Page 366 - Organic Electronics in Sensors and Biotechnology

Luminescent Conjugated Polymers for Staining and Characterization of Amyloid Deposits 343

C 96
normal soluble prion protein PrP . It has also been found that intra-
venous injection or oral administration of preformed fibrils from dif-
ferent sources can result in accelerated amyloidosis through a prionlike
mechanism. 97, 98 Hence, an environment enriched with fibrillar mate-
rial could act as a risk factor for amyloid diseases. In addition, prions
can occur as multiple strains, giving rise to different symptoms and
96
incubation periods. The prion strain phenomenon is believed to be
associated with multiple morphologies of the prion aggregates, and
the specific properties of a prion strain are encoded in the tertiary or
quaternary structure of the aggregates.
The presence of highly organized and stable fibrillar deposits,
amyloid plaques, in the organs of patients suffering from protein
deposition diseases led initially to the reasonable postulate that this
material is the causative agent of the various disorders. However, as
mentioned in Sec. 9.3.2, more recent findings have raised the possibil-
ity that precursors to amyloid fibrils, such as low-molecular-weight
oligomers and/or structured protofibrils, are the real pathogenic species,
at least in neurodegenerative diseases. For instance, the severity of cog-
nitive impairment in Alzheimer’s disease (AD) correlates with the levels
of low-molecular-weight species of aggregated A-beta peptide (Aβ),
including small oligomers, rather than with the amyloid burden. 99
Genetic evidence also supports the theory that the precursor aggregates
are the pathogenic species. As seen from in vitro experiments, the intro-
duction of an aggressive mutation in the protein favors the formation of
prefibrillar states, and this mutation is associated with a heritable early
100
onset of Alzheimer’s disease. In addition, the most highly infective
form of the mammalian prion protein has been identified as an oligomer
of about 20 molecules, indicating that such small aggregates are the most
effective initiators of transmissible spongiform encephalopathies. 101

9.3.3 Methods for Detection and Structural
Characterization of Amyloid Fibrils
The formation and presence of amyloid fibrils or amyloid plaques
can be visualized by small amyloid ligands dyes, such as derivatives
102
of Congo red and thioflavins. These dyes bind selectively to protein
aggregates having an extensive cross β-pleated sheet conformation
and sufficient structural regularity. Hence, the presence of mature
amyloid fibrils is easily detected by these dyes, seen as an enhanced
fluorescence (thioflavins) or green-yellow birefringence under cross-
polarized light (Congo red) from the dye. However, these dyes are
not able to recognize prefibrillar species, and amyloid fibrils of
diverse morphological origin, as seen for prion strains, cannot be
separated. Furthermore, amyloid ligands are not selective for a dis-
tinct protein and cannot differentiate between amyloid subtypes. The
classification of amyloid subtypes is made by immunohistochemical
stains utilizing antibodies that are selective for the peptide/protein