A genome-wide RNAi screen for Neuroblast cell cycle exit in Drosophila. Catarina Homem, Juergen Knoblich. IMBA, Vienna, Austria.

   During development, Drosophila neural stem cells, the Neuroblasts (NBs), divide asymmetrically to self-renew and to generate a differentiated Ganglion Mother Cell (GMC). The GMC divides once more to generate two post-mitotic neurons or glia. Drosophila NBs undergo multiple rounds of divisions generating hundreds of neurons, which make up the nervous system of the fly. Drosophila NBs divide throughout development, but stop dividing and disappear just before entering adult stages. To maintain the correct number and type of neurons, it is essential to precisely regulate the time at which neurogenesis ceases. If these stem cells miss this proliferation stop this can lead to uncontrolled proliferation and tumor formation. Although several factors influencing neural proliferation have been identified, the underlying molecular mechanism scheduling the end of progenitor divisions remains enigmatic. NBs stop proliferating during pupal stages, suggesting that NB cell cycle exit happens as a response to either differential pupal nutritional status or to a stage specific humeral signal. By doing ex-vivo co-cultures of larval and pupal NBs we found that even in absence of extrinsic signaling pupal NBs exit cell cycle with similar timing to in vivo. This and other experiments show that NB proliferation termination is indeed cell intrinsically regulated. To identify novel genes regulating NB proliferation and division termination we performed a genome wide in vivo RNAi screen using central brain NBs as a model. In our assay we co-express Luciferase and RNA hairpins specifically in a subset of Nbs and their respective progeny. We then measure luminescence amounts in the adult head to assess Nb number and lineage sizes. From this screen we have identified approximately 80 genes that cause increases in NB size, NB number, NB life-span or their progeny number. Among these we find known NB tumor suppressors like Brat, Numb and Miranda. We also find previously uncharacterized tumor suppressors and 15 new regulators of NB growth and life-span.