Uncovering the hidden lives of complex sugars with cutting-edge imaging

Dr Liang Wu is a Wellcome Trust Sir Henry Dale Fellow at the Rosalind Franklin Institute. Since joining the Franklin in 2020, Dr Wu’s team has been investigating the role and function of enzymes in the production of heparan sulfates – long, complex chains of sugar molecules found in every cell of the body.

Despite being associated primarily with nutrition, sugars and other carbohydrates play a varied and important role in human biology, from regulating tissue growth to fighting off infection. In a new review, Dr Wu and Franklin colleagues explore how advanced imaging technologies are transforming research into glycans – the family of sugar-based molecules to which heparan sulfates belong – by enabling in-depth study within living cells.

Why are scientists trying to learn more about the production and modification of glycans?

Molecules are fundamental to life, and the importance of molecules such as DNA and proteins is well known. But what’s less well known is that sugars also play a hugely important role in how our cells and tissues operate. They aren’t just a source of energy – as a class of molecule, they are just as important as DNA, RNA and proteins. We need to increase our understanding of the processes behind complex sugar structures. 

Where are the gaps in our knowledge?

We don’t know as much about complex sugars as we do about other molecules, and that is partly because they don’t follow the same templated rules for construction as molecules like DNA or proteins. There isn’t an obvious underlying mechanism for how they are constructed enzymatically. Our new article tries to piece together the link between what we term ‘traditional’ methods, which have looked at the steps taken by individual enzymes in building and modifying glycans, and emerging imaging techniques such as cryo-electron tomography (cryo-ET) and super-resolution microscopy, which can give us a more unified picture of how networks of enzymes operate together.

How might these new insights lead to benefits in health and medicine?

Again, I’d suggest the role of sugars in human health and disease is underappreciated. The dysregulation of sugars is involved in many disease processes. An obvious example is cancer, in which the arrangements or ‘motifs’ of sugars on cancer cell surfaces can change quite dramatically, similar to patterns of proteins. That’s due to changes in the underlying enzymatic processes involved in sugar construction – and because sugars are crucial in things like communication between cells, it can allow cancer cells to promote aggressive growth and evade our immune systems.

Another example is viruses. A lot of cell surface sugars are involved in viral interactions, and so certain motifs are used as targets by viruses to bind on to cells and carry out their infection processes. If we can understand how different sugar patterns are encoded through this ‘conveyor belt’ of enzymatic machinery, we may be able to adjust those patterns to correct the elements that are causing problems. It could open up a completely new avenue of therapeutic intervention.

How are new imaging techniques contributing to breakthroughs in glycan research – including those being developed at the Franklin such as cryo-ET?

Cryo-ET is starting to become more mature as a technique, and essentially its power is in the ability to visualise biological systems at near-molecular resolution – and, importantly, within human cells and tissues in their natural state, which we call ‘in situ’ rather than ‘in vitro’. We are able to preserve samples in this state by quickly freezing them, and then we use a technique called focused ion beam (FIB) milling to cut away a window that allows us to see into a particular region of the cell and work out what’s going on. 

At the Franklin, we are developing ‘tomography-in-a-box’ in collaboration with Thermo Fisher Scientific, which aims to streamline the sample preparation and imaging process while improving the resolution attainable and increasing the volume attainable. This development has led to the first pseudoatomic structure of the human ribosome in cells.

In terms of glycosylation, which is the name for the enzymatic process in which sugars attach themselves to other molecules inside a cell or on its surface, looking at samples in situ allows us to view the conveyor belt of arrayed enzymes working together. And that’s only possible at the kinds of resolutions we’re able to achieve using modern electron microscopy. It’s really empowering to be doing this work at the Franklin because there are so many exciting imaging, analysis and design technologies – and a critical mass of expertise – under one roof.

What are the next steps in this area of research?

We’re at the stage where we can see things quite clearly now. However, a lot of the enzymes we’re interested in are still on the small side, which makes them more difficult to study. The frontier in the next few years is developing a biochemical toolkit to help us analyse and annotate our data so we can identify points of interest and more easily find the enzymatic reactions we need to be looking at.

We’ll also look to integrate other techniques and approaches available at the Franklin beyond imaging – for example, cross-linking mass spectrometry – to complement the information from cryo-ET images and tell us more about the spatial arrangements of molecules.

Finally, we don’t just want to be able to see these arrayed networks of enzymes – we need to investigate the functional implications of all the interactions to get the most impact out of the data.

The review article ‘Structural glycobiology – from enzymes to organelles’ is published in the journal Biochemical Society Transactions.