Axon guidance by gradients of a target-derived component

Theoretical analysis of gradient detection by growth cones

Gradients of diffusible and substrate-bound molecules play an important role in guiding axons to appropriate targets in the developing nervous system. Although some of the molecules involved have recently been identified, little is known about the physical mechanisms by which growth cones sense gradients. This article applies the seminal Berg and Purcell (1977) model of gradient sensing to this problem. The model provides estimates for the statistical fluctuations in the measurement of concentration by a small sensing device. By assuming that gradient detection consists of the comparison of concentrations at two spatially or temporally separated points, the model therefore provides an estimate for the steepness of gradient that can be detected as a function of physiological parameters. The model makes the following specific predictions. (a) It is more likely that growth cones use a spatial rather than temporal sensing strategy. (b) Growth cone sensitivity increases with the concentration of ligand, the speed of ligand diffusion, the size of the growth cone, and the time over which it averages the gradient signal. (c) The minimum detectable gradient steepness for growth cones is roughly in the range 1-10%. (d) This value varies depending on whether a bound or freely diffusing ligand is being sensed, and on whether the sensing occurs in three or two dimensions. The model also makes predictions concerning the role of filopodia in gradient detection.

Orientation of chemotactic cells and growth cones: models and mechanisms

A model is proposed for an amplification step in chemotactically sensitive cells or growth cones that accounts for their extraordinary directional sensitivity. It is assumed that cells have an intrinsic pattern forming system that generates the signals for extension of filopods and lamellipods. An external signal such as a graded cue is assumed to impose some directional preference onto the pattern formed. According to the model, a saturating, self-enhancing reaction is coupled with two antagonistic reactions. One antagonist equilibrates rapidly over the whole cell, causing competition between different surface elements of the cell cortex for activation. It will be won by those cortical regions of the cell that are exposed to the highest concentrations of the external graded cues. The second antagonistic reaction is assumed to act more locally and has a longer time constant. It causes a destabilization of peaks after they have formed. While the total activated area on the cell surface is maintained, the disappearance of some hot spots allows the formation of new ones, preferentially at positions specified by the actual external guiding signal. Computer simulations show that the model accounts for the highly dynamic behaviour of chemotactic cells and growth cones. In the absence of external signals, maxima of the internal signals emerge at random positions and disappear after some time. Travelling waves or oscillations in counter phase can emerge on the cell cortex, in agreement with observations reported in the literature. In other ranges of parameters, the model accounts for the generation of a stable cell polarity.

Positional specificities of retinal growth cones in the mouse superior colliculus

In the developing retinotectal system, repulsive topographic tectal cues have been demonstrated to contribute to the final mapping. Here, we describe a novel response of nasal axons to growth-promoting cues expressed by anterior tectal cells. In in vitro experiments, contact of fibres from the nasal (but not temporal) pole of the mouse retina with anterior (but not posterior) tectal membranes leads to their adopting very elongated and filopodial morphologies, and to increase their growth rates. As previously demonstrated, fibres from the temporal pole of the retina are collapsed by posterior tectal membranes in vitro. In addition, a study of retinal growth cone morphologies in vivo, at early stages of target invasion, shows that growth cones of nasal fibres have streamlined morphologies, usually indicative of active elongation growth modes, in the anterior part of the embryonic mouse tectum, and more elaborate morphologies posteriorly. Vice versa, temporal fibres have mainly elaborate growth cones anteriorly, and collapsed growth cones posteriorly. These experiments demonstrate that nasal retinal fibres respond preferentially to permissive or growth-promoting cues in the embryonic mouse tectal environment, both in vitro and in vivo. This phenomenon might contribute to ingrowth of retinal fibres in their target area, and to promote the homing of nasal fibres towards the posterior aspect of the tectum, which is their normal target region.

Neuronal growth cone collapse triggers lateral extensions along trailing axons

Axonal outgrowth is generally thought to be controlled by direct interaction of the lead growth cone with guidance cues, and, in trailing axons, by fasciculation with pioneer fibers. Responses of axons and growth cones were examined as cultured retinal ganglion cell (RGC) axons encountered repellent cues. Either contact with cells expressing ephrins or mechanical probing increased the probability of lead growth cone retraction. Lateral extension of filopodia and lamellipodia hundreds of microns behind the lead growth cone was correlated with its collapse. Transmission electron microscopy showed that some of the lateral extensions originate from the pioneer axon, whereas others represent growth cones of defasciculating trailing axons.

Mechanisms involved in development of retinotectal connections: roles of Eph receptor tyrosine kinases, NMDA receptors and nitric oxide

Axons of retinal ganglion cells exhibit a specific pattern of connections with the brain. Within each visual nucleus in the brain, retinal connections are topographic such that axons from neighboring ganglion cells have neighboring synapses. Research is beginning to shed light on the mechanisms responsible for development of topographic connections in the visual system. Much of this research is focused on the axonal connections of the retina with the tectum. In vivo and in vitro experiments indicate that the pattern of retinotectal connections develops in part due to positional labels carried by the growing retinal axons and by the tectal cells. Evidence suggests that gradients of Eph receptor tyrosine kinases serve as positional labels on the growing retinal axons, and gradients of ligands for these receptors serve as positional labels in the tectum. Blocking expression of EphA3, a receptor tyrosine kinase, in the developing retina resulted in disruption of the topography of the retinotectal connections, further supporting the role of these, molecules. Although positional labels appear to be important, other mechanisms must also be involved. The initial pattern of retinotectal connections lacks the precision seen in the adult. The adult pattern of connections arises during development by activity dependent refinement of a roughly ordered prepattern. The refinement process results in elimination of projections to the wrong side of the brain, to non-visual nuclei and to inappropriate regions within a nucleus. Blocking NMDA receptors during the period of refinement preserved anomalous retinotectal projections, which suggests that elimination of these projections is mediated by NMDA receptors. Furthermore, tectal cells normally express high levels of nitric oxide synthase (NOS) during the period of refinement, and blocking nitric oxide (NO) synthesis also preserved inappropriate projections. Thus, both NMDA receptors and NO appear to be involved in refinement. Blocking NMDA receptor activation reduced NOS activity in tectal cells, which suggests the possibility that NO is the downstream mediator of NMDA function related to refinement. A quantitative comparison of blocking NMDA receptors, NO synthesis or both showed that all three treatments have comparable effects on refinement. This indicates that the role of NMDA receptor activation relative to refinement may be completely mediated through nitric oxide. Quantitative analysis also suggests that other mechanisms not involving NMDA receptors or NO must be involved in refinement. Other mechanisms appear to include cell death.

Eph receptors and ephrins in neural development

Ephrins, ligands for the Eph family of receptor tyrosine kinases, are pivotal players in many developmental phenomena in both the central and peripheral nervous systems. Ephrins appear to act typically, but not exclusively, as repellents throughout development to influence axon pathfinding and topographic mapping, as well as restricting cell migration and intermingling. Recent findings are beginning to characterize the function and signaling of ephrins, as well as major roles for them in other tissues.

Osteoblast recruitment and bone formation enhanced by cell matrix-associated heparin-binding growth-associated molecule (HB-GAM)

Bone has an enormous capacity for growth, regeneration, and remodeling. This capacity is largely due to induction of osteoblasts that are recruited to the site of bone formation. The recruitment of osteoblasts has not been fully elucidated, though the immediate environment of the cells is likely to play a role via cell- matrix interactions. We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation. Intriguingly, N-syndecan, which acts as a receptor for HB-GAM, is expressed by osteoblasts/osteoblast precursors, whose ultrastructural phenotypes suggest active cell motility. The hypothesis that HB-GAM/N-syndecan interaction mediates osteoblast recruitment, as inferred from developmental studies, was tested using osteoblast-type cells that express N-syndecan abundantly. These cells migrate rapidly to HB-GAM in a haptotactic transfilter assay and in a migration assay where HB-GAM patterns were created on culture wells. The mechanism of migration is similar to that previously described for the HB-GAM-induced migratory response of neurons. Our hypothesis that HB-GAM/N-syndecan interaction participates in regulation of osteoblast recruitment was tested using two different in vivo models: an adjuvant-induced arthritic model and a transgenic model. In the adjuvant-induced injury model, the expression of HB-GAM and of N-syndecan is strongly upregulated in the periosteum accompanying the regenerative response of bone. In the transgenic model, the HB-GAM expression is maintained in mesenchymal tissues with the highest expression in the periosteum. The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness. HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

Roger Sperry and his chemoaffinity hypothesis

In the early 1940s, Roger Sperry performed a series of insightful experiments on the visual system of lower vertebrates that led him to draw two important conclusions: When optic fibers were severed, the regenerating fibers grew back to their original loci in the midbrain tectum to re-establish a topographical set of connections; and the re-establishment of these orderly connections underlay the orderly behavior of the animal. From these conclusions, he inferred that each optic fiber and each tectal neuron possessed cytochemical labels that uniquely denoted their neuronal type and position and that optic fibers could utilize these labels to selectively navigate to their matching target cell. This inference was subsequently formulated into a general explanation of how neurons form ordered interconnections during development and became known as the chemoaffinity hypothesis. The origins of this hypothesis, the controversies that surrounded it for several decades and its eventual acceptance, are discussed in this article.

Microlithographic determination of axonal/dendritic polarity in cultured hippocampal neurons

High resolution substrates, created using patterned self-assembled monolayers, are shown to direct axonal and dendritic process extension at the level of a single hippocampal neuron. Axons and dendrites were identified using morphological characteristics and immunocytochemical markers. Patterns were formed on glass coverslips from a co-planar monolayer of cell adhesive aminosilanes and non-adhesive fluorinated silanes. On patterned surfaces, the percentage of the total number of cells attached to the 0.71 mm2 substrate field with compliance to the 25-micron diameter 'somal adhesion site' reached 41 +/- 7% (mean +/- S.D., 428 cells counted). A total of 76 +/- 11% of cells that adhered to a somal attachment site developed a lone process > or = 100 microns oriented in the direction of the continuous aminosilane pathway which was shown to express axonal markers. Cells on either the fluorinated silane, which is non-permissive for neurite outgrowth, or localized on an aminosilane region only 5 microns wide failed to extend major processes. This approach is amenable to a variety of industry standard fabrication techniques and may be used to study the role of fine scale spatial cues in neuronal development and synapse formation.

Mathematical guidance for axons

Axon guidance by gradients plays an important role in wiring up the developing nervous system. Growth cones seem to sense a concentration difference across their spatial extent, and convert this into a signal to move up or down a gradient. In this article, a simple mathematical framework is developed to understand when and where gradient detection can occur as a function of gradient shape. This framework is applied to two examples:the guidance of axons by target-derived diffusible factors in vivo and in collagen gels, and guidance by substrate-bound gradients of optimal shape, as might be relevant in the retinotectal system.Two distinct spatial limits on guidance emerge: I mm for a target-derived diffusible gradient, and I cm for a substrate-bound gradient.

Lipids are required for directional pollen-tube growth

Successful pollination and fertilization are absolute requirements for sexual reproduction in higher plants. Pollen hydration, germination and penetration of the stigma by pollen tubes are influenced by the exudate on wet stigmas and by the pollen coat in species with dry stigmas. The exudate allows pollen tubes to grow directly into the stigma, whereas the pollen coat establishes the contact with the stigma. Pollen tubes then grow into the papillae, which are covered by a cuticle. The components of the exudate or pollen coat that are responsible for pollen tube penetration are not known. To discover the role of the exudate, we tested selected compounds for their ability to act as functional substitutes for exudate in the initial stages of pollen-tube growth on transgenic stigmaless tobacco plants that did not produce exudate. Here we show that lipids are the essential factor needed for pollen tubes to penetrate the stigma, and that, in the presence of these lipids, pollen tubes will also penetrate leaves. We propose that lipids direct pollen-tube growth by controlling the flow of water to pollen in species with dry and wet stigmas.

Axon guidance: stretching gradients to the limit

Neuronal growth cones, the sensory-motile structures at the tips of developing axons, navigate to their targets over distances that can be many times greater than their diameter. They may accomplish this impressive task by following spatial gradients of axon guidance molecules in their environment (Bonhoeffer & Gierer, 1984; Tessier-Lavigne & Placzek, 1991; Baier & Bonhoeffer, 1994). We calculate the optimal shape of a gradient and the distance over which it can be detected by a growth cone for two competing mechanistic models of axon guidance. The results are surprisingly simple: Regardless of the mechanism, the maximum distance is about 1 cm. Since gradients and growth cones have coevolved, we suggest that the shape of the gradient in situ will predict the mechanism of gradient detection. In addition, we show that the experimentally determined dissociation constants for receptor-ligand complexes implicated in axon guidance are about optimal with respect to maximizing guidance distance. The relevance of these results to the retinotectal system is discussed.

Single neuron mosaics of the drosophila gigas mutant project beyond normal targets and modify behavior

gigas is a lethal mutant that differentiates enlarged cells, including the nucleus. This trait manifests only after the completion of the mitotic program. We have taken advantage of this phenotype to test in vivo the capacity of normal target cells to arrest the growth of mutant sensory axons. Single neuron connectivity changes have been analyzed in mosaics after horseradish peroxidase retrograde tracings. A mutant mechanoreceptor neuron, growing over a genetically normal substrate, contacts its normal target, and in addition projects to novel areas of the CNS. The mutant axon does terminate its growth eventually, and the new additional targets that are reached correspond to mechanoreceptor domains in other ganglia, indicating that this territorial constraint is operational in the mutant. gigas neurons maintain their stereotyped profile and represent an expanded version of the normal branching pattern. The ultrastructure of the invading projections does not reveal gliotic or necrotic reactions from the new cell contacts. The functional consequences of the connectivity changes produced by the mutant mechanoreceptors have been studied in grooming behavior. Mosaic flies carrying a single gigas mechanoreceptor show modified, albeit context-coherent, grooming responses after stimulation of the mutant bristle, whereas the response from neighboring normal sensory neurons remains unchanged. All of these experiments indicate that target recognition and growth arrest are two dissectible processes of neural development, and they highlight the autonomous features of the growth cone during pathfinding.

Ephrin-A5 (AL-1/RAGS) is essential for proper retinal axon guidance and topographic mapping in the mammalian visual system

Ephrin-A5 (AL-1/RAGS), a ligand for Eph receptor tyrosine kinases, repels retinal axons in vitro and has a graded expression in the superior colliculus (SC), the major midbrain target of retinal ganglion cells. These properties implicate ephrin-A5 in the formation of topographic maps, a fundamental organizational feature of the nervous system. To test this hypothesis, we generated mice lacking ephrin-A5. The majority of ephrin-A5-/- mice develop to adulthood, are morphologically intact, and have normal anterior-posterior patterning of the midbrain. However, within the SC, retinal axons establish and maintain dense arborizations at topographically incorrect sites that correlate with locations of low expression of the related ligand ephrin-A2. In addition, retinal axons transiently overshoot the SC and extend aberrantly into the inferior colliculus (IC). This defect is consistent with the high level of ephrin-A5 expression in the IC and the finding that retinal axon growth on membranes from wild-type IC is inhibited relative to that on membranes from ephrin-A5-/- IC. These findings show that ephrin-A5 is required for the proper guidance and mapping of retinal axons in the mammalian midbrain.

Organization of the medial pulvinar nucleus in the macaque

BACKGROUND

The medial pulvinar appears to subserve the integration of associative cortical information and projects to visuomotor-related cortex. In contrast to the other pulvinar subdivisions, the medial pulvinar is a polymodal structure. Therefore, we studied the structural organization of the medial pulvinar to determine how it differs from the surrounding unimodal nuclei.

METHODS

Nissl-stained sections were examined to determine the boundaries of, and the distribution of neuronal sizes within, the medial pulvinar. In addition, Golgi-impregnated neurons were examined and drawn for analysis. Only rhesus monkey specimens were used, and the material had been prepared previously for other studies.

RESULTS

Projection neurons have round to oval somata and moderate numbers of primary dendrites that extend for short distances before branching into many secondary branches. Two variations of projection neurons (P1 and P2) were distinguished on the basis of the diameters of their dendritic tree. Both varieties have short dendrites that radiate in all directions. They differ in that P2 cells have longer second tier dendrites than P1 cells. Three types of local circuit neurons, tufted, radiating and varicose, were distinguished on the basis of their dendritic morphology. Four types of afferent fibers were identified. Type 1 afferents form cone-shape terminal arbors. Type 2 afferents are similar to those reported for retinal or cortical terminals. Type 3 afferents are of medium thickness and of an unknown origin. Type 4 afferents are thin and have small varicosities consistent with previously described cortical afferents. Afferent fibers are predominantly oriented along the mediolateral axis of the nucleus. We observed putative contacts between some afferents and local circuit neurons and between local circuit neurons and projection neurons.

CONCLUSIONS

Medial pulvinar neurons are generally smaller and rounder than those found in the adjacent pulvinar nuclei. These results provide additional evidence for structural distinctions between thalamic nuclei having different functions. However, the observed differences are subtle. In addition, the data in this report provide morphological evidence that cortical signals are likely to be integrated by means of the circuitry located within the nucleus.

Development and regeneration of the retinotectal map in goldfish: a computational study

We present a simple computational model to study the interplay of activity-dependent and intrinsic processes thought to be involved in the formation of topographic neural projections. Our model consists of two input layers which project to one target layer. The connections between layers are described by a set of synaptic weights. These weights develop according to three interacting developmental rules: (i) an intrinsic fibre-target interaction which generates chemospecific adhesion between afferent fibres and target cells; (ii) an intrinsic fibre-fibre interaction which generates mutual selective adhesion between the afferent fibres; and (iii) an activity-dependent fibre-fibre interaction which implements Hebbian learning. Additionally, constraints are imposed to keep synaptic weights finite. The model is applied to a set of eleven experiments on the regeneration of the retinotectal projection in goldfish. We find that the model is able to reproduce the outcome of an unprecedented range of experiments with the same set of model parameters, including details of the size of receptive and projective fields. We expect this mathematical framework to be a useful tool for the analysis of developmental processes in general.

A compartment-based, asymmetric representation of the retina in an induced projection to the olfactory cortex

Displacing the optic nerve into the telencephalon in adult Rana pipiens induces a projection to olfactory cortex. We have examined the topographic organization of this projection anatomically by injecting a mixture of biotin dextran (BDA) with 3H-amino acids into the affected eye immediately after making cuts across defined sectors of the nerve fiber layer to trace the complementary patterns of anterograde migration of BDA and 3H label in the cut and intact retinal axons, respectively. Fibers from the temporal side of the optic disc terminated in an oblique band along the posterior two-thirds or more of the ectopic projection field. In contrast, fibers arising in the nasal retina terminated in a parallel strip occupying the anterior one-third or less of the field. Varying the location of the cuts within each hemiretina did not reveal any further organization along the nasotemporal or dorsoventral axes of the retina. The retinal location of the cells involved in this projection was further studied with injections of wheat germ agglutinin conjugated to horseradish peroxidase into the olfactory cortex. Ganglion cells labeled by retrograde transport were found throughout the retina, but they were much more numerous on the temporal side, having a mean spatial density 3.7-7.4 times greater in the temporal hemiretina, whereas the overall ganglion cell density (labeled plus unlabeled) was roughly the same in the two halves of the retina. These data provide an example of a permanent projection in which the overall representation of the retina, though nontopological, is polarized in one axis (nasotemporal) and, therefore, compartmentally organized.

Diffusion in axon guidance

Axon guidance by target-derived diffusible factors plays an important role in the development of the nervous system. This paper considers the constraints imposed on this process by the mathematics of diffusion. A point source continuously producing a factor into an infinite three-dimensional volume is considered as a model for both the in vivo and in vitro situation. Basic constraints for effective guidance are assumed to be that the concentration falls between certain maximum and minimum limits, and that the percentage change in concentration across the width of the growth cone exceeds a certain minimum value. The evolution of the shape of the gradient over time is analysed. Using biologically reasonable parameter values, it is shown that the maximum range over which growth cone guidance by a diffusible factor is possible for large times (several days) after the start of the production of the factor is 500-1000 microm. This maximum distance is independent of the diffusion constant of the diffusing molecule, applies to both chemoattractants and chemorepellents, and agrees with experimental data. At earlier times, however, the constraints may be satisfied for distances up to several millimetres. The time it takes for this maximum guidance distance to fall to the asymptotic value depends on the diffusion constant. This time is a few hours for a small molecule but as much as a few days for a large molecule. The model therefore predicts that guidance over distances larger than 1000 microm is possible if the start of production of the factor is carefully matched to the time when guidance is required.

En-2 regulates the expression of the ligands for Eph type tyrosine kinases in chick embryonic tectum

The retinotectal projection map is organized in a precise retinotopic order, so that the temporo-nasal axis of the retina corresponds to the rostro-caudal axis of the tectum. en-1 and en-2, homologues of the Drosophila segment polarity gene engrailed, are expressed in a gradient along the rostro-caudal axis of the tectal anlage, and are suggested to confer caudal characteristics as the results of transplantation and ectopic engrailed (en) expression. Recently the ligands for Eph type receptor tyrosine kinases have been shown to be expressed strongly at the caudal tectum and play a role in retinotectal map formation by repulsing the temporal retinal fibers. Using the system of replication competent retroviral vector, en-2 RCAS (A/B), we misexpressed en-2 on the tectum. Elf-1 or RAGS was induced at the ectopic En-2 sites. The present results have shown that En-2 can regulate expression of both Elf-1 and RAGS. This suggests that the cells which express en at the early stage of tectum development acquire positional specificity as 'caudal' tectum, and these cells may later express the ligands for Eph type receptor tyrosine kinases. Therefore the temporal retinal fibers which have the receptors are repelled when they meet the ligands on the tectum.

Cues intrinsic to the spinal cord determine the pattern and timing of primary afferent growth

We have used organotypic cultures of embryonic rat spinal cord and dorsal root ganglia (DRG) to study the development of central projections of primary sensory afferent axons that express calcitonin gene-related peptide (CGRP). In vivo, small- and medium-diameter CGRP-positive primary afferents terminate in laminae I, II, and V of the spinal cord and do not enter the ventral horn. A similar pattern of CGRP-positive axonal projections was observed in spinal cord slices of Day 16 embryos (E16) maintained in culture for 6 days. Both intact and dissociated DRG neurons showed the same pattern of central arborization, indicating that complex intercellular interactions between DRG neurons are not required for laminar specific targeting. Furthermore, targeting to the dorsal horn and avoidance of the ventral horn was observed in isolated dorsal and ventral hemicords, suggesting that separate mechanisms mediate the avoidance of CGRP-positive axons from the ventral horn and the elaboration of the afferent arbors within the dorsal horn. CGRP-positive afferents can grow into the dorsal horn only during a brief time window. Cultures of age-matched (isochronic) DRG and spinal cord from E14, E16, and E18 animals showed the characteristic pattern of CGRP-positive axon arborization, while cultures from E20 and neonatal animals did not. Heterochronic cultures indicate that it is the age of the spinal cord, and not the age of the DRG, that determines the ability of the CGRP-positive afferents to arborize within the dorsal horn. Together these results demonstrate that cues intrinsic to the spinal cord can direct sensory projections to appropriate locations in the spinal cord.

Dual action of a carbohydrate epitope on afferent and efferent axons in cortical development

During development of the mammalian cerebral cortex, ingrowing afferents from the thalamus take a path that is different from that of axons leaving the cortical plate. Thalamic axons arrive at the cortex at the time before their target cells of layer 4 are generated in the ventricular zone, but they invade the cortex only shortly before these cells have migrated to their final position in the cortex. Growth-promoting molecules are up-regulated in the developing cortical plate during this period. To identify such molecules, we have generated monoclonal antibodies against membrane preparations from rat postnatal cortex. In Western blots, one antibody (mAb 10) recognized a carbohydrate epitope of a glycoprotein with an apparent molecular weight extending from 180 to 370 kDa. Immunohistochemical staining revealed that the staining pattern of mAb 10 at embryonic stages delineates the pathway of thalamocortical axons, with only very faint labeling of the corticofugal pathway. In vitro assays in combination with time-lapse imaging indicated that mAb 10 has opposite effects on the growth of thalamic and cortical axons. The growth speed and axonal elongation of thalamic fibers on postnatal cortical membranes preincubated with mAb 10 was reduced compared with untreated cortical membranes. In contrast, cortical axons grew faster and stopped their growth less frequently after addition of mAb 10 to a cortical membrane substrate. Taken together, these results suggest that a carbohydrate moiety of a membrane-associated glycoprotein plays a role in the segregation of afferent and efferent cortical axons in the white matter. Moreover, the epitope recognized by mAb 10 might also contribute to regulation of the timing of the thalamocortical innervation at later developmental stages.

The molecular biology of axon guidance

Neuronal growth cones navigate over long distances along specific pathways to find their correct targets. The mechanisms and molecules that direct this pathfinding are the topics of this review. Growth cones appear to be guided by at least four different mechanisms: contact attraction, chemoattraction, contact repulsion, and chemorepulsion. Evidence is accumulating that these mechanisms act simultaneously and in a coordinated manner to direct pathfinding and that they are mediated by mechanistically and evolutionarily conserved ligand-receptor systems.

Growth cones and the cues that repel them

Neuronal growth cones establish appropriate connections with their targets during development by responding to both positive and negative guidance cues. The importance of repulsive and inhibitory cues in pathfinding and target selection has now been firmly established at the cellular and molecular levels. Observations in vitro have demonstrated developmentally significant repulsive interactions among various neuronal populations, providing the basis for molecular and functional characterization of several families of molecules that can mediate these guidance events. Analysis of both the expression and function of these molecules in vivo suggests how they, together with positive guidance cues, participate in the dynamic process of growth-cone guidance during both development and axonal regeneration.

In vivo observations of timecourse and distribution of morphological dynamics in Xenopus retinotectal axon arbors

Changes in neuronal structure can contribute to the plasticity of neuronal connections in the developing and mature nervous system. However, the expectation that they would occur slowly precluded many from considering structural changes as a mechanism underlying synaptic plasticity that occurs over a period of minutes to hours. We took time-lapse confocal images of retinotectal axon arbors to determine the timecourse, magnitude, and distribution of changes in axon arbor structure within living Xenopus tadpoles. Images of axons were collected at intervals of 3 min, 30 min, and 2 h over total observation periods up to 8 h. Branch additions and retractions in arbors imaged at 3 or 30 min intervals were confined to shorter branches. Sites of additions and retractions were distributed throughout the arbor. The average lifetime of branches was about 10 min. Branches of up to 10 microns could be added to the arbor within a single 3 min observation interval. Observations of arbors at 3 min intervals showed rapid changes in the structure of branchtips, including transitions from lamellar growth cones to more streamlined tips, growth cone collaps, and re-extension. Simple branchtips were motile and appeared capable of exploratory behavior when viewed in time-lapse movies. In arbors imaged at 2-h intervals over a total of 8 h, morphological changes included longer branches, tens of microns in length. An average of 50% of the total branch length in the arbor was remodeled within 8 h. The data indicate that the elaboration of the arbor occurs by the random addition of branches throughout the arbor, followed by the selective stabilization of a small fraction of the new branches and the retraction of the majority of branches. Stabilized branches can then elongate and support the addition of more branches. These data show that structural changes in presynaptic axons can occur very rapidly even in complex arbors and can therefore play a role in forms of neuronal plasticity that operate on a timescale of minutes.

Organization of retinal ganglion cell axons in the optic fiber layer and nerve of fetal ferrets

Previous authors have hypothesized that retinotopic projections may be influenced by 'preordering' of the axons as they grow towards their targets. In some nonmammalian species, axons are reorganized at or near the optic nerve head to establish a retinotopic order. Data are ambiguous concerning the retinotopy of the mammalian retinal nerve fiber layer and whether fibers become reorganized at the optic nerve head. We have examined this question in fetal and newborn ferrets (Mustela putorius furo) by comparing the arrangement of axons in the retinal nerve fiber layer with that in the optic nerve. Dil or DiA crystals were implanted into fixed tissue in the innermost layers of the retinal periphery, or at a location midway between the periphery and the optic nerve head. Fluorescence labelling was examined in 100-200 microns Vibratome sections, or the eyecup and nerve were photooxidized and 1-2 microns longitudinal or transverse sections were examined. Regardless of fetal age, eccentricity or quadrant of the implant site, a segregation of labelled peripheral axons from unlabelled central ones was not detected within the nerve fiber layer. Axons coursed into the nerve head along the margin of their retinal quadrant of origin, often entering the optic nerve as a radial wedge, thus preserving a rough map of retinal circumference. However, peripheral axons were in no way restricted to the peripheral (nor central) portions of the nerve head or nerve, indicating that the optic axons do not establish a map of retinal eccentricity. Our results demonstrate that (1) the nerve fiber layer is retinotopic only with respect to circumferential position and (2) optic axons are not actively reorganized to establish a retinotopic ordering at the nerve head. The present results suggest that any degree of order present within the optic nerve is a passive consequence of combining the fascicles of the retinal nerve fiber layer; optic axons are not instructed to establish, nor constrained to maintain, a retinotopic order within the optic nerve.

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.

Rostral optic tectum acquires caudal characteristics following ectopic engrailed expression

BACKGROUND

Expression of the homeobox-containing gene Engrailed (En) in an increasing rostral-to-caudal gradient in the dorsal mesencephalon is the earliest known marker for polarity of the chick optic tectum. In heterotopic transplantation experiments, En protein expression correlates well with the subsequent gradient of cytoarchitecture as well as the pattern of retinotectal projections. The En gradient also correlates with the expression of two putative retinal axon-guidance molecules, RAGS and ELF-1, which are Eph-like receptor tyrosine kinase ligands that may function in the establishment of retinotopic projections by excluding temporal axons from the caudal tectum.

RESULTS

To examine the function of En in determining tectal polarity, we used the replication-competent retroviral vector RCAS to misexpress mouse En-1 throughout the chick tectal primordium. Our results show that the rostral portion of the tectum adopts a caudal phenotype: the gradient of cytoarchitectonic differentiation is abolished, and the molecular markers RAGS and ELF-1 are strongly expressed rostrally. In addition, cell membranes from rostral tectum of RCAS En-1-infected embryos preferentially repel temporal axons in in vitro membrane stripe assays.

CONCLUSIONS

These results are consistent with a role for En in determining rostrocaudal polarity of the developing tectum. The demonstration that both RAGS and ELF-1 are upregulated following En misexpression provides a molecular basis for understanding the previous observation, also based on retrovirus-mediated En misexpression, that nasal axons form ectopic connections in rostral tectum, from which temporal axons are excluded.

Selective neurite outgrowth of cultured cortical neurons on specific regions of brain cryostat sections

During development, axons of the mammalian cerebral cortex show a high degree of selectivity in their growth into specific regions of the central nervous system (CNS). A number of studies have shown that growing axons are guided by permissive or inhibitory membrane-bound molecules. Cryostat sections of the developing brain provide a useful assay to investigate possible membrane-bound guidance cues because such cues are retained in their normal in situ locations in specific regions of the CNS. Moreover, cryostat sections can also be subjected to various treatments that affect membrane-bound molecules. Therefore, to determine the ability of such cues to regulate the growth and guidance of cortical neurites into specific brain regions at different stages of development, we used an in vitro assay system in which explants from newborn hamster cortex were plated onto various regions of cryostat sections from developing and adult hamster brain. Neurite outgrowth from cortical explants onto the cryostat sections was visualized with a fluorescent vital dye. Results showed first that cortical neurites grew robustly on neonatal cryostat sections but only sparsely on sections from adult hamster. Second, cortical neurites grew preferentially on regions of the neonatal sections such as the cortex, basal ganglia, brainstem, thalamus, and colliculus, which are either pathways or targets for cortical axons in vivo. In contrast, cortical neurites avoided growing on the cerebellum and olfactory bulb, which are neither targets nor pathways for cortical neurites in vivo. Results also showed that cortical neurites extending onto cortical regions of neonatal sections preferred to grow along the radial axis of the cortex. Finally, heat treatment of the neonatal sections drastically reduced cortical neurite outgrowth. Taken together, these results suggest that the growth and guidance of cortical neurites is influenced by substrate-bound, developmentally regulated, heat-sensitive guidance cues preserved in the cryostat sections.

Antibody that neutralizes myelin-associated inhibitors of axon growth does not interfere with recognition of target-specific guidance information by rat retinal axons

During development, many CNS projection neurons establish topographically ordered maps in their target regions. Myelin-associated inhibitors of neurite growth contribute to the confinement of fiber tracts during development and limit plastic changes after CNS projections have been formed. Neutralization of myelin-associated growth inhibitors leads to an expansion of the retinal innervation of the superior colliculus (SC). In the lesioned adult mammalian CNS, these long projection neurons are usually unable to regrow axons over long distances after lesion due to myelin-associated inhibitors, which interfere with axonal growth in vivo and in vitro. Application of a specific antibody directed against myelin-inhibitors (IN-1) promotes regrowth of corticospinal tract or retinal ganglion cell axons. In the present study, we asked whether application of an antibody to myelin-associated growth inhibitors would lead to disturbances of target-specific axon guidance. To examine this issue, we used an in vitro model, the "stripe assay", to examine the behavior of rat retinal ganglion cell axons on membranes from embryonic and deafferented adult rat SC. On membrane preparations from embryonic rat SC, retinal fibers avoid posterior tectal membranes, possibly due to the presence of a repulsive factor. Nasal retinal axons show a random growth pattern. On membranes prepared from the deafferented adult rat SC, temporal and nasal axons prefer to grow on membranes prepared from their specific target region, which suggests the involvement of target-derived attractive guidance components. The results of the present study show that retinal axons grow significantly faster in the presence of IN-1 antibody that neutralizes myelin-associated growth inhibitors present in the membrane preparations from the adult rat SC. IN-1 antibody, however, does not interfere with specific axonal guidance. This suggests that axonal guidance and specific target finding are independently regulated in retinal axons.

Directional neurite outgrowth and axonal differentiation of embryonic hippocampal neurons are promoted by a neurite outgrowth domain of the B2-chain of laminin

Molecular cues involved in directional neurite outgrowth and axonal differentiation of embryonic hippocampal neurons were studied on substrates coated in a striped 5 microns pattern with synthetic peptides from a neurite outgrowth (RDIAEIIKDI, P1543) and cell attachment (CDPGYIGSR, P364) domain of the B2- and B1-chains of laminin, respectively. Both peptides supported neuronal attachment, but only the B2-chain-derived P1543 promoted expression of a mature neuronal phenotype. Directional neurite outgrowth and axonal differentiation of embryonic hippocampal neurons were selectively induced by striped substrates of the B2-chain-derived P1543. Axonal differentiation was determined by expression of a phosphorylated epitope of the 200 kDa neurofilament protein in the longer "axonal" neurite of the bipolar embryonic hippocampal neurons. Ethanol (100 mM), a neuroactive compound known to delay neuronal development, impaired both directional neurite outgrowth and expression of a phosphorylated epitope of the 200 kDa neurofilament protein on a patterned P1543 substratum. The present results provide direct evidence that a 10 amino acid peptide (P1543), derived from a neurite outgrowth domain of the B2-chain of laminin, may be an axonal guidance and differentiation factor for embryonic hippocampal neurons in vitro.

Chemoattraction of sensory neuron growth cones by diffusible concentration gradients of acetylcholine

Axon guidance cues are critical for the development and repair of both the central and peripheral nervous systems. These cues serve to help select the pathways taken by axon growth cones, by attracting or repulsing them. During development and following injury to the adult peripheral nervous system, neurons must extend processes, often over long distances, through a variety of cellular environments composed of innervated and uninnervated cells, to find, recognize, and synapse on their appropriate targets. The responsibility for recognizing and responding to the extensive number of cues that are encountered as axons elongate falls on the growth cones at the tip of the elongating axons. Cajal (1928) proposed that denervated target cells release diffusible factors that assist in orienting the direction of out-growth of peripheral axons. However, it is only relatively recently that experiments to identify the molecules responsible for serving this function, and the molecular mechanisms by which they function, have begun to bear fruit. Gradients of both substrate-bound and diffusible factors have now been shown to play critical roles in directing axon outgrowth. The present experiments were aimed at determining whether the neurotransmitter acetylcholine (ACh) can act as a chemoattractant for adult sensory neuron growth cones.

Axons of Xenopus neural tube respond to reversals of neural tube orientation

Axonal trajectories of the Kolmer-Agduhr (KA) neurons of Xenopus embryos, were observed after anterior-posterior (A-P) inversions of neural tube grafts to determine whether KA axons follow cell-inherent directional cues, cues from their immediate environment, or rostrocaudal signals from the embryo. KA axons form one of the earliest ascending spinal pathways in Xenopus and are visible in the lateral marginal zone of whole mounts processed for GABA immunoreactivity. Grafts were made at trunk levels at stages 22-24, 3-5 h before the first KA neurons were detectable and prior to axonal out-growth. Embryos were fixed and immunostained 6-36 h later. KA trajectories within and adjacent to reversed grafts were compared to those of nonrotated control grafts and to neural tube lengths comparable in position and in length in unoperated embryos. Most KA axons within rotated grafts followed the graft's orientation. However, others changed direction, taking novel routes, including turning to conform to the orientation of the host embryo. Reorientations were most common near the posterior host/graft interface. Some host KA cells also reoriented, always within a few hundred microns of the graft interface. Taken together, these growth patterns show that most KA axons within the grafts grow normally with respect to the original polarity of the graft neural tube and maintain that direction even into tissue of opposite polarity, suggesting that their routes are mainly determined by cell-intrinsic and/or local tissue factors. However, the reorientation of many other axons, particularly near graft seams, implies that KA axons can respond to local fluctuations in directional or segment identity signals generated in both host and graft after this perturbation.

Axon guidance: following the Eph plan

Recently discovered 'Eph' family receptors and their ligands appear likely to provide the 'cytochemical tags' that Sperry speculated enable axons projecting from the retina to find their correct targets in the brain.

Positional cues that are strictly localized in the telencephalon induce preferential growth of mitral cell axons

In mice, mitral cells are the major efferent neurons of the main olfactory bulb and elongate axons into a very narrow part of the telencephalon to form a fiber bundle referred to as the lateral olfactory tract (LOT). To clarify the mechanisms responsible for guidance of mitral cell axons along this particular pathway, we co-cultured mouse embryo main olfactory bulbs with the telencephalons, and analyzed the pathways taken by mitral cell axons. Ingrowth of mitral cell axons into the telencephalon was observed in those co-cultures in which the olfactory bulbs had been exactly combined to their normal pathway (the LOT position) of the telencephalon. The axons grew preferentially along the LOT position, and formed a LOT-like fiber bundle. When the olfactory bulbs were grafted at positions apart from their normal pathway, however, no mitral cell axons grew into the telencephalon. Neocortical fragments combined with the telencephalon projected fibers into the telencephalon in random directions. These results suggest that the LOT position of the telencephalon offers a guiding pathway for mitral cell axons and that guiding cues for mitral cell axons are extremely localized.

Cellular migration and axonal outgrowth from adult mammalian peripheral nerves in vitro

It is known that following peripheral nerve transections, sheath cells proliferate and migrate to form a bridge between nerve stumps, which may facilitate axonal regeneration. In the present investigations, cellular migration and axonal outgrowth from nerves of adult mice were studied in vitro using collagen gels. During the first 3 days in culture, profuse migration of fibroblasts and macrophages occurred from the ends of sciatic nerve segments, which had been lesioned in situ a few days prior to explantation, but not from segments of normal nerves. The mechanism of cellular activation in the lesioned nerves was not determined, but migration was blocked by suramin, which inhibits the actions of several growth factors. The migrating cells, which form the bridge tissue, may promote axonal regeneration in two ways. Firstly, axonal outgrowth from isolated intercostal nerves was significantly increased in co-cultures with bridges from lesioned sciatic nerves. This stimulatory effect was inhibited by antibodies to 2.5S nerve growth factor. Secondly, the segments of bridge tissue contracted when removed from animals. It is possible that fibroblasts within the bridge exert traction that would tend to pull the lesioned stumps of peripheral nerve together, as in the healing of skin wounds. The traction may also influence deposition of extracellular matrix materials, such as collagen fibrils, which could orient the growth of the regenerating axons toward the distal nerve stump.

A role for gradient en expression in positional specification on the optic tectum

The optic tectum, the primary visual center in non-mammalian vertebrates, receives retinal fibers in a topographically ordered manner. en (en-1 and en-2, homologs of the Drosophila segment polarity gene engrailed) is expressed in the tectal primordium in a rostrocaudal gradient, around the stage when the polarity of the retinotectal projection map is being determined. Here we report that scattered en expression, caused by retroviral gene transfer, perturbed the retinotopic order. Nasal retinal fibers, which normally recognize the caudal side of the tectum (strong en expression side) as a target, arborized at ectopic sites, as if they found their targets, or degenerated. Temporal retinal fibers, which normally recognize the rostral side of the tectum (weak en expression side) as a target, were also affected in some cases by degeneration or prevention of innervation in the tectum. These results suggest that gradient en expression defines the positional identity of the tectum along the rostrocaudal axis.

Crossed and uncrossed retinal axons respond differently to cells of the optic chiasm midline in vitro

In mouse, retinal axon divergence takes place within a cellular specialization localized at the midline of the optic chiasm. To test whether the cells in this locus present cues for differential retinal axon growth, retinal explants were cocultured with cells dissociated from the chiasmatic midline, both taken from day 14-15 embryos, during the principal period of retinal axon divergence. Compared with crossed axons from other retinal regions, axons from ventrotemporal retina, the sole source of uncrossed axons, were shorter, more fasciculated, and fewer in number when growing on chiasm cells. Furthermore, uncrossed axons avoided clusters of chiasm neurons and glia having the composition and arrangement of the midline specialization, but crossed axons readily grew over them. In contrast to the clusters of chiasm cells, however, individual neurons and glia did not elicit differential retinal axon growth. These data demonstrate that cues for divergence derive from cells resident to the chiasm and suggest that cellular interactions among resident midline cells are required to produce these cues.

FGF signaling and target recognition in the developing Xenopus visual system

We report that the growth cones of Xenopus retinal ganglion cells express fibroblast growth factor receptors (FGFRs) and that bFGF stimulates neurite extension from cultured retinal neurons. Furthermore, bFGF is abundant in the developing optic tract but is reduced in the optic tectum. To test whether FGF signaling plays a role in axonal guidance in vivo, bFGF was exogenously applied to the developing optic pathway in "exposed brain" preparations. FGF-treated retinal axons navigate normally through the optic tract, but the majority veer aberrantly at the tectal border and bypass the target. Our results implicate FGF signaling in target recognition and suggest that diminished levels of bFGF in the tectum cause arriving axons to slow their growth.

Topographical organization in the early postnatal corticopontine projection: a carbocyanine dye and 3-D computer reconstruction study in the rat

We have explored basic rules guiding the early development of topographically organized projections, employing the rat corticopontine projection as a model system. Using anterograde in vivo tracing with 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), we studied the distribution of labelled fibers in the pontine nuclei in relation to cortical site of origin during the first postnatal week. Labelled corticopontine fibers enter the pontine nuclei in distinct, sharply defined zones. The putative terminal fibers typically occupy lamella-like subspaces. Related to changes in cortical site of origin, we describe mediolateral, internal to external, and caudorostral distribution gradients in the pontine nuclei. Fibers originating in the anterolateral cortex occupy an internal central core, while implantations at increasing distance from the anterolateral cortex produce 1) more externally located lamellae, and 2) a caudal to rostral shift in fiber location. Previous investigations have shown that pontocerebellar neurons migrate into the ventral pons in a temporal sequence (Altman and Bayer [1987] J. Comp. Neurol. 257:529). The earliest arriving neurons occupy the central core and later arriving neurons settle in more externally and rostrally located subspaces. We hypothesize that the earliest arriving corticopontine fibers grow into the then only available zone of pontocerebellar neurons (central core), attracted by a diffusible chemotropic cue. Later arriving fibers grow into correspondingly later and more externally and rostrally located contingents of pontocerebellar neurons. Thus, we propose that the topographical organization in the early postnatal corticopontine projection is determined by simple temporal and spatial gradients operative within source (cerebral cortex) and target region (pontine nuclei).

Non-uniform distribution of cellular phenotypes in the optic tectum of the leopard frog

Regional specialization in the retina have been described in a number of species. We have investigated whether such specializations can be found in the optic tectum, an area of the brain responsible for the processing of visual information. Using the tectum of Rana pipiens, we have examined the distribution of three different cell types defined on the basis of their immunoreactivity to somatostatin, substance P, and serotonin antibodies. These three immunoreactive cell populations had differing, nonuniform distributions in the optic tectum. Somatostatin-line immunoreactive cells were largely restricted to the caudal one-third of the tectum, whereas both substance P-like immunoreactive (SP-ir) and serotonin-like immunoreactive (5-HT-ir) cells were found unequally represented throughout the tectum. The percentage of SP-ir cells decreased significantly in both the posterior and medial directions from its high in the anterior lateral tectum. Although serotonin-like immunoreactivity was also greatest in the lateral tectum and decreased significantly medially, it was largely constant in the anterior-to-posterior dimension. The populations of SP-ir and 5-HT-ir cells were nonoverlapping. Our results suggest that information may be processed differently in different regions of the optic tectum.

Immunohistological localization of tenascin-C in the developing and regenerating retinotectal system of two amphibian species

The expression pattern of the extracellular matrix molecule tenascin-C was investigated in the retinotectal system of the frog Discoglossus pictus and the salamander Pleurodeles waltl during development and optic nerve regeneration in the adult. In both species, the retina was devoid of tenascin-C immunoreactivity at all ages studied. During development, tenascin-C was distributed in a gradient in the optic nerve, with the highest immunoreactivity in the eye near part of the optic nerve. The myelin-associated glycoprotein was distributed in a gradient with opposite polarity. In Discoglossus, but not Pleurodeles, tenascin-C was detected in the anterior chiasm. In the tectum of both species, tenascin-C was observed in deep cellular and fiber layers but not in the layers receiving optic fibers or proliferative zones. The distribution patterns of tenascin-C were the same during development and in the adult, except for a disappearance of the molecule from the intraocular part of the optic nerve. After lesioning the optic nerve of adult animals, tenascin-C was strongly reexpressed in the intraocular part of the optic nerve but was only weakly upregulated in the distal optic nerve stump. In contrast, a chondroitin sulfate epitope was strongly upregulated in the distal optic nerve stump. These observations suggest that during development, tenascin-C serves as an attenuating barrier for myelinating cells in the optic nerve and contributes to the guidance of growing retinal ganglion cell axons. Due to its sustained expression in the adult, tenascin-C may have similar functions during regeneration of the lesioned adult retinotectal system.

In vitro guidance of retinal ganglion cell axons by RAGS, a 25 kDa tectal protein related to ligands for Eph receptor tyrosine kinases

The results of previous in vitro experiments indicate that a glycosylphosphatidylinositol (GPI)-anchored protein may play an important role in the guidance of temporal retinal axons during the formation of the topographically ordered retinotectal projection. We have purified and cloned a GPI-anchored, 25 kDa glycoprotein that is a good candidate for a molecule involved in this process. During the time of innervation by retinal ganglion cells, this protein is gradedly expressed in the posterior part of the developing tectum. In two different in vitro assay systems, the recombinant protein induces growth cone collapse and repulsion of retinal ganglion cell axons. These phenomena are observed for axons of temporal as well as nasal origin, indicating that an additional activity may be necessary to confer the nasotemporal specificity observed in previous assays. We named the protein RAGS (for repulsive axon guidance signal). The sequence of RAGS shows significant homology to recently identified ligands for receptor tyrosine kinases of the Eph subfamily.