Neurologin Function in Excitatory and Inhibitory Synapses

Neuroligins (NLs) are postsynaptic cell adhesion molecules which by binding to presynaptic neurexins (NRXs) are thought to mediate synapse formation and function. Both NLs and NRXs are discussed in the genetic correlation to Autism. Over-expression of NLs could induce the formation of synaptic contacts with axons in non-neuronal cells and increase the synaptic density and response in cultured neurons, through binding and recruiting NRXs; however, little is known about NL signaling though NRXs or inside the cell. First, we hypothesized that NLs signal through their cytoplasmic region. Over-expression of NL1 with cytoplasmic tail truncation abolished the increase of synaptic density by NL1 full length. By yeast two hybrid screening using NL2 cytoplasmic region, we identified potential interaction partners, of which Necab2 and NRP/B (also named as ectodermal cortex 1, EC1) are two promising candidates and the interactions were confirmed. NL1 or NL2 c-tail truncations partially abolished the change in miniature IPSC, but not the evoked responses. NL c-tail binding partners?ver-expression does not show any change in evoked responses. It suggested that NL cytoplasmic region is important for some neuronal changes but does not contribute to the major phenotype of NLs. And we investigated the contribution of NL-NRX binding by using NL extracellular NRX binding mutants. The mutants abolished the change of the evoked and miniature inhibitory responses from the NL2 wild type, which suggested the inhibitory responses triggered by NL2 go through NRXs. And the slight change of the paired pulse ratio suggested the change of presynaptic calcium through binding. The study suggested that NL2 facilitate the inhibitory synaptic transmission through extracellular region via neurexin binding, possibly by the increase in presynaptic calcium. We also found Brain-specific Angiogenesis Inhibitors (BAIs), a family of G-protein coupled receptors (GPCRs), will bind to NLs extracellularly and may serve as signaling modules binding to NLs. Over-expression of BAIs do not change evoked IPSCs, but Bai1 decreased evoked EPSCs and increased the burst duration in the spontaneous responses, possibly because of some secondary responses. Therefore, we found NL-NRX though NL extracellular region is important for NL2 function in synaptic transmission, and BAIs may be potential signaling molecules of NLs