Structural basis of acetylcholinesterase inhibition by triterpenoidal alkaloids

An aqueous macerate of Ziziphus jujuba reduces long-term spatial memory impairment in D-galactose treated rats: role of anti-inflammatory pathways

Pharmacological treatments against Alzheimer disease provide only symptomatic relief and are associated with numerous side effects. Previous studies showed that a concoction of Ziziphus jujuba leaves possesses anti-amnesic effects in scopolamine-treated rats. More recently, an aqueous macerate of Z. jujuba leaves has been shown to reduce short-term memory impairment in D-galactose-treated rats. However, no study on the effect of an aqueous macerate of Z. jujuba on long-term memory impairment was performed. Therefore, this study evaluates the effect of an aqueous macerate of Z. jujuba on long-term spatial memory impairment in D-galactose-treated rats. Long-term spatial memory impairment was induced in rats by administering D-galactose (350 mg/kg/day, s.c.), once dailyfor 21 days. On the 22nd day, the integrity of this memory was assessed using the Morris water maze task. Rats that developed memory impairment were treated with tacrine (10 mg/kg, p.o.), or aspirin (20 mg/kg, p.o.), or extract (41.5, 83, and 166 mg/kg, p.o.), once daily, for 14 days. At the end of the treatment, memory impairment was once more assessed using the same paradigm. Animals were then euthanized, and some pro-inflammatory cytokine markers were analyzed in the hippocampus or blood. The extract at all doses significantly reduced the latency to attain the platforming of the water maze test. The extract (83 mg/kg) also increased the time spent in the target quadrant during the retention phase. The extract markedly reduced the concentration of pro-inflammatory cytokine markers in the hippocampus and blood. Together, these results suggest that this aqueous extract Z. jujuba reduces long-term spatial memory impairment. This effect may be mediated in part by its anti-inflammatory activity.

Sarcorucinine-D Inhibits Cholinesterases and Calcium Channels: Molecular Dynamics Simulation and In Vitro Mechanistic Investigations

Acetylcholinesterase (AChE) inhibitors and calcium channel blockers are considered effective therapies for Alzheimer’s disease. AChE plays an essential role in the nervous system by catalyzing the hydrolysis of the neurotransmitter acetylcholine. In this study, the inhibition of the enzyme AChE by Sarcorucinine-D, a pregnane type steroidal alkaloid, was investigated with experimental enzyme kinetics and molecular dynamics (MD) simulation techniques. Kinetics studies showed that Sarcorucinine-D inhibits two cholinesterases—AChE and butyrylcholinesterase (BChE)—noncompetitively, with K_i values of 103.3 and 4.66 µM, respectively. In silico ligand-protein docking and MD simulation studies conducted on AChE predicted that Sarcorucinine-D interacted via hydrophobic interactions and hydrogen bonds with the residues of the active-site gorge of AChE. Sarcorucinine-D was able to relax contractility concentration-dependently in the intestinal smooth muscles of jejunum obtained from rabbits. Not only was the spontaneous spasmogenicity inhibited, but it also suppressed K⁺-mediated spasmogenicity, indicating an effect via the inhibition of voltage-dependent Ca²⁺ channels. Sarcorucinine-D could be considered a potential lead molecule based on its properties as a noncompetitive AChE inhibitor and a Ca²⁺ channel blocker.

Chalcone Scaffolds Exhibiting Acetylcholinesterase Enzyme Inhibition: Mechanistic and Computational Investigations

This study was aimed to perform the mechanistic investigations of chalcone scaffold as inhibitors of acetylcholinesterase (AChE) enzyme using molecular docking and molecular dynamics simulation tools. Basic chalcones (C1–C5) were synthesized and their in vitro AChE inhibition was tested. Binding interactions were studied using AutoDock and Surflex-Dock programs, whereas the molecular dynamics simulation studies were performed to check the stability of the ligand–protein complex. Good AChE inhibition (IC₅₀ = 22 ± 2.8 to 37.6 ± 0.75 μM) in correlation with the in silico results (binding energies = −8.55 to −8.14 Kcal/mol) were obtained. The mechanistic studies showed that all of the functionalities present in the chalcone scaffold were involved in binding with the amino acid residues at the binding site through hydrogen bonding, π–π, π–cation, π–sigma, and hydrophobic interactions. Molecular dynamics simulation studies showed the formation of stable complex between the AChE enzyme and C4 ligand.

Kinetic Modeling of Time-Dependent Enzyme Inhibition by Pre-Steady-State Analysis of Progress Curves: The Case Study of the Anti-Alzheimer's Drug Galantamine

The Michaelis–Menten model of enzyme kinetic assumes the free ligand approximation, the steady-state approximation and the rapid equilibrium approximation. Analytical methods to model slow-binding inhibitors by the analysis of initial velocities have been developed but, due to their inherent complexity, they are seldom employed. In order to circumvent the complications that arise from the violation of the rapid equilibrium assumption, inhibition is commonly evaluated by pre-incubating the enzyme and the inhibitors so that, even for slow inhibitors, the binding equilibrium is established before the reaction is started. Here, we show that for long drug-target residence time inhibitors, the conventional analysis of initial velocities by the linear regression of double-reciprocal plots fails to provide a correct description of the inhibition mechanism. As a case study, the inhibition of acetylcholinesterase by galantamine, a drug approved for the symptomatic treatment of Alzheimer’s disease, is reported. For over 50 years, analysis based on the conventional steady-state model has overlooked the time-dependent nature of galantamine inhibition, leading to an erroneous assessment of the drug potency and, hence, to discrepancies between biochemical data and the pharmacological evidence. Re-examination of acetylcholinesterase inhibition by pre-steady state analysis of the reaction progress curves showed that the potency of galantamine has indeed been underestimated by a factor of ~100.

In vitro anticholinesterase, antimonoamine oxidase and antioxidant properties of alkaloid extracts from kola nuts (Cola acuminata and Cola nitida)

Background

The development of cholinesterase (ChE) and monoamine oxidase (MAO) inhibitors for management of neurodegenerative diseases such as Alzheimer’s disease (AD) has come with their undesirable side effects. Hence, research for potent but natural ChE and MAO inhibitors with little or no side effects is essential. This study investigated the potentials of alkaloid extracts from two Cola species as nutraceuticals for prevention and management of AD.

Methods

Alkaloid extracts were obtained from two Cola species (Cola nitida [KN] and Cola acuminata [KA]) by solvent extraction method. The extracts were characterized for their alkaloid contents using gas chromatography (GC). The effects of the extracts on ChE and MAO activities were investigated in vitro. Also, the extracts’ ability to inhibit Fe2+-induced lipid peroxidation in rat brain homogenate, scavenge DPPH and OH radicals, as well as chelate Fe2+ were determined.

Results

GC characterization revealed the presence of augustamine and undulatine as the predominant alkaloids in the extracts. There was no significant (P > 0.05) difference in the inhibitory effects of the extracts on ChE activities. However, KA extract exhibited significantly higher (P < 0.05) MAO inhibitory effect than KN. Also, KA extract inhibited Fe2+- induced malondialdehyde (MDA) production in rat brain homogenate more significantly than KN, while there was no significant difference in DPPH and OH radicals scavenging, as well as Fe2+-chelating abilities of the extracts.

Conclusions

Our findings revealed that KN and KA alkaloid extracts exhibited significant effect in vitro on biological pathways that may contribute to neuroprotection for the management of neurodegenerative diseases.

Alkaloids as a source of potential anticholinesterase inhibitors for the treatment of Alzheimer's disease

Objectives

The inhibition of acetylcholinesterase (AChE), the key enzyme in the breakdown of acetylcholine, is currently the main pharmacological strategy available for Alzheimer's disease (AD). In this sense, many alkaloids isolated from natural sources, such as physostigmine, have been long recognized as acetyl- and butyrylcholinesterase (BChE) inhibitors. Since the approval of galantamine for the treatment of AD patients, the search for new anticholinesterase alkaloids has escalated, leading to promising candidates such as huperzine A. This review aims to summarize recent advances in current knowledge on alkaloids as AChE and BChE inhibitors, highlighting structure–activity relationship (SAR) and docking studies.

Key findings

Natural alkaloids belonging to the steroidal/triterpenoidal, quinolizidine, isoquinoline and indole classes, mainly distributed within Buxaceae, Amaryllidaceae and Lycopodiaceae, are considered important sources of alkaloids with anti-enzymatic properties. Investigations into the possible SARs for some active compounds are based on molecular modelling studies, predicting the mode of interaction of the molecules with amino acid residues in the active site of the enzymes. Following this view, an increasing interest in achieving more potent and effective analogues makes alkaloids good chemical templates for the development of new cholinesterase inhibitors.

Summary

The anticholinesterase activity of alkaloids, together with their structural diversity and physicochemical properties, makes them good candidate agents for the treatment of AD.

Investigation of the in vitro and ex vivo acetylcholinesterase and antioxidant activities of traditionally used Lycopodium species from South America on alkaloid extracts

ETHNOPHARMACOLOGICAL RELEVANCE

The study was aimed at evaluating medicinal and therapeutic potentials of two Lycopodiaceae species, Lycopodium clavatum (L.) and Lycopodium thyoides (Humb. & Bonpl. ex Willd), both used in South American folk medicine for central nervous system conditions. Alkaloid extracts were evaluated for chemical characterization, acetylcholinesterase and antioxidant activities.

MATERIALS AND METHODS

The alkaloid extracts obtained by alkaline extraction were determined for each species by GC/MS examination. The evaluation of the anticholinesterase and the antioxidant activities of the extracts were tested by determining in vitro and ex vivo models. Effects on acetylcholinesterase (AChE) were tested in vitro using rat brain homogenates and ex vivo after a single administration (25, 10 and 1mg/kg i.p.) of the alkaloid extracts in mice. The in vitro antioxidant effects were tested for the 2-deoxyribose degradation, nitric oxide (NO) interaction, 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity and total reactive antioxidant potential (TRAP). After an acute administration (25 and 10mg/kg i.p.) of the extracts in middle-aged (12 months) mice, the antioxidant effects were estimated through the thiobarbituric acid reactive substances test (TBARS), and the antioxidant enzymes activities for catalase (CAT) and superoxide dismutase (SOD) were measured.

RESULTS

AChE activity was inhibited in vitro by the alkaloid-enriched extracts of both Lycopodium species in a dose and time-dependent manner in rat cortex, striatum and hippocampus. A significant inhibition was also observed in areas of the brain after acute administration of extracts, as well as decreased lipid peroxidation and increased CAT activity in the cortex, hippocampus and cerebellum. A moderate antioxidant activity was observed in vitro for the extracts. Chemically, the main alkaloids found for the two species were lycopodine and acetyldihidrolycopodine.

CONCLUSION

This study showed that the biological properties of the folk medicinal plants Lycopodium clavatum and Lycopodium thyoides include AChE inhibitory activity and antioxidant effects, two possible mechanisms of action in Alzheimer's related processes.

Acetonic extract of Buxus sempervirens induces cell cycle arrest, apoptosis and autophagy in breast cancer cells

Plants are an invaluable source of potential new anti-cancer drugs. Here, we investigated the cytotoxic activity of the acetonic extract of Buxus sempervirens on five breast cancer cell lines, MCF7, MCF10CA1a and T47D, three aggressive triple positive breast cancer cell lines, and BT-20 and MDA-MB-435, which are triple negative breast cancer cell lines. As a control, MCF10A, a spontaneously immortalized but non-tumoral cell line has been used. The acetonic extract of Buxus sempervirens showed cytotoxic activity towards all the five studied breast cancer cell lines with an IC(50) ranging from 7.74 µg/ml to 12.5 µg/ml. Most importantly, the plant extract was less toxic towards MCF10A with an IC(50) of 19.24 µg/ml. Fluorescence-activated cell sorting (FACS) analysis showed that the plant extract induced cell death and cell cycle arrest in G0/G1 phase in MCF7, T47D, MCF10CA1a and BT-20 cell lines, concomitant to cyclin D1 downregulation. Application of MCF7 and MCF10CA1a respective IC(50) did not show such effects on the control cell line MCF10A. Propidium iodide/Annexin V double staining revealed a pre-apoptotic cell population with extract-treated MCF10CA1a, T47D and BT-20 cells. Transmission electron microscopy analyses indicated the occurrence of autophagy in MCF7 and MCF10CA1a cell lines. Immunofluorescence and Western blot assays confirmed the processing of microtubule-associated protein LC3 in the treated cancer cells. Moreover, we have demonstrated the upregulation of Beclin-1 in these cell lines and downregulation of Survivin and p21. Also, Caspase-3 detection in treated BT-20 and T47D confirmed the occurrence of apoptosis in these cells. Our findings indicate that Buxus sempervirens extract exhibit promising anti-cancer activity by triggering both autophagic cell death and apoptosis, suggesting that this plant may contain potential anti-cancer agents for single or combinatory cancer therapy against breast cancer.

Screening for acetylcholinesterase inhibitory activity in plant extracts from Argentina

Plants are a potential source of bioactive compounds and offer a promising strategy for the treatment of neurological disorders such as Alzheimer's disease. The inhibitory effect of 73 native and naturalized plants collected from the central region of Argentina on acetylcholinesterase (AChE) was tested using microplate and TLC assays after solvent fractionation of complete ethanol extracts obtained from the plants. Organic fractions obtained from extracts of Achyrocline tomentosa (Asteraceae), Eupatorium viscidum (Asteraceae), Ruprechtia apetala (Polygonaceae) Trichocline reptans (Asteraceae) and Zanthoxylum coco (Rutaceae) presented strong inhibition of AChE (higher than 80%) at 1 mg/mL, with R. apetala and T. reptans being the most potent, showing complete inhibition of the enzyme. Their IC(50) values were 0.0779 and 0.1118 mg/mL, respectively. Aqueous fractions did not show any inhibitory activity on the enzyme. These results suggest that the most effective extracts deserve further investigation with the aim of obtaining new molecules for the treatment of neurodegenerative disorders.

Molecular interactions of cholinesterases inhibitors using in silico methods: current status and future prospects

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a low amount of acetylcholine (ACh) in hippocampus and cortex. Acetylcholinesterase (AChE) is one of the most important enzymes in many living organisms including human being and other vertebrates, insects like mosquitoes, among others. Several reports have been published where it has been clearly shown that the genesis of amyloid protein plaques associated with AD is connected to modifications of both AChE and butyrylcholinesterase (BChE), since the plaque is significantly decreased in AD patients using cholinesterase inhibitors (ChEIs). This review gives some examples of these inhibitors discovered during past couple of years that have shown very prominent interactions at the active site triad of the proteins as well as different other parts of the active site like, peripheral anionic site (PAS), oxyanionic hole, anionic subsite or acyl binding pocket (ABP). Most of the inhibition and their interactions have been visualized by X-ray crystallography, but some of the other inhibitors have been studied either by molecular docking or molecular dynamic (MD) simulations or by both the in silico methods. Some of these prominent studies have been crucially observed and reported here.

New Potent Acetylcholinesterase Inhibitors in the Tetracyclic Triterpene Series

A new highly selective inhibitor of acetylcholinesterase (AChE) was discovered by high-throughput screening. Compound 1 was synthesized from a natural product, the N-3-isobutyrylcycloxobuxidine-F 2. A new extraction protocol of this compound is described. The hemisynthesis and optimization of 1 are reported. The analogs of 1 were tested in vitro for the inhibition of both cholinesterases (AChE and BuChE). These compounds selectively inhibited AChE. Extensive molecular docking studies were performed with 2 and AChE employing Discover Biosym software to rationalize the binding interaction. The results suggested that ligand 2 binds simultaneously to both catalytic and peripheral sites of AChE.

New natural cholinesterase inhibiting and calcium channel blocking quinoline alkaloids

During this study, one new coumarin; 7-O-beta-D-glucopyranoside-2H-1-benzopyran-2-one (1) and three quinoline alkaloids; 3-hydroxy, 2, 2, 6-trimethyl-3, 4, 5, 6-tetrahydro-2H-pyrano[3,2-c] quinoline 5-one (2), ribalinine (3) and methyl isoplatydesmine (4) were isolated from the aerial parts of Skimmia laureola and their structures established by spectroscopic studies. Compounds 2-4 were found to be linear mixed type inhibitors of acetylcholinesterase (K(i) = 110.0, 30.0 and 30.0 microM, respectively). Compounds 2 and 3 were also found to be linear mixed type inhibitors of butyrylcholinesterase, while compound 4 was a noncompetitive inhibitor of the enzyme (K(i) = 90.0, 70.0 and 19.0 microM, respectively). The inhibition of acetyl- and butyryl-cholinesterase enzymes persists as the most promising therapeutic strategy for activating the impaired cholinergic functions in Alzheimer's disease and related dementias. Compound 4 also showed dose-dependent spasmolytic activity in the isolated rabbit jejunum intestinal preparation by relaxing the spontaneous (EC50 = 0.1 mg/mL) and K(+)-induced contractions (EC50 = 0.4 mg/mL), suggesting that the spasmolytic effect of compound 4 is mediated through the blockade of voltage-dependent Ca2+ channels.

Acetylcholinesterase inhibitors from plants and fungi

This review describes 183 compounds obtained from plants and fungi which have been shown to inhibit acetylcholinesterase. The mechanism of action of cholinesterase, together with the binding sites, and, where this is known, the mode of action of inhibitors is described. The relative activities of the different compounds are recorded. The strongest inhibitors are generally alkaloids although some meroterpenoids from fungi have also been found to be active and display better selectivity.