Mechanism Of Neuroligin 4 Regulating Synaptic Development At The Drosophila Neuromuscular Junction

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 Drosophila melanogaster,mutations in the neuroligin 1-3 genes have been reported to induce synapse developmental defects at neuromuscular junctions(NMJs),but the role of neuroligin 4(dnlg4)in synapse development has not been determined.We here reported the generation of an independent null allele of dnlg4,characterized the role of Drosophila neuroligin 4(DNlg4)in synaptic development and functions at NMJs and dissected the underlying mechanism.We found that the DNlg4 is different from DNlgl-3 in that it presynaptically regulates NMJ synapse development.Loss of dnlg4 resulted in reduced growth of NMJs with fewer synaptic boutons,and some structral defects of synapse at NMJs.The morphological defects caused by dnlg4 mutant were 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 dnlg4 mutants,and immunoprecipitation and pull down assay showed that DNlg4 physically interacted with Tkv in vivo through its acetylcholinesterase-like domain.Our study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv.