How do neurons in the brain decide to refine their synaptic connections in vivo?

Project Summary The formation of functional neural circuits is critical for the proper functioning of the brain. To establish the most efficient synaptic circuits, synaptic connections must be refined by neural activity during development. However, the manner and molecules by which synapse refinement is regulated remain to be elucidated. We have established mouse in vivo systems, in which neural activity can be conditionally controlled, and showed that inactive synaptic connections are eliminated during development, but they are eliminated only when there are other active connections with which to compete. This suggests that active connections send a "punishment" signal to inactive ones and instruct them to leave by triggering "elimination" signals within the inactive synapses. Active connections are kept by the presence of "stabilization" signals. By performing various screens, we have identified that the tyrosine kinases Pyk2 and JAK2 serve as "elimination" signals of inactive synaptic connections during development. Pyk2 and JAK2 are turned on at inactive synapses in response to "punishment" signals sent from active synapses to drive inactive synapse elimination. Furthermore, we found that a cell adhesion molecule SIRPa from postsynaptic neurons serves as a "stabilization" signal for active synapses. Together, we propose that neurons calculate the balance between the "elimination" signals (Pyk2/JAK2) and the "stabilization" signals (SIRPa) and determine whether to eliminate or stabilize their synaptic connections. Interestingly, our recent preliminary data suggest that the elimination signals Pyk2 and JAK2 have different roles, with Pyk2 in synapse elimination and JAK2 in axon elimination. Additionally, we found that astrocytic ensheathing of synapses also plays important roles in synapse elimination. To further uncover the molecular mechanisms underlying synapse refinement in vivo, we propose to: Aim 1. Investigate the differential roles of Pyk2 and JAK2 for synapse and axon refinement and how SIRPa regulates their activity. Aim 2: Visualize the elimination signals and their regulation of activity-dependent synapse/axon refinement in vivo. Aim 3: Examine the role(s) of astrocytic sheaths in regulating the elimination signals and synapse refinement. Our project will molecularly delineate how neurons decide to establish functional synaptic connections in the mammalian brain. Many forms of mental illness including autism and schizophrenia are associated with abnormal alterations in synapse refinement. Thus, our studies should also yield novel insights into the etiology and treatment of such disorders.