Advances in small molecule selective ligands for heteromeric nicotinic acetylcholine receptors

Light-Activated Pharmacological Tools for Exploring the Cholinergic System

Cholinergic transmission plays a critical role in both the central and peripheral nervous systems, affecting processes such as learning, memory, and inflammation. Conventional cholinergic drugs generally suffer from poor selectivity and temporal precision, leading to undesired effects and limited therapeutic efficacy. Photopharmacology aims to overcome the limitations of traditional drugs using photocleavable or photoswitchable ligands and spatiotemporal patterns of illumination. Spanning from muscarinic and nicotinic modulators to cholinesterase inhibitors, this review explores the development and application of light-activated compounds as tools for unraveling the role of cholinergic signaling in both physiological and pathological contexts, while also paving the way for innovative phototherapeutic approaches.

Structural Determinants of Oxantel Analogs Reveal Modulatory Selectivity of α3β2 and α4β2 Neuronal Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs), ligand-gated ion channels involved in key physiological processes, show pharmacological diversity across receptor subtypes and species. The structurally similar anthelmintic compounds pyrantel, morantel, and oxantel differentially affect the α3β2 and α4β2 nAChR subtypes. Mutation analysis located the modulator binding sites to β(+)/α(-) interface pockets, homologous to the orthosteric agonist sites. We present here the synthesis and pharmacological characterization of 10 oxantel analogs with various phenyl substituents, planarity, and N-methylation, thereby elucidating the structural determinants of nAChR allosteric modulation by oxantel. Two-electrode voltage-clamp in Xenopus laevis oocytes expressing α3β2 and α4β2, respectively, revealed that selectivity and pharmacological profiles were most severely affected by the position of the hydroxy group (meta in oxantel) and the nature of the phenyl substituent. Oxantel is a PAM for α3β2 receptors, with EC50 = 3.9 μM and E max = 1.98 (relative to ACh alone, EC50 = 3.4 μM), but a NAM for α4β2 receptors, with EC50 = 200 μM and E max = 0.75 (relative to ACh alone, EC50 = 1.1 μM). Examples of large changes in modulatory activity of the analogs include the o-OH in 2a, resulting in a α3β2-selective PAM (EC50 = 0.061 μM and E max = 2.08), and the p-OH in 2c elucidated stricter requirement for activity at α3β2 (EC50 = 5.8 μM and E max = 1.01) compared to α4β2 (EC50 = 96 μM and E max = 0.88). These results, rationalized by in-silico docking studies, highlight distinct analog selectivity between the two subtypes and fine-tuning their pharmacological profiles.

Targeting cryptic allosteric sites of G protein-coupled receptors as a novel strategy for biased drug discovery

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and are highly effective targets for therapeutic drugs. GPCRs couple different downstream effectors, including G proteins (such as Gi/o, Gs, G12, and Gq) and β-arrestins (such as β-arrestin 1 and β-arrestin 2) to mediate diverse cellular and physiological responses. Biased signaling allows for the specific activation of certain pathways from the full range of receptors' signaling capabilities. Targeting more variable allosteric sites, which are spatially different from the highly conserved orthosteric sites, represents a novel approach in biased GPCR drug discovery, leading to innovative strategies for targeting GPCRs. Notably, the emergence of cryptic allosteric sites on GPCRs has expanded the repertoire of available drug targets and improved receptor subtype selectivity. Here, we conduct a summary of recent progress in the structural determination of cryptic allosteric sites on GPCRs and elucidate the biased signaling mechanisms induced by allosteric modulators. Additionally, we discuss means to identify cryptic allosteric sites and design biased allosteric modulators based on cryptic allosteric sites through structure-based drug design, which is an advanced pharmacotherapeutic approach for treating GPCR-associated diseases.

Structural insights into the activation mechanism of the human zinc-activated channel

The zinc-activated channel (ZAC) is an atypical mammalian cys-loop receptor (CLR) that is activated by zinc ions and protons, allowing cations to pass through. The molecular mechanism that ligands use to activate ZAC remains elusive. Here, we present three cryo-electron microscopy reconstructions of human ZAC (hZAC) under different conditions. These three hZAC structures display highly similar conformations to one another, forming symmetrical homo-pentamers with a central ion-conduction pore. The hZAC protomer comprises an extracellular domain (ECD) and a transmembrane domain (TMD), sharing more structural similarity with anion-permeable CLRs, such as glycine receptors and type A γ-aminobutyric acid receptors. Notably, hZAC possesses a distinctive C-tail that establishes a disulfide bond with the loop M2-M3 in the TMD and occupies what is typically the canonical neurotransmitter orthosteric site in other mammalian CLRs. Moreover, the tip of the cys-loop creates an unprecedented orthosteric site in hZAC. The binding of Zn2+ triggers a conformational shift in the cys-loop, which presumably prompts the loop M2-M3 to move and open the channel gate. This study sheds light on the assembly of the channel, its structural features, and the process of signal transduction in hZAC.

Spinosad blocks CHRNA5 mediated EGFR signaling pathway activation to inhibit lung adenocarcinoma proliferation

Lung adenocarcinoma (LUAD) is the leading cause of cancer death worldwide, with high incidence and low survival rates. Nicotinic acetylcholine receptors play an important role in the progression of LUAD. In this study, a screening of 17 nicotinic acetylcholine receptor allosteric agents revealed that spinosad effectively suppressed the proliferation of LUAD cells. The experiments demonstrated that spinosad induced cell cycle arrest in the G1 phase and stimulated apoptosis, thereby impeding the growth of LUAD and enhancing the responsiveness to gefitinib in vitro and vivo. Mechanistic insights obtained through transcriptome sequencing, Co-IP, and protein immunoblots indicated that spinosad disrupted the interaction between CHRNA5 and EGFR, thereby inhibiting the formation of downstream complexes and activation of the EGFR signaling pathway. The supplementation of exogenous acetylcholine showed to mitigate the inhibition of LUAD cell proliferation induced by spinosad. This study elucidates the therapeutic effects and mechanisms of spinosad in LUAD, and offers a theoretical and experimental foundation for novel LUAD treatments.

Selective Potentiation of the (α4)3(β2)2 Nicotinic Acetylcholine Receptor Response by NS9283 Analogues

NS9283, 3-(3-pyridyl)-5-(3-cyanophenyl)-1,2,4-oxadiazole, is a selective positive allosteric modulator of (α4)3(β2)2 nicotinic acetylcholine receptors (nAChRs). It has good subtype selective therapeutic potential afforded by its specific binding to the unique α4-α4 subunit interface present in the (α4)3(β2)2 nAChR. However, there is currently a lack of structure activity relationship (SAR) studies aimed at developing a class of congeners endowed with the same profile of activity that can help consolidate the druggability of the α4-α4 subunit interface. In this study, new NS9283 analogues were designed, synthesized, and characterized for their ability to selectively potentiate the ACh activity at heterologous (α4)3(β2)2 nAChRs vs nAChR subtypes (α4)2(β2)3, α5α4β2, and α7. With few exceptions, all the NS9283 analogues exerted positive modulation of the (α4)3(β2)2 nAChR ACh-evoked responses. Above all, those modified at the 3-cyanophenyl moiety by replacement with 3-nitrophenyl (4), 4-cyanophenyl (10), and N-formyl-4-piperidinyl (20) showed the same efficacy as NS9283, although with lower potency. Molecular dynamics simulations of NS9283 and some selected analogues highlighted consistency between potentiation activity and pose of the ligand inside the α4-α4 site with the main interaction being with the complementary (-) side and induction of a significant conformational change of the Trp156 residue in the principal (+) side.

Nicotinic acetylcholine receptors in cancer: Limitations and prospects

Nicotinic acetylcholine receptors (nAChRs) have long been considered to solely mediate neurotransmission. However, their widespread distribution in the human body suggests a more diverse physiological role. Additionally, the expression of nAChRs is increased in certain cancers, such as lung cancer, and has been associated with cell proliferation, epithelial-to-mesenchymal cell transition, angiogenesis and apoptosis prevention. Several compounds that interact with these receptors have been identified as potential therapeutic agents. They have been tested as drugs for treating nicotine addiction, alcoholism, depression, pain and Alzheimer's disease. This review focuses on nAChR-mediated signalling in cancer, presenting opportunities for the development of innovative nAChR-based anticancer drugs. It displays the differences in expression of each nAChR subunit between normal and cancer cells for selected cancer types, highlighting their possible involvement in specific cases. Antagonists of nAChRs that could complement existing cancer therapies are summarised and critically discussed. We hope that this review will stimulate further research on the role of nAChRs in cancer potentially leading to innovative cancer therapies.