Prions: Protein Aggregation and Infectious Diseases - Physiological ...

egion in mouse chromosome 2 (479). Hybridization studies
demonstrated 0.002 Prnp gene sequences per ID 50
unit in purified prion fractions, strongly suggesting that a
nucleic acid encoding PrP Sc cannot constitute a component
of the infectious prion particle (370). This is a major
feature that distinguishes prions from viruses, including
retroviruses which carry cellular oncogenes and “satellite
viruses” bearing coat proteins from viruses that had previously
infected the host.
D. PrP Amyloid
The discovery of PrP 27–30 in fractions enriched for
scrapie infectivity was accompanied by the identification
of rod-shaped particles in the same fractions (404, 410).
The fine structure of these particles, which had been
originally described by Mertz as “scrapie-associated
fibrils” (344), failed to reveal any regular substructure
characteristic of most viruses (526). Conversely, prion
rods are indistinguishable from many purified amyloids
(114). This analogy was extended when the prion rods
were found to display the tinctorial properties of amyloids
(410). Small amyloidotropic dyes, such as derivatives of
Congo red and thioflavins, bind with various degrees of
selectivity to protein aggregates having an extensive cross
-pleated sheet conformation and sufficient structural
regularity and give rise to an enhanced fluorescence (thioflavins)
or apple-green birefringence under cross-polarized
light (Congo red) (47, 138). However, these dyes are
not suitable for recognizing prefibrillary species and amyloid
deposits of diverse morphological origin. Advancement
in this direction has been provided by luminescent
conjugated polymers (LCPs), a recently developed, novel
class of amyloidotropic dyes (216, 366, 367).
The amyloid plaques in the brains of humans and
other animals with prion disease contain PrP, as determined
by immunoreactivity and amino acid sequencing
(45, 132, 265, 426, 488). Solubilization of PrP 27–30 into
liposomes with retention of infectivity (169) suggests that
large PrP polymers are not required for infectivity, although
it is certainly possible that PrP Sc oligomers may
constitute nucleation centers pivotal to prion replication
(238, 288). To systematically evaluate the relationship
between infectivity, converting activity, and the size of
various PrP Sc -containing aggregates, Caughey and coworkers
(464) partially disaggregated PrP Sc . The resulting
species were fractionated by size and analyzed by light
scattering and nondenaturing gel electrophoresis. Intracerebral
inoculation of the different fractions into hamsters
revealed that with respect to PrP content, infectivity
peaked markedly with 17–27 nm (300–600 kDa) particles.
These results suggest that nonfibrillar particles, with
masses equivalent to 14–28 PrP molecules, are the most
efficient initiators of TSE disease (464). As with other
AGGREGATION AND INFECTIOUS DISEASE 1109
diseases characterized by protein aggregation, such as
Alzheimer’s disease and other amyloidoses, the formation
of large amyloid fibrils might be a protective process that
sequesters the more dangerous subfibrillar oligomers of
the amyloidogenic peptide or protein into relatively innocuous
deposits.
Prion-like amyloids also exist in lower eukaryotes
such as yeast. Fungal prions are non-PrP related molecules
that include HET-s, Ure2p, and Sup35 proteins,
which can adopt both nonamyloid and self-perpetuating
amyloid structures. In contrast to PrP Sc , the conversion of
these proteins into their prion-like conformations has
been shown to have important physiological functions in
yeast. The conversion of Ure2p and Sup35 into their amyloid
forms (URE3 and PSI, respectively) regulates the
transcription and translation of specific yeast genes (237,
492). Aggregated HET-s regulates heterokaryon incompatibility,
a fungal self/non-self recognition phenomenon that
prevents various forms of parasitism (509). So far there
have been only a few reported instances of mammalian
proteins that are functionally regulated in a nonpathological
way by interconversion between nonamyloid and amyloid
forms. A remarkable example is the synthesis of
melanin, which involves the formation of amyloid structures
(158). In addition, it has been proposed that proteins
involved in establishing long-term memory might do so by
converting reversibly to and from an amlyoid-like state
(455, 457).
E. Formation of PrP Sc
Physiol Rev VOL 89 OCTOBER 2009 www.prv.org
It remains to be established whether any form of
PrP C can act as a substrate for PrP Sc formation, or
whether a restricted subset of PrP molecules are precursors
for PrP Sc (511). It is also unknown whether reactive
transition states between the two exist. Several experimental
results argue that PrP molecules destined to
become PrP Sc exit to the cell surface prior to their conversion
into PrP Sc (59, 99, 494). Similar to other GPIanchored
proteins, PrP C appears to localize in cholesterolrich,
nonacidic, detergent-insoluble membranes known as
rafts (26, 195, 253, 456, 495). Within the raft compartment,
GPI-anchored PrP C is apparently either converted into
PrP Sc or partially degraded (343, 495). Chemical and enzymatic
treatment of purified PrP 27–30 leads to the release
of glycolipid components, suggesting that PrP Sc could be
tethered to the membrane by a GPI anchor (481).
The role of the GPI membrane anchor in the formation
of PrP Sc in vivo has been addressed by Chesebro et
al. (108), who have established a transgenic mouse model
expressing anchorless, and hence secreted, PrP. When
these transgenic mice were subsequently infected with
protease-resistant PrP Sc , they developed significant amyloid
plaque pathology in the brain, but clinical manifesta-