Specific inhibition of pathological prion protein accumulation by small interfering RNAs

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part II: strategies for therapeutics development

Introduction: The cellular prion protein (PrP^C), some of its derivatives (especially PrP N-terminal N1 peptide and shed PrP), and PrP^C-containing exosomes have strong neuroprotective activities, which have been reviewed in the companion article (Part I) and are briefly summarized here.Areas covered: We propose that elevating the extracellular levels of a protective PrP form using gene therapy and other approaches is a very promising novel avenue for prophylactic and therapeutic treatments against prion disease, Alzheimer's disease, and several other neurodegenerative diseases. We will dissect the pros and cons of various potential PrP-based treatment options and propose a few strategies that are more likely to succeed. The cited references were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.Expert opinion: Concurrent knockdown of celllular PrP expression and elevation of the extracellular levels of a neuroprotective PrP N-terminal peptide via optimized gene therapy vectors is a highly promising broad-spectrum prophylactic and therapeutic strategy against several neurodegenerative diseases, including prion diseases, Alzheimer's disease and Parkinson's disease.

PrPC knockdown by liposome-siRNA-peptide complexes (LSPCs) prolongs survival and normal behavior of prion-infected mice immunotolerant to treatment

Prion diseases are members of neurodegenerative protein misfolding diseases (NPMDs) that include Alzheimer's, Parkinson's and Huntington diseases, amyotrophic lateral sclerosis, tauopathies, traumatic brain injuries, and chronic traumatic encephalopathies. No known therapeutics extend survival or improve quality of life of humans afflicted with prion disease. We and others developed a new approach to NPMD therapy based on reducing the amount of the normal, host-encoded protein available as substrate for misfolding into pathologic forms, using RNA interference, a catabolic pathway that decreases levels of mRNA encoding a particular protein. We developed a therapeutic delivery system consisting of small interfering RNA (siRNA) complexed to liposomes and addressed to the central nervous system using a targeting peptide derived from rabies virus glycoprotein. These liposome-siRNA-peptide complexes (LSPCs) cross the blood-brain barrier and deliver PrP siRNA to neuronal cells to decrease expression of the normal cellular prion protein, PrPC, which acts as a substrate for prion replication. Here we show that LSPCs can extend survival and improve behavior of prion-infected mice that remain immunotolerant to treatment. LSPC treatment may be a viable therapy for prion and other NPMDs that can improve the quality of life of patients at terminal disease stages.

Anti-prion Protein Antibody 6D11 Restores Cellular Proteostasis of Prion Protein Through Disrupting Recycling Propagation of PrPSc and Targeting PrPSc for Lysosomal Degradation

PrP^Sc is an infectious and disease-specific conformer of the prion protein, which accumulation in the CNS underlies the pathology of prion diseases. PrP^Sc replicates by binding to the cellular conformer of the prion protein (PrP^C) expressed by host cells and rendering its secondary structure a likeness of itself. PrP^C is a plasma membrane anchored protein, which constitutively recirculates between the cell surface and the endocytic compartment. Since PrP^Sc engages PrP^C along this trafficking pathway, its replication process is often referred to as "recycling propagation." Certain monoclonal antibodies (mAbs) directed against prion protein can abrogate the presence of PrP^Sc from prion-infected cells. However, the precise mechanism(s) underlying their therapeutic propensities remains obscure. Using N2A murine neuroblastoma cell line stably infected with 22L mouse-adapted scrapie strain (N2A/22L), we investigated here the modus operandi of the 6D11 clone, which was raised against the PrP^Sc conformer and has been shown to permanently clear prion-infected cells from PrP^Sc presence. We determined that 6D11 mAb engages and sequesters PrP^C and PrP^Sc at the cell surface. PrP^C/6D11 and PrP^Sc/6D11 complexes are then endocytosed from the plasma membrane and are directed to lysosomes, therefore precluding recirculation of nascent PrP^Sc back to the cell surface. Targeting PrP^Sc by 6D11 mAb to the lysosomal compartment facilitates its proteolysis and eventually shifts the balance between PrP^Sc formation and degradation. Ongoing translation of PrP^C allows maintaining the steady-state level of prion protein within the cells, which was not depleted under 6D11 mAb treatment. Our findings demonstrate that through disrupting recycling propagation of PrP^Sc and promoting its degradation, 6D11 mAb restores cellular proteostasis of prion protein.

Dual MicroRNA to Cellular Prion Protein Inhibits Propagation of Pathogenic Prion Protein in Cultured Cells

Prion diseases are fatal transmissible neurodegenerative disorders affecting humans and various mammals. In spite of intensive efforts, there is no effective cure or treatment for prion diseases. Cellular forms of prion protein (PrP^C) is essential for propagation of abnormal isoforms of prion protein (PrP^Sc) and pathogenesis. The effect of an artificial dual microRNA (DmiR) on PrP^C suppression and resultant inhibition of prion replication was determined using prion-infectible cell cultures: differentiated C2C12 culture and primary mixed neuronal and glial cells culture (MNGC). Processing of DmiR by prion-susceptible myotubes, but not by reserve cells, in differentiated C2C12 culture slowed prion replication, implying an importance of cell type-specific PrP^C targeting. In MNGC, reduction of PrP^C with DmiR was effective for suppressing prion replication. MNGC lentivirally transduced with non-targeting control miRNAs (scrambled) reduced prion replication at a level similar to that with a synthetic analogue of viral RNA, poly I:C. The results suggest that a synergistic combination of the immunostimulatory RNA duplexes (miRNA) and PrP^C silencing with DmiR might augment a therapeutic potential of RNA interference.

Treatment of SMB-S15 Cells with Resveratrol Efficiently Removes the PrP(Sc) Accumulation In Vitro and Prion Infectivity In Vivo

Prion diseases are transmissible and invariably fatal neurodegenerative disorders, which still lack of efficacious prophylactic and therapeutic tools. Our previous study has proposed that the natural phytoalexin, resveratrol, can reduce the amounts of PrP(Sc) in a scrapie-infected cell line SMB-S15. To address its anti-prion efficacy, the inhibitive activity of resveratrol on prion accumulation in vitro and prion infectivity in vivo was analyzed in the present study. Exposure of SMB-S15 cells to various concentrations of resveratrol (0.25 to 200 μM) reduced and even removed cellular PrP(Sc) in a dose-dependent manner, with EC50 0.61 μM. Meanwhile, PrP(Sc) signals in SMB-S15 cells treated with 5 and 10 μM resveratrol maintained undetectable after drug withdrawal, indicating that the removal of PrP(Sc) in SMB-S15 cells by resveratrol is irreversible. Furthermore, the lysates of SMB-S15 cells exposed to 10 μM resveratrol for 2 and 7 days were intracerebrally inoculated into CD1 mice. All mice (n = 9) infected with SMB-S15 cells without treatment of resveratrol appeared typical experimental scrapie symptoms from 155 to 228 day post inoculation (dpi), while all mice (n = 9) inoculated with SMB-S15 cells treated with resveratrol for 7 days maintained healthy by the end of observations (284 dpi). PrP-specific Western blots and neuropathological tests did not identify PrP(Sc) or prion disease-associated pathological abnormality in the brains of mice inoculated with 7-day resveratrol-treated SMB-S15 cells. It indicates that the prion infectivity of SMB-S15 onto CD1 mice is eradicated by 1-week resveratrol treatment. Sensitivity of PrP(Sc) to resveratrol highlights its potential role in prion therapeutics.

Therapeutic approaches for prion disorders

Prion diseases are lethal for both humans and animals, and affected individuals die after several months following a rapid disease progression. Although researchers have attempted for decades to develop effective therapeutics for the therapy of human prion disorders, until now no efficient drug has been available on the market for transmissible spongiform encephalopathy (TSE) treatment or cure. Approximately 200 patients worldwide have died or suffer from variant Creutzfeldt-Jakob disease (CJD). Incidences for sporadic and familial CJD are approximately 1.5-2 per million per year and one per 10 million per year, respectively, in Europe. This review summarizes classical and modern trials for the development of effective anti-TSE drugs, introduces potential effective delivery systems, such as lentiviral and adeno-associated virus systems for antiprion components, including antibodies and siRNAs, and presents vaccination trials. Most of the antiprion drugs target prion protein PrP(c) and/or PrP(Sc). Alternative targets are receptors and coreceptors for PrP, that is, the 37/67-kDa laminin receptor and heparan sulfate proteoglycanes. We review clinical trials for the treatment of TSEs and describe hindrances and chances for a breakthrough in therapy of prion disorders.

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.

Modelling neurodegeneration in prion disease - applications for drug development

Prion diseases are a group of neurodegenerative diseases that affect mammals, including humans and ruminants such as sheep. They are believed to be caused by the conversion of the prion protein (PrP), a host expressed protein, into a toxic form (PrP(sc)). PrP(sc) accumulates in the brain, resulting in neuronal loss and the typical spongiform appearance of the brain. So far, there are no effective therapies available for prion diseases. This review discusses possible therapies for prion diseases and the models available for advancing research into the disease.

Increased locomotor activity and non-selective attention and impaired learning ability in SD rats after lentiviral vector-mediated RNA interference of Homer 1a in the brain

Our previous studies found that Homer 1a, a scaffolding protein localized at the post-synaptic density (PSD) of glutamatergic excitatory synapses, is significantly down-regulated in the brain of spontaneous hypertensive rats (SHR), an animal model of attention deficit hyperactivity disorder (ADHD). Furthermore, a first-line treatment drug for ADHD, methylphenidate, can up-regulate the expression of Homer 1a. To investigate the possible role of Homer 1a in the etiology and pathogenesis of ADHD, a lentiviral vector containing miRNA specific for Homer 1a was constructed in this study. Intracerebroventricular injection of this vector into the brain of Sprague Dawley (SD) rats significantly decreased Homer 1a mRNA and protein expression levels. Compared to their negative controls, these rats displayed a range of abnormal behaviors, including increased locomotor activity and non-selective attention and impaired learning ability. Our results indicated that Homer 1a down-regulation results in deficits in control over behavioral output and learning similar to ADHD.

Anti-prion activity of Brilliant Blue G

Background

Prion diseases are fatal neurodegenerative disorders with no effective therapy currently available. Accumulating evidence has implicated over-activation of P2X7 ionotropic purinergic receptor (P2X7R) in the progression of neuronal loss in several neurodegenerative diseases. This has led to the speculation that simultaneous blockade of this receptor and prion replication can be an effective therapeutic strategy for prion diseases. We have focused on Brilliant Blue G (BBG), a well-known P2X7R antagonist, possessing a chemical structure expected to confer anti-prion activity and examined its inhibitory effect on the accumulation of pathogenic isoforms of prion protein (PrPres) in a cellular and a mouse model of prion disease in order to determine its therapeutic potential.

Principal Findings

BBG prevented PrPres accumulation in infected MG20 microglial and N2a neural cells at 50% inhibitory concentrations of 14.6 and 3.2 µM, respectively. Administration of BBG in vivo also reduced PrPres accumulation in the brains of mice with prion disease. However, it did not appear to alleviate the disease progression compared to the vehicle-treated controls, implying a complex role of P2X7R on the neuronal degeneration in prion diseases.

Significance

These results provide novel insights into the pathophysiology of prion diseases and have important implications for the treatment.

Intracerebral Infusion of Antisense Oligonucleotides Into Prion-infected Mice

Mice deficient for the cellular prion protein (PrP^C) do not develop prion disease; accordingly, gene-based strategies to diminish PrP^C expression are of interest. We synthesized a series of chemically modified antisense oligonucleotides (ASOs) targeted against mouse Prnp messenger RNA (mRNA) and identified those that were most effective in decreasing PrP^C expression. Those ASOs were also evaluated in scrapie-infected cultured cells (ScN2a) for their efficacy in diminishing the levels of the disease-causing prion protein (PrP^Sc). When the optimal ASO was infused intracerebrally into FVB mice over a 14-day period beginning 1 day after infection with the Rocky Mountain Laboratory (RML) strain of mouse prions, a prolongation of the incubation period of almost 2 months was observed. Whether ASOs can be used to develop an effective therapy for patients dying of Creutzfeldt–Jakob disease remains to be established.

Oligomeric-induced activity by thienyl pyrimidine compounds traps prion infectivity

Accumulation of PrP(Sc), an abnormal form of cellular prion protein (PrP), in the brain of animals and humans leads to fatal neurodegenerative disorders known as prion diseases. Limited protease digestion of PrP(Sc) produces a truncated form called PrP(27-30) that retains prion infectivity and is the main marker of disease targeted in most diagnostic tests. In the search for new anti-prion molecules, drug-screening assays on prion-infected murine cells have been oriented toward decreasing levels of PrP(27-30). In contrast, we screened for drugs promoting multimers of PrP(27-30), illustrating a possible stabilization of mouse PrP(Sc) species, because recent studies aiming to characterize the conformational stability of various prion strains showed that stable recombinant amyloids produced more stable prion strain, leading to longest incubation time. We identified a family of thienyl pyrimidine derivatives that induce SDS-resistant dimers and trimers of PrP(27-30). Bioassays performed on mice brain homogenates treated with these compounds showed that these thienyl pyrimidine derivatives diminished prion infectivity in vivo. Oligomeric-induced activity by thienyl pyrimidine compounds is a promising approach not only to understanding the pathogenesis of prions but also for prion diagnostics. This approach could be extended to other neurodegenerative "prionopathies," such as Alzheimer's, Huntington, or Parkinson's diseases.

Diphenylpyrazole-derived compounds increase survival time of mice after prion infection

Transmissible spongiform encephalopathies (TSEs) represent a group of fatal neurodegenerative disorders that can be transmitted by natural infection or inoculation. TSEs include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. The emergence of a variant form of CJD (vCJD), which has been associated with BSE, produced strong pressure to search for effective treatments with new drugs. Up to now, however, TSEs have proved incurable, although many efforts have been made both in vitro and in vivo to search for potent therapeutic and prophylactic compounds. For this purpose, we analyzed a compound library consisting of 10,000 compounds with a cell-based high-throughput screening assay dealing with scrapie-infected scrapie mouse brain and ScN(2)A cells and identified a new class of inhibitors consisting of 3,5-diphenylpyrazole (DPP) derivatives. The most effective DPP derivative showed half-maximal inhibition of PrP(Sc) formation at concentrations (IC(50)) of 0.6 and 1.2 μM, respectively. This compound was subsequently subjected to a number of animal experiments using scrapie-infected wild-type C57BL/6 and transgenic Tga20 mice. The DPP derivative induced a significant increase of incubation time both in therapeutic and prophylactic experiments. The onset of the prion disease was delayed by 37 days after intraperitoneal and 42 days after oral application, respectively. In summary, we demonstrate a high in vitro efficiency of DPP derivatives against prion infections that was substantiated in vivo for one of these compounds. These results indicate that the novel class of DPP compounds should comprise excellent candidates for future therapeutic studies.

From high-throughput cell culture screening to mouse model: identification of new inhibitor classes against prion disease

Transmissible spongiform encephalopathies (TSE) or prion diseases belong to a category of fatal and so far untreatable neurodegenerative conditions. All prion diseases are characterized by both degeneration in the central nervous system (CNS) in humans and animals and the deposition and accumulation of Proteinase K-resistant prion protein (PrP(res)). Until now, no pharmaceutical product has been available to cure these diseases or to alleviate their associated symptoms. Here, a cell-culture screening system is described that allows for the large-scale analysis of the PrP(res) inhibitory potential of a library of compounds and the identification of structural motifs leading potent compounds able to cause PrP(res) clearance at the cellular level. Based on different scrapie-infected cell lines, 10,000 substances were tested, out of which 530 potential inhibitors were identified. After re-screening and validation using a series of dilutions, 14 compounds were identified as the most effective. These 14 compounds were then used for therapeutic studies in a mouse bioassay to test and verify their in vivo potency. Two compounds exhibited therapeutic potential in the mouse model by significantly extending the survival time of intracerebrally infected mice, when treated 90 days after infection with scrapie.

HEPES inhibits the conversion of prion protein in cell culture

HEPES is a well-known buffering reagent used in cell-culture medium. Interestingly, this compound is also responsible for significant modifications of biological parameters such as uptake of organic molecules, alteration of oxidative stress mechanisms or inhibition of ion channels. While using cell-culture medium supplemented with HEPES on prion-infected cells, it was noticed that there was a significant concentration-dependent inhibition of accumulation of the abnormal isoform of the prion protein (PrP(Sc)). This effect was present only in live cells and was thought to be related to modification of the PrP environment or biology. These results could modify the interpretation of cell-culture assays of prion therapeutic agents, as well as of previous cell biology results obtained in the field using HEPES buffers. This inhibitory effect of HEPES could also be exploited to prevent contamination or propagation of prions in cell culture.

Mouse neuronal cells expressing exogenous bovine PRNP and simultaneous downregulation of endogenous mouse PRNP using siRNAs

Prion diseases, which are called transmissible spongiform encephalopathies (TSEs), comprise a group of fatal infectious neurodegenerative disorders. Investigation of prion strains and generation of species dependent TSE model are necessary to understand pathogenesis of the disease. To establish a BSE-specific in vitro cell culture model, N2a and GT1 mouse neuronal cell lines were generated to express the bovine prion protein by transfection of the bovine prion gene (Prnp). In addition, the endogenous mouse prion protein was suppressed in N2a, NbP, GT1 and GbP cell lines using the siRNA duplexes, siRNA1 and siRNA2 that target the N- and C-termini of murine Prnp, respectively. Both siRNA1 and siRNA2 effectively decreased murine prion protein levels by more than 80% and the downregulation efficacy was increased in siRNA dose-dependent manner. The greatest downregulation was observed 48 h after siRNA delivery. The moPrnp knockdown NbP and GbP cell lines and the Prnp-targeting siRNA technique established in the present study would be useful tools for dissecting the basic mechanisms of prion infection, especially for BSE.

Applications of RNA interference-based gene silencing in animal agriculture

Classical genetic selection, recently aided by genomic selection tools, has been successful in achieving remarkable progress in livestock improvement. However, genetic selection has led to decreased genetic diversity and, in some cases, acquisition of undesirable traits. In order to meet the increased demands of our expanding population, new technologies and practices must be developed that contend with zoonotic and animal disease, environmental impacts of large farming operations and the increased food and fibre production needed to feed and clothe our society. Future increases in productivity may be dependent upon the acquisition of genetic traits not currently encoded by the genomes of animals used in standard agricultural practice, thus making classical genetic selection impossible. Genetic engineering of livestock is commonly used to produce pharmaceuticals or to impart enhanced production characteristics to animals, but has also demonstrated its usefulness in producing animals with disease resistance. However, significant challenges remain because it has been more difficult to produce animals in which specific genes have been removed. It is now possible to modify livestock genomes to block expression of endogenous and exogenous genes (such as those expressed following virus infection). In the present review, we discuss mechanisms of silencing gene expression via the biology of RNA interference (RNAi), the technology of activating the RNAi pathway and the application of this technology to enhance livestock production through increased production efficiency and prevention of disease. An increased demand for sustainable food production is at the forefront of scientific challenges and RNAi technology will undoubtedly play a key role.

Downregulation of lentivirus-mediated ILK RNAi on tractional force generation in human retinal Müller cells

AIM

To investigate the effect of lentivirus-mediated integrin-linked kinase (ILK) RNA interference (RNAi) on human retinal Müller cells transdifferentiation into contractile myofibroblasts.

METHODS

A lentiviral vector expressing ILK-specific shRNA was constructed and introduced into cultured retinal Müller cells. Silencing of the ILK gene was identified by real time RT-PCR and Western blot. The Müller cell phenotype change was confirmed by immunodetection of alpha-smooth muscle actin (alpha-SMA) stress fiber formation. The generation of tractional force was assessed using a tissue culture assay with cells incubated in three-dimensional collagen gels; cell migration was determined by the Boyden chamber method, using 10% FBS as a chemotactic factor.

RESULTS

Significant decreases in ILK mRNA and protein expression were detected in Müller cells carrying lentiviral ILK-shRNA vector. Cells treated with anti-ILK siRNA showed less alpha-SMA stress fiber formation under hypoxic conditions or cell subcultivation. Lentiviral ILK-shRNA vector transfection also significantly reduced cell migration and cell-mediated gel contraction.

CONCLUSION

Lentivirus-mediated ILK RNAi decreased cell migration and contractile force generation by inhibiting alpha-SMA stress fiber formation in human retinal Müller cells. This tool might be useful to treat ocular fibroproliferative diseases associated with transdifferentiated Müller cells.

Utility of RNAi-mediated prnp gene silencing in neuroblastoma cells permanently infected by prions: potentials and limitations

Prion diseases are incurable, transmissible neurodegenerative disorders in humans and animals. Because the disease-associated isoform of prion protein, PrP(Sc), is conformationally converted from cellular prion protein, PrP(C), knockdown of PrP(C) expression by RNA interference (RNAi) implicates therapy for prion diseases. In this study, introduction of small interfering (si) and small hairpin (sh) RNAs targeting the prion protein gene (prnp) transcripts triggered specific gene silencing and reduced the PrP(C) level in both prion-free and -infected neuroblastoma cell lines. Furthermore, this approach suppressed PrP(Sc) formation and ultimately eliminated PrP(Sc) from prion-infected cell lines. However, prolonged culture of cured cells resulted in reappearance of PrP(Sc) in the cell population, presumably by de novo PrP(Sc) formation from residual PrP(C) uncontrolled by RNAi and PrP(Sc) remained under the detection limit. Protein misfolding cyclic amplification assays further confirmed that lysate of cured cells was sufficient to support PrP(Sc) propagation. Our data not only suggest a potential treatment option but also implicate a caveat for using an RNAi approach for prion diseases. These findings provide critical information required to advance RNAi-based prevention and therapy for prion diseases of humans and animals.

Aptamers against prion proteins and prions

Prion diseases are fatal neurodegenerative and infectious disorders of humans and animals, characterized by structural transition of the host-encoded cellular prion protein (PrP(c)) into the aberrantly folded pathologic isoform PrP(Sc). RNA, DNA or peptide aptamers are classes of molecules which can be selected from complex combinatorial libraries for high affinity and specific binding to prion proteins and which might therefore be useful in diagnosis and therapy of prion diseases. Nucleic acid aptamers, which can be chemically synthesized, stabilized and immobilized, appear more suitable for diagnostic purposes, allowing use of PrP(Sc) as selection target. Peptide aptamers facilitate appropriate intracellular expression, targeting and re-routing without losing their binding properties to PrP, a requirement for potential therapeutic gene transfer experiments in vivo. Elucidation of structural properties of peptide aptamers might be used as basis for rational drug design, providing another attractive application of peptide aptamers in the search for effective anti-prion strategies.

Therapy for prion diseases: Insights from the use of RNA interference

Insights into the molecular basis and the temporal evolution of neurotoxicity in prion disease are increasing, and recent work in mice leads to new avenues for targeting treatment of these disorders. Using lentivirally mediated RNA interference (RNAi) against native prion protein (PrP), White et al. report the first therapeutic intervention that results in neuronal rescue, prevents symptoms and increases survival in mice with established prion disease.(1) Both the target and the timing of treatment here are crucial to the effectiveness of this strategy: the formation of the neurotoxic prion agent is prevented at a point when diseased neurons can still be saved from death. But the data also give new insights into the timing of treatment in the context of the pattern of spread of prion infection throughout the brain, with implications for developing the most effective treatments.

Recent advances in prion chemotherapeutics

The transmissible spongiform encephalopathies are rapidly progressive and invariably fatal neurodegenerative diseases for which there are no proven efficacious treatments. Many approaches have been undertaken to find ways to prevent, halt, or reverse these prion diseases, with limited success to date. However, as both our understanding of pathogenesis and our ability to detect early disease increases, so do our potential therapeutic targets and our chances of finding effective drugs. There is increasing pressure to find effective decontaminants for blood supplies, as variant Creutzfeldt Jakob Disease (vCJD) has been shown to be transmissible by blood, and to find non-toxic preventative therapies, with ongoing cases of Bovine Spongiform Encephalopathy (BSE) and the spread of Chronic Wasting Disease (CWD). Within the realm of chemotherapeutic approaches, much research has focussed on blocking the conversion of the normal form of prion protein (PrP(c)) to its abnormal counterpart (PrP(res)). Structurally, these chemotherapeutic agents are often polyanionic or polycyclic and may directly bind PrP(c) or PrP(res), or act by redistributing, sequestering, or down-regulating PrP(c), thus preventing its conversion. There are also some polycationic compounds which proport to enhance the clearance of PrP(res). Other targets include accessory molecules such as the laminin receptor precursor which influences conversion, or cell signalling molecules which may be required for pathogenesis. Of recent interest are the possible neuroprotective effects of some drugs. Importantly, there is evidence that combining compounds may provide synergistic responses. This review provides an update on current testing methods, therapeutic targets, and promising candidates for chemical-based therapy.

Prion expression is activated by Adenovirus 5 infection and affects the adenoviral cycle in human cells

The prion protein is a cell surface glycoprotein whose physiological role remains elusive, while its implication in transmissible spongiform encephalopathies (TSEs) has been demonstrated. Multiple interactions between the prion protein and viruses have been described: viruses can act as co-factors in TSEs and life cycles of different viruses have been found to be controlled by prion modulation. We present data showing that human Adenovirus 5 induces prion expression. Inactivated Adenovirus did not alter prion transcription, while variants encoding for early products did, suggesting that the prion is stimulated by an early adenoviral function. Down-regulation of the prion through RNA interference showed that the prion controls adenovirus replication and expression. These data suggest that the prion protein could play a role in the defense strategy mounted by the host during viral infection, in a cell autonomous manner. These results have implications for the study of the prion protein and of associated TSEs.

Efficient knockdown of human prnp mRNA expression levels using hybrid hammerhead ribozymes

Prion diseases are invariably fatal infectious diseases of the central nervous system. The prion protein has been identified as the underlying causative agent as PrP knockout mice (prnp(0/0)) are resistant to infection. This suggests that a significant reduction in the expression levels of PrP(c) should interrupt disease progression. Accomplishing this in vivo, upon presentation of symptoms, requires a mechanism that significantly reduces prnp mRNA levels while lacking potential side effects that may be cytotoxic or lethal to the host. Hybrid hammerhead ribozymes (HyHamRzs) include both a helicase recruitment signal and a tRNA(Val) promoter. HyHamRzs offer a means of highly specific and significant mRNA cleavage. In this study, data demonstrate increased activity granted to HamRzs by the addition of the helicase recruitment signal. Results show that three different HyHamRzs, targeting different locations along the full length prnp mRNA, reduced expression levels by greater than 95% relative to the control. It is postulated that HyHamRzs, modified to enhance serum stability and delivered intravenously to neurons by forming a complex with the modified rabies virus G protein (RVG), may offer a potential gene therapy strategy.

Targeting prion proteins in neurodegenerative disease

BACKGROUND

Spongiform neurodegeneration is the pathological hallmark of individuals suffering from prion disease. These disorders, whose manifestation is sporadic, familial or acquired by infection, are caused by accumulation of the aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc). Although usually rare, prion disorders are inevitably fatal and transferrable by infection.

OBJECTIVE

Pathology is restricted to the central nervous system and premortem diagnosis is usually not possible. Yet, promising approaches towards developing therapeutic regimens have been made recently.

METHODS

The biology of prion proteins and current models of neurotoxicity are discussed and prophylactic and therapeutic concepts are introduced.

RESULTS/CONCLUSIONS

Although various promising drug candidates with antiprion activity have been identified, this proof-of-concept cannot be transferred into translational medicine yet.

Single treatment with RNAi against prion protein rescues early neuronal dysfunction and prolongs survival in mice with prion disease

Prion diseases are fatal neurodegenerative conditions for which there is no effective treatment. Prion propagation involves the conversion of cellular prion protein, PrP(C), to its conformational isomer, PrP(Sc), which accumulates in disease. Here, we show effective therapeutic knockdown of PrP(C) expression using RNAi in mice with established prion disease. A single administration of lentivirus expressing a shRNA targeting PrP into each hippocampus of mice with established prion disease significantly prolonged survival time. Treated animals lived 19% and 24% longer than mice given an "empty" lentivirus, or not treated, respectively. Lentivirally mediated RNAi of PrP also prevented the onset of behavioral deficits associated with early prion disease, reduced spongiform degeneration, and protected against neuronal loss. In contrast, mice receiving empty virus or no treatment developed early cognitive impairment and showed severe spongiosis and neuronal loss. The focal use of RNAi therapeutically in prion disease further supports strategies depleting PrP(C), which we previously established to be a valid target for prion-based treatments. This approach can now be used to define the temporal, quantitative, and regional requirements for PrP knockdown for effective treatment of prion disease and to explore mechanisms involved in predegenerative neuronal dysfunction and its rescue.

Prnp knockdown in transgenic mice using RNA interference

RNA interference has become a widely used approach to perform gene knockdown experiments in cell cultures and more recently transgenic animals. A designed miRNA targeting the prion protein mRNA was built and expressed using the human PRNP promoter. Its efficiency was confirmed in transfected cells and it was used to generate several transgenic mouse lines. Although expressed at low levels, it was found to downregulate the endogenous mouse Prnp gene expression to an extent that appears to be directly related with the transgene expression level and that could reach up to 80% inhibition. This result highlights the potential and limitations of the RNA interference approach when applied to disease resistance.

A cell line infectible by prion strains from different species

It has been shown previously that ovine prion protein (PrP(C)) renders rabbit epithelial RK13 cells permissive to the multiplication of ovine prions, thus providing evidence that species barriers can be crossed in cultured cells through the expression of a relevant PrP(C). The present study significantly extended this observation by showing that mouse and bank vole prions can be propagated in RK13 cells that express the corresponding PrP(C). Importantly, the respective molecular patterns of abnormal PrP (PrP(res)) and, where examined, the neuropathological features of the infecting strains appeared to be maintained during the propagation in cell culture. These findings indicate that RK13 cells can be genetically engineered to replicate prion strains faithfully from different species. Such an approach may facilitate investigations of the molecular basis of strain identity and prion diversity.

Susceptibility of cell substrates to PrPSc infection and safety control measures related to biological and biotherapeutical products

Concerns over the potential for infectious prion proteins to contaminate human biologics and biotherapeutics have been raised from time to time. Transmission of the pathogenic form of prion protein (PrP(Sc)) through veterinary vaccines has been observed, yet no human case through the use of vaccine products has been reported. However, iatrogenic transmissions of PrP(Sc) in humans through blood components, tissues and growth hormone have been reported. These findings underscore the importance of reliable detection or diagnostic methods to prevent the transmission of prion diseases, given that the number of asymptomatic infected individuals remains unknown, the perceived incubation time for human prion diseases could be decades, and no cure of the diseases has been found yet. A variety of biochemical and molecular methods can selectively concentrate PrP(Sc) to facilitate its detection in tissues and cells. Furthermore, some methods routinely used in the manufacturing process of biological products have been found to be effective in reducing PrP(Sc) from the products. Questions remain unanswered as to the validation criteria of these methods, the minimal infectious dose of the PrP(Sc) required to cause infection and the susceptibility of cells used in gene therapy or the manufacturing process of biological products to PrP(Sc) infections. Here, we discuss some of these challenging issues.

Octapeptide repeat region of prion protein (PrP) is required at an early stage for production of abnormal prion protein in PrP-deficient neuronal cell line

An abnormal isoform of prion protein (PrP(Sc)), which is composed of the same amino acids as cellular PrP (PrP(C)) and has proteinase K (PK)-resistance, hypothetically converts PrP(C) into PrP(Sc). To investigate the region important for PrP(Sc) production, we examined the levels of PrP(Sc) in PrP gene-deficient cells (HpL3-4) expressing PrP(C) deleted of various regions including the octapeptide repeat region (OR) or hydrophobic region (HR). After Chandler or Obihiro prion infection, PrP(Sc) was produced in HpL3-4 cells expressing wild-type PrP(C) or PrP(C) deleted of HR at an early stage and further reduced to below the detectable level, whereas cells expressing PrP(C) deleted of OR showed no PrP(Sc) production. The results suggest that OR of PrP(C) is required for the early step of efficient PrP(Sc) production.

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

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

Nonpsychoactive cannabidiol prevents prion accumulation and protects neurons against prion toxicity

Prion diseases are transmissible neurodegenerative disorders characterized by the accumulation in the CNS of the protease-resistant prion protein (PrPres), a structurally misfolded isoform of its physiological counterpart PrPsen. Both neuropathogenesis and prion infectivity are related to PrPres formation. Here, we report that the nonpsychoactive cannabis constituent cannabidiol (CBD) inhibited PrPres accumulation in both mouse and sheep scrapie-infected cells, whereas other structurally related cannabinoid analogs were either weak inhibitors or noninhibitory. Moreover, after intraperitoneal infection with murine scrapie, peripheral injection of CBD limited cerebral accumulation of PrPres and significantly increased the survival time of infected mice. Mechanistically, CBD did not appear to inhibit PrPres accumulation via direct interactions with PrP, destabilization of PrPres aggregates, or alteration of the expression level or subcellular localization of PrPsen. However, CBD did inhibit the neurotoxic effects of PrPres and affected PrPres-induced microglial cell migration in a concentration-dependent manner. Our results suggest that CBD may protect neurons against the multiple molecular and cellular factors involved in the different steps of the neurodegenerative process, which takes place during prion infection. When combined with its ability to target the brain and its lack of toxic side effects, CBD may represent a promising new anti-prion drug.

Divergent expression of cellular prion protein on blood cells of human and nonhuman primates

BACKGROUND

Four recent transmissions of variant Creutzfeldt-Jakob disease infection by transfusion highlight the need for detailed understanding of blood-related prion pathogenesis. Nonhuman primates are the most relevant models of human prion diseases.

STUDY DESIGN AND METHODS

Quantitative flow cytometry with monoclonal antibodies FH11, 3F4, and 6H4 against different parts of the normal cellular form of the prion protein (PrP(C)) was used to evaluate its expression on blood cells of humans, chimpanzees, cynomolgus macaques, rhesus macaques, squirrel monkeys, and microcebe lemurs.

RESULTS

Chimpanzees, rhesus macaques, and squirrel monkeys displayed a much higher quantity of total blood cell membrane PrP(C) than humans, due to a markedly higher expression of PrP(C) on their red blood cells (RBCs). In contrast, cynomolgus macaques and lemurs demonstrated substantially lower levels of membrane PrP(C) due to the lack of significant PrP(C) expression on RBCs and platelets (PLTs). All species displayed PrP(C) on white blood cells (WBCs), with the highest levels found on human cells. Only humans, chimpanzees, and to a lesser degree rhesus macaques expressed PrP(C) on PLTs.

CONCLUSION

If PrP(C) contributes to the propagation or transport of prion infectivity in blood, the differences reported here need to be considered when extrapolating results of transmission studies in primate models to blood and blood components in humans.

Prion proteins: a biological role beyond prion diseases

The biological role of the scrapie isoform of prion protein (PrP(Sc)) as an infectious agent in numerous human and non-human disorders of the central nervous system is well established. In contrast, and despite decades of intensive research, the physiological function of the endogenous cellular form of the prion protein (PrP(C)) remains elusive. In mammals, the ubiquitous expression of PrP(C) suggests biological functions other than its pathological role in propagating the accumulation of its misfolded isotype. Other functions that have been attributed to PrP(C) include signal transduction, synaptic transmission and protection against cell death through the apoptotic pathway. More recently, immunoregulatory properties of PrP(C) have been reported. We review accumulating in vitro and in vivo evidence regarding physiological functions of PrP(C).

PrPc does not mediate internalization of PrPSc but is required at an early stage for de novo prion infection of Rov cells

We have studied the interactions of exogenous prions with an epithelial cell line inducibly expressing PrPc protein and permissive to infection by a sheep scrapie agent. We demonstrate that abnormal PrP (PrPSc) and prion infectivity are efficiently internalized in Rov cells, whether or not PrPc is expressed. At odds with earlier studies implicating cellular heparan sulfates in PrPSc internalization, we failed to find any involvement of such molecules in Rov cells, indicating that prions can enter target cells by several routes. We further show that PrPSc taken up in the absence of PrPc was unable to promote efficient prion multiplication once PrPc expression was restored in the cells. This observation argues that interaction of PrPSc with PrPc has to occur early, in a specific subcellular compartment(s), and is consistent with the view that the first prion multiplication events may occur at the cell surface.

Hot spots in prion protein for pathogenic conversion

Prion proteins are key molecules in transmissible spongiform encephalopathies (TSEs), but the precise mechanism of the conversion from the cellular form (PrP(C)) to the scrapie form (PrP(Sc)) is still unknown. Here we discovered a chemical chaperone to stabilize the PrP(C) conformation and identified the hot spots to stop the pathogenic conversion. We conducted in silico screening to find compounds that fitted into a "pocket" created by residues undergoing the conformational rearrangements between the native and the sparsely populated high-energy states (PrP*) and that directly bind to those residues. Forty-four selected compounds were tested in a TSE-infected cell culture model, among which one, 2-pyrrolidin-1-yl-N-[4-[4-(2-pyrrolidin-1-yl-acetylamino)-benzyl]-phenyl]-acetamide, termed GN8, efficiently reduced PrP(Sc). Subsequently, administration of GN8 was found to prolong the survival of TSE-infected mice. Heteronuclear NMR and computer simulation showed that the specific binding sites are the A-S2 loop (N159) and the region from helix B (V189, T192, and K194) to B-C loop (E196), indicating that the intercalation of these distant regions (hot spots) hampers the pathogenic conversion process. Dynamics-based drug discovery strategy, demonstrated here focusing on the hot spots of PrP(C), will open the way to the development of novel anti-prion drugs.

Cellular prion protein regulates beta-secretase cleavage of the Alzheimer's amyloid precursor protein

Proteolytic processing of the amyloid precursor protein (APP) by beta-secretase, beta-site APP cleaving enzyme (BACE1), is the initial step in the production of the amyloid beta (Abeta) peptide, which is involved in the pathogenesis of Alzheimer's disease. The normal cellular function of the prion protein (PrP(C)), the causative agent of the transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease in humans, remains enigmatic. Because both APP and PrP(C) are subject to proteolytic processing by the same zinc metalloproteases, we tested the involvement of PrP(C) in the proteolytic processing of APP. Cellular overexpression of PrP(C) inhibited the beta-secretase cleavage of APP and reduced Abeta formation. Conversely, depletion of PrP(C) in mouse N2a cells by siRNA led to an increase in Abeta peptides secreted into the medium. In the brains of PrP knockout mice and in the brains from two strains of scrapie-infected mice, Abeta levels were significantly increased. Two mutants of PrP, PG14 and A116V, that are associated with familial human prion diseases failed to inhibit the beta-secretase cleavage of APP. Using constructs of PrP, we show that this regulatory effect of PrP(C) on the beta-secretase cleavage of APP required the localization of PrP(C) to cholesterol-rich lipid rafts and was mediated by the N-terminal polybasic region of PrP(C) via interaction with glycosaminoglycans. In conclusion, this is a mechanism by which the cellular production of the neurotoxic Abeta is regulated by PrP(C) and may have implications for both Alzheimer's and prion diseases.

Comparison of gene silencing in human vascular cells using small interfering RNAs

BACKGROUND

Gene silencing achieved through small interfering RNA (siRNA) transfection represents a promising approach to vascular gene therapy. Here we characterize the behavior of RNA interference (RNAi) in vascular biology by comparing the RNAi response to single- and multigene siRNA transfections in vitro in human vascular cells.

STUDY DESIGN

The strength and specificity of multigene silencing in cultured human coronary artery smooth muscle and human coronary artery endothelial cells (HCASMC/HCAEC) were assessed by quantitative reverse transcription-polymerase chain reaction (QRT-PCR) and Western blot after transfection singly or simultaneously with siRNAs targeting glyceraldehyde-3-phosphate dehydrogenase, the myristoylated alanine-rich C kinase substrate, and cadherin 11. RNAi response to low-dose (0.25 to 10 nM) siRNA transfection was characterized between the two cell types by QRT-PCR and fluorescence-activated cell sorter analysis.

RESULTS

Powerful and specific silencing of all targets was observed in both cell types after multigene siRNA transfections, but with a reduction in effect compared with single-gene siRNA transfections. Multigene messenger RNA (mRNA) reductions in HCAECs exceeded those achieved in HCASMCs, and superior mRNA silencing and siRNA delivery were observed in HCAECs after low-dose siRNA transfections.

CONCLUSIONS

Multigene silencing by siRNA stands as a promising nonviral approach for manipulating gene expression in human vascular cells. Under our in vitro conditions, endothelial cells were more susceptible to siRNA transfection and gene silencing than vascular smooth muscle cells. RNAi technology could potentially find use in the development of siRNA cocktails for application to vein bypass grafts or for modulating endothelial cell function in other forms of vascular disease.

Ribonucleic acid interference for neurological disorders: candidate diseases, potential targets, and current approaches

OBJECTIVE

Ribonucleic acid (RNA) interference (RNAi) is a conserved evolutionary defense mechanism that is gaining utility for therapeutic application by modulating gene expression or silencing disease-causing genes.

METHODS

This strategy has recently achieved success in mammalian cells via synthetic small interfering RNA or short hairpin RNA expressed in vectors for gene delivery. The vector-based RNAi strategy has particular potential because of the possibility of targeted gene delivery, long-term gene expression, and the potential means of penetrating the blood-brain barrier.

RESULTS

RNAi-based approaches have been proposed for a variety of neurological disorders, including dominant genetic diseases, neurodegenerative diseases, malignant brain tumors, pain, and viral-induced encephalopathies.

CONCLUSION

This review summarizes the current approaches of the RNAi strategy for neurological disorders, focusing on potential targets for therapeutic intervention.

Thioaptamer interactions with prion proteins: sequence-specific and non-specific binding sites

Binding of nucleic acids to the prion protein (PrP) created a conundrum that required distinguishing between non-specific interactions and biologically important polynucleotides. In the process of developing selective ligands for PrP, we found using a single-stranded DNA thioaptamer library that the binding of thioaptamers to PrP occurs on at least two different sites on the protein. Selection against recombinant (rec) PrP of Syrian hamster (SHa) sequence 90-231 folded into an alpha-helical-rich conformation identified a 12-base consensus sequence within a series of 20 thioaptamers, all of which consist of 40 bases. Each thioaptamer was comprised of both normal and thio-dA modified bases. One thioaptamer designated 97 bound to recSHaPrP with affinity of 0.58(+/-0.1) nM; lower affinities for bovine (Bo), and human (Hu) were found, establishing that binding is dependent on the primary structure of PrP. High affinity binding of thioaptamer 97 to PrP was found to be mediated through the dodecyl sequence GACACAAGCCGA within the consensus region with five critical backbone modifications 5' to each dA residue. A control oligonucleotide with an equivalent number of phosphorothioates to thioaptamer 97 and a scrambled consensus sequence could not distinguish among the three PrP sequences. Control oligonucleotides bearing non-selected sequences bound to PrP at a sequence-independent DNA-binding site. In contrast, the high-affinity binding of thioaptamer 97 to PrP depends on (1) backbone modifications, (2) oligonucleotide sequence, and (3) PrP sequence.

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

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

Lentivector-mediated RNAi efficiently suppresses prion protein and prolongs survival of scrapie-infected mice

Prion diseases are fatal neurodegenerative diseases characterized by the accumulation of PrP(Sc), the infectious and protease-resistant form of the cellular prion protein (PrP(C)). We generated lentivectors expressing PrP(C)-specific short hairpin RNAs (shRNAs) that efficiently silenced expression of the prion protein gene (Prnp) in primary neuronal cells. Treatment of scrapie-infected neuronal cells with these lentivectors resulted in an efficient and stable suppression of PrP(Sc) accumulation. After intracranial injection, lentiviral shRNA reduced PrP(C) expression in transgenic mice carrying multiple copies of Prnp. To test the therapeutic potential of lentiviral shRNA, we used what we believe to be a novel approach in which the clinical situation was mimicked. We generated chimeric mice derived from lentivector-transduced embryonic stem cells. Depending on the degree of chimerism, these animals carried the lentiviral shRNAs in a certain percentage of brain cells and expressed reduced levels of PrP(C). Importantly, in highly chimeric mice, survival after scrapie infection was significantly extended. Taken together, these data suggest that lentivector-mediated RNA interference could be an approach for the treatment of prion disease.

Suppression of prion protein in livestock by RNA interference

Given the difficulty of applying gene knockout technology to species other than mice, we decided to explore the utility of RNA interference (RNAi) in silencing the expression of genes in livestock. Short hairpin RNAs (shRNAs) were designed and screened for their ability to suppress the expression of caprine and bovine prion protein (PrP). Lentiviral vectors were used to deliver a transgene expressing GFP and an shRNA targeting PrP into goat fibroblasts. These cells were then used for nuclear transplantation to produce a cloned goat fetus, which was surgically recovered at 81 days of gestation and compared with an age-matched control derived by natural mating. All tissues examined in the cloned fetus expressed GFP, and PCR analysis confirmed the presence of the transgene encoding the PrP shRNA. Most relevant, Western blot analysis performed on brain tissues comparing the transgenic fetus with control demonstrated a significant (>90%) decrease in PrP expression levels. To confirm that similar methodologies could be applied to the bovine, recombinant virus was injected into the perivitelline space of bovine ova. After in vitro fertilization and culture, 76% of the blastocysts exhibited GFP expression, indicative that they expressed shRNAs targeting PrP. Our results provide strong evidence that the approach described here will be useful in producing transgenic livestock conferring potential disease resistance and provide an effective strategy for suppressing gene expression in a variety of large-animal models.

Silencing the activity and proliferative properties of the human EagI Potassium Channel by RNA Interference

EagI potassium channels are natively expressed in the mammalian brain as well as in many cancer cell lines and tumor tissues. The role of EagI in malignant transformation has been suggested by several experiments, but the lack of specific EagI inhibitors has made it difficult to examine the influence of the channel on oncogenesis and its potential as a therapeutic target. We have used short interfering RNA to test the effects of EagI reduction on the behavior of tumor cells in vitro. By generating and optimizing an EagI-specific short interfering RNA system, we were able to study the effects of EagI depletion on several cancer cell lines that endogenously express this protein. We show here that our short interfering RNA sequences act specifically on EagI, reproducibly induce a significant decrease in the proliferation of tumor cell lines, and do not trigger any observable nonspecific responses.