Gene expression profile in prion protein-deficient fibroblasts in culture

The Multifaceted Functions of Prion Protein (PrPC) in Cancer

The cellular prion protein (PrPC) is a glycoprotein anchored to the cell surface by glycosylphosphatidylinositol (GPI). PrPC is expressed both in the brain and in peripheral tissues. Investigations on PrPC's functions revealed its direct involvement in neurodegenerative and prion diseases, as well as in various physiological processes such as anti-oxidative functions, copper homeostasis, trans-membrane signaling, and cell adhesion. Recent findings have revealed the ectopic expression of PrPC in various cancers including gastric, melanoma, breast, colorectal, pancreatic, as well as rare cancers, where PrPC promotes cellular migration and invasion, tumor growth, and metastasis. Through its downstream signaling, PrPC has also been reported to be involved in resistance to chemotherapy and tumor cell apoptosis. This review summarizes the variance of expression of PrPC in different types of cancers and discusses its roles in their development and progression, as well as its use as a potential target to treat such cancers.

Gene Expression Profile Associated with Asmt Knockout-Induced Depression-Like Behaviors and Exercise Effects in Mouse Hypothalamus

Sleep disorder caused by abnormal circadian rhythm is one of the main symptoms and risk factors of depression. As a known hormone regulating circadian rhythms, melatonin (MT) is also namely N-acetyl-5-methoxytryptamine. N-acetylserotonin methyltransferase (Asmt) is the key rate-limiting enzyme of MT synthesis and has been reportedly associated with depression. Although 50–90% of patients with depression have sleep disorders, there are no effective treatment ways in the clinic. Exercise can regulate circadian rhythm and play an important role in depression treatment. In the present study, we showed that Asmt knockout induced depression-like behaviors, which were ameliorated by swimming exercise. Moreover, swimming exercise increased serum levels of MT and 5-hydroxytryptamine (5-HT) in Asmt knockout mice. In addition, the microarray data identified 10 differentially expressed genes (DEGs) in KO mice compared with WT mice and 29 DEGs in KO mice after swimming exercise. Among the DEGs, the direction and magnitude of change in epidermal growth factor receptor pathway substrate 8-like 1 (Eps8l1) and phospholipase C-β 2 (Plcb2) were confirmed by qRT-PCR partly. Subsequent bioinformatic analysis showed that these DEGs were enriched significantly in the p53 signaling pathway, long-term depression and estrogen signaling pathway. In the protein–protein interaction (PPI) networks, membrane palmitoylated protein 1 (Mpp1) and p53-induced death domain protein 1 (Pidd1) were hub genes to participate in the pathological mechanisms of depression and exercise intervention. These findings may provide new targets for the treatment of depression.

Physiological role of Prion Protein in Copper homeostasis and angiogenic mechanisms of endothelial cells

The Prion Protein (PrP) is mostly known for its role in prion diseases, where its misfolding and aggregation can cause fatal neurodegenerative conditions such as the bovine spongiform encephalopathy and human Creutzfeldt–Jakob disease. Physiologically, PrP is involved in several processes including adhesion, proliferation, differentiation and angiogenesis, but the molecular mechanisms behind its role remain unclear. PrP, due to its well-described structure, is known to be able to regulate copper homeostasis; however, copper dyshomeostasis can lead to developmental defects. We investigated PrP-dependent regulation of copper homeostasis in human endothelial cells (HUVEC) using an RNA-interference protocol. PrP knockdown did not influence cell viability in silenced HUVEC (PrPKD) compared to control cells, but significantly increased PrPKD HUVEC cells sensitivity to cytotoxic copper concentrations. A reduction of PrPKD cells reductase activity and copper ions transport capacity was observed. Furthermore, PrPKD-derived spheroids exhibited altered morphogenesis and their derived cells showed a decreased vitality 24 and 48 hours after seeding. PrPKD spheroid-derived cells also showed disrupted tubulogenesis in terms of decreased coverage area, tubule length and total nodes number on matrigel, preserving unaltered VEGF receptors expression levels. Our results highlight PrP physiological role in cellular copper homeostasis and in the angiogenesis of endothelial cells.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

Role of hypoxia‑mediated cellular prion protein functional change in stem cells and potential application in angiogenesis (Review)

Cellular prion protein (PrP^C) can replace other pivotal molecules due to its interaction with several partners in performing a variety of important biological functions that may differ between embryonic and mature stem cells. Recent studies have revealed major advances in elucidating the putative role of PrP^C in the regulation of stem cells and its application in stem cell therapy. What is special about PrP^C is that its expression may be regulated by hypoxia-inducible factor (HIF)-1α, which is the transcriptional factor of cellular response to hypoxia. Hypoxic conditions have been known to drive cellular responses that can enhance cell survival, differentiation and angiogenesis through adaptive processes. Our group recently reported hypoxia-enhanced vascular repair of endothelial colony-forming cells on ischemic injury. Hypoxia-induced AKT/signal transducer and activator of transcription 3 phosphorylation eventually increases neovasculogenesis. In stem cell biology, hypoxia promotes the expression of growth factors. According to other studies, aspects of tissue regeneration and cell function are influenced by hypoxia, which serves an essential role in stem cell HIF-1α signaling. All these data suggest the possibility that hypoxia-mediated PrP^C serves an important role in angiogenesis. Therefore, the present review summarizes the characteristics of PrP^C, which is produced by HIF-1α in hypoxia, as it relates to angiogenesis.

Cellular prion protein (PrPC) in the development of Merlin-deficient tumours

Loss of function mutations in the neurofibromatosis Type 2 (NF2) gene, coding for a tumour suppressor, Merlin, cause multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas. These tumours may occur sporadically or as part of the hereditary condition neurofibromatosis Type 2 (NF2). Current treatment is confined to (radio) surgery and no targeted drug therapies exist. NF2 mutations and/or Merlin inactivation are also seen in other cancers including some mesothelioma, breast cancer, colorectal carcinoma, melanoma and glioblastoma. To study the relationship between Merlin deficiency and tumourigenesis, we have developed an in vitro model comprising human primary schwannoma cells, the most common Merlin-deficient tumour and the hallmark for NF2. Using this model, we show increased expression of cellular prion protein (PrP^C) in schwannoma cells and tissues. In addition, a strong overexpression of PrP^C is observed in human Merlin-deficient mesothelioma cell line TRA and in human Merlin-deficient meningiomas. PrP^C contributes to increased proliferation, cell-matrix adhesion and survival in schwannoma cells acting via 37/67 kDa non-integrin laminin receptor (LR/37/67 kDa) and downstream ERK1/2, PI3K/AKT and FAK signalling pathways. PrP^C protein is also strongly released from schwannoma cells via exosomes and as a free peptide suggesting that it may act in an autocrine and/or paracrine manner. We suggest that PrP^C and its interactor, LR/37/67 kDa, could be potential therapeutic targets for schwannomas and other Merlin-deficient tumours.

Loss of prion protein induces a primed state of type I interferon-responsive genes

The cellular prion protein (PrPC) has been extensively studied because of its pivotal role in prion diseases; however, its functions remain incompletely understood. A unique line of goats has been identified that carries a nonsense mutation that abolishes synthesis of PrPC. In these animals, the PrP-encoding mRNA is rapidly degraded. Goats without PrPC are valuable in re-addressing loss-of-function phenotypes observed in Prnp knockout mice. As PrPC has been ascribed various roles in immune cells, we analyzed transcriptomic responses to loss of PrPC in peripheral blood mononuclear cells (PBMCs) from normal goat kids (n = 8, PRNP+/+) and goat kids without PrPC (n = 8, PRNPTer/Ter) by mRNA sequencing. PBMCs normally express moderate levels of PrPC. The vast majority of genes were similarly expressed in the two groups. However, a curated list of 86 differentially expressed genes delineated the two genotypes. About 70% of these were classified as interferon-responsive genes. In goats without PrPC, the majority of type I interferon-responsive genes were in a primed, modestly upregulated state, with fold changes ranging from 1.4 to 3.7. Among these were ISG15, DDX58 (RIG-1), MX1, MX2, OAS1, OAS2 and DRAM1, all of which have important roles in pathogen defense, cell proliferation, apoptosis, immunomodulation and DNA damage response. Our data suggest that PrPC contributes to the fine-tuning of resting state PBMCs expression level of type I interferon-responsive genes. The molecular mechanism by which this is achieved will be an important topic for further research into PrPC physiology.

Unaltered Prion Pathogenesis in a Mouse Model of High-Fat Diet-Induced Insulin Resistance

Epidemiological, clinical, and experimental animal studies suggest a strong correlation between insulin resistance and Alzheimer’s disease. In fact, type-2 diabetes is considered an important risk factor of developing Alzheimer’s disease. In addition, impaired insulin signaling in the Alzheimer’s disease brain may promote Aβ production, impair Aβ clearance and induce tau hyperphosphorylation, thereby leading to deterioration of the disease. The pathological prion protein, PrP^Sc, deposits in the form of extracellular aggregates and leads to dementia, raising the question as to whether prion pathogenesis may also be affected by insulin resistance. We therefore established high-fat diet-induced insulin resistance in tga20 mice, which overexpress the prion protein. We then inoculated the insulin-resistant mice with prions. We found that insulin resistance in tga20 mice did not affect prion disease progression, PrP^Sc deposition, astrogliosis or microglial activation, and had no effect on survival. Our study demonstrates that in a mouse model, insulin resistance does not significantly contribute to prion pathogenesis.

Behind the Link between Copper and Angiogenesis: Established Mechanisms and an Overview on the Role of Vascular Copper Transport Systems

Angiogenesis critically sustains the progression of both physiological and pathological processes. Copper behaves as an obligatory co-factor throughout the angiogenic signalling cascades, so much so that a deficiency causes neovascularization to abate. Moreover, the progress of several angiogenic pathologies (e.g. diabetes, cardiac hypertrophy and ischaemia) can be tracked by measuring serum copper levels, which are being increasingly investigated as a useful prognostic marker. Accordingly, the therapeutic modulation of body copper has been proven effective in rescuing the pathological angiogenic dysfunctions underlying several disease states. Vascular copper transport systems profoundly influence the activation and execution of angiogenesis, acting as multi-functional regulators of apparently discrete pro-angiogenic pathways. This review concerns the complex relationship among copper-dependent angiogenic factors, copper transporters and common pathological conditions, with an unusual accent on the multi-faceted involvement of the proteins handling vascular copper. Functions regulated by the major copper transport proteins (CTR1 importer, ATP7A efflux pump and metallo-chaperones) include the modulation of endothelial migration and vascular superoxide, known to activate angiogenesis within a narrow concentration range. The potential contribution of prion protein, a controversial regulator of copper homeostasis, is discussed, even though its angiogenic involvement seems to be mainly associated with the modulation of endothelial motility and permeability.

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.

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.

PrP(C) regulates epidermal growth factor receptor function and cell shape dynamics in Neuro2a cells

The prion protein (PrP) plays a key role in prion disease pathogenesis. Although the misfolded and pathologic variant of this protein (PrP(SC)) has been studied in depth, the physiological role of PrP(C) remains elusive and controversial. PrP(C) is a cell-surface glycoprotein involved in multiple cellular functions at the plasma membrane, where it interacts with a myriad of partners and regulates several intracellular signal transduction cascades. However, little is known about the gene expression changes modulated by PrP(C) in animals and in cellular models. In this article, we present PrP(C)-dependent gene expression signature in N2a cells and its implication in the most overrepresented functions: cell cycle, cell growth and proliferation, and maintenance of cell shape. PrP(C) over-expression enhances cell proliferation and cell cycle re-entrance after serum stimulation, while PrP(C) silencing slows down cell cycle progression. In addition, MAP kinase and protein kinase B (AKT) pathway activation are under the regulation of PrP(C) in asynchronous cells and following mitogenic stimulation. These effects are due in part to the modulation of epidermal growth factor receptor (EGFR) by PrP(C) in the plasma membrane, where the two proteins interact in a multimeric complex. We also describe how PrP(C) over-expression modulates filopodia formation by Rho GTPase regulation mainly in an AKT-Cdc42-N-WASP-dependent pathway.

Transcriptomic analysis brings new insight into the biological role of the prion protein during mouse embryogenesis

The biological function of the Prion protein remains largely unknown but recent data revealed its implication in early zebrafish and mammalian embryogenesis. To gain further insight into its biological function, comparative transcriptomic analysis between FVB/N and FVB/N Prnp knockout mice was performed at early embryonic stages. RNAseq analysis revealed the differential expression of 73 and 263 genes at E6.5 and E7.5, respectively. The related metabolic pathways identified in this analysis partially overlap with those described in PrP1 and PrP2 knockdown zebrafish embryos and prion-infected mammalian brains and emphasize a potentially important role for the PrP family genes in early developmental processes.

Proteomic consequences of expression and pathological conversion of the prion protein in inducible neuroblastoma N2a cells

Neurodegenerative diseases are often associated with misfolding and deposition of specific proteins in the nervous system. The prion protein, which is associated with transmissible spongiform encephalopathies (TSEs), is one of them. The normal function of the cellular form of the prion protein (PrP(C)) is mediated through specific signal transduction pathways and is linked to resistance to oxidative stress, neuronal outgrowth and cell survival. In TSEs, PrP(C) is converted into an abnormally folded isoform, called PrP(Sc), that may impair the normal function of the protein and/or generate toxic aggregates. To investigate these molecular events we performed a two-dimensional gel electrophoresis comparison of neuroblastoma N2a cells expressing different amounts of PrP(C) and eventually infected with the 22L prion strain. Mass spectrometry and peptide mass fingerprint analysis identified a series of proteins with modified expression. They included the chaperones Grp78/BiP, protein disulfide-isomerase A6, Grp75 and Hsp60 which had an opposite expression upon PrPC expression and PrP(Sc) production. The detection of these proteins was coherent with the idea that protein misfolding plays an important role in TSEs. Other proteins, such as calreticulin, tubulin, vimentin or the laminin receptor had their expression modified in infected cells, which was reminiscent of previous results. Altogether our data provide molecular information linking PrP expression and misfolding, which could be the basis of further therapeutic and pathophysiological research in this field.

Brain transcriptional stability upon prion protein-encoding gene invalidation in zygotic or adult mouse

BACKGROUND

The physiological function of the prion protein remains largely elusive while its key role in prion infection has been expansively documented. To potentially assess this conundrum, we performed a comparative transcriptomic analysis of the brain of wild-type mice with that of transgenic mice invalidated at this locus either at the zygotic or at the adult stages.

RESULTS

Only subtle transcriptomic differences resulting from the Prnp knockout could be evidenced, beside Prnp itself, in the analyzed adult brains following microarray analysis of 24 109 mouse genes and QPCR assessment of some of the putatively marginally modulated loci. When performed at the adult stage, neuronal Prnp disruption appeared to sequentially induce a response to an oxidative stress and a remodeling of the nervous system. However, these events involved only a limited number of genes, expression levels of which were only slightly modified and not always confirmed by RT-qPCR. If not, the qPCR obtained data suggested even less pronounced differences.

CONCLUSIONS

These results suggest that the physiological function of PrP is redundant at the adult stage or important for only a small subset of the brain cell population under classical breeding conditions. Following its early reported embryonic developmental regulation, this lack of response could also imply that PrP has a more detrimental role during mouse embryogenesis and that potential transient compensatory mechanisms have to be searched for at the time this locus becomes transcriptionally activated.

Cellular prion protein and caveolin-1 interaction in a neuronal cell line precedes Fyn/Erk 1/2 signal transduction

It has been reported that cellular prion protein (PrPc) is enriched in caveolae or caveolae-like domains with caveolin-1 (Cav-1) participating to signal transduction events by Fyn kinase recruitment. By using the Glutathione-S-transferase (GST)-fusion proteins assay, we observed that PrPc strongly interacts in vitro with Cav-1. Thus, we ascertained the PrPc caveolar localization in a hypothalamic neuronal cell line (GN11), by confocal microscopy analysis, flotation on density gradient, and coimmunoprecipitation experiments. Following the anti-PrPc antibody-mediated stimulation of live GN11 cells, we observed that PrPc clustered on plasma membrane domains rich in Cav-1 in which Fyn kinase converged to be activated. After these events, a signaling cascade through p42/44 MAP kinase (Erk 1/2) was triggered, suggesting that following translocations from rafts to caveolae or caveolaelike domains PrPc could interact with Cav-1 and induce signal transduction events.

New insights into cellular prion protein (PrPc) functions: the "ying and yang" of a relevant protein

The conversion of cellular prion protein (PrP(c)), a GPI-anchored protein, into a protease-K-resistant and infective form (generally termed PrP(sc)) is mainly responsible for Transmissible Spongiform Encephalopathies (TSEs), characterized by neuronal degeneration and progressive loss of basic brain functions. Although PrP(c) is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. Recent studies have confirmed its participation in basic physiological processes such as cell proliferation and the regulation of cellular homeostasis. Other studies indicate that PrP(c) interacts with several molecules to activate signaling cascades with a high number of cellular effects. To determine PrP(c) functions, transgenic mouse models have been generated in the last decade. In particular, mice lacking specific domains of the PrP(c) protein have revealed the contribution of these domains to neurodegenerative processes. A dual role of PrP(c) has been shown, since most authors report protective roles for this protein while others describe pro-apoptotic functions. In this review, we summarize new findings on PrP(c) functions, especially those related to neural degeneration and cell signaling.

Function of PrPC (1-OPRD) in biological activities of gastric cancer cell lines

Approximately 10–15% of the human prion disease is inherited and one of the important genetic mutations occurs in the octapeptide repeat region of prion protein gene. One of the variants, one octapeptide repeat deletion (1-OPRD), existed in several gastric cancer cell lines and its mutation frequency was higher in gastric cancer cases. However, the biological functions of it remain unknown. Wild-type and mutation forms of PrP^C were cloned and transfected into gastric cancer cells. Cell apoptosis, adhesion, invasion, multidrug resistance (MDR) and proliferation were, respectively, investigated. Different expressed genes were screened by gene array and proved by PT-PCR. Further, luciferase report assay was used to explore the transcriptional activation of target genes. Forced overexpression PrP^C (1-OPRD) could promote the gastric cancer cells SGC7901 growth through facilitating G1- to S-phase transition in the cell cycle. PrP^C (1-OPRD) could also inhibit apoptosis, and promote adhesion, invasion and MDR in SGC7901. However, it exhibited no significant difference between wild-type PrP^C (1-OPRD) and PrP^C on apoptosis, invasion or MDR effects. Further experiments indicated that PrP^C (1-OPRD) could trigger the transactivation of cyclinD3 besides cyclinD1 to promote cell transition and proliferation. Overexpression of PrP^C (1-OPRD) might promote the proliferation of gastric cancer cells at least partially through transcriptional activation of cyclinD3 to accelerate the G1-/S-phase transition. The promoting proliferation effect of PrP^C (1-OPRD) was more than that of wild-type PrP^C. However, they showed no difference on apoptosis, adhesion, invasion or MDR effects of gastric cancer cells.

[Recent progress in bioinformatics for microarray analysis]

Following the completion of the Human Genome Project in 2003, we were able to clarify the comprehensive profile of the whole human genome on DNA microarray. KeyMolnet is a bioinformatics tool for analyzing molecular interactions on the curated knowledge database. It promotes genome-based drug discovery research aimed at mining the most relevant molecular target to personalized medicine. Multiple sclerosis (MS) is an inflammatory demyelinating disease with a relapsing-remitting clinical course, affecting exclusively the human central nervous system white matter. By DNA microarray, we identified a set of differentially expressed genes in T lymphocytes between MS and healthy subjects, and between acute relapse and complete remission. Hierarchical clustering of discriminator genes established the molecular classification of MS subgroups, associated with the therapeutic response to interferon-beta. The molecular network of the genes involved in development of MS and induction of acute relapse of MS was related to NF-kappaB-regulated gene expression. Prion diseases are an intractable neurodegenerative disease, mediated by an abnormal prion protein PrPSc. The protein conformational conversion from the cellular prion protein PrPC to PrPSc requires an as yet unidentified species-specific chaperone named "Protein X". By protein microarray, we identified a set of novel PrPC interactors, serving as the candidate for X. The molecular network of PrPC and interactors was closely associated with signaling pathways essential for cell survival, differentiation, proliferation and apoptosis. Thus, the molecular network analysis is highly valuable to extract biological implications from a huge amount of microarray data.

The brain proteome profile is highly conserved between Prnp-/- and Prnp+/+ mice

The aim of this study is to compare the proteome of Prnp-/- (Zürich I) gene-ablated mouse brains with wild-type mouse brains. Fluorescence two-dimensional-difference gel electrophoresis (DIGE) and isotope-coded protein labeling (ICPL) were applied for brain homogenates. Similar quantitative protein profiles (<or=1.5-fold change, P<0.05) were obtained by quantitative two-dimensional-DIGE analysis of whole brain homogenates of two developmental stages (days 1 and 67). Complementary ICPL-liquid chromatography-mass spectrometry experiments (n=5) of whole brain homogenates and synaptosomes were performed. The abundance ratios of all proteins quantified did not exceed the statistical spread determined (<2-fold). Therefore, both of the two-dimensional-DIGE and ICPL analyses demonstrated a highly conserved quantitative protein profile in brain homogenates of Prnp-/- and Prnp+/+ mice.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Physiological role of the cellular prion protein (PrPc): protein profiling study in two cell culture systems

The physiological role of the cellular prion protein (PrP (c)) is still not fully understood. Current evidence strongly suggests that PrP (c) overexpression in different cell lines sensitizes cells to apoptotic stimuli through a p53 dependent pathway. On the other hand, an expression of PrP (c) in PrP (c)-deficient cells undergoing apoptosis exhibited repeatedly antiapoptotic effects. Therefore, the presence/absence and/or the level of PrP (c) expression seem to be critical for the fluctuation between PrP (c)'s pro- and antiapoptotic properties. The present study examined whether an overexpression of PrP (c) itself, without addition of any apoptotic agent, can lead to proteome changes that might account for the higher responsiveness to apoptotic stimuli. Beyond this, we examined whether the sole introduction of PrP (c) into PrP (c)-deficient cells could be sufficient to up-regulate antiapoptotic proteins capable of mitigating apoptosis. For this purpose, we used two cell lines, one expressing [human embryonic kidney (HEK) 293 cells] and the other lacking (mouse neuronal PrP (c)-deficient cells) endogenous PrP (c). Protein profiling following transient PrP (c) overexpression in HEK 293 cells revealed a major PrP (c) involvement in regulation of proteins participating in energy metabolism and cellular homeostasis, whereas transient introduction of PrP (c) into mouse neuronal PrP (c)-deficient cells resulted mainly in the regulation of proteins involved in protection against oxidative stress and apoptosis. In addition, we report for the first time that PrP (c) overexpression influenced the regulation of several proteins known to have contributory roles in the pathogenesis of Alzheimer disease (AD). Revealing the correlation between presence/absence and/or different levels of PrP (c) expression with the regulation of certain cellular proteins might further contribute to our understanding of the complex role of PrP (c) in cell physiology.

Protein microarray analysis identifies human cellular prion protein interactors

AIMS

To obtain an insight into the function of cellular prion protein (PrPC), we studied PrPC-interacting proteins (PrPIPs) by analysing a protein microarray.

METHODS

We identified 47 novel PrPIPs by probing an array of 5000 human proteins with recombinant human PrPC spanning amino acid residues 23-231 named PR209.

RESULTS

The great majority of 47 PrPIPs were annotated as proteins involved in the recognition of nucleic acids. Coimmunoprecipitation and cell imaging in a transient expression system validated the interaction of PR209 with neuronal PrPIPs, such as FAM64A, HOXA1, PLK3 and MPG. However, the interaction did not generate proteinase K-resistant proteins. KeyMolnet, a bioinformatics tool for analysing molecular interaction on the curated knowledge database, revealed that the complex molecular network of PrPC and PrPIPs has a significant relationship with AKT, JNK and MAPK signalling pathways.

CONCLUSIONS

Protein microarray is a useful tool for systematic screening and comprehensive profiling of the human PrPC interactome. Because the network of PrPC and interactors involves signalling pathways essential for regulation of cell survival, differentiation, proliferation and apoptosis, these observations suggest a logical hypothesis that dysregulation of the PrPC interactome might induce extensive neurodegeneration in prion diseases.

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.

Cellular prion protein promotes proliferation and G1/S transition of human gastric cancer cells SGC7901 and AGS

The function of cellular prion protein (PrP(C)), the essential protein for the pathogenesis and transmission of prion diseases, is still largely unknown. The putative roles of PrP(C) are thought to be related to cell signaling, survival, and differentiation. In a previous study, we showed that PrP(C) was overexpressed in gastric cancer tissues. In the present report, we show that ectopic expression of PrP(C) could promote tumorigenesis, proliferation, and G1/S transition in gastric cancer cells. Furthermore, CyclinD1, a protein related to cell cycle, was shown to be significantly up-regulated by PrP(C) at both mRNA and protein levels. PI3K/Akt pathway mediated above PrP(C) signal since PrP(C) increased the expression of phosphorylated Akt, and the specific inhibitor of Akt, LY294002, could markedly suppress growth of SGC7901 and transactivation of CyclinD1 induced by PrP(C). Octapeptide repeat region played a vital role in this function, as deletion of this region abolished or reduced these effects. Collectively, this study demonstrates that overexpression of PrP(C) might promote the tumorigenesis and proliferation of gastric cancer cells at least partially through activation of PI3K/Akt pathway and subsequent transcriptional activation of CyclinD1 to regulate the G1/S phase transition, in which octapeptide repeat region might be an indispensable region.

Cellular prion protein promotes invasion and metastasis of gastric cancer

Cellular prion protein (PrPc) is a glycosylphosphatidylinositol (GPI) -anchored membrane protein that is highly conserved in mammalian species. PrPc has the characteristics of adhesive molecules and is thought to play a role in cell adhesion and membrane signaling. Here we investigated the possible role of PrPc in the process of invasiveness and metastasis in gastric cancers. PrPc was found to be highly expressed in metastatic gastric cancers compared to nonmetastatic ones by immunohistochemical staining. PrPc significantly promoted the adhesive, invasive, and in vivo metastatic abilities of gastric cancer cell lines SGC7901 and MKN45. PrPc also increased promoter activity and the expression of MMP11 by activating phosphorylated ErK1/2 in gastric cancer cells. MEK inhibitor PD98059 and MMP11 antibody (Ab) significantly inhibited in vitro invasive and in vivo metastatic abilities induced by PrPc. N-terminal fragment (amino acid 24-90) was suggested to be an indispensable region for signal transduction and invasion-promoting function of PrPc. Taken together, the present work revealed a novel function of PrPc that the existence of N-terminal region of PrPc could promote the invasive and metastatic abilities of gastric cancer cells at least partially through activation of MEK/ERK pathway and consequent transactivation of MMP11.

Cell expression of a four extra octarepeat mutated PrPCmodifies cell structure and cell cycle regulation

RK13 cell lines generated to express bovine PrP(C) with a four extra octarepeat insertional mutation (Bo-10ORPrP(C)) show partially insoluble PrP(C) and lower rates of cell growth when compared to either the same cells expressing wild type Bo-6ORPrP(C) or the original RK13 cell line. The expression of Bo-10ORPrP(C) in cell cultures was also associated with changes in cell size and reorganization of the actin cytoskeleton. This last process was reversed by Clostridium difficile toxin-B, a specific inhibitor of small GTPase proteins. Further, in clones expressing Bo-10ORPrP(C), increased proportions of cells at cell cycle stage G2/M were observed. Proteasome inhibitors caused a further expansion of G2/M-stage cells that was more marked in cell lines expressing Bo-10ORPrP(C) than those expressing Bo-6ORPrP(C), while this effect was minimal or null in the original RK13 cell line. Hence, the presence of Bo-10ORPrP(C) in RK13 cells promotes cell cycle arrest at G2/M, and the effect is amplified by proteasome inhibition. These findings suggest a role for PrP(C) in cell morphology and cell cycle regulation, and open new avenues for understanding the mechanisms underlying PrP mutation-associated diseases.

Comparative genomic organization of the human and bovine PRNP locus

We sequenced a 208-kb BAC clone spanning the bovine prion protein (PRNP) locus, and compared the genomic structure with that of human. As a result, we determined the precise breakpoint between the two syntenic genomes, located on the 5' UTR of the PRNP gene, and discovered two highly repetitive sequences near the breakpoint. Further analysis demonstrated that the genomic structure of three genes, PRNP, PRND, and RASSF2, within the syntenic region of the bovine genome is highly conserved in order and orientation. The PRNT locus was not found in bovine but is conserved in several primates, including human. Moreover, we confirmed that the bovine RASSF2 is composed of 10 exons, as is the human gene, showing some difference from a previous report. Our findings may provide useful clues for understanding the evolutional process in the PRNP locus and also the mechanism that allows TSE from cattle to infect humans.

A neuronal cell line that does not express either prion or doppel proteins

Prions have been extensively studied since they represent a new class of infectious agents, the pathogenic prion protein (PrPSc). However, a central question on the physiological function of the normal prion protein (PrPC) remains unresolved. A cell model which was previously established from Rikn mice (PrP-/-) remains problematic because of its ectopic expression of the doppel (Dpl) which may have a neurotoxic effect. Here we established neuronal cell lines from Zürich I (PrP-/-) which do not express Dpl protein and ICR mice (PrP+/+) by transfecting with plasmid encoding for the large T antigen of SV40. The transformed cells have shown neuronal characteristics and, thus, these cell lines may provide a useful model to explore the function of neuronal PrPC.

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.

Gene Expression Profile Following Stable Expression of the Cellular Prion Protein

1. The cellular prion protein (PrPC) is expressed widely in neural and nonneural tissues at the highest level in neurons in the central nervous system (CNS). 2. Recent studies indicated that transgenic mice with the cytoplasmic accumulation of PrPC exhibited extensive neurodegeneration in the cerebellum, although the underlying mechanism remains unknown. To identify the genes whose expression is controlled by over-expression of PrPC in human cells, we have established a stable PrPC-expressing HEK293 cell line designated P1 by the site-specific recombination technique. 3. Microarray analysis identified 33 genes expressed differentially between P1 and the parent PrPC-non-expressing cell line among 12,814 genes examined. They included 18 genes involved in neuronal and glial functions, 5 related to production of extracellular matrix proteins, and 2 located in the complement cascade. 4. Northern blot analysis verified marked upregulation in P1 of the brain-specific protein phosphatase 2A beta subunit (PPP2R2B), a causative gene of spinocerebellar ataxia 12, and the cerebellar degeneration-related autoantigen (CDR34) gene associated with development of paraneoplastic cerebellar degeneration. 5. These results indicate that accumulation of PrPC in the cell caused aberrant regulation of a battery of the genes important for specific neuronal function. This represents a possible mechanism underlying PrPC-mediated selective neurodegeneration.

Quantitative analysis of the cardiac fibroblast transcriptome-implications for NO/cGMP signaling

Cardiac fibroblasts regulate tissue repair and remodeling in the heart. To quantify transcript levels in these cells we performed a comprehensive gene expression study using serial analysis of gene expression (SAGE). Among 110,169 sequenced tags we could identify 30,507 unique transcripts. A comparison of SAGE data from cardiac fibroblasts with data derived from total mouse heart revealed a number of fibroblast-specific genes. Cardiac fibroblasts expressed a specific collection of collagens, matrix proteins and metalloproteinases, growth factors, and components of signaling pathways. The NO/cGMP signaling pathway was represented by the mRNAs for alpha(1) and beta(1) subunits of guanylyl cyclase, cGMP-dependent protein kinase type I (cGK I), and, interestingly, the G-kinase-anchoring protein GKAP42. The expression of cGK I was verified by RT-PCR and Western blot. To establish a functional role for cGK I in cardiac fibroblasts we studied its effect on cell proliferation. Selective activation of cGK I with a cGMP analog inhibited the proliferation of serum-stimulated cardiac fibroblasts, which express cGK I, but not higher passage fibroblasts, which contain no detectable cGK I. Currently, our data suggest that cGK I mediates the inhibitory effects of the NO/cGMP pathway on cardiac fibroblast growth. Furthermore the SAGE library of transcripts expressed in cardiac fibroblasts provides a basis for future investigations into the pathological regulatory mechanisms underlying cardiac fibrosis.

Ageing and exposure to oxidative stress in vivo differentially affect cellular levels of PrP in mouse cerebral microvessels and brain parenchyma

The biological function of cellular prion protein PrPc has not been established, despite in vitro studies suggesting antioxidant activity or link to signal transduction pathways. In this study, mice were exposed to hyperoxia to establish whether oxidative stress affected prion expression in vivo. C57Bl/6J mice aged 6, 18, and 24 months, maintained under normoxic conditions, exhibited age-related increases in PrPc in both cerebral microvessels and in microvessel-depleted brain homogenate. We demonstrate that PrPc is differentially affected by exposure to hyperoxia in vivo for 1 (24 h) or 2 (48 h) days, or for 1 day hyperoxia, followed by 1 day normoxia. Brain parenchymal cells from 6-month-old mice exposed to 1 day hyperoxia showed elevation of a glycosylated approximately 36 kDa form, whereas in 24-month-old mice cellular prion level was substantially reduced. Extending hyperoxia from 1 to 2 days resulted in significantly reduced PrPc level, regardless of age. Parenchymal PrPc is substantially elevated in 6-month-old mice, but declines in 18- and 24-month-old animals following 1 day hyperoxia. By contrast, PrPc content in cerebral microvessels from 6-month-old mice declined after a 2 day exposure to hyperoxia, while microvessels from 24-month-old brains showed elevated prion levels 24 h after hyperoxia. Moreover, unglycosylated 25-30 kDa PrPc, and a previously undescribed 50-64 kDa band containing at least some glycosylated protein, predominated in microvessels with lesser content of the glycosylated approximately 36 kDa form. Cellular content of these unglycosylated forms was correlated with age, while the response to hyperoxia was evident in both unglycosylated and glycosylated forms of the protein following 1 and 2 day exposures. The observed elevation of the 25-30 and 50-64 kDa bands of microvessel PrPc is not sustainable following 1 day hyperoxia, but returns to near normoxic levels within 24 h after hyperoxia. We also show in a knockout mouse for methionine sulfoxide reductase (MsrA), the enzyme responsible for reducing methionine sulfoxide back to methionine, and a regulator of cellular antioxidant defence, that following hyperoxia brain PrPc in the null mutant is elevated relative to PrPc content in the parent strain. Our results show up-regulated PrPc expression or reduced turnover in response to age-related, and hyperoxia-induced oxidative stress.

EAAT4 mRNA expression is preserved in the cerebellum of prion protein-deficient mice

To study the mechanism underlying the selective degeneration of Purkinje cells in the cerebellum of the Nagasaki (Ngsk) prion protein-deficient (PrP(-/-)) mice, the mRNA levels of glutamate transporter EAAT4, the marker highly specific for Purkinje cell synapses, were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. EAAT4 mRNA was expressed in the cerebellum of PrP(-/-) mice presenting with cerebellar ataxia, at the levels identical to those in the cerebellum of non-ataxic PrP(+/-) mice. Furthermore, EAAT4 mRNA was identified in the cerebrum of both PrP(-/-) and PrP(+/-) mice, although its levels were much lower than those in the cerebellum. These results indicate that Purkinje cell degeneration found in the cerebellum of PrP(-/-) mice is not primarily caused by glutamate neurotoxicity, although it remains to be investigated whether preserved expression of EAAT4 might represent a compensatory mechanism for protecting against Purkinje cell degeneration in the PrP(-/-) mice cerebellum.

M129V variation in the prion protein may influence cognitive performance

Correlations between general intelligence (g) and brain volume are about 0.40, and the correlation between g and white matter volume has been reported to be largely due to genetic factors. Establishing that the correlation between brain volumes and cognitive abilities is mediated by shared genetic factors is only the first step in unveiling the relation between them. We have recently shown that methionine at codon 129 in the prion protein is associated with white matter reduction in a group of healthy volunteers and schizophrenic patients. The present study examines the influence of the same genetic variation on psychometric cognitive performance measurements in 335 community-based healthy volunteers. The polymorphism was associated with Full Scale IQ (genotype: F=4.38, df=2/317, P=0.013; allele: F=8.04, df=1/658, P=0.005), as measured by HAWIE-R (German version of the Wechsler Adult Intelligence Scale, Revised). Genotype accounted for 2.7% of the total variability in Full Scale IQ (partial eta2=0.027). An exploratory analysis revealed association with several HAWIE-R subscales; the association with the Digit Symbol subtest remained significant after correction for multiple testing. In summary, we deliver evidence for an association of a common genetic variation in the prion protein gene with cognitive performance. However, independent replications are needed before firm conclusions can be drawn.

Involvement of caveolae and caveolae-like domains in signalling, cell survival and angiogenesis

Caveolae, the flask-shaped membrane invaginations abundant in endothelial cells, have acquired a prominent role in signal transduction. Evidence, that events occurring in caveolae participate in cell survival and angiogenesis, has been recently substantiated by the identification of two novel caveolar constituents: prostacyclin synthase (PGIS) and the cellular form of prion protein (PrP(c)). We have shown that PGIS, previously described as an endoplasmic reticulum component, is bound to caveolin-1 (cav-1) and localized in caveolae in human endothelial cells. By generating prostacyclin, PGIS is involved in angiogenesis. Previous observations regarding the localization of PrP(c) in caveolae-like membrane domains (CLDs) have been recently confirmed and extended. It has been demonstrated that PrP(c) is bound to cav-1 and, by recruiting Fyn kinase, can participate in signal transduction events connected to cell survival and differentiation. The new entries of PGIS and PrP(c) in caveolar components place caveolae and CLDs at the centre of a network, where cells decide whether to proliferate or differentiate and whether to survive or to suicide by apoptosis.

Methionine homozygosity at codon 129 in the prion protein is associated with white matter reduction and enlargement of CSF compartments in healthy volunteers and schizophrenic patients

Twin studies point toward a substantial heritability in individual variations in the size of the human brain. However, the etiology is largely unknown. The prion protein (gene name: PRNP) aids cellular resistance to oxidative stress and neurodegeneration and is involved in neurodevelopment. This study examines the influence of a polymorphism in the PRNP gene on brain morphology in 47 healthy males and 43 male schizophrenic patients. All subjects underwent identical MRI scanning sessions followed by segmentation in cerebrospinal fluid (CSF), gray and white matter tissue, and genotyping for a biallelic polymorphism in PRNP (Met129Val). Genotype and allele frequencies did not differ between schizophrenic patients and controls but the polymorphism was associated with white matter tissue reduction (P = 0.024) and enlargement of CSF compartments (P = 0.039). These findings suggest that homozygosity for methionine at codon 129 is associated with decreased white matter tissue and larger CSF volume in right-handed male healthy volunteers and schizophrenic patients. This, however, being a novel finding, should warrant further investigation.

The normal cellular prion protein is strongly expressed by myeloid dendritic cells

Abnormal isoforms of the prion protein (PrP(Sc)) that cause prion diseases are propagated and spread within the body by "carrier" cell(s). Cells of the immune system have been strongly implicated in this process. In particular, PrP(Sc) is known to accumulate on follicular dendritic cells (FDCs) in individuals affected by variant Creutzfeld-Jakob disease. However, FDCs do not migrate widely and the natural history of prion disorders suggests other cells may be required for the transport of PrP(Sc) from the site of ingestion to lymphoid organs and the central nervous system. Substantial evidence suggests that the spread of PrP(Sc) requires bone marrow-derived cells that express normal cellular prion protein (PrP(C)). This study examined the expression of PrP(C) on bone marrow-derived cells that interact with lymphoid follicles. High levels of PrP(C) are present on myeloid dendritic cells (DCs) that surround the splenic white pulp. These myeloid DCs are ontologically and functionally distinct from the FDCs. Consistent with these observations, expression of PrP(C) was strongly induced during the generation of mature myeloid DCs in vitro. In these cells PrP(C) colocalized with major histocompatibility complex class II molecules at the level of light microscopy. Furthermore, given the close anatomic and functional connection of myeloid DCs with lymphoid follicles, these results raise the possibility that myeloid DCs may play a role in the propagation of PrP(Sc) in humans.