The activity of extracellular matrix metalloproteinases (MMPs) can shape synaptic structure and functional neurotransmission during neural development. Dysregulation of MMPs has been causally implicated in neurodevelopmental disorders such as Fragile X syndrome (FXS). In vivo, MMP regulation includes the secretion of endogenous tissue inhibitors of MMPs (TIMPs). However, the general roles of MMP regulation by TIMPs at the synapse remain elusive, due both to the redundancy of the mammalian MMP/TIMP system and the nesting of TIMP genes within synapsin genes. Drosophila has only two MMPs regulated by a single TIMP, making it possible to dissect the entire matrix metalloproteome in a tractable genetic model. Using CRISPR/Cas9 genome editing, we have created the first precise disruption of TIMP in order to study its neurological phenotypes. We discovered that loss of TIMP results in significant overelaboration of neuromuscular synapses, resembling the synaptic abnormalities seen in FXS disease models. Using cell-targeted RNAi, we have found bidirectional contributions of TIMP for maintaining synaptic morphology. Analysis of multiple trans-synaptic signals has revealed a mechanism where TIMP selectively tunes BMP signaling to control synaptic expansion. We are now extending these results using engineered TIMP alleles that will allow us to visualize its expression and differentiate its functional domains. To link our genetic results with biochemical data, we have optimized and begun testing a protocol for measuring MMP activity at individual synapses. Together, these genetic, biochemical, and neurobiological approaches demonstrate an important role for TIMP in regulating synaptic development, with potential applications to neurodevelopmental disease states.