'Caffeine Receptor' Solved: Structure Of Important Neurological Receptor Defined

The study was published on October 2, 2008, in Science Express, an advance, online publication of selected research papers from the journal Science.

"We are developing a robust platform for studying human G protein-coupled receptor structure and function," Raymond Stevens, a Scripps Research scientist and professor. "This work lays a strong foundation for understanding drug-receptor interactions. We expect to continue our work and develop a deep understanding as to how drugs interact with the broader class. The findings—and our future research—could one day lead to the development of a novel class of therapeutics with improved pharmaceutical properties."

The new study defined the structure of the human A_2A adenosine receptor—sometimes referred to as the "caffeine receptor"—which falls in the larger family of G protein-coupled receptors (GPCR).

"Last year, we determined the structure of the β2-adrenergic G protein-coupled receptor with multiple ligands," said Stevens. "The big question then was—is it going to be another 10 years until we get the next new receptor? The answer is 'no.' It has taken less than a year to determine this new structure. Our expectation is that even more will come out in the next few years."

Because of the importance of GPCRs to health and medicine and previous lack of knowledge about their structure, the Stevens lab's 2007 research solved the structure of the β2-adrenergic G protein-coupled receptor and was selected as one of the top 10 breakthroughs of the year by Science magazine.

"The National Institutes of Health supports programs specifically designed to develop technology to elucidate the structures of membrane proteins such as G-protein coupled receptors, which are critical for almost all aspects of health and disease," said Jeremy M. Berg, director of the NIH's National Institute of General Medical Sciences. "The recent successes with the new methods foreshadow exciting future advances in determining the structures of other medically important proteins."

In the new study, the Stevens laboratory worked with the IJzerman laboratory at the Leiden/Amsterdam Center for Drug Research in The Netherlands, to illuminate the A_2A adenosine receptor. This receptor is blocked by methylxanthines like caffeine, which prevents the binding of other naturally occurring ligands. Interestingly, there is evidence that coffee drinkers have a lower risk of Parkinson's disease.

Because membrane proteins like adenosine receptors have been notoriously difficult to crystallize—a key step in determining the structure of a molecule through the technique of x-ray crystallography—the scientists bound the A_2A adenosine receptor to a high-affinity antagonist, ZM241385. ZM241385, which had been developed as a potential drug to combat Parkinson's disease, stabilizes the receptor.

With the two molecules bound together, the scientists were able to obtain crystals of the complex, and determine its structure.

A Big Surprise

The crystallographic model of the A_2A receptor bound to ZM241385 reveals features distinct from previously reported GPCR structures.

With over one thousand members, G protein-coupled receptors are one of the most diverse protein families in the human genome. They transduce or convert extracellular stimuli into intracellular signals through a number of signaling pathways including neurotransmitters, light, hormones, lipids, and proteins. Because of their diverse signaling pathways, approximately one third, and perhaps as many as half, of currently marketed drugs are designed to target these receptors.

Extracellular adenosine plays an important role in physiology and initiates most of its effects through the activation of four GPCR subtypes, A₁, A_2A, A_2B, and A₃. Each of these four receptors plays an essential role in responding to adenosine in the central nervous system in pain regulation, cerebral blood flow, basal ganglia functions, respiration, and sleep.

In the new study, the A_2A adenosine-ligand bound structure suggests that there is no general receptor binding pocket conserved across the adenosine receptor family. Rather, the pocket itself can vary in position and orientation, yielding more opportunity for receptor diversity and ligand selectivity.

"A big surprise for us seeing the structure was that the ligand was in an extended conformation and pointed perpendicular to the membrane, interacting with the extracellular loops," Stevens said.

This feature can be seen as the rationale for A_2A receptor selectivity and may help in the design of new chemical entities with increased selectivity for this important drug target.

Stevens has focused on the structural studies of G protein-coupled receptors for almost two decades. He is the director of the RoadMap Joint Center for Innovative Membrane Protein Technologies (2003), and a co-investigator of the PSI-2 Accelerated Technology Center for Gene to 3D Structures (2005), both projects in collaboration with the Peter Kuhn laboratory at Scripps Research and funded by the National Institutes of Health, focused on the development of technologies to accelerate the study of human membrane protein structural biology.

The other authors of the study, The 2.6 Å Crystal Structure of a Human A2A Adenosine Receptor Bound to an Antagonist, are Veli-Pekka Jaakola, Mark T. Griffith, Michael A. Hanson, Vadim Cherezov, and Ellen Y.T. Chien of The Scripps Research Institute and J. Robert Lane and Adriaan P. IJzerman of the Leiden/Amsterdam Center for Drug Research, The Netherlands.

The study was supported by the National Institutes of Health, Pfizer, and the Dutch Top Institute Pharma.