Drosophila MAGE controls neural precursor proliferation in postembryonic neurogenesis

Necdin, a member of the MAGE family, is expressed abundantly in postmitotic neurons and is required for their differentiation and survival. In mammals, the MAGE family consists of more than 30 genes, whereas only one MAGE gene exists in the genome of nonmammalian vertebrates such as zebrafish and chicken. These nonmammalian MAGE genes are expressed in developing nervous system, and the primary structures of the encoded proteins resemble those of necdin-like MAGE proteins. Fruit fly Drosophila also carries a single necdin-like MAGE gene, which is highly expressed in neural stem cells (neuroblasts) during nervous system development. In the present study, we investigated the function of MAGE in Drosophila neurogenesis in vivo using an RNA interference (RNAi) –mediated gene knockdown system. Ubiquitous knockdown of Drosophila MAGE by double-stranded RNA injection into embryos was lethal at early stages of organogenesis. MAGE was then knocked down in developing mushroom bodies by RNAi-mediated gene silencing using the OK107-GAL4 driver. MAGE RNAi increased the population of proliferative neural precursors in larval mushroom bodies. At the pupal stage, RNAi-mediated MAGE knockdown led to a significant enlargement of the mushroom bodies as a result of increased neuronal population, presumably by accelerating the asymmetric division of neural stem cells. MAGE RNAi mushroom bodies of adult flies showed neurodegenerative changes such as vacuolation and nuclear DNA breaks, implying that supernumerary neurons undergo apoptosis during postpupal development. These results suggest that evolutionally conserved necdin-like MAGE is involved in both neural stem cell proliferation and neuronal survival during nervous system development.