Investigating CD47-SIRPa as novel protective signals during CNS synaptic pruning
Microglia, the brain’s resident immune cells and phagocytes, are emerging as critical regulators of developingsynaptic circuits in the healthy brain. Recent studies from our lab and others indicate that microglia engulfsynapses in the developing brain; however, how microglia know which synapses to target remains a majoropen question. Our previous work demonstrates that microglia-mediated pruning underlies developmentalsynaptic refinement, an essential process required for the formation of mature circuits in which weak orinappropriate synapses are eliminated and remaining connections are maintained and strengthened. We foundthat microglial engulfment of presynaptic inputs is activity-dependent and driven by complement moleculesC1q and C3, and microglial complement receptor CR3. These molecules are innate immune “eat me” signalsknown for promoting macrophage phagocytosis of apoptotic cells or debris, and mice lacking these signalsexhibit reduced microglial engulfment of synaptic inputs and impaired refinement. This suggests that microgliamediatedpruning may be analogous to the removal of non-self material by phagocytes in the immune system.However, we do not yet know how microglia precisely determine which inputs to engulf and which to avoid, animportant decision regarding the specificity needed to sculpt precise, mature connections. We propose thatprotective “don’t eat me” signals are required to prevent inappropriate microglial engulfment of necessaryconnections during synaptic refinement, just as they prevent inappropriate engulfment of healthy self-cells byphagocytes during an immune response. Our preliminary data support this hypothesis, as “don't eat me”signals CD47 and SIRPα are present in the developing brain and required to prevent excess microglialengulfment of synaptic inputs. We will investigate the anatomical, functional, and behavioral abnormalities inmice lacking CD47 and SIRPα to better understand the consequences of excess microglial engulfment. We willalso investigate whether and how these “don’t eat me” signals are regulated by activity to determine if theydirect microglia to engulf specific synapses in an activity-dependent manner. Finally, as “don’t eat me” signalsare known to be downregulated in the brains of patients with neurodegenerative diseases, we will examinewhether these molecules are dysregulated in mouse models of Huntingon’s disease (HD) and could therebyunderlie synapse loss caused by aberrant microglial engulfment. This study would be the first to demonstratethat synaptic protection is required to prevent inappropriate microglial engulfment of necessary connections during development. This research program will provide insight not only into the mechanisms regulating microglial engulfment of specific synapses, but also into possible mechanisms underlying synapse loss in CNS neurodegenerative diseases.