Progressive dysfunction and loss of neurons ultimately culminates in the symptoms and eventual fatality of prion disease, yet the pathways and mechanisms that lead to neuronal degeneration remain elusive. Here, we used RNAseq to profile transcriptional changes in microdissected CA1 and thalamus brain tissues from prion infected mice. Numerous transcripts were altered during clinical disease, whereas very few transcripts were reliably altered at pre-clinical time points. Prion altered transcripts were assigned to broadly defined brain cell types and we noted a strong transcriptional signature that was affiliated with reactive microglia and astrocytes. While very few neuronal transcripts were common between the CA1 and thalamus, we described transcriptional changes in both regions that were related to synaptic dysfunction. Using transcriptional profiling to compare how different neuronal populations respond during prion disease may help decipher mechanisms that lead to neuronal demise and should be investigated with greater detail.

Fibrinogen Mitigates Prion-Mediated Platelet Activation and Neuronal Cell Toxicity

Prion peptide (PrP) misfolds to infectious scrapie isoform, the β pleat-rich insoluble fibrils responsible for neurodegeneration and fatal conformational diseases in humans. The amino acid sequence 106–126 from prion proteins, PrP(106–126), is highly amyloidogenic and implicated in prion-induced pathologies. Here, we report a novel interaction between PrP(106–126) and the thrombogenic plasma protein fibrinogen that can lead to mitigation of prion-mediated pro-thrombotic responses in human platelets as well as significant decline in neuronal toxicity. Thus, prior exposure to fibrinogen-restrained PrP-induced rise in cytosolic calcium, calpain activation, and shedding of extracellular vesicles in platelets while it, too, averted cytotoxicity of neuronal cells triggered by prion peptide. Interestingly, PrP was found to accelerate fibrin-rich clot formation, which was resistant to plasmin-mediated fibrinolysis, consistent with enhanced thrombus stability provoked by PrP. We propose that PrP-fibrinogen interaction can be clinically exploited further for prevention and management of infectious prion related disorders. Small molecules or peptides mimicking PrP-binding sites on fibrinogen can potentially mitigate PrP-induced cellular toxicity while also preventing the negative impact of PrP on fibrin clot formation and lysis.

The protease-sensitive N-terminal polybasic region of prion protein modulates its conversion to the pathogenic prion conformer

Conversion of normal prion protein (PrP^C) to the pathogenic PrP^Sc conformer is central to prion diseases such as Creutzfeldt-Jakob disease and scrapie; however, the detailed mechanism of this conversion remains obscure. To investigate how the N-terminal polybasic region of PrP (NPR) influences the PrP^C-to-PrP^Sc conversion, we analyzed two PrP mutants: ΔN6 (deletion of all six amino acids in NPR) and Met4-1 (replacement of four positively charged amino acids in NPR with methionine). We found that ΔN6 and Met4-1 differentially impacted the binding of recombinant PrP (recPrP) to the negatively charged phospholipid 1-palmitoyl-2-oleoylphosphatidylglycerol, a nonprotein cofactor that facilitates PrP conversion. Both mutant recPrPs were able to form recombinant prion (recPrP^Sc) in vitro, but the convertibility was greatly reduced, with ΔN6 displaying the lowest convertibility. Prion infection assays in mammalian RK13 cells expressing WT or NPR-mutant PrPs confirmed these differences in convertibility, indicating that the NPR affects the conversion of both bacterially expressed recPrP and post-translationally modified PrP in eukaryotic cells. We also found that both WT and mutant recPrP^Sc conformers caused prion disease in WT mice with a 100% attack rate, but the incubation times and neuropathological changes caused by two recPrP^Sc mutants were significantly different from each other and from that of WT recPrP^Sc. Together, our results support that the NPR greatly influences PrP^C-to-PrP^Sc conversion, but it is not essential for the generation of PrP^Sc. Moreover, the significant differences between ΔN6 and Met4-1 suggest that not only charge but also the identity of amino acids in NPR is important to PrP conversion.

The aminoglycoside G418 hinders de novo prion infection in cultured cells

The study of prions and the discovery of candidate therapeutics for prion disease have been facilitated by the ability of prions to replicate in cultured cells. Paradigms in which prion proteins from different species are expressed in cells with low or no expression of endogenous prion protein (PrP) have expanded the range of prion strains that can be propagated. In these systems, cells stably expressing a PrP of interest are typically generated via coexpression of a selectable marker and treatment with an antibiotic. Here, we report the unexpected discovery that the aminoglycoside G418 (Geneticin) interferes with the ability of stably transfected cultured cells to become infected with prions. In G418-resistant lines of N2a or CAD5 cells, the presence of G418 reduced levels of protease-resistant PrP following challenge with the RML or 22L strains of mouse prions. G418 also interfered with the infection of cells expressing hamster PrP with the 263K strain of hamster prions. Interestingly, G418 had minimal to no effect on protease-resistant PrP levels in cells with established prion infection, arguing that G418 selectively interferes with de novo prion infection. As G418 treatment had no discernible effect on cellular PrP levels or its localization, this suggests that G418 may specifically target prion assemblies or processes involved in the earliest stages of prion infection.

Aβ receptors specifically recognize molecular features displayed by fibril ends and neurotoxic oligomers

Several cell-surface receptors for neurotoxic forms of amyloid-β (Aβ) have been described, but their molecular interactions with Aβ assemblies and their relative contributions to mediating Alzheimer's disease pathology have remained uncertain. Here, we used super-resolution microscopy to directly visualize Aβ-receptor interactions at the nanometer scale. We report that one documented Aβ receptor, PrPC, specifically inhibits the polymerization of Aβ fibrils by binding to the rapidly growing end of each fibril, thereby blocking polarized elongation at that end. PrPC binds neurotoxic oligomers and protofibrils in a similar fashion, suggesting that it may recognize a common, end-specific, structural motif on all of these assemblies. Finally, two other Aβ receptors, FcγRIIb and LilrB2, affect Aβ fibril growth in a manner similar to PrPC. Our results suggest that receptors may trap Aβ oligomers and protofibrils on the neuronal surface by binding to a common molecular determinant on these assemblies, thereby initiating a neurotoxic signal.

AKR1B10 negatively regulates autophagy through reducing GAPDH upon glucose starvation in colon cancer

Autophagy is considered to be an important switch for facilitating normal to malignant cell transformation during colorectal cancer development. Consistent with other reports, we found that the membrane receptor Neuropilin1 (NRP1) is greatly upregulated in colon cancer cells that underwent autophagy upon glucose deprivation. However, the mechanism underlying NRP1 regulation of autophagy is unknown. We found that knockdown of NRP1 inhibits autophagy and largely upregulates the expression of aldo-keto reductase family 1 B10 (AKR1B10). Moreover, we demonstrated that AKR1B10 interacts with and inhibits the nuclear importation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and then subsequently represses autophagy. Interestingly, we also found that an NADPH-dependent reduction reaction could be induced when AKR1B10 interacts with GAPDH, and the reductase activity of AKR1B10 is important for its repression of autophagy. Together, our findings unravel a novel mechanism of NRP1 in regulating autophagy through AKR1B10.

G392E neuroserpin causing the dementia FENIB is secreted from cells but is not synaptotoxic

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a progressive neurodegenerative disease caused by point mutations in the gene for neuroserpin, a serine protease inhibitor of the nervous system. Different mutations are known that are responsible for mutant neuroserpin polymerization and accumulation as inclusion bodies in many cortical and subcortical neurons, thereby leading to cell death, dementia and epilepsy. Many efforts have been undertaken to elucidate the molecular pathways responsible for neuronal death. Most investigations have concentrated on analysis of intracellular mechanisms such as endoplasmic reticulum (ER) stress, ER-associated protein degradation (ERAD) and oxidative stress. We have generated a HEK-293 cell model of FENIB by overexpressing G392E-mutant neuroserpin and in this study we examine trafficking and toxicity of this polymerogenic variant. We observed that a small fraction of mutant neuroserpin is secreted via the ER-to-Golgi pathway, and that this release can be pharmacologically regulated. Overexpression of the mutant form of neuroserpin did not stimulate cell death in the HEK-293 cell model. Finally, when treating primary hippocampal neurons with G392E neuroserpin polymers, we did not detect cytotoxicity or synaptotoxicity. Altogether, we report here that a polymerogenic mutant form of neuroserpin is secreted from cells but is not toxic in the extracellular milieu.

Non-cell autonomous astrocyte-mediated neuronal toxicity in prion diseases

Under normal conditions, astrocytes perform a number of important physiological functions centered around neuronal support and synapse maintenance. In neurodegenerative diseases including Alzheimer’s, Parkinson’s and prion diseases, astrocytes acquire reactive phenotypes, which are sustained throughout the disease progression. It is not known whether in the reactive states associated with prion diseases, astrocytes lose their ability to perform physiological functions and whether the reactive states are neurotoxic or, on the contrary, neuroprotective. The current work addresses these questions by testing the effects of reactive astrocytes isolated from prion-infected C57BL/6J mice on primary neuronal cultures. We found that astrocytes isolated at the clinical stage of the disease exhibited reactive, pro-inflammatory phenotype, which also showed downregulation of genes involved in neurogenic and synaptogenic functions. In astrocyte-neuron co-cultures, astrocytes from prion-infected animals impaired neuronal growth, dendritic spine development and synapse maturation. Toward examining the role of factors secreted by reactive astrocytes, astrocyte-conditioned media was found to have detrimental effects on neuronal viability and synaptogenic functions via impairing synapse integrity, and by reducing spine size and density. Reactive microglia isolated from prion-infected animals were found to induce phenotypic changes in primary astrocytes reminiscent to those observed in prion-infected mice. In particular, astrocytes cultured with reactive microglia-conditioned media displayed hypertrophic morphology and a downregulation of genes involved in neurogenic and synaptogenic functions. In summary, the current study provided experimental support toward the non-cell autonomous mechanisms behind neurotoxicity in prion diseases and demonstrated that the astrocyte reactive phenotype associated with prion diseases is synaptotoxic.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

Highly infectious prions are not directly neurotoxic

Prions are infectious agents which cause rapidly lethal neurodegenerative diseases in humans and animals following long, clinically silent incubation periods. They are composed of multichain assemblies of misfolded cellular prion protein. While it has long been assumed that prions are themselves neurotoxic, recent development of methods to obtain exceptionally pure prions from mouse brain with maintained strain characteristics, and in which defined structures-paired rod-like double helical fibers-can be definitively correlated with infectivity, allowed a direct test of this assertion. Here we report that while brain homogenates from symptomatic prion-infected mice are highly toxic to cultured neurons, exceptionally pure intact high-titer infectious prions are not directly neurotoxic. We further show that treatment of brain homogenates from prion-infected mice with sodium lauroylsarcosine destroys toxicity without diminishing infectivity. This is consistent with models in which prion propagation and toxicity can be mechanistically uncoupled.

Mutant prion proteins increase calcium permeability of AMPA receptors, exacerbating excitotoxicity

Prion protein (PrP) mutations are linked to genetic prion diseases, a class of phenotypically heterogeneous neurodegenerative disorders with invariably fatal outcome. How mutant PrP triggers neurodegeneration is not known. Synaptic dysfunction precedes neuronal loss but it is not clear whether, and through which mechanisms, disruption of synaptic activity ultimately leads to neuronal death. Here we show that mutant PrP impairs the secretory trafficking of AMPA receptors (AMPARs). Specifically, intracellular retention of the GluA2 subunit results in synaptic exposure of GluA2-lacking, calcium-permeable AMPARs, leading to increased calcium permeability and enhanced sensitivity to excitotoxic cell death. Mutant PrPs linked to different genetic prion diseases affect AMPAR trafficking and function in different ways. Our findings identify AMPARs as pathogenic targets in genetic prion diseases, and support the involvement of excitotoxicity in neurodegeneration. They also suggest a mechanistic explanation for how different mutant PrPs may cause distinct disease phenotypes.

Genetic prion diseases are degenerative brain disorders caused by mutations in the gene encoding the prion protein (PrP). Different PrP mutations cause different diseases, including Creutzfeldt-Jakob disease, fatal familial insomnia and Gerstmann-Sträussler-Scheinker syndrome. How mutant PrP causes neuronal death and how different mutants encode distinct disease phenotypes is not known. Here we show that mutant PrP alters the subunit composition of glutamate AMPA receptors, promoting cell surface exposure of GluA2-lacking, calcium-permeable receptors, ultimately increasing neuronal vulnerability to excitotoxic cell death. We also demonstrate that the underlying molecular mechanism is the formation of a GluA2 subunit-PrP complex which is retained in the neuronal secretory pathway. PrP mutants associated with clinically different genetic prion diseases have distinct effects on GluA2 trafficking, depending on their tendency to misfold and aggregate in different intracellular organelles, indicating a possible contribution of this mechanism to the disease phenotype.

Enhanced phosphorylation of PERK in primary cultured neurons as an autonomous neuronal response to prion infection

Conversion of cellular prion protein (PrP^C) into the pathogenic isoform of prion protein (PrP^Sc) in neurons is one of the key pathophysiological events in prion diseases. However, the molecular mechanism of neurodegeneration in prion diseases has yet to be fully elucidated because of a lack of suitable experimental models for analyzing neuron-autonomous responses to prion infection. In the present study, we used neuron-enriched primary cultures of cortical and thalamic mouse neurons to analyze autonomous neuronal responses to prion infection. PrP^Sc levels in neurons increased over the time after prion infection; however, no obvious neuronal losses or neurite alterations were observed. Interestingly, a finer analysis of individual neurons co-stained with PrP^Sc and phosphorylated protein kinase RNA-activated-like endoplasmic reticulum (ER) kinase (p-PERK), the early cellular response of the PERK-eukaryotic initiation factor 2 (eIF2α) pathway, demonstrated a positive correlation between the number of PrP^Sc granular stains and p-PERK granular stains, in cortical neurons at 21 dpi. Although the phosphorylation of PERK was enhanced in prion-infected cortical neurons, there was no sign of subsequent translational repression of synaptic protein synthesis or activations of downstream unfolded protein response (UPR) in the PERK-eIF2α pathway. These results suggest that PrP^Sc production in neurons induces ER stress in a neuron-autonomous manner; however, it does not fully activate UPR in prion-infected neurons. Our findings provide insights into the autonomous neuronal responses to prion propagation and the involvement of neuron-non-autonomous factor(s) in the mechanisms of neurodegeneration in prion diseases.

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.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

Discrimination of Prion Strain Targeting in the Central Nervous System via Reactive Astrocyte Heterogeneity in CD44 Expression

Prion diseases or transmissible spongiform encephalopathies are fatal, progressive, neurodegenerative, protein-misfolding disorders. Prion diseases may arise spontaneously, be inherited genetically or be acquired by infection and affect a variety of mammalian species including humans. Prion infections in the central nervous system (CNS) cause extensive neuropathology, including abnormal accumulations of misfolded host prion protein, vacuolar change resulting in sponge-like (spongiform) appearance of CNS tissue, neurodegeneration and reactive glial responses. Many different prion agent strains exist and these can differ based on disease duration, clinical signs and the targeting and distribution of the neuropathology in distinct brain areas. Reactive astrocytes are a prominent feature in the prion disease affected CNS as revealed by distinct morphological changes and upregulation of glial fibrillary acidic protein (GFAP). The CD44 antigen is a transmembrane glycoprotein involved in cell-cell interactions, cell adhesion and migration. Here we show that CD44 is also highly expressed in a subset of reactive astrocytes in regions of the CNS targeted by prions. Astrocyte heterogeneity revealed by differential CD44 upregulation occurs coincident with the earliest neuropathological changes during the pre-clinical phase of disease, and is not affected by the route of infection. The expression and distribution of CD44 was compared in brains from a large collection of 15 distinct prion agent strains transmitted to mice of different prion protein (Prnp) genotype backgrounds. Our data show that the pattern of CD44 upregulation observed in the hippocampus in each prion agent strain and host Prnp genotype combination was unique. Many mouse-adapted prion strains and hosts have previously been characterized based on the pattern of the distribution of the spongiform pathology or the misfolded PrP deposition within the brain. Our data show that CD44 expression also provides a reliable discriminatory marker of prion infection with a greater dynamic range than misfolded prion protein deposition, aiding strain identification. Together, our data reveal CD44 as a novel marker to detect reactive astrocyte heterogeneity during CNS prion disease and for enhanced identification of distinct prion agent strains.

The Development of Neuronal Polarity: A Retrospective View

In 1988, Carlos Dotti, Chris Sullivan, and I published a paper on the establishment of polarity by hippocampal neurons in culture, which continues to be frequently cited 30 years later (Dotti et al., 1988). By following individual neurons from the time of plating until they had formed well developed axonal and dendritic arbors, we identified the five stages of development that lead to the mature expression of neuronal polarity. We were surprised to find that, before axon formation, the cells pass through a multipolar phase, in which several, apparently identical short neurites undergo periods of extension and retraction. Then one of these neurites begins a period of prolonged growth, becoming the definitive axon; the remaining neurites subsequently become dendrites. This observation suggested that any of the initial neurites were capable of becoming axons, a hypothesis confirmed by later work. In this Progressions article, I will try to recall the circumstances that led to this work, recapture some of the challenges we faced in conducting these experiments, and consider why some of today's neuroscientists still find this paper relevant.

Replication of multiple system atrophy prions in primary astrocyte cultures from transgenic mice expressing human α-synuclein

Glial cytoplasmic inclusions (GCIs) containing aggregated and hyperphosphorylated α-synuclein are the signature neuropathological hallmark of multiple system atrophy (MSA). Native α-synuclein can adopt a prion conformation that self-propagates and spreads throughout the brain ultimately resulting in neurodegeneration. A growing body of evidence argues that, in addition to oligodendrocytes, astrocytes contain α-synuclein inclusions in MSA and other α-synucleinopathies at advanced stages of disease. To study the role of astrocytes in MSA, we added MSA brain homogenate to primary cultures of astrocytes from transgenic (Tg) mouse lines expressing human α-synuclein. Astrocytes from four Tg lines, expressing either wild-type or mutant (A53T or A30P) human α-synuclein, propagated and accumulated α-synuclein prions. Furthermore, we found that MSA-infected astrocytes formed two morphologically distinct α-synuclein inclusions: filamentous and granular. Both types of cytoplasmic inclusions shared several features characteristic of α-synuclein inclusions in synucleinopathies: hyperphosphorylation preceded by aggregation, ubiquitination, thioflavin S–positivity, and co-localization with p62. Our findings demonstrate that human α-synuclein forms distinct inclusion morphologies and propagates within cultured Tg astrocytes exposed to MSA prions, indicating that α-synuclein expression determines the tropism of inclusion formation in certain cells. Thus, our work may prove useful in elucidating the role of astrocytes in the pathogenic mechanisms that feature in neurodegeneration caused by MSA prions.

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

BACKGROUND

The prion protein (PrP) is known to bind certain soluble aggregates of the amyloid β-protein (Aβ), and two regions of PrP, one centered around residues 19-33, and the other around 87-112, are thought to be particularly important for this interaction. When either of these sequences are grafted into a human IgG the resulting antibodies react with disease-associated PrP conformers, whereas the parental b12 IgG does not.

METHODS

Human antibodies containing grafts of PrP 19-33 or 87-112 were prepared as before (Solforosi et al., 2007) and tested for their ability to recognize synthetic and Alzheimer's disease (AD) brain-derived Aβ. Since aqueous extracts of AD brain contain a complex mixture of active and inactive Aβ species, we also assessed whether PrP-grafted antibodies could protect against neuritotoxicity mediated by AD brain-derived Aβ. For these experiments, human iPSC-derived neurons were grown in 96-well plates at 5000 cells per well and on post-induction day 21, AD brain extracts were added +/- test antibodies. Neurons were imaged for 3 days using an IncuCyte live-cell imaging system, and neurite number and density quantified.

RESULTS

Grafted antibodies bound a significant portion of aggregated Aβ in aqueous AD extracts, but when these antibodies were co-incubated with neurons treated with brain extracts they did not reduce toxicity. By contrast, the PrP fragment N1 did protect against Aβ.

CONCLUSIONS

These results further demonstrate that not all Aβ oligomers are toxic and suggest that PrP derivatives may allow development of agents that differentially recognize toxic and innocuous Aβ aggregates.

Altered Domain Structure of the Prion Protein Caused by Cu2+ Binding and Functionally Relevant Mutations: Analysis by Cross-Linking, MS/MS, and NMR

The cellular isoform of the prion protein (PrP^C) serves as precursor to the infectious isoform (PrP^Sc), and as a cell-surface receptor, which binds misfolded protein oligomers as well as physiological ligands such as Cu²⁺ ions. PrP^C consists of two domains: a flexible N-terminal domain and a structured C-terminal domain. Both the physiological and pathological functions of PrP depend on intramolecular interactions between these two domains, but the specific amino acid residues involved have proven challenging to define. Here, we employ a combination of chemical cross-linking, mass spectrometry, NMR, molecular dynamics simulations, and functional assays to identify residue-level contacts between the N- and C-terminal domains of PrP^C. We also determine how these interdomain contacts are altered by binding of Cu²⁺ ions and by functionally relevant mutations. Our results provide a structural basis for interpreting both the normal and toxic activities of PrP.

Identification of anti-prion drugs and targets using toxicity-based assays

Prion diseases are untreatable and invariably fatal, making the discovery of effective therapeutic interventions a priority. Most candidate molecules have been discovered based on their ability to reduce the levels of PrP^Sc, the infectious form of the prion protein, in cultured neuroblastoma cells. We have employed an alternative assay, based on an abnormal cellular phenotype associated with a mutant prion protein, to discover a novel class of anti-prion compounds, the phenethyl piperidines. Using an assay that monitors the acute toxic effects of PrP^Sc on the synapses of cultured hippocampal neurons, we have identified p38 MAPK as a druggable pharmacological target that is already being pursued for the treatment of other human diseases. Organotypic brain slices, which can propagate prions and mimic several neuropathological features of the disease, have also been used to test inhibitory compounds. An effective anti-prion regimen will involve synergistic combination of drugs acting at multiple steps of the pathogenic process, resulting not only in reduction in prion levels but also suppression of neurotoxic signaling.

Prion neurotoxicity

Although the mechanisms underlying prion propagation and infectivity are now well established, the processes accounting for prion toxicity and pathogenesis have remained mysterious. These processes are of enormous clinical relevance as they hold the key to identification of new molecular targets for therapeutic intervention. In this review, we will discuss two broad areas of investigation relevant to understanding prion neurotoxicity. The first is the use of in vitro experimental systems that model key events in prion pathogenesis. In this context, we will describe a hippocampal neuronal culture system we developed that reproduces the earliest pathological alterations in synaptic morphology and function in response to PrP^Sc . This system has allowed us to define a core synaptotoxic signaling pathway involving the activation of NMDA and AMPA receptors, stimulation of p38 MAPK phosphorylation and collapse of the actin cytoskeleton in dendritic spines. The second area concerns a striking and unexpected phenomenon in which certain structural manipulations of the PrP^C molecule itself, including introduction of N-terminal deletion mutations or binding of antibodies to C-terminal epitopes, unleash powerful toxic effects in cultured cells and transgenic mice. We will describe our studies of this phenomenon, which led to the recognition that it is related to the induction of large, abnormal ionic currents by the structurally altered PrP molecules. Our results suggest a model in which the flexible N-terminal domain of PrP^C serves as a toxic effector which is regulated by intramolecular interactions with the globular C-terminal domain. Taken together, these two areas of study have provided important clues to underlying cellular and molecular mechanisms of prion neurotoxicity. Nevertheless, much remains to be done on this next frontier of prion science.

Prions activate a p38 MAPK synaptotoxic signaling pathway

Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrP^Sc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrP^C by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrP^Sc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrP^Sc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrP^C mediates the synaptotoxic effects of both PrP^Sc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.

Prion diseases are a group of fatal neurodegenerative disorders that includes Creutzfeldt-Jakob disease and kuru in humans, and bovine spongiform encephalopathy in cattle. The infectious agent, or prion, that transmits these diseases is a naked protein molecule, the prion protein (PrP), which is an altered form of a normal, cellular protein. Although a great deal is known about how prions propagate themselves and transmit infection, the process by which they actually cause neurons to degenerate has remained mysterious. Here, we have used a specialized neuronal culture system to dissect the cellular and molecular mechanisms by which prions damage synapses, the structures that connect nerve cells and that play a crucial role in learning, memory, and neurological disease. Our results define a stepwise molecular pathway underlying prion synaptic toxicity, which involves activation of glutamate neurotransmitter receptors, influx of calcium ions into the neuron, and stimulation of specific mitogen-activated protein kinases, which attach phosphate groups to proteins to regulate their activity. We demonstrate that specific drugs, as well as a dominant-negative kinase mutant, block these steps and thereby prevent the synaptic degeneration produced by prions. Our results provide new insights into the pathogenesis of prion diseases, they uncover new drug targets for treating these diseases, and they allow us to compare prion diseases to other, more common neurodegenerative disorders like Alzheimer’s disease.

Prion acute synaptotoxicity is largely driven by protease-resistant PrPSc species

Although misfolding of normal prion protein (PrP^C) into abnormal conformers (PrP^Sc) is critical for prion disease pathogenesis our current understanding of the underlying molecular pathophysiology is rudimentary. Exploiting an electrophysiology paradigm, herein we report that at least modestly proteinase K (PK)-resistant PrP^Sc (PrP^res) species are acutely synaptotoxic. Brief exposure to ex vivo PrP^Sc from two mouse-adapted prion strains (M1000 and MU02) prepared as crude brain homogenates (cM1000 and cMU02) and cell lysates from chronically M1000-infected RK13 cells (MoRK13-Inf) caused significant impairment of hippocampal CA1 region long-term potentiation (LTP), with the LTP disruption approximating that reported during the evolution of murine prion disease. Proof of PrP^Sc (especially PrP^res) species as the synaptotoxic agent was demonstrated by: significant rescue of LTP following selective immuno-depletion of total PrP from cM1000 (dM1000); modestly PK-treated cM1000 (PK+M1000) retaining full synaptotoxicity; and restoration of the LTP impairment when employing reconstituted, PK-eluted, immuno-precipitated M1000 preparations (PK+IP-M1000). Additional detailed electrophysiological analyses exemplified by impairment of post-tetanic potentiation (PTP) suggest possible heightened pre-synaptic vulnerability to the acute synaptotoxicity. This dysfunction correlated with cumulative insufficiency of replenishment of the readily releasable pool (RRP) of vesicles during repeated high-frequency stimulation utilised for induction of LTP. Broadly comparable results with LTP and PTP impairment were obtained utilizing hippocampal slices from PrP^C knockout (PrPo/o) mice, with cM1000 serial dilution assessments revealing similar sensitivity of PrPo/o and wild type (WT) slices. Size fractionation chromatography demonstrated that synaptotoxic PrP correlated with PK-resistant species >100kDa, consistent with multimeric PrP^Sc, with levels of these species >6 ng/ml appearing sufficient to induce synaptic dysfunction. Biochemical analyses of hippocampal slices manifesting acute synaptotoxicity demonstrated reduced levels of multiple key synaptic proteins, albeit with noteworthy differences in PrPo/o slices, while such changes were absent in hippocampi demonstrating rescued LTP through treatment with dM1000. Our findings offer important new mechanistic insights into the synaptic impairment underlying prion disease, enhancing prospects for development of targeted effective therapies.

Misfolding of the normal prion protein (PrP^C) into disease-associated conformations (PrP^Sc) is the critical initiating step for prion diseases. Similar to other neurodegenerative disorders, progressive failure of brain synapses is considered a primary deleterious event underpinning prion disease evolution. Our current understanding of the underlying mechanisms associated with synaptic failure is rudimentary contributing to difficulties in developing effective treatments. Herein we report the use of an electrophysiology paradigm that allowed us to demonstrate that at least modestly proteinase K (PK)-resistant PrP^Sc species from two mouse-adapted prion strains (M1000 and MU02) are directly synaptotoxic causing significant acute impairment of hippocampal CA1 region long-term potentiation (LTP). Of note, the LTP disruption approximated that reported in prion animal models. Additional detailed analyses provided novel pathophysiological insights suggesting possible heightened pre-synaptic vulnerability to the acute synaptotoxicity through impairment of replenishment of the readily releasable pool of neurotransmitter vesicles, while biochemical analyses demonstrated reduced levels of multiple key pre-and post-synaptic proteins. Broadly similar acute synaptic dysfunction and dose-response susceptibility were observed in slices from mice not expressing PrP^C albeit with minor but noteworthy differences in electrophysiological and biochemical findings. Our study offers important new mechanistic insights into the synaptic impairment underlying prion disease, enhancing prospects for development effective therapies.

Acute Neurotoxicity Models of Prion Disease

Prion diseases are phenotypically diverse, transmissible, neurodegenerative disorders affecting both animals and humans. Misfolding of the normal prion protein (PrP^C) into disease-associated conformers (PrP^Sc) is considered the critical etiological event underpinning prion diseases, with such misfolded isoforms linked to both disease transmission and neurotoxicity. Although important advances in our understanding of prion biology and pathogenesis have occurred over the last 3-4 decades, many fundamental questions remain to be resolved, including consensus regarding the principal pathways subserving neuronal dysfunction, as well as detailed biophysical characterization of PrP^Sc species transmitting disease and/or directly associated with neurotoxicity. In vivo and in vitro models have been, and remain, critical to furthering our understanding across many aspects of prion disease patho-biology. Prion animal models are arguably the most authentic in vivo models of neurodegeneration that exist and have provided valuable and multifarious insights into pathogenesis; however, they are expensive and time-consuming, and it can be problematic to clearly discern evidence of direct PrP^Sc neurotoxicity in the overall context of pathogenesis. In vitro models, in contrast, generally offer greater tractability and appear more suited to assessments of direct acute neurotoxicity but have until recently been relatively simplistic, and overall there remains a relative paucity of validated, biologically relevant models with heightened reliability as far as translational insights, contributing to difficulties in redressing our knowledge gaps in prion disease pathogenesis. In this review, we provide an overview of the spectrum and methodological diversity of in vivo and in vitro models of prion acute toxicity, as well as the pathogenic insights gained from these studies.

DLP1-dependent mitochondrial fragmentation and redistribution mediate prion-associated mitochondrial dysfunction and neuronal death

Mitochondrial malfunction is a universal and critical step in the pathogenesis of many neurodegenerative diseases including prion diseases. Dynamin-like protein 1 (DLP1) is one of the key regulators of mitochondrial fission. In this study, we investigated the role of DLP1 in mitochondrial fragmentation and dysfunction in neurons using in vitro and in vivo prion disease models. Mitochondria became fragmented and redistributed from axons to soma, correlated with increased mitochondrial DLP1 expression in murine primary neurons (N2a cells) treated with the prion peptide PrP^106-126 in vitro as well as in prion strain-infected hamster brain in vivo. Suppression of DLP1 expression by DPL1 RNAi inhibited prion-induced mitochondrial fragmentation and dysfunction (measured by ADP/ATP ratio, mitochondrial membrane potential, and mitochondrial integrity). We also demonstrated that DLP1 RNAi is neuroprotective against prion peptide in N2a cells as shown by improved cell viability and decreased apoptosis markers, caspase 3 induced by PrP^106-126 . On the contrary, overexpression of DLP1 exacerbated mitochondrial dysfunction and cell death. Moreover, inhibition of DLP1 expression ameliorated PrP^106-126 -induced neurite loss and synaptic abnormalities (i.e., loss of dendritic spine and PSD-95, a postsynaptic scaffolding protein as a marker of synaptic plasticity) in primary neurons, suggesting that altered DLP1 expression and mitochondrial fragmentation are upstream events that mediate PrP^106-126 -induced neuron loss and degeneration. Our findings suggest that DLP1-dependent mitochondrial fragmentation and redistribution plays a pivotal role in PrP^Sc -associated mitochondria dysfunction and neuron apoptosis. Inhibition of DLP1 may be a novel and effective strategy in the prevention and treatment of prion diseases.

The function of the cellular prion protein in health and disease

The essential role of the cellular prion protein (PrP^C) in prion disorders such as Creutzfeldt-Jakob disease is well documented. Moreover, evidence is accumulating that PrP^C may act as a receptor for protein aggregates and transduce neurotoxic signals in more common neurodegenerative disorders, such as Alzheimer's disease. Although the pathological roles of PrP^C have been thoroughly characterized, a general consensus on its physiological function within the brain has not yet been established. Knockout studies in various organisms, ranging from zebrafish to mice, have implicated PrP^C in a diverse range of nervous system-related activities that include a key role in the maintenance of peripheral nerve myelination as well as a general ability to protect against neurotoxic stimuli. Thus, the function of PrP^C may be multifaceted, with different cell types taking advantage of unique aspects of its biology. Deciphering the cellular function(s) of PrP^C and the consequences of its absence is not simply an academic curiosity, since lowering PrP^C levels in the brain is predicted to be a powerful therapeutic strategy for the treatment of prion disease. In this review, we outline the various approaches that have been employed in an effort to uncover the physiological and pathological functions of PrP^C. While these studies have revealed important clues about the biology of the prion protein, the precise reason for PrP^C's existence remains enigmatic.

MicroRNA-16 targets mRNA involved in neurite extension and branching in hippocampal neurons during presymptomatic prion disease

The mechanisms that lead to neuronal death in neurodegenerative diseases are poorly understood. Prion diseases, like many more common disorders such as Alzheimer's and Parkinson's diseases, are characterized by the progressive accumulation of misfolded disease-specific proteins. The earliest changes observed in brain tissue include a reduction in synaptic number and retraction of dendritic spines, followed by reduced length and branching of neurites. These pathologies are observable during presymptomatic stages of disease and are accompanied by altered expression of transcripts that include miRNAs. Here we report that miR-16 localized within hippocampal CA1 neurons is increased during early prion disease. Modulating miR-16 expression in mature murine hippocampal neurons by expression from a lentivirus, thus mimicking the modest increase seen in vivo, was found to induce neurodegeneration. This was characterized by retraction of neurites and reduced branching. We performed immunoprecipitation of the miR-16 enriched RISC complex, and identified associated transcripts from the co-immunoprecipitated RNA (Ago2 RIP-Chip). These transcripts were enriched with predicted binding sites for miR-16, including the validated miR-16 targets APP and BCL2, as well as numerous novel targets. In particular, genes within the neurotrophin receptor mediated MAPK/ERK pathway were potentially regulated by miR-16; including TrkB (NTRK2), MEK1 (MAP2K1) and c-Raf (RAF). Increased miR-16 expression in neurons during presymptomatic prion disease and reduction in proteins involved in MAPK/ERK signaling represents a possible mechanism by which neurite length and branching are decreased during early stages of disease.

An inter-domain regulatory mechanism controls toxic activities of PrPC

The normal function of PrP^C, the cellular prion protein, has remained mysterious since its first description over 30 years ago. Amazingly, although complete deletion of the gene encoding PrP^C has little phenotypic consequence, expression in transgenic mice of PrP molecules carrying certain internal deletions produces dramatic neurodegenerative phenotypes. In our recent paper, ¹ we have demonstrated that the flexible, N-terminal domain of PrP^C possesses toxic effector functions, which are regulated by a docking interaction with the structured, C-terminal domain. Disruption of this inter-domain interaction, for example by deletions of the hinge region or by binding of antibodies to the C-terminal domain, results in abnormal ionic currents and degeneration of dendritic spines in cultured neuronal cells. This mechanism may contribute to the neurotoxicity of PrP^Sc and possibly other protein aggregates, and could play a role in the physiological activity of PrP^C. These results also provide a warning about the potential toxic side effects of PrP-directed antibody therapies for prion and Alzheimer's diseases.

In vivo prion models and the disconnection between transmissibility and neurotoxicity

The primary causative event in the development of prion diseases is the misfolding of the normal prion protein (PrP^C) into an ensemble of altered conformers (herein collectively denoted as PrP^Sc) that accumulate in the brain. Prominent amongst currently unresolved key aspects underpinning prion disease pathogenesis is whether transmission and toxicity are sub-served by different molecular species of PrP^Sc, which may directly impact on the development of effective targeted treatments. The use of murine models of prion disease has been of fundamental importance for probing the relationship between hypothesised "neurotoxic" and "transmissible" PrP^Sc and the associated kinetic profiles of their production during disease evolution, but unfortunately consensus has not been achieved. Recent in vivo studies have led to formulation of the "two-phase" hypothesis, which postulates that there is first an exponential increase in transmitting PrP^Sc species followed by an abrupt transition to propagation of neurotoxic PrP^Sc species. Such observations however, appear inconsistent with previous in vivo murine studies employing detailed time-course behavioural testing, wherein evidence of neurotoxicity could be detected early in disease progression. This review analyses the contributions of in vivo murine models attempting to provide insights into the relationship between transmitting and neurotoxic PrP^Sc species and explores possible refinements to the "two-phase hypothesis", that better accommodate the available historical and recent evidence.

Identification of Anti-prion Compounds using a Novel Cellular Assay

Prion diseases are devastating neurodegenerative disorders with no known cure. One strategy for developing therapies for these diseases is to identify compounds that block conversion of the cellular form of the prion protein (PrP^C) into the infectious isoform (PrP^Sc). Most previous efforts to discover such molecules by high-throughput screening methods have utilized, as a read-out, a single kind of cellular assay system: neuroblastoma cells that are persistently infected with scrapie prions. Here, we describe the use of an alternative cellular assay based on suppressing the spontaneous cytotoxicity of a mutant form of PrP (Δ105-125). Using this assay, we screened 75,000 compounds, and identified a group of phenethyl piperidines (exemplified by LD7), which reduces the accumulation of PrP^Sc in infected neuroblastoma cells by >90% at low micromolar doses, and inhibits PrP^Sc-induced synaptotoxicity in hippocampal neurons. By analyzing the structure-activity relationships of 35 chemical derivatives, we defined the pharmacophore of LD7, and identified a more potent derivative. Active compounds do not alter total or cell-surface levels of PrP^C, and do not bind to recombinant PrP in surface plasmon resonance experiments, although at high concentrations they inhibit PrP^Sc-seeded conversion of recombinant PrP to a misfolded state in an in vitro reaction (RT-QuIC). This class of small molecules may provide valuable therapeutic leads, as well as chemical biological tools to identify cellular pathways underlying PrP^Sc metabolism and PrP^C function.