The binding pocket into which big dynorphin, or BigDyn, settles ought to be obvious. But in fact, this pocket is easy to miss, or at least it seems that way to scientists who would like to design painkilling medications. These scientists want to know how BigDyn, an opioid neuropeptide, interacts with a membrane-embedded receptor called acid-sensing ion channel (ASIC). BigDyn and ASIC, a proton-gated cation channel, work together to send pain signals through the brain. If the fit between them were better understood, scientists could find ways to interfere with the BigDyn-ASIC interaction, which has been implicated not only in pain but also neuronal death following ischemic stroke.

In a new study, scientists based at the University of Copenhagen employed a broad array of technologies to unravel the details of where BigDyn binds to ASIC and how BigDyn modulates ASIC’s activity. The scientists, led by Stephan Pless, PhD, professor of drug design and pharmacology, reported their findings March 31 in the Proceedings of the National Academy of Sciences, in an article titled, “Mechanism and site of action of big dynorphin on ASIC1a.”

“We demonstrate that BigDyn binding results in an ASIC1a closed resting conformation that is distinct from open and desensitized states induced by protons,” the article’s authors wrote. “Using alanine-substituted BigDyn analogs, we find that the BigDyn modulation of ASIC1a is primarily mediated through electrostatic interactions of basic amino acids in the BigDyn N terminus.”

To arrive at these findings, the scientists experimentally manipulated the peptide and the receptor using technologies such as electrophysiology, genetically encoded cross-linkers, as well as CRISPR. Normally, the biological interaction within the brain cell occurs very fast, but their approach allowed the researchers to trap the interactions and map them.

Perhaps the most interesting finding was the identification of ASIC’s acidic pocket as the binding site for BigDyn. “Neutralizing acidic amino acids in the ASIC1a extracellular domain reduces BigDyn effects, suggesting a binding site at the acidic pocket,” the article’s authors detailed. “This is confirmed by photocrosslinking using the noncanonical amino acid azidophenylalanine.”

ASIC’s acidic pocket, the scientists suggested, could be an important site for the development of ASIC-targeting therapeutics.

“We have mapped exactly how and where BigDyn binds to this receptor, which can cause a pain signal to be sent inside the body,” said Pless. “BigDyn is the most potent regulator of this particular receptor discovered in the human body so far. The painkillers that we use today affect other types of receptors. This means that our discovery could pave the way for a new type of painkilling medicine via this receptor, potentially helping to circumvent some of the typical adverse effects of opioids.”

“We know that both the receptor and BigDyn are upregulated in patients with inflammation and chronic pain,” added Nina Braun, a postdoctoral researcher in the Pless laboratory. “This means that there are many more of them than under normal conditions. And that, in theory at least, can lead to more pain and the risk of long-lasting negative effects on brain health. This means that our result could have implications for these diseases in terms of drug development.”

Other researchers have previously shown that if the ASIC receptor is knocked out, it is possible to reduce pain in mouse models. This underlines the potential of the new findings from the University of Copenhagen.

Pless and colleagues are looking forward to exploiting the mechanism pharmacologically in their coming studies. They hope to find relevant compounds that could show potential for pain reduction for these vulnerable patient-groups.

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