Dual Mechanisms for Shedding of the Cellular Prion Protein

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

The multiple functions of PrPC in physiological, cancer, and neurodegenerative contexts

Cellular prion protein (PrP^C) is a highly conserved glycoprotein, present both anchored in the cell membrane and soluble in the extracellular medium. It has a diversity of ligands and is variably expressed in numerous tissues and cell subtypes, most notably in the central nervous system (CNS). Its importance has been brought to light over the years both under physiological conditions, such as embryogenesis and immune system homeostasis, and in pathologies, such as cancer and neurodegenerative diseases. During development, PrP^C plays an important role in CNS, participating in axonal growth and guidance and differentiation of glial cells, but also in other organs such as the heart, lung, and digestive system. In diseases, PrP^C has been related to several types of tumors, modulating cancer stem cells, enhancing malignant properties, and inducing drug resistance. Also, in non-neoplastic diseases, such as Alzheimer's and Parkinson's diseases, PrP^C seems to alter the dynamics of neurotoxic aggregate formation and, consequently, the progression of the disease. In this review, we explore in detail the multiple functions of this protein, which proved to be relevant for understanding the dynamics of organism homeostasis, as well as a promising target in the treatment of both neoplastic and degenerative diseases.

Prion Protein in Stem Cells: A Lipid Raft Component Involved in the Cellular Differentiation Process

The prion protein (PrP) is an enigmatic molecule with a pleiotropic effect on different cell types; it is localized stably in lipid raft microdomains and it is able to recruit downstream signal transduction pathways by its interaction with various biochemical partners. Since its discovery, this lipid raft component has been involved in several functions, although most of the publications focused on the pathological role of the protein. Recent studies report a key role of cellular prion protein (PrP^C) in physiological processes, including cellular differentiation. Indeed, the PrP^C, whose expression is modulated according to the cell differentiation degree, appears to be part of the multimolecular signaling pathways of the neuronal differentiation process. In this review, we aim to summarize the main findings that report the link between PrP^C and stem cells.

Changes in cellular prion protein expression, processing and localisation during differentiation of the neuronal cell line CAD 5

BACKGROUND INFORMATION

Cellular prion protein (PrP^C ) is infamous for its role in prion diseases. The physiological function of PrP^C remains enigmatic, but several studies point to its involvement in cell differentiation processes. To test this possibility, we monitored PrP^C changes during the differentiation of prion-susceptible CAD 5 cells, and then we analysed the effect of PrP^C ablation on the differentiation process.

RESULTS

Neuronal CAD 5 cells differentiate within 5 days of serum withdrawal, with the majority of the cells developing long neurites. This process is accompanied by an up to sixfold increase in PrP^C expression and enhanced N-terminal β-cleavage of the protein, which suggests a role for the PrP^C in the differentiation process. Moreover, the majority of PrP^C in differentiated cells is inside the cell, and a large proportion of the protein does not associate with membrane lipid rafts. In contrast, PrP^C in proliferating cells is found mostly on the cytoplasmic membrane and is predominantly associated with lipid rafts. To determine the importance of PrP^C in cell differentiation, a CAD 5 PrP^-/- cell line with ablated PrP^C expression was created using the CRISPR/Cas9 system. We observed no considerable difference in morphology, proliferation rate or expression of molecular markers between CAD 5 and CAD 5 PrP^-/- cells during the differentiation initiated by serum withdrawal.

CONCLUSIONS

PrP^C characteristics, such as cell localisation, level of expression and posttranslational modifications, change during CAD 5 cell differentiation, but PrP^C ablation does not change the course of the differentiation process.

SIGNIFICANCE

Ablation of PrP^C expression does not affect CAD 5 cell differentiation, although we observed many intriguing changes in PrP^C features during the process. Our study does not support the concept that PrP^C is important for neuronal cell differentiation, at least in simple in vitro conditions.

Prion protein is required for tumor necrosis factor α (TNFα)-triggered nuclear factor κB (NF-κB) signaling and cytokine production

The expression of normal cellular prion protein (PrP) is required for the pathogenesis of prion diseases. However, the physiological functions of PrP remain ambiguous. Here, we identified PrP as being critical for tumor necrosis factor (TNF) α-triggered signaling in a human melanoma cell line, M2, and a pancreatic ductal cell adenocarcinoma cell line, BxPC-3. In M2 cells, TNFα up-regulates the expression of p-IκB-kinase α/β (p-IKKα/β), p-p65, and p-JNK, but down-regulates the IκBα protein, all of which are downstream signaling intermediates in the TNF receptor signaling cascade. When PRNP is deleted in M2 cells, the effects of TNFα are no longer detectable. More importantly, p-p65 and p-JNK responses are restored when PRNP is reintroduced into the PRNP null cells. TNFα also activates NF-κB and increases TNFα production in wild-type M2 cells, but not in PrP-null M2 cells. Similar results are obtained in the BxPC-3 cells. Moreover, TNFα activation of NF-κB requires ubiquitination of receptor-interacting serine/threonine kinase 1 (RIP1) and TNF receptor-associated factor 2 (TRAF2). TNFα treatment increases the binding between PrP and the deubiquitinase tumor suppressor cylindromatosis (CYLD), in these treated cells, binding of CYLD to RIP1 and TRAF2 is reduced. We conclude that PrP traps CYLD, preventing it from binding and deubiquitinating RIP1 and TRAF2. Our findings reveal that PrP enhances the responses to TNFα, promoting proinflammatory cytokine production, which may contribute to inflammation and tumorigenesis.

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.

Cellular prion protein (PrPC) in the development of Merlin-deficient tumours

Loss of function mutations in the neurofibromatosis Type 2 (NF2) gene, coding for a tumour suppressor, Merlin, cause multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas. These tumours may occur sporadically or as part of the hereditary condition neurofibromatosis Type 2 (NF2). Current treatment is confined to (radio) surgery and no targeted drug therapies exist. NF2 mutations and/or Merlin inactivation are also seen in other cancers including some mesothelioma, breast cancer, colorectal carcinoma, melanoma and glioblastoma. To study the relationship between Merlin deficiency and tumourigenesis, we have developed an in vitro model comprising human primary schwannoma cells, the most common Merlin-deficient tumour and the hallmark for NF2. Using this model, we show increased expression of cellular prion protein (PrP^C) in schwannoma cells and tissues. In addition, a strong overexpression of PrP^C is observed in human Merlin-deficient mesothelioma cell line TRA and in human Merlin-deficient meningiomas. PrP^C contributes to increased proliferation, cell-matrix adhesion and survival in schwannoma cells acting via 37/67 kDa non-integrin laminin receptor (LR/37/67 kDa) and downstream ERK1/2, PI3K/AKT and FAK signalling pathways. PrP^C protein is also strongly released from schwannoma cells via exosomes and as a free peptide suggesting that it may act in an autocrine and/or paracrine manner. We suggest that PrP^C and its interactor, LR/37/67 kDa, could be potential therapeutic targets for schwannomas and other Merlin-deficient tumours.

Common therapeutic strategies for prion and Alzheimer's diseases

A number of unexpected pathophysiological connections linking different neurodegenerative diseases have emerged over the past decade. An example is provided by prion and Alzheimer's diseases. Despite being distinct pathologies, these disorders share several neurotoxic mechanisms, including accumulation of misfolded protein isoforms, stress of the protein synthesis machinery, and activation of a neurotoxic signaling mediated by the cellular prion protein. Here, in addition to reviewing these mechanisms, we will discuss the potential therapeutic interventions for prion and Alzheimer's diseases that are arising from the comprehension of their common neurodegenerative pathways.

The Pathogenic A116V Mutation Enhances Ion-Selective Channel Formation by Prion Protein in Membranes

Prion diseases are a group of fatal neurodegenerative disorders that afflict mammals. Misfolded and aggregated forms of the prion protein (PrP(Sc)) have been associated with many prion diseases. A transmembrane form of PrP favored by the pathogenic mutation A116V is associated with Gerstmann-Sträussler-Scheinker syndrome, but no accumulation of PrP(Sc) is detected. However, the role of the transmembrane form of PrP in pathological processes leading to neuronal death remains unclear. This study reports that the full-length mouse PrP (moPrP) significantly increases the permeability of living cells to K(+), and forms K(+)- and Ca(2+)-selective channels in lipid membranes. Importantly, the pathogenic mutation A116V greatly increases the channel-forming capability of moPrP. The channels thus formed are impermeable to sodium and chloride ions, and are blocked by blockers of voltage-gated ion channels. Hydrogen-deuterium exchange studies coupled with mass spectrometry (HDX-MS) show that upon interaction with lipid, the central hydrophobic region (109-132) of the protein is protected against exchange, making it a good candidate for inserting into the membrane and lining the channel. HDX-MS also shows a dramatic increase in the protein-lipid stoichiometry for A116V moPrP, providing a rationale for its increased channel-forming capability. The results suggest that ion channel formation may be a possible mechanism of PrP-mediated neurodegeneration by the transmembrane forms of PrP.

Characterization of prion protein function by focal neurite stimulation

The cellular prion protein (PrP^C), encoded by the PRNP gene, is a ubiquitous glycoprotein, which is highly expressed in the brain. This protein, mainly known for its role in neurodegenerative diseases, is involved in several physiological processes including neurite outgrowth. By using a novel focal stimulation technique, we explored the potential function of PrP^C, in its soluble form, as a signaling molecule. Thus, soluble recombinant prion proteins (recPrP) encapsulated in micro-vesicles were released by photolysis near the hippocampal growth cones. Local stimulation of wild-type growth cones with full-length recPrP induced neurite outgrowth and rapid growth cone turning towards the source. This effect was shown to be concentration dependent. Notably, PrP^C-knockout growth cones were insensitive to recPrP stimulation, but this property was rescued in PrP-knockout growth cones expressing GFP-PrP. Taken together, our findings indicate that recPrP functions as a signaling molecule, and that its homophilic interaction with membrane-anchored PrP^C might promote neurite outgrowth and facilitate growth cone guidance.

Shedding light on prion disease

Proteolytic processing regulates key processes in health and disease. The cellular prion protein (PrP(C)) is subject to at least 3 cleavage events, α-cleavage, β-cleavage and shedding. In contrast to α- and β-cleavage where there is an ongoing controversy on the identity of relevant proteases, the metalloprotease ADAM10 represents the only relevant PrP sheddase. Here we focus on the roles that ADAM10-mediated shedding of PrP(C) and its pathogenic isoform (PrP(Sc)) might play in regulating their physiological and pathogenic functions, respectively. As revealed by our recent study using conditional ADAM10 knockout mice (Altmeppen et al., 2015), shedding of PrP seems to be involved in key processes of prion diseases. These aspects and several open questions arising from them are discussed. Increased knowledge on this topic can shed new light on prion diseases and other neurodegenerative conditions as well.

Regulation of the α-secretase ADAM10 at transcriptional, translational and post-translational levels

A tremendous gain of interest in the biology of ADAM10 emerged during the past 15 years when it has first been shown that this protease was able to target the α-site of the β-amyloid precursor protein (βAPP) and later confirmed as the main physiological α-secretase activity. However, beside its well-established implication in the so-called non-amyloidogenic processing of βAPP and its probable protective role against Alzheimer's disease (AD), this metalloprotease also cleaves many other substrates, thereby being implicated in various physiological as well as pathological processes such as cancer and inflammation. Thus, in view of possible effective therapeutic interventions, a full comprehension of how ADAM10 is up and down regulated is required. This review discusses our current knowledge concerning the implication of this enzyme in AD as well as its more recently established roles in other brain disorders and provides a detailed up-date on its various transcriptional, translational and post-translational modulations.

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.

Prion protein scrapie and the normal cellular prion protein

Prions are infectious proteins and over the past few decades, some prions have become renowned for their causative role in several neurodegenerative diseases in animals and humans. Since their discovery, the mechanisms and mode of transmission and molecular structure of prions have begun to be established. There is, however, still much to be elucidated about prion diseases, including the development of potential therapeutic strategies for treatment. The significance of prion disease is discussed here, including the categories of human and animal prion diseases, disease transmission, disease progression and the development of symptoms and potential future strategies for treatment. Furthermore, the structure and function of the normal cellular prion protein (PrP(C)) and its importance in not only in prion disease development, but also in diseases such as cancer and Alzheimer's disease will also be discussed.

Unfolded Protein Response and Macroautophagy in Alzheimer's, Parkinson's and Prion Diseases

Proteostasis are integrated biological pathways within cells that control synthesis, folding, trafficking and degradation of proteins. The absence of cell division makes brain proteostasis susceptible to age-related changes and neurodegeneration. Two key processes involved in sustaining normal brain proteostasis are the unfolded protein response and autophagy. Alzheimer’s disease (AD), Parkinson’s disease (PD) and prion diseases (PrDs) have different clinical manifestations of neurodegeneration, however, all share an accumulation of misfolded pathological proteins associated with perturbations in unfolded protein response and macroautophagy. While both the unfolded protein response and macroautophagy play an important role in the prevention and attenuation of AD and PD progression, only macroautophagy seems to play an important role in the development of PrDs. Macroautophagy and unfolded protein response can be modulated by pharmacological interventions. However, further research is necessary to better understand the regulatory pathways of both processes in health and neurodegeneration to be able to develop new therapeutic interventions.

Loss of Cellular Sialidases Does Not Affect the Sialylation Status of the Prion Protein but Increases the Amounts of Its Proteolytic Fragment C1

The central molecular event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrPC), which is a sialoglycoprotein, into the disease-associated, transmissible form denoted PrPSc. Recent studies revealed a correlation between the sialylation status of PrPSc and incubation time to disease and introduced a new hypothesis that progression of prion diseases could be controlled or reversed by altering the sialylation level of PrPC. Of the four known mammalian sialidases, the enzymes that cleave off sialic acid residues, only NEU1, NEU3 and NEU4 are expressed in the brain. To test whether cellular sialidases control the steady-state sialylation level of PrPC and to identify the putative sialidase responsible for desialylating PrPC, we analyzed brain-derived PrPC from knockout mice deficient in Neu1, Neu3, Neu4, or from Neu3/Neu4 double knockouts. Surprisingly, no differences in the sialylation of PrPC or its proteolytic product C1 were noticed in any of the knockout mice tested as compared to the age-matched controls. However, significantly higher amounts of the C1 fragment relative to full-length PrPC were detected in the brains of Neu1 knockout mice as compared to WT mice or to the other knockout mice. Additional experiments revealed that in neuroblastoma cell line the sialylation pattern of C1 could be changed by an inhibitor of sialylatransferases. In summary, this study suggests that targeting cellular sialidases is apparently not the correct strategy for altering the sialylation levels of PrPC, whereas modulating the activity of sialylatransferases might offer a more promising approach. Our findings also suggest that catabolism of PrPC involves its α-cleavage followed by desialylation of the resulting C1 fragments by NEU1 and consequent fast degradation of the desialylated products.

Proteomic identification of mammalian cell surface derived glycosylphosphatidylinositol-anchored proteins through selective glycan enrichment

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are an important class of glycoproteins that are tethered to the surface of mammalian cells via the lipid GPI. GPI-APs have been implicated in many important cellular functions including cell adhesion, cell signaling, and immune regulation. Proteomic identification of mammalian GPI-APs en masse has been limited technically by poor sensitivity for these low abundance proteins and the use of methods that destroy cell integrity. Here, we present methodology that permits identification of GPI-APs liberated directly from the surface of intact mammalian cells through exploitation of their appended glycans to enrich for these proteins ahead of LC-MS/MS analyses. We validate our approach in HeLa cells, identifying a greater number of GPI-APs from intact cells than has been previously identified from isolated HeLa membranes and a lipid raft preparation. We further apply our approach to define the cohort of endogenous GPI-APs that populate the distinct apical and basolateral membrane surfaces of polarized epithelial cell monolayers. Our approach provides a new method to achieve greater sensitivity in the identification of low abundance GPI-APs from the surface of live cells and the nondestructive nature of the method provides new opportunities for the temporal or spatial analysis of cellular GPI-AP expression and dynamics.

The alpha secretase ADAM10: A metalloprotease with multiple functions in the brain

Proteins belonging to the 'A Disintegrin And Metalloproteinase' (ADAM) family are membrane-anchored proteases that are able to cleave the extracellular domains of several membrane-bound proteins in a process known as 'ectodomain shedding'. In the central nervous system, ADAM10 has attracted the most attention, since it was described as the amyloid precursor protein α-secretase over ten years ago. Despite the excitement over the potential of ADAM10 as a novel drug target in Alzheimer disease, the physiological functions of ADAM10 in the brain are not yet well understood. This is largely because of the embryonic lethality of ADAM10-deficient mice, which results from the loss of cleavage and signaling of the Notch receptor, another ADAM10 substrate. However, the recent generation of conditional ADAM10-deficient mice and the identification of further ADAM10 substrates in the brain has revealed surprisingly numerous and fundamental functions of ADAM10 in the development of the embryonic brain and also in the homeostasis of adult neuronal networks. Mechanistically, ADAM10 controls these functions by utilizing unique postsynaptic substrates in the central nervous system, in particular synaptic cell adhesion molecules, such as neuroligin-1, N-cadherin, NCAM, Ephrin A2 and A5. Consequently, a dysregulation of ADAM10 activity is linked to psychiatric and neurological diseases, such as epilepsy, fragile X syndrome and Huntington disease. This review highlights the recent progress in understanding the substrates and function as well as the regulation and cell biology of ADAM10 in the central nervous system and discusses the value of ADAM10 as a drug target in brain diseases.

Prion Protein Does Not Confer Resistance to Hippocampus-Derived Zpl Cells against the Toxic Effects of Cu2+, Mn2+, Zn2+ and Co2+ Not Supporting a General Protective Role for PrP in Transition Metal Induced Toxicity

The interactions of transition metals with the prion protein (PrP) are well-documented and characterized, however, there is no consensus on their role in either the physiology of PrP or PrP-related neurodegenerative disorders. PrP has been reported to protect cells from the toxic stimuli of metals. By employing a cell viability assay, we examined the effects of various concentrations of Cu2+, Zn2+, Mn2+, and Co2+ on Zpl (Prnp-/-) and ZW (Prnp+/+) hippocampus-derived mouse neuronal cells. Prnp-/- Zpl cells were more sensitive to all four metals than PrP-expressing Zw cells. However, when we introduced PrP or only the empty vector into Zpl cells, we could not discern any protective effect associated with the presence of PrP. This observation was further corroborated when assessing the toxic effect of metals by propidium-iodide staining and fluorescence activated cell sorting analysis. Thus, our results on this mouse cell culture model do not seem to support a strong protective role for PrP against transition metal toxicity and also emphasize the necessity of extreme care when comparing cells derived from PrP knock-out and wild type mice.

Taking advantage of physiological proteolytic processing of the prion protein for a therapeutic perspective in prion and Alzheimer diseases

Prion and Alzheimer diseases are fatal neurodegenerative diseases caused by misfolding and aggregation of the cellular prion protein (PrP(C)) and the β-amyloid peptide, respectively. Soluble oligomeric species rather than large aggregates are now believed to be neurotoxic. PrP(C) undergoes three proteolytic cleavages as part of its natural life cycle, α-cleavage, β-cleavage, and ectodomain shedding. Recent evidences demonstrate that the resulting secreted PrP(C) molecules might represent natural inhibitors against soluble toxic species. In this mini-review, we summarize recent observations suggesting the potential benefit of using PrP(C)-derived molecules as therapeutic agents in prion and Alzheimer diseases.

The sheddase ADAM10 is a potent modulator of prion disease

The prion protein (PrP^C) is highly expressed in the nervous system and critically involved in prion diseases where it misfolds into pathogenic PrP^Sc. Moreover, it has been suggested as a receptor mediating neurotoxicity in common neurodegenerative proteinopathies such as Alzheimer's disease. PrP^C is shed at the plasma membrane by the metalloprotease ADAM10, yet the impact of this on prion disease remains enigmatic. Employing conditional knockout mice, we show that depletion of ADAM10 in forebrain neurons leads to posttranslational increase of PrP^C levels. Upon prion infection of these mice, clinical, biochemical, and morphological data reveal that lack of ADAM10 significantly reduces incubation times and increases PrP^Sc formation. In contrast, spatiotemporal analysis indicates that absence of shedding impairs spread of prion pathology. Our data support a dual role for ADAM10-mediated shedding and highlight the role of proteolytic processing in prion disease.

DOI:http://dx.doi.org/10.7554/eLife.04260.001

Prion proteins are anchored to the surface of brain cells called neurons. Normally, prion proteins are folded into a specific three-dimensional shape that enables them to carry out their normal roles in the brain. However, they can be misfolded into a different shape known as PrP^Sc, which can cause Creutzfeldt-Jakob disease and other serious conditions that affect brain function and ultimately lead to death.

The PrP^Sc proteins can force normal prion proteins to change into the PrP^Sc form, so that over time this form accumulates in the brain. They are essential components of infectious particles termed ‘prions’ and this is why prion diseases are infectious: if prions from one individual enter the brain of another individual they can cause disease in the recipient. The UK outbreak of variant Creutzfeldt-Jakob disease in humans in the 1990s is thought to be due to the consumption of meat from cattle with a prion disease known as mad cow disease.

An enzyme called ADAM10 can cut normal prion proteins from the surface of neurons. However, it is not clear whether ADAM10 can also target the PrP^Sc proteins and what impact this may have on the development of prion diseases.

Here, Altmeppen et al. studied mutant mice that were missing ADAM10 in neurons in the front portion of their brain. These mice had a higher number of normal prion proteins on the surface of their neurons than normal mice did. When mice missing ADAM10 were infected with prions, more PrP^Sc accumulated in their brain and disease symptoms developed sooner than when normal mice were infected. This supports the view that mice with higher numbers of prion proteins are more vulnerable to prion disease. However, disease symptoms did not spread as quickly to other parts of the brain in the mice missing ADAM10. This suggests that by releasing prion proteins from the surface of neurons, ADAM10 helps PrP^Sc proteins to spread around the brain.

Recently, it has been suggested that prion proteins may also play a role in Alzheimer's disease and other neurodegenerative conditions. Therefore, Altmeppen et al.'s findings may help to develop new therapies for other forms of dementia. The next challenge is to understand the precise details of how ADAM10 works.

DOI:http://dx.doi.org/10.7554/eLife.04260.002

Prion protein promotes kidney iron uptake via its ferrireductase activity

Brain iron-dyshomeostasis is an important cause of neurotoxicity in prion disorders, a group of neurodegenerative conditions associated with the conversion of prion protein (PrP(C)) from its normal conformation to an aggregated, PrP-scrapie (PrP(Sc)) isoform. Alteration of iron homeostasis is believed to result from impaired function of PrP(C) in neuronal iron uptake via its ferrireductase activity. However, unequivocal evidence supporting the ferrireductase activity of PrP(C) is lacking. Kidney provides a relevant model for this evaluation because PrP(C) is expressed in the kidney, and ∼370 μg of iron are reabsorbed daily from the glomerular filtrate by kidney proximal tubule cells (PT), requiring ferrireductase activity. Here, we report that PrP(C) promotes the uptake of transferrin (Tf) and non-Tf-bound iron (NTBI) by the kidney in vivo and mainly NTBI by PT cells in vitro. Thus, uptake of (59)Fe administered by gastric gavage, intravenously, or intraperitoneally was significantly lower in PrP-knock-out (PrP(-/-)) mouse kidney relative to PrP(+/+) controls. Selective in vivo radiolabeling of plasma NTBI with (59)Fe revealed similar results. Expression of exogenous PrP(C) in immortalized PT cells showed localization on the plasma membrane and intracellular vesicles and increased transepithelial transport of (59)Fe-NTBI and to a smaller extent (59)Fe-Tf from the apical to the basolateral domain. Notably, the ferrireductase-deficient mutant of PrP (PrP(Δ51-89)) lacked this activity. Furthermore, excess NTBI and hemin caused aggregation of PrP(C) to a detergent-insoluble form, limiting iron uptake. Together, these observations suggest that PrP(C) promotes retrieval of iron from the glomerular filtrate via its ferrireductase activity and modulates kidney iron metabolism.

PrP(C) signalling in neurons: from basics to clinical challenges

The cellular prion protein PrP(C) was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP(Sc), itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP(C) is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP(Sc)-mediated toxicity. Converging evidence have highlighted that neuronal, GPI-anchored PrP(C) is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP(C) in a neuronal context. It is now well-established that PrP(C) can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP(C) with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP(Sc). Recently, amyloid Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP(C) in neurons, summarize how their subversion by PrP(Sc) or Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

Scube2 enhances proteolytic Shh processing from the surface of Shh-producing cells

All morphogens of the Hedgehog (Hh) family are synthesized as dual-lipidated proteins, which results in their firm attachment to the surface of the cell in which they were produced. Thus, Hh release into the extracellular space requires accessory protein activities. We suggested previously that the proteolytic removal of N- and C-terminal lipidated peptides (shedding) could be one such activity. More recently, the secreted glycoprotein Scube2 (signal peptide, cubulin domain, epidermal-growth-factor-like protein 2) was also implicated in the release of Shh from the cell membrane. This activity strictly depended on the CUB domains of Scube2, which derive their name from the complement serine proteases and from bone morphogenetic protein-1/tolloid metalloproteinases (C1r/C1s, Uegf and Bmp1). CUB domains function as regulators of proteolytic activity in these proteins. This suggested that sheddases and Scube2 might cooperate in Shh release. Here, we confirm that sheddases and Scube2 act cooperatively to increase the pool of soluble bioactive Shh, and that Scube2-dependent morphogen release is unequivocally linked to the proteolytic processing of lipidated Shh termini, resulting in truncated soluble Shh. Thus, Scube2 proteins act as protease enhancers in this setting, revealing newly identified Scube2 functions in Hh signaling regulation.

Detection of the GPI-anchorless prion protein fragment PrP226* in human brain

BACKGROUND

The accumulation of the misfolded forms of cellular prion protein, i.e. prions (PrPSc), in the brain is one of the crucial characteristics of fatal neurodegenerative disorders, called transmissible spongiform encephalopathies (TSEs). Cellular prion protein is normally linked to the cell surface by the glycosylphosphatidylinositol (GPI) anchor. There is accumulating evidence that the GPI-anchorless prion protein may act as an accelerator of formation and propagation of prions. In the TSE affected human brain we have previously discovered a novel GPI-anchorless prion protein fragment, named PrP226*, which ends with the tyrosine 226. This fragment can be labeled specifically by the monoclonal antibody V5B2.

METHODS

We developed a DELFIA based assay for quick and sensitive detection of the PrP226* fragment in human brain tissue homogenates. By calculating the ratio between the signals of native (N) and denatured (D) samples applied to the assay we were able to observe significant difference between 24 TSE affected brains and 10 control brains. The presence of PrP226* in brain tissue was confirmed by western blot.

RESULTS

Our results demonstrate that PrP226* is present in small quantities in healthy human brain, whereas in degenerated brain it accumulates in prion aggregates, proportionally to PrPSc. Samples with high D/N ratio generally comprised more proteinase K resistant PrP, while no correlation was found between the quantity of PrP226* and standard classification of Creutzfeldt-Jakob disease (CJD).

CONCLUSIONS

In the present study we show that the PrP226* fragment accumulates in prion aggregates and after being released from them by a denaturation procedure, could serve as a proteinase K digestion independent biomarker for human TSEs. The PrP226* assay described in this paper offers a tool to follow and study this unique anchorless PrP fragment in various parts of human brain and possibly also in other tissues and body fluids.

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.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Prion disease and the innate immune system

Prion diseases or transmissible spongiform encephalopathies are a unique category of infectious protein-misfolding neurodegenerative disorders. Hypothesized to be caused by misfolding of the cellular prion protein these disorders possess an infectious quality that thrives in immune-competent hosts. While much has been discovered about the routing and critical components involved in the peripheral pathogenesis of these agents there are still many aspects to be discovered. Research into this area has been extensive as it represents a major target for therapeutic intervention within this group of diseases. The main focus of pathological damage in these diseases occurs within the central nervous system. Cells of the innate immune system have been proven to be critical players in the initial pathogenesis of prion disease, and may have a role in the pathological progression of disease. Understanding how prions interact with the host innate immune system may provide us with natural pathways and mechanisms to combat these diseases prior to their neuroinvasive stage. We present here a review of the current knowledge regarding the role of the innate immune system in prion pathogenesis.

Separate mechanisms act concurrently to shed and release the prion protein from the cell

The cellular prion protein (PrP (C) ) is attached to the cell membrane via its glycosylphosphatidylinositol (GPI)-anchor and is constitutively shed into the extracellular space. Here, three different mechanisms are presented that concurrently shed PrP (C) from the cell. The fast α-cleavage released a N-terminal fragment (N1) into the medium and the extreme C-terminal cleavage shed soluble full-length (FL-S) PrP and C-terminally cleaved (C1-S) fragments outside the cell. Also, a slow exosomal release of full-length (FL) and C1-fragment (C1) was demonstrated. The three separate mechanisms acting simultaneously, but with different kinetics, have to be taken into consideration when elucidating functional roles of PrP (C) and also when processing of PrP (C) is considered as a target for intervention in prion diseases. Further, in this study it was shown that metalloprotease inhibitors affected the extreme C-terminal cleavage and shedding of PrP (C) . The metalloprotease inhibitors did not influence the α-cleavage or the exosomal release. Taken together, these results are important for understanding the different mechanisms acting in parallel in the shedding and cleavage of PrP (C) .

α-Cleavage of cellular prion protein

The cellular prion protein (PrP^C) is subjected to various processing under physiological and pathological conditions, of which the α-cleavage within the central hydrophobic domain not only disrupts a region critical for both PrP toxicity and PrP^C to PrP^Sc conversion but also produces the N1 fragment that is neuroprotective and the C1 fragment that enhances the pro-apoptotic effect of staurosporine in one report and inhibits prion in another. The proteases responsible for the α-cleavage of PrP^C are controversial. The effect of ADAM10, ADAM17, and ADAM9 on N1 secretion clearly indicates their involvement in the α-cleavage of PrP^C, but there has been no report of direct PrP^C α-cleavage activity with any of the three ADAMs in a purified protein form. We demonstrated that, in muscle cells, ADAM8 is the primary protease for the α-cleavage of PrP^C, but another unidentified protease(s) must also play a minor role. We also found that PrP^C regulates ADAM8 expression, suggesting that a close examination on the relationships between PrP^C and its processing enzymes may reveal novel roles and underlying mechanisms for PrP^C in non-prion diseases such as asthma and cancer.

Ion channels induced by the prion protein: mediators of neurotoxicity

Prion diseases comprise a group of rapidly progressive and invariably fatal neurodegenerative disorders for which there are no effective treatments. While conversion of the cellular prion protein (PrP(C)) to a β-sheet rich isoform (PrP(Sc) ) is known to be a critical event in propagation of infectious prions, the identity of the neurotoxic form of PrP and its mechanism of action remain unclear. Insights into this mechanism have been provided by studying PrP molecules harboring deletions and point mutations in the conserved central region, encompassing residues 105-125. When expressed in transgenic mice, PrP deleted for these residues (Δ105-125) causes a spontaneous neurodegenerative illness that is reversed by co-expression of wild-type PrP. In cultured cells, Δ105-125 PrP confers hypersensitivity to certain cationic antibiotics and induces spontaneous ion channel activity that can be recorded by electrophysiological techniques. We have utilized these drug-hypersensitization and current-inducing activities to identify which PrP domains and subcellular locations are required for toxicity. We present an ion channel model for the toxicity of Δ105-125 PrP and related mutants and speculate how a similar mechanism could mediate PrP(Sc)-associated toxicity. Therapeutic regimens designed to inhibit prion-induced toxicity, as well as formation of PrP(Sc) , may prove to be the most clinically beneficial.

The toxicity of a mutant prion protein is cell-autonomous, and can be suppressed by wild-type prion protein on adjacent cells

Insight into the normal function of PrP^C, and how it can be subverted to produce neurotoxic effects, is provided by PrP molecules carrying deletions encompassing the conserved central region. The most neurotoxic of these mutants, Δ105–125 (called ΔCR), produces a spontaneous neurodegenerative illness when expressed in transgenic mice, and this phenotype can be dose-dependently suppressed by co-expression of wild-type PrP. Whether the toxic activity of ΔCR PrP and the protective activity or wild-type PrP are cell-autonomous, or can be exerted on neighboring cells, is unknown. To investigate this question, we have utilized co-cultures of differentiated neural stem cells derived from mice expressing ΔCR or wild-type PrP. Cells from the two kinds of mice, which are marked by the presence or absence of GFP, are differentiated together to yield neurons, astrocytes, and oligodendrocytes. As a surrogate read-out of ΔCR PrP toxicity, we assayed sensitivity of the cells to the cationic antibiotic, Zeocin. In a previous study, we reported that cells expressing ΔCR PrP are hypersensitive to the toxic effects of several cationic antibiotics, an effect that is suppressed by co-expression of wild type PrP, similar to the rescue of the neurodegenerative phenotype observed in transgenic mice. Using this system, we find that while ΔCR-dependent toxicity is cell-autonomous, the rescuing activity of wild-type PrP can be exerted in trans from nearby cells. These results provide important insights into how ΔCR PrP subverts a normal physiological function of PrP^C, and the cellular mechanisms underlying the rescuing 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.

A naturally occurring C-terminal fragment of the prion protein (PrP) delays disease and acts as a dominant-negative inhibitor of PrPSc formation

The cellular prion protein (PrPC) undergoes constitutive proteolytic cleavage between residues 111/112 to yield a soluble N-terminal fragment (N1) and a membrane-anchored C-terminal fragment (C1). The C1 fragment represents the major proteolytic fragment of PrPC in brain and several cell types. To explore the role of C1 in prion disease, we generated Tg(C1) transgenic mice expressing this fragment (PrP(Δ23-111)) in the presence and absence of endogenous PrP. In contrast to several other N-terminally deleted forms of PrP, the C1 fragment does not cause a spontaneous neurological disease in the absence of endogenous PrP. Tg(C1) mice inoculated with scrapie prions remain healthy and do not accumulate protease-resistant PrP, demonstrating that C1 is not a substrate for conversion to PrPSc (the disease-associated isoform). Interestingly, Tg(C1) mice co-expressing C1 along with wild-type PrP (either endogenous or encoded by a second transgene) become ill after scrapie inoculation, but with a dramatically delayed time course compared with mice lacking C1. In addition, accumulation of PrPSc was markedly slowed in these animals. Similar effects were produced by a shorter C-terminal fragment of PrP(Δ23-134). These results demonstrate that C1 acts as dominant-negative inhibitor of PrPSc formation and accumulation of neurotoxic forms of PrP. Thus, C1, a naturally occurring fragment of PrPC, might play a modulatory role during the course of prion diseases. In addition, enhancing production of C1, or exogenously administering this fragment, represents a potential therapeutic strategy for the treatment of prion diseases.

Lack of a-disintegrin-and-metalloproteinase ADAM10 leads to intracellular accumulation and loss of shedding of the cellular prion protein in vivo

Background

The cellular prion protein (PrP^C) fulfils several yet not completely understood physiological functions. Apart from these functions, it has the ability to misfold into a pathogenic scrapie form (PrP^Sc) leading to fatal transmissible spongiform encephalopathies. Proteolytic processing of PrP^C generates N- and C-terminal fragments which play crucial roles both in the pathophysiology of prion diseases and in transducing physiological functions of PrP^C. A-disintegrin-and-metalloproteinase 10 (ADAM10) has been proposed by cell culture experiments to be responsible for both shedding of PrP^C and its α-cleavage. Here, we analyzed the role of ADAM10 in the proteolytic processing of PrP^C in vivo.

Results

Using neuron-specific Adam10 knockout mice, we show that ADAM10 is the sheddase of PrP^C and that its absence in vivo leads to increased amounts and accumulation of PrP^C in the early secretory pathway by affecting its posttranslational processing. Elevated PrP^C levels do not induce apoptotic signalling via p53. Furthermore, we show that ADAM10 is not responsible for the α-cleavage of PrP^C.

Conclusion

Our study elucidates the proteolytic processing of PrP^C and proves a role of ADAM10 in shedding of PrP^C in vivo. We suggest that ADAM10 is a mediator of PrP^C homeostasis at the plasma membrane and, thus, might be a regulator of the multiple functions discussed for PrP^C. Furthermore, identification of ADAM10 as the sheddase of PrP^C opens the avenue to devising novel approaches for therapeutic interventions against prion diseases.

Metalloproteases and Proteolytic Processing

Proteolytic enzymes constitute around 2% of the human genome and are involved in all stages of cell and organism development from fertilization through to cell death. In the human genome the major classes of peptidases are represented by cysteine-, serine- and metalloenzymes, which possess a wide spectrum of substrate specificity and physiological functions. The identification of many novel peptidases from genome sequencing programmes has suggested potential new therapeutic targets. In addition, several well characterised peptidases were recently shown to possess new and unexpected biological roles in neuroinflammation, cancer and angiogenesis, cardiovascular diseases and neurodegeneration. This chapter will briefly characterize the main classes of metallopeptidases and their roles in health and disease. Particular attention will be paid to the angiotensin-converting enzyme (ACE), neprilysin (NEP) and adamalysin (ADAM) families of proteases and their pathophysiological roles with a particular emphasis on cancer and neurodegeneration. The roles and mechanisms of protein shedding which primarily involve the ADAMs family of metallopeptidases will be explained using amyloid protein precursor (APP) processing cascades as a well characterized example. The therapeutic significance of modulating (activating or inhibiting) metallopeptidase activity will be a particular focus of this chapter.

Role of phospholipases in fungal fitness, pathogenicity, and drug development - lessons from cryptococcus neoformans

Many pathogenic microbes, including many fungi, produce phospholipases which facilitate survival of the pathogen in vivo, invasion and dissemination throughout the host, expression of virulence traits and evasion of host immune defense mechanisms. These phospholipases are either secreted or produced intracellularly and act by physically disrupting host membranes, and/or by affecting fungal cell signaling and production of immunomodulatory effectors. Many of the secreted phospholipases acquire a glycosylphosphatidylinositol sorting motif to facilitate membrane and/or cell wall association and secretion. This review focuses primarily on the role of two members of the phospholipase enzyme family, phospholipase B (Plb) and phosphatidylinositol (PI)-specific phospholipase C (PI-C/Plc), in fungal pathogenesis and in particular, what has been learnt about their function from studies performed in the model pathogenic yeast, Cryptococcus neoformans. These studies have revealed how Plb has adapted to become an important part of the virulence repertoire of pathogenic fungi and how its secretion is regulated. They have also provided valuable insight into how the intracellular enzyme, Plc1, contributes to fungal fitness and pathogenicity – via a putative role in signal transduction pathways that regulate the production of stress-protecting pigments, polysaccharide capsule, cell wall integrity, and adaptation to growth at host temperature. Finally, this review will address the role fungal phospholipases have played in the development of a new class of antifungal drugs, which mimic their phospholipid substrates.

Zinc metalloproteinases and amyloid Beta-Peptide metabolism: the positive side of proteolysis in Alzheimer's disease

Alzheimer's disease is a neurodegenerative condition characterized by an accumulation of toxic amyloid beta- (Aβ-)peptides in the brain causing progressive neuronal death. Aβ-peptides are produced by aspartyl proteinase-mediated cleavage of the larger amyloid precursor protein (APP). In contrast to this detrimental "amyloidogenic" form of proteolysis, a range of zinc metalloproteinases can process APP via an alternative "nonamyloidogenic" pathway in which the protein is cleaved within its Aβ region thereby precluding the formation of intact Aβ-peptides. In addition, other members of the zinc metalloproteinase family can degrade preformed Aβ-peptides. As such, the zinc metalloproteinases, collectively, are key to downregulating Aβ generation and enhancing its degradation. It is the role of zinc metalloproteinases in this "positive side of proteolysis in Alzheimer's disease" that is discussed in the current paper.

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.

Characterization of the prion protein in human urine

The presence of the prion protein (PrP) in normal human urine is controversial and currently inconclusive. This issue has taken a special relevance because prion infectivity has been demonstrated in urine of animals carrying experimental or naturally occurring prion diseases, but the actual presence and tissue origin of the infectious prion have not been determined. We used immunoprecipitation, one- and two-dimensional electrophoresis, and mass spectrometry to prove definitely the presence of PrP in human urine and its post-translational modifications. We show that urinary PrP (uPrP) is truncated mainly at residue 112 but also at other residues up to 122. This truncation makes uPrP undetectable with some commonly used antibodies to PrP. uPrP is glycosylated and carries an anchor which, at variance with that of cellular PrP, lacks the inositol-associated phospholipid moiety, indicating that uPrP is probably shed from the cell surface. The detailed characterization of uPrP reported here definitely proves the presence of PrP in human urine and will help determine the origin of prion infectivity in urine.

Redox control of prion and disease pathogenesis

Imbalance of brain metal homeostasis and associated oxidative stress by redox-active metals like iron and copper is an important trigger of neurotoxicity in several neurodegenerative conditions, including prion disorders. Whereas some reports attribute this to end-stage disease, others provide evidence for specific mechanisms leading to brain metal dyshomeostasis during disease progression. In prion disorders, imbalance of brain-iron homeostasis is observed before end-stage disease and worsens with disease progression, implicating iron-induced oxidative stress in disease pathogenesis. This is an unexpected observation, because the underlying cause of brain pathology in all prion disorders is PrP-scrapie (PrP(Sc)), a beta-sheet-rich conformation of a normal glycoprotein, the prion protein (PrP(C)). Whether brain-iron dyshomeostasis occurs because of gain of toxic function by PrP(Sc) or loss of normal function of PrP(C) remains unclear. In this review, we summarize available evidence suggesting the involvement of oxidative stress in prion-disease pathogenesis. Subsequently, we review the biology of PrP(C) to highlight its possible role in maintaining brain metal homeostasis during health and the contribution of PrP(Sc) in inducing brain metal imbalance with disease progression. Finally, we discuss possible therapeutic avenues directed at restoring brain metal homeostasis and alleviating metal-induced oxidative stress in prion disorders.

Glimepiride reduces the expression of PrPc, prevents PrPSc formation and protects against prion mediated neurotoxicity in cell lines

BACKGROUND

A hallmark of the prion diseases is the conversion of the host-encoded cellular prion protein (PrP(C)) into a disease related, alternatively folded isoform (PrP(Sc)). The accumulation of PrP(Sc) within the brain is associated with synapse loss and ultimately neuronal death. Novel therapeutics are desperately required to treat neurodegenerative diseases including the prion diseases.

PRINCIPAL FINDINGS

Treatment with glimepiride, a sulphonylurea approved for the treatment of diabetes mellitus, induced the release of PrP(C) from the surface of prion-infected neuronal cells. The cell surface is a site where PrP(C) molecules may be converted to PrP(Sc) and glimepiride treatment reduced PrP(Sc) formation in three prion infected neuronal cell lines (ScN2a, SMB and ScGT1 cells). Glimepiride also protected cortical and hippocampal neurones against the toxic effects of the prion-derived peptide PrP82-146. Glimepiride treatment significantly reduce both the amount of PrP82-146 that bound to neurones and PrP82-146 induced activation of cytoplasmic phospholipase A(2) (cPLA(2)) and the production of prostaglandin E(2) that is associated with neuronal injury in prion diseases. Our results are consistent with reports that glimepiride activates an endogenous glycosylphosphatidylinositol (GPI)-phospholipase C which reduced PrP(C) expression at the surface of neuronal cells. The effects of glimepiride were reproduced by treatment of cells with phosphatidylinositol-phospholipase C (PI-PLC) and were reversed by co-incubation with p-chloromercuriphenylsulphonate, an inhibitor of endogenous GPI-PLC.

CONCLUSIONS

Collectively, these results indicate that glimepiride may be a novel treatment to reduce PrP(Sc) formation and neuronal damage in prion diseases.

Glypican-1 mediates both prion protein lipid raft association and disease isoform formation

In prion diseases, the cellular form of the prion protein, PrP^C, undergoes a conformational conversion to the infectious isoform, PrP^Sc. PrP^C associates with lipid rafts through its glycosyl-phosphatidylinositol (GPI) anchor and a region in its N-terminal domain which also binds to heparan sulfate proteoglycans (HSPGs). We show that heparin displaces PrP^C from rafts and promotes its endocytosis, suggesting that heparin competes with an endogenous raft-resident HSPG for binding to PrP^C. We then utilised a transmembrane-anchored form of PrP (PrP-TM), which is targeted to rafts solely by its N-terminal domain, to show that both heparin and phosphatidylinositol-specific phospholipase C can inhibit its association with detergent-resistant rafts, implying that a GPI-anchored HSPG targets PrP^C to rafts. Depletion of the major neuronal GPI-anchored HSPG, glypican-1, significantly reduced the raft association of PrP-TM and displaced PrP^C from rafts, promoting its endocytosis. Glypican-1 and PrP^C colocalised on the cell surface and both PrP^C and PrP^Sc co-immunoprecipitated with glypican-1. Critically, treatment of scrapie-infected N2a cells with glypican-1 siRNA significantly reduced PrP^Sc formation. In contrast, depletion of glypican-1 did not alter the inhibitory effect of PrP^C on the β-secretase cleavage of the Alzheimer's amyloid precursor protein. These data indicate that glypican-1 is a novel cellular cofactor for prion conversion and we propose that it acts as a scaffold facilitating the interaction of PrP^C and PrP^Sc in lipid rafts.

The prion diseases are unique in that their infectious nature is not dependent on nucleic acid but is instead attributed to a misfolded protein, the prion protein. This misfolded prion protein is capable of inducing the misfolding of the normal form of the prion protein that is present on the surface of neurons and other cells in the body. However, the site in the cell at which this misfolding occurs and whether other proteins are involved remains controversial. We have addressed these questions by investigating how the normal form of the prion protein is targeted to specialised domains on the plasma membrane termed cholesterol-rich lipid rafts. We show that targeting is due, in part, to a particular heparin sulfate proteoglycan called glypican-1. Significantly, reducing the levels of glypican-1 in an infected cell line reduced the accumulation of misfolded prion protein. We propose that glypican-1 acts as a scaffold facilitating the favourable interaction of the misfolded, infectious form of the prion protein with the normal cellular form within cholesterol-rich lipid rafts. Our results indicate that glypican-1 is intimately involved in the misfolding of the prion protein, the critical event in the pathogenesis of prion diseases such as Creutzfeldt-Jakob disease in humans.

Influence of ADAM10 on prion protein processing and scrapie infectiosity in vivo

Both the cellular prion protein (PrP(c)) and the amyloid precursor protein (APP) are physiologically subjected to complex proteolytic processing events. While for APP the proteinases involved--alpha-, beta- and gamma-secretase--have been identified in vitro and in vivo, the cleavage of PrP(c) by now has been linked only to the shedding activity of the metalloproteinase ADAM10 and/or ADAM17 in cell culture. Here we show that neuronal overexpression of the alpha-secretase ADAM10 in mice reduces all PrP(c) species detected in the brain instead of leading to enhanced amounts of specific cleavage products of PrP(c). Additionally, the incubation time of mice after scrapie infection is significantly increased in mice moderately overexpressing ADAM10. This indicates that overexpression of ADAM10 rather influences the amount of the cellular prion protein than its processing in vivo.

Role of ADAMs in the ectodomain shedding and conformational conversion of the prion protein

The cellular prion protein (PrP^C) is essential for the pathogenesis and transmission of prion diseases. PrP^C is bound to the plasma membrane via a glycosylphosphatidylinositol anchor, although a secreted, soluble form has also been identified. Previously we reported that PrP^C is subject to ectodomain shedding from the membrane by zinc metalloproteinases with a similar inhibition profile to those involved in shedding the amyloid precursor protein. Here we have used gain-of-function (overexpression) and loss-of-function (small interfering RNA knockdown) experiments in cells to identify the ADAMs (adisintegrin and metalloproteinases) involved in the ectodomain shedding of PrP^C. These experiments revealed that ADAM9 and ADAM10, but not ADAM17, are involved in the shedding of PrP^C and that ADAM9 exerts its effect on PrP^C shedding via ADAM10. Using dominant negative, catalytically inactive mutants, we show that the catalytic activity of ADAM9 is required for its effect on ADAM10. Mass spectrometric analysis revealed that ADAM10, but not ADAM9, cleaved PrP between Gly²²⁸ and Arg²²⁹, three residues away from the site of glycosylphosphatidylinositol anchor attachment. The shedding of another membrane protein, the amyloid precursor protein β-secretase BACE1, by ADAM9 is also mediated via ADAM10. Furthermore, we show that pharmacological inhibition of PrP^C shedding or activation of both PrP^C and PrP^Sc shedding by ADAM10 overexpression in scrapie-infected neuroblastoma N2a cells does not alter the formation of proteinase K-resistant PrP^Sc. Collectively, these data indicate that although PrP^C can be shed through the action of ADAM family members, modulation of PrP^C or PrP^Sc ectodomain shedding does not regulate prion conversion.