Zfh1, a somatic motor neuron transcription factor, regulates axon exit from the CNS

Motor neurons are defined by their axon projections, which exit the CNS to innervate somatic or visceral musculature, yet remarkably little is known about how motor axons are programmed to exit the CNS. Here, we describe the role of the Drosophila Zfh1 transcription factor in promoting axon exit from the CNS. Zfh1 is detected in all embryonic somatic motor neurons, glia associated with the CNS surface and motor axons, and one identified interneuron. In zfh1 mutants, ventral projecting motor axons often stall at the edge of the CNS, failing to enter the muscle field, despite having normal motor neuron identity. Conversely, ectopic Zfh1 induces a subset of interneurons--all normally expressing two or more "ventral motor neuron transcription factors" (e.g. Islet, Hb9, Nkx6, Lim3)--to project laterally and exit the CNS. We conclude that Zfh1 is required for ventral motor axon exit from the CNS.