Motor behavioral and neuropathological deficits in mice deficient for normal prion protein expression.

It has been difficult to reconcile the absence of pathology and apparently normal behavior of mice lacking prion protein (PrP), referred to as Prnp(0/0) mice, with a mechanism of prion pathogenesis involving progressive loss of PrP(C)-mediated neuroprotection. However, here we report that Prnp(0/0) mice exhibit significant age-related defects in motor coordination and balance compared with mice expressing wild type Prnp on a syngeneic background, and that the brains of behaviorally-impaired Prnp(0/0) mice display the cardinal neuropathological hallmarks of spongiform pathology and reactive astrocytic gliosis that normally accompany prion disease. Consistent with the appearance of cerebellar ataxia as an early symptom in patients with Gerstmann-Sträussler-Scheinker syndrome (GSS), an inherited form of human prion disease, motor coordination and balance defects manifested in a transgenic (Tg) mouse model of GSS considerably earlier than the onset of end-stage neurodegenerative disease. Our results are consistent with a mechanism in which loss of normal PrP(C) function is an important pathological component of prion diseases.

GFP-tagged PrP supports compromised prion replication in transgenic mice.

The ability of green fluorescent protein (GFP)-prion protein (PrP) fusions to support prion propagation has not been demonstrated. Here, we show that while transgenic mice expressing PrP tagged at its amino terminus with enhanced GFP, referred to as EGFPrP-N, supported prion replication, disease onset was prolonged, the brains of diseased mice did not exhibit typical disease neuropathology and disease-associated EGFPrP-N lacked the full spectrum of biochemical properties normally associated with PrP(Sc). Co-expression of wild-type PrP and EGFPrP-N substantially reduced prion incubation times and resulted in accumulation of protease-resistant EGFPrP(Sc)-N in the brains of transgenic mice as well as chronically infected cultured cells, suggesting that wild-type PrP rescued a compromised amino terminal function in EGFPrP-N. While our results show that EGFPrP(C)-N adopts a conformation necessary for the production of infectious prions, the synergistic interaction of wild-type and EGFPrP-N underscores the importance of the amino terminus in modulating prion pathogenesis.

Immunodetection of disease-associated mutant PrP, which accelerates disease in GSS transgenic mice.

The absence of infectivity-associated, protease-resistant prion protein (PrP(Sc)) in the brains of spontaneously sick transgenic (Tg) mice overexpressing PrP linked to Gerstmann-Sträussler Scheinker syndrome, and the failure of gene-targeted mice expressing such PrP to develop disease spontaneously, challenged the concept that mutant PrP expression led to spontaneous prion production. Here, we demonstrate that disease in overexpressor Tg mice is associated with accumulation of protease-sensitive aggregates of mutant PrP that can be immunoprecipitated by the PrP(Sc)-specific monoclonal antibody designated 15B3. Whereas Tg mice expressing multiple transgenes exhibited accelerated disease when inoculated with disease-associated mutant PrP, Tg mice expressing mutant PrP at low levels failed to develop disease either spontaneously or following inoculation. These studies indicate that inoculated mutant PrP from diseased mice promotes the aggregation and accumulation of pre-existing pathological forms of mutant PrP produced as a result of transgene overexpression. Thus, while pathological mutant PrP possesses a subset of PrP(Sc) characteristics, we now show that the attribute of prion transmission suggested by previous studies is more accurately characterized as disease acceleration.