Butyrylcholinesterase activity in the rat forebrain and upper brainstem: Postnatal development and adult distribution

The structural modification and biological evaluation of tetrahydrothienopyridine derivatives as selective BChE inhibitors

A series of tetrahydrothienopyridine derivatives have been designed, synthesized, and evaluated as selective BChE inhibitors. Compounds were analyzed via HRMS, 1H NMR, and 13C NMR. The inhibitory effects were evaluated according to the method of Ellman et al. 6n was the most potent and selective inhibitor against BChE (eeAChE IC50 = 686.4 ± 478.6 μM, eqBChE IC50 = 10.5 ± 5.0 nM, SI = 6.5*104, hBChE IC50 = 32.5 ± 6.5 nM). Cell-based assays have confirmed the low neurotoxicity of 6a and 6n and their moderate neuroprotective effects. Compounds 6a and 6n provide novel chemical entities for the treatment of Alzheimer's disease.

Identification and molecular characterization of propionylcholinesterase, a novel pseudocholinesterase in rice

Cholinesterase is consisting of acetylcholinesterase (AChE) and pseudocholinesterase in vertebrates and invertebrates. AChE gene has been identified in several plant species, while pseudocholinesterase gene has not yet been found in any plant species. In this study, we report that the AChE gene paralog encodes propionylcholinesterase (PChE), a pseudocholinesterase in rice. PChE was found to be located adjacent to AChE (Os07g0586200) on rice chromosome 7 and designated as Os07g0586100. Phylogenetic tree analysis showed a close relationship between rice AChE and PChE. PChE-overexpressing rice had higher hydrolytic activity toward propionylthiocholine than acetylthiocholine and showed extremely low activity against butyrylthiocholine. Therefore, the PChE gene product was characterized as a propionylcholinesterase, a pseudocholinesterase. The rice PChE displayed lower sensitivity to the cholinesterase inhibitor, neostigmine bromide, than electric eel, maize, and rice AChEs. The recombinant PChE functions as a 171 kDa homotetramer. PChE was expressed during the later developmental stage, and it was found be localized in the extracellular spaces of the rice leaf tissue. These results suggest that the rice plant possesses PChE, which functions in the extracellular spaces at a later developmental stage. To the best of our knowledge, this study provides the first direct evidence and molecular characterization of PChE in plants.

Synthesis and Initial Characterization of a Reversible, Selective 18F-Labeled Radiotracer for Human Butyrylcholinesterase

PURPOSE

A neuropathological hallmark of Alzheimer's disease (AD) is the presence of amyloid-β (Aβ) plaques in the brain, which are observed in a significant number of cognitively normal, older adults as well. In AD, butyrylcholinesterase (BChE) becomes associated with A_β aggregates, making it a promising target for imaging probes to support diagnosis of AD. In this study, we present the synthesis, radiochemistry, in vitro and preliminary ex and in vivo investigations of a selective, reversible BChE inhibitor as PET-tracer for evaluation as an AD diagnostic.

PROCEDURES

Radiolabeling of the inhibitor was achieved by fluorination of a respective tosylated precursor using K[¹⁸F]. IC₅₀ values of the fluorinated compound were obtained in a colorimetric assay using recombinant, human (h) BChE. Dissociation constants were determined by measuring hBChE activity in the presence of different concentrations of inhibitor.

RESULTS

Radiofluorination of the tosylate precursor gave the desired radiotracer in an average radiochemical yield of 20 ± 3 %. Identity and > 95.5 % radiochemical purity were confirmed by HPLC and TLC autoradiography. The inhibitory potency determined in Ellman's assay gave an IC₅₀ value of 118.3 ± 19.6 nM. Dissociation constants measured in kinetic experiments revealed lower affinity of the inhibitor for binding to the acylated enzyme (K₂ = 68.0 nM) in comparison to the free enzyme (K₁ = 32.9 nM).

CONCLUSIONS

The reversibly acting, selective radiotracer is synthetically easily accessible and retains promising activity and binding potential on hBChE. Radiosynthesis with ¹⁸F labeling of tosylates was feasible in a reasonable time frame and good radiochemical yield.

Development of potent reversible selective inhibitors of butyrylcholinesterase as fluorescent probes

Brain butyrylcholinesterase (BChE) is an attractive target for drugs designed for the treatment of Alzheimer's disease (AD) in its advanced stages. It also potentially represents a biomarker for progression of this disease. Based on the crystal structure of previously described highly potent, reversible, and selective BChE inhibitors, we have developed the fluorescent probes that are selective towards human BChE. The most promising probes also maintain their inhibition of BChE in the low nanomolar range with high selectivity over acetylcholinesterase. Kinetic studies of probes reveal a reversible mixed inhibition mechanism, with binding of these fluorescent probes to both the free and acylated enzyme. Probes show environment-sensitive emission, and additionally, one of them also shows significant enhancement of fluorescence intensity upon binding to the active site of BChE. Finally, the crystal structures of probes in complex with human BChE are reported, which offer an excellent base for further development of this library of compounds.

Characterization of organophosphate-induced brain injuries in a convulsive mouse model of diisopropylfluorophosphate exposure

Organophosphate (OP) compounds constitute a class of highly toxic molecules, characterized by irreversible cholinesterase (ChE) inhibition. Being either pesticides or chemical warfare agents, they present a major health issue in some countries, as well as a terrorist or military threat. Prompted by the need for suitable animal models to test novel medical countermeasures, we developed a new convulsive mouse model of OP poisoning using diisopropylfluorophosphate (DFP). Using electrocorticography (ECoG), we analyzed seizure and status epilepticus (SE) occurrences, as well as relative power of ECoG frequency band modifications after DFP injection in male Swiss mice. Next, we investigated DFP effect on ChE inhibition. Histological changes on neuronal activity and neuronal damage were examined by c-Fos immunolabeling and Fluoro-Jade C staining. We showed that mice exposed to DFP presented electrocorticographic seizures that rapidly progressed to SE within 20 minutes. Lasting >8 hours, DFP-induced SE was associated with major power spectrum modifications in seizing DFP animals compared to control animals. Seizures and SE development were concomitant with profound ChE inhibition and induced massive neuronal degeneration. Presenting all hallmarks of convulsive OP poisoning, we showed that our mouse model is valuable for studying pathophysiological mechanisms and preclinical testing of newly available therapeutic molecules.

Development of an in-vivo active reversible butyrylcholinesterase inhibitor

Alzheimer’s disease (AD) is characterized by severe basal forebrain cholinergic deficit, which results in progressive and chronic deterioration of memory and cognitive functions. Similar to acetylcholinesterase, butyrylcholinesterase (BChE) contributes to the termination of cholinergic neurotransmission. Its enzymatic activity increases with the disease progression, thus classifying BChE as a viable therapeutic target in advanced AD. Potent, selective and reversible human BChE inhibitors were developed. The solved crystal structure of human BChE in complex with the most potent inhibitor reveals its binding mode and provides the molecular basis of its low nanomolar potency. Additionally, this compound is noncytotoxic and has neuroprotective properties. Furthermore, this inhibitor moderately crosses the blood-brain barrier and improves memory, cognitive functions and learning abilities of mice in a model of the cholinergic deficit that characterizes AD, without producing acute cholinergic adverse effects. Our study provides an advanced lead compound for developing drugs for alleviating symptoms caused by cholinergic hypofunction in advanced AD.

New pharmacological approaches to the cholinergic system: an overview on muscarinic receptor ligands and cholinesterase inhibitors

The cholinergic system is expressed in neuronal and in non-neuronal tissues. Acetylcholine (ACh), synthesized in and out of the nervous system can locally contribute to modulation of various cell functions (e.g. survival, proliferation). Considering that the cholinergic system and its functions are impaired in a number of disorders, the identification of new pharmacological approaches to regulate cholinergic system components appears of great relevance. The present review focuses on recent pharmacological drugs able to modulate the activity of cholinergic receptors and thereby, cholinergic function, with an emphasis on the muscarinic receptor subtype, and additionally covers the cholinesterases, the main enzymes involved in ACh hydrolysis. The presence and function of muscarinic receptor subtypes both in neuronal and non-neuronal cells has been demonstrated using extensive pharmacological data emerging from studies on transgenic mice. The possible involvement of ACh in different pathologies has been proposed in recent years and is becoming an important area of study. Although the lack of selective muscarinic receptor ligands has for a long time limited the definition of therapeutic treatment based on muscarinic receptors as targets, some muscarinic ligands such as cevimeline (patents US4855290; US5571918) or xanomeline (patent, US5980933) have been developed and used in pre-clinical or in clinical studies for the treatment of nervous system diseases (Alzheimer' and Sjogren's diseases). The present review focuses on the potential implications of muscarinic receptors in different pathologies, including tumors. Moreover, the future use of muscarinic ligands in therapeutic protocols in cancer therapy will be discussed, considering that some muscarinic antagonists currently used in the treatment of genitourinary disease (e.g. darifenacin, patent, US5096890; US6106864) have also been demonstrated to arrest tumor progression in nude mice. The involvement of muscarinic receptors in nociception also is over-viewed. In fact, muscarinic agonists such as vedaclidine, CMI-936 and CMI-1145 have been demonstrated to have analgesic effects in animal models comparable or more pronounced to those produced by morphine or opiates. Likewise, the crucial role of cholinesterases (acetylcholinesterase and butirylcholinesterase) in neural transmission is discussed, as large number of drugs inhibiting cholinesterase activity have become of increasing relevance particularly for the treatment of neurodegenerative disorders. Herein we summarize the current knowledge of the cholinesterase inhibitors with particular attention to recent patents for Alzheimer's disease drugs.

Developmental adaptation of central nervous system to extremely high acetylcholine levels

Acetylcholinesterase (AChE) is a key enzyme in termination of fast cholinergic transmission. In brain, acetylcholine (ACh) is produced by cholinergic neurons and released in extracellular space where it is cleaved by AChE anchored by protein PRiMA. Recently, we showed that the lack of AChE in brain of PRiMA knock-out (KO) mouse increased ACh levels 200-300 times. The PRiMA KO mice adapt nearly completely by the reduction of muscarinic receptor (MR) density. Here we investigated changes in MR density, AChE, butyrylcholinesterase (BChE) activity in brain in order to determine developmental period responsible for such adaptation. Brains were studied at embryonal day 18.5 and postnatal days (pd) 0, 9, 30, 120, and 425. We found that the AChE activity in PRiMA KO mice remained very low at all studied ages while in wild type (WT) mice it gradually increased till pd120. BChE activity in WT mice gradually decreased until pd9 and then increased by pd120, it continually decreased in KO mice till pd30 and remained unchanged thereafter. MR number increased in WT mice till pd120 and then became stable. Similarly, MR increased in PRiMA KO mice till pd30 and then remained stable, but the maximal level reached is approximately 50% of WT mice. Therefore, we provide the evidence that adaptive changes in MR happen up to pd30. This is new phenomenon that could contribute to the explanation of survival and nearly unchanged phenotype of PRiMA KO mice.

Chronic exposure to cigarette smoke during gestation results in altered cholinesterase enzyme activity and behavioral deficits in adult rat offspring: potential relevance to schizophrenia

Prenatal cigarette smoke exposure (PCSE) has been associated with physiological and developmental changes that may be related to an increased risk for childhood and adult neuropsychiatric diseases. The present study investigated locomotor activity and cholinesterase enzyme activity in rats, following PCSE and/or ketamine treatment in adulthood. Pregnant female Wistar rats were exposed to 12 commercially filtered cigarettes per day for a period of 28 days. We evaluated motor activity and cholinesterase activity in the brain and serum of adult male offspring that were administered acute subanesthetic doses of ketamine (5, 15 and 25 mg/kg), which serves as an animal model of schizophrenia. To determine locomotor activity, we used the open field test. Cholinesterase activity was assessed by hydrolysis monitored spectrophotometrically. Our results show that both PCSE and ketamine treatment in the adult offspring induced increase of locomotor activity. Additionally, it was observed increase of acetylcholinesterase and butyrylcholinesterase activity in the brain and serum, respectively. We demonstrated that animals exposed to cigarettes in the prenatal period had increased the risk for psychotic symptoms in adulthood. This also occurs in a dose-dependent manner. These changes provoke molecular events that are not completely understood and may result in abnormal behavioral responses found in neuropsychiatric disorders, such as schizophrenia.

Japanese quail acute exposure to methamidophos: experimental design, lethal, sub-lethal effects and cholinesterase biochemical and histochemical expression

We exposed the Japanese quail (Coturnix coturnix japonica) to the organophosphate methamidophos using acute oral test. Mortality and sub-lethal effects were recorded in accordance to internationally accepted protocols. In addition cholinesterases were biochemically estimated in tissues of the quail: brain, liver and plasma. Furthermore, brain, liver and duodenum cryostat sections were processed for cholinesterase histochemistry using various substrates and inhibitors. Mortalities occurred mainly in the first 1-2h following application. Sub-lethal effects, such as ataxia, ruffled feathers, tremor, salivation and reduced or no reaction to external stimuli were observed. Biochemical analysis in the brain, liver and plasma indicates a strong cholinesterase dependent inhibition with respect to mortality and sub-lethal effects of the quail. The histochemical staining also indicated a strong cholinesterase inhibition in the organs examined and the analysis of the stained sections allowed for an estimation and interpretation of the intoxication effects of methamidophos, in combination with tissue morphology visible by Haematoxylin and Eosin staining. We conclude that the use of biochemistry and histochemistry for the biomarker cholinesterase, may constitute a significantly novel approach for understanding the results obtained by the acute oral test employed in order to assess the effects of methamidophos and other chemicals known to inhibit this very important nervous system enzyme.

Butyrylcholinesterase and the cholinergic system

The cholinergic system plays important roles in neurotransmission in both the peripheral and central nervous systems. The cholinergic neurotransmitter acetylcholine is synthesized by choline acetyltransferase (ChAT) and its action terminated by acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). The predominance of AChE has focused much attention on understanding the relationship of this enzyme to ChAT-positive cholinergic neurons. However, there is ample evidence that BuChE also plays an important role in cholinergic regulation. To elucidate the relationship of BuChE to neural elements that are producing acetylcholine, the distribution of this enzyme was compared to that of ChAT in the mouse CNS. Brain tissues from 129S1/SvImJ mice were stained for BuChE and ChAT using histochemical, immunohistochemical and immunofluorescent techniques. Both BuChE and ChAT were found in neural elements throughout the CNS. BuChE staining with histochemistry and immunohistochemistry produced the same distribution of labeling throughout the brain and spinal cord. Immunofluorescent double labeling demonstrated that many nuclei in the medulla oblongata, as well as regions of the spinal cord, had neurons that contained both BuChE and ChAT. BuChE-positive neurons without ChAT were found in close proximity with ChAT-positive neuropil in areas such as the thalamus and amygdala. BuChE-positive neuropil was also found closely associated with ChAT-positive neurons, particularly in tegmental nuclei of the pons. These observations provide further neuroanatomical evidence of a role for BuChE in the regulation of acetylcholine levels in the CNS.

Pharmacophore-based virtual screening and density functional theory approach to identifying novel butyrylcholinesterase inhibitors

AIM

To identify the critical chemical features, with reliable geometric constraints, that contributes to the inhibition of butyrylcholinesterase (BChE) function.

METHODS

Ligand-based pharmacophore modeling was used to identify the critical chemical features of BChE inhibitors. The generated pharmacophore model was validated using various techniques, such as Fischer's randomization method, test set, and decoy set. The best pharmacophore model was used as a query in virtual screening to identify novel scaffolds that inhibit BChE. Compounds selected by the best hypothesis in the virtual screening were tested for drug-like properties, and molecular docking study was applied to determine the optimal orientation of the hit compounds in the BChE active site. To find the reactivity of the hit compounds, frontier orbital analysis was carried out using density functional theory.

RESULTS

Based on its correlation coefficient (0.96), root mean square (RMS) deviation (1.01), and total cost (105.72), the quantitative hypothesis Hypo1 consisting of 2 HBA, 1 Hy-Ali, and 1 Hy-Ar was selected as the best hypothesis. Thus, Hypo1 was used as a 3D query in virtual screening of the Maybridge and Chembridge databases. The hit compounds were filtered using ADMET, Lipinski's Rule of Five, and molecular docking to reduce the number of false positive results. Finally, 33 compounds were selected based on their critical interactions with the significant amino acids in BChE's active site. To confirm the inhibitors' potencies, the orbital energies, such as HOMO and LUMO, of the hit compounds and 7 training set compounds were calculated. Among the 33 hit compounds, 10 compounds with the highest HOMO values were selected, and this set was further culled to 5 compounds based on their energy gaps important for stability and energy transfer. From the overall results, 5 hit compounds were confirmed to be potential BChE inhibitors that satisfied all the pharmacophoric features in Hypo1.

CONCLUSION

This study pinpoints important chemical features with geometric constraints that contribute to the inhibition of BChE activity. Five compounds are selected as the best hit BchE-inhibitory compounds.

Quantum dot labeling of butyrylcholinesterase maintains substrate and inhibitor interactions and cell adherence features

Butyrylcholinesterase (BChE) is the major acetylcholine hydrolyzing enzyme in peripheral mammalian systems. It can either reside in the circulation or adhere to cells and tissues and protect them from anticholinesterases, including insecticides and poisonous nerve gases. In humans, impaired cholinesterase functioning is causally involved in many pathologies, including Alzheimer's and Parkinson's diseases, trait anxiety, and post stroke conditions. Recombinant cholinesterases have been developed for therapeutic use; therefore, it is important to follow their in vivo path, location, and interactions. Traditional labeling methods, such as fluorescent dyes and proteins, generally suffer from sensitivity to environmental conditions, from proximity to different molecules or special enzymes which can alter them, and from relatively fast photobleaching. In contrast, emerging development in synthesis and surface engineering of semiconductor nanocrystals enable their use to detect and follow molecules in biological milieus at high sensitivity and in real time. Therefore, we developed a platform for conjugating highly purified recombinant human BChE dimers (rhBChE) to CdSe/CdZnS quantum dots (QDs). We report the development and characterization of highly fluorescent aqueous soluble QD-rhBChE conjugates, present maintenance of hydrolytic activity, inhibitor sensitivity, and adherence to the membrane of cultured live cells of these conjugates, and outline their advantageous features for diverse biological applications.

Visualization of exogenous delivery of nanoformulated butyrylcholinesterase to the central nervous system

Butyrylcholinesterase (BChE) is an efficient bioscavenger of highly toxic organophosphorus poisons and nerve agents. However, BChE administered into the periphery does not provide significant protection of the central nervous system (CNS) due to rejection by the blood-brain barrier. In this study, we evaluated the feasibility of delivering BChE to the CNS by packing it into a block ionomer complex of nanoscale size with a cationic poly(l-lysine)-graft-poly(ethylene oxide) (PLL-g-PEO) copolymer. The multimolecular structure of BChE/PLL-g-PEO complexes was further reinforced by formation of cross-links between the polymer chains. The resulting cross-linked complexes were stable against dilution without significant loss of BChE enzymatic activity. In some cases the BChE was labeled with fluorescent IRDye 800CW before it was incorporated into nanoparticles. BChE/PLL-g-PEO complexes were injected into mice intramuscularly and intravenously. In vivo imaging showed incorporation of the fluorescently labeled BChE in brain. Activity assays showed that BChE remained active in the brain at 72-h post-injection. It was concluded that nanocomplexes can deliver the 340 kDa BChE tetramer to the brain.