Scientists at the Scripps Research Institute (TSRI) and Vanderbilt University say they have created the most detailed 3D picture yet of a membrane protein that is linked to learning, memory, anxiety, pain, and brain disorders such as schizophrenia, Parkinson's, Alzheimer’s, and autism.
“This receptor family is an exciting new target for future medicines for treatment of brain disorders,” said P. Jeffrey Conn, Ph.D., Lee E. Limbird Professor of Pharmacology and director of the Vanderbilt Center for Neuroscience Drug Discovery. He along with Raymond Stevens, Ph.D., a professor in the department of integrative structural and computational biology at TSRI, were senior authors of the study (“Structure of a Class C GPCR Metabotropic Glutamate Receptor 1 Bound to an Allosteric Modulator”) that appears in Science. “This new understanding of how drug-like molecules engage the receptor at an atomic level promises to have a major impact on new drug discovery efforts.”
The mGlu1 receptor, which helps regulate the neurotransmitter glutamate, belongs to a superfamily of molecules known as G protein-coupled receptors (GPCRs). After binding molecules outside the cell, GPCRs trigger a specific response inside the cell. More than one-third of therapeutic drugs target GPCRs, including allergy and heart medications, drugs that target the central nervous system, and antidepressants.
When the Stevens group decided to pursue the structure of mGlu1 and other key members of the mGlu family, it was natural that the scientists reached out to the researchers at Vanderbilt. “They are the best in the world at understanding mGlu receptors,” said Dr. Stevens. “By collaborating with experts in specific receptor subfamilies, we can reach our goal of understanding the human GPCR superfamily and how GPCRs control human cell signaling.”
The task was made more difficult because there was no template for mGlu1 from closely related GPCR proteins to guide the researchers.
“mGlu1 belongs to class C GPCRs, of which no structure has been solved before,” said TSRI graduate student Chong Wang, a first author of the new study with TSRI graduate student Huixian Wu. “This made the project much harder. We could not use other GPCRs as a template to design constructs for expression and stabilization or to help interpret diffraction data. The structure was so different that old school methods in novel protein structure determination had to be used.”
The team decided to try to determine the structure of mGlu1 bound to novel “allosteric modulators” of mGlu1 contributed by the Vanderbilt group. Allosteric modulators bind to a site far away from the binding site of the natural activator (in this case, presumably the glutamate molecule), but change the shape of the molecule enough to affect receptor function. In the case of allosteric drug candidates, the hope is that the compounds affect the receptor function in a desirable, therapeutic way.
The team proceeded to apply a combination of techniques, including X-ray crystallography, structure-activity relationships, mutagenesis and full-length dimer modeling. At the end of the study, they had achieved a high-resolution image of mGlu1 in complex with one of the drug candidates, as well as a deeper understanding of the receptor's function and pharmacology.
“We determined the 2.8 Å resolution structure of the human mGlu1 receptor seven-transmembrane (7TM) domain bound to a negative allosteric modulator FITM,” wrote the investigators. “The modulator binding site partially overlaps with the orthosteric binding sites of Class A GPCRs, but is more restricted compared to most other GPCRs.”
The findings show that mGlu1 possesses structural features both similar to and distinct from those seen in other GPCR classes, but in ways that would have been impossible to predict in advance.
“The mGlu1 receptor structure now provides a solid platform for much more reliable modeling of closely related receptors, some of which are equally important in drug discovery,” said Vsevolod “Seva” Katritch, Ph.D., assistant professor of molecular biology at TSRI and a co-author of the paper.