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See online version for legend and references.494 Cell 153, April 11, 2013 ©2013 Elsevier Inc. DOI http://dx.doi.org/10.1016/j.cell.2013.03.031

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SnapShot: Axon GuidanceR. Jeroen Pasterkamp1 and Alex L. Kolodkin2

1Department of Neuroscience and Pharmacology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, 3584 CG Utrecht, The Netherlands2Department of Neuroscience, HHMI, The Johns Hopkins University School of Medicine, Baltimore, MD 21212, USA

This SnapShot surveys general axon guidance mechanisms, focusing on ligands and receptors that direct general cellular categories of neuronal process guidance. These modes of neuronal guidance function during embryogenesis and postnatal development, and many are phylogenetically conserved at both cellular and molecular levels (Kolodkin and Tessier-Lavigne, 2011). Axon guidance cues bind to their cognate receptors, triggering signaling events near the growth cone cell membrane and downstream signal transduction pathways that converge on the cytoskeleton. Protein kinases, small GTPases, cytoskeleton-associated proteins, and the proteins that regulate their activity all play central roles in these signaling cascades. Shown in the box are additional molecular mechanisms that regulate, diversify, and expand the effects of axon guidance proteins. These examples depicted here provide a basic foundation for understanding the full range of molecules and mechanisms that direct the establishment of neuronal connectivity.

Chemoattraction and RepulsionGuidance of axons across the central nervous system (CNS) midline involves long-range attractive and repulsive guidance cues (Dickson and Zou, 2010; Derijck et al., 2010). Commissural neurons in the dorsal vertebrate spinal cord, shown here in cross-section, extend commissural axons ventrally that are attracted at long range by the secreted protein netrin, which is synthesized in the floor plate. A second secreted protein also made by the floor plate, Slit, serves to repel commissural axons only after they have crossed the midline, preventing recrossing and allowing subsequent comississural axon anterior extension. The netrin receptor DCC and the Slit receptor Robo both initiate intracellular signaling events that can affect cytoskeletal reorganization (Bashaw and Klein, 2010), often through regulation of Rho GTPases through the action of GEFs and GAPs such as Trio, Dock180, srGAP, and Sos. Other cytosolic signaling components, including the tyrosine kinase Abl, participate in these signaling events, as do a range of signaling molecules and extracellular cues, including morphogens, not shown here (Bashaw and Klein, 2010).

Surround RepulsionAxons are often directed away from inappropriate intermediate targets by the action of repulsive cues that function in trans to provide permissive pathways through which axons can extend. The secreted repellent semaphorin 3A (Sema3A) serves such a function for peripheral axon projections (Tran et al., 2007). For example, mouse trigeminal sensory projections to regions adjacent to the developing eye are constrained by surrounding Sema3A; similar constraint of peripheral axon extension is directed by Sema3A at the level of the segmented somites of the developing mouse embryo. Transmembrane Semas can serve similar functions (not shown). Semas, with the exception of most subclass-3-secreted semaphorins, employ holoreceptor complexes containing Plexins as ligand-binding and -signaling receptors. Class 3 Semas include Sema3A, and generally require neuropilin (Nrp) as a ligand-binding coreceptor. Plexin activation promotes Ras/RapGAP activity intrinsic to the plexin cytoplasmic domain and regulates downstream molecules, including GEFs (LARG, RhoGEF, and FARP) (Pasterkamp, 2012). These signaling events alter the growth cone cytoskeleton, usually resulting in repulsive guidance.

Selective FasciculationIndividual axons encounter a myriad of intermediate axonal targets that complicates the selective bundling, or fasciculation, of any individual extending axon with its correct trajectory. Selective fasciculation with individual axon pathways allows axons to identify and extend along appropriate bundles, often leading to the establishment of axon scaf-folds that provide a template for subsequent CNS and PNS wiring. These fasciculation events can be mediated by homophilic cell adhesion molecules. The example here in the developing insect nervous system shows that individual axons extending from identified neurons selectively fasciculate to form specific nerve bundles. The red and orange axons express the same adhesion molecule, the Ig superfamily protein fasciclin II (FasII), resulting in the formation of a single pioneer axon pathway, the MP1 pathway, critical for CNS wiring (Lin et al., 1994). Ig superfamily cell adhesion molecules mediate selective axonal fasciculation from worms to mammals (Kolodkin and Tessier-Lavigne, 2011).

Topographic MappingA common and important neuronal projection strategy involves the retention within target of nearest neighbor relations between axons of neurons with neighboring cell bodies: so called “topographic” mapping. A classic example of this wiring pattern is the projection of retinal ganglion cell (RGC) axons to subcortical targets (SC/tectum) in the vertebrate midbrain (Feldheim and O’Leary, 2010). This allows preservation of the two-dimensional retinal image onto midbrain targets, as shown here for four RGC neurons located in the nasal (N), temporal (T), dorsal (D), and ventral (V) retina, which project to the posterior (P), anterior (A), ventral, and dorsal SC/tectum, respectively. A low-N to high-T gradient of EphA receptor tyrosine kinase (RTK) expression in RGCs is matched by a low-A to high-P gradient of ephrin A expression in the SC/tectum. Topographic mapping along the D-V axis also involves guidance cue gradients, shown here for EphB receptors expressed by RGCs and cognate ephrin-B-attractive ligands expressed in the SC/tectum (not shown are countergradients of Wnts and their receptors that participate in D-V topographic mapping). Eph receptors activate a range of RTK downstream signaling pathways (not shown) in addition to GEFs (ephexin, Vav) and GAPs (α-chimaerin) (Bashaw and Klein, 2010).

Self-AvoidanceMany neurons confront the daunting task of distinguishing their own axons and dendrites from those of adjacent neurons. This comes into play when individual axons in a bundle branch to innervate multiple targets, when neuronal dendrites elaborate complex morphologies, and when sensory neurons maximize coverage of receptive fields. A convergent evolutionary solution to this complex problem involves the expression in any one neuron of a small subset of protein isoforms derived from a single gene that is capable of gener-ating a vast array of distinct protein isoforms, providing each neuron with a unique identity (Zipursky and Sanes, 2010). These molecules, DSCAMs in insects and protocadherins (Pcdhs) in mammals, have the ability to mediate homophilic adhesion events. These interactions subsequently activate intracellular signaling pathways, thus far uncharacter-ized, that result in repulsive interactions leading to neuronal process self-avoidance but allowing overlap with nonself processes extending from other neurons. Shown here is DSCAM-mediated promotion of axon branching in the fly mushroom body, resulting from the expression on each axon of a small subset of the thousands of possible DSCAM protein isoforms generated by stochastic DSCAM RNA-splicing events. Also shown is Pcdh-mediated self-avoidance by dendrites of individual starburst amacrine cells (SACs) in the mammalian retina, here resulting from expression of a subset of Pcdh isoforms generated by use of alternative promoters (Lefebvre et al., 2012). For DSCAMs and Pcdhs, molecular diagrams show how multiple isoforms are generated, as are proposed models for how these molecules homophilically interact.

AbbreviationsAbl, Abelson murine leukemia virus oncogene; DCC, deleted in colorectal cancer; DSCAM, Down syndrome cell adhesion molecule; FAK, focal adhesion kinase; FARP, FERM-, RhoGEF-, and pleckstrin-domain-containing protein; GAP, GTPase-activating protein; GEF, guanine nucleotide exchange factor; GSK3, glycogen synthase kinase 3; Ig, immu-noglobulin; LARG, leukemia-associated RhoGEF; miRNA, microRNA; PAK, p21-activated kinase; PI3K, phosphoinositide-3 kinase; PKA, protein kinase A; Robo, roundabout; ROCK, Rho-associated kinase; SC, superior colliculus ; Sos, son of sevenless; srGAP, Slit-Robo RhoGTPase-activating protein; TF, transcription factor; Vav, Vav oncogene.

RefeRences

Bashaw, G.J., and Klein, R. (2010). Signaling from axon guidance receptors. Cold Spring Harb. Perspect. Biol. 2, a001941.

Derijck, A.A., Van Erp, S., and Pasterkamp, R.J. (2010). Semaphorin signaling: molecular switches at the midline. Trends Cell Biol. 20, 568–576.

Dickson, B.J., and Zou, Y. (2010). Navigating intermediate targets: the nervous system midline. Cold Spring Harb. Perspect. Biol. 2, a002055.

Feldheim, D.A., and O’Leary, D.D. (2010). Visual map development: bidirectional signaling, bifunctional guidance molecules, and competition. Cold Spring Harb. Perspect. Biol. 2, a001768.

Kolodkin, A.L., and Tessier-Lavigne, M. (2011). Mechanisms and molecules of neuronal wiring: a primer. Cold Spring Harb. Perspect. Biol. 3. Published online December 1, 2010. http://dx.doi.org/10.1101/cshperspect.a001727.

494.e1 Cell 153, April 11, 2013 ©2013 Elsevier Inc. DOI http://dx.doi.org/10.1016/j.cell.2013.03.031

SnapShot: Axon GuidanceR. Jeroen Pasterkamp1 and Alex L. Kolodkin2

1Department of Neuroscience and Pharmacology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, 3584 CG Utrecht, The Netherlands2Department of Neuroscience, HHMI, The Johns Hopkins University School of Medicine, Baltimore, MD 21212, USA

Lefebvre, J.L., Kostadinov, D., Chen, W.V., Maniatis, T., and Sanes, J.R. (2012). Protocadherins mediate dendritic self-avoidance in the mammalian nervous system. Nature 488, 517–521.

Lin, D.M., Fetter, R.D., Kopczynski, C., Grenningloh, G., and Goodman, C.S. (1994). Genetic analysis of Fasciclin II in Drosophila: defasciculation, refasciculation, and altered fasciculation. Neuron 13, 1055–1069.

Pasterkamp, R.J. (2012). Getting neural circuits into shape with semaphorins. Nat. Rev. Neurosci. 13, 605–618.

Tran, T.S., Kolodkin, A.L., and Bharadwaj, R. (2007). Semaphorin regulation of cellular morphology. Annu. Rev. Cell Dev. Biol. 23, 263–292.

Zipursky, S.L., and Sanes, J.R. (2010). Chemoaffinity revisited: dscams, protocadherins, and neural circuit assembly. Cell 143, 343–353.

494.e2 Cell 153, April 11, 2013 ©2013 Elsevier Inc. DOI http://dx.doi.org/10.1016/j.cell.2013.03.031