Almost 35% of drugs approved by the Food and Drug Administration work by targeting G protein-coupled receptors (GPCRs), proteins embedded in cell membranes that allow cells to communicate with each other. Adhesion G protein-coupled receptors (aGPCRs) are the second largest family of these receptors in humans. As their name suggests, they help cells adhere, or stick to each other, and send signals inside the body.
These receptors are involved in many processes, such as how tissues grow, how the immune system works, and how organs form. Problems with aGPCRs can also lead to diseases like cancer, brain disorders, and growth issues. Despite the obviously important role they play in the body, however, there are no drugs approved to target aGPCRs because they are large, complex, and difficult to study.
New research from the University of Chicago combines two powerful imaging techniques to study the complete structure of a common aGPCR, including how its long and complex extracellular region interacts with the transmembrane region embedded in the cell surface. The different positions and movements of the extracellular region appear to be an important way to activate the receptor.
“This opens up new opportunities for drugging adhesion GPCRs, because now we are showing that the extracellular region is communicating with the transmembrane region,” said Demet Araç, PhD, Associate Professor of Biochemistry and Molecular Biology at UChicago and senior author of the new study. The results were published this month in Nature Communications.
Capturing new images and new configurations
The extracellular region of an aGPCR extends from the cell membrane into space outside the cell, where it can bind to molecules and receptors from other cells. It consists of several domains, including the GPCR Autoproteolysis INducing (GAIN) domain, which can cleave itself into two pieces.
The common understanding of how to activate an aGPCR is that a ligand from outside the cell attaches to one of the extracellular domains and exerts force that separates the GAIN domain from its other piece, a peptide called the tethered agonist (TA) that remains attached to the transmembrane region. When the TA is separated, it can move and interact with the transmembrane region to initiate signaling, but a growing body of biochemistry research shows that many aGPCR functions don’t rely on this cleavage-dependent mechanism. Separating the GAIN domain is also irreversible, leaving the receptor in a constant “on” state, which may be harmful for the cell. Sometimes a cell may need to toggle a receptor on and off, so there must be some other way of doing it.
Araç’s lab has been working for 11 years to reveal the structure of full-length aGPCRs, hoping to learn how incoming signals are transmitted from outside to inside the cell. These receptors are notoriously difficult to fully understand because the extracellular regions have many complex and distinct configurations. Graduate student Szymon Kordon, PhD, led the new study, picking up the work of a previous student to capture images of the complete structure of Latrophilin3, an aGPCR involved in developing brain synapses that has also been linked to attention deficit hyperactivity disorder and several cancers.
Kordon and Araç optimized generation and purification of Latrophilin3 and captured initial electron microscopy images, but they faced numerous challenges to get a good picture of the receptor. They then worked with Antony Kossiakoff, PhD, the Otho S.A. Sprague Distinguished Service Professor of Biochemistry and Molecular Biology at UChicago, to create a synthetic antibody that could attach to the aGPCR. This antibody stabilized the extracellular region and lent it a distinctive shape that allowed Kordon to capture the full receptor structure using cryo-electron microscopy (cryo-EM), an imaging technique that freezes cells and molecules for a snapshot. The resulting images became the first known structure of a complete aGPCR.
