Targeted domain-specific mutations and cell type–specific rescue experiments show that L1CAM/Neuroglian is required for axon–axon interactions and yield insights into the cellular mechanisms controlling establishment of the complex mushroom body architecture.

Highlights d Epidermal Nrg167 and the neuronal Nrg180 isoform interact with and stabilize each other d Epidermal Nrg167 supports dendrite growth and promotes dendrite enclosure d Epidermal Nrg167 prevents dendrite bundling by blocking Nrg180-Nrg180 interactions d Internalization of surface Nrg180 dissociates dendritedendrite binding

Adaptive decision-making depends on the formation of novel memories. In Drosophila , the mushroom body (MB) is the site of associative olfactory long-term memory (LTM) storage. However, due to the sparse and stochastic representation of olfactory information in Kenyon cells (KCs), genetic access to individual LTMs remains elusive. Here, we develop a cAMP response element (CRE)-activity–dependent memory engram label (CAMEL) tool that genetically tags KCs responding to the conditioned stimulus (CS). CAMEL activity depends on protein-synthesis–dependent aversive LTM conditioning and reflects the time course of CRE binding protein 2 (CREB2) activity during natural memory formation. We demonstrate that inhibition of LTM-induced CAMEL neurons reduces memory expression and that artificial optogenetic reactivation is sufficient to evoke aversive behavior phenocopying memory recall. Together, our data are consistent with CAMEL neurons marking a subset of engram KCs encoding individual memories. This study provides new insights into memory circuitry organization and an entry point towards cellular and molecular understanding of LTM storage.