Pseudocholinesterase activity in the central nervous system

Analysis of Acetylcholinesterase Activity in Cell Membrane Microarrays of Brain Areas as a Screening Tool to Identify Tissue Specific Inhibitors

Acetylcholinesterase (AChE) is responsible for hydrolyzing the acetylcholine neurotransmitter, bringing an end point to cholinergic neurotransmission. Thus, AChE is the primary target of a wide spectrum of compounds used as pesticides, nerve agents or therapeutic drugs for neurodegenerative diseases such as Alzheimer’s disease (AD). This enzyme is heterogeneously distributed in the brain showing different activity depending on the nervous region. Therefore, the aim of this work is to report a novel technology that enables the simultaneous determination of tissue specific AChE activity, as well as the analysis and screening of specific inhibitors, by using cell membrane microarrays. These microarrays were composed of cell membranes, isolated from 41 tissues, organs and brain areas, that were immobilized over a slide, maintaining the functionality of membrane proteins. To validate this platform, demonstrating its usefulness in drug discovery as a high throughput screening tool, a colorimetric protocol to detect the membrane-bound AChE activity was optimized. Thus, rat cortical and striatal AChE activities were estimated in presence of increased concentrations of AChE inhibitors, and the donepezil effect was assessed simultaneously in 41 tissues and organs, demonstrating the major potential of this microarray’s technology.

Some Recent Trends in Neuropathology

In his presidential address to the first International Congress of Neuropathology in Rome, Gozzano (1952) raised the question of the future of neuropathology. In his opinion neuropathology was a relatively young branch of pathology and had started with the discoveries of Golgi, Cajal, Weigert and Nissl. Golgi invented “la reazione nera” in 1873; it was applied to the nervous system only a decade later. The first Weigert preparations for myelin were demonstrated in 1884 (Wallenberg, 1925). Nissl, inspired by Weigert's success with aniline dyes in nerve fibres, introduced his method for nerve cells in 1885, using first magenta red, later methylene blue and finally toluidine blue. Cajal made his first scientific appearance in 1889, at a meeting of the German Society of Anatomists.

Cholinergic signaling in myelination

There is a long history of research on acetylcholine (ACh) function in myelinating glia, but a resurgence of interest recently as a result of the therapeutic potential of manipulating ACh signaling to promote remyelination, and the broader interest in neurotransmitter signaling in activity-dependent myelination. Myelinating glia express all the major types of muscarinic and nicotinic ACh receptors at different stages of development, and acetylcholinesterase and butyrylcholinesterase are highly expressed in white matter. This review traces the history of research on ACh signaling in Schwann cells, oligodendrocytes, and in the myelin sheath, and summarizes current knowledge on the intracellular signaling and functional consequences of ACh signaling in myelinating glia. Implications of ACh in diseases, such as Alzheimer's disease, multiple sclerosis, and white matter toxicity caused by pesticides are considered, together with an outline of major questions for future research. GLIA 2017;65:687-698.

Butyrylcholinesterase K variant associated with higher enzyme activity in the temporal cortex of elderly patients

There is evidence to suggest an involvement of the K variant of the butyrylcholinesterase gene (BCHE) in dementia. We have examined the relationship between BCHE genotype and butyrylcholinesterase (BuChE) activity in autopsy brain tissue. We studied 164 autopsy cases, 144 with dementia and 20 controls, including 13 K homozygotes and 48 K heterozygotes, from three centres: Newcastle, Oxford and London. Mean BuChE activity in temporal cortex was 37% higher in K homozygotes than in wild-type homozygotes. Linear regression analysis, controlling for gender, diagnosis, age at death and study centre, showed that the number of BCHE-K alleles was associated with increasing BuChE activity (p=0.009).

Sarin (nerve agent GB)-induced differential expression of mRNA coding for the acetylcholinesterase gene in the rat central nervous system

We carried out a time-course study on the effects of a single intramuscular (i.m.) dose (0.5x LD(50)) of sarin (O-isopropyl methylphosphonofluoridate), also known as nerve agent GB, on the mRNA expression of acetylcholinesterase (AChE) in the brain of male Sprague-Dawley rats. Sarin inactivates the enzyme AChE which is responsible for the breakdown of the neurotransmitter acetylcholine (ACh), leading to its accumulation at ACh receptors and overstimulation of the cholinergic system. Rats were treated with 50 microg/kg of sarin (0.5x LD(50)) in 1 mL saline/kg and terminated at the following time points: 1 and 2 hr and 1, 3, and 7 days post-treatment. Control rats were treated with normal saline. Total RNA was extracted, and northern blots were hybridized with cDNA probes for AChE and 28S RNA (control). Poly-A RNA from both treated and control cortex was used for reverse transcription-polymerase chain reaction (RT-PCR)-based verification of the data from the northern blots. The results obtained indicate that a single (i.m.) dose of sarin (0.5x LD(50)) produced differential induction and persistence of AChE mRNA levels in different regions of the brain. Immediate induction of AChE transcripts was noted in the brainstem (126+/-6%), cortex (149+/-4%), midbrain (153+/-5%), and cerebellum (234+/-2%) at 1 hr. The AChE expression level, however, increased over time and remained elevated after a decline at 1 day in the previously shown more susceptible brainstem. The transcript levels remained elevated at a later time point (3 days) in the midbrain, after a dramatic decline at day 1 (110+/-2%). In the cortex, transcript levels came down to control values by day 1. The cerebellum also showed a decline of the elevated levels observed at 2 hr (275+/-2%) to control values by day 1. RT-PCR analysis of the AChE transcript at 30 min in the cortex showed an induction to 213+/-3% of the control level, confirming the expression pattern obtained by the northern blot data. The immediate induction followed by the complex pattern of the AChE mRNA time-course in the CNS may indicate that the activation of both cholinergic-related and unrelated functions of the gene plays an important role in the pathological manifestations of sarin-induced neurotoxicity.

Engineering of human cholinesterases explains and predicts diverse consequences of administration of various drugs and poisons

The acetylcholine hydrolyzing enzyme, acetylcholinesterase, primarily functions in nerve conduction, yet it appears in several guises, due to tissue-specific expression, alternative mRNA splicing and variable aggregation modes. The closely related enzyme, butyrylcholinesterase, most likely serves as a scavenger of toxins to protect acetylcholine binding proteins. One or both of the cholinesterases probably also plays a non-catalytic role(s) as a surface element on cells to direct intercellular interactions. The two enzymes are subject to inhibition by a wide variety of synthetic (e.g., organophosphorus and carbamate insecticides) and natural (e.g., glycoalkaloids) anticholinesterases that can compromise these functions. Butyrylcholinesterase may function, as well, to degrade several drugs of interest, notably aspirin, cocaine and cocaine-like local anesthetics. The widespread occurrence of butyrylcholinesterase mutants with modified activity further complicates this picture, in ways that are only now being dissected through the use of site-directed mutagenesis and heterologous expression of recombinant cholinesterases.

Tissue distribution of human acetylcholinesterase and butyrylcholinesterase messenger RNA

Cholinesterase inhibitors occur naturally in the calabar bean (eserine), green potatoes (solanine), insect-resistant crab apples, the coca plant (cocaine) and snake venom (fasciculin). There are also synthetic cholinesterase inhibitors, for example man-made insecticides. These inhibitors inactivate acetylcholinesterase and butyrylcholinesterase as well as other targets. From a study of the tissue distribution of acetylcholinesterase and butyrylcholinesterase mRNA by Northern blot analysis, we have found the highest levels of butyrylcholinesterase mRNA in the liver and lungs, tissues known as the principal detoxication sites of the human body. These results indicate that butyrylcholinesterase may be a first line of defense against poisons that are eaten or inhaled.

Neurological cholinesterases in the normal brain and in Alzheimer's disease: relationship to plaques, tangles, and patterns of selective vulnerability

Butyrylcholinesterase (BChE) and an altered form of acetylcholinesterase (AChE) accumulate in the plaques and tangles of Alzheimer's disease (AD). The sources for these plaque- and tangle-bound cholinesterases have not been identified. We now report that AChE and BChE activities with pH preferences and inhibitor selectivities identical to those of plaque- and tangle-bound cholinesterases are found in the astrocytes and oligodendrocytes of control and AD brains. These glial-type cholinesterases are selectively inhibited by indolamines and protease inhibitors. In control brains glial-type cholinesterases appear confined to the intracellular space, whereas in patients with AD they decorate plaques and tangles as well. In control and AD brains AChE-positive glia are distributed throughout the cortical layers and subcortical white matter, whereas BChE-positive glia reach high densities only in the deep cortical layers and white matter. In non-AD control brains, the ratio of BChE to AChE glia was higher in entorhinal and inferotemporal cortex, two regions with a high susceptibility to the pathology of AD, than in primary somatosensory and visual cortex, two areas with a relatively lower susceptibility to the disease process. There was no age-related differences in the density or distribution of cholinesterase-positive glia. In comparison with age-matched control specimens, AD brains had a significantly higher density of BChE glia and a lower density of AChE glia in entorhinal and inferotemporal regions but not in the primary somatosensory or visual areas. These results suggest that glia constitute a likely source for the cholinesterase activity of plaques and tangles and that a high ratio of BChE- to AChE-positive glia may play a permissive or causative role in the neuropathology of AD.

Correlation between concentration of cholinesterases and the resistance of animals to organophosphorus compounds

The molar concentrations of the catalytic sites of serum cholinesterase (ChE--EC 3.1.1.8.) and cholinesterases (ChEs-acetylcholinesterase (AChE)--EC 3.1.1.7. and ChE) from brain and perfused liver of male birds, rats, swine and sheep were determined. A positive correlation between the molar concentrations of the catalytic sites of ChEs and the resistance of the animals to some organophosphorus compounds (OPhCs) was found. In addition, the present study also showed that the difference of the molar concentrations of catalytic sites of ChEs in the brain, blood serum and liver can cause varied resistance to some OPhCs.

Modified properties of serum cholinesterases in primary carcinomas

Cholinesterases were characterized in the serum of 77 treated and 11 untreated patients having primary carcinomas of various tissue origins and 21 healthy volunteers which served as controls. In most of the samples, pseudocholinesterase (BuChE) accounted for almost all cholinesterase (ChE) activity and was inhibited by the organophosphorous poison tetraisopropyl pyrophosphoramide (iso-OMPA). In samples from the tumor-bearing patients, ChE degraded 733 +/- 59 nmole acetylcholine/h/mg protein, lower than the 960 +/- 175 nmole/hour/mg levels measured in controls. Tumor serum ChE exhibited elevated sensitivity to 1,5-bis-(4-allyldimethyl ammonium phenyl)-pentan-3-one dibromide (BW), the selective bisquaternary inhibitor of "true" acetylcholinesterase (AChE), with no correlation to age, sex, staging of tumor, presence of metastases or the specific treatment protocol, and with a different distribution pattern from the decrease in ChE specific activity or the sensitivity to iso-OMPA. In sucrose gradients, ChE sedimented as 12S in controls whereas in tumor serum samples from treated patients an additional component of 6 to 7 S, inhibited by both iso-OMPA and BW, also was detected. However, the ChE activity in serum of patients with diagnosed carcinomas before surgery and medical treatment appeared to be nondistinguishable from controls. These findings suggest that the modified properties of serum cholinesterases in carcinoma patients are not the result of the tumor itself, but that the common therapy protocols used in the treatment of primary carcinomas may cause the appearance of soluble ChE activity with properties of both AChE and BuChE, which accumulates in the serum.

Modes of attachment of acetylcholinesterase to the surface membrane

Acetylcholinesterase (AChE) occurs in multiple molecular forms differing in their quaternary structure and mode of anchoring to the surface membrane. Attachment is achieved by post-translational modification of the catalytic subunits. Two such mechanisms are described. One involves attachment to catalytic subunit tetramers, via disulfide bridges, of a collagen-like fibrous tail. This, in turn, interacts, primarily via ionic forces, with a heparin-like proteoglycan in the extracellular matrix. A second such modification involve the covalent attachment of a single phosphatidylinositol molecule at the carboxyl-terminus of each catalytic subunit polypeptide; the diacylglycerol moiety of the phospholipid serves to anchor the modified enzyme hydrophobically to the lipid bilayer of the plasma membrane. The detailed molecular structure of these two classes of acetylcholinesterase are discussed, as well as their biosynthesis and mode of anchoring.

Molecular forms of butyrylcholinesterase in the human neocortex during development and degeneration of the cortical cholinergic system

The total levels of butyrylcholinesterase (BChE) activity and, more specifically, the distribution of BChE molecular forms were measured in the human neocortex during fetal development. Both the amount of total activity and the abundance of the different molecular forms (G1 and G4) remained relatively constant between gestational ages of 8-22 weeks and were similar to those observed in samples of cortex from aged brain. In addition, in both Alzheimer-type and parkinsonian dementia, the levels of total BChE activity as well as the relative abundance of the G1 and G4 molecular forms were similar to those observed in control tissue. Hence, both the levels of total activity and the distribution of molecular forms did not change significantly either during fetal development or in the neurodegenerative disorders of Alzheimer-type and parkinsonian dementias. Because these situations are accompanied by changes in the cortical cholinergic system (including an increase and decrease in levels of the G4 form of acetylcholinesterase, respectively), it is concluded that, at least in the human neocortex, BChE is unrelated to cholinergic neurotransmission associated with subcortical cholinergic projection fibres.

Molecular biological search for human genes encoding cholinesterases

Cholinesterases (ChEs) are highly polymorphic proteins, capable of rapidly hydrolyzing the neurotransmitter acetylcholine and involved in terminating neurotransmission in neuromuscular junctions and cholinergic synapses. In an attempt to delineate the structure and detailed properties of the human protein(s) and the gene(s) coding for the acetylcholine hydrolyzing enzymes, a human cDNA coding for ChE was isolated by use of oligodeoxynucleotide screening of cDNA libraries. For this purpose, a method for increasing the effectiveness of oligonucleotide screening by introducing deoxyinosine in sites of codon ambiguity and using tetramethyl-ammonium salt washes to remove false-positive hybrids was employed. The resulting isolated 2.4-kilobase (kb) cholinesterase cDNA sequences encode for the entire mature secretory protein, preceded by an N-terminal signal peptide. The human ChE primary sequence shows almost no homology to other serine hydrolases, with the exception of a hexapeptide at the active site. In contrast, it displays extensive homology with acetylcholinesterase form Torpedo californica and Drosophila melanogaster as well as with bovine thyroglobulin. These extensive homologies probably suggest the need of the entire coding sequence for the physiological function(s) fulfilled by the enzyme and further suggest a common, unique, ancestral gene for these cDNAs. In turn, the cDNA was used as a probe to isolate genomic DNA sequences for the 5'-region of the human ChE gene. The genomic DNA fragment encoding part of the 5'-region of ChEcDNA was detected by DNA blot hybridization, enriched 70-fold by gel electrophoresis and electroelution, cloned in lambda phage and isolated. Sequencing of the cloned DNA revealed that it did indeed include part of the 5'-region of ChEcDNA, starting at an adjacent 5'-position to the nucleotides coding for the initiator methionine, and ending with an EcoRI restriction site inherent to the ChEcDNA sequence. The isolated fragment of the human cholinesterase gene is currently employed to complete the structural characterization of this and related genes.

Molecular forms of acetylcholinesterase and butyrylcholinesterase in the aged human central nervous system

The distribution of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) molecular forms and their solubility characteristics were examined, using density gradient centrifugation, in various regions of the postmortem human CNS. Total AChE activity varied extensively (50-fold) among the regions investigated, being highest in the telencephalic subcortical structures (caudate nucleus and nucleus of Meynert); intermediate in the substantia nigra, cerebellum, and spinal cord; and least in the fornix and cortical regions (hippocampus and temporal and parietal cortex). Total BChE activity was, in contrast, much more evenly distributed, with only a threefold variation between the regions studied. Although the patterns of molecular forms of each enzyme were broadly similar among the different areas, regional variations in the distribution and abundance of the various forms of AChE were much greater than those of BChE. Thus, although the tetrameric G4 form of AChE constituted the majority of the total AChE activity in all regions examined, the ratio of the G4 form to the monomeric G1 form, the latter of which constituted the majority of the remaining activity, varied markedly, ranging from 21 in the caudate nucleus to 1.7 in the temporal cortex. In addition to the G4 and G1 forms of AChE, the dimeric G2 form was observed in the nucleus of Meynert and a fast-sedimenting (16S) species was found in samples of both the parietal cortex and spinal cord. In contrast, the G4 and G1 forms of BChE were the only molecular species observed in the different areas and the G4:G1 ratio varied from 3.3 in the substantia nigra to 0.9 in the temporal cortex. Regarding the solubility characteristics of the individual AChE and BChE molecular forms, the majority of the G4 form of AChE was extractable only in the presence of detergent, indicating a predominantly membrane-bound localization of this species. The smaller AChE forms (G1 and G2) and both the G1 and G4 forms of BChE were all relatively evenly distributed between soluble and membrane-bound species. These findings are discussed in relation to neurochemical and neuroanatomical, particularly cholinergic, features of the regions examined.

Polymorphism of acetylcholinesterase in discrete regions of the developing human fetal brain

The molecular forms and membrane association of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) and pseudocholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) were determined in the presence of protease inhibitors in dissected regions of developing human fetal brain, as compared with parallel areas from mature brain. All areas contained substantial cholinesterase activities, of which acetylcholinesterase accounted for almost all the activity. Two major forms of acetylcholinesterase activity, sedimenting at 10-11S and 4-5S, respectively, were detected on sucrose gradients and possessed similar catalytic properties, as judged by their individual Km values toward [3H]acetylcholine (ca. 4 X 10(-4) M). The ratio between these forms varied by up to four- to fivefold, both between different areas and within particular areas at various developmental stages, but reached similar values (about 5:2) in all areas of mature brain. Acetylcholinesterase activity was ca. 35-50% low-salt-soluble and 45-65% detergent-soluble in various developmental stages and brain areas, with an increase during development of the detergent-soluble fraction of the light form. In contrast, pseudocholinesterase activity was mostly low-salt-soluble and sedimented as one component of 10-11S in all areas and developmental stages. Our findings suggest noncoordinate regulation of brain acetylcholinesterase and pseudocholinesterase, and indicate that the expression of acetylcholinesterase forms within embryonic brain areas depends both on cell type composition and on development.

Characterization of activities and forms of cholinesterases in human primary brain tumors

The activities and molecular forms of cholinesterases were studied in a collection of primary brain tumors consisting of primarily gliomas and meningiomas, together with samples of forebrain taken postmortem from patients suffering from diseases unrelated to the nervous system. Both types of tumors, as well as normal forebrain, contained substantial amounts of cholinesterase activity and some gliomas contained exceptionally high levels. In both normal forebrain and meningiomas, acetylcholinesterase (acetylcholine hydrolase; EC 3.1.1.7) accounted for almost all the cholinesterase activity, but in almost all gliomas elevated pseudocholinesterase (acylcholine acylhydrolase; EC 3.1.1.8) could be detected. The cholinesterase activity of both normal forebrain and gliomas migrated on sucrose gradients as a major component of 10-11 S together with a minor component of 4-5 S. In meningiomas a light (4.5 S) form was the principal component.

Substance P hydrolysis by human serum cholinesterase

Highly purified human serum cholinesterase (EC 3.1.1.8, also known as pseudocholinesterase and butyrylcholinesterase) had peptidase activity toward substance P. Digestion of substance P was monitored by high performance liquid chromatography, which separated three product peptides. The cleavages occurred sequentially. The first peptide to appear as Arg1-Pro2. The Km for this hydrolysis was 0.3 mM; maximum activity was 7.9 nmol min-1 mg-1 of protein, which corresponded to a turnover number of 0.6 min-1. A second cleavage yielded Lys3-Pro4. A third cleavage occurred at the C-terminal, where the amide was removed from Met11 to yield a peptide containing residues 5-11. Both the peptidase and esterase activities of the enzyme were completely inhibited by the anticholinesterase agent, diisopropylfluorophosphate. Substance P inhibited the hydrolysis of benzoylcholine (a good ester substrate) with a KI of 0.17 mM, indicating that substance P interacted with cholinesterase rather than with a trace contaminant. Peptidase and amidase activities for serum cholinesterase are novel activities for this enzyme. It was demonstrated previously that the related enzyme acetylcholinesterase (EC 3.1.1.7) catalyzed the hydrolysis of substance P, but at entirely different cleavage sites from those reported in the present work. Since butyrylcholinesterase is present in brain and muscle, as well as in serum, it may be involved in the physiological regulation of substance P.

Topographical distribution of acetylcholinesterase and butyrylcholinesterase in the cerebral hemisphere of the garden lizard, Calotes versicolor

Differential patterns of distribution of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are observed in the cerebral nuclei and fiber tracts of Calotes versicolor. The majority of the nuclei reveal a positive reaction for AChE varying from moderate to intense. In contrast, almost all the nuclei are found negative to BChE, except for the area of the nucleus commissurae anterioris and of the paleostriatum where the positive activity is due to BChE positive fibers. The medial forebrain bundle demonstrates intense activity for AChE, while the lateral forebrain bundle and commissures present a negative picture. Interestingly, all the fiber bundles and commissures show an intense reaction for BChE. The nature of nuclei and fiber tracts in relation to the differential patterns of AChE and BChE is discussed.

Histochemical mapping of acetylcholinesterase and butyrylcholinesterase in the medulla oblongata and pons of squirrel (Funambulus palmarum)

The distribution of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) has been investigated in a series of sections passing through the medulla oblongata and pons of the squirrel brain. A comparison of the two enzymes has given an interesting picture of their selective localization in the different nuclei. Marked AChE activity has been observed in the cranial nerve nuclei. BChE activity in various nuclei of the medulla oblongata and pons is variable and occurs diffusely between the cells. Possible reasons pertaining to marked variation in AChE and BChE contents of various nuclei and fiber tracts have been discussed.

Enzymatic patterns in reptilian brain. Histochemical characterization of the optic tectum

The enzymatic patterns present in the optic tectum of 4 species belonging to different reptilian orders seem related to the degree of structural and functional organization reached by the nervous centre, as in other vertebrates. In particular the AChE localization in reptiles is representative of a evolutionary sequence in the distribution of this enzyme in the optic tectum along the tetrapode series.

Cholinesterase patterns in the cerebellum of reptiles

In the cerebellum of four species belonging to the three main reptilian orders the histochemical localization of cholinesterases has been studied. The use of different substrate-inhibitor combinations permits to record the distribution patterns of acetylcholinesterase and pseudocholinesterase, mainly revealed as butyrylcholinesterase activity. From the neurological point of view it is interesting to note that acetylcholinesterase activity shows three different distribution patterns in reptilian cerebellum, thus confirming the characteristic variability previously noticed in the cerebellar cortex of other vertebrates.

Topographical distribution of acetylcholinesterase and butyrylcholinesterase in the diencephalon and mesencephalon of the garden lizard (Calotes versicolor)

The present report incorporates the histochemical mapping of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) among the various nuclei and fiber tracts of the diencephalon and mesencephalon of Calotes veriscolor. The various nuclei, for both enzymes, present varying degrees of staining, ranging from negative nuclei, on the one hand, to mild and intense staining on the other hand. Almost all of the fiber tracts reveal intense activity in BChE preparations, while they demonstrate mild and moderate activities for AChE. The nature of the various nuclei in relation to enzymatic patterns is discussed.

Histochemical characterization of cholinesterase activity in the frog brain with special reference to its localization on the wall of blood vessels

The nature of the cholinesterase activity present on the wall of blood vessels in amphibian brain has been studied histochemically. It is concluded that the enzyme involved is acetylcholinesterase rather than butyrylcholinesterase, which more frequently occurs in the blood vessels of the brain of other vertebrates. The histochemical results are in agreement with most biochemical data concerning substrate specificity and inhibitor response for both acetylcholinesterase and butyrylcholinesterase. The two main hypotheses put forward by others in order to explain cholinesterase activity in brain vessels are discussed briefly.