Essential role of heparan sulfates in axon navigation and targeting in the developing visual system

Molecular mechanisms of proteoglycan-mediated semaphorin signaling in axon guidance

The precise assembly of a functional nervous system relies on axon guidance cues. Beyond engaging their cognate receptors and initiating signaling cascades that modulate cytoskeletal dynamics, guidance cues also bind components of the extracellular matrix, notably proteoglycans, yet the role and mechanisms of these interactions remain poorly understood. We found that Drosophila secreted semaphorins bind specifically to glycosaminoglycan (GAG) chains of proteoglycans, showing a preference based on the degree of sulfation. Structural analysis of Sema2b unveiled multiple GAG-binding sites positioned outside canonical plexin-binding site, with the highest affinity binding site located at the C-terminal tail, characterized by a lysine-rich helical arrangement that appears to be conserved across secreted semaphorins. In vivo studies revealed a crucial role of the Sema2b C-terminal tail in specifying the trajectory of olfactory receptor neurons. We propose that secreted semaphorins tether to the cell surface through interactions with GAG chains of proteoglycans, facilitating their presentation to cognate receptors on passing axons.

Xenopus retinal ganglion cell axon extension is unaffected by 5-HT 1B/D receptor activation during visual system development

Activating 5-HT 1B/D receptors with the agonist Zolmitriptan was previously shown to facilitate Xenopus retinal ganglion cell (RGC) axon extension from ectopic eye primordia transplanted to the ventral fin. To determine if 5-HT 1B/D receptor activation influenced entopic RGC axonal outgrowth toward the optic tectum during typical visual system development, we reared embryos in 50 μΜ Zolmitriptan then visualized optic tracts with anterograde HRP labeling. Zolmitriptan did not significantly alter entopic RGC extension in the contralateral brain. Consequently, RGC axon extension in ectopic but not entopic locations is influenced by altering serotonergic signaling .

Glycans and Carbohydrate-Binding/Transforming Proteins in Axon Physiology

The mature nervous system relies on the polarized morphology of neurons for a directed flow of information. These highly polarized cells use their somatodendritic domain to receive and integrate input signals while the axon is responsible for the propagation and transmission of the output signal. However, the axon must perform different functions throughout development before being fully functional for the transmission of information in the form of electrical signals. During the development of the nervous system, axons perform environmental sensing functions, which allow them to navigate through other regions until a final target is reached. Some axons must also establish a regulated contact with other cells before reaching maturity, such as with myelinating glial cells in the case of myelinated axons. Mature axons must then acquire the structural and functional characteristics that allow them to perform their role as part of the information processing and transmitting unit that is the neuron. Finally, in the event of an injury to the nervous system, damaged axons must try to reacquire some of their immature characteristics in a regeneration attempt, which is mostly successful in the PNS but fails in the CNS. Throughout all these steps, glycans perform functions of the outermost importance. Glycans expressed by the axon, as well as by their surrounding environment and contacting cells, encode key information, which is fine-tuned by glycan modifying enzymes and decoded by glycan binding proteins so that the development, guidance, myelination, and electrical transmission functions can be reliably performed. In this chapter, we will provide illustrative examples of how glycans and their binding/transforming proteins code and decode instructive information necessary for fundamental processes in axon physiology.

Glycosylation in Axonal Guidance

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

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.

Loss of perlecan heparan sulfate glycosaminoglycans lowers body weight and decreases islet amyloid deposition in human islet amyloid polypeptide transgenic mice

Islet amyloid is a pathologic feature of type 2 diabetes (T2D) that is associated with β-cell loss and dysfunction. These amyloid deposits form via aggregation of the β-cell secretory product islet amyloid polypeptide (IAPP) and contain other molecules including the heparan sulfate proteoglycan perlecan. Perlecan has been shown to bind amyloidogenic human IAPP (hIAPP) via its heparan sulfate glycosaminoglycan (HS GAG) chains and to enhance hIAPP aggregation in vitro. We postulated that reducing the HS GAG content of perlecan would also decrease islet amyloid deposition in vivo. hIAPP transgenic mice were crossed with Hspg2Δ3/Δ3 mice harboring a perlecan mutation that prevents HS GAG attachment (hIAPP;Hspg2Δ3/Δ3), and male offspring from this cross were fed a high fat diet for 12 months to induce islet amyloid deposition. At the end of the study body weight, islet amyloid area, β-cell area, glucose tolerance and insulin secretion were analyzed. hIAPP;Hspg2Δ3/Δ3 mice exhibited significantly less islet amyloid deposition and greater β-cell area compared to hIAPP mice expressing wild type perlecan. hIAPP;Hspg2Δ3/Δ3 mice also gained significantly less weight than other genotypes. When adjusted for differences in body weight using multiple linear regression modeling, we found no differences in islet amyloid deposition or β-cell area between hIAPP transgenic and hIAPP;Hspg2Δ3/Δ3 mice. We conclude that loss of perlecan exon 3 reduces islet amyloid deposition in vivo through indirect effects on body weight and possibly also through direct effects on hIAPP aggregation. Both of these mechanisms may promote maintenance of glucose homeostasis in the setting of T2D.

Therapeutic Potential of Neurotrophins for Repair After Brain Injury: A Helping Hand From Biomaterials

Stroke remains the leading cause of long-term disability with limited options available to aid in recovery. Significant effort has been made to try and minimize neuronal damage following stroke with use of neuroprotective agents, however, these treatments have yet to show clinical efficacy. Regenerative interventions have since become of huge interest as they provide the potential to restore damaged neural tissue without being limited by a narrow therapeutic window. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), and their high affinity receptors are actively produced throughout the brain and are involved in regulating neuronal activity and normal day-to-day function. Furthermore, neurotrophins are known to play a significant role in both protection and recovery of function following neurodegenerative diseases such as stroke and traumatic brain injury (TBI). Unfortunately, exogenous administration of these neurotrophins is limited by a lack of blood-brain-barrier (BBB) permeability, poor half-life, and rapid degradation. Therefore, we have focused this review on approaches that provide a direct and sustained neurotrophic support using pharmacological therapies and mimetics, physical activity, and potential drug delivery systems, including discussion around advantages and limitations for use of each of these systems. Finally, we discuss future directions of biomaterial drug-delivery systems, including the incorporation of heparan sulfate (HS) in conjunction with neurotrophin-based interventions.

Diverse functions of protein tyrosine phosphatase σ in the nervous and immune systems

Tyrosine phosphorylation is a common means of regulating protein functions and signal transduction in multiple cells. Protein tyrosine phosphatases (PTPs) are a large family of signaling enzymes that remove phosphate groups from tyrosine residues of target proteins and change their functions. Among them, receptor-type PTPs (RPTPs) exhibit a distinct spatial pattern of expression and play essential roles in regulating neurite outgrowth, axon guidance, and synaptic organization in developmental nervous system. Some RPTPs function as essential receptors for chondroitin sulfate proteoglycans that inhibit axon regeneration following CNS injury. Interestingly, certain RPTPs are also important to regulate functions of immune cells and development of autoimmune diseases. PTPσ, a RPTP in the LAR subfamily, is expressed in various immune cells and regulates their differentiation, production of various cytokines and immune responses. In this review, we highlight the physiological and pathological significance of PTPσ and related molecules in both nervous and immune systems.

Heparan Sulfate Proteoglycans as Emerging Players in Synaptic Specificity

Neural circuits consist of distinct neuronal cell types connected in specific patterns. The specificity of these connections is achieved in a series of sequential developmental steps that involve the targeting of neurites, the identification of synaptic partners, and the formation of specific types of synapses. Cell-surface proteins play a critical role in each of these steps. The heparan sulfate proteoglycan (HSPG) family of cell-surface proteins is emerging as a key regulator of connectivity. HSPGs are expressed throughout brain development and play important roles in axon guidance, synapse development and synapse function. New insights indicate that neuronal cell types express unique combinations of HSPGs and HS-modifying enzymes. Furthermore, HSPGs interact with cell type-specific binding partners to mediate synapse development. This suggests that cell type-specific repertoires of HSPGs and specific patterns of HS modifications on the cell surface are required for the development of specific synaptic connections. Genome-wide association studies have linked these proteins to neurodevelopmental and neuropsychiatric diseases. Thus, HSPGs play an important role in the development of specific synaptic connectivity patterns important for neural circuit function, and their dysfunction may be involved in the development of brain disorders.

Proteoglycans and axon guidance: a new relationship between old partners

Neural circuits are formed with great precision during development. Accumulated evidence over the past three decades has demonstrated that growing axons are navigated toward their targets by the combined actions of attractants and repellents together with their receptors. It has long been known that proteoglycans, glycosylated proteins possessing covalently attached glycosaminoglycans, play a critical role in axon guidance; however, the molecular mechanisms by which proteoglycans regulate axon behaviors remain largely unknown. Glycosaminoglycans such as heparan sulfate and chondroitin sulfate are large linear polysaccharides composed of repeating disaccharide units that are highly modified by specific sulfation and epimerization. Recent biochemical and molecular biological studies have identified the enzymes that are involved in the biosynthesis of glycosaminoglycans. Interestingly, many mutants lacking glycosaminoglycan-synthesizing enzymes or proteoglycans in several model organisms show defects in specific nerve tract formation. In parallel, detailed biochemical studies have identified the molecular interactions between axon guidance molecules and glycosaminoglycans that have specific modification in their sugar chains. This review summarizes the structure and function of axon guidance molecules and glycosaminoglycans, and then tries to combine the knowledge from these studies to understand the role of proteoglycans from a new vantage point. Deciphering the sugar code is important for understanding the complicated nature of proteoglycans in axon guidance. Neural circuits are formed by the combined actions of axon guidance molecules. Proteoglycans play critical roles in regulating axon guidance through the interaction between signaling molecules and glycosaminoglycan chains attached to the core protein. This paper summarizes the structure and functions of axon guidance molecules and glycosaminoglycans and reviews the molecular mechanisms by which proteoglycans regulate axon guidance from a new vantage point. This article is part of the 60th Anniversary special issue.

Heparan sulfate proteoglycans: a sugar code for vertebrate development?

Heparan sulfate proteoglycans (HSPGs) have long been implicated in a wide range of cell-cell signaling and cell-matrix interactions, both in vitro and in vivo in invertebrate models. Although many of the genes that encode HSPG core proteins and the biosynthetic enzymes that generate and modify HSPG sugar chains have not yet been analyzed by genetics in vertebrates, recent studies have shown that HSPGs do indeed mediate a wide range of functions in early vertebrate development, for example during left-right patterning and in cardiovascular and neural development. Here, we provide a comprehensive overview of the various roles of HSPGs in these systems and explore the concept of an instructive heparan sulfate sugar code for modulating vertebrate development.

Summary: This Review article examines the role of heparan sulfate proteoglycans in vertebrate development and explores the concept of an instructive 'sugar code' for modulating development.

NF-Protocadherin Regulates Retinal Ganglion Cell Axon Behaviour in the Developing Visual System

Cell adhesion molecules play a central role in mediating axonal tract development within the nascent nervous system. NF-protocadherin (NFPC), a member of the non-clustered protocadherin family, has been shown to regulate retinal ganglion cell (RGC) axon and dendrite initiation, as well as influencing axonal navigation within the mid-optic tract. However, whether NFPC mediates RGC axonal behaviour at other positions within the optic pathway remains unclear. Here we report that NFPC plays an important role in RGC axonogenesis, but not in intraretinal guidance. Moreover, axons with reduced NFPC levels exhibit insensitivity to Netrin-1, an attractive guidance cue expressed at the optic nerve head. Netrin-1 induces rapid turnover of NFPC localized to RGC growth cones, suggesting that the regulation of NFPC protein levels may underlie Netrin-1-mediated entry of RGC axons into the optic nerve head. At the tectum, we further reveal a function for NFPC in controlling RGC axonal entry into the final target area. Collectively, our results expand our understanding of the role of NFPC in RGC guidance and illustrate that this adhesion molecule contributes to axon behaviour at multiple points in the optic pathway.

Diverse functions of perlecan in central nervous system cells in vitro

Therapeutic treatment targeting one cell type is considered ineffective in remedying any injury to the central nervous system (CNS). Perlecan, a multi-functional, heparan sulfate proteoglycan, shows diverse effects on distinct cell types, suggesting that it is one of the candidates that can augment the regenerative mechanisms in the injured CNS. Therefore, we examined the functions of perlecan in CNS cells in vitro by using perlecan purified from bovine kidney. Perlecan-coated cell culture plates, unlike their type I/III collagen-coated counterparts, did not inhibit the adhesion of neural stem/progenitor cells (NS/PCs) and neurons. The coated perlecan and the perlecan added to the culture medium suppressed astrocyte proliferation; however, perlecan added to the medium promoted NS/PC proliferation. Neurons were promoted to extend their neurites on the perlecan-coated substrate, and perlecan added to the medium also showed a similar effect. NS/PC proliferation and neurite extension is a major regenerative reaction in CNS injury, whereas excess proliferation of astrocytes cause hypertrophy of glial scars, which repels neurons. Our in vitro study suggests that perlecan is an attractive candidate to promote regenerative mechanisms and to suppress reactions that hamper regenerative processes in cases of CNS injury.

Connecting the retina to the brain

The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed.

FGF-2-responsive and spinal cord-resident cells improve locomotor function after spinal cord injury

The adult central nervous system has only a limited capacity for axonal regeneration. In this study, fibroblast growth factor-2 (FGF-2) was injected once into the spinal cord tissue around the injury site immediately after complete spinal cord transection in rats. This treatment markedly improved the locomotor function of the animals. Histological analysis demonstrated that tissue composed of FGF-2-induced fibronectin-positive cells (FIFs) had infiltrated the injury site and filled large cystic cavities, into which numerous axons with growth-associated protein-43 immunoreactivity penetrated. The FIFs could also be cultured from the intact spinal cord tissue, demonstrating that they were resident in the non-injured spinal cord. They had a spindle-shaped morphology and enhanced expression of mRNAs of N-cadherin and neurotrophic factors, suggesting the beneficial properties of the FIFs for axonal regeneration. Thus, these results argue for the continual use of autologous transplantation as a novel and promising cell therapy for the treatment of spinal cord injury.

Y-P30 promotes axonal growth by stabilizing growth cones

The 30-amino acid peptide Y-P30, generated from the N-terminus of the human dermcidin precursor protein, has been found to promote neuronal survival, cell migration and neurite outgrowth by enhancing the interaction of pleiotrophin and syndecan-3. We now show that Y-P30 activates Src kinase and extracellular signal-regulated kinase (ERK). Y-P30 promotes axonal growth of mouse embryonic stem cell-derived neurons, embryonic mouse spinal cord motoneurons, perinatal rat retinal neurons, and rat cortical neurons. Y-P30-mediated axon growth was dependent on heparan sulfate chains. Y-P30 decreased the proportion of collapsing/degenerating growth cones of cortical axons in an Src and ERK-dependent manner. Y-P30 increased for 90 min in axonal growth cones the level of Tyr418-phosphorylated Src kinase and the amount of F-actin, and transiently the level of Tyr-phosphorylated ERK. Levels of total Src kinase, actin, GAP-43, cortactin and the glutamate receptor subunit GluN2B were not altered. When exposed to semaphorin-3a, Y-P30 protected a significant fraction of growth cones of cortical neurons from collapse. These results suggest that Y-P30 promotes axonal growth via Src- and ERK-dependent mechanisms which stabilize growth cones and confer resistance to collapsing factors.

Heparan sulfate glycosaminoglycans modulate migration and survival in zebrafish primordial germ cells

Early in embryonic development, primordial germ cells (PGCs) are specified and migrate from the site of their origin to where the gonad develops, following a specific route. Heparan sulfate glycosaminoglycans (HS-GAGs) are ubiquitous in extracellular matrix and the cell surface and have long been speculated to play a role during the migration of PGCs. In line with this speculation, whole-mount immunohistochemistry revealed the existence of HS-GAGs in the vicinity of migrating PGCs in early zebrafish embryos. To examine the roles of HS-GAGs during PGC migration, zebrafish heparanase 1 (hpse1), which degrades HS-GAGs, was cloned and overexpressed specifically in PGCs. The guidance signal for the migration of PGCs was disrupted with the overexpression of hpse1, as cluster formation and marginal localization at the blastoderm were significantly perturbed at 6 hours postfertilization. Furthermore, the number of PGCs was significantly decreased with the lack of vicinal HS-GAGs, as observed in the whole-mount in situ hybridization and quantitative PCR of the PGC marker gene vasa. Terminal deoxynucleotidyl transferase dUTP nick-end labeling indicated significantly increased apoptosis in PGCs overexpressing hpse1, suggesting that HS-GAGs contribute to the maintenance of PGC survival. In conclusion, HS-GAGs play multifaceted roles in PGCs during migration and are required both for guidance signals and multiplication of PGCs.

Proteoglycans in the central nervous system: role in development, neural repair, and Alzheimer's disease

Proteoglycans (PGs) are major components of the cell surface and extracellular matrix and play critical roles in development and maintenance of the central nervous system (CNS). PGs are a family of proteins, all of which contain a core protein to which glycosaminoglycan side chains are covalently attached. PGs possess diverse physiological roles, particularly in neural development, and are also implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). The main functions of PGs in the CNS are reviewed as are the roles of PGs in brain injury and in the development or treatment of AD.

Dynamic responses of Xenopus retinal ganglion cell axon growth cones to netrin-1 as they innervate their in vivo target

Netrin-1 influences retinal ganglion cell (RGC) axon pathfinding and also participates in the branching and synaptic differentiation of mature RGC axons at their target. To investigate whether netrin also serves as an early target recognition signal in the brain, we examined the dynamic behavior of Xenopus RGC axons soon after they innervate the optic tectum. Time-lapse confocal microscopy imaging of RGC axons expressing enhanced yellow fluorescent protein demonstrated that netrin-1 is involved in early axon branching, as recombinant netrin-1 halted further advancement of growth cones into the tectum and induced back branching. RGC growth cones exhibited differential responses to netrin-1 that depended on the degree of differentiation of the axon and the developmental stage of the tadpole. Netrin-1 decreased the total number of branches on newly arrived RGC growth cones at the target, but increased the dynamic branching of more mature arbors at the later developmental stage. To further explore the response of axonal growth cones to netrin, Xenopus RGC axons were followed in culture by time-lapse imaging. Exposure to netrin-1 rapidly increased the forward advancement of the axon and decreased the size and expanse of the growth cone, while also inducing back branching. Taken together, the differential in vivo and in vitro responses to netrin-1 suggest that netrin alone is not sufficient to induce the cessation of growth cone advancement in the absence of a target but can independently modulate axon branching. Collectively, our findings reveal a novel role for netrin on RGC axon branch initiation as growth cones innervate their target.

Growth factors: a role in guiding axons?

A remarkable finding to emerge in recent years is that the early brain neuroepithelium is highly patterned before axonogenesis begins. Growth factors are among a variety of classes of molecules whose regionalized expression divides the early brain into molecularly distinct domains. Thus, when axons first grow to their synaptic targets, growth factor signalling may help them to navigate. This review discusses recent studies that reveal that growth factors can act as chemoattractants and repellents and that growth factor signalling is important for target entry. These new findings raise the compelling idea that growth factors play an active role in axon navigation.

LAR receptor tyrosine phosphatases and HSPGs guide peripheral sensory axons to the skin

BACKGROUND

Peripheral axons of somatosensory neurons innervate the skin early in development to detect touch stimuli. Embryological experiments had suggested that the skin produces guidance cues that attract sensory axons, but neither the attractants nor their neuronal receptors had previously been identified.

RESULTS

To investigate peripheral axon navigation to the skin, we combined live imaging of developing zebrafish Rohon-Beard (RB) neurons with molecular loss-of-function manipulations. Simultaneously knocking down two members of the leukocyte antigen-related (LAR) family of receptor tyrosine phosphatases expressed in RB neurons, or inhibiting their function with dominant-negative proteins, misrouted peripheral axons to internal tissues. Time-lapse imaging indicated that peripheral axon guidance, rather than outgrowth or maintenance, was defective in LAR-deficient neurons. Peripheral axons displayed a similar misrouting phenotype in mutants defective in heparan sulfate proteoglycan (HSPG) production and avoided regions in which HSPGs were locally degraded.

CONCLUSIONS

HSPGs and LAR family receptors are required for sensory axon guidance to the skin. Together, our results support a model in which peripheral HSPGs are attractive ligands for LAR receptors on RB neurons.

Two promoters with distinct activities in different tissues drive the expression of heparanase in Xenopus

In Xenopus laevis embryos, heparanase, the enzyme that degrades heparan sulfate, is synthesized as a preproheparanase (XHpaL) and processed to become enzymatically active (XHpa active). A short nonenzymatic heparanase splice variant (XHpaS) is also expressed. Using immunohistochemistry, Western blot, and heparanase promoter analysis, we studied the dynamic developmental expression of the three heparanases. Our results indicate that (1) all three isoforms are maternally expressed; (2) XHpaS is a developmental variant; (3) in the early embryo, heparanase is localized to both the plasma membrane and the nucleus; (4) several tissues express heparanase, but expression in the developing nervous system is most evident; (5) two promoters with distinct activities in different tissues drive heparanase expression; (6) Oct binding transcription factors may modulate heparanase promoter activity in the early embryo. These data argue that heparanase is expressed widely during development, but localization and levels are finely regulated.

Xenopus sonic hedgehog guides retinal axons along the optic tract

The role of classic morphogens such as Sonic hedgehog (Shh) as axon guidance cues has been reported in a variety of vertebrate organisms (Charron and Tessier-Lavigne [2005] Development 132:2251-2262). In this work, we provide the first evidence that Xenopus sonic hedgehog (Xshh) signaling is involved in guiding retinal ganglion cell (RGC) axons along the optic tract. Xshh is expressed in the brain during retinal axon extension, adjacent to these axons in the ventral diencephalon. Retinal axons themselves express Patched 1 and Smoothened co-receptors during RGC axon growth. Blocking Shh signaling causes abnormal ventral pathfinding, and targeting errors at the optic tectum. Misexpression of exogenous N-Shh peptide in vivo also causes pathfinding errors. Retinal axons grown in culture respond to N-Shh in a dose-dependent manner, either by decreasing extension at lower concentrations, or retracting axons in the presence of higher doses. These data suggest that Shh signaling is required for normal RGC axon pathfinding and tectal targeting in the developing visual system of Xenopus. We propose that Shh serves as a ventral optic tract repellent that helps to define the caudal boundary for retinal axons in the diencephalon, and that this signaling is also required for initial target recognition at the optic tectum.

Moving away from the midline: new developments for Slit and Robo

In most tissues, the precise control of cell migration and cell-cell interaction is of paramount importance to the development of a functional structure. Several families of secreted molecules have been implicated in regulating these aspects of development, including the Slits and their Robo receptors. These proteins have well described roles in axon guidance but by influencing cell polarity and adhesion, they participate in many developmental processes in diverse cell types. We review recent progress in understanding both the molecular mechanisms that modulate Slit/Robo expression and their functions in neural and non-neural tissue.

E3 ligase Nedd4 promotes axon branching by downregulating PTEN

Regulated protein degradation via the ubiquitin-proteasome system (UPS) plays a central role in building synaptic connections, yet little is known about either which specific UPS components are involved or UPS targets in neurons. We report that inhibiting the UPS in developing Xenopus retinal ganglion cells (RGCs) with a dominant-negative ubiquitin mutant decreases terminal branching in the tectum but does not affect long-range navigation to the tectum. We identify Nedd4 as a prominently expressed E3 ligase in RGC axon growth cones and show that disrupting its function severely inhibits terminal branching. We further demonstrate that PTEN, a negative regulator of the PI3K pathway, is a key downstream target of Nedd4: not only does Nedd4 regulate PTEN levels in RGC growth cones, but also, the decrease of PTEN rescues the branching defect caused by Nedd4 inhibition. Together our data suggest that Nedd4-regulated PTEN is a key regulator of terminal arborization in vivo.

Roles of heparan sulfate in mammalian brain development current views based on the findings from Ext1 conditional knockout studies

Development of the mammalian central nervous system proceeds roughly in four major steps, namely the patterning of the neural tube, generation of neurons from neural stem cells and their migration to genetically predetermined destinations, extension of axons and dendrites toward target neurons to form neural circuits, and formation of synaptic contacts. Earlier studies on spatiotemporal expression patterns and in vitro function of heparan sulfate (HS) suggested that HS is functionally involved in various aspects of neural development. Recent studies using knockout of genes involved in HS biosynthesis have provided more physiologically relevant information as to the role of HS in mammalian neural development. This chapter reviews the current understanding of the in vivo function of HS deduced from the phenotypes of conditional Ext1 knockout mice.

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.

Heparan sulfate regulates ephrin-A3/EphA receptor signaling

Increasing evidence indicates that many signaling pathways involve not only ligands and receptors but also various types of coreceptors and matrix components as additional layers of regulation. Signaling by Eph receptors and their ephrin ligands plays a key role in a variety of biological processes, such as axon guidance and topographic map formation, synaptic plasticity, angiogenesis, and cancer. Little is known about whether the ephrin-Eph receptor signaling system is subject to such additional layers of regulation. Here, we show that ephrin-A3 binds to heparan sulfate, and that the presence of cell surface heparan sulfate is required for the full biological activity of ephrin-A3. Among the ephrins tested, including ephrin-A1, -A2, -A5, -B1, and -B2, only ephrin-A3 binds heparin or heparan sulfate. Ephrin-A3-dependent EphA receptor activation is reduced in mutant cells that are defective in heparan sulfate synthesis, in wild-type cells from which cell surface heparan sulfate has been removed, and in the hippocampus of conditional knockout mice defective in heparan sulfate synthesis. Ephrin-A3-dependent cell rounding is impaired in CHO cells lacking heparan sulfate, and cortical neurons lacking heparan sulfate exhibit impaired growth cone collapse. In contrast, cell rounding and growth cone collapse in response to ephrin-A5, which does not bind heparan sulfate, are not affected by the absence of heparan sulfate. These results show that heparan sulfate modulates ephrin/Eph signaling and suggest a physiological role for heparan sulfate proteoglycans in the regulation of ephrin-A3-dependent biological processes.

Retina development in zebrafish requires the heparan sulfate proteoglycan agrin

Recent studies from our laboratory have begun to elucidate the role of agrin in zebrafish development. One agrin morphant phenotype that results from agrin knockdown is microphthalmia (reduced eye size). To begin to understand the mechanisms underlying the role of agrin in eye development, we have analyzed retina development in agrin morphants. Retinal differentiation is impaired in agrin morphants, with retinal lamination being disrupted following agrin morpholino treatment. Pax 6.1 and Mbx1 gene expression, markers of eye development, are markedly reduced in agrin morphants. Formation of the optic fiber layer of the zebrafish retina is also impaired, exhibited as both reduced size of the optic fiber layer, and disruption of retinal ganglion cell axon growth to the optic tectum. The retinotectal topographic projection to the optic tectum is perturbed in agrin morphants in association with a marked loss of heparan sulfate expression in the retinotectal pathway, with this phenotype resembling retinotectal phenotypes observed in mutant zebrafish lacking enzymes for heparan sulfate synthesis. Treatment of agrin morphants with a fibroblast growth factor (Fgf) receptor inhibitor, rescue of the retinal lamination phenotype by transplantation of Fgf8-coated beads, and disruption of both the expression of Fgf-dependent genes and activation of ERK in agrin morphants provides evidence that agrin modulation of Fgf function contributes to retina development. Collectively, these agrin morphant phenotypes provide support for a crucial role of agrin in retina development and formation of an ordered retinotectal topographic map in the optic tectum of zebrafish.

Transforming growth factor-beta1 induces heparan sulfate 6-O-endosulfatase 1 expression in vitro and in vivo

Transforming growth factor (TGF)-beta1 plays an important role in the development of pulmonary fibrosis. In this study we examined the relationship between TGF-beta1 stimulation and the expression of heparan sulfate (HS) 6-O-endosulfatase 1 (Sulf1) in cultured normal human lung fibroblasts (NHLFs) and in murine lungs in vivo. By removing 6-O-sulfates from specific HS intrachain sites on the cell surface, Sulf1 has been shown to modulate the activities of many HS binding growth factors and morphogens including fibroblast growth factor (FGF)-2. Real time reverse transcription-PCR analysis revealed that TGF-beta1 increased Sulf1 expression in NHLFs in a dose- and time-dependent manner which was accompanied by a decrease in 6-O-sulfated disaccharides as revealed by high performance liquid chromatography analysis. Decreased ERK activation after FGF-2 stimulation was observed in TGF-beta1-treated NHLFs compared with control cells without changes in HS-dependent FGF-2 binding or FGF-2.FR1c complex formation. To study the function of Sulf1, negative control or Sulf1-specific small interference RNA (siRNA)-transfected NHLFs were stimulated with TGF-beta1. Enhanced Smad2/3 phosphorylation and elevated total Smad2 protein level were observed in Sulf1 siRNA-transfected cells and were accompanied by enhanced expression of alpha-smooth muscle actin and fibronectin. In addition, Sulf1 siRNA transfection enhanced the anti-proliferative effect of TGF-beta1. Finally Sulf1 expression was up-regulated in the lungs of mice treated with adenovirus encoding active TGF-beta1. Taken together, our data indicate that Sulf1 is a TGF-beta1-responsive gene both in vitro and in vivo and may function as a negative regulator of TGF-beta1-induced fibrogenesis.

Loss of short dystrophin isoform Dp71 in olfactory ensheathing cells causes vomeronasal nerve defasciculation in mouse olfactory system

The Duchenne muscular dystrophy (DMD) gene encodes dystrophin, which is a protein defective in DMD patients, as well as a number of shorter isoforms, which have been shown to be expressed in various non-muscle, primarily neural, tissues. As of yet, the physiological function of the various dystrophin isoforms is not fully understood. In the present study, we investigated the neurological phenotype that arises in the DMD-null mice, where expression of all dystrophin isoforms had been disrupted. We demonstrate that vomeronasal axons in the DMD-null mice are defasciculated, and some of the defasciculated vomeronasal axons aberrantly entered into the main olfactory bulb, which indicates that the product(s) of the DMD gene plays an important role in vomeronasal nerve organization. Through western blot and immunofluorescence analyses, we determined that the dystrophin isoform Dp71 was exclusively expressed in the mouse olfactory system: mainly in the olfactory ensheathing cells (OECs), an olfactory system-specific glia cell that ensheaths fascicles of the olfactory nerve. In the OECs, Dp71 was co-localized with beta-dystroglycan, utrophin, laminin, and perlecan. Since beta-dystroglycan and perlecan expression was decreased in the OECs of DMD-null mice, we hypothesize that Dp71 expressed in the OECs participates in fasciculation of the vomeronasal nerve, most likely through interactions with extracellular matrix.

Two heparanase splicing variants with distinct properties are necessary in early Xenopus development

Heparanase is an endoglycosidase that cleaves heparan sulfate (HS) side chains from heparan sulfate proteoglycans (HSPGs) present in extracellular matrix and cell membranes. Although HSPGs have many functions during development, little is known of the role of the enzyme that degrades HS, heparanase. We cloned and characterized the expression of two heparanase splicing variants from Xenopus laevis and studied their function in early embryonic development. The heparanase gene (termed xHpa) spans over 15 kb and consists of at least 12 exons. The long heparanase (XHpaL) cDNA encodes a 531-amino acid protein, whereas the short splicing variant (XHpaS) results in a protein with the same open reading frame but missing 58 amino acids as a consequence of a skipped exon 4. Comparative studies of both isoforms using heterologous expression systems showed: 1) XHpaL is enzymatically active, whereas XHpaS is not; 2) XHpaL and XHpaS interact with heparin and HS; 3) both proteins traffic through the endoplasmic reticulum and Golgi apparatus, but XHpaL is secreted into the medium, whereas XHpaS remains associated with the membrane as a consequence of the loss of three glycosylation sites; 4) overexpression of XHpaS but not XHpaL increases cell adhesion of glioma cells to HS-coated surfaces; 5) XHpaL and XHpaS mRNA and protein levels vary as development progresses; 6) specific antisense knock-down of both XHpaL and XHpaS, but not XHpaL alone, results in failure of embryogenesis to proceed. Interestingly, rescue experiments suggest that the two heparanases regulate the same developmental processes, but via different mechanisms.

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.

The retinal ganglion cell axon's journey: insights into molecular mechanisms of axon guidance

The developing visual system has proven to be one of the most informative models for studying axon guidance decisions. The pathway is composed of the axons of a single neuronal cell type, the retinal ganglion cell (RGC), that navigate through a series of intermediate targets on route to their final destination. The molecular basis of optic pathway development is beginning to be elucidated with cues such as netrins, Slits and ephrins playing a key role. Other factors best characterised for their role as morphogens in patterning developing tissues, such as sonic hedgehog (Shh) and Wnts, also act directly on RGC axons to influence guidance decisions. The transcriptional basis of the spatial-temporal expression of guidance cues and their cognate receptors within the developing optic pathway as well as mechanisms underlying the plasticity of guidance responses also are starting to be understood. This review will focus on our current understanding of the molecular mechanisms directing the early development of functional connections in the developing visual system and the insights these studies have provided into general mechanisms of axon guidance.

Proteoglycans as modulators of axon guidance cue function

Organizing a functional neuronal network requires the precise wiring of neuronal connections. In order to find their correct targets, growth cones navigate through the extracellular matrix guided by secreted and membrane-bound molecules of the slit, netrin, ephrin and semaphorin families. Although many of these axon guidance molecules are able to bind to heparan sulfate proteoglycans, the role of proteoglycans in regulating axon guidance cue function is only now beginning to be understood. Recent developmental studies in a wide range of model organisms have revealed a crucial role for heparan sulfate proteoglycans as modulators of key signaling pathways in axon guidance. In addition, emerging evidence indicates an essential role for chondroitin sulfate proteoglycans in modifying the guidance function of semaphorins. It is becoming increasingly clear that extracellular matrix molecules, rather than just constituting a structural scaffold, can critically influence axon guidance cue function in development, and may continue to do so in the injured adult nervous system.

Heparan sulfate regulation of progenitor cell fate

Currently there is an intense effort being made to elucidate the factors that control stem and progenitor cell fate. Developments in our understanding of the FGF/FGFR pathway and its role as an effector of stem cell pluripotency have heightened expectations that a therapeutic use for stem cells will move from a possibility to a probability. Mounting evidence is revealing the molecular mechanisms by which fibroblast growth factor (FGF) signaling, together with a large number of other growth and adhesive factors, is controlled by the extracellular sugar, heparan sulfate (HS). What has resulted is a novel means of augmenting and thus regulating the growth factor control of stem and progenitor cell fate. Here, we review the numerous bioactivities of HS, and the development of strategies to implement HS-induced control of cell fate decisions.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Slits contribute to the guidance of retinal ganglion cell axons in the mammalian optic tract

RGC axons extend in the optic tracts in a manner that correlates with the expression in the hypothalamus and epithalamus of a soluble factor inhibitory to RGC axon outgrowth. Additionally, although the RGC axons extend adjacent to the telencephalon, they do not normally grow into this tissue. Here, we show that slit1 and slit2, known chemorepellents for RGC axons expressed in specific regions of the diencephalon and telencephalon, help regulate optic tract development. In mice lacking slit1 and slit2, a subset of RGC axons extend into the telencephalon and grow along the pial surface but not more deeply into this tissue. Surprisingly, distinct guidance errors occur in the telencephalon of slit1 -/-; slit2 +/- and slit1/2 -/- embryos, suggesting that the precise level of Slits is critical for determining the path followed by individual axons. In mice lacking both slit1 and slit2, a subset of RGC axons also project aberrantly into the epithalamus, pineal and across the dorsal midline. However, many axons reach their primary target, the superior colliculus. This demonstrates that Slits play an important role in directing the guidance of post-crossing RGC axons within the optic tracts but are not required for target innervation.

Heparan sulfate proteoglycans and the emergence of neuronal connectivity

With the identification of the molecular determinants of neuronal connectivity, our understanding of the extracellular information that controls axon guidance and synapse formation has evolved from single factors towards the complexity that neurons face in a living organism. As we move in this direction - ready to see the forest for the trees - attention is returning to one of the most ancient regulators of cell-cell interaction: the extracellular matrix. Among many matrix components that influence neuronal connectivity, recent studies of the heparan sulfate proteoglycans suggest that these ancient molecules function as versatile extracellular scaffolds that both sculpt the landscape of extracellular cues and modulate the way that neurons perceive the world around them.

Signaling mechanisms underlying Slit2-induced collapse of Xenopus retinal growth cones

Slits mediate multiple axon guidance decisions, but the mechanisms underlying the responses of growth cones to these cues remain poorly defined. We show here that collapse induced by Slit2-conditioned medium (Slit2-CM) in Xenopus retinal growth cones requires local protein synthesis (PS) and endocytosis. Slit2-CM elicits rapid activation of translation regulators and MAP kinases in growth cones, and inhibition of MAPKs or disruption of heparan sulfate blocks Slit2-CM-induced PS and repulsion. Interestingly, Slit2-CM causes a fast PS-dependent decrease in cytoskeletal F-actin concomitant with a PS-dependent increase in the actin-depolymerizing protein cofilin. Our findings reveal an unexpected link between Slit2 and cofilin in growth cones and suggest that local translation of actin regulatory proteins contributes to repulsion.

Molecular mechanisms of optic axon guidance

Axon guidance is one of the critical processes during vertebrate central nervous system (CNS) development. The optic nerve, which contains the axons of retinal ganglion cells, has been used as a powerful model to elucidate some of the mechanisms underlying axon guidance because it is easily manipulated experimentally, and its function is well understood. Recent molecular biology studies have revealed that numerous guidance molecules control the development of the visual pathway. This review introduces the molecular mechanisms involved in each critical step during optic axon guidance. Axonal projections to the optic disc are thought to depend on adhesion molecules and inhibitory extracellular matrices such as chondroitin sulfate. The formation of the head of the optic nerve and the optic chiasm require ligand-receptor interactions between netrin-1 and the deleted in colorectal cancer receptor, and Slit proteins and Robo receptors, respectively. The gradient distributions of ephrin ligands and Eph receptors are essential for correct ipsilateral projections at the optic chiasm and the topographic mapping of axons in the superior colliculus/optic tectum. The precise gradient is regulated by transcription factors determining the retinal dorso-ventral and nasal-temporal polarities. Moreover, the axon guidance activities by Slit and semaphorin 5A require the existence of heparan sulfate, which binds to numerous guidance molecules. Recent discoveries about the molecular mechanisms underlying optic nerve guidance will facilitate progress in CNS developmental biology and axon-regeneration therapy.

Morphogens as growth cone signalling molecules

Morphogen signalling among cells is one of the most important mechanisms underlying the progressive patterning of embryos. Members of the hedgehog (Hh), wingless (Wnt), transforming growth factor-beta (TGFbeta), and fibroblast growth factor (Fgf) families of extracellular signalling molecules act as morphogens. Recent studies have demonstrated that members of these four families of proteins, secreted by well-characterised organiser centres in the central nervous system (CNS) as floor plate or midbrain-hindbrain boundary, are reused at later developmental stages to control axon growth. Here, we have summarised the evidence for this novel idea with a particular emphasis on those related to Shh and Wnt signalling-the object of some works in our laboratory.

Proteoglycan control of cell movement during wound healing and cancer spreading

By virtue of their multifunctional nature, proteoglycans (PGs) are thought to govern the process of cell movement in numerous physiological and pathological contexts, spanning from early embryonic development to tumour invasion and metastasis. The precise mode by which they influence this process is still fragmentary, but evidence is accruing that they may affect it in a multifaceted manner. PGs bound to the plasma membrane mediate the polyvalent interaction of the cell with matrix constituents and with molecules of the neighbouring cells' surfaces; they modulate the activity of receptors implicated in the recognition of these components; and they participate in the perception and convergence of growth- and motility-promoting cues contributed by soluble factors. Through some of these interactions several PGs transduce to pro-motile cells crucial intracellular signals that are likely to be essential for their mobility. A regulated shedding of certain membrane-intercalated PGs seems to provide an additional level of control of cell movement. Coincidentally, matrix-associated PGs may govern cell migration by structuring permissive and non-permissive migratory paths and, when directly secreted by the moving cells, may alternatively create favourable or hostile microenvironments. To exert this latter, indirect effect on cell movement, matrix PGs strongly rely upon their primary molecular partners, such as hyaluronan, link proteins, tenascins, collagens and low-affinity cell surface receptors, whereas a further finer control is provided by a highly regulated proteolytic processing of the PGs accounted by both the migrating cells themselves and cells of their surrounding tissues. Overall, PGs seem to play an important role in determining the migratory phenotype of a cell by initiating, directing and terminating cell movement in a spatio-temporally controlled fashion. This implies that the "anti-adhesive and/or "anti-migratory" properties that have previously been assigned to certain PGs may be re-interpreted as being a means by which these macromolecules elaborate haptotaxis-like mechanisms imposing directionality upon the moving cells. Since these conditions would allow cells to be led to given tissue locations and become immobilized at these sites, a primary function may be ascribed to PGs in the dictation of a "stop or go" choice of the migrating cells.

Sugar codes for axons?

Proteoglycans are complex macromolecules with the potential for extraordinary diversity. Several recent studies have demonstrated important roles for heparan sulfate and chondroitin sulfate proteoglycans (HSPGs and CSPGs) in axon pathfinding and have linked HSPGs to specific signaling pathways. More speculatively, there are hints of a "sugar code," in which specific sugar modifications might act instructively in guidance decisions. This raises the intriguing possibility that the complexity of neuronal wiring may in part reflect the complexity of proteoglycan modifications.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

Here we review mechanisms and molecules that necessitate protection and oppose axonal growth in the injured spinal cord, representing not only a cast of villains but also a company of therapeutic targets, many of which have yet to be fully exploited. We next discuss recent progress in the fields of bridging, overcoming conduction block and rehabilitation after spinal cord injury (SCI), where several treatments in each category have entered the spotlight, and some are being tested clinically. Finally, studies that combine treatments targeting different aspects of SCI are reviewed. Although experiments applying some treatments in combination have been completed, auditions for each part in the much-sought combination therapy are ongoing, and performers must demonstrate robust anatomical regeneration and/or significant return of function in animal models before being considered for a lead role.

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.

Mechanisms of axon guidance in the developing nervous system

The human brain assembles an incredible network of over a billion neurons. Understanding how these connections form during development in order for the brain to function properly is a fundamental question in biology. Much of this wiring takes place during embryonic development. Neurons are generated in the ventricular zone, migrate out, and begin to differentiate. However, neurons are often born in locations some distance from the target cells with which they will ultimately form connections. To form connections, neurons project long axons tipped with a specialized sensing device called a growth cone. The growing axons interact directly with molecules within the environment through which they grow. In order to find their targets, axonal growth cones use guidance molecules that can either attract or repel them. Understanding what these guidance cues are, where they are expressed, and how the growth cone is able to transduce their signal in a directionally specific manner is essential to understanding how the functional brain is constructed. In this chapter, we review what is known about the mechanisms involved in axonal guidance. We discuss how the growth cone is able to sense and respond to its environment and how it is guided by pioneering cells and axons. As examples, we discuss current models for the development of the spinal cord, the cerebral cortex, and the visual and olfactory systems.