Fibroblast growth factors redirect retinal axons in vitro and in vivo

Roles of Fibroblast Growth Factors in the Axon Guidance

Fibroblast growth factors (FGFs) have been widely studied by virtue of their ability to regulate many essential cellular activities, including proliferation, survival, migration, differentiation and metabolism. Recently, these molecules have emerged as the key components in forming the intricate connections within the nervous system. FGF and FGF receptor (FGFR) signaling pathways play important roles in axon guidance as axons navigate toward their synaptic targets. This review offers a current account of axonal navigation functions performed by FGFs, which operate as chemoattractants and/or chemorepellents in different circumstances. Meanwhile, detailed mechanisms behind the axon guidance process are elaborated, which are related to intracellular signaling integration and cytoskeleton dynamics.

Ca2+ homeostasis maintained by TMCO1 underlies corpus callosum development via ERK signaling

Transmembrane of coiled-coil domains 1 (TMCO1) plays an important role in maintaining homeostasis of calcium (Ca²⁺) stores in the endoplasmic reticulum (ER). TMCO1-defect syndrome shares multiple features with human cerebro-facio-thoracic (CFT) dysplasia, including abnormal corpus callosum (CC). Here, we report that TMCO1 is required for the normal development of CC through sustaining Ca²⁺ homeostasis. Tmco1^-/- mice exhibit severe agenesis of CC with stalled white matter fiber bundles failing to pass across the midline. Mechanistically, the excessive Ca²⁺ signals caused by TMCO1 deficiency result in upregulation of FGFs and over-activation of ERK, leading to an excess of glial cell migration and overpopulated midline glia cells in the indusium griseum which secretes Slit2 to repulse extension of the neural fiber bundles before crossing the midline. Supportingly, using the clinical MEK inhibitors to attenuate the over-activated FGF/ERK signaling can significantly improve the CC formation in Tmco1^-/- brains. Our findings not only unravel the underlying mechanism of abnormal CC in TMCO1 defect syndrome, but also offer an attractive prevention strategy to relieve the related agenesis of CC in patients.

ADGRL3, FGF1 and DRD4: Linkage and Association with Working Memory and Perceptual Organization Candidate Endophenotypes in ADHD

Attention deficit hyperactivity disorder (ADHD) is a highly heritable neurobehavioral disorder that affects children worldwide, with detrimental long-term consequences in affected individuals. ADHD-affected patients display visual-motor and visuospatial abilities and skills that depart from those exhibited by non-affected individuals and struggle with perceptual organization, which might partially explain impulsive responses. Endophenotypes (quantifiable or dimensional constructs that are closely related to the root cause of the disease) might provide a more powerful and objective framework for dissecting the underlying neurobiology of ADHD than that of categories offered by the syndromic classification. In here, we explore the potential presence of the linkage and association of single-nucleotide polymorphisms (SNPs), harbored in genes implicated in the etiology of ADHD (ADGRL3, DRD4, and FGF1), with cognitive endophenotypes related to working memory and perceptual organization in 113 nuclear families. These families were ascertained from a geographical area of the Caribbean coast, in the north of Colombia, where the community is characterized by its ethnic diversity and differential gene pool. We found a significant association and linkage of markers ADGRL3-rs1565902, DRD4-rs916457 and FGF1-rs2282794 to neuropsychological tasks outlining working memory and perceptual organization such as performance in the digits forward and backward, arithmetic, similarities, the completion of figures and the assembly of objects. Our results provide strong support to understand ADHD as a combination of working memory and perceptual organization deficits and highlight the importance of the genetic background shaping the neurobiology, clinical complexity, and physiopathology of ADHD. Further, this study supplements new information regarding an ethnically diverse community with a vast African American contribution, where ADHD studies are scarce.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

Familiar growth factors have diverse roles in neural network assembly

The assembly of neuronal circuits during development depends on guidance of axonal growth cones by molecular cues deposited in their environment. While a number of families of axon guidance molecules have been identified and reviewed, important and diverse activities of traditional growth factors are emerging. Besides clear and well recognized roles in the regulation of cell division, differentiation and survival, new research shows later phase roles for a number of growth factors in promoting neuronal migration, axon guidance and synapse formation throughout the nervous system.

Common cues wire the spinal cord: Axon guidance molecules in spinal neuron migration

Topographic arrangement of neuronal cell bodies and axonal tracts are crucial for proper wiring of the nervous system. This involves often-coordinated neuronal migration and axon guidance during development. Most neurons migrate from their birthplace to specific topographic coordinates as they adopt the final cell fates and extend axons. The axons follow temporospatial specific guidance cues to reach the appropriate targets. When neuronal or axonal migration or their coordination is disrupted, severe consequences including neurodevelopmental disorders and neurological diseases, can arise. Neuronal and axonal migration shares some molecular mechanisms, as genes originally identified as axon guidance molecules have been increasingly shown to direct both navigation processes. This review focuses on axon guidance pathways that are shown to also direct neuronal migration in the vertebrate spinal cord.

Receptor Tyrosine Kinases and Phosphatases in Neuronal Wiring: Insights From Drosophila

Tyrosine phosphorylation is at the crossroads of many signaling pathways. Brain wiring is not an exception, and several receptor tyrosine kinases (RTKs) and tyrosine receptor phosphates (RPTPs) have been involved in this process. Considerable work has been done on RTKs, and for many of them, detailed molecular mechanisms and functions in several systems have been characterized. In contrast, RPTPs have been studied considerably less and little is known about their ligands and substrates. In both families, we find redundancy between different members to accomplish particular wiring patterns. Strikingly, some RTKs and RPTPs have lost their catalytic activity during evolution, but not their importance in biological processes. In this regard, we have to keep in mind that these proteins have multiple domains and some of their functions are independent of tyrosine phosphorylation/dephosphorylation. Since RTKs and RPTPs are enzymes involved not only in early stages of axon and dendrite pathfinding but also in synapse formation and physiology, they have a potential as drug targets. Drosophila has been a key model organism in the search of a better understanding of brain wiring, and its sophisticated toolbox is very suitable for studying the function of genes with pleiotropic functions such as RTKs and RPTPs, from wiring to synaptic formation and function. In these review, we mainly cover findings from this model organism and complement them with discoveries in vertebrate systems.

Fibroblast growth factor acts upon the transcription of phospholipase C genes in human umbilical vein endothelial cells

Besides the control of calcium levels, the phosphoinositide-specific phospholipases C (PI-PLCs), the main players in the phosphoinositide signalling pathway, contribute to a number of cell activities. The expression of PI-PLCs is strictly tissue specific and evidence suggests that it varies under different conditions, such as tumour progression or cell activation. In previous studies, we obtained a complete panel of expression of PI-PLC isoforms in human umbilical vein endothelial cells (HUVEC), a widely used experimental model for endothelial cells (EC), and demonstrated that the expression of the PLC genes varies under inflammatory stimulation. The fibroblast growth factor (FGF) activates the PI-PLC γ1 isoform. In the present study, PI-PLC expression in FGF-treated HUVEC was performed using RT-PCR, observed 24 h after stimulation. The expression of selected genes after stimulation was perturbed, suggesting that FGF affects gene transcription in PI signalling as a possible mechanism of regulation of its activity upon the AkT-PLC pathway. The most efficient effects of FGF were recorded in the 3-6-h interval. To understand the complex events progressing in EC might provide useful insights for potential therapeutic strategies. The opportunity to manipulate the EC might offer a powerful tool of considerable practical and clinical importance.

Developmental origin of patchy axonal connectivity in the neocortex: a computational model

Injections of neural tracers into many mammalian neocortical areas reveal a common patchy motif of clustered axonal projections. We studied in simulation a mathematical model for neuronal development in order to investigate how this patchy connectivity could arise in layer II/III of the neocortex. In our model, individual neurons of this layer expressed the activator-inhibitor components of a Gierer-Meinhardt reaction-diffusion system. The resultant steady-state reaction-diffusion pattern across the neuronal population was approximately hexagonal. Growth cones at the tips of extending axons used the various morphogens secreted by intrapatch neurons as guidance cues to direct their growth and invoke axonal arborization, so yielding a patchy distribution of arborization across the entire layer II/III. We found that adjustment of a single parameter yields the intriguing linear relationship between average patch diameter and interpatch spacing that has been observed experimentally over many cortical areas and species. We conclude that a simple Gierer-Meinhardt system expressed by the neurons of the developing neocortex is sufficient to explain the patterns of clustered connectivity observed experimentally.

Fibroblast growth factor 2 regulates adequate nigrostriatal pathway formation in mice

Fibroblast growth factor 2 (FGF-2) is an important neurotrophic factor that promotes survival of adult mesencephalic dopaminergic (mDA) neurons and regulates their adequate development. Since mDA neurons degenerate in Parkinson's disease, a comprehensive understanding of their development and maintenance might contribute to the development of causative therapeutic approaches. The current analysis addressed the role of FGF-2 in mDA axonal outgrowth, pathway formation, and innervation of respective forebrain targets using organotypic explant cocultures of ventral midbrain (VM) and forebrain (FB). An enhanced green fluorescent protein (EGFP) transgenic mouse strain was used for the VM explants, which allowed combining and distinguishing of individual VM and FB tissue from wildtype and FGF-2-deficient embryonic day (E)14.5 embryos, respectively. These cocultures provided a suitable model to study the role of target-derived FB and intrinsic VM-derived FGF-2. In fact, we show that loss of FGF-2 in both FB and VM results in significantly increased mDA fiber outgrowth compared to wildtype cocultures, proving a regulatory role of FGF-2 during nigrostriatal wiring. Further, we found in heterogeneous cocultures deficient for FGF-2 in FB and VM, respectively, similar phenotypes with wider fiber tracts compared to wildtype cocultures and shorter fiber outgrowth distance than cocultures completely deficient for FGF-2. Additionally, the loss of target-derived FGF-2 in FB explants resulted in decreased caudorostral glial migration. Together these findings imply an intricate interplay of target-derived and VM-derived FGF signaling, which assures an adequate nigrostriatal pathway formation and target innervation.

The Xenopus retinal ganglion cell as a model neuron to study the establishment of neuronal connectivity

Neurons receive inputs through their multiple branched dendrites and pass this information on to the next neuron via long axons, which branch within the target. The shape the neuron acquires is thus the key to its proper functioning in the neural circuit in which it participates. Both axons and dendrites grow in a directed fashion to their target partner neurons by responding to a large number of molecular cues in the milieu through which they extend. They then go through the process of synaptogenesis, first choosing a neuron on which to synapse, and then the appropriate subcellular location. How a neuron acquires its unique shape, establishes and modifies appropriate synaptic connectivity, and the molecular signals involved, are key questions in developmental neurobiology. Such questions of nervous system wiring are being pursued actively with a variety of different animal models and neuron types, each with its own unique advantages. Among these, the developing retinal ganglion cell (RGC) of the South African clawed frog, Xenopus laevis, has proven particularly fruitful for revealing the secrets of how axons and dendrites acquire their final morphology and connectivity. In this review, we describe how this system can be used to understand the multiple molecular events that instruct the incorporation of RGCs into the neural circuit that controls vision.

Changes in fibroblast growth factor-2 and FGF receptors in the frog visual system during optic nerve regeneration

We have previously shown that application of fibroblast growth factor-2 (FGF-2) to cut optic nerve axons enhances retinal ganglion cell (RGC) survival in the adult frog visual system. These actions are mediated via activation of its high affinity receptor FGFR1, enhanced BDNF and TrkB expression, increased CREB phosphorylation, and by promoting MAPK and PKA signaling pathways. The role of endogenous FGF-2 in this system is less well understood. In this study, we determine the distribution of FGF-2 and its receptors in normal animals and in animals at different times after optic nerve cut. Immunohistochemistry and Western blot analysis were conducted using specific antibodies against FGF-2 and its receptors in control retinas and optic tecta, and after one, three, and six weeks post nerve injury. FGF-2 was transiently increased in the retina while it was reduced in the optic tectum just one week after optic nerve transection. Axotomy induced a prolonged upregulation of FGFR1 and FGFR3 in both retina and tectum. FGFR4 levels decreased in the retina shortly after axotomy, whereas a significant increase was detected in the optic tectum. FGFR2 distribution was not affected by the optic nerve lesion. Changes in the presence of these proteins after axotomy suggest a potential role during regeneration.

Regulation of axonal growth and neuromuscular junction formation by neuronal phosphatase and tensin homologue signaling

Axonal growth and synaptogenesis are sequential events of neuronal development. Phosphatase and tensin homologue (PTEN) is expressed in motor neurons, and its disruption leads to continued axonal extension, even upon muscle contact, leading to synaptogenic suppression. Thus PTEN is involved in target-mediated cessation of axonal growth and subsequent synaptic differentiation.

During the development of the vertebrate neuromuscular junction (NMJ), motor axon tips stop growing after contacting muscle and transform into presynaptic terminals that secrete the neurotransmitter acetylcholine and activate postsynaptic ACh receptors (AChRs) to trigger muscle contraction. The neuron-intrinsic signaling that retards axonal growth to facilitate stable nerve–muscle interaction and synaptogenesis is poorly understood. In this paper, we report a novel function of presynaptic signaling by phosphatase and tensin homologue (PTEN) in mediating a growth-to-synaptogenesis transition in neurons. In Xenopus nerve–muscle cocultures, axonal growth speed was halved after contact with muscle, when compared with before contact, but when cultures were exposed to the PTEN blocker bisperoxo (1,10-phenanthroline) oxovanadate, axons touching muscle grew ∼50% faster than their counterparts in control cultures. Suppression of neuronal PTEN expression using morpholinos or the forced expression of catalytically inactive PTEN in neurons also resulted in faster than normal axonal advance after contact with muscle cells. Significantly, interference with PTEN by each of these methods also led to reduced AChR clustering at innervation sites in muscle, indicating that disruption of neuronal PTEN signaling inhibited NMJ assembly. We thus propose that PTEN-dependent slowing of axonal growth enables the establishment of stable nerve–muscle contacts that develop into NMJs.

Reelin is required for class-specific retinogeniculate targeting

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto neurons within subnuclei of the lateral geniculate nucleus (LGN) [i.e., the dorsal LGN (dLGN), ventral LGN (vLGN), and intergeniculate leaflet (IGL)]. Distinct classes of RGCs project to these subnuclei: the dLGN is innervated by image-forming RGCs, whereas the vLGN and IGL are innervated by non-image-forming RGCs. To explore potential mechanisms regulating class-specific LGN targeting, we sought to identify differentially expressed targeting molecules in these LGN subnuclei. One candidate targeting molecule enriched in the vLGN and IGL during retinogeniculate circuit formation was the extracellular matrix molecule reelin. Anterograde labeling of RGC axons in mutant mice lacking functional reelin (reln(rl/rl)) revealed reduced patterns of vLGN and IGL innervation and misrouted RGC axons in adjacent non-retino-recipient thalamic nuclei. Using genetic reporter mice, we further demonstrated that mistargeted axons were from non-image-forming, intrinsically photosensitive RGCs (ipRGCs). In contrast to mistargeted ipRGC axons, axons arising from image-forming RGCs and layer VI cortical neurons correctly targeted the dLGN in reln(rl/rl) mutants. Together, these data reveal that reelin is essential for the targeting of LGN subnuclei by functionally distinct classes of RGCs.

Dynamic expression of axon guidance cues required for optic tract development is controlled by fibroblast growth factor signaling

Axons are guided to their targets by molecular cues expressed in their environment. How is the presence of these cues regulated? Although some evidence indicates that morphogens establish guidance cue expression as part of their role in patterning tissues, an important question is whether morphogens are then required to maintain guidance signals. We found that fibroblast growth factor (FGF) signaling sustains the expression of two guidance cues, semaphorin3A (xsema3A) and slit1 (xslit1), throughout the period of Xenopus optic tract development. With FGF receptor inhibition, xsema3A and xslit1 levels were rapidly diminished, and retinal ganglion cell axons arrested in the mid-diencephalon, before reaching their target. Importantly, direct downregulation of XSema3A and XSlit1 mostly phenocopied this axon guidance defect. Thus, FGFs promote continued presence of specific guidance cues critical for normal optic tract development, suggesting a second later role for morphogens, independent of tissue patterning, in maintaining select cues by acting to regulate their transcription.

Weaving the neuronal net with target-derived fibroblast growth factors

Fibroblast growth factors (FGFs) are a large family of secreted growth factors that are involved in the development, regeneration and repair of various tissues. In the nervous system, FGFs have been implicated in early developmental processes, such as neural induction, proliferation and patterning. Accumulating data indicate that FGFs are also important for the formation of functional neural networks. The role of FGFs in axon guidance, target recognition and synaptic differentiation as target-derived factors, and how they cooperate with cell adhesion molecules that are also involved in the wiring of the nervous system are the focus of this review.

Electroporation of cDNA/Morpholinos to targeted areas of embryonic CNS in Xenopus

Background

Blastomere injection of mRNA or antisense oligonucleotides has proven effective in analyzing early gene function in Xenopus. However, functional analysis of genes involved in neuronal differentiation and axon pathfinding by this method is often hampered by earlier function of these genes during development. Therefore, fine spatio-temporal control of over-expression or knock-down approaches is required to specifically address the role of a given gene in these processes.

Results

We describe here an electroporation procedure that can be used with high efficiency and low toxicity for targeting DNA and antisense morpholino oligonucleotides (MOs) into spatially restricted regions of the Xenopus CNS at a critical time-window of development (22–50 hour post-fertilization) when axonal tracts are first forming. The approach relies on the design of "electroporation chambers" that enable reproducible positioning of fixed-spaced electrodes coupled with accurate DNA/MO injection. Simple adjustments can be made to the electroporation chamber to suit the shape of different aged embryos and to alter the size and location of the targeted region. This procedure can be used to electroporate separate regions of the CNS in the same embryo allowing separate manipulation of growing axons and their intermediate and final targets in the brain.

Conclusion

Our study demonstrates that electroporation can be used as a versatile tool to investigate molecular pathways involved in axon extension during Xenopus embryogenesis. Electroporation enables gain or loss of function studies to be performed with easy monitoring of electroporated cells. Double-targeted transfection provides a unique opportunity to monitor axon-target interaction in vivo. Finally, electroporated embryos represent a valuable source of MO-loaded or DNA transfected cells for in vitro analysis. The technique has broad applications as it can be tailored easily to other developing organ systems and to other organisms by making simple adjustments to the electroporation chamber.

Initiation to end point: the multiple roles of fibroblast growth factors in neural development

From a wealth of experimental findings, derived from both in vitro and in vivo experiments, it is becoming clear that fibroblast growth factors regulate processes that are central to all aspects of nervous system development. Some of these functions are well known, whereas others, such as the roles of these proteins in axon guidance and synaptogenesis, have been established only recently. The emergent picture is one of remarkable economy, in which this family of ligands is deployed and redeployed at successive developmental stages to sculpt the nervous system.

Fibroblast growth factor blocks Sonic hedgehog signaling in neuronal precursors and tumor cells

The Sonic hedgehog (Shh) and FGF signaling pathways regulate growth and differentiation in many regions of the nervous system, but interactions between these pathways have not been studied extensively. Here, we examine the relationship between Shh and FGF signaling in granule cell precursors (GCPs), which are the most abundant neural progenitors in the cerebellum and the putative cell of origin for the childhood brain tumor medulloblastoma. In these cells, Shh induces a potent proliferative response that is abolished by coincubation with basic FGF. FGF also inhibits transcription of Shh target genes and prevents activation of a Gli-responsive promoter in fibroblasts, which suggests that it blocks Shh signaling upstream of Gli-mediated transcription. FGF-mediated inhibition of Shh responses requires activation of FGF receptors and of ERK and JNK kinases, because it can be blocked by inhibitors of these enzymes. Finally, FGF promotes differentiation of GCPs in vitro and in vivo and halts proliferation of tumor cells from patched (ptc) mutant mice, a model for medulloblastoma. These findings suggest that FGF is a potent inhibitor of Shh signaling and may be a useful therapy for tumors involving activation of the hedgehog pathway.

Midline radial glia translocation and corpus callosum formation require FGF signaling

Midline astroglia in the cerebral cortex develop earlier than other astrocytes through mechanisms that are still unknown. We show that radial glia in dorsomedial cortex retract their apical endfeet at midneurogenesis and translocate to the overlaying pia, forming the indusium griseum. These cells require the fibroblast growth factor receptor 1 (Fgfr1) gene for their precocious somal translocation to the dorsal midline, as demonstrated by inactivating the Fgfr1 gene in radial glial cells and by RNAi knockdown of Fgfr1 in vivo. Dysfunctional astroglial migration underlies the callosal dysgenesis in conditional Fgfr1 knockout mice, suggesting that precise targeting of astroglia to the cortex has unexpected roles in axon guidance. FGF signaling is sufficient to induce somal translocation of radial glial cells throughout the cortex; furthermore, the targeting of astroglia to dorsolateral cortex requires FGFr2 signaling after neurogenesis. Hence, FGFs have an important role in the transition from radial glia to astrocytes by stimulating somal translocation of radial glial cells.

Regulated expression of FLRT genes implies a functional role in the regulation of FGF signalling during mouse development

Within the mammalian genome, there are many multimember gene families that encode membrane proteins with extracellular leucine rich repeats which are thought to act as cell adhesion or signalling molecules. We previously showed that the members of the NLRR gene family are expressed in a developmentally restricted manner in the mouse with NLRR-1 being expressed in the developing myotome. The FLRT gene family shows a similar genomic layout and predicted protein secondary structure to the NLRRs so we analysed expression of the three FLRT genes during mouse development. FLRTs are glycosylated membrane proteins expressed at the cell surface which localise in a homophilic manner to cell-cell contacts expressing the focal adhesion marker vinculin. Each member of the FLRT family has a distinct, highly regulated expression pattern, as was seen for the NLRR family. FLRT3 has a provocative expression pattern during somite development being expressed in regions of the somite where muscle precursor cells migrate from the dermomyotome and move into the myotome, and later in myotomal precursors destined to migrate towards their final destination, for example, those that form the ventral body wall. FLRT3 is also expressed at the midbrain/hindbrain boundary and in the apical ectodermal ridge, regions where FGF signalling is known to be important, suggesting that the role for FLRT3 in FGF signalling identified in Xenopus is conserved in mammals. FLRT1 is expressed at brain compartmental boundaries and FLRT2 is expressed in a subset of the sclerotome, adjacent to the region that forms the syndetome, suggesting that interaction with FGF signalling may be a general property of FLRT proteins. We confirmed this by showing that all FLRTs can interact with FGFR1 and FLRTs can be induced by the activation of FGF signalling by FGF-2. We conclude that FLRT proteins act as regulators of FGF signalling, being induced by the signal and then able to interact with the signalling receptor, in many tissues during mouse embryogenesis. This process may, in part, be dependent on homophilic intercellular interactions between FLRT molecules.

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.

Multiple signaling pathways regulate FGF-2-induced retinal ganglion cell neurite extension and growth cone guidance

Growth cones use cues in their environment in order to grow in a directed fashion to their targets. In Xenopus laevis, fibroblast growth factors (FGFs) participate in retinal ganglion cell (RGC) axon guidance in vivo and in vitro. The main intracellular signaling cascades known to act downstream of the FGF receptor include the mitogen-activated protein kinase (MAPK), phospholipase Cgamma (PLCgamma) and phosphotidylinositol 3-kinase (PI3K) pathways. We used pharmacological inhibitors to identify the signaling cascade(s) responsible for FGF-2-stimulated RGC axon extension and chemorepulsion. The MAPK, PI3K and PLCgamma pathways were blocked by U0126, LY249002 and U73122, respectively. D609 was used to test a role for the phosphotidylcholine-PLC (PC-PLC) pathway. We determined that the MAPK and two PLC pathways are required for FGF-2 to stimulate RGC neurite extension in vitro, but the response of axons to FGF-2 applied asymmetrically to the growth cone depended only on the PLC pathways.

Voltage-gated potassium channels regulate the response of retinal growth cones to axon extension and guidance cues

Xenopus retinal ganglion cell growth cones express various voltage-gated potassium (Kv) channels. We showed previously that 4-aminopyridine and tetraethylammonium have different effects on the outward currents of embryonic Xenopus retinal ganglion cells. Therefore, we asked whether these Kv channel inhibitors differentially regulate the response of retinal ganglion cell growth cones to extrinsic cues. First, we tested the role of Kv channels in axon extension mediated by a substrate bound cue and found that 4-aminopyridine blocked, whereas tetraethylammonium enhanced basal extension on laminin. Yet, when the growth cones were stimulated to extend with application of soluble growth factors, both inhibitors resulted in a return to the basal extension rates observed in the presence of laminin alone. Second, we asked if Kv channels modulate the response of retinal ganglion cell growth cones to a guidance cue, the chemorepellent fibroblast growth factor-2. When presented in a gradient to one side of the growth cone, fibroblast growth factor-2 repulsed retinal ganglion cell growth cones in the presence of 4-aminopyridine but not tetraethylammonium. These data argue that tetraethylammonium- and 4-aminopyridine-sensitive Kv channels differ in the manner by which they regulate the response of retinal ganglion cell axons to extension and guidance cues. Non-ratiometric calcium imaging indicated that differences in the ability of tetraethylammonium- and 4-aminopyridine-sensitive Kv channels to regulate calcium activity within the growth cone may underlie their unique modulation of growth cone behaviour.

Controlling cell behavior electrically: current views and future potential

Direct-current (DC) electric fields are present in all developing and regenerating animal tissues, yet their existence and potential impact on tissue repair and development are largely ignored. This is primarily due to ignorance of the phenomenon by most researchers, some technically poor early studies of the effects of applied fields on cells, and widespread misunderstanding of the fundamental concepts that underlie bioelectricity. This review aims to resolve these issues by describing: 1) the historical context of bioelectricity, 2) the fundamental principles of physics and physiology responsible for DC electric fields within cells and tissues, 3) the cellular mechanisms for the effects of small electric fields on cell behavior, and 4) the clinical potential for electric field treatment of damaged tissues such as epithelia and the nervous system.

Axon and Schwann Cell Partnership During Nerve Regrowth

Regeneration of peripheral nerve involves an essential contribution by Schwann cells (SCs) in collaboration with regrowing axons. We examined such collaboration between new axons and Schwann cells destined to reform peripheral nerve trucks in a regeneration chamber bridging transected rat sciatic nerves. There was a highly intimate "dance" between axons that followed outgrowing and proliferating SCs. Axons without SCs only grew short distances and almost all axon processes had associated SC processes. When regeneration chambers were infused through an external access port with local mitomycin, a mitosis inhibitor, SC proliferation, migration and subsequent axon regrowth were dramatically reduced. Adding laminin to mitomycin did not reverse this regenerative lag and indicated that SCs provide more than laminin synthesis alone. Laminin infused alone supplemented endogenous laminin and facilitated first SC then axon regrowth. "Wrong way" misdirected axons were associated with misdirected SC processes and were more numerous in bridges exposed to mitomycin, but were fewer in laminin supplemented bridges. Later, by 21 days, there was myelinated axon repopulation of regenerative bridges but those exposed to mitomycin alone at early time points had substantial impairments in axon investment. Reforming peripheral nerve trucks involves a very close and intimate relationship between axons and SCs that must proliferate and migrate, facilitated by laminin.

Matrix metalloproteinases are required for retinal ganglion cell axon guidance at select decision points

Axons receive guidance information from extrinsic cues in their environment in order to reach their targets. In the frog Xenopus laevis, retinal ganglion cell (RGC) axons make three key guidance decisions en route through the brain. First, they cross to the contralateral side of the brain at the optic chiasm. Second, they turn caudally in the mid-diencephalon. Finally, they must recognize the optic tectum as their target. The matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase (ADAM) families are zinc (Zn)-dependent proteolytic enzymes. The latter functions in axon guidance, but a similar role has not yet been identified for the MMP family. Our previous work implicated metalloproteinases in the guidance decisions made by Xenopus RGC axons. To test specifically the importance of MMPs, we used two different in vivo exposed brain preparations in which RGC axons were exposed to an MMP-specific pharmacological inhibitor (SB-3CT), either as they reached the optic chiasm or as they extended through the diencephalon en route to the optic tectum. Interestingly, SB-3CT affected only two of the guidance decisions, with misrouting defects at the optic chiasm and tectum. Only at higher concentrations was RGC axon extension also impaired. These data implicate MMPs in the guidance of vertebrate axons, and suggest that different metalloproteinases function to regulate axon behaviour at distinct choice points: an MMP is important in guidance at the optic chiasm and the target, while either a different MMP or an ADAM is required for axons to make the turn in the mid-diencephalon.

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.

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.