Investigating the roles of cell adhesion molecules in synapse formation and function

Recent findings have revealed a crucial contribution of the adhesion molecule neuroligin-1 to the precise organization and regulation of intercellular synaptic connections within the central nervous system, and disruption of neuroligin-1 signaling in vivo fosters cognitive abnormalities. Despite considerable recent progress, several uncertainties remain regarding the exact synaptic function of neuroligin-1. Principle among these uncertainties is whether neuroligin-1 primarily promotes initiation of de novo synaptic connections or maturation of functional, pre-existent connections. To begin to address this, experiments must be devised that are capable of dissociating activity-dependent and -independent effects of neuroligin-1 signaling on pre- and postsynaptic compartments. An additional uncertainty is how and when synapses containing neuroligin-1 are specified as either excitatory or inhibitory. Elucidating these synapse specification cascades will prove crucial in defining the contribution of neuroligin-1 to overall network balances of excitation and inhibition that guide proper cognitive development. A final uncertainty is how alternate adhesion complexes may coordinate with neuroligin-1 to initiate or maintain synaptic connections. Differentiating redundant from complementary functions among adhesion systems will help reconcile unresolved discrepancies between in vitro and in vivo experiments and ultimately provide a clearer understanding of synapse formation and function in vivo. Herein I detail significant new findings clarifying each of these uncertainties. Utilizing a specific transfection protocol, I first demonstrate that neuroligin-1 is capable of robustly inducing presynaptic differentiation independent of proper postsynaptic development and synaptic activity. Second, employing both multi-molecular perturbations and a delimited biological model of the synapse, I show that the postsynaptic scaffolding molecule PSD95 specifically acts downstream of neuroligin-1-mediated synapse initiation. Third, the model synapse is again employed to differentiate between separate synaptic functions of neuroligin-1 and alternate adhesion molecule SynCAM1. Building from these distinct synaptic functions, I provide preliminary evidence that SynCAM1 matures inactive neuroligin-1-initiated synapses. Fourth, I present the first direct evidence that neuroligin-1 contributes to dendritic morphogenesis in mammalian neurons, consistent with recent findings within the Xenopus system. Collectively, these results evince a robust capacity of neuroligin-1 in initial stages of synaptogenesis and contribute to a new theory of neuroligin-1 function in both activity-dependent synapse initiation and activity-dependent synapse maturation