Regulation of dendrite morphogenesis by Nanos and Pumilio. Balpreet Bhogal, Elizabeth Gavis. Department of Molecular Biology, Princeton University, Princeton, NJ.

   The organization of dendritic arbors dictates how effectively a neuron can receive and integrate extracellular signals. Although many factors have been identified that are essential for establishing the size and complexity of dendritic arbors, much less is known about how the branching patterns are subsequently refined and maintained during neuronal development. Dendritic arborization (da) neurons in the Drosophila larval peripheral nervous system have provided a powerful genetic system to identify factors required for dendrite morphogenesis. The translational repressors Nanos (Nos) and Pumilio (Pum) are required to maintain dendritic arborization during late larval development in the highly branched class IV da neurons. Recent genetic analyses from our lab suggest that proper dendrite morphogenesis of class IV da neurons requires regulation of the apoptotic pathway by Nos and Pum. We found that genetically reducing apoptotic gene function suppresses the dendritic defects observed in class IV da neurons with reduced nos function. Time-lapse confocal microscopy revealed that suppression of the dendritic defects observed in nos-deficient class IV da neurons is most likely due to a restoration of branch growth and a reduction in branch retraction. A fluorescent reporter is currently being utilized to determine whether caspase activity is increased in nos and pum mutant larvae. We are concurrently determining if Nos and Pum directly regulate the pro-apoptotic factor Head involution defective (Hid). Hid expression is elevated in nos and pum mutant larvae and genetic analysis revealed that reducing Hid expression suppresses the dendritic arborization defects observed in nos-deficient class IV da neurons. We have also demonstrated that Pum binds to the 3' untranslated region (UTR) of hid mRNA in vitro. We propose that Nos and Pum modulate nonapoptotic caspase function through regulation of hid in order to maintain dendritic complexity of larval class IV da neurons prior to metamorphosis.