Dscam1 promotes blood cell survival in Drosophila melanogaster through a dual role in blood cells and neurons

Down Syndrome Cell Adhesion Molecule 1 (Dscam1) is a receptor-like cell adhesion molecule that is conserved across the animal kingdom, but its roles in hematopoiesis remain unknown. Dscam1 related genes in vertebrates and invertebrates are key regulators of neuron morphogenesis and neuronal tiling. In Drosophila, Dscam1 in addition has roles in blood cells (hemocytes) in innate immunity and phagocytosis of pathogens. Given the anatomical and functional role of peripheral sensory neurons as microenvironments for resident hematopoietic sites in the Drosophila larva, we sought to investigate the role of Dscam1 in this context. Interestingly, we find that Dscam1 fills the role of a previously anticipated factor in neuron-hemocyte communication that supports trophic survival: tissue specific silencing of Dscam1 by in vivo RNAi in sensory neurons leads to neuron reduction, which in turn results in reduced hemocyte numbers due to apoptosis. Dscam1 silencing in hemocytes also results in a reduction of hemocytes and increased apoptosis. This cell-autonomous effect of Dscam1 silencing can be mimicked by RNAi silencing of dreadlocks (dock), suggesting that intracellular Dscam1 signaling relies on the adapter protein Dock in this system. Our findings reveal a dual role for Dscam1 in Drosophila hematopoiesis, by promoting survival of the sensory neuron microenvironments that in turn support hemocyte survival, and by promoting survival of hemocytes cell-autonomously. It will be interesting to explore possible functions of vertebrate Dscam1 related genes such as DSCAML1 in blood cells and their trophic survival.