The cellular prion protein PrPc is expressed in human enterocytes in cell-cell junctional domains

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.

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

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

Involvement of the Prion Protein in the Protection of the Human Bronchial Epithelial Barrier Against Oxidative Stress

Aim: Bronchial epithelium acts as a defensive barrier against inhaled pollutants and microorganisms. This barrier is often compromised in inflammatory airway diseases that are characterized by excessive oxidative stress responses, leading to bronchial epithelial shedding, barrier failure, and increased bronchial epithelium permeability. Among proteins expressed in the junctional barrier and participating to the regulation of the response to oxidative and to environmental stresses is the cellular prion protein (PrP^C). However, the role of PrP^C is still unknown in the bronchial epithelium. Herein, we investigated the cellular mechanisms by which PrP^C protein participates into the junctional complexes formation, regulation, and oxidative protection in human bronchial epithelium. Results: Both PrP^C messenger RNA and mature protein were expressed in human epithelial bronchial cells. PrP^C was localized in the apical domain and became lateral, at high degree of cell polarization, where it colocalized and interacted with adherens (E-cadherin/γ-catenin) and desmosomal (desmoglein/desmoplakin) junctional proteins. No interaction was detected with tight junction proteins. Disruption of such interactions induced the loss of the epithelial barrier. Moreover, we demonstrated that PrP^C protection against copper-associated oxidative stress was involved in multiple processes, including the stability of adherens and desmosomal junctional proteins. Innovation: PrP^C is a pivotal protein in the protection against oxidative stress that is associated with the degradation of adherens and desmosomal junctional proteins. Conclusion: Altogether, these results demonstrate that the loss of the integrity of the epithelial barrier by oxidative stress is attenuated by the activation of PrP^C expression, where deregulation might be associated with respiratory diseases.

Inflammatory mediators reduce surface PrPc on human BMVEC resulting in decreased barrier integrity

The cellular prion protein (PrP^c) is a surface adhesion molecule expressed at junctions of various cell types including brain microvascular endothelial cells (BMVEC) that are important components of the blood-brain barrier (BBB). PrP^c is involved in several physiological processes including regulation of epithelial cell barrier function and monocyte migration across BMVEC. BBB dysfunction and disruption are significant events in central nervous system (CNS) inflammatory processes including HIV neuropathogenesis. Tumor necrosis factor (TNF)-α and vascular endothelial growth factor (VEGF) are two inflammatory factors that have been implicated in the processes that affect BBB integrity. To examine the effect of inflammation on PrP^c expression in BMVEC, we used these mediators and found that TNF-α and VEGF decrease surface PrP^c on primary human BMVEC. We also showed that these factors decrease total PrP^c protein as well as mRNA, indicating that they regulate expression of this protein by de novo synthesis. To determine the effect of PrP^c loss from the surface of BMVEC on barrier integrity, we used small hairpin RNAs to knockdown PrP^c. We found that the absence of PrP^c from BMVEC causes increased permeability as determined by a fluorescein isothiocyanate (FITC)-dextran permeability assay. This suggests that cell surface PrP^c is essential for endothelial monolayer integrity. To determine the mechanism by which PrP^c downregulation leads to increased permeability of an endothelial monolayer, we examined changes in expression and localization of tight junction proteins, occludin and claudin-5, and found that decreased PrP^c leads to decreased total and membrane-associated occludin and claudin-5. We propose that an additional mechanism by which inflammatory factors affect endothelial monolayer permeability is by decreasing cell-associated PrP^c. This increase in permeability may have subsequent consequences that lead to CNS damage.

The nucleo-junctional interplay of the cellular prion protein: A new partner in cancer-related signaling pathways?

The cellular prion protein PrP(c) plays important roles in proliferation, cell death and survival, differentiation and adhesion. The participation of PrP(c) in tumor growth and metastasis was pointed out, but the underlying mechanisms were not deciphered completely. In the constantly renewing intestinal epithelium, our group demonstrated a dual localization of PrP(c), which is targeted to cell-cell junctions in interaction with Src kinase and desmosomal proteins in differentiated enterocytes, but is predominantly nuclear in dividing cells. While the role of PrP(c) in the dynamics of intercellular junctions was confirmed in other biological systems, we unraveled its function in the nucleus only recently. We identified several nuclear PrP(c) partners, which comprise γ-catenin, one of its desmosomal partners, β-catenin and TCF7L2, the main effectors of the canonical Wnt pathway, and YAP, one effector of the Hippo pathway. PrP(c) up-regulates the activity of the β-catenin/TCF7L2 complex and its invalidation impairs the proliferation of intestinal progenitors. We discuss how PrP(c) could participate to oncogenic processes through its interaction with Wnt and Hippo pathway effectors, which are controlled by cell-cell junctions and Src family kinases and dysregulated during tumorigenesis. This highlights new potential mechanisms that connect PrP(c) expression and subcellular redistribution to cancer.

PrPC expression and prion seeding activity in the alimentary tract and lymphoid tissue of deer

The agent responsible for prion diseases is a misfolded form of a normal protein (PrPC). The prion hypothesis stipulates that PrPC must be present for the disease to manifest. Cervid populations across the world are infected with chronic wasting disease, a horizontally-transmissible prion disease that is likely spread via oral exposure to infectious prions (PrPCWD). Though PrPCWD has been identified in many tissues, there has been little effort to characterize the overall PrPC expression in cervids and its relationship to PrPCWD accumulation. We used immunohistochemistry (IHC), western blot and enzyme-linked immunosorbent assay to describe PrPC expression in naïve white-tailed deer. We used real-time, quaking-induced conversion (RT-QuIC) to detect prion seeding activity in CWD-infected deer. We assessed tissues comprising the alimentary tract, alimentary-associated lymphoid tissue and systemic lymphoid tissue from 5 naïve deer. PrPC was expressed in all tissues, though expression was often very low compared to the level in the CNS. IHC identified specific cell types wherein PrPC expression is very high. To compare the distribution of PrPC to PrPCWD, we examined 5 deer with advanced CWD infection. Using RT-QuIC, we detected prion seeding activity in all 21 tissues. In 3 subclinical deer sacrificed 4 months post-inoculation, we detected PrPCWD consistently in alimentary-associated lymphoid tissue, irregularly in alimentary tract tissues, and not at all in the brain. Contrary to our hypothesis that PrPC levels dictate prion accumulation, PrPC expression was higher in the lower gastrointestinal tissues than in the alimentary-associated lymphoid system and was higher in salivary glands than in the oropharyngeal lymphoid tissue. These data suggest that PrPC expression is not the sole driver of prion accumulation and that alimentary tract tissues accumulate prions before centrifugal spread from the brain occurs.

Cell Biology of Prion Protein

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

Functions of the Prion Protein

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

PrP(c) deficiency and dasatinib protect mouse intestines against radiation injury by inhibiting of c-Src

BACKGROUND & AIM

Despite extensive study of the contribution of cell death and apoptosis to radiation-induced acute intestinal injury, our knowledge of the signaling mechanisms involved in epithelial barrier dysfunction remains inadequate. Because PrP(c) plays a key role in intestinal homeostasis by renewing epithelia, we sought to study its role in epithelial barrier function after irradiation.

DESIGN

Histology, morphometry and plasma FD-4 levels were used to examine ileal architecture, wound healing, and intestinal leakage in PrP(c)-deficient (KO) and wild-type (WT) mice after total-body irradiation. Impairment of the PrP(c) Src pathway after irradiation was explored by immunofluorescence and confocal microscopy, with Caco-2/Tc7 cells. Lastly, dasatinib treatment was used to switch off the Src pathway in vitro and in vivo.

RESULTS

The decrease in radiation-induced lethality, improved intestinal wound healing, and reduced intestinal leakage promoted by PrP(c) deficiency demonstrate its involvement in acute intestinal damage. Irradiation of Cacao2/Tc7 cells induced PrP(c) to target the nuclei associated with Src activation. Finally, the protective effect triggered by dasatinib confirmed Src involvement in radiation-induced acute intestinal toxicity.

CONCLUSION

Our data are the first to show a role for the PrP(c)-Src pathway in acute intestinal response to radiation injury and offer a novel therapeutic opportunity.

Recombinant mouse prion protein alone or in combination with lipopolysaccharide alters expression of innate immunity genes in the colon of mice

The objectives of this study were to test whether recombinant mouse (mo)PrP alone or in combination with LPS or under simulated endotoxemia would affect expression of genes related to host inflammatory and antimicrobial responses. To test our hypotheses colon tissues were collected from 16 male mice (FVB/N strain) and mounted in an Ussing chamber. Application of moPrP to the mucosal side of the colon affected genes related to TLR- and NLR- signaling and antimicrobial responses. When LPS was added on the mucosal side of the colon, genes related to TLR, Nlrp3 inflammasome, and iron transport proteins were over-expressed. Addition of LPS to the serosal side of the colon up-regulated genes related to TLR- and NLR-signaling, Nlrp3 inflammasome, and a chemokine. Treatment with both moPrP and LPS to the mucosal side of the colon upregulated genes associated with TLR, downstream signal transduction (DST), inflammatory response, attraction of dendritic cells to the site of inflammation, and the JNK-apoptosis pathway. Administration of moPrP to the mucosal side and LPS to the serosal side of the colon affected genes related to TLR- and NLR-signaling, DST, apoptosis, inflammatory response, cytokines, chemokines, and antimicrobial peptides. Overall this study suggests a potential role for moPrP as an endogenous 'danger signal' associated with activation of colon genes related to innate immunity and antibacterial responses.

The cellular prion protein PrPc is a partner of the Wnt pathway in intestinal epithelial cells

We reported previously that the cellular prion protein (PrP(c)) is a component of desmosomes and contributes to the intestinal barrier function. We demonstrated also the presence of PrP(c) in the nucleus of proliferating intestinal epithelial cells. Here we sought to decipher the function of this nuclear pool. In human intestinal cancer cells Caco-2/TC7 and SW480 and normal crypt-like HIEC-6 cells, PrP(c) interacts, in cytoplasm and nucleus, with γ-catenin, one of its desmosomal partners, and with β-catenin and TCF7L2, effectors of the canonical Wnt pathway. PrP(c) up-regulates the transcriptional activity of the β-catenin/TCF7L2 complex, whereas γ-catenin down-regulates it. Silencing of PrP(c) results in the modulation of several Wnt target gene expressions in human cells, with different effects depending on their Wnt signaling status, and in mouse intestinal crypt cells in vivo. PrP(c) also interacts with the Hippo pathway effector YAP, suggesting that it may contribute to the regulation of gene transcription beyond the β-catenin/TCF7L2 complex. Finally, we demonstrate that PrP(c) is required for proper formation of intestinal organoids, indicating that it contributes to proliferation and survival of intestinal progenitors. In conclusion, PrP(c) must be considered as a new modulator of the Wnt signaling pathway in proliferating intestinal epithelial cells.

Common themes in PrP signaling: the Src remains the same

The ability of the cellular prion protein (PrP^C) to trigger intracellular signals appears central to neurodegeneration pathways, yet the physiological significance of such signals is rather puzzling. For instance, PrP^C deregulation disrupts phenomena as diverse as synaptic transmission in mammals and cell adhesion in zebrafish. Although unrelated, the key proteins in these events -the NMDA receptor (NMDAR) and E-cadherin, respectively- are similarly modulated by the Src family kinase (SFK) Fyn. These observations highlight the importance of PrP^C-mediated Fyn activation, a finding reported nearly two decades ago. Given their complex functions and regulation, SFKs may hold the key to intriguing aspects of PrP biology such as its seemingly promiscuous functions and the lack of strong phenotypes in knockout mice. Here we provide a mechanistic perspective on how SFKs might contribute to the uncertain molecular basis of neuronal PrP phenotypes affecting ion channel activity, axon myelination and olfactory function. In particular, we discuss SFK target proteins involved in these processes and the role of tyrosine phosphorylation in the regulation of their activity and cell surface expression.

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.

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.

Conserved roles of the prion protein domains on subcellular localization and cell-cell adhesion

Analyses of cultured cells and transgenic mice expressing prion protein (PrP) deletion mutants have revealed that some properties of PrP -such as its ability to misfold, aggregate and trigger neurotoxicity- are controlled by discrete molecular determinants within its protein domains. Although the contributions of these determinants to PrP biosynthesis and turnover are relatively well characterized, it is still unclear how they modulate cellular functions of PrP. To address this question, we used two defined activities of PrP as functional readouts: 1) the recruitment of PrP to cell-cell contacts in Drosophila S2 and human MCF-7 epithelial cells, and 2) the induction of PrP embryonic loss- and gain-of-function phenotypes in zebrafish. Our results show that homologous mutations in mouse and zebrafish PrPs similarly affect their subcellular localization patterns as well as their in vitro and in vivo activities. Among PrP’s essential features, the N-terminal leader peptide was sufficient to drive targeting of our constructs to cell contact sites, whereas lack of GPI-anchoring and N-glycosylation rendered them inactive by blocking their cell surface expression. Importantly, our data suggest that the ability of PrP to homophilically trans-interact and elicit intracellular signaling is primarily encoded in its globular domain, and modulated by its repetitive domain. Thus, while the latter induces the local accumulation of PrPs at discrete punctae along cell contacts, the former counteracts this effect by promoting the continuous distribution of PrP. In early zebrafish embryos, deletion of either domain significantly impaired PrP’s ability to modulate E-cadherin cell adhesion. Altogether, these experiments relate structural features of PrP to its subcellular distribution and in vivo activity. Furthermore, they show that despite their large evolutionary history, the roles of PrP domains and posttranslational modifications are conserved between mouse and zebrafish.

Relevance of TNBS-colitis in rats: a methodological study with endoscopic, histologic and Transcriptomic [corrected] characterization and correlation to IBD

Background

Rectal instillation of trinitrobenzene sulphonic acid (TNBS) in ethanol is an established model for inflammatory bowel disease (IBD). We aimed to 1) set up a TNBS-colitis protocol resulting in an endoscopic and histologic picture resembling IBD, 2) study the correlation between endoscopic, histologic and gene expression alterations at different time points after colitis induction, and 3) compare rat and human IBD mucosal transcriptomic data to evaluate whether TNBS-colitis is an appropriate model of IBD.

Methodology/Principal Findings

Five female Sprague Daley rats received TNBS diluted in 50% ethanol (18 mg/0.6 ml) rectally. The rats underwent colonoscopy with biopsy at different time points. RNA was extracted from rat biopsies and microarray was performed. PCR and in situ hybridization (ISH) were done for validation of microarray results. Rat microarray profiles were compared to human IBD expression profiles (25 ulcerative colitis Endoscopic score demonstrated mild to moderate colitis after three and seven days, but declined after twelve days. Histologic changes corresponded with the endoscopic appearance. Over-represented Gene Ontology Biological Processes included: Cell Adhesion, Immune Response, Lipid Metabolic Process, and Tissue Regeneration. IL-1α, IL-1β, TLR2, TLR4, PRNP were all significantly up-regulated, while PPARγ was significantly down-regulated. Among genes with highest fold change (FC) were SPINK4, LBP, ADA, RETNLB and IL-1α. The highest concordance in differential expression between TNBS and IBD transcriptomes was three days after colitis induction. ISH and PCR results corresponded with the microarray data. The most concordantly expressed biologically relevant pathways included TNF signaling, Cell junction organization, and Interleukin-1 processing.

Conclusions/Significance

Endoscopy with biopsies in TNBS-colitis is useful to follow temporal changes of inflammation visually and histologically, and to acquire tissue for gene expression analyses. TNBS-colitis is an appropriate model to study specific biological processes in IBD.

Endogenous cellular prion protein regulates contractility of the mouse ileum

BACKGROUND

Cellular prion protein (PrP(C) ) is expressed in the enteric nervous system (ENS), however, its physiological role has not been identified. Studies suggest that PrP(C) can function as a metal-binding protein, as absence of the protein has been linked to altered copper metabolism and atypical synaptic activity. Because copper is known to modulate smooth muscle relaxation, we tested the hypothesis that PrP(C) deficiency would alter intestinal contractility.

METHODS

We examined electrically evoked ileal contractility in Prnp(-/-) or wild type littermate mice and the effects of copper or copper chelation. PrP(C) expression was studied in whole mount ileal preparations of mice and guinea pigs by immunohistochemistry.

KEY RESULTS

Relative to wild type mice, ileal tissues of Prnp(-/-) mice exhibited reduced electrical field stimulation (EFS)-evoked contractility. Furthermore, EFS-induced relaxation, as a percentage of that induced by a nitric oxide donor, was enhanced. Addition of a copper donor to the organ bath increased, whereas the addition of a copper chelator inhibited, nitric oxide donor-induced ileal relaxation in Prnp(-/-) mice. PrP(C) was expressed on nerve fibers or terminals, and some cell bodies in the myenteric and submucosal plexuses of wild type mice. PrP(C) colocalized with a neuron-specific ectonucleotidase, nucleoside triphosphate diphosphohydrolase 3 (NTPDase3), but to only a limited extent with GFAP, a marker of enteric glia. Guinea pigs expressed PrP(C) in nerve fibers or terminals and enteric glia in the myenteric and submucosal plexuses.

CONCLUSIONS & INFERENCES

Our findings suggest that PrP(C) , which is abundant in the ENS, has a role in the regulation of ileal contractility.

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.

Detection and control of prion diseases in food animals

Transmissible spongiform encephalopathies (TSEs), or prion diseases, represent a unique form of infectious disease based on misfolding of a self-protein (PrP^C) into a pathological, infectious conformation (PrP^Sc). Prion diseases of food animals gained notoriety during the bovine spongiform encephalopathy (BSE) outbreak of the 1980s. In particular, disease transmission to humans, to the generation of a fatal, untreatable disease, elevated the perspective on livestock prion diseases from food production to food safety. While the immediate threat posed by BSE has been successfully addressed through surveillance and improved management practices, another prion disease is rapidly spreading. Chronic wasting disease (CWD), a prion disease of cervids, has been confirmed in wild and captive populations with devastating impact on the farmed cervid industries. Furthermore, the unabated spread of this disease through wild populations threatens a natural resource that is a source of considerable economic benefit and national pride. In a worst-case scenario, CWD may represent a zoonotic threat either through direct transmission via consumption of infected cervids or through a secondary food animal, such as cattle. This has energized efforts to understand prion diseases as well as to develop tools for disease detection, prevention, and management. Progress in each of these areas is discussed.

Endogenous prion protein attenuates experimentally induced colitis

Although the cellular prion protein (PrP(C)) is expressed in the enteric nervous system and lamina propria, its function(s) in the gut is unknown. Because PrP(C) may exert a cytoprotective effect in response to various physiologic stressors, we hypothesized that PrP(C) expression levels might modulate the severity of experimental colitis. We evaluated the course of dextran sodium sulfate (DSS)-induced colitis in hemizygous Tga20 transgenic mice (approximately sevenfold overexpression of PrP(C)), Prnp(-/-) mice, and wild-type mice. On day 7, colon length, disease severity, and histologic activity indices were determined. Unlike DSS-treated wild-type and Prnp(-/-) animals, PrP(C) overexpressing mice were resistant to colitis induction, exhibited much milder histopathologic features, and did not exhibit weight loss or colonic shortening. In keeping with these results, pro-survival molecule expression and/or phosphorylation levels were elevated in DSS-treated Tga20 mice, whereas pro-inflammatory cytokine production and pSTAT3 levels were reduced. In contrast, DSS-treated Prnp(-/-) mice exhibited increased BAD protein expression and a cytokine expression profile predicted to favor inflammation and differentiation. PrP(C) expression from both the endogenous Prnp locus or the Tga20 transgene was increased in the colons of DSS-treated mice. Considered together, these findings demonstrate that PrP(C) has a previously unrecognized cytoprotective and/or anti-inflammatory function within the murine colon.

Desmosome assembly and cell-cell adhesion are membrane raft-dependent processes

The aim of our study was to investigate the association of desmosomal proteins with cholesterol-enriched membrane domains, commonly called membrane rafts, and the influence of cholesterol on desmosome assembly in epithelial Madin-Darby canine kidney cells (clone MDc-2). Biochemical analysis proved an association of desmosomal cadherin desmocollin 2 (Dsc2) in cholesterol-enriched fractions that contain membrane raft markers caveolin-1 and flotillin-1 and the novel raft marker ostreolysin. Cold detergent extraction of biotinylated plasma membranes revealed that ∼60% of Dsc2 associates with membrane rafts while the remainder is present in nonraft and cholesterol-poor membranes. The results of immunofluorescence microscopy confirmed colocalization of Dsc2 and ostreolysin. Partial depletion of cholesterol with methyl-β-cyclodextrin disturbs desmosome assembly, as revealed by sequential recordings of live cells. Moreover, cholesterol depletion significantly reduces the strength of cell-cell junctions and partially releases Dsc2 from membrane rafts. Our data indicate that a pool of Dsc2 is associated with membrane rafts, particularly with the ostreolysin type of membrane raft, and that intact membrane rafts are necessary for desmosome assembly. Taken together, these data suggest cholesterol as a potential regulator that promotes desmosome assembly.

Prion interaction with the 37-kDa/67-kDa laminin receptor on enterocytes as a cellular model for intestinal uptake of prions

Enterocytes, a major cell population of the intestinal epithelium, represent one possible barrier to the entry of prions after oral exposure. We established a cell culture system employing enterocytes from different species to study alimentary prion interaction with the 37-kDa/67-kDa laminin receptor LRP/LR. Human, bovine, porcine, ovine, and cervid enterocytes were cocultured with brain homogenates from cervid, sheep, and cattle suffering from chronic wasting disease (CWD), scrapie, and bovine spongiform encephalopathy (BSE), respectively. PrP(CWD), ovine PrP(Sc), and PrP(BSE) all colocalized with LRP/LR on human enterocytes. PrP(CWD) failed to colocalize with LRP/LR on bovine, porcine, and ovine enterocytes. Ovine PrP(Sc) colocalized with the receptor on bovine enterocytes, but failed to colocalize with LRP/LR on cervid and porcine enterocytes. PrP(BSE) failed to colocalize with the receptor on cervid and ovine enterocytes. These data suggest possible oral transmissibility of CWD and sheep scrapie to humans and may confirm the oral transmissibility of BSE to humans, resulting in zoonotic variant Creutzfeldt-Jakob disease. CWD might not be transmissible to cattle, pigs, and sheep. Sheep scrapie might have caused BSE, but may not cause transmissible spongiform encephalopathy in cervids and pigs. BSE may not be transmissible to cervids. Our data recommend the enterocyte model system for further investigations of the intestinal pathophysiology of alimentary prion infections.

PrPs: Proteins with a purpose: Lessons from the zebrafish

The best-known attribute of the prion protein (PrP) is its tendency to misfold into a rogue isoform. Much less understood is how this misfolded isoform causes deadly brain illnesses. Neurodegeneration in prion disease is often seen as a consequence of abnormal PrP function yet, amazingly little is known about the normal, physiological role of PrP. In particular, the absence of obvious phenotypes in PrP knockout mice has prevented scientists from answering this important question. Using knockdown approaches, we previously produced clear PrP loss-of-function phenotypes in zebrafish embryos. Analysis of these phenotypes revealed that PrP can modulate E-cadherin-based cell-cell adhesion, thereby controlling essential morphogenetic cell movements in the early gastrula. Our data also showed that PrP itself can elicit homophilic cell-cell adhesion and trigger intracellular signaling via Src-related kinases. Importantly, these molecular functions of PrP are conserved from fish to mammals. Here we discuss the use of the zebrafish in prion biology and how it may advance our understanding of the roles of PrP in health and disease.

Regulation of embryonic cell adhesion by the prion protein

Prion proteins (PrPs) are key players in fatal neurodegenerative disorders, yet their physiological functions remain unclear, as PrP knockout mice develop rather normally. We report a strong PrP loss-of-function phenotype in zebrafish embryos, characterized by the loss of embryonic cell adhesion and arrested gastrulation. Zebrafish and mouse PrP mRNAs can partially rescue this knockdown phenotype, indicating conserved PrP functions. Using zebrafish, mouse, and Drosophila cells, we show that PrP: (1) mediates Ca⁺²-independent homophilic cell adhesion and signaling; and (2) modulates Ca⁺²-dependent cell adhesion by regulating the delivery of E-cadherin to the plasma membrane. In vivo time-lapse analyses reveal that the arrested gastrulation in PrP knockdown embryos is due to deficient morphogenetic cell movements, which rely on E-cadherin–based adhesion. Cell-transplantation experiments indicate that the regulation of embryonic cell adhesion by PrP is cell-autonomous. Moreover, we find that the local accumulation of PrP at cell contact sites is concomitant with the activation of Src-related kinases, the recruitment of reggie/flotillin microdomains, and the reorganization of the actin cytoskeleton, consistent with a role of PrP in the modulation of cell adhesion via signaling. Altogether, our data uncover evolutionarily conserved roles of PrP in cell communication, which ultimately impinge on the stability of adherens cell junctions during embryonic development.

Unlike conventional pathogens, prions are infectious particles devoid of nucleic acids and composed entirely of a misfolded host protein, PrP. It is widely assumed that the neurodegeneration observed in prion disorders may be related to an aberrant function of PrP in the misfolded state. However, the normal physiological function of PrP remains poorly understood, mainly owing to the absence of clear phenotypes in mice lacking PrP. Here, we show that when PrP is depleted in zebrafish embryos, dramatic phenotypes ensue, severely affecting the development of early and late (neural) structures. We examined the mechanisms responsible for some of these defects, and found that fish and mammalian PrPs play conserved roles in cell–cell communication, by directly mediating cell adhesion and by triggering cellular signals that further modulate the function of other adhesion molecules. In the early zebrafish embryo, these activities control not only tissue integrity and cell morphology, but also the complex cellular movements that give rise to germ layers. This study describes—to our knowledge—the first known in vivo function of PrP and its molecular cellular basis, which may provide helpful insights into the role of PrP in the adult brain and its proposed connections to prion-induced neurotoxicity.

Knockdown experiments in zebrafish embryos reveal that prion proteins, otherwise known for their involvement in lethal neurodegenerative disease, play important roles in cell-cell adhesion and signaling during embryonic development.

A deleted prion protein that is neurotoxic in vivo is localized normally in cultured cells

The prion protein (PrP) possesses sequence-specific domains that endow the molecule with neuroprotective and neurotoxic activities, and that may contribute to the pathogenesis of prion diseases. To further define critical neurotoxic determinants within PrP, we previously generated Tg(DeltaCR) mice that express a form of PrP harboring a deletion of 21 amino acids within the central domain of the protein [Li et al., EMBO J. 26 (2007), 548]. These animals exhibit a neonatal lethal phenotype that is dose-dependently rescued by co-expression of wild-type PrP. In this study, we examined the localization and cell biological properties of the PrP(DeltaCR) protein in cultured cells to further understand the mechanism of PrP(DeltaCR) neurotoxicity. We found that the distribution of PrP(DeltaCR) was identical to that of wild-type PrP in multiple cell lines of both neuronal and non-neuronal origin, and that co-expression of the two proteins did not alter the localization of either one. Both proteins were found in lipid rafts, and both were localized to the apical surface in polarized epithelial cells. Taken together, our results suggest that PrP(DeltaCR) toxicity is not a result of mislocalization or aggregation of the protein, and more likely stems from altered binding interactions leading to the activation of deleterious signaling pathways.

The cellular prion protein PrP(c) is involved in the proliferation of epithelial cells and in the distribution of junction-associated proteins

Background

The physiological function of the ubiquitous cellular prion protein, PrP^c, is still under debate. It was essentially studied in nervous system, but poorly investigated in epithelial cells. We previously reported that PrP^c is targeted to cell–cell junctions of polarized epithelial cells, where it interacts with c-Src.

Methodology/Findings

We show here that, in cultured human enterocytes and in intestine in vivo, the mature PrP^c is differentially targeted either to the nucleus in dividing cells or to cell–cell contacts in polarized/differentiated cells. By proteomic analysis, we demonstrate that the junctional PrP^c interacts with cytoskeleton-associated proteins, such as gamma- and beta-actin, alpha-spectrin, annexin A2, and with the desmosome-associated proteins desmoglein, plakoglobin and desmoplakin. In addition, co-immunoprecipitation experiments revealed complexes associating PrP^c, desmoglein and c-Src in raft domains. Through siRNA strategy, we show that PrP^c is necessary to complete the process of epithelial cell proliferation and for the sub-cellular distribution of proteins involved in cell architecture and junctions. Moreover, analysis of the architecture of the intestinal epithelium of PrP^c knock-out mice revealed a net decrease in the size of desmosomal junctions and, without change in the amount of BrdU incorporation, a shortening of the length of intestinal villi.

Conclusions/Significance

From these results, PrP^c could be considered as a new partner involved in the balance between proliferation and polarization/differentiation in epithelial cells.

Coexpression of wild-type and mutant prion proteins alters their cellular localization and partitioning into detergent-resistant membranes

Transmissible spongiform encephalopathies (TSEs) are a group of diseases of infectious, sporadic and genetic origin, found in higher organisms and caused by the pathological form of the prion protein. The inheritable subgroup of TSEs is linked to insertional or point mutations in the prion gene prnp, which favour its misfolding and are passed on to offspring in an autosomal-dominant fashion. The large majority of patients with these diseases are heterozygous for the prnp gene, leading to the coexpression of the wild-type (wt) (PrP(C)) and the mutant forms (PrPmut) in the carriers of these mutations. To mimic this situation in vitro, we produced Fischer rat thyroid cells coexpressing PrPwt alongside mutant versions of mouse PrP including A117V, E200K and T182A relevant to the human TSE diseases Gestmann-Sträussler-Scheinker (GSS) disease and familial Creutzfeldt-Jakob disease (fCJD). We found that coexpression of mutant PrP with wt proteins does not affect the glycosylation pattern or the biochemical characteristics of either protein. However, FRET and co-immunoprecipitation experiments suggest an interaction occurring between the wt and mutant proteins. Furthermore, by comparing the intracellular localization and detergent-resistant membrane (DRM) association in single- and double-expressing clones, we found changes in the intracellular/surface ratio and an increased sequestration of both proteins in DRMs, a site believed to be involved in the pathological conversion (or protection thereof) of the prion protein. We, therefore, propose that the mutant forms alter the subcellular localization and the membrane environment of the wt protein in co-transfected cells. These effects may play a role in the development of these diseases.

Subcellular localization of prion proteins and the 37 kDa/67 kDa laminin receptor fused to fluorescent proteins

The 37 kDa/67 kDa laminin receptor LRP/LR acts as a receptor for both PrPc and PrPSc at the cell surface. Here, we further analyzed the subcellular localization of fluorescent labeled prion protein (PrP) and laminin receptor (LRP/LR) molecules. We show that EGFP-PrP is localized at the cell surface and in a perinuclear compartment, respectively. In contrast, a DsRed-DeltaSP-PrP mutant lacking the signal peptide is almost exclusively found in the nucleus but does not colocalize with heterochromatin. Interestingly, LRP-DsRed efficiently colocalizes with EGFP-PrP in the perinuclear compartment and LRP-ECFP partly colocalizes with DsRed-DeltaSP-PrP in the nucleus, respectively. We conclude that the interactions of PrP and LRP/LR are not restricted to the cell surface but occur also in intracellular compartments suggesting a putative role of LRP/LR in the trafficking of PrP molecules.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Fonctions de la protéine prion PrPc

It is now well established that both normal and pathological (or scrapie) isoforms of prion protein, PrPc and PrPsc respectively, are involved in the development and progression of various forms of neurodegenerative diseases, including scrapie in sheep, bovine spongiform encephalopathy (or "mad cow disease") and Creutzfeldt-Jakob disease in human, collectively known as prion diseases. The protein PrPc is highly expressed in the central nervous system in neurons and glial cells, and also present in non-brain cells, such as immune cells or epithelial and endothelial cells. Identification of the physiological functions of PrPc in these different cell types thus appears crucial for understanding the progression of prion diseases. Recent studies highlighted several major roles for PrPc that may be considered in two major domains : (1) cell survival (protection against oxidative stress and apoptosis) and (2) cell adhesion. In association with cell adhesion, distinct functions of PrPc were observed, depending on cell types : neuronal differentiation, epithelial and endothelial barrier integrity, transendothelial migration of monocytes, T cell activation. These observations suggest that PrPc functions may be particularly relevant to cellular stress, as well as inflammatory or infectious situations.

Human cellular prion protein hPrPC is sorted to the apical membrane of epithelial cells

Propagation of the scrapie isoform of the prion protein (PrP(Sc)) depends on the expression of endogenous cellular prion (PrP(C)). During oral infection, PrP(Sc) propagates, by conversion of the PrP(C) to PrP(Sc), from the gastrointestinal tract to the nervous system. Intestinal epithelium could serve as the primary site for PrP(C) conversion. To investigate PrP(C) sorting in epithelia cells, we have generated both a green fluorescent protein (EGFP) or hemagglutinin (HA) tagged human PrP(C) (hPrP(C)). Combined molecular, biochemical, and single living polarized cell imaging characterizations suggest that hPrP(C) is selectively targeted to the apical side of Madin-Darby canine kidney (MDCKII) and of intestinal epithelia (Caco2) cells.

Novel aspects of prions, their receptor molecules, and innovative approaches for TSE therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.

Junctional expression of the prion protein PrPC by brain endothelial cells: a role in trans-endothelial migration of human monocytes

The conversion of prion protein (PrPC) to its protease-resistant isoform is involved in the pathogenesis of prion diseases. Although PrPC is highly expressed in neurons and other cell types, its physiological function still remains elusive. Here, we describe how we evaluated its expression, subcellular localization and putative function in brain endothelial cells, which constitute the blood-brain barrier. We detected its expression in microvascular endothelium in mouse brain sections and at intercellular junctions of freshly isolated brain microvessels and cultured brain endothelial cells of mouse, rat and human origin. PrPC co-localized with the adhesion molecule platelet endothelial cell adhesion molecule-1 (PECAM-1); moreover, both PrPC and PECAM-1 were present in raft membrane microdomains. Using mixed cultures of wild-type and PrPC-deficient mouse brain endothelial cells, we observed that PrPC accumulation at cell-cell contacts was probably dependent on homophilic interactions between adjacent cells. Moreover, we report that anti-PrPC antibodies unexpectedly inhibited transmigration of U937 human monocytic cells as well as freshly isolated monocytes through human brain endothelial cells. Significant inhibition was observed with various anti-PrPC antibodies or blocking anti-PECAM-1 antibodies as control. Our results strongly support the conclusion that PrPC is expressed by brain endothelium as a junctional protein that is involved in the trans-endothelial migration of monocytes.

Cellular Prion Protein in the Bovine Mammary Gland Is Selectively Expressed in Active Lactocytes

The cellular prion protein (PrPc) is a highly conserved glycoprotein with a still enigmatic physiological function. It is mainly expressed in the central nervous system but accumulating data suggest that PrPc is also found in a broad spectrum of non-neuronal tissue. Here we investigated the cell-type-related PrPc expression in the bovine mammary gland by using immunohistochemistry (IHC), ELISA, Western blot, and real-time RT-PCR. Specific immunostaining of serial sections revealed that PrPc is selectively localized in mammary gland epithelial cells. Particularly strong expression was found at the basolateral surface of those cells showing active secretion. Results obtained by RT-PCR and ELISA complemented IHC findings. No correlation was found between the level of PrPc expression and other parameters such as age of the animals under study or stage of lactation.

Intranasal immunization of Balb/c mice against prion protein attenuates orally acquired transmissible spongiform encephalopathy

To test whether prion protein (PrP) specific secretory immunoglobulin A (sIgA) can be induced and protect against oral transmission of spongiform encephalopathy (SE) we immunized Balb/c mice either intragastrically or intranasally (i.n.) with a recombinant PrP-fragment (PrP90-231) and cholera toxin (CT) adjuvant. Since PrP90-231 was rapidly digested in intestinal lavage, aprotinin was added to some vaccine formulations. While an anti-CT response was elicited via both routes, solely i.n. immunization without aprotinin induced PrP-specific sIgA. They recognize predominantly PrP-oligomers as the antigen was aggregated in the vaccine formulations. Challenge experiments showed that the immune response induced by our protocol could not prevent disease, but increases the median survival of the animals. We conclude that PrP-specific sIgA reduce the infectivity of the inoculum and that complete protection against transmission of SE should be achievable by optimized immunization regimens.

Bovine prion is endocytosed by human enterocytes via the 37 kDa/67 kDa laminin receptor

Some forms of transmissible spongiform encephalopathies result from oral infection. We have thus analyzed the early mechanisms that could account for an uptake of infectious prion particles by enterocytes, the major cell population of the intestinal epithelium. Human Caco-2/TC7 enterocytes cultured on microporous filters were incubated with different prion strains and contaminated brain homogenates in the apical compartment. Internalization of infectious particles was analyzed by Western blotting and immunofluorescence. We observed internalization by enterocytes of prion particles from bovine spongiform encephalopathy brain homogenates but not from mouse-adapted scrapie-strain brain homogenates or purified bovine spongiform encephalopathy scrapie-associated fibrils. Bovine prion particles were internalized via endocytosis within minutes of infection and were associated with subapical vesicular structures related to early endosomes. The endocytosis of the infectious bovine PrP(Sc) was reduced by preincubating the cells with an anti-LRP/LR blocking antibody, identifying the 37 kDa/67 kDa laminin receptor (LRP/LR), which is apically expressed in Caco-2/TC7 cells, as the receptor for the infectious prion protein. Altogether, our results underscore a potential role of enterocytes in the absorption of bovine prions during oral infection through specific LRP/LR-dependent endocytosis.

The increase in mucin exocytosis and the upregulation of MUC genes encoding for membrane-bound mucins induced by the thiol-activated exotoxin listeriolysin O is a host cell defence response that inhibits the cell-entry of Listeria monocytogenes

In vivo Listeria monocytogenes infection results in the massive release of mucus by goblet cells into the lumen of the intestine. We have previously reported that apical infection by L. monocytogenes is followed by listeriolysin O (LLO)-dependent stimulation of mucus exocytosis, and the upregulation of the MUC genes. Here, we report that L. monocytogenes EGD wild-type bacteria enter cultured human polarized, mucin-secreting, HT29-MTX cells apically by an InlA-dependent mechanism. The LLO-induced increase in mucin secretion together with an increase in transcription of the MCU4 and MUC12 genes encoding for membrane-bound mucins, results in the inhibition of the cell-entry of L. monocytogenes into mucin-secreting, HT29-MTX cells. Moreover, we report that sialic acid residues in mucins are crucial for the inhibition of L. monocytogenes internalization. Based on these findings, we suggest that the LLO-induced mucin exocytosis and upregulation of the MUC genes encoding for membrane-bound mucins constitute a host cell defence response that inhibits the cell-entry of L. monocytogenes.

Protease-resistant human prion protein and ferritin are cotransported across Caco-2 epithelial cells: implications for species barrier in prion uptake from the intestine

Foodborne transmission of bovine spongiform encephalopathy (BSE) to humans as variant Creutzfeldt-Jakob disease (CJD) has affected over 100 individuals, and probably millions of others have been exposed to BSE-contaminated food substances. Despite these obvious public health concerns, surprisingly little is known about the mechanism by which PrP-scrapie (PrP(Sc)), the most reliable surrogate marker of infection in BSE-contaminated food, crosses the human intestinal epithelial cell barrier. Here we show that digestive enzyme (DE) treatment of sporadic CJD brain homogenate generates a C-terminal fragment similar to the proteinase K-resistant PrP(Sc) core of 27-30 kDa implicated in prion disease transmission and pathogenesis. Notably, DE treatment results in a PrP(Sc)-protein complex that is avidly transcytosed in vesicular structures across an in vitro model of the human intestinal epithelial cell barrier, regardless of the amount of endogenous PrP(C) expression. Unexpectedly, PrP(Sc) is cotransported with ferritin, a prominent component of the DE-treated PrP(Sc)-protein complex. The transport of PrP(Sc)-ferritin is sensitive to low temperature, brefeldin-A, and nocodazole treatment and is inhibited by excess free ferritin, implicating a receptor- or transporter-mediated pathway. Because ferritin shares considerable homology across species, these data suggest that PrP(Sc)-associated proteins, in particular ferritin, may facilitate PrP(Sc) uptake in the intestine from distant species, leading to a carrier state in humans.

Expression of Prion Protein in the Gut of Mice Infected Orally with the 301V Murine Strain of the Bovine Spongiform Encephalopathy Agent

Transmissible spongiform encephalopathies (TSEs) are characterized by the accumulation of an abnormal, disease-associated prion protein (PrP(d)). Expression of its normal cellular counterpart (PrP(c)) by the host is a pre-requisite for the spread of infection to the central nervous system and the development of disease. Moreover, cells expressing PrP(c) at specific sites such as the gastrointestinal tract might be regarded as the initial point of PrP(c)-PrP(d) conversion after infection by the oral route. In this study, inbred mice of the I/M strain were infected orally with the 301V murine strain of the bovine spongiform encephalopathy agent. The expression of PrP(c) and the accumulation of PrP(d) in the intestine was then investigated immunohistochemically, together with the variations in immunoreactivity that resulted from different pretreatments of the tissue. After proteinase K (PK) pretreatment, abnormal PrP was still detectable only in the gut-associated lymphoid tissue (GALT) of clinically affected mice and, to a much more limited degree, in the enteric nervous system (ENS). Cellular PrP that disappeared after PK treatment was particularly conspicuous in the ENS and present to a lesser extent in the GALT of all mice examined after inoculation with 301V or with normal brain homogenates, as well as in uninoculated controls. These findings suggested that not all PrP found in infected mice was PrP(d) and that part of the PrP(d) was sensitive to PK treatment. Reactivity to PrP antibody 1A8 was consistently found in the absorptive epithelium of the intestinal villi, with or without PK pretreatment. However, epithelial immunolabelling was comparable in inoculated and uninoculated mice and was also consistently seen in PrP "knockout" mice used as controls. It is therefore concluded that immunohistochemically detectable accumulation of PrP(d) in the gut of mice is a relatively late event in the pathogenesis of experimental infection in this model and that the immunoreactivity observed in the intestinal epithelium does not correspond to PrP expression. While enterocytes may still play a role in the uptake of infection from the intestinal lumen, the results do not suggest that these cells are a site of initial accumulation of PrP(d).

Molecular characterization, phylogenetic relationships, and developmental expression patterns of prion genes in zebrafish (Danio rerio)

Prion diseases are characterized by the accumulation of a pathogenic misfolded form of a prion protein (PrP) encoded by the Prnp gene in humans. In the present study in zebrafish, two transcripts and the corresponding genes encoding prion proteins, PrP1 and PrP2, related to human PrP have been characterized with a relatively divergent deduced amino acid sequence, but a well preserved overall organization of structural prion protein motifs. Whole-mount in situ hybridization analysis performed during embryonic and larval development showed a high level of PrP1 mRNA spatially restricted to the anterior floor-plate of the central nervous system and in ganglia. Transcripts of prp2 were detected in embryonic cells from the mid-blastula transition to the end of the segmentation period. From 24 h postfertilization up to larval stages, prp2 transcripts were localized in distinct anatomical structures, including a major expression in the brain, eye, kidney, lateral line neuromasts, liver, heart, pectoral fins and posterior intestine. The observed differential developmental expression patterns of the two long PrP forms, prp1 and prp2, and the short PrP form prp3, a more divergent prion-related gene previously identified in zebrafish, should contribute to understanding of the phylogenetic and functional relationships of duplicated prion gene forms in the fish genome. Together, the complex history of prion-related genes, reflected in the deduced structural features, conserved amino acid sequence and repeat motifs of the corresponding proteins, and the presence of differential developmental expression patterns suggest possible acquisition or loss of prion protein functions during vertebrate evolution.

Prion-associated Increases in Src-family Kinases

The prion diseases result from the generation and propagation of an abnormal conformer of the prion protein. It is unclear how this molecular event disrupts neuronal function and viability. Current evidence argues it is not due to loss of normal prion protein activity or direct toxic effects of the abnormal conformer. Both the normal and abnormal prion proteins are glycosylphosphatidylinositol-linked membrane proteins. Conversion to the abnormal isoform results in the formation and accumulation of prion protein aggregates. Because aggregation of glycosylphosphatidylinositol-linked proteins activates Src-family kinases, the activation status and levels of the Src-family kinases in prion disease were investigated. Elevations of Src-family kinases were found in a cell culture model and two separate animal models of prion disease. The elevations in Src kinases preceded the onset of symptoms and occurred concurrently with the appearance of detergent-insoluble prion protein. In addition, the total level of kinases phosphorylated at tyrosine residues associated with activation was increased. Similar alterations were not present in brain homogenates from presymptomatic animals early in the disease course, prion protein-ablated animals, or end-stage Tg2576 mice overexpressing mutant amyloid precursor protein. Identification of similar elevations in cell culture and animal model systems suggests the elevations are a specific response to the presence of the disease-associated conformer. Abnormal regulation of these signal transduction cascades may be a key element in the cellular pathology of the prion diseases.

Cellular prion protein is expressed in a subset of neuroendocrine cells of the rat gastrointestinal tract

Prion diseases are believed to develop from the conformational change of normal cellular prion protein (PrPc) to a pathogenic isoform (PrPsc). PrPc is present in both the central nervous system and many peripheral tissues, although protein concentration is significantly lower in non-neuronal tissues. PrPc expression is essential for internalization and replication of the infectious agent. Several works have pointed to the gastrointestinal (GI) tract as the principal site of entry of PrPsc, but how passage through the GI mucosa occurs is not yet known. Here we studied PrPc expression using Western blot, RT-PCR, and immunohistochemistry in rat GI tract. PrPc mRNA and protein were detected in corpus, antrum, duodenum, and colon. Immunoreactivity was found in scattered cells of the GI epithelium. With double immunofluorescence, these cells have been identified as neuroendocrine cells. PrPc immunostaining was found in subsets of histamine, somatostatin (Som), ghrelin, gastrin (G), and serotonin (5HT) cells in stomach. In small and large bowel, PrPc cells co-localized with subpopulations of 5HT-, Som-, G-, and peptide YY-immunolabeled cells. Our results provide evidence for a possible and important role of endocrine cells in the internalization of PrPsc from gut lumen.

Compartmentalization of Prion Isoforms Within the Reproductive Tract of the Ram1

Cellular prion protein (Prp(C)) is a glycoprotein usually associated with membranes via its glycosylphosphatidylinositol (GPI) anchor. The trans-conformational form of this protein (Prp(SC)) is the suggested agent responsible for transmissible neurodegenerative spongiform encephalopathies. This protein has been shown on sperm and in the reproductive fluids of males. Antibodies directed against the C-terminal sequence near the GPI-anchor site, an N-terminal sequence, and against the whole protein showed that the Prp isoforms were compartmentalized within the reproductive tract of the ram. Immunoblotting with the three antibodies showed that the complete protein and both N- and C-terminally truncated and glycosylated isoforms are present within cauda epididymal fluid and seminal plasma. Moreover, we demonstrate that in these fluids, the Prp(C) isoforms are both in a soluble state as well as associated with small membranous vesicles (epididymosomes). We also report that only one major glycosylated 25 kDa C-terminally truncated Prp(C) isoform is associated with sperm from the testis, cauda epididymis, and semen, and this form is also present in the sperm cytoplasmic droplets that are released during maturation. In sperm, this C-terminal truncated form was found to be associated with membrane lipid rafts present in the mature sperm, suggesting a role for it in the terminal stages of sperm maturation.

Prion infection of epithelial Rov cells is a polarized event

During prion infections, the cellular glycosylphosphatidylinositol-anchored glycoprotein PrP is converted into a conformational isoform. This abnormal conformer is thought to recruit and convert the normal cellular PrP into a likeness of itself and is proposed to be the infectious agent. We investigated the distribution of the PrP protein on the surface of Rov cells, an epithelial cell line highly permissive to prion multiplication, and we found that PrP is primarily expressed on the apical side. We further show that prion transmission to Rov cells is much more efficient if infectivity contacts the apical side, indicating that the apical and basolateral sides of Rov cells are not equally competent for prion infection and adding prions to the list of the conventional infectious agents (viruses and bacteria) that infect epithelial cells in a polarized manner. These data raise the possibility that apically expressed PrP may be involved in this polarized process of infection. This would add further support for a crucial role of PrP at the cell surface in prion infection of target cells.

Expression of Prnp mRNA (Prion Protein Gene) in Mouse Spermatogenic Cells

The Prnp (prion protein) gene, which encodes a soluble protein anchored to the cell surface by glycosylphosphatidylinositol (GPI), might be involved in cell-to-cell interaction. The expression of Prnp is strongly observed not only in the brain, but also in non-neuronal tissues. In order to examine the Prnp expression sites in mouse testes, we carried out Northern blot and in situ hybridization analyses. By Northern blot analysis, two kinds of Prnp transcripts (major band of 2.2 kb, and minor band of 1.1 kb) were detected in testes. The 2.2-kb transcript was observed in testes throughout the postnatal development, whereas the 1.1-kb transcript was observed in testes from 2 to 70 weeks old. In situ hybridization analysis showed that the positive signals for Prnp mRNAs were predominantly observed in spermatogenic cells, but not in somatic cells such as Sertoli cells, Leydig cells and peritubular myoid cells. The signals were observed moderately in spermatogonia, and strongly in spermatocytes and round spermatids, but not in elongate spermatids and spermatozoa. These results suggest that Prnp may be involved in germ cell differentiation during mammalian spermatogenesis.