Cellular and Molecular Mechanisms of Inhibitory Synapse Development

PROJECT SUMMARY Synapses are the primary sites of cell-cell contact in the nervous system and are required for the proper function of neural circuits. Signaling molecules expressed on the neuronal cell surface during early development instruct synapse formation and specify the identity of synapses as excitatory or inhibitory. Proper balance between excitation and inhibition is essential for proper nervous system function. Perturbations to this process are implicated in a variety of neurological and neurodevelopmental disorders including autism, epilepsy, and schizophrenia. The proposed research will reveal key biological mechanisms of inhibitory synapse formation and suggest pathways to target dysregulated excitatory-inhibitory balance in various neurological disorders. Despite a robust understanding of the molecular mechanisms of excitatory synapse formation, relatively little is known about inhibitory synapse formation. Previous work indicates that the cleaved transmembrane protein Semaphorin 4D (Sema4D), signaling via its receptor PlexinB1, rapidly (~30 mins) promotes the formation of new inhibitory synapses in the adult brain. In addition, Sema4D exclusively promotes inhibitory synapse formation while having no effect on excitatory synapses, a property which thus far appears to be unique to Sema4D. Further, previous work showed that treatment with Sema4D increases seizure resistance in adult mice in vivo, suggesting that selectively increasing inhibitory synapse density may have therapeutic applications. This proposal aims to elucidate the biological mechanisms by which new inhibitory synapses are formed in the central nervous system by utilizing the unique ability of Sema4D to rapidly promote inhibitory synapse formation. Using a live imaging approach, the proposed experiments seek to determine how elements of the pre-and postsynaptic specialization coordinate in real time to rapidly assemble new synapses under the control of Sema4D. This approach will also provide insights into poorly understood processes which have been observed during synapse formation, such as the splitting of presynaptic and postsynaptic protein assemblies. Finally, these experiments will reveal intracellular signaling pathways which mediate Sema4D-dependent inhibitory synapse formation by manipulating signaling downstream of the PlexinB1 receptor.