American Scientist

The surface of every cell of the human body has proteins that snake in and out of its membrane. These proteins are receptors that are responsible for picking up signals from the environment—everything from smells to hormones to neurotransmitters. In humans, an estimated 1,000 different members are in this family of proteins, called G-protein–coupled receptors (GPCRs), and they are one of the most common drug targets: More than one-third of all medications approved by the U.S. Food and Drug Administration (FDA) act by binding to GPCRs. But an entire aspect of the receptors has been largely overlooked. The receptor proteins interact with a second group of proteins in the body called receptor activity-modifying proteins (RAMPs), in a way that appears to be far more widespread than anyone previously knew.

RAMPs bind to and change the shape of GPCRs. There are only three known RAMPs, and there were just a few published examples of interactions between the two protein families—until now. In a paper published September 18, 2019, in Science Advances, molecular biologist Thomas Sakmar, of Rockefeller University in New York, found 10 more GPCRs that interact with RAMPs. And he says that’s likely just the tip of the iceberg.

“If one of three RAMPs is in almost every cell, and there are 100 receptors in the cell, it’s likely that the interaction is more widespread than the sporadic case report,” Sakmar says. The findings, which he and graduate student Emily Lorenzen plan to expand upon, have big implications for drug discovery. Because most pharmaceuticals are tested on cell lines, which usually lack RAMPs, researchers can only test them against the known GPCRs—something that wouldn’t necessarily translate to more complicated receptors in living humans.

The first RAMP was discovered in the 1990s, when researchers at Glaxo Wellcome (now GlaxoSmithKline) were trying to clone a GPCR receptor that seemed to keep disappearing. After investigating more closely, they discovered that what they were cloning wasn’t the receptor at all but a transmembrane protein that—by binding to the receptor—completely changed its pharmacology. Today, a few drugs on the market target these GPCR/RAMP complexes, but Sakmar and Lorenzen believe their discovery will pave the way for many more.

Lorenzen first came up with the idea of searching for additional RAMPs as she was beginning her dissertation research at Rockefeller University. Sakmar asked her to look into the interaction between just one GPCR and the RAMP known to be associated with it for her PhD project. But when she started her research for the project, something got her attention. The more she learned, the more she became convinced that the picture was much bigger than her assigned GPCR, and that interactions between the two families of proteins may be far more common.

She then proposed a project to try to assess how widespread the GPCR/RAMP interactions really are. But the huge variety of GPCR proteins, combined with the additional variation that RAMPs introduce, meant Lorenzen and Sakmar were looking at a massive puzzle with thousands of possible permutations.

Lorenzen recruited the expertise of Jochen Schwenk and his research team with the Science for Life Laboratory at the KTH Royal Institute of Technology in Stockholm, Sweden. Schwenk and his colleagues have developed antibodies to almost every human protein, and Lorenzen wanted to use those to capture and visualize GPCRs, RAMPs, and the GPCR/RAMP complexes. The researchers used an approach based on an array that contained color-coded magnetic nanobeads: There can be up to 500 different colors of beads, each one coupled to its own antibody, and each of those antibodies binds to a specific GPCR or RAMP. When the color-coded antibodies are mixed with a solution of GPCRs and RAMPs, they simultaneously pull out the proteins from the solution and label them.

In their small sample of 23 GPCRs, the team found 10 new receptor-RAMP complexes. “To be honest, I would have been happy just finding a couple. Finding so many was really exciting,” Lorenzen says. She notes that the implications for drug development are extensive since, to date, most therapeutics have focused on single receptors, and therefore few pharmaceuticals have targeted protein-protein complexes. “I see it as this broad landscape of potential that could be used for a huge variety of diseases,” she says.

Sakmar notes that until now scientists had been targeting cell-surface GPCRs in isolation. But there could be novel ways to attack disease using receptor-RAMP complexes as the target. “We’re applying a new technology—this bead-array technology—to study an entire family of related receptors at the same time instead of one by one,” Sakmar says. “We hope it could become a platform for drug discovery.”

Steven Foord led the Glaxo Wellcome research in the 1990s in which RAMPs were first discovered. Now a consultant based in Philadelphia, he thinks Lorenzen’s work is important. “It gives you a methodology for testing multiple combinations of proteins that might not otherwise have been tested,” he says. Beyond that, it also opens up the opportunity to better characterize a group of GPCRs called orphan receptors—proteins that have been described but have no known function. “They’re really important,” he says. “They’re in humans, sharks, and lampreys and have been conserved throughout evolution, but we still don’t know what switches them on, why they’re there, or what they do.”

It may well be that the reason the orphan receptors have been so difficult to understand is because the actual receptor may not be a GPCR in isolation but a receptor-RAMP complex. Characterizing those complexes could finally open up understanding of what they do and how they might be used as new drug targets. “These receptors that we don’t know what they do? There will be drugs in there,” Foord says,“things we’re completely missing right now.”