Hunchback functions in the post-mitotic larval MDN to restrict axon outgrowth, synapse formation, and backward locomotion

During neurodevelopment, a single progenitor cell can generate many different neuron types. As these neurons mature, they form unique morphologies, integrate into neural circuits, and contribute to behavior. However, the integration of these developmental events is understudied. Here, we show that the same transcription factor is important for both the generation of neuronal diversity and maintaining mature neuronal identity, providing novel insights on how the generation of neuronal identity and morphology are coordinated. We utilized a previously characterized larval locomotor circuit in Drosophila, where activation of the Moonwalker Descending Neuron (MDN) triggers backward locomotion via its presynaptic connection with the premotor neuron A18b. MDN expresses the temporal transcription factor Hunchback (Hb), which has a well-characterized role in neural progenitors. Loss of Hb in the post-mitotic MDN increases axon/dendrite branching, leading to additional functional synapses on A18b and increasing backward locomotion. We conclude that the endogenous function of Hb is to restrain axon/dendrite outgrowth, including limiting MDN-A18b synapses, thereby dampening backward locomotion. Our work provides insights on how a transcription factor can have different functions throughout life - i.e. Hb generates neuronal diversity in the progenitor and regulates neuronal connectivity in the mature neuron to generate an appropriately tuned behavior.