What ligand-gated ion channels can tell us about the allosteric regulation of G protein-coupled receptors

Allosteric Modalities for Membrane-Bound Receptors: Insights from Drug Hunting for Brain Diseases

Medicinal chemists are accountable for embedding the appropriate drug target profile into the molecular architecture of a clinical candidate. An accurate characterization of the functional effects following binding of a drug to its biological target is a fundamental step in the discovery of new medicines, informing the translation of preclinical efficacy and safety observations into human trials. Membrane-bound proteins, particularly ion channels and G protein-coupled receptors (GPCRs), are biological targets prone to allosteric modulation. Investigations using allosteric drug candidates and chemical tools suggest that their functional effects may be tailored with a high degree of translational alignment, making them molecular tools to correct pathophysiological functional tone and enable personalized medicine when a causative target-to-disease link is known. We present select examples of functional molecular fine-tuning of allosterism and discuss consequences relevant to drug design.

Distinct Signaling Patterns of Allosteric Antagonism at the P2Y1 Receptor

Traditionally, G protein-coupled receptor antagonists are classified as competitive or noncompetitive and surmountable or insurmountable based on functional antagonism. P2Y1 receptor (P2Y1R) structures showed two antagonists binding to two spatially distinct sites: nucleotide MRS2500 (orthosteric, contacting the helical bundle) and urea BPTU (allosteric, on the external receptor surface). However, the nature of their P2Y1R antagonism has not been characterized. Here we characterized BPTU antagonism at various signaling pathways activated by structurally diverse agonists. BPTU rightward shifted the concentration-response curves of both 2-methylthioadenosine 5'-diphosphate trisodium salt and MRS2365 (5'-diphosphates) in some signaling events, such as extracellular signal-regulated kinase 1/2 and label free, in a parallel manner without affecting the maximum agonist effect (Emax) but antagonized insurmountably (suppressed agonist Emax) in signaling events such as guanosine 5'-3-O-(thio)triphosphate binding and β-arrestin2 recruitment. However, with dinucleotide Ap4A as an agonist, BPTU suppressed the Emax insurmountably in all signaling pathways. By comparison, MRS2500 behaved as surmountable antagonist rightward-shifting concentration-response curves of all three agonists in a parallel manner for all signaling pathways measured. Thus, we demonstrated a previously undocumented phenomenon that P2Y1R antagonism patterns could vary in different signaling pathways, which could be related to conformational selection, signaling amplification, and probe dependence. This phenomenon may apply generally to other receptors considering that antagonism by a specific ligand is often not compared at multiple signaling pathways. Thus, antagonism can be surmountable or insurmountable depending on the signaling pathways measured and the agonists used, which should be of broad relevance to drug discovery and disease treatment.