Acetylcholinesterase enhances neurite growth and synapse development through alternative contributions of its hydrolytic capacity, core protein, and variable C termini

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

Overexpression of muscle specific kinase increases the transcription and aggregation of acetylcholine receptors in Xenopus embryos

Muscle specific kinase (MuSK) mediates agrin-induced acetylcholine receptor (AChR) aggregation on muscle membrane at the neuromuscular junction (NMJ). To examine whether MuSK enhances NMJ formation during embryonic development in vivo, the level of expression of MuSK was manipulated in Xenopus embryos and the functional consequence at the NMJ was assessed. We found that overexpression of MuSK enhanced the formation of NMJ by increasing the aggregation of AChRs at innervated regions in developing embryos. The area of AChR aggregation increased by approximately 2-fold in MuSK injected embryos during the critical stages of NMJ formation. Interestingly, overexpression of MuSK in Xenopus embryos was found to induce the level of AChR transcript. Deletion of the Kringle domain in the MuSK construct did not attenuate the observed induction of AChR transcription and aggregation. Taken together, our findings provide the first demonstration that increased level of MuSK expression in vivo significantly elevate the aggregation and transcription of AChR at the NMJ in developing Xenopus embryos.

Expression of recombinant human acetylcholinesterase in transgenic tomato plants

Enzyme therapy for the prevention and treatment of organophosphate poisoning depends on the availability of large amounts of cholinesterases. Transgenic plants are being evaluated for their efficiency and cost-effectiveness as a system for the bioproduction of therapeutically valuable proteins. Here we report production of a recombinant isoform of human acetylcholinesterase in transgenic tomato plants. Active and stable acetylcholinesterase, which retains the kinetic characteristics of the human enzyme, accumulated in tomato plants. High levels of specific activity were registered in leaves (up to 25 nmol min(-1) mg protein(-1)) and fruits (up to 250 nmol min(-1) mg protein(-1)).

Modified testicular expression of stress-associated "readthrough" acetylcholinesterase predicts male infertility

Male infertility is often attributed to stress. However, the protein or proteins that mediate stress-related infertility are not yet known. Overexpression of the "readthrough" variant of acetylcholinesterase (AChE-R) is involved in the cellular stress response in a variety of mammalian tissues. Here, we report testicular overexpression of AChE-R in heads, but not tails, of postmeiotic spermatozoa from mice subjected to a transient psychological stress compared with age-matched control mice. Transgenic mice overexpressing AChE-R displayed reduced sperm counts, decreased seminal gland weight, and impaired sperm motility compared with age-matched nontransgenic controls. AChE-R was prominent in meiotic phase spermatocytes and in tails, but not heads, of testicular spermatozoa from AChE-R transgenic mice. Head-localized AChE-R was characteristic of human sperm from fertile donors. In contrast, sperm head AChE-R staining was conspicuously reduced in samples from human couples for whom the cause of infertility could not be determined, similar to the pattern found in transgenic mice. These findings indicate AChE-R involvement in impaired sperm quality, which suggests that it is a molecular marker for stress-related infertility.

A cyclic AMP-regulated negative feedforward system for neuritogenesis revealed in a neuroblastomaxglioma hybrid cell line

We examined the role of second messengers during the neuritogenesis that accompanies neuronal differentiation in a neuroblastomaxglioma hybrid cell line (NG108-15). NG108-15 cells extended neurites after treatment with dibutyryl cyclic AMP. This dibutyryl cyclic AMP treatment evoked the synthesis of voltage-dependent Ca(2+) channel proteins in the cells. The number of neurites was decreased by Ca(2+) influx under condition of high K(+). Interestingly, the increase of neurites stimulated by dibutyryl cyclic AMP and the decrease of neurites caused by high K(+) were both reversible. This is the first study to demonstrate that cyclic AMP regulates a negative feedforward system for neuritogenesis, which links with Ca(2+) signaling. Such a dual role of cyclic AMP may play an important part in precise neurite targeting.

Changes in neuronal acetylcholinesterase gene expression and division of labor in honey bee colonies

Division of labor in honey bee colonies is highlighted by adult bees making a transition at 2-3 wk of age from working in the hive to foraging for nectar and pollen outside. This behavioral development involves acquisition of new tasks that may require advanced learning capabilities. Because acetylcholinesterase (AChE) hydrolyzes acetylcholine, a major neurotransmitter associated with learning in the insect brain, we searched for changes in AChE expression in the brain during bee behavioral development. Biochemical aspects of the AChE protein were similar in foragers and "nurse" bees that work in the hive tending brood. However, catalytic AChE activity was significantly lower in foragers. Cloning of bee AChE cDNA enabled mRNA analysis, which demonstrated that the forager-related decrease in AChE activity was associated with decreased AChE mRNA levels. This was particularly apparent in the mushroom bodies, a brain region known to be involved with olfactory and visual learning and memory. In addition, treatment with the AChE-inhibitor metrifonate improved performance in an olfactory-learning assay. These findings demonstrate long-term, naturally occurring developmental downregulation of AChE gene expression in the bee brain, and suggest that this genomic plasticity can contribute to facilitated learning capabilities in forager bees.

Regional localization and developmental profile of acetylcholinesterase-evoked increases in [(3)H]-5-fluororwillardiine binding to AMPA receptors in rat brain

In addition to its role in hydrolyzing the neurotransmitter acetylcholine, the synaptically enriched enzyme acetylcholinesterase (AChE) has been reported to play an important role in the development and remodelling of neural processes and synapses. We have shown previously that AChE causes an increase in binding of the specific AMPA receptor ligand (S)-[(3)H]-5-fluorowillardiine ([(3)H]-FW) to rat brain membranes. In this study we have used quantitative autoradiography to investigate the regional distribution and age-dependence of AChE-evoked increases in the binding of [(3)H]-FW in rat brain. Pretreatment of rat brain sections with AChE caused a marked enhancement of [(3)H]-FW binding to many, but not all, brain areas. The increased [(3)H]-FW binding was blocked by the specific AChE inhibitor BW 284c51. The maximal potentiation of [(3)H]-FW binding occurred at different developmental age-points in different regions with a profile consistent with the peak periods for synaptogenesis in any given region. In addition to its effects on brain sections, AChE also strongly potentiated [(3)H]-FW binding to detergent solubilized AMPA receptors suggesting a direct action on the receptors themselves rather than an indirect effect on the plasma membrane. These findings suggest that modulation of AMPA receptors could provide one molecular mechanism for the previously reported effects of AChE on synapse formation, synaptic plasticity and neurodegeneration.

Acetylcholinesterase--new roles for an old actor

The discovery of the first neurotransmitter--acetylcholine--was soon followed by the discovery of its hydrolysing enzyme, acetylcholinesterase. The role of acetylcholinesterase in terminating acetylcholine-mediated neurotransmission made it the focus of intense research for much of the past century. But the complexity of acetylcholinesterase gene regulation and recent evidence for some of the long-suspected 'non-classical' actions of this enzyme have more recently driven a profound revolution in acetylcholinesterase research. Although our understanding of the additional roles of acetylcholinesterase is incomplete, the time is ripe to summarize the evidence on a remarkable diversity of acetylcholinesterase functions.

Direct evidence for an adhesive function in the noncholinergic role of acetylcholinesterase in neurite outgrowth

Acetylcholinesterase (AChE) can promote neurite outgrowth through a mechanism that is independent of its role in hydrolyzing the neurotransmitter acetylcholine. It has been proposed that this neuritogenic capacity of AChE may result from its intrinsic capacity to function in adhesion. In this study we directly tested this hypothesis using neuroblastoma cell lines that have been engineered for altered cell-surface expression of AChE. Using a microtiter-plate adhesion assay and the electrical cell-substrate impedance-sensing (ECIS) method, we demonstrate that the level of cell-substratum adhesion of these cells directly correlates with their level of AChE expression. Furthermore, this adhesion is blocked by either an anti-AChE antibody or a highly specific AChE inhibitor (BW284c51), both of which have also been shown to block neurite outgrowth. In addition, cells that overexpress AChE showed enhanced neurite initiation. By employing cell lines with different levels of AChE expression in two types of cell-substratum adhesion assays, our current studies provide evidence for an adhesive function for AChE. These results, together with the fact that AChE shares sequence homology and structural similarities with several known cell adhesion molecules, support the hypothesis that AChE promotes neurite outgrowth, at least in part, through an adhesive function.

Muscular dystrophy alters the processing of light acetylcholinesterase but not butyrylcholinesterase forms in liver of Lama2(dy) mice

In order to know whether the histopathological changes of liver, which accompany muscular dystrophy, affect the synthesis of cholinesterases, the distribution and glycosylation of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) forms in normal (NL) and dystrophic Lama2(dy) mouse liver (DL) were investigated. About half of liver AChE, and 25% of BuChE were released with a saline buffer (fraction S(1)), and the rest with a saline-Brij 96 buffer (S(2)). Abundant light (G(2)(A) and G(1)(A)) AChE (87%) and BuChE (93%) forms, and a few G(4)(H) and G(4)(A) ChE species were identified in liver. The dystrophic syndrome had no effect on solubilization or composition of ChE forms. Most of the light AChE and BuChE species (>95%) were bound by octyl-Sepharose, while most light AChE forms (80%), but not BuChE isoforms (15%), were retained in phenyl-agarose. About half of the AChE dimers lost their amphiphilic anchor with phosphatidylinositol-specific phospholipase C (PIPLC), and the fraction of PIPLC-resistant species increased in DL. AChE T and R transcripts were detected by reverse transcriptase-polymerase chain reaction (RT-PCR) of liver RNA. ChE components of liver, erythrocyte, and plasma were distinguished by their amphiphilic properties and interaction with lectins. The dystrophic syndrome increased the liver content of the light AChE forms with Lens culinaris agglutinin (LCA) reactivity. The abundance of ChE tetramers in plasma and their small amount in liver suggest that after their assembly in liver they are rapidly secreted, while the light species remain associated to hepatic membranes.

Regulation of the rat oligodendroglia cell line OLN-93 by antisense transfection of butyrylcholinesterase

Butyrylcholinesterase (BChE) is a glial cell marker with unknown function. For neuroepithelial cells, BChE has been shown to regulate cell division and expression of the postmitotic marker acetylcholinesterase (AChE), while similar studies are lacking for glial cells. By transducing an antisense-5'BChE cDNA expression vector via calcium phosphate precipitation, we have analyzed the effect of BChE inhibition on proliferation and differentiation of rat oligodendroglia-derived OLN-93 cells. OLN-93 cells were chosen because they are highly proliferative, while expressing markers of differentiated oligodendrocytes (Richter-Landsberg and Heinrich, 1996). First, we established that OLN-93 cells do express BChE protein, albeit chiefly in an inactive state, and that BChE was decreased by antisense-5'BChE transfection. Cell proliferation was also strongly diminished, protein kinase C (PKCalpha) was upregulated, and expression of cytoskeletal and cell surface proteins was altered. In particular, immunoreactivities of the intermediate filament proteins vimentin and the cell adhesion protein F11 were detected, indicating that BChE-inhibited OLN-93 cells have shifted toward an astrocytic phenotype. These data support a role of the glia marker BChE in CNS glial cell proliferation and differentiation, achieved via a nonenzymatic mechanism. The possible biomedical impact of BChE protein, e.g., on CNS nerve regeneration, is briefly discussed.

Excess "read-through" acetylcholinesterase attenuates but the "synaptic" variant intensifies neurodeterioration correlates

Acute stress increases the risk for neurodegeneration, but the molecular signals regulating the shift from transient stress responses to progressive disease are not yet known. The "read-through" variant of acetylcholinesterase (AChE-R) accumulates in the mammalian brain under acute stress. Therefore, markers of neurodeterioration were examined in transgenic mice overexpressing either AChE-R or the "synaptic" AChE variant, AChE-S. Several observations demonstrate that excess AChE-R attenuates, whereas AChE-S intensifies, neurodeterioration. In the somatosensory cortex, AChE-S transgenics, but not AChE-R or control FVB/N mice, displayed a high density of curled neuronal processes indicative of hyperexcitation. In the hippocampus, AChE-S and control mice, but not AChE-R transgenics, presented progressive accumulation of clustered, heat shock protein 70-immunopositive neuronal fragments and displayed a high incidence of reactive astrocytes. Our findings suggest that AChE-R serves as a modulator that may play a role in preventing the shift from transient, acute stress to progressive neurological disease.

Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models

Vulnerable periods during the development of the nervous system are sensitive to environmental insults because they are dependent on the temporal and regional emergence of critical developmental processes (i.e., proliferation, migration, differentiation, synaptogenesis, myelination, and apoptosis). Evidence from numerous sources demonstrates that neural development extends from the embryonic period through adolescence. In general, the sequence of events is comparable among species, although the time scales are considerably different. Developmental exposure of animals or humans to numerous agents (e.g., X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury, or chlorpyrifos) demonstrates that interference with one or more of these developmental processes can lead to developmental neurotoxicity. Different behavioral domains (e.g., sensory, motor, and various cognitive functions) are subserved by different brain areas. Although there are important differences between the rodent and human brain, analogous structures can be identified. Moreover, the ontogeny of specific behaviors can be used to draw inferences regarding the maturation of specific brain structures or neural circuits in rodents and primates, including humans. Furthermore, various clinical disorders in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may also be the result of interference with normal ontogeny of developmental processes in the nervous system. Of critical concern is the possibility that developmental exposure to neurotoxicants may result in an acceleration of age-related decline in function. This concern is compounded by the fact that developmental neurotoxicity that results in small effects can have a profound societal impact when amortized across the entire population and across the life span of humans.

Acquisition of neuronal proteins during differentiation of NG108-15 cells

The differentiated type of neuroblastomaxglioma hybrid cell line, NG108-15, has widely been used in in vitro studies instead of primary-cultured neurons. Here we examined whether NG108-15 cells can be used as a model for studying the neuronal differentiation process. We compared the expression of neuronal proteins (neurofilament 200 (NF200), phosphorylated-NF200 (p-NF200), microtubule associated protein 2, synaptophysin, syntaxin 1, choline acetyltransferase, and acetylcholinesterase (AChE)) and a glial protein (vimentin) between undifferentiated and differentiated NG108-15 cells by immunocytochemistry and immunoblot analysis. The expression of all neuronal proteins, with the exception of NF200 and p-NF200, was positive in differentiated cells, but almost negative in undifferentiated cells. On the other hand, cytoskeletal intermediate filaments (NF200 and p-NF200) for neurons and that (vimentin) for glia were present in both undifferentiated and differentiated cells. Furthermore, a high expression of AChE mRNA was confirmed in differentiated cells by reverse transcription-PCR analysis. Our results showed that even though the expression of cytoskeletal filaments does not change during differentiation of NG108-15 cells, these cells during differentiation can serve as an appropriate tool for investigating and understanding the mechanisms involved in neuronal development and differentiation.

Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models

Vulnerable periods during the development of the nervous system are sensitive to environmental insults because they are dependent on the temporal and regional emergence of critical developmental processes (i.e., proliferation, migration, differentiation, synaptogenesis, myelination, and apoptosis). Evidence from numerous sources demonstrates that neural development extends from the embryonic period through adolescence. In general, the sequence of events is comparable among species, although the time scales are considerably different. Developmental exposure of animals or humans to numerous agents (e.g., X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury, or chlorpyrifos) demonstrates that interference with one or more of these developmental processes can lead to developmental neurotoxicity. Different behavioral domains (e.g., sensory, motor, and various cognitive functions) are subserved by different brain areas. Although there are important differences between the rodent and human brain, analogous structures can be identified. Moreover, the ontogeny of specific behaviors can be used to draw inferences regarding the maturation of specific brain structures or neural circuits in rodents and primates, including humans. Furthermore, various clinical disorders in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may also be the result of interference with normal ontogeny of developmental processes in the nervous system. Of critical concern is the possibility that developmental exposure to neurotoxicants may result in an acceleration of age-related decline in function. This concern is compounded by the fact that developmental neurotoxicity that results in small effects can have a profound societal impact when amortized across the entire population and across the life span of humans.

Evidence for the direct role of acetylcholinesterase in neurite outgrowth in primary dorsal root ganglion neurons

Dorsal root ganglion (DRG) neurons show a transient peak expression of acetylcholinesterase (AChE) during periods of axonal outgrowth prior to synaptogenesis, suggesting that AChE has a non-enzymatic role during development. We have previously shown that perturbation of cell surface AChE in cultured embryonic rat DRG neurons results in decreased neurite outgrowth and neurite detachment. In this report, we demonstrate a direct correlation between endogenous AChE content and neurite outgrowth in primary DRG neurons. Adenoviral vectors were constructed using full-length rat AChE(T) cDNA in either the sense or antisense orientations to overexpress or knock down AChE expression, respectively. Treatment with the sense-expressing vector produced a 2.5-fold increase in AChE expression and a 2-fold increase in neurite length compared with either untreated or null virus-treated control cells. Conversely, treatment with the antisense-expressing vector reduced AChE expression by 40% and resulted in a reduction in neurite length of similar magnitude. We also observed that overexpression of AChE resulted in greater branching at the distal tips of each primary neurite as well as an increase in cell body size. These findings further indicate that AChE expressed on the axonal surface of developing DRG neurons may modulate their adhesive properties and thereby support axonal development.

Neuronal-glial interactions mediated by interleukin-1 enhance neuronal acetylcholinesterase activity and mRNA expression

Cholinergic dysfunction in Alzheimer's disease has been attributed to stress-induced increases in acetylcholinesterase (AChE) activity. Interleukin-1 (IL-1) is overexpressed in Alzheimer's disease, and stress-related changes in long-term potentiation, an ACh-related cerebral function, are triggered by interleukin-1. Microglial cultures (N9) synthesized and released IL-1 in response to conditioned media obtained from glutamate-treated primary neuron cultures or PC12 cells. This conditioned media contained elevated levels of secreted beta-amyloid precursor protein (sAPP). Naive PC12 cells cocultured with stimulated N9 cultures showed increased AChE activity and mRNA expression. These effects on AChE expression and activity could be blocked by either preincubating the glutamate-treated PC12 supernatants with anti-sAPP antibodies or preincubating naive PC12 cells with IL-1 receptor antagonist. These findings were confirmed in vivo; IL-1-containing pellets implanted into rat cortex also increased AChE mRNA levels. Neuronal stress in Alzheimer's disease may induce increases in AChE expression and activity through a molecular cascade that is mediated by sAPP-induced microglial activation and consequent overexpression of IL-1.

Role of acetylcholinesterase in the development of axon tracts within the embryonic vertebrate brain

In the developing vertebrate brain, acetylcholinesterase (AChE) expression coincides temporally with axon tract formation. Although AChE promotes neurite outgrowth in vitro, the role of this molecule in the development of axon tracts in vivo is unknown. To address this question, we examined the effects of the AChE inhibitor, BW284C51, on the formation of the early scaffold of axon tracts in the embryonic Xenopus brain. In exposed Xenopus brain preparations, axons elongate and establish a normal topography of axon tracts. However, when brains were exposed to BW284C51, the thickness of the major longitudinal axon tract, the tract of the post-optic commissure decreased in a dose-dependent manner. When BW284C51 was removed from the culture media axon tract development returned to normal within 5 h. These findings provide the first evidence for a non-classical role of AChE in the initial formation of axon tracts within the developing vertebrate brain.

Tracking cholinergic pathways from psychological and chemical stressors to variable neurodeterioration paradigms

Cholinergic hyperexcitation can be induced by both acute psychological stress and exposure to acetylcholinesterase inhibitors. Both factors are known risk factors for delayed neurodeterioration processes such as Alzheimer's disease and Parkinson's disease. Recent publications on the involvement of cholinergic pathways in these and other neurodeterioration syndromes are reviewed.

Neuronal differentiation in PC12 cells is inhibited by chlorpyrifos and its metabolites: is acetylcholinesterase inhibition the site of action?

Developmental expression of AChE has been associated with neuronal differentiation (P. G. Layer and E. Willbold, Prog. Histochem. Cytochem. 29, 1-94, 1995). In this study we used pheochromocytoma (PC12) cells, a noncholinergic cell line, rich in acetylcholinesterase (AChE) activity, to examine the effects of cholinesterase-inhibiting pesticides on neural differentiation. The experimental paradigm was focused on whether alterations in cholinesterase (ChE) activity by a pesticide or its metabolites would affect neurite outgrowth, a morphological marker of neuronal differentiation. Results indicated that (1) in controls, both total ChE and AChE activities were significantly increased in NGF-primed PC12 cells compared to NGF-unprimed cells, while the basal expression of butyrylcholinesterase (BuChE) activity was much lower (1.3-7% of total ChE activity) in either the presence or the absence of NGF; (2) an increase in AChE activity was highly correlated (r(2) = 0.99) with the extension of neurite outgrowth, suggesting a link between the expression of AChE activity and the elaboration of neurite outgrowth; (3) NGF increased neurite outgrowth in a time- and concentration-dependent manner; and (4) either chlorpyrifos (CPF) or its metabolites (CPF oxon and TCP) inhibited NGF-induced neurite outgrowth (branches per cell, fragments per cell, total neurite outgrowth per cell) in PC12 cells. These data suggest that the expression of AChE activity is associated with the extension of neurite outgrowth. Both enzyme activity and neurite branching were disrupted by CPF oxon; however, CPF and its other metabolite TCP (1 microgram/ml) caused inhibition of neurite outgrowth in the absence of ChE inhibition, suggesting an alternative mechanism(s) may be involved in pesticide-induced inhibition of differentiation.

Characterization of molecular forms of acetyl- and butyrylcholinesterase in human acoustic neurinomas

Acoustic neurinomas were sequentially extracted with saline and saline-Triton X-100 buffers. Detergent was required to detach the bulk of acetylcholinesterase (AChE), but butyrylcholinesterase (BuChE) was mostly released with saline buffer. Sedimentation analysis and hydrophobic chromatography revealed that neurinomas contain principally amphiphilic AChE tetramers, dimers and monomers, and hydrophilic BuChE tetramers. The AChE dimers and monomers remained amphiphilic after incubation with phosphatidylinositol-specific phospholipase C (PIPLC), after or without prior treatment with alkaline hydroxylamine, which shows that, in contrast to the meningioma AChE dimers and monomers, the neurinoma isoforms are devoid of glycolipid. Neurinoma AChE reacted with the antibodies HR2 and AE1 raised against AChE from human brain or erythrocyte, whereas BuChE bound to a sheep antiserum.

Increased butyrylcholinesterase levels in microsomal membranes of dystrophic Lama2dy mouse muscle

The proportions and the glycosylation of butyrylcholinesterase (BuChE) forms in vesicles rich in sarcoplasmic reticulum from normal (NMV) and dystrophic (DMV) muscle were analyzed, using merosin-deficient dystrophic mice. BuChE activity in DMV was two- to threefold that in NMV. Globular amphiphilic G1A, G2A, and G4A and hydrophilic G4H BuChE forms were identified in NMV and DMV. The amount of G2A forms increased sevenfold in DMV, and the other forms increased about twofold. The higher BuChE level in DMV might reflect a maturational defect, with dystrophy preventing the down-regulation of BuChE with muscle development. About half of G1A, G2A, and G4H BuChE forms in NMV or DMV bound to Lens culinaris agglutinin (LCA), a higher fraction to wheat germ agglutinin (WGA), and little to Ricinus communis agglutinin (RCA). Most of the G4A forms in NMV or DMV bound to LCA or WGA; those from NMV failed to bind to RCA, whereas most of the variants in DMV bound to it, suggesting that the excess of tetramers in DMV is mainly RCA-reactive. The differential interaction of lectins with BuChE components from muscle microsomes, serum, and nerves confirmed that the microsomal BuChE was muscle-intrinsic. The results provide clues regarding the alterations that dystrophy produces in the biosynthesis of BuChE forms in muscle.

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Manipulations of ACHE gene expression suggest non-catalytic involvement of acetylcholinesterase in the functioning of mammalian photoreceptors but not in retinal degeneration

To explore role(s) of acetylcholinesterase (AChE) in functioning and diseased photoreceptors, we studied normal (rd/+) and degenerating (rd/rd) murine retinas. All retinal neurons, expressed AChEmRNA throughout fetal development. AChE and c-Fos mRNAs peaked at post-natal days 10-12, when apoptosis of rd/rd photoreceptors begins. Moreover, c-Fos and AChEmRNA were co-overexpressed in rd/rd mice producing transgenic human (h), and host (m) AChE, but not in rd/+ mice. However, mAChE overexpression also occurred in transgenics expressing human serum albumin. Drastic variations in AChE catalytic activity were ineffective during development. Neither transgenic excess nor diisopropylfluorophosphonate (DFP) inhibition (80%) affected the rd phenotype; nor did DFP exposure induce photoreceptor degeneration or affect other key cholinergic proteins in rd/+ mice, unlike reports of adult mice and despite massive induction under DFP of c-Fos70 years). Therefore, the extreme retinal sensitivity to AChE modulation may reflect non-catalytic function(s) of AChE in adult photoreceptors. These findings exclude AChE as causing the rd phenotype, suggest that its primary function(s) in mammalian retinal development are non-catalytic ones and indicate special role(s) for the AChE protein in adult photoreceptors.

Antisense technologies have a future fighting neurodegenerative diseases

Our growing understanding of the role that unfavorable patterns of gene expression play in the etiology of neurodegenerative disease emphasizes the need for strategies to selectively block the biosynthesis of harmful proteins in the brain. Antisense technologies are ideally suited to this purpose. Tailor-designed to target specific RNA, antisense oligonucleotides and ribozymes offer tools to suppress the production of proteins mediating neurodegeneration. Although technical limitations must still be overcome, the antisense approach represents a novel and exciting strategy for intervention in diseases of the central nervous system.

Comparative neurochemical effects of repeated methyl parathion or chlorpyrifos exposures in neonatal and adult rats

Several studies have reported higher sensitivity based on lethality in young animals compared to adults following acute exposure to organophosphorus insecticides (OPs). We propose that age-related differences in sensitivity to OPs may differ qualitatively and quantitatively with different OPs and varying exposure conditions (e. g., high vs. low dose, acute vs. repeated). To test this hypothesis, we treated neonatal (7 days of age) and adult (90 days of age) rats with either methyl parathion (MPS) or chlorpyrifos (CPF) daily for 14 days and measured neurochemical endpoints {cholinesterase (ChE) inhibition, total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) and muscarinic M2 subtype-preferential ([(3)H]AF-DX 384) binding} in frontal cortex and striatum at timepoints both during (1 day after the 7(th) and 14(th) dose) and after (8 days after the 14(th) dose) exposures. Repeated CPF exposures were associated with relatively similar degrees of ChE inhibition between the age groups during dosing but more extensive inhibition was noted in adults after termination of exposures. Relatively similar changes in muscarinic receptor binding were also noted between age groups following CPF exposures. Moreover, the degree of muscarinic receptor binding reduction relative to ChE inhibition appeared similar in both age groups following CPF exposures. In contrast, ChE activity and muscarinic receptor binding were generally more reduced in neonatal relative to adult brain regions following repeated MPS exposures. Furthermore, the relationship between the degree of ChE inhibition and the reduction in cortical muscarinic receptor binding appeared different between the age groups, i.e., more extensive reduction was noted in neonates compared to adults with a given level of ChE inhibition. We conclude that OP-selective differences in in vivo ChE sensitivity, differential rates of enzyme recovery following inhibition, and age-dependent differences in muscarinic receptor adaptations can all influence the nature of age-related susceptibility to OPs.

Characterization of acetylcholinesterase and butyrylcholinesterase forms in normal and dystrophic Lama2dy mouse heart

In searching for possible differences in acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) forms of dystrophic heart, the properties of ChE species in normal (NH) and dystrophic Lama2dy mouse heart (DH) were investigated. BuChE predominated over AChE. Loosely- and tightly-bound ChEs were released with saline (extract S1) and saline-Triton X-100 buffers (S2). About 50% of AChE, and 25% of BuChE, in NH or DH was measured in S1, and the rest in S2. Asymmetric AChE forms A12 (15%) and A8 (11%), globular hydrophilic G(H)4 (8%), amphiphilic G(A)4 (15%), and G(A)2+G(A)1 (51%) AChE species, and BuChE forms G(H)4 (13%), G(A)4 (3%), and G(A)2+G(A)1 (84%) were identified in NH and DH. Most of the asymmetric and G(A)4 AChE species were bound to Triticum vulgaris (WGA) or Ricinus communis (RCA) agglutinins. About half of G(H)4 and G(A)2+G(A)1 AChE were bound to WGA, and less (10%) to RCA. Variable amounts of G(H)4+G(A)4 (60%), and G(A)2+G(A)1 (75%) BuChE bound to WGA, and 50 and 10% to RCA. The lack of structural differences between ChE species in NH and DH indicates that, in contrast to the ChE forms in mouse skeletal muscle, the biosynthesis of ChE components in heart is not disturbed by dystrophy.

Genetic analysis of collagen Q: roles in acetylcholinesterase and butyrylcholinesterase assembly and in synaptic structure and function

Acetylcholinesterase (AChE) occurs in both asymmetric forms, covalently associated with a collagenous subunit called Q (ColQ), and globular forms that may be either soluble or membrane associated. At the skeletal neuromuscular junction, asymmetric AChE is anchored to the basal lamina of the synaptic cleft, where it hydrolyzes acetylcholine to terminate synaptic transmission. AChE has also been hypothesized to play developmental roles in the nervous system, and ColQ is also expressed in some AChE-poor tissues. To seek roles of ColQ and AChE at synapses and elsewhere, we generated ColQ-deficient mutant mice. ColQ-/- mice completely lacked asymmetric AChE in skeletal and cardiac muscles and brain; they also lacked asymmetric forms of the AChE homologue, butyrylcholinesterase. Thus, products of the ColQ gene are required for assembly of all detectable asymmetric AChE and butyrylcholinesterase. Surprisingly, globular AChE tetramers were also absent from neonatal ColQ-/- muscles, suggesting a role for the ColQ gene in assembly or stabilization of AChE forms that do not themselves contain a collagenous subunit. Histochemical, immunohistochemical, toxicological, and electrophysiological assays all indicated absence of AChE at ColQ-/- neuromuscular junctions. Nonetheless, neuromuscular function was initially robust, demonstrating that AChE and ColQ do not play obligatory roles in early phases of synaptogenesis. Moreover, because acute inhibition of synaptic AChE is fatal to normal animals, there must be compensatory mechanisms in the mutant that allow the synapse to function in the chronic absence of AChE. One structural mechanism appears to be a partial ensheathment of nerve terminals by Schwann cells. Compensation was incomplete, however, as animals lacking ColQ and synaptic AChE failed to thrive and most died before they reached maturity.

Morphogenic role for acetylcholinesterase in axonal outgrowth during neural development

Acetylcholinesterase (AChE) is the enzyme that hydrolyzes the neurotransmitter acetylcholine at cholinergic synapses and neuromuscular junctions. However, results from our laboratory and others indicate that AChE has an extrasynaptic, noncholinergic role during neural development. This article is a review of our findings demonstrating the morphogenic role of AChE, using a neuronal cell culture model. We also discuss how these data suggest that AChE has a cell adhesive function during neural development. These results could have additional significance as AChE is the target enzyme of agricultural organophosphate and carbamate pesticides as well as the commonly used household organophosphate chlorpyrifos (Dursban). Prenatal exposure to these agents could have adverse effects on neural development by interfering with the morphogenic function of AChE.

Human osteogenesis involves differentiation-dependent increases in the morphogenically active 3' alternative splicing variant of acetylcholinesterase

The extended human acetylcholinesterase (AChE) promoter contains many binding sites for osteogenic factors, including 1,25-(OH)2 vitamin D3 and 17beta-estradiol. In differentiating osteosarcoma Saos-2 cells, both of these factors enhanced transcription of the AChE mRNA variant 3' terminated with exon 6 (E6-AChE mRNA), which encodes the catalytically and morphogenically active E6-AChE isoform. In contrast, antisense oligodeoxynucleotide suppression of E6-AChE mRNA expression increased Saos-2 proliferation in a dose- and sequence-dependent manner. The antisense mechanism of action was most likely mediated by mRNA destruction or translational arrest, as cytochemical staining revealed reduction in AChE gene expression. In vivo, we found that E6-AChE mRNA levels rose following midgestation in normally differentiating, postproliferative fetal chondrocytes but not in the osteogenically impaired chondrocytes of dwarf fetuses with thanatophoric dysplasia. Taken together, these findings suggest morphogenic involvement of E6-AChE in the proliferation-differentiation balance characteristic of human osteogenesis.

Functional redundancy of acetylcholinesterase and neuroligin in mammalian neuritogenesis

Accumulated evidence attributes noncatalytic morphogenic activitie(s) to acetylcholinesterase (AChE). Despite sequence homologies, functional overlaps between AChE and catalytically inactive AChE-like cell surface adhesion proteins have been demonstrated only for the Drosophila protein neurotactin. Furthermore, no mechanism had been proposed to enable signal transduction by AChE, an extracellular enzyme. Here, we report impaired neurite outgrowth and loss of neurexin Ialpha mRNA under antisense suppression of AChE in PC12 cells (AS-ACHE cells). Neurite growth was partially rescued by addition of recombinant AChE to the solid substrate or by transfection with various catalytically active and inactive AChE variants. Moreover, overexpression of the homologous neurexin I ligand, neuroligin-1, restored both neurite extension and expression of neurexin Ialpha. Differential PCR display revealed expression of a novel gene, nitzin, in AS-ACHE cells. Nitzin displays 42% homology to the band 4.1 protein superfamily capable of linking integral membrane proteins to the cytoskeleton. Nitzin mRNA is high throughout the developing nervous system, is partially colocalized with AChE, and increases in rescued AS-ACHE cells. Our findings demonstrate redundant neurite growth-promoting activities for AChE and neuroligin and implicate interactions of AChE-like proteins and neurexins as potential mediators of cytoarchitectural changes supporting neuritogenesis.

Mutation in the human acetylcholinesterase-associated collagen gene, COLQ, is responsible for congenital myasthenic syndrome with end-plate acetylcholinesterase deficiency (Type Ic)

Congenital myasthenic syndrome (CMS) with end-plate acetylcholinesterase (AChE) deficiency is a rare autosomal recessive disease, recently classified as CMS type Ic (CMS-Ic). It is characterized by onset in childhood, generalized weakness increased by exertion, refractoriness to anticholinesterase drugs, and morphological abnormalities of the neuromuscular junctions (NMJs). The collagen-tailed form of AChE, which is normally concentrated at NMJs, is composed of catalytic tetramers associated with a specific collagen, COLQ. In CMS-Ic patients, these collagen-tailed forms are often absent. We studied a large family comprising 11 siblings, 6 of whom are affected by a mild form of CMS-Ic. The muscles of the patients contained collagen-tailed AChE. We first excluded the ACHE gene (7q22) as potential culprit, by linkage analysis; then we mapped COLQ to chromosome 3p24.2. By analyzing 3p24.2 markers located close to the gene, we found that the six affected patients were homozygous for an interval of 14 cM between D3S1597 and D3S2338. We determined the COLQ coding sequence and found that the patients present a homozygous missense mutation, Y431S, in the conserved C-terminal domain of COLQ. This mutation is thought to disturb the attachment of collagen-tailed AChE to the NMJ, thus constituting the first genetic defect causing CMS-Ic.

Muscarinic acetylcholine receptors activate the acetylcholinesterase gene promoter

The acetylcholinesterase (AChE) gene promoter contains several overlapping binding sites for Sp1 and Egr-1 transcription factors. Cotransfection experiments and promoter assays showed that Egr-1 can potently activate transcription from the human AChE promoter. Muscarinic acetylcholine receptors (mAChR) rapidly activate, via protein kinase C-mediated signaling, expression of the Egr-1 gene, leading to dramatically increased nuclear concentrations of Egr-1 protein, and to increased binding of Egr-1 to specific DNA recognition sequences. These mAChR-induced increases are followed by increased transcription from the human AChE promoter. In vivo studies with intraventricular infusions of the cholinergic immunotoxin 192 IgG saporin showed more than 80% decrease of AChE activity in cholinergic target areas of the hippocampus and brain cortex. The results are compatible with a combination of decreased AChE activity in degenerating subcortical cholinergic projections, and additional decreases in postsynaptic AChE gene expression. Together our data show that mAChR can activate transcription from the AChE promoter via increased synthesis of Egr-1. The results suggest a feedback mechanism by which the AChE gene is activated by cholinergic neurotransmission, possibly leading to increased formation of AChE protein and accelerated degradation of acetylcholine at cholinergic synapses. This possibility suggests testing of cholinomimetic compounds currently in development for the treatment of Alzheimer's disease for their potential ability to increase AChE gene expression.

Position effect variegations and brain-specific silencing in transgenic mice overexpressing human acetylcholinesterase variants

Position effect variegations as well as brain-specific silencing were observed in novel transgenic mouse pedigrees expressing human acetylcholinesterase (AChE) variants. Muscle AChE activities varied between 1.6- and 350-fold of control in these lines, one carrying insertion-inactivated InE6-AChE and two with 'readthrough' I4/E5 AChE, all under control of the ubiquitous CMV promoter. In contrast, brain AChE levels remained within a range of 1.5-fold over control, suggesting an upper limit of brain AChE which is compatible with life.

Acute stress facilitates long-lasting changes in cholinergic gene expression

Acute traumatic stress may lead to post-traumatic stress disorder (PTSD), which is characterized by delayed neuropsychiatric symptoms including depression, irritability, and impaired cognitive performance. Curiously, inhibitors of the acetylcholine-hydrolysing enzyme acetylcholinesterase may induce psychopathologies that are reminiscent of PTSD. It is unknown how a single stressful event mediates long-term neuronal plasticity. Moreover, no mechanism has been proposed to explain the convergent neuropsychological outcomes of stress and of acetylcholinesterase inhibition. However, acute stress elicits a transient increase in the amounts released of the neurotransmitter acetylcholine and a phase of enhanced neuronal excitability. Inhibitors of acetylcholinesterase also promote enhanced electrical brain activity, presumably by increasing the survival of acetylcholine at the synapse. Here we report that there is similar bidirectional modulation of genes that regulate acetylcholine availability after stress and blockade of acetylcholinesterase. These calcium-dependent changes in gene expression coincide with phases of rapid enhancement and delayed depression of neuronal excitability. Both of these phases are mediated by muscarinic acetylcholine receptors. Our results suggest a model in which robust cholinergic stimulation triggers rapid induction of the gene encoding the transcription factor c-Fos. This protein then mediates selective regulatory effects on the long-lasting activities of genes involved in acetylcholine metabolism.

Transgenic acetylcholinesterase induces enlargement of murine neuromuscular junctions but leaves spinal cord synapses intact

Acetylcholinesterase (AChE) produced by spinal cord motoneurons accumulates within axo-dendritic spinal cord synapses. It is also secreted from motoneuron cell bodies, through their axons, into the region of neuromuscular junctions, where it terminates cholinergic neurotransmission. Here we show that transgenic mice expressing human AChE in their spinal cord motoneurons display primarily normal axo-dendritic spinal cord cholinergic synapses in spite of the clear excess of transgenic over host AChE within these synapses. This is in contrast to our recent observation that a modest excess of AChE drastically affects the structure and long-term functioning of neuromuscular junctions in these mice although they express human AChE in their spinal cord, but not muscle. Enlarged muscle endplates with either exaggerated or drastically shortened post-synaptic folds then lead to a progressive neuromotor decline and massive amyotrophy (Andres et al., 1997). These findings demonstrate that excess neuronal AChE may cause distinct effects on spinal cord and neuromuscular synapses and attribute the late-onset neuromotor deterioration observed in AChE transgenic mice to neuromuscular junction abnormalities.