In the march of metastasis, a molecular trail of crumbs guides some cancer cells from the primary tumor to establish new colonies within the body. Blocking the cells’ ability to follow the trail might halt metastasis but could also meddle with an intricate cellular signaling system critical to immune response. Purdue University scientists are deciphering this signaling system to better understand how it could be used to address multiple diseases, including cancer.
Recent work, published in Nature, focused on a specific transaction inside the cell but is broadly applicable to how cells respond to signals from the endocrine system, a hormonal messaging system that influences metabolism, growth and reproduction and helps the body maintain homeostasis.
“There are multiple pathways inside a cell that are triggered by this messaging system and when they don’t work together properly, it promotes disease,” said research lead John Tesmer, the Walther Distinguished Professor in Cancer Structural Biology in the College of Science and a member of the Purdue Institute for Cancer Research. “Some of these pathways are useful, so ideally, we shouldn’t just turn off the signal at the source. But maybe we can find compounds that elicit a more nuanced response inside the cell, such as by preserving good pathways and dampening those that are bad.”
Tesmer’s work is part of Purdue’s presidential One Health initiative, which involves research at the intersection of human, animal and plant health and well-being.
When cells in the body are threatened by pathogens or tissue damage, they secrete chemokines — small proteins that summon immune cells to move toward the source of the signal. As chemokines encounter cells, they bind to a receptor that spans the cell membrane, which triggers a response inside the cell. Ordinarily, these triggered receptors are gradually pulled into the cell, a mechanism that ensures a cell can respond to changes in the amount of chemokine in its environment.
Tesmer’s team studied one such receptor, atypical chemokine receptor 3 (ACKR3), which is part of a much larger class of G protein-coupled receptors (GPCRs) that stud the surface of animal, plant and fungi cells, allowing these cells to sense their environment.
GPCRs are an attractive target for drug discovery because drugs need only bind to the cell surface receptor, without entering the cell, to effect a change inside the cell. By some estimates, more than 30% of drugs approved by the Food and Drug Administration target GPCRs. While many drugs take advantage of the receptors, they function as part of a vast sequence of molecular steps that isn’t fully understood. ACKR3 offers a good example of the complexity of the system and the potential rewards of decoding it.
ACKR3 can be thought of as a flexible tube of protein that weaves its way through the cell membrane seven times to make a barrel-shaped bundle. One end of the barrel protrudes out of the cell while the other is inside. The ends form pockets to which molecules or proteins can bind. A chemokine binding to the outer pocket of ACKR3 changes the shape of the inner pocket. Like ringing a doorbell, this change in conformation activates the receptor, calling for attention from within.
A series of steps inside the cell answers the call, and with each come options that can prompt a different cellular response from the same activated receptor. First, a protein from the GPCR kinase (GRK) family arrives and attaches phosphate molecules to molecular tails dangling from the inner end of the bundle. The pattern of phosphates, known as a “barcode,” can depend on which GRK protein answers the call. The barcode in turn influences the next step, in which a protein from the so-called arrestin family binds to the barcode, cuing the cell’s response.
During metastasis, some cancer cells produce excess ACKR3 receptors that allow them to more easily follow the trail of chemokines to distant organs for colonization. With a better understanding of what happens after ACKR3 is activated, it might be possible to control this process in cancerous cells without hampering how the immune system responds to infection.
In the Nature paper, the team looked at the barcodes installed by two different GRKs, GRK2 and GRK5, into ACKR3 and how they affect the interactions of two different arrestins, arrestin2 and arrestin3, with the receptor. For each combination, they obtained structural data for the configurations formed with ACKR3 using cryogenic electron microscopy.
And indeed, both the barcode and the arrestin mattered. GRK2 puts its barcode near the ends of the molecular tails on the receptor pocket creating a loose, dynamic binding site for the arrestins, whereas GRK5 put its barcode closer to the pocket of the receptor, prompting a tight, more rigid interaction between the arrestins and ACKR3. Because arrestin3 is unable to bind to the cell surface like arrestin2, it also results in more dynamic interactions. These differences can readily be detected by signaling machinery inside the cell.
“We’re trying to get at the reason why it matters which GRK is putting the barcodes in. In particular, is there anything there that would help us understand how they differentially engage downstream machinery?” Tesmer said. “And what we found is that, for ACKR3, it’s not so much the barcode sequence itself that matters, it’s the region, the location of the barcode that matters more in terms of the arrestin-receptor configuration. This emphasizes the amazing complexity of signaling that can occur after triggering just a single receptor but also represents an opportunity for researchers to devise therapeutic approaches that might shut down one set of barcode-specific pathways but preserve others that are beneficial.”
Reference: Chen Q, Schafer CT, Mukherjee S, et al. Effect of phosphorylation barcodes on arrestin binding to a chemokine receptor. Nature. 2025;643(8070):280-287. doi: 10.1038/s41586-025-09024-9
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