13 August 2012. Two new studies describe how neurexin, a molecule implicated in schizophrenia by genetic association, interacts with other molecules to assemble synapses during development and fine-tunes their signals in maturity. Together, the studies suggest that defects in neurexin or its binding partners can alter how signals are routed through the brain.
The first study, published August 5 in Nature Neuroscience, reports that neurexin cooperates with Syd-1 to organize the proteins on both sides of the synapse during development. Led by Stephan Sigrist of Freie Universität Berlin, Germany, the study reports that neurexin is destabilized in Drosophila lacking Syd-1, leading to postsynaptic abnormalities. The second study, published online August 2 in Science, reports a role for neurexin and its binding partner, neuroligin, in synaptic signaling after development is complete. Led by Joshua Kaplan at Harvard Medical School, Boston, the study reported that neurexin-neuroligin binding sharpened the timing of neurotransmitter release in the worm Caenorhabditis elegans.
Deletions of neurexin found in schizophrenia, autism, and other psychiatric disorders (see SRF related news story; SRF Live Discussion), along with mutations in neuroligin found in autism (e.g., Zhang et al., 2009), have intensified research into the basic science of these adhesion molecules. Neurexin typically pokes out of the tip of an emerging axon, and binds to neuroligin, a molecule located on the surface of the contacted neuron. Researchers are now delving into how this neurexin-neuroligin partnership governs the assembly of both sides of the synapse, including the localization of neurotransmitter-containing vesicles on the presynaptic side, and the clustering of receptors to receive these chemical messages on the postsynaptic side. The new studies find this partnership is crucial for coupling these two sides effectively.
A trans-synaptic assembler
In the Nature Neuroscience study, first authors David Owald, Omid Khorramshahi, and Varun Gupta studied synapse assembly in vivo at the Drosophila neuromuscular junction. This venue has advantages over mammalian systems because the latter seem to have compensatory processes that make it tricky to discern neurexin’s role in synapse assembly (Varoqueaux et al., 2006). But in Drosophila, removing neurexin (Nrx-1) or neuroligin (Nlg-1) leads to severe deficits (Li et al., 2007). Because previous work had found that Syd-1, a cytoplasmic scaffolding protein, promoted presynaptic assembly, the researchers tested whether it works in concert with neurexin and neuroligin. It seems to—synaptic bouton numbers (a measure of synapse formation) were similarly decreased in Syd-1, Nrx-1, and Nlg-1 single mutants compared to controls, and the numbers were not further reduced in double mutants (Syd-1 with Nrx-1, or Syd-1 with Nlg-1), suggesting these proteins work along the same pathway.
The researchers next found that Syd-1 helped to cluster Nrx-1 and Nlg-1. In Syd-1 mutants, fluorescently labeled Nrx-1 intensity was 30 percent of that found in controls, which was taken as a sign of fragmented Nrx-1 clusters. Likewise, Nlg-1 clustering decreased in Syd-1 mutants. Nlg-1 clusters were recovered, however, when Nrx-1 was overexpressed in Syd-1 mutants. This and other experiments led to a picture of trans-synaptic cooperation: Syd-1 binds to the cytoplasmic end of Nrx-1, recruits it to the active zone of a developing synapse, and stabilizes it enough to form clusters; these Nrx-1 clusters then promote clustering of Nlg-1 across the synapse.
But it’s not all about the presynaptic side telling the postsynaptic side what to do. Sigrist's group found that Nlg-1 clustering in turn stabilized existing presynaptic clusters of Syd-1 and a related protein called Liprin-α, which would otherwise disintegrate. On its own postsynaptic turf, Nlg-1 clustering also guided the proper formation of glutamate receptors: without Nlg-1 clusters, GluR2B subunits were incorporated into glutamate receptors before GluR2A subunits—a reversal of what normally happens.
The report in Science adds an interesting twist by finding that neurexin signaling can influence even mature synapses. First author Zhitao Hu and colleagues studied the Nrx-1−Nlg-1 partnership in C. elegans, in which the usual location of these molecules is reversed, with Nrx-1 on the postsynaptic side and Nlg-1 on the presynaptic side. Using electrophysiology, the researchers asked whether the neurexin-neuroligin interaction mediated a previously reported suppression of neurotransmitter release observed upon inactivating a muscle microRNA (miR-1). When they combined the miR-1 mutation with the Nrx-1 or Nlg-1 mutation, this eliminated the release defect, suggesting that the miR-1 mutation in muscle needs working versions of Nrx-1 and Nlg-1 to carry out its effects on the presynaptic terminal.
To follow up on this clue, the researchers looked for changes in single Nrx-1 and Nlg-1 mutants. Though they did not turn up evidence for alterations in synapse formation or morphology that might reflect aberrant development, they did find that Nrx-1 or Nlg-1 inactivation increased quantal content, the number of synaptic vesicles released in response to an action potential, relative to controls. Looking at Nlg-1 mutants more carefully, they found that the ensuing currents recorded on the postsynaptic side were larger and decayed more slowly than controls, consistent with a slower and more prolonged release of synaptic vesicles. To see if this effect might also be at work in mammalian synapses, the researchers reanalyzed published data from synapses recorded in mouse triple knockouts lacking neuroligin-1, -2, and -3 (Varoqueaux et al., 2006). There, they uncovered a similar slowing of synaptic responses that stemmed from presynaptic changes.
Further experiments suggested that Nlg-1 normally sharpens these synaptic responses by inhibiting exocytosis of those synaptic vesicles far from the calcium channels—the stragglers that are released only when enough calcium drifts their way. The researchers found that a redistribution of presynaptic proteins involved in exocytosis could underlie this change. This further emphasizes that the precise location of synaptic proteins could matter very much for how synapses ultimately work, with even subtle changes in position leading to garbled consequences for synaptic communication.—Michele Solis.
Owald D, Khorramshahi O, Gupta VK, Banovic D, Depner H, Fouquet W, Wichmann C, Mertel S, Eimer S, Reynolds E, Holt M, Aberle H, Sigrist SJ. Cooperation of Syd-1 with Neurexin synchronizes pre- with postsynaptic assembly. Nat Neurosci. 2012 Aug 5. Abstract
Hu Z, Hom S, Kudze T, Tong XJ, Choi S, Aramuni G, Zhang W, Kaplan JM. Neurexin and Neuroligin Mediate Retrograde Synaptic Inhibition in C. elegans. Science. 2012 Aug 2. Abstract