Genetic analysis of axonal guidance in the zebrafish embryo

In the vertebrate embryo, different populations of spinal motor neurons initially share common paths, but at choice points diverge onto a cell-type-specific path. Elegant embryonic manipulations have provided evidence that motor growth cones respond to cues along their trajectory to select their cell-type-specific path. However, little is known about guidance cues that direct pathway selection of motor growth cones locally at choice points. The zebrafish embryo is an excellent model system in which to study pathway selection of spinal motor neurons. As in other vertebrates, zebrafish spinal motor neurons have distinct identities and targets. Each somitic hemisegment is typically innervated by three pioneering motor neurons. On their way toward their synaptic targets, their growth cones encounter choice points, where they are confronted with trajectory choices. In unplugged mutant embryos, growth cones reach the somite choice point, but make inappropriate pathway decisions. We have previously shown that the unplugged gene acts as an adaxial cell-derived cue controlling pathway choice of motor axons. As the first motor growth cones approach the choice point, adaxial cells migrate away, suggesting that unplugged activity influences growth cones by a contactindependent mechanism. Our results suggested a model by which the unplugged gene encodes or is essential to produce a somite derived signal that elicits differential guidance decisions in motor growth cones. To test this model and to identify the molecular nature of the unplugged gene, we positionally cloned the unplugged gene. We will present ongoing molecular analysis of the unplugged gene to provide insights how unplugged activity controls motor axon guidance.