Unusual Modifications of Neuropeptides Play Important Roles in the Central Nervous System

Understanding how neuropeptides function in cellular communication is important to understanding what happens when cell signaling goes wrong, as is the case in many diseases.

Cell-to-cell signaling neuropeptides containing a rare structural modification, a D-amino acid residue, may play a variety of roles throughout the central nervous system, researchers report in a new study.

Beckman Institute Postdoctoral Fellow James Checco
“If we hope to cure diseases, we need to know how the central nervous system actually works,” said Beckman Institute Postdoctoral Fellow James Checco (pictured), who led the new research with Jonathan Sweedler, a professor of chemistry.

“Understanding the structure of a given cell-to-cell signaling molecule, what receptor it binds to, and what activity that leads to is very critical if you want to design a drug that could either block or mimic that interaction,” said Beckman Institute Postdoctoral Fellow James Checco, who led the new research with Jonathan Sweedler, a professor of chemistry. “So we want to study what these cell-to-cell signaling molecules are, and what specific post-translational modifications are important for various functions—both in receptor activation as well as biological stability.”

Checco, who earned his Ph.D. in chemistry with a focus on chemical biology from the University of Wisconsin-Madison in 2015, and began his Beckman fellowship in 2016, focuses on the functional roles and biochemical signaling pathways for neuropeptides that contain rare D-amino acid residues. Most peptides and proteins are made up of chains of amino acid residues in the “L” configuration, indicating a specific arrangement of atoms in three-dimensional space. However, some rare peptides contain amino acid residues in the “D” configuration, a mirror image of the common L-amino acid residues (similar to how your left hand is the mirror image of your right hand), Checco explained. However, this subtle difference in arrangement can have profound effects on the biological function of the molecule. The incorporation of D-amino acids into peptides to enhance specific properties has been utilized by pharmaceutical companies in designing pharmaceutical drugs for years, but the presence and function of endogenous D-amino acids in our neuropeptides is not well known.

D-amino acid-containing peptides (DAACPs) are very difficult to detect, Checco said, noting that these molecules have been detected in diverse animals in multiple phyla, and that there is compelling evidence suggesting that DAACPs exist in humans, although none have been detected so far.

But studying these changes on the molecular level is a challenge. Peptides that contain L-amino acids and those with D-amino acids have the same chemical composition, so many common experimental techniques will not easily distinguish between them. In order to better understand neuropeptides with D-amino acids, Checco studied a receptor which specifically recognizes D-amino acid-containing neuropeptides present in Aplysia californica, a marine mollusk and a popular model organism for neuroscience.

Checco was the lead author on the paper, “Molecular and Physiological Characterization of a Receptor for D-Amino Acid-Containing Neuropeptides,” published in March in ACS Chemical Biology.

“We can see that the receptor is expressed in many areas (though not in every cell) … which suggests it plays a variety of different functions,” he said.

In addition, Checco’s study revealed that neuropeptides with D-residues are much more stable to peptide-degrading enzymes present in the CNS than peptides with only L-residues, which has important implications for their function.

Understanding how neuropeptides function in cellular communication is important to understanding what happens when cell signaling goes wrong, as is the case in many diseases.

“If we hope to cure diseases, we need to know how the central nervous system actually works,” Checco said. Figuring out how the molecules are modified in cells is a key part of the process, he said.  Sweedler has been studying the structures of neuropeptides for decades and has discovered several DAACPs.

“Checco’s research is the most comprehensive study ever published on a DAACP signaling system including its receptor. The work is impressive and exciting,” Sweedler said.

Checco used equipment at Beckman’s Microscopy Suite, the Protein Sciences Facility, and the Illinois Neuroproteomics Center on Cell-Cell Signaling.

Beckman collaborators on the study included Sweedler and Elena V. Romanova, Beckman senior research scientist. Sweedler leads the Cellular and Molecular Foundations of Intelligent Behavior Group, which is in the Intelligent Systems theme, which he co-chairs. In addition, Rachel H. Roberts-Galbraith, Department of Cell and Developmental Biology in the School of Molecular and Cellular Biology at Illinois; and Peter M. Yau, director of the Protein Sciences Facility at the Roy J. Carver Biotechnology Center at Illinois, are co-authors, along with collaborators from Nanjing University, China.

Funding for the research came from the National Institutes of Health, the National Resource for Aplysia, and the National Natural Science Foundation of China.