View Program - 63rd Annual Drosophila Research Conference

Microcephaly is a failure to achieve proper brain size and neuron number during development. Most microcephaly-linked genes function either at the mitotic spindle or in DNA damage repair (DDR). Spindle proteins are thought to control brain size via their functions in neural stem cell (NSC) mitosis, whereas DDR proteins are thought to suppress DNA damage and cell death. However, since few microcephaly genes are well-studied in neurogenesis, we set out to characterize microcephaly genes in Drosophila brains. Here, we investigated the microcephaly gene Traip, known to function in DDR, and found that Traip promotes proper brain size by suppressing DNA bridges in mitosis. We show that traip^- flies have microcephaly-like brain defects, including fewer neurons and marked loss of NSCs via caspase-dependent cell death. traip^- NSCs have normal levels of DNA damage in interphase, suggesting that Traip does not fit the canonical model of DDR functions in microcephaly. In contrast, traip^- NSCs are frequently aneuploid, multinucleate, or have micronuclei, suggesting mitotic failure. Live fluorescence microscopy of traip^- NSCs revealed frequent mitotic DNA bridges, providing a possible explanation for the observed nuclear defects via chromosome fragmentation and cytokinesis failure. We characterized fluorescence-tagged Traip transgenes: in interphase, Traip is nuclear, whereas in mitosis Traip localizes on spindles, furrow, and midbody. A Traip variant lacking the nuclear localization signal is absent from the nucleus, but localizes properly in mitosis and rescues traip^- brain phenotypes, showing that a mitotic Traip function is sufficient to suppress microcephaly. Together, our work challenges current thinking about the relationships between DDR, mitosis and microcephaly by showing that, instead of repairing DNA damage during interphase, Traip primarily functions to preserve NSC genome integrity by resolving mitotic DNA bridges. Now, using Traip as a model microcephaly gene, we are using whole brain imaging and 3D analysis to screen for suppressors and uncover downstream pathways that mediate microcephaly phenotypes. To date, we have found roles for MAPK and Toll signaling, abscission kinase, and caspase-dependent cell death. We are now testing whether these pathways also mediate the phenotypes of other microcephaly genes, including both DDR and mitotic spindle genes, and targeting these pathways as potential therapeutic targets to minimize neuron loss in microcephaly.

Traip controls brain size via suppression of mitotic DNA bridges