Neuroligin

Neuroligin (NLGN), a type I membrane protein, is a cell adhesion protein on the postsynaptic membrane that mediates the formation and maintenance of synapses between neurons. Neuroligins act as ligands for β-Neurexins, which are cell adhesion proteins located presynaptically. Neuroligin and β-neurexin 'shake hands', resulting in the connection between two neurons and the production of a synapse. Neuroligins also affect the properties of neural networks by specifying synaptic functions, and they mediate signalling by recruiting and stabilizing key synaptic components. Neuroligins interact with other postsynaptic proteins to localize neurotransmitter receptors and channels in the postsynaptic density as the cell matures. Additionally, neuroligins are expressed in human peripheral tissues and have been found to play a role in angiogenesis. In humans, alterations in genes encoding neuroligins are implicated in autism and other cognitive disorders. Neuroligin (NLGN), a type I membrane protein, is a cell adhesion protein on the postsynaptic membrane that mediates the formation and maintenance of synapses between neurons. Neuroligins act as ligands for β-Neurexins, which are cell adhesion proteins located presynaptically. Neuroligin and β-neurexin 'shake hands', resulting in the connection between two neurons and the production of a synapse. Neuroligins also affect the properties of neural networks by specifying synaptic functions, and they mediate signalling by recruiting and stabilizing key synaptic components. Neuroligins interact with other postsynaptic proteins to localize neurotransmitter receptors and channels in the postsynaptic density as the cell matures. Additionally, neuroligins are expressed in human peripheral tissues and have been found to play a role in angiogenesis. In humans, alterations in genes encoding neuroligins are implicated in autism and other cognitive disorders. Neuroligins bind with the aid of Ca2+ to the α-neurexin LNS (laminin, neurexin and sex hormone-binding globulin-like folding units) domains and to the β-neurexin LNS domain which then establishes a heterophilic trans-synaptic recognition code. Through the observation of the crystal structure of neuroligin-1, it was determined that neuroligin-1 forms a protein dimer when two neurexin-1 beta monomers bind to the neuroligin-1's two opposite surfaces. This forms a heterotetramer, which contains an interface for binding Ca2+. The interaction of neuroligin and neurexin to form a heterotetramer is monitored by alternatively spliced sites located near the binding interface for Ca2+ in both the neuroligin-1 and the neurexin-1 beta. Subsequently, the presence of native neuroligin dimers was confirmed in neurons through biochemical detection, which included heterodimers composed of different neuroligin species, increasing the potential heterogeneity of endogenous neuroligin core dimer complexes. The extracellular domain of NLGN consists mostly of a region that is homologous to acetylcholinesterases, but the amino acids important for catalysis in AChE are not conserved in NLGN, which lack esterase activity. Furthermore, this AChE homologous region is crucial for the proper function of NLGN. Neuroligins have been identified in both vertebrates and invertebrates, including humans, rodents, chickens, Drosophila melanogaster, Caenorhabditis elegans, honeybees and Aplysia. Three genes for neuroligin expression have been found in mice and rats, while humans express five genes. Drosophila express four genes, honeybees express five genes, and both C. elegans and Aplysia express a single gene for neuroligin. The known neuroligin genes in Homo sapiens include NLGN1, NLGN2, NLGN3, NLGN4X and NLGN5 (also known as NLGN4Y). Each gene has been found to have unique influences on synaptic transmission. Expression of neuroligins may differ between species. Neuroligin 1 is expressed specifically in the CNS at excitatory synapses. In humans, expression of neuroligin 1 is low before birth and increases between postnatal days 1-8 and remains high through adulthood. This postnatal increase during active synaptogenesis corresponds to increased expression of postsynaptic density protein-95 (PSD-95). Neuroligin 2 is mainly concentrated at inhibitory synapses in the CNS, but in mice and humans it may also be expressed in tissues such as the pancreas, lung, endothelia, uterus and colon. Neuroligin 3 is expressed in CNS neurons, as well as a variety of glial cells in mice and rats and the brain, heart, skeletal muscle, placenta and pancreas in humans. Neuroligin 4X, found only in humans, is expressed in the heart, liver, skeletal muscle, pancreas and low levels in the brain. Neuroligin 5 (or 4Y), located on the Y chromosome, is only 19 amino acids different from neuroligin 4X. Neuroligin mRNA are present in human endothelial cells from large blood vessels and in Dorsal Root Ganglions. Alternative splicing, a modification that occurs after transcription of mRNA, regulates neuroligins’ binding selectivity for α- or β-neurexins as well as the function of synapses. Alternative splicing in neuroligins occurs in the main functional domain, the acetylcholinesterase-homologous region. Because neuroligin has two conserved splice sites in this region, sites A and B, up to four different isoforms are possible for each neuroligin gene. Neurexins also undergo alternative splicing, and certain splice variants of neuroligins and neurexins are more selective for one another. Specific pairing of splice variants also affects synaptic function. For example, neuroligins lacking the B splice insert and β-neurexins with the S4 insert promote differentiation of inhibitory, GABAergic synapses. On the other hand, neuroligins with the B insert and β-neurexins lacking the S4 insert promote differentiation of excitatory, glutamatergic synapses. The A insert may promote neuroligin localization and function at inhibitory synapses, but the mechanisms are unknown. Neurexin and neuroligin work together to gather and maintain the cytoskeleton components needed to localize synaptic vesicles. Neurexin is necessary for containing the voltage-gated Ca2+ channels that are required for the release of vesicles, while neuroligin binds neurexin in order to localize the necessary neurotransmitter receptors and proteins for postsynaptic specialization. At the postsynaptic site, neuroligins are networked to specialized proteins that stimulate specific neurotransmitter receptors and channels to densely occupy specialized regions of the postsynaptic terminal during the maturation of the synapse. Because all developing synapses contain neurexins and neuroligins, developing cells can make many different connections to other cells. Neuroligin is sufficient to form new functional presynaptic terminals in vitro. However, evidence suggests that additional adhesion molecules, such as immunoglobulin-domain and cadherin family proteins, mediate the initial contact between the axons and dendrites for a synapse. Neurexins and neuroligins then reinforce the contact.