Ryk-mediated Wnt repulsion regulates posterior-directed growth of corticospinal tract
Abstract and Figures
Guidance cues along the longitudinal axis of the CNS are poorly understood. Wnt proteins attract ascending somatosensory axons to project from the spinal cord to the brain. Here we show that Wnt proteins repel corticospinal tract (CST) axons in the opposite direction. Several Wnt genes were found to be expressed in the mouse spinal cord gray matter, cupping the dorsal funiculus, in an anterior-to-posterior decreasing gradient along the cervical and thoracic cord. Wnts repelled CST axons in collagen gel assays through a conserved high-affinity receptor, Ryk, which is expressed in CST axons. Neonatal spinal cord secretes diffusible repellent(s) in an anterior-posterior graded fashion, with anterior cord being stronger, and the repulsive activity was blocked by antibodies to Ryk (anti-Ryk). Intrathecal injection of anti-Ryk blocked the posterior growth of CST axons. Therefore, Wnt proteins may have a general role in anterior-posterior guidance of multiple classes of axons.
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... Other embryonic guidance molecules retain their inhibitory effect on the growth cone and some of these are expressed after injury to the mature CNS. Wnt proteins are good examples, as they repel the growth cone of descending axons during development (103). The decreasing concentration of Wnt proteins direct the axons from the brain in caudal direction towards the spinal cord. ...
Mature neurons in the human central nervous system (CNS) fail to regenerate after injuries. This is a common denominator across different aetiologies, including multiple sclerosis, spinal cord injury and ischemic stroke. The lack of regeneration leads to permanent functional deficits with a substantial impact on patient quality of life, representing a significant socioeconomic burden worldwide. Great efforts have been made to decipher the responsible mechanisms and we now know that potent intra- and extracellular barriers prevent axonal repair. This knowledge has resulted in numerous clinical trials, aiming to promote neuroregeneration through different approaches. Here, we summarize the current understanding of the causes to the poor regeneration within the human CNS. We also review the results of the treatment attempts that have been translated into clinical trials so far.
... This finding is in line with a study showing that the Drosophila homolog of cwn-2, Wnt-5, and the receptor tyrosine kinase Derailed/Ryk mediate axonal repulsion away from the posterior commissure (Yoshikawa et al., 2003). Wnts also repel descending motor corticospinal axons in mice through Ryk (Liu et al., 2005). However, CWN-2/Wnt has also been shown to be an attractant for processes of the GABAergic head neurons RMED and RMEV, acting through MIG-1 and CFZ-2 Frizzled receptors (Song et al., 2010), which we found to be dispensable for FLP field size restriction. ...
The complete and non-redundant coverage of sensory tissues by neighboring neurons enables effective detection of stimuli in the environment. How the neurites of adjacent neurons establish their boundaries to achieve this completeness in coverage remains incompletely understood. Here, we use distinct fluorescent reporters to study two neighboring sensory neurons with complex dendritic arbors, FLP and PVD, in C. elegans . We quantify the sizes of their dendritic fields, and identify CWN-2/Wnt and LIN-17/Frizzled as a ligand and receptor that regulate the relative dendritic field sizes of these two neurons. Loss of either cwn-2 or lin-17 results in complementary changes in the size of the dendritic fields of both neurons; the FLP arbor expands, while that of PVD shrinks. Using an endogenous knock-in mNeonGreen-CWN-2/Wnt, we find that CWN-2/Wnt is localized along the path of growing FLP dendrites. Dynamic imaging shows a significant braking of FLP dendrite growth upon CWN-2/Wnt contact. We find that LIN-17/Frizzled functions cell-autonomously in FLP to limit dendritic field size and propose that PVD fills the space left by FLP through contact-induced retraction. Our results reveal that interactions of dendrites with adjacent dendrites and with environmental cues both shape the boundaries of neighboring dendritic fields.
Highlights
▫ Secreted Wnt CWN-2 and cell-autonomous activity of neuronal LIN-17/Frizzled receptors restrict FLP dendritic field sizes
▫ Endogenously tagged CWN-2/Wnt is punctate and visible in the same plane of growing FLP dendrites
▫ Growth of developing FLP dendrites is inhibited upon contact with extracellular CWN-2/Wnt and with PVD dendrites
... Extrasomatic regulation, such as axonal translation and protein modifications, which play a critical role in axon guidance, pruning and stabilization 42,43 , might leverage these ground-state soma-derived axonal projection programs to allow for fine target specificity and circuit assembly. While the current study focuses on the molecular and anatomical delineation of ET neurons across cortical areas, similar mechanisms are likely at play to regulate projections to distinct spinal cord segments and brainstem vs. spinal cord projections within sensorimotor cortex 10, 19,[44][45][46] . Despite the potential metabolic drawbacks of "exuberant" growth followed by area-specific pruning, such axonal refinement may serve to allow activity-dependent interactions between proximal and distal sites within transient circuits 47 . ...
Layer 5 extratelencephalic (ET) neurons are a main class of neocortical projection neurons that predominate in the motor cortex and send their axon to the pons and spinal cord, and collaterals to the thalamus and multiple deep subcerebral structures. Precise connectivity of ET neurons is critical for fine motor control; they are central to loss of function upon spinal cord injury and specifically degenerate in select neurodegenerative disorders. ET neurons consist of several types of cells with distinct laminar and areal locations, molecular identities, connectivities, and functions. Within layer 5 of the cortex, two cardinal subtypes of ET neurons have been identified: ETlower neurons, which express Slco2a1 and project to distal targets including the spinal cord, ETupper neurons, which express Nprs1 or Hpgd and project more proximally to the pons and thalamus. Despite their critical function, how these neuronal subtypes emerge during development and acquire their area-specific distributions remains unaddressed. Here, using combinations of anatomical labeling, MAPseq mapping, and single-nucleus transcriptomics across developing cortical areas, we reveal that these two subtypes of ET neurons are present at birth along opposite antero-posterior cortical gradients. We first characterize area-specific developmental axonal dynamics of ETlower and ETupper neurons and find that the latter can emerge by pruning of subsets of ETlower neurons. We next identify area- and ET neuron type-specific developmental transcriptional programs to identify key target genes in vivo. Finally, we reprogram ET neuron area-specific connectivity from motor to visual by postnatal in vivo combinatorial knockout of three key type-specific transcription factors. Together, these findings delineate the functional transcriptional programs controlling ET neuron diversity across cortical areas and provide a molecular blueprint to investigate and direct the developmental emergence of corticospinal motor control.
... This could result in attractive gradients forming of other guidance cues over time and space in the spinal cord, or differential ratios of repulsive and attractive cues forming over time that could affect axon growth and arborization in the spinal cord. Ryk-Wnt signaling has been shown to function as guidance cues through a rostro-caudal spatial gradient in the spinal cord (Liu et al., 2005). This signal could additionally help promote posterior-bound fasciculation and its gradual dissipation may cause greater CS axon arborization in the posterior spinal gray matter. ...
Axon fasciculation is thought to be a critical step in neural circuit formation and function. Recent studies have revealed various molecular mechanisms that underlie axon fasciculation; however, the impacts of axon fasciculation, and its corollary, defasciculation, on neural circuit wiring remain unclear. Corticospinal (CS) neurons in the sensorimotor cortex project axons to the spinal cord to control skilled movements. In rodents, the axons remain tightly fasciculated in the brain and traverse the dorsal funiculus of the spinal cord. Here we show that plexinA1 (PlexA1) and plexinA3 (PlexA3) receptors are expressed by CS neurons, whereas their ligands, semaphorin-5A (Sema5A) and semaphorin-5B (Sema5B) are expressed in the medulla at the decussation site of CS axons to inhibit premature defasciculation of these axons. In the absence of Sema5A/5B-PlexA1/A3 signaling, some CS axons are prematurely defasciculated in the medulla of the brainstem, and those defasciculated CS axons aberrantly transverse in the spinal gray matter instead of the spinal dorsal funiculus. In the absence of Sema5A/Sema5B-PlexA1/A3 signaling, CS axons, which would normally innervate the lumbar spinal cord, are unbundled in the spinal gray matter, and prematurely innervate the cervical gray matter with reduced innervation of the lumbar gray matter. In both Sema5A/5B and PlexA1/A3 mutant mice (both sexes), stimulation of the hindlimb motor cortex aberrantly evokes robust forelimb muscle activation. Finally, Sema5A/5B and PlexA1/A3 mutant mice show deficits in skilled movements. These results suggest that proper fasciculation of CS axons is required for appropriate neural circuit wiring and ultimately affect the ability to perform skilled movements.
Significance Statement:
Axon fasciculation is believed to be essential for neural circuit formation and function. However, whether and how defects in axon fasciculation affect the formation and function of neural circuits remain unclear. Here we examine whether the transmembrane proteins semaphorin-5A (Sema5A) and semaphorin-5B (Sema5B), and their receptors, plexinA1 (PlexA1) and plexinA3 (PlexA3) play roles in the development of corticospinal circuits. We find that Sema5A/Sema5B and PlexA1/A3 are required for proper axon fasciculation of corticospinal neurons. Furthermore, Sema5A/5B and PlexA1/A3 mutant mice show marked deficits in skilled motor behaviors. Therefore, these results strongly suggest that proper corticospinal axon fasciculation is required for the appropriate formation and functioning of corticospinal circuits in mice.
... When investigating these interactions in mice, it was found that blocking the Ryk receptor with anti-Ryk antibodies inhibited the growth of these axons in the caudal direction, thus reinforcing the importance of these molecular cues in the development of the CST. 23 Another important guidance cue that has been found to be implicated in CST outgrowth is IGF-I. In contrast to the repulsive mechanisms associated with Wnt-Ryk interactions, IGF-I provides an attractive cue upon interaction with the IGF-IR receptor found on motor neurons and ultimately promotes CST growth. ...
Spinal cord injury (SCI) severely diminishes quality of life and presents patients with a substantial financial burden. The lack of a curative treatment has guided efforts toward identifying potential regenerative treatments. Neural stem/progenitor cell (NSPC) transplantation represents a promising strategy for the regeneration of the injured spinal cord due to the ability of these cells to replace neural cells lost post-injury. However, the transplant-derived oligodendrocytes and neurons need to be able to associate and integrate within the appropriate endogenous circuits to guarantee optimal functional recovery. To date, the integration of these transplant-derived cells has lacked specificity and remains a challenge. As such, it appears that the transplanted cells will require additional guidance cues to instruct the cells where to integrate. In the present review, we propose a variety of combinatorial techniques that can be used in conjunction with NSPC transplantation to direct the cells toward particular circuits of interest. We begin by introducing distinct molecular signatures that assist in the formation of specific circuits during development, and highlight how favorable molecular cues can be incorporated within the cells and their environment to guide the grafted cells. We also introduce alternative methods including task-specific rehabilitation, galvanotaxis, and magnet-based tools, which can be applied to direct the integration of the grafted cells toward the stimulated circuits. Future research examining these combinatorial efforts may serve to improve outcomes following SCI.
... And finally, once the CST has crossed the midline, it starts travelling along the spinal cord and yet other factors, such as ephrins (Coonan et al., 2001;Dottori et al., 1998;Leighton et al., 2001), molecules of the WNT signalling pathway (Y. Liu et al., 2005) and even an insulin growing factor (Özdinler et al., 2006) guide the fibres down their path and help them in finding their correct positioning. Welniarz et al., 2007). ...
In mammals, the proper execution of fine voluntary movements relies on complex, but highly organized neuronal networks connecting various regions of the brain, such as the cerebral cortex, basal ganglia, pontine nuclei, cerebellum and thalamus. Understanding the genetic and molecular mechanisms underlying the neuronal organization of these circuits may improve our knowledge of how motor networks are normally established during development and affected in neurodevelopmental diseases. Among others, subcortically projecting Layer V Pyramidal Neurons (LVPNs) are central to this circuit.We have previously shown that the genetic loss of the transcription factor Nr2f1 in the developing neocortex, the evolutionary most recent region of the cerebral cortex, affects areal organization and molecular specification of LVPNs, leading to defective voluntary motor functions in both mouse models and human NR2F1 haploinsufficient patients. To further assess the contribution of Nr2f1 in the establishment of cortico-subcortical networks, we used two independent Nr2f1 conditional mutant mouse lines and investigated electrophysiological, morphological and connectivity features of LVPNs at different developmental stages.Our electrophysiological and morphological data reveal that postnatal mutant LVPNs are characterized by increased intrinsic excitability and reduced dendrite complexity, indicating that Nr2f1 plays a key role in LVPN functional maturation during cortical development. Moreover, genetic tracing of LVPN projections in mutant brains shows abnormal topographic mapping between the cortex and pontine nuclei, implying that LVPNs need to acquire their proper areal identity to establish normal subcortical projections. Overall, our data indicate that Nr2f1 is involved in the establishment of functional and structural properties of LVPNs, as well as in the topographic organization of cortico-pontine projections, first players of the cortico-ponto-cerebellar circuit involved in fine motor skills.
... Interestingly, WNT5A, which is transduced by RYK, was previously identified as a repulsive cue in both murine corticospinal tract axon pathfinding (Keeble et al., 2006;Liu et al., 2005) and in Drosophila midline axon crossing (Yoshikawa et al., 2003). However, in the midgut epithelium, filopodial protrusions grow towards the source of WNT5A, indicating that WNT5A acts as an attractant. ...
http://deepblue.lib.umich.edu/bitstream/2027.42/175365/2/DownloadCombinedArticleAndSupplmentPdf.pdf
Precise control of morphogen signaling levels is essential for proper development. An outstanding question is: what mechanisms ensure proper morphogen activity and correct cellular responses? Previous work has identified Semaphorin (SEMA) receptors, Neuropilins (NRPs) and Plexins (PLXNs), as positive regulators of the Hedgehog (HH) signaling pathway. Here, we provide evidence that NRPs and PLXNs antagonize Wnt signaling in both fibroblasts and epithelial cells. Further, Nrp1/2 deletion in fibroblasts results in elevated baseline Wnt pathway activity and increased maximal responses to Wnt stimulation. Notably, and in contrast to HH signaling, SEMA receptor-mediated Wnt antagonism is independent of primary cilia. Mechanistically, PLXNs and NRPs act downstream of Dishevelled (DVL) to destabilize beta-catenin (CTNNB1) in a proteosome-dependent manner. Further, NRPs, but not PLXNs, act in a GSK3beta/CK1-dependent fashion to antagonize Wnt signaling, suggesting distinct repressive mechanisms for these SEMA receptors. Overall, this study identifies SEMA receptors as novel Wnt pathway antagonists that may also play larger roles integrating signals from multiple inputs.
Layer 5 extratelencephalic (ET) neurons are a main class of neocortical projection neurons that predominate in the motor cortex and send their axon to the pons and spinal cord, and collaterals to the thalamus and multiple deep subcerebral structures. Precise connectivity of ET neurons is critical for fine motor control; they are central to loss of function upon spinal cord injury and specifically degenerate in select neurodegenerative disorders. ET neurons consist of several types of cells with distinct laminar and areal locations, molecular identities, connectivities, and functions. Within layer 5 of the cortex, two cardinal subtypes of ET neurons have been identified: “ET lower” neurons, which express Slco2a1 and project to distal targets including the spinal cord, and “ET upper“ neurons, which express Nprs1 or Hpgd and project more proximally to the pons and thalamus. Despite their critical function, how these neuronal subtypes emerge during development and acquire their area-specific distributions remains unaddressed. Here, using combinations of anatomical labeling, MAPseq mapping, and single-nucleus transcriptomics across developing cortical areas, we reveal that these two subtypes of ET neurons are present at birth along opposite antero-posterior cortical gradients. We first characterize area-specific developmental axonal dynamics of ET lower and ET upper neurons and find that the latter can emerge by pruning of subsets of ET lower neurons. We next identify area- and ET neuron type-specific developmental transcriptional programs to identify key target genes in vivo. Finally, we reprogram ET neuron area-specific connectivity from motor to visual by postnatal in vivo combinatorial knockout of three key type-specific transcription factors. Together, these findings delineate the functional transcriptional programs controlling ET neuron diversity across cortical areas and provide a molecular blueprint to investigate and direct the developmental emergence of corticospinal motor control.
Wilms tumors present as an amalgam of varying proportions of three tissues normally located within the developing kidney, one being the multipotent nephron progenitor population. While incomplete differentiation of the nephron progenitors is widely-considered the underlying cause of tumor formation, where this barrier occurs along the differentiation trajectory and how this might promote therapeutic resistance in high-risk blastemal-predominant tumors is unclear. Comprehensive integrated analysis of genomic datasets from normal human fetal kidney and high-risk Wilms tumors has revealed conserved expression of genes indicative of podocyte lineage differentiation in tumors of all subtypes. Comparatively upregulated expression of several of these markers, including the non-canonical WNT ligand WNT5A, was identified in tumors with the relapse-associated mutation SIX1/2-Q177R. These findings highlight the shared progression of cellular differentiation towards the podocyte lineage within Wilms tumors and enhancement of this differentiation program through promotion of non-canonical WNT/planar cell polarity signaling in association with SIX1/2-Q177R.
By using the polymerase chain reaction with degenerate oligonucleotides based on highly conserved motifs held in common between all members of the protein tyrosine kinase (PTK) family, a PTK-related sequence was isolated from murine peritoneal macrophage cDNA. Full-length clones have been isolated that encompass the entire coding region of the mRNA, and the predicted amino acid sequence indicates that the protein encoded has the structure of a growth factor receptor PTK (RTK). We have dubbed this molecule RYK (for related to tyrosine kinase). The RYK-encoded protein bears a transmembrane domain, with a relatively small (183 amino acid) extracellular domain, containing five potential N-linked glycosylation sites. The intracellular domain of RYK is unique among the broader family of RTKs and has several unusual sequence idiosyncrasies in some of the most highly conserved elements of the PTK domain. These sequence differences call into question the potential catalytic activity of the RYK protein.
Members of the Eph family of tyrosine kinase receptors have been implicated in the regulation of developmental processes and, in particular, axon guidance in the developing nervous system. The function of the EphA4 (Sek1) receptor was explored through creation of a null mutant mouse. Mice with a null mutation in the EphA4 gene are viable and fertile but have a gross motor dysfunction, which is evidenced by a loss of coordination of limb movement and a resultant hopping, kangaroo-like gait. Consistent with the observed phenotype, anatomical studies and anterograde tracing experiments reveal major disruptions of the corticospinal tract within the medulla and spinal cord in the null mutant animals. These results demonstrate a critical role for EphA4 in establishing the corticospinal projection.
This third edition of the standard reference on the nervous system of the rat is a complete and updated revision of the 1994 second edition. All chapters have been extensively updated, and new chapters added covering early segmentation, growth factors, and glia. The book is now aligned with the data available in the Rat Brain in Stereotaxic Coordinates, making it an excellent companion to this bestselling atlas. Physiological data, functional concepts, and correlates to human anatomy and function round out the new edition. Designed to be used in conjunction with the bestselling Rat Brain in Stereotaxic Coordinates. New to this edition is inclusion of physiological data, functional concepts, and correlates to human anatomy and function in each chapter. Contains new chapters on early segmentation of the central nervous system, growth factors and glia.
Commissural axons cross the nervous system midline and then turn to grow alongside it, neither recrossing nor projecting back into ventral regions. In Drosophila, the midline repellent Slit prevents recrossing: axons cross once because they are initially unresponsive to Slit, becoming responsive only upon crossing. We show that commissural axons in mammals similarly acquire responsiveness to a midline repellent activity upon crossing. Remarkably, they also become responsive to a repellent activity from ventral spinal cord, helping explain why they never reenter that region. Several Slit and Semaphorin proteins, expressed in midline and/or ventral tissues, mimic these repellent activities, and midline guidance defects are observed in mice lacking neuropilin-2, a Semaphorin receptor. Thus, Slit and Semaphorin repellents from midline and nonmidline tissues may help prevent crossing axons from reentering gray matter, squeezing them into surrounding fiber tracts.
It is rapidly becoming accepted that the vertebrate neural tube, in particular the hindbrain, develops into a segmented structure. After segment formation, cells in the neural tube do not cross segmental boundaries, and segment-specific gene expression is observed. However, it is not known what positional cues instruct the neural tube to express genes in this restricted manner. We have cloned a murine homeobox-containing gene, Hox-2.9, whose expression in the neural tube at E9.5 is restricted to a segment of the hindbrain known as rhombomere 4. A study of its expression pattern earlier in development revealed that prior to the start of neurulation (E7.5) Hox-2.9 is expressed within a posterior to the embryonic mesoderm that will participate in hindbrain formation. With the onset of neurulation, expression then becomes detectable in the neural plate as well, but only in the part that overlies the Hox-2.9-expressing mesoderm; it is not detected in the more anterior neuroectoderm that will form the future midbrain and forebrain. On the basis of these findings, we propose that the mesoderm is providing cues that serve to instruct the overlying neuroectoderm with respect to its position along the anteroposterior axis and that Hox-2.9 participates in or reflects this process. As neurulation continues and individual segments form, a second phase of expression is detected in the neural tube in which high levels of Hox-2.9 transcripts become restricted to rhombomere 4. Hox-2.9 expression is also detected in the developing branchial arch units of the hindbrain region, in a pattern that suggests to us that here, too, mesoderm is providing a localized signal that induces Hox-2.9 expression, in this case in endoderm of the pharynx and in superficial ectoderm. In general, we interpret the expression patterns of Hox-2.9 in the hindbrain region as suggesting that the specific mechanisms of pattern formation in mammals are fundamentally similar to those of amphibians and avians - i.e. anteroposterior positional information is acquired by mesoderm, mesoderm induces positional values within (neuro-) ectoderm and endoderm, and both events occur within a restricted window of time.
Mek4 and Sek are tyrosine kinases with expression patterns in the mouse embryo that suggest important functions in early development. However, like all Eph family kinases, both were identified as orphan receptors without known ligands. We show that Mek4 and Sek soluble receptor-alkaline phosphatase fusion proteins can be used in a procedure termed RAP in situ to identify regions of ligand expression in the mouse embryo. Based on this spatial information, a cDNA expression library was prepared, and was screened with the fusion proteins to identify Eph ligand family-1 (ELF-1). In cell lines and embryos, ELF-1 is membrane bound by a phosphatidylinositol tail, a feature that may account for unique biological functions. Its sequence is homologous with B61, a ligand for the Eck kinase, defining a family of related ligands. The expression domains of ELF-1, Mek4 and Sek indicate potential roles in embryonic patterning.
The neural cell adhesion molecule N-CAM has been proposed to function in the guidance of outgrowing axons in the peripheral and central nervous system. Light microscopic observations have shown that the embryonic form of N-CAM (200-230 kDa) is present in the ventralmost part of the dorsal funiculus during corticospinal tract (CST) ingrowth in the first postnatal week (Joosten, Dev. Brain Res., 78 (1994) 226-236). Here, the subcellular localization of the embryonic form of N-CAM (E-NCAM) is determined by pre-embedding staining on vibratome sections and by postembedding immunogold-labelling on Epon embedded spinal cord sections. The electron microscopical observations indicate that E-NCAM is present on the outer membrane of CST growth cones as well as other unmyelinated axons which are present in the ventralmost part of the dorsal funiculus. Furthermore, E-NCAM is localized in an irregular patchy way on the outer side of the axonal membrane of small unmyelinated, later arriving CST axons. From these results it may be deduced that E-NCAM is involved in CST tract formation through guidance of outgrowing pioneer CST growth cones along other unmyelinated axons and through mediation of axon fasciculation of later arriving CST axons.
During nervous system development, spinal commissural axons project toward floor plate cells and trochlear motor axons extend away from these cells. Netrin-1, a diffusible protein made by floor plate cells, can attract spinal commissural axons and repel trochlear axons in vitro, but its role in vivo is unknown. Netrin-1 deficient mice exhibit defects in spinal commissural axon projections that are consistent with netrin-1 guiding these axons. Defects in several forebrain commissures are also observed, suggesting additional guidance roles for netrin-1. Trochlear axon projections are largely normal, predicting the existence of additional cues for these axons, and evidence is provided for a distinct trochlear axon chemorepellent produced by floor plate cells. These results establish netrin-1 as a guidance cue that likely collaborates with other diffusible cues to guide axons in vivo.
Neural cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) have been implicated in both the fasciculation and guidance of axons, but direct genetic evidence of a role for neural IgCAMs in axon guidance in vertebrates is lacking. The L1 subfamily of vertebrate neural IgCAMs function as both homophilic and heterophilic receptors for a variety of cell-surface and extracellular ligands and may signal through intracellular kinases or by recruitment of the fibroblast growth factor receptor. L1 itself has been implicated in many neural processes and is expressed widely in the embryonic and adult nervous systems. In humans, mutations in the L1 gene are linked with a spectrum of brain disorders, including loss of the corticospinal tract, but the mechanistic basis for these disorders is unknown.
We show that mice that do not express L1 have defects in the guidance of axons of the corticospinal tract, a major motor control pathway projecting from the cortex to the spinal cord. Although the pathway to the caudal medulla appears normal, a substantial proportion of axons fail to cross the midline to the opposite dorsal column as normal. In adults, this results in a reduced decussation and in large numbers of axons projecting ipsilaterally. There is also a varying, but reduced, number of corticospinal axons in the dorsal columns of the spinal cord. These do not project beyond cervical levels. We show that these are defects in axon guidance, because they arise during the early stages of the development of the decussation. The presence of a ligand for L1, CD24, specifically at the point of decussation suggests a mechanism in which L1 functions to guide corticospinal axons across the midline.
L1 function is necessary for the guidance of corticospinal axons across the pyramidal decussation in mice. Some of the defects in the corticospinal tract of humans with mutations in L1 could be due to errors in axon guidance at the pyramidal decussation.
The corticospinal tract (CST) plays an important role in the control of voluntary movements. Although the development of the CST has been studied extensively in other species, limited information is available on its development in mice. In the present study, the growth of corticospinal axons was characterized in developing mice using Phaseolus vulgaris leucoagglutinin (PHA-L). Our results indicate that the leading CST axons reach the 8th cervical segment at postnatal day (PD) 2, the 7th thoracic segment at PD4, the 13th thoracic segment at PD7, and the 5th lumbar segment at PD9. The arrival of corticospinal axons at the distal lumbar cord at PD9 was further confirmed by retrograde tracing using fast blue (FB). A waiting period of 2-3 days exists after the leading CST axons pass a particular segment before sending collaterals into the gray matter of that segment. The CST continues to increase in size in lower thoracic and lumbar areas up to PD14 when its adult appearance is achieved. In this study, the date of animal's sacrifice was used as the specific postnatal date to demonstrate the growth of the CST. This definition gives a more reliable indication of the exact location of the CST at a specific developmental time point since the CST continues to grow after tracer injections and since the dye is transported much faster than axonal growth. We suggest that these findings can be used as a template for studies on both normal and transgenic mice where some developmental significance is given to the CST.