A novel method of generating neuronal cell lines from gene-knockout mice to study prion protein membrane orientation

Prion infection in cells is abolished by a mutated manganese transporter but shows no relation to zinc

The cellular prion protein has been identified as a metalloprotein that binds copper. There have been some suggestions that prion protein also influences zinc and manganese homeostasis. In this study we used a series of cell lines to study the levels of zinc and manganese under different conditions. We overexpressed either the prion protein or known transporters for zinc and manganese to determine relations between the prion protein and both manganese and zinc homeostasis. Our observations supported neither a link between the prion protein and zinc metabolism nor any effect of altered zinc levels on prion protein expression or cellular infection with prions. In contrast we found that a gain of function mutant of a manganese transporter caused reduction of manganese levels in prion infected cells, loss of observable PrP(Sc) in cells and resistance to prion infection. These studies strengthen the link between manganese and prion disease.

The effect of chicken extract on ERK/CREB signaling is ApoE isoform-dependent

It is unclear how the nutritional supplement chicken extract (CE) enhances cognition. Human apolipoprotein E (ApoE) can regulate cognition and this isoform-dependent effect is associated with the N-methyl-d-aspartate receptor (NMDAR). To understand if CE utilizes this pathway, we compared the NMDAR signaling in neuronal cells expressing ApoE3 and ApoE4. We observed that CE increased S896 phosphorylation on NR1 in ApoE3 cells and this was linked to higher protein kinase C (PKC) activation. However, ApoE4 cells treated with CE have lowered S897 phosphorylation on NR1 and this was associated with reduced protein kinase A (PKA) phosphorylation. In ApoE3 cells, CE increased calmodulin kinase II (CaMKII) activation and AMPA GluR1 phosphorylation on S831. In contrast, CE reduced CaMKII phosphorylation and led to higher de-phosphorylation of S831 and S845 on GluR1 in ApoE4 cells. While CE enhanced ERK/CREB phosphorylation in ApoE3 cells, this pathway was down-regulated in both ApoE4 and mock cells after CE treatment. These results show that CE triggers ApoE isoform-specific changes on ERK/CREB signaling.

Prion protein (PrP) gene-knockout cell lines: insight into functions of the PrP

Elucidation of prion protein (PrP) functions is crucial to fully understand prion diseases. A major approach to studying PrP functions is the use of PrP gene-knockout (Prnp (-/-)) mice. So far, six types of Prnp (-/-) mice have been generated, demonstrating the promiscuous functions of PrP. Recently, other PrP family members, such as Doppel and Shadoo, have been found. However, information obtained from comparative studies of structural and functional analyses of these PrP family proteins do not fully reveal PrP functions. Recently, varieties of Prnp (-/-) cell lines established from Prnp (-/-) mice have contributed to the analysis of PrP functions. In this mini-review, we focus on Prnp (-/-) cell lines and summarize currently available Prnp (-/-) cell lines and their characterizations. In addition, we introduce the recent advances in the methodology of cell line generation with knockout or knockdown of the PrP gene. We also discuss how these cell lines have provided valuable insights into PrP functions and show future perspectives.

Prion protein expression alters APP cleavage without interaction with BACE-1

The prion protein (PrP) and the beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE-1) are both copper binding proteins, but are associated with two separate neurodegenerative diseases. The role of BACE-1 in the formation of beta-amyloid has made it a major target in attempts to reduce the formation of beta-amyloid in Alzheimer's diseases. However, the suggestion that PrP, normally associated with prion diseases, binds to BACE-1 and reduces its activity has led to the suggestion that the study of this interaction could be of considerable importance to Alzheimer's disease. We therefore undertook to investigate the possible interaction of these two proteins physically and at the level of transcription, translation and APP cleavage. Our findings suggest that mature PrP and BACE-1 do not physically interact, but that altered PrP expression results in altered BACE-1 protein expression and promoter activity. Additionally, overexpression of PrP results in increased cleavage of APP in contrast to previous datas suggesting a reduction. Our findings suggest that any relation between PrP and BACE-1 is indirect. Altered expression of PrP causes changes in the expression of many other proteins which may be as a result of altered copper metabolism.

Development of cysteine-free fluorescent proteins for the oxidative environment

Molecular imaging employing fluorescent proteins has been widely used to highlight specific reactions or processes in various fields of the life sciences. Despite extensive improvements of the fluorescent tag, this technology is still limited in the study of molecular events in the extracellular milieu. This is partly due to the presence of cysteine in the fluorescent proteins. These proteins almost cotranslationally form disulfide bonded oligomers when expressed in the endoplasmic reticulum (ER). Although single molecule photobleaching analysis showed that these oligomers were not fluorescent, the fluorescent monomer form often showed aberrant behavior in folding and motion, particularly when fused to cysteine-containing cargo. Therefore we investigated whether it was possible to eliminate the cysteine without losing the brightness. By site-saturated mutagenesis, we found that the cysteine residues in fluorescent proteins could be replaced with specific alternatives while still retaining their brightness. cf(cysteine-free)SGFP2 showed significantly reduced restriction of free diffusion in the ER and marked improvement of maturation when fused to the prion protein. We further applied this approach to TagRFP family proteins and found a set of mutations that obtains the same level of brightness as the cysteine-containing proteins. The approach used in this study to generate new cysteine-free fluorescent tags should expand the application of molecular imaging to the extracellular milieu and facilitate its usage in medicine and biotechnology.

Activation and repression of prion protein expression by key regions of intron 1

Expression of the prion protein is necessary for infection with prion diseases. Altered expression levels may play an important role in susceptibility to infection. Therefore, understanding the mechanisms that regulate prion protein expression is of great importance. It was previously shown that expression of the prion protein is to some degree regulated by an alternative promoter within intron 1. Studies using GFP and luciferase reporter systems were undertaken to determine key sites for the repression and activation of expression of the prion protein driven by intron 1. We identified a region within intron 1 sufficient to drive prion protein expression. Our findings highlight two potential repressor regions. Both regions have binding sites for the known repressor Hes-1. Hes-1 overexpression caused a dramatic decrease in PrP protein expression. Additionally, we have identified Atox-1 as a transcription factor that upregulates prion protein expression. These findings clearly indicate that intron 1 plays a key role in regulation of prion protein expression levels.

Investigation of PrPC metabolism and function in live cells : methods for studying individual cells and cell populations

Prion protein (PrP)(C) expression levels and protein localization are known to be affected by factors such as metal ions and oxidative stress. By the development of a green fluorescent protein (GFP)-PrP(C) fusion protein, the movement of PrP can be followed in real time. Furthermore, alterations in cellular metabolism can be detected while cells are still viable. The internalization response of PrP to 20 microM manganese (Mn) in divalent metal ion-depleted media is used to demonstrate the movement of GFP-tagged proteins in live cells and real time. A live cell microtiter plate assay shows that PrP null cells are less capable of dealing with Mn-induced oxidative stress. In addition, this chapter outlines several complementary techniques for studying live cells and GFP fusion proteins.

Prion protein gene-deficient cell lines: powerful tools for prion biology

Prion diseases are zoonotic infectious diseases commonly transmissible among animals via prion infections with an accompanying deficiency of cellular prion protein (PrP(C)) and accumulation of an abnormal isoform of prion protein (PrP(Sc)), which are observed in neurons in the event of injury and disease. To understand the role of PrP(C) in the neuron in health and diseases, we have established an immortalized neuronal cell line HpL3-4 from primary hippocampal cells of prion protein (PrP) gene-deficient mice by using a retroviral vector encoding Simian Virus 40 Large T antigen (SV40 LTag). The HpL3-4 cells exhibit cell-type-specific proteins for the neuronal precursor lineage. Recently, this group and other groups have established PrP-deficient cell lines from many kinds of cell types including glia, fibroblasts and neuronal cells, which will have a broad range of applications in prion biology. In this review, we focus on recently obtained information about PrP functions and possible studies on prion infections using the PrPdeficient cell lines.

Regulation of prion protein expression by noncoding regions of the Prnp gene

Expression of the cellular prion protein is necessary for the transmission and propagation of prion diseases. Increasing the level of prion protein expression decreases the incubation period for these diseases. Therefore, understanding the regulation of prion protein expression could be critical for treating or preventing these diseases. We investigated the regulation of prion protein expression by the promoter and noncoding regions of the bovine and murine Prnp genes. We determined that expression is modulated by intron 1 and exon 1. In the absence of intron1, exon 1 inhibited activity of the promoter. However, intron 1 demonstrated promoter-like activity and possessed a TATA box. In addition, we identified an alternative transcript present in the brains of cattle and mice that lacks exon 1. Taken together, these results show that intron 1 and exon 1 play a critical role in the regulation of prion protein expression. Because switching off prion protein expression has been shown to arrest prion disease, these regions present novel targets for intervention in the disease process.

Copper-dependent co-internalization of the prion protein and glypican-1

Heparan sulfate chains have been found to be associated with amyloid deposits in a number of diseases including transmissible spongiform encephalopathies. Diverse lines of evidence have linked proteoglycans and their glycosaminoglycan chains, and especially heparan sulfate, to the metabolism of the prion protein isoforms. Glypicans are a family of glycosylphosphatidylinositol-anchored, heparan sulfate-containing, cell-associated proteoglycans. Cysteines in glypican-1 can become nitrosylated by endogenously produced nitric oxide. When glypican-1 is exposed to a reducing agent, such as ascorbate, nitric oxide is released and autocatalyses deaminative cleavage of heparan sulfate chains. These processes take place while glypican-1 recycles via a non-classical, caveolin-associated pathway. We have previously demonstrated that prion protein provides the Cu2+ ions required to nitrosylate thiol groups in the core protein of glypican-1. By using confocal immunofluorescence microscopy and immunomagnetic techniques, we now show that copper induces co-internalization of prion protein and glypican-1 from the cell surface to perinuclear compartments. We find that prion protein is controlling both the internalization of glypican-1 and its nitric oxide-dependent autoprocessing. Silencing glypican-1 expression has no effect on copper-stimulated prion protein endocytosis, but in cells expressing a prion protein construct lacking the copper binding domain internalization of glypican-1 is much reduced and autoprocessing is abrogated. We also demonstrate that heparan sulfate chains of glypican-1 are poorly degraded in prion null fibroblasts. The addition of either Cu2+ ions, nitric oxide donors, ascorbate or ectopic expression of prion protein restores heparan sulfate degradation. These results indicate that the interaction between glypican-1 and Cu2+-loaded prion protein is required both for co-internalization and glypican-1 self-pruning.

Prion protein reduces both oxidative and non-oxidative copper toxicity

The prion protein is a membrane tethered glycoprotein that binds copper. Conversion to an abnormal isoform is associated with neurodegenerative diseases known as prion diseases. Expression of the prion protein has been suggested to prevent cell death caused by oxidative stress. Using cell based models we investigated the potential of the prion protein to protect against copper toxicity. Although prion protein expression effectively protected neurones from copper toxicity, this protection was not necessarily associated with reduction in oxidative damage. We also showed that glycine and the prion protein could both protect neuronal cells from oxidative stress. Only the prion protein could protect these cells from the toxicity of copper. In contrast glycine increased copper toxicity without any apparent oxidative stress or lipid peroxidation. Mutational analysis showed that protection by the prion protein was dependent upon the copper binding octameric repeat region. Our findings demonstrate that copper toxicity can be independent of measured oxidative stress and that prion protein expression primarily protects against copper toxicity independently of the mechanism of cell death.

Copper binding is the governing determinant of prion protein turnover

The cellular isoform of the prion protein (PrP(c)) is located at the cell membrane, anchored externally by a glycosylphosphatidylinositol (GPI) anchor. It is a copper (Cu) binding glycoprotein with a rapid basal turnover. Previous studies have shown that exposure of cells to Cu causes internalisation of PrP(c) in vitro. In this study, we show that physiological levels of Cu promote internalisation of PrP(c). Interaction between PrP(c) and Cu was found to be the overriding factor in stimulating the internalisation response with other metals showing no effect. Deletion mutation studies have shown that two domains are essential for copper-induced internalisation to occur. These two domains are the octameric repeat region, encompassing amino acids 51-89, and the palindromic region, amino acids 112-119 with the sequence AGAAAAGA. The decrease in detectable levels of PrP(c) at the cell surface following Cu treatment was found to be the result of rapid internalisation rather than loss into the surrounding environment. These results have implications for both normal metabolism of PrP(c) and the possible mechanism of conversion of PrP(c) to PrP(sc).

BSE and vCJD cause disturbance to uric acid levels

Bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) are two new members of the family of neurodegenerative conditions termed prion diseases. Oxidative damage has been shown to occur in prion diseases and is potentially responsible for the rapid neurodegeneration that is central to the pathogenesis of these diseases. An important nonenzymatic antioxidant in the brain is uric acid. Analysis of uric acid in the brain and cerebrospinal fluid (CSF) of cases of BSE and CJD showed a specific reduction in CSF levels for both BSE and variant CJD, but not sporadic CJD. Further studies based on cell culture experiments suggested that uric acid in the brain was produced by microglia. Uric acid was also shown to inhibit neurotoxicity of a prion protein peptide, production of the abnormal prion protein isoform (PrP(Sc)) by infected cells, and polymerization of recombinant prion protein. These findings suggest that changes in uric acid may aid differential diagnosis of vCJD. Uric acid could be used to inhibit cell death or PrP(Sc) formation in prion disease.