Prions and Prion Diseases: Current Perspectives Chapter Abstracts
Chapter 1. A Functional Role for a Copper Binding Prion Protein
Andrew R. Thompsett and David R. Brown
Abstract
It is now widely accepted that the normal cellular prion protein
(PrPC) binds copper. While the extent, affinities
and sites of binding are still disputed evidence suggests that PrP binds four coppers in the highly conserved histidine
rich repeat domain and possibly at a fifth site outside the repeat region. Here we review the characteristics of the
cupro-prion protein in relation to the normal function of the PrP and discuss a role for PrP in the relief of neuronal
oxidative stress. Various mechanisms including copper signaling, metal ion homeostasis, and roles for PrP as a copper
chaperone and enzymatic superoxide dismutase are considered to explain how PrP may perform this function. Finally, loss of
PrP function is compared to the disease state and related to other neurological disorders in which copper is implicated.
Chapter 2. Binding and Conversion Reactions Between Prion Protein Isoforms
Byron Caughey, Jay R. Silveira, Jonathan O. Speare and Gerald S. Baron
Abstract
A fundamental process in the transmissible spongiform encephalopathies (TSE) or prion diseases is the conversion
of PrPC to abnormal and usually protease-resistant forms such as
PrPSc. A variety of in vitro experiments have shown that TSE-associated forms of PrP can cause
PrPC to convert to forms that are similarly protease-resistant. These
observations have shown that pathologic forms of PrP have at least limited capacity to propagate themselves, which would
be necessary for them to be infectious proteins. However, it has not been shown that new infectivity is generated in
any such reactions. Nonetheless, PrP conversion reactions are highly specific and may account, at least in part, for
TSE species barriers and the propagation of strains. Such
in vitro conversion systems have yielded insights into the molecular mechanisms of TSE disease and are being exploited as screens for anti-TSE drugs and as bases for diagnostic tests.
Chapter 3. The Prion Protein in Cell Culture
Jeremy P. Brockes and Nnennaya Kanu
Abstract
Cell culture models have afforded an accessible context in which to study the biosynthesis and trafficking of the
prion protein (PrP) with respect to its glycosylation, conformation, membrane topology, aggregation and disulfide
bonding. These mechanisms may impinge directly or indirectly on the propensity of the normal form of PrP, referred to as
PrPC to undergo conversion to the disease-associated isoform,
PrPSc during de novo infection of cells, or in a stably
infected cell. Studies in cell culture have also delineated several mechanisms by which an infected cell might convert
a neighbouring target cell, and have specifically suggested cell contact as an important aspect of this process.
Chapter 4. PrP Deletion Mutants
Surachai Supattapone and Judith R. Rees
Abstract
A number of investigators have successfully used deletion mutagenesis of the prion protein (PrP) to investigate
the mechanisms of prion propagation, PrP folding, and
PrPSc-induced neurotoxicity. Several of these studies have
shown that PrP is comprised of at least three distinct domains. The N-terminal domain spanning residues 23-88
facilitates efficient prion propagation. The central, hydrophobic region spanning residues 89-140 is amyloidogenic and
mediates neurotoxicity. The C-terminal domain is structured and helps prevent
PrPC from misfolding. Other deletion
mutagenesis studies have shown that a 106 amino acid molecule, PrP
(D23-88, D141-176) can form infectious miniprions in
transgenic (Tg) mice with an artificial transmission barrier.
In vitro translation experiments demonstrated that deletion of the signal sequence spanning residues 1-22 increases production of the pathogenic topologic variant
CtmPrP, indicating that CtmPrP is generated by post-translational translocation into endoplasmic reticulum membranes.
Chapter 5. Targeting the Murine PrP Gene
Rona M. Barron and Jean C. Manson
Abstract
The prion protein (PrP) is known to be central to the Transmissible Spongiform Encephalopathies (TSE) and
point mutations and polymorphisms in the PrP gene have a major effect in defining the incubation time of TSE disease
and the susceptibility of the host to TSE infection. In mice, polymorphisms at amino acids 108 and 189 of PrP have
been implicated in the control of TSE incubation time. In humans, point mutations in the PrP protein are linked to
the occurrence of familial forms of TSE disease, such as P102L Gerstmann-Straussler-Scheinker Syndrome (P102L GSS).
In order to study the role of these point mutations and polymorphisms in PrP in TSE disease, we have
developed several transgenic lines in which specific mutations have been introduced into the endogenous murine PrP protein by gene targeting. These transgenic mice express PrP at the same level and under the same regulatory controls as
wild type mice, and all transgenic lines are directly comparable as the mutations have been introduced on the same
genetic background. Inoculation of these mice has shown that amino acids 108 and 189 in murine PrP do indeed
control scrapie incubation time in mice, and that the 101L mutation alters the susceptibility of the mice to several strains
of TSE agent from different species.
Chapter 6. Transgenic Mouse Models of Prion Diseases
Karah Nazor and Glenn C. Telling
Abstract
Manipulation of prion protein (PrP) genes by transgenesis in mice has provided important insights into mechanisms
of prion propagation and the molecular basis of prion strains and species barriers. Despite these advances, our
understanding of these unique pathogens is far from complete. Transgenic approaches will doubtless remain one of the
cornerstones of investigations into the prion diseases in the coming years which will include mechanistic studies of prion
pathogenesis and prion transmission barriers. Transgenic models will also be important tools for the evaluation of potential
therapeutic agents.
Chapter 7. Peripheral Pathogenesis of Prion Diseases
Adriano Aguzzi
Abstract
Prion neuroinvasion consists of an ordered sequence of events resulting in infection of the central nervous
system (CNS). Successful oral challenge requires transepithelial migration of prions, which may be accomplished by M-cells.
Depletion of lymphocytes from the intestinal mucosa by ablation of
a4b7 integrins does not prevent pathogenesis,
yet mice exhibiting reduced number of Peyer's patches are virtually uninfectible orally. After gaining access to the
body from peripheral sites, prions colonize lymphoid organs of mice, humans, and sheep: the failure of
peripherally administered prions to elicit disease in immune deficient mice indicates that this is crucial for pathogenesis.
B-lymphocytes are required for neuroinvasion upon intraperitoneal administration, probably (but not necessarily
only) because they provide lymphotoxins to secondary lymphoid organs, thereby maintaining follicular dendritic cells
(FDCs): genetic or pharmacological interference with lymphotoxin signaling effectively impairs pathogenesis. The
sympathetic nervous system appears to be involved in prion transfer to brain, since sympathectomy delays or prevents
pathogenesis, whereas sympathetic hyperinnervation accelerates it. Various components of the complement system are modifiers
of neuroinvasion efficiency, and their pharmacological or genetic ablation interferes with neuroinvasion. Although
isogenic prions are immunologically inert, expression of
anti-PrPC antibodies in transgenic mice has uncovered that
autoreactivity to PrPC does not necessarily tolerize B-cells, and that sustained
anti-PrPC IgM titers can prevent peripheral
prion pathogenesis.
Chapter 8. Immunological Advances in Prion Diseases
R. Anthony Williamson
Abstract
The only known component of prions, the infectious agents causing transmissible spongiform encephalopathies
(TSEs), is an abnormally folded conformer
(PrPSc) of the cellular prion protein,
PrPC. During prion propagation,
PrPSc appears to act as a molecular template, sequestering endogenous
PrPC and somehow triggering its conformational
conversion, thereby yielding additional molecules of
PrPSc. Presently, there are no effective approaches to either prevent or
treat prion infections. However, antibodies specifically binding to certain regions of PrP have emerged as a class of
potent inhibitors of prion replication, seemingly capable of resolving infection both
in vitro and in vivo. Mechanistically, these molecules likely operate either by hindering
PrPC-PrPSc interactions, or by stabilizing
PrPC in its native conformation, thereby preventing its structural reconfiguration. These findings indicate that the immune system, which
remains largely impassive throughout the course of a natural prion infection, may if stimulated correctly, yield
meaningful protection from exposure to prions. Significantly, however, antibodies are generally excluded from central
nervous system (CNS), and are therefore likely to be limited in their ability to readily reverse established prion infections
that have spread to, or originated within, these tissues. Looking to the future, antibodies that efficiently inhibit
prion propagation are now being harnessed as tools with which to rationally identify small molecules possessing
equivalent anti-prion properties, but that may more readily gain access to the CNS.
Chapter 9. Prnd and the Doppel Protein
David Westaway
Abstract
Doppel (Dpl) is a PrP-related protein encoded by the
Prnd gene downstream of Prnp. Like PrPC, Dpl is a GPI-anchored protein with two N-glycosylation sites and a globular C-terminus containing three
a-helices and two short b-strands, and both proteins bind copper ions
in vitro. Dpl is distinguished from PrPC by a shorter N-terminal region (lacking both octarepeats and an optional transmembrane region), a kink in helix B, and a different location
and stoichiometry of Cu binding site. In vivo, Dpl is apparently unable to be converted to a
PrPSc-like form. Also, whereas mouse
PrPC is expressed abundantly in the CNS and is present in several peripheral tissues, Dpl expression in adult
is restricted almost exclusively to the testis. When expressed in the CNS, however, Dpl produces neurodegeneration
with similarities to phenotypes produced by some mutant forms of
PrPC, and furthermore, the neurotoxic effects of Dpl
are blocked by expression of wild type
PrPC. These findings, as well as an unexpected parallel between Familial
Creutzfeldt-Jakob disease mutations and conserved residues in Dpl suggest cross-talk between Dpl and
PrPC in certain physiological and pathogenic situations. Defining the basis for the overlapping and complementary functions of Dpl and
PrP isoforms comprises an important challenge for future research.
Chapter 10. Cellular Control of Prion Formation and Propagation in Yeast
Yury O. Chernoff
Abstract
Yeast prions provide a model for understanding both the molecular mechanisms of mammalian amyloidoses and
the general principles of protein-based inheritance. In yeast, initial prion formation is induced by protein
overproduction and facilitated in a cascade-like fashion by pre-existing prion isoforms of unrelated proteins, containing
prion-forming domains of similar amino acid composition. Pre-existing yeast prions also increase aggregation and toxicity of
the heterologous polyglutamine proteins, thus manifesting themselves as susceptibility factors for other aggregation disorders.
Formation and propagation of yeast prions is modulated by various cellular regulatory networks, including the
stress-defense systems (heat shock proteins [Hsps] and the ubiquitin pathway), cytoskeleton, and functional partners of
the specific prion-forming proteins. The very ability of yeast prions to spread in cell divisions or by cytoplasmic
exchanges, that turns them into "malignant" subcellular "tumors" and distinguishes prions from non-perpetuating protein
aggregates, requires a certain level of Hsp activity. Thus, prions have "learned" how to use the cellular stress-defense systems
to their own advantage. Stresses and chemical agents can cure yeast cells of prions by altering the Hsp levels or
activities. The mechanism of prion curing by guanidine hydrochloride is specifically discussed. Curing yeast cells of prions
via chaperone manipulations suggests an approach for development of new potential anti-prion treatments.
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