Netrin-1 Role in Nociceptive Neuron Sprouting through Activation of DCC Signaling in a Rat Model of Bone Cancer Pain

BACKGROUND

Bone cancer pain (BCP) is a common primary or metastatic bone cancer complication. Netrin-1 plays an essential role in neurite elongation and pain sensitization. This study aimed to determine the role of netrin-1 from the metastatic bone microenvironment in BCP development and identify the associated signaling pathway for the strategy of BCP management.

METHODS

The rat BCP model was established by intratibial implantation of Walker 256 cells. Von Frey filaments measured the mechanical pain threshold. Movement-induced pain was assessed using limb use scores. Expressions of associated molecules in the affected tibias or dorsal root ganglia (DRG) were measured by immunofluorescence, immunohistochemistry, real-time quantitative polymerase chain reaction, or western blotting. Transduction of deleted in colorectal cancer (DCC) signaling was inhibited by intrathecal injection of DCC-siRNA.

RESULTS

In BCP rats, the presence of calcitonin gene-related peptide (CGRP)-positive nerve fibers increased in the metastatic bone lesions. The metastatic site showed enrichment of well-differentiated osteoclasts and expressions of netrin-1 and its attractive receptor DCC. Upregulation of DCC and increased phosphorylation levels of focal adhesion kinase (FAK) and Rac family small GTPase 1/Cell division cycle 42 (Rac1/Cdc42) were found in the DRG. Intrathecal administration of DCC-siRNA led to a significant reduction in FAK and Rac1/Cdc42 phosphorylation levels in the DRG, decreased nociceptive nerve innervation, and improved pain behaviors.

CONCLUSIONS

Netrin-1 may contribute to the activation of the BCP by inducing nociceptive nerve innervation and improving pain behaviors.

Activation of Multiple Eph Receptors on Neuronal Membranes Correlates with The Onset of Traumatic Optic Neuropathy

Background

Optic neuropathy (ON) is a major cause of irreversible blindness, yet the molecular determinants that contribute to neuronal demise have not been fully elucidated. Several studies have identified 'ephrin signaling' as one of the most dysregulated pathways in the early pathophysiology of ON with varied etiologies. Developmentally, gradients in ephrin signaling coordinate retinotopic mapping via repulsive modulation of cytoskeletal dynamics in neuronal membranes. Little is known about the role ephrin signaling played in the post-natal visual system and its correlation with the onset of optic neuropathy.

Methods

Postnatal mouse retinas were collected for mass spectrometry analysis for Eph receptors. Optic nerve crush (ONC) model was employed to induce optic neuropathy, and proteomic changes during the acute phase of neuropathic onset were analyzed. Confocal and super-resolution microscopy determined the cellular localization of activated Eph receptors after ONC injury. Eph receptor inhibitors assessed the neuroprotective effect of ephrin signaling modulation.

Results

Mass spectrometry revealed expression of seven Eph receptors (EphA2, A4, A5, B1, B2, B3, and B6) in postnatal mouse retinal tissue. Immunoblotting analysis indicated a significant increase in phosphorylation of these Eph receptors 48 hours after ONC. Confocal microscopy demonstrated the presence of both subclasses of Eph receptors in the inner retinal layers. STORM super-resolution imaging combined with optimal transport colocalization analysis revealed a significant co-localization of activated Eph receptors with injured neuronal processes, compared to uninjured neuronal and/or injured glial cells, 48 hours post-ONC. Eph receptor inhibitors displayed notable neuroprotective effects after 6 days of ONC injury.

Conclusions

Our findings demonstrate the functional presence of diverse Eph receptors in the postnatal mammalian retina, capable of modulating multiple biological processes. Pan-Eph receptor activation contributes to the onset of neuropathy in ONs, with preferential activation of Eph receptors on neuronal processes in the inner retina following optic nerve injury. Notably, Eph receptor activation precedes neuronal loss. We observed neuroprotective effects upon inhibiting Eph receptors. Our study highlights the importance of investigating this repulsive pathway in early optic neuropathies and provides a comprehensive characterization of the receptors present in the developed retina of mice, relevant to both homeostasis and disease processes.

Axonal Guidance Using Biofunctionalized Straining Flow Spinning Regenerated Silk Fibroin Fibers as Scaffold

After an injury, the limited regenerative capacity of the central nervous system makes the reconnection and functional recovery of the affected nervous tissue almost impossible. To address this problem, biomaterials appear as a promising option for the design of scaffolds that promote and guide this regenerative process. Based on previous seminal works on the ability of regenerated silk fibroin fibers spun through the straining flow spinning (SFS) technique, this study is intended to show that the usage of functionalized SFS fibers allows an enhancement of the guidance ability of the material when compared with the control (nonfunctionalized) fibers. It is shown that the axons of the neurons not only tend to follow the path marked by the fibers, in contrast to the isotropic growth observed on conventional culture plates, but also that this guidance can be further modulated through the biofunctionalization of the material with adhesion peptides. Establishing the guidance ability of these fibers opens the possibility of their use as implants for spinal cord injuries, so that they may represent the core of a therapy that would allow the reconnection of the injured ends of the spinal cord.

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

Altered Circulating microRNAs in Patients with Diabetic Neuropathy and Corneal Nerve Loss: A Pilot Study

An alteration in circulating miRNAs may have important diagnostic and therapeutic relevance in diabetic neuropathy. Patients with type 2 diabetes mellitus (T2DM) underwent an assessment of neuropathic symptoms using Douleur Neuropathique 4 (DN4), the vibration perception threshold (VPT) using a Neurothesiometer, sudomotor function using the Sudoscan, corneal nerve morphology using corneal confocal microscopy (CCM) and circulating miRNAs using high-throughput miRNA expression profiling. Patients with T2DM, with (n = 9) and without (n = 7) significant corneal nerve loss were comparable in age, gender, diabetes duration, BMI, HbA1c, eGFR, blood pressure, and lipid profile. The VPT was significantly higher (p < 0.05), and electrochemical skin conductance (p < 0.05), corneal nerve fiber density (p = 0.001), corneal nerve branch density (p = 0.013), and corneal nerve fiber length (p < 0.001) were significantly lower in T2DM patients with corneal nerve loss compared to those without corneal nerve loss. Following a q-PCR-based analysis of total plasma microRNAs, we found that miR-92b-3p (p = 0.008) was significantly downregulated, while miR-22-3p (p = 0.0001) was significantly upregulated in T2DM patients with corneal nerve loss. A network analysis revealed that these miRNAs regulate axonal guidance and neuroinflammation genes. These data support the need for more extensive studies to better understand the role of dysregulated miRNAs’ in diabetic neuropathy.

The iPSC perspective on schizophrenia

Over a decade of schizophrenia research using human induced pluripotent stem cell (iPSC)-derived neural models has provided substantial data describing neurobiological characteristics of the disorder in vitro. Simultaneously, translation of the results into general mechanistic concepts underlying schizophrenia pathophysiology has been trailing behind. Given that modeling brain function using cell cultures is challenging, the gap between the in vitro models and schizophrenia as a clinical disorder has remained wide. In this review, we highlight reproducible findings and emerging trends in recent schizophrenia-related iPSC studies. We illuminate the relevance of the results in the context of human brain development, with a focus on processes coinciding with critical developmental periods for schizophrenia.

CRMP4-mediated fornix development involves Semaphorin-3E signaling pathway

Neurodevelopmental axonal pathfinding plays a central role in correct brain wiring and subsequent cognitive abilities. Within the growth cone, various intracellular effectors transduce axonal guidance signals by remodeling the cytoskeleton. Semaphorin-3E (Sema3E) is a guidance cue implicated in development of the fornix, a neuronal tract connecting the hippocampus to the hypothalamus. Microtubule-associated protein 6 (MAP6) has been shown to be involved in the Sema3E growth-promoting signaling pathway. In this study, we identified the collapsin response mediator protein 4 (CRMP4) as a MAP6 partner and a crucial effector in Sema3E growth-promoting activity. CRMP4-KO mice displayed abnormal fornix development reminiscent of that observed in Sema3E-KO mice. CRMP4 was shown to interact with the Sema3E tripartite receptor complex within detergent-resistant membrane (DRM) domains, and DRM domain integrity was required to transduce Sema3E signaling through the Akt/GSK3 pathway. Finally, we showed that the cytoskeleton-binding domain of CRMP4 is required for Sema3E's growth-promoting activity, suggesting that CRMP4 plays a role at the interface between Sema3E receptors, located in DRM domains, and the cytoskeleton network. As the fornix is affected in many psychiatric diseases, such as schizophrenia, our results provide new insights to better understand the neurodevelopmental components of these diseases.

The Dynamics of Axon Bifurcation Development in the Cerebral Cortex of Typical and Acallosal Mice

The corpus callosum (CC) is a major interhemispheric commissure of placental mammals. Early steps of CC formation rely on guidance strategies, such as axonal branching and collateralization. Here we analyze the time-course dynamics of axonal bifurcation during typical cortical development or in a CC dysgenesis mouse model. We use Swiss mice as a typical CC mouse model and find that axonal bifurcation rates rise in the cerebral cortex from embryonic day (E)17 and are reduced by postnatal day (P)9. Since callosal neurons populate deep and superficial cortical layers, we compare the axon bifurcation ratio between those neurons by electroporating ex vivo brains at E13 and E15, using eGFP reporter to label the newborn neurons on organotypic slices. Our results suggest that deep layer neurons bifurcate 32% more than superficial ones. To investigate axonal bifurcation in CC dysgenesis, we use BALB/c mice as a spontaneous CC dysgenesis model. BALB/c mice present a typical layer distribution of SATB2 callosal cells, despite the occurrence of callosal anomalies. However, using anterograde DiI tracing, we find that BALB/c mice display increased rates of axonal bifurcations during early and late cortical development in the medial frontal cortex. Midline guidepost cells adjacent to the medial frontal cortex are significant reduced in the CC dysgenesis mouse model. Altogether these data suggest that callosal collateral axonal exuberance is maintained in the absence of midline guidepost signaling and might facilitate aberrant connections in the CC dysgenesis mouse model.

Glycosylation in Axonal Guidance

How millions of axons navigate accurately toward synaptic targets during development is a long-standing question. Over decades, multiple studies have enriched our understanding of axonal pathfinding with discoveries of guidance molecules and morphogens, their receptors, and downstream signalling mechanisms. Interestingly, classification of attractive and repulsive cues can be fluid, as single guidance cues can act as both. Similarly, guidance cues can be secreted, chemotactic cues or anchored, adhesive cues. How a limited set of guidance cues generate the diversity of axonal guidance responses is not completely understood. Differential expression and surface localization of receptors, as well as crosstalk and spatiotemporal patterning of guidance cues, are extensively studied mechanisms that diversify axon guidance pathways. Posttranslational modification is a common, yet understudied mechanism of diversifying protein functions. Many proteins in axonal guidance pathways are glycoproteins and how glycosylation modulates their function to regulate axonal motility and guidance is an emerging field. In this review, we discuss major classes of glycosylation and their functions in axonal pathfinding. The glycosylation of guidance cues and guidance receptors and their functional implications in axonal outgrowth and pathfinding are discussed. New insights into current challenges and future perspectives of glycosylation pathways in neuronal development are discussed.

Editorial: Implications for BACE1 Inhibitor Clinical Trials: Adult Conditional BACE1 Knockout Mice Exhibit Axonal Organization Defects in the Hippocampus

BACE1 is the rate-limiting enzyme for the production of the Aβ peptide that forms amyloid plaques in Alzheimer's disease (AD). Small molecule inhibitors of BACE1 are being tested in clinical trials for AD, but the safety and efficacy of BACE1 inhibition has yet to be fully explored. Knockout of the Bace1 gene in the germline of mice causes multiple neurological phenotypes, suggesting that BACE1 inhibition could be toxic. However, these phenotypes could be the result of BACE1 deficiency during development rather than due to the lack of BACE1 function in the adult. To address this problem, we generated tamoxifen-inducible conditional BACE1 knockout mice in which the Bace1 gene may be deleted in the whole body of the adult at will. Importantly, the adult conditional BACE1 knockout mice largely lack phenotypes, indicating that many BACE1 functions are not required in the adult organism. However, a germline phenotype was observed after BACE1 knockout in the adult: reduced length and disorganization of the hippocampal mossy fiber infrapyramidal bundle comprised of axons of dentate gyrus granule cells. The infrapyramidal bundle abnormality correlated with reduced proteolytic processing of the neural cell adhesion protein CHL1 that is involved in axonal guidance. We conclude that BACE1 inhibition in the adult mouse brain does not lead to the phenotypes associated with BACE1 deficiency during embryonic and postnatal development. However, adult conditional BACE1 knockout mice also suggest that BACE1 inhibitor drugs may disrupt the organization of an axonal pathway in the hippocampus, an important structure for learning and memory. Here, I review the adult conditional BACE1 knockout results and consider their implications for BACE1 inhibitor clinical trials.

Classic axon guidance molecules control correct nerve bridge tissue formation and precise axon regeneration

The peripheral nervous system has an astonishing ability to regenerate following a compression or crush injury; however, the potential for full repair following a transection injury is much less. Currently, the major clinical challenge for peripheral nerve repair come from long gaps between the proximal and distal nerve stumps, which prevent regenerating axons reaching the distal nerve. Precise axon targeting during nervous system development is controlled by families of axon guidance molecules including Netrins, Slits, Ephrins and Semaphorins. Several recent studies have indicated key roles of Netrin1, Slit3 and EphrinB2 signalling in controlling the formation of new nerve bridge tissue and precise axon regeneration after peripheral nerve transection injury. Inside the nerve bridge, nerve fibroblasts express EphrinB2 while migrating Schwann cells express the receptor EphB2. EphrinB2/EphB2 signalling between nerve fibroblasts and migrating Schwann cells is required for Sox2 upregulation in Schwann cells and the formation of Schwann cell cords within the nerve bridge to allow directional axon growth to the distal nerve stump. Macrophages in the outermost layer of the nerve bridge express Slit3 while migrating Schwann cells and regenerating axons express the receptor Robo1; within Schwann cells, Robo1 expression is also Sox2-dependent. Slit3/Robo1 signalling is required to keep migrating Schwann cells and regenerating axons inside the nerve bridge. In addition to the Slit3/Robo1 signalling system, migrating Schwann cells also express Netrin1 and regenerating axons express the DCC receptor. It appears that migrating Schwann cells could also use Netrin1 as a guidance cue to direct regenerating axons across the peripheral nerve gap. Engineered neural tissues have been suggested as promising alternatives for the repair of large peripheral nerve gaps. Therefore, understanding the function of classic axon guidance molecules in nerve bridge formation and their roles in axon regeneration could be highly beneficial in developing engineered neural tissue for more effective peripheral nerve repair.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.

Directional selectivity of afferent neurons in zebrafish neuromasts is regulated by Emx2 in presynaptic hair cells

The orientation of hair bundles on top of sensory hair cells (HCs) in neuromasts of the lateral line system allows fish to detect direction of water flow. Each neuromast shows hair bundles arranged in two opposing directions and each afferent neuron innervates only HCs of the same orientation. Previously, we showed that this opposition is established by expression of Emx2 in half of the HCs, where it mediates hair bundle reversal (Jiang et al., 2017). Here, we show that Emx2 also regulates neuronal selection: afferent neurons innervate either Emx2-positive or negative HCs. In emx2 knockout and gain-of-function neuromasts, all HCs are unidirectional and the innervation patterns and physiological responses of the afferent neurons are dependent on the presence or absence of Emx2. Our results indicate that Emx2 mediates the directional selectivity of neuromasts by two distinct processes: regulating hair bundle orientation in HCs and selecting afferent neuronal targets.

Plexin-Semaphorin Signaling Modifies Neuromuscular Defects in a Drosophila Model of Peripheral Neuropathy

Dominant mutations in GARS, encoding the ubiquitous enzyme glycyl-tRNA synthetase (GlyRS), cause peripheral nerve degeneration and Charcot-Marie-Tooth disease type 2D (CMT2D). This genetic disorder exemplifies a recurring paradigm in neurodegeneration, in which mutations in essential genes cause selective degeneration of the nervous system. Recent evidence suggests that the mechanism underlying CMT2D involves extracellular neomorphic binding of mutant GlyRS to neuronally-expressed proteins. Consistent with this, our previous studies indicate a non-cell autonomous mechanism, whereby mutant GlyRS is secreted and interacts with the neuromuscular junction (NMJ). In this Drosophila model for CMT2D, we have previously shown that mutant gars expression decreases viability and larval motor function, and causes a concurrent build-up of mutant GlyRS at the larval neuromuscular presynapse. Here, we report additional phenotypes that closely mimic the axonal branching defects of Drosophila plexin transmembrane receptor mutants, implying interference of plexin signaling in gars mutants. Individual dosage reduction of two Drosophila Plexins, plexin A (plexA) and B (plexB) enhances and represses the viability and larval motor defects caused by mutant GlyRS, respectively. However, we find plexB levels, but not plexA levels, modify mutant GlyRS association with the presynaptic membrane. Furthermore, increasing availability of the plexB ligand, Semaphorin-2a (Sema2a), alleviates the pathology and the build-up of mutant GlyRS, suggesting competition for plexB binding may be occurring between these two ligands. This toxic gain-of-function and subversion of neurodevelopmental processes indicate that signaling pathways governing axonal guidance could be integral to neuropathology and may underlie the non-cell autonomous CMT2D mechanism.

It is all about the support - The role of the extracellular matrix in regenerating axon guidance

Although it is known for long time that the peripheral nervous system has the capacity for self-regeneration, the molecular mechanisms by which Schwann cells and extracellular matrix (ECM) guide the injured axons to regrow along their original path, remains a poorly understood process. Due to the importance of ECM molecules during development, constitutive mutant organisms display increased lethality, therefore, conditional or inducible strategies have been used to increase the survival of the organisms and allow the study of the role of ECM proteins. In a recent report published in Neuron, Isaacman-Beck and colleagues (2015) used these pioneering genetic studies on zebrafish combined with in vivo fluorescent imaging, to investigate the micro-environmental conditions required for targeted regeneration of the dorsal motor nerve of zebrafish larvae after laser-transection. A candidate gene approach targeting lh3 basal laminar collagen substrates revealed that the lh3 substrate col4α5 regulates dorsal nerve regeneration by destabilizing misdirected axons. Col4α5 was upregulated in a small population of lh3 expressing Schwann cells located ventrally and ventro-laterally to the injury site and found to co-localize with the molecule slit guidance ligand 1 (slit1a). Capitalizing on the crucial observations of mistargeted regeneration of dorsal nerves in mutant larvae, they put forward a model in which Schwann cells shape an environment that allows and directs axonal regeneration to their original synaptic target. In the light of Isaacman-Beck and colleagues (2015) findings, we will review how their study contributes to the research field, and comment on its potential implications for promoting nerve regeneration after injury.

Segregation of motor and sensory axons regenerating through bicompartmental tubes by combining extracellular matrix components with neurotrophic factors

Segregation of regenerating motor and sensory axons may be a good strategy to improve selective functionality of regenerative interfaces to provide closed-loop commands. Provided that extracellular matrix components and neurotrophic factors exert guidance effects on different neuronal populations, we assessed in vivo the potential of separating sensory and motor axons regenerating in a bicompartmental Y-type tube, with each branch prefilled with an adequate combination of extracellular matrix and neurotrophic factors. The severed rat sciatic nerve was repaired using a bicompartmental tube filled with a collagen matrix enriched with fibronectin (FN) and brain-derived neurotrophic factor (BDNF) encapsulated in poly-lactic co-glycolic acid microspheres (FN + MP.BDNF) in one compartment to preferentially attract motor axons and collagen enriched with laminin (LM) and nerve growth factor (NGF) and neurotrophin-3 (NT-3) in microspheres (LM + MP.NGF/NT-3) in the other compartment for promoting sensory axons regeneration. Control animals were implanted with the same Y-tube with a collagen matrix with microspheres (MP) containing PBS (Col + MP.PBS). By using retrotracer labelling, we found that LM + MP.NGF/NT-3 did not attract higher number of regenerated sensory axons compared with controls, and no differences were observed in sensory functional recovery. However, FN + MP.BDNF guided a higher number of regenerating motor axons compared with controls, improving also motor recovery. A small proportion of sensory axons with large soma size, likely proprioceptive neurons, was also attracted to the FN + MP.BDNF compartment. These results demonstrate that muscular axonal guidance can be modulated in vivo by the addition of fibronectin and BDNF.

Topographical and Electrical Stimulation of Neuronal Cells through Microwrinkled Conducting Polymer Biointerfaces

The development of smart biointerfaces combining multiple functions is crucial for triggering a variety of cellular responses. In this work, wrinkled organic interfaces based on the conducting polymer poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) are developed with the aim to simultaneously convey electrical and topographical stimuli to cultured cells. The surface wrinkling of thin films on heat-shrink polymer sheets allows for rapid patterning of self-assembled anisotropic topographies characterized by micro/sub-microscale aligned wrinkles. The developed interfaces prove to support the growth and differentiation of neural cells (SH-SY5Y, human neuroblastoma) and are remarkably effective in promoting axonal guidance, by guiding and stimulating the neurite growth in differentiating cells. Electrical stimulation with biphasic pulses delivered through the conductive wrinkled interface is found to further promote the neurite growth, demonstrating the suitability of such interfaces as platforms for conveying multiple stimuli to cells and tissues.

Genetic landscape of sporadic vestibular schwannoma

OBJECTIVE Vestibular schwannoma (VS) is a benign tumor with associated morbidities and reduced quality of life. Except for mutations in NF2, the genetic landscape of VS remains to be elucidated. Little is known about the effect of Gamma Knife radiosurgery (GKRS) on the VS genome. The aim of this study was to characterize mutations occurring in this tumor to identify new genes and signaling pathways important for the development of VS. In addition, the authors sought to evaluate whether GKRS resulted in an increase in the number of mutations. METHODS Forty-six sporadic VSs, including 8 GKRS-treated tumors and corresponding blood samples, were subjected to whole-exome sequencing and tumor-specific DNA variants were called. Pathway analysis was performed using the Ingenuity Pathway Analysis software. In addition, multiplex ligation-dependent probe amplification was performed to characterize copy number variations in the NF2 gene, and microsatellite instability testing was done to investigate for DNA replication error. RESULTS With the exception of a single sample with an aggressive phenotype that harbored a large number of mutations, most samples showed a relatively low number of mutations. A median of 14 tumor-specific mutations in each sample were identified. The GKRS-treated tumors harbored no more mutations than the rest of the group. A clustering of mutations in the cancer-related axonal guidance pathway was identified (25 patients), as well as mutations in the CDC27 (5 patients) and USP8 (3 patients) genes. Thirty-five tumors harbored mutations in NF2 and 16 tumors had 2 mutational hits. The samples without detectable NF2 mutations harbored mutations in genes that could be linked to NF2 or to NF2-related functions. None of the tumors showed microsatellite instability. CONCLUSIONS The genetic landscape of VS seems to be quite heterogeneous; however, most samples had mutations in NF2 or in genes that could be linked to NF2. The results of this study do not link GKRS to an increased number of mutations.

Role of Rb during Neurogenesis and Axonal Guidance in the Developing Olfactory System

The Retinoblastoma protein, Rb, was shown to regulate distinct aspects of neurogenesis in the embryonic and adult brain besides its primary role in cell cycle control. It is still unknown, however, whether Rb is required for tissue morphogenesis and the establishment of synaptic connections between adjacent tissues during development. We have investigated here the role of Rb during development of the olfactory system (OS), which heavily relies on reciprocal interactions between the olfactory epithelium (OE) and the olfactory bulb (OB). We show that mice carrying a telencephalic-specific deletion of Rb display several neurogenic defects in the OS during late development. In the OE, loss of Rb leads to ectopic proliferation of late-born progenitors (Tuj-1+), abnormal radial migration and terminal maturation of olfactory sensory neurons (OSNs). In the OB, deletion of Rb causes severe lamination defects with loss of clear boundaries between distinct layers. Importantly, starting around E15.5 when OB glomerulogenesis is initiated, many OSNs axons that project along the olfactory nerve layer (ONL) fail to properly innervate the nascent bulb, thus resulting in partial loss of connectivity between OE-OB and gradual neuronal degeneration in both tissues peaking at birth. This deficiency correlates with deregulated expressions of two key chemo-repellant molecules, Robo2/Slit1 and Nrp2/Sema3F that control the formation of dorsal-ventral topographic map of OSNs connections with OB glomeruli. This study highlights a critical requirement for Rb during neurogenesis and the establishment of proper synaptic connections inside the OS during development.

Dishevelled attenuates the repelling activity of Wnt signaling during neurite outgrowth in Caenorhabditis elegans

Wnt proteins regulate axonal outgrowth along the anterior-posterior axis, but the intracellular mechanisms that modulate the strength of Wnt signaling in axon guidance are largely unknown. Using the Caenorhabditis elegans mechanosensory PLM neurons, we found that posteriorly enriched LIN-44/Wnt acts as a repellent to promote anteriorly directed neurite outgrowth through the LIN-17/Frizzled receptor, instead of controlling neuronal polarity as previously thought. Dishevelled (Dsh) proteins DSH-1 and MIG-5 redundantly mediate the repulsive activity of the Wnt signals to induce anterior outgrowth, whereas DSH-1 also provides feedback inhibition to attenuate the signaling to allow posterior outgrowth against the Wnt gradient. This inhibitory function of DSH-1, which requires its dishevelled, Egl-10, and pleckstrin (DEP) domain, acts by promoting LIN-17 phosphorylation and is antagonized by planar cell polarity signaling components Van Gogh (VANG-1) and Prickle (PRKL-1). Our results suggest that Dsh proteins both respond to Wnt signals to shape neuronal projections and moderate its activity to fine-tune neuronal morphology.

Attractive and permissive activities of semaphorin 5A toward dorsal root ganglion axons in higher vertebrate embryos

Elongation of the efferent fibers of dorsal root ganglion (DRG) neurons toward their peripheral targets occurs during development. Attractive or permissive systems may be involved in this elongation. However, the molecular mechanisms that control it are largely unknown. Here we show that class 5 semaphorin Sema5A had attractive/permissive effects on DRG axons. In mouse embryos, Sema5A was expressed in and around the path of DRG efferent fibers, and cell aggregates secreting Sema5A attracted DRG axons in vitro. We also found that ectopic Sema5A expression in the spinal cord attracted DRG axons. Together, these findings suggest that Sema5A functions as an attractant to elongate DRG fibers and contributes to the formation of the early sensory network.

Cyclin I and p35 determine the subcellular distribution of Cdk5

The atypical cyclin-dependent kinase 5 (Cdk5) serves an array of different functions in cell biology. Among these are axonal guidance, regulation of intercellular contacts, cell differentiation, and prosurvival signaling. The variance of these functions suggests that Cdk5 activation comes to pass in different cellular compartments. The kinase activity, half-life, and substrate specificity of Cdk5 largely depend on specific activators, such as p25, p35, p39, and cyclin I. We hypothesized that the subcellular distribution of Cdk5 activators also determines the localization of the Cdk5 protein and sets the stage for targeted kinase activity within distinct cellular compartments to suit the varying roles of Cdk5. Cdk5 localization was analyzed in murine kidney and brain slices of wild-type and cyclin I- and/or p35-null mice by immunohistochemistry and in cultured mouse podocytes using immunofluorescence labeling, as well as cell fractionation experiments. The predominance of cyclin I mediates the nuclear localization of Cdk5, whereas the predominance of p35 results in a membranous localization of Cdk5. These findings were further substantiated by overexpression of cyclin I and p35 with altered targeting characteristics in human embryonic kidney 293T cells. These studies reveal that the subcellular localization of Cdk5 is determined by its specific activators. This results in the directed Cdk5 kinase activity in specific cellular compartments dependent on the activator present and allows Cdk5 to serve multiple independent roles.

Hedgehog: Multiple Paths for Multiple Roles in Shaping the Brain and Spinal Cord

Since the discovery of the segment polarity gene Hedgehog in Drosophila three decades ago, our knowledge of Hedgehog signaling pathway has considerably improved and paved the way to a wide field of investigations in the developing and adult central nervous system. Its peculiar transduction mechanism together with its implication in tissue patterning, neural stem cell biology, and neural tissue homeostasis make Hedgehog pathway of interest in a high number of normal or pathological contexts. Consistent with its role during brain development, misregulation of Hedgehog signaling is associated with congenital diseases and tumorigenic processes while its recruitment in damaged neural tissue may be part of the repairing process. This review focuses on the most recent data regarding the Hedgehog pathway in the developing and adult central nervous system and also its relevance as a therapeutic target in brain and spinal cord diseases.

Pontine malformation, undecussated pyramidal tracts, and regional polymicrogyria: a new syndrome

BACKGROUND

Horizontal gaze palsy and progressive scoliosis is caused by mutations in the ROBO3 gene, which plays a role in axonal guidance during brain development. Horizontal gaze palsy and progressive scoliosis is characterized by the congenital absence of conjugate lateral eye movements with preserved vertical gaze and progressive scoliosis as well as dysgenesis of brainstem structures and ipsilateral projection of the pyramidal tract.

PATIENT

A 4-year, 11-month, girl presented with psychomotor retardation and autistic traits. Magnetic resonance imaging revealed hypoplasia and malformation of the ventral portion of the pons and medulla oblongata. Diffusion tensor imaging revealed the absence of decussation of the bilateral pyramidal tracts. These findings were similar to the typical findings for horizontal gaze palsy and progressive scoliosis. However, restriction of horizontal eye movement was minimal, and bilateral polymicrogyria were also noted in the occipitotemporal cortex in the present patient. These findings have not been previously reported in patients with horizontal gaze palsy and progressive scoliosis. No mutations in the ROBO3, SLIT1, SLIT2, NTN1, SEMA3 A, or SEMA3 F genes were identified.

CONCLUSION

This child may have a disorder caused by an unidentified factor, other than a mutation in the genes analyzed, involved in corticogenesis, axonal guidance, and brainstem morphogenesis.

Evidence for dysregulation of axonal growth and guidance in the etiology of ASD

Current theories concerning the cause of autism spectrum disorders (ASDs) have converged on the concept of abnormal development of brain connectivity. This concept is supported by accumulating evidence from functional imaging, diffusion tensor imaging, and high definition fiber tracking studies which suggest altered microstructure in the axonal tracts connecting cortical areas may underly many of the cognitive manifestations of ASD. Additionally, large-scale genomic studies implicate numerous gene candidates known or suspected to mediate neuritic outgrowth and axonal guidance in fetal and perinatal life. Neuropathological observations in postmortem ASD brain samples further support this model and include subtle disturbances of cortical lamination and subcortical axonal morphology. Of note is the relatively common finding of poor differentiation of the gray–white junction associated with an excess superficial white matter or “interstitial” neurons (INs). INs are thought to be remnants of the fetal subplate, a transient structure which plays a key role in the guidance and morphogenesis of thalamocortical and cortico-cortical connections and the organization of cortical columnar architecture. While not discounting the importance of synaptic dysfunction in the etiology of ASD, this paper will briefly review the cortical abnormalities and genetic evidence supporting a model of dysregulated axonal growth and guidance as key developmental processes underlying the clinical manifestations of ASD.

Optogenetic-mediated increases in in vivo spontaneous activity disrupt pool-specific but not dorsal-ventral motoneuron pathfinding

Rhythmic waves of spontaneous electrical activity are widespread in the developing nervous systems of birds and mammals, and although many aspects of neural development are activity-dependent, it has been unclear if rhythmic waves are required for in vivo motor circuit development, including the proper targeting of motoneurons to muscles. We show here that electroporated channelrhodopsin-2 can be activated in ovo with light flashes to drive waves at precise intervals of approximately twice the control frequency in intact chicken embryos. Optical monitoring of associated axial movements ensured that the altered frequency was maintained. In embryos thus stimulated, motor axons correctly executed the binary dorsal-ventral pathfinding decision but failed to make the subsequent pool-specific decision to target to appropriate muscles. This observation, together with the previous demonstration that slowing the frequency by half perturbed dorsal-ventral but not pool-specific pathfinding, shows that modest changes in frequency differentially disrupt these two major pathfinding decisions. Thus, many drugs known to alter early rhythmic activity have the potential to impair normal motor circuit development, and given the conservation between mouse and avian spinal cords, our observations are likely relevant to mammals, where such studies would be difficult to carry out.

Two specific populations of GABAergic neurons originating from the medial and the caudal ganglionic eminences aid in proper navigation of callosal axons

The corpus callosum (CC) plays a crucial role in interhemispheric communication. It has been shown that CC formation relies on the guidepost cells located in the midline region that include glutamatergic and GABAergic neurons as well as glial cells. However, the origin of these guidepost GABAergic neurons and their precise function in callosal axon pathfinding remain to be investigated. Here, we show that two distinct GABAergic neuronal subpopulations converge toward the midline prior to the arrival of callosal axons. Using in vivo and ex vivo fate mapping we show that CC GABAergic neurons originate in the caudal and medial ganglionic eminences (CGE and MGE) but not in the lateral ganglionic eminence (LGE). Time lapse imaging on organotypic slices and in vivo analyses further revealed that CC GABAergic neurons contribute to the normal navigation of callosal axons. The use of Nkx2.1 knockout (KO) mice confirmed a role of these neurons in the maintenance of proper behavior of callosal axons while growing through the CC. Indeed, using in vitro transplantation assays, we demonstrated that both MGE- and CGE-derived GABAergic neurons exert an attractive activity on callosal axons. Furthermore, by combining a sensitive RT-PCR technique with in situ hybridization, we demonstrate that CC neurons express multiple short and long range guidance cues. This study strongly suggests that MGE- and CGE-derived interneurons may guide CC axons by multiple guidance mechanisms and signaling pathways.

Map formation in the olfactory bulb by axon guidance of olfactory neurons

The organization of representations in the brain has been observed to locally reflect subspaces of inputs that are relevant to behavioral or perceptual feature combinations, such as in areas receptive to lower and higher-order features in the visual system. The early olfactory system developed highly plastic mechanisms and convergent evidence indicates that projections from primary neurons converge onto the glomerular level of the olfactory bulb (OB) to form a code composed of continuous spatial zones that are differentially active for particular physico-chemical feature combinations, some of which are known to trigger behavioral responses. In a model study of the early human olfactory system, we derive a glomerular organization based on a set of real-world, biologically relevant stimuli, a distribution of receptors that respond each to a set of odorants of similar ranges of molecular properties, and a mechanism of axon guidance based on activity. Apart from demonstrating activity-dependent glomeruli formation and reproducing the relationship of glomerular recruitment with concentration, it is shown that glomerular responses reflect similarities of human odor category perceptions and that further, a spatial code provides a better correlation than a distributed population code. These results are consistent with evidence of functional compartmentalization in the OB and could suggest a function for the bulb in encoding of perceptual dimensions.

Global expression of NGF promotes sympathetic axonal growth in CNS white matter but does not alter its parallel orientation

Axonal regeneration is normally limited after injuries to CNS white matter. Infusion of neurotrophins has been successful in promoting regenerative growth through injured white matter but this growth generally fails to extend beyond the infusion site. These observations are consistent with a chemotropic effect of these factors on axonal growth and support the prevailing view that neurotrophin-induced axonal regeneration requires the use of gradients, i.e., gradually increasing neurotrophin levels along the target fiber tract. To examine the potential of global overexpression of neurotrophins to promote, and/or modify the orientation of, regenerative axonal growth within white matter, we grafted nerve growth factor (NGF) responsive neurons into the corpus callosum of transgenic mice overexpressing NGF throughout the CNS under control of the promoter for glial fibrillary acidic protein. One week later, glial fibrillary acidic protein and chondroitin sulfate proteoglycan immunoreactivity increased within injured white matter around the grafts. NGF levels were significantly higher in the brains of transgenic compared with non-transgenic mice and further elevated within injury sites compared with the homotypic region of the non-injured side. Although there was minimal outgrowth from neurons grafted into non-transgenic mice, extensive parallel axonal regeneration had occurred within the corpus callosum up to 1.5 mm beyond the astrogliotic scar (the site of maximum NGF expression) in transgenic mice. These results demonstrate that global overexpression of neurotrophins does not override the constraints limiting regenerative growth to parallel orientations and suggest that such factors need not be presented as positive gradients to promote axonal regeneration within white matter.

Different signals control laminar specificity of commissural and entorhinal fibers to the dentate gyrus

The factors governing the characteristic laminated termination of hippocampal afferents are essentially unknown. Principally, diffusible factors of the target region, membrane-bound molecules on the ingrowing afferent fibers and on the postsynaptic target cells as well as molecules of the extracellular matrix (ECM), may play a role. Using slice cocultures as a model, we show that hyaluronic acid, an ECM molecule, is essential for the segregated, layer-specific termination of entorhinal fibers but not of commissural afferents to the mouse dentate gyrus. Laminar specificity of the latter, in contrast, is determined by the position of the postsynaptic granule cells. Thus, malpositioning of the granule cells in slice cultures from reeler mutant mice altered the projection of commissural fibers from cocultured wild-type hippocampus. In contrast, commissural fibers from reeler mouse hippocampus formed a normal, sharply delineated projection to the inner molecular layer of cocultured wild-type dentate gyrus, precluding a cell-autonomous effect of the reeler mutation on commissural neurons. Interestingly enough, entorhinal fibers formed their normal, sharply delineated projection in cocultured reeler dentate gyrus despite the malpositioning of the target granule cells. Because hyaluronan-associated molecules are likely to control the segregated termination of entorhinal fibers, we compared immunolabeling for neurocan and chondroitin sulfate in sections from reeler and wild-type mice and found it similar in both genotypes. Together these results show that different mechanisms underlie the formation of commissural and entorhinal fiber layers during the development of the dentate gyrus.

Miswiring of limbic thalamocortical projections in the absence of ephrin-A5

Axon guidance cues of the ephrin ligand family have been hypothesized to regulate the formation of thalamocortical connections, but in vivo evidence for such a role has not been examined directly. To test whether ephrin-mediated repulsive cues participate in sorting the projections originating from distinct thalamic nuclei, we analyzed the organization of somatosensory and anterior cingulate afferents postnatally in mice lacking ephrin-A5 gene expression. Projections from ventrobasal and laterodorsal nuclei to their respective sensory and limbic cortical areas developed normally. However, a portion of limbic thalamic neurons from the laterodorsal nucleus also formed additional projections to somatosensory cortical territories, thus maintaining inappropriate dual projections to multiple cortical regions. These results suggest that ephrin-A5 is not required for the formation of normal cortical projections from the appropriate thalamic nuclei, but rather acts as a guidance cue that restricts limbic thalamic axons from inappropriate neocortical regions.

Target-dependent sexual differentiation of a limbic-hypothalamic neural pathway

Neural pathways between sexually dimorphic forebrain regions develop under the influence of sex steroid hormones during the perinatal period, but how these hormones specify precise sex-specific patterns of connectivity is unknown. A heterochronic coculture system was used to demonstrate that sex steroid hormones direct development of a sexually dimorphic limbic-hypothalamic neural pathway through a target-dependent mechanism. Explants of the principal nucleus of the bed nuclei of the stria terminalis (BSTp) extend neurites toward explants of the anteroventral periventricular nucleus (AVPV) derived from male but not female rats. Coculture of BSTp explants from male rats with AVPV explants derived from females treated in vivo with testosterone for 9 d resulted in a high density of neurites extending from the BSTp to the AVPV explant, as was the case when the BSTp explants were derived from females and the AVPV explants were derived from males or androgen-treated females. These in vitro findings suggest that during the postnatal period testosterone induces a target-derived, diffusible chemotropic activity that results in a sexually dimorphic pattern of connectivity.

Malformation of the functional organization of somatosensory cortex in adult ephrin-A5 knock-out mice revealed by in vivo functional imaging

The molecular mechanisms that coordinate the functional organization of the mammalian neocortex are largely unknown. We tested the involvement of a putative guidance label, ephrin-A5, in the functional organization of the somatosensory cortex by quantifying the functional representations of individual whiskers in vivo in adult ephrin-A5 knock-out mice, using intrinsic signal optical imaging. In wild-type mice ephrin-A5 is expressed in a gradient in the somatosensory cortex during development. In adult ephrin-A5 knock-out mice, we found a spatial gradient of change in the amount of cortical territory shared by individual whisker functional representations across the somatosensory cortex, as well as a gradient of change in the distance between the functional representations. Both gradients of change were in correspondence with the developmental expression gradient of ephrin-A5 in wild-type mice. These changes involved malformations of the cortical spacing of the thalamocortical components, without concurrent malformations of the intracortical components of individual whisker functional representations. Overall, these results suggest that a developmental guidance label, such as ephrin-A5, is involved in establishing certain spatial relationships of the functional organization of the adult neocortex, and they underscore the advantage of investigating gene manipulation using in vivo functional imaging.

Spatial distributions of guidance molecules regulate chemorepulsion and chemoattraction of growth cones

It is generally assumed that gradients of chemotropic molecules are instrumental to the wiring of the nervous system. Recently, two members of the secreted class III semaphorin protein family have been implicated as repulsive (Sema3A) and attractive (Sema3C) guidance molecules for cortical axons (). Here, we show that stabilized gradients of increasing semaphorin concentrations elicit stereotyped responses from cortical growth cones, independent of the absolute concentration and the slope of these gradients. In contrast, neither repulsive effects of Sema3A nor attractive effects of Sema3C were observed when axons were growing toward decreasing semaphorin concentrations. Thus, growth cone guidance by gradients of chemotropic molecules is robust and reproducible, because it is primarily independent of the exact dimensions of the gradients.

Specification of distinct dopaminergic neural pathways: roles of the Eph family receptor EphB1 and ligand ephrin-B2

Dopaminergic neurons in the substantia nigra and ventral tegmental area project to the caudate putamen and nucleus accumbens/olfactory tubercle, respectively, constituting mesostriatal and mesolimbic pathways. The molecular signals that confer target specificity of different dopaminergic neurons are not known. We now report that EphB1 and ephrin-B2, a receptor and ligand of the Eph family, are candidate guidance molecules for the development of these distinct pathways. EphB1 and ephrin-B2 are expressed in complementary patterns in the midbrain dopaminergic neurons and their targets, and the ligand specifically inhibits the growth of neurites and induces the cell loss of substantia nigra, but not ventral tegmental, dopaminergic neurons. These studies suggest that the ligand-receptor pair may contribute to the establishment of distinct neural pathways by selectively inhibiting the neurite outgrowth and cell survival of mistargeted neurons. In addition, we show that ephrin-B2 expression is upregulated by cocaine and amphetamine in adult mice, suggesting that ephrin-B2/EphB1 interaction may play a role in drug-induced plasticity in adults as well.

Membrane-associated molecules guide limbic and nonlimbic thalamocortical projections

Membrane-associated signals expressed in restricted domains of the developing cerebral cortex may mediate axon target recognition during the establishment of thalamocortical projections, which form in a highly precise manner during development. To test this hypothesis, we first analyzed the outgrowth of thalamic explants from limbic and nonlimbic nuclei on membrane substrates prepared from limbic cortex and neocortex. The results show that different thalamic fiber populations are able to discriminate between membrane substrates prepared from target and nontarget cortical regions. A candidate molecule that could mediate selective choice in the thalamocortical system is the limbic system-associated membrane protein (LAMP), which is an early marker of cortical and subcortical limbic regions (Pimenta et al.,1995) that can promote outgrowth of limbic axons. Limbic thalamic and cortical axons showed preferences for recombinant LAMP (rLAMP) in a stripe assay. Incubation of cortical membranes with an antibody against LAMP prevented the ability of limbic thalamic fibers to distinguish between membranes from limbic cortex and neocortex. Strikingly, nonlimbic thalamic fibers also responded to LAMP, but in contrast to limbic thalamic fibers, rLAMP inhibited branch formation and acted as a repulsive axonal guidance signal for nonlimbic thalamic axons. The present studies indicate that LAMP fulfills a role as a selective guidance cue in the developing thalamocortical system.

Involvement of distinct pioneer neurons in the formation of layer-specific connections in the hippocampus

During neural development, specific recognition molecules provide the cues necessary for the formation of initial projection maps, which are reshaped later in development. In some systems, guiding cues for axonal pathfinding and target selection are provided by specific cells that are present only at critical times. For instance, the floor plate guides commissural axons in the spinal cord, and the subplate is involved in the formation of thalamocortical connections. Here we study the development of entorhinal and commissural connections to the murine hippocampus, which in the adult terminate in nonoverlapping layers. We show that two groups of pioneer neurons, Cajal-Retzius cells and GABAergic neurons, form layer-specific scaffolds that overlap with distinct hippocampal afferents at embryonic and early postnatal stages. Furthermore, at postnatal day 0 (P0)-P5, before the dendrites of pyramidal neurons develop, these pioneer neurons are preferential synaptic targets for hippocampal afferents. Birthdating analysis using 5'-bromodeoxyuridine (BrdU) pulses showed that most such early-generated neurons disappear at late postnatal stages, most likely by cell death. Together with previous studies, these findings indicate that distinct pioneer neurons are involved in the guidance and targeting of different hippocampal afferents.

Dual action of a ligand for Eph receptor tyrosine kinases on specific populations of axons during the development of cortical circuits

The structural basis of cortical columns are radially oriented axon collaterals that form precise connections between distinct cortical layers. During development, these connections are highly specified from the initial outgrowth of collateral branches. Our previous work provided evidence for positional cues confined to individual layers that induce and/or prevent the formation of axon collaterals in specific populations of cortical neurons. Here we demonstrated with in situ hybridization techniques that mRNA of the Eph receptor tyrosine kinase EphA5 and one of its ligands, ephrin-A5, are present in distinct cortical layers, at a time when intrinsic connections are being formed in the cortex. Axonal guidance assays indicate that ephrin-A5 is a repellent signal for a populations of axons that in vivo avoid the cortical layer expressing ephrin-A5. In contrast to its established role as a repulsive axonal guidance signal, ephrin-A5 specifically mediates sprouting of those cortical axons that target the ephrin-A5-expressing layer in vivo. These results identify a novel function of ephrin-A5 on axonal arbor formation. The laminar distribution and the dual action on specific populations of axons suggest that ephrin-A5 plays a role in the assembly of local cortical circuits.

Retroviral misexpression of engrailed genes in the chick optic tectum perturbs the topographic targeting of retinal axons

We have investigated the role of the homeodomain transcription factor genes En-1 and En-2, homologs of the Drosophila segment polarity gene engrailed, in regulating the development of the retinotopic map in the chick optic tectum. The En proteins are distributed in a gradient along the rostral-caudal axis of the developing tectum, with highest amounts found caudally. Previous evidence suggests that En-1 and En-2 may regulate the polarity of the rostral-caudal axis of the tectum and the subsequent topographic mapping of retinal axons. We have tested this hypothesis by using a recombinant replication-competent retrovirus to overexpress the En-1 or En-2 genes in the developing tectum. Anterograde labeling with the axon tracer Dil was used to analyze the topographic mapping of retinal axons after the time that the retinotectal projection is normally topographically organized. Overexpression of either En-1 or En-2 perturbed the topographic targeting of retinal axons. In En-infected tecta, nasal retinal axons form an abnormally diffuse projection with numerous aberrant axons, branches, and arbors found at topographically incorrect locations, colocalized with domains of viral infection. In contrast, temporal axons did not form a diffuse projection or discrete aberrant arbors; however, many temporal axons were stunted and ended aberrantly rostral to their appropriate TZ, or in other cases either did not enter the tectum or formed a dense termination at its extreme rostral edge. These findings indicate that En-1 and En-2 are involved in regulating the development of the retinotopic map in the tectum. Furthermore, they support the hypothesis that En genes regulate the polarity of the rostral-caudal axis of the tectum, most likely by controlling the expression of retinal axon guidance molecules.

The behavior of optic axons on substrate gradients of retinal basal lamina proteins and merosin

To study the behavior of optic axons to continuously changing concentrations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The following growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with increasing concentration of the substrate. On shallow basal lamina gradients, the axons also showed a directional response that resulted in guidance to higher substrate concentrations. (2) Axons that grew down gradients, i.e., from high to low substrate concentrations, became shorter and more fasciculated with decreasing concentrations of the substrate. On gradients of merosin, a significant alteration in the axonal growth direction toward higher substrate concentrations was detected. Axons heading down gradients never U turned to higher substrate concentrations. (3) Axons confronted with discontinuous substrates were confined to the borders of the substrate exclusively, whereas axons confronted with substrate gradients were able to cross into the territory beyond the substrate. (4) The growth patterns of axons on substrate gradients of basal lamina proteins and merosin were similar but not identical, indicating that axons may respond to substrate gradients dependent on its chemical composition. The present results show that substrate gradients can regulate length and fasciculation of neurites and have a limited capability to direct axons to higher substrate concentrations.