Serum Response Factor-Mediated Gene Regulation in a Drosophila Visual Working Memory

BACKGROUND Navigation through the environment requires a working memory for the chosen target and path integration facilitating an approach when the target becomes temporarily hidden. We have previously shown that this visual orientation memory resides in the ellipsoid body, which is part of the central complex in the Drosophila brain. Former analysis of foraging and ignorant mutants have revealed that a hierarchical PKG and RSKII kinase signaling cascade in a subset of the ellipsoid-body ring neurons is required for this type of working memory in flies. RESULTS Here we show that mutants in the ellipsoid body open (ebo) gene, which encodes the actin-binding protein Exportin 6, exhibit excessive nuclear accumulation of actin during development and in the adult brain. ebo mutants lack the orientation memory independent of the structural defect in the ellipsoid-body neuropil, and EBO activity in any type of adult ring neurons is sufficient for orientation-memory function. Moreover, genetic interaction studies revealed that nuclear actin accumulation in ebo mutants inhibits the Drosophila coactivator myocardin-related transcription factor A (dMRTF) and therefore the transcriptional activator serum response factor (dSRF). dSRF also functions in different ring neurons, suggesting that it regulates abundance of a diffusible factor that enables a working memory in ellipsoid-body ring neurons. CONCLUSIONS To date, SRF has only been implicated in longer forms of memory formation like synaptic long-term potentiation and depression. This study provides the first evidence that SRF-mediated gene regulation is also required for a working memory that lasts only for a few seconds.