16-P014 Cordon-bleu is a developmentally regulated, novel actin nucleator that plays a role in epithelial ciliogenesis

genes that control motor neuron induction, subtype identity, and target specificity. Mice were mutagenized with ENU and outcrossed to an HB9-GFP transgenic reporter line that expresses GFP in spinal motor neurons and their axons. Litters were analyzed by fluorescence microscopy for recessive mutations that affect motor neuron induction and motor axon projection pattern. We have screened 137 F1 lines and identified 7 lines that have striking motor neuron defects. These mutations include novel regulators of the Shh pathway that affect the specification of motor neurons as well as mutations that affect later aspects of motor neuron development such as subtype identity, axon guidance and axonal fasciculation. One mutation that affects motor neuron specification is an allele of Wdr19, the homologue of Chlamydomonas IFT144, an IFT A complex protein. Like other IFT mutations that affect Shh signaling, Wdr19 mutants lack normal cilia, but the effect on Shh signaling is unique. A second motor neuron induction mutant disrupts Kif7, a homologue of the Drosophila kinesin-like protein Costal2. Kif7 acts as both a core component of the Shh pathway and a ciliary motor. A third mutant that affects motor axon pathfinding is an allele of HoxC9. This mutant has too many motor axons following the pathway of the intercostal nerves as well as abnormal thoracic motor axons projecting into the forelimb. Hoxc9 is required for the specification of thoracic-level motor neurons and restricts limb-specific motor neurons from the thorax.