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There is growing concern that bovine spongiform encephalopathy (BSE) may have passed from cattle to humans. We report here that transgenic (Tg) mice expressing bovine (Bo) prion protein (PrP) serially propagate BSE prions and that there is no species barrier for transmission from cattle to Tg(BoPrP) mice. These same mice were also highly susceptible to a new variant of Creutzfeldt-Jakob disease (nvCJD) and natural sheep scrapie. The incubation times (Ϸ250 days), neuropathology, and disease-causing PrP isoforms in Tg(BoPrP)Prnp 0/0 mice inoculated with nvCJD and BSE brain extracts were indistinguishable and differed dramatically from those seen in these mice injected with natural scrapie prions. Our findings provide the most compelling evidence to date that prions from cattle with BSE have infected humans and caused fatal neurodegeneration.

Prion diseases can be infectious, sporadic and genetic. The infectious forms of these diseases, including bovine spongiform encephalopathy and Creutzfeldt-Jakob disease, are usually characterized by the accumulation in the brain of the transmissible pathogen, an abnormally folded isoform of the prion protein (PrP) termed PrPSc. However, certain inherited PrP mutations appear to cause neurodegeneration in the absence of PrPSc, working instead by favoured synthesis of CtmPrP, a transmembrane form of PrP. The relationship between the neurodegeneration seen in transmissible prion diseases involving PrPSc and that associated with ctmPrP has remained unclear. Here we find that the effectiveness of accumulated PrPSc in causing neurodegenerative disease depends upon the predilection of host-encoded PrP to be made in the ctmPrP form. Furthermore, the time course of PrPSc accumulation in transmissible prion disease is followed closely by increased generation of CtmPrP. Thus, the accumulation of PrPSc appears to modulate in trans the events involved in generating or metabolising CtmPrP. Together, these data suggest that the events of CtmPrP-mediated neurodegeneration may represent a common step in the pathogenesis of genetic and infectious prion diseases.

Alzheimer's disease (AD) is characterized by progressive neurodegeneration and cerebral accumulation of the β-amyloid peptide (Aβ), but it is unknown what makes neurons susceptible to degeneration. We report that the TGF-β type II receptor (TβRII) is mainly expressed by neurons, and that TβRII levels are reduced in human AD brain and correlate with pathological hallmarks of the disease. Reducing neuronal TGF-β signaling in mice resulted in age-dependent neurodegeneration and promoted Aβ accumulation and dendritic loss in a mouse model of AD. In cultured cells, reduced TGF-β signaling caused neuronal degeneration and resulted in increased levels of secreted Aβ and β-secretase-cleaved soluble amyloid precursor protein. These results show that reduced neuronal TGF-β signaling increases age-dependent neurodegeneration and AD-like disease in vivo. Increasing neuronal TGF-β signaling may thus reduce neurodegeneration and be beneficial in AD. IntroductionAlzheimer's disease (AD) is a progressive neurodegenerative disease that leads to loss of cognitive function in a large number of elderly people. The human AD brain is characterized by the accumulation of β-amyloid peptide (Aβ) in extracellular plaques and hyperphosphorylated tau in intracellular neurofibrillary tangles. In addition, there is degeneration of synapses and dendrites and a progressive loss of neurons involved in memory processes (1). The cause of this degeneration in AD remains unknown, and no effective treatments are available.Survival of neurons is dependent on extracellular signals from neurotrophic factors and related factors with trophic activity (reviewed in ref.2). Levels of the neurotrophin nerve growth factor (NGF) and its receptor, tropomyosin receptor kinase A (TRKA), as well as levels of brain-derived neurotrophic factor (BDNF) and its receptor, TRKB, are lower in human AD brains than in nondemented controls (2-5). It was therefore proposed that a deficiency in neurotrophic factor signaling would promote neurodegeneration and cognitive dysfunction in AD, but this hypothesis has not been tested using specific genetic inhibition of neurotrophic factor signaling in a mouse model for AD (reviewed in refs. 2, 6).

Prion diseases are caused by an infectious protein (20,25). These invariably fatal illnesses cannot be cured using routine antimicrobial agents, and materials contaminated with prions cannot be disinfected by conventional methods. Therefore, it is important to identify compounds that can be used either as therapeutic or disinfecting reagents for prion diseases. Ongoing epidemics of new variant Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (BSE) in the United Kingdom highlight the urgency of this task.We recently reported that branched polyamines could purge scrapie-infected neuroblastoma (ScN2a) cells of PrP Sc , the disease-causing isoform of the prion protein (33). The ability of these compounds to eliminate PrP Sc from ScN2a cells depended upon a highly branched structure and a high surface density of primary amino groups. The most potent compounds identified were generation 4.0 polyamidoamine (PAMAM) and polypropyleneimine (PPI) dendrimers. Dendrimers are branched polyamines manufactured by a repetitive divergent growth technique, allowing the synthesis of successive, welldefined "generations" of homodisperse structures. In the current study, we demonstrate that branched polyamines cure prion-infected cells and identify the site and mechanism of polyamine-mediated prion clearance. We also demonstrate that these compounds can be employed in a rapid and simple assay to discriminate between different prion strains in vitro. MATERIALS AND METHODSChemical compounds. High-molecular-weight polyethyleneimine (PEI) was purchased from Fluka. SuperFect transfection reagent was purchased from Qiagen. All other polyamines were purchased from Sigma-Aldrich. Fluoresceinlabeled PPI was synthesized by mixing 30 mg of fluorescein isothiocyanate (FITC) with 1 mg of PPI generation 4.0 in 2 ml of ethanol overnight at 4°C. Labeled PPI was separated from residual, unreacted FITC using a Sephadex P-2 column.Cultured cells. Cultures of ScN2a cells were maintained as described previously (33). Cytotoxicity after treatment with polyamines was assessed in ScN2a cells by the following four methods: (i) examination of morphology under phase contrast microscopy, (ii) observation of growth curves and cell counts for 3 weeks after treatment, (iii) vital staining of living cells with 0.4% trypan blue (SigmaAldrich), and (iv) assay of dehydrogenase enzymes with 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) (Sigma-Aldrich). For ScN2a cells treated with either PAMAM or PPI generation 4.0 continuously for 1 week, the 50% toxic dose was ϳ50 g/ml.To prepare samples for infectivity assays, 100-mm-diameter plates (Falcon) of confluent cells were washed three times with 5 ml of phosphate-buffered saline, scraped into 2 ml of phosphate-buffered saline, and homogenized by repeated extrusion through a 26-gauge needle. Prion infectivity was determined by intracerebral inoculation of 30 l of cell homogenate into Tg(MoPrP)4053 mice. Mice were observed for clinical signs of scrapie, and a subset of diagnoses were confir...

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