In the course of scrapie, a transmissible spongiform encephalopathy caused by an unconventional agent, a normal cellular protein is converted to an abnormal form that copurifies with infectivity and aggregates to form deposits of amyloid. We have used immunocytochemistry and methods that enhance detection of amyloidogenic proteins to investigate the types ofcells in the central nervous system which are involved in the formation of the abnormal scrapieassociated protein. We show that this protein accumulates in astrocytes prior to the cardinal neuropathological changes in scrapie-astrogliosis, vacuolation, neuron loss, and amyloid deposition. These rindings implicate the astrocyte in the formation of the scrapie isoform of the prion protein and amyloid in scrapie and suggest that this cell type might also be involved in the replication of the scrapie agent.Infection by the scrapie agent, and by other unconventional agents, is accompanied by conversion of a 33-to 37-kDa cellular protein referred to as Cp33-37 or PrPc (cellular isoform of the prion protein) to an altered form (Sp33-37 or PrPSc) that is more resistant to proteolysis and forms fibrils characteristic of all transmissible spongiform encephalopathies (1, 2). Aggregated PrPSc also copurifies with infectivity and is a major component of the amyloid deposits in diseases induced by unconventional agents (3-6).The type of cell in the central nervous system in which the conversion of PrPc to PrPSc takes place has not been defined but has been generally assumed to be the neuron, as the normal cellular isoform is found predominantly in this cell type (7-11). In our studies of the molecular basis for pathological changes in scrapie, we have used differential hybridization screening of cDNA libraries constructed from mRNAs isolated from the brains of infected animals to identify genes whose expression increases during infection (12). Subsequent sequencing and homology searches have revealed three genes [encoding glial fibrillary acidic protein (GFAP; refs. 13 and 14), apolipoprotein E, and cathepsin D (J.F.D., H. Minnigan, R.I.C., J. N. Whitaker, R. Race, T. Rustan, W. Frey II, and A.T.H., unpublished work)] that are activated in scrapie. Because increased expression in all three cases occurs in astrocytes, we became curious about the role the astrocyte might play in amyloid formation in scrapie, and therefore reexamined infected tissues with more sensitive techniques for detecting amyloid and its major component in scrapie, PrPsc. By examining tissue sections from animals early in the course of infection, we found that PrPSc accumulates in astrocytes rather than neurons. In this report we describe this discovery and the relationship of PrPsc accumulation in astrocytes to astrocytosis, amyloid deposition, vacuolation, and neuronal death. MATERIALS AND METHODSInfection of Mice and Histological Procedures. For most experiments C57BL/6J mice were inoculated intracerebrally with brain homogenate of mice infected with the 22L strain of scrapie (five animals) o...

With the rationale that the neuropathological similarities between scrapie and Alzheimer's disease reflect convergent pathological mechanisms involving altered gene expression, we set out to identify molecular events involved in both processes, using scrapie as a model to study the time course of these changes. We differentially screened a cDNA library constructed from scrapie-infected mice to identify mRNAs that increase or decrease during disease and discovered in this way two mRNAs that are increased in scrapie and Alzheimer's disease. These mRNAs were subsequently shown by sequence analysis to encode apolipoprotein E and cathepsin D (EC 3.4.23.5). Using in situ hybridization and immunocytochemistry to define the cellular and anatomic pathology of altered gene expression, we found that in both diseases the increase in apolipoprotein E and cathepsin D mRNAs and proteins occurred in activated astrocytes. Iit scrapie, the increase in gene expression occurred soon after the amyloid-forming abnormal isoform of the prion protein has been shown to accumulate in astrocytes. In Alzheimer's disease, the increased expression of cathepsin D also occurred in association with ,-amyloid. These studies reveal some of the molecular antecedents of neuropathological changes in scrapie and Alzheiner's disease and accord new prominence to the role of astrocytes in neurodegenerative conditions.

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