by With the help of some sea slugs, University of Nebraska-Lincoln chemists discovered that one of the smallest possible modifications to a biomolecule can produce one of the biggest possible consequences: directing the firing of neurons.
Their discovery came from investigating peptides, the short chains of amino acids that can transmit signals between cells, including neurons, that inhabit the central nervous systems and bloodstreams of most animals. Like many other molecules, an amino acid in a peptide can adopt one of two forms that have the same atoms, with the same connectivity, but in mirror-image orientations: L and D.
Chemists often think of these two orientations as the left and right hands of a molecule. The L orientation is by far the most common in peptides, to the point that it is considered the default. But when enzymes reverse the L to a D, the seemingly small face can turn, say, a potentially therapeutic molecule into a toxic one, or vice versa.
Now, Husker chemists James Checco, Baba Yussif and Cole Blasing have revealed an entirely new role for this molecular mirroring. For the first time, the team showed that the orientation of a single amino acid — in this case, one of dozens found in a sea slug’s neuropeptide — can dictate the likelihood that the peptide will activate one neuron receptor over another. Because different types of receptors are responsible for different neuronal activities, the finding points to another means by which a brain or nervous system can regulate the labyrinthine communication between its cells that sustains life.
“We discovered a new way that biology works,” said Czeko, an assistant professor of chemistry at Nebraska. “It’s nature’s way of helping to make sure the peptide goes into one signaling pathway over another. And understanding more about that biology will help us be able to exploit it for future applications.”
Checco’s interest in neuropeptide signaling dates back to his time as a postdoctoral researcher, when he came across the first study showing evidence of a peptide with a D-amino acid activating a neuron receptor in slugs. This particular receptor responded to the peptide only when it contained the D-amino acid, making its flip from L to D similar to an on/off switch.
Eventually, Checco himself will recognize a second such receptor. Unlike the one that had originally sparked his interest, Checco’s receptor responded to both a peptide containing all L-amino acids and the same peptide with a single D. But the receptor was also more responsive to the all-L peptide, activating when introduced in lower concentrations than its D-containing counterpart. Instead of an on/off switch, Checco seemed to have come up with something closer to a dimmer.
“We were left wondering: Is this the whole story?” Cheko said. “What’s really going on? Why make this D molecule if it’s even worse at activating the receptor?”
The newest findings of the team, detailed in the magazine Proceedings of the National Academy of Sciences, it hints at a response inspired by a hypothesis. Perhaps, the team thought, there were other receptors in the sea slug sensitive to this D-containing peptide. If so, perhaps some of those receptors would respond differently to it.
Yussif, a PhD candidate in chemistry, went to work looking for sea slug receptors whose genetic blueprints resembled what Checco had uncovered. He eventually narrowed down a list of candidates, which the team cloned and managed to express in cells before inserting them into the same D-containing peptide as before. One of the receptionists answered. But this receptor — in a near-mirror image performance of Checco’s original — responded much more favorably to the D-containing peptide than its all-L counterpart.
“You can see a pretty dramatic change,” Checco said, “where now D is, in fact, much more potent than L in activating this new receptor.”
In fact, the team realized that the orientation of this single amino acid directs its peptide to activate either receptor. In the all-L condition, the neurotransmitter preferred Checco’s prototype. When that particular L became a D, on the other hand, he went for Yussif’s new candidate.
Central nervous systems rely on different types of neurotransmitters to send different signals to different receptors, with dopamine and serotonin among the most familiar to humans. Given the radical complexity and subtlety of signaling in many animals, however, Checco said it stands to reason that they could evolve equally sophisticated ways to fine-tune the signals sent by even a single neuropeptide.
“These kinds of communication processes have to be very, very tightly regulated,” Checco said. “You have to make the right molecule. It has to be released at the right time. It has to be released in the right place. It has to degrade, actually, over a certain amount of time so you don’t have too much signaling.
“So you have all this regulation,” he said, “and now this is a whole new level.”
Unfortunately for Checco and others like him, natural peptides containing D-amino acids are difficult to identify using the instruments readily available in most laboratories. He suspects this is one reason why, at least to date, no D-containing peptides have been found in humans. He also suspects that this will change — and that, when it does, it could help researchers better understand both the function and dysfunction of signaling in the brain associated with the disease.
“I think it’s possible to find peptides with this kind of modification in humans,” Checco said. “And that will potentially open up new therapeutic avenues in terms of that particular target. Understanding more about how these things work could be exciting there.”
Meanwhile, Checco, Yussif and Blasing, a senior majoring in biochemistry and chemistry, are busy trying to answer other questions. For starters, they wonder whether an all-L versus D-containing peptide—even those that are equally likely to activate a receptor—might activate that receptor in different ways, with different cellular consequences. And the search for receptors won’t stop, either.
“This is one receptor system, but there are others,” Checco said. “So I think we want to start expanding and discovering new receptors for more of these peptides to really get a bigger picture of how this modification affects signaling and function.
“What I really want to do long-term with this project,” he said, “is to get a better idea, across biology, of what this modification does.”
