Synaptic neurexin-1 assembles into dynamically regulated active zone nanoclusters.

Neurexins are well-characterized presynaptic cell adhesion molecules that engage multifarious postsynaptic ligands and organize diverse synapse properties. However, the precise synaptic localization of neurexins remains enigmatic. Using super-resolution microscopy, we demonstrate that neurexin-1 forms discrete nanoclusters at excitatory synapses, revealing a novel organizational feature of synaptic architecture. Synapses generally contain a single nanocluster that comprises more than four neurexin-1 molecules and that also includes neurexin-2 and/or neurexin-3 isoforms. Moreover, we find that neurexin-1 is physiologically cleaved by ADAM10 similar to its ligand neuroligin-1, with ∼4-6% of neurexin-1 and ∼2-3% of neuroligin-1 present in the adult brain as soluble ectodomain proteins. Blocking ADAM10-mediated neurexin-1 cleavage dramatically increased the synaptic neurexin-1 content, thereby elevating the percentage of Homer1(+) excitatory synapses containing neurexin-1 nanoclusters from 40-50% to ∼80%, and doubling the number of neurexin-1 molecules per nanocluster. Taken together, our results reveal an unexpected nanodomain organization of synapses in which neurexin-1 is assembled into discrete presynaptic nanoclusters that are dynamically regulated via ectodomain cleavage.