Redox Properties of Small Molecules Essential for Multiple Reactivities with Pathological Factors in Alzheimer's Disease

Synthesis and biological evaluation of indane-based fluorescent probes for detection of amyloid-β aggregates in Alzheimer's disease

In this article, the development of fluorescent imaging probes for the detection of Alzheimer's disease (AD)-associated protein aggregates is described. Indane derivatives with a donor-π-acceptor (D-π-A) structure were designed and synthesized. The probes were evaluated for their ability to bind to β-amyloid (Aβ) protein aggregates, which are a key pathological hallmark of AD. The results showed that several probes exhibited significant changes in fluorescence intensity at wavelengths greater than 600 nm when they were bound to Aβ aggregates compared to the Aβ monomeric form. Among the tested probes, four D-π-A type indane derivatives showed promising binding selectivity to Aβ aggregates over non-specific proteins such as bovine serum albumin (BSA). The molecular docking study showed that our compounds were appropriately located along the Aβ fibril axis through the hydrophobic tunnel structure. Further analysis revealed that the most active compound having dimethylaminopyridyl group as an election donor and dicyano group as an electron acceptor could effectively stain Aβ plaques in brain tissue samples from AD transgenic mice. These findings suggest that our indane-based compounds have the potential to serve as fluorescent probes for the detection and monitoring of Aβ aggregation in AD.

Designing multi-target-directed flavonoids: a strategic approach to Alzheimer's disease

The underlying causes of Alzheimer's disease (AD) remain a mystery, with multiple pathological components, including oxidative stress, acetylcholinesterase, amyloid-β, and metal ions, all playing a role. Here we report a strategic approach to designing flavonoids that can effectively tackle multiple pathological elements involved in AD. Our systematic investigations revealed key structural features for flavonoids to simultaneously target and regulate pathogenic targets. Our findings led to the development of a highly promising flavonoid that exhibits a range of functions, based on a complete structure-activity relationship analysis. Furthermore, our mechanistic studies confirmed that this flavonoid's versatile reactivities are driven by its redox potential and direct interactions with pathogenic factors. This work highlights the potential of multi-target-directed flavonoids as a novel solution in the fight against AD.

Methods for analyzing the coordination and aggregation of metal-amyloid-β

The misfolding and aggregation of amyloid-β (Aβ) peptides are histopathological features found in the brains of Alzheimer's disease (AD). To discover effective therapeutics for AD, numerous efforts have been made to control the aggregation of Aβ species and their interactions with other pathological factors, including metal ions. Metal ions, such as Cu(II) and Zn(II), can bind to Aβ peptides forming metal-bound Aβ (metal-Aβ) complexes and, subsequently, alter their aggregation pathways. In particular, redox-active metal ions bound to Aβ species can produce reactive oxygen species leading to oxidative stress. In this review, we briefly illustrate some experimental approaches for characterizing the coordination and aggregation properties of metal-Aβ complexes.

Synthesis and neuroprotective effects of H2S-donor-peptide hybrids on hippocampal neuronal cells

Hydrogen sulfide (H₂S) has emerged as an endogenous signaling molecule that functions in many physiological and pathological processes of human cells in health and disease, including neuromodulation and neuroprotection, inflammation, angiogenesis, and vasorelaxation. The limited clinical applications of current H₂S donors have led to the development of H₂S donor hybrid compounds that combine current H₂S donors with bioactive molecules. Finely tuned multi-targeting hybrid molecules have been shown to have complementary neuroprotective effects against reactive oxygen species (ROS)-induced oxidative stress. In this study, we developed hybrid molecules combining a dithiolethione-based slow-releasing H₂S donor that exerts neuroprotective effects, with the tripeptides glycyl-L-histidyl-l-lysine (GHK) and L-alanyl-L-cystinyl-l-glutamine (ACQ), two natural products that exhibit powerful antioxidant effects. In particular, a hybrid combination of a dithiolethione-based slow-releasing H₂S donor and ACQ exhibited significant neuroprotective effects against glutamate-induced oxidative damage in HT22 hippocampal neuronal cells. This hybrid remarkably suppressed Ca²⁺ accumulation and ROS production. Furthermore, it efficiently inhibited apoptotic neuronal cell death by blocking apoptosis-inducing factor release and its translocation to the nucleus. These results indicate that the hybrid efficiently inhibited apoptotic neuronal cell damage by complementary neuroprotective actions.

Multipronged diagnostic and therapeutic strategies for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and a major contributor to dementia cases worldwide. AD is clinically characterized by learning, memory, and cognitive deficits. The accumulation of extracellular amyloid β (Aβ) plaques and neurofibrillary tangles (NFTs) of tau are the pathological hallmarks of AD and are explored as targets for clinical diagnosis and therapy. AD pathology is poorly understood and there are no fully approved diagnosis and treatments. Notwithstanding the gap, decades of research in understanding disease mechanisms have revealed the multifactorial nature of AD. As a result, multipronged and holistic approaches are pertinent to targeting multiple biomarkers and targets for developing effective diagnosis and therapeutics. In this perspective, recent developments in Aβ and tau targeted diagnostic and therapeutic tools are discussed. Novel indirect, combination, and circulating biomarkers as potential diagnostic targets are highlighted. We underline the importance of multiplexing and multimodal detection of multiple biomarkers to generate biomarker fingerprints as a reliable diagnostic strategy. The classical therapeutics targeting Aβ and tau aggregation pathways are described with bottlenecks in the strategy. Drug discovery efforts targeting multifaceted toxicity involving protein aggregation, metal toxicity, oxidative stress, mitochondrial damage, and neuroinflammation are highlighted. Recent efforts focused on multipronged strategies to rationally design multifunctional modulators targeting multiple pathological factors are presented as future drug development strategies to discover potential therapeutics for AD.

Advancements in the development of multi-target directed ligands for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial irreversible neurological disorder which results in cognitive impairment, loss of cholinergic neurons in synapses of the basal forebrain and neuronal death. Exact pathology of the disease is not yet known however, many hypotheses have been proposed for its treatment. The available treatments including monotherapies and combination therapies are not able to combat the disease effectively because of its complex pathological mechanism. A multipotent drug for AD has the potential to bind or inhibit multiple targets responsible for the progression of the disease like aggregated Aβ, hyperphosphorylated tau proteins, cholinergic and adrenergic receptors, MAO enzymes, overactivated N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor etc. The traditional approach of one disease-one target-one drug has been rationalized to one drug-multi targets for the chronic diseases like AD and cancer. Thus, over the last decade research focus has been shifted towards the development of multi target directed ligands (MTDLs) which can simultaneously inhibit multiple targets and stop or slow the progression of the disease. The MTDLs can be more effective against AD and eliminate any possibility of drug-drug interactions. Many important active pharmacophore units have been fused, merged or incorporated into different scaffolds to synthesize new potent drugs. In the current article, we have described various hypothesis for AD and effectiveness of the MTDLs treatment strategy is discussed in detail. Different chemical scaffolds and their synthetic strategies have been described and important functionalities are identified in the chemical scaffold that have the potential to bind to the multiple targets. The important leads identified in this study with MTDL characteristics have the potential to be developed as drug candidates for the effective treatment of AD.

Mechanistic Insight into the Design of Chemical Tools to Control Multiple Pathogenic Features in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia and is characterized by memory loss and cognitive decline. Approximately 50 million people worldwide are suffering from AD and related dementias. Very recently, the first new drug targeting amyloid-β (Aβ) aggregates has been approved by the United States Food and Drug Administration, but its efficacy against AD is still debatable. Other available treatments temporarily relieve the symptoms of AD. The difficulty in discovering effective therapeutics for AD originates from its complicated nature, which results from the interrelated pathogenic pathways led by multiple factors. Therefore, to develop potent disease-modifying drugs, multiple pathological features found in AD should be fully elucidated.Our laboratory has been designing small molecules as chemical tools to investigate the individual and interrelated pathologies triggered by four pathogenic elements found in the AD-affected brain: metal-free Aβ, metal-bound Aβ, reactive oxygen species (ROS), and acetylcholinesterase (AChE). Aβ peptides are partially folded and aggregate into oligomers, protofibrils, and fibrils. Aβ aggregates are considered to be neurotoxic, causing membrane disruption, aberrant cellular signaling, and organelle dysfunction. In addition, highly concentrated metal ions accumulate in senile plaques mainly composed of Aβ aggregates, which indicates that metal ions can directly interact with Aβ. Metal binding to Aβ affects the aggregation and conformation of the peptide. Moreover, the impaired homeostasis of redox-active Fe(II/III) and Cu(I/II) induces the overproduction of ROS through Fenton chemistry and Fenton-like reactions, respectively. Dysregulated ROS prompt oxidative-stress-damaging biological components such as lipids, proteins, and nucleic acids and, consequently, lead to neuronal death. Finally, the loss of cholinergic transmission mediated by the neurotransmitter acetylcholine (ACh) contributes to cognitive deficits observed in AD.In this Account, we illustrate the design principles for small-molecule-based chemical tools with reactivities against metal-free Aβ, metal-bound Aβ, ROS, and AChE. More importantly, mechanistic details at the molecular level are highlighted with some examples of chemical tools that were developed by our group. The aggregation of metal-free Aβ can be modulated by modifying amino acid residues responsible for self-assembling Aβ or disassembling preformed fibrils. To alter the aggregation and cytotoxicity profiles of metal-bound Aβ, ternary complexation, metal chelation, and modifications onto metal-binding residues can be effective tactics. The presence and production of ROS are able to be controlled by small molecules with antioxidant and metal-binding properties. Finally, inhibiting substrate access or substrate binding at the active site of AChE can diminish its activity, which restores the levels of ACh. Overall, our rational approaches demonstrate the feasibility of developing small molecules as chemical tools that can target and modulate multiple pathological factors associated with AD and can be useful for gaining a greater understanding of the multifaceted pathology of the disease.