There was a time when scientists believed in the paradigm of “one gene – one protein – one function”. The actual situation is both more complex and more elegant. Just as researchers today know that one gene can encode multiple protein variants, small sugar chains protruding from the surface of proteins can create even more variation. Researchers are only beginning to understand the meaning of this third language of life – glycosylation. A new study reveals that the naturally occurring sugar chains regulate peptide hormones and thus potentially how the body is balanced. By recreating these changes in the laboratory, researchers may be able to fine-tune the medicine of the future.
The peptide hormone insulin has saved millions of lives over a century. In recent years, insulin has been joined by several other biologically active peptides, so that today peptides represent one of the largest and most promising classes of drug candidates to counteract nervous system and metabolic disorders. The therapeutic potential of peptides results from their enormous importance in regulating physiological processes in the body. The challenge is that peptides are often unstable and therefore degrade rapidly. New research identifies a key factor affecting their function and stability.
“Recent studies have shown that some peptide hormones in the body have small sugar chains linked to their surface (glycosylation). We therefore decided to investigate how many, and when we realized the extent, we started to produce an atlas to help future research in studying the function of peptides and potentially guide drug design. One of the glycosylated peptide hormones we found in this study was GLP-1, which is used in treating people with diabetes and obesity. In our study we show that in animal models the small sugar molecules make GLP-1 stay in circulation 10 times longer, so the impact of these small sugar chains is quite remarkable,” says Katrine T. Schjoldager, Associate Professor, Copenhagen Center for Glycomics, University of Copenhagen.
Surprisingly many sugar chains
Long chains with more than 50 amino acids are called proteins, and shorter chains are called peptides. They include a large class of biologically active molecules such as peptide hormones and neuropeptides that regulate physiological processes such as blood glucose, appetite, anxiety, inflammation and blood pressure. Now, however, research shows that the activity and stability of peptides may be affected by small sugar chains on their surface.
“In glycosylation, the cell attaches small sugar chains to specific amino acids in proteins, and this plays a major role in regulating the function of proteins, including peptide hormones. We wanted to understand how widespread O-glycosylation is among peptide hormones and therefore systematically searched proteins from pig and rat organs for these sugar chains,” explains a main author, Thomas Daugbjerg Madsen, a PhD student at the Copenhagen Center for Glycomics, University of Copenhagen.
The task Thomas Daugbjerg Madsen and his colleagues took on was quite extensive, especially because the peptide hormones had to be extracted from the various organs of the animals. They then analysed the small peptides by using mass spectrometry – a method for isolating protein fragments based on size and charge. The researchers could thus determine whether the peptides had sugar chain modifications and their location on the peptide.
“Surprisingly we found sugar chains on more than one third of all known peptide hormones, and since not all peptide hormones are expressed in the biological material we studied, we estimate that many more can be found,” says Thomas Daugbjerg Madsen.
Extremely good starting-point
The researchers found the sugar chain modifications in humans, pigs and rats in almost identical protein sequences, suggesting that natural selection has retained these modifications and that they are functionally significant.
“Modifying peptide hormones, such as GLP-1, PYY and secretin, increased the stability tremendously and protected the peptides from degradation by proteases and concurrently reduced their receptor activity slightly, which indicates that the body can use the glycosylation as a regulatory mechanism. In other words, the body can use the sugar chains to increase and decrease the stability and potency of the hormones,” explains Katrine T. Schjoldager.
The research group found the same regulatory mechanism last year when they studied another peptide hormone, atrial natriuretic peptide (ANP). This is secreted by muscle cells into the bloodstream, where it causes blood vessels to dilate and has other effects. Here, too, the O-glycans made the peptide hormone more stable and slightly less potent. The researchers now hope to be able to transfer this effect to drugs so that they become more stable.
“This atlas, which indicates both the glycosylations found and those we consider to be probable, has not only vastly improved our understanding of the significance of glycosylations in the body. We also have an extremely good starting-point for improving the stability and effectiveness of existing and future peptide-based drugs, so that we mimic the natural physiological effects more closely,” says Katrine T. Schjoldager.
“An atlas of O-linked glycosylation on peptide hormones reveals diverse biological roles” has been published in Nature Communications. “Discovery of O-glycans on atrial natriuretic peptide (ANP) that affect both its proteolytic degradation and potency at its cognate receptor” has been published in the Journal of Biological Chemistry. In 2017, the Novo Nordisk Foundation awarded a grant to Katrine T. Schjoldager for the project Novel Proteoforms of Peptide Hormones Provide Exciting Options for Improving Drug Design.