A cell's most important conversations happen at its surface, the interface where biological messages are received and responded to. Controlling which signals get through, and how loudly they are heard, are sugar-coated sensors called proteoglycans. 

Composed of a core protein and long sugar chains, these big signalling molecules sit on the cell surface or are deposited into the space surrounding cells. They bind to growth factors or chemicals that fight infections, allowing cells to respond to a changing environment. 

Despite proteoglycans’ clear importance, their unique structure means they are hard to analyse using traditional methods, like mass spectrometry, which identifies and measures proteins. Chemical biologist Ben Schumann endeavoured to change this. 

“Proteoglycans are vital for the growth of most of our organs - alterations in these molecules are lethal in developing embryos," he describes. “Although studies have identified just about a hundred in human cells, there are likely many more. At the moment, it’s a clunky process to identify just one proteoglycan at a time, work out its structure and what it’s doing. I wanted to try and streamline this.”

Filling in the blanks: tagging proteoglycans

In research published today in Nature Chemical Biology, Ben’s team at the Crick and Imperial College, led by Zhen Li and Himanshi Chawla, worked out a method to characterise and track proteoglycans using ‘click chemistry’, which involves joining molecules together permanently. 

“Instead of focusing on the whole proteoglycan molecule, we targeted one of the steps in making it,” explains Himanshi. “Using the ‘bump and hole engineering technique, where we modify a ‘hole’ in an enzyme and a ‘bump’ in a sugar, we altered the enzyme that glues together a sugar and protein to form the proteoglycan, so that it would accept a bumped version of the sugar.” 

Crucially, this modified sugar contains a chemical tag which means it can be traced by using click chemistry. For instance, scientists can attach a fluorescent molecule to ‘see’ the molecule by imaging, or a molecule acting like an anchor to isolate and further study it. The enzyme adds the modified sugar to the protein, creating a tagged proteoglycan whose behaviour can now be studied. 

“This technique allowed us to fill in the blanks,” says Zhen. “The modified enzyme and sugar were successfully incorporated into normal mammalian cellular processes, showing that the technique doesn’t alter their biology.” 

Next steps: tracking proteoglycans

Now that proteoglycans can be tracked more easily, Ben sees a world of opportunity. “Researchers could tag these molecules in different contexts to see what they’re doing, such as in organ development,” he says. “We could also alter proteoglycan function by replacing the sugar chain with a different biological or synthetic molecule in what I’m thinking of as ‘designer proteoglycans’.” 

Ben is now taking this chemical toolkit with him to TUD Dresden University of Technology in Germany, where he plans to investigate how proteoglycans help tissues develop into complex organs.

He also believes the technique holds promise for fighting tumours. “I’m hopeful this tracking system could help us understand and even modify what signals a cancer cell is picking up, perhaps by introducing a designer proteoglycan that can’t respond to usual cancer growth drivers,” says Ben. “This might one day help us find better treatments.”

Ben’s work was majorly funded by the Biotechnological and Biological Sciences Research Council UK.