Prion strain- and species-dependent effects of antiprion molecules in primary neuronal cultures

Transmissible spongiform encephalopathies (TSE) arise as a consequence of infection of the central nervous system by prions and are incurable. To date, most antiprion compounds identified by in vitro screening failed to exhibit therapeutic activity in animals, thus calling for new assays that could more accurately predict their in vivo potency. Primary nerve cell cultures are routinely used to assess neurotoxicity of chemical compounds. Here, we report that prion strains from different species can propagate in primary neuronal cultures derived from transgenic mouse lines overexpressing ovine, murine, hamster, or human prion protein. Using this newly developed cell system, the activity of three generic compounds known to cure prion-infected cell lines was evaluated. We show that the antiprion activity observed in neuronal cultures is species or strain dependent and recapitulates to some extent the activity reported in vivo in rodent models. Therefore, infected primary neuronal cultures may be a relevant system in which to investigate the efficacy and mode of action of antiprion drugs, including toward human transmissible spongiform encephalopathy agents.     

In vitro synergistic anti-prion effect of cholesterol ester modulators in combination with chlorpromazine and quinacrine

Our studies on the role of cholesterol in prion infection/replication showed that brains and peripheral cells of sheep susceptible-to or suffering-from Scrapie were characterized by an altered cholesterol homeostasis, and that drugs affecting cholesterol ester pool were endowed with selective anti-prion activity in N2a cell lines infected with the 22L and RML prion strains. In these prion-infected N2a cell lines, we now report increased anti-prion activity of dual-drug combinations consisting of cholesterol ester modulators associated with prion inhibitors. Synergism was obtained with the cholesterol ester modulators everolimus, pioglitazone, progesterone, and verapamil associated with the anti-prion chlorpromazine, and with everolimus and pioglitazone associated with the anti-prion quinacrine. In addition, comparative lipid analyses in prion-infected vs. uninfected N2a cells, demonstrated a derangement of type and distribution of cholesterol ester, free cholesterol, and triglyceride pools in the infected cells. Single-drug treatments differently affected synthesis of the various lipid forms, whereas combined drug treatments appeared to restore a lipid profile similar to that of the untreated-uninfected cells. We conclude that the anti-prion synergistic effects of cholesterol ester modulators associated with the cholesterol-interfering anti-prion drugs chlorpromazine and quinacrine may arise from the ability of combined drugs to re-establish lipid homeostasis in the prion-infected cells. Overall, these data suggest that inhibition of prion replication can be readily potentiated by combinatorial drug treatments and that steps of cholesterol/cholesterol ester metabolism may represent suitable targets.     

A Camelid Anti-PrP Antibody Abrogates PrPSc Replication in Prion-Permissive Neuroblastoma Cell Lines

The development of antibodies effective in crossing the blood brain barrier (BBB), capable of accessing the cytosol of affected cells and with higher affinity for PrP^Sc would be of paramount importance in arresting disease progression in its late stage and treating individuals with prion diseases. Antibody-based therapy appears to be the most promising approach following the exciting report from White and colleagues, establishing the “proof-of-principle” for prion-immunotherapy. After passive transfer, anti-prion antibodies were shown to be very effective in curing peripheral but not central rodent prion disease, due to the fact that these anti-prion antibodies are relatively large molecules and cannot therefore cross the BBB. Here, we show that an anti-prion antibody derived from camel immunised with murine scrapie material adsorbed to immunomagnetic beads is able to prevent infection of susceptible N2a cells and cure chronically scrapie-infected neuroblastoma cultures. This antibody was also shown to transmigrate across the BBB and cross the plasma membrane of neurons to target cytosolic PrP^C. In contrast, treatment with a conventional anti-prion antibody derived from mouse immunised with recombinant PrP protein was unable to prevent recurrence of PrP^Sc replication. Furthermore, our camelid antibody did not display any neurotoxic effects following treatment of susceptible N2a cells as evidenced by TUNEL staining. These findings demonstrate the potential use of anti-prion camelid antibodies for the treatment of prion and other related diseases via non-invasive means.     

KDEL-tagged anti-prion intrabodies impair PrP lysosomal degradation and inhibit scrapie infectivity

Transmissible spongiform encephalopathy or prion diseases are fatal neurodegenerative disorders characterized by the conversion of the cellular prion protein (PrPC) into the infectious scrapie isoform (PrPSc). We have recently demonstrated that anti-prion intrabodies targeted to the lumen of the endoplasmic reticulum provide a simple and effective means to inhibit the transport of PrPC to the cell surface. Here, we report that they completely block the traffic of mature full-length PrPC molecules, impair prion lysosomal degradation, and interfere with the early phase of scrapie formation. Since anti-prion intrabodies efficiently block PrPSc accumulation in vitro, we investigated whether they could also antagonize scrapie infectivity in vivo. We found that mice intracerebrally injected with KDEL-8H4-NGF-differentiated PC12 cells infected with scrapie neither develop scrapie clinical signs nor brain damage. Furthermore, no protease-resistant PrPSc is detectable in brains of inoculated animals. These results indicate that anti-prion intrabody strategy may be effective against prion infection.     

Poly-L-histidine inhibits prion propagation in a prion-infected cell line

ABSTRACT

Transmissible spongiform encephalopathies (TSEs) are a group of lethal neurodegenerative diseases involving the structural conversion of cellular prion protein (PrP^C) into the pathogenic isoform (PrP^Sc) for which no effective treatment is currently available. Previous studies have implicated that a polymeric molecule with a repeating unit, such as pentosane polysulfate and polyamidoamide dendrimers, exhibits a potent anti-prion activity, suggesting that poly-(amino acid)s could be a candidate molecule for inhibiting prion propagation. Here, by screening a series of poly-(amino acid)s in a prion-infected neuroblastoma cell line (GT^FK), we identified poly-L-His as a novel anti-prion compound with an IC₅₀ value of 1.8 µg/mL (0.18 µM). This potent anti-prion activity was specific to a high-molecular-weight poly-L-His and absent in monomeric histidine or low-molecular-weight poly-L-His. Solution NMR data indicated that poly-L-His directly binds to the loop region connecting Helix 2 and Helix 3 of PrP^C and sterically blocks the structural conversion toward PrP^Sc. Poly-L-His, however, did not inhibit prion propagation in a prion-infected mouse when administered intraperitoneally, suggesting that the penetration of blood-brain barrier and/or the chemical stability of this polypeptide must be addressed before its application in vivo. Taken together, this study revealed the potential use of poly-L-His as a novel treatment against TSEs. (203 words)     

Anti-prion activity found in beetle grub hemolymph of Trypoxylus dichotomus septentrionalis

No remedies for prion disease have been established, and the conversion of normal to abnormal prion protein, a key event in prion disease, is still unclear. Here we found that substances in beetle grub hemolymph, after they were browned by aging for a month or heating for hours, reduced abnormal prion protein (PrP) levels in RML prion-infected cells. Active anti-prion components in the hemolymph were resistant to protease treatment and had molecular weights larger than 100 kDa. Aminoguanidine treatment of the hemolymph abolished its anti-prion activity, suggesting that Maillard reaction products are enrolled in the activity against the RML prion. However, levels of abnormal PrP in RML prion-infected cells were not decreased by incubation with the Maillard reaction products formed by amino acids or bovine serum albumin. The anti-prion components in the hemolymph modified neither cellular or cell-surface PrP levels nor lipid raft or autophagosome levels. The anti-prion activity was not observed in cells infected with 22 L prion or Fukuoka-1 prion, suggesting the anti-prion action is prion strain-dependent. Although the active components of the hemolymph need to be further evaluated, the present findings imply that certain specific chemical structures in the hemolymph, but not chemical structures common to all Maillard reaction products, are involved in RML prion formation or turnover, without modifying normal PrP expression. The anti-prion components in the hemolymph are a new tool for elucidating strain-dependent prion biology.     

Efficient transmission of two different sheep scrapie isolates in transgenic mice expressing the ovine PrP gene

We produced transgenic mice expressing the sheep prion protein to obtain a sensitive model for sheep spongiform encephalopathies (scrapie). The complete open reading frame, with alanine, arginine, and glutamine at susceptibility codons 136, 154, and 171, respectively, was inserted downstream from the neuron-specific enolase promoter. A mouse line, Tg(OvPrP4), devoid of the murine PrP gene, was obtained by crossing with PrP knockout mice. Tg(OvPrP4) mice were shown to selectively express sheep PrP in their brains, as demonstrated in mRNA and protein analysis. We showed that these mice were susceptible to infection by sheep scrapie following intracerebral inoculation with two natural sheep scrapie isolates, as demonstrated not only by the occurrence of neurological signs but also by the presence of the spongiform changes and abnormal prion protein accumulation in their brains. Mean times to death of 238 and 290 days were observed with these isolates, but the clinical course of the disease was strikingly different in the two cases. One isolate led to a very early onset of neurological signs which could last for prolonged periods before death. Independently of the incubation periods, some of the mice inoculated with this isolate showed low or undetectable levels of PrPsc, as detected by both Western blotting and immunohistochemistry. The development of experimental scrapie in these mice following inoculation of the scrapie infectious agent further confirms that neuronal expression of the PrP open reading frame alone is sufficient to mediate susceptibility to spongiform encephalopathies. More importantly, these mice provide a new and promising tool for studying the infectious agents in sheep spongiform encephalopathies.     

A Bovine Cell Line That Can Be Infected by Natural Sheep Scrapie Prions

Cell culture systems represent a crucial part in basic prion research; yet, cell lines that are susceptible to prions, especially to field isolated prions that were not adapted to rodents, are very rare. The purpose of this study was to identify and characterize a cell line that was susceptible to ruminant-derived prions and to establish a stable prion infection within it. Based on species and tissue of origin as well as PrP expression rate, we pre-selected a total of 33 cell lines that were then challenged with natural and with mouse propagated BSE or scrapie inocula. Here, we report the successful infection of a non-transgenic bovine cell line, a sub-line of the bovine kidney cell line MDBK, with natural sheep scrapie prions. This cell line retained the scrapie infection for more than 200 passages. Selective cloning resulted in cell populations with increased accumulation of PrP^res, although this treatment was not mandatory for retaining the infection. The infection remained stable, even under suboptimal culture conditions. The resulting infectivity of the cells was confirmed by mouse bioassay (Tgbov mice, Tgshp mice). We believe that PES cells used together with other prion permissive cell lines will prove a valuable tool for ongoing efforts to understand and defeat prions and prion diseases.     

Establishment of an SV40 large T antigen-immortalized bovine brain cell line and its neuronal differentiation by dibutyryl-cyclic AMP

Immortalized bovine brain cell lines provide ideal in vitro cellular infection models for bovine spongiform encephalopathies (BSEs) caused by prions without enduring species barrier. We have established an immortalized brain cell line (FBBC-1 cells) from primary cultures of cryopreserved fetal bovine brain tissues after transfection with SV40 large T antigen. FBBC-1 cells are stable after passaging to >100 population doublings after single cell cloning, with a generation time of 24 h. After the treatment with dibutyryl-cyclic AMP, the cells ceased proliferation and extended neurite-like processes that were immunostained with the antibody against tubulin βIII, a marker of immature neurons. Upregulation of tubulin βIII expression was confirmed by immunoblotting. These bovine cells expressed cellular prion protein and its processed smaller C1 fragment, and may provide an in vitro means of propagating cattle BSE prion.     

The GLN family of murine endogenous retroviruses contains an element competent for infectious viral particle formation

Several families of endogenous retroviruses (ERVs) have been identified in the mouse genome, in several instances by in silico searches, but for many of them it remains to be determined whether there are elements that can still encode functional retroviral particles. Here, we identify, within the GLN family of highly reiterated ERVs, one, and only one, copy that encodes retroviral particles prone to infection of mouse cells. We show that its envelope protein confers an ecotropic host range and recognizes a receptor different from mCAT1 and mSMIT1, the two previously identified receptors for other ecotropic mouse retroviruses. Electron microscopy disclosed viral particle assembly and budding at the cell membrane, as well as release of mature particles into the extracellular space. These particles are closely related to murine leukemia virus (MLV) particles, with which they have most probably been confused in the past. This study, therefore, identifies a new class of infectious mouse ERVs belonging to the family Gammaretroviridae, with one family member still functional today. This family is in addition to the two MLV and mouse mammary tumor virus families of active mouse ERVs with an extracellular life cycle.     

Differential expression in human and mouse cells of human immunodeficiency virus pseudotyped by murine retroviruses.

Expression of cell surface CD4 influences susceptibility of cells to human immunodeficiency virus (HIV) infection; however, some CD4-positive human and mouse cells are still resistant to HIV infection. To search for mechanisms of resistance to HIV independent of CD4 expression, HIV expression was studied in human and mouse cells normally resistant to HIV infection by introducing infectious virus by transfection of HIV DNA or infection with HIV pseudotyped with amphotropic or polytropic murine leukemia viruses. The results indicated that even when barriers to viral entry were bypassed, mouse NIH 3T3 cells and Dunni cells still showed a marked reduction in number of cells expressing HIV compared with the human cells studied, although the intensity of immunostaining of individual positive mouse cells was indistinguishable from that seen on permissive human cell lines. CD4 expression in mouse cells or human brain or skin cells did not influence the number of HIV foci observed after transfection with HIV DNA or infection with pseudotyped HIV. These results suggested that in addition to a block in the usual HIV fusion and entry process, CD4-positive mouse cells differed from human cells in exhibiting partial resistance to HIV infection which acted at a postpenetration step in the infection cycle. This resistance was partially overcome when mouse cells were infected by direct exposure to human lymphocytes producing HIV pseudotyped by amphotropic murine leukemia virus.     

Endogenous retroviruses as potential hazards for vaccines

Retroviruses are classified as exogenous or endogenous according to their mode of transmission. Generally, endogenous retroviruses (ERVs) are not pathogenic in their original hosts; however, some ERVs induce diseases. In humans, a novel gammaretrovirus was discovered in patients with prostate cancer or chronic fatigue syndrome. This virus was closely related to xenotropic murine leukemia virus (X-MLV) and designated as xenotropic murine leukemia virus-related virus (XMRV). The origin and transmission route of XMRV are still unknown at present; however, XMRV may be derived from ERVs of rodents because X-MLVs are ERVs of inbred and wild mice. Many live attenuated vaccines for animals are manufactured by using cell lines from animals, which are known to produce infectious ERVs; however, the risks of infection by ERVs from xenospecies through vaccination have been ignored. This brief review gives an overview of ERVs in cats, the potential risks of ERV infection by vaccination, the biological characteristics of RD-114 virus (a feline ERV), which possibly contaminates vaccines for companion animals, and the methods for detection of infectious RD-114 virus.     

Immunization with Recombinant Prion Protein Leads to Partial Protection in a Murine Model of TSEs through a Novel Mechanism

Transmissible spongiform encephalopathies are neurodegenerative diseases, which despite fervent research remain incurable. Immunization approaches have shown great potential at providing protection, however tolerance effects hamper active immunization protocols. In this study we evaluated the antigenic potential of various forms of recombinant murine prion protein and estimated their protective efficacy in a mouse model of prion diseases. One of the forms tested provided a significant elongation of survival interval. The elongation was mediated via an acute depletion of mature follicular dendritic cells, which are associated with propagation of the prion infectious agent in the periphery and in part to the development of humoral immunity against prion protein. This unprecedented result could offer new strategies for protection against transmissible encephalopathies as well as other diseases associated with follicular dendritic cells.     

Proteolytic inactivation of the bovine spongiform encephalopathy agent

Thermostable proteases have been investigated for their ability to provide a novel biological solution to decontamination of prion agents responsible for transmissible spongiform encephalopathies (TSEs). Proteases were identified that digested total mouse brain homogenate (MBH) protein from uninfected mice. These proteases were then evaluated for digestion of BSE (301V) infectious MBH over a range of pH and temperatures, screened for loss of anti-prion antibody 6H4 immunoreactivity and protease-treated infectious MBH assessed in mouse bioassay using VM mice. Despite a number of proteases eliminating all 6H4-immunoreactive material, only the subtilisin-enzyme Properase showed a significant extension in incubation period in mouse bioassays following a 30-min incubation at 60 degrees C and pH 12. These results demonstrate the potential of the method to provide a practical solution to the problems of TSE contamination of surgical instruments and highlight the inadequacy of using Western blot for assessment of decontamination/inactivation of TSE agents.     

Metal complexes with superoxide dismutase-like activity as candidates for anti-prion drug

Various compounds were evaluated for ability to inhibit the formation of the abnormal protease-resistant form of prion protein (PrP-res) in two cell lines infected with different prion strains. Examination of the structure-activity relationships indicated that compounds with copper-selective chelating ability and whose copper complexes have high SOD-like activity are candidates for anti-prion drug.