A multi enzyme study reviewing the role of target enzymes in Alzheimer’s disease and unveiling potential inhibitors with insights on their present and future assessment

Jitendra Joshi N., Raja Sekhar Reddy A.

Alzheimer's disease (AD) remains a significant global health challenge, necessitating the exploration of novel therapeutic strategies. AD is a neurodegenerative disease characterized by the formation of amyloid-β (Aβ) plaques and neurofibrillary tangles composed of tau protein. It is stated to be a cause of multiple mechanistic pathways and biochemical events, and hence, targeting only one mechanism at a time cannot suffice for all-round therapy. A multi-target drug ligand strategy can thus be employed in such a setting to modulate multiple drug targets simultaneously. Glycogen synthase kinase-3β (GSK-3β) is an enzyme involved in tau hyperphosphorylation, neuroinflammation, maintaining neuronal plasticity, and various cell processes. Thus, inhibiting GSK-3β itself may help in the control of some of the key factors stated to be responsible for Alzheimer's disease pathophysiology. This review comprehensively evaluates the potential of GSK-3β inhibitors as multi-target drug ligands in AD therapeutics. We discuss the molecular mechanisms of GSK-3β in AD pathology and review the preclinical and clinical evidence supporting the efficacy of GSK-3β inhibitors in ameliorating cognitive decline and pathological hallmarks of AD. Furthermore, we explore the challenges associated with targeting GSK-3β, such as selectivity, blood–brain barrier penetration, and adverse effects. Additionally, we highlight recent advances in the development of novel GSK-3β inhibitors with improved pharmacokinetic properties and multitargeting capabilities. Finally, we discuss the future directions and potential clinical implications of GSK-3β inhibitors as multi-target drug ligands in the complex landscape of AD therapeutics.

Starr L.A., McKay L.E., Peter K.N., Seyfarth L.M., Berkowitz L.A., Caldwell K.A., Caldwell G.A.

Differential RNA editing by adenosine deaminases that act on RNA (ADARs) has been implicated in several neurological disorders, including Parkinson’s disease (PD). Here, we report results of a RNAi screen of genes differentially regulated in adr-2 mutants, normally encoding the only catalytically active ADAR in Caenorhabditis elegans, ADR-2. Subsequent analysis of candidate genes that alter the misfolding of human α-synuclein (α-syn) and dopaminergic neurodegeneration, two PD pathologies, reveal that reduced expression of xdh-1, the ortholog of human xanthine dehydrogenase (XDH), is protective against α-synuclein-induced dopaminergic neurodegeneration. Further, RNAi experiments show that WHT-2, the worm ortholog of the human ABCG2 transporter and a predicted interactor of XDH-1, is the rate-limiting factor in the ADR-2, XDH-1, WHT-2 system for dopaminergic neuroprotection. In silico structural modeling of WHT-2 indicates that the editing of one nucleotide in the wht-2 mRNA leads to the substitution of threonine with alanine at residue 124 in the WHT-2 protein, changing hydrogen bonds in this region. Thus, we propose a model where wht-2 is edited by ADR-2, which promotes optimal export of uric acid, a known substrate of WHT-2 and a product of XDH-1 activity. In the absence of editing, uric acid export is limited, provoking a reduction in xdh-1 transcription to limit uric acid production and maintain cellular homeostasis. As a result, elevation of uric acid is protective against dopaminergic neuronal cell death. In turn, increased levels of uric acid are associated with a decrease in ROS production. Further, downregulation of xdh-1 is protective against PD pathologies because decreased levels of XDH-1 correlate to a concomitant reduction in xanthine oxidase (XO), the form of the protein whose by-product is superoxide anion. These data indicate that modifying specific targets of RNA editing may represent a promising therapeutic strategy for PD.

Thies J.L., Willicott K., Craig M.L., Greene M.R., DuGay C.N., Caldwell G.A., Caldwell K.A.

Oxidative stress is a contributing factor to Parkinson’s disease (PD). Considering the prevalence of sporadic PD, environmental exposures are postulated to increase reactive oxygen species and either incite or exacerbate neurodegeneration. We previously determined that exposure to the common soil bacterium, Streptomyces venezuelae (S. ven), enhanced oxidative stress and mitochondrial dysfunction in Caenorhabditis elegans, leading to dopaminergic (DA) neurodegeneration. Here, S. ven metabolite exposure in C. elegans was followed by RNA-Seq analysis. Half of the differentially identified genes (DEGs) were associated with the transcription factor DAF-16 (FOXO), which is a key node in regulating stress response. Our DEGs were enriched for Phase I (CYP) and Phase II (UGT) detoxification genes and non-CYP Phase I enzymes associated with oxidative metabolism, including the downregulated xanthine dehydrogenase gene, xdh-1. The XDH-1 enzyme exhibits reversible interconversion to xanthine oxidase (XO) in response to calcium. S. ven metabolite exposure enhanced XO activity in C. elegans. The chelation of calcium diminishes the conversion of XDH-1 to XO and results in neuroprotection from S. ven exposure, whereas CaCl2 supplementation enhanced neurodegeneration. These results suggest a defense mechanism that delimits the pool of XDH-1 available for interconversion to XO, and associated ROS production, in response to metabolite exposure.

Wang S., Sudan R., Peng V., Zhou Y., Du S., Yuede C.M., Lei T., Hou J., Cai Z., Cella M., Nguyen K., Poliani P.L., Beatty W.L., Chen Y., Cao S., et. al.

Genetic studies have highlighted microglia as pivotal in orchestrating Alzheimer's disease (AD). Microglia that adhere to Aβ plaques acquire a transcriptional signature, "disease-associated microglia" (DAM), which largely emanates from the TREM2-DAP12 receptor complex that transmits intracellular signals through the protein tyrosine kinase SYK. The human TREM2R47H variant associated with high AD risk fails to activate microglia via SYK. We found that SYK-deficient microglia cannot encase Aβ plaques, accelerating brain pathology and behavioral deficits. SYK deficiency impaired the PI3K-AKT-GSK-3β-mTOR pathway, incapacitating anabolic support required for attaining the DAM profile. However, SYK-deficient microglia proliferated and advanced to an Apoe-expressing prodromal stage of DAM; this pathway relied on the adapter DAP10, which also binds TREM2. Thus, microglial responses to Aβ involve non-redundant SYK- and DAP10-pathways. Systemic administration of an antibody against CLEC7A, a receptor that directly activates SYK, rescued microglia activation in mice expressing the TREM2R47H allele, unveiling new options for AD immunotherapy.

Pourtaher H., Hasaninejad A., Iraji A.

The objective of this study was to design new polysubstituted pyrrole derivatives as selective acetylcholinesterase (AChE) inhibitors to target Alzheimer's disease. In this context, a highly efficient, one-pot, sequential, multi-component synthesis of a diverse range of polysubstituted pyrroles was developed through a sequential domino strategy by the condensation of amines with 1,1-bis(methylthio)-2-nitroethene (BMTNE), Knovenagle reaction of arylglyoxals with malono derivatives and subsequent Michael addition and intramolecular cyclization reaction in EtOH at reflux. Thirty-nine synthesized compounds were evaluated as AChE and butyrylcholinesterase (BChE) inhibitors. Among the synthesized compounds, compound 4ad (IC50 = 2.95 ± 1.31 µM) was the most potent and selective AChE inhibitor with no significant inhibition against butyrylcholinesterase BChE. A kinetic study of 4ad revealed that this compound inhibited AChE in an uncompetitive mode. Based on a molecular modeling study, compound 4ad due to its small size properly fitted into the active site of AChE compared to BChE and stabilized by H-bond and hydrophobic interactions with the critical residues of the AChE binding pocket. Consequently, it was proposed that the 4ad derivative can be an ideal lead candidate against AD with a simple and practical operation of synthetic procedures.

Karaca Ş., Osmaniye D., Sağlık B.N., Levent S., Ilgın S., Özkay Y., Karaburun A.Ç., Kaplancıklı Z.A., Gundogdu-Karaburun N.

The use of dual acetylcholinesterase (AChE)–monoamine oxidase B (MAO-B) inhibitors is a new approach in the treatment of Alzheimer disease (AD).

Yan N., Shi X., Tang L., Wang D., Li X., Liu C., Liu Z.

Glycogen synthase kinase 3β (GSK-3β) catalyses the hyperphosphorylation of tau protein in the Alzheimer's disease (AD) pathology. A series of novel thieno[3,2-c]pyrazol-3-amine derivatives were designed and synthesised and evaluated as potential GSK-3β inhibitors by structure-guided drug rational design approach. The thieno[3,2-c]pyrazol-3-amine derivative 16b was identified as a potent GSK-3β inhibitor with an IC50 of 3.1 nM in vitro and showed accepted kinase selectivity. In cell levels, 16b showed no toxicity on the viability of SH-SY5Y cells at the concentration up to 50 μM and targeted GSK-3β with the increased phosphorylated GSK-3β at Ser9. Western blot analysis indicated that 16b decreased the phosphorylated tau at Ser396 in a dose-dependent way. Moreover, 16b effectively increased expressions of β-catenin as well as the GAP43, N-myc, and MAP-2, and promoted the differentiated neuronal neurite outgrowth. Therefore, the thieno[3,2-c]pyrazol-3-amine derivative 16b could serve as a promising GSK-3β inhibitor for the treatment of AD.

Sharma S., Chauhan N., Paliwal S., Jain S., Verma K., Paliwal S.

Abstract: Alzheimer’s disease (AD) is an emerging major health and socioeconomic burden worldwide. It is characterized by neuronal loss, memory loss and cognitive impairment in the aging population. Despite several scientific advancements over the past five decades, the underlying molecular mechanism of the disease progression is yet unknown. Glycogen synthase kinase-3β (GSK-3β) has huge implications on the brain function, causing molecular pathologies, neuronal damage and impairment of brain performance in AD. It is one of the key players in signaling pathways for normal brain functioning and a critical molecular link between amyloid-beta (Aβ) and tau neurofibrillary tangles (NFTs). GSK-3β activation is driven by phosphorylation of tau(τ) protein which results in disruption of neuronal synaptic activities and the formation of neuronal plaques. Although the accumulation of Aβ plaques and intracellular tangles of hyperphosphorylated tau protein has been well established as neuropathological hallmarks of the disease, the molecular mechanism has not been unraveled. This review focuses on the role of GSK-3β in the molecular mechanisms participating in the manifestation and progression of AD. The review also suggests that GSK-3β inhibitors can be used as potential therapeutic targets for amelioration of AD.

Haji Ali S., Osmaniye D., Sağlık B.N., Levent S., Özkay Y., Kaplancıklı Z.A.

Alzheimer’s disease (AD) is a slowly progressive neurodegenerative disease that causes dementia in people aged 65 and over. In the present study, a series of thiadiazole hybrid compounds with benzothiazine derivatives as acetylcholinesterase inhibitors were developed and evaluated for their biological activity. The AChE and BChE inhibition potentials of all compounds were evaluated by using the in vitro Ellman method. The biological evaluation showed that compounds 3i and 3j displayed significant inhibitory activity against AChE. Compounds 3i and 3j showed IC50 values of 0.027 µM and 0.025 µM against AChE, respectively. The reference drug donepezil (IC50 = 0.021 µM) also showed significant inhibition against AChE. Further docking simulation also revealed that these compounds (3i and 3j) interacted with the active site of the enzyme similarly to donepezil. The antioxidant study revealed that compounds 3i and 3j exhibited greater antioxidant effects. An in vitro blood–brain barrier permeability study showed that compounds 3i and 3j are promising compounds against AD. The cytotoxicity study of compounds 3i and 3j showed non-cytotoxic with an IC50 value of 98.29 ± 3.98 µM and 159.68 ± 5.53 µM against NIH/3T3 cells, respectively.

Jiang X., Liu C., Zou M., Xie H., Lin T., Lyu W., Xu J., Li Y., Feng F., Sun H., Liu W.

Acetylcholinesterase (AChE) inhibitors are currently the first-line drugs approved by the FDA for the treatment of Alzheimer's disease (AD). However, a short effective-window limits their therapeutic benefits. Clinical studies have confirmed that the combination of AChE inhibitors and neuroprotective agents exhibits better anti-AD effects. We have previously reported that the dual AChE/GSK3β (Glycogen synthase kinase 3β) modulators have both neuroprotective effects and cognitive impairment-improvement effects. In this study, we characterized a new backbone of the AChE/GSK3β inhibitor 11c . It was identified as a highly potent AChE inhibitor and was found superior to donepezil, the first-line drug for the treatment of AD. In vivo studies confirmed that 11c significantly inhibited the activity of AChE in the brain but had little effect on the activity of AChE in the intestine. This advantage of 11c was expected to reduce the peripheral side effects caused by donepezil. Furthermore, biomarker studies have shown that 11c also improved the levels of acetylcholine and synaptophysin in the brain and exhibited neuroprotective effects. Preliminary in vivo and in vitro research results underline the exciting potential of compound 11c in the treatment of AD. A series of isonicotinamides have been synthesized as AChE and GSK3 dual inhibitors. Compound with better AChE inhibitory activity than donepezil was identified in vitro and in vivo. Compound 11c significantly inhibited the activity of AChE in the brain but had little effect on the activity of AChE in the intestine. 11c improved the levels of acetylcholine and synaptophysin in the brain and exhibited neuroprotective effects. Demonstrated reversal of scopolamine- or Aβ42-induced learning and memory impairment in ICR mice.

Liu W., Liu X., Tian L., Gao Y., Liu W., Chen H., Jiang X., Xu Z., Ding H., Zhao Q.

Alzheimer's disease (AD) is a chronic and progressive neurodegenerative disease, characterized by irreversible cognitive impairment, memory loss and behavioral disturbances, ultimately leading to death. Glycogen synthase kinase 3β (GSK-3β) and dual-specificity tyrosine phosphorylation regulated kinase1A (DYRK1A) have gained a lot of attention for its role in tau pathology. To search for potential dual GSK-3β/DYRK1A inhibitors, we focused on harmine, a natural β -carboline alkaloid, which has been extensively studied for its various biological effects on the prevention of AD. In this study, a new series of harmine derivatives were designed, synthesized and evaluated as dual GSK-3β/DYRK1A inhibitors for their multiple biological activities. The in vitro results indicated that most of them displayed promising activity against GSK-3β and DYRK1A. Among them, compound ZDWX-25 showed potent inhibitory effects on GSK-3β and DYRK1A with IC 50 values of 71 and 103 nM, respectively. Molecular modelling and kinetic studies verified that ZDWX-25 could interact with the ATP binding pocket of GSK-3β and DYRK1A. Western blot analysis revealed that ZDWX-25 inhibited hyperphosphorylation of tau protein in okadaic acid (OKA)-induced SH-SY5Y cells. In addition, ZDWX-25 showed good blood-brain barrier penetrability in vitro. More importantly, ZDWX-25 could ameliorate the impaired learning and memory in APP/PS1/Tau transgenic mice. These results indicated that the harmine-based compounds could be served as promising dual-targeted candidates for AD. • Synthesis of novel harmine derivatives as kinase inhibitors for the treatment of AD. • ZDWX-25 inhibits GSK-3β (IC 50 = 71 nM) and DYRK1A (IC 50 = 103 nM). • Effects of ZDWX-25 on the protein expression levels in okadaic acid-induced SH-SY5Y cells. • ZDWX-25 ameliorates the impaired learning and memory in APP/PS1/Tau transgenic mice.

Bowroju S.K., Penthala N.R., Lakkaniga N.R., Balasubramaniam M., Ayyadevara S., Shmookler Reis R.J., Crooks P.A.

• Synthesis of a novel series of 2-hydroxybenzylamine-deoxyvasicinone hybrid analogs. • Analogs evaluated as inhibitors of AChE, BuChE, as inhibitors of Aβ 1-42 aggregation. • Lead analogs were 40-fold more selective for inhibition of AChE over BuChE. • Active compounds bind to a new allosteric pocket (site B) of AChE. • Dual-acting lead compounds for the development of highly effective anti-AD drugs. A series of novel 2 - hydroxybenzylamine-deoxyvasicinone hybrid analogs ( 8a-8n ) have been synthesized and evaluated as inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), and as inhibitors of amyloid peptide (Aβ 1-42 ) aggregation, for treatment of Alzheimer’s disease (AD). These dual acting compounds exhibited good AChE inhibitory activities ranging from 0.34 to 6.35 µM. Analogs 8g and 8n were found to be the most potent AChE inhibitors in the series with IC 50 values of 0.38 µM and 0.34 µM, respectively. All the analogs ( 8a-8n ) exhibited weak BuChE inhibitory activities ranging from 14.60 to 21.65 µM. Analogs 8g and 8n exhibited BuChE with IC 50 values of 15.38 µM and 14.60 µM, respectively, demonstrating that these analogs were greater than 40-fold more selective for inhibition of AChE over BuChE. Additionally, compounds 8g and 8n were also found to be the best inhibitors of self-induced Aβ 1-42 peptide aggregation with IC 50 values of 3.91 µM and 3.22 µM, respectively; 8g and 8n also inhibited AChE-induced Aβ 1-42 peptide aggregation by 68.7% and 72.6%, respectively. Kinetic analysis and molecular docking studies indicate that analogs 8g and 8n bind to a new allosteric pocket (site B) on AChE. In addition, the observed inhibition of AChE-induced Aβ 1-42 peptide aggregation by 8n is likely due to allosteric inhibition of the binding of this peptide at the CAS site on AChE. Overall, these results indicate that 8g and 8n are examples of dual-acting lead compounds for the development of highly effective anti-AD drugs.

Demuro S., Di Martino R.M., Ortega J.A., Cavalli A.

Protein kinases (PKs) have been recognized as central nervous system (CNS)-disease-relevant targets due to their master regulatory role in different signal transduction cascades in the neuroscience space. Among them, GSK-3β, FYN, and DYRK1A play a crucial role in the neurodegeneration context, and the deregulation of all three PKs has been linked to different CNS disorders with unmet medical needs, including Alzheimer’s disease (AD), Parkinson’s disease (PD), frontotemporal lobar degeneration (FTLD), and several neuromuscular disorders. The multifactorial nature of these diseases, along with the failure of many advanced CNS clinical trials, and the lengthy approval process of a novel CNS drug have strongly limited the CNS drug discovery. However, in the near-decade from 2010 to 2020, several computer-assisted drug design strategies have been combined with synthetic efforts to develop potent and selective GSK-3β, FYN, and DYRK1A inhibitors as disease-modifying agents. In this review, we described both structural and functional aspects of GSK-3β, FYN, and DYRK1A and their involvement and crosstalk in different CNS pathological signaling pathways. Moreover, we outlined attractive medicinal chemistry approaches including multi-target drug design strategies applied to overcome some limitations of known PKs inhibitors and discover improved modulators with suitable blood–brain barrier (BBB) permeability and drug-like properties.

McDade E., Llibre-Guerra J.J., Holtzman D.M., Morris J.C., Bateman R.J.

Alzheimer disease (AD) prevention trials hold the promise to delay or prevent cognitive decline and dementia onset by intervening before significant neuronal damage occurs. In recent years, the first AD prevention trials have launched and are yielding important findings on the biology of targeting asymptomatic AD pathology. However, there are limitations that impact the design of these prevention trials, including the translation of animal models that recapitulate key stages and multiple pathological aspects of the human disease, missing target validation in asymptomatic disease, uncertain causality of the association of pathophysiologic changes with cognitive and clinical symptoms, and limited biomarker validation for novel targets. The field is accelerating advancements in key areas including the development of highly specific and quantitative biomarker measures for AD pathology, increasing our understanding of the course and relationship of amyloid and tau pathology in asymptomatic through symptomatic stages, and the development of powerful interventions that can slow or reverse AD amyloid pathology. We review the current status of prevention trials and propose key areas of needed research as a call to basic and translational scientists to accelerate AD prevention. Specifically, we review (1) sporadic and dominantly inherited primary and secondary AD prevention trials, (2) proposed targets, mechanisms, and drugs including the amyloid, tau, and inflammatory pathways and combination treatments, (3) the need for more appropriate prevention animal models and experiments, and (4) biomarkers and outcome measures needed to design human asymptomatic prevention trials. We conclude with actions needed to effectively move prevention targets and trials forward.

Cho H., Kim S., Sim J., Yang S., An H., Nam M., Jang D., Lee C.J.

Monoamine oxidase (MAO) is believed to mediate the degradation of monoamine neurotransmitters, including dopamine, in the brain. Between the two types of MAO, MAO-B has been believed to be involved in dopamine degradation, which supports the idea that the therapeutic efficacy of MAO-B inhibitors in Parkinson’s disease can be attributed to an increase in extracellular dopamine concentration. However, this belief has been controversial. Here, by utilizing in vivo phasic and basal electrochemical monitoring of extracellular dopamine with fast-scan cyclic voltammetry and multiple-cyclic square wave voltammetry and ex vivo fluorescence imaging of dopamine with GRABDA2m, we demonstrate that MAO-A, but not MAO-B, mainly contributes to striatal dopamine degradation. In contrast, our whole-cell patch-clamp results demonstrated that MAO-B, but not MAO-A, was responsible for astrocytic GABA-mediated tonic inhibitory currents in the rat striatum. We conclude that, in contrast to the traditional belief, MAO-A and MAO-B have profoundly different roles: MAO-A regulates dopamine levels, whereas MAO-B controls tonic GABA levels. The inhibition of two forms of an enzyme that modulate key processes in the brain has different benefits for patients with Parkinson’s disease than previously thought. Monoamine oxidase (MAO) is present in the brain as MAO-A and MAO-B, both of which were thought to be involved in dopamine degradation. MAO inhibitors are used to limit dopamine degradation in Parkinson’s disease and depression, improving symptoms by increasing levels of usable dopamine. In experiments on rats, Hyun-U Cho at Hanyang University, Seoul, South Korea, and coworkers have shown that MAO-A, but not MAO-B, affects dopamine degradation. The team found that MAO-B instead mediates the synthesis of a key neurotransmitter, GABA, the upregulation of which is linked to Parkinson’s motor symptoms. Taking MAO-B inhibitors may be addressing these symptoms, explaining why patients show improvement.