Cellular behaviors regulating tangential migration of facial branchiomotor neurons in the zebrafish embryo

Development of the nervous system is characterized by intricate processes like neuronal migration, axon outgrowth and guidance and synaptic connections to their targets. These processes involve navigation of the migrating neuronsoraxonal growth cones through a complex extracellular environment in the neuron tube. Signals generated through cell-cell interactions between the migrating neurons, surrounding cells and the extracellular matrix likely play a major role in neuronal migration. We show here that transient axonal glycoprotein1 (TAG1) is necessary for the migration of the facial branchiomotor neurons (FBMNs) in the zebrafish hindbrain. We also show that TAG1 exhibits genetic interactions with the extracellular matrix protein laminin1 (LAMA1) and the transmembrane protein strabismus (STBM) to regulate FBMN migration. To gain insight into the underlying cellular mechanisms, we examined the dynamic cellular behaviors of GFP-expressing FBMNs in wild type and morphant embryos using time lapse microscopy. FBMNs in control and morphant embryos move at comparable speeds, but differ in directionality. Knockdown of any of the three genes leads to similar deficits in caudally directed migratory behavior without affecting mediolateral migratory behavior. Together, the genetic and behavioral analysis suggests strongly that TAG1, LAMA1 and STBM regulate the same cellular processes necessary for motor neuron migration.