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1.  Prions and lymphoid organs 
Prion  2013;7(2):157-163.
Prion colonization of secondary lymphoid organs (SLOs) is a critical step preceding neuroinvasion in prion pathogenesis. Follicular dendritic cells (FDCs), which depend on both tumor necrosis factor receptor 1 (TNFR1) and lymphotoxin β receptor (LTβR) signaling for maintenance, are thought to be the primary sites of prion accumulation in SLOs. However, prion titers in RML-infected TNFR1−/− lymph nodes and rates of neuroinvasion in TNFR1−/− mice remain high despite the absence of mature FDCs. Recently, we discovered that TNFR1-independent prion accumulation in lymph nodes relies on LTβR signaling. Loss of LTβR signaling in TNFR1−/− lymph nodes coincided with the de-differentiation of high endothelial venules (HEVs)—the primary sites of lymphocyte entry into lymph nodes. These findings suggest that HEVs are the sites through which prions initially invade lymph nodes from the bloodstream. Identification of HEVs as entry portals for prions clarifies a number of previous observations concerning peripheral prion pathogenesis. However, a number of questions still remain: What is the mechanism by which prions are taken up by HEVs? Which cells are responsible for delivering prions to lymph nodes? Are HEVs the main entry site for prions into lymph nodes or do alternative routes also exist? These questions and others are considered in this article.
doi:10.4161/pri.23536
PMCID: PMC3609124  PMID: 23357827
prions; lymph nodes; high endothelial venules; lymphotoxin beta receptor; tumor necrosis factor receptor 1; peripheral prion replication; neuroinvasion; follicular dendritic cell
2.  Genetic Depletion of Complement Receptor CD21/35 Prevents Terminal Prion Disease in a Mouse Model of Chronic Wasting Disease1 
The Complement System has been shown to facilitate peripheral prion pathogenesis. Mice lacking Complement receptors CD21/35 partially resist terminal prion disease when infected intraperitoneally with mouse-adapted scrapie prions. Chronic wasting disease (CWD) is an emerging prion disease of captive and free-ranging cervid populations that, like scrapie, has been shown to involve the immune system, which probably contributes to their relatively facile horizontal and environmental transmission. Here we show that mice overexpressing the cervid prion protein and susceptible to CWD (Tg(cerPrP)5037 mice) but lack CD21/35 expression completely resist clinical CWD upon peripheral infection. CD21/35 deficient Tg5037 mice exhibit greatly impaired splenic prion accumulation and replication throughout disease, similar to CD21/35 deficient murine PrP mice infected with mouse scrapie. TgA5037;CD21/35-/- mice exhibited little or no neuropathology and deposition of misfolded, protease-resistant PrP associated with CWD. CD21/35 translocates to lipid rafts and mediates a strong germinal center response to prion infection that we propose provides the optimal environment for prion accumulation and replication. We further propose a potential role for CD21/35 in selecting prion quasi-species present in prion strains that may exhibit differential zoonotic potential compared to the parental strains.
doi:10.4049/jimmunol.1201579
PMCID: PMC3478448  PMID: 23002439
ChronicWasting Disease; Complement; Prions
3.  Crystallization and preliminary X-ray diffraction analysis of prion protein bound to the Fab fragment of the POM1 antibody 
The complex of MoPrP(120–232) and Fab POM1 has been crystallized (space group C2, unit-cell parameters a = 83.68, b = 106.9, c = 76.25 Å, β = 95.6°). Diffraction data to 2.30 Å resolution have been collected using synchrotron radiation.
Prion diseases are neurodegenerative diseases that are characterized by the con­version of the cellular prion protein PrPc to the pathogenic isoform PrPsc. Several antibodies are known to interact with the cellular prion protein and to inhibit this transition. An antibody Fab fragment, Fab POM1, was produced that recognizes a structural motif of the C-terminal domain of mouse prion protein. To study the mechanism by which Fab POM1 recognizes and binds the prion molecule, the complex between Fab POM1 and the C-terminal domain of mouse prion (residues 120–232) was prepared and crystallized. Crystals of this binary complex belonged to the monoclinic space group C2, with unit-cell parameters a = 83.68, b = 106.9, c = 76.25 Å, β = 95.6°.
doi:10.1107/S1744309111026273
PMCID: PMC3212364  PMID: 22102029
prions; antibodies; POM1
4.  Follicular Dendritic Cells Emerge from Ubiquitous Perivascular Precursors 
Cell  2012;150(1):194-206.
Summary
The differentiation of follicular dendritic cells (FDC) is essential to the remarkable microanatomic plasticity of lymphoid follicles. Here we show that FDC arise from ubiquitous perivascular precursors (preFDC) expressing platelet-derived growth factor receptor β (PDGFRβ). PDGFRβ-Cre-driven reporter gene recombination resulted in FDC labeling, whereas conditional ablation of PDGFRβ+-derived cells abolished FDC, indicating that FDC originate from PDGFRβ+ cells. Lymphotoxin-α-overexpressing prion protein (PrP)+ kidneys developed PrP+ FDC after transplantation into PrP mice, confirming that preFDC exist outside lymphoid organs. Adipose tissue-derived PDGFRβ+ stromal-vascular cells responded to FDC maturation factors and, when transplanted into lymphotoxin β receptor (LTβR) kidney capsules, differentiated into Mfge8+CD21/35+ FcγRIIβ+PrP+ FDC capable of trapping immune complexes and recruiting B cells. Spleens of lymphocyte-deficient mice contained perivascular PDGFRβ+ FDC precursors whose expansion required both lymphoid tissue inducer (LTi) cells and lymphotoxin. The ubiquity of preFDC and their strategic location at blood vessels may explain the de novo generation of organized lymphoid tissue at sites of lymphocytic inflammation.
doi:10.1016/j.cell.2012.05.032
PMCID: PMC3704230  PMID: 22770220
5.  BSE-associated Prion-Amyloid Cardiomyopathy in Primates 
Emerging Infectious Diseases  2013;19(6):985-988.
Prion amyloidosis occurred in the heart of 1 of 3 macaques intraperitoneally inoculated with bovine spongiform encephalopathy prions. This macaque had a remarkably long duration of disease and signs of cardiac distress. Variant Creutzfeldt-Jakob disease, caused by transmission of bovine spongiform encephalopathy to humans, may manifest with cardiac symptoms from prion-amyloid cardiomyopathy.
doi:10.3201/eid1906.120906
PMCID: PMC3713817  PMID: 23735198
BSE; cardiomyopathy; prions; PrPSc; vCJD; variant Creutzfeldt-Jakob disease; bovine spongiform encephalopathy; primates
6.  The complex PrPc-Fyn couples human oligomeric Aβ with pathological tau changes in Alzheimer’s disease 
Amidst controversy, the cellular form of the prion protein PrPc has been proposed to mediate oligomeric Aβ-induced deficits. In contrast, there is consistent evidence that the Src kinase Fyn is activated by Aβ oligomers and leads to synaptic and cognitive impairment in transgenic animals. However, the molecular mechanism by which soluble Aβ activates Fyn remains unknown. Combining the use of human and transgenic mouse brain tissue as well as primary cortical neurons, we demonstrate that soluble Aβ binds to PrPc at neuronal dendritic spines in vivo and in vitro where it forms a complex with Fyn, resulting in the activation of the kinase. Using the antibody 6D11 to prevent oligomeric Aβ from binding to PrPc, we abolished Fyn activation and Fyn-dependent tau hyperphosphorylation induced by endogenous oligomeric Aβ in vitro. Finally we showed that gene dosage of Prnp regulates Aβ-induced Fyn/tau alterations. Altogether, our findings identify a complete signaling cascade linking one specific endogenous Aβ oligomer, Fyn alteration and tau hyperphosphorylation in cellular and animal models modeling aspects of the molecular pathogenesis of Alzheimer’s disease.
doi:10.1523/JNEUROSCI.1858-12.2012
PMCID: PMC3568961  PMID: 23175838
amyloid-beta; oligomer; prion protein; Fyn; Alzheimer’s disease; transgenic
7.  Prions, prionoids and pathogenic proteins in Alzheimer disease 
Prion  2013;7(1):55-59.
Like patients with prion disease, Alzheimer patients suffer from a fatal, progressive form of dementia. There is growing evidence that amyloid-β (Aβ) aggregates may be transmissible similar to prions, at least under extreme experimental conditions. However, unlike mice infected with prion protein (PrP) prions, those inoculated with Aβ do not die. The transmission of Aβ and PrP thus differs conspicuously in the neurological effects they induce in their hosts, the difference being no less than a matter of life and death. Far from being a mere academic nuance, this distinction between Aβ and PrP begs the crucial questions of what, exactly, controls prion toxicity and how prion toxicity relates to prion infectivity.
doi:10.4161/pri.23061
PMCID: PMC3609051  PMID: 23208281
Alzheimer’s disease; PrP; amyloid-β; pathogenic proteins; prionoids; prions; tau
8.  Prion Pathogenesis Is Faithfully Reproduced in Cerebellar Organotypic Slice Cultures 
PLoS Pathogens  2012;8(11):e1002985.
Prions cause neurodegeneration in vivo, yet prion-infected cultured cells do not show cytotoxicity. This has hampered mechanistic studies of prion-induced neurodegeneration. Here we report that prion-infected cultured organotypic cerebellar slices (COCS) experienced progressive spongiform neurodegeneration closely reproducing prion disease, with three different prion strains giving rise to three distinct patterns of prion protein deposition. Neurodegeneration did not occur when PrP was genetically removed from neurons, and a comprehensive pharmacological screen indicated that neurodegeneration was abrogated by compounds known to antagonize prion replication. Prion infection of COCS and mice led to enhanced fodrin cleavage, suggesting the involvement of calpains or caspases in pathogenesis. Accordingly, neurotoxicity and fodrin cleavage were prevented by calpain inhibitors but not by caspase inhibitors, whereas prion replication proceeded unimpeded. Hence calpain inhibition can uncouple prion replication from its neurotoxic sequelae. These data validate COCS as a powerful model system that faithfully reproduces most morphological hallmarks of prion infections. The exquisite accessibility of COCS to pharmacological manipulations was instrumental in recognizing the role of calpains in neurotoxicity, and significantly extends the collection of tools necessary for rigorously dissecting prion pathogenesis.
Author Summary
Transmissible spongiform encephalopathies (TSEs) are a group of fatal protein misfolding diseases causing neurodegeneration in vivo. TSEs are unique in that the infectious agent termed ‘prion’ consists of a misfolded protein lacking sequence specific nucleic acids. Prion-infected cultured cells do not develop visible pathological changes, and this has hampered mechanistic studies of prion-induced neurodegeneration. Here, we have developed a prion-induced neurodegeneration model that uses cultured slices of living brain tissue. Such slices display all the classical hallmark of prion disease, namely prion replication, inflammation, spongiform changes and neurodegeneration. Neurotoxicity is blocked by anti-prion drugs by reducing prion replication. We demonstrate for the first time an involvement of calcium-regulated cysteine proteases called calpains in driving neurotoxicity. We find that the proteolytic processing of the calpain substrate is induced by prion infection and blocked by calpain inhibitors without prion replication being affected. The assay system developed here allows for precise dissection of the mechanisms of prion-induced degeneration with pharmacological means.
doi:10.1371/journal.ppat.1002985
PMCID: PMC3486912  PMID: 23133383
9.  Genetic Depletion of Complement Receptors CD21/35 Prevents Terminal Prion Disease in a Mouse Model of Chronic Wasting Disease 
The complement system has been shown to facilitate peripheral prion pathogenesis. Mice lacking complement receptors CD21/35 partially resist terminal prion disease when infected i.p. with mouse-adapted scrapie prions. Chronic wasting disease (CWD) is an emerging prion disease of captive and free-ranging cervid populations that, similar to scrapie, has been shown to involve the immune system, which probably contributes to their relatively facile horizontal and environmental transmission. In this study, we show that mice overexpressing the cervid prion protein and susceptible to CWD (Tg(cerPrP)5037 mice) but lack CD21/35 expression completely resist clinical CWD upon peripheral infection. CD21/35-deficient Tg5037 mice exhibit greatly impaired splenic prion accumulation and replication throughout disease, similar to CD21/35-deficient murine prion protein mice infected with mouse scrapie. TgA5037;CD21/35−/− mice exhibited little or no neuropathology and deposition of misfolded, protease-resistant prion protein associated with CWD. CD21/35 translocate to lipid rafts and mediates a strong germinal center response to prion infection that we propose provides the optimal environment for prion accumulation and replication. We further propose a potential role for CD21/35 in selecting prion quasi-species present in prion strains that may exhibit differential zoonotic potential compared with the parental strains.
doi:10.4049/jimmunol.1201579
PMCID: PMC3478448  PMID: 23002439
10.  Cerebrospinal fluid biomarker supported diagnosis of Creutzfeldt–Jakob disease and rapid dementias: a longitudinal multicentre study over 10 years 
Brain  2012;135(10):3051-3061.
To date, cerebrospinal fluid analysis, particularly protein 14-3-3 testing, presents an important approach in the identification of Creutzfeldt–Jakob disease cases. However, one special point of criticism of 14-3-3 testing is the specificity in the differential diagnosis of rapid dementia. The constant observation of increased cerebrospinal fluid referrals in the national surveillance centres over the last years raises the concern of declining specificity due to higher number of cerebrospinal fluid tests performed in various neurological conditions. Within the framework of a European Community supported longitudinal multicentre study (‘cerebrospinal fluid markers’) we analysed the spectrum of rapid progressive dementia diagnoses, their potential influence on 14-3-3 specificity as well as results of other dementia markers (tau, phosphorylated tau and amyloid-β1–42) and evaluated the specificity of 14-3-3 in Creutzfeldt–Jakob disease diagnosis for the years 1998–2008. A total of 29 022 cerebrospinal fluid samples were analysed for 14-3-3 protein and other cerebrospinal fluid dementia markers in patients with rapid dementia and suspected Creutzfeldt–Jakob disease in the participating centres. In 10 731 patients a definite diagnosis could be obtained. Protein 14-3-3 specificity was analysed for Creutzfeldt–Jakob disease with respect to increasing cerebrospinal fluid tests per year and spectrum of differential diagnosis. Ring trials were performed to ensure the comparability between centres during the reported time period. Protein 14-3-3 test specificity remained high and stable in the diagnosis of Creutzfeldt–Jakob disease during the observed time period across centres (total specificity 92%; when compared with patients with definite diagnoses only: specificity 90%). However, test specificity varied with respect to differential diagnosis. A high 14-3-3 specificity was obtained in differentiation to other neurodegenerative diseases (95–97%) and non-neurological conditions (91–97%). We observed lower specificity in the differential diagnoses of acute neurological diseases (82–87%). A marked and constant increase in cerebrospinal fluid test referrals per year in all centres did not influence 14-3-3 test specificity and no change in spectrum of differential diagnosis was observed. Cerebrospinal fluid protein 14-3-3 detection remains an important test in the diagnosis of Creutzfeldt–Jakob disease. Due to a loss in specificity in acute neurological events, the interpretation of positive 14-3-3 results needs to be performed in the clinical context. The spectrum of differential diagnosis of rapid progressive dementia varied from neurodegenerative dementias to dementia due to acute neurological conditions such as inflammatory diseases and non-neurological origin.
doi:10.1093/brain/aws238
PMCID: PMC3470713  PMID: 23012332
rapid dementia; Creutzfeldt–Jakob disease; cerebrospinal fluid; 14-3-3; specificity; neurodegeneration; differential diagnosis in dementia
11.  Evaluation of OPEN Zinc Finger Nucleases for Direct Gene Targeting of the ROSA26 Locus in Mouse Embryos 
PLoS ONE  2012;7(9):e41796.
Zinc finger nucleases (ZFNs) enable precise genome modification in a variety of organisms and cell types. Commercial ZFNs were reported to enhance gene targeting directly in mouse zygotes, whereas similar approaches using publicly available resources have not yet been described. Here we report precise targeted mutagenesis of the mouse genome using Oligomerized Pool Engineering (OPEN) ZFNs. OPEN ZFN can be constructed using publicly available resources and therefore provide an attractive alternative for academic researchers. Two ZFN pairs specific to the mouse genomic locus gt(ROSA26)Sor were generated by OPEN selections and used for gene disruption and homology-mediated gene replacement in single cell mouse embryos. One specific ZFN pair facilitated non-homologous end joining (NHEJ)-mediated gene disruption when expressed in mouse zygotes. We also observed a single homologous recombination (HR)-driven gene replacement event when this ZFN pair was co-injected with a targeting vector. Our experiments demonstrate the feasibility of achieving both gene ablation through NHEJ and gene replacement by HR by using the OPEN ZFN technology directly in mouse zygotes.
doi:10.1371/journal.pone.0041796
PMCID: PMC3435328  PMID: 22970113
12.  Lymphotoxin, but Not TNF, Is Required for Prion Invasion of Lymph Nodes 
PLoS Pathogens  2012;8(8):e1002867.
Neuroinvasion and subsequent destruction of the central nervous system by prions are typically preceded by a colonization phase in lymphoid organs. An important compartment harboring prions in lymphoid tissue is the follicular dendritic cell (FDC), which requires both tumor necrosis factor receptor 1 (TNFR1) and lymphotoxin β receptor (LTβR) signaling for maintenance. However, prions are still detected in TNFR1−/− lymph nodes despite the absence of mature FDCs. Here we show that TNFR1-independent prion accumulation in lymph nodes depends on LTβR signaling. Loss of LTβR signaling, but not of TNFR1, was concurrent with the dedifferentiation of high endothelial venules (HEVs) required for lymphocyte entry into lymph nodes. Using luminescent conjugated polymers for histochemical PrPSc detection, we identified PrPSc deposits associated with HEVs in TNFR1−/− lymph nodes. Hence, prions may enter lymph nodes by HEVs and accumulate or replicate in the absence of mature FDCs.
Author Summary
Prions are unique infectious agents thought to be composed entirely of an abnormal conformer of the endogenous prion protein. Prions cause a severe neurological disorder in humans and other animals known as prion disease. Though prion disease can arise spontaneously or from genetic mutations in the gene encoding the prion protein, many cases of prion disease arise due to peripheral exposure to the infectious agent. In these cases, prions must journey from the gastrointestinal tract and/or the bloodstream to the brain. Prions often colonize secondary lymphoid organs prior to invading the nervous system via neighboring peripheral nerves. Prion accumulation in follicular dendritic cells found in germinal centers of lymphoid organs is thought to be a crucial step in this process. However, prion colonization of lymph nodes, in contrast to spleen, does not depend on follicular dendritic cells, indicating that other mechanisms must exist. Here, we identify the signaling pathway required for follicular dendritic cell-independent prion colonization of lymph nodes, which also controls the differentiation of high endothelial venules, the primary entry point for lymphocytes into lymph nodes. Importantly, prions could be found within these structures, indicating that high endothelial venules are required for prion entry and accumulation in lymph nodes.
doi:10.1371/journal.ppat.1002867
PMCID: PMC3415451  PMID: 22912582
13.  Aerosols 
Prion  2011;5(3):138-141.
We and others have recently reported that prions can be transmitted to mice via aerosols. These reports spurred a lively public discussion on the possible public-health threats represented by prion-containing aerosols. Here we offer our view on the context in which these findings should be placed. On the one hand, the fact that nebulized prions can transmit disease cannot be taken to signify that prions are airborne under natural circumstances. On the other hand, it appears important to underscore the fact that aerosols can originate very easily in a broad variety of experimental and natural environmental conditions. Aerosols are a virtually unavoidable consequence of the handling of fluids; complete prevention of the generation of aerosols is very difficult. While prions have never been found to be transmissible via aerosols under natural conditions, it appears prudent to strive to minimize exposure to potentially prion-infected aerosols whenever the latter may arise—for example in scientific and diagnostic laboratories handling brain matter, cerebrospinal fluids, and other potentially contaminated materials, as well as abattoirs. Equally important is that prion biosafety training be focused on the control of, and protection from, prion-infected aerosols.
doi:10.4161/pri.5.3.16851
PMCID: PMC3226037  PMID: 21778819
prion; prion transmission; scrapie; chronic wasting diseases; CWD; Creutzfeldt-Jacob-disease; CJD; TSE; aerosol; pathogens; allergens
14.  ZyFISH: A Simple, Rapid and Reliable Zygosity Assay for Transgenic Mice 
PLoS ONE  2012;7(5):e37881.
Microinjection of DNA constructs into fertilized mouse oocytes typically results in random transgene integration at a single genomic locus. The resulting transgenic founders can be used to establish hemizygous transgenic mouse lines. However, practical and experimental reasons often require that such lines be bred to homozygosity. Transgene zygosity can be determined by progeny testing assays which are expensive and time-consuming, by quantitative Southern blotting which is labor-intensive, or by quantitative PCR (qPCR) which requires transgene-specific design. Here, we describe a zygosity assessment procedure based on fluorescent in situ hybridization (zyFISH). The zyFISH protocol entails the detection of transgenic loci by FISH and the concomitant assignment of homozygosity using a concise and unbiased scoring system. The method requires small volumes of blood, is scalable to at least 40 determinations per assay, and produces results entirely consistent with the progeny testing assay. This combination of reliability, simplicity and cost-effectiveness makes zyFISH a method of choice for transgenic mouse zygosity determinations.
doi:10.1371/journal.pone.0037881
PMCID: PMC3362593  PMID: 22666404
15.  Five Questions on Prion Diseases 
PLoS Pathogens  2012;8(5):e1002651.
doi:10.1371/journal.ppat.1002651
PMCID: PMC3342938  PMID: 22570608
16.  Spongiform encephalopathy in transgenic mice expressing a point mutation in the β2–α2 loop of the prion protein 
Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases attributed to misfolding of the cellular prion protein, PrPC, into a β-sheet-rich, aggregated isoform, PrPSc. We previously found that expression of mouse PrP with the two amino acid substitutions S170N and N174T, which result in high structural order of the β2–α2 loop in the NMR structure at pH 4.5 and 20 °C, caused transmissible de novo prion disease in transgenic mice. Here we report that expression of mouse PrP with the single-residue substitution D167S, which also results in a structurally well-ordered β2–α2 loop at 20 °C, elicits spontaneous PrP aggregation in vivo. Transgenic mice expressing PrPD167S developed a progressive encephalopathy characterized by abundant PrP plaque formation, spongiform change, and gliosis. These results add to the evidence that the β2–α2 loop has an important role in intermolecular interactions, including that it may be a key determinant of prion protein aggregation.
doi:10.1523/JNEUROSCI.3504-11.2011
PMCID: PMC3205959  PMID: 21957246
17.  Microglial ablation and lipopolysaccharide preconditioning affects pilocarpine-induced seizures in mice 
Neurobiology of disease  2010;39(1):85-97.
Activated microglia have been associated with neurodegeneration in patients and in animal models of Temporal Lobe Epilepsy (TLE), however their precise functions as neurotoxic or neuroprotective is a topic of significant investigation. To explore this, we examined the effects of pilocarpine induced seizures in transgenic mice where microglia/macrophages were conditionally ablated. We found that unilateral ablation of microglia from the dorsal hippocampus did not alter acute seizure sensitivity. However, when this procedure was coupled with lipopolysaccharide (LPS) preconditioning (1 mg/kg given 24 hours prior to acute seizure), we observed a significant pro-convulsant phenomenon. This effect was associated with lower metabolic activation in the ipsilateral hippocampus during acute seizures, and could be attributed to activity in the mossy fiber pathway. These findings reveal that preconditioning with LPS 24 hours prior to seizure induction may have a protective effect which is abolished by unilateral hippocampal microglia/macrophage ablation.
doi:10.1016/j.nbd.2010.04.001
PMCID: PMC2875881  PMID: 20382223
Microglia; seizure; epilepsy; lipopolysaccharide; status epilepticus; glucose; metabolism; positron emission tomography; pilocarpine; inflammation; mouse; FDG; imaging; immune system
18.  Sheep with Scrapie and Mastitis Transmit Infectious Prions through the Milk▿  
Journal of Virology  2010;85(2):1136-1139.
Prions are misfolded proteins that are infectious and naturally transmitted, causing a fatal neurological disease in humans and animals. Prion shedding routes have been shown to be modified by inflammation in excretory organs, such as the kidney. Here, we show that sheep with scrapie and lentiviral mastitis secrete prions into the milk and infect nearly 90% of naïve suckling lambs. Thus, lentiviruses may enhance prion transmission, conceivably sustaining prion infections in flocks for generations. This study also indicates a risk of prion spread to sheep and potentially to other animals through dietary exposure to pooled sheep milk or milk products.
doi:10.1128/JVI.02022-10
PMCID: PMC3020009  PMID: 21084475
19.  Engulfment of cerebral apoptotic bodies controls the course of prion disease in a mouse strain–dependent manner 
The Journal of Experimental Medicine  2010;207(10):2271-2281.
Progressive accumulation of PrPSc, a hallmark of prion diseases, occurs when conversion of PrPC into PrPSc is faster than PrPSc clearance. Engulfment of apoptotic bodies by phagocytes is mediated by Mfge8 (milk fat globule epidermal growth factor 8). In this study, we show that brain Mfge8 is primarily produced by astrocytes. Mfge8 ablation induced accelerated prion disease and reduced clearance of cerebellar apoptotic bodies in vivo, as well as excessive PrPSc accumulation and increased prion titers in prion-infected C57BL/6 × 129Sv mice and organotypic cerebellar slices derived therefrom. These phenotypes correlated with the presence of 129Sv genomic markers in hybrid mice and were not observed in inbred C57BL/6 Mfge8−/− mice, suggesting the existence of additional strain-specific genetic modifiers. Because Mfge8 receptors are expressed by microglia and depletion of microglia increases PrPSc accumulation in organotypic cerebellar slices, we conclude that engulfment of apoptotic bodies by microglia may be an important pathway of prion clearance controlled by astrocyte-borne Mfge8.
doi:10.1084/jem.20092401
PMCID: PMC2947076  PMID: 20837697
20.  The Strain-Encoded Relationship between PrPSc Replication, Stability and Processing in Neurons is Predictive of the Incubation Period of Disease 
PLoS Pathogens  2011;7(3):e1001317.
Prion strains are characterized by differences in the outcome of disease, most notably incubation period and neuropathological features. While it is established that the disease specific isoform of the prion protein, PrPSc, is an essential component of the infectious agent, the strain-specific relationship between PrPSc properties and the biological features of the resulting disease is not clear. To investigate this relationship, we examined the amplification efficiency and conformational stability of PrPSc from eight hamster-adapted prion strains and compared it to the resulting incubation period of disease and processing of PrPSc in neurons and glia. We found that short incubation period strains were characterized by more efficient PrPSc amplification and higher PrPSc conformational stabilities compared to long incubation period strains. In the CNS, the short incubation period strains were characterized by the accumulation of N-terminally truncated PrPSc in the soma of neurons, astrocytes and microglia in contrast to long incubation period strains where PrPSc did not accumulate to detectable levels in the soma of neurons but was detected in glia similar to short incubation period strains. These results are inconsistent with the hypothesis that a decrease in conformational stability results in a corresponding increase in replication efficiency and suggest that glia mediated neurodegeneration results in longer survival times compared to direct replication of PrPSc in neurons.
Author Summary
Prion diseases are a group of infectious fatal neurodegenerative diseases that affect animals including humans. This unique infectious agent is the result of a post-translational conformational change of the normal form of the prion protein, PrPC, to an infectious form of the prion protein, PrPSc. Different strains of the infectious agent result in characteristic incubation periods and neuropathological features within a single host species. These strain-specific differences in disease outcome are likely due to strain-specific conformations of PrPSc, though the mechanisms by which different conformation can affect prion strain properties are not understood. The aim of this study was to investigate the relationship between the biochemical properties of PrPSc to the corresponding neuropathological characteristics of eight hamster-adapted prion strains. Our findings indicate that PrPSc from short incubation period strains were more efficiently replicated, had a more stable conformation, and were observed to be more resistant to clearance from the soma of neurons compared to prion strains with a relatively long incubation period. These results suggest the progression of prion disease is influenced by the balance between replication and clearance of PrPSc in neurons.
doi:10.1371/journal.ppat.1001317
PMCID: PMC3060105  PMID: 21437239
21.  Aerosols Transmit Prions to Immunocompetent and Immunodeficient Mice 
PLoS Pathogens  2011;7(1):e1001257.
Prions, the agents causing transmissible spongiform encephalopathies, colonize the brain of hosts after oral, parenteral, intralingual, or even transdermal uptake. However, prions are not generally considered to be airborne. Here we report that inbred and crossbred wild-type mice, as well as tga20 transgenic mice overexpressing PrPC, efficiently develop scrapie upon exposure to aerosolized prions. NSE-PrP transgenic mice, which express PrPC selectively in neurons, were also susceptible to airborne prions. Aerogenic infection occurred also in mice lacking B- and T-lymphocytes, NK-cells, follicular dendritic cells or complement components. Brains of diseased mice contained PrPSc and transmitted scrapie when inoculated into further mice. We conclude that aerogenic exposure to prions is very efficacious and can lead to direct invasion of neural pathways without an obligatory replicative phase in lymphoid organs. This previously unappreciated risk for airborne prion transmission may warrant re-thinking on prion biosafety guidelines in research and diagnostic laboratories.
Author Summary
Prions, which are the cause of fatal neurodegenerative disorders termed transmissible spongiform encephalopathies (TSEs), can be experimentally or naturally transmitted via prion-contaminated food, blood, milk, saliva, feces and urine. Here we demonstrate that prions can be transmitted through aerosols in mice. This also occurs in the absence of immune cells as demonstrated by experiments with mice lacking B-, T-, follicular dendritic cells (FDCs), lymphotoxin signaling or with complement-deficient mice. Therefore, a functionally intact immune system is not strictly needed for aerogenic prion infection. These results suggest that current biosafety guidelines applied in diagnostic and scientific laboratories ought to include prion aerosols as a potential vector for prion infection.
doi:10.1371/journal.ppat.1001257
PMCID: PMC3020930  PMID: 21249178
22.  Efficient Generation of Multipotent Mesenchymal Stem Cells from Umbilical Cord Blood in Stroma-Free Liquid Culture 
PLoS ONE  2010;5(12):e15689.
Background
Haematopoiesis is sustained by haematopoietic (HSC) and mesenchymal stem cells (MSC). HSC are the precursors for blood cells, whereas marrow, stroma, bone, cartilage, muscle and connective tissues derive from MSC. The generation of MSC from umbilical cord blood (UCB) is possible, but with low and unpredictable success. Here we describe a novel, robust stroma-free dual cell culture system for long-term expansion of primitive UCB-derived MSC.
Methods and Findings
UCB-derived mononuclear cells (MNC) or selected CD34+ cells were grown in liquid culture in the presence of serum and cytokines. Out of 32 different culture conditions that have been tested for the efficient expansion of HSC, we identified one condition (DMEM, pooled human AB serum, Flt-3 ligand, SCF, MGDF and IL-6; further denoted as D7) which, besides supporting HSC expansion, successfully enabled long-term expansion of stromal/MSC from 8 out of 8 UCB units (5 MNC-derived and 3 CD34+ selected cells). Expanded MSC displayed a fibroblast-like morphology, expressed several stromal/MSC-related antigens (CD105, CD73, CD29, CD44, CD133 and Nestin) but were negative for haematopoietic cell markers (CD45, CD34 and CD14). MSC stemness phenotype and their differentiation capacity in vitro before and after high dilution were preserved throughout long-term culture. Even at passage 24 cells remained Nestin+, CD133+ and >95% were positive for CD105, CD73, CD29 and CD44 with the capacity to differentiate into mesodermal lineages. Similarly we show that UCB derived MSC express pluripotency stem cell markers despite differences in cell confluency and culture passages.
Further, we generated MSC from peripheral blood (PB) MNC of 8 healthy volunteers. In all cases, the resulting MSC expressed MSC-related antigens and showed the capacity to form CFU-F colonies.
Conclusions
This novel stroma-free liquid culture overcomes the existing limitation in obtaining MSC from UCB and PB enabling so far unmet therapeutic applications, which might substantially affect clinical practice.
doi:10.1371/journal.pone.0015689
PMCID: PMC3012708  PMID: 21209896
23.  Preclinical Deposition of Pathological Prion Protein in Muscle of Experimentally Infected Primates 
PLoS ONE  2010;5(11):e13906.
Prion diseases are transmissible fatal neurodegenerative disorders affecting humans and animals. A central step in disease progression is the accumulation of a misfolded form (PrPSc) of the host encoded prion protein (PrPC) in neuronal and non-neuronal tissues. The involvement of peripheral tissues in preclinical states increases the risk of accidental transmission. On the other hand, detection of PrPSc in non-neuronal easy-accessible compartments such as muscle may offer a novel diagnostic tool. Primate models have proven invaluable to investigate prion diseases. We have studied the deposition of PrPSc in muscle and central nervous system of rhesus monkeys challenged with sporadic Creutzfeldt-Jakob disease (sCJD), variant CJD (vCJD) and bovine spongiform encephalopathy (BSE) in preclinical and clinical stage using biochemical and morphological methods. Here, we show the preclinical presence of PrPSc in muscle and central nervous system of rhesus monkeys experimentally infected with vCJD.
doi:10.1371/journal.pone.0013906
PMCID: PMC2978702  PMID: 21085647
24.  Prion protein and Aβ-related synaptic toxicity impairment 
EMBO Molecular Medicine  2010;2(8):306-314.
Alzheimer's disease (AD), the most common neurodegenerative disorder, goes along with extracellular amyloid-β (Aβ) deposits. The cognitive decline observed during AD progression correlates with damaged spines, dendrites and synapses in hippocampus and cortex. Numerous studies have shown that Aβ oligomers, both synthetic and derived from cultures and AD brains, potently impair synaptic structure and functions. The cellular prion protein (PrPC) was proposed to mediate this effect. We report that ablation or overexpression of PrPC had no effect on the impairment of hippocampal synaptic plasticity in a transgenic model of AD. These findings challenge the role of PrPC as a mediator of Aβ toxicity.
See accompanying article: http://dx.doi.org/10.1002/emmm.2010000868.
doi:10.1002/emmm.201000082
PMCID: PMC2962809  PMID: 20665634
Alzheimer's disease; amyloid; prion protein; synaptic plasticity
25.  Experimental Verification of a Traceback Phenomenon in Prion Infection▿  
Journal of Virology  2010;84(7):3230-3238.
The clinicopathological phenotypes of sporadic Creutzfeldt-Jakob disease (sCJD) correlate with the allelotypes (M or V) of the polymorphic codon 129 of the human prion protein (PrP) gene and the electrophoretic mobility patterns of abnormal prion protein (PrPSc). Transmission of sCJD prions to mice expressing human PrP with a heterologous genotype (referred to as cross-sequence transmission) results in prolonged incubation periods. We previously reported that cross-sequence transmission can generate a new prion strain with unique transmissibility, designated a traceback phenomenon. To verify experimentally the traceback of sCJD-VV2 prions, we inoculated sCJD-VV2 prions into mice expressing human PrP with the 129M/M genotype. These 129M/M mice showed altered neuropathology and a novel PrPSc type after a long incubation period. We then passaged the brain homogenate from the 129M/M mouse inoculated with sCJD-VV2 prions into other 129M/M or 129V/V mice. Despite cross-sequence transmission, 129V/V mice were highly susceptible to these prions compared to the 129M/M mice. The neuropathology and PrPSc type of the 129V/V mice inoculated with the 129M/M mouse-passaged sCJD-VV2 prions were identical to those of the 129V/V mice inoculated with sCJD-VV2 prions. Moreover, we generated for the first time a type 2 PrPSc-specific antibody in addition to type 1 PrPSc-specific antibody and discovered that drastic changes in the PrPSc subpopulation underlie the traceback phenomenon. Here, we report the first direct evidence of the traceback in prion infection.
doi:10.1128/JVI.02387-09
PMCID: PMC2838106  PMID: 20089646

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