Precise apposition of presynaptic and postsynaptic domains is a fundamental property of all neuronal circuits. Experiments in vitro suggest that Neuroligins and Neurexins function as key regulatory proteins in this process. In a genetic screen, we recovered several mutant alleles of Drosophila neuroligin 1 (dnlg1) that cause a severe reduction in bouton numbers at neuromuscular junctions (NMJs). In accord with reduced synapse numbers, these NMJs show reduced synaptic transmission. Moreover, lack of postsynaptic DNlg1 leads to deficits in the accumulation of postsynaptic glutamate receptors, scaffold proteins, and subsynaptic membranes, while increased DNlg1 triggers ectopic postsynaptic differentiation via its cytoplasmic domain. DNlg1 forms discrete clusters adjacent to postsynaptic densities. Formation of these clusters depends on presynaptic Drosophila Neurexin (DNrx). However, DNrx binding is not an absolute requirement for DNlg1 function. Instead, other signaling components are likely involved in DNlg1 transsynaptic functions, with essential interactions organized by the DNlg1 extracellular domain but also by the cytoplasmic domain.

The subunit composition of postsynaptic non-NMDA-type glutamate receptors (GluRs) determines the function and trafficking of the receptor. Changes in GluR composition have been implicated in the homeostasis of neuronal excitability and synaptic plasticity underlying learning. Here, we imaged GluRs in vivo during the formation of new postsynaptic densities (PSDs) at Drosophila neuromuscular junctions coexpressing GluRIIA and GluRIIB subunits. GluR composition was independently regulated at directly neighboring PSDs on a submicron scale. Immature PSDs typically had large amounts of GluRIIA and small amounts of GluRIIB. During subsequent PSD maturation, however, the GluRIIA/GluRIIB composition changed and became more balanced. Reducing presynaptic glutamate release increased GluRIIA, but decreased GluRIIB incorporation. Moreover, the maturation of GluR composition correlated in a site-specific manner with the level of Bruchpilot, an active zone protein that is essential for mature glutamate release. Thus, we show that an activity-dependent, site-specific control of GluR composition can contribute to match pre- and postsynaptic assembly.

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...