Lay Summary

Tissue stem cells are long-lived cells in our bodies that enable organs to regenerate. For example, the lining of our intestines fully renews over a course of 4-5 days, thanks to the function of stem cells nestled within the epithelium. These stem cells respond to particular external cues from their local environment that instruct them to divide and produce two different types of progeny cells. One daughter cell proceeds to develop into highly specialised gut cells. The other daughter cell maintains stem-cell characteristics. The latter process, known as stem cell “self-renewal”, also depends on the ability of these cells to maintain the ends of their chromosomes, the telomeres. (Note that in non-stem cells, telomeres shorten with every round of cell division, which causes these cells to age.) Intriguingly, stem cell maintenance mechanisms operate in many cancers, for example bowel cancer.

We take a multi-angled approach to understand the molecular details of stem cell maintenance. Structural biology, a discipline used to visualise cellular components at atomic or near-atomic detail, is at the heart of our work. In the same way as you can learn about the mechanisms of a machine by studying how it is built, you can learn about how proteins function by studying their detailed structures. With these insights in hand, we hope to uncover new ways to keep cancer cells in check.

We combine a range of approaches to understand the molecular details of how stem and cancer cells maintain their ability to divide over long periods of time. Structural biology teaches us about these processes at great detail. (Images modified from Mariotti et al., 2016)

Research Projects

Macromolecular acrobatics of tankyrase

The tankyrase proteins are part of a larger group of enzymes that can tag other proteins and themselves with elaborate, chain-like flags known as poly(ADP-ribose). To achieve that, tankyrase first needs to capture its targets, bring them into the correct orientation for tagging and then perform the tagging reaction itself. All these steps need to be carefully coordinated to prevent tankyrase from acting at the wrong time or in the wrong place. We aim to use sophisticated structural biology techniques to visualise the molecular details of tankyrase in action. We next probe new insights from this work by performing experiments in human tissue culture cells.

Proteins consist of modules (“domains”) that fulfil specialised functions. In tankyrase, so-called ankyrin repeat clusters (ARCs) recognise particular features in other proteins, enabling tankyrase to communicate with these proteins. In some cases, this requires the formation of large chain-like fibre structures that arise from the mutual interaction of many tankyrase molecules via their sterile alpha motif (SAM) domains. A biochemical tagging reaction known as poly(ADP-ribosyl)ation is performed by a catalytic PARP domain. (Images modified from Guettler et al., 2011; Mariotti, Templeton and Ranes et al., 2016; Pollock et al., 2017; Pillay and Mariotti et al., 2022)

How do key signalling proteins in stem and cancer cells collaborate?

Proteins do not exist in isolation. Most proteins are social and function in the context of multi-protein complexes, in which different members collaborate with each other to achieve a common goal. We aim to understand the precise details of how complexes relevant to stem and cancer cells operate. We take a particular interest in learning about how tankyrase can modulate such complexes.

We aim to re-build complex cellular signalling processes in the test tube, using purified proteins. This reduces the complexity of the signalling processes as they occur in the cell. By modulating one factor at a time, we can decipher how protein complexes function.

Top: The beta-catenin destruction complex is an example for collaborative proteins. It's components beta-catenin and APC are among the most frequently mutated proteins in cancer. The complex is bossed around by tankyrase; we aim to understand how. (Images modified from Mariotti et al., 2017) Bottom: The image, which was created by our multi-talented PhD students Saira Sakalas and Yexin Xie, shows armadillos (another name for beta-catenin) being captured by a fishing net, which signifies the beta-catenin destruction complex.

Novel approaches to inhibit tankyrase function

We used a technique known as nuclear magnetic resonance (NMR) spectroscopy to map the binding site of a small molecule (a "fragment" - a building block of a larger drug-like molecule) to a substrate binding domain of tankyrase. (Image modified from Pollock et al., 2019)

Despite being an enzyme, tankyrase likely fulfils functions that do not depend on its catalytic activity. Our previous work has demonstrated that the ability of tankyrase to bind to other proteins in the cell via its amino-terminal substrate binding modules is critical for both its catalytic and non-catalytic activities. In collaboration with Professor Ian Collins (ICR Division of Cancer Therapeutics), we develop novel drug-like molecules that block the interaction of tankyrase with its binding partners and substrates.