With the help of some sea slugs, chemists at the University of Nebraska-Lincoln have discovered that one of the smallest conceivable modifications to a biomolecule can lead to one of the greatest imaginable results: directing the activation of neurons.
Their discovery came from examining peptides, which are short chains of amino acids that can transmit signals between cells, including neurons, populating the central nervous system and bloodstream of most animals. Like many other molecules, an amino acid in a peptide can adopt one of two forms that feature the same atoms, with the same connectivity, but in mirror-image directions: L and D.
Chemists often think of these two directions as the left and right hand of a molecule. The L orientation is the most common in peptides, to the extent that it is considered the default. But when enzymes flip an L into a D, a seemingly simple flip 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 reversal. For the first time, the team has shown that the orientation of a single amino acid — in this case, one of dozens found in a sea slug neuropeptide — can dictate the likelihood of the peptide activating one neuron’s receptor versus another. Because different types of receptors are responsible for different neuronal activities, the findings point to another means by which the brain or nervous system might regulate the labyrinthine life-sustaining connections between its cells.
“We’ve discovered a new way in which biology works,” said Chico, assistant professor of chemistry at Nebraska. “It’s nature’s way of helping to make sure that the peptide goes into one signaling pathway versus the other. Understanding more about this biology will help us be able to take advantage of it in 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 D-amino acid peptide that activates neuronal receptors in sea slugs. This particular receptor only responded to the peptide when it contained a D-amino acid, making its L to D flip akin to an on/off switch.
In the end, Checco himself would define a second such future. In contrast to the one that initially intrigued him, the Checco 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, active when presented to concentrations smaller than that of its L-containing counterpart. D. Instead of an on/off switch, Checco seems to have found something more akin to a dimmer.
“We are left wondering: Is this the whole story?” Checco said. “What’s really going on? Why make this D molecule if it’s worse at activating the receptor?”
The team’s latest findings, detailed in the journal Proceedings of the National Academy of Sciences, Hint to an answer inspired by a hypothesis. The team may have thought that there are other receptors in the sea slug that are sensitive to that D-containing peptide. If so, some of those receptors may have responded to it differently.
Youssef, a chemistry PhD student, went to work looking for sea slug receptors whose genetic blueprints were similar to those discovered by Checco. He eventually narrowed down the list of candidates, which the team then cloned and was able to express in cells before introducing them to the same D-containing peptide as before. One of the receivers responded. But this receptor—in a mirror-image performance of the original Checco—responded much better to the D-containing peptide than its L-type counterpart.
“You can see a very exciting shift,” Chico said, “where D is now, in fact, much more powerful than L in activating this new receptor.”
In fact, the team realized that the direction of this single amino acid was directing the peptide to activate one receptor or the other. In its full L state, the neurotransmitter favored Checco origin. When the L turned into a D, on the other hand, it went for Joseph’s new candidate instead.
Central nervous systems rely on different types of neurotransmitters to send different signals to different receptors, the best known of which are dopamine and serotonin. Given the extreme complexity and subtlety of signaling in many animals, Checco said it made sense that they would develop equally sophisticated ways to fine-tune the signals sent by even a single neuropeptide.
“These communication processes have to be very, very structured,” Chico said. “You need to make the right molecule. It needs to be released at the right time. It needs to be released in the right location. It has to, in fact, degrade in a certain amount of time, so you don’t have too many signals.”
He said, “So you have all these regulations, and now that’s a whole new level of it.”
Unfortunately for Checco and others like him, it is difficult to identify naturally occurring D-amino acid peptides using devices readily available in most labs. He suspects it is one of the reasons, at least to date, that no D-containing peptides have been found in humans. He also suspects this will change — and when it does, it could help researchers better understand the function and disease-related dysfunction of signals in the brain.
“I think it’s likely that we’ll find peptides with this kind of modification in humans,” Chico said. “And that could potentially open up new therapeutic avenues in relation to this specific goal. Understanding more about how these things work could be exciting there.”
Meanwhile, Checco, Yussif, and Blasing, a double major in biochemistry and chemistry, are busy trying to answer other questions. For starters, they wonder if all-L vs. D-containing peptides—even those with equal potential to activate a receptor—might activate that receptor in different ways, with different cellular consequences. And the search for receptors will not stop either.
“This is one of the receptor systems, but there are other receptors,” Chico 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.
“Where I really want to take this project forward in the long term,” he said, “is to get a better idea, across all of biology, of what this modification does.”