Molecular docking of four β-amyloid1–42 fragments on the α7 nicotinic receptor: delineating the binding site of the Aβ peptides

Neuroprotective amyloid β N-terminal peptides differentially alter human α7- and α7β2-nicotinic acetylcholine (nACh) receptor single-channel properties

BACKGROUND AND PURPOSE

Oligomeric amyloid β 1-42 (oAβ1-42) exhibits agonist-like action at human α7- and α7β2-containing nicotinic receptors. The N-terminal amyloid β1-15 fragment (N-Aβ fragment) modulates presynaptic calcium and enhances hippocampal-based synaptic plasticity via α7-containing nicotinic receptors. Further, the N-Aβ fragment and its core sequence, the N-amyloid-beta core hexapeptide (N-Aβcore), protect against oAβ1-42-associated synapto- and neurotoxicity. Here, we investigated how oAβ1-42, the N-Aβ fragment, and the N-Aβcore regulate the single-channel properties of α7- and α7β2-nicotinic receptors.

EXPERIMENTAL APPROACH

Single-channel recordings measured the impact of acetylcholine, oAβ1-42, the N-Aβ fragment, and the N-Aβcore on the unitary properties of human α7- and α7β2-containing nicotinic receptors expressed in nicotinic-null SH-EP1 cells. Molecular dynamics simulations identified potential sites of interaction between the N-Aβ fragment and orthosteric α7+/α7- and α7+/β2- nicotinic receptor binding interfaces.

KEY RESULTS

The N-Aβ fragment and N-Aβcore induced α7- and α7β2-nicotinic receptor single-channel openings. Relative to acetylcholine, oAβ1-42 preferentially enhanced α7β2-nicotinic receptor single-channel open probability and open-dwell times. Co-application with the N-Aβcore neutralized these effects. Further, administration of the N-Aβ fragment alone, or in combination with acetylcholine or oAβ1-42, selectively enhanced α7-nicotinic receptor open probability and open-dwell times (compared to acetylcholine or oAβ1-42).

CONCLUSIONS AND IMPLICATIONS

Amyloid-beta peptides demonstrate functional diversity in regulating α7- and α7β2-nicotinic receptor function, with implications for a wide range of nicotinic receptor-mediated functions in Alzheimer's disease. The effects of these peptides on α7- and/or α7β2-nicotinic receptors revealed complex interactions with these subtypes, providing novel insights into the neuroprotective actions of amyloid β-derived fragments against the toxic effects of oAβ1-42.

Prediction of the 3D conformation of a small peptide vaccine targeting Aβ42 oligomers

The original etiology of Alzheimer's disease (AD) is the deposition of amyloid-beta (Aβ) proteins, which starts from the aggregation of the Aβ oligomers. The optimal therapeutic strategy targeting Aβ oligomer aggregation is the development of AD vaccines. Despite the fact that positive progress has been made for experimental attempts at AD vaccines, the physicochemical and even structural properties of these AD vaccines remain unclear. In this study, through immunoinformatic and molecular dynamics (MD) simulations, we first designed and simulated an alternative of vaccine TAPAS and found that the structure of the alternative can reproduce the 3D conformation of TAPAS determined experimentally. Meanwhile, immunoinformatic methods were used to analyze the physicochemical properties of TAPAS, including immunogenicity, antigenicity, thermal stability, and solubility, which confirm well the efficacy and safety of the vaccine, and validate the scheme reliability of immunoinformatic and MD simulations in designing and simulating the TAPAS vaccine. Using the same scheme, we predicted the 3D conformation of the optimized ACI-24 peptide vaccine, an Aβ peptide with the first 15 residues of Aβ42 (Aβ1-15). The vaccine was verified once to be effective against both full-length Aβ1-42 and truncated Aβ4-42 aggregates, but an experimental 3D structure was absent. We have also explored the immune mechanism of the vaccine at the molecular level and found that the optimized ACI-24 and its analogues can block the growth of either full-length Aβ1-42 or truncated Aβ4-42 pentamer by contacting the hydrophobic residues within the N-terminus and β1 region on the contact surface of either pentamer. Additionally, residues (D1, D7, S8, H13, and Q15) were identified as the key residues of the vaccine to contact either of the two Aβ oligomers. This work provides a feasible implementation scheme of immunoinformatic and MD simulations for the development of AD small peptide vaccines, validating the power of the scheme as a parallel tool to the experimental approaches and injecting molecular-level information into the understanding and design of anti-AD vaccines.

Specific interaction from different Aβ42 peptide fragments to α7nAChR-A study of molecular dynamics simulation

CONTEXT

Existing researches confirmed that β amyloid (Aβ) has a high affinity for the α7 nicotinic acetylcholine receptor (α7nAChR), associating closely to Alzheimer's disease. The majority of related studies focused on the experimental reports on the neuroprotective role of Aβ fragment (Aβx), however, with a lack of investigation into the most suitable binding region and mechanism of action between Aβ fragment and α7nAChR. In the study, we employed four Aβ1-42 fragments Aβx, Aβ1-16, Aβ10-16, Aβ12-28, and Aβ30-42, of which the first three were confirmed to play neuroprotective roles upon directly binding, to interact with α7nAChR.

METHODS

The protein-ligand docking server of CABS-DOCK was employed to obtain the α7nAChR-Aβx complexes. Only the top α7nAChR-Aβx complexes were used to perform all-atom GROMACS dynamics simulation in combination with Charmm36 force field, by which α7nAChR-Aβx interactions' dynamic behavior and specific locations of these different Aβx fragments were identified. MM-PBSA calculations were also done to estimate the binding free energies and the different contributions from the residues in the Aβx. Two distinct results for the first three and fourth Aβx fragments in binding site, strength, key residue, and orientation, account for why the fourth fails to play a neuroprotective role at the molecular level.

Quantitative assessment of oligomeric amyloid β peptide binding to α7 nicotinic receptor

BACKGROUND AND PURPOSE

Progressive dysfunction of cholinergic transmission is a well-known characteristic of Alzheimer's disease (AD). Amyloid β (Aβ) peptide oligomers are known to play a central role in AD and are suggested to impair the function of the cholinergic nicotinic ACh receptor α7 (α7nAChR). However, the mechanism underlying the effect of Aβ on α7nAChR function is not fully understood, limiting the therapeutic exploration of this observation in AD. Here, we aimed to detect and characterize Aβ binding to α7nAChR, including the possibility of interfering with this interaction for therapeutic purposes.

EXPERIMENTAL APPROACH

We developed a specific and quantitative time-resolved FRET (TR-FRET)-based binding assay for Aβ to α7nAChR and pharmacologically characterized this interaction.

KEY RESULTS

We demonstrated specific and high-affinity (low nanomolar) binding of Aβ to the orthosteric binding site of α7nAChR. Aβ binding was prevented and reversed by the well-characterized orthosteric ligands of α7nAChR (epibatidine, α-bungarotoxin, methylylcaconitine, PNU-282987, S24795, and EVP6124) and by the type II positive allosteric modulator (PAM) PNU-120596 but not by the type I PAM NS1738.

CONCLUSIONS AND IMPLICATIONS

Our TR-FRET Aβ binding assay demonstrates for the first time the specific binding of Aβ to α7nAChR, which will be a crucial tool for the development, testing, and selection of a novel generation of AD drug candidates targeting Aβ/α7nAChR complexes with high specificity and fewer side effects compared to currently approved α7nAChR drugs.

LINKED ARTICLES

This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Noncholinesterase effects induced by organophosphate pesticides and their relationship to cognitive processes: implication for the action of acylpeptide hydrolase

Organophosphate pesticides have been classically described as inhibitors of acetylcholinesterase (AChE) activity in insects and invertebrates. However, there is now more evidence supporting the hypothesis that these compounds also act through noncholinergic pathways, especially those related to cognitive processes. The enzyme acylpeptide hydrolase was identified as a new target for organophosphate pesticides. This enzyme is more sensitive than AChE to some organophosphates (OP), including dichlorvos, which is the parent compound for metrifonate, a therapeutic agent used in the treatment of cognitive impairment associated to Alzheimer's disease. Therefore, there is some doubt as to whether the mechanism of action of this drug is mediated by a potentiation of cholinergic transmission. However, the direct action of acylpeptide hydrolase in cognitive processes and the physiological and molecular mechanisms underlying subacute exposure to OP have yet to be demonstrated. This review deals with evidence demonstrating the existence of mechanisms of actions of OP, which are independent of cholinergic pathway potentiation and which have an effect on cognitive processes. In addition, the possible participation of the enzyme acylpeptide hydrolase in these processes is also discussed. Finally, the possibility of using this enzyme activity as a new biomarker for exposure to OP is considered.

The ?-amyloid protein of Alzheimer?s disease binds to membrane lipids but does not bind to the ?7 nicotinic acetylcholine receptor

Accumulation of the amyloid protein (Abeta) in the brain is an important step in the pathogenesis of Alzheimer's disease. However, the mechanism by which Abeta exerts its neurotoxic effect is largely unknown. It has been suggested that the peptide can bind to the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). In this study, we examined the binding of Abeta1-42 to endogenous and recombinantly expressed alpha7nAChRs. Abeta1-42 did neither inhibit the specific binding of alpha7nAChR ligands to rat brain homogenate or slice preparations, nor did it influence the activity of alpha7nAChRs expressed in Xenopus oocytes. Similarly, Abeta1-42 did not compete for alpha-bungarotoxin-binding sites on SH-SY5Y cells stably expressing alpha7nAChRs. The effect of the Abeta1-42 on tau phosphorylation was also examined. Although Abeta1-42 altered tau phosphorylation in alpha7nAChR-transfected SH-SY5Y cells, the effect of the peptide was unrelated to alpha7nAChR expression or activity. Binding studies using surface plasmon resonance indicated that the majority of the Abeta bound to membrane lipid, rather than to a protein component. Fluorescence anisotropy experiments indicated that Abeta may disrupt membrane lipid structure or fluidity. We conclude that the effects of Abeta are unlikely to be mediated by direct binding to the alpha7nAChR. Instead, we speculate that Abeta may exert its effects by altering the packing of lipids within the plasma membrane, which could, in turn, influence the function of a variety of receptors and channels on the cell surface.

High content screen microscopy analysis of A beta 1-42-induced neurite outgrowth reduction in rat primary cortical neurons: neuroprotective effects of alpha 7 neuronal nicotinic acetylcholine receptor ligands

beta-Amyloid peptide 1-42 (A beta(1-42)) is generated from amyloid precursor protein (APP) and associated with neurodegeneration in Alzheimer's disease (AD). A beta(1-42) has been shown to be cytotoxic when incubated with cultured neurons. However, APP transgenic mice over-expressing A beta(1-42) do not show substantial loss of neurons, despite deficits in learning and memory. It is thus emerging that A beta(1-42)-induced memory deficits may involve subtler neuronal alternations leading to synaptic deficits, prior to frank neurodegeneration in AD brains. In this study, high content screen (HCS) microscopy, an advanced high-throughput cellular image processing and analysis technique, was utilized in establishing an in vitro model of A beta(1-42)-induced neurotoxicity utilizing rat neonatal primary cortical cells. Neurite outgrowth was found to be significantly reduced by A beta(1-42) (300 nM to 30 microM), but not by the scrambled control peptide control, in a time- and concentration-dependent manner. In contrast, no reduction in the total number of neurons was observed. The A beta(1-42)-induced reduction of neurite outgrowth was attenuated by the NMDA receptor antagonist memantine and the alpha 7 nicotinic acetylcholine receptor (nAChR) selective agonist PNU-282987. Interestingly, the alpha 7 nAChR antagonist methyllycaconitine also significantly prevented reduction in A beta(1-42)-induced neurite outgrowth. The observed neuroprotective effects could arise either from interference of A beta(1-42) interactions with alpha 7 nAChRs or by modification of receptor-mediated signaling pathways. Our studies demonstrate that reduction of neurite outgrowth may serve as a model representing A beta(1-42)-mediated neuritic and synaptic toxicity, which, in combination of HCS, provides a high-throughput cell-based assay that can be used to evaluate compounds with neuroprotective properties in neurons.

Fully flexible docking models of the complex between α7 nicotinic receptor and a potent heptapeptide inhibitor of the β-amyloid peptide binding

The heptapeptide IQTTWSR (IQ), recently reported as inhibitor of the beta-amyloid (Abeta) binding to nicotinic acetylcholine receptors (nAChrs), was docked to the homology model of the alpha7 nicotinic acetylcholine receptor. The most representative models were further subjected to molecular dynamics simulations. The data obtained here suggest that Abeta needs highly specific structural motifs to bind to the alpha7nAChR. These structural motifs are located principally in the upper and lower surroundings of loop C, including loop F and sheets beta1, beta2, beta6, beta9, and beta10 of the receptor. Overall, these results suggest that IQ can be mimicked by more bioavailable, stable compounds that would be helpful for the understanding of the Abeta binding site and its dynamics, and for the design of novel agents to be used as an effective alternative against Alzheimer's disease.

Synthesis and evaluation of [125I]I-TSA as a brain nicotinic acetylcholine receptor α7 subtype imaging agent

INTRODUCTION

Some in vitro investigations have suggested that the nicotinic acetylcholine receptor (nAChR) alpha7 subtype is implicated in Alzheimer's disease, schizophrenia and others. Recently, we developed (R)-3'-(5-bromothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one (Br-TSA), which has a high affinity and selectivity for alpha7 nAChRs. Therefore we synthesized (R)-3'-(5-[125I]iodothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one ([125I]I-TSA) and evaluated its potential for the in vivo detection of alpha7 nAChR in brain.

METHODS

In vitro binding affinity of I-TSA was measured in rat brain homogenates. Radioiodination was accomplished by a Br-I exchange reaction. Biodistribution studies were undertaken in mice by tail vein injection of [(125)I]I-TSA. In vivo receptor blocking studies were carried out by treating mice with methyllycaconitine (MLA; 5 nmol/5 mul, i.c.v.) or nonradioactive I-TSA (50 micromol/kg, i.v.).

RESULTS

I-TSA exhibited a high affinity and selectivity for the alpha7 nAChR (K(i) for alpha7 nAChR = 0.54 nM). Initial uptake in the brain was high (4.42 %dose/g at 5 min), and the clearance of radioactivity was relatively slow in the hippocampus (alpha7 nAChR-rich region) and was rather rapid in the cerebellum (alpha7 nAChR poor region). The hippocampus to cerebellum uptake ratio was 0.9 at 5 min postinjection, but it was increased to 1.8 at 60 min postinjection. Although the effect was not statistically significant, administration of I-TSA and MLA decreased the accumulation of radioactivity in hippocampus.

CONCLUSION

Despite its high affinity and selectivity, [125I]I-TSA does not appear to be a suitable tracer for in vivo alpha7 nAChR receptor imaging studies due to its high nonspecific binding. Further structural optimization is needed.