Synaptic proteomics identifies cathepsin B as a regulator of synapse remodelling during learning.

Learning requires dynamic changes in synaptic protein composition. Most synaptic proteomic studies capture endpoint snapshots following training, overlooking molecular changes occurring during learning itself. We previously established TurboID proximity proteomics as an approach to capture protein-level changes during learning in the nervous system of Caenorhabditis elegans . Here, we apply this strategy to synapses. Trained synapses exhibited a distinct proteome compared with mock-trained controls, with pathways enriched for neurotransmission, synaptic reorganisation, protein trafficking, and autophagy. Enrichment of autophagy proteins led us to investigate the cathepsin B orthologue CPR-4, a previously uncharacterised candidate for learning identified exclusively in trained synapses. Functional validation identified CPR-4 as a novel regulator of memory: CPR-4 is required for learning, and the absence of CPR-4 results in trained synaptic proteomes that are more like those of mock-trained synapses. Our findings support a model through which CPR-4 promotes synaptic remodelling required for learning through targeted protein clearance.