Commissural axon pathfinding on the contralateral side of the vertebrate midline
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In bilaterally-symmetric organisms, the transfer of sensory information from one side of the body to the other occurs through commissures formed by interneurons that extend axons across the midline. Although recent studies have identified short- and long-range guidance mechanisms that direct the growth of vertebrate commissural axons toward and across the ventral midline, little is known about the pathfinding behavior of these axons on the contralateral side of the spinal cord. Here, we delineate the contralateral commissural pathway in higher vertebrates and show that the segregation of distinct cell-surface guidance receptors to crossed commissural axon segments influences the ability of growth cones to interpret navigational cues on the contralateral side of the floor plate.;In Chapter 1, we demonstrate that during the period of commissural axon pathfinding, transmembrane ephrins (B-class ephrins) are expressed in the floor plate and in a dorsal region of the mouse spinal cord. We further show that the corresponding receptors (B-class Eph receptors) are distributed in a non-overlapping ventral domain containing longitudinally-projecting axons, including decussated segments of commissural axons. Soluble forms of B-class ephrins bind to, and promote the collapse of, commissural axons in vitro. Collectively, these data suggest a role for repulsive B-class Eph receptor-ephrin interactions in constraining the orientation/position of longitudinal axons in the mouse spinal cord.;Chapter 2 provides additional support for this interpretation. In this study, we characterize, for the first time, commissural axon pathfinding on the contralateral side of the chick and mouse spinal cord. In both vertebrate species, most commissural axon populations follow a complex contralateral pathway that culminates in a rostral turn within a dorsal (chick) or intermediate (mouse) region of the spinal cord. Within these regions, EphB-expressing axons execute a final turn and assemble into a discrete longitudinal fiber tract at a conserved boundary of B-class ephrin expression. In vitro perturbation of endogenous EphB-ephrinB interactions results in the misprojection of commissural axons into dorsal regions of B-class ephrin expression. Taken together, these observations suggest that B-class ephrins act locally to specify the dorsoventral position of a longitudinal commissural axon tract.;In Chapter 3, we examine a role for L1, a neural IgCAM expressed exclusively on crossed commissural axon segments, in pathfinding decisions at the midline. A sharp upregulation of L1 on crossed commissural axon segments may enable these axons to respond to cues that influence pathfinding events on the contralateral side of the spinal cord. To explore this possibility in vivo, we used cis-acting MATH1 regulatory sequences to express L1 on commissural axons/growth cones prior to crossing the ventral midline. Although precocious L1 expression did not interfere with the ability of commissural axons to project ventrally toward the ventral midline, projection errors were observed in the vicinity of the floor plate. Axons often piled-up at, or misprojected for short-distances along the ipsilateral floor plate boundary. Furthermore, we observed a shift in the proportion of decussated axons that projected in a caudal direction at the contralateral margin of the floor plate. These observations support a function for L1 in mediating the response of crossed commissural axons to guidance cues that they encounter upon emerging from the floor plate.
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