Gene Expression Is Required for Correct Axon Guidance

As axons form connections in the developing brain, they often change course before they reach their final target. An outstanding question concerning changes in axon guidance is whether autonomous gene expression directs axons to their targets. If autonomous gene expression is required during axon guidance, then at least some aspects of guidance may be irreversible. Once commissural axons cross the midline in the central nervous system (CNS), they usually make guidance decisions that differ from those made on the ipsilateral side. Here I explore whether a change in gene expression is involved in postcommissural axon guidance. Grasshopper serotonergic neurons were microinjected with a fluorescent tracer dye and either a translation blocker or a transcription blocker. After 24 hr of development, the guidance decisions of these microinjected axons were assayed. If the growth cones had already made a postcommissural guidance choice after crossing the midline, they continued growth even when gene expression was inhibited. If growth cones had just crossed the midline and not yet reached the choice point, they made a distinct guidance error when gene expression was inhibited. These results show that there is a discrete step in which gene expression plays a critical role in postcommissural axon guidance.