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Prion diseases are characterized by the conversion of the soluble protease-sensitive host-encoded prion protein (PrPC) into its aggregated, protease-resistant, and infectious isoform (PrPSc). One of the earliest events occurring in cells following exposure to an exogenous source of prions is the cellular uptake of PrPSc. It is unclear how the biochemical properties of PrPSc influence its uptake, although aggregate size is thought to be important. Here we show that for two different strains of mouse prions, one that infects cells (22L) and one that does not (87V), a fraction of PrPSc associated with distinct sedimentation properties is preferentially taken up by the cells. However, while the fraction of PrPSc and the kinetics of uptake were similar for both strains, PrPSc derived from the 87V strain was disaggregated more rapidly than that derived from 22L. The increased rate of PrPSc disaggregation did not correlate with either the conformational or aggregate stability of 87V PrPSc, both of which were greater than those of 22L PrPSc. Our data suggest that the kinetics of disaggregation of PrPSc following cellular uptake is independent of PrPSc stability but may be dependent upon some component of the PrPSc aggregate other than PrP. Rapid disaggregation of 87V PrPSc by the cell may contribute, at least in part, to the inability of 87V to infect cells in vitro. [ABSTRACT FROM AUTHOR]

The secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrPC) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrPC trafficking and tested whether this process controls PrPC-dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrPC, induced PrPC endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrPC; however, heterologous expression of PrPC reconstituted both PKA and ERK1/2 activation. In contrast, a PrPC mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrPC endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis. [ABSTRACT FROM AUTHOR]

A key event in the transmissible spongiform encephalopathies (TSEs) is the formation of aggregated and protease-resistant prion protein, PrP-res, from a normally soluble, protease-sensitive and glycosylated precursor, PrP-sen. While amino acid sequence similarity between PrP-sen and PrP-res influences both PrP-res formation and cross-species transmission of infectivity, the influence of co- or post-translational modifications to PrP-sen is unknown. Here we report that, if PrP-sen and PrP-res are derived from different species, PrP-sen glycosylation can significantly affect PrP-res formation. Glycosylation affected PrP-res formation by influencing the amount of PrP-sen bound to PrP-res, while the amino acid sequence of PrP-sen influenced the amount of PrP-res generated in the post-binding conversion step. Our results show that in addition to amino acid sequence, co- or post-translational modifications to PrP-sen influence PrP-res formation in vitro. In vivo, these modifications might contribute to the resistance to infection associated with transmission of TSE infectivity across species barriers. [ABSTRACT FROM AUTHOR]

Transmissible spongiform encephalopathies are characterized by spongiosis, astrocytosis and accumulation of PrPSc, an isoform of the normal host protein PrPC. The exact cell types responsible for agent propagation and pathogenesis are still uncertain. To determine the possible role of astrocytes, we generated mice devoid of murine PrP but expressing hamster PrP transgenes driven by the astrocyte­specific GFAP promoter. After inoculation with hamster scrapie, these mice accumulated infectivity and PrPSc to high levels, developed severe disease after 227 ± 5 days and died 7 ± 4 days later. Therefore, astrocytes could play an important role in scrapie pathogenesis, possibly by an indirect toxic effect on neurons. Interestingly, mice expressing the same transgenes but also endogenous murine PrP genes propagated infectivity without developing disease. [ABSTRACT FROM AUTHOR]

Prion diseases are fatal, transmissible neurodegenerative diseases of the central nervous system. An abnormally protease-resistant and insoluble form (PrP(Sc)) of the normally soluble protease-sensitive host prion protein (PrP(C)) is the major component of the infectious prion. During the course of prion disease, PrP(Sc) accumulates primarily in the lymphoreticular and central nervous systems. Recent studies have shown that co-infection of prion-infected fibroblast cells with the Moloney murine leukemia virus (Mo-MuLV) strongly enhanced the release and spread of scrapie infectivity in cell culture, suggesting that retroviral coinfection might significantly influence prion spread and disease incubation times in vivo. We now show that another retrovirus, the murine leukemia virus Friend (F-MuLV), also enhanced the release and spread of scrapie infectivity in cell culture. However, peripheral co-infection of mice with both Friend virus and the mouse scrapie strain 22L did not alter scrapie disease incubation times, the levels of PrP(Sc) in the brain or spleen, or the distribution of pathological lesions in the brain. Thus, retroviral co-infection does not necessarily alter prion disease pathogenesis in vivo, most likely because of different cell-specific sites of replication for scrapie and F-MuLV.

The transmissible spongiform encephalopathy (TSE) diseases are a group of rare, fatal, and transmissible neurodegenerative diseases that include kuru and Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep, transmissible mink encephalopathy (TME), and chronic wasting disease (CWD) in mule deer and elk. Over the last 20 yr, they have gone from a fascinating but relatively obscure group of diseases to one that is a major agricultural and economic problem as well as a threat to human health. The shift in the relative impact of the TSE diseases began in the late 1970s when the United Kingdom altered the process by which animal carcasses were rendered to provide a protein supplement (i.e., meat and bone meal) to sheep, cattle, and other livestock. Several years later a new disease was recognized in the British cattle population. The pathological and immunohistochemical characteristics of the disease clearly placed it among the TSEs. The new disease was named bovine spongiform encephalopathy (BSE) by the scientific community and "mad cow disease" by the less-than-scientific press. At its peak in the UK, several thousand cattle a year were diagnosed with BSE, and millions of cattle were slaughtered. Introduction of the specified offals ban as well as banning the practice of feeding ruminants to other ruminants has led to a drastic decrease in the number of yearly BSE cases in the UK (less than 500 in 2003), and the epidemic is clearly on the wane. However, BSE has now spread throughout the rest of Europe, as well as to Japan, Russia, Canada, and Israel and thus remains a worldwide problem.A primary concern following the identification of BSE in 1985 was that it might cross species barriers to infect humans. Initially, it was thought that transmission of BSE to humans was unlikely, given that humans appeared to be resistant to scrapie, an animal TSE that had been endemic in British sheep for centuries. However, a few years after BSE was first recognized, a previously unknown form of CJD (variant CJD or vCJD) was identified in young people in Great Britain. The hypothesis that vCJD was the consequence of exposure of humans to BSE has now been supported by several different studies, and over 140 cases of vCJD have been confirmed.