Page 364 - Organic Electronics in Sensors and Biotechnology

Luminescent Conjugated Polymers for Staining and Characterization of Amyloid Deposits 341

processes dependent on a nucleation step, addition of preformed
fibrillar species to a sample of a protein under aggregation condi-
tions causes the lag phase to be shortened and finally abolished
when the rate of the aggregation process is no longer limited by the
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need for nucleation. Furthermore, changes in experimental condi-
tions can also reduce or eliminate the length of the lag phase, again
assumed to result from a situation wherein nucleation is no longer
rate-limiting. Therefore, the absence of a lag phase does not inevita-
bly imply the absence of a nucleated growth mechanism, but it may
simply be that the time required for fibril growth is sufficiently slow
relative to the nucleation process and that the latter is no longer the
slowest step in the conversion of the monomeric protein into amy-
loid fibrils.
It is clear that the lag phase in fibril formation is an important event
in which a variety of prefibrillar species are formed, including β-sheet
rich oligomers and protofibrils clustered as spherical beads (2 to 5 nm
in diameter) with β-sheet structure. The past decade has seen very
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substantial efforts directed toward identifying, isolating, and charac-
terizing these prefibrillar species that are present in solution prior to
the appearance of fibrils, both because of their likely role in the mecha-
nism of fibril formation and because of their implication as the toxic
species involved in neurodegenerative disorders (see Sec. 9.3.2). Fur-
thermore, the differing features of the aggregation processes, described
in the previous paragraphs, reveal that polypeptide chains can adopt a
multitude of conformational states. Therefore, it is not surprising that
both the prefibrillar species and the fibrillar end products of amyloid
fibril formation are characterized by morphological and structural
diversity. Hence, techniques for detecting of variety of protein aggregates
and methods for studying the molecular details of these aggregates are
of great interest. This statement will become even more evident when
the pathological events underlying the diseases associated with amy-
loid fibril formation are being discussed.

9.3.2 Protein Aggregation Diseases
A broad range of human diseases arise from the failure of a specific
peptide or protein to adopt, or remain in, its native functional confor-
mation. These pathological conditions are generally referred to as
protein misfolding diseases and include pathological states in which
an impairment in the folding efficiency of a given protein results in a
reduction of the effective concentration of a functional protein that is
available to play its normal role, as seen in cystic fibrosis. However,
the largest group of misfolding diseases is associated with the con-
version of peptides or proteins to amyloid fibrils. The amyloid fibrils
are further assembled into higher-order structures that pathologically
are termed amyloid plaques when they accumulate extracellularly,
whereas the term intracellular inclusions has been suggested as more