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New Insights into Adhesion GPCRs and Potential Drug Targets

Recent studies have revealed that approximately 35% of pharmaceuticals approved by the Food and Drug Administration (FDA) act on G protein-coupled receptors (GPCRs), vital proteins located in cell membranes that facilitate communication between cells. Among these, adhesion G protein-coupled receptors (aGPCRs) form the second largest receptor family in the human body. As their name implies, these receptors play a crucial role in cell adhesion and intracellular signaling.

aGPCRs are integral to numerous biological processes, influencing tissue growth, immune responses, and organ development. Disruptions or malfunctions in these receptors can contribute to various health issues, including cancer, neurological disorders, and developmental abnormalities. Despite their significance, current pharmacological interventions do not target aGPCRs due to their large and intricate structures, making them challenging to investigate.

Research conducted at the University of Chicago has recently leveraged innovative imaging technologies to elucidate the full structure of a common aGPCR. This research encompasses how the receptor’s extensive and complex extracellular segment interacts with its transmembrane domain embedded within the cell membrane. The findings suggest that the positional shifts and movements of the extracellular region may play a key role in activating 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,” stated Demet Araç, PhD, an Associate Professor of Biochemistry and Molecular Biology at UChicago and the principal investigator of this study. The findings were published in the journal Nature Communications.

Innovative Imaging Techniques Yield New Insights

The extracellular region of aGPCRs extends from the cell’s membrane into the extracellular environment, where it can interact with various molecules and other cellular receptors. This region contains several structural domains, including the GAIN (GPCR Autoproteolysis Inducing) domain, which has the ability to cleave itself into two segments.

Traditionally, it was thought that aGPCR activation occurs when a ligand binds to an extracellular domain, thereby exerting a force that separates the GAIN domain from a tethered agonist (TA) peptide. The TA then moves and interacts with the transmembrane domain to trigger signaling. However, increasing evidence shows that many functions of aGPCR might not depend on this irreversible cleavage mechanism. This raises questions about how cells might need a way to modulate receptor activity dynamically.

Over the past 11 years, Araç’s laboratory has aimed to elucidate the complete structure of aGPCRs to understand how signals are conveyed from outside to inside cells. This research has been challenging due to the diverse and complex configurations of the extracellular regions. Graduate student Szymon Kordon led this endeavor, continuing prior work to capture images of the entire structure of Latrophilin3, a well-studied aGPCR associated with brain synapse development and linked to conditions like attention deficit hyperactivity disorder and certain cancers.

Working alongside Kordon, Araç optimized the methods for generating and purifying Latrophilin3 to obtain the necessary electron microscopy images. To address the challenges in visualizing the receptor, they collaborated with Antony Kossiakoff, PhD, a professor at UChicago, to develop a synthetic antibody that could bind to the aGPCR. This antibody provided stability to the extracellular region and shaped it in a way that permitted Kordon to utilize cryo-electron microscopy (cryo-EM) for detailed imaging. The resulting data yielded the first complete structure of an aGPCR.

The cryo-EM images illustrated that the GAIN domain adopted various positions relative to the cell membrane. Each of these positional configurations established distinct contact points with the transmembrane region, leading researchers to hypothesize that these different states could facilitate communication with the cell without complete separation of the GAIN domain. To explore this idea further, they collaborated with Reza Vafabakhsh, PhD, and Kristina Cechova, PhD, from Northwestern University, to conduct further experiments that tracked movements of the extracellular segments.

Using Förster resonance energy transfer (FRET) imaging, which can detect energy transfer between closely positioned molecules, Cechova and her team attached fluorescent markers to key regions of both the extracellular and transmembrane areas of the receptor. This enabled them to observe its movements in response to external adhesion forces. Their observations confirmed that different conformational states corresponded to varying receptor signaling activities.

“Different conformational states correlated to different signaling activity of the receptor,” Kordon remarked, underscoring the functional importance of these States in cellular signaling. His exceptional contributions to this research earned him the Best Dissertation Award from the University of Chicago’s Department of Biochemistry and Molecular Biology.

Prospects for Targeting aGPCRs in Drug Development

With a clearer understanding of aGPCR structure and function, Araç indicated that there is now promise for developing drugs that target these receptors, akin to other receptor types. One potential strategy is to engineer antibodies similar to those utilized in this study, but designed to modulate receptor activity rather than solely for imaging purposes. Given the distinct shapes and structures of aGPCRs, such antibodies could be tailored for precise targeting. With 33 different aGPCRs already identified in humans, there are abundant avenues for further exploration.

“This could be the future of drugging adhesion GPCRs,” Araç asserted, highlighting that since extracellular regions vary significantly among aGPCRs, it may be possible to create targeted drugs that minimize undesirable side effects by avoiding binding to other receptor types.

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