The Nogo-66 Receptors NgR1 and NgR3 Are Required for Commissural Axon Pathfinding

Abstract

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.

Semaphorin7A patterns neural circuitry in the lateral line of the zebrafish

“…In summary, we propose that Sema7A mediates four key aspects of the topology of the arborization network: (1) the node degree complexity along the organ’s axis; (2) the transition of arbors from bare terminals to loops; (3) the preference for forming loops containing four nodes with 30-40 µm of arbors; and (4) the preference of bare terminals to adopt higher curvature.…”

“…To measure the accuracy of the extended axonal projections in finding the ectopic Sema7A sec source, we defined three parameters: (1) the source path, denoted as the linear distance from the point of arborization to the boundary of an ectopically expressing Sema7A sec cell; (2) the projection path, representing the linear distance from the point of arborization to the terminus of the extended projection; and (3) the projection proximity, calculated as the linear distance from the terminus of the extended axonal projection to the nearest boundary of the ectopically expressing Sema7A sec cell (Fig. 3B).…”

“…The three-dimensional skeletonized network traces allowed us to accurately delineate the microcircuit topology across individuals, developmental stages, and genotypes from living animals (Figure 2D, E). To quantitatively analyze the arbor network, we defined five topological attributes: (1) nodes, denoted as junctions where arbors branch, converge, or cross; (2) loops, comprising minimal arbor cycles forming topological holes; (3) bare terminals, defined as arbors with free ends; (4) the node degree, calculated as the number of first-neighbor nodes incident to a node; and (5) the arbor curvature, measured as the total curvature of a bare terminal (Figure 3A). We quantified these attributes to accurately determine how the circuit topology is established during maturation of the sensory organ in control larvae and how it is impacted by sema7a deficit.…”

“…Pathfinding axons are directed to their appropriate synaptic targets by a variety of guidance cues. While approaching or traversing the target field, the growth cones of migrating axons encounter secreted or cell surface-attached ligands and respond by steering toward or away from their sources (1)(2)(3)(4). Fine-grained control of these factors is critical for the establishment of proper neural circuitry.…”

Looking for Guidance – Models and Methods to Study Axonal Navigation

“…This method not only permitted to visualize the crossing of commissural axons to the contralateral side of the CNS in real time, but also to follow the subsequent rostral turning (Movie 1). The comparison of these events in control and experimentally manipulated tissue allows even for the distinction between manipulations of neurons versus environment (Baeriswyl et al, 2021;Dumoulin et al, 2021;Vaccaro et al, 2022). This ex vivo culture system also shed light on the dynamics of dI1 growth cones at an intermediate target as well as the interaction between the guidepost cells and commissural growth cones by filopodial contacts in the chicken embryo (Dumoulin et al, 2021).…”