Research
Pattern formation in the Drosophila visual system
The Drosophila eye is a useful model system in which to study cell-cell signaling and cell fate determination. The Hedgehog, Epidermal growth factor, Notch, and Wingless pathways play important roles in setting up the ordered pattern of photoreceptor differentiation. Photoreceptors extend axons directly into the brain, where they must find the right synaptic partners to set up the visual circuitry. We are using this system to address several basic questions.
1. Movement of signaling molecules between cells
Signaling proteins must convey information between cells by traveling in the extracellular space. Surprisingly, we discovered that both the Hedgehog and Spitz signaling molecules are modified with fatty acids by the same enzyme, Rasp, and that these modifications are essential for their function (Lee et al., Dev. Biol. 2001; Lee and Treisman, Curr. Biol. 2001; Miura et al., Dev. Cell 2006). How can hydrophobic modifications enhance the function of diffusible molecules? For Spitz, the answer seems to be that the fatty acid restricts its diffusion, allowing enough Spitz to build up close to the source to activate the receptor on nearby cells (Miura et al., Dev. Cell 2006; Steinhauer et al., in preparation). We are still investigating how lipid-modified proteins can be released to act over a longer range. We would also like to know whether this mechanism is used to regulate the activity of other classes of signaling proteins.
Lipid modifications on secreted molecules regulate their range of action.
2. Signal transduction
Receptor activation triggers a complex cascade of events that ultimately result in a cellular response. These events may be compartmentalized within the cell. The epidermal growth factor receptor (EGFR) is endocytosed and eventually degraded when it binds its ligand, Spitz. We have found mutations that interrupt this endocytic process and also reduce EGFR signaling, suggesting that signaling may occur on endosomes rather than at the plasma membrane (Miura et al., Development 2008; Legent et al., in preparation). One consequence of endocytosis is proteolytic cleavage of the receptor; we are testing whether the cleaved cytoplasmic domain can function in parallel to the canonical downstream signaling pathway.
Mutations that disrupt endocytosis of the EGFR can affect its signaling capacity.
3. Gene expression
The ultimate output of cell-cell signaling is changes in gene expression. Transcriptional activation and repression require not only sequence-specific transcription factors, but also more general complexes that alter chromatin structure or recruit the basal transcriptional machinery. We have found that some subunits of such complexes act as adaptors that allow them to contribute to transcriptional regulation by specific signaling pathways (Collns and Treisman, Genes Dev. 2000; Treisman, Development 2001; Janody et al., Development 2003; Janody et al., Genetics 2004; Carrera et al., PNAS 2008; Carrera et al., Mol. Cell. Biol. 2008). Gene expression can also be regulated post-transcriptionally. We showed that a specific subset of transcripts, including MAP kinase, which codes for a critical signal transduction component, are spliced by a novel mechanism that requires the exon junction complex (Roignant and Treisman, Cell 2010). Many of the genes regulated by this mechanism are located in heterochromatin, raising the intriguing possibility of a connection between chromatin structure and splicing.
Genes in heterochromatin, such as MAP kinase, require a novel splicing mechanism.
4. Synapse formation
Establishment of the correct neural circuitry requires growing axons to recognize the appropriate cells as synaptic partners. The photoreceptors that mediate color vision provide us with a simple system in which to examine this process. We have found that the receptor tyrosine phosphatase LAR is required for ultraviolet-sensitive photoreceptors to form synapses with the correct partners, and that it signals by a novel mechanism in this process (Hofmeyer et al., PNAS 2006; Hofmeyer et al., PNAS 2009). We are interested in understanding the pathway upstream and downstream of LAR, and also in identifying other molecules that provide targeting information to the color photoreceptors (Astigarraga et al., J. Neurosci. 2010). We are also beginning to study axon regeneration in the visual system.
The R7 and R8 photoreceptors, which mediate color vision, form synapses in two different layers.
