DNA Damage-dependent Regulation and Function of akt-1 in Caenorhabditis elegans

The roundworm Caenorhabditis elegans possesses a single, conserved phosphatidylinositol 3-kinase (PI3K) signaling pathway that regulates somatic developmental decisions and lifespan through the Insulin-like receptor tyrosine kinase (RTK) DAF-2, the class I PI3K AGE-1 and the 3-phosphoinositide-dependent protein kinase 1 (PDK1) homologue PDK-1. This pathway ultimately controls the action of Akt homologues on the forkhead transcription factor DAF-16. The C. elegans Akt orthologue akt-1 also negatively regulates the DNA damage-dependent apoptosis of worm germ cells by indirectly interfering with activation of the key transcription factor CEP-1, the sole homologue of p53 in the worm. Because upstream regulation by RTK/PI3K signaling is known to couple with downstream Akt kinase activity, I hypothesized that the worm daf-2/age-1/pdk-1 pathway would function upstream of akt-1/Akt in response to DNA damage. Surprisingly, this was not the case: daf-2/InsR and pdk-1/PDK1 do not function upstream of akt-1/Akt and instead promote DNA damage-induced germ cell apoptosis independently of CEP-1/p53 by regulating the B cell lymphoma (Bcl2) homologue CED-9 and the Apoptotic protease-activating factor 1 (Apaf1)-like adapter protein CED-4, respectively. Furthermore, PDK-1/PDK1 promotes germ cell apoptosis by a mechanism that does not include changes in the subcellular localization or absolute levels of CED-4/Apaf1, but does require the presence of CEP-1/p53. Therefore, daf-2/RTK, pdk-1/PDK1, and cep-1/p53 co-operate from independent pathways to drive germ cell death. The separation of worm Akt function from canonical RTK/PI3K regulation is consistent with the ability of AKT-1 to function without major changes in phosphorylation at threonine 350, a site homologous to Thr308 in mammals. Since this modification is an essential step in the activation of Akt proteins by PDK1, it is likely that damage-dependent germline activity of AKT-1 is controlled by a novel mechanism that does not involve phosphorylation by PDK-1 on key regulatory sites. These data argue that C. elegans re-arranges single homologous components of a signalling pathway to respond to different stimuli in vivo. Finally, I present data identifying the C. elegans ataxia and telangectasia and Rad3-related (ATR) kinase homologue ATL-1 as a potential target of AKT-1. Collectively, my work has uncovered a novel DNA damage-dependent pathway that allows AKT-1 to control CEP-1/p53-dependent apoptosis.Ph