Fluorescence Detection and Inhibition Mechanisms of DNTPH on Aβ42 Oligomers Characterized as Products in the Four Stages of Aggregation

Aβ42 aggregation was implicated in the pathogenesis of Alzheimer's disease (AD) without effective treatment available currently. Future efforts in clinical trials should instead focus on applying those antiamyloid treatment strategies to the preclinical stage and "the earlier, the better". How to identify and inhibit Aβ42 oligomers in the different stages of aggregation is therefore becoming the key to controlling primary aggregation and consequent AD development. Aggregation-induced emission probe DNTPH was demonstrated recently, enabling detection of amyloid at wavelengths up to 710 nm and exhibiting strong inhibitory effects on Aβ fibrosis at low dose. However, the detection and inhibition mechanisms of Aβ oligomers at various early stages of aggregation remain unknown. To this end, we built four different morphologies of Aβ42 pentamers characterized by products in monomeric aggregate (P_M), primary nucleation (P_P), secondary nucleation (P_S), and fibril stages (P_F) to explore the distinguishable ability and inhibition mechanisms of DNTPH with different concentrations upon binding. The results showcased that DNTPH does detect the four different Aβ42 oligomers with conspicuous fluorescence (λ_{P_M} = 657 nm, λ_{P_P} = 639 nm, λ_{P_S} = 630 nm, and λ_{P_F} = 648 nm) but fails to distinguish them, indicating that additional improvements are required further for the probe to achieve it. The inhibition mechanisms of DNTPH on the four Aβ42 aggregation are however of amazing differences. For P_M and P_P, aggregation was inhibited by altering the secondary structural composition, i.e., by decreasing the β-sheet and toxic turn (residues 22-23) probabilities, respectively. For P_S, inhibition was achieved by segregating and keeping the two disordered monomeric species (P_SM) away from the ordered secondary seed species (P_SF) and consequently blocking further growth of the P_SF seed. The inhibition mechanism for P_S is first probed and proposed so far, as far as we know, and the corresponding aggregation stage of P_S is the most important one among the four stages. The inhibition of P_F was triggered by distorting the fibril chains, disrupting the ordered fibril surface for the contact of monomers. In addition, the optimal inhibitory concentrations of DNTPH for P_M, P_P, and P_F were determined to be 1:3, while for P_S, it was 1:5. This outcome offers a novel perspective for designing drugs targeting Aβ42 oligomers at different aggregation stages.