The Thompson Lab is interested in how tissues control their growth and form during development. We use the fruit fly Drosophila as a model system for investigating this problem. The size and shape of tissues is determined by the coordinated behaviour of individual cells during development. This coordination is achieved by intercellular signalling pathways that control the growth and division of cells. We are conducting an in vivo RNAi screen in the fly wing to identify new components of these pathways, with a view to gaining a more complete understanding of how signalling pathways control tissue growth and form.
The Wnt pathway
The Wnt pathway is a developmentally regulated pathway that controls tissue growth and patterning in the fly. The range over which the Wnt ligand Wingless spreads is crucial for determining the size of the fly wing. Neil Pearson has identified a large number of RNAi lines targeting novel genes that produce Wnt-related phenotypes when expressed in the fly wing. Neil is currently testing whether these RNAi lines interact genetically with Wingless in the fly eye. Lines that score positively in both the wing and eye will be selected for further analysis. Neil aims to generate null mutants in selected candidate genes and determine the molecular function of the gene product in Wnt signalling.The Hippo pathway
The Hippo pathway is a new tumour suppressor pathway that controls the proliferation and apoptosis of cells. Together with Nic Tapon's lab, we are interested in identifying new components of the this pathway and how it is regulated during development. Eliana Lucas has identified an RNAi line causing apoptosis of cells in the fly wing, a phenotype closely resembling gain of function of the Hippo pathway. This RNAi line targets a novel gene encoding a protein that binds to Hippo in a yeast-2-hybrid assay. Eliana has generated a mutant in this gene, which confirms the RNAi phenotype. Eliana is also currently characterising the interaction between these two proteins in co-immunoprecipitation assays and performing genetic epistasis analysis to determine whether this novel gene acts genetically upstream or downstream of Hippo. Ruth Brain and Barry Thompson have identified a second novel Hippo pathway component that causes tissue overgrowth when knocked down by RNAi. The RNAi phenotype closely resembles RNAi of hippo in both the fly wing and eye. Alice Genevet in Nic Tapon's lab has generated a mutant in this gene that confirms the RNAi phenotype. Alice has also found that the protein product of this gene binds to the Hippo pathway components Expanded and Merlin (homologues of the human NF2 tumour suppressor). Alice is currently exploring how these proteins bind, whether they are transcriptionally co-regulated, and whether they act synergistically as tumour suppressors.
The Insulin/TOR pathway
The Insulin/TOR pathway regulates cell growth in response to nutrition. Alterations in this pathway change the size of cells, as well as the size of tissues. Clara Sidor has identified an RNAi line causing an increase in both cell and tissue size. This phenotype resembles that of RNAi for negative regulators of the Insulin/TOR pathway such as the PTEN tumour suppressor. Clara is currently generating a mutant in this gene to confirm the RNAi phenotype and to allow a more detailed phenotypic characterisation.
Epithelial polarity and shape
Most of the adult fly develops from epithelial tissues. Epithelial cells are polarised in their apical-basal axis and adopt particular shapes that determine the morphology of the overall tissue. Epithelial organisation allows signals to control development along a single, two dimensional plane. Understanding epithelial polarity and shape is therefore essential to understanding the development of growth and form of tissues. Furthermore, most human tumours are epithelial in origin and progression to metastasis requires that cells escape from the epithelium. Georgina Fletcher has conducted an RNAi screen for defects in epithelial organisation in the ovarian follicle cell epithelium. Georgina has identified a number of RNAi lines that cause loss of epithelial polarity and has confirmed these phenotypes by generating mutants in these genes. Interestingly, several of these RNAi lines are known to form a single protein complex that controls endocytosis. Georgina is currently investigating the links between endocytosis and polarity in these cells.
Figure 1. An in vivo RNAi screen for signalling pathways controlling tissue growth. Mis-regulation of Wnt, Hippo, or Insulin/TOR signalling can lead to defects in tissue size. Each pathway has characteristic effects upon cell size or cell fate that enables their phenotypes to be distinguished. The phenotype of selected signalling components is shown at the top. A schematic diagram of each pathway is shown at the bottom, we have identified novel compounds in each of these pathways.
Figure 2. RNAi of Lgl in the developing Drosophila wing causes loss of epithelial polarity (bottom) and tumour formation (top). Wild-type is shown on the left. Lgl RNAi (Lgl-IR) is shown on the right.
For a list of refereed research papers, see Publications (in navigation on left).
