Axon guidance at the midline choice point

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Abstract
The central nervous system (CNS) of higher organisms is bilaterally‐symmetric. The transfer of information between the two sides of the nervous system occurs through commissures formed by neurons that project axons across the midline to the contralateral side of the CNS. Interestingly, these axons cross the midline only once. Other neurons extend axons that never cross the midline; they project exclusively on their own (ipsilateral) side of the CNS. Thus, the midline is an important choice point for several classes of pathfinding axons. Recent studies demonstrate that specialized midline cells play critical roles in regulating the guidance of both crossing and non‐crossing axons at the ventral midline of the developing vertebrate spinal cord and the Drosophila ventral nerve cord. For example, these cells secrete attractive cues that guide commissural axons over long distances to the midline of the CNS. Furthermore, short‐range interactions between guidance cues present on the surfaces of midline cells, and their receptors expressed on the surfaces of pathfinding axons, allow commissural axons to cross the midline only once and prevent ipsilaterally‐projecting axons from entering the midline. Remarkably, the molecular composition of commissural axon surfaces is dynamically‐altered as they cross the midline. Consequently, commissural axons become responsive to repulsive midline guidance cues that they had previously ignored on the ipsilateral side of the midline. Concomitantly, commissural axons lose responsiveness to attractive guidance cues that had initially attracted them to the midline. Thus, these exquisitely regulated guidance systems prevent commissural axons from lingering within the confines of the midline and allow them to pioneer an appropriate pathway on the contralateral side of the CNS. Many aspects of midline guidance are controlled by mechanistically and evolutionarily‐conserved ligand‐receptor systems. Strikingly, recent studies demonstrate that these receptors are modular; the ectodomains determine ligand recognition and the cytoplasmic domains specify the response of an axon to a given guidance cue. Despite rapid and dramatic progress in elucidating the molecular mechanisms that control midline guidance, many questions remain.