Inhibition of FGF receptor activity in retinal ganglion cell axons causes errors in target recognition

A GFP-based genetic screen reveals mutations that disrupt the architecture of the zebrafish retinotectal projection

The retinotectal projection is a premier model system for the investigation of molecular mechanisms that underlie axon pathfinding and map formation. Other important features, such as the laminar targeting of retinal axons, the control of axon fasciculation and the intrinsic organization of the tectal neuropil, have been less accessible to investigation. In order to visualize these processes in vivo, we generated a transgenic zebrafish line expressing membrane-targeted GFP under control of the brn3c promoter/enhancer. The GFP reporter labels a distinct subset of retinal ganglion cells (RGCs), which project mainly into one of the four retinorecipient layers of the tectum and into a small subset of the extratectal arborization fields. In this transgenic line, we carried out an ENU-mutagenesis screen by scoring live zebrafish larvae for anatomical phenotypes. Thirteen recessive mutations in 12 genes were discovered. In one mutant, ddl, the majority of RGCs fail to differentiate. Three of the mutations, vrt, late and tard, delay the orderly ingrowth of retinal axons into the tectum. Two alleles of drg disrupt the layer-specific targeting of retinal axons. Three genes, fuzz, beyo and brek, are required for confinement of the tectal neuropil. Fasciculation within the optic tract and adhesion within the tectal neuropil are regulated by vrt, coma, bluk, clew and blin. The mutated genes are predicted to encode molecules essential for building the intricate neural architecture of the visual system.

Morphogen signaling at the vertebrate growth cone: A few cases or a general strategy?

Axon navigation relies on the competence of growth cones to sense and interpret attractive and repulsive guidance cues present along their trajectory. For most neurons, this process is mediated by a limited number of conserved families of ligand-receptor signaling systems, including Ephrin/Eph, Netrins/DCC-Unc5, Slits/Robo, and Semaphorins/Plexin-Neuropilin. Recent studies have demonstrated that some neurons respond also to well-known secreted signaling molecules, best known for their roles as morphogens, such as BMP7, SHH, FGF8, and Wnt. Thus, retina ganglion cell axon navigation is influenced by FGF, SHH, and possibly BMP signaling. Similarly, commissural neurons in the spinal cord respond sequentially to the activity of BMP, SHH, and Wnt to extend toward and away from their intermediate target, the floor plate. The data that support this conclusion will be summarized and how morphogens may signal at the growth cone will be discussed.

When sugars guide axons: insights from heparan sulphate proteoglycan mutants

Although there have previously been hints that heparan sulphate proteoglycans (HSPGs) are important for axon guidance, as they are for many other biological processes, there has been little in vivo evidence for interaction with known axon-guidance pathways. Genetic analyses of fly, mouse, nematode and zebrafish mutants now confirm the role of HSPGs in axon guidance and are beginning to show that they might have a key role in modulating the action of axon-guidance ligands and receptors.

Axon Sorting in the Optic Tract Requires HSPG Synthesis by ext2 (dackel) and extl3 (boxer)

Retinal ganglion cell (RGC) axons are topographically ordered in the optic tract according to their retinal origin. In zebrafish dackel (dak) and boxer (box) mutants, some dorsal RGC axons missort in the optic tract but innervate the tectum topographically. Molecular cloning reveals that dak and box encode ext2 and extl3, glycosyltransferases implicated in heparan sulfate (HS) biosynthesis. Both genes are required for HS synthesis, as shown by biochemical and immunohistochemical analysis, and are expressed maternally and then ubiquitously, likely playing permissive roles. Missorting in box can be rescued by overexpression of extl3. dak;box double mutants show synthetic pathfinding phenotypes that phenocopy robo2 mutants, suggesting that Robo2 function requires HS in vivo; however, tract sorting does not require Robo function, since it is normal in robo2 null mutants. This genetic evidence that heparan sulfate proteoglycan function is required for optic tract sorting provides clues to begin understanding the underlying molecular mechanisms.

Proteoglycans as cues for axonal guidance in formation of retinotectal or retinocollicular projections

Understanding the formation of neuronal circuits has long been one of the basic problems in developmental neurobiology. Projections from the retina to their higher center, the optic tectum in nonmammalian vertebrates and the superior colliculus in mammals, are most amenable to experimental approaches; thus, much information has been accumulated about the mechanisms of axonal guidance. The retinal axons navigate along the appropriate pathway with the help of a series of guidance cues. Although much of the work has focused on proteinaceous factors, proteoglycans have been identified as playing important roles in retinal axon guidance. Chondroitin sulfate proteoglycans and heparan sulfate proteoglycans are involved in essential decisions of axon steering along the pathway. However, it has not been determined whether diversity of the carbohydrate chains results in differential effects and how their diversity is recognized by growth cones, which represent an important area of future research.

PTPmu signaling via PKCdelta is instructive for retinal ganglion cell guidance

The receptor protein tyrosine phosphatase (RPTP) PTPmu mediates distinct cellular responses in nasal and temporal retinal ganglion cell (RGC) axons. PTPmu is permissive for nasal RGC neurite outgrowth and inhibitory to temporal RGCs. In addition, PTPmu causes preferential temporal growth cone collapse. Previous studies demonstrated that PTPmu associates with the scaffolding protein RACK1 and the protein kinase C-delta (PKCdelta) isoform in chick retina and that PKCdelta activity is required for PTPmu-mediated RGC outgrowth. Using in vitro stripe and collapse assays, we find that PKCdelta activity is required for both inhibitory and permissive responses of RGCs to PTPmu, with higher levels of PKCdelta activation associated with temporal growth cone collapse and repulsion. A potential mechanism for differential PKCdelta activation is due to the gradient of PTPmu expression in the retina. PTPmu is expressed in a high temporal, low nasal step gradient in the retina. In support of this, overexpression of exogenous PTPmu in nasal neurites results in a phenotypic switch from permissive to repulsive in response to PTPmu. Together, these results suggest that the differential expression of PTPmu within the retina is instructive for RGC guidance and that the magnitude of PKCdelta activation in response to PTPmu signaling results in the distinct cellular behaviors of nasal and temporal RGCs.

Inhibition of netrin-mediated axon attraction by a receptor protein tyrosine phosphatase

During axon guidance, the ventral guidance of the Caenorhabditis elegans anterior ventral microtubule axon is controlled by two cues, the UNC-6/netrin attractant recognized by the UNC-40/DCC receptor and the SLT-1/slit repellent recognized by the SAX-3/robo receptor. We show here that loss-of-function mutations in clr-1 enhance netrin-dependent attraction, suppressing ventral guidance defects in slt-1 mutants. clr-1 encodes a transmembrane receptor protein tyrosine phosphatase (RPTP) that functions in AVM to inhibit signaling through the DCC family receptor UNC-40 and its effector, UNC-34/enabled. The known effects of other RPTPs in axon guidance could result from modulation of guidance receptors like UNC-40/DCC.

Fibroblast growth factor-2 gene delivery stimulates axon growth by adult retinal ganglion cells after acute optic nerve injury

Basic fibroblast growth factor (or FGF-2) has been shown to be a potent stimulator of retinal ganglion cell (RGC) axonal growth during development. Here we investigated if FGF-2 upregulation in adult RGCs promoted axon regrowth in vivo after acute optic nerve injury. Recombinant adeno-associated virus (AAV) was used to deliver the FGF-2 gene to adult RGCs providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to a 10-fold increase in the number of axons that extended past 0.5 mm from the lesion site compared to control nerves. Detection of AAV-mediated FGF-2 protein in injured RGC axons correlated with growth into the distal optic nerve. The response to FGF-2 upregulation was supported by our finding that FGF receptor-1 (FGFR-1) and heparan sulfate (HS), known to be essential for FGF-2 signaling, were expressed by adult rat RGCs. FGF-2 transgene expression led to only transient protection of injured RGCs. Thus the effect of this neurotrophic factor on axon extension could not be solely attributed to an increase in neuronal survival. Our data indicate that selective upregulation of FGF-2 in adult RGCs stimulates axon regrowth within the optic nerve, an environment that is highly inhibitory for regeneration. These results support the hypothesis that key factors involved in axon outgrowth during neural development may promote regeneration of adult injured neurons.

XSEB4R, a novel RNA-binding protein involved in retinal cell differentiation downstream of bHLH proneural genes

RNA-binding proteins play key roles in the post-transcriptional regulation of gene expression but so far they have not been studied extensively in the context of developmental processes. We report on the molecular cloning and spatio-temporal expression of a novel RNA-binding protein, XSEB4R, which is strongly expressed in the nervous system. This study is focused on the analysis of Xseb4R in the context of primary neurogenesis and retinogenesis. To study Xseb4R function during eye development, we set up a new protocol allowing in vivo lipofection of antisense morpholino oligonucleotides into the retina. The resulting XSEB4R knockdown causes an impairment of neuronal differentiation, with an increase in the number of glial cells. By contrast, our gain-of-function analysis demonstrates that Xseb4R strongly promotes neural differentiation. We also showed a similar function during primary neurogenesis. Consistent with this proneural effect, we found that in the open neural plate Xseb4R expression is upregulated by the proneural gene XNgnr1, as well as by the differentiation gene XNeuroD, but is inhibited by the Notch/Delta pathway. Altogether, our results suggest for the first time a proneural effect for a RNA-binding protein involved in the genetic network of retinogenesis.

Corpus callosum deficiency in transgenic mice expressing a truncated ephrin-A receptor

The A-class of the erythropoietin-producing hepatocellular carcinoma cell-derived (EphA) tyrosine kinase receptors and their ligands, the A-ephrins, play critical roles in the specification of topographic axon projection maps during development. In this study, the role of the EphA subfamily in callosal projections was investigated using transgenic mice expressing a kinase deletion mutant of EphA5. In approximately half of these transgenic mice, cerebral cortical neurons in various cortical regions (primary and secondary somatosensory cortices and frontal as well as visual areas) failed to project to the contralateral cortex. When commissural axons were examined with DiI labeling, few callosal fibers were found to traverse the midline in both the adult and neonatal transgenic mice. This defect in callosal development correlates with the expression of the transgene, because neurons in the superficial layers of the motor cortex, where transgene expression is low, show normal contralateral projection through the corpus callosum. In addition, multiple EphA receptors are expressed in callosal neurons and ephrin-A5 stimulates neurite outgrowth of callosal neurons in vitro. The midline glia structures important for callosal axon midline crossing appear normal in the transgenic mice, suggesting that the defects are unrelated to defective guidance structures at the midline. These observations suggest critical functions for EphA receptor in establishing callosal connections during brain development.

B-type Eph receptors and ephrins induce growth cone collapse through distinct intracellular pathways

Forward and reverse signaling mediated by EphB tyrosine kinase receptors and their transmembrane ephrin-B ligands play important roles in axon pathfinding, yet little is known about the intracellular pathways involved. Here we have used growth cones from the ventral (EphB receptor-bearing) and dorsal (ephrin-B-bearing) embryonic Xenopus retina to investigate the signaling mechanisms in both forward and reverse directions. We report that unclustered, but not clustered, EphB2 ectodomains trigger fast (5-10 min) transient collapse responses in growth cones. This collapse response is mediated by low levels of intracellular cyclic GMP and requires proteasome function. In contrast, clustered, but not unclustered, ephrin-B1 ectodomains cause slow (30-60 min) growth cone collapse that depends on high cGMP levels and is insensitive to inhibition of the proteasomal pathway. Upon receptor-ligand binding, endocytosis occurs in the reverse direction (EphB2-Fc into dorsal retinal growth cones), but not the forward direction, and is also sensitive to proteasomal inhibition. Endocytosis is functionally important because blocking of EphB2 internalization inhibits growth cone collapse. Our data reveal that distinct signaling mechanisms exist for B-type Eph/ephrin-mediated growth cone guidance and suggest that endocytosis provides a fast mechanism for switching off signaling in the reverse direction.

Fibroblast growth factors redirect retinal axons in vitro and in vivo

Growth factors have been shown previously to participate in the process of axon target recognition. We showed that fibroblast growth factor receptor (FGFR) signaling is required for Xenopus laevis retinal ganglion cell (RGC) axons to recognize their major midbrain target, the optic tectum [neuron 17 (1996), 245]. Therefore, we have hypothesized that a change in expression of a fibroblast growth factor (FGF) at the entrance of the optic tectum, the border between the diencephalon and mesencephalon, may serve as a signal to RGC axons that they have reached their target. To determine whether RGC axons can sense changes in FGF levels, we asked whether they altered their behavior upon encountering an ectopic source of FGF. We found that in vivo RGC growth cones avoided FGF-misexpressing cells along their path, and that FGF-2 directly repelled RGC growth cones in an in vitro growth cone turning assay. These data support the idea that RGC axons can sense changes in FGF levels, and as such provide a mechanism by which FGFR signaling is involved in RGC axon target recognition.

N-cadherin mediates retinal lamination, maintenance of forebrain compartments and patterning of retinal neurites

The complex, yet highly ordered and predictable, structure of the neural retina is one of the most conserved features of the vertebrate central nervous system. In all vertebrate classes, retinal neurons are organized into laminae with each neuronal class adopting specific morphologies and patterns of connectivity. Using genetic analyses in zebrafish, we demonstrate that N-cadherin (Ncad) has several distinct and crucial functions during the establishment of retinal organization. Although the location of cell division is disorganized in embryos with reduced or no Ncad function, different classes of retinal neurons are generated. However, these neurons fail to organize into correct laminae, most probably owing to compromised adhesion between retinal cells. In addition, amacrine cells exhibit exuberant and misdirected outgrowth of neurites that contributes to severe disorganization of the inner plexiform layer. Retinal ganglion cells also exhibit defects in process outgrowth, with axons exhibiting fasciculation defects and adopting incorrect ipsilateral trajectories. At least some of these defects are likely to be due to a failure to maintain compartment boundaries between eye, optic nerve and brain. Although in vitro studies have implicated Fgf receptors in modulating the axon outgrowth promoting properties of Ncad, most aspects of the Ncad mutant phenotype are not phenocopied by treatments that block Fgf receptor function.

Possible involvement of a fibroblast growth factor 9 (FGF9)-FGF receptor-3-mediated pathway in adult pig retinal ganglion cell survival in vitro

The expression and potential roles of fibroblast growth factors (FGF) and their cognate FGF receptors (FGFR) in adult mammalian retinal ganglion cells (RGC) are poorly known. We show that FGFR-3 and FGFR-4 are especially pronounced on RGC and amacrine cell bodies in adult pig inner retinae both in vivo and in vitro. Western blotting revealed distinct profiles for each receptor. Expression of each FGFR and effects of the preferred ligand for FGFR-3, FGF9, upon RGC survival and neurite outgrowth were examined in primary retinal cell cultures: whereas there was no stimulation of neuritogenesis, RGC survival was promoted in a dose-dependent manner (ED(50) approximately 500 pg/ml, mean maximal increase of 60%) and could be completely blocked by addition of FGF9 neutralising antibody. Experiments with three additional FGF (FGF1, FGF2, and FGF4) showed no stimulation of RGC survival above control levels. Taken together, these data suggest that the ligand-receptor couple FGF9-FGFR-3 may function to promote survival of adult mammalian RGC, and their application might be beneficial in retinal degenerative diseases such as glaucoma.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

Establishing the trochlear motor axon trajectory: role of the isthmic organiser and Fgf8

Formation of the trochlear nerve within the anterior hindbrain provides a model system to study a simple axonal projection within the vertebrate central nervous system. We show that trochlear motor neurons are born within the isthmic organiser and also immediately posterior to it in anterior rhombomere 1. Axons of the most anterior cells follow a dorsal projection, which circumnavigates the isthmus, while those of more posterior trochlear neurons project anterodorsally to enter the isthmus. Once within the isthmus, axons form large fascicles that extend to a dorsal exit point. We investigated the possibility that the projection of trochlear axons towards the isthmus and their subsequent growth within that tissue might depend upon chemoattraction. We demonstrate that both isthmic tissue and Fgf8 protein are attractants for trochlear axons in vitro, while ectopic Fgf8 causes turning of these axons away from their normal routes in vivo. Both inhibition of FGF receptor activation and inhibition of Fgf8 function in vitro affect formation of the trochlear projection within explants in a manner consistent with a guidance function of Fgf8 during trochlear axon navigation.

Chondroitin sulfate disrupts axon pathfinding in the optic tract and alters growth cone dynamics

Little is known about the cues that guide retinal axons across the diencephalon en route to their midbrain target, the optic tectum. Here we show that chondroitin sulfate proteoglycans are differentially expressed within the diencephalon at a time when retinal axons are growing within the optic tract. Using exposed brain preparations, we show that the addition of exogenous chondroitin sulfate results in retinal pathfinding errors. Retinal axons disperse widely from their normal trajectory within the optic tract and extend aberrantly into inappropriate regions of the forebrain. Time-lapse analysis of retinal growth cone dynamics in vivo shows that addition of exogenous chondroitin sulfate causes intermittent stalling and increases growth cone complexity. These results suggest that chondroitin sulfate may modulate the guidance of retinal axons as they grow through the diencephalon towards the optic tectum.

Metalloproteases and guidance of retinal axons in the developing visual system

Axonal growth cones read cues in their environment that guide them to their target. Metalloproteases have been implicated in vitro in modulating the interaction of these cues with receptors in the growth cone. To determine whether metalloprotease function is important in the guidance of vertebrate axons in vivo, we applied hydroxamate-based metalloprotease inhibitors to Xenopus retinal ganglion cell (RGC) axons as they extended through the optic tract. In the presence of two different inhibitors, 0.5-20 microm N-[(2R)-2(hydroxamideocarbonylmethyl)-4-methylpantanoyl]-l-tryptophan methylamide and batimastat, RGC axons made similar dose-dependent guidance errors. Most axons failed to make the expected caudal turn in the diencephalon and continued straight, growing aberrantly toward and across the dorsal midline. As a result, few RGC axons innervated their midbrain target, the optic tectum. Similarly, if the inhibitors were applied after the axons made the turn, many failed to grow into the optic tectum and instead turned to grow along its anterior border. Interestingly, in many instances pathfinding defects were observed in the absence of problems with axon extension, although outgrowth was impaired at the higher doses of the inhibitors. These data provide compelling in vivo evidence that metalloproteases are important for both axon guidance and extension in the developing visual system.

Neurotrophic factors and axonal growth

Neuronal morphological differentiation is regulated by numerous polypeptide growth factors (neurotrophic factors). Recently, significant progress has been achieved in clarifying the roles of neurotrophins as well as glial cell line-derived neurotrophic factor family members in peripheral axon elongation during development. Additionally, advances have been made in defining the signal transduction mechanisms employed by these factors in mediating axon morphological responses. Several studies addressed the role of neurotrophic factors in regenerative axon growth and suggest that signaling mechanisms in addition to those triggered by receptor tyrosine kinases may be required for successful peripheral nervous system regeneration. Finally, recent investigations demonstrate that neurotrophic factors can enhance axon growth after spinal cord injuries.

Changes in expression of fibroblast growth factor receptors during development of the mouse retinofugal pathway

Retinal axons undergo several changes in organization as they pass through the region of the optic chiasm and optic tract. We used immunocytochemistry to examine the possible involvement of fibroblast growth factor receptors (FGFR) in these changes in retinal axon growth. In the retina, at all ages examined, prominent staining for FGFR was seen in the optic fiber layer and at the optic disk. At embryonic day 15 (E15), FGFR immunoreactivity was also detected in the ganglion cell layer, as defined by immunoreactivity for islet-1. At later developmental stages (E16 to postnatal day 0), FGFR were found in the optic fiber layer and the inner plexiform layer. In the ventral diencephalon, immunostaining for FGFR was first detected at E13 in a group of cells posterior to the chiasm. These cells appeared to match the neurons that are immunopositive for the stage-specific embryonic antigen-1 (SSEA-1). FGFR staining was also found on the retinal axons at E13. At E14-E16, when most axons are growing across the chiasm and the tract, a dynamic pattern of FGFR immunoreactivity was observed on the retinal axons. The staining was reduced when axons reached the midline but was increased when axons reached the threshold of the optic tract. These results suggest that axon growth and fiber patterning in distinct regions of the retinofugal pathway are in part controlled by a regulated expression of FGFR. Furthermore, the axons with elevated FGFR expression in the optic tract have a posterior border of rich FGFR expression in the lateral part of the diencephalon. This region overlaps with a lateral extension of the SSEA-1-positive cells, suggesting a possible relation of these cells to the elevated expression of FGFR.

Pathfinding and error correction by retinal axons: the role of astray/robo2

To address how the highly stereotyped retinotectal pathway develops in zebrafish, we used fixed-tissue and time-lapse imaging to analyze morphology and behavior of wild-type and mutant retinal growth cones. Wild-type growth cones increase in complexity and pause at the midline. Intriguingly, they make occasional ipsilateral projections and other pathfinding errors, which are always eventually corrected. In the astray/robo2 mutant, growth cones are larger and more complex than wild-type. astray axons make midline errors not seen in wild-type, as well as errors both before and after the midline. astray errors are rarely corrected. The presumed Robo ligands Slit2 and Slit3 are expressed near the pathway in patterns consistent with their mediating pathfinding through Robo2. Thus, Robo2 does not control midline crossing of retinal axons, but rather shapes their pathway, by both preventing and correcting pathfinding errors.

Functional regeneration of chronically injured sensory afferents into adult spinal cord after neurotrophin gene therapy

Lesioned axons within the dorsal roots fail to regenerate through the peripheral nerve transition zone and into the spinal cord. This regenerative failure leads to a persistent loss of sensory function. To induce axonal growth across this barrier, we used recombinant adenovirus to express fibroblast growth factor-2 (FGF2), nerve growth factor (NGF), L1 cell adhesion molecule (L1), or beta-galactosidase (LacZ) within the endogenous glia of the dorsal spinal cord 16 d after injury. Expression of either FGF2 or NGF, but not L1 or LacZ, induced robust axonal regeneration into normal as well as ectopic locations within the dorsal spinal cord. This regeneration led to near-normal recovery of thermal sensory function. Functional recovery and the majority of regenerating axons within the dorsal horn disappeared with recutting of the sensory roots. Injections of adenovirus encoding NGF, but not FGF2, also resulted in extensive sprouting of noninjured sensory axons, which we previously demonstrated could cause hyperalgesia and chronic pain. Thus, neurotrophic factor gene therapy administered as late as 16 d after injury may serve as a useful treatment to elicit recovery after dorsal root avulsion; however, the choice of neurotrophin is important to induce selective regeneration of damaged axons.

Receptor protein tyrosine phosphatases regulate retinal ganglion cell axon outgrowth in the developing Xenopus visual system

Receptor protein tyrosine phosphatases (RPTPs) are regulators of axon outgrowth and guidance in a variety of different vertebrate and invertebrate systems. Three RPTPs, CRYP-alpha, PTP-delta, and LAR, are expressed in overlapping but distinct patterns in the developing Xenopus retina, including expression in retinal ganglion cells (RGCs) as they send axons to the tectum (Johnson KG, Holt CE. 2000. Expression of CRYP-alpha, LAR, PTP-delta, and PTP-rho in the developing Xenopus visual system. Mech Dev 92:291-294). In order to examine the role of these RPTPs in visual system development, putative dominant negative RPTP mutants (CS-CRYP-alpha, CS-PTP-delta, and CS-LAR) were expressed either singly or in combination in retinal cells. No effect was found on either retinal cell fate determination or on gross RGC axon guidance to the tectum. However, expression of these CS-RPTP constructs differentially affected the rate of RGC axon outgrowth. In vivo, expression of all three CS-RPTPs or CS-PTP-delta alone inhibited RGC axon outgrowth, while CS-LAR and CS-CRYP-alpha had no significant effect. In vitro, expression of CS-CRYP-alpha enhanced neurite outgrowth, while CS-PTP-delta inhibited neurite outgrowth in a substrate-dependent manner. This study provides the first in vivo evidence that RPTPs regulate retinal axon outgrowth.

Optimization of a peptide/non-cationic lipid gene delivery system for effective microinjection into chicken embryo in vivo

Here we report the characterization and optimization of a peptide/non-cationic lipid gene delivery system that successfully produces high levels of gene expression when delivered by microinjection into chicken embryos in vivo. In addition to plasmid DNA, the delivery complex consisted of four components: 1) a "condensing" peptide with both hydrophobic and cationic amino acid segments; 2) a "fusogenic" peptide with both membrane insertion and amphipathic helical segments; 3) a relatively short-chain phosphatidylcholine (14:1 cis-9); and 4) polyethyleneglycol conjugated to dioleoylphosphatidylethanolamine through a disulfide linkage. Optimum amounts of each component were determined by measuring expression of a luciferase reporter gene following a 24-hour incubation with chick embryo fibroblast (CEF) cells in culture. When relatively low amounts of condensing peptide, fusogenic peptide, or lipid were assembled into the complexes, relatively large concentrations of complex were required to reach maximum gene expression. When the amounts of peptide or lipid were increased, less complex was required to achieve maximum expression, but expression fell substantially with higher amounts of added complex. The polyethyleneglycol component significantly increased gene expression. With some preparations, luciferase activities in the CEF cells reached 1x10(10) relative light units per second per mg protein within 24 hours. Following the optimization experiments with the CEF cells, formulations containing low levels, intermediate levels, and high levels of the delivery system components were assembled with green fluorescent protein plasmid DNA, then microinjected into somite regions of chicken embryos in vivo. It was found that intermediate levels of the components gave the most reliable formulations for inducing localized gene expression in the somitic cells.

Fibroblast growth factor-2 promotes axon branching of cortical neurons by influencing morphology and behavior of the primary growth cone

Interstitial branching is an important mechanism for target innervation in the developing CNS. A previous study of cortical neurons in vitro showed that the terminal growth cone pauses and enlarges in regions from which interstitial axon branches later develop (Szebenyi et al., 1998). In the present study, we investigated how target-derived signals affect the morphology and behaviors of growth cones leading to development of axon branches. We used bath and local application of a target-derived growth factor, FGF-2, on embryonic pyramidal neurons from the sensorimotor cortex and used time-lapse digital imaging to monitor effects of FGF-2 on axon branching. Observations of developing neurons over periods of several days showed that bath-applied FGF-2 significantly increased growth cone size and slowed growth cone advance, leading to a threefold increase in axon branching. FGF-2 also had acute effects on growth cone morphology, promoting rapid growth of filopodia within minutes. Application of FGF-2-coated beads promoted local axon branching in close proximity to the beads. Branching was more likely to occur when the FGF-2 bead was on or near the growth cone, suggesting that distal regions of the axon are more responsive to FGF-2 than other regions of the axon shaft. Together, these results show that interstitial axon branches can be induced locally through the action of a target-derived growth factor that preferentially exerts effects on the growth cone. We suggest that, in target regions, growth factors such as FGF-2 and other branching factors may induce formation of collateral axon branches by enhancing the pausing and enlargement of primary growth cones that determine future branch points.

Heparan sulfate proteoglycans in the nervous system: their diverse roles in neurogenesis, axon guidance, and synaptogenesis

Development of the mammalian nervous system involves generation of neurons from neural stem cells, migration of generated neurons toward genetically determined locations, extension of axons and dendrites, and establishment of neuronal connectivity. Recent progresses revealed diverse role of heparan sulfate proteoglycans in these processes. This article reviews our current knowledge about the functional roles of heparan sulfate proteoglycans in three critical events in mammalian neural development, namely neurogenesis, axon guidance, and synapse development.

Soluble N-cadherin stimulates fibroblast growth factor receptor dependent neurite outgrowth and N-cadherin and the fibroblast growth factor receptor co-cluster in cells

A chimeric molecule consisting of the extracellular domain of the adhesion molecule, N-cadherin, fused to the Fc region of human IgG (NCAD-Fc) supports calcium-dependent cell adhesion and promotes neurite outgrowth following affinity-capture to a tissue culture substrate. When presented to cerebellar neurons as a soluble molecule, the NCAD-Fc stimulated neurite outgrowth in a manner equivalent to that seen for N-cadherin expressed as a cell surface glycoprotein. Neurons expressing a dominant-negative version of the fibroblast growth factor (FGF) receptor did not respond to soluble NCAD-Fc. In cells transfected with full-length N-cadherin and the FGF receptor, antibody-clustering of N-cadherin resulted in a co-clustering of the FGF receptor to discrete patches in the cell membrane. The data demonstrate that the ability of N-cadherin to stimulate neurite outgrowth can be dissociated from its ability to function as a substrate associated adhesion molecule. The N-cadherin and the FGF receptor co-clustering in cells provides a basis for the neurite outgrowth response stimulated by N-cadherin being dependent on FGF receptor function.

The EGF and FGF receptors mediate neuroglian function to control growth cone decisions during sensory axon guidance in Drosophila

Cell adhesion molecules (CAMs) implement the process of axon guidance by promoting specific selection and attachment to substrates. We show that, in Drosophila, loss-of-function conditions of either the Neuroglian CAM, the FGF receptor coded by the gene heartless, or the EGF receptor coded by DER display a similar phenotype of abnormal substrate selection and axon guidance by peripheral sensory neurons. Moreover, neuroglian loss-of-function phenotype can be suppressed by the expression of gain-of-function conditions of heartless or DER. The results are consistent with a scenario where the activity of these receptor tyrosine kinases is controlled by Neuroglian at choice points where sensory axons select between alternative substrates for extension.

Growth cone steering by receptor tyrosine phosphatase delta defines a distinct class of guidance cue

Receptor-type tyrosine phosphatases (RPTPs) are involved in pathfinding decisions by elongating axons, but how they function in these decisions remains unclear. A vertebrate RPTP, PTP-delta, is a neurite-promoting homophilic adhesion molecule; here we demonstrate chemoattraction of CNS growth cones by a locally applied gradient of soluble PTP-delta. The attractive effect of PTP-delta was abolished by inhibition of tyrosine phosphatase activity, but in contrast to other guidance proteins was unaffected by inhibition of cyclic nucleotide activities. Gradients of PTP-delta or of laminin-1 also promoted increases in the speed of growth cone migration, but laminin-1 did not steer growth cones. Our results indicate that PTP-delta is a chemoattractant for vertebrate CNS neurons in vitro and suggest that it represents a distinct class of guidance protein from those previously defined. Further, our data indicate that growth cone attraction is mechanistically distinct from increases in the speed of growth cone movement.

Slit2 is a repellent for retinal ganglion cell axons

We set out to isolate inhibitory guidance cues that affect retinal ganglion cell (RGC) axons in vitro and that could potentially be involved in RGC pathfinding decisions. Here we describe the biochemical purification of an RGC growth cone collapsing factor from bovine brain membranes and its identification as Slit2. Recombinant human Slit2 collapses and repels RGC growth cones from all quadrants of the chick retina. In the developing mouse visual system, slit2 is expressed in the eye, in the optic stalk, and in the ventral diencephalon. Slit2 expression is strong in anterior ventral diencephalic structures but is absent from the ventral midline where the optic chiasm forms. The putative receptors for Slits, robo1 and robo2, are expressed in the inner retinal layer in which RGCs are located. A comparison of the expression patterns of Slit2 and retinal axon trajectories suggests that slit2 acts as a short range repellent for retinal ganglion cell axons.

Ace/Fgf8 is required for forebrain commissure formation and patterning of the telencephalon

Fibroblast growth factors (Fgfs) form a large family of secreted signalling proteins that have a wide variety of roles during embryonic development. Within the central nervous system (CNS) Fgf8 is implicated in patterning neural tissue adjacent to the midbrain-hindbrain boundary. However, the roles of Fgfs in CNS tissue rostral to the midbrain are less clear. Here we examine the patterning of the forebrain in zebrafish embryos that lack functional Fgf8/Ace. We find that Ace is required for the development of midline structures in the forebrain. In the absence of Ace activity, midline cells fail to adopt their normal morphology and exhibit altered patterns of gene expression. This disruption to midline tissue leads to severe commissural axon pathway defects, including misprojections from the eye to ectopic ipsilateral and contralateral targets. Ace is also required for the differentiation of the basal telencephalon and several populations of putative telencephalic neurons but not for overall regional patterning of forebrain derivatives. Finally, we show that ace expression co-localises with anterior neural plate cells that have previously been shown to have forebrain patterning activity. Removal of these cells leads to a failure in induction of ace expression indicating that loss of Ace activity may contribute to the phenotypes observed when anterior neural plate cells are ablated. However, as ace mutant neural plate cells still retain at least some inductive activity, then other signals must also be produced by the anterior margin of the neural plate.

Dosage-sensitive, reciprocal genetic interactions between the Abl tyrosine kinase and the putative GEF trio reveal trio's role in axon pathfinding

The Abelson tyrosine kinase (Abl) is integrated into signal transduction networks regulating axon outgrowth. We have identified the Drosophila trio gene through a mutation that exacerbates the Abl mutant phenotype. Drosophila Trio is an ortholog of mammalian Trio, a protein that contains multiple spectrin-like repeats and two Dbl homology (DH) domains that affect actin cytoskeletal dynamics via the small GTPases Rho and Rac. Phenotypic analysis demonstrates that trio and Abl cooperate in regulating axon outgrowth in the embryonic central nervous system (CNS). Dosage-sensitive interactions between trio and Abl, failed axon connections (fax), and enabled (ena) indicate that Trio is integrated into common signaling networks with these gene products. These observations suggest a mechanism by which Abl-mediated signaling networks influence the actin cytoskeleton in neuronal growth cones.

Genetic analysis of ephrin-A2 and ephrin-A5 shows their requirement in multiple aspects of retinocollicular mapping

Ephrin-A2 and -A5 are thought to be anteroposterior mapping labels for the retinotectal/retinocollicular projection. Here, gene disruptions of both these ephrins are characterized. Focal retinal labeling reveals moderate map abnormalities when either gene is disrupted. Double heterozygotes also have a phenotype, showing an influence of absolute levels. In vitro assays indicate these ephrins are required for repellent activity in the target and also normal responsiveness in the retina. In double homozygotes, anteroposterior order is almost though not completely lost. Temporal or nasal retinal labelings reveal quantitatively similar but opposite shifts, with multiple terminations scattered widely over the target. These results indicate an axon competition mechanism for mapping, with a critical role for ephrins as anteroposterior topographic labels. Dorsoventral topography is also impaired, showing these ephrins are required in mapping both axes.

Micro-scale chromophore-assisted laser inactivation of nerve growth cone proteins

Directed growth cone movement is crucial for the correct wiring of the nervous system. This movement is governed by the concerted actions of cell surface receptors, signaling proteins, cytoskeleton-associated molecules, and molecular motors. In order to investigate the molecular basis of growth cone motility, we applied a new technique to functionally inactivate proteins: micro-scale Chromophore-Assisted Laser Inactivation [Diamond et al. (1993) Neuron 11:409-421]. Micro-CALI uses laser light of 620 nm, focused through microscope optics into a 10-microm spot. The laser energy is targeted via specific Malachite green-labeled, non-function-blocking antibodies, that generate short-lived protein-damaging hydroxyl radicals [Liao et al. (1994) Proc Natl Acad Sci USA 91:2659-2663]. Micro-CALI mediates specific loss of protein function with unachieved spatial and temporal resolution. Combined with time-lapse video microscopy, it offers the possibility to induce and observe changes in growth cone dynamics on a real time base. We present here the effects of the acute and localized inactivation of selected growth cone molecules on growth cone behavior and morphology. Based on our observations, we propose specific roles for these proteins in growth cone motility and neurite outgrowth.

Degeneration and regeneration of ganglion cell axons

The retino-tectal system has been used to study developmental aspects of axon growth, synapse formation and the establishment of a precise topographic order as well as degeneration and regeneration of adult retinal ganglion cell (RGC) axons after axonal lesion. This paper reviews some novel findings that provide new insights into the mechanisms of developmental RGC axon growth, pathfinding, and target formation. It also focuses on the cellular and molecular cascades that underlie RGC degeneration following an axonal lesion and on some therapeutic strategies to enhance survival of axotomized RGCs in vivo. In addition, this review deals with problems related to the induction of regeneration after axonal lesion in the adult CNS using the retino-tectal system as model. Different therapeutic approaches to promote RGC regeneration and requirements for specific target formation of regenerating RGCs in vitro and in vivo are discussed.

Fibroblast growth factor receptor-mediated rescue of x-ephrin B1-induced cell dissociation in Xenopus embryos

The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, have been implicated in regulating cell adhesion and migration during development by mediating cell-to-cell signaling events. Genetic evidence suggests that ephrins may transduce signals and become tyrosine phosphorylated during embryogenesis. However, the induction and functional significance of ephrin phosphorylation is not yet clear. Here, we report that when we used ectopically expressed proteins, we found that an activated fibroblast growth factor (FGF) receptor associated with and induced the phosphorylation of ephrin B1 on tyrosine. Moreover, this phosphorylation reduced the ability of overexpressed ephrin B1 to reduce cell adhesion. In addition, we identified a region in the cytoplasmic tail of ephrin B1 that is critical for interaction with the FGF receptor; we also report FGF-induced phosphorylation of ephrins in a neural tissue. This is the first demonstration of communication between the FGF receptor family and the Eph ligand family and implicates cross talk between these two cell surface molecules in regulating cell adhesion.

Graded interference with FGF signalling reveals its dorsoventral asymmetry at the mid-hindbrain boundary

Signalling by fibroblast growth factors (FGFs) at the mid-hindbrain boundary (MHB) is of central importance for anteroposterior neural patterning from the isthmic organiser. Graded suppression of FGF signalling by increasing amounts of a dominant negative FGF receptor provides evidence that in addition to anteroposterior patterning, FGF signalling is also involved in patterning along the dorsoventral axis at the MHB. FGF signalling at the MHB is required for the activation of the HH target gene spalt at the MHB. Our results indicate that FGF signalling mediates the competence of the MHB to activate spalt in response to SHH. This interdependence of the two signalling pathways is also found in the outbudding optic vesicle where HH requires functional FGF signalling to activate spalt in the proximal eye region.

The receptor tyrosine phosphatase CRYPalpha promotes intraretinal axon growth

Retinal ganglion cell axons grow towards the optic fissure in close contact with the basal membrane, an excellent growth substratum. One of the ligands of receptor tyrosine phosphatase CRYPalpha is located on the retinal and tectal basal membranes. To analyze the role of this RPTP and its ligand in intraretinal growth and guidance of ganglion cell axons, we disrupted ligand- receptor interactions on the retinal basal membrane in culture. Antibodies against CRYPalpha strongly reduced retinal axon growth on the basal membrane, and induced a dramatic change in morphology of retinal growth cones, reducing the size of growth cone lamellipodia. A similar effect was observed by blocking the ligand with a CRYPalpha ectodomain fusion protein. These effects did not occur, or were much reduced, when axons were grown either on laminin-1, on matrigel or on basal membranes with glial endfeet removed. This indicates that a ligand for CRYPalpha is located on glial endfeet. These results show for the first time in vertebrates that the interaction of a receptor tyrosine phosphatase with its ligand is crucial not only for promotion of retinal axon growth but also for maintenance of retinal growth cone lamellipodia on basal membranes.

CAMs and FGF cause a local submembrane calcium signal promoting axon outgrowth without a rise in bulk calcium concentration

Binding of basic fibroblast growth factor (bFGF) and cell adhesion molecules to the nerve cell membrane promotes axon outgrowth. This response can be blocked by antagonists of voltage-gated calcium channels, yet no change of cytosolic calcium concentration in the growth cone can be detected upon binding of the growth factor bFGF or the cell adhesion molecule L1. Using barium as a charge carrier, we show that bFGF and L1 open a calcium influx pathway in growth cones of rat sensory neurons without changing the membrane voltage. L1 does not activate influx in cells expressing a dominant negative mutant of the fibroblast growth factor receptor (FGFR) tyrosine kinase. FGFR-activated influx is blocked by specific antagonists of L- and N-type voltage-gated calcium channels and by an inhibitor of diacylglycerol lipase. We propose that both L1 and bFGF act via the FGFR to generate polyunsaturated fatty acids which in turn cause calcium channels to flicker open and shut. Short-lived domains of raised calcium at the cytosolic mouth of open channels activate axon outgrowth without raising bulk cytosolic calcium concentration. In confirmation of this model, the rapidly-acting calcium buffer BAPTA is significantly more effective at blocking FGF-induced axon outgrowth when compared with the slower buffer EGTA. Generation of short-lived calcium domains may provide a crucial mechanism for axon guidance during development and for promoting regeneration of damaged axons.

Retinotectal ligands for the receptor tyrosine phosphatase CRYPalpha

The cell adhesion molecule-like tyrosine phosphatase CRYPalpha is localized on retinal axons and their growth cones. We present evidence that two isoforms of this type IIa phosphatase, CRYPalpha1 and CRYPalpha2, have extracellular ligands along the developing retinotectal pathway. Using alkaline phosphatase fusion proteins containing the CRYPalpha1 ectodomain, we detect a prominent ligand on basement membranes of the early retina, optic stalk, and chiasm. A second ligand is observed in the endfeet region of radial processes in the developing stratum opticum, the site of initial retinal axon invasion. This latter ligand binds CRYPalpha2 preferentially. Further ligand interactions are detected for both CRYPalpha protein isoforms in retinorecipient tectal laminae and on retinal fibers themselves. CRYPalpha thus has cell- and matrix-associated ligands along the entire retinotectal projection. Moreover, these ligands appear to be heterotypic and interact with CRYPalpha through both its immunoglobulin and fibronectin type III regions. The anteroposterior levels of the ligands are relatively uniform within the retina and tectum, suggesting that the CRYPalpha protein within retinal axons does not directly recognise topographically graded guidance cues. We propose that CRYPalpha may have a permissive role in promoting retinal axon growth across the eye and tectum and that its functions are modulated temporally and spatially by isoform-specific interactions with cell- and matrix-associated ligands.

BMPs as mediators of roof plate repulsion of commissural neurons

During spinal cord development, commissural (C) neurons, located near the dorsal midline, send axons ventrally and across the floor plate (FP). The trajectory of these axons toward the FP is guided in part by netrins. The mechanisms that guide the early phase of C axon extension, however, have not been resolved. We show that the roof plate (RP) expresses a diffusible activity that repels C axons and orients their growth within the dorsal spinal cord. Bone morphogenetic proteins (BMPs) appear to act as RP-derived chemorepellents that guide the early trajectory of the axons of C neurons in the developing spinal cord: BMP7 mimics the RP repellent activity for C axons in vitro, can act directly to collapse C growth cones, and appears to serve an essential function in RP repulsion of C axons.

Fibroblast growth factor signaling regulates growth and morphogenesis at multiple steps during brain development

The fibroblast growth factor (FGF) family comprises several members with distinct patterns of expression in the developing central nervous system. FGFs regulate the early specification and the subsequent growth of central nervous system regions. These different actions require the coordinated activation of distinct sets of target genes by FGFs at the appropriate stage of development. The role of FGF2 in the growth and morphogenesis of the cerebral cortex is reviewed in detail. The cellular and molecular mechanisms that underlie the action of FGF2 on cortical development are discussed.

Requirement for the zebrafish mid-hindbrain boundary in midbrain polarisation, mapping and confinement of the retinotectal projection

The organizer at the midbrain-hindbrain boundary (MHB organizer) has been proposed to induce and polarize the midbrain during development. We investigate the requirement for the MHB organizer in acerebellar mutants, which lack a MHB and cerebellum, but retain a tectum, and are mutant for fgf8, a candidate inducer and polarizer. We examine the retinotectal projection in the mutants to assay polarity in the tectum. In mutant tecta, retinal ganglion cell (RGC) axons form overlapping termination fields, especially in the ventral tectum, and along both the anterior-posterior and dorsal-ventral axis of the tectum, consistent with a MHB requirement in generating midbrain polarity. However, polarity is not completely lost in the mutant tecta, in spite of the absence of the MHB. Moreover, graded expression of the ephrin family ligand Ephrin-A5b is eliminated, whereas Ephrin-A2 and Ephrin-A5a expression is leveled in acerebellar mutant tecta, showing that ephrins are differentially affected by the absence of the MHB. Some RGC axons overshoot beyond the mutant tectum, suggesting that the MHB also serves a barrier function for axonal growth. By transplanting whole eye primordia, we show that mapping defects and overshooting largely, but not exclusively, depend on tectal, but not retinal genotype, and thus demonstrate an independent function for Fgf8 in retinal development. The MHB organizer, possibly via Fgf8 itself, is thus required for midbrain polarisation and for restricting axonal growth, but other cell populations may also influence midbrain polarity.