Precise control of organ growth is crucial during animal development. An important candidate for growth control in Drosophila wing imaginal discs is Decapentaplegic (Dpp), a member of the TGF beta signaling pathway and a morphogen. In the wing imaginal discs Dpp is secreted from a group of source cells and spreads in the tissue to form a gradient of concentration.
A possible growth control mechanism by Dpp has been proposed by Ortrud Wartlick in the lab: Science, 331:1154-1159. This study showed that it is not the absolute levels of Dpp that are important for controlling growth, but the percentage change of Dpp over time that cells perceive (dC/Cdt). More precisely, when the amount of Dpp that cells perceive increases by a constant alpha percentage (alpha= 50%), cells divide. And dC/Cdt is the speed with which alpha is reached.
In order to challenge this growth control model, a condition will be used, for which growth control is altered.
Overgrowth suppressor gene lethal(2) giant discs (l(2)gd) encodes a member of an uncharacterized protein family, whose members contain one C2 domain and four repeats of the DM14 domain. It has been previously shown that an overgrowth suppressor gene lethal(2) giant discs (l(2)gd) is required to restrict the activity of Notch to the D/V boundary (Klein T, 2003). Moreover, l(2)gd is a critical regulator of endocytosis and acts in the degradative pathway (Gallagher C.M. et al., 2006, Troost T. et al., 2012).
Mutant l(2)gd discs overgrow and can reach a final size up to 4 times bigger than wild type. Nevertheless, cells in the mutant discs don’t lose their apico-basal polarity. They remain in an epithelial 2D layer and we therefore can use tools developed in wild type to measure and analyse the gradient in these discs.
Moreover, the variation of apical cell area with time in lgd mutant discs is similar to wild type, thus the overgrowth of these discs is not due to increased cell size but to overproliferation.
The goal of my project is to understand the overgrowth phenotype of l(2)gd mutants. The strategies that I will be using are:
- analyse the growth of mutant discs as a function of time
- analyse the Dpp gradient profile as a function of time
- address the relationship between gradient and growth and compare to wild type
- investigate how the l(2)gd phenotype is linked with the role of l(2)gd in manipulating Notch activity
- question the influence of the function of l(2)gd in endocytosis on the kinetics of Dpp transport.
By analyzing overgrowth mutants, I would also like to address two questions: what determines the size prior pupariation of the wing imaginal disc and how do cells detect the relative increase in Dpp signaling levels.