Figure 1. Axons within the nerve cord of vertebrates and invertebrates choose whether to cross or not cross the midline. (a) In the vertebrate neural tube, commissural axons (C) cross the floor plate (FP) cells that lie at the ventral midline (yellow shading). Once across the midline these axons turn to join a longitudinal tract (L) on the opposite side and do not recross the midline. The floor plate cells produce a variety of signals to regulate the outgrowth of these axons. (b) In the segmental Drosophila nerve cord, axons make the same decision. The majority of axons are commissural axons (C) and cross the midline in either an anterior (AC) or posterior (PC) commissural axon tract. After crossing the midline these axons also join a longitudinal pathway (L). A smaller population of axons never cross the midline and extend within the longitudinal tracts on one side of the nerve cord. At the midline of the fly nerve cord are a specialized group of cells (yellow shading) that provide many of the same signals as does the floor plate in vertebrates.

Figure 2. Structures of selected molecules that mediate axon guidance decisions at the midline. A variety of ligands and receptors that are necessary to guide axons at the midline have been identified (as described in the text). These schematics illustrate the types of domains contained within these molecules. (The structures are not drawn to scale.)

Figure 3. Commissural axons switch their behaviour at the midline. Once across the midline they are no longer attracted to it. This change in axon sensitivity is in part due to changes in the molecular composition of the axon surface. (a) Drosophila axons that do not cross the midline continually express Robo and are sensitive to the midline repellent Slit. In commissural axons Comm prevents Robo from reaching the cell surface prior to crossing. Comm activity is reduced in these axons after crossing and Robo surface levels increase. (b) Vertebrate commissural axons switch cell surface antigens after midline crossing. Rodent commissural axons initially express TAG-1 but then switch to express L1 once across the midline. (c) Vertebrate commissural axons also become sensitive to the repellent midline signals Slit, Sema3 and ephrin-B after midline crossing. The increase in ephrin sensitivity is due to a local activation of Eph receptor translation in the crossed axons. Eph receptor activity not only drives these axons away from the midline but also dictates where they turn into a longitudinal tract. A lateral patch of ephrin expression prevents axons continuing too far dorsally and directs them to turn longitudinally.

Figure 4. The Rac/Rho/Cdc42 GTPases have a central role in coupling axon guidance signals to changes in the actin cytoskeleton. In their active GTP-bound state the GTPases promote actin reorganization. Activation of Rac and Cdc42 promotes axon extension, while activation of Rho stimulates stress fibre formation and prevents outgrowth. Thus signals promoting outgrowth increase the activity of Rac and Cdc42, while repellent signals increase Rho activity. The guidance receptors active at the central nervous system midline regulate the GTPases either through direct interactions, as proposed for the plexins, or by regulating the activity of guanine nucleotide exchange factors (GEFs) or GTPase-activating proteins (GAPs) that promote or inhibit, respectively, formation of the active GTP-bound state. Ephexin and RhoGEF stimulate Rho activity while the srGAPs reduce Cdc42 activity. The GTPases control the state of the actin cytoskeleton through a number of effector molecules, including PAK, MLCK and ROCK. The Robo receptor also acts on the actin cytoskeleton via its ability to regulate Enabled (Ena). This activity is modulated by the Abelson tyrosine kinase (Abl).