Human acetylcholinesterase binds to mouse laminin-1 and human collagen IV by an electrostatic mechanism at the peripheral anionic site

Non-neuronal Role of Acetylcholinesterase in Bone Development and Degeneration

Acetylcholinesterase (AChE), an enzyme catalyzing the degradation of acetylcholine, plays an important suppressive role in the cholinergic regulation by terminating the action of acetylcholine. The expression of acetylcholinesterase and other cholinergic components is not restricted to only brain and nerve tissues but can also be found in non-neuronal tissues like the immune system and bone tissue. Primary identification of these components has been achieved. However, the information about their specific functions and underlying molecular mechanisms in bone remains scattered. Here, the physiological process of bone development, homeostasis, and degeneration are introduced. Next, the cholinergic system and its expression in bone tissue is documented. Among them, special attention goes to AChE, as the structure of this enzyme suggests diverse binding affinities, enabled by a peripheral site and a catalytic site. The peripheral site supports the non-enzymatic function of AChE in non-neuronal systems. Based on recent studies, the non-neuronal roles of acetylcholinesterase, both enzymatically and non-enzymatically, in bone development, homeostasis and degeneration are summarized briefly together with potential mechanisms to support these functions. We conclude that AChE may be a potential therapeutic target for bone diseases like osteoporosis.

The multiple biological roles of the cholinesterases

It is tacitly assumed that the biological role of acetylcholinesterase is termination of synaptic transmission at cholinergic synapses. However, together with its structural homolog, butyrylcholinesterase, it is widely distributed both within and outside the nervous system, and, in many cases, the role of both enzymes remains obscure. The transient appearance of the cholinesterases in embryonic tissues is especially enigmatic. The two enzymes' extra-synaptic roles, which are known as 'non-classical' roles, are the topic of this review. Strong evidence has been presented that AChE and BChE play morphogenetic roles in a variety of eukaryotic systems, and they do so either by acting as adhesion proteins, or as trophic factors. As trophic factors, one mode of action is to directly regulate morphogenesis, such as neurite outgrowth, by poorly understood mechanisms. The other mode is by regulating levels of acetylcholine, which acts as the direct trophic factor. Alternate substrates have been sought for the cholinesterases. Quite recently, it was shown that levels of the aggression hormone, ghrelin, which also controls appetite, are regulated by butyrylcholinesterase. The rapid hydrolysis of acetylcholine by acetylcholinesterase generates high local proton concentrations. The possible biophysical and biological consequences of this effect are discussed. The biological significance of the acetylcholinesterases secreted by parasitic nematodes is reviewed, and, finally, the involvement of acetylcholinesterase in apoptosis is considered.

Acetylcholinesterase inhibition ameliorates retinal neovascularization and glial activation in oxygen-induced retinopathy

AIM

To investigate whether inhibition of acetylcholinesterase (AChE) by donepezil ameliorate aberrant retinal neovascularization (RNV) and abnormal glial activation in oxygen-induced retinopathy (OIR).

METHODS

A mouse model of RNV was induced in postnatal day 7 (P7) mice by exposure to 75% oxygen. Donepezil was administrated to P12 mice by intraperitoneal injection. Expression and localization of AChE in mouse retinas were determined by immunofluorescence. RNV was evaluated by paraffin sectioning and hematoxylin and eosin (HE) staining. Activation of retinal Müller glial cells were examined by immunoblot of glial fibrillary acidic protein (GFAP). rMC-1, a retinal Müller cell line, was used for in vitro study. Expression of hypoxia-induced factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF) were determined by Western-blot analysis, enzyme-linked immunosorbent assay (ELISA) or immunostaining.

RESULTS

Aberrant RNV and glial activation was observed after OIR. Of note, retinal AChE was mainly expressed by retinal Müller glial cells and markedly increased in OIR mice. Systemic administration of donepezil significantly reduced RNV and abnormal glial activation in mice with OIR. Moreover, ischemia-induced HIF-1α accumulation and VEGF upregulation in OIR mouse retinas and cultured rMC-1 were significantly inhibited by donepezil intervention.

CONCLUSION

AchE is implicated in RNV with OIR. Inhibition of AChE by donepeizl is likely to be a potential therapeutic approach for retinal neovascular diseases.

Solar light induces the release of acetylcholine from skin keratinocytes affecting melanogenesis

Cholinergic system conducts signal transmission in brain and muscle. Besides nervous system, the nonneuronal functions of cholinergic system have been proposed in various tissues. The expression of cholinergic proteins and release of acetylcholine in human skin have been reported, but its mechanism and influence on dermatological functions is not elucidated. Here, the expression profile of cholinergic markers was further investigated in skin and keratinocyte. The expression levels of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), vesicular acetylcholine transporter (VAChT), and synaptophysin, were upregulated during differentiation of keratinocytes. In cultured keratinocytes, a transient exposure of solar light induced the release of acetylcholine, which was mediated by intracellular Ca²⁺ mobilization. The light-induced acetylcholine release was mediated by the present of opsin. The light-induced melanogenesis was inhibited by acetylcholine or AChE inhibitor in melanocyte in vitro and mouse skin ex vivo. These results indicated that the potential role of cholinergic system could be a negative regulator in skin pigmentation.

Cholinergic control of bone development and beyond

There is ample evidence that cholinergic actions affect the health status of bones in vertebrates including man. Nicotine smoking, but also exposure to pesticides or medical drugs point to the significance of cholinergic effects on bone status, as reviewed here in Introduction. Then, we outline processes of endochondral ossification, and review respective cholinergic actions. In Results, we briefly summarize our in vivo and in vitro studies on bone development of chick and mouse [1,2], including (i) expressions of cholinergic components (AChE, BChE, ChAT) in chick embryo, (ii) characterisation of defects during skeletogenesis in prenatal ChE knockout mice, (iii) loss-of-function experiments with beads soaked in cholinergic components and implanted into chicken limb buds, and finally (iv) we use an in vitro mesenchymal 3D-micromass model that mimics cartilage and bone formation, which also had revealed complex crosstalks between cholinergic, radiation and inflammatory mechanisms [3]. In Discussion, we evaluate non-cholinergic actions of cholinesterases during bone formation by considering: (i) how cholinesterases could function in adhesive mechanisms; (ii) whether and how cholinesterases can form bone-regulatory complexes with alkaline phosphatase (ALP) and/or ECM components, which could regulate cell division, migration and adhesion. We conclude that cholinergic actions in bone development are driven mainly by classic cholinergic, but non-neural cycles (e.g., by acetylcholine); in addition, both cholinesterases can exert distinct ACh-independent roles. Considering their tremendous medical impact, these results bring forward novel research directions that deserve to be pursued.

Nanoporous gold electrode for ultrasensitive detection of neurotoxin fasciculin

Acetylcholinesterase (AChE), an efficient biocatalyst known to hydrolyze the neurotransmitter acetylcholine, could be inactivated in the presence of insecticides, nerve agents or other drug inhibitors to thus result in disrupted neurotransmission. Improvement in the peripheral cholinergic function, as well as overall cognition and neuronal functions of an exposed system could be achieved if the mechanisms of inhibitions are deactivated in a controlled fashion and with rapid response time. Herein, we proposed to develop a simple AChE biosensor capable to realize the rapid detection of neurotoxins. Our approach uses a nanoporous gold film (NPGF) and reduced graphene oxide-tin dioxide nanoparticle (RGO-SnO₂) nanocomposite to define the highly active electrode interface where the electrochemical monitoring of the interaction between AChE and its target molecule, fasciculin, could take place. Our results demonstrate that the established biosensor had the ability to monitor fasciculin concentrations at the ultra-low limit of detection of 8 pM, an inhibition rate of 8% and within only 30min of electrochemical exposure. Our study provides a convenient technology for the rapid and ultrasensitive detection of neurotoxins and has the potential for large applicability to other drugs or toxins screening.

Knockdown of Butyrylcholinesterase but Not Inhibition by Chlorpyrifos Alters Early Differentiation Mechanisms in Human Neural Stem Cells

Butyrylcholinesterase (BChE) is the evolutionary counterpart to acetylcholinesterase (AChE). Both are expressed early in nervous system development prior to cholinergic synapse formation. The organophosphate pesticide chlorpyrifos (CPF) primarily exerts toxicity through the inhibition of AChE, which results in excess cholinergic stimulation at the synapse. We hypothesized that the inhibition of AChE and BChE by CPF may impair early neurogenesis in neural stem cells (NSCs). To model neurodevelopment in vitro, we used human NSCs derived from induced pluripotent stem cells (iPSCs) with a focus on the initial differentiation mechanisms. Over the six days of NSC differentiation, the BChE activity and mRNA expression significantly increased, while the AChE activity and expression remained unchanged. The CPF treatment (10 μM) caused 82% and 92% inhibition of AChE and BChE, respectively. The CPF exposure had no effect on the cell viability or the expression of the differentiation markers HES5, DCX, or MAP2. However, the shRNA-knockdown of the BChE expression resulted in the decreased or delayed expression of the transcription factors HES5 and HES3. BChE may have a role in the differentiation of NSCs independent of, or in addition to, its enzymatic activity.

Active and peripheral anionic sites of acetylcholinesterase have differential modulation effects on cell proliferation, adhesion and neuritogenesis in the NG108-15 cell line

The classical enzymatic role of acetylcholinesterase (AChE) is to terminate impulse transmission at cholinergic synapses through rapid hydrolysis of acetylcholine (ACh). Inactivation of this enzyme's catalytic site is the primary mechanism of acute toxicity of OP insecticides (e.g. parathion, chlorpyrifos). There is now sufficient evidence to suggest that AChE has a neurotrophic function that may be altered by organophosphate (OP) exposure, resulting in defects of neuronal growth and development, though the clarification of the mechanisms involved require further in vitro investigation. In the present study, the mouse neuroblastoma×rat glioma hybrid NG108-15 cell line was used to investigate the differential effects between inhibition of the catalytic site and peripheral anionic site (PAS) of acetylcholinesterase (AChE) on cell adhesion, proliferation and neuritogenesis, in the presence and absence of human red blood cell (hRBC) AChE (ED3.1.1.7). AChE active-site inhibitor paraoxon (PO; 0.1-1.0μM), when added to NG108-15 cells grown on AChE-coated plates, had no effect on cell proliferation, but exerted a significant reduction in strongly adherent viable cells accompanied by mostly short process formations, with 18% of cells considered to be neuritogenic, similar to that observed on uncoated plates. In contrast, PO had no significant effect on cell adhesion and proliferation of NG108-15 cells on uncoated plates. The PAS-ligand thioflavin-T (Th-T; 0.5-25μM), however, decreased cell adhesion and proliferation, on both uncoated and ACh-E coated plates, with less magnitude on AChE-coated plates. Taken together, these results suggest that strong cell adherence and neuritogenesis are sensitive to PO in this cell culture model, with no impact on proliferation, in the presence of membrane bound AChE-coating, while there is no sensitivity to PO on uncoated plates. On the other hand, binding of Th-T directly to the PAS affects both cell adherence and proliferation, with less magnitude in the presence of membrane-bound AChE. The current study indicates that PO is deleterious in neural development during critical periods of strong cell adhesion and differentiation, interfering with AChE trophic function.

Interaction of acetylcholinesterase with neurexin-1β regulates glutamatergic synaptic stability in hippocampal neurons

Background

Excess expression of acetylcholinesterase (AChE) in the cortex and hippocampus causes a decrease in the number of glutamatergic synapses and alters the expression of neurexin and neuroligin, trans-synaptic proteins that control synaptic stability. The molecular sequence and three-dimensional structure of AChE are homologous to the corresponding aspects of the ectodomain of neuroligin. This study investigated whether excess AChE interacts physically with neurexin to destabilize glutamatergic synapses.

Results

The results showed that AChE clusters colocalized with neurexin assemblies in the neurites of hippocampal neurons and that AChE co-immunoprecipitated with neurexin from the lysate of these neurons. Moreover, when expressed in human embryonic kidney 293 cells, N-glycosylated AChE co-immunoprecipitated with non-O–glycosylated neurexin-1β, with N-glycosylation of the AChE being required for this co-precipitation to occur. Increasing extracellular AChE decreased the association of neurexin with neuroligin and inhibited neuroligin-induced synaptogenesis. The number and activity of excitatory synapses in cultured hippocampal neurons were reduced by extracellular catalytically inactive AChE.

Conclusions

Excessive glycosylated AChE could competitively disrupt a subset of the neurexin–neuroligin junctions consequently impairing the integrity of glutamatergic synapses. This might serve a molecular mechanism of excessive AChE induced neurodegeneration.

The Leu-Arg-Glu (LRE) adhesion motif in proteins of the neuromuscular junction with special reference to proteins of the carboxylesterase/cholinesterase family

Short linear motifs confer evolutionary flexibility on proteins as they can be added with relative ease allowing the acquisition of new functions. Such motifs may mediate a variety of signalling functions. The adhesion-mediating Leu-Arg-Glu (LRE) motif is enriched in laminin beta 2, and has been observed in other proteins, including members of the carboxylesterase/cholinesterase family. It acts as a stop signal for growing axons in the developing neuromuscular junction, binding to the voltage-gated calcium channel. In this bioinformatic analysis, we have investigated the presence of the motif in proteins of the neuromuscular junction, and have also examined its structural position and potential for ligand interaction, as well as phylogenetic conservation, in the carboxylesterase/cholinesterase family. The motif was observed to occur with a significantly higher frequency than expected in the UniProt/Swiss-Prot database, as well as in four individual species (human, mouse, Caenorhabditis elegans and Drosophila melanogaster). Examination of its presence in neuromuscular junction proteins showed it to be enriched in certain proteins of the synaptic basement membrane, including laminin, agrin, acetylcholinesterase and tenascin. A highly significant enrichment was observed in cytoskeletal proteins, particularly intermediate filament proteins and members of the spectrin family. In the carboxylesterase/cholinesterase family, the motif was observed in four conserved positions in the protein structure. It is present in the majority of mammalian acetylcholinesterases, as well as acetylcholinesterases from electric fish and a number of invertebrates. In insects, it is present in the ace-2, rather than in the synaptic ace-1, enzyme. It is also observed in the cholinesterase-like adhesion molecules (neuroligins, neurotactin and glutactin). It is never seen in butyrylcholinesterases, which do not mediate cell adhesion. In conclusion, the significant enrichment of the motif in certain classes of protein, as well as its conserved presence and structural positioning in one protein family, suggests that it has specific functions both in cell adhesion in the neuromuscular junction and in maintaining the structural integrity of the cytoskeleton.

Mouse acetylcholinesterase enhances neurite outgrowth of rat R28 cells through interaction with laminin-1

The enzyme acetylcholinesterase (AChE) terminates synaptic transmission at cholinergic synapses by hydrolyzing the neurotransmitter acetylcholine, but can also exert 'non-classical', morpho-regulatory effects on developing neurons such as stimulation of neurite outgrowth. Here, we investigated the role of AChE binding to laminin-1 on the regulation of neurite outgrowth by using cell culture, immunocytochemistry, and molecular biological approaches. To explore the role of AChE, we examined fiber growth of cells overexpressing different forms of AChE, and/or during their growth on laminin-1. A significant increase of neuritic growth as compared with controls was observed for neurons over-expressing AChE. Accordingly, addition of globular AChE to the medium increased total length of neurites. Co-transfection with PRIMA, a membrane anchor of AChE, led to an increase in fiber length similar to AChE overexpressing cells. Transfection with an AChE mutant that leads to the retention of AChE within cells had no stimulatory effect on neurite length. Noticeably, the longest neurites were produced by neurons overexpressing AChE and growing on laminin-1, suggesting that the AChE/laminin interaction is involved in regulating neurite outgrowth. Our findings demonstrate that binding of AChE to laminin-1 alters AChE activity and leads to increased neurite growth in culture. A possible mechanism of the AChE effect on neurite outgrowth is proposed due to the interaction of AChE with laminin-1.

Haemophilus influenzae protein E binds to the extracellular matrix by concurrently interacting with laminin and vitronectin

Nontypeable Haemophilus influenzae (NTHi) causes otitis media and is commonly found in patients with chronic obstructive pulmonary disease (COPD). Adhesins are important for bacterial attachment and colonization. Protein E (PE) is a recently characterized ubiquitous 16 kDa adhesin with vitronectin-binding capacity that results in increased survival in serum. In addition to PE, NTHi utilizes Haemophilus adhesion protein (Hap) that binds to the basement-membrane glycoprotein laminin. We show that most clinical isolates bind laminin and that both Hap and PE are crucial for the NTHi-dependent interaction with laminin as revealed with different mutants. The laminin-binding region is located at the N-terminus of PE, and PE binds to the heparin-binding C-terminal globular domain of laminin. PE simultaneously attracts vitronectin and laminin at separate binding sites, proving the multifunctional nature of the adhesin. This previously unknown PE-dependent interaction with laminin may contribute to NTHi colonization, particularly in smokers with COPD.

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.

RanBPM is an acetylcholinesterase-interacting protein that translocates into the nucleus during apoptosis

Acetylcholinesterase (AChE) expression may be induced during apoptosis in various cell types. Here, we used the C-terminal of AChE to screen the human fetal brain library and found that it interacted with Ran-binding protein in the microtubule-organizing center (RanBPM). This interaction was further confirmed by coimmunoprecipitation analysis. In HEK293T cells, RanBPM and AChE were heterogeneously expressed in the cisplatin-untreated cytoplasmic extracts and in the cisplatin-treated cytoplasmic or nuclear extracts. Our previous studies performed using morphologic methods have shown that AChE translocates from the cytoplasm to the nucleus during apoptosis. Taken together, these results suggest that RanBPM is an AChE-interacting protein that is translocated from the cytoplasm into the nucleus during apoptosis, similar to the translocation observed in case of AChE.

Nonenzymatic role of acetylcholinesterase in neuritic sprouting: regional changes in acetylcholinesterase and choline acetyltransferase after neonatal 6-hydroxydopamine lesions

Acetylcholinesterase (AChE) is postulated to play a nonenzymatic role in the development of neuritic projections. We gave the specific neurotoxin, 6-OHDA to rats on postnatal day (PN) 1, a treatment that destroys noradrenergic nerve terminals in the forebrain while producing reactive sprouting in the brainstem. AChE showed profound decreases in the forebrain that persisted in males over the entire phase of major synaptogenesis, from PN4 through PN21; in the brainstem, AChE was increased. Parallel examinations of choline acetyltransferase, an enzymatic marker for cholinergic nerve terminals, showed a different pattern of 6-OHDA-induced alterations, with initial decreases in both forebrain and brainstem in males and regression toward normal by PN21; females were far less affected. The sex differences are in accord with the greater plasticity of the female brain and its more rapid recovery from neurotoxic injury; our findings indicate that these differences are present well before puberty. These results support the view that AChE is involved in neurite formation, unrelated to its enzymatic role in cholinergic neurotransmission. Further, the results for choline acetyltransferase indicate that early depletion of norepinephrine compromises development of acetylcholine systems, consistent with a trophic role for this neurotransmitter.

Interaction of acetylcholinesterase with the G4 domain of the laminin alpha1-chain

Although the primary function of AChE (acetylcholinesterase) is the synaptic hydrolysis of acetylcholine, it appears that the protein is also able to promote various non-cholinergic activities, including cell adhesion, neurite outgrowth and amyloidosis. We have observed previously that AChE is able to bind to mouse laminin-111 in vitro by an electrostatic mechanism. We have also observed that certain mAbs (monoclonal antibodies) recognizing AChE's PAS (peripheral anionic site) inhibit both laminin binding and cell adhesion in neuroblastoma cells. Here, we investigated the interaction sites of the two molecules, using docking, synthetic peptides, ELISAs and conformational interaction site mapping. Mouse AChE was observed on docking to bind to a discontinuous, largely basic, structure, Val(2718)-Arg-Lys-Arg-Leu(2722), Tyr(2738)-Tyr(2739), Tyr(2789)-Ile-Lys-Arg-Lys(2793) and Val(2817)-Glu-Arg-Lys(2820), on the mouse laminin alpha1 G4 domain. ELISAs using synthetic peptides confirmed the involvement of the AG-73 site (2719-2729). This site overlaps extensively with laminin's heparin-binding site, and AChE was observed to compete with heparan sulfate for laminin binding. Docking showed the major component of the interaction site on AChE to be the acidic sequence Arg(90)-Glu-Leu-Ser-Glu-Asp(95) on the omega loop, and also the involvement of Pro(40)-Pro-Val(42), Arg(46) (linked to Glu(94) by a salt bridge) and the hexapeptide Asp(61)-Ala-Thr-Thr-Phe-Gln(66). Epitope analysis, using CLiPS technology, of seven adhesion-inhibiting mAbs (three anti-human AChE, one anti-Torpedo AChE and three anti-human anti-anti-idiotypic antibodies) showed their major recognition site to be the sequence Pro(40)-Pro-Met-Gly-Pro-Arg-Arg-Phe(48) (AChE human sequence). The antibodies, however, also reacted with the proline-containing sequences Pro(78)-Gly-Phe-Glu-Gly-Thr-Glu(84) and Pro(88)-Asn-Arg-Glu-Leu-Ser-Glu-Asp(95). Antibodies that recognized other features of the PAS area but not the Arg(90)-Gly-Leu-Ser-Glu-Asp(95) motif interfered neither with laminin binding nor with cell adhesion. These results define sites for the interaction of AChE and laminin and suggest that the interaction plays a role in cell adhesion. They also suggest the strong probability of functional redundancy between AChE and other molecules in early development, particularly heparan sulfate proteoglycans, which may explain the survival of the AChE-knockout mouse.

Morphoregulation by acetylcholinesterase in fibroblasts and astrocytes

Acetylcholinesterase (AChE) terminates neurotransmission at cholinergic synapses by hydrolysing acetylcholine, but also has non-enzymatic morphoregulatory effects on neurons such as stimulation of neurite outgrowth. It is widely expressed outside the nervous system, but its function in non-neuronal cells is unclear. Here we have investigated the distribution and function of AChE in fibroblasts and astrocytes. We show that these cells express high levels of AChE protein that co-migrates with recombinant AChE but contains little catalytic activity. Fibroblasts express transcripts encoding the synaptic AChE-T isoform and its membrane anchoring peptide PRiMA-I. AChE is strikingly distributed in arcs, rings and patches at the leading edge of spreading and migrating fibroblasts and astrocytes, close to the cell-substratum interface, and in neuronal growth cones. During in vivo healing of mouse skin, AChE becomes highly expressed in re-epithelialising epidermal keratinocytes 1 day after wounding. AChE appears to be functionally important for polarised cell migration, since an AChE antibody reduces substratum adhesion of fibroblasts, and slows wound healing in vitro as effectively as a beta1-integrin antibody. Moreover, elevation of AChE expression increases fibroblast wound healing independently of catalytic activity. Interestingly, AChE surface patches precisely co-localise with amyloid precursor protein and the extracellular matrix protein perlecan, but not focal adhesions or alpha-dystroglycan, and contain a high concentration of tyrosine phosphorylated proteins in spreading cells. These findings suggest that cell surface AChE, possibly in a novel signalling complex containing APP and perlecan, contributes to a generalised mechanism for polarised membrane protrusion and migration in all adherent cells.

Acetylcholinesterase readthrough peptide shares sequence similarity to the 28-53 peptide sequence of the acetylcholinesterase adhesion-mediating site and competes for ligand binding in vitro

It has been reported that unlike the more commonly expressed splice variants, the embryonic and stress-associated readthrough form of acetylcholinesterase (AChE-R) is unable to promote cell adhesion and neurite outgrowth. We investigated the possibility that the unique AChE-R C-terminal peptide (ARP) might be responsible for this difference, either by binding to AChE itself and inactivating the adhesion-mediating site or by competing with AChE for ligand binding. Synthetic peptides representing the ARP, a scrambled version of the ARP, and sequences of the previously identified adhesion-mediating site on AChE were used in in vitro binding and neuroblastoma cell-spreading assays. It was observed that the ARP was able to bind to laminin-1, identified previously as an in vitro AChE ligand and, to a lesser extent, to collagen IV and to AChE itself. ARP-AChE binding was, however, of very low affinity and was not significantly affected by peripheral site inhibitors, suggesting that inactivation of the AChE adhesion site is not the reason for AChE-R's antiadhesive character. On the other hand, the ARP competed with AChE and the adhesion site peptides for binding to laminin in vitro, and the ARP was observed to inhibit cell spreading in neuroblastoma cells grown on laminin. Monoclonal antibodies recognizing the known AChE adhesion site reacted with the ARP, suggesting structural similarities. These were borne out by an examination of sequence alignments of the ARP and the 28-53 AChE sequence. The ARP contains part of the PPxxxxRFxPPEP motif seen in AChEs and cholinesterase-domain proteins, and both it and the 37-53 sequence bear some resemblance to collagen and collagen-like proteins. It therefore appears likely that the ARP's structural similarity to the AChE adhesion-mediating site is the basis for the observed competition for ligand binding and might account for the antiadhesive characteristics of AChE-R.

Acetylcholinesterase in cell adhesion, neurite growth and network formation

The expression of acetylcholinesterase is not restricted to cholinergically innervated tissues and relates to both neurotransmission and multiple biological aspects, including neural development, stress response and neurodegenerative diseases. Therefore, the classical function of acetylcholinesterase has to be distinguished from its non-classical, e.g. enzymatic from non-enzymatic, functions. Here, the roles of acetylcholinesterase in cell adhesion, promoting neurite outgrowth and neural network formation are reviewed briefly, together with potential mechanisms to support these functions. Part of these functions may depend on the structural properties of acetylcholinesterase, for example, protein-protein interactions. Recent findings have revealed that laminin-1 is an interaction partner for acetylcholinesterase. The binding of acetylcholinesterase to this extracellular matrix component may allow cell-to-cell recognition, and also cell signalling via membrane receptors. Studies using monolayer and 3D spheroid retinal cultures, as well as the acetylcholinesterase-knockout mouse, have been instrumental in elaborating the non-classical functions of acetylcholinesterase.

Acetylcholinesterase modulates neurite outgrowth on fibronectin

Acetylcholinesterase (AChE) has been reported to be involved in the modulation of neurite outgrowth. To understand the role played by different domains, we transfected neuroblastoma cells with three constructs containing the invariant region of AChE, differing in the exon encoding the C-terminus and therefore in AChE cellular fate and localization. All isoforms increased neurite extension, suggesting the involvement of the invariant domain [A. De Jaco, G. Augusti-Tocco, S. Biagioni, Alternative AChE molecular forms exhibit similar ability to induce neurite outgrowth, J. Neurosci. Res. 70 (2002) 756-765]. The peripheral anionic site (PAS) is encoded by invariant exons and represents the domain involved in non-cholinergic functions of AChE. Masking of PAS with fasciculin results in a significant decrease of neurite outgrowth in all clones overexpressing AChE. A strong reduction was also observed when clones were cultured on fibronectin. Treatment of clones with fasciculin, therefore masking PAS, abolished the fibronectin-induced reduction. The inhibition of the catalytic site cannot revert the fibronectin effect. Finally, when clones were cultured on fibronectin in the presence of heparin, a ligand of fibronectin, the inhibitory effect was completely reversed. Our results indicate that PAS could directly or indirectly mediate AChE/fibronectin interactions.

Nonenzymatic functions of acetylcholinesterase splice variants in the developmental neurotoxicity of organophosphates: chlorpyrifos, chlorpyrifos oxon, and diazinon

BACKGROUND

Organophosphate pesticides affect mammalian brain development through mechanisms separable from the inhibition of acetylcholinesterase (AChE) enzymatic activity and resultant cholinergic hyperstimulation. In the brain, AChE has two catalytically similar splice variants with distinct functions in development and repair. The rare, read-through isoform, AChE-R, is preferentially induced by injury and appears to promote repair and protect against neurodegeneration. Overexpression of the more abundant, synaptic isoform, AChE-S, enhances neurotoxicity.

OBJECTIVES

We exposed differentiating PC12 cells, a model for developing neurons, to 30 microM chlorpyrifos (CPF) or diazinon (DZN), or CPF oxon, the active metabolite that irreversibly inhibits AChE enzymatic activity, in order to determine whether they differentially induce the formation of AChE-S as a mechanistic predictor of developmental neurotoxicity. We then administered CPF or DZN to neonatal rats on postnatal days 1-4 using daily doses spanning the threshold for AChE inhibition (0-20%); we then evaluated AChE gene expression in forebrain and brainstem on post-natal day 5.

RESULTS

In PC12 cells, after 48 hr of exposure, CPF, CPF oxon, and DZN enhanced gene expression for AChE-R by about 20%, whereas CPF and DZN, but not CPF oxon, increased AChE-S expression by 20-40%. Thus, despite the fact that CPF oxon is a much more potent AChE inhibitor, it is the native compound (CPF) that induces expression of the neurotoxic AChE-S isoform. For in vivo exposures, 1 mg/kg CPF had little or no effect, but 0.5 or 2 mg/kg DZN induced both AChE-R and AChE-S, with a greater effect in males.

CONCLUSIONS

Our results indicate that nonenzymatic functions of AChE variants may participate in and be predictive of the relative developmental neurotoxicity of organophosphates, and that the various organophosphates differ in the degree to which they activate this mechanism.

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.

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.

Determinants of functionality in the ubiquitin conjugating enzyme family

The E2 enzymes are key enzymes in the ubiquitin and ubiquitin-like protein ligation pathways. To understand the functionality of the different E2 enzymes, we analyzed 190 protein sequences and 211 structures and electrostatic potentials. Key findings include: The ScUbc1 orthologs are defined by a C-terminal UBA domain. An N-terminal sequence motif that is highly conserved in all E2s except for Cdc34 orthologs is important for the stabilization of the L7 loop and is likely to be involved in E1 binding. ScUbc11p has a different electrostatic potential from E2-Cp and other proteins with which it has high sequence similarity but different functionality. All the E2s known to ubiquitinate histones have a negative potential. The members of the NCUBE family have a positive electrostatic potential, although its form is different from that of the SUMO conjugating E2s. The specificities of only the ScUbc4/Ubc5 and ScUbc1p orthologs are reflected in their L4 and L7 loops.

Acetylcholinesterase in Hirschsprung's disease

The association between the congenital absence of colonic ganglion cells and an increased acetylcholinesterase (AChE) expression in the affected tissue is of diagnostic importance in Hirschsprung's disease (HSCR). Investigation of AChE's function in development may also help unravel some of the complex pathophysiology in HSCR. Normal nerves do not stain for AChE, but increased AChE expression is associated with the hypertrophied extrinsic nerve fibres of the aganglionic segment in HSCR. Although a high degree of histochemical diagnostic accuracy exists, results are not always uniform, and false positives and false negatives are reported. False negative results are primarily related to age, and an absence of AChE reaction does not exclude HSCR in neonates within the first 3 weeks after birth. AChE staining results may lack uniformity, resulting in a number of technical modifications that have been made to improve standardization of AChE staining. At least two distinct histological patterns are described, types A and B. The interpretation of increased AChE staining patterns in ganglionated bowel at the time of surgical pull-through remains a problem in patients with HSCR. The development of rapid staining techniques has helped to identify normal ganglionated bowel with greater certainty. The presence of fine AChE neurofibrils in the ganglionated segment has contributed to the debate surrounding intestinal neuronal dysplasia. Quantitative assay of cholinesterase activity confirms the pattern of histochemical staining. AChE is particularly increased in relation to butrylcholinesterase, with one molecular form, the G4 tetrameric form, predominating. It is likely that the raised levels of AChE in aganglionic tissue are the transcriptional consequence of the abnormalities in signalling molecules that characterize HSCR. Evidence suggests that this AChE is functioning in a nonenzymatic capacity to promote cell adhesion and differentiation and that the hypertrophied nerves and neurofibrils may be the result of this increased AChE expression.

Characterization of acetylcholinesterase expression and secretion during osteoblast differentiation

Although best known for its role in cholinergic signalling, a substantial body of evidence suggests that acetylcholinesterase (AChE) has multiple biological functions. Previously, we and others identified AChE expression in areas of bone that lacked expression of other neuronal proteins. More specifically, we identified AChE expression at sites of new bone formation suggesting a role for AChE as a bone matrix protein. We have now characterised AChE expression, secretion and adhesive function in osteoblasts. Using Western blot analysis, we identified expression of two AChE species in osteoblastic cells, a major species of 68 kDa and less abundant species of approximately 55 kDa. AChE colocalised with the Golgi apparatus in osteoblastic cells and was identified in osteoblast-conditioned medium. Further analyses revealed differentiation-dependent secretion by osteoblasts, with AChE secretion levels corresponding with alkaline phosphatase activity. AChE expression by osteoblastic cells was also found to be regulated by mechanical strain both in vitro and in vivo. Finally, we investigated the possibility of a functional role for AChE in osteoblast adhesion. Using specific inhibitors, blockade of sites thought to be responsible for AChE adhesive properties caused a concentration-dependent decrease in osteoblastic cell adhesion, suggesting that AChE is involved in regulating cell-matrix interactions in bone.

Mouse acetylcholinesterase interacts in yeast with the extracellular matrix component laminin-1β

Acetylcholinesterase (AChE) is likely to have roles other than the hydrolysis of acetylcholine, e.g., related to developmental processes like neurite outgrowth, differentiation and adhesion. Here, we investigated whether AChE can function as a heterophilic cell adhesion molecule and searched for proteins interacting with it. Using the yeast two-hybrid method and a mouse brain cDNA library, we have identified an interaction between a partial cDNA encoding the globular domain IV of laminin chain beta1 and the amino acids 240-503 of mouse AChE. Biochemical co-immunoprecipitation assays confirmed the genetic results. We suggest that AChE, by interacting with laminin-1, is able to exert changes in adhesion signaling pathways.

Identification of a structural site on acetylcholinesterase that promotes neurite outgrowth and binds laminin-1 and collagen IV

The cell adhesion and neurite outgrowth-promoting function of acetylcholinesterase has been localised to the area of the peripheral anionic site. In order to precisely determine the site involved, we used synthetic peptides representing sequences of the peripheral anionic site and its surrounds, and investigated their binding to a panel of monoclonal antibodies that inhibit cell adhesion/neurite outgrowth and/or to recognise the peripheral anionic site. Binding to laminin-1 and collagen IV was also investigated. A relationship between recognition of the sequence 37-50, representing a surface loop adjacent to the peripheral anionic site, and the degree of inhibition of cell adhesion was observed; both laminin-1 and collagen IV also bound this loop with high affinity. Neurite outgrowth on coverslips coated with this peptide was similar to those coated with acetylcholinesterase itself. Adhesion-inhibiting antibodies also recognised the omega loop 69-96, as did laminin-1 and collagen IV. Laminin also bound the sequences 55-66 and 340-353, recognised by the antibodies to varying degrees, but collagen did not. All these peptides were able to promote neurite outgrowth to some degree. No binding to the amyloid-binding omega loop 275-304 by the ligands was observed, nor did the antibodies recognise this consistently. No relationship was observed between the degree of inhibition of acetylcholinesterase and inhibition of neurite outgrowth by the antibodies from which we conclude that the neurite outgrowth function is non-cholinergic. In conclusion, we have identified a specific conformational structure on acetylcholinesterase, comprising adjacent surface loops between residues 37-50 and 69-96, with additional involvement of the sequences 55-66 and 340-353, that mediates cell adhesion and neurite outgrowth.

Functional idiotypic mimicry of an adhesion- and differentiation-promoting site on acetylcholinesterase

Acetylcholinesterase mediates cell adhesion and neurite outgrowth through a site associated with the peripheral anionic site (PAS). Monoclonal antibodies raised to this site block cell adhesion. We have raised anti-idiotypic antibodies to one of these antibodies. The anti-idiotypic antibodies recognized the immunogenic antibody and non-specific mouse IgG, but not acetylcholinesterase. Five antibodies (out of 143 clones, an incidence of 3.5%) were able to promote neurite outgrowth in human neuroblastoma cells in vitro in a similar manner to acetylcholinesterase itself, suggesting that these antibodies carry an internal image of the neuritogenic site. Two of the antibodies were significantly more effective (P < 0.01) than acetylcholinesterase in this regard. The antibodies also bound specifically to mouse laminin-1 and human collagen IV, as does acetylcholinesterase. This binding was displaced by unlabelled antibody, as well as by acetylcholinesterase itself, indicating competition with acetylcholinesterase. We have also investigated the development of anti-anti-idiotypic antibodies in mice in vivo, and have observed that four of these (out of 318 clones, an incidence of 1.26%) mimic the idiotypic antibody and abrogate adhesion in neuroblastoma cells. We have thus demonstrated functional mimicry of the neuritogenic site on acetylcholinesterase in anti-idiotypic antibodies, enhancement of this activity in one antibody, and mimicry of the idiotypic antibody site in anti-anti-idiotypic antibodies. Implications of these findings for differentiation-promoting cancer therapy are discussed.

Making connections: cholinesterase-domain proteins in the CNS

Recent studies have highlighted novel functions of a group of cell adhesion molecules during nervous system development. Members of this protein family are characterized by an extracellular domain with sequence homology to cholinesterases and include the neuroligins, synaptic cell adhesion molecules recently implicated in autism, and neurotactin, a cell surface receptor involved in axonal pathfinding. Although these proteins have a structural organization similar to the enzyme acetylcholinesterase, the cholinesterase domain lacks enzymatic activity and functions as a protein-protein interaction motif. This protein family provides a striking example of how the function of a catalytically active domain has evolved to mediate receptor-ligand interactions that regulate morphogenetic processes during development of the nervous system.