Azucena Lemus

Asymptomatic deer excrete infectious prions in faeces.

Infectious prion diseases—scrapie of sheep and chronic wasting disease (CWD) of several species in the deer family—are transmitted naturally within affected host populations. Although several possible sources of contagion have been identified in excretions and secretions from symptomatic animals, the biological importance of these sources in sustaining epidemics remains unclear. Here we show that asymptomatic CWD-infected mule deer (Odocoileus hemionus) excrete CWD prions in their faeces long before they develop clinical signs of prion disease. Intracerebral inoculation of irradiated deer faeces into transgenic mice overexpressing cervid prion protein (PrP) revealed infectivity in 14 of 15 faecal samples collected from five deer at 7–11 months before the onset of neurological disease. Although prion concentrations in deer faeces were considerably lower than in brain tissue from the same deer collected at the end of the disease, the estimated total infectious dose excreted in faeces by an infected deer over the disease course may approximate the total contained in a brain. Prolonged faecal prion excretion by infected deer provides a plausible natural mechanism that might explain the high incidence and efficient horizontal transmission of CWD within deer herds, as well as prion transmission among other susceptible cervids.

Protease-sensitive synthetic prions.

Prions arise when the cellular prion protein (PrPC) undergoes a self-propagating conformational change; the resulting infectious conformer is designated PrPSc. Frequently, PrPSc is protease-resistant but protease-sensitive (s) prions have been isolated in humans and other animals. We report here that protease-sensitive, synthetic prions were generated in vitro during polymerization of recombinant (rec) PrP into amyloid fibers. In 22 independent experiments, recPrP amyloid preparations, but not recPrP monomers or oligomers, transmitted disease to transgenic mice (n = 164), denoted Tg9949 mice, that overexpress N-terminally truncated PrP. Tg9949 control mice (n = 174) did not spontaneously generate prions although they were prone to late-onset spontaneous neurological dysfunction. When synthetic prion isolates from infected Tg9949 mice were serially transmitted in the same line of mice, they exhibited sPrPSc and caused neurodegeneration. Interestingly, these protease-sensitive prions did not shorten the life span of Tg9949 mice despite causing extensive neurodegeneration. We inoculated three synthetic prion isolates into Tg4053 mice that overexpress full-length PrP; Tg4053 mice are not prone to developing spontaneous neurological dysfunction. The synthetic prion isolates caused disease in 600–750 days in Tg4053 mice, which exhibited sPrPSc. These novel synthetic prions demonstrate that conformational changes in wild-type PrP can produce mouse prions composed exclusively of sPrPSc.

Bioluminescence imaging of Aβ deposition in bigenic mouse models of Alzheimer's disease

Transgenic (Tg) mouse models of Alzheimers disease have served as valuable tools for investigating pathogenic mechanisms related to Aβ accumulation. However, assessing disease status in these animals has required time-consuming behavioral assessments or postmortem neuropathological analysis. Here, we report a method for tracking the progression of Aβ accumulation in vivo using bioluminescence imaging (BLI) on two lines of Tg mice, which express luciferase (luc) under control of the Gfap promoter as well as mutant human amyloid precursor protein. Bigenic mice exhibited an age-dependent increase in BLI signals that correlated with the deposition of Aβ in the brain. Bioluminescence signals began to increase in 7-mo-old Tg(CRND8:Gfap-luc) mice and 14-mo-old Tg(APP23:Gfap-luc) mice. When Tg(APP23:Gfap-luc) mice were inoculated with brain homogenates from aged Tg(APP23) mice, BLI detected the accelerated disease onset and induced Aβ deposition at 11 mo of age. Because of its rapid, noninvasive, and quantitative format, BLI permits the objective repeated analysis of individual mice at multiple time points, which is likely to facilitate the testing of Aβ-directed therapeutics.

Measuring prions by bioluminescence imaging

Prions are infectious proteins that cause fatal neurodegenerative diseases. Because astrocytic gliosis marked by the deposition of fibrils composed of GFAP is a prominent feature of prion disease, we asked whether GFAP might be used as a surrogate marker for prions. To interrogate this posit, we inoculated prions into transgenic (Tg) mice expressing luciferase (luc) under the GFAP gene (Gfap) promoter, denoted Tg(Gfap-luc) mice. Weekly noninvasive, bioluminescence imaging (BLI) detected an increase in light emitted from the brains of Tg(Gfap-luc) mice at ≈55 d after inoculation and ≈62 d before neurologic deficits appeared. To determine whether BLI could be used as a proxy bioassay for prion infectivity, we performed endpoint titrations of prions in Tg(Gfap-luc) mice. BLI bioassays were as or more sensitive than those determined by the onset of neurological dysfunction, and were completed in approximately half the time. Our studies argue that BLI is likely to be a suitable surrogate for measuring prion infectivity, and might be useful in the study of Tg mouse models for other neurodegenerative illnesses.

Spontaneous generation of anchorless prions in transgenic mice

Some prion protein mutations create anchorless molecules that cause Gerstmann–Sträussler–Scheinker (GSS) disease. To model GSS, we generated transgenic mice expressing cellular prion protein (PrPC) lacking the glycosylphosphatidyl inositol (GPI) anchor, denoted PrP(ΔGPI). Mice overexpressing PrP(ΔGPI) developed a late-onset, spontaneous neurologic dysfunction characterized by widespread amyloid deposition in the brain and the presence of a short protease-resistant PrP fragment similar to those found in GSS patients. In Tg(PrP,ΔGPI) mice, disease onset could be accelerated either by inoculation with brain homogenate prepared from spontaneously ill animals or by coexpression of membrane-anchored, full-length PrPC. In contrast, coexpression of N-terminally truncated PrP(Δ23–88) did not affect disease progression. Remarkably, disease from ill Tg(PrP,ΔGPI) mice transmitted to mice expressing wild-type PrPC, indicating the spontaneous generation of prions.

Human prion strain selection in transgenic mice

Transgenic (Tg) mice expressing chimeras of mouse and human prion proteins (PrPs) have shorter incubation periods for Creutzfeldt‐Jakob disease (CJD) prions than mice expressing full‐length human PrP. Increasing the sequence similarity of the chimeric PrP to mouse PrP, by reverting human residues to mouse, resulted in a Tg line, denoted Tg22372, which was susceptible to sporadic (s) CJD prions in ∼110 days.

A New Transgenic Mouse Model of Gerstmann–Sträussler–Scheinker Syndrome Caused by the A117V Mutation of PRNP

Gerstmann–Sträussler–Scheinker syndrome (GSS) is a genetic prion disease typified clinically by the development of progressive ataxia and dementia, and histopathologically by the presence of prion protein (PrP) amyloid plaques in the CNS, especially within the cerebellum. Several mutations of the PrP gene (PRNP) are associated with GSS, but only the P102L mutation has been convincingly modeled in transgenic (Tg) mice. To determine whether other mutations carry specific GSS phenotypic information, we constructed Tg mice that express PrP carrying the mouse homolog of the GSS-associated A117V mutation. Tg(A116V) mice express approximately six times the endogenous levels of PrP, develop progressive ataxia by ∼140 d, and die by ∼170 d. Compared with a mouse model of transmissible Creutzfeldt–Jakob disease (CJD), the ataxia of Tg(A116V) mice is more prominent, and the course of disease is more protracted, paralleling that observed in human disease. Neuropathology includes mild scattered vacuolation and prominent, mainly cerebellar localized, thioflavin S-positive PrP plaques comprised of full-length PrPA116V. In some mice, more prominent vacuolation or a noncerebellar distribution of PrP plaques was evident, suggesting some variability in phenotype. The biophysical properties of PrP from Tg(A116V) mice and human GSS(A117V) revealed a similarly low fraction of insoluble PrP and a weakly protease-resistant ∼13 kDa midspan PrP fragment, not observed in CJD. Overall, Tg(A116V) mice recapitulate many clinicopathologic features of GSS(A117V) that are distinct from CJD, supporting PrPA116V to carry specific phenotypic information. The occasional variation in histopathology they exhibit may shed light on a similar observation in human GSS(A117V).

Conformation-Dependent High-Affinity Monoclonal Antibodies to Prion Proteins

Prion diseases are fatal, neurodegenerative illnesses caused by the accumulation of PrPSc, an aberrantly folded isoform of the normal, cellular prion protein. Detection of PrPSc commonly relies on immunochemical methods, a strategy hampered by the lack of Abs specific for this disease-causing isoform. In this article, we report the generation of eight mAbs against prion protein (PrP) following immunization of Prnp-null mice with rPrP. The eight mAbs exhibited distinct differential binding to cellular prion protein and PrPSc from different species as well as PrP-derived synthetic peptides. Five of the eight mAbs exhibited binding to discontinuous PrP epitopes, all of which were disrupted by the addition of 2-ME or DTT, which reduced the single disulfide bond found in PrP. One mAb F20-29 reacted only with human PrP, whereas the F4-31 mAb bound bovine PrP; the K D values for mAbs F4-31 and F20-29 were ~500 pM. Binding of all five conformation-dependent mAbs to PrP was inhibited by 2-ME in ELISA, Western blots, and histoblots. One conformation-dependent mAb F4-31 increased the sensitivity of an ELISA-based test by nearly 500-fold when it was used as the capture Ab. These new conformation-dependent mAbs were found to be particularly useful in histoblotting studies, in which the low backgrounds after treatment with 2-ME created unusually high signal-to-noise ratios.

Convection-Enhanced Delivery of AAV2-PrPshRNA in Prion-Infected Mice

Prion disease is caused by a single pathogenic protein (PrPSc), an abnormal conformer of the normal cellular prion protein PrPC. Depletion of PrPC in prion knockout mice makes them resistant to prion disease. Thus, gene silencing of the Prnp gene is a promising effective therapeutic approach. Here, we examined adeno-associated virus vector type 2 encoding a short hairpin RNA targeting Prnp mRNA (AAV2-PrP-shRNA) to suppress PrPC expression both in vitro and in vivo. AAV2-PrP-shRNA treatment suppressed PrP levels and prevented dendritic degeneration in RML-infected brain aggregate cultures. Infusion of AAV2-PrP-shRNA-eGFP into the thalamus of CD-1 mice showed that eGFP was transported to the cerebral cortex via anterograde transport and the overall PrPC levels were reduced by ∼70% within 4 weeks. For therapeutic purposes, we treated RML-infected CD-1 mice with AAV2-PrP-shRNA beginning at 50 days post inoculation. Although AAV2-PrP-shRNA focally suppressed PrPSc formation in the thalamic infusion site by ∼75%, it did not suppress PrPSc formation efficiently in other regions of the brain. Survival of mice was not extended compared to the untreated controls. Global suppression of PrPC in the brain is required for successful therapy of prion diseases.

Bioluminescence imaging of A beta deposition in bigenic mouse models of Alzheimer's disease (vol 108, pg 2528, 2011)

Author(s): Watts, Joel C; Giles, Kurt; Grillo, Sunny K; Lemus, Azucena; DeArmond, Stephen J; Prusiner, Stanley B