Structural Basis of Small Molecule Targetability of Monomeric Tau Protein

Abstract

The therapeutic targeting of intrinsically disordered proteins (IDPs) by small molecules has been a challenge due to their heterogeneous conformational ensembles. A potential therapeutic strategy to alleviate the aggregation of IDPs is to maintain them in their native monomeric state by small molecule binding. This study investigates the structural basis of small molecule druggability of native monomeric Tau whose aggregation is linked to the onset of Tauopathies such as Alzheimer's disease. Initially, two available monomeric conformational ensembles of a shorter Tau construct K18 (also termed Tau4RD) were analyzed which revealed striking structural differences between the two ensembles, while similar number of hot spots and small molecule binding sites were identified on monomeric Tau ensembles as on tertiary folded proteins of similar size. Remarkably, some critical fibril forming sequence regions of Tau (V306-K311, V275-K280) participated in hot spot formation with higher frequency compared to other regions. As an example of small molecule binding to monomeric Tau, it was shown that methylene blue (MB) bound to monomeric K18 and full-length Tau selectively with high affinity (K = 125.8 nM and 86.6 nM, respectively) with binding modes involving Cys291 and Cys322, previously reported to be oxidized in the presence of MB. Overall, our results provide structure-based evidence that Tau can be a viable drug target for small molecules and indicate that specific small molecules may be able to bind to monomeric Tau and influence the way in which the protein interacts among itself and with other proteins.

Exploring IDP–Ligand Interactions: Tau K18 as a Test Case

“…Methylene Blue did not show any apparent interaction with tau K18 M in 2D [ 1 H-15 N] SF-HMQC NMR spectra (Figure 1), whereas an interaction was identified by MST (K D 50 +/− 4 nM) ( Figure 2). Previously, it has been reported that Methylene Blue interacts with wild type tau K18(C291,C322) with a K D of 125.8 +/− 5.4 nM, as determined by MST [48], which is broadly consistent with the value determined in our study. Previous investigations of wild type tau K18 have also suggested that the Cys residues may be involved in binding Methylene Blue [25,48].…”

“…Previously, it has been reported that Methylene Blue interacts with wild type tau K18(C291,C322) with a K D of 125.8 +/− 5.4 nM, as determined by MST [48], which is broadly consistent with the value determined in our study. Previous investigations of wild type tau K18 have also suggested that the Cys residues may be involved in binding Methylene Blue [25,48]. However, our MST results using mutated tau K18 (tau K18 M ), which lacks these Cys residues, showed a similar interaction suggesting that the Cys residues are not critical.…”

Novel Small Molecules Targeting the Intrinsically Disordered Structural Ensemble of α-Synuclein Protect Against Diverse α-Synuclein Mediated Dysfunctions

“…Indeed, in a related study, we applied HT-CM-SPR screening for small molecules interacting with monomeric full-length tau, which resulted in the discovery of a variety of compounds which were able to reduce the aggregation of tau in vitro and in a cell model 23 . Hence, these studies add to the accumulating evidence that small molecules can bind to IDPs, such as αSyn 20 , tau 23,53 and the Aβ peptide 54 , despite their overall lack of persistent 3D structures.…”

Computer-Aided Drug Design of β-Secretase, γ-Secretase and Anti-Tau Inhibitors for the Discovery of Novel Alzheimer’s Therapeutics

“…Pose prediction accuracy studies on GSK3β inhibitors demonstrated that cognate docking calculations successfully reproduced crystallographic poses with RMSD less than 2 Å, while cross-docking calculations produced poses that deviate from X-ray poses providing valuable insights into computational structure-based design [156]. The structural basis of targetability of monomeric Tau by small molecules was studied using molecular docking indicating the ability of small molecules, like methylene blue, to bind to monomeric Tau and influence interactions of the protein with itself and other proteins [157]. Computational pharmacophore models, molecular docking, and MD simulations were employed to identify hyperphosphorylated sites of Tau including multiple serine sites changing the flanking sequence of R1/R2 repeats, which affects binding of Tau to microtubules [158].…”