After infection and a prolonged incubation period, the scrapie agent causes a degenerative disease of the central nervous system in sheep and goats. Six lines of evidence including sensitivity to proteases demonstrate that this agent contains a protein that is required for infectivity. Although the scrapie agent is irreversibly inactivated by alkali, five procedures with more specificity for modifying nucleic acids failed to cause inactivation. The agent shows heterogeneity with respect to size, apparently a result of its hydrophobicity; the smallest form may have a molecular weight of 50,000 or less. Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term "prion" is proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids. Knowledge of the scrapie agent structure may have significance for understanding the causes of several degenerative diseases.

http://science.sciencemag.org/content/sci/216/4542/136.full.pdf

Novel proteinaceous infectious particles cause scrapie

Nobel Lecture: Prions

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Straussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.

https://science.sciencemag.org/content/sci/252/5012/1515.full.pdf

Molecular biology of prion diseases

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within ...

The Microbiota-Gut-Brain Axis

Strains of transmissible spongiform encephalopathies are distinguished by differing physicochemical properties of PrPSc, the disease-related isoform of prion protein, which can be maintained on transmission to transgenic mice. 'New variant' Creutzfeldt-Jakob disease (CJD) has strain characteristics distinct from other types of CJD and which resemble those of BSE transmitted to mice, domestic cat and macaque, consistent with BSE being the source of this new disease. Strain characteristics revealed here suggest that the prion protein may itself encode disease phenotype.

Molecular analysis of prion strain variation and the aetiology of 'new variant' CJD

Transmissible mink encephalopathy (TME) has been transmitted to Syrian golden hamsters, and two strains of the causative agent, HYPER (HY) and DROWSY (DY), have been identified that have different biological properties. During scrapie, a TME-like disease, an endogenous cellular protein, the prion protein (PrPC), is modified (to PrPSc) and accumulates in the brain. PrPSc is partially resistant to proteases and is claimed to be an essential component of the infectious agent. Purification and analysis of PrP from hamsters infected with the HY and DY TME agent strains revealed differences in properties of PrPTME sedimentation in N-lauroylsarcosine, sensitivity to digestion with proteinase K, and migration in polyacrylamide gels. PrPC and HY PrPTME can be distinguished on the basis of their relative solubilities in detergent and protease sensitivities. PrPTME from DY-infected brain tissue shared solubility characteristics of PrP from both uninfected and HY-infected tissue. Limited protease digestion of PrPTME revealed strain-specific migration patterns upon polyacrylamide gel electrophoresis. Prolonged proteinase K treatment or N-linked deglycosylation of PrPTME did not eliminate such differences but demonstrated the PrPTME from DY-infected brain was more sensitive to protease digestion than HY PrPTME. Antigenic mapping of PrPTME with antibodies raised against synthetic peptides revealed strain-specific differences in immunoreactivity in a region of the amino-terminal end of PrPTME containing amino acid residues 89 to 103. These findings indicate that PrPTME from the two agent strains, although originating from the same host, differ in composition, conformation, or both. We conclude that PrPTME from the HY and DY strains undergo different posttranslational modifications that could explain differences in the biochemical properties of PrPTME from the two sources. Whether these strain-specific posttranslational events are directly responsible for the distinct biological properties of the HY and DY agent strains remains to be determined.

Biochemical and physical properties of the prion protein from two strains of the transmissible mink encephalopathy agent.

Eighty-three percent of hamsters inoculated at birth with JC virus, a human papovavirus isolated from brain tissue of a case of progressive multifocal leukoencephalopathy, developed malignant gliomas within 6 months. Three brain tumors have been serially transplanted as subcutaneous tumors. JC virus was isolated from five of seven tumors tested. Cells from four tumors were cultivated in vitro. These cells contained an intranuclear antigen with the characteristics of a T antigen, and this antigen was antigenically related to SV40 T antigen. Although virus was not recovered from extracts of serially cultured tumor cells, JC virus was rescued when one tumor cell line was fused with permissive cells.

Human papovavirus (JC): induction of brain tumors in hamsters.

Experimental transmission of the Stetsonville, Wisconsin, U.S.A. source of transmissible mink encephalopathy (TME) to outbred Syrian golden hamsters resulted in two distinct syndromes, termed hyper (HY) and drowsy (DY), that diverge by the third hamster passage. The syndromes differed with respect to clinical signs, incubation period, brain titre, brain lesion profile and pathogenicity in mink. HY hamster TME had an incubation period of 65 ± 1 days and was characterized by clinical signs of hyperaesthesia and cerebellar ataxia. Lethargy and the absence of hyperexcitability or cerebellar ataxia were representative of DY hamster TME which had an incubation period of 168 ± 2 days. At endstage, HY and DY infected animals had brain titres of 109.5 LD50/g and 107.4 LD50/g of tissue, respectively, indicating that the replication kinetics of these two strains is different. Hamster TME passaged back into mink revealed that only DY retained mink pathogenicity. This suggests that the DY agent is the major mink pathogen in the Stetsonville TME source that is also pathogenic in hamsters after a long incubation period. The HY agent is likely to be a minor component of the original TME mink brain that replicates more rapidly than DY agent in hamsters, but alone is non-pathogenic in mink. The presence of the HY and DY strains of agent that retain their biological characteristics on repeated hamster passage in the Stetsonville TME source requires that the informational molecule encoding these transmissible agents has the capacity to account for this biological diversity.

Identification of two biologically distinct strains of transmissible mink encephalopathy in hamsters.

Scrapie and transmissible mink encephalopathy were studied in hamsters; clinical signs, pathology, and the replication of the agents of each disease in brain and spleen were compared. The most noticeable clinical sign in scrapie-affected hamsters was a distinct cerebellar ataxia beginning 16 weeks after inoculation. Ataxia was not prominent in animals affected with transmissible mink encephalopathy; these animals gradually became more and more lethargic. The pathology in the central nervous system in both diseases consisted of astrocytic hypertrophy, microvacuolation of the neuropil, and neuronal degeneration. The scrapie agent appeared to have a greater effect on nuclear masses, especially those present in brain stem and the central white matter of the cerebellum. The earliest lesions in both diseases were detected near pia arachnoid surfaces and adjacent to the ventricular system. These initial sites of involvement suggest that the cerebrospinal fluid may be an important route by which inocula are disseminated to susceptible cells after intracerebral inoculation. Both agents multiplied rapidly in brain, reaching titers > 108 LD50/0.05 ml before the onset of clinical signs. Titers in spleen were 4-6 logs lower than titers in brain at every point measured during the asymptomatic or clinical course of disease.

Comparison of Scrapie and Transmissible Mink Encephalopathy in Hamsters. II. Clinical Signs, Pathology, and Pathogenesis

Chronic wasting disease (CWD) is an emerging prion disease of deer and elk. The risk of CWD transmission to humans following exposure to CWD-infected tissues is unknown. To assess the susceptibility of nonhuman primates to CWD, two squirrel monkeys were inoculated with brain tissue from a CWD-infected mule deer. The CWD-inoculated squirrel monkeys developed a progressive neurodegenerative disease and were euthanized at 31 and 34 months postinfection. Brain tissue from the CWD-infected squirrel monkeys contained the abnormal isoform of the prion protein, PrP-res, and displayed spongiform degeneration. This is the first reported transmission of CWD to primates.

/pdf/interspecies-transmission-of-chronic-wasting-disease-prions-2z9rwfexte.pdf

Richard F. Marsh

Papers

Biochemical and physical properties of the prion protein from two strains of the transmissible mink encephalopathy agent.

Human papovavirus (JC): induction of brain tumors in hamsters.

Identification of two biologically distinct strains of transmissible mink encephalopathy in hamsters.

Comparison of Scrapie and Transmissible Mink Encephalopathy in Hamsters. II. Clinical Signs, Pathology, and Pathogenesis

Interspecies Transmission of Chronic Wasting Disease Prions to Squirrel Monkeys (Saimiri sciureus)