UC Berkeley

How do different animals grow to be different sizes? The answer is programmed in their genes, encoding for the myriad of protein-based signaling pathways that regulate growth of cells, tissues, and the entire body. Whereas many developmental signaling pathways have been found to control cell-fate, some pathways have been discovered that specifically regulate growth. How exactly these signaling pathways control growth is still not fully understood and there are likely still undiscovered components. In addition, knowing the normal mechanisms cells use to coordinate growth can help us understand how it becomes misregulated in human diseases such as cancer. The goal of this thesis was to identify novel genes controlling tissue growth and characterize their function, using the imaginal discs in Drosophila melanogaster as a model tissue.

In collaboration with a colleague in the lab, I identified Crumbs as a regulator of cell competition, a phenomenon that affects the survival of cells in growing tissues. Crumbs is a transmembrane protein known to control apical cell polarity and binds homophilically on adjacent cells. Cells that express higher levels of Crumbs die when in contact with cells that express lower levels of Crumbs. Therefore, Crumbs binding on adjacent cells may be a general mechanism for how cells "sense" competitive ability. I contributed by sequencing alleles of crumbs and finding that mutant cells behave as supercompetitors.

I identified the Fbxl7 gene as a novel regulator of Fat and Hippo signaling. The atypical cadherin Fat localizes in a planar polarized manner in cells of the wing imaginal disc, which prevents the membrane localization of an atypical myosin named Dachs. It was not previously known how Fat keeps Dachs off of the membrane. I showed that Fbxl7 associates with the Fat intracellular domain and prevents Dachs from localizing to membrane. Fbxl7 is conserved in humans and may be a novel tumor suppressor.

I identified a role for autophagy members in controlling tissue growth of the eye imaginal disc. Autophagy allows the recycling of cytoplasmic components during starvation via double membrane vesicles called autophagosomes. Atg2 and Atg18 form a complex and are critical for autophagosome formation. Loss of these genes in the eye imaginal disc disrupts formation of autophagosomes in developing photoreceptors and increases growth in the adult eye. Surprisingly, mutations in other autophagy genes do not produce the same phenotype, implying Atg2 and Atg18 may regulate growth through autophagy-independent mechanisms.

I developed a unique genetic system called "CoinFLP" to facilitate the identification additional growth genes, as well as characterize cell-cell interactions occurring with tissues. CoinFLP uses stochastic recombination in a developmentally controlled fashion to produce patches of mutant and wild-type tissue in imaginal discs at predictable ratios. I used CoinFLP to conduct a large-scale screen and identified geminin as a novel regulator of cell competition. In addition, I used CoinFLP to manipulate gene expression in two separate populations and directly visualize cell-cell contacts between them.

In conclusion, I have described three novel components regulating growth in Drosophila imaginal discs, and established a new genetic tool to identify and characterize additional regulators of tissue growth.