N-Terminal Extensions Retard Aβ42 Fibril Formation but Allow Cross-Seeding and Coaggregation with Aβ42
† Department
of Biochemistry and Structural Biology, Lund University, P O Box 124, 221 00 Lund, Sweden
‡ Laboratory
for Neurodegenerative Research, Conway Institute of Biomedical and
Biomolecular Research, University College
Dublin, Belfield, Dublin 4, Republic of Ireland
§ Laboratory
for Neurodegenerative Research, Ann Romney Center for Neurologic Diseases, Brigham and Women’s Hospital and Harvard Medical
School, Boston, Massachusetts 02115, United States
∥ Department
of Chemistry, Cambridge University, Lensfield Road, Cambridge, CB2 1EW, United
Kingdom
⊥ Department
of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Amyloid β-protein (Aβ) sequence length variants with varying aggregation
propensity coexist in vivo, where coaggregation and cross-catalysis
phenomena may affect the aggregation process. Until recently, naturally
occurring amyloid β-protein (Aβ) variants were believed to begin at or
after the canonical β-secretase cleavage site within the amyloid
β-protein precursor. However, N-terminally extended forms of Aβ (NTE-Aβ)
were recently discovered and may contribute to Alzheimer’s disease.
Here, we have used thioflavin T fluorescence to study the aggregation
kinetics of Aβ42 variants with N-terminal extensions of 5–40 residues,
and transmission electron microscopy to analyze the end states. We find
that all variants form amyloid fibrils of similar morphology as Aβ42,
but the half-time of aggregation (t1/2) increases
exponentially with extension length. Monte Carlo simulations of model
peptides suggest that the retardation is due to an underlying general
physicochemical effect involving reduced frequency of productive
molecular encounters. Indeed, global kinetic analyses reveal that
NTE-Aβ42s form fibrils via the same mechanism as Aβ42, but all
microscopic rate constants (primary and secondary nucleation,
elongation) are reduced for the N-terminally extended variants. Still,
Aβ42 and NTE-Aβ42 coaggregate to form mixed fibrils and fibrils of
either Aβ42 or NTE-Aβ42 catalyze aggregation of all monomers. NTE-Aβ42
monomers display reduced aggregation rate with all kinds of seeds
implying that extended termini interfere with the ability of monomers to
nucleate or elongate. Cross-seeding or coaggregation may therefore
represent an important contribution in the in vivo formation of
assemblies believed to be important in disease.
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