Fat and tissue growth

In early 2006, Harvey moved back to Australia to set up a group at the Peter MacCallum Cancer Centre and continue his work on Salvador-Warts-Hippo signalling. "Our major focus at the time was to fill out the pathway [as only five components were then known]," Harvey says.

"Initially we looked for a transmembrane receptor for the pathway that might signal to one or more of the so-far all intracellular components." Taking a candidate approach, Harvey looked for plasma membrane proteins previously shown to regulate tissue growth in Drosophila and came across Fat, an atypical member of the cadherin family of adhesion proteins.

According to Harvey, mutations in Fat were first discovered in flies in the 1920s with dominant alleles that affected wing size and abdomen shape.

"Then, in the late 1980s, Fat was shown to regulate tissue growth and cell proliferation, before cloning of the gene in the 90s identified it as a cadherin, but still no one knew how it worked to regulate organ development."

Cadherins span the membranes of adjacent cells to stick them together and form a barrier, but are also important for signalling to intracellular pathways.

In the first part of his presentation at the Hunter meeting, Harvey will discuss how they identified Fat as a component of the SWH pathway. "Based on earlier work by others, we did genetic studies in our flies and also stained Fat-mutant tissues for different target genes of the SWH pathway such as cyclin E, which drives cell proliferation, and DIAP1, which inhibits apoptosis."

There were strong genetic interactions between Fat and multiple components in the pathway, and animals lacking Fat were phenotypically similar to those with depressed SWH signalling.

Further studies revealed a potential mechanism by which Fat regulates SWH pathway activity. It seemed that Fat regulates the apical membrane localisation of an intracellular protein called Expanded, another upstream regulatory protein in the pathway associated with the plasma membrane.

"We are not sure at the moment how all this happens," Harvey says. "Expanded sits at the apical membrane of developing cells and presumably acts as some sort of bridge between the surface proteins and either the actin cytoskeleton or downstream signalling proteins. We therefore think that Fat restricts organ size by stimulating downstream SWH pathway components to limit transcription."

The current goal of Harvey's group is to nut out the entire SWH pathway. "We now want to know exactly how Fat signals to the downstream components, but also how Fat itself is regulated. Evidence is mounting in both Drosophila and mammals that the SWH pathway is controlled by cell adhesion, but we do not know how.

"Fat is a likely candidate to mediate adhesion dependent-signalling given that it is a cadherin, but other proteins might also be involved, such as another cadherin, Dachsous."