Kegg Pathway: Axon guidance

J Comp Neurol. 2009 Oct 13; 518(2): 119-136
Kotani T, Murata Y, Ohnishi H, Mori M, Kusakari S, Saito Y, Okazawa H, Bixby JL, Matozaki T

PTPRO is a receptor-type protein tyrosine phosphatase (PTP) with a single catalytic domain in its cytoplasmic region and multiple fibronectin type III-like domains in its extracellular region. In the chick, PTPRO mRNA has been shown to be particularly abundant in embryonic brain, and PTPRO is implicated in Axon growth and guidance during embryonic development. However, the temporal and spatial expression of PTPRO protein in the mammalian CNS, particularly in the juvenile and adult mammalian brain, has not been evaluated in any detail. By immunohistofluorescence analysis with a monoclonal antibody to PTPRO, we show that PTPRO is widely expressed throughout the mouse brain from embryonic day 16 to postnatal day 1, while expression is largely confined to the olfactory bulb (OB) and olfactory tubercle in the adult brain. In the OB, PTPRO protein is expressed predominantly in the external plexiform layer, the granule cell layer, and the glomerular layer (GL). In these regions, expression of PTPRO is predominant in interneurons such as gamma-aminobutyric acid (GABA)-ergic or calretinin (CR)-positive granule cells. In addition, PTPRO is expressed in GABAergic, CR-positive, tyrosine hydroxylase-positive, or neurocalcin-positive periglomerular cells in the GL. Costaining of PTPRO with other neuronal markers suggests that PTPRO is likely to be localized to the dendrites or dendritic spines of these olfactory interneurons. Thus, PTPRO might participate in regulation of dendritic morphology or synapse formation of interneurons in the adult mouse OB. J. Comp. Neurol. 518:119-136, 2010. (c) 2009 Wiley-Liss, Inc.

Dev Dyn. 2009 Nov 18; 238(12): 3285-3296
Beaubien F, Cloutier JF

The Slitrk family of transmembrane proteins is composed of six members that are highly expressed in the nervous system. To date, the function of Slitrks during development of the nervous system has yet to be defined. The high homology between the extracellular region of Slitrks and the repulsive Axon guidance molecules Slits suggests that Slitrks may regulate Axon outgrowth during development. To begin to evaluate their role during development, we have examined the expression of the Slitrk genes in the developing murine nervous system using in situ hybridization. Here, we show that despite some overlap in expression, the Slitrks display distinct patterns of expression in the olfactory system, the eye, forebrain structures, the cerebellum, the spinal cord, and dorsal root ganglia. These diverse patterns of expression suggest that Slitrk family members may have different functions during development of the nervous system. Developmental Dynamics 238:3285-3296, 2009. (c) 2009 Wiley-Liss, Inc.

J Biol Chem. 2009 Nov 13;
Esselens C, Malapeira J, Colome N, Casal C, Rodriguez-Manzaneque JC, Canals F, Arribas J

Metastasis is a sequential process that allows cells to move from the primary tumor and grow elsewhere. Because of their ability to cleave a variety of extracellular signaling and adhesion molecules, metalloproteases have been long considered key components of the metastatic program. However, the function of certain metalloproteases, such as ADAMTS1, is not clear and seems to depend on the cellular environment and/or the stage of tumor progression. In order to characterize the function of ADAMTS1, we performed two alternative proteomic approaches, Difference Gel Electrophoresis (DIGE) and Stable Isotopes Labeling by Amino Acids in Cell Culture (SILAC), to identify novel substrates of the metalloprotease. Both techniques showed that overexpression of ADAMTS1 leads to the release of semaphorin 3C from the extracellular matrix. Although semaphorins are well- known regulators of Axon guidance, accumulating evidence shows that they may also participate in tumor progression. Here, we show that the cleavage of semaphorin 3C induced by ADAMTS1 promotes the migration of breast cancer cells, indicating that the co-expression of these molecules in tumors may contribute to the metastatic program.

Neuron. 2009 Nov 12; 64(3): 355-66
Wizenmann A, Brunet I, Lam JS, Sonnier L, Beurdeley M, Zarbalis K, Weisenhorn-Vogt D, Weinl C, Dwivedy A, Joliot A, Wurst W, Holt C, Prochiantz A

Engrailed transcription factors regulate the expression of guidance cues that pattern retinal Axon terminals in the dorsal midbrain. They also act directly to guide Axon growth in vitro. We show here that an extracellular En gradient exists in the tectum along the anterior-posterior axis. Neutralizing extracellular Engrailed in vivo with antibodies expressed in the tectum causes temporal Axons to map aberrantly to the posterior tectum in chick and Xenopus. Furthermore, posterior membranes from wild-type tecta incubated with anti-Engrailed antibodies or posterior membranes from Engrailed-1 knockout mice exhibit diminished repulsive activity for temporal Axons. Since EphrinAs play a major role in anterior-posterior mapping, we tested whether Engrailed cooperates with EphrinA5 in vitro. We find that Engrailed restores full repulsion to Axons given subthreshold doses of EphrinA5. Collectively, our results indicate that extracellular Engrailed contributes to retinotectal mapping in vivo by modulating the sensitivity of growth cones to EphrinA.

Semin Cell Dev Biol. 2009 Nov 10;
Ahmed S, Goh WI, Bu W

Filopodia and lamellipodia are dynamic actin-based structures that determine cell shape and migration. Filopodia are thought to sense the environment and direct processes such as Axon guidance and neurite outgrowth. Cdc42 is a small GTP-binding protein and member of the RhoGTPase family. Cdc42 and its effector IRSp53 (insulin receptor phosphotyrosine 53kDa substrate) have been shown to be strong inducers of filopodium formation. IRSp53 consists of an I-BAR (inverse-Bin-Amphiphysin-Rvs) domain, a Cdc42-binding domain and an SH3 domain. The I-BAR domain of IRSp53 induces membrane tubulation of vesicles and dynamic membrane protrusions lacking actin in cells. The IRSp53 SH3 domain interacts with proteins that regulate actin filament formation e.g. Mena, N-WASP, mDia1 and Eps8. In this review we suggest that the mechanism for Cdc42-driven filopodium formation involves coupling I-BAR domain-induced membrane protrusion with SH3 domain-mediated actin dynamics through IRSp53.

Biochem Soc Trans. 2009 Dec; 37(Pt 6): 1228-32
Fantin A, Maden CH, Ruhrberg C

Blood vessels and neurons share guidance cues and cell-surface receptors to control their behaviour during embryogenesis. The transmembrane protein NRP1 (neuropilin 1) is present on both blood vessels and nerves and binds two structurally diverse ligands, the class 3 semaphorin SEMA3A and an isoform of the vascular endothelial growth factor VEGF-A termed VEGF(165) (VEGF(164) in mice). In vitro, SEMA3A competes with VEGF(164) for binding to NRP1 to modulate the migration of endothelial cells and neuronal progenitors. It was therefore hypothesized that NRP1 signalling controls neurovascular co-patterning by integrating competing VEGF(164) and SEMA3A signals. However, SEMA3A, but not VEGF(164), is required for Axon patterning of motor and sensory nerves, and, vice versa, VEGF(164) rather than SEMA3A is required for blood vessel development. Ligand competition for NRP1 therefore does not explain neurovascular congruence. Instead, these ligands control different aspects of neurovascular patterning that have an impact on cardiovascular function. Thus SEMA3A/NRP1 signalling guides the NCC (neural crest cell) precursors of sympathetic neurons as well as their Axonal projections. In addition, VEGF(164) and a second class 3 semaphorin termed SEMA3C contribute to the remodelling of the embryonic pharyngeal arch arteries and primitive heart outflow tract by acting on endothelium and NCCs respectively. Consequently, loss of either of these NRP1 ligands disrupts blood flow into and out of the heart. Multiple NRP1 ligands therefore co-operate to orchestrate cardiovascular morphogenesis.

J Neurosci. 2009 Nov 11; 29(45): 14066-76
Tremblay M, Fugère V, Tsui J, Schohl A, Tavakoli A, Travençolo BA, Costa Lda F, Ruthazer ES

Radial glia in the developing optic tectum express the key guidance molecules responsible for topographic targeting of retinal Axons. However, the extent to which the radial glia are themselves influenced by retinal inputs and visual experience remains unknown. Using multiphoton live imaging of radial glia in the optic tectum of intact Xenopus laevis tadpoles in conjunction with manipulations of neural activity and sensory stimuli, radial glia were observed to exhibit spontaneous calcium transients that were modulated by visual stimulation. Structurally, radial glia extended and retracted many filopodial processes within the tectal neuropil over minutes. These processes interacted with retinotectal synapses and their motility was modulated by nitric oxide (NO) signaling downstream of neuronal NMDA receptor (NMDAR) activation and visual stimulation. These findings provide the first in vivo demonstration that radial glia actively respond both structurally and functionally to neural activity, via NMDAR-dependent NO release during the period of retinal Axon ingrowth.

J Neurosci. 2009 Nov 4; 29(44): 13981-91
Woo S, Rowan DJ, Gomez TM

Adhesion controls growth cone motility, yet the effects of Axon guidance cues on adhesion site dynamics are poorly understood. Here we show that ephrin-A1 reduces retinal ganglion cell (RGC) Axon outgrowth by stabilizing existing adhesions and inhibiting new adhesion assembly. Ephrin-A1 activates focal adhesion kinase (FAK) in an integrin- and Src-dependent manner and the effects of ephrin-A1 on growth cone motility require FAK activation. We also find that FAK is expressed in a high temporal to low nasal gradient in RGCs, similar to EphA receptors, and that balanced FAK activation is necessary for optimal Axon outgrowth. Last, we find that FAK is required for proper topographic positioning of retinal Axons along the anterior-posterior axis of the optic tectum in both Xenopus and zebrafish, a guidance decision mediated in part by A-type ephrins. Together, our data suggest that ephrin-A1 controls growth cone advance by modulating adhesive point contacts through FAK activation and that graded FAK signaling is an important component of ephrin-A-mediated retinotopic mapping.

Dev Neurobiol. 2009 Nov 2;
Rishal I, Michaelevski I, Rozenbaum M, Shinder V, Medzihradszky KF, Burlingame AL, Fainzilber M

Localized changes in the composition of Axonal cytoplasm (axoplasm) are critical for many biological processes, including Axon guidance, responses to injury, neurite outgrowth, and Axon-glia interactions. Biochemical and molecular studies of these mechanisms have been heavily focused on in vitro systems because of the difficulty of obtaining subcellular extracts from mammalian tissues in vivo. As in vitro systems might not replicate the in vivo situation, reliable methods of axoplasm extraction from whole nerve would be helpful for mechanistic studies on Axons. Here we develop and evaluate a new procedure for preparation of axoplasm from rat peripheral nerve, based on incubation of separated short segements of nerve fascicles in hypotonic medium to separate myelin and lyse nonAxonal structures, followed by extraction of the remaining Axon-enriched material. We show that this new procedure reduces serum and glial cell contamination and facilitates proteomic analyses of Axonal contents. (c) 2009 Wiley Periodicals, Inc. Develop Neurobiol, 2009.

Neuron. 2009 Oct 29; 64(2): 188-99
Katsuki T, Ailani D, Hiramoto M, Hiromi Y

In the developing nervous system, distribution of membrane molecules, particularly Axon guidance receptors, is often restricted to specific segments of Axons. Such localization of membrane molecules can be important for the formation and function of neural networks; however, how this patterning within Axons is achieved remains elusive. Here we show that Drosophila neurons in culture establish intra-Axonal patterns in a cell-autonomous manner; several membrane molecules localize to either proximal or distal Axon segments without cell-cell contacts. This distinct patterning of membrane proteins is not explained by a simple temporal control of expression, and likely involves spatially controlled vesicular targeting or retrieval. Mobility of transmembrane molecules is restricted at the boundary of intra-Axonal segments, indicating that the Axonal membrane is compartmentalized by a barrier mechanism. We propose that this intra-Axonal compartmentalization is an intrinsic property of Drosophila neurons that provides a basis for the structural and functional development of the nervous system.

Neuron. 2009 Oct 29; 64(2): 150-2
Wright AP, Zinn K

In this issue of Neuron, Katsuki and colleagues show that cell-autonomous mechanisms divide Drosophila Axons into proximal and distal compartments. Axon guidance receptors selectively localize to one compartment. A diffusion barrier exists near the compartment boundary, suggesting that it may have properties like those of the Axon initial segment in mammalian neurons.

Cereb Cortex. 2009 Oct 27;
Gopal PP, Simonet JC, Shapiro W, Golden JA

Mammalian forebrain development requires extensive migration, yet the mechanisms through which migrating neurons sense and respond to guidance cues are not well understood. Similar to the Axon growth cone, the leading process and branches of neurons may guide migration, but the cytoskeletal events that regulate branching are unknown. We have previously shown that loss of microtubule-associated protein Lis1 reduces branching during migration compared with wild-type neurons. Using time-lapse imaging of Lis1(+/-) and Lis1(+/+) cells migrating from medial ganglionic eminence explant cultures, we show that the branching defect is not due to a failure to initiate branches but a defect in the stabilization of new branches. The leading processes of Lis1(+/-) neurons have reduced expression of stabilized, acetylated microtubules compared with Lis1(+/+) neurons. To determine whether Lis1 modulates branch stability through its role as the noncatalytic beta regulatory subunit of platelet-activating factor (PAF) acetylhydrolase 1b, exogenous PAF was applied to wild-type cells. Excess PAF added to wild-type neurons phenocopies the branch instability observed in Lis1(+/-) neurons, and a PAF antagonist rescues leading process branching in Lis1(+/-) neurons. These data highlight a role for Lis1, acting through the PAF pathway, in leading process branching and microtubule stabilization.

Brain. 2009 Oct 25;
Muramatsu R, Nakahara S, Ichikawa J, Watanabe K, Matsuki N, Koyama R

Proper Axonal targeting is fundamental to the establishment of functional neural circuits. The hippocampal mossy fibres normally project towards the CA3 region. In the hippocampi of patients with temporal lobe epilepsy and related animal models, however, mossy fibres project towards the molecular layer and produce the hyperexcitable recurrent networks. The cellular and molecular mechanisms underlying this aberrant Axonal targeting, known as mossy fibre sprouting, remain unclear. Netrin-1 attracts or repels Axons depending on the composition of its attraction-mediating receptor, deleted in colorectal cancer, and its repulsion-mediating receptor, uncoordinated-5, on the growth cone; but the roles of netrin-1-dependent guidance in pathological conditions are largely unknown. In this study, we examined the role of netrin-1 and its receptors in mossy fibre guidance and report that enhanced neuronal activity changes netrin-1-mediated cell targeting by the Axons under hyperexcitable conditions. Netrin-1 antibody or Dcc ribonucleic acid interference attenuated mossy fibre growth towards CA3 in slice overlay assays. The Axons were repelled from CA3 and ultimately innervated the molecular layer when hyperactivity was pharmacologically introduced. We first hypothesized that a reduction in netrin-1 expression in CA3 underlies the phenomenon, but found that its expression was increased. We then examined two possible activity-dependent changes in netrin-1 receptor expression: a reduction in the deleted in colorectal cancer receptor and induction of uncoordinated-5 receptor. Hyperactivity did not affect the surface expression of the deleted in colorectal cancer receptor on the growth cone, but it increased that of uncoordinated-5A, which was suppressed by blocking cyclic adenosine monophosphate signalling. In addition, Dcc knockdown did not affect hyperactivity-induced mossy fibre sprouting in the slice cultures, whereas Unc5a knockdown rescued the mistargeting. Thus, netrin-1 appears to attract mossy fibres via the deleted in colorectal cancer receptor, while it repels them via cyclic adenosine monophosphate-induced uncoordinated-5A under hyperexcitable conditions, resulting in mossy fibre sprouting.

Nat Cell Biol. 2009 Nov; 11(11): 1325-31
Jones CA, Nishiya N, London NR, Zhu W, Sorensen LK, Chan AC, Lim CJ, Chen H, Zhang Q, Schultz PG, Hayallah AM, Thomas KR, Famulok M, Zhang K, Ginsberg MH, Li DY

Slit-Roundabout (Robo) signalling has a well-understood role in Axon guidance. Unlike in the nervous system, however, Slit-dependent activation of an endothelial-specific Robo, Robo4, does not initiate a guidance program. Instead, Robo4 maintains the barrier function of the mature vascular network by inhibiting neovascular tuft formation and endothelial hyperpermeability induced by pro-angiogenic factors. In this study, we used cell biological and biochemical techniques to elucidate the molecular mechanism underlying the maintenance of vascular stability by Robo4. Here, we demonstrate that Robo4 mediates Slit2-dependent suppression of cellular protrusive activity through direct interaction with the intracellular adaptor protein paxillin and its paralogue, Hic-5. Formation of a Robo4-paxillin complex at the cell surface blocks activation of the small GTPase Arf6 and, consequently, Rac by recruitment of Arf-GAPs (ADP-ribosylation factor- directed GTPase-activating proteins) such as GIT1. Consistent with these in vitro studies, inhibition of Arf6 activity in vivo phenocopies Robo4 activation by reducing pathologic angiogenesis in choroidal and retinal vascular disease and VEGF-165 (vascular endothelial growth factor-165)-induced retinal hyperpermeability. These data reveal that a Slit2-Robo4-paxillin-GIT1 network inhibits the cellular protrusive activity underlying neovascularization and vascular leak, and identify a new therapeutic target for ameliorating diseases involving the vascular system.

Cell Adh Migr. 2009 Oct 27; 3(4):
Nasarre C, Koncina E, Labourdette G, Cremel G, Roussel G, Aunis D, Bagnard D

Semaphorin 3A (Sema3A) is a secreted guidance molecule initially described in the nervous system. This protein is able to control Axon growth but also effects on endothelial cells migration. Here, we report that Sema3A acts as a chemorepellent factor for the rat C6 glioma cells and 3 different human glioma cell lines. Interestingly, Sema3A triggered a chemoattractive response in a fourth human glioma cell line. The nature of the receptor complex ensuring the appropriate signaling was dissected in C6 cells by using function blocking antibodies and gain- or loss-of function experiments using recombinant receptors. Our results demonstrate that neuropilin-1, neuropilin-2 and PlexinA1 are necessary to trigger cell repulsion. The selective blockade of neuropilin-1 or Plexin-A1 switched the chemorepulsive effect of Sema3A into a chemoattractive one. Strikingly, blocking Neuropilin-2 suppressed Sema3A-induced cell migration while overexpression of neuropilin-2 was able to convert the chemorepulsive effect of Sema3A into a chemoattractive one. Our results not only provide additional evidence for a biological function of Sema3A in glioma migration but also reveal part of the receptor complex involved. Hence, our study describes a receptor-based plasticity in cancer cells leading to opposite migration behavior in response to the same extracellular signal.

Development. 2009 Nov; 136(22): 3801-10
Kennerdell JR, Fetter RD, Bargmann CI

Wnt signaling through Frizzled proteins guides posterior cells and Axons in C. elegans into different spatial domains. Here we demonstrate an essential role for Wnt signaling through Ror tyrosine kinase homologs in the most prominent anterior neuropil, the nerve ring. A genetic screen uncovered cwn-2, the C. elegans homolog of Wnt5, as a regulator of nerve ring placement. In cwn-2 mutants, all neuronal structures in and around the nerve ring are shifted to an abnormal anterior position. cwn-2 is required at the time of nerve ring formation; it is expressed by cells posterior of the nerve ring, but its precise site of expression is not critical for its function. In nerve ring development, cwn-2 acts primarily through the Wnt receptor CAM-1 (Ror), together with the Frizzled protein MIG-1, with parallel roles for the Frizzled protein CFZ-2. The identification of CAM-1 as a CWN-2 receptor contrasts with CAM-1 action as a non-receptor in other C. elegans Wnt pathways. Cell-specific rescue of cam-1 and cell ablation experiments reveal a crucial role for the SIA and SIB neurons in positioning the nerve ring, linking Wnt signaling to specific cells that organize the anterior nervous system.

J Neurosci. 2009 Oct 21; 29(42): 13190-201
Paulus JD, Willer GB, Willer JR, Gregg RG, Halloran MC

Multiple molecular cues guide neuronal Axons to their targets during development. Previous studies in vitro have shown that mechanical stimulation also can affect Axon growth; however, whether mechanical force contributes to Axon guidance in vivo is unknown. We investigated the role of muscle contractions in the guidance of zebrafish peripheral Rohon-Beard (RB) sensory Axons in vivo. We analyzed several mutants that affect muscle contraction through different molecular pathways, including a new mutant allele of the titin a (pik) gene, mutants that affect the hedgehog signaling pathway, and a nicotinic acetylcholine receptor mutant. We found RB Axon defects in these mutants, the severity of which appeared to correlate with the extent of muscle contraction loss. These Axons extend between the muscle and skin and normally have ventral trajectories and repel each other on contact. RB peripheral Axons in muscle mutants extend longitudinally instead of ventrally, and the Axons fail to repel one another on contact. In addition, we showed that limiting muscle movements by embedding embryos in agarose caused similar defects in peripheral RB Axon guidance. This work suggests that the mechanical forces generated by muscle contractions are necessary for proper sensory Axon pathfinding in vivo.

Biophys J. 2009 Oct 21; 97(8): 2316-26
Zeev-Ben-Mordehai T, Mylonas E, Paz A, Peleg Y, Toker L, Silman I, Svergun DI, Sussman JL

Amalgam (Ama) is a secreted neuronal adhesion protein that contains three tandem immunoglobulin domains. It has both homophilic and heterophilic cell adhesion properties, and is required for Axon guidance and fasciculation during early stages of Drosophila development. Here, we report its biophysical characterization and use small-angle x-ray scattering to determine its low-resolution structure in solution. The biophysical studies revealed that Ama forms dimers in solution, and that its secondary and tertiary structures are typical for the immunoglobulin superfamily. Ab initio and rigid-body modeling by small-angle x-ray scattering revealed a distinct V-shaped dimer in which the two monomer chains are aligned parallel to each other, with the dimerization interface being formed by domain 1. These data provide a structural basis for the dual adhesion characteristics of Ama. Thus, the dimeric structure explains its homophilic adhesion properties. Its V shape suggests a mechanism for its interaction with its receptor, the single-pass transmembrane adhesion protein neurotactin, in which each "arm" of Ama binds to the extracellular domain of neurotactin, thus promoting its clustering on the outer face of the plasma membrane.

Genomics. 2009 Oct 14;
Kunapuli P, Lo K, Hawthorn L, Cowell JK

The LGI1 gene suppresses invasion in glioma cells and predisposes to epilepsy. In a gene expression array comparison between parental cells and T98G cell clones forced to express LGI1, we demonstrate that the canonical Axon guidance pathway is the most significantly affected. In particular, aspects of Axon guidance that involve reorganization of the actin cytoskeleton, which is also involved in cell movement and invasion, were affected. Analysis of actin fiber organization using fluorescence microscopy demonstrated that different T98G cell clones expressing the exogenous LGI1 gene show high levels of stress fibers compared with controls. Since stress fiber formation is associated with loss of cell mobility, we used scratch wound assays to demonstrate that LGI1-expressing clones show a significant reduction in cell mobility. LGI1 reexpression also resulted in loss of the PDGFRA and EGFR proteins, suggesting a rapid turnover of these receptors despite increased mRNA levels for PDGFRA. LGI1 suppression of invasion is associated with loss of ERK/MAPK1 activation. LGI1 is a secreted protein, and when the culture supernatant from cells expressing FLAG- and GFP-tagged proteins were applied to parental T98G cells, ERK/MAPK1 phosphorylation and cell mobility was suppressed, demonstrating that the LGI1 protein acts as a suppressive agent for cell movement in this assay. These observations support a previous suggestion that LGI1 can reduce cellular invasion in in vitro assays and, as a secreted agent, may be developed as a means of treating metastatic cancer. In addition, this observation provides a mechanistic link for LGI1's common role in metastasis and epilepsy development.

Brain Res. 2009 Oct 13;
Wang J, Wang L, Zhao H, Chan SO

The localization of an Axon growth inhibitory molecule Nogo and its receptor (NgR) was investigated in the mouse spinal cord during prenatal development of the commissural pathway. Using the antibody N18, an intense signal for Nogo was localized largely on radial glia processes that are immunoreactive to RC2 antibody during the major period of commissural Axon growth and was gradually reduced towards the end of gestation. The glial processes ramified extensively in the ventral funiculus and resided within the interfascicular space between the longitudinally projecting Axons. Axonal localization of Nogo was observed on the premidline segment of commissural Axons and on Axons in the dorsal and ventral funiculi, but only at the earliest stage of pathway development. Nogo signals were initially weak on the glial processes during the period of Axon crossing in the floor plate but was elevated when the decussation is finished. NgR was expressed on the commissural Axons; the expression pattern is spatially regulated, being low in the premidline and midline courses but is upregulated when the Axons leave the floor plate. These expression patterns raise the possibilities that the glial-specific form of Nogo may be involved in the guidance of commissural Axons by (i) preventing recrossing of Axons across the midline through an upregulation of Axonal NgR and (ii) partitioning Axons in the ventral funiculus into longitudinal fascicles.