After many years of trying, Paul Miller and Radu Aricescu of the Structural Biology Division (STRUBI) at the WTCHG have solved the first X-ray crystal structure of a neurotransmitter receptor that plays a vital role in neurological disorders such as epilepsy, insomnia and anxiety, and mediates the action of antidepressants, general anaesthetics and alcohol.
‘This is one of the longest sought-after molecular structures in neuroscience,’, says Aricescu, ‘facilitating the interpretation of decades of pharmacological research.’
The human brain is the most powerful information processor known. It contains around a hundred billion neurons with over a trillion connections between them, channeling vast quantities of information. Such a machine requires carefully balanced rules if it is to operate smoothly, including complex feedback control systems to manage the level of activity.
The neurotransmitter gamma-aminobutyric acid, known as GABA, damps down activity in neurons by binding to GABAA receptors in the nerve cell membrane. This means that GABAA receptors are the brakes on the brain: they spread calm and regulate excitement. When these regulators fail, brain activity increases, which leads to a range of debilitating illnesses affecting tens of millions of people worldwide.
The STRUBI researchers used their understanding from many years of research into various forms of human GABAA receptors to construct one that, unusually, they were able to crystallise successfully. They then used the powerful X-ray beam at the Diamond Light Source to obtain diffraction data from the crystal.
Their findings, published in the journal Nature, revealed a large structure consisting of five protein subunits that cross from one side of the cell membrane to the other and create a gated channel. Visualising the receptor at high resolution made it possible to identify the areas on the molecule where GABA and other inhibitory molecules bind. This helps to explain how that binding opens the channel so that chloride ions can enter the cell, making it less sensitive to excitatory inputs.
‘This is a valuable result,’ says Miller, ‘because GABAA receptors are the principal mediators of rapid inhibitory synaptic transmission in the human brain.’ GABAA receptors are the targets of a wide range of drugs including benzodiazepines (such as Valium), and the world’s most commonly used intravenous general anaesthetics, propofol and etomidate. The drunkard’s slurred speech and staggering gait is also due to the action of ethanol at GABAA receptors. The new structure has made it possible to pinpoint the effects on this receptor, and other structurally-related proteins, of numerous genetic mutations that cause conditions such as epilepsy and insomnia.
Moreover, the research team crystallised the receptor bound to a small molecule, benzamidine, not previously known to bind at this site. Benzamidine promotes the opening of the chloride channel, inhibiting the activity of the target cell. These structural data now open a new avenue for the rational design of drugs that modulate the action of GABAA receptors. With greater understanding of the binding sites of a variety of molecules that target the receptor expected to emerge in the near future, it may become possible to design novel drugs that specifically address the broad range of GABAA receptor-dependent neurological conditions.
‘Human GABAA receptor subunits are encoded by 19 different genes’ says Aricescu. ‘Although the structure we report can be considered groundbreaking, we have only scratched the surface of an enormously complex system. But we are extremely optimistic because most of the technologies we developed along the way should be transferable to other members of the same receptor family and facilitate rapid progress of the field.’