Nonneuronal cellular prion protein

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

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

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

The highly conserved, N-terminal (RXXX)8 motif of mouse Shadoo mediates nuclear accumulation

The prion protein (PrP)-known for its central role in transmissible spongiform encephalopathies-has been reported to possess two nuclear localization signals and localize in the nuclei of certain cells in various forms. Although these data are superficially contradictory, it is apparent that nuclear forms of the prion protein can be found in cells in either the healthy or the diseased state. Here we report that Shadoo (Sho)-a member of the prion protein superfamily-is also found in the nucleus of several neural and non-neural cell lines as visualized by using an YFP-Sho construct. This nuclear localization is mediated by the (25-61) fragment of mouse Sho encompassing an (RXXX)8 motif. Bioinformatic analysis shows that the (RXXX)n motif (n=7-8) is a highly conserved and characteristic part of mammalian Shadoo proteins. Experiments to assess if Sho enters the nucleus by facilitated transport gave no decisive results: the inhibition of active processes that require energy in the cell, abolishes nuclear but not nucleolar accumulation. However, the (RXXX)8 motif is not able to mediate the nuclear transport of large fusion constructs exceeding the size limit of the nuclear pore for passive entry. Tracing the journey of various forms of Sho from translation to the nucleus and discerning the potential nuclear function of PrP and Sho requires further studies.

Enhanced susceptibility of T lymphocytes to oxidative stress in the absence of the cellular prion protein

The cellular prion glycoprotein (PrP(C)) is ubiquitously expressed but its physiologic functions remain enigmatic, particularly in the immune system. Here, we demonstrate in vitro and in vivo that PrP(C) is involved in T lymphocytes response to oxidative stress. By monitoring the intracellular level of reduced glutathione, we show that PrP(-/-) thymocytes display a higher susceptibility to H(2)O(2) exposure than PrP(+/+) cells. Furthermore, we find that in mice fed with a restricted diet, a regimen known to increase the intracellular level of ROS, PrP(-/-) thymocytes are more sensitive to oxidative stress. PrP(C) function appears to be specific for oxidative stress, since no significant differences are observed between PrP(-/-) and PrP(+/+) mice exposed to other kinds of stress. We also show a marked evolution of the redox status of T cells throughout differentiation in the thymus. Taken together, our results clearly ascribe to PrP(C) a protective function in thymocytes against oxidative stress.

Prion infection of differentiated neurospheres

Until now only a few cell lines have been proved able to propagate prions and only limited prion strains have been replicated in cell models. Neurosphere lines isolated from the brains of mice at embryonic day 14 grow as aggregates and contain CNS stem cells. Others authors have previously reported that cultured neurospheres expressing cellular prion protein (PrP(C)) can be infected with prions. As potential neural progenitors the neurosphere cultures are supposed to differentiate into neurons and astrocytes which represent the main cell types infected by prions in vivo. Here we study the ability of undifferentiated and differentiated neurospheres to replicate several prion strains. Neurosphere cultures were isolated from 129/ola, FVB, Prnp(0/0) and Tga20 mice, which over-express murine PrP. We were not able to detect PrP(res) accumulation in dividing neurosphere cultures after prion exposure to two different mouse adapted scrapie inocula (RML and 22L). In contrast, with differentiated neurosphere cultures expressing PrP(C) (129/ola, FVB and Tga20) a successful PrP(Res) amplification was observed in very short time experiments when infected with the same inocula, implying that cell differentiation improve prion replication in these cultured cells. The mouse BSE adapted inocula (301C) was not amplified in these neurosphere cultures neither before nor after differentiation, suggesting that these cell cultures showed a differential prion strain susceptibility. These results suggest that differentiated neurosphere cultures can complement prion bioassays in mouse models.

Oral Delivery of Pathogens from the Intestine to the Nervous System

Most therapeutic agents are delivered orally. Consequently, the major classes of therapeutically useful chemicals are partially lipophilic, small molecular weight compounds. They have reasonable permeability coefficient values across cell membranes, including those of intestinal epithelia and vascular endothelia. In contrast, large molecular weight biotechnology compounds have limited usefulness by non-injected routes as a consequence of their low membrane permeability and variable solubility. However, a wide range of infectious agents have developed strategies or have hijacked physiological routings in order to enter the host by the oral route. Efforts to address such issues have refreshed interest in mechanisms by which different types of payloads (including particulates and microorganisms) translocate across gut epithelia and then distribute to target tissues. Special attention is given to the potential role of the enteric nervous system and its plasticity.

Enhanced susceptibility of Prnp-deficient mice to kainate-induced seizures, neuronal apoptosis, and death: Role of AMPA/kainate receptors

Normal physiologic functions of the cellular prion protein (PrPc) are still elusive. This GPI-anchored protein exerts many functions, including roles in neuron proliferation, neuroprotection or redox homeostasis. There are, however, conflicting data concerning its role in synaptic transmission. Although several studies report that PrPc participates in NMDA-mediated neurotransmission, parallel studies describe normal behavior of PrPc-mutant mice. Abnormal axon connections have been described in the dentate gyrus of the hippocampi of PrPc-deficient mice similar to those observed in epilepsy. A study indicates increased susceptibility to kainate (KA) in these mutant mice. We extend the observation of these studies by means of several histologic and biochemical analyses of KA-treated mice. PrPc-deficient mice showed increased sensitivity to KA-induced seizures in vivo and in vitro in organotypic slices. In addition, we show that this sensitivity is cell-specific because interference experiments to abolish PrPc expression increased susceptibility to KA in PrPc-expressing cells. We indicate a correlation of susceptibility to KA in cells lacking PrPc with the differential expression of GluR6 and GluR7 KA receptor subunits using real-time RT-PCR methods. These results indicate that PrPc exerts a neuroprotective role against KA-induced neurotoxicity, probably by regulating the expression of KA receptor subunits.

Interaction between the cellular prion (PrPC) and the 2P domain K+ channel TREK-1 protein

The cellular prion protein (PrP(C)) is a highly conserved protein throughout the evolution of mammals and therefore is thought to play important cellular functions. Despite decades of intensive researches, the physiological function of PrP(C) remains enigmatic. Differently, in particular pathological contexts, generally referred as transmissible spongiform encephalopathies, a conformational isoform of PrP(C), i.e., PrP(Sc), is considered the causative agent of these diseases. In this study, we investigated putative PrP(C) cellular functions through the identification of PrP(C) protein interactants. Using a bacterial two-hybrid approach, we identified a novel interaction between PrP(C) and a two-pore potassium channel protein, TREK-1. This interaction was further verified in transfected eukaryotic cells using co-immunoprecipitation and confocal microscopic analysis of the fluorescent transfected proteins. Importantly, in the cerebellar cortex, the endogenous PrP(C) and TREK-1 proteins exhibited co-localization signals in correspondence of the Purkinje cells. Furthermore, a deletion mapping study defined the carboxyl-terminal regions of the two proteins as the possible determinants of the PrP(C)-TREK-1 interaction. Our results indicated a novel PrP(C) interacting protein and suggested that this complex might be relevant in modulating a variety of electrophysiological-dependent cellular responses.

Overexpression and significance of prion protein in gastric cancer and multidrug-resistant gastric carcinoma cell line SGC7901/ADR

In our previous work, cellular prion protein (PrPc) was identified as an upregulated gene in adriamycin-resistant gastric carcinoma cell line SGC7901/ADR compared to its parental cell line SGC7901. Here we investigate the expression of PrPc in gastric cancer and whether it was involved in multidrug resistance (MDR) of gastric cancer. We demonstrated that PrPc was ubiquitously expressed in gastric cancer cell lines and tissues. PrPc conferred resistance of both P-glycoprotein (P-gp)-related and P-gp-nonrelated drugs on SGC7901, which was accompanied by decreased accumulation and increased releasing amount of adriamycin in PrPc-overexpressing cell line. Inhibition of PrPc expression by antisense or RNAi technology could partially reverse multidrug-resistant phenotype of SGC7901/ADR. PrPc significantly upregulated the expression of the classical MDR-related molecule P-gp but not multidrug resistance associated protein and glutathione S-transferase pi. The PrPc-induced MDR could be partially reversed by P-gp inhibitor verapamil. PrPc could also suppress adriamycin-induced apoptosis and alter the expression of Bcl-2 and Bax, which might be another pathway contributing to PrPc-related MDR. The further study of the biological functions of PrPc may be helpful for understanding the mechanisms of occurrence and development of clinical gastric carcinoma and PrPc-related MDR and developing possible strategies to treat gastric cancer.

Extracellular copper ions regulate cellular prion protein (PrPC) expression and metabolism in neuronal cells

The physiological functions of cellular prion protein (PrP(C)) remain unclear. It has been demonstrated that PrP(C) is a copper binding protein and proposed that its functions could be strictly linked to copper metabolism and neuroprotection. The aim of this study was to clarify how extracellular copper modifies PrP(C) expression and metabolism in cultured neurones. We reported here that copper delivered at physiological concentrations significantly decreases PrP(C) mRNA expression in GN11 neurones. Moreover, copper increases the release of PrP(C) into the culture medium. These results indicate that extracellular copper strongly affects the amount of cellular PrP and might represent an interesting strategy to decrease the expression of PrP(C) in neurones and its conversion in the pathological isoform PrP(Sc).

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.

Immunologically induced, complement-dependent up-regulation of the prion protein in the mouse spleen: follicular dendritic cells versus capsule and trabeculae

The expression of the prion protein (PrP) in the follicular dendritic cell network of germinal centers in the spleen is critical for the splenic propagation of the causative agent of prion diseases. However, a physiological role of the prion protein in the periphery remains elusive. To investigate the role and function of PrP expression in the lymphoid system we treated naive mice i.v. with preformed immune complexes or vesicular stomatitis virus. Immunohistochemistry and Western blot analysis of the spleen revealed that 8 days after immunization, immune complexes and vesicular stomatitis virus had both induced a strong increase of PrP expression in the follicular dendritic cell network. Remarkably, this up-regulation did not occur in mice that lack an early factor of the complement cascade, C1q, a component which has been shown previously to facilitate early prion pathogenesis. In addition to the variable PrP level in the germinal centers, we detected steady and abundant PrP expression in the splenic capsule and trabeculae, which are structural elements that have not been associated before with PrP localization. The abundant trabeculo-capsular PrP expression was also evident in spleens of Rag-1-deficient mice, which have been shown before to be incapable of prion expansion. We conclude that trabeculocapsular PrP is not sufficient for splenic prion propagation. Furthermore, our observations may provide important clues for a physiological function of the prion protein and allow a new view on the role of complement and PrP in peripheral prion pathogenesis.

Stimulation of cellular prion protein expression by TSH in human thyrocytes

The cellular isoform of prion protein (PrP(C)) is a cell-surface glycosyl-phosphatidylinositol-anchored protein which is ubiquitously expressed on the cell membrane. It may function as a cell receptor or as a cell adhesion molecule. Thyroid follicles, obtained from patients with Graves' disease at thyroidectomy, were cultured in F-12/RPMI-1640 medium supplemented with 0.5% fetal bovine serum and bovine thyroid stimulating hormone (bTSH). Northern blot analyses revealed that bTSH increased the steady-state expression levels of PrP mRNA in a time- and dose-dependent manner. This increase was reproduced by dibutyryl-cAMP and 12-decanoylphorbol-13-acetate. The mRNA expression was greater in thyroid follicles in suspension culture than in thyrocytes cultured in a monolayer. These findings suggest that TSH stimulates PrP mRNA expression in thyrocytes through the protein kinase A and C pathways. The greater mRNA expression in thyroid follicles than in monolayer cells suggests that PrP(C) may be involved in structure formation or maintenance of thyroid follicles.

Prion diseases: contribution of high-resolution immunomorphology

The transmisible spongiform encephalopathies or prion diseases are fatal neurological diseases that occur in animals and humans. They are characterized by the accumulation in the cerebral tissue of the abnormal form of prion protein (PrPsc) produced by a post-translational event involving conformational change of its normal cellular counterpart (PrPc). In this short review, we present some results on the biology of prion proteins which have benefited from morphological approaches combining the electron microscopy techniques and the immunodetection methods. We discuss data concerning in particular the physiological function of the normal cellular prion prion (PrPc) which have allowed to open up new vistas on prion diseases, the biogenesis of amyloid plaque and the cellular site involved in the prion protein conversion process.