Genetic control of caspase dependent/independent cell death induction mechanisms via stress kinase JNK Laboratory for Cell Recovery Mechanisms Programmed cell death (apoptosis) is observed in various processes that occur in the bodies of animals. In the nervous system, neurons, which are produced in excess before there is a clear need or service for them, participate in a selection process determining the formation of precise neural circuits. Mutants in which programmed cell death is not observed have been identified among nematode C.elegans and Drosophila, have helped us to understand the genetic mechanisms controlling cell death. The reaper gene induces cell death and was first discovered in Drosophila. It contributes to the regulation of the number of neuroblasts in the central nervous system. In imagoes, Reaper expression is induced in response to various stress stimuli and induces cell death by activating caspases. In the present study, TRAF family molecule was identified, through genetic screening using Drosophila, as a gene that contributes to cell death through Reaper. TRAF family molecules have been identified, in mammals, as those that transmit signals downstream from tumor necrosis factor (TNF). We ascertained that Drosophila TRAF1 (DTRAF1) induces cell death by activating stress kinase JNK. IAP (DIAP1) promotes degradation of DTRAF1, and Reaper promotes degradation of DIAP1 thereby switching on the stress signal from DTRAF1 to JNK . JNK is activated by various kinds of stress and is known to induce apoptosis. In the present study, we elucidated the genetic relationship between the activation of stress kinase and the cell death induction mechanism, which is likely to facilitate further cell death signaling research in vivo. In addition, as a result of research into caspase-dependent apoptosis, we learned that cell death processes that are not dependent on caspase also contribute to various neurodegenerative disorders. In a joint study with Dr. AIGAKI Toshiro, the Tokyo Metropolitan University, Faculty of Science, we overexpressed genes in the compound eyes of Drosophila, and then screened for cell death inducer genes, examining the shrinkage of compound eyes (which resulted from cell death induction) as an index. By means of this screening, we identified the first TNF family "Eiger" gene in invertebrates. Cell death due to Eiger is not caspase-dependent; rather, it uses a JNK-dependent signal pathway. The TNF family proteins have various functions contributing roles in various diseases and have been studied as cell death factors, which trigger apoptosis in vertebrates. The discovery of this family, in the present study with Drosophila, which is an invertebrate, indicates that cell death factor has been conserved throughout evolution. Since Eiger was expressed in nervous tissue, further studies may be expected to identify new physiological functions and cell death signals related to TNF family molecules in the nervous system, using Drosophila, which are especially suitable for genetic research. Kuranaga, E., Kanuka, H., Igaki, T., Sawamoto, K., Ichijo, H., Okano, H., and Miura, M.: Reaper-mediated inhibition of DIAP1-induced Drosophila TRAF1 degradation leads to JNK activation. Nature Cell Biol. 4, 705-710, 2002 Igaki, T., Kanda, H., Yamamoto-Goto, Y., Kanuka, H., Kuranaga, E., Aigaki, T., and Miura, M.: Eiger, a TNF superfamily ligand that triggers the Drosophila JNK pathway. EMBO J. 21, 3009-3018, 2002 Figure 1 : Mechanism by which the Reaper activates stress kinase to JNK. Eye-specific overexpression of Eiger resulted in a small-eye phenotype(B), compared with wild-type eye(A). This phenotype was suppressed by a reduction of JNK gene dosage(C), and was almost completely rescued by an inhibition of the JNK activity(D). Figure 2 : Cell death induction by Eiger, and the contribution of JNK to Eiger-induced cell death.