Anchors Away — Of Plaques and Pathology in Prion Disease
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《新英格兰医药杂志》
The accumulation of conformationally altered cellular proteins is a common feature underlying the pathogenesis of neurodegenerative conditions such as Alzheimer's disease and the prion diseases. However, the proteins that cause prion diseases are uniquely infectious. The neuropathological profiles of Alzheimer's disease and certain prion diseases such as variant Creutzfeldt–Jakob disease (the human manifestation of bovine spongiform encephalopathy) involve the aggregation of the etiologic protein into amyloid plaques, but the relationship between plaque formation and disease has been the focus of debate. A recent study by Chesebro and colleagues1 weighs in on one side.
Studies of transgenic mice and cell-free systems have been enormously useful for uncovering the mechanism of prion replication — in which a disease-associated version of the prion protein (PrP), referred to as PrPSc, coerces the normal form of the protein, referred to as PrPC, to adopt the disease-associated conformation. Previous in vitro studies2,3 showed that an artificially mutant version of PrP that lacks the terminal sequence for the addition of the glycosylphosphatidylinositol (GPI) lipid anchor that normally tethers PrP to the external surface of the cell membrane (Figure 1A) is capable of adopting a protease-resistant conformation that resembles PrPSc. To determine whether this mutant PrP can support prion propagation and PrPSc production in vivo, Chesebro et al.1 produced transgenic mice expressing "anchorless" PrP.
Figure 1. The Structure, Synthesis, and Consequences of Wild-Type and Mutant Prion Protein.
The endogenous prion protein (PrPC) is a cellular protein expressed in the central nervous system. Once synthesized, it is processed through the removal of N-terminal and C-terminal hydrophobic peptides (Panel A). Removal of the C-terminal peptide serves as a signal for the addition of a glycosylphosphatidylinositol (GPI) lipid anchor (purple circle), which tethers PrP to the cell surface. The proliferation of infectious, disease-causing prions is achieved through the conversion of PrPC into the disease-specific conformation (PrPSc). Available evidence suggests that the interaction between PrPSc and PrPC and, hence, prion conversion occurs on the plasma membrane or in endosomes. Chesebro and coworkers1 recently described a transgenic mouse that expresses a mutant PrP that lacks the C-terminal signal sequence (Panel B). The resulting PrP is secreted, since it lacks a GPI lipid anchor. In transgenic mice, the inoculation of prions is followed by the accumulation of numerous amyloid plaques throughout the brain, but the level of prion replication is lower than that in wild-type mice. The normal life span of these transgenic mice as compared with the shorter life span of the prion-injected wild-type mice suggests that amyloid plaques are not neurotoxic and that the ability of the mutant protein to produce PrPSc is compromised.
As expected, anchorless PrP was not expressed on the surface of cells derived from transgenic mice; instead, it was secreted (Figure 1B). When inoculated with prions, the mice accumulated mutant PrP in the form of abundant amyloid plaques throughout the brain as early as 70 days after inoculation. But despite their accumulation of amyloid-forming, protease-resistant PrP (in many cases at levels higher than those of PrPSc found in the brains of clinically sick wild-type mice), the transgenic mice had no neurologic symptoms of prion disease up to 600 days after prion inoculation — long after inoculated control mice died of the disease.
One hypothesis to explain the dissociation of clinical disease and the accumulation of disease-associated PrP is that neurons die in response to neurotoxic signaling events mediated by GPI-anchored PrPC after the accumulation of PrPSc. The data, however, provide support for an alternative hypothesis in which both amyloid-forming, protease-resistant PrP and PrPSc are produced in the brains of prion-infected transgenic mice expressing the mutant protein (Figure 1B). In this scenario, PrPSc is neurotoxic, whereas the accumulation of amyloid-forming PrP is relatively benign. In support of this interpretation, titers of infectivity in the brain tissues of asymptomatic transgenic mice were at most 10 percent of those found in clinically sick wild-type mice. Moreover, the infectious titers decreased as amyloid plaques accumulated — an unexpected finding and one that suggests that the absence of clinical disease in transgenic mice resulted from relatively inefficient conversion of mutant protein to PrPSc. In support of this notion, PrPSc did not accumulate, nor did clinical disease develop, in transgenic mice inoculated with brain extracts from these asymptomatic, prion-infected mice.
This scenario raises the possibility that neurologic disease failed to develop in transgenic mice because of inefficient accumulation of PrPSc. Low-level, asymptomatic replication of prions is not without precedent. For example, hamster prions persist in the brains of inoculated mice for prolonged periods without producing disease.4
The level of transgene expression in the mutant mice produced by Chesebro et al.1 supports the possibility of subclinical prion propagation; the mutant mice expressed the anchorless PrP at half the level of PrP normally found in the brains of wild-type mice. Previous studies have shown an inverse relationship between the level of expression of PrP transgenes and the duration of prion incubation, and so it is possible that symptomatic prion disease might eventually develop in transgenic mice expressing higher levels of the mutant protein.
The findings of Chesebro et al.1 are in accordance with long-standing observations that the amyloidogenic properties of PrP are unrelated to the formation of PrPSc and the infectivity of prions.5 More generally, they have important implications for other neurodegenerative diseases that involve amyloid deposition in humans. Although strategies geared toward inhibiting the conversion of PrPC to PrPSc remain at the forefront of therapeutic approaches to prion disorders, increasing acceptance that the formation of plaques containing -amyloid may be peripheral or even irrelevant to the pathogenesis of Alzheimer's disease — with neurotoxicity being mediated by -amyloid protofibrils — suggests that abrogating the activity of these protofibrils might be more effective than targeting amyloid formation.
Source Information
From the Department of Microbiology, Immunology and Molecular Genetics, University of Kentucky, Lexington.
References
Chesebro B, Trifilo M, Race R, et al. Anchorless prion protein results in infectious amyloid disease without clinical scrapie. Science 2005;308:1435-1439.
Rogers M, Yehiely F, Scott M, Prusiner SB. Conversion of truncated and elongated prion proteins into the scrapie isoform in cultured cells. Proc Natl Acad Sci U S A 1993;90:3182-3186.
Kocisko DA, Come JH, Priola SA, et al. Cell-free formation of protease-resistant prion protein. Nature 1994;370:471-474.
Race R, Chesebro B. Scrapie infectivity found in resistant species. Nature 1998;392:770-770.
Wille H, Prusiner SB, Cohen FE. Scrapie infectivity is independent of amyloid staining properties of the N-terminally truncated prion protein. J Struct Biol 2000;130:323-338.(Glenn Telling, Ph.D.)
Studies of transgenic mice and cell-free systems have been enormously useful for uncovering the mechanism of prion replication — in which a disease-associated version of the prion protein (PrP), referred to as PrPSc, coerces the normal form of the protein, referred to as PrPC, to adopt the disease-associated conformation. Previous in vitro studies2,3 showed that an artificially mutant version of PrP that lacks the terminal sequence for the addition of the glycosylphosphatidylinositol (GPI) lipid anchor that normally tethers PrP to the external surface of the cell membrane (Figure 1A) is capable of adopting a protease-resistant conformation that resembles PrPSc. To determine whether this mutant PrP can support prion propagation and PrPSc production in vivo, Chesebro et al.1 produced transgenic mice expressing "anchorless" PrP.
Figure 1. The Structure, Synthesis, and Consequences of Wild-Type and Mutant Prion Protein.
The endogenous prion protein (PrPC) is a cellular protein expressed in the central nervous system. Once synthesized, it is processed through the removal of N-terminal and C-terminal hydrophobic peptides (Panel A). Removal of the C-terminal peptide serves as a signal for the addition of a glycosylphosphatidylinositol (GPI) lipid anchor (purple circle), which tethers PrP to the cell surface. The proliferation of infectious, disease-causing prions is achieved through the conversion of PrPC into the disease-specific conformation (PrPSc). Available evidence suggests that the interaction between PrPSc and PrPC and, hence, prion conversion occurs on the plasma membrane or in endosomes. Chesebro and coworkers1 recently described a transgenic mouse that expresses a mutant PrP that lacks the C-terminal signal sequence (Panel B). The resulting PrP is secreted, since it lacks a GPI lipid anchor. In transgenic mice, the inoculation of prions is followed by the accumulation of numerous amyloid plaques throughout the brain, but the level of prion replication is lower than that in wild-type mice. The normal life span of these transgenic mice as compared with the shorter life span of the prion-injected wild-type mice suggests that amyloid plaques are not neurotoxic and that the ability of the mutant protein to produce PrPSc is compromised.
As expected, anchorless PrP was not expressed on the surface of cells derived from transgenic mice; instead, it was secreted (Figure 1B). When inoculated with prions, the mice accumulated mutant PrP in the form of abundant amyloid plaques throughout the brain as early as 70 days after inoculation. But despite their accumulation of amyloid-forming, protease-resistant PrP (in many cases at levels higher than those of PrPSc found in the brains of clinically sick wild-type mice), the transgenic mice had no neurologic symptoms of prion disease up to 600 days after prion inoculation — long after inoculated control mice died of the disease.
One hypothesis to explain the dissociation of clinical disease and the accumulation of disease-associated PrP is that neurons die in response to neurotoxic signaling events mediated by GPI-anchored PrPC after the accumulation of PrPSc. The data, however, provide support for an alternative hypothesis in which both amyloid-forming, protease-resistant PrP and PrPSc are produced in the brains of prion-infected transgenic mice expressing the mutant protein (Figure 1B). In this scenario, PrPSc is neurotoxic, whereas the accumulation of amyloid-forming PrP is relatively benign. In support of this interpretation, titers of infectivity in the brain tissues of asymptomatic transgenic mice were at most 10 percent of those found in clinically sick wild-type mice. Moreover, the infectious titers decreased as amyloid plaques accumulated — an unexpected finding and one that suggests that the absence of clinical disease in transgenic mice resulted from relatively inefficient conversion of mutant protein to PrPSc. In support of this notion, PrPSc did not accumulate, nor did clinical disease develop, in transgenic mice inoculated with brain extracts from these asymptomatic, prion-infected mice.
This scenario raises the possibility that neurologic disease failed to develop in transgenic mice because of inefficient accumulation of PrPSc. Low-level, asymptomatic replication of prions is not without precedent. For example, hamster prions persist in the brains of inoculated mice for prolonged periods without producing disease.4
The level of transgene expression in the mutant mice produced by Chesebro et al.1 supports the possibility of subclinical prion propagation; the mutant mice expressed the anchorless PrP at half the level of PrP normally found in the brains of wild-type mice. Previous studies have shown an inverse relationship between the level of expression of PrP transgenes and the duration of prion incubation, and so it is possible that symptomatic prion disease might eventually develop in transgenic mice expressing higher levels of the mutant protein.
The findings of Chesebro et al.1 are in accordance with long-standing observations that the amyloidogenic properties of PrP are unrelated to the formation of PrPSc and the infectivity of prions.5 More generally, they have important implications for other neurodegenerative diseases that involve amyloid deposition in humans. Although strategies geared toward inhibiting the conversion of PrPC to PrPSc remain at the forefront of therapeutic approaches to prion disorders, increasing acceptance that the formation of plaques containing -amyloid may be peripheral or even irrelevant to the pathogenesis of Alzheimer's disease — with neurotoxicity being mediated by -amyloid protofibrils — suggests that abrogating the activity of these protofibrils might be more effective than targeting amyloid formation.
Source Information
From the Department of Microbiology, Immunology and Molecular Genetics, University of Kentucky, Lexington.
References
Chesebro B, Trifilo M, Race R, et al. Anchorless prion protein results in infectious amyloid disease without clinical scrapie. Science 2005;308:1435-1439.
Rogers M, Yehiely F, Scott M, Prusiner SB. Conversion of truncated and elongated prion proteins into the scrapie isoform in cultured cells. Proc Natl Acad Sci U S A 1993;90:3182-3186.
Kocisko DA, Come JH, Priola SA, et al. Cell-free formation of protease-resistant prion protein. Nature 1994;370:471-474.
Race R, Chesebro B. Scrapie infectivity found in resistant species. Nature 1998;392:770-770.
Wille H, Prusiner SB, Cohen FE. Scrapie infectivity is independent of amyloid staining properties of the N-terminally truncated prion protein. J Struct Biol 2000;130:323-338.(Glenn Telling, Ph.D.)