Evidence of presynaptic location and function of the prion protein

Zinc ions trigger the prion protein liquid-liquid phase separation

Prion diseases are characterized by the misfolding and conversion of the monomeric prion protein (PrP) to a multimeric aggregated pathogenic form, known as PrPSc. We and others have recently shown that biomolecular condensates formed via liquid-liquid phase separation of PrP can undergo maturation to solid-like species that resemble pathological aggregates, and this process is modulated by DNA, RNA, and oxidative conditions. Conversely, the most well-studied ligand of PrP, copper ions, induce liquid-like condensates of PrP that accumulate Cu2+in vitro, and live PrPC-expressing cells show condensation at the cell surface as triggered by physiologically relevant conditions of Cu2+ and protein concentrations. Since PrP can also bind to Zn2+ through its intrinsically disordered N-terminal domain, though with different affinities and binding modes than Cu2+, we hypothesized that Zn2+ could modulate PrP phase separation differently from copper ions. Using an appropriate buffer with negligible metal ion binding, as well as relevant pH, ionic strength, molecular crowding, and Zn2+ concentrations, we show that recombinant PrP undergoes phase separation with Zn2+. Furthermore, we show that metal ion-induced condensation of PrP is dependent on the N-terminal domain (residues 23-90). In vitro Fluorescence Recovery After Photobleaching (FRAP) experiments and thioflavin T aggregation kinetics support key differences in the molecular properties of PrP:Zn2+versus PrP:Cu2+ phase separated states. FRAP analysis indicated that both Cu2+ and Zn2+ promote liquid-like PrP condensates; however, PrP:Zn2+condensates exhibit a faster recovery. Cu2+ pronouncedly inhibits seed-induced PrP misfolding, whereas Zn2+ provides a milder delay in PrP aggregation. Our findings provide insights on Zn2+-induced phase separation of PrP, supporting a variety of previously proposed functions of PrP in metal sequestering and uptake, processes that could be effectively regulated through biomolecular condensation.

A Copper-Binding Peptide with Therapeutic Potential against Alzheimer's Disease: From the Blood-Brain Barrier to Metal Competition

Alzheimer's disease (AD) is the most common form of dementia worldwide. AD brains are characterized by the accumulation of amyloid-β peptides (Aβ) that bind Cu2+ and have been associated with several neurotoxic mechanisms. Although the use of copper chelators to prevent the formation of Cu2+-Aβ complexes has been proposed as a therapeutic strategy, recent studies show that copper is an important neuromodulator that is essential for a neuroprotective mechanism mediated by Cu2+ binding to the cellular prion protein (PrPC). Therefore, in addition to metal selectivity and blood-brain barrier (BBB) permeability, an emerging challenge for copper chelators is to prevent the formation of neurotoxic Cu2+-Aβ species without perturbing the neuroprotective Cu2+-PrPC interaction. Previously, we reported the design of a tetrapeptide (TP) that withdraws Cu2+ from Aβ(1-16) and impacts the Cu2+-induced aggregation of Aβ(1-40). In this study, we improved the drug-like properties of TP in a BBB model, evaluated the metal selectivity of the optimized peptide (TP*), and tested its effect on Cu2+ coordination to PrPC and proteins involved in copper trafficking, such as copper transporter 1 and albumin. Our results show that changing the stereochemistry of the first residue prevents TP degradation in the BBB model and coadministration of TP with a peptide that increases BBB permeability allows its passage through the BBB model. TP* is highly selective toward Cu2+ in the presence of Zn2+ ions, transfers Cu2+ to copper-trafficking proteins, and forms a ternary TP*-Cu2+-PrP species that does not perturb the physiological conformation of PrP and displays only a minor impact in the neuroprotective Cu2+-dependent interaction of PrPC with the N-methyl-d-aspartate receptor. Overall, these results show that TP* displays desirable features for a copper chelator with therapeutic potential against AD. Moreover, this is the first study that explores the effect of a Cu2+ chelator with therapeutic potential for AD on Cu2+ coordination to PrPC (an emerging key player in AD pathology), integrating recent knowledge about metalloproteins involved in AD with the design of copper chelators against AD.

Prions: structure, function, evolution, and disease

Prions are proteinaceous infectious particles implicated in fatal neurodegenerative disorders known as prion diseases. Herein, we provide an overview of prion biology, emphasizing the structural, functional, and evolutionary aspects of prions, along with their potential applications in protein engineering. Understanding the structure-function relationships of both healthy and disease-associated prion proteins enables a deeper understanding of the mechanisms of prion-induced neurotoxicity. Furthermore, we describe how insights into prion evolution have begun to shed light on their ancient origins and evolutionary resilience, offering deeper insights into the potential roles of prions in primordial chemical processes.

Histamine stimulates human microglia to alter cellular prion protein expression via the HRH2 histamine receptor

Although the cellular prion protein (PrPC) has been evolutionarily conserved, the role of this protein remains elusive. Recent evidence indicates that PrPC may be involved in neuroinflammation and the immune response in the brain, and its expression may be modified via various mechanisms. Histamine is a proinflammatory mediator and neurotransmitter that stimulates numerous cells via interactions with histamine receptors 1-4 (HRH1-4). Since microglia are the innate immune cells of the central nervous system, we hypothesized that histamine-induced stimulation regulates the expression of PrPC in human-derived microglia. The human microglial clone 3 (HMC3) cell line was treated with histamine, and intracellular calcium levels were measured via a calcium flux assay. Cytokine production was monitored by enzyme-linked immunosorbent assay (ELISA). Western blotting and quantitative reverse transcription-polymerase chain reaction were used to determine protein and gene expression of HRH1-4. Flow cytometry and western blotting were used to measure PrPC expression levels. Fluorescence microscopy was used to examine Iba-1 and PrPC localization. HMC3 cells stimulated by histamine exhibited increased intracellular calcium levels and increased release of IL-6 and IL-8, while also modifying PrPC localization. HMC3 stimulated with histamine for 6 and 24 hours exhibited increased surface PrPC expression. Specifically, we found that stimulation of the HRH2 receptor was responsible for changes in surface PrPC. Histamine-induced increases in surface PrPC were attenuated following inhibition of the HRH2 receptor via the HRH2 antagonist ranitidine. These changes were unique to HRH2 activation, as stimulation of HRH1, HRH3, or HRH4 did not alter surface PrPC. Prolonged stimulation of HMC3 decreased PrPC expression following 48 and 72 hours of histamine stimulation. HMC3 cells can be stimulated by histamine to undergo intracellular calcium influx. Surface expression levels of PrPC on HMC3 cells are altered by histamine exposure, primarily mediated by HRH2. While histamine exposure also increases release of IL-6 and IL-8 in these cells, this cytokine release is not fully dependent on PrPC levels, as IL-6 release is only partially reduced and IL-8 release is unchanged under the conditions of HRH2 blockade that prevent PrPC changes. Overall, this suggests that PrPC may play a role in modulating microglial responses.

The prion protein is required for normal responses to light stimuli by photoreceptors and bipolar cells

The prion protein, PrPC, is well known as an essential susceptibility factor for neurodegenerative prion diseases, yet its function in normal, healthy cells remains uncertain. A role in synaptic function has been proposed for PrPC, supported by its cell surface expression in neurons and glia. Here, in mouse retina, we localized PrPC to the junctions between photoreceptors and bipolar cells using synaptic proteins EAAT5, CtBP2, and PSD-95. PrPC localized most densely with bipolar cell dendrites synapsing with cone photoreceptors. In two coisogenic mouse strains, deletion of the gene encoding PrPC, Prnp, significantly altered the scotopic and/or photopic electroretinographic (ERG) responses of photoreceptors and bipolar cells. Cone-dominant pathways showed the most significant ERG changes. Retinal thickness, quantitated by high-resolution optical coherence tomography (OCT), and ribbon synapse morphology were not altered upon deletion of PrPC, suggesting that the ERG changes were driven by functional rather than structural alterations.

Dopaminergic neurodegeneration in Gerstmann-Sträussler-Scheinker (P102L) disease: insights from imaging and pathological examination

Gerstmann-Sträussler-Scheinker (GSS) disease is an inherited prion disease characterized by dementia, cerebellar ataxia, and painful sensory disturbances. GSS is pathologically defined by the presence of amyloid plaques comprised of prion protein predominantly localized in the cerebral cortex, cerebellar cortex, and basal ganglia, resulting from mutations in the prion protein gene. This study investigated five cases of GSS P102L [GSS caused by a leucine (L) substitution of proline (P) at position 102 of the prion protein gene] with L-dopa-resistant extrapyramidal symptoms and reduced dopamine transporter single-photon emission computed tomography (DAT-SPECT) uptake. Clinical findings revealed diverse manifestations, with all cases exhibiting parkinsonism, and four patients had a vertical gaze palsy. Notably, all patients showed reduced striatal DAT-SPECT uptake, indicating neurodegeneration of the nigrostriatal system. Autopsy findings in one case confirmed prion protein plaques and dopaminergic neuron loss in the substantia nigra of a patient with GSS P102L. Additionally, reduced DAT immunostaining was observed in the putamen compared with a control. While previous studies have identified reduced DAT-SPECT and positron emission tomography uptake in Creutzfeldt-Jakob disease and fatal familial insomnia owing to nigrostriatal neurodegeneration induced by abnormal prion protein deposition, similar phenomena in GSS P102L have not been reported. This study provides support for a correlation between abnormal prion protein deposition and nigrostriatal system degeneration in GSS P102L. Our results reveal the importance of considering GSS P102L in cases of atypical Parkinsonism and abnormal DAT-SPECT results, which would serve as a valuable indicator for subsequent prion genetic testing.

Involvement of the nigrostriatal system in Gerstman-Sträussler-Scheinker disease with the PRNP-P102L mutation

INTRODUCTION

Gerstmann-Sträussler-Scheinker disease (GSS) is an autosomal-dominant inherited prion disease most often associated with the human prion protein gene (PRNP)-P102L mutation. Although patients manifest considerable phenotypic heterogeneity, the involvement of the nigrostriatal system has not been well-studied.

METHODS

We performed dopamine transporter single-photon emission computed tomography (DAT-SPECT) using 123I-ioflupane to investigate the nigrostriatal system function in nine patients with the PRNP-P102L mutation. We also examined the pathological findings in another patient whose predominant feature was ataxia and who died 5 years after disease onset.

RESULTS

Striatum uptake of 123I-ioflupane indicated by specific binding ratio (SBR) values was significantly reduced in two patients. The DAT-SPECT examination was performed 6 months after disease onset in one of these patients who manifested rapidly developing cognitive decline mimicking Creutzfeldt-Jakob disease. DAT-SPECT was also performed 9 years after disease onset in another patient who manifested the conventional features of GSS involving ataxia and dementia in the initial phase but showed akinetic mutism at the examination time. Another patient examined 2 years after disease onset who predominantly manifested ataxia showed marginally abnormal SBR values. An autopsy case showed moderate neuronal loss in the substantia nigra, and the degree of neuronal loss was similar in most other parts of the brain.

CONCLUSION

Nigrostriatal system involvement may occur in patients with GSS associated with the PRNP-P102L mutation, even though parkinsonism is not the predominant feature.

Differential expression of cellular prion protein (PrPC) in mouse hepatitis virus induced neuroinflammation

The cellular prion protein (PrPC) is an extracellular cell membrane protein. Due to its diversified roles, a definite role of PrPC has been difficult to establish. During viral infection, PrPC has been reported to play a pleiotropic role. Here, we have attempted to envision the function of PrPC in the neurotropic m-CoV-MHV-RSA59-induced model of neuroinflammation in C57BL/6 mice. A significant upregulation of PrPC at protein and mRNA levels was evident in infected mouse brains during the acute phase of neuroinflammation. Furthermore, investigation of the effect of MHV-RSA59 infection on PrPC expression in specific neuronal, microglial, and astrocytoma cell lines, revealed a differential expression of prion protein during neuroinflammation. Additionally, siRNA-mediated downregulation of prnp transcripts reduced the expression of viral antigen and viral infectivity in these cell lines. Cumulatively, our results suggest that PrPC expression significantly increases during acute MHV-RSA59 infection and that PrPC also assists in viral infectivity and viral replication.

Brain Organoids: A Game-Changer for Drug Testing

Neurological disorders are the second cause of death and the leading cause of disability worldwide. Unfortunately, no cure exists for these disorders, but the actual therapies are only able to ameliorate people's quality of life. Thus, there is an urgent need to test potential therapeutic approaches. Brain organoids are a possible valuable tool in the study of the brain, due to their ability to reproduce different brain regions and maturation stages; they can be used also as a tool for disease modelling and target identification of neurological disorders. Recently, brain organoids have been used in drug-screening processes, even if there are several limitations to overcome. This review focuses on the description of brain organoid development and drug-screening processes, discussing the advantages, challenges, and limitations of the use of organoids in modeling neurological diseases. We also highlighted the potential of testing novel therapeutic approaches. Finally, we examine the challenges and future directions to improve the drug-screening process.

Proteome landscape and interactome of voltage-gated potassium channel 1.6 (Kv1.6) of the murine ophthalmic artery and neuroretina

The voltage-gated potassium channel 1.6 (Kv1.6) plays a vital role in ocular neurovascular beds and exerts its modulatory functions via interaction with other proteins. However, the interactome and their potential roles remain unknown. Here, the global proteome landscape of the ophthalmic artery (OA) and neuroretina was mapped, followed by the determination of Kv1.6 interactome and validation of its functionality and cellular localization. Microfluorimetric analysis of intracellular [K+] and Western blot validated the native functionality and cellular expression of the recombinant Kv1.6 channel protein. A total of 54, 9 and 28 Kv1.6-interacting proteins were identified in the mouse OA and, retina of mouse and rat, respectively. The Kv1.6-protein partners in the OA, namely actin cytoplasmic 2, alpha-2-macroglobulin and apolipoprotein A-I, were implicated in the maintenance of blood vessel integrity by regulating integrin-mediated adhesion to extracellular matrix and Ca2+ flux. Many retinal protein interactors, particularly the ADP/ATP translocase 2 and cytoskeleton protein tubulin, were involved in endoplasmic reticulum stress response and cell viability. Three common interactors were found in all samples comprising heat shock cognate 71 kDa protein, Ig heavy constant gamma 1 and Kv1.6 channel. This foremost in-depth investigation enriched and identified the elusive Kv1.6 channel and, elucidated its complex interactome.

Copper drives prion protein phase separation and modulates aggregation

Prion diseases are characterized by prion protein (PrP) transmissible aggregation and neurodegeneration, which has been linked to oxidative stress. The physiological function of PrP seems related to sequestering of redox-active Cu2+, and Cu2+ dyshomeostasis is observed in prion disease brain. It is unclear whether Cu2+ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. This study indicates that Cu2+ promotes PrP condensation in live cells at the cell surface and in vitro through copartitioning. Molecularly, Cu2+ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H2O2, triggered liquid-to-solid transition of PrP:Cu2+ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrPC initially protected against Cu2+ cytotoxicity but led to PrPC aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevents copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Diminished Neuronal ESCRT-0 Function Exacerbates AMPA Receptor Derangement and Accelerates Prion-Induced Neurodegeneration

Endolysosomal defects in neurons are central to the pathogenesis of prion and other neurodegenerative disorders. In prion disease, prion oligomers traffic through the multivesicular body (MVB) and are routed for degradation in lysosomes or for release in exosomes, yet how prions impact proteostatic pathways is unclear. We found that prion-affected human and mouse brain showed a marked reduction in Hrs and STAM1 (ESCRT-0), which route ubiquitinated membrane proteins from early endosomes into MVBs. To determine how the reduction in ESCRT-0 impacts prion conversion and cellular toxicity in vivo, we prion-challenged conditional knockout mice (male and female) having Hrs deleted from neurons, astrocytes, or microglia. The neuronal, but not astrocytic or microglial, Hrs-depleted mice showed a shortened survival and an acceleration in synaptic derangements, including an accumulation of ubiquitinated proteins, deregulation of phosphorylated AMPA and metabotropic glutamate receptors, and profoundly altered synaptic structure, all of which occurred later in the prion-infected control mice. Finally, we found that neuronal Hrs (nHrs) depletion increased surface levels of the cellular prion protein, PrPC, which may contribute to the rapidly advancing disease through neurotoxic signaling. Taken together, the reduced Hrs in the prion-affected brain hampers ubiquitinated protein clearance at the synapse, exacerbates postsynaptic glutamate receptor deregulation, and accelerates neurodegeneration.SIGNIFICANCE STATEMENT Prion diseases are rapidly progressive neurodegenerative disorders characterized by prion aggregate spread through the central nervous system. Early disease features include ubiquitinated protein accumulation and synapse loss. Here, we investigate how prion aggregates alter ubiquitinated protein clearance pathways (ESCRT) in mouse and human prion-infected brain, discovering a marked reduction in Hrs. Using a prion-infection mouse model with neuronal Hrs (nHrs) depleted, we show that low neuronal Hrs is detrimental and markedly shortens survival time while accelerating synaptic derangements, including ubiquitinated protein accumulation, indicating that Hrs loss exacerbates prion disease progression. Additionally, Hrs depletion increases the surface distribution of prion protein (PrPC), linked to aggregate-induced neurotoxic signaling, suggesting that Hrs loss in prion disease accelerates disease through enhancing PrPC-mediated neurotoxic signaling.

Prion Protein Octarepeat Domain Forms Transient β-Sheet Structures upon Residue-Specific Binding to Cu(II) and Zn(II) Ions

Misfolding of the cellular prion protein (PrP^C) is associated with the development of fatal neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs). Metal ions appear to play a crucial role in PrP^C misfolding. PrP^C is a combined Cu(II) and Zn(II) metal-binding protein, where the main metal-binding site is located in the octarepeat (OR) region. Thus, the biological function of PrP^C may involve the transport of divalent metal ions across membranes or buffering concentrations of divalent metal ions in the synaptic cleft. Recent studies have shown that an excess of Cu(II) ions can result in PrP^C instability, oligomerization, and/or neuroinflammation. Here, we have used biophysical methods to characterize Cu(II) and Zn(II) binding to the isolated OR region of PrP^C. Circular dichroism (CD) spectroscopy data suggest that the OR domain binds up to four Cu(II) ions or two Zn(II) ions. Binding of the first metal ion results in a structural transition from the polyproline II helix to the β-turn structure, while the binding of additional metal ions induces the formation of β-sheet structures. Fluorescence spectroscopy data indicate that the OR region can bind both Cu(II) and Zn(II) ions at neutral pH, but under acidic conditions, it binds only Cu(II) ions. Molecular dynamics simulations suggest that binding of either metal ion to the OR region results in the formation of β-hairpin structures. As the formation of β-sheet structures can be a first step toward amyloid formation, we propose that high concentrations of either Cu(II) or Zn(II) ions may have a pro-amyloid effect in TSE diseases.

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.

Early Changes in Transcriptomic Profiles in Synaptodendrosomes Reveal Aberrant Synaptic Functions in Alzheimer’s Disease

Alzheimer’s disease (AD) is one of the most prevalent neurodegenerative disorders characterized by the progressive decline of cognitive functions, and is closely associated with the dysfunction of synapses, which comprise the basic structure that mediates the communication between neurons. Although the protein architecture and machinery for protein translation at synapses are extensively studied, the impact that local changes in the mRNA reservoir have on AD progression is largely unknown. Here, we investigated the changes in transcriptomic profiles in the synaptodendrosomes purified from the cortices of AD mice at ages 3 and 6 months, a stage when early signatures of synaptic dysfunction are revealed. The transcriptomic profiles of synaptodendrosomes showed a greater number of localized differentially expressed genes (DEGs) in 6-month-old AD mice compared with mice 3 months of age. Gene Ontology (GO) analysis showed that these DEGs are majorly enriched in mitochondrial biogenesis and metabolic activity. More specifically, we further identified three representative DEGs in mitochondrial and metabolic pathways—Prnp, Cst3, and Cox6c—that regulate the dendritic spine density and morphology in neurons. Taken together, this study provides insights into the transcriptomic changes in synaptodendrosomes during AD progression, which may facilitate the development of intervention strategies targeting local translation to ameliorate the pathological progression of AD.

Neurogranin and Neurofilament Light Chain as Preclinical Biomarkers in Scrapie

Prion diseases are diagnosed in the symptomatic stage, when the neuronal damage is spread throughout the central nervous system (CNS). The assessment of biological features that allow the detection of asymptomatic cases is needed, and, in this context, scrapie, where pre-symptomatic infected animals can be detected through rectal biopsy, becomes a good study model. Neurogranin (Ng) and neurofilament light chain (NfL) are proteins that reflect synaptic and axonal damage and have been studied as cerebrospinal fluid (CSF) biomarkers in different neurodegenerative disorders. In this study, we evaluated Ng and NfL both at the protein and transcript levels in the CNS of preclinical and clinical scrapie-affected sheep compared with healthy controls and assessed their levels in ovine CSF. The correlation between these proteins and the main neuropathological events in prion diseases, PrPSc deposition and spongiosis, was also assessed. The results show a decrease in Ng and NfL at the protein and gene expression levels as the disease progresses, and significant changes between the control and preclinical animals. On the contrary, the CSF levels of NfL increased throughout the progression of the disease. Negative correlations between neuropathological markers of prion disease and the concentration of the studied proteins were also found. Although further research is needed, these results suggest that Ng and NfL could act as biomarkers for neurodegeneration onset and intensity in preclinical cases of scrapie.

Prion Protein: The Molecule of Many Forms and Faces

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

Prion Protein on Human Leukocytes Is Reduced in Iron Deficiency - Possible Implications for Age-related Macular Degeneration?

MATERIALS AND METHODS

Patients presenting to the department of ophthalmology of the Medical University of Graz for reasons unrelated to prion diseases were enrolled. Parameters of iron metabolism, including ferritin and soluble transferrin receptor were measured by routine laboratory tests. Serum prion protein was determined by enzyme-linked immunosorbent assay. Surface prion protein on CD14⁺ monocytes and CD4⁺ T cells was analyzed by fluorescence activated cell sorting.

RESULTS

95 patients were enrolled. Soluble transferrin receptor correlated significantly with prion protein levels on CD14⁺POM1⁺ monocytes (P = .001, r = -0.7) and on CD4⁺POM1⁺ T cells (P = .01, r = -0.62).

CONCLUSION

Our findings suggest a connection between the physiological function of the prion protein and iron metabolism in humans.

Impairment of cerebellar long-term depression and GABAergic transmission in prion protein deficient mice ectopically expressing PrPLP/Dpl

Prion protein (PrP^C) knockout mice, named as the “Ngsk” strain (Ngsk Prnp^0/0 mice), show late-onset cerebellar Purkinje cell (PC) degeneration because of ectopic overexpression of PrP^C-like protein (PrPLP/Dpl). Our previous study indicated that the mutant mice also exhibited alterations in cerebellum-dependent delay eyeblink conditioning, even at a young age (16 weeks of age) when neurological changes had not occurred. Thus, this electrophysiological study was designed to examine the synaptic function of the cerebellar cortex in juvenile Ngsk Prnp^0/0 mice. We showed that Ngsk Prnp^0/0 mice exhibited normal paired-pulse facilitation but impaired long-term depression of excitatory synaptic transmission at synapses between parallel fibres and PCs. GABA_A-mediated inhibitory postsynaptic currents recorded from PCs were also weakened in Ngsk Prnp^0/0 mice. Furthermore, we confirmed that Ngsk Prnp^0/0 mice (7–8-week-old) exhibited abnormalities in delay eyeblink conditioning. Our findings suggest that these alterations in both excitatory and inhibitory synaptic transmission to PCs caused deficits in delay eyeblink conditioning of Ngsk Prnp^0/0 mice. Therefore, the Ngsk Prnp^0/0 mouse model can contribute to study underlying mechanisms for impairments of synaptic transmission and neural plasticity, and cognitive deficits in the central nervous system.

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

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

EFFECT OF ORAL COPPER SUPPLEMENTATION ON SUSCEPTIBILITY IN WHITE-TAILED DEER (ODOCOILEUS VIRGINIANUS) TO CHRONIC WASTING DISEASE

Chronic wasting disease (CWD) is an infectious disease, but reported associations suggest several metals-especially copper (Cu) and manganese-potentially play a role in this and other prion diseases. To assess the utility of dietary Cu supplementation in protecting white-tailed deer (Odocoileus virginianus) from CWD, we compared incidence and disease course among individuals naturally exposed to CWD while being maintained on sustained-release Cu boluses or unsupplemented (control). Oral Cu supplementation increased liver tissue Cu concentrations compared to controls but did not affect susceptibility to CWD or survival after natural exposure in the captive white-tailed deer we studied. Over the 27 mo study, 89% (8/9) of the Cu-supplemented deer and 86% (6/7) of control deer became CWD-infected. Survival to 27 mo postexposure did not differ between Cu-supplemented and control deer: model-averaged survival probabilities to 27 mo were 0.45-0.47 for all combinations of Cu treatment and PRNP gene haplotype presence. The PRNP gene haplotype influenced the probability of deer remaining biopsy negative for at least 17 mo but did not affect overall susceptibility.

Prion protein-Semisynthetic prion protein (PrP) variants with posttranslational modifications

Deciphering the pathophysiologic events in prion diseases is challenging, and the role of posttranslational modifications (PTMs) such as glypidation and glycosylation remains elusive due to the lack of homogeneous protein preparations. So far, experimental studies have been limited in directly analyzing the earliest events of the conformational change of cellular prion protein (PrP^C ) into scrapie prion protein (PrP^Sc ) that further propagates PrP^C misfolding and aggregation at the cellular membrane, the initial site of prion infection, and PrP misfolding, by a lack of suitably modified PrP variants. PTMs of PrP, especially attachment of the glycosylphosphatidylinositol (GPI) anchor, have been shown to be crucially involved in the PrP^Sc formation. To this end, semisynthesis offers a unique possibility to understand PrP behavior invitro and invivo as it provides access to defined site-selectively modified PrP variants. This approach relies on the production and chemoselective linkage of peptide segments, amenable to chemical modifications, with recombinantly produced protein segments. In this article, advances in understanding PrP conversion using semisynthesis as a tool to obtain homogeneous posttranslationally modified PrP will be discussed.

The Link of the Prion Protein with Ca2+ Metabolism and ROS Production, and the Possible Implication in Aβ Toxicity

The cellular prion protein (PrP^C) is an ubiquitous cell surface protein mostly expressed in neurons, where it localizes to both pre- and post-synaptic membranes. PrP^C aberrant conformers are the major components of mammalian prions, the infectious agents responsible for incurable neurodegenerative disorders. PrP^C was also proposed to bind aggregated misfolded proteins/peptides, and to mediate their neurotoxic signal. In spite of long-lasting research, a general consensus on the precise pathophysiologic mechanisms of PrP^C has not yet been reached. Here we review our recent data, obtained by comparing primary neurons from PrP-expressing and PrP-knockout mice, indicating a central role of PrP^C in synaptic transmission and Ca²⁺ homeostasis. Indeed, by controlling gene expression and signaling cascades, PrP^C is able to optimize glutamate secretion and regulate Ca²⁺ entry via store-operated channels and ionotropic glutamate receptors, thereby protecting neurons from threatening Ca²⁺ overloads and excitotoxicity. We will also illustrate and discuss past and unpublished results demonstrating that Aβ oligomers perturb Ca²⁺ homeostasis and cause abnormal mitochondrial accumulation of reactive oxygen species by possibly affecting the PrP-dependent downregulation of Fyn kinase activity.

Cellular prion protein transcriptionally regulated by NFIL3 enhances lung cancer cell lamellipodium formation and migration through JNK signaling

Tumor invasion and metastasis are the major causes of treatment failure and mortality in lung cancer patients. In this study, we identified a group of genes with differential expression in in situ and invasive lung adenocarcinoma tissues by expression profiling; among these genes we further characterized the association of the upregulation of PRNP, the gene encoding cellular Prion protein (PrPc), with lung adenocarcinoma invasiveness. Immunohistochemistry on clinical specimens showed an association of PrPc expression with invasive but not in situ lung adenocarcinoma. Consistently, the expression of PrPc was higher in the highly invasive than in the lowly invasive lung adenocarcinoma cell lines. Knockdown of PrPc expression in cultured lung adenocarcinoma cells decreased their lamellipodium formation, in vitro migration and invasion, and in vivo experimental lung metastasis. Phosphorylation of JNKs was found to correlate with PrPc expression and the inhibition of JNKs suppressed the PrPc-induced up-regulation of lamellipodium formation, cell migration, and invasion. Moreover, we identified the nuclear factor, interleukin 3 regulated (NFIL3) protein as a transcriptional activator of the PRNP promoter. Accordingly, NFIL3 promoted lung cancer cell migration and invasion in a PrPc-dependent manner. High NFIL3 expression in clinical specimens of lung adenocarcinoma was also associated with tumor invasiveness. Overall, our observations suggest that the NFIL3/PrPc axis, through regulating lamellipodium formation and cell mobility via JNK signaling, plays a critical role in lung cancer invasiveness and metastasis.

Structural Consequences of Copper Binding to the Prion Protein

Prion, or PrP^Sc, is the pathological isoform of the cellular prion protein (PrP^C) and it is the etiological agent of transmissible spongiform encephalopathies (TSE) affecting humans and animal species. The most relevant function of PrP^C is its ability to bind copper ions through its flexible N-terminal moiety. This review includes an overview of the structure and function of PrP^C with a focus on its ability to bind copper ions. The state-of-the-art of the role of copper in both PrP^C physiology and in prion pathogenesis is also discussed. Finally, we describe the structural consequences of copper binding to the PrP^C structure.

Copper Binding Regulates Cellular Prion Protein Function

The cellular prion protein (PrP^C), mainly known for its role in neurodegenerative diseases, is involved in several physiological processes including neuritogenesis. In addition, its ability to bind copper or zinc has been suggested for its role in metal homeostasis. Although PrP^C has been known as a copper-binding molecule, little is known about how copper can affect PrP^C physiological functions. By combining genomic approaches, cellular assays, and focal stimulation technique, we found that PrP^C neuritogenesis function is directly influenced by N-terminal copper-binding amino acids. Several recombinant mouse PrP (recMoPrP) mutants at N-terminal copper-binding sites were produced, and primary hippocampal cultures were treated either in bulk or exposed near the hippocampal growth cones (GC) of single neurons in local stimulation manner. While focal stimulation of GC with wild-type recMoPrP induced neurite outgrowth and rapid GC turning toward the source, N-terminal mutants fail to support this effect. Indeed, disrupting all the copper-binding sites at the N-terminus of PrP^C was toxic to neurons indicating that these regions are crucial for the protein function. Mutants at both octarepeat and non-octarepeat region abolished the neuritogenesis effect. Altogether, our findings indicate the crucial role of copper-binding sites in maintaining the neuritogenesis function in PrP, suggesting a potential link between loss-of-function of the protein and disease initiation.

Molecular Features of the Zn2+ Binding Site in the Prion Protein Probed by 113Cd NMR

The cellular prion protein (PrP^C) is a zinc-binding protein that contributes to the regulation of Zn²⁺ and other divalent species of the central nervous system. Zn²⁺ coordinates to the flexible, N-terminal repeat region of PrP^C and drives a tertiary contact between this repeat region and a well-defined cleft of the C-terminal domain. The tertiary structure promoted by Zn²⁺ is thought to regulate inherent PrP^C toxicity. Despite the emerging consensus regarding the interaction between Zn²⁺ and PrP^C, there is little direct spectroscopic confirmation of the metal ion's coordination details. Here, we address this conceptual gap by using Cd²⁺ as a surrogate for Zn²⁺. NMR finds that Cd²⁺ binds exclusively to the His imidazole side chains of the repeat segment, with a dissociation constant of ∼1.2 mM, and promotes an N-terminal-C-terminal cis interaction very similar to that observed with Zn²⁺. Analysis of ¹¹³Cd NMR spectra of PrP^C, along with relevant control proteins and peptides, suggests that coordination of Cd²⁺ in the full-length protein is consistent with a three- or four-His geometry. Examination of the mutation E199K in mouse PrP^C (E200K in humans), responsible for inherited Creutzfeldt-Jakob disease, finds that the mutation lowers metal ion affinity and weakens the cis interaction. These findings not only provide deeper insight into PrP^C metal ion coordination but they also suggest new perspectives on the role of familial mutations in prion disease.

Prion Seeds Distribute throughout the Eyes of Sporadic Creutzfeldt-Jakob Disease Patients

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common prion disease in humans and has been iatrogenically transmitted through corneal graft transplantation. Approximately 40% of sCJD patients develop visual or oculomotor symptoms and may seek ophthalmological consultation. Here we used the highly sensitive real-time quaking-induced conversion (RT-QuIC) assay to measure postmortem prion seeding activities in cornea, lens, ocular fluid, retina, choroid, sclera, optic nerve, and extraocular muscle in the largest series of sCJD patient eyes studied by any assay to date. We detected prion seeding activity in 100% of sCJD eyes, representing three common sCJD subtypes, with levels varying by up to 4 log-fold among individuals. The retina consistently showed the highest seed levels, which in some cases were only slightly lower than brain. Within the retina, prion deposits were detected by immunohistochemistry (IHC) in the retinal outer plexiform layer in most sCJD cases, and in some eyes the inner plexiform layer, consistent with synaptic prion deposition. Prions were not detected by IHC in any other eye region. With RT-QuIC, prion seed levels generally declined in eye tissues with increased distance from the brain, and yet all corneas had prion seeds detectable. Prion seeds were also present in the optic nerve, extraocular muscle, choroid, lens, vitreous, and sclera. Collectively, these results reveal that sCJD patients accumulate prion seeds throughout the eye, indicating the potential diagnostic utility as well as a possible biohazard.IMPORTANCE Cases of iatrogenic prion disease have been reported from corneal transplants, yet the distribution and levels of prions throughout the eye remain unknown. This study probes the occurrence, level, and distribution of prions in the eyes of patients with sporadic Creutzfeldt-Jakob disease (sCJD). We tested the largest series of prion-infected eyes reported to date using an ultrasensitive technique to establish the prion seed levels in eight regions of the eye. All 11 cases had detectable prion seeds in the eye, and in some cases, the seed levels in the retina approached those in brain. In most cases, prion deposits could also be seen by immunohistochemical staining of retinal tissue; other ocular tissues were negative. Our results have implications for estimating the risk for iatrogenic transmission of sCJD as well as for the development of antemortem diagnostic tests for prion diseases.

Sporadic Creutzfeldt-Jakob disease

Sporadic Creutzfeldt-Jakob disease (CJD), the most common human prion disease, is generally regarded as a spontaneous neurodegenerative illness, arising either from a spontaneous PRNP somatic mutation or a stochastic PrP structural change. Alternatively, the possibility of an infection from animals or other source remains to be completely ruled out. Sporadic CJD is clinically characterized by rapidly progressive dementia with ataxia, myoclonus, or other neurologic signs and, neuropathologically, by the presence of aggregates of abnormal prion protein, spongiform change, neuronal loss, and gliosis. Despite these common features the disease shows a wide phenotypic variability which was recognized since its early descriptions. In the late 1990s the identification of key molecular determinants of phenotypic expression and the availability of a large series of neuropathologically verified cases led to the characterization of definite clinicopathologic and molecular disease subtypes and to an internationally recognized disease classification. By showing that these disease subtypes correspond to specific agent strain-host genotype combinations, recent transmission studies have confirmed the biologic basis of this classification. The introduction of brain magnetic resonance imaging techniques such as fluid-attenuated inversion recovery and diffusion-weighted imaging sequences and cerebrospinal fluid biomarker assays for the detection of brain-derived proteins as well as real-time quaking-induced conversion assay, allowing the specific detection of prions in accessible biologic fluids and tissues, has significantly contributed to the improved accuracy of the clinical diagnosis of sporadic CJD in recent years.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

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.

Prion protein stabilizes amyloid-β (Aβ) oligomers and enhances Aβ neurotoxicity in a Drosophila model of Alzheimer's disease

The cellular prion protein (PrP^C) can act as a cell-surface receptor for β-amyloid (Aβ) peptide; however, a role for PrP^C in the pathogenesis of Alzheimer's disease (AD) is contested. Here, we expressed a range of Aβ isoforms and PrP^C in the Drosophila brain. We found that co-expression of Aβ and PrP^C significantly reduces the lifespan, disrupts circadian rhythms, and increases Aβ deposition in the fly brain. In contrast, under the same conditions, expression of Aβ or PrP^C individually did not lead to these phenotypic changes. In vitro studies revealed that substoichiometric amounts of PrP^C trap Aβ as oligomeric assemblies and fragment-preformed Aβ fibers. The ability of membrane-anchored PrP^C to trap Aβ as cytotoxic oligomers at the membrane surface and fragment inert Aβ fibers suggests a mechanism by which PrP^C exacerbates Aβ deposition and pathogenic phenotypes in the fly, supporting a role for PrP^C in AD. This study provides a second animal model linking PrP^C expression with Aβ toxicity and supports a role for PrP^C in AD pathogenesis. Blocking the interaction of Aβ and PrP^C represents a potential therapeutic strategy.

The phospholipase A2 pathway controls a synaptic cholesterol ester cycle and synapse damage

The cellular prion protein (PrP^C) acts as a scaffold protein that organises signalling complexes. In synaptosomes, the aggregation of PrP^C by amyloid-β (Aβ) oligomers attracts and activates cytoplasmic phospholipase A₂ (cPLA₂), leading to synapse degeneration. The signalling platform is dependent on cholesterol released from cholesterol esters by cholesterol ester hydrolases (CEHs). The activation of cPLA₂ requires cholesterol released from cholesterol esters by cholesterol ester hydrolases (CEHs), enzymes dependent upon platelet activating factor (PAF) released by activated cPLA₂ This demonstrates a positive feedback system in which activated cPLA₂ increased cholesterol concentrations, which in turn facilitated cPLA₂ activation. PAF was also required for the incorporation of the tyrosine kinase Fyn and cyclooxygenase (COX)-2 into Aβ-PrP^C-cPLA₂ complexes. As a failure to deactivate signalling complexes can lead to pathology, the mechanisms involved in their dispersal were studied. PAF facilitated the incorporation of acyl-coenzyme A:cholesterol acyltransferase (ACAT)-1 into Aβ-PrP^C-cPLA₂-COX-2-Fyn complexes. The esterification of cholesterol reduced cholesterol concentrations, causing dispersal of Aβ-PrP^C-cPLA₂-COX-2-Fyn complexes and the cessation of signalling. This study identifies PAF as a key mediator regulating the cholesterol ester cycle, activation of cPLA₂ and COX-2 within synapses, and synapse damage.

Role of cellular prion protein in interneuronal amyloid transmission

Several studies have indicated that certain misfolded amyloids composed of tau, β-amyloid or α-synuclein can be transferred from cell to cell, suggesting the contribution of mechanisms reminiscent of those by which infective prions spread through the brain. This process of a 'prion-like' spreading between cells is also relevant as a novel putative therapeutic target that could block the spreading of proteinaceous aggregates throughout the brain which may underlie the progressive nature of neurodegenerative diseases. The relevance of β-amyloid oligomers and cellular prion protein (PrP^C) binding has been a focus of interest in Alzheimer's disease (AD). At the molecular level, β-amyloid/PrP^C interaction takes place in two differently charged clusters of PrP^C. In addition to β-amyloid, participation of PrP^C in α-synuclein binding and brain spreading also appears to be relevant in α-synucleopathies. This review summarizes current knowledge about PrP^C as a putative receptor for amyloid proteins and the physiological consequences of these interactions.

Prion Proteins Without the Glycophosphatidylinositol Anchor: Potential Biomarkers in Neurodegenerative Diseases

Prion protein (PrP) is a biomolecule that is involved in neuronal signaling, myelinization, and the development of neurodegenerative diseases. In the cell, PrP is shed by the ADAM10 protease. This process generates PrP molecules that lack glycophosphatidylinositol anchor, and these molecules incorporate into toxic aggregates and neutralize toxic oligomers. Due to this dual role, these molecules are important biomarkers for neurodegenerative diseases. In this review, we present shed PrP as a potential biomarker, with a focus on PrP226*, which may be the main biomarker for predicting neurodegenerative diseases in humans.

Monomeric amyloid-β reduced amyloid-β oligomer-induced synapse damage in neuronal cultures

Alzheimer's disease is a progressive neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) in the brain. Aβ oligomers are believed to cause synapse damage resulting in the memory deficits that are characteristic of this disease. Since the loss of synaptic proteins in the brain correlates closely with the degree of dementia in Alzheimer's disease, the process of Aβ-induced synapse damage was investigated in cultured neurons by measuring the loss of synaptic proteins. Soluble Aβ oligomers, derived from Alzheimer's-affected brains, caused the loss of cysteine string protein and synaptophysin from neurons. When applied to synaptosomes Aβ oligomers increased cholesterol concentrations and caused aberrant activation of cytoplasmic phospholipase A₂ (cPLA₂). In contrast, Aβ monomer preparations did not affect cholesterol concentrations or activate synaptic cPLA₂, nor did they damage synapses. The Aβ oligomer-induced aggregation of cellular prion proteins (PrP^C) at synapses triggered the activation of cPLA₂ that leads to synapse degeneration. Critically, Aβ monomer preparations did not cause the aggregation of PrP^C; rather they reduced the Aβ oligomer-induced aggregation of PrP^C. The presence of Aβ monomer preparations also inhibited the Aβ oligomer-induced increase in cholesterol concentrations and activation of cPLA₂ in synaptosomes and protected neurons against the Aβ oligomer-induced synapse damage. These results support the hypothesis that Aβ monomers are neuroprotective. We hypothesise that synapse damage may result from a pathological Aβ monomer:oligomer ratio rather than the total concentrations of Aβ within the brain.

Human S100A5 binds Ca2+ and Cu2+ independently

Background

S100A5 is a calcium binding protein found in a small subset of amniote tissues. Little is known about the biological roles of S100A5, but it may be involved in inflammation and olfactory signaling. Previous work indicated that S100A5 displays antagonism between binding of Ca²⁺ and Cu²⁺ ions-one of the most commonly cited features of the protein. We set out to characterize the interplay between Ca²⁺ and Cu²⁺ binding by S100A5 using isothermal titration calorimetry (ITC), circular dichroism spectroscopy (CD), and analytical ultracentrifugation (AUC).

Results

We found that human S100A5 is capable of binding both Cu²⁺ and Ca²⁺ ions simultaneously. The wildtype protein was extremely aggregation-prone in the presence of Cu²⁺ and Ca²⁺. A Cys-free version of S100A5, however, was not prone to precipitation or oligomerization. Mutation of the cysteines does not disrupt the binding of either Ca²⁺ or Cu²⁺ to S100A5. In the Cys-free background, we measured Ca²⁺ and Cu²⁺ binding in the presence and absence of the other metal using ITC. Saturating concentrations of Ca²⁺ or Cu²⁺ do not disrupt the binding of one another. Ca²⁺ and Cu²⁺ binding induce structural changes in S100A5, which are measurable using CD spectroscopy. We show via sedimentation velocity AUC that the wildtype protein is prone to the formation of soluble oligomers, which are not present in Cys-free samples.

Conclusions

S100A5 can bind Ca²⁺ and Cu²⁺ ions simultaneously and independently. This observation is in direct contrast to previously-reported antagonism between binding of Cu²⁺ and Ca²⁺ ions. The previous result is likely due to metal-dependent aggregation. Little is known about the biology of S100A5, so an accurate understanding of the biochemistry is necessary to make informed biological hypotheses. Our observations suggest the possibility of independent biological functions for Cu²⁺ and Ca²⁺ binding by S100A5.

A roadmap for investigating the role of the prion protein in depression associated with neurodegenerative disease

The physiological properties of the native, endogenous prion protein (PrP(C)) is a matter of concern, due to its pleiotropic functions and links to neurodegenerative disorders and cancer. In line with our hypothesis that the basic function of PrP(C) is to serve as a cell surface scaffold for the assembly of signaling modules, multiple interactions have been identified of PrP(C) with signaling molecules, including neurotransmitter receptors. We recently reported evidence that PrP(C) may modulate monoaminergic neurotransmission, as well as depressive-like behavior in mice. Here, we discuss how those results, together with a number of other studies, including our previous demonstration that both inflammatory and behavioral stress modulate PrP(C) content in neutrophils, suggest a distributed role of PrP(C) in clinical depression and inflammation associated with neurodegenerative diseases. An overarching understanding of the multiple interventions of PrP(C) upon physiological events may both shed light on the pathogenesis of, as well as help the identification of novel therapeutic targets for clinical depression, Prion and Alzheimer's Diseases.

Copper- and Zinc-Promoted Interdomain Structure in the Prion Protein: A Mechanism for Autoinhibition of the Neurotoxic N-Terminus

The function of the cellular prion protein (PrP^C), while still poorly understood, is increasingly linked to its ability to bind physiological metal ions at the cell surface. PrP^C binds divalent forms of both copper and zinc through its unstructured N-terminal domain, modulating interactions between PrP^C and various receptors at the cell surface and ultimately tuning downstream cellular processes. In this chapter, we briefly discuss the molecular features of copper and zinc uptake by PrP^C and summarize evidence implicating these metal ions in PrP-mediated physiology. We then focus our review on recent biophysical evidence revealing a physical interaction between the flexible N-terminal and globular C-terminal domains of PrP^C. This interdomain cis interaction is electrostatic in nature and is promoted by the binding of Cu²⁺ and Zn²⁺ to the N-terminal octarepeat domain. These findings, along with recent cellular studies, suggest a mechanism whereby NC interactions serve to regulate the activity and/or toxicity of the PrP^C N-terminus. We discuss this potential mechanism in relation to familial prion disease mutations, lethal deletions of the PrP^C central region, and neurotoxicity induced by certain globular domain ligands, including bona fide prions and toxic amyloid-β oligomers.

The cholesterol ester cycle regulates signalling complexes and synapse damage caused by amyloid-β

Cholesterol is required for the formation and function of some signalling platforms. In synaptosomes, amyloid-β (Aβ) oligomers, the causative agent in Alzheimer's disease, bind to cellular prion proteins (PrP^C) resulting in increased cholesterol concentrations, translocation of cytoplasmic phospholipase A₂ (cPLA₂, also known as PLA2G4A) to lipid rafts, and activation of cPLA₂ The formation of Aβ-PrP^C complexes is controlled by the cholesterol ester cycle. In this study, Aβ activated cholesterol ester hydrolases, which released cholesterol from stores of cholesterol esters and stabilised Aβ-PrP^C complexes, resulting in activated cPLA₂ Conversely, cholesterol esterification reduced cholesterol concentrations causing the dispersal of Aβ-PrP^C complexes. In cultured neurons, the cholesterol ester cycle regulated Aβ-induced synapse damage; cholesterol ester hydrolase inhibitors protected neurons, while inhibition of cholesterol esterification significantly increased Aβ-induced synapse damage. An understanding of the molecular mechanisms involved in the dispersal of signalling complexes is important as failure to deactivate signalling pathways can lead to pathology. This study demonstrates that esterification of cholesterol is a key factor in the dispersal of Aβ-induced signalling platforms involved in the activation of cPLA₂ and synapse degeneration.

Cell Biology of Prion Protein

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP^C remains elusive but may depend upon its cellular localization. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP^Sc, suggesting that molecular species of PrP distinct from PrP^Sc may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP^Sc but rather ^CtmPrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP^Sc and that associated with ^CtmPrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP^C biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.

Functions of the Prion Protein

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP^C have gained much interest over the past two decades. Research aiming at unraveling PrP^C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection and, more recently, in the context of Alzheimer's disease. It must now be admitted that PrP^C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP^C in cell signaling events. Our aim here is to provide an overview of our current understanding of PrP^C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.

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.

Insights into the mechanisms of copper dyshomeostasis in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a severe neuromuscular disease characterised by a progressive loss of motor neurons that usually results in paralysis and death within 2 to 5 years after disease onset. The pathophysiological mechanisms involved in ALS remain largely unknown and to date there is no effective treatment for this disease. Here, we review clinical and experimental evidence suggesting that dysregulation of copper homeostasis in the central nervous system is a crucial underlying event in motor neuron degeneration and ALS pathophysiology. We also review and discuss novel approaches seeking to target copper delivery to treat ALS. These novel approaches may be clinically relevant not only for ALS but also for other neurological disorders with abnormal copper homeostasis, such as Parkinson's, Huntington's and Prion diseases.

The biological function of the cellular prion protein: an update

The misfolding of the cellular prion protein (PrP^C) causes fatal neurodegenerative diseases. Yet PrP^C is highly conserved in mammals, suggesting that it exerts beneficial functions preventing its evolutionary elimination. Ablation of PrP^C in mice results in well-defined structural and functional alterations in the peripheral nervous system. Many additional phenotypes were ascribed to the lack of PrP^C, but some of these were found to arise from genetic artifacts of the underlying mouse models. Here, we revisit the proposed physiological roles of PrP^C in the central and peripheral nervous systems and highlight the need for their critical reassessment using new, rigorously controlled animal models.

Almost a century of prion protein(s): From pathology to physiology, and back to pathology

Prions are one of the few pathogens whose name is renowned at all population levels, after the dramatic years pervaded by the fear of eating prion-infected food. If now this, somehow irrational, scare of bovine meat inexorably transmitting devastating brain disorders is largely subdued, several prion-related issues are still unsolved, precluding the design of therapeutic approaches that could slow, if not halt, prion diseases. One unsolved issue is, for example, the role of the prion protein (PrP^C), whole conformational misfolding originates the prion but whose physiologic reason d'etre in neurons, and in cells at large, remains enigmatic. Preceded by a historical outline, the present review will discuss the functional pleiotropicity ascribed to PrP^C, and whether this aspect could fall, at least in part, into a more concise framework. It will also be devoted to radically different perspectives for PrP^C, which have been recently brought to the attention of the scientific world with unexpected force. Finally, it will discuss the possible reasons allowing an evolutionary conserved and benign protein, as PrP^C is, to turn into a high affinity receptor for pathologic misfolded oligomers, and to transmit their toxic message into neurons.