PROJECT SUMMARY A gap in knowledge remains regarding how structural organization of synapses, robustly established during development, is also flexibly modified during learning to sustain behaviors. Knowledge on how the cell biology of synapses is established and altered in the actuation of memories is of critical importance in our aspiration to understand how the building blocks of the nervous system come together to produce its functional output, behaviors. The key scientific premise of my research program is that our understanding of how complex behaviors emanate from the molecular building blocks of synapses could be meaningfully expanded by simultaneously interrogating structure-function relationships across scales, bridging knowledge on the molecular composition of individual synapses within single cells, the organization of single cells within circuits, and the coordinated activity of circuits within the brain of behaving animals. Achieving this integrated view is a challenge of critical importance best summarized in the BRAIN Working Group Report to the NIH Director, and requires new approaches that allow the integration of knowledge across 1) spatial scales, from the subcellular architecture of the synapse to the cellular architecture of the circuits governing behaviors, and 2) temporal scales, from the events leading to synaptic assembly during development, to the events leading to synaptic plasticity during learned behaviors. To address this challenge, my lab established strategic collaborations with network scientists, microscopists and computational biologists and pioneered integrated approaches that enable unprecedented access to the cell biology of the synapse in the thermotaxis circuit of C. elegans. Using these approaches we discovered concepts that reframed how we understand the cell biology of the synapse in vivo, and now position us to address three fundamental questions in neuroscience: 1) How is synaptic specificity achieved during the development of the nerve ring neuropil? 2) How are changing metabolic needs met at synapses to sustain function? 3) How are synapses modified to form memories and underpin behaviors? We propose to answer these questions for the thermotaxis circuit to define fundamental mechanisms that govern how synapses are precisely assembled, maintained, and modified to sustain behavior. Importantly, the resulting integrated understanding across scales will yield new concepts regarding the interplay between the programs that robustly establish synaptic architecture during development, and the plasticity programs that govern synaptic change to facilitate behavior. We anticipate, because of the molecular conservation of the examined pathways, that advancements in our understanding based on these innovations will result in transposable lessons of broad biological significance.