Insights into the Control of Growth and Axon Guidance by the Drosophila Insulin Receptor
Li, Caroline Rita
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The Drosophila insulin receptor (DInR) regulates a diverse array of biological processes, including growth, axon guidance, and sugar homeostasis. Growth regulation by DInR is mediated by the adaptor protein Chico, the Drosophila homolog of vertebrate Insulin-Receptor-Substrate (IRS) proteins. In contrast, DInR regulation of photoreceptor axon guidance in the developing visual system is mediated by the SH2/SH3 domain adaptor protein Dreadlocks (Dock). In vitro studies by others suggested that different parts of DInR interact with different adaptor proteins: five NPXY motifs, one situated in the juxtamembrane region and four in the signaling C-terminal tail (C-tail), were important for interaction with Chico. Yeast two-hybrid assays suggested that a different region in the DInR C-tail interacts with Dock. To test whether these sites are required for growth or axon guidance in the animal, in vivo add-back type experiments were conducted. A panel of DInR proteins, in which the putative Chico and Dock interaction sites had been mutated individually or in combination, were tested for their ability to rescue viability, growth, and axon guidance defects of dinr mutant flies. Sites important for viability were identified. In addition, mutation of all five NPXY motifs drastically decreased growth in both male and female adult flies, but did not affect photoreceptor axon guidance, showing that different binding sites on DInR control growth and axon guidance. Unexpectedly, mutation of both putative Dock binding sites, either individually or in combination, did not lead to defects in photoreceptor axon guidance. Finally, none of the seven putative ligands for DInR, the Drosophila insulin-like peptides (dilps), was required for directing photoreceptor axon guidance, although we found that dilp1, -2, -3, -4, and/or -5 are required for controlling whole animal allometry. Importantly, we showed that the developmental delay exhibited by dinr mutants is not a factor underlying their photoreceptor axon guidance defects. Together, these studies confirmed the role of Chico-interacting regions of DInR in regulation of growth in vivo. They demonstrated that DInR is necessary to control axon guidance in vivo and showed that this role is not simply a function of developmental timing. However, they leave open the mechanisms activating DInR in regulating axon targeting.