Synapse Maturation by Activity-Dependent Ectodomain Shedding of SIRP

Formation of functional synaptic connections is critical for proper functioning of the brain. After initial synaptic differentiation, synapses are maturated and stabilized by neural activity to establish appropriate synaptic connections. During maturation, presynaptic boutons enlarge, more synaptic vesicles accumulate to the presynaptic terminal, the number of active zones and postsynaptic densities increases, and the shape of spines changes in response to synaptic activity. However, the molecular mechanisms underlying activitydependent synapse maturation remain to be elucidated. Using the ability to cluster synaptic vesicles in cultured neurons as a bioassay, we have purified molecules that can organize presynaptic terminals from developing brains. This purification revealed two peaks of activity that induced synaptic vesicle clustering. One peak contains FGF22, and we have shown that FGFs promote differentiation of cerebellar, neuromuscular, and hippocampal synapses. The other peak, on which we focus here, contains the extracellular domain of signal regulatory protein α (SIRPα), a transmembrane immunoglobulin superfamily member. SIRPα is highly expressed in the hippocampus around the time of synapse maturation. It is localized in dendrites and concentrated at synapses. Overexpression of SIRPα in cultured hippocampal neurons promotes maturation of presynaptic terminals on the SIRPα-expressing neurons; this effect is inhibited by globally suppressing neural activity. Interestingly, the extracellular domain of SIRPα is cleaved and shed in response to neuronal activation, and the application of shed SIRPα to hippocampal neurons promotes synaptic vesicle clustering. Conditional SIRPα KO mice show defects in presynaptic maturation. From these preliminary results, we propose the following model: After initial synapse formation by axon-dendrite contacts, neurotransmitter release from the presynaptic terminal induces the cleavage of postsynaptic SIRPα, and the shed ectodomain of SIRPα in turn promotes the maturation of the presynaptic terminal. To test this hypothesis, we propose to:

• Aim 1: Determine whether ectodomain shedding is required for the presynaptic effect of SIRPα

• Aim 2: Investigate the role of neural activity for SIRPα-dependent presynaptic maturation

• Aim 3: Examine the importance of ectodomain shedding of SIRPα for presynaptic maturation in vivo

We will use molecular and cellular biological, biochemical, imaging and electrophysiological approaches. Through these studies we should understand the molecular mechanisms underlying functional synapse establishment in the hippocampus by neural activity. Many forms of neurological disorders are associated with abnormal alterations of synapses in the hippocampus. Furthermore, SIRPα is implicated in depression, and a SIRPα receptor CD47 is implicated in Alzheimer's disease and schizophrenia. Thus, our studies will help design strategies to prevent and treat such neurological disorders. In future studies, we will use conditional SIRPα KO mice to investigate the in vivo role of SIRPα and its ectodomain shedding in neurological disorders.