Structural analysis of human GABAA receptors

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The human brain is the most powerful biological processor on the planet. It is also extremely complicated. It contains around one-hundred billion brain cells and over a trillion connections between those brain cells which channel the flow of vast quantities of information. Clearly such a complicated system requires rules if it is to operate smoothly. It requires a regulatory system. And the targets of our research, gamma-aminobutyric acid Type-A receptors, termed GABA-A-Rs, are a most important part of this regulatory system.GABA-A-Rs are proteins expressed at the surface of nerve cells. They exist throughout the brain to allow nerve cells to respond to the neurotransmitter gamma-aminobutyric acid, called GABA. The neurotransmitter GABA acts upon GABA-A-Rs to send a stop signal to nerve cells. This means GABA-A-Rs are the brakes on the brain. They spread calm. They regulate excitement. And so it is no surprise that when these regulators fail the result is heightened brain activity, which leads to a range of debilitating illnesses such as insomnia, anxiety-disorders and epilepsy, disorders that affect tens of millions of people worldwide. Fortunately there are treatments available, most famously benzodiazepines which reinforce the calming influence of GABA-A-Rs. Unfortunately, because GABA-A-Rs are vital to so many different processes in the brain benzodiazepines have side-effects. Furthermore, the many roles that GABA-A-Rs play in brain function are far from fully resolved. For example, GABA-A-Rs are also targets for general anaesthetics and for the brains home-grown anti-stress molecules, the neurosteroids. They are also targets in depression and chronic pain, and are being investigated as therapeutics in schizophrenia, stroke and alcohol addiction. Thus, it is vital to understand as fully as possible how these proteins operate in order to better understand how GABA-A-Rs contribute to brain function.One way to do this is to analyse the structures of GABA-A-Rs. Obtaining three-dimensional maps of GABA-A-Rs at high (atomic or near-atomic) resolution provides vital information on, for example, how these biological machines respond to the neurotransmitter GABA to internally reorganisation themselves to open a built-in gate that lets ions flow across the membranes of nerve cells. Such maps will also reveal how GABA-A-Rs bind clinically relevant drugs and natural ligands in the human brain. Finally, there are different populations of GABA-A-Rs in the human brain, with each population having unique structures, unique functions, unique expression profiles, and unique pharmacology. So obtaining structural data for each of the distinct GABA-A-Rs populations will unveil their distinguishing features and how these features imbue unique properties. Furthermore, detailed structures permit visualisation of drugs bound to GABA-A-Rs, which will inform on design of improved therapeutics in the future.However, obtaining high-resolution structures of GABA-A-Rs is extremely challenging. Large quantities of highly pure GABA-A-Rs in a stable form are required, something that is especially difficult because GABA-A-Rs are very delicate proteins. Fortunately in our laboratory we have state-of-the-art technologies for protein production, purification, screening, and structure determination. That is why this year (2014) we were successful in publishing the first ever high resolution structure of a GABA-A-R, revealing data on its mode of operation and on the drug-binding cavities it contains. However, a single structure is only a snap-shot. To gain a better understanding of protein function many alternative structures (snap shots) of these biological machines in action are required, as well as of other subtypes. We believe that by obtaining more GABA-A-R structures in the future we will substantially enhance the understanding of how GABA-A-Rs impact on brain function and inform pharmaceutical enterprise on how to design improved therapeutics.