Small molecule-mediated therapeutic approaches to target Tau and Alzheimer's disease.

The aggregation of tau protein is one of the hallmarks for Alzheimer’s disease, resulting in neurodegeneration. The peptidomimetics strategy to prevent tau aggregation is more specific over other small molecules. In the present study, we analyzed the effect of amyloid-β-derived peptidomimetics for inhibiting heparin-induced tau aggregation in vitro. These peptides and their derivatives were known to prevent aggregation of amyloid-β. KLVFF is a hydrophobic sequence of the pentapeptide that prevented tau aggregation as observed by thioflavin S fluorescence, transmission electron microscopy, and circular dichroism spectroscopy. P4 and P5 also prevented assembly of tau into aggregates and formed short fibrils. The β-sheet breaker LPFFD was however ineffective in preventing tau aggregation. The peptides further demonstrated reversal of tau-induced cytotoxicity in a dose-dependent manner. Our results suggested that these peptides can also be used to inhibit tau aggregation and also, toxicity induced by tau could be considered as potential molecules that have an effect on tau as well as amyloid-β.

Amyloid fibrils of tau are increasingly accepted as a cause of neuronal death and brain atrophy in Alzheimer's disease (AD). Diminishing tau aggregation is a promising strategy in the search for efficacious AD therapeutics. Previously, our laboratory designed a six-residue, nonnatural amino acid inhibitor D-TLKIVW peptide (6-DP), which can prevent tau aggregation in vitro. However, it cannot block cell-to-cell transmission of tau aggregation. Here, we find D-TLKIVWC (7-DP), a d-cysteine extension of 6-DP, not only prevents tau aggregation but also fragments tau fibrils extracted from AD brains to neutralize their seeding ability and protect neuronal cells from tau-induced toxicity. To facilitate the transport of 7-DP across the blood-brain barrier, we conjugated it to magnetic nanoparticles (MNPs). The MNPs-DP complex retains the inhibition and fragmentation properties of 7-DP alone. Ten weeks of MNPs-DP treatment appear to reverse neurological deficits in the PS19 mouse model of AD. This work offers a direction for development of therapies to target tau fibrils.

Small-molecule Tau aggregation inhibitors are under investigation as potential therapeutic agents against Alzheimer disease. Many such inhibitors have been identified in vitro, but their potency-driving features, and their molecular targets in the Tau aggregation pathway, have resisted identification. Previously we proposed ligand polarizability, a measure of electron delocalization, as a candidate descriptor of inhibitor potency. Here we tested this hypothesis by correlating the ground state polarizabilities of cyanine, phenothiazine, and arylmethine derivatives calculated using ab initio quantum methods with inhibitory potency values determined in the presence of octadecyl sulfate inducer under reducing conditions. A series of rhodanine analogs was analyzed as well using potency values disclosed in the literature. Results showed that polarizability and inhibitory potency directly correlated within all four series. To identify putative binding targets, representative members of the four chemotypes were added to aggregation reactions, where they were found to stabilize soluble, but SDS-resistant Tau species at the expense of filamentous aggregates. Using SDS resistance as a secondary assay, and a library of Tau deletion and missense mutants as targets, interaction with cyanine was localized to the microtubule binding repeat region. Moreover, the SDS-resistant phenotype was completely dependent on the presence of octadecyl sulfate inducer, but not intact PHF6/PH6* hexapeptide motifs, indicating that cyanine interacted with a species in the aggregation pathway prior to nucleus formation. Together the data suggest that flat, highly polarizable ligands inhibit Tau aggregation by interacting with folded species in the aggregation pathway and driving their assembly into soluble but highly stable Tau oligomers.

Moving Beyond Amyloid in Alzheimer's Therapeutics

Heat shock proteins regulates Tau protein aggregation in Alzheimer's disease.

INTRODUCTIONAs aggregation underpins Tau toxicity, aggregation inhibitor peptides may have disease‐modifying potential. They are therefore currently being designed and target either the 306VQIVYK311 aggregation‐promoting hotspot found in all Tau isoforms or the 275VQIINK280 aggregation‐promoting hotspot found in 4R isoforms. However, for any Tau aggregation inhibitor to potentially be clinically relevant for other tauopathies, it should target both hotspots to suppress aggregation of Tau isoforms, be stable, cross the blood‐brain barrier, and rescue aggregation‐dependent Tau phenotypes in vivo.METHODSWe developed a retro‐inverso, stable D‐amino peptide, RI‐AG03 [Ac‐rrrrrrrrGpkyk(ac)iqvGr‐NH2], based on the 306VQIVYK311 hotspots which exhibit these disease‐relevant attributes.RESULTSUnlike other aggregation inhibitors, RI‐AG03 effectively suppresses aggregation of multiple Tau species containing both hotspots in vitro and in vivo, is non‐toxic, and suppresses aggregation‐dependent neurodegenerative and behavioral phenotypes.DISCUSSIONRI‐AG03 therefore meets many clinically relevant requirements for an anti‐aggregation Tau therapeutic and should be explored further for its disease‐modifying potential for Tauopathies.HighlightsOur manuscript describes the development of a novel peptide inhibitor of Tau aggregation, a retro‐inverso, stable D‐amino peptide called RI‐AG03 that displays many clinically relevant attributes. We show its efficacy in preventing Tau aggregation in both in vitro and in vivo experimental models while being non‐toxic to cells. RI‐AG03 also rescues a biosensor cell line that stably expresses Tau repeat domains with the P301S mutation fused to Cer/Clo and rescues aggregation‐dependent phenotypes in vivo, suppressing neurodegeneration and extending lifespan.Collectively our data describe several properties and attributes of RI‐AG03 that make it a promising disease‐modifying candidate to explore for reducing pathogenic Tau aggregation in Tauopathies such as Alzheimer's disease. Given the real interest in reducing Tau aggregation and the potential clinical benefit of using such agents in clinical practice, RI‐AG03 should be investigated further for the treatment of Tauopathies after validation in mammalian models.Tau aggregation inhibitors are the obvious first choice as Tau‐based therapies as much of Tau‐mediated toxicity is aggregation dependent. Indeed, there are many research efforts focusing on this therapeutic strategy with aggregation inhibitors being designed against one of the two aggregation‐promoting hotspots of the Tau protein. To our knowledge, RI‐AG03 is the only peptide aggregation inhibitor that inhibits aggregation of Tau by targeting both aggregation‐promoting hotspot motifs simultaneously. As such, we believe that our study will have a significant impact on drug discovery efforts in this arena.

We are analyzing the physiological function of Tau protein and its abnormal pathological behavior when this protein is self-assemble into pathological filaments. These aggregates of Tau protein are the main components in many diseases such as Alzheimer's disease (AD). Recent studies suggest that Tau acquires complex oligomeric conformations which may be toxic. In this review, we emphasized the possible phenomena implicated in the formation of these oligomers. Studies with chemical inductors indicates that the microtubule-binding domain is the most important region involved in Tau aggregation and showed the requirement of a pre-arrange Tau in abnormal conformation to promote self-assembly. Transgenic animal models and AD neuropathology studies showed that post-translational modifications are also implicated in Tau aggregation and neural cell death during AD development. Therefore, we analyzed some events that could be present during Tau aggregation. Finally, we included a brief discussion of the possible relation between glucose metabolism dysfunction in AD, and data of Tau aggregation by using aggregation inhibitors. In conclusion, the process Tau aggregation deserves further investigations to design possible therapeutic targets to inhibit the toxicity of these aggregates and it is possible that could be extended to other diseases with similar etiology.

Mutations in the presenilin 1 (PS1) gene are responsible for the early onset of familial Alzheimer disease (FAD). Accumulating evidence shows that PS1 is involved in gamma-secretase activity and that FAD-associated mutations of PS1 commonly accelerate Abeta(1-42) production, which causes Alzheimer disease (AD). Recent studies suggest, however, that PS1 is involved not only in Abeta production but also in other processes that lead to neurodegeneration. To better understand the causes of neurodegeneration linked to the PS1 mutation, we analyzed the development of tau pathology, another key feature of AD, in PS1 knock-in mice. Hippocampal samples taken from FAD mutant (I213T) PS1 knock-in mice contained hyperphosphorylated tau that reacted with various phosphodependent tau antibodies and with Alz50, which recognizes the conformational change of PHF tau. Some neurons exhibited Congo red birefringence and Thioflavin T reactivity, both of which are histological criteria for neurofibrillary tangles (NFTs). Biochemical analysis of the samples revealed SDS-insoluble tau, which under electron microscopy examination, resembled tau fibrils. These results indicate that our mutant PS1 knock-in mice exhibited NFT-like tau pathology in the absence of Abeta deposition, suggesting that PS1 mutations contribute to the onset of AD not only by enhancing Abeta(1-42) production but by also accelerating the formation and accumulation of filamentous tau.

Tau aggregation is linked to multiple neurodegenerative disorders that are collectively termed tauopathies. Small molecules are powerful probes of the aggregation process, helping to reveal the key steps and serving as diagnostics and reporters. Moreover, some of these small molecules may have potential as therapeutics. This review details how small molecules and chemical biology have helped to elucidate the mechanisms of tau aggregation and how they are being used to detect and prevent tau aggregation. In addition, we comment on how new insights into tau prions are changing the approach to small molecule discovery.

In Vivo Spreading of Tau Pathology

Tau phosphorylation plays an important role in brain physiology and pathology. During foetal development, it supports microtubule dynamics and neuroplasticity, whereas in Alzheimer’s disease (AD), it drives pathological tau aggregation and tangle formation. In this multicentre study (n = 462), we measured plasma phosphorylated-tau217 in healthy newborns, premature infants, patients with AD and healthy controls across various age groups. Plasma phosphorylated-tau217 levels were significantly higher in newborns compared to healthy individuals of any age group and even exceeded levels observed in patients with AD. In newborns, plasma phosphorylated-tau217 levels inversely correlated with perinatal factors such as gestational age. Longitudinal analysis of preterm infants demonstrated a decline in serum phosphorylated-tau217 levels over the first months of life, approaching levels observed in young adults. In contrast, elevated plasma phosphorylated-tau217 in older individuals was associated with AD pathology. Our findings corroborate the crucial role of tau phosphorylation in early brain development. However, in AD, tau phosphorylation transitions into a pathological mechanism. The high levels of blood-based phosphorylated-tau217 observed at birth and subsequent clearance might indicate distinct regulatory mechanisms that prevent tau aggregation in early life. Further studies are needed to explore the shared mechanisms of tau phosphorylation in newborns and AD.

Tau protein stabilizes microtubules in neuronal cells and maintains cell structure and function [1]. Post-translational modifications of tau, such as phosphorylations, lead to tau detachment from microtubules and subsequent cell death. Modified tau aggregates into cytotoxic structures, leading to neurodegeneration. Currently, neurodegeneration remains without a cure, but several molecular strategies have been employed towards targeting phosphorylation and aggregation of tau protein. Small molecules have been tested as potential aggregation inhibitors. For example, dopamine receptor agonist compounds were effective inhibitors of tau aggregation in vitro [2]. The large molecules, such as antitau antibodies, were also explored for their ability to inhibit phosphorylation of tau or its aggregation. The antitau antibodies, targeting R-repeat epitopes, effectively reduced tau aggregation in vitro [3], and modulated tau phosphorylation by GSK-3β protein kinase [4]. The details regarding the inhibition with various small and large molecules will be described, and new therapeutic strategies discussed.

Pioglitazone prevents tau oligomerization

Synthesis and evaluation of 1,2,3,4-tetrahydro-1-acridone analogues as potential dual inhibitors for amyloid-beta and tau aggregation

Tauopathies, such as Alzheimer's disease, are characterized by the misfolding and progressive accumulation of the microtubule associated protein tau. Chaperones, tasked with maintaining protein homeostasis, can become imbalanced with age and contribute to the progression of neurodegenerative disease. Cyclophilins are a promising pool of underinvestigated chaperones with peptidyl-prolyl isomerase activity that may play protective roles in regulating tau aggregation. Using a Thioflavin T fluorescence-based assay to monitor in vitro tau aggregation, all eight cyclophilins, which include PPIA to PPIH prevent tau aggregation, with PPIB, PPIC, PPID, and PPIH showing the greatest inhibition. The low thermal stability of PPID and the strong heparin binding of PPIB undermines the simplistic interpretation of reduced tau aggregation. In a cellular model of tau accumulation, all cyclophilins, except PPID and PPIH, reduce insoluble tau. PPIB, PPIC, PPIE, and PPIF also reduce soluble tau levels with PPIC exclusively protecting cells from tau seeding. Overall, this study demonstrates cyclophilins prevent tau fibril formation and many reduce cellular insoluble tau accumulation with PPIC having the greatest potential as a molecular tool to mitigate tau seeding and accumulation.