Notum coordinates synapse development via extracellular regulation of Wnt Wingless trans-synaptic signaling

Synaptogenesis requires orchestrated communication between pre- and postsynaptic cells via coordinated trans-synaptic signaling across the extracellular synaptomatrix. The first discovered Wnt signaling ligand Drosophila Wingless (Wg; Wnt-1 in mammals) plays critical roles in synaptic development, regulating synapse architecture as well as functional differentiation. Here, we investigate synaptogenic functions of the secreted extracellular deacylase Notum, which restricts Wg signaling by cleaving an essential palmitoleate moiety. At the glutamatergic neuromuscular junction (NMJ) synapse, we find Notum secreted from the postsynaptic muscle acts to strongly modulate synapse growth, structural architecture, ultrastructural development and functional differentiation. In notum nulls, we find upregulated extracellular Wg ligand and nuclear trans-synaptic signal transduction, as well as downstream misregulation of both pre- and postsynaptic molecular assembly. Structural, functional and molecular synaptogenic defects are all phenocopied by Wg over-expression, suggesting Notum acts solely through inhibiting Wg trans-synaptic signaling. Moreover, these synaptic development phenotypes are suppressed by genetically correcting Wg levels in notum null mutants, indicating that Notum normally functions to coordinate synaptic structural and functional differentiation via negative regulation of Wg trans-synaptic signaling in the extracellular synaptomatrix.