Steroid hormone-dependent glial-neuronal interaction promotes brain development during <i>Drosophila</i> metamorphosis

Steroid hormones regulate various aspects of brain development in metazoans. In the fruit fly <i>Drosophila melanogaster</i>, the primary steroid hormone ecdysone enters the central nervous system (CNS) via Ecdysone Importer (EcI) in the blood-brain barrier (BBB) and induces brain development during metamorphosis. However, our understanding of the exact cell types that require ecdysone during CNS transformation is still limited. Here, we report that ecdysone-dependent glial-neuronal interaction promotes brain development during <i>Drosophila</i> metamorphosis. Unexpectedly, disrupting ecdysone signaling in glial cells caused more severe defects in CNS transformation than in neurons, suggesting an essential role of glia in ecdysone-dependent brain development. When <i>ecdysone receptor</i> was knocked down in mushroom body (MB) neurons, pruning of their larval-specific axonal lobes was disrupted as reported previously. In contrast, knockdown of <i>EcI</i> in the MB did not induce any discernible deficiency in neuronal remodeling, suggesting dispensability of EcI-mediated ecdysone entry into the MB. Consistent with this, the neuronal remodeling defects induced by <i>EcI</i> knockdown in the BBB were rescued by glial cell-specific overexpression of a transforming growth factor-β ligand <i>myoglianin</i> and an engulfment receptor <i>draper</i>, both of which are upregulated in glial cells in an ecdysone-dependent manner. Collectively, our findings suggest that ecdysone is primarily required in glial cells for CNS transformation during metamorphosis, elucidating a hormone-dependent glial-neuronal interaction that drives brain development.