The role of L1 in axon pathfinding and fasciculation

Krüppel-like factor 7 deficiency disrupts corpus callosum development and neuronal migration in the developing mouse cerebral cortex

Krüppel-like Factor 7 (KLF7) is a zinc finger transcription factor that has a critical role in cellular differentiation, tumorigenesis, and regeneration. Mutations in Klf7 are associated with autism spectrum disorder, which is characterized by neurodevelopmental delay and intellectual disability. Here we show that KLF7 regulates neurogenesis and neuronal migration during mouse cortical development. Conditional depletion of KLF7 in neural progenitor cells resulted in agenesis of the corpus callosum, defects in neurogenesis, and impaired neuronal migration in the neocortex. Transcriptomic profiling analysis indicated that KLF7 regulates a cohort of genes involved in neuronal differentiation and migration, including p21 and Rac3. These findings provide insights into our understanding of the potential mechanisms underlying neurological defects associated with Klf7 mutations.

Interaction of L1CAM with LC3 Is Required for L1-Dependent Neurite Outgrowth and Neuronal Survival

The neural cell adhesion molecule L1 (also called L1CAM or CD171) functions not only in cell migration, but also in cell survival, differentiation, myelination, neurite outgrowth, and signaling during nervous system development and in adults. The proteolytic cleavage of L1 in its extracellular domain generates soluble fragments which are shed into the extracellular space and transmembrane fragments that are internalized into the cell and transported to various organelles to regulate cellular functions. To identify novel intracellular interaction partners of L1, we searched for protein-protein interaction motifs and found two potential microtubule-associated protein 1 light-chain 3 (LC3)-interacting region (LIR) motifs within L1, one in its extracellular domain and one in its intracellular domain. By ELISA, immunoprecipitation, and proximity ligation assay using L1 mutant mice lacking the 70 kDa L1 fragment (L1-70), we showed that L1-70 interacts with LC3 via the extracellular LIR motif in the fourth fibronectin type III domain, but not by the motif in the intracellular domain. The disruption of the L1-LC3 interaction reduces L1-mediated neurite outgrowth and neuronal survival.

Embryonic ethanol exposure disrupts craniofacial neuromuscular integration in zebrafish larvae

Forming a vertebrate head involves the meticulous integration of multiple tissue types during development. Prenatal alcohol exposure is known to cause a variety of birth defects, especially to tissues in the vertebrate head. However, a systematic analysis of coordinated defects across tissues in the head is lacking. Here, we delineate the effects of ethanol on individual tissue types and their integration during craniofacial development. We found that exposure to 1% ethanol induced ectopic cranial muscle and nerve defects with only slight effects on skeletal pattern. Ectopic muscles were, however, unaccompanied by ectopic tendons and could be partially rescued by anesthetizing the larvae before muscle fibers appeared. This finding suggests that the ectopic muscles result from fiber detachment and are not due to an underlying muscle patterning defect. Interestingly, immobilization did not rescue the nerve defects, thus ethanol has an independent effect on each tissue even though they are linked in developmental time and space. Time-course experiments demonstrated an increase in nerve defects with ethanol exposure between 48hpf-4dpf. Time-lapse imaging confirmed the absence of nerve pathfinding or misrouting defects until 48hpf. These results indicate that ethanol-induced nerve defects occur at the time of muscle innervation and after musculoskeletal patterning. Further, we investigated the effect of ethanol on the neuromuscular junctions of the craniofacial muscles and found a reduced number of postsynaptic receptors with no significant effect on the presynaptic terminals. Our study shows that craniofacial soft tissues are particularly susceptible to ethanol-induced damage and that these defects appear independent from one another. Thus, the effects of ethanol on the vertebrate head appear highly pleiotropic.

L1CAM variants cause two distinct imaging phenotypes on fetal MRI

Data on fetal MRI in L1 syndrome are scarce with relevant implications for parental counseling and surgical planning. We identified two fetal MR imaging patterns in 10 fetuses harboring L1CAM mutations: the first, observed in 9 fetuses was characterized by callosal anomalies, diencephalosynapsis, and a distinct brainstem malformation with diencephalic–mesencephalic junction dysplasia and brainstem kinking. Cerebellar vermis hypoplasia, aqueductal stenosis, obstructive hydrocephalus, and pontine hypoplasia were variably associated. The second pattern observed in one fetus was characterized by callosal dysgenesis, reduced white matter, and pontine hypoplasia. The identification of these features should alert clinicians to offer a prenatal L1CAM testing.

Wnt/β-Catenin Target Genes in Colon Cancer Metastasis: The Special Case of L1CAM

Simple Summary

The Wnt/β-catenin cell–cell signaling pathway is one of the most basic and highly conserved pathways for intercellular communications regulating key steps during development, differentiation, and cancer. In colorectal cancer (CRC), in particular, aberrant activation of the Wnt/β-catenin pathway is believed to be responsible for perpetuating the disease from the very early stages of cancer development. A large number of downstream target genes of β-catenin-T-cell factor (TCF), including oncogenes, were detected as regulators of CRC development. In this review, we will summarize studies mainly on one such target gene, the L1CAM (L1) cell adhesion receptor, that is selectively induced in invasive and metastatic CRC cells and in regenerating cells of the intestine following injury. We will describe studies on the genes activated when the levels of L1 are increased in CRC cells and their effectiveness in propagating CRC development. These downstream targets of L1-signaling can serve in diagnosis and may provide additional targets for CRC therapy.

Abstract

Cell adhesion to neighboring cells is a fundamental biological process in multicellular organisms that is required for tissue morphogenesis. A tight coordination between cell–cell adhesion, signaling, and gene expression is a characteristic feature of normal tissues. Changes, and often disruption of this coordination, are common during invasive and metastatic cancer development. The Wnt/β-catenin signaling pathway is an excellent model for studying the role of adhesion-mediated signaling in colorectal cancer (CRC) invasion and metastasis, because β-catenin has a dual role in the cell; it is a major adhesion linker of cadherin transmembrane receptors to the cytoskeleton and, in addition, it is also a key transducer of Wnt signaling to the nucleus, where it acts as a co-transcriptional activator of Wnt target genes. Hyperactivation of Wnt/β-catenin signaling is a common feature in the majority of CRC patients. We found that the neural cell adhesion receptor L1CAM (L1) is a target gene of β-catenin signaling and is induced in carcinoma cells of CRC patients, where it plays an important role in CRC metastasis. In this review, we will discuss studies on β-catenin target genes activated during CRC development (in particular, L1), the signaling pathways affected by L1, and the role of downstream target genes activated by L1 overexpression, especially those that are also part of the intestinal stem cell gene signature. As intestinal stem cells are highly regulated by Wnt signaling and are believed to also play major roles in CRC progression, unravelling the mechanisms underlying the regulation of these genes will shed light on both normal intestinal homeostasis and the development of invasive and metastatic CRC.

The Atoh7 remote enhancer provides transcriptional robustness during retinal ganglion cell development

The retinal ganglion cell (RGC) competence factor ATOH7 is dynamically expressed during retinal histogenesis. ATOH7 transcription is controlled by a promoter-adjacent primary enhancer and a remote shadow enhancer (SE). Deletion of the ATOH7 human SE causes nonsyndromic congenital retinal nonattachment (NCRNA) disease, characterized by optic nerve aplasia and total blindness. We used genome editing to model NCRNA in mice. Deletion of the murine SE reduces Atoh7 messenger RNA (mRNA) fivefold but does not recapitulate optic nerve loss; however, SEdel/knockout (KO) trans heterozygotes have thin optic nerves. By analyzing Atoh7 mRNA and protein levels, RGC development and survival, and chromatin landscape effects, we show that the SE ensures robust Atoh7 transcriptional output. Combining SE deletion and KO and wild-type alleles in a genotypic series, we determined the amount of Atoh7 needed to produce a normal complement of adult RGCs, and the secondary consequences of graded reductions in Atoh7 dosage. Together, these data reveal the workings of an evolutionary fail-safe, a duplicate enhancer mechanism that is hard-wired in the machinery of vertebrate retinal ganglion cell genesis.

The Spectrin-Actin-Based Periodic Cytoskeleton as a Conserved Nanoscale Scaffold and Ruler of the Neural Stem Cell Lineage

Through three-dimensional STORM super-resolution microscopy, we resolve the spectrin-actin-based membrane cytoskeleton of neural stem cells (NSCs) and NSC-derived neurons, astrocytes, and oligodendrocytes. We show that undifferentiated NSCs are capable of forming patches of locally periodic, one-dimensional (1D) membrane cytoskeleton with ~180 nm periodicity. Such periodic structures become increasingly ordered and long-ranging as the NSCs mature into terminally differentiated neuronal and glial cell types, and, during this process, distinct 1D periodic ‘‘strips’’ dominate the flat 2D membranes. Moreover, we report remarkable alignment of the periodic cytoskeletons between abutting cells at axon-axon and axon-oligodendrocyte contacts and identify two adhesion molecules, neurofascin and L1CAM, as candidates to drive this nanoscale alignment. We thus show that a conserved 1D periodic membrane cytoskeletal motif serves as a nanoscale scaffold and ruler to mediate the physical interactions between cell types of the NSC lineage.

Interference with the Cannabinoid Receptor CB1R Results in Miswiring of GnRH3 and AgRP1 Axons in Zebrafish Embryos

The G protein-coupled cannabinoid receptors type 1 (CB1R) and type 2 (CB2R), and their endocannabinoid (eCBs) ligands, have been implicated in several aspects of brain wiring during development. Here we aim to assess whether interfering with CB1R affects development, neuritogenesis and pathfinding of GnRH and AgRP neurons, forebrain neurons that control respectively reproduction and appetite. We pharmacologically and genetically interfered with CB1R in zebrafish strains with fluorescently labeled GnRH3 and the AgRP1 neurons. By applying CB1R antagonists we observed a reduced number of GnRH3 neurons, fiber misrouting and altered fasciculation. Similar phenotypes were observed by CB1R knockdown. Interfering with CB1R also resulted in a reduced number, misrouting and poor fasciculation of the AgRP1 neuron’s axonal projections. Using a bioinformatic approach followed by qPCR validation, we have attempted to link CB1R functions with known guidance and fasciculation proteins. The search identified stathmin-2, a protein controlling microtubule dynamics, previously demonstrated to be coexpressed with CB1R and now shown to be downregulated upon interference with CB1R in zebrafish. Together, these results raise the likely possibility that embryonic exposure to low doses of CB1R-interfering compounds could impact on the development of the neuroendocrine systems controlling sexual maturation, reproduction and food intake.

Mutations in MAGEL2 and L1CAM Are Associated With Congenital Hypopituitarism and Arthrogryposis

CONTEXT

Congenital hypopituitarism (CH) is rarely observed in combination with severe joint contractures (arthrogryposis). Schaaf-Yang syndrome (SHFYNG) phenotypically overlaps with Prader-Willi syndrome, with patients also manifesting arthrogryposis. L1 syndrome, a group of X-linked disorders that include hydrocephalus and lower limb spasticity, also rarely presents with arthrogryposis.

OBJECTIVE

We investigated the molecular basis underlying the combination of CH and arthrogryposis in five patients.

PATIENTS

The heterozygous p.Q666fs*47 mutation in the maternally imprinted MAGEL2 gene, previously described in multiple patients with SHFYNG, was identified in patients 1 to 4, all of whom manifested growth hormone deficiency and variable SHFYNG features, including dysmorphism, developmental delay, sleep apnea, and visual problems. Nonidentical twins (patients 2 and 3) had diabetes insipidus and macrocephaly, and patient 4 presented with ACTH insufficiency. The hemizygous L1CAM variant p.G452R, previously implicated in patients with L1 syndrome, was identified in patient 5, who presented with antenatal hydrocephalus.

RESULTS

Human embryonic expression analysis revealed MAGEL2 transcripts in the developing hypothalamus and ventral diencephalon at Carnegie stages (CSs) 19, 20, and 23 and in the Rathke pouch at CS20 and CS23. L1CAM was expressed in the developing hypothalamus, ventral diencephalon, and hindbrain (CS19, CS20, CS23), but not in the Rathke pouch.

CONCLUSION

We report MAGEL2 and L1CAM mutations in four pedigrees with variable CH and arthrogryposis. Patients presenting early in life with this combined phenotype should be examined for features of SHFYNG and/or L1 syndrome. This study highlights the association of hypothalamo-pituitary disease with MAGEL2 and L1CAM mutations.

Vascular co-option in brain metastasis

Vascular co-option by brain metastasis-initiating cells has been demonstrated as a critical step in organ colonization. The physical interaction between the cancer cell and the endothelial cell is mediated by integrins and L1CAM and could be involved in aggressive growth but also latency and immune evasion. The key involvement of vascular co-option in brain metastasis has created an emerging field that aims to identify critical targets as well as effective inhibitors with the goal of preventing brain metastases.

A Human Induced Pluripotent Stem Cell-Derived Tissue Model of a Cerebral Tract Connecting Two Cortical Regions

Cerebral tracts connect separated regions within a brain and serve as fundamental structures that support integrative brain functions. However, understanding the mechanisms of cerebral tract development, macro-circuit formation, and related disorders has been hampered by the lack of an in vitro model. Here, we developed a human stem cell-derived model of cerebral tracts, which is composed of two spheroids of cortical neurons and a robust fascicle of axons linking these spheroids reciprocally. In a microdevice, two spheroids of cerebral neurons extended axons into a microchannel between the spheroids and spontaneously formed an axon fascicle, mimicking a cerebral tract. We found that the formation of axon fascicle was significantly promoted when two spheroids extended axons toward each other compared with axons extended from only one spheroid. The two spheroids were able to communicate electrically through the axon fascicle. This model tissue could facilitate studies of cerebral tract development and diseases.

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A cerebral tract model was generated from iPS cell-derived cortical spheroids

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Two spheroids were spontaneously connected with an axon fascicle in a microdevice

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An axon fascicle electrically connected two cortical spheroids

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Knockdown of L1CAM disrupted axon fascicle formation in the model tissue

Potential of Protein-based Anti-metastatic Therapy with Serpins and Inter α-Trypsin Inhibitors

In this review we summarize the principles of anti-metastatic therapy with selected serpin family proteins, such as pigment epithelial-derived factor (PEDF) and maspin, as well as inter α-trypsin inhibitor (IαIs) light chains (bikunin) and heavy chains (ITIHs). Case-by-case, antimetastatic activity may be dependent or independent of the protease-inhibitory activity of the corresponding proteins. We discuss the incidence of target deregulation in different tumor entities, mechanisms of deregulation, context-dependent functional issues as well as in vitro and in vivo target validation studies with transfected tumor cells or recombinant protein as anti-metastatic agents. Finally, we comment on possible clinical evaluation of these proteins in adjuvant therapy.

Proteolytic cleavage of transmembrane cell adhesion molecule L1 by extracellular matrix molecule Reelin is important for mouse brain development

The cell adhesion molecule L1 and the extracellular matrix protein Reelin play crucial roles in the developing nervous system. Reelin is known to activate signalling cascades regulating neuronal migration by binding to lipoprotein receptors. However, the interaction of Reelin with adhesion molecules, such as L1, has remained poorly explored. Here, we report that full-length Reelin and its N-terminal fragments N-R2 and N-R6 bind to L1 and that full-length Reelin and its N-terminal fragment N-R6 proteolytically cleave L1 to generate an L1 fragment with a molecular mass of 80 kDa (L1-80). Expression of N-R6 and generation of L1-80 coincide in time at early developmental stages of the cerebral cortex. Reelin-mediated generation of L1-80 is involved in neurite outgrowth and in stimulation of migration of cultured cortical and cerebellar neurons. Morphological abnormalities in layer formation of the cerebral cortex of L1-deficient mice partially overlap with those of Reelin-deficient reeler mice. In utero electroporation of L1-80 into reeler embryos normalised the migration of cortical neurons in reeler embryos. The combined results indicate that the direct interaction between L1 and Reelin as well as the Reelin-mediated generation of L1-80 contribute to brain development at early developmental stages.

The immunoglobulin-like superfamily member IGSF3 is a developmentally regulated protein that controls neuronal morphogenesis

The establishment of a functional brain depends on the fine regulation and coordination of many processes, including neurogenesis, differentiation, dendritogenesis, axonogenesis, and synaptogenesis. Proteins of the immunoglobulin-like superfamily (IGSF) are major regulators during this sequence of events. Different members of this class of proteins play nonoverlapping functions at specific developmental time-points, as shown in particular by studies of the cerebellum. We have identified a member of the little studied EWI subfamily of IGSF, the protein IGSF3, as a membrane protein expressed in a neuron specific- and time-dependent manner during brain development. In the cerebellum, it is transiently found in membranes of differentiating granule cells, and is particularly concentrated at axon terminals. There it co-localizes with other IGSF proteins with well-known functions in cerebellar development: TAG-1 and L1. Functional analysis shows that IGSF3 controls the differentiation of granule cells, more precisely axonal growth and branching. Biochemical experiments demonstrate that, in the developing brain, IGSF3 is in a complex with the tetraspanin TSPAN7, a membrane protein mutated in several forms of X-linked intellectual disabilities. In cerebellar granule cells, TSPAN7 promotes axonal branching and the size of TSPAN7 clusters is increased by downregulation of IGSF3. Thus IGSF3 is a novel regulator of neuronal morphogenesis that might function through interactions with multiple partners including the tetraspanin TSPAN7. This developmentally regulated protein might thus be at the center of a new signaling pathway controlling brain development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 75-92, 2017.

Conditional deletion of L1CAM in human neurons impairs both axonal and dendritic arborization and action potential generation

Patzke et al. create human embryonic stem cell–derived neurons that enable the generation of conditional loss-of-function mutations of L1CAM. Deletion of L1CAM impairs axonal elongation, dendritic arborization, and action potential generation.

Hundreds of L1CAM gene mutations have been shown to be associated with congenital hydrocephalus, severe intellectual disability, aphasia, and motor symptoms. How such mutations impair neuronal function, however, remains unclear. Here, we generated human embryonic stem (ES) cells carrying a conditional L1CAM loss-of-function mutation and produced precisely matching control and L1CAM-deficient neurons from these ES cells. In analyzing two independent conditionally mutant ES cell clones, we found that deletion of L1CAM dramatically impaired axonal elongation and, to a lesser extent, dendritic arborization. Unexpectedly, we also detected an ∼20–50% and ∼20–30% decrease, respectively, in the levels of ankyrinG and ankyrinB protein, and observed that the size and intensity of ankyrinG staining in the axon initial segment was significantly reduced. Overexpression of wild-type L1CAM, but not of the L1CAM point mutants R1166X and S1224L, rescued the decrease in ankyrin levels. Importantly, we found that the L1CAM mutation selectively decreased activity-dependent Na⁺-currents, altered neuronal excitability, and caused impairments in action potential (AP) generation. Thus, our results suggest that the clinical presentations of L1CAM mutations in human patients could be accounted for, at least in part, by cell-autonomous changes in the functional development of neurons, such that neurons are unable to develop normal axons and dendrites and to generate normal APs.

Stathmin is enriched in the developing corticospinal tract

Understanding the intra- and extracellular proteins involved in the development of the corticospinal tract (CST) may offer insights into how the pathway could be regenerated following traumatic spinal cord injury. Currently, however, little is known about the proteome of the developing corticospinal system. The present study, therefore, has used quantitative proteomics and bioinformatics to detail the protein profile of the rat CST during its formation in the spinal cord. This analysis identified increased expression of 65 proteins during the early ingrowth of corticospinal axons into the spinal cord, and 36 proteins at the period of heightened CST growth. A majority of these proteins were involved in cellular assembly and organization, with annotations being most highly associated with cytoskeletal organization, microtubule dynamics, neurite outgrowth, and the formation, polymerization and quantity of microtubules. In addition, 22 proteins were more highly expressed within the developing CST in comparison to other developing white matter tracts of the spinal cord of age-matched animals. Of these differentially expressed proteins, only one, stathmin 1 (a protein known to be involved in microtubule dynamics), was both highly enriched in the developing CST and relatively sparse in other developing descending and ascending spinal tracts. Immunohistochemical analyses of the developing rat spinal cord and fetal human brain stem confirmed the enriched pattern of stathmin expression along the developing CST, and in vitro growth assays of rat corticospinal neurons showed a reduced length of neurite processes in response to pharmacological perturbation of stathmin activity. Combined, these findings suggest that stathmin activity may modulate axonal growth during development of the corticospinal projection, and reinforces the notion that microtubule dynamics could play an important role in the generation and regeneration of the CST.

The role of T-cadherin in axonal pathway formation in neocortical circuits

Cortical efferent and afferent fibers are arranged in a stereotyped pattern in the intermediate zone (IZ). Here, we studied the mechanism of axonal pathway formation by identifying a molecule that is expressed in a subset of cortical axons in the rat. We found that T-cadherin (T-cad), a member of the cadherin family, is expressed in deep-layer cell axons projecting to subcortical structures, but not in upper layer callosal axons projecting to the contralateral cortex. Ectopic expression of T-cad in upper layer cells induced axons to project toward subcortical structures via the upper part of the IZ. Moreover, the axons of deep-layer cells in which T-cad expression was suppressed by RNAi projected towards the contralateral cortex via an aberrant route. These results suggest that T-cad is involved in axonal pathway formation in the developing cortex.

Living scaffolds for neuroregeneration

Neural tissue engineers are exploiting key mechanisms responsible for neural cell migration and axonal path finding during embryonic development to create living scaffolds for neuroregeneration following injury and disease. These mechanisms involve the combined use of haptotactic, chemotactic, and mechanical cues to direct cell movement and re-growth. Living scaffolds provide these cues through the use of cells engineered in a predefined architecture, generally in combination with biomaterial strategies. Although several hurdles exist in the implementation of living regenerative scaffolds, there are considerable therapeutic advantages to using living cells in conjunction with biomaterials. The leading contemporary living scaffolds for neurorepair are utilizing aligned glial cells and neuronal/axonal tracts to direct regenerating axons across damaged tissue to appropriate targets, and in some cases to directly replace the function of lost cells. Future advances in technology, including the use of exogenous stimulation and genetically engineered stem cells, will further the potential of living scaffolds and drive a new era of personalized medicine for neuroregeneration.

Semaphorin 3A activates the guanosine triphosphatase Rab5 to promote growth cone collapse and organize callosal axon projections

Axon guidance (pathfinding) wires the brain during development and is regulated by various attractive and repulsive cues. Semaphorin 3A (Sema3A) is a repulsive cue, inducing the collapse of axon growth cones. In the mammalian forebrain, the corpus callosum is the major commissure that transmits information flow between the two hemispheres, and contralateral axons assemble into well-defined tracts. We found that the patterning of callosal axon projections in rodent layer II and III (L2/3) cortical neurons in response to Sema3A was mediated by the activation of Rab5, a small guanosine triphosphatase (GTPase) that mediates endocytosis, through the membrane fusion protein Rabaptin-5 and the Rab5 guanine nucleotide exchange factor (GEF) Rabex-5. Rabaptin-5 bound directly to Plexin-A1 in the Sema3A receptor complex [an obligate heterodimer formed by Plexin-A1 and neuropilin 1 (NP1)]; Sema3A enhanced this interaction in cultured neurons. Rabaptin-5 bridged the interaction between Rab5 and Plexin-A1. Sema3A stimulated endocytosis from the cell surface of callosal axon growth cones. In utero electroporation to reduce Rab5 or Rabaptin-5 impaired axon fasciculation or caused mistargeting of L2/3 callosal projections in rats. Overexpression of Rabaptin-5 or Rab5 rescued the defective callosal axon fasciculation or mistargeting of callosal axons caused by the loss of Sema3A-Plexin-A1 signaling in rats expressing dominant-negative Plexin-A1 or in NP1-deficient mice. Thus, our findings suggest that Rab5, its effector Rabaptin-5, and its regulator Rabex-5 mediate Sema3A-induced axon guidance during brain development.

Semi-automatic quantification of neurite fasciculation in high-density neurite images by the neurite directional distribution analysis (NDDA)

BACKGROUND

Bundling of neurite extensions occur during nerve development and regeneration. Understanding the factors that drive neurite bundling is important for designing biomaterials for nerve regeneration toward the innervation target and preventing nociceptive collateral sprouting. High-density neuron cultures including dorsal root ganglia explants are employed for in vitro screening of biomaterials designed to control directional outgrowth. Although some semi-automated image processing methods exist for quantification of neurite outgrowth, methods to quantify axonal fasciculation in terms of direction of neurite outgrowth are lacking.

NEW METHOD

This work presents a semi-automated program to analyze micrographs of high-density neurites; the program aims to quantify axonal fasciculation by determining the orientational distribution function of the tangent vectors of the neurites and calculating its Fourier series coefficients ('c' values).

RESULTS

We found that neurite directional distribution analysis (NDDA) of fasciculated neurites yielded 'c' values of ≥∼0.25 whereas branched outgrowth led to statistically significant lesser values of <∼0.2. The 'c' values correlated directly to the width of neurite bundles and indirectly to the number of branching points.

COMPARISON WITH EXISTING METHODS

Information about the directional distribution of outgrowth is lost in simple counting methods or achieved laboriously through manual analysis. The NDDA supplements previous quantitative analyses of axonal bundling using a vector-based approach that captures new information about the directionality of outgrowth.

CONCLUSION

The NDDA is a valuable addition to open source image processing tools available to biomedical researchers offering a robust, precise approach to quantification of imaged features important in tissue development, disease, and repair.

Serpins promote cancer cell survival and vascular co-option in brain metastasis

Brain metastasis is an ominous complication of cancer, yet most cancer cells that infiltrate the brain die of unknown causes. Here, we identify plasmin from the reactive brain stroma as a defense against metastatic invasion, and plasminogen activator (PA) inhibitory serpins in cancer cells as a shield against this defense. Plasmin suppresses brain metastasis in two ways: by converting membrane-bound astrocytic FasL into a paracrine death signal for cancer cells, and by inactivating the axon pathfinding molecule L1CAM, which metastatic cells express for spreading along brain capillaries and for metastatic outgrowth. Brain metastatic cells from lung cancer and breast cancer express high levels of anti-PA serpins, including neuroserpin and serpin B2, to prevent plasmin generation and its metastasis-suppressive effects. By protecting cancer cells from death signals and fostering vascular co-option, anti-PA serpins provide a unifying mechanism for the initiation of brain metastasis in lung and breast cancers.

Loss of neural recognition molecule NB-3 delays the normal projection and terminal branching of developing corticospinal tract axons in the mouse

Neural recognition molecule NB-3 is involved in neural development and synapse formation. However, its role in axon tract formation is unclear. In this study, we found that the temporal expression of NB-3 in the deep layers of the motor cortex in mice was coincident with the development of the corticospinal tract (CST). Clear NB-3 immunoreactivity in the CST trajectory strongly suggested that NB-3 was expressed specifically in projecting CST axons. By tracing CST axons in NB-3−/− mice at different developmental stages, we found that these axons were capable of projecting and forming a normal trajectory. However, the projection was greatly delayed in NB-3−/− mice compared with wild-type (WT) mice from the embryonic to postnatal stages, a period that is coincident with the completion of the CST projection in mice. Subsequently, although their projection was delayed, CST axons in NB-3−/− mice gradually completed a normal projection. By stage P21, the characteristics of CST projections in NB-3−/− mice were not statistically different from those in WT mice. In addition, we found that the branching of CST axons into spinal gray matter also was delayed in NB-3−/− mice. The CST innervation area in the spinal gray matter of NB-3−/− mice was greatly reduced in comparison with WT mice until P30 and gradually became normal by P45. These data suggest that NB-3 is involved in the normal projection and terminal branching of developing CST axons.

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Neural cell adhesion molecule, NCAM, regulates thalamocortical axon pathfinding and the organization of the cortical somatosensory representation in mouse

To study the potential role of neural cell adhesion molecule (NCAM) in the development of thalamocortical (TC) axon topography, wild type, and NCAM null mutant mice were analyzed for NCAM expression, projection, and targeting of TC afferents within the somatosensory area of the neocortex. Here we report that NCAM and its α-2,8-linked polysialic acid (PSA) are expressed in developing TC axons during projection to the neocortex. Pathfinding of TC axons in wild type and null mutant mice was mapped using anterograde DiI labeling. At embryonic day E16.5, null mutant mice displayed misguided TC axons in the dorsal telencephalon, but not in the ventral telencephalon, an intermediate target that initially sorts TC axons toward correct neocortical areas. During the early postnatal period, rostrolateral TC axons within the internal capsule along the ventral telencephalon adopted distorted trajectories in the ventral telencephalon and failed to reach the neocortex in NCAM null mutant animals. NCAM null mutants showed abnormal segregation of layer IV barrels in a restricted portion of the somatosensory cortex. As shown by Nissl and cytochrome oxidase staining, barrels of the anterolateral barrel subfield (ALBSF) and the most distal barrels of the posteromedial barrel subfield (PMBSF) did not segregate properly in null mutant mice. These results indicate a novel role for NCAM in axonal pathfinding and topographic sorting of TC axons, which may be important for the function of specific territories of sensory representation in the somatosensory cortex.

Pinceau organization in the cerebellum requires distinct functions of neurofascin in Purkinje and basket neurons during postnatal development

Basket axon collaterals synapse onto the Purkinje soma/axon initial segment (AIS) area to form specialized structures, the pinceau, which are critical for normal cerebellar function. Mechanistic details of how the pinceau become organized during cerebellar development are poorly understood. Loss of cytoskeletal adaptor protein Ankyrin G (AnkG) results in mislocalization of the cell adhesion molecule Neurofascin (Nfasc) at the Purkinje AIS and abnormal organization of the pinceau. Loss of Nfasc in adult Purkinje neurons leads to slow disorganization of the Purkinje AIS and pinceau morphology. Here, we used mouse conditional knock-out techniques to show that selective loss of Nfasc, specifically in Purkinje neurons during early development, prevented maturation of the AIS and resulted in loss of Purkinje neuron spontaneous activity and pinceau disorganization. Loss of Nfasc in both Purkinje and basket neurons caused abnormal basket axon collateral branching and targeting to Purkinje soma/AIS, leading to extensive pinceau disorganization, Purkinje neuron degeneration, and severe ataxia. Our studies reveal that the Purkinje Nfasc is required for AIS maturation and for maintaining stable contacts between basket axon terminals and the Purkinje AIS during pinceau organization, while the basket neuron Nfasc in combination with Purkinje Nfasc is required for proper basket axon collateral outgrowth and targeting to Purkinje soma/AIS. Thus, cerebellar pinceau organization requires coordinated mechanisms involving specific Nfasc functions in both Purkinje and basket neurons.

The expression and activity of β-catenin in the thalamus and its projections to the cerebral cortex in the mouse embryo

Background

The mammalian thalamus relays sensory information from the periphery to the cerebral cortex for cognitive processing via the thalamocortical tract. The thalamocortical tract forms during embryonic development controlled by mechanisms that are not fully understood. β-catenin is a nuclear and cytosolic protein that transduces signals from secreted signaling molecules to regulate both cell motility via the cytoskeleton and gene expression in the nucleus. In this study we tested whether β-catenin is likely to play a role in thalamocortical connectivity by examining its expression and activity in developing thalamic neurons and their axons.

Results

At embryonic day (E)15.5, the time when thalamocortical axonal projections are forming, we found that the thalamus is a site of particularly high β-catenin mRNA and protein expression. As well as being expressed at high levels in thalamic cell bodies, β-catenin protein is enriched in the axons and growth cones of thalamic axons and its growth cone concentration is sensitive to Netrin-1. Using mice carrying the β-catenin reporter BAT-gal we find high levels of reporter activity in the thalamus. Further, Netrin-1 induces BAT-gal reporter expression and upregulates levels of endogenous transcripts encoding β-actin and L1 proteins in cultured thalamic cells. We found that β-catenin mRNA is enriched in thalamic axons and its 3'UTR is phylogenetically conserved and is able to direct heterologous mRNAs along the thalamic axon, where they can be translated.

Conclusion

We provide evidence that β-catenin protein is likely to be an important player in thalamocortcial development. It is abundant both in the nucleus and in the growth cones of post-mitotic thalamic cells during the development of thalamocortical connectivity and β-catenin mRNA is targeted to thalamic axons and growth cones where it could potentially be translated. β-catenin is involved in transducing the Netrin-1 signal to thalamic cells suggesting a mechanism by which Netrin-1 guides thalamocortical development.

The role of NrCAM in neural development and disorders--beyond a simple glue in the brain

NrCAM is a neuronal cell adhesion molecule of the L1 family of immunoglobulin super family. It plays a wide variety of roles in neural development, including cell proliferation and differentiation, axon growth and guidance, synapse formation, and the formation of the myelinated nerve structure. NrCAM functions in cell adhesion and modulates signaling pathways in neural development through multiple molecular interactions with guidance and other factors. Alterations in NrCAM structure/expression are associated with psychiatric disorders such as autism and drug addiction and with tumor progression. The mechanisms of NrCAM participation in development and how these might be perturbed in disorders are reviewed.

Human L1CAM carrying the missense mutations of the fibronectin-like type III domains is localized in the endoplasmic reticulum and degraded by polyubiquitylation

Any mutations in the human neural cell adhesion molecule L1 (hL1CAM) gene might cause various types of serious neurological syndromes in humans, characterized by increased mortality, mental retardation, and various malformations of the nervous system. Such missense mutations often cause severe abnormalities or even fatalities, and the reason for this may be a disruption of the adhesive function of L1CAM resulting from a misdirection of the degradative pathway. Transfection studies using neuroblastoma N2a cells demonstrated that hL1CAM carrying the missense mutations in the fibronectin-like type III (FnIII) domains most likely is located within the endoplasmic reticulum (ER), but it is less well expressed on the cell surface. One mutant, L935P, in the fourth FnIII domain, was chosen from six mutants (K655 and G698 at Fn1, L935P and P941 at Fn4, W1036 and Y1070 at Fn5) in the FnIII domains to study in detail the functions of hL1CAM(200 kDa) , such as the intracellular traffic and degradation, because only a single band at 200 kDa was detected in the hL1CAM(L935P) -transfected cells. hL1CAM(200 kDa) is expressed predominantly in the ER but not on the cell surface. In addition, this missense mutated hL1CAM(200 kDa) is polyubiquitylated at some sites in the extracellular domain and thus becomes degraded by proteasomes via the ER-associated degradation pathway. These observations demonstrate that the missense mutations of hL1CAM in the FnIII domain may cause the resultant pathogenesis because of a loss of expression on the cell surface resulting from misrouting to the degradative pathway.

NrCAM deletion causes topographic mistargeting of thalamocortical axons to the visual cortex and disrupts visual acuity

NrCAM is a neural cell adhesion molecule of the L1 family that has been linked to autism spectrum disorders, a disease spectrum in which abnormal thalamocortical connectivity may contribute to visual processing defects. Here we show that NrCAM interaction with neuropilin-2 (Npn-2) is critical for semaphorin 3F (Sema3F)-induced guidance of thalamocortical axon subpopulations at the ventral telencephalon (VTe), an intermediate target for thalamic axon sorting. Genetic deletion of NrCAM or Npn-2 caused contingents of embryonic thalamic axons to misproject caudally in the VTe. The resultant thalamocortical map of NrCAM-null mutants showed striking mistargeting of motor and somatosensory thalamic axon contingents to the primary visual cortex, but retinogeniculate targeting and segregation were normal. NrCAM formed a molecular complex with Npn-2 in brain and neural cells, and was required for Sema3F-induced growth cone collapse in thalamic neuron cultures, consistent with a vital function for NrCAM in Sema3F-induced axon repulsion. NrCAM-null mice displayed reduced responses to visual evoked potentials recorded from layer IV in the binocular zone of primary visual cortex (V1), particularly when evoked from the ipsilateral eye, indicating abnormal visual acuity and ocularity. These results demonstrate that NrCAM is required for normal maturation of cortical visual acuity, and suggest that the aberrant projection of thalamic motor and somatosensory axons to the visual cortex in NrCAM-null mutant mice impairs cortical functions.

Engineered in vitro/in silico models to examine neurite target preference

Research on spinal cord injury (SCI) repair focuses on developing mechanisms to allow neurites to grow past an injury site. In this article, we observe that numerous divergent paths (i.e., spinal roots) are present along the spinal column, and hence guidance strategies must be devised to ensure that regrowing neurites reach viable targets. Therefore, we have engineered an in vitro micropatterned model in which cultured E7 dorsal root ganglia (DRG) explants may enter alternate pathways (?roots?) along a branching micropattern. Alongside this in vitro model, we have developed an in silico simulation that we validate by comparison with independent experiments. We find in both in silico and in vitro models that the probability of a neurite entering a given root decreases exponentially with respect to the number of roots away from the DRG; consequently, the likelihood of neurites reaching a distant root can be vanishingly small. This result represents a starting point for future strategies to optimize the likelihood that neurites will reach appropriate targets in the regenerating nervous system, and provides a new computational tool to evaluate the feasibility and expected success of neurite guidance in complex geometries.

Polarized targeting of L1-CAM regulates axonal and dendritic bundling in vitro

Proper axonal and dendritic bundling is essential for the establishment of neuronal connections and the synchronization of synaptic inputs, respectively. Cell adhesion molecules of the L1-CAM (L1-cell adhesion molecule) family regulate axon guidance and fasciculation, neuron migration, dendrite morphology, and synaptic plasticity. It remains unclear how these molecules play so many different roles. Here we show that polarized axon-dendrite targeting of an avian L1-CAM protein, NgCAM (neuron-glia cell adhesion molecule), can regulate the switch of bundling of the two major compartments of rat hippocampal neurons. Using a new in-vitro model for studying neurite-neurite interactions, we found that expressed axonal NgCAM induced robust axonal bundling via the trans-homophilic interaction of immunoglobulin domains. Interestingly, dendritic bundling was induced by the dendritic targeting of NgCAM, caused by either deleting its fibronectin repeats or blocking activities of protein kinases. Consistent with the NgCAM results, expression of mouse L1-CAM also induced axonal bundling and blocking kinase activities disrupted its axonal targeting. Furthermore, the trans-homophilic interaction stabilized the bundle formation, probably through recruiting NgCAM proteins to contact sites and promoting guided axon outgrowth. Taken together, our results suggest that precise localization of L1-CAM is important for establishing proper cell-cell contacts in neural circuits.

Human genetic disorders of axon guidance

This article reviews symptoms and signs of aberrant axon connectivity in humans, and summarizes major human genetic disorders that result, or have been proposed to result, from defective axon guidance. These include corpus callosum agenesis, L1 syndrome, Joubert syndrome and related disorders, horizontal gaze palsy with progressive scoliosis, Kallmann syndrome, albinism, congenital fibrosis of the extraocular muscles type 1, Duane retraction syndrome, and pontine tegmental cap dysplasia. Genes mutated in these disorders can encode axon growth cone ligands and receptors, downstream signaling molecules, and axon transport motors, as well as proteins without currently recognized roles in axon guidance. Advances in neuroimaging and genetic techniques have the potential to rapidly expand this field, and it is feasible that axon guidance disorders will soon be recognized as a new and significant category of human neurodevelopmental disorders.

Fasciculation and defasciculation of neurite bundles on micropatterned substrates

We describe experiments of fasciculation, i.e., bundling, of chick sensory neurites on 2D striped substrates. By Fourier decomposition, we separate left-going and right-going neurite components from in vitro images, and we find first that neurite bundles orient toward preferred angles with respect to the stripe direction, and second that in vitro bundles travel in leftward and rightward directions nearly uninterrupted by crossings of bundles traveling in the opposing direction. We explore mechanisms that lead to these behaviors, and summarize implications for future models for neurite outgrowth and guidance.

Requirement of cannabinoid CB(1) receptors in cortical pyramidal neurons for appropriate development of corticothalamic and thalamocortical projections

A role for endocannabinoid signaling in neuronal morphogenesis as the brain develops has recently been suggested. Here we used the developing somatosensory circuit as a model system to examine the role of endocannabinoid signaling in neural circuit formation. We first show that a deficiency in cannabinoid receptor type 1 (CB(1)R), but not G-protein-coupled receptor 55 (GPR55), leads to aberrant fasciculation and pathfinding in both corticothalamic and thalamocortical axons despite normal target recognition. Next, we localized CB(1)R expression to developing corticothalamic projections and found little if any expression in thalamocortical axons, using a newly established reporter mouse expressing GFP in thalamocortical projections. A similar thalamocortical projection phenotype was observed following removal of CB(1)R from cortical principal neurons, clearly demonstrating that CB(1)R in corticothalamic axons was required to instruct their complimentary connections, thalamocortical axons. When reciprocal thalamic and cortical connections meet, CB(1)R-containing corticothalamic axons are intimately associated with elongating thalamocortical projections containing DGLβ, a 2-arachidonoyl glycerol (2-AG) synthesizing enzyme. Thus, 2-AG produced in thalamocortical axons and acting at CB(1)Rs on corticothalamic axons is likely to modulate axonal patterning. The presence of monoglyceride lipase, a 2-AG degrading enzyme, in both thalamocortical and corticothalamic tracts probably serves to restrict 2-AG availability. In summary, our study provides strong evidence that endocannabinoids are a modulator for the proposed 'handshake' interactions between corticothalamic and thalamocortical axons, especially for fasciculation. These findings are important in understanding the long-term consequences of alterations in CB(1)R activity during development, a potential etiology for the mental health disorders linked to prenatal cannabis use.

Podocalyxin is a novel polysialylated neural adhesion protein with multiple roles in neural development and synapse formation

Neural development and plasticity are regulated by neural adhesion proteins, including the polysialylated form of NCAM (PSA-NCAM). Podocalyxin (PC) is a renal PSA-containing protein that has been reported to function as an anti-adhesin in kidney podocytes. Here we show that PC is widely expressed in neurons during neural development. Neural PC interacts with the ERM protein family, and with NHERF1/2 and RhoA/G. Experiments in vitro and phenotypic analyses of podxl-deficient mice indicate that PC is involved in neurite growth, branching and axonal fasciculation, and that PC loss-of-function reduces the number of synapses in the CNS and in the neuromuscular system. We also show that whereas some of the brain PC functions require PSA, others depend on PC per se. Our results show that PC, the second highly sialylated neural adhesion protein, plays multiple roles in neural development.

Cellular strategies of axonal pathfinding

Axons follow highly stereotyped and reproducible trajectories to their targets. In this review we address the properties of the first pioneer neurons to grow in the developing nervous system and what has been learned over the past several decades about the extracellular and cell surface substrata on which axons grow. We then discuss the types of guidance cues and their receptors that influence axon extension, what determines where cues are expressed, and how axons respond to the cues they encounter in their environment.

L1 and CHL1 Cooperate in Thalamocortical Axon Targeting

Neural cell adhesion molecule close homolog of L1 (CHL1) is a regulator of topographic targeting of thalamic axons to the somatosensory cortex (S1) but little is known about its cooperation with other L1 class molecules. To investigate this, CHL1(-/-)/L1(-/y) double mutant mice were generated and analyzed for thalamocortical axon topography. Double mutants exhibited a striking posterior shift of axons from motor thalamic nuclei to the visual cortex (V1), which was not observed in single mutants. In wild-type (WT) embryos, L1 and CHL1 were coexpressed in the dorsal thalamus (DT) and on fibers along the thalamocortical projection in the ventral telencephalon and cortex. L1 and CHL1 colocalized on growth cones and neurites of cortical and thalamic neurons in culture. Growth cone collapse assays with WT and mutant neurons demonstrated a requirement for L1 and CHL1 in repellent responses to EphrinA5, a guidance factor for thalamic axons. L1 coimmunoprecipitated with the principal EphrinA5 receptors expressed in the DT (EphA3, EphA4, and EphA7), whereas CHL1 associated selectively with EphA7. These results implicate a novel mechanism in which L1 and CHL1 interact with individual EphA receptors and cooperate to guide subpopulations of thalamic axons to distinct neocortical areas essential for thalamocortical connectivity.

Role of the cytoplasmic domain of the L1 cell adhesion molecule in brain development

Mutations in the human L1CAM gene cause X-linked hydrocephalus and MASA (Mental retardation, Aphasia, Shuffling gait, Adducted thumbs) syndrome. In vitro studies have shown that the L1 cytoplasmic domain (L1CD) is involved in L1 trafficking, neurite branching, signaling, and interactions with the cytoskeleton. L1cam knockout (L1(KO)) mice have hydrocephalus, a small cerebellum, hyperfasciculation of corticothalamic tracts, and abnormal peripheral nerves. To explore the function of the L1CD, we made three new mice lines in which different parts of the L1CD have been altered. In all mutant lines L1 protein is expressed and transported into the axon. Interestingly, these new L1CD mutant lines display normal brain morphology. However, the expression of L1 protein in the adult is dramatically reduced in the two L1CD mutant lines that lack the ankyrin-binding region and they show defects in motor function. Therefore, the L1CD is not responsible for the major defects observed in L1(KO) mice, yet it is required for continued L1 protein expression and motor function in the adult.

Nuclear factor I-A represses expression of the cell adhesion molecule L1

BACKGROUND

The neural cell adhesion molecule L1 plays a crucial role in development and plasticity of the nervous system. Neural cells thus require precise control of L1 expression.

RESULTS

We identified a full binding site for nuclear factor I (NFI) transcription factors in the regulatory region of the mouse L1 gene. Electrophoretic mobility shift assay (EMSA) showed binding of nuclear factor I-A (NFI-A) to this site. Moreover, for a brain-specific isoform of NFI-A (NFI-A bs), we confirmed the interaction in vivo using chromatin immunoprecipitation (ChIP). Reporter gene assays showed that in neuroblastoma cells, overexpression of NFI-A bs repressed L1 expression threefold.

CONCLUSION

Our findings suggest that NFI-A, in particular its brain-specific isoform, represses L1 gene expression, and might act as a second silencer of L1 in addition to the neural restrictive silencer factor (NRSF).

The role of cell adhesion molecules in axon growth and guidance

During development, axons elongate along the correct path toward their final targets. Growing axons maintain adhesive interactions with specific environmental cues via cell adhesion molecules (CAMs). The axon-environment adhesion must be dynamically controlled, both temporally and spatially, to enable the axons to navigate and migrate correctly. In this way, CAMs play a central role in mediating contact-dependent regulation of motile behavior of the axons. This chapter examines the mechanisms underlying how CAMs control axon growth and guidance, with a particular focus on intracellular signaling, trafficking, and interactions with the actin cytoskeleton.

Genetic deletion of Lsamp causes exaggerated behavioral activation in novel environments

The limbic system-associated membrane protein (LAMP) is a GPI-anchored cell adhesion molecule expressed heavily in limbic and limbic-associated regions of the developing and adult brain. Experimental studies show that LAMP promotes the growth of limbic neurons and guides the projections of limbic fibers. In order to examine the functional consequences of disrupting limbic circuit assembly, we generated a mouse line in which the Lsamp gene encoding LAMP was deleted. Basic neuroanatomical organization and sensory and motor development are normal in Lsamp(-/-) mice. The most profound change in behavior in both male and female Lsamp(-/-) mice is a heightened reactivity to novelty exhibited in several behavioral tests. Lsamp(-/-) mice display hyperactivity in a novel arena and both sexes habituate to the same activity levels as their wild type littermates, but at different rates. In the elevated plus maze, Lsamp(-/-) mice exhibit increased total arm entries, with a bias towards the open arms; they spend more time in the open arms and have a substantial increase in the amount of risk assessment in unprotected areas of the maze. In the y-maze, Lsamp(-/-) mice exhibit characteristic hyperactivity and a decreased level of spontaneous alternation during the period when their novelty-induced hyperactivity is at its peak. We hypothesize that Lsamp(-/-) mice may not simply exhibit a decrease in anxiety, but may have a heightened, and possibly maladaptive, response to novel environmental stressors. Genetic deletion of Lsamp may thus cause circumscribed changes in the fine connectivity of specific circuits that underlie these behaviors.

Close homolog of L1 and neuropilin 1 mediate guidance of thalamocortical axons at the ventral telencephalon

We report a cooperation between the neural adhesion molecule close homolog of L1 (CHL1) and the semaphorin 3A (Sema3A) receptor, neuropilin 1 (Npn1), important for establishment of area-specific thalamocortical projections. CHL1 deletion in mice selectively disrupted the projection of somatosensory thalamic axons from the ventrobasal (VB) nuclei, causing them to shift caudally and target the visual cortex. At the ventral telencephalon, an intermediate target with graded Sema3A expression, VB axons were caudally shifted in CHL1- embryos and in Npn1(Sema-/-) mutants, in which axons are nonresponsive to Sema3A. CHL1 colocalized with Npn1 on thalamic axons, and associated with Npn1 through a sequence in the CHL1 Ig1 domain that was required for Sema3A-induced growth cone collapse. These results identify a novel function for CHL1 in thalamic axon responsiveness to ventral telencephalic cues, and demonstrate a role for CHL1 and Npn1 in establishment of proper targeting of specific thalamocortical projections.

Alteration of the cell adhesion molecule L1 expression in a specific subset of primary afferent neurons contributes to neuropathic pain

The cell adhesion molecule L1 (L1-CAM) plays important functional roles in the developing and adult nervous systems. Here we show that peripheral nerve injury induced dynamic post-transcriptional alteration of L1-CAM in the rat dorsal root ganglia (DRGs) and spinal cord. Sciatic nerve transection (SCNT) changed the expression of L1-CAM protein but not L1-CAM mRNA. In DRGs, SCNT induced accumulation of the L1-CAM into the surface of somata, which resulted in the formation of immunoreactive ring structures in a number of unmyelinated C-fiber neurons. These neurons with L1-CAM-immunoreactive ring structures were heavily colocalized with phosphorylated p38 MAPK. Western blot analysis revealed the increase of full-length L1-CAM and decrease of fragments of L1-CAM after SCNT in DRGs. Following SCNT, L1-CAM-immunoreactive profiles in the dorsal horn showed an increase mainly in pre-synaptic areas of laminae I–II with a delayed onset and colocalized with growth-associated protein 43. In contrast to DRGs, SCNT increased the proteolytic 80-kDa fragment of L1-CAM and decreased full-length L1-CAM in the spinal cord. The intrathecal injection of L1-CAM antibody for the extracellular domain of L1-CAM inhibited activation of p38 MAPK and emergence of ring structures of L1-CAM immunoreactivity in injured DRG neurons. Moreover, inhibition of extracellular L1-CAM binding by intrathecal administration of antibody suppressed the mechanical allodynia and thermal hyperalgesia induced by partial SCNT. Collectively, these data suggest that the modification of L1-CAM in nociceptive pathways might be an important pathomechanism of neuropathic pain.

Serotonin modulates the response of embryonic thalamocortical axons to netrin-1

Modifying serotonin (5-HT) abundance in the embryonic mouse brain disrupts the precision of sensory maps formed by thalamocortical axons (TCAs), suggesting that 5-HT influences their growth. We investigated the mechanism by which 5-HT influences TCAs during development. 5-HT(1B) and 5-HT(1D) receptor expression in the fetal forebrain overlaps with that of the axon guidance receptors DCC and Unc5c. In coculture assays, axons originating from anterior and posterior halves of the embryonic day 14.5 dorsal thalamus responded differently to netrin-1, reflecting the patterns of DCC and Unc5c expression. 5-HT converts the attraction exerted by netrin-1 on posterior TCAs to repulsion. Pharmacological manipulation of 5-HT(1B/1D) receptors and intracellular cAMP showed the signaling cascade through which this modulation occurs. An in vivo correlate of altered TCA pathfinding was obtained by transient manipulation of 5-HT(1B/1D) receptor expression abundance in the dorsal thalamus by in utero electroporation. These data demonstrate that serotonergic signaling has a previously unrecognized role in the modulation of axonal responsiveness to a classic guidance cue.

Neural recognition molecules of the immunoglobulin superfamily: signaling transducers of axon guidance and neuronal migration

Recognition molecules of the immunoglobulin superfamily have important roles in neuronal interactions during ontogeny, including migration, survival, axon guidance and synaptic targeting. Their downstream signal transduction events specify whether a cell changes its place of residence or projects axons and dendrites to targets in the brain, allowing the construction of a dynamic neural network. A wealth of recent discoveries shows that cell adhesion molecules interact with attractant and repellent guidance receptors to control growth cone and cell motility in a coordinate fashion. We focus on the best-studied subclasses, the neural cell adhesion molecule NCAM and the L1 family of adhesion molecules, which share important structural and functional features. We have chosen these paradigmatic molecules and their interactions with other recognition molecules as instructive for elucidating the mechanisms by which other recognition molecules may guide cell interactions during development or modify their function as a result of injury, learning and memory.

The role of neuronal complexes in human X-linked brain diseases

Beyond finding individual genes that are involved in medical disorders, an important challenge is the integration of sets of disease genes with the complexities of basic biological processes. We examine this issue by focusing on neuronal multiprotein complexes and their components encoded on the human X chromosome. Multiprotein signaling complexes in the postsynaptic terminal of central nervous system synapses are essential for the induction of neuronal plasticity and cognitive processes in animals. The prototype complex is the N-methyl-D-aspartate receptor complex/membrane-associated guanylate kinase-associated signaling complex (NRC/MASC) comprising 185 proteins and embedded within the postsynaptic density (PSD), which is a set of complexes totaling approximately 1,100 proteins. It is striking that 86% (6 of 7) of X-linked NRC/MASC genes and 49% (19 of 39) of X-chromosomal PSD genes are already known to be involved in human psychiatric disorders. Moreover, of the 69 known proteins mutated in X-linked mental retardation, 19 (28%) encode postsynaptic proteins. The high incidence of involvement in cognitive disorders is also found in mouse mutants and indicates that the complexes are functioning as integrated entities or molecular machines and that disruption of different components impairs their overall role in cognitive processes. We also noticed that NRC/MASC genes appear to be more strongly associated with mental retardation and autism spectrum disorders. We propose that systematic studies of PSD and NRC/MASC genes in mice and humans will give a high yield of novel genes important for human disease and new mechanistic insights into higher cognitive functions.

Mice lacking L1 cell adhesion molecule have deficits in locomotion and exhibit enhanced corticospinal tract sprouting following mild contusion injury to the spinal cord

L1 is a member of the immunoglobulin superfamily of cell adhesion molecules that is associated with axonal growth, including formation of the corticospinal tract (CST). The present study describes the effects of L1 deletion on hindlimb function in locomotion, and examines the role of L1 in recovery and remodeling after contusive spinal cord injury (SCI) in mice. Uninjured adult L1 knockout (Y/-) mice had impaired performance on locomotor tests compared with their wild-type littermates (Y/+). Anterograde tracing demonstrated that CST axons project to thoracic, but not lumbar, levels of the spinal cord of Y/- mice, and revealed a diversion of these fibers from their position in the base of the dorsal columns. Retrograde tracing also revealed reduced numbers of descending projections from paraventricular hypothalamus and red nuclei to the lumbar spinal cord in Y/- mice. SCI at the mid-thoracic level produced a lesion encompassing the center of the spinal cord, including the site of the dorsal CST and surrounding gray matter (GM). The injury caused lasting deficits in fine aspects of locomotion. There was no effect of genotype on final lesion size or the growth of axons into the lesion area. However, injured Y/- mice demonstrated a robust expansion of CST projections throughout the GM of the cervical and thoracic spinal cord rostral to the lesion compared with Y/+ littermates. Thus, L1 is important for the development of multiple spinal projections and also contributes to the restriction of CST sprouting rostral to the site of a SCI in adults.