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We have characterized families of phenylpiperazine (PP) compounds, studying their relative activity with α7 and α9* nicotinic acetylcholine receptors (nAChRs) and focusing on the effects of side groups on the phenyl ring (R1) and the effects of different alkyl groups on the base nitrogen. In this study, we evaluated the impact of methyl substitution on the piperazine ring, which introduced a chiral center, enabling the generation and separation of stereoisomers. Methyl groups were added to either the C2 or C3 positions on the piperazine of the α9α10 agonist/α7 partial agonist PA-EMPP. Additions at the C3 position greatly reduced activity, while additions at the C2 position had selective effects on either α7 or α9/α10 activity. The 2-methyl S and R isomers of PA-EMPP contain a second chiral center at the nitrogen. Notably, replacing the terminal substitution with N,N-dimethyl abolished α9/α910 agonist activity, rendering the compound selective for α7. We also tested 2M isomers of the α9α10 agonist pCN-EMPP and obtained similar enantioselective activity as observed with the PA-EMPP isomers. Compounds were studied for their ability to reduce the ATP-dependent release of IL-1β from monocytes, one aspect of the cholinergic anti-inflammatory activity. Results were consistent with their apparent activation or antagonism of α9* receptors. These findings underscore the critical role of chirality and structural modifications in fine-tuning receptor selectivity, offering valuable insights for the rational design of selective nicotinic therapeutics.
Host-derived succinate accumulates in the airways during bacterial infection. Here, we show that luminal succinate activates murine tracheal brush (tuft) cells through a signaling cascade involving the succinate receptor 1 (SUCNR1), phospholipase Cβ2, and the cation channel transient receptor potential channel subfamily M member 5 (TRPM5). Stimulated brush cells then trigger a long-range Ca2+ wave spreading radially over the tracheal epithelium through a sequential signaling process. First, brush cells release acetylcholine, which excites nearby cells via muscarinic acetylcholine receptors. From there, the Ca2+ wave propagates through gap junction signaling, reaching also distant ciliated and secretory cells. These effector cells translate activation into enhanced ciliary activity and Cl- secretion, which are synergistic in boosting mucociliary clearance, the major innate defense mechanism of the airways. Our data establish tracheal brush cells as a central hub in triggering a global epithelial defense program in response to a danger-associated metabolite.
Recently, we discovered a cholinergic mechanism that inhibits the adenosine triphosphate (ATP)-dependent release of interleukin-1 beta (IL-1 beta) by human monocytes via nicotinic acetylcholine receptors (nAChRs) composed of alpha 7, alpha 9 and/or alpha 10 subunits. Furthermore, we identified phosphocholine (PC) and dipalmitoylphosphatidylcholine (DPPC) as novel nicotinic agonists that elicit metabotropic activity at monocytic nAChR. Interestingly, PC does not provoke ion channel responses at conventional nAChRs composed of subunits alpha 9 and alpha 10. The purpose of this study is to determine the composition of nAChRs necessary for nicotinic signaling in monocytic cells and to test the hypothesis that common metabolites of phosphatidylcholines, lysophosphatidylcholine (LPC) and glycerophosphocholine (G-PC), function as nAChR agonists. In peripheral blood mononuclear cells from nAChR gene-deficient mice, we demonstrated that inhibition of ATP-dependent release of IL-1 beta by acetylcholine (ACh), nicotine and PC depends on subunits alpha 7, alpha 9 and alpha 10. Using a panel of nAChR antagonists and siRNA technology, we confirmed the involvement of these subunits in the control of IL-1 beta release in the human monocytic cell line U937. Furthermore, we showed that LPC (C16:0) and G-PC efficiently inhibit ATP-dependent release of IL-1 beta. Of note, the inhibitory effects mediated by LPC and G-PC depend on nAChR subunits alpha 9 and alpha 10, but only to a small degree on alpha 7. In Xenopus laevis oocytes heterologously expressing different combinations of human alpha 7, alpha 9 or alpha 10 subunits, ACh induced canonical ion channel activity, whereas LPC, G-PC and PC did not. In conclusion, we demonstrate that canonical nicotinic agonists and PC elicit metabotropic nAChR activity in monocytes via interaction of nAChR subunits alpha 7, alpha 9 and alpha 10. For the metabotropic signaling of LPC and G-PC, nAChR subunits alpha 9 and alpha 10 are needed, whereas alpha 7 is virtually dispensable. Furthermore, molecules bearing a PC group in general seem to regulate immune functions without perturbing canonical ion channel functions of nAChR.
