Activity-independent and subunit-specific recruitment of functional AMPA receptors at neurexin/neuroligin contacts

A combination of cell culture and animal studies has recently shown that adhesion between neurexins and neuroligins played important roles in synapse initiation, maturation, and function. Binding of neurexin-1β to neuroligin-1 triggers the postsynaptic clustering of the scaffold postsynaptic density protein 95, but the composition and timing of accumulation of glutamate receptors at those nascent contacts remain unclear. Using glutamate iontophoresis and patch-clamp recordings, we identified functional AMPA receptors (AMPARs) and NMDA receptors at postsynaptic density protein 95 clusters induced by neurexin-1β coated microspheres on primary hippocampal neurons. The recruitment of AMPARs occurred as early as 2 h after initial contact, and was not blocked by TTX/2-amino-5-phosphovaleric acid (APV) treatment. The differential recruitment of recombinant subunits GluR1 and GluR2, as well as the absence of rectification in voltage/current curves, further indicate that neurexin/neuroligin contacts primarily recruit GluR2-containing AMPARs. Finally, by using glutamate un-caging and calcium imaging, we show that AMPARs participate in calcium entry at neurexin-1β induced post-synapses, most likely through the activation of voltage-gated calcium channels. Such rapid and activity-independent accumulation of functional AMPARs at neurexin-1β-induced postsynapses points to a new role of AMPARs in synaptogenesis.