Understanding the spatial and functional relationship between cadherins and neuroligins at synapses

Cadherins and Neuroligins (NLs) are two of the most extensively studied cell adhesion molecules (CAMs) at synapses and have previously been shown to localize to synapses and exert a key role during their development. Despite this, their spatial and functional relationship with respect to one another has not been studied to date. In the present study, we examine the spatial and functional relationship of cadherin and NL isoforms at glutamatergic and GABAergic synapses in cultured hippocampal neurons. Analysis of the synaptic distribution of N-cadherin and NL1 and NL2 in hippocampal cultures, confirm previous studies demonstrating the enrichment of NL2 at GABAergic synapses and enrichment of NL1 and N-cadherin at glutamatergic synapses. We have also observed subsets of GABAergic synapses that express both N-cadherin and NL2 as well as glutamatergic synapses that only express either NL1 or N-cadherin. These groups of glutmatergic and GABAergic synapses may represent a specific subtype of synapse, or may reflect the differential localization of these adhesion molecules during synapse formation. Moreover, using a combination of overexpression and knockdown analysis we demonstrate that NL1 and N-cadherin promote the formation of synapses, in part, by a common pathway. Indeed, knocking down these proteins individually results in approximately 50% reduction in glutamatergic synapse density with a similar reduction upon combined knockdown. In addition, functional compensation assays demonstrate that NL1 expression can fully rescue synapse loss that is due to knockdown of N-cadherin expression. Furthermore, N-cadherin expression can partially rescue synapse loss that is due to knockdown of NL1 expression. Together this work demonstrates that these two cell adhesion proteins act in concert to regulate excitatory synapse formation. Specifically, we show that N-cadherin acts upstream of NL1 to promote synapse formation and that NL1 is a limiting factor in this pathway.Medicine, Faculty ofGraduat