Neuroligin 4 regulates synaptic growth via the bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

Abstract

The neuroligin (Nlg) family of neural cell adhesion molecules is thought to be required for synapse formation and development and has been linked to the development of autism spectrum disorders in humans. In <i>Drosophila melanogaster</i>, mutations in the <i>neuroligin 1-3</i> genes have been reported to induce synapse developmental defects at neuromuscular junctions (NMJs), but the role of <i>neuroligin 4</i> (<i>dnlg4</i>) in synapse development has not been determined. Here, we report that the <i>Drosophila</i> neuroligin 4 (DNlg4) is different from DNlg1-3 in that it presynaptically regulates NMJ synapse development. Loss of <i>dnlg4</i> results in reduced growth of NMJs with fewer synaptic boutons. The morphological defects caused by <i>dnlg4</i> mutant are associated with a corresponding decrease in synaptic transmission efficacy. All of these defects could only be rescued when DNlg4 was expressed in the presynapse of NMJs. To understand the basis of DNlg4 function, we looked for genetic interactions and found connections with the components of the bone morphogenetic protein (BMP) signaling pathway. Immunostaining and Western blot analyses demonstrated that the regulation of NMJ growth by DNlg4 was due to the positive modulation of BMP signaling by DNlg4. Specifically, BMP type I receptor thickvein (Tkv) abundance was reduced in <i>dnlg4</i> mutants, and immunoprecipitation assays showed that DNlg4 and Tkv physically interacted <i>in vivo</i> Our study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv.