Ino80 is required for ecdysone-dependent regulation of PI3K/Akt signaling during Drosophila development. Sarah Neuman, Robert Ihry, Arash Bashirullah. University of Wisconsin-Madison, Madison, WI.

   During metamorphosis, many larval tissues, including the larval salivary glands, undergo programmed cell death in response to a pulse of the steroid hormone ecdysone. Ecdysone binding to its receptor initiates a cascade of gene expression that culminates in tissue-specific induction of the IAP antagonists reaper (rpr) and hid involution defective (hid). We have identified a mutant allele of the chromatin remodeling protein Ino80 that displays defects in this salivary gland cell death response. Ino80 mutant salivary glands do not initiate caspase activation despite induction of the death activators rpr and hid. Our results indicate that PI3K/Akt signaling is not properly downregulated in Ino80 mutant salivary glands, resulting in dramatic increases in phospho-Akt levels. We demonstrate that constitutive activation of PI3K/Akt signaling is sufficient to resist ectopic expression of death activators, and that ecdysone signaling is normally required to inhibit PI3K/Akt signaling prior to the death response in salivary glands. Critical negative regulators of PI3K/Akt signaling, including PTEN and S6K, are induced in response to ecdysone, and the positive regulator PDK1 is repressed in response to ecdysone. Our data suggests that Ino80 is required for efficient induction of the DHR3-dependent mid-prepupal ecdysone response and that a defect in this cascade disrupts the timely inhibition of PI3K/Akt signaling. These results highlight the role of the ecdysone hierarchy in the regulation of PI3K/Akt signaling during development.