Signal Transduction
How do cells communicate with each other during development to determine their positions within the organism and to take on the appropriate fates? The Drosophila eye is a useful model system in which to answer such questions. Mutations that disrupt the ordered pattern of photoreceptor differentiation are easy to identify and have led us to uncover some novel signaling mechanisms. Current projects related to cell signaling focus on the Epidermal growth factor receptor (EGFR) signaling pathway, which is essential for photoreceptor differentiation, and the Wingless (Wg) pathway, which antagonizes it.

EGFR pathway
EGFR signaling regulates cell differentiation, proliferation and survival. Our work has shown that the range of action of the ligand Spitz is limited by fatty acid modification, which tethers it to the plasma membrane to increase its local concentration (Miura et al., Dev. Cell 2006). Surprisingly, Spitz is synthesized as a transmembrane precursor, but the transmembrane domain inhibits its ability to signal. We resolved this paradox by showing that cleavage of the transmembrane domain is necessary to relocalize Spitz from the apical surface of the cell to the basolateral surface, where the active lipid-modified form is released (Steinhauer et al., J. Cell Sci. 2013). Once bound to Spitz, the EGFR is endocytosed and eventually degraded. However, blocking its progression through the endocytic pathway can curtail signaling, suggesting that it is active on endosomes (Miura et al., Development 2008). Surprisingly, one ESCRT complex component that promotes EGFR signaling does so independently of its effect on endocytosis (Legent et al., Development 2015). We are currently investigating whether cleavage of the EGFR during endocytosis enables its cytoplasmic domain to enter the nucleus and regulate transcription, as described for its mammalian homologues.
Downstream of the receptor, a kinase cascade leads to the regulation of transcription factor activity. We identified a novel adaptor subunit necessary for activation of the most upstream kinase, Raf (Roignant et al., Genes Dev. 2006). The final kinase, MAPK, is encoded by a gene with very large introns located in heterochromatin. We found that its splicing requires three subunits of the exon junction complex, which was previously thought to affect only post-splicing steps of RNA metabolism (Roignant and Treisman, Cell 2010). Phosphorylation by MAPK controls the activity of transcription factors such as the activator Pointed and the repressor Capicua. We recently identified the ubiquitin ligase that degrades phosphorylated Capicua, and found that a separate degradation mechanism establishes the basal level of Capicua (Suisse et al., Development 2017).

Wingless pathway
Wingless signaling is mediated by stabilization of beta-catenin, a protein that both acts as an adherens junction component and also enters the nucleus to activate the transcription of target genes. We found that phosphorylation by Casein kinase 1 alpha is essential to destabilize beta-catenin in the absence of Wg (Legent et al., Genetics 2012). In the nucleus, beta-catenin interacts with the co-activator Pygopus. We found that the missing link that allows Pygopus to activate transcription is recruitment of the mediator complex through its Med12 and Med13 subunits (Carrera et al., PNAS 2008). In recent studies, we found that Wg signaling is sensitive to calcium levels in the endoplasmic reticulum, and have traced this effect to sequestration of beta-catenin by misfolded E-cadherin (Suisse et al., in preparation). Changes in calcium concentrations can also dramatically affect cell growth and cell mixing; we are investigating the mechanism of these effects.