Dominant roles of the polybasic proline motif and copper in the PrP23-89-mediated stress protection response

Prion Protein Endoproteolysis: Cleavage Sites, Mechanisms and Connections to Prion Disease

Highly abundant in neurons, the cellular prion protein (PrPC) is an obligatory precursor to the disease-associated misfolded isoform denoted PrPSc that accumulates in the rare neurodegenerative disorders referred to either as transmissible spongiform encephalopathies (TSEs) or as prion diseases. The ability of PrPC to serve as a substrate for this template-mediated conversion process depends on several criteria but importantly includes the presence or absence of certain endoproteolytic events performed at the cell surface or in acidic endolysosomal compartments. The major endoproteolytic events affecting PrPC are referred to as α- and β-cleavages, and in this review we outline the sites within PrPC at which the cleavages occur, the mechanisms potentially responsible and their relevance to pathology. Although the association of α-cleavage with neuroprotection is well-supported, we identify open questions regarding the importance of β-cleavage in TSEs and suggest experimental approaches that could provide clarification. We also combine findings from in vitro cleavage assays and mass spectrometry-based studies of prion protein fragments in the brain to present an updated view in which α- and β-cleavages may represent two distinct clusters of proteolytic events that occur at multiple neighbouring sites rather than at single positions. Furthermore, we highlight the candidate proteolytic mechanisms best supported by the literature; currently, despite several proteases identified as capable of processing PrPC in vitro, in cell-based models and in some cases, in vivo, none have been shown conclusively to cleave PrPC in the brain. Addressing this knowledge gap will be crucial for developing therapeutic interventions to drive PrPC endoproteolysis in a neuroprotective direction. Finally, we end this review by briefly addressing other cleavage events, specifically ectodomain shedding, γ-cleavage, the generation of atypical pathological fragments in the familial prion disorder Gerstmann-Sträussler-Scheinker syndrome and the possibility of an additional form of endoproteolysis close to the PrPC N-terminus.

Altered energy metabolism in Fatal Familial Insomnia cerebral organoids is associated with astrogliosis and neuronal dysfunction

Fatal familial insomnia (FFI) is a rare neurodegenerative disease caused by a dominantly inherited single amino acid substitution (D178N) within the prion protein (PrP). No in vitro human brain tissue model for this disease has previously been available. Consequently, how this mutation exerts its damaging effect on brain cells is still unknown. Using CRISPR-Cas9 engineered induced pluripotent stem cells, we made D178N cerebral organoids and compared these with isotype control organoids. We found that, in the absence of other hallmarks of FFI, the D178N organoids exhibited astrogliosis with cellular oxidative stress. Abnormal post-translational processing of PrP was evident but no tissue deposition or propagation of mis-folded PrP isoforms were observed. Neuronal electrophysiological function was compromised and levels of neurotransmitters, particularly acetylcholine and GABA, altered. Underlying these dysfunctions were changes in cellular energy homeostasis, with substantially increased glycolytic and Krebs cycle intermediates, and greater mitochondrial activity. This increased energy demand in D178N organoids was associated with increased mitophagy and depletion of lipid droplets, in turn resulting in shifts of cellular lipid composition. Using a double mutation (178NN) we could confirm that most changes were caused by the presence of the mutation rather than interaction with PrP molecules lacking the mutation. Our data strongly suggests that shifting biosynthetic intermediates and oxidative stress, caused by an imbalance of energy supply and demand, results in astrogliosis with compromised neuronal activity in FFI organoids. They further support that many of the disease associated changes are due to a corruption of PrP function and do not require propagation of PrP mis-folding.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

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.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Reduced SOD2 expression does not influence prion disease course or pathology in mice

Prion diseases are progressive, neurodegenerative diseases affecting humans and animals. Also known as the transmissible spongiform encephalopathies, for the hallmark spongiform change seen in the brain, these diseases manifest increased oxidative damage early in disease and changes in antioxidant enzymes in terminal brain tissue. Superoxide dismutase 2 (SOD2) is an antioxidant enzyme that is critical for life. SOD2 knock-out mice can only be kept alive for several weeks post-birth and only with antioxidant therapy. However, this results in the development of a spongiform encephalopathy. Consequently, we hypothesized that reduced levels of SOD2 may accelerate prion disease progression and play a critical role in the formation of spongiform change. Using SOD2 heterozygous knock-out and litter mate wild-type controls, we examined neuronal long-term potentiation, disease duration, pathology, and degree of spongiform change in mice infected with three strains of mouse adapted scrapie. No influence of the reduced SOD2 expression was observed in any parameter measured for any strain. We conclude that changes relating to SOD2 during prion disease are most likely secondary to the disease processes causing toxicity and do not influence the development of spongiform pathology.

Biological Functions of the Intrinsically Disordered N-Terminal Domain of the Prion Protein: A Possible Role of Liquid-Liquid Phase Separation

The mammalian prion protein (PrPC) is composed of a large intrinsically disordered N-terminal and a structured C-terminal domain, containing three alpha-helical regions and a short, two-stranded beta-sheet. Traditionally, the activity of a protein was linked to the ability of the polypeptide chain to adopt a stable secondary/tertiary structure. This concept has been extended when it became evident that intrinsically disordered domains (IDDs) can participate in a broad range of defined physiological activities and play a major functional role in several protein classes including transcription factors, scaffold proteins, and signaling molecules. This ability of IDDs to engage in a variety of supramolecular complexes may explain the large number of PrPC-interacting proteins described. Here, we summarize diverse physiological and pathophysiological activities that have been described for the unstructured N-terminal domain of PrPC. In particular, we focus on subdomains that have been conserved in evolution.

N-Terminal Regions of Prion Protein: Functions and Roles in Prion Diseases

The normal cellular isoform of prion protein, designated PrP^C, is constitutively converted to the abnormally folded, amyloidogenic isoform, PrP^Sc, in prion diseases, which include Creutzfeldt-Jakob disease in humans and scrapie and bovine spongiform encephalopathy in animals. PrP^C is a membrane glycoprotein consisting of the non-structural N-terminal domain and the globular C-terminal domain. During conversion of PrP^C to PrP^Sc, its 2/3 C-terminal region undergoes marked structural changes, forming a protease-resistant structure. In contrast, the N-terminal region remains protease-sensitive in PrP^Sc. Reverse genetic studies using reconstituted PrP^C-knockout mice with various mutant PrP molecules have revealed that the N-terminal domain has an important role in the normal function of PrP^C and the conversion of PrP^C to PrP^Sc. The N-terminal domain includes various characteristic regions, such as the positively charged residue-rich polybasic region, the octapeptide repeat (OR) region consisting of five repeats of an octapeptide sequence, and the post-OR region with another positively charged residue-rich polybasic region followed by a stretch of hydrophobic residues. We discuss the normal functions of PrP^C, the conversion of PrP^C to PrP^Sc, and the neurotoxicity of PrP^Sc by focusing on the roles of the N-terminal regions in these topics.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

Prion protein N1 cleavage peptides stimulate microglial interaction with surrounding cells

Microglia act as the protective immune cell of the brain. By surveying the tissue to identify and rectify problems, they function to maintain the health of brain cells. The prion protein N-terminal cleavage fragment, N1, has demonstrated neuroprotective activities in vitro and in vivo. This study aimed to elucidate whether N1 could modulate microglial function and, if so, determine the consequences for the surrounding tissue. Using a mixed neuronal lineage and microglia co-culture system, we showed that N1 stimulation changed overall morphology and metabolism, suggesting enhanced cellular viability. Furthermore, N1 induced an increase in Cxcl10 secretion in the co-cultures. Recombinant Cxcl10, administered exogenously, mediated the changes in the mixed neuronal lineage culture morphology and metabolism in the absence of microglia, but no effect of Cxcl10 was observed on microglia cultured on their own. Direct cell-to-cell contact was required for N1 to influence microglia in the co-cultures, and this was linked with restructuring of microglial membrane composition to include a higher GM1 content at interaction sites with surrounding cells. Our findings show that N1 can play a regulatory role in microglial function in the context of an inter-connected network of cells by changing both cellular interaction sites and cytokine secretion.

The curious case of opossum prion: a physicochemical study on copper(ii) binding to the bis-decarepeat fragment from the protein N-terminal domain

The opossum is a peculiar model of immunity to prion diseases. Here we scrutinised the bis-decarepeat peptide sequence of the opossum prion (Op_bis-deca) protein by a multitechnique approach, with a combined experimental (potentiometry, UV-visible, circular dichroism, NMR and EPR spectroscopy, quartz crystal microbalance with dissipation monitoring and confocal microscopy) and simulation (DFT calculations) approach. Results showed that the macrochelate structures formed upon the binding to Cu(ii) by the analogous bis-octarepeat peptide sequence of human prion (Hu_bis-octa) are not found in the case of Op_bis-deca. At physiological pH and equimolar amount of copper ions, the [CuLH-2] is the major species formed by Op_bis-deca. In this species one imidazole and two amide nitrogen atoms are involved in metal coordination and its stability constant value is lower than that of the analogous species formed by Hu_bis-octa, due to the presence of an extra proline residue. Moreover, the study on the interaction of the peptides or the peptide/Cu(ii) complexes with the model cell membranes made of supported lipid bilayers disclosed different levels of interaction, monitored by the viscoelastic changes of the membranes, which exhibited a similar viscoelastic response at the interface of the two complexes, while in the absence of Cu(ii), the Hu_bis-octa/SLB interface was more viscoelastic than the Op_bis-deca one.

Prion protein modulates endothelial to mesenchyme-like transition in trabecular meshwork cells: Implications for primary open angle glaucoma

Endothelial-to-mesenchyme-like transition (Endo-MT) of trabecular meshwork (TM) cells is known to be associated with primary open angle glaucoma (POAG). Here, we investigated whether the prion protein (PrP^C), a neuronal protein known to modulate epithelial-to-mesenchymal transition in a variety of cell types, is expressed in the TM, and plays a similar role at this site. Using a combination of primary human TM cells and human, bovine, and PrP-knock-out (PrP^−/−) mouse models, we demonstrate that PrP^C is expressed in the TM of all three species, including endothelial cells lining the Schlemm’s canal. Silencing of PrP^C in primary human TM cells induces aggregation of β1-integrin and upregulation of α-smooth muscle actin, fibronectin, collagen 1A, vimentin, and laminin, suggestive of transition to a mesenchyme-like phenotype. Remarkably, intraocular pressure is significantly elevated in PrP^−/− mice relative to wild-type controls, suggesting reduced pliability of the extracellular matrix and increased resistance to aqueous outflow in the absence of PrP^C. Since PrP^C is cleaved by members of the disintegrin and matrix-metalloprotease family that are increased in the aqueous humor of POAG arising from a variety of conditions, it is likely that concomitant cleavage of PrP^C exaggerates and confounds the pathology by inducing Endo-MT-like changes in the TM.

The molecular basis of interaction domains of full-length PrP with lipid membranes

PrP-lipid membrane interactions are critical to PrP structural conversion and neurotoxicity, but its molecular mechanism remains unclear. A two-dimensional histogram of force-distance curves and a worm-like chain model revealed three binding regions at the PrP N-terminal, providing the molecular basis for understanding the interactions between full-length PrP and lipid membranes.

The prion protein in neuroimmune crosstalk

The cellular prion protein (PrP^C) is a medium-sized glycoprotein, attached to the cell surface by a glycosylphosphatidylinositol anchor. PrP^C is encoded by a single-copy gene, PRNP, which is abundantly expressed in the central nervous system and at lower levels in non-neuronal cells, including those of the immune system. Evidence from experimental knockout of PRNP in rodents, goats, and cattle and the occurrence of a nonsense mutation in goat that prevents synthesis of PrP^C, have shown that the molecule is non-essential for life. Indeed, no easily recognizable phenotypes are associate with a lack of PrP^C, except the potentially advantageous trait that animals without PrP^C cannot develop prion disease. This is because, in prion diseases, PrP^C converts to a pathogenic "scrapie" conformer, PrP^Sc, which aggregates and eventually induces neurodegeneration. In addition, endogenous neuronal PrP^C serves as a toxic receptor to mediate prion-induced neurotoxicity. Thus, PrP^C is an interesting target for treatment of prion diseases. Although loss of PrP^C has no discernable effect, alteration of its normal physiological function can have very harmful consequences. It is therefore important to understand cellular processes involving PrP^C, and research of this topic has advanced considerably in the past decade. Here, we summarize data that indicate the role of PrP^C in modulating immune signaling, with emphasis on neuroimmune crosstalk both under basal conditions and during inflammatory stress.

Prion protein cleavage fragments regulate adult neural stem cell quiescence through redox modulation of mitochondrial fission and SOD2 expression

Neurogenesis continues in the post-developmental brain throughout life. The ability to stimulate the production of new neurones requires both quiescent and actively proliferating pools of neural stem cells (NSCs). Actively proliferating NSCs ensure that neurogenic demand can be met, whilst the quiescent pool makes certain NSC reserves do not become depleted. The processes preserving the NSC quiescent pool are only just beginning to be defined. Herein, we identify a switch between NSC proliferation and quiescence through changing intracellular redox signalling. We show that N-terminal post-translational cleavage products of the prion protein (PrP) induce a quiescent state, halting NSC cellular growth, migration, and neurite outgrowth. Quiescence is initiated by the PrP cleavage products through reducing intracellular levels of reactive oxygen species. First, inhibition of redox signalling results in increased mitochondrial fission, which rapidly signals quiescence. Thereafter, quiescence is maintained through downstream increases in the expression and activity of superoxide dismutase-2 that reduces mitochondrial superoxide. We further observe that PrP is predominantly cleaved in quiescent NSCs indicating a homeostatic role for this cascade. Our findings provide new insight into the regulation of NSC quiescence, which potentially could influence brain health throughout adult life.

Prion Peptides Are Extremely Sensitive to Copper Induced Oxidative Stress

Copper(II) binding to prion peptides does not prevent Cu redox cycling and formation of reactive oxygen species (ROS) in the presence of reducing agents. The toxic effects of these species are exacerbated in the presence of catecholamines, indicating that dysfunction of catecholamine vesicular sequestration or recovery after synaptic release is a dangerous amplifier of Cu induced oxidative stress. Cu bound to prion peptides including the high affinity site involving histidines adjacent to the octarepeats exhibits marked catalytic activity toward dopamine and 4-methylcatechol. The resulting quinone oxidation products undergo parallel oligomerization and endogenous peptide modification yielding catechol adducts at the histidine binding ligands. These modifications add to the more common oxidation of Met and His residues produced by ROS. Derivatization of Cu-prion peptides is much faster than that undergone by Cu-β-amyloid and Cu-α-synuclein complexes in the same conditions.

Diverse functions of the prion protein - Does proteolytic processing hold the key?

Proteolytic processing of the cellular and disease-associated form of the prion protein leads to generation of bioactive soluble prion protein fragments and modifies the structure and function of its cell-bound form. The nature of proteases responsible for shedding, α-, β-, and γ-cleavage of the prion protein are only partially identified and their regulation is largely unknown. Here, we provide an overview of the increasingly multifaceted picture of prion protein proteolysis and shed light on physiological and pathological roles associated with these cleavages. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Analysis of Cellular Prion Protein Endoproteolytic Processing

Like numerous proteins of various structural and functional classes, the glycosylphosphatidylinositol (GPI)-anchored cellular prion protein (PrP^C) has been recognized to undergo endoproteolytic processing for decades, a phenomenon observed in various cultured cell lines, as well as human and several animal tissue extracts. Despite this, the physiological significance of PrP^C proteolytic cleavage has not yet been entirely elucidated. Experimental evidence suggests independent normal biological functions of the full-length and truncated PrP^C species, as well as probable links of endoproteolysis to prion disease transmission susceptibility, pathogenesis, and toxicity. The accurate characterization of constitutive PrP^C processing, through the method outlined in this chapter, is therefore an important tool in order to investigate the biological relevance of the alternative cleavage events.

Prion Diseases

Prion diseases are a group of invariably fatal and transmissible neurodegenerative disorders that are associated with the misfolding of the normal cellular prion protein, with the misfolded conformers constituting an infectious unit referred to as a "prion". Prions can spread within an affected organism by directly propagating this misfolding within and between cells and can transmit disease between animals of the same and different species. Prion diseases have a range of clinical phenotypes in humans and animals, with a principle determinant of this attributed to different conformations of the misfolded protein, referred to as prion strains. This chapter will describe the different clinical manifestations of prion diseases, the evidence that these diseases can be transmitted by an infectious protein and how the misfolding of this protein causes disease.

Reduced Hyperpolarization-Activated Current Contributes to Enhanced Intrinsic Excitability in Cultured Hippocampal Neurons from PrP(-/-) Mice

Genetic ablation of cellular prion protein (PrP^C) has been linked to increased neuronal excitability and synaptic activity in the hippocampus. We have previously shown that synaptic activity in hippocampi of PrP-null mice is increased due to enhanced N-methyl-D-aspartate receptor (NMDAR) function. Here, we focused on the effect of PRNP gene knock-out (KO) on intrinsic neuronal excitability, and in particular, the underlying ionic mechanism in hippocampal neurons cultured from P0 mouse pups. We found that the absence of PrP^C profoundly affected the firing properties of cultured hippocampal neurons in the presence of synaptic blockers. The membrane impedance was greater in PrP-null neurons, and this difference was abolished by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker ZD7288 (100 μM). HCN channel activity appeared to be functionally regulated by PrP^C. The amplitude of voltage sag, a characteristic of activating HCN channel current (I_h), was decreased in null mice. Moreover, I_h peak current was reduced, along with a hyperpolarizing shift in activation gating and slower kinetics. However, neither HCN1 nor HCN2 formed a biochemical complex with PrP^C. These results suggest that the absence of PrP downregulates the activity of HCN channels through activation of a cell signaling pathway rather than through direct interactions. This in turn contributes to an increase in membrane impedance to potentiate neuronal excitability.

Prion protein "gamma-cleavage": characterizing a novel endoproteolytic processing event

The cellular prion protein (PrP(C)) is a ubiquitously expressed protein of currently unresolved but potentially diverse function. Of putative relevance to normal biological activity, PrP(C) is recognized to undergo both α- and β-endoproteolysis, producing the cleavage fragment pairs N1/C1 and N2/C2, respectively. Experimental evidence suggests the likelihood that these processing events serve differing cellular needs. Through the engineering of a C-terminal c-myc tag onto murine PrP(C), as well as the selective use of a far-C-terminal anti-PrP antibody, we have identified a new PrP(C) fragment, nominally 'C3', and elaborating existing nomenclature, 'γ-cleavage' as the responsible proteolysis. Our studies indicate that this novel γ-cleavage event can occur during transit through the secretory pathway after exiting the endoplasmic reticulum, and after PrP(C) has reached the cell surface, by a matrix metalloprotease. We found that C3 is GPI-anchored like other C-terminal and full length PrP(C) species, though it does not localize primarily at the cell surface, and is preferentially cleaved from an unglycosylated substrate. Importantly, we observed that C3 exists in diverse cell types as well as mouse and human brain tissue, and of possible pathogenic significance, γ-cleavage may increase in human prion diseases. Given the likely relevance of PrP(C) processing to both its normal function, and susceptibility to prion disease, the potential importance of this previously underappreciated and overlooked cleavage event warrants further consideration.

The Prion Protein N1 and N2 Cleavage Fragments Bind to Phosphatidylserine and Phosphatidic Acid; Relevance to Stress-Protection Responses

Internal cleavage of the cellular prion protein generates two well characterised N-terminal fragments, N1 and N2. These fragments have been shown to bind to anionic phospholipids at low pH. We sought to investigate binding with other lipid moieties and queried how such interactions could be relevant to the cellular functions of these fragments. Both N1 and N2 bound phosphatidylserine (PS), as previously reported, and a further interaction with phosphatidic acid (PA) was also identified. The specificity of this interaction required the N-terminus, especially the proline motif within the basic amino acids at the N-terminus, together with the copper-binding region (unrelated to copper saturation). Previously, the fragments have been shown to be protective against cellular stresses. In the current study, serum deprivation was used to induce changes in the cellular lipid environment, including externalisation of plasma membrane PS and increased cellular levels of PA. When copper-saturated, N2 could reverse these changes, but N1 could not, suggesting that direct binding of N2 to cellular lipids may be part of the mechanism by which this peptide signals its protective response.

MEK1 transduces the prion protein N2 fragment antioxidant effects

The prion protein (PrP(C)) when mis-folded is causally linked with a group of fatal neurodegenerative diseases called transmissible spongiform encephalopathies or prion diseases. PrP(C) normal function is still incompletely defined with such investigations complicated by PrP(C) post-translational modifications, such as internal cleavage, which feasibly could change, activate, or deactivate the function of this protein. Oxidative stress induces β-cleavage and the N-terminal product of this cleavage event, N2, demonstrates a cellular protective response against oxidative stress. The mechanisms by which N2 mediates cellular antioxidant protection were investigated within an in vitro cell model. N2 protection was regulated by copper binding to the octarepeat domain, directing the route of internalisation, which stimulated MEK1 signalling. Precise membrane interactions of N2, determined by copper saturation, and involving both the copper-co-ordinating octarepeat region and the structure conferred upon the N-terminal polybasic region by the proline motif, were essential for the correct engagement of this pathway. The phenomenon of PrP(C) post-translational modification, such as cleavage and copper co-ordination, as a molecular "switch" for activation or deactivation of certain functions provides new insight into the apparent multi-functionality of PrP(C).

Neutron reflectometry studies define prion protein N-terminal peptide membrane binding

The prion protein (PrP), widely recognized to misfold into the causative agent of the transmissible spongiform encephalopathies, has previously been shown to bind to lipid membranes with binding influenced by both membrane composition and pH. Aside from the misfolding events associated with prion pathogenesis, PrP can undergo various posttranslational modifications, including internal cleavage events. Alpha- and beta-cleavage of PrP produces two N-terminal fragments, N1 and N2, respectively, which interact specifically with negatively charged phospholipids at low pH. Our previous work probing N1 and N2 interactions with supported bilayers raised the possibility that the peptides could insert deeply with minimal disruption. In the current study we aimed to refine the binding parameters of these peptides with lipid bilayers. To this end, we used neutron reflectometry to define the structural details of this interaction in combination with quartz crystal microbalance interrogation. Neutron reflectometry confirmed that peptides equivalent to N1 and N2 insert into the interstitial space between the phospholipid headgroups but do not penetrate into the acyl tail region. In accord with our previous studies, interaction was stronger for the N1 fragment than for the N2, with more peptide bound per lipid. Neutron reflectometry analysis also detected lengthening of the lipid acyl tails, with a concurrent decrease in lipid area. This was most evident for the N1 peptide and suggests an induction of increased lipid order in the absence of phase transition. These observations stand in clear contrast to the findings of analogous studies of Ab and ?-synuclein and thereby support the possibility of a functional role for such N-terminal fragment-membrane interactions.

Effects of FlAsH/tetracysteine (TC) Tag on PrP proteolysis and PrPres formation by TC-scanning

Protein-protein interactions associated with proteolytic processing and aggregation are integral to normal and pathological aspects of prion protein (PrP) biology. Characterization of these interactions requires the identification of amino acid residues involved. The FlAsH/tetracysteine (FlAsH/TC) tag is a small fluorescent tag amenable to insertion at internal sites in proteins. In this study, we used serial FlAsH/TC insertions (TC-scanning) as a probe to characterize sites of protein-protein interaction between PrP and other molecules. To explore this application in the context of substrate-protease interactions, we analyzed the effect of FlAsH/TC insertions on proteolysis of cellular prion protein (PrPsen) in in vitro reactions and generation of the C1 metabolic fragment of PrPsen in live neuroblastoma cells. The influence of FlAsH/TC insertion was evaluated by TC-scanning across the cleavage sites of each protease. The results showed that FlAsH/TC inhibited protease cleavage only within limited ranges of the cleavage sites, which varied from about one to six residues in width, depending on the protease, providing an estimate of the PrP residues interacting with each protease. TC-scanning was also used to probe a different type of protein-protein interaction: the conformational conversion of FlAsH-PrPsen to the prion disease-associated isoform, PrPres. PrP constructs with FlAsH/TC insertions at residues 90-96 but not 97-101 were converted to FlAsH-PrPres, identifying a boundary separating loosely versus compactly folded regions of PrPres. Our observations demonstrate that TC-scanning with the FlAsH/TC tag can be a versatile method for probing protein-protein interactions and folding processes.

Immunotherapeutic approaches in prion disease: progress, challenges and potential directions

Therapeutic trials utilizing animal models of prion disease have explored a variety of compounds and a number of approaches with varying success, including several immunotherapeutic strategies, such as passive immunization through the delivery of viruses carrying nucleic acid inserts encoding prion protein-specific immunoglobulin. Targeted, organ-specific cellular production of therapeutic proteins is a relatively unexplored approach in the treatment of neurodegeneration despite many successful experimental outcomes in animal models and human trials of other diseases of the CNS. Emphasizing studies utilizing mouse models of disease, this review outlines developments and limitations of immunological approaches to the treatment of prion diseases. In addition, the authors discuss the potential of an experimental therapeutic strategy, utilizing hybridoma cells injected directly into the CNS to establish long-term production of anti-prion antibodies in vivo within the organ associated with the greatest pathogenic change in prion disease, the brain.

Prion processing: a double-edged sword?

The events leading to the degradation of the endogenous PrP(C) (normal cellular prion protein) have been the subject of numerous studies. Two cleavage processes, α-cleavage and β-cleavage, are responsible for the main C- and N-terminal fragments produced from PrP(C). Both cleavage processes occur within the N-terminus of PrP(C), a region that is significant in terms of function. α-Cleavage, an enzymatic event that occurs at amino acid residues 110 and 111 on PrP(C), interferes with the conversion of PrP(C) into the prion disease-associated isoform, PrP(Sc) (abnormal disease-specific conformation of prion protein). This processing is seen as a positive event in terms of disease development. The study of β-cleavage has taken some surprising turns. β-Cleavage is brought about by ROS (reactive oxygen species). The C-terminal fragment produced, C2, may provide the seed for the abnormal conversion process, as it resembles in size the fragments isolated from prion-infected brains. There is, however, strong evidence that β-cleavage provides an essential process to reduce oxidative stress. β-Cleavage may act as a double-edged sword. By β-cleavage, PrP(C) may try to balance the ROS levels produced during prion infection, but the C2 produced may provide a PrP(Sc) seed that maintains the prion conversion process.

Prion protein at the crossroads of physiology and disease

The presence of the cellular prion protein (PrP(C)) on the cell surface is critical for the neurotoxicity of prions. Although several biological activities have been attributed to PrP(C), a definitive demonstration of its physiological function remains elusive. In this review, we discuss some of the proposed functions of PrP(C), focusing on recently suggested roles in cell adhesion, regulation of ionic currents at the cell membrane and neuroprotection. We also discuss recent evidence supporting the idea that PrP(C) may function as a receptor for soluble oligomers of the amyloid β peptide and possibly other toxic protein aggregates. These data suggest surprising new connections between the physiological function of PrP(C) and its role in neurodegenerative diseases beyond those caused by prions.

The N-terminal, polybasic region is critical for prion protein neuroprotective activity

Several lines of evidence suggest that the normal form of the prion protein, PrP(C), exerts a neuroprotective activity against cellular stress or toxicity. One of the clearest examples of such activity is the ability of wild-type PrP(C) to suppress the spontaneous neurodegenerative phenotype of transgenic mice expressing a deleted form of PrP (Δ32-134, called F35). To define domains of PrP involved in its neuroprotective activity, we have analyzed the ability of several deletion mutants of PrP (Δ23-31, Δ23-111, and Δ23-134) to rescue the phenotype of Tg(F35) mice. Surprisingly, all of these mutants displayed greatly diminished rescue activity, although Δ23-31 PrP partially suppressed neuronal loss when expressed at very high levels. Our results pinpoint the N-terminal, polybasic domain as a critical determinant of PrP(C) neuroprotective activity, and suggest that identification of molecules interacting with this region will provide important clues regarding the normal function of the protein. Small molecule ligands targeting this region may also represent useful therapeutic agents for treatment of prion diseases.

Acute exposure to prion infection induces transient oxidative stress progressing to be cumulatively deleterious with chronic propagation in vitro

Neuronal loss is a pathological feature of prion diseases for which increased reactive oxygen species (ROS) and consequent oxidative stress is one proposed mechanism. The processes underlying ROS production in prion disease and the precise relationship to misfolding of the prion protein remain obscure. Using cell culture models of prion infection we found that cells demonstrate a rapid, prion protein (PrP) dependent, increase in intracellular ROS following exposure to infectious inoculum. ROS production correlated with internalisation and increased intracellular protease resistant PrP (PrP(Res)). The ROS increase was predominantly lysosomal in origin but not sustained, with cells adapting within 48 hours. Overall ROS levels remained normal in the chronically prion infected cell population; however a subpopulation characterised by loss of membrane phosphatidylserine asymmetry exhibited highly peroxidised intracellular aggregates that localised with PrP and intense caspase activation. These apoptotic cells showed increased ROS closely correlating with increased PrP(Res). Our findings demonstrate that a PrP-dependent, transient, increase in intracellular ROS is characteristic of acute cellular prion infection, while chronic phases of prion infection in vitro are associated with a significant subpopulation manifesting apoptosis accompanying heightened oxidative stress and increased PrP(Res) burden. Such observations strengthen the direct links between heightened ROS and ongoing prion propagation with eventual cellular demise.

The role of zinc in Alzheimer's disease

Zinc, the most abundant trace metal in the brain, has numerous functions, both in health and in disease. Zinc is released into the synaptic cleft of glutamatergic neurons alongside glutamate from where it interacts and modulates NMDA and AMPA receptors. In addition, zinc has multifactorial functions in Alzheimer's disease (AD). Zinc is critical in the enzymatic nonamyloidogenic processing of the amyloid precursor protein (APP) and in the enzymatic degradation of the amyloid-β (Aβ) peptide. Zinc binds to Aβ promoting its aggregation into neurotoxic species, and disruption of zinc homeostasis in the brain results in synaptic and memory deficits. Thus, zinc dyshomeostasis may have a critical role to play in the pathogenesis of AD, and the chelation of zinc is a potential therapeutic approach.

Anionic phospholipid interactions of the prion protein N terminus are minimally perturbing and not driven solely by the octapeptide repeat domain

Although the N terminus of the prion protein (PrP(C)) has been shown to directly associate with lipid membranes, the precise determinants, biophysical basis, and functional implications of such binding, particularly in relation to endogenously occurring fragments, are unresolved. To better understand these issues, we studied a range of synthetic peptides: specifically those equating to the N1 (residues 23-110) and N2 (23-89) fragments derived from constitutive processing of PrP(C) and including those representing arbitrarily defined component domains of the N terminus of mouse prion protein. Utilizing more physiologically relevant large unilamellar vesicles, fluorescence studies at synaptosomal pH (7.4) showed absent binding of all peptides to lipids containing the zwitterionic headgroup phosphatidylcholine and mixtures containing the anionic headgroups phosphatidylglycerol or phosphatidylserine. At pH 5, typical of early endosomes, quartz crystal microbalance with dissipation showed the highest affinity binding occurred with N1 and N2, selective for anionic lipid species. Of particular note, the absence of binding by individual peptides representing component domains underscored the importance of the combination of the octapeptide repeat and the N-terminal polybasic regions for effective membrane interaction. In addition, using quartz crystal microbalance with dissipation and solid-state NMR, we characterized for the first time that both N1 and N2 deeply insert into the lipid bilayer with minimal disruption. Potential functional implications related to cellular stress responses are discussed.