Neurexins and Neuroligins
Two of the most studied synaptic adhesion molecules are Neurexins and Neuroligins. Neurexins were discovered as receptors for alpha-Latrotoxin, a toxic component of black-widow spider venom that binds to presynaptic receptors causing a massive release of neurotransmitter. Type-1 transmembrane proteins, Neurexins can be classified as alpha or beta, which differ in size and extracellular amino acid sequence. Neuroligins are ligands for Neurexins that are expressed at the postsynaptic density. Also type-1 transmembrane proteins, Neuroligins bind both alpha and beta Neurexins. Although known to promote synaptogenesis in vitro, knockout experiments showed that the Neurexin-Neuroligin complex is critically required for synaptic function but not formation. Recent studies suggest that Neurexin-Neuroligin interactions may promote synaptic dendritogenesis. In addition, Neurexin-1 was found to bind LRRTM2 to induce presynaptic differentiation and postsynaptic assembly.