Elevated PrPC Expression Predisposes to Increased HSV-1 Pathogenicity

PrPC is a ubiquitously expressed glycophos-phatidylinositol-linked cell-surface glycoprotein found primarily in neural tissue. Although its normal function has not been established, there is evidence suggesting that PrPC is involved in cell signalling and cellular homeostasis. This suggests that variation in neuronal expression levels of this protein contributes towards pathogenicity induced by neurotropic agents. We have investigated the pathological response to infection with herpes simplex virus type-1 (HSV-1) in strains of mice that express different levels of PrPC. Prnp−/− mice fail to express PrPC due to an interruption in the open reading frame of the Prnp gene, whilst tg19 and tga20 mice express approximately 5 and 10 times more PrPC protein, respectively, than wild-type animals. Mice that express normal or increased levels of PrPC protein were more susceptible to acute HSV-1 infection than Prnp−/− mice. Following ear pinna inoculation with HSV-1 SC16, the order of susceptibility was tga20>tg19>wild-type> Prnp−/−. This trend was reversed when latent virus was assessed. Prnp−/− mice expressed significantly higher levels of latency-associated transcript-positive neurons in various tissues when compared with wild-type, tg19 and tga20 mice. Collectively, our data show that acute HSV-1 replication proceeds more efficiently in neuronal tissue that expresses PrPC protein and lends support to the view that this protein is involved in regulation of neurotropic viral pathogenesis. This suggests that interference of PrPC expression, or possible biochemical pathways associated with its function, may serve as an effective means of limiting the pathogenesis of acute HSV-1 infection.

Differential responses of neuronal and spermatogenic cells to the doppel cytotoxicity

Although structurally and biochemically similar to the cellular prion (PrP(C)), doppel (Dpl) is unique in its biological functions. There are no reports about any neurodegenerative diseases induced by Dpl. However the artificial expression of Dpl in the PrP-deficient mouse brain causes ataxia with Purkinje cell death. Abundant Dpl proteins have been found in testis and depletion of the Dpl gene (Prnd) causes male infertility. Therefore, we hypothesize different regulations of Prnd in the nerve and male productive systems. In this study, by electrophoretic mobility shift assays we have determined that two different sets of transcription factors are involved in regulation of the Prnd promoter in mouse neuronal N2a and GC-1 spermatogenic (spg) cells, i.e., upstream stimulatory factors (USF) in both cells, Brn-3 and Sp1 in GC-1 spg cells, and Sp3 in N2a cells, leading to the expression of Dpl in GC-1 spg but not in N2a cells. We have further defined that, in N2a cells, Dpl induces oxidative stress and apoptosis, which stimulate ataxia-telangiectasia mutated (ATM)-modulating bindings of transcription factors, p53 and p21, to Prnp promoter, resulting the PrP(C) elevation for counteraction of the Dpl cytotoxicity; in contrast, in GC-1 spg cells, phosphorylation of p21 and N-terminal truncated PrP may play roles in the control of Dpl-induced apoptosis, which may benefit the physiological function of Dpl in the male reproduction system.

Gene expression resulting from PrPC ablation and PrPC overexpression in murine and cellular models

The cellular prion protein (PrP(C)) plays a key role in prion diseases when it converts to the pathogenic form scrapie prion protein. Increasing knowledge of its participation in prion infection contrasts with the elusive and controversial data regarding its physiological role probably related to its pleiotropy, cell-specific functions, and cellular-specific milieu. Multiple approaches have been made to the increasing understanding of the molecular mechanisms and cellular functions modulated by PrP(C) at the transcriptomic and proteomic levels. Gene expression analyses have been made in several mouse and cellular models with regulated expression of PrP(C) resulting in PrP(C) ablation or PrP(C) overexpression. These analyses support previous functional data and have yielded clues about new potential functions. However, experiments on animal models have shown moderate and varied results which are difficult to interpret. Moreover, studies in cell cultures correlate little with in vivo counterparts. Yet, both animal and cell models have provided some insights on how to proceed in the future by using more refined methods and selected functional experiments.

Follicular dendritic cell-specific prion protein (PrP) expression alone is sufficient to sustain prion infection in the spleen

Prion diseases are characterised by the accumulation of PrP^Sc, an abnormally folded isoform of the cellular prion protein (PrP^C), in affected tissues. Following peripheral exposure high levels of prion-specific PrP^Sc accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the CNS. Expression of PrP^C is mandatory for cells to sustain prion infection and FDC appear to express high levels. However, whether FDC actively replicate prions or simply acquire them from other infected cells is uncertain. In the attempts to-date to establish the role of FDC in prion pathogenesis it was not possible to dissociate the Prnp expression of FDC from that of the nervous system and all other non-haematopoietic lineages. This is important as FDC may simply acquire prions after synthesis by other infected cells. To establish the role of FDC in prion pathogenesis transgenic mice were created in which PrP^C expression was specifically “switched on” or “off” only on FDC. We show that PrP^C-expression only on FDC is sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrP^C-expression is specifically ablated only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues. The demonstration that Prnp-ablation only on FDC blocked splenic prion accumulation without apparent consequences for FDC status represents a novel opportunity to prevent neuroinvasion by modulation of PrP^C expression on FDC.

Prion diseases are infectious neurological disorders and are considered to be caused by an abnormally folded infectious protein termed PrP^Sc. Soon after infection prions accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the brain where they cause damage to nerve cells. Cells must express the normal cellular prion protein PrP^C to become infected with prions. However, whether FDC are infected with prions or simply acquire them from other infected cells is unknown. To establish the role of FDC in prion disease PrP^C expression was specifically “switched on” or “off” only on FDC. We show that PrP^C-expressing FDC alone are sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrP^C-expression is switched off only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues.

Hypoxia-inducible factor-1 α regulates prion protein expression to protect against neuron cell damage

The human prion protein fragment, PrP (106-126), may contain a majority of the pathological features associated with the infectious scrapie isoform of PrP, known as PrP(Sc). Based on our previous findings that hypoxia protects neuronal cells from PrP (106-126)-induced apoptosis and increases cellular prion protein (PrP(C)) expression, we hypothesized that hypoxia-related genes, including hypoxia-inducible factor-1 alpha (HIF-1α), may regulate PrP(C) expression and that these genes may be involved in prion-related neurodegenerative diseases. Hypoxic conditions are known to elicit cellular responses designed to improve cell survival through adaptive processes. Under normoxic conditions, a deferoxamine-mediated elevation of HIF-1α produced the same effect as hypoxia-inhibited neuron cell death. However, under hypoxic conditions, doxorubicin-suppressed HIF-1α attenuated the inhibitory effect on neuron cell death mediated by PrP (106-126). Knock-down of HIF-1α using lentiviral short hairpin (sh) RNA-induced downregulation of PrP(C) mRNA and protein expression under hypoxic conditions, and sensitized neuron cells to prion peptide-mediated cell death even in hypoxic conditions. In PrP(C) knockout hippocampal neuron cells, hypoxia increased the HIF-1α protein but the cells did not display the inhibitory effect of prion peptide-induced neuron cell death. Adenoviruses expressing the full length Prnp gene (Ad-Prnp) were utilized for overexpression of the Prnp gene in PrP(C) knockout hippocampal neuron cells. Adenoviral transfection of PrP(C) knockout cells with Prnp resulted in the inhibition of prion peptide-mediated cell death in these cells. This is the first report demonstrating that expression of normal PrP(C) is regulated by HIF-1α, and PrP(C) overexpression induced by hypoxia plays a pivotal role in hypoxic inhibition of prion peptide-induced neuron cell death. These results suggest that hypoxia-related genes, including HIF-1α, may be involved in the pathogenesis of prion-related diseases and as such may be a therapeutic target for prion-related neurodegenerative diseases.

[Antisense oligonucleotides as potential drugs for prophylaxis of prion infections]

Prion mRNA translation inhibition by antisense oligodeoxynucleotides (asODN) incorporated into immunoliposomes was investigated. It was shown that asODN complementary to cap region, start-codon region and a part of open reading frame can decrease the prion expression by 80% in L1210 cell line and by 60% in prion-replicating organs of laboratory rats. These results give grounds for further research asODN to be used as a means of prevention and treatment of prion infections.

[Stable inhibition of human prion protein through a retrovirus-based RNAi system]

Prion leads to fatal transmissible spongiform encephalopathies. Cellular prion protein (PrPc) is necessary in prion disease. At present, it is demonstrated that PrPc plays a protective role in several carcinomas, such as gastric and breast cancer. We designed four 19-nt siRNAs according to cDNA sequence of human PrPc and constructed retrovirus-based RNAi vectors. We evaluated the inhibitive effect of these sequences on HuPrPc (human PrPc) and selected out three sequences with stable and efficient inhibition. And the efficiency of si626 reached more than 85%, which effect was significant. Next, we performed cell invasion assays of PC3M-si292 and PC3M-si626 in which PrPc was inhibited. And it showed that the cell invasive ability decreased in PrPc knock-down cell lines. This will make preparations for the further research on gene therapy of prion diseases and PrPc related carcinoma treatment and PrPc could be considered as a potential therapeutic target molecule in prostate cancer treatment.

Effect of copper on the de novo generation of prion protein expressed in Pichia pastoris

The prion protein (PrP) is the key protein implicated in diseases known as transmissible spongiform encephalopathies. PrP has been shown to be a metallo-protein that binds copper (Cu), and copper might have a role in the normal function of the protein. Conversely, PrP expression in yeast led us to suggest that the protein might be involved in the regulation of Cu homeostasis. In the presence of excess Cu in the growth medium, PrP expression limited the increase of the total number of Cu atoms per cell to a maximum of 14-fold compared with mock control cells, which showed a 52-fold increased intracellular Cu level. Conclusively, we suggest that PrP expression itself has a regulatory or buffering function for the cellular Cu level in yeast cells, most likely due to binding of Cu to the multiple Cu binding sites.

[Effect of pentosan polysulphate (SP-54) on the level of prion physiological form in rat tissues]

It is established that except for already known influence of pentosan polysulphate (SP-54) on the expression of pathological prion, this preparation has an inhibiting effect in respect of physiological prion. Moreover, the reduction of concentration of physiologycal prion is registered in the central and peripheral organs of the prion-replicating system. It was also shown that inhibition of the studied protein leads to the growth in copper and zinc concentration in the proper organs and tissues, but at the same time activity of Cu/Zn-dependent superoxide dismutase does not change.

Activation of Prn-p gene and stable transfection of Prn-p cDNA in leukemia MEL and neuroblastoma N2a cells increased production of PrP(C) but not prevented DNA fragmentation initiated by serum deprivation

Prion protein (PrP(C)) via its isoform PrP(SC) is involved in the pathogenesis of transmissible spongiform encephalopathies (TSEs). We observed that murine erythroleukemia (MEL) cells arrested in phase G(1) undergo transcriptional activation of Prn-p gene. Here, we explored the potential role of activation of Prn-p gene and cytosolic accumulation of PrP(C) in growth arrest, differentiation, and apoptotic DNA fragmentation by stably transfecting MEL and N2a cells with Prn-p cDNA. Stably transfected MEL cells (clones # 6, 12, 20, 38, and 42) were assessed for growth and differentiation, while clones N2a13 and N2a8 of N2a cells for growth and apoptosis by flow cytometry using Annexin V and propidium iodide (PI). Our results indicate that (a) Induction of terminal differentiation of stably transfected MEL cells led to growth arrest, activation of Prn-p gene, concomitant expression of transfected Prn-p cDNA, suppression of bax gene, cytosolic accumulation of PrP(C), and DNA fragmentation. The latter was also induced in non-differentiated MEL cells growing under serum-free conditions; (b) similarly, serum deprivation promoted growth arrest, apoptosis/necrosis associated with DNA fragmentation in parental N2a and N2a13 cells that produced relative high level of PrP(C) and not PrP(SC). These data indicate that activation of Prn-p gene and expression of transfected Prn-p cDNA in cells of both hematopoietic and neuronal origin occurred concomitantly, and led to cytosolic accumulation of PrP(C) and DNA damage induced by serum deprivation. PrP(C) production failed to protect DNA fragmentation induced by serum deprivation. The question how does PrP(C) contribute to growth arrest and DNA fragmentation is discussed.

Inducible overexpression of wild-type prion protein in the muscles leads to a primary myopathy in transgenic mice

The prion protein (PrP) level in muscle has been reported to be elevated in patients with inclusion-body myositis, polymyositis, dermatomyositis, and neurogenic muscle atrophy, but it is not clear whether the elevated PrP accumulation in the muscles is sufficient to cause muscle diseases. We have generated transgenic mice with muscle-specific expression of PrP under extremely tight regulation by doxycycline, and we have demonstrated that doxycycline-induced overexpression of PrP strictly limited to muscles leads to a myopathy characterized by increased variation of myofiber size, centrally located nuclei, and endomysial fibrosis, in the absence of intracytoplasmic inclusions, rimmed vacuoles, or any evidence of a neurogenic disorder. The PrP-induced myopathy correlates with accumulation of an N-terminal truncated PrP fragment in the muscle, and the muscular PrP displayed consistent mild resistance to protease digestion. Our findings indicate that overexpression of wild-type PrP in skeletal muscles is sufficient to cause a primary myopathy with no signs of peripheral neuropathy, possibly due to accumulation of a cytotoxic truncated form of PrP and/or PrP aggregation.

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.

Ovine plasma prion protein levels show genotypic variation detected by C-terminal epitopes not exposed in cell-surface PrPC

Ovine PBMCs (peripheral blood mononuclear cells) express PrP(C) [cellular PrP (prion-related protein)] and have the potential to harbour and release disease-associated forms of PrP during scrapie in sheep. Cell-surface PrP(C) expression by PBMCs, together with plasma PrP(C) levels, may contribute to the regulatory mechanisms that determine susceptibility and resistance to natural scrapie in sheep. Here, we have correlated cell-surface PrP(C) expression on normal ovine PBMCs by FACS with the presence of PrP(C) in plasma measured by capture-detector immunoassay. FACS showed similar levels of cell-surface PrP(C) on homozygous ARR (Ala136-Arg154-Arg171), ARQ (Ala136-Arg154-Gln171) and VRQ (Val136-Arg154-Gln171) PBMCs. Cell-surface ovine PrP(C) showed modulation of N-terminal epitopes, which was more evident on homozygous ARR cells. Ovine plasma PrP(C) levels showed genotypic variation and the protein displayed C-terminal epitopes not available in cell-surface PrP(C). Homozygous VRQ sheep showed the highest plasma PrP(C) level and homozygous ARR animals the lowest. For comparison, similar analyses were performed on normal bovine PBMCs and plasma. PrP(C) levels in bovine plasma were approx. 4-fold higher than ovine homozygous ARQ plasma despite similar levels of PBMC cell-surface PrP(C) expression. Immunoassays using C-terminal-specific anti-PrP monoclonal antibodies as capture and detector reagents revealed the highest level of PrP(C) in both ovine and bovine plasma, whilst lower levels were detected using N-terminal-specific monoclonal antibody FH11 as the capture reagent. This suggested that a proportion of plasma PrP(C) was N-terminally truncated. Our results indicate that the increased susceptibility to natural scrapie displayed by homozygous VRQ sheep correlates with a higher level of plasma PrP(C).

Comparative study of PrPc expression in rat, monkey, and cow gastrointestinal tract

The gastrointestinal tract (GIT) appears to be the main site of entry for the pathological isoform of prions (PrP(sc)). To understand how the PrP(sc) internalization process occurs, it is important to characterize the cell types that express normal prion protein (PrP(c)) along the GIT. To do so, we studied the distribution of PrP(c) in the rat, monkey, and cow GIT. Using Western blot analysis, we found that PrP(c) is expressed in all digestive regions of the three species. Immunoreactivity for PrP(c) was found throughout the GIT in epithelial cells sharing the neuroendocrine (NE) phenotype. Immunostained cells appeared scattered throughout the epithelium of fundic and pyloric glands as well as in intestinal villi and crypts.

Green fluorescent protein as a reporter of prion protein folding

Background

The amino terminal half of the cellular prion protein PrP^c is implicated in both the binding of copper ions and the conformational changes that lead to disease but has no defined structure. However, as some structure is likely to exist we have investigated the use of an established protein refolding technology, fusion to green fluorescence protein (GFP), as a method to examine the refolding of the amino terminal domain of mouse prion protein.

Results

Fusion proteins of PrP^c and GFP were expressed at high level in E.coli and could be purified to near homogeneity as insoluble inclusion bodies. Following denaturation, proteins were diluted into a refolding buffer whereupon GFP fluorescence recovered with time. Using several truncations of PrP^c the rate of refolding was shown to depend on the prion sequence expressed. In a variation of the format, direct observation in E.coli, mutations introduced randomly in the PrP^c protein sequence that affected folding could be selected directly by recovery of GFP fluorescence.

Conclusion

Use of GFP as a measure of refolding of PrP^c fusion proteins in vitro and in vivo proved informative. Refolding in vitro suggested a local structure within the amino terminal domain while direct selection via fluorescence showed that as little as one amino acid change could significantly alter folding. These assay formats, not previously used to study PrP folding, may be generally useful for investigating PrP^c structure and PrP^c-ligand interaction.

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.

Altered expression of the prion gene in rat astrocyte and neuron cultures treated with prion peptide 106-126

Neuronal degeneration and astrogliosis are hallmarks of prion disease. Synthetic prion protein (PrP) peptide 106-126 (PrP106-126) can induce death of neurons and proliferation of astrocytes in vitro and this neurotoxic effect depends on the expression of cellular PrP (PrPC) and is hence believed to be PrP(C) -mediated. To further elucidate the involvement of PrPC in PrP106-126-induced neurotoxicity, we determined the expression of PrP mRNA in primary culture of rat cortical neuron cells, cerebellar granule cells, and astrocytes following treatment with 50 microM of PrP106-126 scrambled PrP106-126 by quantitative real-time RT-PCR. As shown by MTT test, PrP106-126 induced significant death of neuron cells and marked proliferation of astrocytes after 10 days of treatment. Under the same treatment regimens, the level of PrP gene expression was significantly down-regulated in cortical neuron cell cultures and cerebellar granule cell cultures and was up-regulated in astrocyte cultures. The altered PrP gene expression occurred as early as 3 days after the treatment. After 10 days of treatment, while the cultured cortical neurons underwent further apoptosis, their expression of PrP gene started to recover gradually. These findings indicate that PrP 106-126 regulates transcription of the PrP gene and this activity is associated with its neurotoxicity in primary rat neuronal cultures.

Cell type-specific transgene expression of the prion protein in Xenopus intermediate pituitary cells

The cellular form of prion protein (PrPC) is anchored to the plasma membrane of the cell and expressed in most tissues, but predominantly in the brain, including in the pituitary gland. Thus far, the biosynthesis of PrPC has been studied only in cultured (transfected) tumour cell lines and not in primary cells. Here, we investigated the intracellular fate of PrPCin vivo by using the neuroendocrine intermediate pituitary melanotrope cells of the South-African claw-toed frog Xenopus laevis as a model system. These cells are involved in background adaptation of the animal and produce high levels of its major secretory cargo proopiomelanocortin (POMC) when the animal is black-adapted. The technique of stable Xenopus transgenesis in combination with the POMC gene promoter was used as a tool to express Xenopus PrPC amino-terminally tagged with the green fluorescent protein (GFP-PrPC) specifically in the melanotrope cells. The GFP-PrPC fusion protein was expressed from stage-25 tadpoles onwards to juvenile frogs, the expression was induced on a black background and the fusion protein was subcellularly located mainly in the Golgi apparatus and at the plasma membrane. Pulse-chase metabolic cell labelling studies revealed that GFP-PrPC was initially synthesized as a 45-kDa protein that was subsequently stepwise glycosylated to 48-, 51-, and eventually 55-kDa forms. Furthermore, we revealed that the mature 55-kDa GFP-PrPC protein was sulfated, anchored to the plasma membrane and cleaved to a 33-kDa product. Despite the high levels of transgene expression, the subcellular structures as well as POMC synthesis and processing, and the secretion of POMC-derived products remained unaffected in the transgenic melanotrope cells. Hence, we studied PrPC in a neuroendocrine cell and in a well-defined physiological context.

Isolation and characterization of full-length recombinant cattle PrPC protein

Full-length Bos taurus PrPC protein was obtained in the eu- and prokaryotic expression systems. Immunoblotting and indirect enzyme immunoassay demonstrated high specificity and antigenic activity of full-length proteins in the reactions with monoclonal antibodies (anti-SAF-32 and VRQ-84). Membrane location of recombinant PrPC protein in insect cells was shown by immunofluorescent analysis.

Skin-derived dendritic cells acquire and degrade the scrapie agent following in vitro exposure

The accumulation of the scrapie agent in lymphoid tissues following inoculation via the skin is critical for efficient neuroinvasion, but how the agent is initially transported from the skin to the draining lymph node is not known. Langerhans cells (LCs) are specialized antigen-presenting cells that continually sample their microenvironment within the epidermis and transport captured antigens to draining lymph nodes. We considered LCs probable candidates to acquire and transport the scrapie agent after inoculation via the skin. XS106 cells are dendritic cells (DCs) isolated from mouse epidermis with characteristics of mature LC cells. To investigate the potential interaction of LCs with the scrapie agent XS106 cells were exposed to the scrapie agent in vitro. We show that XS106 cells rapidly acquire the scrapie agent following in vitro exposure. In addition, XS106 cells partially degrade the scrapie agent following extended cultivation. These data suggest that LCs might acquire and degrade the scrapie agent after inoculation via the skin, but data from additional experiments demonstrate that this ability could be lost in the presence of lipopolysaccharide or other immunostimulatory molecules. Our studies also imply that LCs would not undergo maturation following uptake of the scrapie agent in the skin, as the expression of surface antigens associated with LC maturation were unaltered following exposure. In conclusion, although LCs or DCs have the potential to acquire the scrapie agent within the epidermis our data suggest it is unlikely that they become activated and stimulated to transport the agent to the draining lymph node.

Charged bipolar suramin derivatives induce aggregation of the prion protein at the cell surface and inhibit PrPSc replication

The conversion of the cellular prion protein (PrPc) into a pathogenic isoform (PrP(Sc)) is one of the underlying events in the pathogenesis of the fatal transmissible spongiform encephalopathies (TSEs). Numerous compounds have been described to inhibit prion replication and PrP(Sc) accumulation in cell culture. Among these, the drug suramin induces aggregation and re-targeting of PrPc to endocytic compartments. Plasma membrane and sites of conversion into PrP(Sc) are thereby bypassed. In the present study, a library of suramin analogues was tested as a potential class of new anti-prion compounds and the molecular mechanisms underlying these effects were analysed. Treatment of prion-infected neuroblastoma cells with compounds containing symmetrical aromatic sulfonic acid substitutions inhibited de novo synthesis of PrP(Sc) and induced aggregation and reduction of the half-life of PrPc without downregulating PrPc cell surface expression. Half-molecule compounds lacking the symmetrical bipolar structure or the anionic groups had no effect on PrP(Sc) synthesis or PrPc solubility. Cell surface expression of PrPc was necessary for the activity of effective compounds. Suramin derivatives did not induce aggregation of PrPc when transport along the secretory pathway was compromised, suggesting that their effects occur at a post trans-Golgi network (TGN) site, possibly close to the compartment of conversion into PrP(Sc). In vitro studies with recombinant PrP demonstrated that the inhibitory effect correlated with direct binding to PrP and induction of insoluble PrP aggregates. Our data reveal an anti-prion effect that differs from those characterising other sulphated polyanions and is dependent on the presence of the symmetrical anionic structure of these molecules.

Expression of cellular prion protein in the frontal and occipital lobe in Alzheimer's disease, diffuse Lewy body disease, and in normal brain: an immunohistochemical study

Cellular prion protein (PrP(c)) is a glycoprotein expressed at low to moderate levels within the nervous system. Recent studies suggest that PrP(c) may possess neuroprotective functions and that its expression is upregulated in certain neurodegenerative disorders. We investigated whether PrP(c) expression is altered in the frontal and occipital cortex in two well-characterized neurodegenerative disorders--Alzheimer's disease (AD) and diffuse Lewy body disease (DLBD)--compared with that in normal human brain using immunohistochemistry and computerized image analysis. The distribution of PrP(c) was further tested for correlation with glial reactivity. We found that PrP(c) was localized mainly in the gray matter (predominantly in neurons) and expressed at higher levels within the occipital cortex in the normal human brain. Image analysis revealed no significant variability in PrP(c) expression between DLBD and control cases. However, blood vessels within the white matter of DLBD cases showed immunoreactivity to PrP(c). By contrast, this protein was differentially expressed in the frontal and occipital cortex of AD cases; it was markedly overexpressed in the former and significantly reduced in the latter. Epitope specificity of antibodies appeared important when detecting PrP(c). The distribution of PrP(c) did not correlate with glial immunoreactivity. In conclusion, this study supports the proposal that regional changes in expression of PrP(c) may occur in certain neurodegenerative disorders such as AD, but not in other disorders such as DLBD.

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.

Hypoglycemia enhances the expression of prion protein and heat-shock protein 70 in a mouse neuroblastoma cell line

Cellular prion protein (PrP(C)) expression can be regulated by heat-shock stress, and we designed the present study to determine whether hypoglycemia could affect PrP(C) expression. RT-PCR and Western blotting were used to measure the expression of PrP(C) and heat-shock protein (Hsp70) in mouse neuroblastoma (N18) cells cultured 3 hr to 3 days in media deprived of 97.5% (L) or 75% (M) of its glucose. Hypoglycemia caused a concomitant time-dependent and glucose dose-dependent increase in PrP(C) and Hsp70. In addition, hypoglycemia also increased phosphorylated c-Jun N-terminal kinase (JNK) protein levels in a time-dependent manner. The upregulation of PrP(C) and Hsp70 under hypoglycemic conditions was disrupted by the specific JNK inhibitor SP600125. It was also found from in vitro studies that hypoglycemic conditions induced higher levels of PrP(C) promoter activity in PrP(C) promoters containing a heat-shock element (HSE) than in PrP(C) promoters lacking HSE. We propose that hypoglycemia-increased PrP(C) expression might be due to JNK phosphorylation of a heat-shock transcriptional factor, which then interacts with HSE in the promoter of PrP(C).

Protection from cytosolic prion protein toxicity by modulation of protein translocation

Failure to promptly dispose of undesirable proteins is associated with numerous diseases. In the case of cellular prion protein (PrP), inhibition of the proteasome pathway can generate a highly aggregation-prone, cytotoxic form of PrP implicated in neurodegeneration. However, the predominant mechanisms that result in delivery of PrP, ordinarily targeted to the secretory pathway, to cytosolic proteasomes have been unclear. By accurately measuring the in vivo fidelity of protein translocation into the endoplasmic reticulum (ER), we reveal a slight inefficiency in PrP signal sequence function that generates proteasomally degraded cytosolic PrP. Attenuating this source of cytosolic PrP completely eliminates the dependence on proteasomes for PrP degradation. This allows cells to tolerate both higher expression levels and decreased proteasomal capacity without succumbing to the adverse consequences of misfolded PrP. Thus, the generation of potentially toxic cytosolic PrP is controlled primarily during its initial translocation into the ER. These results suggest that a substantial proportion of the cell's constitutive proteasomal burden may consist of proteins that, like PrP, fail to cotranslationally enter the secretory pathway with high fidelity.

Upregulation of cellular prion protein (PrPc) after focal cerebral ischemia and influence of lesion severity

The pathological isoform of the prion protein (PrP(Sc)) has been identified to mediate transmissible spongiform encephalopathies like Creutzfeldt-Jakob disease (CJD). In contrast, the physiological function of the normal cellular prion protein (PrP(c)) is not yet understood. Recent findings suggest that PrP(c) may have neuroprotective properties and that its absence increases susceptibility to oxidative stress and neuronal injury. To determine whether PrP(c) is part of the cellular response to neuronal injury in vivo, we investigated PrP(c) regulation after severe and mild focal ischemic brain injury in mice using the thread occlusion stroke model. Western Blot and ELISA analysis showed a significant upregulation of PrP(c) in the ischemic hemisphere at 4 and 8h after onset of permanent focal ischemia, which was no longer detectable at 24h after lesion induction when compared to control animals. In contrast, transient focal ischemia (60 min) did only lead to slightly but not significantly elevated PrP(c) levels in the ischemic hemisphere when compared to controls. These results demonstrate that cerebral PrP(c) is upregulated early in response to focal cerebral ischemia. The extent of upregulation, however, seems to depend on the severity of ischemia and may therefore reflect the extent of ischemia induced neuronal damage. Given the known neuroprotective effects of PrP(c) in vitro, ischemia-induced upregulation of cerebral PrP(c) supports the hypothesis that, as part of an early adaptive cellular response to ischemic brain injury, PrP(c) may be involved in the regulation of ischemia-induced neuronal cell death in vivo.

Prion protein accumulation and neuroprotection in hypoxic brain damage

The function of the normal conformational isoform of prion protein, PrP(C), remains unclear although lines of research have suggested a role in the cellular response to oxidative stress. Here we investigate the expression of PrP(C) in hypoxic brain tissues to examine whether PrP(C) is in part regulated by neuronal stress. Cases of adult cerebral ischemia and perinatal hypoxic-ischemic injury in humans were compared with control tissues. PrP(C) immunoreactivity accumulates within neuronal processes in the penumbra of hypoxic damage in adult brain, and within neuronal soma in cases of perinatal hypoxic-ischemic injury, and in situ hybridization analysis suggests an up-regulation of PrP mRNA during hypoxia. Rodents also showed an accumulation of PrP(C) in neuronal soma within the penumbra of ischemic lesions. Furthermore, the infarct size in PrP-null mice was significantly greater than in the wild type, supporting the proposed role for PrP(C) in the neuroprotective adaptive cellular response to hypoxic injury.

Prion protein (PrPc) immunocytochemistry and expression of the green fluorescent protein reporter gene under control of the bovine PrP gene promoter in the mouse brain

Expression of the cellular prion protein (PrP(c)) by host cells is required for prion replication and neuroinvasion in transmissible spongiform encephalopathies. As a consequence, identification of the cell types expressing PrP(c) is necessary to determine the target cells involved in the cerebral propagation of prion diseases. To identify the cells expressing PrP(c) in the mouse brain, the immunocytochemical localization of PrP(c) was investigated at the cellular and ultrastructural levels in several brain regions. In addition, we analyzed the expression pattern of a green fluorescent protein reporter gene under the control of regulatory sequences of the bovine prion protein gene in the brain of transgenic mice. By using a preembedding immunogold technique, neuronal PrP(c) was observed mainly bound to the cell surface and presynaptic sites. Dictyosomes and recycling organelles in most of the major neuron types also exhibited PrP(c) antigen. In the olfactory bulb, neocortex, putamen, hippocampus, thalamus, and cerebellum, the distribution pattern of both green fluorescent protein and PrP(c) immunoreactivity suggested that the transgenic regulatory sequences of the bovine PrP gene were sufficient to promote expression of the reporter gene in neurons that express immunodetectable endogenous PrP(c). Transgenic mice expressing PrP-GFP may thus provide attractive murine models for analyzing the transcriptional activity of the Prnp gene during prion infections as well as the anatomopathological kinetics of prion diseases.

Neuron-specific mRNA complexity responses during hippocampal apoptosis after traumatic brain injury

In an effort to understand the complexity of genomic responses within selectively vulnerable regions after experimental brain injury, we examined whether single apoptotic neurons from both the CA3 and dentate differed from those in an uninjured brain. The mRNA from individual active caspase 3(+)/terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling [TUNEL(-)] and active caspase 3(+)/TUNEL(+) pyramidal and granule neurons in brain-injured mice were amplified and compared with those from nonlabeled neurons in uninjured brains. Gene analysis revealed that overall expression of mRNAs increased with activation of caspase 3 and decreased to below uninjured levels with TUNEL reactivity. Cell type specificity of the apoptotic response was observed with both regionally distinct expression of mRNAs and differences in those mRNAs that were maximally regulated. Immunohistochemical analysis for two of the most highly differentially expressed genes (prion and Sos2) demonstrated a correlation between the observed differential gene expression after traumatic brain injury and corresponding protein translation.

Different behavior toward bovine spongiform encephalopathy infection of bovine prion protein transgenic mice with one extra repeat octapeptide insert mutation

In humans, insert mutations within the repetitive octapeptide region of the prion protein gene (Prnp) are often associated with familial spongiform encephalopathies. In this study, transgenic mice expressing bovine PrP (boTg mice) bearing an additional octapeptide insertion to the wild type (seven octapeptide repeats instead of six) showed an altered course of bovine spongiform encephalopathy (BSE) infection, reflected as reduced incubation times when compared with boTg mice expressing similar levels of the wild-type six-octapeptide protein. In both boTg mouse lines (bo6ORTg and bo7ORTg), incubation times were affected drastically depending on transgene expression levels and the inoculum used. In accordance with the lack of an interspecies barrier to BSE infection, we detected the typical signs of CNS spongiform degeneration by histopathological analysis and the presence of the bovine prion PrP(res) by Western blot or immunohistochemical analyses. When 7OR-PrP(res) was propagated in bo7ORTg mice, a similar earlier onset of clinical signs was observed compared with bo6ORTg mice. Proteins PrP(C) and PrP(res) containing seven octapeptides (7OR-PrP(C) and 7OR-PrP(res)) showed similar protease sensitivity and insolubility in nondenaturing detergents to homologous 6OR-PrP(C) and 6OR-PrP(res). In addition, bo7ORTg mice showed a higher sensitivity than bo6ORTg mice for detecting prion infection in specimens previously diagnosed as negative by conventional biochemical techniques. In the absence of clinical signs of disease, 7OR-PrP(res) could be detected as early as 120 d after inoculation by immunohistochemical and Western blot analyses. These findings may help us improve the current mouse bioassays and understand the role of the octapeptide repeat region in susceptibility to disease.

Susceptibility of common fibroblast cell lines to transmissible spongiform encephalopathy agents

The risk of contamination of tissue culture cells with transmissible spongiform encephalopathy (TSE) agents as a result of the use of animal products as medium components has been considered to be low, in part, because only a few brain-derived cell lines have been reported to be susceptible to TSE infection. In the present study, we demonstrate that the common laboratory fibroblast cell lines NIH/3T3 and L929, which express low levels of cellular mouse prion protein, are susceptible to infection with mouse-adapted scrapie. Our results show that the susceptibility of a cell line to TSE infection cannot be predicted on the basis of its tissue origin or its level of expression of the cellular prion protein, and they suggest that any cell line expressing normal host prion protein could have the potential to support propagation of TSE agents. Thus, testing of cells for TSE susceptibility might be necessary for all cell lines that are routinely used in vaccine production and in other medical applications.

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

The physiological function of PrPc, the cellular isoform of prion protein, still remains unclear, although it has been established, in vitro or by using nerve cells, that it can homodimerize, bind copper, or interact with other proteins. Expression of PrPc was demonstrated as necessary for prion infection propagation. Considering the importance of the intestinal barrier in the process of oral prion infectivity, we have analyzed the expression of PrPc in enterocytes, which represent the major cell population of the intestinal epithelium. Our study, conducted both on normal human intestinal tissues and on the enterocytic cell line Caco-2/TC7, shows for the first time that PrPc is present in enterocytes. Interestingly, we found that this glycosylphosphatidylinositol-anchored glycoprotein was localized in cholesterol-dependent raft domains of the upper lateral membranes of enterocytes, beneath tight junctions, in cell-cell junctional domains. We observed that PrPc, E-cadherin, and Src co-localized in adherens junctions and that PrPc was co-immunoprecipitated with Src kinase but not with E-cadherin. Alteration of cell polarity after cholesterol depletion or loosening of the cell-cell junctions after EGTA treatment rapidly impaired membrane targeting of PrPc. Overall, our results point out the signaling of cell-cell contacts as a putative role for PrPc in epithelial cells.

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.

Determinants of the in vivo folding of the prion protein. A bipartite function of helix 1 in folding and aggregation

Misfolding of the mammalian prion protein (PrP) is implicated in the pathogenesis of prion diseases. We analyzed wild type PrP in comparison with different PrP mutants and identified determinants of the in vivo folding pathway of PrP. The complete N terminus of PrP including the putative transmembrane domain and the first beta-strand could be deleted without interfering with PrP maturation. Helix 1, however, turned out to be a major determinant of PrP folding. Disruption of helix 1 prevented attachment of the glycosylphosphatidylinositol (GPI) anchor and the formation of complex N-linked glycans; instead, a high mannose PrP glycoform was secreted into the cell culture supernatant. In the absence of a C-terminal membrane anchor, however, helix 1 induced the formation of unglycosylated and partially protease-resistant PrP aggregates. Moreover, we could show that the C-terminal GPI anchor signal sequence, independent of its role in GPI anchor attachment, mediates core glycosylation of nascent PrP. Interestingly, conversion of high mannose glycans to complex type glycans only occurred when PrP was membrane-anchored. Our study indicates a bipartite function of helix 1 in the maturation and aggregation of PrP and emphasizes a critical role of a membrane anchor in the formation of complex glycosylated PrP.

Overexpression of PrPc triggers caspase 3 activation: potentiation by proteasome inhibitors and blockade by anti-PrP antibodies

We examined the influence of cellular prion protein (PrPc) in the control of cell death in stably transfected HEK293 cell line and in the PrPc-inducible Rov9 cells. PrPc expression in stably transfected HEK293 human cells did not modify basal apoptotic tonus but drastically potentiated staurosporine-stimulated cellular toxicity and DNA fragmentation as well as caspase 3-like activity and immunoreactivity. An identical staurosporine-induced caspase 3 activation was observed after doxycycline in the PrPc-inducible Rov9 cell line. Interestingly, proteasome inhibitors increase PrPc-like immunoreactivity and unmasked a basal caspase 3 activation. Conversely, we show that anti-PrPc antibodies sequestrate PrPc at the cell surface and drastically lower PrPc-dependent caspase activation. We suggest that intracellular PrPc could sensitize human cells to pro-apoptotic phenotype and that blockade of PrPc internalization could be a track to prevent intracellular toxicity associated with PrPc overexpression.

[Prion diseases]

Creutzfeldt-Jakob disease, kuru, Gerstmann Sträussler Scheinker syndrome and fatal familial insomnia in humans, as well as scrapie and bovine spongiform encephalopathy, in animals, are fatal disorders of the central nervous system that are part of the group of transmissible spongiform encephalopathies, (TSE) or prion diseases. Neuronal intracellular spongiosis and the accumulation of abnormal, protease resistant prion protein in the nervous central system characterize TSE. The conformational change of a host protein, prion protein, into a pathological isoform is the key pathogenetic event in TSE. Despite their relative rarity, prion diseases have a great impact on the scientific community and society in general. There are two major reasons: first, the heretical hypothesis of a disease transmitted by an "infectious protein" in the absence of nucleic acid, the basis of the conformational transmissibility concept; second, the panic originated from the appearance of new variant Creutzfeldt-Jakob disease and the evidence linking it to the exposure of humans to bovine spongiform encephalopathy via food contaminated by affected bovine tissue. Novel therapeutic approaches are examined.

Distribution of intraneuronal immunoreactivity for the prion protein in human prion diseases

Intraneuronal prion protein (PrP) immunoreactivity (INIR), which might represent the non-pathological, cellular form of PrP, needs to be distinguished from disease-associated deposits specific for prion disease (PrD). In adjacent sections of PrD and control brains we applied pretreatments, one of which enhances the immunoreactivity of disease-associated PrP, and another that enhances INIR. We observed an inverse correlation between the proportion of neurons with INIR and the intensity of disease-associated PrP immunoreactivity and severity of lesions. Additionally, we found large intracytoplasmic inclusion-like bodies in ballooned neurons in PrD cases. We noted that the 3F4 (epitope: amino acids 109-112) anti-PrP antibody labels more INIR than antibodies directed against amino acids 23-85 (BG4) or 140-180 (KG9) in PrD cases, in contrast to controls, but all antibodies immunolabel more INIR in PrD brains. The up-regulation of PrP might represent an early loss of function of the non-pathological form of PrP, in parallel with a neurotoxic effect of accumulating disease-associated isoform, as part of the pathogenesis of prion diseases.

A marked disparity between the expression of prion protein and its message by neurones of the CNS

Expression of the normal cellular form of prion protein is both necessary and rate-limiting in the spread of prion disease, yet its cellular expression in vivo is poorly understood. To optimise immunohistochemical labelling of this protein in mouse brain, we have developed novel antibodies that recognise cellular prion protein in glutaraldehyde-fixed tissue. Expression was found to be predominantly neuronal, and to differ between different classes of neurone. Thus, neurones immunoreactive for GABA expressed very high levels of normal prion protein; most projection neurones expressed much lower levels, particularly on their axons in the major fibre tracts, and some neurones (e.g. those positive for dopamine) displayed no detectable prion protein. In marked contrast, all neurones, even those that were immunonegative, expressed high levels of message for prion protein, shown by non-radioactive in situ hybridisation. Glia expressed very low levels of message, and undetectable levels of prion protein. We conclude that the steady-state level of prion protein, which differs so markedly between different neuronal types, is primarily controlled post-transcriptionally, possibly by differences in protein trafficking or degradation. These marked differences in the way different neurones produce and/or degrade their normal cellular prion protein may influence the selective spread and neurotoxic targeting of prion diseases within the CNS.

Properties of the cellular prion protein expressed in Xenopus oocytes

The cellular prion protein (PrPC) from different species can be reproducibly expressed in Xenopus oocytes following injection of in vitro transcribed mRNAs. The level of PrPC accumulation increases with the amount of RNA injected and with the duration of incubation. PrPC expressed in oocytes is similar in size and abundance to PrPC protein in mouse brain and >100 ng of PrPC is expressed per oocyte allowing complete experiments to be carried out in single living cells. The protein is glycosylated, fully protease sensitive and expressed on the cell surface. Xenopus oocytes therefore provide a useful model system for the study of prion proteins and their associated disease processes.

G1-dependent prion protein expression in human glioblastoma cell line T98G

Human glioblastoma cell line T98G produced a cellular form of prion protein (PrP(C)), and we confirmed expression of PrP mRNA by RT-PCR. Immunoblot analysis of whole cell lysate revealed one major (35 kDa) and two faint bands (31, 25 kDa) that reacted with monoclonal anti-human PrP antibody 3F4. Cells treated with tunicamycin produced only a 25 kDa band, representing a deglycosylated form of PrP. Similarly, peptide: N-glycosidase F treatment of whole cell lysate altered the Asn-linked form to the deglycosylated form. When T98G cells were cultured for a longer period, the amount of PrP(C) per cell increased on Day 4 to 16 in a time-dependent manner. When the cells were cultured at high cell-density, the cells on Day 4 produced the same amount of PrP(C) as those on Day 16 of the usual culture. Moreover, in a serum-free medium, cells cultured at a low cell-density produced the same amount of PrP(C) as those cultured at the high cell-density. These results demonstrate that PrP(C) production in T98G cells was dependent on the phase of the cell cycle, probably the G1 phase.

Respiratory function in mice lacking or overexpressing the prion protein

We investigated a possible involvement of the prion protein in ventilatory control in four groups of mice, those deficient for the prion protein (PrP(c)), those overexpressing the prion protein, and two groups of genetically and age-matched controls. Ventilatory patterns of unrestrained mice were measured in a whole-body plethysmograph. Between each genotype and its control, we compared ventilation at rest and the ventilatory response to moderate hypoxia (10-12% O2), hyperoxia and hyperoxic hypercapnia. Mice lacking or overexpressing PrP(c) and their respective controls showed similar ventilatory patterns at rest and similar chemosensory responses when awake and under urethane anesthesia. Our results do not support the view that PrP(c) may play any significant role in basal ventilation or in the chemosensory ventilatory control of adult mice.

[Spongiform encephalitis]

The authors present a personal interpretation of the pathogeny of prionic processes, showing the role played by heredity. They also present the very surprising fact that a protein codified by a mutant gene can act as transmissible agent in a serious illness opening new chapters to the pathology of a series of important diseases. This is a completely new perspective in the biology.

Cell biological studies of the prion protein

Studying PrPC and PrPSc in cell culture systems is advantageous because such systems contain all the organelles, membranes, and molecular cofactors that are likely to play an important role in the biology of the proteins. Using cultured cells expressing PrPC, we have discovered that this isoform constitutively cycles between the cell surface and an endocytic compartment, a process that is mediated by clathrin-coated pits and a putative PrPC receptor. We have also constructed stably transfected lines of CHO cells that express PrP molecules carrying mutations that are associated with familial prion diseases. The mutant PrP molecules in these cells are spontaneously converted to the PrPSc state, a phenomenon which has allowed us to analyze several key features of prion formation.

Functional characterization of the human prion protein promoter in neuronal and endothelial cells

Human prion diseases such as Creutzfeld-Jakob disease and kuru are of major medical and biological importance because of their fatal course, epidemic potential, and unique pathophysiology. Endogenous expression of the normal cellular prion protein (PrP(C)) is necessary for infection and prion replication. However, knowledge of human PrP(C) gene regulation is rudimentary. We therefore cloned1543 bp of the 5' untranslated and promoter region of the PrP gene. Using transient transfection assays, the full-length promoter and serial deletion mutants subcloned in a luciferase reporter vector were analyzed in neuronal (KELLY) and endothelial (EA.hy926) cell lines, which both express PrP(C) as shown by RT/PCR. Analysis of promoter constructs in KELLY cells indicated two activating regions at -131/-284 and -1303/-1543, relative to the 3'-terminal end of exon 1, and also two repressing elements at -254/-567 and -567/-909 in neuronal cells. In EA.hy926 cells, activating elements were identified at -131/-284 and -284/-567, and one repressing region was localized at -567/-909. In addition, transcriptional start sites were determined by 5'-RACE reaction and RNase protection assay, revealing one major transcriptional start site located at -47 (in KELLY cells), -53 (in human thalamus) and at about -55 (in EA.hy926 cells).

Normal cellular prion protein is preferentially expressed on subpopulations of murine hemopoietic cells

We studied the expression of normal cellular prion protein (PrP(C)) in mouse lymphoid tissues with newly developed mAbs to PrP(C). Most of the mature T and B cells in the peripheral lymphoid organs do not express PrP(C). In contrast, most thymocytes are PrP(C+). In the bone marrow, erythroid cells and maturing granulocytes are PrP(C+). Approximately 50% of the cells in the region of small lymphocytes and progenitor cells also express PrP(C). Most of these PrP(C+) cells are CD43(+), but B220(-), surface IgM(-) (sIgM(-)), and IL-7R(-), a phenotype that belongs to cells not yet committed to the B cell lineage. Another small group of the PrP(C+) cell are B220(+), and some of these are also sIgM(+). The majority of the B220(+) cells, however, are PrP(C-). Therefore, PrP(C) is preferentially expressed in early bone marrow progenitor cells and subsets of maturing B cells. Supporting this interpretation is our observation that stimulation of bone marrow cells in vitro with PMA results in a decrease in the number of PrP(C+)B220(-) cells with a corresponding increase of sIgM(+)B220(high) mature B cells. This result suggests that the PrP(C+)B220(-) cells are potential progenitors. Furthermore, in the bone marrow of Rag-1(-/-) mice, there are an increased number of PrP(C+)B220(-) cells, and most of the developmentally arrested pro-B cells in these mice are PrP(C+). Collectively, these results suggest that PrP(C) is expressed preferentially in immature T cells in the thymus and early progenitor cells in the bone marrow, and the expression of PrP(C) is regulated during hemopoietic differentiation.

Human FDC express PrPc in vivo and in vitro

Prion diseases are fatal neurodegenerative disorders caused by accumulation of abnormal prion protein (protease-resistant prion, PrPres). PrPres accumulation is also detected in lymphoid organs after peripheral infection. Several studies suggest that follicular dendritic cells (FDC) could be the site of PrPres retention and amplification. Here we show that human follicular dendritic cells can express normal cellular prion protein (PrPc) both in situ and in vitro. When tonsillar cryosections were treated with anti-PrP antibody, the label was found on some very delicate cell extensions inside the lymphoid follicles, especially in the germinal centres. These extensions react with DRC1 antibody, used frequently to label FDC. Other structures labelled with anti-PrP antibody were the keratinocytes. To confirm the ability of FDC to synthesise PrPc, we isolated FDC by a non-enzymatic procedure and cultured them. By cytochemistry and flow cytometry it was clearly shown that FDC do produce PrPc.

The pattern of prion-related protein expression in the gastrointestinal tract

Prion diseases or transmissible spongiform encephalopathies have been shown to be communicated by oral ingestion of the infectious agent. However, the exact route of transmission is still unknown. In order to better understand the pathophysiology of these diseases, it is crucial to identify cell types of peripheral tissues in which the infectious agent may propagate. Since expression of cellular prion protein (PrPc) is a prerequisite for prion replication, we determined the expression of PrPc in the mucosa of the gastrointestinal tract using immunohistochemistry. Expression of PrPc was negative or weak in the neck region of the gastric mucosa and moderate to strong in crypts of both the small and the large bowel. PrPc was found to be upregulated in the mucosa of patients with Helicobacter pylori gastritis. In contrast, PrPc staining appeared to be downregulated in patients with inflammatory disorders of the large bowel and it remained moderate to strong in inflammatory disorders of the small bowel. Our results support the notion that epithelial cells of the gastrointestinal tract may represent a possible target for prion entry and replication.

Dominant-negative inhibition of prion formation diminished by deletion mutagenesis of the prion protein

Polymorphic basic residues near the C terminus of the prion protein (PrP) in humans and sheep appear to protect against prion disease. In heterozygotes, inhibition of prion formation appears to be dominant negative and has been simulated in cultured cells persistently infected with scrapie prions. The results of nuclear magnetic resonance and mutagenesis studies indicate that specific substitutions at the C-terminal residues 167, 171, 214, and 218 of PrP(C) act as dominant-negative, inhibitors of PrP(Sc) formation (K. Kaneko et al., Proc. Natl. Acad. Sci. USA 94:10069-10074, 1997). Trafficking of substituted PrP(C) to caveaola-like domains or rafts by the glycolipid anchor was required for the dominant-negative phenotype; interestingly, amino acid replacements at multiple sites were less effective than single-residue substitutions. To elucidate which domains of PrP(C) are responsible for dominant-negative inhibition of PrP(Sc) formation, we analyzed whether N-terminally truncated PrP(Q218K) molecules exhibited dominant-negative effects in the conversion of full-length PrP(C) to PrP(Sc). We found that the C-terminal domain of PrP is not sufficient to impede the conversion of the full-length PrP(C) molecule and that N-terminally truncated molecules (with residues 23 to 88 and 23 to 120 deleted) have reduced dominant-negative activity. Whether the N-terminal region of PrP acts by stabilizing the C-terminal domain of the molecule or by modulating the binding of PrP(C) to an auxiliary molecule that participates in PrP(Sc) formation remains to be established.