Spinal cord regeneration is very inefficient in humans, causing paraplegia and quadriplegia. Studying model organisms that can regenerate the spinal cord in response to injury could be useful for understanding the cellular and molecular mechanisms that explain why this process fails in humans. Here, we use Xenopus laevis as a model organism to study spinal cord repair. Histological and functional analyses showed that larvae at pre-metamorphic stages restore anatomical continuity of the spinal cord and recover swimming after complete spinal cord transection. These regenerative capabilities decrease with onset of metamorphosis. The ability to study regenerative and non-regenerative stages in Xenopus laevis makes it a unique model system to study regeneration. We studied the response of Sox2/3 expressing cells to spinal cord injury and their function in the regenerative process. We found that cells expressing Sox2 and/or Sox3 are present in the ventricular zone of regenerative animals and decrease in non-regenerative froglets. Bromodeoxyuridine (BrdU) experiments and in vivo time-lapse imaging studies using green fluorescent protein (GFP) expression driven by the Sox3 promoter showed a rapid, transient and massive proliferation of Sox2/3+ cells in response to injury in the regenerative stages. The in vivo imaging also demonstrated that Sox2/3+ neural progenitor cells generate neurons in response to injury. In contrast, these cells showed a delayed and very limited response in non-regenerative froglets. Sox2 knockdown and overexpression of a dominant negative form of Sox2 disrupts locomotor and anatomical-histological recovery. We also found that neurogenesis markers increase in response to injury in regenerative but not in non-regenerative animals. We conclude that Sox2 is necessary for spinal cord regeneration and suggest a model whereby spinal cord injury activates proliferation of Sox2/3 expressing cells and their differentiation into neurons, a mechanism that is lost in non-regenerative froglets.

Functional neural circuits depend on the establishment of specific connections between neurons and their target cells. To this end, many axons have to travel long distances to reach their target cells during development. Studies addressing the molecular mechanisms of axon guidance have to overcome the complexity of subpopulation-specific requirements with respect to pathways, guidance cues, and target recognition. Compared to the brain, the relatively simple structure of the spinal cord provides an advantage for experimental studies of axon guidance mechanisms. Therefore, the so far best understood model for axon guidance is the dI1 population of dorsal interneurons of the spinal cord. They extend their axons ventrally towards the floor plate. After midline crossing, they turn rostrally along the contralateral floor-plate border. Despite the fact that the trajectory of dI1 axons seems to be rather simple, the number of axon guidance molecules involved in the decisions taken by these axons is bewildering. Because guidance molecules and mechanisms are conserved throughout the developing nervous system, we can generalize what we have learned about the navigation of the floor plate as an intermediate target for commissural axons to the brain.

During neural circuit formation, axons navigate several choice points to reach their final target. At each one of these intermediate targets, growth cones need to switch responsiveness from attraction to repulsion in order to move on. Molecular mechanisms that allow for the precise timing of surface expression of a new set of receptors that support the switch in responsiveness are difficult to study in vivo. Mostly, mechanisms are inferred from the observation of snapshots of many different growth cones analyzed in different preparations of tissue harvested at distinct time points. However, to really understand the behavior of growth cones at choice points, a single growth cone should be followed arriving at and leaving the intermediate target. Existing ex vivo preparations, like cultures of an “open‐book” preparation of the spinal cord have been successfully used to study floor plate entry and exit, but artifacts prevent the analysis of growth cone behavior at the floor plate exit site. Here, we describe a novel spinal cord preparation that allows for live imaging of individual axons during navigation in their intact environment. When comparing growth cone behavior in our ex vivo system with snapshots from in vivo navigation, we do not see any differences. The possibility to observe the dynamics of single growth cones navigating their intermediate target allows for measuring growth speed, changes in morphology, or aberrant behavior, like stalling and wrong turning. Moreover, observation of the intermediate target—the floor plate—revealed its active participation and interaction with commissural axons during midline crossing.

As reflected by the term morphogen, molecules such as Shh and Wnts were identified based on their role in early development when they instruct precursor cells to adopt a specific cell fate. Only much later were they implicated in neural circuit formation. Both in vitro and in vivo studies indicated that morphogens direct axons during their navigation through the developing nervous system. Today, the best understood role of Shh and Wnt in axon guidance is their effect on commissural axons in the spinal cord. Shh was shown to affect commissural axons both directly and indirectly via its effect on Wnt signaling. In fact, throughout neural circuit formation there is cross-talk and collaboration of Shh and Wnt signaling. Thus, although the focus of this review is on the role of Shh in neural circuit formation, a separation from Wnt signaling is not possible.

Nogo-66 receptors (NgR1-3) are glycosylphosphatidyl inositol-linked proteins that belong to the leucine-rich repeat superfamily. Through binding to myelin-associated inhibitors, NgRs contribute to the inhibition of axonal regeneration after spinal cord injury. Their role in limiting synaptic plasticity and axonal outgrowth in the adult CNS has been described previously, but not much is known about their role during the development of the nervous system. Here, we show that NgR1 and NgR3 mRNAs are expressed during spinal cord development of the chicken embryo. In particular, they are expressed in the dI1 subpopulation of commissural neurons during the time when their axons navigate toward and across the floorplate, the ventral midline of the spinal cord. To assess a potential role of NgR1 and NgR3 in axon guidance, we downregulated them using in ovo RNAi and analyzed the trajectory of commissural axons by tracing them in open-book preparations of spinal cords. Our results show that loss of either NgR1 or NgR3 causes axons to stall in the midline area and to interfere with the rostral turn of postcrossing axons. In addition, we also show that NgR1, but not NgR3, requires neuronal PlexinA2 for the regulation of commissural axon guidance. SIGNIFICANCE STATEMENT Over the last decades, many studies have focused on the role of NgRs, particularly NgR1, in axonal regeneration in the injured adult CNS. Here, we show a physiological role of NgRs in guiding commissural axons during early development of the chicken spinal cord in vivo . Both NgR1 and NgR3 are required for midline crossing and subsequent turning of postcrossing axons into the longitudinal axis of the spinal cord. NgR1, but not NgR3, forms a receptor complex with PlexinA2 during axon guidance. Overall, these findings provide a link between neural regenerative mechanisms and developmental processes.