PrP-dependent cell adhesion in N2a neuroblastoma cells

Evaluation of cold atmospheric plasma for the decontamination of flexible endoscopes

BACKGROUND

Despite adherence to standard protocols, residues including live microorganisms may remain on the various surfaces of reprocessed flexible endoscopes. Prions are infectious proteins notoriously difficult to eliminate.

AIM

We tested the potential of cold atmospheric plasma (CAP) for the decontamination of flexible endoscope various surfaces, measuring total proteins and prion-residual infectivity as an indicator of efficacy.

METHODS

New PTFE endoscope channels and metal test surfaces spiked with test soil or prion-infected tissues were treated using different CAP-generating prototypes. Surfaces were then examined for the presence of residues using very sensitive fluorescence epi-microscopy. Prion residual infectivity was determined using the wire implant animal model and a new, more sensitive cell infectivity assay.

FINDINGS

A CAP jet applied perpendicularly at close range on flat test surfaces removed soil within 3 minutes but left microscopic residues and failed to eliminate prion infectivity according to the wire implant animal assay. The longitudinal gas flow from CAP prototypes developed for the treatment of long channels led to the displacement and sedimentation of residual soil towards the distal end, when applied alone. Observations of the plasma inside glass tubes showed temporal and spatial heterogeneity within a limited range. After standard enzymatic manual pre-wash, "CAP-activated" gas effluents prevented prion transmission from treated endoscope channels according to our prion infectivity cell assay.

CONCLUSION

CAP shows promising results as a final step for surgical surfaces decontamination. Optimising CAP delivery could further enhance CAP efficacy, offering a safe, chemical-free alternative for the reprocessing of all luminal flexible endoscope surfaces.

Sensitive microscopic quantification of surface-bound prion infectivity for the assessment of surgical instrument decontamination procedures

BACKGROUND

Pathogenic prions (PrP^Sc) are amyloid-rich hydrophobic proteins which bind avidly to surgical surfaces and represent some of the most difficult targets during the reprocessing of reusable surgical instruments. In-vitro methods to amplify and detect the presence of otherwise undetectable prion contamination are available, but they do not measure associated infectivity. Most of these methods rely on the use of proteinase K, however this can lead to the loss of a substantial portion of PrP^Sc, potentially producing false negatives.

AIM

To develop a sensitive in-situ method without proteinase treatment for the dynamic quantification of amyloid accumulation in N2a #58 cells following 22L-prion infection from infected tissues and spiked stainless-steel surfaces.

METHODS

We spiked cultures of N2a #58 cells with the 22L prion strain in solution or dried on stainless-steel wires and directly measured the accumulation of prion amyloid aggregates over several passages using highly sensitive fluorescence microscopy.

FINDINGS

We demonstrated a 10-log dynamic range using our method to test residual prion infectivity, that was validated to show variable decontamination efficacy against prions from commercially available cleaning chemistries.

CONCLUSIONS

The new cell-based infectivity method presented here avoids partial or possibly total proteinase K digestion of PrP^Sc in samples for greater sensitivity, in addition to low cost, no ethical concerns, and adaptability to detect different prion strains. This method can be used to test cleaning chemistries' efficacy with greater sensitivity than measuring total residual proteins, which may not correlate with residual prion infectivity.

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.

Amyloid β Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms

Amyloid β (Aβ) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that Aβ disrupts the Cu-dependent interaction between cellular prion protein (PrP^C) and N-methyl-d-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that Aβ competes for Cu with PrP^C; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrP^C is cleaved by α-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the α-cleaved His111. In this study, the competition for Cu(II) between Aβ(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of Aβ(1-16) on Cu(II) coordination to PrP is highly site-specific: Aβ(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-Aβ(1-16) complex. In contrast, Aβ(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the α-cleaved His111 site, Aβ(1-16) yields at least two different ternary complexes depending on the ratio of PrP/Cu(II)/Aβ. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of Aβ, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of Aβ on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which Aβ might affect Cu-dependent functions of PrP^C, such as NMDAR modulation.

The Role of Cellular Prion Protein in Promoting Stemness and Differentiation in Cancer

Simple Summary

Aside from its well-established role in prion disorders, in the last decades the significance of cellular prion protein (PrP^C) expression in human cancers has attracted great attention. An extensive body of work provided evidence that PrP^C contributes to tumorigenesis by regulating tumor growth, differentiation, and resistance to conventional therapies. In particular, PrP^C over-expression has been related to the acquisition of a malignant phenotype of cancer stem cells (CSCs) in a variety of solid tumors, encompassing pancreatic ductal adenocarcinoma, osteosarcoma, breast, gastric, and colorectal cancers, and primary brain tumors as well. According to consensus, increased levels of PrP^C endow CSCs with self-renewal, proliferative, migratory, and invasive capacities, along with increased resistance to anti-cancer agents. In addition, increasing evidence demonstrates that PrPc also participates in multi-protein complexes to modulate the oncogenic properties of CSCs, thus sustaining tumorigenesis. Therefore, strategies aimed at targeting PrP^C and/or PrP^C-organized complexes could be a promising approach for anti-cancer therapy.

Abstract

Cellular prion protein (PrP^C) is seminal to modulate a variety of baseline cell functions to grant homeostasis. The classic role of such a protein was defined as a chaperone-like molecule being able to rescue cell survival. Nonetheless, PrP^C also represents the precursor of the deleterious misfolded variant known as scrapie prion protein (PrP^Sc). This variant is detrimental in a variety of prion disorders. This multi-faceted role of PrP is greatly increased by recent findings showing how PrP^C in its folded conformation may foster tumor progression by acting at multiple levels. The present review focuses on such a cancer-promoting effect. The manuscript analyzes recent findings on the occurrence of PrP^C in various cancers and discusses the multiple effects, which sustain cancer progression. Within this frame, the effects of PrP^C on stemness and differentiation are discussed. A special emphasis is provided on the spreading of PrP^C and the epigenetic effects, which are induced in neighboring cells to activate cancer-related genes. These detrimental effects are further discussed in relation to the aberrancy of its physiological and beneficial role on cell homeostasis. A specific paragraph is dedicated to the role of PrP^C beyond its effects in the biology of cancer to represent a potential biomarker in the follow up of patients following surgical resection.

Prion Protein at the Leading Edge: Its Role in Cell Motility

Cell motility is a central process involved in fundamental biological phenomena during embryonic development, wound healing, immune surveillance, and cancer spreading. Cell movement is complex and dynamic and requires the coordinated activity of cytoskeletal, membrane, adhesion and extracellular proteins. Cellular prion protein (PrPC) has been implicated in distinct aspects of cell motility, including axonal growth, transendothelial migration, epithelial-mesenchymal transition, formation of lamellipodia, and tumor migration and invasion. The preferential location of PrPC on cell membrane favors its function as a pivotal molecule in cell motile phenotype, being able to serve as a scaffold protein for extracellular matrix proteins, cell surface receptors, and cytoskeletal multiprotein complexes to modulate their activities in cellular movement. Evidence points to PrPC mediating interactions of multiple key elements of cell motility at the intra- and extracellular levels, such as integrins and matrix proteins, also regulating cell adhesion molecule stability and cell adhesion cytoskeleton dynamics. Understanding the molecular mechanisms that govern cell motility is critical for tissue homeostasis, since uncontrolled cell movement results in pathological conditions such as developmental diseases and tumor dissemination. In this review, we discuss the relevant contribution of PrPC in several aspects of cell motility, unveiling new insights into both PrPC function and mechanism in a multifaceted manner either in physiological or pathological contexts.

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.

Prion Protein Family Contributes to Tumorigenesis via Multiple Pathways

A wealth of evidence suggests that proteins from prion protein (PrP) family contribute to tumorigenesis in many types of cancers, including pancreatic ductal adenocarcinoma (PDAC), breast cancer, glioblastoma, colorectal cancer, gastric cancer, melanoma, etc. It is well documented that PrP is a biomarker for PDAC, breast cancer, and gastric cancer. However, the underlying mechanisms remain unclear. The major reasons for cancer cell-caused patient death are metastasis and multiple drug resistance, both of which connect to physiological functions of PrP expressing in cancer cells. PrP enhances tumorigenesis by multiple pathways. For example, PrP existed as pro-PrP in most of the PDAC cell lines, thus increasing cancer cell motility by binding to cytoskeletal protein filamin A (FLNa). Using PDAC cell lines BxPC-3 and AsPC-1 as model system, we identified that dysfunction of glycosylphosphatidylinositol (GPI) anchor synthesis machinery resulted in the biogenesis of pro-PrP. In addition, in cancer cells without FLNa expression, pro-PrP can modify cytoskeleton structure by affecting cofilin/F-actin axis, thus influencing cancer cell movement. Besides pro-PrP, we showed that GPI-anchored unglycosylated PrP can elevate cell mobility by interacting with VEGFR2, thus stimulating cell migration under serum-free condition. Besides affecting cancer cell motility, overexpressed PrP or doppel (Dpl) in cancer cells has been shown to increase cell proliferation, multiple drug resistance, and angiogenesis, thus, proteins from PrP gene family by affecting important processes via multiple pathways for cancer cell growth exacerbating tumorigenesis.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Spectroscopic and Theoretical Study of Cu(I) Binding to His111 in the Human Prion Protein Fragment 106-115

The ability of the cellular prion protein (PrP^C) to bind copper in vivo points to a physiological role for PrP^C in copper transport. Six copper binding sites have been identified in the nonstructured N-terminal region of human PrP^C. Among these sites, the His111 site is unique in that it contains a MKHM motif that would confer interesting Cu^I and Cu^II binding properties. We have evaluated Cu^I coordination to the PrP(106–115) fragment of the human PrP protein, using NMR and X-ray absorption spectroscopies and electronic structure calculations. We find that Met109 and Met112 play an important role in anchoring this metal ion. Cu^I coordination to His111 is pH-dependent: at pH >8, 2N1O1S species are formed with one Met ligand; in the range of pH 5–8, both methionine (Met) residues bind to Cu^I, forming a 1N1O2S species, where N is from His111 and O is from a backbone carbonyl or a water molecule; at pH <5, only the two Met residues remain coordinated. Thus, even upon drastic changes in the chemical environment, such as those occurring during endocytosis of PrP^C (decreased pH and a reducing potential), the two Met residues in the MKHM motif enable PrP^C to maintain the bound Cu^I ions, consistent with a copper transport function for this protein. We also find that the physiologically relevant Cu^I-1N1O2S species activates dioxygen via an inner-sphere mechanism, likely involving the formation of a copper(II) superoxide complex. In this process, the Met residues are partially oxidized to sulfoxide; this ability to scavenge superoxide may play a role in the proposed antioxidant properties of PrP^C. This study provides further insight into the Cu^I coordination properties of His111 in human PrP^C and the molecular mechanism of oxygen activation by this site.

Cu^I coordination to the His111 site in the HuPrP protein is highly dependent on the pH: at pH <5, two methionine (Met) residues bind Cu^I; in the range of pH 5−8, both Met residues remain coordinated, forming a 1N1O2S species, with N from His111 and O from a backbone carbonyl or a water molecule; at pH >8, 2N1O1S species are formed with only a Met ligand. The Cu^I-1N1O2S species activates dioxygen, and in this process, the Met residues are partially oxidized to sulfoxide.

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.

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.

The prion protein inhibits monocytic cell migration by stimulating β1 integrin adhesion and uropod formation

The broad tissue distribution and evolutionary conservation of the glycosylphosphatidylinositol (GPI)-anchored prion protein (PrP, also known as PRNP) suggests that it plays a role in cellular homeostasis. Given that integrin adhesion determines cell behavior, the proposed role of PrP in cell adhesion might underlie the various in vitro and in vivo effects associated with PrP loss-of-function, including the immune phenotypes described in PrP(-/-) mice. Here, we investigated the role of PrP in the adhesion and (transendothelial) migration of human (pro)monocytes. We found that PrP regulates β1-integrin-mediated adhesion of monocytes. Additionally, PrP controls the cell morphology and migratory behavior of monocytes: PrP-silenced cells show deficient uropod formation on immobilized VCAM and display bleb-like protrusions on the endothelium. Our data further show that PrP regulates ligand-induced integrin activation. Finally, we found that PrP controls the activation of several proteins involved in cell adhesion and migration, including RhoA and its effector cofilin, as well as proteins of the ERM family. We propose that PrP modulates β1 integrin adhesion and migration of monocytes through RhoA-induced actin remodeling mediated by cofilin, and through the regulation of ERM-mediated membrane-cytoskeleton linkage.

To develop with or without the prion protein

The deletion of the cellular form of the prion protein (PrP^C) in mouse, goat, and cattle has no drastic phenotypic consequence. This stands in apparent contradiction with PrP^C quasi-ubiquitous expression and conserved primary and tertiary structures in mammals, and its pivotal role in neurodegenerative diseases such as prion and Alzheimer's diseases. In zebrafish embryos, depletion of PrP ortholog leads to a severe loss-of-function phenotype. This raises the question of a potential role of PrP^C in the development of all vertebrates. This view is further supported by the early expression of the PrP^C encoding gene (Prnp) in many tissues of the mouse embryo, the transient disruption of a broad number of cellular pathways in early Prnp^−/− mouse embryos, and a growing body of evidence for PrP^C involvement in the regulation of cell proliferation and differentiation in various types of mammalian stem cells and progenitors. Finally, several studies in both zebrafish embryos and in mammalian cells and tissues in formation support a role for PrP^C in cell adhesion, extra-cellular matrix interactions and cytoskeleton. In this review, we summarize and compare the different models used to decipher PrP^C functions at early developmental stages during embryo- and organo-genesis and discuss their relevance.

Deficiency of prion protein induces impaired autophagic flux in neurons

Normal cellular prion protein (PrP^C) is highly expressed in the central nervous system. The Zürich I Prnp-deficient mouse strain did not show an abnormal phenotype in initial studies, however, in later studies, deficits in exploratory behavior and short- and long-term memory have been revealed. In the present study, numerous autophagic vacuoles were found in neurons from Zürich I Prnp-deficient mice. The autophagic accumulation in the soma of cortical neurons in Zürich I Prnp-deficient mice was observed as early as 3 months of age, and in the hippocampal neurons at 6 months of age. Specifically, there is accumulation of electron dense pigments associated with autophagy in the neurons of Zürich I Prnp-deficient mice. Furthermore, autophagic accumulations were observed as early as 3 months of age in the CA3 region of hippocampal and cerebral cortical neuropils. The autophagic vacuoles increased with age in the hippocampus of Zürich I Prnp-deficient mice at a faster rate and to a greater extent than in normal C57BL/6J mice, whereas the cortex exhibited high levels that were maintained from 3 months old in Zürich I Prnp-deficient mice. The pigmented autophagic accumulation is due to the incompletely digested material from autophagic vacuoles. Furthermore, a deficiency in PrP^C may disrupt the autophagic flux by inhibiting autophagosome-lysosomal fusion. Overall, our results provide insight into the protective role of PrP^C in neurons, which may play a role in normal behavior and other brain functions.

Roles of the cellular prion protein in the regulation of cell-cell junctions and barrier function

The cellular prion protein was historically characterized owing to its misfolding in prion disease. Although its physiological role remains incompletely understood, PrP(C) has emerged as an evolutionary conserved, multifaceted protein involved in a wide-range of biological processes. PrP(C) is a GPI-anchored protein targeted to the plasma membrane, in raft microdomains, where its interaction with a repertoire of binding partners, which differ depending on cell models, mediates its functions. Among identified PrP(C) partners are cell adhesion molecules. This review will focus on the multiple implications of PrP(C) in cell adhesion processes, mainly the regulation of cell-cell junctions in epithelial and endothelial cells and the consequences on barrier properties. We will show how recent findings argue for a role of PrP(C) in the recruitment of signaling molecules, which in turn control the targeting or the stability of adhesion complexes at the plasma membrane.

Prion protein and cancers

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

The Functional Role of Prion Protein (PrPC) on Autophagy

Cellular prion protein (PrP^C) plays an important role in the cellular defense against oxidative stress. However, the exact protective mechanism of PrP^C is unclear. Autophagy is essential for survival, differentiation, development, and homeostasis in several organisms. Although the role that autophagy plays in neurodegenerative disease has yet to be established, it is clear that autophagy-induced cell death is observed in neurodegenerative disorders that exhibit protein aggregations. Moreover, autophagy can promote cell survival and cell death under various conditions. In this review, we describe the involvement of autophagy in prion disease and the effects of PrP^C.

Alzheimer's disease and prion protein

Alzheimer's disease (AD) is a devastating neurodegenerative disease with progressive loss of memory and cognitive function, pathologically hallmarked by aggregates of the amyloid-beta (Aβ) peptide and hyperphosphorylated tau in the brain. Aggregation of Aβ under the form of amyloid fibrils has long been considered central to the pathogenesis of AD. However, recent evidence has indicated that soluble Aβ oligomers, rather than insoluble fibrils, are the main neurotoxic species in AD. The cellular prion protein (PrP(C)) has newly been identified as a cell surface receptor for Aβ oligomers. PrP(C) is a cell surface glycoprotein that plays a key role in the propagation of prions, proteinaceous infectious agents that replicate by imposing their abnormal conformation to PrP(C) molecules. In AD, PrP(C) acts to transduce the neurotoxic signals arising from Aβ oligomers, leading to synaptic failure and cognitive impairment. Interestingly, accumulating evidence has also shown that aggregated Aβ or tau possesses prion-like activity, a property that would allow them to spread throughout the brain. In this article, we review recent findings regarding the function of PrP(C) and its role in AD, and discuss potential therapeutic implications of PrP(C)-based approaches in the treatment of AD.

Amyloid beta precursor protein and prion protein have a conserved interaction affecting cell adhesion and CNS development

Genetic and biochemical mechanisms linking onset or progression of Alzheimer Disease and prion diseases have been lacking and/or controversial, and their etiologies are often considered independent. Here we document a novel, conserved and specific genetic interaction between the proteins that underlie these diseases, amyloid-β precursor protein and prion protein, APP and PRP, respectively. Knockdown of APP and/or PRNP homologs in the zebrafish (appa, appb, prp1, and prp2) produces a dose-dependent phenotype characterized by systemic morphological defects, reduced cell adhesion and CNS cell death. This genetic interaction is surprisingly exclusive in that prp1 genetically interacts with zebrafish appa, but not with appb, and the zebrafish paralog prp2 fails to interact with appa. Intriguingly, appa & appb are largely redundant in early zebrafish development yet their abilities to rescue CNS cell death are differentially contingent on prp1 abundance. Delivery of human APP or mouse Prnp mRNAs rescue the phenotypes observed in app-prp-depleted zebrafish, highlighting the conserved nature of this interaction. Immunoprecipitation revealed that human APP and PrP(C) proteins can have a physical interaction. Our study reports a unique in vivo interdependence between APP and PRP loss-of-function, detailing a biochemical interaction that considerably expands the hypothesized roles of PRP in Alzheimer Disease.

Oxidative stress impairs autophagic flux in prion protein-deficient hippocampal cells

We previously reported that autophagy is upregulated in Prnp-deficient (Prnp ( 0/0) ) hippocampal neuronal cells in comparison to cellular prion protein (PrP (C) )-expressing (Prnp (+/+) ) control cells under conditions of serum deprivation. In this study, we determined whether a protective mechanism of PrP (C) is associated with autophagy using Prnp ( 0/0) hippocampal neuronal cells under hydrogen peroxide (H 2O 2)-induced oxidative stress. We found that Prnp ( 0/0) cells were more susceptible to oxidative stress than Prnp (+/+) cells in a dose- and time-dependent manner. In addition, we observed enhanced autophagy by immunoblotting, which detected the conversion of microtubule-associated protein 1 light chain 3 β (LC3B)-I to LC3B-II, and we observed increased punctate LC3B immunostaining in H 2O 2-treated Prnp ( 0/0) cells compared with H 2O 2-treated control cells. Interestingly, this enhanced autophagy was due to impaired autophagic flux in the H 2O 2-treated Prnp ( 0/0) cells, while the H 2O 2-treated Prnp (+/+) cells showed enhanced autophagic flux. Furthermore, caspase-dependent and independent apoptosis was observed when both cell lines were exposed to H 2O 2. Moreover, the inhibition of autophagosome formation by Atg7 siRNA revealed that increased autophagic flux in Prnp (+/+) cells contributes to the prosurvival effect of autophagy against H 2O 2 cytotoxicity. Taken together, our results provide the first experimental evidence that the deficiency of PrP (C) may impair autophagic flux via H 2O 2-induced oxidative stress.

Requirement of cellular prion protein for intestinal barrier function and mislocalization in patients with inflammatory bowel disease

BACKGROUND & AIMS

Cell adhesion is one function regulated by cellular prion protein (PrP(c)), a ubiquitous, glycosylphosphatidylinositol-anchored glycoprotein. PrP(c) is located in cell-cell junctions and interacts with desmosome proteins in the intestinal epithelium. We investigated its role in intestinal barrier function.

METHODS

We analyzed permeability and structure of cell-cell junctions in intestine tissues from PrP(c) knockout (PrP(c-/-)) and wild-type mice. PrP(c) expression was knocked down in cultured human Caco-2/TC7 enterocytes using small hairpin RNAs. We analyzed colon samples from 24 patients with inflammatory bowel disease (IBD).

RESULTS

Intestine tissues from PrP(c-/-) mice had greater paracellular permeability than from wild-type mice (105.9 ± 13.4 vs 59.6 ± 10.1 mg/mL fluorescein isothiocyanate-dextran flux; P < .05) and impaired intercellular junctions. PrP(c-/-) mice did not develop spontaneous disease but were more sensitive than wild-type mice to induction of colitis with dextran sulfate (32% mortality vs 4%, respectively; P = .0033). Such barrier defects were observed also in Caco-2/TC7 enterocytes following PrP(c) knockdown; the cells had increased paracellular permeability (1.5-fold over 48 hours; P < .001) and reduced transepithelial electrical resistance (281.1 ± 4.9 vs 370.6 ± 5.7 Ω.cm(2); P < .001). Monolayer shape and cell-cell junctions were altered in cultures of PrP(c) knockdown cells; levels of E-cadherin, desmoplakin, plakoglobin, claudin-4, occludin, zonula occludens 1, and tricellulin were decreased at cell contacts. Cell shape and junctions were restored on PrP(c) re-expression. Levels of PrP(c) were decreased at cell-cell junctions in colonic epithelia from patients with Crohn's disease or ulcerative colitis.

CONCLUSIONS

PrP(c) regulates intestinal epithelial cell-cell junctions and barrier function. Its localization is altered in colonic epithelia from patients with IBD, supporting the concept that disrupted barrier function contributes to this disorder.

Neuroprotective effects of the cellular prion protein in autoimmune optic neuritis

Although the pathologic role of the prion protein in transmissible spongiform encephalopathic diseases has been widely investigated, the physiologic role of the cellular prion protein (PrP(C)) is not known. Among the many functions attributed to PrP(C), there is increasing evidence that it is involved in cell survival and mediates neuroprotection. A potential role in the immune response has also been suggested. However, how these two functions interplay in autoimmune disease is unclear. To address this, autoimmune optic neuritis, a model of multiple sclerosis, was induced in C57Bl/6 mice, and up-regulation of PrP(C) was observed throughout the disease course. In addition, compared with wild-type mice, in PrP(C)-deficient mice and mice overexpressing PrP(C), histopathologic analysis demonstrated that optic neuritis was exacerbated, as indicated by axonal degeneration, inflammatory infiltration, and demyelination. However, significant neuroprotection of retinal ganglion cells, the axons of which form the optic nerve, was observed in mice that overexpressed PrP(C). Conversely, mice lacking PrP(C) demonstrated significantly more neurodegeneration. This suggests that PrP(C) may have a neuroprotective function independent of its role in regulating the immune response.

Infectious prion protein alters manganese transport and neurotoxicity in a cell culture model of prion disease

Protein misfolding and aggregation are considered key features of many neurodegenerative diseases, but biochemical mechanisms underlying protein misfolding and the propagation of protein aggregates are not well understood. Prion disease is a classical neurodegenerative disorder resulting from the misfolding of endogenously expressed normal cellular prion protein (PrP(C)). Although the exact function of PrP(C) has not been fully elucidated, studies have suggested that it can function as a metal binding protein. Interestingly, increased brain manganese (Mn) levels have been reported in various prion diseases indicating divalent metals also may play a role in the disease process. Recently, we reported that PrP(C) protects against Mn-induced cytotoxicity in a neural cell culture model. To further understand the role of Mn in prion diseases, we examined Mn neurotoxicity in an infectious cell culture model of prion disease. Our results show CAD5 scrapie-infected cells were more resistant to Mn neurotoxicity as compared to uninfected cells (EC(50)=428.8 μM for CAD5 infected cells vs. 211.6 μM for uninfected cells). Additionally, treatment with 300 μM Mn in persistently infected CAD5 cells showed a reduction in mitochondrial impairment, caspase-3 activation, and DNA fragmentation when compared to uninfected cells. Scrapie-infected cells also showed significantly reduced Mn uptake as measured by inductively coupled plasma-mass spectrometry (ICP-MS), and altered expression of metal transporting proteins DMT1 and transferrin. Together, our data indicate that conversion of PrP to the pathogenic isoform enhances its ability to regulate Mn homeostasis, and suggest that understanding the interaction of metals with disease-specific proteins may provide further insight to protein aggregation in neurodegenerative diseases.

Cellular prion protein promotes glucose uptake through the Fyn-HIF-2α-Glut1 pathway to support colorectal cancer cell survival

Cellular prion protein (PrPc) is a glycosylphosphatidylinositol-anchored membrane protein that has various physical functions, including protection against apoptotic and oxidative stress, cellular uptake of copper ions, transmembrane signaling, and adhesion to the extracellular matrix. In this study, we show that PrPc is highly expressed in colorectal adenocarcinomas. Transcriptome profiling of PrPc-depleted DLD-1 cells revealed downregulation of glucose transporter 1 (Glut1). PrPc is shown to be involved in regulating Glut1 expression through the Fyn-HIF-2α pathway. As Glut1 is the natural transporter of glucose and is required for the high glycolytic rate seen in colorectal tumors, silencing of PrPc reduced the proliferation and survival rate of colorectal cancer cells in vitro. In vivo, knockdown of PrPc by hydrodynamic injection with a cocktail of PrPc-shRNA-encoding plasmids also inhibited tumorigenicity in a xenograft model in nude mice. In summary, our data characterize a novel molecular mechanism that links PrPc expression to the regulation of glycolysis. Targeting PrPc will therefore be a promising strategy to overcome the growth and survival advantage in colorectal tumors.

The depletion of α and β PrP from complex mixtures

Prion disorders occur when endogenous prion protein (PrP(C)) undergoes a conformational change from a predominantly α-helix-rich structure to an insoluble β-sheet-rich structure (PrP(Sc)). The resulting PrP(Sc) then in some way facilitates the progressive transformation of nearby PrP(C) to PrP(Sc). In time this results in the deposition of insoluble PrP(Sc) aggregates in the brain; aggregate deposition is irreversible and is ultimately fatal. Prion diseases are transmissible orally or through transplantation (including blood transfusion). Current diagnostic methods are limited in that they lack the ability to distinguish qualitatively between PrP(C) and PrP(Sc). PrP has been shown to bind divalent cations including copper and zinc, these cations are toxic and thus of limited use in the removal of PrP from solutions destined for administration to subjects. We have immobilised Fe(3+) to an inert Sepharose resin; this resin was capable of quantitatively removing endogenous and recombinant PrP(C) and recombinant β PrP from complex solutions. The low toxicity of Fe(3+) suggests that the resin described in this report may be of practical use in the depletion of PrP from blood products destined for human use.

The octarepeat region of hamster PrP (PrP51-91) enhances the formation of microtubule and antagonize Cu(2+)-induced microtubule-disrupting activity

Prion protein (PrP) is considered to associate with microtubule and its major component, tubulin. In the present study, octarepeat region of PrP (PrP51-91) was expressed in prokaryotic-expressing system. Using GST pull-down assay and co-immunoprecipitation, the molecular interaction between PrP51-91 and tubulin was observed. Our data also demonstrated that PrP51-91 could efficiently stimulate microtubule assembly in vitro, indicating a potential effect of PrP on microtubule dynamics. Moreover, PrP51-91 was confirmed to be able to antagonize Cu(2+)-induced microtubule-disrupting activity in vivo, partially protecting against Cu(2+) intoxication to culture cells and stabilize cellular microtubule structure. The association of the octarepeat region of PrP with tubulin may further provide insight into the biological function of PrP in the neurons.

Prion protein ablation increases cellular aggregation and embolization contributing to mechanisms of metastasis

Cellular Prion Protein (PrP(C)) is a cell surface protein highly expressed in the nervous system, and to a lesser extent in other tissues. PrP(C) binds to the extracellular matrix laminin and vitronectin, to mediate cell adhesion and differentiation. Herein, we investigate how PrP(C) expression modulates the aggressiveness of transformed cells. Mesenchymal embryonic cells (MEC) from wild-type (Prnp(+/+)) and PrP(C)-null (Prnp(0/0)) mice were immortalized and transformed by co-expression of ras and myc. These cells presented similar growth rates and tumor formation in vivo. When injected in the tail vein, Prnp(0/0)ras/myc cells exhibited increased lung colonization compared with Prnp(+/+)ras/myc cells. Additionally, Prnp(0/0)ras/myc cells form more aggregates with blood components than Prnp(+/+)ras/myc cells, facilitating the arrest of Prnp(0/0)ras/myc cells in the lung vasculature. Integrin alpha(v)beta(3) is more expressed and activated in MEC and in transformed Prnp(0/0) cells than in the respective Prnp(+/+) cells. The blocking of integrin alpha(v)beta(3) by RGD peptide reduces lung colonization in transformed Prnp(0/0) cells to similar levels of those presented by transformed Prnp(+/+) cells. Our data indicate that PrP(C) negatively modulates the expression and activation of integrin alpha(v)beta(3) resulting in a more aggressive phenotype. These results indicate that PrP(C) may have main implications in modulating metastasis formation.

Glycosylation modification of human prion protein provokes apoptosis in HeLa cells in vitro

We investigate the correlation between the glycosylation modified prion proteins and apoptosis. The wild-type PRNP gene and four PRNP gene glycosylated mutants were transiently expressed in HeLa cells. The effect of apoptosis induced by PrP mutants was confirmed by MTT assay, Hochest staining, Annexin-V staining and PI staining. ROS test detected ROS generation within the cells. The mitochondrial membrane potential was analyzed by the flow cytometry. The expression levels of Bcl-xL, Bax, cleaved Caspase-9 proteins were analyzed by Western Blot. The results indicated that the expressed non-glycosylated PrP in HeLa cells obviously induced apoptosis, inhibited the growth of cells and reduced the mitochondrial membrane potential, and more ROS generation and low levels of the apoptosis-related proteins Bcl-xL, the activated the cleaved Caspase-9 proteins were found. The apoptosis induced by non-glycosylated PrP demonstrates that its underlying mechanism correlates with the mitochondria-mediated signal transduction pathway.

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

p53-Dependent transcriptional control of cellular prion by presenilins

The presenilin-dependent gamma-secretase processing of the beta-amyloid precursor protein (betaAPP) conditions the length of the amyloid beta peptides (Abeta) that accumulate in the senile plaques of Alzheimer's disease-affected brains. This, together with an additional presenilin-mediated epsilon-secretase cleavage, generates intracellular betaAPP-derived fragments named amyloid intracellular domains (AICDs) that regulate the transcription of several genes. We establish that presenilins control the transcription of cellular prion protein (PrP(c)) by a gamma-secretase inhibitor-sensitive and AICD-mediated process. We demonstrate that AICD-dependent control of PrP(c) involves the tumor suppressor p53. Thus, p53-deficiency abolishes the AICD-mediated control of PrP(c) transcription. Furthermore, we show that p53 directly binds to the PrP(c) promoter and increases its transactivation. Overall, our study unravels a transcriptional regulation of PrP(c) by the oncogene p53 that is directly driven by presenilin-dependent formation of AICD. Furthermore, it adds support to previous reports linking secretase activities involved in betaAPP metabolism to the physiology of PrP(c).

Fishing for prion protein function

The prion protein is infamous for its role in devastating neurological diseases, but its normal, physiological function has remained mysterious. A new study uses the experimentally tractable zebrafish model to obtain fresh clues to this puzzle.

The LCB2 subunit of the sphingolip biosynthesis enzyme serine palmitoyltransferase can function as an attenuator of the hypersensitive response and Bax-induced cell death

Previous results showed that expression of the gene encoding the LONG-CHAIN BASE2 (LCB(2)) subunit of serine palmitoyltransferase (SPT), designated BcLCB(2), from nonheading Chinese cabbage (Brassica campestris ssp. chinensis) was up-regulated during hypersensitive cell death (HCD) induced by the Phytophthora boehmeriae elicitor PB90. Overexpression of BcLCB(2) in Nicotiana tabacum leaves suppressed the HCD normally initiated by elicitors and PB90-triggered H(2)O(2) accumulation. BcLCB(2) also functioned as a suppressor of mouse Bcl-2 associated X (Bax) protein-mediated HCD and cell death caused by Ralstonia solanacearum. BcLCB(2) overexpression suppressed Bax- and oxidant stress-triggered yeast cell death. Reactive oxygen species (ROS) accumulation induced by Bax was compromised in BcLCB(2)-overexpressing yeast cells. The findings that NbLCB(2) silencing in Nicotiana benthamiana enhanced elicitor-triggered HCD, combined with the fact that myriocin, a potent inhibitor of SPT, had no effect on Bax-induced programmed cell death, suggested that suppression of cell death was not involved in the dominant-negative effect that resulted from BcLCB(2) overexpression. A BcLCB(2) mutant assay showed that the suppression was not involved in SPT activity. The results suggest that plant HCD and stress-induced yeast cell death might share a common signal transduction pathway involving LCB(2), and that LCB(2) protects against cell death by inhibiting ROS accumulation, this inhibition being independent of SPT activity.

LRP1 controls biosynthetic and endocytic trafficking of neuronal prion protein

The trafficking of normal cellular prion protein (PrPC) is believed to control its conversion to the altered conformation (designated PrPSc) associated with prion disease. Although anchored to the membrane by means of glycosylphosphatidylinositol (GPI), PrPC on neurons is rapidly and constitutively endocytosed by means of coated pits, a property dependent upon basic amino acids at its N-terminus. Here, we show that low-density lipoprotein receptor-related protein 1 (LRP1), which binds to multiple ligands through basic motifs, associates with PrPC during its endocytosis and is functionally required for this process. Moreover, sustained inhibition of LRP1 levels by siRNA leads to the accumulation of PrPC in biosynthetic compartments, with a concomitant lowering of surface PrPC, suggesting that LRP1 expedites the trafficking of PrPC to the neuronal surface. PrPC and LRP1 can be co-immunoprecipitated from the endoplasmic reticulum in normal neurons. The N-terminal domain of PrPC binds to purified human LRP1 with nanomolar affinity, even in the presence of 1 μM of the LRP-specific chaperone, receptor-associated protein (RAP). Taken together, these data argue that LRP1 controls both the surface, and biosynthetic, trafficking of PrPC in neurons.

The chemistry of copper binding to PrP: is there sufficient evidence to elucidate a role for copper in protein function?

There has been an enormous body of literature published in the last 10 years concerning copper and PrP (prion protein). Despite this, there is still no generally accepted role for copper in the function of PrP or any real consensus as to how and to what affinity copper associates with the protein. The present review attempts to look at all the evidence for the chemistry, co-ordination and affinity of copper binding to PrP, and then looks at what effect this has on the protein. We then connect this evidence with possible roles for PrP when bound to copper. No clear conclusions can be made from the available data, but it is clear from the present review what aspects of copper association with PrP need to be re-investigated.

Prion proteins: Physiological functions and role in neurological disorders

Stanley Prusiner was the first to promote the concept of misfolded proteins as a cause for neurological disease. It has since been shown by him and other investigators that the scrapie isoform of prion protein (PrP(Sc)) functions as an infectious agent in numerous human and non-human disorders of the central nervous system (CNS). Interestingly, other organ systems appear to be less affected, and do not appear to lead to major co-morbidities. The physiological function of the endogenous cellular form of the prion protein (PrP(C)) is much less clear. It is intriguing that PrP(c) is expressed on most tissues in mammals, suggesting not only biological functions outside the CNS, but also a role other than the propagation of its misfolded isotype. In this review, we summarize accumulating in vitro and in vivo evidence regarding the physiological functions of PrP(C) in the nervous system, as well as in lymphoid organs.

Prion proteins: a biological role beyond prion diseases

The biological role of the scrapie isoform of prion protein (PrP(Sc)) as an infectious agent in numerous human and non-human disorders of the central nervous system is well established. In contrast, and despite decades of intensive research, the physiological function of the endogenous cellular form of the prion protein (PrP(C)) remains elusive. In mammals, the ubiquitous expression of PrP(C) suggests biological functions other than its pathological role in propagating the accumulation of its misfolded isotype. Other functions that have been attributed to PrP(C) include signal transduction, synaptic transmission and protection against cell death through the apoptotic pathway. More recently, immunoregulatory properties of PrP(C) have been reported. We review accumulating in vitro and in vivo evidence regarding physiological functions of PrP(C).

The cellular prion protein (PrP(C)): its physiological function and role in disease

Prion diseases are caused by conversion of a normal cell-surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is infectious in the absence of nucleic acid. Although a great deal has been learned about PrP(Sc) and its role in prion propagation, much less is known about the physiological function of PrP(C). In this review, we will summarize some of the major proposed functions for PrP(C), including protection against apoptotic and oxidative stress, cellular uptake or binding of copper ions, transmembrane signaling, formation and maintenance of synapses, and adhesion to the extracellular matrix. We will also outline how loss or subversion of the cytoprotective or neuronal survival activities of PrP(C) might contribute to the pathogenesis of prion diseases, and how similar mechanisms are probably operative in other neurodegenerative disorders.

Bcl‐2 overexpression delays caspase‐3 activation and rescues cerebellar degeneration in prion‐deficient mice that overexpress amino‐terminally truncated prion

Prnp knockout mice that overexpress an amino-truncated form of PrPc (deltaPrP) are ataxic and display cerebellar cell loss and premature death. Studies on the molecular and intracellular events that trigger cell death in these mutants may contribute to elucidate the functions of PrPc and to the design of treatments for prion disease. Here we examined the effects of Bcl-2 overexpression in neurons on the development of the neurological syndrome and cerebellar pathology of deltaPrP. We show that deltaPrP overexpression activates the stress-associated kinases ERK1-2 in reactive astroglia, p38 and the phosphorylation of p53, which leads to the death of cerebellar neurons in mutant mice. We found that the expression of deltaPrP in cell lines expressing very low levels of PrPc strongly induces the activation of apoptotic pathways, thereby leading to caspase-3 activation and cell death, which can be prevented by coexpressing Bcl-2. Finally, we corroborate in vivo that neuronal-directed Bcl-2 overexpression in deltaPrP mice (deltaPrP Bcl-2) markedly reduces caspase-3 activation, glial activation, and neuronal cell death in cerebellum by improving locomotor deficits and life expectancy.

Copper and zinc promote interactions between membrane-anchored peptides of the metal binding domain of the prion protein

The prion protein (PrP) has been identified as a metalloprotein capable of binding multiple copper ions and possibly zinc. Recent studies now indicate that prion self-recognition may be an important factor in both the normal function and misfunction of this protein. We have developed fluorescently labeled models of the prion protein that allow prion-prion interactions and metal binding to be investigated on the molecular level. Peptides encompassing the full metal binding region were anchored to the surface of small unilamellar vesicles, and PrP-PrP interactions were monitored by fluorescence spectroscopy as a function of added metal. Both Cu2+ and Zn2+ were found to cause an increase in the level of PrP-PrP interactions, by 117 and 300%, respectively, whereas other metals such as Ni2+, Co2+, and Ca2+ had no effect. The binding of either of these cofactors appears to act as a switch that induces PrP-PrP interactions in a reversible manner. Both glutamine and tryptophan residues, which occur frequently in the metal binding region of PrP, were found to be important in mediating PrP-PrP interactions. Experiments demonstrate that tryptophan residues are also responsible for the low level of PrP-PrP interactions observed in the absence of Cu2+ and Zn2+, and this is further supported by molecular modeling. Overall, our results indicate that PrP may be a bifunctional molecule capable of responding to fluctuations in both neuronal Cu2+ and Zn2+ levels.

The role of the cellular prion protein in the immune system

Prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed widely in the immune system, in haematopoietic stem cells and mature lymphoid and myeloid compartments in addition to cells of the central nervous system. It is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocyte. Furthermore, antibody cross-linking of surface PrP modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signalling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, recent work has suggested that PrP is required for self-renewal of haematopoietic stem cells. Here, we consider the evidence for a distinctive role for PrP(C) in the immune system and what the effects of anti-prion therapeutics may be on immune function.