Mechanistic Insights into the Allosteric Modulation of Opioid Receptors by Sodium Ions

The idea of sodium ions altering G-protein-coupled receptor (GPCR) ligand binding and signaling was first suggested for opioid receptors (ORs) in the 1970s and subsequently extended to other GPCRs. Recently published ultra-high-resolution crystal structures of GPCRs, including that of the δ-OR subtype, have started to shed light on the mechanism underlying sodium control in GPCR signaling by revealing details of the sodium binding site. Whether sodium accesses different receptor subtypes from the extra- or intracellular sides, following similar or different pathways, is still an open question. Earlier experiments in brain homogenates suggested a differential sodium regulation of ligand binding to the three major OR subtypes, in spite of their high degree of sequence similarity. Intrigued by this possibility, we explored the dynamic nature of sodium binding to δ-OR, μ-OR, and κ-OR by means of microsecond-scale, all-atom molecular dynamics (MD) simulations. Rapid sodium permeation was observed exclusively from the extracellular milieu, and following similar binding pathways in all three ligand-free OR systems, notwithstanding extra densities of sodium observed near nonconserved residues of κ-OR and δ-OR, but not in μ-OR. We speculate that these differences may be responsible for the differential increase in antagonist binding affinity of μ-OR by sodium resulting from specific ligand binding experiments in transfected cells. On the other hand, sodium reduced the level of binding of subtype-specific agonists to all OR subtypes. Additional biased and unbiased MD simulations were conducted using the δ-OR ultra-high-resolution crystal structure as a model system to provide a mechanistic explanation for this experimental observation

Ligand-Based Discovery of a New Scaffold for Allosteric Modulation of the μ‑Opioid Receptor

With the hope of discovering effective analgesics with fewer side effects, attention has recently shifted to allosteric modulators of the opioid receptors. In the past two years, the first chemotypes of positive or silent allosteric modulators (PAMs or SAMs, respectively) of μ- and δ-opioid receptor types have been reported in the literature. During a structure-guided lead optimization campaign with μ-PAMs BMS-986121 and BMS-986122 as starting compounds, we discovered a new chemotype that was confirmed to display μ-PAM or μ-SAM activity depending on the specific substitutions as assessed by endomorphin-1-stimulated β-arrestin2 recruitment assays in Chinese Hamster Ovary (CHO)-μ PathHunter cells. The most active μ-PAM of this series was analyzed further in competition binding and G-protein activation assays to understand its effects on ligand binding and to investigate the nature of its probe dependence

Versatility of the Curcumin Scaffold: Discovery of Potent and Balanced Dual BACE‑1 and GSK-3β Inhibitors

The multitarget approach has gained increasing acceptance as a useful tool to address complex and multifactorial maladies such as Alzheimer’s disease (AD). The concurrent inhibition of the validated AD targets β-secretase (BACE-1) and glycogen synthase kinase-3β (GSK-3β) by attacking both β-amyloid and tau protein cascades has been identified as a promising AD therapeutic strategy. In our study, curcumin was identified as a lead compound for the simultaneous inhibition of both targets; therefore, synthetic efforts were dedicated to obtaining a small library of novel curcumin-based analogues, and a number of potent and balanced dual-target inhibitors were obtained. In particular, <b>2</b>, <b>6</b>, and <b>7</b> emerged as promising drug candidates endowed with neuroprotective potential and brain permeability. Notably, for some new compounds the symmetrical diketo and the β-keto–enol tautomeric forms were purposely isolated and tested <i>in vitro</i>, allowing us to gain insight into the key requirements for BACE-1 and GSK-3β inhibition