Neuroligin I: a cell adhesion molecule that recruits PSD-95 and NMDA receptors by distinct mechanisms during synaptogenesis

Background: The cell adhesion molecule pair neuroligin I (NlgI) and beta-neurexin (beta-NRX) is a powerful inducer of postsynaptic differentiation of glutamatergic synapses in vitro. Because NlgI induces accumulation of two essential components of the postsynaptic density (PSD) - PSD-95 and NMDA receptors (NMDARs) - and can physically bind PSD-95 and NMDARs at mature synapses, it has been proposed that NlgI recruits NMDARs to synapses through its interaction with PSD-95. However, PSD-95 and NMDARs are recruited to nascent synapses independently and it is not known if NlgI accumulates at synapses before these PSD proteins. Here, we investigate how a single type of cell adhesion molecule can recruit multiple types of synaptic proteins to new synapses with distinct mechanisms and time courses. Results: NlgI was present in young cortical neurons in two distinct pools before synaptogenesis, diffuse and clustered. Time-lapse imaging revealed that the diffuse NlgI aggregated at, and the clustered NlgI moved to, sites of axodendritic contact with a rapid time course. Using a patching assay that artificially induced clusters of Nlg, the time course and mechanisms of recruitment of PSD-95 and NMDARs to those Nlg clusters were characterized. Patching Nlg induced clustering of PSD-95 via a slow palmitoylation-dependent step. In contrast, NMDARs directly associated with clusters of NlgI during trafficking. NlgI and NMDARs were highly colocalized in dendrites before synaptogenesis and they became enriched with a similar time course at synapses with age. Patching of NlgI dramatically decreased the mobility of NMDAR transport packets. Finally, NlgI was biochemically associated with NMDAR transport packets, presumably through binding of NMDARs to MAGUK proteins that, in turn, bind NlgI. This interaction was essential for colocalization and co-transport of NlgI with NMDARs. Conclusion: Our results suggest that axodendritic contact leads to rapid accumulation of NlgI, recruitment of NMDARs co-transported with NlgI soon thereafter, followed by a slower, independent recruitment of PSD-95 to those nascent synapses.