Neuron-produced Activin supports hematopoiesis in the Drosophila larva. Kalpana Makhijani², Brandy Alexander², Sophia Petraki², Michael O'Connor⁴, Katja Brückner^1,2,3. 1) Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research; 2) Department of Cell and Tissue Biology; 3) Department of Anatomy, University of California San Francisco, San Francisco, CA; 4) Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, MN.

   The Peripheral Nervous System (PNS) has been identified as a functional component of hematopoietic microenvironments and other stem cell niches, both in vertebrates and invertebrates. However, it remains largely unknown how sensory neurons and their inputs direct hematopoiesis or immune responses. To address these questions at the molecular and cellular level, we study the role of the PNS as a microenvironment in the hematopoietic pockets of the Drosophila larva. In this system, hemocytes reside in direct physical contact with segmentally repeated sensory PNS clusters and are induced to proliferate in these microenvironments (Makhijani et al. 2011). Using mutants, cell ablation, and other genetic manipulations that disrupt the PNS or generate ectopic neurons, we demonstrate that larval hemocytes functionally depend on the PNS regarding their localization and trophic survival. GRASP demonstrates direct neuron-hemocyte and glia-hemocyte contacts. Hypothesizing that Drosophila larval hematopoiesis is molecularly controlled by the PNS microenvironment, we screened key signaling pathways by in vivo RNAi and identified TGF- related Activin (Act) as a PNS signal that supports larval hemocytes. Using cell type specific RNAi and dominant-negative transgene expression, we demonstrate complementary roles of PNS neuron-expressed Act, and hemocyte-autonomous Activin signaling through baboon (babo), punt (put) and dSmad2, all of which are required to control resident hemocyte adhesion/localization and number. Drosophila larval hematopoiesis shows parallels with vertebrate self-renewing tissue macrophages and hematopoiesis in the bone marrow niche. In the future, this system can be used to identify further, constitutive or neuronal activity-dependent mechanisms by which the PNS regulates hematopoiesis or immune responses.