Cytosolic prion protein (PrP) is not toxic in N2a cells and primary neurons expressing pathogenic PrP mutations

Differential interactome mapping of aggregation prone/prion-like proteins under stress: novel links to stress granule biology

BACKGROUND

Aberrant stress granules (SGs) are emerging as prime suspects in the nucleation of toxic protein aggregates. Understanding the molecular networks linked with aggregation-prone proteins (prion protein, synuclein, and tau) under stressful environments is crucial to understand pathophysiological cascades associated with these proteins.

METHODS

We characterized and validated oxidative stress-induced molecular network changes of endogenous aggregation-prone proteins (prion protein, synuclein, and tau) by employing immunoprecipitation coupled with mass spectrometry analysis under basal and oxidative stress conditions. We used two different cell models (SH-SY5Y: human neuroblastoma and HeLa cell line) to induce oxidative stress using a well-known inducer (sodium arsenite) of oxidative stress.

RESULTS

Overall, we identified 597 proteins as potential interaction partners. Our comparative interactome mapping provides comprehensive network reorganizations of three aggregation-prone hallmark proteins, establish novel interacting partners and their dysregulation, and validates that prion protein and synuclein localize in cytoplasmic SGs. Localization of prion protein and synuclein in TIA1-positive SGs provides an important link between SG pathobiology and aggregation-prone proteins. In addition, dysregulation (downregulation) of prion protein and exportin-5 protein, and translocation of exportin-5 into the nucleus under oxidative stress shed light on nucleocytoplasmic transport defects during the stress response.

CONCLUSIONS

The current study contributes to our understanding of stress-mediated network rearrangements and posttranslational modifications of prion/prion-like proteins. Localization of prion protein and synuclein in the cytoplasmic SGs provides an important link between stress granule pathobiology and aggregation-prone proteins. In addition, our findings demonstrate nucleocytoplasmic transport defects after oxidative stress via dysregulation and nuclear accumulation of exportin-5.

A tetracationic porphyrin with dual anti-prion activity

Prions are deadly infectious agents made of PrPSc, a misfolded variant of the cellular prion protein (PrPC) which self-propagates by inducing misfolding of native PrPC. PrPSc can adopt different pathogenic conformations (prion strains), which can be resistant to potential drugs, or acquire drug resistance, hampering the development of effective therapies. We identified Zn(II)-BnPyP, a tetracationic porphyrin that binds to distinct domains of native PrPC, eliciting a dual anti-prion effect. Zn(II)-BnPyP binding to a C-terminal pocket destabilizes the native PrPC fold, hindering conversion to PrPSc; Zn(II)-BnPyP binding to the flexible N-terminal tail disrupts N- to C-terminal interactions, triggering PrPC endocytosis and lysosomal degradation, thus reducing the substrate for PrPSc generation. Zn(II)-BnPyP inhibits propagation of different prion strains in vitro, in neuronal cells and organotypic brain cultures. These results identify a PrPC-targeting compound with an unprecedented dual mechanism of action which might be exploited to achieve anti-prion effects without engendering drug resistance.

Mutant Prion Protein Endoggresomes are Hubs for Local Axonal Organelle-Cytoskeletal Remodeling

Dystrophic axons comprising misfolded mutant prion protein (PrP) aggregates are a characteristic pathological feature in the prionopathies. These aggregates form inside endolysosomes -called endoggresomes-, within swellings that line up the length of axons of degenerating neurons. The pathways impaired by endoggresomes that result in failed axonal and consequently neuronal health, remain undefined. Here, we dissect the local subcellular impairments that occur within individual mutant PrP endoggresome swelling sites in axons. Quantitative high-resolution light and electron microscopy revealed the selective impairment of the acetylated vs tyrosinated microtubule cytoskeleton, while micro-domain image analysis of live organelle dynamics within swelling sites revealed deficits uniquely to the MT-based active transport system that translocates mitochondria and endosomes toward the synapse. Cytoskeletal and defective transport results in the retention of mitochondria, endosomes, and molecular motors at swelling sites, enhancing mitochondria-Rab7 late endosome contacts that induce mitochondrial fission via the activity of Rab7, and render mitochondria dysfunctional. Our findings point to mutant Pr Pendoggresome swelling sites as selective hubs of cytoskeletal deficits and organelle retention that drive the remodeling of organelles along axons. We propose that the dysfunction imparted locally within these axonal micro-domains spreads throughout the axon over time, leading to axonal dysfunction in prionopathies.

The prion protein and its ligands: Insights into structure-function relationships

The prion protein is a multifunctional protein that exists in at least two different folding states. It is subject to diverse proteolytic processing steps that lead to prion protein fragments some of which are membrane-bound whereas others are soluble. A multitude of ligands bind to the prion protein and besides proteinaceous binding partners, interaction with metal ions and nucleic acids occurs. Although of great importance, information on structural and functional consequences of prion protein binding to its partners is limited. Here, we will reflect on the structure-function relationship of the prion protein and its binding partners considering the different folding states and prion protein fragments.

Calcineurin Controls Cellular Prion Protein Expression in Mouse Astrocytes

Prion diseases arise from the conformational conversion of the cellular prion protein (PrP^C) into a self-replicating prion isoform (PrP^Sc). Although this process has been studied mostly in neurons, a growing body of evidence suggests that astrocytes express PrP^C and are able to replicate and accumulate PrP^Sc. Currently, prion diseases remain incurable, while downregulation of PrP^C represents the most promising therapy due to the reduction of the substrate for prion conversion. Here we show that the astrocyte-specific genetic ablation or pharmacological inhibition of the calcium-activated phosphatase calcineurin (CaN) reduces PrP^C expression in astrocytes. Immunocytochemical analysis of cultured CaN-KO astrocytes and isolation of synaptosomal compartments from the hippocampi of astrocyte-specific CaN-KO (ACN-KO) mice suggest that PrP^C is downregulated both in vitro and in vivo. The downregulation occurs without affecting the glycosylation of PrP^C and without alteration of its proteasomal or lysosomal degradation. Direct assessment of the protein synthesis rate and shotgun mass spectrometry proteomics analysis suggest that the reduction of PrP^C is related to the impairment of global protein synthesis in CaN-KO astrocytes. When WT-PrP and PrP-D177N, a mouse homologue of a human mutation associated with the inherited prion disease fatal familial insomnia, were expressed in astrocytes, CaN-KO astrocytes showed an aberrant localization of both WT-PrP and PrP-D177N variants with predominant localization to the Golgi apparatus, suggesting that ablation of CaN affects both WT and mutant PrP proteins. These results provide new mechanistic details in relation to the regulation of PrP expression in astrocytes, suggesting the therapeutic potential of astroglial cells.

Endosomal sorting drives the formation of axonal prion protein endoggresomes

The pathogenic aggregation of misfolded prion protein (PrP) in axons underlies prion disease pathologies. The molecular mechanisms driving axonal misfolded PrP aggregate formation leading to neurotoxicity are unknown. We found that the small endolysosomal guanosine triphosphatase (GTPase) Arl8b recruits kinesin-1 and Vps41 (HOPS) onto endosomes carrying misfolded mutant PrP to promote their axonal entry and homotypic fusion toward aggregation inside enlarged endomembranes that we call endoggresomes. This axonal rapid endosomal sorting and transport-dependent aggregation (ARESTA) mechanism forms pathologic PrP endoggresomes that impair calcium dynamics and reduce neuronal viability. Inhibiting ARESTA diminishes endoggresome formation, rescues calcium influx, and prevents neuronal death. Our results identify ARESTA as a key pathway for the regulation of endoggresome formation and a new actionable antiaggregation target to ameliorate neuronal dysfunction in the prionopathies.

The Role of Cellular Prion Protein in Cancer Biology: A Potential Therapeutic Target

Prion protein has two isoforms including cellular prion protein (PrP^C) and scrapie prion protein (PrP^Sc). PrP^Sc is the pathological aggregated form of prion protein and it plays an important role in neurodegenerative diseases. PrP^C is a glycosylphosphatidylinositol (GPI)-anchored protein that can attach to a membrane. Its expression begins at embryogenesis and reaches the highest level in adulthood. PrP^C is expressed in the neurons of the nervous system as well as other peripheral organs. Studies in recent years have disclosed the involvement of PrP^C in various aspects of cancer biology. In this review, we provide an overview of the current understanding of the roles of PrP^C in proliferation, cell survival, invasion/metastasis, and stem cells of cancer cells, as well as its role as a potential therapeutic target.

Activation of Src family kinase ameliorates secretory trafficking in mutant prion protein cells

Fatal familial insomnia (FFI), genetic Creutzfeldt-Jakob disease (gCJD), and Gerstmann-Sträussler-Scheinker (GSS) syndrome are neurodegenerative disorders linked to prion protein (PrP) mutations. The pathogenic mechanisms are not known, but increasing evidence points to mutant PrP misfolding and retention in the secretory pathway. We previously found that the D178N/M129 mutation associated with FFI accumulates in the Golgi of neuronal cells, impairing post-Golgi trafficking. In this study we further characterized the trafficking defect induced by the FFI mutation and tested the 178N/V129 variant linked to gCJD and a nine-octapeptide repeat insertion associated with GSS. We used transfected HeLa cells, embryonic fibroblasts and primary neurons from transgenic mice, and fibroblasts from carriers of the FFI mutation. In all these cell types, the mutant PrPs showed abnormal intracellular localizations, accumulating in the endoplasmic reticulum (ER) and Golgi. To test the efficiency of the membrane trafficking system, we monitored the intracellular transport of the temperature-sensitive vesicular stomatite virus glycoprotein (VSV-G), a well-established cargo reporter, and of endogenous procollagen I (PC-I). We observed marked alterations in secretory trafficking, with VSV-G accumulating mainly in the Golgi complex and PC-I in the ER and Golgi. A redacted version of mutant PrP with reduced propensity to misfold did not impair VSV-G trafficking, nor did artificial ER or Golgi retention of wild-type PrP; this indicates that both misfolding and intracellular retention were required to induce the transport defect. Pharmacological activation of Src family kinase (SFK) improved intracellular transport, suggesting that mutant PrP impairs secretory trafficking through corruption of SFK-mediated signaling.

Transcriptional signature of prion-induced neurotoxicity in a Drosophila model of transmissible mammalian prion disease

Prion diseases are fatal transmissible neurodegenerative conditions of humans and animals that arise through neurotoxicity induced by PrP misfolding. The cellular and molecular mechanisms of prion-induced neurotoxicity remain undefined. Understanding these processes will underpin therapeutic and control strategies for human and animal prion diseases, respectively. Prion diseases are difficult to study in their natural hosts and require the use of tractable animal models. Here we used RNA-Seq-based transcriptome analysis of prion-exposed Drosophila to probe the mechanism of prion-induced neurotoxicity. Adult Drosophila transgenic for pan neuronal expression of ovine PrP targeted to the plasma membrane exhibit a neurotoxic phenotype evidenced by decreased locomotor activity after exposure to ovine prions at the larval stage. Pathway analysis and quantitative PCR of genes differentially expressed in prion-infected Drosophila revealed up-regulation of cell cycle activity and DNA damage response, followed by down-regulation of eIF2 and mTOR signalling. Mitochondrial dysfunction was identified as the principal toxicity pathway in prion-exposed PrP transgenic Drosophila. The transcriptomic changes we observed were specific to PrP targeted to the plasma membrane since these prion-induced gene expression changes were not evident in similarly treated Drosophila transgenic for cytosolic pan neuronal PrP expression, or in non-transgenic control flies. Collectively, our data indicate that aberrant cell cycle activity, repression of protein synthesis and altered mitochondrial function are key events involved in prion-induced neurotoxicity, and correlate with those identified in mammalian hosts undergoing prion disease. These studies highlight the use of PrP transgenic Drosophila as a genetically well-defined tractable host to study mammalian prion biology.

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.

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.

Photoexcited Toluidine Blue Inhibits Tau Aggregation in Alzheimer's Disease

The aggregates of microtubule-associated protein Tau are considered as a major hallmark of Alzheimer's disease. Tau aggregates accumulate intracellularly leading to neuronal toxicity. Numerous approaches have been targeted against Tau protein aggregation, which include application of synthetic and natural compounds. Toluidine blue is a basic dye of phenothiazine family, which on irradiation with a 630 nm light gets converted into a photoexcited form, leading to generation of singlet oxygen species. Methylene blue is the parent compound of toluidine blue, which has been reported to be potent against tauopathy. In the present work, we studied the potency of toluidine blue and photoexcited toluidine blue against Tau aggregation. Biochemical and biophysical analyses using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, ThS fluorescence, circular dichroism spectroscopy, and electron microscopy suggested that toluidine blue inhibited the aggregation of Tau in vitro. The photoexcited toluidine blue potentially dissolved the matured Tau fibrils, which indicated the disaggregation property of toluidine blue. The cell biology studies including the cytotoxicity assay and reactive oxygen species (ROS) production assay suggested toluidine blue to be a biocompatible dye as it reduced ROS levels and cell death. The photoexcited toluidine blue modulates the cytoskeleton network in cells, which was supported by immunofluorescence studies of neuronal cells. The studies in a UAS Tau E14 transgenic Drosophila model suggested that photoexcited toluidine blue was potent to restore the survival and memory deficits of Drosophila. The overall finding of our studies suggested toluidine blue to be a potent molecule in rescuing the Tau-mediated pathology by inhibiting its aggregation, reducing the cell death, and modulating the tubulin levels and behavioral characteristics of Drosophila. Thus, toluidine blue can be addressed as a potent molecule against Alzheimer's disease.

Cyclophillin A deficiency accelerates RML-induced prion disease

Prion diseases typically involve brain deposition of abnormally folded prion protein, which is associated with activated glia and increased cytokine production. Cyclophilin A (CypA) is a ubiquitous protein with peptidyl prolyl cis-trans isomerase activity, which regulates protein folding, and can be secreted by cells in response to inflammatory stimuli. On the basis of in vitro studies, CypA was proposed to mediate glial activation during prion infection. To investigate the role of CypA in vivo, we inoculated CypA^+/+, CypA^+/- and CypA^-/- mice with the RML prion strain, and recorded the time to onset of neurological signs and to terminal disease, and the astrocyte and microglia response at presymptomatic and symptomatic stages. Time to onset of disease and survival were significantly shorter in CypA-deficient mice than CypA-expressing controls. CypA-deficient mice had significantly greater microglial activation in the presymptomatic stage, and analysis of anti- and pro-inflammatory microglial markers indicated a shift towards a pro-inflammatory phenotype. There was no difference in astrocyte activation. This suggests that CypA contributes to dampening the pro-inflammatory microglial response during the early stage of RML-induced prion disease.

Rapid degradation of progressive ankylosis protein (ANKH) in craniometaphyseal dysplasia

Mutations in the progressive ankylosis protein (NP_473368, human ANKH) cause craniometaphyseal dysplasia (CMD), characterized by progressive thickening of craniofacial bones and widened metaphyses in long bones. The pathogenesis of CMD remains largely unknown, and treatment for CMD is limited to surgical intervention. We have reported that knock-in mice (AnkKI/KI) carrying a F377del mutation in ANK (NM_020332, mouse ANK) replicate many features of CMD. Interestingly, ablation of the Ank gene in AnkKO/KO mice also leads to several CMD-like phenotypes. Mutations causing CMD led to decreased steady-state levels of ANK/ANKH protein due to rapid degradation. While wild type (wt) ANK was mostly associated with plasma membranes, endoplasmic reticulum (ER), Golgi apparatus and lysosomes, CMD-linked mutant ANK was aberrantly localized in cytoplasm. Inhibitors of proteasomal degradation significantly restored levels of overexpressed mutant ANK, whereas endogenous CMD-mutant ANK/ANKH levels were more strongly increased by inhibitors of lysosomal degradation. However, these inhibitors do not correct the mislocalization of mutant ANK. Co-expressing wt and CMD-mutant ANK in cells showed that CMD-mutant ANK does not negatively affect wt ANK expression and localization, and vice versa. In conclusion, our finding that CMD mutant ANK/ANKH protein is short-lived and mislocalized in cells may be part of the CMD pathogenesis.

A Sequence-Dependent DNA Condensation Induced by Prion Protein

Different studies indicated that the prion protein induces hybridization of complementary DNA strands. Cell culture studies showed that the scrapie isoform of prion protein remained bound with the chromosome. In present work, we used an oxazole dye, YOYO, as a reporter to quantitative characterization of the DNA condensation by prion protein. We observe that the prion protein induces greater fluorescence quenching of YOYO intercalated in DNA containing only GC bases compared to the DNA containing four bases whereas the effect of dye bound to DNA containing only AT bases is marginal. DNA-condensing biological polyamines are less effective than prion protein in quenching of DNA-bound YOYO fluorescence. The prion protein induces marginal quenching of fluorescence of the dye bound to oligonucleotides, which are resistant to condensation. The ultrastructural studies with electron microscope also validate the biophysical data. The GC bases of the target DNA are probably responsible for increased condensation in the presence of prion protein. To our knowledge, this is the first report of a human cellular protein inducing a sequence-dependent DNA condensation. The increased condensation of GC-rich DNA by prion protein may suggest a biological function of the prion protein and a role in its pathogenesis.

Fatal familial insomnia and sporadic fatal insomnia

Fatal familial insomnia (FFI) and sporadic fatal insomnia (sFI), or thalamic form of sporadic Creutzfeldt-Jakob disease MM2 (sCJDMM2T), are prion diseases originally named and characterized in 1992 and 1999, respectively. FFI is genetically determined and linked to a D178N mutation coupled with the M129 genotype in the prion protein gene (PRNP) at chromosome 20. sFI is a phenocopy of FFI and likely its sporadic form. Both diseases are primarily characterized by progressive sleep impairment, disturbances of autonomic nervous system, and motor signs associated with severe loss of nerve cells in medial thalamic nuclei. Both diseases harbor an abnormal disease-associated prion protein isoform, resistant to proteases with relative mass of 19 kDa identified as resPrP^TSE type 2. To date at least 70 kindreds affected by FFI with 198 members and 18 unrelated carriers along with 25 typical cases of sFI have been published. The D178N-129M mutation is thought to cause FFI by destabilizing the mutated prion protein and facilitating its conversion to PrP^TSE. The thalamus is the brain region first affected. A similar mechanism triggered spontaneously may underlie sFI.

Transgenic mice recapitulate the phenotypic heterogeneity of genetic prion diseases without developing prion infectivity: Role of intracellular PrP retention in neurotoxicity

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 (CJD), Gerstmann-Sträussler-Scheinker (GSS) syndrome and fatal familial insomnia (FFI). The reason for this variability is not known. It has been suggested that prion strains with unique self-replicating and neurotoxic properties emerge spontaneously in individuals carrying PrP mutations, dictating the phenotypic expression of disease. We generated transgenic mice expressing the FFI mutation, and found that they developed a fatal neurological illness highly reminiscent of FFI, and different from those of similarly generated mice modeling genetic CJD and GSS. Thus transgenic mice recapitulate the phenotypic differences seen in humans. The mutant PrPs expressed in these mice are misfolded but unable to self-replicate. They accumulate in different compartments of the neuronal secretory pathway, impairing the membrane delivery of ion channels essential for neuronal function. Our results indicate that conversion of mutant PrP into an infectious isoform is not required for pathogenesis, and suggest that the phenotypic variability may be due to different effects of mutant PrP on intracellular transport.

The small heat shock proteins αB-crystallin (HSPB5) and Hsp27 (HSPB1) inhibit the intracellular aggregation of α-synuclein

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. Proteostasis is preserved in the face of stress by a complex network of cellular machinery, including the small heat shock molecular chaperone proteins (sHsps), which act to inhibit the aggregation and deposition of misfolded protein intermediates. Despite this, the pathogenesis of several neurodegenerative diseases has been inextricably linked with the amyloid fibrillar aggregation and deposition of α-synuclein (α-syn). The sHsps are potent inhibitors of α-syn aggregation in vitro. However, the limited availability of a robust, cell-based model of α-syn aggregation has, thus far, restricted evaluation of sHsp efficacy in the cellular context. As such, this work sought to establish a robust model of intracellular α-syn aggregation using Neuro-2a cells. Aggregation of α-syn was found to be sensitive to inhibition of autophagy and the proteasome, resulting in a significant increase in the proportion of cells containing α-syn inclusions. This model was then used to evaluate the capacity of the sHsps, αB-c and Hsp27, to prevent α-syn aggregation in cells. To do so, we used bicistronic expression plasmids to express the sHsps. Unlike traditional fluorescent fusion constructs, these bicistronic expression plasmids enable only individual transfected cells expressing the sHsps (via expression of the fluorescent reporter) to be analysed, but without the need to tag the sHsp, which can affect its oligomeric structure and chaperone activity. Overexpression of both αB-c and Hsp27 significantly reduced the intracellular aggregation of α-syn. Thus, these findings suggest that overexpressing or boosting the activity of sHsps may be a way of preventing amyloid fibrillar aggregation of α-syn in the context of neurodegenerative disease.

Prion-induced neurotoxicity: Possible role for cell cycle activity and DNA damage response

Protein misfolding neurodegenerative diseases arise through neurotoxicity induced by aggregation of host proteins. These conditions include Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, motor neuron disease, tauopathies and prion diseases. Collectively, these conditions are a challenge to society because of the increasing aged population and through the real threat to human food security by animal prion diseases. It is therefore important to understand the cellular and molecular mechanisms that underlie protein misfolding-induced neurotoxicity as this will form the basis for designing strategies to alleviate their burden. Prion diseases are an important paradigm for neurodegenerative conditions in general since several of these maladies have now been shown to display prion-like phenomena. Increasingly, cell cycle activity and the DNA damage response are recognised as cellular events that participate in the neurotoxic process of various neurodegenerative diseases, and their associated animal models, which suggests they are truly involved in the pathogenic process and are not merely epiphenomena. Here we review the role of cell cycle activity and the DNA damage response in neurodegeneration associated with protein misfolding diseases, and suggest that these events contribute towards prion-induced neurotoxicity. In doing so, we highlight PrP transgenic Drosophila as a tractable model for the genetic analysis of transmissible mammalian prion disease.

Domain-Specific Activation of Death-Associated Intracellular Signalling Cascades by the Cellular Prion Protein in Neuroblastoma Cells

The biological functions of the cellular prion protein remain poorly understood. In fact, numerous studies have aimed to determine specific functions for the different protein domains. Studies of cellular prion protein (PrP(C)) domains through in vivo expression of molecules carrying internal deletions in a mouse Prnp null background have provided helpful data on the implication of the protein in signalling cascades in affected neurons. Nevertheless, understanding of the mechanisms underlying the neurotoxicity induced by these PrP(C) deleted forms is far from complete. To better define the neurotoxic or neuroprotective potential of PrP(C) N-terminal domains, and to overcome the heterogeneity of results due to the lack of a standardized model, we used neuroblastoma cells to analyse the effects of overexpressing PrP(C) deleted forms. Results indicate that PrP(C) N-terminal deleted forms were properly processed through the secretory pathway. However, PrPΔF35 and PrPΔCD mutants led to death by different mechanisms sharing loss of alpha-cleavage and activation of caspase-3. Our data suggest that both gain-of-function and loss-of-function pathogenic mechanisms may be associated with N-terminal domains and may therefore contribute to neurotoxicity in prion disease. Dissecting the molecular response induced by PrPΔF35 may be the key to unravelling the physiological and pathological functions of the prion protein.

Ubiquitin-specific protease 14 modulates degradation of cellular prion protein

Prion diseases are fatal neurodegenerative disorders characterized by the accumulation of prion protein (PrP^C). To date, there is no effective treatment for the disease. The accumulated PrP, termed PrP^Sc, forms amyloid fibrils and could be infectious. It has been suggested that PrP^Sc is abnormally folded and resistant to proteolytic degradation, and also inhibits proteasomal functions in infected cells, thereby inducing neuronal death. Recent work indicates that the ubiquitin-proteasome system is involved in quality control of PrP^C. To reveal the significance of prion protein ubiqitination, we focused on ubiquitin-specific protease 14 (USP14), a deubiqutinating enzyme that catalyzes trimming of polyubiquitin chains and plays a role in regulation of proteasomal processes. Results from the present study showed that treatment with a selective inhibitor of USP14 reduced PrP^C, as well as PrP^Sc, levels in prion-infected neuronal cells. Overexpression of the dominant negative mutant form of USP14 reduced PrP^Sc, whereas wildtype USP14 increased PrP^Sc in prion-infected cells. These results suggest that USP14 prevents degradation of both normal and abnormal PrP. Collectively, a better understanding about the regulation of PrP^Sc clearance caused by USP14 might contribute greatly to the development of therapeutic strategies for prion diseases.

Transgenic fatal familial insomnia mice indicate prion infectivity-independent mechanisms of pathogenesis and phenotypic expression of disease

Fatal familial insomnia (FFI) and a genetic form of Creutzfeldt-Jakob disease (CJD¹⁷⁸) are clinically different prion disorders linked to the D178N prion protein (PrP) mutation. The disease phenotype is determined by the 129 M/V polymorphism on the mutant allele, which is thought to influence D178N PrP misfolding, leading to the formation of distinctive prion strains with specific neurotoxic properties. However, the mechanism by which misfolded variants of mutant PrP cause different diseases is not known. We generated transgenic (Tg) mice expressing the mouse PrP homolog of the FFI mutation. These mice synthesize a misfolded form of mutant PrP in their brains and develop a neurological illness with severe sleep disruption, highly reminiscent of FFI and different from that of analogously generated Tg(CJD) mice modeling CJD¹⁷⁸. No prion infectivity was detectable in Tg(FFI) and Tg(CJD) brains by bioassay or protein misfolding cyclic amplification, indicating that mutant PrP has disease-encoding properties that do not depend on its ability to propagate its misfolded conformation. Tg(FFI) and Tg(CJD) neurons have different patterns of intracellular PrP accumulation associated with distinct morphological abnormalities of the endoplasmic reticulum and Golgi, suggesting that mutation-specific alterations of secretory transport may contribute to the disease phenotype.

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 (CJD) and fatal familial insomnia (FFI). The reason for this variability is not known, but assembly of the mutant PrPs into distinct aggregates that spread in the brain by promoting PrP aggregation may contribute to the disease phenotype. We previously generated transgenic mice modeling genetic CJD, clinically identified by dementia and motor abnormalities. We have now generated transgenic mice carrying the PrP mutation associated with FFI, and found that they develop severe sleep abnormalities and other key features of the human disorder. Thus, transgenic mice recapitulate the phenotypic differences seen in humans. The mutant PrPs in FFI and CJD mice are aggregated but unable to promote PrP aggregation. They accumulate in different intracellular compartments and cause distinct morphological abnormalities of transport organelles. These results indicate that mutant PrP has disease-encoding properties that are independent of its ability to self-propagate, and suggest that the phenotypic heterogeneity may be due to different effects of aggregated PrP on intracellular transport. Our study provides new insights into the mechanisms of selective neuronal dysfunction due to protein aggregation.

Prion-induced and spontaneous formation of transmissible toxicity in PrP transgenic Drosophila

Prion diseases are fatal transmissible neurodegenerative diseases of various mammalian species. Central to these conditions is the conversion of the normal host prion protein PrP(C) into the abnormal prion conformer PrP(Sc). Mature PrP(C) is attached to the plasma membrane by a glycosylphosphatidylinositol anchor, whereas during biosynthesis and metabolism cytosolic and secreted forms of the protein may arise. The role of topological PrP(C) variants in the mechanism of prion formation and prion-induced neurotoxicity during prion disease remains undefined. In the present study we investigated whether Drosophila transgenic for ovine PrP targeted to the plasma membrane, to the cytosol or for secretion, could produce transmissible toxicity following exposure to exogenous ovine prions. Although all three topological variants of PrP were efficiently expressed in Drosophila, cytosolic PrP was conformationally distinct and required denaturation before recognition by immunobiochemical methods. Adult Drosophila transgenic for pan neuronally expressed ovine PrP targeted to the plasma membrane, to the cytosol or for secretion exhibited a decreased locomotor activity after exposure at the larval stage to ovine prions. Proteinase K-resistant PrP(Sc) was detected by protein misfolding cyclic amplification in prion-exposed Drosophila transgenic for membrane-targeted PrP. Significantly, head homogenate from all three variants of prion-exposed PrP transgenic Drosophila induced a decreased locomotor activity when transmitted to PrP recipient flies. Drosophila transgenic for PrP targeted for secretion exhibited a spontaneous locomotor defect in the absence of prion exposure that was transmissible in PrP transgenic flies. Our data are consistent with the formation of transmissible prions in PrP transgenic Drosophila.

Prion protein and cancers

The normal cellular prion protein, PrP(C) is a highly conserved and widely expressed cell surface glycoprotein in all mammals. The expression of PrP is pivotal in the pathogenesis of prion diseases; however, the normal physiological functions of PrP(C) remain incompletely understood. Based on the studies in cell models, a plethora of functions have been attributed to PrP(C). In this paper, we reviewed the potential roles that PrP(C) plays in cell physiology and focused on its contribution to tumorigenesis.

Cytosolic PrP can participate in prion-mediated toxicity

Prion diseases are characterized by a conformational change in the normal host protein PrPC. While the majority of mature PrPC is tethered to the plasma membrane by a glycosylphosphatidylinositol anchor, topological variants of this protein can arise during its biosynthesis. Here we have generated Drosophila transgenic for cytosolic ovine PrP in order to investigate its toxic potential in flies in the absence or presence of exogenous ovine prions. While cytosolic ovine PrP expressed in Drosophila was predominantly detergent insoluble and showed resistance to low concentrations of proteinase K, it was not overtly detrimental to the flies. However, Drosophila transgenic for cytosolic PrP expression exposed to classical or atypical scrapie prion inocula showed a faster decrease in locomotor activity than similar flies exposed to scrapie-free material. The susceptibility to classical scrapie inocula could be assessed in Drosophila transgenic for panneuronal expression of cytosolic PrP, whereas susceptibility to atypical scrapie required ubiquitous PrP expression. Significantly, the toxic phenotype induced by ovine scrapie in cytosolic PrP transgenic Drosophila was transmissible to recipient PrP transgenic flies. These data show that while cytosolic PrP expression does not adversely affect Drosophila, this topological PrP variant can participate in the generation of transmissible scrapie-induced toxicity. These observations also show that PrP transgenic Drosophila are susceptible to classical and atypical scrapie prion strains and highlight the utility of this invertebrate host as a model of mammalian prion disease. Importance: During prion diseases, the host protein PrPC converts into an abnormal conformer, PrPSc, a process coupled to the generation of transmissible prions and neurotoxicity. While PrPC is principally a glycosylphosphatidylinositol-anchored membrane protein, the role of topological variants, such as cytosolic PrP, in prion-mediated toxicity and prion formation is undefined. Here we generated Drosophila transgenic for cytosolic PrP expression in order to investigate its toxic potential in the absence or presence of exogenous prions. Cytosolic ovine PrP expressed in Drosophila was not overtly detrimental to the flies. However, cytosolic PrP transgenic Drosophila exposed to ovine scrapie showed a toxic phenotype absent from similar flies exposed to scrapie-free material. Significantly, the scrapie-induced toxic phenotype in cytosolic transgenic Drosophila was transmissible to recipient PrP transgenic flies. These data show that cytosolic PrP can participate in the generation of transmissible prion-induced toxicity and highlight the utility of Drosophila as a model of mammalian prion disease.

Increased expression of p62/SQSTM1 in prion diseases and its association with pathogenic prion protein

Prion diseases are neurodegenerative disorders characterized by the aggregation of abnormally folded prion protein (PrP^Sc). In this study, we focused on the mechanism of clearance of PrP^Sc, which remains unclear. p62 is a cytosolic protein known to mediate both the formation and degradation of aggregates of abnormal proteins. The levels of p62 protein increased in prion-infected brains and persistently infected cell cultures. Upon proteasome inhibition, p62 co-localized with PrP^Sc, forming a large aggregate in the perinuclear region, hereafter referred to as PrP^Sc-aggresome. These aggregates were surrounded with autophagosome marker LC3 and lysosomes in prion-infected cells. Moreover, transient expression of the phosphomimic form of p62, which has enhanced ubiquitin-binding activity, reduced the amount of PrP^Sc in prion-infected cells, indicating that the activation of p62 could accelerate the clearance of PrP^Sc. Our findings would thus suggest that p62 could be a target for the therapeutic control of prion diseases.

Neuropathogenesis of prion disease

ABSTRACT: Although much is known about prion diseases (characterized by a post-translational misfolding of the prion protein [PrP]) and their neuropathology and molecular pathology, the fundamental cause of illness, the basic neuropathogenesis, remains uncertain. There are three broad considerations discussed in this review: the possible loss of normal PrP function, the possible direct toxicity of the abnormally folded PrP and a harmful interaction between the normal and abnormal protein. In considering these possibilities, there are difficulties, including the facts that the relevant normal functions of the PrP are somewhat uncertain and that there are a number of possible toxic species of abnormal protein. In addition to the possible interactions of normal and abnormal PrP in prion disease, PrP may play a role in the neuropathogenesis of other diseases (such as Alzheimer’s disease).

Viability assays for cells in culture

Manual cell counts on a microscope are a sensitive means of assessing cellular viability but are time-consuming and therefore expensive. Computerized viability assays are expensive in terms of equipment but can be faster and more objective than manual cell counts. The present report describes the use of three such viability assays. Two of these assays are infrared and one is luminescent. Both infrared assays rely on a 16 bit Odyssey Imager. One infrared assay uses the DRAQ5 stain for nuclei combined with the Sapphire stain for cytosol and is visualized in the 700 nm channel. The other infrared assay, an In-Cell Western, uses antibodies against cytoskeletal proteins (α-tubulin or microtubule associated protein 2) and labels them in the 800 nm channel. The third viability assay is a commonly used luminescent assay for ATP, but we use a quarter of the recommended volume to save on cost. These measurements are all linear and correlate with the number of cells plated, but vary in sensitivity. All three assays circumvent time-consuming microscopy and sample the entire well, thereby reducing sampling error. Finally, all of the assays can easily be completed within one day of the end of the experiment, allowing greater numbers of experiments to be performed within short timeframes. However, they all rely on the assumption that cell numbers remain in proportion to signal strength after treatments, an assumption that is sometimes not met, especially for cellular ATP. Furthermore, if cells increase or decrease in size after treatment, this might affect signal strength without affecting cell number. We conclude that all viability assays, including manual counts, suffer from a number of caveats, but that computerized viability assays are well worth the initial investment. Using all three assays together yields a comprehensive view of cellular structure and function.

Anchorless 23-230 PrPC interactomics for elucidation of PrPC protective role

Accumulation of conformationally altered cellular proteins (i.e., prion protein) is the common feature of prions and other neurodegenerative diseases. Previous studies demonstrated that the lack of terminal sequence of cellular prion protein (PrPC), necessary for the addition of glycosylphosphatidylinositol lipid anchor, leads to a protease-resistant conformation that resembles scrapie-associated isoform of prion protein. Moreover, mice overexpressing the truncated form of PrPC showed late-onset, amyloid deposition, and the presence of a short protease-resistant PrP fragment in the brain similar to those found in Gerstmann-Sträussler-Scheinker disease patients. Therefore, the physiopathological function of truncated_/anchorless 23-230 PrPC (Δ23-230 PrPC) has come into focus of attention. The present study aims at revealing the physiopathological function of the anchorless PrPC form by identifying its interacting proteins. The truncated_/anchorless Δ23-230 PrPC along with its interacting proteins was affinity purified using STrEP-Tactin chromatography, in-gel digested, and identified by quadrupole time-of-flight tandem mass spectrometry analysis in prion protein-deficient murine hippocampus (HpL3-4) neuronal cell line. Twenty-three proteins appeared to interact with anchorless Δ23-230 PrPC in HpL3-4 cells. Out of the 23 proteins, one novel protein, pyruvate kinase isozymes M1/M2 (PKM2), exhibited a potential interaction with the anchorless Δ23-230 form of PrPC. Both reverse co-immunoprecipitation and confocal laser-scanning microscopic analysis confirmed an interaction of PKM2 with the anchorless Δ23-230 form of PrPC. Furthermore, we provide the first evidence for co-localization of PKM2 and PrPC as well as PrPC-dependent PKM2 expression regulation. In addition, given the involvement of PrPC in the regulation of apoptosis, we exposed HpL3-4 cells to staurosporine (STS)-mediated apoptotic stress. In response to STS-mediated apoptotic stress, HpL3-4 cells transiently expressing 23-230-truncated PrPC were markedly less viable, were more prone to apoptosis and exhibited significantly higher PKM2 expressional regulation as compared with HpL3-4 cells transiently expressing full-length PrPC (1-253 PrPC). The enhanced STS-induced apoptosis was shown by increased caspase-3 cleavage. Together, our data suggest that the misbalance or over expression of anchorless Δ23-230 form of PrPC in association with the expressional regulation of interacting proteins could render cells more prone to cellular insults-stress response, formation of aggregates and may ultimately be linked to the cell death.

Synaptic dysfunction in prion diseases: a trafficking problem?

Synaptic dysfunction is an important cause of neurological symptoms in prion diseases, a class of clinically heterogeneous neurodegenerative disorders caused by misfolding of the cellular prion protein (PrP^C). Experimental data suggest that accumulation of misfolded PrP^C in the endoplasmic reticulum (ER) may be crucial in synaptic failure, possibly because of the activation of the translational repression pathway of the unfolded protein response. Here, we report that this pathway is not operative in mouse models of genetic prion disease, consistent with our previous observation that ER stress is not involved. Building on our recent finding that ER retention of mutant PrP^C impairs the secretory trafficking of calcium channels essential for synaptic function, we propose a model of pathogenicity in which intracellular retention of misfolded PrP^C results in loss of function or gain of toxicity of PrP^C-interacting proteins. This neurotoxic modality may also explain the phenotypic heterogeneity of prion diseases.

Epitope scanning indicates structural differences in brain-derived monomeric and aggregated mutant prion proteins related to genetic prion diseases

Genetic Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia and prion protein cerebral amyloid angiopathy are clinically and neuropathologically distinct neurodegenerative diseases linked to mutations in the PRNP gene encoding the cellular prion protein (PrPC). How sequence variants of PRNP encode the information to specify these disease phenotypes is not known. It is suggested that each mutation produces a misfolded variant of PrPC with specific neurotoxic properties. However, structural studies of recombinant PrP did not detect major differences between wild-type and mutant molecules, pointing to the importance of investigating mutant PrPs from mammalian brains. We used surface plasmon resonance and a slot-blot immunoassay to analyse the antibody-binding profiles of soluble and insoluble PrP molecules extracted from the brains of transgenic mice modelling different prion diseases. By measuring the reactivity of monoclonal antibodies against different PrP epitopes, we obtained evidence of conformational differences between wild-type and mutant PrPs, and among different mutants. We detected structural heterogeneity in both monomeric and aggregated PrP, supporting the hypothesis that the phenotype of genetic prion diseases is encoded by mutant PrP conformation and assembly state.

PrP(C) regulates epidermal growth factor receptor function and cell shape dynamics in Neuro2a cells

The prion protein (PrP) plays a key role in prion disease pathogenesis. Although the misfolded and pathologic variant of this protein (PrP(SC)) has been studied in depth, the physiological role of PrP(C) remains elusive and controversial. PrP(C) is a cell-surface glycoprotein involved in multiple cellular functions at the plasma membrane, where it interacts with a myriad of partners and regulates several intracellular signal transduction cascades. However, little is known about the gene expression changes modulated by PrP(C) in animals and in cellular models. In this article, we present PrP(C)-dependent gene expression signature in N2a cells and its implication in the most overrepresented functions: cell cycle, cell growth and proliferation, and maintenance of cell shape. PrP(C) over-expression enhances cell proliferation and cell cycle re-entrance after serum stimulation, while PrP(C) silencing slows down cell cycle progression. In addition, MAP kinase and protein kinase B (AKT) pathway activation are under the regulation of PrP(C) in asynchronous cells and following mitogenic stimulation. These effects are due in part to the modulation of epidermal growth factor receptor (EGFR) by PrP(C) in the plasma membrane, where the two proteins interact in a multimeric complex. We also describe how PrP(C) over-expression modulates filopodia formation by Rho GTPase regulation mainly in an AKT-Cdc42-N-WASP-dependent pathway.

Levels of the Mahogunin Ring Finger 1 E3 ubiquitin ligase do not influence prion disease

Prion diseases are rare but invariably fatal neurodegenerative disorders. They are associated with spongiform encephalopathy, a histopathology characterized by the presence of large, membrane-bound vacuolar structures in the neuropil of the brain. While the primary cause is recognized as conversion of the normal form of prion protein (PrP^C) to a conformationally distinct, pathogenic form (PrP^Sc), the cellular pathways and mechanisms that lead to spongiform change, neuronal dysfunction and death are not known. Mice lacking the Mahogunin Ring Finger 1 (MGRN1) E3 ubiquitin ligase develop spongiform encephalopathy by 9 months of age but do not become ill. In cell culture, PrP aberrantly present in the cytosol was reported to interact with and sequester MGRN1. This caused endo-lysosomal trafficking defects similar to those observed when Mgrn1 expression is knocked down, implicating disrupted MGRN1-dependent trafficking in the pathogenesis of prion disease. As these defects were rescued by over-expression of MGRN1, we investigated whether reduced or elevated Mgrn1 expression influences the onset, progression or pathology of disease in mice inoculated with PrP^Sc. No differences were observed, indicating that disruption of MGRN1-dependent pathways does not play a significant role in the pathogenesis of transmissible spongiform encephalopathy.

Disease-associated mutations in the prion protein impair laminin-induced process outgrowth and survival

Prions, the agents of transmissible spongiform encephalopathies, require the expression of prion protein (PrP(C)) to propagate disease. PrP(C) is converted into an abnormal insoluble form, PrP(Sc), that gains neurotoxic activity. Conversely, clinical manifestations of prion disease may occur either before or in the absence of PrP(Sc) deposits, but the loss of normal PrP(C) function contribution for the etiology of these diseases is still debatable. Prion disease-associated mutations in PrP(C) represent one of the best models to understand the impact of PrP(C) loss-of-function. PrP(C) associates with various molecules and, in particular, the interaction of PrP(C) with laminin (Ln) modulates neuronal plasticity and memory formation. To assess the functional alterations associated with PrP(C) mutations, wild-type and mutated PrP(C) proteins were expressed in a neural cell line derived from a PrP(C)-null mouse. Treatment with the laminin γ1 chain peptide (Ln γ1), which mimics the Ln binding site for PrP(C), increased intracellular calcium in cells expressing wild-type PrP(C), whereas a significantly lower response was observed in cells expressing mutated PrP(C) molecules. The Ln γ1 did not promote process outgrowth or protect against staurosporine-induced cell death in cells expressing mutated PrP(C) molecules in contrast to cells expressing wild-type PrP(C). The co-expression of wild-type PrP(C) with mutated PrP(C) molecules was able to rescue the Ln protective effects, indicating the lack of negative dominance of PrP(C) mutated molecules. These results indicate that PrP(C) mutations impair process outgrowth and survival mediated by Ln γ1 peptide in neural cells, which may contribute to the pathogenesis of genetic prion diseases.

Effect of divalent metals on the neuronal proteasomal system, prion protein ubiquitination and aggregation

The role of normal cellular prion protein (PrP) remains to be fully elucidated; however, the protein is crucial for the infection and progression of prion diseases. Recent evidence indicates that PrP is a metalloprotein since the octapeptide repeat sequences in the protein have high affinity for various divalent cations and the binding sites appear to play a role in the pathogenesis of prion diseases. In our present study, we tested several divalent metals including manganese and cadmium and determined their effects on protein degradation and protein aggregation in mouse neuronal cells expressing PrP. Cadmium was more neurotoxic than manganese following 24h exposure. Manganese did not show any significant effect on the inhibition of proteasomal activity or formation of high molecular weight ubiquitinated PrPs. Interestingly, treatment with cadmium profoundly inhibited proteasomal activity, which resulted in greatly increased formation of high molecular weight ubiquitinated PrPs. Immunohistochemical analysis also revealed a dramatic increase in formation of oligomers after cadmium treatment. Cadmium also increased the formation of ubiquitinated PrP, but it did not lead to the formation of proteinase-K resistant PrP. Collectively, our results show that a divalent metal, cadmium affects proteasomal function and PrP aggregation, which promote neurotoxicity.

Mutant PrP suppresses glutamatergic neurotransmission in cerebellar granule neurons by impairing membrane delivery of VGCC α(2)δ-1 Subunit

How mutant prion protein (PrP) leads to neurological dysfunction in genetic prion diseases is unknown. Tg(PG14) mice synthesize a misfolded mutant PrP which is partially retained in the neuronal endoplasmic reticulum (ER). As these mice age, they develop ataxia and massive degeneration of cerebellar granule neurons (CGNs). Here, we report that motor behavioral deficits in Tg(PG14) mice emerge before neurodegeneration and are associated with defective glutamate exocytosis from granule neurons due to impaired calcium dynamics. We found that mutant PrP interacts with the voltage-gated calcium channel α(2)δ-1 subunit, which promotes the anterograde trafficking of the channel. Owing to ER retention of mutant PrP, α(2)δ-1 accumulates intracellularly, impairing delivery of the channel complex to the cell surface. Thus, mutant PrP disrupts cerebellar glutamatergic neurotransmission by reducing the number of functional channels in CGNs. These results link intracellular PrP retention to synaptic dysfunction, indicating new modalities of neurotoxicity and potential therapeutic strategies.

Expression of mutant or cytosolic PrP in transgenic mice and cells is not associated with endoplasmic reticulum stress or proteasome dysfunction

The cellular pathways activated by mutant prion protein (PrP) in genetic prion diseases, ultimately leading to neuronal dysfunction and degeneration, are not known. Several mutant PrPs misfold in the early secretory pathway and reside longer in the endoplasmic reticulum (ER) possibly stimulating ER stress-related pathogenic mechanisms. To investigate whether mutant PrP induced maladaptive responses, we checked key elements of the unfolded protein response (UPR) in transgenic mice, primary neurons and transfected cells expressing two different mutant PrPs. Because ER stress favors the formation of untranslocated PrP that might aggregate in the cytosol and impair proteasome function, we also measured the activity of the ubiquitin proteasome system (UPS). Molecular, biochemical and immunohistochemical analyses found no increase in the expression of UPR-regulated genes, such as Grp78/Bip, CHOP/GADD153, or ER stress-dependent splicing of the mRNA encoding the X-box-binding protein 1. No alterations in UPS activity were detected in mutant mouse brains and primary neurons using the Ub^G76V-GFP reporter and a new fluorogenic peptide for monitoring proteasomal proteolytic activity in vivo. Finally, there was no loss of proteasome function in neurons in which endogenous PrP was forced to accumulate in the cytosol by inhibiting cotranslational translocation. These results indicate that neither ER stress, nor perturbation of proteasome activity plays a major pathogenic role in prion diseases.

Experimental approaches to the interaction of the prion protein with nucleic acids and glycosaminoglycans: Modulators of the pathogenic conversion

The concept that transmissible spongiform encephalopathies (TSEs) are caused only by proteins has changed the traditional paradigm that disease transmission is due solely to an agent that carries genetic information. The central hypothesis for prion diseases proposes that the conversion of a cellular prion protein (PrP(C)) into a misfolded, β-sheet-rich isoform (PrP(Sc)) accounts for the development of (TSE). There is substantial evidence that the infectious material consists chiefly of a protein, PrP(Sc), with no genomic coding material, unlike a virus particle, which has both. However, prions seem to have other partners that chaperone their activities in converting the PrP(C) into the disease-causing isoform. Nucleic acids (NAs) and glycosaminoglycans (GAGs) are the most probable accomplices of prion conversion. Here, we review the recent experimental approaches that have been employed to characterize the interaction of prion proteins with nucleic acids and glycosaminoglycans. A PrP recognizes many nucleic acids and GAGs with high affinities, and this seems to be related to a pathophysiological role for this interaction. A PrP binds nucleic acids and GAGs with structural selectivity, and some PrP:NA complexes can become proteinase K-resistant, undergoing amyloid oligomerization and conversion to a β-sheet-rich structure. These results are consistent with the hypothesis that endogenous polyanions (such as NAs and GAGs) may accelerate the rate of prion disease progression by acting as scaffolds or lattices that mediate the interaction between PrP(C) and PrP(Sc) molecules. In addition to a still-possible hypothesis that nucleic acids and GAGs, especially those from the host, may modulate the conversion, the recent structural characterization of the complexes has raised the possibility of developing new diagnostic and therapeutic strategies.

Cell type-specific neuroprotective activity of untranslocated prion protein

Background

A key pathogenic role in prion diseases was proposed for a cytosolic form of the prion protein (PrP). However, it is not clear how cytosolic PrP localization influences neuronal viability, with either cytotoxic or anti-apoptotic effects reported in different studies. The cellular mechanism by which PrP is delivered to the cytosol of neurons is also debated, and either retrograde transport from the endoplasmic reticulum or inefficient translocation during biosynthesis has been proposed. We investigated cytosolic PrP biogenesis and effect on cell viability in primary neuronal cultures from different mouse brain regions.

Principal Findings

Mild proteasome inhibition induced accumulation of an untranslocated form of cytosolic PrP in cortical and hippocampal cells, but not in cerebellar granules. A cyclopeptolide that interferes with the correct insertion of the PrP signal sequence into the translocon increased the amount of untranslocated PrP in cortical and hippocampal cells, and induced its synthesis in cerebellar neurons. Untranslocated PrP boosted the resistance of cortical and hippocampal neurons to apoptotic insults but had no effect on cerebellar cells.

Significance

These results indicate cell type-dependent differences in the efficiency of PrP translocation, and argue that cytosolic PrP targeting might serve a physiological neuroprotective function.

The hydrophobic core region governs mutant prion protein aggregation and intracellular retention

Approx. 15% of human prion diseases have a pattern of autosomal dominant inheritance, and are linked to mutations in the gene encoding PrP (prion protein), a GPI (glycosylphosphatidylinositol)-anchored protein whose function is not clear. The cellular mechanisms by which PrP mutations cause disease are also not known. Soon after synthesis in the ER (endoplasmic reticulum), several mutant PrPs misfold and become resistant to phospholipase cleavage of their GPI anchor. The biosynthetic maturation of the misfolded molecules in the ER is delayed and, during transit in the secretory pathway, they form detergent-insoluble and protease-resistant aggregates, suggesting that intracellular PrP aggregation may play a pathogenic role. We have investigated the consequence of deleting residues 114–121 within the hydrophobic core of PrP on the aggregation and cellular localization of two pathogenic mutants that accumulate in the ER and Golgi apparatus. Compared with their full-length counterparts, the deleted molecules formed smaller protease-sensitive aggregates and were more efficiently transported to the cell surface and released by phospholipase cleavage. These results indicate that mutant PrP aggregation and intracellular retention are closely related and depend critically on the integrity of the hydrophobic core. The discovery that Δ114–121 counteracts misfolding and improves the cellular trafficking of mutant PrP provides an unprecedented model for assessing the role of intracellular aggregation in the pathogenesis of prion diseases.

Non-infectious aggregates of the prion protein react with several PrPSc-directed antibodies

The key event in the pathogenesis of prion diseases is the conformational conversion of the normal prion protein (PrP) (PrP(C)) into an infectious, aggregated isoform (PrP(Sc)) that has a high content of beta-sheet. Historically, a great deal of effort has been devoted to developing antibodies that specifically recognize PrP(Sc) but not PrP(C), as such antibodies would have enormous diagnostic and experimental value. A mouse monoclonal IgM antibody (designated 15B3) and three PrP motif-grafted monoclonal antibodies (referred to as IgG 19-33, 89-112, and 136-158) have been previously reported to react specifically with infectious PrP(Sc) but not PrP(C). In this study, we extend the characterization of these four antibodies by testing their ability to immunoprecipitate and immunostain infectious and non-infectious aggregates of wild-type, mutant, and recombinant PrP. We find that 15B3 as well as the motif-grafted antibodies recognize multiple types of aggregated PrP, both infectious and non-infectious, including forms found in brain, in transfected cells, and induced in vitro from purified recombinant protein. These antibodies are exquisitely selective for aggregated PrP, and do not react with soluble PrP even when present in vast excess. Our results suggest that 15B3 and the motif-grafted antibodies recognize structural features common to both infectious and non-infectious aggregates of PrP. Our study extends the utility of these antibodies for diagnostic and experimental purposes, and it provides new insight into the structural changes that accompany PrP oligomerization and prion propagation.

Interactions of prion protein with intracellular proteins: so many partners and no consequences?

Prion protein (PrP) plays a key role in the pathogenesis of transmissible spongiform encephalopathies (TSEs)--fatal diseases of the central nervous system. Its physiological function as well as exact role in neurodegeneration remain unclear, hence screens for proteins interacting with PrP seem to be the most promising approach to elucidating these issues. PrP is mostly a plasma membrane-anchored extracellular glycoprotein and only a small fraction resides inside the cell, yet the number of identified intracellular partners of PrP is comparable to that of its membranal or extracellular interactors. Since some TSEs are accompanied by significantly increased levels of cytoplasmic PrP and this fraction of the protein has been found to be neurotoxic, it is of particular interest to characterize the intracellular interactome of PrP. It seems reasonable that at elevated cytoplasmic levels, PrP may exert cytotoxic effect by affecting the physiological functions of its intracellular interactors. This review is focused on the cytoplasmic partners of PrP along with possible consequences of their binding.

Mutant prion protein expression is associated with an alteration of the Rab GDP dissociation inhibitor alpha (GDI)/Rab11 pathway

The prion protein (PrP) is a glycosylphosphatidylinositol-anchored membrane glycoprotein that plays a vital role in prion diseases, a class of fatal neurodegenerative disorders of humans and animals. Approximately 20% of human prion diseases display autosomal dominant inheritance and are linked to mutations in the PrP gene on chromosome 20. PrP mutations are thought to favor the conformational conversion of PrP into a misfolded isoform that causes disease by an unknown mechanism. The PrP mutation D178N/Met-129 is linked to fatal familial insomnia, which causes severe sleep abnormalities and autonomic dysfunction. We showed by immunoelectron microscopy that this mutant PrP accumulates abnormally in the endoplasmic reticulum and Golgi of transfected neuroblastoma N2a cells. To investigate the impact of intracellular PrP accumulation on cellular homeostasis, we did a two-dimensional gel-based differential proteomics analysis. We used wide range immobilized pH gradient strips, pH 4-7 and 6-11, to analyze a large number of proteins. We found changes in proteins involved in energy metabolism, redox regulation, and vesicular transport. Rab GDP dissociation inhibitor alpha (GDI) was one of the proteins that changed most. GDI regulates vesicular protein trafficking by acting on the activity of several Rab proteins. We found a specific reduction in the level of functional Rab11 in mutant PrP-expressing cells associated with impaired post-Golgi trafficking. Our data are consistent with a model by which mutant PrP induces overexpression of GDI, activating a cytotoxic feedback loop that leads to protein accumulation in the secretory pathway.

Prevention of intracellular degradation of I2020T mutant LRRK2 restores its protectivity against apoptosis

Leucine-rich repeat kinase 2 (LRRK2) is the causal gene for autosomal dominant familial Parkinson's disease. We have previously reported a novel molecular feature characteristic to I2020T mutant LRRK2: higher susceptibility to post-translational degradation than the wild-type LRRK2. In the present study, we demonstrated that the protective effect of I2020T LRRK2 against hydrogen peroxide-induced apoptosis was impaired in comparison with the wild-type molecule. When the intracellular level of the protein had been allowed to recover by treatment with proteolysis inhibitors, the protective effect of I2020T LRRK2 against apoptosis was increased. We further confirmed that a decrease in the intracellular protein level of WT LRRK2 by knocking down resulted in a reduction of protectivity against apoptosis. These results suggest that higher susceptibility of I2020T mutant LRRK2 to intracellular degradation than the wild-type molecule may be one of the mechanisms involved in the neurodegeneration associated with this LRRK2 mutation.

Context-dependent perturbation of neural systems in transgenic mice expressing a cytosolic prion protein

We analyzed the relationship between pathogenic protein expression and perturbations to brain anatomy and physiology in a genetic model of prion disease. In this model, the mouse line 1D4, neuropathology is promoted by accumulation of a cytosolic form of the prion protein (cyPrP). CyPrP distribution was determined and compared with anatomical magnetic resonance imaging (MRI) data, a form of functional MRI based on manganese labeling, and immediate early gene mapping with an antibody to c-Fos. Significant discrepancies between 1D4 and control mice became apparent well in advance of overt behavioral pathology in the mutant mice. Alterations to brain structure and function in the mutants varied among brain regions, however, and differed strikingly even among regions with the highest levels of cyPrP expression. In the cerebellum, gross neurodegeneration was accompanied by increased Mn(2+)-enhanced MRI signal, raising the possibility that compensatory mechanisms act to preserve cerebellar function in the face of massive atrophy. In the hippocampus of 1D4 mice, no significant structural alterations were observed, but both Mn(2+)-enhanced MRI and c-Fos data indicated perturbations to neurophysiology. In the neocortex, there were no clear neural activity differences between 1D4 and control animals, but mutant mice showed significant reduction in cortical thickness. Our finding that distinct combinations of anatomical and functional abnormalities accompanied cyPrP overexpression in different parts of the brain indicates the importance of context in conditioning effects of protein pathogens, and exemplifies the notion that neurodegenerative phenotypes extend beyond cell death and the immediate consequences of atrophy for particular neural systems.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

Spontaneous generation of prion infectivity in fatal familial insomnia knockin mice

A crucial tenet of the prion hypothesis is that misfolding of the prion protein (PrP) induced by mutations associated with familial prion disease is, in an otherwise normal mammalian brain, sufficient to generate the infectious agent. Yet this has never been demonstrated. We engineered knockin mice to express a PrP mutation associated with a distinct human prion disease, fatal familial insomnia (FFI). An additional substitution created a strong transmission barrier against pre-existing prions. The mice spontaneously developed a disease distinct from that of other mouse prion models and highly reminiscent of FFI. Unique pathology was transmitted from FFI mice to mice expressing wild-type PrP sharing the same transmission barrier. FFI mice were highly resistant to infection by pre-existing prions, confirming infectivity did not arise from contaminating agents. Thus, a single amino acid change in PrP is sufficient to induce a distinct neurodegenerative disease and the spontaneous generation of prion infectivity.

Cytosolic prion protein induces apoptosis in human neuronal cell SH-SY5Y via mitochondrial disruption pathway

Different neurodegenerative disorders like prion disease, is caused by protein misfolding conformers. Reverse-transfected cytosolic prion protein (PrP) and PrP expressed in the cytosol have been shown to be neurotoxic. To investigate the possible mechanism of neurotoxicity due to accumulation of PrP in cytosol, a PrP mutant lacking the signal and GPI (CytoPrP) was introduced into the SH-SY5Y cell. MTT and trypan blue assays indicated that the viability of cells expressing CytoPrP was remarkably reduced after treatment of MG-132. Obvious apoptosis phenomena were detected in the cells accumulated with CytoPrP, including loss of mitochondrial transmembrane potential, increase of caspase-3 activity, more annexin V/PI-double positive-stained cells and reduced Bcl-2 level. Moreover, DNA fragmentation and TUNEL assays also revealed clear evidences of late apoptosis in the cells accumulated CytoPrP. These data suggest that the accumulation of CytoPrP in cytoplasm may trigger cell apoptosis, in which mitochondrial relative apoptosis pathway seems to play critical role.