Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

Acetylcholinesterase involvement in apoptosis

To date, more than 40 different types of cells from primary cultures or cell lines have shown AChE expression during apoptosis and after the induction apoptosis by different stimuli. It has been well-established that increased AChE expression or activity is detected in apoptotic cells after apoptotic stimuli in vitro and in vivo, and AChE could be therefore used as a marker of apoptosis. AChE is not an apoptosis initiator, but the cells in which AChE is overexpressed undergo apoptosis more easily than controls. Interestingly, cells with downregulated levels of AChE are not sensitive to apoptosis induction and AChE deficiency can protect against apoptosis. Some tumor cells do not express AChE, but when AChE is introduced into a tumor cell, the cells cease to proliferate and undergo apoptosis more readily. Therefore, AChE can be classified as a tumor suppressor gene. AChE plays a pivotal role in apoptosome formation, and silencing of the AChE gene prevents caspase-9 activation, with consequent decreased cell viability, nuclear condensation, and poly (adenosine diphosphate-ribose) polymerase cleavage. AChE is translocated into the nucleus, which may be an important event during apoptosis. Several questions still need to be addressed, and further studies that address the non-classical function of AChE in apoptosis are needed.

Counter-regulation of cholinesterases: differential activation of PKC and ERK signaling in retinal cells through BChE knockdown

The ubiquitous cholinesterase (ChE) enzymes, functioning in the termination of acetylcholine mediated neural transmission, are also reported to have additional functions. Through application of siRNAs against butyrylcholinesterase (BChE) in R28 cells, a retinal cell line with pluripotent properties, a counter-regulation between ChEs was revealed. BChE knock down resulted in an up-regulation of not only acetylcholinesterase (AChE), but also altered the signaling status of PKC and ERK. Knockdown of BChE modified ERK signaling most notably through ERK1/2 proteins, together with the transcription activator P90RSK1 and c-fos. Stimulation of the R28 cell line by forskolin revealed that ChEs are involved in an intricate cross talk between different signaling pathways. Forskolin-stimulated R28 cells displayed a robust cholinergic response, as detected by both electrophysiology and ChE expression, and changed the activation status of PKC/ERK signaling pathways. The findings in R28 cells show that ChE expressions are inversely co-regulated and act through the transcription factors c-fos and P90RSK1. Since R28 cells have the capacity to differentiate into different cell types through stimulation of signaling pathways, ChEs are likely to be associated with cell fate determination, rather than just terminating cholinergic responses.

Gender-related differences in circadian rhythm of rat plasma acetyl- and butyrylcholinesterase: effects of sex hormone withdrawal

The role of acetylcholinesterase (AChE) in the termination of the cholinergic response through acetylcholine (ACh) hydrolysis and the involvement of plasma butyrylcholinesterase (BuChE), mainly of hepatic origin, in the metabolism of xenobiotics with ester bonds is well known. Besides, BuChE has a crucial role in ACh hydrolysis, especially when selective anticholinesterases inhibit AChE. Herein, we analyzed the gender-related differences and the circadian changes of rat plasma cholinesterases. Plasma and liver cholinesterase activities were evaluated in control or 2-30-day castrated adult male and female rats. Plasma and liver AChE activities did not differ between genders and were not influenced by sex hormone deprivation. BuChE plasma activity was 7 times greater in female, reflecting gender differences in liver enzyme expression. Castration increased liver and plasma BuChE activity in male, while reduced it in female, abolishing gender differences in enzyme activity. Interestingly, female AChE and BuChE plasma activities varied throughout the day, reaching values 27% and 42% lower, respectively, between 2 p.m. and 6 p.m. when compared to the morning peaks at 8 a.m. Castration attenuated daily female BuChE oscillation. On the other hand, male plasma enzymes remained constant throughout the day. In summary, our results show that liver and plasma BuChE, but not AChE, expression is influenced by sex hormones, leading to high levels of blood BuChE in females. The fluctuation of female plasma BuChE during the day should be taken into account to adjust the bioavailability and the therapeutic effects of cholinesterase inhibitors used in cholinergic-based conditions such Alzheimer's disease.

Termination and beyond: acetylcholinesterase as a modulator of synaptic transmission

Termination of synaptic transmission by neurotransmitter hydrolysis is a substantial characteristic of cholinergic synapses. This unique termination mechanism makes acetylcholinesterase (AChE), the enzyme in charge of executing acetylcholine breakdown, a key component of cholinergic signaling. AChE is now known to exist not as a single entity, but rather as a combinatorial complex of protein products. The diverse AChE molecular forms are generated by a single gene that produces over ten different transcripts by alternative splicing and alternative promoter choices. These transcripts are translated into six different protein subunits. Mature AChE proteins are found as soluble monomers, amphipatic dimers, or tetramers of these subunits and become associated to the cellular membrane by specialized anchoring molecules or members of other heteromeric structural components. A substantial increasing body of research indicates that AChE functions in the central nervous system go far beyond the termination of synaptic transmission. The non-enzymatic neuromodulatory functions of AChE affect neurite outgrowth and synaptogenesis and play a major role in memory formation and stress responses. The structural homology between AChE and cell adhesion proteins, together with the recently discovered protein partners of AChE, predict the future unraveling of the molecular pathways underlying these multileveled functions.

Prenatal exposure to the acetylcholinesterase inhibitor methanesulfonyl fluoride alters forebrain morphology and gene expression

Methanesulfonyl fluoride (MSF) is a CNS-selective acetylcholinesterase (AChE) inhibitor, currently being developed and tested for the treatment of symptoms of Alzheimer's disease. We have previously confirmed that a single in utero exposure to MSF at clinically appropriate doses inhibits AChE activity in fetal rat brain by 20%, and when administered throughout gestation, MSF achieves a 40% level of inhibition. Here, we show that rats chronically exposed in utero to MSF display marked sex-specific differences in morphological development of the cerebral cortical layers compared with controls at 7 days of age. Forebrain size and cortical thickness were increased in females and decreased in males. An analysis of gene expression in neonate brain on the day of birth revealed sex-specific differential expression of over 25 genes, including choline acetyltransferase (ChAT), which were affected by prenatal MSF exposure. Many of these genes are associated with sexual differentiation and brain development, while others are involved in more generalized cellular and metabolic processes. The changes observed in cortical morphology and gene expression suggest a critical developmental role for AChE in the fetal nervous system, most likely through its effect on cholinergic neurotransmission.

Possible influence ofBCHE locus of butyrylcholinesterase on stature and body mass index

Butyrylcholinesterase activity has been shown to be positively associated with weight and body mass index (BMI). The present study was carried out to search for an association between variants of the BCHE gene and weight, stature, and BMI on the basis of means and variances compared between nonusual variants and their respective usual controls. Individuals bearing the atypical mutation (N = 52) did not differ from their usual phenotype controls (N = 104) in these parameters. The BCHE*U/BCHE*K individuals (N = 222) presented a significantly higher BMI variance than their BCHE*U/BCHE*U controls (N = 222, F = 1.40, P = 0.012). This higher BMI variance does not seem to be an isolated effect of the K mutation, but appears to be the result of an interaction between the K allele and the usual allele, since no such difference in variance was detected between BCHE*K/BCHE*K individuals (N = 23) and their BCHE*U/BCHE*U (N = 23) controls. These data may suggest a relation between variability in the BCHE locus itself and BMI. Individuals with the BCHE UF phenotype (N = 45) showed a significantly higher mean stature (about 3 cm more; P = 0.02) than their controls with the usual phenotype (N = 135). A role in cell proliferation has been proposed for BCHE, and since growth depends on the number of mitoses, it is not unexpected that variants of this enzyme may influence body stature in different ways. This study reports the first data on the relation of BCHE alleles to anthropometric characters.

Genomic dissection reveals locus response to stress for mammalian acetylcholinesterase

The mammalian acetylcholinesterase (ACHE) locus was investigated using computational predictive methods and experiments of reverse transcription polymerase chain reaction (RT-PCR). Computational analysis identified two genes downstream to ACHE, an inversely oriented arsenite resistance gene homologue (ARS), and a novel previously unidentified gene (PIX), co-oriented with ACHE. Experimental evidence shows coregulation of murine ACHE and ARS following confined swim, indicating coordinated locus response to stress, that is possibly mediated by altered cholinergic neurotransmission.

Knockout of one acetylcholinesterase allele in the mouse

One allele of the AChE gene (ACHE) was knocked out in embryonic stem (ES) cells by homologous recombination. The targeting vector contained 2 kb of a TK gene cassette for negative selection, 884 bp of ACHE including exon 1, 1.6 kb of a Neo(r) gene cassette for positive selection, 5.2 kb of the ACHE Bam HI fragment including exon 6, and 3 kb of Bluescript. The use of this vector deleted exons 2-5, which removed 93% of the ACHE coding sequence including the signal peptide, the active site serine, and the histidine and glutamic acid of the catalytic triad. The gene targeting vector was transfected into ES cells by electroporation. Colonies resistant to G418 and gancyclovir were screened for homologous recombination by Southern blotting. Out of 200 colonies, four were found to have undergone homologous recombination. These four ACHE (+/-) ES cell lines were expanded to provide cells for microinjection into C57Bl/6 mouse blastocysts. The injected blastocysts were implanted into pseudopregnant CD/l white mice. More than 200 injected blastocysts were transferred into 20 mice. More than 65 mice were born, of which 11 were chimeras. Chimeras were identified by their black and agouti coat color. Littermates were all black. Thus far, seven male chimeras have been bred with more than 130 C57Bl/6 females to generate 26 agouti mice out of 199 living offspring. This demonstrated that the ACHE (+/-) ES cells contributed to the germline. Offspring with agouti coat color have a 50% chance of carrying the knockout allele. The 26 agouti offspring were screened for an ACHE (+/-) genotype by tail biopsy PCR. Ten out of 26 agouti mice are heterozygous ACHE knockout mice, and they are healthy and alive at 29 days of age. We expect a phenotype to appear in nullizygous animals.

Gestational exposure to chlorpyrifos: apparent protection of the fetus?

Previous studies have shown that, in general, young, postnatal animals are more sensitive than adults to the toxic effects of anticholinesterase (antiChE) pesticides. Paradoxically, often fetal brain cholinesterase (ChE) is less inhibited than maternal brain after gestational exposure to an antiChE, presumably due to placental and fetal detoxification of the antiChE. The present investigation was designed to study selected toxicokinetic and toxicodynamic factors surrounding the toxicity of chlorpyrifos (CPF; [O,O'-diethyl O-3,5,6-trichloro-2-pyridyl] phosphorothionate) in pregnant rats dosed repeatedly or singly during late gestation. Dams were dosed daily (po) with CPF in corn oil (0 or 7 mg/kg) on gestational days (GD) 14 to 18. Animals were euthanized at 2 to 120 h after the last dose and tissues were collected for enzyme analysis. Using this dosing regimen, we found that (1) the time of maximal ChE inhibition was the same (i.e., 5-10 h after dosing) for both maternal and fetal brain, (2) the degree of fetal brain ChE inhibition was 4.7 times less than maternal brain inhibition, and (3) the detoxification potential (i.e., carboxylesterase and chlorpyrifos-oxonase) of the fetal tissues was very low compared to the maternal tissues. A separate group of experiments showed that if pregnant dams received only one oral dose of 7 or 10 mg/kg CPF on GD18, the degree of ChE inhibition in the fetal brain was comparable to the maternal brain ChE inhibition. Taking into consideration the net increase (more than fourfold) in fetal brain ChE activity from GD14 to 18 in control animals, and the fact that maternal brain ChE was inhibited more than fetal brain ChE only in a repeated-dosing regimen, we conclude that the fetus is not genuinely protected from the toxic effects of a given dose of CPF. We propose that fetal brain ChE is simply able to recover more fully between each dose as compared to maternal brain ChE, giving the illusion that the fetal compartment is less affected than the maternal compartment.