Understanding the Kinetic Roles of the Inducer Heparin and of Rod-like Protofibrils during Amyloid Fibril Formation by Tau Protein

Abstract

Background:The kinetic role of heparin and of intermediates populated during Tau protein aggregation is not fully understood. Results: Heparin contributes to the initial steps of fibrillation, whereas protofibrillar intermediates accumulate transiently. Conclusion: Heparin is involved in nucleation, and the transient rod-like protofibrils are off-pathway species. Significance: Understanding the kinetic role of heparin and the protofibrils has implications for the development of therapies for tauopathies.

Evidence for the Existence of a Secondary Pathway for Fibril Growth during the Aggregation of Tau

“…6d). Since the kinetic role of heparin is restricted mainly to the rate-limiting steps of aggregation, 57 it is likely that the equilibrium nucleus is composed of three protein molecules held together by one heparin molecule. Future studies will address the important question of whether heparin, in addition to affecting the initial equilibrium between the monomer and the trimeric nucleus, also has roles in modulating growth by both the primary and the secondary pathways.…”

“…This dependence yields only the stoichiometry of binding 68 and not the binding constant, because the binding of heparin to tau is tight with a submicromolar dissociation constant. 57,69,70 The In all the plots, the broken line is a least-squares fit to an exponential equation, Δ = ae kt , and the black continuous line is a least-squares fit to a quadratic equation, Δ = at 2 . The insets in all four panels show that the data fit well to Eq.…”

“…4,16 This result is in accordance with previous results, which also suggested that tau aggregates via an NDP mechanism. 53,[55][56][57] ThT fluorescence was chosen as the spectroscopic probe to study the kinetics of aggregation over a 10-fold range of protein concentrations, as the ThT fluorescence-monitored kinetics were highly reproducible as seen by the small spread in the data (Figs. 1 and 2).…”

What Can the Kinetics of Amyloid Fibril Formation Tell about Off-pathway Aggregation?

“…The hyperbolic (concave) profiles exhibiting no inflection point or lag phase in the absence of seeding (Fig. 1a) are frequently reported in the literature during the aggregation of, for example, serum albumin (19,20), transthyretin (21,22), ␤ 2 -microglobulin (23), the four-repeat domain of Tau (24), apolipoprotein (25), and amyloid-␤ variants (26,27). This type of result is explained by the CLM as the result of predominant primary nucleation over the autocatalytic processes and is also well fitted by the "Ockham's razor" minimalistic model (18,28).…”

Alternative Conformations of the Tau Repeat Domain in Complex with an Engineered Binding Protein

“…7). Heparin is known to greatly accelerate the nucleation step of Tau aggregation (49,50). Under these conditions, the capacity of substoichiometric TP4 concentrations to interfere with the formation and/or growth of nuclei might be exceeded.…”