Trans-Synaptic Interaction of GluRδ2 and Neurexin through Cbln1 Mediates Synapse Formation in the Cerebellum

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The MAM domain-containing GPI anchor proteins MDGA1 and MDGA2 are Ig superfamily adhesion molecules composed of six IG domains, a fibronectin III domain, a MAM domain, and a GPI anchor. MDGAs contribute to the radial migration and positioning of a subset of cortical neurons during early neural development. However, MDGAs continue to be expressed in postnatal brain, and their functions during postnatal neural development remain unknown. Here, we demonstrate that MDGAs specifically and with a nanomolar affinity bind to neuroligin-2, a cell-adhesion molecule of inhibitory synapses, but do not bind detectably to neuroligin-1 or neuroligin-3. We observed no cell adhesion between cells expressing neuroligin-2 and MDGA1, suggesting a cis interaction. Importantly, RNAi-mediated knockdown of MDGAs increased the abundance of inhibitory but not excitatory synapses in a neuroligin-2-dependent manner. Conversely, overexpression of MDGA1 decreased the numbers of functional inhibitory synapses. Likewise, coexpression of both MDGA1 and neuroligin-2 reduced the synaptogenic capacity of neuroligin-2 in an artificial synapse-formation assay by abolishing the ability of neuroligin-2 to form an adhesion complex with neurexins. Taken together, our data suggest that MDGAs inhibit the activity of neuroligin-2 in controlling the function of inhibitory synapses and that MDGAs do so by binding to neuroligin-2.inhibitory synapse formation | synaptic cell adhesion | autism | schizophrenia R ecent studies of synapse formation have uncovered a multitude of synaptic adhesion molecules, and human genetic studies have implicated many of these molecules in neuropsychiatric and neurodevelopmental disorders (1-4). However, little is known about the specific pathophysiological mechanisms by which dysfunctions of synaptic adhesion molecules contribute to these complex disorders.Neurexins and neuroligins (NLs) are arguably the most extensively studied synaptic adhesion molecules (1). They are dispensable for initial synapse establishment but act in an isoformdependent manner to specify the maturation of either excitatory or inhibitory synapses (5). There are four NL members in rodents (NL1-NL4) that show distinct synaptic localizations and functions (5). NL2, in particular, has received considerable attention because of its unique localization and function at inhibitory synapses (6). For instance, NL2 controls perisomatic inhibitory synapse maturation together with gephyrin and collybistin, which regulate GABA receptor clustering on neurons (7,8). Moreover, NL2 exhibits differential functions at different types of inhibitory synapses on the same postsynaptic neuron (9). All four NLs likely mediate synapse-promoting activities through direct interactions with presynaptic neurexins, but NLs also perform additional functions in synapse validation that are independent of their binding to neurexins (10).MAM domain-containing GPI anchor proteins (MDGAs), also termed "GPIMs" or "MAMDCs," initially were identified in tumor cells (11). The two homologous MDGA ...

Section: Discussionmentioning

confidence: 99%

MDGAs interact selectively with neuroligin-2 but not other neuroligins to regulate inhibitory synapse development

Lee

Kim

Lee³

et al. 2012

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139

“…5 and 6). Apart from Nxph, prototypical binding partners of Nrxn are neuroligins (13,14), LRRTMs (15,16), and cerebellin/GluRδ2 (17,18). Physical binding has also been reported for GABA A R subunits (37); dystroglycan (68); and, very recently, calsyntenin-3 (67).…”

Section: Discussionmentioning

confidence: 99%

“…We have addressed this problem by studying neurexins and their interaction partners (5,6). Several aspects make neurexins candidates to couple local recognition/adhesion events to synaptic function: first, both extracellularly longer α-neurexins (α-Nrxn) and shorter β-neurexins (β-Nrxn) are able to induce functional synapses (7,8); second, at least α-Nrxn are essential for synaptic transmission at excitatory and inhibitory terminals (9,10); and third, α-and β-Nrxn are highly polymorphic, mostly presynaptic molecules (11,12) that interact with transsynaptic binding partners like neuroligins (13,14), LRRTMs (15, 16), or cerebellin/GluRδ2 (17,18).…”

mentioning

confidence: 99%

Modulation of synaptic function through the α-neurexin–specific ligand neurexophilin-1

Born

Breuer

Wang

et al. 2014

Neurotransmission at different synapses is highly variable, and cell-adhesion molecules like α-neurexins (α-Nrxn) and their extracellular binding partners determine synapse function. Although α-Nrxn affect transmission at excitatory and inhibitory synapses, the contribution of neurexophilin-1 (Nxph1), an α-Nrxn ligand with restricted expression in subpopulations of inhibitory neurons, is unclear. To reveal its role, we investigated mice that either lack or overexpress Nxph1. We found that genetic deletion of Nxph1 impaired GABA B receptor (GABA B R)-dependent short-term depression of inhibitory synapses in the nucleus reticularis thalami, a region where Nxph1 is normally expressed at high levels. To test the conclusion that Nxph1 supports presynaptic GABA B R, we expressed Nxph1 ectopically at excitatory terminals in the neocortex, which normally do not contain this molecule but can be modulated by GABA B R. We generated Nxph1-GFP transgenic mice under control of the Thy1.2 promoter and observed a reduced short-term facilitation at these excitatory synapses, representing an inverse phenotype to the knockout. Consistently, the diminished facilitation could be reversed by pharmacologically blocking GABA B R with CGP-55845. Moreover, a complete rescue was achieved by additional blocking of postsynaptic GABA A R with intracellular picrotoxin or gabazine, suggesting that Nxph1 is able to recruit or stabilize both presynaptic GABA B R and postsynaptic GABA A R. In support, immunoelectron microscopy validated the localization of ectopic Nxph1 at the synaptic cleft of excitatory synapses in transgenic mice and revealed an enrichment of GABA A R and GABA B R subunits compared with wild-type animals. Thus, our data propose that Nxph1 plays an instructive role in synaptic short-term plasticity and the configuration with GABA receptors. synaptic transmission | thalamus | autism | neuroligin | ultrastructure

“…bind to both α-and β-neurexins because they interact with the sixth LNS domain; strikingly, these interactions are differentially regulated by alternative splicing of neurexins at SS#4 in the sixth LNS domain (24,(26)(27)(28)(29)(30)(31). In addition to these ligands, neurexins were shown to bind to neurexophilin, dystroglycan, and CIRL/ latrophilin (32)(33)(34)(35).…”

Section: Significancementioning

confidence: 99%

“…Neurexins are known to bind to a large number of ligands, of which neuroligins, leucine-rich repeat transmembrane proteins, and cerebellins are best-characterized (22)(23)(24)(25)(26)(27)(28). All of these ligands…”

mentioning

confidence: 99%

Cartography of neurexin alternative splicing mapped by single-molecule long-read mRNA sequencing

Treutlein¹,

Gökçe

Quake

et al. 2014

293

262

Neurexins are evolutionarily conserved presynaptic cell-adhesion molecules that are essential for normal synapse formation and synaptic transmission. Indirect evidence has indicated that extensive alternative splicing of neurexin mRNAs may produce hundreds if not thousands of neurexin isoforms, but no direct evidence for such diversity has been available. Here we use unbiased long-read sequencing of full-length neurexin (Nrxn)1α, Nrxn1β, Nrxn2β, Nrxn3α, and Nrxn3β mRNAs to systematically assess how many sites of alternative splicing are used in neurexins with a significant frequency, and whether alternative splicing events at these sites are independent of each other. In sequencing more than 25,000 full-length mRNAs, we identified a novel, abundantly used alternatively spliced exon of Nrxn1α and Nrxn3α (referred to as alternatively spliced sequence 6) that encodes a 9-residue insertion in the flexible hinge region between the fifth LNS (laminin-α, neurexin, sex hormone-binding globulin) domain and the third EGF-like sequence. In addition, we observed several larger-scale events of alternative splicing that deleted multiple domains and were much less frequent than the canonical six sites of alternative splicing in neurexins. All of the six canonical events of alternative splicing appear to be independent of each other, suggesting that neurexins may exhibit an even larger isoform diversity than previously envisioned and comprise thousands of variants. Our data are consistent with the notion that α-neurexins represent extracellular protein-interaction scaffolds in which different LNS and EGF domains mediate distinct interactions that affect diverse functions and are independently regulated by independent events of alternative splicing.N eurons form highly specific and complex patterns of synaptic connections that underlie all brain function (1-5). Such specific connections require trillions of chemically differentiated synapses whose identity may be shaped by interactions of specific pre-and postsynaptic signaling molecules, especially cell-adhesion molecules. The genome size is insufficient to encode such diversity, but a mixture of combinatorial expression patterns that pair different synaptic cell-adhesion molecules with each other and of distinct alternative splicing patterns that amplify the number of cell-adhesion molecules into a large number of isoforms may generate the number of transsynaptic interactions needed to account for the enormous diversity of synaptic connections. In Drosophila melanogaster, alternative splicing of the mRNAs encoding the Down syndrome cell-adhesion molecule can generate nearly 20,000 protein isoforms whose structures specify axon bundling but not synapse formation (6). In mammals, alternative splicing of neurexin and some protocadherin mRNAs can also produce thousands of isoforms, which may at least in the case of neurexins be involved in synapse formation (7-10).Neurexins are type I membrane proteins that were discovered as presynaptic receptors for α-latrotoxin (8, 9). Six prin...