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

merized into rod-shaped particles with an ultrastructural
appearance of amyloid (Fig. 1C) containing 50% -sheet
and 20% -helix content (103, 176, 435). In contrast to
PrP 27–30 , neither purified PrP C nor PrP Sc forms aggregates
detectable by electron microscopy (338, 379).
2. Models describing the replication of prions
Transgenic mouse studies have provided genetic
(412) and biochemical (343) evidence that the conversion
of PrP C to PrP Sc occurs through the formation of a PrP C /
PrP Sc complex. However, such a complex has never been
isolated to purity. Consequently, it is unclear whether
PrP C binds to one or more additional macromolecules
during the process of PrP C to PrP Sc conversion. The formation
of a PrP C /PrP Sc complex may be facilitated when
the amino acid sequences of the two PrP isoforms are
identical. For instance, hamster PrP differs from mouse
PrP at 16 of 254 positions, perhaps explaining the strong
species barrier between mice and hamsters. Differences
in amino acid sequence delay the onset of prion disease
by prolonging the incubation time (446). As the sequence
specificity of PrP Sc formation is more important for the
conversion to protease resistance than for the initial binding
steps, additional intermolecular interactions between
PrP C and PrP Sc may be required to complete the process
of conversion to the protease-resistant state (228).
The level of PrP C expression appears to be directly
proportional to the rate of PrP Sc formation and, thus, inversely
related to the length of the incubation time. Prnp o/o
mice are resistant to prions and do not propagate scrapie
infectivity (84, 407, 439). Mice hemizygous (Prnp /o ) for
ablation of the Prnp gene have prolonged incubation times
when inoculated with mouse prions (83, 326).
According to the “protein only” hypothesis, there
are two models to explain the conformational conversion
of PrP C into PrP Sc . The “template-directed refolding”
hypothesis predicates an instructionist role for
PrP Sc on PrP C (Fig. 2A). Alternatively, the seeded nucleation
hypothesis suggests that PrP Sc exists in equilibrium
with PrP C . In a nondisease state, such an equilibrium
would be heavily shifted toward the PrP C conformation,
such that only minute amounts of PrP Sc
would coexist with PrP C . If this were the case, PrP Sc
could not possibly represent the infectious agent, since
it would be ubiquitous. According to this “seeded nucleation”
hypothesis (238), the infectious agent would
consist of a highly ordered aggregate of PrP Sc molecules.
The aggregated state would be an intrinsic property
of infectivity. Monomeric PrP Sc would be harmless,
but it might be prone to incorporation into nascent
PrP Sc aggregates (Fig. 2B) (19).
The recently developed PMCA (92, 93, 196, 432,
434) is designed to mimic PrP Sc autocatalytic replication
(Fig. 3). In the PMCA, PrP Sc is amplified in a cyclic
AGGREGATION AND INFECTIOUS DISEASE 1111
Physiol Rev VOL 89 OCTOBER 2009 www.prv.org
FIG. 2. Models for the conformational conversion of PrP C into
PrP Sc . A: the “refolding” or template assistance model postulates an
interaction between exogenously introduced PrP Sc and endogenous
PrP C , which is induced to transform itself into further PrP Sc . A highenergy
barrier may prevent spontaneous conversion of PrP C into PrP Sc .
B: the “seeding” or nucleation-polymerization model proposes that PrP C
and PrP Sc are in a reversible thermodynamic equilibrium. Only if several
monomeric PrP Sc molecules are mounted into a highly ordered seed, can
further monomeric PrP Sc be recruited and eventually aggregate to amyloid.
Fragmentation of PrP Sc aggregates increases the number of nuclei,
which can recruit further PrP Sc and thus results in apparent replication
of the agent.
manner by incubating small amounts of PrP Sc -
containing brain homogenate with uninfected, PrP C -
containing brain homogenate, resulting in the conversion
of PrP C to PrP Sc . The latter forms aggregates that
are sonicated to generate multiple smaller units functioning
as a seed for the continued formation of new
PrP Sc aggregates.