Secreted tissue inhibitor of matrix metalloproteinase restrictstrans-synaptic signaling to coordinate synaptogenesis

Synaptogenesis is coordinated by trans-synaptic signals that traverse the specialized synaptomatrix between pre- and postsynaptic cells. Matrix metalloproteinase (Mmp) activity sculpts this environment, balanced by secreted tissue inhibitors of Mmp (Timp). We use the reductionist Drosophila matrix metalloproteome to test consequences of eliminating all Timp regulatory control of Mmp activity at the neuromuscular junction (NMJ). Using in situ zymography, we find Timp limits Mmp activity at the NMJ terminal and shapes extracellular proteolytic dynamics surrounding individual synaptic boutons. In newly-generated timp null mutants, NMJs exhibit architectural overelaboration with supernumerary synaptic boutons. With cell-targeted RNAi and rescue studies, we find that postsynaptic Timp limits presynaptic architecture. Functionally, timp nulls exhibit compromised synaptic vesicle cycling, with reduced, lower fidelity activity. NMJ defects manifest in impaired locomotor function. Mechanistically, we find that Timp limits BMP trans-synaptic signaling and the downstream synapse-to-nucleus signal transduction. Pharmacologically restoring Mmp inhibition in timp nulls corrects BMP signaling and synaptic properties. Genetically restoring BMP signaling in timp nulls corrects NMJ structure and motor function. Thus, Timp inhibition of Mmp proteolytic activity restricts BMP trans-synaptic signaling to coordinate synaptogenesis.