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Neurexins not only bind to neuroligin. Additional binding partners of neurexin are dystroglycan and neuroexophilins. [1] Dytroglycan is Ca2+ dependant and binds preferentially to α-neurexins on LNS domains that lack splice inserts. In mice, a deletion of dystroglycan causes long-term potentiation impairment and developmental abnormalities similar to muscular dystrophy; however baseline synaptic transmission is normal. Neuroexophilins are Ca2+ independent and bind exclusively to α-neurexins on the second LNS domain. The increased startle responses and impaired motor coordination of neuroexophilin knockout mice indicates that neuroexophilins have a functional role in certain circuits. [2] The significance of the relationship between neurexin and dystroglycan or neuroexophilins is still unclear.
Neurexins are diffusely distributed in neurons and become concentrated at presynaptic terminals as neurons mature. There exists a trans-synaptic dialog between neurexin and neuroligin, meaning neuroligin can induce the expression of neurexin and vise versa. [3] This bi-directional trigger aids in the formation of synapses and is a key component to modifying the neuronal network. Over-expression of either of these proteins causes an increase in synapse forming sites, thus providing evidence that neurexin plays a functional role in synaptogenesis. [4] Conversely, the blocking of β-neurexin interactions reduces the number of excitatory and inhibitory synapses. It is not clear how exactly neurexin promotes the formation of synapses. One possibility is that actin is polymerized on the tail end of β-neurexin, which traps and stabilizes accumulating synaptic vesicles. This forms a forward feeding cycle, where small clusters of β-neurexins recruit more β-neurexins and scaffolding proteins to form a large synaptic adhesive contact.[4]
The trans-synaptic dialog between neurexin and neuroligin organizes the apposition of pre- and post-synaptic machinery by recruiting scaffolding proteins and other synaptic elements such as NMDA receptors, CASK, and synaptotagmin, all of which are necessary for a synapse to exist.
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