| Summary: | Cell adhesion molecules (CAMs) have emerged as important players in synapse development; however, the precise roles of these proteins at newly formed contacts remain unknown. In this thesis, I begin by providing an overview of synaptic structure and development, as well as a review of our current understanding of how key CAMs and associated proteins fit into this framework.
In the second chapter, I demonstrate that members of the postsynaptically localized neuroligin (NL) family of CAMs, including NL1, NL2 and NL3, can trigger the formation of excitatory and inhibitory presynaptic terminals, and that while NL1 is enriched at excitatory contacts, NL2 localizes primarily to inhibitory sites. Neuroligin-mediated enhancement of inhibitory synapse density is blocked by a fusion protein containing the extracellular domain of the presynaptic neuroligin binding partner, neurexin-1β. Furthermore, overexpression of postsynaptic density-95 (PSD-95), a postsynaptic binding partner of neuroligins, results in a shift of NL2 from inhibitory to excitatory synapses. These findings reveal that multiple neuroligins control the number of inhibitory and excitatory synapses, and that localization of NL2 can be altered by scaffolding proteins.
In the third chapter, I examine the mechanisms by which NL2 and NL3 are recruited to inhibitory and excitatory synapses, respectively. To this end, I assessed the roles of PSD-95 and gephyrin, a postsynaptic scaffolding molecule localized exclusively to inhibitory synapses, in localizing NL2 and NL3. Knockdown of gephyrin results in a shift of NL2 from inhibitory to excitatory synaptic contacts, while knockdown of PSD-95 leads to a shift of NL2 and NL3 from excitatory to inhibitory contacts. Deletion of a discrete region within the C-terminus of NL2 reveals that the intracellular tail is required for the normal synaptic clustering of this protein. Together, these data suggest that intracellular mechanisms are involved in the synaptic targeting of different neuroligin family members.
Overall, these results demonstrate an important role for neuroligins in the development of glutamatergic and GABAergic synapses, and indicate that postsynaptic scaffolding molecules modulate the targeting of neuroligins to distinct postsynaptic compartments. The final chapter of the thesis provides a general discussion relating these findings to other recent advancements in the field.
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