Cooperation of Syd-1 with Neurexin synchronizes pre- with postsynaptic assembly

Abstract

Understanding the process of synapse assembly in molecular and cell-biological detail is a prerequisite for understanding neural circuit development and activity-mediated remodeling, and thus is important for unraveling learning and memory processes (structural plasticity) [1][2][3] . Functional chemical synapses are characterized by two apposed compartments that must be coestablished with high spatiotemporal precision: the presynaptic active zone, where regulated and rapid fusion of neurotransmitter-filled synaptic vesicles takes place, and the postsynaptic density (PSD), which embeds neurotransmitter receptors.Glutamatergic neuromuscular junction (NMJ) terminals of Drosophila larvae grow to meet the requirements of the growing muscle fibers, a process in which new synapses are continuously added 4 (where a synapse is defined as a single active zone opposed by a single PSD 1 ). In vivo imaging has shown that presynaptic Syd-1 and Liprin-α clusters initiate active zone formation 5 . On the postsynaptic side, initial PSD growth depends on incorporation of a glutamate receptor (GluR) containing the GluRIIA subunit. Later PSD maturation is marked by the incorporation of GluRIIB-containing receptor complexes 6 . Synapse assembly is concluded at presynaptic active zones by the incorporation of the active zone scaffold component Bruchpilot (BRP) 5 .Coordinating synapse assembly requires signaling across the synaptic cleft, which separates pre-from postsynaptic membranes. Transsynaptic cell adhesion molecules are obvious candidates for coupling active zone and PSD assembly. In vitro, the Neurexin-Neuroligin (Nrx-Nlg) complex can mediate the trans-synaptic signaling required for synapse assembly 7,8 . How this signaling axis integrates with the additional assembly machinery, however, has remained largely unclear.Here, we provide evidence of a dual role for Syd-1 in early assembly of NMJ synapses. It retains Liprin-α clusters at active zones and is important for clustering of presynaptic Nrx-1, likely through a direct and PDZ-domain-dependent interaction. Consequently, Syd-1 is also needed for clustering of postsynaptic Nlg1, which organizes postsynaptic assembly. Coincident action of Syd-1 with Nrx-1-Nlg1 appears to allow active zone scaffolds to pass through an initial, still fragile assembly phase. We suggest that binding between Syd-1 and Nrx-1-Nlg1 is a means to coordinate pre-with postsynaptic assembly. Our study shows an example of how coincident action of a presynaptic active zone scaffold protein and a trans-synaptic cell adhesion protein module can spatiotemporally orchestrate synapse assembly. RESULTSInitially described in cell culture systems (for a review, see ref. 9), interaction between mammalian presynaptic Nrx proteins and postsynaptic Nlg molecules was proposed to be important for proper synapse assembly. However, genetic ablation of three Nlg (Nlgn) genes in mice 10 does not result in a substantial structural phenotype, potentially reflecting a strong capacity for compensatory processes in vivo.Nonethe...

Transmission, Development, and Plasticity of Synapses

“…Mutants of syd-1 and liprin-⍺ have irregular T-bars, which may be small, misshapen, ectopic, or enlarged multi-T-bars, consistent with a role in regulating CAZ assembly (Kaufmann et al 2002;Owald et al 2010). Syd-1 clusters Liprin-⍺ at the CAZ along with the transsynaptic adhesion protein Neurexin (Nrx), which is important for transsynaptic signaling for PSD organization and apposition (Owald et al , 2012. A Drosophila ortholog of Piccolo, a core CAZ protein previously thought to exist only in vertebrates, was recently identified and named fife.…”

“…Two groups have reported that loss of nrx-1 results in an increase in A-type receptors, suggesting that Nrx-1 normally inhibits A-type receptor clustering (Li et al 2007;Chen et al 2010). In contrast, live imaging of GluR incorporation revealed that mutants for nrx-1, as well as interacting partners nlg-1 and syd-1, are deficient in early GluRIIA incorporation (Owald et al 2012). While newly formed GluR clusters are typically rich in GluRIIA, these mutants have new clusters that are rich in GluRIIB.…”

“…These irregularities of synaptic ultrastructure are typically accompanied by defects in AZ spacing, where multiple Brp puncta cluster opposite single enlarged or fused GluRs. Mutations in several synaptic genes exhibit variations of this phenotype, including genes encoding the CAZ components syd-1 and Liprin-⍺ (Kaufmann et al 2002;Fouquet et al 2009;Owald et al 2010), transsynaptic adhesion genes nrx-1, nlg-1, and the teneurins ten-m and ten-a (Li et al 2007;Banovic et al 2010;Owald et al 2012), and components of the postsynaptic actin cytoskeleton (Pielage et al 2006;Blunk et al 2014). There is evidence of cooperation between many of these genes, which begins to provide a picture of how synaptic organization is coordinated across the synaptic cleft.…”

“…Syd-1 forms a complex with Nrx-1 and is important for the clustering of Nrx-1 and postsynaptic Nlg-1. Nlg-1 is in turn involved in stabilizing Syd-1-Liprin-⍺ clusters in the initial stages of AZ formation (Owald et al , 2012.…”

Drosophila Syncrip modulates the expression of mRNAs encoding key synaptic proteins required for morphology at the neuromuscular junction

“…In this analysis, the top GO annotation cluster corresponds to neuron/cell projection morphogenesis or development ( Table 2). msp-300 (Elhanany- Tamir et al 2012;Volk 2013;Morel et al 2014), syd-1 (Owald et al 2010(Owald et al , 2012, nrx-1 (Li et al 2007;Zeng et al 2007;Owald et al 2012), and futsch (Roos et al 2000;Zhang et al 2001) were among the top hits, while hiw (Wan et al 2000;McCabe et al 2004;Wu et al 2005;Collins et al 2006), dlg1 (Lahey et al 1994;Budnik et al 1996;Guan et al 1996;Tejedor et al 1997;Thomas et al 1997), and α-spec (Featherstone et al 2001;Pielage et al 2005Pielage et al , 2006 were also found to associate with Syp ( Fig. 4A; Table 1; Supplemental Table 1; Supplemental Fig.…”