Orientation-Inspired Perspective on Molecular Inhibitor of Tau Aggregation by Curcumin Conjugated with Ruthenium(II) Complex Scaffold

Transition Metal Complexes as Therapeutics: A New Frontier in Combatting Neurodegenerative Disorders through Protein Aggregation Modulation

Neurodegenerative disorders (NDDs) are a class of debilitating diseases that progressively impair the protein structure and result in neurological dysfunction in the nervous system. Among these disorders, Alzheimer's disease (AD), prion diseases such as Creutzfeldt-Jakob disease (CJD), and Parkinson's disease (PD) are caused by protein misfolding and aggregation at the cellular level. In recent years, transition metal complexes have gained significant attention for their potential applications in diagnosing, imaging, and curing these NDDs. These complexes have intriguing possibilities as therapeutics due to their diverse ligand systems and chemical properties and can interact with biological systems with minimal detrimental effects. This review focuses on the recent progress in transition metal therapeutics as a new era of hope in the battle against AD, CJD, and PD by modulating protein aggregation in vitro and in vivo. It may shed revolutionary insights into unlocking new opportunities for researchers to develop metal-based drugs to combat NDDs.

Advances in Cholinesterase Inhibitor Research-An Overview of Preclinical Studies of Selected Organoruthenium(II) Complexes

Cholinesterase (ChE) inhibitors are crucial therapeutic agents for the symptomatic treatment of certain chronic neurodegenerative diseases linked to functional disorders of the cholinergic system. Significant research efforts have been made to develop novel derivatives of classical ChE inhibitors and ChE inhibitors with novel scaffolds. Over the past decade, ruthenium complexes have emerged as promising novel therapeutic alternatives for the treatment of neurodegenerative diseases. Our research group has investigated a number of newly synthesized organoruthenium(II) complexes for their inhibitory activity against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Three complexes (C1a, C1-C, and C1) inhibit ChE in a pharmacologically relevant range. C1a reversibly inhibits AChE and BChE without undesirable peripheral effects, making it a promising candidate for the treatment of Alzheimer's disease. C1-Cl complex reversibly and competitively inhibits ChEs, particularly AChE. It inhibits nerve-evoked skeletal muscle twitch and tetanic contraction in a concentration-dependent manner with no effect on directly elicited twitch and tetanic contraction and is promising for further preclinical studies as a competitive neuromuscular blocking agent. C1 is a selective, competitive, and reversible inhibitor of BChE that inhibits horse serum BChE (hsBChE) without significant effect on the peripheral neuromuscular system and is a highly species-specific inhibitor of hsBChE that could serve as a species-specific drug target. This research contributes to the expanding knowledge of ChE inhibitors based on ruthenium complexes and highlights their potential as promising therapeutic candidates for chronic neurodegenerative diseases.

Nanomaterials that Aid in the Diagnosis and Treatment of Alzheimer's Disease, Resolving Blood-Brain Barrier Crossing Ability

As a form of dementia, Alzheimer's disease (AD) suffers from no efficacious cure, yet AD treatment is still imperative, as it ameliorates the symptoms or prevents it from deteriorating or maintains the current status to the longest extent. The human brain is the most sensitive and complex organ in the body, which is protected by the blood-brain barrier (BBB). This yet induces the difficulty in curing AD as the drugs or nanomaterials that are much inhibited from reaching the lesion site. Thus, BBB crossing capability of drug delivery system remains a significant challenge in the development of neurological therapeutics. Fortunately, nano-enabled delivery systems possess promising potential to achieve multifunctional diagnostics/therapeutics against various targets of AD owing to their intriguing advantages of nanocarriers, including easy multifunctionalization on surfaces, high surface-to-volume ratio with large payloads, and potential ability to cross the BBB, making them capable of conquering the limitations of conventional drug candidates. This review, which focuses on the BBB crossing ability of the multifunctional nanomaterials in AD diagnosis and treatment, will provide an insightful vision that is conducive to the development of AD-related nanomaterials.

Molecular Deformation Is a Key Factor in Screening Aggregation Inhibitor for Intrinsically Disordered Protein Tau

Direct inhibitor of tau aggregation has been extensively studied as potential therapeutic agents for Alzheimer's disease. However, the natively unfolded structure of tau complicates the structure-based ligand design, and the relatively large surface areas that mediate tau-tau interactions in aggregation limit the potential for identifying high-affinity ligand binding sites. Herein, a group of isatin-pyrrolidinylpyridine derivative isomers (IPP1-IPP4) were designed and synthesized. They are like different forms of molecular "transformers". These isatin isomers exhibit different inhibitory effects on tau self-aggregation or even possess a depolymerizing effect. Our results revealed for the first time that the direct inhibitor of tau protein aggregation is not only determined by the previously reported conjugated structure, substituent, hydrogen bond donor, etc. but also depends more importantly on the molecular shape. In combination with molecular docking and molecular dynamics simulations, a new inhibition mechanism was proposed: like a "molecular clip", IPP1 could noncovalently bind and fix a tau polypeptide chain at a multipoint to prevent the transition from the "natively unfolded conformation" to the "aggregation competent conformation" before nucleation. At the cellular and animal levels, the effectiveness of the inhibitor of the IPP1 has been confirmed, providing an innovative design strategy as well as a lead compound for Alzheimer's disease drug development.

Mechanistic insight into the disruption of Tau R3-R4 protofibrils by curcumin and epinephrine: an all-atom molecular dynamics study

The accumulation of Tau protein aggregates is a pathological hallmark of tauopathy, including chronic traumatic encephalopathy (CTE). Inhibiting Tau aggregation or disrupting preformed Tau fibrils is considered one of the rational therapeutic strategies to combat tauopathy. Previous studies reported that curcumin (Cur, a molecule of a labile natural product) and epinephrine (EP, an important neurotransmitter) could effectively inhibit the formation of Tau fibrillar aggregates and disassociate preformed fibrils. However, the underlying molecular mechanisms remain elusive. In this study, we performed multiple molecular dynamics simulations for 17.5 μs in total to investigate the influence of Cur and EP on the C-shaped Tau protofibril associated with CTE. Our simulations show that the protofibrillar pentamer is the smallest stable Tau R3-R4 protofibril. Taking the pentamer as a protofibril model, we found that both Cur and EP molecules could affect the shape of the Tau pentamer by changing the β2-β3 and β7-β8 angles, leading to a more extended structure. Cur and EP display a disruptive effect on the local β-sheets and the formation of hydrogen bonds, and thus destabilize the global protofibril structure. The contact number analysis shows that Cur has a higher binding affinity with the Tau pentamer than EP, especially in the nucleating segment PHF6. Hydrophobic, π-π and cation-π interactions together facilitate the binding of Cur and EP with the Tau pentamer. Cur exhibits stronger hydrophobic and π-π interactions with Tau than EP, and EP displays a stronger cation-π interaction. Our findings provide molecular insights into the disruptive mechanisms of the Tau R3-R4 protofibrils by curcumin and epinephrine, which may be useful for the design of effective drug candidates for the treatment of CTE.

Homocysteine-targeting compounds as a new treatment strategy for diabetic wounds via inhibition of the histone methyltransferase SET7/9

In hypoxia and hyperglycemia, SET7/9 plays an important role in controlling HIF-1α methylation and regulating the transcription of HIF-1α target genes, which are responsible for angiogenesis and wound healing. Here, we report the Ir(III) complex Set7_1a bearing acetonitrile (ACN) ligands as a SET7/9 methyltransferase inhibitor and HIF-1α stabilizer. Interestingly, Set7_1a could engage SET7/9 and strongly inhibit SET7/9 activity, especially after preincubation with homocysteine (Hcy), which is elevated in diabetes. We hypothesize that Set7_1a exchanges ACN subunits for Hcy to disrupt the interaction between SET7/9 and SAM/SAH, which are structurally related to Hcy. Inhibition of SET7/9 methyltransferase activity by Set7_1a led to reduced HIF-1α methylation at the lysine 32 residue, causing increased HIF-1α level and recruitment of HIF-1α target genes that promote angiogenesis, such as VEGF, GLUT1, and EPO, in hypoxia and hyperglycemia. Significantly, Set7_1a improved wound healing in a type 2 diabetic mouse model by activating HIF-1α signaling and downstream proangiogenic factors. To our knowledge, this is the first Hcy-targeting iridium compound shown to be a SET7/9 antagonist that can accelerate diabetic wound healing. More importantly, this study opens a therapeutic avenue for the treatment of diabetic wounds by the inhibition of SET7/9 lysine methyltransferase activity.

Animal trials have demonstrated the potential of a new drug strategy to heal the wounds associated with diabetes, especially in the feet,which often lead to chronic damage, sometimes treatable only by amputation. Leung CH and Lin L at the University of Macau, China, and Ma DL at the Hong Kong Baptist University tested the new therapy on a mouse model of type 2 diabetes. The treatment uses a homocysteine-targeting metal complex that inhibits a key enzyme SET7/9 involved in the processes that cause diabetic wounds. The treatment activated a molecular signalling cascade involved in generating the new blood vessels needed for wounds to heal. It could help address the urgent need for better treatments for this serious problem.

Structurally Distinct Polymorphs of Tau Aggregates Revealed by Nanoscale Infrared Spectroscopy

Aggregation of the tau protein plays a central role in several neurodegenerative diseases collectively known as tauopathies, including Alzheimer's and Parkinson's disease. Tau misfolds into fibrillar β sheet structures that constitute the paired helical filaments found in neurofibrillary tangles. It is known that there can be significant structural heterogeneities in tau aggregates associated with different diseases. However, while structures of mature fibrils have been studied, the structural distributions in early-stage tau aggregates is not well-understood. In the present study, we use atomic force microscopy-IR to investigate nanoscale spectra of individual tau fibrils at different stages of aggregation and demonstrate the presence of multiple fibrillar polymorphs that exhibit different secondary structures. We further show that mature fibrils contain significant amounts of antiparallel β sheets. Our results are the very first application of nanoscale infrared spectroscopy to tau aggregates and underscore the promise of spatially resolved infrared spectroscopy for investigating protein aggregation.

Dynamic-Inspired Perspective on the Molecular Inhibitor of Tau Aggregation by Glucose Gallates Based on Human Neurons

A molecular inhibitor of tau protein aggregation offers an attractive therapeutic possibility as disease-modifying treatment of Alzheimer's disease. However, the ineffectiveness as well as adjoint toxicity due to superficial understanding of the inhibition mechanism has hindered drug development. Conventional approaches for screening drug ligands rely on compatible docking with the well-defined structure of a protein receptor. Therefore, the design of tau aggregation inhibitors has been inevitably hindered by the unstructured, highly dynamic nature of the tau protein. This paper suggested a new strategy for reducing tau aggregation through a dynamic process of conformational isomerization. A group of glucose gallate derivatives were selected as tau aggregation inhibitors. These star-shaped molecules have a biocompatible glucose core surrounded by several gallic acid polyphenol arms, which can bind to peptide chains at different sites, probably through hydrogen bonds and π-π stacking. Theoretically, by elevating the saddle point on the potential energy surfaces (PES) of proteins, the barrier in the dynamic pathway of peptide isomerization, glucose gallates effectively inhibit tau aggregation through a dynamic mechanism. A tau cell model based on human neurons was constructed. For the first time, we confirmed that the moderate thermodynamic binding of the molecular ligand to the tau peptide chain can not only prevent the isomerization of the peptide chain leading to aggregation but also avoid toxicity resulting from the dissociation of tau from microtubules.

Mechanisms of melatonin binding and destabilizing the protofilament and filament of tau R3-R4 domains revealed by molecular dynamics simulation

The accumulation of β-amyloid (Aβ) and tau protein is considered to be an important pathological characteristic of Alzheimer's disease (AD). Failure of medicine targeting Aβ has drawn more attention to the influence of tau protein and its fibrillization on neurodegeneration. Increasing evidence shows that melatonin (Mel) can effectively inhibit the formation of tau fibrils and disassemble preformed tau fibrils. However, the underlying mechanism is poorly understood. In this work, we investigated the kinetics of melatonin binding and destabilizing the tetrameric protofilament and octameric filament of tau R3-R4 domains by performing microsecond all-atom molecular dynamics simulations. Our results show that Mel is able to disrupt the C-shaped structure of the tau protofilament and filament, and destabilizes the association between N- and C-termini. Mel predominantly binds to β1 and β6-β8 regions and favors contact with the elongation surface, which is dominantly driven by hydrogen bonding interactions and facilitated by other interactions. The strong π-π stacking interaction of Mel with Y310 impedes the intramolecular CH-π interaction between I308 and Y310, and the cation-π interaction of Mel with R379 interferes with the formation of the D348-R379 salt bridge. Moreover, Mel occupies the protofilament surface in the tetrameric protofilament and prevents the formation of intermolecular hydrogen bonds between residues K331 and Q336 in the octameric filament. Our work provides molecular insights into Mel hindering tau fibrillization or destabilizing the protofilament and filament, and the revealed inhibitory mechanisms provide useful clues for the design of efficient anti-amyloid agents.

Metal-Curcumin Complexes in Therapeutics: An Approach to Enhance Pharmacological Effects of Curcumin

Curcumin, an active component of the rhizome turmeric, has gained much attention as a plant-based compound with pleiotropic pharmacological properties. It possesses anti-inflammatory, antioxidant, hypoglycemic, antimicrobial, neuroprotective, and immunomodulatory activities. However, the health-promoting utility of curcumin is constrained due to its hydrophobic nature, water insolubility, poor bioavailability, rapid metabolism, and systemic elimination. Therefore, an innovative stride was taken, and complexes of metals with curcumin have been synthesized. Curcumin usually reacts with metals through the β-diketone moiety to generate metal–curcumin complexes. It is well established that curcumin strongly chelates several metal ions, including boron, cobalt, copper, gallium, gadolinium, gold, lanthanum, manganese, nickel, iron, palladium, platinum, ruthenium, silver, vanadium, and zinc. In this review, the pharmacological, chemopreventive, and therapeutic activities of metal–curcumin complexes are discussed. Metal–curcumin complexes increase the solubility, cellular uptake, and bioavailability and improve the antioxidant, anti-inflammatory, antimicrobial, and antiviral effects of curcumin. Metal–curcumin complexes have also demonstrated efficacy against various chronic diseases, including cancer, arthritis, osteoporosis, and neurological disorders such as Alzheimer’s disease. These biological activities of metal–curcumin complexes were associated with the modulation of inflammatory mediators, transcription factors, protein kinases, antiapoptotic proteins, lipid peroxidation, and antioxidant enzymes. In addition, metal–curcumin complexes have shown usefulness in biological imaging and radioimaging. The future use of metal–curcumin complexes may represent a new approach in the prevention and treatment of chronic diseases.

Curcumin as Scaffold for Drug Discovery against Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are one of major public health problems and their impact is continuously growing. Curcumin has been proposed for the treatment of several of these pathologies, such as Alzheimer's disease (AD) and Parkinson's disease (PD) due to the ability of this molecule to reduce inflammation and aggregation of involved proteins. Nevertheless, the poor metabolic stability and bioavailability of curcumin reduce the possibilities of its practical use. For these reasons, many curcumin derivatives were synthetized in order to overcome some limitations. In this review will be highlighted recent results on modification of curcumin scaffold in the search of new effective therapeutic agents against NDs, with particular emphasis on AD.

The cocrystal of 3-((4-(3-isocyanobenzyl) piperazine-1-yl) methyl) benzonitrile with 5-hydroxy isophthalic acid prevents protofibril formation of serum albumin

The formation of amyloid-like fibrils is a central problem in biophysical chemistry and medicine. Fibril formation and their deposition in various tissues and organs are associated with many human diseases. Searching for molecules able to prevent the formation of fibrils is, therefore, necessary. In this work, we examined the potential of a cocrystal (SS3) of 3-((4-(3-isocyanobenzyl) piperazine-1-yl) methy) benzonitrile with 5-hydroxy isophthalic acid, to prevent fibrillation of human serum albumin. We found that the cocrystal strongly bound to human serum albumin (HSA) with association constant (K_a) of 5.8 ± 0.7 × 10⁵ M^-1. The SS3 binding was found to cause small alterations in both secondary and tertiary structure of the protein. Transmission electron microscopy showed that the cocrystal completely prevented the formation of worm-like protofibrils by HSA at SS3/HSA molar ratio of 1:1. The molecule was found to prevent the aggregation in a concentration dependent manner. It was also observed that most of protein in the presence of SS3 remained in soluble state and the secondary structure contained native-like α-helical structure. Therefore, we conclude that the cocrystal effectively prevented conversion of HSA into worm-like protofibril. These finding suggest that combination of molecules in the form of cocrystal or other stable combination could pave a way for the development of drugs against amyloidosis.Communicated by Ramaswamy H. Sarma.