Barry Thompson: Projects

In order to construct tissues of particular forms, cells must be able to orient their behaviour relative to one another. We examine the molecular determinants of cell polarity and how these determinants orient cell behaviour in vivo. The relevance of this work for human disease is illustrated by the fact that defects in systems of cell polarity can lead to developmental abnormalities and tumour formation.

We focus particularly on the epithelial tissues of the fruit fly, Drosophila, which offers the possibility of live-imaging of epithelial tissue development as well as powerful genetic analysis. We also apply computational modelling of cell polarity and tissue morphogenesis to understand the principles that govern polarisation of determinants and how these determinants can influence cell shape, cell division, cell migration and tissue morphogenesis.

When epithelial cells become invasive

Most human cancers arise in epithelial tissues, which progress to metastasis when epithelial cells manage to invade their surroundings and become migratory. The fruit fly, Drosophila, offers an excellent model system for epithelial invasion: the migration of a cluster of 'border cells'.

Border cells are a group of 6-8 epithelial cells that are specified in the follicular epithelium that surrounds each egg chamber of the Drosophila ovary. The border cells subsequently become invasive and migrate from their initial location at the anterior pole of the egg chamber towards the posterior pole, where the oocyte is located. During migration of the border cell cluster, the epithelial cells retain their apical-basal polarity - similar to migrating clusters of human cancer cells.

We sought to understand how invasive migration occurs in border cells. We focussed on the actin cytoskeleton, which is crucial for cell migration and is highly polarised along the apical-basal axis of each border cell. In particular, the actin cytoskeleton is most prominent and dynamically active at the basal surface of each border cell, such that it forms a rim around the entire cluster.

Figure 1

Figure 1. Invasive migration of Drosophila border cells. A. Border cells (green) migrate across the egg chamber during stage 9 of oogenesis. F-actin is labelled in red, nuclei in blue. B. Apical-basal polarity in border cell clusters. F-actin (red) is polarised around the outer rim of the cluster. Hippo pathway components (green) are localised to inner membranes, where they act to restrict F-actin polymerisation. (Click to view larger image)

It was clear that this apical-basal polarisation of the actin cytoskeleton relies on the fundamental molecular determinants of apical-basal polarity, such as the apical Cdc42-aPKC-Par6 complex. Yet, how these molecules control the actin cytoskeleton was not known.

We have now identified a key role for the Hippo signalling pathway in linking determinants of apical-basal polarity with polarisation of the actin cytoskeleton. We find the upstream components of the Hippo pathway co-localise with apical polarity determinants and mediate a signalling cascade involving the Ena/VASP family of proteins that restricts activation of actin polymerisation to the basal side of the cell.

When Hippo signalling is disrupted, the F-actin cytoskeleton is no longer correctly polarised and accumulates abnormally around the entire plasma membrane to disrupt the collective migration of the border cell cluster.

Intruigingly, there is also a role for the nuclear Hippo pathway effector, Yorkie/YAP, in sensing events at the plasma membrane and providing feedback regulation of migration. Consequently, overexpressing Yorkie/YAP causes border cells to accellerate their invasive migration.

These findings identify a novel role for the Hippo pathway in cell polarity and collective cell migration in vivo. These findings have relevance for human cancer, where the Hippo pathway has already been implicated in controlling stem cell proliferation and tissue growth. Further examination of this pathway in cancer should now also consider its potential role in promoting invasive migration of cancer cells.

A new Hippo pathway component

We recently reported the identification of a novel component of the Hippo pathway: Mask. The Mask protein contains multiple ankyrin repeats which bind to the Yki/YAP transcriptional activator. In addition Mask features a KH domain that can bind to nucleic acids. The protein also contains nuclear import and export sequences and appears to be regulated in a highly similar fashion to Yki/YAP proteins. Mask is also essential for Yki/YAP proteins to induce target gene transcription to normal levels.

We have been able to co-purify a complex of Mask, Yki and Scalloped (a Yki-binding transcription factor) by pulling down a promoter DNA sequence from a Yki target gene. Thus, we propose that Mask acts in the nucleus to promote Yki-mediated transcription.

The discovery of Mask provides a new entry point to investigate how Yki/YAP proteins are physiologically regulated in vivo. To date, the only known mechanism of Yki/YAP regulation has been phosphorylation by the Warts/LATS kinase. However, Mask does not appear to be regulated by Warts/LATS activity, which suggests that other signals must be important for regulating both Yki/YAP and Mask in different contexts.

Since Yki/YAP proteins are often nuclear in human epithelial stem cells and cancers and can strongly drive cell proliferation, understanding precisely how the activity of these proteins is regulated is a key goal in understanding cancer.

Barry Thompson

barry.thompson@crick.ac.uk
+44 (0)20 379 61337

  • Qualifications and history
  • 2004 PhD, Cambridge University, UK
  • 2004 Postdoctoral Fellow, European Molecular Biology Laboratory, Germany
  • 2006 Visiting Scientist, Research Institute of Molecular Pathology, Austria
  • 2007 Established lab at the London Research Institute, Cancer Research UK
  • 2015 Group Leader, the Francis Crick Institute, London, UK