Allosteric Modulation of Cys-loop Receptors

The Cys-loop receptor superfamily includes the GABAA, GABA C, glycine, and serotonin receptors as well as the nicotinic acetylcholine receptors (nAChRs). Cys-loop receptors are important drug targets for Parkinson's disease, Alzheimer's disease, and nicotine addiction. They are also targets of general anesthetics. Understanding the mechanisms of allosteric modulation for Cys-loop receptors has implications for the design of novel therapeutics for the treatment of pain, inflammation, and neurological disease. I employed a combination of computational and experimental approaches to understand allosteric modulation of these receptors. Four major contributions resulted from my graduate research: 1) NMR structures of the transmembrane (TM) domains of the alpha7 and alpha4beta2 nAChRs as well as the alpha1 glycine receptor were resolved to provide a scaffold for rationalizing drug-binding sites and drug action. While all structures revealed the typical four-helix bundle, differences were observed which could affect drug binding and allosteric modulation. 2) Computational and experimental results showed that the general volatile anesthetic halothane bound to both alpha7 and alpha4beta2 nAChRs, despite different sensitivities of these receptors to halothane. NMR data also revealed that volatile anesthetics halothane and isoflurane bound to the EC end of the beta2 TM domain, but only at the IC end of the alpha7 TM domain. 3) We not only revealed the drug binding sites but also determined that the binding site at the EC end of the TM domain is functionally relevant. 4) Several factors critical to allosteric modulation in Cys-loop receptors were identified. Applying the perturbation-based Markovian transmission model to GLIC, we identified signaling pathways of agonist-induced channel gating. Using NMR, we identified a link between protein dynamics changes and allosteric modulation. Molecular dynamics simulations suggested that asymmetric binding of the anesthetic propofol to GLIC facilitated the transition from an open- to a closed-channel structure. The study provides evidence that ligand-induced asymmetry facilitates conformational transitions.