In vitro characterization of urea derivatives to inhibit alpha-synuclein early-stage aggregation

Novel strategies for targeting tau oligomers in neurodegenerative diseases

Tau protein is a soluble microtubule-associated protein enriched in neurons, is mainly distributed in the central nervous system, and is responsible for stabilizing neurons. Tau maintains nerve cell morphology and internal transport by binding to normal microtubules. In neurodegenerative diseases, such as Alzheimer's disease (AD), tau proteins undergo aberrant phosphorylation, resulting in their removal from microtubules and the formation of neurofibrillary tangles (NFTs), which are key pathological features. In contrast to the late formation of non-soluble NFTs, early, smaller, soluble tau oligomers (tauO) with disseminated toxicity are considered necessary in neurodegenerative disorders, such as the primary form of tau toxicity in the AD process. Although an increasing number of studies are focusing on tauO, there are still problems to be solved, mainly concerning the molecular and inhibitory mechanisms of tauO toxicity. In this paper, we summarize the new strategies for the molecular mechanisms of tauO toxicity, detection methods, and interventions in the last five years. An outlook on these new strategies and the challenges that may be foreseen is presented to provide new directions for future applications in the clinical treatment of neurodegenerative diseases.

Evaluation of Alpha-Synuclein and Tau Antiaggregation Activity of Urea and Thiourea-Based Small Molecules for Neurodegenerative Disease Therapeutics

Alzheimer's disease (AD) and Parkinson's disease (PD) are multifactorial, chronic diseases involving neurodegeneration. According to recent studies, it is hypothesized that the intraneuronal and postsynaptic accumulation of misfolded proteins such as α-synuclein (α-syn) and tau, responsible for Lewy bodies (LB) and tangles, respectively, disrupts neuron functions. Considering the co-occurrence of α-syn and tau inclusions in the brains of patients afflicted with subtypes of dementia and LB disorders, the discovery and development of small molecules for the inhibition of α-syn and tau aggregation can be a potentially effective strategy to delay neurodegeneration. Urea is a chaotropic agent that alters protein solubilization and hydrophobic interactions and inhibits protein aggregation and precipitation. The presence of three hetero atoms (O/S and N) in proximity can coordinate with neutral, mono, or dianionic groups to form stable complexes in the biological system. Therefore, in this study, we evaluated urea and thiourea linkers with various substitutions on either side of the carbamide or thiocarbamide functionality to compare the aggregation inhibition of α-syn and tau. A thioflavin-T (ThT) fluorescence assay was used to evaluate the level of fibril formation and monitor the anti-aggregation effect of the different compounds. We opted for transmission electron microscopy (TEM) as a direct means to confirm the anti-fibrillar effect. The oligomer formation was monitored via the photoinduced cross-linking of unmodified proteins (PICUP). The anti-inclusion and anti-seeding activities of the best compounds were evaluated using M17D intracellular inclusion and biosensor cell-based assays, respectively. Disaggregation experiments were performed with amyloid plaques extracted from AD brains. The analogues with indole, benzothiazole, or N,N-dimethylphenyl on one side with halo-substituted aromatic moieties had shown less than 15% cutoff fluorescence obtained with the ThT assay. Our lead molecules 6T and 14T reduced α-syn oligomerization dose-dependently based on the PICUP assays but failed at inhibiting tau oligomer formation. The anti-inclusion effect of our lead compounds was confirmed using the M17D neuroblastoma cell model. Compounds 6T and 14T exhibited an anti-seeding effect on tau using biosensor cells. In contrast to the control, disaggregation experiments showed fewer Aβ plaques with our lead molecules (compounds 6T and 14T). Pharmacokinetics (PK) mice studies demonstrated that these two thiourea-based small molecules have the potential to cross the blood-brain barrier in rodents. Urea and thiourea linkers could be further improved for their PK parameters and studied for the anti-inclusion, anti-seeding, and disaggregation effects using transgenic mice models of neurodegenerative diseases.

Discovery of small molecule benzothiazole and indole derivatives tackling tau 2N4R and α-synuclein fibrils

Tau and α-synuclein aggregates are the main histopathological hallmarks present in Alzheimer's disease (AD), Parkinson's disease (PD), and other neurodegenerative disorders. Intraneuronal hyperphosphorylated tau accumulation is significantly connected to the degree of cognitive impairment in AD patients. In particular, the longest 2N4R tau isoform has a propensity to rapidly form oligomers and mature fibrils. On the other hand, misfolding of α-synuclein (α-syn) is the characteristic feature in PD and dementia with Lewy bodies (DLB). There is a strong crosstalk between the two prone-to-aggregation proteins as they coprecipitated in some brains of AD, PD, and DLB patients. Simultaneous targeting of both proteinaceous oligomers and aggregates is still challenging. Here, we rationally designed and synthesized benzothiazole- and indole-based compounds using the structural hybridization strategy between the benzothiazole N744 cyanine dye and the diphenyl pyrazole Anle138b that showed anti-aggregation activity towards 2N4R tau and α-syn, respectively. The anti-aggregation effect of the prepared compounds was monitored using the thioflavin-T (ThT) fluorescence assay, while transmission electron microscopy (TEM) was employed to detect fibrils upon the completion of a time-course study with the ThT assay. Moreover, the photo-induced crosslinking of unmodified protein (PICUP) assay was used to determine the formation of oligomers. Specifically, compounds 46 and 48 demonstrated the highest anti-aggregation activity by decreasing the ThT fluorescence to 4.0 and 14.8%, respectively, against α-syn. Although no noticeable effect on 2N4R tau oligomers, 46 showed promising anti-oligomer activity against α-syn. Both compounds induced a significantly high anti-aggregation effect against the two protein fibrils as visualized by TEM. Moreover, compound 48 remarkably inhibited α-syn inclusion and cell confluence using M17D cells. Collectively, compounds 46 and 48 could serve as a basic structure for further optimization to develop clinically active AD and PD disease-modifying agents.

1,4-Diurea- and 1,4-Dithiourea-Substituted Aromatic Derivatives Selectively Inhibit α-Synuclein Oligomer Formation In Vitro

Parkinson's disease (PD) is the second most common neurodegenerative disease, affecting the elderly population worldwide. In PD, the misfolding of α-synuclein (α-syn) results in the formation of inclusions referred to as Lewy bodies (LB) in midbrain neurons of the substantia nigra and other specific brain localizations, which is associated with neurodegeneration. There are no approved strategies to reduce the formation of LB in the neurons of patients with PD. Our drug discovery program focuses on the synthesis of urea and thiourea compounds coupled with aminoindole moieties to abrogate α-syn aggregation and to slow down the progression of PD. We synthesized several urea and thiourea analogues with a central 1,4-phenyl diurea/thiourea linkage and evaluated their effectiveness in reducing α-syn aggregation with a special focus on the selective inhibition of oligomer formation among other proteins. We utilized biophysical methods such as thioflavin T (ThT) fluorescence assays, transmission electron microscopy (TEM), photoinduced cross-linking of unmodified proteins (PICUP), as well as M17D intracellular inclusion cell-based assays to evaluate the antiaggregation properties and cellular protection of our best compounds. Our results identified compound 1 as the best compound in reducing α-syn fibril formation via ThT assays. The antioligomer formation of compound 1 was subsequently superseded by compound 2. Both compounds selectively curtailed the oligomer formation of α-syn but not tau 4R isoforms (0N4R, 2N4R) or p-tau (isoform 1N4R). Compounds 1 and 2 failed to abrogate tau 0N3R fibril formation by ThT and atomic force microscopy. Compound 2 was best at reducing the formation of recombinant α-syn fibrils by TEM. In contrast to compound 2, compound 1 reduced the formation of α-syn inclusions in M17D neuroblastoma cells in a dose-dependent manner. Compound 1 may provide molecular scaffolds for the optimization of symmetric molecules for its α-syn antiaggregation activity with potential therapeutic applications and development of small molecules in PD.

Evaluation of N- and O-Linked Indole Triazines for a Dual Effect on α-Synuclein and Tau Aggregation

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder underlying dementia in the geriatric population. AD manifests by two pathological hallmarks: extracellular amyloid-β (Aβ) peptide-containing senile plaques and intraneuronal neurofibrillary tangles comprised of aggregated hyperphosphorylated tau protein (p-tau). However, more than half of AD cases also display the presence of aggregated α-synuclein (α-syn)-containing Lewy bodies. Conversely, Lewy bodies disorders have been reported to have concomitant Aβ plaques and neurofibrillary tangles. Our drug discovery program focuses on the synthesis of multitarget-directed ligands to abrogate aberrant α-syn, tau (2N4R), and p-tau (1N4R) aggregation and to slow the progression of AD and related dementias. To this end, we synthesized 11 compounds with a triazine-linker and evaluated their effectiveness in reducing α-syn, tau isoform 2N4R, and p-tau isoform 1N4R aggregation. We utilized biophysical methods such as thioflavin T (ThT) fluorescence assays, transmission electron microscopy (TEM), photoinduced cross-linking of unmodified proteins (PICUP), and M17D intracellular inclusion cell-based assays to evaluate the antiaggregation properties and cellular protection of our best compounds. We also performed disaggregation assays with isolated Aβ-plaques from human AD brains. Our results demonstrated that compound 10 was effective in reducing both oligomerization and fibril formation of α-syn and tau isoform 2N4R in a dose-dependent manner via ThT and PICUP assays. Compound 10 was also effective at reducing the formation of recombinant α-syn, tau 2N4R, and p-tau 1N4R fibrils by TEM. Compound 10 reduced the development of α-syn inclusions in M17D neuroblastoma cells and stopped the seeding of tau P301S using biosensor cells. Disaggregation experiments showed smaller Aβ-plaques and less paired helical filaments with compound 10. Compound 10 may provide molecular scaffolds for further optimization and preclinical studies for neurodegenerative proteinopathies.

5-Nitro-1,2-benzothiazol-3-amine and N-Ethyl-1-[(ethylcarbamoyl)(5-nitro-1,2-benzothiazol-3-yl)amino]formamide Modulate α-Synuclein and Tau Aggregation

Protein misfolding results in a plethora of known diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, transthyretin-related amyloidosis, type 2 diabetes, Lewy body dementia, and spongiform encephalopathy. To provide a diverse portfolio of therapeutic small molecules with the ability to reduce protein misfolding, we evaluated a set of 13 compounds: 4-(benzo[d]thiazol-2-yl)aniline (BTA) and its derivatives containing urea (1), thiourea (2), sulfonamide (3), triazole (4), and triazine (5) linker. In addition, we explored small modifications on a very potent antioligomer 5-nitro-1,2-benzothiazol-3-amine (5-NBA) (compounds 6-13). This study aims to define the activity of BTA and its derivatives on a variety of prone-to-aggregate proteins such as transthyretin (TTR81-127, TTR101-125), α-synuclein (α-syn), and tau isoform 2N4R (tau 2N4R) through various biophysical methods. Thioflavin T (ThT) fluorescence assay was used to monitor fibril formation of the previously mentioned proteins after treatment with BTA and its derivatives. Antifibrillary activity was confirmed using transmission electron microscopy (TEM). Photoreactive cross-linking assay (PICUP) was utilized to detect antioligomer activity and lead to the identification of 5-NBA (at low micromolar concentration) and compound 13 (at high concentration) as the most promising in reducing oligomerization. 5-NBA and not BTA inhibited the inclusion formation based on the cell-based assay using M17D neuroblastoma cells that express inclusion-prone αS-3K::YFP. 5-NBA abrogated the fibril, oligomer, and inclusion formation in a dose-dependent manner. 5-NBA derivatives could be the key to mitigate protein aggregation. In the future, the results made from this study will provide an initial platform to generate more potent inhibitors of α-syn and tau 2N4R oligomer and fibril formation.

Discovery of 4-aminoindole carboxamide derivatives to curtail alpha-synuclein and tau isoform 2N4R oligomer formation

Alzheimer's disease (AD) is a multifactorial, chronic neurodegenerative disease characterized by the presence of extracellular β-amyloid (Aβ) plaques, intraneuronal neurofibrillary tangles (NFTs), activated microglial cells, and an inflammatory state (involving reactive oxygen species production) in the brain. NFTs are comprised of misfolded and hyperphosphorylated forms of the microtubule-binding protein tau. Interestingly, the trimeric form of the 2N4R splice isoform of tau has been found to be more toxic than the trimeric 1N4R isoform in neuron precursor cells. Few drug discovery programs have focused on specific tau isoforms. The present drug discovery project is centered on the anti-aggregation effect of a series of seventeen 4- or 5-aminoindole carboxamides on the 2N4R isoform of tau. The selection of the best compounds was performed using α-synuclein (α-syn). The anti-oligomer and -fibril activities of newly synthesized aminoindole carboxamide derivatives were evaluated with biophysical methods, such as thioflavin T fluorescence assays, photo-induced cross-linking of unmodified proteins, and transmission electron microscopy. To evaluate the reduction of inclusions and cytoprotective effects, M17D neuroblastoma cells expressing inclusion-forming α-syn were treated with the best amide representatives. The 4-aminoindole carboxamide derivatives exhibited a better anti-fibrillar activity compared to their 5-aminoindole counterparts. The amide derivatives 2, 8, and 17 exerted anti-oligomer and anti-fibril activities on α-syn and the 2N4R isoform of tau. At a concentration of 40 μM, compound 8 reduced inclusion formation in M17D neuroblastoma cells expressing inclusion-prone αSynuclein3K::YFP. Our results demonstrate the potential of 4-aminoindole carboxamide derivatives with regard to inhibiting the oligomer formation of α-syn and tau (2N4R isoform) for further optimization prior to pre-clinical studies.