Pharmacophore Mapping Combined with dbCICA Reveal New Structural Features for the Development of Novel Ligands Targeting α4β2 and α7 Nicotinic Acetylcholine Receptors

The neuronal nicotinic acetylcholine receptors (nAChRs) belong to the ligand-gated ion channel (GLIC) group, presenting a crucial role in several biological processes and neuronal disorders. The α4β2 and α7 nAChRs are the most abundant in the central nervous system (CNS), being involved in challenging diseases such as epilepsy, Alzheimer's disease, schizophrenia, and anxiety disorder, as well as alcohol and nicotine dependencies. In addition, in silico-based strategies may contribute to revealing new insights into drug design and virtual screening to find new drug candidates to treat CNS disorders. In this context, the pharmacophore maps were constructed and validated for the orthosteric sites of α4β2 and α7 nAChRs, through a docking-based Comparative Intermolecular Contacts Analysis (dbCICA). In this sense, bioactive ligands were retrieved from the literature for each receptor. A molecular docking protocol was developed for all ligands in both receptors by using GOLD software, considering GoldScore, ChemScore, ASP, and ChemPLP scoring functions. Output GOLD results were post-processed through dbCICA to identify critical contacts involved in protein-ligand interactions. Moreover, Crossminer software was used to construct a pharmacophoric map based on the most well-behaved ligands and negative contacts from the dbCICA model for each receptor. Both pharmacophore maps were validated by using a ROC curve. The results revealed important features for the ligands, such as the presence of hydrophobic regions, a planar ring, and hydrogen bond donor and acceptor atoms for α4β2. Parallelly, a non-planar ring region was identified for α7. These results can enable fragment-based drug design (FBDD) strategies, such as fragment growing, linking, and merging, allowing an increase in the activity of known fragments. Thus, our results can contribute to a further understanding of structural subunits presenting the potential for key ligand-receptor interactions, favoring the search in molecular databases and the design of novel ligands.

A short framework-III (mini-M-2) conotoxin from the venom of a vermivorous species, Conus archon, inhibits human neuronal nicotinic acetylcholine receptors

The venoms of Conus snails contain neuroactive peptides named conotoxins (CTXs). Some CTXs are nicotinic acetylcholine receptor (nAChRs) antagonists. nAChRs modulate the release of neurotransmitters and are implicated in several pathophysiologies. One venom peptide from Conus archon, a vermivorous species from the Mexican Pacific, was purified by RP-HPLC and its activity on human α7, α3β2, and α7β2 nAChRs was assessed by the two-electrode voltage clamp technique. At 36.3 µM the purified peptide (F27-1, renamed tentatively ArchIIIA) slowly reversibly inhibited the ACh-induced response of the hα7 subtype by 44.52 ± 5.83%, while it had low or no significant effect on the response of the hα3β2 and hα7β2 subtypes; the EC₅₀ of the inhibiting effect was 45.7 µM on the hα7 subtype. This peptide has 15 amino acid residues and a monoisotopic mass of 1654.6 Da (CCSALCSRYHCLPCC), with three disulfide bridges and a free C-terminus. This sequence with a CC-C-C-CC arrangement (framework III) belongs to the M superfamily of conotoxins, corresponding to the mini-M´s (M-1-M-3) conotoxins; due to its size and inter-Cys spacings it is an M-2 conotoxin. This toxin is a novel mini-M conotoxin affecting ligand-gated ion channels, like the maxi-M CTX ψ-conotoxins and α-MIIIJ conotoxin (nAChRs blockers). This peptide seems to be homologous to the reg3b conotoxin (from Conus regius) with an identity of 93.3%, differing only in the third residue in the sequence, serine for threonine, both uncharged polar residues. We obtained, in silico, a probable 3D structure, which is consistent with its effect on neuronal subtypes.

JWX-A0108, a positive allosteric modulator of α7 nAChR, attenuates cognitive deficits in APP/PS1 mice by suppressing NF-κB-mediated inflammation

Neuroinflammation plays an early and prominent role in the pathology of Alzheimer's disease (AD). Studies have shown that cholinergic lesion is a contributor for the pathophysiology of AD. The α7 nicotinic acetylcholine receptors (nAChRs), a subtype of nAChRs, are abundantly expressed in the brain regions related to cognition and memory, such as hippocampus and frontal cortex. The α7 nAChR is rapidly activated and desensitized by agonists. JWX-A0108 is a type I positive allosteric modulator (PAM) of α7 nAChR, which mainly enhances agonist-evoked peak currents. Here, we used the Morris Water Maze to evaluate the effect of JWX-A0108 on cognition and memory functions in APP/PS1 mice, and the mechanism related to anti-inflammatory effect. The results showed that JWX-A0108 could improve the learning and memory function of APP/PS1 transgenic mice in Morris water maze, decrease the expression of IL-1β, TNF-α, IL-6 in the brain and lower the phosphorylation level of IκBα (Ser32/36) and NF-κB p65 (Ser536), decrease the expression of Iba1, the microglia activation marker. Nissl staining showed that the CA3 and DG regions of hippocampus were damaged in APP/PS1 mice, which was improved by JWX-A0108. All of these effects of JWX-A0108 were reversed by MLA (α7 nAChR specific blocker). Taken together, the results reveal that JWX-A0108 improved the learning and memory function of APP/PS1 mice by enhancing the anti-inflammatory effect of the endogenous choline system through α7 nAChR, inhibited the activation of the NF-κB signaling pathway by inhibiting IκB phosphorylation, and ultimately inhibited inflammatory responses.

The interplay between α7 nicotinic acetylcholine receptors, pannexin-1 channels and P2X7 receptors elicit exocytosis in chromaffin cells

Pannexin-1 (Panx1) forms plasma membrane channels that allow the exchange of small molecules between the intracellular and extracellular compartments, and are involved in diverse physiological and pathological responses in the nervous system. However, the signaling mechanisms that induce their opening still remain elusive. Here, we propose a new mechanism for Panx1 channel activation through a functional crosstalk with the highly Ca²⁺ permeable α7 nicotinic acetylcholine receptor (nAChR). Consistent with this hypothesis, we found that activation of α7 nAChRs induces Panx1-mediated dye uptake and ATP release in the neuroblastoma cell line SH-SY5Y-α7. Using membrane permeant Ca²⁺ chelators, total internal reflection fluorescence microscopy in SH-SY5Y-α7 cells expressing a membrane-tethered GCAMP3, and Src kinase inhibitors, we further demonstrated that Panx1 channel opening depends on Ca²⁺ signals localized in submembrane areas, as well as on Src kinases. In turn, Panx1 channels amplify cytosolic Ca²⁺ signals induced by the activation of α7 nAChRs, by a mechanism that seems to involve ATP release and P2X7 receptor activation, as hydrolysis of extracellular ATP with apyrase or blockage of P2X7 receptors with oxidized ATP significantly reduces the α7 nAChR-Ca²⁺ signal. The physiological relevance of this crosstalk was also demonstrated in neuroendocrine chromaffin cells, wherein Panx1 channels and P2X7 receptors contribute to the exocytotic release of catecholamines triggered by α7 nAChRs, as measured by amperometry. Together these findings point to a functional coupling between α7 nAChRs, Panx1 channels and P2X7 receptors with physiological relevance in neurosecretion.

Nurr1, Pitx3, and α7 nAChRs mRNA Expression in Nigral Tissue of Rats with Pedunculopontine Neurotoxic Lesion

Background and Objectives: The knowledge that the cholinergic neurons from pedunculopontine nucleus (PPN) are vulnerable to the degeneration in early stages of the Parkinson disease progression has opened new perspectives to the development of experimental model focused in pontine lesions that could increase the risk of nigral degeneration. In this context it is known that PPN lesioned rats exhibit early changes in the gene expression of proteins responsible for dopaminergic homeostasis. At the same time, it is known that nicotinic cholinergic receptors (nAChRs) mediate the excitatory influence of pontine-nigral projection. However, the effect of PPN injury on the expression of transcription factors that modulate dopaminergic neurotransmission in the adult brain as well as the α7 nAChRs gene expression has not been studied. The main objective of the present work was the study of the effects of the unilateral neurotoxic lesion of PPN in nuclear receptor-related factor 1 (Nurr1), paired-like homeodomain transcription factor 3 (Pitx3), and α7 nAChRs mRNA expression in nigral tissue. Materials and Methods: The molecular biology studies were performed by means of RT-PCR. The following experimental groups were organized: Non-treated rats, N-methyl-D-aspartate (NMDA)-lesioned rats, and Sham operated rats. Experimental subjects were sacrificed 24 h, 48 h and seven days after PPN lesion. Results: Nurr1 mRNA expression, showed a significant increase both 24 h (p < 0.001) and 48 h (p < 0.01) after PPN injury. Pitx3 mRNA expression evidenced a significant increase 24 h (p < 0.001) followed by a significant decrease 48 h and seven days after PPN lesion (p < 0.01). Finally, the α7 nAChRs nigral mRNA expression remained significantly diminished 24 h, 48 h (p < 0.001), and 7 days (p < 0.01) after PPN neurotoxic injury. Conclusion: Taking together these modifications could represent early warning signals and could be the preamble to nigral neurodegeneration events.

Nicotine inhibits activation of microglial proton currents via interactions with α7 acetylcholine receptors

Alpha 7 subunits of nicotinic acetylcholine receptors (nAChRs) are expressed in microglia and are involved in the suppression of neuroinflammation. Over the past decade, many reports show beneficial effects of nicotine, though little is known about the mechanism. Here we show that nicotine inhibits lipopolysaccharide (LPS)-induced proton (H⁺) currents and morphological change by using primary cultured microglia. The H⁺ channel currents were measured by whole-cell patch clamp method under voltage-clamp condition. Increased H⁺ current in activated microglia was attenuated by blocking NADPH oxidase. The inhibitory effect of nicotine was due to the activation of α7 nAChR, not a direct action on the H⁺ channels, because the effects of nicotine was cancelled by α7 nAChR antagonists. Neurotoxic effect of LPS-activated microglia due to inflammatory cytokines was also attenuated by pre-treatment of microglia with nicotine. These results suggest that α7 nAChRs in microglia may be a therapeutic target in neuroinflammatory diseases.

Activation of α7 nicotinic acetylcholine receptors increases intracellular cAMP levels via activation of AC1 in hippocampal neurons

The activation of α7 nAChRs has been shown to improve hippocampal-dependent learning and memory. However, the molecular mechanism of α7 nAChRs' action remains elusive. We previously reported that activation of α7 nAChRs induced a prolonged enhancement of glutamatergic synaptic transmission in a PKA-dependent manner. Here, we investigated any connection between the activation of the α7 nAChR and cAMP signaling in hippocampal neurons. To address this question, we employed a FRET-based biosensor to measure the intracellular cAMP levels directly via live cell imaging. We found that application of the α7 nAChR-selective agonist choline, in the presence of the α7 nAChR positive allosteric modulator PNU-120596, induced a significant change in emission ratio of F535/F470, which indicated an increase in intracellular cAMP levels. This choline-induced increase was abolished by the α7 nAChR antagonist MLA and the calcium chelator BAPTA, suggesting that the cAMP increase depends on the α7 nAChR activation and subsequent intracellular calcium rise. The selective AC1 inhibitor CB-6673567 and siRNA-mediated deletion of AC1 both blocked the choline-induced cAMP increase, suggesting that calcium-dependent AC1 is required for choline's action. Furthermore, α7 nAChR activation stimulated the phosphorylation of synapsin, which serves as a downstream effector to regulate neurotransmitter release. Our findings provide the first direct evidence to link activation of α7 nAChRs to a cAMP rise via AC1, which defines a new signaling pathway employed by α7 nAChRs. Our study sheds light into potential molecular mechanisms of the positive cognitive actions of α7 nAChR agonists and development of therapeutic treatments for cognitive impairments.