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The "modified host protein" model of scrapie proposes that the transmissible agent is composed of the degradation-resistant protein, Sp33-37, and that clinical and pathologic signs result from neurotoxic accumulations of this protein. Sp33-37 is an abnormal, amyloidogenic isoform of the normally occurring cellular protein Cp33-37. This study investigated the tissue distribution of Cp33-37 in hamster. In brain, Cp33-37 was most concentrated in the hippocampal formation. Immunohistochemical studies localized Cp33-37 to neurons and surrounding neuropil in hippocampus; septal, caudate, and thalamic nuclei; dorsal root ganglia cells; and large-diameter dorsal root axons. In non-neuronal hamster tissues, Cp33-37 was detected in circulating leukocytes, heart, skeletal muscle, lung, intestinal tract, spleen, testis, ovary, and some other organs. The presence of Cp33-37 in extracerebral tissues indicates that its function is not unique to brain. These results indicate that the molecular substrate for the production of Sp33-37, the scrapie agent, and scrapie amyloid is present in a variety of cerebral and extracerebral sites.

No abstract

Scrapie is a slow infection of the nervous system which progresses in the absence of any apparent immune response. The recent development of a large-scale purification protocol for scrapie prions made it possible to obtain substantial quantities of electrophoretically purified prion protein (PrP 27-30) and we report here on the successful production of a rabbit antiserum to PrP 27-30. The antiserum reacted with PrP 27-30 and several lower molecular weight proteins as shown by Western blots; it did not react with protein preparations from uninfected brains. Discrete structures in the subependymal region of scrapie-infected hamster brains were stained immunocytochemically. These same structures also stained with Congo red dye and showed green birefringence with polarized light, a characteristic of purified prion rods. This staining pattern suggests that they are amyloid plaques.

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...