December 20, 2013
New X-Ray Techniques Let Researchers Map Crucial Drug Targets
Researchers have used one of the brightest X-ray sources in the world to map the three-dimensional structure of an important cellular gatekeeper known as a G protein-coupled receptor, or GPCR, in a more natural state than has been previously possible. The new technique, described Friday in the journal Science, is a major leap forward in exploring GPCRs, a vast, hard-to-study family of proteins that plays a key role in human health and is targeted by an estimated 40 percent of modern medicines.
"For the first time we have a room-temperature, high-resolution structure of one of the most difficult to study but medically important families of membrane proteins," said lead researcher Vadim Cherezov, a pioneer in GPCR research at The Scripps Research Institute.
In the experiment, researchers examined the human serotonin receptor, which plays a role in learning, mood and sleep and is the target of drugs that combat obesity, depression and migraines. The scientists prepared crystallized samples of the receptor in a fatty gel that mimics its environment in the cell.
With a newly designed injection system, they streamed the gel into the path of the Linac Coherent Light Source (LCLS) X-ray pulses, which hit the crystals and produced patterns used to reconstruct a high-resolution, 3D model of the receptor.
The technique eliminates one of the biggest hurdles in the study of GPCRs – the fact they are particularly difficult to crystallize in the large sizes needed for conventional X-ray studies at synchrotrons.
Because LCLS is millions of times brighter than the most powerful synchrotrons and produces ultrafast snapshots, it allows researchers to use tiny crystals and collect data in the instant before any damage sets in. Furthermore, the samples do not have to be frozen to protect them from X-ray damage, and can be examined in a more natural state.
"This is one of the niches that LCLS is perfect for," said Staff Scientist Sébastien Boutet, a staff member at the Department of Energy's SLAC National Accelerator Laboratory and a co-author of the report.
"With really challenging proteins like this you often need years to develop crystals that are large enough to study at synchrotron X-ray facilities."
Cherezov said even after samples of a GPCR are crystallized and imaged, it can take several months to optimize the crystal size and collect enough synchrotron X-ray data to produce structural information.
The experiment demonstrates that an LCLS experiment using smaller crystals can potentially condense that timeline to a matter of days, he said.
Disorders linked to GPCRs include hypertension, asthma, schizophrenia and Parkinson's disease. Because of their vital role in regulating cells' signaling and response mechanisms and their importance to human health, advances in receptor-related research garnered the 2012 Nobel Prize in Chemistry.
To date, scientists have only been able to map the structures of a relative handful of the estimated 800 GPCRs in humans. Although the human serotonin structure had been determined earlier with conventional methods, that effort took much longer and showed the receptor in a less realistic environment.
The more accurate the structure, the better scientists can use it to tailor effective drug treatments without side effects, meaning this new technology could eventually lead to an efficient platform for designing drugs based on GPCR structures, the researchers said.
"I view these recent experiments as just the beginning," Cherezov said.
"Now it is time to start making a serious impact on the field of structural biology of G protein-coupled receptors and other challenging membrane proteins and complexes. The pace of structural studies in this field is breathtaking, and there is still a lot unknown."
The team has also studied two other GPCRs and a membrane enzyme, and will begin follow-up research next month.