Inhibition of neurite outgrowth from chick sympathetic neurons by cholinesterase inhibitors is not mediated by binding to cholinesterases

The Antidiabetic Drug Liraglutide Minimizes the Non-Cholinergic Neurotoxicity of the Pesticide Mipafox in SH-SY5Y Cells

Organophosphorus (OPs) compounds have been widely used in agriculture, industry, and household, and the neurotoxicity induced by them is still a cause of concern. The main toxic mechanism of OPs is the inhibition of acetylcholinesterase (AChE); however, the delayed neuropathy induced by OPs (OPIDN) is mediated by other mechanisms such as the irreversible inhibition of 70% of NTE activity (neuropathy target esterase) that leads to axonal degeneration. Liraglutide is a long-lasting GLP-1 analog clinically used as antidiabetic. Its neurotrophic and neuroprotective effects have been demonstrated in vitro and in experimental models of neurodegenerative diseases. As in OPIDN, axonal degeneration also plays a role in neurodegenerative diseases. Therefore, this study investigated the protective potential of liraglutide against the neurotoxicity of OPs by using mipafox as a neuropathic agent (at a concentration able to inhibit and age 70% of NTE activity) and a neuronal model with SH-SY5Y neuroblastoma cells, which express both esterases. Liraglutide protected cells against the neurotoxicity of mipafox by increasing neuritogenesis, the uptake of glucose, the levels of cytoskeleton proteins, and synaptic-plasticity modulators, besides decreasing the pro-inflammatory cytokine interleukin 1β and caspase-3 activity. This is the first study to suggest that liraglutide might induce beneficial effects against the delayed, non-cholinergic neurotoxicity of OPs.

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.

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

Acetylcholinesterase (EC 3.1.1.7; AChE) is known to induce neurite outgrowth and differentiation, but its ligands are as yet unknown. Laminin-1 and collagen IV were investigated as potential ligands for AChE. We observed specific saturable binding of biotinylated human AChE to mouse laminin and human collagen, with K(d) values of 4.9482 nM (SE 0.3145 nM) and 1.1617 nM (SE 0.1921 nM) respectively. Peripheral anionic site inhibitors (fasciculin, BW284c51, propidium and gallamine) also significantly reduced binding with fasciculin being the most effective. Significant reductions in AChE-laminin and AChE-collagen interactions were produced by a monoclonal anti-AChE antibody known to react with the peripheral anionic site, and a partial reduction with an antibody that partially recognises the site. Self-association of AChE was also observed (K(d)=16.3235 nM; SE 5.8120 nM); increasing markedly at low pH, but not significantly affected by either inhibitors or antibodies, suggesting a non-specific aggregation phenomenon. Binding to laminin and collagen was significantly reduced by increasing ionic strength and decreasing pH, indicating a dominant role for electrostatic interactions, and suggesting that the site may be different from the hydrophobic site identified for the AChE-amyloid interaction.

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.

Modulation of acetylcholinesterase and voltage-gated Na(+) channels in choline acetyltransferase- transfected neuroblastoma clones

Neurotransmitters appear early in the developing embryo and may play a role in the regulation of neuronal differentiation. To study potential effects of acetylcholine production in neuronal differentiation, we used the FB5 subclone of N18TG2 murine neuroblastoma cells stably transfected with cDNA for choline acetyltransferase. We tested whether the forced acetylcholine production can modify the expression or the cellular localization of different neuronal markers. We studied the activity, localization, and secretion of acetylcholinesterase in view of its possible role in the modulation of the morphogenetic action of acetylcholine and of its proposed role of a regulator of neurite outgrowth. FB5 cells are characterized by a high level of acetylcholinesterase, predominantly released into the culture medium. Acetylcholinesterase secretion into the medium was lower in choline acetyltransferase-transfected clones than in nontransfected and antisense-transfected controls. Moreover, sequential extraction of acetylcholinesterase revealed that detergent-extracted, i.e., membrane-associated, activity was higher in the transfected clones expressing choline acetyltransferase activity than in both control groups. These observations suggest that a shift occurs in the utilization of acetylcholinesterase in choline acetyltransferase-transfected clones from a secretion pathway to a pathway leading to membrane localization. In addition, the choline acetyltransferase-positive clones showed higher densities of voltage-gated Na(+) channels and enhanced high-affinity choline uptake, suggesting the accomplishment of a more advanced differentiated neuronal phenotype. Finally, binding experiments demonstrated the presence of muscarinic acetylcholine receptors in all examined clones. This observation is consistent with the proposed existence of an autocrine loop, which may be important for the enhancement in the expression of neurospecific traits.