Drosophila Neuroligin 1 Promotes Growth and Postsynaptic Differentiation at Glutamatergic Neuromuscular Junctions

Abstract

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.

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

“…Previously, we found that muscle (Mef2-Gal4-driven) expression of GFP-tagged Nlg1 lacking its C terminus (UAS-Nlg1∆Cyto GFP (ref. 12) , Fig. 1a) results in marked under-growth of NMJs (Fig.…”

“…Nonetheless, severe deficits in the in vivo assembly of Drosophila NMJ synapses [11][12][13] were reported for mutants of Nrx-1 and Nlg1 (for domain organization, see Fig. 1a).…”

“…We previously showed that Nrx-1 and Nlg1 behave nonadditively: the reduced NMJ bouton numbers in Nlg1 mutants are not further diminished in a Nlg1,Nrx-1 double mutant 12 . To test whether Syd-1 genetically interacts with Nrx-1 or Nlg1, we constructed the respective double mutants.…”

“…To test whether Syd-1 genetically interacts with Nrx-1 or Nlg1, we constructed the respective double mutants. NMJ size is reduced in Nlg1, Nrx-1 and Syd-1 single mutants, as well as in Liprin-α single mutants 5,11,12,18 . As expected, all these single mutants showed significantly reduced bouton numbers as compared to controls (Fig.…”

“…Apart from reduced NMJ size, all three mutants (Nlg1, Nrx-1 and Syd-1) show defects in the organization of the remaining active zones 5,11,12 . We compared their active zone organization by using stimulated emission depletion (STED 19 ) microscopy at ~80 nm resolution (instead of the ~250 nm of a standard confocal microscope) and electron microscopy.…”