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

Analysis of tail coiling activity of zebrafish (Danio rerio) embryos allows for the differentiation of neurotoxicants with different modes of action

In (eco)toxicology, there is a critical need for efficient methods to evaluate the neurotoxic potential of environmental chemicals. Recent studies proposed analysis of early coiling activity in zebrafish embryos as a powerful tool for the identification of neurotoxic compounds. In order to demonstrate that the analysis of early tail movements of zebrafish embryos allows for the discrimination of neurotoxicants acting via different mechanisms, the present study investigated the effects of four different neurotoxicants on the embryogenesis (fish embryo toxicity test) and early tail coiling movements of zebrafish embryos. Cadmium predominantly increased the frequency of tail coiling at the late pharyngula stage. Dichlorvos delayed embryonic development and caused convulsive tail movements resulting in prolonged duration of tail coils. Embryos exposed to teratogenic concentrations of fluoxetine and citalopram displayed absence of spontaneous tail movements at 24 h post-fertilization. In contrast, a non-teratogenic test concentration of citalopram decreased coiling frequency at multiple time points. Results demonstrated that the analysis of tail coiling movements of zebrafish embryos has the potential to discriminate neurotoxic compounds with different primary modes of action. In addition, chemical-induced effects on coiling activity were shown to potentially overlap with effects on embryogenesis. Further studies are needed to clarify the interplay of unspecific developmental toxicity of neurotoxic chemicals and effects resulting from specific neurotoxic mechanisms.

Ascending midbrain dopaminergic axons require descending GAD65 axon fascicles for normal pathfinding

The Nigrostriatal pathway (NSP) is formed by dopaminergic axons that project from the ventral midbrain to the dorsolateral striatum as part of the medial forebrain bundle. Previous studies have implicated chemotropic proteins in the formation of the NSP during development but little is known of the role of substrate-anchored signals in this process. We observed in mouse and rat embryos that midbrain dopaminergic axons ascend in close apposition to descending GAD65-positive axon bundles throughout their trajectory to the striatum. To test whether such interaction is important for dopaminergic axon pathfinding, we analyzed transgenic mouse embryos in which the GAD65 axon bundle was reduced by the conditional expression of the diphtheria toxin. In these embryos we observed dopaminergic misprojection into the hypothalamic region and abnormal projection in the striatum. In addition, analysis of Robo1/2 and Slit1/2 knockout embryos revealed that the previously described dopaminergic misprojection in these embryos is accompanied by severe alterations in the GAD65 axon scaffold. Additional studies with cultured dopaminergic neurons and whole embryos suggest that NCAM and Robo proteins are involved in the interaction of GAD65 and dopaminergic axons. These results indicate that the fasciculation between descending GAD65 axon bundles and ascending dopaminergic axons is required for the stereotypical NSP formation during brain development and that known guidance cues may determine this projection indirectly by instructing the pathfinding of the axons that are part of the GAD65 axon scaffold.

Developmental aspects of the cholinergic system

Beyond its importance in sustaining or modulating different aspects of the activity of the central nervous system (CNS), the cholinergic system plays important roles during development. In the current review, we focus on the developmental aspects associated with major components of the cholinergic system: Acetylcholine, choline acetyltransferase, vesicular acetylcholine transporter, high-affinity choline transporter, acetylcholinesterase, nicotinic and muscarinic receptors. We describe when and where each one of these components is first identified in the CNS and the changes in their levels that occur during the course of prenatal and postnatal development. We also describe how these components are relevant to many events that occur during the development of the CNS, including progenitor cells proliferation and differentiation, neurogenesis, gliogenesis, neuronal maturation and plasticity, axonal pathfinding, regulation of gene expression and cell survival. It will be noticed that evidence regarding the developmental aspects of the cholinergic system comes mostly from studies that used agonists, such as nicotine, and antagonists, such as hemicholinium-3. Studies using immunohistochemistry and genetically altered mice also provided valuable information.

The aryl acylamidase activity is much more sensitive to Alzheimer drugs than the esterase activity of acetylcholinesterase in chicken embryonic brain

The appearance of cholinergic trait often precedes synaptogenesis, indicating the involvement of cholinesterase proteins in nervous system development, particularly so acetylcholinesterase (AChE). In addition to AChE's acclaimed esterase activity, its lesser known non-cholinergic functions have gained much attention, because of AChE protein expression in areas other than cholinergic innervations; one such function could be exerted by its associated aryl acylamidase (AAA) activity. In this study, an attempt has been made in profiling esterase and AAA activities of AChE at different developmental stages of the chick embryo, e.g. at embryonic day 6 (E6), E9, E12, E15 and E18. AAA activity showed a correlated expression with esterase activity at all stages, but the relative ratios of AAA to esterase activity were higher at younger stages. The inhibition of AAA activity was shown to be more sensitive towards Huperzine, Donepezil whereas inhibition of esterase activity was sensitive to Tacrine and DFP. Remarkably, the major Alzheimer drugs- Huperzine and Donepezil, much more strongly inhibited AAA activity of AChE at younger developmental stages whose IC50 values are 0.01 muM and 0.1 muM respectively. In the case of BW284c51, inhibition was more pronounced at older stages and IC50 value was 0.1 muM. Since in Alzheimer's disease (AD), embryonic forms of AChE have been reported to reappear, a possible role of AAA activity in the pathogenesis of AD should be considered.

Exposure to Diazinon Alters In Vitro Retinogenesis: Retinospheroid Morphology, Development of Chicken Retinal Cell Types, and Gene Expression

Developing embryos are more vulnerable than adults to acute cholinergic intoxication by anticholinesterases, including organophosphorus pesticides. These agents affect the process of neural development itself, leading to permanent deficits in the architecture of the nervous system. Recent evidence on direct roles of acetylcholinesterase (AChE) on neuronal differentiation provides additional grounds for investigating the developmental toxicity of anticholinesterases. Therefore, the effect of the organophosphate diazinon on the development of chick retinal differentiation was studied by an in vitro reaggregate approach. Reaggregated spheres from dissociated retinal cells of the E6 chick embryo were produced in rotation culture. During the whole culture period of 10 days, experimental cultures were supplemented with different concentrations of the pesticide, from 20 to 120 microM diazinon. The pesticide-treated spheres were reduced in size, and their outer surface was irregular. More importantly, inner structural distortions could be easily traced because the structure of control spheroids can be well characterized by a histotypical arrangement of laminar parts homologous to the normal retina. Acetylcholinesterase activity in diazinon-treated spheres was reduced when compared with controls. As a dramatic effect of exposure to the pesticide, inner plexiform layer (IPL)-like areas in spheroids were not distinguishable anymore. Similarly, photoreceptor rosettes and Müller radial glia were strongly decreased, whereas apoptosis was stimulated. The expression of transcripts for choline-acetyltransferase and muscarinic receptors was affected, revealing an effect of diazinon on the cholinergic system. This further proves the significance of cholinesterases and the cholinergic system for proper nervous system development and shows that further studies of debilitating diazinon actions on development are necessary.

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.

Expression and putative role of lactoseries carbohydrates present on NCAM in the rat primary olfactory pathway

Primary olfactory neurons project axons from the olfactory neuroepithelium lining the nasal cavity to the olfactory bulb in the brain. These axons grow within large mixed bundles in the olfactory nerve and then sort out into homotypic fascicles in the nerve fiber layer of the olfactory bulb before terminating in topographically fixed glomeruli. Carbohydrates expressed on the cell surface have been implicated in axon sorting within the nerve fiber layer. We have identified two novel subpopulations of primary olfactory neurons that express distinct alpha-extended lactoseries carbohydrates recognised by monoclonal antibodies LA4 and KH10. Both carbohydrate epitopes are present on novel glycoforms of the neural cell adhesion molecule, which we have named NOC-7 and NOC-8. Primary axon fasciculation is disrupted in vitro when interactions between these cell surface lactoseries carbohydrates and their endogenous binding molecules are inhibited by the LA4 and KH10 antibodies or lactosamine sugars. We report the expression of multiple members of the lactoseries binding galectin family in the primary olfactory system. In particular, galectin-3 is expressed by ensheathing cells surrounding nerve fascicles in the submucosa and nerve fiber layer, where it may mediate cross-linking of axons. Galectin-4, -7, and -8 are expressed by the primary olfactory axons as they grow from the nasal cavity to the olfactory bulb. A putative role for NOC-7 and NOC-8 in axon fasciculation and the expression of multiple galectins in the developing olfactory nerve suggest that these molecules may be involved in the formation of this pathway, particularly in the sorting of axons as they converge towards their target.

Postnatal development of the basolateral complex of rabbit amygdala: a stereological and histochemical study

The aim of the study was to estimate developmental changes in the rabbit basolateral complex (BLC) by stereological and histochemical methods. Material consisted of 45 brains of New Zealand rabbits (aged from 2 to 180 days, P2 to P180) of both sexes, divided into nine groups. The following parameters were estimated: volume of the cerebral hemisphere; volume of the whole BLC and of particular BLC nuclei; neuronal density and total number of neurons in these nuclei. Developmental changes in acetylcholinesterase (AChE) activity in the BLC were also examined. The volume of the cerebral hemisphere increased until P30, whereas volumes of nuclei increased for longer--until P90. The density of neurons in all nuclei studied reached the level characteristic for an adult animal at about P30. The total number of neurons in the dorsolateral division of the lateral nucleus (Ldl) stabilized the earliest--between P30 and P60, whereas in the ventromedial division of the lateral nucleus (Lvm), basomedial (BM) and basolateral (BL) nuclei the number stabilized later--between P60 and P90. AChE activity appears minimal in the BLC on P2, reaches a maximum on P30 and then decreases to the level characteristic of an adult animal on P60. AChE activity was greater in BL than in other nuclei in all age groups. Reaching adult AChE activity 1 month earlier than the total number of neurons in the BLC may indicate a role of the cholinergic system in BLC maturation.

Higher expression of alpha7 nicotinic acetylcholine receptors in human fetal compared to adult brain

Neuronal nicotinic acetylcholine receptors are thought to be involved in regulation of several processes during neurogenesis of the brain. In this study the expression of the alpha7 nicotinic acetylcholine receptor subtype was investigated in human fetal (9-11 weeks of gestation), middle-aged (28-51 years) and aged (68-94 years) medulla oblongata, pons, frontal cortex, and cerebellum. The specific binding of the alpha7 receptor antagonist [(125)I]alpha-bungarotoxin was significantly higher in fetal than in both middle-aged and aged medulla oblongata and aged pons. No significant decrease in [(125)I]alpha-bungarotoxin binding sites was observed from fetal to adult cortex and cerebellum. The alpha7 mRNA expression was significantly higher in all fetal brain regions investigated, except for aged cortex, than in corresponding middle-aged and aged tissue. The high expression of alpha7 nicotinic acetylcholine receptors in fetal compared to adult brain supports the view that these receptors play an important role during brain development.

Acetylcholinesterase promotes neurite elongation, synapse formation, and surface expression of AMPA receptors in hippocampal neurones

Here we show that chronic application of low concentrations (0.01-0.05 U/ml) or a single application of 1-5 U/ml acetylcholinesterase (AChE) promotes the extension of neuronal processes, synapse formation, and alpha-amino-3-hydroxy-5-methylisoxazolepropionate receptor (AMPAR) surface expression in both embryonic and postnatal hippocampal cultures. The total number of AMPARs was unchanged but the proportion of receptors that were surface-expressed, predominantly at synapses, was approximately doubled following AChE treatment. Blockade of the peripheral anionic site of endogenous AChE in the cultures dramatically reduced neurite outgrowth but did not alter the appearance of synaptic markers SV2a and PSD95. These results indicate that AChE is necessary for normal dendrite and axon formation in hippocampal neurones and suggest that it may also play a role in excitatory synapse development, plasticity, and remodelling.

AMPA receptor potentiation by acetylcholinesterase is age-dependently upregulated at synaptogenesis sites of the rat brain

We have used radioligand binding to synaptic membranes from distinct rat brain regions and quantitative autoradiography to investigate the postnatal evolution of acetylcholinesterase (AChE)-evoked up-regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in CNS areas undergoing synaptogenesis. Incubation of synaptosomal membranes or brain sections with purified AChE caused a developmentally modulated enhancement in the binding of [3H]-(S)-AMPA and the specific AMPA receptor ligand [3H]-(S)-5-fluorowillardiine, but did not modify binding to kainate neither N-methyl-D-aspartate receptors. In all postnatal ages investigated (4, 7, 14, 20, 27, 40 days-old and adult rats), AChE effect on binding was concentration-dependent and blocked by propidium, BW 284c51, diisopropylfluorophosphonate and eserine, therefore requiring indemnity of both peripheral and active sites of the enzyme. AChE-mediated enhancement of [3H]-fluorowillardiine binding was measurable in all major CNS areas, but displayed remarkable anatomical selectivity and developmental regulation. Autoradiograph densitometry exhibited distinct temporal profiles and peaks of treated/control binding ratios for different cortices, cortical layers, and nuclei. Within the parietal, occipital and temporal neocortices, hippocampal CA1 field and cerebellum, AChE-potentiated binding ratios peaked in chronological correspondence with synaptogenesis periods of the respective AMPA-receptor containing targets. This modulation of AMPA receptors by AChE is a molecular mechanism able to transduce localized neural activity into durable modifications of synaptic molecular structure and function. It might also contribute to AChE-mediated neurotoxicity, as postulated in Alzheimer's disease and other CNS disorders.

Alternative acetylcholinesterase molecular forms exhibit similar ability to induce neurite outgrowth

Several groups have reported that acetylcholinesterase (AChE), through a mechanism not involving its catalytic activity, may have a role in fiber elongation. These observations were performed on experimental systems in which acetylcholine synthesis was active. Because neurite outgrowth can be modulated by neurotransmitters, we used the N18TG2 neuroblastoma line, which is defective for neurotransmitter production, to evaluate whether AChE may modulate neurite sprouting in nonenzymatic ways. To avoid the possibility that differences between transfected and mock-transfected clones may be due to the selection procedure, N18TG2 cells were previously subcloned, and the FB5 subclone was used for transfections. We performed transfections of FB5 cells with three distinct constructs encoding for the glycosylphosphoinositol-anchored AChE form, the tetrameric AChE form, and a soluble monomeric AChE form truncated in its C-terminus. A morphometric analysis of retinoic acid-differentiated clones was also undertaken. The results revealed that higher AChE expression following transfection brings about a greater ability of the clones to grow fibers with respect to nontransfected or mock-transfected cells irrespective of the used construct. Having observed no differences between the morphology of the transfected clones, we tested the possibility that the culture substrate can affect the capability of the clones to extend fibers. Also in this case we revealed no differences between the clones cultured on uncoated or collagen-pretreated dishes. These data indicate that alternative AChE molecular forms that differ in their C-teminal region exhibit similar ability to induce fiber outgrowth and suggest that the protein region responsible for this role is located in the invariant portion of the AChE molecule.

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

A non-cholinergic, trophic action of acetylcholinesterase on hippocampal neurones in vitro: molecular mechanisms

In this study neurite outgrowth from cultured hippocampal neurones was increased by addition of acetylcholinesterase acting in a non-cholinergic manner. Only monomeric acetylcholinesterase, a form of acetylcholinesterase dominant in development, increased neurite outgrowth (3-10 U/ml); moreover this effect was not blocked by active site blockers (echothiophate and galanthamine) but was sensitive to the addition of peripheral site blockers (fasciculin and BW284c51). It appears therefore that acetylcholinesterase has alternative, non-cholinergic functions, one of which could be in development, via a peripheral site. The possibility of a causal relationship between neurite outgrowth and calcium influx was explored using a spectrum of acetylcholinesterase variants, inhibitors and calcium channel blockers. Acetylcholinesterase regulation of outgrowth was shown to depend on an influx of extracellular calcium specifically via the L-type voltage-gated calcium channel. In summary, we propose that, independent of its catalytic activity, a selective form of acetylcholinesterase has a role in the development of hippocampal neurones via a selective voltage-gated calcium channel.

Development of the motor nervous system in ascidians

The motor nervous system of adult ascidians consists of neurons forming the cerebral ganglion from which axons run out directly to the effectors, i.e., muscular and ciliary cells. In this study, we analyzed the development of the motor fibers, correlating this with organ differentiation during asexual reproduction in Botryllus schlosseri. We used a staining method for acetylcholinesterase, whose reaction product is visible with both light and electron microscopy and which labels entire nerves, including their thin terminals, making them identifiable between tissues. While the cerebral ganglion is forming, the axons elongate and follow stereotypical pathways to reach the smooth muscle cells of the body, the striated muscle of the heart, and the ciliated cells of the branchial stigmata and the gut. A strict temporal relation links the development of the local neural network with its target organ, which is approached by nerves before the effector cells are fully differentiated. This process occurs for oral and cloacal siphons, branchial basket, gut, and heart. Axons grow through the extracellular matrix and arrive at their targets from different directions. In some cases, the blood sinuses constitute the favorite roads for growing axons, which seem to be guided by a mechanism involving contact guidance or stereotropism. The pattern of innervation undergoes dynamic rearrangements and a marked process of elimination of axons, when the last stages of blastogenesis occur. The final pattern of motor innervation seems to be regulated by axon withdrawal, rather than apoptosis of motor neurons.

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.

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.

Expression of cholinergic system molecules during development of the chick nervous system

There are suggestions of the participation of nicotinic acetylcholine receptors (nAChRs), the acetylcholine degradation enzyme, acetylcholinesterase (AChE), and the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), in the development of the nervous system. In this study, we aimed at comparing the development of some subunits of the nAChRs, AChE, and ChAT in the chick nervous system by standard immunohistochemical methods. The expression of all molecules investigated here appeared very early in ganglia (embryonic day 3.5-4), persisting into posthatching, except for ChAT, which is not detected after hatching in ganglia. A differential development was observed for nAChR subunits, with these receptors appearing around embryonic day 6 in some sites. The time-course of development of different nAChR subunits revealed several instances of transient expression (such as in the cerebellum), increasing expression (such as in the nucleus spiriformis lateralis), and diminishing expression into posthatching stages (such as in the oculomotor and throclear nuclei). Expression of AChE and ChAT also starts around embryonic day 6 in some structures and follows mainly increasing time-courses in the chick brain. The results of this study reveal a developmentally regulated expression of cholinergic system-related molecules in the chick nervous system and characterize differential time-courses of expression for nAChR subunits, AChE, and ChAT during development.