Silvia Arber, Herwig Baier, Greg Bashaw, Heike Blockus, Frank Bradke, Katja Brose (editor), Pico Caroni, Valérie Castellani, Frédéric Charron, Alain Chédotal (organizer), Sonia Garel, Martyn Goulding, Randal Hand, Artur Kania, Ruediger Klein, Alex L. Kolodkin (organizer), Martin Meyer, Alain Prochiantz, Alvaro Sagasti, Dietmar Schmucker
The recent meeting entitled “The Future of Axon Guidance,” May 27–May 31, 2013, provided the neural development field with an opportunity to take stock of progress in an area of active neuroscience research critical for understanding basic principles that underlie the elaboration of complex neural circuits. Building on past advances, neuroscientists presented work that greatly improves our understanding of novel guidance cue ligand-receptor relationships, complex connectivity events mediated by these cues, generation of neural circuits that result from these events, and, finally, certain behaviors that depend absolutely upon the fidelity with which these connections are established. Given the high significance of the experiments described at this meeting, we are fortunate to be able to conclude that the future of research in the area of axon guidance is assured.
Keywords: Axon, development, connection, circuit, guidance, nervous system, plasticity, regeneration
In 2005, at a previous meeting in les Treilles entitled “Axon Guidance and Regeneration,” the organizers of this present meeting, Drs. Alain Chédotal and Alex Kolodkin, presented their findings. They also learned of advances in the identification of guidance cues and their receptors, how these receptors signal to establish neural circuits, and molecular mechanisms essential for successful neuronal regeneration. This present meeting allowed Principle Investigators, many who are now leaders in the field, to present work with the potential to fulfill the promise of the advances presented at Les Treilles in 2005. Building on past advances, neuroscientists presented work that greatly improves our understanding of novel guidance cue ligand-receptor relationships, complex connectivity events mediated by these cues, generation of neural circuits that result from these events, and, finally, certain behaviors that depend absolutely upon the fidelity with which these connections are established.
Prior to the elaboration of complex neural trajectories, neuronal migrations of many types must successfully occur, positioning neurons so that their axons and dendrites extend in correct directions. Dr. Sonia Garel showed how the formation of thalamocortical projections depends upon interactions with a subpopulation of migrating “corridor” neurons within the developing basal ganglia. Further, she showed that resident microglia in the brain mediate postnatal circuit remodeling by regulating neuronal migration and axon positioning in the developing brain, thereby demonstrating an interplay between the development of neural and immune systems during forebrain morphogenesis. As development of cortical neurons proceeds to the stage where axons and dendritic projections are established, a range of cues influences both process guidance and neuronal plasticity. Dr. Alain Prochiantz presented work exploring key roles in neuronal guidance and plasticity played neuronal uptake of homeodomain transcription factors, and he also demonstrated that the timing of neuronal interactions with these transcription factors is critical for their effects on neural development.
Guidance at the central nervous system midline, from flies to humans, requires that axons extending toward the central nervous system (CNS) midline change their trajectories upon crossing the midline so as not to get stuck. Crossing the embryonic fly and mouse CNS midlines utilizes a phylogenetically conserved suite of guidance cues, receptors, and intracellular signaling molecules, and at this meeting several presentations expanded upon our understanding of this fundamental guidance event. Dr. Greg Bashaw showed that receptors for the canonical CNS midline axonal repellent Slit, called Robo receptors, undergo novel heteromultimeric associations to allow crossing of the midline by commissural axons, and that these may involve non-autonomous interactions among extending axons. This theme of novel heteromultimeric guidance cue receptor interactions at the CNS midline was continued by Dr. Valérie Castellani, who in addition to discussing PlexinA1 receptor collaboration with the cell adhesion molecule NCAM, also showed that PlexinA1 can serve as a receptor for the Slit2 midline repellent, in addition to its classical role as a semaphorin receptor–defining a novel plexin receptor function critical for normal postcrossing behavior of commissural axons in the embryonic mouse spinal cord. Dr. Alain Chédotal addressed the evolutionary conservation of vertebrate Robo receptor function and called into question the notion that the Robo3 receptor in mouse responds to the Slit chemorepellents. Instead, he showed that Robo3 and the netrin receptor DCC interact, both in vitro and in vivo, suggesting intriguing roles for mammalian Robo3 in the regulation of commissural axon guidance at the CNS midline. Finally, Dr. Frédéric Charron discussed work on how commissural axons switch their response to the midline attractant Shh once they cross the CNS midline, defining intracellular signaling pathways necessary for Shh to be both an attractant and a repellent. Taken together, these studies unveil complexity and novel guidance cue functions that greatly enhance our understanding of axon guidance events at intermediate targets during neural development.
The signaling receptors that mediate axon and dendrite guidance signaling utilize a variety of poorly understood mechanisms. Dr. Rüdiger Klein presented evidence that clustering of Eph receptors is critical for mediating ephrin signaling, and moreover, that the leucine-rich-repeat receptor Flrt3 interacts with both Eph and Robo1 receptors to mediate Slit-induced attraction of axons during development: again, raising the theme of complex heteromultimeric receptors serving key roles in neuronal process guidance. In the context of helping neurons maintain separation of axonal processes following branching, and doing so without adverse affects on adjacent axons, Dr. Dietmar Schmucker presented his group’s latest observations on the function of the Dscam proteins—multiple protein isoforms generated from the single genetic locus that encodes a homophilic adhesion protein which, upon contact with the same isoform, leads to adhesive interactions that stimulate subsequent repulsion. They find that expression of multiple non-matching Dscam isoforms within a single neuron controls the extent of growth cone sprouting, an essential event for forming stereotypic axon collaterals, revealing a novel cell-intrinsic use of cell surface receptor diversity. An interesting and novel guidance event involves roles played by skin cells in the development and maintenance of sensory axon endings, and Dr. Alvaro Sagasti presented evidence showing that skin cells function in zebrafish much the same way as glial cells in other contexts. Skin cells produce heparan sulfate proteoglycans that can attract sensory axons to the periphery, leading to dramatic changes in skin cell morphology that include tight ensheathment of innervating axons. Since these skin cells phagocytose axon debris during developmental axon pruning and following injury, this defines an interesting model system for investigating these phenomena. Dr. Frank Bradke also presented work on axon guidance during development that has implications for neuronal regeneration. Increasing axon stability with the microtubule crosslinking reagent taxol leads to promotion of axon extension and, in the context of neuronal injury, reduction in retraction bulbs and also growth on myelin. A taxol-like compound that crosses the blood brain barrier, epothiloneB, also reduces glial scar formation following neuronal injury, providing a promising avenue for promoting neuronal regeneration. Dr. Pico Caroni presented data linking regulation of the actin cytoskeleton by the protein b-adducin to hippocampal dendritic spine turnover following exposure to enriched environments, and coupled with assessments of inhibitory neuronal circuits and memory consolidation, provided support for neuronal cytoskeletal component alteration influencing synaptic function and neuronal plasticity.
Analysis of circuit formation involving motor axon projections in the mammalian spinal cord was presented by three investigators. Dr. Artur Kania showed that synergistic responses by motor axons to netrin and ephrin guidance cues are critical for the selection of spinal motor axon trajectories into the developing limb. This work defines novel roles for the netrin receptor Unc5 in motor axon guidance, and it begins to address why functional redundancy amongst guidance cues is essential for refining axon guidance decisions. Dr. Silvia Arber presented novel anatomical and functional observations revealing organizational and functional principles underlying brainstem neural circuit formation critical for control of spinal motor neurons that innervate limb muscles. This work shows the existence of segregated activity maps that allow for motor program diversification at the brainstem level, pointing the way toward a new understanding of motor circuit control. Dr. Martyn Goulding discussed data relating to the evolution of flexor/extensor motor system, defining two distinct populations of interneurons that differentially regulate motor circuits to allow for normal alternating gate and appropriate activation of distinct motor neuron output. In addition to spinal motor circuitry, the elaboration of visual system circuits was addressed. Dr. Alex Kolodkin showed that in the mouse, a transmembrane guidance cue, semaphorin 6A, and its receptor, plexinA2, selectively regulate the morphology of inhibitory starburst amacrine cells in the mouse retina, regulating the establishment of motion-detection circuitry during visual system development. In the zebrafish, Dr. Herwig Baier utilized a range of in vivo neuronal tracing approaches to characterize tectal neuronal subtypes and determine that there is little pruning of initial retinal axon ganglion cell projections to the tectum. In combination with novel prey capture behavior assays, this work allows for a sophisticated assessment of brainstem-mediated modular control of visually evoked motion responses. Finally, Dr. Martin Meyer, also utilizing the zebrafish as a model for visual system development, addressed the selectivity of individual neuronal responses to directional motion using live calcium imaging to examine how tectal neurons process direction and orientation-selective inputs. They find that cells in the tectum do not exhibit a particular directional preference per se, and instead translate directional space into overlapping cardinal representations. Together, these studies on the organization and function of spinal motor and visual system circuitry demonstrate how axon guidance information ultimately encodes behavioral responses. Future work will certainly investigate how more complex circuits and behaviors are established, bringing to fruition the promise of earlier work on axon guidance and current efforts in the field.
The organizers wish to thank the Les Treilles Foundation for supporting this enlightening meeting. We imagine that the promising future of axon guidance research will suggest that another meeting of this sort is warranted to present progress achieved and to define future directions for work on neuronal process guidance.
Alain Chédotal, Alex Kolodkin