Cynthia Czajkowski - US grants

DESCRIPTION (provided by applicant): Gamma-aminobutyric acid type A receptors (GABAARs) mediate synaptic inhibition in the brain and the actions of several clinically important drugs, including benzodiazepines, barbiturates, ethanol and anesthetics. Several mutations in the receptor are linked to inherited forms of epilepsy. The long-term goal of our research program is to understand the function and pharmacological modulation of the GABAAR in terms of its molecular structure. Work during the current project period significantly advanced our understanding of the structure and dynamics of the GABA and benzodiazepine (BZD) binding sites. Experiments proposed herein build on this information to advance our understanding, on a structural level, of how GABA binding triggers channel gating and how BZD binding is coupled to receptor modulation. We propose to test the following hypotheses: 1) that intra- and inter-subunit salt bridges relay GABA binding site movements in the extracellular domain to gating movements in the transmembrane channel domain by connecting rigid-body protein blocks, 2) that residues in Loop 2 are involved in GABAAR activation and desensitization, 3) that residues in Loop 9 at non-binding site interfaces comprise a `hinge'element important for coupling GABA binding to gating, and 4) that BZD binding modulates GABAAR function by triggering movements in the extracellular domain that are transduced to the transmembrane helices via residues in Loop 2, Loop 7, Loop 9, pre-M1 and M2-M3 regions of the 11 and 32 subunits. The approach combines site-directed mutagenesis, disulfide crosslinking, mutant cycle analysis, substituted cysteine accessibility method, patch-clamping and kinetic analysis. The experiments will be interpreted with the aid of recently elucidated atomic-level structures to gain a deeper understanding of the molecular mechanisms underlying the function of GABAARs and related receptors. The central role played by GABAARs in brain function make this basic research directly relevant to human health. The relevance of the research proposed herein extends beyond the GABAA receptor itself. GABAA receptors are members of a family of receptors that function as ligand-gated ion channels, and their activity regulates information flow throughout the brain. Defects in these channels lead to a wide variety of diseases, and they are the targets of a large number of clinically used drugs. Improvements in our understanding of how these channels work at a molecular level will improve our ability to predict the actions of drugs that act on these channels, to design safer and more effective drugs, to develop better therapeutic strategies, and to understand the etiology of disease-causing mutations. The research proposed here will increase our understanding of how one type of ion channel, the GABAA receptor, functions in health and disease and will establish testable hypotheses for elucidating how other related ligand-gated ion channels function.

DESCRIPTION (provided by applicant): Benzodiazepines (BDZs) exert anxiolytic and anti-epileptic effects in the central nervous system (CNS) by allosteric modulation of the GABAA receptor Cl- current (IGABA). Because of the widespread clinical utility of these drugs, understanding the mechanism(s) by which BDZs alter ion channel function is an important pharmacological inquiry. To identify the structural determinants underlying BDZ modulation of kinetic transitions of channel activation, one must first determine the effects of BDZs on identified, wild-type GABAA receptors. This will be approached using rapid drug application techniques and single-channel recordings to test which of the microscopic kinetic rate constants are altered by modulatory BDZs in a1 B2g2 receptors. We will make use of mutations in the GABA binding site, the pore region, and the extracellular loop to tease apart the contributions of BDZs to binding/unbinding and gating/desensitization of IGABA. We will also make use of tethered tandem subunits to functionally separate the contributions of each of the two GABA binding sites to these processes. The studies outlined in this application will help to establish the functional mechanisms of action for clinically relevant drugs on a major CNS variant of the GABAA receptor, and aid in establishing testable hypotheses regarding structure-function relationships in this and other ligand-gated ion channels.

DESCRIPTION (provided by applicant): Gamma-aminobutyric acid type A receptors (GABAARs) mediate the majority of synaptic inhibition in the brain and are modulated by a variety of clinically important drugs, such as benzodiazepines, barbiturates, steroids, anesthetics and anti-convulsants. Furthermore, GABAAR mutations have recently been linked to epilepsy. The long-term goal of our research program is to understand the function of the GABAAR in terms of its molecular structure. While recent crystallographic advances have provided valuable structural models of the GABAAR, achieving a full understanding of function also requires knowledge of protein dynamics. GABAARs exist in at least three interconvertible states with distinct functions: inactive/closed, active/open, and desensitized/closed. Very little is known about the protein motions that occur during these structural transitions, which are regulated by neurotransmitter and drug binding. We will use fluorescence recording of site-specific labels in GABAARs expressed in Xenopus oocytes to study the structural rearrangements underlying activation, desensitization, and drug modulation as they occur in real time. We propose to study 1) agonist, partial-agonist and antagonist induced rearrangements, 2) global protein motions, 3) pentobarbital induced structural changes and 4) protein motions during desensitization. The experiments will be interpreted with the aid of recently elucidated atomic- level structures to gain a deeper understanding of the molecular mechanisms underlying the function of GABAARs and their relatives. We cannot hope to predict the actions of a drug or ligand or predict the outcome of a disease-causing mutation in the GABAAR without dissecting the movements in the protein that mediate its function. The research proposed here utilizes an innovative new approach that will enable us to learn how GABAARs function in health and disease states. PUBLIC HEALTH RELEVANCE: The opening and closing of ligand-gated ion channels, which lie in the membranes of nerve cells, regulate information flow throughout the brain. Defects in these channels lead to wide variety of diseases, such as myasthenia, hyperekplexia and epilepsy. These channels are also the targets of a number of clinically used drugs, including muscle relaxants, sedative-hypnotics, anti-convulsants, anxiolytics and anesthetics. We cannot hope to predict the actions of a drug, design safer and more effective drugs, develop better therapeutic strategies or predict the outcome of a disease-causing mutation without knowledge of how these channels work at a molecular level. The research proposed here utilizes an innovative new approach that will increase our understanding of how one type of ion channel, the GABAAR, functions in health and disease and will establish testable hypotheses for elucidating how other related ligand-gated ion channels function.

Chemical signaling in the brain relies on rapid opening and closing of ligand-gated ion channels (LGICs) in the membranes of nerve cells. Members of the pentameric LGIC superfamily include nicotinic acetylcholine receptors (nAChR), serotonin-type-3 receptors (5HT3R), gamma-amino butyric acid type A receptors (GABAAR) and glycine receptors. Defects in these channels lead to a variety of neurological diseases and psychiatric disorders and a number of therapeutic drugs, including muscle relaxants, sedative-hypnotics, anticonvulsants, anxiolytics, intravenous and volatile anesthetics, anti-emetics, drugs for nicotine addiction and drugs to treat Alzheimer¿s disease target these channels. For these receptors, binding of neurotransmitter in the extracellular ligand-binding domain triggers opening of an intrinsic ion channel more than 50¿ away in the transmembrane domain of the receptor. Although we know a fair amount about the structure of these receptors, the mechanisms by which the binding of neurotransmitter triggers channel opening are still under debate and our understanding of the protein motions underlying this process limited. The general plan of this proposal is to investigate the binding-to-gating motions in the prokaryotic pLGIC homologs from Gloeobacter violaceus (GLIC) using site-directed spin labels and electron paramagnetic resonance spectroscopy (SDSLEPR) and to test these motions in the GABAAR using an array of biochemical and electrophysiological approaches including voltage clamping, mutant cycle analysis, cysteine cross-linking, disulfide trapping and structural modeling. We will focus on three key regions: the extracellular binding domain (EBD), the gating interface and the transmembrane channel domain (TCD). These studies will build on our previous work and will provide new insights into how neurotransmitters activate pLGICs and how allosteric drugs modulate their activity. A deeper understanding of how these channels work at a molecular level will improve our ability to predict the actions of drugs and ligands that act on these channels, design safer and more effective drugs, develop better therapeutic strategies, and understand the etiology of disease-causing mutations.

Project Summary Benzodiazepines (BZDs) are widely prescribed drugs and exert their anxiolytic, muscle relaxant, sedative, and anticonvulsant actions by binding to GABA-A receptors (GABARs) and potentiating GABA-induced currents. Since their synthesis over 50 years ago, scientists have searched for endogenous ligands in the brain that bind to the BZD binding site and modulate GABAR activity. Recent evidence suggests a 10 kDa protein named `Diazepam binding inhibitor' (DBI) is the brain's endogenous BZD (endozepine). Depending on the brain region, DBI can potentiate or inhibit GABAR activity suggesting that the brain can modulate GABAR-mediated neuronal inhibition by controlling the levels of DBI and its cleavage products. Little, however, is known about how DBI levels are regulated, how it is processed, or how DBI exerts its positive versus negative effects on GABAR activity. Experiments proposed will address these fundamental questions, which will validate DBI's role as an endogenous BZD and provide new mechanistic insights into how endogenous BZDs regulate GABA- mediated inhibition in the brain. Decreases in GABAergic neurotransmission are linked to numerous neurological and mental health disorders such as insomnia, epilepsy, anxiety, autism, Fragile X and schizophrenia. Whether regional changes in DBI levels or DBI cleavage products contribute to human disease is unknown. Our findings will open up new areas of scientific inquiry and have the potential to reveal new targets for therapeutic drug development.