Neuroligin-1 performs neurexin-dependent and neurexin-independent functions in synapse validation

Postsynaptic neuroligins are thought to perform essential functions in synapse validation and synaptic transmission by binding to, and dimerizing, presynaptic α- and Β-neurexins. To test this hypothesis, we examined the functional effects of neuroligin-1 mutations that impair only α-neurexin binding, block both α- and Β-neurexin binding, or abolish neuroligin-1 dimerization. Abolishing α-neurexin binding abrogated neuroligin-induced generation of neuronal synapses onto transfected non-neuronal cells in the so-called artificial synapse-formation assay, even though Β-neurexin binding was retained. Thus, in this assay, neuroligin-1 induces apparent synapse formation by binding to presynaptic α-neurexins. In transfected neurons, however, neither α- nor Β-neurexin binding was essential for the ability of postsynaptic neuroligin-1 to dramatically increase synapse density, suggesting a neurexin-independent mechanism of synapse formation. Moreover, neuroligin-1 dimerization was not required for either the non-neuronal or the neuronal synapse-formation assay. Nevertheless, both α-neurexin binding and neuroligin-1 dimerization were essential for the increase in apparent synapse size that is induced by neuroligin-1 in transfected neurons. Thus, neuroligin-1 performs diverse synaptic functions by mechanisms that include as essential components of α-neurexin binding and neuroligin dimerization, but extend beyond these activities.