The Drosophila Chromodomain Protein Kismet Activates Steroid Hormone Receptor Transcription to Govern Axon Pruning and Memory In Vivo.

Axon pruning is critical for sculpting precise neural circuits. Although axon pruning has been described in the literature for decades, relatively little is known about the molecular and cellular mechanisms that govern axon pruning in vivo. Here, we show that the epigenetic reader Kismet (Kis) is required for developmental axon pruning in Drosophila mushroom bodies. Kis binds to cis-regulatory elements of the steroid hormone receptor ecdysone receptor (ecr) gene and is necessary for activating expression of EcR-B1. Kis promotes the active H3K36 di- and tri-methylation and H4K16 acetylation histone marks at the ecr locus. We show that transgenic EcR-B1 can rescue axon pruning and memory defects associated with loss of Kis and that the histone deacetylase inhibitor SAHA also rescues these phenotypes. EcR protein abundance is the cell-autonomous, rate-limiting step required to initiate axon pruning in Drosophila, and our data suggest this step is under the epigenetic control of Kis.